Power Transformers Overview

Introduction

Powertech Transformers (PTT) is owned by Power Technologies Limited (Powertech), one of the largest power electrical group in Southern Africa and our empowerment partner Power Matla. The power transformer business, which originated in the Swedish ASEA stable was established in South Africa in 1944 and has become dominant in that market. For a period from 1988 to 1992 the company was known as Brown Boveri Technologies (BBT) before the name changed to ABB Powertech Transformers in 1997. On 26 March 2008, the Competition Commission Tribunal gave the go-ahead for Powertech to purchase the 50% share that ABB held in the company and the name was changed to Powertech Transformers. With capacity in excess of 10,000MVA per annum and access to the most advanced technology available, the company is ideally positioned to service the whole sub-Saharan Africa power transformer and shunt reactor market. Each year more than 120 units with ratings from 20 to 800MVA and voltages of up to 500kV are produced.

Design

In practical transformer design the manufacturer has a choice between core type and shell type concepts. We only manufacture core-type transformers.

The majority of our transformers are designed according to the “TrafoStarTM” design which is the common ABB concept used for the design and manufacture of power transformers. It is based on the best practices and know-how acquired over several decades coupled with its global manufacturing experience.

The concept incorporates common electrical and mechanical design standards, global processes and quality norms. Every TrafostarTM transformer is built from standardised, service proven components and modules ensuring flexible, dependable and tailor-made transformer designs. The high reliability of these transformers ensures maximum availability, lower maintenance costs and reduced life-cycle costs.

Powerful and sophisticated software tools are used to develop the mechanical and electrical design of all transformers. These tools help in ensuring optimum electrical design and determining eddy current losses and short circuit forces accurately. A 3 dimensional modelling software (Pro-Engineer) used in developing the mechanical design enables designers to view the virtual prototype of a transformer (including tanking of the corecoil assembly) on the computer screen before production commences.

Windings

Our windings are designed to ensure high short circuit withstand capability, uniform surge voltage distribution and effective heat dissipation which are critical to transformer reliability.

The disc type design of the high voltage windings ensures better short circuit withstand capability, uniform surge voltage distribution and effective heat dissipation.

These windings can be used with different designs for both small and large power transformers up to the highest voltage levels.

Using the latest developments in winding technology, we use “voltage stabilised” disc windings for the high voltage side. The individual winding turns in each group of discs are interleaved thereby increasing the winding series capacitance to get uniform voltage distribution.

Core Construction

Cold-rolled grain oriented silicon steel laminations are used to ensure low no-load losses and suppression of vibrations resulting in lower noise levels. Using stepped sheet widths helps in achieving an almost circular cross section of the core.

Advanced step-lap core construction technique achieve precise core stacking with step lap joints at the corners. This technique also helps in stacking at precise angles thereby preventing the formation of air gaps in the core joints, which otherwise could lead to increased losses and noise generation.

Core form structure is used without any through bolts in the limb or yoke portion. The core limb is wound with conducting resin impregnated glass tape to give rigidity to core and to get a minimum uniform circular section for inner winding. To avoid internal mechanical stresses, the core is set upright in the stacking device itself.

Assembly

Both the medium and large transformer assembly lines at Powertech Transformers are equipped with motorised personnel platforms capable of vertical and horizontal travel which allow the work of assembly and connecting up to be completed with minimum strain on personnel.

The core and windings are assembled using insulation material of the highest quality. Where required, correctly positioned magnetic shunts ensure the collection of stray flux preventing unwanted hotspots in the clamping structure and tank walls.

Jointing, both in the winding and external, is by high frequency welding, which is extremely rapid and reliable. These joints are taped and the leads fixed in position to withstand any stresses during test and service condition.

Processing

Windings are dried under vacuum and, on large units, oil impregnated before assembly. The complete active part is processed in an autoclave to obtain less than 0,5% moisture content. Within the prescribed time following the drying cycle, pressure is applied to the windings and the unit is tanked and filled, under vacuum, with heated, degassed and purified oil.

Cooling

On offer is a wide range of cooling methods such as ONAN, ONAF, OFAF, OFWF or combinations thereof. Forced cooling can be activated by temperature or loading pattern.

Tanks

Computerised tank designs are fabricated from mild steel plate. Welds are dye-penetrant tested to ensure that there are no leaks and specific joints, which may be stressed during handling and transport, are ultrasonically tested. Stiffeners are added at calculated intervals to make the tanks vacuum and over-pressure proof. Tank interiors are painted a light colour to improve visibility. The tank exterior is treated and painted to customer requirements. Nitrile O-ring gaskets for pipe joints and inspection covers are a standard feature in order to avoid oil leaks developing later during service. Tank main covers are generally welded, but these can be offered as bolted covers if specifically requested. Where required, magnetic shielding is applied to the tank interior to prevent heating. Only accessories from approved suppliers are used on our transformers.

Quality and Testing

Quality is built into every Powertech Transformers' product and acceptance testing is one of the final steps of the quality control measures. Powertech Transformers has SABS ISO 9001-2008 accreditation and has a proven quality control system. Furthermore the company has ISO 14001-2004 accreditation for its environmental awareness.

Our testing facility includes an impulse generator, precision power analyser for transformer loss measurement, PD analyser and temperature data logger to carry out routine and type tests accurately.

A special motor-generator set-up is used to test transformers at 50Hz and 60Hz similar to the exact operating frequency which will enable us to obtain accurate results.

In addition to all routine tests, special and type tests that can be carried out in-house include:

  • Temperature rise test
  • Lightning & switching impulse test
  • Induced over-voltage test with PD measurements
  • Measurement of zero sequence impedance
  • Noise levels

Modern data collecting systems and high-end testing instruments allow easy access to accurate results obtained from our test measurements.

Our transformers are subjected to all the necessary routine tests in accordance with the IEC, ensuring they withstand conditions beyond normal operating conditions. The extremes to which the transformers are tested are a measure of the highest standards in quality and reliability built into every transformer

Transport, Installation, Commissioning and Services

Powertech Transformers closely supervises the transport, installation and commissioning of its product to ensure customer satisfaction. Transformers are transported either oil or nitrogen filled. Once installed, transformers are filled under vacuum with highly purified transformer oil. We also offers a wide range of services including, analysis of transformer oil samples, specialist technical advice and tap changer maintenance as well as transformer repairs.

Quality Control

Vision

To be the number 1 supplier of transformers into our targeted markets.

Quality Policy Statement

Powertech Transformers (Power) is committed to serve its customers and meet their needs and expectations in the design, manufacture and supply of reliable and innovative technologies and products using modern management, engineering and material sciences.

Powertech Transformers (Power) is committed to continuing improvement of its products and services to achieve increased customer satisfaction as well as to ensure compliance with the requirements of the Quality Management System and its continuing improvement.
 

Our ultimate goal is to exceed our customers' expectations continually by delivering the highest quality services. Powertech Transformers (Power) complies with our quality policy by:

  • Consistent management focus on quality;
  • Continually improving the effectiveness of our Quality Management System;
  • Motivating our employees to take ownership of their work;
  • Understanding our customers goals and embracing them;
  • Providing on-going training to advance the skills of our greatest asset, our employees;
  • Identifying and solving problems to avoid compromising the quality of our services.

Powertech Transformers (Power) places a premium on excellence of service. We clearly appreciateand respect today's market forces that mandate delivery of quality services and products. We know that our survival as a company depends on providing the highest quality of service on every engagement.

Company Quality Objectives

  • To reduce the overall Test Failures
  • Reduce test failures of units> 132kV
  • Reduce test failures of units < 132kV
  • To improve the On Time Deliveries of transformer units
  • To reduce the materiaI related Cost of Poor Quality (COPQ)

Safety, Health & Environment 

Top management normally establish SHE objectives and targets for the company. The objectives are normally included in the SHE policy.

Objectives shall:

  • Be measureable;
  • Include commitments to prevent pollution;
  • Include commitments to prevent injury and ill health;
  • Include commitments to compliance with applicable legal requirements;
  • Include commitments to continual improvement;
  • Take into account the significant environmental aspects;
  • Take into account high safety and health risks;
  • Consider technological options;
  • Consider financial, operational and business requirements;
  • Consider the views of interested parties.