Reyyan Ahmad Khan

Background: Grid-tied inverters play an efficient role in the integration of renewable energy resources with utility grids. Motivation: However, the interconnection between power converters and the grid has been seen to be responsible for various stability issues such as weak grid, and under weak grid conditions the injection of power to the grid becomes a challenging task due to continuously varying grid impedance affecting the stability margins as a result. Additionally, the grid impedance-related issues boost the voltage harmonics which further devalue its performance. These voltage harmonics propagate through the Phase-Locked Loop (PLL) circuit to the control unit of the inverter which in turn amplifies the low order harmonics of the inverter. Method: The aim of this research is to introduce a novel strategy that decreases the effect of grid impedance-variations on the performance and stability of an inverter. Hence, an adaptive hybrid mode control technique consisting of two parts is proposed in this research. The current regulator part is implemented in a synchronous reference frame for its gain and time parameters to improve the performance, stability, and response time. The adaptive harmonic compensators are implemented in a stationary reference frame for harmonic compensation purposes. This adaptive nature of harmonic compensator can effectively work in the case of frequency variation where the fixed value based harmonic compensator fails. Results: The adaptive harmonic compensators improve the performance by reducing total harmonic distortion (THD), reduce computation and improve stability when the grid has distorted voltage, variation in grid impedance and frequency. Impact and utility: Our results show that the system becomes less sensitive to grid impedance variations which makes the proposed technique very relevant to the stability performance applications.

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.

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.

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.

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.

There is an energy crisis in Pakistan due to the gap between demand and supply of electrical energy. This gap between demand and supply of electrical energy is hampering its economic development. In order to overcome this issue there is a need to increase the generation capacity of the country by integrating renewable energy resources in the system. In this paper the working and functioning of H-11 grid system is tested using the power world simulator software. First the grid system is modeled by incorporating the values of the power transformers and the transmission system of the grid. When the grid system is simulated the grid is tested for the four different scenarios of the Distributed Renewable Energy Generation integration. Our simulations show that Distributed Renewable Energy Generation (DREG) improves the voltage profile, reduce feeder losses and increases the reliability of the system by providing power near the consumer at the distribution level. Simulation shows that the optimal placement of DREG is at bus no 7, in case of 1MW (25% of the total feeder load). Simulation results and the comparative analysis of the respective DREG integration show that the H-11 grid is a good option for the integration of DREG.