Abstract
This work is focused on a step-down switched-inductor hybrid dc-dc converter (SIHDC) integrated in a small power wind energy conversion system (WECS). The converter has two roles, to maintain the wind turbine at the maximum power point by controlling the electric generator loading, and to charge a high power density supercapacitor, which is part of a hybrid storage unit that also contains rechargeable batteries. The paper points out the steady-state analysis of the converter in continuous and discontinuous current modes, specific for this application. The stability investigation is presented in detail, including the influence of the electrical generator. The parameterized small-signal transfer function of the control variable derived in this paper can be used to quickly obtain the information needed to evaluate the stability of any similar WECS. A current controller was designed to achieve a stable operation in continuous current mode, and afterwards the stability was checked for discontinuous current mode. A 5kW step-down SIHDC prototype was built and extensively tested, both in laboratory, by using a wind turbine emulator, and in a small power (5kW) WECS industrial platform. Experimental results obtained under real operating conditions confirm the theoretical analysis and laboratory tests, showing that SIHDC converter is a valid solution for low power wind energy conversion systems.
Highlights
The main goal of a wind energy conversion system (WECS) is to get the maximum available power from the wind turbine at each moment
During operation in DISCONTINUOUS CURRENT MODE (DCM), the duty cycle is automatically controlled at the value expressed by (9), in which instantaneous input and output voltages are imposed by application, and the average input current is required by the wind turbine maximum power point tracking (MPPT) algorithm
In a small power WECS with hybrid energy storage, the integrated step-down switched-inductor hybrid dc-dc converter (SIHDC) is suitable for controlling the output power of the electric generator through converter average input current, in order to maintain the operation at the maximum power point
Summary
The main goal of a wind energy conversion system (WECS) is to get the maximum available power from the wind turbine at each moment. The use of a step-down SIHDC in a WECS was previously reported [15], but an in-depth investigation of the converter integration into the system has not been published in the literature This investigation is necessary because the operation can become instable if the stability is not correctly evaluated by taking into consideration the parameters of the electric generator. According to many market forecasts, a high global growth rate is expected in the future years, which can lead to further price reduction and continuous investment in key research projects focused on characteristics improvement Due to their high-power density, low maintenance requirements and ability to withstand extreme conditions, the supercapacitors are especially attractive for renewable energy applications [16].
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