Temporary Overvoltage (TOV)

1.1. Phase-to-Earth Faults

The primary reason for the generation of TOV is a phase-to-earth fault. The phase-to-earth fault may rise the voltage (i.e., overvoltage) in the other two healthy phases. The amplitude of this overvoltage is depending on the earthing of the system neutral and the fault location.

1.2. Load Rejection

 Load rejection at the remote end of the long transmission line may give overvoltage in the system (it is called the Ferranti effect) and it may affect the components connected on the source side. The amplitude of overvoltage due to load rejection depends on many factors such as load types, the layout of the system after disconnection, and the phase angle difference between the networks (the worst case is phase opposition).

1.3. Resonance

 The TOV due to resonance and Ferro resonance occur when there is an interaction between large capacitive elements (i.e., cables and series compensated lines) and inductive elements (i.e., transformers and shunt reactors). The amplitude of overvoltage due to resonance can be extremely high value and therefore, it is not considered for the selection of surge arrester or insulation design. However, this overvoltage can be limited by de-tuning the system by changing the configuration of the system or adding damping resistors.

2. TOV Simulation Study and Arrester Selection

 For the TOV study, the transient simulation can be performed for the phase-to-earth fault, line, and load rejection conditions. From the TOV simulation studies, the peak overvoltage can be determined. Then, the rating of the surge arrester can be selected by the following relations,

V_TOV_Simulation is the peak overvoltage observed from the TOV simulation.

V_TOV_data is the TOV capability selected from the TOV characteristics of the arrester. 

The safety ratio of 15 % can be considered as indicated in IEC 60099-5. 

The rated voltage of the surge arrester must be higher than V_r_min.

References

• IEEE. "IEEE Standard for Insulation Coordination-Definitions, Principles, and Rules." IEEE Std C62. 82.1-2010 (Revisión de IEEE Std 1313.1-1996) (2011).

• International Electrotechnical Commission. "Insulation co-ordination—Part 2: Application guidelines." International Standard, IEC (2018): 60071-2.

• IEC, TR. "60071-4: 2004, Insulation co-ordination–Part 4.

• Arresters—Part, Surge. "5: Selection and Application recommendations." Standard IEC (2013): 60099-5.