PT Primary Connections 2

[timm33333]

If we feed a transformer-relay (digital) with Y-Y connected PT’s, with primary Y of PT’s ungrounded and secondary Y grounded; will the fact that the primary of PT’s is not grounded cause malfunction of the relay? Thanks

 

[timm33333]

DC current out at terminal 6, pass through sensing resistor, transformer star point, NGR, ground soil; and then back into terminal 7.

 

[LiteYear]

The diagram shows a CT (EFCT-X) monitoring the current. Not sure how the manufacturer expects DC to properly test this setup.

The CT is actually deliberately designed to be ignorant of the test current. Notice how the current path travels in both directions through the CT in order to cancel the effect. The current is monitored by the device internally. The CT is there for traditional earth fault detection (hence the EFCT designation). This is a combined earth fault detection and NGR monitor device.

There seems to be some scepticism about these devices. FWIW, we've been using them in underground coal mines in Australia for years. The latest update to the applicable standards actually requires their use in some situations. Startco are not the only manufacturers. They are bread and butter devices in our industry.

 

[stevenal]

I can see that DC could be used to monitor NGR continuity. I don't think this injection, filtered by the CT, will result in tripping the breaker.

 

[LiteYear]

The device is designed to trip the breaker via its trip coil or undervoltage coil. If the NGR goes out of spec, the device's internal relays change state, closing or interrupting the circuit to the trip or u/v coil respectively, and the breaker opens its main contacts. Often the devices are paired with a contactor instead, and can then just interrupt the contactor's main coil.