statcom based voltage stability

7/28/2019 Statcom Based Voltage Stability

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1.2 FLOW OF POWER IN AN AC SYSTEM

At present, many transmission facilities confront one or more limiting

network parameters plus inability to direct power flow at will.

In ac power systems, given the insignificant electrical storage, the

electrical generation and load must balance all the times. To some extent, the

electrical system is self-regulating. If generation is less than load, the voltage and

frequency drop, and there by the load, goes down to equal the generation minus

the transmission losses. However, there is only a few percent margins for such a

self-regulation. If vo l tage is propped up with reactive power support, them the

load will go up, and consequently frequency will keep dropping, and the system

will collapse. Alternatively, if there is inadequate reactive power, the system can

have voltage collapse.

The basic requirement of power system is to meet the demand that variescontinuously. That is, the amount of power divided by the power companies

must be equal to that of consumers need.

The power transmitted over an AC transmission line is a function of the

line impedance, the magnitude of the sending and receiving and voltages and

the phase angle voltages between voltages. The compensators have been provided

to control any one of the function variable.

Traditional techniques of reactive line compensation and step like voltage

adjustment are generally used to alter these parameters to achieve power

transmission control. Fired and mechanically switched shunt and series reactive

compensation are employed to modify the natural impedance characteristics of

transmission line in order to establish the desired effective impedance between

the sending and receiving ends to meet power transfer requirements. Voltage

regulating and phase shifting transformers with mechanical tap changing gears

are also used to minimize voltage variation and control power flow. These

conventional methods provide adequate control under steady state and slowly

changing conditions, but are largely ineffective in handling dynamic disturbance.

The power systems can be effectively utilized with prudent use of F ACTS

technology on a selective, as needed basis.

FACTS technology opens up new opportunities for controlling power

and enhancing the usable capacity of present, as well as new and upgraded lines.

These opportunities arise through the ability of FACTS controllers to control the

interrelated parameter that govern the operation of transmission systems. These

constraints cannot be overcome while maintaining the required system reliability,

by mechanical means with lowering the usable transmission capacity. By

providing added flexibility, FACTS controllers can enable a line to carry power

closer to its thermal ratings. Mechanical switches need to be supplemented by

rapid-response power electronics.

In this scenario, the FACTS technology opens up new opportunity to


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control the power by controlling the initial parameter that governs the operation

of transmission system.

1.3 AC SYSTEM SCENARIO

Flow in AC lines is generally uncontrollable. As a result of the lack of

control in AC lines the following disadvantages are present in AC systems:

1. The power flow in AC lines (except short lines of lengths below 150

km) is limited by stability considerations. The expression for power flow

in a lossless AC line with voltage magnitude v at sending and receiving

end is given by: Zc and denote the characteristic impedance and electrical distance.

Note that peak power transfer capability is

The normal power flow in a line is kept much below the peak value. This

margin (or reserve) is required to maintain system security under

contingency conditions. The fact implies that the lines may operate

normally at power levels much below their thermal limits.

2. The AC transmission network requires dynamic reactive power control to

maintain sat isfactory voltage profi le under varying load

c o n d i t i o n s a nd transient disturbances. The voltage profile of a long

line with the two ends maintained at voltage magnitude v for different

loading conditions.

3. AC lines while providing synchronizing (restoring) torque for oscillating

generator rotors may contribute negative damping torque which results in

un-damped power oscillations.

4. The increases in load levels are accompanied by higher reactive powerconsumption in the line reactances. In case of mismatch in the reactive

power balance in the system, this can result in voltage instability and

collapse.

Recent developments involving deregulation and restructuring of Power

industry, are aimed at isolating the supply of electrical energy (a product)

from the service involving transmission from generating stations to load centers.

This approach is feasible only if the operation of AC transmission lines is made

flexible by introducing fast acting high power solid-state controllers using

thyristor or GTO valves. This led to the development of FACTS technology.


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1.4 PROBLEM OF VOLTAGE STABILITY

Voltage stability is the ability of a power system to maintain adequate

voltage magnitude so that when the system nominal load is increased, the actual

power transferred to that load will increase. The main cause of voltage instabilityis the inability of the power system to meet the demand for reactive power.

Voltage instability s the cause of system voltage collapse, in which the system

voltage decays to a level from which it is unable to recover. Voltage collapse

may lead to partial or full power interruption in the system.

There are two types of voltage stability based on simulation time;

static voltage stability and dynamic voltage stability. Static analysis

involves computationally less extensive than dynamic analysis. Static voltage

stability is ideal for the bulk of studies in which a voltage stability limit for many

pre-contingency and post-contingency cases must be determined. Providing

adequate reactive power support at the appropriate location solves voltageinstability problems. There are many reactive compensation devices used by

the utilities for this purpose, each of which has its own characteristics and

limitations. However, the utility would like to achieve this with the most

beneficial compensation device.

Voltage stability is one of the biggest problems in power systems.

Engineers and researchers have met with the purpose of discussing and trying to

consolidate a definition regarding to voltage stability, besides proposing

techniques and methodologies for their analysis. Most of these techniques are

based on the search of the point in which the system

s Jacobin becomes singular;

this point is referred as the point of voltage collapse or maximum load ability

point. The series and shunt compensation are able to increase the maximum

transfer capabilities of power network .Concerning to voltage stability, such

compensation has the purpose of injecting reactive power to maintain the voltage

magnitude in the nodes close to the nominal values, besides, to reduce line

currents and therefore the total system losses. At the present time, thanks to the

development in the power electronics devices, the voltage magnitude in some

node of the system can be adjusted through sophisticated and versatile devices

named FACTS. One of them is the static synchronous compensator (STATCOM).

1.4.1. VOLTAGE STABILITY ENHANCEMENT

Voltage stability (instability/collapse) is a totally different form of power

system dynamic problem. Contrary to the loss of electromechanical stability,

voltage instability is a possible consequence of progressive increase in load until

the point of collapse is reached, beyond which little can be done except to

prepare for system restoration. The collapse phenomenon is typically slow, over

several minutes, depending on the time-varying behavior of the loads.

The following conventional corrective actions are possible;

Reserve reactive support must be used, i.e. switched shunt capacitors and


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SVC

Network control actions coordinate system LTCs, recluse lines automatically, useHVDC station reactive power control capabilities.

Load control: automatic under voltage load shedding or operator initiated load

Shedding. Generator control action: remove generation to mit igate a transmissionsystem overload, add local generation or trade real power for reactive power

on critical generation.

FACTS studies on easing voltage instability problems have been confined,

so far, to the application of the SVC and the more recent alternative, the

STATCOM.

A more difficult form of voltage instability, sometimes referred to

as transient voltage instability is becoming an increasing problem. Thisform of voltage instability is the long recognized problem of induction motorinstability. Induction motor instability is an increasing problem as transmissionsystem becomes more heavily loaded. Following a system fault, certain induction

motors may either be already stalled or absorb a disproportional high reactive

power compared with active power in their recovery to operating speed. In the

absence of established solutions, certain FACTS devices (like the STATCOM),

which are fast acting and have the potential for high short time overload ratings,

may be helpful.


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1.5 LOAD FLOW STUDIES

1.5 INTRODUCTION TO LOAD-FLOW

Load-flow studies are probably the most common of all power system analysiscalculations. They are used in planning studies to determine if and when specific

elements will become overloaded. Major investment decisions begin with

reinforcement Strategies based on load-flow analysis. In operating studies, load-flow

analysis is used to ensure that each generator runs at the optimum operating point;

demand will be met without overloading facilities; and maintenance plans can

proceed without undermining the security of the system.

The objective of any load-flow program is to produce the following

information:

Voltage magnitude and phase angle at each bus.

Real and reactive power flowing in each element.

Reactive power loading on each generator.

The above objectives are achieved by supplying the load-flow program with

the Following information:

Branch list of the system connections i.e., the impedance of each element, sending-end and receiving-end node. Lines and transformers are represented by their

-

equivalent models.

Voltage magnitude and phase-angle at one bus, which is the reference point for therest of the system.

Real power generated and voltage magnitude at each generator bus. Real and reactive power demanded at each load bus.

The foregoing information is generally available since it either involves

readily Known data (impedances etc.) or quantities which are under the control of

power system Personnel (active power output and excitation of generators.) Simply

stated the load- flow problem is as follows:

at any bus there are four quantities of interest: V, , P, and Q.

if any two of these quantities are specified, the other two must not be specifiedotherwise we end up with more unknowns than equations.

1.6 Load Flow

Load flow solution is a solution of the network under steady state condition

subjected to certain inequality constraints under which the system operates. These

constraints can be in the form of load magnitude, bus voltages, reactive power

generation of the generators, tap settings of a tap-changing transformer etc. The loadflow solution gives the bus voltages and phase angles, hence the power injection at all


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the buses and power flow through interconnecting transmission lines can be easily

calculated. Load flow solution is essential for designing a new power system as well as

for planning an extension or o p e r a t i o n of the existing one for varying demand. `

These analyses require number of load flow solutions under both normal andabnormal (outage of transmission line or outage of some generators) operating

conditions. Load flow solution also gives the initial state of the system when the

transient behavior of the system is to be studied. The load flow solution of the power

system mainly requires the following calculations/steps:

1. Formulation of equations for the given network

2. Suitable mathematical technique for the solution of the equations

Under steady state condition, the network equations will be in the form of

simple algebraic equations. The loads and generations are continuously changing in a

real power system, but for solving load flow it is assumed that loads and generations arefixed at a particular value over a suitable period of time. E.g. half an hour or

monthly, depending upon data.

1.7 Bus Classification:

In a power system each bus or node is associated with four quantities, real and

reactive powers, bus voltage magnitudes and its phase angles. In a load flow solution

two out of four quantities are specified and the remaining two are to be calculated

through the solution of the equations. The buses are classified into the following three

types depending upon the quantities specified.

PQ bus: At this bus the real and reactive components of power are specified.It is desired to find out the voltage magnitude V and phase angle through theload flow solution. Voltage at load bus can be allowed to vary within a

prescribed value e.g. 5%. It is also known as the load bus.

PV bus: Here the voltage magnitude corresponding to the generator voltage Vand real power PG corresponding to its ratings is specified. It is required to

find out the reactive power generation QG and the phase angle of the bus. It isalso known as the Generator bus or voltage-controlled bus.

Slack/Swing or reference bus: Here the voltage magnitude V and phase angle is specified. This will take care of the additional power generation requiredand transmission losses. It is required to find the real and reactive power

generations (PG, QG) at this bus.This is called the slack (or swing, or

reference) bus and since P and Q are unknown, V and must bespecified. Usually, an angle of = 0 is used at the slack bus and all other busangles are expressed with respect to slack.

Load flow solution can be achieved by any iterative methods. There are many

kinds of iterative methods but as per the literature review the Newton-Raphson

method is normally applied. In the load flow problem as explained above, twovariables are specified at each bus and the remaining variables are obtained through


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load flow solutions.

The additional variables to be specified for load flow solution are the tap

settings of regulating transformers, capacitances, resistances etc. If the specified

variables are allowed to vary in a region constrained by practical considerations (upperand lower limits of real and reactive generations, bus voltage limits and range of

transformer tap settings), these results in load flow solutions each pertaining to one set

of values of specified variables.

1.8 CLASSICAL LOAD FLOW METHODS:

These are classified as:

1. GaussSeidel method2. Fast-Decoupled-Load-Flowmethod.

3. NewtonRaphson method

1.8.1Gauss-Siedal Method:

In numerical algebra, the GaussSeidel method, also known as the Liebmannmethod or the method of successive displacement, is an iterative method used to solve a

linear system of equations. It is named after the German mathematicians Carl

Friedrich Gauss and Philipp Ludwig von Seidel, and is similar to the Jacobian

method. Though it can be applied to any matrix with non-zero elements on the

diagonals, convergence is only guaranteed if the matrix is either diagonally dominant,

or symmetric and positive definite.

Description:

Given a square system of n linear equations with unknown Value of x:

Where,

Then A can be decomposed into a lower triangular component L*, and a strictly upper

triangular component U:

A=L+U

Where,


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The system of linear equations may be rewritten as:

The GaussSeidel method is an iterative technique that solves the left hand side of thisexpression for x, using previous value for x on the right hand side. Analytically, this

may be written as:

However, by taking advantage of the triangular form of L*, the elements of x(k+1)can be computed sequentially using forward substitution:

The procedure is generally continued until the changes made by iteration are belowsome tolerance. The element-wise formula for the GaussSeidel method is extremelysimilar to that of the Jacobian method.

The computation of xi (k+1) uses only the elements of x (k+1) that have already

been computed, and only the elements of x (k) that have yet to be advanced to iteration

k+1. This means that, unlike the Jacobian method, only one storage vector is required as

elements can be overwritten as they are computed, which can be advantageous for very

large problems.

However, unlike the Jacobian method, the computations for each element cannot be

done in parallel. Furthermore, the values at each iteration are dependent on the orderof the original equations .The convergence properties of the GaussSeidel method aredependent on the matrix A. Namely, the procedure is known to converge if either:

A is symmetric positive-definite, or

A is strictly or irreducibly diagonally dominant.

The GaussSeidel method sometimes converges even if these conditions are notsatisfied.

1.8.2 Fast Decoupled Load Flow Method:

It is a reliable and fastest method in obtaining convergence this method with branches of

high (r/x) rations could not solve problems with regard to non-convergence and longexecution time.


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1.8.3 Newton-Raphson load flow (NRLF) Method:

Calculation of Jacobian:For an N-bus power system there will be n equations for real power injection i

Pand n-equations for reactive power11 injection Qi. . (1) (2)

..... (3)The number of equations to be solved depends upon the specifications we

have. If the total number of buses is n and number of generator buses is m then the

number of equations to be solved will be number of known Pis and number of knownQis. In the above conditions number of known Pis are n-1 and the number of knownQis are (n-m), therefore the total number of simultaneous equations will be 2*n-m-1,and number of unknown quantities are also 2*n-m-1. Unknowns to be calculated are

power angles () at all the buses except slack (i.e. n-1) and bus voltages (V) at loadbus (i.e. n-m). The following method known as Newton- Raphson method is used for

solving the unknown quantities.

The problem formulation is as follows: Real power terms will be calculated for all the buses except slack bus and reactive power

terms will be calculated for all load buses. In the above equation

And

is the Jacobian Matrix...(4)The elements of the Jacobian matrix can be calculated using the following equations


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(5)

Procedure for this iterative method is for the given system first the Y-bus matrix hasto be formed.

Where,

Y is a bus admittance matrix

G is real part of Y-bus matrix

B is imaginary part of Y-bus

Y= G+j B

The resistance and reactance of each line have been given for any system from which the

admittance matrix can be formed.

1.9 Iterative Algorithm for N-R Method:

1. With voltage and angle (usually = 0) at slack bus fixed, assume voltagemagnitude and power angles at PQ buses and at all PV buses. Generally flat voltagestart will be used.

2. Compute i Pfor all buses except slack bus and i Q for all PQ buses using Eq. (3). Ifall the values are less than the prescribed tolerance, stop the iterations.

3. If the convergence criterion is not satisfied, evaluate elements of the jacobian using Eq.(5).


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4. Solve the Eq. (2) for correction vector.5. Update voltage angles and magnitudes by adding the corresponding changes to the

previous values and return to step 2.


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CHAPTER 2

FLEXIBLE AC TRANSIMISSION SYSTEMS

These are alternating current transmission systems incorporating power

electronic- based and other static controllers to enhance controllabil ity and

increase power transfer capability. FACTS do not indicate a particular controllers

but a host of controllers which the system planner can choose based on both

technical considerations and cost benefit analysis.

OBJECTIVES OF FACTSThe main objectives of introducing FACTS are:

1. Regulation of power flows in prescribed transmission routes.

2. Secure loading of lines nearer their contributing to emergency control

3. Prevention of cascading outages by contributing to emergency control

4. Improving the stability of the system.

Power Flow in Parallel Paths

Consider a very simple case of power flow through two parallel

paths (possibly corridors of several lines) from a surplus generation area, shown as

an equivalent generator on the left, to a deficit generation area on the right.

Without any control, power flow is based on the inverse of the varioustransmission line impedances. Apart from ownership and contractual issues over

which lines carry how much power, it is likely that the lower impedance line may

become overloaded and thereby limit the loading on both paths even though the

higher impedance path is fully loaded. There would not be an incentive to

upgrade current capacity of the overloaded path, because this would further

decrease the impedance and the investment would be self-defeating particularly

if the higher impedance path already has enough capacity.

Fig (b) shows the same two paths, but one of these has HVDC transmission.

With HVDC, power flows as ordered by the operator, because with HVDC

power electronics converters power is electronically controlled. Also, becausepower is electronically controlled, the HVDC line can be used to its full thermal

capacity if adequate converter capacity is provided. Furthermore, an HVDC line,

because of its high-speed control, can also help the parallel ac transmission line to

maintain stability. However, HVDC is expensive for general use, and is usually

considered when long distances are involved, such as the Pacific DC Inter tie on

which power flows as ordered by the operator.

As alternative FACTS controllers, fig(c) and (d) show one of the

transmission lines with different types of series types FACTS controllers. By

means of controlling impedance, or series injection of appropriate voltage a

FACTS controller can control the power flow as required. Maximum power flowcan in fact be limited to its rated limit under contingency conditions when this


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line is expected to carry more power due to the loss of a parallel line.

Fig 1.1 Power flow in parallel paths a) ac power flow with parallel paths b) power flow control

with hvdc c) power flow control with variable impedance d) power flow control with variable phase

angle

2.1 FACTS CONTROLLERS

A power Electronic based system and other static equipment that provide

control of one or more AC transmission system parameters.

FACTS devices or controllers are used for the dynamic control of voltage,

impedance and phase angle of high voltage AC transmission lines. Below, thedifferent main types of FACTS devices are described:

Shunt connected controllers Series connected controllers Combined series-series controllers Combined series-shunt controllers


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2.1.1 SERIES CONNECTED CONTROLLERS

The series controller could be variable impedance, such as capacitor, reactor

etc (or) power electronics based variable source (or) a combination of these. In

principle, all series controllers inject voltage in series with the line. The seriescontroller could be variable impedance, such as capacitor, reactor, etc., or power

electronics based variable source of main frequency, sub synchronous and

harmonic frequencies to serve the desired need. In principle, all series controllers

inject voltage in series with the line. Even variable impedance multiplied by the

current flow through it, represents an injected series voltage in the line. 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.

i. Thyristors controlled series capacitor(TCSC)

ii. Thyristor switched series capacitor(TSSC)

iii. Static synchronous series compensator(SSSC)

2.1.1.1 THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC)

A capacitive reactance compensator which consists of a series capacitor

bank shunted by a thyirstor-controlled reactor in order to provide a smoothly

variable series capacitive reactance. The TCSC may be a single, large unit, or may

consist of several equal or different-sized smaller capacitors in order to achieve a

superior performance.

Figure 2.1.1 TCSC Layout

The TCSC is based on thyristors without the gate turn-off capability. It is an

alternative to SSSC above and like an SSSC, it is a very important FACTS

controller. A variable reactor such as a Thyristors-controlled reactor (TCR) isconnected across a series capacitor. When the TCR firing angle is 180 degrees, the


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reactor becomes non- conducting and the series capacitor has its normal impedance.

As the firing angle is advanced from 180 degrees to less than 180 degrees, the

capacitive impedance increases. At the other end, when the TCR firing angle is 90

degrees, the reactor becomes fully conducting, and the total impedance becomes

inductive, because the reactive impedance is designed to be much lower than theseries capacitor impedance.

2.1.1.2. THYRISTOR SWITCHED SERIES CAPACITOR (TSSC)

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.

Figure 2.1.2 TSSC LAYOUT

Instead of continuous control of capacitive impedance, this approach of

switching inductors at firing angle of 90 degrees or 180 degrees but withoutfiring angle control, could reduce cost and losses of the controller.

It is reasonable to arrange one of the modules to have thyristors control,

while others could be thyristors switched.

2.1.1.3 STATIC SYNCHRONOUS SERIES CAPACITOR (SSSC)

A static synchronous generator operated without an external electric

energy source as a series compensator whose output voltage is in quadrature with,

and controllable independently of, the line current for the purpose of increasing or

decreasing the overall reactive voltage drop across the line and thereby controlling

the transmitted electric power.

Figure 2.1.3 SSSCThe SSSC may include transiently rated energy storage or energy


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absorbing devices to enhance the dynamic behavior of the power system

by additional temporary active power compensation, to increase or decrease

momentarily, the overall active (resistive) voltage drop across the line. SSSC is one

the most important FACTS controllers.

It is like a STATCOM, except that the output ac voltage is in series with

the line. It can be based on a voltage-sourced converter or current-sourced

converter. Without an extra energy source, SSSC can only inject a variable voltage,

which is 90 degrees leading or lagging the current.

Usually the injected voltage in series would be quite small compared to

the line voltage, and the insulation to ground would be quite high. With and

appropriate insulation between the primary and the secondary of the transformer,

the converter equipment is located at the ground potential unless the entire

converter equipment is located on a platform duly insulated from ground.

Battery- storage or superconducting magnetic storage can also be connected

to a series controller to inject a voltage vector of variable angel in series with the

line.


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reactance is varied in a continuous manner by partial-conduction control of the

thyristor valve.

Figure 2.2.2 TCR

Layout

TCR is a subset of SVC in which conduction time and hence, current in shunt

reactor is controlled by a thyristor-based AC switch with firing control.

2.1.2.3 THYRISTOR SWITCHED REACTOR (TSC)

Figure 2.2.3 TCSC Layout

A shunt-connected, thyristor-switched inductor whose effective reactance

is varied in a stepwise manner by full- or zero-conduction operation of the

thyristor valve.

TSR is made up of several shunt-connected inductors which are switched in

and out by thyristor switches without any firing angle.

TSC is also a subset of SVC in which thyristors based ac switches are used to

switch in and out shunt capacitors units, in order to achieve the required stepchange in the reactive power supplied to the system. Unlike shunt reactors, shunt


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capacitors cannot be switched continuously with variable firing angle control.

2.1.2.4 STATIC VAR COMPENSATOR

A shunt-connected static var generator or absorber 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 based

of thyristors without the gate turn-off capability. SVC is considered by some as a

lower cost alternative to STATCOM.

This is a general term for a thyristors-controlled or thyristors-switched

reactor, and/or thyristors-switched capacitor or combination. SVC is based on

thyristors without the gate turn-off capability. It includes separate equipment for

leading and lagging vars the thyristors-controlled or thyristors-switched reactor forabsorbing reactive power and thyristors-switched capacitor for supplying thereactive power.

2.1.3 COMBINED SERIES-SHUNT CONTROLLERS

This could be combination of separate shunt and series controllers, which are

controlled in a coordinated manner, or a Unified Power Flow Controller with

series and shunt elements in principle, combined shunt and series controllers inject

current into the system with shunt part of controller and voltage in series in the

line with series part of controller. However, when the shunt and series controllers

are unified, there can be real power exchange between the series and shunt

controllers via the power link.

In principle, combined shunt and series controllers inject current into

the system with the shunt part of the controller and voltage in series in the line with

the series part of the controller. However, when the shunt and series

controllers are unified, there can be a real power exchange between the series and

shunt controllers via the power link.

COMBINED SERIES-SERIES CONTROLLERS

This could be a combination of separate series controllers, which

are controlled in a coordinated manner, in a multi line transmission system.

The real power transfer capability of the unified series-series controller, referred to

as interline power flow controller, makes it possible to balance both the real and

reactive power flow in the lines and thereby maximize the utilization of the

transmission system.

2.1.4 UNIFIED POWER FLOW CONTROLLER (UPFC)

A combination of static synchronous compensator (STATCOM) and a

static series compensator (SSSC) which are coupled via a common dc link, toallow bidirectional flow of real power between the series output terminals of the


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SSSC and the shunt output terminals of the STATCOM, and are controlled

to provide concurrent real and reactive series line compensation without an

external electric energy source. The UPFC, by means of angularly unconstrained

series voltage injection, is able to control, concurrently or selectively, the

transmission line voltage,impedance, and angle or, alternatively, the real andreactive power flow in the line. The UPFC may also provide independently

controllable shunt reactive compensation.

Figure 2.3 UPFC Layout

This is a complete controller for controlling active and reactive power

control through the line, as well as line voltage control.

In UPFC, which combines a STATCOM and an SSSC, the active power

for the series unit is obtained from the line itself via the shunt unit STATCOM; the

latter is also used for voltage control with control of its reactive power. This is a

complete controller for controlling active and reactive power control through the

line, as well as line voltage control. Additional storage such as a superconducting

magnet connected to the dc link via an electronic interface would provide the means

of further enhancing the effectiveness of the UPFC. As mentioned before, the

controlled exchange of real power with an external source, such as storage, is

much more effective in control of system dynamics than modulation of the power

transfer within a system.

2.2 BENEFITS OF FACTS CONTROLLERS

Increase the loading capability of lines to their thermal capabilities. Increase the system security through raising the transient stability limit. Control of power flow as ordered. Provide secure tie line connections.


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Provide greater flexibility in setting new generation. Reduce loop flows. Reduce reactive power flows.2.3 Comparison of various facts devices:

TABLE

FACTS DEVICE TYPE OF

CONNECTION

FUNCTION

Static Var compensator Shunt Var compensation, steady

state and dynamic stability

STATCOM Shunt Generating or absorbingthe reactive power

SSSC

Static Synchronous Series

Compensator

Series Controlling transmitted

electric power by increasing

or decreasing reactive

voltage drop

TCSC

Thyristor Controlled

Series Capacitor

Series Capacitive reactance

compensator in continuous

manner

UPFC Twoport Terminal voltage control,

phase angle regulation, seriesline compensation

RELATIVE IMPORTANCE OF CONTROLLABLE PARAMETERS

Control of the line impedance X (e.g., with a thyristor-controlled seriescapacitor)can provide a powerful means of current control.

when the angle is not large, which is often the case, control of X or theangle substantiallyprovides the control of active power.

Control of angle, which in turn controls the driving voltage, provides apowerful means of controlling the current flow and hence active power flow

when the angle is not large.

Injecting a voltage in series with the line, and perpendicular to the currentflow, can increase or decrease the magnitude of current flow. Since the current

flow lags the driving voltage by 90 degrees, this means injection of reactive

power in series, can provide a powerful means of controlling the current, and

hence the active power when the angle is not large.

Injecting voltage in series with the line and with any phase angle with respect


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to the driving voltage can control the magnitude and the phase of the line

current. This means that injecting a voltage phasor with variable phase angle can

provide a powerful means of precisely controlling the active and reactive power

flow. This requires injection of both active and reactive power in series.

Because the per unit line impedance is usually a small fraction of the linevoltage, the MVA rating of a series controller will often be a small fraction of

the throughput line MAVA.

When the angle is not large, controlling the magnitude of one or the otherline voltages can be a very cost-effective means for the control of reactive power

flow through the interconnection.

Combination of the line impedance control with a series controller andvoltage regulation with a shunt controller can also provide a cost-effective

means to control both the active and reactive power flow between the twosystems.


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CHAPTER 3

STATIC COMPENSATOR(STATCOM)

It is a device connected in derivation, composed of a coupling transformer

that serves of link between the electrical power system and the voltage

synchronous controller (VSC) that generates the voltage wave comparing it to

the one of the electric system to realize the exchange of reactive power. The

control system of the STATCOM adjusts at each moment the inverse voltage so

that the current injected in the network is in quadrature to the network voltage, in

these conditions P=0 and Q=0.

Static synchronous compensator (STATCOM) is a voltage-source

converter based device, which converts a DC input voltage into an AC outputvoltage in order to compensate the active and reactive needs of the system.

STATCOM has better characteristics than SVC; when the system voltage drops

sufficiently to force the STATCOM output to its ceiling, its maximum reactive

power output will not be affected by the voltage magnitude. Therefore, it exhibits

constant current characteristics when the voltage is low under the limit.

A schematic diagram and STATCOM characteristic are shown in Figures below

Figure 3.1 structure of statcom


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Figure 3.2 Typical VI characteristics of stat COM

Thus, when operating at its voltage limits, the amount of reactive power

compensation provided by the STATCOM is more than the most-common

compensating FACTS controller, namely the Static Var Compensator (SVC). This

is because at a low voltage limit, the reactive power drops off as the square of

the voltage for the SVC, where Mvar=f(BV2), but drops off linearly with the

STATCOM, where Mvar=f(VI). This makes the reactive power controllability of

the STATCOM superior to that of the SVC, particularly during times of system

distress.

3.1 STATCOM OPERATING PRINCIPLE

The STATCOM generates a balanced 3-phase voltage whose magnitude and

phase can be adjusted rapidly by using semiconductor switches. The STATCOM is

composed of a voltage-source inverter with a dc capacitor, coupling transformer,

and signal generation and control circuit.

The voltage source inverter for the transmission STATCOM operates in

multi- bridge mode to reduce the harmonic level of the output current. Fig. below

shows a single-phase equivalent circuit in which the STATCOM is controlled bychanging the phase angle between the inverter output voltage and the bus voltage at

the common point connection point. The inverter voltage Vi is assumed to be in

phase with the ac terminal voltage Vt .


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Figure 3.3 statcom

The STATCOM supplies reactive powers to the ac system if the magnitudeof Vi is greater than that of Vt. It draws reactive power from the ac system if the

magnitude of Vt is greater than that if Vi.

There can be a little active power exchange between the STATCOM and

the EPS. The exchange between the inverter and the AC system can be

controlled adjusting the output voltage angle from the inverter to the voltage

angle of the AC system. This means that the inverter cannot provide active power

to the AC system form the DC accumulated energy if the output voltage of the

inverter goes before the voltage of the AC system. On the other hand, the inverter

can absorb the active power of the AC system if its voltage is delayed in respect tothe AC system voltage.


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Using the classical equations that describe the active and reactive power

flow in a line in terms of Vi and Vs, the transformer impedance (which can be

assumed as ideal) and the angle difference between both bars, we can define P and

Q. The angle between the Vs and Vi in the system is d. When the STATCOM

operates with d=0 we can see how the active power send to the system devicebecomes zero while the reactive power will mainly depend on the voltage module.

This operation condition means that the current that goes through the transformer

must have a +/-90 phase difference to Vs. In other words, if Vi is bigger than Vs,

the reactive will be send to the STATCOM of the system (capacitive operation),

originating a current flow in this direction. In the contrary case, the reactive will be

absorbed from the system through the STATCOM (inductive operation) and the

current will flow in the opposite direction. Finally if the modules of Vs and Vi are

equal, there wont be nor current nor reactive flow in the system.

Thus, we can say that in a stationary state Q only depends on the

module difference between Vs and Vi voltages. The amount of the reactive power isproportional to the voltage difference between Vs and Vi.

3.2 MODELLING OF STATCOM:

Statcom is a shunt connected reactive power compensation device that is

capable of generating/absorbing reactive power and in which the output can be

varied to control the specific parameters of an electric power system. It is in general

a solid state switching converter capable of generating independently

controllable reactive power at its output terminal.

The statcom is placed in the bus m and is represented by a shunt

reactive current source is as shown in fig below

Figure 3.4 statcom under variable susceptance model

With the statcom the output power Pe of the machine can be written and is

positive when oscillates in between zero and .

The equation of power can be modulated by modulating the shunt reactive current I.


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The modeling of the statcom can be done in various methods

1. Variable susceptance method

2. Firing angle method

3. Transformer tapping and firing angle method

3.2.1 Shunt Variable Susceptance Model:

In practice the SVC can be seen as an adjustable reactance with either firing angle limits. The equivalent circuit shown in figure is used to derive the SVC non-

linear power equations and the linearised equations required by Newtons method.

With reference to the figure , the current drawn by the SVC is

And the reactive power drawn by the SVC which is also the reactive power injected

at bus k, is The linearised equations is given where the equivalent susceptanceis to be taken the state variable.

The changing susceptance represents the total SVC susceptance necessary to

maintain the nodal voltage magnitude at the specified value.

Once the level of compensation has been computed then the thyristor

firing angle can be calculated. However, the additional calculation requires an

iterative solution because the SVC s u s c e p t a n c e and thyristor firing angle are

non linearly related.

3.3 TYPICAL STATCOM APPLICATIONS

Utilities with weak grid knots or fluctuating reactive loads

Unbalanced loads

Arc furnaces

Wind farms

Wood chippers


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Welding operations

Car crushers & shredders

Industrial mills

Mining shovels & hoists

Harbor cranes

3.4 MAIN ADVANTAGES OF STATCOM

Continuous and dynamic voltage control

High dynamic and very fast response time

Enables grid code compliance

Maximum reactive current over extended voltage range

High efficiency

Single phase control for unbalanced loads


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CHAPTER 4

INTRODUCTION TO MATLAB

4.1 INTRODUCTION

MATLAB is a software package for high-performance numerical

computation and visualization it provides an interactive environment with

hundred of built-in function for its own high level programming language. The

name MAT LAB stands for MATrix laboratory.

MATLABs built-in functions provide excellent tool for linear

algebra computation, data analysis, signal processing, optimization, andnumerical solutions of ODES, quadrature, and many type of scientific

computation. Most of these functions use state-of-the art algorithm. These are

numerous functions for 2-D and 3- D-graphics as well as for animation also, for

those who cant do without their FORTRAN or C courses, MATLAB evenprovides an external interface to fun those programs from within MATLAB even

provides an external interface to fun those programs from within MATLAB. The

user however is not limited to the built-in functions, he can write his own

function in the MATLAB language once written, and these functions behave just

like the built in functions MAT labs language is very easy to learn and to use.

There are several optional toolboxes available from the developers ofthe MATLAB. These toolboxes are collections written for special applications

such as symbolic computations, image processing, statics control system designand Neural Networks. The basic building block of MATLAB is the matrix. The

fundamental data type is the array. Vectors, scalars, real matrices and complex

matrices are all automatically handled as special cases of the basic data-type.

What is more, you almost never have to declare the dimensions of the matrix.

MATLAB simply loves matrices and matrix operations. The built in functions are

optimized for vectors operations. Consequently, vectorised commands or

commands or codes run much fast in mat lab.


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4.2 MATLAB WINDOW:

On all UNIX system Macs, and pc, mat lab through three Basic windows, they are:

Command w i n d o w : This is the main window it is characterized by MATLABCommand prompt >>: when you launch the application program, MATLAB

puts you i n th is window. All commands , including those fo r runn ing

user wr i t t en programs, are typed in this window at the MATLAB prompt.

Graphics Window: The outputs of all graphic commands are typed in thecommand window and are f l u s h e d t o the graphic or figure window, a

separate grey window with white back ground c o l o r . The user can create as

many figure windows, as the system memory will allow.

Edit Window: This is where you edit, write, create, and save your own programsin files called M-files. We can use any text editor to carry out these tasks. On the

most systems, such as PCs and Macs, MATLAB provides its built in editor. Onother systems, you can invoke the edit window by typing the standard file editor

command that at the MATLAB prompt following special character !. Theexclamation character prompts MATLAB to return the control temporarily to

the local operation system, which executes the commands following the !character. After editing is completed, the control is returned to MATLAB.

Input-Output: MATLAB supports interactive computation taking input from thescreen, and flushing the output to the screen. In addition, it can read input files and

write output files. The following features hold for all forms of input-output.

Data Type: The fundamental data type in MATLAB is the array. Itencompasses several distinct data objects, integer, double, matrices, character

string, and cells. In most cases, however, we never have to worry about the data

type or the data object declaration. For example there is no need to declare variable,

MATLAB automatically sets the variable to be real.

Dimensioning: Dimensioning is automatic in MATLAB. No dimensioningstatements are required for vectors or arrays. We can find the dimensions of an

existing matrix or a vector with size and length commands.

Case sensitivity: MATLAB is case sensitive, that is, it differentiates betweenlower case and upper case letters. Thus a and an are different variables. Most

MATLAB commands and built-in function calls are typed in lower case letters. We

can turn case sensitivity on and off with case sensitive command.

Output Display: the output of every command is displayed a screen unlessMATLAB is directed otherwise. A semicolon at the end of a command

suppress the screen output, expect for the graphics and on-line help command. The

following facilities are providing for controlling the screen output.

Paged Output: To direct the MATLAB to show one screen of output at a time,type more on the MATLAB prompt. Without it, MATLAB flushes the entire


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output at once, without regard to the speed at which we read.

Output format: Though computations inside the MATLAB are performed usingthe double precision, the appearance of floating point numbers on the screen is

controlled by the output format in use. There are several different screen outputformats.

Command History: MATLAB severs previously typed commands in buffer.These commands can be called with the up arrow key .This helps in editing

previous commands. You can also recall previous command by typing the first few

characters and then pressing the up-arrow key. On most UNIX systems,

MATLAB command line editor also understands the standard maces key

buildings.

FILE TYPE:MATLAB has 3 types for strong information:

M-files: M-files are standard ASCII text files, with an extension to thefilename. There are low types of these files: script files and functions. Most

programs we write in MATLAB are saved as M-files. All built-in function

in MATLAB are M-files, most of which reside on our computer in

precompiled format. Some built in function are provided with secure code in

readable M-file so they can be copied and modified.

Mat-files: Mat-files are binary data-files with mat extensions to thefilename. Mat fills are created by MATLAB can read. Can be loaded into

MATLAB with the load command.

Mex-files are MATLAB: Mat files callable FORTRAN and Cprograms, with a Mex extension to the filename; u s e of these files

requires some experience with MATLAB and a lot of patience.


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4.3 PROBLEM EVALUATION:

4.3.1 IEEE 5 BUS WITHOUT USING THE STATCOM

The IEEE 5 BUS SYSTEM Data

Figure 4.1 IEEE 5bus

Mat lab Results:

TABLE 2

For iteration =4Without Statcom

Bus no. Voltage magnitude Voltage angle

1 1.0600 0

2 1.0000 -2.0554

3 0.9873 -4.6268

4 0.9842 -4.9466

5 0.9717 -5.7563

4

2


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4.3.1.1 IEEE 5 BUS USING WITH SINGLE STATCOM SINGLE STATCOM

Figure 4.2 IEEE 5 bus with statcom

Mat lab results with single statcom

The statcom is placed at the bus no:3 .At the bus 3 we are placing the statcom

for maintaining the voltage stability. TABLE 3

With Statcom

Bus no. Voltage magnitude Voltage angle

1 1.0600 0

2 1.0000 -2.0534

3 1.0000 -4.8379

4 0.9944 -5.1073

5 0.9752 -5.7975

Injected reactive power at bus no:3is QSVC = -0.2047 Mvar p.u

4

2


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4.3.1.2 USING THE MULTIPLE STATCOM

Figure 4.3 IEEE 5 bus with Multiple Statcom

Mat lab results:Table 4

for iter=5 multiple statcom

With Multiple Statcom

Bus no. Voltagemagnitude

Voltage angle

1 1.0600 0

2 1.0000 -2.0549

3 1.0000 -4.8355

4 1.0000 -5.2094

5 0.9771 -5.8262

Injected reactive power at bus no: 3 and 4are QSVC = -0.0180 and -0.2331 Mvar

p.u.


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4.3.2 IEEE 14 BUS SYSTEM IEEE 14 BUS SYSTEM

4.3.2.1 WITHOUT USING THE STATCOM

Figure 4.4 IEEE 14bus


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MAT LAB RESULTS

Ieee 14 bus without using the statcom

Mat lab results without using the statcom for iter=12

TABLE 5

Bus no Voltage magnitude VM Voltage angle VA

1 1.0600 0

2 1.0000 0.6486

3 1.0000 -3.3780

4 0.9900 -7.2978

5 0.9956 -6.2474

6 1.000 -17.1069

7 0.9895 -16.2977

8 1.0000 -16.4584

9 0.9853 -20.2564

10 0.9812 -21.4684

11 0.9865 -20.0745

12 0.9881 -19.1840

13 0.9807 -19.6256

14 0.9648 -22.3504


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4.3.2.2USING SINGLE STATCOM

IEEE 14 BUS WITH USING THE SINGLE STATCOM

Figure 4.5 IEEE 14 bus with statcom


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Mat lab results by placing the single statcom at bus 14 for iter=12

TABLE 6


1 1.0600 0

2 1.0000 1.3388

3 1.0000 -1.4316

4 1.0047 -3.7057

5 1.0089 -3.0453

6 1.0000 -7.8704

7 0.9930 -7.03898 1.0000 -7.0389

9 0.9838 -8.8289

10 0.9786 -8.9877

11 0.9854 -8.5796

12 0.9866 -8.9359

13 0.9872 -9.2189

14 1.0000 -11.0470

Injected reactive power at bus no:4 is QSVC = -0.2221 Mvar p.u


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4.3.2.2USING MULTIPLE STATCOM

IEEE 14 BUS SYSTEM WITH MULTIPLE STATCOM

Figure 4.6 IEEE 14 bus with multiple statocm


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TABLE 7

Mat lab results for multi statcom at bus 4 and 14 for iter=17


1 1.0600 0

2 1.0000 1.3415

3 1.0000 -1.4435

4 1.0000 -3.6365

5 1.0060 -3.0054

6 1.0000 -7.8546

7 0.9911 -6.9891

8 1.0000 -6.9891

9 0.9822 -8.7840

10 0.9733 -8.9477

11 0.9848 -8.5506

12 0.9866 -8.9214

13 0.9872 -9.2057

14 1.0000 -11.0426

Injected reactive power at bus no:4 and 14 are QSVC = 0.1133 and -0.2280 Mvar

p.u


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5. CONCLUSION:

In this thesis, various aspects regarding voltage stability have been presented

and the importance to maintain voltage profile has been discussed.

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). In this thesis we had discussed about the

STATCOM modeling and analysis when connected to a bus and made it to maintain a

flat voltage profile of the full range of operation when there is a need. There by the

reactive power compensation was successfully done in the particular transmission

whenever it is required.

The power flow and the voltage profile in various transmission lines alongwith and without the placement of STATCOM in a specific transmission line is

obtained in order to improve the system performance by using the load flow studies

using MAT lab software.

Hence our objective to maintain voltage stability have been successfully

achieved with the incorporation of Static Synchronous Compensator (STATCOM).


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FUTURE SCOPE

The project has been performed on a 5-bus and 14 bus power system using

Newton raphson method with a single and multiple STATCOMs.

However, it can be extended to any bus system with single and multiple

STATCOMs if necessary. Along with this thesis optimization can also be done.


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Bibliography:

[1] IEEE FACTS working group 15.05.15, FACTS Application, December 1995.

[2] M. J. Lautenberg, M. A. Pai, and K. R. Padiyar, Hopf Bifurcation Control inPower System with Static Var Compensators, Int. J. Electric Power and EnergySystems, vol. 19, no. 5, 1997, pp. 339347.

[3] N. G. Hingorani and Laszlo Gyugyi, UNDERSTANDINGFACTS.

[4] C. A. Canizares and Z. T. Faur, Analysis of SVC and TCSC Controllers inVoltage Collapse, IEEE Trans. on Power Systems, vol. 14, no. 1, February 1999, pp.15865.

[5] C. A. Canizares. Power Flow and Transient Stability Models of FACTSControllers for Voltage and Angle Stability Studies. In Proc. of IEEE/PES WinterMeeting, Singapore, January 2000.

[6] G. Hingorani and L. Gyugi, Understanding FACTS: Concepts and Technology of

Flexible AC Transmission Systems. IEEE Press, 999.

[7] Modern Power System Anaylsis., Dillon.P.kothari

[8] C. A. Canizares, UWPFLOW: Continuation and Direct Methods to Locate FoldBifurcation in AC/DC/FACTS Power Systems. University of Waterloo, November

1999

[9] Ambriz-Perez, H. (2000)Advanced SVC Models for Newton-Raphson Load Flow

and Newton Optimal Power Flow Studies. IEEE Trans. On Power Systems, vol.15,

No:1, February 2000, pp 129-136.


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APPENDIX

IEEE 5 BUS SYSTEM

IEEE 5 bus system load and line data of the system

Impedance and line charging

Bus code impedance line charging

1-2 0.02+j0.06 0+j0.030

1-3 0.08+j0.24 0+j0.025

2-3 0.06+j0.18 0+j0.020

2-4 0.06+j0.18 0+j0.020

2-5 0.04+j0.18 0+j0.015

3-4 0.01+j0.18 0+j0.010

4-5 0.08+j0.24 0+j0.025

Table 8

Buscode

Assumedbusvoltage

Generation load

1 1.06+j0.0 slack 0 0

2 1.00+j0.0 40 30 20 10

3 1.00+j0.0 0 0 45 15

4 1.00+j0.0 0 0 40 5

5 1.00+j0.0 0 0 60 10


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LINE DATA

IEEE 14 BUS SYSTEMS:

Table 9

From bus To bus RESISTANCE(P.U)

Reactance (p.u.) Line

charging

1 3 0.04699 0.19797 0.0438

1 4 0.05811 0.17632 0.0374

1 5 0.05695 0.17388 0.034

2 1 0.01938 0.05917 0.0528

2 5 0.05403 0.22304 0.0492

3 4 0.06701 0.17103 0.0346

4 5 0.01335 0.04211 0.0128

4 7 0.00 0.20912 0.00

4 9 0.00 0.55618 0.00

5 6 0.00 0.25202 0.00

6 1

0.09498 0.1989 0.00

6 1

0.12291 0.25581 0.00

6 1

0.06615 0.13027 0.00

7 8 0.00 0.17615 0.00

7 9 0.00 0.11001 0.00

9 1

0.03181 0.08450 0.00

9 1

0.12711 0.27038 0.00

10 1

0.8205 0.19207 0.00

12 1

0.22092 0.19988 0.00

13 1

0.17903 0.34802 0.00


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Load data IEEE 14 bus system

Table 10

Busno.

PGenerated

(p.u.)

QGenerated

(p.u.)

PLoad

(P.U.

QLoad

(P.U.

BUSTYPE

QGenerated

MAX(p.u.

QGenerated

MIN1 2.32 0.00 0.00 0.00 2 10.0 -10.02 0.4 -0.424 0.2170 0.1270 1 0.5 -0.4

3 0.00 0.00 0.9420 0.1900 2 0.4 0.00

4 0.00 0.00 0.4780 0.00 3 0.00 0.005 0.00 0.00 0.0760 0.0160 3 0.00 0.006 0.00 0.00 0.1120 0.0750 2 0.24 -0.067 0.00 0.00 0.00 0.00 3 0.00 0.008 0.00 0.00 0.00 0.00 2 0.24 -0.06

9 0.00 0.00 0.2950 0.1660 3 0.00 0.0010 0.00 0.00 0.0900 0.0580 3 0.00 0.0011 0.00 0.00 0.0350 0.0180 3 0.00 0.0012 0.00 0.00 0.0610 0.0160 3 0.00 0.0013 0.00 0.00 0.1350 0.0580 3 0.00 0.0014 0.00 0.00 0.1400 0.0500 2 0.00 0.00

statcom based voltage stability

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