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.

Tech Briefs Magazine

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

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