Buchholz Relays for Transformers

Buchholz Relays





The construction of Buchholz relay below is generally as per Indian standards and conditions. However the basic principal and working remain the same. 

What is a Buchholz Relay?


The Buchholz Relay is one of the most essential protective devices to protect equipment's such as Transformer, Capacitors, On load Tap Changers etc. The Buchholz Relay detects abnormal working of the components inside the equipment, and consequently minimizes the spread of any damage to the equipment's active part by disconnecting faulty apparatus before large scale damage is caused by a fault to the apparatus or to other connected apparatus.


Construction of the Buchholz Relay


The relay comprises mainly of an oil-tight housing, which contains the alarm and tripping devices consisting of two buckets hinged on a frame, one in the upper part and one, fitted with a flap, in the lower part.
The flanges on the body permit an easy connection of the relay to the tubes and the wide inspection windows with graduated scale show the volume of gas accumulated inside the relay.

A drain screw is located on the lower part of the body to drain the oil.

The upper (alarm) & lower (trip) bucket is provided with a tilt activated switch. When the relay is filled with oil, the switch is not affected due to buoyancy of the oil & moment due to counterweights.
The terminations of the switches are connected to four terminals (two for alarm and two for trip). The terminals and the protective wire connections are housed in the terminal box.

Operation of the Relay


During normal operation of a transformer the Buchholz Relay is completely filled with oil. Buoyancy and the moment due to counterweights keep the buckets in their original top position.
The relay can help identify the following faults.

 Small failures : Such as loss of insulations between coils; which generate small gas bubbles due to burning of the insulation material. These gas bubbles travel towards the conservator. When the bubbles reach the relay, the gas accumulates (trapped) on the top part of the relay body. The gas (visible through the inspection windows) accumulation reduces the volume of the oil inside the body, as a result, the upper (alarm) bucket rotates on its hinge, and this causes the mercury switch to operate an alarm circuit.
The lower bucket does not change its position, because when the gas reaches the upper inside wall of the pipe it can escape into the conservator. Hence, minor fault in the transformer tank will not trigger the lower switching assembly and will not trip the transformer.

 Heavy failures : In the event of a serious fault in the transformer the development of gas bubbles is fast/violent and in uncontrolled way. The speed of the oil, which flows between the tank and the conservator, increases suddenly (oil surge).
In the relay this surge impinges on the flap fitted on the lower bucket (located in front of the central hole for the oil passage). The flap detects the increase of the speed over a prefixed value (which is adjustable by flap). In the event the liquid velocity exceeds a specific value the lower bucket is forced down which causes the mercury switch (fitted below the lower bucket) to operate the trip circuit, which trips the transformer.
As the liquid flow rate decreases, or the level of the liquid rises, the bucket returns to its original position.

 Oil leakages : Reduce the oil level inside the conservator until it is empty: without oil pressure, the oil inside the body of the relay flows out and causes the rotation of the buckets, setting in action the alarm contact first and the trip contact successively.

 Penetration of air inside the tank : Generally happens due to bad working of the forced oil circulation pumps. It has the same effect of the small failures: small bubbles of air, which accumulates on top part of the relay, set the alarm contact in action.

Testing Before Commissioning


A test system is provided in the Buchholz Relay that allows the functional test of the upper and lower switching system as follows.

Testing of Alarm Circuit :
A small Gas Release Cock, provided on the top of the relay, can be used for, collection of accumulated gas for further analysis and checking the operation of the alarm circuit by means of an air as follows. d Screw the test pump/air bottle to the test Cock.
d Open the Cock & pump air gently into the Buchholz Relay until the upper switching system (alarm) operates.

Testing of Trip Circuit :
A test valve for checking the operation of the tripping circuit by means of air under pressure is provided at the top of the relay. To test the operation of the trip element.
d Open the valve controlling the test pump/air bottle quickly so that air or inert gas rushes in, impinges on the flap and, operates the lower switching (trip) system.


Mounting Of The Relay


The Relay is mounted on the connecting pipe between the transformer and the conservator. While installing the Relay into pipeline kindly take care of the following points.
  1. The pipe should be arranged in such a position so that it allows easy flow of gas (arising from faults in the transformer) to the relay.
  2. The pipe should be connected at the highest point on the transformer cover and should not extend into the transformer.
  3. The pipe should not contain any right-angle elbows.
  4. The Pipe diameter should correspond to the diameter of the hole for the passage of oil of the Relay.
  5. The pipe must be arranged to slope upwards towards the conservator at an angle between 1 to 9 degrees to the horizontal. (If nothing is specified while ordering, relay is supplied at 5 degree pipe angle). The part of the pipe preceding the Relay should be straight for the length equal to at least 5 pipe diameters; the part of the pipe leading to the conservator immediately adjacent to the relay should be straight for a length equal to at least 3 pipe diameters.
  6. Mount the relay as close as possible to the tank in the pipeline between transformer and conservator.
  7. For the successful operation of the relay the axes of the two flanges of relay should remain horizontal. Machined surface on the relay makes this aligning possible with the help of spirit level.
  8. The air valve at the top of the relay should be below the bottom of the conservator.
  9. See that the relay is positioned with the arrow pointing towards the conservator.
  10. If the transformer is provided with an explosion vent or similar attachment, it must be sealed in such a way that any gas liberated by the transformer does not accumulate in the vent; otherwise the operation of the alarm bucket will be delayed. 11 Ensure that the slot on the Test Key remains in the T/L (Transit / Lock) position during storage or loose transportation of the relay.

Commissioning of the relay

Once the relay has been mounted properly, kindly follow below mentioned procedure to put the relay into service.
1 If there is a service lock on the relay unlock it.
2 Unscrew gas release cock's protective cap. Open the gas release cock to allow the relay to fill up with oil fully. (The filling up of oil in the relay and the position of the buckets are seen through the inspection windows.)
3 When the relay is fully filled with oil & when extra oil emerges out of the gas release cock, close the gas release cock & Screw the protective cap.
4 Connect the Alarm & Trip wires as mentioned in “wiring”.
5 Check liquid level in the conservator.


Sizing of Buchholz Relay

1.  In India there are three pipe diameters used for connecting the transformer to the conservator ie
  1. 25 mm
  2. 50 mm
  3. 75 mm
Depending on the size of the pipe dia used the buchholz relay is generally ordered.

Maintenance of Relays

Relay should be tested at every one yearly interval for the following
  1. Alarm and trip contacts should be working correctly.
  2. There should be no gas accumulation in the relays. If there is gas then chemical analysis should be done to find the nature of the fault.
  3. If the gas is clear air, then transformer should be inspected for any leakages. 

References

1. Indian Standard for Buchholz Relays - IS 3637.

Manan Shah
Partner 
Press-N-Forge
email : admin@shree-ji.com

For over 25 years, Press-n-Forge Company has been a trusted manufacturer of Buchholz Relays, specializing in high-quality electrical equipment protection. With a strong commitment to reliability and safety, Press-n-Forge has established itself as a leading provider in the industry. Their expertise and experience ensure that their Buchholz Relays adhere to international standards while meeting the specific needs of customers worldwide. Count on Press-n-Forge for top-notch Buchholz Relays that safeguard your electrical equipment with precision and efficiency.



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