INTRODUCTION:

CIRCUIT BREAKERS

INTRODUCTION:

It is a common experience at homes to see a spark in the switch when putting off a slightly heavy load. This spark is seen because the air gap between the contacts of switch gets ionised and starts conducting and this spark is extinguished by the natural flow of air. But at higher system voltages the natural flow of air is not sufficient to extinguish this spark (also known as arc) which assumes dangerous magnitudes. Also a fuse cannot be used because it cannot do the switching function. All these factors necessitate the use of special device in the power system that is switchgear (Circuit breaker), a contact system with suitable instruments of arc quenching medium and operated by high speed mechanisms.

With the ever-increasing demand for energy, power generation, transmission and distribution have become very important and circuit breaker as a vital component in the transmission system achieves greater significance.

A circuit breaker is defined as a mechanical device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specific time and breaking currents under specific abnormal circuit conditions such as those of short circuit. The circuit serves two basic purposes:

1. Switching during normal operating conditions for the purpose of operation and maintenance.

2. Switching during abnormal conditions such as short circuit and interrupting fault currents.

The first function mentioned above is relatively simple as it involves normal currents, which are easy to interrupt. The second function is complex as the fault currents are relatively high and they should be interrupted automatically with a short time of the order of few cycles. In order to avoid damage to the equipment in power system, protective relays are used. This relay senses the fault and sends signal to the circuit breaker to open. The circuit breaker opens and isolates faulty sections.

CLASSIFICATION OF CIRCUIT BREAKERS:

A) based on voltage:

Circuit breakers can be arbitrarily grouped using many criteria such as: the intended voltage application

1.Low voltage circuit breakers for applications less than 1000 V

2.High voltage circuit breakers for applications greater than 1000V

B) based on location:

The circuit breakers are classified based on the location of the installation as

1. Outdoor Circuit Breakers

2. Indoor Circuit BreakersC) based on external design

From the point of view of their physical structural design, the outdoor circuit breakers can be identified as either dead tank or live tank type circuit breakers.

Dead tank circuit breakers are defined as A switching device in which a vessel(s) at ground potential surrounds and contains the interrupters and the insulating media . Dead tank circuit breakers require multiple low voltage bushing type current transformers at both, the line side and the load side of it.

Live tank circuit breaker is defined as A switching device in which the vessel(s) housing the interrupter(s) is at a potential above the ground . The live tank circuit breakers are cheaper (with no current transformer) and they require less mounting space.

D) based on interrupting media:

In the evolutionary process of the circuit breaker technology, the main factor that have dictated the overall design parameters of the design are, the interrupting and insulating medium that is used.

The choice of air and oil, as the interrupting media, was made at the turn of the century. But the two newer technologies, one using vacuum and the other sulphur hexafluoride (SF6) constitute todays leading technologies.

Air Blast circuit breakers:

In the design of air blast circuit breakers the interrupting process is initiated by establishing the arc between two receding contacts and by simultaneously opening a pneumatic valve which produces a blast of high pressure air that sweeps the arc column subjecting it to the intense cooling effects of the air flow and thereby quenching the arc.

Oil circuit breakers:

In oil circuit breaker the arc drawn across the contacts is contained inside the interrupting pot and thus the hydrogen bubble, formed by the vaporised oil (gas) due to arc energy is also contained inside the chamber. As the contacts continue to move a series of such pots will be opened, when ever the moving contact rod moves away from the fixed contact allowing the pressurised gas blows along the arc column and subjecting it to intense cooling. Thus arc is quenched.

Vacuum circuit breakers:

Vacuum circuit breakers are most popular breakers in the distribution voltage range 33 kV and below today due to their compactness and least maintenance.

The vacuum circuit breaker employs the principle of contact separation under vacuum. Though there will be an initial arc due to field and thermionic emissions, soon it will die away as there is no further ionization because of vacuum.

SF6 gas circuit breakers:

SF6 gas circuit breakers are the most widely used ones today in transmission voltage range 66 kV and above. SF6 gas breakers have many advantages such as high reliability, compactness and less maintenance.

SF6 is an electronegative gas and is excellent for use as an arc-quenching and insulating medium. The principle is similar to that of ABCBs.

VACUUM CIRCUIT BREAKERS:

Although the advantages of interrupting the arc in a vacuum were recognised as early as the 19th century, they did not find wide application till a few years ago. This was because the knowledge of problems in material science, vacuum technology and plasma physics was not sufficiently advanced to provide solution to the many technological problems encountered in design and construction of a reliable vacuum circuit breaker.

High vacuum has two outstanding properties.

1) The highest insulating strength known.

2) When an A.C circuit is opened by the separation of contacts in a vacuum, interruption occurs at the first current zero with the dielectric strength across the contacts building up at the rate of thousands of times higher than that obtained with conventional circuit breakers.

These properties obviously make the vacuum circuit breakers more efficient, less bulky and cheaper.

Vacuum Arc:

When contact separation takes place in air, the ionised molecules air are probably the main carriers of electric charges and responsible of low breakdown value. In vacuum arc the neutral atoms, ions and electrons must ultimately come from the electrodes themselves, and not from the medium in which the arc is drawn. As the current carrying contacts are parted, the current concentrates at a few local high spots on the contact surfaces. Normal conduction through metal ceases when the last bridge between the two contacts is vaporised. In the vacuum arc the emission occurs only at the cathode spots and not from the entire surface of the cathode. For this reason vacuum arc is also known as the cold cathode arc.

Vacuum Arc Stability:

It has been found that the arc stability depends on:

1) The contact material and its vapour pressure

2) Circuit parameters such as voltage, current, capacitance and inductance.

In low current circuits most of the evaporation takes place at dielectric points known as cathode spots, at higher currents the gas evaporates from cathode and anode spots.

Vacuum Switch Construction:

The vacuum circuit breaker is a very simple device as compared to the oil or air blast circuit breakers. Two contacts are maintained inside an insulated vacuum-sealed container. One is fixed and the other may be moved through a short distance. The metallic shield surrounds the contact and protects the insulating container. A typical assembly of vacuum switch is shown in figure below.

It consists of two sub assemblies:

a) Vacuum chamber

b) Operating mechanism.

Vacuum Chamber:

It is made up of synthetic material such as urethane foam which is enclose d in an outer glass fibre reinforced plastic tube or of simple glass or porcelain, two contacts, a metal sheet and a metal bellows are sealed inside the chamber. For reasons already mentioned the contacts must be pure and thoroughly degassed.

The metallic bellows, generally made of stainless steel is used to move the lower contact and provides a gap of the order of 5 to10mm depending upon the application of the switch. The design of the bellows is of particular significance because the life of the switch depends upon the ability of this part to perform repeated operations satisfactorily.

One end of the fixed contact is brought out of the chamber to which the external connections can be made. Similarly provision is made with the lower contacts also for external connection but it is firmly joined to the operating rod of the mechanism.

Operating Mechanism:

The lower end is fixed to a spring operated or a solenoid operated mechanism so that the metallic bellows inside the chamber are moved upward and downward during closing and opening operations respectively. The contact movement should be such as to avoid bounce. However there should be sufficient pressure to allow the responsible wipe for a good connection between the two contacts.

SF6 GAS CIRCUIT BREAKERS:

FEATURES OF SF6 GAS BREAKERS:

SF6 is about 5 times heavier than air. It is chemically very stable, odor less, inert, non-inflammable and non-toxic. The gas has a high dielectric strength and out standing arc-quenching characteristics.

In SF6 the arc voltage remains low until immediately before current zero so that the arc energy does not attain a high value. Moreover the arc time constant for SF6 is also very low. Furthermore, SF6 and its decomposition products are electronegative, permitting electron capture at relatively high temperature. Thus the dielectric strength rises rapidly and enables the breaker to withstand the recovery voltage even under extreme switching conditions. Owing to the low contact erosion in SF6 and almost negligible decomposition of the gas in arc, the breaker can be operated for several years without having to be opened for the purpose of overhauling.

DIELECTRIC PROPERTY OF SF6 GAS:At atmospheric pressure the dielectric strength of SF6 is about 2.5 times that of air. This gas is strongly Electro-negative, which means that free electrons are readily moved from a discharge by the formation of -ve ions through processes by which a free electron is attached to a neutral gas molecule. The attachment may occur in two ways

(a). As direct attachment SF6+e=SF6

(b). As dissociative attachment

SF6+e=SF5+F-

The resulting ions, which are heavy and relatively immobile, are thus ineffective as current carriers so that ionized SF6 has a high an electric strength as unionized gases such as N2 at equal density.

QUENCHING PROPERTIES OF SF6 GAS:

The extinction of A.C arc at the instant of current zero is primarily influenced by the speed with which the dielectric strength in the contact gap regenerates immediately before and after the passage of current zero. Its efficacy as an arc-quenching medium can be explained by the low dynamic time constant of arcs drawn in it.

SF6 has a favourable thermal characteristic which is a function of temperature i.e. the thermal conductivity is low between 3000k and 7000k. The low time constant of SF6 is due to its ability for free electrons to be captured by molecules of SF6 gas. These SF6 ions surround the arc and form an insulating barrier. This reduces the diameter of arc column and hence results in reduction of time constant, which aids arc quenching.

Mayers equation for the limiting value of recovery voltage after the current has passed through zero above which arc restrikes is given by

E = Ea / 2*(3*(Awo)2 A=H.

Ea = arc voltage

Wo = 2*(*f where f = natural frequency of the mains

H = arc time constant

Since H is 100 times smaller for SF6, than air, for the same value of limiting voltage the natural frequency of mains may be 100 times greater. In other words SF6 breaker can withstand severe RRRV, and thus are most suitable for short line faults without switching resistors, and can interrupt capacitive currents with out restriking.

BEHAVIOUR OF SF6 GAS IN ARC

The high temperature of arc causes all molecular gases, including SF6 to be decomposed into atoms, electrons and ions. These atomic components do not recombine completely to the original SF6 gas on cooling. They form low molecular gaseous sulphur fluorides and compounds with the contact metals. Extensive tests have shown that the percentage of gaseous decomposition products is extremely small. These products and any other secondary gaseous reaction products are removed from the gas circuit by filters containing activated aluminium oxide when the gas is pumped back into the high-pressure tank. The bimetal fluorides are deposited as a thin non-conductive and harmless layer

ESSENTIAL PARTS OF SF6 CIRCUIT BREAKER:

TANK:

In case of dead tank breakers the distance between the conductor and earthed parts inside the tank is very much reduced due to better insulating properties of SF6 gas. Even at atmospheric pressure, the insulation distances are sufficient to with stand nearly twice the rated voltage to earth. Special neoprene gaskets are used in the inspection covers for ensuring protection against leaks. The rotating/axial shaft which transmits mechanical motion from mechanism outside of the tank to inside moving parts is sealed by Special dynamic seal arrangements which are unaffected by change in ambient temperature.

INTERRUPTER UNITS:

Organic insulation Poly-tetrafluoroethylene that is resistant to arcing and produces negligible gas contamination is generally used for arcing nozzles. Because of its superior arc interrupting ability SF6 gas flow in the orifice is very small, also the flow producing pressures require for arc extinction are only 1/3 to 1/2 of the values required for air.

The important parts of the interrupter are

1) Main reservoir containing pressurised gas

2) Blast valve and control mechanism

3) Piping for gas under pressure.

4) Axial flow interrupter units

5) Tripping spring

Capacitor units are placed across each brake to ensure equal voltage distribution.

OPERATING MECHANISM:

In operation the tripping spring drives the moving contacts and simultaneously opens the valve of the pressure reservoir. The gas under pressure flows into breaking chambers and extinguishes the arc. At the end of the operation a mechanism releases the valve of the pressure reservoir which is closed by the action of a set of springs.

OPERATINGMECHANISMS:

INTRODUCTION:

Circuit interrupters are primarily mechanical devices for closing and opening electrical circuits. Because breakers are depended on to protect power systems, high reliability is an essential requirement. Breakers must remain closed until called on to open, even when short circuit currents exert strong magnetic opening forces. During their closed periods they must continue to carry normal load currents without excessive power loss (or) heating. They must be able to close against very high momentary currents and remain closed.

All of these requirements and difficulties, which are highly challenging to circuit breaker designers, have led to the use of various types of operating means. These have included spring and latches, pneumatic mechanism, hydraulic mechanism and mechanisms with other sources of power.

SPRING MECHANISMS:This type of mechanism is commonly found in some medium voltage outdoor type circuit breakers and practically in all medium voltage indoor type breakers.

As its name suggests the energy of this mechanism is stored in the closing springs. The stored energy is available for closing the circuit breaker upon command following the release of a closing latch.

The spring mechanism in its simplest form consists of a charging motor and a charging ratchet, a closing cam, closing springs and a toggle linkage. The charging motor and ratchet assembly provides automatic recharging of the closing springs immediately following the closing contact sequence. To release the spring energy either an electrically operated solenoid closing coil (or) a manual closing lever is operated.

Opening of the contacts can be initiated either electrically (or) manually. However the manual operation is generally provided only for maintenance purposes. When the tripping command is given the trip latch is released freeing the trip roller carrier. The opening springs, which are connected, to the main operating shaft provide the necessary energy to open the contacts of the circuit breakerPNEUMATIC MECHANISMS:

This type of mechanism is best suited for air blast circuit breakers because the pressurized air is already used for insulating and interrupting.

In this mechanism a piston is used to drive the closing linkages and to charge a set of opening springs. To close the circuit breaker, high-pressure air is applied to the underside of the piston by opening a three-way valve, the piston moves upwards transmitting the closing force through a toggle arrangement. In addition to closing the contacts the mechanism charges a set of opening springs and once the contacts are closed a trip latch is engaged to hold the breaker in the closed position.

Opening is achieved by energizing trip solenoid which in turn releases the trip latch thus allowing the discharge of the opening springs which forces the contacts to the open position.

HYDRAULIC MECHANISMS:

Hydraulic mechanisms are in reality only a variation of the pneumatic operation, the energy in most cases is stored in a nitrogen gas accumulator and the incompressible hydraulic fluid becomes a fluid operating link that is interposed between the accumulator and a linkage system.

They operate at much higher pressures than the pneumatic cylinders. Leakage is an important factor because the fluid is conserved and recycled.

BUSHINGS:

These contain SF6 gas at a pressure of 2kgs/sq.cm under very much simpler than the condenser bushing. They contain a hollow conductor, a fi00000xing forge, the upper and lower porcelain insulators and the springs, which holds the assembly together. The SF6 gas in bushing communicates with that in the tank through small holes in the upper part of the hollow conductor. The gas in the bushing is thus unaffected by any disturbances in the tank at the instant the current is broken.

GAS SYSTEM:

A compressor sends the gas back after each break to the high-pressure reservoir. Being a closed circuit, no gas escapes to the atmosphere. An auxiliary reservoir of SF6 gas 14kg/sq.cm is located below each tank, containing enough gas for 4 consecutive breakers without the need for starting up the compressor.

PUFFER TYPE BREAKER:

The above figure illustrates the mode of operation of a puffer type breaker. The drawings show the contacts in the closed position, opening of the contacts with the gas being initially compressed, the instant of arc extinction with the hot gases flowing through the hallow contacts and the fully open position. Among the known puffer type breakers this design has the following outstanding properties.

Taking full advantage of this high dielectric strength of SF6 gas uses a relatively small contact clearance. Together with short contact travel from contact separation to the extinguishing position, this results in a small arc energy, high extinguishing ability and short interrupting time. A wide clearance between the arc and the insulation material, the transport of plasma and arcing products to regions with zero or low dielectric stress and a contact clearance not bridged by insulation material make use of the full dielectric properties of SF6 gas during and after arc extinction.

SYSTEM REQUIREMENTS OF CIRCUIT BREAKERS

With the phenomenal growth of power systems (EHV&HV) the circuit breakers are called upon to perform complex duties. Besides the ability to handle short circuits, recent studies into this field have thrown much light on switching duties, which often place several demands on breakers.

Short Circuit (Terminal Fault):

These are the faults that may develop anywhere on the power systems. Usually the short circuit is all most inductive and the restriking voltage appearing across the contacts at the instant of arc interruption is characterized by an oscillation, and the frequency of which ranges from few cycles to few hundreds of cycles. It is on this frequency and the peak value, depends the magnitude of severity imposed on circuit breakers.

As the short circuit occurs, the short circuit current attains high value. The circuit breaker contacts start separating after the operation of protective relays. The rms. value of current at the instant of contact separation is called breaking current of circuit breaker and is expressed in kA. If a circuit breaker closes on existing fault, the current would increase to a highest value during the first half cycle. This peak value is called making current.

Now let us consider the voltage between contacts. When contacts are closed the voltage between them is zero. As the circuit contacts separate the voltage across contacts increases due to an increase in arc resistance. Finally when arc gets extinguished a high frequency voltage transient appears across the contacts. This high frequency transient voltage tries to restrike the arc. Hence it is called transient recovery voltage. If the dielectric strength of the medium between the contacts does not build up faster than the rate of rise of transient recovery voltage, the breakdown takes place causing reestablishment of the arc if the contact space breaks down within a period of one-fourth of cycle of the initial arc extinction, the phenomenon is called reignition. If breakdown occurs after one-fourth of cycle it is called restrike.

L

Vs(t) B

C

F

Vn

B = Breaker

Vn = Voltage of source

L = Inductance on source side

F = Fault at terminal of B

Vs(t) = Voltage across breaker

t = Time in micro sec

Vs(t)Simplified TRV form

2Vn

t

Short Line faults (kilometric faults):

The fault occurring between a distance of few kilometers to a few hundred kilometers from the circuit breaker are called short line faults. Such faults are characterized by high frequency of restriking voltage of the order of 10 to 100 kHz depending upon the length of the line and the location of the fault.

wL

Vs(t) wl

Vn

C V1 V2

wL = impedance of source = 2( fL

( 1= impedance of l km length of line

l = Length of line between breaker and the fault, km

lam = impedance per km length of line

(2Vn

Vs(t)

Fig. Condition representing Short-Line Fault

Interruption of Low Magnetising current (Current Chopping):

When interrupting low inductive currents such as magnetizing currents of transformer, shunt reactor, the rapid deionization of contact space and blast effect may cause the current to be interrupted before its natural zero. This phenomenon of the interruption of current before current zero is called current chopping. At the moment of current interruption the energy stored in inductance is diverted to the capacitance.

Such a transient voltage having high Rate of Rise of Recovery Voltage appears across the contacts, unless the arc continues. If it restrikes a further chop may occur or several chops may occur before the current is finally interrupted, some times circuit breaker may fail to clear.

L

B

Vn V

C L

i

Circuit diagram for illustrating interruption of low inductive currents.

Fig

Switching of Capacitor Banks:

While opening capacitor banks the reignition and restrike can occur in an interrupter. Capacitor banks are connected in the network to provide reactive power at leading power factor. The voltage across a capacitor cannot change instantaneously .The current supply to the capacitor is generally of small order and the circuit breaker can interrupt such currents invariably at the first current zero. Due to 90 degrees phase difference, the voltage across the capacitor is maximum. The recovery voltage of the order of twice Emax appears across circuit breaker pole after half cycle of current zero. Therefore, a restrike is possible. If a restrike occurs the voltage across the interrupter raises upto 4 p.u due to one restrike and upto 6 p.u with second restrike, thus causing insulation failure. Hence, the circuit breakers used for capacitors duty should be restrike free.

Switching of Unloaded Transmission Lines and Unloaded Cables:

Unloaded transmission lines and unloaded under ground power cables take capacitive currents. The magnitudes of capacitive currents encountered in practice are:

1. Unloaded lines: Charging current from 10A-500A.

2. Under ground cables: Charging current from 10A-500A.

During the opening operation, the restrike phenomenon is possible as discussed in above case. The circuit breaker used for a particular application should be capable of performing opening and closing operation without getting damaged.

Phase Opposition Switching:

When two systems are to be synchronised, it may happen that the breaker opens on non-synchronous conditions. If V1 and V2 are not in synchronism during opening of breaker the likely waveform of transient recovery voltage is as shown in the figure below. Under certain conditions, the voltage across a pole may reach three times phase voltage or in extreme cases it may reach twice line to line voltage. The circuit breakers used for synchronising should be capable of opening satisfactorily under non-synchronous condition. In such cases the restrike voltage may be higher than that of short circuit duties.

Vs(t)

t Occurrence of Switching Surges and methods to control them:

In extra-high-voltage networks the expenses for the insulation of equipment and lines constitute a major item in the overall costs for the system. The insulation requirements of electrical equipment are determined by the transient over voltages. Over voltages occur due to several causes, important causes are:

Lightening surges - originate with a lighting stroke on overhead line or outdoor equipment. The wave travels along conductors and reaches the substations. The external clearances of the electrical installations are designed to with stand lighting surges of specified magnitudes. For voltage upto 275kV rms phase to phase, the design of installation and apparatus insulation with respect to clearances voltage stress on surface of conductors, insulation design, impulse with stand level etc. are generally based on the required basic lighting impulse with stand level.

Switching surges occur with the opening or closing of long lines under various switching operations due to transient exchange of energy between the inductance and capacitance.

Occurrence of Over Voltages:

Over voltages mainly occur due to the closing of unloaded long transmission lines. When circuit breaker is closed on unloaded line, the voltage is suddenly applied to the transmission line. This gives rise to a travelling voltage wave, which gets reflected and the successive reflected waves give rise to voltage as high as six times normal voltage. Hence for EHV & UHV lines, this voltage reaches very high magnitude.

Switching voltages are proportional to the system voltages. These voltages are composed of two elements, one being steady state portion, a function of operating frequency and other being transient.

The influence of various parameters on the magnitude of the course of closing surge voltage can be investigated. Short circuit power, line length, compensation degree, pre-loading, periodical scatter of circuit breaker pole, size of closing resistor can be investigated thus in their influence on the surge voltage factors.

Steady State Voltage increase:

The short circuit line of the supply system and the length of the compensation degree of line i.e. ratio of inductive to capacitive reactive power Ql / Qc are decisive for steady state, power frequency voltage increase of line.

Influence of Short Circuit Energy on the Supply Side of the line:

If a long overhead line or cable line with high capacitance on the discharge end is suddenly discharged by switching off the circuit breaker or a unloaded line at the end is closed, a power frequency voltage increase takes place due to capacitive charging current causes a negative voltage drop. Thus voltage Va at the beginning of the line becomes greater than the Voltage Vg of the system or generator.

Ka =Ua / Ug

Ka

Pk

Ua = Surge voltage

Ug = Generator voltage

Pk = Short circuit power

Influence of Line Length:

Based on the capacitive charging current, which flows in an unloaded line, an inductive voltage drop occurs over the longitudinal reactances of line. Thus the voltage Ve at the line end becomes higher than the voltage Va at the beginning of the line. This process is called Ferrantic effect.

Kij

Kij = Ue / Ua

Ue = voltage at line end

Ug = voltage of generator

L = Length of line in km

Influence of Compensation Degree:

The power frequency steady state voltage increase and thus the total surge voltage factor can be limited effectively by using compensating reactors. There by the capacitive charging current of the line is compensated, where by the inductive voltage drops can be decreased in the system.

Kij = surge voltage factor

Kij

Ql / Qc = compensation degree

Ql / Qc

Transient Voltage Increase:

Transient surge voltages are voltage-balancing processes as a consequence of internal energy balancing processes, as they occur in case of sudden faulty or power frequency changes of state. The closing of unloaded line also belongs to this category.

At the point of view of a line a voltage surge runs in the direction of line end. If this wave encounters the open line end after a running time T, its amplitude is increased there to double the value by reflection. This new state extends now to the starting of the line. Since, there, the magnitude of the voltage is specified by the supply system, a voltage drop occurs on the line in the form of a discharge wave. This wave is again reflected after running time T, at the open line end, so that after a renewed reflection of the wave the line is fully discharged to the starting of the line and the total process begins again. Thus surge voltages are occurred.

Higher surge voltages can occur, if the line is charged at the moment of closing. Such a preloading is considerable in case of three-phase reclosing. A further voltage increase occurs due to unavoidable synchronization fault during closing of three circuit breaker poles and the scatter/ leakage due to various prearcing lengths during closing movement of circuit breaker contacts.

Influence of Electrical Preloading on the line:

In three-phase short circuit interruption of a line one should reckon with very high surge voltages due to remaining charges on the line. If, at the moment of closing, the system voltage of a phase reaches the peak value, it exhibits the opposite polarity like the preloading of the line.

If the line is equipped with compensation reactors, a balancing process with the natural frequency of the oscillating circuit formed from the capacitance of the line and the inductance of compensation reactors. These oscillations are damped only slightly and have decay time in the order of few seconds. Therefore, high surge voltages can occur here, if in case of reclosing, after comparatively short rest period, the peak value of the system voltage and the maximum value of line voltage coincides with opposite polarity.

The figure shows the influence of electrical preloading of a line,

Influence of Periodical Scatter of circuit breaker pole:

Determined by the component tolerances, non uniformities in the electrical prearcing during the closing moment of circuit breaker contacts, temperature differences, small leakages occur at the time of closing of circuit breaker poles. Larger overshoot factors and thus higher surge voltage factors occur as a result of inductive and capacitive coupling between the phases. The figure below shows the influence of periodical scatter of circuit breaker pole.

Influence of size of closing resistor:

Surge voltages can be decreased by using a closing resistor, which is connected in parallel to each interrupter of a power circuit breaker and can be closed by means of auxiliary contacts. Surge voltages are reduced because these resistors connect generator voltage step-wise to the line. Th e figure below describes the influence of resistance value of closing resistors on the surge voltages.

Influence of Closing Duration of Closing resistor:

The surge voltages decrease with increasing closing duration of the resistor, since the balancing processes during closing of the resistor die down more, the longer the resistor dampers them. With regard to heating of resistors it is however strived to select the closing duration as short as possible, mostly a range of 8 to 10ms is selected.

Influence of Closing Moment (Synchronous closing):

If the voltage difference over the individual circuit breaker poles, i.e. between line and the supply system, in the closing moment of line circuit breaker is minimum or zero, then the occurrence of surge voltages can be prevented. This closing or switching is generally denoted as synchronous closing.

Frequency Distribution of magnitude of surge voltages:

Hitherto only the maximum values of the surge voltages were considered which occur during switching-in of the line in the most unfavorable moment. These maximum values are however seldom. Most of the connections result in lower surge voltages, since it is switched in always in the most favorable moment. Besides, the maximum value occurs only at one conductor, while the two conductors show lower values. In order to obtain an accurate picture of the real voltages is of importance.

In order to obtain an idea of frequency distribution of the magnitude of closing surge voltage between the conductors and earth at the end of a long unloaded line, the expected total frequency curve for a definite system is estimated.

Methods of Surge voltage limitations:

It is evident that the surge voltage factors can assume high values in case of closing of open-ended line. However, for the economical insulation determination of high voltage units, the definite values of surge voltage factors K for the conductor-earth voltage, which are comparatively low, shouldnt be exceeded.

U = 362kV K = 2.4

U = 550kV K = 2.2

U = 765kV K = 2

The accurate information on electrical closing duration of closing resistors is required for evaluating the thermal load of the resistor body and in determining the efficiency of resistor to limit switching over voltages.

The voltage endurance of the main and auxiliary interrupter is obtained as a function of time. The figure below shows the influence of contact gap of main and auxiliary interrupter on the system voltage.

The subsequent figure shows the evaluation at a circuit breaker pole for 550kV with four interrupter points per pole in two different operating conditions:

Fig. a : With lowest contact speed, low hydraulic pressure and high gas pressure.

Fig. b : With highest contact speed, high hydraulic pressure and low gas pressure.

Closing to Preloaded Line:-

The maximum over voltages of an open line occur during rapid reclosing after a three pole opening. Thereby the critical case is the closing in the moment, when the system voltage exhibits the opposite polarity due to charge on the line.

The voltage endurance curves of the circuit breaker with the maximum contact speed are investigated, since the shortest reaction times and thus the maximum switching over voltages occur.

Closing to Phase Opposition:-

The following switching operation of the closing in phase opposition has great importance for the structural arrangement of the closing resistor. The high voltage determines the number of required resistor bodies and thus the model and the costs.

The two high voltage systems are coupled with each other, a synchronization of the systems should be carried out before closing the systems, and thus both the voltages occurring at the terminals of the circuit breaker do not show any phase shift against each other.

If a fault occurs thereby, in the extreme case, the circuit breaker can be closed at 180 degrees phase opposition, thus, the doubled voltage stress occurs. This gives the maximum thermal and electrical stress in the resistor bodies of closing resistor.

The voltage surge at 180 degrees phase opposition is plotted in figure in the voltage endurance curve of the circuit breaker with the minimum contact speed, which exhibits the maximum closing duration. The maximum closing duration of the closing resistor can be read by selecting a corresponding connecting point considering the prearcing.

Closing to short-circuit:-

During closing of a circuit breaker with closing resistors to a short-circuited line the full system voltage is at closing resistor. The energy converted in the resistor thereby is only about 1/4th of that in case of a closing to full phase opposition.

Thermal Loading of Closing Resistors:

It is important for the operation of power circuit breakers with closing resistors in system to know accurately the thermal capacity of resistor bodies, since a overloading and as a consequence of that a failure of the circuit breaker can occur in case of very high switching frequency.

Material Properties of resistor Bodies: The most important component of the closing resistor is the resistor bodies. It has the task to provide an ohmic resistance with possibly high dielectric strength and thermal capacity.

It offers three materials for this purpose:

1. Metal: They have high temperature resistance and high thermal capacity.

2. Ceramic bound metal oxide: They are used as extremely voltage dependent resistors in lightning arresters.

3. Ceramic bound carbon

Temperature Coefficient of resistor:

The resistance of most metals increases with the temperature. However, carbon shows a negative temperature coefficient, i.e. the ohmic resistance decreases with increasing temperature. Since carbon is used in the closing resistors, this characteristic has attained great prominence.

Voltage Dependence of the resistor:

In addition to the dynamic temperature coefficient, there are two more phenomenons that influence the resistance value of the ceramic and carbon resistors:

1. Voltage waveform

2. Voltage levelSynchronous Switching:

INTRODUCTION:

Synchronous switching has gained a great deal of relevance not only because of their potential for increasing reliability and for making a contribution to improve the overall power quality of the electrical systems, but also for economic reasons. These concepts can be instrumental in minimizing the use of auxiliary components, such as pre-insertion resistors, in reducing equipment wear and unnecessary maintenance and thus the total cost of ownership throughout the full life time of the equipment.

Synchronous switching:Opening (or) closing the contacts of a circuit breaker is normally done in a totally random fashion and consequently, as it has been described before transient current and voltage disturbances may appear in the electrical system. A typical way for controlling this transient behaviour has been to add discrete components such as resistors, capacitors, reactors, surge arresters, combinations of the above to the terminals of the circuit breaker nevertheless, in many cases it would be possible to control these transients with out the addition of external components but by operating the circuit breaker in synchronism with either the current (or) the voltage oscillations, depending upon the switching operations at hand. This means that for example, the opening of the contacts should occur at a current zero when interrupting short circuit currents, (or) that the closing of the contacts should take place at voltage zero when energizing capacitor bank.

Ideally and to obtain the greatest benefits, synchronous switching should be done using circuit breakers that are capable of independent pole operation. However for those designs where all three poles are operated in unison the implementation of controlled switching concepts will require the development of specially designed circuit breakers which are provided with suitable methods for staggering the pole operating sequences.

To select the operating characteristics of a circuit breaker which have the most direct impact for synchronized switching requires a clear definition and understanding of the cause and effective relationship that exists between the mechanical operation of the circuit breaker and the behaviour of the electrical system

Synchronous capacitance switching:

For capacitance switching, the primary concern is not as much the interruption of capacitive currents because, due to the inherent characteristics of vacuum and SF6 gas circuit breakers, the problems associated with restrikes, found with earlier technologies, have been greatly reduced and today, indeed restrike are a very rare occurrence. On the other hand failures are often reported which are the direct result of inrush currents and over voltages that have propagated themselves into lower voltage networks causing damage especially to the electronic equipment connected to the circuit.

Closing control:

In order to completely eliminate the over voltages produced by the closure of a circuit breaker onto a capacitor bank, it is required that there be a zero voltage difference across the contacts of the circuit breaker, naturally this is not always possible simply because some deviation from optimum operating conditions is to be expected. We know that the over voltages can be reduced to acceptable limits when the closing of the contacts occur within one millisecond either before (or) after the voltage zero point. The significance of this requirement is better appreciated when considered in conjunction with the gap withstand capability of the contacts. where the absolute value of a sinusoidal voltage is plotted in conjunction with the slope of an assumed gap withstand characteristic. As it can be seen in the fig. The point where the pre-strike takes place corresponds to the intersection of the two curves.

To increase the rate of change of the withstand capability in an SF6 gas circuit breaker, any of the following three options either individually (or) in combination may be applied.

Increase the gas operating pressure.

Reduce the electric field stress in the contact region.

Increase the closing velocity of the circuit breaker contacts.

Increasing the gas operating pressure, in many cases, is not a viable solution because of the possibility of SF6 liquefaction at low temperatures. Nevertheless it should be kept in mind that at voltages above 362kV the systems are grounded almost without exception and therefore our imaginary non-ideal circuit breaker may be acceptable.

For vacuum circuit breakers, and since presently they are used almost exclusively at system voltages in the range of 15kV to 38kV, the minimum rate of change of the gap withstand does not present any problem. The precise points where the contacts of each pole must close depend upon the system connections. When the capacitors and the system neutrals are grounded then the optimum point to close the contacts is each pole independently at the voltage zero of the corresponding phase.

When the capacitors are connected in an ungrounded system it is possible to close the first pole at random, since there will be no current flow with only one pole closed. The second pole and the third pole must then close at their respective voltage zero. Another alternative would be to close two poles simultaneously at a voltage zero and then close the third pole at its corresponding voltage zero.

SYNCHRONOUS REACTOR SWITCHING:For reactor switching operation the basic needs are the opposite of those considered to be desirable for capacitance switching, that is closing the circuit is not as important as is opening.

Closing control:

Typically, most high voltage circuit breakers will pre-strike during a closing operation and as a result of this pre-strike an over voltage that generally is less than1.5 p.u. is produced. In this case synchronized closing from the point of view of switching over voltages is considered to be unnecessary since this voltage, by no means should be considered to be excessive. Furthermore if the closing is synchronized with a voltage zero condition, this would result in a high asymmetric current which may develop excessive mechanical stresses with in the turns of the reactor being switched on. Additionally if in a grounded circuit the closing of the contacts takes place at a voltage zero, it is possible that an excessive zero-sequence may flow thus raising the possibility of the zero-sequence relays being activated.

If synchronous closing is to be considered, it would be preferable to close the contacts at maximum voltage, which makes the task relatively easy since there is a natural tendency for the contacts to do just that, plus the fact that near the peak of the voltage its rate of change of voltage is basically zero.

A unique condition that is worth mentioning because of the significant benefits that can be attained from synchronous closing, is when rapid reclosing of a shunt reactor compensated line is required. Reclosing implies that current interruption has just taken place and since following interruption a trapped charge will be left on the unloaded line. In this case the voltage across the circuit breaker will show a significant beat as shown in fig.10.6 due to the frequency difference between the line and load sides. At the source side of the circuit breaker the voltage oscillates with the power frequency while at load side the frequency of the oscillations may be as low as one-half that of the power system frequency or as high as to approach the power frequency, it only depends upon the degree of compensation, the higher the compensation, the lower the frequency.

DIS-ADVANTAGES:

Since synchronization should be made at a beat minimum, where the voltage across the contacts is relatively small, it is evident that the degree of complexity for detecting this zero voltage condition across the contacts has greatly increased, thus making this task extremely difficult. This is further complicated by the fact that the variable beat frequency creates a high degree of uncertainty for predicting the voltage zero across the circuit breaker contacts.

Synchronous Transformer Switching: Basically speaking, the switching of an unloaded transformer is no different than switching a reactor, that is the voltages and currents involved in opening and closing the circuit of the transformer generally have the same characteristics of those produced by the switching of reactors. However, for this application the most critical variable is the transformers inrush current, which, in some occasions can reach magnitudes that approach those of the short circuit current. The magnitude of the inrush current depends on the transformer and on the status of its magnetic characteristics of the core of the transformer and on the status of the magnetic flux remnants at the instant when the circuit is energized. The severity of the energizing process is greater for transformers that have high remnants, than for those that arc completely demagnetized. It follows then that for full synchronization it is necessary to detect the remnants level prior to the energizing of the transformer (or) alternately that all openings of the transformer circuit be made synchronously so that the remnants conditions are controlled and can be well defined for the next closing operation.

Synchronous Short circuitcurrent switching:

Synchronous switching of short circuit currents is a desirable feature from the point of view of reducing contact erosion, which translates in extending the life of the circuit breaker.

Closing control:

The aim of synchronous closing would be to reduce contact erosion by reducing the arcing time during closing, which is due to pre-arcing across the contacts. The benefits that may be achieved by synchronous closing must be kept into perspective since contact erosion during a closing operation is significantly less than during interruption. Unless the rate of change of the gap dielectric capability is extremely slow the pre-arcing time is bound to be considerably shorter than the interrupting arcing time. Further more it should be considered that due to the low instantaneous values of current at closing the energy input would significantly lower than that which is seen during interruption.

The optimum switching angle for reducing (or) eliminating pre-arcing would be at voltage zero; nevertheless, this may present a problem because the maximum current peak is reduced under these conditions due to the maximum asymmetry which is produced when the current flow, in an inductive circuit, is initiated at a voltage zero. As a consequence of high current peak the Electro- mechanical stresses imposed on the circuit breaker are the highest. This translates into higher output energy requirements for the operating mechanism and is general larger structures for the circuit breaker.

Condition Monitoring of circuit breaker:

Circuit breaker constitute an important and critical component of the electrical system, they are the last line of defense and consequently proper and reliable operation is paramount to the quality of power delivered, to the promotion of customer satisfaction and most of all to the safety and integrity of the system.

To sustain the confidence on this critical piece of equipment comprehensive maintenance programs have been established. These maintenance programs follow established standard guidelines and the recommendation of the manufacture, which generally are based on this operating experience. A more logical approach may be to continually evaluate the condition of those components that through experience have been identified as being the most likely to fail and those whose failure could provoke a severe damage that would disrupt the service.

Historically, most of the circuit breaker failures that have been observed in the field can be attributed to mechanical problems and difficulties related to the auxiliary control circuits. A number of studies, such as those made by CIGRE provide with an excellent insight into the failure statistics of the components of a circuit breaker. The report indicates as shown in the figure below that 70% of the major failures in the circuits are of mechanical nature, 19% are related to auxiliary and control circuits and 11% can be attributed to electrical problems involving the interrupters (or) the current path of the circuit breakers.

These statistics can be used as a guideline for the selection of those components that should be monitored. Although the most desirable option would be to develop a system that constantly monitors critical components and which is able to detect any deterioration that may occur over time and to predict, in a pro-active way, impending failures of mechanical components. This task however has proven to be rather exclusive. Simpler schemes may provide adequate protection, but naturally, a final choice should be based on an evaluation of the benefits against the complexity and the difficulty of implementing the specially required monitoring function.

Choice Of Monitored parameters:

There are a significant number of parameters that can be chosen for monitoring, there are as well a variety of methods each having varying degrees of complexity for executing the monitoring functions. The optimum system would be one that selects the most basic and important functions and thus minimizes the number of parameters that are to be monitored and yet it maximizes the effectiveness of the evaluation of the system that is being monitored.

In addition to optimizing the number of monitored parameters, the methods used to do the monitoring should be kept as simple and straight forward as possible. It will be desirable, if not essential, from the point of view of availability, cost and operating experience, that commercially available transducers that are used in any related industry should be given preference and used where at all feasible.

Mechanical parameters:

Some of the most likely parameters to be monitored because of their significance and the simplicity of the monitoring scheme are given below.

Contact travel distance and velocity.

Point of contact separation and contact touch.

Contact travel distance and velocity:This could easily be considered as the most important function being monitored. It provides dynamic information about the operating components of the circuit breaker as a whole including not only mechanical links but also the interrupter contacts. The information that can be extracted from these measurements is always extremely valuable for judging the overall status of the circuit breaker and fortunately this is probably one of the simplest and easiest functions to monitor.

The contact or breaker travel measurement can be easily consummated by monitoring the displacement or the rotation of the output shaft of the operating mechanism. The closing or opening velocities then can be obtained by finding the derivative with respect to time of displacement measurement. This mathematical manipulation most likely would be done electronically with the assistance of a central processing unit where all the signals would be collected for evaluation and data storage.

Contact make and contact break:

Contact makes and contact break indications can be obtained either by direct or indirect methods. If a direct indication is desired it can be obtained, when the measurement is made under load conditions by monitoring the voltage across the contacts. For no load operations, or when indirect measurements are made, the methods that are used to determine contact displacement are applicable. The major drawback of this approach is that it fails to take into account the changes in the making or breaking of the contacts that may occur as a result of possible contact erosion.

Electrical parameters:

Dielectric failures and interrupter failures represent a high percentage of the listed reasons for circuit breaker problems. Although, many of these failures would take place without any prior warning, there are some cases where it would be possible to anticipate an impending failure based on some conditions which are generally well known and predictable as is the case with high levels of corona, high leakage currents, high moisture content and low insulating gas density. While some of these parameters can be monitored with only reasonable efforts, there are others which are difficult to monitor while the circuit breaker is energized and in service. Some of the significant electrical components that could be monitored are discussed below.

Contact erosion and interrupter wear.

Monitoring contact erosion and interrupter wear has a strong, direct influence upon the required maintenance frequency therefore, it is not only desirable, but beneficial to accurately evaluate the condition of the interrupters rather than to rely on the presently used method of simply adding the interrupted currents until the estimated accumulated duty that is given by applicable standards, or by the manufacturers recommendations, is reached.

Measurements of contact erosion, or interrupter wear cannot be made directly, but it can be done conveniently by indirect methods using Measurement of current and arcing time. The interrupted current can be measured using conventional instrumentation, such as current transformers, which are generally available in circuit breaker as a standard component. The arcing time can be determined by optical detection of the arc, by measurement of the arc voltage, or simply by estimating the point of contact separation using the information given by the contact travel transducer and the duration of current flow from this time until it is interrupted.

The product of the current and elapsed time from contact separation to current extinction gives a parameter to which the interrupter wear can be related. It is assumed that sufficient data has been collected during development tests relating contact erosion and nozzle ablation to ampere seconds of arcing, and therefore by keeping track of the accumulated ampere seconds an adequate appraisal of the interrupter condition can be made.

Gas density

For SF6 circuit breakers gas density rather than gas pressure is the parameter that should be monitored. To do this it is possible to use commercially available temperature compensated pressure switches or alternatively the density may be determined by electronically processing separate pressure and temperature signals. These signals can be combined by an algorithm representing the well-known equation of state for the gas. Any deviation from a constant density line will indicate that there is a gas leak in the system and unless a massive catastrophic failure occurs, slow leaks can be alarmed and protective action can be implemented. The selection of the corresponding constant density line depends on the initial filling conditions of the circuit breaker.