Presentation on Surge Arresters

2

Surge Arresters and Recent Developments

Why Surge Arresters ?

Aggressor

ProtectorPotential

victim

Surge Arrester Applications

Definition as per IEEE standards

As per IEEE standards, Surge / Lightning Arrester is“ a protective device for limiting

surge voltages on the equipment by diverting surge current and returning the device to its original status. It is capable of repeating these functions as specified”

Over Voltage ProtectionBasic function of a surge arrester is to limit over-voltages to acceptable limits

Causes for over-voltage / Surges in a system

Main causes for over-voltages in electrical system :

1. Lightning over-voltages ( micro seconds )

2. Switching over-voltages ( milli seconds )

3. Temporary Over-voltages ( Seconds )

Over-voltages & electrical system

Electrical equipments like transformers,motors are designed to withstand pre-determined values of impulse voltages.

Higher levels & multiple strikes of impulsevoltages cause the insulation to break downthus creating chaos in the system.

Over-voltages in electrical system

Protection of electrical devises are madepossible by connecting protective devises -Surge Arresters in parallel.

These devises should have variableresistance as a function of the voltagemagnitude to divert over-voltage to ground.

Gapless Zinc Oxide Surge Arresters arethe latest protective devises used to keepthe system voltages well within limits.

Current absorbed by Lightning arrester

Surge Voltage on Transformer

65kA

Equipment BIL

Surge Arresters / Over-voltage protection• Basic function of a surge arrester is to limit over-

voltages to acceptable limits.

– Acceptable limit means • Limiting over voltages to below BIL of equipment being

protected. • Protecting consumers and their equipmentSources of over-voltages:

– Temporary over-voltages: Earth faults, etc.. – Transient overvoltages, (impulses):

• lightning• switching

Surge arresters basic principles:Usually installed between phase

and earth/neutral

Under normal operating conditions, acts as an insulator• When subjected to an transient over-voltage,

it switches and diverts current to ground, thus limiting the over-voltage.

• Returns to insulator function: – ..if energy rating is not exceeded– damaged by internal short circuit.

• Must be thermally stable and able to withstand datasheet defined temporary over-voltages for period from seconds to hours, (depending on system).

Typical application:

Transformer protection

Earth

Arrester

HV

Expensive transformer damagesCost of TRAFO replacement

Cost of outage

Cost of oil clean up

Cost of arrester

Evolution of surge arrester

Arcing HornsBasic arcing horns are:

Simplest of Surge Arresters.

Can be provided with all equipments withbushings.

Difficult to maintain spacing / setting.

Difficult to monitor

SiC ArrestersSilicon Carbide Gapped arresters, assembled with SiC resistors and plate spark gaps:

Creates short circuit to earth when the voltage rises due to the spark gaps.

Series connection of SiC resistors limits the follow current from power supply - arcs disappear across gaps when next current zero occurs

Difficult to maintain gaps and monitor

Metal Oxide ArrestersZinc Oxide Gapless arresters, assembled by stacking ZnO blocks and placing same in insulating housings:

Extremely non-linear characteristics and so do not require any spark gaps.

Current starts to flow through arrester before the over voltage reaches the peak value.

Reduces the over voltage faster than the sparked gap arrester.

TWO SIGNIFICANT DEVELOPMENTSFollowing two significant fundamentaldevelopments occurred in the arrestertechnology:

SiC resistors and plate spark gaps werereplaced by Metal oxide resistors withoutplate gaps.

Housings of the arresters made ofporcelain were replaced with new onesmade of polymer material.

Arrester with Porcelain housing

Arrester ClassificationArresters can be classified based on following:

Based on type of housinga) Porcelain Housed Arrestersb) Polymeric Housed Arresters

Based on Energy HandlingDistribution class Station class I to IV

Surge Arrester Major ComponentsThe major components of LA’s are:

• Outer housing - porcelain or polymeric.• ZnO Blocks.• Springs to keep blocks in place.• Spacers / Heat sinks • Pressure Relief vents • End caps for preventing moisture entry.• Terminals for connecting to line & earth.• Mounting clamps

Outer Housing

Main functions are:

• Provide the contained space in which the ZnO Blocks are assembled.

• Ensure heat transfer from the blocks tooutside atmosphere.

• Have necessary parameters to ensurewithstand of electrical system properties.

• Prevent tracking and flashover.

Zinc Oxide Blocks

HEART of the Surge Arrester:

• Must remain as a non-conducting path during the normal operating voltage &ensure only over-voltages are conducted to earth.

• Minimum leakage current & have good heatdissipation.

• Quick response - absorb incoming surgeswithout time delay.

• Maintain good thermal stability for long life.

ZnO blocks

Electrode

Collar

1. Characterised by Uref, Ures, KV/mm field strength, electrode metallization, collar coating.

1. Quality of block determines Long Duration current2. Diameter = energy handling (assumes same

quality)3. Height = voltage. Typically 3kV =40mm.

2. Electrode is usually aluminium or silver based, used to optimise electrical contact between blocks.

3. Insulating collar used to prevent surface flashover

Microstructure of ZnO materialIdeal material structure

ZnOZnO

A = ZnO B = Spinel C = Bismuth

Optimum structure, perfectly homogeneous, i.e.

no weak points/paths

Homogeneity / thermal behaviour of the block:

Inhomogeneous - Homogeneous

High current impulse, (or thermal runaway)

Weak paths detected in “1”

Excessive local temperature ride along weak paths in samples “1” – leading to fracture & short circuit failure

What is thermal balance ?

ZnO Element Contd...

ARRESTER MANUFACTURING PROCESS

RAW MATERIAL

MIXING & SPRAY DRYING

PRESSING SINTERING LAPPING GLASS COLLARING

ANNEALING METALSPRAY TESTING

( BLOCKS )

ASSEMBLY TESTING PACKING

Ultimate test of varistors and arrester: Independent type testing,

IEC 60099-4 Surge Arrester Type Testing• Insulation withstand tests• Residual voltage tests• Long duration current withstand test• Operating duty test• Power frequency voltage versus time curve• Short circuit (pressure relief) test• Partial discharge test

Latest edition of IEC 60099-4 released 2006

Insulation withstand testInsulation w ithstand test of the arrester

housing:

Arrester housing should be able to withstand the lightning impulse protection level of arrester multiplied by 1.3

Design feature: Flash over distance of the housing

Samples tested: Arrester housing with insulating

core.

Surge arresters / Overvoltage protectionBasic function of a surge arrester is to limit

overvoltages to acceptable limits

Residual Voltage testResidual voltage testing of pro-rated test

samples:

The purpose of the measurement of residual voltage is to obtian the maximum voltages for a given design for all specified currents and waveforms.

Design feature: Related to disc specification

Samples tested: Prorated arrester samples.

Residual voltage determines protection level of arrester when subjected to certain wave forms. Its output also sets the limits for reference voltage routine testing in the factory.

IEC 60-1; Standard lightning waveform definition:

T1/T2;

8/20 waveform

8 µs T1T2 = 20µs

Current absorbed by Lightning arrester

Surge Voltage on Transformer

Equipment BIL

Standard phase to phase lightning insulation levels

Note: Taking the example of the 12kV system, the decision for lightning rating is dependant on statistical likely-hood of lightning, type of neutral earthing and type

of over-voltage protection used as standard.

Residual voltage dropIn order to ensure no loss of residual voltage the surge

arrester should be mounted directly on the transformer.

If it is not then quite a dramatic increase in effective residual voltage will occur. The actual loss will depend on the size of the impulse in kA and the frequency of the wave form.

For example if the arrester is 4m from the transformer and the incoming impulse is 5kA on a 8/20 then the rise in residual voltage will be 5kV. If the impulse is 20kA then the loss will be 20kV.

If the incoming wave is 4/10 then the rise will be 10kV on a 5kA impulse

Influence of surge arrester placementPotential difference across an inductor is given by the equation:

For a transformer lead the inductance L is approx 1μH per metre length of lead wire.

The lead length used to calculate inductance L is twice the transformer lead length (H) because the travelling wave sees the transformer as a near open circuit and is reflected back to the arrester.

e.g. for a 20kA 8/20μs impulse with a 4m transformer lead length the potential rise is calculated as:

Inductance L = 2 x 4m x 1μH/m = 8 μH

kVs

kAHV

208208 Rise Voltage

=

×=µ

µ

dtdiLV =

Lead Length from arrester to transformer = H

EarthThus we can estimate the voltage rise beyond a surge arrester with various lead lengths and surge kA as follows for a typical 8/20 lightning impulse

5kA 10kA 15kA 20kA 40kA0.5m 0.6kV 1.2kV 1.8kV 2.5kV 5.0kV1m 1.2kV 2.5kV 3.7kV 5.0kV 10kV2m 2.5kV 5.0kV 7.5kV 10kV 20kV4m 5kV 10kV 15kV 20kV 40kV6m 7.5kV 15kV 23kV 30kV 60kV

N.B.: For 4/10μs impulses such as the 65kA high current the voltage rises are doubled because the dt time value is half that of an 8/20 μs impulse

Transformer degradationWell designed, well

made, well maintained Poorly designed, poorly made, poorly maintained

Time in Years

BIL

90

75

60

0 5 10 15 20

Normal max operating point for transformer oil is about 80˚C. For every 6˚C above 92˚C rate of aging doubles.

Breathing system maintenance can have very significant bearing on insulation life as moisture content, acidity and oxygen content dramatically effect aging.

Energy AbsorptionLightning is a multi-component event called a flash, (1-2s)A single transfer of current from cloud to ground is called a stroke, (µs)

Positive or negative

Long stroke

First short stroke±i High current

operating duty

Long duration

I peak

Long duration current withstand

Test to confirm that arrester can withstand rated long

duration line discharge duty.

The purpose of the measurement of LD test is to verify the arrester ability to withstand multiple long duration impulses, i.e. Switching / multipulse induced overvoltages while energised at power frequency voltage.

Design feature: Energy handling, thermal stability.

Samples tested: Prorated arrester samples.

Long Duration Current test sequence

Example test for 5kA arrester:

Check reference voltage

Inject 18 impulses of “200A(min), 2ms duration”

Verify Uref is within 5% of original value

Operating duty testTest to confirm that arrester can withstand a combination of stresses

that an arrester is faced with in service while energised at power frequency voltage.

The main requirement to pass this test is that the arrester is able to cool down in between impulses while under power frequency voltage, i.e. Thermal run away does not occur.

Design feature: Impulse stability, energy handling,

thermal stability.

Samples tested: Prorated arrester samples.

Operating Duty Test

20 shotsPreconditioning

5kA 8/20µs

Rated voltage

Continuous Operating voltage

65kA 4/10µs

1. Measure Residual Voltage

2. Energise to 1.2 times MCOV

3. Precondition with 20 shots of 5kA, 8/20µs4. One shot of 65kA, 4/10µs for 5Ka arrester (100kA for 10Ka arrester)

5. Heat to 60˚C

6. Second shot of 65kA, 4/10µs

7. Apply rated voltage for 10 seconds

8. Apply continuous operating voltage for 30 minutes

9. Measure residual voltage again. Must be within 5% of original value

Revision of Standard

IEC 60099-4, 2006

finally caught up with

State-of-the-ArtSurge Arrester

Technology

New Requirements

Moisture Ingress Test;

Weather Ageing Tests;

(Tracking & Erosion Performanceof Housing)

Short Circuit Tests (Test procedure still informative but commonly accepted with distribution type arresters, more realistic failure simulation)

Routine testing: PD < 10pC,

Aging addressed.

Tightness Test (Moisture ingress)IEC 9.7.9

• Terminal torque Pre-conditioning• Thermo-mechanical pre-conditioning

(4 x 24 hours)• Boiling in salty water

Why is this important?

Screens poor seals;

(after mechanical stressing)

Not every design can pass this test

-25°C +60°C

+45°C-40°C

42 h

Moisture Ingress Test

• Mechanical changes to be reported

• Increase of power loss < 20%

• Partial discharges not exceeding 10pC

• Change of residual voltage @nom. discharge current not exceeding 5%

• No breakdown visible in voltage and current oscillograms

Porcelain Failure due to moisture ingress

Moisture Ingress paths

Moisture Ingress paths

Moisture ingress results in internal flashover / surge arrester failure

No tracking or erosion

Salt fog test

• No tracking

• No erosion through entirethickness of external coating

• No puncturing of sheds and housing

• Decrease of reference voltage should not exceed 5%

• Partial discharges should not exceed 10 pC

Product materials testing

Tracking and Erosion Resistance, TERT.• TERT is used a measure of the tracking and erosion resistance

of materials. • It is a good measure used to rank materials relative

performance, at the screening stage. Though this ranking may not always reflect exact ranking in the field, it is considered a good screening test.

Two tert tests;-Step test: Voltage is increased each hour and contaminant rate

increased periodically-Constant Voltage test: Voltage and contaminant flow rate maintained

for set time

In either test method, the material must be non-tracking with eventual failure by erosion only. Flame failure is not allowed.

Tracking and Erosion Resistance, TERT.

Tracking and Erosion test set-up (TERT)

Conductivecontaminant(ammoniumchloride)

ElectrodesElectrodes

Tert plaque (6mm thick)

TERT - Key Failure Mechanisms

Tracking failure(rapid process)

Erosion failure (slow process)

Sample - Front View Side View Sample - Front View Side View

1000 hours continuous salt fog at continuous operating voltage UcSamples energised at Uc in an enclosed chamber 10m^3A mist is generated with a salt concentration of between1 -10kg/m^3Test run for 1000hr min

No tracking and erosion should occur. Outcome?

FAIL FAIL

Weather Ageing Test: 1000hr salt fog

Short Circuit Test

• No violent shattering

• No fragments outside enclosure(except soft polymeric material,fragments less than 10 g,pressure relief vents, ...)

• No open flames after 2 min

After short circuit test withElectrical pre-failingmethod

Short Circuit Test

Failure modes

Risk: Fire, danger, collateral damage

Specific Advantages of Polymeric Housing

• No internal air space, no chance of moisture ingress.• Vandal proof.• Non-explosive failure mode.• High thermal conductivity. Rapid heat dissipation.• Low weight and small size.• Suitable for pollution environment.• Resistant to transport damage and careless handling.• Stud, pedestal or bracket mounted.• Good for abnormal service conditions like enhanced external

insulation where operation is under high or low temperature oraltitudes over 1000m etc

• Easy to install.

Porcelain vs. polymeric

Porcelain disadvantages• Poor resistance to moisture

penetration• Heavier and brittle (note cracked

shed)• Short circuit pressure relief only

for EHV and Station class arresters

• Thermally insulating design reduces energy handling and TOV performance of block gets affected

• Poor pollution flashover performance.

Porcelain housed arrester Contd...

Arrester Selection

For a network with solid earthed neutral, following is the formula for selection of Maximum Continuos operating voltage of arrester

1.4 x UmUc = --------------

1.28 x _/3

- Um : Maximum Voltage- Phase voltage does not exceed 1.4 p.u- Factor 1.28 considered assuming maximum time for clearance for earth fault is 3 sec

For more details contact:anitagupta@ieema.org