Modeling and Experimental Verification of Adaptive 100% Stator Ground Fault Protection Schemes for Synchronous Generators

Salient pole synchronous generators as the main component of an electricity generation station should be carefully maintained and their operation has to be monitored such that any damage on them is avoided. Otherwise, the generating station might experience frequent shut downs which results in electricity generation interruptions and high costs associated with repairing and compensation of lack of energy. In this sense, many protective schemes focusing on a variety of synchronous generator faults have already been proposed and are still modified and developed to further enhance the quality of protection. In this thesis, synchronous generator stator windings to ground fault is studied as one of the most common and crucial faults in these machines. Numerous methods of stator winding to ground fault protection schemes are also reported in the literature. Third harmonic differential voltage and sub-harmonic schemes are studied in this research. A novel adaptive scheme for both methods is modelled and implemented in a comprehensive lab scale set-up where a real generation unit is scaled down including all different components and apparatus. The simulation model is also established based on simultaneous finite element analysis (FEA) and coupled magnetic circuit to assist with system configuration design and parameter selections. The adaptive scheme is proved to be capable of detecting stator windings to ground faults based on actual experimental data. Finally, the proposed adaptive scheme is compared against other available non-adaptive protection schemes currently used in industrial relays. Several important performance evaluation criteria in protection schemes such as sensitivity and security of operation referred to as reliability are considered. It is shown that the adaptive scheme offers higher reliability than other schemes which emphasizes its credibility and applicability

Stator winding fault diagnosis in synchronous generators for wind turbine applications

Wind turbine manufacturers have introduced to the market a variety of innovative concepts and configurations for generators to maximize energy capture, reduce costs and improve reliability of wind energy. For the purpose of improving reliability and availability, a number of diagnostic methods have been developed. Stator current signature analysis (SCSA) is potentially an effective technique to diagnose faults in electrical machines, and could be used to detect and diagnose faults in wind turbines. In this study, an investigation was conducted into the application of SCSA to detect stator inter-turn faults in an excited synchronous generator and a permanent magnet synchronous generator. It was found from simulation results that, owing to disruption of magnetic field symmetry and imbalance between the current flowing in the shorted turn and the corresponding diametrically opposite turn in the winding, certain harmonic components in the stator current clearly increased as the number of shorted turns increased. The findings are helpful to detect faults involving only a few turns without ambiguity, in spite of the difference in the configuration of the generators. As expected, because of the different type, configuration and operational condition of the two generators studied, detecting faults through the generator current signature requires a particular approach for each generator type

Static and dynamic eccentricity fault diagnosis of large salient pole synchronous generators by means of external magnetic field

Although synchronous generators are robust and long-lasting equipment of power plants, consistent electricity production depends on their health conditions. Static and dynamic eccentricity faults are among the prevalent faults that may have a costly effect. Although several methods have been proposed in the literature to detect static and dynamic eccentricity faults in salient pole synchronous generators (SPSGs), they are non-sensitive to a low degree of failure and require a predefined threshold to recognise the fault occurrence that may vary based on machine configuration. This article presents a detailed magnetic analysis of the SPSGs with static and dynamic eccentricity faults by focusing on the external magnetic field. The external magnetic field was measured using two search coils installed on the backside of the stator yoke. Also, advanced signal processing tools based on wavelet entropy were used to analyse the induced electromotive force (emf) in search coils to extract the fault index. The proposed index required no threshold to recognise the starting point of fault occurrence and was sensitive to a low degree of fault. It was also non-sensitive to load variation and noise that may induce a false alarm

Integrated electromechanical wind turbine control for power system operation and load reduction

With the penetration level of wind power in electric power networks increasing rapidly all over the world, modern wind turbines are challenged to provide the same grid services as conventional synchronous power plants. The dynamic interaction between wind turbines and grid has to be assessed first before replacing large amount of conventional power plants by wind power. Over the last few years many power system operators have revised their grid codes and established more demanding requirements for wind power connection. In the past, when wind turbines were small, they were allowed to simply disconnect during a grid fault/disturbance. However, as wind turbine size has increased considerably, their fault ride-through capability has to be improved if the penetration of wind power is to be further increased. Wind turbine design and control need to be improved to optimize the compatibility of wind power and the grid. Among the various requirements that wind turbines have to meet, fault ride-through is of great importance and a very challenging one. Grid faults cause transients not only in the electrical system, but also in the wind turbine mechanical system. The dynamic performance of wind turbines is determined by both mechanical and electrical systems. From the mechanical point of view, the grid disturbance adds extra loads on wind turbine components. Severe grid faults may even lead to wind turbine emergency shut-down. From the electrical point of view, wind farms may lose power generation during a grid fault, which deteriorates the fault impact and slows down the fault recovery. Advanced control and active damping is required to improve wind turbine operation and assist it to remain connected during a grid fault. The novelty of this research is the study of the interaction between mechanical and electrical systems of the wind turbine. The detailed modelling of both the wind turbine mechanical and electrical dynamics not only helps to identify possible problems that wind turbines encounter during grid faults, but also allows adopting a combined approach to design the wind turbine controller. This thesis aims at improving the wind turbine fault ride-through capability and the ability of wind turbine to provide network support during grid disturbances. The main contents are as follows: The detailed model of wind turbine and grid including wind turbine mechanical model, wind turbine controller, synchronous and induction generator model, doubly fed induction generator (DFIG) controller and a generic network model are presented; A wind turbine fault ride-through strategy considering structural loads alleviation is proposed; A controller for asymmetrical fault ride-through of DFIG wind turbines is presented; The effect of having Power System Stabilizer (PSS) on wind turbine is investigated. A multi-band PSS controller for DFIG wind turbine is demonstrated.With the penetration level of wind power in electric power networks increasing rapidly all over the world, modern wind turbines are challenged to provide the same grid services as conventional synchronous power plants. The dynamic interaction between wind turbines and grid has to be assessed first before replacing large amount of conventional power plants by wind power. Over the last few years many power system operators have revised their grid codes and established more demanding requirements for wind power connection. In the past, when wind turbines were small, they were allowed to simply disconnect during a grid fault/disturbance. However, as wind turbine size has increased considerably, their fault ride-through capability has to be improved if the penetration of wind power is to be further increased. Wind turbine design and control need to be improved to optimize the compatibility of wind power and the grid. Among the various requirements that wind turbines have to meet, fault ride-through is of great importance and a very challenging one. Grid faults cause transients not only in the electrical system, but also in the wind turbine mechanical system. The dynamic performance of wind turbines is determined by both mechanical and electrical systems. From the mechanical point of view, the grid disturbance adds extra loads on wind turbine components. Severe grid faults may even lead to wind turbine emergency shut-down. From the electrical point of view, wind farms may lose power generation during a grid fault, which deteriorates the fault impact and slows down the fault recovery. Advanced control and active damping is required to improve wind turbine operation and assist it to remain connected during a grid fault. The novelty of this research is the study of the interaction between mechanical and electrical systems of the wind turbine. The detailed modelling of both the wind turbine mechanical and electrical dynamics not only helps to identify possible problems that wind turbines encounter during grid faults, but also allows adopting a combined approach to design the wind turbine controller. This thesis aims at improving the wind turbine fault ride-through capability and the ability of wind turbine to provide network support during grid disturbances. The main contents are as follows: The detailed model of wind turbine and grid including wind turbine mechanical model, wind turbine controller, synchronous and induction generator model, doubly fed induction generator (DFIG) controller and a generic network model are presented; A wind turbine fault ride-through strategy considering structural loads alleviation is proposed; A controller for asymmetrical fault ride-through of DFIG wind turbines is presented; The effect of having Power System Stabilizer (PSS) on wind turbine is investigated. A multi-band PSS controller for DFIG wind turbine is demonstrated

Effective algorithms for real-time wind turbine condition monitoring and fault-detection

Reliable condition monitoring (CM) can be an effective means to significantly reduce wind turbine (WT) downtime, operations and maintenance costs and plan preventative maintenance in advance. The WT generator voltage and current output, if sampled at a sufficiently high rate (kHz range), can provide a rich source of data for CM. However, the electrical output of the WT generator is frequently shown to be complex and noisy in nature due to the varying and turbulent nature of the wind. Thus, a fully satisfactory technique that is capable to provide accurate interpretation of the WT electrical output has not been achieved to date. The objective of the research described in this thesis is to develop reliable WT CM using advanced signal processing techniques so that fast analysis of non-stationary current measurements with high diagnostic accuracy is achieved. The diagnostic accuracy and reliability of the proposed techniques have been evaluated using data from a laboratory test rig where experiments are performed under two levels of rotor electrical asymmetry faults. The experimental test rig was run under fixed and variable speed driving conditions to investigate the kind of results expected under such conditions. An effective extended Kalman filter (EKF) based method is proposed to iteratively track the characteristic fault frequencies in WT CM signals as the WT speed varies. The EKF performance was compared with some of the leading WT CM techniques to establish its pros and cons. The reported experimental findings demonstrate clear and significant gains in both the computational efficiency and the diagnostic accuracy using the proposed technique. In addition, a novel frequency tracking technique is proposed in this thesis to analyse the non-stationary current signals by improving the capability of a continuous wavelet transform (CWT). Simulations and experiments have been performed to verify the proposed method for detecting early abnormalities in WT generators. The improved CWT is finally applied for developing a new real-time CM technique dedicated to detect early abnormalities in a commercial WT. The results presented highlight the advantages of the improved CWT over the conventional CWT to identify frequency components of interest and cope with the non-linear and non-stationary fault features in the current signal, and go on to indicate its potential and suitability for WT CM.</div

Finite element electromagnetic analysis of generator transient performance

Imperial Users onl

Analysis of Ball Bearing Defects in Synchronous Machines using Electrical Measurements

Rolling element bearings are used in most electrical machines, especially for small and medium size applications. Under non-ideal operating conditions, ball bearing condition degrades by fatigue, ambient vibration, misalignment, overloading, contamination, corrosion from water or chemicals, improper lubrication, shaft currents and residual stress left from the bearing manufacturing process. All of these conditions eventually lead to increased vibration and acoustic noise during machine operation which at some point in time results in unexpected bearing failure. Over the years, a great number of publications have been devoted to the detection of mechanical faults, including rolling element bearing defects and torsional defects, in electrical machines based on Electrical Signature Analysis (ESA). It has been observed that these faults can affect either the stator to rotor air-gap distribution or the running speed of the machine, which can be reflected in the signature of the electrical signals. However, the physical link between the mechanical degradation and the electrical signature is still not explained well. A multi-physics model is developed by joining the detailed mechanical model of a rotor bearing system and the electrical model of a synchronous machine in this research. This combined model is capable of describing the transmission of information originating from bearing faults and their impact on the variations of the measured electrical signals. The electrical machine model is developed based on winding function approach and its validity is demonstrated by a more accurate Finite Element Method (FEM) model. The mechanical model consists of a high fidelity rotor-bearing system with detailed nonlinear ball bearing model and a flexible finite element shaft model. It is validated using the housing vibration data collected from some experiments. Generalized roughness bearing anomalies are linked to load torque ripples and airgap variations, while being related to current signature by phase and amplitude modulation. Considering that the induced characteristic signatures are usually subtle broadband changes in the current spectra, these signatures are easily affected by input power quality variations, machine manufacturing imperfections and environmental noise. In this research, a new algorithm is proposed to isolate the influence of the external disturbances of power quality, machine manufacturing imperfections and environmental noise, and to improve the effectiveness of applying the ESA for generalized roughness bearing defects. The results show that the proposed method is effective in analyzing the generalized roughness bearing anomaly in synchronous machines. Furthermore, the electrical signatures are analyzed in a synchronous machine with bearing defects. The proposed fault detection method employs a Zoomed Fast Fourier Transform (ZFFT) and Principal Component Analysis (PCA) and it is also tested on the available experimental data. The results show that amplitude induced electrical harmonics are related to the level of vibration, and the electrical signatures are affected heavily by other variables, such as power quality and load fluctuation. The proposed method is shown to be effective on detecting generalized roughness bearing defects in synchronous machines

Dynamic Phasor Modeling of Type 3 Wind Farm including Multi-mass and LVRT Effects

The proportion of power attributable to wind generation has grown significantly in the last two decades. System impact studies such as load flow studies and short circuit studies, are important for planning before integration of any new wind generation into the existing power grid. Short circuit modelling is central in these planning studies to determine protective relay settings, protection coordination, and equipment ratings. Numerous factors, such as low voltage situations, power electronic switching, control actions, sub-synchronous oscillations, etc., influence the response of wind farms to short circuit conditions, and that makes short circuit modelling of wind farms an interesting, complex, and challenging task. Power electronics-based converters are very common in wind power plants, enabling the plant to operate at a wide range of wind speeds and provide reactive power support without disconnection from the grid during low voltage scenarios. This has led to the growth of Type 3 (with rotor side converter) and Type 4 (with stator side full converter) wind generators, in which power electronics-based converters and controls are an integral part. The power electronics in these generators are proprietary in nature, which makes it difficult to obtain the necessary information from the manufacturer to model them accurately in planning studies for conditions such as those found during faults or low voltage ride through (LVRT) periods. The use of power electronic controllers also has led to phenomena such as sub-synchronous control interactions in series compensated Type 3 wind farms, which are characterized by non-fundamental frequency oscillations. The above factors have led to the need to develop generic models for wind farms that can be used in studies by planners and protection engineers. The current practice for short circuit modelling of wind farms in the power industry is to utilize transient stability programs based on either simplified electromechanical fundamental frequency models or detailed electromagnetic time domain models. The fundamental frequency models are incapable of representing the majority of critical wind generator fault characteristics, such as during power electronic switching conditions and sub-synchronous interactions. The detailed time domain models, though accurate, demand high levels of computation and modelling expertise. A simple yet accurate modelling methodology for wind generators that does not require resorting to fundamental frequency based simplifications or time domain type simulations is the basis for this research work. This research work develops an average value model and a dynamic phasor model of a Type 3 DFIG wind farm. The average value model replaces the switches and associated phenomena by equivalent current and voltage sources. The dynamic phasor model is based on generalized averaging theory, where the system variables are represented as time varying Fourier coefficients known as dynamic phasors. The two types models provide a generic type model and achieve a middle ground between conventional electromechanical models and the cumbersome electromagnetic time domain models. The dynamic phasor model enables the user to consider each harmonic component individually; this selective view of the components of the system response is not achievable in conventional electromagnetic transient simulations. Only the appropriate dynamic phasors are selected for the required fault behaviour to be represented, providing greater computational efficiency than detailed time domain simulations. A detailed electromagnetic transient (EMT) simulation model is also developed in this thesis using a real-time digital simulator (RTDS). The results obtained with the average value model and the dynamic phasor model are validated with an accurate electromagnetic simulation model and some state-of-the-art industrial schemes: a voltage behind transient reactance model, an analytical expression model, and a voltage dependent current source model. The proposed RTDS models include the effect of change of flux during faulted conditions in the wind generator during abnormal system conditions instead of incorrectly assuming it is a constant. This was not investigated in previous studies carried out in the real-time simulations laboratory at the University of Saskatchewan or in various publications reported in the literature. The most commonly used LVRT topologies, such as rotor side crowbar circuit, DC-link protection scheme, and series dynamic braking resistance (SDBR) in rotor and stator circuits, are investigated in the short circuit studies. The RTDS model developed uses a multi-mass (three-mass) model of the mechanical drive train instead of a simple single-mass model to represent torsional dynamics. The single mass model considers the blade inertia, the turbine hub, and the generator as a single lumped mass and so cannot reproduce the torsional behaviour. The root cause of sub-synchronous frequencies in Type 3 wind generators is not well understood by system planners and protection engineers. Some literature reports it is self excitation while others report it is due to sub-synchronous control interactions. One publication in the stability literature reports on a small signal analysis study aimed at finding the root cause of the problem, and a similar type of analysis was performed in this thesis. A linearized model was developed, which includes the generator model, a three mass drive train, rotor side converter, and the grid side converter represented as a constant voltage source. The linear model analysis showed that the sub-synchronous oscillations are due to control interactions between the rotor side controller of the Type 3 wind power plant and the series capacitor in the transmission line. The rotor side controls were tuned to obtain a stable response at higher levels of compensation. A real-time simulation model of a 450 MW Type 3 wind farm consisting of 150 units transmitting power via 345 kV transmission line was developed on the RTDS. The dynamic phasor method is shown to be accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for different sub- synchronous oscillation frequencies

Real time digital simulation and testing of generator protection elements.

Masters Degree, University of KwaZulu-Natal, Durban.Power system protection is designed to identify and isolate the system from any type of fault or abnormal condition which may endanger the equipment and operation of the system as a whole. Ground faults are the most common types of faults in generators and can damage the stator winding severely. Stator winding protection therefore becomes one of the crucial protection functions in generator protection. The grounding method used plays an imperative role in determining which protection functions are to be employed on the generator. This thesis reviews different types of stator winding faults that occur for a generator and how the generator is protected against these faults using different types of protection system. It also presents how the different types of generator grounding affect generator protection schemes, focusing on high and low impedance grounding. The development of real time digital simulators has greatly improved the simulation and testing of protection studies. In the past, mathematical models were not fully compatible for the representation of the complete synchronous generator stator. The Real Time Digital Simulator (RTDS) has developed a synchronous generator phase domain model which allows for simulation of generator stator internal faults. This thesis illustrates the suitability of the third harmonic voltage protection scheme against stator internal faults. An overview of abnormal conditions that occur on a generator was also reviewed, how they affect the generator and their protection systems. The thesis focused on reverse power, over-excitation, and differential and current unbalance protection. The loss of field excitation in synchronous generators also largely contributes to voltage instability. The large consumption of reactive power and rapid changes in the system components leads to severe damage of the generator and jeopardizes system stability. This thesis looks into loss of field excitation events and how their impacts can be reduced by using the R-X protection scheme. It also illustrates results based on closed loop testing conducted using hardware generator protection relay and the models developed on the RTDS. The simulation and testing of generator protection functions were proved to be theoretically and practically correct which could be used as a guideline for improvements in protection studies