Patent Application
20200144807
A growing development in aircraft propulsion is to employ electrical components to distribute thrust and achieve ultra high effect bypass ratios, as well as other airframe level benefits such as improved lift to drag ratio. This type of distributed electrical propulsion can be supplemented with electrical energy storage. This type of propulsion system can offer improvements in specific fuel consumption, emissions, and noise. Some of these engine types involve connecting multiple electrically driven propeller to a battery and multiple gas turbine generators, often in a DC microgrid, which allows the battery and generator to share power loading while providing more freedom in operating frequencies than a AC grid would. The onboard microgrid can also enable additional aircraft capabilities for high power electrical loads used in defense and other applications.
In DC electric systems, such as these as others, there is the possibility of a short circuit between the positive and negative DC lines which can lead to a catastrophic failure of the system. If the fault is not cleared very quickly (e.g. less than 100 ms), the system could experience damage. Many existing types of fault protection may not be effective in these DC electrical systems.
Current-limiting fuses provide low cost, easy-to-install, compact, fast and reliable over-current protection for electrical systems from distribution networks, to switching power supplies. Being the most fail-safe and compact solution, since 1950, current-limiting fuse technology has evolved its speed, power rating, and adapting to more extreme working conditions, to protect semiconductor devices or equipment in the new power electronics era. However, limited by thermal physics, fuses' non-controllable nature makes them difficult to address the very basic requirements from transportation DC applications, which are generally demanding for product size, temperature rise, power cycling capability, and precise protection over comparatively low fault current to distribution networks (typically, kAs in battery systems versus tens of kAs in distribution networks).
Circuit breakers are also widely used for short-circuit protection. Their ability to be reset is a major advantage against fuses. Moreover, circuit breakers feature a lower on-state voltage drop in the closed position as well as a galvanic separation in the open state. However, when a fault is detected, breakers operate more slowly than current-limiting fuses due to the large mechanical time constant. In DC networks, the presence of arcs leads to contact erosion and arcing chamber fatigue, i.e. a shorter lifetime and high maintenance costs. A longer time to react to a large fault current leads to higher let-thru current, which will ultimately stress the downstream circuit they are intended to protect.
Hybrid protection systems are being developed to overcome the limitations of fuses and circuit breakers but have limitations of their own. Hybrid relays often employ a combination of solid state relay and mechanical relay. Though they more quickly than circuit breakers may not react quickly enough for a hybrid electrical system. As well hybrid relays are often designed to be normally open and thus main power can be lost if the auxiliary power controlling the relay is lost. Pyrofuses are also being developed. Pyrofuses react very quickly but cut the main power line and cannot be reset to restore power, and are thus not suitable as a primary means of fault clearance in a system where maintaining electrical power is mission critical (e.g. aviation).
It would be advantageous to have a robust protection architecture using normally closed components that enable power to be maintained in the event of an auxiliary power/control power failure, yet react quickly enough to the fault to protect the system.
A fault protection architecture in a DC electrical system, the architecture which may include a fault protection circuit positioned in series between a DC source and a DC rail, the fault protection circuit itself may include a hybrid relay; and a pyrofuse in series with the hybrid relay; the hybrid relay having a predetermined triggering condition and a known clearing time; wherein the hybrid relay triggers when the predetermined triggering condition is met on the DC rail; the pyrofuse having a second predetermined triggering condition, wherein the second triggering condition is set such that the pyrofuse trigger is on a delay at least equal to the clearing time of the hybrid relay.
In one embodiment, the predetermined triggering condition may be defined at least as a function of a current threshold. In a further embodiment, the predetermined triggering condition may be defined at least as a function of time. In another embodiment the pyrofuse may be self-triggering. In another embodiment, a current sensing device may be operable connected to the DC source and the hybrid relay. In yet another embodiment a current sensing device, a controller and at least one power supply, the current sensing device operably connected to the DC rail and the controller, the controller operably connected to the power supply and a trigger of the pyrofuse. In yet another embodiment the power supply may be an uninterruptible power supply. An even further embodiment may include a bus selector. At least one power supply may include a non-critical bus and a critical auxiliary power bus, the bus selector connected to the non-critical bus or the critical auxiliary power bus with a switchable mechanism. In another embodiment the pyrofuse may include a pyroswitch arranged in parallel with a conventional fuse. In yet a further embodiment the current sensing device may be a Hall Effect sensor, shunt sensor, or Rogowski coil, In another embodiment the hybrid relay may include a solid-state relay electrically coupled in parallel to a mechanical relay installed in series with the DC rail.
A method of protecting a DC electrical system may include providing a hybrid relay and pyrofuse in series between a DC source and DC rail; subjecting the DC rail to an overcurrent; triggering the hybrid relay in response to the overcurrent; triggering the pyrofuse subsequent to the triggering of the hybrid relay; thereby breaking a conduction path between the DC source and the DC rail. The triggering of the pyrofuse may be delayed by a predetermined time greater than a clearing time of the hybrid relay.
In one embodiment the hybrid relay may include opening the relay. In another embodiment the pyrofuse may be self-triggering. In still another embodiment triggering the pyrofuse further may include sensing the overcurrent and based on the sensed overcurrent applying a power from a power supply to the pyrofuse. In a further embodiment applying power from a power supply may include selecting between a non-critical power bus and a critical auxiliary power bus as the power supply based at least upon availability. In some embodiments the pyrofuse may include a pyroswitch arranged in parallel with a fuse, and triggering the pyrofuse may further include permanently disconnecting the DC source from the DC rail in the pyroswitch and subsequently blowing the fuse. Still yet another embodiment may include a current limiting fuse, and the step of triggering the pyrofuse further may include tripping the current limiting fuse in response to an overcurrent; creating a voltage drop across the current limiting fuse; applying a voltage across a pyroswitch in response to the voltage drop; and, triggering the pyrofuse in response to the voltage.
A fault protection circuit, may include a closed bias relay; and a pyrofuse in series with the relay; the relay having a triggering overcurrent; wherein the relay opens when the triggering overcurrent may be met in the relay; the pyrofuse triggering on a delay with respect to the triggering overcurrent. The delay may be at least greater than a clearing time of the closed relay.
The following will be apparent from elements of the figures, which are provided for illustrative purposes.
The present application discloses illustrative (i.e., example) embodiments. The claimed inventions are not limited to the illustrative embodiments. Therefore, many implementations of the claims will be different than the illustrative embodiments. Various modifications can be made to the claimed inventions without departing from the spirit and scope of the disclose. The claims are intended to cover implementations with such modifications.
For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to a number of illustrative embodiments in the drawings and specific language will be used to describe the same.
The present disclosure is directed to systems and methods for fault protection in hybrid electrical systems.
This protection architecture uses a Hybrid Relay as the primary protection device for overcurrent resulting from DC line-to-line fault with Pyrofuse added in series for redundancy. The pyrofuse may be designed to be self-triggered, or to be controlled by an uninterruptible power supply (UPS), which provides a backup to the hybrid relay in case of the device failure. Furthermore, the inclusion of the backup pyrofuse enables the selection of a normally closed hybrid relay, which implies main power is maintained in the event of an auxiliary power/control power failure. This setup could be very desirable/advantageous for aerospace applications that are safety critical, or especially in defense applications when the system can serve the function of “battle ready mode” more robustly.
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Although examples are illustrated and described herein, embodiments are nevertheless not limited to the details shown, since various modifications and structural changes may be made therein by those of ordinary skill within the scope and range of equivalents of the claims.
