Quasi-Z DC-DC Converter Emergency State Prediction
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Abstract
The article is devoted to the task of prediction the time of converters trouble-free operation for a relatively new class of topology that is used in MicroGrid with renewable energy sources. The DC-DC converter, considered in the article, contains a quasi-Z inverter as an intermediate AC link. A method has been developed for determining the time of DC-DC converter trouble-free operation when changing the parameters of a quasi-Z inverter circuit. This method is based on modeling the operation of the DC-DC converter, determining the time dependence of the change in the converter diagnostic indicator when changing the circuit elements values and approximating the time dependence by the polynomial function. The function of changing the average value of the output voltage from time while changing the inductance and capacitances values of quasi-Z inverter is chosen as the diagnostic indicator.
The duration of trouble-free operation is determined by the moment when the graph of approximated function reaches upper of lower boundary of the range of acceptable scheme parameter. The upper and lower boundaries are defined by national standard 13109-97 and are equal +/-10% from nominal voltage of electrical grid.
Approximation of the function that is built on the base of measured values was done using the software Advanced Grapher. In order to estimate how real data are close to the derived by approximation function the coefficient of determination R2 is used.
It is shown that the error in determining the time of trouble-free operation depends on the approximation function of the diagnostic indicator time dependence. E.g., for linear approximation of time dependence of the output voltage mean value when the inductance LqZ changes the difference between predicted and real trouble-free operation time is 5,6 hours. Prediction error is 18,06% that is non-acceptable. When approximation is done using parabolic function the determination coefficient is 0,9952. That means that approximation almost completely answers to real data. Time estimation error is 1 hour that is 0,25%, i.e. in comparison with the linear approximation the error decreases by 12,06-16,87%.
For the deviation of other scheme parameters it is expedient to use linear approximation for trouble-free operation time. The approximation error lies in the range 0,02…15,74%. The exactness of prediction of trouble-free operation time is 96,82…99,75% that is satisfactory result for the most practical diagnostic tasks.
The proposed method for estimation of trouble-free operation time is perspective for use in the systems of DC-DC converters diagnostics with the intermediate quasi-Z inverter link.
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