University of South Wales Master of Sciences Thesis
Standalone hybrid generation system for the remote area of Thar, Pakistan
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- Fig. 2.26. Multi-string PV topology with high-frequency transformer-based isolation
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Standalone hybrid generation system for the remote area of Thar, Pakistan Fig. 2.25 Three-phase PV topology with line-frequency transformer Modern inverters tend to use a high-frequency transformer for galvanic isolation. This technology results in entirely new designs, such as the printed circuit board (PCB) integrated magnetic components (Kjaer et al. 2005). One such design is shown in Figure 9, where the transformers are embedded in high-frequency DC-DC converters. Such a topology is also very useful for multi-string configurations, where each of the strings can be connected to a common DC bus and then converted to grid compatible AC by using a single DC-AC inverter. Fig. 2.26. Multi-string PV topology with high-frequency transformer-based isolation The most generalized form of power electronics topology for the PV application is the DC-DC converter with embedded high-frequency transformer, along with the DC-AC inverter as shown in Figure 9. In general, the DC-DC converter controller does the MPPT and voltage boost. The power flow control to the utility and the sinusoidal unity power factor current-injection to the utility are produced by the DC-AC inverter controller. A simplified block diagram of the PV system with the power electronics and control is given in Figure 10. Standalone hybrid generation system for the remote area of Thar, Pakistan Fig. 2.27 Generalized power electronics and control of a PV system The power electronics circuits shown in Figure 10 consist of a DC-DC converter and a three-phase inverter. The DC-DC converter is based on current-source full-bridge inverter with an embedded high-frequency transformer and rectifier. The current-source input stage is beneficial since it reduces the requirement for the filter capacitor in parallel with the PV strings. Furthermore, the diodes included in the rectifiers are current-commutated, involving low-reverse recovery of the diodes and low voltage stress (Kjaer et al. 2005). The voltage from the PV string is first converted into a high- frequency AC; The transformer secondary voltage is then rectified using a full-bridge diode rectifier. The rectified DC is then converted into micro-grid compatible AC and connected to the utility by a three-phase voltage-source inverter. Tracking the maximum power point (MPP) of a PV array is usually an essential part of a PV system. Over the years, many MPPT methods have been developed and implemented. These methods vary in complexity, required sensors, convergence speed, cost, range of effectiveness, implementation hardware, popularity, etc. The names of some of these methods are hill climbing, perturb and observe, incremental conductance, fractional open-circuit voltage, fractional short-circuit current, fuzzy logic and neural network control, ripple correlation control, current sweep, DC-link capacitor droop control, load-current or load-voltage maximization, and dP/dV or dP/dI feedback control. The detailed overview of these MPPT methods can be found in T. Esram and P. L. Chapman’s “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques.” In Figure 10, a simple but effective method for the MPPT is shown. By measuring the string voltage and current, the PV array output power is calculated and compared to the actual PV array output power. Depending on the result of the comparison, the duty cycle is changed to control the input current for the current-source inverter, accordingly. This process is repeated until the maximum power point has been reached. Other types of MPPT controllers can also be developed within the same controller framework. Furthermore, additional controllers can be designed to control the amplitude of the high-frequency AC voltage at the primary of the transformer. There are two basic control modes for the grid-connected inverters. One is constant current control; the other is constant power control. It is still debatable if an inverter should be allowed to regulate voltage during grid-connected operation. The current IEEE 1547 standard does not allow distributed generation to actively regulate voltage, while some people in the industry suggest that voltage regulation may have some positive impact on the grid (Ye et al. 2006). Control of the utility- connected inverter is shown with constant power control (see Figure 10). Many functions to manage |