OVER VOLTAGE The voltage waves having magnitude more than its normal value. In other words, sudden rise to an excessive or abnormal value It remains for a very short duration of time.

CAUSES OF OVER VOLTAGES The over voltages occur due to Lightning Switching Faults Travelling Waves

TYPES OF OVER VOLTAGES Over voltage is classified into two types.They are External Over voltages Internal Over voltages

EXTERNAL OVER VOLTAGES External Over voltages are caused due to Direct lightning strokes Indirect lightning strokes

INTERNAL OVER VOLTAGES Internal Over voltages are caused due to Switching operation Fault condition Sudden release of load in the network.

DIRECT LIGHTNING STROKES Here the discharge or the current path is directly from the cloud to the overhead line. The voltage will be in millions which can cause flash over or puncture of insulators. This wave may reach to the substation and damage the equipments because of excessive stress produced.

INDIRECT LIGHTNING STROKES These strokes are due to electro statically induced charges on the conductors due to the presence of charged clouds. Maximum over voltages in the transmission lines are caused by indirect lightning strokes.

SWITCHING OPERATION The switching over voltage occurs during opening and closing EHV AC lines, breaking inductive loads, capacitive loads which damage the electrical equipments. During resonance, substantial over voltages might develop.

FAULT CONDITION Interruption of the fault current causes over voltages. The magnitude of voltage depends on the amount of fault current and the speed with which the fault is cleared. This will damage transformers, cables and motors supplied from the lines.

SUDDEN RELEASE OF LOAD Switching off of the capacitor banks produce over voltages. Adjustable Speed Drives and Solid State Motor Controllers are quite sensitive to voltage rises. ASD might shut down the motor due to over voltage beyond maximum tolerance.

SURGE DIVERTER AND SURGE ABSORBER

SURGE DIVERTER Ideal characterstics: Should not carry current at normal voltage (breakdown voltage > normal or abnormal fundamental frequency voltage) At transient voltage the peak value > spark overvoltage After breakdown , it should be capable to carry discharge current Diverter interrupt as quickly as possible when transient voltage reduces below breakdown value

SURGE DIVERTER OPERATION

CLASSIFICATION Station type -can carry 10kA -voltage range 3.3kV to 123kV - large power stations and substations Line type arrester -can carry 5kA -voltage range 3.3kV to 245kV - transformers, power stations and substations Distribution type and secondary type surge arresters: -can carry 2.5kA and 1.5kA -voltage range 3.3kV - distribution transformers

TYPES OF ARRESTER Horn gap arrester Rod gap arrester Multigap arrester Expulsion type arrester Valve type lightning arrester Metal oxide lightning arrester

HORN GAP ARRESTER

ADVANTAGE Doesnot cause SC as surge is over The follow current is maintain to small value with the help of series resistance

DISADVANTAGE Setting of Horn gap should be changed Time in this type of arrester is slow Harmful to employ Due to external factors it not work properly

ROD GAP ARRESTER

ROD GAP SYSTEM VOLTAGE(kV) ROD GAP(cm) 33 66 132 275 22.66 35.56 66.04 124.46

MULTIGAP ARRESTER

EXPULSION TYPE ARRESTOR

ADVANTAGE Cheaper and install easily After surge is over the current is interrupt immmediately

DISADVANTAGE Cannot protect expensive station equipments Perform only limited number of operation

VALVE TYPE ARRESTER

VOLT – AMPERE CHARACTERSTICS

TEST ON LIGHTNING ARRESTER 1/50 impulse spark over test Wave front impulse spark over test Impulse current withstand test Switching impulse voltage test Discharge capability of durability

SURGE ABSORBER

SURGE ABSORBER & TRAVELLING WAVES

SURGE ABSORBER : A Protective device. Main function : Can reduce the steepness of wavefront of a surge. Absorb the energy in travelling wave.

ELIMINATION OF SURGES : SURGE DIVERTER : It divert the surge to the earth. SURGE ABSORBER : It absorbs energy contained in the surge.

TYPES : Surge absorber using capacitor : using only capacitor. using resistor and capacitor. another type : using parellel combination of inductor and resistor. Ferranti Surge absorber : using inductive coil.

SURGE ABSORBER USING CAPACITOR: LINE CAPACITOR GROUND

USING RESISTOR &CAPACITOR : LINE CAPACITOR RESISTOR GROUND

PARELLEL COMBINATION : RESISTOR LINE INDUCTOR

FERRANTI SURGE ABSORBER : Inductive coil magnetically coupled. Current induced in the metal sheild. Absorber is enclosed in a boiler plate tank, and the vaccum is impregnated. the energy transferred through mutual induction. It is dissipated in the form of heat in the dissipator.

Figure 1: METAL SHIELD LINE

Figure 2 : METAL SHIELD LINE

FIELD OF APPLICATIONS: Placed near the rotating machines. Across series reactors.

TRAVELLING WAVES : WHAT IS IT MEANT BY? Fault on the power system cause transients that propagate along the transmission lines as waves. They continue to travel along the power system until they dimnished due to impedance.

WAVE FORM 1:

WAVE FORM 2 :

EFFECTS : Attenuation Distortion Wave shape modification Discontinuity of operation.

IS IT NECESSARY TO AVOID THIS : Yes it is necessary to nullify it, So as to preserve the wave shape Transmit the signal with strength Fidelity Without interferance.

TRW DETECTION SYSTEM : Capacitive coupling voltage transformer(CCVT) Fault transient interface unit(FTIU) GPS reciever.

OVERVOLTAGE PROTECTIVE DEVICES & ITS SELECTION

Protective Devices: Mitigate surge over voltage and over current of limited duration. Activated in case of exceeding nominal voltage/current. Used to detect abnormal conditions & to take corrective actions. Single component or complex design. Increasing in need

Characteristics ..... Protective devices are usually divided into over voltage elements. & over current elements.

Over voltage components: Two types of over voltage components are voltage switching components & voltage limiting components

Types of over voltage protective devices: Air gap protectors Surge Protector Gas discharge tubes Semi-conductor protective devices Fuses Heat coils Self restraining current limiting devices Fusible links

Air gap protectors Connected between each wire of a line & earth. Limits the voltage Inexpensive Frequent replacement

Surge Protector : A surge protector is designed to protect electrical devices from voltage spikes. It diverts excess voltage/current to grounding wire. Limit the voltage supplied to an electric device.

Installed In… power distribution panels, process control systems, communications systems, And other heavy-duty industrial systems.

Gas discharge tubes: Connected between each wire of a line & earth. Limiting purpose. Compact No need of Frequent replacement

Semi-conductor protective devices: Primary protective devices & Secondary protective devices

Primary protective devices: Carbon electrodes Benefits: elimination of circuit noise Disadvantage: higher costs low current handling capability Gas discharge tubes Benefits: elimination of spark discharge Disadvantage: low current handling capability

Secondary protective devices: Depending on location it may designed to operate in level as low as 1V. Fast-acting Easily damaged unless co-ordinated correctly

Fuses: Connected in series with each wire of a line Disconnect when excessive current flows No protection against lightining surges Manual replacement.

Heat coils: Connected in series with each wire of a line Have some fusible components Does not give adequate protection Manual replacement.

Self restraining current limiting devices: Connected in series with each wire of a line Increase their resistance when over voltage occurs Operates automatically

INSULATION CO-ORDINATION

DEFINITION: The term insulation co-ordination is the correlation of the insulation of electrical equipment and the lines with the characteristics of protective devices such that the insulation of the whole power system is protected from excessive over voltage.

In power system various equipments like transformers, circuit breakers ,etc. have a different breakdown voltage and volt-time characteristics. The insulation for the various protective devices must be properly coordinated. Mainly used to select the suitable values for different components in a power system.

Curve A- volt-time curve for the protective devices Curve B- volt-time curve for the equipment to be protected Any insulation having a withstand voltage strength in excess of the insulation strength.

VOLT-TIME CURVE: It shows the relation between the crest flashover voltage and time to flashover for a series of impulse applications. In this waves are in same shape but different peak values are applied to the insulation.

INSULATION LEVEL: In order to protect the equipment from overvoltage, its necessary to fix the insulation level for the system. BASIC IMPULSE INSULATION LEVEL(B.I.L) is in terms of withstanding voltages of apparatus and lines. It is expressed in impulse crest voltage with a standard wave not longer than 1.2/50microsec wave.

PROBLEMS IN INSULATION LEVEL: Selection of a suitable insulation level. Assurance that the breakdown strength of all insulation in the system will be equal or exceeds the selected level. Selection of protective device will give the good protection for apparatus.

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