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MDP Digital Time Overcurrent Relay
GE Power Management
1 INTRODUCTION
GEK-100682D
1
1.3 APPLICATION
Time-overcurrent relays are used for the protection of feeders, transmission lines, alternating-current machines
and transformers, and numerous other applications where accurate measurement of current and timing is nec-
essary. To ensure proper coordination with a minimum of circuit isolation, the operating time of associated pro-
tective devices should be considered when selecting a time-current characteristic for a particular application.
Four basic time-current characteristics are available for the MDP relay:
1.
INVERSE / BS142 INVERSE: The inverse time current characteristic curves are shown in the following
diagrams:
•
Figure 2–4: INVERSE TIME CURVE on page 2–5.
•
Figure 2–5: LONG INVERSE TIME CURVE on page 2–6.
•
Figure 2–8: BS142 INVERSE TIME CURVE (1 A MODELS ONLY) on page 2–9.
The BS142 inverse curve applies to the 1 A model relays only. These relays are generally applied where
the short circuit current magnitude is dependent largely upon the system generating capacity at the time of
the fault.
2.
VERY INVERSE: The very inverse time-current characteristic shown in Figure 2–6: VERY INVERSE TIME
CURVE on page 2–7 is generally applied where the magnitude of short circuit current flowing through any
given relay is more dependent upon the location of the fault relative to the relay than on the system gener-
ation setup at the time of the fault.
3.
EXTREMELY INVERSE: The extremely inverse time-current characteristic shown in Figure 2–7:
EXTREMELY INVERSE TIME CURVE on page 2–8 is preferred for applications where sufficient time
delay must be provided to allow a re-energized circuit to pick up an accumulated cold load without unnec-
essary tripping on inrush currents. Distribution feeder circuits are a good example of such applications, and
the extremely-inverse characteristic is best suited to such applications because it more nearly approxi-
mates typical power fuse and fuse cutout characteristics.
The general practice for grounded distribution system protection is to use three-phase overcurrent functions for
protection against interphase faults and a ground/residual overcurrent function for single phase-to-ground
faults. The use of a separate ground-overcurrent function is advantageous because it can generally be
adjusted to provide faster and more sensitive protection for single phase-to-ground faults than the phase over-
current functions. Typical connections for such an application are shown in Figure 4–2: TYPICAL EXTERNAL
CONNECTIONS, GROUND CURRENTS and Figure 4–3: TYPICAL EXTERNAL CONNECTIONS, RESIDUAL
CURRENTS on pages 4–2 and 4–3.
The pickup setting of the MDP time unit should be chosen so that it operates for all short circuits in the pro-
tected zone and, when possible, also provides backup protection for short circuits in the immediately adjacent
system element. The time unit pickup should be set low enough to ensure that the minimum fault current is at
least 1.5 times the setting.
The time delay adjustment of the time unit should be chosen to assure selectivity with the protection on the
adjacent system elements. This adjustment should be made for the condition that yields maximum fault current
at the relay location. The time delay is determined by the adjacent relay operating time for this condition, plus a
coordinating time allowance that includes the adjacent circuit breaker maximum operating time and a safety
factor to accommodate any uncertainties. Since the MDP time unit has insignificant overtravel, the only relay
variation that needs consideration in the safety factor is the tolerance on the time curves. A 0.17 second safety
factor is generally used if the relay time is determined by selecting a time dial setting from the time curves. This
safety factor can be reduced to 0.07 second if the time unit is instead set to the desired time by accurate tests.
