Over the years, we have all gained valuable field
experiences that provide insight into how and
why equipment fails. I have created the following
list of top five general causes for switchgear failures and how
they can be prevented, based upon my particular experiences.
I hope that this information can spark creative thinking from
our readers to prevent future switchgear failures due to their
own unique field observations.
Loose and faulty connections cause an increase of resistance
at that localized point. The increased resistance causes
increased heat in accordance with Ohm’s law, P = I2R . The
increase in heat will escalate until complete thermal failure
of the connection occurs or the nearby insulation fails
resulting in a fault. One major insurance carrier estimates
that approximately 25 percent of all electrical failures occur
due to loose connections.
The solution to avoiding these types of failures is to perform
regular infrared inspections of all switchgear. Infrared
viewing ports should be installed and medium-voltage
switchgear should have ports that also pass ultraviolet light
so that corona cameras can be used to inspect for corona and
surface partial discharge activity. Figure 1 shows a thermal
image of a loose switchgear connection that could lead to
future failure if not repaired.
A whole series of articles could be written about these
types of problems, but we will focus just on the most common
culprits here. Low-voltage insulation is fairly simple
and is not subject to the same voltage stresses as mediumvoltage
switchgear insulation. Keeping low-voltage insulation
failures in check mostly involves keeping the insulation
dry and clean and ensuring clearances are adequate.
systems are much
more complex due to the
greater voltage stresses
that exist. Areas within
the switchgear that are
overstressed will initially
fail over a small portion
of the insulation but will
then escalate over time
until complete failure occurs.
The most likely areas
for these problems to occur
- Jumper cables – Unshielded jumper cables are used
to connect switches to transformers in unit substation
design; connect potential transformers, control power
transformers, and surge arresters to the bus; and connect
transformer coils and taps together. Anytime these cables
come in contact with ground, other phases, or even other
types of insulation used to support them, they will likely
develop stresses at those points that will lead to future
failure (see figure 2).
- Bus – Most switchgear designs use simple insulation
barriers to separate compartments and to support the
bus. These barriers often have small air gaps between the
bus and insulation. The localized voltage stress is great
enough to cause the air to break down and this partial
breakdown will lead to eventual failure (see figure 3).
- Cable Terminations – Cable terminations are difficult to
build properly. These components transition the shielded
cable that has nearly perfect inherent voltage stress
control to a connection to the bus. Many things can go
wrong if great care is not taken during the termination
construction, and the proper voltage stress relief details
are not realized. Additionally, the portion of the termination
nearest to the connection essentially consists of
unshielded insulation, so care must be taken to ensure
adequate clearances in that area. All of these locations
within the termination can create localized partial insulation
breakdown that will lead to failure(see figure 4).
Localized partial insulation failures are known as partial
discharges. Fortunately, the partial discharge activity creates
detectable signals which are the early warning signs of future
complete insulation failure. The solution to preventing
medium-voltage switchgear failures begins with utilizing
the hand-held partial discharge detector equipped with an
ultrasonic sensor to detect surface insulation defects and a
transient earth voltage sensor to detect internal insulation
Water intrusion or immersion due to natural disasters or
accidents can create instant short circuits, long term insulation
damage, and long term metallic component corrosion,
among other complications. Medium-voltage switchgear
that is exposed to high humidity conditions will absorb
moisture, and voltage stresses will attack the hydrophobic
insulation surfaces which were designed to inhibit moisture
absorption (see figure 5).
For medium-voltage switchgear, using the partial discharge
detector described above will prevent long term
moisture-related insulation faults while the infrared camera
can detect abnormal heating of corroded connections.
Racking in a closed circuit breaker onto an energized
bus can quickly cause severe personal injury or death and
immediate severe equipment damage (see figure 6). Additionally,
the breaker may not always line up properly or may
encounter other difficulties as it is being racked, and these
problems can cause a sudden severe fault. Unfortunately,
the traditional act of breaker racking requires personnel to
manually perform this task directly in harm’s way.
The solution to this problem is to always make sure that
mechanical and electrical interlocks are functional and all
breaker and cell components are properly inspected and
serviced. To ensure personnel safety, strong consideration
should be given to employment of a remote circuit breaker
racking device such as shown in figure 6.
Faulty Ground Fault Protection
Unlike the items above, a defective ground fault protective
device will not create a fault itself. However, it will not offer
protection from an arcing ground fault which is a common
failure mode in 480Y/277V solidly-grounded switchgear.
These types of faults are very destructive but do not draw
high enough currents to trip breakers or cause fuses to open
and can persist until catastrophic failure of the switchgear
occurs (see figure 7).
The solution to this problem requires an outage and
manually testing the ground fault protection system by
current injection. Just as important is to pay close attention
to ensure that the equipment is properly installed. Sensor
polarities must be tested when applicable and the neutral
ground connection must be located in the correct position
so that the sensors will detect fault currents properly. A list
of recommended test procedures can be found in the NETA
ATS and MTS standards.