lighting strokes lighting strokes lighting arrestor

Dr.-Ing. Getachew Biru

TYPES OF OVERVOLTAGE

Over voltage stressing a power system can be classified into two main types 1. External overvoltage: generated by

atmospheric disturbances like lightning. 2. Internal Overvoltages: generated by

changes in the operating conditions of the network. Internal over voltages can be divided into

(a) switching overvoltages and (b) temporary overvoltages.

Lightning Prone Regions

Danger due to Lightning Strokes

110 kV

data

TV 400/230 V

ABC Company

mobile phone

approx. 1,900,000 lightning strokes in Germany per year*

telephone

Lightning Production • Warm, low pressure air moving through

cool, high pressure air produces static electricity.

• The friction of moving air particles within the cloud causes ionization and charges.

• As the separation of charge proceeds in the cloud, the potential difference between the centers of charges‘ increases and the vertical electric field along the cloud also increases.

Lightning Production


• The lower part of a thundercloud is usually negatively charged.

• The upward area is usually positively charged.


• Continuation of downward step leader

• Further electric field intensification

• Strong upward positive streamer generation


•Positive upward streamer meets the downward step leader

•Conducting path forms

•Potential is equalised by the "return stroke"

•Visible lightning flash

Facts about Lightning

• A strike can produce on average 100 Mega volts of Electricity.

• Current of up to 100, kilo amperes • Can generate 54,000 oF • The height of the thundercloud dipole above

earth may reach 5 km in tropical regions. • Lightning strikes somewhere on the Earth

every second • Kills hundreds of people every year and

causes a huge damage to properties and equipments.


Typical 50 kA

HIGH VOLTAGE di & dv dt dt

HIGH ENERGY

• The current pulse rises to its crest in a few micro seconds and decays over a period of tens or hundreds of microseconds.

Typical characteristics of Lightning Pulse

Instantaneous Power Over one Megawatt

Total Energy Over 250 Kilojoules Sound Pressure 90 Atmospheres at 500m away Temperature 30,000°K+ (5 times Sun

Surface) Rise Time 0.1 to 5 Microseconds Average Current 35 kA Duration 300 Microseconds + Repeats Channel Length 5 km

How Transients Enter your Equipment

Conductive coupling

Magnetic coupling Capacitive coupling

Electromagnetic Coupling

How Transients Enter your Equipment

Capacitive coupling is where the transient voltage is coupled due to the inherent capacitance between two circuits

Magnetic coupling occurs when magnetic field of a current carrying conductor induces lightning current on to an adjacent conductor

Galvanic coupling is a direct electrical connection

Lightning Surges Direct lightning strike:

L1 L2 L3

PEN

20 kV

1b

1b Induced voltage in loops

IT network

power supply

1

1 Striking of external lightning protection system, process structure (in industrial plants), cables etc.

Rst

1a

1a Voltage drop at the earthling resistance Rst

Distant lightning Strike:

2a

2a Strike in to medium- voltage overhead lines

2c Fields of the lightning strike

2c

2b

2b Surge travelling waves on overhead lines Due lightning

Traveling Wave/ Overvoltage Protection

• Lightning hits mid-span • Surge causes traveling

voltage wave • Current divides and then

propagates

Lightning Strokes

Induced voltage on the line

Zo-Surge impedance of the line

Lightning current magnitude

Direct Stroke V = I Zc /2 Z0= √(L/C) L henry/m C farad/m Typically: Zc= 350 ohm

BIL = ICZo /2

Lighting Strokes

Lighting Arrestor

What exactly does a lightning arrester do? • It Does Divert the Lightning to

Ground • It Does not Absorb the Lightning • It Does not Stop the Lightning • It Does Clamp (limit) the Voltage

produced by the Lightning • It Only protects equipment

electrically in parallel with it.

Lighting Arrestor

The Metal Oxide Varistor (MOV) Disk is a Semiconductor that is sensitive to Voltage. At normal Voltages the MOV disk is an insulator and will not conduct current. but at higher voltages caused by lightning it becomes a conductor .

Lighting Arrestor

Lighting Arrestor Ratings The highest phase to earth voltage: the power frequency highest voltage applied to the arrestor. U𝑝𝑝𝑝𝑝 = 2𝑈𝑈𝑠𝑠

√3

Us-system voltage Continuous operating voltage (Maximum continuous operating voltage MCOV) Uc: the power frequency phase voltage which the arrestor can be operated at without any type of restriction. It is greater than the highest continuously occurring voltage by at least 5%.

Lighting Arrestor Ratings

The rated voltage of an arrestor Ur: characterizes the capability of the arrestor to deal with temporary overvoltages in the system. It can be only applied for a very short period of time (10-100 seconds). The leakage current is around 1 mA. Ur=1.25 Uc.

Earthling System

Combination of horizontal strip and vertical rods - Linear - Radial

lighting strokes lighting strokes lighting arrestor

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