Page 652 - Industrial Power Engineering and Applications Handbook

Surge arresters: application and selection 18/61 7
especially with low V,,, at steep surges and a high energy and the housing at the manufacturing stage. Different
absorption capability. Only a station class surge arrester manufacturers have adopted different methods for
is normally preferred for such applications, and where achieving this. One such method is providing a barrier
steep-fronted surges are envisaged, these arresters too of an insulating material not affected by heat and arcing,
may be supplemented with a surge capacitor. Table 18.8 such as an FRP (fibre reinforced plastic) tube between
also provides data for motor protection station class surge the housing and the ZnO elements.
arresters. For 11 kV voltage systems, refer to Table 18.9.
Since a surge arrester is normally an engineered item to
suit a particular application, these data are only for general 18.10 Assessing the condition of an
reference. For exact application and type of installation, arrester
it is always advisable to consult the manufacturer.
To ensure adequate safety for a system and its terminal
18.9 Pressure relief facility equipment against overvoltages and voltage surges it is
essential to ascertain the soundness of the arrester at
A surge arrester is a sealed unit to save it from all regular intervals. It should be possible to do this when it
atmospheric hazards. It is normally filled with air. is in service without taking it off from the lines. If
Explosions of surge arresters have been noticed during deterioration of the ZnO elements is detected it may
service. Explosion of a porcelain housing is dangerous, need more frequent future services or replacement of the
as the shell splinters can cause great damage to nearby arrester. It can now be planned well in advance. The
bushings, insulators and other equipment and also requirement is similar to ascertaining the condition of
maintenance personnel if working in the vicinity. power capacitors when in service (Section 26.2). Like a
It is possible that ZnO elements may break down in capacitor, an arrester deteriorates too with time due to
time due to thermal cracking as a result of system TOVs degradation of the dielectric strength of its ZnO elements.
occurring frequently or existing for long durations. or ZnO is a highly non-linear resistor element. The success
while clearing a lightning or a switching surge and even of an arrester will depend upon its low, continuous resistive
subsequent to that. Breakdown of ZnO elements into leakage current Figure 18.4(a) to maintain low loss and
splinters may collide with the main porcelain housing. low heating over years of continuous operation. When
But most damage is caused by a flashover between the the ZnO blocks start to deteriorate which is a slow process
ZnO elements and the side walls of the housing, which as discussed earlier, the leakage current starts rising from
may result in puncture or crackdown. It may lead to a its original level. The rise in current is rich in third
cascading effect and cause an eventual short-circuit within harmonic component due to the non-linear characteristic
the housing and result in a very heavy ground fault current of the ZnO blocks. Other reasons for degradation in the
( V, / \;3 Zg) through it. dielectric properties and a rise in current may be one or
.
The fault current may cause a flashover and rises in more of the following:
the internal temperature and pressure of the housing. It
is extremely important to make provision to release this Ingress of moisture through the seals, although Silicagel
pressure, otherwise it may lead to an explosion of the is provided beneath the arrester sealing to absorb the
housing, scattering splinters like bullets in the vicinity. moisture.
This is dangerous and must be avoided. The pressure Failure of ZnO elements during or after clearing a
relief capacity must be such that there will be no violent few surges.
explosion of the housing during a failure. Although it Premature ageing of the ZnO elements.
may shatter, the arrester’s fractured pieces should not Temperature variations. The rise in I, is rapid at higher
fall beyond a circle of a radius equal to its height, operating temperatures.
somewhat similar to the properties of safety glass Frequent system voltage variations. and
windscreens used in a car. International specifications Being continuously energized
recommend that on a pressure build-up, the hot gases
will escape through the pressure relief diaphragm or the A rise in leakage current is not desirable and is indicative
housing may simply collapse as a result of thermal shock. of deterioration of the ZnO blocks, which may lead to
An arrester that is vented must be quickly replaced. failure. It is therefore necessary to monitor the leakage
This is achieved through a pressure relief system in current and detect a possible failure beforehand and take
the form of a pressure relief diaphragm at the top of the corrective steps in advance. The maximum safe leakage
housing. The pressure relief system is designed for the current is specified by the arrester manufacturer as a
system fault level. To test the pressure relief system, relation between I, and its third harmonic component,
IEC 60099-4 and IEC 60099-1 have also specified the 13r. 13T is expressed in terms of IZnO as discussed later. It
test current of the same magnitude as the fault current of varies with deterioration of the arrester and is used as a
the system, but for a very short duration, of the order of reference parameter to assess the arrester’s condition.
0.2 second or so. This brief period is enough to burst or As the actual leakage current measured through IZnO
shatter the housing of the arrester, hence a longer duration starts rising and approaches the maximum leakage current
is of no relevance. in healthy condition closer monitoring of the arrester
The practice for heavy-duty arresters is to eliminate becomes essential, to avoid an abrupt failure or explosion.
the causes of internal flashover between the ZnO elements Refer to Table 18.10 to monitor the condition of the