4-24
Protection and Control
BE1-951
for which you are interested. To plot the negative sequence time current characteristics on the same plot
for the ground devices, you need to multiply the pickup value by the multiplier for phase-to-ground faults.
Table 4-16. Fault type Multipliers
Fault Type
Multiplier
Ph-Ph
m = 1.732
Ph-Ph-G
m > 1.732
Ph-G
m = 3
3-Phase
m = Infinity
For example, a down-stream phase 51 element has a pickup of 150 amperes. The up-stream 51Q element
has a pickup of 200 amperes. To check the coordination between these two elements for a phase-to-phase
fault, the phase overcurrent element would be plotted normally with pickup at 150 amperes. The 51Q
element would be shifted to the right by the appropriate factor m. Thus, the characteristic would be plotted
on the coordination graph with pickup at: (200 amperes)*1.732 = 346 amperes.
Generally, for coordination with down-stream phase overcurrent devices, phase-to-phase faults are the most
critical to consider. All other fault types result in an equal or greater shift of the time current characteristic
curve to the right on the plot.
Delta/Wye Transformer Application
Often, the phase relays on the delta side of a delta/wye transformer must provide backup protection for faults
on the wye side. For faults not involving ground, this is not a problem since the phase relays will see 1.0
per unit fault current for three-phase faults and 2/
4
3 (1.15) per unit fault current for phase-to-phase faults.
However, for faults involving ground, the sensitivity is reduced because the zero sequence components are
trapped in the delta not seen by the delta-side phase relays. The phase relays will see only 1/
4
3 (0.577)
per unit current for phase-to-ground faults.
Negative sequence overcurrent protection is immune to the effect caused by the zero sequence trap and
30 degrees phase shift provided by the delta/wye transformer. For a phase-to-ground fault, the magnitude
of the negative sequence components is 1/3 the magnitude of the total fault current. On a per unit basis,
this is true for the fault current on the delta side of the transformer as well. (The previous statement specifies
per unit since the actual magnitudes will be adjusted by the inverse of the voltage ratio of the delta/wye
transformer.) Thus, backup protection for phase-to-ground faults on the wye side of the transformer can be
obtained by using negative sequence overcurrent protection on the delta side with the pickup sensitivity set
at 1/3 per unit of the magnitude of the phase-to-ground fault for which you wish to have backup protection.
Generator Application
Generators have a maximum continuous rating for negative sequence current. This is typically given in
terms of percent of stator rating. When using the 46 time current characteristic curve, the user should
convert the I
2
rating data to actual secondary current at the relay. This value, plus some margin (if
appropriate), should then be entered into the pickup setting. For example, generator ratings of 5A of full-load
current (at the relay terminals) and 10 percent continuous I
2
, converts to 0.25A. Therefore, the minimum
pickup setting for the 46 curve should be set at a value below 0.50A. Continuous (I
2
)2 ratings for generators
are typically in the range of 3 to 15 percent of their full-load current rating.
