Low Voltage Ride Through Control Strategy Research Articles

Impacts of a LVRT Control Strategy of Offshore Wind Farms on the HTS Power Cable

High temperature superconducting (HTS) power cables are a potential solution for the grid integration of offshore wind farms since the HTS cable can conduct bulk wind power at low voltage levels. However, the transient current through the HTS cable in cases of low voltage ride through (LVRT) operation has a negative impact on the HTS cable operation due to the quenching phenomenon. This paper analyzes the impact of LVRT control strategies on the HTS cable operation. In addition, a coordinated control of wind turbines for LVRT improvement of an offshore wind farm is proposed. The feasibility of the HTS cable application for the grid connection of offshore wind farms is also discussed in this study. The proposed controller is designed for the wind turbine generator based on a type-4 permanent magnet synchronous generator. In the proposed controller, the transient current through the HTS cable is reduced by regulating the machine side power during fault conditions. The feasibility of the proposed controller is validated in the PSCAD/EMTDC program (Manitoba Hydro International Ltd., Winnipeg, Manitoba, Canada, version 4.2.1). The effects of transient current on the cable temperatures and resistances are analyzed in this study. Simulation results show that the proposed control strategy could reduce the transient current and temperature rise of the HTS cable.

Crowbarless Symmetrical Low-Voltage Ride Through Based on Flux Linkage Tracking For Brushless Doubly Fed Induction Generators

The significant benefits of brushless doubly fed induction generators (BDFIGs) are higher reliability and lower maintenance cost due to the lack of a brush gear. Recent studies on BDFIGs have shown promising applications in wind power generation. The low-voltage ride-through (LVRT) capability of wind turbines is one of the most important properties specified in the grid codes of many countries. A crowbarless LVRT control strategy based on flux linkage tracking for a BDFIG under symmetrical voltage dips is proposed in this article. A novel and simple controller in static reference frame is presented to implement flux linkage tracking. To both suppress the control winding (CW) current and increase power winding (PW) reactive current, a time-sharing control and CW current-first control solution is proposed. In the most severe case, where the BDFIG rotor speed is 100% of the maximum rotor speed and under 100% symmetrical voltage dips, the experimental results show that the CW current peak can be limited to 2.01 p.u. of the CW current rating and the torque ripple is small during the fault. The experimental results also show the possibility of the BDFIG to ride through the severest voltage dips without extra hardware, such as a crowbar.

A Control Strategy for a Three-Phase Grid Connected PV System under Grid Faults

This paper proposes a Low-Voltage Ride-Through control strategy for a three-phase grid-connected photovoltaic (PV) system. At two stages, the topology is considered for the grid-tied system fed by a photovoltaic generator with a boost converter followed by a three-phase voltage source inverter. A flexible control strategy is built for the proposed system. It accomplishes the PV converter operations under the normal operating mode and under grid faults (symmetrical and asymmetrical grid voltage sag). The boost converter is controlled via an incremental conductance maximum power point tracking technique to maximize the PV generator power extraction. In the case of voltage sag, the implemented control strategy provides a switch between MPPT mode and non-MPPT mode to ensure the protection of the power converters. Theoretical modeling and simulation studies were performed, and significant results are extracted and presented to prove the effectiveness of the proposed control algorithm.

Stability Analysis and Enhancement of Type-4 Wind Turbines Connected to Very Weak Grids Under Severe Voltage Sags

Modern grid codes require wind turbines (WTs), e.g., type-4 WTs, to have low voltage ride-through (LVRT) capability, and recently, great attention has been attracted on the stability issues of WTs during the period of the LVRT. In this paper, we investigate the influences of the LVRT control on the stability of type-4 WTs under severe voltage sags in very weak grids. First, by proposing a novel quasi-static modeling method, we explore the existence of the WT's equilibrium during the LVRT. It is shown that the WT can lose the equilibrium under certain circumstances, leading to instability of the whole WT system. Furthermore, we use the small-signal analysis to illustrate that even there exists an equilibrium, the WT can become linearly unstable due to the dynamics of the current references generated by the LVRT control. We propose an enhanced LVRT control strategy to deal with these instability problems. Simulations verify our analysis and effectiveness of the proposed control strategy.

Dynamic Stability Analysis and Improved LVRT Schemes of DFIG-Based Wind Turbines During a Symmetrical Fault in a Weak Grid

With the increasing penetration of the wind power, the stability issues of the weak AC grid-connected doubly fed induction generator (DFIG)-based wind turbines during low-voltage ride through (LVRT) cannot be neglected. In order to explore the instability mechanism of DFIG system during weak grid fault, the small signal state-space model is established in this paper. The results of the modal analysis show that the dominant unstable poles are mainly impacted by the phase-locked loop (PLL), rotor current control loop, and terminal voltage during the fault, where the PLL is the dominant factor. Furthermore, the impact of each factor on the system dynamic stability is comprehensively evaluated, which indicates that the controller bandwidth under normal grid condition is no longer applicable to the fault condition due to the interaction between the controller and grid. Then, the optimal current proportion which can significantly improve the system stability is deduced. Finally, based on the analysis, this paper proposes the improved LVRT control schemes from two aspects of either injecting active current or decreasing PLL bandwidth to enhance the small signal stability of the system. The effectiveness of the proposed LVRT control strategies is validated by the simulation and experiments.

Low‐voltage ride through control strategy of virtual synchronous generator based on the analysis of excitation state

Virtual synchronous generator (VSG) possesses the advantage of friendly interaction with power grid by simulating synchronous generator characteristics. However, its low-voltage ride through (LVRT) capability is insufficient. The excessive output current of VSG easily causes wind turbines to break away from the power grid, which will exacerbate the negative impact of grid fault. Thus, a new LVRT control strategy is proposed based on the analysis of excitation state for VSG. The droop characteristic, reactive power loop and active power loop of the VSG are improved, respectively, by specifically analysing the response characteristics of VSG. Moreover, the additional current loop is redesigned to assist the system operating in the under excitation state and suppress unbalanced currents without changing the original VSG characteristics. Furthermore, a new orientation method is adopted to accelerate the transient process and achieve better transient performance. It is worth noting that the proposed control strategy does not need switch control algorithm with smooth handoff algorithm under grid fault, and it can deal with both symmetric and asymmetric grid voltage drop problems at the same time. The correctness and feasibility of proposed scheme are verified by rigorous theoretical deduction and complete simulation verification.

Control Strategy for Grid-Connected Three-Phase Inverters During Voltage Sags to Meet Grid Codes and to Maximize Power Delivery Capability

Inverter-based distributed generation plays a vital role in the stability and reliability of new power systems. Under voltage sags, these systems must remain connected to the electrical network according to the stringent requirements of grid codes (GCs). Low-voltage ride-through (LVRT) control strategies are becoming a common trend in power electronics research. However, previous studies of these control strategies have not dealt with the different possible scenarios presented by new GCs, and many of them focus on a very limited number of control objectives. In this study, an algorithm to maximize the converter capabilities was developed and subjected to experimental tests during different voltage sags. In this research, based on unbalanced voltage drops of several severity levels, six different cases of current injection are identified while taking into consideration the restrictions imposed by GCs. The research results represent a further step toward the development of flexible controllers adaptable to the environments of intelligent electricity grids with high integration of distributed generation.

Low-Voltage Ride-Through Control Strategy for a Grid-Connected Energy Storage System

This paper presents a low-voltage ride-through (LVRT) control strategy for grid-connected energy storage systems (ESSs). In the past, researchers have investigated the LVRT control strategies to apply them to wind power generation (WPG) and solar energy generation (SEG) systems. Regardless of the energy source, the main purpose of the LVRT control strategies is to inject reactive power into the grid depending on the grid-code regulations using the grid-side inverter; the proposed LVRT control strategy for grid-connected ESSs also has the same purpose. However, unlike the WPG and SEG systems having unidirectional power flow, grid-connected ESSs have a bidirectional power flow. Therefore, the charging condition of the grid-connected ESSs should be considered for the LVRT control strategy. The proposed LVRT control strategy for grid-connected ESSs determines the injection quantity of the active and reactive currents, and the strategy depends on the voltage drop ratio of the three-phase grid. Additionally, in this paper, we analyzed the variations of the point of common coupling (PCC) voltage depending on the phase of the reactive current during the charging and discharging conditions. The validity of the proposed LVRT control strategy is verified and the variations of the PCC voltage of the grid-connected ESS are analyzed by simulation and experimental results.

Low-Voltage Ride-Through Control Strategy for a Virtual Synchronous Generator Based on Smooth Switching

The grid-connected inverter with virtual synchronous generator (VSG) control technology can improve the friendliness of a distributed power supply to the power grid. However, its low-voltage ride-through (LVRT) capability is insufficient, which results in difficulties in limiting the current and provide reactive power support. A new LVRT control strategy based on the smooth switching is proposed in this paper. In this strategy, the voltage source mode of VSG is transformed into current source mode to limit the output current and provide reactive power support through the proportional resonance current control algorithm under grid fault. Furthermore, the feedback tracking synchronization strategy of the phase angle is employed to realize the smooth switching between two modes. When the grid fault recovers, it can directly switch back to grid-connected operation mode through a delay module without an additional algorithm. The simulation results verify the correctness and feasibility of the proposed control strategy.

Coordinated Control Strategy for a Hybrid Wind Farm with DFIG and PMSG under Symmetrical Grid Faults

This paper presents a coordinated control strategy for a hybrid wind farm with doubly-fed induction generator (DFIG)- and direct-driven permanent-magnet synchronous generator (PMSG)-based wind turbines under symmetrical grid faults. The proposed low-voltage ride-through (LVRT) strategy is based on a novel current allocation principle and is implemented for individual DFIG- or PMSG-based wind turbines. No communication equipment between different wind power generators is required. By monitoring the local voltages and active power outputs of the corresponding wind generators, the proposed control strategy can control the hybrid wind farm to provide the maximum reactive power to support the grid voltage during a symmetrical grid fault. As a result, the reduction in the active power output from the hybrid wind farm can be decreased, which also helps avoid generator over-speed issues and supply active power support for the power grid. In addition, the reactive current upper limits of DFIG- and PMSG-based sub-wind farms are investigated by considering different active power outputs and different grid voltage dip depths, and the feasible regions of the two types of sub-wind farms for meeting the LVRT requirements are further studied. Finally, the effectiveness of the proposed coordinated LVRT control strategy for the hybrid wind farm is validated by simulation and experimental results.

Modelling and simulation of the photovoltaic power station considering the LVRT and HVRT

With the increasing capacity of the photovoltaic (PV) power station, the flexibility and security of power grid face significant challenges. It is necessary for the PV power station to have the capability of voltage ride through (VRT). Based on the newest technical rule (Q/GDW 1617-2015), the control strategies of the low VRT (LVRT) and the high VRT (HVRT) are investigated and improved here by fitting and analysing of the field test data of the PV plant. The improved LVRT control strategy is able to accurately simulate the active power ramp recovery process of the field test data and is able to provide the dynamic voltage support by the reactive power control. The proposed HVRT control strategy makes the PV power station absorb a certain amount of reactive power to restrict the voltage within specific ranges required by the HVRT grid codes. By comparing with the requirements of the technical rule for connecting PV power station to power grid (Q/GDW 1617-2015) and the simulation results conducted on Matlab/Simulink platform, the proposed LVRT and HVRT control strategies are validated.

LVRT Control Strategy of DFIG Based Wind Turbines Combined the Active and Passive Protections

Due to its advantages, Doubly Fed Induction Generator (DFIG) is the most used generator in the wind power area. However, the DFIG provides electrical power at a constant frequency even if the rotor speed varies, also it allows a better capture of wind energy. Although the DFIG has a high sensitivity regarding the electrical faults, which brings up many challenges in terms of compliance: Power provider and electrical operator (in terms of production continuity and quality of energy). Actually, the grid connection requirements impose strict rules to respect for Low Voltage Ride Through (LVRT) and grid support capabilities following the Grid Code (GC). Therefore, it’s crucial for wind turbines (WT) to propose an advanced control. In fact, when detecting voltage dips, WTs must stay connected to the grid to provide the required reactive power in order to have a safe and reliable operation. The objective of this article is to propose a new LVRT strategy able to keep WTs connected to the grid despite severe voltage dips. The principle of this strategy is to make a connection between the level of dips and the optimal solution to overcome the fault. For this reason, the proposed strategy is based on the combination of two solutions (an active and a passive method): The first aims to improve the control strategy to mitigate the over-current at low voltage dips. The second is applied for severe voltage dips using protection circuits: Series Dynamic Braking Resistor (SDBR) and DC-Chopper. LVRT control strategy of DFIG based wind turbines combined the active and passive protections

Synchronous Flux Weakening Control With Flux Linkage Prediction for Doubly-Fed Wind Power Generation Systems

Under grid-voltage dips, there exist dc and negative sequence components in the stator and rotor flux of doubly-fed induction generator (DFIG). As a result, higher transient overcurrent is generated in the rotor. To enhance the low-voltage ride-through (LVRT) ability of the DFIG, a synchronous flux weaken control strategy with flux linkage prediction is proposed to suppress the transient overcurrent. In the proposed control strategy, the deadbeat predictive control is used to realize rapid synchronization and weak interaction between the stator and rotor flux by flux linkage prediction under grid-voltage dips. A series of research is carried out on a typical 1.5-MW DFIG system, and comparisons are made with the LVRT control strategy based on proportional-resonant (PR) controller to validate the proposed control strategy. The results indicate that the proposed control strategy is effective in suppressing overcurrent in the stator and rotor and reducing oscillations in torque, which largely improves the performance of the DFIG during grid-voltage dips.

Research on the Low Voltage Ride-through Control Strategy based on State Feedback Linearization

Based on the analysis of on the dynamic mathematical model of double-fed wind turbines, exact non-linear state feedback linearization control method is adopted to design the control strategy of the LVRT (low voltage ride-through). Firstly, we select the output functions according to control target, and formulate the nonlinear system including input and output. Then, by using linearization the state feedback and using proper coordinate transforming, we convert this nonlinear system to the linear and controllable system. Finally, we finish the control strategy of the control system through the optimized linear control strategy. This control strategy only depend on the parameters of the wind turbines, and strengthen the robust of the parameters of control strategies.

해상풍력 연계용 HVDC의 DC전압 안정화를 위한 DC Link의 발전기측 컨버터 제어 전략

This paper presents DC voltage recovery time improvement method in DC link of High Voltage Direct Current(HVDC) with offshore wind farm. The wind farm should be satisfied Low Voltage Ride Through(LVRT) control strategy when grid faults occur. The LVRT control strategy indicates actions which have to be executed according to the voltage dip ratio and the fault duration. However, The LVRT control strategy makes between wind farm and power system through DC Link voltage when grid fault occurs. The de-loading scheme is one of the method to control the DC voltage. But de-loading scheme need to long DC voltage recovery time. Thus, this paper proposes an improved de-loading scheme and we analysis DC voltage and active power reference through a simulation.

Low-Voltage Ride-Through Operation of Power Converters in Grid-Interactive Microgrids by Using Negative-Sequence Droop Control

Due to the increasing penetration level of microgrids (MGs), it becomes a critical issue for MGs to help sustaining power system stability. Therefore, ancillary services, such as the low-voltage ride-through (LVRT) capability should be incorporated in MGs in order to guarantee stable operation of the utility grid during grid faults. In this paper, a LVRT control strategy based on positive/negative sequence droop control is proposed for grid-interactive MGs to ride-through voltage sags with not only inductive/resistive, but also complex line impedance. By using the proposed control strategy, MGs can support the grid voltage, make profits, and also ride-through the voltage dip during the whole fault period. A two-layer hierarchical control strategy is proposed in this paper. The primary controller consists of voltage and current inner loops, a conventional droop control and a virtual impedance loop, while the secondary controller is based on a positive/negative sequence droop scheme which is able to coordinate the power injection during voltage sags. Experimental results are obtained to verify the effectiveness of the proposed control strategy.

Control Strategy to Maximize the Power Capability of PV Three-Phase Inverters During Voltage Sags

Under voltage sags, grid-tied photovoltaic inverters should remain connected to the grid according to low-voltage ride-through requirements. During such perturbations, it is interesting to exploit completely the distributed power provisions to contribute to the stability and reliability of the grid. In this sense, this paper proposes a low-voltage ride-through control strategy that maximizes the inverter power capability by injecting the maximum-rated current during the sag. To achieve this objective, two possible active power situations have been considered, i.e., high- and low-power production scenarios. In the first case, if the source is unable to deliver the whole generated power to the grid, the controller applies active power curtailment to guarantee that the maximum rated current is not surpassed. In the second case, the maximum allowed current is not reached, thus, the control strategy determined the amount of reactive power that can be injected up to reach it. The control objective can be fulfilled by means of a flexible current injection strategy that combines a proper balance between positive- and negative-current sequences, which limits the inverter output current to the maximum rated value and avoid active power oscillations. Selected experimental and simulation results are reported in order to validate the effectiveness of the proposed control strategy.

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.

Discussion on “A Genetic Algorithm-Based Low Voltage Ride-Through Control Strategy for Grid Connected Doubly Fed Induction Wind Generators”

The authors are commended for their effort in presenting an interesting control scheme for enhancement of low voltage ride-through (LVRT) capability of wind turbines with doubly fed induction generator (DFIG). A fuzzy controller, tuned by genetic algorithm, is proposed.

Closure to Discussion on “A Genetic Algorithm-Based Low Voltage Ride-Through Control Strategy for Grid Connected Doubly Fed Induction Wind Generators”

Regarding the second comment, it should be pointed out that the only reason the response of the conventional control system is cited in Fig. 11 is to show that with the proposed modification of the conventional control system, the DFIG manages to ameliorate its overall response and ride-through the fault without any auxiliary hardware. There is no point to compare the effectiveness of the two systems, as in real cases, the conventional control system cannot ride through the fault by itself, but it also requires a crowbar in order to protect the DFIG. As mentioned in the introduction, during the crowbar activation, the DFIG is converted to a conventional squirrel-cage induction generator absorbing a large amount of reactive power from the grid. On the contrary, using the proposed method, the rotor side converter is not disabled during the fault, so it is possible to supply reactive power to the grid in order to support the voltage recovery, as imposed by several grid codes. In the case studied, the grid voltage, being supported by the proposed control system, recovers quickly, so there is no need for large amounts of reactive power from the DFIG after the fault. However, it is obvious that while the ac voltage remains below its reference value, the DFIG delivers the required amount of reactive power to the grid, in order to support the grid voltage. Once again, we emphasize that Fig. 11 shows a no-realistic response of a DFIG, because in a real case, the control system would protect the wind turbine through the crowbar just after the occurrence of the fault.