Transformer Tap Changer PDF

CHAPTER 5: TRANSFORMER TAP CHANGER A tap changer is a device fitted to power transformers for regulation of the output voltage to required levels. This is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropria...

CHAPTER 5: TRANSFORMER TAP CHANGER A tap changer is a device fitted to power transformers for regulation of the output voltage to required levels. This is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropriate transformer/s. Supply authorities are under obligation to their customers to maintain the supply voltage between certain limits. Tap changers offer variable control to keep the supply voltage within these limits. About 96% of all power transformers today above 10MVA incorporate on load tap changers as a means of voltage regulation. Tap changers can be on load or off load. On load tap changers generally consist of a diverter switch and a selector switch operating as a unit to effect transfer current from one voltage tap to the next. It was more than 60 years ago on load tap changers were introduced to power transformers as a means of on load voltage control. Tap changer posses’ two fundamental features: (a) Some form of impedance is present to prevent short circuiting of the tapped selection. 1  (b) A duplicate circuit is provided so that the load current can be carried by one circuit whilst switching is being carried out on the other. The impedance mentioned above can either be resistive or reactive. The tap changer with a resistive type of impedance uses high speed switching, whereas the reactive type uses slow moving switching. High speed resistor switching is now the most popular method used worldwide. The tapped portion of the winding may be located at one of the following locations, depending upon the type of winding: (a) At the line end of the winding; (b) In the middle of the winding; (c) At the star point. The most common type of arrangements is the last two. This is because they give the least electrical stress between the tap changer and earth; along with subjecting the tapings to less physical and electrical stress from fault currents entering the line terminals. At lower voltages the tap changer may be located at either the low voltage or high voltage windings. Tap changers can be connected to the primary or secondary side windings of the transformer depending on: Current rating of the transformer Insulation levels present Type of winding within the transformer (eg. Star, delta or autotransformer) Position of tap changer in the winding Losses associated with different tap changer configurations eg. Coarse tap or reverse winding Step voltage and circulating currents Cost Physical size 2  On-load tap changer Current Maintenance Strategies of Transformer Tap Changers The frequency of maintenance to on load tap changers is dependent on the condition of the diverter switch and the necessity to maintain the motor drive unit. Maintenance of the diverter switch should be carried out on a cyclic basis, but on transformers where frequency of tap change is high, maintenance may be necessary before the cyclic maintenance becomes due. A certain period should not be exceeded between inspections. When considering inspection periods, serious consideration should be given to the breaking of circulating current which in some cases may exceed the load current. The diverter switch and tap selector is the only internal moving parts in a transformer. The diverter switch does the entire on load making and breaking of currents, whereas the tap selector preselects the tap to which the diverter switch will transfer the load current. The tap selector operates off load and therefore needs no maintenance. However experience has shown that in some circumstances inspection of selector switches becomes necessary where contacts become misaligned or contact braids in fact fatigue and break. 3 Next is a list taken from on what should be carried out during tap changer maintenance. Replace contacts in older type tap changers. Modern tap changers rarely require contact replacement; this depends on the characteristics of the tap changer in question. The frequency of diverter switch and motor drive unit inspections can usually be obtained from manufacturer manuals or previous maintenance experience. Measuring and recording contact consumption during inspection will give a reasonably accurate life expectancy of the contacts at that present load condition. Therefore this should be done on a regular basis. Transition resistors should be checked for continuity and value as an open circuited resistor can result in excessive contact wear. Need to equalize rotation lag between the diverter switch and the motor drive unit to ensure minimum spring energization in the energy accumulator springs. The function of relays, interlocks, limit switches and switches should be checked as well as remote indication of tap position. Drive shafts and gearboxes must be inspected for radial and axial wear. A large percentage of tap change failures are as a result of drive shaft faults. Replace transformer oil with clean, dry oil. Cleaning is only carried out with transformer oil not solvents. Carbon and copper deposits are generally found on horizontal surfaces of the diverter switch as small convection currents in the oil are established each tap change. This results in the carbon being deposited on top of the diverter. 4 Load tap changer The Load Tap Changer (LTC) is a mechanical switching device and is the most expensive and vulnerable accessory on a power transformer. They cause more failures and outages than any other component of a power transformer. The function of a LTC is to change the turns ratio without interrupting the load current. LTC failures are categorized as electrical, mechanical, and thermal but most are mechanical at the beginning and develop into electrical faults. This occurs due to problems on the contacts, transition resistors and insulation breakdowns. The LTC can be evaluated on-line without affecting its normal operation by using a combination of acoustic emission and vibration techniques. Acoustic Emission assessment is based on the fact that no acoustic activity is expected from inside the LTC compartment if the tap changer is not being operated and if it is in good condition. Vibration techniques consist of obtaining the signature of one operation of the tap changer and performing the comparison of its characteristics (time, amplitude, energy, etc.) with another signature obtained sometime in the future or with a sister unit in the same operation. When using a combination of both techniques, the evaluation of the out-of-service condition of the tap changer, is performed using acoustic emission; whereas the evaluation during an operation is by vibration techniques. An example of the type of faults detected using Acoustic Emission can be seen on the image below; the main tank and LTC compartment were instrumented and 5 monitored simultaneously; an acoustic emission source was detected and located inside the LTC compartment. It can be seen that the temperature on the LTC is 30°C higher than the main tank. Overhaul revealed overheated contacts. On-load tap-changers (OLTC) On-load tap-changers are used for changing tappings on energized transformer windings. ABB has manufactured tap-changers since 1910 with more than 30,000 UC/UZ/UB in service Type UCG and VUCG on-load tap-changers The UC types of on-load tap-changers come in a wide range of models with a rating suitable for every application. They are usually mounted inside of the transformer tank, suspended from the transformer cover. The UC types operate with conventional arc quenching in oil and the VUC types operate with arc quenching in 6 vacuum interrupters. UCG and VUCG are the smallest of the UC types, covering up to 400 kV and 500 MVA. Type UBB on-load tap-changers The UBB type of on-load tap-changer is designed for mounting inside of the transformer tank, but contains features usually found with the on-tank type of tap-changer. This combination of the best qualities from both designs of tap-changer, makes the UBB model unique in its design and operation. Primarily designed for the medium sized transformer, the unit is compact and less costly than the equivalent diverter switch type of tap-changer. Type UCL on-load tap-changers The UC types of on-load tap-changers come in a wide range of models with a rating suitable for every application. They are usually mounted inside of the transformer tank, suspended from the transformer cover. The UC types operate with conventional arc quenching in oil and the VUC types operate with arc quenching in 7 vacuum interrupters. UCL is the medium size of the UC types, covering up to 525 kV and 1000 MVA. Up To 150 MVA , 230 kV Class(On load tap changer) Tap-changer with single phase transformer 8  On Load Tap Changer Specifications: 11 kV / 22 kV / 33 kV Class, 200 Amps Rating, 7 / 9 / 11 / 17 Positions, Linear Type, Bolt On Type, Best suited for Distribution Transformers. Model Numbers : L-11, L-22, L-33 Off Circuit Tap Changer An example of Off Circuit Tap Switch of 11 kV Class, 3500 Amps, 5 Position, 3 Phase, Selector Type, Suitable for Vertical Mounting with common Operating Handle. 9  Off Circuit Tap Switch for Power Transformers Featured here is Off Circuit Tap Switch, 132 kV Class (650 BIL), 600 A, 5 Position, 3 Phase, Selector Type, Suitable for Vertical Mounting. Specifiations: 66 / 110 / 132 / 220 kV Class, 300 / 600 / 800 Amps, 5 / 7 / 9 / 11 Positions, Selector / Bridge Type, Horizontal / Vertical (preffered) Mounting, With Geneva Wheel based Bevel Gear Mechanism, Options of Operating Handle with Dial Switch / Limit Switch / Set of Potential Free Contacts / 5 Digit Operation Counter. Off Circuit Tap Switch - Rotary Side Base Type 10 Specifications: 11 kV / 22 kV / 33 kV Class, 30 / 60 / 100 Amps Rating, 5 / 7 / 9 Positions, Bridge Type, Suitable for Horizontal Mounting, With Universal Coupling / Square Slot Coupling, With T Type Handle / Wheel Type Handle Applications: Best suited for Distribution Transformers. Off Circuit Tap Switch Cum Voltage Changeover Switch A Combination Switch where in Off Circuit Tap Switch as well as Voltage Changeover Switch are fixed in a common frame. This picture shows a design suitable for Cover Mounted Transformers. However, various models are available for tank mounted transformers also. Specifications: 3.3/6.6/11/22/33 kV Class, 30/60/100/150/ 250 A, /7/9 Positions (Tap Switch), 3/6 Decks, Horizontal / Vertical Mounting Motorized Off Circuit Tap Switch with Tank & Bushing 11  A Stand Alone Type Motorized Tap Changer, with Oil Tank and Bushings. This unit is installed in a fully automated transformer test laboratory. Motorized Off Circuit Tap Switch for Testing Transformers Featured here is a set of two Motorized Tap Changers. 1. 160 kV Class, 1600 Amps, 8 Position, 3 Phase. 2. 36 kV Class, 1600 Amps, 6 Position, 3 Phase. Off Circuit Tap Switch - Rotary Tie Rod Type Specifications: 11/22/33/44 kV Class, 30/60/100/150/250/500/600 A, 5/7/9/ 11 Positions, Bridge Type, Suitable for Horizontal Mounting, With Universal Coupling / Square Slot Coupling / Ball & Socket Coupling, With T Type / Wheel Type Handle Suitable for Distribution Transformers 12 CHAPTER 6: TRANSFORMERS Instrument Transformers are used to transform high voltages or currents to values which are unified or can be measured safely, while incurring low internal losses. The voltage or current of the secondary winding is identical to the value on the primary side in phase and ratio, except for the error of the transformer. Current Transformer – an instrument transformer intended to have its primary winding connected in series with the conductor carrying the current to be measured. It should reproduce in its secondary circuit definite ratio of current of its primary circuit with the phase relationship substantially preserved. For precautionary measure, secondary terminals of CTs should not in any case be left open while it is in operation. Types of Current Transformer 1. Wound type – Primary winding consisting of one or more turns mechanically encircling the core/s. The primary and secondary windings are completely insulated for their respective voltage ratings Permanently assembled on a laminated core Higher accuracy at lower ratios 2. Bushing Type Annular core and secondary winding insulated form the core and permanently assembled No primary winding and no insulation for a primary winding Use on a fully insulated conductor as the primary winding The core encircles an equipment bushing such as CBs, transformers, etc thru which the conductor passes to form the primary turn. 13  3. Window type/Doughnut type Secondary winding is completely insulated and permanently assembled on an iron core. No primary winding as an integral part of the structure Primary insulation is provided in window, where the primary conductor passes and serves as the primary winding Voltage Transformers Primary winding is connected in parallel with the power supply circuit, the voltage of which is measured or controlled Must reproduce in its secondary circuit a definite ratio of voltage of its primary circuit with the same phase relationship Protection --- should always be protected by fuses or miniature circuit breakers at its secondary windings and should be as close as possible to the transformer Precautions---secondary terminals of VTs should not be in any case be short circuited while it is in operation Inductive Voltage Transformers Low power transformer in which the secondary voltage is proportional and in phase with the primary voltage Selected according to: Primary and secondary rated voltages Accuracy class Rated output of the secondary windings in order to meet the requirements of the devices they are connected to Capacitive Voltage Transformers Used in high voltage applications Designed to based on the capacitive divider principle 14  Can be used for coupling high frequency power line carrier systems Combined instrument transformer (CT and VT) Epoxy Resin Insulated Instrument Transformers Main insulation of these medium-voltage instrument transformers is epoxy resin cast in high vacuum and with superior dielectric and mechanical properties. Instrument transformers can be divided according to place of installation. Classification of Transformer According to Size Distribution Transformer – Used for transferring power from primary distribution circuit to a secondary distribution circuit. Distribution transformers usually use copper or aluminum conductors and are wound around a magnetic core to transform current from one voltage to another. Distribution transformers come in two types; dry- type and liquid. The Dry Type Distribution Transformers are usually smaller and do not generate much heat and can be located in a confined space at a customer's location. The liquid type usually has oil which surrounds the transformer core and conductors to cool and electrically insulate the transformer (see also Oil Filled Transformers). The liquid distribution transformer types are usually the larger and 15 need more than air to keep them from overheating thus the oil insulator. There are common application of Distribution Transformers for the following purpose: Transmission lines, Radar systems, Photocopying machines, Tool Machines, Data processing equipment, Telecommunications systems, Test control and measuring systems, Safety alarm and lighting plants etc. Dry Type Transformer - Are those whose core and coils are gaseous or dry compound insulating medium. Oil Filled Transformers Power Transformer - Transferring power in any part of the system between the generator and primary distribution circuit. 16 220KV/110KV/35KV POWER TRANSFORMER Power Transformer Applications Power transformers are used for many applications including to step down the high transmission voltage in power lines to a level that can be used by industrial business and household usage. Transformers are designated by low, medium and high side operating voltage, and sized by the capacity of the volts and amperes being carried. The power transformer has been the backbone for the electrical distribution system for many years. Power transformers are extremely reliable and are known for their high efficiency. Power Distribution Utilizes Power Transformers In the distribution of electrical power, the power transformer works two ways; 1) it transform the voltage up to a higher level at the generating source, then the electricity travels via transmission lines at this high voltage level to its destination where it is then transformed, lowering the voltage back to a lower level for use in business industries and homes. Power transformers convert an alternating current power to a voltage supply that is compatible with electronic devices, such as electric motors, cell phones, telephones, answering machines, computers, radios and etc. Power transformers are used to change electric voltage and thus they also 17 reduce the dependence upon batteries and provide charging energy for rechargeable batteries from available alternating current power supplies. Power Transformer Physical Makeup Power transformers usually consist of insulated copper wire (or aluminum wire) wound on an iron core and consist of two windings which is referred to as a primary winding and a secondary winding. In a toroidal power transformer, a primary winding is first wound on a toroid (doughnut) shaped core, covering nearly the entire circumference of the core. One or more secondary windings are then wound on top of the primary windings, with a high-voltage insulating layer separating the primary and secondary windings. The insulation layer provides a protection layer capable of withstanding high-voltage insulation testing and high operating potential differences between the primary and secondary windings. Power transformers also use paper or cellulose materials insulation positioned between or around the various conductive paths. Mineral oil is also commonly used as an insulator in power transformers which also works as a coolant. Classification of Transformer According to Location INDOOR TRANSFORMER 18 - Is one which because of construction, must be protected in weather. Usually dry type or non –flammable oil immersed type. OUTDOOR TRANSFORMER - Is of weather – resistant construction from the weather. They are usually mineral oil – immersed type. STATION TYPE TRANSFORMER -Are those designated for installation in a power station or substation. They are usually those that have voltages above 34.5 kV in any of the windings. 19  STATION TRANSFORMER (Liquid-filled Transformers Station type (75 to 15000 kVA) A critical function of any electricity generation and distribution system is the stepping up or down of voltage at various points in the delivery network. The voltage is stepped up or stepped down at substations using Station Type Transformers. UNIT SUBSTATION TRANSFORMER - Is one which is mechanically and electrically connected to, and coordinates in design, with one or more switchgear or motor control assemblies or combination thereof. 20 CLASSIFICATIONS OF UNIT SUBSTATION TRANSFORMER 1. PRIMARY UNIT SUBSTATION – One with voltage section above 1000V 2. SECONDARY UNIT SUBSTATION – One with voltage section below 1000V. NETWORK TRANSFORMER -Is design for use in a vault to feed a variable capacity system is interconnected secondaries. PADMOUNTED TRANSFORMER - Is an outdoor type used as a part of an underground distribution system. They are mounted on a foundation pad. 21  POLE – TYPE TRANSFORMER A transformer mounted on an electrical service pole, usually at the level of the overhead cables but occasionally at ground level. Pole-mounted transformers are the common breadbox transformers used for converting distribution voltage to the 120/240 volt power used by homes and low-volume commercial installations. VAULT – TYPE TRANSFORMER - Is constructed as to be suitable for occasional submerged operation in water under specified conditions of time and external pressure. 22 Type of Transformer According to their Manner of Operation 1. Power transformer (see above discussion) 2. Auto – transformer 500 MVA Single-phase autotransformers (Mitsubishi Electric's largest autotransformer manufactured to date is a 1,000 MVA, 1050/525kV single-phase autotransformer, manufactured in 1993. This is one of the world's largest autotransformers.) 23  A transformer with a single winding, having a large number of connections, or taps. Used to deliver a precise voltage to the high- tension primary circuit. In some rural applications, power companies distribute deliberately-incorrect voltage, for example, starting at a slightly-overvoltage 254V (instead of 240) and 127V (instead of 120) in order to compensate for losses over long distribution lines. An autotransformer can be used to provide a slight boost (or step-up) to correct an under voltage condition or buck (step-down) to correct an overvoltage condition. Some autotransformers are equipped with many taps and automatic switchgear to allow them to act as automatic voltage regulators, to maintain a steady voltage at the customers' service during a wide range of load conditions. A special form of autotransformer called a "zigzag" is used to provide grounding (earthing) on three-phase systems that otherwise have no connection to ground (earth). A zig-zag transformer provides a path for current that is common to all three phases (so-called "zero sequence" current). 3. Booster transformer Capacity - 100 kVA to 2000 kVA Input voltage/Output voltage - As required Application - Both Indoor and outdoor Tapping - Offload tap changer or on load tap changer Winding – Copper 24 Booster Transformers – Their windings are electrically independent, one winding being connected in series with one system in order to alter its voltage. The other winding is connected in parallel with its associated systems. CURRENT / VOLTAGE TRANSFORMER PRODUCTS METERING CURRENT TRANSFORMER - The accuracy of metering CT depends on load currents; the saturation level is much longer than the protection ct to provide protection of the metering facilities during fault conditions. CLASS X CURRENT TRANSFORMER - Design requirements for current transformers for general protection purposes are frequently laid out in terms of knee – point emf, exciting current at the knee point and secondary winding resistance. 25  CORE – BALANCE CURRENT TRANSFORMER - Is normally a ring type, through the center of which is passed the three core cable or three single core cables of a three phase system. Earth faults relay secondary winding and are energized only when there is a residual current in the primary system. (Other current transformer products … Source: Koncar) AGE - OUTDOOR INSTALLATION APE - OUTDOOR INSTALLATION 26 UNA - INDOOR INSTALLATION ASA - INDOOR INSTALLATION VPV - OUTDOOR INSTALLATION VPA - INDOOR INSTALLATION 27  VOLTAGE TRANSFORMER - Is an instrument transformer intended to have its primary winding connected in parallel with the power supply circuit, the voltage of which is to be measured or controlled. INDUCTIVE VOLTAGE TRANSFORMER 28 - Are low power transformers which the secondary voltage, in which for all practical purposes is proportional to and in phase with the primary voltage. - Insulator can be porcelain or composite (silicone) with thermal oil expansion is compensated by stainless steel bellows. It is also oil level indicator as shown in the picture. CAPACITIVE VOLTAGE TRANSFORMERS - Design to work based on the capacitive divider principle. This kind of voltage transformer is known as the capacitive voltage transformer, it has added advantage that they can be used for coupling high frequency power line carrier system. - Capacitive Voltage Transformers isolate the measuring instruments, meter, relays, protections, etc., from the high voltage power circuit and produce a scaled replica of the current, with the possibility of transmitting high frequency signals through the high voltage power lines. Coupling Capacitors are used for coupling high frequency communication signals and correspond to the capacitive part of a capacitive voltage transformer. 29 CHAPTER 7: CIRCUIT BREAKERS and DISCONNECTORS Circuit Breakers are mechanical switching devices capable of making, carrying and breaking currents under normal conditions and also making, carrying for a specified time, and breaking currents under specified abnormal conditions such as those of short circuit. The medium in which circuit interruption is performed maybe designated by a suitable prefix, for example: air-blast circuit breaker, gas circuit breaker, oil circuit breaker, vacuum circuit breaker, etc. Breakers for larger interrupting capacities and higher voltages were made by increasing the size of tank, head of oil, length of stroke clearance, and improved insulation. Circuit breakers are rated by voltage, insulation level, and current, interrupting capabilities, transient recovery voltage, interrupting time and trip delay. The High Insulating Quality of liquid and the gases formed during the arcing was the found effective in quenching the arc and preventing its reestablishment “after the current had passed through the zero”. Regardless of the medium of arc quenching and insulation, each circuit breaker unit consists of the following construction elements: Main contact at system voltage Insulation between main contact and ground potential (porcelain, oil, gas, etc) Operating and supervisory devices as well as accessories out of reach of the system voltage life zone. An insulated link between operating device and main contact. Requirements of a Circuit Breaker Capable of making, carrying and breaking currents under normal conditions. Carry for specified time and breaking currents under specified abnormal conditions. Able to withstand the transient voltages which appear across the contacts immediately after the current flow ceases It must extinguish the arc during opening without delay. PHYSICS OF CIRCUIT BREAKER ARCS 30  When the two current-carrying contacts in a circuit breaker are closed, the actual electrical contact is made at a number of points on the two surfaces, the number of points being a function of the pressure holding the surface together. When starting to separate the contacts, the number of points is reduced and the contact resistance increases, giving rise to an increase in temperature due to the I²R losses produced. This temperature is high enough to cause emission from the contact surfaces, thus tending to maintain the current flow. In addition, as soon as final separation is completed and the contacts are physically separated, the voltage across this gap that has now 31 high resistance will cause further ionization because of the potential gradient. Both these effects will produce electrons to continue the current flow across the gap and therefore to draw an arc. HISTORY OF DEVELOPMENT In the early days of electrification (1890) switches were of the hand-operated type. AIR SWITCHES Using currents and voltages, spring, action driving mechanisms were developed to reduce contact burning by faster- opening operation. Later, main contacts were fitted with arcing contacts of special material and shape, which opened after and closed before the main contact. Oil Circuit Breaker 32  Around 1900, in order to cope with the new requirements for “interrupting capacity”, ac switches were immersed in a tank of oil. Oil is very effective in quenching the arc and establishing the open break after current zero. Deion grids, oil blast features, pressure tight joints and vents, new operating mechanisms, and multiple interrupters were introduced over several decades to make the OCB a reliable apparatus for system voltages up to 362kV. Minimum-Oil Circuit Breaker These breakers were developed after 1930, are used mainly in Europe, and make use of special low oil volume interrupting chambers of extra-light weight. By means of current dependent oil streams in different directions and supported by oil injection, the arc is cooled and extinguished effectively. The interrupters are mounted on porcelain or molded-resin supports, thus avoiding oil as an insulating medium to ground. Air Blast Circuit Breaker Further increase of system voltages and generating capacities triggered the search for faster and stronger circuit breakers utilizing oil less arc interruption. After 1940, the air blast circuit breaker was developed, making use of good insulating and arc quenching properties of dry and clean compressed air. 33 Magnetic Air Circuit Breaker Uses a combination of a strong magnetic field (coil or soft iron plates) with a special arc chute to lengthen the arc until the system voltage cannot maintain the arc circuitry any longer. This interrupting principle is applied mainly in the distribution voltage range in metal-clad switchgear. SF6 CIRCUIT BREAKER The excellent arc quenching and insulating properties of SF6 gas (Sulfur Hexafluoride) stimulated this breaker development in the early 1950s. Both live and dead tank designs were introduced in the 1960s in outdoor circuit breakers from15 to 800kV. 34  The SF6 breaker provides an alternative to oil and vacuum for metal clad and metal enclosed switchgear up to 38kV enclosed switchgear up to 38kV Vacuum Circuit Breaker This has been the most recent advancement in new arc interrupting and breaker development. Vacuum bottle interrupters are designed for higher system voltage, current, and interrupting ratings. Increased application of vacuum circuit breakers spreads at distributions systems both in metal-clad and metal enclosed switchgear. Standard Requirement of Circuit Breaker The standards require that a power circuit breaker shall: Perform at or within its interrupting rating without emitting flame. That, at the end of any performance within its interrupting rating, the circuit breakers shall be in substantially the same mechanical condition in as at the beginning. It recognized, however, that after a breaker performs its duty cycle at or near its interrupting rating, the breaker may have its interrupting ability materially reduced and should be inspected and repaired if necessary. Operating times and contact travel of breakers have to be coordinated properly. 35 TYPES OF CIRCUIT BREAKER DEAD TANK Is where the arc chamber is surrounded by insulating medium insulating the live parts from earth The main insulators are the bushings, the guide for the moving contacts and the insulating drive members of the moving contacts. Basically an “American” design using SF6 as the interrupting medium. A typical dead tank circuit breaker design by ABB–USA (previously Westinghouse). 36  Live Tank Circuit Breaker Basically “European” designs using SF6 as the interrupting medium. Its obvious difference with the dead tank design is the absence of a current transformer as an integral part of the breaker. The main components are the insulators constituting the tank, the insulating of the support collumns and the contact driven rods. Major advantage of Live Tank is lower cost especially at the higher voltage ratings. While the major disadvantage is that it requires extremely mounted current transformer. 37  GANGED OPERATED VS IPO BREAKERS Gang Operated – A circuit breaker consist of three (3) separate poles with common operating mechanism, suitable only for three-pole reclosing. IPO Independent Pole Operated (IPO) – A circuit breaker consists of three (3) separate poles with individual operating mechanism, suitable for both single and three pole reclosing operation. 38 CLASSIFICATION OF CIRCUIT BREAKERS & THEIR PRINCIPLE OF ARC QUENCHING 1. OIL CIRCUIT BREAKER It uses oil for arc quenching/extinction. It is the hydrogen contents that quenches the arc not the oil. Due to high heat conductivity of the hydrogen gas released by the arc and its high pressure, so long as the hot gases decomposition did not explode at the surface of the oil Therefore, with increasing arc power, the arcing contacts had to be immersed deeper in the oil vessel, and the breaking capacity of bulk oil breakers was notably increased by the use of an insulating oil, rigid arch chamber surrounding the contact tulip. SPEED OF BREAK The arcing time must be kept minimum so that the arc energy and burning contact is reduced. It has been found that it is extremely difficult to reduce the arcing time during short circuit interruption considerably below 20m/sec as the current can be extinguished only in the current zeros. Thus the optimum speed break is how ever dependent upon functions other than short circuit performance. Typical Speed Break of Commercial Oil Circuit Breakers: 12KV Circuit Breaker 2 to 3 m/sec 36KV Circuit Breaker 4 to 5 m/sec 145KV Circuit Breaker 6 to 9 m/sec SPEED OF MAKE The speed of make should be chosen so that the closing operation is “not hesitant” free from contact bounce so that the arcing is minimized. Thus avoid generation of high gas pressure and contact burning which tend to prevent contact engagement. Typical Speed Make of Commercial Oil Circuit Breakers: 12KV Circuit Breaker 2 to 3 m/sec 36KV Circuit Breaker 4 to 5 m/sec 145KV Circuit Breaker 6 to 9 m/sec MAINTENANCE OF OIL CIRCUIT BREAKER: 39 The suggested lubrication is as follows: DESCRIPTION PERIODICAL MAINTENANCE LUBRICATION LUBRICATION Outer surface of roller Do not lubricate After 5 yrs service,clean service, clean with light and grease with light coat Slide surface of hooker Do not lubricate Linkage 1 yr application of machine oil Drum shaft of tank Every used, light application machine oil Manual closing screw 2. AIR BLAST CIRCUIT BREAKER ABB are using pressure in excess of 10-20 bars. At this pressure the dielectric strength of compressed air is comparable with that of clean oil. Therefore, air at high pressure is used inside the arc chamber or interrupting chamber as insulation. For all design, the compressed air have to be dried to reduced the moisture content. The low moisture content almost eliminate the usual corrosion processed associated with electrolytic reaction between dissimilar metal. As the contact separate, compressed air is blown through the nozzle-shaped contacts, so quenching the arc and establishing the insulating gap. Air compressor, storage and distribution systems provide airblast breakers with clean, dry, compressed air. There is one type of breaker that draws the arc in open air, saving the expensive high pressure porcelain housing of the unit. It needs a longer contact travel and more space, because of lower dielectric strength. The visible gap is advantageous, but the noise of compressed air expanding to the atmosphere is disturbing. Therefore, full air flow is released for short circuit currents only. This type of air nozzles may be insulated or metallic; The latter is often used as electrodes. Optimum nozzle and electrode geometry depend on the type of fault to be interrupted. 40 3. Gas Circuit Breaker ( SF6 CIRCUIT BREAKER ) The feature of SF6 are extremely high ionization energy molecules and its electro-negativity (electron attachment), both dominated at a quite lowtemperature when proper arc has ceased. This results in the low electrical conductivity of SF6 at low temperatures, lower than electricalconductivity for hydrogen and nitrogen. The bonding energy of SF6 molecules is 2.3 times that of N2, but dissociation occurs in Six energetically equal steps at successive collisions, each of them needing only one sixth of total amount of bonding energy, the maximum thermal conduction appears at lower temperature in SF6 (around 2000 Deg K) than in Fluorine and Nitrogen has therefore a very short thermal time constant. PROPERTIES OF SF6 GAS Physical Property Colorless, odorless, non-toxic, non-flammable Specific density is 5 times that of air at atmospheric and room temperature Sonic Speed is remarkably low (140m/sec) , approx. 40% that of Air. Chemical Property it is chemically inert up to 150 Deg C and will not attack metals, plastic and other substances commonly used in the construction of high voltage circuit breaker components. However, at high temperature caused by arcs, it decomposed into various components which in the presence of moisture is corrosive both to metal and glass. Together with vaporized metal from the arcing contacts, the decomposed gas form a powder which has good insulating properties when not exposed to moisture. The breaker contact must therefore be designed with wiping action to ensure self cleaning of the current carrying surfaces. To prevent moisture at the breaking chamber, it is equipped with a molecular sieve absorbing the moisture. Electrical Property -The dielectric strength of SF6 gas at a pressure of 3ATM absolute is comparable to that of insulating insulating oil. 41  The dielectric strength and gas flow of this gas is limited by its triple point at 38ATM at 45deg. C. At 18ATM it liquefies by 14 deg. Celsius Maintenance of Gas Circuit Breaker The contacts should be renewed when the specific electrical endurance deemed expired. Check main contact surfaces. The surfaces should be smoothed by a fine file and cleaned. Check arcing contacts wear and the condition of the surfaces. If the contact is reusable, clean them with cloth, then apply very slightly contact grease to them. Replace with new ones if the wear is excessive. Replace blast nozzle if the internal surface is extremely worn. Check that blast nozzle fixing ring is tightly fastened. Replacement of absorbent is obligatory on each internal inspection of the breaking chamber. Replace O-ring gaskets whenever joints are disassembled. 4. VACUUM CIRCUIT BREAKERS The interruption is simple and fast.Vacuum Circuit Breakers are for use in applications on circuits, within their voltage, continuous- current ratings, and interrupting capability. The interrupting ampere rating of the breakers must be as great as, or greater than, the maximum short-circuit current they may be called on to interrupt. These new high speed vacuum breakers reduce chance of line burn down with approximately 2-cycles (plus relay time) interruption of higher current faults. When contacts are opened, an arc is drawn between the contact surfaces. It is rapidly moved around the slotted contacts surfaces by self-induced magnetic force which prevent gross contact erosion and formation of hot spots on the surface. The arc burns in an ionized metal vapor which continually leaves the contact area condenses on the surrounding metal shield. At current zero, the arc extinguishes: vapor production ceases. Very rapid dispersion, cooling, recombination, and deionization of metal vapor plasma together with the fast 42  condensation of metal vapor products cause by vacuum to be quickly restored. Hence, the opened contacts withstand the TRV. Advantage s of Vacuum Circuit Breaker 1. They are entirely self contained, need no supplies of gas or liquid and emit no flames of gas. 2. They require no maintenance and in most applications their life will be as long as the circuit breaker in which they are applied. 3. They may be used in any orientation. 4. They are not flammable. 5. They have no need for capacitors or resistors to interrupt short line faults. 6. They require a relative small mechanical energy to be operated. 7. They are silent in operation. Principal points to consider when selecting circuit breaker 1. Maximum operating voltage and location 2. Height of installation above sea level 3. Maximum operating current occuring at location 4. Maximum short-circuit current occuring at location 5. System frequency 6. Duration of short-circuit current 7. Switching cycle 8. Particular operational and climatic conditions Safety First Before starting to work on any breaker, be sure to check the following: 1. Breaker has been tripped off. 2. The disconnects are open on both sides of breaker. 3. Breaker is checked dead with glow indicator. 4. All control circuits are open 5. If pneumatically operated, close air shut valve. 6. Ground all bushings. 7. Breaker, disconnects and control circuits are tagged. 8. There is no feedback from other devices. 43 9. Rope or tape enclosure is placed around breaker. 10. Proper ladders and tools are available. 11. When checking breaker internal adjustments with breaker closed, be sure breaker cannot trip open by blocking the trip free trigger and holding latch. Classifications of Circuit Breakers According to Medium of Interruption/Arc Extinction Oil Circuit Breaker - Could either be the bulk-oil or minimum oil type. Its arc control or interrupter makes use of the oil pressure generated by the gas created by the arc during the interruption process. 44  MAGNETIC AIR CIRCUIT BREAKER - It interrupts high fault current in the normal atmosphere under the influence of a strong magnetic field which acts to force the arc deep into a specially designed arc chute. AIR – BLAST CIRCUIT BREAKER - Uses compressed air to operate the breaker as well as to provide the medium for arc interruption. It is directed across the path of the arc chute where the arc products are cooled and de-ionized to effect interruption. 45  VACUUM CIRCUIT BREAKER - It consists of 3-main parts, a pair of butt contacts, vapor condensing shield, and bellows which allows the movement of one of the contacts. All these elements are sealed in a vacuum tight enclosure “vacuum-bottle”. Arc interruption takes place at the first current zero due to the short time of the vacuum gap and its dielectric strength. Description: 46  – ZW8-12, ZW8-12G series high-voltage vacuum circuit breaker is a kind of outdoor high-voltage switch equipment with 50Hz three-phase alternating current and 12kV rated voltage. – It is mainly applied to break and close the load current, overload and short-circuit in country power net, city power net and mini type power system. SF6 CIRCUIT BREAKER - SF6 gas became the preferred medium interruption for circuit breakers. The gas is a very stable compound, has high insulating qualities, good interruption properties, inert, non- flammable, non – toxic and odorless. 47  SOLENOID MECHANISM CIRCUIT BREAKER - The operating mechanism is solenoid if the breaker closed by the electrical power of a solenoid. The closing control and power supply is either 125/250 V DC or 230 AC. The tripping voltage is either 48/125/250V DC. PNEUMATIC CIRCUIT BREAKER - The operating mechanism is pneumatic if the breaker closed by the action of a compressed air power of a closing air cylinder or piston. Disconnectors 48  Horizontal break knee type disconnector The horizontal break knee type disconnector consists of three poles. Each pole consists of a base, one rotating insulator and two support insulators on which the main blade is mounted. The main blade is made of 2 aluminum tubes with a hinge in the middle and has silver-plated copper contacts at the jaw end. The main contacts are of the "reverse-loop" design, which makes them suitable for very high short circuit currents. 49  The pantograph disconnector consists of three poles. Each pole consists of one support insulator, one rotating insulator, the pantograph mechanism and a counter contact. Equipped with four aluminum arms to ensure a rigid construction with a very high short circuit rating. 50  The vertical break disconnector consists of three poles. Each pole consists of a frame, one rotating insulator and two support insulators on which the main blade is mounted. The main contacts are of the “reverse-loop” design, which makes them suitable for very high short circuit currents. Double-break Disconnectors Each pole consists of a frame, two supporting insulators at each end and one rotating insulator in the center, on which the main blade is mounted. The contacts are self-cleansing, which makes the disconnector suitable for installation in areas with severe climatic conditions. 51  The horizontal center-break disconnector consists of three poles. Each pole consists of a frame, two rotating support insulators and a main blade that moves in a horizontal plane. The contacts are self-cleansing, which makes the disconnector suitable for installation in areas with severe climatic conditions. 52