Autotransformer Zero sequence current

[anggap]

Hi all,

Recently there was single line to ground fault in my plant, below is the current based on the actual record on related relays:

* TR1 and TR2 LV winding are connected to ground with low resistance NGR (1000A
* Neutral point of generators are connected to ground with high resistance NGR (10A)
* TR3 is single winding autotransformer with no tertiary winding, the neutral point is solid grounded

Based on coordination relay study on easy power, it supposed to be no current flow at feeder 7A Bus B during fault at feeder 1B's downstream. But actually there is zero sequence current flow at that feeder which caused breaker 7B tripped by 50N relay.
note: in simulation I modeled TR-3 with two winding Yyn Transformer

Is it wrong to modeled the transformer (TR3) with two winding Yyn Transformer? because there is no single winding autotransformer in easy power.
I modeled it with two winding Yyn Transformer because the zero sequence circuit are the same.

Please advise

Thanks

 

[waross]

I've checked the old drawing and I found that actually the T1-T6 terminals are delta connected.

That would do it.
That is probably an option for use when the primary does not supply a good neutral ground connection at your site.

A. Delta connection and floating wye point.
B. No delta connection or a broken delta connection and a grounded wye point.
Pick one.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[anggap]

I checked the winding connection, it was delta connected.
What is tertiary winding connection in autotransformer actually used for? And what will be the effect if the delta circuit is changed or modified into broken delta?

 

[davidbeach]

One thing it does is allow the wye winding to become a zero sequence source and trip things unexpectedly... Properly accounted for in the development of the relay settings that zero sequence can be quite useful, but when it comes in from left field it may be less desirable.

 

[itsmoked]

I just gotta say you guys are amazing.

anggap; Well asked and supported.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The delta winding balances the voltages in the event that you must run with the wye point floating.
You may have a delta winding or a grounded wye point, but not both together.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Definitely can be both together. All of the bulk power auto transformers on our system have a delta tertiary and a grounded wye point. That's not a problem when everything is properly modeled and the accounted for. On the other hand, they're on the transmission system which is always much better balanced than the distribution side of things.

 

[waross]

I accept that David.
Another factor that I overlooked.
Both a delta and a grounded wye may be used intentionally to balance an unbalanced line.
On the other hand, I have had to deal with so many problems with grounded wye-delta connections that I tend to forget the transmission side of things.
And despite my aversion to the Wye-delta I once tried unsuccessfully to have a utility install an unloaded wye delta bank to correct very badly unbalanced voltages at a large sawmill.
By the way, how do you account for the back feed when calculating upstream single line to ground faults?
Is the figure of 1/3 of the three phase Available Short Circuit Current valid?
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Everything attached to the transmission system is modeled in sufficient detail that I don’t work in rules of thumb. Fortunately I generally get to pass the messy world of distribution off onto others. But I get into it enough to know that zero sequence sources that aren’t also strong positive sequence sources are abominations. Particularly when they sneak into want should be a non-directional, radial world. Like what happened to the OP.

 

[waross]

Thanks David.
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[anggap]

Thanks all for the answer, I have one more question: What If I remove neutral-ground connection of a single winding autotransformer, can it transform zero sequence current? or it's zero sequence circuit will be open circuit like Yy transformer?

 

[prc]

The zero sequence equivalent circuit of auto transformer can be represented as in attachment. A four terminal circuit. When you open tertiary as broken delta, T terminal will be open, increasing the zero sequence reactance. When you open neutral, O terminal will be opened and entire zero sequence will be out.
Some times it will be necessary to open out the tertiary in service. In a large refinery, we had supplied a dozen of 174 MVA 220/33 KV two winding transformers with tertiary. After further expansion, it was found,33 kV breaker capacity was exceeding. To reduce the LG fault level, tertiary was opened out and one end grounded. Similar situation may arise if you find in a series of transformers, tertiary weak to LG faults and failing. Open out the tertiary.

 

[anggap]

The zero sequence equivalent circuit of auto transformer can be represented as in attachment. A four terminal circuit. When you open tertiary as broken delta, T terminal will be open, increasing the zero sequence reactance. When you open neutral, O terminal will be opened and entire zero sequence will be out.
Some times it will be necessary to open out the tertiary in service. In a large refinery, we had supplied a dozen of 174 MVA 220/33 KV two winding transformers with tertiary. After further expansion, it was found,33 kV breaker capacity was exceeding. To reduce the LG fault level, tertiary was opened out and one end grounded. Similar situation may arise if you find in a series of transformers, tertiary weak to LG faults and failing. Open out the tertiary.

Thanks for your explanation. So, if there is no tertiary winding and neutral terminal isolated to ground, then zero sequence current can't pass through H to L terminal?