Primary VT fuse failure and the consequences

[Blake1981]

thread238-120636

Dear all,

I am new to this site as I recently discovered it when searching for some information on the subject. After reading a very helpful post and link I have a few more questions on the subject. I have recently returned to electrical engineering after a few years in project management so sorry if im a little rusty.

I work on an oil refinery and a few weeks back we had many drives trip out on thermal overload due to high voltage on the 11kV supply. After some investigation we found that a primary VT fuse had blown. The voltage reference for the tap changer on the supply transformer is connected to phase A and C of this VT.

From the Basler paper referenced in the previous post, the voltage across the A and C phase secondary should be 1/route3 that of the primary phase to neutral voltage multiplied by the turns ratio of course. This is turn gives a low voltage reference to the tap changer which then tapped up and cause the over voltage.

My questions are these.

1) Hows in the 1/route3 calculated? (table 4 in the paper). I can calculate the zero, neg and pos voltages but i dont understand how you calculate line volts on the secondary with one phase fuse blown on the primary?

2)The tapchangers are very old but they still have a relay that is designed to drop out when no voltage is seen from the VT. In this case the transformer stays on tap because it has detected a no voltage rather than a low voltage signal. However, with 1 fuse blown i am guessing that there is enough voltage on the seconday to hold the relay in and the tapchanger has seen a low voltage signal and has tapped up to compesate. Has anybody seen this before?

If anyone gets back to me it will be much appreciated.

Thanks

Blake

 

[alehman]

Do you know how the VT's are connected? wye-wye, open-delta, etc?


Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Blake1981]

Alan,

Thanks for your quick response! From the schematic its conneced wye - wye but with no neutral connections.

Each secondary winding is split as follows

11000/165/110 V

Closing rectifiers are powers from the 165V connections. Only voltmeters are connected to the 110V connections.

Ive requested information on the core build but im not expecting much since they are very old!

Thanks

Blake

 

[rasevskii]

Install a fuse-failure supervision relay that supervises all 3 phase secondary voltages, and blocks the tap changers and any other voltage dependent protection that may malfunction on phase loss, such as distance and impedance protections. Standard practice in Europe. See the ABB website for what is available, for example. Or Siemens, etc. etc.

rasevskii

 

[Blake1981]

Thanks Rasevski, im actually going to replace the tap changers with solid state kit that already has this functionality. The questions were more for my own engineering knowledge and whether i can relay the behaviour to anywhere else on site.

Cheers

 

[emarch]

You may find helpful this Calculator. It is a free download

http://www.eurosmc.com/web_ing/Calculator/smc_calculator_reg_i.asp

 

[Blake1981]

Dear all,

Thanks for your replies. I managed to get my head around this whole situation and much appreciate your help.

I do however have one final question:

As I mentioned in my first thread, the tap changer tap right up when it saw a low voltage signal. So I was in a possible with high volts on the board but a low reading on the meter (i.e one primary fuse had blown). The next morning one of the ops guys saw the voltmeter on moving round all over the place from 0kV to 9kV. Has anyone experienced this kind of behaviour on old volt meters under high voltage?

Thanks again for you help, ill continue to check the site and give my input if valuable.

Blake

 

[rasevskii]

Check for a bad fuseholder, corrosion, etc. also on the secondary fuses, if any. It can be a bad contact on the sliding contacts of drawer type PT units, common on some types of metalclad gear. This can cause all kinds of problems wirh any kind of voltage regulators.

rasevskii