Transformer Tripping Problem

http://cr4.globalspec.com/thread/85265

Again lot more details are required like inter posing CT ratios, relay settings and its calculation, pre commissioning check results etc for us to really give our opinion. The best way forward would be to get a competent commissioning engineer to carry out primary injection test and stability test on the system to confirm healthiness of primary equipments and also secondary relay connections. Assuming that pre commissioning trails were carried out diligently and relay settings correctly programmed, the problem may probably be due to a defective main or inter posing CT.

The differential relays are very sensitive to fluctuations of current . The problem which you are referring happens when a transmission line trips. The three transformers are in parallel operation and one independent as you stated. The bus system is not clerar from your discription. The BHEL transformers one of the best available in Indian Make and do not give any problems as such. The line tripping are normally due to line fault or load shedding or grid failure due to various reasons . The tripping may be happening when line triping happens due to line fault which causes unbalance in the system which may the reason for tripping. You should also check up your control system for any loose contact in the CT wiring. Read the link below for a better understaning of operation of differential relays.
http://xnet.rrc.mb.ca/janaj/differential_protection.htm

The differential relay protection scheme if designed & installed properly should be immune to any through faults (i.e. faults outside its zone of protection) or grid disturbances. It is only sensitive and fast operating (deliberately so, as it is a unit protection) for faults inside its zone of protection.

The differential relays are designed for the protection of transformers and are supposed to be protected from external disturbances.But even the technical manulas of ABB the manufacturer states and I quote " The use of restraining windings in differential relay design permits more sensitive relay settings.This affords greater protection than is possible against plain over current relays whose trip settings must be high enough to prevent undesiered operation due to current transformer performance under heavvy fault current." The relays are normally designed to work perfectly under well defined system design, like proper relay cordination time settings and properly designed C Ts and Power panels . The protection works perfectly under such circumstances. In the present case the relays are not supposed to trip under the designed parameters as there is no fault occurs in transformer or down stream distribution as per the original thread. So the cause for the mal functioning and solution must be in relay cordination, faults in wirings such as loose connections or even proper design of Power panel and quality of the CTs or the relay itsef. The solution needs through check of the panel. I had experienced similar problems and solved.

Exactly. Biased differential protection as discussed above ensures better stabilty for through faults but high speed clearance for internal faults and that is the reason why it is preferred as primary unit protection for transformers. The tripping as indicated by OP should not happen with high speed biased differential protection. That is the reason I had advised that primary injection tests, stability tests, secondary injection tests etc needs to be carried out to trouble shoot the problem.