Research on Low Voltage Ride through Strategy for Power Electronic Transformer

Power Electronic Transformer(PET) can realize flexible control of AC/DC hybrid distribution network and efficient absorption of new energy. During a grid fault, if PET can adopt a superior low-voltage ride-through(LVRT) strategy, the operational safety of PET itself and the stability of AC/DC hybrid grid can be improved significantly. Moreover, PET exhibits obvious controlled characteristics during LVRT operation, which brings difficulties for grid fault analysis and relay protection setting calculation. Therefore, it is urgent to study the LVRT strategy and fault equivalent model of PET, so as to provide guarantee for the safe and stable operation of the system and lay a theoretical foundation for grid fault analysis. In this paper, a LVRT strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety, based on which the fault current analytical expression of PET is obtained by deducing and analyzing the control model in the bidirectional rotating coordinate system. And the corresponding short-circuit current equivalent calculation model of PET is established. Finally, the correctness of the proposed LVRT strategy and the short-circuit calculation equivalent model is verified by simulation.

Dynamic voltage restorer (DVR) is conventionally utilized in the electrical distribution system to mitigate the voltage sag and swell on point of common coupling of sensitive consumers. In this paper, a novel DVR based on power electronic transformer (PET) is proposed to mitigate the symmetrical and asymmetrical voltage sag and voltage swell. PET is connected between DC power supply and multilevel inverter to provide transitive DC voltage for DC-link of DVR. PET has effectually and flexibly created relevant DC voltage with required magnitude and low distortion. The performance of suggested DVR based on PET has been evaluated under occurrence of different disturbances in the electrical distribution system. Ultimately, the simulation results have confirmed that DVR-PET can significantly mitigate the symmetrical and asymmetrical voltage sag and voltage swell on sensitive consumers.

Introduction. The integration of Renewable Energy Sources (RESs), particularly Wind Energy Conversion Systems (WECS), is vital for reducing reliance on fossil fuels and addressing climate change. However, this transition poses challenges, including ensuring grid stability in the face of intermittent RESs. Compliance with grid codes is crucial, with a focus on Low Voltage Ride Through (LVRT) capability. Problem. The intermittent nature of RESs, specifically in Permanent Magnet Synchronous Generator (PMSG) based WECS, presents challenges to grid stability during voltage dips. Goal. To enhance voltage stability and LVRT capability in PMSG-based WECS by integrating a Dynamic Voltage Restorer (DVR) with an energy storage device. This involves regulating the input DC voltage to the DVR using a type 2 fuzzy controller, adapting intelligently to changing conditions. Methodology. DVR, powered by an energy storage device, is strategically integrated with WECS. A type 2 fuzzy controller regulates the DC voltage to DVR. The rectified WECS output undergoes processing through an isolated flyback converter. A 31-level Cascaded H-Bridge Multilevel Inverter (CHBMLI) with PI control ensures high-quality AC output. Results. The validation of developed system is executed using MATLAB/Simulink revealing a reduced Total Harmonic Distortion (THD) value of 1.8 %, ensuring significance in LVRT capability. Originality. The strategic integration of DVR with PMSG-based WECS, addresses the LVRT challenges. The use of type 2 fuzzy controller for intelligent voltage regulation and a sophisticated multilevel inverter contributes to the uniqueness of proposed solution. Practical value. The developed system provides benefits by ensuring reliable LVRT capability in PMSG-based WECS with reduced THD of 1.8 % indicating improved grid compatibility. References 26, tables 5, figures 20.

The doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) is a mature and promising renewable energy solution amid growing global energy demand. As supply reliability and system resiliency become increasingly important, low voltage ride-through (LVRT) capability has become mandatory for such systems. During a voltage dip at the AC bus, where the WECS delivers power, it must stay connected and aid voltage recovery by supplying reactive power, as per local grid standards. While ON-grid LVRT strategies have been extensively researched, OFF-grid LVRT operations have received little attention. For greater source reliability, WECSs should also operate in OFF-grid mode, where similar voltage dip issues can arise. This paper proposes a hybrid LVRT strategy for DFIG-WECS in OFF-grid mode. A battery energy storage system (BESS) maintains the DC bus voltage, while a grid-forming converter (GFC) holds the AC bus voltage with its limited current capacity. The DFIG rotor disconnects from the rotor side converter (RSC) and connects to a crowbar during the voltage fall and rise period. Between these two periods, the DFIGRSC provides reactive power. The method is validated using a MATLAB/SIMULINK model in an OPAL-RT real-time simulation, demonstrating that under different wind velocities, the OFF-grid DFIG-WECS withstands different levels of voltage dips while supporting voltage recovery. Keywords-battery energy storage system (BESS), doubly fed induction generator (DFIG), grid forming converter (GFC), low voltage ride through (LVRT), vector control (VC), wind energy conversion system (WECS)

The rapid expansion of offshore wind energy and its integration into weak power grids have introduced significant challenges to system stability, particularly under fault conditions that cause voltage sags. Ensuring adequate Low Voltage Ride Through has become essential for maintaining grid reliability and preventing large-scale disconnections of wind farms.To address voltage sags caused by short-circuit faults during the grid integration of large-capacity offshore wind farms via weak networks, this paper establishes a simulation model of a permanent magnet synchronous generator (PMSG)-based offshore wind farm in the PSAT platform. Taking the Critical Clearing Time (CCT) as the core evaluation index, the Low Voltage Ride-Through (LVRT) capability is systematically assessed. By simulating short-circuit disturbances, the transient voltage response at the point of common coupling is analyzed, followed by a comparative evaluation of the voltage support effects of three dynamic reactive power compensation devices: Static Var Compensator (SVC), Static Synchronous Compensator (STATCOM), and Unified Power Flow Controller (UPFC). Results indicate that the rational configuration of dynamic reactive power compensation effectively mitigates voltage drops, significantly extends the CCT, and enhances transient stability margins. The distinct response characteristics and support efficacies of different devices are clarified, providing a theoretical foundation for optimizing LVRT strategies and selecting compensation equipment in offshore wind integration.

In the AC/DC hybrid microgrids equipped with power electronic transformer (PET), different AC grids and DC grids are connected through multiport PET to operate the entire network in a secure and economic manner. This paper proposes a decentralized optimal power flow model, considering the multiport coordinated control strategy of PET, for running autonomous AC/DC hybrid microgrids. To optimize the power flow in the network, the steady state model of PET is established, including voltage source converter power injection model and Kriging model. By introducing a secondary droop control layer can realize the autonomous operation of AC/DC hybrid microgrids and increase the utilization efficiency of distributed generations. Then using the analytical target cascading approach, each AC/DC sub-microgrid is regarded as different autonomous subjects. The distributed PET is as power coupling node and information interaction node to decompose the optimal problem of AC/DC hybrid microgrids equipped with PET which is solved in parallel. The obtained subproblems for these grid levels are solved with a nested solution process. The simulation results verify the correctness and effectiveness of the proposed model in this paper.

With the increasing proportion of new energy generation in the power system, power conversion system (PCS) has got more used in wind and photovoltaic power generation. So the low voltage ride through (LVRT) ability of PCS has got more attention. This paper describes the LVRT control methods of the high-power PCS in detail and gives a current suppression method by blocking PWM wave which can quickly suppress the overcurrent generated at the moment of voltage drop and voltage recovery, so that PCS can realize LVRT stably. The experimental results on PCS have verified the feasibility and effectiveness of the proposed control method which can adopt to high-power PCS.

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.

Enhancement of DFIG-Wind Turbine’s LVRT capability using novel DVR based Odd-nary Cascaded Asymmetric Multi-Level Inverter

In order to analyze the control strategy of the low voltage ride through (LVRT) of DFIG during the asymmetric voltage sag, it is necessary to analyze the transient performance of a DFIG during the asymmetric voltage sag. In this paper, analyzed the influence of the asymmetric grid voltage to DFIG and the analysis method of the asymmetric voltage sag, and on the basis of positive and negative sequence mathematical model, analyzed the composition of stator output active and reactive power under the condition of asymmetric grid voltage. And built a DFIG asymmetric voltage drop simulation model of 1.5MW in MATLAB/Simulink, the simulation results shows that the stator voltage, current, active power and reactive power all present a double frequency ripple during the asymmetric voltage sag, consistent with theoretical analysis. It can provide theoretical basis for double-fed motor control strategy of asymmetric LVRT.

This paper presents the development and performance capability of a comprehensive Low voltage ride through (LVRT) control scheme that makes use of both the DC chopper and the current limiting based on the required reactive power during fault time. The study is conducted on an 8.5 MW single stage PV power plant (PVPP) connected to the Rwandan grid. In the event of fault disturbance, this control scheme helps to overcome the problems of excessive DC-link voltage by fast activation of the DC chopper operation. At the same instance, AC current is limited to the maximum rating of the inverter as a function of the injected reactive current. This helps overcome AC-over- current that may possibly lead to damage or disconnection of the inverter. The control scheme also ensures voltage support and power balance through the injection of reactive current as per grid code requirements. Selected simulations using MATLAB are carried out in the events of different kinds of fault caused voltage dips. Results demonstrate the effectiveness of the proposed LVRT control scheme.

Power electronic transformer (PET) with modular cascaded structure has become a flexible interface between AC and DC grids. Nonetheless, the notable 2^nd-order ripple power introduced from the AC source requires considerable energy storage in the module capacitors, which is likely to degrade the overall power density and increase system cost. In this paper, a power channel based four-winding PET (PC-PET) is proposed to address such an issue. Owing to the phase-crossing power channels, the low-frequency ripples can cancel each other through magnetic flux counteracting. To improve the accuracy of power decoupling, the effect compromised by inconsistent windings is considered. Accordingly, a proportional-resonant (PR) control based power decoupling strategy for PC-PET incorporated with power balance is proposed to decouple the three-phase low-frequency ripple powers, and minimize the voltage ripple in module capacitors. Additionally, the improvement of power density is discussed based on analyses on power losses, capacitor volume and transformer volume. Simulation and experimental results verified the validity of proposed control strategies.

ABSTRACTThe Low Voltage Ride Through (LVRT) is an important requirement in fault conditions for any power system specially microgrids. When the fault occurs, the overcurrent and unbalanced voltage are yielded and these result in serious damages in inverter-based Distributed Generators (DGs) in the microgrid. To prevent this phenomenon, many types of research have been guided to limit the current in faulty conditions and to improve and balance the PCC voltage to be without any distortions. In this paper in islanded mode with reactive power injection during the fault occurrence, the LVRT method is proposed. This reactive power is injected to compensate the voltage dip and simultaneously limit the overcurrents in faulty phases. Also using this method, the faultless phases are not degraded and have no overvoltages. It is noticeable that two inverters in this paper assumed with varying droop gains and so the current and power of them must be divided relative to their droop gains both in faulty and faultless conditions. The simulation results are based on MATLAB Simulink which shows the system improvement in faulty conditions using this method.

The modular cascaded power electronic transformers (PETs) designed for interfacing AC and DC grids are mostly composed of active front end (AFE) and dual active bridges (DABs). Due to the remarkable difference between primary side and secondary side capacitor voltages of DAB at start-up, high input-inrush current will occur if the phase-shift-control (PSC) of DAB is directly employed, which can pose damage to power devices. This paper proposes a novel soft start-up method for DABs in PET with fast pre-charge process. The duty cycle of primary bridge is dynamically adjusted to precisely limit the peak of input-inrush current, and the regulating range is also enlarged.

The Low Voltage ride Through (LVRT) capability of Wind Turbines (WTs) determines the connectivity of WT to the grid based on both voltage sag magnitude and duration locally measured at Point of Common Coupling (PCC), in order to protect the WT against overloading out of its tolerable apparent power. If the WT is still inside its safe and secure operating condition and there is still some standby capacity available in it to support the grid, the WT should remain connected to the power system regardless of voltage sag magnitude and duration until the WT apparent power exceeds its nominal value. In case of islanding, which often is accompanied with a low voltage drop, the WT may be improperly disconnected while operates with less than half of its nominal apparent power. This situation necessitates investigation or perhaps a revision of LVRT grid code to be efficient for all possible incidents, not only Short Circuit (SC) faults, but also cascading events and islanding.