Page 640 - Industrial Power Engineering and Applications Handbook

Surge arresters. application and selection 18/605
influences of the different kinds of surges that may appear 18.6.1 TOV capability and selection of
in the system and which must be taken into account, rated voltage, V,
while engineering a surge protection scheme for such a
system or a part of it. TOV is considered only to select the MCOV and the
rated voltage, V,, of the surge arrester. This is a reference
parameter to define the operating characteristics of an
18.6 Selection of a gapless surge arrester. It plays no part in deciding the protective level
arrester of the arrester, which is solely dependent on the transient
conditions of the system, as discussed later. V,. is used to
make the right choice of an arrester and its energy
To provide the required level of surge protection for absorption capability to ensure that it does not fail under
equipment or a power system against possible transient the system's prospective transient conditions.
frequency voltage surges, ZnO gapless surge arresters To determine the level of TOVs and their duration, it
are the latest in the field of insulation coordination. We is essential to analyse all the possible TOVs the system
pro\;ide below a procedure to select the most appropriate may generate during actual operation, and then decide
type and sizc of a ZnO surge arrester for the required on the mosl crucial of them. as shown in Table 18.5.
insulation coordination. Based on the discussions above, Surge arrester manufacturers provide their TOV capability
the f'ollowing will form the basic parameters to arrive at curves in the shape of TOV strength versus duration of
the most appropriate choice: the TOV. Afew typical TOV capability curves are shown
in Figure 18.16(a) for distribution class and Figure
Service conditions: As for other equipment (e.g. motors, 18.16(b) for station class surge arresters. They indicate
transformers or switchgears) a surge arrester is the ratio between V, and Vr which may vary from
influenced too by unfavourable operating conditions manufacturer to manufacturer and is termed the TOV
such as noted in Table 18.4. strength factor K. For curve 18.16(b) i1 is typically
Unfavourable operating conditions will require a
derating in the rating of a surge arrester or special v, = 0.8 v,
surface treatment and better clearances. Refer to the Generalizing this for different condition4 of TOV and
manufacturer for the required measures and/or deratings.
Mechanical soundness: Such as strength to carry the make of surge arrester,
weight of conductor and the stresses so caused, pressure V, = K ' V,
of wind and, in extremely cold climates, the weight
of ice. For each kind of TOV and its duration, a corresponding
Maximum continuous operating voltage (MCOV) V,: factor (K) is obtained and with this is determined the
This voltage is selected so that the highest system required rating, V,. of the arrester. The most crucial TOV
voltage, VI,,, as in column 2, Tables 13.2 or 13.3, may be selected as the rating of the arrester. If it is not a
when applied to the arrester is less th?! or equal to standard rating as in the manufacturer's catalogue as
the arrester MCOV, that is, V, 2 Vln/y'3. shown in Tables 18.9 and 18.11, one may select the next
The BIL of the equipment being protected (Section higher rating available. A simple procedure is outlined
18.3). in Table 18.5 for a quick reference.
'l'he arrester's nominal discharge current (In): This
ifies an arrester and is the peak value of a lightning
current impulse wave (8/20 ps) that may pass through To determine TOV
the arrester for which it i5 designed. It may be one of The main cause^ of TOV, a\ discu\\ed above, may be
the following:
one or more of the following.
1.5, 2.5, S, IO and 20 kA
Table 18.4 Standard operating conditions Load rejection (Section 24.6.2).
Resonance and ferro-resonance effects (Sections 24.3.1
Pirrcirnrrrr.\ Staridrird conditions and 2 and 20.2.1(2)).
. .- ~~~~ Ground fault: Section 20. I. It is essential to know the
I Ambient temperature -40°C to + 40°C grounding conditions to determine the OV factor as
2 Sl stem frequency 48-62 HI below:
3 Altitude IO00 m Amplitude: As discussed in Section 20.1, grounding
1 Seismic conditionc Locations prone to experience an conditions, besides influencing the ground fault current,
earthquake of magnitude M = 5 or a also raise the system voltage in the healthy phases.
ground acceleration of 0.1 g and more It is established that for an isolated ur resonant
(Section 14.6) grounded system this can rise to 1.73 times and for
5 Pollution/contamination Due to excessive rain humidity, smoke, a solidly grounded system up to 1.4 times the rated
dirt and corrosive surroundings etc., voltage. When system parameters such as Ro, X,, and
wlricli may influence the arrester's X, are known a more accurate assessment of this
porcelain housing outer burface, hence factor can be made by using OV curves as shown in
the insulating properties ofthe arrester. Figure 20.16.
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