Abstract: This paper introduces the development and application of a portable negative-sequence harmonic compensation device (NCRT) for three-phase four-wire system. The device adopts advanced harmonic detection and compensation technology, which can detect and compensate harmonic current in real time and improve power quality effectively. The excellent performance of the device in reducing harmonic current, improving power factor and reducing power loss is verified by field tests and application cases. The device has the advantages of portability, high efficiency and reliability, which is suitable for various power systems and electrical equipment, and is of great significance for improving the stability and reliability of power systems.
Keywords: harmonic compensation device; real-time detection; improve power quality
Content:
2. The principle of three-phase four-wire reactive harmonic compensation
3. Principle of output compensation current of portable reactive harmonic compensation device
3.1 Main circuit of portable reactive harmonic compensation device
3.2 Negative sequence current control in power grid
4. Device Experimental Application
Since the 1980s, the issue of power quality has gradually attracted widespread attention from the international community. Electricity, as an economical, practical, clean, easy-to-control and convertible energy form, is a special product provided by the power sector to power users. However, harmonic and reactive power problems are becoming increasingly serious. The existence of harmonics not only reduces the efficiency of power production, transmission and utilization, but also causes electrical equipment to overheat, generate vibration and noise, accelerate insulation aging, shorten service life, and even cause failure or burnout. Faced with such great harm caused by harmonics, measures must be taken to suppress the harmonics of the power system to ensure the safe operation of the power system and the reliable operation of various electrical appliances connected to the power grid. In recent years, scholars from various countries have conducted extensive research on harmonic issues. They have deeply explored the generation, propagation and control technology of harmonics through theoretical analysis, simulation calculation and experimental research. At present, harmonic control technology has made significant progress. This paper aims to deeply study the harmonic problem and its control technology.
2. The principle of three-phase four-wire reactive harmonic compensation
Take the public transformer area with a transformer capacity of 315kVA and a compensation capacity of 30% as an example.
The input reactive power range of the inverter is -7.5 ~ +7.5kVar, and the three-phase reactive power is independently controllable, which can balance and compensate the negative sequence current generated by the unbalanced load. The overall output reactive power range of the device is -7.5 ~ +97.5kVar, and the output current is continuously and steplessly adjustable within this range. Therefore, compared with the step-by-step switching capacitor bank method, the device has high reactive power control accuracy and fast adjustment speed, which can make the system have a higher power factor and more stable voltage, and can balance the three-phase power to make the three-phase current basically consistent, further reducing the system network loss and improving the system voltage stability.
3. Principle of output compensation current of portable reactive harmonic compensation device
3.1 Main circuit of portable reactive harmonic compensation device
The portable harmonic compensation device includes main circuit, drive circuit, current tracking circuit and instruction operation circuit. The main circuit adopts four-phase full-bridge voltage-type PWM converter; electrolytic capacitor is used to store DC side energy; the function of filter capacitor is to filter out DC side burrs; the function of incoming line inductance is to generate compensation current by the difference between the output voltage of PWM converter and grid voltage.
On the basis of TSC reactive power compensation, RNCT keeps the reactive power at PCC constant at 0 in most of the time. The shorter the switching interval of TSC, the better the reactive power compensation effect. When the switching interval of TSC is long, the reactive power of the system exceeds the compensation range of RNCT. At this time, RNCT outputs the maximum reactive power (-7.5kVar or +7.5kVar).
3.2 Negative sequence current control in power grid
After analysis, the RNCT output current only contains negative sequence components, but not positive sequence reactive components. After installing RNCT, it only contains positive sequence active components, but not negative sequence components and positive sequence reactive components. Among them, the three-phase currents of the system are completely equal, and the rated current is about 0.6A, and the maximum current of the system is reduced by 40%. This shows that after installing RNCT, the current of the largest phase in the three-phase system can be transferred to the other two phases, thereby making the three-phase currents equal. This three-phase unbalanced current balancing method reduces the loss of transformers and cables by reducing the maximum current of the system, that is, reducing the system line loss.
4. Device Experimental Application
A three-phase four-wire portable reactive harmonic compensation device was developed and the compensation performance was tested. The following data is the effect of the active power filter working in two states to compensate the load under the same condition. Before compensation, the load current contains a large number of 2nd to 50th harmonic components, which causes the current waveform on the grid side to be seriously distorted.
