HV FUSE Analysis
Abstract: the driving part of pure electric vehicle and the power supply of high voltage accessory system.
All of them are power battery power supply. In order to protect the safety of vehicles and passengers, the relevant power battery power supply circuit adopts the corresponding fuse as the short-circuit protection measure. In this paper, the
To check from the fuse life, the impact current on the fuse, fuse points
In terms of breaking capacity, the selection principle and verification method of DC high voltage fuse for pure electric vehicle are described.
The power battery power supply voltage of pure electric vehicles is mostly 200 ~ 400 V, except for the power
In addition to the main fuse, there are also high-voltage circuits of other accessories, such as automobile air-conditioning system, heating system, DC / DC system (which converts the power battery voltage to 14 V to provide low-voltage power supply for the whole vehicle, and its function is similar to that of the generator). All circuits need to be connected in series with DC high-voltage fuses for circuit protection.
At this stage, ev special purpose vehicle grade fuses have been launched, but the selection surface is still
It is narrow.The breaking capacity and protection characteristics of the domestic DC fuse can be full
It meets IEC (International Electrotechnical Standardization Organization) or other general standards, and has little difference with imported products for the same purpose.However, there are still some gaps in ROHS certification, extreme condition test and automatic production of series products.
The price of DC high-voltage fuse is slightly higher, so it is necessary to protect the circuit of each system effectively and prevent the abnormal fusing of fuse.This paper will focus on DC.
1 Introduction of conventional high voltage system
The internal structure, charging system and thermal management system of power battery are not considered
On the premise of unification, the design circuit of high voltage accessory system of general pure electric vehicle is shown in Figure 1.It can be seen from Figure 1 that the main circuit of power supply needs one total fuse, and the other subsystems need to be equipped with fuses separately.Generally speaking, at least 4-5 DC series fuses with rated voltage above 400 V can meet the basic functional requirements of vehicles.Figure 1 design circuit of high voltage accessory system for pure electric vehicle
2 Basic principles for selection of DC high voltage fuse
It is mainly to confirm the rated voltage and current of fuse. The rated voltage of fuse should be higher than the maximum voltage of power battery. The selection of rated current (fuse capacity) is referred to formula (1) (1)
In the formula: In – rated current of fuse; IR – load current of protection circuit; K1 – correction coefficient of load form; K2 – correction coefficient of temperature.
The load form correction factor K1 is mainly based on the load characteristics, considering the power change, current ripple, starting and closing instantaneous impulse current and other factors. Under general conditions, the stable operation load is selected as 0.75. If the current fluctuates greatly during the working process of the load, it is recommended that K1 be selected as 0.6. Generally, the temperature correction coefficient K2 can be directly calculated according to the temperature change rate, or according to the ambient temperature used by the fuse and the temperature rise curve of the fuse,Reasonable selection of K2, pure electric vehicle has no obvious high temperature producing area, generally K2 is 0.6 .
When passing through different current, the temperature rise effect is different.
3 Life calculation and verification of fuse life calculation reference fuse load current wave
Shape and I 2 Curve t
Line, I 2 Curve t
The general form of the line is shown in Figure 2 (with a certain brand 40A
DC high voltage fuse as an example).Fig. 2 Curve t line diagram of 40A fuse I 2 of a brand
According to figure 2, theoretically, when the passing current is 50% of the rated current of the fuse, the fuse can ensure continuous operation without abnormal fusing.The actual load waveform is usually not a steady linear load. For different load curves, it needs to be calculated according to formula (2).（ 2）
If the current changes periodically, select any number of cycles to calculate the I 2T meter,
The calculated curve I 2 t should be in the area below the lowest curve.
Generally speaking, the current fluctuation mainly exists in the initial load startup or power rise
Area, can start from the load, quickly increase the load power until stable, grasp the current waveform from the beginning to the load stability, and estimate I 2T, the same requirements
I 2 Curve t line is in the area below figure 2.
Figure 3 is the curve drawn by calculating I 2T according to a specific load, which can be used as a reference.In Figure 3, the red curve is the actual current I 2T, and the red curve is always below the green curve.
The actual life verification of fuse still needs to be carried out on the test bench or with the actual vehicle
The t-theory calculation of I 2 is only for reference.Fig. 3 measured I 2
T curve 4 impact current on fuse After the fuse model is preliminarily determined, it is necessary to connect the
Peak current within.
Figure 4 shows the time current characteristics of a brand 35A fuse. On the basis of Figure 4, the duration and peak value of peak current are compared.Figure 4 (left) time current characteristics of a brand 35A fuse Figure 5 (right) measured impulse current
Fig. 5 shows the impulse current waveform of load measured by oscilloscope and current transformer, 1V corresponding to current value of 25A.The black waveform is the waveform measured by the oscilloscope current probe, which has exceeded the probe range and has no reference significance. From the blue waveform, it can be calculated that the peak current of the impulse current is 590 a, and the whole peak duration is 0.4
ms 。The peak is depicted in the time current characteristic diagram of the primary fuse, as shown in Figure 4.
Through comparison, it can be confirmed that the impulse current in the load has actually exceeded the capacity of the primary fuse to withstand the peak current. If it is used for a long time, it is easy to cause the fuse to fuse abnormally.On the contrary, if the impulse current value does not exceed the time current characteristic curve of the fuse, it can be considered that the primary fuse is suitable for the impulse current of the load.
5 The breaking capacity and short-circuit current of the fuse should be greater than the expected short-circuit current in the protection circuit. The expected short-circuit current passing through the lead resistance of the power battery voltage and the load circuit, the internal resistance of the power supply, the number of connection terminals or transfer points can be simplified . Single calculation.The line resistance and internal resistance of power supply can be obtained by calculation or measurement, and the connection terminal is generally taken as 3 ~ 5m Ω.In general, the expected short-circuit current is calculated .
The ability of closing and breaking fuse should be verified.
Before the test and verification, it is necessary to evaluate the bit prone to short circuit in the whole load circuit
Then set the short-circuit point at this position, connect the corresponding equipment, and measure the voltage waveform at both ends of the fuse and the actual short circuit of the whole load circuit in the process of short circuit
Current and other parameters.
Figure 6 shows the picture of fuse selected before short circuit test. The short circuit circuit is a / C circuit, and the model of fuse for test is PEC 30A / 450VDC.
The short circuit process of the fuse is divided into three sections.That is: ① in the initial stage, melting
When the voltage at both ends of the breaker is 0, there is no current flowing through the load circuit; ② in the fusing stage, the load circuit is short circuited, and the fuse starts to pull out the arc process; ③ after the completion of the fusing, after the completion of the fusing, the fuse will start to pull out the arc,The voltage at both ends of the fuse is the power supply voltage.
Starting from the process of arc drawing and arc extinguishing, the whole fusing process is less than 2 ms, and the breaking speed of fuse is ideal.After the breaking test, remove the measuring equipment and check the appearance of the fuse, mainly including whether there are cracks and whether the carrier is burnt
Erosion and other phenomena.
If the appearance is good, it is necessary to further dissect the inside of the fuse, check the fusing condition of the melt, and check the bonding change of the arc extinguishing material.
Figure 7 shows the situation after the fuse test of this type of fuse
After the short-circuit breaking process, the appearance of the fuse glass tube is good, the quartz sand is still loose, the melt is effectively fused, and the carrier is not affected by the short-circuit current, which indicates that the short-circuit current of the load is within the breaking capacity of the fuse and meets the design requirements.