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

[bdn2004]

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?

 

[bdn2004]

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.

 

[stevenal]

Good points rcwilson.

 

[odlanor]

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 .

 

[bdn2004]

Odlandor,

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