Along with the increasing complexity of electric power systems,
there is an increment on the quality standards of the electric
energy supplied. The power quality refers also to the continuity
of the service, which is greatly affected by failures that
produce outages of transmission and distribution lines. Failures
in distribution networks significantly affect the cost and quality
of power supplied [1], and there is a continuous search for actions
to reduce the possibility of failures.
One method to improve the reliability of power distribution
systems is by replacing overhead bare conductors with covered
conductors (CC) in primary feeders [2]. The use of covered conductors
in distribution systems in not new; in some countries
these conductors have been used for more than 20 years. However,
its use has become more attractive in several parts of the
world due to the impact that CC has on the reliability of medium-
voltage feeders. The CC can consist of up to three XLPE extruded
layers: the first layer is a semi-conducting strand shield,
the second layer is the insulation layer, and the third layer is a
track-resistant covering layer. The construction of a three layer
cable is presented in Figure 1. CC is used in two common ways
on medium line voltage systems (5–69 kV) [3]:
a) CC system using pin post ceramic or polymeric insulators
and
b) spacer cable system (SCS), which is the system analyzed
in this article.
The SCS is a compact design for distribution feeders that can
be installed in heavily treed areas or lines with tight right-ofway
in order to reduce power interruptions and outages. In the
SCS the phase covered conductors are supported from a highstrength
messenger grounded wire by insulating spacers, as
shown in Figure 2.
Several successful cases have been reported around the
world, where the failure rates have been considerably reduced
by installing SCS. An example of good performance of SCS is
shown in Figure 3, where the system average interruption duration
index for a SCS in a period of five years is compared with
the same index for a system with CC and ceramic pin post insulators
(hybrid system), and for a system with bare aerial conductors.
The data are from a distribution system located in the zone
of Orizaba in the Mexican province of Veracruz, located 100
km from the coast. Although initial cost of the SCS is higher
than the bare conductors system, the SCS has proven to be a
long-term benefit for the electric utility when they are installed
inland, and its use is under increasing growth
However, there are some important issues with the same
SCS in zones with highly polluted conditions. SCS installed in
coastal areas, where an increase in the reliability of the lines was
required, were not successful. Distribution lines with SCS in
the coast of the same province had problems during the second
coastal storm after their installation [4]. Tracking and erosion on
the surface of spacers and covered conductors damaged these
components in a similar form as reported in a previous publication
[5] and shown in Figure 4. Some hooks broke apart from the
spacer, and as a result the conductors dropped.
Short circuits between phase conductors occurred because of
lost spacers. Some conductors dropped while energized, representing
a risk to those around the line [4]. After these events,
the Mexican electric utility (Federal Commission of Electricity,
CFE for its acronym in Spanish) decided to replace the lines
with SCS by lines with hybrid systems.
It is well known that electrical tracking on the SCS insulating
materials is associated with the high electric field concentration
around the contact area between the covered conductor and the
spacer [5]–[9]. The high field is a consequence of the zero-angle
contact points (wedge effect), and it is usually considered to be
affected by the dielectric compatibility of the materials. However,
it has been shown that even when the materials in contact (covered conductor and spacer) are the same, the field is considerably
higher compared to the field seen on the surface of the
conductor away from the spacer, as shown in Figure 5. In addition,
conductive pollution plays a significant role in the increase
of electric field, as it will be shown later.
Compatibility tests, in which the tracking resistance of spacers
and covered conductors is measured, become very important
to evaluate the performance of these systems under polluted
conditions. Recent investigations have pointed out the importance
of this test to evaluate the performance of SCS in the field
[5], [9] and the need to clarify certain conditions during the test
in order to confirm that such a test will reproduce the field operation
properly [5]. the finite element method through software Comsol Multiphysics
® 4.4. The maximum electric field appears in the seat area of
the hook for phase A, since the instantaneous maximum peak
voltage is considered on this conductor. The maximum electric
field is 4.3 × 105 V/m when