Hi-pot Test Voltage for new busbar added to old 11kV Board

[khodges]

We have installed a 1970's vintage BVP17 type 11kV switchboard, made up of 17 CB's, manufactured by GEC (same as Reyrolle etc), and are in the process of connecting 2 new panels to the end of the board, with new 11kV busbar extensions. The original busbar was hi-pot tested at 27kV for 1 minute, being 225% of nominal voltage + 2kV ... but I suspect this voltage applied to the now aged busbar insulation will destroy the old insulation, while the new will test okay.
What voltage should be used in this case, ie; on aged buswork, and what international standards contain this information?

 

[ausphil]

khodges,

we generally perform hv tests on 11kV busbars of similar and older vintage swgr, one each phase to earth (with the other phases earthed) at 24kV (which is about 90% of the ac test voltage as per the standard that it would have been purchased and designed to)for one minute.

we don't really rely on the hv test to tell us much however, and do PD and DDF tests, phase to phase and phase to earth, to tell us how the equipment is going.

it can be time consuming, but it gives you a reasonable indication.

I don't think you will find any standard that will really be dealing with equipment that is 30-70 years old, but if your equipment is to perform admirably in service, then the test voltages should still apply. If you are happy for it to not give you the performance that a new piece of equipment would give, and you want to nurse the equipment along, then by all means reduce the test voltages. I would make sure that you do some diagnostic measurements to give you a baseline for future readings though, and these are done at relatively low levels - up to 10kV prestress for PD phase to earth.

Phil

 

[HVTester]

I agree with ausphil. We use exactly the same principle and in some cases also install online PD capacitors witch we monitor every 6 months.

 

[tipsyengine101]

This looks like patching an old garment with new cloth,or better still putting new wine in old wine skins.

The concern here should be safety first!

What voltage levels do we expect when a fault occurs?

would it be okay if the whole systems fails becos the insulation level was not uniform, and perhaps cause injury to service personnel?

Have you considered applying new insulation on the bus-bars, or better still contact GEC.

 

[ausphil]

It is quite a common practice to extend busbars when extra supplies are required from an existing substation.

Gear from the 1970s may well not be at the end of its life, and it would not be feasible to replace a whole switchboard, just to get 2 extra feeders out of the substation. Physical life and electrical life are usually two vastly different things.

That is the concept of life assessment (you are assessing the remaining electrical life of the equipment). It is not a decision based on gut feeeling (with the mention of the word safety) and the 1970 age profile, it is based on the quality of insulation. We have just replaced our 2 oldest transformers on our system, not because of their age, but because of a determination that the insulation is at the end of its electrical life - these have been in reliable service since 1929.

Metalclad switchgear is a different beast to a transformer, but the concepts of assessment are similar, and the assessment needs to be done by people with relevant experience in insulation assessment.

The having different bulk insulation levels within the one piece of connected equipment is not as issue, providing the minimum level is above the expected highest voltages seen on the system (be them overvoltages at power frequency, or switching or lightning impulses).

 

[tipsyengine101]

from Ausphil:
"providing the minimum level is above the expected highest voltages seen on the system (be them overvoltages at power frequency, or switching or lightning impulses "

That's the determining factor, and it applies to the old and new hardware. In other words can the old cloth withstand the maximum stress available that the new patch will withstand?

Safety is still the underlying principle!

 

[ausphil]

I think we are arguing the same point.

Although I don't agree (semantically) that "safety is the underlying principle". Withstand of system conditions (or proper fault containment under failure) is the underlying priciple, safety is an issue.

 

[tipsyengine101]

We are on the same point, but differ on our approach to safety!

If u were installing this for others to operate and maintain
safety might be an issue which u may choose not to attend to.
But if you were to operate and maintain this system then safety will be an undelying principle right from the start!

When we design or do installation for others, we need to do all this believing we are shall be the ones to operate and maintain it. This is not only being proactive, but practicing responsibility instead of just saying it.