Three-phase Distribution System Research Articles

A generalized power flow analysis for distribution systems with high penetration of distributed generation

In this paper, the element incidence matrix has been extended to develop a comprehensive three-phase distribution system power flow program for radial topology. Three-phase overhead or underground primary feeders and double-phase or single-phase line sections near the end of the feeder laterals have been considered. Unbalanced loads with different types including constant power, constant current and constant impedance are modeled at the system buses. Substation voltage regulator (SVR) consisting of three single phase units connected in wye or two single-phase units connected in open delta are modeled to satisfy the desired voltage level along the feeder. The mathematical model of distributed generation (DG) connected as PQ and PV buses are integrated into the power flow program to simulate the penetration of DGs in the distribution systems. The proposed method has been tested and compared with different IEEE test feeders result. The developed algorithm has been used to study the impact of both SVR and high penetration of DG on voltage profile and system power losses.

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.

Design of Delta Primary - Transposed zigzag Secondary (DTz) Transformer to Minimize Harmonic Currents on the Three-phase Electric Power Distribution System

The delta primary - transposed zigzag secondary (DTz) transformer has been designed and used to reduce the bad impacts of the harmonic in the distribution power system. The DTz transformer is constructed with delta connection in primary winding and the three transposed windings at the different core legs of secondary winding. The harmonic reduction method of the DTz transformer applies two basic principles. The first principle is to inhibit electromagnetic energy of the harmonic currents by cancelling the phase polarity on the secondary winding. The second is to insulate the remaining of the mmf induction from harmonic current loads and minimize to circulate in the delta windings on the primary side. The triplen harmonics currents generated on the primary and secondary winding of DTz transformer are simulated in this paper. Both balanced and unbalanced loads of the three-phase distribution system are examined. The experiment shows that the total THD current in the secondary winding when balanced loads are applied is about 70.8 %, and in the primary side is 24.3 %. While for unbalanced loads, the average THD in secondary winding is 68.44 % and in delta winding is 26.4 %. It means the DTz transformer has a filter-ability to reduce about 42 - 46 % THD for both balanced and unbalanced loads. By comparing the computer simulation results and data measurements through experiment in the laboratory, it is proved that the use of the proposed DTz transformer is one of the methods to reduce harmonic currents and inhibit them to enter to the supply system.

Comparisons between the three-phase current injection method and the forward/backward sweep method

This paper presents comparisons of two power flow methodologies for distribution system analysis: the three-phase current injection method – TCIM and the forward/backward sweep – FBS. These techniques were applied for large scale three-phase distribution systems, the advantages and drawbacks are emphasized and their computational performances are presented. The results presented in this work can be helpful to decide which one of the tested methods is the most suitable to solve a particular electrical system.

Analysis of Transformer-Isolated Diode Rectifier Circuits in Three-Phase Rectangular-Waveform Distribution System

This paper presents an analysis of the steady and transient characteristics of capacitor-input-type diode rectifier circuits connected to a delta-star transformer in a three-phase rectangular-waveform distribution system. Equations are derived for the input current, output voltage, and total input power factor of the rectifier circuits. The total input power factor can be improved from 0.91 to 0.98 by using the delta-star transformer. Further, design methods are developed for both the input reactor and output capacitor of the rectifier circuits. A small input reactor is desirable for obtaining a large total input power factor and a constant output voltage. The output capacitor can be designed to obtain the required output voltage variation in steady and transient states. The effectiveness of the proposed analysis is experimentally verified by using a 2.4kW prototype.

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.

The Effects of Mutual Coupling and Transformer Connection Type on Frequency Response of Unbalanced Three Phase Electrical Distribution System

In this paper, a novel harmonic modeling technique by utilizing the concept of multi-terminal components is presented and applied to frequency scan analysis in multiphase distribution system. The proposed modeling technique is based on gathering the same phase busses and elements as a separate group (phase grouping technique, PGT) and uses multi-terminal components to model three-phase distribution system. Using multi- terminal component and PGT, distribution system elements, particularly, lines and transformers can effectively be modeled even in harmonic domain. The proposed modeling technique is applied to a test system for frequency scan analysis in order to show the frequency response of the test system in single and three-phase conditions. Consequently, the effects of mutual coupling and transformer connection types on three-phase frequency scan responses are analyzed for symmetrical and asymmetrical line configurations.

Analysis of the Neutral Current for Two-Step-Type Poles in Distribution Lines

One-step-type poles and two-step-type poles are used in KEPCO's distribution system. An unbalanced current may flow through the neutral wire in three-phase four-wire distribution systems due to unbalanced loads. Generally, the power line and the communication line are installed in close proximity due to the topography of Korea. By means of electrostatic induction, electromagnetic induction, and harmonic induction, the power line induces damaging voltage and current in the communication line. This paper calculates the neutral current in KEPCO's distribution system by using the electromagnetic transients program (EMTP) by composing various simulated conditions, and the results are verified by vector analysis.

Three-Leg Voltage Source Converter Integrated with T-connected Transformer as Three-phase Four-wire Distribution Static Compensator for Power Quality Improvement

In this article, a new topology of a distribution static compensator is proposed for power quality improvement in a three-phase four-wire distribution system. A three-leg voltage source converter based distribution static compensator is integrated with a T-connected transformer for the compensation of reactive power for voltage regulation or for power factor correction, along with load balancing, elimination of harmonics currents, and neutral current compensation at the point of common coupling. The T-connected transformer is used for providing a path to the zero-sequence current in a three-phase four-wire distribution system. In order to reduce the voltage rating of the voltage source converter, the T-connected transformer is designed to have secondary windings for integrating the distribution static compensator. This transformer connection provides the selection of an “off-the-shelf” voltage source converter for this application, and it also provides isolation for the voltage source converter system. The kVA rating of the transformers in the T-connected configuration is comparable to that of a similar zig-zag transformer, and there are only two single-phase transformers required in the T-connected configuration. This reduces the complexity and also cost of the distribution static compensator system. The performance of the proposed distribution static compensator system is validated through simulations using MATLAB software with its Simulink and Power System Blockset toolboxes.

Reduced Rating VSC With a Zig-Zag Transformer for Current Compensation in a Three-Phase Four-Wire Distribution System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A detailed investigation is made into the causes, standards, and remedies of the excessive neutral current. A reduced-rating voltage-source converter with a zig-zag transformer as a distribution static compensator is proposed for power-quality improvement in the three-phase four-wire distribution system. The proposed DSTATCOM is employed for the compensation of reactive power, harmonics currents, neutral current, load balancing and the voltage regulation at the point of common coupling. The zig-zag transformer is used for providing a path to the zero-sequence current. The performance of the DSTATCOM is validated through extensive simulations using MATLAB software with its Simulink and power system blockset toolboxes. </para>

Star-hexagon transformer and non-isolated three-leg VSC based three-phase four-wire DSTATCOM

A new three-phase four-wire DSTATCOM (distribution static compensator) is proposed for power quality improvement in the distribution system. The DSTATCOM is based on a star/hexagon transformer and three-leg VSC (voltage source converter). It compensates harmonic current, reactive power and balances the load along with neutral current compensation. Each of the three single phase transformers has three windings and the secondary windings are connected in star/hexagon configuration for interfacing to a three-phase four-wire power distribution system. The IGBT (insulated gate bipolar transistor) based VSC is a capacitor supported and it is controlled for the required compensation of load currents. The DC bus voltage of the VSC is regulated even during dynamic load variations. The DSTATCOM is tested for power factor correction and voltage regulation along with neutral current compensation, harmonic elimination and load balancing under linear loads as well as non-linear loads. The performance of the three-phase four-wire DSTATCOM is validated using MATLAB software with its Simulink and SimPower System (SPS) tool boxes.

A Novel Structure for Three-Phase Four-Wire Distribution System Utilizing Unified Power Quality Conditioner (UPQC)

This paper presents a novel structure for a three-phase four-wire (3P4W) distribution system utilizing unified power quality conditioner (UPQC). The 3P4W system is realized from a three-phase three-wire system where the neutral of series transformer used in series part UPQC is considered as the fourth wire for the 3P4W system. A new control strategy to balance the unbalanced load currents is also presented in this paper. The neutral current that may flow toward transformer neutral point is compensated by using a four-leg voltage source inverter topology for shunt part. Thus, the series transformer neutral will be at virtual zero potential during all operating conditions. The simulation results based on MATLAB/Simulink are presented to show the effectiveness of the proposed UPQC-based 3P4W distribution system.

Star/Hexagon Transformer Based Three-Phase Four-Wire DSTATCOM for Power Quality Improvement

A new topology of DSTATCOM (distribution static compensator) is proposed for power quality improvement in three-phase four-wire distribution systems. A three-leg VSC (Voltage Source Converter) is integrated with a star/hexagon transformer for the compensation of reactive power for voltage regulation or for power factor correction along with load balancing, elimination of harmonics currents and neutral current compensation. The star/hexagon connected transformer provides a path to the zero sequence current in a three-phase four-wire distribution system. In order to optimize the voltage rating of the VSC, the star/hexagon transformer is designed to have a suitable voltage rating for the secondary windings for integrating the three-leg VSC. This transformer connection provides the selection of 'off the shelf' VSC for this application and it also provides isolation for the VSC system. The performance of the proposed DSTATCOM system is validated through simulations using MATLAB software with its Simulink and Power System Block set (PSB) toolboxes.

A T-Connected Transformer and Three-leg VSC Based DSTATCOM for Power Quality Improvement

In this paper, a new three-phase four-wire distribution static compensator (DSTATCOM) based on a T-connected transformer and a three-leg voltage source converter (VSC) is proposed for power quality improvement. The T-connected transformer connection mitigates the neutral current and the three-leg VSC compensates harmonic current, reactive power, and balances the load. Two single-phase transformers are connected in T-configuration for interfacing to a three-phase four-wire power distribution system and the required rating of the VSC is reduced. The insulated gate bipolar transistor (IGBT) based VSC is supported by a capacitor and is controlled for the required compensation of the load current. The DC bus voltage of the VSC is regulated during varying load conditions. The DSTATCOM is tested for power factor correction and voltage regulation along with neutral current compensation, harmonic elimination, and balancing of linear loads as well as nonlinear loads. The performance of the three-phase four-wire DSTATCOM is validated using MATLAB software with its Simulink and power system blockset toolboxes.

Control Strategies for Load Compensation Using Instantaneous Symmetrical Component Theory Under Different Supply Voltages

In this paper, the theory of instantaneous symmetrical components is applied to explore various control strategies of load compensation, under different supply voltages. When the supply voltages are balanced and sinusoidal, all of these strategies converge to the same compensation characteristics. However, when the supply voltages are not balanced sinusoids, these control strategies result in different degrees of compensation in harmonics, power factor, neutral current, and compensator ratings. These control strategies are discussed in detail and a comparative study of their performance in terms of the rms value, total harmonic distortion, power factor of source currents, and compensator ratings is presented. Based on this study, it is possible to select the best strategy to meet the required load compensation characteristics for available supply voltages. A three-phase four-wire distribution system supplying an unbalanced and nonlinear load is considered for simulation study. The detailed simulation results using MATLAB are presented to support the proposed compensation strategies.

Loss Partitioning and Loss Allocation in Three-Phase Radial Distribution Systems With Distributed Generation

In this paper, the concepts related to loss partitioning among the phase currents in three-phase distribution systems are revisited in the light of new findings identified by the authors. In particular, the presence of a paradox in the classical loss partitioning approach, based on the use of the phase-by-phase difference between the input and output complex power, is highlighted. The conditions for performing effective loss partitioning without the occurrence of the paradox are thus established. The corresponding results are then used to extend the branch current decomposition loss allocation method for enabling its application to three-phase unbalanced distribution systems with distributed generation. Several numerical examples on a three-phase line with grounded neutral and on the modified IEEE 13-node test system are provided to assist the illustration and discussion of the novel conceptual framework.

H∞ Static Control Law for a Three-phase Shunt Active Power Filter

The aim of this work is to design a static feedback gain used to force a three-phase shunt active power filter to act as a current source capable of delivering the appropriate harmonics demanded by non-linear loads. This filter is connected in parallel to a three-phase power distribution system in order to keep the current supplied by the latter almost sinusoidal. The control design is performed using the H ∞ suboptimal control theory based on linear matrix inequalities (LMIs), with both partial and full state feedback. Numerical simulations will be presented in order to investigate the advantages of the full state feedback structure over the partial state feedback one, and to compare the performances of the corresponding H ∞ static control laws.

Steady State Analysis of Resin Molded Type Voltage-Current Sensor for Real-Time Observation of Power Factor in Power Distribution System

We have proposed the voltage-current sensor embedded in a ceramic insulator for the real-time observation of the power factor in three-phase power distribution systems and carried out the finite element analysis to investigate its characteristics and verify its feasibility. However, the ceramic sensor has problems in cost and a burning process in manufacturing to result in the deformation from the designed form. In the paper, we propose a resin molded type of voltage-current sensor with dual search coils for the current sensor part based on the finite element analysis and discuss its steady state electromagnetic behaviors theoretically and experimentally.

Characteristics of Voltage Swell in Multigrounded Systems

The effects of multigrounded neutral (MGN) in three-phase distribution systems are not easy to analyze due to the presence of a fourth conductor-the neutral and its multigrounded arrangement. As a result, the mechanism of voltage swell in MGN feeders becomes difficult to understand. The amount of voltage swell in a MGN feeder is affected by unique factors such as the current in the neutral wire and the neutral-to-phase voltage induction. The purpose of this letter is to reveal and quantify all factors that contribute to voltage swell and substation ground potential rise in an MGN system. The results are expected to help utility engineers to understand the impact of MGN on distribution system operations and to assist their deployment of the MGN schemes

Control Methods and Compensation Characteristics of a Series Active Filter for a Neutral Conductor

Due to the advance of information technologies, a large number of electronic products such as personal computers have been connected to power distribution systems in commercial buildings. Hence, voltage distortion on utility outlets and excessive neutral current on distribution lines have arisen and lead to a number of serious problems in the distribution system. Two control methods and the related compensation characteristics of a series active filter connected to the neutral conductor are presented in this paper. The distinct functions of the proposed active filter are the mitigation of the third-harmonic voltage and the neutral current in a three-phase four-wire distribution system in a building. The required power of the proposed active filter is less than 10% of that of the harmonic-producing loads. A control method of the dc capacitor voltage on the active filter circuit is also described. It is clarified through experiments that one of the two functions of the active filter can be realized selectively and the dc capacitor voltage of the active filter can be regulated to a desired value