Page 313 - Handbook of Electrical Engineering

FAULT CALCULATIONS AND STABILITY STUDIES 299

Table 11.5. Companion of reactor configurations

Case No. of reactors Reactance (pu) Current rating (amps) Product Cost factor
N X r I IX r NIX r
A 5 0.0425 1049.8 44.62 223.10
B 3 0.00952 2099.6 19.99 59.97
C 3 0.02856 1049.8 29.98 89.95

Table 11.5 compares the cases in terms of cost, but without the cost of the extra switchgear
being included. The cost of a circuit breaker would be in the same order of magnitude as its associ-
ated reactor.
When the cost of the switchgear is taken into account, cases B and C become closer in cost,
and possibly either is more expensive than case A. Case A does not require extra switchgear.

Occasionally it is desirable to interconnect isolated power generating stations, e.g. offshore
platforms or desert gathering stations. Although this often seems a good idea when considering
improved power availability and minimising redundancy and spare generators, it frequently causes
difficult fault level problems. However, these problems can sometimes be solved by using reactors or
transformers in the interconnecting cables or overhead lines. Figure 11.14 shows an interconnection
of two offshore platforms.
Even when the reactors are inserted, it may be necessary to impose operational restrictions on
the system configuration, e.g. it may not be permissible to have all the generators connected when
the interconnector is in service. This aspect may be overcome to some extent by introducing a system
of electrical or mechanical interlocks.
Reactors are usually a solution to progressive problems. They should not be designed into a
new system.
Reactors may be iron-cored or air-cored. Iron-cored units are preferred but care has to be
taken in their design so that they do not become saturated when fault currents pass through them.
If the fault current exceeds about three times their rated current then air-cored units become more
economically attractive.

They may be of dry-type or liquid-immersed construction, the latter tending to be most common
because:-

• They are more suitable for outdoor locations.
• They have a high factor of safety with regard to internal flashover.
• They have a tank, which tends to retain all magnetic fluxes inside the unit. This is important
when the location of the reactor is being considered. The radiation of the flux can cause eddy
current heating in adjacent steelwork and magnetic interference with other nearby electrical and
electronic circuits.
• They have high thermal capacity and can therefore absorb the fault current heat more efficiently.
• The manufacturer can use standard tank and cooling designs that would normally be used for
transformers.