Fault Ride-through Control Strategy Research Articles

Fault Ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter

Fault ride-through is a prerequisite for ensuring continuous operation of a variable-speed pumped storage unit with a full-size converter (FSC-VSPU) and providing support for the renewable energy and power grid. This paper proposes low-voltage ride through (LVRT) and high-voltage ride through (HVRT) strategies for FSC-VSPU to address this issue. Firstly, the structure of FSC-VSPU and its control strategy under power generation and pumping conditions are described. Subsequently, the fault characteristics of the FSC-VSPU under different operating conditions are analyzed. More stringent fault ride-through technical requirements than those for wind turbines are proposed. On this basis, the fault ride-through strategies of combining fast power drop, rotor energy storage control, power anti-regulation control, dynamic reactive current control, low power protection, and DC crowbar circuit are proposed. Simulation case studies conducted in PSCAD/EMTDC verify the correctness of the theoretical analysis and the effectiveness of the LVRT and HVRT strategies in this paper.

AC fault ride-through control strategy of MMC-UHVDC system with hierarchical connection mode

The ultra-high voltage direct current (UHVDC) transmission composed of modular multilevel converters (MMC) is an important technology for large-scale centralized transmission of renewable energy. In UHVDC system, temporary faults in the AC power grid system are a high probability fault, and the fault control strategy affects the safety and reliability of system operation. This paper studies on the condition of AC power grid faults occurring at the receiving converter station. Firstly, study the system characteristics after the occurrence of AC faults, and use theoretical analysis to derive the trend of DC voltage changes of each converter valve. Then, an AC fault ride-through control strategy with high power transmission capability is proposed with hierarchical connection structure, the strategy controls the system to synchronously reduce DC voltage and AC active power after a fault, maximizing the retention of the system’s transmission capacity during the occurrence of faults, thereby reduce power shock in the system. Finally, a simulation model of the dual ended system has built based on the PSCAD simulation platform. The simulation results show that when a single-phase ground fault and a three-phase ground fault occur in the high valve group at the receiving station, the system can retain about 83% and 50% of the transmission capacity during the fault period, respectively. Meanwhile, there is no serious overvoltage or overcurrent phenomenon in the system. The simulation results verified the effectiveness of the proposed control strategy.

Investigation of Transient Stability in Power System with Improved FRT Capable Solar PV Inverters

Most regions of the world have seen tremendous growth in distributed energy resources, especially photovoltaic (PV) generation. The penetration of PV generation into the power grid is high and hence it has a major impact in power system. The integration of solar PV plant into the power system has both negative as well as positive impacts on transient stability of the power system during faults. In this work, the proposed current amplitude limiting based Fault Ride Through (FRT) control algorithm is implemented in large scale power systems and enhancement of transient stability is also studied in terms of synchronous machine side and grid side parameters namely rotor speed deviation, load angle, relative rotor angle, active and reactive power. The transient stability of large-scale Grid Connected solar PV (GCPV) system is discussed in this article, when the system is incorporated with the proposed FRT control strategy. The large scale GCPV system is said to be stable, if the angular displacement between the machines in the system stays within predetermined limits. The parameters considered for transient analysis on the synchronous machine side are the rotor angle, rotor speed, and Critical Clearing Time. The proposed model is simulated in MATLAB Simulink and validated in 1 kW hardware setup of grid connected PV system which performs satisfactorily in terms of system parameters and transient stability improvement.

Dual-mode Switching Fault Ride-through Control Strategy for Self-synchronous Wind Turbines

The installed capacity of renewable energy generation has continued to grow rapidly in recent years along with the global energy transition towards a 100% renewable-based power system. At the same time, the grid-connected large-scale renewable energy brings significant challenges to the safe and stable operation of the power system due to the loss of synchronous machines. Therefore, self-synchronous wind turbines have attracted wide attention from both academia and industry. However, the understanding of the physical operation mechanisms of self-synchronous wind turbines is not clear. In particular, the transient characteristics and dynamic processes of wind turbines are fuzzy in the presence of grid disturbances. Furthermore, it is difficult to design an adaptive fault ride-through (FRT) control strategy. Thus, a dual-mode switching FRT control strategy for self-synchronous wind turbines is developed. Two FRT control strategies are used. In one strategy, the amplitude and phase of the internal potential are directly calculated according to the voltage drop when a minor grid fault occurs. The other dual-mode switching control strategy in the presence of a deep grid fault includes three parts: vector control during the grid fault, fault recovery vector control, and self-synchronous control. The proposed control strategy can significantly mitigate transient overvoltage, overcurrent, and multifrequency oscillation, thereby resulting in enhanced transient stability. Finally, simulation results are provided to validate the proposed control strategy.

Dynamic equivalent of inverter-based distributed generations for large voltage disturbances

As the differences among inverter-based distributed generators (DGs) in the active distribution network (ADN), the aggregated dynamic model under large disturbances cannot effectively represent transient characteristics of the ADN. To overcome this issue, in this paper, a generic dynamic equivalent model of DGs is proposed. First, a simulation platform for multiple DGs is built based on electromagnetic transient simulation tool. The simulation results show that the aggregated transient characteristics under large disturbances are mainly determined by the fault ride-through control strategy of each inverter. Based on this, a control strategy is proposed to characterize the differences between inverters, and the equivalent model is derived. To achieve engineering applications, the equivalent structure is simplified, and multiple disturbance curves are used to identify the parameters. The validity and generalization ability of the proposed model is verified by simulation analysis under different disturbance strengths.

Development of grid-side converter-based FRT control and protection in a grid-connected solar photovoltaic park system under different control modes

In a grid-connected solar photovoltaic system, voltage dips on the grid side, increased grid current, and overshoot in the inverter’s dc-link voltage are all noticed during grid disturbances. The grid code stipulates that in order to protect the cascaded power converter systems from high currents and overvoltages, the solar PV system disconnects from the grid anytime the voltage level at PCC falls below a particular standard nominal value. This study proposes combined GSC-based fault ride-through (FRT) and protection control strategies which can provide independent real and reactive power control for the inverter for effective FRT in photovoltaic parks interconnected with a large power system during grid faults. The proposed approach operates in three modes—voltage control, power factor control, and reactive power control modes. The developed protection modules in the PV system consist of over/undervoltage protection, voltage sag detection, and overcurrent detection. The inverter-fed real–reactive power control technique limits grid overcurrent by modifying active power injection into the grid and stabilizes grid voltage during faults by injecting reactive current. The performance of the proposed FRT and protection control strategy is studied through the simulation of 75 MW PV park in the EMTP platform and the experimental setup of a 5 kVA grid-connected inverter-fed solar PV system. The simulation results and hardware discussion presented in this study validate the performance of the proposed FRT scheme in comparison with conventional control schemes during grid faults.

Power surplus mechanism and fault ride through strategies of the hybrid cascaded UHVDC system

Hybrid cascaded dc technique has the advantages of long-distance and large capability transmission of the line commutated converter(LCC) based HVDC, and also the advantages of supplying multiple receiving ends, flexible operation and good system supporting ability of the voltage source converter (VSC) based HVDC. It is a good solution for the efficient use of large scale clean energy and the construction of energy internet. Chinese Baihetan-jiangsu UHVDC project is the first hybrid cascaded UHVDC (HS-UHVDC) project in the world. Power Surplus is a serious problem for the HS-UHVDC, which can cause overvoltage of the VSCs, making the system fail to ride-through the ac faults. This paper reveals the power surplus mechanisms of the HS-UHVDC system. Two energy dissipation devices, that is the controllable arrester scheme and the thyristors &blocking diodes are proposed and compared in detail. Then the ac fault ride-through control strategies and the parameter design method of the energy dissipation devices are proposed. It is shown that with the proposed energy dissipation method, the VSC overvoltage can be suppressed and the ac ride-through can be realized, thus solving the power surplus problemofHS-UHVDC effectively.

The impact of DFIG control schemes on the negative-sequence based differential protection

Negative-sequence currents at the transmission line terminals are widely adopted in differential protection to achieve excellent sensitivity. The underlying principle is that the traditional power sources (synchronous generators) can be treated as inductive reactance in the negative-sequence network. However, for the transmission lines connected to Doubly-Fed Induction Generator (DFIG) based wind parks (WPs), the negative-sequence currents are controlled by fault-ride-through (FRT) solutions. This paper first analyzes the phasor characteristics between negative-sequence voltage and current of a DFIG-based WP under three typical FRT solutions, and then evaluates their impact on the dynamic performance of negative-sequence differential protection elements (87LQ). The analysis indicates that a certain FRT control strategy would make DFIG-based WP provide capacitive current into the negative-sequence network. This impacts the sensitivity and even results in maloperation as shown for the first time in this work. The analytical results are validated with detailed time domain models and simulations in Simulink.

Multimode Inverter Control Strategy for LVRT and HVRT Capability Enhancement in Grid Connected Solar PV System

Grid Connected Photo Voltaic (GCPV) system should be susceptible to grid faults and load curtailment without disconnection and supports in grid stability. During grid faults, there is an increase in dc link voltage, dip in grid voltage which leads to over-current on the grid side. Similarly, when demand is suddenly removed, the voltage at the PCC rises above its nominal value. This leads to possible damage in the PV inverters and hence need to disconnection of GCPV system leading to islanding scenarios. Hence, the GCPV system need to be equipped with Fault Ride Through (FRT) capability to address the issues related to low voltage and high voltage conditions in the grid side. In this work, multimode inverter control strategy is proposed with FRT capability according to grid code compliance. An improved current control technique is proposed as FRT based protection strategy during grid faults and sudden removal of load. According to severity of faults, a fault level detection is developed which triggers the control strategies to generate the reference real and reactive power with respect to the grid code compliance. The performance analysis of the proposed FRT control strategy is implemented in 100 kW two-stage GCPV system in MATLAB/Simulink 2018b environment and tested under low voltage and high voltage ride through conditions. The proposed control strategy is implemented in hardware setup of 1 kW GCPV system. The experimental results prove the effectiveness of the proposed inverter FRT based control strategy in enhancing the system parameters during system disturbances under different modes of operation.

Dual Control Strategy for Grid-tied Battery Energy Storage Systems to Comply with Emerging Grid Codes and Fault Ride Through Requirements

Battery energy storage systems (BESSs) need to comply with grid code and fault ride through (FRT) requirements during disturbances whether they are in charging or discharging mode. Previous literature has shown that constant charging current control of BESSs in charging mode can prevent BESSs from complying with emerging grid codes such as the German grid code under stringent unbalanced fault conditions. To address this challenge, this paper proposes a new FRT-activated dual control strategy that consists of switching from constant battery current control to constant DC-link voltage control through a positive droop structure. The results show that the strategy ensures proper DC-link voltage and current management as well as adequate control of the positive- and negative-sequence active and reactive currents according to the grid code priority. It is also shown that the proposed FRT control strategy is tolerant to initial operating conditions of BESS plant, grid code requirements, and fault severity.

Research on fault ride-through control strategy for DC microgrid with high-proportion renewable generation

DC microgrids with high-proportion renewable generation have been popular in the new era of the energy revolution. To improve the fault ride-through capability of DC microgrid, a fault ride-through scheme based on battery storage is proposed. A feed-forward term based on a nonlinear disturbance observer is introduced, and a fault ride-through control strategy based on battery storage under the improved droop control is designed for DC microgrid, which can effectively suppress the DC voltage fluctuations, shorten the voltage recovery time, thereby realizing the fault ride-through of DC microgrid under short-circuit fault in the DC branch. Finally, a model of DC microgrid with high-proportion renewable generation is established in Simulink, and simulation results verify the proposed scheme and the designed fault ride-through control strategy.

Advanced Control Strategy With Voltage Sag Classification for Single-Phase Grid-Connected Photovoltaic System

This article develops a fault ride-through control strategy for achieving low-voltage ride through in single-phase grid-connected photovoltaic (PV) systems (GCPVS). The proposed control system adapts a neural network classifier for islanding classification and model predictive control for achieving the control of the two-stage PV system. This control scheme takes advantage of the nonlinear nature of the power converters and develops a cost function-based approach to achieve fast and efficient control. In addition, the proposed controller provides voltage support to the grid during voltage sags by injecting minimum reactive current within the threshold. The operation of the proposed control strategy is verified by performing simulation tests on a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4\ {\text{kW}}$</tex-math></inline-formula> GCPVS by creating a sag type of fault in the utility. Further, laboratory experiments were carried out with the developed controller. The results ensure that the proposed control system adheres to the grid requirements by enabling voltage support during grid faults.

Chopperless Fault Ride-Through Control for DC Microgrids

The dc microgrid should remain connected to the ac utility grid to support the grid stability during faults. However, the bidirectional power flow between the utility grid and the dc microgrid will cause either high-voltage or low-voltage faults at the dc bus. It is difficult to control a large number of heterogeneous sources at the dc bus to ride through the fault, as well as exporting reactive power or reactive current to the utility grid. This article proposes a chopperless fault ride-through control strategy for the dc microgrid. The proposed control strategy utilizes the controllability of dc microgrids without using chopper circuit on the dc side to balance the system power. Following goals are achieved by this control strategy: 1) maximizing the output reactive power to the utility grid; 2) reducing the dc bus voltage ripples; 3) realizing the decentralized emergency power control among different distributed generators. The effectiveness of the proposed control method has been validated through related case studies in both simulation and hardware-in-loop (HIL) tests.

Improved Continuous Fault Ride Through Control Strategy of DFIG-Based Wind Turbine During Commutation Failure in the LCC-HVDC Transmission System

The commutation failure fault usually occurs in the line-commutated-converter-based high-voltage direct current transmission system. When commutation failure fault occurs, the voltage of sending alternate current (ac) system changes rapidly, and the connected doubly fed induction generator (DFIG)-based wind turbine may be tripped. Thus, the fault ride through (FRT) control strategy of DFIG should be investigated for enhancing the stability of the sending ac system. However, the voltage of the sending ac system during commutation failure is not changed in rectangular in shape, besides, the voltage presents the “first reduce then rise” characteristic, which is not considered in the existing FRT control strategies. In order to realize the continuous FRT of DFIG during commutation failure, the stator flux and electromotive force when the stator voltage changes continuously have been analyzed for the first time in this article. Furthermore, based on the analysis results, an improved continuous FRT control strategy is proposed. The simulation and experiment results validate the effectiveness of the proposed method. The proposed control strategy is not only suitable for the commutation failure condition, but also for the scenario with continuous voltage variation during grid fault, which indicates that the proposed method is general.

Pilot protection of hybrid MMC DC grid based on active detection

Considering the advantages and limitations of traditional identification method, combined with the strategy of active detection, the principle of DC grid pilot protection based on active detection is proposed to improve the sensitivity and reliability of hybrid MMC DC grid protection, and to ensure the accurate identification of fault areas in DC grid. By using the DC fault ride-through control strategy of the hybrid sub-module MMC, the fault current at the converter station DC terminal is limited. Based on the high controllability of hybrid MMC, sinusoidal fault detection signals with the same frequency are injected into the line at each converter station. Based on model recognition, the capacitance model condition is satisfied by the detected signals at both ends during external faults whereas not satisfied during internal faults. The Spearman correlation coefficients is then introduced, and the correlation discriminant of capacitance model is constructed to realize fault area discrimination of DC grid. The simulation results show that the active detection protection scheme proposed in this paper can accurately identify the fault area of DC grid, and is not affected by fault impedance and has low sampling rate requirement.

Modified STATCOM control strategy for fault ride-through capability enhancement of grid-connected PV/wind hybrid power system during voltage sag

This paper proposes the employment of Static Synchronous Compensator (STATCOM) in reactive power compensation to enhance the Fault Ride-Through (FRT) capability and improve the dynamic performance of a grid-connected PV/wind hybrid power system during the transient grid disturbances. The hybrid power system consisting of 9 MW Doubly Fed Induction Generator (DFIG)-based wind farm and 1 MW PV station is integrated with 100 MVAR STATCOM at the Point of Common Coupling (PCC) bus. The dynamic performance of the PV/wind hybrid power system with the proposed STATCOM controller is analyzed and compared with another FRT control strategy during a grid voltage sag. The FRT control strategy is based on the injection of reactive power from the hybrid system to enhance the FRT capability during the grid faults, and also activation of the outer crowbar protection system to protect the DFIG. On the other hand, the proposed STATCOM controller adjusts the PCC bus voltage during the grid disturbances by dynamically controlling the amount of reactive power injected to or absorbed from the electrical grid. Modeling and simulation of the proposed hybrid power system have been implemented using MATLAB/SIMULINK software. The effectiveness of both the proposed STATCOM controller and the FRT control strategy is evaluated during a 50% grid voltage sag. The simulation results illustrate that the STATCOM controller decreases significantly the level of voltage drop during the voltage sag, maintains the injected active power from the PV station at its rated value, and protects effectively the PV DC-link voltage from overvoltage. Moreover, when the STATCOM controller is employed, the injected active power from the wind farm is improved considerably and the oscillations of the DFIG rotor speed are reduced efficiently during the fault. Furthermore, the comparison confirms the superior dynamic performance of the STATCOM controller in enhancement the FRT capability as compared with the FRT control strategy.

Asymmetrical Fault Ride-Through Control Strategy for Rotor-Side Converter of DFIG

This article proposes an asymmetrical fault ride-through (FRT) control strategy for the rotor-side converter (RSC) of a doubly fed induction generator (DFIG). The mathematical model of the DFIG is presented in the positive and negative rotating synchronous reference frames. To prevent the RSC from overcurrent under asymmetrical fault conditions, both positive- and negative-sequence components of the rotor current are reduced. Considering the voltage capacity limitation of the RSC, the rotor negative-sequence voltage of the DFIG is limited by multiplying by a proportional coefficient. With the proposed control strategy, the DFIG can ride through an asymmetrical fault smoothly even when the imbalanced degree of the grid voltage is equal to 100%. Simulations on a 2-MW DFIG system validate the effectiveness of the proposed asymmetrical FRT control strategy.

Fault ride-through control strategy of H-bridge cascaded energy storage system

The cascaded energy storage system has received extensive attention in areas such as new energy consumption, maintaining stable operation of the power grid, and supporting black start due to its advantages such as high access voltage level, large single unit capacity, and fast dynamic response rate. This paper aimed to improve the fault ride-through capability of the cascaded energy storage system, and proposed a fault ride-through control method. Firstly, the mathematical model of the cascaded energy storage system was established, and then the rapid detection method of the abnormal state of the power grid and the fault ride-through method were analyzed, and finally the simulation analysis was performed. The feasibility and correctness of the control method can be seen from the experiments.

Transient Characteristics of Synchronverters Subjected to Asymmetric Faults

Since the large fault inrush current threatens the safety of power electronic devices, the synchronverter should have the fault ride-through (FRT) capability and should be equipped with a reliable protection system. Therefore, it is important to study the transient fault characteristics of the synchronverter and propose its fault current estimation method. In this paper, by establishing the equivalent model of synchronverter, an asymmetric fault current analysis method using the instantaneous symmetric component method is proposed. In addition, the current components, peak value, peak time, time required for reaching two times of the rated current, and main influencing factors of the asymmetric fault current are also analyzed. It is found that the influencing factors mainly include the control loops of synchronverter, asymmetric fault type, line impedances, and fault occurring moment. Moreover, the time required to reach two times of the rated current is approximately within the range of one hundredth of a microsecond to five milliseconds. These fault current characteristics impose strict requirements on the response speed of FRT control strategies. Finally, the simulation and experiment results verified the correctness of the proposed analysis method.

A DC fault ride‐through and energy dissipation scheme for hybrid MMC‐MTDC integrating wind farms with overhead lines

To integrate bulk wind power, using modular multilevel converter (MMC) based DC grids is an effective solution to improve the flexibility and reliability of the power grid. This paper analyses a bipolar four-terminal MMC based DC grid to integrate wind farms with overhead lines. Since the overhead lines are prone to DC faults, the hybrid MMC is adopted in the system to ride through DC faults. The fault ride-through control strategies are designed, which remain continuous operation of wind turbines under DC faults. Besides, an adaptive dissipation control strategy associated with chopper resistances is designed to absorb surplus energy. Finally, simulations based on PSCAD/EMTDC platform verify the fault ride through control of hybrid MMC based DC grid and proves the effectiveness of the energy dissipation strategies.