DC Fast Charging Overvoltage Protection
Before providing power to the EV battery, most DC fast charging stations communicate with the vehicle to detect how much charge is left in the battery to determine how much power to provide. Control units communicate between the EV and the charger as well as to the driver via a display on the charger.
Because chargers are typically located outdoors, they are subject to voltage transients from which they must be protected to ensure that they are operating properly. Electrical surges are the result of sudden releases of energy that was previously stored, or induced by other means such as heavy inductive loads or lightning strikes. This energy is carried to the EVSE on the power supply lines. Repeatable transients are frequently caused by the switching of reactive circuit components. Random transients, on the other hand, are often caused by lightning and ESD, which generally occur unpredictably and may require elaborate monitoring to be measured accurately, especially if induced at the circuit board level.
The most suitable type of transient suppressor depends on the intended application; some applications require the use of both primary and secondary protection devices. The function of the transient suppressor is to limit the maximum instantaneous voltage that can develop across the protected loads. The choice depends on various factors but ultimately comes down to a tradeoff between the cost of the suppressor and the level of protection needed.
When it is used to protect sensitive circuits, the length of time a transient suppressor requires to begin functioning is extremely important. If the suppressor is slow-acting and a fast-rising transient spike appears on the system, the voltage across the protected load can rise to a damaging level before suppression kicks in. In a DC charging system, a metal oxide varistor (MOV) or high-power Transient Voltage Suppressor (TVS) diode is usually the best type of suppression device. Other types of protectors — such as gas discharge tubes, protection thyristors, and multi-layered varistors (MLV) or combinations of suppression devices — can also be used.
Varistors (Figure 5) are voltage-dependent, nonlinear devices with electrical characteristics similar to back-to-back Zener diodes. They are made primarily of zinc oxide with small additions of other metal oxides such as bismuth, cobalt, manganese and others. The MOV is sintered during manufacturing into a ceramic semiconductor with a crystalline microstructure that allows it to dissipate very high levels of transient energy across the entire bulk of the device. Therefore, MOVs are typically used for the suppression of lightning induced transients and other high energy transients.
TVS diodes are used to protect semiconductor components from high-voltage transients. Their p-n junctions have a larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.
DC fast chargers require protection against ground faults on both the input and output sides. A ground fault is an inadvertent contact between an energized conductor and ground or the equipment frame. The return path of the fault current is through the grounding system and any equipment or person that becomes part of that system. Ground faults are frequently the result of insulation breakdown, and they are the type of electrical fault that most often is the source of electrical shock. Wet and dusty environments, such as those found around an outdoor vehicle charging station, require extra diligence in design and maintenance to minimize the risk of ground faults.
The isolation transformer inside the charger separates the input AC power from the output DC power; therefore, the output side is not grounded. Instead, a ground-fault monitor is installed on the output side to detect any earth leakage and shut off power immediately. The ground-fault monitor is used by installing a ground-reference module between the two buses to establish a neutral point. The ground-fault relay (Figure 6) uses this neutral point as a reference to detect low-level ground faults.
Although there are many types of ground-fault protection devices for use on grounded or ungrounded systems and different applications, they can usually be simplified down to just a few different methods of operation. Current transformers (CTs) are typically used in conjunction with an AC-current-based ground-fault protection device. The CT (Figure 7) detects leakage current flowing outside the intended conductors; if it is outside of the tolerances set on the protection device, the device will trip to prevent damage to the system.
The IEC 60364-7-722 standard calls for every connection point on the input side of the charging station to be fitted with a residual-current device (RCD) with rated residual current ≤30 mA. The output side needs protection in the event of a DC fault current ≥6 mA. This protection can be provided by using a Type B RCD installed separately on each side of the installation.
In order to weather harsh environmental conditions over the long term while ensuring the safety of EV drivers and the general public, the DC charging stations of tomorrow must be protected from overcurrents, overvoltages, over-temperature, and ground faults. Even as new designs for these stations evolve, the need for protection will remain constant. To stay current with new protection approaches, designers must constantly re-educate themselves about circuit protection options.
This article was written by Tim Patel, Global EV Charging Business Development Manager at Littelfuse, Inc. (Chicago, IL). For more information, visit here.
- “Ground faults” are known as “earth faults” in some countries.