Three-phase Four-wire Network Research Articles

Current control of distributed generation power inverters for losses reduction in the distribution network

The main objective of the paper is to evaluate the power losses reduction that can be achieved in a low voltage distribution network when the inverters associated to the distributed generation are used as active power filters to compensate the non active currents produced by inductive and unbalanced loads. For that purpose, a hysteresis current control has been proposed and implemented. A real distribution threephase four-wire network has been simulated considering a large penetration of distributed generation. Power losses and the efficiency of the network have been quantified for different compensation cases.

БАЛАНСИРОВКА ФАЗНЫХ НАГРУЗОК В НИЗКОВОЛЬТНЫХ ТРЕХФАЗНЫХ ЧЕТЫРЕХПРОВОДНЫХ СЕТЯХ

The imbalance of phase loads is a natural negative property of a low-voltage three-phase four-wire network due to uneven connection to the phases and the difference in individual consumer demand for electricity. The imbalance of phase loads in the network nodes leads to a decrease in the quality of electricity, overestimated technical losses in the network and to current overloads in one of the phases of power lines. In the paper, balancing of phase loads in the network is performed using a heuristic algorithm based on average hourly readings of loads and voltages readings of consumer smart meters. The effectiveness of the algorithm was evaluated by reducing losses, maximum currents in the phase wires and the neutral wire in a real feeder of the electrical network

STUDY OF RESONANCE PHENOMENA WHEN CONNECTING 0.38 KV BATTERIES OF TRANSVERSE COMPENSATION CAPACITORS TO THE NETWORK

The article deals with the operation of capacitor banks as part of regulated capacitor units connected to the 0.38 kV electrical network of power transformers of 10/0.4 kV consumer substations. It is known that capacitor banks are used to compensate for the reactive power of electrical receivers and equalize the load of phases in three-phase four-wire networks of 0.30 kV by creating reverse sequence currents equal in magnitude, but rotated by 1800 relative to the load currents due to the inclusion of a different number of capacitors between the network phases. The wide presence of non-linear electrical receivers in electric networks causes the occurrence of higher harmonic components of voltage and current, which cause an increase in the current in capacitors, since the reactive resistance of capacitors decreases with increasing frequency. At the same time as the current in the capacitors increases, which can be regulated with the help of constructive measures, in unfavorable cases resonance phenomena may occur in the networks. Widespread use of capacitor banks creates risks of resonance of voltages and currents both at the main frequency of three-phase alternating current of 50 Hz and at frequencies of 150, 250, 350 and 550 Hz due to higher harmonic components of voltage and current. The obtained results indicate that for the resonance of currents to occur at the main frequency for power transformers of different capacities, the power of the capacitor banks should be from 0.02 kV×Ar per 1 kV×A of power (for a 630 kV×A transformer) to 0.034 kV×Ar (for a 25 kV×A transformer), and for the occurrence of voltage resonance, respectively, from 18.7 kV×Ar/kV×A to 26 kV×Ar/kV×A. In addition, calculations show that to increase the power factor even by a small amount (from 0.9 to 0.95), it is necessary to install capacitor banks of at least 0.14 kV×Ar/kV×A, and to increase the power factor from 0, 7 to 0.95 - only 0.66 kV×Ar/kV×A.

DETERMINATION OF MEASUREMENT ERROR OF ELECTRICAL ENERGY QUALITY INDICATORS BY THE CORRELATION METHOD

The problem of energy conservation is urgent not only for Ukraine but for the whole world. One of the main causes of non-productive energy costs in the energy sector is high losses in power grids. The problem of electricity quality is the focus of many researchers and practitioners of energy and electrification. The co-supply of single-phase and three-phase electrical energy receivers from a three-phase four-wire network is widespread in the power supply systems of industrial enterprises. In such cases, there is almost always a phase asymmetry of the load, as well as significant currents in the zero wire. These factors lead to a decrease in the quality of the voltage and are the cause of economic damage. Organization of hardware control of electricity quality indicators, organizational and economic mechanism of influence on the cause of deterioration of electricity quality, development of methods and technical methods of distortion are important for normalization of electricity quality. The error of the correlation method for measuring the unbalanced voltage of a three-phase network based on the preliminary conversion of a three-phase voltage to a two-phase voltage with subsequent decomposition into orthogonal components using a discrete Fourier transform is estimated.

Analytical Investigation of the Properties of Transients in Unbalanced Three-Phase Four-Wire Networks

In this paper, the theoretical properties of transients in three-phase networks, including the fourth wire and unbalanced source, are systematically investigated by resorting to several analytical tools. First, a Cartesian space vector is introduced to provide the geometrical three-dimensional representation of a three-phase voltage/current transient. It is shown that a voltage/current transient can be always represented as a three-dimensional trajectory leaving one plane (corresponding to the previous steady state) towards another plane (corresponding to the new steady state at the end of the transient). The quantity driving the trajectory from one plane to another is the Park/Clarke zero component. Second, the Clarke transformation is used to study the original three-phase circuit as a superposition of two independent transients: the transient of the Clarke space vectors and the transient of the Clarke zero components. Third, the Park transformation is used to evaluate the dq0 components of transient voltages/currents. Since the Park transformation is related to the Clarke transformation through a simple frequency shift, the effects on the frequency content of the steady-state and transient waveforms are put into evidence. Three case studies are presented and solved to prove the theoretical results.

An efficient compensation of modified DSTATCOM for improving microgrid operation

This paper proposes a design for a modified distribution static compensator (DSTATCOM) with a proposed current controller for microgrid (MG) applications. The modified DSTATCOM is incorporated in a three-phase four-wire network system to deal accurately at grid connected mode with balanced, unbalanced and excursion load for both nonlinear and inductive loads. Moreover, it handles the stability issues of the MG at fault caused islanding incidents. The design criteria of the modified DSTATCOM parameters is presented and discussed. The hysteresis current controller is introduced to drive the voltage source converter (VSC) and its reference voltage signal is generated based on the instantaneous symmetrical components theory (ISCT). Matlab/Simulink software is used to verify the performance of the modified DSTATCOM with the proposed controller under different load types and loading conditions for grid connected mode and islanded mode of MG operation. Based on the simulation results, the power factor is improved and the non-linear harmonics currents are compensated therefore the harmonic distortion of the supply currents and voltage are reduced, moreover the stability operation at fault cause islanding is maintained. The results show the superiority of the proposed modification of DSTATCOM in dealing with unbalanced, distorted and bad power factor system in comparing with conventional DSTATCOM.

Three-Phase Four-Wire OPF-Based Collaborative Control of PV Inverter and ESS for Low-Voltage Distribution Networks With High Proportion PVs

The use of photovoltaic reactive power and energy storage active power can solve the problems of voltage violation, network loss, and three-phase unbalance caused by photovoltaic connection to low-voltage distribution networks. However, the three-phase four-wire structure of the low-voltage distribution network brings difficulties to power flow calculation. In order to achieve photovoltaic utilization through optimal power flow, a photovoltaic-energy storage collaborative control method for low-voltage distribution networks based on the optimal power flow of a three-phase four-wire system is proposed. Considering the amplitude and phase angle of voltage and current, a three-phase four-wire node admittance matrix was used to establish the network topology of the low-voltage distribution network. Also, to minimize the network loss, the three-phase unbalance and voltage deviation. a multi-objective optimization model based on three-phase four-wire network topology was established considering the voltage constraints, reverse power flow constraints and neutral line current constraints. Through improving the node admittance matrix and model convexity, the complexity of solving the problem is reduced. The CPLEX algorithm package was used to solve the problem. Based on a 21-bus three-phase four-wire low-voltage distribution network, a 24-h multi-period simulation was undertaken to verify the feasibility and effectiveness of the proposed scheme.

Distributed Power Quality Conditioning System for Three-Phase Four-Wire Low Voltage Networks

This paper presents distributed power quality conditioning system to compensate current and voltage distortions in three-phase four-wire networks caused by unbalanced non-linear single-phase loads. The proposed conditioning system consists of several hybrid filter units installed in various nodes for compensation of excessive neutral currents and voltage distortions in a selected area of the distribution network. The system is open and it can be easily modified by installation of new filter units. A novel hybrid filter design procedure based on filter frequency characteristics optimization in the parameters of passive and active parts is presented. A digital system of control signal computation for active filters based on the use of modern methods of spectral analysis is considered. The proposed digital control system enables selective compensation of fundamental and harmonic components. The mathematical model of the proposed power quality conditioning system is developed in the MatLab software. The simulated results show that the presented conditioning system reduces the level of neutral conductor currents and the voltage unbalance as well ensures harmonic compensation in three-phase four-wire networks.

Distributed selective harmonic mitigation and decoupled unbalance compensation by coordinated inverters in three-phase four-wire low-voltage networks

Considering the high penetration of distributed generators in low-voltage grids, establishing a coordinated operation of their grid-tied inverters has become imperative to move towards the implementation of smart grids. Yet, knowing that mitigation of power quality issues such as reactive power, current unbalance and harmonics is of importance within such a context, this work proposes a master/slave control approach, which uses a communication means of low-bandwidth, to flexibly coordinate four-leg inverters dispersed in three-phase four-wire networks. Their coordination is attained by means of a current-based approach, allowing the sharing of active, reactive, harmonic and unbalance currents drawn by loads. The control strategy also regulates the power flow at the point of common coupling of the low-voltage network, while proportionally steering inverters according to their nominal capabilities. In addition to the offering of selective harmonic mitigation, the control approach provides distributed and decoupled unbalance compensation, in partial or total portion, based on concepts from the Conservative Power Theory. Consequently, extraction of sequence components or implementation of virtual impedance control loops are not required. The proposed strategy is evaluated based on multiple simulation results, considering the CIGRE's European low-voltage distribution benchmark, including six distributed inverters, as well as linear and nonlinear loads.

Increase of Reactive Power Compensation Efficiency on the Basis of Individual Phase Compensation Devices

The purpose of this article is to establish an effective method for compensating reactive power in four-wire networks voltage up to 1000 V with a solidly grounded neutral. The method and the device for independent individual phase reactive power compensation are presented. The known devices and methods for symmetric reactive power compensation in three-phase four-wire networks are ineffective. Under certain circumstances, probability of emergency mode increases. Asymmetrical and unbalanced load on network phases dominate in three-phase four-wire networks. As a result, current in the neutral wire increases several times. This can lead to thermal destruction of the neutral wire and failure of power supply cable. By reducing load unbalance in network phases and eliminating effects of undercompensation and overcompensation, the method of independent individual phase reactive power compensation makes it possible to reduce the current in the neutral. The method of independent individual phase compensation is preferred when using devices reactive power compensation in three-phase networks with solidly grounded neutral. This method allows to increase transfer capacity, to reduce losses, and also reduces chance of emergency mode. Results of introduction of devices for individual phase reactive power compensation into operation prove the efficiency of the proposed method

New method for calculating the susceptances of a balancing capacitive compensator for a three-phase four-wire distribution network

Nowadays, there is an increasing development of equipment type Custom Power Devices (CPDs) designed to ensure a high level of electrical power quality at the Point of Common Coupling (PCC). Most of these devices are built as advanced power electronics applications, such as the Switching Power Converters (SPCs) category. However, for a wide range of applications in consumer installations and electricity distribution operators, such as power factor correction, load balancing, voltage regulation, or flicker mitigation, the reactive power compensators based on passive circuit elements, Reactive Power Compensators (RPCs) are successfully used. The most efficient RPCs are those built so that they may adapt to variable regimes, known as Static Var Compensators (SVCs). Two of the usual functions of a SVC, namely power factor improvement and load balancing in a three-phase distribution network, can be achieved simultaneously by using an unbalanced SVC built in the version of Adaptive Balancing Reactive Compensator (ABRC). The paper briefly–outlines the mathematical model and operation of a Balancing Reactive Compensator (BRC) for a three-phase four-wire network. Then, moves on to develop a direct method of sizing based on equations that directly link the susceptances’ values of the compensator with the real and imaginary components of load sequence currents. The problem is to determine the weight of these components, meaning their degree of compensation, by requiring only capacitive or null susceptances and avoiding capacitive over-compensation of positive sequence currents. Thus, a particular case of BRC, i.e. a Balancing Capacitive Compensator (BCC) is obtained, which keeps the same functions, but has the advantage of reducing costs by eliminating high-power single-phase coils and by simplifying the compensation control system. The paper also presents the numerical results and the conclusions of a case study on the application of the method, performed using two computational and modeling software tools. Experimental laboratory determinations and modeling under nonsinusoidal conditions complete the analysis. The results demonstrate the correctness of the new method of solving the sizing problem and once again validate the unbalanced capacitive compensation as an efficient method of power factor improvement and load balancing at a PCC belonging to a three-phase four-wire distribution network.

Control strategy for improving voltage quality in residential power distribution network consisting of roof-top photovoltaic-wind hybrid systems, battery storage and electric vehicles

In this paper, a combined voltage control scheme is presented for improving the voltage profile, fluctuations, and imbalance of balanced and unbalanced residential distribution networks with a high penetration level (100% penetration level) of rooftop photovoltaic-wind turbine hybrid generation systems (PVWHSs) and electric vehicles (EVs). The combined control scheme includes both local and central control methods. Local controls in the inverter of EVs and PVWHSs of homes try to maintain their point of connection (POC) voltage within the acceptable range. Then the central controller does a complementary action for voltage improvement, by remotely controlling a battery energy storage (BES) and the network on-load tap changer (OLTC).The proposed control scheme is tested in DIgSILENT PowerFactory15.1.7 environment based on the weather and daily load data (wind speed, solar radiation, temperature, active and reactive power consumption of residential homes) during a year with a 1-min resolution in a three-phase four-wire network. The obtained results demonstrate the desired performance of the proposed scheme on voltage profile improvement, reducing voltage fluctuations, decreasing voltage imbalances of the phases and reduced number of tap operations, in the face of fast growth of distributed energy resources (DERs) penetration in the residential networks.

Iterative Method for Determining the Values of the Susceptances of a Balancing Capacitive Compensator

To increase the electrical power quality, in the last decades, an intense development in the last decades of high-performance equipment built as advanced power electronics applications, such as the compensators from Switching Power Converter category, has taken place. For all that, Reactive Power Compensators (RPC) based on passive circuit elements, such as Static var Compensators (SVCs), still occupy a wide range of applications in customer and installations of the distribution system installations. The functions of power factor (PF) improvement and load balancing in a three-phase distribution network can be achieved with an unbalanced SVC, known as the Adaptive Balancing Reactive Compensator (ABRC). Presenting first the mathematical model of the initial sizing and the working mechanism of a Balancing Reactive Compensator (BRC) for a three-phase four-wire network, this article develops a compensator resizing algorithm through an iterative change of the initial sizing to transform the compensator into a Balancing Capacitive Compensator (BCC), which keeps the same functions. By using two computational and modeling software tools, a case study on the application of the method was carried out, demonstrating the availability of the sizing problem solution and validating the unbalanced capacitive compensation as an efficient way to PF improving and load balancing in a PCC (Point of Common Coupling).

From the Balancing Reactive Compensator to the Balancing Capacitive Compensator

Nowadays, improving the power quality at the Point of Common Coupling (PCC) between the consumers’ installations and the distribution system operators’ installations depends more and more on the use of specialized equipment, able to intervene in the network to eliminate or diminish the disturbances. The reactive power compensators remain valid solutions for applications in consumer and electricity distribution, in those situations when the criterion regarding the costs of installing and operating the equipment is more important than the ones related to the reaction speed or the control accuracy. This is also the case of the equipment for power factor improvement and load balancing in a three-phase distribution network. The two functions can be achieved simultaneously by using an unbalanced static var compensator, known as an adaptive balancing compensator, achieved by adjusting the equivalent parameters of circuits containing single-phase coils and capacitor banks. The paper presents the mathematical model for the sizing and operation of a balancing reactive compensator for a three-phase four-wire network and then presents some resizing methods to convert it into a balancing capacitive compensator, having the same functions. The mathematical model is then validated by a numerical application, modelling with a specialized software tool, and by experimental laboratory determinations. The paper contains strong arguments to support the idea that a balancing capacitive compensator becomes a very advantageous solution in many industrial applications.

Mitigation of Power Quality Problems Using Custom Power Devices: A Review

Electrical power quality (EPQ) in distribution systems is a critical issue for commercial, industrial and residential applications. The new concept of advanced power electronic based Custom Power Devices (CPDs) mainly distributed static synchronous compensator (D-STATCOM), dynamic voltage restorer (DVR) and unified power quality conditioner (UPQC) have been developed due to lacking the performance of traditional compensating devices to minimize power quality disturbances. This paper presents a comprehensive review on D-STATCOM, DVR and UPQC to solve the electrical power quality problems of the distribution networks. This is intended to present a broad overview of the various possible DSTATCOM, DVR and UPQC configurations for single-phase (two wire) and three-phase (three-wire and four-wire) networks and control strategies for the compensation of various power quality disturbances. Apart from this, comprehensive explanation, comparison, and discussion on D-STATCOM, DVR, and UPQC are presented. This paper is aimed to explore a broad prospective on the status of D-STATCOMs, DVRs, and UPQCs to researchers, engineers and the community dealing with the power quality enhancement. A classified list of some latest research publications on the topic is also appended for a quick reference.

Investigation of the interaction between step voltage regulators and large‐scale photovoltaic systems regarding voltage regulation and unbalance

The number of installed photovoltaic (PV) systems has been increasing in an unprecedented rate every year throughout the world. In some situations, the PV systems can provide more power than the demand in certain parts of a network and cause reverse power flow, which the traditional distribution networks are not designed for. Consequently, these instances may result in adverse interactions between PV systems and voltage regulation devices such as step voltage regulators (SVRs). This can potentially drive voltage magnitudes beyond acceptable limits and possibly damage consumers’ appliances. In the literature, such interaction cases with consideration of realistic three-phase four-wire unbalanced networks have not yet been reported. Therefore, this study analyses the interaction between SVR and PV systems in a real-world network by utilising the quasi-static time-series technique for a long-term statistical assessment. This evaluation focuses on examining voltage regulation issues in an unbalanced three-phase four-wire network along with an open-delta SVR configuration, and the corresponding solution is proposed to resolve the concerning issues. The results of this study will provide valuable information on interaction characteristics to academia and utilities for assessing large-scale PV system integration.

DSTATCOM topologies for three-phase high power applications

This paper proposes distribution static compensator topologies for application in three-phase high power distribution systems. These topologies overcome both the voltage and current rating constraints imposed by the semiconductor switches by the use of three single-phase multi-secondary transformers. Five topologies, namely, four leg-group, three leg-group split-capacitor, H bridge-group, three leg-group and two leg-group are suggested. These can effectively compensate unbalance, harmonics and low power factor of the load currents. Apart from the use of minimum number of current sensors and a single capacitor, the control circuit is very simple. It provides the electrical isolation and security deemed essential for high voltage systems. A detailed comparison is made for these topologies. Based on the hardware requirement and compensation quality, the four leg-group and three leg-group topologies are shown to be superior than the other structures for application in three phase four-wire and three-wire systems respectively. They are validated through extensive simulation studies using PSCAD in a three-phase four-wire power network with non-linear unbalanced load. Experimental verification is conducted for H bridge-group topology on a single phase system using a DSP-based prototype laboratory model.

Current Harmonics Cancellation in Three-Phase Four-Wire Systems by Using a Four-Branch Star Filtering Topology

This paper presents a new solution for filtering current harmonics in three-phase four-wire networks. The original four-branch star (FBS) filter topology presented in this paper is characterized by a particular layout of single-phase inductances and capacitors, without using any transformer or special electromagnetic device. Via this layout, a power filter, with two different and simultaneous resonance frequencies and sequences, is achieved-one frequency for positive-/negative-sequence and another one for zero-sequence components. This filter topology can work either as a passive filter, when only passive components are employed, or as a hybrid filter, when its behavior is improved by integrating a power converter into the filter structure. The paper analyzes the proposed topology, and derives fundamental concepts about the control of the resulting hybrid power filter. From this analysis, a specific implementation of a three-phase four-wire hybrid power filter is presented as an illustrative application of the filtering topology. An extensive evaluation using simulation and experimental results from a DSP-based laboratory prototype is conducted in order to verify and validate the good performance achieved by the proposed FBS passive/hybrid power filter.

Distribution of non-sinusoidal currents in parallel conductors used in three-phase four-wire networks

This paper presents an analysis of the distribution of non-sinusoidal currents among parallel-connected cables in low-voltage, three-phase TN–S systems. The cables are assumed to be laid on metallic cable trays in free space, which means that the influence of earth is neglected. An iterative algorithm is developed which can accurately estimate the distribution of line and neutral currents among parallel-connected cables at various harmonic frequencies provided that the impedance matrix of the whole cable system is given. The impedance matrices of the cable systems are calculated by a Finite Element Method (FEM) model, because – as shown – the approximate analytical equations presented in the literature introduce significant errors at frequencies above the 7th harmonic. On the contrary, a FEM model can take into account the variation with frequency of the ohmic resistance, self and mutual inductance of a cable system. The FEM model is further used to estimate the temperature at the surface of the cables. It is shown that the non-uniform distribution of the current among parallel-connected cables can be very high and that it increases with frequency and with the number of parallel cables. The proposed model is used to estimate the cable arrangement that yields almost uniform current distribution without having to transpose the cables. Examples of the distribution of the current of two common non-sinusoidal loads are presented for various cable configurations. The proposed calculation method is validated through comparison with the measured distribution of the current among eighteen 300 mm 2 single-core cables in an industrial distribution substation. Measurements and calculations of the surface temperature of the individual cables showed also very good agreement which further validates the method.

Voltage Multiple-Zero-Crossings at Buses Feeding Large Triac-Controlled Loads

Triac-controlled loads operating in three-phase four-wire electric networks produce large zero-sequence current harmonics that cause large currents in the neutral conductor. Under certain conditions, the phase-to-neutral voltage (PTNV) at the bus feeding a large triac-controlled load is distorted to such a degree that multiple zero crossings (MZC) appear in the voltage waveform. This situation may cause malfunction of the triac control systems or any other control systems that are based on the detection of voltage zero crossings. The influence of various parameters of the triac-controlled load and of the cable feeding it, on the appearance of voltage MZC, is investigated. A simulation model is built with alternative transients program version of Electromagnetic Transients Program and is validated by comparison with measurements taken at a real theatrical lighting installation controlled by triacs. The occurrence of MZC in the voltage waveform is also explained by an analytical approximation. It is shown that certain cable configurations should be avoided when feeding large triac-controlled loads, or otherwise, MZC will appear in the PTNV.