CONTENTS
TOPIC NAME PAGE NUMBER
List of Figures i
List of Tables ii
Abstract 9
1. Introduction
1.1.Introduction 11
1.2.
Objective of the project 12
1.3.
Overview of the thesis 12
1.4.
Literature Survey 12
2.
Power quality
2.1.Introduction 15
2.2.
Need For PQ Improvement 17
2.3.
Power Quality Standards 18
2.4.
Power Quality Terminology 19
2.5.
About PQ disturbances 21
2.6.
Improving techniques 22
3. Active Power Filters
3.1.Introduction 31
3.3.
Three phase Active Filters 33
4. DSTATCOM
4.1.Introduction 43
4.2.
Principle and operation 44
4.3.
Control Scheme for the DSTATCOM 45
4.4.
Phasor diagram 48
4.5
Mathematical Modeling 49
5. MATLAB Simulation
5.1.Introduction 52
5.2.
Proposed circuit-Single Line Diagram 52
5.3.
Circuit description 54
6. Results and Future Scope
6.1.Case Studies 66
6.2.
Conclusion 77
6.3.
Future Scope 78
References
LIST OF FIGURES
2.1. Improving power quality by distortion elimination 23
2.2. Principle of input converter to eliminate distortion loads 23
on the power network. 3.1. Current fed type AF 33
3.2 .Voltage fed type AF 33
3.3. Shunt-type AF 34
3.4 .Series-types AF 34
3.5 Hybrid filter 35
3.6 Unified Power Quality Conditioner 35
3.7 Configuration of the three phase, three wire Active filtering system 39
3.8 Control block of Sample and Hold circuit's harmonic reference template 40
3.9 Method used to capture IP. 41
4.1 Schematic Diagram of a DSTATCOM 44
4.2 Indirect PI controller 46
4.3 Phase-Modulation of the control signal 46
4.4 Phasor diagram for shunt voltage controller 48
5.1 single line diagram of the test system for DSTATCOM 53
5.2 Simulink model of D-STATCOM test system 54
5.2.1 3-phase Functional Block 55
5.2.2 Simulink model of D-STATCOM 56
5.2.4 TRIGGER BLOCK Simulation Model 57
5.2.5 Functional Block of Discrete PI Controller 58
5.2.6 Functional Block of 3-ph Breaker 59
5.2.7 Functional Block of DC Voltage Source 59
5.2.8 Functional Block of #-ph Sequence Analyser 60
5.2.9 Functional Block of Unit Delay 60
5.2.10 Functional Block of 3-ph Transformer 61
5.2.11 Functional Block of Breaker 62
5.2.12 Functional Block of Discrete PWM Generator 63
5.2.13 Functional Block of Universal Bridge 64
6.1 Simulation model for SAG Formation 66
6.2 Voltage Vrms at the load point without DSTATCOM 67
6.3 Simulation Model for SAG Mitigation 68
6.4 Voltage Vrms at the load point with DSTATCOM 69
6.5 Simulation model for SWELL Formation 70
6.6 Voltage Vrms at the Load point without DSTATCOM 71
6.7 Simulation model for SWELL mitigation 72
6.8 voltage Vrms at the load point with DSTATCOM 73
6.9 Simulation model for voltage interruption formation 74
6.10 Voltage Vrms at the Load point without DSTATCOM 75
6.11 Simulation model for Voltage interruption mitigation 76
6.12 Voltage Vrms at the load point with D-STATCOM energy storage of 40.7KV 77
LIST OF TABLES
2.1 Voltage Characteristics as Published by Goteborg Energi 18 3.1. IEEE 519 Voltage Limits 31
ABSTRACT
A Power quality problem is an occurrence manifested as a nonstandard voltage, current or frequency that results in a failure or a mis-operation of end user equipments. Utility distribution networks, sensitive industrial loads and critical commercial operations suffer from various types of outages and service interruptions which can cost significant financial losses. With the restructuring of power systems and with shifting trend towards distributed and dispersed generation, the issue of power quality is going to take newer dimensions.
In developing countries like India, where the variation of power frequency and many such other determinants of power quality are themselves a serious question, it is very vital to take positive steps in this direction .The present work is to identify the prominent concerns in this area and hence the measures that can enhance the quality of the power are recommended.
This work describes the techniques of correcting the supply voltage sag, swell and interruption in a distributed system. At present, a wide range of very flexible controllers, which capitalize on newly available power electronics components, are emerging for custom power applications. Among these, the distribution static compensator and the dynamic voltage restorer are most effective devices, both of them based on the VSC principle. A DVR injects a voltage in series with the system voltage and a D-STATCOM injects a current into the system to correct the voltage sag, swell and interruption. Comprehensive results are presented to assess the performance of each device as a potential custom power solution.
In this project we are discussing the effects of using DSTATCOM in the power system during fault conditions. DSTATCOM means Distribution Static Compensator. It consists of a two-level voltage source converter (VSC) a DC energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. In this paper we are mitigating the faults like voltage sag during single line to ground fault and voltage swell. STATCOM is a static VAR generator, whose output is varied so as to maintain or control specific parameters of the electric power system.
INTRODUCTION
1.1 Introduction
With the advent of power semiconductor switching devices, like thyristors, GTO's (Gate Turn off thyristors), IGBT's (Insulated Gate Bipolar Transistors) and many more devices, control of electric power has become a reality. Such power electronic controllers are widely used to feed electric power to electrical loads, such as adjustable speed drives (ASD's), furnaces, computer power supplies, HVDC systems etc.
The power electronic devices due to their inherent non-linearity draw harmonic and reactive power from the supply. In three phase systems, they could also cause unbalance and draw excessive neutral currents. The injected harmonics, reactive power burden, unbalance, and excessive neutral currents cause low system efficiency and poor power factor.
In addition to this, the power system is subjected to various transients like voltage sags, swells, flickers etc. These transients would affect the voltage at distribution levels. Excessive reactive power of loads would increase the generating capacity of generating stations and increase the transmission losses in lines. Hence supply of reactive power at the load ends becomes essential.
Power Quality (PQ) has become an important issue since many loads at various distribution ends like adjustable speed drives, process industries, printers; domestic utilities, computers, microprocessor based equipments etc. have become intolerant to voltage fluctuations, harmonic content and interruptions.
Power Quality (PQ) mainly deals with issues like maintaining a fixed voltage at the Point of Common Coupling (PCC) for various distribution voltage levels irrespective of voltage fluctuations, maintaining near unity power factor power drawn from the supply, blocking of voltage and current unbalance from passing upwards from various distribution levels, reduction of voltage and current harmonics in the system and suppression of excessive supply neutral current.
Conventionally, passive LC filters and fixed compensating devices with some degree of variation like thyristor switched capacitors, thyristor switched reactors were employed to improve the power factor of ac loads. Such devices have the demerits of
fixed compensation, large size, ageing and resonance. Nowadays equipments using power semiconductor devices, generally known as active power filters (APF's), Active Power Line Conditioners (APLC's) etc. are used for the power quality issues due to their dynamic and adjustable solutions. Flexible AC Transmission Systems (FACTS) and Custom Power products like STATCOM (Static synchronous compensator), DVR (Dynamic Voltage Restorer), etc. deal with the issues related to power quality using similar control strategies and concepts. Basically, they are different only in the location in a power system where they are deployed and the objectives for which they are deployed.
1.2 Objective of the project
To improve the power quality of a distribution system by injecting the required amount of currents to the distribution system from the storage element through DSTATCOM .
.
1.3 Overview the Thesis
The complete project thesis is divided into six chapters as follows.
Chapter 1 provides the introduction of the project and defines the objective of the project. Chapter 2 provides a brief summary of power quality, assets and improvement techniques.
Chapter 3 deals with active power filters.
Chapter 4 deals with operational concepts of DSTATCOM. Chapter 5 provides the simulations diagrams and results.
Chapter 6 deals with conclusion of the thesis and future scope for this project.
1.4 Literature Survey
Power electronic based power processing offers higher efficiency, compact size and better controllability. But on the flip side, due to switching actions, these systems behave as non-linear loads [1-3]. Therefore, whenever, these systems are connected to the utility, they draw non-sinusoidal and/or lagging current from the source. As a result these systems pose themselves as loads having poor displacement as well as distortion
factors. Hence they draw considerable reactive volt-amperes from the utility and inject harmonics in the power networks.
Until now, to filter these harmonics and to compensate reactive power at factory level, only capacitor and passive filters were used. Passive filters have been widely used for the harmonic and reactive power mitigation in the power lines earlier. They are suitable for only eliminating only few harmonics, large size, ageing and resonance. More recently, new PWM based converters for motor control are able to provide almost unity power factor operations. This situation leads to two observations: on one hand, there is electronic equipment which generates harmonics and, on the other hand, there is unity power factor motor drive system which doesn't need power factor correction capacitor. Also, we cannot depend on this capacitor to filter out those harmonics. This is one of the reasons that the research is being done in the area of APF and less pollutant drives.
Loads, such as, diode bridge rectifier or a thyristor bridge feeding a highly inductive load, presenting themselves as current source at point of common coupling (PCC), can be effectively compensated by connecting an APF in shunt with the load [4-6]. On the other hand, there are loads, such as Diode Bridge having a high dc link capacitive filter. These types of loads are gaining more and more importance mainly in forms of AC to DC power supplies and front end AC to DC converters for AC motor drives. For these types of loads APF has to be connected in series with the load [4, 7].
The voltage injected in series with the load by series APF is made to follow a control law such that the sum of this injected voltage and the input voltage is sinusoidal. Thus, if utility voltages are non-sinusoidal or unbalanced, due to the presence of other clients on the same grid, proper selection of magnitude and phase for the injected voltages will make the voltages at load end to be balanced and sinusoidal.
The shunt APF acts as a current source and inject a compensating harmonic current in order to have sinusoidal, in-phase input current and the series APF acts as a voltage source and inject a compensating voltage in order to have sinusoidal load voltage. The developments in the digital electronics, communications and in process control system have increased the number of sensitive loads that require ideal sinusoidal supply voltage for their proper operation. In order to meet limits proposed by standards it is necessary to include some sort of compensation. In the last few years, solutions based
on combination of series active and shunt active filter have appeared [8-9]. Its main purpose is to compensate for supply voltage and load current imperfections, such as sags, swells, interruptions, imbalance, flicker, voltage imbalance, harmonics, reactive currents, and current unbalance [10-16]. This combination of series and shunt APF is called as Unified Power Quality Conditioner (UPQC). In most of the articles control techniques suggested are complex requiring different kinds of transformations. The control technique presented here is very simple and does not require any transformation.
POWER QUALITY
2.1 INTRODUCTION
Power quality is defined as the concept of powering and grounding sensitive equipment in a matter that is suitable to the operation of that equipment.
There are many different reasons for the enormous increase in the interest in power quality. Some of the main reasons are:
• Electronic and power electronic equipment has especially become much more sensitive. Equipment has become less tolerant of voltage quality disturbances, production processes have become less tolerant of incorrect of incorrect operation of equipment, and companies have become less tolerant of production stoppages. The main perpetrators are interruptions and voltage dips, with the emphasis in discussions and in the literature being on voltage dips and short interruptions. High frequency transients do occasionally receive attention as causes of equipment malfunction.
• Equipment produces more current disturbances than it used to do. Both low and high power equipment is more and more powered by simple power electronic converters which produce a broad spectrum of distortion. There are indications that the harmonic distortion in the power system is rising, but no conclusive results are obtained due to the lack of large scale surveys.
• The deregulation of the electricity industry has led to an increased need for quality indicators. Customers are demanding, and getting, more information on the voltage quality they can expect.
• Also energy efficient equipment is an important source of power quality disturbance. Adjustable speed drives and energy saving lamps are both important sources of waveform distortion and are also sensitive to certain type of power quality disturbances. When these power quality problems become a barrier for the large scale introduction of environmentally friendly sources and users’ equipment, power quality becomes an environmental issue with much wider consequences than the currently merely economic issues.
2.2 NEED FOR POWER QUALITY IMPROVEMENT
1. Equipment has become less tolerant of voltage quality disturbances, production processes have become less tolerant of incorrect of incorrect operation of equipment, and companies have become less tolerant of production stoppages. Note that in many discussions only the first problem is mentioned, whereas the latter two may be at least equally important .All this leads to much higher costs than before being associated with even a very short duration disturbance. The main perpetrators are interruptions and voltage dips, with the emphasis in discussions and in the literature being on voltage dips and short interruptions. High frequency transients do occasionally receive attention as causes of equipment malfunction but are generally not well exposed in the literature.
2. Equipment produces more current disturbances than it used to do. Both low and high power equipment is more and more powered by simple power electronic converters which produce a broad spectrum of distortion. There are indications that the harmonic distortion in the power system is rising, but no conclusive results are obtained due to the lack of large scale surveys.
3. The deregulation of the electricity industry has led to an increased need for quality indicators. Customers are demanding, and getting, more information on the voltage quality they can expect. Some issues of the interaction between deregulation and power quality are discussed.
4. Also energy efficient equipment is an important source of power quality disturbance. Adjustable speed drives and energy saving lamps are both important sources of waveform distortion and are also sensitive to certain type of power quality disturbances. When these power quality problems become a barrier for the large scale introduction of environmentally friendly sources and users’ equipment, power quality becomes an environmental issue with much wider consequences than the currently merely economic issues.
2.3 POWER QUALITY STANDARDS 2.3.1 PURPOSE OF STANDARDIZATION
Standards that define the quality of the supply have been present for decades already. Almost any country has standards defining the margins in which frequency and voltage are allowed to vary. Other standards limit harmonic current and voltage distortion, voltage fluctuations, and duration of an interruption. There are three reasons for developing power quality standards.
2.3.2THE EUROPEAN VOLTAGE CHARATERISTICS STANDARD
European standard 50160 [80] describes electricity as a product, including its shortcomings. It gives the main characteristics of the voltage at the customer's supply terminals in public low-voltage and medium-voltage networks under normal operating conditions. Some disturbances are just mentioned, for others a wide range of typical values are given, and for some disturbances actual voltage characteristics are given.
Voltage variation: Standard EN 50160 gives limits for some variations. For each of these
variations the value is given which shall not be exceeded for 95% of the time. The measurement should be performed with a certain averaging window. The length of this window is 10 minutes for most variations; thus very short time scales are not considered in the standard. The following limits for the low-voltage supply are given in the document:
• Voltage magnitude: 95% of the 10-minute averages during one week shall be within
±10% of the nominal voltage of 230 V.
TABLE 2.1 Voltage Characteristics as Published by Goteborg Energi
Phenomenon Basic Level
Magnitude variations Voltage shall be between 207 and 244 V
Voltage unbalance Up to 2%
Voltage fluctuations Not exceeding the flicker curve
2.4 Power quality Terminology
DSTATCOM means Distribution Static Compensator. STATCOM is a static VAR generator, whose output is varied so as to maintain or control specific parameters of the electric power system.
SAG is a decrease in rms voltage or currents to between 0.1 to 0.9 p.u at the power frequency for duration of from 0.5 cycles to 1 minute.
Balanced Sag is an equal drop in the rms value of voltage in the three-phases of a three-phase system or at the terminals of three-phase equipment for duration up to a few minutes.
Voltage dip is sudden reduction in the supply voltage by a value of more than 10% of the reference value, fallowed by a voltage recovery after a short period of time.
Unbalanced Fault is a short circuit or open circuit fault in which not all three phases are equally involved.
Voltage Tolerance it is the immunity of a piece of equipment against voltage magnitude variations (Sags, Swells and Interruptions) and short duration over voltages.
Duration (of Voltage Sag) it is the time during which the voltage deviates significantly from the ideal voltage.
Critical Distance is the distance at which a short-circuit fault will lead to a voltage sag of a given magnitude for a given load position.
Current Disturbance it is a variation of event during which the current in the system or at the equipment terminals deviates from the ideal sine wave.
Voltage Disturbance it is a variation of event during which the voltage in the system or at the equipment terminals deviates from the ideal sine wave.
Power Quality it is the study or description of both voltage and current disturbances. Power quality can be seen as the combination of voltage quality and current quality.
Interruption is the voltage event in which the voltage is zero during a certain time. The time during which the voltage is zero is referred to as the “duration” of the interruption. (OR) A voltage magnitude event with a magnitude less than 10% of the nominal voltage.
Over Voltage is an abnormal voltage higher than the normal service voltage, such as might be caused from switching and lightning surges. (OR) Abnormal voltage between two points of a system that is greater than the highest value appearing between the same two points under normal service conditions.
Under Voltage is a voltage event in which the rms voltage is outside its normal operating margin for a certain period of time. (OR) A voltage magnitude event with a magnitude less than the nominal rms voltage, and a duration exceeding 1 minute.
Swell it is a momentary increase in the rms voltage or current to between 1.1 and 1.8pu delivered by the mains, outside of the normal tolerance, with a duration of more than one cycle and less than few seconds.
Recovery Time is the time interval needed for the voltage or current to return to its normal operating value, after a voltage or current event.
Fault is an event occurs on the power system and it effects the normal operation of the power system.
Voltage Fluctuation is a special type of voltage variation in which the voltage shows changes in the magnitude and/or phase angle on a time scale of seconds or less. Severe voltage fluctuations lead to light flicker.
Voltage Source Converters (VSC)
A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-speed drives, but can also be used to mitigate voltage dips. The VSC is used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual.
2.5 About power Quality Disturbances 2.5.1 Voltage sags
Major Causes: Faults, Starting of large loads, and brown-out recovery.
Major Consequences: Shorts, accelerated aging, loss of data or stability,
Process interrupts, etc.
2.5.2 Capacitor Switching Transients
Major Causes: A power factor correction method
Major Consequences: Insulation breakdown or spark over, semi conductor device damage, shorts, accelerated aging, loss of data
or stability. 2.5.3 Harmonics
Major Consequences: Equipment Over heating, high voltage/current, Protective device operations.
2.5.4 Lightning Transients
Major Causes: Lightning strikes
Major Consequences: Insulation breakdown or spark over, semi conductor device damage, shorts, accelerated aging, loss of data
or stability. 2.5.5 High Impedance faults
Major Causes: Fallen conductors, trees (fail to establish a permanent return
path)
Major Consequences: Fire, threats to personal safety.
2.6.(i) Principles for improving power quality
From the discussion already presented, it is evident that for improving power
quality, the steps given in fig (4) have to be taken. As also pointed out, the appropriate decomposition of power for purposes of both identification and control of the distortion elimination by filters has to be achieved. Since it is essential to use clear and consistent terminology, the term non-active power filter will be used for equipment that eliminates non-active power. The actual types of these filters are to be discussed in a further chapter of this paper.
Figure 2.1: Improving power quality by distortion elimination.
The non-active power filters to be used can be divided into the classes of input converters, dynamic filters and tuned impedance filters. Theses principles and the control requirements will now be discussed shortly.
Figure 2.2: Principle of input converter to eliminate distortion loads on the power network.
2.6 IMPROVEMENT TECHNIQUES
To improve the power quality, some devices need to be installed at a suitable location. These devices are called custom power devices, which make sure that customers get pre specified quality and reliability of supply. The compensating devices compensate a load, i.e its power factor, unbalance conditions or improve the power quality of
Identify distortion by using Appropriate power theory
Decide on method of Distortion elimination Equipment with appropriate power electronic input converters.
(Dynamic input filters)
Dynamic filters for distortion
elimination
Tuned impedance
filters Type of filter:SVC, PWM (Series or parallel) hybrid, undefined
supplied voltage, etc. some of the power quality improving techniques are given as below.
2.6.1 HARMONICS
Harmonic Filters may be used to mitigate, and in some cases, eliminate problems created by power system harmonics. Non-linear loads such as rectifiers, converters, home electronic appliances, and electric arc furnaces cause harmonics giving rise to extra losses in power equipment such as transformers, motors and capacitors. They can also cause other, probably more serious problems, when interfering with control systems and electronic devices. Installing filters near the harmonic sources can effectively reduce harmonics. For large, easily identifiable sources of harmonics, conventional filters designed to meet the demands of the actual application are the most cost efficient means of eliminating harmonics. These filters consist of capacitor banks with suitable tuning reactors and damping resistors. For small and medium size loads, active filters, based on power electronic converters with high switching frequency, may be a more attractive solution.
Benefits
• Eliminates harmonics
• Improved Power Factor
• Reduced Transmission Losses
• Increased Transmission Capability
• Improved Voltage Control
• Improved Power Quality
Other applications
• Shunt Capacitors
2.6.2 VOLTAGE FLICKERS
Voltage flicker can become a significant problem for power distributors when large motor loads are introduced in remote locations. Installation of a series capacitor in the feeder strengthens the network and allows such load to be connected to existing lines, avoiding more significant investment in new substations or new distribution lines.
The use of the MiniCap on long distribution feeders provides self-regulated reactive power compensation that efficiently reduces voltage variations during large motor starting.
Benefits
• Reduced voltage fluctuations (flicker) • Improved voltage profile along the line • Easier starting of large motors
• Self-regulation
2.6.3 BOTTLENECKS
Bottlenecks may be relieved by the use of Series Compensation. Longer lines tend to have stability-constrained capacity limitations as opposed to the higher thermal constraints of shorter lines. Series Compensation has the net effect of reducing transmission line series reactance, thus effectively reducing the line length. Series Compensation also offers additional power transfer capability for some thermal-constrained bottlenecks by balancing the loads among the parallel lines. The power transfer between two-area interconnected systems is limited to 1500MW due to stability constraints. Additional electricity can be delivered between them if Series Compensation is applied to increase the maximum stability limits.
Benefits
• Increased Power Transfer Capability • Additional flexibility in Grid Operation • Improved Grid Voltage Control
• Lower Transmission Losses • Improved Transient Stability
Other applications
• Power Flow Control
2.6.4 SHUNT CAPACITORS
Regulation of the power factor to increase the transmission capability and reduce transmission losses. Shunt capacitors are primarily used to improve the power factor in transmission and distribution networks, resulting in improved voltage regulation, reduced network losses, and efficient capacity utilization. Figure shows a plot of terminal voltage versus line loading for a system that has a shunt capacitor installed at the load bus. Improved transmission voltage regulation can be obtained during heave power transfer conditions when the system consumes a large amount of reactive power that must be replaced by compensation. At the line surge impedance loading level, the shunt capacitor would decrease the line losses by more than 35%. In distribution and industrial systems, it is common to use shunt capacitors to compensate for the highly inductive loads, thus achieving reduced delivery system losses and network voltage drop.
Benefits
• Improved power factor • Reduced transmission losses • Increased transmission capability • Improved voltage control
• Improved power quality
Other applications
• Harmonic Filters
2.6.5 SHUNT REACTOR
The primary purpose of the shunt reactor is to compensate for capacitive charging voltage, a phenomenon getting more prominent for increasing line voltage. Long high-voltage transmission lines and relatively short cable lines (since a power cable has high capacitance to earth) generate a large amount of reactive power during light power transfer conditions which must be absorbed by compensation. Otherwise, the receiving terminals of the transmission lines will exhibit a “voltage rise” characteristic and many types of power equipment cannot withstand such over voltages. A better fine tuning of the reactive power can be made by the use of a tap changer in the shunt reactor. It can be possible to vary the reactive power between 50 to 100 % of the needed power.
Benefits
• Simple and robust customer solution with low installation costs and minimum maintenance
• No losses from an intermediate transformer when feeding reactive compensation from a lower voltage level.
• No harmonics created which may require filter banks.
2.6.6 SVC
Static VAR Compensators are used in transmission and distribution networks mainly providing dynamic voltage support in response to system disturbances and balancing the reactive power demand of large and fluctuating industrial loads. A Static VAR Compensator is capable of both generating and absorbing variable reactive power continuously as opposed to discrete values of fixed and switched shunt capacitors or reactors. Further improved system steady state performance can be obtained from SVC applications. With continuously variable reactive power supply, the voltage at the SVC bus may be maintained smoothly over a wide range of active power transfers or system loading conditions. This entails the reduction of network losses and provision of adequate power quality to the electric energy end-users.
The traction system is a major source of unbalanced loads. Electrification of railways, as an economically attractive and environmentally friendly investment in infrastructure, has introduced an unbalanced and heavy distorted load on the three-phase transmission grid. Without compensation, this would result in significant unbalanced voltage affecting most neighboring utility customers. The SVC can elegantly be used to counteract the unbalances and mitigate the harmonics such that the power quality within the transmission grid is not impaired.
Static Var Compensators are mainly used to perform voltage and reactive power regulation. However, when properly placed and controlled, SVCs can also effectively counteract system oscillations. A SVC, in effect, has the ability to increase the damping factor (typically by 1-2 MW per Mvar installed) on a bulk power system which is experiencing power oscillations. It does so by effectively modulating its reactive power
output such that the regulated SVC bus voltage would increase the system damping capability.
SVC is used most frequently for compensation of disturbances generated by the Electrical Arc Furnaces (EAF). With a well-designed SVC, disturbances such as flicker from the EAF are mitigated Flicker, the random variation in light intensity from incandescent lamps caused by the operating of nearby fluctuating loads on the common electric supply grid, is highly irritating for those affected. The random voltage variations can also be disturbing to other process equipment fed from the same grid. The proper mitigation of flicker is therefore a matter of power quality improvement as well as an improvement to human environment.
Benefits
• Increased Power Transfer Capability • Additional flexibility in Grid Operation • Improved Grid Voltage Stability
• Improved Grid Voltage Control
• Improved Power Factor
Other applications
• Power Oscillation Damping
• Power Quality (Flicker Mitigation, Voltage Balancing)
• Grid voltage support
2.6.7 STATCOM
STATCOM, when connected to the grid, can provide dynamic voltage support in response to system disturbances and balance the reactive power demand of large and fluctuating industrial loads. A STATCOM is capable of both generating and absorbing variable reactive power continuously as opposed to discrete values of fixed and switched shunt capacitors or reactors. With continuously variable reactive power supply, the voltage at the STATCOM bus may be maintained smoothly over a wide range of system operation conditions. This entails the reduction of network losses and provision of sufficient power quality to the electric energy end-users.
STATCOM® is an effective method used to attack the problem of flicker. The unbalanced, erratic nature of an electric arc furnace (EAF) causes significant fluctuating
reactive power demand, which ultimately results in irritating electric lamp flicker to neighboring utility customers. In order to stabilize voltage and reduce disturbing flicker successfully, it is necessary to continuously measure and compensate rapid changes by means of extremely fast reactive power compensation.
STATCOM® uses voltage source converters to improve furnace productivity similar to a traditional SVC while offering superior voltage flicker mitigation due to fast response time. Similar to SVC, the STATCOM can elegantly be used to restore voltage and current balance in the grid, and to mitigate voltage fluctuations generated by the traction loads.
Benefits
• Increased Power Transfer Capability • Additional flexibility in Grid Operation • Improved Grid Voltage Stability
• Improved Grid Voltage Control
• Improved Power Factor • Eliminated Flicker
• Harmonic Filtering • Voltage Balancing • Power Factor Correction
• Furnace/mill Process Productivity Improvement
Other applications
• Power Quality (Flicker Mitigation, Voltage Balancing) • Grid Voltage Support
ACTIVE POWER FILTER
Shunt Active Filters
3.1 Introduction
The various nonlinear loads like Adjustable Speed Drives (ASD’s), bulk rectifiers, furnaces, computer supplies, etc. draw non sinusoidal currents containing harmonics from the supply which in turn causes voltage harmonics. Harmonic currents result in increased power system losses, excessive heating in rotating machinery, interference with nearby communication circuits and control circuits, etc.
It has become imperative to maintain the sinusoidal nature of voltage and currents in the power system. Various international agencies like IEEE and IEC have issued standards, which put limits on various current and voltage harmonics. The limits for various current and voltage harmonics specified by IEEE-519 for various frequencies are given in Table 3.1 and Table 3.2.
Table 3.1
IEEE 519 Voltage Limits
The objectives and functions of active power filters have expanded from reactive power compensation, voltage regulation, etc. to harmonic isolation between utilities and consumers, and harmonic damping throughout the distribution as harmonics propagate through the system. Active power filters are either installed at the individual consumer premises or at substation and/or on distribution feeders. Depending on the compensation objectives, various types of active power filter topologies have evolved, a proper briefing provided in further.
Bus Voltage
Minimum Individual
Harmonic Components (%) Maximum
THD (%)
69 kV and below 3 5
115 kV to 161 kV 1.5 2.5
Table 3.2
IEEE 519 Current Limits
SCR=Isc/Il h<11 11 to 17 17 to 23 23 to 35 35<h THD <20 4.0 2.0 1.5 0.6 0.3 5.0 20 - 50 7.0 3.5 2.5 1.0 0.5 8.0 50 -100 10.0 4.5 4.0 1.5 0.7 12.0 100 - 1000 12.0 5.5 5.0 2.0 1.0 15.0 >1000 15.0 7.0 6.0 2.5 1.4 20.0
3.2 Classifications of Active Power Filters
3.2.1 Converter based classification
Current Source Inverter (CSI) Active Power Filter (Fig 3.1) and Voltage Source Inverter Active Power Filter (VSI) (Fig 3.2) are two classifications in this category. Current Source Inverter behaves as a nonsinusoidal current source to meet the harmonic current requirement of the nonlinear loads. A diode is used in series with the self-commutating device (IGBT) for reverse voltage blocking. However, GTO-based configurations do not need the series diode, but they have restricted frequency of switching. They are considered sufficiently reliable, but have higher losses and require higher values of parallel ac power capacitors. Moreover, they cannot be used in multilevel or multistep modes to improve performance in higher ratings.
The other converter used as an AF is a voltage-fed PWM inverter structure, as shown in Fig 3.2. It has a self-supporting dc voltage bus with a large dc capacitor. It has become more dominant, since it is lighter, cheaper, and expandable to multilevel and multistep versions, to enhance the performance with lower switching frequencies. It is more popular in UPS-based applications, because in the presence of mains, the same Inverter bridge can be used as an AF to eliminate harmonics of critical nonlinear loads.
3.2.2 Topology based Classification
AF’s can be classified based on the topology used as series or shunt filters, and unified power quality conditioners use a combination of both. Combinations of active series and passive shunt filtering are known as hybrid filters. Fig 3.3 is an example of an
active shunt filter, which is most widely used to eliminate current harmonics, reactive power compensation (also known as STATCOM, and balancing unbalanced currents. It is mainly used at the load end, because current harmonics are injected by nonlinear loads. It injects equal compensating currents, opposite in phase, to cancel harmonics and/or reactive components of the nonlinear load current at the point of connection. It can also be used as a static VAR generator (STATCOM) in the power system network for stabilizing and improving the voltage profile.
Fig 3.1 Current fed type AF Fig 3.2 Voltage fed type AF
Fig 3.4 shows the basic block of a stand-alone active series filter. It is connected before the load in series with the mains, using a matching transformer, to eliminate voltage harmonics, and to balance and regulate the terminal voltage of the load or line. It has been used to reduce negative-sequence voltage and regulate the voltage on three-phase systems. It can be installed by electric utilities to compensate voltage harmonics and to damp out harmonic propagation caused by resonance with line impedances and passive shunt compensators.
Fig 3.4 shows the basic block of a stand-alone active series filter. It is connected before the load in series with the mains, using a matching transformer, to eliminate voltage harmonics, and to balance and regulate the terminal voltage of the load or line. It has been used to reduce negative-sequence voltage and regulate the voltage on three-phase systems. It can be installed by electric utilities to compensate voltage harmonics and to
damp out harmonic propagation caused by resonance with line impedances and passive shunt compensators.
Fig 3.5 shows the hybrid filter, which is a combination of an active series filter and passive shunt filter. It is quite popular because the solid-state devices used in the active series part can be of reduced size and cost (about 5% of the load size) and a major part of the hybrid filter is made of the passive shunt L–C filter used to eliminate lower order harmonics. It has the capability of reducing voltage and current harmonics at a reasonable cost.
Fig 3.3 Shunt-type AF Fig 3.4 Series-type AF
Fig 3.5 Hybrid filter Fig 3.6 Unified Power Quality Conditioner Fig 3.6 shows a unified power quality conditioner (also known as a universal AF), which is a combination of active shunt and active series filters. The dc-link storage element (either inductor or dc-bus capacitor) is shared between two current-source or voltage-source bridges operating as active series and active shunt compensators. It is used in single-phase as well as three-phase configurations. It is considered an ideal AF, which eliminates voltage and current harmonics and is capable of giving clean power to critical and harmonic-prone loads, such as computers, medical equipment, etc. It can balance and regulate terminal voltage and eliminate negative-sequence currents. Its main drawbacks are its large cost and control complexity because of the large number of solid-state devices involved.
3.2.3 Supply-System-Based Classification
This classification of AF’s is based on the supply and/or the load system having single-phase (two wire) and three-phase (three wire or four wire) systems. There are many nonlinear loads, such as domestic appliances, connected to single-phase supply systems. Some three-phase nonlinear loads are without neutral, such as ASD’s, fed from three-wire supply systems. There are many nonlinear single-phase loads distributed on four-wire three-phase supply systems, such as computers, commercial lighting, etc. Hence, AF’s may also be classified accordingly as two-wire, three-wire, and four-wire types.
1)Two-Wire AF’s:
Two-wire (single phase) AF’s are used in all three modes as active series, active shunt, and a combination of both as unified line conditioners. Both converter configurations, current-source PWM bridge with inductive energy storage element and voltage-source PWM bridge with capacitive dc-bus energy storage elements, are used to form two-wire AF circuits. In some cases, active filtering is included in the power conversion stage to improve input characteristics at the supply end.
2)Three-Wire AF’s:
Three-phase three-wire nonlinear loads, such as ASD’s, are major applications of solid-state power converters and, lately, many ASD’s, etc., incorporate AF’s in their front-end design. A large number of publications have appeared on three-wire AF’s with different configurations. All the configurations shown in Figs 3.1–3.6 are developed, in three-wire AF’s, with three wires on the ac side and two wires on the dc side. Active shunt AF’s are developed in the current-fed type (Fig 3.1) or voltage-fed type with single-stage (Fig 3.2) or multi-step/multilevel and multi-series configurations. Active shunt AF’s are also designed with three single-phase AF’s with isolation transformers [18] for proper voltage matching, independent phase control, and reliable compensation with unbalanced systems. Active series filters are developed for stand-alone mode (Fig 3.4) or hybrid mode with passive shunt filters (Fig 3.5). The latter (hybrid) has become quite popular to reduce the size of power devices and cost of the overall system. A combination of active series and active shunt is used for unified power quality conditioners (Fig 3.6) and universal filters.
3) Four-Wire AF’s:
A large number of single-phase loads may be supplied from three-phase mains with neutral conductor. They cause excessive neutral current, harmonic and reactive power burden, and unbalance. To reduce these problems, four-wire AF’s have been attempted. They have been developed as: 1) active shunt mode with current feed and voltage feed; 2) active series mode; and 3) hybrid form with active series and passive shunt mode.
3.2.4 Compensated Variable Based Classification
(1) Harmonic Compensation (2) Multiple Compensation
This is the most important system parameter requiring compensation in power systems and it is subdivided into voltage- and current-harmonic compensation. The compensation of voltage and current harmonics is interrelated.
Different combinations of the above systems can be used to improve the effectiveness of filters. The following are the most frequently used combinations.
· Harmonic currents with Reactive power compensation. · Harmonic voltages with Reactive power compensation. · Harmonic currents and voltages.
· Harmonic currents and voltages with reactive-power compensation.
3.2.5 Voltage Type Vs Current Type APF’s
A clear trend for preferred type of APF’s does not exist. A choice depends on
source of distortion at the specified bus, equipment cost, and amount of correction desired.
Voltage-type has an advantage in that they can be readily expanded in parallel to increase their combined rating. Their combined switching rate can be increased if they are carefully controlled so that their individual switching times do not coincide. Therefore, using parallel voltage-type converters with out increasing individual converter switching rates can eliminate higher order harmonics. Voltage type converters are lighter and less expansive than current-type converters.
The main drawback of voltage-type converters lies in the increased complexity of their control system. For systems with several connected in parallel, this complexity is greatly increased.
Current-type converters have advantages of excellent current controllability, easy protection and high reliability over Voltage source APF. More over CSI topology has superior characteristics compared to VSI topology in terms of direct injected current, which result in a faster response in time varying load environment and lower dc energy
storage requirement. The drawback of the current source APF is larger power losses of the dc-link inductor. However, the current-type active power filter will become more attractive when the super conducting coils are available in the future. Losses are less important in low- power applications but very important in high power applications. Since they are easily expandable, voltage type APF’s are likely to be used for network wide compensation. Current type APF’s will continue to popular for single-node distortion problems. In other words, electric utility interest will likely to be focused on voltage type converters, while industrial users likely to use both type of converters.
3.3 Operation of Three Phase Active Power Filters
In recent years, the power quality of the AC main system has become a great concern due to the rapidly increased number of electronic equipment. In order to reduce the harmonic contamination in power lines and improve the transmission efficiency Active power filters become essential. A current source is connected in parallel with nonlinear load and controlled to generate the harmonic currents needed for the load. The basic configuration of a three-phase three-wire active power filter is shown in Fig 3.7. The diode bridge rectifier is used as an ideal harmonic generator to study the performance of the Active filter. The current-controlled voltage-source inverter (VSI) is shown connected at the load end. This PWM inverter consists of six switches with antiparallels diode across each switch. The capacitor is designed in order to provide DC voltage with acceptable ripples. In order to assure the filter current at any instant, the DC voltage Vdc must be at least equal to 3/2 of the peak value of the line AC mains voltage.
Fig 3.7 Configuration of the three phase, three wire Active filtering system.
Three aspects have to be considered in the design of APF.
a) The parameters of the inverter such as inverter switches and the values of the link inductances.
b) Modulation method used and
c) The control method used to generate the harmonic reference template.
3.3.1 Sample and Hold circuit’s method for harmonic reference template
This method is simple, eliminates complicated transformations and mathematical operations such as multiplications and divisions, and permits good transient response. Fig 3.8 shows implementation of Sample and Hold method.
Fig 3.8 Control block of Sample and Hold circuit's harmonic reference template
The current in each phase of the load is filtered to get the fundamental phase current. A "Sample and Hold" circuit, synchronized with the peak value of the phase-to-neutral voltage, allows to get three dc signals, which are proportional to the amplitude of the active component of the current for each phase. Three dc signals, with the information of the total active power in the load, are averaged to balance the system. Then, by multiplying the averaged dc signal for a set of balanced reference waveforms (in phase with the mains voltages), three in phase balanced currents for each phase are obtained. Finally, these currents are subtracted from the real load currents to get the compensation currents. These harmonic are then able to correct the harmonic distortion, the power factor and the unbalances of the load.
Let to assume that IL is the total load current in one phase. This current contains
basically three components. IL = IP + IQ + IH
Where IP, IQ and IH are the fundamental active, reactive and harmonic currents
respectively. The APF will eliminate IQ and IH by subtracting IP from IL. Extraction of IP
First the load currents sensed and filtered to eliminate the (IH) and then the total
fundamental currents (one for each phase) are obtained. These currents have to be separated in their active and reactive components.
Where
IP = I cosφ
However the angle "φ " does need to be known, because the term "I cosφ " can be obtained from the time function of the fundamental when the main voltage reaches the its maximum value. Fig 3.9 explains graphically the idea.
Fig 3.9 Method used to capture IP.
IP is captured and "stored" until the next sample of IP is obtained to replace the
old one. This action is executed with the help of "Sample and Hold" circuits, which are synchronized with the synchronization pulses to trigger the S&H are generated through the "zero-crossing" signals, obtained from the set of quadrature voltages". These "in-quadrature voltages" are generated in the control block with the DZ0 connection, signal transformer.
The control circuit is also has the capability to avoid flickers and transient phenomena in the source, produced by sudden changes in load current. To do this, control system makes soft variation of IP during these moments. However, this action will require
having the energy storage components in the APF. Hence the design of the control system has to take in account the characteristics of the power filter.
DSTATCOM
DSTATCOM
4.1 Introduction
Modern power systems are complex networks, where hundreds of generating stations and thousands of load centers are interconnected through long power transmission and distribution networks. Even though the power generation in most countries is fairly reliable, the quality of power is not so reliable. Power distribution system should provide their customers with an uninterrupted flow of energy at smooth sinusoidal voltage at the contracted magnitude level and frequency. Power system especially distribution systems have numerous non linear loads, which significantly affect the quality of power supplies. This ends up producing many power quality problems. Apart from non linear loads events like capacitor switching, motor starting and unusual faults could also inflict power quality problems.
Power quality problem is defined as any manifested problem in voltage /current or leading to frequency deviations that result in failure or mis-operation of customer equipment. Voltage sags and swells are among the many power quality problems the industrial process has to face. Voltage sags are most severe.
During the past few decades, power industries has proved that adverse impacts on the power quality can be mitigated or avoided by conventional means, and that techniques using fast controlled force commutated power electronics are even more effective. Power quality compensators can be categorized into two main types. One is shunt connected compensation device, which has dominates the most common application-power harmonics elimination. The other is the series connected type, which has an edge over the shunt type for correcting the distorted system side voltage and voltage sags caused by power transmission system faults.
The DSTATCOM is a power quality device, which can protect these industries against the sags and swells. Usually sags and swells are related to remote faults. A DSTATCOM compensates for these voltage disturbances provided that supply grid does not get disconnected entirely through breaker trips. It can exchange both active and reactive power with the distribution system by varying the amplitude and phase angle of the converter voltage with respect to the line terminal voltage.
4.2 Principle and Operation of DSTATCOM
A D-STATCOM (Distribution Static Compensator), which is schematically depicted in Figure 4.1, consists of a two-level Voltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the dc voltage across the storage device into a set of three-phase ac output voltages. These voltages are in phase and coupled with the ac system through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective control of active and reactive power exchanges between the D-STATCOM and the ac system. Such configuration allows the device to absorb or generate controllable active and reactive power.
The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes:
1. Voltage regulation and compensation of reactive power; 2. Correction of power factor; and
3. Elimination of current harmonics.
Here, such device is employed to provide continuous voltage regulation using an indirectly controlled converter.
Fig. 4.1 Schematic Diagram of a DSTATCOM
Figure-4.1 the shunt injected current Ish corrects the voltage sag by adjusting the voltage drop across the system impedance Zth. The value of Ish can be controlled by adjusting the output voltage of the converter. The shunt injected current Ish can be written as,
Ish = IL – Is = IL – (Vth – VL)/ Zth --- 4.1
--- 4.2
The complex power injection of the D-STATCOM can be expressed as,
--- 4.3
It may be mentioned that the effectiveness of the D-STATCOM in correcting voltage sag depends on the value of Zth or fault level of the load bus. When the shunt injected current Ish is kept in quadrature with VL, the desired voltage correction can be achieved without injecting any active power into the system. On the other hand, when the value of Ish is minimized, the same voltage correction can be achieved with minimum apparent power injection into the system.
4.2.1 Voltage Source Converter
A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-speed drives, but can also be used to mitigate voltage dips. The VSC is used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual. The converter is normally based on some kind of energy storage, which will supply the converter with a DC voltage. The solid-state electronics in the converter is then switched to get the desired output voltage. Normally the VSC is not only used for voltage dip mitigation, but also for other power quality issues, e.g. flicker and harmonics.
4.3 Control Scheme for the DSTACOM
The aim of the control scheme is to maintain constant voltage
magnitude at the point where a sensitive load is connected, under system disturbances. The control system only measures the r.m.s voltage at the load point, i.e., no reactive power measurements are required. The VSC switching strategy is based on a sinusoidal PWM technique which offers simplicity and good response. Since custom power is a
relatively low-power application, PWM methods offer a more flexible option than the Fundamental Frequency Switching (FFS) methods favored in FACTS applications. Besides, high switching frequencies can be used to improve on the efficiency of the converter, without incurring significant switching losses.
The controller input is an error signal obtained from the reference voltage and the value rms of the terminal voltage measured. Such error is processed by a PI controller the output is the angle δ, which is provided to the PWM signal generator. It is important to note that in this case, indirectly controlled converter, there is active and reactive power exchange with the network simultaneously: an error signal is obtained by comparing the reference voltage with the rms voltage measured at the load point. The PI controller process the error signal generates the required angle to drive the error to zero, i.e., the load rms voltage is brought back to the reference voltage.
Fig. 4.2 Indirect PI controller
The sinusoidal signal Vcontrol is phase-modulated by means of the angle . i.e., VA = Sin (ωt +δ), --- 4.4
VB= Sin(ωt+δ-2π/3), --- 4.5 VC = Sin (ωt +δ+2π/3). --- 4.6
The modulated signal Vcontrol is compared against a triangular signal in order to generate the switching signals for the VSC valves. The main parameters of the sinusoidal PWM scheme are the amplitude modulation index of signal, and the frequency modulation index of the triangular signal. The amplitude index is kept fixed at 1 pu, in order to obtain the highest fundamental voltage component at the controller output.
--- 4.7
is the peak amplitude of control signal, is the peak amplitude of triangular signal.
The switching frequency is set at 1080 Hz. The frequency modulation index is given by, mf = fs/f1= 1080/60 = 18
Where f1 is the fundamental frequency.
The modulating angle is applied to the PWM generators in phase A. The angles for phases B and C are shifted by 240 degrees and 120 degrees, respectively. It can be seen in that the control implementation is kept very simple by using only voltage measurements as the feedback variable in the control scheme. The speed of response and robustness of the control scheme are clearly shown in the simulation results.
4.4 Phasor diagram of DSTATCOM
√
Fig 4.4 Phasor diagram for shunt voltage controller Solid Lines Without phase angle jump
Dashed Lines with phase angle jump
The large increase in active power injected with increasing phase-angle jump is explained in fig 4.4. The injected voltage is the required voltage rise at the load due to the injection of a current into the source impedance. This injected voltage is the difference between the normal operating voltage and the sag voltage as it would be without controller. The injected current is the injected voltage divided by the source impedance. In phasor terms, the argument (angle, Direction) of the injected current is the argument of the injected voltage minus the argument of the source impedance. The source impedance is normally mainly reactive. Incase of sag without phase-angle jump, the injected current is also mainly reactive. A phase-angle jump causes a rotation of the injected voltage as indicated in the fig 4.4. This leads to a rotation of the injected current away from the imaginary axis. From the fig 4.4 it becomes obvious that this will quickly cause a serious increase in the active part of the current (i.e. the projection of the current on the load voltage). The change in the reactive part of the current is small, so is the change in current magnitude.
4.5 Mathematical Modeling
The load voltage during the sag can be seen as the superposition of the voltage due to the system and the voltage change due to the controller. The former is the voltage as it would have been without a controller. The former is the injected current.
Assume that the voltage with out controller is
Vsag = V cosӨ + jV sinӨ --- 4.8
The load voltage is again equal to 1 pu:
Vload = 1+j0 --- 4.9
The required change in voltage due to the injected current is the difference between the load voltage and the sag voltage:
∆V = 1- V cosӨ - jV sinӨ --- 4.10 The change in voltage must be obtained by injecting a current equal to
I cont = P – jQ --- 4.11
With P the active power and Q the reactive power injected by the controller. The active power will determine the requirements for energy storage. Let the impedance seen by the shunt controller (Source impedance in parallel with load impedance) be equal to
Z = R+ jX --- 4.12 The effect of the injected current is a change in voltage according to
∆V = Icont Z = (R + jX)(P – jQ) --- 4.13
The required voltage increase (4.10) and the achieved increase (4.13) have to be equal. This gives the fallowing expression for the injected complex power:
P - jQ = (1-VcosӨ - jVsinӨ) --- 4.14 R + jX
Splitting the complex power in a real and an imaginary part, gives expressions for active and reactive power:
P = R(1 – V cosӨ) - VXsinӨ --- 4.15 R2 + X2
Q = RV sinӨ + X (1-V cosӨ) --- 4.16 R2 + X2
The Main limitation of the shunt controller is that the source impedance becomes very small for faults at the same voltage level close to the load. Mitigting such Sags through a shunt controller is impractical as it would require very large currents. We therefore only consider faults upstream of the supply transformer. The minimum value of the source impedance is the transformer impedance.
The current rating of the controller is determined by both active and reactive power. From 4.15 and 4.16 we find for the absolute value of the injected current:
MATLAB SIMULATION
MATLAB Simulation
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MATLAB provides a number of features for documenting and sharing your work. You can integrate your MATLAB code with other languages and applications, and distribute your MATLAB algorithms and applications.
Key Features
• High-level language for technical computing
• Development environment for managing code, files, and data
• Interactive tools for iterative exploration, design, and problem solving
• Mathematical functions for linear algebra, statistics, Fourier analysis, filtering, optimization, and numerical integration.
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• Functions for integrating MATLAB based algorithms with external applications and languages, such as C, C++, FORTRAN, Java, COM, and Microsoft Excel.
The below figure shows the test system used to carry out the various DSTATCOM simulations.
Fig 5.1 single line diagram of the test system for DSTATCOM
Where Vs = 3-ph Voltage generator, Zs = Source Impedance, Vdc = DC voltage Source,
1,2 and 3 are known as feeders or buses to which we connect tharious electric Equipments,
L1, L2 are the loads connected to the system, S1 is the switch
Switch S1 is used to attain various situations in the power system, i.e. sags, swells.
In the actual practice we are using CB(Circuit breaker instead of switch), by varying the intial positions of the CB . We are performing the tests by using DSTATCOM.
5.3 Circuit Description
Fig 5.2 Simulink model of D-STATCOM test system
Figure 5.2 shows the basic simulink model of D-STATCOM test system which consists of 3-phase source it produce required 3-ph voltage for supplying to loads which is having ph to ph 230 KV with phase angle of phase A is 0 deg and frequency 0 Hz and this generator internally connected as Yn, with source resistance(RS) 0.1 ohms and source inductance 0.758H . Here the loads are also 3-ph series RL loads where we neglect the capacitance (3-ph Source Function Block shown in fig 5.2.1).
Fig 5.2.1 3-phase Functional Block
DSTATCOM is the static var generator connecter in shunt to the line to mitigate the faults that are occurring in power system in the distribution side. This is connected through CB which is normally in OPEN position, when the fault occurs at that time this CB closes and DSTATCOM comes into picture it mitigates the losses that are occurred.
DSTATCOM consists of a DC voltage source of 19.5KV magnitude, breakers normally in closed position, Universal bridge used to convert the DC stored energy into 3-ph AC voltage and this is helpful to mitigate the faults occurred in the Power system. And it consists of CONTROLLER, Discrete PWM generator and Delay unit. The controller circuit also consist a trigger circuit. Figure 5.2.2 showa alla these blocks that are in the DSTATCOM.