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:
- Check all associated relay panels and log protection flags
- Visually inspect the transformer
- 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.
- Low oil level alarm
- Gas build up and oil surge (Buchholz trip)
- Winding temperature (overheating)
- Oil temperature (overheating)
- Malfunction of an earthing compensator (differential/restricted earth fault)
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.
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.
The most important function of the Buchholz is to trip the transformer when:
- Internal fault causes a surge of gas or oil from inside the transformer, or
- 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.
The winding temperature indicator is to:
- Start auxiliary cooling fans and/or oil pumps
- Activate an over-temperature alarm, and
- 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:
- An overload of the transformer causing heat increase, or
- 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.
The temperature values as shown in Figure 3 are typical values and may vary on different transformers. Care should be taken when testing cooling fans and pumps to ensure that the control circuits are restored to normal operation after testing is complete.
This protection has two main purposes:
- To initiate an oil over-temperature alarm, and
- To initiate oil over-temperature tripping of the transformer circuit breakers.
The alarm and trip settings on this protection are set lower than the winding temperature gauge. This is due to the fact that the heat generated by the windings is dissipated through the cooling medium (oil) and so the alarm setting on the oil gauge (95° C) roughly corresponds to the alarm setting on the winding gauge (120° C) (see Figure 3).
Oil and winding protection can be used singularly or both together, they are used for the same purpose. One acts as a backup for the other, ensuring efficient protection of the transformer. Where a transformer is not fitted with pumps and fans, usually only an oil temperature alarm is fitted.
As mentioned previously, an alarm or trip on either oil or winding temperature protection, must be viewed seriously.
Where a transformer trips on winding or oil temperature the transformer should not be re-energized until approval is given by the relevant person in substation control center.
An earthing compensator transformer is used on power transformers with a delta secondary supplying feeders or a delta tertiary winding which supplies reactors or station supply transformers (See Figure 4 below).
The earthing compensator provides an earth reference on the delta winding, facilitating the detection of earth faults on connected apparatus.
Earthing compensator transformers are used:
- To allow a return path for earth currents in the event of faults on the circuits connected to the winding
- To reduce fault current level
- To allow measurement of earth currents for various types of transformer protection, and
- To limit the rise in voltage on the sound phases in the event of a fault.
The earthing compensator transformer is an integral part of the operation of the main transformer. If it becomes defective or for some reason malfunctions, the main transformer must also be taken out of service.
Transformers are designed for operation over a range of voltages which ensure that the core is not over excited. Over-excitation of the core, due to operation at higher than design voltages, causes magnetic saturation of the core leading to overheating and possible damage to the core.
Transformers should always be operated within the normal design range of voltages.
The magnetic flux in the core is proportional to the voltage applied to the winding divided by the impedance of the winding. The flux in the core increases with either increasing voltage or decreasing frequency.
During startup or shutdown of generator-connected transformers, or following a load rejection, the transformer may experience an excessive ratio of volts to hertz, that is, become overexcited.
When a transformer core is overexcited, the core is operating in a non-linear magnetic region, and creates harmonic components in the exciting current. A significant amount of current at the 5th harmonic is characteristic of overexcitation.
- Switching Operator’s Manual – Transmission Switching by Horizon Power
- Transformer Protection Principles – General Electric