GE P645

3 PHASE OVERCURRENT PROTECTION
Phase current faults are faults where fault current flows between two or more phases of a power system. The fault
curr
ent may be between the phase conductors only or, between two or more phase conductors and earth.
Although not as common as earth faults (single phase to earth), phase faults are typically more severe.
3.1 PHASE OVERCURRENT PROTECTION FOR POWER TRANSFORMERS
A fault external to a power transformer can result in damage to the transformer. If the fault is not cleared
pr
omptly, the resulting overload on the transformer can cause severe overheating and failure. Overcurrent
elements may be used to clear the transformer from a faulted bus or line before the transformer is damaged.
For faults internal to the transformer, the overcurrent protection is less effective because sensitive settings and
fast operation times are usually not possible.
Sensitive settings are not possible because the pickup should allow overloading of the transformer when required.
Fast operating times are not possible because of the grading required with respect to downstream overcurrent
relays.
3.2 PHASE OVERCURRENT PROTECTION IMPLEMENTATION
The product provides three overcurrent elements for backup phase overcurrent protection. Each element provides
four
-stages of non-directional or directional three-phase overcurrent protection with independent time delay
characteristics. You can select the overcurrent element operating quantity for each of the elements with the
setting cells Overcurrent 1, Overcurrent 2 and Overcurrent 3 in the OVERCURRENT column of the relevant settings
group. You can set each element as T1, T2, T3, T4, T5, HV winding, LV winding or TV winding. The HV, LV
and TV windings comprise the vector sums of the CT inputs assigned to that particular winding.
All overcurrent and directional settings apply to all three phases but are independent for each of the four stages.
You may set the overcurrent element as directional only if the three phase VT input is available in the P643/5 and if
the two single phase VT inputs are available in the P642. You may assign the VT to either of the HV, LV or TV
windings. Therefore, overcurrent directional elements are available for the current inputs associated with the
winding that has the VT input assigned.
The first two stages of overcurrent protection have time-delayed characteristics which you can set as IDMT or DT.
The third and fourth stages have definite time characteristics only.
Stages 1, 2 provide a choice of operate and reset characteristics, where you can select between:
● A range of standard IDMT (Inverse Definite Minimum Time) curves
● DT (Definite Time)
This is achieved using the cells
● I>(n) Function for the overcurrent operate characteristic
● I>(n) Reset Char for the overcurrent reset characteristic (IEEE curves only)
where (n) is the number of the stage.
The IDMT-capable stages (1 and 2) also provide a Timer Hold facility. This is configured using the cells I>(n) tReset,
where (n) is the number of the stage. This is not applicable for curves based on the IEEE standard.
Note:
Stages 3 and 4 can have definite time characteristics only.
Chapter 9 - Current Protection Functions P64x
202 P64x-TM-EN-1.3