Ct Saturation Detection - GE L90 Instruction Manual

CHAPTER 10: THEORY OF OPERATION
with a standard deviation that is estimated from the sum of the squares of the differences between the data samples and
the sine function that is used to fit them. This error has a spectrum of frequencies. Current transformer saturation is
included with noise and transient error. The error for noise, harmonics, transients, and current transformer saturation is
computed as follows. First, the sum of the squares of the errors in the data samples is computed from the sum of squares
information for the present phaselet:
Then fundamental magnitude is computed as follows for the same phaselet:
Finally, the local adaptive restraint term is computed as follows, for each local current:
Another source of the measurement errors is clock synchronization error, resulting in a clock uncertainty term. The L90
algorithm accounts for two terms of synchronization error corresponding to:
• Raw clock deviation computed from time stamps — There are several effects that cause it to not track exactly. First,
the ping-pong algorithm inherently produces slightly different estimates of clock deviation at each terminal. Second,
because the transmission of time stamps is spread out over several packets, the clock deviation estimate is not up to
date with other information it is combined with. Channel asymmetry also contributes to this term. The clock deviation
computation is indicated in equation 10.15 as θ
channels and then the average of absolute values is computed. If GPS compensation is used, then GPS clock
compensation is subtracted from the clock deviation.
• Startup error — This term is used to estimate the initial startup transient of PFLLs. During startup conditions, a
decaying exponential is computed to simulate envelope of the error during startup.
The clock uncertainty is expressed as:
Eventually, the local clock error is computed as:
The local squared adaptive restraint is computed from all local current sources (1 to 4) and is obtained as follows:

10.1.17 CT saturation detection

Current differential protection is inherently dependent on adequate CT performance at all terminals of the protected line,
especially during external faults. CT saturation, particularly when it happens at only one terminal of the line, introduces a
spurious differential current that can cause the differential protection to misoperate.
The L90 applies a dedicated mechanism to cope with CT saturation and ensure security of protection for external faults.
The relay dynamically increases the weight of the square of errors (the so-called 'sigma') portion in the total restraint
quantity, but for external faults only. The following logic is applied:
• First, the terminal currents are compared against a threshold of 3 pu to detect overcurrent conditions that can be
caused by a fault and can lead to CT saturation
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
. If two channels are used, clock deviation is computed for both
i
OVERVIEW
Eq. 10-28
Eq. 10-29
Eq. 10-30
Eq. 10-31
Eq. 10-32
Eq. 10-33
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10-13