Transfer switch device and method
View Patent ↗This disclosure is concerned with devices and methods for voltage source transfer switching that reduces or eliminates transformer saturation due to DC flux built up during a transfer event. First and second voltage sources (primary and alternate) are connectable to a load via corresponding switches. A transformer is connected downstream of the switches. A controller operates the switches according to various transfer methods wherein a switching time is determined to minimize downstream saturation current.
1. A method of switching first and second voltage sources, comprising:
disconnecting the first voltage source from a load:
computing in substantially real time volt-second areas from waveforms of the first and second voltage sources;
assigning a polarity to each volt-second area based on the polarity of the corresponding voltage waveform;
determining a switching time based on when absolute values of the volt-second areas of the first and second voltage sources are approximately equal and the polarities are opposite; and
connecting the second voltage source to the load at the determined switching time so that downstream saturation current is minimized.
2. A method of switching first and second voltage sources, comprising:
disconnecting the first voltage source from a load;
computing in substantially real time volt-second areas from voltage waveforms of the first and second voltage sources;
assigning a polarity to each volt-second area based on the polarity of the corresponding voltage waveform;
summing absolute values of the volt-second areas under the waveforms of the first and second voltage sources;
determining a switching time when the sum of the absolute values of the volt-second areas of the first and second voltage sources are approximately equal to an absolute value of a volt-second area of a complete half cycle of the second voltage waveform when their polarities are the same; and
connecting the second voltage source to the load at the determined switching time.
3. A power source system, comprising:
a first switch connectable to a first voltage source;
a second switch connectable to a second voltage source;
a controller operationally connected to the first and second switches to selectively activate and deactivate the first and second switches to 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 transformer,
the controller configured to, upon a predetermined condition:
deactivate the first switch to disconnect the first voltage source from the load, compute substantially in real time volt-second areas under voltage waveforms of the first and second voltage sources, and
assign a polarity to each volt-second area based on the polarity of the corresponding voltage waveform;
determine a switching time current based on when absolute values of the areas under the waveforms of the first and second voltage sources are approximately equal and their polarities are opposite; and
connect the second voltage source to the load at the determined switching time to minimize downstream saturation.
4. A transfer switch system, comprising:
a first switch connectable to a first voltage source;
a second switch connectable to a second voltage source;
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 transformer, wherein upon a predetermined condition, the controller:
deactivates the first switch to disconnect the first voltage source from the load,
computes in substantially real time volt-second areas under voltage waveforms of the first and second voltage sources,
assigns a polarity to each volt-second area based on the polarity of the corresponding voltage;
determines a switching time to minimize downstream saturation current based on the sum of the absolute values of the areas under the waveforms of the first and second voltage sources being approximately equal to the absolute value of the area under the waveform of a complete half cycle of the second voltage, and their polarities being the same; and
connects the second voltage source to the load at the determined switching time.
5. An uninterruptible power supply 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:
deactivates the first switch to disconnect the first voltage source from the transformer;
computes volt-second areas under voltage waveforms of the first and second voltage sources in real time;
assigns a polarity to each volt-second area based on the polarity of the corresponding voltage;
determines a switching time to minimize downstream saturation current when the absolute values of the areas under the waveforms of the first and second voltage sources when they are approximately equal and their polarities are opposite; and
connects the second voltage source to the transformer at the determined switching time.
6. An uninterruptible power supply 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:
deactivates the first switch to disconnect the first voltage source from the transformer; computes volt-second areas under voltage waveforms of the first and second voltage sources in real time;
assigns a polarity to each volt-second area based on the polarity of the corresponding voltage;
determines a switching time to minimize downstream saturation current when the sum of the absolute values of the areas under the waveforms of the first and second voltage sources are approximately equal to the absolute value of the area under the waveform of a complete half cycle of the second voltage, and when their polarities are the same; and
connects the second voltage source to the transformer at the determined switching time.