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New Transformer Design Provides A Global Solution
ransformer designs typically
take on one of two configurations,
both of which are widely used
throughout the world. One
convention is the standard practice
in Europe, Australia and other IEC
markets utilizing medium voltage
switchgear in close proximity to the
transformer, in what is commonly
known as compact Medium Voltage/
Low Voltage Distribution Centers
(MV/LV Centers). This technology,
characterized by the use of Ring Main
Unit (RMU) switchgear, incorporates
high voltage (HV) primary switching
and overcurrent protection in a
separate compartment or unit
adjacent to and connected by cables
or buss to the transformer.
In this configuration, low
voltage (LV) cable are protected
by LV fuses or more often LV breaker
panels, which are adjacent to or
part of the transformer LV
compartment. Typically the entire
package is compartmentalized into
an indoor or outdoor kiosk to ensure
public safety and improve aesthetics.
This system, utilizing a separate
switch unit, can significantly increase
the size, and subsequently, the cost
of the installation.
Pad-mounted Configuration
A second convention is the
more familiar U.S. style substation
or pad-mounted transformer. This
configuration evolved in the late 1950s
and early 1960s as environmental,
reliability and aesthetic concerns led
to the widespread use of underground
power distribution in the United
States, and ultimately many other
global utility markets. This trans-
former configuration, often referred
to as an ANSI design, incorporates
overcurrent protection and primary
switching into the transformer
cubicle. Combining bay-o-net fusing
with backup, current-limiting fusing
provides primary fault protection
and both transformer overload and
secondary fault protection.
Three-Phase Applications
Bay-o-net fusing and later,
screened, loadbreak cable termina-
tions enabled a size reduction of
the pad-mounted transformer and
improved functionality. Growing
demand for three-phase applications
and/or 3-wire systems necessitated
the use of gang-operated, oil-
immersed sectionalizing switches
to control ferroresonance, and single-
and two-phase feeding of three-phase
loads. Extensive testing and develop-
ment efforts demonstrated that
sectionalizing switches could be
ganged efficiently, occupy minimal
space, and were relatively easy to
operate. Additionally, the switches
could be operated many times without
significantly degrading the dielectric
strength of the insulating fluid and
subsequently, shortening the life of
the transformer. In the ensuing 40
years, literally millions of pad-mount-
ed transformers (both single-and
three-phase) have been installed
by utilities throughout the world.
Transformer life has matched
design expectations, and the use of
oil-immersed switches and protection
(e.g. fuses and arresters) has proven
safe and reliable.
Until the 1990s, both switch tech-
nology and transformer convention
remained largely unchanged. However,
privatization and other utility industry
changes shifted the focus on
distribution systems from heavy
infrastructure to that of flexibility, cost
containment and improved reliability.
Global utilities began looking beyond
conventional technologies and
markets for equipment and best
practices, which would meet their
changing needs and priorities.
New Hybrid Transformer
In the past 4-5 years, Cooper
Power Systems has worked closely
with utilities in several markets
outside the United States to design
transformers which provide the
functionality of RMU switchgear, while
taking advantage of the smaller size
integral components and flexibility
of the ANSI style pad-mounted
transformer. Design objectives for
these hybrid transformers included
maintaining the reliability and safety
attributes of the RMU switchgear, the
use of shielded (screened) cable
terminations, the use of pad-mounted
components to provide switching
capabilities, and the use of HV fusing
to provide primary fault protection.
The result of these efforts was a new
transformer design, the compact
pad-mounted substation, which is
functionally equivalent to the
conventional European MV/LV Center.
Variations of the compact
substation have been built and
provided for customers in Spain,
China and the United Kingdom.
New Transformer Design
Provides A Global Solution
System Reliability
by Larry Bucholtz, Manager, Line Installation and Protection Products
Kostos Argiropoulos, Director, Eastern Hemisphere Sales
Nick Vassiliou, Senior Sales Engineer
Eugene Knabe, Product Manager, Over-Current Protection Devices,
FCIs and Single-Phase Overhead Switches
Figure 1 RMU loop scheme Despite its growing popularity,
the compact pad-mounted substation
has been limited to specific applica-
tions and uses. The primary limita-
tions are a function of higher system
fault currents and the use of three-
wire systems common to IEC
distribution networks.
Pad-mounted transformers
originally developed in the 1960s were
developed for 4-wire systems common
in North America although utilities in
certain regions of North America use a
3-wire system. There are differences
between the 4-wire systems and the
common IEC based systems. Some of
these typical differences include:
The 4-wire systems in North
American market are better suited to
single-phase and mixed single-phase
circuits while the IEC systems are
predominantly three-phase, 3-wire
with delta connected transformer
primaries. The 4-wire system often
uses line-to-ground rated devices
such as fuses to protect distribution
transformers. IEC systems typically
require phase-to- phase rated
protective devices and/or ganged
fuses, since faults require multiple
phase involvement on this type of
distribution system.
The 3-wire delta systems are
prone to ferroresonance associated
with single-phase switching. Due to
this concern, switching is in most
cases three-phase ganged.
Fault currents found in 4-wire
systems typically range from a few
hundred amperes to 10,000 plus
amperes. Thus, for most North
American distribution systems typical
fault current levels are limited to
10 kA or less. In many IEC based
countries fault current levels are in
the range of 12.5 kA to 16 kA, because
the load density requires the use of
larger cables and shorter cable runs.
In the US and other markets
where 4-wire systems are used,
practices generally require the cable
to be separated from the equipment
and/or grounded (earthed) before
power system technicians can work
on the gear. In IEC countries, switches
are used to both control the power
source to the transformer and to earth
(ground) the transformer. The former
calls for separable connectors and
other connection devices that allow
disconnection. The latter often leaves
the cables in place, relying on switch
status indicators and checks of the
equipment to be sure
it is not energized.
These different
system design and
operating practices
require the compo-
nents used on the
systems to vary
somewhat. As a
result, changes to the
components used in
the compact substa-
tion were required
before wide accept-
ance of this system
design could be
adopted.
With these design challenges in
mind, Cooper Power Systems design
and manufacturing teams set out to
develop switch and overcurrent
components which would meet these
needs. Design objectives were to
combine the reliability of the
existing products, with the higher
performance and operational require-
ments for the desired components.
In doing so, the opportunity would
be to revolutionize the compact
pad-mounted substation concept as
an economical, size reduced and
functionally equivalent distribution
system asset.
The result of these multi-year
research and development efforts is
the current introduction of fully rated
IEC sectionalizing switches, and near
term introduction of the three-phase
MagneX
TM
Interrupter. These products,
which are unique to Cooper Power
Systems, and include several patent
pending innovations, will significantly
extend the range of applications for
the compact pad-mounted substation.
In addition, the operational benefits of
the components offer the opportunity
to revolutionalize the compact pad-
mounted substation concept as an
economical, size reduced and
functionally equivalent distribution
system component.
The new sectionalizing switch is
an upgraded and improved version
of the four position sectionalizing
switch which has been manufactured
by Cooper Power Systems, and used
by OEMs for 30 plus years. The new
switch offers separate versions that
meet all IEC, (60265-1998) and
ANSI (C37.71-2001) requirements. The
IEC rated switches include higher
momentary withstand ratings (16 kV -
two seconds), increased switching
capabilities (630A