Chargers for electric cars. Can they deteriorate the quality of power supply?

Introduction

At present, it is difficult to imagine societies functioning smoothly without car transport. The number of cars in individual countries is counted in millions, and the vast majority of them are vehicles powered by processed fuels obtained from oil (petrol, diesel). Due to the limited quantity of this natural resource, the rising costs of its extraction and the world’s ecological trends, there is a need to reduce its consumption. Moreover, public awareness concerning pollutant emissions is also at a much higher level than it was several years ago. All this drives the search for alternative solutions to the fuels currently in use, particularly in the area of car transport.

The number of electric vehicles (not only passenger ones) on the roads will grow. This implies the necessity to expand the infrastructure for charging such vehicles. This article presents some of the selected and most important threats to the quality of electricity posed by the rapid increase in the number of electric vehicle charging stations.

Choice of chargers’ power

The number of charging points for electric vehicles is growing from year to year. However, if the devices are to operate reliably, it is necessary to plan the investment carefully based on accurate data. The installation of an electric vehicle charging station is more complicated than connecting e.g. an electric heater. In addition to meeting the basic requirements for electric shock and fire protection, electromagnetic compatibility requirements should also be met, which ensure that chargers can operate reliably in the power grid. The compatibility requirements will mainly depend on the type and design of electric vehicle charging stations.

Currently produced chargers for electric cars can be divided into several basic types, which are defined by the IEC 61851-1 Standard [1]. Based on the power supplied to the car, the chargers can be divided into:

standard (also known as slow) chargers with powers up to approx. 3.7 kW for single-phase power supply and power up to about 11 kW for three-phase power supply,
semi-fast with approximately 7 kW in single-phase power and 22 kW for three phases,
fast (sometimes called super chargers) with power up to 150 kW (and in the future even 300 kW).

The power of the charger is directly related to the speed of charging the car. Before installing the charger, the basic question is what power is available at the point of the planned installation in order to install possibly the fastest charger, and at the same time to prevent the charger from adversely affecting other devices powered from a common connection point. To answer the question, an excellent solution is to use one of the PQM series power quality analyzers. They allow you to record the power consumption within a certain time (preferably a week) and establish the load profile. It is very important, because the load peak can occur at a different time than speculated. Thanks to the wide range of accessories, the analyzers can be easily installed in the dashboard without the necessity to switch off the power.

Adding a single 3.7 kW charger (car charging time 8 to 12 hours) to an existing system is unlikely to cause many power efficiency problems, even in a typical household. Assuming that the car is charged overnight, the power supply should be sufficient. It can be observed from the graph in Fig. 1, where you can see that the power consumed at night is negligible. In this case, we can even talk about a positive aspect for the energy system, i.e. power consumption during the period of its overproduction.

Fig. 1. Diagram of power consumption in a typical household

However, adding a few charging points may already have a negative impact on the system stability. The addition of a 150 kW charger (charging time of approx. 40 minutes) is only possible where a large power reserve is provided. For companies that plan to install several charging points, probably a better option would be to install several 11 kW chargers than a single 72 kW charger. This will allow a number of cars to be charged in a reasonable time of 1 to 2 hours instead of one vehicle in 30 minutes. In Fig. 2 you can see that in the company described in the example the peak power consumption is between 7:00 pm and 8:00 pm. Therefore, during the day there should be no problem to recharge even a few cars if only the total power does not exceed 30...40 kW.

Fig. 2. Three-phase power diagram for a production plant

It is thus important to carry out a thorough load analysis for the existing network before installing a charging point. Without this, the installed chargers may not be able to operate in the hours when they are most needed. Furthermore, the maximum power consumption contracted with the energy supplier may be exceeded. Most likely, it will result in financial penalties.

Potential network disruption

The quality parameters in the power supply are provided in the EN 50160 Standard [2]. In addition, manufacturers of charging stations should meet the requirements of the IEC 61000 multi-sheet standard [3] with regard to the emission of higher harmonics into the mains and the limitation of voltage fluctuations and flicker. In theory, chargers should be designed in such a way that they do not significantly interfere with the mains supply. However, electric cars are powered with DC current from built-in batteries. Therefore, charging is also done with direct current – either from the charging station itself or rectifying is done in the vehicle. In both cases there are power electronic components involved which non-linear character will cause more or less interference.

Still, there may be charging devices that for some reason do not meet the requirements and generate more interference to the mains.

Again, Sonel power quality analyzers come useful. In particular, the PQM-707 model with a built-in touch screen. Thanks to its autonomy, it does not need an external computer to operate, and everything can be done in the analyzer when it is connected to the charging station. The impact of the charger(s) on the mains supply can be verified quickly and easily by measuring:

harmonics in current and voltage (together with THD coefficients) – rectifiers usually generate harmonics by drawing a distorted current, as can be seen in the current flow in Fig. 3. If the harmonics in the power system exceed the limits set in the regulations, it may be necessary to disconnect the problematic charger, which will certainly cause frustration to car users. It is worth remembering that the presence of voltage and current harmonics is a natural consequence of commonly used electronic devices, so it is worth checking their level even before installing the charging station. In the case of high harmonics content in the voltage, it may turn out that after connecting the charger, the limits may be exceeded. And even if they are not, a high harmonics level can cause various negative effects in the mains, such as overloading of the neutral line, poorer performance of devices, vibrations or difficult starting of motors.

Fig. 3. Example diagram of the current and voltage waveforms of a single-phase charger at 2.2 kW

unbalance – if three-phase chargers are used, this problem should not exist. But when using multiple single-phase chargers with higher power output, this can result in unbalance of currents and voltages. Even if the chargers are installed in different phases, it may not be possible to force all of them to work at the same time, at the same load. This will result in a certain level of unbalance, which has a negative impact on three-phase consumers, causing, for example, higher energy consumption and increased heat loss in motor windings, which shortens their service life.
voltage dips – high-power chargers will cause voltage drops in the mains, which may cause a number of problems with functioning of devices in conditions of reduced voltage or cause a decrease in their efficiency. In addition, in extreme cases, the permissible voltage levels set by regulations may be exceeded.
reactive power – chargers as converters generate capacitive reactive power in most cases. Since excess reactive power is not desirable, and exceeding the contractual values may result in increased financial charges, it is worth controlling the power level.
active power – assuming that peak power demand measurements have been carried out before the installation of the charging station, it is worthwhile to carry out tests again after its installation. With time, more consumers may be connected to the mains, which, as mentioned earlier, may cause problems with power supply efficiency.

Conclusion

Electromobility can revolutionise road transport. Chargers are constantly being improved to make sure that their electrical parameters are as good as possible. However, the uncontrolled expansion of the electric car supply point network can disrupt or even destabilise the functioning power network. That is why it is so important to plan investments well and monitor quality parameters, so that the chargers in operation do not interfere with other electrical devices. Among other things, the PQM series power quality analyzers have been created for such tasks, and they can obviously help their users to solve many other problems occurring in the power network.

References

[1] IEC 61851-1 Electric vehicle conductive charging system - Part 1: General requirements
[2] EN 50160 Voltage characteristics of electricity supplied by public electricity networks
[3] IEC 61000-3-2 Electromagnetic compatibility (EMC) - Part 3-2 - Limits - Limits for harmonic current emissions (equipment input current ≤ 16 A per phase), IEC 61000-3-3 Electromagnetic compatibility (EMC) - Part 3-3 - Limits - Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems, for equipment with rated current ≤ 16 A per phase and not subject to conditional connection, IEC 61000-3-11 - Electromagnetic compatibility (EMC) - Part 3-11: Limits - Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems - Equipment with rated current ≤ 75 A and subject to conditional connection, IEC 61000-3-12 Electromagnetic compatibility (EMC) - Part 3-12: Limits - Limits for harmonic currents produced by equipment connected to public low-voltage systems with input current > 16 A and ≤ 75 A per phase