sky wires, poises, harmonics and unbalanced

By: Mr. Viren B. PandyaM.E. (Electrical Power System)Lecturer (EE)Institute of TechnologyNirma University

Sky Wires, Counter-poise, Sky Wires, Counter-poise, Harmonics and Unbalanced Harmonics and Unbalanced Conditions in Power SystemConditions in Power System

INSTITUTE OF TECHNOLOGY

2 03/09/2008 Prepared by:Viren B. Pandya

Quick Look at Presentation ContentSky Wires/Ground Wires/Shielding Wires

Counter-poise

Harmonics in Power System and Machines

Balanced Condition of Three-phase Power System with Phasor Diagrams

Unbalanced Condition of Three-phase Power System with Phasor Diagrams


Sky Wires To provide protection against overvoltages due

to lightning strokes Ground wire is a conductor run parallel to the

main conductor of the of overhead transmission line supported on the same tower and earthed at every equally and regularly spaced towers

It is run above the main conductor of the line The ground wire shields the transmission line

conductor from induced charges, from clouds as well as from a lightning discharge


Configurations of Sky Wires

h

P P

G

s

h

•The shielding angle s=30 was considered adequate for tower heights of 30 m or less•The shielding wires may be one or more depending on the tower type of the tower used


Counter-poise When a line is shielded, the lightning strikes either the tower or

the ground wire. The part for drainage of the charge and lightning current is (a) through the tower frame to ground, (b) through the ground line in opposite directions from the point of striking

The instantaneous potential to which tower top can rise is

0

1

TT

T

S

I ZV

Z

Z

where Zwhere ZTT = surge impedance of the tower = surge impedance of the tower

ZZSS = surge impedance of the ground wire = surge impedance of the ground wire

If the surge impedance of the tower, which is the effective tower footing resistance, is reduced, the surge voltage developed is also reduced considerably. This is accomplished by provision of counter-poise wires connected to the tower legs at the tower footing or foundation


Counter-poise Counter-poise wires are wires buried in the ground

at a depth of 0.5 to 1.0 m, running parallel to the transmission line conductors and connected to the tower legs

These wires may be 50 to 100 m long. These are found to be more effective than driven rods and the surge impedance of the tower may be reduced to as low as 25 ohm

The depth does not materially affect the resistance of the counter-poise and it is only necessary to bury

them to a depth enough to prevent theft


Harmonics in Electrical Machines and Power System Harmonics are the undesirable waveforms

having frequencies in integer-multiples of fundamental power frequency of voltage or current

For 50 Hz power frequency sinusoidal voltage or current, 2nd ,3rd ,4th , 5th ,….nth harmonic frequencies are 100, 150, 200, 250,….50xn respectively in Hz


Distortion in Fundamental Waveform


Sources of Harmonics The distortion comes from electronic and nonlinear

devices, principally loads. Examples of Non-linear loads• Electronic lighting ballasts• Adjustable speed drives• Electric welding equipment• Solid state rectifiers• Industrial process controls• Uninterruptible Power Supplies(UPS)

systems• Saturated transformers• Computer system


Negative Effects of Harmonics Harmonics Overheating and premature failure of

distribution transformers

(Increasing iron and copper losses or eddy currents due to stray flux losses)

Overheating and mechanical oscillations in the motor-load system

(Producing rotating magnitude field, which is opposite to the fundamental magnitude field)

Overheating and damage of neutral conductors

Trouble Harmonics: 3rd, 9th, 15th …

(A 3-phase 4-wire system: single phase harmonic will add Rather than cancel on the neutral conductor)


Measure of Harmonic Content Total Harmonic Distortion(THD) -contribution of all

harmonic frequency currents to the fundamental current. This THD value is expressed as a percentage of the fundamental current

sum of sqares of amplitudeof all harmonics%THD 100%

squareof amplitudeof fundamental

Individual Harmonic Distortion (IHD): ratio of the harmonic component to the fundamental component. This value is sometimes used to track the effect of each individual harmonic and examine its magnitude


Harmonic Contribution for different Loads


Harmonics in Three-phase System


Harmonics in Three-phase Induction Motor It may introduce Crawling of three-phase induction motor i.e. the tendency of motor to run at 1/7 speed of synchronous speed due to predominance of seventh harmonic.Third harmonics do not generate any rotating field, hence no harmonic torque contributed by thoseFifth harmonic produces negative rotation in turn producing somewhat braking action what could be neglected as compared to forward torque


Mitigation Effects of Harmonics

Use of delta zigzagzigzag transformer to eliminate 3rd harmonic component

For thyristor controlled devices, using more number of firing pulses reduces harmonic content, however it increases pulsation losses

Use of Harmonic Filters Harmonic Filters with variable frequency drives

(a) Passive filter- Combination of L & C (b) Active filter- blends static devices

Using high permeability material


Balanced Three-phase System

To transmit power in bulk, three phase system is most economical world widely used compared to single phaseThree phase balanced system:- all voltage or current phasors are having equal amplitudes as well as equal phase angle diff. of 120 degree.Star and Delta connections are commonly used for generation, transmission and distribution of powerThree phase power is in star or delta fashions is :

3 L LP 3V I cos ,

where notaions have usual meanings


Phasor Diagrams & Waveform for Balanced Systems


Causes of Unbalanced in Power System Unsymmetrical short circuit faults

(a) L-G

(b) L-L-G

(c) L-L

Overloading on of three phases of 3-phase, 4 wire star connected system

Open conductor fault

(a) one conductor open

(b) two conductors open

Harmonics


Importance of Neutral Wire Neutral wire helps derive single phase load from three phase star connectionPresence of neutral wire help provide provision of sensing of earth fault current (zero sequence current) for protectionWith a Y-connected system, a neutral wire is needed in case one of 1- phase loads gets to fail open (or be turned off), in order to keep the phase voltages at the load from changing This is not necessary (or even possible!) in a Δ-connected circuitDelta provides path for 3rd harmonic components and 3rd harmonic does not appear on LINE voltages and currents.Under unbalanced condition neutral wire carries thrice 3rd harmonic current in star connection


Advantage of Delta over Star ConnectionThe greatest advantage of the Δ-connected source is its fault

tolerance. It is possible for one of windings in a Δ-connected three-phase source to fail open without affecting load voltage or current!

The only consequence of a source winding failing open for a Δ-connected source is increased phase current in the remaining windings.


Advantage of Delta over Star ConnectionWith a Δ-connected load, two of the resistances suffer reduced voltage while one remains at the original line voltage, 208. A Y-connected load suffers an even worse fate with the same winding failure in a Y-connected source


Advantage of Delta over Star ConnectionIn this case, two load resistances suffer reduced voltage while the third loses supply voltage completely!


Application of Star & Delta ConnectionAll the synchronous generators/alternators are

star connected to have neutral wire for protection purposeDelta connection is deployed for secondary of step up power transformer at power station for further transmission of bulk power over large distanceOpen delta connection or V-V connection of three phase transformerStar connected rotor of slip ring induction motor to achieve high starting torque


At brink of completion, we get to know..Shielding wire and counter-poise applicationThe production, causes & remedies of harmonics in power system and electrical machines in connection with star and delta configurationBalanced/Symmetrical three-phase system overviewCauses of unbalanced in power systemComparison of star and delta connected systems from point of view of tolerance under unbalanced conditions


Thank You

sky wires, poises, harmonics and unbalanced

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