GE L90, Инструкция по эксплуатации

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5-284 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
GROUPED ELEMENTS CHAPTER 5: SETTINGS
5
The differential current is produced as an unbalance current between the ground current of the neutral CT (IG) and the
neutral current derived from the phase CTs (IN = IA + IB + IC):
Igd = |IG + IN| = |IG + IA + IB + IC| Eq. 5-29
The relay automatically matches the CT ratios between the phase and ground CTs by re-scaling the ground CT to the
phase CT level. The restraining signal ensures stability of protection during CT saturation conditions and is produced as a
maximum value between three components related to zero, negative, and positive-sequence currents of the three phase
CTs as follows:
Irest = max(IR0, IR1, IR2) Eq. 5-30
The zero-sequence component of the restraining signal (IR0) is meant to provide maximum restraint during external
ground faults, and therefore is calculated as a vectorial difference of the ground and neutral currents:
IR0 = |IG - IN| = |IG - (IA +IB +IC)| Eq. 5-31
The equation above brings an advantage of generating the restraining signal of twice the external ground fault current,
while reducing the restraint below the internal ground fault current. The negative-sequence component of the restraining
signal (IR2) is meant to provide maximum restraint during external phase-to-phase faults and is calculated as follows:
IR2 = |I_2| or IR2 = 3 x |I_2| Eq. 5-32
Following complete de-energization of the windings (all three phase currents below 5% of nominal for at least five cycles),
the relay uses a multiplier of 1 in preparation for the next energization. The multiplier of 3 is used during normal operation;
that is, two cycles after the winding has been energized. The lower multiplier is used to ensure better sensitivity when
energizing a faulty winding.
The positive-sequence component of the restraining signal (IR1) is meant to provide restraint during symmetrical
conditions, either symmetrical faults or load, and is calculated according to the following algorithm:
1 If |I_1| > 2 pu of phase CT, then
2
If |I_1| > |I_0|, then IR1 = 3 x (|I_1| - |I_0|)
3 else IR1 = 0
4 else IR1 = |I_1|/8
Under load-level currents (below 150% of nominal), the positive-sequence restraint is set to 1/8th of the positive-sequence
current (line 4). This is to ensure maximum sensitivity during low-current faults under full load conditions. Under fault-level
currents (above 150% of nominal), the positive-sequence restraint is removed if the zero-sequence component is greater
than the positive-sequence (line 3), or set at the net difference of the two (line 2).
The raw restraining signal (Irest) is further post-filtered for better performance during external faults with heavy CT
saturation and for better switch-off transient control:
Igr(k) = max(Irest(k),
α
x Igr(k - 1)) Eq. 5-33
where
k represents a present sample
k – 1 represents the previous sample
α is a factory constant ( α < 1)
The equation above introduces a decaying memory to the restraining signal. Should the raw restraining signal (Irest)
disappear or drop significantly, such as when an external fault gets cleared or a CT saturates heavily, the actual
restraining signal (Igr(k)) does not reduce instantly but keeps decaying decreasing its value by 50% each 15.5 power
system cycles.
Having the differential and restraining signals developed, the element applies a single slope differential characteristic with
a minimum pickup as shown in the following logic diagram.