Application of local variations and maximum sensitivities selection for optimal placement of shunt capacitor banks under nonsinusoidal operating conditions

A new iterative algorithm is proposed for optimal sizing and placement of fixed and switched capacitor banks in radial distribution lines in the presence of linear and nonlinear loads. The method is a combination of maximum sensitivities selection (to improve convergence by selecting candidate buses) and local variations (to improve accuracy in the permissible solution region) and is implemented using the HARMFLOW codes. Objective functions include minimum energy losses, peak losses and capacitor costs while IEEE-519 power quality limits are used as constraints. The 18 bus IEEE distorted system is used to investigate the advantages and limitations of the proposed algorithm and compare its performance with the maximum sensitivities selection (MSS) method. The contributions of the proposed algorithm, as compared to prior publications, are the consideration of harmonic couplings and reactions of actual nonlinear loads, improved convergence and more accurate solutions.     

Experimental analysis of a DC bucking motor blockinggeomagnetically induced currents

An experimental study investigates the performance of a DC bucking motor for the blocking of geomagnetically induced currents (GICs) within a 3 kVA/416 V three-phase distribution feeder. This blocking method consists of connecting a separately excited DC motor between neutral and ground of a Y-grounded transformer consisting of a bank of three single-phase transformers. Losses and currents of the transformer and the blocking device are measured at no-load, rated-load, unbalanced-load and line-to-neutral short circuit conditions without and with GICs. Thereafter, the DC, AC and total λ-i characteristics of one single-phase transformer of the above-mentioned transformer bank are simultaneously measured using an arbitrary waveform generator, a power amplifier and a digital data acquisition network. These λ-i characteristics (where the DC bias current is a parameter) are then used to model a single-phase transformer under DC bias current conditions (e.g., assumed GICs) and to calculate voltages and currents of a single-phase transformer when subjected to GICs. The simulated signals are then compared with measured time functions     

Using equidistant vector-based hysteresis current regulators in DFIG wind generation systems

This paper proposes a novel equidistant vector-based hysteresis current regulator (VBHCR) in the rotor-side converter (RSC) of DFIG-based wind generation systems. The Γ-form equivalent circuit is used for the machine modelling, with the discrete formulation of the RSC output voltage. The overall vector control scheme is then explored and the control structure of the proposed equidistant VBHCR is presented. When compared to the commonly used PI current regulators, the proposed VBHCR exhibits several advantages such as very fast transient response, simple hardware implementation, satisfactory steady-state performance, and intrinsic robustness to machine parameters variations. Moreover, fixed hysteresis bands are replaced with equidistant bands in order to limit the instantaneous variations of the switching frequency and reduce the maximum switching frequency of the RSC. Detailed simulation studies are carried out for a 1.5 MW DFIG-based wind generator to examine the operation of the proposed current regulator under various operating conditions and demonstrate its superiority over the standard PI current regulator.     

Low and high voltage ride-through of DFIG wind turbines using hybrid current controlled converters

Doubly fed induction generators have been recognized as the dominant technology used in wind generation systems. However, this type of wind generator is very sensitive to the drop/rise in the supply voltage and without efficient “ride-through” strategy, continuous operation of DFIG may fail due to destructive overcurrents in the rotor winding or large overvoltages in the dc-link capacitor. This paper introduces a hybrid current control scheme, implemented in the rotor-side and grid-side converters of DFIG, to enhance low and high voltage ride-through capacities of DFIG-based wind turbines. The proposed control scheme is constituted of two switching strategies integrated with a supervisory control unit: standard PI current controllers for normal operating conditions and vector-based hysteresis current controllers for DFIG protection during severe voltage sag/swell conditions. Time-domain simulation studies are carried out to examine the effectiveness of the proposed ride-through strategy under various types of grid disturbances. It is shown that the proposed controller constrains the rotor current and dc-link voltage within the safety limits of DFIG and as a result, the wind generator can comply with the strict low/high voltage ride-through requirements stipulated by modern grid codes.     

Transformer magnetizing current and iron-core losses in harmonicpower flow

The problem of modifying the harmonic power flow analysis to permit the inclusion of nonlinear anisotropic transformers and computation of their iron-core (and copper) losses are discussed. The nonlinear model used for transformers is capable of simulating saturation of iron-cores, anisotropy of laminations, and the iron-core and copper losses associated with anisotropic transformers, but simple enough to be included in the harmonic power flow algorithm without deteriorating its convergence properties. The dependency of iron-core losses on the maximum value of the total (fundamental and harmonic) flux density and the fact that iron-core losses are a function of the waveform of the induced voltage-that is its harmonic phase shifts with respect to the fundamental phase angle-are included. The induced voltage is transformed from frequency domain to time domain: the instantaneous induced voltage and the computed λ-i characteristics are employed to compute the instantaneous magnetizing and core-loss currents. Thereafter a transformation is made from time domain back to frequency domain to compute the fundamental and harmonic components of the above-mentioned currents. Therefore, transformer harmonic couplings are properly modeled and included in the analysis. The modified harmonic load flow formulation is applied to a balanced three-phase feeder consisting of a grounded-wye, grounded-wye nonlinear anisotropic transformer and linear and nonlinear loads     

Utilization of DFIG on an Islanded Power Generation and Distribution System

The utilization of wind generation equipment, such as DFIGs （double fed induction generators）, interconnected to islanded power generation and distribution systems is investigated in order to determine their effects on the overall system operating characteristics and stability. The use of a stable power station （with high speed machines） will be critical in achieving fast and reliable transient response to network events, in particular, when large transient loads are expected on a continuous basis, i.e., industrial mining and mineral processing equipment. Simulation results of this paper assist in understanding how small power stations and wind generation equipment respond to large transients in an islanded network. In particular, detailed simulations and analyses will be presented on impacts of distributed wind generation units （1.5 MW DF1G） on the stability of a small weak network. The novelty of this paper is on detailed analyses and simulation of weak networks with interconnects DFIG＇s including their impacts on system stability under various transient operating conditions.     

Theoretical and experimental analyses of photovoltaic systems with voltageand current-based maximum power-point tracking

Detailed theoretical and experimental analyses are presented for the comparison of two simple, fast and reliable maximum power-point tracking (MPPT) techniques for photovoltaic (PV) systems: the voltage-based (VMPPT) and the current-based (CMPPT) approaches. A microprocessor-controlled tracker capable of online voltage and current measurements and programmed with VMPPT and CMPPT algorithms is constructed. The load of the solar system is either a water pump or resistance. "Simulink" facilities are used for simulation and modeling of the novel trackers. The main advantage of this new MPPT, compared with present trackers, is the elimination of reference (dummy) cells which results in a more efficient, less expensive, and more reliable PV system.     

Microprocessor-controlled new class of optimal battery chargers for photovoltaic applications

A simple, fast and reliable technique for charging batteries by solar arrays is proposed. The operating point of a battery is carefully forced near the maximum power point of solar cells under all environmental (e.g., insolation, temperature, degradation) conditions. Optimal operation of solar arrays is achieved using the Voltage-Based Maximum Power Point Tracking (VMPPT) technique and the charger operating point is continuously adjusted by changing the charging current. An optimal solar battery charger is designed, simulated and constructed. Experimental and theoretical results are presented and analyzed. The main advantages of the proposed solar battery charger as compared with conventional ones are shorter charge time and lower cost.     

Large signal nonlinear model of anisotropic transformers fornonsinusoidal operation. II. Magnetizing and core-loss currents

For pt.I see ibid., vol.6, no.1, p.174 (1991). No-load currents and voltages of saturated single-phase transformers are computed based on given λ-i and harmonic core-loss characteristics. A circuit analysis approach is used to satisfy the nonlinear equations involved in an iterative manner. The theoretical and experimental analyses confirm that for the calculation of the iron-core losses under nonsinusoidal operating conditions the principle of superposition cannot be applied: the iron-core losses are dependent not only on the amplitudes of the fundamental and the harmonics of the flux density, but also on the phase shifts of the harmonics with respect to the fundamental. For the computation of the ohmic losses at no-load the skin effects within the windings are taken into account. To confirm the validity of the computed results some of them are compared with measurements     

Discussion of "Measured efficiency improvements of induction motors with thyristor/triac controllers"

For original paper see Fuchs and Hanna (IEEE Trans. Energy Conversion, vol.17, p.437-44, December 2002). The present authors discuss several queries relating to that paper.