JUMPER OPENING IN HV SIDE OF TRANSFORMER

[appunni]

Sir,
Consider a 66/11kv transformer (star-star earthed at both side neutrals) in a solidly earthed system. Also consider one 11/0.44kv Delta/star transformer is connected to 11kv side of the 66/11kv transformer. The supply is fed from 66kv side of the 66/11kv transformer and this is stepped down to 440volts by 11/0.44kv transformer for ditribution. The 66/11kv transformer is of core type.
Suppose 'A' phase on 66kv side of the transformer became disconnected by jumper opening of the 'A' phase of the line connected to the transformer. In this situation, I think the flux due to 'B' and 'C' phase currents will flow through limb of the 'A' phase of transformer core. But in the 66kv side, there will be only two phases and the 'A' phase in the 11kv side will carry current due to the flux in 'A' phase core due to the currents in 'B' and 'C' phases in the 66kv. That is, in 11kv side there will be 3 phase currents. But the current in 66kv will increase in order to supply current in 'A' phase in 11kv side. If earth fault relay was connected in 66kv side cts, it will trip if the residual current is above pick up value due to the load in 11kv side.
Am I correct? I want to know whether my analysis is correct. Please help me with your valuable comments.
with regards,
appunni

 

[jghrist]

There will be no zero-seq current in the 11 kV or 66 kV if all load is on the 11-0.44 kV Dyn distribution transformer. All unbalanced load at 0.44 kV becomes pos- and neg-seq at 11 kV. With no zero-seq current in the 66 kV, and phase A open, the sum of phase B and phase C current and flux will be zero, so there will be no flux in phase A and no current.

If you did have zero-seq current in the 11 kV, and the 66-11 kV transformer were of 3-legged core design, there would be no path for the zero-seq flux in the core. It would flow in the air and tank (possibly causing tank heating).

 

[jbartos]

Suggestion to appunni (Electrical) Jun 26, 2003 marked ///\\Consider a 66/11kv transformer (star-star earthed at both side neutrals) in a solidly earthed system.
///This connection propagates the zero sequence current, if it exists, i.e. if the load is unbalanced.\\ Also consider one 11/0.44kv Delta/star transformer is connected to 11kv side of the 66/11kv transformer.
///This connection does not propagate the zero sequence current whether the secondary load is balanced or unbalanced.\\ The supply is fed from 66kv side of the 66/11kv transformer and this is stepped down to 440volts by 11/0.44kv transformer for ditribution. The 66/11kv transformer is of core type.
Suppose 'A' phase on 66kv side of the transformer became disconnected by jumper opening of the 'A' phase of the line connected to the transformer. In this situation, I think the flux due to 'B' and 'C' phase currents will flow through limb of the 'A' phase of transformer core. But in the 66kv side, there will be only two phases and the 'A' phase in the 11kv side will carry current due to the flux in 'A' phase core due to the currents in 'B' and 'C' phases in the 66kv.
///The upstream power supply will see this situation as an unbalanced three phase load. The zero sequence current can materialize if the upstream power supply connection permits it, i.e. grounded y-connection of transformer or generator exists.\\ That is, in 11kv side there will be 3 phase currents.
///If y-connection loses one line, ther will only be one phase voltage and current available downstream.\\ But the current in 66kv will increase in order to supply current in 'A' phase in 11kv side. If earth fault relay was connected in 66kv side cts, it will trip if the residual current is above pick up value due to the load in 11kv side.
///Yes. If will be clear, if you draw a diagram of this situation and formulate mathematical relationships.\\\

 

[appunni]

Thank you to jghrist and jbartos for your valuable replies. I think there will be zero zequence magnetising current in 66kv side if one phase losses. As the magnitude of zero zequence magnetising current is less, the residual current will be less than earth fault pickup setting and therefore, the earth fault relay will not pickup . Am I correct ? Suppose the open phase touches ground, what will be the effect? Please explain.
With regards,
appunni

 

[jghrist]

You're probably right about the magnetizing current, but it won't be more than 3% of full load.

If the open phase touches the ground, sparks will fly because the 11-0.44 kV delta windings connected to the open phase will act as a voltage divider between the intact phases. Voltage on the open phase will be 1/2 the normal voltage.
[tt]
__________ A 6.35 kV <0° |
| C |
| C |
| C_______ B open 3.175 kV <30° | 11 kV <150°
| C |
| C |
| C_______ C 6.35 kV <120° V
| C
| C
| C
|__|
[/tt]

 

[appunni]

Sir, thanks for your reply. But I was telling about jumper opening on 66kv side and not on 11kv side. I request for reply.
with regards,
appunni

 

[jghrist]

The voltage will be present on the 66 kV side as well.