| DC arc fault problem cannot be ignored in photovoltaic power generation system.The problem often results in long time ignition anda great financial loss because of its random fault points,a variety of fault types and occupying a large area.Circuit breaker protection is not suitable for arc fault protection in combiner box,so it is necessary to put forward a new method to distinguish different fault types and to develop an online monitoring system with proper protection strategies.There are many research about the time-frequency domain detection of PV DC series arc fault,while the impact of environmental factors on the detection of the parallel arc fault still can not be excluded.Therefore,we build a DC arc fault test platform to study the time/frquency characteristics of different types of arc faults in various position,besides the environmental parameters such as ambient temperature and light intensity are taken into account.In this paper,newly recognition methods of PV DC arc faults are combination of the characteristics of integrated time domain and frequency domain as well as temperature and radiation intensity,besides,the three-stage time-frequency analysis method and multi-string comparison method.Three-stage time-frequency analysis method is applied to distinguish the different time and frequency domain properties of the third stage DC voltage and current from both the combiner box and inverter.That is to say,through the combination of feature vectors "average" and "standard deviation" extracted in time domain,and the feature vectors "fast discrete Fourier transform" extracted in frequency domain,at last,through the dual range division of time domain and frequency domain,the difference between different types of PV DC arc fault is identified.Moreover,multi-string comparison method is used in field experiment of PV DC arc fault,and the experiment was carried out at the test base of the PV system.Four fault points represent four types of arc fault,namely a series arc fault in the combiner box,an in-line series arc fault,a parallel arc fault in the combiner box and an in-line parallel arc fault.In the experiment,simultaneously acquiring the output voltage and current waveform of fault and normal branch or terminal,and the arc itself.When the time domain characteristics are analysed,relevant current harmonics are extracted for the FFT analysis,too.Finally,the different fault points is distinguished by the time domain threshold TSV(28)[6.2158,5.5155](the mean value of the DC side of the inverter and that of the combiner box)and the range of the frequency domain threshold of 60kHz(for example,the value of fault point b is 1000~1500,and th e fault point d value is 6000~8000).The results show that the above two self-proposed recognition methods work well to some extent,and the time-frequency domain characteristics of simulation are almost consistent with the experiment.The different types of PV DC arc fault is possible to be distinguished. |
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