Subsynchronous resonance (SSR) is one of the major obstacles for the wide spread of high degrees (60% and higher) of series capacitor compensation. Recently, a new obstacle, namely Subsynchronous Interaction (SSI) has been added to the list after the Zorillo Gulf wind farm incident in Texas in October 2009. SSI is due to the interaction between large Doubly Fed Induction Generator (DFIG)-based wind farms and series capacitor compensated transmission systems. In integrated power systems incorporating series capacitor compensated transmission lines and high penetration of wind energy conversion systems, especially DFIG-based wind farms, SSR and SSI could occur concurrently as a result of some system contingences. Therefore, mitigating SSR and SSI is an important area of research and development targeting at developing practical and effective countermeasures. This thesis reports the results of digital time-domain simulation studies that are carried out to investigate the potential use of offshore and DFIG-based wind farms for simultaneous mitigation of SSR and SSI. This is achieved through introducing supplemental control signals in the reactive power control loops of the grid side converters of the DFIG wind turbines or the HVDC onshore Modular Multilevel Converter (MMC) connecting the offshore wind farm to the grid. In this context, two supplemental controls designated as Supplemental Controls I and II are examined. Supplemental Control I introduces a signal in the HVDC onshore converter to damp both SSR and SSI oscillations. On the other hand, Supplemental Control II introduces a signal in the HVDC onshore converter for damping SSR oscillations and another signal in the grid side converters of the DFIG wind turbines for damping SSI oscillations. Time-domain simulations are conducted on a benchmark model using the ElectroMagnetic Transients program (EMTP-RV). The results of the investigations have demonstrated that the presented two supplemental controls are very effective in mitigating the SSR and SSI phenomena at different system contingencies and operating conditions.

Subsynchronous Resonance (SSR) phenomena in a thermal-electric power system with series-capacitor-compensated transmission lines may cause damaging torsional oscillations in the shaft of the turbine-generator. This thesis deals with a wide-range multi-mode stabilization of single-machine and multi-machine SSR systems using output feedback excitation control. Chapter 1 summarizes the SSR countermeasures to date. Chapter 2 presents a unified electro-mechanical model for SSR studies, illustrates the torsional interaction between the electrical and mechanical systems, and demonstrates that multi-mass representation of the turbine-generator must be used for SSR studies. For the control design, a reduced order model is desirable. For the model reduction, modal analysis is applied to identify the excitable torsional modes, and a mass-spring equivalencing technique to retain only the unstable modes is developed in Chapter 3. Using the reduced order one: machine models, linear optimal excitation controls are designed in Chapter 4. The controls are further simplified by examining the eigenvalue sensitivity, and the results are tested on the linear and nonlinear full models. In Chapter 5, the stabilization technique is further extended and applied to a two-machine system and a three-4nachine system. The stabilizers still can be designed one machine at a time using a one-machine equivalent for each machine by retaining only the path with the strongest interacting current and the critical electrical resonance frequency as seen by the machine. To coordinate all controllers for the entire system, an iterative process is developed. The controllers thus designed are tested on linear and nonlinear full models. From both eigenvalue analysis and nonlinear dynamic performance tests of the one-machine, two-machine, and three-machine systems, a conclusion is drawn in Chapter 6 that the excitation controls thus designed by the methods developed in this thesis can effectively stabilize single-machine and multi-machine SSR, systems over a wide range of capacitor compensation. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Various computer programs are currently used by electric utilities to determine if potential subsynchronous resonance problems exist which can impact turbine-generators. One of the most popular of these is the frequency scanning program. The representative transmission system input data for these programs are generally based on constant temperature and frequency. However, as conductor temperatures and applied frequencies fluctuate, the resistances also change. This thesis investigates the effects that resistance variations, due to temperature and frequency, have on frequency scanning results. The maximum resistance change (increased and decreased) from the standard value is determined and applied to the transmission lines in four study system cases. The frequency scan output for the modified cases is used to determine if torsional interaction has become more severe. It is found that, under extreme conditions, the net system damping can decrease by over one hundred percent.

Three aspects of Subsynchronous Resonance (SSR) related problems in power systems are addressed in this dissertation which aims at contributing to a better understanding of these problems. Subsynchronous Resonance (SSR) problems in series compensated steam-turbine power systems co-exist with the beneficial effects provided by the series capacitors. Since the early 1930s, numerous researchers have addressed issues relating to these problems. The development of a generalized frequency scan method for analyzing SSR in a Single-Machine Infinite-Bus (SMIB) power system equipped with fixed series capacitor compensation is presented. This method overcomes shortcomings present in the traditional frequency scan technique which is widely used in power system analysis. It has been noticed that there are nonlinear dynamic phenomena in power systems which can not be explained by linear system theory. This includes limited oscillations in a power system when it experiences SSR at a frequency close to one of the system modes. The phenomenon can be explained by Hopf bifurcations. This dissertation presents an analysis for a high dimensional model of a SMIB power system equipped with fixed series capacitor compensation. The results obtained can lead to a more precise understanding of this phenomenon than those available to date which use perturbation methods and highly simplified second-order power system models. Compared with fixed series capacitor compensation in power systems, the newly developed Thyristor Controlled Series Compensation (TCSC) scheme has some well known advantages with regard to flexible power system control. It has been noted that vernier mode TCSC operation can provide for SSR mitigation. In this thesis, such beneficial effect is demonstrated and analyzed for a simplified North-Western American Power System (NWAPS) model, based on EMTP simulations. Issues relating to modelling and simulation of power system and TCSC are addressed. / Graduation date: 1995

A problem of interest in the power industry is the mitigation of severe torsional oscillations induced in turbine-generator shaft systems due to Subsynchronous Resonance (SSR). SSR occurs when a natural frequency of a series compensated transmission system coincides with the complement of one of the torsional modes of the turbine-generator shaft system. Under such circumstances, the turbine-generator shaft system oscillates at a frequency corresponding to the torsional mode frequency and unless corrective action is taken, the torsional oscillations can grow and may result in shaft damage in a few seconds. <p> This thesis reports the use of a supplementary controller along with the Voltage Source Converter (VSC) HVDC back-to-back active power controller to damp all SSR torsional oscillations. In this context, investigations are conducted on a typical HVAC/DC system incorporating a large turbine-generator and a VSC HVDC back-to-back system. The generator speed deviation is used as the stabilizing signal for the supplementary controller. <p>The results of the investigations conducted in this thesis show that the achieved control design is effective in damping all the shaft torsional torques over a wide range of compensation levels. The results and discussion presented in this thesis should provide valuable information to electric power utilities engaged in planning and operating series capacitor compensated transmission lines and VSC HVDC back-to-back systems.

A problem of interest in the power industry is the mitigation of severe torsional oscillations induced in turbine-generator shaft systems due to Subsynchronous Resonance (SSR). SSR occurs when a natural frequency of a series compensated transmission system coincides with the complement of one of the torsional modes of the turbine-generator shaft system. Under such circumstances, the turbine-generator shaft system oscillates at a frequency corresponding to the torsional mode frequency and unless corrective action is taken, the torsional oscillations can grow and may result in shaft damage in a few seconds. <p> This thesis reports the use of a supplementary controller along with the Voltage Source Converter (VSC) HVDC back-to-back active power controller to damp all SSR torsional oscillations. In this context, investigations are conducted on a typical HVAC/DC system incorporating a large turbine-generator and a VSC HVDC back-to-back system. The generator speed deviation is used as the stabilizing signal for the supplementary controller. <p>The results of the investigations conducted in this thesis show that the achieved control design is effective in damping all the shaft torsional torques over a wide range of compensation levels. The results and discussion presented in this thesis should provide valuable information to electric power utilities engaged in planning and operating series capacitor compensated transmission lines and VSC HVDC back-to-back systems.

Electric power utilities are faced with the challenge of meeting increasing demand for electric power whilst many factors prevent traditional remedies such as the expansion of transmission networks and the construction of new generating facilities. Due to issues of environment, health and rights-of-way, the construction of new generating plants and transmission lines were either excessively delayed or prevented in many parts of the world in past years. An alternative resides in loading the existing transmission network beyond its present operating region but below its thermal limit, which would ensure no degradation of the system. This alternative approach has been possible with the emergence of Flexible AC Transmission Systems (FACTS) technology. The FACTS concept involves the incorporation of power-electronic controlled devices into AC power transmission systems in order to safely extend the power-transfer capability closer of these systems to their stability limits. One member of the family of FACTS series compensators is the Static Synchronous Series Compensator (SSSC), and this thesis considers the use of the SSSC to carry out closed-loop control of AC power flow in a transmission system. Although the SSSC has the potential to enhance the operation of power systems, the introduction of such a device can cause adverse interactions with other power system equipment or existing network resonances. This thesis examines the interaction between high-level power flow controllers implemented around the SSSC and a particular form of system resonance, namely subsynchronous resonance (SSR) between a generator turbine shaft and the electrical transmission network. The thesis initially presents a review of the background theory on SSR and then presents a review of the theory and operation of two categories of SSSC, namely the reactance-controlled SSSC and the quadrature voltage-controlled SSSC. The two categories of SSSC are known to have different SSR characteristics, and hence this thesis considers the impact on the damping of subsynchronous torsional modes of additional controllers introduced around both categories of SSSC to implement AC power flow control. The thesis presents the development of the mathematical models of a representative study system, which is an adaptation of the IEEE First Benchmark system for the study of SSR to allow it to be used to analyse the effect of closed-loop power flow control on SSR stability. The mathematical models of the study system are benchmarked against proven and accepted dynamic models of the study system. The investigations begin by examining the effect of a reactance-controlled SSSC-based power flow controller on the damping of torsional modes with an initial approach to the design of the control gains of the power flow controller which had been proposed by others. The results show how the nature and extent of the effects on the damping of the electromechanical modes depend on both the mode in which the power flow controller is operated and its controller response times, even for the relatively-slow responding controllers that are obtained using the initial controller design approach. The thesis then examines the impact of a reactance-controlled SSSC-based power flow controller on the damping of torsional modes when an improved approach is used to design the gains of the power flow controller, an approach which allows much faster controller bandwidths to be realised (comparable to those considered by others). The results demonstrate that for both of the modes in which the power flow controller can be operated, there is a change in the nature and extent of the power flow controller’s impact on the damping of some the torsional modes when very fast controller response times are used. Finally, the thesis investigates the impact of a quadrature voltage-controlled SSSC-based power flow controller on the damping of torsional modes in order to compare the influence of the design of both Vsssc-controlled and Xsssc-controlled SSSC-based power flow controllers on torsional mode damping for different power flow controller response times. The results obtained indicate that a Vsssc-controlled SSSC-based power flow controller allows a larger range of SSR stable operating points as compared to a Xsssc-controlled SSSC-based power flow controller. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2012.

Κύριος στόχος της εργασίας αυτής είναι η κατανόηση της διαδικασίας της μοντελοποίησης Συστήματος Ηλεκτρικής Ενέργειας, και πως η διαδικασία αυτή βοηθά στην αντιμετώπιση του φαινομένου της υποσύγχρονης αντίδρασης. Το φαινόμενο αυτό εμφανίζεται όταν ΣΗΕ στο οποίο ενυπάρχει αντιστάθμιση σε σειρά συνδέεται με ατμοστρόβιλο. Αντικείμενο μελέτης της εργασίας αυτής είναι με ποιο τρόπο η εν σειρά αντιστάθμιση συντελεί στην εμφάνιση του φαινομένου. Μελετώνται αναλυτικές υπολογιστικές μέθοδοι διάγνωσης του, και τέλος προτείνονται τρόποι αντιμετώπισης. / The main target of this project is the understanding of modeling an electrical system, and how can this help in the specific case of sub synchronous resonance studies. This phenomenon occurs when a series compensated electrical system is connected with a steam turbine, sometimes resulting in torsional stress. In addition, computational methods for the study of the phenomenon are presented. Last, but not least, countermeasures are suggested.

This thesis investigates the phenomenon of subsynchronous resonance (SSR) in meshed series compensated AC/DC systems in the presence of operational uncertainties. The main contribution of this research is the novel application of risk assessment methods to SSR studies. In terms of network topology, future electric power transmission networks alongside the current power networks are expected to be meshed, and consequently, exposed to relative low risk of subsynchronous resonance. However, power systems are increasingly operated closer to stability limits in order to enhance efficiency and economics of their use. These stressed operating conditions may contribute to the deterioration in the system reliability. Uncertainty associated with the loads will also further diversify in the future due to new type of devices connected to the network whilst the integration of stochastic renewable generation sources will add another layer of uncertainty to system operation. There is a growing necessity to explore the challenges created by the increased uncertainty in generation and loads, and quantify risk to keep a balance between avoiding potentially catastrophic systems failures and mitigating for extremely rare event. This research work introduces risk assessment in subsynchronous resonance studies. Two indices are developed to quantify the severity of dynamic instability and transient torque amplification. Using these indices generators in the network can be ranked based on potential exposure to SSR. Following the development of indices, a methodology is proposed to evaluate the risk of SSR. The developed methodology takes into account the severity of SSR problem and probabilities of different contingencies and different operating conditions of a turbine generator. A robust investigation, into the effect of uncertainties on both aspects of SSR with symmetrical and asymmetrical compensation schemes, is also performed. The results of the analysis reveal that a critically compensated system in normal meshed network configuration may become dynamically unstable with as low as ±5% uncertainty in mechanical parameters. The critical compensation level with asymmetrical compensation in normal network configuration and each contingency becomes higher. It is also shown that the risk level assessed with the developed methodology does not change under the influence of ±5% uncertainties in the mechanical parameters. After establishing, that risk, based approach provides a better picture of all credible scenarios and risk of SSR in compensated power network, a methodology based on risk evaluation of SSR for selecting an optimal combination of TCSCs and fixed series capacitors for compensation of transmission lines is presented. This proposed methodology maximizes the use of fixed capacitors whilst maintaining the risk of SSR within an acceptable level in all credible contingencies and operating conditions.

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