Analysis of 800V high voltage technology for new energy vehicles

In the process of promoting new energy vehicles, they are faced with the problems of short driving range, difficulty in charging, and slow charging. The charging efficiency is improved by increasing the current and increasing the system voltage. However, high current will cause high heat loss of components, so increasing the system voltage has become the mainstream choice to improve efficiency. As the core component of new energy vehicles, the electric drive system is the key to reflecting the performance and core competitiveness of automobile products. At present, domestic and foreign brands such as Volkswagen, BMW, Mercedes-Benz, BYD, Geely, and Great Wall have made layouts in high-voltage platforms. The 800 V electric drive system based on the high-voltage platform has also become a key technology that the industry focuses on research.

Development trend of 800V high voltage electric drive technology

On September 4, 2019, Porsche released its first pure electric sports car, the new Tayca. Among them, the first models released are the new Taycan Turbo S and the new Taycan Turbo. Both models are "Porsche E-Performance", representing the highest performance of Porsche's pure electric mass-produced Taycan series. At present, the common electric vehicle system voltage is 400 V. The new Porsche Taycan is the first mass-produced model with a system voltage of 800 V. This model adopts a dual-motor four-wheel drive configuration (Table 1). It is equipped with 800 V technology derived from the Le Mans champion racing car 919 Hybrid, a dual permanent magnet synchronous motor and a rear axle two-speed transmission, taking into account the needs of both performance and driving range. The 800 V three-electric system has low power consumption and a built-in booster to increase continuous output power, increase charging power, shorten charging time, and reduce system mass. Both front and rear drive motors use AC permanent magnet synchronous motors, adopt HairPin hairpin winding technology, have a slot fill rate of up to 70%, and are partially laser welded. Porsche announced that Taycan can support more than 10 consecutive launch starts without torque output derating, and its motor thermal performance design capability is good.

Table 1 Porsche Taycan electric drive system technical indicators

On December 2, 2020, Hyundai Motor Group launched the world's first electric vehicle dedicated modular platform E-GMP (Electric-Global Modular Platform, E-GMP). The platform adopts an 800 V voltage electrical architecture, bidirectional charging, and a charging power of up to 350kW. It can be charged to 80% within 18 minutes, and can travel 100km in 5 minutes. Hyundai Motor said that its Integrated Charge Control Unit (ICCU) is the world's first patented technology that uses a motor and inverter to increase 400 V to 800 V, and achieves stable charging of 800 V batteries with a 400 V fast charging pile. In 2021, ZF, BYD, Geely, BAIC, Changan, GAC, Dongfeng, Xiaopeng and others followed up and released the 800 V high-voltage platform architecture, and the models are expected to start mass production in 2022. The 800 V high-voltage electric drive system is about to usher in explosive growth.

What are the advantages compared to 400V systems?

First, the charging power can be higher, eliminating anxiety about charging time. The industry generally believes that 500A is the limit of automotive-grade wiring harness connectors. If the current is higher, the complexity of electrical system design will increase significantly, which means that the charging power of about 200kW under the 400V system will become the limit of many vehicle designs; and the 800V high-voltage system can break the limit to 400kW. In this case, if the long-range vehicle battery is charged at 100kWh@20%-80%, it only takes 9 minutes, which is basically the same as the time it takes to refuel a traditional fuel vehicle, completely eliminating anxiety about charging time.

Second, the fast charging system has low cost. There are also fast charging based on 400V system on the market, but the 800V high-voltage system can achieve lower system cost in high-power charging applications. Table 1 shows a qualitative comparison of the vehicle assembly cost of the 400V system and the 800V high-voltage system, which is further reflected as follows: In the short term, 800V charging is in the charging power range of more than 250kW, and in the long term, 800V charging is in the charging power range of more than 150kW. The 800V high-voltage system has obvious system cost advantages.

Table 1 Vehicle assembly cost under fast charging application

Third, fast charging has low charging loss. Compared with the 400V system, the 800V high-voltage system has a small charging current, which can reduce battery loss, wiring harness loss, and charging pile loss, thus achieving energy-saving charging.

Fourth, the vehicle's driving energy consumption is low, and a longer driving range can be achieved with the same battery capacity, or the battery capacity can be reduced and the assembly cost can be lowered with the same driving range.

Compared with the 400V system, the 800V high-voltage system battery, electric drive and other high-voltage components have small currents, and the losses of related components and wiring harnesses can be reduced; secondly, with the introduction of the third-generation semiconductor silicon carbide technology, the energy consumption of various high-voltage components, especially electric drive components, can be greatly reduced, enabling energy-saving driving of the vehicle.

From 400V to 800V, which parts and components need to be upgraded?

When car companies apply 800V platform architecture, they need to have higher requirements on the voltage resistance, loss and heat resistance of their core three-electric technology and power devices:

1. Motor

Specifically, there are the following points:

Generation of shaft voltage: The motor controller is powered by a variable frequency power supply, which contains high-order harmonic components. The inverter, stator winding, and housing form a loop, generating an induced voltage, called common mode voltage, which generates a high-frequency current in this loop. Due to the principle of electromagnetic induction, an induced voltage is formed at both ends of the motor shaft, which becomes a shaft voltage, which is generally unavoidable.

The rotor, motor shaft and bearing form a closed loop. The bearing balls and the inner surface of the raceway are in point contact. If the shaft voltage is too high, it is easy to break through the oil film and form a loop. The shaft current will cause bearing corrosion.

The application of SiC in 800V inverters results in high voltage change frequency, increased shaft current, and increased requirements for bearing corrosion protection.

At the same time, due to the increase in voltage/switching frequency, the insulation/EMC protection level requirements inside the 800V motor are improved.

800V SiC application causes shaft current to increase significantly and increases the risk of oil film breakdown

2. Electronic control

Taking Si-IGBT as an example, its withstand voltage is 650V at 450V. If the automotive electrical architecture is upgraded to 800V, considering factors such as switching voltage and switch overload, the corresponding power semiconductor withstand voltage level needs to reach 1200V. However, under high voltage, the switching/conduction loss of Si-IGBT increases sharply, facing the problem of rising costs and reduced energy efficiency.

In addition, SiC power devices are also used in vehicle chargers, charging piles, etc., and have the advantages of achieving high power density and optimizing the total system cost. Its technology can effectively improve the overall efficiency of the 800V electric drive system motor and electronic control, and meet the compatibility and reliability requirements of the application.

Power semiconductors in vehicles and charging piles are expected to shift from Si-based to SiC

3. Battery

The fast charging performance of power batteries is constrained by the negative electrode. On the one hand, the layered structure of graphite materials means that lithium ions can only enter from the end face, resulting in a long ion transmission path; on the other hand, the graphite electrode has a low potential, and the graphite electrode is highly polarized under high-rate fast charging, and the potential is easily reduced to below 0V, resulting in lithium precipitation. Therefore, it is imperative to improve the fast charging performance of the battery negative electrode.

4. Connector + wiring harness

The upgrade of platform architecture from 400V to 800V requires the reselection of connectors, and the number of connectors may increase (adding high-power fast charging interfaces); under the same power conditions, the voltage increases, the current decreases, the cable pressure resistance increases, and the volume decreases.

5. Filter system

It mainly includes capacitors and magnetic rings. The original filtering system is designed based on a 400V architecture. After upgrading to 800V, the EMC radiation will change, and the vehicle filtering system needs to be redesigned.

6. Relay

Upgrading to the 800V platform requires that the voltage resistance of relays be improved, and some existing relays are compatible with high voltages.