Basler Electric BE1-51/27R - Manual (Page 61)

9137200999 Rev F 55
INST 2 Control Setting
For this adjustment, repeat INST 1 Control Setting, but substitute instantaneous 2 output terminals (11
and 12) and the front panel INST 2 controls. Because neutral sensing does not apply to this pickup point,
ignore Steps 9 through 15.
Setting the Relay
General
Phase time overcurrent elements (BE1-51/27R relays) can be coordinated with external protection in a
conventional manner by assuming that the pickup current is 25 percent of nominal. This is the case when
the restraint voltage is 25 percent or less. When the restraint voltage exceeds 25 percent, pickup current
exceeds 25 percent of nominal and the relay is slower than if pickup current was fixed at 25 percent.
Assuming a fixed 25 percent pickup simplifies calculations and provides an added safety margin because
the BE1-51/27R relay is the last step in the coordination for an external fault.
Neutral (ground) time overcurrent and all instantaneous overcurrent functions operate independently of
the voltage control circuits. Therefore, these elements (overcurrent functions) can be set in the normal
manner. They are set independently of each other except that the instantaneous setting is 1 to 40 times
the associated time overcurrent element pickup. The instantaneous element pickup should be set for at
least 120 percent of the maximum current that can be seen by the relay for an external fault where
operation is not desired.
Neutral time overcurrent pickup must be set above the maximum expected normal unbalance. In addition,
for residually connected elements, added relay current can result from dissimilar errors in the three
current transformers. These errors are greater for the lower accuracy class current transformers. A
neutral element pickup of ten percent of the circuit rating will ordinarily be above non-fault unbalances for
a solidly grounded system. Lower settings are appropriate and safe for impedance grounded systems or
for ungrounded protected equipment connections.
When choosing the time delay setting in a generator back-up application, for a motor starting application,
or during acceleration after a fault is cleared, the limiting condition may be the percentage of generator
rating that is motor load and not coordination for an external fault. The percentage of generator rating that
is motor load needs to be considered when setting the relay.
A nominal phase time overcurrent pickup (with rated restraint voltage) of 200 percent of generator rating
can prevent undesirable tripping during a severe recoverable swing. This pickup is still low enough to
provide sufficient sensitivity for faults.
Relay Setting Concepts
Figure 35 plots fault current in multiples of generator rating (assuming no voltage regulator boosting) and
relay pickup as a percent of nominal. The relay tap must be selected so that pickup in multiples of
generator rating is less than the fault current shown in the Figure 35 example.
Restraint voltage (V) is proportional to the drop across external reactance (X
E
), and decays along with the
fault current (I) that is developed by the generator. Initial voltage is 40 percent of rated, so the phase time
overcurrent pickup is also 40 percent of nominal. At about 0.04 seconds after fault inception, restraint
voltage drops below 25 percent and the pickup current flattens to 25 percent. During the shaded portion
of the graph, the relay operates slower than it would if the pickup current was constant at 25 percent of
nominal. This occurs for a minor interval compared to the total relay operating time, so the increased
pickup has a negligible effect on operating time.
BE1-51/27R Tests and Adjustments