Causes of switching overvoltage; inductive current interruption

Switching overvoltages are transient overvoltages produced when breakers or switches change the network configuration. They are the most important overvoltages for EHV and UHV lines (above about 400 kV), where they set the insulation levels.

Major causes

1. Energising (closing) a long unloaded line. The step voltage starts travelling waves that double at the open end. With trapped charge from an earlier opening, the overvoltage can reach about 3 pu.
2. High-speed reclosing on a line with trapped charge.
3. Interruption of small inductive currents. Transformer magnetising current or reactor current is chopped before its natural zero.
4. Interruption of capacitive currents. De-energising unloaded lines, cables or capacitor banks can cause restrikes and voltage escalation.
5. Fault initiation and fault clearing, including out-of-phase switching.
6. Load rejection, together with the Ferranti effect and generator overspeed. (Usually counted as a temporary overvoltage.)
7. Ferroresonance during single-pole switching of unloaded transformers.

Overvoltage due to interruption of inductive current

When a breaker opens an unloaded transformer or reactor, the current is small (a few amperes). The arc becomes unstable and the breaker may cut the current abruptly at a value $i_0$ before its natural zero. This is current chopping.

  Vs ~--- CB ---+-----------+
            i0  |           |
               C stray     L (transformer)
                |           |
  --------------+-----------+--- ground

• At the instant of chopping, the magnetic energy in the transformer inductance is $\tfrac{1}{2} L i_0^2$.
• The current in $L$ cannot change suddenly, so it flows into the stray capacitance $C$ of the winding and bushing. Energy oscillates between $L$ and $C$ at

$$f = \frac{1}{2\pi\sqrt{LC}}$$

• When all the energy is in $C$:

$$\tfrac{1}{2} C V_m^2 = \tfrac{1}{2} L i_0^2 + \tfrac{1}{2} C V_0^2$$

where $V_0$ is the capacitor voltage at the chopping instant. When $V_0$ is small:

$$V_m \approx i_0 \sqrt{\frac{L}{C}}$$

The surge impedance $\sqrt{L/C}$ of a transformer is very high (tens of kΩ). So chopping only 5–10 A can produce more than 200 kV.

Effects: the high voltage appears across the breaker contacts and can cause restrikes, with the voltage escalating, and it stresses the transformer insulation.

Remedies:

• Resistance switching: a resistor across the breaker contacts damps the oscillation and absorbs the energy.
• Surge arresters at the transformer terminals.
• Breakers designed with low chopping levels.