Design and implementation of phase lock loop algorithm in a static compensator

Dissertation for Master's Degree in Power Electricity

Power Systems Trend

Abstract:

Reactive power compensation has always been one of the important research topics in most universities and research centers. Meanwhile, the new generation of compensators (FACTS) in order to control reactive power, stability and increase the capacity of lines has been the focus of the power industry and research centers. In this thesis, the control structure of STATCOM (Static Reactive Power Compensator) is investigated. In this structure, a phase-locked loop (PLL) is used to estimate the frequency and voltage angle of the power grid. The purpose of using this controller is to stabilize and synchronize the reactive power compensator with the consumer network. Then, in order to evaluate the compensator, two average and phasor models have been used. The simulation results show that the controller designed with PLL has been able to control the reactive power in the power grid well.

Keyword: reactive power static compensator, phase lock loop, frequency estimation, angle

Chapter One

Introduction

1-1 Preface

Operating voltage phase angle is one of the important information for better performance. devices. Including: controlled dc to ac converters, VAR static compensators, cycloconverters, active harmonic filters and other systems

energy storage coupled with the electrical system. This information may be used to coordinate power devices on and off, calculate and control active and reactive power distribution, or to convert feedback variables into a reference format suitable for control purposes. Angular information is usually obtained using some kind of phase-locked loop (PLL[1]). In addition to operating applications, PLL methods are also used in motor control to estimate the electric angular velocity of the rotor. In the above applications, the locking quality directly affects the performance of the control loops. Line cut, voltage imbalance, phase drop and frequency changes are common conditions faced by operating system equipment. Any PLL used in such a situation should not only be fast in locking the system voltages and remain locked, but also should have a low distortion output. Our intention here is to use the PLL system only to solve control problems.

The rapid development of power electronics technology provides opportunities to develop new power devices to improve the modeling of real power systems. During the last decade, a number of control devices known as flexible AC transmission systems technology (FACTS[2]) have been invented and used. FACTS devices can be used to control load spreading, voltage regulation, increase transient stability and damp power fluctuations. FACTS devices can be used as series, parallel or a combination of these two types of controllers.  

Prior to 1970, high-power line thyristors were used with capacitors and inductors in various circuit forms to produce variable output such as parallel static VAR compensation (SVC[3]) and controlled series capacitor with connected series thyristors (TCSC[4]), as well as commercially available switched thyristor capacitors. TSC [5]) and Thyristor Controlled Inductors (TSR [6]) which have been widely used to regulate AC voltage in power systems by reactive power injection control. With the advent of high-voltage GTOs [7] and other power semiconductors with internal turn-off capability such as IGBT [8], a new generation of power electronic equipment has been used in switching circuits of converters. Voltage source converters are used to generate and absorb reactive power without the use of AC capacitors or solar banks.. The new generation and the most important need of converters in FACTS controllers are static synchronous compensation (STATCOM [9]), static synchronous series compensation (SSSC [10]) and a combination of two unique power distribution controllers, all of which work based on voltage source converters.  

The lack of reactive power in power networks causes voltage drop and voltage fluctuations. Considering the expansion and complexity of power networks, reactive power compensators are of particular importance in order to prevent voltage collapse. Among the reactive power compensators, shunt capacitor, SVC, and STATCOM have attracted the attention of researchers due to their flexibility and controllability. But the results show that if SVC and STATCOM are installed on site, they increase voltage stability and increase power transmission ability. One of the types of equipment of the second generation of FACTS devices used for dynamic parallel compensation in transmission lines is the synchronous static compensator or STATCOM. which nowadays is considered and used due to its many advantages compared to its first generation type, SVC. Unlike SVC, in which the reactive power exchanged with the grid is supplied by inductors and capacitors, in STATCOM, this reactive power is supplied without using AC capacitors or inductors. 

1-2 History of PLL and STATCOM

Early research into what we know as a phase-locked loop dates back to 1932, when British researchers developed another method, the homodyne, for Howard Armstrong's superheterodyne receiver in The homodyne or synchrodyne system is a local oscillator set to the desired frequency of the input power and multiplied by the input signal. A phase-locked loop (PLL) is a circuit that allows the frequency and phase of its loop oscillator to be controlled by an external reference signal. The frequency of the loop oscillator can be equal to the reference frequency or a multiple thereof. If the reference signal is taken from an oscillator, other frequencies can be obtained with the frequency stability of the oscillator. This is the basis of frequency synthesizers. If the frequency of the reference signal is variable, the frequency of the loop oscillator follows the input frequency. But it was the late 1960s when PLL was perfected and it was widely used in the power electronics industry. STATCOM is the first parallel controller based on the power converter. The original idea of ??STATCOM was presented by Mr. Gayogi in 1976. Instead of extracting reactive power directly from the energy storage components, the STATCOM basically circulates the power with the connected grid. Therefore, the reactive components used in STATCOM can be much smaller than SVC components. In 1995, the first STATCOM with power has been installed in the "Sullivan" substation belonging to the "Tennessee Valley Authority" (TVA[11]) in the northeastern state of Tennessee, USA. The project was co-sponsored by the Electric Industry Research Institute and TVA, and was designed and built by Westinghouse Electric Company. TVA is a federally owned utility with an installed generating capacity of more than 30,000 megawatts that supplies electric power to other utilities and industrial customers in seven states through 16,000 miles (25,806 km) of transmission lines.

1-3 parts of STATCOM system

1-3-1 power converter

The converters used in active filters and FACTS controllers include a DC part as well as an AC output. The AC output contains a component whose amplitude and frequency can be adjusted. Depending on the type of converter (voltage source or current source), AC output and DC part are controlled and equipped. In a voltage source converter (VSI[12]), the DC part is capacitive and in a current source converter (CSI[13]), the DC part is self-contained. In order to achieve the desired range and phase of the AC output, it is necessary to switch the switches of the converter at appropriate times. Depending on whether the desired output is current or voltage, different modulation methods can be used for switching the converter. For example, if the goal is to create a reference current at the output of the converter, the hysteresis method is suitable, and if the goal is to create a reference voltage at the output of the converter, pulse width modulation methods can be used.