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Compensation method
The sophisticated method for unbalance protection is to compensate
the natural unbalance current. The compensation is triggered
manually when commissioning. The phasors of the unbalance current
and one phase current are recorded. This is because one polarizing
measurement is needed. When the phasor of the unbalance current is
always related to IL1, the frequency changes or deviations have no
effect on the protection.
After recording the measured unbalance current corresponds the
zero-level and therefore, the setting of the stage can be very
sensitive.
Compensation and location
The most sophisticated method is to use the same compensation
method as mentioned above, but the add-on feature is to locate the
branch of each faulty element or to be more precise, the broken fuse.
This feature is implemented to the stage I
stage I
>>> can still function as normal unbalance protection stage
0
with compensation method. Normally, the I
alarming stage while stage I
The stage I
>>>> should be set based on the calculated unbalance
0
current change of one faulty element. This can be easily calculated.
However, the setting must be, say 10% smaller than the calculated
value, since there are some tolerances in the primary equipment as
well as in the relay measurement circuit. Then, the time setting of
I
>>>> is not used for tripping purposes. The time setting specifies,
0
how long the device must wait until it is certain that there is a faulty
element in the bank. After this time has elapsed, the stage I
makes a new compensation automatically, and the measured
unbalance current for this stage is now zero. Note, the automatic
compensation does not effect on the measured unbalance current of
stage I
>>>.
0
If there is an element failure in the bank, the algorithm checks the
phase angle of the unbalance current related to the phase angle of the
phase current I
. Based on this angle, the algorithm can increase the
L1
corresponding faulty elements counter (there are six counters).
The user can set for the stage I
elements, e.g. if set to three elements, the fourth fault element will
issue the trip signal.
The fault location is used with internal fused capacitor and filter
banks. There is no need to use it with fuseless or external fused
capacitor and filter banks, nor with the reactor banks.
Protection Functions
>>>>, while the other
0
>>>> could be set as an
0
>>> will trip the circuit-breaker.
0
>>>> the allowed number of faulty
0
857-UM001A-EN-P – July 2009
3-41
>>>>
0
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