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?
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
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
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
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
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
The document listed gives a good overview of surge arrestor performance.
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RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
They are MOV, I have the wrong symbol?
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
This will reduce refleted voltage wave back to origin.
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
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
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
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
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
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
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
And burnt2X thanks for the great info.
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
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
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
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
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
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
RE: Diverting xfmr lighting arrestor discharge through a ground CT & relay
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
I made a mistake, the primary is a 15kV ungrounded.