ABB Relion REG670, Applications Manual

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Fault resistance
The performance of distance protection for single phase-to-earth faults is very
important, because normally more than 70% of the faults on transmission lines are
single phase-to-earth faults. At these faults, the fault resistance is composed of three
parts: arc resistance, resistance of a tower construction, and tower-footing resistance.
The arc resistance can be calculated according to Warrington's formula:
1.4
28707 L
Rarc
I
×
=
EQUATION1456 V1 EN
(Equation 104)
Where:
L
represents the length of the arc (in meters). This equation applies for the distance protection
zone 1. Consider approximately three times arc foot spacing for zone 2 to get a reasonable
margin against the influence of wind.
I is the actual fault current in A.
In practice, the setting of fault resistance for both phase-to-earth RFPE and phase-to-
phase RFPP should be as high as possible without interfering with the load impedance
in order to obtain reliable fault detection.
8.3.3 Series compensation in power systems
The main purpose of series compensation in power systems is virtual reduction of line
reactance in order to enhance the power system stability and increase loadability of
transmission corridors. The principle is based on compensation of distributed line
reactance by insertion of series capacitor (SC). The generated reactive power provided
by the capacitor is continuously proportional to the square of the current flowing at the
same time through the compensated line and series capacitor. This means that the
series capacitor has a self-regulating effect. When the system loading increases, the
reactive power generated by series capacitors increases as well. The response of SCs
is automatic, instantaneous and continuous.
The main benefits of incorporating series capacitors in transmission lines are:
• Steady state voltage regulation and raise of voltage collapse limit
• Increase power transfer capability by raising the dynamic stability limit
• Improved reactive power balance
• Increase in power transfer capacity
• Reduced costs of power transmission due to decreased investment costs for new
power lines
8.3.3.1 Steady state voltage regulation and increase of voltage collapse limit
A series capacitor is capable of compensating the voltage drop of the series inductance
in a transmission line, as shown in figure
106 . During low loading, the system voltage
Section 8
1MRK 502 071-UEN -
Impedance protection
228 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual