If the voltage gradient is not supplied by the cable manufacturer for the configuration used, it can be calculated using relevant equations and methods derived from IEEE 575 “Guide for Bonding Sheaths and Shields of Single-Conductor Power Cables Rated 5 to 500 kV”:

Once the voltage gradient is known for 1 km at 1000 A, the voltage that will appear at the open end of a segment during a fault event can also be calculated. It is important to determine this voltage level because the SVL voltage rating (Uc) needs to be set just higher so that the arrester does not conduct during a fault event. Should the arrester conduct in this instance, it would need a much higher energy handling capability than generally available for distribution type arresters. If it is found later in the sizing process that a lower level Uc is needed, a transient analysis will determine the SVL’s proper Uc and energy rating.

Assuming that the margin of protection will be adequate, then the Uc rating of the SVL will be greater than or equal to the voltage at the open point (Eopen), as follows:

Uc ? Eopen= voltage gradient x segment

length x max. expected fault current
where voltage gradient is V/km/1000A, length is in km and fault current is expressed in kA. For example, if a voltage gradient on a particular system is 200V/km/kA and the line is 2 km long with a potential of 17.5 kA, then the minimum acceptable Uc rating for the SVL would be 7000 V. Note that if the line were only 1 km long, the SVL’s minimum Uc would be half that of the 2 km long line and could be a minimum of 3500 V.

Fig. 9: Current conduction through properly sized SVL.
Fig.

Figure 9 shows the current flow through an appropriately rated SVL on a 1 km line with the above-mentioned voltage gradient and fault current. It can be seen that only some microamps flow through the SVL, which is exactly what is desired. However if the same SVL is applied to a similar line of 2 km length, the current through the SVL would be significant (as in Figure 10) and the immediate temperature rise to failure is shown in Figure 11.

Fig. 10: Current through improperly sized SVL with peak levels in 600 A range per half cycle.

Fig. 11: Temperature rise of improperly sized SVL showing imminent failure if breaker does not immediately interrupt fault.

Therefore, when determining the proper Uc ratings for SVLs, one cannot select one rating for all link boxes unless the lengths of all segments are equal. Moreover, if the SVL is chosen correctly, it will not be required to absorb any significant level of energy during a system fault.

Arresters

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