Hi List,
We have a 1200kVAR Ungrounded Double Wye Capacitor bank connected to a 400Amp vacuum contactor suitable for this purpose. The bank contain six (6) monophase 1386V, 1phase, 200kVAR, internally fused capacitors. Système voltage is ungrounded delta 2400V. Since the capacitor itself is already protected internally, i am looking for a simple way to protect the bank for overcurrent, overvoltage and unbalance, without buying an expensive capacitor protection digital relay.

Thank you for your help,
Pascal

 

Responding to Pascal Poirier's Sun, Jan 19, 6:49pm query:

I'm not quite sure of my interpretation of your query. Please confirm or clarify:

1) Doe this capacitor bank "control" system power factor? In other words is it switched daily, weekly, monthly, or rarely?

1) Do you want additional bank protective relaying to trip the vacuum contactor as backup to the internal fuse?

2) Do you want additional relaying to trip the contactor when an internal fuse operates?

3) Are you trying to duplicate all or some of the functions of a "digital" relay?

4) Instead of a "digital" relay, what about the old but still reliable "induction disk" type?

5) Are you concerned about Inrush? Harmonics? Overvoltage?

6) Is this installation in France? Or elsewhere using French equipment and/or system standards?

Regards,
Phil Corso, PE
(Boca Raton, FL) [[email protected]] {[email protected]}

 

In order to clarify my question:

>1) Doe this capacitor bank "control" system power factor? In other words
>is it switched daily, weekly, monthly, or rarely?

No control, switched rarely....

>1) Do you want additional bank protective relaying to trip the vacuum
>contactor as backup to the internal fuse?

Vaccuum contactor fuses will provide backup for overcurrent...

>2) Do you want additional relaying to trip the contactor when an
>internal fuse operates?

I guess that's the purpose for "Neutral to Neutral" current sensing in this ungrounded Y-Y arrangement.

>3) Are you trying to duplicate all or some of the functions of a
>"digital" relay?
>
>4) Instead of a "digital" relay, what about the old but still reliable
>"induction disk" type?

I have some unused Westinghouse IQ-1000, Voltage relay, old Ge IAC 50/51 relay, etc. I am just trying to use them.

>5) Are you concerned about Inrush? Harmonics? Overvoltage?

I'am aware of these potentials problems...

>6) Is this installation in France? Or elsewhere using French equipment
>and/or system standards?

No... Quebec, CANADA, where ungrounded 2400V delta distribution system is common (at least in old industrials plants....)

We bought a setup of MV motor starters and MV vacuum breakers on the "used" market. Two Single Wye 600kVAR capacitor bank were included in the deal. Each Capacitor bank encloser (space, bus ampacity, isolation/ground switch, etc.) have been design to accept 3 more 200kVAR capacitor to have an 1200kVAR ungrounded double wye bank. I want to used all the capacitors (6x 200kVAR) in the same encloser, and then switch on a 1200kVAR power factor correction capacitor bank that won't turn off (2 or 3 times a year, for mill shutdowns). I am just trying to get some advice on choosing protection devices, phases and neutral CT ratio, pick-up and time settings, etc., because i am not really familiar with this particular installation.

Thank you, Pascal

 

You're probably aware of the "perceived" benefits of ungrounded systems as well as their problems. If I'm wrong in the preceeding statement or the following scenario, please correct me:

I presume that you are considering ground-fault protection to automatically:
1) remove the faulted component quickly; and
2) identify which of the two banks failed. Do you intend to interconnect the wye point of each bank, and then use it in a quasi-differential scheme? If yes, it will be troublesome! Even if the neutrals were left separated, there probably won't be enough current to reliably operate the typical overcurrent relay, for example, the 50/51 you mentioned. Nor will it be significantly improved if you add a residual connected 50N/51N. The reason is that the only source of the ground-fault current is the current available in the MV system's line to ground capacitance, with all components considered! The voltage relay will certainly detect a ground-fault but it won't be helpful in locating the fault's location.

In my opinion, if the MV load consists of large motors, then they will probably have a high enough power factor to preclude (economic) correction via the capacitor(s). You'll have to work out the numbers. Furthermore, you don't want 1,200 kVAR of capacitance connected to an unloaded MV system!

In conclusion, I don't know how large the MV system is but you might want to consider a permanent neutral (preferably a restricted one) rounding scheme... especially if the MV system is very old. If you want additional information, I'll be glad to help.

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]
{[email protected]}