buchholz electrical relay of a transformer Operation and precautions

[smartpagelinks groupid=’7′ style=’boxlink’ linkcolor=’#0066bf’ linkhover=’#dd0000′ linkback=’#ffffff’]

Introduction to Buchholz relay

The buchholz electrical relay is a protective device intimately connected with the structure of the transformers to detect the non-electrical effects of fault conditions. This relay is applicable to oil immersed with conservator type transformer in which the transformer tank is completely filled with oil and a pipe connects the transformer tank to an auxiliary tank or “conservator” which acts as an expansion chamber. It is developed in 1921 and it is the first device which operates on gas parameters for all the faults including electrical. In simple it does not differentiate between the current, voltage and power to detect the fault in the transformer. As specified above it is a gas operated relay.

Definition of buchholz relay

Whenever a fault in transformer develops slowly, heat is produced locally, which begins to decompose solid or liquid insulated materials and thus to produce inflammable gas and oil flow. This phenomenon has been used to trigger the gas protection relay or popularly known as buchholz electrical relay.

buchholz relay protection Faults

  • Earth Fault
  • Winding Short Circuit
  • Phase to Phase Fault

Construction of buchholz relay

It is placed in between the transformer tank and conservative tank pipe line. The excess oil from transformer tank flows from transformer tank to conservative tank and when it is pressurized the oil force will be high. It consists of two floats one is ‘upper float’ and the second one is ‘lower float’. The upper float operates the alarm switch and the lower float operates to provide trip signal to circuit breaker. These are two are connected to a switch which is either mercury switch or magnetic switch (now a days).

Operation of buchholz relay

Figure shows a buchholz relay connected into the pipes leading into the conservator tank and arranges to detect gas produced in the transformer tank.

buchholz electrical relay operation

buchholz electrical relay operation

For Slow developing faults

As the gas circulates for a minor fault the oil level fails in the transformer tank which will increase the gas pressure above the oil in the relay compartment. This high gas pressure is trapped in the upper portion and it pressurizes the upper float and it operates mercury switch sounding an alarm. This is a slow process for slowly developing faults.

For severe fault in transformer

When a more serious fault occurs within the transformer during which intense heating takes place, an intense liberation of gases results.  These gases rush towards the conservator and create a rise in pressure in the transformer tank due to which the oil is forced through the connecting pipe to the conservator. The oil flow develops a force on the lower float shown in the figure and over tips it causing it contacts to complete the trip circuit of the transformer breaker. Operation of the upper float indicates an incipient fault and that of the lower float a serious fault.

Faults and preventive methods of buchholz electrical relay

The Buchholz relay may become operative not only during faults within the transformer.  For instance when oil is added to a transformer, air may get in together with oil, accumulate under the relay cover and thus cause a false operation of the gas relay.  For this reason when the ‘Gas’ alarm signal is energized the operators must take a sample of the gas from the relay, for which purpose a special clock is provided.  Gases due to faults always have color and an odor and are inflammable.

     The lower float may also falsely operate if the oil velocity in the connection pipe though not due to internal faults, is sufficient to tip over the float.  This can occur in the event of an external short circuit when over currents flowing through the windings over-heat copper and the oil and cause the oil to expand.  If mal-operation of buchholz electrical relay due to overloads or external short circuits is experienced it may be necessary that the lower float is adjusted for operation for still higher velocities.

Precautions need to be taken in buchholz installation

  • The conductor connection contacts to the terminals on the cover must have paper insulation as rubber insulation may be damaged by the oil.
  • The floats must be tested for air tightness by for example, submerging them in hot oil to create a surplus pressure in them.
  • The relay cover and the connection pipe should have slope of 1.5 to 3 percent and not have any protruding surface to ensure unrestricted of the gases into the conservator.

Advantages of buchholz relay

  • It is the simplest form of transformer protection.
  • It detects the slow developing faults much earlier compared to all other protections of transformer.
  • It operated on gas pressure as that there will not be any harmonics affect on this relay.

Disadvantages of buchholz

  • It is only suitable for oil immersed transformers as it operates on the oil and gas pressure.
  • The device can detect the faults only based on oil levels so precautions should be taken against fault protection.
  • The faults which will not impact the oil pressure should be protected by other protection devices.