Selective coordination of solid-state circuit breakers and mechanical circuit breakers in electrical distribution systems
In an electrical distribution system including a solid-state circuit breaker (SSCB) and one or more downstream mechanical circuit breakers (CBs), a solid-state switching device in the SSCB is repeatedly switched ON and OFF during a short circuit event, to reduce a root-mean-square (RMS) value of the short circuit current. The resulting pulsed short circuit current is regulated in a hysteresis control loop, to limit the RMS to a value low enough to prevent the SSCB from tripping prematurely but high enough to allow one of the downstream mechanical CBs to trip and isolate the short circuit. Pulsing is allowed to continue for a maximum short circuit pulsing time. Only if none of the downstream mechanical CBs is able to trip to isolate the short circuit within the maximum short circuit pulsing time is the SSCB allowed to trip.
1. A method of selectively coordinating a solid-state circuit breaker (SSCB) among one or more downstream mechanical circuit breakers (CBs) in an electrical distribution system, comprising:
determining whether a short circuit current is flowing through the SSCB and one or more downstream mechanical CBs in the electrical distribution system;
if the short circuit current is determined to be flowing, pulsing the short circuit current by repeatedly switching a solid-state switching device, through which the short circuit current is flowing in the SSCB, ON and OFF;
limiting a root-mean-square (RMS) value of the pulsed short circuit current to a value low enough to prevent the SSCB from tripping prematurely and high enough to allow one of the downstream mechanical CBs to trip and isolate the short circuit; and
switching the solid-state switching device in the SSCB OFF permanently if none of the downstream mechanical CBs trips to isolate the short circuit after a predetermined short circuit pulsing time.
2. The method of claim 1 , wherein limiting the RMS value of the pulsed short circuit current comprises regulating the pulsed short circuit current in a hysteresis current control loop.
3. The method of claim 1 , further comprising restricting pulsing of the short circuit current to times in each cycle of a line voltage waveform that a magnitude of the line voltage is less than a predetermined voltage threshold.
4. The method of claim 1 , further comprising:
counting the number of pulses generated during pulsing the short circuit current; and
switching the solid-state switching device in the SSCB OFF permanently if it is determined that the number of pulses counted corresponds to a time exceeding the predetermined short circuit pulsing time.
5. The method of claim 1 , wherein the predetermined short circuit pulsing time is set by a controller in the SSCB and is adjustable depending on time-current characteristics of one or more of the one or more downstream mechanical CBs.
6. The method of claim 1 , wherein the RMS value of the pulsed short circuit current is set by a controller in the SSCB and is adjustable depending on time-current characteristics of one or more of the one or more downstream mechanical CBs.
7. The method of claim 1 , further comprising;
sensing the short circuit current on a load side of the SSCB;
determining a rate of change of the short circuit current based on the sensed short circuit current;
determining whether a system impedance of the electrical distribution system is significantly inductive based on the determined rate of change of the short circuit current; and
if it is determined that the system impedance is significantly inductive, waiting for a line voltage to decay below a predetermined voltage threshold before each time the solid-state switching device is switched back ON during pulsing the short circuit current.
8. The method of claim 7 , further comprising:
if the system impedance is determined to be significantly inductive, counting a number of inductive pulses that a surge protection device in the SSCB is exposed to during pulsing the short circuit current; and
switching the solid-state switching device in the SSCB OFF if the number of inductive pulses counted exceeds a predetermined maximum inductive pulse count correlating to a safe operating area of the surge protection device.