Dynamic modeling of six-pulse thyristor rectifier in a synchronous generator static excitation system

Abstract

The main task of the synchronous generator excitation system is to supply the necessary voltage for the generator excitation winding. The excitation voltage is a DC voltage and it is used to create a flux in the excitation winding of the generator and consequently to produce a magnetic field to produce electrical energy by a synchronous generator. In the excitation system, the rectifier output dc voltage is provided as the reference input of the excitation system with the output voltage signal of the synchronous generator that passes through a voltage converter and enters a comparator. After passing through a PI controller and a saturation block, the obtained error signal enters the generator excitation system and provides its winding excitation voltage. It should be noted that this is also used as a synchronous generator reactive power controller. In this simulation, two ohmic-self-capacitive loads are considered for the system, one of the loads is directly connected to the three-phase output of the generator and the other one is of the previous type, but is connected to the output of the synchronous generator by a three-phase breaker.

Key words: excitation system, rectifier, controller, thyristor,

Chapter One

1-1 - Synchronous machines

History and structure

The synchronous machine has always been one of the most important elements of the power grid and has played a key role in the production of electrical energy and other special applications. The synchronous generator has a history of more than a hundred years. The first developments of the synchronous generator occurred in the 1880s. In early prototypes such as direct current machines, there were one or two pairs of coils on the rotating armature, the ends of which were connected to the slip rings, and the fixed poles on the stator provided the excitation field. This design was called the so-called foreign pole. In the following years, another example in which the location of the field and armature was moved was noticed. This example, which was the initial form of synchronous generator, was known as internal pole generator and found a suitable place in the electricity industry. Various forms of magnetic poles and field windings were used on the rotor, while the stator winding was single-phase or three-phase. The researchers soon realized that the optimal mode is obtained from the combination of three alternating currents with a phase difference relative to each other. The stator consisted of three pairs of coils that were connected to the star connection point on one side and to the transmission line on the other side. Hasselwander built the first three-phase synchronous generator in 1887, which produced a power of about 2.8 kW at a speed of 960 rpm (frequency of 32 Hz). This machine had a fixed three-phase armature and a four-pole wound rotor that provided the required excitation field. This generator was used to supply local loads. In 1891, for the first time, the combination of generator and long transmission line was successfully tested in order to supply distant loads. The electrical energy produced by this generator was transferred by a three-phase transmission line from Lafen to the Frankfurt International Exhibition at a distance of 175 km. The phase-to-phase voltage was 95 volts, the phase current was 1400 amps, and the nominal frequency was 40 Hz. The rotor of this generator, which was designed for a speed of 150 rpm, had 32 poles. Its diameter was 1752 mm and its effective length was 380 mm. The excitation current was supplied by a direct current machine. Its stator had 96 grooves in which a copper rod with a diameter of 29 mm was placed in each groove. Since the skin effect was not known until then, the stator winding consisted of one pole per pole/phase. The efficiency of this generator was 96.5%, which was excellent compared to the technology of that time. This generator was designed and built by Charles Brown. In the beginning, most synchronous generators were designed to be connected to water turbines, but after the development of powerful steam turbines, the need for compatible high-speed turbogenerators was felt. In response to this need, the first turbine in one of the important fields in the discussion of synchronous generators is the insulation system. The primary insulating materials used were natural materials such as fibers, cellulose, silk, linen, wool and other natural fibers. Also, natural resins obtained from plants and crude oil compounds were used to make insulating materials. In 1908, research on artificial insulation was started by Dr. Bykland.. During the First World War, asphaltic resins, called bitumen, were used for the first time along with mica pieces to insulate the grooves in the stator windings of turbogenerators. These pieces were surrounded by high quality cellulose paper on both sides. In this method, the stator windings were first covered with cellulose strips and then with two layers of linen strips. The coils were heated in a chamber and then placed under vacuum. After a few hours, dry and porous insulation was obtained. Then, under vacuum, a large amount of hot bitumen was poured on the coils. Next, the chamber was filled with dry nitrogen gas at a pressure of 550 kPa, and after a few hours, the nitrogen gas was discharged and the windings were cooled and hardened at ambient temperature. This process was called VPI. In the late 1940s, General Electric Company chose epoxy compounds to improve the stator winding insulation system. As a result of this research, a so-called resin-rich system was introduced, in which resin was used in strips or varnish between layers. In the 1940s to 1960s, along with the increase in the capacity of generators and as a result of the increase in thermal stress, the number of insulation faults increased dramatically. After the investigation, it was found that the cause of most of these errors is the phenomenon of tape separation or cracking. This phenomenon was caused by the uncoordinated expansion and contraction of the copper conductor and the iron core. To solve this problem after World War II, Westinghouse company researchers started laboratory work on new polyesters and released a system with the brand name Thermoelastic. The next generation of insulators, which were used in the first half of the 1950s, were fiberglass papers. Further, in 1955, a type of insulation resistant to partial discharge was obtained from the combination of 50% fiberglass strands and 50% PET strands, which was covered on the conductor, and then by heating in special furnaces, PET melted and covered the fiberglass. This insulation was used in one or more layers depending on the need. The said insulation entered the market under the general name of Polyglass and the brand name of Douglas. The most important stresses on insulation are thermal stresses. Therefore, insulation systems have always been closely related to cooling systems. Cooling in early generators was done by air. The best result obtained with this cooling method was a 200 MVA generator with a speed of 1800 rpm, which was installed in 1932 in the Brooklyn area of ??New York. But with the increase in the capacity of the generators, the need for a more effective cooling system was felt. The idea of ??hydrogen cooling was first proposed in 1915 by Max Schuler. His efforts to build such a system started in 1928 and in 1936 he made the first prototype with a speed of 3600 rpm. In 1937, General Electric launched the first commercial hydrogen-cooled turbogenerator. This technology became popular in Europe after 1945. In the 1950s and 1960s, various direct cooling methods such as cooling the stator windings with gas, oil, and water emerged, until by the mid-1960s, most large generators were water-cooled. The advent of direct cooling technology increased the capacity of generators by 1500 MVA.

One ??of the outstanding developments that took place in the 1960s was the production of the first commercial superconducting material, niobium-titanium, which received much attention in the following decades. Developments of the 1970s In this decade, an important development occurred in the generator insulation process. Before 1975, most insulators were impregnated with volatile organic compound soluble resins. In this process, the said compounds were evaporated and released into the atmosphere. Due to the establishment of environmental laws and the beginning of the green movement in the early 1970s, severe restrictions were imposed on the release of these substances, which resulted in their removal from this process. As a result, the use of environmentally friendly materials in the production and repair of electric machines was considered. The use of water-based resins was one of the first suggestions, but a more comprehensive solution that is still common today was the use of solid adhesives. In the same direction, the production of solvent-free rich resin mica strips was also developed. Another important development of this decade was the emergence of superconducting generators. A superconducting machine generally consists of a superconducting field coil and a copper armature coil. The rotor core is generally not iron, because the iron is saturated due to the high intensity of the field produced by the field winding.