Ride-through Research Articles

Optimized Series Dynamic Braking Resistor for LVRT of Doubly-Fed Induction Generator With Uncertain Fault Scenarios

The terminal-connected series dynamic braking resistor (SDBR) is applied to assist the low-voltage ride-through (LVRT) of the doubly-fed induction generator (DFIG). With the fault current and switch-in of the SDBR, the stator voltage oscillates, thus the constant stator voltage drop assumption is invalid and the effect of the converter current control is weakened with the changing voltage-oriented reference frame. In this paper, the xy frame of the point of common coupling is applied to the converter control to avoid oscillation of the reference frame. The analytical expression of fault current with the SDBR and constant converter current control is derived. To evaluate the LVRT effect, the analytical analysis of the LVRT transient is carried out. The resistance of the SDBR is optimized based on an index combining the capabilities of the DFIG to provide the active power support and damp the electromagnetic torque oscillation. The uncertainties of the fault scenario are considered in the optimization algorithm by applying the probabilistic method. Simulation results show that the improved LVRT effect of the DFIG is realized with optimization to the SDBR resistance and its switch-in criterion.

Requirement Analysis of Wind Turbine High Voltage Ride-through Capability Detection and Simulation

With the number of wind farms connected to the power system increasing rapidly, the impact of wind power generation system on power grid system stability is becoming increasingly prominent. Especially when the power grid has transient fault, there will be problems such as reverse power flow, large-area power failure of wind farm and so on. This will directly lead to the power system instability and seriously affect the power grid security and stability. Especially in the stage of voltage recovery after the fault is eliminated, the low-voltage fault of the power grid might lead to a sharp rise in the voltage of power grid and overvoltage on the high-voltage line connected to the power grid with wind farm, resulting in the disconnection of the wind power grid connected system. However, China’s current standards do not specify the high-voltage crossing of wind power grid connected system, especially the high-voltage crossing detection device, detection method and detection procedure. Therefore, it is urgent to study the detection technology of wind power high voltage crossing and put forward the detection method. There are many reasons for high-voltage power grid fault, and the interaction between power grid and inverter is more complex in high-voltage fault. If the type test method is used to simulate various high-voltage fault conditions in power system and carry out the test and study on the HVRT( high voltage ride through) performance of wind turbine, the human and material cost is huge and there are many potential safety hazards. At the same time, the grid connection performance of wind turbines are largely related to the control strategy of the controller. Therefore, the semi physical simulation method can be used to research the HVRT detection technology of wind turbine. This can effectively avoid the safety problem of large current and high voltage in the primary circuit, and realize the physical connection between the real controller and the flexible high voltage fault platform model through hardware in the loop simulation. It can simulate the wind turbine capability of high-voltage ride through under various high-voltage fault conditions. It not only has low test cost, but also can deeply study the interaction between power grid and power grid.

High voltage ride-through of doubly-fed induction generator based on grid side and rotor side converters cooperative control

In order to further improve the doubly-fed wind turbine high voltage ride-through ability, this article is different from the traditional control research only considering the rotor side converter in the power grid failure in stator flux dynamic changes of active and reactive power decoupling, the influence of the coupling current and the additional quantity of the stator flux changes as the feedforward compensation component in the current reference value. On this basis, an improved control strategy is also proposed for the grid-side converter, which takes into account the power imbalance between the two ends of the DC bus and the voltage mutation of the grid. Simulation in power grid voltage surge, coordinated control based on the grid-side converter and rotor-side converter control strategy of doubly-fed wind power generator of high voltage ride-through the active power and reactive power of wind power unit, the rotor over current, DC bus voltage fluctuations, etc all have better inhibition, effectively improve the doubly-fed wind turbine high voltage crossing ability.

A Cooperative Approach of Frequency Regulation Through Virtual Inertia Control and Enhancement of Low Voltage Ride-through in DFIG-based Wind Farm

Doubly-fed induction generator (DFIG)-based wind farms (WFs) are interfaced with power electronic converters. Such interfaces are attributed to the low inertia generated in the WFs under high penetration and that becomes prevalent in a fault scenario. Therefore, transient stability enhancement along with frequency stability in DFIG-based WFs is a major concern in the present scenario. In this paper, a cooperative approach consisting of virtual inertia control (VIC) and a modified grid-side converter (GSC) approach for low voltage ride-through (LVRT) is proposed to achieve fault ride-through (FRT) capabilities as per the grid code requirements (GCRs) while providing frequency support to the grid through a synthetic inertia. The proposed approach provides LVRT and reactive power compensation in the system. The participation of the VIC in a rotor-side converter (RSC) provides frequency support to the DFIG-based WFs. The combined approach supports active power compensation and provides sufficient kinetic energy support to the system in a contingency scenario. Simulation studies are carried out in MATLAB/Simulink environment for symmetrical and unsymmetrical faults. The superiority of the proposed scheme is demonstrated through analysis of the performance of the scheme and that of a series resonance bridge-type fault current limiter (SR-BFCL).

Coordinated Low Voltage Ride-Through of MMC-HVDC Transmission System and Wind Farm With Distributed Braking Resistors

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission is becoming a trend in offshore wind-farm integration. However, the large DC-side energy dissipation equipment, which is utilized to dissipate surplus wind power under grid fault conditions, will largely increase the cost of MMC-HVDC systems. To reduce the cost, a novel low voltage ride-through (LVRT) strategy is proposed in this paper. When a grid fault occurs and the DC voltage exceeds the limit, the sending end converter is controlled to reduce the AC voltage of the wind farm. The LVRT of the wind generators will be activated, and the output active power of the wind farm is reduced. With this coordination, the DC-side energy dissipation equipment only needs to dissipate surplus power in the early stage of the grid fault before the output active power of the wind farm drops. Therefore, the heat generated by braking resistors can be significantly reduced. On this basis, the braking resistors can be distributed into the submodules of the receiving end converter (REC) station. The LVRT problem can be solved without building an individual energy dissipation station. Using the proposed coordination strategy, the construction cost of the MMC-HVDC system with offshore wind farm integration can be significantly reduced.

Comprehensive Evaluation of the Transient Reactive Power Support Capability of New Energy Stations Based on TOPSIS

In order to effectively evaluate the reactive power support capability of new energy stations, an evaluation system is established with the high voltage ride through(HVRT) capability, low voltage ride through (LVRT) capability and reactive power support capability selected as the evaluation index. And then, a comprehensive evaluation method to effectively evaluate the transient reactive support capability of new energy stations is proposed in this paper. The weight of a single index was determined by the coefficient of variation method and then the comprehensive evaluation index was determined based on TOPSIS(Technique for Order Preference by Similarity to an Ideal Solution). Finally, the rationality and effectiveness of the proposed evaluation method are verified by the measured data of a wind farm.

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.

Voltage Restoration Control for Microgrid With Recurrent Wavelet Petri Fuzzy Neural Network

This study presents a voltage restoration control (VRC) based on battery energy storage system (BESS), which can be used for both supporting power source and voltage compensation. Voltage restoration is an important task for the power control of microgrid during utility disturbances. One of the disturbances is caused by short circuit on power line of the microgrid, which may lead to voltage sag and even blackout of the microgrid system. To tackle this problem, the recurrent wavelet petri fuzzy neural network (RWPFNN) controller is proposed in this study for the VRC of BESS to provide fast control response to mitigate the transient impact. Moreover, to examine the compliance with the requirements of low voltage ride through (LVRT) of the photovoltaic (PV) plant and investigate the performance of the proposed VRC, the microgrid built in Cimei Island in Penghu Archipelago, Taiwan, is investigated. Furthermore, the PV system, the wind turbine generator (WTG) system and the BESS are connected to the same point of common coupling (PCC) with separated step-up transformers in the microgrid. In addition, the diesel generators provide the main power sources and form the isolated microgrid system. Through the hardware in the loop (HIL) mechanism, which is built using OPAL-RT real-time simulator, with two floating-point digital signal processors (DSPs), the effectiveness of proposed intelligent controllers can be verified and demonstrated.

Experimental Study on Low Voltage Ride-Through of DFIG-Based Wind Turbine

This paper presents an architecture of the Doubly-Fed Induction Generator based on Wind Energy Conversion System (DFIG-WECS) to study with the Low Voltage Ride Through (LVRT) capability. The proposed architecture consists of three main parts namely the wind turbine, electric generator, control systems. The wind turbine and gearbox are modeled by a servo motor driven by using an inverter system. The electric generator uses the Wound Rotor Induction Generator (WRIG). The control system composes of the rotor-side converter (RSC) and the grid-side converter (GSC). The integration of the digital signal processor (DSP) with the Simulink/Matlab software of the vector control scheme is implemented to control both RSC and GSC. Besides, to protect the overcurrent of the generator’s rotor and overvoltage of the DC-link, the crowbar and chopper circuits and the logical scheme between the crowbar and the RSC are also proposed. To approve the control algorithms implemented in DSP as hardware in the loops, the proposed 0.8kW DFIG-WESC system has been performed under the experimental environment considering LVRT requirement in German grid code.

Multi-Functional PV Inverter With Low Voltage Ride-Through and Constant Power Output

Renewable photovoltaic (PV) energy is a primary contributor to sustainable power generation in microgrids. However, PV grid-tied generators remain functional as long as the grid voltage and the input PV source remain normal. Abnormal conditions like transient grid sags or solar irradiation flickering can make the grid-tied inverter go offline. Simultaneous shut down of PV generators residing in the distribution grid may lead to an overall grid instability or outage. Therefore, PV generators must be equipped with fault-ride-through mechanisms in order to remain connected and operational during faults. This paper presents a PV-inverter with low-voltage-ride-through (LVRT) and low-irradiation (LR) compensation to avoid grid flickers. The single-phase inverter rides through the voltage sags while injecting reactive power into the grid. The proposed control strategy ensures a steady DC-link voltage and remains connected to the grid during AC-side low voltage and DC-side low-irradiation faults. Unlike other PV inverters, the controller maintains the maximum-power-point-tracking (MPPT) in all conditions. LVRT, constant power output, and robust MPPT are the noticeable features of the proposed system. Frequency analysis, simulations, and a laboratory prototype validate the proposed control strategy.

Renewable Microgrid Protection Strategy Coordinating with Current-based Fault Control

The renewable microgrid (RMG) is a critical way to organize and utilize new energy. Its control and protection strategies during the fault are the core technologies to ensure the safe operation and stability of the system. The traditional protection principles are unsuitable for RMGs due to the flexibility of RMG operation, the complexity of RMG topology, and the variety of fault control strategies of inverter-interfaced distributed generators (IIDGs). The traditional fault component protection principle is affected by the low voltage ride-through (LVRT) control strategy and will fail in some scenarios. In order to make the fault component protection principle available in every scenario, a current-based fault control strategy is proposed. Specific fault characteristics are generated by the grid-feeding IIDGs during the fault so they can be equivalent to the open circuits, and the fault models in additional network can be simplified. By analyzing the fault characteristics, an RMG protection strategy based on the current-based fault control of IIDGs is presented. The fault directions of feeders can be distinguished and the fault feeder can be located accurately in both grid-connected and islanded RMGs. Then, the grid-feeding IIDGs can transit to LVRT mode smoothly. Thus, IIDGs are considered comprehensively in terms of coordinating with fault control and fault characteristic generation. Finally, the experimental results of the hardware platform prove the effectiveness of the proposed current-based fault control strategy, and the simulation results based on PSCAD/EMTDC verify the correctness of the protection strategy.

Improved Particle Swarm Optimization Based Selective Harmonic Elimination and Neutral Point Balance Control for Three-Level Inverter in Low-Voltage Ride-Through Operation

In high-power applications, selective harmonic elimination pulsewidth modulation (SHEPWM) method is widely used to eliminate low-order harmonics in three-level inverters with low power losses. However, it faces the challenges that the conventional optimization methods for solving harmonic elimination equations in SHEPWM are easy to be trapped in local optimum. Moreover, the neutral point (NP) voltage of three-level inverter cannot be balanced under low-voltage ride-through (LVRT) condition with conventional SHEPWM method. To address these problems, a SHEPWM scheme is proposed, which is on the basis of an improved particle swarm optimization (IPSO) algorithm to solve the harmonic elimination equations and an enhanced NP balance control method in LVRT operation. First, an IPSO algorithm is proposed to calculate the switching angles of the transcendental equations for harmonic elimination. The IPSO algorithm defines a nonlinear negative exponential inertia weight considering the current and optimal fitness value, which can dynamically adjust the local and global searching speed and step size according to the actual situation, greatly improving the convergence speed and solution accuracy and avoiding falling into the local optimum. Then, by increasing the degree of freedom of the output current direction and combining with the NP voltage deviation, an enhanced NP voltage balance control method of SHEPWM in LVRT operation is proposed. The validity of the proposed method is proved by simulation and experimental results.

A Method for Real-Time Prediction of the Probability of Voltage Sag Duration Based on Harmonic Footprint

In this paper, a novel method for real-time prediction of voltage sag duration is proposed. It is based on the recently introduced new characteristic of voltage sag, named harmonic footprint, and is formulated using a logistic regression model. The concept is mathematically formulated and statistically analyzed using an extensive set of real grid measurement data which are recorded in distribution grids. Furthermore, the proposed method is applied as a part of an advanced grid-tie converter control. It is included in previously developed methods for fast sag detection and magnitude of voltage sag prediction. The algorithm is applied to the control of grid-tie converters used in distributed generators and tested with real/grid measurement data in the IEEE 13-bus test grid by simulations and in the IEEE 33-bus test grid using a hardware-in-the-loop (HIL) microgrid laboratory testbed. It is shown that this method can prevent unnecessary tripping of distributed generators (DG) and improve low-voltage ride-through (LVRT) support. In addition, the model has the potential to be applied to a wide range of devices or algorithms for the protection, monitoring, and control systems of distribution grids.

Formulation of Low Voltage Ride-Through (Lvrt) Curve Using Discrete Optimization Strategy with Reliability Constraints for High Penetration of Wind Power - a Case Study in Taipower System

Formulation of Low Voltage Ride-Through (Lvrt) Curve Using Discrete Optimization Strategy with Reliability Constraints for High Penetration of Wind Power - a Case Study in Taipower System

Evaluating Reactive Power-Based Voltage Support for the Low Voltage Ride Through Capability of the Distributed Energy Resource

Most of the grid codes generally require the Distributed Energy Resources (DERs) in order to remain connected throughout short-circuit grid faults and to provide the grid voltage support through reactive power-based regulation via additional Reactive Current Injection (aRCI) during the Low Voltage Ride Through (LVRT). It has been found out that the X/R ratio of the DER connection affects the effectiveness of the voltage support in improving the voltage sags. In this paper, the theoretical analysis of the system's X/R ratio impact on the effectiveness of the investigated voltage support is given. Furthermore, with the help of a MATLAB Simulink-based simulation study, the theoretical analysis is verified. The research is meant to substantiate a clear understanding of the effect of low X/R ratio to the voltage support in improving voltage sag. Finally, the rising number of the DERs on the distribution grid has been linked to the weakening of the grid strength, which often causes the system instability following the post-LVRT state. From the simulation, it has been found out that on a weak grid, the instability during the post-LVRT period could be prevented through correctly adjusted reactive power-based voltage regulation.

A Distributed Control Scheme Using SiC-Based Low Voltage Ride-Through Compensator for Wind Turbine Generators.

As the penetration of renewable energy power generation, such as wind power generation, increases low voltage ride-through (LVRT), control is necessary during grid faults to support wind turbine generators (WTGs) in compensating reactive current to restore nominal grid voltages, and maintain a desired system stability. In contrast to the commonly used centralized LVRT controller, this study proposes a distributed control scheme using a LVRT compensator (LVRTC) capable of simultaneously performing reactive current compensation for doubly-fed induction generator (DFIG)-, or permanent magnet synchronous generator (PMSG)-based WTGs. The proposed LVRTC using silicon carbide (SiC)-based inverters can achieve better system efficiency, and increase system reliability. The proposed LVRTC adopts a digital control scheme and dq-axis current decoupling algorithm to realize simultaneous active/reactive power control features. Theoretical analysis, derivation of mathematical models, and design of the control scheme are initially conducted, and simulation is then performed in a computer software environment to validate the feasibility of the system. Finally, a 2 kVA small-scale hardware system with TI’s digital signal processor (DSP) as the control core is implemented for experimental verification. Results from simulation and implementation are in close agreement, and validate the feasibility and effectiveness of the proposed control scheme.

Low voltage ride through enhancement in DFIG based WECS by hybrid method

ABSTRACT This paper proposes a hybrid technique for the enhancement of Low Voltage Ride Through (LVRT) capacity in a Doubly Fed Induction Generator (DFIG) based Wind Energy Conversion System (WECS). The proposed hybrid technique is the joint execution of S-shaped binary version of Whale Optimisation Algorithm (S-bWOA) and Random Decision Forest (RDF), hence it called S-bWOA-RDF method. The major objective is to ensure the LVRT capacity in the WECS at voltage drop including failure stages. S-bWOA model is used to identify the optimal solutions for offline mode as an available search location under objective function and generates the training dataset. In light of proficient dataset, RDF performs and predicts better feasible machine side with grid side converter control signals. The proposed technique is activated in MATLAB/Simulink site, and the efficiency of the proposed S-bWORDF method is compared with the existing methods, such as Random Decision Forest and Whale Optimisation Algorithm. The IAE and ITAE of the proposed and existing methods are also analysed. The IAE and ITAE of the proposed hybrid technique are identified as 5.0071 and 54.0475. The comparison results demonstrate the efficiency of the S-bWORDF method and confirm its potential for enhancing the LVRT of DFIG based WECS.

An internal parallel capacitor control strategy for DC-link voltage stabilization of PMSG-based wind turbine under various fault conditions

In recent years, wind energy conversion systems (WECSs) have been growing rapidly. Due to various advantages, a permanent magnet synchronous generator (PMSG) is an appealing solution among different types of wind generators. As wind power penetration level in the grid increases, wind power impacts the grid and vice versa. The most essential concerns in the system are voltage sag and swell, and grid code compliance, particularly for low voltage ride-through (LVRT) and high voltage ride-through (HVRT) capability, is a pressing necessity. This paper presents a parallel capacitor (PC) control strategy to enhance the LVRT and HVRT capability of PMSG. Furthermore, this study presents a method for the sizing of a PC system for the reduction of the overvoltage of the DC-link during voltage sags and swell. Fast Fourier transform analysis is used to determine the total harmonic distortion (THD) for the injected current into the grid. The obtained results illustrate the effectiveness of the proposed system in keeping the DC-link voltage below the limit, power quality improvement, and increasing the LVRT and HVRT capability. Models of wind turbine, PMSG, and PC control system are built using MATLAB/SIMULINK software.

Enhancement in fault ride through capabilities with inertia control for DFIG-wind energy conversion system

Doubly fed induction generator (DFIG)–based wind energy conversion system (WECS) has been found very effective in harnessing wind power. Conferring to grid code requirements, DFIG needs to stay connected to the grid under fault conditions. This implicates the requirement of enhancing the transient stability through augmenting the low-voltage ride through (LVRT) capability of DFIG. There are many software and hardware solutions for the improvement of LVRT. Fault current limiter is one of the frequently used hardware solutions for the argumentation of fault ride through requirements. Instead of LVRT, DFIG-based WECS also faces low inertia problems at fault condition. In this article, a combined approach of virtual inertia control with an external retrofit adaptive hysteresis control–based bridge-type fault current limiter (AHC-BFCL) is proposed to improve the LVRT capability as well as the inertia of DFIG-WECS. The results are compared with that of a conventional logic control-BFCL and proportional–integral-BFCL.

Low-Voltage Ride Through Enhancement of Both DFIG and SEIG-Based Wind Power Plants by RC-Type Solid-State Fault Current Limiter

The low-voltage ride-through (LVRT) is the main requirement of grid codes for integration of wind power plants (WPPs) to the power system. This paper proposes a RC-type solid state fault current limiter (SSFCL) to enhance the LVRT requirement of WPPs. It is based on the RL-type SSFCL in which its configuration is changed to satisfy the LVRT requirements. To evaluate the proposed RC-type SSFCL performance, extended time domain simulations are executed in PSCAD/EMTDC software environment. Also, the performance of the RC-type and RL-type SSFCLs are compared under both symmetrical and asymmetrical fault conditions to show the efficiency of the proposed RC-type SSFCL. Both doubly-fed induction generator (DFIG) and self-excited induction generator (SEIG) are used to examine the RC-type SSFCL performance. Based on simulation results, the RC-type SSFCL provides an effective hardware scheme to augment the LVRT requirements of both DFIG and SEIG based WPPs and outperforms the RL-type SSFCL.