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The increasing penetration of distributed renewable energy sources (RES) requires appropriate control techniques in order to remain interconnected and contribute in a proper way to the overall grid stability, whenever disturbances occur. In addition, the disconnection of RES due to synchronization problems must be avoided as this may result in penalties and loss of energy generation to RES operators. The control of RES mainly depends on the synchronization algorithm, which should be fast and accurately detect the grid voltage status (e.g., phase, amplitude, and frequency). Typically, phase-locked loop (PLL) synchronization techniques are used for the grid voltage monitoring. The design and performance of PLL directly affect the dynamics of the RES grid side converter (GSC). This paper presents the characteristics, design guidelines and features of advanced state-of-the-art PLL-based synchronization algorithms under normal, abnormal and harmonically-distorted grid conditions. Experimental tests on the selected PLL methods under different grid conditions are presented, followed by a comparative benchmarking and selection guide. Finally, corresponding PLL tuning procedures are discussed.

The presence of dc offset and harmonics/interharmonics in grid voltage input signal of phaselocked loop (PLL) results in inaccurate controller response. The inaccuracies are due to the low and high frequency oscillations that appear in the PLL estimated phase, amplitude and frequency. The importance of DC offset and harmonic/interharmonic rejection capability for PLLs can be appreciated by international standards that impose strict limitations for grid-tied converters. The suppression of fundamental frequency oscillations caused by DC offset in the input signal must be carried out without compromising the dynamic response of the system. The use of low pass filters for example results in undesirable, slow response. This paper proposes an accurate and fast decoupling of fundamental frequency oscillations using a mathematic cancellation decoupling cell. Higher frequency oscillations generated by harmonics/interharmonics are eliminated by a different compensation network (HCN) that is also proposed in this paper. The performance of conventional techniques is limited because they eliminate only specifically selected harmonics. The proposed HCN module, however, eliminates any number of harmonics/interharmonics present in the grid with the least computational complexity and without any prior knowledge. Furthermore, its advanced features provide accurate synchronization under any abnormal grid condition at the lowest computational complexity when compared to existing state-of-the-art PLLs. The advanced performance of the proposed Harmonic-Interharmonic-DC-Offset (HIHDO) PLL is verified through simulation and experimental results.

The large-scale integration of renewable energy sources require routine planning. Renewable sources are expected to be a key element of future power systems. With many challenges ahead, thinking out of the box is necessary in order to equip the power system with additional advanced functionalities and capabilities to combat the anticipated challenges. The associated equipment of distributed generation can be seen as a grid asset upgrade because of the increased number of elements connected to the grid, such as grid side converters (GSC) of photovoltaics and battery storage systems. It therefore makes sense to think of ways to diversify the role of distributed renewables in a way that benefits the grid, consumers, and prosumers in terms of power quality. In parallel with the diversified role of distributed renewables, this approach can be also thought of as a systematic process of developing, operating, maintaining and upgrading the grid. The work proposes an advanced, multi-function GSC control technique for improving the distribution grid power quality by enabling the injection of asymmetric, DC and harmonic currents in case it is needed. Injecting for example such currents locally through the GSC of a residential photovoltaic system can compensate undesired characteristics of the prosumer loads in order to allow the flow of symmetric and high quality currents between the prosumer and the distribution grid (irrespective of the load conditions). The proposed control technique consists of a Phase-Locked Loop (PLL) necessary for synchronization purposes and advanced PQ and current controllers necessary for injecting the reference currents. Experiments and simulation results are presented to validate the performance of proposed control technique.

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