This paper presents a fundamental study of voltage collapses that occur on a post-fault trajectory of a stressed power system in seconds after large disturbances. The focus of the study are voltage collapses that are induced by certain load models. Using an n-machine-N-bus power system model, the paper explicitly shows that the voltage collapse is caused by the non-existence of a real, positive solution for a load voltage magnitude in different areas of a relative rotor angle space when the load is of non-linear type. These "areas without voltage solution" are denoted as Voltage Impasse Regions (VIR) and are mathematically characterized as trigonometric functions of (n-1) relative rotor angles. Once the post-fault trajectory enters a VIR, voltage magnitude solutions become complex or negative, the algebraic Jacobian becomes singular, and the behaviour of a system becomes undefined. The case study has been carried out using a simple 3-machine-1-load system with static load models. In the study, VIR appeared and enlarged as the non-linear (constant power and constant current) load increased. Furthermore, the non-convergence of time-domain solution occurred exactly at VIR, thereby confirming that the problem is of structural nature.

Maintaining automatic reserve capacities is essential for a sustainable and reliable power system. Today, many power systems experience more frequent frequency deviations coming from increased power variations. This implies an increased utilization of automatic reserves. To decrease frequency deviations, one can increase the automatic reserve capacities. However, the solution tends to be costly and ineffective. Therefore, it is urgent to develop better solutions to cease this trend. Here we have designed new proactive control-room strategies to decrease the need for automatic reserves. We design strategies for a process called Re-Scheduling of Generation and for the Tertiary Frequency Control process. The new control-room strategies are tested using an intra-hour model comparing already used strategies against new ones. It is shown that the historical used strategies are well executed. Nevertheless, results show that the proactive TFC-strategy using a forecasted frequency as control parameter would improve system security significantly.

The aim of this research work is to analyse subsynchronous control interaction (SSCI) in doubly-fed induction generators (DFIGs) and to design a supplementary control technique for the mitigation of SSCI. A mathematical model of the DFIG is derived and linearized in order to perform an eigenvalue analysis. This analysis pinpoints the parameters of the system which are sensitive in making sub-synchronous modes unstable and hence are responsible for causing SSCI. A power oscillation damper (POD) is designed using a residue method to make the DFIG system immune to the SSCI. The POD control signal acts as a supplementary control, which is fed to the controller of the grid-side converter (GSC). The POD signal is applied to different summation junctions of the GSC controller in order to determine the best placement of the POD for effective mitigation of SSCI and for the increased damping of the system.

This paper investigates the optimal placement and design of a Power Oscillation Damping (POD) controller within an embedded VSC-MTDC network. As VSC-MTDC networks are expected to play an eminent role in interconnecting AC grids, there is a growing need to optimize the placement and design of supplementary controllers in the MTDC converters, in order to maximize their benefit to the connected AC systems. Based on small signal stability analysis of the system, the location of damping controller within the MTDC network is selected according to the eigenvalue sensitivity of poorly damped oscillatory modes. On the other hand, the design of the damping controller is based on Modal Linear Quadratic Gaussian (MLQG) control that uses Wide Area Measurement Signals (WAMS). The results show the ability of the controller to enhance the damping in the system in case of disturbances, as well as to improve the small signal stability by boosting the damping of inter-area modes.

Inaccurate line loss predictions leads to additional regulation costs for Transmission System Operators (TSOs) that place energy bids at the day-ahead market to account for these losses. This paper presents a line loss prediction model design, applicable with the TSOs forecast conditions, that can reduce additional expenditure due to inaccurate predictions. The model predicts line losses for the next day per bidding area in relation to prognosis data on electrical demand, supply, renewable energy generation and regional exchange flows. Linear regression analysis can extract these relation factors, known as line loss rates, and derive a line loss prediction with increased accuracy and precision. Required input data is available at the power exchange markets apart from future exchange flows, which instead have been modelled as an optimisation problem and predicted by linear programming. Simulations performed on the Swedish National Grid for 2015 demonstrate the models performance and adequacy for TSO application.

This paper examines the application of a hybrid HVDC link in a two area power system with the purpose of increasing the inter-regional power transfer. A hybrid HVDC system combines both LCCs and VSCs, and hence it is capable of combining the benefits of both converter technologies, such as reduced cost and power losses due to the LCCs, and ability to connect to weak AC grids due to the VSCs. The mathematical model of the power system including the HVDC link is presented. The increase in inter-area power transfer is demonstrated and compared to the case when the hybrid HVDC link is not used. Furthermore, the transient stability of the AC/DC power system was enhanced using auxiliary controllers for Power Oscillation Damping (POD). The results show the ability of the hybrid HVDC link to increase the unidirectional inter-area power transfer, while enhancing the transient stability of the power system.

This paper proposes an efficient solution approach based on Benders' decomposition to solve a network-constrained ac unit commitment problem under uncertainty. The wind power production is the only source of uncertainty considered in this paper, which is modeled through a suitable set of scenarios. The proposed model is formulated as a two-stage stochastic programming problem, whose first-stage refers to the day-ahead market, and whose second-stage represents real-time operation. The proposed Benders' approach allows decomposing the original problem, which is mixed-integer nonlinear and generally intractable, into a mixed-integer linear master problem and a set of nonlinear, but continuous subproblems, one per scenario. In addition, to temporally decompose the proposed ac unit commitment problem, a heuristic technique is used to relax the inter-temporal ramping constraints of the generating units. Numerical results from a case study based on the IEEE one-area reliability test system (RTS) demonstrate the usefulness of the proposed approach.

Multi-terminal HVDC (MTDC) networks are being contemplated for large scale integration of renewable energy sources, as well as for the interconnection of asynchronous AC systems. A hybrid MTDC network that combines line commutated converters (LCCs) and voltage source converters (VSCs) can combine the benefits of both technologies. This paper presents a mathematical model of an AC/DC power system with an embedded hybrid MTDC network interconnection. Small signal stability analysis of the power system is conducted based on the linearization of the model. The impact of VSC controller gains on the dominant modes in the system is investigated. The contributions of the converters and generators to different modes of the system are investigated based on the participation matrix analysis. Auxiliary controllers are applied at the converters for the purpose of damping power oscillations in case of disturbances. The results of small signal stability analysis are validated by time-domain simulations in MATLAB.

The automatic frequency containment reserve (FCR-N) is in place to keep the electric frequency within the interval 50.0 +/-0.1 Hz during normal operation. This function is mainly provided by a number of hydropower plants where the turbine governor is set to control the discharge in proportion to the measured frequency deviation. In later years it has been shown that the disturbance damping is very low in an interval around 1160 Hz and it is believed that proper tuning of the turbine governors that provide FCR-N can help mitigating this problem. New regulator settings have been suggested to improve the performance of the FCR-N, yet keeping the system robust and the wear on participating units at a minimum. It is now desired to investigate the possible effects of new governor settings on the overall power system frequency response. In a word, the overall performance for new governor settings are tested in a large scale power system model in this thesis paper. The frequency response with the newly suggested governor settings have been investigated when introducing a disturbance into the system. Secondly, the effects of the new governor settings on electro-mechanical oscillations are also investigated.

Multi-terminal HVDC systems are an attractive option to connect offshore wind farms to onshore grids. Although scheduling the multi-terminal HVDC system is based on forecasted wind power, the forecasted values may differ from their real time ones. This paper presents a new controller based on multi-agent system which optimally tries to follow the variations of real time wind power outputs. Since a fast optimal power flow algorithm is needed, a convexified AC-OPF model which can be efficiently solved through interior point methods (IPMs) is embedded into the proposed online controller. Simulations are carried out and validated using GAMS platform and MATLAB/Simulink.