In order to meet the high demand for power transmission capacity, some power companies have installed series capacitors on power transmission lines.  This allows the impedance of the line to be lowered, thus yielding increased transmission capability.  The series capacitor makes sense because it’s simple and could be installed for 15 to 30% of the cost of installing a new line, and it can provide the benefits of increased system stability, reduced system losses, and better voltage regulation.

Protective distance relays, which make use of impedance measurements in order to determine the presence and location of faults, are “fooled” by installed series capacitance on the line when the presence or absence of the capacitor in the fault circuit is not known a priori.  This is because the capacitance cancels or compensates some of the inductance of the line and therefore the relay may perceive a fault to be in its first zone when the fault is actually in the second or third zone of protection.  Similarly, first zone faults can be perceived to be reverse faults! Clearly this can cause some costly operating errors.

The general approach of interest is a method leading to the determination of the values of series L and C of the line at the time of the fault.  This is done by analyzing the synchronous and subsynchronous content of the V and I signals seperately which provides adequate information to compute the series L and C of the line.

Introduction - The Relaying Problems Associated With Series Capacitor
As modern transmission systems become more and more heavily loaded, the benefits of series compensation for many of the grid’s transmission lines become more obvious.  Clearly, adding series compensation is one of the cheapest, simplest ways of increasing transmission line capacity and system stability, lowering losses, and improving voltage regulation.1  Unfortunately, the series capacitor can undermine the effectiveness of many of the protection schemes used for long distance transmission lines.

The introduction of the capacitance in series with the line reactance adds certain complexities to the effective application of impedance based distance relays.  The relay will attempt to look at the ratio of voltage to current to determine the distance to the fault in order to decide if the fault is in or out of its zone of protection.

It is of course possible to correct the settings of the relay when it is know that the capacitor is always going to be part of the fault circuit.  However, that is not always known.  By canceling some of the line’s series inductance, the series capacitor can make remote forward faults look as if they are in zone one of the relay when the capacitor is switched into the transmission line circuit and the relay setting rules are based on no capacitor in the fault loop (i.e. they can cause the relay to “overreach”).

Under these conditions, close-in faults can appear to be reverse faults due to voltage reversal (voltage inversion).2  More specifically, if we look at a plot of the apparent impedance seen versus the distance from the relay, we see the condition shown in figure I.1.  The case depicted in figure I.1 represents a line with 50% compensation.  (Line impedance is jX and cap impedance is -j(0.5*X).

It is clear from the plot that when the relay has been set according to a line with no series compensation, it
will see many of the faults on the line as reverse faults and will not operate at all.  Faults at almost 150% of the line will appear to be zone 1 faults as well. Clearly, some other scheme must be used to protect these lines.

One approach is to slow down the operation of the relay so that the capacitor protection system in use (MOV and/or spark gap and/or circuit breaker) will have time to operate and remove the capacitor (or short circuit its terminals) from service.  Then the traditional impedance (mho) relay will function properly. Unfortunately, extending the fault clearing time can lead to instability in the system.


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