Equipment BIL Rating Concerns

[milwaukeebob]

Hello,
I was recently brought into a project that involves installing a new piece of electrical equipment into an existing substation. The new equipment (a harmonic filter bank) is being installed on the 24.9kV of the substation. The substation is located at ~7150 feet above sea-level and according to the existing drawings has a BIL rating of 150kV at that elevation (actually it's probably a little higher than that but the design and installation documents indicate minimum 150kV BIL).

Although the new equipment has not been built yet, I've been informed by the OEM that their system is 38kV Class with a 150kV BIL but this rating is at an elevation of <1000m (<3280'). The derated BIL at 7150' feet is approximately 132kV for the new equipment. This is the focus of my concern. Please note my decades of experience up to this point has been at elevations <<1000m (<3280') so I've never had to worry about derates in this regard.

The new equipment will have surge arresters rated 27kV, 22kV MCOV (GE# 9L11XPM027S) at the line input to the gear AND there will be another set of same rating arrestors outside the gear on the exposed 24.9kV line disconnect feeding the equipment. Note the existing xfmr feeding the system is solidly grounded.

My question is this - will these arresters provide adequate protection for the new equipment which has a derated BIL of 132kV? Regardless of the presence of arresters, I'm used to matching the new equipment BIL level to the requirements of the existing system.

Thanks in advance for any feedback and guidance on my concerns.

 

[Kiribanda]

You have mentioned that your system is SOLIDLY grounded.
In that case how did you select 22kV as the MOV for you arresters irrespective of the height above sea level issue?

 

[milwaukeebob]

Kiribanda, Thanks. These arresters were not selected be me. I will have to ask the EPC Contractor who is responsible for integrating this into the existing substation. This Contractor is also the original designer of the Substation. I will have to ask them the question. Unfortunately, I came into the project late.

 

[che12345]

I have the following opinion for your consideration.
1. Existing drawing (= original) design indicates for solidly grounded 24.9kV system, min 150kV BIL at 7150ft.
2. New equipment Installed with 27kV, 22kV MCOV arresters...is rated rated 38kV 150kV BIL < 1000m. Derated to 132kV BIL at 7150ft.
3. Irrespective of the arresters, it would be "better" to have the new equipment rated to the same level of the existing 150kV BIL; perhaps at a HIGHER cost !.
4. Ask the new equipment OEM for advice/reasons for offering 132kV BIL , where the original design indicates min 150kV BIL at 7150ft.
Che Kuan Yau (Singapore)

 

[Kiribanda]

1) First you have to agree with the correct MCOV depending on the grounding method with the contractor.
The parameter is the MCOV.
2) The arresters have non-linear resisters which starts to conduct at a specified overvoltage and therefore no BIL is
defined. Hence it is my understanding that surge arresters are not required to be de-rated according to the elevation.

 

[cuky2000]

The surge arrester insulation withstand is easily misunderstood. For seal arresters, the altitude has negligible impact on the internal BIL, However, the altitude impacts the external surge arrester housing
Arrester housing withstand values are not BIL; they are the voltage withstand of the housing when the internal components of the arrester are removed. The voltage across the housing can never exceed the protective levels. Thus, the external insulation is self-protected if its withstand strength is higher than the protective levels corrected for installation altitude. See the table below for additional
details.

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