Minimization of the DC Component in Transformer less Three-Phase Grid-Connected Photovoltaic Inverters
The dc component can cause line-frequency power ripple, dc-link voltage ripple, and a further second-order harmonic in the ac current. This paper has proposed 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.
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 transformer 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 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.
4) The corrosion of grounding wire in substations is intensified due to the dc component.
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. However, the performance of the conventional integral method based on fundamental-period integral degrades under line-frequency variations. In this project a new integral method based on the sliding window iteration algorithm and double time integral is presented. This method is effective in the extraction of the dc component even for currents with frequency fluctuations and harmonics.
TOOLS AND SOFTWARE USED:
- MP LAB
Qingzeng Yan, Xiaojie Wu, Xibo Yuan, Member, IEEE, Yiwen Geng, and Qi Zhang, “Minimization of the DC Component in Transformerless Three-Phase Grid-Connected Photovoltaic Inverters”, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 7, JULY 2015.