GE 869 Instruction Manual page 266

PROTECTION
4–134
BLOCK (THERMAL MODEL BLOCK)
Range: FlexLogic operand
Default: Off
The thermal model can be blocked by any asserted FlexLogic operand. While the
blocking signal is applied, the element remains running and updates the thermal
memory, but the states of the Thermal Trip OP and Thermal Alarm OP operands will
reset. When the element blocking signal is removed, the element logic is based on the
new value of the thermal capacity and updates the status of the Thermal Trip OP and
Thermal Alarm OP operands.
The following procedure, along with the previous figure, illustrates the construction of
the voltage overload curves.
1. Draw a curve for the running overload thermal limit. The curve is one that has been
selected in the relay as an Overload Curve.
2. Determine the point of intersection between the Overload Curve and the vertical line
corresponding to the per-unit current value of VD Accel. Intersect @ MIN V (see point
2).
3. Determine the locked rotor thermal limit point for the minimum voltage motor start.
The coordinates of this point are the per-unit current value of VD Stall Current @ Min
Volts and the time value of VD Safe Stall Time @ Min V (see point 1).
4. The line connecting points 1 and 2 constructs the acceleration curve for the system
voltage level defined by the VD Min Motor Volts setting. The acceleration time-current
curve for the minimum voltage starting is calculated from the following equation:
– I is a variable multiplier of the motor rated current (values between I
– I is a multiplier of the rated motor current (FLA) specified by the VD Stall Current
1
@ Min V setting,
– t is a time value specified by the VD Safe Stall Time @ Min V setting,
1
– I is a multiplier of the rated motor current (FLA) specified by the VD Accel.
2
Intersect @ Min V setting, and
– t is a time coordinate of the intersection point between the thermal model curve
2
and the vertical line corresponding to the per-unit current value of the VD Accel.
Intersect @ Min V setting.
5. Determine the point of intersection between the Overload Curve and the vertical line
corresponding to the multiplier of the rated current value of the VD Accel. Intersect @
100% V setting (see point 4).
6. Draw the locked rotor thermal limit point for the 100% voltage motor start. The
coordinates of this point are the multiplier of the rated current value (FLA) of the VD
Stall Current @ 100% V setting and the time value of the VD Safe Stall Time @ 100% V
setting (see point 3).
7. The line connecting points 3 and 4 constructs the acceleration curve for the motor
rated system voltage. The acceleration time-current curve for the rated voltage
starting is calculated from the same equations, but the setpoints associated with the
100% voltage starting are applied.
8. The line connecting points 1, 3 and 5 represent the motor safe stall conditions for any
system voltage from the minimum to 110% of rated. Ideally, all the points on this line
are characterized by the same thermal limit (I
impedance at reduced voltage is greater than the impedance at full voltage. As such,
the higher terminal voltages tend to reduce I
2
t), but the equivalent starting
2 2
t. The rate of I t reduction is dictated by
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
Eq. 25
and I ),
1 2