Conventional harmonic compensation techniques have several drawbacks, such as: 1) additional hardware needs; 2) complex control structure; 3) difficulties in parameter tuning; 4) issues in applicability with the reference frame of the fundamental current controller. In addition, most of them are not good enough in terms of compensating harmonics under the light load condition, which can be mainly attributed to the inaccurate detection of harmonics. In order to cope with the abovementioned problems, a novel digital lock-in amplifier (DLA)-based harmonic compensation method is proposed in this article. Since the proposed method can detect harmonics very accurately without additional hardware, it is simple, robust, and easy to implement. The detected amplitude and phase information of a certain harmonic are used to reconstruct the harmonic and it is used to compensate that harmonic through a negative feedback loop. The theoretical background of the DLA, mathematical analysis, simulation, and experimental results are presented to corroborate the validity and feasibility of the proposed harmonic compensation method. A 5-kW single-phase grid connected inverter is built to verify the performance of the proposed harmonic compensation method by a comparison with those of conventional harmonic compensation methods.

The power quality of grid-connected inverters (GCIs) is attracting a lot of attention as the number of distributed generation systems with renewable energy sources are increased. It is well known that the harmonics present in the grid have a lot of detrimental effects on the equipment and devices connected to a power system network such as heating and derating. These effects lead to problems in terms of safety and lifespan. Therefore, harmonic standards such as IEEE 519 and P1547 suggest that GCIs should achieve total harmonic distortion (THD) of the output current that is less than 5%. However, it is not easy to achieve the current THD of GCIs that is less than 5% with only an output filter under distorted grid conditions. In this paper, a harmonic compensation method using a lock-in amplifier (LIA) is proposed. Due to the outstanding performance of the LIA, accurate information of the harmonics can be obtained. This information is used to eliminate the harmonics by the simple PI controller. To demonstrate the superiority of the proposed method, it is applied to a 5 kW single phase GCI connected to the real grid. The results obtained are compared with those of the conventional harmonic compensation methods.

This letter proposes a virtual oscillator phase-locked loop (VO-PLL) that integrates an Andronov-Hopf oscillator to improve synchronization in distorted grid. VO-PLL replaces the conventional proportional integral (PI) and resettable-integrator in traditional PLL, with a nonlinear Hopf oscillator known for its ability to sustain orbitally stable limit cycle oscillations at specific frequency. Inherent nonlinear dynamics of oscillator results in fast dynamic performance, harmonic rejection, and improved frequency tracking. Experimental tests show strong evidence for the effectiveness of the proposed VO-PLL, demonstrating its potential to enhance grid synchronization and imitate the operation of a higher order PLL under grid disturbances.

The performance of Single Phase Grid Connected Inverters (SPGCIs) get degraded due to several detrimental factors such as the grid voltage harmonics, the dead time effect and the turn ON/OFF of the switches and so on, which cause the harmonics at the output of Grid Connected Inverters (GCIs). Therefore, it is not easy to satisfy the harmonic standards such as IEEE 519 and P1547 without the help of harmonic compensator. In this paper, the Digital Lock-in Amplifier (DLA) based harmonic compensation is proposed, which makes it possible to extract the amplitude and phase information of the harmonics from the output current and compensate it by using a simple PI controller in a feedforward manner. In order to show the superior performance of the proposed harmonic compensation technique, it is compared with those of conventional harmonic compensation methods in terms of the effectiveness of harmonic elimination, complexity and implementation. The validity of the proposed harmonic compensation techniques is verified through the PSIM simulation and experiments with a 5kW SPGCI.

Due to the recent increasing use of nonlinear loads and Grid Connected Inverters (GCIs), the harmonics are also increasing in the grid. The detrimental effects of the harmonics in the grid include the heating in the equipment and conductors, the misfiring in variable speed drives, the torque pulsations in motors and the asynchronization of the GCIs. Therefore, the standard such as IEEE 519 and P1547 forces the GCIs to meet a certain level of output quality in terms of harmonics, phase and frequency variation. In order to inject the pure active power to the grid, the synchronization between the grid and a GCI by a highperformance Phase Locked Loops (PLLs) is essential. Several kinds of PLL methods to synchronize the GCI under the distorted grid condition have been proposed. However, their performances are significantly degraded under the distorted grid condition, especially with a DC off-set and harmonics. In this paper a novel Digital Lock-in Amplifier (DLA) based PLL is proposed to improve the performance of the PLL under the highly distorted grid condition including the power quality events. Since the proposed DLA-PLL is composed of a robust Phase Sensitive detector (PSD), it is immune to any frequency component except the fundamental and the DC off-set present in the grid can be completely rejected. The superiority of the proposed DLA-PLL is proved by the simulation and experiments by comparing it to the results obtained with the six kinds of conventional PLL methods widely used under distorted grid condition.

Objective: The objective of this study is to calculate and analyze the economic parameters of three main DC power sources i.e. Thermoelectric Generator, Transformer Rectifier, and solar photovoltaic (PV) Solar System, which are widely used in impressed current cathodic protection (ICCP) of underground pipelines. Methodology/Analysis : Initial or capital cost, operation and maintenance costs of the afore-mentioned DC sources are computed, for a practical ICCP system of high transmission underground pipeline over a period. Present worth analysis technique of economic analysis with inflation adjustment using a spreadsheet is adopted to compute these values. The analysis period of 20 years is used for present worth calculations of these DC sources. Findings: The cost efficiency of DC power sources for an ICCP system have been discussed in very few research papers. The calculations are made for either only one DC source at a time or the comparison is based on calculation without inflation adjustment. This study, in line with the currently adopted methodologies, provides a far easier and more comprehensive comparison and analysis of the most economical DC source for the ICCP system of underground metallic pipelines. Novelty/ Improvement: DC power sources play a pivotal role in an ICCP system that helps to mitigate the rate of corrosion in underground pipelines. This study contributes towards choosing the costeffective DC Power source for an ICCP system of underground pipelines. The results are drawn using practical data. This subsequently helps in mitigating the rate of corrosion in these pipelines. Keywords: Economic analysis; corrosion; DC power source; impressed current cathodic protection; underground pipelines