Research On Method Of Harmonic Compensation Based On Digital Technology

With the constant improvement of industry, agriculture and living standards of the people,the need of power supply is increasing fleetly. And at the same time the criterion of quality and reliability of power supply is also becoming more and more strict.People are paying more attention to the electric quality. However, most of electric equipment such as frequency conversion air-condition are non-linear load.And they are the source of harmonic.The public electric wire netting will be populated inevitably if current with harmonic is injected into it. And this will result in a lot of damage as: Influence the work of the voltage transformer; Influence the dependability of relay protection; Accelerate the aging of metallization membrane condenser; Increase consumption on electricity transmission etc.Two kinds of countermeasures to the above-mentioned problems come into being with the development of electric and electronic technology. One way is to connect a special-purpose power changer paratactically in the end of the public load of the main electric wire netting. And make compensation to the harmonic of the current. This is the so called active filter. Another way is to put power transform equipment between the rectifier and capacitance of the electronic device itself, this is the active power factor correct circuit. With the fast development of the digital signal processor (DSP), It can already meet the requirement of real-time control, especially can adjust control parameters, such as gaining and bandwidth through software. Thus it will make the debug to the system more convenient and will not be influenced by noise of the circuit. Distortion in the transmittance process of analog signal will also be preventedThe digital power factor correction and harmonic compensation are"multiplier" correct technology which is formed by 2 control loops,namely voltage loop and current loop. The voltage loop controls the output voltage through regulating on the average input current, while the loop of current controls the inductance current to make it follow the input voltage waveform.The scheme is based on the boost-circuit. The power stage devices includes: Inductance, capacity, diode, IGBT and etc. The object of the design of control loop is to calculate the PI parameter of the voltage loop and current loop. The first step of is to calculate the loop gain equation, the second step is to analyse the high frequency approximation of the power stage. And in the third step we get the PI parameters.The simulation of system is based on MATLAB/SIMULINK.Adopt to the top-down design method. The main block diagram includes: voltage source, rectification bridge, inductance, capacity, load, IGBT switch , current and voltage sensor, pi controls block ,pwm block and etc. Among them the PI controls block is formed by voltage loop"DoVoltageRegulation"and current loop"DoCurrentControl4Hcm".After all the modules are designed and connected. The dynamic simulation will be carried on. Through it the dynamic characteristic of system can be observed. The rationality and feasibility of the structure will also be evaluated. The main content of analyse is: input current waveforms, FFT of the input current, signal of control loop and power stage devices.The scheme of hardware is based on dsp-TMS320x2406. The main reason for selecting 2406 for use is the abundant resources of it. It make a possibility to put harmonic compensation and motor control into one system finally. The harmonic compensation model consist 5 parts: power circuit, cpu peripheral circuit, the signal measures the circuit, drive and protection circuit and communication circuit. The key device of the protection circuit is the gate drive IC IR2121 with current detection. The protection circuitry detects over-current in driven power transistor and limits the gate drive voltage. And at the same time the corresponding pin outputs the signal of protecting. Though the optical-coupled isolation circuit the signal will be transmitted to 2406.The writing and debugging of software is finished in CCS2. The cpu resources witch is used include analog-to-digital converter, event manager, and system of interrupt. The main function is realized in the interrupt trigged by the AD complete .Two of them are"DoCurrentControl4Hcm"of current loop and"DoVoltageRegulation"of voltage loop.Finally after experiment we analyse the influence of the harmonic compensation system on: input current waveform, output voltage waveform and the harmonic of input current. The result of experiment testified the correctness of above-mentioned and the parameter which is designed .It is convenient to adjust parameters of the controlling algorithm in the course of debugging. This embodies the fine performance and flexibility of the digital control system. Meanwhile, There is still expansion space for the whole system. With abundant resources of cpu, harmonic compensation and motor control will be put into one system, and work in phase. There will be very wide prospect on the field of motor control.