Calculating Relay Contactor Ratings

Question

Calculating Relay Contactor Ratings

cjcoronel21 (Electrical)

(OP)

26 Apr 16 13:53

Hey all,

I am having trouble determine why a relay is rated for 10kV and not anything else. I only ask this question because we have an issue at a site where we are using utility sized inverters with corresponding combiner PV ties. These inverters will send a signal to the combiner PV ties when the voltage is to low to activate the internal relay in the Combiner PV Ties to open and send the circuit to ground. This is a bipolar PV system.

I have found that the combiner PV ties have 6kV and 10kV relays and a majority of the 6kV relays were at one point replaces. I would like to determine how someone can calculate why the 6kV relays are burning out through.

Thanks

Skogsgurra (Electrical) · Answer 1 · 2016-04-27 11:59:02

Not so sure what you mean. Kilovolt relays are not common. And I have never seen one in a PV installation. For medium voltage, like 6 and 10 kV, Ross relays are used.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

ScottyUK (Electrical) · Answer 2 · 2016-04-27 18:13:29

Can you clarify whether you are talking about vacuum contactors which are frequently used to control medium voltage motors in the high kW to low MW power range, or HV relays of the type made by Gigavac (and others) which can switch a few amps up to a few tens of amps at several kV?

Do you have a schematic diagram of this circuit? Remember that all we know about this installation is what you put on the screen!

cjcoronel21 (Electrical) · Answer 3 · 2016-04-28 17:46:15

This is a bi-polar system connected to a inverter with inputs of +1000VDC and -1000VDC. The Gigavacs are attached to a fuse holder (wire in blue). What i have encountered is that some of these Gigavacs whose contacts were rated for 6kV were changed at some point. The remaining Gigavacs are rated 10kV and they have never been replaced. And these Combiner PV Ties are connected in Daisy chain formation to the inverter controls. I know that the 6kV Gigavacs should not be there b/c of my experience with their failures and the need to replace them. However what i do not know is the energy transfer from the inverter to these gigavac and why it should be rated for 10kV.

Thank you all!

FreddyNurk (Electrical) · Answer 4 · 2016-04-29 00:25:47

The Gigavac contactors, as I understand it, are only capable of breaking the current in one direction only, rather than both ways (as may be found on battery storage installations, along with much higher fault current). It is possible, although I'm guessing, that the reason the higher voltage rating units are working but the others aren't, is that they're being asked to open when the current is flowing in the wrong direction. Of course, it might be for completely unrelated reasons too.
Also appears that the 6kV / 10kV might be a BIL rating or something similar, rather than nominal voltage, hence the questions on size.

That isolation point in between 1kV of DC doesn't appear to be well placed to me either.

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Calculating Relay Contactor Ratings