22-10-2016, 10:56 AM
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Abstract: The dc component can affect the operating point of the transformers in the power system. The transformer cores are driven into unidirectional saturation with consequent larger excitation current. The service lifetime of the transformer is reduced as a result with further increased hysteresis and eddy current losses and noise. The transformer reduces the inrush currents and maintains required voltages. But low-frequency (50 or 60 HZ) transformer is bulky, Heavy, and expensive and its power loss brings down the overall system efficiency.. PV array is connected to the grid via a three phase voltage-source two-level inverter and an LCL filter replacing the transformer. The limit for dc component in the grid-side ac currents, e.g., below 0.5% of the rated current. The dc component can cause line-frequency power ripple, dc-link voltage ripple, and a further second-order harmonic in the ac current. The dc component injected to the grid can affect the normal operation of the loads connected to the grid, for example, causing torque ripple and extra loss in ac motors. This paper has an effective solution to minimize the dc component in three-phase ac currents and developed a software-based approach to mimic the blocking capacitors used for the dc component minimization, the so-called virtual capacitor. The “virtual capacitor” is achieved by adding an integral of the dc component in the current feedback path. A method for accurate extraction of the dc component based on double time integral, as a key to achieve the control, has been devised and approved effective even under grid-frequency variation and harmonic conditions. A proportional-integral-resonant controller is further designed to regulate the dc and line-frequency component in the current loop to provide precise control of the dc current.
INTRODUCTION
GRID-CONNECTED photovoltaic (PV) systems often include a line transformer between the power converter and the grid. The transformer guarantees galvanic isolation between the grid and the PV systems, thus fulfilling safety standards. Fur—there more, it ensures that no direct current (dc) is injected to the grid [1]. However, the low-frequency (50 or 60 Hz) transformer is bulky, heavy, and expensive and its power loss brings down the overall system efficiency [1]–[5].
The dc component can have negative impacts on the power system in the following ways :
1) The dc component can affect the operating point of the transformers in the power system. The transformer cores are driven into unidirectional saturation with consequent larger excitation current. The service lifetime of the trans-former is reduced as a result with further increased hysteresis and eddy current losses and noise.
2) The dc component can circulate between inverter phase legs as well as among inverters in a paralleled configuration. The dc component circulation affects the even current and loss distribution among paralleled inverters.
3) The corrosion of grounding wire in substations is intensi- fied
In the proposed control strategy, an accurate dc component measurement and extraction is critical. Several methods can be used to measure the dc component, e.g., by shunt resistors, voltage transformers, mutual coupled inductors, and integral methods, etc. Among them, only the integral methods do not need extra hardware.
CONCLUSION
This paper has presented an effective method to minimize the dc component in a three-phase transformerless grid-connected PV system. The dc component can introduce line-frequency power ripple in the system and further cause dc-link voltage ripple and second-order harmonics in the ac currents. A software-based “virtual capacitor” approach has been implemented to minimize the dc component via a feed-forward of the dc component. The dc component can be accurately obtained using the sliding window iteration and double time integral even under frequency variation and harmonic conditions. A PIR controller has been designed to enable the precise regulation of both the dc and line-frequency components in the d-q frame
The proposed method can be well adopted in the existing PV systems for dc component minimization by adding software programs for dc-component extraction, dc-component feed-forward term as well as the resonant controller in the current control loops.