Low Voltage Ride Through Test Research Articles

Implementation Strategy of Test Facility Based on Auto-Transformer for LVRT/HVRT Evaluation of Large-Scale Wind Turbine

In accordance with South Korea’s recent 2030 Carbon Neutral Plan, an 8GW offshore wind farm is planned for construction in the South-west Sea. Therefore, it is expected that large-scale wind turbines will be installed, and these turbines must operate stably, even when there are instantaneous voltage fluctuations in the power system. The grid code is described for the low-voltage-ride-through (LVRT) and high-voltage-ride-through (HVRT) functions, and a test facility that can perform both LVRT and HVRT tests is essential. In the case of LVRT/HVRT test facilities developed by the existing RLC (impedance component) method, it may be difficult to test large-scale wind turbines due to problems such as power quality, frequent failures and narrow short-circuit capacity ranges. Therefore, to solve such problems, this paper proposes an LVRT/HVRT test facility of the autotransformer type, which is capable of outputting the desired voltage range by changing the wiring method and tap position. Specifically, in order to implement the test facility of the autotransformer type, which is able to output the desired voltage range by changing the wiring method and tap according to the LVRT/HVRT test status, this paper presents an impedance determination algorithm (two-step layer impedance determination algorithm) of auto-transformer based on the fault-current analysis and operation strategy at a real LVRT/HVRT testing evaluation facility.

Influences on the LVRT behavior of DFIG wind turbine systems

Grid faults introduce highly dynamic electrical and mechanical loads to a wind turbine (WT). Especially WTs with a direct grid connection like the doubly-fed induction generator (DFIG) are strongly affected. The behavior of a WT during grid faults can be tested in low voltage ride through tests (LVRT). But there are numerous influencing factors on the behavior of the DFIG during a LVRT which have not yet been fully investigated. The pre-fault operating point of the DFIG, the grid inductance, the pre-fault phase angle of the grid voltage, the fall time of the voltage as well as the start and end values of the voltage drop affect the electromagnetic torque and the short circuit current of the generator. Therefore, many LVRT test results for DFIGs are neither comparable nor representative. In this paper it is shown that the peaks of electromagnetic torque and currents during LVRTs can be reduced. A low pre-fault torque and rotational speed, a high grid inductance and a slow voltage drop can minimize the impact of a grid fault. The rotational speed is especially critical because it influences the slip of the DFIG and, thus, has an influence on the dynamics of the fault.

Research on HIL-based HVRT and LVRT automated test system for photovoltaic inverters

As an important capability of the photovoltaic grid-connected inverter, high voltage ride-through (HVRT) and low voltage ride-through (LVRT) capability are related to the stability of the whole power grid system. Therefore, according to the requirements of HVRT and LVRT standards, an automatic test and analysis system of the photovoltaic inverter is designed based on a hardware-in-the-loop (HIL) simulation platform, which saves time and human resources. Firstly, this paper analyzes the specific requirements for HVRT and LVRT capabilities in Q/GDW 1617-2015. Secondly, a HIL simulation platform is built. Thirdly, an automatic test and analysis system is developed based on Python and m language using RT-LAB and Matlab software, which can realize the automation of HVRT and LVRT test, data collection and storage, data processing and standard index calculation, and test results report generation of the photovoltaic inverter under different steady-state conditions. Finally, the correctness of the system is verified on the HIL simulation platform.

Signature POWIs for DFIG under LVRT conditions: Analysis, experiments and recommendations

As one of the important characteristics of voltage sags, the point on wave of initiation (POWI) has been widely studied in its influence on sag sensitive devices. However, it is less mentioned in the research and test standards related to the low voltage ride through (LVRT) of doubly fed inductor generators (DFIGs). To fill this gap, a comprehensive study for the influence of POWIs together with the phase angle jump (PAJ) is carried out in this paper. Taking rotor current as the evaluation index, mechanism analysis and experimental verifications are conducted under different asymmetrical sags. The results demonstrate that POWIs affect the LVRT response through the transient dc stator flux and rotor current. The two POWIs with the greatest and lightest impact within half a cycle are defined as the signature POWIs, which depends on the sag type, sag magnitude and PAJ. Analysis under the signature POWIs, especially the most severe POWI, can more accurately evaluate the LVRT response of DFIGs. Besides, the PAJ will have a significant detrimental impact together with the POWI, manifested as the increase of the negative ac and transient dc rotor current. Suggestions for improving the LVRT testing standard are further put forward, that performing tests within the range of ± 15° around the most severe POWI.

Study of stability after low voltage ride-through caused by phase-locked loop of grid-side converter

In a power system with a high percentage of converter coupled generation, dynamic characteristics of the converter affects the stability of the whole power system. The interaction of the various control loops of the grid-side converter, e.g., the phase-locked loop and the DC link voltage control loop, dominates the various dynamic characteristics of the grid-side converter, including its low voltage ride-through ability. In this paper, the phase-locked loop is modeled and analyzed and a stability criterion for the converter, depending on the grid impedance, is obtained. According to this criterion, a common low voltage ride-through test device based on the shunt impedance voltage sag generator is analyzed. The results show that, and explain why, the test device cannot reproduce the characteristics of the practical grid, which is also verified by experiments using a controller-Hardware-in-the-loop system.

Transient modelling of doubly‐fed induction generator based wind turbine on full operation condition and rapid starting period based on low voltage ride‐through testing

Transient simulation model of doubly-fed induction generator-based wind turbine has a complex model structure and considerable control parameters. An accurate simulation model is especially important for studying the static and transient characteristics of doubly-fed induction generator-based wind turbine and the grid-connected system. Based on the measured data of low voltage ride-through testing, the integrated parameter identification method is proposed here. Combined with the time-domain simulation analysis, curve fitting is used to determine the control strategy of doubly-fed induction generator-based wind turbine. The improved genetic algorithm is used to identify the generator parameters, power control parameters, and resistance of Crowbar of doubly-fed induction generator-based wind turbine. Combined with the electrical parameters provided by the wind turbine manufacturer, electromagnetic transient model on full operation condition and rapid starting period, corresponding to a measured doubly-fed induction generator-based wind turbine in West Inner Mongolia Power Grid, is then established, which can operate in the full wind speed range and quickly enter the stable state within 1 s after start-up. Comparison of simulation results and measured data shows that the established electromagnetic transient simulation model using parameters identified by the proposed method can accurately reflect the key characteristics of doubly-fed induction generator-based wind turbine.

Impact of Phase Angle Jump on DFIG Under LVRT Conditions: Challenges and Recommendations

The increased penetration of wind power has made its low voltage ride through (LVRT) capability more crucial than ever. Currently, the LVRT performance of wind generators is verified using experimental voltage sags with different magnitudes and durations. The actual fault, however, also causes the phase angle jump (PAJ) of the grid voltage. In this paper, the impact of PAJ on the LVRT capability of doubly fed inductor generator (DFIG) under three-phase fault is analyzed. To achieve this, detailed analytical calculations and experimental verifications are conducted. The results demonstrate that the PAJ imposes more challenges on the LVRT by intensifying the rotor overcurrent through the natural stator flux. As a result, DFIGs that have passed the existing LVRT test may still fail to comply with the grid code requirements in practice. To tackle this issue, an improved test procedure is recommended. The new procedure considers the impedance angle of the target system as an additional test condition, thus can reveal the LVRT performance of DFIG under the potential PAJ.

Parameter identification and modelling of photovoltaic power generation systems based on LVRT tests

With the increasing usage of photovoltaic (PV) generation systems, it is of great relevance to develop effective models to characterise the dynamic behaviours of actual PV systems under different failures and operation modes. In general, three test items are required to identify the three types of parameters, namely, the low-voltage ride-through (LVRT) control parameters, PV array parameters, and DC voltage loop parameters. To simplify the test items and steps needed for parameter identification, an appropriate identification and modelling method for a PV generation system is proposed on the basis of an LVRT test. This LVRT field test is conducted on a large PV system in North China. The three groups of parameters are identified with the test data. Specifically, the equivalent resistance of the collection line of a PV array is considered to accurately simulate the dynamic process of DC voltage. Lastly, the effectiveness of the proposed method is verified through a comparison of simulation results and field test results.

Low-Voltage Ride-Through Remote Testing Method for Offshore Wind Turbines

Grid-connected wind turbines (WTs) are required to have low-voltage ride-through (LVRT) capability. On-site LVRT test is a convincing certificating method. To tackle the limited accessibility and harsh conditions of on-site test for offshore WTs, this paper proposes a remote testing method, which places the testing device on shore to produce the specified fault voltages through a subsea cable to the WT. For further eliminating the effects of introduced cable on LVRT test, a stepwise voltage sag generation strategy is applied to suppress the overvoltage and maintain required voltage sags. First, the LVRT testing requirements and voltage sag specification need to be considered in a practical test are introduced. The stepwise generation strategy and its implementation procedure based on the designed testing device are presented. Second, the effects of submarine cable on the WT-side voltage are analyzed under different generation stages of voltage sags. Next, the testing device’s impedance configuration method, based on the derived residual voltage expression, is given to generate the required steady-state voltage sags. Finally, simulation results indicate that with the proposed method, the effects of cable can be eliminated and the required grid faults can be emulated for remote LVRT test.

Research and Application of Low Voltage Ride through (LVRT) and the Power Grid Adaptability Testing Device for Frequency Converter

In order to study the response characteristics of converters applied in the field of renewable power integration and motor speed regulation when grid fault appears, the traditional LVRT schemes and effects of converters are analyzed, and the integrated design of AC and DC sources scheme based on power electronics technology is proposed.The three-phase four-wire topology AC and DC sources and adaptive fuzzy PI control algorithm is adopted to realize the real-time adjustment of the DC output voltage and dynamic control of the three-phase or single-phase AC output voltage. Meanwhile, the dynamic control includes the drop amplitude, the drop time, the frequency, and the Harmonic. The 40KW LVRT and grid adaptability testing device for converter is developed. The laboratory test proved that the dynamic response time of the device is less than 5ms, the harmonic content of the AC output voltage is less than 1%, and the protection mechanism is complete and reliable. The upper computer sets parameters offline or online, tests circularly and operates conveniently.

Analysis and elimination method of the effects of cables on LVRT testing for offshore wind turbines

The current state, characteristics and necessity of the low voltage ride through (LVRT) on-site testing for grid-connected offshore wind turbines are introduced firstly. Then the effects of submarine cables on the LVRT testing are analysed based on the equivalent circuit of the testing system. A scheme for eliminating the effects of cables on the proposed LVRT testing method is presented. The specified voltage dips are guaranteed to be in compliance with the testing standards by adjusting the ratio between the current limiting impedance and short circuit impedance according to the steady voltage relationship derived from the equivalent circuit. Finally, simulation results demonstrate that the voltage dips at the high voltage side of wind turbine transformer satisfy the requirements of testing standards.

On-Site LVRT Testing Method for Full-Power Converter Wind Turbines

Low-voltage ride-through (LVRT) capability is now required for all large-scale wind turbines (WTs) connected to the grid. This paper presents an on-site LVRT testing method for full-power converter (FPC) WTs. The new method makes use of the already installed WTs to emulate the grid and produce a given fault for the WT under test. To guarantee the safety of grid, both the test and grid emulation WTs are disconnected from the network and operate in stand-alone mode during the test. The system configuration, testing procedure, and control strategies are studied in the paper. Practical issues including the capability of emulated grid, WT overspeed limitation, network impedance, and fault conditions emulation are discussed. A wind farm with typical 2-MW FPC permanent magnet synchronous generator (PMSG) WTs is modeled in MATLAB. Simulation results show that with only proper modifications of a WT control algorithm, it is feasible to conduct on-site a series of LVRT tests for the FPC WT safely under given conditions.

The Research of Mobile LVRT Testing Device in High Altitude Area

For high-altitude environment particularity, the device was designed for high-altitude correction. The equivalent stress analysis of particle system is researched to meet seismic requirementsneeds of automotive mobile devices. A new auto-switch is designed by improving the traditional tap-switch for automatic switching cable in the car. It is first used in a car as a automatically switch and greatly reduces the manual wiring workload. The field test results show that the device can assume a high altitude solar motion detector demand of the low voltage ride through (LVRT) .

Comparative Study on Low Voltage Ride-Through Test Methods for Grid-Connected Photovoltaic Inverter

The large scale photovoltaic plants have more effects on the stability of regional power grid with the rapid increase of photovoltaic generation ratio in some areas. According to the grid guideline, the photovoltaic inverter is required to maintain the running status even the grid voltage drops and needs to have the capability to contribute towards the stability of power grid voltage and to support the grid voltage ride through. To test the ability of photovoltaic inverter low voltage ride through (LVRT), the test platform to simulate all kinds of fault for voltage drop needs to be set up. By the low voltage through technical requirements, this paper puts forward three kinds of design for test platform and analyzes respective work principle and characteristics, in order to provide optimization solution for the low voltage ride through test work.

Analysis of Low Voltage Ride-Through Capability for Variable Speed Wind Turbines Using Experimental and Measured Data

With an increasing amount of wind energy installed, the behavior of wind turbines during grid disturbances becomes more and more important. This paper introduces the basic structures of doubly-fed wind turbines and direct-drive permanent magnet synchronous wind turbines and analyses the capability of low voltage ride through (LVRT) of the wind turbines. Finally this paper elaborates the principle and the procedures of the LVRT tests. The results of the tests show that: Doubly-fed wind turbines and direct-drive permanent magnet synchronous wind turbines are equipped with LVRT capability. The turbines are able to operate normally without disconnection and provide reactive power to support grid restoration during the dip. The direct-drive permanent magnet synchronous wind turbines are superior to the doubly-fed wind turbines on LVRT since a full-power converter is used.

Safety Design of Wind Turbine’s Mobile Low Voltage Ride through Test Device

Wind turbine’s Low Voltage Ride Through(LVRT) ability must be verified by site-test, safety design was key factor of wind turbine’s mobile LVRT test device design and development. Based on research of protection scope, design principal of safety design, this paper realized five aspects of safety design: switchgear selection, relay configuration and setting value, routine operation, self-check failure lock, operation regulations. Safety design realized by this paper did not affect normal test process, colud provided perfect protection to operator, wind farm facilities and test device. Test device complied with this design had been used in wind turbine site-test and perfected well.