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






RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation
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.
RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation
http://literature.rockwellautomation.com/idc/group...
RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation
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?
RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation
RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation
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.
RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation
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.
RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation
RE: Capacitive Trip Device (CTD) for Medium Voltage Vista Switch - Arc Flash Mitigation