Buchholz Relay - Construction, Working

What is a Buchholz relay?

Buchholz relay is a safety device which is generally used in large oil immersed transformers (rated more than 500 kVA). It is a type of oil and gas actuated protection relay. It is used for the protection of a transformer from the faults occurring inside the transformer, such as impulse breakdown of the insulating oil, insulation failure of turns etc.

Working principle of buchholz relay

Whenever a fault occurs inside the transformer, such as insulation failure of turns, breakdown of core or excess core heating, the fault is accompanied by production of excess heat. This excess heat decomposes the transformer insulating oil which results in production of gas. The generation of gases depend on intensity the of fault. Gas bubbles tend to flow in upward direction towards conservator and hence they are collected in the buchholz relay which is placed on the pipe connecting the transformer tank and conservator.


Buchholz relay consists of an oil filled chamber. There are two hinged floats, one at the top and other at the bottom in the chamber. Each float is accompanied by a mercury switch. The mercury switch on the upper float is connected to an alarm circuit and that on the lower float is connected to an external trip breaker. The construction of a  buchholz relay is shown in the figure.

How does a Buchholz Relay work?

Whenever a minor fault occurs inside the transformer, heat is produced by the fault currents. The produced heat causes decomposition of transformer oil and gas bubbles are produced. These gas bubbles flow in upward direction and get collected in the buchholz relay. The collected gas displaces the oil in buchholz relay and the displacement is equivalent to the volume of gas collected. The displacement of oil causes the upper float to close the upper mercury switch which is connected to an alarm circuit. Hence, when minor fault occurs, the connected alarm gets activated. The collected amount of gas indicates the severity of the fault occurred. During minor faults the production of gas is not enough to move the lower float. Hence, during minor faults, the lower float is unaffected.
During major faults, like phase to earth short circuit, the heat generated is high and a large amount of gas is produced. This large amount of gas will similarly flow upwards, but its motion is high enough to tilt the lower float in the buccholz relay. In this case, the lower float will cause the lower mercury switch which will trip the transformer from the supply, i.e. transformer is isolated from the supply.

Advantages of Buchholz Relay

  • Buchholz relay indicates the internal faults due to heating and it helps in avoiding the major faults.
  • Severity of the fault can be determined without even dismantling the transformer.
  • If a major fault occurs, the transformer can be isolated with the help of buchholz relay to prevent accidents.


Parallel Operation of Transformers

Sometimes, it becomes necessary to connect more than one transformers in parallel, for example, for supplying excess load of the rating of existing transformer. If two or more transformers are connected to a same supply on the primary side and to a same load on the secondary side, then it is called as parallel operation of transformers.

Necessity of Parallel Operation of Transformers

Why parallel operation of transformers is needed?
  • Increased Load: When load is increased and it exceeds the capacity of existing transformer, another transformer may be connected in parallel with the existing transformer to supply the increased load.
  • Non-availability of large transformer: If a large transformer is not available which can meet the total requirement of load, two or more small transformers can be connected in parallel to increase the capacity.
  • Increased reliability: If multiple transformers are running in parallel, and a fault occurs in one transformer, then the other parallel transformers still continue to serve the load. And the faulty transformer can be taken out for the maintenance.
  • Transportation is easier for small transformers: If installation site is located far away, then transportation of smaller units is easier and may be economical.
parallel operation of single phase transformers

Conditions for Parallel Operation

When two or more transformers are to be operated in parallel, then certain conditions have to be met for proper operation. These conditions are - 
  • Voltage ratio of all connected transformers must be same.
    If the voltage ratio is not same, then the secondaries will not show equal voltage even if the primaries are connected to same busbar. This results in a circulating current in secondaries, and hence there will be reflected circulating current on the primary side also. In this case, considerable amount of current is drawn by the transformers even without load.
  • The per unit (pu) impedance of each transformer on its own base must be same.
    Sometimes, transformers of different ratings may be required to operate in parallel. For, proper load sharing, voltage drop across each machine must be same. That is, larger transformer has to draw equivalent large current. That is why per unit impedance of the connected transformers must be same.
  • The polarity of all connected transformers must be same in order to avoid circulating currents in transformers. Polarity of a transformer means the instantaneous direction of induced emf in secondary. If polarity is opposite to each other, huge circulating current flows.
  • The phase sequence must be identical of all parallel transformers.
    This condition is relevant to poly-phase transformers only. If the phase sequences are not same, then transformers can not be connected in parallel.
  • The short-circuit impedances should be approximately equal (as it is very difficult to achieve identical impedances practically).


Cooling Methods of a Transformer

No transformer is truly an 'ideal transformer' and hence each will incur some losses, most of which get converted into heat. If this heat is not dissipated properly, the excess temperature in transformer may cause serious problems like insulation failure. It is obvious that transformer needs a cooling system. Transformers can be divided in two types as (i) dry type transformers and (ii) oil immersed transformers. Different cooling methods of transformers are - 
  • For dry type transformers
    1. Air Natural (AN)
    2. Air Blast
  • For oil immersed tranformers
    1. Oil Natural Air Natural (ONAN)
    2. Oil Natural Air Forced (ONAF)
    3. Oil Forced Air Forced (OFAF)
    4. Oil Forced Water Forced  (OFWF)

Cooling methods for Dry type Transformers

Air Natural or Self air cooled transformer

This method of transformer cooling is generally used in small transformers (upto 3 MVA). In this method the transformer is allowed to cool by natural air flow surrounding it.

Air Blast

For transformers rated more than 3 MVA, cooling by natural air method is inadequate. In this method, air is forced on the core and windings with the help of fans or blowers. The air supply must be filtered to prevent the accumulation of dust particles in ventilation ducts. This method can be used for transformers upto 15 MVA.

Cooling methods for Oil Immersed Transformers

Oil Natural Air Natural (ONAN)

Cooling of transformer - Oil Natural Air Natural - ONAN
 This method is used for oil immersed transformers. In this method, the heat generated in the core and winding is transferred to the oil. According to the principle of convection, the heated oil flows in the upward direction and then in the radiator. The vacant place is filled up by cooled oil from the radiator. The heat from the oil will dissipate in the atmosphere due to the natural air flow around the transformer. In this way, the oil in transformer keeps circulating due to natural convection and dissipating heat in atmosphere due to natural conduction. This method can be used for transformers upto about 30 MVA.

Oil Natural Air Forced (ONAF)

Cooling of transformer - Oil Natural Air Forced - ONAF
 The heat dissipation can be improved further by applying forced air on the dissipating surface. Forced air provides faster heat dissipation than natural air flow. In this method, fans are mounted near the radiator and may be provided with an automatic starting arrangement, which turns on when temperature increases beyond certain value. This transformer cooling method is generally used for large transformers upto about 60 MVA.

Oil Forced Air Forced (OFAF)

In this method, oil is circulated with the help of a pump. The oil circulation is forced through the heat exchangers. Then compressed air is forced to flow on the heat exchanger with the help of fans. The heat exchangers may be mounted separately from the transformer tank and connected through pipes at top and bottom as shown in the figure. This type of cooling is provided for higher rating transformers at substations or power stations.
Cooling of transformer - Oil Forced Air Forced - OFAF

Oil Forced Water Forced (OFWF)

This method is similar to OFAF method, but here forced water flow is used to dissipate hear from the heat exchangers. The oil is forced to flow through the heat exchanger with the help of a pump, where the heat is dissipated in the water which is also forced to flow. The heated water is taken away to cool in separate coolers. This type of cooling is used in very large transformers having rating of several hundreds MVA.
Cooling of Transformer - Oil Forced Water Forced - OFWF


Instrument Transformers - CT and PT

Instrument Transformers

How will you measure AC currents and voltages of very high magnitude? You will need the measuring instruments having higher range, which literally mean huge instruments. Or there's another way, using the transformation property of AC currents and voltages. You can transform the voltage or current down with a transformer whose turns ratio is accurately known, then measuring the stepped down magnitude with a normal range instrument. The original magnitude can be determined by just multiplying the result with the transformation ratio. Such specially constructed transformers with accurate turns ratio are called as Instrument transformers. These instruments transformers are of two types - (i) Current Transformers (CT) and (ii) Potential Transformers (PT).

Current Transformers (CT)

Current transformers are generally used to measure currents of high magnitude. These transformers step down the current to be measured, so that it can be measured with a normal range ammeter. A Current transformer has only one or very few number of primary turns. The primary winding may be just a conductor or a bus bar placed in a hollow core (as shown in the figure). The secondary winding has large number turns accurately wound for a specific turns ratio. Thus the current transformer steps up (increases) the voltage while stepping down (lowering) the current.
 Now, the secondary current is measured with the help of an AC ammeter. The turns ratio of a transformer is NP / NS = IS / IP

Current instrument transformer CT
One of the common application of a current transformer is in a 'Digital Clamp Meter'.
Generally, current transformers are expressed in their primary to secondary current ratio. A 100:5 CT would mean the secondary current of 5 amperes when primary current is 100 amperes. The secondary current rating is generally 5 amperes or 1 ampere, which is compatible with standard measuring instruments.

Potential Transformer (PT)

Potential transformers are also known as voltage transformers and they are basically step down transformers with extremely accurate turns ratio. Potential transformers step down the voltage of high magnitude to a lower voltage which can be measured with standard measuring instrument. These transformers have large number of primary turns and smaller number of secondary turns.
A potential transformer is typically expressed in primary to secondary voltage ratio. For example, a 600:120 PT would mean the voltage across secondary is 120 volts when primary voltage is 600 volts.

potential instrument transformer PT