Basics of Total Harmonics Distortion (THD) in EV fast chargers | What causes THD & How to mitigate

Total Harmonics Distortion (THD) is one of the least discussed items in the eMobility world but is super crucial for EV charging operations and the electrical grid. This article may give an overview of what is THD, what the acceptable THD levels are in EV chargers, how it impacts EV drivers/ EV charging station operators (CPO), and electrical grids, and importantly how to deal with this issue.

What is Total Harmonics Distortion (THD):

Total Harmonic Distortion (THD) is a measurement used to quantify the level of distortion in an electrical system or device, particularly in the context of power systems, electronic circuits and also in audio systems.

In an ideal electrical system, our electrical grids provide alternating power (AC) with a common frequency of 50Hz (in most parts of the world). However, in practice, it is never at fundamental frequency and there are several distortions (noise/pollution) caused by different devices connected to it.

Various power supplies, Electric Motors and Drives, Lighting Systems, and even loudspeakers can create significant amounts of harmonic distortion to the grid. This means THD is not new; this has always been in electrical systems. However, with the addition of EV charging stations, the amount of total harmonic distortion just got bigger.

Total Harmonics Distortion in EV chargers:

Our electrical grids provide alternating current (AC) that is converted into Direct current (DC) by the converter modules inside the fast chargers. During this conversation process, there will be rapid switching actions within the converter modules that create non-sinusoidal currents, leading to harmonic distortion. These harmonics, which are multiples of the base frequency, can interfere with the proper functioning of electrical components in EV chargers.

Here are some factors contributing to THD in EV chargers:

1. Rectification: EV chargers typically incorporate rectifiers to convert AC power from the grid to DC power for charging the vehicle's battery. Rectifiers can introduce harmonic distortion due to their nonlinear operation, especially when they are operated near their maximum capacity.

2. Inverter: Some EV chargers also incorporate inverters, particularly in bidirectional chargers or chargers with energy storage capabilities. Inverters can introduce harmonic distortion when converting DC power from the battery back to AC power for grid interaction.

3. Power Factor Correction (PFC): Many modern EV chargers incorporate Power Factor Correction circuits to improve the power factor and efficiency of the charger. However, the operation of PFC circuits can also introduce harmonic distortion if not properly designed and controlled.

4. Switching Frequency: The switching frequency of power electronics used in EV chargers, such as insulated gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs), can also contribute to harmonic distortion. Higher switching frequencies may result in higher-frequency harmonics in the output waveform.

5. Charging Profiles: Different charging profiles or modes (e.g., fast charging, trickle charging) may have varying levels of harmonic distortion depending on the charger's design and operation.

Impacts of THD on EV chargers:

Unchecked harmonic distortion can cause significant damage to EV power cables, resulting in excessive heating, voltage distortion, and increased electromagnetic emissions that can disrupt nearby devices. Continuous exposure to these harmonics weakens the insulation of the cables, ultimately reducing their lifespan.

Harmonic distortion not only impacts EV power cables but also increases electromagnetic interference (EMI) and can potentially affect electronic vehicle components. Additionally, it interacts with the inductive and capacitive properties of the cables, leading to voltage fluctuations that reduce charging efficiency. The choice of conductive material, insulation type, and physical design of the cable all play a role in determining its susceptibility to harmonic distortion. Thicker insulation may provide better heat resistance but could also increase vulnerability to capacitive effects and alter the way the cable interacts with harmonics.

Beyond EV power cables, harmonic distortion in EV chargers can result in overheating of transformers and neutral wires, potentially causing circuit breakers to trip due to elevated currents. This distortion can also degrade overall power quality, leading to harmonic-related power losses and shortening the lifespan of electrical components. Charging multiple EVs simultaneously can further increase harmonic distortion, potentially pushing EV charger components beyond their operational limits.

While IEEE standards suggest a total harmonic distortion (THD) lower than 5%, certain EV chargers have been found to exhibit THD levels as high as 11.6%.

(Source: EV Engineering)

Implications for EV charging Operators (CPO) & EV drivers:

Harmonic distortion presents significant challenges for EV chargers, introducing non-linearities that impact the efficient utilization of power from the grid and leading to suboptimal charging performance. The persistent presence of these distortions over time results in wear and tear, necessitating frequent power cable replacements and repairs.

Moreover, EMI-induced disruptions, a direct consequence of harmonic distortion, pose a threat to the stability of vehicle systems. These disruptions can cause unpredictable behaviors in vehicles, such as erratic dashboard displays, unintended acceleration or braking, and false alarm activations. Additionally, they can result in malfunctions in infotainment systems, faulty door locks, loss of communication with key FOBs, and impaired GPS and navigation systems.

Mitigation strategies for EV chargers

To address the effects of harmonic distortion on EV power cables, a comprehensive design approach is necessary. While establishing clear power factor limits for EV chargers can enhance overall efficiency, directly tackling harmonic content is equally important. Incorporating active inter-harmonic detection into EV charger specifications effectively manages and reduces the impact of harmonic distortion.

Other key mitigation measures and design considerations include:

- Optimizing converter designs to limit harmonic generation through devices with fewer switching actions or soft-switching techniques, thereby improving efficiency, conserving energy, and reducing risks associated with harmonic distortion.
- Implementing smart guides for on-board chargers to optimize the charging process and maintain consistency across different charging sessions.
- Utilizing passive filters when connected to non-linear loads to reduce the influence of specific harmonic frequencies, resulting in a cleaner power line flow and enhanced charging efficiency.
- Employing PFC circuits to ensure that current waveforms closely align with voltage waveforms, thereby enhancing power quality.

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