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| A Novel Linearization Control Strategy to Improve Dynamic Performance of DC Solid-State Transformers under All Working Conditions |
| Li Zhixiang, Pei Yunqing Wang Laili, Liu Jiahao, Gao Yunyao |
| School of Electrical Engineering Xi’an Jiaotong University Xi’an 710049 China |
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Abstract The control system of the DC solid-state transformer with the input-series-output-parallel dual active bridge converters typically consists of output voltage control and input voltage sharing control. In the conventional control strategy, the loop gain characteristics of each control loop change considerably with the load and other working conditions, and the dynamic performance is ignored in the controller parameter design. This paper proposes a novel linearization control strategy to achieve the linearization of the loop models of both the output voltage control and input voltage sharing control, thereby simplifying the design of controller parameters and enhancing the adaptability of the converter.
Firstly, the control command output by the PI controller of the output voltage control loop is changed from the phase shift command to a current command. A secondary-side current command is generated after the control command is superimposed with the output current. Then, the control command output by the PI controller of the input voltage sharing control loop is changed from the phase shift increment to the primary-side current increment. The method for superimposing the control commands of the two loops is changed from the phase-shift superposition to the primary-side current superposition. Finally, the phase shift ratio of each DAB is calculated based on the input current command generated by the control system, which is no longer directly output by the PI controller.
In simulations, frequency sweep analyses are conducted on the output voltage control loop and input voltage sharing control loop. With the conventional control strategy, the closed-loop gain exhibits a high resonance peak with light loads. This peak disappears with heavier loads, but the bandwidth is low. In contrast, the frequency characteristics of the control loops change little with the load using the proposed linearization control strategy.
An experimental prototype of a DC solid-state transformer composed of three DAB units is built. Experimental results show that under full-load conditions, the output voltage’s response speed is slow using the conventional control strategy. When the load is reduced to 10%, the overshoot significantly increases, indicating a noticeable decrease in stability. In contrast, with the proposed linearization control strategy, the output voltage’s step response waveforms under various load conditions roughly overlap, demonstrating high stability and fast response. Moreover, when switching from half-load to full-load, the voltage drop and recovery time are significantly lower than those using the conventional control strategy.
The conclusions are as follows. (1) Using the conventional control strategy, the closed-loop gain's resonance frequency-domain indicators (peak, frequency bandwidth) and time-domain indicators (overshoot, rise time, and adjustment time) vary significantly with operating conditions. These dynamic performance indicators show slight variation using the proposed control strategy, facilitating the comprehensive optimization of dynamic performance. (2) The proposed control strategy enhances the suppression capability of the DC solid-state transformer against load disturbances using output current information for load compensation during the linearization process.
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Received: 09 September 2023
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2024, Vol. 39 
