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Section 8
Impedance protection
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Line current as a function of time
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Capacitor current as a function of time
IEC06000614 V1 EN-US
Figure 206:
MOV protected capacitor with examples of capacitor voltage and
corresponding currents
The impedance apparent to distance IED is always reduced for the amount of
capacitive reactance included between the fault and IED point, when the spark gap
does not flash over, as presented for typical cases in figure 205. Here it is necessary
to distinguish between two typical cases:
•
Series capacitor only reduces the apparent impedance, but it does not cause
wrong directional measurement. Such cases are presented in figure
50% compensation at 50% of line length and 33% compensation located on
33% and 66% of line length. The remote end compensation has the same
effect.
•
The voltage inversion occurs in cases when the capacitor reactance between
the IED point and fault appears bigger than the corresponding line reactance,
Figure 205, 80% compensation at local end. A voltage inversion occurs in IED
point and the distance IED will see wrong direction towards the fault, if no
special measures have been introduced in its design.
The situation differs when metal oxide varistors (MOV) are used for capacitor
overvoltage protection. MOVs conduct current, for the difference of spark gaps,
only when the instantaneous voltage drop over the capacitor becomes higher than
the protective voltage level in each half-cycle separately, see figure 206.
Extensive studies at Bonneville Power Administration in USA ( ref. Goldsworthy,
D,L "A Linearized Model for MOV-Protected series capacitors" Paper 86SM357–
8 IEEE/PES summer meeting in Mexico City July 1986) have resulted in
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MOV current as a function of time
Line distance protection REL670 2.2 IEC
1MRK 506 369-UEN B
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