HV Connectiors for Substations

[babdoc]

Hi!

I'm working in new substation project construction (in Ireland), and one of my tasks is to choose the HV TERMINAL CONNECTORS. Since this is my first time on this issue I have a couple of doubts that I would like to be clarified:

I've a HV Circuit Breaker with a 224 mm terminal pad with 9 holes (see attached image, please). The terminal pad for the HV Terminal connector that i should choose must cover all (or most) the surface or the Circuit Breaker terminal pad?

Some of my coleagues say no, and others say yes. One of my coleagues say that i should choose a HV terminal connector with a plate just to cover the holes area of the HV Circuit breaker terminal pad (a plate with 125mmx125mm with 9 holes) and others say that I should choose a HV terminal connector with a bigger plate in order to cover most of the HV Circuit Breaker terminal pad surface. But no one explain me why?

I think, if the HV Terminal Connector is the same size (or approximately) of the Circuit Breaker Terminal pad, the current that will flow, will be evenly distributed throughout the surface. Selecting a smaller HV Terminal Connector (for example, a 125x125 mm pad) will get more focused. I do not know if over time, if you choose a HV terminal smaller than the surface of the circuit breaker terminal, it will accumulate debris and cause hot spots. I do not know, I'll probably be saying some nonsense and I'd like to hear your opinion.

My best regards,

 

[Power0020]

It depends on the design load current.

As per IEC-62271:301

For copper terminals, a maximum contact terminal current density of about 0.32 A/Sq.mm is allowed and 0.17 A/Sq.mm for Aluminium.

If you have a bimetal connection, chose the one with lowest allowable current destiny (Aluminium)

Exclude the holes area from calculation.

 

[stevenal]

I think the idea that current density across the surface will be uniform is questionable. I expect the density increases near the holes where the clamping force is greatest. And toward the left and right edges away from the holes it will be least. The extra unclamped area will also be a place for corrosion to begin.

 

[babdoc]

stevenal: I think I agree with your opinion. Today (before I read your reply) I've talk with the HV connectros supplier and he told that I should choose the bigest connector instead of the smaller one, because the current density will be "more" uniform across the Circuit Breaker terminal pad. He told me, if I choose the smaller one, the current will be more concentrated in a smaller area, and this could cause, "hot spots" in the clamping zone, in the future, also he told me that waste can accumulate in the edges of Circuit Breaker terminal pad. In conclusion, i don't know what to do

Power0020: thank you for pint me that IEC, I will check that. The HV supplier, assured me that whichever option I choose the required nominal current will be ensured. And the price of the connector will be the same.

 

[qman5]

Just ask the supplier to recommend you a clamp for the maximum ampacity you foresee. You can always go and calculate this rating, but calculating hole diameter and the effect on ampacity... don't worry about this for now. Trust your supplier (until he's wrong)

 

[basilasq1]

Keep in mind one more issue that we previously experienced for a 240kV breaker terminal pad. A smaller pad will have a greater temp rise for higher ampacity. This is especially an issue with breakers which have the max operating temp of limited (ref. IEEE 62271-1, Table 5.1). You may use IEEE 605 for calculating temp rise and ampacity of your terminal pad.

Our utility standardizes terminals pad requirements to be 6 Hole NEMA Pads (HNP)for ampacities of >= 3000A and 4HNP for <3000A.

 

[babdoc]

Hello!

Please find attached the two solutions propose by the manufacturer of the HV Connectors. Both of the assure the necessary ampacity.

After read all your comments, I've discuss this issue with my colleagues and I cannot find a unanimous choose.

One of my colleagues told me exactly what stevenal said, namely that if I choose the HV connector with a larger terminal (solution 2), it is expected that the current density increases in the tightening zone and that in the Left and right edges, between the HV connector terminal and the Circuit Breaker terminal will accumulate debris over time.

Another colleague of mine told me that if I choose solution 1, and since the HV connector pad is smaller than the CB terminal pad, there will be a higher current concentration in the tightening area of the CB terminal pad. This will cause an area with a higher temperature than the areas not covered by the HV connector (left and right edges od the CB terminal pad). On the other hand, the uncovered areas will accumulate debris.

In conclusion, what should be the best solution?

 

[cuky2000]

Both connectors are suitable for the intended application.

Joint compounds are recommended to prevent formation of oxides on the cleaned metal surfaces and to prevent moisture from entering tile connection thus reducing the chances of corrosion. Also apply tightening all bolts with a torque wrench in accordance with manufacturer's instructions.

 

[prc]

I would prefer solution 2 as it offer more surface area for transferring heat to outside air and hence may give slightly cooler joint.It is true current transfer capacity will be same for both solutions as the transfer will be entirely through the bolt pressure area only ie near to bolts.

 

[Andiri812]

The smaller palm also has 3 times the thickness of the larger palm... 45mm vs 15. This would also have an effect on the amount of warping that would occur away from the bolting areas especially if using alluminium.
Jointing compound is a must and definitley correct torque values and correct tightening sequence on a 9 hole pad.
Selection of fastener as far as expansion coefficient of the type of metal used and belville washers to spread the area of the fastener pressure more uniformally.