Circuit Breakers Technology

Circuit Breakers Technology • A circuit breaker is a piece of equipment which can Make or Break a circuit either manually or by remote control under normal conditions. • Break a circuit automatically under fault condition • Make a circuit either manually or by a remote control under fault or no-fault condition

Operating Principle • Circuit Breaker consists of Fixed and Moving contacts called Electrodes • Under normal operating conditions these contacts or electrodes remain closed and will not open automatically unless the system becomes faulty .These contacts can be opened manually or by remote control. • When a fault occurs in a circuit the trip coils of the circuit breaker get energized and the moving contacts are instantaneously pulled apart by some mechanism ,thus opening the circuit.

Simplified Diagram of a Circuit Breaker Control

Electric Arc of a C.B.

Electric Arc of a C.B. • When contacts of a circuit breaker starts separating, the contact resistance starts increasing. This increases the I2R losses, which is heat produced in the air gap between the two electrodes. • This heat increases the energy of the electrons in the contact areas and the ionized particles try to maintain the current when contacts are separated. This flow of charged particles from one contact to another is called electric arc. • The medium surrounding the arc also contains ions . • Due to this charged particles the arc continues even if the contacts are separated. • Even if the voltage (i.e., the potential gradient) across the arc is less, the arc may continue due to the ionization of the medium.

ARC in AC and DC circuits • DC arcs are to be interrupted by increasing the resistance between the contacts, in which resistance of the arc is increased so that the arc voltage can no longer maintain the current and the arc is practically extinguished. • Size of a DC circuit breaker increases as the voltage level increases.  AC arcs current reduces to zero in each cycle (2 times)  If the circuit breaker contacts are opened at a time when the current is passed through the zero, and the dielectric strength of the medium is rapidly built up so that the arc cannot strike again, then the arc can successfully be extinguished. Therefore, the size of a AC circuit breaker can be small compared to the same voltage rating of a DC circuit breaker.

Principles of Arc extinction (extinguishing) • For a low voltage circuit breaker, separating the contacts such that the arc resistance increases to a very high value, the P.D. between the contacts can not maintain the arc current (High Resistance Method). For a high voltage circuit breaker, this method is impractical since a separation of many meters will be required. • The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized ,the arc extinction is facilitated .This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts (Low Resistance Method).

Important terms  Arc Voltage – It is the voltage that appears across the contacts of a circuit breaker during the arcing period as the contacts are opened.  Recovery voltage – It is the normal frequency voltage that appears across the contacts of a circuit breaker after final arc extinction.  Rate of rise of restriking voltage (RRRV) – It is the rate of increase of a transient voltage across contacts. The RRRV depends upon: 1) Natural frequency of oscillation. 2) Recovery voltage.

Restriking Voltage • It is the transient voltage that appears across the contacts at or near current zero during arcing period. • If dielectric strength rise is greater than the rise of restriking voltage, then the arc will not restrike again.

Current Chopping • It is the phenomena of current interruption before natural current zero is reached. It occurs in air blast circuit breaker because they retain same extinguishing power irrespective of the magnitude of current to be interrupted. • When interrupting low inductive current, e.g. magnetizing current of a transformer, a rapid deionizing effect causes current to fall below its zero value before natural current zero is called current chopping.

Current Chopping

Resistance Switching

Resistance Switching The switching Resistor (R) is connected in parallel with the CB contacts. Current chopping produces high voltage oscillations which can be prevented by this method. During arc interruption CB contacts separate first and after arc gets extinguished ‘S’ opens depending upon the time delay provided to it. When the fault occurs the CB contacts open and arc is struck between them. Since R is in parallel with CB contacts ,a part of arc current flows through this resistance so arc current decreases and deionization rate increases. The arc resistance also increases so current through R increases. This processes continues till the arc current is insufficient to maintain the arc.

Circuit Breaker Rating  Breaking capacity – It is the r.m.s. current that a Circuit Breaker is capable of breaking at a given recovery voltage and under specified conditions.  Making Capacity – The peak value of current (including DC component) during the first cycle of current wave after closure of a circuit breaker is known as making capacity. The making capacity of a CB = 2.55 × symmetrical breaking capacity.  Short time rating – It is the period for which the CB is able to carry a fault current while remaining closed.  Normal current rating – It is the r.m.s. value of current which the CB is capable of carrying continuously at its rated frequency under rated specified conditions.

Requirements of Circuit Breaker

Classification of Circuit Breakers

Air Break Circuit Breakers Operation: In this Circuit Breaker the arc is elongated using arc runners and arc splitters so as to increase the resistance of the arc (see Figure for Air-Break-Circuit-Breaker). This increases the voltage required to maintain the arc and if the available voltage cannot sustain the arc ,the arc is baisically extinguished. At current zero , on the sine wave-shape, the recovery voltage across the contacts becomes less than the arc voltage and the arc gets extinguished (i.e., the voltage across the contacts is not capable to maintain the arc).

Air Break Circuit Breakers The magnetic energy stored in the protected circuit inductance at current zero is zero, hence arc interruption is easier. Since the contact resistance of a circuit breaker is practically zero, then the power loss across the circuit breaker’s closed contacts (I2R, V2/R, VI) is also zero. When the circuit breaker’s contacts progressively open, the arcresistance increases whereas the arc-current decreases. During this process, the power loss across the circuit breaker’s separating contacts (I2R, V2/R, VI) increases to a certain limit, but only to be decreased to zero when the arc is completely extinguished and the contacts are fully opened.

Air Break Circuit Breakers Used For low voltage and low current levels As voltage level increases, the size of the breaker becomes larger so not convenient for higher voltage and higher current levels. Air is used as a medium to extinguish the arc which has inferior extinguishing properties compared to SF6 or Vacuum circuit breakers. Operating control is manual as well as automatic. It is used up to 6.6 kV with a breaking capacity of up to 15MVA. Suitable for repeated economic operation because the medium of arc-extinction is air, so it is commonly used in Industrial Switchgears and Auxiliary switchgear in Generating Stations

Air-Break-Circuit-Breaker

Air Blast Circuit Breakers In this breaker, a high pressure air blast is used as an arc quenching medium. The contacts are opened and a flow of air blast is maintained by opening the blast valve. The air blast cools the arc and takes away the arcing products to atmosphere . This rapidly increases the dielectric strength of the medium between the contacts and the arc is extinguished and the flow of current is interrupted.

Air Blast Circuit Breaker(Radial Flow)

Air Blast Circuit Breaker(Radial Flow) Advantages  High speed of operation  Short arcing time  High speed reclosing  Less weight as compared to oil circuit breakers  Much less maintenance  No possibility of explosion

Disadvantages  Cost is more  Complete compressed air charging system installation is required  These breakers are more sensitive to RRRV.  Highly skilled persons are required for operation and maintenance ,

Air Blast Circuit Breakers (Axial Flow)

Axial Flow ABCB Operation:  Air is admitted in to the arc extinction chamber to push back the moving contact. This air blast takes away the ionized gases along with it. Eventually the arc is extinguished, since the high pressure replaced air has higher dielectric strength.  The design is such that the high pressure air expands into the low pressure (atmospheric) zone.  The air at high speed also removes heat from the arc, diameter of the arc is reduced, thus the arc is quenched.. Uses: 1.Arc Furnaces 2.Traction Systems

Modification of Air Blast Circuit Breakers

General Advantages of Air Blast Circuit Breakers

General Disadvantages of Air Blast Circuit Breakers

General Applications of Air Blast Circuit Breakers

Vacuum Circuit Breakers When the two contacts of this type of circuit breakers are separated in vacuum, an arc is struck and hot spots are formed on the surface of the contacts. These hot spots produce metal vapor in plasma. The amount of metal vapor in plasma depends on how rapidly the vapor is emitted from the contact surface which in turn depends on the arc current. Since the current is of an alternating nature, it crosses the zero values several times, so the rate of vapor emission also becomes zero, and the vapor that is previously emitted gets condensed. During this process the dielectric strength builds up rapidly and the restriking of arc is eventually prevented.

Vacuum Circuit Breakers

Vacuum Circuit Breakers Construction –  It consists of a fixed contact, a moving contact and an arc shield mounted inside a vacuum chamber.  The arc shield prevents deterioration of the internal dielectric strength by rapidly condensing the emitted metal vapour formed due to the hot spots on the plates.  The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of any vacuum-leakage.  A glass or ceramic vessel is used as an outer insulating body for interrupting chamber. Applications –  In outdoor applications where maintenance required is minimum.  In high voltage power Circuits from 22 KV to 66 kV.

Vacuum Circuit Breakers Advantages:  Compact in size  Reliable and long life  Heavy fault can be interrupted effectively  No gas is generated after arc extinction operation  Operation is not noisy  Arc energy is low  No risk of fire

Disadvantages: Vacuum has to be maintained at desired level always

Summary: When the two contacts of a circuit breaker are separated in vacuum, arc is struck and hot spots are formed on the surface of the contacts. These hot spots produce metal vapour and plasma. At current zero the rate of vapor emission becomes zero. The vapor already emitted is condensed. During this process the dielectric strength builds up and the restriking of arc is prevented.

Oil Circuit Breakers

Bulk Oil Circuit Breaker

Oil Circuit Breakers (Working) • Under normal working conditions, the fixed and moving contacts are closed. On the occurrence of a fault, the moving contacts come down and an arc is struck between the contacts. The oil between the contacts gets decomposed and hydrogen gas bubble is formed around the contacts. The hydrogen gas cools the arc and the turbulence effect causes deviation and lengthening of the arc. The deionization of the medium between the contacts takes place and at some critical length of the gap between the two contacts , the arc is extinguished.

• The hydrogen gas bubble produces a very high pressure in the oil tank. The tank is therefore made strong to withstand such a large pressure. The oil moves upwards when hydrogen bubble is formed. The air is present between the oil level and the tank top surface and acts as cushion and absorbs the mechanical shock produced due to upward oil movement during the arcing.

Bulk Oil Circuit Breakers • This breaker makes use of oil for quenching the arc. • The circuit breaker which uses more oil or which is bulky is called bulk oil circuit breaker. • The construction is simple and it consists of fixed and moving contacts enclosed in a strong weather-tight earthed tank containing oil up to a certain level and an air cushion above the oil level. Application: These breakers are used up to 11 KV with an interrupting capacity of 250MVA.

Bulk Oil Circuit Breakers Advantages

Disadvantages

• oil has high dielectric strength • Oil absorbs arc energy while decomposing • Good cooling property of the gas formed due to decomposition • It acts as an insulator between the live parts and earth.

• Long arcing time • Do not permit high speed of interruption • Arc interruption control can be obtained only by increasing the length of the arc.

Low & Minimum Oil circuit Breakers Operation: • When the arcing contacts are separated in the minimal oil chamber, arc is formed. • The heat of the arc decomposes the minimal oil and insulating gases are formed in the arc extinction device. The useful gases expand due to heating of the arc. The gases flowing near the arcing contact zone cause cooling and splitting of the arc and the arc is effectively extinguished.

Low & Minimum Oil circuit Breakers • The supporting outer chamber and top chambers are made of good insulating porcelain. Hence, the clearance between the live parts is small and requires less quantity of oil, hence the breaker is called Minimum Oil Circuit Breaker. The chambers are completely filled with oil. The oil from the upper chamber does not come to lower chamber. • The fixed and moving contacts are enclosed in a quenching chamber. The moving main contact makes a late pressure contact with the fixed main contact, while the arcing contacts make an early sliding contact to each other. The common operating rod (gang) is operated by operating mechanism for all the three poles to operate simultaneously. • The voltage ratings are from 3.6kV to 420 KV, therefore minimum oil circuit breakers are available in for all voltages and highest breaking capacities and they are preferred in almost all protection schemes.

Low & Minimum Oil circuit Breakers Advantages Requires less quantity of oil Requires smaller space Maintenance is less Cost per breaking capacity in MVA is less Suitable for both manual and automatic operation

Disadvantages Possibility of fire and explosion. Difficult to remove gases from the space between contacts. Oil deteriorates rapidly due to carbonization Smaller quantity of oil, so carbonization increases.

Maintenance of Oil Circuit Breakers

SF6 circuit Breakers (Properties of SF6 gas)  Electronegative – It is the ability of an atom to attract and hold electrons.  Sulfur hexafluoride SF6 is electronegative gas. It forms negative ions, their negative ions are very heavy and immobile, so they do not flow easily.  SF6 gas has high dielectric strength when regaining deionization by attracting and holding the free electrons caused by the arc.  Rate of rise of dielectric strength is very high.  Can be liquefied and stored in steel tanks  Dielectric strength increase linearly with pressure.  Gas is inert, therefore contacts will not get eroded.  Gas is inflammable , Colorless ,odorless, Non-toxic and very heavy.  Thermally stable up to 55 degrees

Sulfur Hexafluoride Circuit Breakers

Operation of SF6 Circuit Breakers • Under normal operating conditions the contacts are closed . • On occurrence of fault contacts are opened. The movable contact moves away from the fixed contact. • The arc is struck between the fixed and moving contacts. • High pressure SF6 gas now flows over the arc and it absorbs the free electrons from the arc. • This builds up the dielectric strength across the gap very fast and the arc is practically extinguished.

Advantages and Disadvantages of SF6 Breakers Advantages

Disadvantages

 Silent operation, compact size  Very short arcing time  No risk of fire  No reduction in dielectric strength due to operation  No current chopping problem  Can interrupt larger currents  Suitable for explosive environment due to totally enclosed body

 Costly  Requires conditioning of SF6 gas from time to time  SF6 gas is suffocating ,so its leakage can cause suffocation of the persons in surrounding areas.  Special facilities are required for transporting gas  Additional equipment are required for reconditioning

Circuit Breaker Controls The following are some different types of control equipment and devices required for the successful operation of circuit breakers:  Relays – These are required devices to give a trip signal to circuit breakers in case of fault conditions. Different types of relays are available like over current, over voltage ,under voltage, loss of excitation, reverse power,.. etc.  Sensor equipment are also required to check the condition of circuit breakers arc extinguishing medium.  Among the controls are the pressure sensors to sense the pressure of the air in case of air blast circuit breakers .  In case of Sulfur hexafluoride circuit breakers also the pressure sensors are required.  In case of vacuum circuit breakers sensors are also required devices to check the vacuum level inside the sealed interrupting vacuum chamber of the breaker.

Isolators

Isolators Isolator (disconnecting switch) operates under no load condition. It does not have any current breaking capacity or current making capacity. Isolator is not even used for breaking load currents. Isolators are used in addition to circuit breakers, and are provided on each side of every circuit breaker to provide isolation and enable maintenance. Sequence of operation • While opening –Open circuit breaker first and then isolators • While closing –Close isolators first and then close circuit breakers

Reclosers Application of Reclosers (Circuit Breakers) in distribution systems requires selection of its ratings such as minimum trip current, continuous current, symmetrical interrupting current etc. For a single phase system, single phase Reclosers can be used whereas for a three phase system, one three phase Recloser or three single phase Reclosers can be used. Reclosers have to be selected by considering the following factors: • Voltage Rating. • Continuous current Rating: This is the maximum load current the Recloser has to carry. • Maximum Symmetrical Interrupting Rating: The maximum symmetrical fault current should not exceed this rating. • Minimum Tripping current (sensitivity of Recloser ): This is the minimum fault current that a Recloser will clear. It is equal to two times the continuous current rating with usually tolerance of ±10%.

Example on Recloser Selection Consider a three phase distribution system with a single phase tap as shown in the Figure below. Maximum load on this single phase tap is 40A and that on three phase line is 200A. Fault currents at F1,F2, F3 and F4 are also shown. Select the ratings of all Reclosers.

Automatic Reclosing Many faults (80-90%) in the overhead distribution system like flash over of insulators, crow faults, temporary tree contacts, etc. are temporary in nature. Thus, taking a feeder or line permanent outage may lead to unnecessary long loss of service to customers. Hence, many utilities use fast automatic Reclosers for an overhead radial feeder without synchronous machines or with minimum induction motor load. Presence of synchronous machines will require additional problem of Synchro-check to be addressed. The almost universal practice is to use three and occasionally four attempts by Reclosers to restore service before lock out .

Automatic Reclosing Subsequently, energization is initiated by manual intervention. The initial reclosure can be of high speed (0.2 - 0.5sec) or delayed speed (3 - 5 sec). This allows for the deionization time for fault arc. If the temporary fault is cleared, then the service is restored. Otherwise, the relay again trips the circuit breaker. There may be one or two additional time delayed reclosures are programmed on the reclosing relay. Typical triping schedule might be instantaneous, followed by 30 sec, or 35 sec, followed by 15 sec. If the circuit still continues to trip, the fault is declared as permanent and the Recloser is locked out. Reclosers use three phase and single phase oil or vacuum circuit breakers for overhead distribution lines. With underground network, faults tend to be more often permanent and Reclosers are not recommended. In case of large synchronous motors, distributed generators or induction motor loads, it is recommended that sufficient time is allowed for under frequency relays to trip these sources of back emf out-of-the-circuit.

Sectionalizers

Sectionalizers Sectionalizers are circuit breakers used in the bus bars so that a fault on any section of the bus bar will not cause complete shutdown of power. The central circuit breaker between the two sections of the bus bars in the previous shown Figure is acting as a sectionalizer breaker. Advantages:  If a fault occurs on any section of the bus bars, then that section is isolated from other sections without affecting the whole system.  Fault current is much lower than in case of un sectionalized system, as the fault is only fed from one section.  Repairing and maintenance on one section can be carried out by de energizing that section only, eliminating the possibility of complete shutdown of the whole system.

Fuses Fuse is a device used in circuits for protecting electrical equipment against overloads and /or short circuits. Fuse element or fuse wire is that part of the fuse device which melts when an excessive current flows in the circuit and thus isolates the faulty device from the supply circuit. Desirable qualities of fuse elements: 1.Low melting point 2.Low ohmic losses 3.High conductivity 4.Free from deterioration due to oxidation 5.Low cost

Thermal Characteristics of Fuses As the magnitude of the fault current increases in a circuit, melting time of fuse link reduces. It should be obvious that larger magnitude currents will lead to higher power dissipation (I2R) in the fuse and hence faster rise in temperature of the melting element. This would imply that melting time of the fuse should be inversely proportional to magnitude of square of the fault current. The relationship between the magnitude of the current that causes melting and the time needed for it to melt is given by the fuse's melting time current characteristics (TCC). To cover a wide range of currents and operating times, TCC is plotted on a log-log paper.

Current-Voltage-Time relationship of NonCurrent Limiting Expulsion Fuse

Current-Voltage-Time relationship of Current Limiting Fuse

Non-Current Limiting Expulsion Fuse The expulsion type fuse is used where expulsion gases cause no problem, such as in overhead circuits and equipment. These fuses can be termed as current zero awaiting types; and the function of interrupting medium is similar to that of an ac oil or SF6 circuit breakers. The temperature of the arc is in the order of 4000-5000Ko. At this temperature, special materials located in close proximity to the fuse element rapidly create gases. Preferred gas-generating-materials are fiber, melamine, boric acid and liquids such as oil or carbon tetrachloride. These expulsion gases help to create a high pressure turbulent medium surrounding the arc, thus when the current does reach to the zero point and the arc channel reduces to a minimum; the ablated gases rapidly mix with remaining ionized gas and thereby deionize them as well as remove them from the ‘arc area'. In turn, this leads to rapid build up of dielectric strength that can withstand the transient recovery voltage (TRV) and steady state power system voltage.

Non-Current Limiting Vacuum Fuse Vacuum fuse is a non expulsive fuse but still a current zero awaiting type. The design, operation and current-voltage-time relationship of this fuse closely matches with that of an expulsion fuse. The main difference is that it is a completely sealed unit and no expulsion action. Interruption occurs because of rapid dielectric strength build up that occurs in the vacuum after current zero is reached. The operation of this type of fuses is somewhat similar to the operation of vacuum circuit breaker.

Current Limiting Fuse Basically, the current limiting fuses attempt to constrict the arc and it is cooled by sand. A typical current limiting fuse is shown in the figure . In this case, the fusible element is very long to stretch the arc. The fusible element is completely surrounded by a filler material, typically silica sand to contain the arc as well as maintaining a very high pressure in the long restricted arc area caused by the practically simultaneous melting of the full length of the element. This then allows the fuse to produce a very high resistance in the circuit in a very short period of time (typically hundreds of µsec).