fenomena tegangan lebih.pdf

8/14/2019 fenomena tegangan lebih.pdf

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Contents

1

Te an an Lebih Sur a Petir 2

Tegangan Lebih Surja Penyambungan3

Korona4

www.themegallery.com


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Jenis Te an an Lebih

Over Voltage

Internal

Externalovervoltages:

overvoltages :generated by changes

atmospheric

disturbances. Of

conditions of the

network. Internal,lightning is the mostcommon and the

divided into (a)switching overvoltages


.

overvoltages


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LOGOTegangan Lebih Petir (Lightning

Lightning is produced in an attempt by nature to maintain ad namic balance between the ositivel char ed

Marshall, 1973

ionosphere and the negatively charged earth

Over fair-weather areas there is a downward transfer of positive-

Lewis, 1965

.counteracted by thunderstorms, during which positive chargesare transferred upward in the form of lightning.



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Te an an Lebih Petir

During thunderstorms, positive and negative charges are separated

layer of a cloud and rain in the lower part. The cloud becomesnegatively charged and has a larger layer of positive charge at its

. ,

difference between the concentrations of charges increases and thevertical electric field along the cloud also increases. The total

otential difference between the two main char ed enters ma var from 100 to 1000 MV. Only a part of the total charge-severalhundred coulombs-is released to earth by lightning; the rest isconsumed in inter cloud discharges. The height of the thunderclouddipole above earth may reach 5 km in tropical regions.



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LOGOPelimpahan Muatan Petir

The lightning discharge through air occurs as one of theorms o s reamer rea own o ong a r gaps. e c anne

to earth is first established by a stepped discharge called aleader stroke. The leader is generally initiated by a

breakdown between polarized water droplets at the cloudbase caused by the high electric field, or a dischargebetween the negative charge mass in the lower cloud andthe positive charge pocket below it. The developmentstages of a lightning flash are depicted in the followingfi ure:


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Pelim ahan Muatan Petir

As the downward leader approaches the earth, an upward

leader begins to proceed from earth before the former ,a point referred to as the striking point. This is the start of thereturn stroke, which progresses upward like a traveling wave

.current reaching the order of tens of kilo amperes occurs,which is responsible for the known damage of lightning. Theve oc ty o progress on o t e return sto e s very g anmay reach half the speed of light. The corresponding current

heats its path to temperatures up to 20.000 0 C, causing theexplosive air expansion that is heard as thunder. The currentpulse rises to its crest in a few microseconds and decays over a eriod of tens or hundreds of microseconds Ra aller

www.themegallery.com1980) .


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Te an an Sur a Petir

The most severe lightning stroke is that which strikes a phaseconductor on the transmission line as it produces the highestovervoltage for a given stroke current. The lightning stroke injectsits current into a termination impedance Z, which in this case is half the line surge impedance Zo since the current will flow in bothdirections as shown in the following figure. Therefore, thevoltage

surge magnitude at the striking point is



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Te an an Sur a Petir

e g tn ng current magn tu e s rare y ess t an(Berger et al., 1975) and thus, for a typical overhead linesurge impedance Zo of 300 , the lightning surge voltagewill probably have a magnitude in excess of 1500 kV. Theequation assumes that the impedance of the lightningchannel itself is much lar er than Zo indeed it is

believed to range from 100 to 3000 . The equation alsoindicates that the lightning voltage surge will have

. ,however, the shapes and magnitudes of lightning surge

waves get modified by their reflections at points of .


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LOGO Tegangan Lebih Penyambungan

w c ng vervo ageWith the steady increase in transmission voltages needed to fulfill therequired increase in transmitted powers, switching surges havebecome the governing factor in the design of insulation for EHV and

systems. n t e meant me, g tn ng overvo tages come as asecondary factor in these networks. There are two fundamentalreasons for this shift in relative importance from lightning to switchingsurges as g er ransm ss on vo ages are ca e or: Overvoltages produced on transmission lines by lightning strokes

are only slightly dependent on the power system voltages. As a,

decrease as the latter is increased. External insulation has its lowest breakdown strength under surges

- ,switching surge.

IEC recommendations, all equipment designed for operatingvoltages above 300 kV should be tested under switching impulses(i.e., laboratory-simulated switching surges)


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Pen ebab Timbuln a Te an anLebih Penyambungan

1. Energization of transmission lines and cables.The following specific switching operations are some of themost common in this category:a. Ener ization of a line that is o en circuited at the far end

b. Energization of a line that is terminated by an unloadedtransformer.

-.transformer

2. Energization of a line.charges trapped by previous line interruptions when high-speed reclosures are used.


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LOGO Penyebab Timbulnya Tegangan

e enyam ungan3. Load rejection.

This is effected by a circuit breaker opening at the far end of .sending end in what is called a line dropping operation.

. . All switching operations involving an element of thetransmission network will produce a switching surge. Of par cu ar mpor ance, owever, are e o ow ng opera ons:

a. Switching of high-voltage reactors

b. Switching of transformers that are loaded by a reactor on their tertiary winding

c. Switching of a transformer at no load

5. Fault initiation and clearing.


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LOGO Penyalaan Saluran Transmisi

anpa e an


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LOGOPenyalaan Saluran Transmisi

where


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Te an an Lebih Tem orer

Temporary overvoltages (i.e., sustained

overvoltages) differ from transient switching ,typically from a few cycles to a few seconds. They

oscillations at a frequency equal or close to thepower frequency. The event which caused temporaryover voltage are: Load Rejection

Ground Fault Harmonic overvolta es due to Ma netic Saturation


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Load Re ection

When a transmission line or a large inductive load that is fed froma power station is suddenly switched off, the generator will speedup and the busbar voltage will rise.

The amplitude of the overvoltage can be evaluatedapproximately

E is the volta e behind the transient reactance constant Xs is the transient reactance of the generator in series with thetransformer reactance.

Xc the equivalent capacitive input reactance of the system


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Ferranti Effect

The Ferranti effect of an uncompensated transmission line is given by:

where Vr and Vs are the receivin -end and sendin -end volta es

respectively, and t is the line length (km). o is the phase shiftconstant of the line per unit length. It is equal to the imaginary partof m, where Z and Y are the impedance and admittance of the line

per unit length. For a lossless line , & = o m where L and C are theinductance and capacitance of the line per unit length. o has a value

of about 6 0 per 100 km at normal power frequency (Diesendorf,1974).


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