Introduction to FACTS
The AC transmission system has various limits classified as static limits and dynamic limits. These inherent power system limits restrict the power transaction, which lead to the underutilization of the existing transmission resources. Traditionally, fixed or mechanically switched shunt and series capacitors, reactors and synchronous generators were being used to solve much of the problem. However, there are restrictions as to the use of these conventional devices. Desired performance was not being able to achieve effectively. Wear and tear in the mechanical components and slow response were the heart of the problems. There was greater need for the alternative technology made of solid state devices with fast response characteristics. The need was further fueled by worldwide restructuring of electric utilities, increasing environmental and efficiency regulations and difficulty in getting permit and right of way for the construction of overhead transmission lines. This, together with the invention of Thyristor switch (semiconductor device), opened the door for the development of power electronics devices known as Flexible AC Transmission Systems (FACTS) controllers. The path from historical Thyristor based FACTS controllers to modern state-of-the-art voltage source converters based FACTS controllers, was made possible due to rapid advances in high power semiconductors devices. FACTS controllers have been in use in utilities around the world since 1970s, when the first utility demonstration of first family of FACTS named as Static VAR Compensator (SVC) was accomplished. Since then the large effort was put in research and development of FACTS controllers.
FACTS technology provides the opportunity to [13]
Increase loading capacity of transmission lines.
Prevent blackouts.
Improve generation productivity.
Reduce circulating reactive power.
Improves system stability limit.
Reduce voltage flicker.
Reduce system damping and oscillations.
Control power flow so that it flows through the designated routes.
Basic definitions
Flexibility of electric power transmission:
“The ability to accommodate changes in the electric transmission system or operating conditions while maintaining sufficient steady state and transient margin”.
Flexible AC transmission system (FACTS):
“Alternating current transmission system incorporating power electronics based and other static controllers to enhance controllability and increase power transfer capability”.
FACTS controller:
“A power electronic based system and other static equipment that provide control of one or more AC transmission system parameters”. [14]
Objectives of FACTS
The concept of FACTS was established in order to solve the problem which was emerging in power systems in the late 1980s as there are restrictions on the construction of transmission line and to promote power growth of import and export.
The main objectives behind FACTS based controllers are;
Power transfer capability of transmission systems is to be increased
The power flow is to be kept at the designated route
The first objective indicates the power flow in a given transmission line can be increased up to its thermal limits.
The second objective indicates that the flow of power in the line can be restricted to select proper transmission corridors by controlling current in the line.
If these two objectives are fulfilled there will be significant increase in the utilization of new and existing transmission lines. It will promote the deregulation of power system and there will be minimum requirement for new transmission lines. In order to implement these objectives, high power compensators and controllers are required. [7]
Benefits and cost
Primarily, the FACTS controllers provide voltage support at critical buses in the system (with shunt connected controllers) and regulate power flow in critical lines (with series connected controllers). Both voltage and power flow are controlled by the combined series and shunt controller (UPFC). The power electronic control is quite fast and this enables regulation both under steady state and dynamic conditions (when the system is subjected to disturbances). The benefits due to FACTS controllers are listed below.
They contribute to optimal system operation by reducing power losses and improving voltage profile.
The power flow in critical lines can be enhanced as the operating margins can be reduced due to fast controllability. In general, the power carrying capacity of lines can be increased to values up to the thermal limits (imposed by current carrying capacity of the conductors).
The transient stability limit is increased thereby improving dynamic security of the system and reducing the incidence of blackouts caused by cascading outages.
The steady state or small signal stability region can be increased by providing auxiliary stabilizing controllers to damp low frequency oscillations.
FACTS controllers such as TCSC can counter the problem of sub synchronous resonance (SSR) experienced with fixed series capacitors connected in lines evacuating power from thermal power stations (with turbo generators).
The problem of voltage fluctuations and in particular, dynamic over voltages can be overcome by FACTS controllers.
The capital investment and the operating costs (essentially the cost of power losses and maintenance) are offset against the benefits provided by the FACTS controllers and the `payback period’ is generally used as an index in the planning. The major issues in the deployment of FACTS controllers are (a) the location (b) ratings (continuous and short term) and (c) control strategies required for the optimal utilization. Here, both steady-state and dynamic operating conditions have to be considered. Several systems studies involving power flow, stability, short circuit analysis are required to prepare the specifications. The design and testing of the control and protection equipment is based on Real Time Digital Simulator (RTDS) or physical simulators. It is to be noted that a series connected FACTS controller (such as TCSC) can control power flow not only in the line in which it is connected, but also in the parallel paths (depending on the control strategies). [8]
Generation of reactive power compensation [15]
.First Generation; Mechanically switched devices are:
Fixed shunt reactor (FR)
Fixed shunt capacitor (FC)
Mechanical switched shunt reactor (MSR)
Mechanical switched shunt capacitor (MSC)
.Second Generation; Thyristor-based devices are:
Thyristor controlled Reactor (TCR)
Thyristor switched capacitor (TSC)
Static VAR compensator (SVC)
Thyristor switched series compensator (Capacitor or reactors) (TSSC/TSSR)
Thyristor controlled series compensator capacitors or reactors (TCSC/TCSR).
Thyristor controlled braking resistors (TCBR)
Thyristor controlled phase shifting transformers (TCPST)
Line commutated converter compensator (LCC)
.Third Generation; Converter-based devices:
Static synchronous compensator (SATCOM)
Static Synchronous Series compensator (SSSC)
Unified power flow controller (UPFC)
Interline power flow controller (IPFC)
Self-commutated compensator (SCC)
Figure 3.1: Overview of conventional and FACTS devices
Classification of FACTS devices
The classification of the FACTS Controllers done on the bases of their types of arrangement in the Power system
Figure 3.2: Classification of FACTS devices
Series FACTS controllers:
These FACTS controller could be variable impedance such as capacitor, reactor or a power electronics based variable source which in principle injects a voltage in series with the line as illustrated in fig. As long as the voltage is in phase quadrature with the line current, the series controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well.
Applications:
The main application of series compensators are
Reduction of voltage fluctuations within defined limits during changing power transmissions
Improvement of oscillation damping of the system
Limitations of short circuit currents in networks or substations
Shunt FACTS controllers:
These FACTS controller could be variable impedance such as capacitor, reactor or a power electronics based variable source which is shunt connected to the line in order to inject variable current , as shown in figure. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes variable reactive power. Any other phase relationship will involve handling of real power as well.
Applications:
The primary function of shunt connected compensator is to provide reactive power compensation. The main application of these type of controllers in transmission, distribution and networks are
Reduction in unwanted reactive power flows and reduction in losses
Compensation of consumers and power quality improvement in those applications where huge demand fluctuations occur such as industrial machines, metal melting plants, railway or underground train system
Improvement of transient stability
Combine series-series FACTS controllers:
These controllers are the combination of separate series FACTS controllers, which are controlled in a coordinated manner in a multi-line transmission system, as illustrated in figure. This configuration provides independent series reactive power compensation for each line but also transfers real power among the lines via power link. The presence of power link between series controllers names this configuration as “Unified Series-Series controller”
Combined series –shunt FACTS controllers:
These controllers are the combination of separate shunt and series FACTS controllers, which are controlled in a coordinated manner or a unified power flow controller, with series and shunt elements. When shunt and series FACTS controllers are unified there can be real power exchange between series and shunt controllers via power link.
FACTS Application
FACTS controllers can be used for various applications to enhance power system performance. Once of the greatest advantages of using FACTS controllers is that it can be used in all the three states of power system, namely Steady state, Transient and Post transient steady state. However, the conventional devices find little application during system transient or contingency condition.
steady state application:
Various steady state applications of FACTS controllers includes voltage control (low and high), increase of thermal loading, post-contingency voltage control, loop flows control, reduction in short circuit level and power flow control. SVC and STATCOM can be used for voltage control while TCSC is more suitable for loop flow control and for power flow control.
Congestion management
Congestion management id a serious concern for independent system operator (ISO) in present deregulated electricity markets as it can arbitrarily increase the prices and hinder the free electricity trade. FACTS devices like TCSC, TCPAR (Thyristor controlled Phase Angle Regulator) and UPFC can help to reduce congestion smooth locational marginal price (LMP) and to increase the social welfare by redirecting power from congested interface to underutilize line.
ATC improvement
In many deregulated market, the power transaction between buyer and seller is allowed based on calculation of ATC. Low ATC signifies that the network is unable to accommodate further transaction and hence does not promote free competition. FACTS controllers like TCSC, TCPAR and UPFC can help to improve ATC by allowing more power transactions.
Reactive power and voltage control
The use of shut FACTS controllers like SVC and STATCOM for reactive power and voltage control is well known.
Loading margin improvement
Several blackouts in many part of the world occur mainly due to voltage collapse at the maximum load ability point. Series and shut compensations are generally used to increase the maximum transfer capabilities of power networks. The recent advancement in FACTS controllers have allowed them to be used more efficiently for increasing the loading margin in the system.
Power flow balancing and control
FACTS controllers especially TCSC, SSSC and UPFC, enable the load flow on parallel circuits and different voltage levels to be optimized and controlled with a minimum of power wheeling, the best possible utilization of the lines, and a minimizing of overall system losses at the same time.
Dynamic applications
Transient stability enhancement
Transient instability is caused by large disturbances such as tripping of a major transmission line or a greater and problem can be seen from the first swing of the angle. FACS devices can resolve the problem by providing fast and rapid response during the first swing to control voltage and power flow in the system.
Oscillation damping
Electromechanical oscillations have been observed in many power systems worldwide and may lead to partial power interruption if not controlled. Initially, power system stabilizer (PSS) is used for oscillation damping in power system. Now this function can be more effectively handled by proper placement and setting of SVC, STATCOM and TCSC.
Dynamic voltage control
Shunt FACTS controllers like SVC and STATCOM as well as UPFC can be utilized for dynamic control of voltage during system contingency and save the system from collapse and black out.
SSR elimination
Sub synchronous resonance (SSR) is a phenomenon which can be associated with series compensation under certain adverse conditions. TCSC have dynamic characteristics that differ drastically at frequencies outside the operating frequency range and hence is used in Stode, Sweden for the elimination of SSR in the power system.
Power system interconnection
Interconnection of power system is becoming increasingly widespread as part of power exchange between countries as well as regions within countries in many parts of the world .There are numerous examples of interconnection of remotely separated region within one country. In case of long distance AC transmission , as in interconnected power systems care has to be taken for safeguarding of synchronism as well as stable system voltages, particularly in conjunction with system fault .With series compensation, bulk AC power transmission over distances of more than 1,000 km are a reality today. With the advent of TCSC, further potential as well as flexibility is added to AC power transmission. [16]
Introduction to static synchronous compensators (STATCOM)
Over the last couple of decades, researchers and engineers have made path breaking research on FACTS devices and by virtue of which, many STATCOM controllers based on self-commutated solid state voltage source converter (VSC) have been developed and commercially put in operation to control system dynamics under stressed conditions. STATCOM is qualitatively superior then line commutating static VAR compensator (SVC) and so this controller has given many names as Static compensator advanced static VAR compensator, advanced static VAR generator or static VAR generator, static condenser, synchronous solid state VAR compensator, VSC-based SVC or self-commutated SVC or static synchronous compensator, static condenser (STATCON).
With the advent of voltage-source converter (VSC) technology built upon self-commutating controllable solid state switches has ushered a new family of FACTS controllers such as static synchronous compensators (STATCOM) and unified power flow controller (UPFC) have been developed. The self-commutating VSC, called as DC-to-AC converter, is the backbone of these controllers being employed to regulate reactive current by generation and absorption of controllable reactive power with various solid-state switching techniques. [17]
The Static Synchronous Compensator (STATCOM) is a shunt connected reactive compensation equipment which is capable of generating and/or absorbing reactive power whose output can be varied so as to maintain control of specific parameters of the electric power system. The STATCOM provides operating characteristics similar to a rotating synchronous compensator without the mechanical inertia, due to the STATCOM employ solid state power switching devices it provides rapid controllability of the three phase voltages, both in magnitude and phase angle. STATCOM provide voltage support to buses by modulating bus voltages during dynamic disturbances in order to provide better transient characteristics, improve the transient stability margins and to damp out the system oscillations due to these disturbances.
Definition
STATCOM is defined by IEEE as “a self-commutated switching power converter supplied from an appropriate electric energy source and operated to produce a set of adjustable multiphase voltage, which may be coupled to an AC power system for the purpose of exchanging independently controllable real and reactive power.”
When two AC sources of same frequency are connected through a series inductance, active power flows from leading source to lagging source and reactive power flows from higher voltage magnitude AC source to lower voltage magnitude AC source. Active power flow is determined by the phase angle difference between the sources and the reactive power flow is determined by the voltage magnitude difference between the sources. Hence, STATCOM can control reactive power flow by changing the fundamental component of the converter voltage with respect to the AC bus bar voltage both phase wise and magnitude wise.
Basic Circuit Configuration of STATCOM
The STATCOM has been defined as per CIGRE/IEEE with following three operating structural components.
First component is Static: based on solid state switching devices with no rotating components; second component is Synchronous: analogous to an ideal synchronous machine with 3 sinusoidal phase voltages at fundamental frequency; third component is Compensator: provided with reactive compensation [15]
The typical connection of STATCOM to AC bus is shown in Figure 3.5. That consists of the coupling transformer, input filter, Voltage Source Converter and a controller.
Figure 3.5: Connection of STATCOM with AC bus
The STATCOM is a static compensator is composed of inverters with a capacitor in its dc side, coupling transformers, and a control system. The inverters are, in conventional STATCOMs, switched with a single pulse per period and the transformers are connected in order to provide harmonic minimization. The equipment action is made through the continuous and quick control of capacitive or inductive reactive power. Its output voltage is a waveform composed of pulses that approaches a sinusoidal wave. To obtain voltage harmonic content, that clearly agrees with strict standards, without the necessity of filters, it is necessary at least a set of eight inverters and transformers to produce a 48-pulse voltage waveform. Figure 3.21 shows one example of such a STATCOM and Figure 3.23 shows its voltage.
The major attributes of STATCOM
The major attributes of STATCOM over SVC are;
Faster response
Requires less space as bulky passive components (such as reactors) are eliminated
Inherently modular and relocatable
It can be interfaced with real power sources such as battery, fuel cell or SMES (superconducting magnetic energy storage)
A STATCOM has superior performance during low voltage condition as the reactive current can be maintained constant (In a SVC, the capacitive reactive current drops linearly with the voltage at the limit (of capacitive susceptance). It is even possible to increase the reactive current in a STATCOM under transient conditions if the devices are rated for the transient overload. In a SVC, the maximum reactive current is determined by the rating of the passive components reactors and capacitors. [13]
STATCOM voltage sources
In addition to voltage source using batteries and capacitors, STATCOMs can be operated with an inductor, which provides a source of direct current rather than voltage, Figure 3.6. A three-phase, current-source converter then generates a set of three-phase output currents which, by appropriate switching action, lag or lead the system voltages. The basic output current is a square or block wave and harmonic reduction requires PWM, or multi-level or multi-phase techniques, and/or harmonic filters. The energy in the current source can be sustained by drawing energy from the supply system or by using an external energy source. However the losses of a current-sourced converter tend to be higher than those of voltage-sourced converter.
Figure 3.6: Current sourced convertor CSC
A further interesting concept is to design the converter as an AC to AC frequency charge. The “source “can be a three-phase high frequency generator, a resonant circuit (a parallel capacitor and inductor in each phase), or even a transformer which itself connected to the supply system Figure 3.7.
Figure 3.7: power doubling converter arrangement
In this power-doubling arrangement, there are no energy storage components as such; the output connection to the output transformer can be considered to behave as a current source for the converter. However, the input terminals then need to behave as a voltage source and therefore an input filter needs to be connected to the input terminals of the converter. This type of converter requires special device which have both bi-directional current carrying and forward and reverse voltage blocking capabilities. Suitable devices to implement the power doubling arrangement are not yet commercially available, so this scheme is only of theoretical interest at present. [9]
Difference between CSC and VSC
CSC is the lowest cost convertor.
CSC does not have high short circuit current as does VSC.
For CSC the rate of rise of fault current during external or internal faults is limited by the reactor. Whereas in the VSC the capacitor discharges current would rise very rapidly and can damage the valves.
In CSC the valves are not subjected to high dv/dt due to the presence of the AC capacitors.
Interface of CSC with AC system is more complex.
Continuous losses in DC reactor of a CSC are much higher than the losses in the DC capacitor in VSC.
With the presence of capacitors in VSC, which are subjected to commutation charging and discharging, this convertor will produce harmonic voltages at a frequency of resonance between the capacitor and AC system inductances. These harmonics as well as DC reactor can result in over voltages on the valves and transformers.
Wide spread adoption of asymmetrical devices, IGBTs and GTOs, has made VSC a favorable choice. [6]
Figure 3.9: (a) voltage source converter (b) current source converter
Working principle of STATCOM
A STATCOM is comparable to a Synchronous Condenser (or Compensator) which can supply variable reactive power and regulate the voltage of the bus where it is connected. (Synchronous condenser is a salient pole synchronous generator without prime mover).
The equivalent circuit of a Synchronous Condenser (SC) is shown in Figure 3.12, which shows a variable AC voltage source (E) whose magnitude is controlled by adjusting the field current. Neglecting losses, the phase angle (δ) difference between the generated voltage (E) and the bus voltage (V) can be assumed to be zero. By varying the magnitude of E, the reactive current supplied by SC can be varied. When E = V, the reactive current output is zero. When E > V, the SC acts as a capacitor whereas when E < V, the SC acts as an inductor. When δ = 0, the reactive current drawn (Ir) is given by
I_r=(V-E)/X^’
Figure 3.12: A synchronous condenser
A STATCOM (previously called as static condenser (STATCON)) has a similar equivalent circuit as that of a SC. The AC voltage is directly proportional to the DC voltage (Vdc) across the capacitor (see Figure 3.13 which shows the circuit for a single phase STATCOM).
If an energy source (a battery or a rectifier) is present on the DC side, the voltage Vdc can be held constant. The self-commutated switches T1 and T2 (based on say GTOs) are switched on and off once in a cycle. The conduction period of each switch is 1800 and care has to be taken to see that T1 is off when T2 is on and vice versa. The diodes D1 and D2 enable the conduction of the current in the reverse direction. The charge on the capacitors ensures that the diodes are reverse biased. The voltage waveform across PN is shown in Figure 4.14. The voltage V_PN= V_DC/2 when T1 is conducting (T2 is off) and V_PN= 〖-V〗_DC/2 when T2 is conducting (T1 is off).
The switches are synchronized with the supply voltage (V) which is assumed to be sinusoidal of frequency ω. The fundamental component, rms value (E1) is obtained as
E_1=√2/π ∫_0^π▒〖V_DC/2 〗 sin〖θ 〗 dθ= √2/π V_DC
When E1 > V, the STATCOM draws a capacitive reactive current, whereas it is inductive if E1 < V. Note that, to be compatible with the convention used for SVC, the inductive current drawn is assumed to be positive.
At the instant when T1 is switched on and Ir is inductive, the current (Ir) flowing through the circuit is negative (as it is a lagging current) and flows through T1 (as iT1 is negative of Ir). After 900, the current through T1 becomes zero and as Ir rises above zero and becomes positive, the diode D1 takes over conduction. Similar events occur when T2 turns on and off. Thus, both T1 and T2 cease conduction before they are turned off. On the other hand, when Ir is capacitive, the current Ir is positive at the instant of turning on T1 and flows through the diode D1. After 900, the current reverses its sign and flows through T1. At the time of switching off T1, the current through it is at its peak value. Thus, we need self-commutated devices such as GTOs when the STATCOM draws capacitive reactive current. In contrast, T1 and T2 carry peak current at turn on when Ir is inductive.
Note that diode D1 or D2 turns off automatically when the parallel device (T1 or T2) turns off. Also, the capacitors can be charged from the source through the diodes.
Figure 3.13: A single phase STATCOM
Figure 3.14: The waveform of VPN
In comparing SC and STATCOM, we note that while rotation of the DC field winding on the rotor results in the generation of AC voltages in the stator windings through magnetic induction, the synchronous operation of the switches in a STATCOM results in the AC voltage at the output.
Unlike in a SC, this output voltage also contains many harmonics and some solution has to be found to eliminate them.
Unlike in the case of a SC, the capacitors can be charged from the AC side and there is no need of an energy source on the DC side if only reactive current is to be provided in steady state. The losses in the STATCOM can be met from the AC source.
The advantages of a STATCOM over a SC are:
The response is much faster to changing system conditions.
It does not contribute to short circuit current.
It has a symmetric lead-lag capability.
It has no moving parts and hence the maintenance is easier.
It has no problems of loss of synchronism under a major disturbance [13].
Figure 3.15: Reactive power compensation SC & controlled voltage source switching convertor
STATCOM characteristics
STATCOM operating characteristics
The steady state control characteristics of a STATCOM are shown in Figure 3.16. The losses in the STATCOM are neglected and ISTATCOM is assumed to be purely reactive. As in the case of a SVC, the negative current indicates capacitive operation while positive current indicates inductive operation. The limits on the capacitive and inductive currents are symmetric (±Imax).
The positive slope BC is provided for the V-I characteristic to
prevent the STATCOM hitting the limits often and
To allow parallel operation of two or more units.
The reference voltage (Vref ) corresponds to zero current output and generally, the STATCOM is operated close to zero output during normal operating conditions, such that full dynamic range is available during contingencies. This is arranged by controlling the mechanically switched capacitors/reactors connected in parallel with a STATCOM [13].
Figure 3.16: Control characteristics of a STATCOM
Transient response
Because the operation of a STATCOM is based on the generation of the sinusoidal voltage, its response to transient disturbances inherently good and extremely rapid. The steady state operating condition of a STATCOM is dependent on the system voltage (and impedance) and the STATCOM source voltage and its coupling impedance. Thus in Figure 3.17, with an open circuit system voltage VS1 slightly larger than the target voltage of STATCOM steady state characteristics, the STATCOM draws a small capacitive current I1. In order to generate this current the STATCOM source voltage VC1 must be slightly higher than the target voltage.
If now the system voltage is depressed, due to a fault, to a value VS2, the point of intersection of the system characteristics and the STATCOM controlled characteristic demands a current I2. Initially, before there has been any change of STATCOM source voltage, the STATCOM current increases substantially from I1 to I’2 (given by the intersection of the system characteristics and the natural STATCOM characteristics; this is increased by the control action to the required value I2 by an increase of source voltage to VS2, normally within one half cycle.
Figure 3.17: Response of a STATCOM to a system voltage changes
Figure 3.18 illustrates how a STATCOM responds to voltage disturbances. Prior to the voltage dip, the STATCOM is operating at about its rated lagging current. A dip of system voltage suddenly occurs, to about 50% of its steady state value. This STATCOM inherently responds to this disturbance by generating a capacitive current to support the system voltage but, even on the natural characteristics, there would be a capacitive overload current. To prevent this, the STATCOM control system defects the sudden change and reduce the target voltage to limit the STATCOM current to its rated capacitive value.
Figure 3.18: Response of a STATCOM to a depression system voltage
When the fault is cleared and the system voltage is recovers to its pre-fault value, this will trend to cause an inductive overload current in the STATCOM. Again the STATCOM control system is able to detect change and adjust the target voltage appropriately to reach rated lagging current. Although there is an unavoidable transient distortion of the STATCOM current at each step change, it can be seen from Figure 3.18 that the changes from inductive to capacitive and capacitive to inductive current to take place within a half cycles. [9]
Optimal Location of STATCOM
Transmission lines are often driven close to or even beyond their thermal limits in order to satisfy the increased electric power consumption and trades due to increase of the unplanned power exchanges. Due to this the bus voltage of load buses falls. This also leads to an increase in Transmission losses of the system. Thus to improve the overall voltage profile we require shunt FACTS. STATCOM is one of the better shunt FACTS available.
PSO, BFO and Plant Growth Optimization etc. are techniques for finding the optimal STATCOM location with objective function as transmission losses.
Particle Swarm Optimization (PSO) is an evolutionary computation technique developed by Eberheart & Kennedy in 1995 and is based on bird flocking and fish schooling. Its simplicity and faster convergence make it an attractive algorithm to employ.
The bacterial foraging optimization (BFO) algorithm is inspired from biomimicry of the e-coli bacteria and is a robust algorithm for non-gradient optimization solution, proposed in 2002 by Kevin M Passino. It consists of four steps: chemotaxis, swarming, reproduction & elimination-dispersal
PLANT GROWTH ALGORITHM is a Bionic random algorithm. According to the plant growth characteristics, an artificial plant growth model is built including leaf growth, branching, phototropism and spatial occupancy. [22]
STATCOM control
Following are the STATCOM control strategies
VSC using 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 voltage is proportional to the voltage Vdc. Therefore Vdc has to be varied for controlling the reactive power.
VSC using IGBT-based PWM inverters.
This type of inverter uses Pulse-Width Modulation (PWM) 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 fixed DC voltage Vdc. The fundamental component of voltage is varied by changing the modulation index of the PWM modulator.
The controller of a STATCOM is used to operate the inverter in such a way that the phase angle between the inverter voltage and the line voltage is dynamically adjusted so that the STATCOM generates or absorbs desired VAR at the point of connection. [20]
Figure 3.21: 48-pulse STATCOM diagram
The control used for the model of STATCOM shown in Figure 3.21 is a very simple one. It uses measurements of voltages and currents at the point where the STATCOM is connected to the AC system bus. These measured signals are worked in two ways as shown in Figure 3.22.
In one way, the voltages are fed to the PLL (phase locked loop) block in order to detect the frequency and phase angle and to generate the synchronizing signal to the switching logic.
In the second way of the control, the voltage is fed together with the measured currents to the “Instantaneous Power Theory” block, in order to calculate the instantaneous imaginary power q. This imaginary power q is compared with a reference q* and the error observed is fed to proportional integral controller block. The proportional-integral controller outputs a signal that gives the leading or lagging phase angle necessary to adjust the voltage on the dc side capacitor, thus controlling the energy flow in or out of it.
The leading or lagging signal is added to the PLL synchronism signal output and delivered to the switch logic control block. [21]
Figure 3.22: Control diagram
The interaction between the AC system voltage and the inverter-composed voltage provides the control of the STATCOM VAR output. When these two voltages are synchronized and have the same amplitude, the active and reactive power output is zero. Figure 3.23 (a) shows this situation. However, if the amplitude of the STATCOM voltage is smaller than that of the system voltage, it produces a current lagging the voltage by 90o (see Figure 3.23 (b)), and the compensator behaves as an inductive load, which reactive value depends on the voltage amplitude. Making the STATCOM voltage higher than the AC system voltage the current will lead the voltage by 90o, (see Figure 3.23(c)), and the compensator behaves as a variable capacitive load. As in the previous case, the reactive power depends on the voltage amplitude. This amplitude control is done through the control of the voltage on the dc capacitor. This voltage is related to the energy stored at the dc capacitor. By lagging or leading the STATCOM voltage, it is possible to charge or discharge the dc capacitor, as a consequence, change the value of the dc voltage and the STATCOM’s operational characteristics.
Figure 3.23: STATCOM 48-pulse voltage and compensating current