Patent Application
20240396324
The present disclosure pertains to devices for managing electrical power and, more particularly, to a remotely switchable circuit breaker that is constructed with a microelectromechanical (MEMS) relay.
There is a need for a remotely switchable circuit breaker for load shedding and “smart grid” power management applications. This has not been possible to date, because the switching components available are too large, too power inefficient and too expensive for an economically viable switchable breaker.
Various suppliers are manufacturing and selling ‘smart circuit breakers’ which can be installed in existing or new load centers in residences and commercial buildings. The goal of smart breakers is to provide existing circuit protection capability while adding the capability to be remotely controlled. These existing devices use semiconductor switches and/or conventional electro-mechanical relays to control the current path.
For both technical and economic reasons, overcurrent protection is usually still provided by conventional breaker technology (thermal or magnetic trip). In the case of breakers made with semiconductors in the primary current path, the forward voltage required by semiconductor switches combined with their high on-resistance leads to untenable heat dissipation levels for all but the lowest current rating breakers (10-15A), and even those require nontrivial thermal design work (including heat sinking, which requires significant space). Exacerbating the situation, an air gap solution is still required in semiconductor breakers, per the UL489 standard, and that is typically provided with an additional conventional electromechanical relay, adding cost and physical size.
Smart breakers made utilizing electromechanical relays are very bulky because the components are too large to fit into a standard breaker housing along with all other required components. The size of breakers is important, because the form factor of load centers is established and there is limited total space for breakers. An oversized breaker consumes two or three pole locations, reducing the total number of circuit breakers that can be installed in the load center.
Beyond the size, the cost of the existing smart breaker products is prohibitive, driven by the cost of the additional switching componentry. Between the cost, size and thermal issues, the market for smart breakers remains largely unsatisfied.
The present disclosure is directed to a hybrid breaker that utilizes a microelectromechanical (MEMS) relay. In accordance with one aspect of the present disclosure, a hybrid breaker for use with a voltage source and a load is provided. The breaker includes a first relay circuit having first and second control input terminals, the first relay circuit coupled in series with the voltage source and load via a line in input terminal coupled to the voltage source, and a line out output terminal coupled to the load, the first relay circuit including a MEMS device; a second relay circuit having first and second control input terminals, the second relay circuit coupled in parallel to the first relay circuit with a line in input terminal coupled to the voltage source, and a line out output terminal coupled to the load; and a control logic circuit having a first pair of control terminals coupled to the first and second control input terminals of the first relay circuit, a second pair of control terminals coupled to the second relay circuit, and at least one current sense input coupled to the voltage source.
This unique device combines low on-resistance (less than 10 milliohms, and preferably less than 1 milliohm, and more preferably less than 100 micro-Ohms) with extremely small size (dimensions less than 1.5 cm diameter by 1 cm thick and preferably less than 10 mm diameter by 4 mm thick). Using this relay as a component within a smart circuit breaker meets several needs:
The MEMS relay of the present disclosure (which is electronically controllable) becomes the primary current-carrying component in the circuit breaker. Because of its small mass, the MEMS relay must be protected from two cases: (1) arcing during opening and/or closing; and (2) extremely high current cases, caused by short circuits or similar phenomena. Circuit breakers are rated to different levels of “High Interrupting Current” (HIC), and the relay of the present disclosure enables at least the 10 kA and 22 kA cases to be addressed.
To prevent arcing during scheduled opening or closing, zero current or zero voltage switching is used respectively. Current and voltage sensors are used to determine the most favorable time for the switch transition to minimize arcing potential.
In the HIC case, the current rises too fast to allow waiting for a zero crossing before opening, so a different solution is utilized. In this HIC case, a solid-state relay (MOSFETS or similar) is wired in parallel with the MEMS relay, and when the logic monitoring the rate of change of current with respect to time of the system detects an HIC condition, the solid-state relay is closed to provide a bypass current path. The MEMS relay is then opened, and once it reaches a safe opening gap for the required voltage standoff the solid-state relay is opened. The current rises at a rate that is determined by the overall circuit impedance and is on the order of 2 A-15 A/μs. Therefore, early detection of a true short circuit (rather than inrush or other normal current fluctuations) and a rapid response to open the circuit is paramount to reducing the overall load seen by the switch during opening. The high speed of the MEMS relay makes this type of design possible at an economic price point, because the solid-state relay must only conduct for the brief time period during which the MEMS relay is moving from its closed position to a minimum contact gap. A solid-state relay consisting of MOSFETs, IGBTs (Insulated Gate Bipolar Transistors) or similar semiconductor devices is utilized to provide a temporary low-impedance channel for current while the MEMS relay switches state.
One significant concern for this hybrid approach is the cost of the semiconductors required, particularly the power MOSFETs, which need to conduct the current during the time the MEMS relay is partially open. These MOSFETs need to dissipate electrical energy in the form of heat. The energy to dissipate is the integral over time of the product of the current and the voltage. The voltage across the MOSFETs is, in turn, the product of the current and the internal resistance of the MOSFETs. This energy is expressed as the number of Joules a MOSFET can tolerate in a single event. The total energy may be reduced by one or both of a) shortening the time during which the MOSFETs must conduct the current and b) reducing the rate of current increase.
These two approaches used separately or in combination permit the use of less capable and therefore less costly semiconductor components in the Solid-State Relay used to carry the current during the MEMS relay opening time.
An alternative approach to the solid-state bypass during the HIC case is the use of a secondary relay. This relay is connected in parallel with the primary relay and is used to take the brunt of the energy during a HIC opening. This design takes advantage of the UL regulatory requirements for circuit breakers that stipulate that a breaker only has to survive two such HIC events. The secondary relay could be built with higher on-resistance and more arc resistant contacts including an arc chute. Its resistance is not critical as it only conducts during the opening time of the primary relay. During a HIC event the primary relay would open ahead of this secondary relay directing all of the arc energy to the latter. Unlike solid-state relays, a mechanical relay does not have PN junctions that impose a minimum voltage for conduction to occur. This allows for a smooth redirection of the current from the primary relay to the secondary relay, since they are already actively sharing current at that time. This reduces inductive kick back and its associated jump in voltage across the primary MEMS relay, which is advantageous to prevent arcing.
During scheduled openings the two relays can be switched using the same timing, which results in zero current crossing opening and zero voltage crossing closing of the relays.
If the MEMS relay is designed to never fail closed during a HIC event, then a third approach could be used where three such relays are wired in parallel. Only one of them will be used at a time. If a HIC event is detected and dealt with by that relay, it is declared (electronically) out of commission and the next one is selected for future HIC events. This meets the UL requirement of surviving two HIC events while also having the advantage of giving the user feedback as to when a replacement might be needed.
Another advantage of the electromechanical bypass over a solid-state approach is that it provides an air gap with a single series element. This reduces overall power consumption of the breaker as the current only flows through a single contact.
In an embodiment, a circuit for use in circuit breakers with a voltage source and a load, includes a first relay circuit having first and second control input terminals. The first relay circuit is coupled in series with the voltage source and the load using one or more line in input terminals coupled to the voltage source and one or more line out output terminals coupled to the load. The first relay circuit includes a MEMS device. The circuit includes a second relay circuit having first and second control input terminals. The second relay circuit is coupled in parallel with the first relay circuit using the one or more line in input terminals coupled to the voltage source and the one or more line out output terminals coupled to the load. The circuit includes a control logic circuit having a first pair of control terminals coupled to the first and second control input terminals of the first relay circuit, a second pair of control terminals coupled to the second relay circuit, and at least one current sense input coupled to the voltage source.
In an embodiment, the control logic circuit includes a third pair of control terminals and the circuit includes a third relay circuit having first and second control input terminals coupled to the third pair of control terminals and one or more line in input terminals coupled to the voltage source and one or more line out output terminals coupled to the load such that the third relay circuit is coupled in parallel with the first and second relay circuits.
In an embodiment, the second and third relay circuits each have a relay design that is different from the MEMS device of the first relay circuit. In an embodiment, the third relay circuit is a sacrificial device. In an embodiment, the second and third relay circuits each include a MEMS relay that is identical to the MEMS device of the first relay circuit.
In an embodiment, the control logic circuit is configured to determine whether the second relay circuit has been damaged by an earlier short circuit. In an embodiment, in response to determining that the second relay circuit has not been damaged, the control logic circuit is configured to control the first and second relay circuits to operate in a closed state during normal operation, detect an increase in current, in response to detecting the increase in current, control the first relay circuit to transition to an open state, delay for a fixed time duration, and after delaying for the fixed time duration, control the second relay circuit to transition to the open state; and
In an embodiment, the control logic circuit is configured to in response to determining that the second relay circuit has been damaged, control the first and third relay circuits to operate in the closed state during normal operation, detect an increase in current, in response to detecting the increase in current, control the first relay circuit to transition to the open state, delay a fixed amount of time duration, and after delaying for the fixed time duration, control the third relay circuit to transition to the open state.
In an embodiment, the second relay circuit includes a solid-state relay circuit. In an embodiment, the second relay circuit is a sacrificial device.
In an embodiment, the control logic circuit is configured to control the first and second relay circuits to operate in a closed state during normal operation, detect an increase in current, in response to detecting the increase in current, preemptively control the first relay circuit to transition to an open state, determine that the increase in current is a true short, and in response to determining that the increase in current is a true short, control the second relay circuit to transition to the open state. In an embodiment, the control logic circuit is configured to determine that the increase in current is a true short by determining that the true short is not an inrush current event.
In an embodiment, the control logic circuit is configured to control the first and second relay circuits to operate in a closed state during normal operation, detect an increase in current, in response to detecting the increase in current, preemptively control the first relay circuit to transition to an open state, determine that the increase in current is an inrush current event, and in response to determining that the increase in current is an inrush current event, control the first relay circuit to transition to the closed state.
In an embodiment, the control logic circuit is configured to control the first and second relay circuits to operate in a closed state during normal operation, detect an increase in current, in response to detecting the increase in current, control the first relay circuit to transition to an open state, delay for a fixed time duration, and after delaying for the fixed time duration, control the second relay circuit to transition to the open state.
In an embodiment, a method includes operating first and second relay circuits in a closed state. The first and second relay circuits are coupled to each other in parallel and the first and second relay circuits are together coupled in series to a voltage source and a load. The first relay circuit includes a MEMS device. The method includes sensing a current from the voltage source, detecting an increase in the current, in response to detecting the increase in current, preemptively controlling the first relay circuit to transition to an open state, determining whether the increase in current is a true short or an inrush current event, and selectively controlling the first relay circuit to transition to the closed state or the second relay circuit to transition to the open state depending on whether the increase in current is an inrush current event or a true short.
In an embodiment, selectively controlling the first relay circuit to transition to the closed state or the second relay circuit to transition to the open state includes in response to determining that the increase in current is an inrush current event, controlling the first relay circuit to transition to the closed state.
In an embodiment, selectively controlling the first relay circuit to transition to the closed state or the second relay circuit to transition to the open state includes in response to determining that the increase in current is a true short, controlling the second relay circuit to transition to the open state.
In an embodiment, the second relay circuit includes a MEMS relay that is identical to the MEMS device of the first relay circuit.
In an embodiment, a system includes a voltage source, a load, and a circuit including a first relay circuit having first and second control input terminals. The first relay circuit is coupled in series with the voltage source and the load using one or more line in input terminals coupled to the voltage source and one or more line out output terminals coupled to the load. The first relay circuit includes a MEMS device. The circuit includes a second relay circuit having first and second control input terminals. The second relay circuit is coupled in parallel with the first relay circuit using the one or more line in input terminals coupled to the voltage source and the one or more line out output terminals coupled to the load. The circuit includes a control logic circuit having a first pair of control terminals coupled to the first and second control input terminals of the first relay circuit, a second pair of control terminals coupled to the second relay circuit, and at least one current sense input coupled to the voltage source.
In an embodiment, the control logic circuit includes a third pair of control terminals and the circuit includes a third relay circuit having first and second control input terminals coupled to the third pair of control terminals. In an embodiment, the third relay circuit is coupled in parallel with the first and second relay circuits, and together the first, second and third relay circuits are coupled in series between the voltage source and the load.
In an embodiment, the second and third relay circuits each include a MEMS relay that is identical to the MEMS device of the first relay circuit. In an embodiment, the second and third relay circuits each have a relay design that is different from the MEMS device of the first relay circuit. In an embodiment, the second and third relay circuits are sacrificial devices. In an embodiment, the second and third relay circuits each include a solid-state relay circuit.
The foregoing and other features and advantages of the present disclosure will be more readily appreciated as the same become better understood from the following detailed description when taken in conjunction with the accompanying drawings, wherein:
In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed implementations. However, one skilled in the relevant art will recognize that implementations may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with breakers, relays, coils, and typical electrical components have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the implementations.
Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.”
Reference throughout this specification to “one implementation” or “an implementation” means that a particular feature, structure, or characteristic described in connection with the implementation is included in at least one implementation. Thus, the appearance of the phrases “in one implementation” or “in an implementation” in various places throughout this specification are not necessarily all referring to the same implementation. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more implementations. It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements or steps.
As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its broadest sense, that is as meaning “and/or” unless the content clearly dictates otherwise.
The headings and Abstract of the Disclosure provided herein are for convenience only and do not interpret the scope or meaning of the implementations.
The present disclosure is directed to a ‘hybrid’ breaker, utilizing both semiconductor and electro-mechanical components. This unique device is enabled by the availability of a MEMS relay that combines low on-resistance (in one instantiation <100 micro-Ohms) with extremely small size (in one instantiation dimensions approximately 10 mm diameter by 4 mm thick.) The semiconductor parts are used to determine when to open or close the MEMS relay. Semiconductors may also be used to protect the MEMS relay from arcing when opening under load. The decision to switch from open to closed or vice versa may be based on measuring an over-current condition caused by a circuit fault, or may be based on a signal coming from an external source. Such a signal could be transmitted over a wired or wireless connection.
The MEMS relay components in this design provide unique value because the miniature size of the MEMS relay allows the inclusion of multiple relays within a standard circuit breaker package. Standard relays are far too large to support this type of design. Keeping the overall circuit breaker to a standard size allows the breaker to be installed as a retrofit into existing circuit panels, greatly increasing the potential market for such a device.
In addition, the opening speed of the MEMS relay is dramatically faster than that of a standard mechanical relay. The small moving mass of the moving part allows it to accelerate extremely quickly. The opening time of the MEMS relay is less than 50 microseconds, whereas standard mechanical relay opening times are typically measured in milliseconds. The opening time of most solid-state relays (SSRs) can be as much as 8 milliseconds because most SSRs are only able to open at a zero crossing, which happens twice in a 1/60 second cycle. The rapid speed of the MEMS relay means that the current has significantly less time to climb before the relay is open. The energy which the relay must dissipate during this high current load is proportional to the current squared times the contact resistance. Thus, limiting the maximum current will reduce total energy in a more than linear fashion.
Electro-mechanical relays can experience arcing during opening or closing, when the gap between the contacts is smaller than the withstand distance required for the voltage being switched. The voltage experienced by the contacts may be significantly higher than the line voltage due to inductance in the load.
The present disclosure includes three possibilities for reducing the likelihood of arcing:
UL testing requires that circuit protection devices survive at least two High Available Fault Current Short Interruption events (“High Interruption Current” or “HIC” events) in which they are shorted while supplied with a high-current line source. In one implementation of Approach 3 above, the circuit breaker may include multiple sacrificial MEMS relays used as interrupting contacts to protect the primary MEMS relays from damage.
In each of the two required events one of the sacrificial MEMS relays may be destroyed. The next MEMS relay would then be brought into service to provide protection from the next event. The circuit breaker is thus prepared to survive n−1 events and to provide protection for n events, where n is the number of MEMS relays included in the package.
In normal operation, the MEMS relay 14 and Isolation Relay (27) are closed, providing a low-impedance path for current to flow through the hybrid breaker 10 to the Line to Load terminal 22 and Neutral to Load terminal 24 terminal, which are seen at the right side of
Current through the hybrid breaker 10 is measured by means of a shunt resistor 26 connected in series with the load. (Note that other means of current measurement may be equally suitable, such as a current transformer or a Rogowski coil.) The Control Logic 16 measures the voltage across the shunt resistor 26 via lines 19 and 21, and with a known value of the shunt resistor 26, it can calculate the current. When the Control Logic 16 determines that it is necessary to open the breaker 10, it proceeds in one of two ways:
In this design, when the Control Logic 16 identifies an HIC event, it will first open the MEMS relay 14, leaving the MEMS Interrupter 34 closed. The voltage across the entire device will rise as the impedance of the MEMS Interrupter 34 is greater than that of the MEMS relay 14. After a defined delay to allow the MEMS relay 14 to open past the breakdown distance, the Control Logic 16 will send a pulse to the MEMS Interrupter 34 to cause it to open. The MEMS Interrupter 34 may be configured with an arc chute 36 (shown in
In
The operation of the hybrid breaker 40 in
When the fault has been corrected and the hybrid breaker 40 is reset, the Control Logic 16 will assume that MEMS Interrupter A 34 has been damaged. It will leave MEMS Interrupter A 34 open and will replace its function with MEMS Interrupter B 42 until a second HIC. After the second HIC, the Control Logic 16 will assume MEMS Interrupter B 42 is damaged and will leave it open, replacing its function with MEMS Interrupter C 44. This pattern will continue for as many interrupters as exist in the breaker. Note that it is possible to measure the contact resistance of the MEMS Interrupters 34, 42, 44, so the Control Logic 16 may be able to use each MEMS Interrupter 34, 42, 44 for more than one HIC until the respective MEMS Interrupter 34, 42, 44 no longer has low enough contact resistance. Circuitry for measuring the Interrupter contact resistance is not shown in
The arc chute 36 is, in one implementation, a stationary set of electrodes that guide the arc away from MEMS Interrupter 34 after it forms when the contacts of the secondary MEMS relay 14 open. The arc travels along the curved arc chute carriers, expanding as they diverge from each other. When it reaches the intermediate arc rails, it is broken into several smaller arcs. Finally, the moving expanding air and its associated energy is expelled from the breaker through a vent (not shown).
Current if not interrupted. This trace illustrates the rapid rise time of the current if the breaker were not opened. The horizontal line up until time T1 represents proper steady state current. T1 is the point at which the short circuit forms. When the short starts, the current rises rapidly, limited only by the sourcing capacity of the power source and the impedance of the short. This curve would continue to rise off the top of the chart.
Primary Relay Opens. The control logic is monitoring the current through the relay. It must distinguish true short circuits which lead to HIC events from small overloads which might trigger a ‘nuisance trip.’ At point T2, the breaker logic has determined that a short circuit is in effect and the logic sends a signal to open the primary relay. The primary relay takes a finite time (from T2 to T3) to open, and by time T3 it is open far enough to withstand the voltage generated across the breaker. Note that the opening time (T3-T2) for a MEMS relay is approximately 50 microseconds, much faster than the opening time for a conventional relay, which may be 10s of milliseconds, hundreds of times slower. During the opening time the current continues to increase. The energy is the integral of the power that is proportional to the square of the current, which is proportional to the time for a linear rise in current. Therefore, the energy is proportional to the cube of the time to open. This is the driver for getting to the fastest possible opening. The secondary relay remains closed, and the current is diverted from the main relay through the secondary relay.
Secondary Relay Opens. The control logic pauses after opening the main relay. It leaves the secondary relay closed until enough time has elapsed to allow the main relay to open far enough to provide adequate standoff distance to prevent arcing at the rated voltage. This minimum delay is the time between T2 and T3. At time T4 the control logic sends a signal to open the secondary relay. The secondary relay also takes time to open, and it is subject to arcing until it is open enough to extinguish any arc. This opening time (T4-T5) is also dependent on the type of relay used for the secondary relay. A MEMS device will open faster, significantly lowering the maximum current generated before the relay opens, and thereby lowering the total energy which must be dissipated in the form of an arc. For a current that rises linearly with time, the energy put into the relay resistance is:
So rise time is squared, time is cubed. Cutting the opening time in half will result in ⅛th the energy.
Current Limited by Relay. This trace shows the current flowing in the controlled situation. As in the first trace, the current starts to rise at time T1. The current continues to rise after the primary relay opens at T3. Between T3 and T5 the current is flowing through the secondary relay. At T5, when the secondary relay opens, the circuit is completely open, and the current drops to 0. Depending on the design of the secondary relay, there will be some current flowing in the form of an arc for a period after the secondary relay starts opening.
In
It can be seen from the descriptions of
It will be appreciated from the foregoing that the small size and low resistance of the MEMS relay allows a standard sized circuit breaker to tolerate much more current than a similarly sized semiconductor breaker. It further allows the control of the relay by means of digital signals, providing additional features compared with a traditional circuit breaker. The size is important because it allows the breaker to be installed in a standard electrical load center, enabling its use as an easily installed retrofit.
It is to be understood that various changes can be made to the disclosure to enhance its utility. The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications, and publications to provide yet further embodiments.
| Number | Date | Country | |
|---|---|---|---|
| 63503777 | May 2023 | US |
