An HVDC transmission system including a rectifierstation and an inverterstation each having a series connection of at least two converters. A by-pass DC breaker is connected in parallel with each converter. A control device is adapted to deblock a blocked converter by starting to control the converter at high delay angle and gradually decreasing the delay angle until substantially all DC current flows through the converter and to then control the by-pass breaker to open at substantially zero current, and to stop the operation of a converter by controlling the converter at gradually increasing delay angle until the voltage across the converter is substantially zero and to then control the converter to be blocked by firing a by-pass pair thereof and to then control the by-pass breaker to close for taking over all the DC current when the voltage between a neutral bus and a pole of a transmission line between the stations is to be increased and reduced, respectively.
A wireless damage detector for ceramic armor plates includes an interrogator and a body armorceramic plate. The interrogator includes an inductive primary coil having a resonant frequency. The plate includes a corresponding inductive secondary coil adapted to cooperate with the primary coil when the interrogator is positioned in an interrogation position wirelessly adjacent the plate. When in the interrogation position, the primary and secondary coils are inductively coupled, that is, form an inductive coupling, when the primary coil is energized at the resonant frequency. The plate includes at least one self-contained frangible continuity circuit electrically connected to the secondary coil. The inductive coupling induces an electrical current flow in the continuity circuit when the circuit is undamaged. A detector cooperates with the primary and secondary coils when the interrogator is in the interrogation position. The detector detects the inductive coupling. An indicator cooperates with the detector.