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Seminar
On
STATIC SYNCHRONOUS COMPENSATOR (STATCOM)
SUBMITTED IN PARTIAL FULFILLMENT OF
REQUIREMENT OF THE DEGREE OF
B.Tech – BACHELOR OF TECHNOLOGY
IN
ELECTRICAL ENGINEERING
Supervised by Submitted by
Dr. G.K. Joshi Dimpal Soni
Head Enroll. No. : - 12/22526
Roll No. :-
B.Tech (Electrical Eng.)
DEPARTMENT OF ELECTRICAL ENGINEERING
M.B.M. ENGINEERING COLLAGE
JAI NARAYAN VYAS UNIVERSITY
JODHPUR
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ACKNOWLEDGEMENT
By the blessings of “Lord Shiva”, the academic outcome in the form of a
seminar
work
on
“STATIC
SYNCHRONOUS
COMPENSATOR
(STATCOM)” could take the shape of reality.
It is privilege for me to express my sincere gratitude towards my esteemed
guide without the support of whom, this would have been very difficult for me to
bring out this seminar in this form. I m grateful to Dr. G.K. Joshi (Head of
Department) for all the valuable guidance, constant moral encouragement
extended at his end.
I like to remember the motivation initiated by My Father Shree Brijesh
Soni and My Mother Smt. Meena Soni whose love and latent blessings are the
basis for me to bring out this seminar.
I am thankful to my friend Deepak Soni for his constant encouragement
and all those who helped me directly or indirectly in my endeavor. This
acknowledgement is intended to be a thanks giving gesture to all those people
involved directly or indirectly with my work.
Wednesday, November 13, 2013
Dimpal Soni
Enroll. No. : - 12/22526 Roll No.: -
B.Tech (Electrical Eng.) Department of Electrical Engineering
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CERTIFICATE
This is to certify that Miss Dimpal Soni, B.tech. Scholar in Electrical
engineering bearing Roll no. 111103365 & Enroll. No. 12/22526 has carried out
her seminar on “STATIC SYNCHRONOUS COMPENSATOR (STATCOM)”
under my supervision.
The work presented in this seminar has not been submitted elsewhere for
award of any other degree or diploma.
Dr. G. K. Joshi
Supervisor
Head
Deptt. Of Electrical Eng.
J.N.V. University, Jodhpur
Counter Signed by
Prof. Manoj Kumar Bhaskar
Deptt. Of Electrical Engineering
J.N.V. University, Jodhpur
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LIST OF TABLES
Table No. Particulars Page No.
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LIST OF FIGURES
Fig. No. Particulars Page No.
1.1 Operational Limits of Transmission lines 2 for different voltage levels
1.2 Overview of Major Facts Devices 3
1.3 SVC building blocks and voltage / current
Characteristic 6
1.4 SVC Outlook 6
1.5 SVC using a TCR and FC 7
1.6 Comparison of the loss characteristics of 7
TSC–TCR, TCR–FC compensators and Synchronous condenser
1.7 SVC of combined TSC and TCR type 8
2.1 Reactive power generation by a STATCOM 10
2.2 STATCOM operating in inductive or
capacitive modes 11
2.3 Current controlled block diagram of STATCOM 11
2.4 Voltage controlled block diagram of STATCOM 12
2.5 Static Synchronous Compensator 14
2.6 Waveform for Operation of Statcom 15
2.7 Two machine system with STATCOM 15
2.8 Transmitted power versus transmission 17
angle characteristic of a STATCOM
2.9 V-I characteristic of a STATCOM 18
2.10 STATCOM structure and voltage / 19
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2.11 6 Pulses STATCOM 20
2.12 STATCOM Equivalent Circuit 21
2.13 Substation with a STATCOM 21
3.1 TCSC Circuit and Characteristics 23
3.2 Principal configuration of DFC 24
3.3 Operational diagram of a DFC 25
3.4 Principle configuration of an UPFC 26
3.5 UPFC functional scheme 27
4.1 Thevenin Equivalent Circuit Diagram of 30
STATCOM: (a) STATCOM Schematic Diagram; (b) STATCOM Equivalent Circuit
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Chapter 1 INTRODUCTION
1-8
1.1 INTRODUCTION 1
1.2 FACTS DEVICES 1
1.2.1 Facts for Transmission System 2
1.3 MAJOR FACTS DEVICES 3
1.4 CONFIGURATION OF FACTS DEVICES 4
1.4.1 Shunt Devices 4 1.4.2 SVC 4 1.5 SVC USING A TCR AND AN FC 6 1.5.1 SVC of the FC/TCR type 7 1.6 SVC USING A TCR AND TSC 8
Chapter 2 STATIC SYNCHRONOUS COMPENSATOR
9-21
(STATCOM)
2.1 INTRODUCTION 9
2.2 STRUCTURE OF STATCOM 9
2.3 CONTROL OF STATCOM 10
2.3.1 Two Modes of Operation 10
2.3.2 Current Controlled STATCOM 11
2.3.3 Voltage Controlled STATCOM 12
2.4 BASIC CONFIGURATION AND PRINCIPLE OF OPERATION 13
2.5 CHARACTERISTICS OF STATCOM 15
2.6 STATCOM V-I CHARACTERISTIC 18
2.7 FUNCTIONAL REQUIREMENTS OF STATCOM 18
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3.1 SERIES DEVICES 22
3.2 TCSC 22
3.2.1 Advantages 23
3.3 DYNAMIC POWER FLOW CONTROLLER 23
3.3.1 (TSC / TSR) 24
3.4 UNIFIED POWER FLOW CONTROLLER 26
3.4.1 OPERATING PRINCIPLE OF UPFC 26
Chapter 4 STATIC SYNCHRONOUS COMPENSATOR POWER
29-33
FLOW MODEL
4.1 STATCOM POWER FLOW MODEL 29
4.2 LINEARISED POWER EQUATION 31
4.3 NEWTON-RAPHSON-ALGORITHM 31
Chapter 5 APPLICATIONS, CONCLUSION AND FUTURE WORK 34-35
5.1 APPLICATIONS OF STATCOM 34
5.2 SCOPE FOR FUTURE RESEARCH 34
5.3 CONCLUSION 35
References………36
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CHAPTER: 1
INTRODUCTION
1.1 INTRODUCTION:-
Flexible AC transmission system (FACTS) controllers are power electronics based controllers. With the applications of FACTS technology, bus voltage magnitude and power flow along the transmission lines can be more flexibly controlled. Among the FACTS controllers, the most advanced type is the controller that employs Voltage Sourced Converter (VSC) as synchronous sources. Representative of the VSC type FACTS controllers are the Static Synchronous Compensator (STATCOM), which is a shunt type controller, the Static Series Compensator (SSSC), which is a series type controller and the Unified Power Flow Controller (UPFC), a combined series-shunt type controller. Of all the VSC the most widely used is the STATCOM. It can provide bus voltage magnitude control. Computation and control of power flow for power systems embedded with STATCOM appear to be fundamental for power system analysis and planning purposes. Power flow studies incorporating STATCOM requires accurate model in solution algorithms.
There are mainly two models of STATCOM which have well tested in power systems. There are the Current Injection Model (CIM) and the Power Injection Model (PIM). The CIM STATCOM has a current source connected in shunt the bus for voltage magnitude control. The PIM models the STATCOM as shunt voltage source behind an equivalent reactance or impedance, which is also referred to as voltage source model (VSM). This steady state power injection model of STATCOM has proved reliable when incorporated in power systems and is well documented. The use of this STATCOM in power system simulators has therefore increased over the last one decade and is therefore adopted implementation in this work with the voltage expressed in rectangular coordinate.
1.2 FACTS DEVICES:-
Flexible AC Transmission Systems, called FACTS, got in the recent years a well known term for higher controllability in power systems by means of power electronic devices. Several FACTS-devices have been introduced for various applications worldwide. A number of new types of devices are in the stage of being introduced in practice.
In most of the applications the controllability is used to avoid cost intensive or landscape requiring extensions of power systems, for instance like upgrades or additions of substations and power lines. FACTS-devices provide a better adaptation to varying operational conditions and improve the usage of existing installations. The basic applications of FACTS-devices are: • Power flow control,
Page | 10 • Voltage control,
• Reactive power compensation, • Stability improvement,
• Power quality improvement, • Power conditioning,
• Flicker mitigation,
• Interconnection of renewable and distributed generation and storages.
1.2.1 FACTS FOR TRANSMISSION SYSTEM:-
Figure 1.1 shows the basic idea of FACTS for transmission systems. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different FACTS devices. It can be seen that with growing line length, the opportunity for FACTS devices gets more and more important.
The influence of FACTS-devices is achieved through switched or controlled shunt compensation, series compensation or phase shift control. The devices work electrically as fast current, voltage or impedance controllers. The power electronic allows very short reaction times down to far below one second.
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1.3 MAJOR FACTS DEVICES:-
The development of FACTS-devices has started with the growing capabilities of power electronic components. Devices for high power levels have been made available in converters for high and even highest voltage levels. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. Figure 1.2 shows a number of basic devices separated into the conventional ones and the FACTS-devices. For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some explanation. The term 'dynamic' is used to express the fast controllability of FACTS-devices provided by the power electronics. This is one of the main differentiation factors from the conventional devices. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. Therefore most of the FACTS-devices can equally be static and dynamic.
Page | 12 The left column in Figure 1.2 contains the conventional devices build out of fixed or mechanically switch able components like resistance, inductance or capacitance together with transformers. The FACTS-devices contain these elements as well but use additional power electronic valves or converters to switch the elements in smaller steps or with switching patterns within a cycle of the alternating current. The left column of FACTS-devices uses Thyristor valves or converters. These valves or converters are well known since several years. They have low losses because of their low switching frequency of once a cycle in the converters or the usage of the Thyristors to simply bridge impedances in the valves.
The right column of FACTS-devices contains more advanced technology of voltage source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or Insulated Gate Commutated Thyristors (IGCT). Voltage Source Converters provide a free controllable voltage in magnitude and phase due to a pulse width modulation of the IGBTs or IGCTs. High modulation frequencies allow to get low harmonics in the output signal and even to compensate disturbances coming from the network. The disadvantage is that with an increasing switching frequency, the losses are increasing as well. Therefore special designs of the converters are required to compensate this.
1.4 CONFIGURATION OF FACTS DEVICES
: 1.4.1 Shunt Devices:The most used FACTS-device is the SVC or the version with Voltage Source Converter called STATCOM. These shunt devices are operating as reactive power compensators. The main applications in transmission, distribution and industrial networks are:
Page | 13 • Reduction of unwanted reactive power flows and therefore reduced network losses.
• Keeping of contractual power exchanges with balanced reactive power.
• Compensation of consumers and improvement of power quality especially with huge demand fluctuations like industrial machines, metal melting plants, railway or underground train systems.
• Compensation of Thyristor converters e.g. in conventional HVDC lines. • Improvement of static or transient stability.
Almost half of the SVC and more than half of the STATCOMs are used for industrial applications. Industry as well as commercial and domestic groups of users require power quality. Flickering lamps are no longer accepted, nor are interruptions of industrial processes due to insufficient power quality. Railway or underground systems with huge load variations require SVCs or STATCOMs.
1.4.2 SVC:
Electrical loads both generate and absorb reactive power. Since the transmitted load varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations or even a voltage depression, at the extreme a voltage collapse.
A rapidly operating Static Var Compensator (SVC) can continuously provide the reactive power required to control dynamic voltage oscillations under various system conditions and thereby improve the power system transmission and distribution stability.
Applications of the SVC systems in transmission systems:
a. To increase active power transfer capacity and transient stability margin b. To damp power oscillations
c. To achieve effective voltage control In addition, SVCs are also used 1. in transmission systems
a. To reduce temporary over voltages b. To damp sub synchronous resonances
Page | 14 2. in traction systems
a. To balance loads
b. To improve power factor c. To improve voltage regulation 3. In HVDC systems
a. To provide reactive power to ac–dc converters 4. In arc furnaces
a. To reduce voltage variations and associated light flicker
Installing an SVC at one or more suitable points in the network can increase transfer capability and reduce losses while maintaining a smooth voltage profile under different network conditions. In addition an SVC can mitigate active power oscillations through voltage amplitude modulation.
SVC installations consist of a number of building blocks. The most important is the Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide controllability. Air core reactors and high voltage AC capacitors are the reactive power elements used together with the Thyristor valves. The step up connection of this equipment to the transmission voltage is achieved through a power transformer.
Fig 1.3 SVC building blocks and voltage / current characteristic
In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor Switched or Controlled Reactors (TSR / TCR). The coordinated control of a combination of these branches varies the reactive power as shown in Figure. The first commercial SVC was installed in 1972 for an electric arc furnace. On transmission level the first SVC was used in 1979. Since then it is widely used and the most accepted FACTS-device.
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Fig. 1.4 SVC Outlook
1.5 SVC USING A TCR AND AN FC:
In this arrangement, two or more FC (fixed capacitor) banks are connected to a TCR (thyristor controlled reactor) through a step-down transformer. The rating of the reactor is chosen larger than the rating of the capacitor by an amount to provide the maximum lagging vars that have to be absorbed from the system. By changing the firing angle of the thyristor controlling the reactor from 90° to 180°, the reactive power can be varied over the entire range from maximum lagging vars to leading vars that can be absorbed from the system by this compensator.
Fig.1.5 SVC using a TCR and FC
Page | 16 The main disadvantage of this configuration is the significant harmonics that will be generated because of the partial conduction of the large reactor under normal sinusoidal steady-state operating condition when the SVC is absorbing zero MVAr. These harmonics are filtered in the following manner. Triplex harmonics are canceled by arranging the TCR and the secondary windings of the step-down transformer in delta connection. The capacitor banks with the help of series reactors are tuned to filter fifth, seventh, and other higher-order harmonics as a high-pass filter. Further losses are high due to the circulating current between the reactor and capacitor banks.
Fig.1.6 Comparison of the loss characteristics of TSC–TCR, TCR–FC compensators and synchronous condenser
These SVCs do not have a short-time overload capability because the reactors are usually of theair-core type. In applications requiring overload capability, TCR must be designed for short-time overloading, or separate thyristor-switched overload reactors must be employed.
1.6 SVC USING A TCR AND TSC:-
This compensator overcomes two major shortcomings of the earlier compensators by reducing losses under operating conditions and better performance under large system disturbances. In view of the smaller rating of each capacitor bank, the rating of the reactor bank will be 1/n times the maximum output of the SVC, thus reducing the harmonics generated by the reactor. In those situations where harmonics have to be reduced further, a small amount of FCs tuned as filters may be connected in parallel with the TCR.
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Fig. 1.7 SVC of combined TSC and TCR type
When large disturbances occur in a power system due to load rejection, there is a possibility for large voltage transients because of oscillatory interaction between system and the SVC capacitor bank or the parallel. The LC circuit of the SVC in the FC compensator. In the TSC–TCR scheme, due to the flexibility of rapid switching of capacitor banks without appreciable disturbance to the power system, oscillations can be avoided, and hence the transients in the system can also be avoided. The capital cost of this SVC is higher than that of the earlier one due to the increased number of capacitor switches and increased control complexity.
CHAPTER: 2
STATIC SYNCHRONOUS COMPENSATOR (STATCOM)
2.1 INTRODUCTION:-
The STATCOM is a solid-state-based power converter version of the SVC. Operating as a shunt-connected SVC, its capacitive or inductive output currents can be controlled independently from its terminal AC bus voltage. Because of the fast-switching characteristic of power converters, STATCOM provides much faster response as compared to the SVC. In
Page | 18 addition, in the event of a rapid change in system voltage, the capacitor voltage does not change instantaneously; therefore, STATCOM effectively reacts for the desired responses. For example, if the system voltage drops for any reason, there is a tendency for STATCOM to inject capacitive power to support the dipped voltages.
STATCOM is capable of high dynamic performance and its compensation does not depend on the common coupling voltage. Therefore, STATCOM is very effective during the power system disturbances.
Moreover, much research confirms several advantages of STATCOM. These advantages compared to other shunt compensators include:
• Size, weight, and cost reduction • Equality of lagging and leading output
• Precise and continuous reactive power control with fast response • Possible active harmonic filter capability
This chapter describes the structure, basic operating principle and characteristics of STATCOM. In addition, the concept of voltage source converters and the corresponding control techniques are illustrated.
2.2 STRUCTURE OF STATCOM:-
Basically, STATCOM is comprised of three main parts (as seen from Figure below): a voltage source converter (VSC), a step-up coupling transformer, and a controller. In a very-high-voltage system, the leakage inductances of the step-up power transformers can function as coupling reactors. The main purpose of the coupling inductors is to filter out the current harmonic components that are generated mainly by the pulsating output voltage of the power converters.
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Fig. 2.1 Reactive power generation by a STATCOM
2.3 CONTROL OF STATCOM
The controller of a STATCOM operates the converter in a particular way that the phase angle between the converter voltage and the transmission line voltage is dynamically adjusted and synchronized so that the STATCOM generates or absorbs desired VAR at the point of coupling connection. Figure 3.4 shows a simplified diagram of the STATCOM with a converter voltage source __1E and a tie reactance, connected to a system with a voltage source, and a Thevenin reactance, XTIEX_THVTH.
2.3.1 Two Modes of Operation
There are two modes of operation for a STATCOM, inductive mode and the capacitive mode. The STATCOM regards an inductive reactance connected at its terminal when the converter voltage is higher than the transmission line voltage. Hence, from the system’s point of view, it regards the STATCOM as a capacitive reactance and the STATCOM is considered to be operating in a capacitive mode. Similarly, when the system voltage is higher than the converter voltage, the system regards an inductive reactance connected at its terminal. Hence, the STATCOM regards the system as a capacitive reactance and the STATCOM is considered to be operating in an inductive mode
Page | 20 .
Fig. 2.2 STATCOM operating in inductive or capacitive modes
In other words, looking at the phasor diagrams on the right of Figure 3.4, when1I, the reactive current component of the STATCOM, leads (THVE−1) by 90º, it is in inductive mode and when it lags by 90º, it is in capacitive mode.
This dual mode capability enables the STATCOM to provide inductive compensation as well as capacitive compensation to a system. Inductive compensation of the STATCOM makes it unique. This inductive compensation is to provide inductive reactance when overcompensation due to capacitors banks occurs. This happens during the night, when a typical inductive load is about 20% of the full load, and the capacitor banks along the transmission line provide with excessive capacitive reactance due to the lower load. Basically the control system for a STATCOM consists of a current control and a voltage control.
2.3.2 Current Controlled STATCOM
Page | 21 Figure above shows the reactive current control block diagram of the STATCOM. An instantaneous three-phase set of line voltages, vl, at BUS 1 is used to calculate the reference
angle, θ, which is phase-locked to the phase a of the line voltage, vla . An instantaneous
three-phase set of measured converter currents, il, is decomposed into its real or direct component,
I1d, and reactive or quadrature component, I1q, respectively. The quadrature component is
compared with the desired reference value, I1q* and the error is passed through an error
amplifier which produces a relative angle, α, of the converter voltage with respect to the transmission line voltage. The phase angle, θ1, of the converter voltage is calculated by adding
the relative angle, α, of the converter voltage and the phase – lock-loop angle, θ. The reference quadrature component, I1q*, of the converter current is defined to be either positive if the
STATCOM is emulating an inductive reactance or negative if it is emulating a capacitive reactance. The DC capacitor voltage, vDC, is dynamically adjusted in relation with the converter
voltage. The control scheme described above shows the implementation of the inner current control loop which regulates the reactive current flow through the STATCOM regardless of the line voltage.
2.3.3 Voltage Controlled STATCOM
In regulating the line voltage, an outer voltage control loop must be implemented. The outer voltage control loop would automatically determine the reference reactive current for the inner current control loop which, in turn, will regulate the line voltage.
Fig. 2.4 Voltage controlled block diagram of STATCOM
Figure shows a voltage control block diagram of the STATCOM. An instantaneous three-phase set of measured line voltages, v1, at BUS 1 is decomposed into its real or direct component,
V1d, and reactive or quadrature component, V1q, is compared with the desired reference value,
V1*, (adjusted by the droop factor, Kdroop) and the error is passed through an error amplifier
which produces the reference current, I1q*, for the inner current control loop. The droop factor,
Kdroop, is defined as the allowable voltage error at the rated reactive current flow through the
Page | 22
2.4 BASIC CONFIGURATION AND PRINCIPLE OF OPERATION
Basically, shunt connected FACTS device can be realized by either a VSC or a CSC. But the VSC topology is preferred because CSC topology is more complex than VSC in both power and control circuits. In CSC such as GTO (Gate Turn Off Thyristor) is used, a diode has to be placed in series with each of the switches. This almost doubles the conduction losses compared with the case of VSC. The DC link energy storage element in CSC topology is inductor where as that in VSC topology is a capacitor. Thus, the efficiency of a CSC is expected to be lower than that of a VSC. The modeled STATCOM using VSC topology is being used in the test system to supply reactive power to increase the transmittable power and to make it more compatible with the prevailing load demand.
Thus, the shunt connected FACTS device should be able to minimize the line over voltage under light load condition and maintain voltage levels under heavy load condition. Two VSC technologies can be used for the VSC. One of them, VSC is constructed with IGBT/GTO-based SPWM inverters. This type of inverter uses sinusoidal Pulse-Width Modulation (SPWM) technique to synthesize a sinusoidal waveform from a DC voltage source with a typical chopping frequency of a few kilohertz. Harmonic voltages are cancelled by connecting filters at the AC side of the VSC.
This type of VSC uses a DC link voltage Vdc. Output voltage is varied by changing the modulation index of the SPWM modulator. Thus modulation index has to be varied for controlling the reactive power injection to the transmission line. In another type VSC is constructed with GTO-based square-wave inverters and special interconnection transformers. Typically four three-level inverters are used to build a 48-step voltage waveform. Special interconnection transformers are used to neutralize harmonics contained in the square waves generated by individual inverters. In this type of VSC, the fundamental component of output voltage is proportional to the voltage Vdc. Therefore Vdc has to be varied for controlling the reactive power.
The shunt controller is like a current source, which draws from or injects current into the system at the point of connection. The shunt controller may be variable impedance, variable source or a combination of these. Variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes reactive power. When system voltage is low, the STATCOM generates reactive power (STATCOM capacitive). When system voltage is high, it absorbs reactive power (STATCOM inductive).
The variation of reactive power is performed by means of a VSC connected on the secondary side of a coupling transformer. The VSC uses forced-commutated power electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage V2 from a DC voltage source. Any other phase relationship will involve handling of real power as well. So, the shunt controller is therefore a good way to control the voltage at and around the point of connection through injection of reactive current (leading or lagging) alone or a combination of active and reactive current for a more effective voltage control and damping of voltage dynamics.
Page | 23 The real power (P) and reactive power (Q) are given by:
Fig. 2.5 Static Synchronous Compensator
E is the line voltage of transmission line. V is the generated voltage of VSC. X is the equivalent reactance of interconnection transformer and filters and δ is the phase angle of E with respect to V.
In steady state operation, the voltage V generated by the VSC is in phase with E (δ=0), so that only reactive power is flowing (P=0). If V is lower than E, Q is flowing from E to V (STATCOM is absorbing reactive power). On the reverse, if V is higher than E, Q is flowing from V to E (STATCOM is generating reactive power).
Since we are using here a VSC based on SPWM inverters hence modulation index is varied for controlling the reactive power injection to the transmission line. A capacitor is connected on the DC side of the VSC acts as a DC voltage source. In steady state the voltage V has to be phase shifted slightly behind E in order to compensate for transformer and VSC losses and to keep the capacitor charged.
Page | 24
Fig. 2.6 Waveform for Operation of Statcom
2.5 CHARACTERISTICS OF STATCOM
The derivation of the formula for the transmitted active power employs considerable calculations. Using the variables defined in Figure below and applying Kirchoffs laws the following equations can be written;
Page | 25 By equaling right-hand terms of the above formulas, a formula for the current I1 is obtained as
Where UR is the STATCOM terminal voltage if the STATCOM is out of operation, i.e. when Iq
= 0. The fact that Iq is shifted by 90◦ with regard to UR can be used to express Iq as
Applying the sine law to the diagram in Figure below the following two equations result
Page | 26 The formula for the transmitted active power can be given as
To dispose of the term UR the cosine law is applied to the diagram in Figure above Therefore,
Fig. 2.8 Transmitted power versus transmission angle characteristic of a STATCOM
With these concepts of STATCOM, it is thus important to utilize these principles in accommodating shunt compensation to any system. Since this thesis only reflects on the voltage control and power increase, the requirements of the STATCOM would be further elaborated.
Page | 27
2.6 STATCOM V-I CHARACTERISTIC:-
A V-I characteristic of a STATCOM is depicted in Fig.2.9 . As can be seen, the STATCOM can supply both the capacitive and the inductive compensation and is able to independently control its output current over the rated maximum capacitive or inductive range irrespective of the amount of ac-system voltage. That is, the STATCOM
can provide full capacitive-reactive power at any system voltage even as low as 0.15 pu. The characteristic of a STATCOM reveals strength of this technology: that it is capable of yielding the full output of capacitive generation almost independently of the system voltage (constant-current output at lower voltages). This capability is particularly useful for situations in which the STATCOM is needed to support the system voltage during and after faults where voltage collapse would otherwise be a limiting factor.
Fig. 2.9 V-I characteristic of a STATCOM
2.7 FUNCTIONAL REQUIREMENTS OF STATCOM:-
The main functional requirements of the STATCOM in this thesis are to provide shunt compensation, operating in capacitive mode only, in terms of the following;
• Voltage stability control in a power system, as to compensate the loss voltage along transmission. This compensation of voltage has to be in synchronism with the AC system regardless of disturbances or change of load.
Page | 28 • Direct voltage support to maintain sufficient line voltage for facilitating increased reactive power flow under heavy loads and for preventing voltage instability
• Reactive power injection by STATCOM into the system
The design phase and implementation phase (as presented in the next chapter) would refer to the theoretical background of STATCOM in providing the requirements
In 1999 the first SVC with Voltage Source Converter called STATCOM (STATic COMpensator) went into operation. The STATCOM has a characteristic similar to the synchronous condenser, but as an electronic device it has no inertia and is superior to the synchronous condenser in several ways, such as better dynamics, a lower investment cost and lower operating and maintenance costs. A STATCOM is build with Thyristors with turn-off capability like GTO or today IGCT or with more and more IGBTs. The static line between the current limitations has a certain steepness determining the control characteristic for the voltage. The advantage of a STATCOM is that the reactive power provision is independent from the actual voltage on the connection point. This can be seen in the diagram for the maximum currents being independent of the voltage in comparison to the SVC. This means, that even during most severe contingencies, the STATCOM keeps its full capability.
In the distributed energy sector the usage of Voltage Source Converters for grid interconnection is common practice today. The next step in STATCOM development is the combination with energy storages on the DC-side. The performance for power quality and balanced network operation can be improved much more with the combination of active and reactive power.
Page | 29 STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either Gate-Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices.
The STATCOM is a very fast acting, electronic equivalent of a synchronous condenser. If the STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc) is larger than bus voltage, Es, then leading or capacitive VARS are produced. If Vs is smaller then Es then lagging or inductive VARS are produced.
Fig 2.11 6 Pulses STATCOM
The three phases STATCOM makes use of the fact that on a three phase, fundamental frequency, steady state basis, and the instantaneous power entering a purely reactive device must be zero.
The reactive power in each phase is supplied by circulating the instantaneous real power between the phases. This is achieved by firing the GTO/diode switches in a manner that maintains the phase difference between the ac bus voltage ES and the STATCOM generated voltage VS. Ideally it is possible to construct a device based on circulating instantaneous power which has no energy storage device (ie no dc capacitor).
A practical STATCOM requires some amount of energy storage to accommodate harmonic power and ac system unbalances, when the instantaneous real power is non-zero. The maximum energy storage required for the STATCOM is much less than for a TCR/TSC type of SVC compensator of comparable rating.
Page | 30
Fig. 2.12 STATCOM Equivalent Circuit
Several different control techniques can be used for the firing control of the STATCOM. Fundamental switching of the GTO/diode once per cycle can be used. This approach will minimize switching losses, but will generally utilize more complex transformer topologies. As an alternative, Pulse Width Modulated (PWM) techniques, which turn on and off the GTO or IGBT switch more than once per cycle, can be used. This approach allows for simpler transformer topologies at the expense of higher switching losses.
The 6 Pulse STATCOM using fundamental switching will of course produce the 6 Nth
harmonics. There are a variety of methods to decrease the harmonics. These methods include the basic 12 pulse configuration with parallel star / delta transformer connections, a complete elimination of 5th and 7th harmonic current using series connection of star/star and star/delta transformers and a quasi 12 pulse method with a single star-star transformer, and two secondary windings, using control of firing angle to produce a 30 degree phase shift between the two 6 pulse bridges. This method can be extended to produce a 24 pulse and a 48 pulse STATCOM, thus eliminating harmonics even further. Another possible approach for harmonic cancellation is a multi-level configuration which allows for more than one switching element per level and therefore more than one switching in each bridge arm. The ac voltage derived has a staircase effect, dependent on the number of levels. This staircase voltage can be controlled to eliminate harmonics.
Page | 31
CHAPTER: 3
OTHER SERIES AND SHUNT DEVICES
3.1 SERIES DEVICES:-
Series devices have been further developed from fixed or mechanically switched compensations to the Thyristor Controlled Series Compensation (TCSC) or even Voltage Source Converter based devices.
The main applications are:
• Reduction of series voltage decline in magnitude and angle over a power line,
• Reduction of voltage fluctuations within defined limits during changing power transmissions, • Improvement of system damping resp. damping of oscillations,
• Limitation of short circuit currents in networks or substations, • Avoidance of loop flows resp. power flow adjustments.
3.2 TCSC:-
Thyristor Controlled Series Capacitors (TCSC) address specific dynamical problems in transmission systems. Firstly it increases damping when large electrical systems are interconnected. Secondly it can overcome the problem of Sub Synchronous Resonance (SSR), a phenomenon that involves an interaction between large thermal generating units and series compensated transmission systems.
The TCSC's high speed switching capability provides a mechanism for controlling line power flow, which permits increased loading of existing transmission lines, and allows for rapid readjustment of line power flow in response to various contingencies. The TCSC also can regulate steady-state power flow within its rating limits.
From a principal technology point of view, the TCSC resembles the conventional series capacitor. All the power equipment is located on an isolated steel platform, including the Thyristor valve that is used to control the behavior of the main capacitor bank. Likewise the control and protection is located on ground potential together with other auxiliary systems. Figure shows the principle setup of a TCSC and its operational diagram. The firing angle and the thermal limits of the Thyristors determine the boundaries of the operational diagram.
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Fig. 3.1 TCSC Circuit and Characteristics
3.2.1 Advantages
Continuous control of desired compensation level Direct smooth control of power flow within the network Improved capacitor bank protection
Local mitigation of sub synchronous resonance (SSR). This permits higher levels of compensation in networks where interactions with turbine-generator torsional vibrations or with other control or measuring systems are of concern.
Damping of electromechanical (0.5-2 Hz) power oscillations which often arise between areas in a large interconnected power network. These oscillations are due to the dynamics of inter area power transfer and often exhibit poor damping when the aggregate power tranfer over a corridor is high relative to the transmission strength.
3.3 DYNAMIC POWER FLOW CONTROLLER:-
A new device in the area of power flow control is the Dynamic Power Flow Controller (DFC). The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched series compensation.
A functional single line diagram of the Dynamic Flow Controller is shown in Figure 3.2. The Dynamic Flow Controller consists of the following components:
• A standard phase shifting transformer with tap-changer (PST) • Series-connected Thyristor Switched Capacitors and Reactors
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3.3.1 (TSC / TSR)
• A mechanically switched shunt capacitor (MSC). (This is optional depending on the system reactive power requirements)
Fig.3.2 Principal configuration of DFC
Based on the system requirements, a DFC might consist of a number of series TSC or TSR. The mechanically switched shunt capacitor (MSC) will provide voltage support in case of overload and other conditions. Normally the reactance of reactors and the capacitors are selected based on a binary basis to result in a desired stepped reactance variation. If a higher power flow resolution is needed, a reactance equivalent to the half of the smallest one can be added.
The switching of series reactors occurs at zero current to avoid any harmonics. However, in general, the principle of phase-angle control used in TCSC can be applied for a continuous control as well. The operation of a DFC is based on the following rules:
• TSC / TSR are switched when a fast response is required.
• The relieve of overload and work in stressed situations is handled by the TSC / TSR.
• The switching of the PST tap-changer should be minimized particularly for the currents higher than normal loading.
• The total reactive power consumption of the device can be optimized by the operation of the MSC, tap changer and the switched capacities and reactors.
In order to visualize the steady state operating range of the DFC, we assume an inductance in parallel representing parallel transmission paths. The overall control objective in steady state would be to control the distribution of power flow between the branch with the DFC and the parallel path. This control is accomplished by control of the injected series voltage.
The PST (assuming a quadrature booster) will inject a voltage in quadrature with the node voltage. The controllable reactance will inject a voltage in quadrature with the throughput current. Assuming that the power flow has a load factor close to one, the two parts of the series voltage will be close to collinear. However, in terms of speed of control, influence on reactive power balance and effectiveness at high/low loading the two parts of the series voltage has
Page | 34 quite different characteristics. The steady state control range for loadings up to rated current is illustrated in Figure 3.3 , where the x-axis corresponds to the throughput current and the y-axis corresponds to the injected series voltage.
Fig3.3. Operational diagram of a DFC
Operation in the first and third quadrants corresponds to reduction of power through the DFC, whereas operation in the second and fourth quadrants corresponds to increasing the power flow through the DFC. The slope of the line passing through the origin (at which the tap is at zero and TSC / TSR are bypassed) depends on the short circuit reactance of the PST.
Starting at rated current (2 kA) the short circuit reactance by itself provides an injected voltage (approximately 20 kV in this case). If more inductance is switched in and/or the tap is increased, the series voltage increases and the current through the DFC decreases (and the flow on parallel branches increases). The operating point moves along lines parallel to the arrows in the figure. The slope of these arrows depends on the size of the parallel reactance. The maximum series voltage in the first quadrant is obtained when all inductive steps are switched in and the tap is at its maximum.
Now, assuming maximum tap and inductance, if the throughput current decreases (due e.g. to changing loading of the system) the series voltage will decrease. At zero current, it will not matter whether the TSC / TSR steps are in or out, they will not contribute to the series voltage. Consequently, the series voltage at zero current corresponds to rated PST series voltage.
Next, moving into the second quadrant, the operating range will be limited by the line corresponding to maximum tap and the capacitive step being switched in (and the inductive steps by-passed). In this case, the capacitive step is approximately as large as the short circuit reactance of the PST, giving an almost constant maximum voltage in the second quadrant.
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3.4 UNIFIED POWER FLOW CONTROLLER:-
The UPFC is a combination of a static compensator and static series compensation. It acts as a shunt compensating and a phase shifting device simultaneously.
Fig3.4. Principle configuration of an UPFC
The UPFC consists of a shunt and a series transformer, which are connected via two voltage source converters with a common DC-capacitor. The DC-circuit allows the active power exchange between shunt and series transformer to control the phase shift of the series voltage. This setup, as shown in Figure 1.21, provides the full controllability for voltage and power flow. The series converter needs to be protected with a Thyristor bridge. Due to the high efforts for the Voltage Source Converters and the protection, an UPFC is getting quite expensive, which limits the practical applications where the voltage and power flow control is required simultaneously.
3.4.1 OPERATING PRINCIPLE OF UPFC
The basic components of the UPFC are two voltage source inverters (VSIs) sharing a common dc storage capacitor, and connected to the power system through coupling transformers. One VSI is connected to in shunt to the transmission system via a shunt transformer, while the other one is connected in series through a series transformer.
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Fig.3.5. UPFC functional scheme
The series inverter is controlled to inject a symmetrical three phase voltage system (Vse), of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is equal only to the losses of the inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point.
The two VSI’s can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power low on the transmission line.
The UPFC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current, ish into the transmission line. The shunt inverter can be controlled in two different modes:
VAR Control Mode: The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the var reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, Vdc, is also required.
Page | 37 Automatic Voltage Control Mode: The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value. For this mode of control, voltage feedback signals are obtained from the sending end bus feeding the shunt coupling transformer.
The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. The actual value of the injected voltage can be obtained in several ways.
Direct Voltage Injection Mode: The reference inputs are directly the magnitude and phase angle of the series voltage. Phase Angle Shifter Emulation mode: The reference input is phase displacement between the sending end voltage and the receiving end voltage. Line Impedance Emulation mode: The reference input is an impedance value to insert in series with the line impedance. Automatic Power Flow Control Mode: The reference inputs are values of P and Q to maintain on the transmission line despite system changes.
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CHAPTER: 4
STATIC SYNCHRONOUS COMPENSATOR POWER FLOW MODEL
4.1 STATCOM POWER FLOW MODEL
The STATCOM is a FACTS controller based on voltage sourced converter (VSC). A VSC generate a synchronous voltage of fundamental frequency, controllable magnitude and phase angle.
If a VSC is shunt-connected to a system via a coupling transformer as shown in Fig. 4.1, the resulting STATCOM can inject or absorb reactive power to or from the bus to which it is connected and thus regulate the bus voltage magnitude. This STATCOM model is known as Power Injection Model (PIM) or Voltage Source Model (VSM). Steady state modeling of STATCOM within the Newton-Raphson method in rectangular co-ordinates is carried out as follows:
The Thevenin equivalent circuit representing the fundamental frequency operation of the switched-mode voltage sourced converter and its transformer is shown in Figure 4.1
(1) is expressed in Norton equivalent form
(2) (2) where
In these expressions, Vk represents bus k voltage and Vstc represents the voltage source inverter.
IN is the Norton’s current while Istc is the inverter’s current. Also, Z SC and Y SC are the
transformer’s impedance and short-circuit admittance respectively. The STATCOM voltage injection V STC bound constraints is as follows:
(3)
Where VSTC min and VSTC max are the STATCOM’s minimum and maximum voltages.
The current expression in (2) is transformed into a power expression by the VSC and power injected into bus k as shown in equations (4) and (5) respectively.
(4)
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Fig. 4.1 Thevenin Equivalent Circuit Diagram of STATCOM: (a) STATCOM Schematic Diagram; (b) STATCOM Equivalent Circuit
Using the rectangular coordinate representation,
Where V STC and δ STC are the STATCOM voltage magnitude and angle respectively. e k and f k
are the real and imaginary parts of the bus voltage respectively.
e STC and f STC are the real and imaginary parts of the STATCOM voltage respectively. The
active and reactive powers for the STATCOM and node k respectively are: (6)
Page | 40 And
(8)
(9)
4.2 LINEARISED POWER EQUATION
A single-phase power network with n-buses is described by 2×(n-1) non-linear equations. The inclusion of one STATCOM model augments the number of equations by two. The solution of the combined system of non-linear equations is carried out by iteration using the full
Newton-Raphson method.
The Jacobian used in conventional power flow is suitably extended to take account of the new elements contributed by the STATCOM. The set of linearised power flow equations for the complete system is
The Jacobian elements in equation (10) are given in table 4.1 ahead.
4.3 NEWTON-RAPHSON-ALGORITHM
1. We assume a suitable solution for all the buses except the slack bus. We assume a flat voltage profile i.e. Vp=1.0+j0.0 for p=1,2,…,n, p≠s, Vs=a+j0.0.
2. We then set a convergence criterion = ε i.e. if the largest of absolute of the residues exceeds ε, the process is repeated, or else its terminated.
3. Set the iteration count K=0. 4. Set the bus count p=1.
Page | 41 6. Calculate the real and reactive powers Pp and Qp respectively, using the equations derived for the same earlier.
7. Evaluate
8. Check if the bus p is a generator bus. If that is the case, compare Qkp with the limits. If it
exceeds the limits, fix the reactive power generation to the corresponding limit and treat the bus as a load bus for that iteration and go to the next step. If lower limit is violated, set Qsp=Qp min. If
the limit is not violated evaluate the voltage residue.
9. Evaluate
10. Increment the bus count by 1, i.e. p = p+1 and finally check if all the buses have been taken into consideration. Or else, go back to step 5.
11. Determine the largest value among the absolute value of residue.
12. If the largest of the absolute value of the residue is less than ε, go to step 17. 13. Evaluate the Jacobian matrix elements.
14. Calculate the voltage increments
15. Calculate the new bus voltage Evaluate cosδ and sinδ
of all voltages.
16. Advance iteration count K=K+1 and go back to step 4. 17. Evaluate bus and line powers and output the results.
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TABLE 1
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CHAPTER: 5
APPLICATIONS, CONCLUSION AND FUTURE WORK
5.1 APPLICATIONS OF STATCOM
Usually a STATCOM is installed to support electricity networks that have a poor power factor and often poor voltage regulation. There are however, other uses, the most common use is for voltage stability. A STATCOM is a voltage source converter (VSC)-based device, with the voltage source behind a reactor. The voltage source is created from a DC capacitor and therefore a STATCOM has very little active power capability. However, its active power capability can be increased if a suitable energy storage device is connected across the DC capacitor. The reactive power at the terminals of the STATCOM depends on the amplitude of the voltage source. For example, if the terminal voltage of the VSC is higher than the AC voltage at the point of connection, the STATCOM generates reactive current; on the other hand, when the amplitude of the voltage source is lower than the AC voltage, it absorbs reactive power. The response time of a STATCOM is shorter than that of an SVC, mainly due to the fast switching times provided by the IGBTs of the voltage source converter. The STATCOM also provides better reactive power support at low AC voltages than an SVC, since the reactive power from a STATCOM decreases linearly with the AC voltage (as the current can be maintained at the rated value even down to low AC voltage).
STATCOM has following applications in controlling power system dynamics. Damping of power system oscillations.
Damping of subsynchronous oscillations. Balanced loading of individual phases.
Reactive compensations of AC-DC converters and HVDC links. Improvement of transient stability margin.
Improvement of steady-state power transfer capacity. Reduction of temporary over-voltages.
Effective voltage regulation and control.
Reduction of rapid voltage fluctuations (flicker control).
5.2 SCOPE FOR FUTURE RESEARCH
Although this research has covered most of the interesting issues and challenges of the advanced STATCOM and several aspects of the integration of ESS into STATCOM, there are certain aspects that might be interesting for future investigations which are given below:
Due to the excessive number of semiconductor devices and passive components, a fault protection scheme to enhance the ride-though capability in various faults scenarios remains as an important challenge
In the investigation of the interface topology, the ES was assumed to be charged to a voltage level that is not higher than the DC-side voltage of the VSC. It might be valuable to
Page | 44 investigate the possibility of charging ES to a higher extent and the related issues such as protection issues.
Research on the CMC based topology with ESS can be implemented for real and reactive power compensation in wind farms with FSIGs or Double Fed Induction Generator
5.3 CONCLUSION
Among FACTS controllers, the shunt controller STATCOM have shown feasibility in terms of cost effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. A comparison between the STATCOM and the SVC is made and based on several aspects it is concluded that a STATCOM is more preferred when compared to SVC and other compensation devices. Instead of directly deriving reactive power from the energy storage components, the STATCOM basically circulates power with the connected network. Therefore, the reactive components used in the STATCOM are much smaller than those in the SVC.
The location of the shunt FACTS device depends on the application for which it is installed. Shunt compensation FACTS devices are installed at the end points of transmission lines (buses) when used for applications, such as bus voltage regulation and improving HVDC link performance, etc. However, from simulation results it is observed that for increasing the power transfer capability of long transmission lines (tie lines connecting two major grids), midpoint of the lines is the best location for shunt connected multi pulse STATCOM device. When connected at the midpoint the real power is improved and the load ability margin. The midpoint sitting of STATCOM also facilitates the independent control of reactive power at both the ends of the transmission line. For a given voltage limit, the midpoint sitting controls a larger reactive power because each side of the STATCOM device addresses only half the line impedance and not the full line impedance as in the case of the transmission line receiving end sitting and sending end sitting. The simulation study shows that a STATCOM with real power capability can improve the real power and enhance load stability margin, damp the power system oscillations ore effectively and stabilize the system faster if the STATCOM-SMES controller is located at the midpoint. Various concepts regarding the FACTS technology and the important features of some of the FACTS devices have been presented. The Newton raphson method has been presented to solve the power flow problem in the power system with static synchronous compensator (STATCOM).
The study of the basic principles of the STATCOM is carried out as well as the basics of reactive power compensation using a STATCOM. A power flow model of the STATCOM is attempted and it is seen that the modified load flow equations help the system in better performance. The bus system shows improved plots and the thus we can conclude that the addition of a STATCOM controls the output of a bus in a robust manner.
Hence our objective to maintain voltage stability has been successfully achieved with the incorporation of Static Synchronous Compensator (STATCOM).
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