95i Inrush Blocking Function - Symap Protection Relay

[Drivesrock]

Hi all.

A Dynamic Postioning drilling rig I'm on has 5 x 5.1 MVA generators per 11kV bus (2) and bus-tie open. Mostly, only two generators run in parallel at a time per bus. This is minumum requirement. The generators have the Symap BCG Protection relays and 95 i Inrush Blocking Relay function is ON. This is only used in the Differential Protection (87) setup.
Last year, customer noticed that in most of the generator relays of both busses, the event logs are being flooded by "2302. 95_i Inrush [date and time] A"- Active and then about 20ms later i - inactive. And repeats regularly every few seconds. Sometimes a few minutes gap appears in the log and then the same cycle for hours and hours while the generator is online. This means the event logs fill up very quickly and effectively have only a few days' worth of events and anything important before that is gone.

The rig is over 10 years in operations but it is not known when this Inrush events logging started. And, the 12-pulse VFD thruster transformers and the service transformers are not being switched all the time. These are the loads I would expect Inrush / 2nd harmonic - at energization. No new major loads have been added.

The Symap manual says the inrush blocking relay pickup is a % of 2nd Harmonic content in the current.
The setting here is 5%. And it has been since the original relay commissioning. It looks like from the service manual that the default value is 20%. What would be a good value?

The changes I have found so far are; about 3 years ago the relays were firmware upgraded and about 18 months ago the AVRs changed from analogue to digital ones. Maybe originally, the bus-tie was closed and three or more generators were always operating in parallel. Still digging.

I'm going to connect a Fluke PQM in the next day or so, according to operations, to register what the harmonics look like for real although the Symap relays have a reading of about 4.2% THD V and I on one of the meter pages.

Any comments or thoughts from anyone?

 

[wcaseyharman]

I’m not familiar with that relay, but two thoughts come to mind.
- what is the 2nd harmonic level the relay is seeing when the harmonic blocking triggers?
- Perhaps you could raise the harmonic blocking value up in steps until it doesn’t trigger, it would surprise me if your system
Had too much more than 5% second harmonic.
- I’ll have to look but typically harmonic blocking is used for transformer differentials, I don’t recall it being needed for generator differential protection as generators don’t have the same issues with high harmonic content during inrush - generally the exciter slowly brings the machine up to voltage. What I’m saying is you might check that you may not need harmonic blocking at all for that application.

 

[Drivesrock]

Feedback

Thanks for your comments wcaseyharman

The Fluke PQM read only some maximum 0.5% 2nd harmonic but the relay records from 5% to 8% when it fills the event log. 5% being the threshold setting to start the 95i blocking function - which is used only in the 87 Differential protection settings. Interestingly, the constant registering is within a generator current range of about 30 to 50 Amps. Overall THDi and THDv were around 3%.

Further checks and this only happens on the newer relays. The manufacturer informs that the newer relays are more sensitive and the 95i 2nd harmonic threshold setting should be raised up to 10 or 15%.

Now for the protection study to be rerun. And we'll have to see in practice, I think, if when closing a transformer breaker the new threshold setting will not be too high such that the generator 87 protection activates.

 

[wcaseyharman]

I wonder if the relay is calculating harmonic current in percent of load current, and when load current is really low the % harmonics is quite high. If so maybe there’s a way to supervise the blocking with current magnitude.

Just a thought.