Reactive Power Control Strategy Research Articles

Analysis of reactive power strategies in HVDC‐connected wind power plant clusters

AbstractOffshore wind power plants (WPPs) built near each other but far from shore usually connect to the main grid by a common high‐voltage DC (HVDC) transmission system. In the resulting decoupled offshore grid, the wind turbine converters and the high‐voltage DC voltage‐source converter share the ability to inject or absorb reactive power. The overall reactive power control dispatch influences the power flows in the grid and hence the associated power losses. This paper evaluates the respective power losses in HVDC‐connected WPP clusters when applying 5 different reactive power control strategies. The case study is made for a 1.2‐GW–rated cluster comprising 3 WPP and is implemented in a combined load flow and converter loss model. A large set of feasible operating points for the system is analyzed for each strategy. The results show that a selection of simulations with equal wind speeds is sufficient for the annual energy production comparison. It is found that the continuous operation of the WPPs with unity power factor has a superior performance with low communication requirements compared with the other conventional strategies. The optimization‐based strategy, which is developed in this article, allows a further reduction of losses mainly because of the higher offshore grid voltage level imposed by the high‐voltage DC voltage‐source converter. Reactive power control in HVDC‐connected WPP clusters change significantly the overall power losses of the system, which depend rather on the total sum of the injected active power than on the variance of wind speeds inside the cluster.

A Large Scale Grid Data Analysis Platform for DSOs

The number of fluctuating distributed energy resources (DER) in electricity grids is continuously rising. Due to the lack of operational information on low-voltage (LV) networks, conservative assumptions are necessary to assess the connection of generators to the grid. This paper introduces the hosting capability (HC) as a measure to assess the amount of DER that can be integrated in LV-feeders. The HC of a feeder is the minimum amount of DER that can be hosted in a feeder without reinforcement needs for a given DER-scenario and for a given admissible voltage rise. The hosting capability assessment was performed on the entire LV-grid data of two Austrian Distribution System Operators (DSOs) with more than 36,000 LV-feeders. In total, 40 HC-scenarios were calculated with varying admissible voltage rise levels, DER-scenarios and reactive power control strategies. It turned out that only few feeder parameters such as the resistance at the end node and the lowest ampacity value of feeders show a high correlation with the calculated HC. Further, the impact of the DER-scenario on the share of voltage and loading constrained feeders is rather limited. The gathered results are suitable to validate equivalent LV-feeders models to perform integrated power flow studies on the transmission and distribution grids. Besides the results obtained for the network data of the two DSOs, a performant, modular and parallelizable tool has been developed to automatically analyze large LV network sets.

Research on Inverter Integrated Reactive Power Control Strategy in the Grid-Connected PV Systems

In order to solve the problems caused by the susceptibility to changing weather conditions and the complex load conditions of photovoltaic (PV) systems, and the fact a single target inverter control strategy cannot effectively mitigate large voltage fluctuations at point of common coupling (PCC), an integrated reactive power control strategy for PV inverters is proposed. According to the weather and load conditions, this strategy is divided into four control modes: normal operation control mode, reverse power control mode, cloudy control mode and night control mode. The four control modes switch between each other under the specific switching rules to ensure an appropriate quantity of reactive power injection or consumption in the PV inverter. The effects of the four control modes in isolated operation were significant. The deviation rate of PCC voltage from its standard value was mitigated at about 2% under the integrated control strategy. Therefore, the proposed control strategy has a high application value in improving power quality and maximizing utilization of PV inverters.

Review of Reactive Power Dispatch Strategies for Loss Minimization in a DFIG-based Wind Farm

This paper reviews and compares the performance of reactive power dispatch strategies for the loss minimization of Doubly Fed Induction Generator (DFIG)-based Wind Farms (WFs). Twelve possible combinations of three WF level reactive power dispatch strategies and four Wind Turbine (WT) level reactive power control strategies are investigated. All of the combined strategies are formulated based on the comprehensive loss models of WFs, including the loss models of DFIGs, converters, filters, transformers, and cables of the collection system. Optimization problems are solved by a Modified Particle Swarm Optimization (MPSO) algorithm. The effectiveness of these strategies is evaluated by simulations on a carefully designed WF under a series of cases with different wind speeds and reactive power requirements of the WF. The wind speed at each WT inside the WF is calculated using the Jensen wake model. The results show that the best reactive power dispatch strategy for loss minimization comes when the WF level strategy and WT level control are coordinated and the losses from each device in the WF are considered in the objective.

Strategy to enhance the low-voltage ride-through in photovoltaic system during multi-mode transition

With the increasing capacity of distributed generation (DG) connected to the power grid, the future generation of photovoltaic (PV) systems are expected to provide a full range of voltage regulation during grid faults in order to enhance the low-voltage ride-through (LVRT) capability of a PV system. In such a condition, the DG should remain connected to the grid for reactive power support, thereby improving voltage profile. This paper aims to propose a control strategy of active and reactive power for a single-stage three-phase grid-connected PV system to enhance the LVRT. The dynamic behaviours of the system were investigated by considering various scenarios such as varying irradiance, local load disconnection, and short circuits, at different locations during the multi-DG operation. Results confirm that the grid-connected PV system is able to remain connected to the power grid during steady-state and transient-state conditions without violating the grid code requirements. The established dynamic behaviour analysis model of the proposed control for grid-connected PV systems can be used in planning an operational strategy for a practical system.

Mitigating techniques for the operational challenges of a standalone hybrid system integrating renewable energy sources

Renewable energy sources (RESs) combined with energy storage can significantly increase the resilience of standalone systems. However, the intermittent nature of renewable energy resources creates a number of operational challenges for standalone systems. In this paper, the potential challenges facing the operation of a standalone hybrid system with solar photovoltaic (PV) and wind turbine energy generation and battery energy storage are addressed. Control strategies for the system sources are developed to mitigate the potential challenges. A droop-based voltage and frequency control strategy is proposed for the battery energy storage to balance the mismatch between the renewable generation and the load and to maintain the system stability. An active and reactive power control strategy with maximum power point tracking (MPPT) is proposed for the PV and wind sources to maximize the generation when there is a lack of energy resources and to supply quality power from the sources. A simulation was completed to determine the viability of the proposed control strategies. Results show that the proposed control strategies are able to regulate the system voltage and frequency within the acceptable ranges irrespective of variations in the sources and the load-demand.

Reactive Power Strategy of Cascaded Delta-Connected STATCOM Under Asymmetrical Voltage Conditions

Cascaded static synchronous compensator (STATCOM) is an effective solution for reactive power support in middle/high voltage conditions, and it has been widely employed to control reactive power in photovoltaic (PV) plants, wind farms, and industrial occasions. In this paper, reactive power control strategy of cascaded delta-connected STATCOM under asymmetrical voltage conditions is investigated. A new phase current reference calculation method is proposed to support reactive power continually under abnormal voltage conditions considering cluster voltage balancing control and phase current limitation. Constrains between voltage and current phasors of STATCOM are deduced. Then, the analytical expressions of phase currents and circulating current references are solved. Therefore, reactive power support and the safe operation of STATCOM can be obtained simultaneously by limiting the peak value of the phase current references. Furthermore, the reactive power support capability of cascaded STATCOM under asymmetrical voltage conditions is explored and compared by combining the proposed references calculation method with the three generalized current references calculation strategies. Finally, simulation and experimental results have been given to verify the theoretical studies.

Reactive Power and Voltage Optimization Control Strategy in Active Distribution Network Based on the Determination of the Key Nodes

The distributed generation which is integrated in the active distribution network changes power flow, bringing new challenges to the voltage control. When voltage limit violation happens, in order to make the voltage return to normal range and improve the voltage quality, a novel voltage control strategy is proposed. Considering the voltage quality and node importance, the electrical closeness centrality and key node contribution degree are defined, and the key nodes are determined by the orders of the key node contribution degree. This paper uses the reactive power compensation devices which are installed at the key nodes coordinated with the reactive power output of the distributed generation to realize the voltage optimization control. The voltage optimization control model is established by taking the minimum power loss as an objective function. Using the particle swarm optimization algorithm solves the model. The simulation results of the improved IEEE-33 bus system verify the effectiveness of the proposed method.

Coordinated control strategy of reactive power for large‐scale wind power transmission by LCC‐HVDC links

Coordinated control strategy of reactive power for large‐scale wind power transmission by LCC‐HVDC links

Reactive power control strategy of DFIG‐based wind farm to mitigate SSO

Wind farms connected with series compensated transmission network suffer from the risk of sub-synchronous oscillation (SSO). Moreover, the strong active and reactive power control ability of doubly fed induction generator (DFIG) exerts non-negligible influence on SSO. In this study, the impacts of adjustable parameters on SSO are studied based on eigenvalue analysis. The results indicate that increasing inductive reactive power produced by DFIG can provide positive damping to the system; furthermore, the universality of this rule is verified in the whole operating region of DFIG. According to the rule, a reactive power control strategy of DFIG is proposed for SSO suppression. The reactive power is suggested to increase gradually once the SSO is detected and stop until SSO is suppressed or the reactive power limit is reached. The proposed control method is a low-cost method, which only need to adjust the reactive power reference value of DFIG to damp SSO. Finally, the time-domain simulation results illustrate the validity and adaptability of the proposed SSO reactive power control strategy.

Research of reactive power control strategy to reduce the reactive power exchange of AC and DC power grid

In order to solve the difficulties of steady-state voltage regulating for ac power grid brought about by surplus reactive power during the small power running for dc project, the general practice is by adding station low reactor or static reactive power compensator to absorb surplus reactive power, or by adjusting the operation mode of ac/dc system to change converter consumption of reactive power, realising the balance of reactive power exchange. Via the detailed analysis and research of auxiliary converter reactive power control strategy (converter and low load reactive power optimisation function) based on different technical route of dc control system, deducing the relation between the converter reactive power consumption and α / γ , current, which show that increasing minimum γ can absorb surplus reactive power, effectively solving ac grid voltage rise during small power running. At the same time, engineering widely used for a low reactor or static reactive power compensator adopted in weak ac system with reactive power balance control strategy are discussed, putting forward the feasible suggestions of reactive power optimisation control strategy to reduce ac/dc power grid reactive power exchange. Experiments proved that the proposed method can effectively solve the surplus reactive power problem during dc low power operation, simplify the converter station reactive power configuration, can be applied in dc project, and has good applicability.

Wind farm HVRT capability improvement based on coordinated reactive power control strategy

Voltage swells after grid fault clearance is one of the major causes of large-scale wind power tripping accidents in China. In this study, reasons that led to overvoltage phenomena were researched, and factors that influence the grid overvoltage during grid fault were quantitatively analysed through simulation. A coordinated reactive power control strategy combined the control of wind turbines with reactive power compensation devices was proposed to prevent the occurrence of high voltage and improve the high-voltage ride through capability of wind power plant. Finally, the effectiveness of the proposed control strategy was indicated by the simulation results in DIgSILENT/PowerFactory.

Combined active and reactive power control strategy to improve power system frequency stability with DFIGs

Combined active and reactive power control strategy to improve power system frequency stability with DFIGs

A New Adaptive Control Strategy of active and reactive power sharing in Islanded Microgrids

This paper proposes a new control sharing method for parallel three-phase inverters in an islanded microgrid. The proposed technique uses adaptive PIDs combined with the communication among the parallel inverters to accurately share active power and reactive power among the inverters via adjusting the desired voltage if there is a distinct difference between line impedance and the load change in the microgrid. Moreover, the paper also presents the response ability of the inverters to maintain the error within the allowed limits as the transmission line is interrupted. The proposed technique has been verified in a microgrid with three parallel distributed generation-inverter units using Matlab/Simulink. In the simulation, as the droop control using the communication information among the inverters, the sharing errors for active power and reactive power are around 0.2% and 0.6%, respectively. As the connection between the microgrid and transmission line is interrupted, the sharing errors for active power and reactive power increase to 0.4% and 2%, respectively. The simulation results have indicated that the proposed technique is superior to the traditional droop control in terms of the accuracy and stability. Therefore, the new proposed technique can be a favor alternative model for active power and reactive power sharing control of parallel inverters in an islanded microgrid.

Novel Reactive Power Control Strategy Based on Constant DC-Capacitor Voltage Control for Reducing the Capacity of Smart Charger for Electric Vehicles on Single-Phase Three-Wire Distribution Feeders

This paper proposes a novel reactive power control strategy to reduce the capacity of the previously proposed smart charger for electric vehicles (EVs) on single-phase three-wire distribution feeders. The proposed reactive power control strategy is based on the constant dc-capacitor voltage control of the grid-connected pulsewidth-modulated rectifier. Any calculation blocks of the load-side active current are not needed. Thus, we offer the simplest reactive power control strategy. The basic principle of the proposed reactive power control strategy is discussed in detail, and then confirmed by a digital computer simulation using the PSIM software. A prototype experimental model is constructed and tested. Experimental result demonstrates that balanced source currents with a power factor of 0.9, which is an acceptable value in Japanese home appliances, are obtained on the secondary side of the pole-mounted distribution transformer during the battery charging operation in the EVs, reducing the capacity of the smart charger by 36% as compared with that of the smart charger with the previously proposed control strategy.

Reactive power control strategy for a wind farm with DFIG

Reactive power control strategy for a wind farm with a doubly fed induction generator (DFIG) is investigated. Under stator flux-oriented (SFO) vector control, the real power of the DFIG is controlled by the q-axis rotor current Iqr and the reactive power is mainly affected by the d-axis rotor current Idr. To examine the effect of Idr on stator reactive power Qs and rotor reactive power Qr, the DFIG is operated under five different operating modes, i.e, the maximum Qs absorption mode, the rotor unity power factor mode, the minimum DFIG loss mode, the stator unity power factor mode, and the maximum Qs generation mode. In pervious works, stator resistance Rs was usually neglected in deriving the d-axis rotor currents Idr for different DFIG reactive power operating modes. In the present work, an iterative algorithm is presented to compute the d-axis rotor currents Idr for the five reactive power control modes. To demonstrate the effectiveness of the proposed iterative algorithm, the rotor current, rotor voltage, stator current, real power and reactive power of a 2.5 MW DFIG operated under the five different operating modes are computed.

An Optimal Reactive Power Control Strategy for UHVAC/DC Hybrid System in East China Grid

During the holidays, the power load of the East China Grid dropped significantly, resulting in excess capacitive reactive power, which makes the grid voltage increase significantly. For ultra-high voltage alternating current/direct current (UHVAC/DC) hybrid system, restricted by the minimum input number of the alternating current filter (ACF) in the dc converter stations, multiple dc stations that operate at a low-power transmission mode will inject a large capacitive reactive power to 500 kV ac grid, leading to a dramatic voltage rise. In addition, the 1000 kV UHVAC system may also transport a large capacitive reactive power to 500 kV ac grid during the holidays. In this paper, an optimal reactive power control strategy based on multi-dc power modulation has been proposed. It includes two steps. First, the minimized reactive power interchange control strategy which minimizes the reactive power interchange (RPI) between the UHVAC grid and the 500 kV ac grid has been executed. After that, if the grid voltage still exceeds the upper limit, the multi-dc power modulation is used to minimize the RPI between the dc converter stations and the 500 kV ac grid. The example analysis shows that the proposed optimal reactive power control strategy can effectively optimize the reactive power distribution in the UHVAC/DC hybrid system, thus reasonably controling the grid voltage.

Comparative Study of PI, RST, Sliding Mode and Fuzzy Supervisory Controllers for DFIG based Wind Energy Conversion System

This work aims to present a comparative study of four controllers for Double Fed Induction Generator (DFIG) based Wind Energy Conversion System (WECS). The DFIG is directly connected to the grid and driven by the rotor through an AC/DC/AC converter. A model was developed for each component (Turbine, DFIG and Rectifier-Filter-Inverter) of the wind system. The PWM control method is applied to the inverter to drive the DFIG from the rotor circuit. To ensure high performance and better enforcement of DFIG, a direct vector control strategy of active and reactive power of the stator has been developed. The synthesis of conventional PI controller and advanced RST, Sliding Mode (SM) and Fuzzy Supervisory (FS) controllers is performed. The system’s performance has been tested and compared according to reference tracking, robustness, and disturbance rejection. A set of simulation studies are carried-out on a WECS model to prove the effectiveness of the proposed controllers design.

Flicker Mitigation via Dynamic Volt/VAR Control of Power-Electronic Interfaced WTGs

The emerging trend for electric utilities to install wind-based generation on a large scale into distribution networks, employing a doubly fed induction generator or full-converter turbine technologies is raising issues for distribution planning engineers with regard to flicker among other power-quality (PQ) issues. This paper investigates dynamic volt/var control, a viable flicker mitigation technique which exploits the reactive power capabilities of modern power-electronic interfaced wind turbine generators. Dynamic volt/var control essentially mitigates flicker by solely dispatching reactive power based on voltage fluctuations within the flicker frequency band. This technique can be incorporated as part of a wind farm multiobjective reactive power-control scheme, which facilitates regulating: 1) the voltage level at the point of common coupling (PCC), employing slow-acting voltage control or power factor control and 2) the flicker level at the PCC with fast-acting dynamic volt/var control. This two-level approach can reduce the wind farm flicker level up to 80% and maintains the voltage level at the PCC well within acceptable bands. The results presented in this paper focus on a typical 25-kV medium-voltage distribution feeder integrating wind-based generation on a large scale. The performance of the two-level controller was tested for varying levels of penetration and wind speed conditions. The applicable standards and utility grid-code requirements are considered throughout.

Offline tuning of dynamic settings considering an online central controller in a wind energy harvesting network

This study aims to define an online reactive power control scheme for a wind energy harvesting network such that it regulates the voltage at the transmission level in a manner comparable to a conventional synchronous plant and hence could be integrated in an existing transmission network hierarchical voltage control scheme. For that purpose, all decentralised elements within the network (wind farms and on load tap changing (OLTC) transformers) should be coordinated. In that sense, a central controller needs to be implemented. Unwanted controller interactions may then arise as the various decentralised controllers dynamically respond to the changing set-points received from a central controller. To mitigate these interactions, this study proposes a novel offline optimisation approach for tuning the dynamic settings (i.e. settings that affect the central controller temporal evolution such as time constant, time delays or dead bands). These settings ensure that the centrally determined set-points can actually be achieved in practice, and unlocking such performance is the principle research contribution of the present study.