1, overview

In metro train, the protection of main circuit of traction and auxiliary system is jointly undertaken by fast fuse and high-speed switch.This design is based on the following considerations:

(1) the high-speed switch has the functions of short-circuit protection, over-current protection, overload protection and under voltage protection, and can be operated frequently

The advantages of.But the performance of high-speed switch short-circuit protection is not ideal to withstand the range.

Short circuit current and breaking overvoltage cannot be limited to the circuit

(2) fast fuse has strong breaking capacity, short breaking time, good current limiting characteristics

I t value is small, breaking overvoltage is low

It can limit the short-circuit current and breaking over-voltage in the range that the circuit can bear. It is the most ideal protection device.However, the fuse can not be reused and must be replaced once.

(3) the probability of short circuit fault is very small.

The combination of high-speed switch and fuse can avoid the trouble of replacing fuse frequently.

Make the advantages of the two complement each other,

So that the circuit can be effectively protected,

When selecting high-speed fuse,

According to the characteristics of the circuit to be protected,

High speed switch and rapid melting are determined respectively

The parameters of high-speed fuse should be matched with high-speed fuse.How to choose correctly, system development, designers must pay attention to and solve practical problems.

2The structure, working principle and characteristics of fast fuse

2.1 Structure of quick fuse

The fuse consists of magnetic shell, conductive plate, melt, quartz sand, arc extinguishing agent and indicator.The material of the melt is pure silver, the shape of which is rectangular sheet and has round hole and narrow neck.As shown in the figure:

Using fast fuse, yes .

Geometry of fuse for fast fuse

2.2 Arc extinguishing principle of fast fuse

The melt of the fast fuse is made of pure silver, which has low resistivity, good ductility and good chemical stability,

Therefore, the melt of the fast fuse can be made into a sheet,

It has a round hole and narrow neck structure.

Short circuit occurs at narrow neck

Therefore, the narrow neck is first fused and divided into many small sections by quartz sand.

In this way, the arc formed by fusing the melt

It is divided into many small sections by quartz sand, and the arc current is small,

The distribution space is small and easy to be absorbed by arc extinguishing agent.Because of quartz

Sand is insulated and an insulator is formed immediately after the arc is extinguished, breaking the circuit.

2.3 Characteristics of fast fuse

2.3.1 Characteristics of inverse time current protection

The fuse has the characteristics of inverse time delay, that is, when the overload current is small, the fusing time is long; when the overload current is large, the fusing time is short.Therefore, in a certain range of overload current and overload time, the fuse will not fuse and can be used continuously.

The fusing characteristic curve of fuse fuse is shown in the figure

1 As shown in Fig.

Figure 1 fusing characteristic curve of fuse

Figure 1 shows that the fuse has the characteristics of inverse time delay, that is, when the overload current is small, the fusing time is long; when the overload current is large, the fuse will fuse

In a certain range of overload current and overload time, the fuse will not fuse. This inverse time limit current protection characteristic is consistent with the protection performance required by the protected circuit.

It can be used continuously.Fuse

Fuse has different fuse characteristic curves, which can be applied to different types of protection objects.

2.3.2 Current limiting characteristics

Because the melt of the fast fuse is a rectangular sheet with a series of round holes and narrow necks,

And filled with quartz sand arc extinguishing medium.

circular

When the short circuit fault occurs, the fault current has not reached the expected short-circuit current,

The arc is fused and the arc is divided into many small segments by quartz sand.In this way, out.

The increase of short-circuit current is limited,

It also accelerates the extinction of the arc

For example, the expected effective value of short circuit fault current in the system is

40ka, using high voltage current limiting fuse, due to the fuse

The fault current will be limited to

Below 10kA.

2.3.3 Strong breaking capacity

Because the melt of the fast fuse is a rectangular sheet with a series of round holes and narrow necks,

And filled with quartz sand arc extinguishing medium.

hair

When a short circuit fault occurs, the narrow neck of the round hole is first fused, the arc is divided into many small sections by quartz sand, and the arc is quickly extinguished.

Because quartz sand is insulated, after the arc is extinguished, the breaking capacity of the breaker is strong, which can be up to

50kA。

The fuse immediately becomes an insulator,

Break the circuit.Therefore, it can melt rapidly

2.3.4 The impact energy of load equipment is small

In case of short circuit fault, the impact energy of load equipment is

W=I2Rt Where, I – short circuit current

– resistance of the circuit;

– the time from the occurrence of a short circuit fault to the interruption of the circuit.

The breaking time of fast fuse is short and has good current limiting effect, so the impact energy of load equipment is small.

The expected current in the system is, for example, a fault

When the high-speed switch is used for short-circuit protection, the load setting is 40 Ka

The impact energy of the equipment is

W1 = I 12rt 1 (where I 1 = 40ka, T1 = 25ms); if high voltage current limiting fuse is used for short circuit protection

When protecting, the impact energy of the load equipment is

W2=I

22Rt

2(i

2=10kA， t

2=10ms）； W2 /W1=1/40 。

2.3.5 Low breaking overvoltage

Since the melt of the fast fuse is a rectangular sheet with a round hole and a narrow neck,

And filled with quartz sand arc extinguishing medium.

Short circuit occurred

In case of obstacle, the narrow neck of round hole is first fused,

The arc is divided into many small segments by quartz sand, and the arc current is small

When it is close to zero, it is extinguished, so the breaking overvoltage is low.When the rated current of the fuse is selected,

The breaking overvoltage is about

Of the rated voltage of the circuit

2.5 Times.

3Selection of fuse

3.1 Selection principle of fuse

(1) the rated voltage of the fast fuse shall be equal to the grid voltage; 2) the fuse shall be able to withstand the normal load and the surge current of the normal load

Starting current, capacitor charging current, etc;

(e.g. excitation peak current of transformer

Electric

(3) the fuse should be able to break the minimum short-circuit current of the protected circuit within the required time;

(4) the breaking capacity of fuse shall be greater than the maximum short-circuit current of the protected circuit;

(3) the fuse shall have good current limiting characteristics to limit the short-circuit current within the scope of the protected circuit; and (6) the breaking over-voltage of the fuse shall be less than the maximum overvoltage that the protected circuit can bear.

(7) fast fuse and high speed switch

3.2 Steps of fuse selection

And load characteristics should be well matched.

(1) according to the rated current and load property of the protected circuit, it is preliminarily determined

Rated current of fuse;

(2) according to the preliminary determination of the rated current of the fuse and referring to the fuse selection manual,

Type selection of fuse;

(3) according to the current protection characteristic curve of the selected fuse

Check whether the requirements of the protected circuit are met,

If not, then

The fuse with higher rated current shall be selected and re inspected until it meets the requirements of the protected circuit.

3.3 Calculation formula of rated current of fuse (primary selection)

(1) protection of steady load without starting process

Stable load without starting process, such as lighting circuit, resistance, electric furnace, etc.

I FN ≥I N

(1)Where: I FN

— Rated current of fuse;

I n — rated current of load circuit.

(2) protection of single long-term working motor

I FN ≥(1.5 ～ 2.5)I

N

II.

Where: I

FN

— Rated current of fuse;

I

N

— Rated current of motor.

If the motor starts frequently, the coefficient in the formula can be appropriately increased to

(3) protection of multiple motors (power supply trunk line)

(4) protection of transformer

(1) according to the rated current of transformer, the rated current of fuse shall be selected preliminarily

Generally take the rated current of transformer

3-1.5 times of ITN

3.4 Examples of fuse selection

(1) fuse for protection transformer

1）Transformer parameters:

Rated voltage: 6 kV, rated capacity: 200 KVA, rated current: I 2) requirements for fuse:

=2 A, short circuit impedance: UD% = 5%.

① Rated voltage of fuse ② fuse must be able to cut off

VFN≥6 kV

The minimum short circuit current of the transformer, i.e

I 0 ＜ I TN/ud%＝ 19.2/5% ＝ 384 A

③ The fuse must be able to withstand Excitation impulse current of transformer,

3）Fuse selection

① It is preliminarily selected according to the rated current of transformer

Determined selection: 40A / 7.2kV

Fuse.

(2) fuse to protect main traction circuit

1）Parameters of traction main circuit:

Rated voltage: DC1500V (voltage variation range

900v to 2000V), rated current: 800A (maximum working current

The rated voltage of IGBT module is 3300v and the rated current is 1200A.

2）Setting of protection circuit

The over-current protection of traction converter is undertaken by high-speed switch, short-circuit protection is undertaken by fast fuse and grid drive circuit. 3）Specific setting of three-level protection parameters:

Over current protection of IGBT

① Design principle of IGBT over current protection circuit:

Over current capacity of IGBT: 10 μ s, 2 times of rated current.

Because the overload resistance of IGBT is only 10 μ s and 2 times of rated current.Therefore, over high speed switch can only be realized by grid driving circuit.

IGBT over-current protection is impossible

The design principle of IGBT over-current protection circuit is: when the over-current value is less than

2 When times the rated current, instantaneous blocking is adopted

When the over-current value is greater than

2 The soft turn off method is adopted when the rated current is twice as high as 2 μ S-5 μ s

The grid voltage is reduced to zero within 0,

To the final back voltage of – 5 V (because the instantaneous blocking pulse voltage will make

di/dt

If it is too large, a higher peak voltage will be induced in the main circuit).② Selection principle of high speed switch:

The rated voltage of high-speed switch shall be equal to the rated voltage of catenary,

It can withstand the highest overvoltage of catenary;

High speed on

The rated current of switch off shall be greater than or equal to the working current of traction converter; the set value of over-current trip of high-speed switch shall be set at 1.2 times of rated current.

Based on the above analysis, the relevant parameters of high-speed switch are selected as follows:

Rated voltage: 1500 V

Rated current: 1200 a

IGBT

Rated impulse withstand voltage

: 18 kV

Trip current setting value: 1400 a rated insulation voltage: 3000 V

③ Selection of quick fuse:

The rated voltage of quick fuse shall be equal to that of catenary;

The rated current of fast fuse should be equal to traction transformer

The working current of the current breaker; the fast fuse should be able to withstand the setting current of high-speed switch tripping;

The fast fuse should be able to break the pull

The minimum short-circuit current of the main circuit; the breaking capacity of the fast fuse should be able to break the maximum short-circuit current of the traction main circuit.Based on the above analysis, the relevant parameters of high-speed switch are as follows:

Rated voltage: 1500 V

Rated current: 1000 a

25ms melting current: 10ms melting current:

≥ 2000 a (trip current 1400 a, ≤ 4000 a

Trip time 25ms)

Rated breaking capacity: 50 Ka

4Precautions for fuse selection

(1) rated voltage of fast fuse shall be equal to grid voltage;

(2) the rated current of the fast fuse must match with the working current of the protected circuit, which will cause high breaking overvoltage;

(1) the tolerance of equipment characteristics should be considered to obtain good protection effect;

No derating, derating

(4) in the protection circuit with fuse and high-speed switch set at the same time,

One of the important parameters affecting the protection characteristics is its intersection

Connected current izy, which is the current at the junction of the protection curve between the high-speed switch and the current limiting fuse, that is, when the current is greater than the current, the protection function is performed by the current limiting fuse; when the current is less than the current, the protection function is performed by the high-speed switch.

The handover current izy shall be determined according to the following principles:

Over current protection setting value ＜

Short circuit current of the circuit

1If the main circuit is not effective in case of fire, whether the main circuit is able to cause fire when the fuse is not effective.

2Fuse is the key safety component in electric vehicle. Its application and selection requirements are different from the traditional industrial fuse and automobile fuse. The main difference is the strict requirements in the application of electric vehicle.

Electric vehicle charging pile fuse

Electric vehicle charging pile fuse, fuse is widely used in high and low voltage distribution system and control system as well

In electrical equipment, as a protector of short circuit and over-current, it is one of the widely used protective devices.’

1. The protection characteristics of the fuse should adapt to the overload characteristics of the protected object. Considering the possible short-circuit current, the fuse with corresponding breaking capacity should be selected;

2. The rated voltage of the fuse should adapt to the line voltage level, and the rated current of the fuse should be greater than or equal to the rated current of the fuse; 3. The melt of fuse should be matched according to the requirements. It is not allowed to increase the melt arbitrarily or replace the melt with other conductors.

Patrol inspection of electric vehicle charging pile fuse:

1. Check whether the rated value of fuse and melt matches with the protected equipment;

2. Check the appearance of the fuse for damage and deformation, and whether the porcelain insulation part has flickering discharge traces; 3. Check whether the contact points of the fuse are in good condition, close contact and overheating; 4 Whether the fuse signal indicator is normal.

. Check whether there is a lack of brake fluid in the brake oil cup, if less, add fluid;

2. Check the brake oil cup and brake oil pipe for oil leakage, and replace them if any;

3. Check whether the brake pads are seriously worn, and replace them if they are seriously worn;

4 Check the brake hub brake pad clearance adjustment (normal yes 2-4mm ）

1. electric vehicles motor The fuse is burnt out and replaced.

2. electric vehicles fuse The power switch is broken. Replace the power switch.Judgment method: turn on the power switch and measure the input end of the power switch with the ohmmeter

If the resistance value is zero, it is normal; if the resistance value is infinite, the power switch is broken.

3 If the accelerator is out of order, use the DC voltage level of the multimeter to measure the output voltage of the accelerator. If there is a voltage output, it will be normal; if there is no voltage output, it will be normal

Abnormal. If there is no voltage output, the accelerator is broken and must be replaced.

4. If the controller is broken, the electronic control must be replaced.Use a multimeter to measure the output voltage of electronic control. If there is output voltage, it will be good, otherwise it will be broken.

5. If the motor is burnt out, replace the motor. 6. The connection wire ends of the motor are loose, so check them again electric vehicles fuse.

1. electric vehicles fuse If the charger fuse is burnt out, all indicator lights of the charger are not on at this time, the fuse must be replaced.

2. electric vehicles fuse If the battery pack is disconnected, connect the battery cable.

3. electric vehicles fuse If the charger plug and battery socket are not connected in place, they should be re connected.

4. electric vehicles fuse If the charger is broken, the fuse of charger is normal, and the output voltage of charger should be zero by measuring with multimeter.

Note: we use intelligent charger.It has the function of under voltage and over-voltage protection

Automatic power off and shutdown.In this case, the motor is first charged and then disconnected.

1 electric vehicles has no power The fuse is broken. Use a multimeter to measure the battery terminal voltage. If there is voltage output, it is normal. If there is no voltage output, the fuse is broken or the battery plug is broken

Or the battery is broken.

2. electric vehicles has no power The wiring plug is loose. Check the power switch connector.

3. If the electric vehicles has no power power switch is broken, use a multimeter to measure the voltage at both ends of the input and output lines of the power switch. If there is normal voltage output, the power switch is normal If there is no voltage output, the power switch is broken [when the battery has voltage output], it shall be repaired or replaced

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.

What

According to the common sense and experience of the author, the main causes of fuse blow are as follows:

A. Overload.The household electricity load is too large, causing overload, so that the fuse is blown.This is especially true in the use of

It is easy to appear when air conditioner, electric heater or other high-power electrical appliances are added.

B. Poor contact.Some families, although the fuse selection is more reasonable, the load is not too big, can be used

Air conditioners, electric heaters, rice cookers and other large power appliances will “trip”.The reason may be that during the installation and replacement of the fuse, the poor contact between the fuse and the plug screw causes the ignition and heating, and the screw fixing the fuse on the porcelain plug and switch knife is oxidized and “burnt out”. C. Short circuit.If the fuse is replaced, a switch on the “trip”, it may be a short circuit.The first is the short circuit.The second is load short circuit, such as electric kettle, rice cooker and other commonly used larger power electrical appliances and commonly used mobile electrical appliances, as well as inferior electrical appliances, are prone to short circuit fault.

are the reasons for the fuse to blow