Ride-through Research Articles

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.

Adaptive super-twisting terminal sliding mode control and LVRT capability for switched reluctance generator based wind energy conversion system

Wind Energy Conversion systems (WECS) based on the Switched Reluctance Generator (SRG) devoid of rotor windings and permanent magnets are becoming more and more an attractive generator in wind energy market. Currently, the applicability of this machine in the WECS is now possible thanks to the mitigation of its drawbacks, mainly its high torque fluctuations and its low power factor. In this paper, an Adaptive Super Twisting Terminal Sliding Mode (AST-TSM) control is proposed for SRG-based WECS interfaced to the grid via a full-scale back-to-back converter. The main purpose of the proposed scheme is controlling the generator speed in order to achieve a Maximum Power Point Tracking (MPPT) with better performances. The second issue addressed consists of exploring the Low Voltage Ride Through (LVRT) capability of SRG-based WECS for compliance with recent grid code requirements. In modern grid codes, wind farms are only allowed to be connected to the grid if they have the capacity to remain in service during a short grid disturbance. As a matter of fact, under grid faults, the main tasks of the LVRT method consist in feeding the required reactive power into the grid in order to facilitate the recovery of the grid voltage. In this work, the investigated LVRT scheme, classified as a software method, is responsible for swapping the back-to back converter duties without external devices under grid voltage sags. Thereby, the generated power is stored as kinetic form, preventing strong overvoltage on the DC-bus. The proposed AST-TSM control scheme and the adopted LVRT method are substantiated by simulation results. All simulation case studies show that the AST-TSM control achieves good performance and extreme robustness as compared with the Proportional Integral (PI) and Integral Backstepping (IBC) controllers. Furthermore, the proposed controller is able to improve the suppression of chattering effect when its parameters are adjusted carefully. Also, simulation under grid faults conditions has proven that the LVRT method is effective for SRG-based WECS.

High-voltage ride-through strategy for wind turbine with fully-rated converter based on current operating range

During grid high-voltage ride-through (HVRT), a wind turbine (WT) with fully-rated converter (FRC) would experience an overvoltage at the DC link, which can threaten the safety of the DC link capacitors and the connected power modules. This paper investigates the process of the DC link voltage rise. Two concepts, current operating range (COR) and performance factor, are introduced to help propose a grid-side converter control strategy, in which the DC link overvoltage is greatly released or eliminated through injection of quantitative reactive current derived from the current margin of fully-rated converter and the grid voltage. The injected levels of reactive current into the grid are determined under different grid overvoltage conditions with reduced magnitude and occurrence of the DC link overvoltage. Thereby, the high-voltage ride-through capability of the fully-rated converter wind turbine is significantly improved. The detailed verification is obtained through simulations using DIgSILENT PowerFactory.

Superimposed components-based distance protection of lines emanating from DFIG-based wind farms

Type-III wind turbines with doubly fed induction generators (DFIGs) are generally equipped with crowbar circuits in practice to realize the low voltage ride-through (LVRT) during the fault period. Under this situation, DFIGs exhibit significantly different fault behavior by contrast with synchronous generators (SGs). This difference poses inevitable challenges to the existing distance protections that are normally designed based on SGs’ fault signature. In this paper, some easily overlooked issues regarding the superimposed components-based distance protection (SCBDP) when applied in lines emanating from DFIG-based wind farms (WFs) are revealed. It is shown that the unified phase angle property in the additional fault network may be jeopardized in the presence of DFIG-based WFs, thereby resulting in the high probability of failure under the in-zone fault conditions or undesirable operation under the out-of-zone fault conditions for the conventional SCBDP. To address these problems, based on the “phase angle congruency” idea, this paper proposes an improved SCBDP to mitigate the adverse impact of the crowbar-activated DFIGs. By adaptively regulating the phase angle of operating voltage, the improved SCBDP can work properly independent from the fault types and the sizes of crowbar resistance. Also, compared with conventional SCBDP, the improved SCBDP shows higher sensitivity and reliability while inheriting its strong tolerance capability against fault resistance. Extensive simulation results under different fault conditions show the expected performance of the improved SCBDP.

Grid-Connected PV System with Reactive Power Management and an Optimized SRF-PLL Using Genetic Algorithm

This paper presents a two-stage grid-connected PV system with reactive power management capability. The proposed model can send phase-shifted current to the grid during a low-voltage ride through (LVRT) to recover the voltage levels of the grid’s feeders. The novelty of the proposed algorithm, unlike the common methods, is that it does not need to disable the maximum power point tracking (MPPT) state while managing active and reactive power injection simultaneously. Moreover, the new method promotes a safety factor by offering overcurrent protection to the PV inverter. The phase-locked loop based on the synchronous reference frame (SRF-PLL) is optimized using a genetic algorithm (GA). The settling time of SRF-PLL’s step response is minimized, and the frequency dynamics are improved to enhance synchronization during LVRT. The system’s performance is tested and verified using MATLAB/Simulink simulations. The obtained results prove the effectiveness of the proposed control algorithm in managing reactive power interventions. The optimized phase-locked loop shows robust performance and is compared to the conventional low-gain PLL to spot the enhancement.

Dynamic Phasor Model of Grid-Connected Photovoltaic System in MATLAB/Simulink Considering Low Voltage Ride through Characteristics for Voltage Stability Analysis of Power System

This paper proposes a model of grid-connected photovoltaic (PV) system with low voltage ride through (LVRT) characteristic in MATLAB/Simulink based on dynamic phasor approach. Unlike all PV system models available in MATLAB/Simulink using detailed model with small capacity, the proposed model focuses on control loops of PV system to simulate a large capacity system that is appropriate for voltage stability analysis of power system installing high penetration level of PV generation. An intuitive and convenient tool for adjusting the LVRT characteristics of the PV system is also developed. The effectiveness of dynamic phasor model and the impact of the LVRT characteristic of the PV system on voltage stability are examined through MATLAB/Simulink simulations conducted on IEEE 9-bus test system. The simulation results provide an insight of how the system integrated with a large PV generation behaves under the disturbances. It also points out that the LVRT of the PV system is very important to the power system voltage stability.

Simulation of a Low-Voltage Direct Current System Using T-SFCL to Enhance Low Voltage Ride through Capability

Owing to the increasing penetration level of distributed energy resources (DER) and direct current (DC) load, the usage of low-voltage direct current (LVDC) systems has expanded to achieve efficient operations. However, because the LVDC system reaches the peak fault current at a faster rate than the alternating current (AC) system, a solution that protects the system components is necessary to maintain system integrity. It is required by the low-voltage ride-through (LVRT) that the DERs maintain their interconnections with the LVDC system and support fault recovery. In this study, a method is proposed to allow the application of the superconducting fault current limiter (SFCL) to reduce the fault current and enhance the LVRT capability. However, when the DER maintain a connection to support fault recovery, the conventional resistive-type SFCL must withstand the burden of high-temperature superconducting (HTSC) operation during fault state dependence on LVRT. Therefore, this study proposes a trigger-type SFCL to reduce the burden of the HTSC element and enhance the LVRT capability. The voltage sag related to the LVRT was improved owing to the SFCL. The proposed solution was confirmed using PSCAD/EMTDC, which is a commercial software.

Modified Super Twisting algorithm based sliding mode control for LVRT enhancement of DFIG driven wind system

The grid-connected Doubly Fed Induction Generator (DFIG) system is exposed to severe grid faults. The DFIG is sensitive to grid disturbances, which lead to consideration in the controller design. This paper proposed the Modified Super Twisting (MST) algorithm for the Low Voltage Ride Through (LVRT) enhancement under voltage sag condition. This proposed algorithm is implemented using the 2nd Order Sliding Mode (SOSM) to control the DFIG based wind generator. The higher-order switching functions are introduced in the SOSM for sliding surface control. Moreover, the Lyapunov analysis for the MST algorithm brings down the chattering amplitude. The advantages of the proposed algorithm are that it reduces the system uncertainties, chattering effect and improves the settling period of the system parameters. The performance of the proposed algorithm is compared with existing algorithms in the literature with the help of MATLAB/SIMULINK. The Hardware-In-Loop (HIL) is used to validate the simulation results, which have been performed on the OPAL-RT setup. In addition, the proposed algorithm is also tested on an equivalent model of the practical Wind Farm (WF). Based on the studies, it is found that the proposed algorithm enhanced the LVRT performance of the single Wind Turbine (WT)-DFIG system as well as the practical WF under transient conditions.

Impact of on-grid solar energy generation system on low voltage ride through capability

This paper represented a control strategy for photovoltaic (PV) system, this control strategy was referred to as a low voltage ride through (LVRT), it had been achieved by using three phase (3-PH) (PV) grid-connected system, where this paper discussed the way to achieve maximum output active power from the solar system, while the solar system remained connected to the grid with voltage decreasing controller techniques and this was valid until certain amount of voltage decreasing which was clarified in the results. The main goal of low voltage ride through depended on injecting reactive power to the grid, amount of the injected reactive power depended on regulations of the grid code using the control of the inverter and the strategy depended on the grid voltage drop amount. MATLAB simulation had been used to achieve what was mentioned above, which led to present various cases of achieving maximum output active power with grid voltage drops by using conventional proportional integral (PI) control of the inverter. Finally, another control method, which was proportional integral genetic algorithm (PI-GA), had been used to improve value of the generated output active power.

계통 연계형 3레벨 TNPC 인버터의 LVRT & HVRT

With the increase in renewable power generation facilities, strict regulations on grid connection standards are being presented. The standards of Low Voltage Ride Through(LVRT) and High Voltage Ride Through(HVRT), which are representative grid connection standards to improve grid stability by maintaining grid connection in low and high voltage situations when a grid failure occurs, are gradually being subdivided. In this paper, the LVRT and HVRT control algorithms suggested in the transmission and distribution system connection regulations of KEPCO(Korea Electric Power Corporation) are applied to the grid-connected 3-level TNPC(T-Type Neutral Point Clamped) inverter. And in the VRT situation, the active and reactive current injection strategies were applied to the grid connected inverter algorithm. In addition, various simulation cases were analyzed, such as LVRT & HVRT conditions of transmission and distribution system. In the distribution system LVRT simulation, the grid voltage operated at 150ms and 1500ms, respectively, within 0.5p.u. and 0.75p.u. In the transmission system simulation, the LVRT operation and the HVRT operation were also simulated. By analyzing various simulation cases, the characteristics of the VRT cases was analyzed in detail and VRT control performance of the designed TNPC inverter was verified.

Adjustable unbalanced current controller for tranformerless PV generation to suppress DC voltage ripples of inverter in low‐voltage ride‐through (LVRT) operation

Low voltage ride-trough (LVRT) feature is compulsory for each grid-connected inverter to support the grid by injecting reactive power in low voltage conditions. It is well known that the operation of an inverter in a normal grid condition is different from its operation in a faulty one. One of the detrimental effects of working in a faulty grid is ripples in the DC bus voltage due to the active power oscillation caused by unbalance voltages. Here, a new control system based on unbalance current injection is proposed to eliminate these ripples. For the analysis, a four-leg inverter is considered in a transformerless microgrid. Then, instantaneous power constraints, which are imposed by the capacity of the inverter, and low voltage ride-through necessities are considered to achieve the required reference currents. In addition, constant power strategy is used as a power control method and essential reference currents for this condition are presented. Moreover, the required reference currents are achieved to eliminate the ripples in the DC bus voltage when the constant power strategy cannot be performed because of current limitation restriction. Finally, simulation results are presented to prove the effectiveness of the proposed controller in different conditions.

A Novel Synchronous Type Variable Speed Wind Turbine and Its Control Strategy Study

Compared with traditional power plants, current main stream wind turbines vary in grid connection interfaces, resulting in deficiencies like weak grid voltage and frequency support capability. In order to solve the problem, a synchronous type variable speed wind turbine is proposed in this paper. The wind turbine has the advantages such as the intrinsic properties of maintaining grid voltage and frequency, no harmonic pollution on grid, black start capability. In the wind turbine drive train, an electrically excited synchronous generator is directly coupled to the grid. A power self‐balance type electromagnetic coupling device is used to connect the synchronous generator rotor shaft and a gearbox high speed shaft. The device is composed of a permanent magnet synchronous generator (PMSG), a back‐to‐back converter and an electromagnetic coupler. The converter with small capacity, supplied by the PMSG which is coaxial with the gearbox high speed shaft, is used to apply the electromagnetic torque of the electromagnetic coupler. The matching relationship between the electromagnetic torque and the turbine mechanical torque, and the power flow in the drive train in different wind speeds are analyzed. Control strategies are proposed for the wind turbine normal operation including start, stop, maximum wind power capture, and low voltage ride through (LVRT). Compared with the main stream wind turbines, blade and tower loads can be improved greatly during LVRT. A 2 MW wind turbine model is built in Matlab/Simulink to verify the control strategies proposed and the wind turbine excellent LVRT performance. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Coordinate Control of Grid Power, Battery SoC and LVRT Protection in Single VSC Tied DFIG

This paper proposes a coordinate control scheme for the single VSC tieddoubly-fed induction generator (DFIG). In this control scheme, both the gridpower and battery SoC (State of Charge) are maintained to provide an un-interrupted power supply. During the continuous operation of DFIG in thesub synchronous region, there is scope for complete battery discharge. Henceto overcome this drawback, the coordinated control scheme maintains thebattery SoC level within the limits. If the SoC falls below the specified lowerlimit, then the proposed scheme curtails the grid power. Instead of dischargingthe battery, the control shifts the battery to charging mode until the safelimit of SoC is attained. During the continuous operation of DFIG in thesuper synchronous region, if the SoC reaches its upper limit, the proposedscheme discharges the extra power to the dump load. Further, this controlscheme also introduces the low voltage ride through (LVRT) aspect accordingto IEGC (Indian electricity grid code) of 15% of nominal voltage and also an enhanced rotor position computation is implemented for the effectiveestimation of rotor position for single VSC tied DFIG. This control makesthe topology more robust and improves the reliability of the system. Theproposed scheme is validated for a test system of 3.7 kW Wound RotorInduction Machine based DG unit and investigations are done in MATLABsimulation.

A protection scheme for active distribution network based on the cooperative control of inverter-interfaced distributed generators

Due to the existence of bidirectional fault currents in active distribution networks (ADN), a key issue of its protection is how to identify the fault direction. In traditional power systems, the positive sequence fault component (PSFC) based protection schemes are widely used, since they have good performance under different types of fault. However, due to the diversity of voltage variants and the influence from low-voltage ride-through (LVRT) requirement, the PSFC impedance angle of an inverter-interfaced distributed generator (IIDG) is difficult to determine. To solve this problem, this paper proposed a solution based on the cooperative design of the IIDG fault control strategy. Within a few power cycles after the fault occurs, each IIDG is controlled to output a specific positive sequence current according to the voltage variant. In this way, the PSFC impedance angle of IIDG is fixed to a given value under any fault scenario, thereby ensuring the applicability of PSFC-based protection in ADN. On this basis, the fault direction identification method and trip logic for the protection device are proposed. The effectiveness of the proposed method is verified by Matlab simulation results. The PSFC impedance angle of IIDG stabilizes at the desired value, and the output current of IIDG is always limited within a safe range during the whole process. Based on the sampled signals in the second power cycle after the occurrence of a fault, the protection device can use the PSFC-based criterion to accurately determine the direction of the fault.

Coordination of mode switching control and SMES unit for low‐voltage ride‐through fulfillment of power electronic transformer

AbstractPower electronic transformer (PET) should have a satisfying low‐voltage ride‐through (LVRT) ability to cope with different kinds and severities of voltage sags. To solve the LVRT issue, this paper proposes to coordinate mode switching control (MSC) and superconducting magnetic energy storage (SMES), and it is to realize the targets of promoting voltage recovery, keeping DC‐link stabilization, and alleviating current inrush. First, the theoretical modeling of a PET incorporating with a SMES unit is implemented, and the PET’s fault transient characterizes as well as the SMES’s potential supportability is analyzed. Then, the coordination philosophy of the MSC and the SMES is presented. For that the voltage sag reaches a certain threshold, the MSC is enabled to remove the PET’s normal control, and the SMES collaboratively operates to lower the DC‐link overvoltage, while avoiding the PET damage. In the process of the coordination control handling the voltage transients, a proper reactive current injection is carried out at the PET’s output stage to assist the voltage recovery, and the PET can possess a better LVRT margin. Using MATLAB, a 3 MW PET with an 8 H/1.2 MJ SMES unit is modeled, and different voltage sag scenarios and utilization ways of the MSC and the SMES are simulated. The comparative analysis results show that the proposed approach realizes a satisfying LVRT for the PET.

IEEE1547-2018 Category3 대응을 위한 연료전지 시스템의 인버터 제어 전략

This paper presents an inverter control scheme for fuel cell systems in IEEE1547-2018 Category3. The current configuration of Grid-Connected fuel cell system has limitations Category3’s LVRT (Low Voltage Ride Through) reponse, additional inverter are needed. The inverter operates in an abnormal state where the grid voltage is unstable, and normally supplies the voltage to BOP (Balance of Plant) of the fuel cell system. Simulated verification of the inverter control scheme was performed to change the voltage source to the BOP from grid to inverter during abnormal conditions.

A PMSG Wind Energy System Featuring Low-Voltage Ride-through via Mode-Shift Control

Low-voltage ride-through (LVRT) and grid support capability are becoming a necessity for grid-tied renewable energy sources to guarantee utility availability, quality and reliability. In this paper, a swap control scheme is proposed for grid-tied permanent magnet synchronous generator (PMSG) MW-level wind turbines. This scheme shifts system operation from maximum power point tracking (MPPT) mode to LVRT mode, during utility voltage sags. In this mode, the rectifier-boost machine-side converter overtakes DC-link voltage regulation independently of the grid-side converter. The latter attains grid synchronization by controlling active power injection into the grid to agree with grid current limits while supporting reactive power injection according to the sag depth. Thus grid code requirements are met and power converters safety is guaranteed. Moreover, the proposed approach uses the turbine-generator rotor inertia to store surplus energy during grid voltage dips; thus, there is no need for extra hardware storage devices. This proposed solution is applied on a converter topology featuring a minimal number of active switches, compared to the popular back-to-back converter topology. This adds to system compatibility, reducing its size, cost and switching losses. Simulation and experimental results are presented to validate the proposed approach during normal and LVRT operation.

Enhancing Doubly Fed Induction Generator Low-Voltage Ride-through Capability Using Dynamic Voltage Restorer with Adaptive Noise Cancellation Technique

Some of the major challenges facing micro-grids (MGs) during their connection with the utility grid are maintaining power system stability and reliability. One term that is frequently discussed in literature is the low-voltage ride-through (LVRT) capability, as it is required by the utility grid to maintain its proper operation and system stability. Furthermore, due to their inherent advantages, doubly fed induction generators (DFIGs) have been widely installed on many wind farms. However, grid voltage dips and distortion have a negative impact on the operation of the DFIG. A dynamic voltage restorer (DVR) is a commonly used device that can enhance the LVRT capability of DFIG compared to shunt capacitors and static synchronous compensator (STATCOM). DVR implements a series compensation during fault conditions by injecting the proper voltage at the point of common coupling (PCC) in order to preserve stable terminal voltage. In this paper, we propose a DVR control method based on the adaptive noise cancelation (ANC) technique to compensate for both voltage variation and harmonic mitigation at DFIG terminals. Additionally, we propose an online control of the DC side voltage of the DVR using pulse width modulation (PWM) rectifier to reduce both the size of the storage element and the solid-state switches of the DVR, aiming to reduce its overall cost. A thorough analysis of the operation and response of the proposed DVR is performed using MATLAB/SIMULINK under different operating conditions of the grid. The simulation results verify the superiority and robustness of the proposed technique to enhance the LVRT capability of the DFIG during system transients and faults.

Doubly Fed Induction Generator Low Voltage Ride Through Improvement Through Modular Multilevel Converter

This paper proposes a novel fault current suppression method for doubly fed induction generator (DFIG) low voltage ride through (LVRT) improvement employing modular multilevel converter (MMC). It is based on the inherent MMC arm impedance and the number of levels to provide transient damping of DFIG rotor and stator currents under severe grid faults, contributing effectively to maintaining the DFIG connection while ensuring controllability by avoiding the protection activation. Neither additional hardware nor control loops are necessary. DFIG dynamic equations under LVRT, including the contribution of the MMC, are presented, and a model is derived for control design. Performance assessment, including symmetrical and asymmetrical fault scenarios for MMC with 3, 21, 51, and 101 levels and comparison with existing and proposed controllers, highlighted that arm impedance and increased MMC level number contributed to DFIG stator and rotor fault current suppression, providing DFIG LVRT improvement under severe grid fault conditions.

Matching Analysis of LVRT Grid Code and Injection Current Dependent Voltage Response of WTC Connected to High Impedance AC Grid

With increasing penetration of wind power, wind turbines are usually required to stay connected and provide reactive current support according to LVRT (low voltage ride through) grid code during grid faults. However, for WTC (wind turbine converter) connected to high impedance AC grid, the voltage dependent current injection required by LVRT grid code may conflict with the injection current dependent voltage response of WTC, resulting in LVRT failure. In this letter, the matching of them is studied in detail. First, the voltage dependent current injection defined by LVRT grid code and injection current dependent voltage response of WTC are developed, respectively. Then the matching of them is revealed from the source-load matching analysis of equivalent circuit. It is identified that there exists constraints on the selection of K-factor to ensure the matching during low voltage grid faults. Simulated results are also presented to verify the analysis.