Zigzag Transformer Research Articles

Tap for Classical HVDC Based on Multilevel Current-Source Inverters

This paper describes an innovative HVDC tap design based on series connections of current-source capacitor-commutated thyristor modules. Besides immunity from commutation failure and autonomous frequency control, the tap has independent active and reactive power control. There is potential of cost reduction because the number of thyristors is reduced by one-third. Switching loss is minimum as the modules switch at 2 pulses per ac cycle. Harmonic elimination by “time shifting and zig-zag transformers” achieves a current total harmonic distortion of about 4%.

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.

VSC with zigzag transformer based electronic load controller for a stand-alone power generation

This paper presents a novel configuration of a voltage and frequency (VF) controller for a stand-alone pico hydro power generating system employing an asynchronous generator (IAG). The proposed VF controller is an electronic load controller (ELC), which consists of a three leg insulated gate bipolar junction transistor (IGBT) based voltage source converter (VSC), a DC chopper and an auxiliary load at its DC bus. The neutral terminal for the consumer loads is created using a zigzag transformer. Three leg VSC is connected to the tertiary winding of the zigzag transformer to select the optimum voltage rating of the DC bus. The control algorithm of the VF controller is developed to control the system VF under varying consumer loads. The proposed system is modelled and simulated in MATLAB using Simulink and power system blockset (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 VF controller.

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>

A Three-Phase Four-Wire Power Filter Comprising a Three-Phase Three-Wire Active Power Filter and a Zig–Zag Transformer

Active power filters (APFs) have been developed to solve the problems of harmonic suppression and reactive power compensation simultaneously, In this paper, a three-phase four-wire power filter comprising a three-phase three-wire APF and a Zig-Zag transformer is developed. A hardware prototype is implemented and tested to verify the performance of the proposed power filter under various ideal and non-ideal power conditions. Experimental results show that this three-phase four-wire power filter has the desired performance under limited zero sequence utility voltage.

부하변동 시의 고조파제거 및 절전기 개발

본 연구에서는 부하의 변동 시에 고품질의 전력을 얻기 위하여 중성선과 전력선에 흐르는 각각의 영상 전류 성분과 제3고조파 전류 성분을 감소시킬 수 있는 Zig-Zag 변압기, 그리고 그 변압기의 2차측에 3상 브리지 정류기와 케패시터로 구성된 단상 능동 필터를 제작하였다. 이 단상 능동 필터를 변압기와 비선형 부하에 연결시켜 각 상에 보상 전류를 발생시킴으로써, 비선형 부하로 인하여 발생하는 제3고조파 성분을 감소시킬 수 있었다.

Analysis of Zig-Zag Transformer Applying in the Three-Phase Four-Wire Distribution Power System

The load unbalance and the nonlinear loads result in a significant neutral current in the three-phase four-wire distribution power system. The Zig-Zag transformer has been proposed to attenuate the neutral current of the three-phase four-wire distribution power system. In this paper, an analysis is carried out and computer simulation is used to evaluate the performance of the Zig-Zag transformer under ideal and nonideal power conditions. The simulation results show that (a) the Zig-Zag transformer can effectively attenuate the neutral current and zero-sequence harmonic currents on the utility side under the balanced utility voltage, (b) the utility side neutral current becomes larger under the unbalanced utility voltage or the distorted utility voltage with zero sequence harmonic components after applying the Zig-Zag transformer, (c) the insertion of an inductor in the utility side of the neutral conductor can alleviate overloading of the neutral current caused by the unbalanced utility voltages and the distorted utility voltages with zero sequence harmonic components.

A New Three-Phase Harmonic-Free Rectification Scheme Based on Zero-Sequence Current Injection

This paper proposes a new three-phase harmonic-free rectification scheme based on a zero-sequence injection technique. The proposed scheme incorporates two half-bridge inverters and two single-phase transformers to actively and individually shape the positive and negative dc rail currents of the rectifier. The shaped zero-sequence harmonic currents are then circulated through the ac side of the rectifier via a zigzag transformer, resulting in pure sinusoidal input currents in the three-phase diode rectifier. A design example along with component ratings calculation are detailed. Experimental results on a 1.5-kVA prototype are provided to validate the proposed technique.

상업용 설비 부하의 중성선 영상전류 저감장치 분석 및 적용에 관한 연구

With the proliferation of nonlinear loads such as commercial facilities in all of modern society high neutral harmonic currents have been observed. High neutral currents in three-phase four-wire distribution systems can cause lots of harmonic problems. The most common solution of neutral harmonic currents reduction is a zero phase sequence filter using zig-zag transformer. This paper analyzes a commercially available neutral line zero sequence currents reduction device and applies the equipment to commercial office loads for eliminating harmonics. Experimental results m compared numerically and graphically with simulation results through the use of MATLAB.

Experimental Investigation Into Operation Under Single-Phasing Condition of a Three-Phase Induction Motor Connected Across a Zigzag Transformer

A negative-sequence current produced during the single-phasing condition of a three-phase induction motor does not allow the motor to start and creates excessive heating while running. To eliminate the negative-sequence current, and thus to operate the motor satisfactorily under a single-phasing condition, the student is to connect a zigzag winding transformer with its star point connected to a system neutral across the motor. This experiment provides a thorough understanding of the sequence network connections under the single-phasing condition of a three-phase induction motor.

조명부하의 중성선 영상전류 저감장치 분석 및 적용에 관한 연구

현대 사회 전반에 걸쳐 조명설비와 같은 비선형 부하가 증가하여, 중성선에는 많은 고조파 전류가 흐른다. 3상 4선식 배전계통을 채용하는 중성선에 과다한 고조파 전류가 흐르면 여러 가지 고조파 장해를 일으킨다. 중성선 고조파 저감 대책으로 지그재그 변압기를 이용하는 영상 필터가 널리 쓰이고 있다. 본 논문에서는 상용화된 중성선 영상 전류 저감장치를 분석하고, 고조파를 저감시키기 위해서 조명 부하에 적용하고자 한다. 실측한 값과 MATLAB으로 시뮬레이션한 값을 수치와 그래프로 비교해 보았다.

Grounding transformer applications and associated protection schemes

Various aspects of the purpose, protection philosophy, application and specifications of different types of grounding transformers and two governmental system case studies illustrating improperly applied grounding transformers and associated ground fault protection systems are discussed. The first case study involves the use of an incorrect value of resistance in grounded neutral wye-broken delta grounding transformers associated with a 2400 V ungrounded system. Detailed calculations are given to correctly size the grounding transformer resistance value necessary to limit the magnitudes of transient overvoltages caused by restriking ground faults. The second case involves ground fault tripping deficiencies caused by utilizing only a single zig-zag grounding transformer with an associated neutral resistor on a multibus 2400 V ungrounded system. It is stressed that a single grounding transformer is not adequate for use with a multibus configuration. A protection scheme for use on a multibus arrangement is presented. >

A Practical Discussion ol Problems in Transformer Differential Protection

Some conditions which have to be met with in protecting transformers by the usual current-differential method produce special problems. Those involving the protection of interconnected wye or zigzag transformers and those involving Scott-connected transformers are dealt with at some length. There is also a discussion of tolerances in the selection of current transformers and the factors to be considered in the selection of relays for the purpose.

A practical discussion of problems in transformer differential protection

Some conditions which have to be met with in protecting transformers by the usual current-differential method produce special problems. Those involving the protection of interconnected wye or zigzag transformers and those involving Scott-connected transformers are dealt with at some length. There is also a discussion of tolerances in the selection of current transformers and the factors to be considered in the selection of relays for the purpose.

Operating experience with Petersen coils on 66-kv system of Metropolitan Edison company

THE Metropolitan Edison Company, operating in the eastern part of Pennsylvania, has a total of 264 miles of 66,000-volt overhead transmission lines, to which interconnections at this voltage with the systems of the Pennsylvania Water and Power Company and the Pennsylvania Power and Light Company add 59 miles, making a total of 323 circuit miles. A single-line diagram of this system is shown in figure 1, which also shows the various types of line construction used, ranging from all-wood construction to all-steel construction, with overhead ground wires and counterpoise. The system neutral is grounded through transformers at West Reading, Middletown, and the Holtwood station of the Pennsylvania Water and Power Company. At West Reading, the grounding bank consists of three single-phase 500-kva 66,000–13,200-volt transformers connected wye-delta, and at Middletown a three-phase zigzag transformer having an equivalent transformer capacity of 1,000 kva. At Holtwood each 66,000-volt line, as shown in figure 1, terminates in a 20,000-kva three-phase 69,000–13,200-volt wye-delta transformer, having the high-voltage neutral grounded through a 300-ohm reactor. The 13,200-volt winding of these transformers is connected to the 13,200-volt station bus.

Operating Experience With Petersen Coils on 66-Kv System of Metropolitan Edison Company

THE Metropolitan Edison Company, operating in the eastern part of Pennsylvania, has a total of 264 miles of 66,000-volt overhead transmission lines, to which interconnections at this voltage with the systems of the Pennsylvania Water and Power Company and the Pennsylvania Power and Light Company add 59 miles, making a total of 323 circuit miles. A single-line diagram of this system is shown in figure 1, which also shows the various types of line construction used, ranging from all-wood construction to all-steel construction, with overhead ground wires and counterpoise. The system neutral is grounded through transformers at West Reading, Middletown, and the Holtwood station of the Pennsylvania Water and Power Company. At West Reading, the grounding bank consists of three single-phase 500-kva 66,000–13,200-volt transformers connected wye-delta, and at Middletown a three-phase zigzag transformer having an equivalent transformer capacity of 1,000 kva. At Holtwood each 66,000-volt line, as shown in figure 1, terminates in a 20,000-kva three-phase 69,000–13,200-volt wye-delta transformer, having the high-voltage neutral grounded through a 300-ohm reactor. The 13,200-volt winding of these transformers is connected to the 13,200-volt station bus.