Session 6(Power Electronic Controllers)

7/29/2019 Session 6(Power Electronic Controllers)

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M. S. Ramaiah School of Advanced Studies, Bangalore

PEMP EMP2506PT12

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Session 6

Session Speaker

Thejaswini R

Power Electronic Controllers

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Session Objectives

Students will be able to learn about:

Introduction

Limitations of Transmission systems

Principle of series and shunt compensation

FACTs controllers and their modelling Static condenser, Static phase shifter

TCR, SVC, TCSC, STATCOM, SSSC, UPFC

Harmonics analysis

Source of power system harmonics

Propagation, modelling, Elimination.

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Session Topics Introduction to power electronic controllers

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Session Topics

Facts

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Introduction

The AC transmission system has various limits classified as

static limits and dynamic limits.

Desired performance was not being able to achieve effectively.

There was greater need for the alternative technology made of

solid state devices with fast response characteristics.

Invention of Thyristor switch (semiconductor device), openedthe door for the development of power electronics devices known as

Flexible AC Transmission Systems (FACTS) controllers.

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Below are some of the drawbacks faced on the presentcommon used devices in present Power Grid

Technology.

1) long Switching Periods

2) High power loss

3) Slow response to system power change

4) Low System Stability

Introduction

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Introduction

The development and growth in the power industry requires the

power transmission systems to be highly stable with maximumthermal limits

For a profitable operation of generation, transmission and

distribution system, the incorporation of advanced technologyand increased efficiency in utilization and control of the existing

power transmission system infrastructure are mandatory

FACTS technology allows us to achieve quality and reliabilityof the transmission systems with minimum investment

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FACTS

It is an evolving technology-based solution envisioned to helpthe utility industry to deal with changes in the power-delivery

business

It is a new integrated concept based on power electronicswitching converters and dynamic controllers to enhance the

system utilization and power transfer capacity as well as the

stability, security, reliability and power quality of AC system

interconnections

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FACTS controllers

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Shunt-connected Controllers

A shunt controller may be a variable impedance or avariable source frequency or it may be a combination of

both

The function of the shunt controller is to inject current into

the transmission line at the point of installation

When the injected current is in-phase quadrature with the

overhead line voltage, this controller controls only supplies

or consumes the variable reactive power

If injected current is not inphase quadrature with the line

voltage, it controls the active power also

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Series-Connected Controller

Series controllers may be a variable impedance or a

variable source frequency

The function of the series controller is to inject voltage in

series with the transmission line

Even though the current that flows through the

transmission line gets multiplied with the variable

impedance, the voltage obtained represents an injected

series voltage in the line

When the injected voltage is inphase quadrature with the

over head line current, this controller controls only thevariable reactive power and if injected voltage is not

inphase quadrature with the line current, it controls active

power also

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Combined shunt and series- connected controllers

By combining individual shunt and series connected

controllers in a coordinated manner, a new type ofcontroller can be obtained

The main function of this controller is to inject the current

into the system and to inject voltage in series with the line

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FACTS key devices

Static Synchronous Compensator (STATCOM)

Static Synchronous Series Compensator (SSSC)

Unified Power Flow Controller (UPFC)

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Static Synchronous Compensator

It is a static synchronous generator as shunt static varcompensator whose capacitive or inductive current can be

controlled independent of the system voltage.

The STATCOM scheme in parallel with AC power gridsystem and is controlled by a dynamic controller

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STATIC SYNCHRONOUS SERIES COMPENSATOR

It is a static synchronous generator operated without an

external energy source as a series compensator.

The output voltage is in quadrature with and controllable

independently of the line current.

It is increase or decrease the overall reactive voltage drop

across the line and thereby controlling the transmitted

electric power.

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Series Compensated Network

Application

Machine initialized for

P=1500 MW

Vt=13.8kV

Three-Phase Series Compensated Network

The 'Model initialization function' defined in the

Model P roperties automatical ly sets the sample time T s

to 50e-6 s

Discrete,

Ts = Ts s.

A

B

C

a

b

c

Series

Comp. 2

A

B

C

a

b

c

Series

Comp. 1

-C-

Pm

Line 2

(300 km)

Line 1

(300 km)

ABCABC

Fault

Breaker

-C-

E

?

Double-click here for info

Open this blockto visualize

recorded signal s

Data Acquisition

A

B

C

a

b

c

CB2

A

B

C

a

b

c

CB1

A

B

a

b

c

B3

A

B

B2

A

B

B1

Pm

E

m

A

B

C

SSM

6*350MVA

13.8 kV

A

B

C

a

b

c

6*350 MVA

13.8/735 kV

ABC

330 Mvar

ABC

330 Mvar

A

B

C

a2b2c2a3b3c3

300 MVA

735/230 kV

A

B

C

30,000 MVA

735 kV

ABC

250 MW

ABC

100 MW

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Series Compensated Network

Latch

1

out

Discrete,

Ts = 5e-005 s.

z

1

v+-

UCsa

Scope1

Scope

>

RL Damp.

30e6

MOV

Energy Threshold

MOV

i+

-

IMOV

c

12

Gap

K Ts

z-1

Discrete-Time

Integrator

Cs

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UNIFIED POWER FLOW CONTROLLER (UPFC)

The UPFC scheme consists of two basic switching power

converter namely shunt and series converters connected to

each other through a dc link capacitor

The shunt converter operates exactly as STATCOM for

reactive power compensation and voltage stabilization

The series converter operates as SSSC to control the real

power flow

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FACTS technology: Opportunities

Control power so that it flows on the desired routes.

Increase loading capacity of transmission lines.

Prevent blackouts.

Improve generation productivity.

Effective use of upgrading/ uprating

A set of hybrid electromagnetic

guiding system[2]19

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Control of Power Systems

FACTS is designed to meet planners, investors and

operators goals without their having to undertake major

system additions

It can remove the following constraints

Transmission stability limit

Voltage limits

Loop flows

It enables different ways of attaining an increase of

power transmission capacity at optimum conditions

Power quality improvement and protecting equipment in

transmission and distribution is another major reason for

the implementation of the FACTS technology

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Benefits

When FACTS controllers are used, the following improvements in the

power system may be obtained:

Stability

Loading capability

Security and reliability

Added flexibility in setting new generation

No need for additional transmission lines

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FACTS for minimizing grid investment

The important incentive for considering FACTS in grid

planning is that it is economically-attractive alternative for

investments in more transmission lines

By considering series compensation from the very

beginning, power transmission between regions can be

planned with a minimum of transmission circuits, there byminimizing costs as well as environmental impact from the

start

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i i li l l i



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Transmission line voltage regulation

The natural active and reactive power flows (Prn and Qrn)

at the receiving end are

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SVC

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Thyristor Controlled Series Compensator

(TCSC)

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STATCOM

Shunt device

Lower rated components

since only carry a

fraction of the line

current

Impacts bus voltage and

reactive power support

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SSSC

Series device

Must have higher rated

transformer and

devices iImpacts active power

flow

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UPFC

Combination ofSTATCOM and SSSC

Mmay control voltage,

impedance, and angle Impacts active and

reactive power flow in

line

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UPFC Topology

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PSS and SVC Application

Transient stability of a two-machine transmission system

with Power System Stabilizers (PSS) and Static Var Compensator (SVC)

Phasors

powergui

-C-no PSS

?

info

m

Pref

Pm

Vf

Turbine &

Regulators M2

m

Pref

Pm

Vf

Turbine &

Regulators M1

System

Show impact of PSS

for 1-phase faul t

Show impact of SVC

for 3-phase faul t

Show compa rison

Detailed vs Phasor

simulation

STOP

Stop Simulation

if loss of synchronism

m

A B C

SVCSVC

(Phasor Type)

SVC

This demo requires the Control System Toolbox

0.809094

Pref2

0.95

Pref1

V

P

PV Mea surements

-C-Multi-Band

Machines

d_theta1_2

w1 w2

Vt1 Vt2

stop

Machine

Signals

Pm

Vf _

m

A

B

C

M2 5000 MVA

Pm

Vf _

m

A

B

C

M1 1000 MVA

A B CLoad

5000 MW

L2 350 kmL1 350 km

PSS

-C-Generic

A B CA B C

Fault Breaker

A

B

C

a

b

c

B3

A

B

C

B2

A

B

C

a

b

c

B1

A

B

C

a

b

c

5000MVA

13.8 kV/500 kV

A

B

C

a

b

c

1000 MVA

13.8 kV/500 kV

w1 w2 (pu)

d_theta1_2 (deg)

Vt1 Vt2 (pu)

V pos, seq.

B1 B2 B3 (pu)

Line power (MW)

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Shunt Compensation

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Shunt Compensation

The active powers at bus 1 and bus 2 are equal ,

The injected reactive power by the capacitor to regulate the

voltage at the mid-point of the transmission line is

calculated as :

The transmitted power can be significantly increased, and

the peak point shifts from =90 to =180

The operation margin and the system stability are

increased by the shunt compensation

The reactive power compensation at the end of the radial

line is especially effective in enhancing voltage stability

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Series Compensation

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Series Compensation

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FACTS A li i d


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FACTS Applications and

ImplementationsTransmission Transfer Capacity Enhancement

Advanced Solutions

Transmission

Link

Enhanced

Power Transfer

and Stability

Line

Reconfiguration

Fixed

Compensation

FACTS

Energy Storage

Better

Protection

Increased

Inertia

Breaking

Resistors Load

Shedding

FACTS

Devices

Traditional Solutions

SVC

STATCOM

TCSC, SSSC

UPFC

Steady State

Issues

Voltage Limits

Thermal Limits

Angular Stability Limits

Loop Flows

Dynamic

Issues

Transient Stability

Damping Power SwingsPost-Contingency Voltage

Control

Voltage Stability

Subsynchronous Res.

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How are Harmonics Produced ?

Power system harmonics: presenting deviations from aperfect sinusoidal-waveform (voltage or current waveform).

The distortion comes from a Nonlinearity caused bysaturation, electronic-switching and nonlinear electricloads, Inrush/Temporal/Arc/Converter/Limiter/ThresholdType Loads.

Figure: 2 [1]

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Why Bother about Harmonics?

50-60% of all electrical Ac Systems in North Americaoperate with non-linear type loads

Power-Quality-PQ Issues & Problems

Damage to Power Factor Correction capacitors

Waveform Distortion can create

SAG/SWELL/NOTCHING/RINGING/

All can cause damage effects to consumer loads and power

systems due to Over-Current/Over-Voltage or Waveform

Distortion

Additional Power/Energy Losses

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Loads Producing Harmonic Currents

Electronic lighting ballasts/Controls

Adjustable speed Motor-Drives

Electric Arc Welding Equipment

Solid state Industrial Rectifiers

Industrial Process Control Systems

Uninterruptible Power Supplies ( UPS )systems

Saturated Inductors/Transformers LAN/Computer Networks

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Current vs. Voltage Harmonics

Harmonic current flowing through the AC Power Systemimpedance result in harmonic voltage-drop at the load busand along the Feeder!!

Figure: 3[3]

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Calculation of THD

THD: Ratio of the RMS of the harmonic content tothe RMS of the Fundamental [3]

Current THD-I

(

Voltage THD-V

(

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Harmonics and Parallel Resonance

Circuit

Harmonic currents produced by variable speed motor-drives:can be amplified up to 10-15 times in parallel resonance circuitformed by the capacitance bank and network inductance [5]

Amplified/intensified harmonic currents: leading to internaloverheating of the capacitor unit.

Higher frequency currents: causing more losses than 60hzcurrents having same amplitude

Parallel resonance circuit and its equivalent circuit41

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Harmonics and Series Resonance Circuit

The voltage of upstream AC Network can be alsodistorted due to series/parallel resonance formed by

capacitance of the capacitor bank and System/load

inductance : Ca cause high harmonic current

circulation through the capacitors[5]

Parallel Resonance can also lead to high voltage distortion.

Series resonance circuit and its equivalent circuit

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Measure Equipments of Harmonics

Digital Oscilloscope:

Wave shape, THD and Amplitude of each harmonic

TrueRMS Multi-Meter:

Giving correct readings for distortion-free sine waves andtypically reading low when the current waveform is

distorted

Use of Harmonic Meters-Single Phase or three Phase

TrueRMS

Multi-Meter43

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Standards for Harmonics Limitation

IEEE/IEC

IEEE 519-1992 Standard: Recommended Practices andRequirements for Harmonic Control in Electrical PowerSystems(Current Distortion Limits for 120v-69kv DS)

Table 1: Current Harmonic Limits

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Standard of Harmonics Limitation

(contd)

IEEE 519-1992 Standard: Recommended Practicesand Requirements for Harmonic Control in ElectricalPower Systems(Voltage Distortion Limits)

Table 2: Voltage Harmonic Limits

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Mitigation Of Harmonics


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Mitigation Of Harmonics

Ranging from variable frequency motor- drive to othernonlinear loads and equipments

Power System Design: Limiting the non-linear load penetration to 30% of

the maximum transformers capacity

Limiting non-linear loads to 15% of thetransformers capacity, when power factor

correction capacitors are installed. Avoiding/Detuning resonant conditions on the AC

System:

hr = resonant frequency as a multiple of thefundamental frequency

kVAsc = short circuit current as the point ofstudy

kVARc = capacitor rating at the system voltage

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Mitigation the Effects of Harmonics


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Mitigation the Effects of Harmonics [1]

(contd)

Delta-Delta and Delta-Wye Transformers Using two separate utility feed transformers with equal

non-linear loads

Shifting the phase relationship to various six-pulse

converters through cancellation techniques

Delta-Delta and Delta-Wye Transformers

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Mitigation the Effects of Harmonics [1]

(contd)

Isolation-Interface Transformers The potential to voltage match by stepping up or

stepping down the system voltage, and by providing aneutral ground reference for nuisance ground faults

The best solution when utilizing AC or DC drives that

use SCR/GTO/SSR.. as bridge rectifiers

Line Isolation-Reactors

More commonly used for their low cost

Adding a small reactor in series with capacitor bank

forms a Blocking series Filter. Use diode bridge rectifier as a front end to avoid severe

harmonic power quality problems

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[(contd) Harmonic-Shunt or Trap Filters:

Used in applications with a high non-linear ratio to system

to eliminate harmonic currents

Sized to withstand the RMS current as well as the value of

current for the harmonics

Providing true distortion power factor correction

Typical Harmonic Trap Filter

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Harmonic Trap Filters (contd)

Tuned to a specific harmonic order such as the 5th, 7th,11th, etc to meet requirements of IEEE 519-1992

Standard

The number of tuned filter-branches depends on the

offending steady-state harmonics to be absorbed and onrequired reactive power level to be compensated

Typical Filter Capacitor Bank50

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Harmonics Filter Types

Isolating harmonic current to protect electrical equipment from

damage due to harmonic voltage distortion Passive Filter-Low cost:

Built-up by combinations of capacitors, inductors (reactors)and resistors

most common and available for all voltage levels Active Power Filter APF:

Inserting negative phase compensating harmonics into theAC-Network, thus eliminating the undesirable harmonicson the AC Power Network.

APF-Used only for for low voltage networks

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Harmonic Filter Types (contd) [7]

Unified Switched Capacitor Compensator USCS:The single line diagram (SLD) of the utilization (single-phase)

or (three-phase- 4-wire) feeder and the connection of the

Unified Switched- Capacitor Compensator (USCS) to the

Nonlinear-Temporal Inrush /Arc type Loads or SMPS-Computer/LAN-Network loads.

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M. S. Ramaiah School of Advanced Studies, Bangalore53

Harmonics Filter Types (contd)

The USCS is a switched/modulated capacitor bank using apulse-width modulated (PWM/SPWM) Switching Strategy.The switching device uses either solid state switch SSR-(IGBTor GTO).

Figure 11[7]

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Need To Evaluate System


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Need To Evaluate System

Harmonics?

The application of capacitor banks in systems where 20%or more of the load includes other harmonic generatingequipment.

The facility has a history of harmonic related problems,including excessive capacitor fuse operation or damage tosensitive metering/relaying/control equipment.

During the Planning/Design stage of any facility

comprising capacitor banks and nonlinear harmonicgenerating equipment.

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When to Evaluate System Harmonics?


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When to Evaluate System Harmonics? [1]

(contd)

In facilities where restrictive Electric Power UtilityCompany Standards/Guidelines limit the harmonicinjection back into their system to very small magnitudes.

Industrial/Commercial Plant expansions that addsignificant harmonic generating nonlinear type equipmentoperating in conjunction with capacitor banks.

When coordinating and planning to add any emergency

standby generator as an alternate/renewable power source

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Need for harmonic filters

Harmonic filters are shunt elements that are used in powersystems for decreasing voltage distortion and for power

factor correction

Harmonic filters reduce distortion by diverting harmonic

currents in low impedance paths. Harmonic filters are designed to be capacitive at

fundamental frequency, so that they are also used for

producing reactive power required by converters and for

power factor correction.

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Flow chart analysis for Harmonic Filter


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Flow chart analysis for Harmonic Filter

using MATLAB

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Unified Power Quality Conditioner

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Solutions to power quality problems

The first approach is called load conditioning, whichensures that the equipment is less sensitive to power

disturbances, allowing the operation even under significant

voltage distortion.

The other solution is to install line conditioning systemsthat suppress or counteracts the power system

disturbances.

A flexible and versatile solution to voltage quality

problems is offered by active power filters. Currently they are based on PWM converters and connect

to low and medium voltage distribution system in shunt or

in series.

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Summary


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Summary

Standard diodes are used for rectification

Fast recovery diodes are must for free wheeling Analyzing a thyristor circuit, start from a diode circuit with

the same topology.

Behavior of the diode circuit is exactly the same as the

thyristor circuit when firing angle is 0. Time- domain behavior of the power electronic circuit is

the combination of consecutive transients of the different

linear circuits.

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M. S. Ramaiah School of Advanced Studies, Bangalore 61

Summary

Different working principle different load Resistive load: current waveform of a resistor is the same

as the voltage waveform

Inductive load with a large inductor: the inductor current

can be considered constant Converters can be controlled by varying conduction timepower devices

Single phase or Three phase converters based on thesupply types

Three phase converts are normally used in high power,high frequency applications

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M. S. Ramaiah School of Advanced Studies, Bangalore 62

Summary

With forced commutation power factor can be improvedand lower order harmonics can be reduced

The load current could be continuous or discontinuousdepending on load-time constant and delay angle.

The operational flexibility and controllability that FACTS

has to offer will be one of the most important tools for the

system operator in the changing utility environment

In view of the various power system limits, FACTS provides

the most reliable and efficient solution.

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Summary

The operational flexibility and controllability that FACTS

has to offer will be one of the most important tools for thesystem operator in the changing utility environment

In view of the various power system limits, FACTS

provides the most reliable and efficient solution.

The high initial cost has been the barrier to its deployment,

which highlight the need to device proper tools and

methods for quantifying the benefits that can be derivedfrom use of FACTS.

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Conclusions

The high initial cost has been the barrier to its deployment,which highlight the need to device proper tools and

methods for quantifying the benefits that can be derived

from use of FACTS

The harmonic distortion principally comes fromNonlinear-Type Loads.

The application of power electronics is causing increased

level of harmonics due to Switching!!

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Summary

Harmonic distortion can cause serious Failure/Damageproblems.

Harmonics are important aspect of power operation thatrequires Mitigation!!

Over-Sizing and Power Filtering methods are commonlyused to limit Overheating Effects of Sustained Harmonics.

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Documents

Session 6(Power Electronic Controllers)