Differential relay question

[joshtrevino]

I need help understanding this differential relay problem.

Problem Statement:

A three phase, delta-wye-connected, 30MVA, 33/11kv transformer is protected by a differential relay. Calculate the relay current setting for faults drawing up to 200 percent of the rated current. The CT ratio on the primary side is 500:5, and that on the secondary side is 2000:5.

Problem Solution:

Primary rated current = 30000/(1.73*33) = 524.88 amps; Secondary rated current = 3*(524.88) = 1574.64 amps

CT current on primary = 524.88*(5/500) = 5.249 amps; CT current on secondary = 1574.64*(5/2000)*(1.73) = 6.818 amps

Relay current at 200% of the rated current = 2*(6.818-5.249) = 3.3138 amps

My question: Why is the CT current from the wye connected secondary multiplied by 1.73 (the square root of 3)???

If anything, I would think that we would need to multiply the primary side by square root of 3 because it is a delta connection.

Please help.

 

[cableguy]

In the old school scheme of things (which is the approach these problems take), when you have a delta-wye transformer, you hook your CT's in wye-delta configuration.

See the example that starts on page 329 of this book:

http://www.osinerg.gob.pe/newweb/uploads/G.../TEXTO%207A.pdf

Your sqrt(3) comes in to play because of that.

That's the old school way to do it though. With modern microprocessor relays, it's all Y connected.

Secondary currents in phase on the differential relay could be provided byconnecting the abc set of CTs in wye or in delta with the ABC set of CTs in

delta or wye, respectively. However, connecting the abc CTs in wye would

result in incorrect operation for external ground faults. Zero-sequence current

supplied by the transformer-grounded wye to external faults in the abc system

can flow through the wye-connected abc CTs to the relay-restraint coil

returning through the operating coil. This is because the zero-sequence

current circulates in the transformer delta and does not flow in the ABC

system to provide proper external fault-balancing restraint. Therefore, the

CTs on wye transformer windings should be connected in delta. This provides

a zero-sequence circulating path within the CT connection so that it cannot

flow in the relays.

 

[sd3232]

cablguy, i looked at the ebook u provided, on pages 42,43,43, ANSI devices numbering, ur book says 87 is voltage and power directional relay, i believe whole table is wrong, its weird but the rest of info in the book is good?

 

[cableguy]

Forgot to mention that. YES, there are errors in this book, and the ANSI table is one of them. It is bad. Don't use that part. That was the worst one I found. I found a few smaller formula errors, but they were pretty obvious. But that ANSI device number table is a huge, huge mistake.

Ironically, in the 2nd edition of this book (this is the 3rd), the table was correct.