Bus duct is an equipment, which serves for transmitting electric power efficiently and effectively by inter-connection between two or more equipments. Bus duct comprises conductors, which are supported on insulators in a non-magnetic, dust tight enclosure by which safety is ensured and likelihood of faults is reduced. Bus ducts are designed based on current, voltage requirement, short circuit capacity, availability of space and aesthetic aspects. The bus duct system has emerged as replacement to cables with the following advantages
- Higher fault current withstanding capabilities
- Enhanced current carrying capacity
- Less voltage drop due to lower impedance
- Simpler way of jointing
- Easy operation and maintenance practices
Types and Ratings
The construction of the bus ducts shall be robust enough to take the load of bus bars, withstand rated fault levels and severe adverse field conditions [4]. Provision shall be made for expansion and contraction of enclosure due to temperature. Under service conditions, the bus duct should not be operated at a temperature that will cause an appreciable change in its electrical and mechanical characteristics. The heat generation normally creates power losses or eddy current losses at the conductor. Flow of higher current will obviously increases the heat generation and stresses over the bus ducts [2]. To facilitate and improve the heat dissipating capacity and to reduce temperature rise, matt black paint is always applied on the surface of bus ducts. The brief classification of various types of air insulated bus ducts is shown in figure.1.
Segregated phase bus ducts (SPB) and Non-segregated phase bus ducts (NSPB) are available only up to 5000A beyond which Isolated phase bus ducts (IPB) are to be used.IPBs are supposed to face very high forces expected in case of short circuit faults. However, NSPBs are also used for excitation connection to large generators as fault current is comparatively low in such cases [4]. The preferred current ratings with reference to type of bus ducts and ratings is indicated in the figure.2 [2].
Factors influencing Temperature
Mostly, electrical apparatus are rated based on maximum operating temperature uttered by the nature of insulating materials used. Excess heat generated can cause accelerated ageing of joints which leads for deterioration. As the temperature increases, it becomes difficult to maintain good condition of bolted Joints [8]. Hence, temperature rise over 400C ambient has become the standard practice to declare temperature rating for bus ducts carrying the full- load current. However, this can be further increased by applying silver plating to a thickness of 6 to 8 microns on the mating surface of joints. The heat caused by electrical losses in a conductor is dissipated primarily by convection and radiation. The amount of heat carried off by conduction through supports and connections is small and ignored in most of cases [8]. For steady state conditions, the rate of heat dissipation is equal to the rate of heat supply.
I²R=Wc×Ac + Wr×Ar Watts
WhereI I = Current in conductor (amp)
R = Resistance of conductor (ohms), for ac or dc at temperature
Wc = Watts loss per sq in due to convection
Wr = Watts loss per sq in due to radiation
Ac = Surface area of conductor (sq in) for convection
Ar = Surface area of conductor (sq in) for radiation
When a temperature rise is considered, the most efficient way of increasing the cross sectional area is nothing but having more surface area. If increase in width of single bars becomes impractical, the number of bars shall be increased rather than the thickness of the single bar [2]. For alternating current (A.C.) bus ducts, skin effect and proximity effect must be taken into account while designing itself [10].
The bus duct which carries A.C. induces an electric field causing skin and proximity effects. These effects play a complex role in determining the current distribution through the cross section of a conductor [3]. The induced e.m.f. is produced in the conductor by its own electric field cutting the conductor. It is denser at the center and becomes less at the surface. For more than one conductor per phase all the conductors together may be considered as forming a large conductor for the purpose of analyzing the skin effect. The phenomenon of uneven distribution of current within the same conductor due to the inductive effect is known as the ‘skin effect’ and leads to increase effective resistance and power loss. The ratio between Ra.c to Rd.c, is the measure of this effect and is known as the ‘skin effect ratio’. The skin and proximity effects are affecting the voltage drop of the conductor and indirectly reduces its current carrying capacity [9].While assessing the temperature rise of bus duct the following factors shall be considered [7].
- Temperature coefficient of resistance
- Skin effect ratio
- Proximity effect ratio
- Effect of the presence of enclosure
- Service conditions
Verification of Temperature-Rise
The temperature-rise test on the individual circuits shall be made at their rated frequency. To pass the desired current any convenient value of the test voltage may be used. The test currents shall be adjusted to be equal in all phases. The test shall be performed for a time sufficient to reach a steady state condition, which generally occurs within 6 to 9 hours, depending on rating and type of bus duct [5].
For low voltage (LV) bus ducts, with system voltage of less than 1000V, bus trunkingunit (BTU) shall consist of at least two joints for a minimum length of 6m. This representative arrangement as per relevant standard shall be mounted at its reference mounting conditions and tested for rated current. The temperature of conductors shall be measured in the middle portion of the BTU length, and at each and every joint that exist in the bus ways under each phase. The temperature of the corresponding parts of the enclosure shall be measured on all free sides. The test shall be continued for a time sufficient to reach a stable condition. In practice, this condition is reached, when the variation at all measured points including the ambient temperature does not exceed 1 K/h [5-6].Thermocouples or thermometers shall be used for temperature measurements.
Horizontal orientation: The bus trunking unit (BTU) designed for LV system shall be supported horizontally at approximately one metre from the floor level. The ambient temperature shall be measured in the immediate vicinity of the center of the BTU, at the same level and at a distance of approximately 1m from both of the longitudinal sides of the enclosure as shown in the figure.3.
Vertical orientation: The Bus Trunking Unit (BTU) shall be arranged vertically, that is with at least four meters in the vertical position and fixed to a rigid structure in accordance with the original manufacturer’s instructions. The ambient temperature shall be measured at 1.5m down from top end of test arrangement at a distance of approximately 1m from each of the longitudinal sides of the enclosure. At the end of the test, the temperature rise shall not exceed the values specified in relevant standard [5].
For high voltage (HV) bus ducts, with system voltage above 1kV and up to and including 36kV, the test assembly shall be three phase unit or single phase unit as applicable with minimum length of 5m. It shall have at least one joint per phase, which is of bolted, clamped or welded [7]. Each end of the bus enclosure will be properly sealed to reduce any heat leakage. The bus arrangement shall be placed about 60cm above floor level. The ambient temperature should be between 100C to 400C. As per IEEE guidelines, the ambient temperature shall be measured by taking the average of minimum three thermometer readings kept on the sides of the metal enclosed (ME) bus centerline at least 30cm away from the enclosure. One of the temperature sensing devices shall be placed at the center of the ME bus and other two shall be placed 60cm inward from the ends of the ME bus enclosure [1]. The test current shall be applied continuously until the temperature of all the bus bar parts and supports are substantially constant. The three successive readings at not less than 30min intervals shall show a maximum variation of ± 10C for HV bus systems. Similarly, for LV bus systems as per IEEE standard, the maximum variation shall not be more than 1K/h to predict the steady state condition. To shorten the test, the test current may be increased for the initial hours. The test may be conducted at reduced voltage, as the eminence is on heating due to the flow of test current. For a successful test, as per the relevant IS (or) IEC (or) IEEE standards the temperature rise limits over reference ambient temperature of 400C shall not exceed the values as given in the below Table-1.
..To be continued
References
1. IEEE Std. C37.23TM -2015, IEEE Standard for Metal-Enclosed Bus
2. V.K.Kanjlia, P.P.Wahi ‘’Manual on Bus duct’’, Pub.319, CBIP
3. Chang-Chou Hwang, Y.H.Jiang, J.J. Chang. “Analysis of electromagnetic and thermal fields for a bus duct system”, Electric Power System Research; Vol.45, 1998.
4. V.Balachandran, “An Introduction to Bus bar Systems” self-published, 1st Edition 2020, ISBN: 978-93-5419-563-1
5. IEC 61439-6:2012, Bus bar Trunking systems, (busways)
6. IEC 61439-1:2020, Low voltage switchgear and Control gear Assemblies
7. IS: 8084-1976 (Reaffirmed 2017), Specification for Interconnecting Bus-bars for AC Voltage above 1 kV up to and including 36 kV
8. MasriMuhammood, Mohamad Kamarol, DahamanIshak and SyafrudinMasri “Temperature Rise Prediction in 3-Phase Bus bar System at 20°C Ambient Temperature”2012 IEEE International Conference on Power and Energy (PECon),
9. Wu Anbo, Chen Degui, Wang Jianhua, Cai Bin, GengYingsan. “Evaluations of Thermal Performance for Air Insulated Bus bar Trunking System by Coupled Magneto-Fluid-Thermal Fields”, National Science Foundation, Vol. 1, 2002.
10. K.C.Agarwal, “Industrial Power Engineering and Applications Handbook”Newnes Power Engineering Series.
G. Girija, M.E. (Power & Energy Systems) from UVCE, Bangalore University, Bangalore. Since, 1998 she is associated with CPRI working as Joint Director, currently at Short Circuit Lab, CPRI, Bangalore. She is having wide experience in of short circuit testing, Performance evaluation of Low Voltage Switchgear and control gear equipment, Distribution transformer and Current Transformers. She is a Member of BIS committee Environmental testing procedures Sectional Committee -LITD 01.
S. Arjuna Rao, M.Tech (Power Systems) NIT Tiruchy, M.B.A from Bangalore University and
PGD from Annamalai University. He joined CPRI in 2007 and currently holding the post of
Engineering Officer Gr.4. His areas of interests include Power System Analysis, LV switchgear and Distribution Transformers. He is a Member of the Institute of Engineers (MIE) and IEEE Professional Member in Power & Energy Society. He has eighteen publications in in the area of Power systems, Distribution Transformers, CT’s & Switchgear. Got experienced in testing of Transformers, LT/HT Panels, Isolators, Instrument transformers, Bus ducts, Circuit Breakers. GIS Switchgear.
Rakesh K G, Degree in Electrical and Electronics Engineering in 2013 from BMS Evening College of Engineering, Bangalore. He has the experience of 5 years (2005-2010) in Electrical industry in Production & Quality Assurance of Oil & Winding Temperature indicator used in transformers. Joined CPRI in 2010 & currently holding the post of Engineering officer Gr. 1. Got involved in testing of Transformers, Panels, Bus ducts, Breakers, GIS Switchgear and Control Gears etc.
N. Mahesawara Rao joined CPRI in 2009 currently holding the post of Engineering Officer Gr.3. He has experienced in the fields of short circuit testing and Evaluation of various
products. He is involved in R& D activities of short circuit laborotroy.His areas of interest
are short circuit testing, design and development of power electronic modules for short circuit applications.
B. R. Vasudevamuthy is currently holding the post of Joint Director in CPRI with more than 25 years of experience in short circuit testing lab. He obtained his BE in Electrical Engineering from Bangalore University. His Areas of Interests are testing and analysis of & Low Voltage switch and control gear assemblies
Swaraj Kumar Das was graduated in ECE from N.I.T (R.E.C) Durgapur, WB. He worked as
Engineer in R & D Centre of M/s. Hindustan Cables Ltd., Hyderabad during 1989 – 1991 then joined CPRI, Short Circuit Laboratory, and Bangalore at the end of 1991. Currently holding the post of Additional Director & heading Short Circuit Laboratory in CPRI with more than 28 years of wide experience in the field of short circuit testing and performance evaluation of LV & HV switchgear and control gear equipment. He is a member of Bureau of Indian Standards ET 34 & ET 07 committee for CT & PT and Low voltage switchgear & control gear assemblies respectively. He has publications in the area of Distribution Transformers, CT’s & LV switch gear and control gear assemblies.
