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Sizing Primary PT's & CT's

Sizing Primary PT's & CT's

Given a 2MVA 480V (delta)/12470V (wye-grounded) generator step-up transformer that will be interconnected to the utility. How to determine the size (rating & quantity) of PT's & CT's in the primary for 27, 59, 81O/U and 67N protective relaying? Is there any literature for this? Also, what is the best method of grounding (primary transformer neutral)for this set-up?

RE: Sizing Primary PT's & CT's

Book:  J.L. Blackburn, "Protective Relaying Principles and Applications"


RE: Sizing Primary PT's & CT's

Blackburn is a good start.

After that you need to determine if you are in the 5A secondary part of the world or in the 1A secondary part of the world.  480V and 12470V make it sound like you are in the 5A secondary part of the world.  2400:5 CTs on the 480V side would be about right, the C rating will depend on the burden and fault X/R.  The high side would probably do with 100:5 CTs, and again the C rating will depend on the burden and X/R.  You could see a very high X/R on the generator side; you will be well served by getting the best CTs you can for the generator side.

PTs - you could select PTs for 120V phase-to-neutral on the secondary or for 120V phase-to-phase on the secondary (or any other voltage you want, but those are the two most common).  In theory, some relays will directly accept the 480V, but that is not a good idea.  Applying the generator output 277V phase-to-neutral to a relay capable of 300V settings allows a maximum overvoltage setting of only 108.3% of nominal.

RE: Sizing Primary PT's & CT's

Thanks for the reply. I ordered J.L. Blackburn's book for my reference.
In the meantime, initial info shows that my primary PT should be 13200/120V or 14400/120V connected open delta for the 27, 59 and 81O/U functions. And for the 67N, it's either a 100/1A residual CT (ZCT) or a CT in series with the neutral. I still have to verify the accuracy, burden and specially the X/R issues on these.

RE: Sizing Primary PT's & CT's

Do I still need three (3) CT's from the lines for the 67N?

RE: Sizing Primary PT's & CT's

Having no idea what type of relay you are using, the following may not apply to your specific application.

A 67N needs one of two polarization sources: In or 3V0.  The element also needs 3I0 as an operate quantity.

The use of only 2 VTs will filter out V0, so you either need to get a third VT or you need to use In as your polarizing quantity.

The operate quantity, 3I0, can come from the residual connection of 3 phase CTs or it can come from the output of a single CT around all three phase conductors.

3V0 as your polarizing quantity can be calculated in a numeric relay if you provide the relay with 3-phase, grounded-wye/wye connected VT secondaries.  For a single function 67N, the 3V0 will have to come from VTs connected grounded-wye/broken delta.  These can be axillary 120-120V VTs connected to the secondary of grounded-wye/grounded-wye main VTs or they can be VTs with the primary connected to the line.  It is also possible to get VTs with two secondary windings so that you can connect one secondary grounded-wye and the other secondary broken-delta.

The instruction manual of the relay you intend to use will provide much additional useful information about the application of the relay.

If you are in the ANSI world, as your voltages suggest, I would advise sticking with 5A CT secondaries unless you have some specific application need to use 1A secondaries.

RE: Sizing Primary PT's & CT's

Thanks David.

After clarifying with the switchgear company, the SEL-351S relay will need three (3) PT's 12000/120V connected wye(grounded)-wye(grounded). The CT that will be connected in series with the neutral will have 5A secondary (100/5A ratio). These are preliminary ratings though and still subject for approval by the utility company.

RE: Sizing Primary PT's & CT's

Just to give our readers an update, the PT's are 7200/120V connected wye-wye and the neutral CT is 100/5A. Utility company has no comment on this.

RE: Sizing Primary PT's & CT's

Ratio is only part of the spec. Please reread David's comments above regarding the C rating.

RE: Sizing Primary PT's & CT's

Here's the latest on this project. The utility company blinks and wants the transformer configuration changed to delta-delta. Relay functions required for this are 27, 59, 81O/U and 59N. My question is since the sensing voltage transformers are in the high side, is it necessary to have the 59N? I think the 27/59 functions will already serve the purpose of the 59N. Am I right?
Also, since the PT's and CT mentioned above cannot be used anymore, can I use a 2-PT 14400/120V connected open delta or a 3-PT 14400/120V connected YG-YG for the 27/59 and 81O/U?

RE: Sizing Primary PT's & CT's

First off, 59N where?  The only possible location for a 59N in your revised system as described is on the generator neutral grounding resistor, more likely a function of the generator protection relay than the interconnection relay.

Why can't the previously mentioned CTs and VTs be used?  There is nothing in changing the GSU to delta-delta that would impact the choice of CTs or VTs, what else changed?

Any time you are using numeric relays, the use of of a 2-VT scheme is penny-wise and pound-foolish, don't do it.  In using 2-VTs you are throwing away perfectly good information, information that could prove to be invaluable in doing fault analysis.

RE: Sizing Primary PT's & CT's

The 27/59, 81O/U and 59N are the relay function being required by the utility on the point of common coupling w/c is on the primary side of the GSU. These are necessary to avoid islanding the generator. Of course, there are more relay functions on the generator side of the system.

The CT mentioned above is for the 67N w/c is no longer required since there's no more neutral on the transformer.
The 3-PT's (7.2kV/120V) can't be used since this is now an ungrounded system. It seems we have to use a 3-PT 14.4kV/120V connected YG-YG.

The 59N or 59G can be connected to a wye/broken delta PT's or on a zig-zag connected grounding transformer or on a single-phase PT connected line to ground (plus 27N). But, these are in addition to the PT's for the other relay functions.

Back to my question, is it necessary to have the 59N even if there are three phase 27 and 59 sensing already? A ground fault on a line will trigger the 27 of that line and/or the 59 of the other two lines. Is the utility asking for a more redundant protection? We already have protection on the GSU both on primary and secondary plus Transfer Trip relaying.      

RE: Sizing Primary PT's & CT's

If you have 3 VTs in wye-wye configuration, the relay can do phase-ground voltages as well as calculate a V0.  A separate VT is not necessary.  I'd missed the change from 12kV VTs to 7.2kV VTs; yes, you would want to go back to higher rated VTs on a system that could be left ungrounded.

In a 2-VT configuration, you would have no means of detecting the voltage shift caused by a ground fault on an ungrounded system.  3 VTs into a numeric relay is a far better solution than 2 VTs for phase voltages plus a third for a 59N type of connection; which would require a stabilizing resistor that isn't necessary when connecting three wye connected VTs to the relay.

What the utility may be looking for with the 59N is the combination on one phase low and two phases high, and that can be done in relay logic.

RE: Sizing Primary PT's & CT's

You're right David. The relay manufacturer confirmed that a 3-VT's connected YG-YG are enough for the 27/59, 81O/U & 59N functions that the utility company is requiring.

Thank you.

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