Patent Application
20190031125
The present disclosure relates to electrical power systems, and more particularly to an electrical power system for a range extended electric vehicle.
Conventional electric vehicles (EVs) rely on a battery, such as a lithium-ion battery, as a sole power source. These types of EVs, however, can only be used for a relatively short duration. To extend the vehicle range, range-extended EVs (RE-EVs) which incorporate an internal combustion engine as a secondary power source to charge the lithium-ion battery and/or operate the vehicle have been introduced.
Lithium-ion batteries are suitable for many direct current (DC) loads in EVs, but they have a linear discharge curve and may not handle certain DC loads adequately, such as pulse (dynamic) loads.
An example vehicle electrical power system includes a battery operable to power a DC load of a vehicle over a DC bus, a capacitor, a multiphase AC machine comprising a plurality of windings, and a power converter. The power converter includes a plurality of power switches and a controller. The controller is configured to, in a first mode, charge the battery over the DC bus by operating the power converter as an active rectifier; and in a second mode, operate the power converter as a buck converter that decreases a voltage from the DC bus, and charge the capacitor from the decreased voltage. The controller is configured to, in a third mode, operate the power converter as a boost converter for the capacitor that increases an output voltage of the DC bus, and provide the increased output voltage to the DC load. The power converter utilizes the plurality of windings when operated as the active rectifier, buck converter, and boost converter.
A method of operating a vehicle electrical power system is also disclosed.
The embodiments, examples, and alternatives of the preceding paragraphs, the claims, or the following description and drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.
The DC loads 14 include at least one base load and at least one pulse (dynamic) load. The at least one base load tends to have a more constant power usage, and may include items such as an export inverter (e.g., for plugging in an AC appliance in a vehicle), or a vehicle HVAC system, for example. The at least one pulse load has a more variable power usage, and may include items such as a traction drive motor of an electric vehicle, a radar, or a laser or microwave-based weapon (e.g., a directed-energy weapon or “DEW”). A traction drive motor, for example, may perform rapid acceleration during which additional voltage is needed on the DC bus 16, and may perform rapid deceleration, potentially in connection with regenerative braking, where excess voltage may be provided on the DC bus 16.
A supercapacitor 18 is provided that can charge and discharge more rapidly than the battery 12 during such conditions. The supercapacitor 18 is operable to charge during an overvoltage condition on the DC bus 16, and discharge during an undervoltage condition on the DC bus 16. As used herein, a “supercapacitor” refers to a high-capacity capacitor with capacitance values much higher than other capacitors, and that typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, and can accept and deliver charge much faster than batteries. In one example, the supercapacitor 18 has a specific density of 3-10 Wh/kg and an energy density of 14-17 Wh/L.
Lithium-ion batteries provide long term energy and have a flat discharge curve, whereas supercapacitors 18 are most effective when a quick charge and/or discharge is needed to support pulse loads. Combining the battery 12 and the supercapacitor 18 satisfies long-term energy requirements of an electric vehicle and also facilitates quick charging/discharging, resulting in reduced battery stress and improved reliability.
The supercapacitor 18 is selectively connected to the positive rail 16A of the DC bus 16 by a switch S1. In particular, when the switch S1 is ON, supercapacitor 18 is connected the positive portion 16A of the DC bus 16 through a neutral N of a multiphase alternating current (AC) machine 26. When the switch S1 is OFF, the supercapacitor 18 is disconnected from the neutral N of AC machine. The switch S1 is operated by a controller 20, through its associated gate drive 22 and a control line 70. In one example the switch S1 is a solid state circuit breaker (SSCB), but it is understood that other switches could be used if desired.
Supercapacitors 18 have a linear discharge curve and require a DC-DC converter to help to recover energy in a low voltage band. A multifunctional converter circuit 40 utilizes stator windings A, B, C of the AC machine 26 to act as a DC-DC converter, and to provide other features as well.
In one example, the AC machine 26 is a brushless electric machine, such as a permanent magnet synchronous machine (PMSM), and the plurality of windings A, B, C are arranged in a wye formation. A PMSM uses rotating permanent magnets to provide an electrical field that induces a current in the plurality of stator windings A, B, C. Of course, other AC machines could be used, such ones that can operate as an axial flux machine, a wound field synchronous machine, or an induction machine.
During typical vehicle operation, the DC loads 14 operate primarily or exclusively from the battery 12. When a charge level of the battery 12 becomes depleted and falls below a charging threshold, the AC machine 26 operates as part of a range extender 28 to charge the battery 12 and extend the range of a vehicle incorporating the electrical power system 10.
The range extender 28 includes a prime mover engine 30 and its associated fuel tank 32. The prime mover engine 30 can be a diesel or gas turbine engine, for example. The prime mover engine 30 drives operation of the AC machine 26 through a rotor 34 that induces an electrical current in the plurality of windings A, B, and C and operates the AC machine 26 as a generator. By providing current to the battery 12 when its charge is depleted, the range of an electric vehicle incorporating the electrical power system 10 can be extended.
The multifunctional converter circuit 40 has a plurality of switching legs, 42A-C, which are illustrated in more detail in
An output of each winding A, B, C is connected to the nodes 50, 52 of its respective switching leg 42. Each power switch 44 is connected in parallel to an associated freewheeling diode 60, and each power switch 46 is also connected in parallel to an associated freewheeling diode 62. The freewheeling diodes 60, 62 form a current path when their respective switches 44, 46 are turned OFF. The controller 20 operates gate drive 22 to control the switches 46, 48 over control lines 66A-C and 67A-C. Although the power switches 44, 46 are shown as being metal-oxide semiconductor field-effect transistors (MOSFETs), it is understood that other types of switches could be used, such as insulated-gate bipolar transistors (IGBTs).
The multifunctional converter circuit 40 includes a DC link capacitor 64 connected across the DC bus 16. In some examples, the DC link capacitor 64 is not a supercapacitor.
The power generating section 11 includes four operating modes. In a first “starting mode” mode, the controller 20 starts the prime mover engine 30 from the battery 12. In a second “active rectification” mode, the controller 20 operates the multifunctional converter circuit 40 as an active rectifier that charges the battery 12 while the prime mover engine 30 operates the AC machine 26 in a generator mode. In a third “buck converter” mode, the controller 20 operates the multifunctional converter circuit 40 as a buck converter that decreases a voltage on the DC bus to charge the supercapacitor 18 (e.g., when a traction motor pulse load is rapidly decelerating and performing regenerative braking). In a fourth “boost converter” mode, controller 20 operates the multifunctional converter circuit 40 as a boost converter for the supercapacitor 18 to increase a DC bus voltage (e.g., during rapid acceleration of a traction motor pulse load). The multifunctional converter circuit 40 utilizes the plurality of windings A, B, C of the multiphase AC machine 26 when operated as the active rectifier, buck converter, and boost converter.
The power generating section 11 enters the first mode when a charge of battery 12 is depleted beneath a charge level threshold, and the controller 20 needs to start the prime mover engine 30 from the battery 12. In the first mode, the controller 20 operates the plurality of switching legs 42A-C of the multifunctional converter circuit 40 as a motor drive pulse-width modulated inverter that converts DC from the battery 12, as received over the positive rail 16A of the DC bus, to AC in the plurality of windings A, B, C. This operates the AC machine 26 in a motoring mode, to rotate rotor 34 and provide electric start of the prime mover engine 30. In one example, the controller 20 uses a field oriented motor control using a known sensorless technique. The controller 20 may optionally use a motor rotor position sensor 68 to perform the engine start in the first mode. The rotor position sensor 68 is operable to detect a position of the rotor 34 of the prime mover engine 30. During engine start, the battery 12 is directly connected to the DC bus 16. In one example, switch S1 is open/OFF during the first mode, which disconnects the neutral N of the AC machine 26 from the supercapacitor 18.
Once the prime mover engine 30 is started and reaches a threshold speed, the power generating section 11 enters the second “active rectification” mode in which the multifunctional converter circuit 40 provides DC power to DC bus 16 to recharge the battery 12. In this mode, the controller 20 performs pulse width modulation on the switching legs 42 to operate the multifunctional converter circuit 40 as a pulse width modulated active rectifier.
During the second mode, the controller 20 utilizes a field oriented control using a known sensorless technique, optionally using rotor position sensor 68. Also, during the second mode, switch S1 is open/OFF, which disconnects the neutral N of the AC machine 26 from the supercapacitor 18.
During the second mode the controller 20 operates the multifunctional converter circuit 40 to utilize the windings A, B, C and function as a boost converter. The controller 20 uses an interleaved technique by parallel connection of three channels of boost converters, with each “channel” corresponding to a current phase on a respective one of the windings A, B, C. The controller 20 performs phase shifting of the pulse width modulation frequencies for each phase by 120° between channels. The interleaved technique significantly reduces input and output current ripple on the DC bus 16 and supercapacitor 18.
In the third “buck converter” mode, the controller 20 operates the switching legs 42 of the multifunctional converter circuit 40 to utilize the plurality of windings A, B, C and function as a buck converter that decreases a voltage from the DC bus 16, and charges the supercapacitor 18 from that decreased voltage. The controller 20 enters the third mode based on the detected voltage on the positive rail 16A of the DC bus being above a first voltage threshold, indicating that, for example, regenerative braking may be occurring to provide an overvoltage condition on the DC bus, potentially beyond what can be absorbed by the battery 12.
In the third mode, the controller 20 controls switch S1 to enter or maintain a closed/ON state to enable its charging. Also, in the third mode, switches 46A-C are turned off and the controller 20 performs pulse width modulation on the upper switches 44A-C, using the multiphase interleaving technique. The interleaving technique is used by phase shifting a pulse width modulation frequency applied to the plurality of windings A, B, C by 120°. Here too, such interleaving significantly reduces input and output current ripple. The prime mover engine 30 is not operating during the third mode.
In the fourth “boost converter” mode, the controller 20 controls switch S1 to enter or maintain a closed/ON state, supercapacitor 18 discharges into the plurality of windings A, B, C, and the controller 20 operates the plurality of switching legs 42 as a boost converter that increases the DC bus output voltage. During the fourth mode, switches 44A-C are turned off, and the controller 20 performs pulse width modulation of the switches 46A-C, also using the interleaving technique described above. The controller 20 enters the fourth mode based on a voltage on the DC bus 16 falling below a second threshold voltage that is lower than the first threshold voltage, and which may occur during rapid acceleration of the pulse load (e.g., rapid acceleration of one or more traction motors). The prime mover engine 30 is not operating during the fourth mode.
The electrical power system includes a plurality of lines 70-80 used by the controller 20 for controlling and/or sensing in the electrical power system 10. Control line 70 is used for controlling an operational state of switch S1. Lines 72 and 78 are used to measure a voltage of the battery 12. Sensing line 74 is used to detect a rotational position of rotor 34 of the prime mover engine 30 from sensor 68. Current sensing line 76 is used to detect and/or measure electrical current of the windings A, B, C from current sensors 77A, 77B, 77C. Although a single current sensing line 76 is schematically shown, it is understood that each winding A, B, C, may have its own current sensing line 76. Lines 78 and 80 are used to measure a voltage of the DC bus.
The electrical power system 10 uses electrostatic, electrochemical, and chemical types of energy. The supercapacitor 18 is an electrostatic device, and in one example has a specific density of 3-10 Wh/kg and an energy density of 14-17 Wh/L. The battery 12 is an electrochemical device, and in one example has a specific density of 100-243 Wh/kg and an energy density of 250-731 Wh/L. The prime mover engine 30 uses the chemical energy of its fuel, and in one example has a specific density of approximately 12,880 Wh/kg and an energy density of approximately 9,500 Wh/L.
The electrical power system 10, by combining the electrostatic characteristics of supercapacitor 18 and the electrochemical characteristics of battery 12, provides improved charge and discharge characteristics compared to prior art systems.
The electrical power system 10 eliminates an external inductor typically used in prior art systems by operating the multifunctional converter circuit 40 to utilize the AC machine's stator windings A, B, C and function as boost and buck converters, which can provide additional benefits such as size/weight reduction, parts count reduction, and cost savings, and can also reduce electromagnetic interference (EMI). The elimination can also increase reliability and simplify thermal management, because eliminating the additional DC-DC boost converter can in some examples also facilitate elimination of a dedicated thermal management system of the eliminated boost converter.
Although a three phase system has been described above that includes three windings A, B, C and three switching legs 42A-B, it is understood that this is only an example and that other quantities of phases could be used if desired (e.g., more than three or less than three).
Also, although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure.
