High Impedance Bus Zone- Surge Arrester in zone

[Marmite]

I'm involved in a project which involves the replacement of an 11kV switchboard on an industrial 11kV underground network. The new switchboard has surge arresters which are within the zone of the high impedance busbar protection scheme. The client has asked whether the bus zone protection operation should be delayed by a few milliseconds to avoid maloperation if the surge arrester operates. The client has seen a SEL paper on the subject of bus bar protection which suggests that this should be done.
I've never come across this practice before. My view was always that surge arresters conduct for transients in the MHz range, and I can't see how they would be transformed by a CT as an operate current or picked up by a high impedance relay.
I'd be interested in your views, and whether the delaying of bus bar protection operation for this reason is practiced in your area.

https://www.google.com.au/url?sa=t&...02548/&usg=AFQjCNHyGsUrnPBPZK8RjedZKwVu2GYsGQ

Regards
Marmite

 

[stevenal]

We had a mis-operation with a 587 during a lightning storm once that we presume was caused by an in zone arrester. While a delay was discussed, the fix we adopted was to raise the pickup voltage. It has operated for years since without issue.
I find it interesting that we take instrument transformers rated for 50 or 60 Hz and attach them to power quality meters that capture high speed transients and high numbered harmonics. Apparently these transformers can supply the waveform.

 

[Mbrooke]

I know this might not contribute to the answer you are looking for... but why are the lightning arrestors on the bus and not the outgoing feeders? IMHO it would be worth moving them there if this is a retrofit.

 

[Marmite]

It's a switchboard that was originally procured for a different project that was canned a few years ago, which is being redeployed. To a large extent we are working with what we have been given. You are right though, it might be worthwhile looking at the practicalities of shifting the arresters to the feeders.
I'm just intrigued as to whether this is a real problem with modern arresters. Maybe for switching transients with a longer duration than lightning?

Regards
Marmite

 

[Mbrooke]

I have never seen a substation with bus LAs around me. I am not denying they exist, but from everything I know and have heard is that a bus LA is asking for inadvertent trips. Sure you can time delay the bus protection, but why do so when it will only increase the incident energy?

 

[cuky2000]

Hi Marmite,
If there is any consolation price, you are no along in this issue. There is a general believe in the industry that relay detection/trip and surge arrester operation are not related since they work in different time range. It is well known that surge arresters transient operate at high frequency in in the microsecond range vs. millisecond for relay detection.
With advance on fast fault detection logic of modern digital relays, there is a growing concerns that nuisance tripping of bus differential relaying is susceptible to misoperation due to mismatch caused probably by discharging through the bus lightning arrester(s), reflection, and refraction and/or skewed sampling of the bus differential relay.
We should take this opportunity to encourage the researchers and relay manufacturers to implement better fault detection algorism to eliminate this type of nuisance tripping instead of adding delays tripping time.

 

[krisys]

I find it interesting that we take instrument transformers rated for 50 or 60 Hz and attach them to power quality meters that capture high speed transients and high numbered harmonics. Apparently these transformers can supply the waveform.

The parameters measured by the power quality meters are still related with power frequency. The CT frequency bandwidth should be accommodating these frequencies upto about 50*f.

But the surge arrestor conduction is for the for the high frequency. for example if the surge voltage rise time is 50µs, the frequency is 1 / (50 x 10^-6)
= 20,000 Hz or 20 kHz

Moreover, the surge arrestor is expected to conduct for a cycle or two only. This conduction is only to discharge the trapped energy which is measured in Joules.

Hence in my opinion it is not correct to draw parallels between the harmonic currents and the surge current.

 

[Mbrooke]

@krisys X2. Harmonic and surge currents are very different in nature.

 

[davidbeach]

There are high-speed bus differential relays that make the internal/external decision in less than half a cycle. Arrestors in a transformer zone are less problematic as transformer differentials are inherently a bit slower. I'd try to leave arrestors associated with lines in the line zone rather than in the bus zone.

 

[stevenal]

Not exactly a bus arrester in our case, but it is within the protected zone. Consider a bus with several line breakers plus a circuit switcher feeding a transformer. The CTs used are bushing CTs of the line breakers plus the bushing CTs of the transformer. Primary arresters installed to protect the transformer lie within this zone.

 

[stevenal]

I mentioned harmonics second since it also shows an instrument transformer's ability to represent non-fundamental values. I mentioned high speed transients first. I'm presently looking at a recording of a transient that is off the ITIC chart with a duration of less than 1E-4s, measured with a VT that according to IEEE standard was designed and rated for 60 Hz. Perhaps the accuracy is not revenue grade at that frequency, but my point is that instrument transformers are transforming the high frequency stuff contrary to the OP's statement above.

 

[jghrist]

There will be a short duration 60 Hz current in the arrester until the arrester MOV resistance recovers after the lightning stroke.