Charging Conundrum

Impact of electric vehicles on power quality

As part of its global climate change commitments, India has pledged to significantly reduce its carbon footprint. This requires reducing the dependence on fossil fuel for transportation and power generation. Electric vehicles (EVs) present a viable solution in this regard as they can help meet both of these requirements and reduce the consumption of fossil fuels, if they draw from renewable energy sources. The charging of EVs presents an excellent outlet for surplus energy produced by renewable energy sources during low demand periods. Further, the fulfilment of India’s goal of an all-electric car fleet by 2030 would reduce the fuel import bill as well as the running cost of vehicles.

However, the increased integration of EVs and charging technology in the grid also poses a few problems, particularly related to power quality. Both home charging and quick charging at charging stations can impact power quality. This can be in the form of non-linear distortions in the grid, addition of heavy single-phase loads, unwarranted peaks in power consumption, or harmonic contamination.

Power quality issues

By design, EV interface subsystems consist of power electronic converters that are non-linear devices with semiconductor-based switching power electronics elements. Thus, EV charging stations can create non-linear distortions and cause failure of electrical equipment.

Meanwhile, charging an EV battery bank at home may lead to additional heavy single-phase loads on the residential low voltage distribution network. Local distribution grids are not built to accommodate the huge spikes in power demand where the EVs are deployed. The risk of overloading local transformers is particularly high during peak hours. Utilities can, however, manage this by modifying customers’ demand using appropriate pricing mechanisms and incentivising charging during non-peak hours.

Voltage unbalance in the low voltage distribution network is caused by asymmetrical consumption and production. It leads to greater power loss in the system, interference with protection systems, degradation in the performance of components on that network, and overheating, even to the point of burnout. Voltage unbalance mainly affects three-phase components, such as transformers, synchronous machines and induction motors, which are designed to keep all three-phase windings carefully balanced. Therefore, it is critical that voltage unbalance in a circuit be immediately identified and dealt with to ensure a smooth operation of the power system and the connected loads.

Further, the input current in EV power electronic converters is generally characterised by high levels of harmonics, which are usually controlled by pulse-width modulation and filtering. Harmonic distortions are also caused by EV battery chargers in the power systems. Hence, the potential for increased levels of harmonic distortion rises with the increasing penetration of EVs in the network.

While a high voltage system may be able to cope with the extra load from EV charging, low voltage infrastructure will be significantly stressed and is bound to affect the reliability of network assets. There is a strong correlation between harmonic levels and EV charging routines. A quick charging rate causes significant voltage harmonics and losses, disrupting the standard limits. Transformer overloading also occurs when charging is concentrated in peak hours.

The way ahead

Going forward, utilities would need to rely heavily on smart grid technologies and advanced metering infrastructure, which can measure electricity consumption in real time and communicate this to the utility via radio frequency or broadband over power line technology to ensure the efficient management of energy supply and demand. Once connected to a smart grid, EV charging can be optimised and EVs can be integrated with other aspects of generation and demand. A smart grid provides the visibility and control needed to mitigate the load impact. Further, the deployment of different charging strategies and the use of voltage controllers can address the issues in the network and mitigate voltage unbalances. For example, EV charging power can be moderated through a decentralised voltage-dependent charging strategy  to reduce the voltage unbalances and improve the overall power quality. Moreover, EV interface devices can be designed to minimise or even eliminate the effects of EVs on the network.

Net, net, EVs are a favourable alternative to reducing the carbon footprint of the transport sector.  The growing popularity of EVs has led to the establishment of charging stations; however, the detrimental impact of EV charging station loads on the distribution network cannot be neglected.

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