Novel Controls of Photovoltaic (PV) Solar Farms

Solar Farms are absolutely idle in the night and even during daytime operate below capacity in early mornings and late afternoons. Thus, the entire expensive asset of solar farms remains highly unutilized. This thesis presents novel technologies for utilization of PV solar farm inverter in nighttime for providing multiple benefits to power systems, as well as accomplishing the same objectives during the daytime from the inverter capacity left after production of real power. The new technology transforms a solar farm inverter functionally into a dynamic reactive power compensator known as STATCOM, and termed PV-STATCOM. A novel coordinated control of PV-STATCOMs is proposed for loss reduction in a distribution network. The saved energy is substantial and can be used for powering several homes annually. The second novel PV-STATCOM control involves a temporary curtailment of real power production and utilization of the available reactive power capacity to prevent the instability of a critical induction motor load. The third novel PVSTATCOM control is employed to significantly enhance the power transfer limit of a long transmission line both in the nighttime and also during daytime even when the solar farm is producing a large amount of real power. A new technique for short circuit current management is developed for a conventional PV solar farm that can potentially solve the problem due to which several solar farms have been denied connectivity in Ontario. This thesis has contributed to two patent applications and presented first time implementations of another two filed patents. A generalized PV solar system model in EMTDC/PSCAD software has been developed and validated with manufacturer\u27s datasheet. Another contribution of this thesis is the first time harmonics impact study of the largest solar farm in Canada, in the distribution utility network of Bluewater Power, in Sarnia, Ontario. This thesis makes a strong case for relaxing the present grid codes to allow solar farms to exercise these novel controls. This technology can open up new avenues for solar farms to earn revenues apart from the sale of real power. This will require appropriate agreements between the regulators, network utilities, solar farm developers and inverter manufacturers

Transient analysis and modelling of multimachine systems with power electronics controllers for real-time application

Electricity usage has grown steadily ever since the first commercial generator came into operation more than one century ago. Power transmission networks too, have grown in size and in operational complexity to be able to handle the large blocks of electricity that travel from generator to consumers round-the-clock and with huge variations. At various stages of the development, state-of-the-art equipment, methods and techniques have been incorporated in the vast array of tools that power systems engineers have at their disposal to keep up with the demands imposed by the planning, management, operation and control of modern power systems. Transient stability has always been an issue of paramount importance in power system planning and operation. Arguably, most of the ideas and concepts associated with power system stability analysis were conceived many years ago. Nonetheless, continuous expansion of the network and the emergence of a new generation of fast acting, multi-purpose power system controllers have called for renewed research efforts in this all-important application area of power systems. In particular, there is growing concern that the power network is becoming more unbalanced, owing to higher operating voltages and a relentless drive for interconnection, and that unbalances may impair the effectiveness of power electronic-based loads and controllers. These are issues that may be difficult to address satisfactorily with conventional transient stability modelling approaches since they are based on the premise that the transmission network observes a perfect balance, even under faulted operating regimes. The study of a limited range of asymmetrical transient stability problems using conventional methods can be achieved, but only with great difficulty, which involves transforming the network into fictitious components (i. e. symmetrical components). This is significant since asymmetrical short-circuit faults constitute the largest percentage of faults that occur in the power network, and network designs based solely on the three-phase short-circuit-to-ground faults result in underengineered networks. Equally important issues are the widespread commissioning of modern power electronics controllers and the lack of suitable models and methods for assessing the impact of such controllers in network-wide operation with particular reference to transient stability and unbalanced operation. The research reported in this thesis addresses these issues and develops a direct time phasedomain model for conducting multimachine transient stability analysis where asymmetrical operating conditions and the impact of modem power electronics controllers are represented. In this simulation environment, AC synchronous and asynchronous generators are represented together with asynchronous motors. The set of non-linear equations describing the machines are solved using discretisation and the trapezoidal rule of integration. The proposed model is compared against an industry standard power system package for cases of symmetrical operation. The generality and versatility of the model is demonstrated when applied to the analysis of symmetrical and asymmetrical power system operations. An important aspect of this research is a drive towards the solution of transient stability in real-time, where the results produced are in actual world time. This is achieved by embedding the model into a commercially available multi-purpose real-time station. To this end, coherency-based synchronous generators equivalent has been developed to enable the solution of multimachine systems in real-time. The equivalent unit is obtained based on the aggregation of the coherent generators using phase-domain techniques. Dynamic loads in the form of asynchronous motors are implemented within the multimachine network. The adverse influences of motor operation on voltage problems in the network under symmetrical and asymmetrical conditions are analysed. Transient analysis of dispersed generation is also considered where the asynchronous machine is operated as a generator alongside synchronous generators. The behaviours of the two type of generators under various networks and operating conditions are presented. Models of power electronics controllers in the direct time phase-domain are also described in this thesis. The generalised models of the Static Var Compensator (SVC), Static Synchronous Compensator (STATCOM), Dynamic Voltage Restorer (DVR) and High Voltage Direct Current-Voltage Source Converter (HVDC-VSC) station are proposed. The SVC comprised of a fix capacitor and a thyristor controlled reactor (TCR) is developed. Here, switching functions are used to represent the operation of the thyristor. Models of STATCOM, DVR and HVDC-VSC station are developed based on the self-commutated voltage source converter (VSC) technology. The VSC is represented by the switching functions of its pulse width modulation (PWM) control, hence, providing a flexible model within the direct time phase-domain approach. The model of the VSC is implemented into the respective power electronics controllers enabling a convenient modular approach to be adopted. The power electronics controllers are incorporated into the multimachine environment for the analysis of transient and power quality related issues

A Hybrid Method of Performing Electric Power System Fault Ride-Through Evaluations on Medium Voltage Multi-Megawatt Devices

This dissertation explores the design and analysis of a Hybrid Method of performing electrical power system fault ride-through evaluations on multi-megawatt, medium voltage power conversion equipment. Fault ride-through evaluations on such equipment are needed in order to verify and validate full scale designs prior to being implemented in the field. Ultimately, these evaluations will help in reducing the deployment risks associated with bringing new technologies into the marketplace. This is especially true for renewable energy and utility scale energy storage systems, where a significant amount of attention in recent years has focused on their ever increasing role in power system security and stability. The Hybrid Method couples two existing technologies together - a reactive voltage divider network and a power electronic variable voltage source - in order to overcome the inherent limitation of both methods, namely the short circuit duty required for implementation. This work provides the background of this limitation with respect to the existing technologies and demonstrates that the Hybrid Method can minimize the fault duty required for fault evaluations. The physical system, control objectives, and operation cycle of the Hybrid Method are analyzed with respect to the overall objective of reducing the fault duty of the system. A vector controller is designed to incorporate the time variant nature of the Hybrid Method operation cycle, limit the fault current seen by the power electronic variable voltage source, and provide regulation of the voltage at the point of common coupling with the device being evaluated. In order to verify the operation of both the Hybrid Method physical system and vector controller, a controller hardware-in-the-loop experiment is created in order to simulate the physical system in real-time against the prototype implementation of the vector controller. The physical system is simulated in a Real Time Digital Simulator and is controlled with the Hybrid Method vector controller implemented on a National Instruments FPGA. In order to evaluate the complete performance of the Hybrid Method, both a synchronous generator and a doubly-fed induction generator are modeled as the device under test in the simulations of the physical system. Finally, the results of the controller hardware-in-the-loop experiments are presented which demonstrate that the Hybrid Method is a viable solution to performing fault ride-through evaluations on multi-megawatt, medium voltage power conversion equipment

Optimised design of isolated industrial power systems and system harmonics

This work has focused on understanding the nature and impact of non-linear loads on isolated industrial power systems. The work was carried out over a period of 8 years on various industrial power systems: off-shore oil and gas facilities including an FPSO, a wellhead platform, gas production platforms, a mineral processing plant and an LNG plant. The observations regarding non-linear loads and electrical engineering work carried out on these facilities were incorporated into the report.A significant literature describing non-linear loads and system harmonics on industrial power systems was collected and reviewed. The literature was classified into five categories: industrial plants and system harmonics, non-linear loads as the source of current harmonics, practical issues with system harmonics, harmonic mitigation strategies and harmonic measurements.Off-shore oil and gas production facilities consist of a small compact power system. The power system incorporates either its own power generation or is supplied via subsea cable from a remote node. Voltage selection analysis and voltage drop calculation using commercially available power system analysis software are appropriate tools to analyse these systems. Non-linear loads comprise DC rectifiers, variable speed drives, UPS systems and thyristor controlled process heaters. All nonlinear loads produce characteristic and non-characteristic harmonics, while thyristor controlled process heaters generate inter-harmonics. Due to remote location, harmonic survey is not a common design practice. Harmonic current measurements during factory acceptance tests do not provide reliable information for accurate power system analysis.A typical mineral processing plant, located in a remote area includes its own power station. The power generation capacity of those systems is an order of magnitude higher than the power generation of a typical off-shore production facility. Those systems comprise large non-linear loads generating current and voltage interharmonics. Harmonic measurements and harmonic survey will provide a full picture of system harmonics on mineral processing plants which is the only practical way to determine system harmonics. Harmonic measurements on gearless mill drive at the factory are not possible as the GMD is assembled for the first time on site.LNG plants comprise large non-linear loads driving gas compressor, however those loads produce integer harmonics. Design by analysis process is an alternative to the current design process based on load lists. Harmonic measurements and harmonic survey provide a reliable method for determining power system harmonics in an industrial power system

A new method for power quality improvement

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A new control method for active power filters in conjunction with a passive filter circuit are presented and analysed in this thesis. A new technique for load modelling is introduced in order to enable the design of compensators to improve the power factor and to reduce harmonic levels in electrical power systems. The principles of analysis, design, operation and control of the new circuit equipped with IGBTs are presented. This enables the compensation of rapidly changing loads and reactive power. A special circuit equipped with IGBTs is able to compensate for the reactive power and harmonic currents of different orders. The important aspect of the present work is based on the compensator control circuit for power factor correction and harmonic elimination, and its application. This new configuration improves the rating of the active power filter, reducing power losses in the switches compared to existing and newly developed active filters. Furthermore, it is very stable in operation and much faster by a factor of 20. The thesis also presents a detailed mathematical modelling of the proposed system with frequency and time domain equations. The frequency response of the proposed system is also discussed. This new proposal has been checked using a dedicated software simulation program, which was specifically developed for this purpose. An experimental set-up has been designed and implemented in order to apply the new method using IGBTs as well as some other devices. This thesis also presents a critical literature survey, which provides a critical overview of previous work relevant to the power quality improvement reactive power compensation and active filtering

Dynamic modelling and simulation of electric power systems using the Newton-Raphson method

The research work presented in this thesis is concerned with the development of a dynamic power flow computer algorithm using Newton's method. It addresses both the development of a positive sequence dynamic power flow algorithm for the dynamic study of balanced power systems and a fully-fledged three-phase dynamic power flow algorithm for the dynamic study of power systems exhibiting a significant degree of either structural or operational unbalance. As a prelude to the research work on dynamic power flows, a three-phase Newton-Raphson power flow algorithm in rectangular co-ordinates with conventional HVDC power plant modelling is presented in this thesis, emphasising the representation of converter control modes. The solution approach takes advantage of the strong numerical solutions for combined HVAC-HVDC systems, where power plant and operational imbalances are explicitly taken into account. The dynamic algorithm is particularly suited to carrying out long-term dynamic simulations and voltage stability assessments. Dynamic model representations of the power plants components and the load tap changing transformer are considered, and to widen the study range of dynamic voltage phenomena using this method, extensions have been made to include induction motor and polynomial load modelling features. Besides, reactive power compensators that base their modus operandi on the switching of power electronic valves, such as HVDC-VSC and the STATCOM are taken into account. The dynamic power flow algorithm has primarily been developed making use of the positive sequence and [dq] representations. Extensions are made to developing a three-phase power flows dynamic algorithm. Test cases for the various dynamic elements developed in this research are presented to show the versatility of the models and simulation tool, including a trip cascading event leading up to a wide-area voltage collaps. Comparisons with the output of a conventional transient stability program carried out where appropriate

Investigation of Control Concepts for High-Speed Induction Machine Drives and Grid Side Pulse-Width Modulation Voltage Source Converters

Control of a low voltage ac/dc/ac converter for high-speed induction machine drive applications has been investigated. Such a configuration can be applied, for example, in microturbines and high-speed spindles. Scalar control is usually applied for the control of high-speed drives especially in the case of very high-speed drives. Indirect rotor-flux-oriented control and direct torque control are designed and compared for the control of an exemplary high-speed induction machine drive. The 2L VSC is the most widely applied converter for high-speed drives. However, the 3L-NPC VSC is an attractive topology if drastically increased switching frequencies are required. A detailed comparison between a 2L VSC and a 3L-NPC VSC as the machine side converter of the exemplary high-speed induction machine drive is carried out. Voltage-oriented control is applied for the control of the grid side PWM active front end converter. In several industrial applications PWM active front end converters commonly operate in parallel to thyristor converter fed dc drives. Behavior of the voltage-oriented controlled active front end converter with L-filter in the presence of a parallel thyristor converter is investigated. The design of the LCL-filter components according to the given maximum grid current harmonics (e.g. IEEE-519) is a complex task. So far a precise and clear design procedure has not been presented. A new procedure to design the grid side filter (L- and LCL-filter) is proposed using the analytical expression of the converter voltage harmonics based on Bessel functions to achieve the compliance with the grid standard of IEEE-519. Voltage-oriented control with active damping is used to control the active front end converter with LCL-filter. A simple method is proposed to design the required lead-lag compensator in the active damping loop

New approach to steady-state and dynamic nonlinear modelling of laminated salient-pole alternator systems

A complete mathematical model for a generator system consisting of an isolated laminated salient-pole alternator, exciter and prime mover is presented, with emphasis on the inherent electromagnetic nonlinearities in the alternator and its exciter. An equivalent circuit, representing the rotor circuits accurately, has been adopted to model the al ternator in the dqo reference frame. A computer program has been developed to calculate the unsaturated parameters of the model using the machine design data. A new approach has been developed to account for the electromagnetic saturation effects on the model reactances. Consequently new saturation factors, based on the machine design particulars have been deri ved. The advantages of these saturation factors, compared with conventional factors, are that both mutual saturation effects between the main and leakage fluxes, and between the direct- and quadratureaxis fluxes are considered. A mathematical nonlinear model, utilising the new saturation factors, is presented for a system containing an isolated laminated salientpole alternator and a direct thyristor static exciter. A digital computer program has been developed to simulate the system. The predicted results, for some steady state and dynamic candi tions, showgood agreement with test results and clear improvement over those obtained if saturation is either neglected or considered using the conventional saturation factors. At high saturation levels, the conventional method of calculating the machine transformer voltages, using static saturated reactances, gives unacceptable errors. A method for calculating these voltages correctly, in models utilising the currents as state space variables, is presented using new derived dynamic saturated reactances. This dynamic reactance concept is presented in a generalised form so that it can be applied to any machine with different saturation factors. The previous mathematical model of the alternator system has been modified according to the dynamic reactance concept, and the computer program has been developed accordingly. The predicted results confirm the need to apply this concept especially to dynamic conditions characterised by high saturation levels. To extend the analysis to a wider range of loading conditions, the alternator has been modelled in the abc reference frame. The unsaturated, static and dynamic saturated reactances of the machine in this reference frame have been obtained using conventional dqo-abc transformation techniques. Starting from the fundamental machine relations, a new set of equations, in the phase reference frame, has been derived employing the new dynamic reactance concept. A comprehensive system consisting of an isolated laminated salientpole alternator, brushless exciter, thyristor divert automatic voltage regulator and a diesel prime mover has been studied. Both the alternator and the exciter have been modelled in the abc frame to comply with the nature of rectifier loading associated with the exci ter. A complete steady state and dynamic mathematical model is presented where the t~r technique has been applied to the dynamic variable topology of the system electrical circuits. The model presented covers all the possible modes of operation associated with the exciter rotating bridge rectifier circuit. A digital computer program has been developed to simUlate the system. The predicted results obtained using the new set of saturation factors in conjunction with the dynamic reactance concept show good agreement with the test results. The study presented confirms the validity of the mathematical models developed for the alternator systems. Also, it supports the metlxxi by which the electromagnetic nonlinearity has been accounted for

Steady state and transient stability analysis of A.C.-D.C. power systems

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