The invention relates generally to point on wave switching and more particularly to real-time prediction of zero crossings of fault currents for use in point on wave switching.
As described in commonly assigned Long et al., U.S. Pat. No. 4,922,363, to apply electromechanical contactors for switching currents in power systems that have available fault currents greater than the interrupting capacity of a contactor, it is necessary to protect the contactor from damage by backing it up with a series device that is sufficiently fast acting to interrupt fault currents prior to the contactor opening at all values of current above the interrupting capacity of the contactor. In control gear, back up fuses are used to provide this function. These fuses must also be capable of interrupting the maximum prospective fault current that can flow during a short circuit. In order to maintain good contactor-fuse coordination, the back up fuse must fully protect the contactor without subjecting the contactor to any time-current zones that may make the contactor vulnerable to damage. Poor contactor-fuse coordination can result if contactor tips open on a fault above their interrupting capacity before the fuse has time to clear since fuses do not have instantaneous trip characteristics. The period of time for a fuse to clear depends on the level of fault current. Optimum contactor-fuse coordination is obtained when the fuse clears a fault just before the contactor tips open. If the contactor tips open before the fuse clears the fault, an arc may continue across the open contact tips until the fuse clears. The arc (in air break contactors) introduces some additional impedance into the circuit that may delay fuse operation.
The challenges discussed in aforementioned Long et al., U.S. Pat. No. 4,922,363 that are associated with contactors are additionally present for other types of switching devices. With knowledge of zero crossings of fault current in a power system, operation of a switching device can be controlled to be at a specific point on the waveform of interest.
It would therefore be desirable to have improved capabilities for predicting zero crossings of fault current in a power system.
Briefly, in accordance with one embodiment of the present invention, a method for predicting zero crossings of fault currents in multi-phase power systems includes sensing a fault current in each respective phase, estimating parameters of a model of each respective fault current, and independently using the estimated parameters for each respective fault current to predict a zero crossing (here and hereinafter meaning at least one zero crossing) of the respective fault current.
In accordance with another embodiment of the present invention, a method for predicting zero crossings of a fault current in a power system includes sensing the fault current; estimating parameters of a model of the fault current; and using the estimated parameters to predict a zero crossing of the fault current by (a) selecting an initial time interval in which a zero crossing is present, (b) identifying a portion of the interval that includes the zero crossing, (c) changing the interval to comprise the identified portion, and (d) determining whether the changed interval provides a desired resolution, and, if not, cycling through elements (b)-(d) until the changed interval provides the desired resolution.
In accordance with another embodiment of the present invention, a method for predicting zero crossings of a fault current in a power system includes sensing the fault current; estimating parameters of a model of the fault current; and using the estimated parameters to predict a zero crossing of the fault current by (a) predicting a predicted post-fault current zero crossing, (b) determining an actual post-fault current zero crossing, (c) determining a difference between the predicted and actual post fault current zero crossing, and (d) using the difference to predict an additional post-fault current zero crossing, the additional crossing occurring subsequent to the predicted crossing.