The electric power system is currently undergoing a period of unprecedented changes. Environmental and sustainability concerns lead to replacement of a significant share of conventional fossil fuel-based power plants with renewable energy resources. This transition involves the major challenge of substituting synchronous machines and their well-known dynamics and controllers with power electronics-interfaced generation whose regulation and interaction with the rest of the system is yet to be fully understood. In this article, we review the challenges of such low-inertia power systems, and survey the solutions that have been put forward thus far. We strive to concisely summarize the laid-out scientific foundations as well as the practical experiences of industrial and academic demonstration projects. We touch upon the topics of power system stability, modeling, and control, and we particularly focus on the role of frequency, inertia, as well as control of power converters and from the demand-side.

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Foundations and Challenges of Low-Inertia Systems (Invited Paper)

The phase-locked loops (PLLs) are probably the most widely used synchronization technique in grid-connected applications. The main challenge that is associated with the PLLs is how to precisely and fast estimate the phase and frequency, when the grid voltage is unbalanced and/or distorted. To overcome this challenge, incorporating moving average filter(s) (MAF) into the PLL structure has been proposed in some recent literature. An MAF is a linear-phase finite-impulse-response filter, which can act as an ideal low-pass filter, if certain conditions hold. The main aim of this paper is to present the control design guidelines for a typical MAF-based PLL. The paper starts with the general description of MAFs. The main challenge associated with using the MAFs is then explained, and its possible solutions are discussed. The paper then proceeds with a brief overview of the different MAF-based PLLs. In each case, the PLL block diagram description is shown, the advantages and limitations are briefly discussed, and the tuning approach (if available) is evaluated. The paper then presents two systematic methods to design the control parameters of a typical MAF-based PLL: one for the case of using a proportional-integral (PI) type loop filter (LF) in the PLL, and the other for the case of using a proportional-integral-derivative (PID) type LF. Finally, the paper compares the performance of a well-tuned MAF-based PLL when using the PI-type LF with the results of using the PID-type LF, which provides useful insights into their capabilities and limitations.

/pdf/moving-average-filter-based-phase-locked-loops-performance-2o4bmfz6rb.pdf

Moving Average Filter Based Phase-Locked Loops: Performance Analysis and Design Guidelines

A control algorithm for a three-phase hybrid power filter is proposed. It is constituted by a series active filter and a passive filter connected in parallel with the load. The control strategy is based on the vectorial theory dual formulation of instantaneous reactive power, so that the voltage waveform injected by the active filter is able to compensate the reactive power and the load current harmonics and to balance asymmetrical loads. The proposed algorithm also improves the behavior of the passive filter. Simulations have been carried out on the MATLAB-Simulink platform with different loads and with variation in the source impedance. This analysis allowed an experimental prototype to be developed. Experimental and simulation results are presented.

Improvement of the Electric Power Quality Using Series Active and Shunt Passive Filters

Active power filters (APFs) have been used to compensate harmonics, reactive current, and negative sequence fundamental frequency current drawn by nonlinear loads. The control of APF is the core issue for their proper operation. The flexibility of selective compensation embedded in the control scheme makes APF versatile for compensation of reactive power, harmonic currents, and unbalance in source currents and their combinations, depending upon the limited rating of voltage source inverter employed as APF. The proposed scheme utilizes neural network-based decomposition of the load current into positive and negative sequence fundamental frequency component, reactive component and harmonic components. The adaline-based current decomposer estimates the reference currents through tracking of unit vectors together with tuning of the weights. The implementation of the control scheme facilitates selective compensation which respects the limited rating of the APF. The simulated results using developed MATLAB model are presented and are validated by experimental results to depict the effectiveness of the proposed control method of APF

Neural Network-Based Selective Compensation of Current Quality Problems in Distribution System

A novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage vector based on a generalization of the delayed signal cancellation method is proposed in this paper. The technique is implemented by sampling and storing the instantaneous αβ voltage vector. A mathematical transformation is then proposed through which the current and delayed voltage vectors are combined. It is shown that the proposed transformation has unity gain for the fundamental-frequency positive-sequence voltage vector, while its gain is equal to zero for some chosen components. Cascaded transformations can then be used for eliminating the fundamental-frequency negative-sequence vector, as well as chosen positive- and negative-sequence harmonic vector components and, thus, for accurately obtaining the fundamental-frequency positive-sequence voltage vector. The output of the last transformation block is input to a synchronous reference frame phase-locked loop for detecting frequency and position of the positive-sequence vector. A proposal for making the scheme frequency adaptive is also presented. The good performance of the proposed method is verified with simulations and experiments by using distorted and unbalanced signals, containing fundamental-frequency as well as positive- and negative-sequence harmonic components. The proposed method frequency adaptation capability is also verified.

A Generalized Delayed Signal Cancellation Method for Detecting Fundamental-Frequency Positive-Sequence Three-Phase Signals

This paper uses data obtained from laboratory measurements of typical home appliances to verify whether these nonlinear loads behave similar to current or voltage-type harmonic sources. Based on the measurements and additional simulations, it is stated that domestic and commercial electronic loads behave essentially like harmonic voltage sources. This behavior helps to explain why reactive shunt compensators and filters may even increase the harmonic current content in the feeders. Additionally, based on field measurements in a residential low-voltage distribution secondary, the share between linear and nonlinear loads, as well as the prevailing kind of their nonlinearity is analyzed. The measurements, made just before and after the installation of a capacitor bank, confirm that the use of shunt compensation increases both voltage and current distortion in the feeder

Characterization and Compensation of Harmonics and Reactive Power of Residential and Commercial Loads

This paper proposes a novel and simple positive sequence detector (PSD), which is inherently self-adjustable to fundamental frequency deviations by means of a software-based PLL (phase locked loop). Since the proposed positive sequence detector is not based on Fortescue's classical decomposition and no special input filtering is needed, its dynamic response may be as fast as one fundamental cycle. The digital PLL ensures that the positive sequence components can be calculated even under distorted waveform conditions and fundamental frequency deviations. For the purpose of validating the proposed models, the positive sequence detector has been implemented in a PC-based power quality monitor and experimental results illustrate its good performance. The PSD algorithm has also been evaluated in the control loop of a series active filter and simulation results demonstrate its effectiveness in a closed-loop system. Moreover, considering single-phase applications, this paper also proposes a general single-phase PLL and a fundamental wave detector (FWD) immune to frequency variations and waveform distortions

Frequency-Adjustable Positive Sequence Detector for Power Conditioning Applications

This paper discusses the main characteristics and presents a comparative analysis of three synchronization algorithms based respectively, on a phase-locked loop, a Kalman filter and a discrete Fourier transform. It will be described the single and three-phase models of the first two methods and the single-phase model of the third one. Details on how to modify the filtering properties or dynamic response of each algorithm will be discussed in terms of their design parameters. In order to compare the different algorithms, these parameters will be set for maximum filter capability. Then, the dynamic response, during input amplitude and frequency deviations will be observed, as well as during the initialization procedure. So, advantages and disadvantages of all considered algorithms will be discussed.

Comparative analysis of Synchronization Algorithms based on PLL, RDFT and Kalman Filter

This paper presents an alternative method to evaluate selectively disturbance compensation. Instantaneous decompositions to be applied directly on voltages and current signals are used to identify the distorting and unbalancing components, and can be used to compensate either nonactive power, power oscillations, harmonics and load imbalances in a different way as provided by the traditional pq theory. Three control strategies of active filters were tested and compared for solving typical power quality problems. Simulation results are presented to show the main differences among the control strategies facing nonlinear, unsymmetrical and variable loads under generic voltage conditions. According to these results, it is shown that some shunt active filtering strategies can even improve the PCC voltages under resonance conditions.

http://ieeexplore.ieee.org/iel5/8658/27439/01221483.pdf

Selective disturbance compensation and comparisons of active filtering strategies

Time at level analysis of discrete events is proposed to characterize voltage sags and swells instead of the simple magnitude vs duration characteristics presently used. Although requiring a little more computer memory and digital processing, the proposed methodology constitutes a significant improvement for the characterization of unpredictable temporary voltage disturbances through simple numerical index. Some potential applications of the new method are: full conformity verification to existent or future standards; direct comparison with load sensitivity curves; implicit data compression for transmission and storage; automatic event classification; events ranking and propagation analysis. An IEEE database of voltage sags and swells was used to test the proposed methodology.

S.M. Deckmann

Papers

Characterization and Compensation of Harmonics and Reactive Power of Residential and Commercial Loads

Frequency-Adjustable Positive Sequence Detector for Power Conditioning Applications

Comparative analysis of Synchronization Algorithms based on PLL, RDFT and Kalman Filter

Selective disturbance compensation and comparisons of active filtering strategies

About voltage sags and swells analysis