Static Var Generator Research Articles

Control strategy of mine-used LCL three-level static var generator

Control strategy of mine-used LCL three-level static var generator

Collaborative optimization allocation of VDAPFs and SVGs for simultaneous mitigation of voltage harmonic and deviation in distribution networks

This paper proposes a collaborative optimization allocation strategy for voltage-detection active power filters (VDAPFs) with multiple functions and static var generators (SVGs) to improve voltage harmonic (VH) and voltage deviation (VD) of distribution networks (DNs). The integrated voltage sensitivity is defined considering VH and VD simultaneously. According to the local characteristic of power quality (PQ), a partition algorithm based on integrated voltage sensitivity is developed to divide a network into several regions. Two methods based on single and integrated sensitivity are presented to determine dominant nodes as candidates of VDAPFs and SVGs in each region. The corresponding allocations of VDAPFs and SVGs are respectively provided considering the two methods. The sites and sizes of VDAPFs and SVGs are optimized through minimizing the total cost including investment and operation costs by improved particle swarm optimization (PSO) algorithm. An integrated evaluation index based on economy and PQ disturbance mitigation effect is defined to evaluate the performance of the two allocations, thereby determining an optimal allocation. The feasibility and effectiveness of the proposed strategy are verified implemented on IEEE 33-bus system and IEEE 69-bus system by comparing the two allocations with each other and with the conventional allocation strategy.

Harmonic circulation and DC voltage instability mechanism of parallel‐SVG system

A parallel static var generator (SVG) system based on cascaded H-bridge topology in wind afflux station is analysed in this study to reveal the mechanism of harmonic current and DC voltage instability due to the phase difference between the PWM carriers. First, the inherent relation of harmonic current and low-frequency disturbance is revealed by carrier-phase-shift pulse-width-modulation (CPS-PWM) theory. Second, to identify the negative impact of low-frequency disturbance, a small-signal circuit model is built to research the electric coupling relation between SVGs and the grid, then pointing out the main factors which affect the harmonic circulation among the two-paralleled SVGs. Third, the output impedance and the transfer function of SVG give an insight into the interaction between SVG and low-frequency resonance characteristics, based on which a virtual resistance method is proposed to shape the resonant peak. Finally, theoretical analysis and system experiment prove that the proposed control strategy can rapidly attenuate low-frequency resonant peaks of the system, avoiding the DC voltage instability in the SVG system when harmonic circulation occurs.

Research on Reactive Power and Voltage Control for Wind Farm Based on coordinate control of DFIGs and SVG

Abstract Wind farm generally uses long-distance transmission mode to transmit electricity, because the grid can not fully support the grid voltage of wind farm. When the wind speed changes, there is a problem of voltage instability at the grid-connection point. However, the reactive power compensation device of the power grid can not meet the requirements of fast reactive power response and compensation. Although doubly-fed induction generators (DFIGs) have fast power decoupling control, its reactive power is limited by active power output. In order to meet the reactive power demand of the power grid, the instability of the voltage at the grid-connection point is solved by coordinating the reactive power output capacity of DFIGs and the Static Var Generator (SVG). This paper proposes a coordinated control method of reactive voltage in wind power plant based on Starsim, which uses the upper computer to send voltage control instructions to control the reactive output of the simulation model in Starsim. The simulation model is built in MATLAB, and the simulation model is put into Starsim. Through the simulation, it is concluded that the control method could stabilize the grid voltage of wind farm.

A New Quantitative Analysis Method for Overvoltage in Sending End Electric Power System With UHVDC

With the development of renewable electric power generation and ultra-high voltage direct current (UHVDC) transmission, the transient overvoltage in the sending end electric power system with UHVDC occurs, but it is lack of quantitative transient voltage assessment index (TVAI) to assess the overvoltage. To deal with the above problems, this paper proposes a new quantitative transient voltage assessment index considering overvoltage and voltage dip in the sending end electric power system with UHVDC, and applies it to analyze different influencing factors of overvoltage and determine the dynamic reactive power compensation capacity. In detail, first, according to the principle of the transient voltage assessment index considering voltage dip and the overvoltage assessment index for the distribution network, a new transient voltage assessment index considering transient overvoltage in the sending end electric power system with UHVDC is proposed with the approximated step function. Then, based on the requirements of the system with UHVDC, the parameters of the proposed transient voltage assessment index are determined. Furthermore, with the absolute and relative sensitivity, the impact of the parameters of voltage dependent current order limiter and current control amplifier on the transient overvoltage is analyzed and compared quantitatively. Finally, the impact of reactive power compensation equipment, such as synchronous condenser, static var compensator, and static var generator, on the transient overvoltage of the system with UHVDC is compared quantitatively, and the reactive compensation capacity needed to maintain transient voltage security and stability is estimated. The two applications show the effectiveness and advantages of the proposed index in the sending end electric power system with UHVDC.

Research on static reactive power generator based on asymmetric distribution network

Negative or zero sequence components are generated when the voltage is asymmetric or harmonic in distribution networks. Meanwhile, the decoupling process of traditional dq transform is complex. To solve the above problems, firstly, the T/4 delay method (T is the period of grid voltage) is presented to separate positive and negative sequence components, which improves the stability of software phase-locked. Then, software phase-locked loop (SPLL) is designed to ensure the instantaneity of reactive current check. Besides, a double-loop control scheme combining proportional integral (PI) controller for DC voltage in outer loop and proportional resonance (PR) controller for AC current in inner loop without decoupling is designed by considering the characteristics of traditional PI and PR without static error regulation. It avoids the complicated decoupling process and improves the real-time performance of the system. Finally, both simulation and experimental results are given to verify the feasibility of design scheme in the static var generator (SVG) system by MATLAB/Simulink and experimental platform based on DSP28335.

Research on static reactive power generator based on asymmetric distribution network

Negative or zero sequence components are generated when the voltage is asymmetric or harmonic in distribution networks. Meanwhile, the decoupling process of traditional dq transform is complex. To solve the above problems, firstly, the T/4 delay method (T is the period of grid voltage) is presented to separate positive and negative sequence components, which improves the stability of software phase-locked. Then, software phase-locked loop (SPLL) is designed to ensure the instantaneity of reactive current check. Besides, a double-loop control scheme combining proportional integral (PI) controller for DC voltage in outer loop and proportional resonance (PR) controller for AC current in inner loop without decoupling is designed by considering the characteristics of traditional PI and PR without static error regulation. It avoids the complicated decoupling process and improves the real-time performance of the system. Finally, both simulation and experimental results are given to verify the feasibility of design scheme in the static var generator (SVG) system by MATLAB/Simulink and experimental platform based on DSP28335.

Frequency‐coupled impedance model‐based sub‐synchronous interaction analysis for direct‐drive wind turbines connected to a weak AC grid

Recently, emerging sub-synchronous oscillation (SSO) issues caused by the interaction between power electronic converters and weak AC grids have provoked serious stability concerns. Impedance-based modelling approaches have been widely employed for stability analysis of such interactions. In this study, a sequence-domain frequency-coupled impedance model (FCIM) and the corresponding stability criterion are proposed. First, a fast identification method of the FCIM is proposed, in which the impedance-frequency curves of the FCIMs at multiple frequencies are measured, followed by identification of their transfer function models. Next, the proposed method is applied to model a practical system, where a SSO event once occurred due to the interactions between direct-drive wind turbines and a weak AC grid. The individual FCIMs of wind turbine generators, steam turbine generators and static var generator in this system are derived. After that, an aggregated FCIM is formed by combining the FCIMs without extra coordinate transformation. Then, the aggregated FCIM-based stability analysis is carried out to investigate the interaction between the wind farms and weak AC grids. In the end, the results of FCIM-based stability analysis are verified by time-domain simulations.

Reactive power optimization configuration for distribution network integrated with microgrids

In recent years, distributed generation (DG) and microgrid develop rapidly in distribution network. With the increase of its penetration, the distribution network is faced with many operation challenges such as voltage violation and voltage flicker, etc. Thus, it is urgent to perform reasonable reactive power planning and optimize the operation strategy of reactive equipment to handle the arisen problems in the distribution network. This paper puts up with a reactive power configuration method, optimizing the operation of shunt capacitor bank (C) and static var generator (SVG). The genetic algorithm (GA) is applied to solve the model. It is proved by the modified IEEE33 system that the proposed model could effectively improve the voltage eligibility rate, reduce the grid loss and achieve better economy, which provides the reference for the reactive power optimization configuration for the future distribution network. The method could be applied in the situation of the integration of microgrid with large capacity generation with randomness.

Power Control and Fault Ride-Through Capability Analysis of Cascaded Star-Connected SVG under Asymmetrical Voltage Conditions

The cascaded H-bridge static var generator (SVG) has been employed to provide reactive power and regulate grid voltages for many years because of its good modularity, easy scalability, and improved harmonic performance. A novel cluster-balancing power control method combining negative-sequence currents and zero-sequence voltage is proposed to redistribute the unbalanced active powers and eliminate the power oscillation under asymmetrical conditions. Simultaneously, the dynamic performance of the SVG power balance control can be improved under asymmetrical conditions with the zero-sequence voltage expression derived in this paper. On the basis of the proposed method, the fault ride through capability of star-connected SVG under asymmetrical conditions is compared among active power oscillation elimination (APOE), reactive power oscillation elimination (RPOE), and balanced positive sequence current (BPSC) injection references calculation strategies from the perspective of the zero-sequence voltage, maximum phase voltage, and maximum phase current. The method provides the theoretical reference for power control under asymmetric conditions and the analysis results show that under asymmetrical conditions, the current of BPSC is minimal and symmetrical, while the RPOE has the least voltage and no zero- sequence voltage needs to be injected. Finally, the results of simulation and experiment have been given to verify the theoretical studies.

PMSG-Based Black-Start Technology and Its Field Tests

It is of great importance for power grids to have black-start capability for rapid recovery, and there is great theoretical significance and practical application value in studying how to use wind farms as the black-start power supply source for power grids with large-scale renewable energy generation. In this paper, a black-start scheme using a permanent-magnet synchronous generator (PMSG)-based wind farm as black-start power supply source is formulated. First, a diesel generator is used as an external supporting power supply for the self-start of a wind power unit (WPU). Then, after all the planned WPUs operate normally, the wind farm with the diesel generator and static var generator (SVG) is used to black start the simulated auxiliary load of a thermal power plant. A field test of the proposed black-start scheme is carried out on an actual wind farm in Jiangsu Province (China). The results of the field test show that wind farms can act as a black-start power supply source for the grid after appropriate technological transformation.

Direct current control strategy of SVG based on dual sequence dq coordinates under asymmetric load condition

In the case of asymmetric loads of power grid, load currents are composed of four components: positive sequence active and reactive components and negative sequence active and reactive components that can pollute the power grid with harmonics and reactive power and interrupt the normal operation of power grid. Therefore, increasing numbers of static var generators (SVGs) are used to stabilize the power grid. In this study, a new type of current compensation control strategy of a three-phase three-wire SVG under an asymmetric load is proposed. According to the symmetric component method and dual dq synchronous transformation theory, the load currents are transformed into four components under dual dq coordinates. Each component is compensated separately by the SVG Simultaneously, the proportional-limiter method is adopted to minimize the harmonics from the SVG by optimizing the waveform of the modulation wave and limiting its amplitude. Finally, the validity and feasibility of the control strategy are verified by simulation and experimental results.

Research of Large-Capacity Low-Cost DC Deicer With Reactive Power Compensation

DC ice melting is an effective technical method against ice disaster on transmission lines. To satisfy the demand for simultaneous de-icing and reactive power compensation for substations, this paper proposed a large-capacity low-cost DC deicer with reactive power compensation, which is mainly composed of rectifier transformer, Static Var Generator (SVG) and diode rectifier. In the proposed topology, SVG and diode rectifier share the rectifier transformer, but rectifier transformer capacity is not increased compared with a single deicer. This paper firstly presents the proposed topology and analyzes its advantages. Then, the rectifier transformer design is introduced, and the SVG control strategy is presented. Simulation results for the proposed device are presented, and an engineering devices with a capacity of 21.8 MVA developed for the 220 kV Fuchong substation is described. The simulation and application results indicate that the proposed devices work well.

Grid connected converters as reactive power ancillary service providers: Technical analysis for minimum required DC-link voltage

Providing solutions for reactive power compensation next to reactive loads is beneficial for the efficiency of the overall grid system. The main reactive power compensation solutions are based on passive VAR generators (like capacitor banks) or active VAR generators (like static VAR generators). Extra use of VAR generators can be avoided by providing reactive power through renewable energy systems or loads, which are connected to the grid through Grid connected Converters (GcCs). In fact, GcCs can be controlled so that, in addition to their main control task, they realize an additional reactive power ancillary service. In this context, this paper investigates the achievement of reactive power ancillary service using three-phase GcCs and presents a detailed analysis regarding the determination of the minimal required DC-link voltage level for proper operation of GcCs as reactive power ancillary service providers. This analysis is done under both steady-state and transient conditions. Several simulation and experimental results are presented to confirm the validity of the obtained analytical results.

Research on the Unbalanced Compensation of Delta-Connected Cascaded H-Bridge Multilevel SVG

The paper presents the application of delta-connected cascaded H-bridge multilevel static var generator (SVG) under unbalanced compensation currents or unbalanced supply voltages. Clustered balancing control for delta-connected SVG can be realized by injecting a zero-sequence current to the delta loop. But zero-sequence current injection may cause the high peak phase current which may break converter switches. The aim of this paper is to analyze the key factors that affect the maximum output current of the SVG with injecting zero-sequence current and acquire the quantitative relationship among unbalance compensation capability, the unbalance degree of the supply voltage, the initial phase of negative-sequence voltage, the unbalance degree of the compensation current, and the initial phase of negative-sequence current. On this foundation, the valid compensation range of delta-connected SVG under unbalanced conditions is obtained. Furthermore, the compensation characteristic of the negative-sequence current is deduced with the certain supply voltage, and the influence of supply voltage variation on the maximum output current for SVG is also considered with the certain compensation current. Finally, the correctness of the relevant theoretical analysis is verified by simulation and experiment.

Applications of Digital-Physical Hybrid Real-Time Simulation Platform in Power Systems

Digital-physical hybrid real-time simulation (hybrid simulation) platform integrates the advantages of both digital simulation and physical simulation by combining the physical simulation laboratory and the real-time digital simulator. Based on a 400 V/50 kVA hybrid simulation platform with 500 kVA short-circuit capacity, the hybrid simulation methodology and a Hausdorff distance based accuracy evaluation method are proposed. The case validation of power system fault recurrence is performed through this platform, and the stability and accuracy are further validated by comparing the hybrid simulation waveform and field-recorded waveform and by evaluating the accuracy with the proposed error index. Two typical application scenarios in power systems are studied subsequently. The static var generator testing shows the hybrid simulation platform can provide system-level testing conditions for power electronics equipment conveniently. The low-voltage ride through standard testing of a photovoltaic inverter indicates that the hybrid simulation platform can be also used for voltage standard testing for various power system apparatus with low cost. With this hybrid simulation platform, the power system simulation and equipment testing can be implemented with many advantages, such as short period of modelling, flexible modification of parameter and network, low cost, and low risk. Based on this powerful tool platform, there will be more application scenarios in future power systems.

Expansion planning for active distribution networks considering deployment of smart management technologies

In this study, a multi-stage long-term expansion planning model for an active distribution network (ADN) is presented, with the aim of minimising the investment and operation cost in a coordinated manner over an established horizon. The planning model optimises the following alternatives: upgrading the capacities of substations, reinforcing and/or constructing cable circuits, placing voltage regulators (VRs) and/or static VAR generators, and determining the connection points for distributed generators (DGs). The investment decisions are optimised over the entire planning horizon which can be further divided into multiple periods, and the operation strategies, e.g. active management of DG as well as ADN topology reconfiguration, are determined according to the profiles of representative scenarios. To relieve the computational burden, the original model is properly simplified as a mixed-integer quadratic constrained programming problem through linearisation and approximation techniques, and the solution optimality is guaranteed after invoking the off-the-shell solver. A 24-node test system is employed to validate the effectiveness of the proposed model.

Suppression Methods for Low Frequency Oscillation of Wind Farm Considering SVG Minimum Output Dead Zone

At present, the research of Static Var Generator (SVG) mainly focuses on the main circuit topology, the optimization of control strategy, the method of selection, and lack of research on the low frequency oscillation of the wind farm caused by SVG minimum reactive output dead zone in practical engineering applications. For this reason, this paper first studies the mechanism of the low frequency oscillation of the wind electric field induced by the SVG minimum output dead zone of the wind farm, and discusses the conditions of this type of oscillation. On this basis, the automatic voltage control system (AVC) optimization model of wind farm is established with the minimum reactive variation of SVG output near the zero point as the optimization goal, and an optimization control strategy of wind farm with SVG minimum output reactive dead zone is proposed. Finally, the correctness of the result is verified by simulation. The results show that, due to the existence of the minimum output dead zone in the wind farm, the large reactive step may occur near the zero point of the SVG output, which causes the voltage low frequency oscillation of the wind farm. The optimization strategy proposed in this paper can significantly suppress the low frequency oscillation by coordinating and optimizing reactive power output near the zero point of multiple SVGs in wind farm.

Decomposition method for coordinated planning of distributed generation and distribution network

With recent development on renewable energy and distributed generation (DG), distribution system expansion planning (DSEP) problem is facing more challenges. This study proposed a multistage coordinate planning model to minimise the present value of construction cost and operating cost over the planning horizon. Installation of DGs, static var generators, and expansion of distribution network were considered simultaneously. The proposed model is a large-scale mixed-integer second-order conic programming (MISOCP) problem containing large amounts of constraints and variables. The MISOCP can be decomposed into: (i) the master problem which determines the construction plan, (ii) and many subproblems which optimise the operating states under different network configurations and different load levels. A modified Benders decomposition method was applied to generate valid cuts from subproblems with integer variables. Results of the proposed decomposition method were compared with those obtained by commercial software on a 24-node distribution system. Numerical experiments showed that the proposed method can solve the integrated DSEP problem efficiently.

A Method for Wind Farm Reactive Power Controlling using DIgSILENT and MATLAB

In order to regulate the voltages of Point of Common Coupling (PCC) and single wind turbine in a wind farm, it is necessary to control the output of reactive power of wind turbines, and in some case, it also need to control static var compensators (SVCs) or static var generators (SVGs) of the wind farm to satisfy the voltage set-point requirement. The accuracy of reactive power control system decides the control level of grid voltage. In order to improve the accuracy, this paper introduces modeling and simulation method of wind farm reactive power control system using DIgSILENT and MATLAB. At first, it introduces the modeling process and co-calling principle of the two kinds of software tools, and then emphasizes that it develops a kind of advanced wind farm reactive power control logic to realize control accuracy improvement in MATLAB, whereas the power quality and distribution logic of reactive power among wind turbines are simulated and assessed. In the end, taking an Inner Mongolia wind farm with 33 double-fed wind turbines as a test application, the wind farm reactive power control model is verified by comparing real wind farm operating data and simulation results, and the result shows that the accuracy of reactive power control system is higher than traditional method.