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Parallel Operation of Transformers With on Load Tap Changer and Photovoltaic Systems With Reactive Power Control
Abstract
In recent years, the number of active grid components for voltage regulation in distribution grids has increased significantly. Besides voltage regulators (VRs), such as transformers with On Load Tap Changers (OLTCs), distributed generators can provide a certain voltage support by means of reactive power control (RPC). The different control entities, OLTC and RPC by photovoltaic (PV) systems, usually operate based on local measurements and control characteristics. Hence, unintended interactions between the control entities cannot be excluded in general. This study analyses the parallel operation of OLTC transformers with a voltage based control and PV systems with different RPC strategies (e.g. watt/power factor control PF(P), volt/var control (Q(V)) in a distribution system environment. The focus is on unintended interactions, such as an increase of OLTC switching operations by PV RPC. The contribution and novelty of this paper is to raise awareness for the likelihood of these unintended interactions and to provide a first methodology to assess the parallel operation of OLTC control and PV RPC in detail. The results show that the impact of PV RPC on the number of OLTC switching operations and the effectiveness in parallel operation can differ considerably between the applied PV RPC strategies.
... Under such a circumstances, an over voltage condition might be converted to under voltage issue and vice versa due to over reaction of all parallel transformer OLTCs. Therefore, coordination of multiple OLTCs is essential to prevent undesired switching actions and their corresponding oscillations in the voltage profile [11]. ...
... Unintended interactions between OLTC and reactive power controller at distributions systems is investigated in [11] for parallel operation of OLTC transformers in PV farms with different reactive power control strategies. The paper highlights reduction of life time of OLTC due to increment in the number of OLTC switching actions [12]. ...
... The life time reduction is the consequence of lack of control coordination, which in our paper is suggested as an advantage of OLTC integration into power plant controller. Reference [11] clearly emphasizes on the contribution of current paper that addresses coordination of OLTC with entire power plant controller to achieve an efficient control strategy and higher OLTC life time as a positive side effect. ...
This paper suggests a new control algorithm for On‐Load Tap Changer (OLTC) of wind farm main transformer that is responsible for voltage regulation of collector network. The conventional control scheme adjusts the voltage of LV side of transformer without considering the wind turbine voltages. The voltage of end‐string wind turbines may reach the upper or lower limits of Over or Under‐Excited operation regions. Voltages close to upper or lower limits bounds the reactive power capability (generation or consumption) of wind farm due to dependency of wind turbine reactive power capability to voltage. Operation of wind turbine at extreme voltages may stress its power electronic devices. Furthermore, minor disturbances at either grid or power plant may trigger the protection system and interrupt generation due to vicinity of operation point to protection thresholds. The suggested technique integrates the OLTC control into the power plant controller and considers both LV side voltage of transformer and wind turbine voltages. By monitoring the wind turbine voltages, the voltage setpoint of OLTC is adjusted to provide voltages closer to middle of range for wind turbines that prevents the wind turbines from reaching the voltage limits. Dynamical simulations conducted in DigSILENT PowerFactory software for both Under‐Excited and Over‐Excited operation regions demonstrate desired performance for the suggested technique.
... On the opposite, synchronous generators present nonzero finite negative-and zero-sequence admittances, and therefore, they generate unbalanced voltages and currents [14]. 2. Power profile: DGs can operate in constant PQ, constant PV (conventional PV bus modeling), or constant P-Q(V) mode [12,13]. The first case does not pose any challenge to the power flow computation since the DG is simply modeled as a negative constant power load. ...
... Power profile: DGs can operate in constant PQ, constant PV (conventional PV bus modeling), or constant P-Q(V) mode [12,13]. The first case does not pose any challenge to the power flow computation since the DG is simply modeled as a negative constant power load. ...
... Therefore, further DG action is required to maintain DG reference voltages. This is achieved by correcting again the DG control variables x dg1 and x dg02 , as follows in Equation (13). Note that Equation (13) is derived from Equation (A1) presented in the Appendix A. ...
Local voltage controllers (LVCs) are important components of a modern distribution system for regulating the voltage within permissible limits. This manuscript presents a sensitivity-based three-phase weather-dependent power flow algorithm for distribution networks with LVCs. More specifically, the proposed algorithm has four distinct characteristics: (a) it considers the three-phase unbalanced nature of distribution systems, (b) the operating state of LVCs is calculated using sensitivity parameters having accelerated convergence, (c) it considers the precise switching sequence of LVCs based on their reaction time delays, and (d) the nonlinear influence of weather variations in the power flow is also taken into consideration. Simulations and validation results presented indicate that the proposed approach outperforms other existing algorithms with respect to the accuracy and speed of convergence, thus making it a promising power flow tool for accurate distribution system analysis.
... Another important challenge is the variety of operational modes of DGs. They may operate in constant voltage mode (modeled as PV nodes) [3] or in droop Q(V) mode according to the newly developed standard IEEE Std 1547-2018 from March 2018 [5]- [7]. In both cases, they interact with the other LVCs. ...
... A lot of effort has been put so far by the researchers to study the optimal operation of LVCs in distribution networks e.g. [5], [7], [13]- [20]. However, the field of power flow in distribution networks with LVCs has not been extensively studied, and a lot of work is still needed to improve the accuracy and computation time of power flow. ...
... The Joule loss ( ) is given in Eq. (7). It refers to the active power losses generated in the conductor due to the current ( ) flow through the conductor resistance ( ). ...
b>Local voltage controllers (LVCs) are important components of a modern distribution system for regulating the voltage within permissible limits. This manuscript presents a sensitivity-based three-phase weather-dependent power flow algorithm for distribution networks with LVCs. This Part I presents the theoretical development of the proposed algorithm, which has four distinct characteristics: a) it considers the three-phase unbalanced nature of distribution systems, b) the operating state of LVCs is calculated using sensitivity parameters, which accelerates the convergence speed of the algorithm, c) it considers the precise switching sequence of LVCs based on their reaction time delays, and d) the nonlinear influence of weather variations in the power flow is also taken into consideration. Simulations and validation results presented in Part II indicate that the proposed approach outperforms other existing algorithms with respect to the accuracy and speed of convergence, thus making it a promising power flow tool for accurate distribution system analysis.
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... Another important challenge is the variety of operational modes of DGs. They may operate in constant voltage mode (modeled as PV nodes) [3] or in droop Q(V) mode according to the newly developed standard IEEE Std 1547-2018 from March 2018 [5]- [7]. In both cases, they interact with the other LVCs. ...
... A lot of effort has been put so far by the researchers to study the optimal operation of LVCs in distribution networks e.g. [5], [7], [13]- [20]. However, the field of power flow in distribution networks with LVCs has not been extensively studied, and a lot of work is still needed to improve the accuracy and computation time of power flow. ...
... The Joule loss ( ) is given in Eq. (7). It refers to the active power losses generated in the conductor due to the current ( ) flow through the conductor resistance ( ). ...
b>Local voltage controllers (LVCs) are important components of a modern distribution system for regulating the voltage within permissible limits. This manuscript presents a sensitivity-based three-phase weather-dependent power flow algorithm for distribution networks with LVCs. This Part I presents the theoretical development of the proposed algorithm, which has four distinct characteristics: a) it considers the three-phase unbalanced nature of distribution systems, b) the operating state of LVCs is calculated using sensitivity parameters, which accelerates the convergence speed of the algorithm, c) it considers the precise switching sequence of LVCs based on their reaction time delays, and d) the nonlinear influence of weather variations in the power flow is also taken into consideration. Simulations and validation results presented in Part II indicate that the proposed approach outperforms other existing algorithms with respect to the accuracy and speed of convergence, thus making it a promising power flow tool for accurate distribution system analysis.
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... Another important challenge is the variety of operational modes of DGs. They may operate in constant voltage mode (modeled as PV nodes) [3] or in droop Q(V) mode according to the newly developed standard IEEE Std 1547-2018 from March 2018 [5]- [7]. In both cases, they interact with the other LVCs. ...
... A lot of effort has been put so far by the researchers to study the optimal operation of LVCs in distribution networks e.g. [5], [7], [13]- [20]. However, power flow in distribution networks with LVCs has not been extensively studied, and scope to improve remains. ...
... The Joule loss ( ) is given in Eq. (7). It refers to the active power losses generated in the conductor due to the current ( ) flow through the conductor resistance ( ). ...
b>Local voltage controllers (LVCs) are important components of a modern distribution system for regulating the voltage within permissible limits. This manuscript presents a sensitivity-based three-phase weather-dependent power flow algorithm for distribution networks with LVCs. This Part I presents the theoretical development of the proposed algorithm, which has four distinct characteristics: a) it considers the three-phase unbalanced nature of distribution systems, b) the operating state of LVCs is calculated using sensitivity parameters, which accelerates the convergence speed of the algorithm, c) it considers the precise switching sequence of LVCs based on their reaction time delays, and d) the nonlinear influence of weather variations in the power flow is also taken into consideration. Simulations and validation results presented in Part II indicate that the proposed approach outperforms other existing algorithms with respect to the accuracy and speed of convergence, thus making it a promising power flow tool for accurate distribution system analysis.
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... In that case, another transformer with the same technical specifications can operate parallel with the existing transformer to meet the demand. Transformers can take 5-6 months to produce, and 2-3 months to repair [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. A transformer operating integrated into the system may be subject to a major failure. ...
... S max (kVA) defines the maximum load. We can load the transformers that will operate in parallel [11]. S 1 ,S 2 ,and S 3 (kVA) define the nominal power of transformers. ...
... Many sites above 11kV operate with parallel infrastructure [4]. This practice provides the practical consequence of allowing a plant to be taken out of service for maintenance and repairs without the inconvenience of interrupting the electrical supply to the consumer. ...
... The benefits of parallel infrastructure have long been discussed from automatic switching between transmission lines to maintain supply [4] to the economic and efficiency benefits [6]. In transmission, the primary interest of parallel networks is to improve the network "capacity, reliability and security" [7] or to share the capacity across several sites. ...
In 2014 then revised in 2019 the European Commission published regulations on the minimum specifications of new Eco Power Transformers. These regulations also required that transformers not meeting these requirements could no longer be installed or sold except under defined exceptions. Typically, almost all networks above 11kV operate with a parallel plant; especially when it comes to transformers. Using models of some transmission/distribution sites and by inputting twelve months of half-hour transformer loading data into these models; the amount of unnecessary losses, due to their parallel orientation, should be quantifiable. Since these sites need to be capable of supporting a varying load profile, they are for considerable periods of time oversized. Using the model, a sequence of taking offline excess plant will definable not only reducing network losses (a reduction in transformer losses of 35% per site is feasibly achieved in the model) but also marginally improve the sustainability of some of these existing sites.
... conventional voltage regulating devices such as On-load tap changer (OLTC), voltage regulator and capacitor bank are slow in response and have a limited number of switching life due to wear and tear of mechanical parts [4]. Smart inverters with reactive power capability and active power curtailment control can solve the problem of voltage fluctuations, and increased switching of the conventional voltage regulating devices [4], [5], [6]. ...
... conventional voltage regulating devices such as On-load tap changer (OLTC), voltage regulator and capacitor bank are slow in response and have a limited number of switching life due to wear and tear of mechanical parts [4]. Smart inverters with reactive power capability and active power curtailment control can solve the problem of voltage fluctuations, and increased switching of the conventional voltage regulating devices [4], [5], [6]. ...
The increasing trend of renewable energy sources (RESs) integration in the distribution systems may cause voltage fluctuations and voltage limit violations. The smart inverters with reactive power capability (RPC) and active power curtailment (APC) control can solve the grid voltage issues. In this paper, three-phase optimal power flow (TPOPF) is applied to dispatch the APC and reactive power injection (RPI) of inverters. The TPOPF takes smart meter measurements as inputs. The non-proportional APC and proportional APC are examined. A fair incentive scheme for APC by the inverters is proposed, appropriate for both utility and renewable energy producers. Similarly, unequal RPI and equal RPI by the inverters to maintain the system voltage are studied. The RPI incentive scheme is analyzed by accounting for the costs incurred in i) overrating of the inverters, ii) power losses, and iii) life reduction due to reactive power supplied by the inverters. The effective location of inverters for APC and RPI in the distribution network is explored. The incentive schemes are implemented in 27-node distribution system and IEEE European LV distribution system.
... In the past decade or more, there has been significant attention on ADNs, with explosive growth in publications researching optimization operation strategies [5][6][7][8][9], energy storage system (ESS) configuration methods [10][11][12][13], new converter controllers [14][15][16], oscillation analysis, Volt-Var control [17][18][19][20], and other related areas. However, there has been very little focus on developing suitable and cost-effective structures specifically designed to enhance the capacity of ADNs in mountainous regions for integrating DGs, including both RoR-SH and PV. ...
Due to the radial network structures, small cross-sectional lines, and light loads characteristic of existing AC distribution networks in mountainous areas, the development of active distribution networks (ADNs) in these regions has revealed significant issues with integrating distributed generation (DGs) and consuming renewable energy. Focusing on this issue, this paper proposes a wide-range thyristor-controlled series compensation (TCSC)-based ADN and presents a deep reinforcement learning (DRL)-based optimal operation strategy. This strategy takes into account the complementarity of hydropower, photovoltaic (PV) systems, and energy storage systems (ESSs) to enhance the capacity for consuming renewable energy. In the proposed ADN, a wide-range TCSC connects the sub-networks where PV and hydropower systems are located, with ESSs configured for each renewable energy generation. The designed wide-range TCSC allows for power reversal and improves power delivery efficiency, providing conditions for the optimization operation. The optimal operation issue is formulated as a Markov decision process (MDP) with continuous action space and solved using the twin delayed deep deterministic policy gradient (TD3) algorithm. The optimal objective is to maximize the consumption of renewable energy sources (RESs) and minimize line losses by coordinating the charging/discharging of ESSs with the operation mode of the TCSC. The simulation results demonstrate the effectiveness of the proposed method.
... Their study emphasized an integrated voltage control approach, where DG functions as VRs, thereby reducing adverse interactions between OLTCs and active devices integrated with DG. The authors proposed a simultaneous operation of OLTC regulation and reactive power supply from PV units (Kraiczy et al., 2018). ...
In recent years, the landscape of power generation has undergone a significant transformation, moving from centralized power plants to decentralized power systems. This shift has been driven by substantial changes in grid architecture, introducing the concept of Distributed Generation (DG), which is now a vital component of electrical power systems, especially within distribution networks (DNs). Integrating DG is crucial for ensuring reliable power generation and reducing power losses. However, the widespread adoption of decentralized technologies presents new challenges for power systems, such as identifying optimal locations for DG installation , configuring protective devices, managing voltage control, and addressing Power Quality (PQ) issues. The variability and unpredictability of Renewable Energy Sources (RES), such as solar and wind power, add to these complexities, requiring careful evaluation and strategic planning. This research's primary contribution lies in its thorough exploration and systematic analysis of various DG types and their integration methods. By identifying and addressing the key challenges of DG integration, this study offers valuable insights and innovative solutions that enhance grid stability and efficiency. Furthermore, the research proposes strategic frameworks for optimal DG deployment, advanced control strategies, and effective regulatory compliance, which are essential for the sustainable development of electrical power systems. The findings underscore the importance of adopting holistic approaches to address the complexities of DG integration, ultimately contributing to a more resilient and sustainable energy future.
... � Generic grids: The work in ref. [150,151] used the term generic to refer to a grid where different parameters can be tweaked to generate various grids. However, the term was synonymous with representative grid in the work of ref. [152]. � Synthetic grids: Grids that are neither models of real grids nor derived from a real-world grid. ...
The development of smart grids, traditional power grids, and the integration of internet of things devices have resulted in a wealth of data crucial to advancing energy management and efficiency. Nevertheless, public datasets remain limited due to grid operators' and companies' reluctance to disclose proprietary information. The authors present a comprehensive analysis of more than 50 publicly available datasets, organised into three main categories: micro‐ and macro‐consumption data, detailed in‐home consumption data (often referred to as non‐intrusive load monitoring datasets or building data) and grid data. Furthermore, the study underscores future research priorities, such as advancing synthetic data generation, improving data quality and standardisation, and enhancing big data management in smart grids. The aim of the authors is to enable researchers in the smart and power grid a comprehensive reference point to pick suitable and relevant public datasets to evaluate their proposed methods. The provided analysis highlights the importance of following a systematic and standardised approach in evaluating future methods and directs readers to future potential venues of research in the area of smart grid analytics.
... Such issues arise due to the variable and intermittent nature of PVs, and they are related to frequent voltage violations and higher losses [3]. The traditional DN control devices, including OLTC, CBs and VRs, preset slow response mechanisms with a limited lifespan due to mechanical parts [4]. However, combined with quick response mechanisms such as PV inverter's reactive power support can achieve promising results in improving DN technical conditions [5][6][7][8][9]. ...
The high expansion of a variable and intermittent nature of distributed generation, such as photovoltaics (PV), can cause technical issues in existing distribution networks (DN). In addition to producing electrical energy, PVs are inverter-based sources, and can help conventional control mechanisms in mitigating technical issues. This paper proposes a multi-stage optimal power flow (OPF)-based mixed-integer non-linear programming (MINLP) model for improving an operation state in LV PV-rich DN. A conventional control mechanism such as on load tap changer (OLTC) is used in the first stage to mitigate overvoltage caused by PVs. The second stage is related to reducing losses in DN using reactive power capabilities from PVs, which defines the optimization problem as a fully centralized observed from the distribution system operator’s (DSO) point of view. The optimization problem is realized under the co-simulation approach in which the power system analyzer and computational intelligence (CI) optimization method interact through an interface. This approach allows keeping the original MINLP model without approximations and using any computational intelligence method. OpenDSS is used as a power system analyzer, while particle swarm optimization (PSO) is used as a CI optimization method in this paper. Detailed case studies are performed and analyzed over a single-day period. To study validation and feasibility, the proposed model is evaluated on the IEEE LV European distribution feeder. The obtained results suggest that a combination of conventional control mechanisms (OLTC) and inverter-based sources (PVs) represent a promising solution for DSO and can serve as an alternative control method in active distribution networks.
... The quality of power is affected by the intermittent nature of PV generation. Voltage variations along light spark triggered through voltage variations are major power quality issues connected to fast PV output variations [8]. Damage is occurring to electric devices linked to the network by the voltage fluctuations and flickers [9] as well as health problems are occurring based on the light flicker. ...
In this manuscript proposed a hybrid Garra Rufa Fish Optimization (GRFO) and Improved Tunicate Swarm Algorithm (ITSA) for improving the power quality of the integrated Photovoltaic (PV) and Plug-in Electric Vehicle (PEV) in Smart Grid (SG) system. The GRFO-ITSA approach is hybrid wrapper of GRFO and ITSA. Commonly it is named as GRFO-ITSA approach. The grid-connected PV-PEV, active power management is performed by the proposed approach. The proposed GRFO approach is used to determine the individual harmonic components and to reduce the recompense currents applied to PVs via PEV converters. The load flow control is performed by ITSA approach, which controls the power among the PVs, and PEVs. Additionally, it satisfies the power demand, and voltage variation. The proposed approach is also to analyze the mutual properties of PVs as well as PEVs on the feeder and transmitting loads, voltage outlines, harmonic alterations of an urban electric power distribution system. Also, the performance of the GRFO-ITSA is implemented on MATLAB site as well as associated with several existing approaches. The GRFO-ITSA have improved the power quality and compensate the harmonics and reactive power of the system. The optimal outcome is obtained by GRFO-ITSA with less computation time.
... In order to reduce voltage deviations and maintain voltage stability, a three-stage hierarchical voltage control method considering network voltage stability is proposed, and individual control gain constraints are developed for each inverter [10]. Ref. [11] analysed the operational characteristics of PV inverters with different control strategies and evaluated the possibility of parallel control of OLTC and PV. In Ref. [12], a distribution network voltage control scheme based on model predictive control is proposed in concert considering distributed generators (DG), ESS and OLTC to keep all bus voltages within the allowed range. ...
The rapid increase of photovoltaic (PV) penetration in active distribution networks (ADN) is posing great challenges to traditional voltage control schemes. A two‐stage voltage control strategy of ADN is proposed by the authors based on the alternating direction method of multipliers (ADMM) with refined power to hydrogen (P2H) model, considering multiple types of PV forms such as residential photovoltaic cluster (RPVC) and small‐scale PV power stations. In the day‐ahead stage, the optimal power flow is performed to determine the optimal scheduling results of on‐load tap changers and capacitor banks etc. In the intra‐day stage, the real‐time voltage control strategy is implemented at the distribution network layer to regulate the power of each type of PV, energy storage systems and P2H to further reduce the voltage deviation. At the customer layer, the residential photovoltaic (RPV) within the RPVC is precisely controlled based on the ADMM algorithm to achieve the minimum voltage deviation at each RPV access point. The proposed strategy is tested on the modified IEEE 33‐bus and IEEE 69‐bus distribution systems, and the simulation results verify its effectiveness in mitigating voltage violations.
... Furthermore, more research has been undertaken into the parallel functioning of DER reactive power support and static voltage support devices (OLTC, SCB) for effective voltage control [6 -10]. Ref. [6] provided a strategy for managing voltage control in the German distributed power system by combining and coordinating the operation of solar PV units with reactive power assistance and an OLTC unit. It has evaluated the switching operations of OLTC units with various events in a day with the existence of a solar PV network, and it demonstrates that OLTC operations are effectively decreased by adjusting the reactive power support of a solar PV unit. ...
Since more distributed energy resources (DER) are being linked to the electrical grid, the current distributed power system (DPS) is encountering additional voltage regulation challenges. Traditionally, on-load tap changers, step voltage regulators, and switched capacitor banks have been used for voltage regulation in DPS. However, these sources are insufficient for voltage regulation in current DPS. As a result, reactive power assistance from a DER unit based on power electronics is intended to adjust the voltage in the DPS. In terms of flexibility, security, reliability, and availability, embedded control systems are now being investigated by researchers for use in power converter-fed DER units. This paper presents a method for constructing an embedded controller unit based on XynergyXS for the power converter controller, which includes reactive power regulation in the DER unit. Furthermore, it proposes dynamic reactive power control in the DER unit for enhanced DPS voltage regulation. The suggested voltage control approach is tested in a MATLAB/Simulink model of a practical 85-bus distributed power system located in the northern Tamilnadu region, India as well as a modified IEEE 33-bus system. The new control approach investigates all potential power grid disruptions. The results reveal that the proposed reactive power control method in DER units enhances network voltage regulation while reducing the number of switching operations performed by static voltage regulating units such as on load tap changers and switched capacitor banks.
... Parallel transformers are essential method to offer N-1 contingency to the overall system. Usually, parallel transformers involve various parameters to be fulfilled, such as transformer impedance must be relatively equal, both transformers using the identical vector groups, the voltage level are within acceptance tolerance and have similar phase sequence [2,12]. Other than that, there are also extra parameters that need to be fulfilled such as, numbers of tap changer position although new technology such as circulating current method may cater for this issue [2]. ...
The research focuses on the impact of parallel two power transformers with and without earthing transformer based on potential earth fault events. Paralleling two power transformers is recommended in medium voltage substation topology to adopt N-1 criteria and minimize interruption time. However, the paralleling transformers may cause other issues such as increasing the level of fault current and oblige additional protection coordination to isolate the exact transformer during the fault. This research will examine and analyze the benefit and consequence after parallel both transformers with and without earthing transformer. The study focuses on 132/22 kV transformers with star-delta vector group that connected to the diverse topology involving earthing transformer before connected to neutral earth resistor. Based on the investigation, it found that the star-delta transformer without earthing transformer at one side of transformers has a higher potential to cause poor isolation during any event of an earth fault due to insulation design. Moreover, the transformer that connected to earthing transformer has a higher possibility to experience higher fault current which indirectly reduces the life span of that earthing transformer as well as power transformer due to unavailability of the earthing transformer at the other transformer.
... Advances in power electronics have helped researchers propose various effective methods to mitigate the problems arising from increased RETs integration. These include different autonomous control schemes for PV inverter [5], on-load tap changers OLTC [6] and custom power devices [7,8]. Although these local control approaches respond much faster at no additional cost, they can potentially lead to demand unnecessary reactive power compensation or insufficient voltage support, which may significantly increase power losses, especially at high PV penetration levels [9]. ...
Increased integration of photovoltaics (PVs) systems and charging stations for electric vehicles (EVs) has led to a substantial increase in the level of voltage unbalance beyond the acceptable limit. Ordinary voltage regulation devices such as on‐load tap changers (OLTCs) and distribution static synchronous compensator (DSTATCOM) are sometimes incapable of adequately addressing this issue without proper coordination with PVs and EVs. This paper presents a novel real‐time optimal coordination scheme to determine the tap position of OLTC, the amount of reactive power to be exchanged by DSTATCOM and a PV inverter, and the phase connection of EVs. The proposed scheme aims to maintain the voltage magnitude and voltage unbalance within the statutory limit while minimising the power losses in an active unbalanced power distribution system. Advanced and hybrid particle swarm optimisation (AHPSO) algorithm is also developed to solve the optimisation problem, and its robustness in comparison with other techniques is verified. The impact of uncoordinated voltage control and proposed control on voltage unbalance and power losses are investigated. Time‐series simulations confirm the significance and scalability of the developed coordination control scheme on IEEE 37‐node and IEEE 123‐node test feeders with real data and different PV penetration levels.
... In typical MV grids, an OLTC is often available at the HV-MV transformer, so that the grid operator can adjust the local grid voltage with an automatic voltage controller. It is important to coordinate the setup of the OLTC to the DER local voltage control [7], which needs to be properly simulated in the planning phase. The parallel approach yields errors in the discrete variables such as tap positions for the following reason. ...
The increasing penetration from intermittent renewable distributed energy resources in distribution grid brings along challenges in grid operation and planning. To evaluate the impact on the grid voltage profile, grid losses, and discrete actions from assets (e.g. transformer tap changes), quasi-static simulation is an appropriate method. Quasi-static time series and Monte-Carlo simulation requires a tremendous number of power flow calculations (PFCs), which can be significantly accelerated with a parallel High-Performance Computing (HPC)-PFC solver. In this paper, we propose a HPC-PFC-solver-based grid simulation (parallel simulation) approach for a multi-core CPU platform as well as a greedy method, which can prevent the errors caused by simultaneous parallel simulation. The performance of the proposed approach and the comparison is demonstrated with two use cases.
... The addition of naively controlled DERs such as solar PV makes this voltage regulation problem harder [1] and can cause LTCs to actuate much more frequently, reducing their lifespan. Depending on the control logic and design parameters, connecting smart inverters can either alleviate [2] these voltage fluctuations or be a source of adverse interactions [3]. ...
As more distributed energy resources (DERs) are connected to the power grid, it becomes increasingly important to ensure safe and effective coordination between legacy voltage regulation devices and inverter-based DERs. In this work, we show how a distribution circuit model, composed of two LTCs and two inverter devices, can create voltage oscillations even with reasonable choices of control parameters. By modeling the four-device circuit as a switched affine hybrid system, we analyze the system's oscillatory behavior, both during normal operation and after a cyber-physical attack. Through the analysis we determine the specific region of the voltage state space where oscillations are possible and derive conditions on the control parameters to guarantee against the oscillations. Finally, we project the derived parameter conditions onto 2D spaces, and describe the application of our problem formulation to grids with many devices.
... Also, it ensures the validity and reliability of the loads on the power grid systems Mahfuz- Ur-Rahman et al. (2018). The proficiency of the energy transfer can be guaranteed based on the gain of voltage sag, swell and power factor Kurtoğlu et al.(2019), Kraiczy and Stetz (2017). Some of the traditional works developed a multilevel converters Behera and Thakur (2017) for increasing the quality of voltage and current in the power systems. ...
The solar Photovoltaic (PV) systems have gained a significant attention due to its easy implementation and availability. In which, the proper energy management should be mainly concentrated for a successful PV power utilization. In the traditional works, various controlling techniques have been developed for reactive power compensation. But, it lacks with the issues related to reduced system performance, increased loss, and high harmonics. To solve these issues, this paper aims to develop a new controlling methodology, named as Synthesis Segmental Multilevel Converter (SSMC) for reactive power compensation in a three phase grid systems. Initially, it extracts the maximum power from the solar PV systems by using an Enhanced Perturb and Observe (EPO) method. The panel separation is done and the three phase power input is given to the SSMC converter, where the synchronization and switching pulse generation processes are performed. During synchronization, integrated techniques such as Proportional Integral (PI), Fuzzy Logic Controller (FLC), and Improved Artificial Neural Network (IANN) techniques are utilized to maintain the voltage, magnitude and phase angle in a same level. Consequently, the Inductance Capacitance (LC) filtering technique is applied to reduce the harmonics distortion in the signal. After that, the Park transformation is used to perform the dq0 to abc transformation, which is implemented for reducing the high volume of error. Finally, the error free signal is fed to the three phase grid system with reduced harmonics also both the simulation and analytical results have been taken for analyzing the performance of the proposed technique.
... In Fig. 9, the total three-phase reactive power of the DG connected at bus 8 is presented. It operates in inductive mode according to droop equation (1), to mitigate the voltage rise caused by the high amount of generated active power [8]. It is observed that the proposed algorithm indicates a consumption of 542 kVar, which is exactly the same as in the dynamic simulation. ...
p> Power flow is an integral part of distribution system planning, monitoring, operation, and analysis. This two-part paper proposes a sensitivity-based three-phase weather-dependent power flow approach for accurately simulating distribution networks with local voltage controllers (LVC). This part II, firstly, presents simulation results of the proposed approach in an 8-Bus and 7-Bus network, which are validated using dynamic simulation. Secondly, simulation results for the IEEE 8500-node network are also presented. An extensive comparison is conducted between the proposed sensitivity-based approach and the other existing power flow approaches with respect to result accuracy and convergence speed. Moreover, the influence of weather and magnetic effects on the power flow results and the LVC states is also investigated. Simulation results confirm that the proposed sensitivity-based approach produces more accurate results than the existing approaches since it considers the actual switching sequence of LVCs as well as the weather and magnetic effects on the network. Moreover, the proposed algorithm exhibits accelerated convergence due to the usage of the sensitivity parameters, which makes it an important tool for distribution system analysis. </p
... Due to overload on the transformer, the voltage regulation of transformer increases, the windings get over heated and also the efficiency drops [3]. These problems can be overcome by connecting the transformers in parallel [4][5][6]. The parallel operation reduces the burden on single transformer; thus, it increases the efficiency and reduces winding failure [7]. ...
An increase in the development of Industries and rapid growth in the population has led to an increase in the power demand in the distribution network. With these increased needs, the existing distribution transformer have become overloaded conditions. Due to overload on the transformer, the efficiency and power factor drops and also it led to increase in the transformer voltage regulation and windings get overheated. This paper presents a novel topology called transformer auto stop-start that will automatically energise and de-energise, one pair of transformers at a kV/V Distribution network. In this way, the proposed technique reduces overall electric losses. Performance of transformer under different load conditions are illustrated by simulation.
... The regulation of voltage in distribution systems (DS) with the high installation of distributed generations (DGs) becomes a great challenge for the system operator. The uncertain nature and the high penetration of renewable energy sources (RESs) cause various voltage problems and excessive operation of conventional devices of voltage control [1], [2]. The conventional devices of voltage regulation in the active DSs such as on-load tap-changer (OLTC), step voltage regulators (SVR), and shunt capacitors (SC) are not fast enough to act with the rapid change of voltages due to the intermittent characteristics of RESs [3], [4]. ...
In recent years, with increasing the penetration of renewable-based distributed generation (DGs), voltage control plays a vital role in operating distribution systems (DS). Furthermore, the traditional voltage control devices are not fast enough to regulate the voltage due to transient events and the intermittent characteristics of renewable energy sources (RESs). On the other hand, because of the fast response of power electronic components, the DG inverter can cope with the intermittent and uncertainty of power generation due to environmental changes. Therefore, this paper proposes a cooperative voltage control scheme to solve the voltage problems associated with high DG penetration. The scheme is developed based on a multi-agent system (MAS) with a distributed control architecture using time coordination between voltage regulators and reactive power control of the renewable-based DGs. The scheme’s objective is to minimize voltage deviations and reduce the stress on the traditional voltage control devices by utilizing the available reactive power of the DGs. Different simulations are carried out and analyzed for various operating conditions over 24 hours using the IEEE 34-node and 123-node test feeders. The simulation results show that the proposed control scheme can successfully reduce the total voltage deviation and decrease the number of tap changes of voltage regulators at different sun profiles.
... The steady state modeling of DGs is usually categorized depending on the generated voltage/current profile as well as the power profile, as follows: [26]. ...
p>This paper presents a comprehensive three-bus equivalent circuit model of three-phase step voltage regulators. The proposed model can be efficiently integrated in the Z-bus power flow method and can accurately simulate any configuration of step voltage regulators. In contrast to the conventional step voltage regulator models that include the tap variables inside the YBUS matrix of the network, the proposed model simulates them in the form of current sources, outside the YBUS matrix. As a result, the re-factorization of the YBUS matrix is avoided after every tap change reducing significantly the computational burden of the power flow. Furthermore, possible convergence issues caused by the low impedance of step voltage regulators are addressed by introducing fictitious impedances, without, however, affecting the accuracy of the model. The results of the proposed step voltage regulator model are compared against well-known commercial softwares such as Simulink and OpenDSS using the IEEE 4-Bus and an 8-Bus network. According to the simulations, the proposed model outputs almost identical results with Simulink and OpenDSS confirming its high accuracy. Furthermore, the proposed 3-bus equivalent model is compared against a recently published conventional step voltage regulator model in the IEEE 8500-Node test feeder. Simulation results indicate that the proposed step voltage regulator model produces as accurate results as the conventional one, while its computation time is significantly lower. More specifically, in the large IEEE 8500-node network consisting of four SVRs, the proposed model can reduce the computation time of power flow around one minute for every tap variation. Therefore, the proposed step voltage regulator model can constitute an efficient simulation tool in applications where subsequent tap variations are required. </p
... Energies 2021,14, 1215 ...
Distribution systems are under constant stress due to their highly variable operating conditions, which jeopardize distribution transformers and lines, degrading the end-user service. Due to transformer regulation, variable loads can generate voltage profiles out of the acceptable bands recommended by grid codes, affecting the quality of service. At the same time, nonlinear loads, such as diode bridge rectifiers without power factor correction systems, generate nonlinear currents that affect the distribution transformer operation, reducing its lifetime. Variable loads can be commonly found at domiciliary levels due to the random operation of home appliances, but recently also due to electric vehicle charging stations, where the distribution transformer can cyclically vary between no-load, rated and overrated load. Thus, the distribution transformer can not safely operate under highly-dynamic and stressful conditions, requiring the support of alternative systems. Among the existing solutions, hybrid transformers, which are composed of a conventional transformer and a power converter, are an interesting alternative to cope with several power quality problems. This article is a review of the available literature about hybrid distribution transformers.
... In Fig. 9, the total three-phase reactive power of the DG connected at bus 8 is presented. It operates in inductive mode according to droop equation (1), to mitigate the voltage rise caused by the high amount of generated active power [8]. It is observed that the proposed algorithm indicates a consumption of 542 kVar, which is exactly the same as in the dynamic simulation. ...
p> Power flow is an integral part of distribution system planning, monitoring, operation, and analysis. This two-part paper proposes a sensitivity-based three-phase weather-dependent power flow approach for accurately simulating distribution networks with local voltage controllers (LVC). This part II, firstly, presents simulation results of the proposed approach in an 8-Bus and 7-Bus network, which are validated using dynamic simulation. Secondly, simulation results for the IEEE 8500-node network are also presented. An extensive comparison is conducted between the proposed sensitivity-based approach and the other existing power flow approaches with respect to result accuracy and convergence speed. Moreover, the influence of weather and magnetic effects on the power flow results and the LVC states is also investigated. Simulation results confirm that the proposed sensitivity-based approach produces more accurate results than the existing approaches since it considers the actual switching sequence of LVCs as well as the weather and magnetic effects on the network. Moreover, the proposed algorithm exhibits accelerated convergence due to the usage of the sensitivity parameters, which makes it an important tool for distribution system analysis. </p
... Reactive power flow fluctuations in the PV integrated DG system happen due to frequently activating/deactivating the capacitors, OLTCs and line VRs [19]. The high level of PV and the variability of solar radiation further trigger increasing of the reactive power flow fluctuations [55]. Some methods are proposed to inhibit reactive power fluctuations in the literature. ...
In order to meet the electricity needs of domestic or commercial buildings, solar energy is more attractive than other renewable energy sources in terms of its simplicity of installation, less dependence on the field and its economy. It is possible to extract solar energy from photovoltaic (PV) including rooftop, ground-mounted, and building integrated PV systems. Interest in rooftop PV system applications has increased in recent years due to simple installation and not occupying an external area. However, the negative effects of increased PV penetration on the distribution system are troublesome. The power loss, reverse power flow (RPF), voltage fluctuations, voltage unbalance, are causing voltage quality problems in the power network. On the other hand, variations in system frequency, power factor, and harmonics are affecting the power quality. The excessive PV penetration also the root cause of voltage stability and has an adverse effect on protection system. The aim of this article is to extensively examines the impacts of rooftop PV on distribution network and evaluate possible solution methods in terms of the voltage quality, power quality, system protection and system stability. Moreover, it is to present a comparison of the advantages/disadvantages of the solution methods discussed, and an examination of the solution methods in which artificial intelligence, deep learning and machine learning based optimization and techniques are discussed with common methods.
... In Fig. 9, the total three-phase reactive power of the DG connected at bus 8 is presented. It operates in inductive mode according to droop equation (1), to mitigate the voltage rise caused by the high amount of generated active power [8]. It is observed that the proposed algorithm indicates a consumption of 542 kVar, which is exactly the same as in the dynamic simulation. ...
p> Power flow is an integral part of distribution system planning, monitoring, operation, and analysis. This two-part paper proposes a sensitivity-based three-phase weather-dependent power flow approach for accurately simulating distribution networks with local voltage controllers (LVC). This part II, firstly, presents simulation results of the proposed approach in an 8-Bus and 7-Bus network, which are validated using dynamic simulation. Secondly, simulation results for the IEEE 8500-node network are also presented. An extensive comparison is conducted between the proposed sensitivity-based approach and the other existing power flow approaches with respect to result accuracy and convergence speed. Moreover, the influence of weather and magnetic effects on the power flow results and the LVC states is also investigated. Simulation results confirm that the proposed sensitivity-based approach produces more accurate results than the existing approaches since it considers the actual switching sequence of LVCs as well as the weather and magnetic effects on the network. Moreover, the proposed algorithm exhibits accelerated convergence due to the usage of the sensitivity parameters, which makes it an important tool for distribution system analysis. </p
This paper presents a novel direct-injection modular universal power flow and quality control topology exclusively using lower power components. In addition to conventional high-voltage applications, it is particularly attractive for the distribution and secondary grids, e.g., in soft open points, down to low voltage as it can exploit the latest developments in low-voltage high-current semiconductors. In contrast to other concepts that do not interface the grid through transformers, it does not need to convert the entire line power but only the injected or extracted power difference. The proposed power flow and quality (f/q) controller comprises a shunt active front end, together with high-frequency links serving as a power supply for a series floating module per phase. Each of the floating modules is in series with one phase of the line, floating with the electric potential of that particular phase, avoiding any ground connection. Omitting bulky and dynamically limited line transformers of conventional universal power flow controllers, the presented direct-injection f/q controller enables exceptionally small size and volume, high power density, high frequency content, and fast response. In contrast to direct-injection concepts with full back-to-back converters, it only needs to handle a fraction of the power. The circuit combines grid-voltage low-current electronics in the shunt unit and low-voltage high-current modules in the floating series injection units. Simulations and experiments demonstrate and validate the concept.
The voltage fluctuations that occur locally in the distribution system caused by high penetration of photovoltaic (PV) power generation based on the carbon neutrality strategy have been regarded as one of the serious operational problems. Assuming that larger amount of PV power sources will be connected in the future distribution system, Volt-var control of PV smart inverters is expected. This will enable more flexible and effective voltage control support by provide custom var response. The authors have investigated the effective voltage control of coordination between tap operation of Step Voltage Regulator (SVR) and Volt-var control of PV smart inverters. In our previous studies, we proposed a Volt-var curve determination method for MW-large-scale PV smart inverters connected to medium-voltage to decrease SVR tap operations and minimize distribution losses. However, there are uncertainties such as PV outputs and loads of low-voltage consumers that cannot be completely and individually determined from the measurements of medium-voltage PV smart inverters. In this study, using a detailed distribution line model that reflects actual power measurement datasets of individual low-voltage customers, effects of reduction in distribution losses, reduction in the number of SVR tap operations, and changes in cumulative reactive power output sharing of PV inverters are evaluated through simulation studies to confirm the effectiveness of the proposed Volt-var setting method.
Solar photovoltaic (PV) systems have gained significant attention due to their easy implementation and availability, where proper energy management should be highly concentrated for a successful PV power utilization. In the traditional works, various controlling techniques have been developed for reactive power compensation. But, it lacks with the issues of reduced system performance, increased loss, and high harmonics. Hence, this paper aims to develop a new controlling methodology, named the Synthesis Segmental Multilevel Converter (SSMC) for reactive power compensation in a three-phase grid system. Initially, it extracts the maximum amount of power from the solar PV systems by using an Enhanced Perturb and Observe (EPO) method. Then, the panel separation is performed and the three-phase power input is given to the SSMC converter, where the synchronization and switching pulse generation processes are performed. During synchronization, integrated techniques such as Proportional Integral (PI), Fuzzy Logic Controller (FLC), and Improved Artificial Neural Network (IANN) are utilized to maintain the voltage, magnitude and phase angle in the same level. Consequently, the Inductance Capacitance (LC) filtering technique is applied to reduce the harmonics distortion in the signal. After that, the Park transformation is used to perform the dq0 to abc transformation, which is implemented for reducing the high volume of error. Finally, the error-free signal is fed to the three-phase grid system with reduced harmonics. During experimentation, both the simulation and analytical results have been taken for analyzing the performance of the proposed technique. Moreover, it is compared to the existing algorithms for proving the betterment of the proposed methodology.
This article develops a two-stage volt-VAr control strategy coordinating the discrete controls of legacy grid devices and the operation of smart inverters (SIs) in power distribution grids with high penetration of photovoltaic generation. The first stage dispatch problem optimally coordinates the tap settings of on-load tap changers,
on
/
off
status of shunt capacitor banks, and SIs’ active power output. The second stage incorporates the SI volt-VAr mode, i.e.,
as per IEEE-1547, and an adaptive volt-VAr droop function,
, to dispatch the SI reactive power generation. Both dispatch problems are formulated as mixed-integer linear programming problems to maintain tractable formulations and reduce the complexity and computational burden that usually arise when solving large-scale optimal power flow problems. The proposed two-stage strategy is tested on a modified IEEE 123-bus system considering different droop settings. The dispatch strategy augmented with
droop outperforms the model with only
droop in stabilizing the PCC voltage while maintaining less active power curtailment.
The voltage rise problem due to the reverse power flow is one of the main obstacles to expanding the photovoltaic systems (PVSs) in distribution networks. In this paper, a decentralized control method for coordinated control of on-load tap changer (OLTC) transformers and PV inverters, is proposed for the voltage regulation of radial distribution networks. In this method, the allowable voltage range is divided into several zones, and appropriate corrective actions are adaptively taken according to each zone. To reduce the tension on the OLTC, first, the reactive power capability of the PV inverters is employed for the voltage regulation, and if it is insufficient, OLTC is used as the last solution. In this regard, the unused capacity of PV inverters is utilized for the reactive power compensation, considering the grid codes reactive power requirements. Furthermore, decentralized control based on a signaling method is used for coordination between OLTC and PVSs, which eliminates the need for communication links. Finally, the effectiveness of the proposed method is validated by the simulation results for the IEEE 33-bus distribution network, and the impact of different reactive power control methods on the number of OLTC operation is investigated.
With the explosive growth of distributed renewable energy connected to the power system, the application of AC-DC hybrid distribution network with power electronic transformer (PET) becomes more and more extensive. An equivalent loss model is formulated from the perspective of the internal topology of PET. Then, a novel PET power flow model is established to obtain the relationship between the three ports. On this basis, reactive power of PV nodes connecting PET and distributed renewable generation is modified by establishing an equivalent impedance matrix, thus the forward-backward sweep load flow calculation method is improved for power flow calculation and volt/var control of distribution network. Different from traditional reactive optimization methods, a multi-objective reactive power optimization model considering the PET and two types of photovoltaic converters is established in this paper, with the targets of minimizing voltage deviation and power loss simultaneously. Finally, the numerical simulation shows the accuracy, rationality, and feasibility of the proposed method.
Considering the rising penetration of diverse types of renewable generating units (RGUs) in modern electricity networks, the active and reactive power management and the coordination of RGUs with different operating states can be intricate. Specifically, the RGUs may not be always allowed to operate with their maximum generation capacity, as they are committed to provide reactive power support to the grid during both normal and emergency conditions. In that case, the optimal control setpoints can be assigned to the RGUs, such that the voltages can be maintained within an acceptable range, the power losses can be minimized and the inherent uncertainties of renewables can be addressed. In this paper, a power management system is designed for the optimal allocation of the active and reactive power flow through the network, and to provide the voltage management locally, considering the service provider’s requirements. Particularly, a combined local/centralized control system is implemented at substation levels to monitor and modify the voltage control setpoints of RGUs in real-time, which can improve the voltage profile in the local networks, minimize the power losses and reduce network congestions by allowing the optimal active power transfer. Based on the simulation results, when the proposed combined local-centralized strategy is implemented, the voltages can be improved for all buses, the congestion constraints of the power lines can be maintained and the power losses can be significantly minimized.
Renewable and sustainable energies such as wind power generation are increasingly integrated into electric grids due to their various technical, environmental, and economic advantages. However, these resources impose more uncertainties on the power system in addition to the uncertainty of load demands. These uncertainties and correlations among them confront operation decisions with serious challenges.
In this paper, a new multi-objective probabilistic approach is proposed for smart voltage control of wind-energy-integrated systems through tap adjustment of power transformers and voltage regulation of generators. The objectives include improving voltage stability margin, voltage profile improvement, and power loss reduction in the presence of wind and demand uncertainties. A data clustering algorithm is employed to handle the uncertainty of the random input variables, and a Cholesky decomposition method is used for modeling the correlation between them. The proposed multi-objective optimization is solved using NSGA-II. The power system has been modeled in DIgSILENT-Powerfactory linked with MATLAB, where the NSGA-II is programmed. The IEEE 14-bus system has been employed to evaluate the performance of the conducted approach. The results demonstrate the effectiveness of the proposed approach in decreasing the losses, increasing the voltage stability margin, and improving the voltage profile of wind-energy-integrated power systems.
This chapter explains the concept of coordinating multiple control devices, using examples from Flexible AC Transmission System (FACTS) technology. FACTS Controllers and smart inverters both perform voltage control at a rapid rate. The chapter presents the issues related to the control coordination of smart photovoltaic (PV) inverters with conventional voltage control equipment. Load flow software with the appropriate models of FACTS Controllers is generally utilized to examine such control interactions. Interactions may occur among the control systems of different voltage control devices such as FACTS Controllers and/or high voltage damping controller converters. Smart inverter functions on the PV inverters are used to provide autonomous control of the point of common coupling (PCC) voltage of each PV system to the desired level. The PCC voltage control is impacted both by active power exchange and reactive power exchange by the smart PV inverter.
The strong mutual influence of Demand Response (DR) and Volt/Var Optimization (VVO) on each other has largely been ignored in most of the previous schemes. Those research studies, which collectively consider DR and VVO, deal with minimal devices. Further, rescheduling of loads participating in DR and phase-specific operations of voltage control devices, such as Capacitor Banks (CBs), Voltage Regulators (VRs), On-Load Tap Changer (OLTC), and inverters of the solar photovoltaic sources, in an active three-phase distribution system significantly cause the unbalance levels. Hence, a comprehensive formulation, which considers DR to reduce peak load, VVO to minimize loss, and unbalance minimization using elastic loads, inverters, OLTC, VRs, and CBs, is proposed in this paper for three-phase unbalanced active distribution systems. Most previous studies deal with the cost while integrating DR and VVO, favoring real power rescheduling. In this research, rescheduling of kVA loads, affecting VVO devices’ operations, is adopted. A load factor-based load shifting index is proposed and utilized for this purpose. The proposed formulation aims to minimize the substation transformer’s current unbalance levels that seem to increase while limiting bus voltage unbalance factors. The proposed scheme is solved using the multi-objective particle swarm optimization and is tested on the modified IEEE 13-bus and IEEE 123-bus test systems. Compared with the Conservation Voltage Reduction (CVR) based method, the obtained results reveal that the proposed formulation minimizes peak load, loss, and unbalances more effectively while managing voltage rise or drop issues more efficiently without source or load curtailments.
This paper presents a comprehensive three-bus equivalent circuit model of three-phase step voltage regulators. The proposed model can be efficiently integrated in the Z-bus power flow method and can accurately simulate any configuration of step voltage regulators. In contrast to the conventional step voltage regulator models that include the tap variables inside the YBUS matrix of the network, the proposed model simulates them in the form of current sources, outside the YBUS matrix. As a result, the re-factorization of the YBUS matrix is avoided after every tap change reducing significantly the computational burden of the power flow. Furthermore, possible convergence issues caused by the low impedance of step voltage regulators are addressed by introducing fictitious impedances, without, however, affecting the accuracy of the model. The results of the proposed step voltage regulator model are compared against well-known commercial softwares such as Simulink and OpenDSS using the IEEE 4-Bus and an 8-Bus network. According to the simulations, the proposed model outputs almost identical results with Simulink and OpenDSS confirming its high accuracy. Furthermore, the proposed 3-bus equivalent model is compared against a recently published conventional step voltage regulator model in the IEEE 8500-Node test feeder. Simulation results indicate that the proposed step voltage regulator model produces as accurate results as the conventional one, while its computation time is significantly lower. More specifically, in the large IEEE 8500-node network consisting of four SVRs, the proposed model can reduce the computation time of power flow around one minute for every tap variation. Therefore, the proposed step voltage regulator model can constitute an efficient simulation tool in applications where subsequent tap variations are required.
This paper proposes an optimal voltage regulation scheme (OVRS) for distribution systems with rich photovoltaic (PV). Various regulation devices are optimally controlled in a coordinated manner: PV inverter, D-STATCOM, step voltage regulator (SVR), and on-load-tap-changer (OLTC). A data structure algorithm is proposed to split the distribution system into layered zones considering the radial structure of the system. The solution process of the proposed scheme is accomplished by a meta-heuristic optimizer. OVRS addresses the voltage violations while yielding a coordinated operation of the various control devices. The proposed OVRS involves three control levels to completely prevent voltage violations. In the first control level, the PV inverter and D-STATCOM mitigate rapidly the local voltage deviation through injecting/absorbing optimized reactive power. The second control level is a decentralized-based control scheme that utilizes the voltage control devices in each zone to handle the voltage violations if any. For each zone, the control devices in the upper-stream zones (parent zones) are managed by the third control level to ensure cooperative control actions. The simulation results on the 119-bus distribution system, with clear, low fluctuation, and high fluctuation of solar radiation profiles, demonstrate the effectiveness of the proposed OVRS.
This article presents an efficient voltage regulation method using capacitive reactive power. Simultaneous operation of photovoltaic power systems with the local grids induces voltage instabilities in the distribution lines. These voltage fluctuations cross the allowable limits on several occasions and cause economic losses. In the proposed method, the reactive power is applied at the load and generated using a capacitor bank. The capacitors are arranged in a binary order of capacitances to enable the 2n equally dispersed combinations. Initially, a strict analytical solution is developed to study the outcome of capacitance connection at load. The capacitance is selected based on the analytical solution of non-linear equations describing voltage deviations in the distribution line connected with photovoltaic systems. The control system is developed for fast and appropriate capacitance selection and a smooth transient process. The experimental setup is developed to verify the theoretical analysis. The measured data shows good agreement with the calculated one, verifying the correctness and accuracy of the proposed method. It is recommended that the reactive power compensation can be applied for a shorter time because the source current enhances substantially as the capacitance is connected to the load. The proposed method can be applied together with the tap-changers functionality, and the capacitance should be disconnected as the tap-changer corrects the transformation ratio.
The integration of large-scale distributed photovoltaics (PVs) in distribution networks (DNs) causes reverse power flow and the voltage violation problem. However, in some rural area, DNs do not have sufficient measuring information and communication ability to meet the challenge. In order to solve the voltage violation problem, this study proposes a voltage profile perception method and a voltage regulation analysis model as an alternative method to power flow calculations to guide the partition and voltage control of DNs. We then propose a novel hierarchical decentralised voltage control method strategy that combines cluster autonomous optimisation and cluster coordination optimisation in different time scales based on the DN partition. The optimal control is conducted for voltage regulation to minimise the amount of reactive power compensation and the active power curtailment under the DN operation constraints. The effectiveness and feasibility of the proposed method are demonstrated via simulation tests on a real 10.5 kV 34-node feeder in China.
ABSTRACT: Local reactive power provision by photovoltaic (PV) inverters can contribute to maintaining the voltage in distribution grids and therefore contribute to reduce grid reinforcement measures, caused by an increased PV penetration. This study investigates the interferences and dynamics of a parallel and autonomous operation of HV/MV on-load tap changer (OLTC) control and autonomous PV inverter control. The focus of this investigation is to identify whether reactive power provided by PV inverters can cause an increase of HV/MV transformer tap changes. The analysis is based on simulations of a real German distribution grid with a high share of PV generation. To incorporate realistic power fluctuations of PV systems in the investigated grid, geographically distributed solar irradiation measurements are used as input data for the simulations. The simulations show that reactive power provided by PV inverters can lead to an increase of reactive power fluctuations over the HV/MV transformer and can thus cause a significant increase of OLTC tap changes. The number of additional tap changes depends on the reactive power control strategy of the PV inverters.
In this paper, the impact of voltage support by PV systems, installed at LV level, on the reactive power exchange at the HV/MV connection point is presented. The analysis was performed using a detailed system model of the smart grid re-gion Seebach (south-eastern Bavaria), encompassing LV and MV level. The simulations were performed for different voltage control strategies (fixed cosϕ, cosϕ(P)-control, Q(U)-control) applied by the low voltage PV systems and for different penetration rates of PV systems with voltage support. However, the results show significant disparities be-tween the investigated control strategies. While the Q(U)-control shows a minor impact on the reactive power exchange at the HV/MV connection point, a fixed cosϕ or cosϕ(P)-control can lead to a significant increase on the reactive power exchange, especially with increasing penetration of controlled low voltage PV systems. Therefore, Q(U) control of low voltage PV systems can be an effective measure for voltage support at the LV level, whilst minimizing the reactive power that is required from upstream voltage levels for compensation purpose.
This thesis analyzes the technical and economic potential of autonomous voltage control strategies for improving distribution grid operation with high shares of photovoltaic (PV) generation. Key issues include:
The simultaneity of local photovoltaic generation and local consumption as well as its influence on reverse power flows.
The theoretical potential of autonomous voltage control strategies to increase a grid’s hosting capacity for additional photovoltaic generation.
Stability analyses of a voltage-dependent combined active and reactive power control strategy for photovoltaic inverters.
The cost savings potential (CAPEX & OPEX) of autonomous voltage control strategies, compared to traditional grid reinforcement measures.
The results suggest that autonomous voltage control strategies can be used to improve the technical and economic distribution grid integration of PV systems. If applied appropriately, these strategies are capable of deferring grid reinforcement measures and hence shifting investment costs to future points in time. Of all investigated autonomous voltage control strategies, the on-load tap changer voltage control and a combined Q(V)/P(V) PV inverter control strategy showed the most promising results, from a technical and an economic perspective.
Power system load modeling is a mature and generally well researched area which, as many other in electrical power engineering at the present time, is going through a period of renewed interest in both industry and academia. This interest is fueled by the appearance of new non-conventional types of loads (power electronic-based, or interfaced through power electronics) and requirements to operate modern electric power systems with increased penetration of non-conventional and mostly intermittent types of generation in a safe and secure manner. As a response to this renewed interest, in February 2010 CIGRE established working group C4.605: “Modelling and aggregation of loads in flexible power networks”. One of the first tasks of the working group was to identify current international industry practice on load modeling for static and dynamic power system studies. For that purpose, a questionnaire was developed and distributed during the summer/autumn of 2010 to more than 160 utilities and system operators in over 50 countries on five continents. This paper summarizes some of the key findings from about 100 responses to the questionnaire received by September 2011 and identifies prevalent types of load models used as well as typical values of their parameters.
A wavelet variability model (WVM) for simulating solar photovoltaic (PV) power plant output given a single irradiance point sensor timeseries using spatio-temporal correlations is presented. The variability reduction (VR) that occurs in upscaling from the single point sensor to the entire PV plant at each timescale is simulated, then combined with the wavelet transform of the point sensor timeseries to produce a simulated power plant output. The WVM is validated against measurements at a 2-MW residential rooftop distributed PV power plant in Ota City, Japan and at a 48-MW utility-scale power plant in Copper Mountain, NV. The WVM simulation matches the actual power output well for all variability timescales, and the WVM compares well against other simulation methods.
The challenges of designing a sustainable future electrical power system with an improved integration of distributed energy units and renewable energy units are tackled in various research and development projects with different approaches. Due to the early but rapidly maturing stage of research on this complex topic many definitions of future concepts are not harmonised yet. The complete picture of the future power system is still under investigation with regard to technical as well as economic aspects.
This thesis provides four main contributions to this research process:
1. A set of adequate definitions for various concepts concerning the integration of distributed energy units (distributed generators, loads and storage) in the electrical power system is proposed. These definitions are required to reach a common understanding because many of them are still under discussion. An extensive review on definitions and concepts is the basis for the proposed structure of aggregation approaches. Also ancillary services are characterised that can be provided by distributed generators by extending the present focus from transmission networks to active distribution networks.
2. A comprehensive overview of the technological control capabilities of distributed generators and the resulting possibilities of providing ancillary services is provided. The technological potential is
investigated by application of a new assessment approach that considers the grid-coupling converter separately with its particular capabilities. An enormous technological potential is identified.
3. The economic potential of a participation of distributed generators in frequency control and reactive power supply is investigated with cost-benefit-analyses that are based on newly developed assessment
approaches. Especially reactive power supply by distributed generators looks very promising. Therefore, a modified droop concept is developed that can reduce the operational costs of reactive power supply in mini-grids considerably.
4. A control system for distributed energy units in ISET’s Design-Centre for Modular Supply Technology is developed. This control system allows demonstrating the technological and economic capabilities of distributed energy units with regard to the provision of ancillary services by hardware experiments.
Optimised reactive power supply is demonstrated with centralised and decentralised control
concepts.
A stability study of distribution networks with a high penetration of distributed generators (DGs) actively supporting the network is presented in this paper. A possible way of mitigating the voltage rise caused by DGs is the local control of reactive power. Among the different possible options, the Q(U) control (reactive power as a function of the voltage) is one of the commonly suggested solutions. However, the Q(U) control can, under certain conditions, lead to instability. This paper summarizes the results of a stability study conducted on a single-inverter system and on a multiinverter system. It shows that the requirements for reaching a stable operation can easily be met for integrated systems but could be a significant constraint for systems relying on communication.
First, we investigate the technical and economic potential of autonomously operating voltage control strategies in low voltage grids with high photovoltaic penetration. The investigated control strategies are based on the reactive power control capabilities of modern photovoltaic inverters and the application of distribution transformers with on-load tap changers. We present a methodology for assessing the technical potential of autonomous voltage control strategies to increase the hosting capacity of low voltage grids and show the results for 40 real German LV grids. Second, we compare the investment and operational costs for conventional grid extension measures with those resulting from the application of autonomous voltage control strategies over a period of 10 years, using real measured load and generation data. Finally, we assess the additional grid loss reduction potential by applying a coordinated control approach, using the reactive power capabilities of the inverters as well as the on-load tap changer. The results clearly highlight the economic benefit of autonomous voltage control strategies by deferring grid reinforcement measures.
The quality and the reliability of the power generated by large grid‐connected photovoltaic (PV) plants are negatively affected by the source characteristic variability. This paper deals with the smoothing of power fluctuations because of geographical dispersion of PV systems. The fluctuation frequency and the Maximum fluctuation registered at a PV plant ensemble are analyzed to study these effects. We propose an empirical expression to compare the fluctuation attenuation because of both the size and the number of PV plants grouped. The convolution of single PV plants frequency distribution functions has turned out to be a successful tool to statistically describe the behavior of an ensemble of PV plants and determine their maximum output fluctuation. Our work is based on experimental 1‐s data collected throughout 2009 from seven PV plants, 20MWp in total, separated between 6 and 360 km.
This paper presents a quasi-static time-series test feeder based on the well-known 34 node test feeder [1]. This test feeder is useful for the evaluation and quantification the distribution system impacts of distributed PV integrations. The load profiles and solar resource profiles used are both publically available. The quasi-static time-series test feeder has been developed using data for the year 2010. The fixed time period between power flow solutions is 1 min making the presented quasi-static time-series test feeder appropriate for use in investigating voltage regulation device operations. A load allocation algorithm is also proposed for assigning spot and distributed loads in the 34 node test feeder to time varying load profiles based on rate class load profiles. A simple model for modeling the real AC power output of a PV system based on discrete global horizontal irradiance (GHI) is also presented. The results of analysis completed using the quasi-static time-series test feeder are shown for various single-site 1MW PV deployment scenarios and voltage regulator compensation settings.
The uptake of variable megawatts from photovoltaics (PV) challenges distribution system operation. The primary problem is significant voltage rise in the feeder that forces existing voltage control devices such as on-load tap-changers and line voltage regulators to operate continuously. The consequence is the deterioration of the operating life of the voltage control mechanism. Also, conventional non-coordinated reactive power control can result in the operation of the line regulator at its control limit (runaway condition). This paper proposes an optimal reactive power coordination strategy based on the load and irradiance forecast. The objective is to minimize the number of tap operations so as not to reduce the operating life of the tap control mechanism and avoid runaway. The proposed objective is achieved by coordinating various reactive power control options in the distribution network while satisfying constraints such as maximum power point tracking of PV and voltage limits of the feeder. The option of voltage support from PV plant is also considered. The problem is formulated as constrained optimization and solved through the interior point technique. The effectiveness of the approach is demonstrated in a realistic distribution network model.
Technically effective and economically efficient voltage control is a major issue in distribution systems with high amounts of installed capacity from dispersed generators, such as photovoltaic. In this paper, the results of an encompassing cost–benefit analysis for different voltage control strategies are presented. The investigated voltage control strategies comprise two different reactive power control methods and one combined reactive power/active power control method, each applied by inverters of utility scale photovoltaic systems. The results are gained by performing 12‐month root‐mean‐square simulations with a 1 min resolution, using the model of a real distribution grid as well as complex generation and load models. The simulations show that local reactive power provision methods as well as temporal active power output curtailment methods are capable of reducing the necessity of a voltage‐driven grid reinforcement. However, the economic benefit of those voltage control strategies highly depends on the parameterization of the respective control algorithm. Copyright © 2013 John Wiley & Sons, Ltd.
This document summarizes the equations and applications associated with the photovoltaic array performance model developed at Sandia National Laboratories over the last twelve years. Electrical, thermal, and optical characteristics for photovoltaic modules are included in the model, and the model is designed to use hourly solar resource and meteorological data. The versatility and accuracy of the model has been validated for flat-plate modules (all technologies) and for concentrator modules, as well as for large arrays of modules. Applications include system design and sizing, 'translation' of field performance measurements to standard reporting conditions, system performance optimization, and real-time comparison of measured versus expected system performance.
The widespread use of distributed generation (DG) relies on methods and techniques aimed at facilitating the network integration of DG. In this context a methodology for the evaluation of the quality and relative merits of these methods and techniques is missing. CIGRE Task Force C6.04.02, which is affiliated with CIGRE Study Committee C6, has addressed this problem by proposing a set of resource and network benchmarks. In the present paper, the benchmark for integrating DG in medium voltage distribution networks is described. The proposed benchmark is representative of a real network while it is also designed for ease of use. The application of the benchmark is described through several case studies that show the impact of DG on power flow and voltage profiles at the medium voltage level
This paper presents modern techniques to study step voltage regulator (SVR) - distributed generation (DG) interactions in distribution systems using electromagnetic transient programs. Digital models represent physical characteristics of both SVR and DG and the associated control systems. These modeling techniques may effectively simulate realistic aspects of SVR and DG real time operations.
Because traditional electric power distribution systems have been designed assuming the primary substation is the sole source of power and short-circuit capacity, DR interconnection results in operating situations that do not occur in a conventional system. This paper discusses several system issues which may be encountered as DR penetrates into distribution systems. The voltage issues covered are the DR impact on system voltage, interaction of DR and capacitor operations, and interaction of DR and voltage regulator and LTC operations. Protection issues include fuse coordination, feeding faults after utility protection opens, impact of DR on interrupting rating of devices, faults on adjacent feeders, fault detection, ground source impacts, single phase interruption on three phase line, recloser coordination and conductor burndown. Loss of power grid is also discussed, including vulnerability and overvoltages due to islanding and coordination with reclosing. Also covered separately are system restoration and network issues.
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