A regenerative active clamp circuit for DC/AC converters with high-frequency isolation in photovoltaic systems

DC/AC converters with high-frequency isolation and bidirectional power flow are extensively used in photovoltaic power systems and small isolated power converters at low and medium power ranges. The main disadvantages of these circuits are: high freewheeling loss of the primary leakage current, limited ZVS range for the lagging leg switches, the effects of the parasitic elements of the systems and others. To avoid these losses an energy recovery circuit based on an active voltage clamper is presented. The control circuit is designed having a number of soft switching transitions. The system has been verified by simulation and a prototype is being tested. In this paper we present an energy recovery system and a modulation sequence for the cycloconverter. The energy recovery system is based on an active voltage clamper; the voltage peaks energy is returned to the source. Furthermore, the presented modulation sequence is designed to have a maximum number of soft-switching transitions independent of the electric variables; which means minimum losses and independence on measurement systems limitations for modulation.Peer ReviewedPostprint (published version

Reliability Analysis Of Low-Frequency Ac Transmission System Topology Of Offshore Wind Power Plants

Many countries and regions of the world are planning to reduce the energy sector\u27s carbon footprint and increase sustainable energy sources. To this end, wind power has become one of their primary renewable energy sources. However, wind power\u27s significant challenges relate to the need for long transmission lines that connect the offshore wind power plants to the onshore grid. The three major transmission configurations and design topologies of High Voltage AC (HVAC) Transmission, High Voltage DC (HVDC) Transmission, and Low-Frequency AC (LFAC) Transmission for offshore wind power resources have been thoroughly discussed both in industry and academia. HVAC is the standard transmission system for short and long distances. In contrast, HVDC is a popular solution for the long-distance transmission of offshore wind power generators. In recent years, LFAC transmission topology at 20Hz has become an alternative solution to HVAC and HVDC transmission systems. The significant advantages of LFAC transmission are the substantial increment of transmissible power over traditional AC transmission systems and the elimination of offshore converter stations. The absence of an offshore converter system renders LFAC transmission less costly compare to the HVDC system. The efficient design and reliability of offshore wind power transmission topologies are essential requirements for the transmission grid\u27s smooth operation. This thesis work extensively investigated and reviewed the LFAC transmission topologies over HVAC and HVDC transmissions topologies of offshore wind power plans. Different methods are used to assess the reliability performance of system designs. In this research, the state of the art of the simulation models for three transmission systems have been developed for reliability analysis of the above three transmission systems topologies using Fault tree analysis (FTA). This research has identified several reliability performance characteristics including minimal cut sets, importance measures, and time-based matrics (i.e, number of failures and mean unavailability) of the transmission systems, and compared these characteristics among three transmission systems. For reliability performance analysis, the time-base metrics, such as mean-unavailability and number of failures of the systems over 10,000 hours of operation, importance measures, or reliability importance measures, such as Critical Importance Measure (CIM) and Risk Reduction Worth (RRW), and Cut Sets have been calculated. The thesis has successfully identified major fault events for all the three transmission systems, and that the large switch is the most critical piece of equipment in the HVAC system, while the AC/DC or DC/AC converter is the most critical piece of equipment in the HVDC system, and the DC/AC converter and Cycloconverter are the most critical components in the LFAC transmission system. Furthermore, to enhance the offshore transmission systems reliability and ensure their smooth operation, effective and reliable offshore wind power generation predictions are critical. To this end, this research work also introduces the necessary offshore wind power forecasting tools

Variable speed generator application on the MOD-5A 7.3 mW wind turbine generator

This paper describes the application of a Scherbiustat type variable speed subsystem in the MOD-5A Wind Turbine Generator. As designed by General Electric Company, Advanced Energy Programs Department, under contract DEN3-153 with NASA Lewis Research Center and DOE, the MOD-5A utilizes the subsystem for both starting assistance in a motoring mode and generation in a controlled airgap torque mode. Reactive power control is also provided. The Scherbiustat type arrangement of a wound rotor machine with a cycloconverter in the rotor circuit was selected after an evaluation of variable speed technologies that followed a system evaluation of drivetrain cost and risk. The paper describes the evaluation factors considered, the results of the evaluations and summarizes operating strategy and performance simulations

High frequency inverter-transformer-cycloconverter system for DC to AC (3-phase) power conversion

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is concerned with a 3-phase multistage high frequency link DC to AC power conversion with a novel inverter-cycloconverter circuitry. The conversion system is composed of a high frequency PWM inverter, step-up high frequency transformer and cycloconverter with bidirectional switching devices. In first stage the DC voltage of the power source , say a submarine battery, is inverted to a system of 3-phase sinusoidally modulated I kHz alternative wave forms. For this purpose a suggested optimized PWM technique for 3-phase inverter operation is adopted, in which harmonic components up to 17 th ( 17 kHz) are eliminated from the inverter output voltages. In the second stage, for DC input isolation from AC output and also for a voltage transformation ( here stepping-up )a high frequency ( size reduced ) transformer is employed. Generalized high frequency operation, influence and side effects of the transformer on overall system design & performance is investigated. In the final stage the 1 kHz -to- 50 Hz conversion process is accomplished by a 3-phase cycloconverter. The proposed "nonlinear modulation strategy" for cycloconverter output voltage and associated harmonic analysis is demonstrated, in which the harmonic components up to 38th (1.9 kHz ) are eliminated from the conversion system output voltage. To assess the suggested functioning principles for the inverter & cycloconverter , the prototype conversion system was developed. Some design criteria and switching device selection are presented, together with different voltage & current wave forms of the prototype system under resistive & inductive load (induction motor) and their respective spectra

THE OUTPUT FREQUENCY SPECTRUM OF A THYRISTOR PHASE-CONTROLLED CYCLOCONVERTER USING DIGITAL CONTROL TECHNIQUES

The principle of operation dictates that the output of a cycloconverter contains some harmonics. For drive applications, the harmonics at best increase losses in the motor and may well cause instability. Various methods of analysing the output waveform have been considered. A Fortran 77 program employing a modified Fourier series, making use of the fact that the input waveforms are sinusoidal, was used to compute the individual harmonic amplitudes. A six pulse three phase to single phase cycloconverter was built and a Z-80 microprocessor was used for the control of firing angles. Phase locked loops were used for timing, and their effect upon the output with changing input frequency and voltage were established. The experimental waveforms are analysed by a FFT spectrum analyser. The flexibility of the control circuit enables the following investigations not easily carry out using traditional analog control circuit. The phase relationship between the cosine timing and reference wave in the cosinusoidal control method was shown to affect the output waveform and hence the harmonic content. There is no clear optimum value of phase and the T.H.D. up to 500Hz remains virtually constant. However, the changes of individual harmonic amplitudes is quite significant. In practice it may not be possible to keep the value of phase constant but it should be considered when comparing control strategies. Another investigation involves the changing of the last firing angle in a half cycle. It shows that the value of firing angles produced by the cosinusoidal control method is desirable. Operation at theoretical maximum output frequency was also demonstrated.Bristol Universit

Structures performance, benefit, cost-study

New technology concepts and structural analysis development needs which could lead to improved life cycle cost for future high-bypass turbofans were studied. The NASA-GE energy efficient engine technology is used as a base to assess the concept benefits. Recommended programs are identified for attaining these generic structural and other beneficial technologies

Active and Reactive Power Control of Wound Rotor Induction Generators by Using the Computer and Driver

In this chapter, a power control system for a wound rotor induction generator has been explained. This power control system has realized a control method using a rotating reference frame fixed on the air-gap flux of the generator. Application of such a system allows control of the active and reactive power of generators independently and stably. So, a two-step process is presented here. The first step is to acquire the complex power expression (and thus the active and reactive power expressions) for an induction machine in space vector notation and in two-axes system. Then, a computer aided circuit is given to realize the power and current control by analyzing them. Also, the results of an experiment given in literature are shown to be able to compare the results