MICOM P923 RELAY 2

[sudiptoghosh]

Alsthom
Micom P923 RELAY in our generator relay panel has tripped on undervoltage as seen from fault records.The relay was set to operate at 80 % voltage with a definite time lag of two seconds.Though it is showing that the relay has tripped on undervoltage in fault records , but when we see the magnitude of the voltage during the fault it is showing around 95 % of the rated voltage (11 kv).So my question is why the relay has tripped on undervoltage when the magnitude of voltage was only 95 % of the rated?.OR is it that the magnitude of volatge during tripping is not being registered correctly during the fault.

 

[dpc]

we see the magnitude of the voltage during the fault it is showing around 95 % of the rated voltage

This voltage magnitude comes from an event report of the relay that tripped or some other measurement?

If this is a new installation, I would verify the PT ratios assumed by the relay against the actual PT ratio. I assume the settings actually in the relay (not on a piece of paper) have been verified?

Also, verify the voltage being seen at the relay. Any chance of a blown PT fuse?

 

[pwrtran]

Does the relay protection element uses the fundamental component but the relay metering element uses total RMS values?

 

[sudiptoghosh]

DPC,
Thanks for the reply:
The voltage magnitude comes from the same relay and not from other measurement devices
Pretty old installation and the chances of blown fuse is absent

 

[ZapSib]

Hello,
did you have a look at the disturbance record?

 

[dpc]

Well, if the voltage magnitude in the trip event report is above the supposed trip set point, either the settings are not what you think they are, or the relay malfunctioned.

 

[DTR2011]

Compare the micom relay voltage to another device, such as a meter.

 

[pwrtran]

As I said before one measures the total RMS the other measures fundamental. In P923 it is called the True RMS that is up to 10th harmonic. That may explain your positive undervoltage was asserted but your phase voltage was measured above the pickup.

See the page from the relay attached.