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2.7 NEGATIVE SEQUENCE ELEMENT (46)
2.7NEGATIVE SEQUENCE ELEMENT (46)
The MIG relay incorporates a negative sequence protection function in order to detect system conditions that can cause
unbalanced three-phase currents in the generator. As previously mentioned, these unbalances can be of higher magnitude
than the load unbalance.
If line currents are not balanced, using the symmetrical components we can verify that negative sequence current
components are generated. This component rotates in the opposite way to the rotor. The flux produced by this current seen
from the rotor, has a frequency that is twice the synchronous speed as a result of the inverse rotation combined with the
2
direct rotation of the rotor. This flux induces currents with twice the nominal frequency, whose pellicular effect makes them
fluctuate through the rotor surface, and in a slighter way, through the winding. The resulting overheating can reach
extremely high in a short time. The result of this fact is a quick damage of isolating elements.
Therefore, the overheating limit for the rotor is defined by:
Where:
K
Constant that depends on the design and size of the generator according to standards (e.g. ANSI C50.13)
t
Time in seconds
I
I
/ FLC
eq
2
I
Negative sequence current (DFT)
2
FLC
Generator's rated current
2-10
MIGII Digital Protection for Electrical Machines
2 PRODUCT DESCRIPTION
GEK-113188A
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