Voltage 11 kV
Starting D.O.L
For this application we have assumed the machine is a CMR motor so the Ith setting is calculated as follows:
I
th
= I
CMR
x (1/CT Ratio)
Wher
e:
I
CMR
= Continuous Motor Rating
Therefore: I
th
= 293 x 1/300 = 0.976 In
Therefore set: I
th
= 0.98 In
2.5.1.2 K COEFFICIENT
The constant K Coefficient is used to incr
ease the influence of negative sequence current on the thermal replica.
This factor should be set equal to the ratio of negative phase sequence, rotor resistance to positive sequence
resistance at rated speed. When an exact setting cannot be calculated, a default setting of 3 should be used. This
is a typical setting and is sufficient for the majority of applications.
Therefore set: K Coefficient = 3
2.5.1.3 THERMAL TIME CONSTANTS
The relay heating and cooling time constants are set in accordance with the stator thermal heating and cooling
time constants. The r
elay heating time constant (Thermal Const T1) should be set equal to, or as close to, the
stator heating thermal time constant, which is obtainable from the motor manufacturer. It is good practice to set
T1 slightly less than the stator heating thermal time constant to allow for relay tolerances. However, this is not
always necessary as the stator thermal time constants provided by the motor manufacturer are usually
conservative.
Thermal Const T2 is automatically applied above 2 I
th
and is used to modify the relay thermal curve during
starting for certain applications. For example, where star/delta starters are being utilized. During normal running,
with the motor connected in delta, the current in the motor winding is only 57% of that monitored by the relay.
However, during starting, with the motor connected in star, the current monitored by the relay is equal to the
current in the motor winding. For this reason, T2 can be used to reduce the operating time of the relay during
starting. For applications where direct on line (DOL) starting is utilized, T2 should be set equal to T1, resulting in one
continuous thermal curve.
It is important to plot the chosen thermal characteristics on a time-current graph to ensure that the cold curve
does not intersect the starting characteristic.
In certain applications the thermal time constants are not available. However, a graphical representation of these
values may be given. In such applications, a stator heating time constant must be chosen, so that when plotted on
a time-current graph, it closely matches the cold withstand curve of the motor.
Cooling Const Tr is the cooling time constant. This setting is important for cyclic operation of the motor, since
precise information of the thermal state of the motor is required during heating and cooling. It is set as a multiple
of T1 and should be set to the nearest value above the motor cooling time constant.
Setting Example:
For this application the stator thermal time constants of the motor are known so the required time constant
settings are as follows:
Therefore set: T1 = 20 minutes
T2 = T1 since machine is DOL starting
Tr to 5 x T1 = 100 mins.
These settings are shown in the figure below.
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 79
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