BIL for 69 kV isolated neutral system

[tem1234]

Hi,

We have a 69 kV isolated neutral station. There is two 69 kV transmission line that are ending at the station. One of them is very old and many fault occur. These fault are not always seen by protection because the system is unearthed and the line is quite long for 69 kV.

We have to had some equipment in the station and we are not sure about the BIL we should use. As per IEC 60071-1, for a 72,5 kV equipment, the standard BIl is 325 kV.

Our standard here is 350 kV BIL for 72,5 kV, but for solidly grounded system or low resistance grounded. Is this BIL is ok for unearthed system? We could easily use switch and CT with higher BIL, but for PT, it's a little bit complicated since standard PT with BIL higher will also have a higher ratio (they are for 120 kV system).

The MCOV (or Uc) of the surge arrester will be higher than 69 kV, probably around 72-74 kV.

Thanks a lot

 

[odlanor]

this insulation class 69kV has 72,5kV max system voltage
transformer winding has 250 kV or 350kV BILs
bushing transformer and other equipments have 350kV BIL
By IEEE
MCOV 57kV refers to 72kV duty-cycle OZn arrester.
MCOV 70kV refers to 90kV duty-cycle OZn arrester.

In contrast to IEEE, a standardized list of arrester rated voltages and MCOV are not provided in IEC.
you should research with surge -arrester manufacturers.

 

[tem1234]

Ok,thanks odlanor. I'll try to ask a manufacturer

 

[Adam1980]

BIL protection level according to IEC 71-1 should be sufficient as it is independent of system configuration.

In medium voltage system the protection level is usually enough especially if arresters are installed according to manufacturers recommendations.

The arrester selection is described in IEC 60099-5. The basic selection criteria is the rated voltage Ur and the continuous operating voltage Uc which corresponds to MCOV in IEEE.

The Uc depends on the earthing system configuration however this is no difference between low impedance and isolated eathing systems since for both the earth fault factor is taken as 1.7 according to IEC.

The last remaining factor in selecting the arrester is the rated residual voltage which should stay within the equipment protection level after considering all factors described in IEC 71-2.