Abstract
This thesis aims to extract d-axis machine parameters of a 214 MVA high-speed turbo• generator using well-established off-line techniques. MATLAB/SIMULINK tool has been utilized to model the generator and perform sudden short-circuit (SSC) tests. Because constant speed operation of the generator is vital consideration for the accuracy of SSC tests for parameter extraction, this thesis has employed an arrangement of 2 DC motors operating alongside a drive system, which is configured to couple the combined motor to drive the shaft of the generator to maintain synchronous speed throughout the test. Simulating the different drive configurations, the SSC oscillographs were captured and transformed into symmetrical envelope waveforms which were then used to extract the operational impedances and time constants of the generator. From the simulation tests, the influence of speed departures on shaft torques and current waveforms were presented and analysed. The results obtained for the variable-speed simulation using the drive system were then validated with those calculated from standard equations, as well as those of the constant-speed mode. The test results confirm that SSC tests can be performed on synchronous machines under rated conditions if the couplings can be appropriately designed to contain the dangerous values of electromagnetic torque that are developed during such tests. In addition, an electronic switch was designed to manage the abnormal field current which results from the introduction of SSC currents. In the end, the switching contraption is seen to reduce the power losses in the machine’s field winding during SSC.
CHAPTER ONE
1.1 Background of Study
INTRODUCTION
Synchronous machines exist both as a generator and as a motor depending on how and/or which of the windings is energized to operate them as well as the type of load it is designed to supply. Nearly all of the electric power used throughout the world is generated by synchronous machines driven by hydro or steam turbines or combustion engines [1]. It is the primary means by which mechanical energy is converted into electrical energy. By further explanation, conventional synchronous machine has the field-winding wound on the rotating member (the rotor), and the armature wound on the stationary member (the stator). A de current, creating a magnetic field that must be rotated at synchronous speed induces a 3-phase set of voltages within the stator windings of a synchronous generator. The excitation of the rotating field-winding can be achieved through a set of slip rings and brushes (external excitation), or from a diode-bridge mounted on the rotor (self-excited or brushless excitation) [2].
Because of increased amount of maintenance and power losses associated with slip rings and brushes, brushless exciters are preferred for large synchronous machines. A brushless exciter is a small ac generator with its field mounted on the stator and its armature circuit mounted on the rotor shaft. The 3-phase output of the exciter generator is rectified to direct current by a 3-phase rectifier circuit also mounted on the shaft of the generator, and is then fed to the main de circuit. To make the excitation of a generator completely independent of any external power sources, a small pilot exciter can be used. A pilot exciter is a small ac generator with permanent magnets mounted on the rotor shaft and a 3-phase winding on the stator. It produces power for the field circuit of the exciter, which in turn controls the field circuit of the main machine.
Elsewhere, it has been defined that a synchronous machine acting as a motor is an ac rotating machine whose speed under steady state condition is proportional
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to the frequency of the current in its armature and this causes the magnetic field created by the armature currents to rotate at the same speed as that created by the field current on the rotor, which is rotating at synchronous speed, resulting to a steady torque.
Synchronous generators have been variously described and classified and more details will be provided much later in Chapter Three. However, it is good to mention here that a general attempt has been made to describe them based on the two types of their rotor structures – round or cylindrical rotor and salient pole rotor types (see Fig. 1); the basic difference being that the later is magnetically unsymmetrical.
Historically, the commercial birth of the synchronous generator, also called alternator, dates back to August 24, 1891 when the first large-scale demonstration of ac power generation was carried out during an international electrical exhibition in Frankfurt, Germany. The success of its adoption since then is based on the feasibility of transmitting ac power over long distances. In 1895, the same technology was applied to the New York Niagara Falls power plant and this marked the end of the great de versus ac duel. Today, tremendous development in machine ratings, insulation components, and design procedures have been recorded, however, the basic constituents of the machine have remained practically unchanged.
The largest and perhaps the most popular of synchronous machines are the three-phase synchronous generators [2]. Though constructing them is relatively more expensive, their higher efficiency is an advantage at very high power ratings. The stator windings are identically distributed over pole-pairs, and the phase axes are spaced 120 apart. The rotor on the other hand, can be salient or cylindrical. Salient pole construction is mostly used in low-speed applications where the diameter to length ratio on the rotor can be made larger to accommodate the high pole number.
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Flux
paths
(a) (b)
Fig. 1. Illustration of Synchronous Machines of (a) salient rotor structures and (b)
round or cylindrical rotor
Generally, the round rotor structure is used for high speed synchronous machines, such as steam turbine generators, while salient pole structure is used for low speed applications, such as hydroelectric generators. In salient pole synchronous generators, the short, pancake-like rotor has separate pole pieces bolted onto the periphery of a spider-web-like hub and the “salient” simply refers to the protruding poles; the alternating arrangement of the pole iron and the inter-polar gap results in preferred directions of magnetic flux paths or magnetic saliency. Non• salient-pole (cylindrical) rotors are utilized in two-or-four-pole machines, and, very rarely, in six-pole machines.
The rotor of a synchronous machine is equipped with a field winding and one or more damper windings, which accounts for the different electrical characteristics unique to rotor windings. In the basic two-pole representation of synchronous generators, the axis of the North Pole is called the direct or d-axis. The quadrature, or q-axis, is defined in the direction 90 electrical degrees ahead of the direct axis. Under no-load operation with only field excitation, the field MMF will be along the d• axis, and the stator internal voltage, will be along the q-axis.
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From the structure of a synchronous generator, the rotor field is a physical winding; while damping windings may just be electrically equivalent windings [1]-[2]. For example, the damping windings for a salient-pole generator represent the damping effect of damping rods distributed on the rotor while for a round rotor generator; the damping function produced is developed by the eddy current inside the whole rotor. In other words, they are just equivalent windings and can be represented by a single or multiple damping windings. These damping windings are made of copper bars short-circuited at both ends and embedded in the head of the pole, close to the face of the pole. The purpose of this winding is to start the motor under its own power as an induction motor, and take it unloaded to almost synchronous speed when the rotor is “pulled in” by the synchronous torque. The winding also serves to damp the oscillations of the rotor around the synchronous speed, and is therefore named damping-winding (also known as amortisseurs).
Synchronous machine can be adapted as motors where constant speed drive is required because of the rotor speed which is proportional to the frequency of excitation or as synchronous condensers in power systems for power factor correction or for control of reactive KVA flow. The latter is made possible because the reactive power generated by an unloaded synchronous machine can be adjusted by controlling the magnitude of the rotor field current. With power electronic variable voltage variable frequency (VVVF) power supplies, synchronous motors, especially those with permanent magnet rotors, are widely used for variable speed drives; this type of motors are known as brushless de motors since they do not need brushes.
Since the synchronous machines have been generally accepted as a three• phase generator for power generation, there is a body of literature on their modeling and the determination of parameters. A few of such studies are mentioned as references [1]-[3]. Much later in this work, Chapter Three has been designed to explain the mathematical model that is based on the equivalent idealised synchronous machine with the rotor equipped with a field winding and three damper windings.
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So far, the background information provided in this section for the generator action of synchronous machines is sufficient to undertake this research work and it is thus structured in the following sections.
1.2 Motivations
Off-line sudden short-circuit (SSC) tests in synchronous generators have continued to generate growing amount of research interests because it offers an advantage in determining some transient parameters in synchronous generators on the basis that their transient behaviour can be accurately captured. Synchronous generator sudden short-circuit tests are also necessary in power system studies to provide protective equipment designed to isolate the faulted generator from the remainder of the system in the appropriate time. The interrupting capacity of the breakers should be designed to withstand the highest value of possible short-circuit currents.
Of primary concern during sudden short-circuit tests is to maintain a constant speed for both the driver and driven machine. To this end, short-circuit measurements for very small synchronous generators do not pose a serious challenge because a separately excited de motor of about two times its rating can be used to drive them without encountering speed departures. However, for very large machines, this is not possible because the electromagnetic torque of the synchronous generator will increase in the event of sudden short-circuit currents, and in turn this will appear as a very large load torque to the driving motor. Consequently, the speed of the motor suddenly drops with such an additional load.
On the other hand, an induction motor is not amenable to drive large synchronous generators because of the difficulty in maintaining their speed constant where possible.
Looking at synchronous motors they are hard to brake. Moreover, if an attempt is made to employ synchronous motors to drive large synchronous generators, the presence of short-circuit currents will result in values as high a three
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times it rated current value leading to higher torque values of up to six or more its rated value and this poses greater mechanical threat to the connecting shaft which can result in its breaking and/or either of the machine being destroyed.
In contemporary practice, linear mapping using the reduced field current method which is an attempt to avoid the dangerous impact of very high short-circuit currents and torque have been attempted. This method is impractical because it assumes a linear operation of the generator by reducing the field current to a safe value which operates to produce short-circuit current values below the machine rated values. It is difficult to accurately predict, based on this assumption, what happens at the non-linear saturation point on the 8-H curve. Even if so many interpolations are taken in the linear region, it is difficult to predict what happens in the non-linear region using this method. Moreover, this kind of approximation can be very inaccurate for extraction of parameters and time constants of large synchronous generators in the transient and sub-transient regions. Hence, SSC under rated conditions drive the machine fluxes into deep saturation.
Separately-excited de motors that operate under constant speed are best choice for driving synchronous generators, but when compared to large synchronous generators – to be considered in this work — they come in relatively small sizes with very low speed and efficiency. Turbo generators (of ratings of several MVA) operate at very high speed otherwise they will not deliver the required power. The biggest and practical sizes of de motor are about 2MW. It has been stated that de motor sizes double or more than of 2MW exist, but this is highly inefficient as they operate with enormous losses.
In some cases, designers prefer to transport large synchronous generators after they have been manufactured to generating sites where they are then randomly tested. But this approach is also not justifiable when the cost of transportation and other logistics are brought into consideration.
In this work, the main motivation is to develop a drive system using a system of coupled separately excited de motors to drive a large synchronous generator
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under short-circuit conditions without altering the speed of both the motor and that of the generator to guarantee the accurate extraction of the d-axis parameters of the synchronous generator. In addition, this work will also consider how to design and implement a switching system to turn off the generator at dangerous levels of short• circuit currents. In the first case, the system to be modeled is in three separate parts and includes all realistic components of the drive system. This enables the accurate
extraction of all the vital information needed to estimate the machine d-axis parameters under transient and steady conditions such as a X‘, Ta T’, and T“. In
the second case, a full-load operation mode of the synchronous generator is assumed so as to allow for the determination of the minimum safest field excitation at which the generator can be operated without compromise of its safety.
1.3 Objectives
The objective of this work is to accurately extract the d-axis machine parameter of a large synchronous generator using coupled DC motors to appropriately set-up a drive system. Non-trivial equations for the coupling shaft will also be derived. These efforts will seek to provide answers to the following research questions:
1. How does one ensure a valid drive system for driving large synchronous generators?
2. What type of motor can be used to conveniently drive large synchronous generators to capture short-circuit oscillographs?
3. What practical measures are needed to ensure that the speed of large synchronous generators remain constant during sudden short-circuit tests? The target then is to ensure that the speed of the drive system must remain
constant at the instant when the short-circuit occurs to the point where sufficient
data for parameter extraction has been captured without altering the speed of the drive system. This hypothesis is on the basis that the speed was constant before the test and therefore must be maintained at a constant value as much as possible in
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the duration of the test. Altering the speed of the machines alters the frequency of the current and by extension, everything about the short-circuit current from which vital machine parameters will be extracted. Moreover, capturing the machine parameters and time constants at rated speed values will offer opportunity to make close comparison with known tests, thereby providing greater confidence for accuracy.
Also, using the MATLAB/SIMULINK” toolbox is a deliberate effort to design a flexible computer programme which can be amenable to various dynamic analyses and is simple enough to permit fast, low-cost and safe testing of the machine model. In this way, it will be possible to study short-circuit fault at various machine conditions and models. Moreover, another advantage of using this method is that it can be done at the factory of the manufacturer; it poses a reduced risk factor to the machine being tested, and provides complete data in the direct axis.
Another objective for this research is to incorporate a control that can regulate the generator field current at SSC current modes. Again, MATLAB/Simulink will be used to achieve this stage of the research after relevant deductions from the machine have been made at full load operation. A number of research questions are raised here:
1. What is the maximum safest (trip-off) field current at which the generator can be excited?
2. How can one automatically switch off the field current if it exceeds a threshold value?
3. How can one implement a switching system to automatically switch off the field current at dangerous SSC rates?
1.4 Three-Phase Short-Circuit Fault in Synchronous Generators: An
Overview
A synchronous generator, connected to an infinite bus whose voltage is the generator rated voltage, has its stable operating point defined by the following rated
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values: armature voltage, apparent power, speed, power factor and excitation voltage. This set of values are maintained during normal operation, and in a situation where they are not, the generator is said to be operating under fault or more commonly, under transient condition. For this very reason, it is important that all the generator ratings and nameplate parameters are always maintained within rated machine values.
Synchronous generator failure is usually caused by external factors such as lightning strikes, heavy rain, strong winds, or contamination of insulators. Most of the faults that occur in synchronous generators are non-symmetric; long-term average short-circuit statistics indicate that 70 percent of synchronous generator faults occur during electrical storms [1], [4].
The procedure for symmetrical short-circuit test of an unloaded generator
using the reduced field current method is described in references [5]-[8] thus: The terminals of the machine at no load, excited by a reduced field current and rotating at rated speed are shorted while the currents in the armature windings and the field windings are recorded.
Physically, short-circuit faults produce severe forces during the operation of large alternators, and to calculate the magnitude of these forces will require the magnitude of the short-circuit currents to be known as a first step [9], although the process is extremely short (usually about 0.1 ~0.3s).
For this reason, the task is to provide protective equipment designed to isolate a faulted generator from extreme impacts of short-circuit currents in the appropriate time. The interrupting capacity of breakers should be designed to accommodate the largest short-circuit currents and hence care must be taken not to base the precision decision simply on the results of a balanced three-phase short• circuit [4]. The circuit protective mechanism should be capable of carrying for a short time the specified short-circuit current. However, the possibility of catastrophic failure exists if the short-circuit currents are not properly calculated and the protection is subjected to fault values that exceed its rating. The stator phase and
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rotor field currents at short-circuit faults take dangerous values, thermally overloading the machine. The critical value of electromagnetic torque, during short circuit transient, equally has to be known by designers to appraise the mechanical strength of the generator.
A look at the sudden short-circuit of a synchronous generator that has initially been operating under open-circuit steady state condition, the machine undergoes a transient in all the three phases and finally ends in a new steady-state condition. Immediately upon short circuit, the de off-set currents appear in all three phases; each with different magnitude since the point at which the short circuit occurs is different magnitude for each phase voltage, displaced 120 apart. The developed symmetrical short-circuit current is limited initially only by the leakage reactance of the machine, and because the air gap flux cannot change instantaneously to counter the demagnetization of the armature short circuit current, currents appear in the field winding as well as in the damper winding in a direction to help the main flux. These currents then decay in accordance with the winding time constants.
1.5 Study Limitations
A major limitation was encountered in this research thesis during the modelling of the drive system using stiffly-coupled DC motors. Because of the novelty of the approach, very little literature evidence exists on its methodology. To overcome this limitation, a number of assumptions were necessary and advanced.
In another instance, the process leading to the development of a switching system was altogether challenging. The first attempt to externally model a current controller for the field circuit using a system of Simulink blocks which comprise time delays, logic gates and signal converters did not operate accordingly as the controlled current appears not to account for the dynamics of the field circuit. A cursory attempt to consider varying the field resistance was trailed as it is not physically achievable given the enormously high occurrence of field current (up to 3 p.u.) during SSC. Similarly, developing and incorporating a MATLAB code inside the
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embedded Simulink block of the synchronous generator, using logical control commands to operate the field current at SSC, did not suffice.
Again, a different approach which required that the integrator which was used to perform the differential component of the voltage equation of the field windings be bounded at some limits during SSC, yet abortive was initiated. It was initially anticipated, and actually undertaken, that the boundary limits will be determined from simulations of the generator based on normal operating conditions. This attempt was mainly impractical because it was later observed that the Simulink integrator block is not time-bound. Eventually, a single switch block from the Simulink library was preferred and configured to realize the control of the high currents appearing in the field windings during SSC.
During the extraction of parameters from SSC current oscilloscope using the symmetrical envelope technique, the transient and subtransient time constants for constant speed and rubber damping drive configurations did not follow after the normal process of plotting I + f’ on semilog paper. This was because the current envelopes did not amend exactly to the conventional MATLAB codes developed for
this purpose. In the end, T, and T were calculated using standard synchronous
machine equations from the values obtained for x, and x, respectively.
1.6 Thesis Outline
The thesis is divided into five chapters. Chapter One has been discussed and is being summarized here. Chapter Two presents a review of previous works on parameter estimation in synchronous generator short-circuit tests which includes analysis and modelling of synchronous generators, methods of solution to short• circuit current analysis of synchronous generators, importance of balanced short circuit fault analysis in synchronous generators, objectives of simulation programs to synchronous machine modeling, identification of synchronous generator parameters and a direction to pattern the current research. Chapter Three highlights the detailed modelling of synchronous generators and DC motors as well as the formulation of a
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suitable interconnecting drive system. It stresses on system identification, model selection, formulation of voltage equation and flux-linkage equations in both ac generator and de motor, description of the drive system and the per-unitization concept. Also, provision was made much later in Chapter Three to discuss the configuration and determine the field excitation for assuming full-load operation of the synchronous generator, design an RL-load model and model a switching mechanism to control the generator field current at high values of SSC. In addition, Chapter Three deals on the computer software implementation of the models earlier developed. Chapter Four begins with a description of the methodology involved in undertaking the simulation tests and also presents the simulation results. Chapter Five is used to conclude and give relevant recommendations.
This material content is developed to serve as a GUIDE for students to conduct academic research
MODELING AND PARAMETER EXTRACTION OFA 214 MVA ‘TURBO-GENERATOR DRIVEN BY DC MOTORS DURING OFF-LINE SHORT-CIRCUIT
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