State-of-the-Art Power Electronics Solutions for Autonomous Electric Transportation Systems

By Vamsi K. Pathipati and Sheldon S. Williamson

With the introduction of various autonomous modes of electric mobility, the face of the global transportation industry is on the verge of a major change. Autonomous electric vehicles (AEVs) are essentially EVs with self-driving capability, propelled by battery-powered electric drivetrains .

Recently, companies such as Tesla, Google, and Uber have developed their own models of autonomous electric vehicles (AEVs) and are continuing to look to expand their horizons. Autonomous electrified urban mass transit modes of transportation, such as buses, trams, and subways, have also been demonstrated successfully in recent times.

Almost all future modes of autonomous electric transportation will need adaptable charging infrastructures to keep them sustainable. There are several proposals in place to power AEVs using either static or dynamic (in-motion) charging, mostly involving wireless inductive power transfer (IPT). Dynamic IPT charging has the potential to overcome the infamous range anxiety issue associated with electric transportation.

In order to understand IPT-based wireless charging systems, including the associated high-frequency (HF) power electronic conversion system, consider the IPT setup. A static IPT system includes a transmitter coil and a pick-up coil onboard the vehicle. The Society of Automotive Engineers (SAE) recently released static wireless power transfer (WPT) standards, which include four levels of power transfer; dynamic charging standards will be released in impending years, based on industry feedback for such systems.

Furthermore, the current WPT standards solely deal with power levels for unidirectional charging, based on AC conductive charging for grid-to-vehicle power flow (SAE J1772). Future WPT standards will address vehicle-to-grid (V2G) power flows as well. As per SAE Standard J2954 for static wireless charging, the power levels are defined as: WPT 1 (3.7 kW), WPT 2 (7.7 kW), WPT 3 (11.0 kW), and WPT 4 (22.0 kW). The AC/DC rectifier converts the HF AC into usable DC power, to charge the on-board battery pack. In the case of static IPT, proximity sensing techniques are used between the transmitter and pick-up coil, to ensure realistic alignment. Practical DC/DC efficiencies in the range of 92-95 percent have been proven for power levels of 3.7 kW, even under misaligned conditions, using intelligent control techniques for the HF AC link.

Typical dynamic IPT charging systems are potentially usable for AEVs. Since continuous power is available using dynamic charging, lower investment may be required in terms of battery storage for AEVs, thus reducing vehicle cost. Two design options exist for dynamic IPT charging. A simple topology uses a high-power driver circuit to power the distributed transmitter coils. This could be an in-road high-power inductive power track/rail, which could be dedicated to a dedicated driving lane. However, this is not an advisable design, since it is not reliable. Since the driver circuit powers all transmitters at once, the primary coils are naturally floating, which leads to wastage of power.

An alternative design includes a modular driver circuit. Each individual distributed driver circuit can be actively controlled and turned on sequentially, only when the vehicle (along with the pick-up coil) is running over it. This can be easily achieved by using exclusive resonant frequencies for transmitting and pick-up coils. Such an arrangement naturally avoids wastage of power (parasitic standby power, when the pickup coil is not required to be powered). Also, being modular, the system is more reliable and less expensive.

Dynamic IPT charging of AEVs poses interesting technical challenges moving forward. Due to its highly inductive nature, IPT systems pose a challenge not only in terms of developmental hurdles, but also presents power quality and demand side management challenges for the ever-evolving smart electric grid. In this regard, plans for smartly coordinated, autonomous microgrids are already in place, to facilitate mass deployment of AEVs and associated IPT charging facilities. Meeting grid power quality requirements, supply-side power factor correction, as well as electromagnetic emission (EMI) requirements at the vehicle level, while charging in motion, represent critical design requirements that need immediate attention.

Dynamic charging systems mostly employ resonant conversion, which when operated close to the resonant frequency, makes the AC grid feed a resistive system load. However, a slight misalignment leads to leakage flux between the coils, which in turn makes the system inductive. A critical issue from the grid standpoint is the prediction of load demand, due to the unknown deployment of AEVs in coming years. This may lead to grid stability problems.

A natural offshoot of the above-mentioned issues includes development of more efficient techniques to reduce reactive power demand, in order to improve overall system efficiency. Also, in order to improve HF AC-link efficiency, single-stage, HF AC/AC conversion techniques have been proposed, such as AC/AC matrix converters. HF AC-link single-stage conversion eliminates the usage of typical 2-stage power conversion, which leads to elimination of bulky DC-link capacitors and large, low-frequency inductors, which are typically used to generate voltages at high frequency from the supply-frequency AC grid.

For active control of grid-side power factor, several power electronic converter topologies and control techniques have been introduced for power factor correction (PFC). Conventional PFC techniques use a well-known topology, which includes a DC/DC boost converter stage. A resonant Z-source single-stage converter for PFC has been proposed. The Z-source stage performs PFC as well as output voltage regulation concurrently. Thus, no additional converter switches/conversion stages are required.

In summary, this article aimed at presenting possible practical solutions to design sustainable solutions for future autonomous modes of electric transportation. At the heart of this technology is wireless power transfer systems and dynamic charging, enabled by the design and development of smart power electronic conversion systems.

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