25kV VT Ground alarm

[Sn00ze]

Hello everyone,

I was going through some old projects and I noticed that in some of them, a VT Ground alarm is used for a 3-ph single winding PT (120V) used for metering. I remember the project engineer said that they don't normally use a Gnd Alarm for 25kV VT's unless there are special considerations.

I have another project now where the project engineer is saying to go ahead and get a Gnd Alarm for a very similar project. That is, a 25kV bus VT

Does anyone have any standard or rule I can refer to? or personal experience as to this preference? What special conditions should i consider in order to justify a Gnd alarm?

I appreciate any insight. thanks!

 

[7anoter4]

In my opinion, you may find an answer in the following article:

http://sedatacenters.com/files/3012/4723/1794/LineToGroundVoltageMonitoring_08262005.pdf

 

[Sn00ze]

Thank you 7anoter4, i'll have some good reading material over the weekend

 

[GroovyGuy]

Hi Snooze,
Is the 25kV system solidly grounded? Can you provide a [very] simple 1-line diagram?
I have used [once] a type 64 relay before to detect grounds on a 25kV solidly grounded incoming Utility feeder. The PT configuration was grounded-wye / broken delta, with the 64 relay across the broken-delta secondary. This relay was used to detect ground-faults on the incoming Utility supply and trip the incoming circuit-breaker. The Customer in this case has a whole bunch of grid converted 4-quadrant inverters and routinely would export power back to the Utility. The Utility was concerned wrt the Customer backfeeding into a ground on the Utility side. Note: the Customer's step-down transformers were delta-wye.


The pessimist sees difficulty in every opportunity. The optimist sees the opportunity in every difficulty.
Winston Churchill

 

[Sn00ze]

Hello GroovyGuy,

Here's a rough single line sketch. VT taps off of the 25kV Bus directly.

and yeah it is solidly grounded.

 

[GroovyGuy]

Hey Snooze,
You have a 25kV solidly grounded system, and you wish to install a 'ground alarm' relay.

I trust that you also have some form of (current-based) ground detection on the system as well. Either residually connected CTs and a 50N/51N relay, or zero sequence CTs and a 50G/51G relay. Of course, I have assumed that your system uses 25kV CBs and not HV fuses for O/C protection, but perhaps this is not the case? I understand that you work in the Utility (substation) side of the business, so I suspect that you will be using SEL multi-function relays for feeder protection (on CBs), n'est pas?

In my limited experience, in industrial power systems, a MV system (ie 25kV) either has a ground-fault, or it does not. There is no middle ground, at least on (shortish) feeders. Of course longer feeders, transformers and motors can be more of a challenge. I take it this is why you are considering a ground-alarm relay on the system.
GG


The pessimist sees difficulty in every opportunity. The optimist sees the opportunity in every difficulty.
Winston Churchill