Abstract
When controlling a Dual-Active-Bridge (DAB) DC/DC converter, the high frequency terminal current is usually measured for use in the current feedback controller. In order to measure that current, a wide bandwidth sensor accompanied with high-speed amplifiers are required. Furthermore, a high Analog-to-Digital sampling rate is also necessary for sampling and processing the measured data. To avoid those expensive requirements, this paper proposes an alternative control method for the DAB converter. In the proposed method, the terminal current is estimated by a reduced-order proportional integral observer. A technique is also proposed to reduce the phase drift effect when the voltages at two terminals are not matched. Afterwards, a combined current feedforward—voltage feedback control system is developed to enhance the system dynamics and to regulate the output voltage. This control system needs only the information of the terminal voltages and no current sensor is required. Experimental results show that the observer can estimate the terminal current very quickly with the accuracy of more than 98 % . In addition, the output voltage is well regulated with a fluctuation of less than ± 2.6 % and a settling time of less than 6.5 ms in the presence of a 30 % load change.
Highlights
In micro-grid applications, line frequency transformers (LFTs), in correspondence with mechanical breakers, are usually employed to interconnect a micro-grid with the main grid
In order to improve the power quality, reliability and stability, there are some possible solutions, such as adopting Flexible Alternating Current Transmission System (FACTS) devices [2] to process the fluctuations at the point of common coupling or using Solid-State-Transformers (SSTs) [3] instead of
Gu (z) is factorized into two parts: Gua (z), which includes zeros located inside the unit circle that will be poles of the feedforward controller; and Guu (z), which consists of all non-minimum phase zeros: Gu (z) = Gua (z) Guu (z)
Summary
In micro-grid applications, line frequency transformers (LFTs), in correspondence with mechanical breakers, are usually employed to interconnect a micro-grid with the main grid. When the low voltage AC (LVAC) utility is not available, the SST should generate AC voltage to supply the LVAC bus, the DAB converter will act as a voltage source to regulate the LVDC bus In this voltage mode, the load current is usually used for enhancing the system dynamics. In the interconnected mode, the LVDC bus is regulated by the grid-tied inverter; the DAB converter should behave as a current source In this context, a feedback current mode controller is usually employed. There was no dynamic model for further control system design since the equations were based on the steady state analysis From another aspect, the techniques reported in [20,21] used observers to estimate the active and reactive powers, and a decoupled current controller was employed to regulate two power components individually. The expensive current sensor is removed and efforts to calibrate that noisy signal are eliminated and economical benefit is achieved
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