Minimum Gap Spacing Rationale & Equations
For switching and lightning type EGLAs, the gap needs to be spaced such that no AC power frequency voltage swings can cause it to spark over. To achieve this, the gap is set to withstand the maximum expected temporary overvoltage level plus a 20% safety margin. The maximum temporary overvoltage of any system can be determined accurately with transient modeling. It can also be estimated based on knowledge of the source transformer’s grounding configuration. This procedure is basically similar to selecting the MCOV rating of standard MOV type arresters. TOV factors recommended for this procedure are presented in Table 1 below:
Because there is no current flow through the arrester prior to flashover of the gap, the arrester does not influence gap flashover voltage. The minimum AC power frequency voltage at which the EGLA’s gap will flashover is:
CFOpf 50% power frequency flashover voltage (kVrms)
Esys Max line-to-line voltage (kVrms)
TOVfactor TOV factor, as per Table 1
SF1 Safety factor (typically 1.2)
The minimum gap spacing can then be set from the minimum critical flashover voltage (U50) as determined in Eq. 1.
Smin is the gap spacing (in inches)
Note: This applies for gaps <79 inches or about 200 cm (for greater spacing a gap factor is used).
Maximum Gap Spacing Rationale & Equations
The maximum gap spacing is set so there is little risk the insulator flashes over during a lightning surge. In order to achieve this, the CFO of the insulator being protected must be known and the CFO of the gap set at a safe level below CFOins. The relationship of the CFOgap and the insulator CFOgap is shown in Fig. 3.
Margin of Protection
Margin of protection is an important part of determining maximum gap spacing and minimum margin of protection is defined as:
MinMOP = CFO90 (U90) of gap ≤ CFO10 (U10) of insulator
Gap CFO90 is the voltage level that represents a 90% probability of gap flashover.
Insulator CFO10 is the voltage level that represents a 10% probability of insulator flashover.
The recommended value for Gap CFO90 is 7.5% above gap CFO. The recommended value for Insulator CFO10 is 7.5% below the insulator CFO.
Given the relationship of maximum gap CFO, as above, once the CFO of an insulator is known, the gap CFO can be calculated as 85% of insulator CFO to meet the minimum margin of protection. It is a generally accepted fact that the impulse breakdown voltage of air is 560 kV/ft or 15kV/inch at STP.
Eq. 3 then describes the relationship of gap spacing (in inches) and gap CFO
Smax is the maximum gap spacing in inches
CFOins CFO of insulator in kV peak
Vbreakdown Pos. impulse breakdown voltage of air (accepted as 15kV/inch at STP)
K Gap factor (1 for rod to plane gaps)
Other Gap Spacing Considerations
Fortunately, if a faster surge arrives at the arrester/insulator, both devices will flashover at higher voltages/ As such, once the spacing is set for a standard wave, it will work for faster waves as well. If the installation is applied at higher altitude, gap spacing must increase as a function of air pressure. Moreover, if the electrodes or the arrester can move relative to one another, a second safety factor must be factored in to the equation to account for such motion.
The gap can also be affected by environmental conditions, but this applies as well to the insulator in parallel with the EGLA. If the gap is set to flashover before the protected insulator, it will remain so under most weather conditions. However, if the parallel insulator is so severely affected by pollution that it flashes over at power frequency, the arrester cannot protect against this.
Lastly, as with all transmission line arresters, lead management is a significant consideration. In the case of an EGLA, the electrodes are generally shorter than leads and electrode management is therefore less a challenge than in the case of the leads of an NGLA.