I. Overview

At present, the energy density of power batteries is increasing, and the capacity of single cells is growing. If high-voltage components experience a short circuit without proper protection, it can lead to component damage or even cause a fire (especially in power batteries), with severe consequences. Therefore, protecting the circuits of high-voltage components is crucial. This article will explain the calculation and selection methods for high-voltage DC fuses in pure electric vehicles, along with a practical example. The electrical topology of an electric vehicle is shown in Figure 1.

Figure 1: Electric Vehicle Electrical Topology Diagram

II. Fuse Selection

2.1 Fuse Classification

According to action characteristics:

General-purpose fuse (gG/gL)

Fast-acting fuse with partial range protection (aR)

Fast-acting fuse with full range protection (gR)

Time-delay type and special fuses

According to shape:

British Standard Fuse: The British-style fuse uses a ceramic housing and a cylindrical tube. It is compact, has strong surge resistance, high cost-effectiveness, low arc voltage, and low power consumption. For fuses rated under 100A, British-style fuses are recommended. The appearance of the BS88 British standard fuse is shown in Figure 2.

Figure 2: BS88 British Standard Fuse

American Standard Fuse: The American-style fuse series features integrated knife blade terminals, and the fuse element is welded in one piece, making it resistant to strong impacts and vibrations. It also has high flame retardancy, high insulation performance, low arc voltage, and low power consumption, making it the preferred choice for electric vehicles. For fuses rated above 100A, American-style fuses are recommended for increased reliability. The appearance of the American standard fuse is shown in Figure 3.

Figure 3: American Standard Fuse

European Standard Fuse: The European-style square fuse uses a ceramic housing and is characterized by low operating temperature, low power loss, and low Joule integral value. It is suitable for applications requiring compact structure and superior performance, especially in manual service switches (MSD). The appearance of the European standard square fuse is shown in Figure 4.

Figure 4: European Standard Square Fuse

French Standard Fuse: The French-style fuse has strong cycling performance, compact size, and unique construction. It features a modular base for easy installation and is suitable for PDU, BDU, small AC drives, and other low-power applications where space is limited. The appearance of the French standard round fuse is shown in Figure 5.

Figure 5: French Standard Round Fuse

2.2 Selection of Fuse Rated Voltage

As a protective device in the circuit, the fuse operates in two stages: "melting" and "breaking." The "melting" process is related to the current, while the "breaking" process is related to the voltage. The fuse voltage can be described as the voltage that this fuse can break and extinguish the arc generated.

There is a difference between AC and DC voltages. For pure electric vehicles, the voltage is DC. The inductance in the circuit generates an induced voltage at the moment the fuse breaks, and the effect of inductance on arc extinction must be considered. The maximum voltage a fuse can withstand must exceed the system voltage. The difference in breaking AC and DC fuses is significant: AC has a sine wave with zero crossing points, making it easier to extinguish arcs; however, DC lacks zero crossing points, requiring the fuse element to vaporize quickly, with the help of quartz sand for arc diffusion, absorption, and cooling, to force arc extinction. Therefore, breaking a DC arc is much more challenging than breaking an AC arc.

For the current selection of fuses in electric vehicles, the first considera

tion is to avoid tripping during normal operation. This is the starting point for selecting the fuse current, as fuses primarily break due to heat accumulation. Continuous current is fundamentally the basis for fuse selection.

The rated current of the fuse can be calculated using the formula:

In=I*K/(Kt*Ke*Kv*Kf*Ka*Kb)

Where:

· In: Rated current of the fuse

· I_nom: Nominal load current

· K: Load correction factor

· Kt: Temperature correction factor

· Ke: Connector thermal conduction coefficient

· Kv: Air cooling correction factor

· Kf: Frequency correction factor

· Ka: Altitude correction factor

· Kb: Fuse housing correction factor

Using the above formula, an initial rated current value for the fuse can be calculated. After preliminary selection, the rated current value should be adjusted according to actual operating conditions, considering factors like overload current duration, magnitude, surge current duration, and magnitude.

III. Example Analysis

Taking a certain pure electric logistics vehicle as a reference, we analyze the industrial fuse selection scheme, using Bussmann fuses as examples.

Table 1: Electrical Parameters of a Certain Electric Vehicle

· K: Load correction factor, depending on the circuit load, an amplification factor K is added. For resistive loads, K is approximately 1.5; for capacitive loads, considering the power-on surge, K is around 2; for electric compressors with high peak current at startup, K is 7-8; for motor circuits, K is 1.2-1.5. The specific values for K are shown in Table 2.

Table 2: K Value Selection Table

· Kt: Temperature correction factor. For electric vehicles, the maximum ambient temperature is generally 60°C. Referring to Figure 6, the temperature correction factor curve, Kt=0.8.

Figure 6: Temperature Correction Factor Curve

· Ke: Connector thermal conduction coefficient, based on the current in each circuit, at 1.3A/mm² being 100%. Referring to Figure 7, the connector thermal conduction coefficient curve, Ke values are shown in Table 3.

Figure 7: Connector Thermal Conduction Coefficient Curve

Table 3: Ke Value Selection Table

Kv: Air cooling correction factor, with the fuse adopting natural convection cooling. Referring to Figure 8, the air cooling correction factor curve, and without additional heat dissipation, Kv=1.

Kf: Frequency correction factor, for DC current below 1000Hz. Referring to Figure 9, the frequency correction factor curve, Kf=1.

Figure 8: Air Cooling Correction Factor Curve

Figure 9: Frequency Correction Factor Curve

Ka: Altitude correction factor. Based on the current operating conditions of electric vehicles, Ka=1.

Kb: Fuse housing correction factor, Kb=1 for ceramic housing, and Kb=0.9 for melamine housing.

Using the formula:

In=I*K/(Kt*Ke*Kv*Kf*Ka*Kb)

The fuse current selection for a melamine housing fuse is as follows:

· OBC Circuit: In = 19.09 * 1 / (0.8 * 0.76 * 1 * 1 * 1 * 0.9) = 34.88A, fuse selection is 40A.

· DCDC Circuit: In = 4.34 * 3 / (0.8 * 0.97 * 1 * 1 * 1 * 0.9) = 18.64A, fuse selection is 20A or 25A.

· Motor Circuit: In = 130.2 * 1.2 / (0.8 * 0.85 * 1 * 1 * 1 * 0.9) = 255.29A, considering the peak current of the motor at 202.5A, fuse selection is 300A.

· A/C Circuit: In = 3.47 * 7 / (0.8 * 1 * 1 * 1 * 1 * 0.9) = 33.74A, fuse selection is 40A.

· PTC Circuit: In = 4.34 * 1.5 / (0.8 * 0.97 * 1 * 1 * 1 * 0.9) = 9.32A, fuse selection is 10A or 15A.

IV. Specific Selection

Using the Bussmann EV series fuses as a reference, specific fuse selection is analyzed in the table below:

Table 4: EV Series Fuse Selection Table

In conclusion, based on the Bussmann EV series fuse as a reference, the voltage rating is 500VDC, which meets the requirements. After checking the current specifications, the fuse selection for each circuit is as follows:

· OBC Circuit: EV10-40-C

· DCDC Circuit: EV10-20-C / EV10-25-C

· Motor Circuit: EV30-300-C

· A/C Circuit: EV10-40-C

· PTC Circuit: EV10-15-C

By referencing the time-current characteristic curves of the EV series, with a breaking capacity of 20KA, the recommended fuses meet the requirements.