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Distributed generation (DG) produces many benefits for the distribution systems that are operated as active systems, even if they have been traditionally designed as passive networks. Then, new technical issues must be addressed, involving both normal and abnormal operating conditions such as short circuits. Photovoltaic systems (PVSs) are among the most diffuse DG and their contributions to the short-circuit operating conditions depend on several factors, such as environmental conditions, PVS inverter control characteristics, self-protection systems, and fault location and type. An analytical model that aims at quickly and accurately analyzing the three-phase short-circuit operating conditions in a distribution system in presence of PVSs is presented in this paper. The model is applied to a typical North American MV network characterized by large penetration of PVSs, investigating the influence of the aforementioned factors.

Waveform distortions are an important issue in distribution systems. In particular, the assessment of very wide spectra, that include also components in the 2–150 kHz range, has recently become an issue of great interest. This is due to the increasing presence of high-spectral emission devices like end-user devices and distributed generation systems. This study proposed a new sliding-window wavelet-modified estimation of signal parameters by rotational invariance technique (ESPRIT) method, particularly suitable for the spectral analysis of waveforms that have very wide spectra. The method is very accurate and requires reduced computational effort. It can be applied successfully to detect spectral components in the range of 0–150 kHz introduced both by distributed power plants, such as wind and photovoltaic generation systems, and by end-user equipment connected to grids through static converters, such as fluorescent lamps.

A high penetration of Photovoltaic (PV) systems into power networks can alter the fault currents and negatively impacts on the operation of the protection systems. In this paper, a new analytical model of the fault contribution of a three-phase PV system equipped with a voltage source converter is proposed. Such a model extends the classical steady-state short-circuit analysis to balanced active power networks including PV systems, with the aim to analyze their impact on the breaking capacity of the interrupting devices. The proposed model: (i) takes into account environmental conditions and VSC current limits; (ii) includes the effects of reactive power injections by a detailed representation of different type of VSC control systems; and (iii) can be easily integrated in a software package for power system analysis. The proposed approach is validated by comparing analytical results with time-domain simulations of the IEEE benchmark PV system. Furthermore, it is used to evaluate the impact of a large penetration of PV systems on the fault currents.

In this paper an optimal strategy is proposed for the real time control of a hybrid microgrid (μG). Hybrid μGs are characterized by the presence of two sections operating in alternating current (AC μG) and in direct current (DC μG), respectively. They include dispatchable and non-dispatchable renewable generation units, battery energy storage systems and controllable loads. These different resources are connected to the AC or DC μGs through static converters that are optimally controlled for the simultaneous improvement of waveform distortions and minimization of the energy costs. In order to show the effectiveness and feasibility of the proposed approach, the control strategy was applied to a hybrid μG derived from the distribution system of an actual industrial facility.

Recent advances in power electronic technology for interfacing dispersed energy resources to distribution grids offer potential solutions to several technical and economical problems in distribution networks. In particular, Dispersed Generation (DG) can be used to provide the usual energy service as well as some ancillary services, such as load following, back-up service, and compensation of Power Quality disturbances. In this paper, a fuel cell-based DG system is proposed for providing energy and ancillary services. The novelty of the system is that it can operate with a ‘system strategy’ (aimed at compensating for the disturbances in the whole system or in some privileged areas of the network) thanks to a suitable control technique of the power electronic interfaces. In this way, the benefits of using the dispersed generation system are maximized. Computer simulations were performed on an actual distribution grid to demonstrate the effectiveness of the compensation actions.

Some of the most remarkable issues related to interharmonic theory and modeling are presented. Starting from the basic definitions and concepts, attention is first devoted to interharmonic sources. Then, the interharmonic assessment is considered with particular attention to the problem of the frequency resolution and of the computational burden associated with the analysis of periodic steady-state waveforms. Finally, modeling of different kinds of interharmonic sources and the extension of the classical models developed for power system harmonic analysis to include interharmonics are discussed. Numerical results for the issues presented are given with references to case studies constituted by popular schemes of adjustable speed drives.

Power quality (PQ) is an important issue due to the presence of an increasing number of perturbing and susceptible loads in modern power systems. In this context PQ evaluation requires advanced methods of signal processing that should be accurate and fast. In this paper, a new, advanced, parametric method is proposed that guarantees an accurate estimation of waveform distortions with reduced computational efforts. The method consists of a modified version of the well-known, parametric, high-resolution, sliding window estimation of signal parameters by rotational invariance technique (ESPRIT) method. The main benefit of the proposed scheme is that it significantly reduces the computational effort required to analyze waveforms by evaluating the frequencies and the damping factors of the spectral components one time or, at most, only a few times. Moreover, the waveform was appropriately sampled again to increase the accuracy of the results; the sampling frequency was chosen heuristically using information about the characteristics of the waveform of the power system. Experiments were conducted and the results were compared also in synthesis data. The results showed that the proposed scheme was effective and efficient.

Discrete Fourier Transform (DFT) and high-performance signal-analysis techniques (i.e., Prony, ESPRIT, root-MUSIC methods) have been proposed to calculate Power Quality indices that characterize waveform distortions in power systems. DFT can be affected by spectral leakage problems that cause inaccuracies in the spectral component calculation while high-performance signal-analysis techniques usually involve onerous computational efforts. Recently, hybrid methods were proposed in the relevant literature which make it possible to calculate PQ indices with accuracy and more acceptable computational efforts. This paper proposes a new three-step hybrid method which applies alternatively DFT and Prony methods to sliding windows. This method does not suffer problems of spectral leakage and guarantees a significant reduction of computational efforts compared with the application of the only Prony method. Experiments are conducted and comparisons are made with traditional methods using both synthetic and measured waveforms. The results confirm the effectiveness and efficiency of the proposed scheme.

The sensitivity to Power Quality (PQ) disturbances is today increasing in micro grids due to the great economic consequences that PQ disturbances can generate. However, micro grids include several distributed energy resources (DER) interconnected to the grid with static converters that can deteriorate the Power Quality levels. Among DER, wind turbine systems (WTSs) are expected to be widely employed due to the forecasted investment cost reduction and economic incentives. WTSs can cause significant time varying voltage and current waveform distortions whose entity depends on the WTS scheme. In case of time varying distortion, standards suggest the use of a windowed Discrete Fourier Transform (DFT) that, however, can suffer of well know spectral leakage problems. Thus, advanced spectral analysis methods should be considered. In this paper, the time varying waveforms distortions caused by different WTSs schemes are analyzed and critically compared applying DFT, some parametric methods (Prony and ESPRIT) and some hybrid methods (ESPRIT-DFT and Prony-DFT) that conjointly use DFT and parametric methods. This paper, which reports the theoretical aspects of the WTS schemes and of the spectral analysis methods, is a companion paper to a paper of the same title, Part II, in which numerical experiments are performed using test waveforms and measured non-stationary voltage and current waveforms of different WTS schemes.

In this paper, we propose a novel scheme that jointly employs sliding-window (SW) Prony and the discrete Fourier transform (DFT) for estimating power-quality indices based on the time-frequency analysis of non-stationary waveforms in power-system disturbances. These indices, recently proposed in the relevant literature, require the separation of the waveform in two parts, i.e., the fundamental component and the disturbance waveform. In the proposed scheme, Prony’s method was applied to calculate only the fundamental component from low-pass, filtered data, while the disturbance waveform is obtained by using DFT. The application of Prony’s method only to the fundamental component reduces the computational effort required by this method significantly and allows the size of the window of the DFT to be set as an integer multiple of the estimated fundamental period, which minimizes the spectral leakage of the DFT. The main advantages of the proposed, new approach include high resolution and accuracy in the estimation of indices and significant reduction of the computational cost. Experiments were conducted and comparisons were made using both synthetic and measured data. The results confirmed the effectiveness and efficiency of the proposed scheme.

In recent years, modern distribution networks are rapidly evolving toward the new concept of Smart Grid (SG) due to the increasing level of penetration of distributed generation (DG) units, storage systems and information and communication technologies. In the framework of low voltage distribution systems, SGs based on either AC or DC grids have been proposed and compared in the technical literature, evidencing their advantages and drawbacks. Only recently, hybrid SGs that include both DC and AC grids have been proposed with the aim of optimizing the whole system behavior. In this paper, an innovative hybrid low voltage SG that includes properly controlled AC and DC grids is analyzed. The DC grids - including storage systems and interconnected to the AC grid through AC/DC PWM-controlled static converters - highly contribute to improve both quality and continuity of supply to AC and DC sensitive loads. The AC grid is optimally controlled to perform several system services (i.e., minimize power losses and optimize voltage profiles at fundamental frequency and at harmonic frequencies). Time domain simulations on a 30-node low voltage hybrid SG are presented and discussed to evidence the effectiveness of the considered solution.

The problem of developing a definition that adequately assesses power-quality (PQ) levels in the presence of distributed generation (DG) is addressed by using proper probabilistic indices for distribution networks. In the planning of new DG installations, these indices take the variation of PQ level into account and use weighting functions properly. They are useful in quantifying the impact of the installation of DG units because they can consider several PQ disturbances simultaneously. Also, they indicate how each PQ disturbance may affect the decision concerning the installation of DG units. Several tests on real distribution networks were performed and discussed in order to show the usefulness of these indices in evidencing the impact of DG on PQ levels. The analysis of the electrical distribution systems on the basis of the considered indices is valuable also to help decide the best allocation and size of distributed generators.

This study considers a distribution system with a number of dispersed generation (DG) units interconnected to the AC grid through power electronic interfaces. Some selected DG units were able to provide energy service and system ancillary services (in particular, voltage regulation and partial compensation or elimination of some power quality disturbances, such as waveform distortions and voltage unbalances) through a proper centralised control system that provides the reference signals to the converters of the DG units in real time. In addition, the problem of time delays owing to data acquisition and digital signal processing for reference signal calculations and the effects of these delays on compensation actions were studied. Several time-domain simulations of an actual distribution system are reported, taking into account different DG units and time-delay scenarios.

The presence of the new liberalized markets has increased the interest in power-quality (PQ) disturbances due to their economic effect. In particular, in the case of disturbances caused by a single event (such as a capacitor switching or voltage sag), the waveform assessment can be difficult due to the rapid variations in waveform spectral component characteristics; these difficulties require a suitable choice of signal-processing techniques for spectral analysis and, in particular, the resort to time-frequency representations. In this paper, the adaptive Prony method is proposed to calculate PQ indices based on a time-frequency analysis of waveforms. Numerical applications on a simulated transient due to capacitor switching, a measured voltage sag, and a test waveform are also presented and discussed in order to investigate the validity of the proposed method.

Discrete Fourier Transform (DFT) is recommended for waveform distortion assessment in IEC Standard framework. One of the most important problems for DFT application is the spectral leakage. It was shown in the literature that the most remarkable errors due to leakage problems in power system waveform assessment are mainly due to the errors in synchronizing fundamental and harmonics. In this paper, a new two-steps signal processing technique based on ESPRIT and DFT methods is proposed. This technique guarantees a fast and accurate synchronization of fundamental and harmonics; it also allows to obtain an accurate evaluation of the low frequency interharmonics. The proposed method is applied on test waveforms in order to demonstrate its effectiveness in terms of reduced computational efforts and accuracy of the results.

Electrical power system robustness is defined as the intrinsic capacity of an electrical power system to maintain assigned disturbance levels when external conditions change. This paper compares two methods for estimating voltage dip severity: the critical distance method and the fault position method. Their differences are not linked to the type of network but to the type of results which we are searching for. This paper proposes a graphical color visualization of the during fault voltage matrices that immediately correlates dip severity with colors defined by a proper scale. New robustness indices are also proposed to synthesize bus performance in terms of affected and exposed areas. Numerical applications refer to real transmission and real distribution systems.

The paper deals with the reactive power control in a MV distribution system in presence of Dispersed Generation (DG). The control variables are the reactive powers of DG units and capacitors, and the tap position of the on-load tap changer of the HV/MV transformer. Single-objective and multi-objective optimization problems are formulated and solved in order to achieve the optimal control, even containing the impact of the waveform distortions. Numerical applications performed on a 109-bus test distribution system give evidence of the effectiveness of the proposed procedure.

The International Electrotechnical Commission (IEC) standards characterize the waveform distortions in power systems with the amplitudes of harmonic and interharmonic groups and subgroups. These groups/subgroups utilize the waveform spectral components obtained from a fixed frequency-resolution discrete Fourier transform (DFT). Using the IEC standards allows for a compromise among the different goals, such as the needs for accuracy, simplification, and unification. In some cases, however, the power-system waveforms are characterized by spectral components that the DFT cannot capture with enough accuracy due to the fixed frequency resolution and/or the spectral leakage phenomenon. This paper investigates the possibility of a group/subgroup evaluation using the following advanced spectrum estimation methods: adaptive Prony, estimation of signal parameters via rotational invariance techniques, and root multiple-signal classification (MUSIC). These adaptive methods use variable lengths of time windows of analysis to ensure the best fit of the waveforms; they are not characterized by the fixed frequency resolution and do not suffer from the spectral leakage phenomenon. This paper also presents the results of the applications of these methods to three test waveforms, to current and voltage waveforms obtained from simulations of a real DC arc-furnace plant, and to waveforms measured at the point of common coupling of the low-voltage network supplying a high-performance laser printer.

The presence of the new liberalized markets has increased interest in power quality disturbances due to their economic value. Customers have more choices; then disturbances are becoming bigger issues and suppliers and customers should regulate them in the frame of power quality contracts. A single global power quality index can be very useful for characterizing the overall level of the voltage waveform quality; in particular, this index can provide global indications regarding different aspects of voltage quality and allows a strong reduction of the measurement data needed to cover all power quality disturbances, in this paper, a global power quality index (GPQI) for discrete disturbances is proposed, based on the discrete severity indicators (DSI). We present the results of a GPQI evaluation of an Italian distribution system on voltage dips, whose measured data are available on the website of the project QuEEN led by CESI Ricerca.

The wide use of the induction motors for industrial and transport applications leads to increase the interest in their monitoring for on-line detection of incipient faults. The traditional diagnostic techniques are based on the stator current or torque spectral analysis by FFT-based methods. However, the well known spectral leakage phenomena of FFT can cause not accurate estimation of the spectral components; moreover, its frequency resolution problems limits a fast detection of the fault condition. In order to solve these problems, this paper presents an high resolution method, the ESPRIT method, to make the fault identification faster and accurate. A proper dynamic model of the induction machine is used to simulate the occurring of a fault condition. Simulation results allow to compare the ESPRIT and FFT-based methods, considering the case of some rotor bars broken.

The method of fault position is useful for characterising power-system performance in the presence of voltage dips due to faults. It is based on short-circuit simulations repeated for all the system nodes and for many points along the system lines: fault voltages that are below a preset threshold are the required voltage dips. These dips are stored in so-called dip matrices which contain only the dips in all the system nodes when faults occur at points along the lines. The paper proposes a new compact analytical formulation of dip matrices for balanced and unbalanced faults in terms of bidimensional vector relations and for site- and system-voltage dip indexes. Compact formulations are very useful tools when several sensitivity analyses have to be conducted to estimate variation of site- and system-voltage dip indexes in relation to possible reinforcement and/or compensation devices. Graphical presentation of dip matrices is also proposed as a valuable tool to ascertain the critical area for system performance. Numerical applications to an actual transmission system are presented to demonstrate the easy applicability of the model.

One of the main problems for waveform distortion assessment that is to reduce the effects of the spectral leakage due to fundamental and harmonics on interharmonic components. In the framework of DFT approaches, line frequency synchronization techniques or algorithms to compensate desynchronization are necessary; alternative approaches as those based on the Prony methods are not sensitive at all to desynchronization but can require not negligible computation burden. In the paper, different proposals by the same authors regarding both DFT based and Prony based advanced methods are recalled and compared to each other. The results of several numerical experiments are reported and analyzed; some of them are in accordance with IEC guidelines and some others are related to more open scenarios.

Nowadays the need for more flexible electric systems, the changing in the regulatory and economic scenarios, the importance of harnessing energy savings and minimising environmental impact have been providing the impetus for the development of a new distribution system into which embedded generation with wind farms is predicted to play a key role. While several advantages result from wind farms presence in distribution systems (reduction of power losses, enlargement of the time horizon for distribution company investments aimed to the network reinforcement, and so on), they can involve technical problems among which the ones related to power quality have received a great interest in the most recent relevant literature. In this paper the problem of calculation of the waveform distortions in distribution systems with wind farms is considered. Taking into account that unavoidable uncertainties affect the modelling input data, a probabilistic harmonic power flow (PHPF) is proposed which properly include the randomness linked to the wind farm presence. The PHPF permits the evaluation of the percentile and maximum values of the probability density functions of the waveform distortion indices that are nowadays the statistical measures of interest in the most recent international standards and recommendations. Numerical applications on a 32-bus test system are also shown

Probabilistic and time-frequency aspects of time varying waveforms in power systems are considered with reference to DC arc furnaces and adjustable speed drives using line commutated inverters. These loads are characterized by waveform whose spectral components are time-varying in amplitude and frequency. The problems of understanding and assessing their waveform distortion is taken into account. Some considerations on the most adequate techniques for the spectral component representation and calculation are effected

This paper investigates the problem of contemporaneously choosing optimal locations and sizes for both shunt capacitors and series voltage regulators in three-phase unbalanced distribution systems. The sizing and placement procedure not only minimizes the power losses along distribution feeders but also makes sure that both capacitors and series regulators will have the minimum possible impact on the harmonic distortion of bus voltages in the system. The fact that distribution systems can operate under unbalanced loading conditions means that the optimization will have to account for any unbalances in the system. A genetic algorithm, which successfully solves the above-described problem, is developed and tested. This paper presents the results of simulations for a 34-bus IEEE distribution test system.

The widespread use of advanced technologies and automation in AC and DC railway systems requires an accurate investigation of the Power Quality disturbances they suffer. In some cases, both railway systems operate in the same transmission network area, so that voltage unbalances and waveform distortions can be complicated by the interactions between the two systems. In this paper, a robust electronic compensation device is proposed in order to solve the interaction problems; the solution is based upon a two-phase inverter with a non-linear and variable structure control system. Several numerical applications give evidence of its rejection action against disturbances and power parameters variations.

Both amplitude and argument of voltage harmonics can affect the electrical component behavior. In the paper an integrated index taking into account both of them is proposed for inclusion in harmonic standards. This index, which refers to the well known peak value of distorted voltage, allows taking into account the main effect of distorted voltages on component life. The time varying nature of harmonics is also considered and, coherently, limits to be assigned to this index are proposed also in probabilistic scenarios.

The problem of the evaluation of the buried cable thermal useful life in presence of harmonic distortion and taking into account the moisture migration is analysed. The evaluation is effected in nondeterministic scenario with a probabilistic approach. Numerical applications referred to medium voltage cables are presented and discussed in order to show the influence of the harmonics and moisture migration on the cable useful life.

There is an increasing interest in standards and technical literature on the probabilistic characterisation of voltage and current harmonics in multiconvertor power systems. Characterisation requires the expression of input data of the harmonic penetration modelling by random variables and application of probabilistic techniques of analysis. A probabilistic method is described for harmonic analysis of a multiconvertor power system which takes into account the interactions between the supply voltage distortions and the converter current harmonics. To reduce computational effort, the proposed method linearises around an expected value region the nonlinear equation describing the system steady-state behaviour. Consequently it offers a faster solution than the one based on probabilistic iterative methods which need Monte Carlo simulation procedures. Numerical applications to a test system are presented and discussed to demonstrate the accuracy and speed of the proposed method.

Harmonic distortion can affect significantly the reliability of electrical plant components by increasing the degradation rate of electrical insulation. This paper presents life models which enable estimation of failure times of the electrical insulation of the main components of industrial electrical plants, i.e. capacitors, cables, motors and transformers, when the prevailing aging stresses are voltage and temperature and harmonics give rise to considerable voltage and current distortion. The models are inserted in a probabilistic framework, which accounts for the inherently random nature of harmonics. It is shown that the degradation rate can become much larger than in the sinusoidal case, especially due to the contribution of harmonics to voltage peak magnification: also current distortion can play an important role. Hence, significant life reduction can be expected in the components which are subjected to harmonics.

The increasing presence of current and voltage harmonics in distribution and transmission systems is well known. The harmonics are due to the wide-spread use of nonlinear loads both for efficient energy utilization in residential, commercial and industrial areas and for increasing the flexibility of power systems; they can seriously damage the system components and operation. Over the past several years electric utilities have experienced and research groups have confirmed the nondeterministic nature of harmonics, due to continual changes in system configurations, in linear load demands and in operating modes of nonlinear loads. Taking into account the time-varying nature of harmonics, it is most useful to introduce random variables and to apply probabilistic techniques of analysis.

This paper describes the various techniques available for the analysis of power system harmonic distortion generated by nonlinear loads having random variation. The techniques may be classified as either direct Monte-Carlo based or detailed, where Monte-Carlo simulation is used with complex convertor models. It is concluded that the methods are able to produce accurate information which stand up to verification against measurement and that the results of the analysis are in the correct form for use with the proposed harmonic standards to limit distortion levels.

Harmonic bursts are short-duration waveform distortions which result from the sudden variations in nonlinear loads; they often cause damage to common power system components. A probabilistic model is proposed that describes these disturbances in terms of suitable stochastic processes. The use of this model results in indices that can be used to characterise the system state. The indices are based on appropriate mean values of random functions which measure the quality degradation in terms of the occurrence of disturbance levels higher than a fixed value. They are particularly suited to the characterisation of the risk related to the severe burst occurrences and can allow a suitable prediction of system behaviour over time. A numerical application is presented to illustrate the physical meaning of these indices. The key feature of the proposed model is that it can be used for both burst amplitude and duration characterisation.

The deregulated market requires a great deal of attention to satisfy reliability, security and optimisation objectives. As is well known, the voltage stability problem may become more and more frequent in this new scenario. Various techniques have been proposed in order to incorporate voltage stability aspects into the classical analysis methods of power systems. However, some technical problems, such as unbalances typical of distribution or transmission systems, require suitable formulations in order to derive critical conditions and their dependence on interest parameters. Hence, a three-phase formulation arises in order to characterise properly the inability of unbalanced power systems to meet heavy load demand. More specifically, a three-phase constrained optimal power flow is proposed in which the load demand at the assigned bus is maximised, simultaneously satisfying power balance equations and various technical constraints. An application is presented with reference to the IEEE 13-bus test system, which highlights the feasibility and the goodness of the proposed technique. Sensitivity analyses are performed in order to capture the dependence of voltage stability on the level of unbalance.

The problem of choosing optimal locations and sizes for shunt capacitors in distribution systems is addressed. The objective of the capacitor placement procedure is not only to minimise the power losses along distribution feeders, but also to make sure that these capacitors will have the minimum possible impact on the harmonic distortion of bus voltages in the system. Furthermore, the fact that distribution systems can operate under unbalanced loading conditions, means that the optimisation will have to account for any unbalances in the system. An optimisation program, which successfully solves the problem is developed and tested. Results obtained by simulation of a distribution test system under various operating conditions, are presented to validate the proposed solution methods.

Passive and active filters have traditionally been used to reduce harmonics in power systems. Only recently combined systems of shunt passive and active filters have been proposed in order to overcome the technical problems of passive filters and to reduce the costs of active ones. The paper tries to overcome some drawbacks of a combined filtering system (modified high-pass filter) which appeared in literature. In particular, improvements in the system structure and in the steady-state design methodology are proposed to obtain a filtering system with both satisfactory compensation characteristics and low costs. Finally, numerical applications are discussed considering a test case.

This is a companion paper to a paper of the same title, Part I, in which the theoretical aspects of some spectrum estimation methods for analysis of nonstationary voltages and currents in power systems with nonlinear loads are analysed. In particular, the Prony model and rootMusic method are taken into account. In this work, numerical applications of the methods presented in Part I are discussed, in order to illustrate their implementation problems and practical applications. To investigate the methods, several experiments were performed using nonstationary voltage and current waveforms in a supply system of a DC arc furnace. For comparison, similar experiments were repeated using the short-time Fourier transform. The comparison proved the effectiveness of the new methods; however, their computational efforts can be much more complex. Conclusions and recommendations are made concerning the framework of each method.

Representation of signals in time and frequency domain has been of interest in power systems for many years, especially in the case of nonstationary signals. Standard tools of waveform distortion analysis are based on the Fourier transform while only recently the Prony and rootMusic methods were tested for this purpose. However, these methods show a strong sensitivity to the number of spectral components when applied to evaluate waveform distortion indices. This paper investigates this problem in order to evidence their optimal number. Numerical simulations on test waveforms and on DC arc furnace current waveforms allow to point out the most suitable choices.