8.4kV PT primary operated ungrounded on 13.8kV system 1

[pwrengrds]

What is your take on the rating provided. The system is connected to a 13.8kV ungrounded system, the PT's are PTG5-1-110-842F, see attached spec sheet. The PT's are connected phase to ground, the max system voltage rating is 15.5kV.

Is the PT with the primary voltage rated at 8400 volts okay to operate at 13800 volts if ground fault occurs?
Does the fault need to be cleared without delay?
Should the PT not be installed on an ungrounded system?

David

 

[Kiribanda]

You have mentioned that your system is UNGROUNDED. Therefore, you cannot use single bushing VTs with
rated voltage 14400V/(1.732)=8400V. Instead you have to install dual terminal VTs with rated
voltage =14,400V, Ratio=120, Catalogue # PTG5-2-110-1442FF with twin fuses. Also you have to make sure
that those VTS are having its rated voltage factor=1.9 for 8Hrs. (No info about voltage factor
in the catalogue given)

 

[Kiribanda]

Sorry, pl. disregard my previous post. It should read as follows.
You have mentioned that your system is UNGROUNDED. Therefore, you cannot use single bushing VTs with
rated voltage 14400V/(1.732)=8400V. Instead you have to install single bushing terminal VTs with rated
voltage =14,400V, Ratio=120, Catalogue # PTG5-1-110-1442F with a single fuse. But most importantly,
you have to make sure that those VTS are having its rated voltage factor=1.9 for 8Hrs so that they
are suitable for an UNGROUNDED system.

 

[davidbeach]

You do need the dual bushing type. But that little note about Maximum System Voltage of 15.5kV means that these can be used on that system without being over-voltaged.

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

 

[pwrengrds]

This is what was installed in 2005 and operating currently. The site currently has a reoccurring ground fault (monthly??) that goes away without being able to find the ground fault. The site does trip off on ground fault (generator and utility). I'm wondering if the PT's can withstand full voltage without failing. (apparently one set did fail sometime earlier, not sure when).

The system is a main power transformer with a 13.8kV delta secondary, a 13.8kV generator with a high resistance ground, and industrial loads connected with delta transformers, no grounding TX's. The site has switchgear with 3 drawers (3 PT's each set) of the PT's mentioned above installed.

When the brochure says maximum system voltage of 15.5kV and a primary voltage of 8.4kV I get mixed signals as to what's allowed.
and then there is this note
..what is the rated value?...is it the 8.4kV?

(a) Two fuse transformers should not be used for Y connections. It is
preferred practice to connect one lead from each voltage transformer
directly to the neutral terminal, using a fuse in the line side of the primary
only. By using this connection a transformer can never be made live from
the line side by reason of a blown fuse in the neutral side. For continuous
operation the transformer primary voltage should not exceed 110% of
rated value.

 

[davidbeach]

Been down that road before. It's a 15,500:221.43V VT. But that rating is just weird, so they call it a 8400:120V VT.

Two bushings, but perhaps just 1 fuse and 1 slug. Maybe two bushings is a bit of overkill and maybe you could get away with a single bushing. But with two bushings there's no worry about mounting and wye-point voltages being radically different.

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

 

[Kiribanda]

Per CSA & ANSI 14490V (15kV) is the maximum system voltage (Um) for which the VT is designed. During operation service voltage cannot go above 15kV
per ANSI/ NEMA. For such 15kV systems the rated primary accuracy voltage of the VTs is 14400V. So you have to use 14400V VTs with ratio 120:1.
Since your system is almost ungrounded, to avoid failures during ground faults
1) The VTs installed should have voltage factor=1.9/ 8Hrs
2) The surge arresters installed should have MCOV=15kV

 

[scottf]

The standard listing the maximum system voltage is misleading for VTs and really has nothing to do with the rating of the VT.

Per IEEE C57.13, 15kV class is 15kV nom system voltage and 15.5kV max system voltage. However, for a VT, the ratings that matter are the rated voltage and the over-voltage factor.

You mention you have a 14400V ungrounded wye system and the VTs are connected line-to-ground...but I'm assuming you really mean line-to-neutral.

The correct VT to use would be one with a primary rating of 8400/14400V Y, which is a 2-bushing unit with a line-to-neutral ratio. The OVF can be spec'd based on your system protection.

The GE PTG-5 is their OEM version of the same style JVM-5C. However, the JVM-5C has the spec rating you need as cat#765x121028. This is a unit rated 8400/14400V with 2 bushings with 2 fuses. As noted above, when applying this unit line-to-neutral, you would remove the fuse from the H2 side and connect directly to the H2 terminal.

Without being too self-promotional, Ritz has an option for this as well as type VZU15-01, cat# 121542001.395103FF. Same note about removing the fuse on the H2 side. Or you could be the un-fused version and mount the fuse on the H1 side external to the VT.

https://ritzusa.com/wp-content/uploads/2020/11/VEU-and-VZU.pdf