A safety device for a DC grid includes a voltage measuring device between a potential line and a reference potential line, and a protective circuit for reducing an electric shock caused by Y-capacitors of the electric DC grid. The protective circuit includes a circuit breaker between the respective potential line and the reference potential line. A plurality of tripping criteria are predefined and the first circuit breaker and/or the second circuit breaker can be actuated so as to close exclusively in the event of all predefined tripping criteria being met as determined by the first voltage measuring device and/or by the second voltage measuring device.

A power system performs a method comprising: applying normal voltages to a load as load voltages; upon detecting a power fault that necessitates applying alternate voltages to the load in place of the normal voltages, disconnecting the normal voltages from the load; while waiting to detect a volt-second balance between the alternate voltages and the load voltages, determining one or more qualified voltages of the alternate voltages that will not saturate the load, and applying the one or more qualified voltages to the load as the load voltages; upon detecting the volt-second balance, additionally applying to the load each alternate voltage that is not the one or more qualified voltages; and after additionally applying, volt-second balancing the alternate voltages with the load voltages.

A power system performs a method comprising: applying normal voltages to a load as load voltages; upon detecting a power fault that necessitates applying alternate voltages to the load in place of the normal voltages, disconnecting the normal voltages from the load; while waiting to detect a volt-second balance between the alternate voltages and the load voltages, determining one or more qualified voltages of the alternate voltages that will not saturate the load, and applying the one or more qualified voltages to the load as the load voltages; upon detecting the volt-second balance, additionally applying to the load each alternate voltage that is not the one or more qualified voltages; and after additionally applying, volt-second balancing the alternate voltages with the load voltages.

A control apparatus including a first operation control circuit configured to open a direct-current switch when a power conversion apparatus is in a waiting state and a first direct-current voltage value is greater than a predetermined first threshold, a clocking circuit configured to, when the direct-current switch is opened, clock an elapsed time after the direct-current switch is opened, and a soundness determination circuit configured to calculate a first difference voltage value that is a difference between the first direct-current voltage value and a second direct-current voltage value when the power conversion apparatus is in the waiting state and the direct-current switch is opened, and the elapsed time is greater than or equal to a predetermined first time, to determine that the direct-current switch is abnormal when the first difference voltage value is less than or equal to the predetermined second threshold.

A control apparatus including a first operation control circuit configured to open a direct-current switch when a power conversion apparatus is in a waiting state and a first direct-current voltage value is greater than a predetermined first threshold, a clocking circuit configured to, when the direct-current switch is opened, clock an elapsed time after the direct-current switch is opened, and a soundness determination circuit configured to calculate a first difference voltage value that is a difference between the first direct-current voltage value and a second direct-current voltage value when the power conversion apparatus is in the waiting state and the direct-current switch is opened, and the elapsed time is greater than or equal to a predetermined first time, to determine that the direct-current switch is abnormal when the first difference voltage value is less than or equal to the predetermined second threshold.

A power system performs a method comprising: applying normal voltages to a load as load voltages; upon detecting a power fault that necessitates applying alternate voltages to the load in place of the normal voltages, disconnecting the normal voltages from the load; while waiting to detect a volt-second balance between the alternate voltages and the load voltages, determining one or more qualified voltages of the alternate voltages that will not saturate the load, and applying the one or more qualified voltages to the load as the load voltages; upon detecting the volt-second balance, additionally applying to the load each alternate voltage that is not the one or more qualified voltages; and after additionally applying, volt-second balancing the alternate voltages with the load voltages.

A power system performs a method comprising: applying normal voltages to a load as load voltages; upon detecting a power fault that necessitates applying alternate voltages to the load in place of the normal voltages, disconnecting the normal voltages from the load; while waiting to detect a volt-second balance between the alternate voltages and the load voltages, determining one or more qualified voltages of the alternate voltages that will not saturate the load, and applying the one or more qualified voltages to the load as the load voltages; upon detecting the volt-second balance, additionally applying to the load each alternate voltage that is not the one or more qualified voltages; and after additionally applying, volt-second balancing the alternate voltages with the load voltages.