ABB RET670 Applications Manual page 237

1MRK504116-UUS C
Application manual
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ANSI06000621 V1 EN
Figure 99: Distance IED on adjacent
power lines are influenced by
the negative impedance
Normally the first zone of this protection must be delayed until the gap flashing has
taken place. If the delay is not acceptable, some directional comparison must also be
added to the protection of all adjacent power lines. As stated above, a good protection
system must be able to operate correctly both before and after gap flashing occurs.
Distance protection can be used, but careful studies must be made for each individual
case. The rationale described applies to both conventional spark gap and MOV
protected capacitors.
Special attention should be paid to selection of distance protection on shorter adjacent
power lines in cases of series capacitors located at the line end. In such case the
reactance of a short adjacent line may be lower than the capacitor reactance and
voltage inversion phenomenon may occur also on remote end of adjacent lines.
Distance protection of such line must have built-in functionality which applies
normally to protection of series compensated lines.
It usually takes a bit of a time before the spark gap flashes, and sometimes the fault
current will be of such a magnitude that there will not be any flashover and the
negative impedance will be sustained. If equation
< < +
X X X X
11 C S 11
EQUATION1898 V1 EN
in figure 100, the fault current will have the same direction as when the capacitor is
bypassed. So, the directional measurement is correct but the impedance measured is
negative and if the characteristic crosses the origin shown in figure
operate. However, if there is a memory circuit designed so it covers the negative
impedance, a three phase fault can be successfully cleared by the distance protection.
As soon as the spark gap has flashed the situation for protection will be as for an
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en06000621_ansi.vsd
IED application
146
100 the IED cannot
Section 3
(Equation 146)
231