Low Voltage Ride Through Control Strategy Research Articles

Low Voltage Ride Through Control Strategy for a Two-Stage Grid-Tied Solar Photovoltaic System with Grid Faults

This paper investigates a low voltage ride-through (LVRT) control strategy for a two-stage grid-tied solar photovoltaic (SPV) system with line-to-ground (L-G) and double line-to-ground (L-L-G) faults. A proportional resonant (PR) controller controls the SPV inverter based on classical and reactive power oscillation-based reference current generating techniques. The proposed method injects reactive power as an ancillary service to provide voltage support and prevent power outages, reduces the DC component injection into the grid currents, and finally injects sinusoidal currents into the grid, avoid voltage oscillations in the DC-link and limit the inverter current by operating the boost converter in non-MPPT mode during L-G & L-LG faults. Experimental results for reference current generation using classical method and proposed method are compared to highlight the proposed controller’s ability to maintain power quality during faults.

Control strategy for current limitation and maximum capacity utilization of grid connected PV inverter under unbalanced grid conditions

Under grid voltage sags, over current protection and exploiting the maximum capacity of the inverter are the two main goals of grid-connected PV inverters. To facilitate low-voltage ride-through (LVRT), it is imperative to ensure that inverter currents are sinusoidal and remain within permissible limits throughout the inverter operation. An improved LVRT control strategy for a two-stage three-phase grid-connected PV system is presented here to address these challenges. To provide over current limitation as well as to ensure maximum exploitation of the inverter capacity, a control strategy is proposed, and performance the strategy is evaluated based on the three generation scenarios on a 2-kW grid connected PV system. An active power curtailment (APC) loop is activated only in high power generation scenario to limit the current’s amplitude below the inverter’s rated current. The superior performance of the proposed strategy is established by comparison with two recent LVRT control strategies. The proposed method not only injects necessary active and reactive power but also minimizes overcurrent with increased exploitation of the inverter’s capacity under unbalanced grid voltage sag.

A New Short-Circuit Current Calculation and Fault Analysis Method Suitable for Doubly Fed Wind Farm Groups.

The transient characteristics of wind farms in groups are quite different; in addition, there is a strong coupling between the wind farms and the grid, and these factors make the fault analysis of the grid with wind farm groups complicated. In order to solve this problem, a mathematical model of the converter is established based on the input-output external characteristics of the converter, and a transient model of a doubly fed wind turbine (DFIG) is presented considering the influence of the low-voltage ride-through control (LVRT) of the converter, and the effect mechanism of the LVRT strategy on the short-circuit current is analyzed. Finally, a short-circuit current calculation model of a doubly fed wind turbine with low-voltage crossing control is established. The interaction mechanism between wind farms during the fault is analyzed, and a short-circuit current calculation method of doubly fed wind farm groups is proposed. RTDS is used to verify the accuracy of the proposed short-circuit current calculation method for doubly fed field groups. On this basis, a method of power grid fault analysis after doubly fed field group access is discussed and analyzed.

A Low Voltage Ride Through Control Strategy of Optical Storage Microgrid

When the distribution network is disturbed or fails, the microgrid connected to it will suddenly leave the distribution network and cause some serious consequences. Aiming at the low voltage ride through (LVRT) capability of the microgrid when the voltage of the distribution network drops, a control strategy of LVRT for the optical storage microgrid based on Quasi-resonant control (QPR) is proposed. During the voltage sag period of the optical storage microgrid, the active power at the DC side is absorbed by the energy-storage device to maintain the voltage stability of the DC bus, and the injection power of the converter is reduced to avoid over-current; The photovoltaic inverter collects the voltage and current at the junction point in the two-phase static coordinate system as the outer loop feedback and the filtered capacitor current as the inner loop feedback to provide the internal equivalent virtual impedance, thus providing the required damping for the filter circuit, and modulates it into a PWM wave to directly control the AC quantity to reduce the impact of the harmonic component on the optical storage microgrid. Finally, the simulation model is built in Matlab/Simulink, The research shows that the control strategy proposed in this paper can effectively maintain the stability of the DC bus voltage and limit the current of the junction point, which confirms the rationality of the LVRT 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.

Research on the influence of load model with distributed PV generation on the voltage stability of receiving-end power grid

The low voltage ride-through (LVRT) characteristics of DPV are first analyzed in this paper, and then the influence of the LVRT control strategies and parameters of PDGs on the grid voltage stability is studied. Finally, the influence of load model with and without DG on grid voltage stability is studied, the simulation results show that it may be too optimistic when new energy sources are not considered.

Low-voltage ride through of multi-port power electronic transformer

A low-voltage ride-through (LVRT) control strategy for the multi-port power electronic transformer (PET) based on power co-regulation is proposed. During the sag and recovery of the grid-side voltage of the medium-voltage ac (MVac) port, the grid-connected active power of the low-voltage ac (LVac) port, rather than the power from external renewable energy sources (e.g., photovoltaic (PV)), is adjusted quickly to rebalance the power flowing across all ports, thereby preventing overcurrent and overvoltage. Moreover, a power-coordinate-frame-based LVRT mode classification is designed, and a total of six LVRT modes are classified to meet the LVRT requirements in all power configuration scenarios of the PET. In this way, the PET is endowed with the LVRT capability in both power-generation and power-consumption states, which is significantly different from traditional power generation systems such as PV or wind power. Furthermore, by optimizing the active power regulation path during LVRT transition, the overcurrent problem caused by the grid-voltage sag-depth detection delay is overcome. Finally, the effectiveness of the proposed control scheme is verified by experiments on a hardware-in-the-loop platform.

Low voltage ride-through control strategy for a wind turbine with permanent magnet synchronous generator based on operating simultaneously of rotor energy storage and a discharging resistance

When the low voltage ride-through(LVRT) method of combining rotor energy storage with a discharging resistance for a wind turbine with permanent magnet synchronous generator(PMSG) is adopted, with the traditional control strategy, there are the shortcomings of the rotor energy storage function not exerted to the maximum extent and the large discharging current flowing through the discharging resistance. On the above shortcomings, a LVRT control strategy for a wind turbine with PMSG based on operating simultaneously of rotor energy storage and a discharging resistance is presented. On the basis of the rotor motion equation, the setting calculation formula of the active power of machine-side converter for maximizing the performance of the rotor energy storage is derived, and the switching method of the discharging resistance is designed. Simulation examples of a wind turbine with PMSG rating of 1.5MW are constructed using PSCAD/EMTDC to compare the proposed LVRT control strategy with the traditional control strategy according to the LVRT requirements of China National Standard for a wind turbine generator system. The simulation results show that the proposed LVRT strategy can not only exert the rotor energy storage function to the maximum extent, but also significantly reduce the discharging current, which verifies the effectiveness of the proposed LVRT control strategy.

Experimental study on short-circuit current characteristics of a photovoltaic system with low voltage ride through capability under a symmetrical fault

Large number of photovoltaic (PV) power plants connected to a power grid can bring significant impacts to fault currents and the operation of protection systems. In this paper, short-circuit current characteristics of a PV system with low voltage ride through (LVRT) capability under a symmetrical fault is studied. PV system short-circuit experiments with different voltage dips at high and low output power levels are designed and conducted. The experiment results provide useful and valuable references for researches of PV system short-circuit current characteristics, modeling and PV system short-circuit current contribution to a power grid. Then, based on the experiment results, steady-state values, peak values and steady state short-circuit currents harmonic characteristics at different conditions are obtained and analyzed. It is found that, the DC component amplitude of a PV system short-circuit current is almost negligible compared with that of the power frequency component, which is different from conventional synchronous generator short-circuit current. Meanwhile, based on the LVRT control strategy and experimental results, the expression of steady-state short-circuit current RMS value is derived and verified, which provides an important basis for short-circuit current calculation of power system with large scale of PV plants.

The Whole-Process Low-Voltage Ride-Through Control Strategy for Photovoltaic Power Generation Based on Adaptive Maximum Power Tracking

The impact of the large-scale disconnection of photovoltaic generators on the grid cannot be ignored, so the low-voltage ride-through problem of photovoltaic generators needs to be solved urgently, especially in the stage after fault recovery, there is a problem that the converter is difficult to switch back to the original control strategy. Therefore, a new control strategy of the whole-process low-voltage ride-through control strategy for photovoltaic power generation based on adaptive maximum power tracking is proposed. In the initial stage of failure, an adaptive maximum power control strategy is proposed. This control strategy transforms maximum power point tracking control into adaptive maximum power control by feedback of voltage drop amplitude. It can control the output of photovoltaic power quickly and effectively and realize the zero-voltage crossing of the photovoltaic grid-connected system. In the stage after fault recovery, the switching function is established by comparing the network voltage with the rated voltage. The operating state of the photovoltaic system can be judged by different output of the switching function. It realizes the fast switching of the photovoltaic side control strategy during the fault recovery stage. Simulation results show that the proposed method can switch the control strategy reliably and realize the whole-process low-voltage crossing of photovoltaic power generation quickly.

Renewable Microgrid Protection Strategy Coordinating with Current-based Fault Control

The renewable microgrid (RMG) is a critical way to organize and utilize new energy. Its control and protection strategies during the fault are the core technologies to ensure the safe operation and stability of the system. The traditional protection principles are unsuitable for RMGs due to the flexibility of RMG operation, the complexity of RMG topology, and the variety of fault control strategies of inverter-interfaced distributed generators (IIDGs). The traditional fault component protection principle is affected by the low voltage ride-through (LVRT) control strategy and will fail in some scenarios. In order to make the fault component protection principle available in every scenario, a current-based fault control strategy is proposed. Specific fault characteristics are generated by the grid-feeding IIDGs during the fault so they can be equivalent to the open circuits, and the fault models in additional network can be simplified. By analyzing the fault characteristics, an RMG protection strategy based on the current-based fault control of IIDGs is presented. The fault directions of feeders can be distinguished and the fault feeder can be located accurately in both grid-connected and islanded RMGs. Then, the grid-feeding IIDGs can transit to LVRT mode smoothly. Thus, IIDGs are considered comprehensively in terms of coordinating with fault control and fault characteristic generation. Finally, the experimental results of the hardware platform prove the effectiveness of the proposed current-based fault control strategy, and the simulation results based on PSCAD/EMTDC verify the correctness of the protection strategy.

Low voltage ride-through control strategy based on cascaded H-bridge that can avoid overmodulation and power backflow

Because of its multi-level output voltage and modular structure, the cascaded H-bridge(CHB) multi-level inverter can expand the system to higher voltage and power levels. It has good development and application prospects in the photovoltaic grid-connected market. Under the power grid fault voltage sag, the system should generate reactive power according to national codes and meet the dynamic performance requirements to maintain the connection with the grid, and achieve the minimum target requirements of low voltage ride through(LVRT). Aiming at the over-modulation and the unique power backflow problem of cascaded H-bridges, a LVRT control algorithm that can avoid over-modulation and power backflow is studied, and MATLAB simulation verification is carried out under common grid faults, which proves the correctness of the theory.

Low voltage ride-through control strategy for virtual synchronous generators based on virtual self-inductive flux linkage

Virtual synchronous generators (VSGs), with the operational characteristics of synchronous generators (SGs), have been employed in renewable energy generation grid-connected systems to solve the problem of insufficient equivalent inertia, which is caused by the high permeability of distributed generation systems in the grid. However, a VSG does not possess low voltage ride-through (LVRT) capability. A novel LVRT control strategy for a VSG based on virtual self-inductive flux linkage is proposed in this paper. First, the electromagnetic transient response mechanism of a SG under grid faults is analyzed in detail. Then, a memory and retention strategy are proposed to simulate the effect of the switch law. Furthermore, the virtual q-axis armature self-inductance is introduced into the VSG and a virtual self-inductive magnetic chain is utilized to block the change of the transient fault current. This helps the inverter adjust the output voltage quickly. In addition, under the premise of meeting the reactive power margin, the reactive power compensation strategy is optimized to achieve a quick response in terms of the reactive power compensation of grid faults, which is helpful for realizing the LVRT of a VSG. Finally, the feasibility and effectiveness of the proposed LVRT method are verified by thorough simulation results.

Active Power Backflow Control Strategy for Cascaded Photovoltaic Solid-State Transformer During Low-Voltage Ride Through

Different from the conventional photovoltaic (PV) inverters, a three-phase PV solid-state transformer (SST) based on the cascaded H-bridge (CHB) topology can be regarded as consisting of three single-phase CHB inverters in essence. During the low-voltage ride through (LVRT), some phases of three-phase PV SST may inversely absorb active power from ac grid affected by negative-sequence voltages, resulting in that the dc bus voltages of all H-bridges in these phases continue to rise, and that the system will shut down due to overvoltage protection. To deal with this problem, in this article, we study the mechanism of active power backflow during LVRT, analyzes and compares the main methods for suppressing active power backflow, and then proposes an LVRT control strategy based on the zero-sequence voltage compensation, which not only ensures that all modules in three-phase PV SST transmit the almost same active power but also can avoid effectively active power backflow problem even if the total output active of PV array is relatively low. The validity of the proposed method is verified by the simulation and experimental results.

A Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems

Due to the high penetration of grid-connected photovoltaic (GCPV) systems, the network operators are regularly updating the grid codes to ensure that the operation of GCPV systems will assist in maintaining grid stability. Among these, low-voltage-ride-through (LVRT) is an essential attribute of PV inverters that allows them to remain connected with the grid during short-term disturbances in the grid voltage. Hence, PV inverters are equipped with control strategies that secure their smooth operation through this ride-through period as per the specified grid code. However, during the injection of reactive power under LVRT condition, various challenges have been observed, such as inverter overcurrent, unbalance phase voltages at the point of common coupling (PCC), overvoltage in healthy phases, oscillations in active, reactive power and dc-link voltage, distortion in injected currents and poor dynamic response of the system. Several strategies are found in the literature to overcome these challenges associated with LVRT. This paper critically reviews the recent challenges and the associated strategies under LVRT conditions in GCPV inverters. The drawbacks associated with the conventional current control strategies are investigated in MATLAB/Simulink environment. The advanced LVRT control strategies are categorized and analyzed under different types of grid faults. The work categorizes the state-of-the-art LVRT techniques on the basis of the synchronization methods, current injection techniques and dc-link voltage control strategies. It is found that state-of-the-art control strategies like OVSS/OCCIDGS provides improved voltage support and current limitation, which results in smooth LVRT operation by injecting currents of enhanced power quality.

Development of HVRT and LVRT Control Strategy for PMSG-Based Wind Turbine Generators

Various challenges are acknowledged in practical cases with high wind power penetration. Fault ride-through (FRT) capability has become the most dominant grid integration requirements for the wind energy conversion system worldwide. The high voltage ride-through (HVRT) and low voltage ride-through (LVRT) performance play a vital role in the grid-friendly integration into the system. In this paper, a coordinated HVRT and LVRT control strategy is proposed to enhance the FRT capability of the permanent magnet synchronous generator (PMSG)-based wind turbine generators (WTG). A dual-mode chopper protection is developed to avoid DC-link overvoltage, and a deadband protection is proposed to prevent oscillations under edge voltage conditions. The proposed strategy can ride through different levels of voltage sags or swells and provide auxiliary dynamic reactive power support simultaneously. The performance of the proposed control scheme is validated through various comparison case tests in PSCAD/EMTDC.

Design and implementation of fuzzy logic based modified real-reactive power control of inverter for low voltage ride through enhancement in grid connected solar PV system

In a grid connected solar PV system, during grid faults, increase in grid current, dip in voltage at point of common coupling and overshoot in dc link voltage in the inverter side are observed. The solar PV system gets disconnected from the grid whenever the grid voltage goes below a certain nominal value in order to safeguard the cascaded power converter systems from high currents and over voltages. This paper proposes an intelligent fuzzy based real and reactive power control of inverter for effective Low Voltage Ride Through (LVRT) capability during grid faults. The proposed fuzzy logic based modified inverter real and reactive power control strategy, stabilizes the grid voltage by injecting reactive current and limits the grid over current by reducing active power flow into the grid. To regulate the overshoot in dc link voltage during grid faults, a braking chopper and dynamic braking resistor strategies are included in the proposed LVRT scheme. The performance of the proposed inverter control strategy is studied through simulation of 100 kW grid connected solar PV system in MATLAB/SIMULINK 2018a platform. The algorithm is validated in real time hardware of 1 kW grid connected solar PV system. The simulation and hardware results validate the superior performance of the proposed inverter LVRT control strategy in comparison with other conventional schemes during grid faults.

Disturbance feedforward control for type-3 wind turbines to achieve accurate implementation of transient control targets during LVRT

The existing low voltage ride-through (LVRT) control strategies usually modify the post-fault current reference to contain some transient and negative-sequence components, to achieve the controllable LVRT operating of type-3 wind turbine (WT) under severe grid fault. However, the modified current reference and post-fault electromotive force (EMF) disturbance behave as the multi-frequency (i.e., 0-, 50- and 100-Hz) components in the synchronous dq-frame, thus the classic PI controller-based current control cannot provide zero-error tracking of the aforesaid reference, and leading to the inaccurate implementation of transient control targets. To solve this problem, based on the relationship between the modified current reference and post-fault EMF, an improved disturbance feedforward control is proposed to simultaneously suppress EMF disturbance and improve the tracking capability of modified current reference. As a result, the transient control targets of the improved LVRT control strategies can be implemented accurately. Moreover, the method is easy to implement. Finally, the proposed method is verified by the simulations on a 1.5-MW type-3 WT.

Impacts of a LVRT Control Strategy of Offshore Wind Farms on the HTS Power Cable

High temperature superconducting (HTS) power cables are a potential solution for the grid integration of offshore wind farms since the HTS cable can conduct bulk wind power at low voltage levels. However, the transient current through the HTS cable in cases of low voltage ride through (LVRT) operation has a negative impact on the HTS cable operation due to the quenching phenomenon. This paper analyzes the impact of LVRT control strategies on the HTS cable operation. In addition, a coordinated control of wind turbines for LVRT improvement of an offshore wind farm is proposed. The feasibility of the HTS cable application for the grid connection of offshore wind farms is also discussed in this study. The proposed controller is designed for the wind turbine generator based on a type-4 permanent magnet synchronous generator. In the proposed controller, the transient current through the HTS cable is reduced by regulating the machine side power during fault conditions. The feasibility of the proposed controller is validated in the PSCAD/EMTDC program (Manitoba Hydro International Ltd., Winnipeg, Manitoba, Canada, version 4.2.1). The effects of transient current on the cable temperatures and resistances are analyzed in this study. Simulation results show that the proposed control strategy could reduce the transient current and temperature rise of the HTS cable.

Crowbarless Symmetrical Low-Voltage Ride Through Based on Flux Linkage Tracking For Brushless Doubly Fed Induction Generators

The significant benefits of brushless doubly fed induction generators (BDFIGs) are higher reliability and lower maintenance cost due to the lack of a brush gear. Recent studies on BDFIGs have shown promising applications in wind power generation. The low-voltage ride-through (LVRT) capability of wind turbines is one of the most important properties specified in the grid codes of many countries. A crowbarless LVRT control strategy based on flux linkage tracking for a BDFIG under symmetrical voltage dips is proposed in this article. A novel and simple controller in static reference frame is presented to implement flux linkage tracking. To both suppress the control winding (CW) current and increase power winding (PW) reactive current, a time-sharing control and CW current-first control solution is proposed. In the most severe case, where the BDFIG rotor speed is 100% of the maximum rotor speed and under 100% symmetrical voltage dips, the experimental results show that the CW current peak can be limited to 2.01 p.u. of the CW current rating and the torque ripple is small during the fault. The experimental results also show the possibility of the BDFIG to ride through the severest voltage dips without extra hardware, such as a crowbar.