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Diverting xfmr lighting arrestor discharge through a ground CT & relay
2

Diverting xfmr lighting arrestor discharge through a ground CT & relay

Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
Most of the installations at this plant have the discharge of the transformer mounted lightning arrestor going through a case ground relay. See the attached picture.

The purpose for this I'm told is that should the arrestor fire, it can help pinpoint the issue via the alarms - that the problem is at the xfmr not somewhere else along the distribution path.

All of these are old installations. What is the parameter on the ground CT that is important to know for this installation, should we duplicate it on a new install? And when the arrestor discharges what is the current that would be flowing? I realize the arrestor works as a switch but does it also act as any kind of impedance?   

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

Seeing the attached picture, the amount of current flowing into the CT primary will be depending on the grounding resistance of your ground rod and the transformer casing -transformer mounting rails-to ground resistance. Whatever LA discharge current occurring on any lightning event will be split between the CT - ground rod path and the transformer casing to ground. (If you have the chance (perhaps during any outage), try measuring your "ground rod resistance" and compare with your measured ground resistance taken from the "transformer casing-rail" section. Then you can calculate the current split, hence the possible current that would be flowing into your CT. Of course you need to assume the lightning impulse voltages or currents.)

My experience with lightning protection installations is that some OEMs provide lightning counters to give a little idea of whether the protection functioned/ was hit. If you find it of use and your company have some money to burn, you can duplicate that setup (if you can find vendors of those ground discharge sensing relays).

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

In my opinion this is a gap or spinter instead of a surge arrester.
Travelling wave voltage due to lightning discharge on line when exceed eletric space voltage of the gap,  current impulse will diverted to ground. the current will evolve for the industrial frequency, if  become a short-circuit to earth detected by CT which will operate a ground relay.
 

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
Odlanor,

Specifically they are ABB XPS Station Class surge arrestors.
The new installation is calling for the transformer to have an insulating material under it, so it should be isolated from the ground except this one connection. The picture is typical of another install, not this one.

The vendor is calling for the arrestors to go to their own ground rod, tied to the system but not through this case ground cicuit. If the arrestors conduct they will cause a phase to ground fault and will trip our breaker, but the breaker is 1/4 mile away, and lots of stuff along this path that could cause this same phase to ground signal. Thus my questions why can't we run this through this CT?
 

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

Is this an air gap as depicted? Or an MOV?

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
Stevenal,

They are MOV, I have the wrong symbol?

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

They are MOV so there is no short circuit to ground. It is a good idea transformer to have an insulating material from transformer , so cable  should be isolated from the transformer tank  and connected direct to ground cable before CT.
This will reduce refleted voltage wave back to origin.
 

 

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

Even if the relay could sense it, your power frequency rated CT will not reproduce the high frequency of an MOV discharge very well. The CT as shown will only sense a faulted arrester or a faulted transformer.

I agree with the vendor. Arresters work best if lead lengths are as short and direct as posible. The extra (high frequency) impedance of your wound primary (I assume) CT will ensure a higher voltage at the transformer terminals in an overvoltage event. Of course you would need to set your arresters on insulators to keep them isolated from the tank if you follow the vendor's advice.

This type of transformer ground protection is called "case ground." IEEE warns: "The system should be tested periodically to
determine that no accidental grounds have been added. Incorrect operations can result from accidental grounds from power tools and transformer auxiliary equipment."

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
Stevenal,

This is the old way they have connected everything likely 1950's vintage when MOVs weren't even invented. With the old type silicon gap arrestor would that have been a concern?

So what you are saying is keep the arrestors isolated from the tank and go directly to ground as the vendor specifies.

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

Please understand that you are trying to protect those transformers from the ill effects of lightning strokes! The best location for those arresters are right beside the protected transformers. You also need to provide as short a discharge lead to ground as possible, no tight turns!

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
Smallgreek,

What is the discharge voltage and current? And how do you calculate this? For example, What if the impulse were 50kA and the resistance to ground is 10 ohms. I tried to read through the paper you reference. I'm not getting it from that.

There is a question of what size this wire should be to the ground rod. On the arrestors themselves it allows for #2 all the way to 1000MCM.  

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
burnt2X,

The surge arrestors are mounted on top of the transformer within about a foot of the high voltage bushings. And they are mounted on the far side, so it will be a straight shot down to the ground.

The vendor is also recommending these be insulated and (3) separate wires to a ground rod. I'm thinking about putting bare  leads in a PVC conduit. They don't tell us the size wire to use though. I would like to have some backup materials saying why a certain size was picked as opposed to some "well that's how the rest of them are done"...

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

Quote:

The vendor is also recommending these be insulated and (3) separate wires to a ground rod. I'm thinking about putting bare  leads in a PVC conduit. They don't tell us the size wire to use though. I would like to have some backup materials saying why a certain size was picked as opposed to some "well that's how the rest of them are done"...
bdn2004,
Regarding wire sizing for lightning ground leads, the criteria being used is "fusing current" capacity of wires and not "current carrying" capacity. E.g. most substation system ground wires utilize 4/0 copper clad steel wires which has a fusing current of about 49kA in 30 cycles (0.5 seconds). If you are sure that the lightning discharge current is lesser in your case, you can use smaller wires, 7-strand #6 (31kA in 0.5 seconds) perhaps.

Please find attached link for fusing currents data for the different sizes of copper-clad steel wires used as ground and OHG wires.

On the decision to use "insulated" versus "bare" wires, I guess you will have to inquire from your vendor why they recommended those "insulated wires" as ground wires when these are not current carrying wires!

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
I think the issue is they want to keep the arrestor leads away from the case ground which is connected to the tank, thus the insulated wires. And could there not be some arcing.

And burnt2X thanks for the great info.  

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

I don't think you'd want to ground SiO2 arresters through the CTs either, but I'm not aware of their characteristics.

The reason for insulating the ground leads is to prevent the core protection from failing to operate properly for a faulted transformer. PVC conduit would do the trick. For the same reason you will need to insulate the arrester bases from the transformer tank they are mounted on.

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

I think you mean silicon carbide not SiO2.  They have been replaced by metal oxide varistors or MOVs.

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

Yep.

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

bdn2004
ABB XPS surge arrester are applied from 3kV to 500kV system.
1- what is  high voltage of transformer ?
2 - what kind of grounded for high voltage system?
3- what are duty cycle or MCOV of XPS surge arrester?
These are the data necessary to answer your qeustions.
 

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

I thought the surge arrestors are to protect the transformer windings.  Isn't the voltage stress on the windings between the energized winding and the tank?  Wouldn't it be better to connect the arrestor directly to the tank to minimize that voltage?  Also, the steel tank probably has better high frequency impedance than the ground wires.

Surge currents through the case ground CT will create a tank to ground voltage rise, but that will not be seen by the windings, unless the winding is externally grounded through a neutral terminal.

A minor problem with case ground CT protection is it also responds to ground faults on auxiliary equipment like a faulted transformer cooling fan.  The CT and relay can't tell the difference between 120V and 115 kV fault current.    

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
Odlanor,


1. The high voltage on the primary is 15kV, the arrestor is rated 18kV RMS.
2. The high voltage side is solidly grounded.
3. MCOV 15.3kV RMS.

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

Good points rcwilson.  

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

bdn2004,
Attached table of the XPS-surge arrester. For Vn (1) = 12.47kV, Vr(3) = 18kV, MCOV (4) =15.3kV,
 in accordance LPT (7) to 40kA-discharge current  and 55.4kV-discharge voltage.
For simplicity we assume a traveling wave of  40KA entering the HV terminal of the transformer.
The arrester will fire 40KA diverting current to ground through the tank, it sends a reflected wave voltage of 55.4KV ,
reverse of the original wave to the input terminal, and send a wave refracted of 55.4kV into the transformer.
The transformer has a BIL of 110kV and can absorb this stress.

The standard lightning impulse waveshape is 1.2/50us.
There exists little doubt that in a actual system, this wave shape has never appeared across a piece of insulation.
For example, the actual voltage at a transformer has an oscillatory waveshape.
It is true that , in general, lighning surges do have short fronts and relative short tails, but the importance in the
standardization  process is that all laboratories can with ease produce this waveshape.

The current in CT to operate ground relay  will be at industrial frequency long times after lightning impulse current,
IF surge arrester do not CUT flow after  lightning impulse stop.

As XPS surge arrester is applied from 3kV to 500kV, vendors  generalize recomendations more important at HV,EHV.


rcwilson,
voltage stress is between the conductor and the isolation of the winding , a phase-to neutral voltage.
winding is externally grounded through a neutral terminal solid grounded.
 
we use to isolate  tank from ground by separate wheels from ground by a isolant material as plastic.
This reduce  ground faults on auxiliary equipment .
 

RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay

(OP)
Odlandor,

I made a mistake, the primary is a 15kV ungrounded.  

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