HV Voltage Transformer - Protection Secondary against Short-Circuits 2

[mikeangel]

Dear friends,

I'm working in a power substation and i've to calculate the protection device of a high voltage VT secondary against short-circuits. Has you certenley know this protection is to prevent the VT lost if a short-circuit occurred.

Could someone please tell me how this protection dimensioning is made (formulas)? What calculations i have to do? Wich device i will apply (fuses or miniature circuit breakers)? what are the main characteristics of the protection device?

Another question. If i have 3 VT (trifasic system) i have to protect each VT secondary individually or only one device protect them all?

best regards
Miguel

 

[scottf]

Most common is to use fuses on the secondary circuit.

The easiest is to fuse based on the thermal burden rating of the VT.

For example, if the secondary is rated 115V and the thermal burden rating is 5000 VA, then use something like a 50 A fuse. You can go lower if you want. The available short-circuit current for a short at the terminals is much higher than 50A for a wound-type VT.

 

[mikeangel]

Thank you very much scottf.

one more question...If i have three VT in a high voltage panel i have to protect each VT secondary individually or only one device could protect them all?

i've read in a paper that secondary fuses should be provided at the first box where the three phases are brought together. The circuit before the first box from the terminal box is constructed to minimize the risk of faults in the circuit. Any fuse in the terminal box is preferable not used as the voltage transformer supervision is difficult to perform then and the fuses in the three phase box is still provided to enable a fusing of the circuits to different loads like protection and metering circuit.



yours sincerely

 

[slavag]

Hi Miguel.
Our practic at the MV and HV: we always used tree-phase MCB
as protection against SC in secondary.
US practic is fuses.
In the IEC and ANSI land ( as I know, of course)instaleed first MCB ( fuses) near to VT as possible.
Why MCB's, becouse aux contacts for the information about status of MCB's.
Regards.
Slava

 

[mikeangel]

Thank you too slavag.

So if i have a 3 VT with:

thermal burden rating: 100VA
secondary voltage:120V

100 VA ÷ 120 V = 0.83 A

i could apply a 1A fuse for each VT or a tree-phase 1A MCB for all VT, it's that correct?

best regards
Miguel

 

[slavag]

I think is too small.
We used MCB 4-6A for this size of VT. MCB used against SC, not as overload.
About fuses,sorry, I don't know.
Regards.
Slava

 

[mikeangel]

Ok Slavag, i get the main ideia,

But why did you chose that MCB value? Do you have a mathematic relation? Could you get me a example with formulas and calculations. Because i have to make a paper for the client and i have to do represent in a excel spread sheet the calculations and formulas.

Thanks a lot slavag

 

[slavag]

Hmmm, now I'm at problems.
Really, I don't know, it's standard design.
I will try check. Maybe it's take time or someone at the forum can help with this issue.
Regards.
Slava

 

[mikeangel]

Ok Slavag thank you for you're quick answers and thank you for your time. I hope you or someone could help me in the next hours, let's see

Thankx

regards
miguel

 

[rcw retired EE]

My reasoning is that secondary fuses or breakers are to protect the transformer and secondary wiring from short circuits not overloads. Design of the circuit protects it from overload. The designer should not connect more equipment than the VT can handle.

A 1 amp rated fuse or breaker would be for overload protection. A 4, 5 or 20 amp would be for short circuit protection. Sizing the protection too small could result in nuisance trips and/or improper operation of the protection and metering systems.

 

[scottf]

rcwilson-

I agree. The rationale for using the thermal burden as a guide is that by sizing it over the thermal burden, you don't limit the application, but use as small of a fuse as possible

To the other questions....my recommendation is to protect each VT separately if using fuses. If using MCBs, you can use a 3-phase unit if desired.

Mikeangel....a VT with a thermal burden of 100 VA sounds pretty small. Remember thermal burden is not the same as the accuracy burden rating. Typically, only a small 600V VT would have a thermal burden as low as 100 VA and normally, those units come with integral primary side fuses.

 

[mikeangel]

rcwilson, thank you for your explanation. I understant what you and slavag and scottf said. you said "A 1 amp rated fuse or breaker would be for overload protection. A 4, 5 or 20 amp would be for short circuit protection" so i still need help in maths/formulas to get this final result 4,5 or 20A. My client wants a detail calculation.

in resume, i have VT with:

thermal burden rating: 100VA
secondary voltage:120V

how do i made the correct dimensioning?

regards
Miguel

 

[mikeangel]

The VT have the follow characteristics:

Areva UEX150
thermal burden rating: 1500VA
primary voltage: 150kV
secondary voltage: 100V

 

[slavag]

Hi.
Actually, VT(PT) is transformer.
As transformer VT (PT) must have short circuit resistance Rk and short circuit reactance Xk. I'm sure possible ask Areva mnf. for this data, is possible calculate according to primary and secondary winding dimensions. Result will be Zk SC impedance. Now, with this data possible calculte
short circuit current. You now what is a lenght of voltage cable from VT(PT) to metering and protection panels and calculate all as standard transformer. Of course include calculation for two-phase and earth(ground) faults.
Hope is show some way for you.
Regards.
Slava

 

[slavag]

Forgot send example.
See attached.

 

[scottf]

MIkeangel-

I just have to say again, there is absolutely no reason for an involved calculation. Size the fuse / miniature breaker slightly higher than the thermal burden. That method is extremely conservative and does not impact the ratings of the VT.

Secondary fusing presents little to no risk of nuisance tripping, like one might be concerned about with primary fusing, i.e. there is no charging current, surge current, etc on the secondary side.

In my opinion, this is a very long discussion on a very simple issue.

 

[slavag]

Hello.
Scottf, I'm not agree with you.
Simple case --- yes.
But you see only protection of VT(PT).
Designer of system must protect also wiring and equpment.
Possible say other:
Install first MCB(fuse) as VT-protection according to thermal burden ( for example according to your post 5000VA-50A, to VT's thermal burden 1500VA-15A) and secondary MCB's ( fuses) according to separate load and distance to panels).
Several rules:
It is possible to protect a voltage transformer from secondary short-circuit by incorporating fuses in the secondary circuits. High voltage fuses on the primary side will not protect the transformers, only the network.
A short-circuit on the secondary windings produces only a few amperes in the primary winding and is not sufficient to rupture a high voltage fuse.
6-10 A is a typical value for safe rupture of the fuses.

Fuses should be provided at the first box where the three phases are brought together.
The circuit from the terminal box to the first box is constructed to minimize the risk of faults in the circuit. It is preferable not to use fuses in the voltage transformer
terminal box, as this will make the supervision of the voltage transformers more difficult. The fuses in the three-phase box enable a differentiated fusing of the circuits to different loads like protection and metering circuits.
The fuses must be selected to give a fast and reliable fault clearance, even for a fault at the end of the cabling. Earth faults and two-phase faults should be checked.
Regards.
Slava

 

[scottf]

Slavaq-

The discussion we're having here is strictly in regards to secondary protection of VTs. In this case, the purpose of the protection is to protect the VT from external shorts. It is indeed simple. Size the protection to slightly higher than thermal burden, put it in the 3-phase junction box, and be done with it. The secondary short-circuit current of a wound-type VT is so high and by standard must be able to handle this current for 1 second, that you basically can't screw it up.

As engineers, we must sometimes resist the urge to make things more complicated than they are.

 

[slavag]

Dear Scott.
For you this issue is very simple, but I learn a lot from this thread and I'm hope not only me. I asked few designers, checked few documents, nothing on the issue.
I wrote recommendation/rules, what I found in few sources
(good sources).
Thanks a lot to you.
Best Regards.
Slava

 

[mikeangel]

By now I think I have informations and sufficient data to prepare a calculation and to present a justificatory analysis to the client.

I think this is an important question. it´s an interesting subject and in the certain form little explained, therefore I think that it was an interesting subject and that it deserves the knowledgement of all us. I already put the question to an ancient teacher and I wait for his comment.

Thank you all, a special word to Slavag and to Scottf.

Slavag:

Thank you for your persistence in this subject and thank you by all your recommendations and rules. To purpose, do you know any good pdf that boards the questions of the calculations of current and voltage transformers (CTs and VTs)?

Scottf

I thank also your contribution for the discussion that i brought to this forum. They were enough clear your answers. Only I do not agree with you when you say that it is a great discussion for simple issue.