Asymmetrical Voltage Sag Research Articles

Low Voltage Ride-Through Capability Solutions for Permanent Magnet Synchronous Wind Generators

Due to the increasing number of wind power plants, several countries have modified their grid codes to include specific requirements for the connection of this technology to the power system. One of the requirements is the ride-through fault capability (RTFC), i.e., the system capability to sustain operation during voltage sags. In this sense, the present paper intends to investigate the behavior of a full-converter wind generator with a permanent magnet synchronous machine during symmetrical and asymmetrical voltage sags. Two solutions to improve the low voltage ride-through capability (LVRT) of this technology are analyzed: discharging resistors (brake chopper) and resonant controllers (RCs). The design and limitations of these solutions and the others proposed in the literature are discussed. Experimental results in a 34 kW test bench, which represents a scaled prototype of a real 2 MW wind conversion system, are presented.

Doubly‐fed induction generator ride‐through fault capability using resonant controllers for asymmetrical voltage sags

Modern grid codes worldwide determine specific requirements for the connection of wind energy conversion systems (WECSs) to the grid. One of the requirements is the ride-through fault capability (RTFC) or low-voltage ride-through (LVRT), which defines that the plants must not be disconnected from the grid at certain levels of voltage sags and also must contribute to network stabilisation. The LVRT of the doubly-fed induction generator (DFIG) technology is a well-known problem, mainly during unbalanced grid voltage conditions. The present study extends the studies presented in the literature, analysing the DFIG behaviour during asymmetrical voltage sags using a frequency-domain modelling. A modified resonant controller is proposed to improve the system response during voltage sags, and the analysis of the RTFC based on the converter limits is carried out. The proposed strategy is validated with simulation results of a 2 MW-WECS and experimental results of a 25 kW scaled test bench.

An Alternative Approach to Locating Voltage Sag Source Side at the Point of Common Coupling Based on Power-Flow Information

This paper proposes a new approach capable of locating the voltage sag source side in the power grid based on power-flow behavior at the common coupling point. Measuring the phase voltages and the phase currents flowing at the interested systems interconnection point, the proposed approach is a suitable solution to locate the side of symmetrical and asymmetrical voltage sag source using a set of Boolean logics. A comparative analysis is performed between the best distance relay fault locator method and the proposed approach after being assessed through a simulated case study in a Brazilian utility and industrial systems. Each simulated system has a distributed generation, active loads and power flowing at any directions. The novelty proposal is justified by the simplicity and robustness. Theoretical analysis and simulation have verified its effectiveness.

The Transient Characteristics Analysis of Doubly-Fed Induction Generator during the Asymmetric Voltage Sag

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

Asymmetric Voltage Sag Control Strategy for Grid-Connected PV System Based on PIR Controller

This paper proposes an improved LVRT control strategy based on PIR controller for grid-connected photovoltaic system under asymmetric voltage sag fault. First, the PIR current controller is designed in the positive synchronous frame without the need to decompose the positive and negative sequence components and the dynamic adjustment method for the current-reference calculation is given in order to gain a constant active power. And then, the principle how control strategies switching between the one in normal state and the other one in fault state is revealed in detail. Finally, simulations and experiment test on a 500kVA grid-connected photovoltaic inverter verify that this control strategy achieves good dynamic performance and it improves the PV inverters’ capacity of riding through asymmetric voltage sag fault effectively.

Induction Motors Performance Study under Various Voltage Sags Using Simulation

The main aim of this paper is to study effects of different voltage sags and their intensity on the behavior of induction motors using a simulation approach. For this purpose, necessary simulation was done in Matlab - Simulink first. Then, to ensure-reasonable accuracy of the simulation, some results of the simulation were compared with the corresponding results measured in the laboratory. Then effects of intensity of symmetric voltage sag (three phase) and its starting point on the behavior of small and large induction motors was studied using simulation and subsequently, the effects of asymmetric voltage sags (single phase and two phase) has also been studied and finally the conclusion of the study was presented.

Three-Phase HFL-DVR With Independently Controlled Phases

Conventional dynamic voltage restorers (DVRs) are connected to the power grid through power-frequency transformers. These bulky and costly transformers cause voltage drop and power losses. In this paper, a high-frequency-link dynamic voltage restorer (HFL-DVR) is proposed based on transformer-isolated topologies. This topology facilitates independent operation conditions for each phase in a three-phase system. It enjoys relatively low cost, low losses, and small size. Also, it is free from transformer inrush currents. Small-signal ac equivalent circuit for the power stage including HFL-DVR is derived based on an averaged modeling approach. Transfer functions are obtained to study the effect of inputs such as dc-link voltage, grid voltage, and the load current on the output of HFL-DVR. In order to obtain acceptable properties such as transient overshoot, setting time, and steady-state error, a PID controller is added to the system. This shows that the effect of disturbances on the output of HFL-DVR can be reduced. The experimental results are obtained from a 220V/50Hz HFL-DVR setup. The simulation and experimental results have been compared to verify theoretical aspect of the proposed DVR for both symmetrical and asymmetrical voltage sag conditions.

Fault ride-through capability enhancement of doubly-fed induction wind generators

Recent fault ride-through (FRT) requirements have proven problematic for variable-speed wind generation systems. A particular problem regarding to doubly-fed induction generators (DFIGs) is that standard proportional integral (PI) current controllers, designed with very limited control bandwidth, cannot eliminate rotor current oscillations that occur during a grid fault. As a consequence, the current in the rotor-side converter can exceed the safety limits of semiconductor switches, which potentially leads to converter failure. This study introduces a hybrid current controller to enhance the FRT capability of DFIGs through keeping the rotor current below the safety limits. The proposed current controller includes two switching strategies: the standard PI current controller for normal operating conditions and a vector-based hysteresis current controller (with very fast transient response) for overcurrent protection during grid faults. Simulation studies are carried out to demonstrate the effectiveness of the proposed hybrid current controller under various symmetrical and asymmetrical voltage sag conditions.

Generalization of Methods for Voltage-Sag Source Detection Using Vector-Space Approach

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper discusses methods for voltage-sag source detection, which are based either on energy, current, or impedance criteria. It is shown that methods known from the literature do not work well, particularly in cases of asymmetrical voltage sags. Therefore, generalized methods for voltage-sag source detection are proposed, using a vector-space approach. All the discussed methods, both the already known and the proposed ones, were tested by applying extensive simulations, as well as laboratory and field tests. The correctness of the proposed generalized methods was verified in this way. The obtained results show that all the proposed methods are highly effective in all cases of voltage sags, whereas the overall effectiveness of those methods known from literature is mainly unacceptable. </para>

Optimized Control Strategy for a Medium-Voltage DVR—Theoretical Investigations and Experimental Results

Most power quality problems in distribution systems are related to voltage sags. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. Dynamic voltage restorers (DVRs) have been proposed to provide higher power quality. Currently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to operate and to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor is used, which makes it impractical for DVRs to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback, an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper, a brief introduction of the basic DVR principle will be given. Next, three standard control strategies will be compared and an optimized control strategy is developed in this paper. Finally, experimental results using a medium-voltage 10-kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

Detection of voltage sag sources based on instantaneous voltage and current vectors and orthogonal Clarke's transformation

Different methods for voltage sag source detection are discussed. They are based either on disturbance energy, voltage-current characteristic or active (real) current component. It is shown that, in the cases of asymmetrical voltage sags, both current-based methods known from literature do not work well. These two methods are therefore generalised using a vector-space approach. Furthermore, three new methods are introduced using orthogonal Clarke's transformation, which can be used to detect the sources of those voltage sags provoked by earth faults. All the discussed methods for voltage sag source detection have been tested by applying extensive simulations, laboratory tests and field testing. The results obtained show the very high effectiveness of the proposed methods, which are superior to the methods known from literature, especially for detecting sources of asymmetrical voltage sags.