Slow Front Overvoltage (SFO)

1. Slow Front Overvoltage

Slow front overvoltage (SFO) is a transient overvoltage and usually it is unidirectional. It takes 20 µs to 5000 µs time (Tp) to reach the peak value and the tail duration (T2) is less than or equal to 20 ms, i.e., (T2 ≤ 20 ms). The SFO shape is shown in Fig. 1.

The SFO can occur due to the following reasons:

Fig. 1. SFO Shape.

1.1. Switching Operations

Switching operations such as energization and re-energization are the main reason for the generation of switching overvoltage (SOV) in all 3-phases of the transmission line. The amplitude of this overvoltage depends on the type of circuit breaker, the short circuit power of the network, the length of the line, and the type of line termination.

1.2. Fault Initiation and Clearing

The fault initiation and fault clearing can generate the SOV in the system. The maximum overvoltage for fault initiation and fault clearing is given by the following relations,

For fault initiation, maximum overvoltage = (2k-1) per unit

For fault clearing, maximum overvoltage = 2.0 per unit

where k is the ground fault factor in per unit of maximum phase voltage of the system.

1.3. Switching of inductive and capacitive currents

Switching the inductive or capacitive currents can generate SOV in the system. Switching of inductive current is interrupting the motor starting current, switching the magnetizing current of the transformer, and shunt reactor. Switching of capacitive current is switching the capacitor banks and unloaded cables. Switching the capacitor banks can generate very abnormal overvoltage in the system.

2. SOV Simulation Study

For the SOV study, transient simulation can be performed for the switching operations and fault conditions. Tabulate the SOV measured at the various locations for all the cases and then the protective ratio for SOV can be calculated by,

where VSOV is the peak overvoltage measured from SOV simulation and BSL is the basic switching 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, and no further improvement is needed for the substation. However, If the calculated protective ratio is less than MPR, the peak overvoltage must be reduced by providing recommendation.


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