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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.

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Diesel generating set load acceptance performance has deteriorated in recent years due to a number of influencing factors. Market forces have demanded that a higher power output is achieved for a given engine displacement, whilst global concern and awareness of climate change has led to the introduction of increasingly stringent emissions legislation. In order to provide satisfactory load acceptance performance, whilst maintaining compliance with the growing restrictions placed on operation, more sophisticated control and protection features are required than would be provided with the traditional standby diesel gen-set package. Presently, diesel gen-set control involves limited interaction between the automatic . voltage regulator (AVR) and engine speed governor, causing issues to arise due to the absence of a co-ordinated control scheme. For example, in the event of a large real power load application, a fast responding AVR can cause excessive engine speed deviation, which may be unacceptable in some applications. The problem is further exacerbated by the introduction of fuel limiting as a method of achieving emissions compliance, restricting the action of the engine speed governor during a transient. This has prompted the research to investigate integration of the various components of a modern diesel gen-set control system. A co-ordinated control approach, in which the AVR aids transient engine speed recovery, has therefore been developed. Testing and evaluation of the integrated diesel gen-set control system was conducted on diesel gen-sets in the Queen's University of Belfast test facility. A number of different integrated control configurations were evaluated to determine the available ifTIprovement in diesel gen set load acceptance performance. The thesis presents results showing that, by utilising a co-ordinated control strategy, a significant overall improvement in the maximum initial load application is achieved, thus meeting market expectation whilst remaining fully emissions compliant. Supplied by The British Library - 'The world's knowledge'

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