M. Tavakoli Bina

Nonlinear Modeling of Temporal Wind Power Variations

Modeling wind speed time series (WSTS) is an essential part of network planning studies in order to generate synthetic wind power time series (WPTS). Hence, this paper proposes a methodology to equip planners with accurate simulation of wind speed and power variations as well as complete temporal dependence structure based on the copula theory. Unlike traditional autoregressive and Markov chain methods, the suggested technique is well-prepared to deal with “nonlinear long-memory temporal dependence” and “non-Gaussian empirical probability distributions” of the WSTS. Meanwhile, the proposed statistical modeling framework is compatible with the scenario-based analysis of active networks as well. Furthermore, a case study for optimal sizing of an autonomous wind/photovoltaic/battery system is presented. The purpose of the presented study is to fully examine the accuracy and effectiveness of the copula-based model of wind generation for planning nonmemoryless power systems. Among a list of commercially available system devices, the optimal number and type of units are calculated ensuring both a minimum 20-year round total system cost and a perfect reliability. The genetic algorithm is used in four wind generation scenarios consisting of real and simulated WPTS. Then, considering the corresponding optimal solutions, the autoregressive moving average (ARMA), nonparametric Markov and proposed copula-based simulations are compared against real data.

Extended modular multilevel converters suitable for medium-voltage and large-current STATCOM applications

This paper discusses a new transformerless shunt static compensator (STATCOM) based on the extended modular multilevel converter (EMMC), to achieve higher performance in a medium-voltage large-current network. It introduces an appropriate control algorithm based on a phase-shifted carrier modulation strategy, to ensure the source-end three-phase currents are sinusoidal and balanced, balancing the voltages of the dc-link capacitors of the modules as well. Compared to the conventional modular multilevel converters (MMC), the EMMC introduces higher reliability and better performance in the distorted large-current systems. Simulation results from a complete model of the proposed STATCOM are presented, confirming the pre-defined objectives. One interesting application for the EMMC-based STATCOM could be the improvement in power quality and performance of the electrified railway power supply systems.

New transformerless medium-voltage STATCOM based on half-bridge cascaded converters

To achieve higher performance in a distorted and unbalanced medium-voltage large-current system, a new shunt compensator is presented based on half-bridge cascaded converters (HBCC). Then, the transformerless HBCC based STATCOM can be controlled for various purposes such as reactive power of STATCOM, simultaneous harmonic cancellation and load balancing procedure. Compared to the conventional modular multilevel converters, the HBCC based STATCOM introduces lower ratings both for active and passive elements, improving the overall efficiency of the converter. A control strategy is proposed to ensure the source-end three phase currents are sinusoidal and balanced. At the same time, the DC capacitors voltages are controlled to operate under balanced condition. One interesting application for the HBCC based STATCOM could be electric traction system. Both power circuit and the proposed control strategy are simulated with PSIM and MATLAB, confirming the pre-defined objectives.

Frequency estimation of distorted signals in power systems using particle extended Kalman filter

In this paper, frequency of distorted signal in power system has been estimated with particle extended Kalman filter. Base of particle algorithm, extended Kalman filter and particle extended Kalman filter are mentioned. For selecting state variables, a nonlinear time-variant sinusoidal signal is developed then a particle extended Kalman filter is applied to detect the frequency variations. Several tests are performed to show the performance of PEKF algorithm. Comparison of PEKF with EKF reveals the PEKF preference. Also, these tests prove the fast speed, good accuracy and robustness against noise. These advantages illustrate that PEKF is more suitable for power system applications.

Novel control structure of single-phase active power filter using recursive least squares estimator considering a distorted environment

In this paper, a new reactive power and harmonic compensation procedure based on the variable forgetting factor-recursive least squares (VFF-RLS) estimator and new phase-locked loop is proposed. For this purpose, the fundamental harmonic component of load current is estimated with VFF-RLS. Independence on order and amplitude of harmonics, fast dynamic and high accuracy are the remarkable features of the designed estimator. As in the case of load changes, the sag/swell and phase jump in grid voltage, optimal compensation is performed in less than half a cycle with a low total harmonic distortion value. In addition, to compensate for an infected power system by varied frequency, a second-order general integrator-frequency-locked loop (SOGI-FLL) control system based on wavelet transform (WT) has been proposed. This phase detector can track the reference phase of a grid in the best manner in the presence of frequency variations. The effectiveness of the proposed method in a single-phase power system has been validated using both simulation and experimental results.

SSSC circuit model for three-wire systems coupled with Delta-connected transformer

This paper presents a new circuit model of SSSC based on state equations in three-wired systems. SSSC is a series compensator of FACTS family. It injects an almost sinusoidal voltage with variable amplitude and is equivalent to an inductive or a capacitive reactance in series with the transmission line. The analysis of a power electronics system is complex, owing to its switching behavior. Since such a system has its special complexities, the need for simpler model is essential, though a more practical one is sometimes appropriate. With simple changes, this papers model shows different states of SSSC, both transient and steady states, in the range of ideal to practical as confirmed by MATLAB simulation. This model is suitable for circuit simulation software such as MATLAB or PSPICE and is proper to control process and control methods. This model can be used as a starting point for further investigations on control methods in the future.

Multivariate modeling of uncharacteristic harmonics using archimedean copulas

This paper introduces “Copulas” analytical tool for multivariate modeling of stochastic harmonic generation mechanism. Additional stochastic harmonics can be modulated through the power inverters at the point of common coupling (PCC) under unbalanced non-linear loads. The proposed multivariate unbalance modeling via copulas is applied to evaluate aggregate harmonics injection at the PCC. Copulas have become a popular analytical tool in multivariate modeling, where recently has been applied in many fields. Here, the contributions of copulas to Monte Carlo method are described. It is first come up with modeling unbalance of three-phase active and reactive powers at a distribution substation. To introduce a firmer basis for the suggested procedure, the investigation is carried out based on the measured data for pursuing further analysis that is associated with simulating statistical correlation between stochastic harmonics and realistic unbalanced conditions for a static compensator (STATCOM) at the point of common coupling (PCC).

A Critical Cross-Examination on Load-Balancing Transformers for Distribution Systems

A load balancing transformer (LBT) has already been suggested for improving the unbalance of three-phase primary currents. Each phase includes one extra pair of coupling windings in addition to the usual primary and secondary windings. Two coupling windings, located on two different phases, are made in series, where the resulting circuit is then reversely paralleled with the secondary winding placed on the third phase. Under unbalanced conditions, the load currents are distributed between the coupling and secondary windings that are supplied through different primary phases. This paper proposes a method in order to define all possible LBTs, which paves the way to cross-examine and select the best connections of windings for the LBT. Starting from the original LBT, it is shown that the best LBT is practically connected, such as a zig-zag winding. Then, a novel idea is suggested in which a controller and some four-quadrant semiconductor switches contribute to the improvement of the performance of the LBT. These switches should control the amount of current transfer from one phase to another. A combinatorial selection problem is arranged to find the best way of switching modulation. Both simulations and experimental works (using a designed 12-kVA laboratory prototype) verify the studied examination and proposals, showing the switch-mode zig-zag LBT as having the best performance.

Neural identification of SSSC based on average model using GAMMA, DNN, RBF and MLP for steady state calculations

The use of exact model of FACTS devices in steady state calculations is complex, due to their switching behavior. However, applying very simple models such as pure inductor/capacitor for FACTS devices leads to inaccurate results in power system studies. In addition in a power market the use of inaccurate models for power system components affects the electrical energy pricing system; while this is very crucial when FACTS devices are used for congestion management of the transmission systems. Average technique provides an appropriate time-domain representation of FACTS devices in which high frequency switching ripples are vanished. But average model can not be directly applied to the power system steady states. Thus this paper extends average model and presents an average-neural model of SSSC as a series FACTS device, which is well-suited for analytical purposes in power system applications. To this extend, design and development of four neural identifiers are performed using the GAMMA, DNN, RBF and MLP. To verify the developed models, the exact solutions obtained from the average model of SSSC are compared with the outcomes of the identifiers.

A new phase sequence detector for the three-phase rotary loads

Automated load balancers can swap output phases of feeders in a distribution substation by the use of matrix converters. In addition, the rotational three phase loads operate clockwise if three phase system has positive sequence. In order to complete the proposed method for balancers, in this paper three different methods is proposed for recognizing phase sequence in power systems. The first evaluated method detects phase sequence by measuring of instantaneous symmetrical components. This method has drawbacks of complexity and low speed in detecting. The second method detects phase sequence by using a phase lock loop (PLL) for metering the power system frequency. The second technique is fast, but PLL that is used in this technique operates incorrectly under unbalanced condition. Finally, the third method is a new technique that is proposed for the first time in this paper which is very fast. This innovative method detects phase swapping by the zero crossing times. The whole methods are simulated by SIMULINK to verify and compare the suggested method with other techniques.