Low Voltage Ride-through Capability Improvement Research Articles

Low-voltage ride-through capability improvement of Type-3 wind turbine through active disturbance rejection feedback control-based dynamic voltage restorer

Abstract Disconnections due to voltage drops in the grid cannot be permitted if wind turbines (WTs) contribute significantly to electricity production, as this increases the risk of production loss and destabilizes the grid. To mitigate the negative effects of these occurrences, WTs must be able to ride through the low-voltage conditions and inject reactive current to provide dynamic voltage support. This paper investigates the low-voltage ride-through (LVRT) capability enhancement of a Type-3 WT utilizing a dynamic voltage restorer (DVR). During the grid voltage drop, the DVR quickly injects a compensating voltage to keep the stator voltage constant. This paper proposes an active disturbance rejection control (ADRC) scheme to control the rotor-side, grid-side and DVR-side converters in a wind–DVR integrated network. The performance of the Type-3 WT with DVR topology is evaluated under various test conditions using MATLAB®/Simulink®. These simulation results are also compared with the experimental results for the LVRT capability performed on a WT emulator equipped with a crowbar and direct current (DC) chopper. The simulation results demonstrate a favourable transient and steady-state response of the Type-3 wind turbine quantities defined by the LVRT codes, as well as improved reactive power support under balanced fault conditions. Under the most severe voltage drop of 95%, the stator currents, rotor currents and DC bus voltage are 1.25 pu, 1.40 pu and 1.09 UDC, respectively, conforming to the values of the LVRT codes. DVR controlled by the ADRC technique significantly increases the LVRT capabilities of a Type-3 doubly-fed induction generator-based WT under symmetrical voltage dip events. Although setting up ADRC controllers might be challenging, the proposed method has been shown to be extremely effective in reducing all kinds of internal and external disturbances.

Active Power Backflow Suppression Strategy of Cascaded PV Solid-State Transformer Under Interphase Short-Circuit Fault Condition

Currently, the grid-tied guidelines all require large and medium-sized photovoltaic (PV) power stations to stay connected to the grid during low-voltage ride through (LVRT). However, three-phase cascaded PV solid-state transformer (SST) has the inherent active power backflow problem during asymmetric voltage sags, causing that H-bridge dc-bus voltages are out of control and that the converter will be disconnected from the grid because of overvoltage fault. The existing control strategy is capable of avoiding active power backflow in theory by injecting appropriate zero-sequence voltage, but the overmodulation range is larger during interphase short-circuit fault, weakening active power backflow suppression effect. For this issue, this article proposes a combined control method of positive–negative-sequence maximum–minimum harmonic zero-sequence voltage injection strategy and adaptive zero-sequence voltage compensation strategy, which can shrink effectively active power backflow range of three-phase cascaded PV SST, increasing its adaptability to different output powers and different grid drop depths, and then improving LVRT capability of the PV power generation system. Finally, a low-voltage and low-power experimental prototype is made to verify the effectiveness and feasibility of the proposed control strategy.

An Optimized LVRT Control Strategy of Cascaded Modular Medium-Voltage Inverter for Large-Scale PV Power Plant

Active power backflow is an inherent problem of three-phase cascaded H-bridge (CHB) photovoltaic (PV) grid-tied inverters during low-voltage ride through (LVRT), probably resulting in no balanced operating point of the system, and the inverter will be shut down and off-grid due to overvoltage fault. Aiming at this issue, this article first reviews the existing control methods and analyzes their limitations in application, that is, they cannot effectively suppress active power backflow under the scenario of deeper voltage drop and lower output power. On this basis, an optimized LVRT control strategy for cascaded modular medium-voltage PV power generation system is proposed, which adopts different control methods to suppress active power backflow under three types of asymmetric grid drop conditions: single-phase short-circuit fault with ground, two-phase short-circuit fault with ground, and two-phase short-circuit fault without ground, enhancing the adaptability of CHB PV grid-connected inverters to different output powers and different drop depths during grid voltage failure and then improving LVRT capability of the system. Finally, the effectiveness and feasibility of the proposed control strategy are verified by experimental results.

A stored energy-based control strategy to improve LVRT capability of HRES using PSO & WCA optimized DVR

The article presented here utilizes the particle swarm optimization (PSO) and water cycle algorithm (WCA) based dynamic voltage restorer (DVR) to augment the low voltage ride through (LVRT) performance of a hybrid renewable energy system (HRES). Moreover, FACT equipment like the dynamic voltage restorer (DVR) is deployed to boost the quick responsiveness of compensation and good transient performance of HRES. Due to numerous disturbances brought on by WECS' nonlinearity, the best tuning of a PI controller for DVR control is investigated to match the LVRT requirement of the HRES. The design incorporates a battery energy storage system (BESS) that is connected over the DC terminals of the DVR to improve the compensating capabilities by regulating the D-Q axis voltage signal to control the voltage source converter gate signals. In comparison to DVR alone, the suggested technique increases the DC-link voltage and PCC voltage by 44% and 4.6 %, respectively. MATLAB simulation results show that the BESS-enabled PSO DVR performs better in all respect for the compensation of voltage sag and makes the system self-LVRT capable.

Improving LVRT capability of microgrid by using bridge-type fault current limiter

A grid-connected microgrid should meet the low voltage ride through (LVRT) requirements. It means that in addition to reactive power injection during low voltage conditions, microgrid has to remain connected to the grid for a predefined time. In this paper, the bridge type fault current limiter (BFCL) is used to improve the low voltage ride through capability of grid-connected microgrids. A new method is proposed to control the BFCL and the voltage-source converters within the microgrid for this purpose. Effectiveness of the proposed method is verified by simulation results and comparison with previous methods.

Enhancing low-voltage ride-through capability of PMSG based on cost-effective fault current limiter and modified WTG control

Series single-resistor based scheme (SSRBS) can achieve a favorable low-voltage ride-through (LVRT) enhancement, also has the advantages of low costs and complexity. Its basic idea is to increase the terminal voltage of wind turbine generator (WTG) during grid faults by inserting a series single-resistor into the grid-side circuit. However, traditional SSRBSs cannot tackle different grid voltage sag events due to the constant resistance of the inserted resistor. Besides, as a device-based scheme, the SSRBSs have weak cost-effectiveness especially, utilizing the superconducting fault current limiter (SFCL). To solve these problems, this paper proposes a cost-effective LVRT scheme by cooperating a resistive-type SFCL and two modified controls. In the cooperative scheme, a surplus power dissipating mechanism based on SFCL is designed, which provides a novel way for SFCL to improve LVRT capability. Moreover, modified reactive power control for WTG is proposed to ensure that SFCL can dissipate constant surplus power irrespective of grid voltage magnitude and to provide reactive power support. Additionally, another modified WTG control is proposed for assisting with the SFCL to dissipate surplus power, which reduces the tape usage and energy-loss of SFCL, thus improving cost-effectiveness. Several cases demonstrate the feasibility and superiority of this scheme.

Salp swarm algorithm‐based optimal control scheme for LVRT capability improvement of grid‐connected photovoltaic power plants: design and experimental validation

Contribution of Photovoltaic (PV) systems is rapidly growing and great attention is given to the design of PV controllers to enhance both the performance of PV systems and the low voltage ride through (LVRT) capability during abnormal operational conditions. This article presents a novel application of the salp swarm algorithm (SSA) in order to optimally tune the PV controllers to enhance the LVRT of grid-connected PV systems. Enhancement of LVRT is indicated in percentage undershoots or overshoots, settling time and steady-state error of voltage response. A control strategy is applied to the DC-DC converter to obtain a maximum power point tracking operation through a proportional-integral (PI)-based open fractional voltage control. The grid side inverter controls both the point of common coupling voltage and the DC-link voltage through PI-based cascaded-voltage control. To get PI controller parameters that guarantee the optimum design of the controllers, the fitness function is optimized by using the SSA. The proposed optimal control scheme is tested under various fault scenarios and compared with other conventional optimization-based PI controllers to examine its validity under PSCAD environment. The effectiveness of the optimal control scheme is verified by comparing the simulation results with the practical results of the PV system.

Enhancement of Dynamic Modeling for LVRT Capability in DFIG-Based Wind Turbines

Low-voltage ride through (LVRT) is important for system compensation and reliable operation of the system during balance and unbalanced voltage dips. In this study, a new LVRT capability method was developed using active and passive compensator in doubly fed induction generator (DFIG)-based wind turbines. While the active compensator provides the control of the rotor-side converter and grid-side converters of DFIG, the passive compensator decreases the stator and rotor over currents and injects reactive power into the network to support the grid voltage DFIG. Besides, rotor electromotive force is developed to improve LVRT capability against not only symmetrical but also asymmetrical faults of DFIG. It was found that the system became stable in a short time and oscillations damped using active and passive compensator modeling.

Analysis and Control of Low-Voltage Ride-Through Capability Improvement for PMSG Based on an NPC Converter Using an Interval Type-2 Fuzzy Logic System

Wind farms must be linked to the grid during the voltage sag. Therefore, the improvement of low-voltage ride-through (LVRT) capability of wind turbine (WT) is a vitally issue. Protection system based on an interval type-2 fuzzy logic system (IT-2 FLC) is presented to improvement of LVRT capability of permanent magnet synchronous generator (PMSG) based on WT during the voltage dip. The presented control system prevents the power converters from the damaging effects of over-voltage during LVRT time. In addition to, the proposed protection system based on IT-2 FLC provides constant DC link voltage, reduces the amplitude of stator fault voltages, and enhances all response of PMSG. The proposed method is applied wind turbine using a 1.5MVA PMSG during different voltage sag types in the MATLAB/Simulation. The effectiveness of presented technique is verified by simulation results.

Low voltage ride-through capability improvement of microgrid using a hybrid coordination control strategy

This paper proposes a hybrid coordination control strategy to improve the low voltage ride-through (LVRT) capability of microgrids. During microgrid external failure, the overcurrent and the voltage sag of the microgrid can be effectively suppressed. Compared with traditional control strategies, the microgrid transient performance can also be enhanced by adopting a generalized control algorithm, performing the active switching of the energy storage (ES) operation mode and triggering the inductance-type flux coupling fault current limiters (FCLs). Among them, according to the hierarchical control structure of the grid-connected inverter, the generalized control algorithm is applied to ES. Through the two degrees of freedom's control principle and inverse plant modeling techniques, the generalized control algorithm can make the control layer of the ES inverter operate in all control targets (i.e., PQ control, droop control and Vf control), with a single control structure. Also, it eliminates the effects of microgrid distortion voltage and ES surge current on the control system of the ES inverter. For the inverter application layer, by introducing microgrid voltage and angular frequency feedforward compensation, the voltage and frequency fluctuation of the microgrid can be suppressed efficiently under the fault conditions. Therefore, with the help of the generalized control algorithm, the active switching function of ES can be carried out successfully, which is the key technology for microgrids to realize their operational mode smooth switching. Meanwhile, the voltage sag and overcurrent of microgrids can be significantly mitigated through generating reactive power from microgrids and injecting it into inductance-type FCLs with lower cost. The related theoretical derivation and simulation analysis under various scenarios (including asymmetric faults) confirm that the proposed control strategy can not only enhance the LVRT capability, but also strengthen the microgrid transient performance.

A Low Voltage Ride Through Scheme for DFIG-Based Wind Farm With SFCL and RSC Control

Taking into consideration the low voltage ride through (LVRT) demand proposed in grid codes, a resistive-type superconducting fault current limiter (R-SFCL) LVRT solution is presented in this paper for doubly fed induction generator (DFIG) based wind turbine generators. The installed position, equivalent modelling of R-SFCL, and technical principle for improving LVRT capability are described. Cooperative control between R-SFCL and two advanced control strategies-reactive power control (RPC) and inductance-emulating control (IEC)-are introduced in this paper to help the DFIG generate reactive power and protect key parameters during grid faults. Simulations and performance evaluation are also conducted compared with the single IEC scheme and single RPC-IEC scheme, which demonstrate the effectiveness and superiority of the proposed scheme in enhancing LVRT capability.

A New Control Strategy based on Reference Values Changing for Enhancing LVRT capability of DFIG in Wind Farm

Low voltage ride through (LVRT) capability enables a doubly fed induction generator (DFIG) – based wind farm (WF) to remain online supporting the electric grid during fault condition. This paper proposes a control-based strategy for improving LVRT capability of DFIG, in which by changing active and reactive powers reference values following a voltage dip (VD) condition. Moreover, the direct power control (DPC) method is modified to adaptively change the reference values based on the severity of VD. In the proposed modified DPC (MDPC) method, the parameters of the PI controllers are tuned using the imperialist competitive algorithm (ICA). For activating/deactivating the proposed strategy, a voltage dip index (VDI) is proposed which is updated within a moving time window including samples measured by phasor measurement unit (PMU). In order to evaluate rout mean square (RMS) value of the voltage from the measured values by PMU, the DFT technique is used. For activating/deactivating the control strategy, two threshold values I± and I² are defined. In the active mode, the active and reactive power are changed to zero and one p.u, while in the deactivate mode theses are changed to one and zero p.u, respectively. Based on the proposed control strategy, during a VD condition, DFIG will be able to smooth the DC-link voltage fluctuations and significantly reduces the oscillations of the stator and rotor currents. The simulation results show the effectiveness of the proposed control strategy for improving LVRT capability of DFIG.

Perturbation compensation‐based non‐linear adaptive control of ESS‐DVR for the LVRT capability improvement of wind farms

This study presents a non-linear adaptive control (NAC) for the energy storage system (ESS) embedded dynamic voltage restorer (DVR) in enhancing the low-voltage ride through (LVRT) capability of wind farms. The proposed NAC features a perturbation observer to estimate and then compensate the real perturbation of the whole system, including parameter uncertainties, measurement noise, and external disturbances such as different grid faults and intermittent wind power. It can achieve an adaptive and robust control without requiring accurate system model and full-state measurements. This control is then applied to the ESS embedded DVR (ESS-DVR) system, in which the ESS can store the blocked wind power for potential power fluctuation suppression. Simulation studies have verified that the proposed control for ESS-DVR can effectively enhance the LVRT capability of wind farms installed with different types of WTGs under different operating conditions. Moreover, its superiority has also been demonstrated by comparing with fixed gains-based conventional vector control and accurate system model-based feedback linearising control.

Low voltage ride-through capability improvement methods for DFIG based wind farm

In this paper the low voltage ride-through (LVRT) capability enhancement of the DFIG based wind farm (WF) is accomplished by using of the existing series devices like the new bridge type fault current limiter (NBFCL), new solid-state fault current limiter (N-SSFCL), resistive solid-state fault current limiter (R-type SSFCL), inductive-resistive type solid-state fault current limiter (SSFCL-LR), and series dynamic braking resistor (SDBR). The LVRT analysis is done among the series devices in terms of LVRT capability improvement performances, implementation feasibility, and cost. A temporary balanced and unbalanced fault was applied in the DFIG based 15MW test model system to demonstrate the LVRT performance among the series devices. In this work, all simulations were executed in Matlab/Simulink software and from all simulation results, it can be stated that the mentioned all series devices can augment the LVRT capability during the fault, however the performance of the NBFCL and the N-SSFCL are most impressive in comparison to others.

Application of Optimized SFCL and STATCOM for theTransient Stability and LVRTCapability Enhancement of Wind Farms

In this paper, the combined superconducting fault current limiter (SFCL) and STATCOM are used to improve transient stability and low-voltage ride-through (LVRT) capability of the wind turbine generation system (WTGS). The SFCL absorbs the accelerating energy of the generator, and therefore, improves the WTGS transient stability. Also, STATCOM helps the restore connecting point voltage to improve LVRT capability by reactive power compensation (RPC) after fault clearing. The optimization problem of SFCL resistance and STATCOM capacity is formulated based on the transient stability and LVRT capability of WTGS. The Simulation study shows the superior improving LVRT capability of combined SFCL and STATCOM over individual SFCL or STATCOM. Also, the required capacity of STATCOM for RPC is significantly reduced. PSCAD/EMTDC software (V4.2) is used for simulation.

Low voltage ride-through capability improvement of photovoltaic systems using a novel hybrid control

This paper proposes an advanced hybrid control method to enhance low voltage ride-through (LVRT) capability of grid connected photovoltaic (PV) power plants. Using the proposed method, when a low voltage fault occurs at the point of common coupling, the grid connected inverter overcurrent and its DC-link overvoltage can be effectively suppressed by tuning the PV array output power and triggering the flux coupling fault current limiters (FCLs). The AC-side voltage sag of the inverter can be significantly mitigated by generating reactive power from the inverter and injecting this into the FCLs. Employing the proposed method can also reduce the regulating quantity of output power from the grid inverter and effectively improve PV system dynamic recovery. DC-link voltage fluctuations can also be significantly smoothed by the application of feed forward compensation and the FCLs. Related theoretical derivation and simulation analysis under various scenarios, including asymmetric faults, confirm that the proposed control method can not only enhance the LVRT capability but also strengthen the transient performance of PV systems.

Application of UIPC to improve power system stability and LVRT capability of SCIG‐based wind farms

This study discusses the possibility of the connection of squirrel cage induction generator (SCIG)-based wind farms (WFs) to power system through unified inter-phase power controller (UIPC) to improve the low-voltage ride-through (LVRT) capability of WF and enhance the power system stability. The UIPC is based on inter-phase power controller (IPC), where its phase-shifting transforms (PSTs) are replaced by two series converters (SECs) and one shunt converter (SHC). The operation of the UIPC is divided into normal and LVRT operation modes. In normal operation mode, the UIPC injects the total active power generated by the WF to power system and controls the power factor of the UIPC connecting point (CP) by reactive power control. In LVRT mode, the WF which is connected through UIPC, acts as a flexible AC transmission system device with capability of active and reactive power controls at CP. In this study, the UIPC model is developed based on phase angles of injected SECs voltage and a unified control scheme including UIPC SECs, UIPC SHC and pitch angle control of wind turbine, is proposed to improve LVRT capability of WF and power system stability during faults. Also, the impact of the simultaneous application of the IPC and static synchronous compensator is studied and compared with the application of the UIPC for connecting the SCIG-based WFs to the power system. The PSCAD/EMTDC simulation results show that the UIPC presents an effective solution for connecting WFs to power system.

Low voltage ride-through capability improvement of doubly fed induction generator using series connected damping resistances

This paper proposes a control strategy to improve low voltage ride through (LVRT) capability of doubly fed induction generator (DFIG)-based wind turbines in response to grid fault occurrences or grid fault clearances to follow the requirements defined by the grid codes. The proposed scheme involves the use of series connected damping resistances (SCDR) and bypass switching devices coupled with rotor side converter (RSC), by which current peaks at both the rotor side and stator side can be reduced. The steady state behaviour under normal condition and dynamic behaviour of DFIG-based wind turbines during grid faults using the proposed strategy is simulated and assessed, and the results are compared with the scheme using crowbar circuit.

Low voltage ride-through capability improvement of doubly fed induction generator using series connected damping resistances

This paper proposes a control strategy to improve low voltage ride through (LVRT) capability of doubly fed induction generator (DFIG)-based wind turbines in response to grid fault occurrences or grid fault clearances to follow the requirements defined by the grid codes. The proposed scheme involves the use of series connected damping resistances (SCDR) and bypass switching devices coupled with rotor side converter (RSC), by which current peaks at both the rotor side and stator side can be reduced. The steady state behaviour under normal condition and dynamic behaviour of DFIG-based wind turbines during grid faults using the proposed strategy is simulated and assessed, and the results are compared with the scheme using crowbar circuit.

Implementation of SVC based on grey theory and fuzzy logic to improve LVRT capability of wind distributed generations

Due to the great absorption of reactive power after voltage drops caused by faults in network, the low-voltage ride through (LVRT) capability of squirrel cage induction generators (SCIGs) in wind farms is a great challenge. If a static VAR compensator (SVC) is installed at the point of common coupling (PCC) of a wind farm with a main network, it can improve the wind generation's LVRT capability with reactive power compensation. In the voltage control loop of a conventional SVC, the voltage actual value of the PCC is compared with the reference voltage value. This paper presents a method for implementation of a SVC based on grey theory and fuzzy logic to improve the LVRT capability of SCIG wind turbines. In this method, instead of the voltage actual value of the PCC, the voltage of the PCC is predicted by the GM (1,1) grey model. Predicted voltage is then compared with reference voltage. After obtaining voltage error, a fuzzy controller with a PI controller in the SVC voltage control loop controls the SVC output. The simulation results are compared for a conventional SVC, fuzzy SVC and fuzzy-grey SVC. These results show the superiority of the fuzzy-grey controller for the SVC in improving the LVRT capability of wind farms with SCIGs.