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For long time, the main concern of consumers in power system was the reliability of supply which means that the continuity of electricity. However, it is not only the reliability that consumers want these days, quality of electricity supply is also very important for consumers. The term, electric power quality, broadly refers to maintaining a nearly sinusoidal bus voltage at specified magnitude and frequency in an uninterrupted manner from the reliability point of view. For a well-designed generating plant, which generates voltages almost perfectly sinusoidal at rated magnitude and frequency, power quality problems start with transmission system and stay applicable until end users in distribution system. The power quality in power system are categorized as temporary phenomena and steady state phenomena The power qualities are characterized in the power system by different terms suggested by Padiyar, K.R.,(2007) under these two categorizes are summarized as follows:
1.2. TEMPORARY PHENOMENA
Transients: Transients are short-duration, high-amplitude pulses superimposed on a normal voltage waveform. They can vary widely from twice the normal voltage to several thousand volts and last from less than a microsecond up to a few hundredths of a second. Transients can be classified as impulsive transients and oscillatory transients. Impulse transients are mainly caused by the impact of lightning strikes to the power system. The typical causes of oscillatory transients are capacitor or transformer energization and converter switching. While impulsive transient is a sudden and has non-power frequency change in voltage and current with a fast rise and decaying time, oscillatory transient has one or more sinusoidal components with frequencies in the range from power frequency (50Hz) to 500 kHz and decays in time.
Short Duration Voltage Variations: Short Duration Voltage Variations are defined as the variations in the supply voltage for durations not exceeding one minute and caused by faults, energization of large loads that having large inrush currents or rapidly varying large reactive power demands of the loads. These are further classified as voltage sags, voltage swells and interruption.
Long Duration Voltage Variations: Long Duration Voltage Variations are defined as the rms variations in the supply voltage at fundamental frequency for exceeding one minute, such as overvoltage, under voltage and sustained interruption. The causes of overvoltage (or under voltage) may be the switching off (or on) of a large load having poor power factor, or the energization of a large capacitor bank or reactors.
Voltage Unbalance: Voltage Unbalance is the condition in which three phase voltages of the supply are not equal in magnitude and may not be equally displaced in time. The primary causes are the single phase loads, open circuit in any one phase of a balanced 3phase loads and unequal loads connected in each phase of a poly phase systems.
Waveform Distortion: Waveform Distortion is defined as steady-state deviation in the voltage or current waveform from an ideal sine wave. These distortions are classified as dc-offset, harmonics and notching. The causes of dc offsets in power systems are geomagnetic disturbances, especially at higher altitudes and half-wave rectifications. These may increase the peak value of the flux in the transformer, pushing it into saturation and resulting in heating in the transformer. Power electronics equipments like UPS, adjustable speed drives injects harmonics in the power systems. Notching is a periodic voltage distortion due to the operation of power converters when current commutates from one phase to another.
Voltage Fluctuations: Voltage Fluctuations are defined as the rapid, systematic and random variations in the supply voltage. This is also called as “Voltage Flicker” and is caused by rapid and large variations in current magnitude of loads having poor power factor such as arc furnaces. These large variations in load current causes severe dip in the supply voltage unless the supply bus is very stiff.
Power Frequency Variations: Power Frequency Variations are the variations that are caused by rapid changes in the load connected to the system, such as the operation of draglines connected to a comparatively low inertia system. Since the frequency is directly related to rotational speeds of the generators, large variations in power frequency may reduce the life span of turbine blades on the shaft connected to the generator.
Although these above terms are not new, customer awareness on power quality has increased. In recent times, power quality issues and custom solutions have generated tremendous amount of interest among power system authorities and engineers. International Electro technical Commission (IEC) and Institute of Electrical and Electronics Engineers (IEEE) have proposed various standards on power quality. This led to more stringent regulations and limits imposed by electricity authorities although they differ from one country to another in a limited extend. Although terms of power quality are valid for transmission and distribution systems, their approach to power quality has different concerns. An engineer of transmission system deals with the control of active and reactive power flow in order to maximize both the loading capability and stability limits of the transmission system. On the other hand, an engineer of distribution system deals with load compensation either by means of individual or group compensation in order to maintain power quality for each load in the distribution system (Sankaran.C, 2002, John J.Paserba et al, 2000). The utilization of power electronic based power conditioning devices brought the solution for these power quality issues in distribution system.
1.2 FACTS CONTROLLERS
In recent years, many multinational software companies and automobile industries established their units in India. In turn, it initiates many other small industries to supply their needs. The growth of these industries is found to be very fast and it pollutes the power system by injecting harmonics into it. These industries need electrical power for its operation. Establishing new power generation unit is not so easy in India due to the initial cost. In addition it has many constraints like fuel constraints, political constraints, economical constraints and technological constraints. This makes to think an alternate solution for the scarcity of power by improving the quality of existing power. Reducing the wastages and improving the quality of available power is equivalent to generation of power. To improve the reliability and deliver energy at the lowest possible cost with improved power quality, power supply industries require increased flexibility in the transmission and in the distribution systems. The power industries are handling these challenges with the power electronics based technology of Flexible AC Transmission systems (FACTS). This term covers the whole family of power electronic controllers, some of which may have achieved maturity within the industries, while some others are yet in the design stage. As Higorani et al (1999) described the various VSC based FACTS controllers are available for power quality improvement.
FACTs has been defined by the IEEE as follows.
“Power electronics based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability”.
In general, FACTs controllers can be classified as follows
Combined series and shunt Controllers
Combined shunt and series Controllers
Based on the power electronic devices used in the controller, the FACTS controllers can be classified as:
(A) Variable impedance type FACTS Controller
(B) Voltage Source Converter (VSC) based FACTS Controller
The variable impedance type controllers include:
(i) Shunt connected- Static Var Compensator (SVC)
(ii) Series Connected-Thyristors Controlled Series Capacitor or Compensator (TCSC)
(iii) Combined shunt and series connected – Thyristors Controlled Phase Shifting Transformer (TCPST) of Static PST
The VSC based FACTS controllers are:
(i) Static synchronous Compensator (STATCOM) (shunt connected)
(ii) Static Synchronous Series Compensator (SSSC) (series connected)
(iii) Interline Power Flow Controller (IPFC) (combined series-series)
(iv) Unified Power Flow Controller (UPFC) (combined shunt-series)
The VSC based FACTS controllers have several advantages over the variable impedance type. VSC based STATCOM response is much faster than a variable impedance type SVC. STATCOM requires less space than SVC for same rating. It can supply required reactive power even at low values of the bus voltage. In addition, a STATCOM can supply active power if it has an energy source or large energy storage at its DC terminals. It can also be designed to have in built, short-term overload capability. The only drawback with VSC based controllers is that it requires use of self-commutating power semiconductor switches such as Gate Turn-off (GTO) thyristors, Insulated Gate Bipolar Transistors (IGBT), Integrated Gate Commutated Thyristors (IGCT). However, the VSC based controllers build with emerging power semiconductor devices using silicon carbide technology will lead to the wide spread use of VSC based controllers in future.
Among FACTs controllers, the shunt controllers have shown feasibility in terms of cost effectiveness in a wide range of problem solving from transmission to distribution levels. For more than a decade, it has been recognized that the transmittable power through transmission lines could be increased and the voltage profile along the transmission line could be controlled by an appropriate amount of compensated reactive power. Moreover, the shunt controller can improve transient stability and can damp power oscillation during a post-fault event. Using a high speed power converter, the shunt controller can further alleviate the flicker problem caused by electrical arc furnaces.
.1 SERIES CONTROLLERS
Static synchronous series compensator (SSSC) is series reactive power compensation devices used in transmission level. The series compensation is obtained by controlling the equivalent impedance of a transmission line, to regulate the power flow through the line. The SSSC can be considered as a static synchronous generator that acts as a series compensator whose output voltage is fully controllable, independent of line current and kept in quadrature with it, with the aim of increasing or decreasing the voltage drop across the line, thus controlling the power flow. The basic structure of a SSSC connected with the network is shown in Figure 1.1.
Figure.1.1 Series Connected SSSC
The SSSC injects a voltage Vq in quadrature with line current. It can provide either capacitive compensation if Vq leads the line current by Ï€/2 rad or inductive compensation if Vq lags line current by Ï€/2 rad. A relatively small active power exchange is required to compensate for coupling transformer and switching losses, and maintain the required DC voltage.
1.2.2 SHUNT CONTROLLERS – STATCOM
The schematic diagram of a STATCOM is shown in Figure.1.2. In principle, all shunt type controllers inject additional current into the system at the point of common coupling (PCC). VSC that uses charged capacitors as the input dc source and produces a 3Ï• ac voltage output in synchronism and in phase with the ac systems. The converter is connected in shunt to a bus by means of the impedance of a coupling transformer. A control on the output voltage of this converter is either lower or higher than the connecting bus voltage, controls the reactive power drawn from or supplied to the connected bus. The impedance of the shunt controller, which is connected to the line causes a variable current to flow and hence represents an injection of current into the line. As long as the injected current is in phase quadrature with the line voltage, the shunt controller can either supply or consume variable reactive power.
Figure.1.2 Shunt Connected STATCOM
A six pulse Voltage Source Converter (VSC) with suitable controller, the phase angle and the magnitude of the AC voltage injected by the VSC can be controlled. The Phase Lock Loop (PLL) ensures that the sinusoidal component of the injected voltage is synchronized (matching in frequency and required phase angle) with the AC bus voltage to which VSC is connected through a coupling inductor. Often, the leakage impedance of the interconnecting transformer serves as the coupling inductor. It also serves as harmonic filter for the voltage injected by the VSC. The injection of harmonic voltages can also be minimized by multi-pulse (12, 24 or 48), and/or multilevel convertors. At low power levels, the pulse width modulation (PWM) technique is sufficient to control the magnitude of the fundamental component of the injected voltage. The high voltage IGBT devices can be switched at high frequency (2 kHz and above) of sinusoidal modulation enables the use of simple LC-low pass filters to reduce harmonic components.
1.2.3 COMBINED SHUNT AND SERIES CONTROLLERS
(a). Unified Power Flow Controller (UPFC):
The Unified Power Flow Controller (UPFC) is the most versatile FACTS controller for the regulation of voltage and power flow in a transmission line. It consists of two-voltage source converters (VSC) in which one connected in shunt and the other one connected in series. The DC capacitors of the two converters are connected as shown in Figure.1.3. the shunt connected converters work as STATCOM and controls the reactive current injected in to the line. Series connected converter work as SSSC and control reactive voltage injected series with the line. The combination of these two converters enables to exchange active power flow between the two converters. The series connected converter can supply or absorb the active power.
Figure 1.3 Schematic of UPFC
The controllable power source on the DC side of the series connected converter, results in the control of both real and reactive power flow in the line at the receiving end of the line. The shunt-connected converter provides the required reactive power and injects the reactive current at the converter bus. Thus, a UPFC has 3 degrees of freedom whereas other FACTS controllers have only one degree of freedom or control variable. The concept of combining two or more converters can be extended to provide flexibility and additional degrees of freedom. A Generalized UPFC refers to the use of three or more converters out of which one shunt connected while the remaining converters are series connected
(b). Interline Power Flow Controller (IPFC):
An Interline Power Flow Controller (IPFC) refers to the configuration of two or more series connected voltage source converters sharing a common DC bus as shown in Figure 1.4. The Interline Power Flow Controller (IPFC) is used reactive (series) compensation of each individual line. In addition to this, the IPFC is capable of exchanging real power between the two or more compensated lines. To achieve this AC side of the series connected VSCs are connected in different lines and on the DC side, all the DC capacitors of individual converters are connected in parallel. This is possible because all the series converters are located inside the substation in close proximity.
Figure 1.4 Schematic of IPFC for two transmission line using two VSC
An IPFC is similar to a UPFC in that the magnitude and phase angle of the injected voltage in the line (main system) can be controlled by exchanging real power with the second line (support system) in which a series converter is connected. The basic difference with a UPFC is that the support system in the UPFC is the shunt converter instead of a series converter. The series converter associated with the main system of one IPFC is termed as the master converter while the series converter associated with the support system is termed as the slave converter. The master converter controls both active and reactive voltage within limits while the slave converter controls the DC voltage across the capacitor and the reactive voltage magnitude.
1.3 APPLICATION FACTS CONTROLLERS IN DISTRIBUTION SYSTEMS
Although the concept of FACTS was developed originally for transmission network, later on this has been extended since last decade for improvement of Power Quality (PQ) in distribution systems operating at low or medium voltages. In the early days, the power quality referred primarily to the uninterrupted power supply at acceptable voltage and frequency. In the modern context, power quality problem is defined as any problem manifested in voltage, current or frequency deviations that result in failure or malfunctioning of customer equipment. However, the increase in the use of computers, microprocessors and power electronic systems has resulted in power quality issues involving transient disturbances in voltage magnitude, waveform and frequency. The nonlinear loads not only cause power quality (PQ) problems but also very sensitive to the voltage deviations. The unbalanced load in the distribution system like single-phase railway loading creates power quality problem at the distribution level. The highly inductive load like arc furnace is a major source of creating power quality problems in distribution network.
Hingorani et al (1999), was the first to propose FACTS controllers for improving power quality in distribution systems. They have called it as Custom Power Devices. These are based on VSC with appropriate controller. Based on the types of connection with the distribution network the custom power devices classifications are given below;
1. Series connected Dynamic Voltage Restorer (DVR)
2. Shunt connected Distribution STATCOM (DSTATCOM)
3. Combined shunt and series connected Unified Power Quality Conditioner (UPQC).
The Dynamic Voltage Restorer (DVR) is a series connected custom power device in the distribution systems. The DVR is analogous to a SSSC in the transmission system. The main function of a DVR is to reduce voltage sags seen by sensitive loads such as semiconductor manufacturing plant or a paper mill. They have been designed to compensate three phase voltage sags up to 35% for duration of time less than half a second (depending on the requirement). If the voltage sag occurs only in one phase as in the case of Single Line to Ground (SLG) faults then the DVR may be designed to provide compensation for sags exceeding 50%. The capacitor is designed to store energy in the range of 0.2 to 0.4 MJ per MW of load served. A DVR is connected in series with the distribution feeder through a transformer. The low voltage winding of the transformer is connected to the converter. If a DVR is used mainly to regulate the voltage at the load bus, it injects a series voltage of the required magnitude if it detects a voltage sag else remains in stand-by mode during which the converter is bypassed or it is not injecting voltage. It is necessary to protect the DVR against the fault currents as in the case of a SSSC. A DVR with IGBT/IGCT devices can be controlled to act as a series active filter to reduce the voltage harmonics on the source side. It is also possible to balance the voltage on the load side by injecting negative and/or zero sequence voltages in addition to harmonic voltages.
The distribution STATCOM (DSTATCOM) is similar to a STATCOM in transmission system that it uses a VSC of the required rating. However, the VSC used in a DSTATCOM is a 6-pulse converter with SPWM or Space Vector Modulated PWM (SVPWM) control over the magnitude of the injected AC voltage while maintaining a constant DC voltage across the capacitor. In DSTATCOM, faster power semiconductor devices such as IGBT or IGCT are used instead of GTO as in STATCOM. The rapid switching capability provided by IGBT (or IGCT) switches enables the use of DSTATCOM for balancing, active filtering and flicker mitigation. The unbalanced system is balanced by injecting negative sequence current to the system. The active filtering is done by injecting harmonic currents in the system. A DSTATCOM can be viewed as a controlled variable current source. If more power that is reactive is required for compensation in distribution system, dynamic capacitor rating is increased. To increase the dynamic rating in the capacitive range, a fixed capacitor can be connected in parallel with DSTATCOM. By connecting energy storage device such as a Superconducting Magnetic Energy Storage (SMES) or a battery charged by a separate charging system on the DC side, it is possible to exchange real power with the network for momentary interruptions or large voltage sags for a limited time.
The combination of shunt and series active filters which are connected on the common DC side as shown in Figure.1.5 used as Unified Power Quality Conditioner. This configuration is inspired by the UPFC in the transmission system. Akagi.H (1996), suggest the possibility of a centralized UPQC at the distribution substation that will provide harmonic isolation between the sub-transmission system and distribution system. The series branch of UPQC provides this harmonic isolation in addition to voltage regulation and imbalance compensation. The shunt branch provides for harmonic and negative sequence current compensation in addition to DC link voltage regulation. A UPQC can be considered as the combination of DSTATCOM and DVR. A DSTATCOM is utilized to eliminate the harmonics from the source currents and balance them in addition to providing reactive power compensation to improve power factor or regulate the load bus voltage (Padiyar.K.R. 2007).
Figure 1.5 Schematic of a Unified Power Quality Controller (UPQC)
The terminology is yet to be standardized. The term `active filters’ or `power conditioners’ is also employed to describe the custom power devices. Irrespective of the name, the trend is to increasingly apply VSC based compensators for power quality improvement.
Development of gate turn off capability of semiconductor switches opened a way to second-generation FACTs controller using voltage source converter (VSC). This VSC can be operated at high switching frequency to provide a faster response. The STATCOM is a shunt connected power converter based compensating device. Van Zyl. A, et.al proposed an idea for Converter based solution to power quality problems on radial distribution lines (1996). This is a first power converter based shunt compensator. The concept of STATCOM was disclosed by Gyuayi,.L (1988). The concept gives the characteristics of VSC that are suitable for grid connected FACTS controller application. In the older version of reactive power compensation device, the reactive power is drawn from energy storage devices such as capacitor in the case of Static Var Compensator (SVC), but in STATCOM power is circulated within the connected network. The energy storage components used in the STATCOM is much smaller in capacity than those used in the SVC.
In 1995, the first +100MVA STATCOM was installed at the Sullivan substation of Tennessee Valley Authority (TVA) in northeastern Tennessee. This device is mainly used to regulate 161kV bus during the daily load variation to reduce the operation of the tap changer of a 1.2GVA – 161kV/500kV transformer. The VSC used in this STATCOM is made up of eight two level VSC resulting a 48 pulse VSC. The output of each VSC is integrated by a complex interface zigzag connected interfacing transformers, because this is a two-level VSC, a series connection of five of gate-turn-off (GTO) thyristor is used as a main switch. The staircase type switching scheme at fundamental frequency (60Hz) was used as a control scheme for this STATCOM. Due to slow switching speed of the GTOs; the firing angles of the output wave form are fixed. Therefore, the amplitude of each output waveform is controlled by exchanging real power of the DC-link capacitor with the power grid. The power quality problem at distribution level like voltage regulation, harmonics reduction, power factor correction, reactive power compensation and unbalance compensations need to be carried out at distribution level.
The DSTATCOM, connected to the grid through the coupling inductor at the point of common coupling (PCC) is controlled in such a way that it exchanges only reactive power with the grid. This is achieved by injecting current in quadrature with the grid voltage. The DSTATCOM is developed from the STATCOM used in transmission system for voltage regulation. Hingorani, N.G,. et al (1999) explored the concept and technology of Flexible AC Transmission Systems. The detailed modeling and average modeling of DSTATCOM and its performance for voltage regulation application is studied by Pierre Giroux et al (2000). This gives the concept of PWM controlled DSTATCOM in dq coordinate system. Sen Sarma. P.S., et al (2001) Analyzed and evaluated the performance of a distribution STATCOM for compensating voltage Fluctuations. Sao, C.K et al (2002) proposed the application of DSTATCOM from voltage regulation to reactive power compensation, power factor correction, mitigation of voltage sag and swell in distribution system and created a benchmark system to test all these performance. This DSTATCOM is controlled by PI controller in dq coordinate using park’s transformation matrix. This work reduced the computation time of the controller by avoiding Inverse Park’s transformation. The application of DSTATCOM is extended to compensate the reactive power for isolated induction generator by Bhim Singh et al (2003). This gave the mathematical modeling of induction generator and DSTATCOM. As the DSTATCOM is suitable for distribution system and stand alone system researcher focused to increase the performance of the controller. The concept of using DSTATCOM as a shunt active filter to reduce the current harmonics in the industrial application and gradually extended to power systems application by Georges, S. et al (2006) and Kannan, H.Y. et al (2008). The concept of Generalized Instantaneous Reactive Power Theory for Three-phase Power Systems is exploited by Akagi, P., et al (1984) and Fang Zheng Peng et al (1996). The concept of instantaneous reactive and real power is brought by them in to the design of controller for closed loop operation of VSC. A Survey of Current Control Techniques for Three-Phase Voltage-Source PWM Converters is brought by Marian P., et al (1998). These current control techniques provided a path way for direct control of VSC output current. Design and Implementation of DSTATCOM for fast load compensation of unbalanced loads was implemented by Wei-Neng Chang et al (2009). The controller for unbalanced system was built by phase sequence method and pulses are generated by current regulated PWM method. The Space Vector Modulation (SVM) PWM technique was an emerging control technique used in Voltage Source Converter (VSC) for controlling its output voltage by Atif Iqbal et al (2010). A New Vector-Based Hysteresis Current Control Scheme for Three-Phase PWM Voltage-Source Inverters was developed by Mansour Mohseni et al (2010). This thesis tries to apply Vector-Based Hysteresis Current Control Scheme for DSTATCOM for power factor improvement.
This research is focusing to use the SVM based PWM technique for DSTATCOM operation in addition to PI controlled SPWM in dq coordinate systems. This also extends the application SVM based HCC from inverter to DSTATCOM.
1.5 PROBLEM STATEMENT
The voltage at distribution systems need to be maintained at 1pu at all conditions. The reactive power control plays an important role in maintaining the bus voltage at 1pu in the distribution bus.Classical reactive power controllers like fixed capacitors, switched capacitors, TCR, SVC etc have slow response and bulky. A DSTATCOM, though a costlier device it has faster response. Hence it is preferred when faster correction of voltages is required. It is required to design specific controllers for voltage regulation, power factor correction and unbalanced system compensations. All the above problems can be solved by installing a DSTATCOM with proper controllers.
The main aim of this thesis is to design and implement the controller for DSTATCOM to improve the power quality namely voltage regulation, voltage sag or swell, reactive power compensation, power factor improvement and unbalance compensation. The controllers presented in this work will aid the design engineers to develop an integrated controller with multiple control objectives. The main objectives of this thesis include
To study the concepts of DSTATCOM and bring out the design procedure of it. To understand the controller principle for various applications and explore it for novel controller design.
To design the new control algorithm namely PI controlled Space Vector Pulse Width modulation method and Study the performance of DSTATCOM for this controller to improve the power quality issues such as voltage regulation, power factor improvement and reactive power compensation. To compare this SVPWM controller performance with the performance of existing Sine Pulse Width Modulation (SPWM) method.
To modify the basic SVPWM method so as to extend its controller to directly control the flow of current of DSTATCOM. This method of controller is called SVPWM based Hysteris Current Controller (HCC) method.
To suggest a new control techniques for unbalanced system compensations using sequence analyzing method and validate its performance for power quality improve improvement.
To explore the design of DSTATCOM components.
To identify the controller for compensating balanced and unbalanced systems.
1.7 THESIS ORGANISATION
This thesis contains seven chapters summarized as follows:
In Chapter 1 need for improving the quality of power is discussed the power quality issues and various Flexible AC Transmission System (FACTS) controllers available for the power quality improvements in the transmission systems and distribution systems. This chapter also includes the review of the literature, outlines the research objectives and the organization of the thesis.
Chapter 2 describes the general method for designing a DSTATCOM for power quality improvement. The DSTATCOM consists of a DC capacitor, a VSC, a coupling inductor and the controller. This chapter gives a method of designing the coupling inductor, the DC capacitor and selecting the power electronic switches for the VSC. It also focuses on analyzing the controllers of DSTATCOM for power quality improvements.
In Chapter 3, the mathematical modeling of a two-level VSC based DSTATCOM is described. This Chapter also presents the PI controlled Sine Pulse Width Modulation (SPWM) and Space Vector PWM (SVPWM) switching techniques for voltage regulation applications. The comparative performance of these switching techniques is carried out. The control logic is developed from the power invariant property of the Park’s transformation of
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