Electric vehicles and the smart power grid
Electric cars, with a history ranging back more than hundred years, are experiencing a renaissance. The rise in fuel costs, improved battery technologies and government incentives are just some of the factors that – in the long run – make Electric Vehicles (EVs) a great choice especially for individual mobility in the growing megacities. China shows an impressive rollout plan, but also European countries are driving this trend. Germany has a plan for 1 million EVs until 2020, France is moving ahead even faster with massive government funding for a plan to install 1 million EV charging spots until 2015 and 2 million EVs until 2020. The increased distribution of EVs also offers new business opportunities for utility providers creating new income sources and – in the future -also allow grid stabilization (cut of peak loads) via the EV’s battery as buffer feeding energy back into the power grid.
The charging of these EV batteries can be done in different ways. The majority of the charging cycles will certainly happen at home or at work (estimation 80%), but public charging spots are required as well to ensure a fine grid of supply. There are different possibilities for charging. The most basic one is to use a single phase power supply of 230V and up to 32A AC via a typical 3kW On-Board-Charger, but also via a 3 phase 400V with up to 63A AC (typical 20kW charger). Charging with these currents is typically taking hours and is combined with parking times of the car (home, work, shopping). A closer equivalent of a gas station for conventional combustion engine cars is the DC charging (or also called fast charging) ranging up to 100kW power, but this has to be possible without damaging the battery pack.
Electric vehicles as part of the smart grid
All of these charging situations in public drive the need for user interaction at the charging spot. For advanced features, the identification of the vehicle itself at the charging spot is required, which is typically realized via Power Line Communication (PLC). In these cases, a variety of data like vehicle identification, current battery status, maximum allowed charge current and number of phases, charging times (e.g. delayed charging start) and overall ‘charged’ electricity amount with associated costs is exchanged. The identification of the car in the network opens up a cross utilityprovider usage of the electricity, similar like roaming in a mobile phone network.
However, this brings up one of the most important requirements: every car has to work with every charging spot. So the standardization does not only apply to the used plug standard, but also to the method of communication of the car with the charging infrastructure.
The PLC is a well suited choice as the connection cable is needed anyway for charging (excluding the possibility for inductive charging, which is also discussed at the moment), it’s robust and does normally not require any additional user interaction.
Power Line Communication (PLC) basics and usability
Power line communication is a long known technique to use the already existing power cable for communication. This method is also used for communication of electricity meters back into the power grid e.g. for remote readout of the consumption data, tariff switching, but also for load management purposes in the future.
The European PLC frequency bands as defined by the CENELEC are divided into the CENELEC-A band (3kHz-95kHz) which is exclusively reserved for energy providers and the CELENEC-B, C and D which are open for end-user applications. The frequency bands in the US cover the frequency range of 10kHz to 490kHz, Japan from 10kHz to 450kHz and China from 90kHz-500kHz.
Picture 1: Frequency table for the European CENELEC bands
This means that there is a common, open frequency range available to be used as a worldwide standard utilizing the CENELEC-B frequency band.
The used modulation scheme has to offer a good compromise between data rate, component cost and robustness, communicating via the often very noise power lines. The relatively basic SFSK (Spaced Frequency Shift Keying) modulation is only offering lower data rates, but is simple to implement. The more advanced OFDM (Orthogonal Frequency Division Multiplexing) modulation offers more communication robustness and higher data rates up to 128kbit/s, But it also demands a higher processing power as the signal is split into several sub signals, which are transmitted in orthogonal separated channels.
Picture 2: Comparison of PLC modulation standards
Two popular communication protocols in the smart grid area are PRIME (higher data rates of up to 128kbps) and G3 (lower data rates), both based on the OFDM standard. Texas Instruments is a principal member of the PRIME consortium, but is also fully supporting G3 based solution.
The used Narrow Band Low Frequency PLC (NB LF PLC) offers an optimal combination of communication bandwidth, robustness and cost and is already used in smart meters. The use in Vehicle to Grid (V2G) communication is expanding the PLC communication to the ‘last meter’ from the charging spot to the electric vehicle.
Higher bandwidth PLC technologies from the consumer electronic area are also competing here, but it is important to also consider the fit for Automotive use (e.g. Q100 qualification) and the long time availability similar to industrial smart meter solutions. Also, a high speed data path from the car to the infrastructure might become available anyway via Wi-Fi or 3G, so the PLC solutions would be mainly used to handle the charging of the vehicle and not to download huge amounts of Infotainment data.
Make it real – PLC system implementation
The not completed standardization process in the Vehicle to Grid (V2G) communication creates the need for a flexible adoption of the PLC system to a final standard. This is the philosophy of the complete 2-chip solution with Analog Front-End (AFE) and the powerful 32-bit microcontroller family TMS320F280xx / Piccolo as interfacing and processing backend. The AFE includes a 10-bit DAC, filter and output amplifier in the transmit channel plus a filter and two Programmable Gain Amplifiers (PGA) in the receive channel. It communicated via a 4-channel interface with the TMS320F280xx microcontroller, which runs the complete protocol stack and the communication to the host processor.
Picture 3: power line modem with the TMS320F28x. For full resolution, click here.
The PLC modem software (plcSUITE) is available as open source library from Texas Instruments. It includes a layered and modular software framework that easily scales to the actual requirements. The physical layer supports basic SFSK modulation as well as the OFDM based PRIME and G3 plus a TI proprietary solution called FlexOFDM. Also the MAC and higher layers of the communication stack are available as modular software modules, which can be added or removed based on the actual user demand. The modular concept of the plcSUITE also offers user application functions to be added and executed on the same microcontroller.
Picture 4: software structure of the Texas Instruments plcSUITE software
This fully programmable solution created the flexibility to easily adapt various different PLC standards using the same hardware platform. The PLC development kit (TMDSPLCKIT-V2 – 599$) reduces the complexity in designing the PLC hardware solution. The kit includes two PLC modems and the complete plcSUITE software framework plus USB JTAG emulation capabilities. It is based on the TMS320F28x controlCARD, so also the used microcontroller derivative can be exchanged via exchanging the controlCARD from the delivered F28335 e.g. to the new F2806x.
Picture 5: PLC evaluation kit from Texas Instruments – TMDSPLCKIT-V2]
The TMS230F2806x microcontroller family
The popular TMS320F280xx Piccolo family of 32-bit microcontrollers is growing further with the announcement of the new TMS320F2806x range. These device extend the Piccolo’s memory range to variants up to 256kB of Flash and 100kB of RAM, keeping the powerful peripherals like the 150ps High Resolution PWM, 3-MSPS 12-bit ADC or CAN, but is enriched with new peripherals like USB 2.0, eCAP or eQEP.
Picture 6: Piccolo TMS320F2806x block diagram
Further valuable extensions are the Floating Point Unit (FPU) and Viterbi & Complex Math Unit (VCU),which includes its own 75 instruction set and can be used to offload the CPU in case of math intensive calculations. For the use in PLC applications, the required FFT Filter is performed by the Complex Math Unit; the Viterbi Unit will decode the signal including Forward Error Correction. The also in the VCU included CRC (Cyclic Redundancy Check) module calculates the required 32-bit CRC in only three cycles. These features of the VCU enable an easy implementation of the PRIME or G3 PLC standard. The required peak MIPS in a typical G3 PLC implementation are reduced by 2.5x (average MIPS reduced by 2x) using the TMS320F2806x and it’s VCU versus an implementation with the TMS28335.
The Control Law Accelerator (CLA) offers an additional CPU core for control oriented applications. So functions like digital power conversion can be offloaded to the CLA creating bandwidth for the PLC function in the main core – and further optimizing system integration. The TMS320F280xx / Piccolo family has a long ranging, strong background in digital power supply systems in the industrial market. Several development tools for DC/DC and AC/DC conversion with PFC (Power Factor Correction) including a powerful library with software solutions (controlSUITE) are available to speed up the design cycle. As the devices are also available in automotive quality, they enable the seamless re-use of this know-how in the automotive market.
The TMS320F2806x processing performance and peripheral set enables the combination of digital power driven EV charger functions – where the Piccolo family is already used – with the vehicle to grid power line communication and therefore helps to drive system cost down.
Summary
The next 10 years will see a much wider distribution of electric vehicles with different types and brands in Europe. This drives the need for standardization of the charging infrastructure – every vehicle has to work with every charging spot. The best possible compromise between required features and cost of implementation has to be considered – especially for the communication of the car with the infrastructure to also enable new upcoming features like controlled charging up to grid stabilization with the EV batteries (charging back into the power grid).
Power Line Communication (PLC) is seen as great fit, standardization is already in progress with several national and international working groups consisting of car OEMs, power utilities and module / component providers.Goal is a worldwide standard for communication, based on an ISO/IEC standard.
The TI PLC system based on the proven TMS320F280xx microcontroller family and the AFE031 front-end offer a flexible, easy-to-implement solution, and is also available in automotive qualified versions.
About the author:
Frank Forster is Marketing Manager Automotive Microcontroller EMEA, Texas Instruments.
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