Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications

The worldwide demand for more diverse and greener energy supply has had a significant impact on the development of wind energy in the last decades. From 2 GW in 1990, the global installed capacity has now reached about 100 GW and is estimated to grow to 1000 GW by 2025. As wind power penetration increases, it is important to investigate its effect on the power system. Among the various technologies available for wind energy conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions because it offers the advantages of reduced mechanical stress and optimised power capture thanks to variable speed operation. This work presents the small-signal modelling and analysis of the DFIG for power system stability studies. This thesis starts by reviewing the mathematical models of wind turbines with DFIG convenient for power system studies. Different approaches proposed in the literature for the modelling of the turbine, drive-train, generator, rotor converter and external power system are discussed. It is shown that the flexibility of the drive train should be represented by a two-mass model in the presence of a gearbox. In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison is made with the conventional synchronous generators (SG) and squirrel-cage induction generators to highlight the differences between the machines. The initialisation of the DFIG dynamic variables and other operating quantities is then discussed. Various methods are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative computations in initial condition mentioned in the literature. The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal analysis is performed for both open-loop and closed-loop situations. The effect of parameters and operating point variations on small signal stability is observed. For the open-loop DFIG, conditions on machine parameters are obtained to ensure stability of the system. For the closed-loop DFIG, it is shown that the generator electrical transients may be neglected once the converter controls are properly tuned. A tuning procedure is proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is performed. It is shown that there is no common mode to the two types of generators. The result confirms that the DFIG does not introduce negative damping to the system, however it is also shown that the overall effect of the DFIG on the power system stability depends on several structural factors and a general statement as to whether it improves or detriorates the oscillatory stability of a system can not be made

Prognostic Reasoner based adaptive power management system for a more electric aircraft

This research work presents a novel approach that addresses the concept of an adaptive power management system design and development framed in the Prognostics and Health Monitoring(PHM) perspective of an Electrical power Generation and distribution system(EPGS).PHM algorithms were developed to detect the health status of EPGS components which can accurately predict the failures and also able to calculate the Remaining Useful Life(RUL), and in many cases reconfigure for the identified system and subsystem faults. By introducing these approach on Electrical power Management system controller, we are gaining a few minutes lead time to failures with an accurate prediction horizon on critical systems and subsystems components that may introduce catastrophic secondary damages including loss of aircraft. The warning time on critical components and related system reconfiguration must permits safe return to landing as the minimum criteria and would enhance safety. A distributed architecture has been developed for the dynamic power management for electrical distribution system by which all the electrically supplied loads can be effectively controlled.A hybrid mathematical model based on the Direct-Quadrature (d-q) axis transformation of the generator have been formulated for studying various structural and parametric faults. The different failure modes were generated by injecting faults into the electrical power system using a fault injection mechanism. The data captured during these studies have been recorded to form a “Failure Database” for electrical system. A hardware in loop experimental study were carried out to validate the power management algorithm with FPGA-DSP controller. In order to meet the reliability requirements a Tri-redundant electrical power management system based on DSP and FPGA has been develope

Integration of a mean-torque diesel engine model into a hardware-in-the-loop shipboard network simulation using lambda tuning

This study describes the creation of a hardware-in-the-loop (HIL) environment for use in evaluating network architecture, control concepts and equipment for use within marine electrical systems. The environment allows a scaled hardware network to be connected to a simulation of a multi-megawatt marine diesel prime mover, coupled via a synchronous generator. This allows All-Electric marine scenarios to be investigated without large-scale hardware trials. The method of closing the loop between simulation and hardware is described, with particular reference to the control of the laboratory synchronous machine, which represents the simulated generator(s). The fidelity of the HIL simulation is progressively improved in this study. First, a faster and more powerful field drive is implemented to improve voltage tracking. Second, the phase tracking is improved by using two nested proportional–integral–derivative–acceleration controllers for torque control, tuned using lambda tuning. The HIL environment is tested using a scenario involving a large constant-power load step. This provides a very severe test of the HIL environment, and also reveals the potentially adverse effects of constant-power loads within marine power systems

Micro grid stability improvements by employing storage

Storage devices can be used in a power gird to store the excess energy when the energy production is high and the demand is low and utilize the stored energy when the produced energy cannot meet the high demands of the consumers. This thesis represents a micro grid consisting of a conventional synchronous generator, as well as renewable energy sources, energy storage, and loads in order to investigate the effective energy flow control and transient stability improvement by employing thermal storage. Thermal storage, unlike electrical one (such as battery) is more environmental friendly, has longer life span, and is more effective in power flow control. In this thesis, resistive and heat pump type thermal storage are proposed and its stability effects on micro grids are evaluated. A suitable model is developed for the thermal storage and the grid’s stability analysis is adopted by using linearization methods. Consequently, by designing an optimal controller for the storage the stability of the micro grid is improved as verified through the simulations

An investigation of turbogenerator dynamics and control

This thesis provides an investigation of the dynamics and control of turbogenerators from a multivariable control viewpoint. The multivariate control framework chosen -Individual Channel Analysis and Design- is particularly appropriate since it encapsulates the dynamical characteristics of the uncontrolled system with a view to exposing the potential and limitations for subsequent closed-loop control. The main contribution of the thesis is a complete new insight into why excitation/governor control with Power System Stabilisers (PSS) has been so successful for the control of turbogenerators connected to an infinite bus provided by the small-signal multivariable analysis framework, Individual Channel Analysis and Design. The multivariable analysis justifies treating the turbogenerator system as a pseudo- Single-Input Single-Output, (SISO) system where the governor loop is first closed and the exciter loop is treated as a SISO system for the prime purpose of rejecting voltage disturbances. The function of the PSS is identified as that of overcoming an awkward switch-back frequency-domain characteristic of the excitation channel so as to permit high-performance excitation channel bandwidths up to 10 rad/sec that otherwise could not be obtained. Thus, in addition to the control requirements of set point regulation of the terminal voltage and shaft speed, the PSS provides for a second control requirement of strong voltage disturbance rejection over the important frequency range of 0 to 10 rad/sec. The PSS control option is also assessed against other control options. Several other results concerning stability robustness to system uncertainties in different system configurations follow from the analysis in a transparent and immediate way

Notional System Report

The objective of this report is to set forth a group of time-domain models for the early-stage design study of shipboard power systems, and to demonstrate their use on various system architectures. The effort stemmed out of an earlier effort in which waveform-level models of three notional architectures – a Medium Voltage AC System, a High-Frequency AC System, and a Medium Voltage DC System were partially developed. Unfortunately, these codes were extremely computationally intense, limiting their usefulness for early design studies in which large numbers of runs, and a degree of user interactiveness, is required.Center for Electromechanic