Fenomena Tegangan LebihFenomena Tegangan Lebih

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Contents

Jenis Tegangan Lebih1 Jenis Tegangan Lebih1

Tegangan Lebih Surja Petir2 g g j

Tegangan Lebih Surja Penyambungan3

Korona4

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Jenis Tegangan Lebihg g

Over Voltage

InternalExternal overvoltages:generated by

Internal overvoltages:generated by changes in the operatinggenerated by

atmospheric disturbances. Ofthese disturbances

in the operating conditions of the network. Internal overvoltages can bethese disturbances,

lightning is the most common and the most severe.

overvoltages can be divided into (a) switching overvoltagesand (b) temporary

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most severe. and (b) temporary overvoltages

LOGO Tegangan Lebih Petir (Lightning Overvoltage)Overvoltage)

Lightning is produced in an attempt by nature to maintain adynamic balance between the positively charged

Marshall, 1973

y p y gionosphere and the negatively charged earth

Over fair-weather areas there is a downward transfer of positivecharges through the global air-earth current This is then

Lewis, 1965

charges through the global air-earth current. This is thencounteracted by thunderstorms, during which positive chargesare transferred upward in the form of lightning.

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Tegangan Lebih Petirg g

During thunderstorms, positive and negative charges are separatedby the movements of air currents forming ice crystals in the upperby the movements of air currents forming ice crystals in the upperlayer of a cloud and rain in the lower part. The cloud becomesnegatively charged and has a larger layer of positive charge at itstop. As the separation of charge proceeds in the cloud, the potentialtop. As the separation of charge proceeds in the cloud, the potentialdifference between the concentrations of charges increases and thevertical electric field along the cloud also increases. The totalpotential difference between the two main charged enters may varyp g y yfrom 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 thundercloudg gdipole above earth may reach 5 km in tropical regions.

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LOGO Pelimpahan Muatan Petir (Lightning Discharge)(Lightning Discharge)

The lightning discharge through air occurs as one of thef f t b kd f l i Th h lforms of streamer breakdown of long air gaps. The channelto earth is first established by a stepped discharge called aleader stroke. The leader is generally initiated by abreakdown between polarized water droplets at the cloudbase caused by the high electric field, or a dischargebetween the negative charge mass in the lower cloud andg gthe positive charge pocket below it. The developmentstages of a lightning flash are depicted in the followingfigure:figure:

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Pelimpahan Muatan Petirp

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Pelimpahan Muatan Petirp

As the downward leader approaches the earth, an upwardleader begins to proceed from earth before the formerreaches earth The upward leader joins the downward one atreaches earth, The upward leader joins the downward one ata point referred to as the striking point. This is the start of thereturn stroke, which progresses upward like a traveling waveon a transmission line At the earthing point a heavy impulseon a transmission line. At the earthing point a heavy impulsecurrent reaching the order of tens of kilo amperes occurs,which is responsible for the known damage of lightning. The

l i f i f h k i hi h dvelocity of progression of the return stoke is very high andmay reach half the speed of light. The corresponding currentheats 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 overa period of tens or hundreds of microseconds (Ragaller,

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p ( g ,1980).

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Tegangan Surja Petirg g j

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 injectsg g g g jits current into a termination impedance Z, which in this case is halfthe line surge impedance Zo since the current will flow in bothdirections as shown in the following figure. Therefore, thevoltagesurge magnitude at the striking point is

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Tegangan Surja Petirg g j

Th li h i i d i l l h 10 kAThe lightning current magnitude is rarely less than 10 kA(Berger et al., 1975) and thus, for a typical overhead linesurge impedance Zo of 300 Ω, the lightning surge voltagewill probably have a magnitude in excess of 1500 kV. Theequation assumes that the impedance of the lightningchannel itself is much larger than ½ Zo; indeed, it isg ; ,believed to range from 100 to 3000 Ω. The equation alsoindicates that the lightning voltage surge will haveapproximately the same shape characteristics In practiceapproximately the same shape characteristics. In practice,however, the shapes and magnitudes of lightning surgewaves get modified by their reflections at points ofdiscontinuity as they travel along transmission linesdiscontinuity as they travel along transmission lines.

LOGO Tegangan Lebih Penyambungan (S it hi O lt )(Switching Overvoltage)With 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 andUHV I h i li h i lUHV systems. In the meantime, lightning overvoltages come as asecondary factor in these networks. There are two fundamentalreasons for this shift in relative importance from lightning to switching

hi h t i i lt ll d fsurges as higher transmission voltages are called for:• Overvoltages produced on transmission lines by lightning strokes

are only slightly dependent on the power system voltages. As aresult their magnitudes relative to the system peak voltageresult, their magnitudes relative to the system peak voltagedecrease as the latter is increased.

• External insulation has its lowest breakdown strength under surgeswhose fronts fall in the range 50- 500 µs which is typical forwhose fronts fall in the range 50- 500 µs, which is typical forswitching 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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Penyebab Timbulnya Tegangan y y g gLebih Penyambungan

1. Energization of transmission lines and cables.The following specific switching operations are some of themost common in this category:a. Energization of a line that is open circuited at the far endg pb. Energization of a line that is terminated by an unloaded

transformer.c Energization of a line through the low-voltage side of ac. Energization of a line through the low voltage side of a

transformer2. Energization of a line.

This means the energization of a transmission line carryingThis means the energization of a transmission line carryingcharges trapped by previous line interruptions when high-speed reclosures are used.

LOGO Penyebab Timbulnya Tegangan L bih P bLebih Penyambungan

3. Load rejection.This is effected by a circuit breaker opening at the far end ofthe line This may also be followed by opening the line at thethe line. This may also be followed by opening the line at thesending end in what is called a line dropping operation.

4 Switching on and off of equipment4. Switching on and off of equipment.All switching operations involving an element of thetransmission network will produce a switching surge. Of

ti l i t h th f ll i tiparticular importance, however, are the following operations:a. Switching of high-voltage reactorsb. Switching of transformers that are loaded by a reactor

on their tertiary windingc. Switching of a transformer at no load

5. Fault initiation and clearing.

LOGO Penyalaan Saluran Transmisi T B bTanpa Beban

LOGO Penyalaan Saluran Transmisi Tanpa BebanTanpa BebanThe circuit performance after switching may be expressed by theThe circuit performance after switching may be expressed by thefollowing differential equation:

The supply voltage us(t) beyond the switching instant is given by:

Equations above can easily be manipulated (e g by usingEquations above can easily be manipulated (e.g., by usingoperational calculus) and the expression for the voltage across theline capacitance takes the form:

LOGO Penyalaan Saluran Transmisi Tanpa BebanTanpa Beban

where

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Tegangan Lebih Temporerg g p

Temporary overvoltages (i.e., sustainedovervoltages) differ from transient switchingovervoltages in that they last for longer durationsovervoltages in that they last for longer durations,typically from a few cycles to a few seconds. Theytake the form of undamped or slightly dampedtake the form of undamped or slightly dampedoscillations at a frequency equal or close to thepower frequency. The event which caused temporaryover voltage are:• Load Rejection• Ferranti Effect• Ferranti Effect• Ground Fault• Harmonic overvoltages due to Magnetic SaturationHarmonic overvoltages due to Magnetic Saturation

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Load Rejectionj

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 voltage behind the transient reactance (constant)g ( )Xs is the transient reactance of the generator in series with the transformer 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 receiving-end and sending-end voltages,g g g ,respectively, and t is the line length (km). βo 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 lineper unit length. For a lossless line , & = o m where L and C are theinductance and capacitance of the line per unit length. βo has a valueof about 60 per 100 km at normal power frequency (Diesendorf,1974).

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