PT connection 1

[ovlac]

Hi All,

We have a 4160V bus for our generators. Three PT’s with two identical secondary windings each are connected in the bus. The PT primaries are Y connected to the 4160V bus. One set of the 100V Y-connected secondary windings were used for synchronisation, metering and protection circuits. The circuit on the other set of secondary windings is what I can’t fully understand. The circuit goes like this:

The windings are marked “a” and “n” on each PT. On Phase A, the “a” terminal is connected to a 3A mini circuit breaker with 25 Ohm resistor as load connected to neutral. The “n” terminal of Phase A is connected to the “n” terminal of Phase B. The “a” terminal of Phase B is connected to the “a” terminal of Phase C and the “n” of Phase C is connected to neutral. In short, this set of winding is series connected in opposing polarities. The mini circuit breaker has an auxiliary contact that will open the blackstart generator circuit breaker in case this mini CB trips off.

What protection does this circuit provide for the blackstart generator? How does it work?

What will happen if the resistor is bypassed to neutral (this is what we found when we actually traced the circuit against the reference diagram)? I assume that the mini CB will trip earlier.

Please help. Links to references would a great help.

Thank you.

 

[waross]

Well, 100 volts into a 25 ohm resistor is 4 amps on my calculator.
200 volts into a 25 ohm resistor is 8 amps.
Are you sure that it is not 250 ohms.
The connection you describe sounds like a zig-zag connection such as is used to derive single phase power from a 3 phase alternator. Not to be confused with a zig-zag grounding transformer.
The "A" phase and "B" phase PTs form an open delta. 100 volts is developed across the open delta. The "C" phase PT develops 100 volts that is in phase but opposite polarity to the open delta voltage.
You will have 200 volts from PT "A", terminal a to PT "C" terminal a.
As I understand it, your circuit would trip the breaker and disable the "Black start" generator any time there was voltage on the 4160 bus.
respectfully

 

[subtech]

This sounds somewhat like an attempt at creating a "broken delta" connection which is typically used to provide a polarizing quantity for directional ground fault relays.
I attempted to diagram the connections that you described, and if the the connections to the Y winding of the "B" phase pt were to be reversed, you would indeed have a typical broken delta. Your mention of the aux contact on the mini circuit breaker and its connection to the gen brkr makes me believe that the intent to your pt connections was to provide some manner of directional ground fault detection/protection for the gen.

 

[subtech]

Duh! My bad.
Forget my reply ovlac. Re-read your post.
Agree with waross.