Unblocking sequence

There are two ways to unblock the converters (e.g. following a persistent DC fault): manually or automatically.

Manual unblocking of a converter is initiated by the operator and takes effect only when the DC current is lower than a specified threshold (set at 0,10 p.u.). This current must then be decreased by lowering the current reference. All the reference signals have a rate of change limiter which limits their variation speed (adjustable).When the operator varies the reference, the effective reference will follow, but at a speed limited by the rate of change limiters.

The automatic disabling takes effect immediately to protect the system. It is controlled by the DC fault protection system.

When the converter is disabled (manually or automatically), the control system cancels the firing pulses to the valves. Additionally, a constant delay angle is imposed permanently.