Secondary arc currents occur after a fault is cleared (typically by circuit breakers) due to inductive or capacitive coupling with healthy circuits. Such currents are often experienced in long transmission lines during single-line-to-ground faults, especially in systems using single-pole tripping.

Switching the arc resistance from the fault resistance to the secondary arc resistance in a simulation involves distinguishing between the fault current phase (high current, low resistance) and the secondary arc phase (low current, high dynamic resistance). This transition occurs immediately after the primary fault current is interrupted by the line circuit breakers but the faulted path remains partially conductive due to ionization or coupling.

The attached project includes a secondary arc model suitable for EMT simulations. For the sake of the exercise, instead of using a simulation event, a single-phase to ground fault is explicitly modeled by an impedance connected to ground.

Ideal switches are added to transition from the fault to the secondary arc stages of the transient. The fault impedance is connected at t = 0 s and disconnected 80 ms later together with the faulted phase of the line, thus emulating the action of the protection relays. The secondary arc is represented by a resistor modulated by a hybrid Modelica dynamic model. When the fault is cleared, the secondary arc model is triggered by a simulation event which in turn closes the corresponding ideal arc switch.

The model describes an arc in air by a differential equation of the arc conductance, which is a function of the arc current and length. The arc is extinguished if the time derivative of the instantaneous arc resistance exceeds a pre-defined limit provided that the arc conductance per length is less than a minimum threshold.