Fast fuse application characteristics

Current passing ability

The rated current of the fast-blow fuse is expressed by the effective value. Generally, the normal passing current is 30% to 70% of the nominal rated current. The fast-blow fuse is used when its end is heated by the semiconductor device and the other end is cooled by the water-cooled busbar, or both sides are cooled by the water-cooled busbar; or forced air-cooling is used to control the temperature rise to maintain the current passing capability.

The connection at the quick-fuse connector in the rectifier directly affects the temperature rise and reliable operation of the quick-blow, which must be kept flat and clean. If the contact surface of the unplated busbar is to remove the oxide layer, a predetermined pressing force is applied during the installation, and it is preferable to elastically deform the contact surface. Parallel fast-acting fuses require a one-to-one detection of the pressure drop across the contact surface.

2. Fast-blower temperature rise and power consumption

The power consumption of the fast fuse is W=ΔUIw; ΔU=f(Iw) where: Iw---operating current; ΔU---the voltage drop of the fast fuse.

The power consumption of the fast fuse has a great relationship with its cold resistance. The use of a fast fuse with a small cold resistance is beneficial to reduce the temperature rise because the current passing capability is mainly limited by the temperature rise. As mentioned earlier, the connection condition at the quick-fuse connector also affects the temperature rise of the fast-blow fuse, requiring that the temperature rise at the quick-fuse connector should not affect the operation of its neighboring devices. The experiment proves that the temperature rise of the fast fuse can be long-term operation when it is lower than 80 °C. The product with stable manufacturing process can still operate for a long time when the temperature rises to 100 °C. The temperature rise of 120 °C is the critical point of the current passing ability. Fast fuses do not operate for long periods of time at °C.

At present, the chemical industry generally uses water-cooled busbars and air-cooled methods to reduce the temperature rise of fast fuses. The water-cooled busbars are especially effective for low-voltage quick-acting fuses such as 400-600V. The temperature difference between the quick-fuse terminal and the water-cooled busbar connection is generally 1.0 to 2.0 °C. Many high-power fast-blow fuses are designed for water-cooled conditions, so users should consult the manufacturer before using them. Air cooling is also an effective method to reduce the temperature rise. According to the wind speed, the wind speed is used to determine the influence of wind speed on the temperature rise of the fast fuse. When the wind speed is about 5m/s, the flow capacity can be increased by 25%. If the wind speed is increased, There will be no obvious effect.

Depending on the fast-blower voltage drop curve provided by the manufacturer and the power dissipation at rated current, measuring the voltage drop between the two terminals of the fast-blow can quickly calculate the actual current of the branch.

In addition, in the same flow conditions, the temperature rise is also related to whether the fast fuse is single or double. In high-power rectifiers manufactured in advanced industrial countries, fast fuses are often used in series with semiconductor devices, such as 700A×2, 1400A×2, and 2500A×2. The double fuse structure fast fuse terminals can be thinned as much as possible to reduce the resistance. There is a type of double-connected quick-blow fuses connected by bolts and connecting plates, and the other type is a structure in which a connecting plate (terminal) is welded with two melts (terminals), and such a structure is relatively advanced. The high-voltage fast-blow fuse has a large internal resistance, especially for products above 800V. Since the outer casing has a certain length and a large surface area, the heat generated by the melt is radiated through the filler and the outer casing, so the voltage is high. The air cooling effect of the fuse is more significant.

3. The choice of breaking ability

The strength of the fuse of the fast fuse largely determines the breaking capacity of the maximum fault current. Secondly, the shape of the metal foil inside the fast-acting fuse, the ability of the filler to adsorb metal vapor and heat, and the electric power of the fuse-molded body all affect the breaking capacity. When designing the rectifier, the phase-to-phase short-circuit current of the “rectifier transformer” should be calculated, and the fast-acting fuse with sufficient breaking capacity should be selected according to this current. The fast fuse with insufficient breaking capacity will continue to arc until it explodes. In severe cases, it will cause AC/DC short circuit, so the rated breaking capacity is a safety indicator.

In addition, the dispersion of product manufacturing is also one of the factors that affect the ability to break.

The problem that is easy to ignore is the power factor of the line in the case of a short-circuit fault, because the amount of arc energy generated when the fast-fuse is disconnected has a great relationship with the magnitude of the inductive reactance of the circuit. When the line power factor cosφ<0.2, the split is broken. The ability has a particularly high demand.

Energy when the fast fuse is disconnected Wo=Wa+Wr+W1

Where: Wa---Arc energy; Wr---resistance energy consumption; W1---the energy released by the line inductance.

When the breaking capacity satisfies the requirements of the "rectifier", it should also be noted that the breaking arc voltage peak (referred to as "transient recovery voltage" in the standard) should not be too high, and should be limited in the manufacture of the fast fuse to make it lower than the semiconductor. The maximum value the device can withstand, or the semiconductor device will be damaged. Therefore, the fuse with the shortest break time is not necessarily the most suitable.

When the fast-blow fuse is used in the DC circuit, since there is no zero-crossing point of the voltage during the DC breaking process, the reliable breaking of the quick-blow fuse is a severe condition, so in general, the fast-blow fuse is used in the DC circuit. Only 60% of the rated voltage of the fast-acting fuse can be used. It is best to use a DC fast-blow fuse.

4.I2t selection

The fuse's fusing time t is related to the magnitude of the fusing current I, which is inversely proportional to the square of the current. Figure 3 shows the relationship between t∞1/I2, which is called the second-ampere characteristic curve of the fuse.

Since various electrical equipment (including the power grid) have a certain overload capacity, when the overload is light, it can be allowed to operate for a long time, and when a certain overload multiple is exceeded, the fuse is required to be blown for a certain period of time. Selecting a fuse to protect against overload and short-circuit must be aware of the overload characteristics of the consumer, making this feature properly within the protection of the fuse's second-ampere characteristic.

The fusing time of the fusing current Io is theoretically infinite, called the minimum melting current or the critical current, that is, the current passing through the melt is less than the critical value and will not be blown. Therefore, the rated current Ie of the melt should be less than Io; the ratio of Io to Ie is usually 1.5 to 2.0, which is called the melting coefficient. This coefficient reflects the different protection characteristics of the fuse when it is overloaded. If the fuse can protect the small overload current, the melting coefficient should be lower; in order to avoid the short-time overcurrent at the start of the motor to melt the melt, the melting coefficient should be Higher.

After the fuser current passing capability meets the requirements of the system short-circuit current, the fault current can be isolated when a short-circuit fault occurs, but whether the semiconductor devices connected in series can be protected must analyze the I2t values ​​of the two. The semiconductor device can be protected only when the I2t value of the fast-acting fuse is smaller than the I2t value of the semiconductor device. In the case of a short-circuit fault, the I2t value is divided into two phases, namely, pre-arc I2t and fuse I2t. The time from the solid state to the liquid state of the melt metal is the pre-arc time, about 1.0 to 2.0 ms, which can be considered as an adiabatic process. The current time integral generated by the fast fuse during this period can be considered as a certain value, which is determined by the design. The pre-arc I2t value is also different for different materials, and it is a constant for each material. When the molten metal becomes vapor, the arc begins to ignite. During the arcing process, the current is reduced from zero to zero. At this stage, I2t is the fuse I2t, which is a variable. This process relies mainly on the corrosion of the filler to absorb energy.

When designing a fast-blow fuse, many measures are taken to satisfy the ever-increasing current rating of the semiconductor device, and the fast-acting fuse cannot be simply selected by arithmetic. Experiments have shown that when the rated current is doubled, the I2t value of the fast-acting fuse is four times that of the original, and the increase in the I2t value of the semiconductor device is much smaller. To make the fast fuse reduce the I2t value is more difficult, only a variety of measures, such as reasonable fuse distribution, shorten the melt length, reduce the arc grid and improve the arc extinguishing ability of the arc extinguishing material. The I2t value is one of the important indicators for selecting fast fuses.

5. Insulation resistance

The index of insulation resistance after breaking the fuse is empirically proven to be important. Potassium and sodium salts were added to a large number of products in the 1990s, and the sodium salt can improve the breaking capacity of the arc grid. However, the insulation resistance of the poorly manufactured fast fuses is mostly lower than 0.3MΩ, and there is even leakage. In special cases, the fault is re-ignited after a period of time, which will cause more faults. A good quality fast fuse (with the addition of potassium salt, sodium salt) should form an insulation resistance of 0.5 MΩ or more after breaking. The fast-blow fuse can reach an insulation resistance of more than 1 to 30 MΩ after 10 minutes of breaking, and it can be considered to have good reliability.

In addition, the use of fast fuses should also consider its life and reliability; insulation resistance index after breaking (> 0.5MΩ); as low as possible transient recovery voltage; do not use products with hidden faults.