Testing of surge diverter has to be explained

1. TESTING OF SURGE DIVERTERS
Contents
• Introduction
• Power Frequency Spark over Test
• Hundred per cent Standard Impulse Spark over
Test
• Front of Wave Spark over Test
• Residual Voltage Test
• High Current Impulse Test on Surge Diverters
• Other Tests
• Conclusion and Reference
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2. Introduction
• In modern practices, surge diverters or lightning
arresters are the most reliable apparatus to project the
power system against transient voltages due to
lightning and switching surges.
• They are invariably used from distribution voltages
(400 V) to highest system transmission voltages of
765 kV or above.
• Hence, testing them precisely in standard laboratories
with standard test procedures are of great importance
in modern power system practice.
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3. • A surge diverter has to be a non-conductor for
operating power frequency voltages.
• It should behave as a short circuit for transient
over voltages of impulse character, discharge
the heavy current, and recover its insulation
without allowing the follow-up of the power
frequency current.
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4. Power Frequency Spark over Test
• This is routine test. The test is conducted using
a series resistance to limit the current in case
a spark over occurs.
• The arrester has to withstand at least 1.5
times the rated value of the voltage for five
successive applications.
• The test is generally done also under dry and
wet conditions.
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5. Hundred per cent Standard Impulse
Spark over Test
• This test is conducted to ensure that the diverter
operates positively when over voltages of
impulse nature occur.
• The impulse generator is adjusted to give the
standard impulse voltage of a preset magnitude
specified in the specifications.
• The arrester has to spark over every time in each
of the ten successive applications. The test is
done with both positive and negative polarity
waveforms.
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6. • Sometimes, the test is done by starting at a
voltage level that does not give flashover at
all, and is repeated in increasing steps of
voltage till hundred per cent flashover occurs.
• The magnitude of the voltage at which
hundred per cent flashover occurs is the
required spark over voltage.
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7. Front of Wave Spark over Test
• In order to ensure that the surge diverter
flashes over for very steep fronted waves of
high peaks, this test is conducted using an
overvoltage having a rate of rise of 1OO kV/
ps, per 12 kV of the rating.
• The estimated maximum steepness of the
waves are specified in standards and
specifications.
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8. • The test is done by conducting hundred
percent spark over voltage test for increasing
magnitudes of the standard impulse wave.
• The time to spark over is measured. The volt-
time characteristic of the diverter is plotted,
and the intersection of the V-I characteristic
and the line with slope of the virtual
steepness of the front gives the front of a
wave spark over voltage.
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9. Residual Voltage Test
• This test is conducted on pro-rated diverters
of ratings in the range 3 to 12 kV only.
• The voltage developed across the Non-Linear
Resistor units (NLR) during the flow of surge
currents through the arrester is-called the
'residual voltage’.
• A pro-rated arrester is a complete, suitably
housed section of an arrester including series
gaps and non-linear series resistors in the
same proportion as in the complete arrester.
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10. • Standard impulse currents of the rated
magnitudes are applied, and the voltage
developed across the diverter is recorded using a
suitable voltage divider and a CRO.
• The magnitudes of the currents are
approximately 0.5,1.0, and 2.0 times the rated
currents.
• From the oscillogram, a graph is drawn between
the current magnitudes and the voltage
developed across the diverter pro-rated unit.
• From the graph, the residual voltage
corresponding to the exact rated current is
obtained.
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11. High Current Impulse Test on Surge
Diverters
• This test is also done on pro-rated diverter units in the
range of 3 to 12 kV.
• A high current impulse wave of 4/10μs of peak value
mentioned in the specifications is applied to a spare unit of
identical characteristics.
• To pass the test, if
(i) the power frequency spark over voltage before and
after the test does not differ by more than 10%,
(ii) the voltage and current waveforms of the diverter
do not differ significantly in the two applications, and
(iii) the non-linear resistance elements in the diverter
do not show any sign of puncture or external flashover.
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12. Other Tests
• The other tests that are normally conducted
on surge diverters are
(i) mechanical tests like porosity test,
temperature cycle tests, and others,
(ii) pressure relief test,
(iii) the voltage withstand test on the
insulator housing of the diverter,
(iv) the switching surge flashover test, and
(v) the pollution tests.
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13. • Conclusion
From these we conclude that know about
varies types of testing on surge diverter and
also its different ratings.
• Reference
“High voltage engineering ” by M S Naidu and
V Kamaraju, Tata McGraw Hill Education, 5th
edition.
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