Jointing Practices of Busbars
There are different ways to make joints of current carrying conductors in bus bar systems. The most commonly used methods in the order of preference are welding, bolting and clamping . In an enclosed bus system, it is always recommended that wherever it is possible to make a welded joint it should be preferred, because bolted or clamped joints apart from being expensive additionally needs access for the periodic maintenance. To make the joints effective, sufficient contact pressure should be maintained with low resistance. The contact pressure should be evenly distributed by the use of pressure plates or washers of adequate area and thickness . A bus bar joint is efficient if the resistance across it is less than or equal to the resistance of an equivalent length without the joint. In an isolated phase bus, the most common method of jointing is by welding. In the non-segregated and segregated phase bus ducts, the method of jointing is by bolting. Whereas, the sandwich bus ducts are preferably joined by clamping, keeping bolting at termination pads. Different types of joints that are applicable to bus ducts are shown in the figure 4. The joints shall be bolted such that the contact pressure considerably not reduced at all temperatures up to rated full load. Always, suggested to remove the irregularities, which might appear on contact faces such as burrs, welding projections. Spring washersare choseneffectively at every bolted joint .
An expansion (flexible) joint is used for indoor or outdoor type bus ducts, fitted with long runs to enable thermal expansions, when carrying rated continuous current. As per design, flexible copper braids of required ampacity will connect the bus conductors as expansion joint at regular intervals. Similarly, welding is the most efficient and preferred form of jointing as it is permanent, maintenance free, does not deteriorate with time and is being extremely economical .
Welding of copper or aluminium bus ways needs an expertise, infrastructure and skilled operators mostly at sites. Wherever, it is not possible to carry out welding, bolting is the preferred option for bus bar joints. Bolted joints also give adequate performances in onerous service conditions and serves for long period. Bolting is expected with low contact resistance, reliability during its period by holding the effects such as corrosion, fretting, thermal cycling, differential expansion, vibration and fatigue.
A study has been carried out, to find the effectiveness of bus way joints and also their temperature rise at the appropriate locations identified while conducting verification temperature rise tests as per IS and IEC standards. Sufficient quantity of T-type thermocouples, data loggers cum monitoring system is used to record timely readings of temperatures while performing type test. The test samples selected under study are of different configurations such as:
- Bus duct consists with one Rigid joint on one Expansion (flexible) Joint.
- Bus duct consists with two Rigid Joints and one Expansion (flexible) Joint
- Bus duct consists with one Rigid joint and Combination of Rigid/Expansion (flexible) Joint
- Bus duct consists with only two Rigid joints.
The configurations of LT & HT bus ducts up to the current rating of 5000A are analyzed to find the effectiveness of joints based on temperature rise irrespective of test current passed. It is quite evident that, the temperature rise values will be more, if the value of the test current is high but, induces the same effect of heat between the rigid and expansion (flexible) joints that exist in the test sample. It was observed that, the expansion (flexible) joints have seen considerably higher temperatures than the rigid joints, in all the test configurations that were investigated. The number of thermocouples for measurement will be twice at the expansion (flexible) joint when compared with the rigid joint under each configuration. Moreover, from the test results it is proven that the middle portion of each bus duct has seen more temperature rise values than at the extreme ends. Out of the three phases the centrally located phase bus bar joints experiences more temperature than the outer phase bus bars due to the proximity effect of the adjacent phase bus bars. The hourly temperature readings are analyzed graphically as shown in the figure 8.
The standards IS-8084, IEC-61439& IEEE C37.23 have only specified the number of joints that should exist over the length of bus ducts under each category but, does not revealed the type and nature of joints such as expansion (or) rigid that should exist in each case. The IS-8084, standard for HV bus ducts states that the length of the Bus duct shall be minimum of 5m length with at least one Joint, whereas IEC 61439 the standard for LV bus duct states the length of bus duct under the test shall be of minimum 6m with at least two joints. The type of joint that should present is not mentioned in the above standards. Hence, the manufacturer/designer having the tractability to design the bus ductsas per end user’s specifications or particular priorities in to consideration, which are cost effective. The standardization might have been made with respect to type of joints also that should exist, like one rigid and one flexible joint for LV bus duct of minimum 6m, similarly for the case of HV bus ducts as applicable. The flexible joint will have the scope for thermal expansion in severe load conditions of the field, hence consideration might have been given, to keep at least one flexible joint.
The effect of rated voltages on the magnetic field is having no affect considering the Maxwell’s equations underElectro Magnetic Field (EMF) theory concepts . The test results of two extreme cases are shown graphically in the figure 5 & figure 6 respectively. Case-I corresponds to the results of LV Non-Segregated phase bus duct with current rating of 3200A with one rigid joint &one expansion (flexible) joint. Likewise, Case-II corresponds to the results of LV Non-Segregated phase bus duct with current rating of 4000A with two rigid joints & one expansion (flexible) joint. The interleaving arrangement is considered significantly in each of the above case.
Infrared thermography is a non-intrusive technique that can detect subtle temperature differences between components. The Thermal image was captured while equipment was under energized state during temperature rise test on bus ducts of various configurations . Thermal imaging enables to assess electrical equipment healthiness and also acts as diagnostic tool to analyze the failures of the test samples. The actual quantitative temperatures of objects can be determined with camera settings that can adjust from the distance to the targeted portion of test sample. This technique has been used for image capture and to analyze the distribution of the heat zones with reference to the hot-spot temperature [12-13]. It was observed that the distribution of the heat is more at the all the joints in the bus bar enclosure. The expansion (flexible) joints have exhibited more heat than the rigid joints. The respective images are shown in the figure.7.
For the case II under study, the hourly readings are measured with ‘T’ type thermocouples fixed at respective locations identified in the LV bus duct of rating 415V, 4000A during temperature rise test. The amount of temperature occurs are resembling the uniform current flow in busways until obtaining the steady state. The data is used to predict temperature variation from the beginning to final steady state condition by passage test current . The current carrying capacity is limited by temperature rise limits i.e., the difference between point of measurement and ambient temperature as specified in appropriate National or International standards. The results shown in the table 2 are relevant to the center located phase bus bar as it exhibited more temperature, also due to skin and proximity effects.
In this article, a brief study has been conducted on bus ducts consists of rigid and expansion (flexible) to monitor the temperature rise at respective points. The hourly test data and absolute and relative temperatures are analyzed by advanced monitoring system and also by thermography. It was witnessed that, the amount of heat dissipation is more at the expansion (flexible) joints by about 10 to 15% than rigid joints in each case. Bus ducts with expansion (flexible) joints can handle extreme loading conditions of field as they are flexible enough to handle thermal expansions. Hence, it is proposed to keep at least one expansion joint considering the benefits with respect to current rating and length of bus duct. Further, the skin and proximity effects can be analyzed respect to consideration interleaving arrangements in bus ducts.
The authors are thankful to the management of Central Power Research Institute, Bengaluru, India for supporting this work.
 S.Thirumurugaveerakumar, M. Sakthivel, S Rajendran “Experimental and Analytical Study of Effect of Forced Convectional Cooling of Bus Duct System in the Prediction of Temperature Rise”International Journal of Applied Engineering Research, Vol.10, 2015, Research India Publications
 F. Delgado, C.J. Renedo, A. Ortiz, I. Fernández, A. Santisteban, “3D thermal model and experimental validation of a low voltage three-phase busduct”,Applied Thermal Engineering (2016)
 S. L. Ho, Y. Li, X. Lin, H. C. Wong, and K. W. E. Cheng, “A 3-D Study of Eddy Current Field and Temperature Rises in a Compact Bus Duct System” IEEE Transactionson Magnetics, VOL. 42, NO. 4, April 2006
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.
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.
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.
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.
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.