Safe electric vehicle charging

Figure 1 : Protection even against storms: Charging devices need to function safely and reliably in all types of weather.
  • Protecting charging infrastructure from transient overvoltages.

Dipl.-Ing. Holger Heckler, Product Marketing Lightning and Surge Protection Trabtech, Phoenix Contact GmbH & Co. KG, Blomberg, Germany; and Dipl.-Ing. Axel Rüther, Electronics Sales Marketing, Phoenix Contact Deutschland GmbH, Blomberg, Germany.

Users and operators of charging stations expect one hundred percent availability from them. When a charging station is out of order, one side has limited mobility, and the other faces lost sales and repair costs. 

Short voltage spikes, known as transient overvoltages, are responsible for a significant proportion of failures. Protective measures to prevent transient overvoltages can help (Figure 1)

Sensitive electronics in charging devices

The equipment in charging devices depends heavily on the type of charge. A battery charging device is needed for all types of charging. For AC charging, the charging device is located in the vehicle, and for DC charging it is located in the stationary charging device. All charging devices have a charging controller. In AC charging devices, residual current devices (RCDs) are used to detect faulty currents, known as residual currents; in DC charging devices, insulation monitoring devices (IMDs) are used. Public charging stations also usually have operator terminals, billing systems and communications equipment. Charging parks include additional equipment for load distribution and for energy management. All of these components react sensitively to transient overvoltages, and must be protected from them (Figure 2).

Figure 2 : Charging station for AC charging in public areas: The sensitive electronic components are protected by overvoltage protection devices of various types.

Hazard due to the effects of lightning and switching overvoltages 

With charging equipment, indirect lightning strikes – which are lighting strikes close by – must be taken into consideration. If the equipment is away from buildings, direct lighting strikes are more unlikely. In addition, comprehensive protection against direct lightning strikes is expensive — a lightning protection system is made up of an arrester, protective devices, a suitable grounding system, a dense potential equalization system, and lightning current carrying-capable surge protective devices (SPDs) of type 1. Therefore, most charging parks and individual charging stations do not include a lightning protection system of this type. Direct lightning strikes on charging devices are simply accepted as a risk.

But the voltage peaks that also occur on the supply and data lines due to indirect lightning strikes are usually significantly higher than the surge voltage strength of the equipment used in the charging devices and the surge voltage strength of the electric vehicle attached to them. 

When planning charging devices, however, malfunctions caused by lightning strikes are not the only thing that need to be taken into account; switching overvoltages from the low-voltage supply system also play a part. Switching overvoltages occur frequently, but do not always cause immediate damage to the equipment. However, if switching overvoltages occur more frequently, then they can age electrical and electronic equipment prematurely. But individual switching overvoltages can also have an amplitude above the insulation or impulse withstand voltage of the equipment.

Therefore, when higher voltage peaks occur due to lightning strikes or switching overvoltages, charging devices and the associated auxiliary equipment, including the electric vehicle itself, can become damaged or even destroyed. Overvoltages cause short circuits, among other things. The damage caused by short circuits in energy technology circuits is often more serious than damage that has been caused by the actual overvoltage event.

When short-circuit currents are being directed to ground (ground leakage currents), conductive parts that are grounded can carry voltages that are dangerous if touched. Conductive parts of charging stations or conductive grounded parts of electric vehicles that can be touched can also be carrying dangerous voltages.

The likelihood of voltages that are dangerous to the touch that were caused by voltage peaks with subsequent ground leakage currents can be effectively lowered by using surge protective devices. Usually, it is sufficient to protect charging devices and electric vehicles in such a way that they can survive indirect lighting strikes and switching overvoltages from the power grid without damage.

Effective protection thanks to overvoltage safety equipment

Overvoltage safety equipment effectively protects against transient events due to overvoltages resulting from lightning strikes (lightning surge voltages), overcurrents resulting from lightning strikes (lighting surge currents) and switching overvoltages. The following protective effects are achieved by using overvoltage safety equipment:

  • Surge currents caused by lighting are “redirected” so that the conductors do not overheat.
  • Overvoltages caused by lightning are limited for neighboring circuits in such a way that the insulation and electric strength of the equipment to be protected is not exceeded and there is no dangerous spark formation or short-circuiting.
  • Switching overvoltages are limited in such a way that premature aging of electronic devices is prevented to the greatest extent possible.

The spatial design and the arrangement of electronic equipment in a charging park has a huge influence on the respective load that can be expected due to surge voltages and surge currents. Therefore, a risk assessment should be carried out for each charging park. This is the only way to ensure that suitable surge protective devices are selected and the best-possible installation locations are found.

Figure 3 : SPD type 2: The VAL-SEC-T2-3S-350-FM devices from Phoenix Contact have been designed for grounded 230/400 V AC power supply systems.

If direct lightning strikes are to be expected, then powerful type 1 overvoltage protection equipment – referred to as lightning current arresters – should be included in the the potential equalization system for the power lines. If no external lightning protection is present or planned, then more cost-effective type 2 overvoltage protection equipment – referred to as surge protective devices – are the optimal solution (Figure 3).

In addition, numerous components are supplied with 24 V DC in the charging stations. The 24 V auxiliary voltage can be protected with type 3 overvoltage protection equipment, for example. In charging parks and at public charging stations, components for communication with a billing system or a building management system are also usually present. For this purpose, for example, an Ethernet connection can be used. Type D1 overvoltage protection equipment would then be recommended to protect this connection (Figure 4).

Charging park and charging station designs can be significantly more complex than in the example above. Therefore, all lines and equipment where a hazard due to overvoltages can be expected must be included in the risk assessment. This includes the following components:

Figure 4: SPD type D1: The DT-LAN-CAT.6+ device from Phoenix Contact protects the Ethernet cables.
  • If it has its own transformer, then there is always a low voltage switching device.
  • In more complex charging devices and charging parks, there are always control lines for energy management.
  • DC fast-charging devices can be equipped with additional central cooling equipment or with cooling equipment in the charging stations.
  • DC storage with buffer batteries and DC/DC or DC/AC converters.

Overvoltage safety equipment increases the availability of the charging infrastructure and also protects the connected electric vehicles. 

Phoenix Contact provides suitable solutions for all application cases with AC or DC charging in the fields of electrical energy technology, measurement and control technology, as well as for data technology and information technology equipment.

Additional information:

www.phoenixcontact-emobility.com/en/

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