High frequency switching power DC operating power supply system

    Abstract: This paper describes the composition of high-frequency switching DC operating power supply systems and the power factor, current sharing method, heat dissipation and dust prevention of high-frequency switching power supply modules, and discusses the future development trends of DC operating power supply systems.

    Keywords: high frequency switching power supply, DC operating power supply

l Introduction

    DC operating power supply system is one of the indispensable secondary equipment in power plants and substations. Its reliability directly affects the safe and reliable operation of power plant and substation equipment. Many of the DC operating power systems currently operating in my country's power plants and substations are still relatively backward and obsolete equipment, with many defects, causing many accidents and causing heavy losses. With the popularization of valve-regulated sealed lead-acid batteries, higher requirements have been put forward for the original DC operating power supply system. Compared with acid-proof explosion-proof batteries and cadmium-nickel alkaline batteries, valve-regulated sealed lead-acid batteries have The following features: no maintenance work such as adding water and adjusting the acid proportion, etc., maintenance-free function; no leakage, no acid mist, no corrosion of equipment, easy to form a complete set of devices; small self-discharge current; long battery life, float charge at 25°C The life span can reach 10 to 15 years; the structure is compact, the sealing is good, and the vibration resistance is good; there is no "memory effect" of cadmium-nickel alkaline batteries. However, valve-regulated sealed lead-acid batteries are more sensitive to temperature and have strict requirements on charging devices. Overcharging and undercharging are not allowed. If an outdated charging device is still used, due to its low voltage and current stabilization accuracy and high ripple coefficient, the life of the valve-controlled sealed battery may be reduced or even the body may be cracked and damaged, paralyzing the entire DC system.

    After the development of communication power supply in recent years, charging devices composed of valve-controlled sealed lead-acid batteries and high-frequency switching power supply modules have been widely used. The high-frequency switching power supply module has the characteristics of small size, light weight, low noise, high voltage stabilization accuracy, small ripple coefficient, and flexible configuration. When used in conjunction with valve-regulated sealed lead-acid batteries, it can increase the reliability and stability of the DC system. . At present, in urban and rural power grid construction and renovation projects, DC operating power supply complete sets composed of high-frequency switching power supply modules and valve-regulated sealed lead-acid batteries have begun to be used in part, which have good results in ensuring reliable operation of the DC system and battery life. Well received by design and operation personnel.

    Oriental Electronic Information Industry Co., Ltd. has been developing intelligent high-frequency switching DC operating power supply systems since 1996. So far, more than 100 sets of DC power supplies are in operation on site.

2. Composition of DC operating power supply system

    There are currently three types of high-frequency switching power supply modules: 5A, 10A and 20A. Depending on the load requirements and battery capacity, multiple modules can be connected in parallel according to the N+l backup principle to form a DC operating power supply system of tens to hundreds of amps. Figure 1 is the schematic block diagram of the DC operating power supply system. This is a single bus wiring method. The module output is connected in parallel with the DC bus and battery pack. The battery is usually in a full float charge state. For a DC operating power supply system with separate control and power buses, there are two wiring methods: one is that the output of all modules is connected in parallel with the battery pack and power bus, and an automatic voltage regulating device is set between the power bus and the control bus. The load is provided by the power bus through the automatic voltage regulating device. The principle is shown in Figure 2. This method requires the automatic voltage regulating device to have high reliability; the other is to divide the module into two groups, one group of output and power bus, The battery pack is connected in parallel, and the other set of output is connected in parallel with the control bus. An automatic voltage regulating device is installed between the power bus and the control bus. Under normal circumstances, the control bus load is provided by the module, and the automatic voltage regulating device is in standby mode due to the back pressure. Only when the AC power outage or all modules of the control bus fail, the automatic voltage regulating device will be put into operation. Its principle block diagram is shown in Figure 3. This wiring method requires both sets of modules to be configured N+l according to the load.

3 Input power factor of high-frequency switching power supply module

    Low input power factor is a common problem in early high-frequency switching power supply modules, which is mainly related to the circuit form used. In early high-frequency switching power supplies, the AC input voltage was rectified and directly applied to both ends of the filter capacitor. Only when the AC input voltage was higher than the voltage across the filter capacitor, the rectifier diode began to conduct electricity, so the input current waveform had a very narrow width. For pulses, the harmonic distortion of the input current is serious, and the power factor is usually only 0.6 to 0.7. This kind of switching power supply module causes harmonic pollution to the power grid, causes electric power pollution, interferes with other electrical equipment, and causes large errors in measuring instruments. In order to reduce the pollution of power supply devices to the power grid, the relevant standards of EMI and EMC have clear regulations on the power factor and harmonic current values ​​of power supply devices of different power levels. Therefore, the power factor of high-frequency switching power supply modules needs to be corrected.

    There are two basic methods of power factor correction, passive power factor correction (PFC) and active power factor correction (APFC). The passive power factor correction method is to add a low-frequency inductor with a large inductance to the input end and reduce the capacity of the filter capacitor to reduce the peak charging current of the filter capacitor. This method is relatively simple. However, the correction effect is not ideal and can only reach about 0.9~0.92. It is generally used for high-frequency switching power supply modules with three-phase input. The active power factor correction method is to add a high-frequency inductor, a diode, a high-frequency switch tube and a corresponding controller to the input end to form a boost converter. The controller collects the AC input voltage signal and current signal to control The switching tube is turned on and off, so that the input current waveform always follows the input voltage waveform, so that the power factor of the high-frequency switching power supply module reaches above 0.99 and the harmonic distortion is less than 5%.

4 Current sharing of high-frequency switching power supply modules

    Different from the phase-controlled charging device, the charging device of the DC operating power supply system composed of high-frequency switching power supply modules generally adopts the N+1 redundant backup method. The power distribution between the modules is realized through the current sharing circuit between the parallel modules. The power between the modules is The degree of balance of distribution mainly depends on the current sharing method. The load in the DC system includes two parts: battery charging current and control bus load current. The battery pack is in a float charge state for a long time, and the charging current is very small. For lead-acid maintenance-free batteries, the float charge current is only about 0.0l of the rated capacity. In addition, the control load is small, and the entire charging device is in a light load state; when the high-voltage circuit breaker When closing, the battery pack provides a closing surge current. The charging device connected in parallel with the battery pack is in a current-limiting protection state due to excessive current. After the closing surge current ends, the charging device recharges the battery, causing a sudden increase in charging current. Therefore, the current sharing circuit needs to ensure that the charging device maintains good current sharing characteristics whether under light load or overload, which is the so-called "full range current sharing". If the current sharing characteristics are not good under light load, some modules may have no current output and remain in no-load operation for a long time, seriously affecting the reliability of the module.

    There are many current sharing methods used in high-frequency switching power supply modules, such as: voltage reduction method, master-slave control method, external control method, average current automatic current sharing method, maximum current automatic current sharing method, etc. Considering the operating characteristics of the DC system charging device and the requirements for voltage/current stabilization accuracy, we adopt the average current automatic current sharing method in the high-frequency switching power supply module. The advantage of this method is that there is no main module and the number of parallel modules Without restrictions, precise distribution of load current and current sharing across the entire load range can be achieved.

5 Heat dissipation and dust prevention of high-frequency switching power supply modules

    The charging device is the heart of the DC system, and its reliability is an important guarantee for the safe operation of the DC system. For charging devices composed of high-frequency switching power supply modules, on the one hand, N+1 redundant backup can be used to effectively extend the mean trouble-free working time of the charging device; on the other hand, the mean trouble-free working time of a single high-frequency switching power supply module must be improved ( i.e. life span). High-frequency switching power supply modules are composed of a large number of resistors, capacitors, power electronic devices, etc. according to a certain circuit method. During the power conversion process, a certain amount of power loss is always generated, and the power loss is usually emitted in the form of heat energy, causing The temperature of the power module rises. Excessive temperature rise has a great impact on the life of the module. The higher the operating temperature of the module, the lower the performance and reliability, and the shorter the service life. Therefore, in addition to adopting a high-reliability circuit method, it is also necessary to choose an appropriate heat dissipation method to effectively reduce the temperature rise of the high-frequency switching power supply module and ensure its service life.

    High-frequency switching power supply modules currently used in power DC systems mainly use two heat dissipation methods: forced air cooling and natural cooling. The advantages of the forced air cooling method are the small size, light weight, and low internal temperature of the module. The disadvantages are the loud noise, the lifespan of the fan itself, and the problems of dust accumulation on the circuit board. The advantage of the natural cooling method is that it is noiseless and does not have the problem of fan life. The disadvantage is that it is large in size and high in cost.

    High-frequency switching power supply modules were first popularized and applied in the communication power supply industry. Some technologies in many high-frequency switching power DC power supplies were also transformed from high-frequency switching power supplies for communication. The heat dissipation methods of the modules also mostly follow the technology of communication power supplies. Whether it is forced air cooling or natural cooling, the cooling air ducts adopt an open structure. However, the working environment of substations is worse than that of communication equipment rooms, and the dust content in the air is very high. Especially for new stations, the civil construction projects are often not completed yet. Since the debugging of relay protection and other devices requires DC power supply, the DC power supply is often put into operation in advance. If effective dust-proof measures are not taken, a large amount of cement dust and other dust will be adsorbed on the circuit boards or components in the power module, causing insulation degradation or even short circuit, causing the module to malfunction.

    The dust formed on the circuit board of the high-frequency switching power supply module is, firstly, dust sucked in by the fan, and secondly, electrostatic adsorption. In order to prevent dust, some switching power supply modules have adopted the following measures:

    Use a dust cover: Installing a dust cover at the air inlet of the module can prevent dust to a certain extent, but it needs to be cleaned frequently, otherwise the ventilation holes on the dust cover will be easily blocked and affect the ventilation and heat dissipation effect. This method is not suitable for use in unattended substations.

    Using natural cooling: This can prevent the fan from inhaling dust, but for heat dissipation needs, many heat dissipation holes must be opened on the module, so the problem of electrostatic adsorption of dust cannot be solved.

    In the process of developing high-frequency switching power supply modules, Oriental Electronic Information Industry Co., Ltd. comprehensively considered the advantages and disadvantages of forced air cooling and natural cooling as well as the conditions of the substation site. The module heat dissipation method adopts temperature-controlled forced air cooling and closed Cooling duct. The fan is controlled by the temperature detection circuit. The fan will only run when the module radiator temperature is higher than the set value. Since the charging device of the DC system has been operating at light load for a long time, it generally only has about 15% of the rated capacity. The radiator temperature is lower than the fan starting temperature and the fan does not work. This heat dissipation method can keep the internal temperature of the module relatively stable and does not change with the external environment and load. The life of the fan can be increased by 2 to 3 times, thereby improving the reliability of the high-frequency switching power supply module. In terms of dust prevention, a completely enclosed cooling air duct is used so that the cooling air flow only passes through the surface of the radiator to isolate the heat dissipation channel from the internal circuit. This can not only prevent dust accumulation on the circuit board, but also improve the heat dissipation effect and fully improve charging. Module's ability to adapt to the environment.

6 Development Trends of DC Systems

    In order to ensure the safe operation of AC and DC power devices such as background machines, automatic devices, transmitters, communication equipment, and protection devices in the substation, in addition to the DC system of the substation, UPS devices and dedicated communication power supply devices also need to be configured. In the past, People have been setting up these three different power sources separately, each equipped with a set of batteries, resulting in high equipment costs, heavy maintenance, low reliability, and low resource utilization.

    With the popularization and application of high-frequency switching power supply technology in DC systems, people have begun to consider how to rationally utilize substation resources, reduce equipment costs and maintenance workload, and improve reliability. At present, some substations are beginning to try to adopt such a combination: using sine wave inverter power supply to replace UPS equipment, using high-power DC/DC converters to replace communication power supply devices, and the inputs of the two devices are directly attached to the busbar of the DC system. When the AC power is normal, the charging device of the DC system provides power for the inverter and DC/DC converter; when the AC power is lost, the battery pack of the DC system provides DC power. The status information of the inverter and DC/DC converter is sent to the monitoring unit of the DC system.

    Using the above method, at least the battery packs and monitoring units in UPS and communication power supply devices can be omitted. In terms of equipment management, only the battery packs of the DC system need to be intelligently managed, thereby reducing system maintenance.

7 Conclusion

    The charging device of the high-frequency switching DC operating power supply system is composed of multiple high-frequency switching power supply modules connected in parallel in N+1 redundant backup mode. In order to reduce harmonic pollution to the power grid, the high-frequency switching power supply module should have a power factor correction function. Considering the load characteristics of the system during operation, the high-frequency switching power supply module should have full-range current sharing characteristics, and the heat dissipation of the module should isolate the heat dissipation channel of the module from the circuit board to prevent electrostatic adsorption and dust accumulation on the circuit board caused by the fan. In order to rationally utilize the power supply device resources of the substation, in the configuration of the DC operating power supply system, the power sine wave inverter and the DC/DC converter are used to provide AC and DC power for communication respectively, reducing the maintenance workload of the substation power supply device. and equipment costs, this integration method will become one of the development directions of DC operating power supply systems in the future.