BUS VT Supplying to Generator Protection/Synchronizing panel - Secondary MCB Tripping 1

[Tahir28]

thread238-200371

Hi,

Referring to the old thread, please note that this is the situation we are also having in one of the old Generator Protection/Synchronization Panel. The Bus VT secondary 4A Z curve MCB supplying to the Generator panel trips instantaneously 2 or 3 out of five times and is stable the first three times the VT supply is switched ON.

@rcwilson had mentioned that the Y phase was grounded "to prevent circulating currents and blown fuses between the two systems"

Could anyone please elaborate this further and whether it is possible that we could be having a trip in our circuit because we changed the grounding to Neutral grounded instead of Y phase grounded which is causing an increase in the inrush/circulating currents causing the MCB to trip sometimes.

Secondly, is there a possibility that the residual magnetism in the Auxiliary transformer PTB increases the inrush currents or the heating in the MCB causes reduction of min. tripping current which leads to tripping after the first few operations, and if so what precautions should we take? Any references regarding this will be much appreciated. I have attached the drawings here for reference.

Thanks in advance.

 

[waross]

A change from phase grounding to neutral grounding should not make any difference unless there is a ground or partial ground in the circuit.
The tripping may be coincidental to the change in grounding.
Possibly a failing breaker.
Surface corrosion/erosion of the trip elements in the breaker. This will slightly decrease the resistance with a corresponding increase in the current and the I²R heating, or in the current in an instantaneous element.
One pole may be becoming "lazy" and closing slightly slower than the other poles.
Possibly a failing/failed suppression device.
Possibly a higher burden on the phase that was previously grounded and is now on the breaker.
The tripping current may be a combination of high inrush of the auxiliary transformer PTB and residual magnetism.
Primary voltage creeping upwards over the years.
A second ground or partial ground on one of the phases. (Check that the grounding on the wye:delta transformer is on the same phase.)
Circulating currents:
Any current in the delta must be supported by current in the wye primary.
The primary current will be of the same order of magnitude as the delta current divided by the transformer ratio.
Current in the delta is proportional to the unbalance of the primary voltages and inversely proportional to the meter burden.
The burden or impedance of the meter connected to the broken delta is important.
With a high impedance meter the circulating current will be negligible, with a low impedance meter element, circulating currents may be an issue.
First, check the ground connections.
Next check the suppression devices/snubbers.
Next, change out the breaker. Consider up-sizing the breaker. The next larger trip rating breaker will offset any voltage setting creep.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Tahir28]

Hi waross,

Thank you much for your response.

It was not a breaker condition issue as we had a trip on the other MCB too which was supplying to another similar panel.

As we had no tripping when we had disconnected the sync isolation PT (PTB), I hope circulating currents might not be an issue (the secondary did have low impedance 59BN relay ~2VA).

We once again took up with the VT factory to change the VT secondary MCB of 4A Z Curve to a 3A C curve (ETAP comparison curves were made) and after discussion, we performed the MCB replacement. The 3A C curve MCB (C60N-C3) did not have any spurious tripping on energization and the circuit is now stable.

Thank you!

 

[waross]

Thank you for sharing your solution with us.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter