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Three scenarios with specialized power electronic solutions

As the electrification of heavy-duty and commercial vehicles becomes more common, charging larger batteries than in electric passenger cars is becoming increasingly important. Since time is money, especially in logistics, the preferred options are to increase the charging capacity or allocate downtime for charging. These advantages lead to three different charging scenarios.

Scenario 1: Charging base and fleet operation

Modern battery technologies and advanced solutions in the field of power semiconductors enable the creation of highly efficient infrastructure. The picture above shows a modern depot that charges for the bus fleet.

Depot charging is the best option for local fleet operation, especially for buses and delivery vehicles. They operate on relatively fixed routes and do not operate at night.

This form of charging provides lower demand for charging power and additional power management options. Including stationary batteries, splitting bus charging time from excess energy time also becomes an option.
Modern electric buses with storage batteries have a capacity of 250 to 500 kWh, which allows them to work in one shift without recharging. Individual depot chargers only need to charge one vehicle overnight, and even with an 80 percent charge from 500kWh in 6 hours, 70kW is sufficient. Of course, this is multiplied by the number of vehicles that must be simultaneously charged for the entire depot.

A typical charger circuit includes an input stage that can adapt the DC link voltage, an output rectifier, and a galvanic isolation stage between them, as shown in Figure 2.

Figure 2: Schematic of a bidirectional charger and recommended components

As a rule, chargers are created on a modular basis from subsystems that can be combined to increase the output power. Most standard designs are 15-60 kW per subsystem, and component selection varies based on power requirements and cooling preferences. While 10 to 15 kW forced-air-cooled units are widely composed of discrete devices, higher-power units use liquid cooling and mostly consist of several power modules.

Parallelizing blocks is another option for increasing output power while creating functional system redundancy. This allows the system to operate at lower power in the event of a single module failure instead of losing the entire system.

Depot charging also opens the door to being used as a secondary network service. Stationary energy storage helps reduce the load on the grid and, during times of high energy demand, even supports the grid. Scheduled charging and load balancing also become an option. Charging times can coincide with periods of excess energy, potentially leading to reduced or even negative energy costs at night.

Fleets with a fixed schedule do not need to be fully charged at the same time. It is also possible to exchange energy between vehicles, and those that should not be in operation can contribute their stored energy. Entire depots as large industrial areas can also become solar power plants.

Scenario 2: Opportunity Fee

Vehicles that follow predetermined routes allow for increased range by adding less energy more often. This is called opportunity charging. This works best when it is fully automated.

There are two recommended solutions for possible charging.

Mechanical systems known as pantographs allow large electrical contacts to travel long distances and safely contact their counterparts. Proven reliable technology, pantographs are widely used in trams and railways. Pantographs are divided into top-down and bottom-up systems depending on the installation location. Bottom-up approaches are mounted on the vehicle and communicate with the station. The top-down mechanics are part of the station and drop down to the car. Figure 3 shows how to set up pantograph charging.

Figure 3: Top down pantograph for possible charging

Infrastructure construction remains limited to the roadside. Thus, such a facility can be built to retrofit existing stations where there is adequate on-site power supply. Since this rarely happens, buffering the station with a battery accumulator is a widespread solution to separate the high power charging of the vehicle from recharging stationary batteries.

Power levels of 125-250 kW are usually used.

Before the charging process begins, the charging voltage and current are equalized between the station and the vehicle’s battery management systems. Due to the high capacity, pantograph charging is always carried out by direct current charging with direct access to the vehicle battery.

For future installations, pantographs are a recommended solution, especially for autonomous vehicles, as they do not require a plug or wire that requires precise use. The systems can easily handle vehicles of different heights and can be designed to tolerate misalignment between the station and the vehicle.

Also popular for mobile devices such as smartphones, consider upgrading wireless power transmission (WPT) to meet the needs of large-scale power transmission. SAE J2594 details wireless power transmission for automotive systems. Wireless charging systems essentially have two independent parts that exchange energy using magnetic flux. In order not to sacrifice too much transmission efficiency, SAE J2594 targets them to achieve a minimum of 80 percent transmission efficiency. Series-compensated resonant circuits operating in the 80-140 kHz frequency range, as shown in Figure 4, can be used to meet this requirement.

Figure 4: Resonant WPT setup with series compensation

Many input rectifier topologies should be considered, including static diode rectifiers as a cost-optimized solution or a thyristor-based version. The Wien rectifier is a common solution due to its excellent electromagnetic damage characteristics, reduced effort required for filtering, and adjustable DC link voltage. At a high switching frequency of 80 to 140 kHz to drive the transmission coil as required by the standard, low switching loss IGBTs or SiC-MOSFETs for the DC-to-DC conversion stage can be considered.

Inductive chargers should be installed in places where they can be run over by vehicles. Unlike pantographs, it has a strong impact on infrastructure, especially in public transport. Therefore, inductive charging is a suitable solution, primarily for semi-public places. For example, airport baggage carts can benefit from wireless power transmission as the power level, energy involved, and topographic conditions match the application’s requirements.

Scenario 3: Individual long-term operation

Traveling along random routes, as required by long-distance logistics, requires individual high-capacity charging, similar to today’s gas stations. This powerful charging must become part of the existing infrastructure to ensure the seamless integration of electric trucks into the mobility sector.

With a DC voltage of up to 1500 V and a maximum charging current of up to 3000 A, charging with a power exceeding 2 MW becomes possible.

With a 2MW charge, 500kWh to drive another 300km can be obtained in around 15 minutes, which is fully covered by the breaks the driver must take to meet legal requirements. However, urban low-voltage 3-phase networks up to 400 V do not support this power level.

In this scenario, a local medium voltage power supply is a must. Although buffering from stationary batteries is a potential option, the storage capacity would be relatively large.

The need to operate from a medium voltage transformer leads to a promising option for chargers in the megawatt mode. Instead of scaling up the structure used for car charging, it makes sense to follow the well-established circuit used in electrolysis. Figure 5 shows the correlating high power setup.

Figure 5: High Power Charging Topology with B12C, also marked as B6C-2P

This approach involves only one power conversion stage and moving the galvanic isolation stage from smaller individual converters to a medium voltage transformer increases the efficiency of the power conversion stage to 99 percent. At the same time, it minimizes the number of resources per installed kW, and the assembly, built from press-package components, reduces space requirements.

When used in the megawatt mode, thyristor-based solutions combine excellent efficiency with unprecedented lifetime and reliability of capsule-type devices.

Such infrastructure systems require a large number of duty cycles and create extreme uptime expectations. Designers should consider both in the early stages of design. Although the technology and topology may seem outdated, the higher efficiency, lower cost, and smaller space requirements make it the obvious choice. This approach will be crucial as future autonomous commercial vehicles require even higher power to further reduce charging times as the driver will not need to rest.

To learn more, download the white paper, Reducing emissions through electrificationcourtesy of Littelfuse, Inc.

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