Anti-jamming technology to improve the functional quality of power grids - Power Circuits - Circuit Diagram

Brand AVX TPSE226M035R0125 Low impedance tantalum capacitor AVX 22

MOS power IC full range

Shunluo Inductance Power Inductor SWPA Series First Class Agent

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

SMD aluminum electrolytic capacitor

1 Introduction With the rapid development of power electronics (PE) technology, people have put forward higher and higher requirements for the reliability, safety and quality of power supply of power systems. However, there are a large number of non-linear loads and impact loads in the power grid, including chemical, metallurgical, coal mines and household appliances, especially high-power converter devices, thyristor rectifiers, electric arc furnaces, etc., resulting in transient shock and reactive power in the power grid. Problems such as higher harmonics and three-phase unbalance are becoming more and more serious, causing pollution to the power grid, increasing energy loss, and degrading the quality of power supply, which is not conducive to the safety and economic operation of power supply, supply and use equipment. In particular, the interference of higher harmonics has constituted a major "public hazard" affecting power quality in the current power grid. Therefore, solving the harmonic suppression and reactive power compensation of the power system and ensuring the quality of the power supply has become a hot topic of concern to everyone.

2 Harm of higher harmonics and requirements of modern control systems The voltage output from the three-phase alternator in the power system is basically sinusoidal, that is, there is almost no DC and higher harmonic components in the waveform. In the case of a fundamental wave, it is a symmetrical component, and the sum of the three-phase vectors is zero, and no electromagnetic field is formed externally. However, the harmonic current component is not zero due to the sum of the three-phase vectors, and can form a strong magnetic field, which has various harmful effects on the power grid.

2.1 Influence on power quality The nonlinear load is a harmonic source that injects harmonic current components that are multiples of the fundamental frequency into the grid. These harmonic currents create harmonic voltage drops on the grid, causing waveform distortion of the grid voltage and current, resulting in degradation of power quality.

2.2 Influence on the distribution network In the non-ferrous metal conductor, the distribution of the fundamental current can be approximated to be uniform throughout the cross section. When the harmonic current is passed, since the skin effect current concentrates on the thin surface of the conductor surface, the resistance of the harmonic current loop is increased, and the effective resistance of the conductor is increased, resulting in an increase in power loss and energy loss of the power grid. Higher harmonics can also cause voltage resonance in the power system, causing high voltage on the line, possibly breaking through the insulation of the line equipment.

2.3 Influence on power factor of power system Since the actual power factor of the equipment is less than the power factor of the equipment under ideal conditions, the higher harmonics increase the power consumed by the power equipment and reduce the power factor of the system.

2.4 The requirements of the variable frequency speed control system The frequency converter of the variable frequency speed control transmission system becomes an important part of the AC transmission because of its high efficiency and energy saving characteristics, but the rectifier bridge of the frequency converter is a non-linear load to the power grid, and its inverter Most of them use PWM technology. When working in switching mode and switching at high speed, a large amount of coupling noise will be generated. The EMI is serious, causing the inverter to operate in a harsh electromagnetic environment. The voltage and current on the input and output sides are more. Higher harmonics. Therefore, the inverter should be operated to prevent external interference, and to prevent it from interfering with the outside world, that is, to achieve so-called electromagnetic compatibility (EMC).

2.5 Requirements for Modern AC Motor Control Systems As the topology of new PE converters continues to emerge, the required computational and control functions are greatly increased. 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 extensive. However, the electromagnetic environment of PE systems and motor control systems tends to be complex, and because of the high operating frequency, the anti-jamming capability 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 higher harmonics

3.1 Installation of 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 non-linear load to be compensated, to provide a low-resistance path for the harmonics, and also to provide the required load. Power, this is the most common and practical method. The device utilizes an inductor and a capacitor energy storage element. According to the resonance principle, the higher harmonics that need to be eliminated are tuned by the filter circuit to cause resonance. In order to obtain the characteristic of minimum impedance at the time of resonance, the harmonics of the specified number of times are effectively eliminated, and the harmonic current is absorbed locally in the harmonic source accessory, so that it is not injected into the power grid. The device has the advantages of low investment, high efficiency and simple structure. It is reliable in operation and easy to maintain, and has low operating cost. It not only filters, but also performs reactive power compensation. Therefore, passive filters are an important means of suppressing harmonics and reactive power compensation that are widely used at present. However, the compensation characteristics of this method are affected by the impedance, frequency and operating conditions of the power grid. They can only suppress the fixed frequency harmonics of a certain number of times, and it is likely to amplify other subharmonics and overload the filter. Even burned. In addition, the LC filter circuit will cause a parallel resonance problem with the system due to changes in the system impedance parameters, with serious consequences and consequences.

3.2 Application of active power filter
APF is a new type of PE device that dynamically suppresses harmonics. The filtering method is: firstly detecting the harmonic current from the compensation object, and then using the controllable power semiconductor device (compensation device) to inject 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 harmonic of the power supply zero, achieving the purpose of real-time compensation of harmonics. It has been proved by experience that APF is an ideal and flexible feasible solution for suppressing harmonics and compensating for reactive power, which will be highlighted below.

4 Active Power Filter (APF)
APF is the most effective PE device for suppressing grid harmonics and compensating reactive power and improving grid power quality. Most APF topologies utilize voltage source inverters, and typically use capacitors as energy storage devices as shown in Figure 1. The DC voltage is converted to an AC voltage by appropriately strobing a controllable power semiconductor switch. Although a single pulse per half cycle can be applied to the composite AC voltage, pulse width modulation (PWM) is commonly used today for the dynamic performance required in most applications.

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