DESIGN AND ANALYSIS OF PRE- INSERTION RESISTOR MECHANISM

DESIGN AND ANALYSIS OF PRE- INSERTION RESISTOR MECHANISM Bhavik Bhesaniya 1, Nilesh J Parekh 2, Sanket Khatri 3 1 Student, Mechanical Engineering, Nirma University, Ahmedabad 2 Assistant Professor, Mechanical Engineering, Hasmukh Goswami College of Engineering, Ahmedabad 3 Student, Mechanical Engineering, Nirma University, Ahmedabad ABSTRACT In electrical system, circuit breaker is used to make or break current for different power system equipments like transformers, reactors, transmission lines, capacitor banks etc. in normal as well as abnormal (short circuits, over voltages and many more) conditions. These conditions create switching over voltages. This can be substantially damped by inserting pre-insertion resistor in circuit. The existing electrically parallel pre-insertion resistor is to be converted in to a new simple and easily controllable electrically series pre insertion resistor mechanism with mechanical feasibility. The mechanism is comprised of two electrical contacts and certain means for controlling the motion of the electrical contact. The motion of two electrical contacts should be controlled to insert the resistor for 10 ms as soon as the circuit breaker is closed and bypass the resistor while opening the circuit breaker. Five different concepts are developed from scratch to insert resistor. Dynamic motion analyses of concepts are carried out in MSC ADAMS for various control parameters. Stress analysis of critical components of final concept which fulfills the requirement of insertion is carried out and subsequent shape modification is followed by final dynamic analysis which aligns with the requirement. Keywords: Pre-Insertion Resistor, Circuit Breaker, Dynamic Analysis 1 INTRODUCTION A circuit breaker (CB) is an apparatus in electrical a system that has the capability to, in the shortest possible time, switch from being an ideal conductor to an ideal insulator and vice-versa. The most important task of a drive operated circuit breaker is to interrupt fault currents and thus protect electric and electronic equipment [1]. The interruption and the Fig 1: Schematic of circuit showing the process of inserting resistor (R) in series with main contacts Subsequent reconnection is called opening and closing respectively. Switching operations create transient voltages which can be damped by inserting a particular value of resistor for certain duration in the circuit during connection [1]. Many switch like mechanisms which insert resistor in series during closing are used in industries. Fig 1 shows completely open position initially. In open position, PIR contacts 1 and main circuit breaker contacts 2 are open. For inserting resistor R in circuit, main breaker contacts 2 are closed first, whereas PIR contact 1 remains open. Resistor R is bypassed after closing PIR contacts 1. Freeman, Froelich and Johnson [2] developed a switch like mechanism ( 1 in Fig. 1) using a linearly moving roller. The roller engages a crank arm that is pivotally connected to movable pre-insertion resistor (PIR) contact of switch mechanism. Variable mechanical advantage obtained from angle between roller and crank causes the PIR contacts to close at high speed just after main circuit breaker contacts ( 2 in Fig. 1) to insert resistor during closing. A piston cylinder coupling establishing rectilinear connection between main circuit breaker contacts and PIR contacts was devised by Muller and Talir [3]. Coupling was designed to provide closing and opening delay of PIR contacts to insert resistor and bypass resistor respectively. Michel Perret [4] invented a switch like mechanism with both PIR contacts movable. Initial pressure suction is used to maintain gap between PIR contacts. This initial maintenance of gap provides closing delay of PIR contacts to insert resistor. Opening delay is provided by resistance provided to motion of Volume 3, Issue 12, December 2015 Page 27

PIR contact via separate spring damper system. Considering the inventions, there is a need to develop a mechanism which can be controlled easily for insertion time. The present work focuses on development of an easy to control mechanism for closing and opening where insertion time can be controlled easily. The specifications to insert a resistor in series for 10 milliseconds (ms) are as under 1. Gap of at least 35 mm should be maintained between PIR contacts until main circuit breaker contacts close; 2. 2. PIR contacts should close at 65.9 ms i.e. after 10 ms of main circuit breaker closing as shown in Fig. 2; 3. PIR contacts should open at 469.2 ms i.e. after 300 ms of main circuit breaker opening as shown in Fig. 2 Fig 2: Input motion curve shows instants of closing and opening of main circuit breaker and PIR switch 2. METHODOLOGY The concepts for new switch mechanisms are developed based on three central ideas which are as under. 1. Only one contact of mechanism is movable, the other is fixed. 2. Both the contacts moving, where motion of one contact is controlled by input motion and the other is controlled by spring. 3. Both the contacts moving, where motion of one contact is controlled partially by input motion and partially by spring damper, the other is controlled by spring. 2.1 Generation of concepts based on first idea 2.1.1 Concept 1 The concept simplifies and replaces the spring with that of damper of the switch mechanism invented by Muller & Talir and patented as US5814782 [3]. As shown in Fig. 3, piston cylinder is used to couple main circuit breaker contacts to PIR contacts. The gap between piston cylinder walls d 1 will delay motion of PIR contacts during opening and closing. Spring used in patented mechanism will only aid to control the closing but the same spring will not control the motion of movable contact during opening. The damper used in concept 1 will control the motion of movable contact while opening and closing both. Fig3: Open and closed position of concept 1 Volume 3, Issue 12, December 2015 Page 28

2.1.2 Concept 2 Fig 4: Open and closed position of concept 2 Input from main circuit breaker is provided to a crank which consists of radial slot connected to movable contact (free slider) via connecting rod as shown in Fig. 4. Fixed contact is not shown in Fig. 4. In the closing process, the movable contact does not move until the crank rotates angle θ 1 (as shown in Fig. 4) and strikes connecting rod to transfer motion. The angle θ 1 is control parameter which delays the motion of PIR contact thus providing delayed closing. The same method is followed in opening which provides opening delay of PIR contacts. 2.2 Generation of concepts based on second idea Fig 5 depicts general working of PIR switch based on second idea, where input motion is provided to front contact and the same motion is transmitted by certain means to back contact also. In the closing process, initially both the contacts move in closing direction without much affecting the gap between them. After some time the back contact is released from the input motion and as shown in Fig.5, the compressed spring will make back contact to return to its original position; mean while the front contact (slider) is moving in closing direction thus closes the switch. In opening, the motion is not transmitted to the back contact so it will remain at its position and only front contact will move and opens the switch. Overlap between contacts will give opening delay. Fig 5: Schematic showing operation of second idea Concept 3 and concept 4 are developed based on second idea 2.2.1 Concept 3 Fig 6 shows the working of concept 3. A crank, push fitted on shaft is connected to front contact and a disc, free on shaft is connected to back contact (contacts are not shown in Fig. 6). Main circuit breaker input drives the crank and also the disc, due to the pin inserted in the disc is extended to the crank as shown in Fig 6. Initially as per idea both the contacts move in same direction maintaining gap even if the motion is transferred. As soon as the pin is lifted off by cam ( as shown in Fig 6), back contact is no longer driven by input and compressed spring makes back contact and thus disc to return to original position. As crank and thus front contact are still in motion the closing occurs. To control insertion time the control parameters are crank and disc radius, the position of cam and stiffness of spring attached to back contact. Volume 3, Issue 12, December 2015 Page 29

2.2.2 Concept 4 Fig 6: Working of concept 3 Fig.7 Working of Concept 4 Fig.8 General schematic of concept 4 Fig 7 shows working of concept 4. The input motion is provided to front contact and same motion is transferred to back contact via collapsible link as shown in Fig 8. Back contact is equipped with a cam profile and is held by back spring and damper. As per the idea, front contact travels in closing direction and the motion is also transferred to collapsible link. When collapsible link strikes to cam of back contact, back contact also starts travelling in closing direction. After sufficient angular travel of collapsible link, back contact is no longer controlled by input motion and is only controllable by back spring. Initial travel of both contacts in closing direction aid to maintain gap between two contacts and closing of PIR contacts is thus delayed. In opening, the motion transferred to the front contact is also transferred to the collapsible link, but design of collapsible link is such that it collapses on cam and regains its shape due to spring. Thus, the back contact does not move in opening. Overlap between two contacts in closed position is kept higher than overlap of main circuit breaker contacts to get opening delay. Control parameters of switch for closing are crank radius of collapsible link, back spring stiffness which holds back contact, horizontal distance between onset of cam and hinge of collapsible link and for opening delay overlap of two contacts. Volume 3, Issue 12, December 2015 Page 30

2.3 General Concepts based on third idea Fig.9 Schematic showing operation of third idea As shown in Fig. 9, 'F' is front contact and 'Back' is back contact. Initially front contact is held by a spring damper system. Back contact is also held by a spring. Input motion is provided to a slider and the same motion is transmitted by some means to back contact. In closing, front contact follows the same input motion only after the slider strikes the front contact stretching the spring attached to it. Due to motion transmitted to the back contact, it moves in closing direction initially and increases or maintains the gap between two contacts. Sometime later, back contact is released from the input motion and due to spring force it travels in opening direction as shown in closing mode. The front contact still moves in closing direction and closing takes place. In the opening process, due to the motion provided to slider it travels in the direction as shown in opening mode. The back contact is held by spring so it will not move from the position. The front contact is acted upon by a spring force and damper as soon as the slider leaves contact with 'front contact' and the motion of the front contact is then controlled by stiffness and damping co-efficient of spring damper system. 2.3.1 Concept 5 Volume 3, Issue 12, December 2015 Page 31

Fig.10 Closing and Opening of Concept 5 Fig 10 shows concept 5 which is based on third idea, where a separate slider is used to take the motion from main circuit breaker during closing and same is transferred to collapsible link. As shown Fig 10(2) front contact does not move until the separate slider strikes front contact, meanwhile the motion transferred to collapsible link strikes back contact and it travels in closing direction too. The rest is followed as per concept 4. Initial travel of both the contacts in closing direction maintains the gap between them and provides delayed closing of PIR contacts. In opening, due to the motion provided to separate slider it moves in opening direction but front contact cannot follow the same velocity as it is controlled by spring and piston cylinder dashpot (The idea here of using the piston cylinder for damping in opening is taken from US patent 6239399 [4], however different damper instead of piston cylinder can be used.). Back contact does not move in opening same as in concept 4. Thus, delayed opening is achieved by setting proper damping. Control parameters of switch for closing are crank radius of collapsible link, back spring stiffness which holds back contact, horizontal distance between onset of cam and hinge of collapsible link(striking distance) and initial gap between separate slider and front contact. For opening delay stiffness of opening spring attached to front contact and damping coefficient are control parameters. 3 RESULT AND DISCUSSION From primary analysis of concepts, it is found that concept 2 requires more space and is complex to control whereas concept 3 involves more parts subjected to wear which creates suspended metal particles in electrical system. Thus concept 2 and concept 3 are dropped here. Dynamic analysis of concept 1, 4 and 5 are as follow. 3.1 Dynamic analysis of concept 1in MSC ADAMS For d 1 = 48 mm, initial gap between contacts = 70 mm, overlap =12 mm, damping co-efficient = 0.5 Ns/mm, Adams contact stiffness = 100000 N/mm, exponent = 2.2, maximum contact damping = 10 Ns/mm and for Aluminum material of all the moving parts, Insertion time of 10 ms and opening delay of 3.65 ms is obtained as shown in Fig 11. For any combination of parameters, the gap between PIR contacts at the instant of closing of main circuit breaker cannot be obtained more than 14 mm. This gap should be at least 35 mm. Thus, concept 1 fails to fulfill the gap requirement and hence it is dropped. Fig.11 Gap between PRI contacts and main circuit s breaker contacts v/s time superimposed Volume 3, Issue 12, December 2015 Page 32

3.2 Dynamic Analysis of Contact 4 Fig 12: Gap between PIR contacts and main circuit breaker contacts v/s time superimposed As shown in Fig 12 insertion time of 7.7 ms and opening delay of 3 ms is obtained for crank radius of 90.5 mm, back spring stiffness and damping of 2 N/mm and 0.050 Ns/mm respectively, horizontal distance between onset of cam and hinge of collapsible link (striking distance) of 2 mm and overlap of 37 mm. All the moving parts are assumed of Aluminum material. Adams contact parameters are taken as of concept 1 for analysis. Insertion time of 10 ms can be obtained by setting back spring stiffness but maximum opening delay obtainable here is 15.6 ms. It is difficult to achieve opening delay of 300 ms, hence concept 4 is dropped here. 3.3 Dynamic Analysis of Contact 5 Parameters like back spring stiffness, crank radius and striking distance (horizontal distance between onset of cam and hinge of collapsible link) are critical parameters to control insertion time. These parameters are varied to understand their effect on travel of back contact. Fig 13: Effect of stiffness on travel of back contact As seen from Fig 13, as the spring stiffness increases travel of back contact decreases as well as it returns to initial position more quickly. As seen from Fig 14, as the crank radius increases, the linear velocity of striking and hence the velocity of back contact decreases. Higher crank radius will generate less impact forces on back contact. As seen from Fig 15, as the striking distance increases striking instance is delayed. However at zero striking distance, the force will be only horizontal thus no vertical reactions will have to be borne by system. To reduce contact forces, bottom collapsible link is assumed to be covered with PU foam (Young s Modulus of 0.5GPa and Poison s ratio of 0.5). ADAMS contact stiffness of 5000 N/mm and maximum contact damping of 100N ms/mm is obtained from Lankarani and Nikravesh [6] model of contact stiffness. Volume 3, Issue 12, December 2015 Page 33

IPASJ International Journal of Mechanical Engineering (IIJME) Volume 3, Issue 12, December 2015 ISSN 2321-6441 From above results, for least impact, maximum possible crank radius considering space is 75 mm. To avoid vertical reaction force in system, appropriate striking distance is 0mm. For analysis, crank radius of 75 mm and striking Fig.14 Effect of crank radius on travel of back contact distance of 0 mm is taken. Setting these two parameters, whole system depends on back contact spring stiffness. Fig.15 Effect of striking distance on travel of back contact To get 10 ms insertion time of resistor, spring stiffness of 6.9 N/mm is obtained from dynamic analysis. Thus stiffness can be varied to obtain variety of insertion time. Opening delay can be easily controlled by opening spring stiffness and damping. As the requirement of insertion time and opening delay can be fulfilled in concept 5, it is selected for further design and analysis. 3.4 Design of Mechanism based on Concept 5 To design the mechanism various forces acting on joints as well as inertia forces are obtained from analysis of concept 5. Fig 16 shows loads on connecting link (as shown in Fig 10) from which stresses at various sections are obtained. The link needs to be non-conducting for electrical feasibility. Volume 3, Issue 12, December 2015 Page 34

Fig. 16: Loads on connecting link Max. Compressive stress at mid section with area of 5 12 mm 2 is 29.57 MPa and tensile stress at joint cross section with area of 5 6 mm 2 is 52.48 MPa. For stresses produced, glass fiber reinforced plastic material is proposed. Fig. 17 Loads on top and bottom collapsible link Fig 17 shows loads on top and bottom collapsible links. Boundary Condition: X-Y displacements made zero at hinge point. Inertia loads and moments are applied at CG of the link. However, inertia force in top collapsible link is neglected due to minimal value. Shapes are modified after pre-stress analysis and final shape is analyzed as shown in Fig 18. For prevailing stresses, alloy steel equivalent to 40NiCr1Mo15 having yield strength of 580Mpa [7] is proposed. Fig. 18 stresses in top and bottom collapsible link Volume 3, Issue 12, December 2015 Page 35

3.4.1 Final analysis of concept 5 Using the dimensions and proposed material, final dynamic analysis is carried out for crank radius of 75 mm, striking distance of 0mm, back spring stiffness of 7.05 N/mm, opening spring stiffness of 1N/mm and gas damping coefficient of 0.985 Ns/mm. Fig 19 shows that concept 5 gives insertion time of 10 ms, opening delay of 300 ms and gap of 51 mm is maintained between PIR contacts until the main circuit breaker contacts are closed. 4. Conclusion Fig.19 Gap between PIR contacts and main circuit breaker contacts v/s time superimposed Five concepts are developed for inserting resistor in series. Two concepts are dropped due to mechanical and electrical feasibility. Dynamic analyses of three concepts are carried out in MSC ADAMS. Two more concepts are dropped as they cannot fulfill specific requirement. Concept 5 is selected for final design of mechanism after analysis. After modifying shapes according to stresses and using the material proposed final dynamic analysis is carried out. Final dynamic analysis gives insertion time of 10 ms, opening delay of 300 ms and maintains gap of 51 mm between PIR contacts until the main contacts are closed. Thus, mechanism based on concept 5 can be used to insert resistor in series. Moreover in concept 5, variety of insertion time can be obtained by just adjusting back spring stiffness. Thus, Concept 5 is very easy to control mechanism. References [1] http://www.abb.com/global/scot/scot245.nsf/veritydisplay/_le/buyers guide hv live tank circuit breaker sed5en.pdf accessed on 29th, April, 2013. [2] W. B. Freeman, K. Froelich, D. S. Johnson, Modular Closing Resistor, US5245145, 1993. [3] R. Muller, J. Talir, Rower circuit breaker having closing resistor, US5814782, 1998. [4] M. Perret, Interrupter with resistor insertion system having long insertion time, US6239399, 2001. [5] W. Daniel, Contact Modeling, Benelux ADAMS User Meeting, 2012. [6] C. Pereira, A. Ramalho, J. Ambrsio, a Critical Overview of Internal and External Cylinder Contact Force Models. [7] V. Bhandari, Design of Machine Elements, 2004. Volume 3, Issue 12, December 2015 Page 36