L-N PT's on a LRG system

[rockman7892]

Is it permissible to have L-N PT's on a MV (12.47kV) LRG or HRG system? I'm looking at a project where there are several (1) 7.2kV:120V PT's that are suggested for replacement in converting system from solidly grounded to LRG. I'm assuming reason is that it is not permitted to have any L-N connection on an LRG system.

Assuming the same would apply for any station service transformers connected L-N on primary?

I'm assuming these single phase connections are L-N and not L-G? The drawings I've seen represent a ground symbol on the other side of PT primary but I take this to be neutral since current has to flow back to neutral of transformer and is not intended to flow on ground system?

There also appear to be some PT's that are a (3) PT configuration shown as 7.2kV:120V. I'm assuming this would be these PT's in a wye configuration? I don't think there is an issue with this wye connection on an LRG system however do these PT's not need to be rated for full L-L voltage instead of L-N voltage?

 

[davidbeach]

The VTs (PTs) can (should) be connected phase-ground, but must now be rated for 12.47kV rather than 7.2kV. If you want to keep the 60:1 ratio, 14400:240 becomes a good choice. It may also be possible that the existing VTs are rated for a continuous 15kV overvoltage condition and would be suitable.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

Line to neutral loads are not allowed unless the neutral is solidly grounded.
The PTs are or should be part of the protection system and are allowed.
A station service transformer is a load and is not allowed to be connected line to neutral on a system that is not solidly grounded.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

PTs are connected Line to ground normally, irrespective of type of system grounding.
PTs have 'Voltage factor' specified in addition to the nominal voltages.
This is in IEC and specifies what overvoltage the PT can withstand on its primary and for hat duration.
Typically, the voltage factor specified for PTs in HRG system is 1.2 continuous and 1.9 for 8 hours. The same is 1.9 for 30s in case of LRG systems.

 

[scottf]

It depends a bit on the standard being used.

As a general answer, 1-bushing VTs can only be used on a solidly grounded WYE system.

2-bushing VTs with L-N ratio (e.g. 7200:120V (60:1)) can be used on non-rigidly grounded WYE systems in the ANSI/IEEE world. The overvoltage factor and H2 bushing insulation can handle a short-time rise in neutral voltage during fault conditions.

 

[Kiribanda]

Is it permissible to have L-N PT's on a MV (12.47kV) LRG or HRG system?

1) HRG Systems: NO. Single phase apperatus (single bushing apperatus) should NOT be connected because on HRG systems,
the COG (EFF) is greater than 80%. Therefore, during a ground fault those apperatus are subjected to overvoltages.
Hence arresters should be specified with the correct MCOV. VTs should have at least 1.9 Voltage factor.
2) LRG Systems: YES provided the COG (EFF) is not greater than 80%. In other words if the system is effectively grounded
then single phase (Single bushing) apperatus are allowed.

 

[rockman7892]

Thanks for the responses.

Would these L-G connection limitations stated above apply to PT's connected in a primary wye or open delta configuration, or only single phase PT's that were connected L-G on one phase?

With an open delta I guess PT's would already be rated for full L-L voltage so probably just answered my own question above. For wye configuration can there still be an elevated L-L voltage across PT during system L-G fault?

Any idea why an existing system would only have one L-G PT on one phase and not the others. Especially on a gen breaker feed that feeds into a paralleling bus? Woulnd't all (3) phases be needed for sync-check?

 

[xnuke]

Yes, there will be L-L voltage across a grounded wye or single phase VT during a L-G fault in a resistance-grounded system, so any VT needs to be rated for L-L voltage. You did answer your own question regarding open delta.

It is common for a sync check to be performed only using a single phase.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[scottf]

Hi Waross-

I think you might have your post above reversed.

 

[waross]

Fixed it Scott.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[rockman7892]

I finally was able to find some existing information on the existing surge arrestors on this system which are rated for 8.2kv MVOV.

With this system now changing to an LRG system would these arrestors need to change to 15kV MVOC?

From what I read above the PT’s should all be connected in wye configuration (w 3 pts) however each PT must be rated for full L-L voltage?

Are insulation coordination studies ever recommended for application of PT’s and Arrestors on resistance grounded systems? If not when are these studies typically useful?

 

[Kiribanda]

If you have found that the existing SA are haing MCOV=8.7kV, then it is
reasonable to assume that your existing 12.47kV netwrok is effectively grounded.
But both solid grounded & LRG systems are effectively groundded if the COG is less than or
equal to 80%. Therefore, unless you do a SC study on your LRG network and
estimate the COG at the arrester location, it is not advisable to change the MCOV to 15kV.
Because the protection margin (PM) offered by the arresters will drop down due to high LPL of
15kV surge arresters.
Also,you can model the system correctly on any EMT program (PSCAD or EMTP etc)
and by putting the SA charteristics, you can estimate the voltage appearing in front of the
transfomer for lightning impulses 10-100kA. It will enable one to calculate the PM.
Insulation Coordination study is that.

 

[rockman7892]

Thanks Kiribanda

I was able to get a better look at the yard and had a couple of additional questions to help my understanding here:

1) The yard had several 7.2kV 25kVA CPT's that have (2) bushings and are connected L-G. Does the fact that the CPT has (2) bushings indicate that the CPT has insulation rated for full 13.8kV? Why would a (2) bushing CPT be used opposed to a single bushing when connected L-G? Because we are not changing system to an LRG system the CPT L-G connection can no longer be used and must be changed to a 13.8kV CPT connected L-L?

2) Likewise there are several 7.2kV single PT's that have (2) bushings and are connected L-G. Does this mean that PT has fully rated insultation for 13.8kV and does not need to be replaced with changing system from solid grounded to LRG? (Assuming 2 bushing is full insulation rating).

3) Do (3) PT's connected in a wye configuration need to have full 13.8kV rating for PT's when applied on an LRG system? I believe so since a ground fault on one phase will elevate neutral point of wye and thus have full L-L voltage on other PT's in wye? Existing PT's connected in wye on system appear to be (2) bushing PT's with 2nd bushing at wye point connected to ground. I'm thinking that is an indication that these PT's are fully rated and adequate in the wye arrangement?

 

[steve_h]

I modeled my system using the PSCAD program and did everything according to the recommendations with the LRG system

 

[Kiribanda]

Sorry, I donot know whether I understood all your questions 100%.
All what I have to say is,
1) Since your system is solidly grounded, the single bushing VTs are to be used connected between line & ground because their
primary voltage is rated for Line/ 1.732. Voltage factor = 1.1/ cont is sufficient.
2) A double bushing VT is used whenever you need line to line voltage for your application. In that case the primary voltage
is rated for full Line to Line voltage. Voltage factor = 1.1/ cont is sufficient.
Both may have the same voltage ratio but with two different secondary voltages.

If the system is HRG then we usually use double bushing VTs connected line & line.
If the system is HRG, then if a single bushing VT is used then either it should be
having rated primary voltage equal to Line to Line with a voltage factor 1.2/ cont
or having rated primary voltage equal to Line/1.732 but with a voltage factor 1.9.

 

[waross]

If you are using a single phase PT for sync check than that may be an exception and may possibly connected line to neutral.
For phase check you generaĺy want to see the phase to neutral phase angle, rather than the phase to ground phase angle, if the angles are not equal.
It depends.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[rockman7892]

So if existing PT's are (2) bushing PT's and connected L-G with either a single PT, or in a wye configuration does this mean that they are potentially rated for the full 12.47kV? If so is this an accepted practice? When connected L-G would the secondary voltage under normal conditions be 69V which then increased to 120V during L-G fault on LRG system, however PT's would be ok to withstand this elevated voltage?

Attached is a photo of PT's I am referencing which are marked with a 7200V rating.

 

[Kiribanda]

Please upload the photos of the name plates of the subject VTs.

 

[rockman7892]

I'll try uploading again here, it didn't work on the other post for some reason.

 

[Kiribanda]

Photos of the name plates of the VTs or data sheet are needed.