Protection Elements - GE Multilin MM200 Quick Start Manual

PROTECTION ELEMENTS

1.3 Protection elements
1.3.1 Thermal protection
16
The primary protective function of the MM200 is the thermal model. The MM200 integrates
stator and rotor heating into a single model. The rate of motor heating is gauged by
measuring the terminal currents. The present value of the accumulated motor heating is
maintained in the Thermal Capacity Used actual value register. When the motor is in
overload, the motor temperature and thermal capacity used will rise. A trip occurs when
the thermal capacity used reaches 100%. When the motor is stopped and is cooling to
ambient, the thermal capacity used decays to zero. If the motor is running normally, the
motor temperature will eventually stabilize at some steady state temperature, and the
thermal capacity used increases or decreases to some corresponding intermediate value,
which accounts for the reduced amount of thermal capacity left to accommodate
transient overloads.
The thermal model consists of four key elements.
• Hot/cold biasing that accounts for normal temperature rise.
• An overload curve that accounts for the rapid heating that occurs during stall,
acceleration, and overload.
• Cooling rate that accounts for heat dissipation.
• Thermal protection reset that controls recovery from thermal trips and lockouts.
Each of these categories are described in the following sub-sections.
1.3.1.1 Overload curve
The overload curve accounts for the rapid motor heating that occurs during stall,
acceleration, and overload. Specifically, the overload curve controls the rate of increase of
Thermal Capacity Used whenever the equivalent motor heating current is greater than
1.01 times the full load current setpoint. The curve is defined by the following equation and
reflects that overload heating largely swamps the cooling, and this heating is primarily due
to resistive losses in the stator and the rotor windings (said losses being proportional to the
square of the current).
=
Trip time
In the above equation,
• The trip time represents the time (in seconds) for the MM200 to trip, given the motor
starts cold and the current is constant.
• The multiplier represents the value of the Curve Multiplier setpoint. This setpoint can
be used to adjust the curve to match the thermal characteristics of the motor.
• Iav represents the equivalent motor heating current in per-unit values on a full load
current base. The value of Iav is limited in this equation to 8.0 to prevent the overload
from acting as an instantaneous element and responding to short circuits.
I
=
AV
Pickup
FLA
´
Curve Multiplier 2.2116623
2
0.02530337 x (Pickup - 1) + 0.05054758 x (Pickup -1)
MM200 MOTOR MANAGEMENT SYSTEM – QUICKSTART GUIDE
QUICKSTART GUIDE
Eq. 1
Eq. 2