Design of hybrid power filter based on RCP

Abstract: In order to solve the problems of long design cycle and high investment cost of digital controller in power system harmonic control, a fast control model is designed based on the study of traditional power filters. An active power filter is added to the traditional filter structure, and the hybrid active power filter is debugged online by using computer-aided software Simulink and TI's DSP development environment CCS, which accurately reduces the harmonic components. The simulation results show that after passing through the filter, the grid voltage distortion factor is reduced to 1.88% (lower than the IEEE-519-1992 standard). The device has been operated in the 35 kV power grid of Wuhan Iron and Steel arc furnace. The operation results show that the device has high reliability and significant filtering effect, and has good engineering application value.

At present, power filters are mostly implemented with digital controllers, which requires engineers to have high software programming capabilities. In this way, most of the filter design cycle will be spent on program writing and optimization. Considering the establishment of mathematical models, algorithm design, and offline debugging, the entire development time will be very long and the cost will increase accordingly.

The design of Rapid Control Prototyping (RCP) reduces the design cycle. By using Simulink's graphical programming method, complex program writing is no longer necessary. For hardware engineers, on-site debugging can be achieved by changing model parameters. For theoretical researchers, they only need to consider the speed and practicality of the algorithm.

Wavelet transform is a fast and effective method for analyzing non-steady-state voltage and current waveforms. Like FFT, wavelet transform decomposes the signal into frequency components. However, discrete wavelet transform (DWT) has variable frequency resolution, which can effectively solve the voltage flicker caused by load mutation, and can track harmonics in real time. This is a useful feature for analyzing transient signals. In addition, wavelet analysis does not need to be performed simultaneously in the entire frequency domain, and the calculation amount is concentrated in a certain frequency range, which reduces the amount of calculation and speeds up the analysis.

This paper models the hybrid active power filter (HAPF) based on Simulink software, uses the Wavelet toolbox to perform harmonic analysis and simulation, generates DSP code from MATLAB/Simulink/Embedded Target for TI C2000, and finally implements the hardware on TMS320F2812 . 1 Rapid Control Model (RCP)

RCP consists of two parts: computer-aided design software Simulink and proprietary hardware TMS320F2812 with real-time operating system, as shown in Figure 1. This graphical programming method replaces the writing of traditional programs and only requires engineers to focus on optimizing functions and performance. The complete system proposed in this paper is carried out in a simulation environment.

Figure 1 Components of RCP

Embedded Target for TI C2000 connects software and hardware. The Simulink toolbox provides various models required in this article, providing an integrated platform for designing, simulating and implementing embedded control systems on general-purpose DSPs. Figure 2 shows the design process.

Figure 2 Design flow chart

By using Embedded Target, efficient DSP code can be generated through CCS (Cede Composer Studio). By connecting the host and DSP through their interfaces, the DSP can be controlled and optimized online. For complex algorithms that require cyclic calculations, the fast execution function of RCP will show great advantages. In view of the prospect of wavelet transform analysis of power system harmonics and the convenience of modeling, the active part control algorithm of this filter uses wavelet transform to analyze power grid harmonics.

2 Wavelet Analysis

2.1 Multi-resolution decomposition method

The implementation of wavelet analysis usually adopts the multiresolution signal decomposition method (Multiresolution Signl Decomposition, MSD), and the high-pass filter h and the low-pass filter g are constructed by wavelet functions respectively, as shown in Figure 3.

Figure 3 Implementation of signal multi-resolution decomposition method by wavelet analysis

Scale 1 in Figure 3 contains information from the Nyquist frequency to 1/4 sampling frequency, scale 2 contains information from 1/4 to 1/8 sampling frequency, and so on for other scales. Wavelet decomposition can be terminated at any scale, and the final smoothed output contains information on all remaining scales. However, the number of decomposition layers of a signal is not arbitrary. A signal of length N can only be decomposed into log2N layers at most.

2.2 Wavelet transform

The wavelet transform of a continuous signal f(t) is defined as:

Among them, is the mother wavelet, a is the expansion factor, and b is the translation factor. Whether it is stretched or contracted in the time domain depends on a.

In discrete wavelet transform, some wavelet coefficients m and n are given, which depend on the order of the scaling factor and the translation factor. Then the discrete wavelet coefficients can be expressed as:

Although this transform is continuous in time, the wavelet form is discrete. The inverse discrete wavelet transform is as follows:

Formula (3): K = (A + B) / 2, A and B are the maximum values ​​(frame values) of a and b respectively.

The choice of mother wavelet is different for different problems, and the selection of mother wavelet has a great influence on the resulting structure. Orthogonal wavelets ensure that the signal can be reconstructed from its transform coefficients, wavelets with symmetric filter coefficients can produce linear phase shifts, and the wavelet group derived by Daubechies covers the field of orthogonal wavelets.

2.3 Model implementation of control algorithm

The Simulink toolbox provides a rich set of mathematical models, from which we select C28xADC, C28x PWM, F2812 eZdsp (DSP code cannot be generated without this module), DWT, and IDWT to form a model as shown in Figure 4.

Figure 4 Control algorithm model including wavelet transform

The Wavelet subsystem integrates Environment Control, Buffer, DWT and IDWT modules to perform harmonic analysis on the sampled and quantized signals and generate compensation voltage command signals, which then control the IGBT shutdown through PWM output signals to reduce harmonics and reactive power compensation. During the simulation, the duty cycle of the C28x PWM is adjusted in real time as needed to generate a suitable output waveform.

3 Hybrid Active Power Filter Modeling

3.1 Hybrid Active Power Filter

At present, LC resonant passive filters (PPF) are mainly used for high-voltage and large-capacity harmonics. These filters also have reactive power compensation functions. Although PPF has the advantages of small initial investment and high operating efficiency, the filtering effect of PPF is greatly affected by the impedance of the power system, and it can only eliminate harmonics of a specific order. The filtering effect is not good for loads with frequently changing harmonic orders.

It may also resonate with the system, causing the LC filter to overload or even burn out. The active power filter (APF) is equivalent to a variable resistor, with a fundamental impedance of 0 and a high impedance state for harmonics. Although APF can overcome the defects of PPF, its installation capacity is limited by the capacity of the switching device. The passive filter and
the active filter are combined to form a hybrid active power filter (HAPF). The active power filter is only used to improve the filtering effect of the passive filter and suppress possible resonance. In this way, the active power filter does not bear the fundamental voltage of the AC power supply, so the device capacity is greatly reduced, usually only about 1/10 of the total capacity of the nonlinear load is required, so that the active power filter can be used in high-power occasions.

Large power supply and distribution stations usually want to filter out harmonics while performing reactive power compensation, which will inevitably increase the technical difficulty and cost of inverter implementation, thus limiting the application of active power filters in large substations. By coupling the inverter output voltage to the inductor and capacitor of the filter branch of the passive filter through a transformer, the active power filter will neither bear the fundamental voltage nor the fundamental current, thereby greatly reducing the capacity of the active power filter.
3.2 Control system structure

In the past, the control part of the active power filter was composed of an industrial computer and a single-chip microcomputer. The industrial computer realizes harmonic detection, analysis, and control signal calculation, while the single-chip microcomputer generates control signals. Due to the processing speed of the single-chip microcomputer, this paper integrates signal sampling, harmonic analysis, and PWM pulse width signal generation in TMS320F2812 to give full play to the computing efficiency of the 32-bit DSP. The control circuit structure is shown in Figure 5.

Figure 5 Control circuit structure

The zero-crossing point of the A-phase voltage is selected as the initial value, and the three-phase current is is measured by the Hall sensor after the initial moment, and the measured value is sent to the DSP. After high-speed A/D conversion, the sampling value is obtained, and then the sampling value is subjected to discrete wavelet transformation to obtain the fundamental value is1 of the three-phase current. The fundamental value is subtracted from the sampling value of the three-phase current to obtain the three-phase harmonic current value ish that the active power filter needs to compensate, and the command signal U=KIsh of the active power filter output compensation voltage can be obtained. Then, the inverter is controlled by the PWM module of the DSP to obtain the desired voltage waveform.

3.3 Hybrid Active Power Filter Simulation Model

The powerful Simulink toolbox includes all the algorithms and peripherals of the C2000 DSP series involved in this paper, which will undoubtedly provide convenient conditions for the simulation design of the controller. The hybrid active power filter model is shown in Figure 6.

Figure 6 Hybrid active power filter model

The three-phase AC voltage source 35 kV, 50 Hz, 500 kVA simulates the power grid and is stepped down to 400 V, 50 Hz through a transformer. The inverter output voltage of the active filter is coupled to the inductance and capacitance of the filter branch of the passive filter through a transformer to reduce the capacity of the active power filter, as shown in Figure 7. B1 and B2 are measuring instruments, respectively, and the nonlinear load consists of an asymmetric rectifier.

Figure 7 Active filter model

4 Experimental Results

DC bus capacitance:

Among them, the rated voltage of the capacitor is Vn=Vc/1.83, the apparent power of the distribution line is Sn=S*n/0.087, and S*n is the power of the capacitor at f=50 Hz.

Minimum filter capacitance:

Where, is the current standard value of the nth harmonic, is the voltage fundamental standard value.

Then, according to formula (6), the filter inductance is obtained:

ωs is a certain sub-angular frequency. According to the above formula, the parameter values ​​of this simulation system are shown in Table 1.

Table 1 System parameter values

The waveforms before and after current compensation are shown in Figure 8. It can be concluded from the waveform that after passive filtering and compensation current, a more accurate three-phase sinusoidal current waveform is obtained.

Figure 8 Waveforms before and after current compensation

After the calculation and analysis of harmonics by the wavelet analysis toolbox and the hybrid active power filter, the distortion coefficient is reduced from 22.50% to 1.88%, which is in line with the IEEE-519-1992 standard, as shown in Figure 9.

Figure 9 Spectrum of phase A voltage before and after filtering

5 Conclusion

Compared with traditional power filters, the fast control model has a short design cycle, low investment cost and obvious filtering effect. The operation results show that the fast model established by using DSP as the controller can accurately track the voltage flicker of the power grid caused by sudden load changes, so as to compensate for harmonics. The equipment has high reliability, strong anti-interference ability, good economic benefits, and is suitable for engineering application and promotion.