transmission system optimization in electrical power

International Journal of Emerging trends in Engineering and Development ISSN 2249-6149

Available online on http://www.rspublication.com/ijeted/ijeted_index.htm Issue 2, Vol.5 (July 2012)

Transmission System Optimization in Electrical

Power system –An Introductory Review

Tarun Mittal1, Amit Bhardwaj

2, Navpreet Singh Tung

3, Vikram Kumar Kamboj

4

1,4Member IEEE

1,2,3M.Tech Scholars, Lovely Professional University, Jalandhar, Punjab, India. 4Assistant Professor and Head of Deptt. (Electrical Engg.), CT Institute of Technology, Jalandhar, India.

___________________________________________________________________________

Abstract- This paper presents the basic principles of transmission system optimization .In recent years greater demands

have been placed on the transmission network and these demands will continue to increase because of the increasing

number of nonutility generators and heightened competition among utilities themselves. Increased demands on transmission

,absence of long term planning, and the need to provide open access to generating companies and customers ,all together

have created tendencies towards less security and reduced quality of supply. So that there is a great need to optimized

transmission system network in order to enhancing its capability ,flexibility and reliability for controlling this deregulated

environment and reducing the power system stresses.

Keywords- Interconnection(IC) ,power-flow(PF), compensation, FACTSs, Loading capability(LC), VSC’s.

___________________________________________________________________________

1. Introduction to Transmission Lines

Transmission lines are basically distributed parameter devices. For the study of fast switching transients, it is

necessary to model them in some detail. For example, the frequency response of a line can be approximated by

cascaded connection of π networks - a lumped parameter model. However for power system dynamic

performance studies involving frequencies below fundamental (synchronous frequency), the representation by a

single π circuit is adequate. As a matter of fact, for studies involving low frequency transients, the transmission

lines can be assumed to be in quasi-steady state - the voltages and currents can be assumed to be sinusoidal with

slowly varying amplitudes and phase angles. A basic assumption in the modelling of three phase transmission

lines is that they are symmetric. This implies that the self impedances of all the three phases are equal. Also, the

mutual impedances between any two phases are equal. An additional assumption is that the line parameters are

constant - the network is linear. It can be shown that, in steady state, a symmetric three phase linear network connected to synchronous generators has only fundamental frequency voltages or currents. On the other hand, a

lack of symmetry leads to unbalanced currents (with negative sequence components) which can result in third

harmonic voltage generation. The symmetry is disturbed during unbalanced faults such as single line to ground

or line to line faults. However, their duration is brief and the presence of harmonics can be neglected.

1.1 Modelling of Transmission Network

A single phase π equivalent of a transmission line is shown in Fig. 1 .1. However it is to be noted that the

coefficient matrices, inductance [L], resistance [R] and capacitance [C] are all 3x3 matrices. These are defined

as

Fig 1.1 A single phase π equivalent of transmission line



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1.2 Transformation to D-Q Components



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2. Tramsmission-System Parameters And Its Loading Capability

The interrelated parameters that governs the operation of transmission systems including series impedance,

shunt impedance, current, voltage, phase angle and the damping of oscillations at various frequencies below the

rated frequency. If we optimised these parameters by suitable techniques for eg. By FACTS CONTROLLERS

an emerging technology, by other compensating devices that is supplemented by high speed power electronics

switches( VSC’s, GTO’s ,IGBT’s)etc we can made possible to enable a line to carry power closer to its thermal

rating.

LOADING CAPABILITY LIMITS OF A TRANSMISSION SYSTEM-

Basically, there are three kinds of limitations:

1. Thermal

2. Dielectric

3. Stability

Thermal capability of an overhead transmission line is a function of the ambient temperature, wind

conditions, conditions of the conductor and ground clearance. It varies perhaps by a factor 2 to 1 due to

variable environment and loading history .The nominal rating of a line is generally decided on a

conservative basis, envisioning a worst ambient environment case scenario .Then there is a possibility

of upgrading a line by changing the conductor to that of higher current rating, which may in turn

require structural upgrading .Finally there is a possibility of converting a single-circuit to a double

circuit line. once the higher current capability is available, then the question arise how it should be

used. Will the extra power actually flow and be controllable?

Dielectric From an insulation point of view, many lines are designed conservatively. For a given

nominal voltage rating it is often possible to increase normal operation by +10% voltage (i.e ,500 KV-

550KV) or even higher. Care is then needed to ensure that dynamic and transient over voltages are with



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in limits. Compensation techniques could be used to ensure acceptable over- voltage and power flow

conditions.

Stability There are number of stability issues that limits the transmission capability. These include:

1. Transient stability

2. Dynamic stability

3. Steady-state stability

4. Frequency collapse

5. Voltage collapse

6. Sub-synchronous resonance

The various transmission system compensation techniques can certainly be used to overcome any

of the stability limits, in which case the ultimate limit would be thermal and dielectric.

2 Basics of Power Transmission Networks

Transmission lines that carry power from the sources to loads, modern power systems are also highly

interconnected for economic reasons. The interconnected systems benefit by (a) exploiting load diversity (b)

sharing of generation reserves and (c) economy gained from the use of large effcient units without sacrificing

reliability. However, there is also a downside to ac system interconnection { the security can be adversely

affected as the disturbances initiated in a particular area can spread and propagate over the entire system

resulting in major blackouts caused by cascading outages.

2.1 CONCEPT OF REACTIVE POWER IN AC TRANSMISSION SYSTEM

In this an understanding of reactive power associated with power transmission networks is developed. To make

transmission networks operate within desired voltage limits, methods of making up or taking away reactive

power— hereafter called reactive-power control—are discussed. Before proceeding further, however, a

thorough understanding of the reactive power in ac systems is necessary. Upon energization, the ac networks

and the devices connected to them create associated time-varying electrical fields related to the applied voltage, as well as magnetic fields dependent on the current flow. As they build up, these fields store energy that is

released when they collapse. Apart from the energy dissipation in resistive components, all energy-coupling

devices, including transformers and energy-conversion devices (e.g., motors and generators), operate based on

their capacity to store and release energy. For the ac circuit shown in Fig. 2.1(a), instantaneous power from the

voltage source to the load Z/–f, in terms of the instantaneous voltage v and current i, is given as

where V and I are the respective root mean square (rms) values of v and i. Equations (2.1) and (2.2) are

pictorially represented in Fig. 2.1(b). Equation (2.2) comprises two double-frequency (2q) components. The first

term has an average value as well as a peak magnitude of VI cosf. This average value is the active power, P,

flowing from the source to the load. The second term has a zero average value, but its peak value is VI sin f.

Written in phasor domain, the complex power in the network in Fig. 2.1(a) is given by



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where P is called the active power, which is measured in watts (W), and Q is called the reactive power, which is

measured in volt–ampere reactive (VAR). Comparing Eqs. (2.3) and (2.2), the peak value of the second

component of instantaneous power in Eq. (2.2) is identified as the reactive power. The reactive power is

essential for creating the needed coupling fields for energy devices. It constitutes voltage and current loading of

circuits but does not result in an average (active) power consumption and is, in fact, an important component in

all ac power networks. In high-power networks, active and reactive powers are measured in megawatts (MW) and MVAR, respectively. Figure 2.1(c) shows a commonly used power triangle. Electromagnetic devices store

energy in their magnetic fields. These devices draw lagging currents, thereby resulting in positive values of Q;

therefore, they are frequently referred to as the absorbers of reactive power. Electrostatic devices, on the other

hand, store electric energy in fields. These devices draw leading currents and result in a negative value of Q;

thus they are seen to be suppliers of reactive power. The convention for assigning signs to reactive power is

different for sources and loads, for which reason readers are urged to use a consistent notation of voltage and

current, to rely on the resulting sign of Q, and to not be confused by absorbers or suppliers of reactive power.

2.2.2 UNCOMPENSATED TRANSMISSION LINES

2.2.2.1 A Simple Case

To develop a good, qualitative understanding of the need for reactive-power control, let us consider a simple

case of a lossless short-transmission line connecting a source Vs to a load ZLᶲ (For simplicity, the line is

represented only by its inductive reactance Xl.) Figure 2.2 shows such a network with its parameters, as well as a

phasor diagram showing the relationship between voltages and currents. From Fig. 2.2(b), it is clear that

between the sending- and the receiving-end voltages, a magnitude variation, as well as a phase difference, is

created. The most significant part of the voltage drop in the line reactance (∆V1=ĪxXl) is due to the reactive

component of the load current, Ix. To keep the voltages in the network at nearly the rated value, two control

actions seem possible:

1. load compensation, and

2. system compensation.

1. Load Compensation It is possible to compensate for the reactive current Ix of the load by adding a parallel capacitive load so that Ic= −Ix. Doing so causes the effective power factor of the combination to become unity.

The absence of Ix eliminates the voltage drop ∆V1, bringing Vr closer in magnitude to Vs; this condition is called

load compensation. Actually, by charging extra for supplying the reactive power, a power utility company

makes it advantageous for customers to use load compensation on their premises. Loads compensated to the

unity power factor reduce the line drop but do not eliminate it; they still experience a drop of ∆V2 from jĪrXl..



89

2. System Compensation To regulate the receiving-end voltage at the rated value, a power utility may install a

reactive-power compensator as shown in Fig. 2.3. This compensator draws a reactive current toovercome both

components of the voltage drop ∆V1 and ∆V2as a consequence of the load current Il. through the line reactance

Xl. To compensate for ∆V2, an additional capacitive current, ∆Ic, over and above Ic that compensates for Ix, is

drawn by the compensator. When ∆ĪcXl = ∆V2, the receiving-end voltage, Vr, equals the sending-end voltage,

Vs. Such compensators are employed by power utilities to ensure the quality of supply to their customers [1].

2.2.2.2 Lossless Distributed Parameter Lines

Most power-transmission lines are characterized by distributed parameters: series resistance, r; series

inductance, l; shunt conductance, g; and shunt capacitance, c—all per-unit (pu) length. These parameters all

depend on the conductors’ size, spacing, clearance above the ground, and frequency and temperature of

operation. In addition, these parameters depend on the bundling arrangement of the line conductors and the

nearness to other parallel lines. The characteristic behaviour of a transmission line is dominated by its l and c

parameters. Parameters r and g account for the transmission losses. The fundamental equations governing the

propagation of energy along a line are the following wave equations:



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These equations are used to calculate voltage and current anywhere on line, at a distance x from the sending end,

in terms of the sending-end voltage an current and the line parameters. In Eqs. (2.4) and (2.5),

Comparing Eqs. (2.7) and (2.8) and taking the directional notation of Fig. 2.4 into account, it is concluded that for a lossless line, Ps = − Pr , as expected. However, Qs≠ Qr because of the reactive-power

absorption/generation in the line. From Eqs. (2.7) and (2.8), the power flow from the sending end to the

receiving end is expressed as

In electrically short power lines, where βa is very small, it is possible to make a simplifying assumption that sinβa = ba or Z0 sin βa= Z0βa = ωla, where ωla=Xl is the total series reactance of a line. This substitution results

in the following well-recognized power equation:



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Accordingly, the maximum power transfer is seen to depend on the line length. When the power-transfer

requirement for a given length of a line increases, higher transmission voltages of Vs and Vr must be selected.

So we examine only those aspects that enhance the understanding of the interplay between voltages on the line

and the resulting reactive-power flows.

2.2.2.3 Symmetrical Lines -When the voltage magnitudes at the two ends of a line are equal, that is, Vs

=Vr = V, the line is said to be symmetrical. Because power networks operate as voltage sources, attempts are

made to hold almost all node voltages at nearly rated values. A symmetrical line, therefore, presents a realistic

situation. From Eqs. (2.7) and (2.8) the following relationships are derived

2.2.2.4 Midpoint Conditions of a Symmetrical Line- The magnitude of the midpoint voltage

depends on the power transfer. This voltage influences the line insulation and therefore needs to be well

understood. For a symmetrical line where the end voltages are held at nominal values, the midpoint voltage

shows the highest magnitude variation. In terms of the midpoint voltage Vm, the receiving-end voltage of a

symmetrical line, from Eq. (2.4), is given as



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2.3 CONCEPT OF POWER FLOW IN AC TRANSMISSION NETWORK-

A large majority of power transmission lines are AC lines operating at different voltages (10 kV to 800 kV). The

distribution networks generally operate below 100 kV while the bulk power is transmitted at higher voltages.

The lines operating at different voltages are connected through transformers which operate at high efficiency.

Traditionally, AC lines have no provision for the control of power flow. The mechanically operated circuit

breakers (CB) are meant for protection against faults (caused by flashovers due to over voltages on the lines or

reduced clearances to ground). A CB is rated for a limited number of open and close operations at a time and

cannot be used for power flow control. (unlike a high power electronic switch such as thyristor, GTO, IGBT,

IGCT, etc.). Fortunately, ac lines have inherent power flow control as the power flow is determined by the

power at the sending end or receiving end. For example, consider a transmission line connecting a generating

station to a load centre in Fig.2.5(a). Assuming the line to be lossless and ignoring the line charging, the power

flow (P) is given by

where X is the series line reactance. Assuming V1 and V2 to be held constants (through voltage regulators at the

two ends), the power injected by the power station determines the flow of power in the line. The difference in

the bus angles is automatically adjusted to enable P = PG (Note that usually there could be more than one line

transmitting power from a generating station to a load centre). If one or more lines trip, the output of the power

station may have to be reduced by tripping generators, so as to avoid overloading the remaining lines in

operation.



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Fig. 2.5(b) shows another situation where a line supplies power to a load located at bus (2). Here also the eq.

(2.16) applies but the power flow in the line is determined by the load supplied. The essential difference

between the two situations is that in Fig. 2.5(a), the load centre is modelled as an infinite bus which can absorb

(theoretically) any amount of power supplied to it from the generating station. This model of the load centre

assumes that the generation available at the load centre is much higher than the power supplied from the remote

power station (obviously, the total load supplied at the load centre is equal to the net generation available at that

bus).

The reliability of the power supply at a load bus can be improved by arranging two (or more) sources of power

as shown in Fig. 2.6. Here, P1 is the output of G1 while P2 is the output of G2 (Note that we are neglecting

losses as before). However, the tripping of any one line will reduce the availability of power at the load bus. This problem can be overcome by providing a line (shown dotted in Fig. 2.6) to interconnect the two power

stations. Note that this results in the creation of a mesh in the transmission network. This improves the system

reliability, as tripping of any one line does not result in curtailment of the load. However, in steady state, P1 can

be higher or lower than PG1 (the output of G1). The actual power flows in the 3 lines forming a mesh are

determined by Kirchhoff's Voltage Law (KVL). In general, the addition of an (interconnecting) line can result in

increase of power flow in a line (while decreasing the power flow in some other line). This is an interesting

feature of AC transmission lines and not usually well understood (in the context of restructuring). In general, it

can be stated that in an uncontrolled AC transmission network with loops (to improve system reliability), the

power flows in individual lines are determined by KVL and do not follow the requirements of the contracts

(between energy producers and customers). In other words, it is almost impossible to ensure that the power flow

between two nodes follows a predetermined path. This is only feasible in radial networks (with no loops), but the reliability is adversely affected as even a single outage can result in load curtailment. Consider two power

systems, each with a single power station meeting its own local load, interconnected by a tie line as shown in

Fig. 2.7(a). In this case, the power flow in the tie line (P) in steady state is determined by the mismatch between

the generation and load in the individual areas. Under dynamic conditions, this power flow is determined from

the equivalent circuit shown in Fig. 2.7(b). If the capacity of the tie is small compared to the size (generation) of

the two areas, the angles ±1 and ±2 are not affected much by the tie line power flow. Thus, power flow in AC tie

is generally uncontrolled and it becomes essential to trip the tie during a disturbance, either to protect the tie line

or preserve system security. In comparison with a AC transmission line, the power flow in a HVDC line in

controlled and regulated. However, HVDC converter stations are expensive and HVDC option is used primarily

for (a) long distance bulk power transmission (b) interconnection of asynchronous systems and (c) underwater

(submarine) transmission. The application of HVDC transmission (using thyristor converters) is also

constrained by the problem of commutation failures affecting operation of multi terminal or multi-feed HVDC systems. This implies that HVDC links are primarily used for point-to-point transmission of power and

asynchronous interconnection (using Back to Back (BTB) links).



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2.4 CONTROL OF POWER FLOW IN AC TRANSMISSION NETWORK-

We may like to control the power flow in a AC transmission line to (a) enhance power transfer capacity and or

(b) to change power flow under dynamic conditions (subjected to disturbances such as sudden increase in load,

line trip or generator outage) to ensure system stability and security. The stability can be affected by growing

low frequency, power oscillations (due to generator rotor swings), loss of synchronism and voltage collapse

caused by major disturbances.

where ±max (30±{40±) is selected depending on the stability margins and the stiffness of the terminal buses to

which the line is connected. For line lengths exceeding a limit, Pmax is less than the thermal limit on the power

transfer determined by the current carrying capacity of the conductors (Note this is also a function of the

ambient temperature). As the line length increases, X increases in a linear fashion and Pmax reduces as shown in

Fig. 2.8



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The series compensation using series connected capacitors increases Pmax as the compensated value of the

series reactance (Xc) is given by

where kse is the degree of series compensation. The maximum value of kse that can be used depends on several

factors including the resistance of the conductors. Typically kse does not exceed 0.7. Fixed series capacitors have

been used since a long time for increasing power transfer in long lines. They are also most economical solutions

for this purpose. However, the control of series compensation using thyristor based FACTS Controllers in

Power Transmission and Distribution switches has been introduced only 10{15 years ago for fast power flow control. The use of Thyristor Controlled Reactors (TCR) in parallel with fixed capacitors for the control of Xc,

also helps in overcoming a major problem of Sub-synchronous Resonance (SSR) that causes instability of

torsional modes when series compensated lines are used to transmit power from turbo generators in steam power

stations. In tie lines of short lengths, the power flow can be controlled by introducing Phase Shifting

Transformer (PST) which has a complex turns ratio with magnitude of unity. The power flow in a lossless

transmission line with an ideal PST (see Fig. 2.9) is given by

Again, manually controlled PST is not fast enough under dynamic conditions. Thyristor switches can ensure fast

control over discrete (or continuous) values of Á, depending on the configuration of PST used. Pmax can also be

increased by controlling (regulating) the receiving end voltage of the AC line. When a generator supplies a unity

power factor load(see fig:2.5b) , the maximum power occurs when the load resistance is equal to the line

reactance. It is to be noted that V2 varies with the load and can be expressed as

By providing dynamic reactive power support at bus (2) as shown in Fig. (2.10), it is possible to regulate the bus

voltage magnitude. The reactive



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Comparing eq. (2.22) with (2.16), it can be seen that the maximum power transfer can be doubled just by

providing dynamic reactive power support at the receiving end of the transmission line. This is in addition to the

voltage support at the sending end. It is to be noted that while steady state voltage support can be provided by

mechanically switched capacitors, the dynamic voltage support requires synchronous condenser or a power

electronic controller such as Static Var Compensator (SVC) or STATic synchronous COMpensator

(STATCOM).

3 TECHNIQUES OR METHODS TO OPTIMIZED OR COMPENSATE

TRANSMISSION SYSTEM NETWORK-

3.1 CONVENTIONAL CONTROL MECHANISMS

The lack of control on active- and reactive-power flow on a given line, embedded in an interconnected ac transmission network, was stated. Also, to maintain steady-state voltages and, in selected cases, to alter the

power-transmission capacity of lines, traditional use of shunt and series impedances was hinted. In a

conventional ac power system, however, most of the controllability exists at generating stations. For example,

generators called spinning reserves maintain an instantaneous balance between power demand and power

supply. These generators, in fact, are purposely operated at reduced power. Also, to regulate the system

frequency and for maintaining the system at the rated voltage, controls are exercised on selected generators

1. Automatic Generation Control (AGC)

The megawatt (MW) output of a generator is regulated by controlling the driving torque, Tm, provided by a

prime-mover turbine. In a conventional electromechanical system, it could be a steam or a hydraulic turbine.

The needed change in the turbine-output torque is achieved by controlling the steam/water input into the turbine.

Therefore, in situations where the output exceeds or falls below the input, a speed-governing system senses the

deviation in the generator speed because of the load-generation mismatch, adjusts the mechanical driving torque to restore the power balance, and returns the operating speed to its rated value. The speed-governor output is

invariably taken through several stages of mechanical amplification for controlling the inlet (steam/water)

valve/gate of the driving turbine. Figure 3.1 shows the basic speed-governing system of a generator supplying

an isolated load. The operation of this basic feedback-control system is enhanced by adding further control

inputs to help control the frequency of a large interconnection. In that role, the control system becomes an

automatic generation control (AGC) with supplementary signals.



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To avoid competing control actions, in a multi generator unit station each speed-governor system is provided

with droop (R) characteristics through a proportional feedback loop (R, Hz/MW). Figure 3.2 shows an AGC on

the principal generating unit with supplementary control. In contrast, the second, third, and remaining

generating units in a multiunit station operate with their basic AGCs. In a complex interconnected system, the

supplementary control signal may be determined by a load-dispatch centre.

2. Excitation Control

The basic function of an exciter is to provide a dc source for field excitation of a synchronous generator. A

control on exciter voltage results in controlling the field current, which, in turn, controls the generated voltage.

When a synchronous generator is connected to a large system where the operating frequency and the terminal

voltages are largely unaffected by a generator, its excitation control causes its reactive power output to change.

In older power plants, a dc generator, also called an exciter, was mounted on the main generator shaft. A control

of the field excitation of the dc generator provided a controlled excitation source for the main generator. In

contrast, modern stations employ either a brushless exciter (an inverted 3-phase alternator with a solid-state

rectifier connecting the resulting dc source directly through the shaft to the field windings of the main generator)

or a static exciter (the use of a station supply with static rectifiers). An excitation-control system employs a

voltage controller to control the excitation voltage. This operation is typically recognized as an automatic

voltage regulator (AVR). However, because an excitation control operates quickly, several stabilizing and protective signals are invariably added to the basic voltage regulator. A power-system stabilizer (PSS) is

implemented by adding auxiliary damping signals derived from the shaft speed, or the terminal frequency, or the

power—an effective and frequently used technique for enhancing small-signal stability of the connected system.

Figure 3.3 shows the functionality of an excitation-control system.

3. Transformer Tap-Changer Control

Next to the generating units, transformers constitute the second family of major power-transmission-system

apparatuses. In addition to increasing and decreasing nominal voltages, many transformers are equipped with

tap-changers to realize a limited range of voltage control. This tap control can be carried out manually or

automatically. Two types of tap changers are usually available: offload tap changers, which perform adjustments when de-energized, and on-load tap changers, which are equipped with current-commutation capacity and are

operated under load. Tap changers may be provided on one of the two transformer windings as well as on

autotransformers. Because tap-changing transformers vary voltages and, therefore, the reactive power flow,

these transformers may be used as reactive-power-control devices. On-load tap-changing transformers are

usually employed to correct voltage profiles on an hourly or daily basis to accommodate load variations. Their

speed of operation is generally slow, and frequent operations result in electrical and mechanical wear and tear.

4. Phase-Shifting Transformers

A special form of a 3-phase–regulating transformer is realized by combining a transformer that is connected in

series with a line to a voltage transformer equipped with a tap changer. The windings of the voltage transformer

are so connected that on its secondary side, phase-quadrature voltages are generated and fed into the secondary

windings of the series transformer. Thus the addition of small, phase-quadrature voltage components to the phase voltages of the line creates phase-shifted output voltages without any appreciable change in magnitude.

A phase-shifting transformer is therefore able to introduce a phase shift in a line. Figure 1.4 shows such an

arrangement together with a phasor diagram. The phasor diagram shows the phase shift realized without an

appreciable change in magnitude by the injection of phase-quadrature voltage components in a 3-phase



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system. When a phase-shifting transformer employs an on-load tap changer, controllable phase-shifting is

achieved. The interesting aspect of such phase shifters is that despite their low MVA capacity, by controlling the

phase shift they exercise a significant real-power control. Therefore, they are used to mitigate circulating power

flows in interconnected utilities. A promising application of these devices is in creating active-power regulation

on selected lines and securing active-power damping through the incorporation of auxiliary signals in their

feedback controllers. From this description, it is easy to visualize that an incremental in-phase component can

also be added in lines to alter only their voltage magnitudes, not their phase. The modification of voltage

magnitudes andor their phase by adding a control voltage is an important concept. It forms the basis of some

of the new FACTS devices discussed in this book. The injected voltage need not be realized through electromagnetic transformer–winding arrangements; instead, by using high-speed semiconductor switches such

as gate turn-off (GTO) thyristors, voltage source inverters (VSIs)—synchronized with the system frequency—

are produced. The application of a VSI to compensate the line voltage drop yields a new, fast, controllable

reactive-power compensator: the static synchronous series compensator (SSSC). The application of a VSI to

inject a phase-quadrature voltage in lines yields a new, fast, controllable phase shifter for active- power control.

Once a synchronized VSI is produced, it is indeed easy to regulate both the magnitude and the phase angle of

the injected voltages to yield a new, unified power-flow controller (UPFC).

3.2 Flexible AC Transmission System

Controllers-

3.2.1 General Description The large interconnected transmission networks (made up of predominantly overhead transmission lines) are

susceptible to faults caused by lightning discharges and decrease in insulation clearances by undergrowth. The

power flow in a transmission line is determined by Kirchhoff's laws for a specified power injections (both active

and reactive) at various nodes. While the loads in a power system vary by the time of the day in general, they

are also subject to variations caused by the weather (ambient temperature) and other unpredictable factors. The

generation pattern in a deregulated environment also tends to be variable (and hence less predictable). Thus, the

power flow in a transmission line can vary even under normal, steady state conditions. The occurrence of a

contingency (due to the tripping of a line, generator) can result in a sudden increase/decrease in the power flow.

This can result in overloading of some lines and consequent threat to system security.

A major disturbance can also result in the swinging of generator rotors which contribute to power swings in

transmission lines. It is possible that the system is subjected to transient instability and cascading outages as individual components (lines and generators) trip due to the action of protective relays. If the system is

operating close to the boundary of the small signal stability region, even a small disturbance can lead to large

power swings and blackouts. The increase in the loading of the transmission lines sometimes can lead to voltage

collapse due to the shortage of reactive power delivered at the load centres. This is due to the increased

consumption of the reactive power in the transmission network and the characteristics of the load (such as

induction motors supplying constant torque).

The factors mentioned in the previous paragraphs point to the problems faced in maintaining economic and

secure operation of large interconnected systems. The problems are eased if sufficient margins (in power

transfer) can be maintained. This is not feasible due to the difficulties in the expansion of the transmission

network caused by economic and environmental reasons. The required safe operating margin can be

substantially reduced by the introduction of fast dynamic control over reactive and active power by high power

electronic controllers. This can make the AC transmission network `flexible' to adapt to the changing conditions caused by contingencies and load variations. Flexible AC Transmission System (FACTS) is defined as

`Alternating current transmission systems incorporating power electronic-based and other static controllers to

enhance controllability and increase power transfer capability' [1,2]. The FACTS controller is defined as `a



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power electronic based system and other static equipment that provide control of one or more AC transmission

system parameters'.

The FACTS controllers can be classified as-

1. Shunt connected controllers

2. Series connected controllers 3. Combined series-series controllers

4. Combined shunt-series controllers

Depending on the power electronic devices used in the control, the FACTS controllers can be classified as-

(A) Variable impedance type

(B) Voltage Source Converter (VSC) - based.

The variable impedance type controllers include:

(i) Static Var Compensator (SVC), (shunt connected)

(ii) Thyristor Controlled Series Capacitor or compensator (TCSC), (series connected)

(iii) Thyristor Controlled Phase Shifting Transformer (TCPST) of Static PST (combined shunt and series)

The VSC based FACTS controllers are:

(i) Static synchronous Compensator (STATCOM) (shunt connected)

(ii) Static Synchronous Series Compensator (SSSC) (series connected)

(iii) Interline Power Flow Controller (IPFC) (combined series-series)

(iv)Unified Power Flow Controller (UPFC) (combined shunt-series)

Some of the special purpose FACTS controllers are

(a) Thyristor Controller Braking Resistor (TCBR)

(b) Thyristor Controlled Voltage Limiter (TCVL)

(c) Thyristor Controlled Voltage Regulator (TCVR) (d) Inter-phase Power Controller (IPC)

(e) NGH-SSR damping

The FACTS controllers based on VSC have several advantages over the variable impedance type. For example,

a STATCOM is much more compact than a SVC for similar rating and is technically superior. It can supply

required reactive current even at low values of the bus voltage and can be designed to have in built short term

overload capability. Also, a STATCOM can supply active power if it has an energy source or large energy

storage at its DC terminals. The only drawback with VSC based controllers is the requirement of using self

commutating power semiconductor devices such as Gate Turn-off (GTO) thyristors, Insulated Gate Bipolar

Transistors (IGBT), Integrated Gate Commutated Thyristors (IGCT). Thyristors do not have this capability and

cannot be used although they are available in higher voltage ratings and tend to be cheaper with reduced losses.

However, the technical advantages with VSC based controllers coupled will emerging power semiconductor de-vices using silicon carbide technology are expected to lead to the wide spread use of VSC based controllers in

future. It is interesting to note that while SVC was the first FACTS controllers (which utilized the thyristor

valves developed in connection with HVDC line commutated convertors) several new FACTS controller based

on VSC have been developed. This has led to the introduction of VSC in HVDC transmission for ratings up to

300 MW.

3.2.2 EMERGING TRANSMISSION NETWORKS

A historic change is overtaking electrical power utility businesses. Customers are demanding their right to

choose electrical energy suppliers from competing



Page 800

vendors—a movement that has arisen from the benefits of lower costs of such services as long-distance

telephone calls, natural-gas purchases, and air travel. The industries embracing these activities have been

recently deregulated, and in these sectors, competition has been introduced. The basic belief is that competition

leads to enhanced efficiency and thus lower costs and improved services. For nearly 100 years, electrical power

utilities worldwide have been vertically integrated, combining generation, transmission, distribution, and

servicing loads. Also, most such utilities have operated as monopolies within their geographic regions. Their method of operation has been ―power at cost,‖ and their principal financers have been governments. Therefore,

to many people the pressure of electrical power utilities to operate efficiently has been missing. Operating the

electrical energy sector competitively requires the unbundling of generation, transmission, and distribution.

Competition is expected to exist among generators as well as retailers. The transmission and distribution (i.e.,

the controlling wires) must, out of necessity, be regulated. The new order requires new agencies taking the

responsibility to link customers (loads) with generators (market operators) and, at the same time, to clearly

understand the limitations and capabilities of power-transmission and -distribution networks [22], [23].

On becoming responsible for its own business, a power-transmission company must make the best use of its

transmission capacity and ensure that transmission losses are reduced to their lowest values. Also, any loss of

transmission capacity means loss of income for the company; therefore, all actions must be taken to ensure that

unwanted circulating power is not clogging the available transmission capacity. In addition, energy congestion

in critical transmission corridors must be avoided to eliminate the risk of missed business opportunities. Finally, to offer the greatest flexibility to market operators, a transmission company must create the maximum safe

operating limits to allow power injection and tapping from its buses without risking stable operation. The

success of a transmission company depends on offering the maximum available transmission capacity (ATC) on

its lines. From the foregoing discussion, it is evident that in the emerging electrical energy business,

transmission companies have a greater need to make their networks more flexible. Fortunately, advances in

power-electronics technology now offer new fast, controllable FACTS controllers to secure the needed

flexibility[15], [22], [23].The subject matter contained in this book is intended to assist engineers seeking

FACTS knowledge and help utilities meet the energy challenge.

Conclusion In this paper,we introduced basic principles and optimization techniques of transmission system which will help

future researchers to dig out the innovative solution for transmission system.

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Page 801

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BIOGRAPHIES-

Tarun Mittal is an M-Tech Scholar in Electrical Engg., School of Electrical and Electronic., Lovely

Professional University, India. He received his B-Tech in Electrical Engg. from PTU Jalandhar. His area of interest involves Power Electronics, FACTS Devices,Transmission System, Application of power electronics in power quality issues. (email -mittal.9009@ gmail.com)

Navpreet Singh Tung is an M-Tech Scholar in Electrical Engg., School of Electrical and Electronics Engineering. , Lovely Professional University, India. He received his B-Tech in Instrumentation and Control from NIT Jalandhar. His area of interest involves Power system planning, Power Converters, Unit Commitment,Application..of..power..electronics in power quality issues. He is an IEEE member.(e-mail: navpreet.tung@ ieee.org).

Amit Bhardwaj was born in district Rohtak, Haryana, India in 1990. He received his B.Tech. degree in

Electrical Engineering from Maharishi Markandeshwar University, Ambala, Haryana, INDIA in 2011. He is currently pursuing M.Tech. in Electrical Engg. from Lovely Professional University, Jalandhar, India . His area of interests are the development of methodologies for the Optimization and Planning of Electrical Power Systems. (e-mail: amitbdwj47@ gmail.com).

Vikram Kumar Kamboj is Ph.D Research scholar in Punjab Technical University, Jalandhar, Punjab (INDIA). Presently, he is working as Assistant Professor and Head of the Department, EEE in CT Institute of Technology, Jalandhar, Punjab, INDIA. His area of interests includes Reliability, Optimization and Planning of Electrical Power System. (e-mail: kamboj.vikram@

gmail.com).

transmission system optimization in electrical power

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