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Virtual Synchronous Power Strategy for Multiple HVDC Interconnections of Multi-Area AGC Power Systems
Abstract
Automatic Generation Control (AGC) in multi-area interconnected power systems is experiencing several adaptions due to increasing level of power converter based components in the system. The concept of Virtual Synchronous Power (VSP) to simulate the dynamic effects of virtual inertia emulations by HVDC links for higher level control applications is introduced and reflected in the multi-area AGC model. By using this proposed combination in AGC model, the dynamic performance of the studied system shows a significant improvement. The proposed formulation is generalized for Multi-areas with multiple HVDC links. The active power loop control in VSP based HVDC link has a second-order characteristic which makes a simultaneous enabling of damping and inertia emulations into the system. Trajectory sensitivities are also used to analyze the effects of VSP’s parameters on the system stability. The effectiveness of the proposed concept on dynamic improvements is tested through Matlab simulation of a four-area system.
... For further details on research successes on VIC readers may see [4] and [16]. It is noted from the literature survey [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] that various control schemes have been suggested for facilitating incorporation of RoCoF in MG system. So far the frequency and rotor angle instability and voltage fluctuations are all the key causes of microgrid destabilization and power blackouts in the past and the present. ...
... VIC based on the derivative method (a derivative of frequency response: df/dt) is considered to be one of the most successful ways to emulate virtual inertia power to boost frequency stability and system inertia. The HVDC linked system was subjected to derivative method-based inertia control, which improved frequency stability [17][18][19][20]. So, to reduce the frequency oscillation of large penetrated RESs in microgrid was investigated by Kerdphol et al. [21]. ...
... For a better understanding of the VID emulation and frequency management of the MG, the mathematical equations for describing the state space representation of the MG have been provided in (17) to (27). ...
Nowadays, there is extensive penetration of renewable energy sources (RESs) in microgrids such as solar power stations (SPS) and wind power stations (WPS). The RESs are power electronic converter-dominated systems that have zero inertia making the microgrid to have very low inertia. Low inertia microgrid has a high rate of change of frequency (RoCoF), and the frequency response is highly volatile. To cope with this issue virtual inertia and damping are emulated into the microgrid. Virtual inertia and damping, i.e., converter with short-term energy storage device (ESD), which delivers and absorbs electrical power depending on the frequency response of microgrid and minimizes the power variation between power generation and power consumption. In this paper virtual inertia and damping are emulated based on a novel two-degree of freedom PID (2DOFPID) controller optimized with African vultures optimization algorithm (AVOA) technique. The meta-heuristic technique, AVOA, tunes the gains of the 2DOFPID controller and also the inertia and damping gain of the virtual inertia and damping control (VIADC) loop. AVOA comes out to be superior to other optimization techniques when compared in terms of convergence rate and quality. The performance of the proposed controller is compared to other conventional control methodology that has demonstrated its better performance. The dynamic response of such a proposed methodology in a microgrid model is verified in an OPAL-RT real-time environmental simulator, i.e., OP4510.
... In fact, frequency and voltage instability are two major phenomena that may cause microgrid instability leading to power blackouts. In order to improve the inertia in microgrid virtual inertia support (VIS) scheme has been used [15][16][17][18]. As such few articles have considered controllable derivative technique-based VIS scheme in recent time using ultracapacitor unit for frequency regulation of power network [16][17][18][19][20][21][22][23][24]. ...
... In order to improve the inertia in microgrid virtual inertia support (VIS) scheme has been used [15][16][17][18]. As such few articles have considered controllable derivative technique-based VIS scheme in recent time using ultracapacitor unit for frequency regulation of power network [16][17][18][19][20][21][22][23][24]. The frequency stability of the high voltage DC-linked system was improved in [16][17][18] by applying the derivative method-based inertia control. ...
... As such few articles have considered controllable derivative technique-based VIS scheme in recent time using ultracapacitor unit for frequency regulation of power network [16][17][18][19][20][21][22][23][24]. The frequency stability of the high voltage DC-linked system was improved in [16][17][18] by applying the derivative method-based inertia control. Sockeel et al. [25] suggested controlling the inertia power using a derivative technique using model predictive control. ...
This paper presents a novel control scheme for combined frequency and voltage stabilization of an islanded multi-generator hybrid microgrid (IHμG). The control concept incorporates an improved virtual inertia support scheme (IVIS) and the recently developed yellow saddle goatfish technique (YSGA) to obtain optimal control parameters. IHμG model consists of an AVR-based voltage compensating loop for synchronous biodiesel generator, wind generator, wave generator, photon exchange membrane fuel cell (PEMFC), and controllable heat pump and freezer. An integer order proportional-integral-derivative (IOPID) controller is leveraged for frequency-voltage stabilization. A comparative response assessment has been performed with/without IVIS. The utilization of YSGA has been justified by comparative assessment with particle swarm optimization, firefly, and sine-cosine techniques. A meticulous performance evaluation of YSGA optimized IOPID control scheme in the IHμG has been conducted through several case studies. Furthermore, the rigorous sturdiness assessment of YSGA optimized IOPID controller was performed under different uncertainties such as: variation of amplifier gain, ±30% variation in demanded loading magnitude, moment of inertia and droop coefficient. Finally, real-time hardware-in-the-loop (HIL) simulation platform is utilized to validate the proposed control approach.
... With the development of HVDC tie-lines and FACTS devices, many researchers were interested in improving the power transfer capabilities by providing flexible operation and control in AGC studies [198][199][200]. HVDC and FACTS devices play a key role in improving the system performance in terms of real power flow, real power loss minimization, and damping ratio improvement resulting in a low number of oscillations, and lesser values of peak overshoots, and quick settling time [201][202][203][204][205]. The right selection of FACTS controller from among the different available options and its ideal location in the PS is a critical decision [206,207]. ...
... Later Saikia et al. proposed an AHVDC system for a three-area LFC system under both conventional and deregulated environments considering cascade controllers [55][56][57]. Also, HVDC studies considering phase-locked loops and virtual inertia are carried out for LFC systems [140,204,205]. The effect of HVDC integration with the numerical values is shown in Table 4. ...
This review article aims to provide an in-depth analysis of the literature along with comprehensive bibliography on automatic generation control (AGC)/load frequency control investigations. Different control perspectives concerning frequency and power control have been featured. Diverse linear, non-linear power system models are discussed under conventional and deregulated environments considering various power generation sources, including conventional, renewable energy sources (RES), and realistic RES. Moreover, AGC literature briefly explains various secondary controllers like integer order, fractional order, intelligent, cascade, and some recently used controllers. To obtain the optimum values of secondary controllers, various optimization strategies such as numerical approach, heuristic, and meta-heuristic techniques for AGC issues are mentioned. Also, AGC studies concerning power transmission, considering inertia and phase-locked loops in high voltage direct current (HVDC) and accurate HVDC models are discussed. Further, AGC literature integrated with flexible alternating current transmission system devices in loaded transmission lines and energy storage devices due to intermittent power generation in RES is deliberated. Furthermore, various performance index criteria (PIC) such as standard PIC and hybrid peak area (HPA)–PIC for controller optimization with algorithms are also conferred.
... The obtained results provided appreciable results in terms of reduction in peak deviation in the frequency and tie-line power, thereby improving stability. In a similar attempt in [12], the authors implemented the virtual synchronous power in HVDC links to emulate inertia in a multi-area power system. The authors reported sufficient improvement in the stability of the system with the proposed virtual synchronous power-based HVDC links. ...
The widespread adoption of power converter-based renewable energy sources (RESs) has led to a significant decline in overall system inertia within interconnected power systems. This reduction in inertia poses a significant challenge, as it increases the susceptibility of the interconnected power system to instability. To address this critical issue, this research proposes an application of virtual inertia control as a means to enhance the frequency stability of interconnected power systems characterized by a high penetration level of RESs. The proposed approach leverages a derivative control technique to enable higher-level virtual inertia emulation. By introducing a second-order characteristic into the virtual inertia control loop, the method emulates inertia, resulting in improved frequency stability and enhanced system resiliency. A dynamic model of microgrid linked is developed and the frequency stability is ensured by a Fractional Order Proportional Integral Derivative (FOPID) controller using Teaching Learning Based Optimisation (TLBO) algorithm. The problem is formulated with Integral Time Squared Error (ITSE) of frequency deviation for determining the controller. The performance of the developed controller is compared against conventional controllers in terms of the stability of the microgrid. In addition, the superiority of TLBO is analysed by comparing it with other well-established algorithms in the literature. The suitability is established under various scenarios of load and renewable uncertainties. Furthermore, the work also includes sensitivity verification of the controller by parametric variation of the range of .
... The outcome was an inadequate frequency response for the system. In order to solve this problem, virtual damping is suggested in [17], and the system's fast settling/stabilizing time is calculated using frequency deviation. In order to examine the dynamic effects of new virtual damping on the system performance, eigenvalue trajectory analysis is presented in [9]. ...
... This power is used most of the time to simulate the dynamic effects of virtual inertia emulations by HVDC links, particularly in AGC systems. A multi-area AGC system is presented in [428], where higher-level control applications are studied. Using this concept in AGC contributed to a clear improvement in the system's dynamic performance. ...
Load frequency control (LFC) is one of the most important tools in power system control. LFC is an auxiliary service related to the short-term balance of energy and frequency of power systems. As such, it allows the acquisition of a central role in enabling electricity exchanges and providing better conditions. The classification of LFC can be carried out from different angles: we can enumerate, among others, the type of control used. The following types of control are presented in this review: classical, optimal, and robust control. More advanced controls can also be used for classification: fuzzy logic control, ANN control, genetic algorithms, PSO control, etc. The influence of renewables and power control tools like FACTS is also considered as a category to be analyzed. The last classifications are related to two important subjects—the influence of DC links on LFC efficiency and the dangers of cyberattacks on the LFC.
... The authors of [20] utilized voltagedependent loads coupled with electric spring technology as smart loads for enhancing the system's inertia. There is evidence that virtual inertia can be controlled and improved by utilizing the virtual synchronous generator (VSG), which improves the voltage and frequency of weak (low-inertia) MGs [21][22][23][24][25]. Similarly, using VSG technology can also help improve the system's stability, allowing for greater control and responsiveness in the network. ...
The increase in renewable energy resources penetration into microgrids weakens the inertia of microgrids and negatively affects their frequency stability. This reduction in inertia reduces the microgrid’s ability to handle disturbances and increases frequency instability risk. The purpose of this paper is to present a virtual inertia (VI) control strategy for an island microgrid with the use of an energy storage system (ESS) to improve inertia and frequency stability of the system. The VI controller parameters are optimized using the differential evolution (DE) algorithm. The proposed VI control strategy can effectively improve microgrid frequency stability and reduce the risk of frequency instability. The results demonstrate that the proposed strategy is effective and can provide fast and reliable virtual inertia support for the microgrid. The proposed VI control approach is verified through simulation results and performs better than the conventional inertial response for frequency stability in islanded microgrids. Virtual inertia control injects/absorbs active power into/from the microgrid during frequency deviations. Different disturbances are applied to demonstrate uncertain load and RES generation behavior. The proposed VI control method is compared with the available literature. In the proposed method, the frequency deviation is better controlled. Furthermore, the results demonstrate a more stable and efficient system operation with the developed VI controller than the existing methods.
... [5][6][7] This technique has been extensively applied and has achieved considerable results, particularly in the field of automatic generation control (AGC). 8,9 From the application characteristics of VSG, it is clear that it will be the key to making full use of new energy sources and ensuring stability in the power system. ...
Virtual synchronous generator (VSG) technology has achieved some results in distributed generation networks and enhanced system stability. For problems that may occur in the system due to ignoring parameter linkages, this paper designs a synergistic controller that establishes the connection between the active frequency and the reactive voltage control loop based on synergy theory. As the output of the synergetic controller, the derived control law u is added to the power frequency control in the form of negative feedback. Two disturbance forms, a step disturbance, and a three-phase short-circuit fault, are set up. The conventional droop control and synergetic droop control results are compared using a MATLAB simulation system. Then, the system parameter variations were studied and analyzed, and the sensitivity of the control system with different values of parameters K1, K2, and K3 in the case of a three-phase short circuit was analyzed, which provides a reference guide for selecting parameters in later control optimization. Finally, the control effectiveness of a multi-machine VSG system was tested. The simulation results show that the proposed method has a pronounced effect on making the system stabilize quickly, which illustrates the effectiveness of this technique.
... To adjust the latter to suppress the oscillation, the difficulty lies in how to quantify the trajectory sensitivity (TS), which be solved with perturbation method or analytical expression [21]. The perturbation method needs 2 time-domain simulations, thus is computationally expensive [22], [23]. The analytical expression finds the TS with one simulation, hence is more effective. ...
In the power systems with high-voltage DC (HVDC) transmission, the angular oscillations among the synchronous generators (SGs) endanger the system security. The pre-fault power of the HVDC changes the power flow and affects the system dynamics, hence may be optimized to improve the system stability. However, the HVDC power does not appear in the system matrix, thus it is difficult to derive the analytical expression of the trajectory sensitivity (TS) for parameter optimization. Based on the power flow equations, the initial value of the TS is newly introduced in the sensitivities of the power angles of the SGs with respect to the intermediate variables and then to the DC powers. The imbalanced matrix of the TS is derived during the time-domain simulation. By setting the objective function as the integral of the square of the relative value of the SGs’ power angle deviation with time, and summing the trajectory sensitivity for all time steps, the gradient information of the objective function to the DC powers is newly derived. The optimization model is solved with the interior point method to adjust the steady-state HVDC powers. The effect of the power optimization on the angular oscillation is verified with the time-domain simulation. The numerical results show that the objective function is reduced by 37.92%, and the oscillation amplitude is reduced by 35%.
... Furthermore, with the advantage of creating pollution-free energy, renewable energy resources are increasingly being incorporated into the MSMA system. To gain a thorough understanding of the system's frequency control, the power system model, which comprises a variety of conventional and renewable energy sources, was coupled with an HVDC connection that ran parallel to the existing AC tie line [11]. The parallel configuration of HVDC and AC tie line might improve inter-area power exchange competency, adding to the stability of the interconnected system [12,13]. ...
In the recent decades, insufficient power output has led to the inclusion of renewable energy sources into microgrid systems. However, uncertainty in renewable energy supply and load variation has an impact on system frequency, influencing the stable operation of a microgrid system. An intelligent controller for continuous electric power is necessary to supplement the system’s reliable operation. This paper proposes the use of cascaded PIDFN controller based on modified differential evolution (MDE). Moreover, in order to facilitate improved power exchange, a modified HVDC link is incorporated in the hybrid renewable energy system (HRES). The system is validated in MATLAB®/SIMULINK using load perturbations and system uncertainties and compared with the conventional HVDC link.
... Several investigations on constructing VI control to improve the frequency response have been carried out in the field of LFC systems. For HVDC interconnected systems, derivative-based VI control was applied [30,31] for frequency stabilization with elevated RES immersion in a two-area LFC system [32]. Moreover, cutting-edge control design techniques, like model predictive control [33], H ∞ robust control [34] and robust coefficient diagram method [35], were employed to implement the VI control, enhancing the system frequency's robustness. ...
The utilization of communication linkages generates delays that degrade system dynamic performance and cause fluctuations in the system frequency while the extensive integration of renewable energy sources may cause further increase in the frequency oscillations owing to low inertia. By considering the inclusion of the virtual inertia and damping (VID) control in load frequency control (LFC) systems with time delays, this article investigates the impacts of the VID control on the stability delay margins (SDMs) of two-area LFC systems. For this purpose, an exact method, which is based on recursively removing exponential terms from the characteristic equation, is first implemented to compute SDMs of the two-area LFC system enhanced by VID control for a wide range of controller gains. Secondly, the effect of the VID parameters on the SDMs is investigated by employing the exact method. Theoretical results clearly illustrate that SDMs significantly increase as VID parameters increase. Furthermore, simulation analysis elucidates that with the VID incorporation, the performance of the LFC system with low inertia is enhanced and the rate of change of the frequency and frequency nadir are remarkably decreased, improving the stability of time-delayed LFC systems.
... In addition to frequency regulation of synchronous generator (SG), for converter-based resources such as RES, HVDC, load and energy storage, the inertia emulation [20][21] and fast active power regulation (FAPI) [22][23] can also be implemented by controlling their grid-connected converters, to support the inertia and frequency of low-inertia power system. Common frequency control methods for converters like droop control [24], virtual inertia (VI) control [25], virtual synchronous machine (VSG) control [21], [26], virtual synchronous power (VSP) control [27][28], can all improve the frequency response of power systems. Therefore, for low-inertia power systems, it is a research focus that fully tap the frequency response capability of source-grid-load-storage links to achieve frequency regulation control and design optimal operation strategies. ...
With the integration of large-scale renewable energy sources (RES) and high voltage direct current (HVDC) based inter-regional transmission, modern power systems are facing low-inertia issues and poor frequency regulation capabilities. This paper conducts a comprehensive review on frequency control and optimal operation methods in this context. First, the frequency characteristic of a low-inertia power system is analyzed covering the whole frequency response process. Secondly, the impacts of RES and HVDC integration on system inertia and frequency performance are studied, and the system frequency characteristics considering RES and HVDC are analyzed. Then, the frequency response modes of various frequency regulation resources are compared from the perspective of “source-grid-load-storage”, including their basic control principles and frequency control strategies. Further, from the operation dispatch perspective, the frequency security-constrained power system optimal operation methods are reviewed. Finally, future research trends in this area are discussed.
... Hence, there is a need to emulate the inertia into the system so that the system's transient stability can be improved. The inertia emulation control (INEC) strategy is proposed recently, which supports the AC network during and following disturbances, with minimal impact on the systems connected beyond the HVDC system's terminals (Rakhshani et al., 2017). ...
This is an edited book, which is the collection of research works published in a Research Topic titled "Sustainable planning and life-cycle thinking of energy infrastructure" in Frontiers in Energy Research. The aim of this collection is to enable the sustainable transformation of energy infrastructure during its implementation, modernization, expansion, decommissioning, and end-of-life from a science, technology, engineering, and policy perspective. A total of 22 research works were included in this edited book and they briefly fall under energy infrastructure dedicated to the power sector and other interdependent and dependent sectors.
... Hence, there is a need to emulate the inertia into the system so that the system's transient stability can be improved. The inertia emulation control (INEC) strategy is proposed recently, which supports the AC network during and following disturbances, with minimal impact on the systems connected beyond the HVDC system's terminals (Rakhshani et al., 2017). ...
The direct coal liquefaction process usually produces a liquefaction residue of about 30% of its coal feed. The effective utilization of the coal liquefaction residue (CLR), which contains about 80% organic matter, is of great significancefor improving the oil yield of the direct liquefaction process and reducing the amount of pollution emitted from this oil-containing organic solid waste. In this study, the CLR fluidized bed pyrolysis process was studied through a fluidized bed reactor pyrolysis experiment and steady-state thermal analysis. The characteristics of CLR were first analyzed, and then the pyrolysis experiment was conducted in a fluidized bed reactor system. The experiment results show that the oil yield is 34.81% at 540°C for an ash-free feedstock using a fluidized bed pyrolysis reactor. Based on the pyrolysis experimental data and the Aspen Plus software platform, the fluidized bed pyrolysis reactor, fractionation column, coke burner, gasifier, and other equipment were modeled to compare four different process schemes. CLR pyrolysis is an endothermic reaction, and its heat is usually supplied by coke combustion, which produces significant CO2 emissions. Case studies of the pyrolysis process were performed in detail in order to reduce CO2 emissions. Thermal efficiency, carbon efficiency, solid waste discharge, and CO2 emissions of the different schemes were compared, and a flexible fluidized pyrolysis (FFP) process coupled with water electrolysis was proposed. The introduction of green hydrogen and green oxygen to the process can realize near-complete utilization of carbon and hydrogen elements in the CLR and produce high-quality liquid fuels and syngas (for further chemical synthesis), and this new process can achieve almost near-zero CO2 emission in the entire unit. This process principle can also be applied to the CO2 emission reduction of organic solid waste pyrolysis, catalytical fluid cracking, fluid coking, and so on.
... The Δu c (s) = Δf(s)(J VSG • s +D VSG ) +d(s) is the stabilizing power change from the VSG controller, which is feedbacked to the MG or RES-based CIG systems. As a result, the dynamic transfer function of the proposed VSG control can be derived in the frequency domain (s) as [20,23,24]: (19) where J VSG and D VSG are the virtual inertia and damping constants, respectively, and T CIG is the time constant of the CIG converter. ...
Effects of forced oscillations (FOs) in well-damped power systems are relatively smaller than those of microgrids (MGs) in which the severity of the FOs may be intensified by converter interfaced generators (CIGs). According to the distinct system characteristics, the FOs in MGs will be challenging problems in future research topics. Without proper controls of the CIGs, the FOs may be exhibited extremely higher amplitude, resulting in the MG instability. Such influences will be exacerbated in a low-inertia MG, which can trigger critical frequency oscillation and system collapse. This paper introduces an extended virtual synchronous generator (VSG) with virtual forced components to attenuate the dynamic FO effects in the presence of a low-inertia MG. Contrastive scenarios, i.e., periodic, combined, full sine, and high-frequency FOs are conducted to validate the performances of VSG control in both stand-alone and interconnected MG environments. Numerical results verify that the extended VSG control provides promising benefits in a low-inertia MG not only for the sake of MG stability improvement but also the further FO suppression.
... The proposed IPS has two areas connected through a high-voltage direct current (HVDC) link see Fig. 1. Its modeling is given in Supplementary Information section titled two-area IPS modelling in the presence of SEVB [15,16,[30][31][32]; the related parameters are given in Table S1. We assumed Ontario state in Canada as the region of operation considering power system resilience issues and potential interconnections. ...
Parked electric vehicles (EVs) participation is thought to be a low-carbon and resilient solution in addressing the load frequency control (LFC) problems in interconnected power systems (IPS). But this participation results in multiple issues such as trust, traceability, and transparency (3Ts) for EV owners in terms of financial compensation, battery effects, and range anxiety; early EV batteries end-of-life (EoL) management by recyclers; and burden on critical materials for new EV batteries manufacturing. Hence, to avoid the highlighted issues while addressing LFC, we propose a product life-extension circular business model and blockchainification of non-digital assets inspired spent EV battery (SEVB) buffer storage system. Two area IPS where areas 1 and 2 having a tie-line power agreement, SEVB storage system, particle swarm optimization (PSO)-based controller, and blockchain-based participatory smart contracts (BC-PSC) are modeled. Resilience Performance, Life Cycle Analysis, and Techno-Economic (RePLiCATE) approach is used for realizing the feasibility. Results showed BC-PSC protocols and PSO effectively obtained the controller's optimal parameters under disruptive scenarios ranging from 5 to 20%. The recovery times went from 18.60 to 46.43 sec. SEVB in LFC produces 18.71% less global warming potential than parked EVs. Revenues for SEVB owners were 122.27%, 100.22%, and 66.67% higher than cash spent on charging at off-peak, mid-peak, and on-peak, respectively.
... The authors of [17] introduced a load frequency control depending on model predictive control design for the studied µG, which depends on the coordination of wind turbines and plug-in hybrid electric vehicles. Sedghi, in [18], used a combination of robust control and fuzzy logic control strategies to regulate the frequency of the system. One of the recent solutions that can overcome the previously mentioned problems in the power system, i.e., organizing the participation of the µG distributed energy sources without causing any stability or feasibility problems, was provided using power electronic devices through which the behavior of the synchronous generator is virtually emulated. ...
High penetration of renewable energy sources into isolated microgrids (µGs) is considered a critical challenge, as µGs' operation at low inertia results in frequency stability problems. To solve this challenge, virtual inertia control based on an energy storage system is applied to enhance the inertia and damping properties of the µG. On the other hand, utilization of a phase-locked loop (PLL) is indispensable for measuring system frequency; however, its dynamics, such as measurement delay and noise generation, cause extra deterioration of frequency stability. In this paper, to improve µG frequency stability and minimize the impact of PLL dynamics, a new optimal frequency control technique is proposed. A whale optimization algorithm is used to enhance the virtual inertia control loop by optimizing the parameters of the virtual inertia controller with consideration of PLL dynamics and the uncertainties of system inertia. The proposed controller has been validated through comparisons with an optimized virtual inertia PI controller which is tuned utilizing MATLAB internal model control methodology and with-based virtual inertia control. The results show the effectiveness of the proposed controller against different operating conditions and system disturbances and uncertainties.
... The virtual inertia control system contains an energy supply, an inverter, and appropriate virtual inertia emulation control unit. Using the derivative technique described in [42,43], the virtual inertia component is constructed by computing the RoCoF for adjusting the excess power to a set-point of the MG-VI system during a disturbance. The virtual damping component is utilized for a speedy settling/stabilization period according to the system's frequency fluctuations. ...
Large-scale deployment of renewable energy sources (RESs) contributes to fluctuations in the system frequency due to their inherent reduced inertia feature. Time delays have emerged as a major source of concern in microgrids (MGs) as a result of the broad adoption of open communication networks since significant delays inevitably reduce the controller's performance and even cause instabilities. In this article, a frequency-domain direct method is used to evaluate the impact of the virtual inertia (VI) control on the stability delay margins of MG with communication delays. By avoiding approximation, the approach first removes transcendental terms from characteristic equations and turns the transcendental characteristic equations into regular polynomials. With this method, roots of the original characteristic equation on the imaginary axis correspond to exactly the positive real roots of the new regular polynomial not including any exponential term. This new polynomial can be used to find out whether the system stability is delay-dependent or not and enables us to compute the stability delay margin for the delay-dependent stability case. The proposed analytical method is utilized for evaluating stability delay margins with regard to system parameters for various values of proportional-integral (PI) gains where the MG is marginally stable. Moreover, the quantitative effect of virtual inertia and damping gains is comprehensively investigated. Based on the results, it is concluded that incorporating VI control enhances stability delay margins and enhances the MG's stability performance. Theoretical delay margin results are verified using time-domain simulations and a quasi-polynomial mapping-based root finder (QPmR) algorithm.
This article demonstrates the practice of real-time laboratory (RT-Lab) in bilateral transactions of AGC studies. Investigations determine the optimal location of superconducting magnetic energy storage (SMES) and interline power flow controller (IPFC) in a thermal-renewable system comprising of realistic dish Stirling system in area-1, 2, and precise wind system in area-3 with the integration of accurate model of high voltage direct current (AHVDC) transmission. In addition, a new cascading controller combining a fractional order proportional integral and tilt integral derivative filter (FOPI-TIDN) and their parameters are enhanced by an emperor penguin optimizer technique considering hybrid peak area integral squared error (HPA-ISE) as a new performance criteria. The superiority of the FOPI-TIDN controller is evaluated over FO-PI and TIDN and better dynamics are observed with FOPI-TIDN. Moreover, system dynamics obtained at optimum gain parameters with HPA-ISE shows superior performance over ISE. Further, the response comparison with RT-Lab shows better dynamics than MATLAB Software responses. The responses of the thermal-renwable system unified with SMES, AHVDC and IPFC show better dynamics over the AC and AC-AHVDC system. Furthermore, investigations suggest the ideal setting of SMES-IPFC in fewer capacity areas.
Long-distance HVDC-LCC (High Voltage DC - Line Commutated Converter) transmission utilizes thyristor-based converters and offers advantages such as high efficiency and low cost compared to AC transmission. However, it also suffers from drawbacks such as thyristor commutation failure (CF), a critical issue leading to power interruptions. The concern of CF intensifies when multiple HVDC-LCC inverters are electrically close, causing multi-infeed interaction. To address this problem, this paper proposes the series voltage conditioner (SeVC) to mitigate CF. The SeVC employs six single-phase Modular Multilevel Converters (MMC) based on full-bridge submodules for a 12-pulse LCC. This paper aims to propose the SeVC to provide a feasible solution for real-world applications by achieving high performance in mitigating CF while operating with reduced rated power well below that of the LCC. Additionally, the proposed control method ensures a reduction of submodule capacitance. Simulation results are presented, comparing the SeVC approach with other methods. The results also include the outstanding performance of the SeVC in mitigating CF within the Brazilian power system with its six HVDC-LCC bipoles.
Accurate and generalized collaborative prediction of multi-cluster renewable energy power generation is both an inevitable trend and urgent demand as the growth of multi-region interconnected power grids with wind and photovoltaic (PV) power. In this paper, a novel heterogeneous spatiotemporal graph convolution network (HSTGCN) is proposed for ultra-short-term multi-modal prediction oriented to wind-PV power, which sufficiently considers spatiotemporal correlations in tens of wind farms or PV stations of each neighboring region and effectively coordinates the heterogeneities of different power generation types in different regions. This approach first designs a dynamical heterogeneous graph structure including modes, nodes, and edges to give a unified framework evolving over time for different interdependencies in the multi-cluster wind and PV sites, and then develops a hierarchical spatiotemporal learning mechanism to enhance representation power for multi-cluster prior information from temporal and spatial dimensions, integrating 2-D CNN with different sizes of filters and GCN embedded a specially designed lightweight graph convolution attention module (GCAM). Experiments including 57 operating wind farms and PV stations from 4 regions distributed over a broad spatial scale demonstrate the generalization and interpretation of HSTGCN compared with other commonly considered benchmarks.
This paper presents a control strategy based on the virtual synchronous generator (VSG) concept to improve the frequency support of converter-interfaced systems. The strategy has the flexibility to set virtual inertia, power-frequency droop, damping, and response speed. In previous VSG designs, the response speed depends on the virtual inertia; therefore, both specifications cannot be selected independently. In the present paper, this limitation is overcome by using a VSG structure able to set the response speed independently of the other design specifications, giving an additional degree of freedom to provide frequency support. In addition, the ability to select the speed at which the VSG draws power from the dc side allows for better use of the available dc power supply. The proposed VSG does not require phase-locked loop to achieve the damping effect; it has a simple implementation and a systematic design procedure. The control strategy is validated in a multimachine case study based on the Argentinian power system, including several VSGs. Aspects such as oscillation damping and improvements in the rate of change of frequency (RoCoF) and frequency nadir are discussed and compared with previous VSGs.
The Inverter-based generator lacks natural inertia, which leads to instability problems in Microgrid frequency. Virtual inertia is a promising control concept that rapidly injects/absorbs active power to/from the power system to ensure the frequency stability of low-inertia systems. This paper proposes an innovative virtual damping stabilizer (VDS) integrated into battery energy storage system (BESS) based virtual inertia and damping control loops. The proposed VDS adds supplementary damping and stabilizing capability to the BESS by properly correcting and controlling its inertia pumped/drawn power. The VDS aims to enhance frequency stability, decrease the necessary BESS large capacity, and lower the initial expense. Furthermore, the proposed VDS helps operate the BESS efficiently by smoothing the output power profile during transient disturbances, consequently elongating the lifetime. The Particle Swarm Optimization (PSO) method is utilized to get the optimal parameters of the proposed VDS. A frequency control model of an isolated microgrid with the proposed controller is presented, considering various operating conditions. Time-domain simulations and experimental validation using OPAL-RT verify the efficacy of the proposed VDS scheme. Sensitivity and eigenvalues analyses assess the VDS's dynamic stability enhancement and robustness against inherent uncertainties in Microgrid parameters. The results demonstrate a superior frequency response and a reduction in the BESS size compared to PSO and model predictive controller (MPC) based virtual inertia and damping strategies.
The transient behavior of Automatic Generation Control (AGC) systems is a critical aspect of power system operation. Therefore, fully extracting the potential of Battery Energy Storage Systems (BESSs) for AGC enhancement is of paramount importance. In light of the challenges posed by diverse resource interconnections and the variability associated, we propose an online optimization scheme that can adapt to changes in an unknown and variable environment. To leverage the synergy between BESSs and Conventional Generators (CGs), we devise a variant of the Area Injection Error (AIE) as a measure to quantify the ramping needs. Based on this measure, we develop a distributed optimization algorithm with adaptive learning rates for the allocation of the ramping reserve. The algorithm restores a larger learning rate for compliance with the ramping needs upon detecting a potentially destabilizing event. We demonstrate the effectiveness and scalability of the proposed scheme through comprehensive case studies. It is shown that the proposed scheme can improve the transient behavior of the AGC system by bridging the gap in ramping capability.
This article examines the effect of virtual inertia and damping (VID) control on the stability regions of time-delayed load frequency control (LFC) systems. Because of the substantial integration of renewable energy sources (RES) and the utilization of an open communication network for sending/receiving data and control signals, the system inertia significantly decreases, and inevitable delays are observed, both of which adversely affect the controller performance and frequency stability. For improving testability of LFC systems including RES and delays, this study proposes the integration of the VID control and determines stability regions using the method of stability boundary locus. The regions’ boundaries are substantiated by time-domain simulations and a quasi-polynomial mapping-based rootfinder (QPmR) algorithm. A wide-ranging evaluation of the VID coefficients’ effects on the stability region and the system’s frequency response are presented. Results evidently divulge that with the integration of the VID control, stability regions get larger, the frequency dynamic is improved, and unstable LFC systems with delays are stabilized.
With the large-scale integration of low-inertia renewable energy sources, increasing the virtual inertia of the power system by using Virtual Synchronous Power (VSP) technology through the control of HVDC converter stations is a promising approach. Considering the non-negligible influence of critical cyber parameters on the stability of the hybrid power system with VSP-based inertial emulation, this paper first proposes a new evaluation framework that fully considers the impact of the measurement period and various communication delays, based on mainstream measuring devices and communication standard. Theoretical results have been carried out for the stability analysis that the characteristics of the physical system and the power modulation controller have been taken into account. Further, we propose a mitigation method through a modified Smith compensator to reduce the impact of communication delays especially the malicious ones. To this end, we conduct numerical simulations on different measurement periods and three types of communication delays. A comparison study is performed on the conventional AC system, the AC/DC system and the hybrid AC/HVDC system with VSP-based inertia emulation. It shows that different measurement periods and communication delays could significantly influence the hybrid AC/HVDC system dynamic performance and even frequency stability. Fortunately, our mitigation method could help increase the robustness of the whole system and achieve frequency control targets even in the case of malicious delays.
Single widespread employment of renewable energy sources (RESs) contributes to the shortage in the inertia of the microgrid (MG). After this, frequency stability may regress as a result of power imbalance or minor load fluctuations. This paper proposes an explicit adaptive modified virtual inertia control (VIC) based on an online Archimedes optimization algorithm (AOA) identifier for MG containing thermal, wind, and solar photovoltaic power plants. The Rung Kutta approach is used to construct the proposed online identifier, which acts as a model of the MG. AOA predicts the coefficients of the online identifier based on the input and output of MG to mimic the frequency deviation of the MG online. AOA estimates online the inertia and damping coefficients of the VIC system with an energy storage device based on online AOA identifier coefficients. The frequency deviation of the MG based on the proposed explicit adaptive modified VIC is compared with the conventional VIC based on fixed parameters and the VIC system based on optimal parameters using AOA offline under mutation in loads, weather-dependent input, and MG parameters using MATLAB/Simulink software. Furthermore, the proposed explicit adaptive modified VIC based on an online AOA identifier is evaluated with the adaptive VIC system based on fuzzy logic control, which adjusts only the inertial gain online. The simulation results demonstrate the capabilities of the proposed explicit adaptive modified VIC to improve the frequency stability and enhance low-inertia islanded MGs with RESs.
This paper investigates the impact of virtual inertia and damping (VID) control on stability regions defined in the gain space of the proportional integral (PI) controller for a micro-grid (MG) with communication time delays. In a MG-centralized controller equipped with communication links, controller performance and the system frequency are adversely affected by time delays in communication channel. For improving the stability of the time-delayed MG, this study considers the VID control which is integrated to reduce the adverse effects of high penetration of renewable energy sources and to emulate the required inertia response and damping into MG having low inertia. Therefore, this paper aims to determine all stabilizing PI controller gains, defining a region in PI controller parameters space of the time-delayed MG system with VID control. The qualitative impact of VID control parameters on the stability regions and MG frequency responses is thoroughly analyzed. Results clearly demonstrate that the inclusion of the VID control loop considerably increases the stability regions and improves the frequency dynamic behavior. Finally, the accuracy of stability region boundaries is verified by quasi-polynomial mapping-based root finder algorithm and time-domain simulations.
With DC microgrids (DC MG) gaining attention, virtual inertia (VI) schemes for stabilizing the system voltage become increasingly important. With decreased inherent energy storage, conventional droop control causes severe fluctuations in the DC bus voltage. Hence, this paper proposes a VI scheme that emulates inertia by attenuating and low-pass filtering the converter’s capacitor current. Since the capacitor’s current is directly proportional to its rate of change of voltage, attenuating the capacitor current restricts its voltage change which would otherwise occur with higher capacitance. Thus attenuating the capacitor current virtually increases the system capacitance. However, since practical capacitors have some parasitic equivalent series resistance (ESR), attenuation of capacitor current cannot guarantee a distortion-less inertial response when ESR is high. It was found that cascading a low pass filter of time constant governed by the capacitance and ESR can compensate for its impact. Time-domain simulations performed in MATLAB/Simulink and experiments carried out in laboratory-scale setup validate the effectiveness of the proposed VI scheme.
The proliferation of inverter-based distributed energy resources (DER) makes the interfaced grid face a fast frequency response, and the resulted time-variant frequency features a considerable fluctuation that degrades the frequency stability. This frequency information can be estimated by the phase-locked loops/frequency-locked loops (PLLs/FLLs) embedded in the DER inverters, yet they are difficult to track this frequency and obtain its derivative quickly. This paper aims to tackle this problem. Firstly, a time-variant parameter model is proposed to reflect an accurate relationship between the frequency derivative and the input signal. Since the input signal is defined with ramp frequency, the given model overcomes the inherent time-invariant restriction of the well-known notch filter/model. Then, a nonlinear estimator is derived by the equality-constraint optimization to obtain the model's parameters. The nonlinear estimator avoids the slow convergence speed and biased time-variant parameter tracking induced by the linear estimation framework. The derived linearized model further proves that the proposed estimator can accurately track ramp frequency within 20 ms. Experimental tests under time-variant frequency disturbances verify the proposed estimator's faster and more accurate estimation performance than the commonly-used/recently published PLLs/FLLs.
An idea toward improving stability and performance of a power grid with numerous distributed generators (DGs) and renewable energy sources (RESs) is to compensate system inertia, damping, and weak dynamic characteristics, virtually. This may be established by grid connected converters (GCCs) with a primary power source and a flexible control system. This setup will then operate to emulate desirable dynamics, by effective shaping of the injected active and reactive powers to the grid. This approach provides a promising solution to tackle the grid stability and performance challenges in the presence of high penetration of DGs/RESs.
In this section, the idea of the GCC-based grid dynamic shaping is explained and its significant role in enhancing the performance and dynamics of renewable integrated power grids is emphasized. An overall scheme for a flexible GCC-based dynamic emulator is briefly introduced in Section 9.1. Section 9.2 addresses GCC-based dynamic shaping emphasizing virtual impedance, virtual Q-droop, virtual inertia, and virtual governor to enhance the power grid stability and performance. Section 9.3 discusses the role of GCC-based dynamic shaping in the grid ancillary service support. The issue is illustrated by two examples for the grid frequency regulation support. Section 9.4 addresses the application of GCC-based dynamic shaping in power grids with HVDC and low-frequency transmission systems. Finally, the chapter is summarized in Section 9.5.
To achieve renewable energy targets, more wind turbine generators (WTGs) are being integrated into many power networks around the world. However, unlike traditional synchronous generators, modern WTGs are driven by power electronic devices which provide almost zero inertia and frequency response to frequency related events such as generator tripping. Wind manufacturers and researchers have been working on synthetic WTG inertia support, which can better utilise the rotational nature of WTGs. However, synthetic inertia alone may be insufficient under certain circumstances to prevent automatic under frequency load shedding after generation tripping, which causes security concerns for network operation. Consequently, WTG active power control (APC, similar to governor control) should be activated to improve network security. However, APC will costly reduce WTG output from the maximum power point and result in financial concerns. Therefore when and how much APC service should be activated becomes an extremely important question, which has not been addressed in the literature. This study develops a new method to quickly estimate frequency response caused by generator tripping hence system operators can use this proposed method to continuously evaluate inertia and headroom competency and accordingly activate the amount of WTG inertia and APC required for reliable system operation.
The voltage source converter (VSC) station is playing a more important role in modern power systems, but the dynamic behavior of the VSC station is quite different from that of the synchronous generator. This paper presents the synchronous generator emulation control (SGEC) strategy for the VSC-HVDC station. The SGEC strategy is divided into the inner control loop and the outer control loop. The inner controller is developed for fast current and voltage regulations. An inertia element is introduced into the frequency-power droop to determine the command reference of the frequency, and the inertia response and the primary frequency regulation are emulated. In addition, the secondary frequency regulation can be achieved by modulating the scheduled power in the SGEC strategy. The time-domain simulation results demonstrate the VSC station with the proposed control strategy can provide desired frequency support to a low-inertia grid. Therefore, the SGEC strategy provides a simple and practical solution for the VSC station to emulate the behavior of a synchronous generator.
A new design of decentralized load-frequency controller for interconnected power systems with ac-dc parallel using Particle Swarm Optimization (PSO) algorithm is proposed in this paper. A HVDC link is connected in parallel with an existing ac tie-line to stabilize the frequency oscillations of the ac system. Any optimum controller selected for load frequency control of interconnected power systems should not only stabilize the power system but also reduce the system frequency and tie line power oscillations and settling time of the output responses. In practice Load Frequency Control (LFC) systems use simple Proportional Integral (PI) or Integral (I) controller parameters are usually tuned based on classical or trial-and-error approaches, they are incapable of obtaining good dynamic performance for various load change scenarios in multi-area power system. For this reason, in this paper the PI and I control parameters are tuned based on PSO algorithm method for the LFC control in the two-area power system. A two area interconnected thermal power system is considered to demonstrate the validity of the proposed controller. The simulation results show that the proposed controller provides better dynamic responses with minimal frequency and tie-line power deviations, quick settling time and guarantees closed-loop stability margin.
An integrated design method for emergency frequency control of interconnected AC/DC power systems with centre of inertia (COI) signals is proposed in this study. It aims to provide emergency power support from reserve-rich area(s) to the disturbed control area(s) through the paralleled AC and DC tie-lines under extreme conditions, when the normal automatic generation control (AGC) is suspended. With the assumption of area COI frequency and the employment of wide-area measurement system (WAMS), the centralised control strategy drives the area COI frequencies to track the overall system COI frequency by virtue of coordination control of emergency generation and high voltage direct current (HVDC) power such that the long-term frequency stability can be promisingly improved. The adaptive backstepping sliding-mode approach holds desired robustness to time-varying generator parameter, modelling error and area load disturbances. Simulations have been conducted on a 3-area-interconnected AC/DC system in computer and the results demonstrate the feasibility and effectiveness of the proposed design method.
The future power systems face several challenges; one of them is the use of high power converters that virtually decouple primary energy source from the AC power grid. An important consequence of this modified the total system inertia and affecting its ability to overcome system frequency's disturbances. The wind power industry has created a controller to enable inertial response on wind turbines generators: Synthetic Inertial. This paper evaluates the effects of the synthetic inertia provided by wind turbines on the total system inertia after a system frequency disturbance. The main contribution of this paper is to demonstrate that during an under-frequency transients on future power systems, the synthetic inertia not completely avoid worse scenarios in terms of under-frequency load shedding. The extra power delivered from a wind turbine during frequency disturbances can increase “momentary” the total system inertia and substantially reduce the rate of change of frequency providing time for the active governors to respond. However, synthetic inertia might not completely avoid under-frequency load shedding.
In electricity grids the frequency of the voltage is stabilized by a combination of the rotational inertia (rotating mass) of synchronous power generators in the grid and a control algorithm acting on the rotational speed of a number of major synchronous power generators. When in future small non-synchronous generation units replace a significant part of the synchronous power generation capacity, the total rotational inertia of the synchronous generators is decreased significantly. This causes large frequency variations that can end up in an unstable grid. A way to stabilize the grid frequency is to add virtual rotational inertia to the distributed generators. A virtual inertia can be attained for any generator by adding a short-term energy storage to it, combined with a suitable control mechanism for its power electronics converter. In this way a generator can behave like a ldquoVirtual Synchronous Generatorrdquo (VSG) during short time intervals, and contribute to stabilization of the grid frequency.
In this paper, the idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed. We call the inverters that are operated in this way synchronverters. Using synchronverters, the well-established theory/algorithms used to control SGs can still be used in power systems where a significant proportion of the generating capacity is inverter-based. We describe the dynamics, implementation, and operation of synchronverters. The real and reactive power delivered by synchronverters connected in parallel and operated as generators can be automatically shared using the well-known frequency- and voltage-drooping mechanisms. Synchronverters can be easily operated also in island mode, and hence, they provide an ideal solution for microgrids or smart grids. Both simulation and experimental results are given to verify the idea.
A generic inertia emulation controller (INEC) scheme for multi-terminal voltage-source-converter (VSC)-based high voltage direct current (HVDC) systems is proposed in this study. The INEC can be incorporated in any grid-side VSC station, allowing the multi-terminal HVDC (MTDC) terminal to contribute an inertial response to connected AC systems during system disturbances, in a fashion similar to synchronous generators. The DC-link capacitors within the MTDC are utilised by the INEC scheme to exchange stored energy with the AC system by varying the overall DC voltage level of the MTDC network within a safe and pre-defined range. A theoretical treatment of the INEC algorithm and its implementation and integration within a conventional VSC control system are presented, and the impact on the total DC capacitance required within the MTDC network to ensure that DC voltages vary within an acceptable range is discussed. The proposed INEC scheme is validated using a MATLAB/SIMULINK model under various changes in demand and AC network faults. The
model incorporates a multi-machine AC power system connected to a MTDC transmission system with multiple converter interfaced nodes. The effectiveness of the INEC in damping post-fault oscillations and in enhancing AC grid frequency stability is also investigated.
Recent advancements in power electronics have made HVDC links and renewable based generation more popular in power systems application with better grid support functionalities like frequency control and inertia emulation tasks. Conventional operation and control strategies are undergoing of different changes and all the infrastructure of future modern power system should efficiently support the delivery of ancillary services in complex scenarios of AC/DC multi-area interconnected system. The AGC system of tomorrow must be able to handle complex interactions between control areas with HVDC links and distributed generation equipment. In such scenario, the effects of wide-area interconnections, PLL (Phase Locked Loop) and frequency measurements cannot be ignored. The dynamics effects of PLL and frequency measurements are very important for HVDC operation. For obtaining an acceptable performance of AC/DC system, the dynamic models of PLL and measurements need to be taken into account. This paper focused on the effects of PLL and frequency measurements in frequency supports of HVDC interconnected system. A novel approach for analyzing the dynamic effects of HVDC links considering PLL effects during coordination with AC system is presented and discussed. The effects of PLL are considered by introducing a second-order function. A Pade approximation method is also introduced for adding the effects of communication delays on AGC operation and the state space models are presented. The proposed model is analyzed for different multi-area test systems which contains parallel AC/DC transmission links.
Due to increasing level of power converter-based component and consequently the lack of inertia, automatic generation control (AGC) of interconnected systems is experiencing different challenges. To cope with this challenging issue, a derivative control-based virtual inertia for simulating the dynamic effects of inertia emulations by HVDC (high-voltage direct current) interconnected systems is introduced and reflected in the multi-area AGC system. Derivative control technique is used for higher level applications of inertia emulation. The virtual inertia will add an additional degree of freedom to the system dynamics which makes a considerable improvement on first overshoot responses in addition to damping characteristics of HVDC links. Complete trajectory sensitivities are used to analyse the effects of virtual inertia and derivative control gains on the system stability. The effectiveness of the proposed concept on dynamic improvements is tested through Matlab simulation of two-area test system for different contingencies.
This paper proposes an improved model of a high voltage dc (HVDC) link capable of providing frequency support to networks with no inertia or near-zero inertia; for instance, ac grids with small synchronous generators. The model is useful to carry out steady-state and dynamic simulations of ac grids described by their positive-sequence representation using phasorial information. The core of the frequency control scheme put forward in this paper uses the angular aperture that exists between the internal phase-shifting angle of the voltage source converter (VSC)-HVDC rectifier and the voltage angle at its ac terminal. This is amenable to power flow regulation in the dc link and, hence, to frequency control in the low-inertia grid. A feature of this model is that the developed VSC-HVDC link model may also be used to feed an island system; for instance, a system with dead load, where the inverter station provides the electrical angular reference. Hence, the inverter acts as a virtual synchronous generator with frequency regulation capabilities as seen from the low-inertia ac grid. The dynamic control scheme that enables the VSC-HVDC to provide frequency control in such operating environments has been comprehensively investigated under a wide range of credible scenarios. Overall, the dynamic system of equations describing the VSC-HVDC and the synchronous generators are discretized using the trapezoidal rule, and the ensuing equations are combined together with the algebraic equations of the ac and dc grids in a linearized reference frame amenable to iterative solutions using the Newton-Raphson method.
LFC (Load frequency control) in power systems is very important to supply reliable electric power with good quality. In general, proportional PI (plus integral) controller is used for the LFC, which is incapable of obtaining good dynamic performance for a wide range of operating conditions. Type-I fuzzy logic controller (T-I FLC) is a sophisticated technique to use, but the designing of membership functions and control rules is an important function.. To achieve satisfactory membership function and control rules, a designer's experience is necessary. This conflict may be checked up byapplying the principle of Type-II fuzzy logic controller (T-II FLC) to use expert knowledge and being adaptive in nature. The dual-mode concept is incorporated in the proposed controller to improve the system performance. In this work, SMES (superconducting magnetic energy storage) unit and parallel alternating current – direct current (AC–DC) tie-lines are included with base model to study their performance under different operating conditions. The simulation results were analyzed using MATLAB 2014, proving that the superior performance of the proposed controller provides very good transient and steady state response when compared to PI and T-I FLC under different operating conditions and parameter variations.
The virtual synchronous generator (VSG) is a control scheme applied to the inverter of a distributed generating unit to support power system stability by imitating the behavior of a synchronous machine. The VSG design of our research incorporates the swing equation of a synchronous machine to express a virtual inertia property. Unlike a real synchronous machine, the parameters of the swing equation of the VSG can be controlled in real time to enhance the fast response of the virtual machine in tracking the steady-state frequency. Based on this concept, the VSG with alternating moment of inertia is elaborated in this paper. The damping effect of the alternating inertia scheme is investigated by transient energy analysis. In addition, the performance of the proposed inertia control in stability of nearby machines in power system is addressed. The idea is supported by simulation and experimental results, which indicates remarkable performance in the fast damping of oscillations.
In comparison of the conventional bulk power plants, in which the synchronous machines dominate, the distributed generator (DG) units have either very small or no rotating mass and damping property. With growing the penetration level of DGs, the impact of low inertia and damping effect on the grid stability and dynamic performance increases. A solution towards stability improvement of such a grid is to provide virtual inertia by virtual synchronous generators (VSGs) that can be established by using short term energy storage together with a power inverter and a proper control mechanism. The present paper reviews the fundamentals and main concept of VSGs, and their role to support the power grid control. Then, a VSG-based frequency control scheme is addressed, and the paper is focused on the poetical role of VSGs in the grid frequency regulation task. The most important VSG topologies with a survey on the recent works/achievements are presented. Finally, the relevant key issues, main technical challenges, further research needs and new perspectives are emphasized.
A generic inertia emulation controller (INEC) scheme for multi-terminal voltage-source-converter (VSC)-based high voltage direct current (HVDC) systems is proposed in this study. The INEC can be incorporated in any grid-side VSC station, allowing the multi-terminal HVDC (MTDC) terminal to contribute an inertial response to connected AC systems during system disturbances, in a fashion similar to synchronous generators. The DC-link capacitors within the MTDC are utilised by the INEC scheme to exchange stored energy with the AC system by varying the overall DC voltage level of the MTDC network within a safe and pre-defined range. A theoretical treatment of the INEC algorithm and its implementation and integration within a conventional VSC control system are presented, and the impact on the total DC capacitance required within the MTDC network to ensure that DC voltages vary within an acceptable range is discussed. The proposed INEC scheme is validated using a MATLAB/SIMULINK model under various changes in demand and AC network faults. The model incorporates a multi-machine AC power system connected to a MTDC transmission system with multiple converterinterfaced nodes. The effectiveness of the INEC in damping post-fault oscillations and in enhancing AC grid frequency stability is also investigated.
Demand response (DR) has proved to be an inevitable part of the future grid. Much research works have been reported in the literature on the benefits and implementation of DR. However, little works have been reported on the impacts of DR on dynamic performance of power systems, specifically on the load frequency control (LFC) problem. This paper makes an attempt to fill this gap by introducing a DR control loop in the traditional LFC model (called LFC-DR) for a single-area power system. The model has the feature of optimal operation through optimal power sharing between DR and supplementary control. The effect of DR communication delay in the controller design is also considered. It is shown that the addition of the DR control loop increases the stability margin of the system and DR effectively improves the system dynamic performance. Simulation studies are carried out for single-area power systems to verify the effectiveness of the proposed method.
Renewable energy sources (RES) like wind and PV are dependent on weather conditions and geographic location and as results their stochastic behavior can significantly influence power systems performance. These effects will be more relevant in case of large-scale penetration of RES. Therefore, modern power plants based on RES should both deliver power as conventional generators and contribute to support the grid services by providing ancillary services and in this way applications of advanced technology are very important to reach this goal. Active power and frequency controls are known as essential ancillary services that should be provided by generation units in large power plants. Therefore, controlling this type of grid interactive power plants is critical issue to achieve large-scale integration of RES in distributed power systems. In consequence, it is necessary to take advantages of new technologies and advanced control concepts in order to configure more intelligent and flexible generation systems, which should be able to improve the performance and stability of grid. A brief review on conventional active power/frequency control issues and complete investigation on adapted scenarios of active power/frequency control considering liberalized markets, high penetration of RES and coexistence of AC and DC networks will be explained in this chapter.
The high penetration of distributed generation, as PV or wind power, has forced the Transmission System Operators (TSOs) to set restrictive requirements for the operation of such systems. As it can be extracted from the forthcoming grid codes drafts, the future distributed generation systems will be requested to have the equivalent performance of a synchronous generator, which is seen from the TSOs as the only solution if a massive integration of renewable in the electrical network should be achieved. In this paper a method for controlling PV grid connected power converters as a synchronous generator, namely Synchronous Power Controller (SPC), is presented. As a difference with previous works this method permits to take advantage of emulating the synchronous behavior meanwhile it is able to get rid of its drawbacks. The main concept of the SPC, as well as some simulation and experimental results will be shown in this paper considering a PV power plant as a study case.
This paper addresses the problem of providing frequency control services, including inertia emulation and primary frequency control, from offshore wind farms connected through a multiterminal HVDC network. The proposed strategy consists of a cascading control mechanism based on dc voltage regulation at the onshore converters and frequency regulation at the offshore converters. The control scheme involves only local measurements and actions avoiding security and reliability issues of control structures based on remote information. The effectiveness of the proposed strategy is illustrated in a test system that consists of two nonsynchronous areas linked by a multiterminal HVDC grid where two offshore wind farms are also connected.
This paper presents a new control strategy for voltage-source converters (VSCs) in the frequency-angle domain which enables dc-link voltage regulation via frequency and load angle adjustment. A major advantage of the proposed controller is emulating the behavior of synchronous machines (SMs) with proper regulation of dc-link voltage which eases integration of VSCs interfacing distributed and renewable generation units into ac systems in the presence of conventional SMs. A cascaded frequency, angle and virtual torque control topology is developed to emulate the mechanical behavior of an SM which offers synchronization power to eliminate the need for a phase-locked-loop after initial converter synchronization, and damping power dynamics to damp power oscillations; and presents frequency dynamics similar to SMs, thus it introduces some inertia to the grid. The controller presents high stability margin and fast dc-link voltage regulation, whereas it can provide frequency support in the ac-side during contingencies. Frequency and voltage amplitude are adjusted by two separate loops. Two different variants are proposed for dc-link voltage control; namely direct dc-link voltage control and indirect dc-link voltage control via a dc-link voltage controller. Small-signal dynamics, analysis, and design process are presented. Both simulation and experimental results are provided to validate the controller effectiveness.
To allow for high penetration of distributed generation and alternative energy units, it is critical to minimize the complexity of generator controls and the need for close coordination across regions. We propose that existing controls be replaced by a two-tier structure of local control operating within a global context of situational awareness. Flatness as an extension of controllability for nonlinear systems is a key to enable planning and optimization at various levels of the grid in this structure. In this study, flatness-based control for automatic generation control (AGC) of a multi-machine system with high penetration of wind energy is investigated. The local control tracks the reference phase which is obtained through economic dispatch at the global control level. As a result of applying the flatness-based method, the n machine system is decoupled into n linear controllable systems in canonical form. Therefore, the control strategy results in a distributed AGC formulation which is significantly easier to design and implement compared to conventional AGC. Practical constraints such as generator ramping rates can be considered in designing the local controllers. The proposed strategy demonstrates promising performance in mitigating frequency deviations and the overall structure could facilitate operation of other nontraditional generators.
This paper presents a comprehensive study on dynamic performance of a more realistic power system with diverse sources in each area and interconnected via parallel AC/DC transmission links. The power generation in each area is based on diverse sources such as thermal, hydro and gas. To carry out the investigations, optimal Automatic Generation Control (AGC) regulators are designed and implemented by considering an example of a two area power system for 1% step load disturbance in one of the control areas. In order to assess the improvement in stability of this realistic power system with DC link in parallel with AC tie-line as interconnection, eigenvalue study is also conducted. It has been shown that the transient response of power system subject to a step load disturbance is sluggish/poor in the presence of hydro as one of the diverse source in the power system. But an appreciable improvement in system dynamic performance is achieved by considering parallel AC/DC links as interconnection between areas rather than using AC tie lines only.
The ever increasing penetration of renewable energy systems (RESs) in today deregulated intelligent power grids, necessitates the use of electrical storage systems. Energy storage systems (ESSs) are helpful to make balance between generation and demand improving the performance of whole power grid. In collaboration with RESs, energy storage devices can be integrated into the power networks to bring ancillary service for the power system and hence enable an increased penetration of distributed generation (DG) units. This paper presents different applications of electrical energy storage technologies in power systems emphasizing on the collaboration of such entities with RESs. The role of ESSs in intelligent micropower grids is also discussed where the stochastic nature of renewable energy sources may affect the power quality. Particular attention is paid to flywheel storage, electrochemical storage, pumped hydroelectric storage, and compressed air storage and their operating principle are discussed as well. The application of each type in the area of power system is investigated and compared to others.
As demonstrated by recent major blackouts, power grids and their associated markets play a vital role in the operation of our society. Understanding how electric generation, transmission, and delivery systems interact and operate is paramount to guaranteeing reliable sources of electricity. Electric Energy Systems offers highly comprehensive and detailed coverage of power systems operations, uniquely integrating technical and economic analyses. The book fully develops classical subjects such as load flow, short-circuit analysis, and economic dispatch within the context of the new deregulated, competitive electricity markets. With contributions from 24 internationally recognized specialists in power engineering, the text also presents a wide range of advanced topics including harmonic load flow, state estimation, and voltage and frequency control as well as electromagnetic transients, fault analysis, and angle stability. A well-needed and updated extension on classical power systems analysis books, Electric Energy Systems provides an in-depth analysis of the most relevant issues affecting the blood-line of our society, the generation and transmission systems for electric energy.
This letter develops a coordination control strategy for wind farms with line commutated converter (LCC) based HVDC delivery to participate in load frequency control. The coordination philosophy is to let the HVDC rectifier sense the grid frequency. If the grid frequency is too high or too low, active power flow through the HVDC link will be ramped down or up by introducing a droop at the rectifier control loop. In turn, wind generation will increase or decrease the blade angles to reduce or increase the captured wind power. This will be done by wind generation pitch controllers. A case study demonstrates the effect of the frequency droop in HVDC control. Simulation results in TSAT are given.
Supplying power to offshore oil platform facilities is a challenging task due to the high power demands and long distances involved. While it is possible to supply power by ac submarine transmission cable to a nearby offshore load, this becomes impractical for loads with long distances and/or high power demands. This paper examines technically and economically two alternatives for subsea transmission: high-voltage alternating current and high-voltage direct current (HVdc). The most effective solution is determined to supply approximately 530 MW of load for several proposed offshore fields (from a few kilometers up to 100 km from the shore). Additionally, this paper sheds some light on voltage source converter technology and how it positively impacts the HVdc setup and efficiency, improving the economics of supplying electrical power to offshore installations.
A photovoltaic (PV) system's output power fluctuates according to the weather conditions. Fluctuating PV power causes frequency deviations in the power utilities when the penetration is large. Usually, an energy storage system (ESS) is used to smooth the PV output power fluctuations and then the smoothed power is supplied to the utility. In this paper, a simple fuzzy-based frequency-control method is proposed for the PV generator in a PV-diesel hybrid system without the smoothing of PV output power fluctuations. By means of the proposed method, output power control of a PV generator considering the conditions of power utilities and the maximizing of energy capture are achieved. Here, fuzzy control is used to generate the PV output power command. This fuzzy control has average insolation, change of insolation, and frequency deviation as inputs. The proposed method is compared with a maximum power point tracking control-based method and with an ESS-based conventional control method. The numerical simulation results show that the proposed method is effective in providing frequency control and also delivers power near the maximum PV power level.
In this paper, a decentralized radial basis function neural network (RBFNN) based controller for load frequency control (LFC) in a deregulated power system is presented using the generalized model for LFC scheme according to the possible contracts. To achieve decentralization, the connections between each control area with the rest of system and effects of possible contracted scenarios are treated as a set of input disturbance signals. The idea of mixed H2/H∞ control technique is used for the training of the proposed controller. The motivation for using this control strategy for training the RBFNN based controller is to take large modeling uncertainties into account, cover physical constraints on control action and minimize the effects of area load disturbances. This newly developed design strategy combines the advantage of the neural networks and mixed H2/H∞ control techniques to provide robust performance and leads to a flexible controller with simple structure that is easy to implement. The effectiveness of the proposed method is demonstrated on a three-area restructured power system. The results of the proposed controllers are compared with the mixed H2/H∞ controllers for three scenarios of the possible contracts under large load demands and disturbances. The resulting controller is shown to minimize the effects of area load disturbances and maintain robust performance in the presence of plant parameter changes and system nonlinearities.
A new design of Multi-Objective Evolutionary Algorithm based decentralized controllers for load–frequency control of interconnected power systems with Governor Dead Band and Generation Rate Constraint nonlinearities, AC–DC parallel tie-lines and Superconducting Magnetic Energy Storage (SMES) units, is proposed in this paper. The HVDC link is used as system interconnection in parallel with AC tie-line to effectively damp the frequency oscillations of AC system while the SMES unit provides bulk energy storage and release, thereby achieving combined benefits. The proposed controller satisfies two main objectives, namely, minimum Integral Squared Error of the system output and maximum closed-loop stability of the system. Simulation studies are conducted on a two area interconnected power system with nonlinearities, AC–DC tie-lines and SMES units. Results indicate that the proposed controller improves the transient responses and guarantees the closed-loop stability of the overall system even in the presence of system nonlinearities and with parameter changes.
A structure-preserved power-frequency slow dynamics simulation model is suggested for interconnected ac/dc power systems with automatic generation control (AGC) consideration, which will be applied to study relevant emergency control in future so that the bulk system viability crisis caused by load-frequency slow dynamics can be released. In the model, the network structure of interconnected power systems is entirely preserved, and the multi-area dynamic load flow (DLF) is developed for simulation. The generator speed governor and rotor dynamics, load-frequency characteristics, simplified models for high voltage direct current (HVDC) transmission and flexible ac transmission systems (FACTS) device thyristor controlled series capacitor (TCSC) suitable for long-term dynamics are considered with their AGC interfaces kept for future emergency-AGC study. However, at this stage, the sub-problem of reactive power and voltage is neglected for modelling simplicity and dc load flow is thus used for network solution. The concept of area centre of inertia (ACOI) is used based on the assumption of uniform frequency in each control area similar to that of the conventional single-area DLF calculation. The application of ACOI concept is attractive because the signal can be obtained from wide-area measurement systems (WAMSs) in real time and used to enhance long-term frequency stability through advanced control in future. The computer test results from 2-area 4-machine and IEEE 30-bus power systems demonstrated the validity and effectiveness of the suggested model and corresponding algorithm.
This paper presents a voltage-source-converter (VSC)-HVDC-supplied industrial system with onsite generation where the VSC-HVDC and generator are taking part in the voltage and frequency control. The reason for using a frequency controller in the converter is to increase the ridethrough capability of the VSC-HVDC-supplied industrial system by exploiting the inertia energy of rotating masses in case of voltage disturbances. The investigation is performed using the PSCAD/EMTDC program and the dynamic performance of the system is evaluated during grid faults and load changes. Three comparisons are done to analyze the system response when the inverter uses different frequency-control strategies, the impact on the system when the turbine operates in different modes, and the VSC-HVDC's ability to mitigate voltage dips from the grid. Simulation results show that the use of a VSC-HVDC for feeding industrial plants can mitigate voltage dips in the grid. Different frequency-control strategies have different responses on the system. The best frequency response is achieved when the turbine is also equipped with a frequency controller.
The paper presents a platform system for the incorporation of flexible ac transmission systems (FACTS) devices. The platform permits detailed electromagnetic transients simulation as it is of manageable size. It manifests some of the common problems for which FACTS devices are used such as congestion management, stability improvement, and voltage support. The platform can be valuable for the validation of reduced order models such as small signal or transient stability models. The paper presents details on the development and validation of a small signal based model with the inclusion of a Unified Power Flow Controller. The validated model is then used successfully for the design of a feedback controller for improved damping.
Small-signal stability analyzed results of an autonomous hybrid renewable energy power generation/energy storage system connected to isolated loads using time-domain simulations is presented in this paper. The companion paper presents frequency-domain analyzed results of the same hybrid system. The proposed renewable energy power generation subsystems include three wind turbine generators (WTGs), a diesel engine generator, two fuel cells (FCs), and a photovoltaic system (PV) while the energy storage subsystems consist of a battery energy storage system and a flywheel energy storage system. An aqua electrolyzer absorbs a part of generated energy from PV or WTGs to generate available hydrogen for FCs. A time-domain approach based on three mathematical models for three studied cases under various operating points and disturbance conditions is performed. It can be concluded from the simulation results that the proposed hybrid power generation/energy storage system feeding isolated loads can be properly operated to achieve system power-frequency balance condition.
The authors describe what automatic generation control (AGC) might
be expected to do, and what may not be possible or expedient for it to
do. The purposes and objectives of AGC are limited by physical elements
involved in the process and, hence, the relevant characteristics of
these elements are described. For reasons given, it is desired that AGC
act slowly and deliberately over tens of seconds or a few minutes. From
a perspective of utility operations, there is no particular economic or
control purpose served by speeding up the AGC action
Simulation and optimization in an AGC system after deregulation
- V Donde
- A Pai
- I A Hiskens
Synchronous power controller of a generation system based on static power converters
- rodriguez