Anti-interference technology to improve power grid functional quality - power supply circuit - circuit diagram

1 Introduction
With the rapid development of power electronics (PE) technology, people have put forward increasingly higher requirements for the reliability, safety and quality of power supply of power systems. However, there are a large number of nonlinear loads and impact loads in the power grid, including chemical industry, metallurgy, coal mining and household appliances, especially high-power converter equipment, thyristor rectifiers, electric arc furnaces and other loads, resulting in transient impact and reactive power in the power grid. Problems such as high-order harmonics and three-phase unbalance are becoming increasingly serious, causing pollution to the power grid, increasing energy losses, and deteriorating the quality of power supply, which is not conducive to the safe and economical operation of power generation, supply, and consumption equipment in the power system. In particular, the interference of high-order harmonics has constituted a major "public hazard" affecting power quality in the current power grid. Therefore, solving harmonic suppression and reactive power compensation in power systems and ensuring the quality of power supply have become hot topics of concern to everyone.

2 The harm of high-order harmonics and the requirements of modern control systems
The waveform of the voltage output by the three-phase alternator in the power system is basically a sine wave, that is, there is approximately no DC and high-order harmonic components in the waveform. As far as the fundamental wave is concerned, it is a symmetrical component, the sum of the three phase vectors is zero, and no electromagnetic field is formed externally. However, because the sum of the three-phase vectors is not zero, the harmonic current component can form a strong magnetic field and have various harmful effects on the power grid.

2.1 Impact on power quality
Nonlinear loads are harmonic sources that inject harmonic current components that are integral multiples of the fundamental frequency into the power grid. These harmonic currents produce harmonic voltage drops on the grid, causing waveform distortion of grid voltage and current, leading to deterioration of power quality.

2.2 Impact on distribution network
In non-ferrous metal conductors, the distribution of fundamental current can be approximately considered to be uniform throughout the entire cross-section. When harmonic currents pass through, the skin effect current is concentrated in the thin layer on the surface of the conductor, which increases the resistance of the harmonic current loop and increases the effective resistance of the conductor, resulting in increased power loss and energy loss in the power grid. High-order harmonics may also cause voltage resonance in the power system, causing high voltage on the line and possibly breaking down the insulation of line equipment.

2.3 Impact on the power factor of the power system
Since the actual power factor of the equipment is smaller than the power factor of the equipment under ideal conditions, high-order harmonics increase the power consumed by electrical equipment and reduce the power factor of the system.

2.4 Requirements of variable frequency speed control system
The frequency converter of the variable frequency speed control transmission system has become an important part of the AC drive due to its high efficiency and energy saving characteristics. However, the rectifier bridge of the frequency converter is a non-linear load for the power grid, and its inverter Most of them use PWM technology. When working in switching mode and performing high-speed switching, a large amount of coupling noise will be generated, and EMI is serious, causing the inverter to operate in a harsh electromagnetic environment. The voltage and current on its input and output sides contain more Higher harmonics. Therefore, when the frequency converter is running, it is necessary to prevent the outside world from interfering with it and prevent it from interfering with the outside world, that is, to achieve the so-called electromagnetic compatibility (EMC).

2.5 Requirements for modern AC motor control systems
As new PE converter topologies continue to emerge, the required amount of calculations and control functions have increased significantly. With the development of high-voltage and large-capacity PE devices, the application of DSP (digital signal processor) control technology will become more and more widespread. However, the electromagnetic environment of PE systems and motor control systems is often complex, and due to the high operating frequency, the anti-interference ability of DSP is usually weaker than that of microprocessors. Therefore, improving the anti-interference ability of DSP and peripheral circuits is closely related to ensuring the reliable operation of the system. The "purification" of the power grid is an important prerequisite for the development and application of modern PE systems and AC motor control systems.

3 Main indicators for suppressing high-order harmonics

3.1 Install AC filter device (passive filter)
In the power distribution system, the traditional method of harmonic suppression and reactive power compensation is to connect the passive power filter in parallel with the nonlinear load that needs to be compensated to provide a low level for harmonics. While blocking the path, it also provides the reactive power required by the load. This is the most common and practical method. The device utilizes inductors and capacitors as energy storage components. According to the principle of resonance, the high-order harmonics that need to be eliminated are tuned through the filter circuit to cause resonance. In order to obtain the characteristics of minimum impedance when resonating, it can effectively eliminate the harmonics of the specified order, and absorb the harmonic current locally near the harmonic source, so as not to inject it into the power grid. The advantages of this device are low investment, high efficiency and simple structure. It has reliable operation, easy maintenance, and low operating cost. It not only plays a filtering role, but also performs reactive power compensation. Therefore, passive filters are currently widely used as an important means to suppress harmonics and reactive power compensation. However, the compensation characteristics of this method are affected by the grid impedance, frequency and operating conditions. It can only suppress certain fixed frequency harmonics, but it is likely to amplify other harmonics and overload the filter. Even burned. In addition, the LC filter circuit will cause parallel resonance problems with the system due to changes in system impedance parameters, with serious impacts and consequences.

3.2 Application of active power filter
APF is a new type of PE device that can dynamically suppress harmonics. The filtering method is: first detect the harmonic current from the compensation object, and then use the controllable power semiconductor device (compensation device) to inject into the power grid the harmonic component (I or U) of the harmonic source with the same amplitude and opposite phase. The harmonic component (I or U) makes the total harmonics of the power supply zero, achieving the purpose of real-time compensation of harmonics. Experience has proven that APF is an ideal and flexible solution for harmonic suppression and reactive power compensation, which will be highlighted below.

4 Active power filter (APF)
APF is the most effective PE device to suppress grid harmonics, compensate for reactive power, and improve grid power supply quality. Most APF topologies utilize voltage source inverters, often with capacitors as energy storage devices as shown in Figure 1. The DC voltage is converted into AC voltage by appropriately gated and controllable power semiconductor switches. Although it is possible to apply a single pulse every half cycle to synthesize the AC voltage, for the dynamic performance required in most applications, pulse width modulation (PWM) is commonly used today.