Sayed Ali Khajehoddin

Addressing DC Component in PLL and Notch Filter Algorithms

This paper presents a method for addressing the dc component in the input signal of the phase-locked loop (PLL) and notch filter algorithms applied to filtering and synchronization applications. The dc component may be intrinsically present in the input signal or may be generated due to temporary system faults or due to the structure and limitations of the measurement/conversion processes. Such a component creates low-frequency oscillations in the loop that cannot be removed using filters because such filters will significantly degrade the dynamic response of the system. The proposed method is based on adding a new loop inside the PLL structure. It is structurally simple and, unlike an existing method discussed in this paper, does not compromise the high-frequency filtering level of the concerned algorithm. The method is formulated for three-phase and single-phase systems, its design aspects are discussed, and simulations/experimental results are presented.

Problems of Startup and Phase Jumps in PLL Systems

An adaptive phase-locked loop (PLL) structure is proposed which offers fast and smooth tracking of phase-angle jumps. Correlatively, it offers soft startup stage and avoids undesired frequency swings caused by phase jumps. The adaptive mechanism adjusts the gain of frequency estimation loop in order to mitigate large transients of frequency during sudden phase angle variations. This reduces the coupling of phase and frequency variables and allows tremendously faster and smoother estimation of both variables. The proposed adaptive mechanism can be applied to different PLL and adaptive notch-filter systems three of which including the enhanced PLL (EPLL), the synchronous reference frame PLL (SRF-PLL), and the second order generalized integrator frequency-locked loop (SOGI-FLL) are studied in this paper.

A Novel topology and control strategy for maximum power point trackers and multi-string grid-connected PV inverters

The number of grid-connected photovoltaic (PV) systems is increasing noticeably. However, the high initial cost of such systems impedes their wide spread commercialization. This paper presents a new multi-string grid-connected converter topology and a simplified control strategy with a minimum number of components that considerably lowers the initial investment and increases the life time of the system. The proposed system consists of two stages. The first stage is a new robust maximum power point (MPP) tracker circuit that can rapidly follow the given reference signal of the MPP under any irradiation or temperature condition. This stage also decouples the output power pulsation from the input power generation to minimize the deviation from the MPP. The multi-string topology enables the circuit to extract the maximum available power from each string independently for partially shaded conditions. The second stage is a current source inverter using a modified modulation strategy to inject a current with minimum harmonic components into the grid at unity power factor. Simulation results are provided to demonstrate the performance of the converter and to prove the validity of the proposed control system.

Derivation and Design of In-Loop Filters in Phase-Locked Loop Systems

This paper addresses the concept of in-loop filters in phase-locked loop (PLL) systems. The in-loop filters are derived from an optimization perspective, and an analytical method to design the controlling parameters of a PLL with in-loop filters is also presented. Such filters can also be selected as conventional window functions in which case they can be tuned to reject certain frequency components similar to the discrete Fourier transform. In this paper, a rigorous method to introduce the concept of in-loop filters and window functions into PLL systems is presented. This method enables smoother estimation of the signal parameters such as phase angle, frequency, and amplitude in the presence of noise and harmonics. The in-loop filters can be adjusted to completely remove specific harmonics. The method is first developed for a single-phase enhanced PLL system and is then extended to three-phase PLLs including the well-known synchronous-reference-frame PLL. Simulation and experimental results are also included.

The Application of the Cascaded Multilevel Converters in Grid Connected Photovoltaic Systems

Unique features of multi-level converters have recently nominated them as significant alternatives for solid-state power converting units, even in the low and medium power range. The fact that multilevel converters need several DC sources in the DC side, makes them attractive for photovoltaic (PV) applications. This paper presents a new control strategy to control cascaded multilevel converters in a multi-string configuration for single phase grid connected systems. Eventually, simulation results are provided to validate the control system under various insolation conditions.

A Power Control Method With Simple Structure and Fast Dynamic Response for Single-Phase Grid-Connected DG Systems

This paper presents a new method for controlling the exchange of power between a single-phase distributed generation system and the grid. Rather than controlling the active and reactive powers separately and through the media of current signal as is done by the conventional techniques, the proposed controller acts directly on the instantaneous power. This eliminates the conventional need for calculating the active and reactive powers; a calculation that involves filtering/phase-shifting and slows down the system responses and adds to computational complexity. This paper first formulates a nonlinear structure from a purely mathematical approach based on minimizing a cost function. The minimization procedure generates a reference for the current signal which is subsequently used in the current control loop. This paper then derives an equivalent linear counterpart for the nonlinear structure. Moreover, it is also shown that the idea of controlling the instantaneous power does not require a separate loop for the current. Having replaced the nonlinear part with its linear equivalent, a control loop that comprises linear time-varying elements is obtained. This paper further develops a linear time-invariant model of the loop for stability and design purposes. The proposed control system is successfully applied to a photovoltaic system and performance evaluation results (using computer simulations and a laboratory experimental setup) are presented. Desired performance and robustness of the proposed method is verified by testing it within different operating conditions.

A Control Design Approach for Three-Phase Grid-Connected Renewable Energy Resources

This paper presents a method to design a control system for a three-phase voltage source converter (VSC) that connects a renewable energy source to the utility grid through an output L-type or LCL-type filter. The well-known abc/dq transformation method creates coupling terms that are visible and can readily be canceled in the L-type filter. Such terms, however, are very complicated when an LCL filter is used. This paper, first revisits the derivation of the decoupling control method for an L-ype output filter and then, for the first time, derives the decoupling terms for an LCL-type filter. Having successfully decoupled the real and reactive power loops, feedback controllers are presented and designed to achieve desirable performance. The proposed controller provides active damping of the LCL resonance mode, robustness with respect to grid frequency, and impedance uncertainty. Moreover, a new controller is designed to improve the startup transient of the system. The methodology used in this paper is inspired from the feedback linearization theory and it provides a clear design method for the nonlinear systems. Simulation results are presented to confirm the analytical results.

A New Phase-Locked Loop System for Three-Phase Applications

This paper presents a new three-phase phase-locked loop (PLL) system that primarily estimates the phase angles, frequency, and magnitudes of the a three-phase input signal and also provides a filtered version of the input. It is then extended to the estimation of sequence components, their magnitudes, and phase angles. As compared with the conventional three-phase PLL, this method does not suffer from errors that are caused by signal unbalance and dc offset. It also provides estimate for several other variables which are not included in the conventional three-phase PLL. As compared with the method of using three independent single-phase enhanced PLLs, the proposed method offers a simpler structure. Moreover, the estimated frequency is more accurate and smoother because it uses the information from all three phases to estimate a single value for frequency. The paper also presents a modification that makes the PLL parameters independent from the input signal amplitude. The same modification is applicable to existing methods such as conventional three-phase PLL. Simulation and experimental results are presented to confirm desirable performance of the proposed method.

A robust power decoupler and maximum power point tracker topology for a grid-connected photovoltaic system

The exponential growth in the number of photovoltaic (PV) system installations verifies that solar power technology has become one of the promising energy resources. This paper introduces new control scheme and converter topology for low to medium power PV applications. Proposed topology provides independent MPPT and power decoupling without the use of bulky electrolytic capacitors. The grid-connected output stage is a current source inverter with a modified modulation strategy to inject a low harmonic current into the grid at unity power factor. Simulation results are presented to show the effectiveness of the proposed method in terms of MPPT, power decoupling and power injection into the grid at a high quality.

A Systematic Approach to DC-Bus Control Design in Single-Phase Grid-Connected Renewable Converters

This paper presents a method for design and control of dc-bus capacitance and transients in a renewable single-phase grid-connected converter. Conventionally, a proportional (P) or proportional-integrating (PI) controller is commonly used and the design stage is performed using trial-error or using a simplified analysis that does not take the dynamics of the current control loop into consideration. This paper proposes 1) a systematic and efficient method for design of dc-bus PI controller gains; and 2) an accurate method for the design of the dc-bus controller gains without neglecting the dynamics of the current control loop. Two main objectives are to have control over the amount of output current harmonics and over the level of bus fluctuations caused by random input power swings. The proposed method is transparent and it provides a convenient and rigorous insight for the designer to properly select the size of dc-bus component and to determine the controller gains.