Friends, Our client has 6.6kV sy 2

[RRaghunath]

Friends,

Our client has 6.6kV system, resistance earthed with current limited to 2000A. He wants this to be changed to 400A one.

Do you think that I need to change surge arrestors in motor feeders. The switch board is with vacuum CBs for transformer feeders and incomers and vacuum contactors for motor feeders.

Thanks in anticipation. Raghunath

 

[DanDel]

What are the ratings of the existing surge arrestors?
Why do you expect to have to change the arrestors; will the change of neutral-grounding resistor value change the phase-to-ground potential during a voltage surge?

 

[busbar]

IEEE Std C62.22-1997 Application of Metal-Oxide Surge Arresters for Alternating-Current Systems does not seem to differentiate arrestor application or ratings in systems fitted with 2000A versus 400A resistors.

 

[RajT]

Not only you need not change the surge arresters due to the change in the NER rating, you do not need surge arrestors at all on vacuum feeders if the feeder cables are armoured.

 

[jbartos]

Suggestion: Normally, the high-resistance and medium-resistance system groundings do not favor any capacitances that may increase the capacitive charging currents and consequently the ground fault currents. The intent is to have those currents as small as possible to reduce damages by fault currents. I recommend keeping only those capacitors that are justified, and remove those that are "nice to have." The surge protection is supposed to be implemented upstream from the system grounding location.

 

[coingy]

Faulty,

There should be no need to change any surge arresters if you change the NER. I assume the request to increase the value of the NER is to limit the fault current to the value of tyhe full rating of the largest Generator of Transformer. They are slighly inductive and this can be a disadvantage with overhead lines exposed to lightening, since travelling waves or implulses are subject to positive reflection and this can stress the insulation of the equipment and cause breakdown. For your case though the value will be

R=VL/ sq3I (I being the full load current of the largest machine) and will be designed to carry their rated currect for no longer than 30 secs.

Hope this helps, Coingy

 

[RRaghunath]

Thanks everybody for the response.

Coingy,

Actually, the 2000A corresponds to the transformer incomer rated current. Thre is no generator connected to the 6.6kV swbd. The system is over 25 years old and as part of system upgrade (changing MOCBs to VCBs etc.) client specified replacement of NER as well.

Thanks once again to everyone. Raghunath

 

[neps3]

RajT

I like to know more about how multiple reignition does not occur in armoured cables. Could you please explain.

Regards

 

[RajT]

My assumption is based on the fact that the plant has surge protection already fitted say on the HV side of the 6.6kV dist transformer (ie the utility supply from the o/head lines say)so any system surges would be taken care of.
All currently available vaccum contactor technology ensures that restrikes within the contactor bottles is eliminated and as such the switchgear manufactures do not recommend that additional surge protection is required on each ourgoing feeder, though engineers often specify the needs and so its more often a question of choice rather then technical needs.
The note on the armoured cable is based on the fact that any votage rise (on any single feeder) due to say a lightning strike on that feeder would be eliminated.The use of armoured cables has nothing to do with multiple reignition.

 

[neps3]

RajT

I was reading your reply on 22nd where you have stated that surge arresters are not required if the cable is armoured. (Above)

While I understand that armour has nothing to do with multiple reignitions, I had to raise the question because of your reply and my understanding.

Anyway we have installed surge arresters on VC switching and this was in 2002. This is a leading manufacturer ABB.

While most manufacturers say that VCs do not require protection I find that the whole reig.. depends on the L & C and also the VC itself. I have had experience in short cable runs where we have not used SAs.

The issue is when a plant fails everyone scratch their heads to find the root cause.

Thank you and regards

 

[RajT]

Kantor,

My apologies for the misunderstanding

 

[neps3]

RajT

Thank you and regards