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Posted on July 14, 2016 by Dr Peter Harrop

Integrating charging and traction systems in electric vehicles: Part 1

Fleetcarma
Researchers at Chalmers University of Technology in Sweden have developed a new type of high-power integrated motor drive and battery charger for electric vehicles. Compared to today's electric vehicle chargers, the new system could shorten the charging time from eight to two hours, and to reduce the cost by around $2,000, according to the developers. Dr Saeid Haghbin at Chalmers proposed the system which uses the components in the traction circuit—such as the electric motor and the inverter—in the charger circuit to reduce the size, weight and price of the on-board charger. In essence, the proposed system uses the motor as a grid-connected generator with extra terminals. Berlin Technical University has been working on integration of power electronics by merging motor inverters and on-board charging and Dr Martin Bruell of Continental recently presented on a "Bidirectional Charge- and Traction-System" which he hopes will be commercialised soonest. Indeed, the bigger picture is to merge many more vehicle input and output power electronic systems. The race is on.
 
Here is a shortened version of the new paper. Bruel writes: The field of fast charging is diverse. Many solutions use dedicated additional costly components for each charging type. To push the E-Mobility market, a new system for charging is proposed comprising of E-Machine, Inverter and Boost DC/DC converter with a minimum of additional components. The new Bidirectional Charge- and Traction-System (BCTS) is capable for traction and all kinds of conductive charging with reduced system costs.
 
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The electric vehicle (EV) market growth is below the expectations from beginning of this decade. Besides pricing per EV, the main technical reason is the limited range. To increase the range in principle two options are possible: Increase battery capacity or increase availability of (ultra) fast charging. An economic charging solution, making as little as possible changes in car and utility necessary is a key enabler for E-Mobility. Several high power charging solutions are available on the market: On the one hand 3-phase AC charging of about 20 kW and up to 43 kW. State-of-the-art chargers comprise always a dedicated rectifier only used in charging mode. The infrastructure usually supports about up to 20 kW only, mainly due to investment costs.
 
Rare 1-phase solutions for medium power are available as well, like a 10 kW one-phase charger for the US market. On the other hand DC charging for 40 kW and above is available as a simple solution in the car for some car manufacturers. Vehicle costs are low, but infrastructure investments are high. Based on this situation the distribution of DC-fast charging stations is not sufficient and growing slowly, except for single-company solutions like Tesla. Latest publications ask for even higher charging power than available today. Higher charging power is a must for long range vehicles due to both, growing battery capacities and the driver's request for short charging times.
 
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The current situation of the fast-charging infrastructure is a classical chicken-and-egg problem. On the one hand the business case for fast-charging stations is difficult, since no EVs are charging. On the other hand there are no EVs, since their travelling without a charging station is not possible. With the Bidirectional Charge- and Traction System we present an attractive solution for E-Mobility to overcome this problem.
 
We introduce here an architecture with high power components covering both, the traction and the charging function. It reuses existing components in the car enabling AC fast charging. This enables reduction of investment costs for charging stations, since no expensive DC charging electronics are necessary. Furthermore, the system is compatible with existing DC charging infrastructure.

System Architecture

Figure 1: Bidirectional Charge- and Traction System (BCTS)
Source Continental
 
The Bidirectional Charge- and Traction-System (BCTS), as shown in Fig. 1, comprises mainly out of a serial connection of the following key components: The E-Machine, the Inverter and a Boost DC/DC Converter (DC Booster). There is no separate unit on board for the charging mode.
 
In traction mode, when no plug is connected to the car, it is an energy-optimized architecture. The battery is connected via the Battery-Link to a DC Booster, which is used to stabilize the DC-Link voltage on an optimized maximal level. In partial load the control strategy reduces the DC-Link voltage to optimize the energy consumption and semiconductor lifetime. Furthermore the inverter costs are reduced significantly, since the system design can be optimized for a lower E-Machine/Inverter current. This also contains a lot of implicit benefits, like lower current, enabling lighter cables and connectors, lower switching transients, reduced electromagnetic interference (EMI) and higher efficiency. To enable High-Power EV applications there is a trend for higher DC-Link voltage levels such as 800V. The BCTS supports this trend".
 
He explained that, in charging mode the main task is the transformation from the 1- or 3-phase AC mains power to DC power as input for the battery. To fulfil the grid requirements, a mains filter and an active rectifier is necessary. Further a DC/DC converter to adapt the rectifier output to the battery's needs. In case of an AC charging system, the charger is on board of the vehicle. In case of a DC charging system, it is inside the charging station. Both competing systems are available today, but either the car manufacturer or the public utility wants to avoid its additional cost. Reusing the inverter for this function has been proposed.
 
In the presented BCTS all three functions are included in components, which are already on board of the vehicle: The E-Machine stator is used as the AC mains filter, the inverter as the 1- or 3-phase rectifier and the DC Booster adapts the DC-Link voltage to the Battery-Link voltage for battery management. In principle, this solution could support AC charging with power as high as the installed continuous traction power depending on additional filter efforts. Counting both, vehicle and charging station, the system cost is optimized.
 
He describes how the BCTS is also capable of ultra=fast DC charging (charging above 100 kW). We propose the connection to the DC-Link (see Fig. 1). This reduces DC charging station costs, since it can operate on a stable voltage and an additional DC/DC converter in the charging station is not necessary. (Such a charging station is not compatible to battery-link charging). Furthermore the charger is compatible to state-of-the-art systems at ~400 V that connect directly to the Battery-Link. So, this architecture is prepared for both, ~400 V and ~800 V DC charging stations.
 
For (ultra) fast charging, the charging cable becomes a major issue due to its weight and stiffness for the user. A transition to higher voltages offers lighter cables. Competing concepts are charging robots or water-cooled cables. The BCTS offers a cost efficient high voltage (~800 V) solution. Since all components are designed for bi-directional usage, an integration of the BCTS into Vehicle-to-Grid (V2G) applications is simply possible from power electronics point of view.
 
The BCTS can be beneficially used in all kinds of battery electric vehicles. In case of hybrid energy sources, a DC-Link management is needed anyway and a DC/DC Converter would be beneficial for the system efficiency. So it is prepared for the integration of a Fuel-Cell Range Extender, different independent battery packages and an ICE-based serial hybrid; a parallel hybrid is already an EV and thus prepared for the BCTS.

Use cases

Figure 2: BCTS Use Cases
Source Continental
 
The BCTS is designed for three use case areas with six main use cases. Fig. 2 shows the energy flow paths for the different use cases. The first area is Driving with the use cases traction and regeneration. In traction mode the battery is discharged via the Battery-Link, the DC Booster converts the voltage to the DC-Link in the range of battery voltage and 800 V output. The DC-Link supplies the energy flow to the inverter which converts the DC to AC for the E-machine. In regeneration mode the direction of the energy flow and the energy conversion in the BCTS components is reverse. Between high SOC and regeneration mode (battery voltage increases due to internal resistance) to low SOC and full acceleration (battery voltage decreases due to internal resistance) the battery voltage has roughly a ratio of 2 (typically 450 V to 270 V). This wide range is covered by the DC Booster and no margin in the Inverter is necessary. The second area is Charging with the use cases AC charging and DC charging.
 
The AC charging mode is split into the sub modes 1-phase charging and 3-phase charging. 1-phase charging uses the standard interfaces at every home. It is possible at both, 230 V and 110 V. The charging power hereby is limited to some kW only, depending on local regulations. 3-phase charging enables higher power with low investment costs in charging stations. All kinds of AC charging modes require a mains filter, a rectifier and a DC voltage adaptation on board. The maximal power is mainly limited by the on-board charger. The energy flow in the BCTS is as following: From the charger plug socket the 1- or 3-phase AC current is fed through the E-Machine via the opened star point. The inductance of the E-Machine is used as filter. Via the DC-Link the AC current reaches the inverter, which rectifies the current. The DC Booster sets the voltage down to the battery needs on the Battery-Link.
 
He says, "For the DC charging mode we propose the connection of the DC socket to the DC-Link so that the energy flow is going through the DC Booster towards the battery. This is beneficial since on the one hand the BCTS gives flexibility for different DC voltages in charging stations. Especially both, 400 V and 800 V charging stations can be supported without any modification on charging station side. On the other hand, the charging time is decreased by using a DC Booster on board. For this use case the charging power is not limited at low battery voltage (Pcharge = Icable · Vbat), since a high voltage is always used between car and station.
 
The third area is Vehicle-to-Grid with the use cases of AC supply mode and DC supply mode. In both modes the direction of the energy flow and the energy conversion in the BCTS components is reverse to the AC charging mode and DC charging mode, respectively. These modes are possible, since no component on board is unidirectional (see Fig.1), since the regeneration mode already requires bidirectional components. (Bidirectional functionality of charging station required).

Comparison with state of the art systems

Figure 3: State of the art EV architecture for traction and charging
Source Continental
 
Today every EV on the market has a 1-phase AC charger on board. This comprises a mains filter, a rectifier (AC/DC) and a DC voltage adaptation (DC/DC). The DC/DC converter in this charger is unidirectional and has usually a galvanic separation. It comprises a DC/AC inverter, a transformer and a AC/DC rectifier (see Fig. 3). The 1-phase charging power is limited in some regions, like central Europe, to e.g. 3.7 kW due to grid regulations, which should avoid strong deviations on the mains. Furthermore, increasing the 1-phase charging power leads to high current ripples on the DC-Link.
 
Several of the first EVs from 2010-2012, the Slow-Chargers, were equipped with the 1-phase AC charging function only. With the introduction of 3-phase AC on-board charger from 2013 on, the charging power for the AC-Chargers increased to 22 kW. Although even 43 kW have been installed the public charging stations often do not supply more than 22 kW, which is sufficient for a full charge of typical today's battery capacities in about one hour.
 
As an alternative for fast charging, the DC-Chargers have a direct connection for DC power supply to the Battery-Link. This keeps the power electronics for the conversion out of the car, but requires the same functionality in the charging station (see Fig. 3). On top, a complex communication is needed for the battery monitoring system.
 
Beside today's Battery-Link charging stations, in near future some 800 V DC charging stations could show up. This requires either a dual voltage interface for the charging station or leads into an incompatibility for some cars. Technically, the DC- Chargers shall be prepared for DC power supply, depending on the features of the charging station and the implemented protocol.
 
DC charging stations are costly and need more space. Today in urban areas, AC charging stations are more often present than DC charging stations. Both, the Slow-Chargers and the DC-Chargers can use the AC charging stations, but with the limited power of the 1-phase on-board charger. Therefore urban public charging often is more efficient for the AC-Chargers.
 
Fig. 3 depicts the state-of-the-art architecture for an EV with an AC on-board charger and the commonly used state-of-the-art DC-charging architecture. DC-Chargers have already been used for V2G applications without extra hardware on vehicle side. Since V2G requires AC power supply, the DC/AC conversion is done with an extra unit off-board. For more see the IDTechEx reports, Power Electronics for Electric Vehicles 2016-2026, Future Powertrains 2016-2026 and Electric Vehicle Traction Motors, Belt Driven & Integrated Starter Generators 2016-2026.
Dr Peter Harrop

Authored By: Dr Peter Harrop

Chairman

Posted on: July 14th 2016