ABB eVM1 Installation And Service Instructions Manual page 88

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14.7.3.5.1 Measurement mode
The Unbalanced load protection function evaluates the measured amount of negative phase
sequence current at the fundamental frequency.
The negative-sequence three phase system L1 - L3 - L2 is superimposed on the three-phase
system that corresponds to the standard phase sequence.
The NPS current is obtained from the three-phase current set (IR, IS, IT) DFT values at the
fundamental frequency.
14.7.3.5.2 Operation criteria
If the calculated negative phase sequence current exceeds the setting threshold value (Is), the
protection function is started and the start signal will be activated. The protection function will
return to passive status and the start signal will be cleared if the NPS current I
the setting threshold value.
When the protection enters the START status the operating time is continuously recalculated
according to the set parameters (Heating Parameter K, Start Value Is) and the negative phase
sequence current value I
If the calculated operating time is exceeded, the function goes into TRIP status and the trip signal
becomes active.
The protection function will exit the TRIP status and the trip signal will be cleared when the NPS
current value falls below 40% of the setting threshold value.
The operating time depends on the calculated negative phase sequence as follows:
where:
t: time until the protective function trips under sustained NPS overcurrent
K: heating parameter of the component
I : calculated negative phase sequence current
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I : start threshold
S
I : Nominal Primary Current value
n
According to the standard, the characteristic is only defined for I
value of the ratio mentioned is higher than 20, the operation time remains constant as the operation
time calculated for I /Is = 20.
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The operating time t must not be shorter than 50 ms.
To avoid reconnection of overheated machines, the machine cannot be reconnected after the trip
event until a configurable Reset Time (t rst) has elapsed. The function remains in BLOCK status
until the reset time has elapsed.
NS: Time to Next Motor Start (s) window in Monitoring Tab shows the remaining time before
Number of Start protection setting Reset Time, t rst, expires and it is assumed that the motor has
had time to cool down.
.
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falls below 95% of
2
/Is in the range up to 20. If the
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