Static transfer switch device and method
View Patent ↗Devices and methods for improved voltage source transfer switching. First and second voltage sources (primary and alternate) are connectable to a load via corresponding first and second switches, which may comprise SCRs. A controller operates the switches according to various transfer methods. In some transfer methods, the timing of the switching operation is critical. For example, switching times may be optimized to reduce or prevent transformer saturation due to the dc flux built up during the transfer event. The controller force commutates off the primary source switch by turning on the appropriate alternate source switch for a brief period of time. In this case, the alternate source switches are “pulsed” (rather than being turned on continuously), so that the switches will naturally commutate off at the next current zero cross. Subsequently, the volt-second balancing control logic will permanently turn on these switches at the appropriate time.
1. A transfer switch system, comprising:
a first switch connectable to a first voltage source;
a second switch connectable to a second voltage source; and
a controller connected to the first and second switches to activate and deactivate the first and second switches to selectively connect the first or the second switch to a load via a transformer;
the controller having inputs for receiving signals representing the voltage levels of the first and second voltage sources and the voltage applied to the load, wherein upon a predetermined condition, the controller temporarily pulses on the second switch and thereafter, turns on the second switch, wherein the controller determines a switching time to minimize downstream saturation current and turns on the second switch at the switching time.
2. The transfer switch system of claim 1 , wherein the controller temporarily pulses the second switch prior to the switching time.
3. The transfer switch system of claim 1 , wherein the controller determines the switching time in response to the signals representing the voltage levels of the first and second voltage sources.
4. A transfer switch system, comprising:
a first voltage source;
a second voltage source;
a first switch connected to the first voltage source;
a second switch connected to the second voltage source;
a transformer having an input connected to the first and second switches to selectively connect the transformer to the first or second voltage source, the transformer having output terminals connectable to a load; and
a controller connected to the first and second switches to activate and deactivate the first and second switches to selectively connect the first or the second switch to the transformer;
the controller receiving signals representing the voltage levels of the first and second voltage sources and the voltage applied to the transformer input, wherein upon a predetermined condition, the controller temporarily pulses on the second switch and thereafter, turns on the second switch, wherein the controller determines a switching time to minimize downstream saturation current and turns on the second switch at the switching time.
5. The transfer switch system of claim 4 , wherein the controller temporarily
pulses the second switch prior to the switching time.
6. The transfer switch system of claim 4 , wherein the controller determines the switching time in response to the signals representing the voltage levels of the first and second voltage sources.
7. A transfer switch system, comprising:
a first silicon controlled rectifier connected to a first voltage source;
a second silicon controlled rectifier connected to a second voltage source; and
a controller connected to the first and second silicon controlled rectifiers to activate and deactivate the first and second silicon controlled rectifiers to selectively connect the first or the second silicon controlled rectifiers to a load;
the controller having inputs for receiving signals representing the voltage levels of the first and second voltage sources and the voltage applied to the load, wherein upon a predetermined condition, the controller temporarily pulses on the second silicon controlled rectifier and thereafter, indefinitely turns on the second silicon controlled rectifier to maintain power to the load.
8. A transfer switch system, comprising:
a first silicon controlled rectifier connected to a first voltage source;
a second silicon controlled rectifier connected to a second voltage source; and
a controller connected to the first and second silicon controlled rectifiers to activate and deactivate the first and second silicon controlled rectifiers to selectively connect the first or the second silicon controlled rectifiers to a load via a transformer;
the controller having inputs for receiving signals representing the voltage levels of the first and second voltage sources and the voltage applied to the load, wherein upon a predetermined condition, the controller temporarily pulses on the second silicon controlled rectifier and thereafter, substantially permanently turns on the second silicon controlled rectifier to maintain power to the load.