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.
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