Executive Summary

Network codes for electric vehicle (EV) charging ensure efficient and safe integration of EVs into the power grid. These codes specify technical and operational requirements for grid connection, focusing on frequency and voltage stability, power quality, and certification. This paper evaluates the current implementation status of relevant requirements for EV charging throughout the world and compares the requirements of different grid codes. Compliance with these codes is crucial for manufacturers to meet product certification, thus enhancing reliability and market acceptance.

Motivation

Network codes establish technical standards and operational guidelines to ensure that electric vehicles (EV) can be integrated into the power grid efficiently and safely. For manufacturers, adhering to network codes is crucial. Compliance not only enhances system stability but also fosters market acceptance. Ideally network codes are streamlined as much as possible to facilitate the world wide EV market. The paper aims to provide a comprehensive understanding of network codes as perceived by the authors long standing experience in given development around the world.

Network code for electric vehicles

Network codes increasingly address electric vehicle charging. The following provides a summary of addressed requirements, including an evaluation of the current implementation state.

Frequency range

The frequency range defines the operational limits within which equipment must stay connected. It specifies the frequencies that equipment must be able to tolerate without any damage [1]. Typical ranges are ±2.5 Hz deviation from the nominal voltage for up to 30 minutes [4]. Since EV charging is inverter based, fulfillment does not pose major challenges to equipment manufacturers.

Rate of Change of Frequency

The Rate of Change of Frequency (RoCoF) is a measure of how quickly the system frequency changes, typically in the case of generation loss, expressed in Hz/s. Network codes specify RoCoF withstand capabilities to ensure that EV charging can remain connected and operational during such events[5]. Usual values are 4Hz/s for 0.25s [4] or 2Hz/s for 0.5s [2]. While inverters are capable of withstanding given values, proper testing is needed from a manufacturer’s perspective.

Limited frequency sensitive mode

Limited Frequency Sensitive Mode (LFSM) is an operational mode for power-generating modules that adjust active power output in response to significant frequency deviations. It includes LFSM-O (overfrequency), which reduces power output when the system frequency exceeds a certain threshold, and LFSM-U (underfrequency), which increases power output when the frequency drops below a specified level. Both are essential for maintaining grid stability during extreme frequency events by providing a rapid response to prevent system collapse [6]. Response occurs outside the normal operation window (typically ±200mHz), with a drop between 2-12% (typically 5%) [2][4]. In order to fulfill LFSM requirements, the grid frequency must be continuously monitored and reacted to, as is already state of the art.

Setpoint steps

Setpoint steps refer to the incremental adjustments made during the charging process. These steps ensure smooth transitions and prevent abrupt changes that could lead to system instability or equipment stress [7],

which is only a requirement in a few countries [2][3][4][12]. Gradual ramping is already possible with existing communication protocols and should be considered by smart charging providers, regardless of regulations.

Grid forming

Grid forming refers to the capability of inverter-based resources, such as electric vehicles, to establish and maintain frequency and voltage in the power grid, similar to traditional synchronous generators. This technology is essential for integrating inverter-based resources into grids with very few or even no synchronous generators. The inverters are controlled to behave as a voltage source, which enhances grid stability and provides inertia [8]. Not part of network codes yet, the European RfG draft sets this requirement for charging stations above 50 kW [4]. It will require adjustments to all newly installed fast charging stations. Although grid forming is in essence a control strategy, it will have to be properly understood and implemented by manufacturers.

Fault ride through / Voltage stability

Fault Ride Through (FRT) is the capability to remain connected and operational during periods of low voltage caused by faults. FRT requirements typically specify the duration and voltage levels that equipment must withstand without tripping [7]. Implementation for EV charging does not pose major challenges.

Reactive power range

The reactive power range specifies the limits in which the charging process is allowed to supply or absorb reactive power [2]. This range is defined in terms of kVA or as a percentage of the maximum reactive power capability. It will support the system to maintain voltage levels within acceptable limits under various operating conditions [9]. Albeit important to local system stability voltage ranges are not part of the RfG draft [4], although already required in some member states[2][12].

Voltage control

Voltage can be locally controlled through the adjustment of reactive power during the electric vehicle (dis-) charging process. Setpoints are either provided by the grid operator or based on a local voltage characteristic curve. For DC-charging above 12 kVA it is already a requirement in the German Network code [2]. For AC charging it is not yet required, since application would have to be done on the vehicle side, with varying requirements for different markets.

Harmonics

Harmonics in electrical networks refer to voltage or current waveforms that are multiples of the fundamental frequency, usually measured by the total harmonic distortion. Managing harmonics is crucial to prevent equipment overheating, reduce losses, and ensure the reliable operation of sensitive devices [11]. In case harmonics are regulated, the calculation follows specific formulas, not possible to cover in this chapter.

On board chargers of some electric vehicles are known for exceeding the set limits and have caused transformer failure at car dealerships in the UK, where numerous vehicles of the same brand are charged at the same main connection point.

Certification

Certification is process of verifying that equipment meets the specified technical and operational standards, introduced above [7]. Currently, most network codes only require a certificate of compliance from the manufacturer involved in the EV charging process, although testing requirements differ [2][4][12]. Independent testing is not done. Still, through self-certification legal accountability is ensured.

Further aspects

Within the paper, the general network aspects are elaborated by analyzing and comparing country specific setups of the network codes in key countries. Focus is given to the new European Regulation for Generators (RfG), and network codes from USA and China. In addition, the implication of smart charging on network code development is highlighted.

References

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