Analysis and Countermeasures of Dual Power Supply Conversion Problems in Railway Signaling

In recent years, with the rapid development of my country's railway industry, the driving speed has been continuously improved, and the requirements for the reliability of power supply for primary loads such as railway communication signals have become increasingly higher. Once a fault occurs on the power supply side, it will inevitably seriously affect driving and cause great losses to the national economy.

my country's current railway design specifications require that primary loads such as communication signals should have two power supplies, which are respectively supplied to power-consuming equipment or low-voltage dual power switching devices. In the current design, the station automatic block and the low-voltage side of the through transformer are generally connected to the dual power switching box at the power-consuming equipment, and then fed to the load from the lower port of the dual power switching box. In this power supply system, the dual power switching device plays a key role in ensuring reliable power supply as the conversion point of the two power supplies. However, there are still many issues worth exploring in terms of how to select dual power switching devices, the settings of dual power switching devices in different distribution systems, the selection of types, and management in design and use.

l Development status of dual power conversion

There are four modes for realizing dual power switching in the engineering field of my country, namely, two contactor type, two circuit breaker type, excitation type special transfer switch and electric type special transfer switch. The initial design was two contactor type, which is a simple circuit formed by overlapping two contactors. This method is less and less used in current projects because of its unreliable mechanical interlocking, easy to generate temperature rise and heating, contact adhesion, and coil burning. It is close to being eliminated; the second is two circuit breaker type (CB level). This dual power conversion circuit is formed by overlapping two circuit breakers, and is equipped with a mechanical interlocking device. It also has short circuit and overcurrent protection functions. However, the mechanical interlocking of this mode is not very reliable and is mostly used in places with large current calculations; the third is an excitation type special transfer switch, which is composed of an excitation contactor plus a controller to form an integral device, with reliable mechanical interlocking. When switching, the electromagnetic coil generates attraction to drive the switch, and the speed is fast. The fourth is composed of an electric switch, the main body of which is a load isolation switch. As an electromechanical integrated switch, the conversion is driven by a motor, the conversion is smooth and fast, and it has a zero-position function (PC level).

According to GB/T 14048.11, dual power switching devices can be divided into two levels: PC level or CB level. PC level: can connect and carry, but not used to disconnect short-circuit current; CB level: equipped with overcurrent release, the main contact can be connected and used to disconnect short-circuit current.

2 Controversies in the current design

The main controversies and misunderstandings about dual power switching devices in current design are as follows

1) How to select CB-level and PC-level dual power conversion devices;

2) Choose a three-pole switch or a four-pole switch for the dual power conversion device;

3) Choose the dual power conversion device as two-position or three-position;

4) Whether the dual power conversion device can be used as an isolation device;

5) Determination of the conversion time of the dual power conversion device;

These issues will be discussed below in order to find solutions suitable for railway communication signal power supply systems.

3. Countermeasures for railway design

The characteristics of the power distribution system and the needs of the load are the basis for selecting the type and various parameters of dual power switching in the design. Obviously, the connection and disconnection capacity is closely related to the characteristics of the system load and the frequency of operation, and needs to be analyzed as appropriate in the design. The national standard GB/T14048.11--2002 stipulates the corresponding use categories according to their different uses (Table 1).

Railway communication signal loads are all inductive loads. At the same time, since the high-voltage side 10kV self-closing line and through line that supply power to the communication signal are equipped with backup automatic switching and reclosing devices, the number of power outages is relatively small except for planned maintenance, which is relatively reliable. Therefore, a dual power switching device that is not frequently operated for inductive loads should be selected in general stations.

After the basic principles are determined, the questions raised in the previous chapter are analyzed one by one as follows:

1) How to choose PC-level and CB-level conversion devices

The CB level itself has thermal magnetic protection, and its main contacts can be connected and used to disconnect short-circuit current. When selecting the CB level, it is only required that the executive circuit breaker of the automatic transfer device is selected correctly, that is, it meets the following conditions: ① The rated breaking current (effective value) of the low-voltage circuit breaker is greater than the expected maximum short-circuit current value (effective value) at this location; ② The selection of the low-voltage circuit breaker and the selection of the low-voltage circuit breaker of the upper and lower outgoing line circuits meet the requirements of power distribution protection selectivity. The PC level only has power conversion function, and no short-circuit and overload protection function. The rated short-time withstand current effective value (1s) of the automatic transfer switch with a rated current less than or equal to 250A is generally around 5 to 12kA; therefore, the design needs to be specifically analyzed and objectively selected based on the conditions and occasions of the entire low-voltage distribution system and the requirements of the load.

In actual applications, in the distribution box or cabinet, the CB-level front end can only be equipped with isolation devices or isolation switches, and there is no need to install short-circuit protection devices; while the PC-level front end needs to be equipped with short-circuit protection devices, and the MCB on the outgoing line circuit in the distribution box or cabinet needs to be cascaded with the MCB installed at the front end.

It should be noted that CB-class conversion devices will produce a series of problems caused by protection functions:

(1) As the number of protection levels increases, it is necessary to ensure selectivity between the upper and lower levels.

(2) Since the input signal of the dual power conversion is taken from the upper port of the power supply line, when the voltage or frequency of the normal power supply is normal, the circuit breaker trips due to overcurrent, causing the load to lose power, and the conversion device will not operate. From this perspective, the protection function of the CB level is not favorable in the operation of the system. However, in the distribution system with large short-circuit capacity, the CB level has certain advantages.

2) Choose a three-pole switch or a four-pole switch

Regarding the selection of three-pole and four-pole switches, it is generally believed that three-pole switches can be used for power supplies of the same type and capacity, such as two public power grids, two generator sets, etc., whose neutral lines are not cut off or have a common neutral line; four-pole switches must be used for automatic conversion between power supplies of different standards and those with greater harmonic current influence; four-pole switches should be used for conversion between two power supplies of different capacities, because there are different zero-sequence voltages on their respective neutral lines. If the neutral line is not disconnected, a circulating current will be formed, which is very harmful to electrical equipment.

The two power sources for railway communication signals are 10kV self-closing line and through line, which generally come from 10kV railway distribution stations along the line. Their superior power source is the local public power grid, so they belong to the same type of power supply. In the application, a three-pole switch can be used unless there are special circumstances.

3) Choose a two-position or three-position switch

During the conversion process of the dual power conversion device, there is one action in place (two-position type) and there is an empty position in the middle (three-position type). The main contact of the two-position switch has only two working positions (normal power position and backup power position), and its conversion action time is faster; the main contact of the three-position switch has three working positions (normal power position, backup power position and zero position). When the zero position is set, the main contact is in neutral. Because the zero position is set, the conversion action time of the three-position switch is slower. The main function of setting the "zero position" is to avoid transient stop of the impact current when the load is high inductive reactance or large motor load. It can be said that the two-position switch has higher reliability than the three-position switch.

According to the characteristics of the primary load of the railway, the two power supplies are required to be switched quickly and the load impact current is not large, so a two-position switch should be selected as much as possible in the design.

4) Can the dual power conversion device be used as an isolation device?

The isolating switch must have the following functions: ① The moving contact can be locked or visible in the disconnected position; ② It has a higher rated impulse withstand voltage (1.25 times); ③ In any case, the limiting leakage current should not exceed 6mA.

From the definition, it can be seen that the main purpose of the dual power switching device in the electrical system is to complete the switching function of the main and backup power supplies in the primary and secondary loads. On the other hand, it also has the possibility of overhaul and maintenance, so the dual power switching device itself cannot meet the requirements of the isolator. At the same time, the importance and structural complexity of the conversion device itself require a condition of no-power maintenance, and an isolation appliance must be set at its front end.

5) Determination of conversion time of dual power conversion device

Different backup power properties and different load conditions have different requirements for conversion time. Dual power conversion device switching time △t = △t1 + △t2

△t1——conversion action time

Δt2——artificial delay time

The operation of the power distribution system is accompanied by the generation and extinction of arcs between the main contacts. In order to ensure the reliable extinction of the arc and prevent the arc crossing between the first and second main contacts from causing a short circuit between the normal and backup power supplies, △t must be greater than the arc existence time.

Secondly, the principle of determining artificial delay is: ① The total action time of the lower transfer switch should be greater than 10 cycles (200ms) of the upper transfer switch. ② If the normal power supply and the backup power supply are equipped with a connecting circuit breaker on the power supply side, the total action time of the transfer switch at this level should be 0.5s greater than the delay setting mutual switching time of the upper contact switch.

Under current production conditions, the operating time range of the transfer switch is roughly shown in the following table.

4 Conclusion

The dual power conversion device is used at the intersection of normal power supply and backup power supply. As the core part of the primary load low-voltage power supply system such as railway communication signal, it plays an important role in ensuring power supply reliability. It is easy to be overlooked as an ordinary switch during design and use, leaving major hidden dangers for the safe operation of the entire system. Therefore, during design and use, it is necessary to comprehensively analyze the power supply, load and other aspects of the system, correctly select the dual power switching device, and effectively meet the load requirements, laying a solid foundation for the safe and reliable operation of the entire system.