Differential Element Characteristics - GE L90 Instruction Manual

9.1 OVERVIEW
The underlying single phase model for compensation for a two and three terminal system are shown below.
Figure 9–5: 2-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION
Figure 9–6: 3-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION
Apportioning the total capacitance among the terminals is not critical for compensating the fundamental power system fre-
quency charging current as long as the total capacitance is correct. Compensation at other frequencies will be approximate.
If the VTs are connected in wye, the compensation is accurate for both balanced conditions (i.e. all positive, negative and
zero sequence components of the charging current are compensated). If the VTs are connected in delta, the compensation
is accurate for positive and negative sequence components of the charging current. Since the zero sequence voltage is not
available, the L90 cannot compensate for the zero sequence current.
The compensation scheme continues to work with the breakers open, provided the voltages are measured on the line side
of the breakers.
For very long lines, the distributed nature of the line leads to the classical transmission line equations which can be solved
for voltage and current profiles along the line. What is needed for the compensation model is the effective positive and zero
sequence capacitance seen at the line terminals.
Finally, in some applications the effect of shunt reactors needs to be taken into account. With very long lines shunt reactors
may be installed to provide some of the charging current required by the line. This reduces the amount of charging current
flowing into the line. In this application, the setting for the line capacitance should be the residual capacitance remaining
after subtracting the shunt inductive reactance from the total capacitive reactance at the power system frequency.
The differential element is completely dependent on receiving data from the relay at the remote end of the line, therefore,
upon startup, the differential element is disabled until the time synchronization system has aligned both relays to a common
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time base. After synchronization is achieved, the differential is enabled. Should the communications channel delay time
increase, such as caused by path switching in a SONET system or failure of the communications power supply, the relay
will act as outlined in the next section.
The L90 incorporates an adaptive differential algorithm based on the traditional percent differential principle. In the tradi-
tional percent differential scheme, the operating parameter is based on the phasor sum of currents in the zone and the
restraint parameter is based on the scalar (or average scalar) sum of the currents in the protected zone - when the operat-
ing parameter divided by the restraint parameter is above the slope setting, the relay will operate. During an external fault,
the operating parameter is relatively small compared to the restraint parameter, whereas for an internal fault, the operating
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Is
Vs
L
R
C/2
C/3
C/3
C/3

9.1.19 DIFFERENTIAL ELEMENT CHARACTERISTICS

L90 Line Current Differential System
9 THEORY OF OPERATION
Ir
Vr
C/2
831793A1.CDR
831019A1.CDR
GE Multilin