The first mass-produced electric vehicles (EVs) hit the road late in 2010 with the introduction of the Nissan Leaf, which remains the world’s top-selling, highway-capable, all-electric car. In the United States, sales of EVs are gaining momentum, with 2017 sales up by 25% over 20161. EVs, however, are still outnumbered by roughly 300 to 1 by vehicles powered by internal combustion engines. EVs are unlikely to become fully mainstream until there is a nationwide network of charging stations that can charge a vehicle quickly enough to get travelers back on the road in a matter of minutes rather than hours.

The charging infrastructure necessary to keep these vehicles on the road has also begun to grow steadily. Market forecasters at Navigant Research are predicting that global sales of DC fast chargers will grow from 19,000 units in 2017 to more than 70,000 in 20262. DC charging systems allow for far faster charging than AC charging systems, which are inherently limited in power based on the capabilities of the charger installed inside of the vehicle (i.e., the onboard charger).

Electric vehicle charging stations — known in North America as Electric Vehicle Supply Equipment (EVSE) or simply as charging stations, charging posts, or charging piles elsewhere — must be engineered to withstand years of harsh environmental conditions such as heat, cold, rain, snow, and even effects due to nearby lightning strikes. In addition, they must ensure the safety of EV drivers who are holding a connector capable of carrying 1,000 DC volts or more. That means the charging station must be protected from overcurrents, overvoltages, overtemperature, and ground faults3. What’s more, the charging infrastructure industry is trying to figure out this emerging application, so there are multiple design approaches and no single set of standards to guide them. This article offers an overview of the mechanisms available for protecting users, vehicles, the public, and DC fast chargers.

An Introduction to DC Fast Charging Systems

To provide context to the discussion of DC fast charging systems, it may be helpful to describe the various AC charging approaches that preceded them.

The first approach, typically intended for use in residential settings, provides 120 VAC (US)/230 VAC (EU) single phase charging, with from 1.4 kW to 1.9 kW of output power. Depending on the capacity of the vehicle’s battery and its discharge level, a full recharge could take anywhere from 12 to 18 hours. The second approach, often used in public parking environments, provides 240 VAC (US)/400 VAC (EU) of single- or three-phase charging, with output power from 4 kW to 19.2 kW. Charging times range from two to six hours. A third approach, which is supported by several European vehicle manufacturers, provides three-phase AC fast charging at power levels of up to 43 kW. All three of these approaches use the vehicle’s onboard charger (AC to DC converter) to charge the vehicle’s battery pack.

In contrast to these approaches, DC fast charging systems are designed to bypass the vehicle’s onboard charging system and connect directly to its battery system. DC fast chargers can provide up to 400 kW of DC output power (typically from 400 VDC to 1000 VDC), converting three-phase AC power sourced from the electrical grid into DC power using highly efficient power semiconductor devices. This high output power can charge fully depleted batteries on most vehicles to 80% of their full charge in 30 minutes or less. Charger system developers around the world are striving to reduce that charging time still further, so that charging takes roughly the same time as filling a traditional vehicle’s gas tank.

An isolation transformer inside the EVSE separates the AC power on its input side from the DC output side. Once the EVSE’s connector is attached to the vehicle, the EVSE performs an automatic safety check of the circuit insulation and checks for any possible short circuits between the charger and vehicle contactors. Once energy begins flowing into the battery, if a malfunction occurs in the vehicle, communication lines in the connector signal the EVSE to open the contact to stop the DC output and indicate an error on the display.

Within the EVSE, power undergoes several conversion stages, each requiring some form of circuit protection:

  • AC input: This requires overcurrent and overvoltage protection, residual-current or ground-fault detection, along with one or more stages of filtering for electromagnetic interference (EMI) purposes.

  • AC-to-DC rectification: This stage converts the positive and negative cycles of the AC input power to just positive voltage.

  • Power Factor Correction (PFC): Sometimes included in the rectifier stage, this stage compensates for energy-storing components (capacitors, inductors, etc.) used in the power converter to minimize the amount of reactive power (or non-useful power) as much as possible.

  • DC-to-DC conversion: This stage uses high-efficiency semiconductors to adjust the DC voltage efficiently to the optimum value(s) for charging.

  • DC output: This stage demands over-current, overvoltage, ground-fault protection and filtering.

  • DC fast charger overcurrent protection.

An overcurrent is any current that exceeds the ampere rating of conductors, equipment, or devices under their conditions of use. The term “overcurrent” includes both overloads and short-circuits. In the United States, overcurrent protection requirements for EV charging stations are based on requirements from the NEC® and UL. In most other parts of the world, they are dictated by the IEC 61851 series of standards or derivatives of those standards.

All electrical systems, including DC chargers, will eventually experience some level of overcurrent. Unless removed in time, even moderate over-currents can quickly overheat system components, damaging insulation, conductors, and equipment; large overcurrents can even melt conductors and vaporize insulation. Very high overcurrents produce magnetic forces capable of bending and twisting bus bars, and uncontrolled overcurrents can damage chargers, leading to fires, poisonous fumes, and explosions that can injure or kill anyone nearby.

AC Input Side Overcurrent Protection

Industrial fuses (Figure 1) are the recommended overcurrent protection device for the charger’s AC input side. A variety of considerations must be weighed in order to select the right fuses for this application.

Current Rating — The AC current (expressed in amps) that the fuse can carry continuously under specified conditions. A number of derating factors are applied to the current rating of a fuse, based on ambient temperature, expected lifetime, and other factors. Generally, these derating factors are useful in analytically determining the amount of current the fuse can carry without nuisance opening.

Voltage Rating — The maximum AC voltage at which the fuse is designed to operate. Fuse voltage ratings must equal or exceed the circuit voltage where the fuses will be installed.

Interrupting Rating — The highest available symmetrical RMS alternating current that the fuse is required to interrupt safely at its rated voltage under standardized test conditions. A fuse must interrupt all overcurrents up to its interrupting rating without damage. Standard power fuses are available with interrupting ratings ranging from 10,000 to 300,000 amps.

Type of Protection and Fuse Characteristics — Time-current characteristics determine how fast a fuse responds to overcurrents. All fuses have inverse time characteristics; that is, the fuse opening time decreases as the magnitude of overcurrent increases. When properly sized, fuses provide both overload and short-circuit protection to system components.

Current Limitation — A current-limiting fuse is designed to open and clear a fault in less than 180 electrical degrees or, in other words, within the first half electrical cycle (0.00833 seconds).

Physical Size — The size of the fuse intended for a given application is another important selection consideration. Although reducing space requirements wherever possible is almost always preferable, other considerations must be taken into account: Does the smallest fuse have the most desirable characteristics for the EVSE? Does the EVSE provide adequate space for maintenance? Do the small fuses being considered coordinate well with the EVSE’s other over-current protection devices?

Indication — Fuses with indicating features offer an easy way to identify which fuse in the system has opened, and which reduces downtime, increases safety, and helps reduce housekeeping or troubleshooting headaches and delays.

Output Side Overcurrent Protection

Given the high level of DC power being fed to the vehicle’s battery, the margin for error for charging properly is very narrow. The most-often overlooked aspect of this overcurrent protection application is protecting the costly power semiconductor devices like MOSFETs, thyristors, and IGBTs used in power converters (inverters, rectifiers, etc.). These devices are typically fabricated from silicon or silicon carbide and have low thermal withstand capacity. They can be greatly affected by the electrical, mechanical, thermal, and environmental stresses they undergo during operation, which can cause them to fail prematurely. When these power semiconductors fail, they can cause catastrophic conditions such as case rupture, fire, and explosion.

Figure 2. The most common styles of high-speed fuses for EV DC charging stations are (l-r) round body, square body, and cylindrical or ferrule.

High-speed fuses (also known as rectifier fuses, ultra-fast acting fuses, ultra-quick fuses, and semiconductor fuses) offer the level of protection these sensitive power semiconductor devices require to withstand these harsh conditions. They are classified based on dimensions, mounting, and origin. The most common styles are North American traditional round body, square body, and cylindrical or ferrule (Figure 2). High-speed fuses offer the short-circuit characteristics required to protect semiconductor devices, including low energy let-through (l2t), low peak currents (lPEAK), low arc voltage, and high heat dissipation. They contain one or more current sensitive elements made of silver, silver-plated copper, copper, etc., each of which has a reduced cross-section at one or more points that provides a measured resistance in each element. The resistance of each element and the number of elements used in each fuse typically determine the fuse’s current rating.

Protection of Power Conversion Devices

Figure 3. Typical three-phase power converter circuit.

Figure 3 represents a typical DC fast charger system made up of several building blocks, including, but not limited to the input protection, input filtering, rectifier, power factor correction, the DC bus or DC link, the DC/DC converter and output protection.

Although protection requirements vary at each location, the main purpose of the fuses in this circuit are to allow the nominal load current and any permissible overload current to go on continuously without interruption. At the same time, the fuses are selected to interrupt any overcurrent fault caused during overload or short-circuit, with minimal let-through energy in order to protect the power semiconductor devices connected in the circuit.

Figure 4. Location of high-speed fuses in rectifiers.

The location of a high-speed fuse in a rectifier circuit depends on the size of the of the system when considering power rating. Figure 4 illustrates the typical location of high-speed fuses in a rectifier circuit.

For smaller power rated devices, high-speed fuses are typically found only on the AC line side in a one-fuseper-phase arrangement. For larger power systems, high-speed fuses are typically located both on the AC line side and individually in series with each power semiconductor device on each arm of the rectifier circuit.

High-speed fuses are used in inverter circuits to prevent line-to-line short circuit fault conditions that can be generated in multiple ways, with the misfiring of transistors being one of the leading causes. Depending on the power rating of the inverter circuit, the location and number of high-speed fuses used in the circuit varies. For low-power applications, the high-speed fuses are typically designed only on the DC bus (one each on positive and negative). For higher-power inverter circuits, fuses can be used both on the DC bus side and individually nearer (in series) to each transistor.

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.

Figure 5. Varistors like this iTMOV Series varistor from Littelfuse have an integrated thermally activated element designed to open in the event of overheating.

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.

Ground-Fault Protection

Figure 6. DC ground-fault relays such as this Littelfuse SE-601 detect DC leakage current and indicate the faulted bus.

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.

Figure 7. Current transformers are often used in conjunction with ground-fault protection devices.

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.

Conclusion

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 .

References

  1. https://arstechnica.com/cars/2018/01/2017-was-the-best-year-ever-for-electric-vehicle-sales-in-the-us/
  2. https://www.navigantresearch.com/news-and-views/global-sales-of-dc-fast-chargers-for-electric-vehicles-are-expected-to-reach-70000-in-2026
  3. “Ground faults” are known as “earth faults” in some countries.

Tech Briefs Magazine

This article first appeared in the April, 2019 issue of Tech Briefs Magazine.

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