Transformers: the heart of the electricity supply industry

When Googling the word ‘transformers’ the Hollywood blockbuster movie about incredible machinery coming to life and saving the world dominates the search. In reality, a transformer is the heart that pumps electricity at a constant supply, saving the world from darkness, writes ESI Africa editor, Nicolette Pombo-van Zyl.

Eskom’s planned load shedding, implemented to keep 80% of South Africa supplied with electricity during times of crisis, such as when there is an increased demand for electricity and a shortage of generation capacity, means constant switching of circuits, and this contributes to the breakdown of switchgear and transformer assets.

This scenario is not ideal, as in the electricity supply industry the performance of transformers and associated assets is a critical aspect in keeping the delivery of electricity at a constant, and is vital to the operation of industrial plants, commercial enterprises, and residential buildings.

However, whilst generation equipment is often blamed for load shedding and outages, unreliability of the distribution networks is often the cause.

Expected service life of transformers

Since the power transformer is one of the most important and expensive items of capital equipment in a distribution network, it is strategic and vitally important to maintain the assets correctly to keep them at optimum performance and extend their service life.

According to Chris Reay, CEO of Engineer Placements and Council Member of the South African Institution of Mechanical Engineering (SAIMechE), it is possible to find units that have been in service for 40 – 50 years, efficiently performing the imperative function of transforming voltages either up or down as required for distribution.

Jan Scholtz, Senior Engineer at Eskom Distribution, confirms that large power transformers (>1MVA) should last between 30 and 40 years. Smaller pole-mounted power transformers may last 15 to 20 years, and miniature substations on cable networks should have a lifespan of between 30 and 40 years.

With this in mind, field workers should note that the transformer will fail if operated above the designated temperature. It is thus essential that the appropriate ventilation is provided, particularly when operating in an enclosed environment.

Scholtz explains that one of the main reasons for large power transformer failure is due to damage as a result of heavy through-faults, bushing failures, tap changer failures and winding paper insulation failure due to old age. Whereas small pole-mounted transformers on rural lines are very susceptible to lightning and vandalism, in coastal areas tank rusting may be problematic if the tanks are not properly treated against rust.

Recommendations for transformer maintenance

The solutions that Scholtz recommends for large power transformers are to:

• regularly change the breather silica gel to prevent moisture ingress,

• perform regular oil analysis in order to early detect any unwanted gas accumulation, and

• repair oil leaks and rust as soon as possible.

For pole-mounted transformers, it is best to install surge arresters at each transformer’s primary bushings.

Unfortunately, there is not much that can be done for miniature substations where failure is due to repeated or sustained cable faults and vandalism. Efforts should, therefore, be around educating the public in terms of equipment vandalism and to fence off the transformers where possible.

The power of transformers

· Transformers are very energy efficient machines in terms of energy in and energy out, often operating at close to 99%.

· They are reliable when correctly maintained.

· To overcome the I2R power losses incurred over long transmission lines between the generation power stations to population regions, very high voltages of up to 740kV are used.

·  Without the use of transformers to increase the voltage, most of the energy from the power station would be lost as heat within the transmission lines. The only alternative would be to increase the transmission line diameter significantly, making them impractically heavy and too expensive to use.

· Transformers are used to reduce the high voltages used for transmission to lower voltages suitable for distribution to consumers.

· Substation switching operations may be required for a number of purposes, including:

· Taking a high voltage feeder out of service.

·  Performing maintenance or replacement of equipment such as transformers.

·  Isolation of other substation equipment including circuit breakers, busbars, capacitor banks, frequency injection equipment, and singular items of HV apparatus such as isolators, earth switches, cable potheads, surge arrestors, current transformers and voltage transformers.

·  Making temporary or permanent reconfiguration of the substation.

The most common substation switching activity is associated with feeders and transformers. The same principles are utilised for switching and isolation of other substation equipment.

The procedure for performing the substation switching operation will be provided by the System Operator (or equivalent) in the form of a Switching Instruction (also known as a ‘Disconnection and Reconnection Instruction’).

However, it is possible to be lulled into a false sense of security through the use of these instructions. Switching operators must have the knowledge and skill to recognise mistakes or omissions in the Switching Instruction, which can occur because of local substation variations and conditions which the System Operator, who is located elsewhere, may be unaware of.

Incorrect substation switching can cause catastrophic failure of equipment, extreme electrical arcing, and explosions, all of which have the potential to cause serious injury or death.

Thus, extensive training is required to provide the theory and methodology for working safely when performing switching operations within the substation environment.

As substations are unmanned installations, regular inspection of the units is required to ensure uninterrupted supply to customers. During the substation inspection, the condition of the equipment, buildings, surroundings, fencing, and gates is checked to confirm that they are in operational condition, safe and secure.

Data collected during routine inspections is fed back into the operational planning so that equipment can be repaired or replaced before it fails. It is therefore crucial that all data observed and done during the inspection is accurately recorded and that this information is provided to the appropriate person or input into an organisational database.

Scholtz advises utilities to perform regular dissolved gas analysis tests on the oil of a large power transformer, to keep the records and actively track the dissolved gas quantities over its service life. ”If you do not measure, you do not know! Always install appropriately rated surge arresters on pole-mounted transformers, even in low lightning areas; it will save you much trouble and cost in the long run,” concludes Scholtz.

The regularity of these inspections is crucial. Many transformers and substations have been neglected for years and only receive attention when a fault occurs.

Further to this, it is ideal to maintain all the components in the circuit to reduce probability of faults affecting the transformer, as the transformer will be subject to high fault levels if the switchgear cannot trip the system in time.

Utilities, industrial plants and commercial property owners are advised to call on the services of experienced consultants to perform audit functions to assess keeping distribution systems at high uptime levels. This should be a standard procedure on an annual basis especially where a high frequency of switching has occurred due to load shedding.

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Transformers 101

Apart from power transformers, there are instrument transformers used in the substation environment that are used to monitor the power levels, or are used for protection purposes to trigger circuit breakers and other current breaking devices.

There are also other inductive coil devices that resemble transformers called ‘reactors’. These are used to limit surge currents, and have maintenance regimes similar to transformers.

Transformers are designed with various types of cooling systems, such as:

ONAN – Oil Natural Air Natural

ONAF – Oil Natural Air Forced

OFAN – Oil Forced Air Natural

OFAF – Oil Forced Air Forced

The main reasons for transformer breakdown:

1. System overload

2. Harmonics

3. Current spikes and transients

4. Operating over-temperature

5. Surges through load switching (such as during load shedding)

6. Incorrect settings in the switchgear

The solutions to the above challenges can be achieved by giving attention to:

1. Circuit breaker settings on both HV and LV sides of the transformer

2. Voltage regulation on the transmission lines

3. Bucholz settings on the transformer

4. Proper earthing of the transformer

5. Ensuring good condition of cable terminals and bushings

6. Temperature control

Preventative solutions that can be put in place, include:

• Preventive maintenance according to a set programme

• Avoiding of regular load shedding

• Keeping the transformer energised when not on load

• Use of infra-red testing to determine overheating areas

• Systems to include auto reclose in case of intermittent faults elsewhere in the circuit.

This article was originally published in ESI Africa Issue 2 2015.

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