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at first what is zero sequence current..
Zero sequence currents, in simple terms, are those found in the neutral conductor. They include the unbalanced 50 Hz currents and the 3rd, 9th, 15th and other triplen harmonic currents.
Zero sequence currents appear in the neutral because they do not cancel in the way that 50Hz currents cancel. This is due to the fact that the zero sequence component on one phase is always in phase with the zero sequence components of the other 2 phases.
we can say 50 Hz current on one phase, on the other hand, is always 120° out of phase with the other phases 50 Hz current which causes their balanced portions to cancel in the neutral.
now,
any method of ground current sensing involves detecting zero sequence currents. For solidly grounded systems, connecting the phase current Transformers together with a common return allows measurement of zero sequence current in the common lead, if there is no neutral load current. The vector sum of the phase currents is equal to the zero sequence ground current.
Under normal load conditions, the vector sum of the three phase currents is near zero. It is not exactly zero as the system capacitive charging current of the load circuit is non-zero. but it is limited value..if it is beyond the limit this zero sequence CT take care of it.
All phase conductors are passed through the window of a single CT referred to as a zero sequence CT. Under normal circumstances, the three phase currents will sum to zero resulting in an output of zero from the zero sequence CT's secondary. If one of the phases were shorted to ground, the sum of the phase currents would no longer equal zero causing a current to flow in the secondary of the zero sequence CT. This current would be detected by the relay as a ground fault.
Regards,
Balaji K
APL,
Wiki User
∙ 12y ago
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