Fast Front Overvoltage (FFO)
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1. Fast Front Overvoltage (FFO)
Fast front overvoltage (FFO) is a transient overvoltage, and it is unidirectional. It takes 0.1 µs to 20 µs time (T1) to reach the peak value and the tail duration (T2) is less than or equal to 300 µs, i.e., (T2 ≤ 300 µs). The FFO shape is shown in Fig. 1.
The FFO can occur due to lightning strokes. The overvoltage due to lightning strokes can be divided into 3 categories:
Shielding failure (SF): Direct strokes to the phase conductors.
Back Flashover (BFO): Strokes to the shield wire or tower which flashes to the phase conductors due to the breakdown of the insulator.
Induced Voltage: Strokes to the ground in close to the line that induces overvoltage in the phase conductors.
Fig. 1. FFO Shape.
2. Lightning Overvoltage Simulation Study
To perform the Lightning overvoltage (LIOV) study, there is a consideration for the long path and short path model in the substation to find the worst-case scenario. The long path is the longest possible option to connect the transmission line (feeder) with the substation transformer which covers most of the components in the substation. A short path is the shortest possible option to connect the transmission line (feeder) and substation transformer. The transient simulation can be performed for the SF and BFO cases for the long path and short path models. The determined peak overvoltage can be tabulated for all the cases and the protective ratio for LIOV can be calculated by,
where VLIOV is the peak overvoltage measured from the LIOV simulation and BIL is the basic lightning impulse insulation level given in IEEE C62.82.1-2010. As per the standard, the minimum protective ratio (MPR) is 1.15. If the calculated protective ratio is higher than MPR, the determined peak overvoltage is already within the limit, i.e., compliance with the standard. However, If the calculated protective ratio is less than MPR, then the determined peak overvoltage must be reduced by providing recommendation.
References
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Imece, Ali F., Daniel W. Durbak, and Hamid Elahi. "Modeling guidelines for fast front transients." IEEE Transactions on Power Delivery 11, no. CONF-950103- (1996).
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