Test Systems Emerging for EV Charging Applications

Article By : Maurizio Di Paolo Emilio

Electric vehicle test systems are certifying the reliability of complex EV powertrains along with related charging interfaces and supply gear.

Power management and battery technologies are advancing via more efficient and flexible designs aimed at the growing electric vehicle sector. Related test and measurement systems must therefore meet more stringent technical requirements.

With that in mind, Keysight Technologies recently launched it SL1200A Series Scienlab Regenerative 3-Phase AC Emulator for EVs and supply equipment (EVSE) charging and grid applications. The system is composed of hardware, software and support services.

Keysight executives stressed the importance of EV test systems used to certify the reliability of increasingly complex EV powertrains. Among the focuses are charging interfaces and EV supply equipment along with grid-edge power converters.

The portable and high-power versions of the SL1200A Series are aimed at users’ growing EV test and measurement requirements. The aim is simulating real-world charging scenarios along with the ability to meet global EV charging standards.

Keysight executives highlighted the importance of test systems capable of certifying the reliability of EV powertrains.

Electric vehicle charging is expected to generate significant demand on power grids. At the same time, energy storage opportunities are emerging through vehicle-to-grid power applications. Renewable energy sources will propel that transition, leading to increased grid complexity. That creates new testing challenges when rolling out charging applications.

Performance, design metrics

Key performance metrics in EV design focus on battery and propulsion systems. Design parameters include power level, conversion efficiency, operating temperature in the vehicle powertrain along with thermal energy dissipation capacity and system packaging.

EV systems must be adapted for high voltage measurements (1,000 V and higher) to ensure safe, reliable operation. Real-world driving conditions present the largest challenges for system testers. Harsh environments range from -30 °C to +60°C.

At the component level, wide-bandgap semiconductors are used in the different power converters inside the EV, as well as in EVSE” and grid-tied inverters, said Kevin Cavell of Keysight’s Automotive and Energy Solutions business unit “Double-pulse test equipment is needed here. WBG modeling and circuit simulation is also a valuable tool.”

Battery testing starts with cell grading. “High-volume manufacturing is needed to supply the large quantities of cells for EVs,” said Julian Tomczyk, a Keysight marketing specialist.

“This manufacturing equipment is needed to take the cells through their initial charge/discharge [or battery formation] and then grade the cells based on quality. While most grading can be done in a few minutes, measuring self-discharge during cell aging is a lengthy test that requires many days, creating large work-in-process inventories stored during the aging,” Tomczyk added.

To help reduce inventory costs, Keysight claims its tool help reduce self-discharge measurement from days to hours, thereby reducing aging while speeding cell grading.

Once cells are fabricated, aged and graded, they are assembled to create a battery. Depending on voltage and power requirements, an individual battery could include thousands of cells. “Battery management systems are put in place to monitor the health of each cell in the battery. Test and measurement equipment is needed at this phase as well, to test the [management system] and the entire battery pack,” Tomczyk said.

A variable frequency drive (VFD) converts DC power from the battery into variable frequency AC used to drive the motors. Large power supplies capable of hundreds of kilowatts of power are needed to emulate the DC side; a machine emulator is necessary to reproduce the motor to surround the device under test. The instruments can run the VFD through many scenarios while measuring input and output power to calculate power conversion efficiency.

Wide-bandgap devices

Using WBG semiconductors yields potential efficiencies of more than 95 percent, greatly extending range, Keysight claims.

Power converters are a key component for leveraging renewable energy for transportation and industrial applications. To facilitate needed advances in power converter design, new WBG semiconductor technologies based on silicon carbide (SiC) and gallium nitride (GaN) are options.

Compared to silicon and GaAs semiconductors, the wider bandgap translates into a greater breakdown voltage and the possibility of operating at high temperatures while reducing radiation susceptibility without losing electrical characteristics.

As temperatures increase, thermal energy of the electrons in the valence band also increases until they reach the necessary energy (at a certain temperature) to jump to the conduction band. In the case of silicon, this temperature is about 150°C. Those values are much higher for WBG devices.

WBG semiconductors provide performance increases as high as 100 times faster than conventional designs along with higher voltage and thermal operation. Those attributes translate into improved efficiency, reduced size and cost. “However, the resulting high-performance power converters are proving difficult to design due to many new challenges when characterizing WBG semiconductors,” said Cavell.

Those hurdles have slowed the design of new converters. With commercial systems in short supply, Keysight said homegrown test systems are used to characterize WBG semiconductors.

“Unfortunately, it is difficult to produce repeatable and reliable measurement results with one-off, homegrown testers,” Tomczyk added. “Unreliable results create additional obstacles for power converter designers when correlating their measurements with the semiconductor’s data sheets.”

Keysight programmed its PD1500A dynamic power device analyzer as a platform for reliable characterization of WBG semiconductors. The company said its analyzer was developed in collaboration with chip manufacturers along with EV and power management designers.

Figure 1: SiC device cross-section.

Electrification of powertrains changes testing requirements by requiring more than just combustion process analysis. Electric and hybrid vehicles may have multiple motors, inverters and battery packs. Hence, all energy sources and loads must be considered.

The challenges facing EV manufacturers and their suppliers vary by specialty, whether it be powertrains, autonomous driving or in-vehicle networks.

“An engineer focused on cell and battery test would address the battery, since it is the fuel required to drive the powertrain,” according to Cavell. “An engineer focused on power conversion would address the power converter [and] variable frequency drive. Each is critical to the proper, efficient, long-range operation of the EV. And each presents its own unique test challenges.”

Charging specs, evolving grid

Meanwhile, EV charging standards continue to proliferate. Keysight notes several specs such as the American and European Combined Charging Standard, the Chinese GB/T spec, the Japanese CHAdeMO framework and, soon, the new Asian standard dubbed ChaoJi.

The regional standards cover EV-EVSE communications, physical plug design, power flow and test scenarios. “Each region has many conformance standards that are dependent on the type of charging—AC, DC, high-power DC,” Cavell added.

EV and equipment manufacturers targeting the global market need to test regional standards in order to address a diverse market. Hence, Keysight promotes an automated test framework addressing different charging standards, including power flow and communications.

Figure 2:  SL1200A Series Scienlab Regenerative 3 Phase AC Emulator

EV adoption is also reshaping grid infrastructure. “Within the automotive industry, the electrification of vehicles is expected to create significant demand on the grid for charging, while also expanding the opportunity for energy storage through vehicle-to-grid power applications,” said Tomczyk. “As the energy mix intensifies, so does the challenge of managing the way we produce, distribute and consume electricity.”

Smart inverters with grid support have emerged as a key enabler for overcoming such challenges. Hence, inverter manufacturers are required to adhere to a specific set of grid compliance and interconnection specs requiring extensive testing.

For example, grid emulation equipment testing is mandatory. Distributed energy resources are moving to higher output voltages to reduce losses, moving to as high as 1,000 volts of AC (VAC). “The goal of higher voltages combined with the requirement to provide grid support functions, such as high-voltage ride-through creates the need to test to even higher than the 1,000-VAC limit,” Tomczyk said.

According to Keysight, “To achieve the high voltages needed to test new inverter [and] control designs, inverter engineers often must either connect multiple power supplies in series or use an external transformer. This leads to costly, complex test setups with an inability to easily expand, along with reduced performance….”

The vendor’s automated testing approach can therefore be configured to regional charging standards and accommodates different physical plugs. Testers must then select the appropriate test standard; the automation software takes it from there.

With the rise of bidirectional power flows, including vehicle-to-grid (V2G) and EVs themselves becoming energy storage systems, standards organizations must specify required tests and testing configurations. Hence, V2G implementations “will add even more complexity, and there will be grid codes and interoperability standards that will need to be tested,” said Cavell.

Vehicle electrification and the transformation of the grid infrastructure is reshaping test and measurement requirements. Instruments must adapt to the presence of high-voltage signals and must withstand harsh environmental conditions. On the software side, the synchronous acquisition of electrical and mechanical data along with a plethora of standards requires a fundamental rethinking for optimizing EV test and measurement.

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