Rotary Operated, Oil Immersed Sectionalising Switches
An Integral Component of Compact Padmounted Transformer Substation
Larry D. Bucholtz, Cooper Power Systems, USA
Frank J. Muench, PE, Cooper Power Systems, USA
The recent introduction and rapid acceptance of the hybrid oil-filled, “compact padmount substation” by
Australian utilities is part of a trend in the global utility market. The compact padmount substation
integrates primary over-current protection and switching into the transformer cubicle to preserve safety
and operational performance, while reducing costs and minimizing space requirements.
Historical Perspective
Transformer 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” utilising medium voltage switchgear in close proximity to the transformer. This technology,
often referred to as Ring Main Unit (RMU) switchgear, incorporates high voltage (HV) primary switching
and over-current protection in a separate compartment or “unit” adjacent to and connected by cables or
buss to the transformer.
In this configuration, low voltage (LV) cables are protected by LV cable 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, utilising a separate switch unit, can significantly increase the size, and subsequently, the cost of
the installation.
A second convention is the U.S. style substation or “padmount” transformer. This configuration evolved
in the late 1950’s and early 1960’s 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 transformer configuration, often referred to as an “ANSI” design, incorporates overcurrent protection and primary switching into the transformer cubicle. Combining bay-o-net fusing with
back up, partial range current limiting fusing provides primary fault protection and both transformer
overload and secondary fault protection. This convention typically does not typically include separate LV
fuses or circuit protection.
The Evolution of “Padmount” Transformers and Associated Components
The first “generation” underground systems in the U.S. effectively “ganged” 1∅ overhead transformers in
an outdoor kiosk. These arrangements were understandably large despite attempts for close spacing, as
walls or fences were needed to ensure public safety and provide utility line crews space to work within.
Incorporation of gang operated air-break switches to allow line sectionalizing, required the kiosks or
enclosures to become even larger to provide adequate space and clearance.
A solution to these issues resulted in a second-generation “underground padmount system” transformer
whereby primary fusing and switching was moved into the air compartment of the transformer. While
this solution was reasonably effective for single phase transformer applications, the protection and
switching still required a relatively large amount of space for three phase applications, and did not provide
an effective methodology to disconnect or isolate all three phases.
Major breakthroughs occurred in evolving underground distribution systems in the 1960’s in the U.S.
This ultimately led to “padmount” transformers which incorporated the innovations of screened primary
separable connectors, primary over-current protection (bay-o-net fuses in series with under-oil current
limiting back-up fuses) and three-phase, gang-operated, oil immersed, rotary loadbreak “sectionalising”
switches into the transformer. The use of separable connectors provided a “visible break and visible
earthing” of the cables from the transformer bushings.
The development and use of primary oil immersed bay-o-net (draw-out expulsion) fuses and oil immersed
switches was significant from the perspective that the devices could be used to provide primary overload
protection and to turn single phase transformers “off and on”. The concept reinforced that under-oil
loadbreak and interrupting devices could be used in transformers without shortening transformer life.
Bay-o-net fusing and later, screened, loadbreak cable terminations enabled a size reduction of the
padmount transformer and improved functionality. Growing demand for three-phase applications and/or
3-wire systems necessitated the use of gang-operated, oil immersed sectionalising switches to control
ferro-resonance, and single and two phase feeding of three phase loads. Extensive testing and
development efforts demonstrated that sectionalising 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 padmount 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.
A Work-horse and Key Distribution System Component
Rotary, oil immersed loadbreak sectionalising switches have been used in hundreds of thousands of
transformer applications for 30+ years. The switch was upgraded in the 1970’s to increase performance
characteristics and provide improved functionality for growing and increasingly diverse utility
applications.
Until the late 1990’s, both switch technology and transformer “convention” remained largely unchanged.
However, privatisation 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.
Recognising these changing needs, Wilson Transformer Company Pty. Ltd., Wodonga, Victoria,
Australia, designed and introduced a new “hybrid” transformer to the Australian utility industry. This
concept provides the functionality of conventional RMU based designs while yielding improvement in
space savings and in life cycle savings via lower installed and operational costs. Central to this concept is
the use of oil-immersed fuses and rotary, loadbreak sectionalising switches mounted in the transformer
cubicle. Improvements to these switches – primarily in the form of momentary withstand ratings of 16kA,
will allow the use of these transformers in most distribution system applications where the RMU’s are
used today. One switch, unique in design and function, is a three or four position “sectionalising” switch
manufactured by Cooper Power Systems, Pewaukee, Wisconsin, U.S.A.
This switch is available in a number of configurations, allowing the end-user utility great flexibility in
configuring transformer feeds and in earthing primary cable. The most common switch configurations
allow users to select the cable source feeding power to the transformer, “feedthru” the transformer (with
or without the load connected), or earth the primary cables. Using two sectionalising switches, users can
achieve the same open/close/earth functionality typically provided by traditional RMU’s. Incorporating
the primary switching (and over-current protection) into the transformer cubicle minimizes space
requirements, requires fewer terminations and simplifies operational activities.
Use of the compact padmount substation transformer is increasing rapidly in many global utility markets.
The RMU configuration is well suited for applications where traditional operational practices are
followed or where installations are required indoors, although typically at a cost premium. Where space
is at a premium, the compact padmount substation is ideally suited. Adding to the attraction of utilising
oil immersed rotary switches is the fact they are suitable (with some limitations) for use with high fire
point, less flammable fluids (such as high molecular weight hydrocarbon and recently introduced
vegetable based dielectric fluids). This is an important consideration as many transformer applications
are indoors, adjacent to buildings or in hazardous environments which may require the use of less
flammable fluids. To reduce the size and cost of the RMU, many manufacturers utilise sulfur hexaflorine
(SF6 ) gas as the insulating medium. SF6 continues to face increasing scrutiny as an ozone depleting
substance and will, at a minimum, face restrictions of use and/or the potential for relatively expensive
monitoring and reclamation processes in the future.
Rotary, oil immersed sectionalising switches, used as an integral component in the compact padmount
substation, provide a time proven, cost effective alternative to conventional ring main
switchgear/transformer applications. Distribution system functionality is, at a minimum maintained, and
in many instances improved, by virtue of simplified distribution system architecture. New versions of
four-position sectionalising switches (to be introduced in coming months) will offer additional operational
functionality and allow utilities to alternate from one source of power to another while minimizing circuit
interruptions.
Key among these new developments are “make before break” versions of the switch. These switches will
provide further opportunities to improve system reliability by eliminating momentary power interruptions.
The combination of these switches, completely deadfront cable terminators, and load and fault overcurrent devices in series with oil immersed current limiting fuses provide for optimal protection, safety
and operational performance at minimal cost.
Bulletin No. 02041, dated 10/02