Low Voltage Ride-through Strategy Research Articles

Low-voltage ride-through strategy for an integrated permanent magnet synchronous generator in a flexible interconnected distribution network

For the permanent magnet synchronous generator (PMSG) integrated into the flexible interconnected distribution network (FIDN), its low-voltage ride-through (LVRT) strategy needs to be designed to enhance the transient operation capability of the FIDN. The design of the LVRT strategy also needs to take the fault location of the FIDN into consideration. To deal with faults occurring on the integrated feeder of the PMSG, the PMSG needs to realize successful LVRT using the hardware protection equipment. To deal with faults occurring on the feeder adjacent to the integrated feeder of the PMSG, the PMSG needs to release its stored energy to temporarily increase its active power output, which is then supplied to the loads on the faulted feeder that are isolated from the fault through the soft open point (SOP). In this paper, a novel LVRT strategy designed for the PMSG integrated into the FIDN is proposed, which includes dual operation modes that are separately applied to different fault locations. If PMSG is on the faulted feeder, the DC-link voltage of the PMSG is maintained with the controllable resistive fault current limiter (CRFCL), while the maximized stored kinetic energy is reserved to enhance the generation efficiency during the LVRT. If PMSG is on the feeder adjacent to the faulted feeder, the control strategy of the converters of the PMSG is adjusted in response to the power regulation goal at the SOP. Meanwhile, the maximum releasable kinetic energy of the PMSG is considered when increasing its active power output. The feasibility and effectiveness of the LVRT strategy for the PMSG are verified based on the numerical analysis.

Instantaneous power theory-fuzzy intelligent controller (IPT-FIC) based improved low voltage ride-through strategy for grid connected photovoltaic system

Instantaneous power theory-fuzzy intelligent controller (IPT-FIC) based improved low voltage ride-through strategy for grid connected photovoltaic system

Low-Voltage Ride-Through Technology in Photovoltaic Power Generation

As global photovoltaic (PV) integration surges, the ramifications of PV systems on the power grid have become increasingly pronounced. When faced with grid disturbances or faults, the abrupt disconnection of PV systems can critically imperil grid stability. In light of this, the low-voltage ride-through (LVRT) technology, pivotal for resilient grid fault recovery, stands out as an essential topic in PV power generation's advancement. This paper initiates a deep dive into the contemporary prerequisites for PV LVRT capabilities and delineates pathways for their effective realization. It then meticulously unpacks the evolution and multifaceted methods associated with the twin primary LVRT strategies: the assimilation of auxiliary apparatuses and the nuanced control of inverters. A critical analysis is presented in the succeeding sections, contrasting these cardinal strategies' relative merits and demerits and their diverse implementation modalities. The paper culminates with insightful recommendations, positioning itself as an invaluable scaffold for subsequent scholarly explorations in PV LVRT paradigms.

Enhanced Excitation Converter With Parallel/Series DC-Link Based on TAB for DFIG to Improve the LVRT Capability Under Severe Grid Faults

The low-voltage ride-through (LVRT) capability of the doubly-fed induction generator (DFIG) based on the conventional excitation converter is insufficient under severe grid faults. The reason is that the dynamic rotor electromotive forces (EMF) will far exceed the dc-link voltage. To solve this issue, this letter proposes an enhanced excitation converter with parallel/series dc-link based on the triple active bridge (TAB) for DFIG. The dc-link of the enhanced excitation converter consists of the TAB and switching circuit, replacing the capacitive-type dc-link. Based on this topology, the dc-link voltage can increase instantaneously during severe grid faults to resist rotor EMF and improve the LVRT capability of DFIG. Compared with existing LVRT schemes, the enhanced excitation converter enables DFIG to achieve LVRT and inject the reactive current requirements of the grid code into the grid under severe grid faults. Finally, experiments verified the effectiveness of the proposed enhanced excitation converter and its control strategy.

A novel low voltage ride-through scheme for DFIG based on the cooperation of hybrid energy storage system and crowbar circuit

To improve the low voltage ride-through (LVRT) capability of DFIG, a novel LVRT scheme based on the cooperation of hybrid energy storage system (HESS) and crowbar circuit is proposed. The HESS composed of superconducting magnetic energy storage (SMES) and batteries is connected in the DC-link bus of DFIG. During grid faults, the HESS is switched to stabilize the DC-link bus voltage of DFIG, the grid-side converter (GSC) of DFIG is switched to inject reactive power to the grid for improving the grid fault voltage and the crowbar circuit is activated in real-time to avoid RSC from being damaging according to the rotor current. During normal operation, the HESS is controlled to deal with power difference between the turbine generation power and power dispatching command, and the GSC of DFIG is controlled to manage the output power of the DFIG with HESS. Finally, the simulations of the DFIG with HESS system is conducted in RTDS platform, and the simulation results show that the proposed scheme can effectively achieve the LVRT of DFIG during gird faults and flexibly manage the DFIG output power with HESS during normal operation.

Low-voltage ride-through strategy for offshore wind turbines based on current relaxation region

This paper proposes a current relation region (CRR)-based low-voltage ride-through (LVRT) strategy for offshore wind turbines (OWTs). The proposed strategy seeks to minimize the detrimental shortfall of OWT's active current on the premise of maintaining the reactive support ability by considering short-time overcurrent capacity of grid-side converter (GSC) during low-voltage fault. Firstly, the extended transient current region (ETCR) is introduced to improve the transient current ability of OWTs with comparison to conventional transient current region (CTCR) based on rated current capacity of GSC. Combined with the introduced ETCR, grid code constraint and wind speed constraint are investigated for detailed description of OWT's transient current boundary of CRR. Furthermore, we design the CRR-based LVRT strategy with look-up tables to facilitate dispatching the transient current instructions to OWTs’ controllers. Finally, extensive case studies based on PSCAD are performed and the results show that the proposed strategy can improve active current and reactive current simultaneously over the conventional strategy. In the scenario of the rated wind speed (11.4 m/s) and critical voltage (0.2 p.u.), this achievement can be clearly seen by the value improvement in the active current and reactive current by 0.328 p.u. and 0.05 p.u., respectively.

A novel model predictive control strategy of D-PMSG wind turbine systems for LVRT based on two-state unloading resistance and super capacitor

For low voltage ride through (LVRT) of the wind power generation system, rotor energy storage and super capacitor are promising options. When the rotor energy storage cooperates with the super capacitor for LVRT, the DC voltage will undulate violently due to the speed fluctuation. To solve this problem, a crowbar circuit topology and its control strategy including unloading resistance and super capacitor are proposed in this paper. During the process of LVRT, the super capacitor and resistor absorb energies together. In doing so, the DC bus voltage fluctuation will be suppressed by changing the value of the unloading resistance. The proposed method can effectively reduce the power of the unloading resistor and the capacity of super capacitor, and further decrease the hardware cost. By adding the penalty term of the current change rate to the cost function using the model predictive control method (MPC), the DC bus voltage fluctuation is reduced with the realization of rotor energy storage. The control performances of the proposed LVRT strategy are validated through simulation results.

A low voltage ride-through strategy for grid-connected PV converters based on variable power point tracking method

A novel low voltage ride through control strategy with variable power tracking trajectory is proposed. The voltage fall amplitude is controlled by feedforward, and the tracking trajectory of power point is adjusted to realize the real-time change of the photoelectric cell voltage. On this basis, the output power of the photovoltaic generation system is controlled quickly and efficiently, and the purpose of power balance in the PV inverter is achieved. Through collaborative control of the grid-tied inverters, the output current of grid-tied inverter can meet the active and reactive power requirements of power grid as much as possible without overing the limit. In this way, the maximized support for the voltage recovery of power grid which contains zero voltage ride through is realized. In the non-standard environment, the proposed control strategy is proven to be effective by the simulation results. Compared with traditional LVRT strategy, the proposed strategy in this paper not only realizes the safe and reliable operation, but also improves the economy of photovoltaic power generation system, without any add additional equipment.

Study on a new fault phase selector in interconnection line with wind turbine based on doubly-fed induction generator’s wind farm

The fault current characteristics of wind turbine based on doubly fed induction generator (DFIGWT) is complicated. The conventional superimposed phase selectors cannot meet the requirements of phase selection in the interconnection line based on DFIGWT’s wind farm. In this paper, according to the low voltage ride-through strategy of DFIGWTs, the shortcomings of traditional superimposed phase selector are analyzed. Then, a new fault phase selector is proposed based on the amplitude ratio between superimposed phase voltage and superimposed phase-to-phase voltage. According to the proportional relationship between the superimposed phase voltage and superimposed phase-to-phase voltage in the interconnection line with DFIGWT-based wind farm, the fault phase selection coefficients are proposed. Based on the proposed fault phase selection coefficients, the fault phases under different fault conditions can be identified. Finally, the effectiveness and reliability of the proposed fault phase selector are validated by simulation. Test results show that the proposed method is not affected by the LVRT control strategies and can meet the phase selection requirements of new energy sources under different LVRT strategies conditions.

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.

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.

A simple and effective grid-supporting low voltage ride-through scheme for single-stage photovoltaic power plants

Due to the ever-increasing impact of grid-connected photovoltaic power plants (GCPPP), compliance with grid-code (GC) requirements has become inevitable. According to current GCs, not only a GCPPP needs to inject reactive current into the network with a drop in the grid’s voltage, but a necessity exists to inject active power into the network during grid fault occurrence and post-faults. This is because the power system faces stability issues in the case of either GCPPP being disconnected from the network or active power not being injected into the grid after fault clearance. In this paper, a braking chopper (BC) is installed and a controller has been designed to enable fulfillment of low voltage ride through (LVRT) criteria for single-stage grid-integrated PV systems. This method proves to be simple, inexpensive, and effective for a wide range of operating conditions and different types of grid-faults. Furthermore, a current limiter has been incorporated for the DC–AC converter that is compatible with the BC controller itself. A general guideline has been devised in this research to evaluate the proposed LVRT strategy for seamless operation of single-stage GCPPPs. Not only does the GCPPP remain connected to the network by adhering to relevant GCs, but it also tracks the maximum power point after fault clearance without any delays. The simulations are carried out in MATLAB/Simulink software and comparisons have been drawn with the conventional approach. The results indicate excellent improvement in transient and steady-state operation of the GCPPP.

Photovoltaic power generation control strategy

This paper establishes the maximum power tracking technology based on the conductance increment method, establishes the PV model in a synchronous rotating coordinate system, and uses the grid voltage-oriented vector control technology to realize the decoupling of the active and reactive power of the PV. In order to meet the low voltage ride-through requirements put forward by the State Grid Corporation of China, this paper proposes a low voltage ride-through strategy for PV.

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.

Low-Voltage Ride Through Strategy for MMC With Y₀/Y₀ Arrangement Transformer Under Single-Line-to-Ground Fault

In the offshore wind farm high-voltage direct-current (HVDC) system, the power delivery capability of the onshore modular multilevel converter (MMC) decreases severely under grid fault, which makes the DC-bus voltage increase rapidly and threatens the safe operation of the system. This paper proposes a low-voltage ride through (LVRT) strategy for MMC with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{Y}_{0}/\text{Y}_{0}$ </tex-math></inline-formula> arrangement transformer under single-line-to-ground (SLG) fault. The influence of different transformer arrangements to the MMC under SLG fault is analyzed. On this basis, a power delivery capability enhanced method is proposed for MMC with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{Y}_{0}/\text{Y}_{0}$ </tex-math></inline-formula> arrangement transformer to take advantage of its control ability on zero sequence current. In addition, an optimized LVRT strategy based on resonant controller is proposed, which has simple control structure and can ride through the SLG fault without DC chopper. The offshore wind farm MMC-HVDC simulation system is established in PSCAD/EMTDC and simulation studies are conducted to validate the effectiveness of the proposed LVRT strategy.

Mechanical Stress Comparison of PMSG Wind Turbine LVRT Methods

The grid code for renewable energy is increasingly strict to circumvent issues with grid stability and reliability. One grid code standard that is enforced for most modern variable speed wind turbines (WTs) is the low-voltage ride-through (LVRT) criteria, where WTs are to be grid-connected during voltage dips. Traditionally, for permanent magnet synchronous generator (PMSG) WTs, LVRT is achieved by using a DC crowbar or DC chopper to dissipate the power difference between the grid and the generator. Alternatively, a popular LVRT strategy proposed by the research community for PMSG-based wind energy conversion system (WECS) is the stored energy in rotor inertia (SEIRI) strategy, which is done by altering the control of the machine-side converter (MSC) with potential cost savings. However, there are some concerns regarding additional mechanical stress to the drivetrain that may pertain to this method. A hybrid LVRT method has been suggested to combine the crowbar and the SEIRI methods to incorporate the benefits from both methods. In this article, we are studying and comparing the electrical and mechanical performance of PMSG WTs operating with the traditional crowbar, the SEIRI, and the hybrid LVRT method. To do so, the electrical and mechanical dynamics of these strategies are simulated using a two-mass drive train model, which is necessary for analyzing WTs under mechanical transient. Finally, the performance of wind farms with power reserves while using an inertia based LVRT method will be investigated to show the impact of the power reserve on the WT's LVRT mechanical dynamics.

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.

Enhancing low-voltage ride-through capability of PMSG based on cost-effective fault current limiter and modified WTG control

Series single-resistor based scheme (SSRBS) can achieve a favorable low-voltage ride-through (LVRT) enhancement, also has the advantages of low costs and complexity. Its basic idea is to increase the terminal voltage of wind turbine generator (WTG) during grid faults by inserting a series single-resistor into the grid-side circuit. However, traditional SSRBSs cannot tackle different grid voltage sag events due to the constant resistance of the inserted resistor. Besides, as a device-based scheme, the SSRBSs have weak cost-effectiveness especially, utilizing the superconducting fault current limiter (SFCL). To solve these problems, this paper proposes a cost-effective LVRT scheme by cooperating a resistive-type SFCL and two modified controls. In the cooperative scheme, a surplus power dissipating mechanism based on SFCL is designed, which provides a novel way for SFCL to improve LVRT capability. Moreover, modified reactive power control for WTG is proposed to ensure that SFCL can dissipate constant surplus power irrespective of grid voltage magnitude and to provide reactive power support. Additionally, another modified WTG control is proposed for assisting with the SFCL to dissipate surplus power, which reduces the tape usage and energy-loss of SFCL, thus improving cost-effectiveness. Several cases demonstrate the feasibility and superiority of this scheme.

Simulation Test of a DC Fault Current Limiter for Fault Ride-Through Problem of Low-Voltage DC Distribution

The low voltage direct current (LVDC) distribution networks are connected with too many kinds of loads and sources, which makes them prone to failure. Due to the small damping value in the DC lines, the fault signal propagates so fast that the impact current with the wave front of millisecond and the transient voltage pose great challenges for fault detection. Even worse, some faults with small currents are difficult to detect and the communication is out of sync, resulting in protection misoperation. These problems have severely affected the new energy utilization. In view of this, a DC fault current limiter (FCL) composed of inductance, resistance, and power electronic switch was designed in this paper. The rising speed of fault current can be decreased by the series inductance and the peak value of the fault current can be limited by series impedance, thus in this way the running time can be gained for fault detection and protection. For distributed energy access, by deducing the short circuit fault characteristic expression of LVDC distribution network, the feasibility of FCL was verified. Based on the structure of the bridge-type alternating current (AC) current limiter, the structure and parameters of the DC FCL were determined according to the fault ride-through target. Then, a low voltage ride-through strategy based on DC FCL was proposed for the bipolar short-circuit fault of LVDC distribution network. Finally, MATLAB/Simulink simulation was used to verify the rationality of the proposed FCL and its ride-through strategy.

A novel low voltage ride through strategy for cascaded power electronic transformer

In view of the operating characteristics for voltage sags of AC side of the power electronic transformer(PET), a low-voltage ride through(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. During the asymmetric voltage sag, the negative sequence current of PET is eliminated to ensure the symmetry of the injected current. According to the degree of positive sequence voltage sag, the reactive current injection is provided to assist in voltage recovery. According to the PET active power condition before the voltage sag, the level and direction of which are maintained as far as possible without exceeding the limit, for which the disturbance to the AC and DC grids is reduced. Finally, the effectiveness of the proposed LVRT strategy is verified by simulation model.