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Stability analysis in the power system is becoming more important than ever as more distributed energy resources penetrate in the system. This study presents a novel load pattern voltage stability index (LPVSI) applicable to transmission and distribution systems. By considering the nominal value of voltages, the power network is converted into a two-bus equivalent system. Then, LPVSI is derived by only the real-time measurement of the voltage and deviation of active and reactive power loads. Also, the assessment of distributed generation's penetration level on unbalanced systems, with maximum loadability and power loss reduction constraints, is performed with regard to daily load variations. The accuracy and efficiency of the proposed indicator are tested on an unbalanced 34-node radial distribution system. Obtained results in comparison with some other papers in the literature demonstrate that the proposed voltage stability index is fast and effective in identifying non-trivial instabilities in the power system networks.

This study presents a method to interface an electromagnetic transient (EMT) simulator with a dynamic phasor (DP) simulation program. The hybrid DP-EMT simulator offers flexibility in deciding the harmonic contents to be preserved in the DP domain; additionally, it offers significant reduction in computing time of large networks compared with EMT simulation. The proposed interfacing method is based on a generic transmission line, which uses specialised numerical techniques to transfer discretised waveform samples between the two simulators. These numerical methods produce the required mapping between EMT simulation samples and their counterpart DPs. This study describes the mathematical foundations of the DP-EMT interface and demonstrates them using an illustrative example. Several large networks are then studied to assess the interface's accuracy and performance in reducing computing time.

A screening method called the compensation impact factor (CIF) is proposed in this study to identify the risk of sub-synchronous resonance (SSR) caused when generators are connected to series compensated transmission lines. First, the definition of CIF is given and the meaning behind this index is explained. Second, the electric damping is used to classify the operating scenario into high SSR risk group and low SSR risk group. Then the correlation between the value of the CIF and the risk classification is analysed and the threshold value of CIF for this classification is calculated by using conditional probability. The proposed method is validated by (a) a 3-bus system with one generator and (b) the IEEE-39-bus system with ten generators. The advantage of applying the proposed method is that only the high-risk scenarios need to be analysed in detail using electromagnetic transient simulation.

This study presents a simple approach to modify the Prony algorithm to extract dominant low-frequency modes present in ring-down oscillations in power systems. The proposed approach is based on the observation that true modes present in the ring-down oscillations appear consistently, irrespective of the order of the Prony model. It is shown that the consistently appearing modes can be extracted using a sorting method. The improved Prony algorithm which has the feature of extracting only the true modes present in the input signal is utilised to propose an oscillation monitoring algorithm in this study. The suitability of the proposed oscillation monitoring algorithm for real-time monitoring of low-frequency inter-area oscillations is demonstrated using synthetic signals and simulated signals of different test systems.

A technique is presented that is suitable for undervoltage load shedding (UVLS) of dominant induction motor loads in power systems. To derive the algorithm, the induction motor voltage-dependent characteristics are formulated along a line parallel to the previously proposed generic dynamic load model. The load model is derived from the third-order induction motor model and is expressed in terms of real power, reactive power and slip, while retaining its dynamic characteristics. For the period following the initial induction motor transients (t>3 s), the model has been further simplified, explicitly representing the real and reactive power coupling in load dynamics. In the event of a voltage unstable situation, at a given voltage, the proposed UVLS criterion can be used to calculate the amount of load to be shed at any time to arrest voltage instability. It is shown that, when the induction motor dynamics are approximated using the first-order generic dynamic load models proposed in literature, the predicted amount of load to shed is approximately 2-3 times the amount obtained using the proposed method. This is owing to the real and reactive power coupling in the load dynamics. The proposed method is general and can be applied to any power system, provided the proportion of induction motor loads is approximately known.

The paper presents a technique for undervoltage load shedding (UVLS) in power systems. The undervoltage load shedding criterion has been developed using a dynamic load model. The parameters of the dynamic load model are estimated online using a nonlinear least squares technique, namely the Gauss-Newton method. The amount of load to be shed is calculated using the parameters of the dynamic load model. In the event of a voltage unstable situation, the proposed undervoltage load shedding criterion can be used to calculate the minimum amount of load to be shed at any point in time to avoid a voltage collapse. The criterion is general and can be applied to any power system.

The paper investigates the mechanism of the three control methods of unified power flow controllers, namely in-phase voltage control, quadrature voltage control and shunt compensation, in improving transient stability of power systems and examines the utilisation of the voltampere ratings by the three control methods. The study is based on a single machine infinite bus system with a unified power flow controller connected in series with the transmission lines. The potential of the three control methods in transient stability enhancement is identified. Analytical results indicate that significant reduction in the transient swing can be obtained with any of the above methods by using a simple proportional feedback of rotor angle deviation. The analysis of the transient energy function shows that the transient stability margin can also be substantially improved with in-phase voltage control and quadrature voltage control, however, to a smaller extent with shunt compensation. The analysis is verified by transient stability simulation. The simulation results show that the quadrature voltage control, apart from being more effective in reducing the transient swing than the in-phase voltage control, incurs a substantially smaller real power flow in the excitation converter. This makes a larger voltampere capability of the excitation converter available for shunt compensation.

A fuzzy logic approach is applied to coordinate three control inputs: in-phase voltage control; quadrature voltage control; and shunt compensation, of a unified power flow controller to improve the transient stability of a power system. The fuzzy rules for the controller are created based on a bang-bang control strategy whose objectives are minimising the first swing, damping the oscillations at an optimum rate and maximising the transient stability margin. The proposed controller also incorporates a technique of reducing the chattering of generator output which naturally results from the application of bang-bang control. Simulation results show that the fuzzy logic based controller provides satisfactory performance, meeting the design objectives. The results also show the robustness of the controller.

A novel method for the design of output feedback controllers for thyristor controlled series compensators (TCSCs) in a meshed power system is developed in the paper. The selection of the output feedback gains for the TCSC controllers is formulated as an optimisation problem and the simulated annealing (SA) algorithm is used to find the solution. Using this method, the conflicting design objectives, such as the improvement in the damping of the critical modes, any deterioration of the damping of the non critical modes and the saturation of the controller actuators, can be simultaneously considered. It is also shown that the proposed SA algorithm can be used to design robust controllers which satisfy the required performance criteria over several operating conditions. The proposed control scheme can be easily implemented as only the measurable signals local to each TCSC location are used to control the TCSCs (decentralised control).

Methods for co-ordinated design of output feedback damping controllers, for thyristor-controlled series compensators (TCSCs) in a meshed power system, are developed in the paper. The locations for installing TCSCs and the co-ordination of TCSC controls are determined by analysing the mode controllability. The analysis of mode observability is used to select the effective output feedback signals local to the TCSC locations. The design of controller gains is divided into two steps. First, initial gains for each controller are calculated according to the co-ordination determined by the analysis of the mode controllability. These initial gains are then optimised by using linear programming to satisfy the given design criteria. It is shown that, by using this procedure, the desired damping of the critical modes can be achieved without significantly deteriorating the damping of the other modes and the stability of the power system. It is also shown that the prolonged saturation in the TCSC actuators and the adverse interaction among TCSC controllers can be avoided. The proposed controller design methods are validated by a case study.

The application of controlled series compensators (CSCs) to increase the damping of dynamic oscillations in multimachine power systems is described. A CSC controller design is presented based on a linearised model of the power system around a given nominal operating point. The pole placement technique is used for calculating the controller feedback gains. A procedure for determining an optimum location for installing a CSC in a power system to provide the most effective damping is investigated. A technique for coordinating CSCs when it is necessary to use more than one CSC in a power system is also discussed. Computer simulations are used to demonstrate the effectiveness of CSCs in increasing the damping of power system dynamic oscillations. A simple power system with nine buses and three synchronous machines is used to demonstrate the design procedure.

The paper investigates the application of parallel simulated annealing for unit commitment problems. Two parallel simulated annealing concepts, speculative computation and serial subset, are applied to a unit commitment problem of ten thermal generators. The authors also propose a combined scheme where speculative computation is used in the initial phase and the serial subset is used in the final phase. The parallel simulated annealing schemes are tested with an example problem and the results show that the parallel schemes can considerably speed up the computation of simulated annealing.