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. The future development trend of DC operating power supply systems is also discussed. Keywords: High-frequency switching power supply, DC operating power supply I. Introduction DC operating power supply systems are indispensable secondary equipment in power plants and substations. Their reliability directly affects the safe and reliable operation of power plant and substation equipment. Many of the DC operating power supply systems currently operating in power plants and substations in China are still outdated and have numerous defects, leading to many accidents and significant losses. With the widespread adoption of valve-regulated sealed lead-acid (VRC) batteries, higher demands have been placed on existing DC operating power supply systems. Compared to acid-proof and explosion-proof batteries and nickel-cadmium alkaline batteries, VRC batteries have the following characteristics: no need for water addition or acid specific gravity adjustment, making them maintenance-free; no leakage, no acid mist, no equipment corrosion, and easy to integrate into complete systems; low self-discharge current; long battery life, with a float charge life of 10-15 years at 25℃; compact structure, good sealing, and good vibration resistance; and no "memory effect" like nickel-cadmium alkaline batteries. However, VRC batteries are highly sensitive to temperature and require strict charging devices, prohibiting overcharging and undercharging. Using outdated charging devices, due to their low voltage and current regulation accuracy and high ripple coefficient, may reduce the lifespan of the VRC batteries or even cause them to crack and break, paralyzing the entire DC system. In recent years, communication power supplies have widely adopted charging devices composed of VRC batteries and high-frequency switching power supply modules. High-frequency switching power supply modules are characterized by their small size, light weight, low noise, high voltage regulation accuracy, low ripple coefficient, and flexible configuration. When used in conjunction with valve-regulated sealed lead-acid batteries, they can increase the reliability and stability of DC systems. Currently, some urban and rural power grid construction and renovation projects have begun to adopt DC operating power supply systems composed of high-frequency switching power supply modules and valve-regulated sealed lead-acid batteries. These systems have shown good performance in ensuring reliable operation of DC systems and extending battery life, receiving positive feedback from designers and operators. Dongfang Electronics Information Industry Co., Ltd. has been developing intelligent high-frequency switching DC operating power supply systems since 1996, and more than one hundred sets of DC power supplies are currently in operation. 2. Composition of DC Operating Power Supply Systems High-frequency switching power supply modules are currently available in 5A, 10A, and 20A capacities. Depending on the load requirements and battery capacity, multiple modules can be connected in parallel according to the N+1 backup principle to form DC operating power supply systems ranging from tens to hundreds of amps. Figure 1 shows the principle block diagram of a DC operating power supply system. This is a single busbar connection method, where the module output is connected in parallel with the DC bus and the battery bank. Normally, the battery is in a fully float-charged state. For DC operating power supply systems with separate control and power buses, there are two wiring methods: One method involves connecting all module outputs in parallel with the battery pack and power bus, and installing an automatic voltage regulator between the power bus and control bus. The load on the control bus is supplied by the power bus via the automatic voltage regulator, as shown in Figure 2. This method requires high reliability from the automatic voltage regulator. The other method divides the modules into two groups: one group's output is connected in parallel with the power bus and battery pack, and the other group's output is connected in parallel with the control bus. An automatic voltage regulator is installed between the power bus and control bus. Under normal conditions, the control bus load is supplied by the modules. The automatic voltage regulator is in standby mode due to reverse voltage. It only activates when there is an AC power outage or all modules on the control bus fail, as shown in Figure 3. This wiring method requires both groups of modules to be configured N+1 according to the load. 3. Input Power Factor of High-Frequency Switching Power Supply Modules Low input power factor was a common problem in early high-frequency switching power supply modules, mainly related to the circuit configuration used. In early high-frequency switching power supplies, the AC input voltage was directly applied across the filter capacitor after rectification. The rectifier diodes only started conducting when the AC input voltage was higher than the voltage across the filter capacitor. Therefore, the input current waveform was a very narrow pulse, resulting in severe harmonic distortion and a power factor typically only 0.6–0.7. This type of switching power supply module caused harmonic pollution to the power grid, creating a power hazard, interfering with other electrical equipment, and causing significant errors in measuring instruments. To reduce the pollution of the power grid by power supply devices, relevant EMI and EMC standards have clear regulations on the power factor and harmonic current values ​​of power supply devices of different power levels. Therefore, it is necessary to correct the power factor of high-frequency switching power supply modules. There are two basic methods for power factor correction: passive power factor correction (PFC) and active power factor correction (APFC). The passive power factor correction method involves adding a low-frequency inductor with a large inductance at the input terminal and reducing the capacitance of the filter capacitor to reduce the spike in the filter capacitor charging current. This method is relatively simple. However, the correction effect is not ideal, only reaching around 0.9 to 0.92, and is generally used in high-frequency switching power supply modules with three-phase input. The active power factor correction method involves adding a high-frequency inductor, a diode, a high-frequency switching transistor, and a corresponding controller to the input terminal to form a boost converter. The controller collects the AC input voltage and current signals and controls the switching transistor's on and off states, ensuring that the input current waveform always follows the input voltage waveform, achieving a power factor of over 0.99 and harmonic distortion of less than 5% for the high-frequency switching power supply module. 4. Current Sharing of High-Frequency Switching Power Supply Modules Unlike phase-controlled charging devices, the charging devices in DC operating power supply systems composed of high-frequency switching power supply modules generally adopt an N+1 redundancy backup method. Power distribution between parallel modules is achieved through a current sharing circuit. The balance of power distribution between modules mainly depends on the current sharing method. The load in a DC system includes two parts: the battery charging current and the control bus load current. The battery bank is in a float charging state for a long time, with a very small charging current. For lead-acid maintenance-free batteries, the float charging current is only about 0.0L of the rated capacity. Combined with a small control load, the entire charging device is under light load. When the high-voltage circuit breaker closes, the battery bank provides a closing inrush current. The charging device connected in parallel with the battery bank is in a current-limiting protection state due to excessive current. After the closing inrush current ends, the charging device replenishes 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 under both light and overload conditions, i.e., "full-range current sharing." If the current sharing characteristics are poor under light load, some modules may have no current output and operate in an unloaded state for a long time, seriously affecting the reliability of the modules. There are many current sharing methods for high-frequency switching power supply modules, such as: buck method, master-slave control method, external control method, automatic average current sharing method, and automatic maximum current sharing method. Considering the operating characteristics of DC system charging devices and the requirements for voltage/current regulation accuracy, we adopted an automatic average 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 is unlimited, enabling precise distribution of load current and current sharing across the entire load range. 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 a crucial 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 redundancy backup can effectively extend the mean time between failures (MTBF) of the charging device; on the other hand, it is essential to improve the MTBF (i.e., lifespan) of a single high-frequency switching power supply module. High-frequency switching power supply modules are composed of a large number of resistors, capacitors, power electronic devices, etc., arranged in a specific circuit configuration. During power conversion, a certain amount of power loss is always generated, and this power loss is usually dissipated as heat, causing the power supply module temperature to rise. Excessive temperature rise significantly affects the module's lifespan; the higher the module's operating temperature, the lower its performance and reliability, and the shorter its lifespan. Therefore, in addition to adopting a highly reliable circuit configuration, it is also necessary to select a suitable heat dissipation method to effectively reduce the temperature rise of the high-frequency switching power supply module and ensure its lifespan. Currently, high-frequency switching power supply modules used in DC power systems mainly employ two heat dissipation methods: forced air cooling and natural cooling. Forced air cooling offers advantages such as small module size, light weight, and low internal temperature, but disadvantages include higher noise levels, fan lifespan issues, and dust accumulation on circuit boards. Natural cooling is noiseless and eliminates fan lifespan concerns, but is characterized by larger size and higher cost. High-frequency switching power supply modules were first adopted in the communication power supply industry, and many technologies in high-frequency switching DC power supplies are derived from communication high-frequency switching power supplies. The heat dissipation methods for these modules largely follow those of communication power supplies, employing open-type air ducts for both forced and natural cooling. However, the working environment in substations is harsher than in communication equipment rooms, with high dust content, especially in newly built substations. Often, DC power supplies are put into operation prematurely before the civil engineering work is completed, due to the need for commissioning relay protection and other devices. Without effective dust prevention measures, large amounts of cement ash and other dust will accumulate on the circuit boards and components inside the power supply module, causing insulation degradation, short circuits, and ultimately, module failure. Dust forming on the circuit board of a high-frequency switching power supply module comes from two sources: dust drawn in by the fan and dust attracted by electrostatic discharge. To prevent dust accumulation, some switching power supply modules employ the following measures: Using dust covers: Installing dust covers at the module's air inlet provides some dust protection, but requires frequent cleaning; otherwise, the ventilation holes on the dust cover can easily become clogged, affecting ventilation and heat dissipation. This method is unsuitable for substations without on-site personnel. Using natural cooling: This avoids dust being drawn in by the fan, but for heat dissipation purposes, many ventilation holes must be opened on the module, thus still not solving the problem of electrostatic dust attraction. In the research and development of high-frequency switching power supply modules, Dongfang Electronics Information Industry Co., Ltd. comprehensively considered the advantages and disadvantages of forced air cooling and natural cooling, as well as the conditions at the substation site. The module's heat dissipation method adopts temperature-controlled forced air cooling and a closed-loop cooling duct. The fan is controlled by a temperature detection circuit; it only operates when the module's heat sink temperature is higher than the set value. Because the DC system's charging device operates under light load for extended periods, typically only at about 15% of its rated capacity, the fan does not operate when the heat sink temperature is lower than the fan's operating temperature. This heat dissipation method maintains a relatively stable internal temperature of the module, unaffected by changes in external environment and load, extending fan life by 2-3 times and thus improving the reliability of the high-frequency switching power supply module. Regarding dust prevention, a completely enclosed heat dissipation duct is used, allowing airflow to pass only through the surface of the heat sink, isolating the heat dissipation channel from the internal circuitry. This prevents dust accumulation on the circuit board while simultaneously improving heat dissipation, significantly enhancing the charging module's environmental adaptability. 6. Development Trends of DC Systems To ensure the safe operation of AC/DC power devices in substations, such as back-end systems, automatic devices, transmitters, communication equipment, and protection devices, in addition to the substation's DC system, UPS devices and dedicated communication power supplies are also required. Previously, these three different power supplies were installed separately, each with its own battery pack, resulting in high equipment costs, heavy maintenance, low reliability, and low resource utilization. With the widespread application of high-frequency switching power supply technology in DC systems, considerations have begun to be given to how to rationally utilize substation resources, reduce equipment costs and maintenance workload, and improve reliability. Currently, some substations are beginning to experiment with a combined approach: using a sinusoidal inverter to replace UPS equipment and a high-power DC/DC converter to replace the communication power supply unit. The inputs of both devices are directly connected to the DC system bus. When AC power is normal, the DC system's charging unit provides power to the inverter and DC/DC converter; when AC power fails, the DC system's battery bank provides DC power. The status information of the inverter and DC/DC converter is sent to the DC system's monitoring unit. Using this approach, at least the battery bank and monitoring unit in the UPS and communication power supply units can be eliminated. In terms of equipment management, only intelligent management of the DC system's battery bank is required, thereby reducing system maintenance. 7 Conclusion The charging unit of the high-frequency switching DC operating power supply system consists of multiple high-frequency switching power supply modules connected in parallel with N+1 redundancy backup. To reduce harmonic pollution to the power grid, the high-frequency switching power supply modules should have power factor correction functionality. 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 module's heat dissipation should isolate the heat dissipation channel from the circuit board to prevent electrostatic adsorption and dust accumulation on the circuit board caused by the fan. To make rational use of the substation's power supply resources, in the configuration of the DC operating power supply system, the power sine wave inverter and DC/DC converter are used to provide AC power for communication and DC power for DC power supply, respectively, reducing the maintenance workload and equipment costs of the substation's power supply equipment. This integrated approach will become one of the future development directions for DC operating power supply systems.