Reactive Power Control Strategy Research Articles

배전계통에서 전압상승 완화를 위한 재생 발전기의 새로운 무효전력제어 전략

배전계통에서 전압상승 완화를 위한 재생 발전기의 새로운 무효전력제어 전략

Distributed control scheme for accurate reactive power sharing with enhanced voltage quality for islanded microgrids

This study presents a distributed control scheme for islanded microgrids to eliminate power sharing errors as well as frequency and voltage deviations based on distributed cooperative control. A consensus algorithm is developed through adaptive regulation of the virtual impedances for accurate power sharing regardless of load variations. The proposed power sharing controller is quite easy to implement with only one simple integral gain. It is achieved without using information about the feeder impedances, load powers or detailed microgrid structure. To remove the deviations in the frequency and voltage magnitude, conventional droop equations are enhanced by adding compensating signals, which are simply determined from the neighbors’ droop information without a PI controller. Therefore, the control complexity is significantly reduced. The control performance is theoretically analyzed using a small-signal state-space model to evaluate the system dynamics and stability. A digital simulation is done using PSIM, and experiments are carried out with a scaled-down microgrid prototype to verify the proposed scheme.

ADMM-based distributed optimal reactive power control for loss minimization of DFIG-based wind farms

In this paper, a distributed optimal reactive power control (DORPC) scheme is proposed for minimizing the total losses of doubly fed induction generator (DFIG)-based wind farms (WFs), including the losses of generators, converters, filters, and networks. The DORPC minimizes total WF losses by optimally coordinating reactive power outputs of the DFIG stator and the grid-side converter. The optimal control problem is solved in a distributed manner by using the consensus alternating direction method of multipliers (ADMM). With the consensus ADMM, the total WF loss optimization problem is transformed into a distributed optimal power flow problem considered with DFIGs’ optimal operation. The optimization problem with local constraints considers the reactive power limit of DFIG-based wind turbines (WTs) and the voltage limits at all WT terminal buses inside the WF. In the DORPC, the optimal control problem is solved by the collector bus station controller and WT controllers in parallel, only with the information exchange between immediate neighbors. It eliminates the need of a central controller and centralized communication, implying better robustness and plug-and-play capability. A WF with 20 DFIG-based WTs was used to validate the proposed DORPC scheme.

Coordinated Optimal Control of Multiple Reactive Power Devices at Different Voltage Levels in UHVDC Near Zone

Affected by different steady-state reactive power output ratios among generators, capacitors and other reactive devices in the end-to-end power grid, voltage collapse may occur due to the failure of the receiving-end AC system, and the problem of voltage stabilization in multi-DC feed systems is particularly common. For suppressing voltage collapse, sufficient dynamic reactive power support is an effective measure, and there are some differences in the dynamic support effect of different reactive power sources. The dynamic reactive power response of the generator and its reactive power margin are two important factors affecting the coordination and optimization of the reactive power of the generator. The comprehensive evaluation index is adopted to optimize the sequencing of the reactive power output of the generator near the DC drop point. A coordinated control method of dynamic and static reactive power for DC near-point systems at different voltage levels is proposed. By controlling the steady-state reactive power output ratio between multiple reactive devices, the node voltage is maintained near the target value, and reactive power control schemes at different voltage levels can be given to meet load changes. Finally, taking the actual situation of Central China Power Grid as an example, the results of different reactive voltage control strategies are compared and analyzed, which proves that the coordinated control strategy of multiple reactive power devices can significantly improve the stability of the receiving grid voltage.

The Reactive Power Support Strategy based on Dual-loop Control for Three-phase Grid-connected Inverter

Renewable energy sources (RESs) generally connected with electric power system via power electronic interface. This paper presents a reactive power and voltage (Q/V) control strategy of three-phase photovoltaic (PV) system to offering reactive power based on the typical dual-loop control topology. It is worth mentioning that control strategy can support reactive power when a low voltage fault occurs in AC bus without additional compensation device. With the help of the decoupling control, the PV array can generate active power as much as possible in variable external solar radiation conditions. The voltage of PV arrays is adopted as the objective, which on account of the easy availability and controllability of voltage, to control output active power. Besides, accurately modeling process from a PV cell to PV array is described in the beginning to acquire the P-V and V-I characteristics of PV arrays, which promote the designment of Q/V control.

An adaptive compensation droop control strategy for reactive power sharing in islanded microgrid

In a parallel distributed generation system, the conventional droop control strategy makes it difficult for the inverter to output reactive power precisely due to the line impedance uncertainty and load fluctuation, which leads to a voltage deviation of the microgrid system. In order to precisely distribute reactive power, this paper advances an adaptive compensation strategy based on fuzzy control. The fuzzy controller detects the load power of AC bus terminal and output adaptive power coefficient to adjust amplitude voltage to generate compensation voltage. The compensation voltage is fed back to the d-axis voltage of the power loop for reactive power redistribution. Because the fuzzy control membership function value is difficult to determine accurately, this paper puts forward a method which combines neural network with fuzzy control to determine membership function values and fuzzy rules. Simulations and experimental results show that the adaptive compensation control strategy can share power accurately with good dynamic characteristics and robustness.

An integrated reactive power control strategy for improving low voltage ride-through capability

Among all renewable energies, wind power is rapidly growing, whereby it has the most participation to supply power. Doubly fed induction generator (DFIG) is the most popular wind turbine, as it can play a very significant role to enhance low voltage ride through (LVRT) capability. Ancillary services such as voltage control and reactive power capability are the main topics in wind power control systems that should be handled profoundly and carefully. The lack of reactive power during fault period can result in instability in generators and/or disconnection of the wind turbine from the power system. The main aims of this study are to illustrate the most effective approaches subject to improve the efficiency, stability, and reliability of wind power plant associated with LVRT capability enhancement. This effectiveness and efficiency are demonstrated by, firstly, comparison between all types of wind turbines, focusing on the ancillary services, after the existing advanced control strategies. According to the literature, there is a consensus that modifying converter-based control topology is the most effective approach to enhance LVRT capability in DFIG-based wind turbine (WT). Therefore, an advanced integrated control strategy is designed by considering the effect of the rotor side converter (RSC) and the grid side converter (GSC). A model of the wind power plant is presented based on the control objectives. MATLAB/Simulink is also used to illustrate the effectiveness of the designed algorithm.

A Novel VSG-Based Accurate Voltage Control and Reactive Power Sharing Method for Islanded Microgrids

Islanded microgrids (IMGs) are more likely to be perturbed by renewable generation and load demand fluctuation, thus leading to system instability. The virtual synchronous generator (VSG) control has become a promising method in the microgrids stability control area for its inertia-support capability. However, the improper power sharing and inaccurate voltage control problems of the distributed generations (DGs) in microgrids still has not been solved with a unified method. This paper proposes a novel VSG equivalent control method named Imitation Excitation Control (IEC). In this method, a multi-objective control strategy for voltage and reactive power in a low voltage grid that considers a non-negligible resistance to reactance ratio (R/X) is proposed. With the IEC method, the voltage drop across feeders is compensated, thus the terminal voltage of each inverter will be regulated, which will effectively stabilize the PCC (point of common coupling) voltage and inhibit the circular current. Meanwhile, this method can realize accurate reactive power tracking the reference value, making it accessible for reactive power scheduling. What is more, the reasonability of the IEC model, namely the equivalent mechanical characteristic and transient process inertia support between VSGs and conventional synchronous generators (SG), is illustrated in this paper. Moreover, steady-state stability is proved by the small-signal modeling method, and the energy required by inertia support is given. Finally, the simulation result validates the effectiveness of the proposed method, and it is also demonstrated that the proposed method outperforms the conventional droop control method.

The Adaptive Sliding Mode Reactive Power Control Strategy for Wind–Diesel Power System Based on Sliding Mode Observer

Wind power may become the key development area of the renewable energy industry planning. Due to the fluctuated wind output power, the system parameter uncertainties and load disturbances, these may lead to the unstable operation of renewable energy power system for the larger voltage deviation. Therefore, the novel control strategy is proposed for the load side converter (LSC) of double-fed induction generator (DFIG) to improve the stability of power system, which is designed by taking advantage of adaptive sliding mode (SM) method and sliding mode observer (SMO). Furthermore, the static synchronous compensator (STATCOM) is also controlled through the designed adaptive SM reactive power controller, which can supply the reactive power compensation for the interconnected wind-diesel power system. Moreover, the LSC state variables need to be estimated by the SMO, which is constructed according to the established mathematical state model. Based on the estimated state variables, the designed reactive power controller is established for LSC by adaptive SM algorithm and fuzzy algorithm. The reactive power controller includes two parts. One is the designed SM controller by taking advantage of the estimated state variables and state control model. The other is micro-regulated part and the control signal is calculated by using fuzzy algorithm according to the system voltage deviation online. Experiment studies are performed by using RTDS to verify the effectiveness of the proposed strategy and assure the stability of wind-diesel power system under different operation conditions.

Long-term voltage stability with large-scale solar-photovoltaic (PV) generation

The dynamic characteristics of power grids have substantially evolved over the last two decades due to the large-scale integration of power-electronic converter (PEC)-interfaced renewable energy sources (RESs). Therefore, the impact of PEC-interfaced RESs on power system stability must be thoroughly examined. This paper comprehensively investigates the long-term voltage stability (LTVS) phenomenon with large-scale solar-photovoltaic (PV) generation. The reactive power characteristics and control schemes of the synchronous generator (SG) and solar-PV system are carefully analysed, as these characteristics significantly affect the voltage stability. First, the LTVS phenomenon is analysed using a simple test system. Time-domain simulations and dynamic reactive power-voltage (QV) curves at critical time-domain snapshots have been used to analyse the trajectory of the system operating point. Moreover, important parameters of the solar-PV system, such as the inverter rating, current limiting strategies, and reactive power gain are also thoroughly examined to characterize their impact on LTVS. The effect of variation in solar-irradiance and ambient temperature on LTVS is also studied in the paper. Finally, the effect of the solar-PV generation on LTVS is investigated using the Nordic test system. This study has shown that solar-PV systems with improved controllers could provide enhanced dynamic reactive power response, hence improve the LTVS. However, large-scale solar-PV systems have both beneficial and adverse impact on stressed power systems, and the nature of the impact depends on the relative loading level of the replaced over excitation limiter (OEL) activated SGs.

Parallel Control of Converters with Energy Storage Equipment in a Microgrid

The converter in a microgrid uses the active power and reactive power (PQ) control strategy when connected to the grid. In the case of failure of large power grid, the converters are required to be connected in parallel under the condition of island to provide power to the load. In this paper, a new control method for the parallel operation of converters based on V/F control is proposed. The V/F control is used to ensure the output voltages have the same amplitude and frequency, then the converters will only produce circulating current caused by phase angle inconsistency. The phase angle self-synchronization strategy is proposed to make sure the phase angle of output voltage of all converters in the system are consistent. First, a large inductor is added to the end of the converter to ignore the line reactance, through this, the measured voltage at the terminal of the converter roughly equals to the voltage of the load, thus, every converter has the same reference of phase angle. Using the proposed phase angle self-synchronization strategy allows the output voltage of every converter to have the same phase angle, so that there is no circulating current between converters, and the power is evenly distributed among the converters. The simulation verification was carried out on the Power Simulation (PSIM) simulation platform, and the experimental verification was implemented on the hardware experimental platform. Both results demonstrate the effectiveness of the proposed strategy. This method is highly reliable and easy to implement, and the circulating current can be reduced effectively.

A Reactive Power Control Scheme for DER-Caused Voltage Rise Mitigation in Secondary Systems

Distributed energy resources (DERs) are a new class of disruptive technologies with rapidly increasing adoption levels due to favorable government policies and subsidies. However, these emerging technologies come with technical challenges for utilities and system operators; for instance, voltage rise issues in residential secondary distribution systems with high penetration of DERs. One of the low-cost mitigation options to alleviate the overvoltage problem is reactive power absorption. Nevertheless, secondary feeders exhibit a high R/X ratio more noticeably than primary distribution systems, requiring large amount of reactive power for voltage regulation that cannot be entirely provided by the DERs alone. It is also necessary to minimize the reactive power drawn from the primary distribution system to prevent any stress on the network and reduce extra system losses, while at the same time ensuring the voltage within admissible limits. To address these critical issues, this paper proposes a distributed reactive power management, where additional source is provided from inexpensive devices such as switchable shunt reactors. The proposed overvoltage mitigation is accompanied by detailed analytical investigation that estimates the minimum amount of required reactive power to manage the voltage with user specified voltage rise tolerances. Case studies have also been conducted on real residential distribution systems subjected to severe voltage rise issues provoked by large adoption of DERs.

Reactive power control strategies for renewable energy systems

A comprehensive review of reactive power control strategies for dealing with large numbers of photovoltaic units connected to the electrical grid is presented.

A comprehensive review of reactive power control strategies for three phase grid connected photovoltaic systems with low voltage ride through capability

Photovoltaic (PV) sources have recently become one of the most mature technologies. With the increasing penetration level and the integration of a PV system into the grid, the stability and reliability of the power networks have been serious concerns. Thus, grid codes are being released by grid operators for low voltage networks under grid faults. Consequently, the Low Voltage Ride Through (LVRT) capability of the grid connected PV system became the most important issue related to grid codes, i.e., more reactive power is injected into the grid during voltage disturbances. In this paper, a comprehensive review of reactive power control strategies for the three-phase PV system has been analyzed to support the grid during voltage sags by providing LVRT capability. The control techniques have been classified into three main categories: Fixed power factor, constant active power control, and constant reactive power control. The results illustrate that the stability of the system is improved when two control techniques are simultaneously implemented. This paper concludes that further research must be carried on LVRT control techniques for the reactive power injection during unbalanced voltage sags.

Broadcast Gossip Algorithms for Distributed Peer-to-Peer Control in AC Microgrids

This paper focuses on a fully distributed peer-to-peer control scheme for voltage regulation and reactive power sharing of multiple inverter-based distributed energy resources (DERs) in ac microgrids. The proposed peer-to-peer control strategy is fully distributed enabled through broadcast gossip communication, where each DER unit only requires local voltage and current measurement from its own and some (but not all) nearby neighbors for the voltage and reactive power sharing control. Employing the broadcast gossip communication protocol with attractive scalability and reliability properties, the control inputs can be updated to restore the voltage magnitudes at the point of common coupling to a desired value ensuring an accurate reactive power sharing for each DER. Since the proposed distributed controllers are implemented on local DERs, the central controller and hierarchy are no longer required. Accordingly, the microgrid system stability is preserved in the peer-to-peer requirements of line switches, which in turn, enables a plug-and-play operation of DERs and their robustness against microgrid topology change scenarios. Simulation studies in a modified IEEE 34-bus test network demonstrate the effectiveness and applicability of the proposed control strategy.

Bi-level decentralized active and reactive power control for large-scale wind farm cluster

This paper proposes a bi-level decentralized active and reactive power control (DARPC) for the large-scale wind farm cluster (WFC) composed of several wind farms. The WFC tracks the active power reference from the transmission system operator (TSO) while controlling the bus voltage of the point of connection (POC), and maintaining the wind turbine (WT) terminal voltages stable in each wind farm. In the upper level, a distributed active and reactive power control scheme based on the consensus protocol is designed for the WFC, which can achieve fair active and reactive power sharing among multiple wind farms, and generates active and reactive power references for each wind farm. In the lower level, a centralized control scheme based on Model Predictive Control (MPC) is proposed, which can effectively regulates active and reactive power outputs of all WTs within the wind farm. The proposed centralized control scheme can maintain WTs terminal voltage close to the rated voltage while tracking the power reference from the upper level control. The DARPC can effectively reduce the computation burden of the WFC controller by distributing the computation and monitoring tasks to several wind farm controllers. Moreover, the communication cost is reduced. A WFC with 8 wind farms and totally 128 WTs was used to validate the proposed DARPC scheme.

Independent active–reactive power control scheme for NSC-drive operated at constant frequency

ABSTRACTThis paper presents a new control scheme for the ac-dc-ac nine-switch converter (NSC) with induction motor operated at constant frequency. The NSC at constant frequency operates optimally and its application for induction motor drive with re-active power compensation is proposed. In industries for various applications, induction motors are operated at constant frequency but at different loading conditions. These motors are operated at lagging power factor. The power distribution company demands high power factor operation and it gives benefits to consumer on operating a system closer to unity. In this paper, a control scheme is proposed to operate NSC drive at unity as well as at leading power factor with sinusoidal input current. The independent active–reactive power control technique is developed. The input–output active power relation is developed such that with the change in load, output active power matches input active power without affecting dc-link voltage. This results in faster dynamic response as compared to conventional dc-link balancing control techniques. With the proposed control scheme, the NSC is operated at desired leading power factor to compensate reactive power at point of common coupling. The simulation and experimental results show the feasibility of the proposed control scheme.

A Flexible Active and Reactive Power Control Strategy of a LV Grid Connected PV System

The aim of this paper is to present a command approach of a typical double-stage grid-connected PV system functioning under normal conditions and Symmetrical Grid Voltage Dips (SGVD). In Normal Operation Mode (NOM), the command developed rises the relatively low solar voltage to a suitable level corresponds to the Maximal Power Point Tracking (MPPT) and regulates, respectively, the dc-link voltage and the output inverter currents. It permits to inject energy into the grid at unity power factor and a current with low harmonic distortion. Under SGVD, the control strategy approach should ensure a stable connection as long as possible and inject some reactive power to support the grid faults. This command calculates dynamically active and reactive powers that inverter can deliver according to the level of voltage dips and inverter rating currents. Those values are used as reference to control output inverter currents, the dc-link voltage and to deactivate the MPPT command. To validate and to prove the effectiveness of the control strategy, some experimental results are also presented.

Utilizing the Dead-Time Effect to Achieve Decentralized Reactive Power Sharing in Islanded AC Microgrids

In islanded ac microgrids, distributed energy storage systems (DESSs) are normally coupled to the grid through voltage source inverters (VSIs). To improve the operation efficiency and to avoid overloading, it is desirable that VSIs can share active and reactive powers in proportion to their respective power ratings. Although accurate active power sharing can be easily guaranteed by frequency droop control, it is difficult to achieve reactive power sharing as desired due to mismatched grid impedances and voltage sensor scaling errors. To overcome this challenge, a decentralized reactive power control scheme is proposed and used in conjunction with the conventional voltage droop control to enhance the reactive power sharing performance. Specifically, the proposed control scheme utilizes the dead-time effect to equalize the power factors of all VSIs. As a result, the reactive power sharing performance is improved in a fully decentralized manner. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed control scheme.

Distributed Voltage Control Based on ADMM for Large-Scale Wind Farm Cluster Connected to VSC-HVDC

A distributed voltage control (DVC) scheme based on the alternating direction method of multipliers (ADMM) is proposed for the large-scale wind farm cluster (WFC) connected to a voltage source converter-high voltage direct current system. The proposed scheme coordinates the reactive power output among several wind farms, and wind turbines (WTs) within each wind farm. This keeps the collector bus voltage of each wind farm and WT terminal voltages within the feasible range, and minimizes active power losses. First, a distributed reactive power control scheme based on the consensus ADMM is designed for the WFC, which can achieve fair reactive power sharing among wind farms with the optimal reactive power utilization ratio, to regulate the collector bus voltage close to the rated voltage. Second, a decentralized reactive power control scheme based on the standard ADMM is designed for each wind farm, which can effectively regulate reactive power outputs of all WTs within the wind farm. The ADMM algorithm is used to reduce the calculation burden of the wind farm controller. Moreover, the proposed decentralized control scheme coordinates the voltage regulation and economical operation. The DVC is able to effectively reduce the computation burden of the WFC and wind farm controllers, and the communication cost is reduced. A WFC with eight wind farms was used to validate the proposed DVC scheme.