Low Voltage Ride Research Articles

Strategies to Increase the Transient Active Power of Photovoltaic Units during Low Voltage Ride Through

Due to a limitation in the magnitude of the three-phase output inverter currents, the output active power of the photovoltaic (PV) unit has been de-rated during low voltage ride through, which brings great instability risk to the power system. With the increase in the penetration rate of new energy, the impact of the power shortage on the system transient stability increases. It is of great significance to analyze the impact of this transient power shortage on system stability. This article explores methods to improve the active power output capability of photovoltaic units during low-breakthrough periods. A transient simulation model of a grid-connected PV generator with low-voltage ride-through (LVRT) capability is presented, under the condition of meeting the overcurrent capacity of the PV inverter and the requirement of dynamic reactive power support supplied by the PV generator specified in the China grid codes (GB/T 19964-2012) during grid fault. An example system with high PV penetration is built. The change principle and influencing factors of PV transient active power output are analyzed. The simulation model is designed in PowerFactory/DIgSILENT, and several types of three-phase voltage sags are performed in simulation to assess the impact of the active current reference calculation method and the maximum inverter output current (Imax) limit value on the PV active power output. According to the three indexes, namely the maximum active power of PV unit during the fault, the power improvement gradient and the power surge after the fault is cleared. Simulation results showed that using the orthogonal decomposition method to calculate the active current reference can make full use of the current capacity of the converter. Setting Imax to 1.1 rated current of photovoltaic inverter (IN) can reduce the cost-effectiveness ratio of the transient active power output of the PV unit. Therefore, we aim to improve the unit’s transient active power output capacity and realize the optimal effect of improving the transient active power shortage of the system.

초대형 풍력발전기의 성능평가를 위한 단권변압기 기반 Low and High Voltage Ride Through Test System 구현

국내에서는 최근 탄소중립선언에 따른 서남해 지역의 해상(Off-Shore)에 8GW이상의 풍력단지를 조성하는 계획을 가지고 있다. 이러한 배경하에 대용량 풍력발전기가 계통에 안정적으로 도입하기 위해서는 순간적인 사고로 인한 순시 전압강하 및 전압상승이 발생하는 경우에도 일정 기준 이상의 연계운전을 지속할 수 있는 능력을 가지고 있어야 한다. 따라서 본 논문에서는 대용량 풍력발전기를 대상으로 일정기준이상 연계운전 능력을 평가 할 수 있는 10MW급 이상의 LVRT(Low voltage Ride Through)/HVRT(Low voltage Ride Through)기능 모의가 가능한 시험장비의 개발 방안을 제안한다. 즉 RLC 방식의 단점을 보완한 단권변압기 기반 탭 변환 방식 시험장비의 권수비 조정 및 탭 변환 모델링과 시험장비의 차동 운영전략을 제시한다. 상기에서 제안한 설계 및 운영 방안을 바탕으로 구현한 30kW급 축소형 LVRT/HVRT 시험장비의 성능시험 결과, 대용량 풍력발전기의 LVRT/HVRT 현장시험용으로 안정적인 적용이 가능함을 확인하였다.

Dynamic Low Voltage Ride through Detection and Mitigation in Brushless Doubly Fed Induction Generators

Brushless doubly-fed induction generators have higher reliability, making them an attractive choice for not only offshore applications but also for remote locations. These machines are composed of two back-to-back voltage source converters: the grid side converter and the rotor side converter. The rotor side converter is typically used for reactive current control of the power winding using the control winding current. A low voltage ride through (LVRT) fault is detected using a hysterisis comparison of the power winding voltage. This approach leads to two problems, firstly, the use of only voltage to detect faults results in erroneous or slow response, and secondly, sub-optimal control of voltage drop because of static reference values for reactive current compensation. This paper solves these problems by using an analytical model of the voltage drop caused by a short circuit. Moreover, using a fuzzy logic controller, the proposed technique employs the voltage frequency in addition to the power winding voltage magnitude to detect LVRT conditions. The analytical model helps in reducing the power winding voltage drop while the fuzzy logic controller leads to better and faster detection of faults, leading to an overall faster response of the system. Simulations in Matlab/Simulink show that the proposed technique can reduce the voltage drop by up to 0.12 p.u. and result in significantly lower transients in the power winding voltage as compared to existing techniques.

LVRT Impact on Tower Loads, Drivetrain Torque and Rotational Speed—Measurement Results of a 2-MW Class DFIG Wind Turbine

With an increasing share in energy production, wind turbines have to fulfill strict grid requirements to support the grid in case of discontinuities. Here LVRT (Low Voltage Ride Through) compliance is one essential part, where the turbine has to stay connected to the grid in case of voltage drops and is not allowed to stop. This paper comprises measured low-voltage events from an LVRT campaign with a focus on the mechanical loads for the drivetrain and tower during these temporary and instant drops of power. Moreover, we analyze the turbine operation itself, like the rotational speed, which is essential to keep the turbine within its operational parameters and design limits.

LVRT Enhancement of a Grid-tied PMSG-based Wind Farm using Static VAR Compensator

This paper presents an efficient Low Voltage Ride Through (LVRT) control scheme for a 10.0MW grid-tied Permanent Magnet Synchronous Generator (PMSG)-based wind farm. The proposed control strategy plans to enhance the power quality and amount of injected power to satisfy the grid code requirements. The proposed approach utilizes a static Shunt Var Compensator (SVC) to enhance the LVRT capability and to improve power quality. It has been observed from the outcomes of the study that the proposed SVC controller ensures safe and reliable operation of the considered PMSG-based power system. The proposed system not only improves power quality but also it provides voltage stability of the Wind Energy Conversion System (WECS) under abnormal/fault conditions. The results show the superiority of the proposed control strategy.

A Comprehensive Modulation Strategy for the Neutral-Point Clamped Wind Power Converter Thermal Management During Low-Voltage Ride Through

The temperature fluctuations in the semiconductor devices within neutral-point clamped (NPC) wind power converter during low-voltage ride-through operation would reduce the lifetime cycle of the converter and jeopardize the reliability of the wind power system. In this article, a thermal management strategy based on carrier-based modulation techniques, i.e., zero-sequence and dipolar injections, is proposed to address this issue. The temperature fluctuations in semiconductor devices among three phases can be reduced and balanced. Besides, the neutral-point voltage balancing can be guaranteed. Simulation and experimental results are presented to verify the feasibility and effectiveness of the proposed technique. Its influences on the converter total power loss and output current harmonics are discussed as well.

Dynamic Behaviour of Wind Turbine Dfig Generator under Grid Fault Conditions (Lvrt – Symmeterical Fault)

Across modern renewable power industry in globally, significant advances was established in the wind industry. Rising limit involves wind generation has to keep up steady activity during matrix shortcoming conditions, can be determined in the low voltage ride through ability. Double Fed Induction generator in wind turbines were normally utilized in wind industry applications, which are delicate to aggravations after force matrix. Thus, various sorts of assurance paths also different regulator strategies are incurred to Double Fed Induction Generator Wind Turbines for Low Voltage Ride Through ability upgrade. Here provides as an exhaustive survey additionally assessment for projected Low Voltage Ride Through arrangements utilized in Double Fed Induction generator wind turbines, of internal control techniques.

PV-Statcom based Smart Inverter in Distribution System

Shrewd PV Inverter can be used to give voltage control during basic framework needs on an every minute of every day premise. In the evening, the whole inverter limit is used for Smart PV Inverter activity. During a basic framework unsettling influence in the daytime, the shrewd inverter suspends its genuine force age work incidentally, and discharges its whole inverter limit with regards to Smart PV Inverter activity. When the aggravation is cleared and the requirement for network voltage control is satisfied, the sun oriented homestead gets back to its pre-unsettling influence genuine force creation. The Low Voltage Ride Through (LVRT) execution of the Smart PV Inverter is shown through both MATLAB reenactments and research center execution utilizing d-SPACE control. This proposed Smart PV Inverter with a reaction season of 1-2 cycles, can offer a comparable support as a genuine Smart PV Inverter in a given application and conceivably look for incomes for offering this assistance.

Considerations for testing full-scale wind turbine nacelles with hardware-in-the-loop

In this paper, pole placement control techniques are utilized to compensate for variations in the torsional dynamics resulting from the different boundary conditions experienced by a wind turbine nacelle when mounted on a test bench. Simulation case studies are used to understand the baseline torsional characteristics of the nacelle coupled to the test bench versus that coupled to the rotor. It was found that the frequency shifts by up to 200% and 400% for the first and second torsional modes, respectively. The feasibility of utilizing pole placement technique to match the torsional characteristics of the turbine on the test bench is investigated using PI and PID controllers. The performance of the tuned controller is then verified under two test scenarios: Low Voltage Ride Through (LVRT), and a highly dynamic turbulent wind input. It is demonstrated that tuning the test bench speed controller can effectively shift the poles of the closed-loop system to match the desired first and second modes of the full turbine and replicate transient field events on the test bench.

Mitigation of harmonics and inter-harmonics with LVRT and HVRT enhancement in grid-connected wind energy systems using genetic algorithm- optimized PWM and fuzzy adaptive PID control

The growing installed wind capacity over the last decade has led many energy regulators to define specific grid codes for wind energy generation systems connecting to the electricity grid. These requirements impose strict laws regarding the Low Voltage Ride Though (LVRT) and High Voltage Ride Though (HVRT) capabilities of wind turbines during voltage disturbances. The main aim of this paper is to propose LVRT and HVRT strategies that allow wind systems to remain connected during severe grid voltage disturbances. Power quality issues associated with harmonics and inter-harmonics are also discussed and a control scheme for the grid-side converter is proposed to make the Wind Energy Conversion System insensitive to external disturbances and parametric variations. The Selective Harmonic Elimination Pulse Width Modulation technique based on Genetic Algorithm optimization is employed to overcome over-modulation problems, reduce the amplitudes of harmonics, and thus reduce the Total Harmonic Distortion in the current and voltage waveforms. Furthermore, to compensate for the fluctuations of the wind speed due to turbulence at the blades of the turbine, a fuzzy Proportional-Integral-Derivative controller with adaptive gains is proposed to control the converter on the generator side.

Novel Switching Frequency FCS-MPC of PMSG for Grid-Connected Wind Energy Conversion System with Coordinated Low Voltage Ride Through

The integration of wind energy systems (WECS) into the power grid through power electronic converters should ensure the high performance of the control system. In spite of several advantages of conventional Finite control set-model predictive controller (FCS-MPC), variable switching frequency and high computational burden are considered its main drawbacks. In this paper, a fast FCS-MPC of a machine side converter (MSC) of direct-driven permanent magnet synchronous generator (PMSG) based wind turbines for wind energy conversion system is proposed. The wind energy conversion system has been realized using a direct driven PMSG and a full-scale back-to-back power converter. The proposed controller is designed to reduce the required calculations in each horizon. In addition, the performance of conventional FCS-MPC is compared with the proposed method, and an improvement in total harmonic distortion spectra and simulation time required even when imposing a lower sampling frequency was found. To overcome the variable switching frequency problem, a modulation algorithm is introduced in the minimization process of modulated FCS-MPC. To keep the proposed system attached to the utility during a fault, a coordinated pitch angle control and low voltage-ride through (LVRT) algorithm is designed and inserted in the vector control of the grid side converter (GSC) to supply reactive power to the grid during fault for ensuring safe operation of the inverter and meeting the grid code requirements. The effectiveness of the proposed controller is illustrated using simulation results under different operating conditions.

A Novel Saturated Amorphous Alloy Core Based Fault Current Limiter for Improving the Low Voltage Ride Through Capability of Doubly-Fed Induction Generator Based Wind Turbines

This article proposes the use of a novel saturated amorphous alloy core-based fault current limiter (SAACFCL) to improve the low voltage ride through (LVRT) capability of doubly-fed induction generator (DFIG) based wind energy systems, especially during voltage sag events. Compared to the traditional cores, which are widely used in fault current limiter (FCL), amorphous alloy core possesses a very narrow B-H loop, which indicates that the SAACFCL requires a smaller dc excitation current, and it will incur low core losses. Under normal conditions, this developed SAACFCL accomplishes low impedance and has a negligible impact on the network operation. During grid faults, a deep voltage sag causes large fault currents that desaturate the SAACFCL core, increasing its impedance, which limits the fault currents and improves the LVRT capability of DFIG systems. The SAACFCL is designed and characterized by ANSYS software. To validate the performance of the SAACFCL, a DFIG-based system equipped with the SAACFCL has been modeled in MATLAB/Simulink. The simulation results with and without the SAACFCL show that the SAACFCL is capable to achieve the LVRT successfully and can meet the grid code requirement. The performance of the SAACFCL has also been compared with that of the saturated iron core FCL (SICFCL) and superconducting FCL (SFCL). The SAACFCL promises better performances than SICFCL and SFCL. A small scale SAACFCL has been designed, developed, and tested in the laboratory to check its performance. The experimental results show that the proposed SAACFCL can effectively reduce the level of fault current and mitigate voltage sag experienced by the DFIG.

Augmentation of Low Voltage Ride through (LVRT) Capability using ANN based electric spring for a Grid-tied Wind Energy Conversion System

Electric Spring (ES) is an innovative approach for distributed voltage control (DVC) whereas at the same time accomplishing nominal demand-side management (DSM) technique for continuously varying renewable energy sources. In order to demonstrate the efficiency of ES when deployed in enormous quantities across the grid-connected power distribution system, it is important to build easy and reliable simulation models of ES that can be integrated into large-scale simulation studies of the power system. The ES study gap identification method defines the ES dynamic simulation methodology that is appropriate for the WECS grid-connected voltage and frequency control studies. The integration of WECS to the electric grid leads to many serious power quality problems. Low voltage ride through (LVRT) capability improvement is one of the important constraints in grid connected WECS. This articleexamines the enhancement of LVRT capability of Doubly Fed Induction Generator (DFIG) based grid connected WECS by usingES. The effectiveness of proposed Artificial Neural Network (ANN) based Electric spring controller is compared with existing PI controller using MATLAB/Simulink software. It is evident that the proposed controller nurtures the trustworthiness of WECS when integrated with existing electric power grid system.

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.

Advanced LVRT Control Scheme for Offshore Wind Power Plant

Offshore wind farms (OWF) are being rapidly integrated to the power systems, which can be primarily attributed to saturation of the mainland onshore area, and high wind speed in offshore sites. However, to maintain stable grid operation, OWFs are required to comply with Low Voltage Ride Through (LVRT) capability. OWF during LVRT period should not only stay grid connected, but also support the grid during the disturbance. In this paper, a new communication-independent LVRT control scheme for HVDC connected OWF has been proposed. Under the proposed strategy, OWFs not only successfully ride through a fault induced voltage dip, but also comply with active power recovery ramp rate during the fault recovery period. Besides complying with LVRT requirement and ramp rate limit, the proposed strategy improves voltage dip induced frequency (VDIF) issue by increasing active power injection from the OWF during LVRT period. The proposed LVRT strategy also exploits the stored energy in HVDC link capacitors in order to reduce the impact of VDIF issue. The proposed and the conventional LVRT strategies for HVDC connected OWF have been implemented and evaluated on IEEE 39 bus benchmark system.

Enhanced LVRT capability of Wind Turbine based on DFIG using Dynamic Voltage Restorer controlled by ADRC-based Feedback control

For grid-connected DFIG-based wind turbine, Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT) capability is vital problem that need to be improved. This paper proposes an Active Disturbance Rejection Control (ADRC) strategy applied to Doubly Fed Induction Generator (DFIG) based Wind turbine (WT), which integrates a Dynamic Voltage Restorer (DVR). The DVR connect in series the DFIG output terminal and the utility grid. The ADRC scheme of the new topology DFIG-based WT with integrated DVR is designed to compensate grid voltage disturbances, which in turn meet LVRT requirement and increase the level of wind power penetration. The performance of this WT-DFIG-DVR structure is investigated in different operating scenarios in order to show the skills of the designed controllers.

A Grid-tied PV Inverter with Sag-severity- independent Low-voltage Ride Through, Reactive Power Support, and Islanding Protection

This paper proposes a grid-tied photovoltaic (PV) inverter capable of low-voltage ride through (LVRT), reactive power support, and islanding protection. Unlike other LVRT inverters, the proposed inverter is independent of sag severity while maintaining the maximum power-point tracking (MPPT) under normal and faulty conditions. The addition of an energy storage buffer stage mitigates the DC-link voltage surge during sags. At the same time, the inverter injects the reactive power during back-to-back sags of variable depths. The control system of the inverter generates the appropriate reference signals for normal, LVRT, and anti-islanding modes while the MPPT continues running. The salient features of the proposed inverter are: (1) active power injection under normal grid conditions; (2) sag-depth independent LVRT with reactive power support; (3) no DC-link fluctuations; (4)continuous MPPT mode; and (5) simultaneous LVRT and anti-islanding support during a grid outage. The inverter demonstrates an uninterrupted operation and seamless transition between various operating modes. Simulations and the experimental prototype have been implemented to validate the efficacy of the proposed PV inverter.

A Specialized System-on-Chip Based Distance Protection for Distribution Grids with Inverter Interfaced Distributed Generation

Legacy protection schemes face new challenges as Inverter Interfaced Distribution Generation (IIDG) significantly changes the transient fault response of the distribution grid. The performance of the protection near the IIDG side is adversely affected by Low Voltage Ride Through (LVRT) and the negative sequence current suppression control characteristics of the IIDG. The operational characteristics of the protection are very different from those of the legacy protection schemes used in the distribution grid. Traditional overcurrent protection schemes cannot meet the requirements of selectivity and sensitivity. This paper analyses the influence of the IIDG on the protection schemes used in the distribution grid. Based on the positive sequence voltage polarization impedance criterion this study proposes a polarizing impedance criterion based on the fault component of positive sequence voltage, which can adaptively follow the fault resistance variation to satisfy the requirements of grid operation. The simulation results show that: (a) the proposed criterion is immune to the adverse effects of the transient characteristics of the IIDG; and, (b) it can adaptively follow the change of fault resistance making it suitable for application in short distribution lines. Using specialized System-on-Chip technology, a new distance protection device has been developed and tested on an industrial site. Simulation results and field tests showed that the new distance protection meets the requirements of the distribution grid with IIDGs.

Autonomous Operation of Grid Synchronizing Breakers with Low Voltage Ride Through (LVRT) Capabilities

With rapidly increasing penetration level of renewable energy sources (RES) in electric grids, it is noteworthy that stable and reliable operation of electric power systems is urgently needed, given the inverter interfaced RES has less inertia compared to conventional rotational generation units. Meanwhile, it should be also noted that grid operation can be categorized as normal and abnormal conditions. For each type of grid operation condition, the inverter interfaces should produce reliable power generation into the grid. Especially during fault operation, low-voltage ride through (LVRT) is a critical operation condition that needs to be achieved in each interface inverter. Furthermore, when the system is operated in severe conditions, the faulty areas should be effectively isolated from the rest of the system. Considering the operation requirements in electric grids, in this paper, an autonomous operation of grid synchronizing breakers is designed and implemented. Particularly, the developed grid synchronizing breakers can autonomously detect grid operation condition by measuring the grid-connected current and grid side voltage. An extended LVRT curve is designed to support the original LVRT curve so that enhanced fault mitigation and isolation can be achieved. In the meantime, the designed circuit breaker can automatically synchronize with the external grid voltage. Therefore, transient process when breaker is turned on or off can be successfully controlled without violating the existing standards. A simulation model is implemented in MATLAB/Simulink to verify the effectiveness of the proposed method.

Improving low-voltage ride-through capability of a multimegawatt DFIG based wind turbine under grid faults

Large integration of doubly-fed induction generator (DFIG) based wind turbines (WTs) into power networks can have significant consequences for power system operation and the quality of the energy supplied due to their excessive sensitivity towards grid disturbances. Under voltage dips, the resulting overcurrent and overvoltage in the rotor circuit and the DC link of a DFIG, could lead to the activation of the protection system and WT disconnection. This potentially results in sudden loss of several tens/hundreds of MWs of energy, and consequently intensifying the severity of the fault. This paper aims to combine the use of a crowbar protection circuit and a robust backstepping control strategy that takes into consideration of the dynamics of the magnetic flux, to improve DFIG’s Low-Voltage Ride Through capability and fulfill the latest grid code requirements. While the power electronic interfaces are protected, the WTs also provide large reactive power during the fault to assist system voltage recovery. Simulation results using Matlab/Simulink demonstrate the effectiveness of the proposed strategy in terms of dynamic response and robustness against parametric variations.