Source-transfer switching system and method

Abstract

A control arrangement and method is provided for source-transfer switching systems including a high-speed control arrangement and high-speed switches to control the transfer of a load from one source to another with minimal transfer delays. The system supplies an electrical load with alternating current from either a first source or a second source via respective first and second switches. The system also includes a controller that samples the voltage waveforms of each of the first and second sources to detect when transfer between the sources is desirable, e.g. power outages, voltage levels that are either too low or too high, or reverse currents that indicate an upstream fault. The controller provides appropriate control signals to control operation of the switches and transfer supply of the load therebetween. In a preferred embodiment, after a transfer decision is made, the control arrangement initiates a transfer of the load from a first source to a second source with no intentional delay via the opening of a first switch and the closing of a second switch. In another embodiment, the control arrangement initiates a transfer by first opening the first switch and then closing the second switch after the current is interrupted through the first switch. In yet another embodiment, transfer is accomplished via an 'open' transition, break before make, when severe disturbances of a first category are detected, and transfer is accomplished via a 'closed' transition, which may involve make-before-break conditions, when less severe disturbances of a second category are detected.