Electromagnetic compatibility (EMC) challenges cut across many areas of power electronics. Solid-state transformers (SSTs) and electric vehicle supply equipment (EVSE), like EV chargers, face different EMC challenges because of their different internal architectures and factors related to their levels of grid integration.
SSTs and EVSE generate a combination of common-mode (CM) and differential-mode (DM) conducted electromagnetic interference (EMI), but their different power conversion architectures mean that CM dominates in SSTs and DM in many EVSE.
For SSTs and EVSE, the root cause of conducted emissions is the same, which is high-frequency power switching. However, they operate with different voltages, frequencies, power conversion architectures, and most importantly, different isolation and safety requirements.
In addition, these systems are susceptible to various sources of radiated EMI, and they can be sources of radiated EMI that can interfere with the operation of nearby systems. The different operating environments mean that different EMC standards apply.
Location, location, location
There aren’t specific EMC standards based on equipment type. The relevant EMC standards for SSTs and EVSE depend on where and how the equipment is used.
There are three primary location classifications for these devices (Figure 1). SSTs tend to be found in electric substations (even those used in AI data centers are often considered to be in a substation environment). EVSEs are found primarily in residential or commercial building or factory and light industrial environments. That means different EMC standards apply.
SSTs are embedded in the grid and work with medium-voltage to lower-voltage isolation issues, grid-side emissions, and power quality concerns. EVSEs are in the lower-voltage areas of the grid and are concerned with user safety, communication with the grid to support grid-interactive functions, and integrity of vehicle-to-charger communication to optimize battery charging.
Standard considerations
SSTs are an emerging technology, and they often must satisfy a hybrid of requirements, including legacy transformer standards like IEEE C57.16 and C62.41, as well as standards like CISPR 11 / EN 55011 for industrial equipment. The IEEE Standards Association is actively developing P3105, a new recommended practice for the design and integration of SSTs in electric grids.
EV chargers are subject to well-established EMC limits like IEC 61851-21-2 for offboard charging. For wireless dc fast charging, SAE J2954 sets criteria for EMC, electromagnetic field (EMF) limits, interoperability, and safety for inductive power transfer.
Generic EMC standards abound within the IEC 61000 and CISPR frameworks for industrial, light industrial and commercial environments. For example, IEC 61000-6-1 specifies immunity requirements for devices operating in residential, commercial, and light-industrial spaces, and IEC 61000-6-2 does the same in industrial settings.
IEC 61000-6-4 ensures that emissions from heavy machinery don’t disrupt nearby communications. IEC 61000-6-3 delineates emissions requirements for commercial and light industrial operations. Various CISPR and FCC requirements also set multiple emissions limits based on equipment type and operating environment.
Isolation and parasitic capacitance
The high-frequency (>100 kHz) transformers in EVSE provide galvanic and safety isolation between the grid and vehicle. The lower voltages compared with an SST mean that designers must focus on parasitic capacitance to control the amount of noise getting into vehicle charge management systems.
SSTs get galvanic isolation from a medium-frequency (tens of kHz) transformer. The large voltage differences between the transformer primary and secondary combined with parasitic inter-winding capacitances can significantly amplify the difficulties of controlling CM noise.
In addition to corrupting communications, SST EMI can cause nuisance tripping of protection relays, induce thermal or dielectric stress on adjacent electrical equipment, and reduce long-term reliability (Figure 2).
Merging and morphing
The EMC distinction between EVSE and SSTs is morphing as the devices are merged in MW-scale dc fast charging applications. In these cutting-edge designs, the EV charger and the SST are being combined into a single, intelligent system (Figure 3).
The combined SST plus EVSE can support bidirectional power flow and grid reactive power compensation. It will be expected to meet the more restrictive conducted and radiated EMC requirements for individual SSTs and EVSE, respectively.
Summary
One of the main challenges with EMC for SSTs and EVSE is that there are no SST or EVSE specific standards. Existing standards are based on the location and other application criteria. That makes it incumbent on designers to have a clear understanding of the requirements for specific designs and application scenarios. The development of systems that combine SST and EV charging won’t necessarily clarify that situation.
References
- Addressing EMC Challenges in Electric Vehicle Supply Equipment, Microwave Journal
- Common-mode Electromagnetic Interference Mitigation for Solid-state Transformers, IEEE Explore
- Conducted Emission Mitigation for Solid-State Transformer using Modulation and Multiple Configuration Techniques, Nottingham Power Electronics, Machines and Control (PEMC) Research Institute
- EMC Considerations for Electric Vehicle Charging, AstrodyneTDI
- EMC Testing for EV Chargers: A Complete Guide, Feyree
- Impact of Electrical Vehicle Charging Stations on the Electric Grid: Lessons Learnt and Challenges, ResearchGate
- In-depth study of the application of solid-state transformer in design of high-power electric vehicle charging stations, IET Electrical Systems in Transportation
- Power modules: One shortcut to EMI compliance, Wolfspeed