Delta wye transfomer during a grounded phase 5

[Mbrooke]

What happens when one or two phases fault to the MGN (multi grounded neutral) of a distribution system with the effected phase fuse blowing (but still grounded) while feeding a delta wye transformer? Will any inductive tank heating take place? Will a 3 limb vs 5 limb core make a difference?


Basically I am entertaining the idea of using a delta wye padmount instead of a conventional 5 limb wye grounded wye grounded, however I am unsure if certain fault scenarios create special protection concerns. Both the over head feeder and underground riser are protected via fuses, so single phasing and shorted phasing is a concern.

 

[Mbrooke]

@waross: X2, this has also been on my mind the whole time.

In so far I'd like to thank everyone for the replies. I've learned a lot in this thread and still am.

 

[jghrist]

Line to line quantities on the delta would never be affected by zero sequence because zero sequence doesn't produce a voltage differential across each winding.

The zero-sequence primary voltage results from the unbalance caused by V_Bn going to zero because of the fault. V_An = V_AB and V_Cn = V_CB.

The thing when I looked over your calculations that I had never thought of before is that the type of load I think could dictate what happens on the secondary of the transformer. The load that you used in your calcs was 100 A load in phase with the voltage on each of the phases, a constant current load, probably single phase. Zero sequence current on the secondary is generated by the change in angle of the secondary voltages and zero sequence current is produced due to the currents not adding up to zero. If you had just a passive load, I don't think you would have seen any zero sequence currents on the secondary because the secondary voltages with just positive and negative voltages would have only produced positive and negative sequence currents. I suppose you might see something similar with constant power loads. I suppose this isn't profound or anything but I guess the type of load you have could determine if you should care what is happening on the high side in respect to tank heating.

The secondary voltage depends only on the primary winding voltages. I have neglected any voltage drop in the transformer winding from load current. There is no zero-sequence secondary voltage because V_AB + V_BC + V_CA= 0. There can be zero-sequence secondary current regardless of the secondary voltage. Consider if there is only load connected from one secondary phase to neutral. The zero-sequence current would be 1/3 of the load current.

 

[HamburgerHelper]

Waross,

Wouldn't you just include some grounding impedance if you were worried about back feeding fault current?

 

[waross]

We are discussing a multiple grounded neutral system.

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

 

[prc]

I have a query with reference to the Westinghouse paper of 1978 posted earlier. It is said (slide 15) when there is a break on A phase primary, Vo will be applied on primary and this cause zero sequence flux resulting in tank heating. Since primary neutral is solidly grounded physically there is no voltage on the A winding and limb of the core. What ever flux generated in B &C limbs can return through the flux empty C limb of the core. Then how zero sequence flux can go to tank to create heating?

 

[Mbrooke]

The whole "no voltage" term is confusing me. While it may be correct; grounded or not, in either delta or wye it forms some type of circuit. In wye its just a shorted winding while the others remain energized and in delta the other phase is feeding that winding.

In fact in wye, I would be concerned about back feeding load heating the winding itself from being fully shorted out. I have never though about this, but it now just ran through my mind.

 

[prc]

As per slide 15,in YN winding A phase is shorted to neutral and no connection to incoming supply as fuse opened out. Hence my view no voltage and flux in limb A.

 

[Mbrooke]

But what about load back feeding it? Think a motor or lots of delta loads. The shorted primary would produce a very low impedance on the secondary allowing back fed current from the load to flow easily.

 

[HamburgerHelper]

PRC,

I think the confusion is due to what you think V0 is. V0 is the same magnitude and angle on all three phases. Whatever is on the third leg that is unconnected will be come mix of zero, negative, and positive sequence flux. The zero sequence flux by definition will be the same in each leg and it can't return through the open or faulted phased leg. So, there will be flux that passes in the third leg but it won't be just the zero sequence flux. The voltage on phase A can be zero, but that is due to all the sequence components adding up to zero for that phase. It is just like how a single line to ground fault has I1=I0=I2 but current is only present on one of those phases. The sequence components in the unfaulted phases add to zero so you have what you expected, only current in the faulted phase.

 

[waross]

Realistically given the great number of delta/wye transformers in service I don't anticipate a problem.
There are problems with a wye/delta connection when the wye neutral is connected.
This may be mitigated by floating the neutral.
Back feeding will be primarily from motor contributions that do not have a neutral connection so the wye/delta issues will be mitigated.
As to secondary voltages during a fault.
With one phase open the two windings connected to the open phase will be in series across line voltage. The voltage division will be determined by the loads on the individual transformers.
With one phase faulted to ground on an ungrounded system the result will be the same.
With one phase faulted to the neutral, the voltages across the windings connected to the faulted phase will drop from line to line to line to neutral voltage.
The phase angles will shift. With B phase faulted the winding at A-B will become A-N and the winding at B-C will become N-C.

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

 

[Mbrooke]

@waross, thanks for the explanation. Even though two limbs are phase shifted there is no actual concern and the transformer can run like this for hours? Reason I keep thinking this is that normally on a 3 limb core everything is 120* apart and thus all net flux is equal to zero. But with two limbs now phase shifted by (30?) degrees I can't see the net flux equaling zero. Unless my understanding of 3 phase transformer theory is off?

 

[Mbrooke]

@Jghrist: what program did you run that simulation through? Or is just hand calculated? Just curious, I'd like to run it on that same program if possible.

 

[jghrist]

Mathcad

 

[Mbrooke]

Thanks

 

[Mbrooke]

On page 47, back feed can still occur on a wye-wye shared core transformer, even without ferroresonance and load back feed?

https://www.ieee.li/pdf/viewgraphs/basic_distribution_engineering_utility_system-operations.pdf

 

[waross]

That's the phantom delta on a three legged core.

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

 

[Mbrooke]

Thanks- and a 5 limb Y-Y will not back feed-right?

 

[waross]

I don't believe so. The 5 limb core eliminates the phantom delta. I am not aware of any other type of back feed.
A delta fed from a wye with the neutral connected has the ability to transfer power from one or two phases to the third phase.
The phase windings in a grounded wye are independent of each other, unless the phantom delta is present.

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