Arrangements to detect and respond to disturbances in electrical power systems
A control arrangement and method is provided for detecting and responding to disturbances in electrical power systems. In a preferred arrangement, an integration is initiated that is based on a comparison of actual voltage of a source and a reference voltage. When the integration exceeds a predetermined value, the source is considered unreliable. Also in a preferred arrangement, a determination is made as to whether or not the disturbance is a downstream fault condition. For example, this is useful for applications where a transfer is made from a first source to a second source when predetermined disturbances are detected. In this manner, the transfer of the load to a second source is avoided which would continue the supply of the downstream fault. Additionally, the arrangement distinguishes between various degrees of disturbances to permit appropriate response based on the severity and type of disturbance. For example, a first immediate response, i.e. without intentional delay, is provided for more severe disturbances while a second delayed response is provided for less severe disturbances. The control arrangements transfers the load to an alternate source of power via the use of a high-speed source-transfer switching system that both avoids undesirable current flow between sources and minimizes undesirable transfer delays.
1. A control arrangement for a source-transfer switching system of the type that includes first and second solid-state switches that are controlled to selectively supply a load circuit connection with an alternating-current waveform via one of the two first and second solid-state switches, the control arrangement comprising:
transfer control means for controlling the first and second solid-state switches to selectively supply the load circuit connection from one of the first and second solid-state switches, said transfer control means further comprising first means for inhibiting a change in the supply of the load circuit connection between the first and second solid-state switches when a fault is detected that is downstream of the control arrangement, said first means further comprising second means for comparing sensed current and voltage at various points along the alternating-current waveform to determine if the sensed current establishes a downstream fault current, said first means further comprising third means operative when said second means does not establish a downstream fault current for delaying determination for a predetermined time interval if the point along the waveform is within a predetermined range of a voltage zero point, and fourth means for determining that a downstream fault current is not present if the second means does not establish a downstream fault current by the end of the time interval.
2. The control arrangement of claim 1 wherein said predetermined range represents a portion of the waveform approaching a voltage zero.
3. The control arrangement of claim 1 wherein the time interval is a portion of a cycle of the waveform.
4. The control arrangement of claim 1 wherein said time interval corresponds to approximately ⅛ cycle of the waveform.
5. A control arrangement for a source-transfer switching system of the type that includes first and second solid-state multi-phase switches that are controlled to selectively supply a multi-phase load circuit connection with a multi-phase alternating-current waveform via one of the two first and second multi-phase solid-state switches, the control arrangement comprising:
transfer control means for controlling the first and second multi-phase solid-state switches to selectively supply the load circuit connection from one of the first and second multi-phase solid-state switches, said transfer control means further comprising first means for inhibiting a change in the supply of the load circuit connection between the first and second multi-phase solid-state switches when a fault is detected that is downstream of the control arrangement, said first means further comprising second means for sensing the phase-to-phase and phase-to-ground voltages and currents as to current magnitude and direction for the multi-phase load-circuit connection at various points along the alternating-current waveform so as to provide indication of downstream, upstream or no determination for each of the sensed currents, said first means further comprising third means responsive to said second means for determining the existence of a downstream fault based on a majority of the indications by the second means.