Grid connected inverter with harmonic suppression based on DSP control

Based on Grid Technology Based on distributed power generation system, and to achieve the output active power harmonic suppression for the purpose of designing a new type of DSP control inverter. This device uses DSP to detect harmonic current of power grid, the speed of DSP to ensure that the system has instantaneity, eliminates the influence of detection delay of the system, so as to realize the instantaneous detection of harmonics; at the same time, through the detection of DSP signal processing, finally realizes the control of the inverter in the launch of IGBT. The experimental results are given and no harmonics under two kinds of situations, this new device in the active power input power at the same time, also has a good inhibitory effect on the harmonic in the power grid. The results show that the device achieves the expected function, and has better stability and practicability.


Introduction
With the wide application of the distributed power generation system, the grid problem has attracted more and more attention, if it is idle to produce energy feedback to the grid, will ease the tense situation in the energy supply and demand. The basic requirements of the distributed power generation system of energy feedback is grid side power factor is 1 and the grid current distortion and phase with the grid voltage, the active power back to grid.
In fact, due to the widespread use of power electronic equipment, the harmonics in power system, the harmonic current of electric equipment of great harm, will reduce the electric power production, transmission and utilization efficiency, so that the electrical equipment overheating, vibration and noise, and the insulation aging and shorten life, and even failure or burned. Harmonics will cause disoperation of relay protection and automatic device, which will lead to confusion of energy metering [1]. If the distributed power generation system can realize the harmonic suppression in the output power at the same time, will greatly improve the quality of power, and makes it more practical and novel.

Structure analysis of grid connected inverter
The high frequency alternating current generated by the high speed permanent magnet generator is rectified to output the stable DC power, and then the DC power is converted into the power supply equipment by the grid connected inverter, or the idle energy is incorporated into the power grid. At present, the grid connected inverter can be divided into parallel, series, series parallel and hybrid according to its connection with the grid. Among them, the parallel inverter is technically mature, and it is a widely used active filter topology. Therefore, the commonly used voltage type three-phase grid connected inverter [2] is adopted. Fig. 1 shows the structure of voltage type three phase grid connected inverter. U is DC input C is voltage filter capacitor L is AC side filter inductance.

Principle analysis of harmonic detection
It is an important premise to realize harmonic suppression in real time to detect harmonics in the current of power grid, and it is also one of the difficult points to be solved in this system. In many harmonic detection methods, instantaneous reactive power theory [3] is the most widely used. It is calculated by the instantaneous active current and instantaneous reactive current p

Analysis of signal processing and control principle
After detecting the harmonic current, it is necessary to process the signal and then control the inverter.  i with the triangle wave, but the c i  is processed by the PI link and then compared with the triangle wave. Such a control system is designed based on the minimization of the c i  control.

System hardware design
The hardware circuit of the digital voltage type grid connected inverter adds a DSP link to the AC side of the voltage type three-phase grid connected inverter in Fig. 1. The switching device adopts independent IGBT, the upper bridge arm had HCPL-type 316J current protection control chip, under the bridge arm with over-current protection function of HCPL-type 3120 control chip, digital control using DSP2407, AC frequency three-phase power [5].

System software design
Because of the harmonic detection algorithm in complex and real-time requirement of the system is high, the general microprocessor, such as 51 series, 96 series speed cannot meet the real-time requirements. So, the previous algorithm consists mostly of hardware, although high real-time, but requires multiple hardware multiplier, the components of high precision, so the circuit is complex, and the cost is high. DSP is a dedicated processor high-speed real-time processing after the analog signal into a digital signal in the new DSP is more suitable for processing large amounts of data, has the performance of high speed, high resolution, strong operation ability etc. Therefore, using DSP to realize harmonic and reactive current detection and calculation can ensure real-time, and the system structure is simple. Fig. 3 shows the flow chart of the main program. As shown in the figure, the whole system sampling, closed-loop control and the algorithm is implemented in DSP2407, the digital control system based on DSP as the core, to complete the A/D sampling, transform and digital lowpass filter, and the filter harmonic inverse transform to obtain a command signal and control signal then, according to the control signal to calculate the corresponding PWM signals.

experimental results analysis
On the basis of the above theoretical analysis, a voltage type three-phase grid connected inverter based on DSP2407 control is designed with 6K W. After debugging in the local power grid, the external power grid is connected to the local power grid after the high power transformer, and the inverter is connected to the beginning of the local power grid. The electric equipment is electric furnace, Rectifier Bridge, electric lamp and so on. When there is no harmonic source in the electrical equipment s U , the device can return the electric energy to the grid when the power factor is 1; and when the harmonic source exists in the electric equipment, the device can output active power and realize harmonic suppression simultaneously. When the load is pure resistance load, its terminal 5 1234567890 ''""  Fig. 4 shows the experimental waveforms without harmonics.

Conclusion
The principle and control scheme of harmonic suppression and grid connected inverter are analyzed. The DSP implementation of harmonic suppression is focused on, and a three-phase grid connected inverter with harmonic suppression based on DSP control is designed. The experimental results show that the device is innovative, widely used, and has good stability.