Bouchloz Relay

• INTRODUCTION

Power Transformers are considered to  be  a  highly  reliable  type  of equipment, yet, in order to ensure the continuity  of  service  that  modern conditions  demand,  protective devices are required. The purpose of such devices is to disconnect faulty apparatus before large-scale damage is caused by a fault to the apparatus or  to  other  connected  apparatus. Such devices generally respond to a change  in  the  current  or  pressure arising from the faults and are used for  either  signaling  or  tripping  the circuits. 

Protective devices in the ideal case must be sensitive to all faults, simple in operation, robust for service and economically  feasible.  Considering liquid  immersed  transformers,  a near-ideal  'protective  device'  is available in the form of Gas and Oil relay  described  here.  The  relay operates on the well-known fact that almost every type of electric fault in a 'liquid  immersed  transformer'  gives rise to gas. This gas is collected in the body of the relay and is used in some way or other to cause the alarm or the tripping circuit to operate. 

The principle of the Gas and Oil relay was first successfully demonstrated and utilized by 'Buchholz' many years back.  In  a  series  of  experiments carried out extensively in Germany it was proved that the Relay is capable of  bringing  to  light  incipient  fault thereby preventing further spreading of the fault and extensive damage and thus saving expensive and protracted repairs. So successful is the principle of  this  Relay  that  despite  the continued search for better protective devices in other electrical fields the Gas-and-Oil Relay is still on its own in providing protection against a variety of faults.

• TYPES OF BOUCHLOZ RELAY

1.  Buchholz Relay with Mercury Switches
2.  Buchholz Relay with Magnetic Switches

• Bouchloz Relay with Mercury Switches

1. Uses mercury in switches which is toxic and also a carcinogen
   substance. Mercury is now prohibited in most parts of the world.
2. Relays with mercury switches are not accepted internationally by
   utilities and OEMs in most countries of the world.
3. Huge variation in gas volume and surge velocity readings from
   one relay to another.
4. Mercury  susceptible  to  oxidation's  resulting  in  no/false  signal
   upon prolonged use.
5. Switch activated by flow of mercury.
6. UN-branded locally made mercury switches prone to rejections in
   incoming, process and final inspection as well as transit.
7. May maloperate in one or more of the following conditions :a. External shocks to a transformer resulting in vibration.b.  Turbulence of oil due to starting of pump in forced cooled
   transformer.c.  Variation in angle of mounting of the Relay.d.  Earthquake of minor intensity.

• Buchholz Relay with Magnetic Switches

1. No use of mercury.
2. Consistent readings of gas volume and surge velocity.
3.  No affect of ageing
4. Switch activated by a magnet.
5. Branded  switches  imported  from  USA/Japan  are  free  from
   rejection in all stages.
6. Worldwide acceptance:
   a. Immune to such vibrations.
   b.  Highly stable and resistant and will not operate due to oil
   pump operation.
   c.  Immune  to  variations  of  angle  as  experienced  in
   transformer mounting.
   d.  Vibration proof to 6g accelerations.

• OPERATION

The  function  of  a  double  element relay will be described here. During normal operation of a transformer the Buchholz  relay  is  completely  filled with oil. Buoyancy and t he moment due t o counterweights keep the floats in their original top positions. In the event of some fault in the interior of the transformer tank, gas bubbles are produced  which  accumulate  in  the Buchholz  relay  on  the  way  to  the conservator. In consequence, the oil level  in  the  relay  enclosure  drops which  in  turn  lowers  the  upper bucket. 

This causes the magnetic switch to operate an alarm signal. 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. 

In case the liquid continues to drop due to loss of oil, the lower bucket also goes down. In consequence, the lower  switching  system  operates  if the level of oil goes below the bottom level  of  the  pipe  connected  to  the relay. 

Alternately in the event the liquid flow exceeds  a  specific  value  (which  is continuously adjustable, by means of a  flap)  the  lower  bucket  is  forced down,  thus  triggering  the  lower switching system to operate.

As the liquid flow rate decreases, or the level of the liquid rises, the bucket returns  to  its  original  position.  The single  element  relay  has  only  Trip element  and  it  responds  to  only  oil surges.  The  method  of  operation  is similar  to  that  described  for  double element relay. Single element relays are suitable for potential transformers and on load lap changers. 

The single element oil Surge relay has been  specifically  designed  for  use with on load tap change equipment and it will by-pass normal amounts of gas  which  are  generated  by  tap change  operations  and  will  only respond to oil surges and loss of oil. 

• APPLICATIONS

Double element relays can be used in detecting minor or major faults in a transformer.  The  alarm  element  will operate, after a specified volume of gas  has  collected  to  give  an  alarm indication.  Examples  of  incipient faults are :

1.  Broken-down core bolt insulation
2. Shorted lamination
3. Bad contacts
4. Overheating of part of winding's

The alarm element will also operate in the event of oil leakage, or if air gets into the oil system. 

The trip element will be operated by an  oil  surge  in  the  event  of  more serious faults such as :

1. Earth faults
2. Winding short circuits
3.  Puncture of bushings
4. Short circuit between phases

The trip element will also be operated if  a  rapid  loss  of  oil  occurs.  Single element relays can be used to detect either incipient or major faults in oil filled  potential  transformers, reactors,  capacitors  etc.  A  special single  element  relay  is  available  for the protection of on load tap-change equipment. 

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