Arc-free switching of hundreds of volts is possible using a MEMS switching device developed by a team of researchers with General Electric Global Research and General Electric Industrial Solutions. The resulting arrays are able to conduct current more efficiently and can open orders of magnitude faster than traditional macroscopic mechanical relays. The prototype system has been used to turn on and off a 3/4-hp motor and, more importantly, to provide arc-less protection in a test simulating a 16,000A fault current. Applications envisioned for this highly-scalable technology range from mobile phone handsets to grid-connected appliances.
Arc-free mechanical switching has been achieved through a combination of fast switching speed and the ability to open the contacts at a forced and momentary artificially induced zero voltage. The fast switching speed is achieved through an electrostatically actuated micromechanical switch that is micrometers in size and that is switched between the open and closed states in microseconds.
“Fast switching speed is critical in order to open the contacts at precisely the instant when a shunt is momentarily activated to provide a near-zero voltage across the mechanical contacts,†explained Chris Keimel, leader of the research team. “The momentary shunt is established around the mechanical switches for a few microseconds by using a pulsed balanced diode bridge. The diode bridge functions to divert the load current momentarily away from the switches, and when properly balanced, the bridge creates a near-zero voltage potential across the switch contacts. When the switches are in the closed state, they can be scaled to carry the current levels associated with steady state, inrush, and momentary transient currents,†Keimel continued.
Very-fast fault current detection and interruption is needed to mitigate damage caused by fault currents. As a result, sensing and control are critical elements of this system. Upon detection of a fault condition, the diode bridge shunt is activated to force a temporary near-zero voltage condition while the switch’s mechanical contacts separate rapidly, commutating current to the diode bridge due to the increasing (nonlinear) switch resistance. A minimal transient voltage spike, on the order of a few tens of millivolts, is induced from the die packaging and system local stray inductance.
The switches open with only a minimal induced transient voltage spike caused by the localized stray inductance. The transient voltage spike is not sufficient to cause an arc across the contacts. After a few microseconds, the switches have fully opened, and the pulsed diode bridge turns off, leaving the micro switches to hold the full system applied voltage. To close the switch safely with voltage present across the contacts, an analogous sequence— pulsing the diode bridge to collapse the voltage across the contacts while they rapidly close—is used.
“The contacts exhibit no visible surface damage after thousands of switching operations when protected in this manner,†Keimel stated. “This technology has the potential to provide next-generation protection capability by enabling fast mechanical switching speeds that limit fault currents up to 100X, reduce fault energies by up to 1,000,000X, switch loads, and enables faulted circuits to be completely arc freeâ€
Fault current interruption takes place in less than 10 microseconds, almost regardless of the prospective fault current magnitude. The properties of the MEMS switch arrays allow the switching mechanism to operate at temperatures in excess of 200 degrees C. The switches also have a vibration tolerance in excess of 1000G. The combination of fast MEMS switching speed, optimized current and voltage handling capacity of the switch arrays, the arc-suppression circuitry, and optimized sensing and control enables a single sensing, control, and switching system to operate in a few tens of microseconds.
“When fault situations occur in electrical power distribution systems, conventional power circuit protection devices, even current-limiting ones, can react too slowly to adequately limit destructive energy dissipation that damages electronic equipment downstream from the fault interrupter. Additionally, even normal-interruption-related discharge plasmas and arc energy can eventually damage the contacts used in breakers and contactors, thereby rendering the devices inoperable,†Keimel observed.
“Our team has developed and demonstrated a micrometer-scaled ultrafast mechanical switch array and integrated the array with fast electrical bypass circuitry to create a system that switches electrical energy, without a significant arc, in a few microseconds. Arc energy between the switching contacts is reduced by a factor of up to 1 million, and this small amount of energy does not damage the MEMS microscale contact gap or the nanosized contact surface topography of the contacts. This ultrafast and arc-free switching system (current sensing, decision logic, control logic, switch opening, and commutation) capability responds to a fault much faster than even a fuse and is completely resettable due to the lack of arc damage,†Keimel concluded.
GE is interested in licensing this technology or jointly-developing commercial systems. The switch mechanisms can be scaled from milliamperes to tens of amperes and from millivolts to hundreds of volts. With further application-specific development, it is expected to be possible that this type of switching could take over from traditional mechanical switching for overcurrent protection and control and transfer switch applications.
The fast switching enables supervising electronics to control multiple switching devices simultaneously allowing, essentially, one processor to control interruption, source transfer, or completely change a distribution scheme’s topology within 20 microseconds, much less than typical mechanical systems that switch within one-half of a power cycle. The switching does not differentiate between dc and ac currents, handling both with equal facility further expanding potential applications.
