Three-phase Power Grid Research Articles

A Novel Fast Open-loop Phase Locking Scheme Based on Synchronous Reference Frame for Three-phase Non-ideal Power Grids

Rapid and accurate phase synchronization is critical for the reliable control of grid-tied inverters. However, the commonly used software phase-locked loop methods do not always satisfy the need for high-speed and accurate phase synchronization under severe grid imbalance conditions. To address this problem, this study develops a novel open-loop phase locking scheme based on a synchronous reference frame. The proposed scheme is characterized by remarkable response speed, high accuracy, and easy implementation. It comprises three functional cascaded blocks: fast orthogonal signal generation block, fast fundamental-frequency positive sequence component construction block, and fast phase calculation block. The developed virtual orthogonal signal generation method in the first block, which is characterized by noise immunity and high accuracy, can effectively avoid approximation errors and noise amplification in a wide range of sampling frequencies. In the second block, which is the foundation for achieving fast phase synchronization within 3 ms, the fundamental-frequency positive sequence components of unsymmetrical grid voltages can be achieved with the developed orthogonal signal construction strategy and the symmetrical component method. The real-time grid phase can be consequently obtained in the third block, which is free from self-tuning closed-loop control and thus improves the dynamic performance of the proposed scheme. The proposed scheme is adaptive to severe unsymmetrical grid voltages with sudden changes in magnitude, phase, and/or frequency. Moreover, this scheme is able to eliminate phase errors induced by harmonics and random noise. The validity and utility of the proposed scheme are verified by the experimental results.

A novel induction motor starting method using superconduction

In this paper, an alternative method for starting up induction motors is proposed, taking into account experimental measurements. The new starting current limitation method is based on using a high-temperature superconductor. A prototype of the superconducting starting current limiter was constructed with a commercially available second-generation high-temperature superconductor YBCO tape, and this was tested with a 55-kW industrial induction motor in a 440-V/60-Hz three-phase power grid. Performance evaluations of the superconducting limiter method (applied to startup of the induction motor) were performed and were compared with a direct-on-line starter and an electronic soft starter. In addition, a computational model was developed and used for electromagnetic torque analysis of the system. As significant characteristics, our method offers the ability to limit the starting current of the induction motor with greater electromagnetic torque, reduced current waveform distortion and therefore lower harmonic pollution during startup when compared to the soft starter method.

Starting Current Limitation Method Using HTS for Induction Motors

The typical starting current for an asynchronous three-phase electric motor is in the range of five to ten times the rated current. To mitigate problems caused by this high current, various methods to start this type of motor exist. As an alternative method for starting up induction motors, a new starting current limitation method is proposed, which is based on using commercially available high-temperature superconductor tape. A prototype of the superconducting starting current limiter was constructed and tested with a 55-kW industrial induction motor, in a 440-V/60-Hz three-phase power grid. As the total starting time can be longer than the time the superconductor tape can support a high current, a shunt reactor receives the starting current after 80 ms to avoid damage to the tape. Performance evaluations of the superconducting limiter method were performed and were compared with a direct-on-line starter and an electronic soft starter. In addition to limiting the starting current, our method offers reduced current waveform distortion and lower harmonic pollution during startup.

Three-Phase Static Var Generator Based on Z-Source Network

The utility model provides a three-phase static var generator based on a Z source network. The three-phase static var generator comprises the Z source network, a main inversion circuit and a detection control module, wherein output ends of the Z source network are connected to a direct current side of the main inversion circuit; output ends of the main inversion circuit are connected to a three-phase power grid in parallel; and the detection control module is used for detecting voltages and currents of the three-phase power grid, the output ends of the main inversion circuit, and the Z source network, controlling the on-off state of a switch device of the main inversion circuit through driving signals, and further realizing the dynamic reactive compensation of the three-phase power grid. Unique X-type combination of small inductors and capacitors of the Z source network is used to replace the topological structure on the direct current side of an existing SVG, the defect that the SVG device is liable to be influenced by the insufficient voltage on the direct current side is overcome, a dead-zone circuit is effectively eliminated, and the output noises caused by the dead time are avoided, so that the SVG provided by the utility model is lower in cost, the reactive power compensation range is wider, the compensation effect is better, and the working reliability of the SVG is substantially improved.

G2V and V2G operation 20 kW Battery Charger

This paper presents a bidirectional on-board battery charger for Electric Vehicles designed to perform both Grid to Vehicle (G2V) and Vehicle to Grid (V2G) operation. The charger can also operate with single or three-phase power grid connection, regulates the battery charging current and presents input unity power factor. A high frequency three-phase transformer has been included in the charger, this providing galvanic isolation.

Control methods for PWM rectifier cooperating with variable speed PM generator

Abstract The paper deals with cooperation between pulse width modulation (PWM) rectifier and variable speed synchronous generator which is applied especially in small water or wind plants. In such applications, the synchronous generator, which is usually permanent magnet (PM) generator, rotates at a variable speed which depends on water or wind energy. Therefore, this energy should be converted to the parameters of the three-phase power grid with the use of a power electronic unit. The main aim of the control strategy is to transfer a maximum possible amount of energy produced by the water turbine or the wind turbine connected to a synchronous generator. The second purpose of the control method is to decrease the amount of higher harmonics of generator currents. The paper describes two basic methods which are used in control systems of the PWM rectifiers. The first one is the sinusoidal PWM method, and the second method relates to the hysteresis switching of the PWM rectifier transistors. A significant part of the paper is devoted to control principles of the PWM rectifier which cooperates with a variable speed PM synchronous generator. Special attention is paid to higher harmonics of PM generator currents with respect to individual methods.

Digital Detection, Control, and Distribution System for Co-Phase Traction Power Supply Application

Unbalance and reactive power currents of electric railway will result in poor power quality of three-phase industrial power grid. A co-phase traction power supply system is introduced in this paper to solve these problems by adopting single-phase ac-dc-ac converter. The converter is connected between two secondary windings of traction transformer with abilities of active power transmission and reactive power compensation. Because of the power conversion, the co-phase supply system can utilize single-phase feeding connection scheme instead of traditional two-phase feeding scheme. An actual 10 MVA/27.5 kV co-phase system with impedance-matching balance transformer is designed, developed, and tested. Its real-time detection, control, and distribution algorithms with field programmable gate array-based implementation are introduced. Some test running results validate the performance of the digital controller and demonstrate the feasibility of the proposed co-phase power supply system.

Adaptive nonlinear control of three-phase shunt active power filters

The problem of controlling three-phase shunt active power filters (SAPF) is addressed in presence of nonlinear loads supplied with three-phase power grids. The SAPF-load system is shown to be modeled, in the (a,β) coordinate frame, by a third-order nonlinear state-space representation. The control objective is twofold: (i) compensating for the current harmonics and the reactive power absorbed by the nonlinear load; (ii) regulating the inverter DC capacitor voltage. To this end, a nonlinear adaptive controller is developed, using the Backstepping design, on the basis of the averaged system model. The controller is made adaptive to compensate for the uncertainty that prevails on the unknown SAPF switching loss power. The average performances of the adaptive controller are formally analyzed using system averaging theory. The theoretical results are confirmed by numerical simulations.

Energy conversion in small water plants with variable speed PM generator

Energy conversion in small water plants with variable speed PM generatorThe Small Hydro Power Plants allow to increase the energy amount from renewable sources, especially from small rivers in mountainous areas. This paper presents a new concept of a energy conversion system for application in a Small Hydropower Plant (SHP) which is based on a permanent magnet generator (PM generator) with a propeller turbine integrated with the generator rotor. The PM generator can work at a variable speed and therefore energy produced by the PM generator has to be converted by means of a power electronic unit to fit to the three-phase power grid parameters. For this concept, dimensions and parameters of the PM generator were specially designed on account of integration with water turbine. The paper precisely describes elements of energy conversion system and also presents the results of numerical tests for chosen working conditions. An original algorithm of control strategy for power electronic unit was used to adjust generated energy to the required parameters of the three-phase grid.

Assessment of harmonic distortion level considering the interaction between distributed three-phase harmonic sources and power grid

An iterative frequency domain-based harmonic analysis method for harmonic distortion assessment for systems with distributed harmonic sources has been proposed. The proposed method, unlike the traditional method, is able to consider harmonic attenuation and diversity due to the interaction between the harmonic sources and power grid. Time domain simulation for systems with distributed adjustable speed drives is used to validate the proposed method. Case studies are conducted to show the merits of the proposed method especially with a large number of distributed harmonic sources.

Transient gate matrices of three-phase modelled power grids

A method of calculating transients in three-phase modelled power grids is presented. First, important devices are modelled by transient device admittance matrices. According to the configuration of the grid, these matrices are put together to form a transient node admittance matrix. Based on this result, transient gate impedance, admittance and hybrid matrices are derived. Their main properties are discussed.