Loosing a phase of a 3-phase system 2

[rockman7892]

If I have a 4.16kV system that is stepped down through a 4.16kV Delta to 480V Y transformer what happens when I blow a fuse on the 4.16kV system?

Obviously I know that all motors on the 480V system (MCC) will single phase and I understand the effects of single phasing. What will be some of the other adverse effects that would occur besides single phasing? Obviously smaller 3-phase 120/208 transformers would be effected as well.

I read somewhere once that when a phase is lost the single phasing motors somehow act to create this missing phase when a fuse blows. I cannot seem to remember where I read this.

 

[jghrist]

The voltage on two secondary phases will be 50% of nominal and will be in phase, not 120° out of phase. They will be 180° out of phase with the third phase. If you draw out the delta-wye phasors you can see this. The open primary bushing will have a voltage halfway between the two intact phases.

This is if there is no load. Connected motors may change the voltage somewhat because of back emf.

 

[rockman7892]

I tried to draw out the phasors as best as I could see them. Case I attached is the normal phasors for all three phases during normal operation and Case II is the phasors for and open ciruict on one of the primary phases. Phase A is open in these examples.

I am assuming that the 180deg phase difference is due to the fact that on the primary since phase A is open then this just becomes a series circuit from phase C throgh the junction at A and back to B. AB and CA are therefore in phase with each other ane are the equivelent of a CB phasor. This CB phasor is 180deg offset from the BC phasor which is at 4.16kV.

On the secondary wye the phasor relationships would be reflected from the primary. Phases A-N and C-N would be in phase with each other at a magnitude of 138V and at an angle of 0deg. Phase B-N would be 277V and would be at and angle of -180deg reflected from the primary.

Am I looking at this correctly?

 

[jghrist]

Am I looking at this correctly?

You pretty much have the right idea. The only problem is the phase relationship between primary and secondary. You need to rotate the secondary 90°. The 30° phase shift between primary and secondary is between Ø-N voltages. You are showing Ø-Ø voltages on the primary; these should be in phase with the secondary Ø-N voltages.

 

[lefty51]

I do know that for 3 phase motors, if you pop a fuse,
( or lose a phase....same difference to the motor ) the motor may continue to run, providing its not loaded up. If its loaded up, it will trip eventually.

 

[rockman7892]

jghirst

I am not following exactly what you mean by rotating the secondary by 90deg? Why would there not be a 30deg phase shift between L-L and L-N voltages?

So from looking at this in the case of a motor if you go take voltage measurments at the motor you will not read 0V L-G on any of the leads but will read 138V and 277V respectivaly. However these are only seperated by 180deg so this is still a single phase case.

If the open circuit were to happen on the 480V side we would read 0V on the open circuit motor lead L-G however we would still have a single phase situation. So the only difference betwwen an open circuit on the primary of a transformer and the secondary are the voltage readings you will see at the loads. The same will apply for load transformers.

 

[automatic2]

I would contend that the missing leg would become the sum of the remaining phasors (in the primary delta). This would leave two intact phasors at 120 seperation, with the third (missing) phasor attaining a 60 degree mippoint relation. Your Y secondary would demonstrate balanced voltage untill current was drawn, upon which your neural should see large unbalanced currents.

 

[davidbeach]

Sorry, that's not how it works. If there is a missing phase on the delta, you have one winding connected phase-phase and two windings connected in series phase-phase, in parallel to the other winding. The only phase angles available are 0[°] and 180[°].

 

[rockman7892]

Would the secondary L-N voltages be 138V, 138V and 277V as I mentioned above?

 

[jghrist]

There is not a phase shift between the primary Ø-Ø voltage and the secondary Ø-N voltage because the primary winding, which is connected across the Ø-Ø voltage, is directly coupled to the Ø-N connected secondary winding.

You are correct about the secondary Ø-N voltages being 138V, 138V, and 277V.

 

[waross]

The motors will act as induction generators. Actually, that is what causes the back EMF in an induction motor. The one good phase will provide excitation and torque. The motors will continue to run on one phase as if they were single phase motors. The induction generator effect will transfer energy back into the missing phase. But, with only one good phase the available HP of the motors will be about 1/3 to 1/2 of normal. What happens next depends on the mechanical load on the motors and the electrical load still connected downstream of the missing phase.
Usually one motor will trip out first and through more load on the other motors, which will quickly follow suit and also trip out. After all motors have tripped off-line, the system will revert to the single phase and 50% voltages described above.
Another possibility is that the load that the motors, acting as induction generators, are trying to support is so large in relation to the motor capacities that voltage drop drops out the motor controls almost immediately.

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

 

[prc]

Let me raise a related issue.A 100 MVA 220/132 kV auto transformer is feeding to a 50 MVA 132/33 KV unit.One phase of 220 Kv opening out due to breaker problem>But system continues to work.What will be the nature of problems faced by transformers?Will there be zerosequence current loading on trfs?

 

[marks1080]

I think the short answer would be Yes in regards to zero sequence current. I think zero sequence current on the secondary of the 2nd transformer would depend on how it was (or wasn't) grounded.

 

[jghrist]

Let me raise a related issue.A 100 MVA 220/132 kV auto transformer is feeding to a 50 MVA 132/33 KV unit.One phase of 220 Kv opening out due to breaker problem>But system continues to work.What will be the nature of problems faced by transformers?Will there be zerosequence current loading on trfs?

Depends on the load and the connection of the 132/33 kV transformer. If all loads are stepped down through delta-wye transformers, then there will be no zero-sequence current. If the 132/33 kV transformer is delta-wye, then the secondary Ø-N voltage will be 19 kV on one phase and 9.5 kV on the other two phases.

 

[rockman7892]

Does zero zequence current not transfer through a Delta Wye transformer?

 

[jghrist]

Does zero zequence current not transfer through a Delta Wye transformer?

No, because the delta winding has no ground/neutral connection.

 

[hdp]

On a bit of a side note, we recently experienced a primary phase loss on a delta-wye 12.47kV/480Y277V service. The building lighting is all 277V. All the lighting continued to operate - the ballasts are MVOLT; ie., they will function on all voltages from 120 through 277. Although operating fine at roughly 135V, you can imagine what the branch circuit loads were doing. After about 10 minutes, we decided to open the main and subsist on generator powered standby circuits.

 

[prc]

I am sorry.Both 100 MVA and 50 MVA units are star/star with solidly grounded neutrals.When one phase was opened out, 33 kV line REF operated after some time and transformer Bucholtz relays operated on alarm.