6 alarms coming from a substation transformer you MUST take very seriously

Transformer alarms

Transformers are the most expensive piece of apparatus in a power substation and therefore must have appropriate protection equipment installed to guard against various faults. This technical article explains a few internal transformer faults that make an alarm in a substation. For substation crew it’s always important to understand what is going on inside a transformer and why.

In order to react properly in faulty situations it’s important for operator to understand how internal protection mechanisms of a transformer work. For example, switching operators must not energize any substation transformer that has tripped off on fault.

When confronted with this situation the substation operators must:

1. Check all associated relay panels and log protection flags
2. Visually inspect the transformer
3. Contact in-charge person from control center and inform of exact details of the fault.

Together with the usual type of protection relays (i.e. overcurrent, earth fault) used elsewhere on the system, transformers have additional protection.

These include alarms and trips that guard against:

1. Low oil level alarm
2. Gas build up and oil surge (Buchholz trip)
3. Winding temperature (overheating)
4. Oil temperature (overheating)
5. Malfunction of an earthing compensator (differential/restricted earth fault)
6. Over-excitation

Important to note!

Unless approval is given by the relevant in-charge person from a substation control center, a switching operator must never attempt to put a substation transformer which has tripped on protection back into service, because of the risk that energizing the transformer could do further damage.

1. Low Oil Level Alarm

Some substation transformers have an inbuilt alarm on the conservator tank level gauge (see Figure 1). If the oil level falls below a preset level an alarm will be triggered. These alarms are usually relayed to the control center.

Two types of oil level indicators are shown on Figure 1 below.

Type 2 is simply an oil level gauge. If no low oil alarm is available on the conservator, a Buchholz gas alarm will also indicate if the oil level is low. This is due to the float arrangement inside the transformer Buchholz relay activating either for a build-up of gas from inside the transformer, or from the oil level reduction in the relay housing.

If flagging indicates a low oil level, investigations must be carried out to find if the problem is sufficient to warrant de-energization of the transformer until the oil level can be increased. If the flags indicate ‘Buchholz gas alarm’ the transformer must be taken out of service as soon as possible and investigations made to find if the cause of the alarm was minimum oil level or gas build up from inside the tank.

Gas in the Buchholz relay should always be removed for analysis.

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2. Buchholz Trip

The most important function of the Buchholz is to trip the transformer when:

1. Internal fault causes a surge of gas or oil from inside the transformer, or
2. The main oil level of the transformer drops below the Buchholz relay.

These points are important because the quicker these faults can be removed, the least amount of damage will result inside the transformer.

For any Buchholz trip alarm, the appropriate technical staff must be brought in to take samples of gas or oil via the Buchholz bleeder valves for chemical analysis. This analysis gives positive indication on the exact type of fault and its extent. Decisions can then be made about the transformers future.

Note that some transformers have an additional Buchholz pressure-activated relay fitted for the tap changer tank.

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3. Winding Temperature

The winding temperature indicator is to:

1. Start auxiliary cooling fans and/or oil pumps
2. Activate an over-temperature alarm, and
3. Initiate a trip of the transformer circuit breakers if the temperature continues to rise

For example, a 20/27MVA transformer will run at 20MVA without any cooling equipment, but at 27MVA with all cooling equipment running.

If a winding temperature alarm is activated it is normally due to either:

1. An overload of the transformer causing heat increase, or
2. Malfunction of cooling equipment causing a heat increase in the transformer.

Inspection of the transformer and its loading will dictate what action needs to be taken. The winding temperature circuit is connected so that extra cooling facilities (fans/pumps) are activated before the alarm/trip function.

See Figure 3 above.