Controlling the power flow in transmission systems has recently gained increased interest. The difficulties of building new lines and the pressure of having a high utilization of existing assets, makes the flexibility of grid systems increasingly important. This master thesis work investigates induction regulators as control devices for active power flow in a transmission system. A small change in angle of the rotor affects both the amplitude and the phase of the voltage. The magnetic coupling in the induction regulator can be controlled by changing the permeability of a thermo magnetic material such as gadolinium and can hence give a second independent controlling parameter. An analytical model and calculations in the FEM software AceTripleC together with Matlab, is used to simulate the influence of the regulators connected to a simple grid in case1, a 400 kV scenario and case 2, a 45 kV scenario. The analysis was carried out on a small transmission system consisting of two parallel transmission lines connected to source and load. The induction regulators are connected to one of the parallel transmission lines. The regulators modelled in case 1 must be able to control the active power flow in the regulated line to vary between 50 and 150 % of the original power flow through this line. This shall be done over a range of 0 to 800 MW transmitted power. The regulators modelled in case 2 must be able to control the active power flow in the regulated line to vary between 0 and 30 MW, if this does not cause the power flow in the parallel line to exceed 30 MW. This shall be done over a range of 0 to 50 MW transmitted power. The regulators are designed as small and inexpensive as possible while still fulfilling requirements regarding the active power flow controllability in the grid, current density in windings and maximum flux density in core and gap. The results indicate that the size of the 400 kV solution has to be reduced to become competitive whereas for the 45 kV solution the relative difference to existing solution is smaller. Advantages with the proposed design over a phase shifting transformer are mainly a simpler winding scheme and the absence of a tap changer.

M. Tech. Electrical Engineering. / Discusses that most real-time applications of synchronised phasor measurement units in power system stability studies are done using RTDS simulator, hardware PMUs and PTP hardware modules with GPS providing the time reference. The major drawbacks of this configuration are both technical and economical. The technical limitations are related to the possible number of outputs that can be used to interface PMU devices, which can be very limited; and also the possible limits on small-time step computation due to the big number of output signals. The economic constraints are mainly due to cost: for a research laboratory, only a limited number of units can be acquired due to budgeting issues. To overcome these difficulties, the realisation of an entirely software-based synchrophasor measurement unit, presents an attractive approach. Such a unit will be capable of deliveringreal-time data by acquiring the voltage and current signals from the Matlab/Simulink simulator, low cost software environment PMUs computing real-time phasors of voltage and current and software only PTP synchronisation protocol. The unit delivers synchrophasors for the application of power system SE and small signal stability analysis, but also taking into consideration the accuracy of the synchronisation protocol on the application of power system SE and small signal stability analysis.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Flexible AC Transmission Systems (FACTS) involves the incorporation of power-electronic controlled devices into ac power transmission systems in order to extend the power-transfer capability of these systems beyond their traditionally accepted boundaries. One particular category of FACTS devices makes use of high-powered voltage source inverters to insert near-sinusoidal ac compensating voltages into the transmission system. This thesis considers this particular category of inverter-based FACTS devices, namely the static synchronous compensator (STATCOM), static synchronous series compensator (SSSC) and unified power flow controller (UPFC). Although the potential for FACTS devices to enhance the operation of power systems is well known, a device such as a UPFC is itself a complicated subsystem of the overall power system. There is therefore also the possibility that the introduction of such devices could cause adverse interactions with other power system equipment or with existing network resonances. This thesis examines the interactions between inverter-based compensators and a particular form of system resonance, that of subsynchronous resonance between a generator turbine shaft and the electrical transmission network. The thesis presents a review of the theory of operation of high-power, multi-pulse inverter topologies actually used in transmission-level FACTS devices. Detailed simulation models are developed of both two-level and three-level multi-pulse inverters. With appropriate controls, simulation models of both the SSSC and STATCOM, and a full UPFC are then developed using these detailed inverter models and the results from these simulation models compared against other results from the literature. These comparisons show favourable agreement between the detailed FACTS models developed in the thesis and those used by other researchers. However, the models presented in this thesis include a more detailed representation of the actual power-electronic circuitry and firing controls of inverter-based FACTS devices than is the case with other models used in the literature. The thesis then examines the issue of whether the introduction of an SSSC to a transmission system could cause subsynchronous resonance (SSR). SSR is a form of dynamic instability that arises when electrical resonances in a series capacitively compensated transmission line interact with the mechanical resonances of a turbo-generator shaft system. The detailed SSSC simulation model developed in the thesis is used to determine the impedance versus frequency characteristics of a transmission line compensated by an SSSC. The results confirm earlier work by others, this time using more detailed and realistic models, in that the introduction of an SSSC is shown to cause subsynchronous resonance. The thesis then considers the addition of supplementary damping controllers to the SSSC to reduce subsynchronous oscillations caused both by the SSSC itself as well as by a combination of conventional series capacitors and an SSSC in a representative benchmark study system. The results show that subsynchronous oscillations in the transmission system compensated solely by an SSSC can successfully be damped out using a single-mode supplementary damping controller for a range of values of SSSC series compensation. However, in the case of the transmission system compensated by both conventional series capacitors and an SSSC, the nature of the subsynchronous oscillations is shown to be complex and strongly multi-modal in character. The thesis then proposes an extension to the single-mode supplementary damping controller structure that is better suited to damping the multi-modal resonances caused when an SSSC and conventional series capacitors are used together to compensate a transmission line. The results obtained from this multi-modal controller indicate that it is able to stabilise SSR for a range of compensation values, but that the controller design needs to be adjusted to suit different values of compensation. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2006.

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The system frequency of a synchronous power system varies with the imbalance of energy supplied and the electrical energy consumed. When large generating blocks are lost, the system undergoes a frequency swing relative to the size of the loss. Limits imposed on the magnitude of frequency deviation† prevent system collapse. Operation of frequency responsive plant to control frequency, results in lower machine efficiencies. Changes to the generation mix on the British transmission system have occurred in the past ten years, when the response requirement was last reviewed. Future increased levels of wind turbines‡ will alter the operational characteristics of the system and warrant investigation. A process to optimise the response requirements while maintaining statutory limits on frequency deviation has been identified. The method requires suitable load and generator models to replicate transmission system performance. A value to substitute for current load sensitivity to frequency has been presented from empirical studies. Traditional coal fired generator models have been improved with additional functions to provide a comparable response with existing units. A novel combined cycle gas turbine model using fundamental equations and control blocks has also been developed. A doubly fed induction generator model, based on existing literature, has been introduced for representing wind turbine behaviour in system response studies. Validation of individual models and the complete system against historic loss events has established confidence in the method. A review of the current system with the dynamic model showed that current primary response requirements are inadequate. The secondary response requirements generally show a slight reduction in the holding levels. Simulations including extra wind generation have shown that there is potential to reduce the primary response requirement in the future. The secondary response requirements are maintained with added wind farms.

Electric power systems are characterized by their immense complexity. The assessment of their security on-line has always been a challenging task. Many possibilities were investigated in the past in an attempt to characterize the secure operating region of a power system. Pattern recognition is thus far the only tool that can take various degrees of network complexity into consideration. / In the present study, an efficient algorithm which learns adaptively the secure operating region is proposed. At each iteration, training operating points are generated sequentially on a piecewise linearly approximated separation surface computed by the one-nearest-neighbor (1-NN) rule. The separation surface so estimated approaches the true one as the number of training points increases. The algorithm not only provides a consistent technique in learning an unknown region, it generates a highly efficient training set. It is found to be effective in reducing the size of the training set without adverse effect to the classifier. / Once the secure region of a power system is available, the task of on-line security monitoring reduces to one of determining whether the current operating point resides in the secure region. As demonstrated in the thesis, both the security status and the security margin of the operating point can be assessed very efficiently. By using the piecewise linearly approximated secure region, the thesis proceeds to give efficient ways of moving an insecure operating point into the secure region. This comprises the problem of security enhancement. / The regionwise methodology via the Voronoi diagram developed in the thesis is also applied to a wide range of problems, such as network planning, coordinating tuning of machine parameters and automatic contingency selection. The major merit is that the dynamics and the nonlinearity of the system no longer present a limitation to solving these problems.