Resonance Detection Method of AC-DC Hybrid Micro-grid Based on Wavelet Transform and Fast Fourier Transform

The grid connection of a mass of distributed power sources in the AC/DC hybrid micro-grid will cause harmonics, voltage sags, voltage fluctuations, and flicker in the grid, which seriously affects the power quality of the grid. Therefore, the power quality problems such as the resonance generated by the AC/DC hybrid micro-grid need to be studied urgently. In this paper, a resonance detection technology based on the combination of wavelet transform (WT) and fast Fourier transform (FFT) is proposed. This method uses wavelet transform to process the high-frequency and low-frequency parts of the signal separately, and eliminates part of the interference signal. The method then uses the FFT to obtain the resonance content. The AC/DC hybrid micro-grid resonance detection method based on WT and FFT can not only determine the time and amplitude of the resonance signal, but also filter the interference signal and improve the accuracy of frequency domain analysis. It is verified by simulation that the algorithm can detect the resonance of the AC/DC hybrid micro-grid.


Introduction
With the further reduction of photovoltaic power generation costs, distributed photovoltaics have developed rapidly.Distributed photovoltaics have the advantages of flexible access, economical environmental protection, and simple maintenance.It has been widely used in remote areas, urban residences, road lighting and other occasions [1][2].Affected by meteorological conditions, the output of photovoltaic power sources has randomness and volatility [3].When the access capacity is too large, it is easy to cause system voltage fluctuations.The photovoltaic connection changes the power flow distribution and voltage distribution of the system, which may cause voltage deviations at the access bus node.The connection of the photovoltaic inverter will also inject harmonic current into the system, which may cause the harmonic voltage content of some nodes to exceed the standard or even cause resonance.Therefore, it has important research value to research the resonance of the AC/DC hybrid micro-grid.WT is adaptive, and the FFT method has the characteristics of fast detection.Therefore, the combination of WT and FFT is suitable for the detection of resonance disturbance in the AC/DC hybrid micro-grid.
The principle of AC/DC hybrid micro-grid resonance is first analyzed, and on this basis, a resonance detection method based on WT and FFT is proposed.The algorithm first uses WT to detect the resonant signal in the power system, and obtains the most obvious detail signal of energy fluctuation, and then uses FFT to further analyze the approximate signal including the resonant signal, and finally obtains the resonant frequency.The simulation proves that the resonance detection method based on the WT and FFT can effectively detect the resonance frequency.

Resonance Analysis
Figure 1 is the typical structure of distributed energy resources access to AC/DC hybrid micro-grid [4][5], where PCC stands for point of common coupling, MT stands for micro-turbine, DG stands for distributed energy, ES stands for energy storage, PV stands for photovoltaic, and FC stands for fuel cell.From Figure 2, the following formulas can be obtained.
  Among them, Z1 and Z2 constitute a typical series resonant tank, and Z3 can be equivalently regarded as a parallel resonant tank.If damping is ignored, the frequencies of series-parallel resonance are equal.The resonant frequency is:

Resonance Detection Method Based on WT and FFT
According to the wavelet analysis theory and the sampling theorem, when the resonant signal in the power system is decomposed in n layers, the frequency band range of the scale n low frequency coefficient is . The scale n must satisfy [6][7] : Among them, s f is the sampling frequency, res f is the natural resonance frequency.In this paper, the sampling frequency of the resonant signal of the power system is 6400 Hz, and the resonance frequency obtained from the parameters in Table 1 and ( 4) is 712.12Hz.According to the calculation of formula (5), it is known that the number of wavelet decomposition layers is 3.
Since the natural resonance point of the resonance generation system coincides with the frequency of a certain interference source, the obtained energy fluctuation is the most obvious detail signal, which corresponds to a certain frequency band range, that is, the resonance frequency range.
The steps of the resonance detection are as follows: 1) Firstly, using WT to analyze the original signal, a series of sub bands can be obtained.And extracting the sub frequency band with the most significant amplitude fluctuations.
2) Then use FFT to analyze the sub frequency band with the most significant amplitude fluctuations extracted in 1).
3) Finally, the resonant frequency of the power system is obtained.

Simulation
To demonstrate the correctness of the resonance detection method, a simulation model is established in MATLAB/Simulink.Apply WT to analyze the resonance signal, extract the low-frequency signal containing the resonance disturbance, and analyze the low-frequency signal through FFT to obtain the resonance frequency.Table 1 shows the system parameters.Fig. 4 shows that the obvious energy fluctuation is the high frequency signal of cd3, that is, the resonance frequency is between 400-800Hz, and the low frequency signal of the upper layer is ca2.
Further FFT analysis is performed on the low-frequency signal ca2 including the resonance signal, and the analysis result is shown in Fig. 5. From Fig. 5, it can be obtained that the resonance frequency is 730.4Hz.The simulation proves that the resonance detection method based on WT and FFT is correct.

Simulation When Large Scale Distributed Energy is Connected
The resonant voltage when large scale distributed energy is connected in the power system is shown in Fig. 6.WT analysis is performed on the resonant voltage of Fig. 6, and the analysis result is shown in Fig. 7. Fig. 7 shows that the obvious energy fluctuation is the high frequency signal of cd3, that is, the resonance frequency is between 400-800Hz, and the low frequency signal of the upper layer is ca2.
Further FFT analysis is performed on the low-frequency signal ca2 including the resonance signal, and the analysis result is shown in Fig. 8. From Fig. 8, the resonance frequency can be obtained as 721.5Hz.The simulation proves that the resonance detection method based on WT and FFT is effective.
From the above analysis, the analysis results of the combination of WT and FFT of the system resonant voltage can be obtained in Table 3.It can be seen from Table 3 that the resonance frequency when large scale distributed energy sources are connected to the grid is lower than that when small scale distributed energy sources are connected to the grid, and the resonance content increases.That is, the grid connection of large scale distributed energy sources will increase the resonance degree of the system.It is verified by simulation that the detection method based on WT and FFT can effectively detect the resonant signal.

Conclusion
In this paper, the resonance model of the AC/DC hybrid micro-grid is established, and the resonance problem is analyzed.A resonance detection method based on WT and FFT is proposed.The detection method is applied to detect the resonance signals when small scale distributed energy is connected.And The detection method is also applied to detect the resonance signals when large scale distributed energy is connected.The simulations prove that the grid connection of large scale distributed energy sources will increase the resonance degree of the system.The resonance detection algorithm can effectively detect the resonance disturbance in the power grid.

Figure 2
Figure2is the resonance analysis model.

Figure 3 .Figure 4 .
Figure 3. Resonant voltage when small scale distributed energy is connected.WT analysis is performed on the resonant voltage of Fig.3, and the analysis result is shown in Fig.4.

Figure 6 .
Figure 6.Resonant voltage when large scale distributed energy is connected.

Table 1 .
System parameters.According to the parameters in Table1, after the three-layer wavelet analysis is performed on the signal, the corresponding frequency band range of each signal is shown in Table2.Ca represents a low frequency signal.Cd represents a high frequency signal.

Table 2 .
AC/DC hybrid micro-grid parameters.Simulation when Small Scale Distributed Energy is Connected The resonant voltage when small scale distributed energy is connected in system is shown in Fig.3.

Table 3 .
Resonance detection results based on WT and FFT.