Assessment and Mitigation of Dynamic Instabilities in Single-Stage Grid-Connected Photovoltaic Systems With Reduced DC-Link Capacitance IEEE Electrical Projects

ABSTRACT:

Single-stage utility-scale photovoltaic (PV) systems are usually interfaced with the host grid via a centralized voltage-source converter (VSC). Recently, and due to their reliability, the dc-link film capacitors are favored over electrolytic types in grid-connected applications. However, the capacitance per unit volume of film capacitors is significantly smaller than electrolytic capacitors. The overall system stability might be compromised by the reduction of the dc-link capacitance, particularly in PV systems that have a dynamic resistance that varies with operating conditions. Using a detailed small-signal model of the grid-connected PV system, it is shown in this paper that the reduction of the dc-link capacitance interferes with the dynamic resistance of the PV array, which eventually leads to instabilities. The minimum dc-link capacitance that preserves the overall system stability is determined. A simple and effective active compensator is developed to mitigate the instabilities with the reduced dc-link capacitance. Detailed time-domain simulations are presented to validate the analytical results and show the proposed compensator’s effectiveness in preserving the system stability.

KEYWORDS:

  1. Active damping
  2. DC-AC power converters
  3. DC-link stabilization
  4. Grid-connected inverters photovoltaic
  5. Single-stage
  6. Small-signal analysis

SOFTWARE: MATLAB/SIMULINK

SCHEMATIC DIAGRAM:

Figure 1. Vector Control Schematic Of The Grid-Connected Vsc.

EXPECTED SIMULATION RESULTS:

Figure 2. Uncompensated System Response Operating In The Cvr, Mpp, And Ccr At T D 0 􀀀 3 S, T D 3 􀀀 4 S, And T D 4 􀀀 7 S, Respectively, As Cdc Decreases From 1 P:U: To 0:6 P:U:

Figure 3. Influence Of Added Active Compensation Loop At T D 1:3s. Under The Ccr Operation.

Figure 4. Compensated System Response Operating In The Cvr, Mpp, And Ccr At T D 0 􀀀 3 S, T D 3 􀀀 4 S, And T D 4 􀀀 7 S, Respectively, As Cdc Decreases From 1 P:U: To 0:6 P:U:

Figure 5. Compensated And Uncompensated Dc-Link Voltage Response To A Single-Phase Ground Fault At T D 1:5 S For 5 Cycles. (A) Under The Ccr And Cdc D 0:6p:U: (B) At Mpp And Cdc D 0:6p:U:

Figure 6. Compensated And Uncompensated Dc-Link Voltage Responses At Cdc D 0:6p:U: Due To The Dc Cable Influence.

Figure 7. Compensated And Uncompensated Dc-Link Voltage Responses At Cdc D 1p:U:

CONCLUSION:

This paper has introduced comprehensive modeling and control of the single-stage grid-connected PV system. The dynamic resistance of the PV arrays is analyzed and defined under different operating regions. It is found that reduced dc-link capacitance affects the dynamic stability of the overall system due to interactions with the dynamic resistance of the PV array. As a result, a new and simple compensator is proposed to stabilize the system with a reduced dc-link capacitance. The small-signal stability analysis of the overall system is performed under different operating conditions. The proposed compensators have the following advantages: 1) it is simple yet effective and can be easily designed using linear analysis tools, 2) it does not affect the steady-state operation of the VSC grid-connected PV system, 3) it improves the damping performance of the dc-link voltage and provides a robust and stable performance at different operating conditions of the PV system, and 4) it facilitates successful low voltage ride-through at different operating conditions.

REFERENCES:

[1] International Renewable Energy Agency. (Mar. 2019). Renewable Capac- ity Statistics. [Online]. Available: https://www.irena.org/-/media/Files/  IRENA/Agency/Publication/2019/Mar/RE_capacity_highlights_2019.pdf

[2] B. Karanayil, V. G. Agelidis, and J. Pou, “Performance evaluation of three-phase grid-connected photovoltaic inverters using electrolytic or polypropylene _lm capacitors,” IEEE Trans. Sustain. Energy, vol. 5, no. 4, pp. 1297_1306, Oct. 2014.

[3] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1184_1194, Sep. 2004.

[4] A. Yazdani, A. R. Di Fazio, H. Ghoddami, M. Russo, M. Kazerani, J. Jatskevich, K. Strunz, S. Leva, and J. A. Martinez, “Modeling guidelines and a benchmark for power system simulation studies of three-phase single-stage photovoltaic systems,” IEEE Trans. Power Del., vol. 26, no. 2, pp. 1247_1264, Apr. 2011.

[5] H. Wang and F. Blaabjerg, “Reliability of capacitors for DC-link applications in power electronic converters_An overview,” IEEE Trans. Ind. Appl., vol. 50, no. 5, pp. 3569_3578, Oct. 2014.

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