Maher A. Azzouz

Real-Time Optimal Voltage Regulation for Distribution Networks Incorporating High Penetration of PEVs

This paper proposes a vehicle-to-grid reactive power support (V2GQ) strategy for optimal coordinated voltage regulation in distribution networks with high distributed generation (DG) penetration. The proposed algorithm employs plug-in electric vehicles (PEVs), DG, and on-load tap changer (OLTC) to satisfy PEV charging demand and grid voltage requirements with relaxed tap operation, and minimum DG active power curtailment. The voltage regulation problem is formulated as a nonlinear programming and consists of three consecutive stages, in which successive stages apply the outputs of their preceding stages as constraints. The first stage aims to maximize the energy delivered to PEVs to assure PEV owner satisfactions. The second stage maximizes the DG extracted active power. Third stage minimizes the voltage deviation from its nominal value utilizing the available PEV and DG reactive powers. The main implicit objective of the third stage problem is relaxing the OLTC tap operation. In addition, the conventional OLTC control is replaced by a proposed centralized controller that utilizes the output of the third stage to set its tap position. Real-time simulations are developed to demonstrate the effectiveness of the proposed optimal coordinated algorithm on a typical distribution network using OPAL-RT real-time simulator (RTS) in a hardware-in-the-loop (HIL) application.

Distance Protection of Lines Connected to Induction Generator-Based Wind Farms During Balanced Faults

Wind farms (WFs) are increasingly integrated with high-voltage (HV) grids, for which distance relaying is normally the protection of choice. This paper reveals some serious defects of distance protection for the lines connected to induction generator (IG)-based WFs during balanced faults. It is shown that for the squirrel cage IG (SCIG) WFs, distance protection becomes insecure, while for the doubly fed IG (DFIG) WFs, the relay performance is utterly unreliable, due to operating scenarios that are unique to such WFs, and are unfamiliar to the existing relaying practices. The detected failures can easily result in unnecessary WF tripping, thus jeopardizing the objectives pursued by the new grid codes that oblige WFs to remain connected to the grid during disturbances. Moreover, a novel modified permissive overreach transfer trip (POTT) scheme along with a fault current classification technique is proposed to address these problems for both types of IG-based WFs, and the accurate nondelayed protection of a distance relay over the entire line length is restored. A comprehensive performance evaluation confirms the findings of this paper and validates the efficacy of the proposed solution for all operating conditions. Results are particularly promising for the DFIG-based WFs with nonzero crowbar resistance, which is the most likely situation confronted by distance relays.

Multiagent Supervisory Control for Power Management in DC Microgrids

This paper proposes multiagent supervisory control for precise power management in isolated dc microgrids. Two power management aspects are considered: 1) equal power sharing, which is realized via a proposed distributed equal power sharing algorithm; and 2) optimal power dispatch, which is achieved through a proposed distributed equal incremental cost (DEIC) algorithm. Both algorithms offer the additional advantage of the ability to restore the average system voltage to its nominal value. The proposed algorithms are based on the application of the average consensus theory along with voltage sensitivity analysis. Each distributed generation (DG) unit exchanges information with its neighbors, thus locally updating its no-load voltage setting to achieve the supervisory control objectives. The incorporation of DG droop-based control renders the proposed algorithms fully distributed with a reduced number of agents. The stability of the proposed algorithms is addressed, as well as the convergence of the proposed DEIC algorithm. Real-time OPAL-RT simulations demonstrate the effectiveness of the proposed algorithms in a hardware-in-the-loop application.

Real-Time Fuzzy Voltage Regulation for Distribution Networks Incorporating High Penetration of Renewable Sources

This paper proposes a coordinated voltage regulation scheme for on-load tap changer (OLTC) and renewable distributed generation (DG) units to provide a proper voltage regulation for active distribution systems. The main motivation of applying fuzzy logic is that it can deal with environments of imperfect information, and thus, it can reduce communication requirements. The proposed regulation scheme consists of three fuzzy-based control algorithms. The first control algorithm is proposed for the OLTC such that it can mitigate the effect of DG units on the voltage profile. The second control algorithm is proposed to provide a DG reactive power sharing, in order to relax the OLTC tap operation. The third control algorithm aims to partially curtail DG active powers to restore a feasible solution from the OLTC prospective. The proposed fuzzy algorithms have the advantage of providing proper voltage regulation with relaxed tap operation, utilizing only the estimated system minimum and maximum voltages. Moreover, it avoids numerical instability and convergence problems associated with centralized approaches, as it does not require to run an optimization algorithm. Real-time simulations are developed to show the effectiveness of the proposed fuzzy algorithms on a typical distribution network, using OPAL-RT real-time simulator.

Three-Phase Fault Direction Identification for Distribution Systems With DFIG-Based Wind DG

Distributed generation (DG) integration necessitates upgrading some distribution system overcurrent relays to directional ones to offer selective protection. The directional feature is conventionally achieved by phase angle comparison between phasors of the fault current and a polarizing quantity, normally a voltage signal. Doubly fed induction generator (DFIG)-based wind turbines constitute an appreciable portion of todays DG power. This paper unveils that conventional directional elements malfunction during three-phase short-circuits when a distribution system incorporates DFIG-based wind DG. The maloperation is due to the exclusive fault behavior of DFIGs, which affects the existing relaying practices. The paper also proposes a fault current classification technique that replaces the conventional directional element during problematic conditions and provides accurate fault direction quickly based on waveshape properties of the current. An extensive performance evaluation using PSCAD/EMTDC simulation of the IEEE 34 bus system corroborates the effectiveness of the proposed method. Results are exceptionally encouraging in the case of resistive crowbar circuits for DFIGs, which is the typical scenario in practice.

A Sequential Power Flow Algorithm for Islanded Hybrid AC/DC Microgrids

This paper proposes a sequential power flow algorithm for hybrid ac/dc microgrids operating in the islanded mode. Unlike in grid-connected systems, variable, rather than fixed, frequency and voltage are utilized for power coordination between the ac and dc microgrids, respectively. The main challenge is to solve the power flow problem in hybrid microgrids while considering the absence of a slack bus and the coupling between the frequency and dc voltage. In the proposed algorithm, the ac power flow is solved using the Newton-Raphson (NR) method, thereby updating the ac variables and accordingly utilizing these variables in a proposed interlinking converter model for the dc problem. This sequential algorithm is iterated until convergence. The proposed algorithm is generic and can include different operational modes not only for the distributed generation units (DGs), but also for the interlinking converters. Detailed time-domain simulations using PSCAD/EMTDC have validated the algorithms accuracy. Its robustness and computational cost are contrasted to those of conventional algorithms.

Addressing IEC Flickermeter Deficiencies by Digital Filtration Inside a Sliding Window

Although globally deployed for flicker assessment, the flickermeter presented by International Electrotechnical Commission (IEC) Standard 61000-4-15 has been proved to suffer from some deficiencies regarding voltage rectangular modulation and interharmonics. The latter results in flickermeter inability to accurately measure flicker for non-incandescent lamps, which are ubiquitous nowadays. The flickermeter inaccuracies tend to arise from its demodulator. So far, few solutions have been provided. Furthermore, the existing solutions deal with only the interharmonic case. This paper introduces a method to address IEC flickermeter deficiencies by developing a digital signal processing block that demodulates the voltage in a real-time manner for voltages that include interharmonics or are affected by rectangular modulation. The suggested demodulator implements discrete Fourier transform inside a sliding window. For rectangular modulation, this approach is used to demodulate the voltage. Meanwhile, for the interharmonics problem, this approach provides the interharmonic frequency. Using the measured frequency, least error squares technique is then employed inside another sliding window to find the interharmonics amplitude. On this basis, a modified flickermeter is devised. Performance of the modified flickermeter is evaluated and its effectiveness is verified using the results obtained from an experimental set-up. The contribution of this paper is not limited to IEC flickermeter modification; the proposed method can be employed for any application requiring interharmonic measurement.

Multivariable Grid Admittance Identification for Impedance Stabilization of Active Distribution Networks

Estimating grid admittance is essential for assessing impedance stability and for designing adaptive controllers for distributed generation (DG) units. This paper proposes a new multivariable grid admittance identification algorithm that involves adaptive model order selection as an ancillary function within inverter-based DG controllers. Cross-coupling between

Optimal coordinated volt/var control in active distribution networks

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Fuzzy-based control of on-load tap changers under high penetration of distributed generators

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