Schweitzer Engineering Laboratories SEL-387E, Инструкция по эксплуатации

Страница: 90 / 506

3-36 Differential, Restricted Earth Fault, Date Code 20020129
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
Figure 3.17: 50Pn3 and 50Pn4 Phase Instantaneous O/C Element, Nontorque Controlled
51Pn – Phase Inverse-Time Element
Figure 3.18 shows the logic for the 51Pn element. The logic compares the magnitudes of phase
input currents IAWn, IBWn, and ICWn to pickup setting 51PnP. If one or more current
magnitudes exceed the pickup level, a logical 1 asserts at one input to the AND gate at the center.
The torque-control SEL
OGIC
control equation 51PnTC determines the other AND input. If
51PnTC is true, Relay Word bit 51Pn asserts and the inverse curve begins timing.
Figure 3.18: 51Pn Phase Inverse-Time O/C Element, Torque Controlled
Four settings define an inverse-time curve: the pickup setting, 51PnP, acts as a horizontal scaling
factor, because the curve formula uses current multiple of pickup as an input; the curve setting,
51PnC, defines the particular curve equation, of which there are 10 (five U.S. and five IEC); the
time-dial setting, 51PnTD, defines the time dial, which scales the curve in a vertical direction to
vary the output timing for a given multiple of pickup; and the reset setting, 51PnRS, defines
whether the curve resets slowly like an electromechanical disk or instantaneously when current
drops below pickup. The phase inverse-time curve looks at all three phase current magnitudes
and times on the basis of the greatest current of the three. It updates this maximum phase current
selection every quarter-cycle.