Evaluation of High Step-up Power Conversion Systems for Large-capacity Photovoltaic Generation Integrated into Medium Voltage DC Grids*

ABSTRACT:

With the increase of dc based renewable energy generation and dc loads, the medium voltage dc (MVDC) distribution network is becoming a promising option for more efficient system integration. In particular, large-capacity photovoltaic (PV)-based power generation is growing rapidly, and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid. Different from traditional ac grid-connected converters, the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios. This is an important issue, yet received limited research so far. In this paper, a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted. The required structural features for such a conversion system are first discussed. Based on these features, the conversion system is classified into four categories by series-parallel connection scheme of power modules. Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance, conversion efficiency, modulation method, control complexity, power density, reliability, and hardware cost. In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.

KEYWORDS:

  1. Photovoltaic generation
  2. Dc-dc conversion
  3. Medium voltage dc grid
  4. Large-capacity
  5. Ultra-high voltage transfer ratio

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1 Topology of PDSR system solution

EXPECTED SIMULATION RESULTS:

Fig. 2 Cycle waves in steady state of the solution of PDSR

Fig. 3 Power step response of the solution of PDSR

Fig. 4 Cycle waves in steady state of the solution of PDDS

Fig. 5 Power step response of the solution of PDDS

Fig. 6 Cycle waves in steady state of the solution of PPDS

Fig. 7 Power step response of the solution of PPDS

CONCLUSION:

 In this paper, the emerging conversion systems for large-scale PV plants integrated into MVDC grid are studied. The required structural features for such a conversion system are discussed. The conversion system can be classified into four categories by series-parallel connection scheme of power modules: PDSR, PPSR, PDDS and PPDS. Features of each connection-scheme are qualitatively analyzed. A solution of PDDS is also proposed in this paper. Through comparison of the proposed solution with the existing solutions of PDSR and PPDS are conducted through module topology analysis, simulation verification, and estimates of efficiency and cost. The system-connection schemes PDDS and PPDS are most promising. Usually, the PDDS scheme leads to high PWM frequency, good response, high power density and high efficiency, but it has a high cost for active switches and has common reliability. On the other hand, the PPDS scheme leads to low PWM frequency, slow response, low power density, and common efficiency. But it has a low cost of active switches and is highly reliable

REFERENCES:

[1] A Q Huang, M L Crow, G T Heydt, et al. The future renewable electric energy delivery and management (FREEDM) system: The energy internet. Proceedings of the IEEE, 2011, 99(1): 133-148.

[2] E Rodriguez-Diaz, J C Vasquez, J M Guerrero. Intelligent dc homes in future sustainable energy systems: When efficiency and intelligence work together. IEEE Consumer Electronics Magazine, 2016, 5(1): 74-80.

[3] M Starke, L M Tolbert, B Ozpineci. AC vs. DC distribution: A Loss comparison. 2008 IEEE/PES Transmission and Distribution Conference and Exposition, 2008, Chicago, IL, USA.

[4] M Ding, Z Xu, W Wang, et al. A review on China’s large-scale PV integration: Progress, challenges and recommendations. Renewable and Sustainable Energy Reviews, 2016, 53(9): 639-652.

[5] F Li, C Li, K Sun, et al. Capacity configuration of hybrid CSP/PV plant for economical application of solar energy. Chinese Journal of Electrical Engineering, 2020, 6(2): 19-29.

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