Abstract
This paper proposes an improved multiloop control strategy for a three-phase four-leg voltage source inverter (VSI) operating with highly unbalanced loads in an autonomous distribution network. The main objective is to balance the output voltages of the four-leg inverter under unbalanced load conditions. The proposed control strategy consists of a proportional-integral (PI) voltage controller and a proportional current loop in each phase. The voltage controller and the current control loop are, respectively, used to regulate the instantaneous output voltage and generate the pulse width modulation (PWM) voltage command with zero steady-state tracking error and fast transient response. A voltage decoupling feedforward path is also used to enhance the system robustness. Since the outer voltage loop operates in the synchronous reference frame, tuning and stability analysis of the PI controller is far from being straightforward. In order to cope with this challenge, the stationary reference frame equivalent of the voltage controller in the rotating frame is derived. Subsequently, a systematic design based on a frequency response approach is provided. Simulation results are also carried out using the DIgSILENT PowerFactory software to verify the effectiveness of the suggested control strategy.
Highlights
Poor operational efficiency, gradual depletion of fossil-fuel resources, and environmental pollution are the main problems associated with traditional power systems [1,2,3]
The evaluation of the steadystate behavior of the proposed control strategy is made for a three-phase balanced load (RA = RB = RC = 8 Ω) and three various unbalanced load cases, including (RA = 10 Ω, RB = 7 Ω, RC = 8 Ω), (RA = RB = 8 Ω, RC = ∞), and (RB = 8 Ω, RA = RC = ∞)
While there exist at least three line cycles transient with the conventional approach, the results show that the suggested controller has a significant ability to balance the output voltages under severe unbalanced load conditions with zero steady-state error and fast dynamic response
Summary
Gradual depletion of fossil-fuel resources, and environmental pollution are the main problems associated with traditional power systems [1,2,3]. These serious issues have led to a considerable attempt to generate power using renewable energy sources (RES) at the low voltage level [4, 5]. The most important distinguishing feature of a voltage source inverter (VSI) is that there is no need to change its controller, when the operation mode is changed [7] It is commonly used in distribution networks, due to its proper capability to operate in both grid-connected and islanded modes [8]. To achieve a stable and secure operation, a number of technical and regulatory issues have to be resolved before VSIs can become commonplace in distribution networks
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