SURGE ARRESTERS

•These were previously called Lightning arresters, which is not the correct name or definition.

•Surge arrester is a device which protects the electrical expensive items from damage; like Transformers or Reactors or other insulators.

    

TYPE OF SUREGE ARRESTERS

Expulsion Type. 

Gap Type.  

Metal oxide Type

METAL OXIDE ARRESTERS

These are also called Non linear resistance type arresters.Normally Aluminum oxide is used for Metal oxide because it is cheap and have good characteristics. Due to non linear voltage / current characteristics it offers low resistance path to the flow of high discharge current thus limiting the voltage across the arrester terminals.

M.O ARRESTERS Parameters

Arrester Rated voltageIt is the maximum permissible voltage which the arrester can withstand, without discharge and without changing its operating characteristics.

  The voltage which appears across the terminal of arrester during discharge. The minimum, the better. The following types of residual voltages are of importance.

Residual voltage of arrester

Switching impulse residual voltage

It is equipment insulation withstand test voltage with a front time larger than 30 µs. Lightning impulse residual voltageIt is 1.2/50 µs wave shape.BS use current wave of form 8/20 µs for residual voltage.

The above are referred to Arrester Currents.  

Temporary over voltage to Earth

The Arrester must be selected with a rated voltage high enough to withstand temporary over voltages which may be produced by various occurances. Line to fault current is the most common occurance

Protective Margin

There should be sufficient margin between various required impulse and Power frequency withstand voltages and arrester voltage. It is margin in % and expressed as: (Insulation withstand voltage – Residual voltage) x 100

Residual voltage

Energy capability

It is the capability of arrester to dissipate a given amount of energy without damage.

Pressure Relief Capability

The ability of arrester to conduct short circuit current without violent disintegration in the event of its failure.

DESIGN OF ARRESTER

1. Highest system voltage is the base for Design parameters. It is defined by IEC and other Standards

2. Temporary over voltages in the system are normally determined by single line to

ground fault. As such Rated voltage will be different

depending on the grounding condition and protection scheme. In case of directly grounded system the voltage of healthy phase may rise 1.4 times phase to earth voltage.

where 1.4 is the Earth factor.

3. In case of ungrounded system Earth factor will be 1.73. Thus the voltage may rise upto 1.73 times the phase to Earth voltage.

3. Check for the abnormal operating conditions, Um.4. Determine continuous operating voltage (COV)

COV =Um/√3

5. Determine Preliminary rated voltage VR0

based on COV6. VR0 = COV/K0 where K0 is Design factor

6. Determine Temporary over voltage at Earth fault

TOVE = KE x COV

7. Select Preliminary Rated Voltage RE based on TOVE

 Determine Rated voltage, RE = TOVE / KT , where KT is arrester TOV capability expressed as multiple of rated voltage.

TESTS

The following Tests are made on Arresters Routine Tests1. Power Frequency voltage spark over test on complete arrester2. Lightning voltage impulse spark over test on complete arrester

Visual examination3. Residual voltage Test (if agreed between Client and

manufacturer)

 

Type Tests

1. Power Frequency Voltage spark over voltage2. Lightning voltage impulse spark over voltage3. Switching voltage impulse spark over voltage4. Spark over voltage/Time curve test5. Residual voltage test

6. Operating duty7. Pressure relief test

CHARACTERISTICS OF SURGE ARRESTERS

System Voltage Nom./Highest

Arrester Rated Voltage

Max.Contin-uous Operating voltage.

Virtual Steepness of

Front for Front of wave

Max. Front of wave S.O.V

Max. Residual Voltage

KV KV KV KV (Peak) KV (Peak) KV (Peak)

11/12 12 10 100 50 43

66 /72.5 60 46 500 250 216

132/145 120 92 930 463 400

220/245 198 156 1200 746 649

System Voltage Nom./Highest

Arrester Rated Voltage

Max.Contin-uous Operating voltage.

Steep current (at 10 KA) 1/20 µs

impulse residual voltage

Lightrning current (at 10 KA) 8/20 impulse residual

voltage

Switching current (at 0.5/1KA)

30/60 residual voltage

KV KV KV KV (Peak) KV (Peak) KV (Peak)

11/12 12 10 40 38 27

66 /72.5 60 46 175 160 130

132/145 120 92 330 295 250

220/245 198 156 545 485 410

DESIGN EXAMPLE

Normal System Voltage

132 KV

Max. system Voltage, Vmax =

=1.1* Vnom KV

=145 KV

Continuous operating voltage, C.O.V

145/1.732 = 84 KV

Earth Factor, KE =1.4

T.O.V == KE * COV

=117.6 KV

KT = 1.08

Preliminary Rated Voltage =

TOV / 1.08

=108.89 KV

Take 10 % margin =108.89*1.1 =119.78 KV

Select Rated Voltage = 120 KV

PROTECTIVE MARGIN

Assume a working on 132 KV protected by a Surge arrester 25 Meters from Transformer.

Assume velocity, V = 300 m / µs

Steepness of Incoming wave =1000 KV/µs

Residual voltage, U1 = 276 KV

Voltage Stress, V2 = =276+2*1000*25/300 KV

= 443 KV

BIL of 132 KV System = 650 KV

Margin= =(650-443)/443*100

=47 %

Thus sufficient margin is available and selected arrester is fine.