Selection and application of fast fuse
1. Overview
In the metro trains, the protection of the main circuit of traction and the auxiliary system is jointly undertaken by a 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-circuits 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
It 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 a high-speed fuse should be matched with a high-speed fuse. How to choose correctly, system development, designers must pay attention to and solve practical problems.
2. The structure, working principle, and characteristics of fast fuse
2.1 Structure of quick fuse
The fuse consists of a 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 a rectangular sheet and has a 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 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.
3. Selection 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 the transformer, the rated current of the 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
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