Capacitive Voltage Transformers and Line Arresters

[ivana24]

I should have to install three 115 kV capacitive voltage transformers inside my substation on line side of 115 kV disconnect switch. Also, I am informed that I have to install three line arresters on the same - line side of 115 kV DS, because CVTs are provided on the line side, not substation side of disconnect switch. Downstream I have power transformer protected by three separate surge arresters.

My question is - why I need these 3 line arresters next to CVTs ?
115 Kv disconnect switch is coupled with earthing switch - and opening of 115 kV disconnect switch means in the same time a closing of earthing switch and I do not see a reason for these 3 line arresters. ( transformer is protected by three separate surge arresters)

 

[scottf]

It is common to place CCVTs outside of arrester protection, as the capacitive nature of the insulation handles surges very well.

 

[odlanor]

if you have DS with earthing switch you do not need line arester close to CCVT because incoming surge are originated from the line and can be vanish to the ground by ES.
Line arrester should be instaled in a position of line bay in such way that it zone of protection covers all line equipments.
For 230kV and above power transformer are protected by independent surge arresters.
For 138kV sometime they only use spinterometers at HV bushing of transformers.

 

[ivana24]

I tried to understand - you said I don't need line arresters for my case.
But if LS-line switch is closed ( ES-open) and NO arresters provided on line side of station - travelling wave is coming to my station LS -CVT - CT- 52- 89B - TR, coming to the transformer ( protected by separate arresters) and all equipment on this way is not protected - such as instrument transformers ?

 

[odlanor]

You always need line arrester! but they dont need be installed at the entrance of line.
They could be installed in the middle of bay
( LS-CVT - CT- surgeArrester- 52- 89B - TR) in a position that its cover distance from it to transformer and from it to beggining of the line.
I said you dont need surge arrester close to transformer less than 230kV, because line arresters could be enough.

 

[jghrist]

You may need line arresters in addition to the transformer arresters. It depends on the distance from the transformer to the incoming line terminal. If lightning comes in on a line, the transformer surge arresters will create a negative surge travelling back to the line terminal. The negative surge will reduce the surge at the line terminal when it gets there. I would put arrester at the transformer in any case. That location protects the transformer best. I'd rather risk damaging a CCVT than a power transformer.

 

[scottf]

As tried to say earlier, CCVTs do not require protection by arresters.

 

[odlanor]

with LS-opened&ES-closed no if DS is opened incoming surge voltage goes to earth, all bay equipments are protected!

with LS-closed If lightning comes in on a line, the line surge arresters will create a negative(refleted wave) surge travelling back to the line terminal protecting ( LS-CVT - CT ) and positive (refrated wave) surge travelling wave moving forward to find the (52- 89B - TR).
all this process of reflexion and refratation should have a value supportable by equipments.

Modern OZn surge arresters are so effective that even installed at entrance they can protect up to transformer.
I know many utilities of subtransmission only adopt arc horne(?) at 138kV- bushing terminal transformer instead of surge arrester.