Characteristics of high-frequency switching power supplies and their applications in power systems

In the power system, DC power supply is used as power supply for relay protection, automatic devices, control operation circuits, lighting and audio signals, and emergency lighting. It is an important equipment in power plants and substations. Accidents caused by DC power failures occur from time to time, so there are high requirements for the reliability and stability of DC power supply. Most traditional DC power supplies use thyristor rectifiers. In recent years, many DC power supply manufacturers have launched intelligent high-frequency switching power supplies. This power supply system has many advantages: safety, reliability, high degree of automation, smaller size and weight, high comprehensive efficiency, and low noise. It meets the needs of power grid development and is worth promoting.

At present, the DC power supply used in my country's power system is gradually changing from traditional phase-controlled power supply to modular high-frequency switching power supply. The working principle of the high-frequency switching power supply rectifier: the AC power supply is connected to the rectifier module, and becomes DC after filtering and three-phase full-wave rectifier, and then connected to the high-frequency inverter circuit to convert DC into high-frequency AC, and finally outputs stable DC after passing through the high-frequency transformer, rectifier bridge and filter. This high-frequency switching power supply is mainly composed of a high-frequency switching charging module, a centralized monitor and a battery pack, among which the charging module and the centralized monitor have a built-in microprocessor and a high degree of intelligence. When the high-frequency switching power supply system is operating normally, the output of the charger runs in parallel with the battery pack to supply power to the regular load, and at the same time, the battery is float-charged to supplement the self-discharge of the battery. When the AC power input is interrupted, the battery pack supplies power to the load to ensure continuous and uninterrupted power supply to the load. When the AC power returns to normal, the system automatically charges the battery evenly and quickly replenishes the energy after the battery is discharged in large quantities.

Compared with traditional phase-controlled power supplies, the main technical indicators of intelligent high-frequency switching power supplies are 1 to 2 levels better than those of departmental standards, and they have the following characteristics.

(1) The phase-controlled power silicon rectifier adopts 1+1 master-slave backup mode, while the high-frequency switching power supply adopts N+1 module redundant parallel combination mode for power supply. That is, if the output current of N modules can meet the charging current requirements, N+1 modules are used for even distribution. Therefore, the system operation reliability can be improved. When individual modules fail, they can be replaced with power on, which will not affect the normal operation of the system and is convenient for expansion and maintenance.

(2) When the thyristor rectifier is operated in floating charge mode, the ripple coefficient of the DC output is large, and accidents such as malfunction of the central signal device and erroneous signal transmission of high-frequency relay protection have occurred. According to the requirements issued by the ministry, the ripple coefficient should not exceed 2%. In addition, when the thyristor rectifier is operated in parallel with the battery, if the ripple coefficient is large, if the floating charge voltage fluctuates or is low, the battery will experience pulsating charging and discharging, which is not good for the battery. The charging device of the high-frequency switching power supply uses multiple intelligent modules in parallel to provide power, which significantly improves the power supply quality and technical parameters. The module uses quasi-resonance technology (or pulse width modulation technology) and current and voltage dual-loop control technology to increase the switching operating frequency, reduce the opening loss, and the output voltage ripple coefficient is very small, generally ≤±0.1% of the rated voltage, thereby preventing the battery from pulsating charging and discharging, extending the battery life, and increasing reliability.

(3) The high-frequency switching power supply rectifier module has a built-in microprocessor, which is the basis for improving the level of equipment management. Under the premise that the DC system fault signal should be as perfect as possible, the wiring is simple and the installation and debugging are fast. In addition to being able to directly display the output current and voltage and various operating conditions of the module on the panel, it can also communicate with the automation network of the power system or the DC class monitoring system of the substation area through the monitoring module to conduct remote monitoring and various operations on the module, realizing the four remote functions. Traditional DC power supplies are generally equipped with current, voltage meters and other special devices on the panel cabinet to monitor the equipment, and these measured values ​​cannot be remotely monitored through the communication port (except for microcomputer models). Even if there is telemetry, it also adopts DC sampling method, with few sampling points, and the signals reflecting various operating conditions are also connected to the light-emitting sign or remote signal screen in the form of contacts. Therefore, the wiring is cumbersome, the degree of automation is low, and it is difficult to realize remote control and remote adjustment functions.

(4) According to the requirements issued by the ministry, the error of the current stabilization accuracy during charging is ≤±5%, and the error of the voltage stabilization accuracy during floating charge is ≤±2%. The high-frequency switching power supply has higher accuracy of voltage and current stabilization, and its error is generally ≤±0.5%, which can avoid overcharging or undercharging the battery and ensure that the battery operates in the best state. Valve-regulated batteries have large capacity, low maintenance, and low discharge rate, and are suitable for large-capacity DC power supplies. From the perspective of principle performance, high-frequency switching modules are suitable for use with valve-regulated batteries.

(5) The high-frequency switching power supply rectifier module has an automatic current equalization function in parallel operation mode. At the same time, it is equipped with overcurrent, overvoltage and instantaneous short-circuit protection, safe and reliable lightning protection measures, and can effectively withstand the output short-circuit impact. In addition, multiple effective measures are taken to prevent high-frequency power supply and harmonics from interfering with the AC power grid side.

(6) The high-frequency switching power supply has a high overall conversion efficiency. The conversion efficiency of most manufacturers reaches more than 90%, while the conversion efficiency of the phase-controlled power supply is generally only 60% to 80%.

Since this power supply system is equipped with a microcomputer-based centralized monitor, it can support multiple communication protocols and communicate with the DC class monitoring system of the dispatching center or substation area to perform four-remote monitoring of the DC system. The main functions are:

(1) Communicate with the monitoring center through the MODEM and telephone network, and read the information of the high-frequency switching power supply from the communication port;

(2) Measure the output current and voltage of the module, the DC bus current and voltage, the output current and voltage of the power supply, the battery charge and discharge current and voltage, etc.;

(3) Control the output current and current stability of the power supply, control the power on and off, etc.;

(4) Control the high-frequency switching power supply to realize automatic conversion between floating charge and equalization charge of the battery;

(5) Control the automatic or manual switching of the silicon chain to ensure the voltage stabilization accuracy of the control bus, thereby ensuring the reliability of the power supply for the microcomputer and transistor protection and preventing false operation of the protection;

(6) Adjust the charging current limit value and the total output current regulated value;

(7) It has local and remote control modes, and uses passwords to allow or prohibit operations to enhance system operation reliability.

At the same time, this system is equipped with a special microcomputer insulation monitoring device, which can display the bus voltage and the insulation resistance of the positive and negative bus to the ground in real time and issue abnormal alarms, inspect the insulation condition of each feedback line, and indicate the specific circuit where the fault occurs. This line selection function greatly facilitates the search for DC grounding.

At present, my country is vigorously implementing unmanned management of substations. Therefore, the selection of equipment will develop in the direction of miniaturization, oil-free, low maintenance or maintenance-free, and high degree of automation. High-frequency switching DC power supply can meet this requirement. After several years of operation test, the performance of this product has gradually matured and stabilized. With superior technical performance and good price-performance ratio, high-frequency switching power supply will become the first choice for DC power supply replacement.