Static Var Generator Research Articles

Analysis of Influence of Grid-Following and Grid-Forming Static Var Generators on High-Frequency Resonance in Doubly Fed Induction Generator-Based Wind Farms

In Doubly Fed Induction Generator (DFIG)-based wind farms with Static Var Generators (SVGs), high-frequency resonance will be more like to occur when an unloaded cable is put into operation, which will threaten the stable operation of the wind farm. To address this issue, the influence of power outer loops on the impedance of grid-connected inverters is considered. Based on harmonic linearization, theoretical models for the sequence impedances of DFIGs, Grid-following (GFL) SVGs, and Grid-forming (GFM) SVGs are established. The correctness of the three models is verified by impedance scanning using the frequency sweep method. Through a comparative analysis of these sequence impedances, it is found that unlike the GFM SVG (which exhibits inductive impedance), the GFL SVG exhibits capacitive impedance in the high-frequency band, which leads to negative damping characteristics in the high-frequency band for the wind farm system with the grid-following SVG; thereby, the risk of high-frequency resonance also increases accordingly. On the contrary, GFM control adopted by SVGs can effectively eliminate the negative damping region in the high-frequency band for wind farms to suppress high-frequency resonance. Meanwhile, for grid-forming SVGs, the parameter variations in power synchronous loops have no significant impact on the suppressing effect of high-frequency resonance for wind farms. Finally, an electromagnetic simulation model for a DFIG-based wind farm system with an SVG is established using the StarSim-HIL (hardware-in-the-loop) experiment platform, and the simulation results validate the correctness of the theoretical analysis.

Comprehensive compensation method for co‐phase power supply in electrified railways based on V/v transformer and electromagnetic single‐phase var compensator

AbstractTo solve the problem of negative‐sequence and reactive power in electrified railway, this paper proposes a comprehensive compensation method based on V/v transformer and electromagnetic single‐phase var compensator (ESVC) for co‐phase power supply. First, based on the principles of static var generator and single‐phase rotary phase shifting transformer, the topology and model of ESVC have been established, and the compensation mechanism has been analysed. Second, the topological structure and mathematical model of co‐phase power comprehensive compensation device (CPCD) are put forward based on the principles of V/v transformer and ESVC, and the operation mode of CPCD in multiple scenarios is designed. Then the strategy of CPCD with double closed loop control is analysed: corresponding compensation mode is selected according to negative‐sequence unbalance degree and expected power factor value. In this strategy, the compensated current output by ESVC is taken as the quantity of external loop control, and the rotation angle of ESVC is taken as the quantity of internal loop control to realize double‐closed loop control of CPCD. Finally, the comprehensive CPCD compensation model is built on the simulation platform and validated the accuracy of the mathematical model and the effectiveness of the control strategy.

Research on the coupling mechanism of transient overvoltage in large-scale new energy station grid connection

Abstract The large-scale new energy station transmission system is relatively weak, and long-distance transmission is prone to transient overvoltage. The hysteresis of the control loop of the Static Var Generator (SVG) can also contribute to the increase of transient overvoltage. Therefore, this article first establishes a photovoltaic power generation model, SVG model, and photovoltaic grid-connected system. Secondly, the transient overvoltage coupling mechanism of photovoltaic grid-connected systems under various faults such as commutation failure, DC blocking, and DC restart was analyzed through PSCAD simulation. At the same time, the changes in transient overvoltage before and after the installation of SVG were compared, and SVG’s reactive power dynamic behavior was analyzed. Finally, the mechanism of SVG assisting in increasing transient overvoltage was summarized.

Coordinated voltage control for large-scale wind farms with ESS and SVG based on MPC considering wake effect

The wake effect reduces the wind speed at downstream wind turbines (WTs), making it necessary for the central controller to collect wind power generation data from each WT. However, wind farms (WFs) face a more complex problem in maintaining the voltage stability at the WT terminal while following the transmission system operator (TSO) instructions due to the information collection as well as the possible data loss during transmission. Therefore, this study proposes a coordinated control method for WTs and multiple power sources based on model predictive control under wake disturbance conditions, aiming to reduce the average voltage deviation in WT terminals and go close to the rated voltage and ensure effective compliance with TSO commands in large-scale WFs. Accordingly, the Jensen wake model was utilized to accurately calculate the available active and reactive power limits for each WT. Energy storage systems and static Var generators were modeled to coordinate and maintain the voltage in all WT terminals within the feasible range, providing peak shaving and valley filling support to reduce wind energy waste and shortfall, thereby enhancing the economic and operational reliability of WF. Further, the effectiveness of the proposed method was validated in MATLAB/Simulink.

Advanced Static Var Generator in the Reactive Power Automatic Compensation System of an Underground Mine with a Thyristor Hoisting Machine

In industrial plants with dynamically varying load profiles, the problem of increased charges for over-contracted reactive power consumption is very common. To eliminate these charges, reactive power compensation systems are used. This article presents a case study of an automatic reactive power compensation system in an underground mine where a thyristor hoisting machine is used. To reduce the higher current harmonics of the hoisting machine and compensate for the reactive power of the PCC (Point of Common Coupling), the main reactive power automatic compensation system and an ASVG (Advanced Static Var Generator) with a nonstandard control loop were used. The article considers aspects of the operation of the ASVG, locally with the hoisting machine, and also as a master-controlled device that is part of the reactive power automatic compensation system. This part of the article is based on measurements taken in a mine. The reactive power automatic compensation system considered in this article has been successfully implemented, improving PQ (Power Quality) and eliminating additional overcompensation charges.

Adaptive transient overvoltage control strategy for wind farm based on data‐driven

AbstractTo achieve the transient overvoltage suppression of doubly fed induction generator (DFIG) terminals and the point of common coupling (PCC) in the wind farm (WF), an adaptive overvoltage control strategy based on data‐driven approach is proposed. Firstly, the PCC reactive power demand and DFIG overvoltage threshold are derived, respectively. Secondly, three regression models are developed through surface fitting to determine three variables: the maximum overvoltage of each DFIG, the PCC maximum overvoltage, and the reactive power demand of each DFIG. Subsequently, the K‐means clustering method is adopted to group all DFIGs, and then the reactive power requirement of the PCC is proportionally distributed to each group of DFIGs. Lastly, a coordinated control strategy of DFIGs and static var generator (SVG) is designed based on the values of three regression models, which can adaptively adjust the reactive power absorbed by each DFIG and SVG. In particular, the de‐load control is adopted to enlarge the maximum reactive power capacity of DFIG. The simulation results show that the proposed control strategy can timely and effectively restore the transient overvoltage of the wind farm to the normal range during the gird failure, ensuring the stable operation of the wind farm.

SVG Loss Estimation Method Based on Extreme Gradient Boosting

Static Var Generator (SVG) consumes large amounts of electrical energy during its operation, generating high equipment operating costs. Therefore, it is necessary to quantify the operating SVG loss. However, the existing methods of loss estimation generally have notable problems, such as large deviations in loss estimation results. In this paper, we propose a method for calculating SVG loss and establish an SVG loss estimation model based on the extreme gradient boosting algorithm under different operating conditions. First, some data obtained by simulation experiments are input into the algorithm model for training, and the hyperparameters of the model are adjusted by the Bayesian optimization algorithm. Then, a new test set is constructed by superimposing the measurement errors with different signal-to-noise ratio noises on the test set data. Finally, the test set is fed into the trained model for testing to validate the effect of the present model. The experimental results show that the model can quickly and accurately achieve loss estimation of SVG devices with good engineering practicality.

Transient power angle stability analysis and improvement strategy for parallel system of virtual synchronous generator and static var generator

SummaryVirtual synchronous generators (VSGs) can actively support the frequency and voltage of the grid and are widely used in new energy power plants. Static var generators (SVGs) can improve the reactive power–voltage drop characteristics of the VSG and increase its ability to cope with complex operating conditions. However, the interaction between the VSG and the SVG can affect the output characteristics of the VSG and thus its transient power angle stability. Therefore, this paper proposes a method for analyzing the transient power angle stability of a VSG–SVG parallel system and its improvement strategy. Firstly, based on the equivalent circuit of the parallel system, the interaction‐coupled power model and the transient power angle characteristic model are derived. On this basis, the influence law of various parameters on the transient stability of VSG is analyzed based on the phase plane method. Secondly, a strategy is proposed to improve the transient power angle stability of the VSG–SVG parallel system based on the adaptive regulation of active power, and the feasibility of the strategy is verified by comparing the phase plane diagrams before and after the improved control. Finally, the effectiveness of the proposed model and strategy is verified by Matlab/Simulink simulation.

Расчет тепловых потерь в комбинированной фильтрокомпенсирующей установке

Purpose: To calculate heat losses in the combined filter-compensating unit and to assess the possibility of using air cooling. Methods: Mathematical and computer modeling are used to calculate heat losses of combined filter-compensating unit with regard to the main parameters of the converter. Results: The paper presents schemes and the main differences of reactive power compensation installations, used nowadays on the electrified sections of railroads. The calculation of thermal losses of combined filter-compensating installation with regard to the main parameters of the Static Var Generator (SVG) converter on the basis of IGBT transistors has been performed. In the software package ANSIS 18.2 a simulation model of the converter cabinet has been developed and the thermal calculation of the cooler has been performed. Practical significance: Results of calculation of thermal losses confirmed the possibility of using air cooling in the presented combined filter-compensating unit with capacity up to 15 Mvar.

A two-stage coordinated line loss reduction model based on elephant herding optimization and second-order cone programming

Aiming at the problem of high line loss caused by the access station of distributed generation (DG), a two-stage coordinated line loss reduction model based on elephant herding optimization (EHO) and second-order cone programming (SOCP) was put forward by using static var generators (SVG) and energy storage system (ESS) to reduce the line loss from reactive and active power aspects, respectively. Firstly, based on the elephant herding optimization, the location model of distributed generation, SVG and ESS was constructed and determined. On this basis, a cooperative line loss reduction model is proposed, which takes into account multiple line loss reduction methods, such as SVG reactive power compensation device and energy storage. By properly dealing with power flow constraints of distributed generation, the influence of operation control mode of distributed generation on optimal distribution and line loss reduction is precisely explained. The constraint is transformed into a second-order cone programming problem by using second-order cone relaxation. In this way, the coordinated reduction of line loss is realized, and the goal of line loss minimization and voltage stability maximization is achieved. Finally, a practical station is taken as an example to verify the correctness of the proposed model and algorithm and the effectiveness of the proposed method.

Research on Harmonic Mitigation Strategy of Static VAR Generator Based on Frequency Adaptation

Offshore wind farms are integrated into the power grid via AC submarine cables, which may cause severe harmonic problems and reduce the power quality of the grid. To address the problem of excessive harmonics in offshore wind power plants (WPPs), this paper proposes a method applying the static var generator (SVG) with frequency adaptive harmonic mitigation function to the offshore WPPs. Firstly, the SVG model and the mechanism of harmonic mitigation are investigated. Secondly, the proportional and resonant control strategy is proposed according to the internal model theorem and applied to the voltage control loop of SVG. The harmonic suppression performance of SVG is studied through the Bode diagram. Thirdly, to adapt to the uncertainty of harmonic frequencies, the recursive least-squares method is introduced to realize the real-time tracking of harmonic frequencies. And the harmonic compensation bandwidth of the SVG is broadened by adjusting the parameters of the controller. Finally, the offshore WPPs in Jiangsu are built based on the Matlab/Simulink platform to verify the effectiveness of the mitigation strategy. Compared with the LC filter, the improved method can effectively mitigate (inter-) harmonics and has strong robustness and adaptability.

Model Predictive Control Based Voltage Regulation Strategy Using Wind Farm as Black-Start Source

A coordinated voltage regulation method based on model predictive control (MPC) is proposed in this paper for utilizing wind farms (WF) as black-start (BS) source to start up a thermal generating unit. The reactive power regulation devices with different dynamic response characteristics including wind turbine generators (WTGs), energy storage system (ESS), and static var generator (SVG) are coordinated by the proposed MPC to handle disturbances caused by ancillary machine start during the BS process. The reactive power sharing between WTGs is optimized to maximize the dynamic reactive power reserve. The capabilities of ESS and SVG in providing sufficient dynamic reactive power against disturbances are also fully exploited, which helps accelerate voltage recovery for low voltage ride through to avoid the tripping incidents of WTGs. The impact of active power on bus voltage variation due to low <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> ratio is also considered. The reactive power and active power of WTGs and ESS are coordinately controlled for handling voltage disturbance without harming frequency control. A WF with 33 WTGs rated 1.5 MW each is used in case studies to demonstrate the enhanced disturbance handling capability of the proposed voltage regulation strategy during BS progress.

Research on High Harmonic Amplification of 10kV Series Static Var Generator

With the continuous development of power electronic devices and science and technology, cables and rectifiers, inverters, DC transmission equipment, and other nonlinear loads have been widely used in power systems. However, this causes harmonic pollution and large capacitive reactive power, which can affect the safe and stable operation of the power system. Based on the analysis of the reasons for the use of static var generator in the power grid and the existing conditions of even harmonic, the impedance analysis method was used to analyze the abnormal noise event of a transformer in a substation. Firstly, the equivalent impedance model diagram is drawn by the equivalent circuit method, and the equivalent impedance value of the system is calculated. A number of harmonic amplifications and the positions of the resonance point are calculated by MATLAB software. Secondly, the correctness of the above theoretical analysis is verified by field test and simulation model built in ETAP software. Finally, because of the abnormal noise of the transformer in a substation caused by harmonic amplification, three harmonic suppression measures are given.

Enhancing Voltage Compliance in Distribution Network Under Cloud and Edge Computing Framework

While high penetration of DER significantly helps facilitate the decarbonization of the electric power grid, it also brings unexpected operational challenges, among which voltage compliance has been a significant concern. To address this issue, the efficient load profile forecast, the operational framework and related strategies are critical challenges that need to be addressed urgently. Hence, this paper presents the cloud-edge computing-based framework to effectively operate a coupled medium-voltage (MV) and low-voltage (LV) distribution network. The high computational efficiency in cloud computing and low data latency in edge computing are presented and explored to coordinate the day-ahead and intraday operations ranging different framework layers. Under the framework, a customer-level forecasting algorithm is employed to predict both day-ahead and real-time load profiles. Next, an optimization model based on unbalanced-three phase optimal power flow is proposed and solved by an efficient and accurate linearization-based approach that considers the controllability of on-line tap changers, distributed static var generator and the PV inverters. Simulations based on an extensive mocked MV-LV distribution network and show the proposed forecasting method is adopted in the real-time operations in terms of high accuracy, and demonstrate the efficiency of the proposed method in enhancing the voltage compliance in the network.

Grid-connected inductor design of static var generator for photovoltaic system

With the access of high-proportion photovoltaic power stations and the development of high permeability power electronic power systems, the problem of distributed photovoltaic access voltage over-limit and power factor is becoming more and more prominent, and the static var generator (SVG) is an important equipment to solve this kind of problem. As a key component of the static var generator, the connecting inductor has the functions of storing energy, bidirectional feeding, adjusting the phase of the output current, and suppressing the high-order harmonic current. It not only affects the dynamic and static response of the device but also restricts the output power, power factor, and DC side voltage of the SVG. At present, the parameter selection of SVG connection inductance is from the perspective of harmonic characteristics and economy, and there is no actual theoretical calculation basis. From the perspective of SVG output capacity and modulation ratio, the upper limit of the value of the connection inductance is derived. Combined with the traditional lower limit of the inductance when the harmonic suppression condition is satisfied, a design method of SVG connection inductance is given. Finally, the simulation results show that the connection inductance value designed by this method is correct and reasonable.

Sub-synchronous Oscillation Control of the DFIG System with SVG Based on Feedback Linearization of RSC

To suppress the subsynchronous oscillation in the doubly-fed induction generator (DFIG) system with Static VAR Generator (SVG), a feedback linearization control (FLC) method for rotor-side converter (RSC) is proposed, and its oscillation effect under small disturbances is analyzed and verified. Firstly, a 22-order time-domain model of a DFIG system with SVG is established. Secondly, combined with the principle of feedback linearization, the corresponding control method is designed for RSC inner loop control. Thirdly, the effects of PI control and feedback linearization control FLC on the system stability are compared when different parameters change under small disturbance. The results show the effectiveness of the proposed controller.

Control strategy of DFIG and SVG cooperating to regulate grid voltage of wind power integration point

To better solve doubly-fed wind farms' voltage stability control problem with static var generator (SVG), this paper proposes and designs a reactive power regulation strategy based on SVG and doubly-fed wind turbines. To stabilize the voltage of the wind farm, SVG is preferred for reactive power compensation when the voltage fluctuates. After the voltage is stable, the reactive power output of SVG and the doubly-fed induction generator (DFIG) is adjusted according to the system state. The DFIG can output enough reactive power and meet the specified power factor conditions. The control mode adopts a real-time tracking mode to monitor the voltage of the control bus. In the model, comprehensive optimization indexes consider the maximum voltage offset, the time-consuming voltage to enter the stable range, the steady-state voltage deviation, and the average voltage offset in voltage fluctuation. The genetic algorithm optimizes the proportional-integral (PI) controller parameters of DFIG and SVG reactive reference values. DFIG and SVG have better response effects to voltage fluctuation than before optimization. Finally, the grid model of DFIG is built, and the simulation results verify the effectiveness and correctness of the proposed method. The cooperative control strategy of SVG and DFIG can adjust the voltage quickly and avoid the long-term reactive power flow between them.

Application of Dynamic Reactive-power Compensator Based on SVG in Coal Mines

With the improvement of automation degree of coal mine mechanical and electrical equipments, such as conveyors, crushers, mining motors, inverters, and a large number of non-linear loads, cause large reactive power consumption, power factor of coal mine power grid, power supply quality and stability, and bring greater influence to coal mine safety production and economic benefits. As a new generation of high voltage dynamic reactive power compensator, SVG (Static Var Generator) has fast response and compound functions, such as compensating reactive power, suppressing harmonic and imbalance with FC(Filter Capacitor), which plays a good role in improving the power quality of coal mine. This paper designs the system parameters of SVG in coal mine and introduces its application.

The Voltage Balance Control Strategy of Static Var Generators DC-Side Capacitors Based on Fuzzy-PI Adaptive Cascaded H-Bridge

A voltage balance control strategy based on the fuzzy adaptive PI (Proportional Integral control) is proposed to address the problem of unbalanced capacitor voltage on the DC side of cascaded H-bridge static var generators (SVG). In the control strategy, fuzzy control is introduced based on PI control. A duty ratio correction is added to all levels’ public active power duty ratios, thereby realizing the balance of DC-side voltages at all levels. A test was conducted by building a single-phase three-H-bridge unit cascaded SVG simulation platform. The results show that the DC-side voltage balance can be controlled by the voltage balance control strategy at all levels effectively. It has the advantages of a simple control structure, double closed-loop control with decoupling of the system’s current DQ, no coupling effect, a fast response speed, and high precision. The voltage balance control strategy can exert a good control effect when the system experiences a sudden voltage imbalance.

A Current-Control Strategy for Grid-Connected Converter Based on Inductance Non-Linear Characteristic Compensation

Inductance is a necessary device for a grid-connected converter (GcC) to attenuate the switching-frequency harmonics in injected grid currents. However, in practice, the inductance decreases with an increase in output current. Especially when the amplitude of sinusoidal currents is higher, the inductance will vary over a wide range as the current value changes in a period. This variation may lead to system instability and cause output current fluctuation. To solve this issue, the model of the GcC with a proportional resonant regulator is firstly built, and the system stability with different current values is analyzed using the Nyquist criterion. The results show that the system stability decreases with an increase in current absolute value. Further, a loop gain compensation unit is embedded into the current regulator to maintain the loop gain constant and ensure the stability of the system under a wide variation range of current values. With this scheme, the compensation unit is only determined by the rated value and the non-linear characteristic of the filter inductance. Therefore, the loop gain compensation unit is independent of the original control system, and the traditional controller parameter design method can also be inherited. Finally, the simulated and experimental results from a 50 A static var generator (SVG) with wide filter inductance variation (using Mega-Flux core) have verified the correctness of the analyses and the effectiveness of the proposed method in this paper.