Page 624 - Industrial Power Engineering and Applications Handbook

Surge arresters: application and selection 18/589
18.1 Surge arresters phere. The discharge of gases affects the surroundings,
particularly nearby equipment. The gas-ejecting
enclosure deteriorates with every operation and.
When surge protection is considered necessary, surge therefore, has only a limited operating life. Moreover.
arresters" may be installed on or near the equipment
being protected. This is a device that limits the high TVs these types of arrester are of specific ratings and an
(transient voltages) generated during a system disturbance excessive surge than the rated may result in its f ai 'I ure.
They are now obsolete in view of their frequent failures
by diverting the excessive part of it to the ground and and erratic behaviour and the availability of a more
reducing the amplitude of the transient voltage wave advanced technology in a metal oxide arrester.
across the equipment to a permissible safe value less 2 Spark gap These have a pair of conducting rods
than the impulse withstand level of the equipment (Tables with an adjustable gap, depending upon the spark-
1 1.6, I4.1,32.1 (A), 13.2. and 1 3.3. The rate of rise (r.r.r.v.)
remains the same, i.e. over voltage of the arrester. Precise protection is not
possible, as the spark-over voltage varies with polarity,
steepness and the shape of the wave. These arresters
- "" (curves (I, and u2 of Figure 17.21) are also now obsolete for the same reasons.
I, t;'
3 Valve or non-linear resistor In this version, a non-
thus shielding the connected equipment from such dan- linear Sic resistance is provided across the gap and
gerous voltage surges. This is achieved by providing a the whole system works like a preset valve for the
conducting path of relatively low surge impedance follow current. The resistance has an extremely low
between the line and the ground. The discharge current value on surge voltages and a very high one during
to the ground through the surge impedance limits the normal operations to cause a near-open circuit. It is
voltage to the ground. During normal service this now easier to interrupt the follow currents.
impedance is high enough to provide a near-open circuit. A non-linear resistor-type gapped surge arrester may
It remains so until a surge voltage occurs and is restored generally consist of three non-linear resistors (NR) in
immediately after discharge of the excess surge voltage. series with the three spark gap assemblies (see Figure
18.1 (a)). The resistance decreases rapidly from a high
Corollary value at low currents to a low value at high currents,
An arrester can be considered a replica of an HRC fuse. While a such that R/ -- constant (Figure 18.1 (b)). Hence, V-/
fuse i\ ;1 cun-ciit limiting device and protects the connected equipment is an almost flat curve, as illustrated in Figure 18.1 (b).
by linuting the prospectlve peak fault currents, Is, (Figure 12.18) Thyrite* and Metrosil* are such materials. The purpose
an arrester is a voltage limiting device and protect, the connected of non-linear resistors is to permit power frequency
cquipinent by limiting the prospective peak aurge voltage, V, (curve follow currents, after the clearance of surge voltages,
3. Figure 17.21 1.
while maintaining a reasonably low protective level
(Vres). Across the spark gaps. known as current limiting
Arresters or diverters are generally of the following
types and the choice between them will depend upon the gaps, are provided high-value resistors (HR) backed
power frequency system voltage, characteristics of the up with HRC fuses. The non-linear resistors have a
voltage surges and the grounding system, i.e. very flat V-/ curve, i.e. they maintain a near-constant
voltage at different discharge currents. The flatness
of the curve provides a small residual voltage and a
(i) Gapped or conventional. and low current. When the switching surge voltage exceeds
(ii) Gapless or metal oxide. the breakdown voltage of the spark gap, a spark-over
takes place and permits the current to flow through
18.1.1 Gapped surge arresters the non-linear resistor NR. Due to the non-linear
characteristics of the resistor, the voltage across the
These are generally of the following types: motor terminals is limited to approximately the dis-
charge commencing voltage (V,,,), which is signi-
I Expulsion These interrupt the follow current by an ficantly below the 3-5 p.u. level. It may be noted that
expulsion action and limit the amplitude of the surge the use of resistor across the spark gap stabilizes the
voltages to the required level. They have low residual breakdown of the spark gap by distributing the surge
or discharge voltages (Vrcs). The arrester gap is housed voltage between the gap and the non-linear resistor.
in a gas-ejecting chamber that expels gases during Figures 17.S(a) and (b) are oscillograms illustrating
spark-over. The arc across the gap is quenched and the effect of a surge arrester in arresting the surge
blown-off by the force of the gases thus produced. voltages caused during a switching operation or a
The enclosure is so designed that after blowing off lightning strike.
the arc it forcefully expels the gases into the atmos- The current limiting gaps, as noted above, in series
with the non-linear resistors make it possible to adjust
the protection level of the surge arrester for different
*Basically they arc surge diverters but conventionally are called values of dircharge currents. They also help to maintain
arresters upto 245 kV lightening surges and beyond 245 kV switching
surges arc found to he more severe, Section 18.3. It ir cwtomary.
therefore, to call an arrester up to 245 kV a lightning arrester and
beyond 245 kV a surge arrester. For ease of reference. we have *Thyrite is a brand name from General Electric. USA.
described them as surge arrestcrs or only arresters for all types. Metrosil is a brand name from a GEC company in the UK