Schweitzer Engineering Laboratories SEL-701, Instruction Manual

Page: 377 / 468

Date Code 20010719
701 Motor Protection Relay
Appendix E
Motor Thermal Element
Introduction
The SEL-701 Relay provides effective motor thermal protection using a patented
protection algorithm. The relay offers three convenient methods to set the thermal
element. They are:
Motor ratings method.
Generic thermal limit curve method.
User thermal limit curve method.
These setting methods are described in detail in Section 4: Settings Calculation .
The three methods accommodate differences in protected motors and in the amount
and type of motor information available. They also offer the majority of relay users at
least one familiar setting method.
While the implementation details of each setting method vary, the fundamental
thermal element is the same for all three, so this generalized discussion applies to all
three methods equally. Regardless of the setting method used, the thermal element
provides motor protection for the following potentially damaging conditions:
Locked Rotor Starts.
Running Overload.
Operation Under Unbalanced Currents.
Too Frequent or Prolonged Starting.
In order to provide integrated protection for all these conditions, the thermal
element:
Continuously maintains a numeric estimate analogous to the heat
energy in the motor.
Adjusts the heat estimate based on the measured positive-sequence
and negative-sequence current flowing in the motor.
Weights the heating effect of negative-sequence current as five times
the heating effect of positive-sequence current when the motor is
running.
Weights the heating effects of positive- and negative-sequence current
equally when the motor is starting.