ABB Relion 670 series - Page 589

t_MinTripDelay
tCutOff
t_MaxTripDelay
IPickupIBase
t (s)
I (A)
1
2
IBase
I
TD1
tr
t
ANSI12000009-1-en.vsd
ANSI12000009 V1 EN-US
Figure 312: Operating characteristic for overload function
As shown in Figure 312 it is possible to define the maximum ( t_MaxTripDelay ) and minimum
(
t_MinTripDelay ) operate time for the function regardless of the level of the measured current. In
addition to this it is also possible to specify the cut-off time below which the IDMT curve defined
by the above equation will be used. Note that tCutOff shall be always set to the smaller value than
t_MaxTripDelay .
The trip time of the stator overload function is calculated by using the integration principle (see
Chapter “Inverse time characteristics” in the Technical Manual for more information). As a service
value from the stator overload function regarding this integration process the Theta parameter,
proportional to the stator heating content, is provided. This parameter has value of 100% when
function gives the trip signal. Thus, in simple words, it represent a percentage of the function trip
level. As soon as the measured current is above the pick-up level
IPickup and no block signal is
present, the output signal PICKUP is activated and the Theta parameter is incremented following
the inverse characteristic given in Figure312. When Theta has reached value 100%, the output
signal TRIP is set. The minimum trip signal duration is defined by the parameter
tPulse . Note also
that the 79M BLOCK signal will remain one as long as Theta > ReclsLevTheta . Thus the parameter
ReclsLevTheta represents the Theta value below which is safe again to re-connect the tripped
generator to the network.
The output signal PICKUP will reset if the measured current falls below reset level or if BLOCK
signal is set to one. However the Theta value is stored in the IED memory and it starts to decay
slowly following a linear reset characteristic defined by the parameter
tReset. The parameter
tReset actually defines the rest time for Theta from value 100 to zero. Note that if an overload
condition happens again, the last Theta value is used as a starting point for the calculation. Thus
the stator winding will be properly protected against varying current levels.
1MRK 502 066-UUS B Section 8
Current protection
583
Technical manual