CIRCUIT BREAKERS EQUIPPED WITH PARALLEL IMPEDANCE

Impedances in parallel with the arc may be either capacitors or resistors, or both, in various combinations. Although such impedances modify the shape of the specified inherent transient recovery voltage, the type and degree of modification in the synthetic test should be the same as in the direct test.

For example, the insertion of a resistor equal to the surge impedance of the line will reduce the line side rate-of-rise to half value. The effect is not as pronounced for a bus fault where a large number of lines are in parallel because their combined surge impedance is much lower than the resistance in parallel with the arc.

Where the shunt impedance is a resistor, particularly if the ohmic value of the resistor is low, the actual peak transient recovery voltage (TRV) in a synthetic test may not attain the value it would in a direct test because of the limited energy available from the voltage source.

Furthermore, the shunt resistor may cause a too rapid decay of the dc voltage following the TRV crest.

In some cases, to meet the TRV requirements of ANSI/IEEE C37.09-1979 [3], it may be possible

1) To adjust the parameters of the voltage circuit to provide the necessary additional energy absorbed by the shunt resistor

2) To switch over to an additional ac voltage source capable of maintaining voltage across the resistor.

An equivalent transient recovery voltage waveform across the terminals of the test circuit breaker can be produced by replacement of resistance at other appropriate places in test circuits.

CONSTRUCTION AND RATINGS OF HIGH VOLTAGE CIRCUIT BREAKERS BASIC INFORMATION

HIGH-VOLTAGE CIRCUIT BREAKER CONSTRUCTION

All high-voltage circuit breakers have

# Contacts that operate at system voltage

# Insulation between main contacts and ground potential (porcelain, oil, or gas)

# Operating and supervisory devices

# Insulated links between the operating devices and the main contacts

Most power circuit breakers are opened and closed automatically by remote control. Various kinds of operating mechanisms are used. Among them are AC or DC solenoids, compressed air, high-pressure oil, springs, or electric motors.

HIGH-VOLTAGE CIRCUIT BREAKER RATINGS

High-voltage circuit breakers are rated by maximum voltage, insulation, maximum continuous and momentary current-carrying capacity, maximum interrupting capacity, transient recovery voltage, interrupting time, and trip delay.

Circuit interruption occurs when a plasma arc with temperatures exceeding 20,000 K appears for a short time interval between the main contacts. This occurs when the current passes zero, and it is determined by the time relationship between the buildup of dielectric strength of the gap between the open contacts and the rise of transient recovery voltage.

The interrupting capacity of a circuit breaker, measured in kilovolt-amperes (kVA), is the product of the phase-to-ground voltage in kilovolts (kV) of the circuit and the interrupting ability, in amperes (A), at stated intervals and for a specific number of operations. The current is the root-mean-square (rms) value existing during the first half-cycle of arc between contacts during the opening stroke.

TYPES OF POWER SYSTEM CIRCUIT BREAKERS BASIC INFORMATION

The five general types of high-voltage circuit breakers are as follows.

1 Oil circuit breakers use standard transformer oil, an effective medium for quenching the arc and providing an open break after current has dropped to zero. There are two general types of oil circuit breakers: dead-tank for the higher voltage ranges and live-tank for lower voltages.

Oil circuit breakers have been improved by adding such features as oil-tight joints, vents, and separate chambers to prevent the escape of oil. Also, improved operating mechanisms prevent gas pressure from reclosing the contacts, making them reliable for system voltages up to 362 kV.

However, above 230 kV, oil-less breakers are more economical.

2 Air-blast circuit breakers were developed as alternatives to oil circuit breakers as voltages increased. They depend on the good insulating and arc-quenching properties of dry and clean compressed air injected into the contact region.

3 Magnetic-air circuit breakers use a combination of strong magnetic field with a special arc chute to lengthen the arc until the system voltage is unable to maintain the arc any longer. They are used principally in power distribution systems.

4 Gas circuit breakers take advantage of the excellent arc-quenching and insulating properties of sulfur hexafluoride (SF6) gas. These outdoor breakers can interrupt system voltages up to 800 kV.

These circuit breakers are typically included in gas insulated substations (GISs) that offer space-saving and environmental advantages over conventional outdoor substations. Gas (SF6) circuit breakers are made with ratings up to 800 kV and continuous cur rent up to 4000 A.

They are alternatives to oil and vacuum breakers for metal-clad and metal-enclosed switchgear up to 38 kV.

5 Vacuum circuit breakers, more accurately termed vacuum-bottle interrupters, are generally used for voltages up to 38 kV and continuous current ratings to 3000 A. They are used for higher system voltage, current, and interrupting ratings, and are typically specified for metal-clad and metal-enclosed switchgear in distribution systems.

HIGH VOLTAGE CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

The logic behind calling 1000 V to 72.5 kV a medium-voltage range is not obvious unless it is compared with the maximum North American grid voltages of 800 kV and more common transmission system voltages of 60 to 500 kV.

Nevertheless, medium-voltage circuit breakers can protect AC generators, some transmission and subtransmission lines, and distribution substations.

The industry classifies circuit breakers in the following way:

# Medium-voltage power (1000 V to 72.5 kV)

# Low-voltage power (1000 V and below)

# Industrial molded case (600 V and below)

# Miniature or branch circuit (240 V and below)

All circuit breakers are electromechanical devices that make and break currents under normal conditions and make, carry for a specified time, and break currents under abnormal conditions such as short circuits. Circuit breakers, like transformers and batteries, are made in a wide range of ratings.

Because the higher-voltage circuit breaker contacts can be damaged or destroyed by the burning action of electric arcs when the contacts of a high-voltage circuit are opened, various methods have been developed to provide an appropriate quenching medium around the contacts that will assist in extinguishing any arcs formed as rapidly as possible.

The names of mediums used for extinguishing the arc are included in the descriptions of the circuit breaker. For example, there are oil circuit breakers, air-blast circuit breakers, and magnetic-air circuit breakers.

The selection of the appropriate method for quenching the arcs depends on the cost-effectiveness and availability of sources of and means for providing air blasts, insulating gas, insulating oil, magnetic fields, or vacuums.

The two basic designs for high-voltage circuit breakers are oil and oil-less. The oil type circuit breaker had been the most popular for outdoor service up to 362 kV, but the air-blast and gas-type versions have been gaining in popularity.

At 550 and 800 Kv, oil-less breakers predominate. For new indoor applications magnetic-air and vacuum circuit breakers predominate, along with some gas-type. Indoor magnetic-air, air-blast, and vacuum breakers have been adapted for outdoor use in the 2.5- to 34.5-kV range by protecting them with metal covers.

It is essential that the correct circuit breakers, fuses, and switches be selected for each power control application because of their importance in the design and function of the overall electrical system.

Immediately upon sensing a short circuit or break in the supply line, fuses and circuit breakers must isolate the sections of the electrical network where the fault occurred, to prevent further damage while permitting the remainder of the network to remain operational.

METAL-ENCLOSED LOW-VOLTAGE POWER CIRCUIT BREAKER SWITCHGEAR BASIC INFORMATION AND TUTORIALS

The design of the high-voltage substation must include consideration for the safe operation and maintenance of the equipment. Switching equipment is used to provide isolation, no load switching, load switching, and/or interruption of fault currents.

The magnitude and duration of the load and fault currents will be significant in the selection of the equipment used. System operations and maintenance must also be considered when equipment is selected.

One significant choice is the decision of single-phase or three-phase operation. High-voltage power systems are generally operated as a three-phase system, and the imbalance that will occur when operating equipment in a single-phase mode must be considered.

Metal-enclosed low-voltage power circuit breaker switchgear indicates a design which contains low voltage ac or dc power circuit breakers in individual grounded metal compartments. The circuit breakers can be either stationary or draw out; manually or electrically operated; fused or unfused; and

either 3-pole, 2-pole or single-pole, construction.

The switchgear may also contain associated control, instruments, metering, protective and regulating equipment as necessary. Definitions, ratings, design and production tests, construction requirements, and guidelines for application, handling, storage, and installation are covered in IEEE C37.20.11.

Low-voltage metal-enclosed switchgear is typically installed in industrial plants, utility and cogeneration facilities, and commercial buildings for the protection and distribution of power for loads such as lighting, machinery, motor control centers, elevators, air conditioning, blowers, compressors, fans, pumps, and motors.

Low-voltage switchgear is available in ac ratings up to 635 V and 5000 A continuous and in dc ratings up to 3200 V and 12000 A continuous. Short-circuit current ratings are available up to 200 kA.

SF 6 POWER CIRCUIT BREAKER BASIC INFORMATION AND TUTORIALS

What Is An SF6 Power Circuit Breaker?

SF6 gas has proven to be an excellent arc quenching and insulating medium for circuit breakers. SF6 is a very stable compound, inert up to about 500 degrees C, non-flammable, non-toxic, odorless, and colorless. At a temperature of about 2000K SF6 has a very high specific heat, and high thermal conductivity, which promotes cooling of the arc plasma just before and at current zero, and thus facilitates quenching of the arc.

The electronegativity behavior of the SF6, that is, the property of capturing free electrons and forming negative ions, results in high dielectric strength and also promotes rapid dielectric recovery of the arc channel after arc quenching. SF6 breakers are available for all voltages up to 1100 kV, continuous currents up to 5000 A for conventional breakers (higher for generator breakers), and shortcircuit interruption up to 80 kA.

SF6 breakers of the indoor type have been incorporated into metal-clad switchgear. Outdoor designs include both dead tank and live tank circuit breakers.

Over the years, SF6 circuit breakers have reached a high degree of reliability; thus they can cope with all known switching phenomena. Their closed-gas system eliminates external exhaust during switching operations and thus perfectly adapts to environmental requirements. Their compact design considerably reduces space requirements and building and installation costs.

In addition, SF6 circuit breakers require very little maintenance. All ratings are economically satisfied by the modular design. Each pole is equipped with one or more interrupters; stored energy, spring, hydraulic, or pneumatic driving mechanisms are provided for each pole or 3-pole unit.

Gas-density monitors are standard. In the closed position, the current flows over the continuous current contacts and the complete volume of the breaker pole is under the same pressure of SF6 gas.

The precompression of the SF6 gas commences with the opening operation. The continuous current contacts separate and the current is transferred to the arcing contacts. At the instant of separation of the arcing contacts, the pressure required to extinguish the arc is reached.

The arc produced is drawn and at the same time exposed to the gas, which escapes through the ring shaped space between the extinction nozzle and the moving arcing contact. The escaping gas has the effect of a double blast in both axial directions.

Until the open position is reached, SF6 gas flows out of the puffer cylinder. The existing overpressure maintains stability of the dielectric strength until the full value of the open contacts at the rated service pressure is reached.

In the case of high-current interruption, arc energy heats the gas, resulting in a pressure rise in the static volume (heating volume) V1. This pressure then quenches the arc at an ensuing current zero. In the low-current case an auxiliary puffer (volume, V2) generates sufficient pressure for interruption.

Necessary force requirements for the mechanical system are therefore drastically reduced. All ancillary equipments, including the oil pump and accumulator associated with the drive, form a modular assembly that is mounted directly on the circuit breaker, thus eliminating installation of piping on the site. The metal-enclosed GIS breaker is provided with the necessary items to fit into the substation arrangement.

The main equipment flanges of the breaker are fitted with contact assemblies to accept the isolator moving contacts. Other equipment modules can be coupled to the same flanges. On the fixed-contact end of the circuit breaker, provision is made for coupling two modules, facilitating the mounting of an extension module to connect the second busbar isolator.

Dead tank SF6 breakers typically employ gas-filled bushings. Such bushings are usually integral to the circuit breaker itself and are not interchangeable with other apparatus bushings.

Electrical grading is provided by a lower throat shield. Ring-type bushing current transformers are located at the base of the bushing. Potential taps are not generally available in SF6 bushings because of the lack of a capacitive grading structure.

Porcelain alternatives, such as composites, have been used to provide greater safety (explosion resistance), easier handling (lighter and nonbrittle), seismic performance (lighter and stronger), and pollution performance.

GENERATOR CIRCUIT BREAKERS BASIC INFORMATION AND PARTS OF GENERATOR CIRCUIT BREAKERS TUTORIALS

What Are Generator Circuit Breakers?

Generator circuit breakers represent another class rated for very high continuous currents and short circuit currents, typically at generator voltages. Generator breakers are incorporated into generator bus ducts and can include other switchgear components for measuring current, detecting faults, and grounding.

Generator breakers are available up to 50 kA nominal current and up to 220 kA interrupting current. Two technologies are employed—air blast at the higher ratings and SF6 self blast at the lower and medium power levels (up to 120 kA). For nominal currents above 20 kA, the generator breaker is usually equipped with a forced cooling system, using water, for example. Generator breakers have been available since the 1960s.

Advantages of using generator breakers include the following:

Reduced station cost by eliminating station transformers and increasing station layout flexibility.

Simplification of operation, especially during commissioning and recommissioning; this is because the generator can be handled as a separate unit, isolated from the main and unit transformers.

Fault protection between the generator and transformer. Two zones of protection are created and generator faults are cleared by the opening of the generator breaker alone.

Unbalanced load protection of the generator.

Protection of the generator from transformer faults.

Reliability/availability increase.

Historically, generator circuit breakers have been of air-blast design with pneumatic operators. This is the technology still used today for large nuclear and fossil fuel power plants (up to 1500 MW), and large pumped storage installations.

The design has a tubular housing and is horizontal. Newer designs utilize SF6 self-blast technology and hydraulic operators. These are rated for application to smaller power plants (gas turbine/cogen, for example) from 60 to 400 MW and smaller pumped storage installations.

PARTS OF GENERATOR CIRCUIT BREAKERS

Outline and interrupter details of a generator air-blast circuit breaker-type DR, 36 Kv, up to 50 kA with forced cooling, 200 kA.

AIR CIRCUIT BREAKER BASIC INFORMATION AND TUTORIALS

What Are Air Circuit Breakers And How Does Air Circuit Breaker Works?

The usual construction of an air circuit breaker makes use of two fixed terminals mounted one above and the other in a vertical plane, which, when the breaker is closed, are bridged under heavy pressure by a bridging member operated by a system of linkages.

Auxiliary and arcing contacts close before and open after the main contacts. The arcing contacts are easily renewable. The breaker is held closed by a latch which may be tripped electrically or mechanically.

Modern breakers are trip-free.

Many breakers use a solid bridging member with spring-mounted self-aligning contacts. The contact surfaces are made of silver so that oxidation will not cause excessive resistance and overheating.

Arcing contacts of modern breakers use a silver-tungsten or copper-tungsten alloy which is arcresistant. The secondary contacts, where used, are usually of copper or silver alloy.

Barriers between poles are generally furnished with breakers on ac and dc circuits 250 V and above, and special arc chutes, quenchers, or deionizing chambers are also used throughout the available lines of air circuit breakers.

These devices are made in different forms by different manufacturers and serve to improve the interrupting performance of the breaker and to shorten the arcing time.

Air-insulated high-voltage electrical equipment is generally covered by standards based on assumed ambient temperatures and altitudes. Ambient temperatures are generally rated over a range from –40°C to +40°C for equipment that is air insulated and dependent on ambient cooling.

At higher altitudes, air density decreases, hence the dielectric strength is also reduced and derating of the equipment is recommended. Operating (strike distances) clearances must be increased to compensate for the reduction in dielectric strength of the ambient air.

Also, current ratings generally decrease at higher elevations due to the decreased density of the ambient air, which is the cooling medium used for dissipation of the heat generated by the load losses associated with load current levels.

OPERATING MECHANISM OF POWER CIRCUIT BREAKER BASIC INFORMATION

A Tutorials On The Operating Mechanism Of Power Circuit Breaker

Opening and closing of power circuit breakers under service conditions is seldom performed manually, since most breakers are installed in systems designed for remote control providing specific redundancy.

Various means of operation are used, such as

(1) dc solenoids,

(2) solenoids operated from an ac source through a dry-type rectifier,

(3) compressed air,

(4) high pressure oil,

(5) charged spring, and

(6) electric motor.

Automatic reclosing of breakers in overhead line feeders is frequently used to restore service quickly after a line trips out because of lighting or other transitory fault. Instantaneous or time-delay reclosing may be provided with a lockout to prevent more than one to several successive reclosures, as desired.

If the fault is cleared before the lockout feature operates, the reclosing device resets itself, permitting a complete cycle of reclosing at a subsequent fault.

The circuit-breaker-operating device has to cope with the increasing requirements in interrupting and current-carrying capability as well as with shorter operating times. Simplicity of design, robustness, and reliability have to ensure safe operation of this vital link between the electrical system controls and the interrupter.

The principle of a pneumatic drive is sketched for an extra high voltage circuit breaker which functions according to the differential piston principle in figure below.

A pneumatic interlocking device in connection with the SF6 gas system ensures that the breaker always remains in the defined open or closed position even on loss of air pressure. Besides opening and closing functions, effective damping of the highly accelerated moving parts is incorporated.

Principle of the drive system for an SF6 outdoor breaker: (a) closed position; (b) open position.

CLOSING OPERATION OF POWER CIRCUIT BREAKERS BASIC INFORMATION

How Power Circuit Breaker Closing Operation Works?

Circuit breakers are designed to perform the closing and reclosing operations as per standard requirements. When operated to close on long lines, extra-high-voltage circuit breakers require special measures to keep switching overvoltages within specified limits.

Such measures may be single or multiple step closing resistors, synchronously closing at the moment of voltage zero, or polarity-controlled-closing, which means closing during the period of equal polarity at the line and source side of the breaker. When operated to close on capacitor banks special measures may be taken to limit transient currents and voltages.

Such measures may be closing resistors; controlled closing at the moment of voltage zero for grounded wye capacitor banks; or controlled closing on ungrounded wye capacitor banks where the first phase is closed at the moment of voltage zero and the other two phases are closed at a point where the voltage difference between the two phases is zero.

When operated to close on power transformers or shunt reactors special measures may be taken to limit inrush transient currents and transient voltages. Such measures may be single or multiple step closing resistors, or controlled closing at the moment of voltage peak.

The magnitude of overvoltages on energizing and reenergizing is influenced by the nature and variables of the power system. Parameters of supply side and line must be taken into account in order to compute the overvoltages or to determine them using transient network analyzers or transient analysis software, such as electromagnetic transients programs (EMTP), power systems computer aided designs (PSCAD), or alternative transient program. (ATP).

For a summary of the magnitude of overvoltages occurring when energizing high-voltage lines, based on numerous studies and measurements in high-voltage networks, see Table 10-9. Surge arresters may also be used to limit switching overvoltages.

Overvoltages Occurring When Energizing High-Voltage Lines

OPENING OPERATION AND DUTY CYCLE OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

How Power Circuit Breaker Operation Works?

Reaction time and speed of modern breakers has increased to reach standard interrupting times of 2 to 5 cycles, with 2 to 3 cycles being common at high voltage. Interrupting time is measured from energizing of the trip coil until the extinguishing of the arc.

The interrupting time during close-open operations may exceed the rated imterrupting time by either 1/2 cycle (for 2 and 3 cycle breakers) or by 1 cycle (for 5 cycle breakers).

The current standard operating duty cycle consists of the following:

Open – T – Close – Open - 3 min – Close – Open

T is defined as either 15 s or 0.3 s depending on whether the circuit breaker is rated for high speed reclosing; this distinction is important in application.

Even circuit breakers rated for high speed reclosing must still be allowed a 0.3-s delay to allow for proper recovery of insulation following the initial fault interruption.

For existing oil and air-magnetic circuit breakers, the standard operating duty cycle was:

Open – 15s – Close - Open.

For additional operations, and/or any close operation in the duty cycle with a time delay of less than 15s after an opening operation, the interrupting rating and related required capabilities of the oil or air magnetic breaker have to be derated.

All operations within a 15-min period are considered part of the same duty cycle and a duty cycle shall have no more than five opening operations.

For guidance on interrupting capability for reclosing service for oil and air-magnetic breakers manufactured after 1960 refer to IEEE C37.0106. Circuit breakers manufactured prior to IEEE C37.7-196010 have different basis of rating.

VOLTAGE RATING AND ISULATION OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

Circuit breakers are built for voltage ratings as defined in IEEE C37.042 and IEEE C37.063. They have to be dimensioned to withstand the maximum voltages as specified. The rated maximum voltage is the upper limit for operation.

For circuit breakers rated in accordance with ANSI C37.06-19874 (or earlier), the range between upper and lower limit is defined by voltage range factor K. Current-interrupting capabilities vary within this range in inverse proportion to the operating voltage.

For circuit breakers rated in accordance with ANSI C37.06-19974 (or later), the current-interrupting capability is a constant Ka value at any voltage equal to or lower than the rated maximum voltage.

The insulation level is determined by the rated withstand test voltages specifying the low frequency voltage (kV, rms) and the impulse voltage (kV, crest). High-voltage breakers must essentially withstand switching surges and both full and chopped-wave lightning impulses.

For multiple-break circuit breakers, equal voltage distribution over the series breaks is achieved by grading capacitors paralleled to the interrupting chambers. Coordination between inner and outside insulation, as well as insulation coordination between interrupters and ground insulation, has to be properly designed to prevent flashover inside the breaker or over the open break.

Outdoor breakers are generally available with special bushings that provide increased creepage distance for installation sites with highly contaminated air. For heavily polluted atmospheres, spray washing of live or deenergized breakers may be an additional measure.

Because of the method of design with enclosed ground insulation, the GIS circuit breaker is not influenced by atmospheric pollution. For installation at altitudes above 3300 ft (1000 m), altitude correction factors have to be applied.

The values of rated maximum voltages and insulation levels are multiplied by these factors to obtain the values for the application. The altitude correction factors are as listed in ANSI/IEEE C37.04-1979.

Correction factors are under discussion in an IEEE Switchgear committee working group and are expected to change. These factors will be published in IEEE C37.100.15. Particular reference is made to the rating structures and preferred ratings for ac high-voltage circuit breakers per the latest standard revisions of IEEE C37.042 and IEEE C37.063.

POWER CIRCUIT BREAKER INTERRUPTION OF CAPACITIVE CURRENTS AND CLOSING ON FAULTS BASIC INFORMATION AND TUTORIALS

Capacitive currents occur during line drooping as well as during disconnecting unloaded cables or capacitor banks. Although, switching of capacitor banks is regarded as a special application, disconnecting of charged lines is a frequent switching operation.

Current chopping may occur at a low instantaneous current value during interruption of capacitive currents, but this does not lead to overvoltages. After interruption of current, the voltage at the line capacitance (L) remains at the peak value of the power frequency voltage, whereas the voltage on the source side (S) oscillates about the driving voltage.

The difference between the two voltages appears across the circuit breaker with an amplitude of more than double the rated voltage. If the circuit breaker cannot withstand this higher voltage restriking may occur.

Restriking is similar to closing transmission lines with trapped charge. After restiking, a transient current flows through the circuit breaker, which is of higher frequency than that of the system and which can again be interrupted during the reignition process.

After reextinction, the line is charged to the potential of the peak value of the equalizing process, whereas the circuit-breaker terminal on the source side (S) recovers to the system voltage. A very high differential voltage appears across the breaker, which may lead to renewed restriking and even switching failures.

Restrike-free interruption of capacitive currents is thus of the utmost importance. Basically, the same phenomenon occurs during disconnection of capacitor banks. To determine the voltage stresses of the circuit breaker, however, the grounding condition of the supply system and capacitor bank and the arrangement of the bank have to be taken into account.

Closing on a Fault.

This directs the stress onto the circuit breaker contact system, particularly as regards the electrodynamic and thermal forces. The current and voltage stress is different during closing on (a) symmetrical or (b) asymmetric short-circuit current.

The deciding factor is the moment of contact touch relative to the phase angle of system voltage. In case contact touch and consequently ignition of the arc occurs at the voltage maximum, the short circuit current will appear symmetrical.

The other extreme case takes place with the moment of closing at voltage zero. Here the asymmetrical short-circuit current contains the maximum dc component. A contact system designed for fast closing operation will be subjected to a shorter arcing time and consequently to reduced contact burning when closing on symmetrical currents. Fast operation is therefore not only important for opening but also for circuit breaker closing.

TERMINAL FAULT, SHORT LINE FAULT AND OUT OF PHASE SWITCHING INTERRUPTING CONDITIONS OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

Terminal Fault.

After interruption of short-circuit current, the recovery voltage oscillates toward the service frequency driving voltage via an initial peak. The natural frequency is determined by the inductance and capacitance of the driving system.

The dc component of the short-circuit current depends on the time constants of the network components like generators, transformers, cables, and high-voltage lines and their reactances of the zero-sequence and the positive sequence networks.

The recovery voltage will accordingly vary depending on the location of the circuit breaker within the network.

Short-Line Fault.

In the case of a short-line fault, a section of line lies between the breaker and the fault location. After the short-circuit current has been interrupted, the oscillation at the line side (L) of the breaker assumes a superimposed “saw-tooth” shape.

The rate of rise of this line oscillation is directly proportional to the effective surge impedance and the time rate of change of current\ (di/dt) at current zero. The component on the supply side (S) basically exhibits the same waveform as a terminal fault.

The circuit breaker is stressed by the difference between these two voltages. Because of the high frequency of the line oscillation, the transient recovery voltage has a very steep initial rate of rise.

Since the initial rate of rise increases with increasing rate of current change, the limiting interrupting capability of many breaker designs is determined by the short-line fault.

Out-of-Phase Switching.

Two network systems with driving voltages E1 and E2 are connected via a high-voltage transmission line. Since the circuit is closed via the closed circuit breaker, the resulting driving voltage is equal to the sum of the two system voltages.

Driving voltage E2 may, for example, exceed voltage E1 by the voltage drop across the transmission line. After opening the breaker, the transient recovery voltages of the disconnected networks oscillate independently.

The circuit breaker is stressed by the difference of these two voltages. In the case of disconnection of long lines, the recovery voltage across the breaker could be increased because of the Ferranti effect, where the voltage of the receiving end can be up to 15% higher than the sending end if the line is lightly loaded.

ALTERNATING CURRENT (AC) INTERRUPTION OF POWER CIRCUIT BREAKERS BASIC INFORMATION

How Power Circuit Breaker Interrupt Alternating Current?

AC interruption occurs at current zero. During the following half-cycle, the recovery voltage will build up across the circuit breaker main contacts. The typical appearance of recovery voltage will differ in inductive, resistive, and capacitive circuits (see Fig. 10-62).

  

FIGURE 10-62 Typical shape of recovery voltage on interruption: (a) induction current; (b) resistive current; (c) capacitive current.

When opening an inductive circuit, the recovery voltage will rise suddenly at a high rate because current interruption occurs at the moment of system voltage peak. This case requires fast building of dielectric strength of the open contact gap.

When interrupting resistive load, current and voltage pass through zero at about the same moment. The recovery voltage will therefore rise at a moderate rate and no particular problems are imposed on the circuit breaker.

At the moment of interruption of capacitive current, the capacitance is fully charged. The recovery voltage rises slowly during the first half cycle but continues to rise to a value twice the system voltage.

This may lead to restrikes, undesired network oscillations, and over voltages. At the moment of fault current interruption the two sections—source side (S) and line side (L)—of the network are decoupled and oscillate independently about their driving voltage.

The difference of these two transients appears across the open contacts of the breaker pole. The behavior of this transient recovery voltage is determined by the circuit parameters.

The still-moving or already fully-open breaker contacts must be able to withstand the recovery voltage.

The most severe stress for the open contact gap is the initial peak and the rate of rise (kV/#s) of the recovery voltage.

If the recovery voltage exceeds the gap insulation, the arc will restrike and current will continue until the next current zero, when interruption will again be attempted. The rate of rise of recovery voltage is a function of the constants of the circuits which supply power through the breaker.

The larger the adjacent capacitance to ground before the major inductance limiting the fault current, the slower will be the rise of the recovery voltage. Some breakers modify the recovery voltage characteristics by limiting the current, modifying its power factor, and so on.

LOW VOLTAGE CIRCUIT BREAKER APPLICATION AND RATINGS BASIC INFORMATION

Application.

Air circuit breakers are used on dc and ac circuits for the protection of general lighting, power, and motor circuits. Distinction is made between various protection classes and different service and ambient conditions.

For selection of a breaker, type and rating, operating speed, selectivity with fuses, and high voltage must be taken into account. Further consideration has to be given to severe or hazardous service conditions like tropical climate or marine- or explosion-proof installations. Reference is made to IEEE C37.1313, C37.1414, and C37.1715; UL-48916; and ANSI C37.1617.

Ratings.

Standard electrically and manually operated breakers are listed in ratings up to and including 5000 A ac and 12,000 A dc. Electrically operated breakers are available in higher current ratings for special applications.

Standard breakers are rated on the basis of a temperature rise on the contacts and terminals not to exceed 50 deg C above an ambient of 40#C (class 90 insulation). Voltage ratings are 254 to 635 V ac and 250 to 3200 V dc.

The short-time current ratings are based on 3-phase symmetric short-circuit currents; the singlephase short-circuit current ratings are 87% of these values. For details, refer to the latest revisions of ANSI C37.1617.

Assembly Variations.

The breakers are usually installed in a metal-enclosed cubicle for dead-front or drawout type of construction. Metal barriers between breakers and busbars provide increased safety in service.

Hand operation by means of a lever is common, even on large breakers. Electric operation by means of a solenoid or motor mechanisms for 48, 125, or 250 V dc, or 120 or 240 V ac is obtainable on all but the smallest sizes of breakers.

Breakers are supplied with an overcurrent trip mechanism which may be of the instantaneous or the time-delay type, or a combination of both. Trip devices are adjustable over a wide range of ratings. Other trip devices and arrangements may be used, for example, undervoltage trips, shunt trips connected to overvoltage, reverse current, or overcurrent relays.

Multiple-pole circuit breakers are commonly used in practically all capacities, one pole being used for each ungrounded line of a circuit, that is, a 2-pole breaker for a 3-wire grounded circuit or a single-pole breaker for a 2-wire grounded circuit.

Breakers can usually be equipped with auxiliary contacts, alarm contacts, push-button control, position indicator, and key interlock. The widely used drawout type of breaker may be moved into and locked in the connected, test, and disconnected positions and/or completely withdrawn. Refer to the latest revisions of IEEE C37.1313, C37.1414, and C37.1715, and ANSI C37.1617.

SYMMETRICAL CURRENT BASIS OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

It has become a widely adopted practice to determine the interrupting capability of circuit breakers in kiloamperes symmetrical. The rated short-circuit current in rms kiloamperes is referred to the rated maximum voltage in kilovolts.

The ratings structure and tables of ratings for ac high-voltage circuit breakers are found in IEEE C37.042 and IEEE C37.063.

The short-circuit current interrupting process is characterized first by an arc appearing between the breaker contacts. The arc contains a high conductivity plasma column originating from the high temperature and related gas ionization (in the case of gas-blast interrupters).

Interruption will occur at current zero and in this case is first determined by successful cooling of the arc (through gas flow) to eliminate the ionized gas conductive path, and then the race to build up dielectric strength of the open contact gap faster than the rise of the power system recovery voltage.

Several specific problems are encountered during the interrupting process of gas-blast interrupters:

1. Arc plasma temperatures exceeding 20,000 K.

2. The turbulent supersonic flow of the quenching gas in a changing flow geometry with speeds ranging from a few hundred meters per second to several thousand meters per second.

3. The interrupter-moving system and its drive accelerates the moving masses in the few thousandths\ of a second to speeds as high as 10 m/s while simultaneously compressing the quenching gas.

4. The stress places on the network system by the current interruption and the recovery voltage.

The interrupting principle of an SF6 puffer-type interrupter is sketched in Fig. 10-60. On opening, the fixed and moving contacts are pulled apart by the operating mechanism. Thus, the fault current is forced to flow along the arc plasma.

  

The contact movement combined with the compression cylinder movement in the opposite direction compresses the quenching gas inside the cylinder. The quenching gas is consequently forced to flow through the contact system, and the insulated nozzle toward the exhaust.

This intensive flow of quenching medium along the arc rapidly removes the energy converted within the arc plasma and transforms the path between the open contacts into an insulating gap.

SHORT CIRCUIT DUTY OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

The short-circuit duty is determined by the maximum short-circuit that the rotating machinery connected to the system at the time of short circuit can pass through the breaker to a point just beyond the breaker, at the instant the breaker contacts open.

The short-circuit current is determined by the characteristics of synchronous and induction machines connected to the system at the time of the short circuit, the impedance between them and the point of short circuit, and the elapsed time between the starting of the short circuit and the parting of the breaker contacts.

In calculating short-circuit currents of high-voltage ac circuit, it is ordinarily sufficiently accurate to take into account only the reactance of the machines and circuits, whereas in low-voltage circuit resistance as well as reactance may enter into the calculation. In dc circuit, resistance only is ordinarily sufficient.

For first approximations, the reactance and typical time-decrement curves of the synchronous machines may be used. For close calculations, the actual reactances and time characteristics of the equipment should be used, and calculation made for single- as well as 3-phase faults.

The “per unit” impedance system and the “internal voltage” method, using “symmetrical components,” are often used in more exact calculations. Programs are available for digital computer studies of system shortcircuit currents, both balanced 3-phase and phase-to-ground.

The interrupting capacity, in kilovolt amperes, is the product of the phase-to-ground voltage, in kilovolts, of the circuit and the interrupting ability, in amperes, at stated intervals and for a specific number of operations. The current taken is the rms value existing during the first half-cycle of arc between contacts during the opening stroke.

PARTS OF SF6 POWER CIRCUIT BREAKERS BASIC INFORMATION

SF6 gas circuit breakers were first developed in the early 1950s by Westinghouse Corporation, following the discovery of the excellent arc quenching and insulating properties of SF6 gas.

Both live tank and dead tank designs were introduced from the late 1950s into the 1960s. SF6 remains the dominant insulating and arc-quenching medium at higher voltages (72.5 kV and above) even today.

Dead tank SF6 gas circuit breakers were incorporated into gas-insulated substations (GIS) up to 800 Kv from the mid-1960s through the present. Gas insulated substations offer space savings and environmental advantages over conventional outdoor substations, using the reduced insulation gap requirements of SF6 gas.

SF6 gas circuit breakers were initially of the two-pressure type, in which high pressure gas for interruption is compressed and stored for later interrupting duty.

Later designs employed the puffer principle, in which interrupting pressure is developed during the contact motion itself, and no high pressure gas is stored.

The latest designs of SF6 gas circuit breakers utilize the arc thermal energy itself to develop the interrupting pressure; these designs are referred to as self-blast or thermal-assist circuit breakers.

Below are the parts of Dead-tank SF6 circuit breaker.

POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

What Are Circuit Breakers?

Courtesy of Siemens

Circuit breakers are mechanical switching devices capable of making and breaking currents under either normal or specified abnormal (short circuit) conditions on the power system. 

Though circuit breakers are primarily defined by their protective capabilities and ratings under abnormal short circuit conditions, they also perform switching duties under a myriad of other system conditions, each of which has its own set of switching stresses.

Circuit breakers are rated primarily by power frequency voltage, insulation levels (BIL, switching impulse, hi-pot voltage), continuous current, short-circuit current, and interrupting time. Reference is made to IEEE C37.1001 for definitions of ratings subjects, and to IEEE C37.042 and IEEE C37.063 for values of ratings typically applied to circuit breakers.

Circuit breakers employ a variety of media for high voltage insulation and/or current interruption. The type of media employed in a specific design is often designated as a prefix in the naming of the circuit breaker, for example, vacuum circuit breaker, or sulfur hexafluoride (SF6) gas circuit breaker.

Circuit breakers are often categorized as being of either “dead tank” or “live tank” design. In the dead tank case, the interrupting contact system is enclosed in a grounded tank, typically surrounded by an insulating fluid (oil) or gas (SF6) (see Figs. 10-53 and 10-54 for examples of dead tank circuit breakers).

The electrical current enters the tank through high voltage entrance bushings, passes through the contact system, and then exits through another high voltage entrance bushing. In the live tank case, the interrupting contact system is supported by insulators at some height above ground potential, but is not contained within a grounded tank system.

There is no grounded tank or enclosure surrounding the live parts. Dead tank design allows the placement of current transformers, which are necessary for protective relaying input signals, around the high voltage entrance bushing.

Live tank design offers no location to place current transformers, and therefore must be independently placed adjacent to the circuit breaker.