In this article we will discuss about:- 1. Introduction to Flexible AC Transmission Systems 2. Concept and Objective of FACTS 3. General Symbols 4. Types of FACTS Controllers 5. Types of FACTS Compensators 6. Structure and Characteristics.
- Introduction to Flexible AC Transmission Systems
- Concept and Objective of FACTS
- General Symbols for FACTS Controllers
- Types of FACTS Controllers
- Types of FACTS Compensators
- Structure and Characteristics of FACTS Controller
1. Introduction to Flexible AC Transmission Systems:
The concept of custom power was introduced by N.G. Hingorani. Like flexible ac transmission systems (FACTS) for transmission systems, the term custom power (CP) pertains to the use of power electronic controllers for distribution systems. Just as FACTS improve the reliability and quality of power transmission by simultaneously enhancing both power transfer volume and stability, the custom power enhances the quality and reliability of power that is delivered to customers. A customer receives a prespecified quality power under this scheme.
The prespecified quality may contain a combination of specifications of the following:
1. Frequency of rare power interruptions.
2. Magnitude and duration of over and under voltages with in specified limits.
3. Low harmonic distortion in the supply voltage.
4. Low phase unbalance.
5. Low flicker in the supply voltage.
6. Frequency of the supply voltage with in specified limits.
There are many custom power devices like current breaking devices that are power electronic based.
It is however to be realized that the use of custom power is still in its infancy. Therefore, there is a limited experience from custom power equipment installations. It is common belief amongst the manufactures and utilities alike that a substantial competitive advantage can be gained if the knowhow of custom power equipment is not made available to others.
2. Concept and Objective of FACTS:
Transmission interconnections are required for efficient utilization of resources. On the other hand, as power interconnections grow and power team for grow, operation become complex.
FACTS (Flexible AC transmission system) technology open new opportunities for controlling power and enhancing the unable capacity of present.
In a typical radial system-
Which is limited by-
(a) Thermal capability of the line
(b) Dielectric capability (Voltage limitation)
(c) Stability limit
A control as power flow can be achieved by adjusting the parameter including series impedance, voltage, phase angle etc. FACTS controllers control the above interrelated parameter to enable the lint to carry power close to its thermal limits.
A well-chosen FACTS-controller can overcome the limitation of a designated transmission line or a corridor.
Possibilities of power flow control in AC line are:
(a) Control of line impedance (with a thyristor controlled series capacitor)
(b) Control of angle (phase angle regulator)
(c) Injecting a voltage in series with the line (may be perpendicular to the current flow or in line or at any angle with respect to driving voltage)
(d) Combination of line impedance control with a series controlled and voltage regulation with a shunt controlled.
FACTS controller achieve the above power flow control. FACTS defined by IEEE as a “power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability.”
3. General Symbols for FACTS Controllers:
FACTS controllers are generally divided into four categories:
i. Series Controllers
ii. Shunt Controllers
iii. Combined Series-Series Controllers
iv. Combined Series-Shunt Controllers
Types of FACTS Controller:
(a) Series Controller:
Series controller could be a variable impedance, such as impedance, such on capacitor, reactor etc.
All series controllers inject a voltage in series with the line. As long on the voltage is in phase quadrature with the line current, series controlled only supplies reactive power. Any other phase relationship will involve real power as well.
Series controller control the current/power flow and damp oscillation.
(b) Shunt Controllers:
Shunt controllers, may be variable impedance, variable sources or a combination of these. Shunt controller inject current into the system at the point of connection.
Shunt controller is good to control the voltage at and ground the point of connection through injection of reactive current (leading/lagging).
(c) Combined Series-Series Controllers:
If could be a combination of separate series controllers, which are controlled in a coordinated manner, in a multi-line transmission system.
(d) Combined Series-Shunt Controllers:
This could be a combination of separate shunt and series controller, which are controlled in a coordinated manner or a unified power flow controller (UPFC) with series and shunt elements.
UPFC can provide effective current/power control along with voltage control.
FACTS controllers may be based on thyristor devices with no gate turn-off or with power devices with gate turn off capability. FACTS controllers can generally accommodate storage, such on capacitors, batteries, and superconducting magnet or may other source of energy can be added in parallel through an electronic interface to refill the converter’s dc storage.
BESS FACTS: Battery Energy Storage System FACTS:
When FACTS are accompanied with battery energy storage.
Types of FACTS Controllers:
FACTS controllers can be either VSC (Voltage Source Convertor) based or CSC (Current Source Convertor). From the overall cost point of view, voltage source convertors are preferred.
Flexibility of Electric Power Transmission System:
The ability to accommodate changes in the electric transmission system or operating conditions while maintaining sufficient steady and transient analysis.
Flexible AC Transmission System (FACTS):
Alternating current transmission system incorporating power electronic based and other static controllers to enhance controllability and increase power transfer capability.
A power-electronic based system and other static equipment that provide control of one of more AC transmission system parameters.
4. Types of FACT Controllers:
1. Shunt Connected Controllers:
i. Static Synchronous Compensator (STATCOM):
A static synchronous generator operated as a shunt connected static VAR compensator whose capacitive or inductive output current can be controlled independent of the ac system voltage.
Its ac output voltage is controlled such that it is just right for the required reactive current flow for any ac bus voltage dc capacitor voltage is automatically adjusted as required.
ii. Static Synchronous Generator (SSG):
SSG is a combination of STATCOM and any energy source to supply/absorb power.
Battery Energy Storage System (BESS) or Superconducting Magnetic Energy Storage (SMES).
A chemical based energy storage system using shunt connected voltage source converters capable of rapidly adjusting the amount of energy which is supplied to or absorbed form an ac system.
A superconducting electromagnetic energy storage device containing electronic convertor that rapidly injects and/or absorbs real and/or reactive power or dynamically controls power flow in an ac system.
c. Static Var Compensator (SVC):
A shunt connected static VAR generator or absorb whose output is adjusted to exchange capacitive or inductive current.
So as to maintain or control specific parameters of the electrical power system (typically bus voltage) SVC is a general term for:
(a) Thyristor controlled (TCR) or thyristor switched reactor (TSR).
(b) Thyristor Switched Capacitor (TSC)
SVC is considered by same as a lower cost alternative to STATCOM.
2. Series Connected Controllers:
(a) Static Synchronous Series Compensator (SSSC):
It is one of the most important FACTS controllers. It is just like a STATCOM except that its output ac voltage is in series with the line.
IPFC – Inter line power flow controlled is also a series controller.
(b) Thyristor Controlled Series Capacitor (TCSC):
A variable vector or a thyristor controlled reactor is connected across a series capacitor.
A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor controlled reactor in order to provide a smoothly variable series capacitive reactance.
A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor switched reactor to provide a stepwise control of series capacitive reactance.
(c) Thyristor Switched Series Capacitor (TCSC):
(d) Thyristor Controlled Series Reactor (TCSR):
An inductive reactance compensator which consists of a series reactor shunted by a thyristor controlled reactor in order to provide a smoothly variable series inductive reactance.
(e) Thyristor Switched Series Reactor (TSSR):
Inductive reactance compensator which consists of a reactor shunted by TCSR (Thyristor controlled switched reactor) in order to provide a step wise control series inductive reactance.
3. Combined Shunt and Series Connected Controllers:
Unified Power Flow Controller (UPFC) is a combination of static synchronous compensator (STATCOM) and a static series compensator (SSSC) that are coupled via a common dc link. DC link allows bidirectional flow of real power between the series output terminals of the SSSC and the shunt output terminals of the STATCOM.
5. Types of FACTS Compensators:
1. Static Var Compensators – SVC and STATCOM:
Static VAR compensator (SVC) and static synchronous compensator (STATCOM) are static VAR generator, whose output is varied so as to maintaining or control specific parameter of the electric power system.
Comparison between STATCOM and SVC:
Functional compensation capability of Statcom and SVC are similar:
i. Statcom function as a shunt connected synchronous voltage source.
ii. SVC function as a shunt connected controlled reactive impedance.
Basically STATCOM has overall superior functional characteristic, better performance and greater application flexibility compared to SVC.
i. Statcom can be operated over its full output current range even at very low, system voltage levels.
ii. Statcom is superior to SCV for providing voltage support under large distribution.
iii. Statcom may have an increased transient rating in both the inductive and capacitive region compared to SVS.
iv. If it is more effective in improving transient stability (first swing stability).
v. Increased stability margin with statcom.
vi. Statcom has the potential of interfacing a suitable energy storage with ac system for real power exchange.
i. Maximum attainable compensating current of SVC decrease linearly with ac system voltage.
ii. Maximum VAR output decreases linearly with square of the voltage.
iii. SVC has no means to increase the VAR generation since the maximum capacitive current it can deliver is determined by the size of the capacitor.
iv. Less effective in improving stability.
v. Less margin
2. Static Series Compensator:
Controllable series line compensator can be applied to achieve full utilization of transmission assets by controlling the power flow in the lines.
3. Variable Impedance Type Series Compensators:
Series compensator is functionally a controlled voltage source connected in series with the transmission line to control its current.
a. Thyristor Switched Series Capacitor (TSSC):
It consists of a number of capacitor, each shunted by an appropriately rated bypass valve composed of a string of reverse paralleled connected thyristors, in series.
The Operating Principle:
Degree of series compensator is controlled in a step-like manner by increasing or decreasing the number of series capacitor inserted. A capacitor is inserted by turning off, and it is by passed by turning on the corresponding thyristor valve.
TSSC can control the degree of compensation by increasing or by passing series capacitor but it cannot change the natural characteristics of the classical series capacitor compensated line, i.e., high degree of TSSC compensation could cause sub-synchronous resonance.
Basic VI characteristic with is series connected components are as follows:
In voltage control mode, capacitive banks are progressively by passed by the thyristor valves to reduce the overall capacitive reactance in a step like manner, as the current is increased from Imin – to Imax.
Thus it maintains a compensating voltage with increased line current. In reactance control mode, TSSC is applied to maintain the maximum rated compensating reactance at any line current upto the rated maximum. In practice, TSSC is operated with a current limiting reactor in series.
b. Thyristor Controlled Series Capacitor (TCSC):
Thyristor controlled series compensating capacitor shunted be a thyristor controlled reactor.
TCSS presents a tuneable parallel LC circuit to the line current that is substantially a constant alternating current source. As the impedance of the controlled reactor, XL(α), is varied from its maximum (α) to its minimum (ωL), the TCSC increase its minimum capacitive impedance XTCSC, min = XC = 1/ωC until parallel resonance at XC = XL(α) is established.
4. Switching Converter Type Series Compensators:
a. Static Synchronous Series Compensator (SSSC):
Voltage sourced converter based series compensator is called static synchronous series compensator (SSSC).
Basic Operating Principle:
It can be explained with reference to conventional series capacitive compensator.
SSSC can provide capacitive or inductive compensatory voltage independent of the line current upto its specified current rating in voltage control mode. In reactance control mode, SSSC is established to maintain the maximum rated capacitive or compensating reactance at any line current upto rated maximum.
b. Voltage and Phase Angle Regulator:
The basic concept of voltage and phase angle regulation is the addition of an appropriate in phase or quadrature component to the prevailing terminal voltage in order to change (increase/decrease) its magnitude or angle to the value derived.
c. Thyristor Controlled Phase Angle Regulator (TCPA):
Phase angle regulation is generally achieved by quadrature voltage injection. Basic thyristor the changer for continues control of output voltage.
6. Structure and Characteristics of FACTS Controller:
I. Variable Impedance Type Static Var Generator:
(a) TCR and TSR (Thyristor Controller and Thyristor Switched Reactor):
It consists of a fixed reactor and bireactional thyristor valve. The current in the reactor can be controlled from maximum (thyristor value closed) to zero (thyristor valve open) by the method of firing delay angle control.
Magnitude of the current in the reactor can be varied continuously from minimum (α = 0) to zero (α, π/2).
If the TCR switching is restricted to a fined delay angle, usually α = 0, then it becomes thyristor switched reactor (TSR).
ILF => amplitude of the fundamental reactor current.
The TSR provides a fined inductive admittance and thus when connected to the ac system the reactive current in if will be proportional to applied voltage.
(b) Thyristor Switched Capacitor (TSC):
It consist of a capacitor, a bidirectional thyristor valve, and a relatively small surge current limiting reactor. This reactor is needed to limit the surge current in the thyristor valve under abnormal operating conditions.
Under steady state condition, when the thyristor valve is closed and the TSC branch is connected to a sinusoidal ac voltage source, v = V sin (at, the current in the branch is given by,
TSC branch can be disconnected at any current zero by prior removal of the gate drive to thyristor. At the current zero crossing the capacitor voltage is at its peak value. Disconnected capacitor stays charged to this value and the voltage across the non-conducting thyristor valve varies between zero and peak-to-peak value of the applied ac voltage.
In the case of TSC, switching in the capacitor must take place at that specific instant in each cycle at which conditions for minimum transients are satisfied, that is, when the voltage across the thyristor valve is zero or minimum. For this reason, a TSC branch can provide only a step-like change in the reactive current it draws (minimum or zero). TSC branch represents a single capacitive admittance which is either connected to, or disconnected from the ac system.
VCmax= Voltage limit
ICmax = Current limit
Bc = admittance of capacitor
(c) Fixed Capacitor, Thyristor Controlled Reactor (FC, TCR) type VAR Generator:
FC, TCR type var generator may be considered essentially to consist of a variable reactor (controlled by delay angle a) and a fixed capacitor, constant capacitive var generation (QC) fixed capacitive is opposed by the variable VAR absorption (QL) of the thyristor- controlled reactor, to yield the total VAR output (Q) required.
At the maximum VAR output, reactor is off, i.e., α = 90°. To decrease the capacitive output, the current in the reactor is increased by decreasing delay angle, α. At zero VAR output, the capacitive and inductive currents become equal and thus capacitive VAR and inductive VARs cancel out.
(d) Thyristor Switched Capacitor, Thyristor Controlled Reactor type VAR Generator (TSC-TCR):
For a given capacitive output range, it typically consists of n TSC branches and one TCR. The number of branches, ‘n’, is determined by practical considerations that include the operating voltage level, maximum. VAR output, current rating of the thyristor values etc.
The total capacitive output range is divided into ‘n’ intervals. In the first interval, the output of the VAR generator is controllable in the zero to Qc max/n range, when O is the total rating provided by all TSC branches.
In all the above four categories, (i.e., TCR and TSR, TSC, FC-TCR, TSC- TCR) static VAR generators generate or absorb controllable reactive power by synchronously switching capacitor and reactor banks in and out of the network.
II. Switching Convertor type VAR Generators:
Aim of this approach is to produce a variable reactive shunt impedance that can be adjusted (continuously or in step-like manner) to meet the compensation requirements of the transmission network. This is based on the possibility of generating controllable reactive power directly, without the use of ac capacitor or reaction.
These (dc to ac or ac to dc) connector are operated on voltage and current sources and they produce reactive power essentially without reactive energy storage components by circuiting ac among the phase of the ac system. Functionally from the stand point of generation, their operation is similar to that of as ideal synchronous machine whose reactive power output is varied excitation control.
They can also exchange real power with the ac system if supplied from an appropriate source. Because of these similarities, they are termed static synchronous generation (SSG.). When an SSG is operated without an energy source; and with appropriate controls to function as a shunt connected reactive compensator, it is termed as Rotating Synchronous Compensator (Condenser) or STATCOM or STATCON.
Basic Operating Principle:
In a conventional rotating synchronous machine, by controlling the excitation of the machine, reactive power flow can be controlled. If the excitation of the machine is controlled so that the corresponding reactive power output maintains or varies a specific parameter of the ac system, then the machine (rotating VAR generator) function as a rotating synchronous compensator (Condenser).
Basic voltage source converter based scheme is through an ac-dc convertor.
DC input voltage is provided by charged capacitor, Cs. Converter produces a set of controllable 3-phase output voltages with the frequency of ac system (K0) and coupled to ac system through a tie reactance. By varying the amplitude of the output voltage produced, reactive power exchange between the convertor and ac system can be controlled in a manner similar to that of rotating synchronous machine.
III. Hybrid VAR Generator:
Hybrid VAR generator is a combination of converter based VAR generation with a fixed capacitor and/or reactor.