MCOV Rating At 72kV Nominal Voltage 2

[ThePunisher]

Hi all,

IEEE standard voltages specifies 69kV system nominal wherein the maximum voltage is specified at 72.5 kV.

However, our application calls for 72kV L-L system nominal voltage. This 72 kV powerline is fed from a 260-72kV Wye-wye (solidly grounded primary and 150A resistor grounded secondary)100MVA power transformer. The transformer has OLTC on the primary with AVR set to maintain 72kV nominal at its secondary.

The lightning arresters installed at the 72kV lines (fed from 150A resistor grounded system) are ABB EXLIM R with MCOV rating of 67kV which I believe is under-rated. For resistance grounded systems, the phase-ground voltages of the unfaulted phases may rise up to line-line (72kV) as the neutral will shift on a bolted line-ground fault. If this happens, the LAs will fail or "conduct" currents to ground resulting to a 3LG fault.

We did not perform any EMTP study on the lines nor calculated the COG of the 72kV system but I believe the conservative theory should be sound enough.

One consultant disagrees with me saying that the ground fault will not be maintained and will be cleared immediately by the protection system within 0.5 seconds. However, I still have concern that the LA would fail. Can anyone help me get some light considering that our nominal system voltage is not standard as published in IEEE Std C62.22-2009.

 

[crshears]

Lightning arresters typically don't begin to conduct in any serious way for simple system overvoltages; perusal of the performance tables provided by the manufacturer should confirm this.

In my experience even the neutral shift caused by ground faults in a three-wire distribution network won't trigger them into conducting; it generally takes the steep-front jolt of a lightning bolt to get them into breakdown conditions. Were it not so, voltage spikes caused by circumstances like major load loss with too many capacitors in service would cause premature lightning arrester failure.

So unless I'm missing something, I don't foresee any with issues with using the specified arresters.

CR

 

[odlanor]

150ohm resistor is at 72.5kV neutral system?

If is true, 67kV MCOV is insufficient. During a ground-fault on one lightning arrester, the others two LA will be submitted to a 1.0*72.kV.(72.5>67).The others LA could be damaged.


If 72.5kV neutral is solidly grounded 67kV MCOV is OK. During a ground-fault on one lightning arrester, the others two LA will be submitted to a 0.80*72.kV.(58<67).The others LA would be OK.

 

[ThePunisher]

Thanks odlanor. The NGR is 150A and hence the ohmic value is 277 ohms at 41.5 kV L-N.

The other problem I have is that the resistance grounded 72kV system is also having 12 MVAR remote shunt capacitors. Assuming the caps are switched in and 72kV is maintained (through transformer AVR). When there is sudden load rejection (due to load tripping), the overvoltage on the system at 60Hz will be high that it might damage the L.A.s.

 

[stevenal]

Your consultant is correct. MCOV (maximum continuous over-voltage)is not the parameter you use to check against a temporary over voltage caused by a rapid clearing fault. Look at the TOV rating. See page 88.

http://www05.abb.com/global/scot/sc...Buyers Guide Edition 10 2014-01 - English.pdf

With a TOV of 97.4 at 1s, I think you have plenty of margin.

 

[dpc]

If you're confident that ground faults will truly clear in 0.5 sec and this looks OK on your TOV curve, I'd be OK with it. A higher MCOV would be conservative, but would reduce transformer protection (a little).

 

[ThePunisher]

Hi Stevenal, I believe you have a point. The MCOV is maximum continuous operating voltage. A line-to-ground fault is not a "continuous" overvoltage but a TOV. If the protection will isolate the fault in 1 second, then the arrester of TOV rating of 97.4 kV at 1 second is ok.

 

[Kiribanda]

Hello ThePunisher,
Here is my input assuming max. voltage=72.5kV.
1) If the 69kV System is SOLIDLY or EFFECTIVELY GROUNDED: The MCOV of the arrester shall be 42kV (rms)and the Arrester rating (Duty Cycle) = 54kV (rms)
2) If the system is low resistance grounded (using 150A resister): In fact a system study has to be performed using such as ETAP/ SKM and then the COG shall be determined. Based on that MCOV to be estimated.
3) If the system is high resitance or isolated grounded: Since the COG=1, then MCOV of the arrester shall be 72.5kV & Arrester rating = 96kV

 

[odlanor]

I'm sorry my old calculation. I assumed LA with airgap OSi. Values are duty cycle for Metal oxide arrester.
I would say YES, an ABB EXLIM R with 67kV-MCOV and 84kV-rated voltage is OK for a 72.5kV system grounded by 277 ohms.
I would say YES, TOV -1s is enough for system with distance relay as remote backup protection, but I prefer TOV-10s.

 

[odlanor]

about 12 MVAR remote shunt capacitors:
you can get some aproximation using Ferranti effect calculation. Finding the voltage rise from sending end for a 60Hz L-km 72kV transmission line if the receiving end voltage is equal to 12MVAr-Xc ohm capacitor bank.
Overvoltage level to which the equipment could be subjected cannot exceed overvoltage whithstand level of equipments.
This applied to transmission line insulation and lightning arrester.
If happened with LA , you must choose another of superior voltage rated class.

 

[stevenal]

odlander,

92.4 kV at 10s. Still looks like plently of margin.

 

[odlanor]

stevenal
I know of no book or standard insulation coordination, which recommends using protective relays to protect surge arrester.
If overvoltage at LA were 67kV I would choose MCOV > 72.5*1.05=76.125 kV. I guess an ABB EXLIM R with 72kV-MCOV and 90kV-rated voltage is OK.
Remember that this is a rough calculation.

 

[stevenal]

odlanor,

I also know of no book or standard that advocates such a thing, and neither do I. Perhaps you misunderstood.
You don't generally apply overcurrent protection based on the arrester although you might review settings if too much compromise is needed on the arrester rating. You generally apply the arrester based on (among many factors) the longest and highest expected transient overvoltage condition; which comes from your fault study assuming the fastest device fails to clear the fault. IEEE C62.22-2009 page 33. "The most common source of TOV is voltage rise on unfaulted phases during a line-to-ground fault."

 

[odlanor]

stevenal,
You are totally right! I'll have to rephrase my concepts!!.
I make a point to reproduce the observations below.
IEEE Std C62.22-2009, Guide for the Application of Metal-Oxide Surge Arrester for alternating current systems (extracts page 30)
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Change in relay setting or use of faster breakers may sometimes allow use of arresters based on MCOV When TOV would otherwise have been decisive
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The duration of overvoltages from line-to-ground faults depends on the relaying adopted short-circuit protection. In the absence of other information,
the following typical values ​​may be used:
-Grounded-neutral systems: TOV duration:
Line protection: 0.2 s
Back-up protection: 1 s (overcurrent relays may be slower)
-Resonant grounded or isolated neutral systems:
Without ground fault clearing: 3 h
With ground fault clearing: 4