SEL SEL-387 Current Differential and Overcurrent Relay

times the TAP setting. This pickup setting is only

exceeded for internal faults.

Restricted Earth-Fault Protection

Apply the REF protection feature to provide sensitive

detection of internal ground faults on grounded wye-con

nected transformer windings and autotransformers. Only

one of the three ABC inputs of Winding 4 is used for

introduction of neutral CT polarizing current. Operating

current is derived from the residual current calculated for

the protected winding. A directional element determines

whether the fault is internal or external. Zero-sequence

current thresholds and selectable CT saturation logic

supervise tripping. One of the Winding 4 inputs is used

for the neutral CT, so a maximum of three winding

inputs may be used for normal differential and overcur

rent protection.

Overcurrent Protection

The SEL-387 has 11 overcurrent elements for each set of

3-phase current inputs, 44 elements total. Nine torque

controlled elements encompass one instantaneous, one

definite-time, and one inverse-time element for each

phase of the negative-sequence and residual currents.

The phase elements operate on the maximum of the

phase currents. The remaining two elements are phase

segregated to assist in phase identification for targeting

purposes or for level sensing type functions. These are

not torque controlled.

Two sets of combined overcurrent elements operate on

the vector sum of the Winding 1 and Winding 2 currents

and the vector sum of the Winding 3 and Winding 4

currents, respectively. The individual currents are scaled

by the appropriate ratio so that the combined current

accurately reflects the primary system current. Inverse

time phase and residual elements are available for each

of the combined currents. These combined elements

offer added flexibility when the relay is applied with

multiple breakers, such as in ring-bus or breaker-and-a

half applications. Different CT ratios are permitted on

the two windings that are summed to create the

combined current. For ideal CTs, these combined

overcurrent functions do not respond to circulating

current passing through both circuit breakers.

The time-overcurrent curves listed in Table 1 have two

reset characteristic choices for each time-overcurrent

element. One choice resets the elements if current drops

below pickup for at least one cycle. The other choice

emulates reset of an electromechanical induction disk

relay