GE CEY51A Instructions Manual page 12

GEK-I27S
Underreach
At reduced voltage the ohmic value at which the mho unit will operate may be
somewhat lower than the calculated value. This "pullback" or reduction in reach is
shown in Figure 2. The unit reach in percent of setting is plotted against the
three-phase fault current for three ohmic-reach tap settings. Note that the fault-
current scale changes with the basic-minimum-reach setting. The mho unit will
operate for all points to the right of the curve. The steady-state curves of
Figure 2 were determined by tests performed with no voltage supplied to the relay
before the fault was applied. The dynamic curves were obtained with full-rated
voltage of 120 volts supplied to the relay before the fault was applied. See Figure
2, Steady-State and Dynamic-Reach Curves for the MHO Unit of the CEY51A Relay.
Memory Action
The dynamic curves of Figure 2 ill ustrate the effect of memory action in the
mho unit, which maintains the polarizing flux for a few cycles following the
inception of the fault. This memory action is particularly effective at low voltage
levels, where it enables the mho unit to operate for low-fault currents. This can
be most forcefully illustrated for a zero-voltage fault by referring to Figure 2. A
zero-vol tage faul tis
1
i kely to be ri ght at the rel ay bus and therefore, to protect
for this faul t, it is imperative that the relay reach zero percent of its setting.
Figure 2 shows that the mho unit, under static conditions, will not see a fault at
0% (zero percent) of the relay setting, regardless of the tap setting. However,
under dynamic conditions when the memory action is effective, Figure 2 shows that an
mho unit with a 3-ohm basic-minimum-reach and 100% tap setting will operate if 130
is greater than 1.5 amperes.
Transient Overreach
The operation of the mho unit under transient conditions at the inception of a
fault is important because the relay is normally connected so that the mho contacts
will trip a circuit breaker independently of any other contacts. The impedance
characteristic of Figure 6 and the steady-state curves of Figure 2 represent steady-
state conditions. If the fault current contains a DC transient, the unit may close
its contacts momentarily even though the impedance being measured is slightly
greater than the calculated steady-state reach. This overreaching tendency will be
at a maximum when a fault occurs at the one instant in either half-cycle that
produces the maximum DC offset of the current wave. The maximum transient overreach
of the mho unit will not exceed 6% of the steady-state reach for line angles up to
85
0 .
Operating Time
The operating time of the mho unit is determined by a number of factors such as
the basic-minimum-reach setting of the unit, fault-current magnitude, ratio of fault
impedance to relay reach, and magnitude of relay vol tage prior to the faul t. The
curves in Figure 7 are for the condition of rated volts prior to the fault. Time
curves are given for four ratios of fault impedance to relay-reach setting. In all
cases, the mho taps were in the 100% position and the angle of maximum torque was
set a t 60
0
1 a g •
See Figure 7, Operating-time Curves for the Mho Unit in the CEY51A Relay.
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