Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation 1

[JTS]

We are designing a power distribution system,12.47kV and 480V,and we are spec'ing an S&C Vista switch for the MV side. It has become our standard that for arc-flash mitigation on the 12.47kV-480V transformer secondary conductors bushing CT's be installed on the low side bushings of the transformer itself. The CT secondaries then run into a relay which is located in the PDC, and a trip circuit is ran from the relay back up to the MV OCPD feeding the transformer. The trip circuit is wetted from a CPT on the line side of the PDC main. In the instance that an event will cause an undervoltage scenario on the CPT a capacitive trip device (CTD) is installed to ensure that the trip circuit maintains its wetting voltage. The problem is that the standard CTD output is 170VDC due to the peak AC input and S&C says that it's maximum external trip is 125VDC. This leaves us with two options.

1. Use the CTD anyway and make sure that a B contact is in line with the trip circuit to ensure that the signal is momentary, and in the event of a trip check all contacts and ensure there has been no damage.
2. Forgo the CTD altogether and use a Voltage Sag Corrector with a 200ms ride through on full voltage loss.

I am leaning towards using the Voltage Sag Corrector however I haven't really seen this done anywhere else, is this a typical setup that you have seen and/or spec'd?

 

[wroggent]

Are you also using S&C's low voltage enclosure (LVE)? Our LVEs had batteries. If you have the LVE, can you use that as a source for your trip circuit instead of the CPT?

Edit: It looks like with option 2 you're still going to get 170Vdc out so I don't see how that's going to help you.

 

[JTS]

We aren't using S&C's LVE. The voltage sag corrector actually outputs a 120VAC. I tried to upload it but it will only let me upload one document, and I can't figure out how to take out the CTD, but here's a link to the Voltage Sag Protector.

http://literature.rockwellautomation.com/idc/groups/literature/documents/td/1608-td001_-en-p.pdf

 

[cranky108]

Of the Vista devices we use, we install batteries to both power the relay ,and the trip circuit.

Most CPD device operated trip coils I have seen are current operated, and not voltage rated. The CPD can not sustain a voltage, but can provide a momentary high current to trip a coil.

What type of relay are you using that does not require a power supply?

 

[JTS]

The relay is a SEL751A and is powered by the same CPT as the trip circuit and will have the same ride through setup either the CTD or the voltage sag corrector. But the relay isn't the issue, it's the input to the shunt trip for the fault interrupters themselves on the Vista. The spec says that it has to be either 120 or 240VAC, but S&C has told us that they can do a special order on this for 125VDC but not 170VDC.

 

[cranky108]

Be careful as I think you are missing something important. The SEL751A requires a battery for power.
S&C Vista's I believe also expect a battery.

There are only a few relays that can be powered with an internal battery, or be powered by a CT, and the 751 is not one of those.

Powering a microprocessor protective relay from an AC, or capacitor backup is not a good idea. The electromechanical relays could do it because they were powered by the CT circuits.

You are looking at trip coil voltage ratings on the Vista, when you should be looking at current requirements of the coil for a capacitor device tripping.

 

[JTS]

SEL751A's don't require a battery and neither does the Vista. The 751A will take both AC and DC power for operation. You can plug one into a wall if you so desired and power it up (obviously this should not be done for an actual installation, but if you want to play with it at your desk it can be done). The Vista trip doesn't come standard with a DC trip, it's a special order.

While batteries offer a reliable source for the trip scheme our customer does not want to add the maintenance, design, and installation costs for a battery system. Capacitive trip devices are widely used and designed for this very instance. It just so happens that S&C maxes their special order shunt trip at 125VDC (only AC is shown in the spec guide). It is understood that a loss of power from the transformer will also turn off the relay. However, the relay is not needed if power is turned off. Where it would be needed is in the event of a fault which in turn could cause a full power loss on the CPT inside of the PDC, which brings us back to the original question.

 

[LionelHutz]

I've seen lots of breaker circuits with 125VDC trip coils controlled from a capacitive trip device producting ~170VDC. It's a common way to build breaker controls when there isn't DC station power available. The trip coil won't stay connected long enough to damage it, even if it did cause the current flow to be higher than rated. Most CTD's have a current limiter in the circuit that will somewhat limit the trip current when a trip happens while AC is present so the capacitor voltage will drop during a trip. If the AC power is lost, a trip will draw enough current to discharge the capacitor.

 

[jghrist]

Could you use a resistance voltage divider on the input of the CTD?