power transformer tripping 1

[Emadshaaban1987]

Dear Folks,
Power transformer 20kv/6.6kv 5MVA ,transformer outgoing tripped by (27),l1=6.2 l2=5.5kv l3=4.3kv ,it seems that the voltage dipping is in one phase only L3
the transformer is protected with many protection units (87T,50,51,64) but non has popped up except 27
-no new load was engaged
-transform tests including IR ohmic and ratio tests was done successfully
-its a real under voltage because downstream feeder has detected this voltage dipping .
the transformer loads are 2MW loading pump, street lighting (feed through 6.6/0.4kv transformer )
any tips

 

[Palletjack]

You have any delay on your 27 relay bit?
If you have any type of fault the voltage will dip. With no delay the 27 will trip the XF

 

[dpc]

Not sure why you need to trip off the transformer on undervoltage. I've never seen that done, except maybe for generator unit transformer that get de-energized along with the generator. Remote faults can cause voltage drop that may operate the UV protection.

 

[cranky108]

Likely the 27 is a substitute for volts/hertz.

Is it possible to have a winding to winding fault? What do the oil tests show?
Or it could be saturation.

 

[dpc]

Is low volts/hertz a problem on a transformer? I've seen overexcitation alarms and trips on transformers but never undervoltage.

 

[cranky108]

Under voltage is usually with some long time delay for this application. May also be part of protection for loss of a phase.

 

[davidbeach]

Any event reports? At 6.6kV the voltages should be 3.8kV to ground and that's what should be measured under most conditions. If the 6.6kV system is ungrounded or impedance grounded it could make sense to look at phase-phase voltages, but those wouldn't normally be shown as just L1, L2, L3. If it is connected to phase-phase voltages we really need a one-line.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[thermionic1]

How about checking the VT's, secondary connections and any fuses. A weak old (possibly wet) fuse can drop quite a bit of voltage.

 

[stevenal]

Winding configuration, such as delta or wye? LTC present?

 

[bacon4life]

The voltage on L2 is 88% of voltage on L1, so the issue (or issues) may not affect only L3. Depending on what your nominal voltage is supposed to be, 6.2 kV on L1 might even be suspect.

Does the trip correspond to starting up the pumping load?

 

[waross]

I suspect a line to ground short on one phase pulled the voltage down.
If the origin of the fault was avian the fault has probably cleared.
A couple of fuses were blown on the three phase metering transformer.
The planr engineer looked at a dead bird on the ground and then up at the transformer.
He then turned to the utility foreman who was directing the crew replacing the fuses.
"Well look at the burn marks.
It flashed on the incoming side.
At least we didn't have to pay to kill the damned bird."


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Emadshaaban1987,
Are you there? Your participation helps in understanding / finding a quick and appropriate solution to the problem. Hope you agree.
The voltages you indicated suggest that the 6.6kV system is ungrounded. Please confirm.
27 relay and tripping the incomer is practiced when you have a second incomer and an autochangeover scheme. Otherwise, 27 should not be wired for trip of incomer (you said transformer trip - 27 should definitely not be wired to transformer primary circuit breaker).
Only transformer protections should be wired to trip the primary breaker and 27 is not a transformer protection.