Proposed Low-voltage Ride-through Strategy Research Articles

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.

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.

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.

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.

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.

Research on Low Voltage Ride through Strategy for 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.

A comprehensive LVRT strategy of two-stage photovoltaic systems under balanced and unbalanced faults

The increase in penetration of photovoltaic (PV) systems into utility grid poses great challenges in terms of system stability, thus, force several countries to update their grid codes to accommodate the low voltage ride-through (LVRT) capability of PV systems to stay connected to the grid and supply the reactive current to support grid voltage during grid faults. In line with this, a comprehensive LVRT strategy is proposed in this paper for grid-connected PV systems under balanced and unbalanced grid faults. The proposed strategy includes the power references calculation method to meet the requirements of grid codes, the active power reduction method to avoid the overvoltage of the dc-link, the peak current limiting method to avoid the overcurrent of the inverter, and three current reference generators implemented by the proposed current reference formulations to ensure the reliable operation of PV systems under different grid faults. The effectiveness of the proposed strategy has been verified in MATLAB/SIMULINK simulation platform and the results show that the proposed LVRT strategy has good performance under different grid faults.

A Smooth LVRT Control Strategy for Single-Phase Two-Stage Grid-Connected PV Inverters

Based on the inherent relationship between dc-bus voltage and grid feeding active power, two dc-bus voltage regulators with different references are adopted for a grid-connected PV inverter operating in both normal grid voltage mode and low grid voltage mode. In the proposed scheme, an additional dc-bus voltage regulator paralleled with maximum power point tracking controller is used to guarantee the reliability of the low voltage ride-through (LVRT) of the inverter. Unlike conventional LVRT strategies, the proposed strategy does not require detecting grid voltage sag fault in terms of realizing LVRT. Moreover, the developed method does not have switching operations. The proposed technique can also enhance the stability of a power system in case of varying environmental conditions during a low grid voltage period. The operation principle of the presented LVRT control strategy is presented in detail, together with the design guidelines for the key parameters. Finally, a 3 kW prototype is built to validate the feasibility of the proposed LVRT strategy.

A control scheme to fulfill the grid-code under various fault conditions in the grid-connected wind turbines

This paper presents systems and algorithms for the low-voltage ride-through control of wind power systems. The low-voltage ride-through (LVRT) requirement which is included in the grid-code regulates wind power systems to maintain their operation and supply reactive power during grid fault. When the grid fault arises some problems occur in wind power systems, such as over voltage and over current by transient conditions or surplus power. This paper proposes the LVRT strategy for a stable operation of wind power systems under the grid-fault condition. Proposed LVRT methods include a fault detection method, a design of dynamic braking resistor, and a flexible power control under various fault conditions. The simulation results show a validity of the proposed LVRT strategy with a 2-MW wind power system model.