Analysis of safety technologies and measures for high voltage devices

Since the development of electric vehicles, everyone is very concerned about whether they are safe. The summit forums organized in various places all involve discussions on battery safety. Are lithium-ion power batteries safe? Are electric vehicles safe? When many experts gave speeches, they counted the safety cases of electric vehicles, especially buses, in the two years from 2015 to 2016. There were fire incidents of electric vehicles from Tesla and domestic manufacturers. These incidents are real, so some experts concluded that China's electric vehicles are unsafe, mainly because lithium-ion batteries are unsafe. The author's point of view is that electric vehicles are safe if safety measures are in place, otherwise they are unsafe.

This article is based on the fact that electric vehicles are based on traditional vehicles. The following discussion is also based on the premise that traditional vehicle safety measures can already guarantee their safety, and discusses the safety technologies and measures of electric vehicles.

1. What is the basis for lithium-ion batteries to be safe and green?

The inventor of lithium cobalt oxide, lithium manganese oxide and lithium iron phosphate positive electrode materials, one of the founders of lithium-ion batteries, known as the "father of lithium batteries" in the industry, is currently a professor of mechanical engineering at the University of Texas at Austin, a famous solid-state physicist, and the 94-year-old John B. Goodenough said, "In the next few years, batteries will achieve significant improvements, including battery safety, price, energy density, service life, and charge and discharge speed.

The author believes that lithium-ion batteries will become increasingly safer in the next few years. The main reasons why lithium-ion batteries are safe and green are:

(1) Compared with gasoline (diesel) and natural gas, lithium-ion batteries will not spontaneously combust or leak under natural (normal) conditions.

(2) Compared with lead-acid batteries, under natural (normal) conditions, they do not pollute the air or soil.

Some experts have repeatedly stressed that lithium-ion batteries are not safe or green. They are talking about events that may occur under conditions beyond "natural (normal) conditions". For example, extreme situations such as poor battery quality, external impact, and charging (discharging) transition.

2. What is the main basis for China's electric vehicles to be safe and environmentally friendly?

(1) China's electric vehicles (new energy vehicles) must use lithium-ion batteries and are not allowed to use lead-acid batteries. Ordinary people already know and understand that lead-acid batteries are not environmentally friendly.

(2) China's electric vehicles (new energy vehicles) must be "pure electric drive" mode. Lithium-ion batteries are a scientific invention of epoch-making significance. The types of power used by China's pure electric vehicles are in sync with the world's scientific and technological progress.

(3) The lithium-ion power batteries purchased by electric vehicle manufacturers are manufactured by manufacturers that are licensed by the government. The quality of their products is endorsed by the government, and the government has strict control over power battery manufacturers.

(4) China's electric vehicles use lithium-ion batteries, which are safer and more environmentally friendly than fuel (gas) vehicles.

3. The quality (safety) of China's power battery pack products is improving with each passing day

Five years ago, electric vehicle production was small-scale, and vehicle production technology was immature. It focused on the verification of engineering application solutions and was in the market verification stage. As shown in Figure 1, the wiring harness integration of the battery pack is relatively simple. The wire extends the pole measurement point to the connector, and then the connector is connected to the LECU. The sampling harness is long and the connector point is short. There is a risk of messy sampling harnesses and short circuits due to harness wear.

Figure 1 Power battery pack five years ago

Five years later, electric vehicle production began to be mass-produced, and the vehicle production technology was basically mature, focusing on the market expansion stage. The degree of component modularization was already very high. It was mainly composed of Module, BMU, PDU, water cooling system, MSD, structural parts, high and low voltage wiring harnesses, and circuit boards were used to replace some sampling lines.

Figure 2 Power battery pack in five years

IV. Introduction to the quality (safety) technology of major high-voltage components of pure electric vehicles in China

The following is a schematic diagram of the high-voltage system of a pure electric vehicle. From the schematic diagram, the main high-voltage components include battery packs, high-voltage boxes, motors, DC/DC, A/C and other five high-voltage components. From an engineering point of view, the safety of electric vehicles can be guaranteed if the quality of high-voltage components and the quality of the connecting parts are in place. The continuous improvement of safety indicators is an eternal topic, and specific issues will be discussed in detail.

Figure 3 Schematic diagram of high voltage system of pure electric vehicle

Electric vehicles are usually equipped with power systems with voltages of more than 300V and currents of more than 200A. This is completely different from the safety level of traditional vehicles, which are only equipped with 12/24V power systems. The high voltage and high current characteristics of electric vehicles may endanger the safety of high-voltage components and personal safety. From an engineering perspective, safety measures must be in place, which are introduced below:

(1) High-voltage distribution box safety technology

Figure 4 Appearance and internal structure of high-voltage distribution box

Anyone who has experience working in a company knows that every factory has a power distribution room. A car is likened to a factory, and the high-voltage distribution box (see Figure 4) is the power distribution room of the factory. The function of the electric vehicle high-voltage box (PDU) is to bridge the power supply and signal transmission between the power battery and various high-voltage equipment, and to detect insulation faults, circuit faults, grounding faults and high-voltage faults of the entire high-voltage system at any time. It is a safeguard measure for the safe transmission of power and electric energy of the entire vehicle system.

The high-voltage distribution box of a pure electric vehicle contains copper bars, circuit breakers, air switches, contactors, soft starters, frequency converters, transformers, contactors, high-voltage relays, fuses, surge protectors, mutual inductors, ammeters, voltmeters, transfer switches, etc.

a) The basic purpose of designing a high-voltage distribution box is to ensure the safety of vehicle operation, the safety of drivers and passengers, and the safety of the vehicle operating environment.

b) Necessity of designing a high-voltage distribution box: As an energy storage device for electric vehicles, power batteries are affected by the small size of the vehicle, very limited available space, and difficult layout. Therefore, in engineering, the contactors and fuses of each circuit are integrated. On the one hand, it can meet the power drive requirements of the whole vehicle, and on the other hand, it is convenient for the maintenance of the power battery system and reduces the need to disassemble the power battery pack.

c) High-voltage distribution box design requirements: compact device layout, easy to disassemble and assemble

d) Safety measures include paying attention to heat dissipation layout to prevent the temperature from rising sharply.

The high-voltage components connected to the high-voltage distribution box include: power battery, motor controller, frequency converter, inverter, electric air conditioner, electric defrost, charging station, etc. At present, cast aluminum housing and connectors are generally used, and the protection level reaches IP67 level. The subsequent development trend is to develop in the direction of small size, light weight and intelligence.

(2) Safety measures for high-voltage connectors and high-voltage wiring harnesses

Traditional connector equipment has been unable to meet its high current and high voltage requirements. Electric vehicle connectors must be high-power connection products. If they do not meet the requirements, electric vehicles will inevitably have fire accidents. Electric vehicle connectors must be automotive-grade products. Industrial-grade products do not meet the requirements. The main indicators are IP level, insulation level, low resistance, error prevention, and locking function.

Figure 5 High voltage connectors of different specifications

A wiring harness is a component that is formed by crimping contact terminals (connectors) made of copper and wires (cables), and then wrapping them with plastic insulators or metal shells to form a wiring harness. Functionally, there are two types of wiring harnesses: power lines that carry power to drive actuators (actuators) and signal lines that transmit sensor input commands. Power lines are divided into low-voltage wiring harnesses (see Figure 5) and high-voltage wiring harnesses (see Figure 7).

Figure 5 Automobile low voltage wiring harness

High-voltage wiring harnesses are cables between connectors in electric vehicles. They are critical connectors that support the safe operation of the entire vehicle. They must be shielded, have an IP rating, and be protected from overheating or burning. The environment in which cars are used is harsh. If they do not meet the requirements, they are the most likely components to spontaneously combust.

Figure 6 Automobile high voltage wiring harness

5. Under the guidance of ISO26262 international standards, Chinese electric vehicles will become safer

ISO26262 is positioned as an international standard for the functional safety of electronic and electrical products in the automotive field, such as specific electrical devices, electronic devices, and programmable electronic devices in the automotive industry. ISO 26262 provides a life cycle (management, development, production, operation, service, and scrapping) concept for automotive safety and provides necessary support in these life cycle stages. The standard covers the overall development process of functional safety (including demand planning, design, implementation, integration, verification, validation, and configuration).

With the development of new energy vehicles in China, the technical level of manufacturers is getting higher and higher. By actively implementing the ISO26262 international standard, China's new energy vehicle safety ecosystem will be formed, and people's concerns about the safety of electric vehicles will gradually be eliminated.