Three-phase Four-wire System Research Articles

Minimum inverter capacity design for LC-hybrid active power filters in three-phase four-wire distribution systems

This study presents a minimum inverter capacity design for three-phase four-wire centre-split inductor-capacitor (LC) coupling hybrid active power filters (LC-HAPFs). Based on its equivalent circuit models in d - q -0 coordinate, the coupling part filtering characteristics of the LC-HAPF without or with neutral inductor can be more clearly illustrated and easily understood, compared with the past analysis based on the generic filter structure. According to the current quality data, the minimum dc-link voltage expressions for the LC-HAPF without and with neutral inductor are deduced and compared. Conventionally, the coupling LC is usually tuned at a higher fifth- or seventh-order harmonic frequency to reduce its cost and size compared with third-order case. When triplen harmonic currents exist significantly, the LC-HAPF with a small tuned neutral inductor can further reduce its minimum dc-link voltage requirement. Thus, the initial cost, switching loss and switching noise of the LC-HAPF can be lowered. Representative simulation and experimental results of the three-phase four-wire LC-HAPF with neutral inductor are presented to verify the filtering characteristics analysis and minimum dc-link voltage expressions, to show the effectiveness of reducing its inverter capacity, switching loss and switching noise in current quality compensation compared with the conventional LC-HAPF.

3P-3W UPQC with zig-zag transformer for 3P-4W Distribution System

In this paper, a new topology is developed for a three-phase four-wire (3P4W) distribution system utilizing a zig-zag transformer along with three-phase three- wire (3P3W) Unified Power Quality Conditioner (UPQC).In this proposed topology the neutral point of the star connected transformer, used for the connection of series active power filter (APF) of 3P3W UPQC, is utilized as a fourth wire for 3P4W distribution system. The zig-zag transformer is connected near the load to compensate the neutral current that may flow toward the neutral point of the series transformer of series APF. The series and shunt APF of 3P-3W UPQC are realized by readily available three-leg voltage source inverters (VSIs). For the mitigation of different power quality problems a control technique based on synchronous reference frame (SRF) theory is used for the control of UPQC. In this control scheme of UPQC, the current/voltage control is applied over the fundamental supply currents/voltages instead of fast changing APFs currents/voltages, there by reducing the computational delay and the required sensors. The performance of the proposed topology of UPQC is analyzed through simulations results using MATLAB software with its Simulink and Power System Block set toolboxes.

RTDS hardware implementation and simulation of SHAF for mitigation of harmonics using p-q control strategy with PI and fuzzy logic controllers

The main objective of this paper is to develop PI and fuzzy controllers to analyze the performance of instantaneous real active and reactive power (p-q) control strategy for extracting reference currents of shunt active filters (SHAFs) under balanced, unbalanced, and balanced non-sinusoidal conditions. When the supply voltages are balanced and sinusoidal, both controllers converge to the same compensation characteristics. However, if the supply voltages are distorted and/or unbalanced sinusoidal, these controllers result in different degrees of compensation in harmonics. The p-q control strategy with PI controller is unable to yield an adequate solution when source voltages are not ideal. Extensive simulations were carried out with balance, unbalanced, and non-sinusoidal conditions. Simulation results validate the superiority of fuzzy logic controller over PI controller. The three-phase four-wire SHAF system is also implemented on a real-time digital simulator (RTDS hardware) to further verify its effectiveness. The detailed simulation and RTDS hardware results are included.

Study of Shunt Active Power Filters Applied to Three-Phase Four-Wire Systems

Study of Shunt Active Power Filters Applied to Three-Phase Four-Wire Systems

Three-phase four-wire hybrid power filter using a smaller power converter

In this paper, a hybrid power filter is proposed to solve problems of line harmonic currents and neutral line currents of three-phase four-wire power distribution system. This hybrid power filter is configured by a there-phase tuned power filter and a three-phase power converter. The tuned power filter is configured by a capacitor and an inductor, and its tuned frequency is setting at the frequency of third-order harmonics. The three-phase power converter has a four legs bridge structure and is connected to the inductor of the tuned power filter in parallel. The three-phase power converter is applied to solve the problems of series/parallel resonance and improve poor filter performance. Furthermore, it also forces the fundamental current of the tuned power filter and the dominant harmonic current of the nonlinear load to flow through the inductor of the tuned power filter. In this way, the voltage rating of the three-phase power converter is reduced because it is serially connected to the power capacitor of the tuned power filter, and the current rating can be diminished because it is connected to the inductor of the tuned power filter in parallel. Hence, the power rating of the power converter applied in the proposed hybrid power filter is minimized. A DSP based prototype is developed and tested to verify the performance of the proposed three-phase four-wire hybrid power filter. Experimental results show that the proposed three-phase four-wire hybrid power filter achieves the expected performance.

Single-phase inverter-based neutral-current suppressor for attenuating neutral current of three-phase four-wire distribution power system

In this study, a neutral-current suppressor is proposed. The proposed neutral-current suppressor comprises a zero-sequence transformer, a single-phase power converter and a rectifier. The proposed zero-sequence transformer is implemented by only two single-phase transformers, which are connected to three-phase lines of the three-phase four-wire distribution power system to supply a zero-sequence current path. The single-phase power converter is connected between the zero-sequence transformer and neutral line. A rectifier with small power rating is connected to the DC bus of single-phase power converter for overcoming the power loss of single-phase power converter to sustain a constant DC voltage. The performance of neutral-current suppression for the zero-sequence transformer is advanced by controlling the single-phase power converter so as to make the neutral load current flow through the proposed neutral-current suppressor. Moreover, the proposed neutral-current suppressor can also avoid the neutral current caused by the unbalanced voltage of three-phase four-wire distribution power systems. Hence, the proposed neutral-current suppressor is an effective solution to the neutral-current problem of three-phase four-wire distribution power systems. Both computer simulation and experimental results verify that the proposed neutral-current suppressor can achieve the expected performance.

Minimisation of VA loading of UPQC in three-phase four-wire distribution system

Unified power quality conditioner (UPQC) is a combination of series and shunt active power filters with a common DC link. This device is used to enhance power quality by injecting required amount of Volt-Ampere (VA) into the system. In view of the cost effectiveness, it is desirable to have VA loading of UPQC without compromising the compensation capability. This paper presents a methodology to minimise VA loading of UPQC, considering the effects of unbalance in load and distortions in both load current and source voltage during the compensation of voltage sag with phase jumps in a distribution system. The minimum VA loading of the UPQC is achieved by injecting a series voltage at an optimum angle using series active power filter with respect to the voltage sag. In this paper, a particle swarm optimisation (PSO)-based technique is used to minimise the VA loading of UPQC. The proposed method has been validated through detailed simulation studies and the results are presented.

Performance of Voltage and Frequency Controller in Isolated Wind Power Generation for a Three-Phase Four-Wire System

This paper deals with a new control algorithm for a voltage and frequency controller (VFC) of an isolated wind energy conversion system (IWECS) using an isolated asynchronous generator to feed three-phase four-wire consumer loads. The reference source currents are estimated for the indirect current control of a voltage source converter (VSC) using the single-phase PQ theory to control the voltage and frequency of IWECS. A three-leg VSC with a battery energy storage system is used as a VFC for an IWECS. A nonisolated star-polygon transformer is used to provide a neutral terminal for four-wire loads and to compensate the neutral current resulting in reduction in rating of VSC. The proposed VFC is implemented using a DSP. Experimental results are presented to demonstrate the additional capabilities of VFC as a load leveler, a load balancer, a harmonic eliminator, and a neutral current compensator.

Research on Methods of Reactive Compensation for Unbalanced Current in Three-Phase Four-Wire Low-Voltage Distribution System

Different condition of unbalanced current of three-phase four-wire low-voltage distribution system is analysed in this paper. The model of unbalanced reactive power compensation is built which is fit for the three-phase four-wire system ,Using phase-to-phase or phase-to-earth capacitance or inductance can transfer active power. Control the neutral current to be zero and three-phase current balanced by means of reactive power compensation. A compensating phase judgment approach is presented, the mathematic model of calculating phase current, phase compensating capacity line current and line-to-line compensating capacity is gave. Simulation result of physical system shows that the neutral current can be controlled to be zero and the phase current balanced. It substantiate that this compensation method is valid and optimize the operation condition of loads.

Control Strategy for Selective Compensation of Power Quality Problems through Three-Phase Four-Wire UPQC

This paper presents a novel control strategy for selective compensation of power quality (PQ) problems, depending upon the limited rating of voltage source inverters (VSIs), through a unified power quality conditioner (UPQC) in a three-phase four-wire distribution system. The UPQC is realized by the integration of series and shunt active power filters (APFs) sharing a common dc bus capacitor. The shunt APF is realized using a three-phase, four-leg voltage source inverter (VSI), while a three-leg VSI is employed for the series APF of the three-phase four-wire UPQC. The proposed control scheme for the shunt APF, decomposes the load current into harmonic components generated by consumer and distorted utility. In addition to this, the positive and negative sequence fundamental frequency active components, the reactive components and harmonic components of load currents are decomposed in synchronous reference frame (SRF). The control scheme of the shunt APF performs with priority based schemes, which respects the limited rating of the VSI. For voltage harmonic mitigation, a control scheme based on SRF theory is employed for the series APF of the UPQC. The performance of the proposed control scheme of the UPQC is validated through simulations using MATLAB software with its Simulink and Power System Block set toolboxes.

A control strategy based on UTT and I Cos&#920 theory of three-phase, fourwire UPQC for power quality improvement

This paper presents a control strategy for a three-phase four-wire Unified Power Quality (UPQC) for an improvement of different power quality (PQ) problems. The UPQC is realized by integration of series and shunt active power filters (APFs) sharing a common dc bus capacitor. The shunt APF is realized using a thee-phase, four leg voltage source inverter (VSI) and the series APF is realized using a three-phase, three legs VSI. A control technique based on unit vector template technique (UTT) is used to get the reference signals for series APF, while ICosO theory is used for the control of Shunt APF. The performance of the implemented control algorithm is evaluated in terms of power-factor correction; load balancing, neutral source current mitigation and mitigation of voltage and current harmonics, voltage sag and swell and voltage dips in a three-phase four-wire distribution system for different combination of linear and non-linear loads. In this control scheme, the current/voltage control is applied over the fundamental supply currents/voltages instead of fast changing APFs currents/voltages, thereby reducing the computational delay and the required sensors. MATLAB/Simulink based simulations results are obtained, which support the functionality of the UPQC.

Parallel Operation of DFIGs in Three-Phase Four-Wire Autonomous Wind Energy Conversion System

This paper deals with a control algorithm for two parallel-operated doubly fed induction generators (DFIGs) driven by wind turbines in a three-phase four-wire autonomous system feeding local loads. The proposed autonomous wind energy conversion system (AWECS) is using back-to-back-connected pulsewidth-modulated insulated-gate-bipolar-transistor-based voltage source converters with a battery energy storage system at their dc link. The system utilizes separate rotor-side converters for each DFIG for maximum power tracking (MPT) through its rotor speed control. However, a common dc bus and a battery bank and stator-side converter are used for voltage and frequency control at the stator terminals of the DFIGs. A delta–star transformer is connected between the stator-side converter and the stator terminals of DFIGs for optimizing the voltage of dc bus, and the load-side neutral is connected to the neutral of the star side of the transformer. The proposed electromechanical system is modeled and simulated in MATLAB using Simulink and Sim Power Systems set toolboxes. The performance of the proposed AWECS is presented to demonstrate its capability of MPT, stator voltage and frequency control, harmonic elimination, load balancing, and load leveling.

Implementation of Neural-Network-Controlled Three-Leg VSC and a Transformer as Three-Phase Four-Wire DSTATCOM

In this paper, a neural-network (NN)-controlled distribution static compensator (DSTATCOM) using a dSPACE processor is implemented for power quality improvement in a three-phase four-wire distribution system. A three-leg voltage-source-converter (VSC)-based DSTATCOM with a zig-zag transformer is used for the compensation of reactive power for voltage regulation or for power factor correction along with load balancing, elimination of harmonic currents, and neutral current compensation at the point of common coupling. The Adaline (adaptive linear element)-based NN is used to implement the control scheme of the VSC. This technique gives similar performance as that of other control techniques, but it is simple to implement and has a fast response and gives nearly zero phase shift. The zig-zag transformer is used for providing a path to the zero-sequence current in a three-phase four-wire distribution system. This reduces the complexity and also the cost of the DSTATCOM system. The performance of the proposed DSTATCOM system is validated through simulations using MATLAB software with its Simulink and Power System Blockset toolboxes and hardware implementation.

Hybrid Multiconverter Conditioner Topology for High-Power Applications

A novel multiconverter conditioner topology and its control stage are proposed in this paper. It is formed by an active conditioner in parallel with a hybrid conditioner composed of an active filter in series with one or more passive filters. This topology allows the reduction of the inverter ratings, constituting an effective solution at high-power levels. Collaborative control strategies are developed for the new topology, which share the compensation objectives between the two converters. These control strategies and the tracking techniques are based on estimating the load current, achieving new algorithms with a reduction in the number of meters in the control stage. The conditioner operates properly in three-phase four-wire systems reducing the harmonic distortion and/or imbalance and attaining the unity displacement power factor. Experimental results are included for the testing of the topology and its control.

A Control Strategy Based on UTT and ISCT for 3P4W UPQC

This paper presents a novel control strategy of a three- phase four-wire Unified Power Quality (UPQC) for an improvement in power quality. The UPQC is realized by integration of series and shunt active power filters (APFs) sharing a common dc bus capacitor. The shunt APF is realized using a thee-phase, four leg voltage source inverter (VSI) and the series APF is realized using a three-phase, three leg VSI. A control technique based on unit vector template technique (UTT) is used to get the reference signals for series APF, while instantaneous sequence component theory (ISCT) is used for the control of Shunt APF. The performance of the implemented control algorithm is evaluated in terms of power-factor correction, load balancing, neutral source current mitigation and mitigation of voltage and current harmonics, voltage sag and swell in a three-phase four-wire distribution system for different combination of linear and non-linear loads. In this proposed control scheme of UPQC, the current/voltage control is applied over the fundamental supply currents/voltages instead of fast changing APFs currents/voltages, there by reducing the computational delay and the required sensors. MATLAB/Simulink based simulations are obtained, which support the functionality of the UPQC. MATLAB/Simulink based simulations are obtained, which support the functionality of the UPQC.

A four-leg unified series–parallel active filter system for periodic and non-periodic disturbance compensation

This paper presents a three-phase four-leg (3P4L) unified series–parallel active filter (USPAF) system, compensating for both periodic and non-periodic disturbances using a generalized non-active power theory (GNAP) based control strategy. The 3P4L USPAF system is realized by the integration of series and parallel active filters (AFs), composed of the two 3P4L voltage source inverters (VSIs) sharing a common dc-link capacitor. The GNAP theory was implemented previously in the parallel AF. In this study, the theory is proposed for the 3P4L USPAF system to compensate non-sinusoidal periodic and non-periodic currents and voltages. Distorted source voltages and unbalanced non-linear load currents compensation were verified simultaneously through the 3P4L USPAF system experimental prototype. Sub-harmonic and stochastic non-periodic current/voltage compensations were analyzed through simulations with Matlab/Simulink software. The simulation and experimental results showed that the theory proposed for the 3P4L USPAF system was applicable to non-active power compensation in three-phase four-wire (3P4W) systems under periodic and non-periodic disturbances.

Digital Signal Processor Implementation of Isolated Reduced-rating Voltage Source Converter Using a Zig-zag Transformer for Three-phase Four-wire Distribution Static Compensator

The proposed distribution static compensator is used to compensate the reactive power for power factor correction or voltage regulation along with load balancing, neutral current compensation, and harmonics current elimination. A zig-zag/star transformer is used to isolate the voltage source converter, and it is operated as a three-phase four-wire distribution static compensator for power quality improvement in a three-phase four-wire system. In order to reduce the current rating of the voltage source converter, the zig-zag configuration is used in the primary windings of the transformer. The voltage source converter is connected to the secondary windings of the zig-zag/star transformer, and this allows the optimum selection of voltage rating of the voltage source converter. Moreover, this transformer connection provides isolation for the voltage source converter system. The distribution static compensator is implemented using the dSPACE DS1104 R&D controller (dSPACE Inc., Wixom, Michigan, USA) and a slave digital signal processor subsystem based on the TMS320F240 digital signal processor (dSPACE Inc., Wixom, Michigan, USA) to validate the design and the developed control algorithm.

Harmonic Mitigation Using a Polarized Ramp-time Current-Controlled Inverter

This paper describes the implementation of a shunt active power filter for a three-phase four-wire system to overcome the power quality problems generated by mixed non-linear loads which are a combination of harmonic , reactive and unbalanced components. The filter consists of a three-phase current-controlled voltage source inverter (CC-VSI) with a filter inductor at the AC output and a DC-bus capacitor. T he CC-VSI is operated to directly control the grid current in order to be sinusoidal and in phase with the grid voltage without sensing the load currents. The switching is controlled by using polarized ramp-time current control, which is based on the concept of zero average current error (ZACE) with a fixed switching frequency. The laboratory experiment results indicate that the filter is able to mitigate the harmonics and the reactive power, so that the grid currents are sinusoidal, in phase with the grid voltages, and symmetric although the grid voltage contains harmonics .

DC Capacitor Voltage Equalization in Neutral Clamped Inverters for DSTATCOM Application

Multilevel neutral point clamped (NPC) inverter systems are increasingly used in load compensation applications. However, the most significant problem associated with these compensators is the capacitor voltage imbalances and drift due to dc components in the zero sequence current, resulting in degradation of tracking performance of the voltage source inverter. This paper proposes a carrier-based pulsewidth modulation control for an inverter-chopper circuit in order to regulate the capacitor voltages to their reference values. To demonstrate the simplicity and effectiveness of the aforementioned control scheme, a three-phase four-wire three-level NPC compensator system is taken as an example. Detailed simulation has been carried out in PSCAD 4.2.1. Experiments are conducted to validate the proposed control scheme.

Three-Dimensional Flux Vector Modulation of Four-Leg Sine-Wave Output Inverters

The time integral of the output voltage vector of a three-phase inverter is often termed the inverter flux vector. This paper addresses the control of a three-phase four-leg sine-wave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that an output filter resonance is actively damped by an output voltage control loop alone. Furthermore, an inverter switching action inherently regulates the output voltage rapidly against dc-bus voltage variations. The flux vector control of sine-wave output inverters finds several applications in three-phase four-wire systems. This paper presents a flux modulation method for three-phase four-leg inverters feeding unbalanced and nonlinear loads. All the necessary steps for the digital implementation of the flux modulator are presented. The switching behavior of the modulator has been evaluated, which is useful for the variable fundamental frequency applications of the inverters. To provide experimental validation, the modulator is implemented as a part of the control system for a stand-alone three-phase four-leg inverter with an LC filter at its output. Control system details are also provided. Experimental results indicate the effectiveness of the modulator and control system in providing balanced voltages at the output of the LC filter even under highly unbalanced conditions with nonlinear loads. The resonance damping and voltage regulation properties of the modulator are also apparent from the experimental results.