Buchholz Relay is a gas actuated relay which is installed in between the pipe connecting conservator to the main tank of transformer. Buchholz relay is used for providing protection to transformer against internal faults. The name of the inventor of this gas actuated relay is Buchholz, This is the reason why these relays are called Buchholz Relay.
Buchholz relay basically has two functions for the protection of transformer. First function is to energizes the alarm circuit when any minor or incipient fault occur in transformer tank. The incipient faults are those faults which slowly developed in transformer tank. Second function of buchholz relay is to energize the trip coil and isolate transformer from supply when any serious fault occur in transformer.
The buchholz relay is used in oil immersed transformers having ratings 750kVA and above. The buchholz relay is not economical for transformer having rating below 750kVA.
The buchholz relay consists of oil tight container which is installed between main tank and conservator tank. The oil tight container contains two mercury switch out of which one mercury switch is connected with alarm circuit and other is connected with trip coil circuit. On the upper part of container, hollow float is installed. On the lower part of oil tight container, Flap valve is installed which is directly in the path of the oil between transformer main tank and conservator tank.
One mercury switch is attached with the upper float and closes the alarm circuit when minor fault occur in transformer. Another mercury switch is attached with the lower flap valve and closes the trip coil circuit and open the breaker contacts, when any serious fault occur in transformer. The container of buchholz rely has test cock which is used to take the sample of gas accumulated in the upper portion of relay container.
Under normal condition, The buchholz relay is completely filled with oil. When minor internal fault occur in the transformer, The heat is produced in transformer. Due to this heat, The oil in transformer tank decomposes and the gas in generated in tank. The 70% part of gas is hydrogen and thus it is lighter in weight. The gas is lighter in weight and goes to upward towards conservator tank through pipe.
The buchholz relay is installed in the pipe between main tank and conservator tank and thus gas goes through the buchholz relay. Because of its lighter weight, Gas is trapped in the upper part of buchholz relay container. Due to this, the oil level falls which further tilts the upper float downward. The mercury switch is attached with upper float and it also moves with upper float and thus it closes the alarm circuit. The alarm gives the indication about the some fault developing inside transformer.
When any serious internal fault occur in transformer, Large amount of gas is produced in transformer main tank. Due to this large amount of gas, Very high pressure is developed inside main tank and the oil inside main tank rushes to the conservator tank. The lower flap valve is in directly path of oil between main tank and conservator tank. Due to the force of heavy rushing oil, The lower flap valve tilts which closes the contacts of mercury switch attached to it. The mercury switch closes the trip coil circuit and initiate the circuit breaker operation to isolate the transformer from supply.
The Buchholz relay has following advantages:
- The buchholz relay has very simple construction. It is a simplest protection as compared to others transformer protections.
- Buchholz relay provides indication of fault at very early stage thus transformer can taken for service before any serious fault occur.
- The buchholz relay has a test cock which is used to test the sample of gas. Thus in most of the cases the fault is detected without opening the transformer.
The buchholz relay has following disadvantages:
- Buchholz relay protection is used for only oil immersed transformers.
- The operating time of buchholz relay is high.
- Buchholz relay only operates when the oil level falls.
- The minium operating time of buchholz relay is 0.1 seconds and average time is 0.2 seconds.
- Sometimes the buchholz relay operate due to bubbles, vibration or mechanical shocks if the mercury switches are kept to sensitive.
The buchholz relay provides protection against internal faults which occur due to many reasons. The buchholz relay provides protection against following faults:
- Buchholz relay is used to give alarm indication when overheating occur in transformer.
- It is used to protect transformer in case of short circuited laminations.
- It is used to provide protection against short circuit between phases.
- Buchholz relay provide protection in case of insulation failure of core bolt.
- It is used to provide protection against short circuited winding fault.
- Buchholz relay is used to protect transformer in case of winding earth faults.