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1 DESIGN & ANALYSIS OF TOUCH PROOF ENCAPSULATED MEDIUM VOLTAGE VACUUM CONTACTOR Abstract JAYSHRI GADHAVE*1, PROF. M. M. HAPSE*2 *1(ME Student S.N.D.COE & RC Yeola) *2(Asst. Prof at S.N.D. COE & RC, Yeola) 1.0 Introduction In an electric power system, switchgear is the combination of electrical disconnector switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply. The technology has been improved over time & there has been substantial growth in design & requirements of circuit breakers for various applications. There are merits & demerits of different types of breakers over each other. Vacuum Circuit breakers are generally operated with the help of mechanical mechanism. Circuit breakers operated with mechanical mechanisms are having limited life because of wear & tear of mechanical linkages. Replacement & maintenance lead to larger down time and become very costlier issue. To overcome these applications based demerits of circuit breaker, contactors came into the pictures. Vacuum contactors are widely used in medium voltage switchgear. In this seminar, primary aim is to understand the different requirements of vacuum contactors and different ways to achieve it. Also, we are going to cover design & simulation analysis of key components & features of medium voltage vacuum contactors like electromagnetic coils, vacuum interrupter, HT fuses, touch proof housing. Switchgear is a generic term which includes all the switching devices associated with mainly power system protection. It also includes all devices associated with control, metering and regulating of electrical power system. Assembly of such devices in a logical manner forms switchgear. We all are familiar with low voltage switches and fuses used in our home appliances. The switch is used to manually open and close the electrical circuit in our home and electrical fuse is used to protect our household electrical circuit from over current and short circuit fault conditions. In same way every electrical circuit including high voltage electrical power system needs switching and protecting devices. But in high voltage and extra high voltage system, this switching and protecting scheme becomes complicated due to high fault current interruption in a safe and secure way. In addition to that from commercial point of view every electrical power system needs measuring, control and regulating arrangement. Collectively the whole system is called switchgear and protection of power system. Published in A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit. Contactors come in many forms with varying capacities and features. Unlike a circuit breaker, a contactor is not 1

2 intended to interrupt a large short circuit current. Contactors range from those having a breaking current of several amperes to thousands of amperes and 24 V to 24 kilovolts. Contactors are used to control electric motors, lighting, heating, capacitor banks, thermal evaporators, and other electrical loads. A medium voltage contactor has three components. These include vacuum interrupters, Power fuses, and electromagnetic coils & touch proof housing. The electromagnet provides the driving force to close and open the contacts inside the vacuum interrupters. The enclosure is a frame housing the high voltage components and the electromagnet will be placed on low voltage side of assembly. Enclosures are made of insulating materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate the contacts and to provide some measure of protection against personnel touching the high voltage parts. Medium voltage vacuum contactors housing will also help to protect high voltage parts against dust, oil, explosion hazards and weather. Touch proof concept is giving important advantage of eliminating possibilities of short circuit and thus provide high degree of safety to operators & equipments. Sometimes an economizer circuit or mechanical latching arrangement is also installed to reduce the power required to keep a contactor closed; an auxiliary contact reduces coil current after the contactor closes. Greater amount of power is required initially to close a contactor than is required to keep it closed. Such a circuit can save a substantial amount of power and allow the energized coil to stay cooler. Economizer circuits are nearly always applied on direct-current contactor coils and on large alternating current contactor coils. A basic contactor will have a coil input (which may be driven by either an AC or DC supply depending on the contactor design). The coil may be energized at the same voltage as a motor the contactor is controlling, or may be separately controlled with a lower coil voltage better suited to control by programmable controllers and lower-voltage pilot devices. Certain contactors have series coils connected in the motor circuit; these are used, for example, for automatic acceleration control, where the next stage of resistance is not cut out until the motor current has dropped. 2.0 Vacuum Contactor Vacuum contactors utilize vacuum bottle encapsulated contacts to extinguish the arc. This arc extinguishing allows the contacts to be much smaller and use less space than air break contacts at higher currents. As the contacts are encapsulated, vacuum contactors can be used fairly in the highly polluted applications, such as mining. Vacuum contactors are only applicable for utilization in AC systems. The AC arc generated upon opening of the contacts will self-extinguish at the zero-crossing of the current waveform (Current Zero), with the vacuum preventing a re-strike of the arc across the open contacts. Vacuum contactors are therefore very efficient at disrupting the energy of an electric arc and are used when relatively fast switching is required, as the maximum break time is determined by the periodicity of the AC waveform. Published in 2.1 IEC utilization categories The current rating of the contactor depends on utilization category. IEC (International Electrotechnical Commission) categories in standard are described as: 2

3 AC-1 : Non-inductive or slightly inductive loads, resistance furnaces AC-2 : Starting of slip-ring motors: starting, switching-off AC-3 : Starting of squirrel-cage motors and switching-off only after the motor is up to speed. AC-4 : Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop. 2.2 Operating principle In low voltage contactors, unlike general-purpose relays, contactors are designed to be directly connected to high-current load devices. Relays tend to be of lower capacity and are usually designed for both normally closed and normally open applications. Devices switching more than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors. Apart from optional auxiliary low current contacts, contactors are almost exclusively fitted with normally open contacts. Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents. When current passes through the electromagnet, a magnetic field is produced, which attracts the moving core of the contactor. The electromagnetic coil draws more current initially, until its inductance increases when the metal core enters the coil. The moving contact is propelled by the moving core; the force developed by the electromagnet holds the moving and fixed contacts together. When the contactor coil is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the contacts. For contactors energized with alternating current, a small part of the core is surrounded with a shading coil, which slightly delays the magnetic flux in the core. The effect is to average out the alternating pull of the magnetic field and so prevent the core from buzzing at twice line frequency. Because arcing and consequent damage occurs just as the contacts are opening or closing, contactors are designed to open and close very rapidly; there is often an internal tipping point mechanism to ensure rapid action. Rapid closing can, however, lead to increase contact bounce which causes additional unwanted open-close cycles. One solution is to have bifurcated contacts to minimize contact bounce; two contacts designed to close simultaneously, but bounce at different times so the circuit will not be briefly disconnected and cause an arc. A slight variant has multiple contacts designed to engage in rapid succession. The first to make contact and last to break will experience the greatest contact wear and will form a highresistance connection that would cause excessive heating inside the contactor. Another technique for improving the life of contactors is contact wipe; the contacts move past each other after initial contact on order to wipe off any contamination. In high voltage contactors, working concept is based on electromagnetic principle similar to low voltage contactors. There is one fixed electromagnet and it will be magnetized once it will supply with control voltage. After magnetizing the electromagnet, it will actuate the closing mechanism of medium voltage contactors and it will make the contact in vacuum interrupter & lead to switch ON the contactors. As a result of this coil fixed part attract the movable part till the control voltage is in ON condition. Published in 3

4 There are two basic types of medium voltage contactors, a) Latch Type b) Non Latch Type. In case of the latch type contactor, electromagnetic coil is used to close the contactor after closing contactor it will get cut off. To hold the contactor in close position, there is one additional external mechanical latch assembly provided. The function of latch assembly is to hold the contactor till tripping command is given. Additional tripping arrangement is provided to trip the mechanical latch arrangement in contactor. These types of contactors are economical, as they will not consume power for long time. Once the contactor gets closed, control power supply to closing coil will get disconnected. These types of contactor are used in the long duty applications. In case of the Non-latch type contactors, there will be coil in coil concept of electromagnetic coil. There will be two separate coils mounted on each other. First coil will be used for Picking Up the contactor to close condition and second coil will be used holding the contactor in close position. If supply to closing coil gets cut off, it will release the contactor to open condition. There will not be latching mechanism provided to hold the contactor. In this type of contactor, there is a requirement of continues control supply. 3.0 Constructional Details The vacuum contactor consists of three Epoxy cast Poles, each individual epoxy cast Pole forms a switching pole. These epoxy cast consist of main current caring components with vacuum bottles are mounted on rigid epoxy base plate to further improve insulation properties. This part is the further attached to a very simple, compact, durable and maintenance free electromagnetic coil and its mechanism. The compact vacuum contactor unit featured with minimum number of components which enhance the mechanical life of the contactor. There are two operating coils in the contactor, both are used to switch as well as hold the vacuum contactor in ON condition. The durable opening springs open the contacts very fast so as to open the circuit effectively to improve the response time of your control circuit. 3.1 Encapsulated Housing Housing assembly also known as Pole assembly or power path assembly. This assembly consists of main housing body made up of Epoxy having two ends cover (front & rear) mounted on both sides. HT fuse and vacuum interrupter has kept inside the epoxy housing, to make it encapsulated. The current transfer path is as follows from top conductor to HRC fuse with fuse holding clamp, then it is transfer from fuse to fuse clamp then vacuum interrupter finally it flows to flexible conductor and bottom conductor. At bottom of the flexible conductor, there is a insulated drive rod which is important components for insulating High voltage and earth parts. All current carrying parts have made of up copper material with silver or Tin plating. Published in 3.2 Vacuum Interrupter Vacuum Interrupter is the heart of switchgear. Basically, the vacuum interrupter is a 4

5 Pair of separable contacts (called "primary contacts") enclosed in a vacuum-tight envelope. The Envelope itself is a ceramic material, with a metal end plate brazed to each end. The metal plates seal the ends and provide support for the parts inside. Various shields inside the envelope provide different types of protection to interrupter parts. 3.3 High Voltage Fuses Current limiting back-up fuse links are the closest in construction to LV cartridge types. However, a longer strip element with many more restrictions is necessary to produce the large number of series arcs needed to interrupt a high voltage. Fuse manufacturers achieve this by coiling the element around a ceramic core with a star shaped cross section. If the adjacent coils are too close, there will be a flashover between them and this sets a practical limit on how long an element can be crammed into a particular cartridge, and Like an LV cartridge fuse link, an HV fuse link of this type has a ceramic body..3.4 Electromagnetic Coil Proportional Solenoids - Included in this category of solenoids are the uniquely designed magnetic circuits that effect analog positioning of the solenoid plunger or armature as a function of coil current. These solenoids, whether axial or rotary, employ a flux carrying geometry that both produces a high starting force (torque), and has a section that quickly begins to saturate magnetically. The resulting force (torque) profile as the solenoid progresses through its operational stroke is nearly flat or descends from a high to a lower value. The solenoid can be useful for positioning, stopping mid-stroke, or for low velocity actuation; especially in a closed loop control system. A uni-directional solenoid would actuate against an opposing force or a dual solenoid system would be self cycling. 4.0 Conclusion In Vacuum Circuit breakers use of mechanical mechanism and linkages has to undergone the wear & tear. Replacement & maintenance lead to larger down time and become very costlier affair. We also studied the use and the different requirements of vacuum contactors and different ways to achieve it in medium voltage switchgear. Vacuum contactors are compact electrical devices which can open and close the circuits quite effectively and efficiently. In this type of arrangement the electrical contacts parts are enclosed in a vacuum, so there are no visible arcing parts, as a result of this, it is very environmental friendly. Vacuum contactor is such a product has grown in popularity and also in size and complex city. Also it provides more safety and high reliability with trouble free operations. 5.0 References Published in [1] Angelo Bortolus, Reginal A. Drake, Micheal Frayne, Multi vacuum contactor control system, Patent no. US B2, Sept

6 [2] Akira Fukashi, Tomico Yamagushi, Vacuum Switch bulb type change over switch fo on load tap changer, Patent no Jan 1997 [3] David D. Miller, Mark W. Eady, Multiphase vacuum switch arrangement including an Electromagnet actuating Mechanism, Patent no Dec [4] Munaf F. Badr Modeling & Simulation of closed loop controlled DC-DC converter fed solenoid coil Journal, Vol. 7 March [5] Radulian, A.; Mocioi, N., Numerical modeling of an electromagnetic actuator for vacuum contactors, IJIRSET, Jan [6] Mueller, A.; Saemann, D. Switching phenomena in medium voltage systems - Good engineering practice on the application of vacuum circuit-breakers and e Index Vol:4, No:6, Contactors, IEEE, Vol. 3, Issue 4, April [7] Long, E.A., An electronic method of controlling multiple reigniting switching Transients in vacuum contactors, IJERA, Vol. 3, Issue 2, [8] Lister, Charles A., Vacuum, SF6 and Air- Break Contactors for Medium Voltage Controllers, CIER, Vol. 2, [9] Ivan Yatchev and Ewen Ritchie., Simulation of Dynamics of a Permanent Magnet Linear Actuator, International Science Published in 6