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V
= CT knee point voltage
k
I
= maximum prospective secondary current for an internal fault
F
When a Metrosil is used for the varistor, it should be selected with the following characteristics:
β
V = CI
where:
V = instantaneous voltage
I = instantaneous current
β = constant, normally in the range 0.20 - 0.25
C = constant.
The C value defines the characteristics of the metrosil, and should be chosen according to the
following requirements:
1. The current through the metrosil at the relay voltage setting should be as low as possible,
preferably less than 30mA for a 1Amp CT and less than 100mA for a 5Amp CT.
2. The voltage at the maximum secondary current should be limited, preferably to 1500Vrms.
Restricted earth fault schemes should be applied with high accuracy CTs whose knee point voltage
V
is chosen according to the equation:
k
≧ 2×V
V
k
S
where V
is the differential stability voltage setting for the scheme.
S
2.1.1.8 Negative Sequence Overcurrent Protection
The negative sequence overcurrent protection (NOC) is used to detect asymmetrical faults
(phase-to-phase and phase-to-earth faults) with high sensitivity in conjunction with phase
overcurrent protection and residual overcurrent protection. It also used to detect load unbalance
conditions.
Phase overcurrent protection is forced to be set to lower sensitivity when the load current is large
but NOC sensitivity is not affected by magnitude of the load current, except in the case of
erroneous negative sequence current due to the unbalanced configuration of the distribution lines.
For some earth faults, small zero sequence current is fed while the negative sequence current is
comparatively larger. This is probable when the fault occurs at the remote end with a small reverse
zero sequence impedance and most of the zero sequence current flows to the remote end.
In these cases, NOC backs up the phase overcurrent and residual overcurrent protection. The NOC
also protects the rotor of a rotating machine from over heating by detecting a load unbalance.
Unbalanced voltage supply to a rotating machine due to a phase loss can lead to increases in the
negative sequence current and in machine heating.
Two independent negative sequence overcurrent elements NOC1 and NOC2 are provided for
tripping and alarm. The NOC1 has selective inverse time and definite time characteristics. The
NOC2 has definite time characteristic only.
Note: NOC1 element that has inverse time or definite time characteristics is discriminated with
NOC1I and NOC1D respectively.
The tripping outputs can be blocked by scheme switches or PLC signals.
Scheme Logic
Figure 2.1.24 and 2.1.25 show the scheme logic of the NOC protection. Two negative sequence
⎯
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6 F 2 S 0 8 4 2
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