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Implementation of a Two-Stage PV System Testbed with Power Reserves for Grid-Support Applications
... where a is an ideality factor; R s and R sh are the series and shunt resistances, respectively; I ph and I s are the photo and diode saturation currents, respectively; and w, given by (5), is calculated with the Lambert W function. The EMT models and the non-linear least squares Levenberg-Marquardt technique utilized for the MPPE are discussed in detail in [4] and [15]. ...
... The battery current, SOC, and the PV output power are defined as output variables to allow the implementation of soft constraints to the respective state variables when designing the MPC. Thus, the hybrid PV plant output power is given by (15), whereas the power reserves are given by (16). ...
... In (23), the tracking cost weights (w y j ) must be tuned so the desired operation performance is achieved. For the hybrid PV plant, the main goal is to track an output power reference defined in (15), hence its weight is set with the highest value. Moreover, some of the weights will always be zero, such as the weights of the battery current (w y 2 ) and the PV output power (w y 5 ), because there is no specific goal for each of them individually. ...
This paper presents an optimal control strategy for operating a solar hybrid system consisting of solar photovoltaic (PV) and a high-power, low-storage battery energy storage system (BESS). A state-space model of the hybrid PV plant is first derived, based on which an adaptive model predictive controller is designed. The controller's objective is to control the PV and BESS to follow power setpoints sent to the the hybrid system while maintaining desired power reserves and meeting system operational constraints. Furthermore, an extended Kalman filter (EKF) is implemented for estimating the battery SOC, and an error sensitivity is executed to assess its limitations. To validate the proposed strategy, detailed EMT models of the hybrid system are developed so that losses and control limits can be quantified accurately. Day-long simulations are performed in an OPAL-RT real-time simulator using second-by-second actual PV farm data as inputs. Results verify that the proposed method can follow power setpoints while maintaining power reserves in days of high irradiance intermittency even with a small BESS storage.
... One can either increase or decrease the maximum power point voltage to reduce the power extracted from a PV module, as shown in the characteristic I-V and P-V curves in Fig. 3. On the one hand, increasing the voltage would allow a wider curtailment range as the duty cycle is improved, given the inverse proportionality between V PV and D, with a fixed V bus . However, controlling the power in that range might be more complex due to the steep power decline when increasing the voltage above the MPP point [11]. On the other hand, reducing the power by reducing the voltage allows smoother control, but reduces the curtailment range as the duty cycle increases. ...
... Note that such lower currents will occur only when the converter is not operating in MPPT mode but in curtailment. Thus, a sudden curtailment requirement will produce a deeper thermal cycle ∆T , since the junction temperature is proportional to the current, as per (7) - (11). ...
The massive deployment of PV systems in residential buildings is causing voltage challenges in low-voltage distribution networks. Worldwide, DSOs started requesting users to curtail power when this is injected back into the grid. Although many commercially available inverters can perform curtailment, the degradation effects of curtailment still have to be investigated. This paper estimated how power curtailment affects the reliability of a boost converter working below MPPT voltages. Using the mission profile method, we determined the conduction and switching losses on the converter switch as the critical component, based on its temperature and current profiles. The results suggest that curtailing the power requires the operation point to move towards lower PV voltages, leading to deeper thermal cycles, therefore reducing the expected lifetime of the converter by up to 80 % for power injection into the grid below 1.5 kW. From the operational perspective, this might require premature replacements compared to operating under MPP conditions, affecting the revenue forecast before the enforcement of curtailment. For power injection above 1.5 kW, the LCoE does not change compared to the case without considering the degradation. However, near zero-injection conditions, the difference in LCoE between considering and not considering replacements increases exponentially up to 135 %.
... Conventional transient stability (TS) software packages that run at the millisecond level are sufficient to simulate synchronous generator dynamic behaviors in a larger network. However, as synchronous generators are being displaced by energy systems powered by IBRs with advanced grid-support functionalities [2], [3], electromagnetic transient (EMT) simulations are required for capturing IBR dynamic responses at the microsecond level. ...
... Conventional transient stability (TS) software packages that run at the millisecond level are sufficient to simulate synchronous generator dynamic behaviors in a larger network. However, as synchronous generators are being displaced by energy systems powered by IBRs with advanced grid-support functionalities [2], [3], electromagnetic transient (EMT) simulations are required for capturing IBR dynamic responses at the microsecond level. ...
This paper presents the development and benchmarking of a novel real-time electromagnetic-transient and transient-stability (EMT-TS) modeling architecture for distribution feeder restoration studies. The work presents for the first time in the literature a real-time EMT-TS testbed in which the grid-forming unit is simulated in EMT domain, operating as the slack bus of the phasor domain while including unbalanced voltage conditions. To evaluate the performance and limitations of the proposed model, an equivalent EMT testbed is developed to serve as a benchmark. First, the co-simulation framework and the domain coupling methodology are introduced. Next, the steady-state operation of the EMT-TS and EMT models are matched. Then, tests are conducted to analyze the transient performance of the proposed EMT-TS model when compared to its EMT benchmark. Results reveal that when utilizing a battery energy storage system (BESS) as the grid-forming unit, the EMT-TS testbed can maintain high accuracy for typical load steps.
With increased integration of renewable resources into the power distribution system,
bidirectional ac/dc converters are gaining popularity in interfacing the utility grid and
energy storage systems. This thesis presents design and modeling of a three-phase
bidirectional PWM ac/dc converter with high power factor, low current THD, and high
efficiency. Design expressions for the passive components based on current ripple,
voltage ripple, and load transients will be proposed. A mathematical model to represent
the dynamic behavior of the converter will be derived using a Switched model and the
Generalized Averaging Method. A cascaded control system with an outer voltage
controller, an inner hysteresis current controller, and a phase-locked loop algorithm will
be designed utilizing the derived converter transfer functions. The designed system will
be set up and simulated using PLECS simulation platform. Simulation results will be
presented to validate the design and to analyze the system performance during
steady-state, start-up, load-transients, and grid-distortion conditions.
The power system is experiencing an ever-increasing integration of photovoltaic power plants (PVPPs), which leads demand on the power system operators to force new requirements to sustain with quality and reliability of the grid. Subsequently, a significant quantity of flexible power point tracking (FPPT) algorithms have been proposed in the literature to enhance functionalities PVPPs. The intention of FPPT algorithms is to regulate the PV power to a specific value imposed by the grid codes and operational conditions. This will inevitably interfere the maximum power point tracking (MPPT) operation of PV systems. Nevertheless, the FPPT control makes PVPPs much more grid-friendly. The main contribution of this paper is to comprehensively compare available FPPT algorithms in the literature from different aspects and provide a benchmark for researchers and engineers to select suitable FPPT algorithms for specific applications. A classification and short description of them are provided. The dynamic performances of the investigated algorithms are compared with experimental tests on a scaled-down prototype. Directions for future studies in this area are also presented.
One of the major concerns associated with the increasing penetration of grid-connected photovoltaic (PV) power plants is the operational challenges (e.g., overloading and overvoltage), imposed due to the variability of PV power generation. A flexible power point tracking (FPPT), which can limit the PV output power to a specific value, has thus been defined in grid-connection regulations to tackle some of the integration challenging issues. However, the conventional FPPT algorithm based on the perturb and observe method suffers from slow dynamics. In this paper, an adaptive FPPT algorithm is thus proposed, which features fast dynamics under rapidly changing environmental conditions (e.g., due to passing clouds), while maintaining low power oscillations in steady-state. The proposed algorithm employs an extra measured sampling at each perturbation to observe the change in the operating condition (e.g., solar irradiance). Afterwards, the voltage-step is adaptively calculated following the observed condition (e.g., transient or steady-state) in a way to improve the tracking performance. Experimental results on a 3-kVA grid-connected single-phase PV system validate the effectiveness of the proposed algorithm in terms of fast dynamics and high accuracy under various operational conditions.
In order for a PV system to provide a full range of ancillary services to the gird, including frequency response, it has to maintain active power reserves. In this paper, a new control scheme for the dc/dc converter of a two-stage PV system is introduced, which permits operation at a reduced power level, estimating the available power (maximum power point - MPP) at the same time. This control scheme is capable of regulating the output power to any given reference, from near-zero to 100% of the available power. The proposed MPP estimation algorithm applies curve fitting on voltage and current measurements obtained during operation to determine the MPP in real time. This is the first method in the literature to use the non-simplified single-diode model for the determination of the MPP and the five model parameters while operating at a curtailed power level. The developed estimation technique exhibits very good accuracy and robustness in presence of noise and rapidly changing environmental conditions. The effectiveness of the control scheme is validated through simulation and experimental tests using a 2 kW PV array and a dc/dc converter prototype at constant and varying irradiance conditions.
Due to the still increasing penetration of grid-connected Photovoltaic (PV) systems, advanced active power control functionalities have been introduced in grid regulations. A power reserve control, where namely the active power from the PV panels is reserved during operation, is required for grid support. In this paper, a cost-effective solution to realize the power reserve for two-stage grid-connected PV systems is proposed. The proposed solution routinely employs a Maximum Power Point Tracking (MPPT) control to estimate the available PV power and a Constant Power Generation (CPG) control to achieve the power reserve. In this method, the solar irradiance and temperature measurements that have been used in conventional power reserve control schemes to estimate the available PV power are not required, and thereby being a sensorless approach with reduced cost. Experimental tests have been performed on a 3-kW two-stage single-phase grid-connected PV system, where the power reserve control is achieved upon demands.
Determination of PV model parameters usually requires time consuming iterative procedures, prone to initialization and convergence difficulties. In this paper, a set of analytical expressions is introduced to determine the five parameters of the single-diode model for crystalline PV modules at any operating conditions, in a simple and straightforward manner. The derivation of these equations is based on a newly found relation between the diode ideality factor and the open circuit voltage, which is explicitly formulated using the temperature coefficients. The proposed extraction method is robust, cost-efficient, and easy-to-implement, as it relies only on datasheet information, while it is based on a solid theoretical background. Its accuracy and computational efficiency is verified and compared to other methods available in the literature through both simulation and outdoor measurements.
Historically, electric power system operators have seen photovoltaic (PV) power systems as potential sources of problems due to intermittency and lack of controllability. However, the flexibility of power electronic inverters allows PV to provide grid-friendly features including volt-VAR control, ramp-rate control, high-frequency power curtailment, and event ride-through. These technologies offer power quality improvements and enable wider penetrations of PV systems. Commercially available smart PV inverters can further provide frequency down-regulation by curtailing power, but they are unable to provide true frequency regulation through active power control (APC) because they are unable to increase power on command. The development of inverters capable of APC for primary and secondary frequency regulation without the need for energy storage has the potential to transform the way grid operators view high PV penetration levels. This paper provides a brief tutorial on APC of PV inverters, summarizes state of the art research in the area, and suggests future research directions.
This paper presents a fast power-setpoint tracking algorithm to enable utility-scale photovoltaic (PV) systems to provide high quality grid services such as power reserves and fast frequency response. The algorithm unites maximum power-point estimation (MPPE) with flexible power-point tracking (FPPT) control to improve the performance of both algorithms, achieving fast and accurate PV power-setpoint tracking even under rapid solar irradiance changes. The MPPE is developed using a real-time, nonlinear curve-fitting approach based on the Levenberg-Marquardt algorithm. A modified adaptive FPPT based on the Perturb and Observe technique is developed for the power-setpoint tracking. By using MPPE to decouple the impact of irradiance changes on the measured PV output power, we develop a fast convergence technique for tracking power-reference changes within three FPPT iterations. Furthermore, to limit the maximum output power ripple, a new design is introduced for the steady-state voltage step size of the adaptive FPPT. The proposed algorithm is implemented on a testbed consisting of a 500 kVA three-phase, single-stage, utility-scale PV system on the OPAL-RT eMEGASIM platform. Results show that the proposed method outperforms the state-of-the-art.
p>Maintaining power reserves in a photovoltaic (PV) system, i.e. operating at a curtailed power level, is a promising method towards grid support functions without energy storage. A major challenge in this approach is to monitor the maximum power (i.e. the available headroom) as it changes over time with the irradiance and temperature. This paper presents a new method to estimate the maximum power point (MPP) power and voltage in real-time, designed for very fast irradiance changes. While keeping reserves, the single-diode PV model equations are applied to voltage and current samples to estimate the conditions and maximum power, rather than using potential erroneous irradiance and temperature sensors. This is the first model-based method to guarantee estimation robustness regardless of the irradiance transient. Furthermore, the operating point perturbation is infrequent and limited, resulting in near-perfect MPPT efficiency at zero reserve levels. The effectiveness of the proposed method is validated via simulations in MATLAB/Simulink on a 10 kW PV system assuming noisy measurements. </p
This paper presents a current control for single phase grid connected inverters. The method allows for inverter active and reactive power control. The method uses the Direct-Quadrature (DQ) synchronous reference frame transformation for single-phase converters. This method transforms an orthogonal pair consisting of the inverter output current and a time shifted version of this current from a stationary frame to a rotating frame synchronous to the fundamental output frequency. Alternatives to using the time shifted current are discussed. The steady state current components in this rotating DQ frame are DC values and thus PI control methods can be used with zero error. A household scale grid-tie inverter is used as an example application of this method. A Simulink simulation model and results where the inverter output is controlled to match a local load's active and reactive power demand is presented.
Advanced grid-friendly controls demonstration project for utility-scale pv power plants
- V Gevorgian
- B O'neill
Highly accurate method for real-time active power reserve estimation for utility-scale photovoltaic power plants
- V Gevorgian
IEEE Std. 1547–2018
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