Impact of Electric Vehicles on Power Quality

Today energy efficiency is a top priority, boosted by a major concern with climatic changes and by the soaring oil prices in countries that have a large dependency on imported fossil fuels. Intending an improvement of energy efficiency, a revolution in the transportation sector is being done and the bet is in the electric mobility.

The Indian Government aspires for a nation with 100% new sales of Electric Vehicles (EVs) by 2030. Some significant strides have been taken towards this aspiration in the past through the launch of NEMMP (National Electric Mobility Mission Plan, 2013) and FAME (Faster Adoption and Manufacturing of Hybrid & Electric Vehicles, 2015). Under NEMMP, 6–7 million EVs/hybrid vehicles have been envisioned to be deployed on Indian roads by the year 2020, while under FAME, the government has been setting aside money to subsidise EV purchases on an annual basis.

Market penetration of EV has been increasing drastically in the recent past. However, EV integration into power grids adds more challenges for power system network. The spread use of EVs will bring consequences to the power grid, mainly in terms of peak load management and power quality, which are associated to the charging stations. It is essential to evaluate potential grid impacts due to EV integration to guarantee consistent grid operation. Though considerable impact studies on power system due to EVs have been undertaken, the impact on power quality has remained almost unattended. Thus, we lay out this blog focusing on the impact of EVs on the Power Quality (PQ).

WHY PQ IS A CONCERN WHEN DEALING WITH EV?

The market for electric vehicles (EVs) is growing and is expected to reach 3.8 million by 2020. EV models are becoming more affordable as a result, which in turn fuels the adoption even further. The charging infrastructure has been growing along, often supported by local governments. But what is interesting to understand is how will the shift to electric vehicles impact the grid? What about PQ problems caused by EV interface device?

EV interface devices employ power electronic converters and these are highly non-linear devices due to their operating principles and the presence of switching power semiconductor elements. Therefore, the input current of the converter generally contains high levels of harmonics. It is evident that significant PQ issues will be caused by malfunctioning of the interface device. The current will be highly distorted and this will impact the local network particularly if there are many EVs having similar problems.

Power Quality is important in order to safeguard the proper functioning of the power grid system and the loads connected to it. PQ requirements should be a characteristic of both parts of the system i.e. the energy supplied by the power grid, as well as the energy consumed by the equipment connected to the grid. The degradation of the power quality is mainly caused by the non-linear current consumption of the battery charging systems.

This reflects in the THD of the consumed current and also in the voltage THD, due to the line impedance. In addition to harmonics there are some more PQ concerns due to EV integration as discussed below:

“Harmonics is one of the prominent PQ issue due to EV integration”

Harmonics: The uncontrolled connection and disconnection of EV into a power distribution system will increase harmonic voltage and current distortions. The unexpected number of EV charging during peak demand hours may affect the overall residential load curve, increase system losses, overload lines and increase harmonics introduction into the system. Besides the harmonics, other power quality problems, as inter-harmonics, noise (electromagnetic interference), momentary interruptions, sags, swells, flicker, notches, and transients can also occur.

Voltage Imbalance: Voltage imbalance in the low voltage distribution network is amongst the main power quality issues caused by electric vehicles. With the expected increase in quantities of EVs, the main connection points and the penetration level remain undetermined; this aspect would result in voltage imbalance in the three phase systems with the current availability of single-phase EV chargers in residential networks.

Power Loss: When charging or discharging electric vehicles, power losses occur in the vehicle and the building systems supplying the vehicle. Predominant losses occur in the power electronics devices used for AC-DC conversion in the vehicle. Large scale connection of PHEV will cause uncertainty in power system operation. Penetration of EVs generates harmonics which would disrupt grid strategies, decrease voltage and increase power losses.

Transformer Loading: The risk of overloading transformers is particularly high during peak hours. Imagine what will happen when all-EV owners in the neighbourhood decide to recharge them at the same time, in the early evening, after returning from work, which is more or less the same time households turn their cooking, cooling and other appliances on. Power electronic appliances increases the harmonics as the load increases, followed by an increase in the transformer temperature. This phenomenon shall lead to wear and tear on the transformer bushings because of the flattened load efforts. Also, EV penetration will enhance transformer load and decreased voltage beyond the acceptable limit.

Figure 1. Impact of EV integration on PQ

Hence, Indian Discoms need to have suitable Time of use/ Time of Day tariffs or similar incentives in place to avoid worsening of existing peak demand slots.

HOW BIG WILL BE THE IMPACT ON PQ?

Electric Vehicle is a new type of load for the grid. The existing grid was not designed for this new type/pattern of load, which corresponds to the battery charging systems of EVs. EVs may be considered as active loads, increasing the demand on the network during charging. They represent a new type of load that introduces new PQ problems. The problems arise from the possibility of simultaneous charging of a large number of vehicles, which can overload the power grid and from the effects of non-sinusoidal current consumption of the battery charging systems.

A report published by the California Energy Commission presents a study about the impact of residential EVs batteries charging systems. It shows that with the use of EV, the THD of the current presents a variation from 3% (at the beginning of charging, with a unitary power factor) to 28.11% (at the end of the charging, with a power factor equal to 0.96). Hence, it is clear that the simultaneous use of a great number of EVs batteries charging systems connected to the electrical distribution grid can cause a significant degradation of the electrical power quality.

Moreover, transformers, which connect every home and business to the grid, are the most vulnerable and affected elements of the system. Most residential transformers are designed to serve between 10-50kVA of load, while a single plug-in vehicle (PEV) with a 240V charging system consumes about 7kVA. If multiple electric car owners use the same distribution transformer (such situation is referred to as clustering), they may cause damage or outages from overloading the equipment or by shortening their normal cool-down period. A single overloaded transformer can, in turn, degrade power quality in other residential feeders.

“EV interface device will affect network loading, voltage profile, phase imbalance and power quality”

To make things worse, transformers do not contain any telemetry systems built in to send information on their health to utilities. In many cases, power companies do not have any system in place informing when the overload occurs.

How serious is the problem? Some studies suggest that higher penetration rate of electric vehicles increase transformers’ loss-of-life factor, even by up to 10,000 times. And this comes with a hefty price tag!

Moreover, poor PQ will raise electrical safety concerns. The SMPS based direct power supply does not provide physical isolation to the users hence vulnerable to accidental contact with live elements causing safety breach. EV charging stations shall use galvanic separation using isolating transformer for ensuring users’ safety. Also, considering the risk involved to users, it is necessary that the components used in charging station should be made of fire retardant halogen free material and conform to available standards.

Thus, the impact is expected to be significant on PQ and power system elements due to the high energy capacity and mass deployment of EVs in the future. EV interface devices may be designed to minimize or even eliminate the effects of EVs on the network fault level and protection system. However, their effects on the network loading, voltage profile, phase imbalance, power quality & safety could be significant and need to be appropriately assessed. Therefore, the impact caused by the spread of EVs will be different and the ones that cannot be neglected.

CONCLUSION

Economic and environmental reasons are making EVs a reality. However, the spread use of EVs will bring consequences to the power grid, mainly in terms of peak load management and power quality. Large deployment of EVs will lead to potential PQ problems for existing power networks, harmonics being prominent. Thus, EVs may have adverse effect on the distribution network if the penetration is not carefully and systematically planned due to its nonlinear nature. It is therefore a need of the hour to study and analyse the effects of EV on power quality before EV hits the grid and hurts it.

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