Fast fault current limiter
AssigneeKoolbridge Solar, Inc.
InventorPaul Wilkinson Dent
Life of this patent
Estimating the term…
Abstract
A fast-operating AC fault current limiter, to limit the fault current let through to downstream equipment during short circuit or low-impedance faults, comprises a series inductor to limit current rise time, a current sensor to sense the instantaneous current, a series AC semiconductor switch to interrupt current flow when it exceeds the maximum desired fault current, and a shunt AC semiconductor switch to catch inductor flyback voltage when the series semiconductor switch is opened. Each of the series and the shunt AC switches comprises two back-to-back MOSFETs. Inventive control of timing of the individual MOSFETs obviates the need for exact simultaneous timing of opening the series switch and closing the shunt switch, which is otherwise required to avoid short circuits.
Claims21
A fault current limiting circuit operative to limit an instantaneous fault current, let through from an alternating current source to a load, to a maximum desired value under fault conditions, comprising:
- an inductor in series with a current path from source to load operative to limit a rate of rise of current under fault conditions;
- a first semiconductor switch in series with the current path from source to load comprising two series transistors connected in reverse and configured to be independently controlled to close and pass alternating current with low voltage drop, or to open to block alternating current flow from said alternating current source;
- a second semiconductor switch in shunt with the current path from said source to said load comprising two series transistors connected in reverse and configured to be independently controlled to maintain current flow through said inductor in one or both of a first direction and a second direction, when said first semiconductor switch is opened, thereby limiting the development of inductor flyback voltages;
- a current sensor operative to sense instantaneous current flowing through said inductor and further operative to output a signal indicative of the instantaneous current;
- and
- a controller operative to receive said output signal from said current sensor and determine a fault condition exists, and further operative to independently control the two transistors of each of said first semiconductor switch and said second semiconductor switch to close said first semiconductor switch and open said second semiconductor switch in the absence of a fault condition so as to pass current from the source to the load, and to limit fault current to a predetermined value when a fault condition exists, by repetitively, and oppositely, opening and closing said first semiconductor switch and said second semiconductor switch in a sequence to control the mean current flow.
A method of limiting instantaneous fault current in an electrical power supply circuit between an alternating current (AC) source and a load, comprising:
- providing an inductor in series with a current path from source to load operative to limit a rate of rise of current under fault conditions;
- sensing an instantaneous current flowing through the inductor;
- if the instantaneous current exceeds a maximum desired value,
- independently controlling two series transistors forming a switch in series with the inductor to block current flow from the source;
- and
- independently controlling two series transistors forming a shunt switch between the inductor and a current return path to the source to maintain current flow through the inductor when the series switch is opened, thereby limiting the development inductor flyback voltages;
- wherein the switch actuations are timed such that the switches are never simultaneously both open or both closed.
Dependencies are inferred from the claim text.