How to prevent and solve the impact of harmonics on high-frequency switching power supplies

In the power system, DC power supply is used as relay protection, automatic devices and primary and secondary equipment operating power supply, and is a very important equipment in power plants and substations. In recent years, accidents caused by DC power failures in the system have occurred from time to time, so there are very high requirements for the reliability and stability of DC power supply. Most traditional DC power supplies use thyristor rectifier type. With the maturity of high-frequency switching power supply technology, high-frequency switching power supplies have gradually begun to replace traditional silicon rectifier chargers in power systems. High-frequency switching power supplies are widely used in power plants, substations, industrial production, transportation and other DC systems and related supporting devices due to their small size, light weight, high efficiency and reliable operation. They are the power core for the normal operation of low-voltage equipment such as circuit breaker opening and closing electricity, backup battery charging and secondary circuit instruments.

Working principle of high frequency switching power supply

The AC power supply is connected to the rectifier module, and is converted into 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 a high-frequency transformer, rectifier bridge and filter.

The high-frequency switching circuit is mainly composed of a rectifier and filter circuit, a full-bridge conversion circuit, a PWM control circuit, a voltage stabilization and voltage limiting circuit, a current stabilization and current limiting circuit, a protection circuit, and an auxiliary power supply circuit.

The three-phase grid (or single-phase) voltage is rectified and filtered after passing through the power switch, and the resulting smooth DC voltage of 520Vdc (300Vdc for single-phase) is supplied to the inverter circuit.

The inverter circuit is mainly composed of a full-bridge conversion circuit consisting of a high-power IGBT module (or field-effect MOSFET module). When the PWM output control signal drives the power modules through the isolation driver, the two diagonal tubes are turned on alternately, generating a high-frequency pulse voltage at the primary of the high-frequency transformer, and the secondary voltage is transformed by the high-frequency transformer and rectified to provide energy to the load.

The output end is connected with feedback circuits such as voltage stabilization, current limiting, current stabilization, and voltage limiting. When placed in the voltage stabilization state, the voltage stabilization and current limiting circuits work. When the output voltage rises or falls, the sampled voltage is compared with the reference voltage through the voltage comparator inside the voltage stabilization circuit, and its error signal voltage is added to the PWM control circuit to make the PWM output pulse width change accordingly, thereby stabilizing the output voltage. If the load current is too high, the current limiting circuit works to limit the output current within the current limiting setting value.

Similarly, in the steady current state, the steady current circuit works to stabilize the output current within the set value, and when overvoltage occurs, the voltage limiting circuit clamps the output voltage at the voltage limit value. When there is an abnormal situation (such as input overvoltage or undervoltage, overcurrent or overheating, etc.), a protection signal is generated and added to the protection control circuit. The protection circuit outputs a voltage and adds it to the PWM circuit, causing the PWM circuit to stop outputting, thereby achieving the protection purpose. Sources of harmonics in power systems

There are many sources of harmonics in power systems. The main ones are as follows:

⑴ Various nonlinear electrical equipment in the system, such as: converter equipment, voltage regulator, electrified railway, arc furnace, fluorescent lamp, household appliances and various electronic energy-saving control equipment. Even if these devices are supplied with ideal sinusoidal voltage, the current they use is nonlinear, that is, harmonic current exists. And the harmonic current generated by these devices will also be injected into the power system, causing harmonic components to be generated in the voltages of various parts of the system. The harmonic content of these devices is determined by their own characteristics and working conditions, and is basically unrelated to the power system parameters, and can be regarded as harmonic constant current sources.

⑵ The nonlinear elements in the power supply system itself are another source of harmonics. These nonlinear elements mainly include transformer excitation branches, thyristor control elements of AC/DC converter stations, thyristor-controlled capacitors, reactor groups, etc.

⑶ For example, fluorescent lamps, household appliances, etc. have small individual capacity, but are large in number and scattered everywhere, and are difficult for the power department to manage. If the current harmonic content of these devices is too large, it will have a serious impact on the power system. The current harmonic content of such equipment should be limited to a certain range when it is manufactured.

⑷ Harmonic potential generated by the generator. When the generator generates harmonic potential, it will also generate harmonic potential. The harmonic potential depends on the structure and working condition of the generator itself, and is basically independent of the external impedance. Therefore, it can be regarded as a harmonic constant voltage source, but its value is very small.

The appearance of harmonics in the power system is a kind of "pollution" to the operation of the power system. They greatly reduce the quality of the sinusoidal waveform of the system voltage and also have a great impact on the high-frequency switching power supply .

Examples of the impact of harmonics on high-frequency switching power supplies

⑴In October 2008, a power supply company replaced the No. 1 main transformer in its 110kV substation.

The power supply of the charger of this station is normally supplied by the main transformer used by 1#, and the 10kV busbar and low-voltage busbar are both in the disconnected position. Since the 10kV I section busbar does not carry the steel plant load, the high-frequency charger operates normally. During the replacement of the 1# main transformer, the 10kV load and low-voltage load of the entire station were all carried by the 2# main transformer, and the 10kV II busbar carried two steel plant loads, and the steel plant was not equipped with a detuning device. When it was reversed to this mode and operated for less than 10 minutes, two high-frequency power modules of the substation burned out, and other 10kV users also reported that their high-frequency power supplies burned out.

(2) In 2006, a large aluminum company under a 220kV substation of a power supply company shut down all its filter devices for harmonic testing. In less than 2 hours, all 6 high-frequency modules of the station's 2# high-frequency charger burned out. Analysis and countermeasures of the impact of harmonics on high-frequency switching power supplies

Since the intelligent DC system based on computers and microprocessors is usually installed near the high-voltage equipment in the substation, the prerequisite for the normal operation of the equipment is that it can withstand the extremely strong electromagnetic interference generated in the substation during normal operation or accident conditions. In addition, since modern high-voltage switches are often integrated with electronic control and protection equipment, the equipment that combines strong and weak current equipment not only needs to be tested for high voltage and high current, but also needs to pass the electromagnetic compatibility test. When the GIS disconnector is operated, it can generate fast transient voltages with a frequency of up to several MHz. This fast transient overvoltage will not only endanger the insulation of equipment such as transformers, but also spread outward through the grounding network, interfering with the normal operation of the DC system and control equipment of the substation. With the improvement of the automation level of the power system, the importance of electromagnetic compatibility technology has become increasingly apparent. Therefore, the high-frequency switching power supply must have a strong ability to resist electromagnetic interference, especially the ability to adapt to lightning strikes, surges, and grid voltage fluctuations, and it must also have sufficient anti-interference ability to electrostatic interference, electric fields, magnetic fields, and electromagnetic waves, to ensure its normal operation and the stability of the power supply to DC equipment.

On the other hand, serious harmonic voltage and current generate electromagnetic interference inside the switching power supply, which causes instability in the internal operation of the switching power supply and reduces the performance of the power supply. Some electromagnetic fields radiate to the surrounding space through the gaps in the switching power supply housing, and propagate through space together with the radiated electromagnetic fields generated by the power lines and DC output lines, causing interference to other high-frequency devices and devices that are sensitive to electromagnetic fields, causing other devices to work abnormally. Therefore, for high-frequency switching power supplies, it is necessary to limit the conducted interference generated by the load lines and power lines, as well as the electromagnetic field interference propagated by radiation, so that devices in the same electromagnetic environment can work normally without interfering with each other.

The electromagnetic compatibility issues caused by high-frequency switching power supplies are quite complicated because they work in a high-voltage and high-current switching state. From the perspective of the electromagnetic compatibility of the entire machine, there are mainly common impedance coupling, line coupling, electric field coupling, magnetic field coupling and electromagnetic wave coupling. The three elements of electromagnetic compatibility are: interference source, propagation path and interfered object. Magnetic field coupling is mainly the coupling of the low-frequency magnetic field generated near the high-current pulse power line to the interference object. Electromagnetic wave coupling is mainly due to the high-frequency electromagnetic waves generated by the pulsating voltage or current, which radiate outward through space and produce coupling to the corresponding interfered object. In fact, each coupling method cannot be strictly distinguished, but the emphasis is different.

In a switching power supply , the main power switch tube works in a high-frequency switching mode at a very high voltage. The switching voltage and switching current are both square waves. The spectrum of the high-order harmonics contained in the square wave can reach more than 1000 times the square wave frequency. At the same time, due to the leakage inductance and distributed capacitance of the power transformer, as well as the non-ideal working state of the main power switch device, high-frequency and high-voltage peak harmonic oscillations are often generated when the high-frequency is turned on or off. The high-order harmonics generated by the harmonic oscillation are transmitted to the internal circuit through the distributed capacitance between the switch tube and the heat sink or radiated to the space through the heat sink and the transformer. The switching diode used for rectification and freewheeling is also an important cause of high-frequency interference. Because the rectifier and freewheeling diodes work in a high-frequency switching state, due to the parasitic inductance of the diode leads, the existence of junction capacitance and the influence of reverse recovery current, it works at a very high voltage and current change rate, generating high-frequency oscillations. Because the rectifier and freewheeling diodes are generally close to the power output line, the high-frequency interference they generate is most likely to be transmitted through the DC output line.

In order to improve the power factor, switching power supplies all use power factor correction circuits. At the same time, in order to improve the efficiency and reliability of the circuit and reduce the electrical stress of the power devices, soft switching technology is widely used. Among them, zero voltage, zero current or zero voltage and zero current switching technology is the most widely used. This technology greatly reduces the electromagnetic interference generated by the switching devices. However, the soft switching lossless absorption circuit mostly uses L and C for energy transfer and uses the unidirectional conductivity of the diode to achieve unidirectional energy conversion. Therefore, the diode in the resonant circuit becomes a major source of electromagnetic interference.

To solve the harmonics of switching power supply, we can start from the following three aspects:

⑴Reduce the interference signal generated by the interference source;

⑵Cut off the propagation path of interference signals;

⑶ Enhance the anti-interference ability of the interfered object.

Through the above research and analysis of electromagnetic compatibility, we know that we must first improve the input and output filter circuits to address the problems of power line harmonic current, power line conducted interference, electromagnetic field radiation interference, etc. The connection method of the output rectifier diode and the position of the filter circuit are adjusted to make the filter circuit closer to the port. The insulation withstand voltage level of the input EMI filter of the power supply is increased to cut off the propagation path of the interference signal.

Secondly, for electrostatic discharge, TVS tubes and corresponding grounding protection are used in the small signal circuits of the current sharing port and the control port, and the electrical distance between the small signal circuit and the housing is increased. Fast transient signals contain a wide spectrum and are easily transmitted into the control circuit in a common mode. The same anti-static method is used to reduce the distributed capacitance of the common mode inductor and strengthen the common mode signal filtering of the input circuit, that is, add common mode capacitance to improve the anti-interference performance of the system. Lightning strikes and surges are important factors that cause devastating damage to switching power supplies and their systems. Therefore, it is also necessary to optimize the lightning protection capabilities of the AC input and DC output ports. For the combined lightning waveform of 1.2/50μs open circuit voltage and 8/20μs short circuit current, the energy is small, so a zinc oxide varistor and a corresponding absorption circuit combination method are used to solve it. In order to ensure the safe operation of the system, common mode and differential mode combined surge suppressors are also equipped on the system's AC incoming line and DC output bus.

Reduce the internal interference of the switching power supply , realize its own electromagnetic compatibility, and improve the stability and reliability of the switching power supply, mainly by appropriately increasing the distance between adjacent lines and adjacent pins to avoid crosstalk and mutual discharge when high voltage is connected. Reduce the area surrounded by high-voltage and high-current circuits, especially the primary side of the transformer and the switch tube, power supply filter capacitor circuit; reduce the area surrounded by the output rectifier circuit and the freewheeling diode circuit and the DC filter circuit, thereby reducing the influence of the transformer leakage inductance and the distributed capacitance of the filter inductor on the switching power supply, and improving the stability of the internal operation of the switching power supply. It has good stability and electromagnetic compatibility, and is suitable for use in DC systems with DC operating power supplies or stand-alone power supplies.