Using the buck converter controlled by sliding mode in order to reduce input harmonics to the static excitation system of the synchronous generator.

Master's Thesis in Electrical Engineering

Power Trend

Abstract

Sliding mode control method is one of the most important non-linear control methods, which is characterized by insensitivity to parameter changes and complete rejection of disturbance and dealing with uncertainty. This controller first brings the system from the initial state using the law of reaching the defined sliding level, which has Lyapunov asymptotic stability, and then brings it to the equilibrium state using the sliding law. Until now, in the research conducted on the method of static feeding of the excitation system, the use of DC/DC reduction converters has not been given special attention.

In this thesis, after the power transformer and rectifier bridge using a buck converter controlled by sliding mode, it is used to reduce the harmonics input to the excitation coil.

In this thesis, we will try to reduce the effects of disturbances (including input voltage change and load change) and adjust the output voltage with very fast dynamics and maximum reduction of harmonics with sliding control method. We will also show the efficiency of this system using SIMULINK/MATLAB.

1- Introduction:

Generators are always one of the most important elements of the power network and play a key role in energy production and other special applications. And it is used for synchronous generators to generate most of the electric power around the world.

In a synchronous generator, a dc current is applied to the rotor coil to produce a rotor magnetic field, then the rotor of the generator is rotated by a main driver, so that a rotating magnetic field is created in the machine. This magnetic field induces a three-phase voltage in the stator windings of the generator. A constant current must be applied to the rotor. Because the rotor rotates, it needs a special arrangement to deliver DC power to its field coils. There are 2 ways to do this:

1- From an external source to the rotor with slip rings and brushes.

2- Providing DC power from a DC power source, which is installed directly on the shaft of the synchronous generator.

A static excitation system due to its similar operation The automatic field voltage regulator behaves so that if the generator voltage decreases, it increases the field current, and on the contrary, if the generator voltage increases, it decreases the field current. In fact, the static excitation system provides the power of the main field of the generator, while the voltage regulator fulfills the power of the excitation field. There are 3 main components in the static excitation system: the control part, the rectifier bridge, and the power transformer, which together control the generator field to achieve the appropriate output voltage.

The DC current injected into the excitation coil must be of very high quality, otherwise the effects of the harmonics input to the excitation coil can also be seen in the shaft of the synchronous generator. In addition to reducing the quality of the power injected into the network, this action increases losses in the system and as a result increases the operating costs.

In this thesis, we want to use a sliding mode controlled buck converter after the power transformer and rectifier bridge to reduce the harmonics input to the excitation coil.

Sliding mode control method It is one of the most important methods of non-linear control, which is characterized by insensitivity to changing parameters and complete rejection of disturbance and dealing with uncertainty. This controller first brings the system from the initial state using the law of reaching the defined sliding level, which has Lyapunov asymptotic stability, and then brings it to the equilibrium state using the sliding law.

Studies show that the DC voltage ripple input to the excitation coil can have adverse effects on the output and shaft of the synchronous generator.On the other hand, non-linear control methods such as the sliding control method have a high ability to fix and adjust the output voltages of DC/DC converters, but despite this, the output voltage still has distortions. The hypothesis of the research is to minimize the harmonics of the output voltage of the converter by using the sliding mode in the buck converter. Finally, using the powerful MATLAB/SIMULINK software, the whole simulation and the part related to filtering will be done.

1-2- Background and records:

The main task of the excitation system is to supply the excitation current to the synchronous machine, in addition to controlling the excitation voltage, it is responsible for the control and protection of a power system.]1[ for There are different methods of stimulating synchronous machines, these methods have been invented one after the other with the advancement of technology and the passage of time, and some of these methods have disadvantages that have made us think of changing the system and improving and improving them [2] The excitation system by changing the dc current of the excitation coil located on the rotor controls the generated driving force of the generator. The current amplitude is also controlled. Reference [3] talks about various stimulation methods, including the static stimulation method, which will be examined in this article. In the design carried out in this thesis, we want to use a DC/DC buck converter to feed the excitation coil. The DC/D converter is a converter that converts the DC hc current of a source to another voltage level, and the output voltage can be higher or lower than the input voltage. Buck converter is a step-down DC-DC converter. The control method used in this thesis for the buck converter is the sliding control method. In reference [4], the PID control method is used, the system in this case has a high response speed and can reduce overshoot. But after the introduction of sliding control in 1977 by Yutkin, the use of this type of controller increased day by day due to the many characteristics that it has in power electronics, so that in 1991, this type of controller was used for the first time in buck converters by Slotin. ]5[

But one of the important things we want to do in this thesis is to use harmonic removal filters in the buck converter controlled by sliding mode so that we can have the maximum harmonic reduction. In reference [6], an RL filter without a DC/DC converter is used to reduce input harmonics to the excitation coil.

In references [7] and [8], a simple design for the use of power electronics for excitation systems was investigated, which does not seem logical due to the harmonics of the input voltage to the excitation coil. In reference [9], a PI controller is used for the converter, but there are still harmonics in the system. In references [10] and [11], Buck-Boost converters are used in the excitation system, all of these designs are without the use of filters and naturally have a high THD output voltage. In this thesis, in addition to using a buck converter controlled by sliding mode, we inject a voltage with very low harmonics into the excitation coil using harmonic elimination filters. There are various methods, these methods have been invented one after the other with the advancement of technology and the passage of time. Some of these methods have disadvantages that made us think of changing and modifying them [2] so that we can increase the efficiency and reduce the harmonics and reduce the THD of the output voltage. Excitation:]3[

In this system, automatic control is used in the static excitation of the synchronous generator in such a way that the terminal voltage is within the operating range of the synchronous machine control without operator excitation.

Voltage regulation under automatic control may be modified by stimulating the active or reactive load current or by different control elements or may exist with different excitation limits in the excitation system.