Abstract
In recent years, a considerable effort has been made to minimize the size of dc-link capacitors in single-phase active front ends (SP-AFEs), to reduce cost, and to increase power density. As a result of the lower energy storage, a high-bandwidth outer dc voltage control loop is required to respond to fast load changes. Linearized modeling is usually performed according to the power-balance method and the control is designed using LTI techniques. This is done assuming negligible voltage ripple at twice the grid frequency, and the model is considered valid up to the grid frequency. However, its precise validity limits are usually unknown and the control design becomes empirical when approaching these boundaries. To overcome this drawback, linear time-periodic (LTP) theory can be exploited, defining the range of validity of the LTI model and providing precise stability boundaries for the dc-link voltage loop. The main result is that LTP models more accurately describe the system behavior and provide superior results compared to the LTI ones. Theoretical analysis, simulations, and extensive experimental tests on a 10-kW converter are presented to validate the claims.
Highlights
T HE use of electrolytic capacitors as energy buffers is the most common approach to stabilize the DC-link voltage in single-phase converters
A large electrolytic capacitor has major disadvantages: low power density and reduced reliability due to the short lifetime expectancy caused by temperature degradation
The paper is organized as follows: Section II provides a description of the active-front-end system and the derivation of the average model; in Section III a brief review of the main features of Linear Time Periodic (LTP) theory is reported; in Section IV the steadystate solutions are evaluated and these are used in Section V to calculate the LTP system, on which the eigenvalue analysis is applied; in Section VI, analytical, simulation and experimental results are presented for a 10 kW prototype, showing how the LTP model can predict the closed loop eigenvalues of the system where the LTI modelling and design approach lose validity
Summary
T HE use of electrolytic capacitors as energy buffers is the most common approach to stabilize the DC-link voltage in single-phase converters. There are other approaches presented in literature, like the Dynamic-Phasor method [27], exploited for the stability analysis of a system comprising a source and a load singlephase converter, or [28], where a three-phase Voltage Source Inverter is analysed both in dq and αβ frames In both cases there is no outer DC-link control, and currents and voltages in these systems have only a dominant component at the grid frequency, which allows the application of these methods. The paper is organized as follows: Section II provides a description of the active-front-end system and the derivation of the average model; in Section III a brief review of the main features of LTP theory is reported; in Section IV the steadystate solutions are evaluated and these are used in Section V to calculate the LTP system, on which the eigenvalue analysis is applied; in Section VI, analytical, simulation and experimental results are presented for a 10 kW prototype, showing how the LTP model can predict the closed loop eigenvalues of the system where the LTI modelling and design approach lose validity
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