MNSPTOC supports both DT and IDMT characteristics. The DT timer characteristics can be selected with "ANSI Def. Time" or "IEC Def. Time" in the Operating curve type setting. The functionality is identical in both cases. When the DT characteristics are selected, the functionality is only affected by the Operate delay time and Reset delay time settings.
The protection relay provides two user-programmable IDMT characteristics curves, "Inv. curve A" and "Inv. curve B".
Current-based inverse definite minimum time curve (IDMT)
In inverse-time modes, the operate time depends on the momentary value of the current: the higher the current, the shorter the operate time. The operate time calculation or integration starts immediately when the current exceeds the set Start value and the START output is activated.
The OPERATE output of the component is activated when the cumulative sum of the integrator calculating the overcurrent situation exceeds the value set by the inverse time mode. The set value depends on the selected curve type and the setting values used.
The Minimum operate time and Maximum operate time settings define the minimum operate time and maximum operate time possible for the IDMT mode. For setting these parameters, a careful study of the particular IDMT curves is recommended.
Inv. curve A
The inverse time equation for curve type A is:
t[s] | Operate time in seconds |
k | Set Machine time Mult |
I 2 | Negative-sequence current |
I r | Set Rated current |
If the negative sequence current drops below the Start value setting, the reset time is defined as:
t[s] | Reset time in seconds |
a | set Cooling time |
b | percentage of start time elapse ( START_DUR) |
When the reset period is initiated, the time for which START has been active is saved. If the fault reoccurs, that is, the negative-sequence current rises above the set value during the reset period, the operate calculations are continued using the saved values. If the reset period elapses without a fault being detected, the operate timer is reset and the saved values of start time and integration are cleared.
Inv. curve B
The inverse time equation for curve type B is:
t[s] | Operate time in seconds |
k | Machine time Mult |
I 2 | Negative-sequence current |
I S | Set Start value |
I r | Set Rated current |
If the fault disappears, the negative-sequence current drops below the Start value setting and the START output is deactivated. The function does not reset instantaneously. Resetting depends on the equation or the Cooling time setting.
- When the negative sequence current drops below start value the subtraction in the denominator becomes negative and the cumulative sum starts to decrease. The decrease in the sum indicates the cooling of the machine and the cooling speed depends on the value of the negative-sequence current. If the sum reaches zero without a fault being detected, the accumulation stops and the timer is reset.
- If the reset time set through the Cooling time setting elapses without a fault being detected, the timer is reset.
The reset period thus continues for a time equal to the Cooling time setting or until the operate time decreases to zero, whichever is less.