Buchholz Relay

 Buchholz Relay

Buchholz relay is a gas-actuated relay installed in oil immersed transformers for protection against all kinds of faults. Named after its inventor, Buchholz, it is used to produce an alarm in case of incipient (i.e. slow-developing) faults in the transformer and to disconnect the transformer from the supply in the event of severe internal faults. It is usually installed in the pipe connecting the conservator to the main tank. It is a universal practice to use Buchholz relays on all such oil immersed transformers having ratings in excess of 750 kVA.

The Buchholz relay is a protective rely for equipment immersed in oil for insulating and cooling purpose. It is intended mainly for transformers or choke coils having a conservator vessel.

The relay responds to the accumulation of gas or air inside the apparatus when the oil level is too low or the flow of oil unusually strong. The relay then either gives a warning signal or disconnects the endangered equipment. The Buchholz relay operates even on very slight faults which are just in process of developing, so that greater damage may be prevented.

Protection Range : 

There is practically no operational fault in transformers or other oilimmersed apparatus to which the Buchholz relay does not respond. Its protective value applies, among others, to the following cases:

1- Flashover between live conductors
2- Flashover between conductor and iron core
3- Earth leakage
4- Insulation breakdown between turns
5- Interruption of a phase or lead connection
6- Excessive heating of the iron core
7- Leakage in the oil container or oil pipes

Construction : 

The Buchholz relay takes the form of a domed vessel placed in the connecting pipe between the main tank and the conservator. The device has two elements.

1- The upper element
The upper element consists of a mercury type switch attached to a float. The upper element of the relay closes the alarm circuit during incipient faults.

2- The lower element
The lower element contains a mercury switch mounted on a hinged type flap located in the direct path of the flow of oil from the transformer to the conservator. The lower element is arranged to trip the circuit breaker in case of severe internal faults.

Other Components : 

The cast-iron cover plate carries below it the whole assembly of the relay elements. It mounts on the top, the oil-tight terminal box with the cable entry, the ceramic insulated connection terminals and the test cock. Through, the cock, the gas collected in the relay can be released. In addition, air can be pumped through the test cock to see if the top float and the mercury switch are operating properly.

Gas Tester : 

By examining the decomposition gases collected in the relay, the gas tester can be provided by the manufacturer, providing the information as about the cause and type of the fault. It prevents the wastage of time for unnecessary dismantling, thus greatly facilitating fault detection.

The gas tester contains two silver nitrate solutions which, on the passage of decomposition gases, form two easily distinguishable precipitates. For instance, the precipitates formed by passing gasified oil through the solutions differs considerably from the one formed by gasified insulating materials, such as paper, cotton, etc. If the tested gas does not form a precipitate, this means that air has collected in the relay.

Mode of Operation : 

All operational faults occurring in an apparatus using oil as insulating medium, with the exception of those due to air lock and oil leakage, are the consequence of local overheating or arcing. This causes oil or other insulating medium adjacent to the fault to form gases. The rate of gas development is dependent upon the nature of the fault and the effect produced on the relay varies accordingly.

In the case of slight faults, the slowly developing gas bubbles accumulate in the upper part of the relay housing, which is constructed as a gas chamber. The Buchholz relay is built into the connecting pipe between the oil tank and the conservator vessel, so that rising gas and air bubbles are trapped by the relay.

Under the normal conditions, the relay is completely filled with oil, but with the accumulation of oil gases, this gas tends to replace the oil, the oil level falls and causes the relay float to tilt a mercury switch, which then either makes or breaks the contact.

In the case of a serious fault with violent gas formation, pressure waves are set up in the oil, forcing it to flow towards the conservator vessel. Should the speed of the oil exceed the adjusted sensitivity of the operating vane, which is coupled to a mercury switch, this switch is already tilted by the pressure wave and disconnects the transformer even before the gases reach the Buchholz relay itself.

The float and mercury switch-tube are connected to each other in such a way that the switch tube tilts with the float into tripping position. The setting range of the operating vane is from 50 to 150 cm/sec oil flow speed. It is usually set at 100 cm/sec response speed when delivered from the factory.

In the single float Buchholz relay, the operating vane is soldered to the float itself. The mercury switches connected to the floats can either operate an alarm or switch off the endangered apparatus altogether; the choice depends upon the particular installation. That is why, small transformers in unattended substations are best provided with the single-float relays, and the transformers simply disconnected when a fault occurs. A warning signal is only useful when a station is manned; then single float relay can also be used or alarm purpose.

The double float relay can be supplied for any type of transformer and with this pattern it is usually found advisable to use the upper float for an alarm. This float responds only when gas or air accumulates or when the oil level sinks. The lower mercury switch tube, on the other hand, is frequently used to disconnect the apparatus or transformer, since it responds to oil loss via the float and to pressure waves via the opening vane. It is however possible, by means of a locking device, to cancel the response of the lower float when the oil level sinks so that disconnection will not take place on account of this particular fault.

Finally, summing up the discussion to a very brief and precise ending, the operation of the Buchholz relay is given as under:

1- In case of incipient faults within the transformer, the heat due to fault causes the decomposition of some transformer oil in the main tank. The products of the decomposition contain more than 70% hydrogen gas. The hydrogen gas being light tries to go into the conservator and in the process gets entrapped in the upper part of the relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and closes the contacts of mercury switch attached to it. This completes the alarm
circuit to sound an alarm.

But it does not call for an immediate removal of the faulty transformer. The reason is that sometimes, the air bubbles in the oil circulation system of a healthy transformer may operate the float. For this reason, float is arranged to sound an alarm; not to trip the transformer. Therefore, on alarm, steps can be taken to verify the gas and its composition.

2- If a serious fault occurs in the transformer, an erroneous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer.

Advantages : 

1. It is the simplest form of transformer protection.
2. It detects the incipient faults at a stage much earlier than is possible with other forms of protection.

Disadvantages : 

1. It can only be used with oil immersed transformers equipped with conservator tanks.
2. The device can detect only faults below oil level in the transformer. Therefore, separate protection is needed for connecting cables.