Unbalanced Source Voltages Research Articles

Implementation and analytical model of three-phase four-switch power factor corrector

This paper presents a three-phase four-switch converter aimed at compensating reactive power in case of distorted and unbalanced source voltages. An original closed-formsolution of line currents, based on the mixed p-z approach is presented. The analytical and experimental results are provided to verify the effectiveness of the proposed control scheme.

Optimization Method of Energy Storage Configuration for Distribution Network with High Proportion of Photovoltaic Based on Source–Load Imbalance

After a high proportion of photovoltaic is connected to the distribution network, it will bring some problems, such as an unbalanced source and load and voltage exceeding the limit. In order to solve them, this paper proposes an optimization method of energy storage configuration for a high-proportion photovoltaic distribution network considering source–load imbalance clustering. Taking the minimum total voltage deviation, the minimum total power loss and the minimum total operating cost as the objective function, and considering various constraints such as power balance constraints and energy storage operation constraints, a mathematical model for energy storage configuration optimization is established. Firstly, the source–load imbalance of the distribution network with a high proportion of photovoltaic is defined. Therefore, according to the 24 h photovoltaic and load data, the 24 h source–load imbalance can be obtained, and the optimal k value can be determined by the elbow rule, so that 24 h a day can be clustered into k periods by the k-means algorithm. Then, the fuzzy comprehensive evaluation method is used to determine the weight factors of each objective function in each period, and three scenes are determined according to the different amount of energy storage. Then, the hybrid particle swarm optimization algorithm proposed in this paper is used to solve the model, and the minimum objective function value, optimal position and optimal capacity of each energy storage grid in each scene are obtained. Finally, it is applied to an example of IEEE33. In the results, the total voltage deviation is increased by more than 10%, the total power loss is increased by more than 8% and the total operating cost is increased by more than 12%, which verifies the effectiveness of the proposed model.

A New Control for Improving the Power Quality Generated by a Three-Level T-Type Inverter

A new controller based on a fractional-order synergetic controller (FOSC) is proposed for a three-level T-type inverter using a shunt active power filter (SAPF). The SAPF is designed to compensate for the reactive power and eliminate the current harmonics caused by non-linear loads, in cases of distorted or unbalanced source voltages. The proposed FOSC technique with the designed parameters and defined macro-variable is a robust control technique that operates well in both transient and steady-state scenarios, ensuring fast convergence and closed-loop system stability. The FOSC technique utilizes a phase-locked loop (PLL) technique on a self-tuning filter (STF) to enhance the SAPF’s ability to compensate current harmonics and reactive power in all situations involving non-linear loads and source voltage variations according to IEEE Std. 519. The proposed control was implemented and verified using Matlab software, where the obtained results were compared with the results of the conventional control based on proportional-integral (PI) controllers in different operating conditions. The results indicate that the proposed FOSC technique outperformed the traditional control in terms of DC voltage tracking and the minimization of the total harmonic distortion of the current.

Control of distribution static compensator with active damped inductor‐capacitor‐inductor filter using dual loop αβ‐frame current controllers for power quality improvement

AbstractA distribution system is connected with many non‐linear loads, and generated harmonies are mitigated by using distribution static compensator (DSTATCOM) which is coupled at the point of common coupling through a voltage source inverter (VSI) and filter circuit. High‐frequency switching ripples are generated along with the output of the VSI during pulse width modulation switching. Inductor‐capacitor‐inductor (LCL) filter is used to suppress the high frequency switching noises. Still, the primary concern about the LCL filter is generation of resonance peak at the resonance frequency. The conventional synchronous reference frame current controller with LCL filter has a complex coupling between and axis; hence decoupling is very difficult. This research article presents a new technique based on active damped dual loop ‐frame current controllers to control the DSTATCOM with LCL filter for achieving improved load compensation. The dual loop controller is enhanced by using capacitor current in the inside loop and grid current at the outside loop with proportional resonant (PR) regulator parallel with harmonic compensator (HC). The proposed method effectively dampens the resonance peak under stationary reference frame The PR controller offers unlimited gain at the fundamental frequency and HC can offer more gain at the specific harmonic frequencies. Hence, characteristics of the PR and HC controllers used together to achieve minimum steady‐state error and the problematic decoupling are ignored. The complete DSTATCOM model with proposed control scheme is discussed with PR and (PR + HC) controllers separately under balanced, unbalanced source voltage. Stability analysis is carried out through Bode and pole‐zero plots to judge the performance of the proposed current controllers. The obtained results, including total harmonic distortion of the source current, are compared with conventional passive damped ‐frame current controller.

Assessment of the influence of voltage unbalance on three-phase operation of power system

In the power system operation, efficient energy utilization is vital. However, with the advent of voltage unbalance, the system is faced with reduced efficiency in the process of energy conversion. This phenomenon occurs due to the asymmetrical behavior of the operating voltage phasor displacement and magnitudes. This occurrence has been attributed to a number of events; such as blown fuse which is as a result of unbalanced loading of the power factor correction capacitors, unbalanced source voltage, unequalled impedance in three-phase system, etc. Though, it is believed that the voltage unbalance phenomenon is more prevalent in the residential and commercial consumer supply. However, its impact on the industrial sector cannot be overemphasized. The power system operation of the induction motor; as the major energy conversion device; is modelled using the Park's transformation. This i s deployed to evaluate the influence of voltage unbalance by adopting the National Electrical Manufacturers Association (NEMA) and institution of Electrical and Electronic Engineering (IEEE) bench marking parameters for evaluating the system’s Voltage Unbalance Factor (VUF). The results show that the motor efficiency drop from 93.2% at 0% of voltage unbalance to 59.51% at 5% of unbalance. This suggests that the power system operation should be closely monitored to ensure that voltage unbalance is eliminated or mitigated by all means; so as to avoid loss of revenue to poor management of the system's power quality.

Mitigation of harmonics and unbalanced source voltage condition in standalone microgrid: positive sequence component and dynamic phasor based compensator with real-time approach

Penetration of Distributed Energy Resources (DER) is in high demand to supply power to the load where the grid is not available. Many of these sources are a single phase source used to form standalone Microgrid (MG). Single phase connectivity of these sources results in an unbalanced source voltage condition (UbSVC). Interfacing power electronic devices also inject the harmonics into Point of Common Coupling (PCC) voltage. The effect of this unbalance and harmonics on the operation of standalone MG is analysed in this paper in a twofold manner. One at a reduced power transfer from DER to load and the other is an error produced in Phase Locked Loop (PLL) operation. Positive Sequence Component (PSC) based and Dynamic Phasor (DP) based compensation techniques are proposed in this paper to mitigate the effect of UbSVC. Simulation validates that both the proposed methods are capable to provide balanced load voltage condition under UbSVC in terms of Voltage Unbalance Factor (VUbF). It also enhances the power transferred from DER to load during an UbSVC. The performance of the proposed compensator during UbSVC and harmonic presence is validated in real-time simulation using Opal-RT and dSPACE simulators.

A grid-tie PV inverter with the ability to improve power quality under unbalanced and distorted source voltage conditions

ABSTRACTThis paper proposes a grid-tie PV inverter that is able to improve power quality under conditions of both distorted and unbalanced source voltage. The presented strategy is based on the instantaneous power theory and uses a second-order generalized integrator-quadrature signal generator (SOGI-QSG). The presented control strategy is aimed at compensating reactive power, eliminating current harmonics, load balancing, and enabling the PV to inject maximum power to the grid. The advantages of the control system are the use of SOGI-QSG adaptive filter and frequency-locked loop (FLL), and removing the low-pass filter and phase-locked loop (PLL). DC–DC and DC–AC converters are utilized for connecting the PV to the grid. The DC–DC and DC–AC converters are responsible for maximum PV power tracking and achieving the control aims, respectively. Using 4-leg converter structure for grid-tie inverter enables achieving the control objectives in 3-phase 4-wire distribution network without any transformer. The presented control strategy is applied to a 3-phase 4-wire distribution network and is simulated in MATLAB/SIMULINK environment. The results of this simulation are then compared with the conventional instantaneous power method in areas including load balancing, reactive power compensation and the elimination of current harmonics, under unbalanced and distorted source voltage conditions.

Operation of Variable-Frequency Drive Motor Systems With Source Voltage Unbalance

This paper describes efficiency measurement in induction motor and variable-frequency drives systems under imposed voltage unbalance at the drive input. A number of drives from different manufacturers are tested with matching sizes of motors and the results are compared. The results highlight differences in the effect of voltage unbalance on drive performance. The results also indicate that intrinsic motor performance has important influence on motor-drive system efficiency.

Speed Control of Matrix Converter-Fed Five-Phase Permanent Magnet Synchronous Motors under Unbalanced Voltages

Five-phase permanent magnet synchronous motors (PMSM) have special applications in which highly accurate speed and torque control of the motor are a strong requirement. Direct Torque Control (DTC) is a suitable method for the driver structure of these motors. If in this method, instead of using a common five-phase voltage source inverter, a three-phase to five-phase matrix converter is used, the low-frequency current harmonics and the high torque ripple are limited, and an improved input power factor is obtained. Because the input voltages of such converters are directly supplied by input three-phase supply voltages, an imbalance in the voltages will cause problems such as unbalanced stator currents and electromagnetic torque fluctuations. In this paper, a new method is introduced to remove speed and torque oscillator factors. For this purpose, motor torque equations were developed and the oscillation components created by the unbalanced source voltage, determined. Then, using the active and reactive power reference generator, the controller power reference was adjusted in such a way that the electromagnetic torque of the motor did not change. By this means, a number of features including speed, torque, and flux of the motor were improved in terms of the above-mentioned conditions. Simulations were analyzed using Matlab/Simulink software.

Extensive Application of UPQC-L for a Dual Point of Common Coupling System with Multiple Loads under Unbalanced Source Condition

—This paper presents a synchronous reference frame (SRF)-based power angle control (PAC) approach for a specific unified power quality conditioner (UPQC) arrangement, termed as UPQC-L for dual point of common coupling system under unbalanced source voltage condition. In contrast to a conventional configuration of UPQC, the static compensator (STATCOM) part is on left side of UPQC-L, while dynamic voltage restorer (DVR) is on the right side. The UPQC-L configuration is specifically adapted for protection of sensitive and critical load types from voltage disturbances. Along with its basic compensation capability of current harmonic compensation and voltage compensation, it efficiently handles reactive power demand from sensitive load. This is achieved with the introduction of PAC concept for effective sharing of reactive power between STATCOM and DVR. The extraction of instantaneous power angle faces difficulty with unbalanced voltage and UPQC-L configuration. Thus, the control algorithm for UPQC-L is designed based on SRF and numerical analysis approach with equal reactive power sharing phenomena by STATCOM and DVR of UPQC-L. UPQC-L with the proposed control approach is successfully implemented in MATLAB/SIMULINK (The Mathworks, Inc. USA) and verified experimentally for mitigation of voltage disturbances (such as sag/swell, unbalancing, and harmonics), current harmonics, and high-reactive power demand.

Implementation of the Predictive Current Control of Multi-Pulse Flexible Topology Thyristor Rectifier Under Unbalanced Source Voltages

Predictive current control strategy has captured much concern recently in power converters. As for the multi-pulse flexible-topology thyristor rectifiers (m-PFTTR), the symmetry of the input line-to-line voltages has great influence upon the implementation of the control strategy. Existing literatures have mainly focused on the implementation under ideal condition, i.e., the input line-to-line voltages, being critical to the predictive current controller, were supposed to be symmetrical. In fact, the unbalance of the input line-to-line voltages usually exists in the windings of the zigzag transformer at front end and the A/D converter of the digital signal processor (DSP). This paper presents a new method to dynamically regulate the input line-to-line voltages to ensure the symmetry of each other. It is realized through the instantaneous correction of the bias values in the course of sampling, and the correction is embodied in the modified control algorithm. The paper includes analyses of the m-PFTTR as well as the control algorithm, simulation and experiments. Simulation and experimental results demonstrate the effectiveness of the presented method.

Search Coil based Online Diagnostics of Transformer Internal Faults

An online approach based on application of search coil for transformer winding fault detection is presented in this paper. Earlier attempts report use of search coil for the detection of transformer interturn fault. This paper extends application of search coil for detection of winding displacement in addition to interturn fault identification. Experimental results based on specially designed 10-kVA transformer are presented to analyze search coil voltages during healthy and faulty conditions. The algorithm detects phase ( $U$ , $V$ , $W$ ), side (LV, HV), and precise location of the fault. Discrimination between interturn fault and winding radial displacement is also achieved by search coil voltage analysis. The results are experimentally verified based on impedance analyzer plots. Experimental analysis indicates that the proposed method is independent of load on the transformer, vector group, nature of winding, balanced or unbalanced source voltage, source voltage harmonics, inrush, and overexcitation conditions.

Management of reactive power sharing & power quality improvement with SRF-PAC based UPQC under unbalanced source voltage condition

This paper is proposed to establish a new control algorithm for UPQC (unified power quality conditioner) to improve power quality and manage effectively equal reactive power sharing between shunt and series inverter of UPQC under unbalanced source voltage condition. The extraction of instantaneous power angle for reactive power sharing faces difficulty with unbalanced source voltage condition. This paper presents a new SRF (synchronous reference frame) based PAC (power angle control) method using decoupled load current parameters for efficient utilization and coordination of UPQC inverters. The proposed controller contributes in improvement of source current and load voltage harmonic profile, provides efficient way of load reactive power compensation and load voltage compensation for sag, swell and unbalanced condition. Effect of source voltage variations in the form of sag, swell or unbalancing on variable power angle estimation and reactive power calculations are also validated through a mathematical analysis. SRF based PAC control approach and PAC based UVT (unit vector template) control approach is adapted for estimating the reference signals of shunt and series inverter respectively and thus reducing the need of extra computation. The simulation and experimental analysis is carried out using Matlab/Simulink software package for computer simulation and a dSPACE based experimental setup for real time verifications.

Experimental Validation of Active Power Filtering with a Simple Robust Control

This article deals with shunt active power filtering for power quality improvement with hysteresis current controllers by two methods of implementation assessed under unfavorable non-linear load and source conditions. Fully digital and simple hybrid hysteresis control strategies are tested based on a numerical integrator proportional DC bus controller combined with phase-locked loop outputs to generate current references. In two cases, reference mains currents are generated to ensure three-phase sinusoidal synchronized waveforms of the mains currents. The experimental evaluation of the robustness control in these two implementation methods is verified using a 10-kVA shunt active power filter in steady state for transitional conditions (step load, switch-on of shunt active power filter) and in the worst case of unbalanced loads (break phase) and source voltages. Several experimental results are presented and discussed to prove the robustness and excellent performance of the proposed simple hybrid technique.

Modelling and Analysis of Controller for Three-Phase Shunt Active Power Filter

Now a days, because of enhancement in high speed computing elements, high voltage controlled semi-conductor devices and emergence of new power circuit topologies, the Voltage Source Converter (VSC) based Active Power Filter (APF) is finding applications for compensating harmonic currents drawn by the various loads. As the active filter consists of semi-conductor devices alone, the response time of this device is very fast and it doesn’t suffer from resonance and other stability problems as faced by the normal passive filters. The Active filters are generally used at harmonic producing loads. Among all APF’s the Shunt Active Power Filter (SAPF) is more prominent at medium/high voltage applications, and also it provides higher efficiency. SAPF in the power circuit can be used either by using the voltage-sourced PWM converter equipped with a DC capacitor or a current-sourced PWM converter equipped with a DC inductor. This paper presents an approach to determine reference compensation currents of the three-phase SAPF under distorted and/or unbalanced source voltages in steady state. It is planned to design a Synchronous Reference Frame (SRF) controller for Voltage Source Converter (VSC) based Shunt active power filter for harmonic compensation. The simulation results have been obtained by using MATLAB/ SIMULINK.

High-Performance Control of Three-Phase Four-Wire DVR Systems using Feedback Linearization

Power quality is a critical issue in distribution systems, where a dynamic voltage restorer (DVR) is commonly used to mitigate the voltage disturbances for loads. This paper deals with a nonlinear control for the three-phase four-wire (3P-4W) DVR under a grid voltage unbalance and nonlinear loads in the distribution system, where a novel control scheme based on the feedback linearization technique is proposed. Through feedback linearization, a nonlinear model of a DVR with a PWM voltage-source inverter (VSI) and LC filters is linearized. Then, the controller design of the linearized model is performed by applying the linear control theory, where the load voltages are kept constant by controlling the d-q-0 axis components of the DVR output voltages. To keep the load voltage unchanged, an in-phase compensation strategy is employed, where the load voltages are recovered to be the same as the previous voltage without a change in the magnitude. With this strategy, the performance of the DVR becomes faster and more stable even under unbalanced source voltages and nonlinear loads. The validity of the proposed control strategy has been verified by simulation and experimental results.

A novel control method for series hybrid active power filter working under unbalanced supply conditions

In this paper a new control algorithm for a Series Hybrid Active Power Filter (SHAPF) is proposed. With the proposed control algorithm, the series active power filter simultaneously compensates for source voltage unbalance and source current harmonics generated by non-linear loads. The proposed control algorithm is based on the generalised instantaneous power theory where the instantaneous inactive power is represented as a second order tensor. The reference voltage is directly associated with three-phase instantaneous voltages and currents and are separated in three-phase co-ordinate systems. Therefore, the calculation of the compensation reference voltage is much simpler than the other available control algorithms. It can be applied for both voltage and current harmonic generating loads connected across balanced and unbalanced source voltages. The mathematical formulation of the proposed control algorithm with its applications to SHAPF is presented. The validity of the proposed control algorithm is verified with extensive experimental study and results are reported.

A DC-Side Current Injection Method for Improving AC Line Condition Applied in the 18-Pulse Converter System

In this paper, a new compensation strategy for receiving clean power of a conventional 18-pulse ac/dc converter formed by three 6-diode bridges is proposed. According to the proposed strategy, a three-phase current-controlled inverter injects the compensation currents into the three positive terminals of the three six-diode bridges. The goal of injecting currents at dc side is to improve the quality of the ac line currents. Compared to the conventional active filter deployed at the ac side, the three-phase inverter used in this paper is with lower kVA rating, and the 18-pulse converter draws nearly sinusoidal currents from the ac main by the proposed compensation strategy. The theoretical compensation command is derived in this paper, and then an approximate approach is recommended to simplify the calculation. A digital-signal processor is employed as a digital controller to calculate the compensation command, and a 3-kW prototype including the 18-pulse converter and the current-controlled inverter is built for evaluation and measurement. Moreover, the performance affected by unbalanced ac source is investigated. The experimental results demonstrate that the proposed method not only improves the line current quality but also mitigates the effects caused by unbalanced source voltages.

A Novel Detecting Algorithm for Harmonics and Reactive Currents under Unbalanced and Non-Sinusoidal Source Voltages

Harmonics and reactive currents detected in precision is the key whether active power filters work well. Methods on detecting affect compensation currents and detecting precision. A novel harmonics and reactive currents detecting algorithm in three phase circuit is proposed. It extracts positive fundamentals of source voltages and load currents using three-phase sinusoid and cosine signals with arbitrary phase, and then detects harmonics, reactive and negative currents by accurately separating positive fundamental active component from load currents. Whether waveforms of voltages are non-sinusoidal or unbalanced, the results can not be affected. Furthermore, it is without phase lock loop. Finally, it is proved to be precision and feasibility from simulation.

The Voltage Output Characteristics of New 24-Pulse Rectification System under Unbalanced Source Voltage

The traditional 6-pulse high-power rectifier has defects such as large harmonic content and low power factor in application. The 24-pulse rectification system based on autotransformer can restrain the harmonics and improve power factor effectively. This paper derivate the input current and output voltage’s characteristics of 24-pulse rectification system under balanced grid. At the same time, the author simulate and analysis various impacts of different situations of unbalanced grid on these two characteristics on the basic of real grid ,which has an important guiding significance on the application of the 24-pulse rectification system.