Hardware And Communication Requirements; Online Estimate Of Measurement Errors - GE L90 Instruction Manual

8 THEORY OF OPERATION
• Outgoing messages over a given channel are treated in the same way as during the channel recovery process. The
special start-up message is sent each time containing only a single time step value.
• When incoming messages begin arriving over a channel, that channel is placed in service and the loop filters are
started up for that channel.
• Whenever the total clock uncertainty is less than a fixed threshold, the phase locking filter is declared locked and differ-
ential protection is enabled.
The average total channel delay in each direction is not critical, provided the total round trip delay is less than 4 power sys-
tem cycles. The jitter is important, and should be less than ±130 µs in each direction. The effect of a difference in the aver-
age delay between one direction and the other depends on the number of terminals. In the case of a 2 or 3 terminal system,
the difference is not critical, and can even vary with time. In the case of a 4 or more terminal system, variation in the differ-
ence limits the sensitivity of the system.
The allowable margin of 130 µs jitter includes jitter in servicing the interrupt generated by an incoming message. For
•
both incoming and outgoing messages, the important parameter is the jitter between when the time stamp is read and
when the message begins to go out or to come in.
• The quality of the crystal driving the clock and software sampling is not critical, because of the compensation provided
by the phase and frequency tracking algorithm, unless it is desired to perform under or over frequency protection.
From the point of view of current differential protection only, the important parameter is the rate of drift of crystal fre-
quency, which should be less than 100 parts per million per minute.
• A 6 Mhz clock with a 16-bit hardware counter is adequate, provided the method is used for achieving the 32-bit resolu-
tion that is described in this document.
• An 8-bit time stamp is adequate provided time stamp messages are exchanged once per cycle.
• A 4-bit message sequence number is adequate.
Depending on the 87L settings, channel asymmetry (the difference in the transmitting and receiving paths channel delay)
cannot be higher than 1 to 1.5 ms if channel asymmetry compensation is not used. However, if the relay detects asymmetry
higher than 1.5 ms, the
87L DIFF CH ASYM DET
are enabled in the
CURRENT DIFFERENTIAL
GE's adaptive elliptical restraint characteristic is a good approximation to the cumulative effects of various sources of error
in determining phasors. Sources of error include power system noise, transients, inaccuracy in line charging current com-
putation, current sensor gain, phase and saturation error, clock error, and asynchronous sampling. Errors that can be con-
trolled are driven to zero by the system. For errors that cannot be controlled, all relays compute and sum the error for each
source of error for each phase. The relay computes the error caused by power system noise, CT saturation, harmonics,
and transients. These errors arise because power system currents are not always exactly sinusoidal. The intensity of these
errors varies with time; for example, growing during fault conditions, switching operations, or load variations. The system
treats these errors as a Gaussian distribution in the real and in the imaginary part of each phasor, 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 tran-
sient 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 pha-
selet:
Then fundamental magnitude is computed as follows for the same phaselet:
GE Multilin

8.1.15 HARDWARE AND COMMUNICATION REQUIREMENTS

FlexLogic™ operand is set high and the event and target are raised (if they
menu) to provide an indication about potential danger.

8.1.16 ONLINE ESTIMATE OF MEASUREMENT ERRORS

⁄
N 2
–
∑
4
--- -
SumSquares
=
1_A k ( )
N
p 0
=
2
( )
I I
= +
1_MAG_A 1_RE_A
L90 Line Differential Relay
1
2
( )
i
( )
1_f_A k p
–
2
( )
I
1_IM_A
8.1 OVERVIEW
8
(EQ 8.21)
(EQ 8.22)
8-11