The rapid decrease in conventional energy resources and their harmful impact on the environment has brought the attention of the researchers towards the use of renewable energy technologies. The renewable energy systems are connected to Direct Current (DC) micro-grids via power electronic converters where the load conditions are unknown and network parameters are uncertain. These conditions call for the use of robust control techniques such as Sliding Mode Control (SMC) in order to regulate the grid voltage. However, SMC has a drawback of operating the power converter at variable switching frequency which results in degrading the power quality. This paper introduces a fixed frequency sliding mode controller that does not suffer from this predicament. A novel double integral type switching manifold is proposed to achieve voltage regulation of a DC micro-grid, in the presence of unknown load demands and un-modeled dynamics of the network. Rigorous mathematical analysis is carried out for the stability of the closed loop system and the technique is experimentally validated on position of a DC micro-grid using a specially designed test rig. For benchmarking purposes, a conventional Proportional Integral (PI) controller is also implemented. An improvement of 2.5% in rise time, 6.7% in settling time and reduction of voltage dip by 31.7% during load transaction is achieved as compared to the PI controller. The experiment confirms the hypothesis that fixed frequency SMC shows better performance than its counterpart in the phase of introduced disturbances.
The rising cost of fossil fuels, their high depleting rate and issues regarding environmental pollution have brought the attention of the researchers towards renewable energy technologies. Different renewable energy resources like wind turbines, fuel cells and solar cells are connected to DC micro grid through controllable power electronic converters. In presence of these diverse generation units, robust controllers are required to ensure good power quality and to regulate grid voltage. This paper presents a sliding mode control based methodology to address the above mentioned challenges. The proposed technique keeps the switching frequency constant so that electromagnetic compatibility (EMC) issues can be solved with conventional filter design. Parallel operation of converter in DC micro gird is considered. Chattering reduction and power quality improvement by harmonic cancellation is proposed. A scaled down hardware for unregulated 11.5 V to 17.5 V input and 24V output is designed and tested.The experimental results show good performance of the controller under different loads and uncertain input voltage conditions. Moreover, the results show the robustness of the closed loop system to sudden variations in load conditions. Furthermore, a significant improvement in power quality is achieved by harmonic cancellation of chattering in the output of the converters.
DC microgrids look attractive in distribution systems due to their high reliability, high efficiency, and easy integration with renewable energy sources. The key objectives of the DC microgrid include proportional load sharing and precise voltage regulation. Droop controllers are based on decentralized control architectures which are not effective in achieving these objectives simultaneously due to the voltage error and load power variation. A centralized controller can achieve these objectives using a high speed communication link. However, it loses reliability due to the single point failure. Additionally, these controllers are realized through proportional integral (PI) controllers which cannot ensure load sharing and stability in all operating conditions. To address limitations, a distributed architecture using sliding mode (SM) controller utilizing low bandwidth communication is proposed for DC microgrids in this paper. The main advantages are high reliability, load power sharing, and precise voltage regulation. Further, the SM controller shows high robustness, fast dynamic response, and good stability for large load variations. To analyze the stability and dynamic performance, a system model is developed and its transversality, reachability, and equivalent control conditions are verified. Furthermore, the dynamic behavior of the modeled system is investigated for underdamped and critically damped responses. Detailed simulations are carried out to show the effectiveness of the proposed controller.
In grid-connected power converter applications, the phase-locked loop (PLL) is probably the most widely used grid synchronization technique, owing to its simple implementation. However, in power grids some very common problems, such as voltage distortion, voltage unbalance, and frequency instability make synchronization a challenging task. The performance of the conventional synchronous reference frame PLL (SRF-PLL) is greatly reduced in the presence of distorted grid conditions. For a polluted grid some advance PLL techniques have been proposed, such as moving average filter PLL (MAF-PLL) and cascaded delayed signal cancellation PLL (CDSC-PLL). These techniques have been mostly evaluated in the presence of odd and even harmonics but the effects of interharmonics on these synchronization techniques still needs to be investigated. In this paper, a detailed performance comparison has been made between SRF-PLL, MAF-PLL, and CDSC-PLL for grid voltages contaminated with interharmonics in the presence of different grid disturbances, such as frequency jump, phase angle jump, and dc offset. The techniques are simulated using Matlab/Simulink. The CDSC-PLL shows excellent performance as compared with other techniques in terms of dynamic response as it settles to frequency step change in a half cycle but the presence of interharmonics greatly reduces its filtering capability. On the other hand, MAF-PLL gives a ripple free behavior in frequency estimation but with a much slower dynamic response as it settles to a frequency step change in more than three cycles. SRF-PLL only performs well under harmonics free grid voltages. INDEX TERMS Grid synchronization, phase locked loop, interharmonics and power quality improvement.
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