Keywords in this article:

Railway, DC fuse.

Key pain points in DC fuse:

The application of DC fuse in electric vehicle is large short circuit current, high rated voltage and small rated current; In photovoltaic power generation, with large periodic fluctuation. For different applications, the design of fuse is very different.

In these applications, the failure of DC overload current breaking is a pain point from many customer.

At present, the research hotspots of DC fuse mainly focus on the arcing process of fuse, simulation model of fuse arcing and hybrid fuse.

Introduction

In the Railway power system, many places need fuse protection to cut off the direct fault current. Typical positions include the main circuit, regenerative DC converter, AC converter, auxiliary loop (air conditioning, lighting, etc.), crowbar system (for braking DC motor) .

In the application of DC circuit protection, the voltage must be determined according to the time constant L / R (L is the loop inductance value, R is the loop resistance value). The time constant and over-current range of DC fuse applied in rail transit is very large. The Bussmann DC fuse with good design can provide enough length for arcing and withstand the huge pressure produced by arcing. Bussmann DC fuse can withstand the influence of arcing, and keep a relatively compact overall dimension of the breaker.

1. Purpose of fuse selection

In many underground rail transit systems, the DC power supply mode of three rail current collector is used in the propulsion circuit of trains. The basic components of current collector are collector shoe and DC fuse. In other rail transit systems, the DC power supply mode of pantograph is often used in the propulsion circuit of trains. There will be a fuse box near the pantograph with DC fuse inside.

Usually, the purpose of this kind of DC fuse is to protect people and equipment from damage caused by insulation problems of power cable connecting current collector (or pantograph) and propulsion circuit, as well as damage caused by motor fault. If there is no fuse protection, it will lead to serious fire.

The safety requirements of rail transit determine that the tube type fuse must be used and the protective action must be completely enclosed.

2, Selection method of fuse

The conditions to ensure the accurate selection of DC fuse are: Parameters of the protected circuit; DC characteristic data of the fuse; And all working conditions of the fuse.

The distance between the train and the substation determines the time constant of the fault circuit and the corresponding fault current amplitude. Therefore, the DC fast blow fuse installed in the three rail current collector or pantograph needs to remove different kinds of faults, such as small overload current, maximum energy breaking (accompanied by large time constant) and high fault current (up to 80 KA).

When fuse protection is selected for DC circuit, the corresponding electrical parameters of the circuit must be known: Time constant for all possible faults and voltage s, maximum and minimum value of fault current, peak value of current, etc.

The selection of fast acting DC fuse also needs to consider all conditions related to its rated current selection, such as: Maximum ambient temperature (usually close to 60 ° C or 65 ° C due to the influence of fuse box), Periodic load or more stable load; Long time overload, Short-term overload must not damage the breaker (usually need to be broken by circuit breaker). 

These conditions are the basic elements for the correct selection of fuses.

2.1 Selection of DC fuse rated voltage

The rated voltage of DC fast blow fuse installed at current collector must be selected according to different time constant and working voltage value. Most three rail systems adopt one of the following two schemes: 

1), DC 600 V ± 20%, time constant L / r-60 MS, but sometimes up to 100 ms. The maximum voltage can reach 750 V;

2), DC 750 V ± 20%, the time constant is 60 ms, but sometimes 100 ms. The maximum voltage is 1050 V;

The rated voltage of DC fast blow fuse installed at pantograph must be selected according to different time constant and operation voltage. Most pantograph systems adopt one of the following two schemes: 

①, In the DC 1500 V type, M is the current sharing coefficient of two current collectors on the same side; from is the environmental temperature coefficient; * '2 is the periodic variable current coefficient; from is the constant current coefficient; and G is the connection coefficient.

The maximum voltage can reach 2100V  with ± 20% time constant L / R ° 50 ms. 

② The maximum voltage can reach 3900 V;

Note (1): the current maximum voltage even exceeds + 20% of the rated voltage. This is due to the regenerative energy of motor braking, So it is not necessary to determine the rated voltage based on this voltage. When the fault occurs, the voltage control will act quickly, and the fault voltage will also avoid falling. This needs to be confirmed with the designer of the three rail system;

Note (2): since the distance between the faulty motor and the next substation changes at any time, its corresponding time constant also changes. When passing through a substation, although the voltage is very high, the fault current is also very large, and the DC fuse can still break the fault when the time constant is very large.

2.2 Selection of DC fuse rated current

2.2.1 Periodic variation of load

The current changes periodically with the repetitive starting, accelerating, uniform speed, braking and stopping of the train. The typical period of 4ms effective value of total current transmitted to EMU is shown in Figure 2, which is shared by two current collectors (two fuses). When calculating the rated current of the DC fuse, the maximum ambient temperature in the fuse box, the periodic variation of the fuse life, the current sharing between two parallel collector shoes, and the overload current 4 in the period from T2 to TT should be considered.

In this example (see Figures 1 and 2), / RMS = dagger + skill. For this application, DC fuses sometimes need to work for hours or even a whole day when one current collector is not working. At this time, the rated current of fuse meets

/rms<W 1.25/rms

2.2.2 Cooperation with circuit breaker

The cooperation with circuit breaker can be divided into two situations: 

One is to cooperate with DC circuit breaker in substation, the other is to cooperate with circuit breaker in vehicle.

The DC fuse rated value can be calculated according to the following conditions:

 ①, Calculate the effective value / RMS of the current waveform limited by the circuit breaker (see Fig. 3); 

②, Calculate the current through the coefficient cf. used by the fuse derating, / blood = / RMS / 0.75, and check the fuse time current characteristic curve to confirm that the fuse is on the front arc current of time such as the action time of the circuit breaker in case of fault current (see Fig. 4),

/(4) the current three conditions are met, and the fuse does not operate when the DC circuit breaker acts.

Figure 4, DC fuse selectivity matching with circuit breaker

Note: 

(1), Due to the need of coordination, sometimes the rated current of the selected DC fuse is higher than the rated current value calculated by normal load; 

(2) The protection purpose of DC fuse is to break open circuit fault or cable insulation fault between circuit breaker and fuse;

Conclusion

High performance DC fuse is an essential protection equipment of rail transit system, which ensures the safe operation of the whole system equipment. Therefore, the high-tech development of electric railway transit system needs more accurate selection of fuse.

Note: Thanks to Wanfang Data provide the original report and the expert who wrote the article.