Distorted Utility Conditions Research Articles

Hedge-Algebra-Based Phase-Locked Loop for Distorted Utility Conditions

This paper presents the first application of the hedge-algebra theory in the field of grid synchronization. For this purpose, an optimized hedge-algebra controller (HAC) is developed and incorporated within the three-phase phase-locked loop (PLL) with moving average filters (MAFs) inside its feedback loop. Optimized fuzziness parameters and linguistic rule base of the HAC are obtained by a genetic algorithm using the integral of absolute error as the performance index during optimization. Calculated optimal parameter values of the HAC depend on the most frequently occurring disturbance in the electric grid. Two different PLL structures are proposed, depending on the types of disturbances occurring in the electric grid. The first structure is the conventional synchronous reference frame PLL with the nonadaptive MAF (i.e., MAF without order adjustment), but with the PI/PID controller in the phase loop replaced by the developed HAC. Such PLL structure is suitable for all analyzed disturbance types, expect for step-changes in the grid frequency. The second PLL structure introduces the adaptive MAF (i.e., MAF with order adjustment) and a new feedback signal in the output stage of the controller to achieve zero steady-state error in the case of step-changes in the grid frequency. The disturbance rejection capability of the two developed PLLs with the HAC (HAC-PLLs) is tested separately and compared experimentally with the PID- and fuzzy-controller-based PLLs.

Control scheme for grid‐tied distributed generation inverter under unbalanced and distorted utility conditions with power quality ancillary services

This study proposes a control scheme with a new fundamental sequence components extractor for grid interfaced distributed generation inverter under polluted grid conditions. The proposed sequence extractor is realised using third-order sinusoidal signal integrator-based frequency adaptive filters. It has an excellent ability to decompose the three-phase voltage and current signals into positive and negative sequence components under distorted grid scenario. The proposed control scheme is immune to frequency variations and is capable of providing power quality ancillary services such as reactive power compensation and harmonics rejection in addition to basic power injection. The performance of the proposed system under various grid disturbances and loading conditions is evaluated through simulation and experimental studies.

Reduction of Input Current Harmonics for Three Phase PWM Converter Systems under a Distorted Utility Voltage

This paper proposes a harmonics reduction technique for the input currents of three phase PWM converters. The quality of the phase angle information on the utility voltage connected to the PWM converters affects their control performance. Under a distorted utility voltage, the extracted phase angle based on the synchronous reference frame PLL method is distorted. This causes large harmonics in the input currents of a PWM converter. In this paper, a harmonics reduction method that makes the input currents in the PWM converter sinusoidal even under distorted utility conditions is proposed. By the proposed method, without additional hardware, the THD (Total Harmonic Distortion) of the input currents can be readily limited to below 5% which is the harmonic current requirements of IEEE std. 519. Its validity is verified by simulations and experimental results.

Tuning of Phase-Locked Loops for Power Converters Under Distorted Utility Conditions

This paper presents a novel approach in the tuning of phase-locked loops (PLLs) for power electronic converters. PLLs are implemented inside a higher level controller to estimate the grid-voltage phase angle and then control the energy transfer between the power converter and the AC mains. The tuning of the PLL is not a trivial task, particularly when considering power-quality phenomena. In a general way, PLLs with a low bandwidth (low-gain PLLs) are required when handling distorted voltages. It is analytically demonstrated in this paper that low-gain PLLs have more tradeoffs than high-gain PLLs (e.g., PLLs for communications); it is not possible to optimize the settling time for a phase jump without making slower the PLL response to frequency variations. Existing tuning methods do not take into account low-gain features, which may result in nonoptimum designs. The proposed PLL tuning methodology is based on inspection of frequency-domain diagrams and, contrary to the other existing tuning methods, takes into account ldquolow-gainrdquo dynamics. It assures an optimized performance in the presence of any kind of disturbances in the grid. From a practical point of view, the proposed tuning procedure is very intuitive for controller designs. Some significant design examples and experimental results, obtained from a discrete implementation (dSpace platform), are provided in order to validate the theoretical approaches.

Three-phase phase-lock loop for distorted utilities

A novel three-phase phase-locked loop (PLL) structure suitable for phase and angular frequency derivation from distorted ac utility voltages is presented. The proposed PLL has a simple structure; a conventional three-phase PLL followed by a proportional-integral (PI)-controlled moving average filter together with a phase-locking algorithm. The objective of the proposed technique is to capture the fundamental phase angle and angular frequency of three-phase clean, distorted, balanced or unbalanced ac utilities. The PLL gives fast, accurate angular frequency and phase locking and is robust to utility distortion such as line notching, random noise, voltage imbalance, phase loss, phase imbalance, harmonics, dc offsets and frequency variation. The analysis presented substantiates the immunity of the proposed PLL to unbalanced and distorted utility conditions. The PLL technique is simulated and digital signal processor (DSP)-implemented for a three-phase system to verify the analytical results. The simulated and experimental results, for numerous utility conditions, demonstrate its phase-tracking ability, whereas the conventional technique fails to lock accurately in highly distorted, three-phase grid-connected operation.

Operation of Static Series Compensator Under Distorted Utility Conditions

The static series compensator (SSC) has shown a significant capability to mitigate voltage dips, which are the most severe problem to sensitive loads. Also, it has been declared that the function of the SSC can be extended to work as a series active filter. This work proposes a moving-average filter to detect the fundamental component of the measured voltages and currents (needed to control the SSC) while using a double vector control algorithm to improve the transient performance of the SSC. This is made in order to accurately control the fundamental voltage component at the load terminals in the case of distorted grid voltage. Furthermore, a selective harmonic compensation strategy is applied to filter out the grid harmonics. The operation of the SSC under distorted utility conditions and voltage dips is discussed. The validity of the proposed controller is verified by experiments, carried out on a 10-kV SSC laboratory setup. Experimental results have shown the ability of the SSC to mitigate voltage dips and harmonics. It is also shown that the proposed controller has improved the transient performance of the SSC even under distorted utility conditions.

A line-interactive UPS system implementation with series-parallel active power-line conditioning for three-phase, four-wire systems

This paper presents a three-phase line-interactive uninterruptible power supply (UPS) system with active series-parallel power-line conditioning capabilities. Synchronous reference frame (SRF)-based controller is used to harmonic and reactive power compensation generated from any configuration of non-linear load. Under normal line conditions the UPS system works with universal filtering capabilities, such as compensating the input currents and output voltages. Two three-phase pulse width modulation (PWM) converters, called series and parallel active filters, are used to perform the series and parallel active power-line compensation. The series active power filter works as sinusoidal current source in phase with the input voltage, drawing from utility sinusoidal and balanced input currents with low total harmonic distortion (THD). The parallel active power filter works as sinusoidal voltage source in phase with the input voltage, providing regulated and sinusoidal output voltages with low THD. Operation of a three-phase phase-locked loop (PLL) structure, used in the proposed line-interactive UPS implementation, is presented and experimentally verified under distorted utility conditions. The performance of the UPS system is evaluated in three-phase, four-wire systems. Digital simulations and experimental results are presented to confirm the theoretical studies.

A three-phase line-interactive UPS system implementation with series-parallel active power-line conditioning capabilities

This paper presents a three-phase line-interactive uninterruptible power supply (UPS) system with series-parallel active power-line conditioning capabilities, using a synchronous reference frame (SRF) based controller, which allows an effective power factor correction, load harmonic current suppression and output voltage regulation. The three-phase UPS system is composed of two active power filter topologies. The first one is a series active power filter, which works as a sinusoidal current source in phase with the input voltage. The other is a parallel active power filter, which works as a sinusoidal voltage source in phase with the input voltage, providing to the load a regulated and sinusoidal voltage with low total harmonic distortion (THD). Operation of a three-phase phase-locked loop (PLL) structure, used in the proposed line-interactive UPS implementation, is presented and experimentally verified under distorted utility conditions. The control algorithm using SRF method and the active power flow through the UPS system are described and analytically studied. Design procedures, digital simulations and experimental results for a prototype are presented to verify the good performance of the proposed three-phase line-interactive UPS system.

A phase tracking system for three phase utility interface inverters

The analysis and design of the phase-locked loop (PLL) system is presented for the phase tracking system of the three phase utility interface inverters. The dynamic behavior of the closed loop PLL system is investigated in both continuous and discrete-time domains, and the optimization method is considered for the second order PLL system. In particular, the performance of the three phase PLL system is analyzed in the distorted utility conditions such as the phase unbalancing, harmonics, and offset caused by the nonlinear load conditions and measurement errors. The tracking errors under these distorted utility conditions are also derived. The phase tracking system is implemented in a digital manner using a digital signal processor (DSP) to verify the analytic results. The design considerations for the phase tracking system are deduced from the analytic and experimental results.

Phase-locked loop for grid-connected three-phase power conversion systems

Analysis and design of a phase-locked loop (PLL) is presented for the power factor control of grid-connected three-phase power conversion systems. The dynamic characteristics of the closed loop PLL system with a second order are investigated in both continuous and discrete-time domains, and the optimisation method is discussed. In particular, the performance of the PLL in the three-phase system is analysed under the distorted utility conditions such as the phase unbalancing, harmonics, and offset caused by nonlinear loads and measurement errors. The PLL technique for the three-phase system is implemented in software of a digital signal processor to verify the analytic results, and the experiments are carried out for various utility conditions. This technique is finally applied to the grid-connected photovoltaic power generation system with the current-controlled PWM inverter as a subpart for generating the current reference of the inverter. The experimental results demonstrate its phase tracking capability in the three-phase grid-connected operation.

Operation of a phase locked loop system under distorted utility conditions

Operation of a phase locked loop (PLL) system under distorted utility conditions is presented. A control model of the PLL system is developed and recommendations are made on tuning of this model specially for operation under common utility distortions such as line notching, voltage unbalance/loss, and frequency variations. The PLL is completely implemented in software without any filters. All analytical results are experimentally verified.