Introduction to Bus-duct

System and Application

Puspak Kumar Mohapatra

Electrical Maintenance
Department
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 Introduction

In electric power distribution, a bus

duct is a grounded sheet metal duct
or also cast resin insulated
containing
either copper or aluminum bus-
bars for the purpose of conducting a
substantial current of electricity . It
is an alternative means of
conducting electricity other
than power cables.

In a power system the reliability for

continuous power supply is one of
the most important aspects. To
ensure this bus-duct is widely used.

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 Installation of Bus-duct

Why Bus Duct required ??

• Bus duct can be installed in most applications where cable or

conduit would normally be used.
• Many people believe bus duct only serves high-amperage
applications. This is a misconception–busway can provide a high
degree of efficiency for both low- and high-amperage situations.
• In order to have low losses and enhanced reliability in the
distribution system, bus-ducts are preferred over cables for
higher current ratings and shorter lengths at different voltage
levels.
• For feeders of moderate ratings, say up to 600/800A, cables are
preferred, while for higher ratings (above 1000A) the preference
is to opt for solid conductors (LT bus systems) on the grounds of
safety, reliability, maintenance, cost, appearance and ease of
handling.

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 Bus-duct Components

Bus- duct is a prefabricated and modular system designed for the

distribution and transportation of electrical energy. Its general
structure consists of a combination of aluminum or copper
conductors with insulation materials in a metal body in accordance
with the standards.
Bus-duct components include:
Conductors conduct electricity: they
are made from aluminum or copper
and vary in size.
Housing : an aluminum or steel
enclosure to contain the conductors.
(non-magnetic enclosure)
Insulating system : made of a
combination of epoxy, polyester film
or air; it separates the conductors
from each other to prevent electrical
faults.
Fittings : such as elbows, offsets help
to properly route bus-duct from one
electrical connection or termination. 5
 Design criteria

The metal-enclosed phase bus ducts are designed for 635 V,

5 kV, 15 kV, 27 kV and 38 kV service in accordance with ANSI
C37.23.

The application of the bus-duct is based upon the following

factors :-

-Rated voltage -Impulse voltage withstand

-Continuous maximum rating -Power frequency

voltage withstand

-Rated short time current rating -Rated insulation level

(kA/sec)
-Duration of the fault -Rated frequency

-Rated momentary peak value of the fault current

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 Short-circuit effects

The purpose is to determine the minimum size of current

carrying conductors and decide on the mounting
arrangement.

• A short circuit results in an excessive current due to low impedance of

the faulty circuit between the source of supply and the fault.
• This excessive current results in excessive heat in the current carrying
conductors, which thus generates electromagnetic effects and electro-
dynamic forces of attraction and repulsion between the conductors and
their mounting structure.
• These forces are distributed uniformly over the length of conductors.
• The effect of a short circuit henceforth requires these two factors
(thermal effects and electro-dynamic forces) to be considered while
designing the size of the current carrying conductors and their
mounting structure, which includes mechanical supports, type of
insulators and type of hardware, in addition to the longitudinal distance
between the supports and the gap between phase to phase conductors.

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 Thermal effects

With normal interrupting devices the fault current lasts for up to 1 sec. This
time is too short to allow heat dissipation from the conductor through
radiation or convection. The total heat generated on a fault will thus be
dissipated by the conductor itself.

• The size of the conductor therefore should be such that its temperature
rise during a fault will maintain its end temperature below the level
where the metal of the conductor will start to soften.
• Aluminum, the most widely used metal for power cables, overhead
transmission and distribution lines or the LT and HT switchgear
assembly and bus duct applications, starts softening at a temperature
of around 180-200 deg. C.
• As a rule, on a fault, a safe temperature rise of 100 deg.C above the
allowable end temperature of 85 deg.C or 90 deg.C of the conductor
during normal service i.e., up to 185 deg.C-190 deg.C during fault
condition is considered secure and taken as the basis to determine the
size of the conductor.

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 Electro-dynamic effects

• The short circuit current is generally asymmetrical and contains a DC

component.
• The DC component, although it lasts for only three or four cycles,
creates a sub transient condition and causes excessive electro-dynamic
forces between the current carrying conductors.

• The mounting structure, bus-bar supports and the fasteners are

subjected to these electrodynamic forces.

• Although this force is only momentary, it may cause permanent damage

to the components and must be considered when designing the current
carrying system and its mounting structure.

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 Type of Bus ducts

Non-segregated phase
bus-ducts

Bus-duct
Enclosure are
classified into segregated phase bus-
various types ducts
depending on
its application
Isolated phase Bus-
ducts

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 Non-segregated Phase bus-duct

• In this construction, all the bus phases are

housed in one metallic enclosure with
adequate spacing between them with air as
medium of insulation between phases.
• Being vivid, it is most widely used
methodology for all types of LT systems.
• These bus-ducts are relatively compact in
comparison with segregated phase bus since
the same are generally used for low voltage
applications and thus needing much lower
electrical clearances between phases and
phase to earth.
• Non-segregated phase bus-ducts to be used
commonly in low voltage distribution with
rated voltages up to 1100 Volts and with
rated continuous currents up to 6300 Amps
with short circuit fault currents of up to 50
KA for 1 second.
• It is applicable in LV PCC to MCC
interconnection.

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 Segregated Phase Bus-duct

• A segregated phase bus-duct is a metal enclosed bus-

duct wherein all the three phase bus-bars are enclosed
in a common enclosure and the all the phases are
segregated by means of non-magnetic metal barriers
preferably made of the same materials as that of the
bus enclosure with degree of protection IP65.

• The barriers are generally welded with the bus-duct

enclosure simulating isolated compartments for each
and every phase thus providing adequate shielding
effect to the bus-bars under short circuit conditions due
to the induced circulating currents in the enclosure
whose phase angle is opposite in nature to that of bus-
bars thus reducing the forces applied on the bus
conductors.

• Besides the above, the segregation barriers also aid

minimizing the phase to phase faults to a great
extent. Bus-bars are generally supported on high creep
porcelain insulators are high quality epoxy resin cast
insulators of suitable rated voltage

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 Segregated Phase Bus-duct

• These Segregated phase bus-ducts are commonly used in Medium voltage

applications with rated voltages from 3.3KV to 33KV and with rated continuous
currents up to 5000 Amps with short circuit fault currents of 50 KA for 1 or 3
seconds.

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 Isolated Phase Bus Duct (IPBD)

The Isolated Phase Bus Duct (IPBD) is the main conduit for power distribution in a
power generation facility. With current carried between the generator and a step-up
transformer solely via the IPBD system, it is the undisputable critical core of any
power generation operation.

• The Isolated Phase Bus (IPB) is used to carry very

large currents, typically between a generator and a
step-up transformer, in power generation facilities.

• Each phase conductor is enclosed in its own

separate grounded metal housing, with each
housing separated from each other by air.
By enclosing conductors in separate housings
there is considerable protection from faults
between the generator and the transformer.

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 Isolated Phase Bus Duct (IPBD)

• Conductors are generally hollow aluminum tubes

or aluminum bars, supported within the housing on
porcelain or polymer insulators. There are generally
two types of housings: continuous and non-continuous
with the latter being the older design requiring more
maintenance.

• IPB systems are designed to carry continuous current

ratings of 3,000 amperes to 45,000 amperes, and rated for
voltages from 5 kV on up to about 38kV. With larger current
ratings, a forced cooled system is used to blow air through
the enclosures in order to maintain ANSI set temperature
limits. The cooling air is then typically re-circulated through a heat exchanger before
reentering the bus.

IPB systems are usually custom designed for a particular plant and can be just as unique to maintain.

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A typical arrangement of IPBD illustrating GT/UT

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 Testing of Bus-duct

• Visual and Mechanical :

Part-1 :- Before and During commissioning

– Compare equipment nameplate data with drawings and specifications.
– Inspect physical and mechanical condition of busway system
– Inspect anchorage, alignment, and grounding.
– Verify correct connection in accordance with single-line diagram.

Part -2 :- Maintenance and testing:

Inspect bolted electrical connections for high resistance using one or more of the
following methods:
– Use of a low-resistance ohmmeter in accordance with Section 2 (Electrical
Tests).
– Verify tightness of accessible bolted electrical connections and bus joints by
calibrated torque-wrench method in accordance with manufacturer’s published
data or Table 100.12 below.
– Perform thermographic survey while into operation
(NOTE: Remove all necessary covers prior to thermographic inspection. Use
appropriate caution, safety devices, and personal protective equipment.)

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Electrical Tests

• Perform resistance measurements through bolted connections and bus joints with
a low-resistance ohmmeter.
• 2. Measure insulation resistance of each busway, phase-to-phase and phase-to-
ground for one minute, in accordance with Table 100.1 below.
• 3. Perform a dielectric withstand voltage test on each busway, phase-to-ground
with phases not under test grounded,
(The test duration must be in accordance with the safety standard being
used. The test time for most standards, including products covered under IEC
60950, is 1 minute. A typical rule of thumb is 110 to 120% of 2U + 1000 V for
1–2 seconds)
• Where no dc test value is shown in Table 100.17, ac value shall be used. The test
voltage shall be applied for one minute.
• Perform a contact-resistance test on each connection point of uninsulated
busway. On insulated busway, measure resistance of assembled busway sections
and compare values with adjacent phases.
• Verify operation of busway space heaters.

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 Standards used for Bus-Duct in GKEL

• IEEE C37.23:2003 – STANDARD FOR METAL ENCLOSED BUS

• IEC 137 – BUSHINGS FOR ALTERNATIVE VOLTAGE > 1KV

• IEC 85 – RECOMMENDATION FOR CLASSIFICATION OF INSULATING MATERIAL

• IEC 270 – PARTIAL DISCHARGE METHODS

• IEC 298 – HV METAL ENCLOSED SWGR & CONTROL GEAR FOR < 72.5 KV

• IEC 529 – IP PROTECTION

• IEC 60- HV TESTS

• IEC 168- POST INSULATORS OF CERAMIC MATERIAL FOR VOLTAGE >1 KV

– PART-1 – METHODS OF TESTING
– PART-2 - SPECIFICATION OF DIMENSION
– PART-3 – GUIDE TO INSULATOR PRACTICE

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