Woodward HighPROtec MCDLV4, Руководство пользователя

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Protective Elements
I2> - Negative-Sequence Overcurrent [51Q]
For activating this function, the parameter
»Measuring Mode« has to be set to »I2« in the parameter set of the
corresponding overcurrent element I[x].
The
negative-sequence
overcurrent protection function (
I2>) is to be seen as an equivalent to the phase overcurrent
protection with the exception that it uses negative-sequence current (I2>) as measured quantities instead of the
three phase currents used by phase overcurrent protection function. The negative-sequence current used by
I2> is
derived from the following well-known symmetrical component transformation:
I
2
=
1
3

I
L1

a
2
I
L2

a I
L3

The pickup set value of a I2> protection function should be set in accordance of the negative-sequence current
occurrence in the protected object.
Besides that, the negative-sequence overcurrent protection function (
I2>) uses the same setting parameters as the
phase overcurrent protection function, like trip and reset characteristics from both IEC/ANSI standards, time
multiplier, etc.
The negative-sequence overcurrent protection function (
I2>) can be used for line, generator, transformer and motor
protection to protect the system from unbalanced faults. Because the
I2> protection function operates on the
negative-sequence current component which is normally absent during load conditions, the
I2> can, therefore, be
set more sensitive than the phase overcurrent protection functions. On the other hand, coordination of negative-
sequence overcurrent protection function in a radial system does not mean automatically very long fault clearing
time for the furthest upstream protection devices, because the tripping time of concerned negative-sequence
overcurrent protection function needs only be coordinate with the next downstream device with the negative-
sequence overcurrent protection function. This makes the
I2> in many cases as an advantageous protection
concept in addition to the phase overcurrent protection function.
If you are using inrush blockings, the tripping delay of the current protection
functions must be at least 30 ms or more in order to prevent faulty trippings.
At the moment of breaker closure, negative-sequence current might be the
result of transients.
707 MCDLV4 DOK-HB-MCDLV4-2E