HVDC circuit breaker operation

An HVDC circuit breaker interrupts direct current in an HVDC line. DC is hard to break because, unlike AC, it has no natural current zero, and the large inductive energy (½ L I²) of the circuit must be absorbed during interruption.

Requirements

1. Create an artificial current zero in the main interrupter.
2. Absorb the stored energy of the system inductance.
3. Withstand the transient recovery voltage that appears after interruption.

Operation (resonance / commutation type)

The common mechanical HVDC breaker has three parallel branches:

        ┌──── Main interrupter (SF6/vacuum) ────┐
 I_dc ──┤                                       ├──►
        ├──── L ──── C ──── (switch S) ─────────┤
        │      commutation / resonant branch    │
        └──── Surge arrester (ZnO) ─────────────┘

1. In normal operation the DC current flows through the main interrupter.
2. On a fault the main contacts open, and an arc forms.
3. A pre-charged capacitor C in the commutation branch is discharged through L (switch S closes), or the arc's negative resistance makes the LC branch oscillate. This injects a current opposite to the line current.
4. The current in the main interrupter is driven to zero, and the arc is extinguished there.
5. The line current is then commutated into the LC branch and charges the capacitor; the voltage rises rapidly.
6. When the voltage reaches the protective level, the surge arrester (energy absorber) conducts, absorbs the stored energy ½ L I² and forces the current to zero.

Hybrid HVDC breaker

Modern breakers (e.g. for multi-terminal VSC-HVDC) use a fast mechanical disconnector with a low-loss IGBT load commutation switch in the main path and a large IGBT main breaker in parallel, plus arresters. They interrupt in a few milliseconds.

Uses

Multi-terminal HVDC systems, metallic return transfer breakers and DC grids. In point-to-point HVDC links, faults are usually cleared by converter control instead.