Bringing EVs into the Smart Grid for Stability and Security

Healthy markets for plug-in hybrid and battery electric vehicles (PHEVs, BEVs) could significantly reduce the environmental burden of tailpipe emissions without sacrificing the freedoms provided by personal transport. EU legislators are among the most enthusiastic advocates of a change to cleaner vehicles, and some European countries have already signaled intentions to ban the sale of new cars and vans with conventional combustion engines by 2040.

Vehicle manufacturers can stimulate market acceptance by making PHEVs and BEVs as easy as possible to use. This includes making charging as straightforward and as safe as charging a mobile phone. Standardizing the physical charging connections can help, although currently three types are specified in the international standard IEC 62196-2. These include the SAE (US Society of Automotive Engineers) J1772 plug, which is widely used in North America and defined as the Type 1 plug in IEC 62196-2. Many European countries prefer the IEC Type 2 plug (with or without an IP rated cover as required in some countries) which can support charging power of up to 43.5 kW, and is able to work with European three-phase AC supplies for fast-charging stations, as well as ordinary single-phase supplies.

Both types of interfaces include safety features to ensure energy can only be delivered when the plug is inserted and secure in order to protect against electrocution should the plug become disconnected before the charging cycle has concluded.

Step 1: Making charging simple and safe

The J1772 specification and IEC 61851-1, which is the global standard for EV charging electrical interfaces, specify basic electrical signaling across a pilot connection between the charging point – or electric vehicle service equipment (EVSE) - and the vehicle’s on-board charger (OBC) electronics. These interactions confirm connection and negotiate power delivery based on criteria such as available ventilation to guard against potential hazards like overheating.

Communication is based on a 1 kHz ±12 V pulse-width modulated (PWM) pilot signal. The EVSE generates the 12 V signal. When the charging plug is properly engaged, the EV places a resistive load that drops the voltage to 9 V. The EVSE then applies PWM and adjusts the duty cycle to indicate its own output current rating. This is the maximum charging current the vehicle is permitted to draw. At the same time, the EVSE closes its output relays to allow charging to begin. At this point the EV applies a lower resistance to the pilot signal line, reducing the voltage to 6 V as an indication that charging is in progress. Figure 1 shows the signal voltages associated with the various EV charging states. State D, when the voltage is reduced to 3 V, indicates that adequate ventilation is available to allow fast charging using the highest possible power.

Figure 1: The EV to be charged places various resistive loads on the pilot signal line to indicate its status.

Because the J1772 interface does not detect when the vehicle is fully charged, charging is terminated when the cable is unplugged. When this occurs, the pilot signal voltage returns to 12 V and the EVSE turns the output off to prevent current from flowing.

J1772 interface reference design

Texas Instruments has produced a reference design for a J1772-compliant EVSE, which leverages features in the MSP430F6736 microcontroller that facilitate control and monitoring of the pilot signal line. These include a highly accurate timer module for generating a PWM signal of the required duty cycle, and a successive approximation register (SAR) analog-to-digital converter (ADC) to read the response of the vehicle on the pilot wire. Because the EVSE’s current rating is essentially determined by the ratings of components such as the output relay and cables and connectors, the value can typically be fixed in firmware for controlling the timer circuit.

To drive the pilot signal across several meters of cable and through the load resistance applied by the vehicle when connected, the reference design uses an OPA171 operational amplifier, taking advantage of its wide supply voltage range of ±18 V, rail-to-rail output, and 475 mA output current rating. The MSP430 microcontroller monitors the output of the OPA171 via a voltage divider, to detect the load resistance applied by the vehicle.

The reference design implements all the electrical functions of a J1772 charging interface, as shown in Figure 2. These include power management to generate ±12 V_DC and 3.3 V logic supplies from the main AC line, and a TPL7407L low-side driver that manages a two-stage output relay. The design also leverages the MSP430F6736’s interruptible general purpose input/output (GPIO) pin, connected to a current transformer through an LM7321 amplifier, to provide protection against potentially dangerous ground faults. Using this interruptible pin enables the system to respond more quickly than is possible by monitoring the output of the current transformer with an ADC.

In addition, the microcontroller’s delta-sigma (ΔΣ) ADCs are used to integrate power metering, leveraging an existing and proven single-phase residential smart meter reference design.

Figure 2: TI reference design implements all the functions needed for a J1772-compliant EV charger.

Step 2: Plugging electric vehicles into the smart grid

Standardizing the charging interface according to specifications like J1772 and IEC 61851-1, to make charging simple and safe, can go a long way towards encouraging greater use of electric vehicles. However, as the numbers of such vehicles in daily use increases, so too will the load on the power grid when they are plugged in to recharge. On the other hand, if charging is managed intelligently, PHEVs/BEVs could support active demand response programs that work to prevent excessive peak loads, and could also be used as storage for surplus renewable energy. Figure 3 illustrates the kind of negotiations that can occur between the vehicle and charging point, leveraging communication with grid management systems to determine energy capacity and tariffing.

Figure 3: Sophisticated communication between the vehicle, smart charger and the grid can ensure charging is completed in time while preserving grid stability. Presentation by Christoph Saalfeld, Daimler AG, Vector Congress 2010.

Moreover, intelligent management of charging makes additional value-added services possible, such as applying cloud-based machine learning algorithms to calculate the most energy efficient route for the next journey and to predict consumption. Both these capabilities deliver value for owners, and help utility companies ensure stability and availability of the grid. Other services that can be offered include dynamic billing so that EV owners can be billed correctly any time they charge at home or at other locations such as at public charging stations, a workplace, or at a friend or family member’s property.

To enable these types of features and services, more sophisticated communication between the vehicle and EVSE is needed. The ISO/IEC 15118 working group has developed specifications for vehicle-to-grid (V2G) communication, leveraging power line communication (PLC) standards when the vehicle is being charged via a cable. In particular, it has chosen the IEEE P1901.2 HomePlug Green PHY (HPGP) broadband PLC specification as the best protocol to ensure robust communication and a high data rate. Operating at frequencies between 2 MHz and 30 MHz, HPGP enables the system to distinguish valid data on a connected pilot line against noise from other nearby sources.

After the charging process is started, communication is established allowing the vehicle and its charge point to exchange information such as control and configuration data, access privileges, time-stamp, tariff information, customer ID and location, and meter readings.

End-to-end V2G communication

Various studies have researched suitable standards for communication between the vehicle and grid management systems. The EU PowerUp project examined the opportunities for using IEC 62056 DLMS/COSEM, leveraging aspects of the protocol such as the PUSH primitive for forwarding information to smart grid management systems such as a load balancing controller. With the addition of EV specific extension, DLMS/COSEM was found to be a suitable protocol for end-to-end V2G communication.

An alternative approach is to use the IEC 61850 protocol, which has been designed to support communications between substation automation systems for purposes such as managing energy flow between renewable electricity resources and consumers. An EV specific extension has been proposed to enable interaction with ISO/IEC 15118 V2G interfaces, and the Fraunhofer Institute for Embedded Systems and Communications technologies (Fraunhofer ESK) has developed a reference system using ISO/IEC 15118 and IEC 61850 standards – as well as HPGP and IPv6 – for V2G communication via a smart charging station, capable of supporting value-added services. Figure 4 shows how this reference design proposes a combination of ISO/IEC 15811 and IEC 61850 protocols to implement end-to-end V2G communication.

Figure 4: End-to-end V2G communication managed by a smart vehicle charging station.

The HomePlug PLC protocols including HomePlug AV, from which HomePlug Green PHY is derived, are designed to enable chip manufacturers to quickly and easily create ICs that support the standards and are ready to be used in a variety of smart home products.

STMicroelectronics’ ST2100 STreamPlug is a system-on-chip that integrates a configurable hardware engine capable of supporting multiple HomePlug AV or HPGP ports. The device is architected to enable a single-chip solution to various smart use cases such as home automation, security, and EV charging. An integrated ARM®9 CPU provides enough processing power to host smart charging applications and protocol stacks like IEC 61850 or DLMS/COSEM, as proposed for communicating with smart grid management systems. Moreover, being designed from the outset as a highly-integrated chip for smart, connected applications, it also contains a hardware cryptographic coprocessor with support for algorithms such as AES, DES/3DES and IPSec. A built-in Ethernet port and color LCD controller enable the device to support a large proportion of smart EV charging functions without additional external devices.

Fortunately, extensive software is available as part of the supporting software development kit (SDK), including an interface layer with core scheduler, system software, and Linux kernel. The interface layer with core scheduler provides APIs to support the system software, which implements the HPGP MAC and other modules. The Linux kernel contains Linux device drivers for controlling the ST2100 interfaces and the hardware platform as a whole.

Figure 5: ST2100 software architecture.

Figure 5 illustrates the ST2100 software architecture. The OK Linux technology supports virtualization, which simplifies application development by allowing multiple applications or operating systems to run side by side on the same processor. A real-time OS could be used, for example, to host latency sensitive functions, while also benefiting from resources in the open-source community to facilitate Linux application development.

Conclusion

Environmental concerns and anti-emissions government policies are among drivers that should encourage a significant shift towards plug-in electric vehicles and dramatically reduce reliance on the internal combustion engine. From the user’s point of view, charging can be simple and safe thanks to standardized interfaces such as SAE J1772 or IEC 61851-1.

Making charging smart is the next step needed to overcome the challenge that widespread use of plug-in EVs presents to grid stability. Suitable communication protocols such as ISO/IEC 15118, HPGP, and IEC 61850 are already available to support end-to-end V2G communication that can help manage demand and balance energy flows, while at the same time delivering extra benefits to vehicle users by providing value-added services.