Transformer Curve Shift

[Fuselink]

IEEE 242 requires 15% margin between the main secondary curve characteristics and the primary fuse. I have seen several expressions in different books.
For example some books just consider .58% curve shift to the left for transformer damage curve and do not consider L-L faults. Like (SKM software and related literature)
Some books require the transformer damage curve to be shifted .15 to the right.
In the first case the closest point of secondary breaker to the primary fuse is the corner of STPU.
Now, I understand that fuse needs to protect the transformer in low current faults that’s why the curve is shifted to the left.
But shifting the transformer curve to the right by .15 percent has already covered by shifting it to the left!
Is this seem right?

 

[dpc]

The 17% shift is to allow for coordination between primary and secondary overcurrent devices for a phase-to-phase fault on the secondary.

The 58% shift you are referring to is for check for transformer through-fault protection by the primary protection for a line-to-ground fault on the secondary side.

These are two different issues.

 

[Fuselink]

Then, what this has to do with shifting the xfrm curve to the right. Suppose if my STPU is 2000A I make sure that fuse curve is on the right side of 3400A line.Right?

 

[dpc]

I suppose shifting the transformer curve 17% to the right will show how well the primary protection protects it for a phase-to-phase through-fault on the secondary, but it seems like a moot point, since the three-phase fault is more demanding on the protection in terms of protecting the transformer.

 

[apowerengr]

Two different issues, the 58% is for delta-wye phase shift through the transformer (assuming it's really delta-wye)with the second setting an allowable time margin between the protecting and protected devices to ensure coordination.

 

[Fuselink]

Thanks for all replies. So the second setting is the margine required between the secondary device and transformer curve damage. What do you mean by true delta-wye? I have heard this before….
Regards,

 

[dpc]

No, the 58% shift is to check the ability of the primary device to protect the transformer for a ground fault on the secondary side (on a delta-wye transformer).

Keep in mind the transformer damage curve only applies to through-faults, where the fault is external to the transformer on the secondary side. If the fault is inside the transformer, the damage curve is not relevant, since the transformer is already damaged.

 

[jghrist]

Two different issues, the 58% is for delta-wye phase shift through the transformer (assuming it's really delta-wye)with the second setting an allowable time margin between the protecting and protected devices to ensure coordination.

Shifting curves left or right on log-log curves changes current values by a constant percent. Shifting curves up or down changes time values by a constant percent. Allowable time margins are generally expressed as a constant time such as 0.3 seconds, not a percent of time.

 

[apowerengr]

Fuselink - my comment was intended to clarify dpc's first response regarding the 58% shift - this is applicable to delta-wye transformers and line-to-ground faults not wye-wye. Since your question did not state what winding configuration was at question or if your question related to a specific winding configuration this seemed to need clarification. As jghrist points out shifting in time and specifying allowable time margins is done with a specific value not a percent. When performing coordination calculations for transformer protection you need to look at the correct devices (primary and secondary if applicable), shifted based on the fault being studied and the transfomrer winding configuration, with your evaluation criteria being the required time margin between the devices to reasonably guarantee "coordination", i.e. the closest device interrupts the fault without affecting the upstream device. Some engineers use different time margins based on the characteristics of the devices being compared, some based on the fault type and probability. You might search the topic of time margins or start a thread asking the "what time margin do you use?" question and see the variety of answers.

 

[Humble2000]

apowerengr ,You are right, I did not mention the transformer config. , its Delta-wye.
I was aware f the 58% curve shift to the left to consider L-G fault. My question was relating 16%-17% curve shift to the right I didn’t know the purpose behind this.
Here dpc, and others suggest that that is required margin (current) , between the secondary protection device and primary protection device. I accepted it; however the curve shift was confusing, because 17% shift would not make that much of a difference in coordination to pick a bigger size fuse and therefore create more margin, so I just ignored the concept of curve shift and stick to the necessary 17% margin between primary and secondary protective curves