VePi Newsletters The Electrical Power Systems Division
The Switchgear & circuit breakers section Number: 3
The rating of a c.b.: In general, the rating of a circuit breaker is the limit of the range of operating characteristics under well defined conditions, this can include, where applicable: rated maximum voltage, rated voltage range factor K, rated frequency, rated continuous current, rated dielectric strength (dry and wet, where applicable, low frequency withstand voltage, dry impulse withstand voltage, dry chopped wave impulse withstand voltage & dry/wet switching impulse withstand voltage, rated standard duty cycle, rated interrupting time, rated permissible tripping delay, rated reclosing time, rated SCC, rated transient recovery voltage, load current switching vs. life of device, rated capacitance current switching, rated line closing switching surge factor, rated out of phase switching current, rated shunt reactor current switching, rated excitation current switching, rated control voltage, rated fluid operating pressure (if hydraulic/pneumatic control). The rated maximum voltage for the different breaker voltage classes in KV: 4.76, 8.25, 15, 25.8, 38 with rated continuous current at 60HZ in A rms: from 600 to 3000 with rated SCC (at rated maximum KV) in KA rms (nominal voltage): 9.130 (4.16KV), 35(7.2KV), 19.3 38(13.8KV), 25(24.6KV), 20(34.5KV). The rated insulation levels, which withstand low frequency and impulse in KV rms and crest, respectively, are: 19 & 60, 26 & 75, 36 & 95 and 70 & 150. The K factor can vary from 1 to 2. The short time ratings (3 seconds) in KA rms are: 44, 25, 37.5 and 31.5. The close and latch rating in KA rms are: 70, 40 and 60. The rated current of a circuit breaker is the maximum value of current in amperes rms which the breaker will carry continuously, without having the temperature of various parts of the breaker as set by the standards being exceeded. Applications at frequencies, other than the design frequency need special consideration and sometimes deration of the breaker capabilities. The close and latch is the ability of the breaker to close and to be maintained closed under faulty conditions i.e. to close against the electromagnetic forces, which is proportional to the square of the phase current (maximum instantaneous value). It is known that in a particular phase, the current is maximum right at the instant the short circuit takes place, after which the current decreases gradually. The current in the first one or two cycles is known as the subtransient current, the next 810 cycles is known as the transient and finally the steady state. The breaking current of the breaker depends upon the instant on the current wave when the contacts begin to open (tripping delay and opening time). In general, the interrupting time (opening time and arcing time), for AC MV CB's, ranges from 5 to 8 cycles. The breaking capacity of a CB is the product of the breaking current and the recovery volt. Asymmetrical current: When SC occurs, the current increases due to the fact of an establishment of a new circuit with lower impedance. For a symmetrical SCC to be produced under the SC PF of zero, the fault has to occur exactly when the normal voltage is maximum. The total SCC is the sum of all sources connected to the circuit. The symmetrical current is essentially at a maximum and decreases (decrements), until a
steadystate value is reached. Most SCC's are not symmetrical, but are offset from the normal current axis for several cycles. If the SC (fault) occurs at point zero on the voltage wave and the PF is zero (fault impedance is purely inductive), the current starts to build up from zero, but cannot follow a normal current axis because the current must lag behind the voltage by 90°. The current is asymmetrical with respect to the original axis. The actual value of the DC component (which causes the offset or asymmetry), would depend on the time at which the SC occurs in the voltage wave and is quantitatively equal and opposite to the value of the steady state symmetrical current wave at time of zero. When the DC current assumes a value equal to the peak value of the symmetrical current, the wave is considered fully offset, (maximum asymmetry). The maximum asymmetrical peak does not generally occur during conditions of maximum asymmetry, it occurs at a fault angle of zero. The fault angle for maximum asymmetry ranges from 0° to 90°. Restriking voltage & recovery voltage: The definition for restriking voltage is the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction. The rate of rise of restriking voltage is the peak value of restriking voltage divided by the time taken to reach the peak value. The recovery voltage is the power frequency (R.M.S. voltage) that appears across the breaker contacts, after the transient oscillation dies out and final extinction of arc has resulted in all poles. The classification of the tests conducted on medium voltage circuit breakers & the purpose of each category: Tests performed on circuit breakers can be divided into five categories, according to the following: Design Tests, Production Tests, Tests after Delivery, Field Tests & Conformance Tests. The purpose of the design (type) tests, is to confirm the adequacy of the design of a particular type of CB to operate satisfactorily under practical conditions. It is intended to work under (examples of tests): rated maximum voltage, rated voltage factor, rated frequency, rated transient recovery voltage, rated interrupting time, rated permissible tripping delay, rated reclosing time, load current switching, rated capacitor switching current, rated line closing surge factor, outofphase switching current tests, shunt reactors, rated excitation current switching, rated control voltage current. Also, rated continuous current carrying capacity (thermal testing): these tests demonstrate that the c.b. can carry its rated continuous current at its rated frequency without exceeding the temperature limits as set by the appropriate standards. Also, rated dielectric strength tests are conducted on the circuit breaker as type tests: it is demonstrated by subjecting the breaker to high potentials both at normal power frequency and high frequency (impulse). The dielectric strength depends upon clearances, bushing material, workmanship and material quality. Generally, the dielectric test at power frequency is performed by applying an a.c. (sine wave voltage)with a crest value of 1.414 times the rated low frequency withstand voltage. This voltage is applied for 60 seconds to different points on the assembly or breaker. For example it is applied to each terminal individually with all other terminals connected together and to ground, this is done while the breaker is opened. With the breaker contacts closed, the test voltage is applied to each phase and the other phases are grounded to the c.b. frame. The breaker is considered to have passed the
test if there is no puncture or flashover of the dielectric material. This is a dry test for indoor installations. Outdoor circuit breakers have to go through a wet dielectric test. The applicable standards (ANSI C37.09 &IEC 56) give the voltage levels and durations (eg. 60KV for 10 sec.). Indoor circuit breakers are also subjected to rated full wave impulse withstand voltage. Both positive and negative impulse voltages have a crest value equal to the rated full wave impulse withstand voltage of the c.b. This value is function of the rated voltage of the c.b., the wave shape is 1.2x50 microsecond. If no damage or flashover is observed the breaker is considered to have passed the test. Chopped wave tests are performed on outdoor c.b. if the breaker voltage rating exceeds a certain level (eg.15.5KV). If the withstandability of the breaker is to be verified for switching, switching impulse tests are performed. They are similar to the full wave tests but with a different wave shape (eg.250/2500 microsecond). Short circuit rating test are also conducted on circuit breakers as type tests. The S.C. rating of a c.b. is proven by an extensive series of tests. To demonstrate the capability of the c.b. to interrupt the maximum specified current without injury to itself (when applied to grounded and ungrounded systems). The c.b. has to interrupt the rated symmetrical current in the presence of abnormal recovery voltage (it is defined in the appropriate standards). The general acceptable conditions of the breaker after successful testing are: the mechanical parts and insulators are to be in the same condition as before the test duty, it is capable of making and breaking its rated normal (full load) current at the rated voltage and shields fitted for bushings or arc control should be intact. Production tests will be conducted on each assembled unit to check for good workmanship and no errors in parts used. They will include, where applicable: nameplate checks, resistors, heaters and coil checks, control and secondary wiring checks, clearance and mechanical adjustment checks, mechanical operations, stored energy system tests, electrical resistance of current path, timing tests, low frequency withstand voltage tests on major insulation components and control/secondary wiring. Tests after delivery are performed to assure that no damage has been inflicted on the breakers during shipment. Field tests are divided into commissioning & startup to ensure that the breaker is in good condition and is suitable for energization & for routine maintenance that is conducted on the breaker at specific intervals during its life time. Conformance tests are certain type tests that are performed on certain breakers in a group of breakers as agreed upon by the purchaser & the manufacturer to reprove conformance of the design with the applicable standards. Different types of interrupting media & their properties: The interrupting media used in medium voltage circuit breakers are: air, oil SF6 and vacuum. The general properties of fluids used in arc extinguishing chambers in m.v. c.b. are: high dielectric strength of the gas or liquid, thermally and chemically stable, noninflammable, high thermal conductivity, low dissociation temperature, short thermal time constant, should not produce conducting material during arcing. Gases used so far in m.v. c.b. can be classified into simple (air) or electronegative (SF6). The main components of an oil c.b & how does it interrupt the arc: They are simple in construction. The major parts of a minimum oil c.b. excluding the poles are the base frame,the drive which is constructed as a stored energy opening and closing mechanism (the operating
mechanism). The opening spring of the stored energy mechanism is charged automatically during the closing action. The closing spring is charged either by means of an electric motor (is built into the drive housing) or by means of a removable crank. The pole constitute of insulating cylinder, arc chamber, fixed, guiding and moving contacts. It also has the gas expansion chamber, terminals, oil sump, oil draining and oil filling plugs and the oil level indicator. Arc Interruption in Oil: on separation of the moving contact from the fixed contact in the arc chamber, the current continues to flow through the vaporizing metallic current paths. The high temperature occurring under such conditions, decomposes the oil (which boils at 658°K), in the immediate vicinity and a gas bubble is formed (under high pressure). It consists of (from outside inward): wet oil vapour, superheated oil vapour, hydrocarbons (C2H2 at around 4000°K), the arc (approximate temperature 7000°K) as shown in fig. 2.7. As can be seen, the arc runs in a mixture of hydrogen (in both molecular and atomic states), carbon and copper vapour. The thermal conductivity is high due to the dissociation of hydrogen molecules into atoms. The thermal energy generated in the arc is primarily dissipated outward through the surrounding gas envelope to the oil. Also, the gas in the arc chamber escapes to the gas expansion chamber, so that a type of heat dissipation by convection is created, thus the rate at which heat is dissipating is increasing. Near current zero, the thermal power generated by the current (in the arc) approaches zero. If the heat dissipation outwards is sufficiently large, the temperature in the arc zone can be reduced in such a manner that the arc would lose conductivity and extinguish. An arc in hydrogen has a short thermal time constant, so that the conditions are favourable for quenching. There are two other situations that may occur under certain conditions: thermal Restriking of Arc, reignition. Thermal restriking is when the postarc current rises again and passes into the next half cycle of SCC, as the arc plasma heats up due to the insufficiency of heat dissipation to make conductance of the arc zone equal to zero. Reignition happens when therestriking voltage of the system causes a renewed formation of the arc, (after completion of the first interruption) and continuation of flow of current. The arcing chamber designs are either of the axial or radial venting type. Often, a combination of both are used in the design of minimum oil, MV CB's. The axial venting process generates high gas pressures and has high dielectric strength. This is used mainly for interruption of low currents. The radial venting is used for high current interruptions, as the gas pressures developed are low and the dielectric strength is low. The higher the current to be interrupted, the larger the gas pressure developed.
The major components of an air circuit breaker and the arc interruption: The basic characteristics for air magnetic circuit breakers are maximum rationalization and constructional simplicity. The major components of such breakers are: the poles, the arc chutes, the base frame, the operating mechanism, The operating mechanism construction and parts are similar to that of the minimum oil CB. The poles include: the fixed and moving arcing contacts, the fixed and moving main contacts, epoxy resin bushings, moving isolating contacts (main disconnects), pneumatic blow nozzles, the connections to the coils arc chute. The arc chutes contain: the blowout coils, the arc splitter plates, the arc runners, supporting insulating plates & the magnet pole pieces. The arc interruption in oil is due to the generation of hydrogen gas because of the decomposition of oil. Arc interruption properties of hydrogen are far superior to air, but air has several advantages which are: fire risk and maintenance difficulties associated with oil CB's are eliminated, arcing products in air are generally removed, whereas oil deteriorates with successive breaking operations, (e.g. formation of carbon) & heavy mechanical stresses set up by gas pressure and oil movement are absent. The arc in the air CB runs in a mixture of nitrogen, oxygen and copper vapour. When the current is greater than 100A, these gases get dissociated into atoms, which changes the characteristics of the arc, on account of the associated change in its thermal conductivity. The outcome of this is the fact that the discharge suddenly contracts and acquires an appreciably higher core temperature. The oxygen gas may remain dissociated, even when the current is in the order of 1 amp. The arc is extinguished by lengthening and increasing the voltage gradient. The arc discharge is moved upward by both thermal and electromagnetic (the blowout coils) effects. The arc is then driven into a chute consisting of splitters. The splitters increase the length of the arc even further, the interspaces between the splitters give improved cooling. Near current zero, the relative high resistance is obtained and the arc quenches. The main components of a vacuum c.b & the arc extinction in vacuum: The most significant characteristics for vacuum circuit breakers are: reduced overall dimensions and weight, long electrical life & low energy requirement for operation. In a vacuum system, pressure is maintained below atmospheric pressure. Pressure is measured in terms of mm of Hg (mercury). One mm of Hg is also known as one torr. The standard atmospheric pressure at 0° is 760 mm of mercury. It is now possible to obtain pressures as low as 108 torr. In a vacuum, the current growth cannot take place prior to breakdown due to formation of electron avalanches. However, if it could be possible to liberate gas in the vacuum by some means, the discharge can take place. In the vacuum arc the neutral atoms, ions and electrons do not come from the medium in which the arc is drawn, rather, they are obtained from the electrodes themselves through the evaporation of their surfaces. The major parts of Vacuum CB's are: the bottle supports, the bottles, as shown in fig. 2.8 below, which include: the fixed contact with fixed stem, the moving contact with moving stem, the bellows, the metallic arcing
chamber at 108 torr of vacuum, the insulators, the mechanical coupling to the operating mechanism, the operating rod, the contact force spring, the operating crank and operating lever, operating mechanism (in its mechanism housing) which includes: the electric spring charging motor, the breaker shaft, the closing spring, the opening spring, ratchet gear, tensioning shaft, coupling rod and any other
auxiliaries required like shunt trip, close release, auxiliary switches, etc. The arc in the vacuum is a metal vapour arc. As the current carrying contacts are separated, cathode spots are formed. For low current (below 10 kA), a highly mobile cathode spot (evenly distributed over the contact surface) is formed and for larger currents, a multiple number of cathode spots (the constricted form of the arc) are formed. These spots constitute the main source of vapour in the arc. In case of constricted arcs, there is the danger of local overheating at the arc roots, which can lead to restrikes. The contacts, therefore, have a hollow cylindrical shape with slits in the body of the contact to divert the current flow away from the axial direction that it would otherwise take. This generates a magnetic force that drives the arc along the circular contact end faces. In this case, the energy released in the arc root is distributed over the whole contact face. Local overheating is thus avoided. The drawing of the arc will be caused by the high electric field between the contacts or by the resistive heating produced at the point of application, or both. The material of the contacts can be copper bismuth alloy, silverbismuth or chromecopper alloy. The emission of electrons from the electrodes can be the result of any combination of the following: field emission, thermionic emission, secondary emission by positive ion bombardment, pinch effect. The high vacuum inside the vacuum interrupter has to be maintained throughout its life. The choice of a suitable contact material is of great importance because each time an arc is switched in vacuum, material is evaporated from the contacts and bound gas is set free. The metal vapour condenses in the form of pure metal onto the contacts and the vapour shields, where it acts as a getter for certain gases. If the getter action exceeds the rate at which the gas is freed, vacuum in the interrupter will be improved. The stability of the arc in the vacuum depends upon the contact material and its vapour pressure (the higher the vapour pressure at low temperature, the better the stability of the arc) and upon the circuit/load parameters, such as voltage, current, inductance and capacitance. It is known that current chopping in air and oil circuit breakers occurs because of instability in the arc column, whereas in the case of vacuum breakers, current chopping is a function of the vapour pressure and the electron emission properties of the contact material. When the arc interruption is over, the space between the electrodes is filled with vapour and plasma. The process by which this residue decays and by which the vacuum gap regains its dielectric strength, is known as the
recovery phenomena. At current zero, the cathode spot extinguishes within 108 seconds and after this the original dielectric strength is established quickly. Certain loads and switching conditions can cause overvoltages and force the use of either surge limiters (limit the magnitude of the overvoltage) or surge capacitors (reduce the rate of rise, lower the surge impedance and may reduce the transient recovery voltage frequency).
Current chopping, multiple reignition & virtual chopping: Current Chopping: is the sudden reduction of current to zero prior to a natural current zero. It is caused by arc instability, high frequency current oscillations and by the interrupter reestablishing dielectric strength, too quickly. This latter problem was overcome through the use of suitable contact material. Multiple Reignition: may occur under certain combinations of capacitance and inductance, on both the load and line sides of the breaker. This may cause transient frequencies and consequent overvoltages. Multiple reignition and its resultant overvoltages take place in the following circumstances: initially the CB interrupts the current in the first zero after separation of contacts, transient recovery voltage must rise at a rate faster than the reestablishment of dielectric strength in the contact gap, so that the reignition will occur (the gap starts to conduct again) & the CB must be in a position to interrupt the high frequency oscillatory current, which flows through the contact gap after the first reignition. Virtual Chopping: occurs in three phase systems and is the result of the reignition of one of the poles, which has previously interrupted. The reignition in one pole, causes high frequency current to flow (induced) in the other two phases. If the breaker extinguishes the arc at the artificial zero (high frequency current) and the magnitude of the power frequency current is at the full load value, virtual chopping is produced and overvoltages that are produced on the system, which are higher than those of current chopping and multiple reignition. Different types of SF6 circuit breakers: The different types are: The Puffer Interrupter, he Magnetic Interrupter & the SelfBlast Interrupter. The puffer type of interrupter uses a piston to compress SF6 gas through a nozzle arranged in such a manner as to exchange, at a high rate, the dielectric medium in the region of the arc. As the ionized gas has the ability to capture free electrons, has high thermal conductivity and has high insulating qualities, the ionized gas can quickly regain its insulating characteristics near current zero. In the magnetic type of interrupter, the arc plasma is moved by magnetic forces into a new region of fresh SF6, (rather than moving the SF6 into the arc plasma region). The higher the current being interrupted, the higher the force of the magnetic field. The interrupting characteristics depend on the rate at which the arc plasma encounters fresh SF6. This is a function of the current being interrupted. The selfblast type of interrupter uses the arc energy to heat the gas and increase its pressure. The gas is then allowed to
expand. With this expansion, the arc extinguishing process takes place in a manner quite similar to that of the puffer interrupter. The main components of an SF6 CB's are: the supports, the interrupters (poles), which consist of cylindrical insulating envelopes, moving and fixed arcing and main contacts, sliding contacts, upper and lower terminals, blow nozzles, operating and insulating connection shafts, activated alumina filter, pressure switches and charging valve/plug, the operating mechanism which includes: the enclosure, the charging motor, the closing and opening springs, closing cam and latch, the tripping latch, any auxiliaries like auxiliary switches, operating push buttons, operation counters and breaker contact indicators. The SF6 gas is colourless, odourless and nontoxic. SF6 is an electronegative gas,which means that it has a high affinity for electrons. Whenever the electron collides with the neutral gas molecule, it is absorbed to form a negative ion, the movement of which is much slower than the free electron. It also has excellent dielectric properties, arc quenching capability and good thermal/chemical characteristics. The dielectric strength is attributed to the large collision crosssection of its molecules and the many elastic collision mechanisms which allow an efficient slowing down of free electrons. The gas not only possesses a good dielectric strength, but it also has the unique property of fast recombination, after the source energizing the spark is removed. SF6 is considered very effective in the arc quenching process. Considering the chemical characteristics, we note first the manner in which SF6 decomposes, with increasing temperature and then recombines as the temperature decreases. The construction of the SF6 molecule is a sulphur atom at the center, electrons are shared with six fluorine atoms, symmetrically arranged. This structure, with its chemical bonds saturated, is chemically inert and highly stable. As the temperature increases, SF6 molecules first dissociate into sulphur and fluorine atoms. This occurs at around 2100K. As the temperature is further elevated,the sulphur gradually ionizes into positive sulphur ions, giving up electrons. These electrons are capturedby the fluorine atoms producing negative fluorine atoms (which are heavier than electrons).As the temperature is increased to 4000K, the energy level is high enough to cause stripping of extra electrons bonded to the fluorine atoms and the medium becomes more conductive. At about 6000K, the medium develops into a conductor by virtue of the abundance of free electrons, stripped from both the sulphur and fluorine atoms. Now, as the temperature in the arc core decreases along with the current, the population of free electrons decreases. At above 6000K the decrease is slow, below 6000K the fluorine atoms begin to capture the free electrons. At 3000K, nearly all of the free electrons are captured by the fluorine atoms producing negative fluorine ions. As the fluorine ions are much slower than the electrons, the current is reduced in proportion to the reduction in speed, when the electrons are captured. As all electrons are captured, the current is reduced to zero. SF6 gas becomes thermally hyperconductive at around 2100K, when molecular dissociation takes place. In any gas, when there is an arc, the majority of the current is carried in a welldefined arc core. Gases within the arc core are more dissociated, providing the source of electrons that carry the current. As the current increases, the temperature of the core increases within significantly lower plateaus of temperatures on either side of the arc core. With SF6, the arc is concentrated into a smaller region and the majority of the medium acts as a heat sink. As the temperature of the arc core falls with the decrease in current, the SF6 ceases to conduct current. The fact that the arc core is well defined, with the majority of the medium in a state of hyperconductivity,
the energy evacuation from the arc core is very efficient.
The thermal energy within the arc core is quickly transferred to the hyperconductive medium and to the surrounding heat sink region. The defining parameters for medium voltage circuit breakers: The defining parameters of medium voltage circuit breakers are: the voltage ratings (nominal, maximum and minimum), the 3phase MVA breaker rating, the rated current, the K factor (Max./Min. ratio), symmetrical interrupting ratings (at maximum, nominal and minimum voltage) in KA, the asymmetrical factor, the short time rating, the close and latch, the insulation level (power frequency, impulse level), the weight, the dimensions, the interrupting medium, the TRV capability, any arcing medium monitoring devices, circuit breaker closing time, tripping time, interrupting time, spring charging time, the control voltages (nominal and range), the spring charging current, close coil current requirement, the trip coil current rating and surges switching capabilities. Home of VePi