Cap Bank & PTs - Ferroresonance?

[amurray]

We recently had an operation that caused 2 primary (2.4kV L-N) PT fuses to blow & are thinking it could be caused by Ferroresonace, but wanted to see if anyone has had similar experience.

A relay mis-operation (no fault) caused several CB to trip that left 2 cap banks (1200kVAR each) connected to a bus with no other source or load. Before the operation caps were online & bus was fed by utility & feeding load. I have attached a sketch to help illustrate the setup. Circuit breakers # 1 & #4 tripped.

Upon trying to re-energize the bus it was discovered that 2 primary PT fuses (1E) were blown (PT#2 on sketch). We know the PT were in-service prior to the mis-operation. Another set of PTs on adjacent bus did not blow fuse, but its fuses are 2E. Unfortunately we have no digital relaying for event records.

Anyone had similar occurrences?

Thanks
Andy

 

[DTR2011]

what relays (protective functions, mfg, type) trip cb 1 & 4? Perhaps it was a fault and relays on 1&4 tripped and didnt target and cb #2 failed to trip. Please provide a bit more information.

 

[amurray]

The trips were initiated by a mistake during relay testing of a transformer 50/51 relay that subsequently operated a LOR & tripping relay.

Thanks
Andy

 

[magoo2]

It's not ferroresonance. You had the capacitors energized and then the 2 breakers tripped left them isolated but with a charge on them. When you re-energized the source, the trapped charge resulted in the higher current which caused the fuses to blow.

You might want to check or replace the fuses that didn't blow. They could be damaged from partial melting.

 

[amurray]

Not sure I see that. During the outage the Caps would have been connected to this otherwise de-energized (isolated) bus with these PT on it. What would make closing either a utility source or load bus CB suddenly make the Cap discharge? The Cap CB (#3 in sketch) was never opened)

Do you think that either closing in the utility source or load bus gave it the sufficient path to discharge? Grounded PT not good enough path?

Thanks
Andy

 

[ssantoshr]

Main reason for ferroresonance during normal switching operation is due to unloading conditions.
The ferroresonance phenomenon could be possible when the PTs are connected WYE grounded on an ungrounded or high resistance grounded systems.
I have read some cases where ferroresonance produced high voltages in three phase power systems had immediately followed the blowing of a fuse, non-simultaneous opening and closing of switch contacts or breaking of the line conductors.
Also be sure to check the condition of the lightning arrestors (if installed).
Did you perform any single phase primary switching at the transformer ??

Regards
Santosh

 

[rasevskii]

It could be that the PTs were damaged by the DC discharge current from the capacitors that occurred when the AC bus voltage disappeared. When the AC bus voltage was restored, the PT fuses blew due the damaged PTs (shorted turns).

I have heard of this phenomena on PTs connected to long HV lines that were switched off, and the remaining DC component discharged itself via the PT primary winding (low DC ohmic resistance).

The solution was to add a resistance winding in series with the copper winding on the PT primary to limit the DC current to a safe value. This was on SF6 GIS switchgear at the 220KV level. I did not see this first hand but heard about this from another supplier.

Others may have better information.

rasevskii

 

[slavag]

Hi.
From my poin of view is not good situation, why capacitor bank was not disconnected from bus in the no voltage situation ( see again post of rasevskii ),27 function will be added.

From other hand possible feroresonance situation too.