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Home / Technical Articles / 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.

5 alarms coming from a substation transformer you MUST pay attention to
5 alarms coming from a substation transformer you MUST pay attention to

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.
From the above information and following consultation with the relevant person from the substation control center, a decision can be made whether or not to re-energise the transformer. Further investigations may be required.

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.

The Type 1 oil level indicator shows the expected oil level at different oil temperatures. The conservator oil level is correct when the conservator gauge temperature is the same as the temperature reading shown on the transformer oil temperature gauge.

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.

Substation transformer oil level gauge
Figure 1 – Substation transformer oil level gauge

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 build up is an indication that some form of overheating or arcing has occurred in the transformer windings due to a fault or maybe due to air trapped in the relay as a result of inadequate bleeding from a previous inspection.

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.

Detailed investigation must be carried out to find the exact cause of a Buchholz trip. The transformer must not be put back into service until the exact corrective measures have been taken.

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.

Buchholz relay
Buchholz relay

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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
Switching operators should be aware that regular inspection of a transformer should be carried out to make sure all auxiliary cooling fans or pumps are in working order. If these items become defective the transformer cannot be run to its full capacity.

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

Transformer temperature control
Figure 3 – Transformer temperature control

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.
Operation of the winding temperature alarm must be treated seriously and immediate attention given to rectifying the problem.

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.

Important Notes!

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.

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4. Oil Temperature

This protection has two main purposes:

  1. To initiate an oil over-temperature alarm, and
  2. 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.

The response to a transformer winding or oil temperature alarm is to check the transformer load and confirm all cooling is functional. Relocating load may be considered as a solution or a decision to temporarily shut the transformer down may be made. This would allow time for the transformer to cool and for further tests if necessary.

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.

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5. Malfunction of an Earthing Compensator

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
Figure 4 – Earthing compensator

Earthing compensator transformers are used:

  1. To allow a return path for earth currents in the event of faults on the circuits connected to the winding
  2. To reduce fault current level
  3. To allow measurement of earth currents for various types of transformer protection, and
  4. 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.

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6. Over-Excitation of Transformers

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.

Normally, transformer is designed to operate at or below a maximum magnetic flux density in the transformer core. Above this design limit the eddy currents in the core and nearby conductive components cause overheating which within a very short time may cause severe damage.

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.

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  1. Switching Operator’s Manual – Transmission Switching by Horizon Power
  2. Transformer Protection Principles – General Electric

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Milione Changala
    Aug 05, 2022

    Dear Sir Edvard

    Your post are very useful and helping us to improve our knowledge. Nevertheless, could you give some information about non oil transformer.. with air insulation.

    Kind Regards.

  2. p nagaraju
    Jun 05, 2021

    What Is BAU transformer fault

  3. Hendarman Suhendro
    Apr 08, 2020

    Very helpful for engineers, thank You for the clear and simple explanation

    Apr 07, 2020

    Very useful…good job

  5. Kelvin Vincent Banda
    Mar 20, 2020

    Very Brief and quite informative, Keep it up.

  6. Oziegbe Eronmene
    Mar 15, 2020

    Nice one engr i real enjoyed your article on power transformer protection. My question is what is the reason /causes of low oil level in a power transformer when the transformer is not have oil leakage issues

  7. Victor I lima
    Sep 19, 2019

    As I only looked for couple minutes i saw lot of things as can be updated as to more aficionados as to what we have today special the turbines as how the fans getting to hot as a new design will helped with the flowing and safety of well being as well more productive as well long lasting and you may consider as to use nitrogen as a cool substitute as to run much more oficion and productive as well lot more lasting and safety features as to run hot and melting water entire I understand how nitrogen works as 100 below zero as I have used it many years as racing aplication with turbos and natural aspirated as to have its balance is no harm and it’s used to cool off the charger if I was your ungenerous I would think really about it as will make it do much productive and will safe in the long run so much money and produce much more as costs as well.

  8. Gys Niesing
    Sep 12, 2019

    Very usefull paper. One question; is it possible in terms of the alarming functions to get a digital or analog output?

  9. Veeramanikandan.s
    Sep 08, 2019

    Thanks this message will useful for engineers

  10. Ankit Dubey
    Aug 09, 2019

    It will very helpful for everone

  11. Olanike Amurawaiye
    Aug 01, 2019

    Very educative and easy to understand thanks

  12. Charles
    Jul 24, 2019

    This is substation operations made easy. The transformer must always be protected from damage. Thanks for sharing

  13. Satyashus
    Jul 23, 2019

    Thanks for sharing very important informations.

  14. Oke , Timothy Obinna.
    Jul 23, 2019

    Your article is more educative than what we learnt in school. May God bless you sir.

  15. R.Nikam
    Jul 23, 2019

    very useful & important information for substation operator.if you communicate information ,then that is easy to prevent accident.

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