Zigzag Transformer Research Articles

ZIG-ZAG GROUNDING TRANSFORMER MODELING FOR ZERO-SEQUNCE IMPEDANCE CALCULATION USING FINITE ELEMENT METHOD

The grounding transformer is one of most important equipment in power energy system. This paper describes the modeling of zig-zag grounding transformer type with varying degrees of complexity. In this paper, the Finite Element model (FEM) of zig-zag grounding transformer with non-linear magnetic characteristic for iron is built using ANSYS software electromagnetic package. A numerical method, based on Finite Element Analysis (FEA), is presented for computing the zero-sequence impedance of grounding transformer. The analysis method is based on the two dimensions (2D) model and this model was solved by using the magnetic vector potential formulation (A).The main purpose of this paper is performing the modeling of the three-phase zig-zag grounding wound core transformer in 2D FEM for any capacity of transformer (100KVA- 1000KVA) and the Finite Element techniques are used for the magnetic field analysis to evaluate the magnetic field and to determine their distribution at any region inside the window and winding. Two types of analyses were performed, including static and transient analysis. The transient analysis in this work is simulated by direct coupling the 2D transformer model with external circuit (voltage sources) .The simulation results prove the analysis' correctness and validity, and the result of zero-sequence impedance of grounding transformer is verified by comparison with experimental result. Those measured in the Diyala transformer factory once the grounding transformer has been built. A good agreement of the computational results with experimental result by using this FEM model of zig-zag grounding transformer allowing us to know the transformer behavior before manufacturing them and, thus reducing the design time and cost.

Zigzag transformer - some new applications with a note to energy efficiency

Zigzag transformer is only used as a grounding transformer in a three-phase delta connected power supply system. Equal DC current flowing through the windings of a zigzag transformer does not produce any core saturation and it may be used as an interfacing device for AC supply with DC in a power system. This property of the transformer may be used to transmit DC and AC power simultaneously through the same transmission line. It may also be used to get three-phase voltage from three-wire, two-phase supply and may be used to maintain three-phase load voltage when any one phase of the three-phase supply line trips. This paper emphasises the role of zigzag transformer as a common interfacing device in all these new applications with some comments on the change of energy efficiency of the power system.

A Zig-Zag Transformer and Three-leg VSC Based DSTATCOM for a Diesel Generator Based Microgrid

Abstract The distribution systems are facing severe power quality problems due to the uncontrolled use of different types of linear and non-linear loads such as solid state controllers which will draw harmonics and reactive currents from ac mains. It may lead to poor power quality at ac sources such as sag, swell, notch, flicker, unbalance etc. The installation of the diesel generator set is widely used in practice to feed power to some crucial equipments and in remote areas. DG sets used for these purposes may be loaded with unbalanced, reactive and non-linear loads such as power supplies in telecommunication system and medical equipments. A Zig-Zag Transformer and three-leg VSC based DSTATCOM is used here to compensate the neutral current, harmonic current, reactive power and unbalanced load. The insulated gate bipolar transistor (IGBT) based VSC is supported by a capacitor and is controlled for the compensation required in the load current. The dc bus voltage of the VSC is regulated during varying load conditions. In this paper, a Synchronous Reference Frame (SRF) controlled three-leg voltage source converter (VSC) based DSTATCOM with a Zig-Zag transformer is used for reactive power, harmonics, unbalanced load current and neutral current compensation in grid connected and islanded mode.

Hyperbolic tangent function‐based least mean‐square control algorithm for distribution static compensator

A variable learning rate least mean-square (LMS) based on hyperbolic tangent function is used to enhance the rate of convergence and to suppress the existing noise in the system for the control of distribution static compensator (DSTATCOM) consisting of a three-leg voltage source converter (VSC) with a zig-zag transformer to eliminate the significant neutral current caused by non-linear unbalanced loads in a three-phase four-wire supply system. Simulation is made for the proposed control algorithm in three-phase four-wire DSTATCOM system and its performance is compared with the conventional Adaline-based LMS control algorithm. It is also shown that the convergence rate of the proposed control algorithm is quite faster in comparison with the conventional control algorithm. A prototype of DSTATCOM is developed and the real-time implementation of the proposed control algorithm is performed on a digital signal processor. Test results have demonstrated the satisfactory performance of DSTATCOM with the proposed control algorithm under linear and non-linear loads.

Neutral current compensation techniques in autonomous wind energy sources

The problem of neutral current compensation has to be addressed in Wind Energy integrated weak micro-grids. This study analyses two types of techniques possible for implementation of neutral current compensation by drawing a comparative research of their application on the same wind energy conversion system. The hybrid micro-grid considered in the study comprises a synchronous diesel generator and an inverter-based wind generator. The results and analysis are conducted by time-domain simulations using MATLAB/Simulink software. The use of electronic power converters that also act as an active neutral current compensator or a zig-zag transformer connected with the load mitigates the problem of neutral current compensation to a large extent. The voltage unbalance factor is also significantly reduced due to neutral current compensation and power quality is enhanced. The results provide the actual operational limits possible under the set of given constraints.

Efficient Grounding for Modular Multilevel HVDC Converters (MMC) on the AC Side

An unbalance voltage between a positive pole and negative pole will exist for a long time in an ungrounded modular multilevel converter (MMC) HVDC transmission system. A zig-zag transformer has been proposed to construct an artificial neutral point and keep voltage balance between the positive pole and the negative pole. An efficient grounding system consists of the zig-zag transformer and the neutral grounding resistor. In this paper, the different impedances, including positive-, negative-, zero-sequence impedance, and dc current impedance, have been analyzed, respectively. Then, the computer simulations have been performed to evaluate the characteristic of the zig-zag transformer under normal voltage, zero-sequence voltage and unbalanced dc voltage, respectively. The simulation results show that 1) the zig-zag transformer can effectively balance the unbalanced voltage in dc side of the MMC under different work conditions; 2) the zero-sequence impedance is low and zero-sequence current is convenient for the detection of a single-phase fault; 3) the power consumption of the neutral grounding resistor is of a light rating, and the cooling of the resistor is unnecessary. The method presented in this paper proves to be an efficient grounding method for the MMC-HVDC transmission system.

A Combined AC+DC Distribution Network Interconnected with Renewable Energy

The combined AC+DC distribution network in this study provides a coupling and decoupling strategy of renewable energy in DC mode by using zig-zag transformer, which makes each line transmit AC electrical power and DC power simultaneously. The proposed scheme is digitally simulated with the help of Simulink software package. Simulation results indicate lower line voltage drop and less active and reactive power loss in steady state; almost similar or even better transient effects in transient state, which demonstrates the feasibility of combined AC+DC distribution network of single line interconnected with renewable energy.

Maintaining balanced three‐phase load voltage during single‐phase auto‐reclosing in medium voltage distribution lines

Single-phase auto-reclosing (SPAR) is used for restoring power supply to clear temporary single-phase to ground fault in medium-voltage distribution systems. A zigzag winding grounding transformer connected at the remotest load bus rebuilds the three-phase voltage during the SPAR dead time. A distribution system having more than one branch needs one zigzag transformer at the remotest end of each branch. Build-up three-phase voltage during SPAR is not balanced. Operation of distributed generator connected to the system during SPAR needs the voltage to be balanced. Magnitude of the negative sequence voltage may be reduced by connecting capacitor in series with the neutrals of each zigzag transformer.

Selective harmonic elimination in multi‐level inverter with zig‐zag connection transformers

This study applies the selective harmonic elimination (SHE) technique to determine the switching angles for a multi-level inverter that cooperated with specially connected transformers. With the ability of directly controlling harmonics, the SHE technique has the adaptability to associate with apparatus which are congenitally immune to specific harmonics, such as the zig-zag connection transformers. In this study, the two sets of primary windings of the transformers are supplied by two six-switch full-bridge inverters with 30 electrical degrees phase shift. Harmonics with orders other than 12h ± 1 (h is positive integral) will not appear in the output voltages in the secondary side of the transformer because they are prohibited by the transformers. Then, the SHE technique with half-wave symmetry is employed to eliminate some selected harmonics with orders equal to 12h ± 1, and adjusts the amplitude of the fundamental. This study obtains a set of precise switching angles through offline calculation over a wide range of modulation indices. A digital signal processor is used to implement online calculation of the switching angles by an approximate method derived from the precise solutions. Some selected analysis and experimental results are demonstrated in this paper. A 4 kW prototype is built to verify the validity of the proposed system.

Power Quality Improvement in Three Phase Four Wire Distribution System with Implementation of Fuzzy Logic Controller Based Adaptive Shunt Active Filter

Non-linear loads and applications of power electronics in 3P4W (three phase four wire) distribution system are the major sources of harmonics. The power quality compensator which mitigates the problem related to Current harmonics is Shunt active filter (SAPF) or passive filter. Active filtering is the most efficient mitigating method adopted for reactive power compensation and mitigation of harmonics. The power quality compensator proposed in this paper is a SAPF which reduces the %THD, compensates reactive power and mitigates neutral current under unbalanced non-linear load condition. A 3-fi MOSFET based 3 leg VSI (Voltage Source Inverter) with DC bus capacitor as energy buffer is employed to realize the SAPF. Fuzzy logic based Direct control method is the propounded control strategy implemented for the estimation of reference current and to keep the voltage of DC capacitor to its reference value. Switching signals for the VSI is generated using Hysteresis current controller. T-connected and Zig-Zag transformers are implemented to reduce the neutral current to about 1/10th of its original value in unbalanced Non-linear load conditions. The propounded control strategy performance is validated and analyzed using MATLAB software

A New Topology for Simultaneous AC-DC Distribution with the same Line

A new topology for simultaneous AC-DC distribution along the same line is proposed in this paper to integrate DC power with the existing alternating current (AC) distribution, which incorporates a zigzag transformer and a boost chopper. The zigzag transformer is utilized to achieve either coupling or decoupling of the AC and DC power without sensible iron-core saturation due to voltage canceling out on one phase. The boost chopper is used to achieve DC power flow and keep the DC ratings at each node. A simulation model of the novel topology is established with PSCAD software, and detailed comparison verifies that the new topology with a zigzag transformer is preferable to that with a common transformer for simultaneous AC-DC distribution scheme, indicating a promising future of the proposed technology.

Analysis and Improved Operation of a PEBB-Based Voltage-Source Converter for FACTS Applications

The demand of power-electronic devices is increasing in several applications, and power-electronic building blocks (PEBBs) are a strategic concept to increase the reliability of the power-electronic converters and to minimize their cost. A possible solution to increase the power rate of these converters is the use of series or parallel connected PEBBs in multipulse configurations. Magnetic elements, such as zig-zag transformers, phase-shifted transformers (pst), or zero-sequence blocking transformers (ZSBT), are used to interconnect the PEBBs. In this paper, the operation of multipulse converters will be analyzed, describing the harmonic cancellation and minimization techniques that could be used in these multipulse converters, focusing on the power-electronics flexible ac transmission systems devices installed at the NYPA Marcy substation. In order to improve the dynamic response of this system, the use of selective harmonic elimination modulation is proposed and implemented.

Rebuilding of three-phase load voltage during single-phase auto reclosing in medium voltage distribution lines

Most faults in medium voltage (MV) distribution lines are temporary single line to ground (LG) faults. Single-phase auto reclosing (SPAR) may be used to remove the LG fault producing highly unbalanced and low voltage across the load during the reclosure dead time. The negative sequence voltage produced may cause severe over heating in three-phase induction motors. It is proposed to connect a zigzag winding grounding transformer at the remotest load bus to rebuild the three-phase voltage during the SPAR dead time. The zigzag transformer neutral should remain disconnected from the ground during normal operation.

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.

Zig-zag transformer for neutral current compensation in an isolated wind energy system

This paper presents a voltage and frequency controller with neutral current compensation in a stand alone wind energy conversion system (WECS) employing an asynchronous generator. The proposed controller consists of a three leg insulated gate bipolar junction transistor (IGBT)-based voltage source converter and a battery energy storage system (BESS) at its DC bus. The neutral terminal for the consumer loads is created using a zigzag transformer with an additional secondary winding. This additional secondary winding of zigzag transformer facilitates to select the optimum voltage rating of the battery system at DC bus. The control algorithm of the controller is developed for bidirectional flow capability of the active power and reactive power control by which it controls the system voltage and frequency under different dynamic conditions such as varying consumer loads and varying wind speeds. The complete system is modelled and simulated in MATLAB using Simulink and PSB toolboxes. Extensive simulation results are presented to demonstrate the capability of the controller as a harmonic eliminator, a load balancer, a neutral current compensator along with voltage and frequency controller.

Analysis, design and control of a two stage improved power quality telecommunication power supply

This paper deals with the power quality improvement at the utility interface of a telecommunication switched mode power supply (SMPS) of 12 kW rating. The first stage of this SMPS consists of a 6-pulse diode bridge converter that performs ac-dc conversion yielding an unregulated dc voltage. In the second stage, two boost rectifiers are connected together which feed to a boost dc-dc full bridge converter connected to the output load. By means of a closed loop control, appropriate value of third harmonic component of the current is injected using a zig-zag transformer to reduce the current harmonics distortion at the utility interface. A detailed design procedure is presented to select various components of the SMPS system.

Maintenance of Three-phase Load Voltage during Single Phase Auto Reclosing in Medium Voltage Radial Distribution Lines

Most faults in medium voltage (MV) distribution lines are temporary line to ground (LG) faults. Three-phase auto reclosing (TPAR) is commonly used to remove this fault with temporary disconnection of all the phases. Multi-shot single-phase auto reclosing (SPAR) may also be used to remove the LG fault. But it produces highly unbalanced and low voltage across the load during the reclosure dead time. It is proposed to connect a zigzag winding grounding transformer at the load bus to maintain the 3-phase load voltage when one phase opens during the SPAR. With low value of grounding resistance the 3-phase voltage during the SPAR dead time becomes approximately balanced. Directional over current relays may be used for the protection. Analysis of a MV radial distribution system having a zigzag transformer connected to the remotest load bus is presented with the computation of voltages during the dead time of SPAR.

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.

Study on the Multiple Voltage Source Inverter for Grid-Connected Large-Scale Wind Energy Conversion System

This paper proposes a novel grid-connected multiple voltage source inverter (VSI) for large-scale wind energy conversion system (WECS) applications. Firstly, the typical configuration, operation principle and mathematical modeling of the direct-drive grid-connected WECS are analyzed in brief. Then, a suitable topology of the grid-connected multiple VSI composed of two six-pulse inverters, two separate zig-zag transformers and a dc-link capacitor is presented. In order to reduce numbers of commutations and to increase dc-link voltage utilization ratio of the proposed grid-connected multiple VSI, a quasi twelve-pulse phase-shift PAM switching strategy is demonstrated. Finally, a simulation test system is set up by using MATLAB/SIMULINK, and the results of simulation validate the effectiveness of the proposed grid-connected multiple VSI.

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.