Transient analysis and modelling of multimachine systems with power electronics controllers for real-time application
Electricity usage has grown steadily ever since the first commercial generator came into operation more than one century ago. Power transmission networks too, have grown in size and in operational complexity to be able to handle the large blocks of electricity that travel from generator to consumers round-the-clock and with huge variations. At various stages of the development, state-of-the-art equipment, methods and techniques have been incorporated in the vast array of tools that power systems engineers have at their disposal to keep up with the demands imposed by the planning, management, operation and control of modern power systems. Transient stability has always been an issue of paramount importance in power system planning and operation. Arguably, most of the ideas and concepts associated with power system stability analysis were conceived many years ago. Nonetheless, continuous expansion of the network and the emergence of a new generation of fast acting, multi-purpose power system controllers have called for renewed research efforts in this all-important application area of power systems. In particular, there is growing concern that the power network is becoming more unbalanced, owing to higher operating voltages and a relentless drive for interconnection, and that unbalances may impair the effectiveness of power electronic-based loads and controllers. These are issues that may be difficult to address satisfactorily with conventional transient stability modelling approaches since they are based on the premise that the transmission network observes a perfect balance, even under faulted operating regimes. The study of a limited range of asymmetrical transient stability problems using conventional methods can be achieved, but only with great difficulty, which involves transforming the network into fictitious components (i. e. symmetrical components). This is significant since asymmetrical short-circuit faults constitute the largest percentage of faults that occur in the power network, and network designs based solely on the three-phase short-circuit-to-ground faults result in underengineered networks. Equally important issues are the widespread commissioning of modern power electronics controllers and the lack of suitable models and methods for assessing the impact of such controllers in network-wide operation with particular reference to transient stability and unbalanced operation. The research reported in this thesis addresses these issues and develops a direct time phasedomain model for conducting multimachine transient stability analysis where asymmetrical operating conditions and the impact of modem power electronics controllers are represented. In this simulation environment, AC synchronous and asynchronous generators are represented together with asynchronous motors. The set of non-linear equations describing the machines are solved using discretisation and the trapezoidal rule of integration. The proposed model is compared against an industry standard power system package for cases of symmetrical operation. The generality and versatility of the model is demonstrated when applied to the analysis of symmetrical and asymmetrical power system operations. An important aspect of this research is a drive towards the solution of transient stability in real-time, where the results produced are in actual world time. This is achieved by embedding the model into a commercially available multi-purpose real-time station. To this end, coherency-based synchronous generators equivalent has been developed to enable the solution of multimachine systems in real-time. The equivalent unit is obtained based on the aggregation of the coherent generators using phase-domain techniques. Dynamic loads in the form of asynchronous motors are implemented within the multimachine network. The adverse influences of motor operation on voltage problems in the network under symmetrical and asymmetrical conditions are analysed. Transient analysis of dispersed generation is also considered where the asynchronous machine is operated as a generator alongside synchronous generators. The behaviours of the two type of generators under various networks and operating conditions are presented. Models of power electronics controllers in the direct time phase-domain are also described in this thesis. The generalised models of the Static Var Compensator (SVC), Static Synchronous Compensator (STATCOM), Dynamic Voltage Restorer (DVR) and High Voltage Direct Current-Voltage Source Converter (HVDC-VSC) station are proposed. The SVC comprised of a fix capacitor and a thyristor controlled reactor (TCR) is developed. Here, switching functions are used to represent the operation of the thyristor. Models of STATCOM, DVR and HVDC-VSC station are developed based on the self-commutated voltage source converter (VSC) technology. The VSC is represented by the switching functions of its pulse width modulation (PWM) control, hence, providing a flexible model within the direct time phase-domain approach. The model of the VSC is implemented into the respective power electronics controllers enabling a convenient modular approach to be adopted. The power electronics controllers are incorporated into the multimachine environment for the analysis of transient and power quality related issues.