ISLANDING DETECTION IN DISTRIBUTED GENERATION

Abstract--

The advancement in new technology like fuel cell, wind turbine, photo voltaic and new innovation in power electronics, customer demands for better power quality and reliability are forcing the power industry to shift for distributed generations. Hence distributed generation (DG) has recently gained a lot of momentum in the power industry due to market deregulations and environmental concerns. Islanding occurs when a portion of the distribution system becomes electrically isolated from the remainder of the power system yet continues to be energized by distributed generators. An important requirement to interconnect a DG to power distributed system is the capability of the DG to detect islanding detection. Failure to trip islanded generators can lead to a number of problems to the generators and the connected loads. The current industry practice is to disconnect all distributed generators immediately after the occurrence of islands. Typically, a distributed generator should be disconnected within 100 to 300 ms after loss of main supply. To achieve such a goal, each distributed generator must be equipped with an islanding detection device, which is also called anti islanding devices like vector surge relay and ROCOF relay.

AN APPLICATION OF OBSERVER FOR POSITION SENSORLESS STEPPER MOTOR DRIVES

Abstract-- 
A control method for stepper motor drives system can be made in open-loop circumstance which mean the system control did not require any feedback input signal in order to run the system. By applying the right sequences of pulses, the stepper motor capable to operate as other motion control. However, the performance of such system cannot be achieved to high level condition and demanded a feedback signal input to compensate the error produced while running the drive system. Therefore, a physical sensor or an encoder is placed in the motor system to obtain the feedback and form a close-loop system for error compensation. Nevertheless, the prices of these instruments are expensive, bulky and also may degrade the system performance. As a result this project presents a sensorless system in stepper motor drive system as an alternative to develop a close-loop system where the input signals are taken from voltage and current of the magnetic flux of the stepper motor. 

A Novel Control Strategy of Three-phase, Four-wire UPQC for Power Quality Improvement

Abstract –

The current paper presents a novel control strategy of a three-phase, four-wire Unified
Power Quality (UPQC) to improve power quality. The UPQC is realized by the integration of series and shunt active power filters (APF) sharing a common dc bus capacitor. The realization of shunt APF is carried out using a three-phase, four-leg Voltage Source Inverter (VSI), and the series APF is realized using a three-phase, three-leg VSI. To extract the fundamental source voltages as reference signals for series APF, a zero-crossing detector and sample-and-hold circuits are used. For the control of shunt APF, a simple scheme based on the real component of fundamental load current (I CosΦ) with reduced numbers of current sensors is applied. The performance of the applied control algorithm is evaluated in terms of power-factor correction, source neutral current mitigation, load balancing, and mitigation of voltage and current harmonics in a three-phase, four-wire distribution system for different combinations of linear and non-linear loads. The reference signals and sensed signals are used in a hysteresis controller to generate switching signals for shunt and series APFs. In this proposed UPQC control scheme, the current/voltage control is applied to the fundamental supply currents/voltages instead of fast-changing APF currents/voltages, thus reducing the computational delay and the required sensors. MATLAB/Simulink-based simulations that support the functionality of the UPQC are obtained.

A Novel Reduced Switching Loss Bidirectional AC/DC Converter PWM Strategy With Feedforward Control for Grid-Tied Microgrid Systems

Abstract—

This paper presents a novel simplified pulse width modulation (PWM) strategy for the bidirectional ac/dc single-phase converter in a microgrid system. Then, the operation mechanism of the novel simplified PWM is clearly explained. The number of switchings of the proposed simplified PWM strategy is one-fourth that of the conventional unipolar PWM and bipolar PWM. Based on the novel simplified PWM strategy, a feasible feedforward control scheme is developed to achieve better rectifier mode and inverter mode performance compared with the conventional dual-loop control scheme. The proposed simplified PWM strategy with the proposed feedforward control scheme has lower total harmonic distortion than the bipolar PWM and higher efficiency than both unipolar and bipolar PWMs. Furthermore, the proposed simplified PWM operated in the inverter mode also has larger available fundamental output voltage VAB than both the unipolar and bipolar
PWMs. A prototype system is constructed and the control scheme is implemented using FPGA Spartan-3E XC3S250E. Both simulation and experimental results verify the validity of the proposed PWM strategy and control scheme.

A New ZCS-PWM Full-Bridge DC–DC Converter With Simple Auxiliary Circuits

Abstract—

A new soft-switching pulsewidth modulated (PWM) full-bridge converter is proposed in this paper. The outstanding feature of the new converter is that it allows its main power switches to operate with zero current switching (ZCS) and with fewer conduction losses than conventional full-bridge converters. This is achieved by using two very simple active auxiliary circuits—one active, the other passive. The paper presents the new converter and then discusses its operation, steady-state characteristics, and design. Experimental results obtained from a 3 kW converter prototype are presented to validate the converter’s performance and the concepts presented in the paper.

A LLC-Type Dual-Bridge Resonant Converter: Analysis, Design, Simulation, and Experimental Results

Abstract—

In this paper, a high-frequency isolated dual-bridge LLC-type resonant converter is proposed. The steady-state analysis of the proposed converter is performed using a modified fundamental harmonics approximation approach, by which the component stress can be obtained quickly without complicated calculation. Necessary and sufficient conditions for zero-voltage switching of all switches are derived too. To illustrate the usefulness of the FHA analysis for a fast design, a design example of a 100 kHz, 200 V input, 40–48 V output 300 W converter is given. Computer simulation and experiment results are included for the purpose of validation. It is shown that this converter is able to maintain zero-voltage switching operation for a wide load range while keeping high efficiency.

A Half-Bridge LLC Resonant Converter Adopting Boost PWM Control Scheme for Hold-Up State Operation

Abstract—

This paper presents a half-bridge LLC resonant converter having a boost pulse width modulation (PWM) converter characteristic for hold-up state operation. The proposed converter is based on a half-bridge LLC resonant converter structure and a single auxiliary switch is added at the primary side. The converter has two different operational characteristics. It shows the same operational characteristic with the conventional LLC resonant converters during nominal state, which is frequency modulation (FM) method. However, when ac line lost and the converter enters into the hold-up time state, which requires wide voltage gain changes, the control method of the proposed converter is changed to the PWM method using the auxiliary switch. Since the proposed converter compensates wide voltage gain variation with PWM method of the auxiliary switch rather than adopting the FM method of main switches, the frequency variation range for the LLC resonant converter is highly reduced in the proposed converter. Therefore, the transformer in the proposed converter can be designed at the optimal operating point and it results in decreased conduction loss of the magnetizing inductor current. Furthermore, the maximum voltage gain of the proposed converter is easily increased by extending the duty ratio of the auxiliary switch. It helps to decrease the link capacitance. To verify the effectiveness of the proposed circuit, operational principle will be explained and experimental results will be presented with following specification. 100 kHz of switching frequency, 250–400 V of input voltage range, 250 V of output voltage, and 75 W output power.

A Four-level Hybrid-Clamped Converter With Natural Capacitor Voltage Balancing Ability

Abstract—

This paper presents a novel four-level hybrid-clamped converter topology which is composed of eight switches and one flying capacitor per phase. The operating principle is introduced and phase-shifted pulse width modulation is used to control this converter. A detailed analysis of the average currents through the flying capacitor and neutral points of the dc-link is presented. Based on the analysis, it can be concluded that the voltages across the flying capacitor and dc-link capacitors can be naturally balanced under ideal and steady-state condition. A low-power three-phase prototype is built up and experimental results are presented to validate the proposed topology and modulation method.

A Cascaded Multilevel Inverter Based on Switched-Capacitor for High-Frequency AC Power Distribution System

Abstract—

The increase of transmission frequency reveals more merits than low- or medium-frequency distribution among different kinds of power applications. High-frequency inverter serves as source side in high-frequency ac (HFAC) power distribution system (PDS). However, it is complicated to obtain a high-frequency inverter with both simple circuit topology and straightforward modulation strategy. A novel switched-capacitor-based cascaded multilevel inverter is proposed in this paper, which is constructed by a switched-capacitor frontend and H-Bridge backend. Through the conversion of series and parallel connections, the switched-capacitor frontend increases the number of voltage levels. The output harmonics and the component counter can be significantly reduced by the increasing number of voltage levels. A symmetrical triangular waveform modulation is proposed with a simple analog implementation and low modulation frequency comparing with traditional multicarrier modulation. The circuit topology, symmetrical modulation, operation cycles, Fourier analysis, parameter determination, and topology enhancement are examined. An experimental prototype with a rated output frequency of 25 kHz is implemented to compare with simulation results. The experimental results agreed very well with the simulation that confirms the feasibility of proposed multilevel inverter.

A Bidirectional High-Frequency-Link Single-phase Inverter: Modulation, Modeling, and Control

Abstract—

This paper proposes a high-performance high-frequency-link (HFL) single-phase inverter. It offers bidirectional two-stage galvanic isolation power conversion without bulky dc link capacitors. An active clamper circuit and corresponding modulation strategy is developed to enable the proposed HFL rectifier to operate in soft-switching modes and be free of voltage spikes during device commutation. A succinct circuit model and high-performance plug-in repetitive control scheme are also developed to enable it to equally function as a high-performance conventional pulse width modulation (PWM) inverter. The experiment results on a 20-kHz HFL inverter prototype demonstrate the efficacy of the soft-switching HFL inverter and its highly promising control performance. The proposed HFL inverter offers a high-reliability, high-efficiency, high-power-density, and high-performance power conversion solution to extensive applications.

Excitation Synchronous Wind Power Generators With Maximum Power Tracking Scheme

Abstract—

This paper presents a novel excitation synchronous wind power generator (ESWPG) with a maximum power tracking scheme. The excitation synchronous generator and servo motor rotor speed tracks the grid frequency and phase using the proposed coaxial configuration and phase tracking technologies. The generator output can thus be directly connected to the grid network without an additional power converter. The proposed maximum power tracking scheme governs the exciter current to achieve stable voltage, maximum power tracking, and diminishing servo motor power consumption. The system transient and static responses over a wide range of input wind power are examined using simulated software. Experimental results from a laboratory prototype ESWPG demonstrate the feasibility of the proposed system.