Preston University

Preston University Subject: Project

6th Spring 2015 Electrical

Chapter No. 01

PROTECTION OF POWER SYSTEM

1.1 INTRODUCTION:

In daily life demand of electrical energy has been increased. It is essential for Lighting, Heating, Domestic equipments, Industrial Machines and electric Traction. In order to secure our precious and costly electrical equipments from being damaged. We have to introduce some protection. Therefore it is required to take out faulty parts from healthy and live systems that run to the system smoothly & properly. So Protection of the system has very important role in power transmission and distribution system. It's provided the facilities of ON/OFF switching in case of any abnormality and need for operation of electric circuit.

All ON/OFF equipments called switch gears. These switch gears consist on Protective devices. For Example Switches, Fuses, isolators, Circuit Breakers and Relays.

During the normal operation switch gear, generator, transmission lines distributor and other installed equipments are used for ON/OFF. But when ever any fault come in Power system e.g. (short circuit, low or high voltage) then very high current flow in the circuits or equipments. If faulty section did not separate from healthy circuit then losses will be more. Therefore in the supply system we need automatic switch gears & these switch gears consist on Relays and Circuit Breakers.

These relays are operating on Current, Voltage, Phase Angles and Frequency of our systems. For Protection system different types of relays used. These all relays need for operation instruments Transformers which are operate the relays. These instruments Transformers are CT (Current Transformer) and VT (voltage Transformer) which are used to operate the relays. Different schemes are used in Protection system but all are play on CT or VT.

The current transformers and voltage transformers are used for transforming the current and voltage to a lower value for the purpose of measurement, protection and control.

For trip circuit auxiliary supply will be need. This auxiliary source is DC. Which got from DC chargers and DC Batteries? These chargers charge the Batteries and provide the dc voltage for control circuit of protection systems. DC Batteries are used if chargers fail.

1.2 REPRESENTATION OF POWER SYSTEM

1.2.1 SINGLE LINE DIAGRAM

Three phase always represent with single Phase. In single Phase have one wire for Phase and another wire for Neutral. When it represent more simple then Neutral wire will not represent. All equipments represented by symbols. This type's of simple figures called single line diagram. Symbols are shown in Figure No. 1.1

Figure No. 1.1

COMPONENT

SYMBOL

ALTERNATE

Ammeter

Wattmeter

Voltmeter

Battery

Capacitor, Feed through

Capacitor, Fixed, Nonpolarized

Capacitor, Fixed, Polarized

Capacitor, Ganged, Variable

Capacitor, General

Capacitor, Variable, Single

Capacitor, Variable, Split-Stator

Fuse

Cell

Air Blast Circuit Breaker

Attenuator, Fixed

Attenuator, Variable

Ground, Chassis

Ground, Earth

Inductor, Air-Core

Inductor, Bifilar

Inductor, Iron-Core

Inductor, Tapped

Inductor, Variable

Integrated Circuit

Lamp, Incandescent

Lamp, Neon

Male Contact, General

Female Contact, General

Positive Voltage Connection

Negative Voltage Connection

Outlet, Utility, 117-V

Outlet, Utility, 234-V

Plug, Utility, 117-V

Plug, Utility, 234-V

Relay Coil

Potentiometer

Delay Line

Relay, DPDT

Relay, DPST

Relay, SPDT

Relay, SPST

Resistor

Fault

Rheostat

Saturable Reactor

Shielding

Signal Generator

Switch, DPDT

Switch, DPST

Switch, Momentary-Contact

Switch, Rotary

Switch, SPDT

Switch, SPST

Terminals, General, Balanced

Terminals, General, Unbalanced

Test Point

Thermocouple

Transformer, Air-Core

Transformer, Iron-Core

Transformer, Tapped Primary

Transformer, Tapped Secondary

1.2.2 Single Line Diagram of Generation using Symbols Shown in Figure No. 1.2

Figure No. 1.2

1.2.3 SINGLE LINE DIAGRAM OF SUBSTATION

A sub-station consists of many sections/bays. The main equipment in a section consists of circuit breakers, isolators or dis-connectors, earth switches, current transformers, surge arrestors, etc. Figure No 1.3 shows a single line diagram of a section at a sub-station identifying different components.

Figure No. 1.3

1.3EHV TRANSMISSION SYSTEM: CONTROL AND PROTECTION

The demand for power is growing rapidly due to increased industrial and agricultural activities in the country. Generating sets of large ratings are being set up to meet this requirement. The generator, which is the source of power in networks, is a major component in electrical installations. Equally important is the transmission system, which is used for distribution and the proper utilization of power generated by power plants. A fault or breakdown in the power plant or transmission network may have far-reaching consequences. It is therefore of paramount importance that the generators and transmission equipment are optimally used and efficiently protected.

Control and protection panels have a functionally important role. They perform the following functions:

(a) They provide facility for centralized control.

(b) They provide a point for centralized supervision at which all vital information relating to controlled equipment is received and assimilated.

(c) They provide for necessary protection and isolation facility of all power circuits like generators, feeders, transformers, bus-coupler, reactors, etc. The control and protection panels provide alarm and trip commands under abnormal conditions and hence function like a watchdog for the system.

1.3.1 TYPES OF EHV SYSTEMS

Two types of EHV transmission systems are generally employed in power system. These are detailed below.

1.3.1.1 ONE-AND-A-HALF BREAKER SYSTEM

It is a two-bus system. An arrangement of two circuits with the associated tie breaker is called a diameter. The diameter can be a line-line or line-transformer or transformer-transformer circuit diameter. Each diameter has three breakers for two circuits, hence the name one-and-a-half breaker system. Normally all the three breakers are closed and power is fed to both the circuits from two buses which operate in parallel. The middle breaker or the tie breaker acts as a bus-coupler for the two circuits.

The main advantage of this configuration is that a fault on any one bus is cleared instantaneously and yet all circuits continue to be fed from the other bus without any supply interruption. The same is true in case of a breaker stuck condition, but with a time delay. The system is otherwise more expensive as it employs a higher number of breakers.

In case of failure of the breaker of any one circuit, the power is fed through the breaker of the second circuit and the tie breaker. Each breaker, therefore, has to have a rating suitable for feeding both the circuits. A typical single line diagram of one-and-

1.3.1.2 TWO MAIN AND A TRANSFER BUS SYSTEM

This system has two main buses and one transfer bus. In case of maintenance on any one breaker, the particular circuit is connected to the system through a transfer bus-coupler or transfer breaker. Complete control and protection is transferred in this case to the transfer breaker through the trip transfer selector switch.

1.4 TYPICAL CIRCUITS IN AN EHV TRANSMISSION SYSTEM

The circuits in a typical EHV system are:

(a) Transmission lines;

(b) Generator transformers;

(c) Inter-connecting transformers;

(d) Feeder transformers;

(e) Reactors;

(f) Tie/transfer bus-coupler;

(g) Bus-coupler; and

(h) Bus bars.

1.5 FAULTS IN POWER SYSTEM

1.5.1 DEFINATION OF FAULTS

In an electric system definition of fault is that cause of defect in any electrical circuit flow of current stop or currents change his path or current value to much increase in full load working limits.

According to American Electrical Institute definition of fault is like this "A wire or cable fault is a partial or total failure in the insulation or continuity of a conductor ".

1.5.2 TYPES OF FAULTS IN POWER SYSTEM

1. Over Current

2. Under Voltage

3. Unbalance

4. Reversed Power

5. Surges

In following detail of above fault

1. OVER CURRENT

This fault is cause of short circuit or corona effect leakage and some time increasing the system load. Over current Relay used for control the fault.

2. UNDER VOLTAGE

This fault is cause of Lines fault and more voltage drop in the machines or failure the Alternator field. Under Voltage relay use for control the fault.

3. UNBALANCE

This fault is cause of one or two Phase ground or short circuit between the two Phases or cut of any one conductor. Due to this between different Phases different current flow. These types of faults called Unbalance fault. Unbalances relay use for control the fault.

4. REVERSED POWER

This type of fault produced only in interconnected system. Generator field when fail then it run like motor and Power return to generator. Means flow of power opposite direction. Generator can burn. Reverse Power Relay used for save the generator.

5. SURGES

Whenever Lightening or in near circuit produce serve fault then produce short live waves of high current and voltage in the lines. These types of fault called Surges. Lighting arrestor and over voltage used for control the surges.

1.6 INTEGRATED PROTECTION AND CONTROL SYSTEMS FOR SUB-STATION

Electrical power utilities have been using discrete electronics and electro-mechanical devices for power system protection, metering and supervisory control. Each device independently acquires and processes the power system data from current and potential transformers, circuit breakers, isolators, tap changers, etc., and performs the assigned function. Such a system suffers from two disadvantages. The first one is the cost associated with each device acquiring power system signals independently. The second and foremost disadvantage is that each device has only the information that it acquires directly for performing its function.

With advancements in computer technology, it has now become possible to introduce micro-computers, digital signal processors, and analogue to digital converters, optical transducer and fiber-optic communication systems to acquire and process electrical power system information in an effective manner and use it in the development of the integrated protection and control system for a sub-station. Such a system not only provides a cost-effective solution to the problems earlier faced by power utilities while using conventional protection and control equipments but adds the good features of MMI, disturbance records and event recording helpful for the post-fault analysis.

The integrated protection and control system provides the following features:

a. Power system protection;

b. Supervisory control and data acquisition;

c. Statistical and revenue metering;

d. Local control;

e. Voltage regulator;

f. Station battery monitoring;

g. Digital fault recording.

Figure 1.4 shows the system architecture for an integrated protection and control system for the sub-station. The above listed functions could be achieved by adopting three-level architecture which has been used to provide information at three distinct rates. The first level is assigned to the protection and metering system. Real time data processing is carried out at the highest rate for the protection and metering functions. For example, a sampling frequency of 800 Hz is required to discretise each current/voltage signal for computing the RMS values of that signal if the processing algorithm so selected is based on 16 samples per cycle. The control and monitoring of sub-station equipment is carried out at a relatively lower speed as compared to the protection system. Similarly the data analysis and archiving function is processed either after the critical event has passed or at a fixed interval of the order of hours. Thus the data is available at the highest rate in the first level (protection and metering), at a lower rate in the second level (control and sub-station monitoring) and at the lowest rate in the third level (data analysis and archiving). Each level incorporates processor(s) to perform the functions that are appropriate for the speed at which data is available at that level. The system is described below.

Figure No. 1.4: Connections to primary equipments

1.7 OBJECTIVE OF PROTECTION SCHEME

The main objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation. Thus, protection schemes must apply a very pragmatic and pessimistic approach to clearing system faults. For this reason, the technology and philosophies utilized in protection schemes are often old and well-established because they must be very reliable.

Protection systems usually comprise five components:

1. Current and voltage transformers to step down the high voltages and currents of the electrical power system to convenient levels for the relays to deal with;

2. Relays to sense the fault and initiate a trip, or disconnection, order;

3. Circuit breakers to open/close the system based on relay and auto recloser commands.

4. Batteries to provide power in case of power disconnection in the system.

5. Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment.

For parts of a distribution system, fuses are capable of both sensing and disconnecting faults.

CHAPTER NO. 02

TRANSFORMERS OF POWER SYSTEM

2.1 HISTORY

The transformer principle was demonstrated in 1831 by Michael Faraday, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee its practical uses. Viable designs would not appear until the 1880s, but within less than a decade, the transformer was instrumental during the "War of Currents" in seeing alternating current systems triumph over their direct current counterparts, a position in which they have remained dominant. Russian engineer Pavel Yablochkov in 1876 invented a lighting system based on a set of induction coils, where primary windings were connected to a source of alternating current and secondary windings could be connected to several "electric candles". The patent claimed the system could "provide separate supply to several lighting fixtures with different luminous intensities from a single source of electric power". Evidently, the induction coil in this system operated as a transformer.

Figure No. 2.1

A Historical Stanley Transformer.

Lucien Gaulard and John Dixon Gibbs, who first exhibited a device with an open iron core called a 'secondary generator' in London in 1882 and then sold the idea to American company Westinghouse. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system.

William Stanley, an engineer for Westinghouse, built the first commercial device in 1885 after George Westinghouse had bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886. Hungarian engineers Zipernowsky, Blthy and Dri from the Ganz company in Budapest created the efficient "ZBD" closed-core model in 1885 based on the design by Gaulard and Gibbs. Their patent application made the first use of the word "transformer. Russian engineer Mikhail Dolivo-Dobrovolsky developed the first three-phase transformer in 1889. In 1891 Nikola Tesla invented the Tesla coil, an air-cored, dual-tuned resonant transformer for generating very high voltages at high frequency. Audio frequency transformers (at the time called repeating coils) were used by the earliest experimenters in the development of the telephone.

While new technologies have made transformers in some electronics applications obsolete, transformers are still found in many electronic devices. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.

2.2 INTRODUCTION

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled electrical conductors. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.

The secondary induced voltage VS is scaled from the primary VP by a factor ideally equal to the ratio of the number of turns of wire in their respective windings:

By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up by making NS more than NP or stepped down, by making it less.

Transformers are some of the most efficient electrical 'machines', with some large units able to transfer 99.75% of their input power to their output. All operate with the same basic principles, though a variety of designs exist to perform specialized roles throughout home and industry.

2.3 INSTRUMENTS TRANSFORMERS

This chapter provides an overview of instrument transformers (current transformers and voltage transformers) which are required for the measurement of electrical parameters and for protection of equipment.

Instruments Transformers are Two Types.

1. Current transformer

2. Voltage transformers

2.3.1 CURRENT TRANSFORMER

A current transformer is a measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers are commonly used in metering and protective relaying, where they facilitate the safe measurement of large currents. The current transformer isolates measurement and control circuit from the high voltages typically present on the circuit being measured.

Figure No.2.2

A CT for operation on a 110kV grid

Current transformers are often constructed by passing a single primary turn (either insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. The CT is typically described by its current ratio from primary to secondary. For example, A 4000:5 CT would provide an output current of 5amperes when the primary was passing 4000amperes. Care must be taken that the secondary winding is not disconnected from its load while current flows in the primary, as this will produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer

a. DESIGN

The most common design of CT consists of a length of wire wrapped many times around a silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore consists of a single 'turn' of conductor, with a secondary of many hundreds of turns.

Figure No. 2.3

Current transformers, designed to be looped around conductors

The CT acts as a constant-current series device with an apparent power burden a fraction of that of the high voltage primary circuit. Hence the primary circuit is largely unaffected by the insertion of the CT. Common secondary are 1 or 5amperes. For example, A 4000:5 CT would provide an output current of 5amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs.

b. USAGE

Current transformers are used extensively for measuring current and monitoring the operation of the power grid. The CT is typically described by its current ratio from primary to secondary. Often, multiple CTs are installed as a "stack" for various uses (for example, protection devices and revenue metering may use separate CTs).

c. CONNECTIONS

Typically, the secondary connection points are labeled as 1S1, 1S2, 2S1, 2S2 and so on, or in the IEEE standard areas, X1...X5, Y1...Y5, and so on. The multi ratio CTs are typically used for current matching in current differential protective relaying applications.

For a three-stacked CT application, the secondary winding connection points are typically labeled Xn, Yn, Zn.

d. SAFETY PRECAUTIONS

Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary, and may permanently affect the accuracy of the transformer.

e. ACCURACY

The accuracy of a CT is directly related to a number of factors including:

burden

rating factor

load

external electromagnetic fields

temperature and

Physical configuration.

i. BURDEN

The burden in a CT metering circuit is essentially the amount of impedance (largely resistive) present. Typical burden ratings for CTs are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 can tolerate up to 0.2 of impedance in the metering circuit before its output current is no longer a fixed ratio to the primary current. Items that contribute to the burden of a current measurement circuit are switch blocks meters and intermediate conductors. This problem can be solved by using CT with 1 ampere secondary which will produce less voltage drop between a CT and its metering devices.

ii. RATING FACTOR

Figure No.2.4

Current transformers used in metering equipment for

Three-phase 400 ampere electricity supply

Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its absolute maximum measurable primary current. Conversely, the minimum primary current a CT can accurately measure is "light load," or 10% of the nominal current (there are, however, special CTs designed to measure accurately currents as small as 2% of the nominal current). This is made possible by the development of more efficient ferrites and their corresponding hysteresis curves. This is a distinct advantage over previous CTs because it increases their range of accuracy, since the CTs are most accurate between their rated current and rating factor.

iii. PHYSICAL CONFIGURATION

Physical CT configuration is another important factor in reliable CT accuracy. While all electrical engineers are quite comfortable with Gauss' Law, there are some issues when attempting to apply theory to the real world. When conductors passing through a CT are not centered in the circular (or oval) void, slight inaccuracies may occur. It is important to center primary conductors as they pass through CTs to promote the greatest level of CT accuracy.

iv. SPECIAL DESIGNS

Specially constructed wideband current transformers are also used (usually with an oscilloscope) to measure waveforms of high frequency or pulsed currents within pulsed power systems. One type of specially constructed wideband transformer provides a voltage output that is proportional to the measured current. Another type (called a Rogowski coil) requires an external integrator in order to provide a voltage output that is proportional to the measured current. Unlike CTs used for power circuitry, wideband CTs are rated in output volts per ampere of primary current.

2.3.2 VOLTAGE TRANSFORMERS

Voltage transformers (VT's) or potential transformers (PT's) are another type of instrument transformer, used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protection relays.

The transformer winding high-voltage connection points are typically labeled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is essential for proper operation of metering and protection relays.

While VT's were formerly used for all voltages greater than 240V primary, modern meters eliminate the need VT's for most secondary service voltages. VT's are typically used in circuits where the system voltage level is above 600 V.

Voltage transformers (VT's) are used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential.

2.3.2.1 CONSTRUCTION

i. CORES

Figure No. 2.5

Laminated core transformer showing edge of laminations at top of Unit.

ii. LAMINATED STEEL CORES

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. The steel has a permeability many times that of free space, and the core thus serves to greatly reduce the magnetizing current, and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy-current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce their magnitude. Thinner laminations reduce losses, but are more laborious and expensive to construct. Thin laminations are generally used on high frequency transformers, with some types of very thin steel laminations able to operate up to 10kHz.

Figure No.2.6

Laminating the core greatly reduces eddy-current losses

One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of "E-I transformer. Such a design tends to exhibit more losses, but is very economical to manufacture. The cut-core or C-core type is made by winding a steel strip around a rectangular form and then bonding the layers together. It is then cut in two, forming two C shapes, and the core assembled by binding the two C halves together with a steel strap. They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance.

A steel core's remanence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remanent magnetism is reduced, usually after a few cycles of the applied alternating current. Over current protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long, overhead power transmission lines, induced currents due to geomagnetic disturbances during solar storms can cause saturation of the core and operation of transformer protection devices.

iii. SOLID CORES

Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above main frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic ceramic materials called ferrites are common. Some radio-frequency transformers also have moveable cores (sometimes called 'slugs') which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

iv. TOROIDAL CORES

Figure No. 2.7

Small transformer with toroidal core

Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or perm alloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited rating.

v. AIR CORES

A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the windings in close proximity to each other, an arrangement termed an "air-core" transformer. The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysterics in the core material. The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution. They have however very high bandwidth, and are frequently employed in radio-frequency applications, for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings.

2.3.2.2 WINDINGS

Windings are usually arranged concentrically to minimize flux leakage

Figure No. 2.8 (B) Figure No. 2.8 (A)

Cut view through transformer windings. White: insulator. Green spiral: Grain oriented silicon steel. Black: Primary winding made of oxygen-free copper. Red: Secondary winding. Top left: Toroidal transformer. Right: C-core, but E-core would be similar. The black windings are made of film. Top: Equally low capacitance between all ends of both windings. Since most cores are (bad) conductors they also need insulation. Bottom: Lowest capacitance for one end of the secondary winding needed for low-power high-voltage transformers. Bottom left: Reduction of leakage inductance would lead to increase of capacitance.

The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enameled magnet wire, such as Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.

High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided litz wire to minimize the skin-effect and proximity effect losses. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor.

For signal transformers, the windings may be arranged in a way to minimize leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interleaved winding.

Both the primary and secondary windings on power transformers may have external connections, called taps, to intermediate points on the winding to allow selection of the voltage ratio. The taps may be connected to an automatic on-load tap changer for voltage regulation of distribution circuits. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar.

Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, one can replace air spaces within the windings with epoxy, thus sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers more suited to damp or dirty environments, but at increased manufacturing cost.

Three-phase oil-cooled transformer with cover cut away. The oil reservoir is visible at the top. Radiative fins aid the dissipation of heat. Figure No.2.9

2.3.2.3 COOLANT

Extended operation at high temperatures is particularly damaging to transformer insulation. Small signal transformers do not generate significant heat and need little consideration given to their thermal management. Power transformers rated up to a few kVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans. Specific provision must be made for cooling high-power transformers, the larger physical size requiring careful design to transport heat from the interior. Some power transformers are immersed in specialized transformer oil that acts both as a cooling medium, thereby extending the lifetime of the insulation, and helps to reduce corona discharge.

The oil-filled tank often has radiators through which the oil circulates by natural convection; large transformers employ forced circulation of the oil by electric pumps, aided by external fans or water-cooled heat exchangers. Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled transformers may be equipped with Buchholz relays, which detect gas evolved during internal arcing and rapidly de-energize the transformer to avert catastrophic failure.

Some "dry" transformers are enclosed in pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas. To ensure that the gas does not leak and its insulating capability deteriorates, the transformer casing is completely sealed. Experimental power transformers in the 2MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.

2.3.2.4 TERMINALS

Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.

2.4 CLASSIFICATION OF TRANSFORMER

The many uses to which transformers are put leads them to be classified in a number of different ways:

By power level: from a fraction of a volt-ampere (VA) to over a thousand MVA;

By frequency range: power-, audio-, or radio frequency;

By voltage class: from a few volts to hundreds of kilovolts;

By cooling type: air cooled, oil filled, fan cooled, or water cooled;

By application function: such as power supply, impedance matching, output voltage and current stabilizer, or circuit isolation;

By end purpose: distribution, rectifier, arc furnace, amplifier output;

By winding turns ratio: step-up, step-down, isolating (near equal ratio), variable.

CHAPTER NO. 03

CURRENT TRANSFORMERS IN PROTECTION SYSTEM

3.1 CT's USING AS MEASURING INSTRUMENTS

Measuring instruments, such as ammeters, voltmeters, kilowatt-hour meters, etc. whether electromechanical or electronic, meet insuperable design problems if faced with the high voltages or high currents commonly used in power systems. Furthermore, the range of currents employed throughout is such that it would not be practical to manufacture instruments on a mass production scale to meet the wide variety of current ranges required.

Current transformers are therefore used with the measuring instruments to:

(a) Isolate the instruments from the power circuits.

(b) Standardize the instruments, usually at 5 amps or 1 amp. The scale of the instrument (according to the C T ratio), then becomes the only non-standard feature of the instrument.

Figure No. 3.1

3.1.1 METER & PILOT LEAD BURDEN

Burden is the load imposed on the secondary of the CT at rated current and is measured in VA (product of volts and amps). The accuracy class applies only to loads at rated VA and below; down to one quarter VA .The burden on the secondary of a CT includes the effect of pilot leads, connections etc, as well as the instrument burden itself.

The Figure No. 3.1 (A) shows the burden imposed on the CT due to a run of pilot wire. It will be seen that a pilot loop of 2.5mm2 wire, 60 meters long (30 meters distance) has a load of 12.5 VA on a 5 amp CT but only 0.5VA on a 1 amp CT.

Figure No.3.1 (A)

3.1.2 ACCURACY CLASS

Accuracy classes for various types of measurement are set out in BSEN /IEC 60044-1. it will be seen that the class designation is an approximate measure of the accuracy, e.g, Class 1 current transformers have ratio error within 1% of rated current. Phase difference is important when power measurements are involved, i.e. Table No. 3.1

TABLE NO. 3.1 (A)

Class

% current ratio at % of ratio current shown below.

Applications

50

120

3

3

3

Ammeters

5

5

5

Approximate Measurements

TABLE NO. 3.1 (B)

Accuracy

% current ratio error at % of rated current shown below

Phase displacement (minutes) at % of rated current shown below

Applications

Class

5

20

100

120

5

20

100

120

0.1

0.4

0.2

0.1

0.1

15

8

5

5

Precision Testing & Measurement

0.2

0.75

0.35

0.2

0.2

30

15

10

10

Precision Grade Meters

0.5

1.5

0.75

0.5

0.5

90

45

30

30

Tarriff kWh Metering

1.0

3.0

1.5

1.0

1.0

180

90

60

60

Commercial kWh Metering

TABLE NO. 3.1 (C)

Accuracy

% current ratio error at % of rated current shown below

Phase displacement (minutes) at % of rated current shown below

Class

5

20

100

120

120

5

20

100

120

120

0.2s

0.75

0.35

0.2

0.2

0.2

30

15

10

10

10

0.5s

1.5

0.75

0.5

0.5

0.5

90

45

30

30

30

When using watt meters, kilowatt-hour meters, VAr meters and Power Factor meters.

Figure No. 3.2

3.1.3 WOUND PRIMARY CT'S

Thus, current transformers for 80 amps and below frequently require more than 1 turn to achieve the desired accuracy class. Considering again the previous example.

Using the same core and by winding 200 secondary turns and 4 primary turns a 50/1 ratio is achieved. The magnetizing ampere turns remains at 2 as before, however the magnetizing current becomes 2 divided by 4 turns or 0.5A and the percentage error is reduced to 1% (approx.).

It is therefore possible to achieve accuracy requirements, without using expensive core materials, by constructing a wound primary transformer. Of course, the cost of the primary winding with its insulation and terminations must be weighed against the cost of the more expensive core which would be required to achieve a 1% accuracy for a ring CT at a 50/1 ratio.

The table below details limits of error for current transformers for special applications and having a secondary current of 5A

3.1.4 AMPERE TURNS RULE

An ideal transformer is based on the Amperes Turns Rule, i.e. Primary Ampere Turns = Secondary Ampere Turns or: IpTp = IsTs (Ts/Tp=Ip/Is)

Thus the current transformation ins in INVERSE proportion to turns whereas voltage transformission is in DIRECT proportion to turns ie Ts/Tp=Vs/Vp

3.1.5 DESIGN CONSIDERATIONS

As in all transformers, errors arise due to a proportion of the primary input current being used to magnetize the core and not transferred to the secondary winding. The proportion of the primary current used for this purpose determines the amount of error.

The essence of good design of measuring current transformers is to ensure that the magnetizing current is low enough to ensure that the error specified for the accuracy class is not exceeded.

In these most common cases the CT is supplied with a secondary winding only, the primary being the cable or bus bar of the main conductor which is passed through the CT aperture in the case of ring CTs (i .e. single primary turn) it should be noted that the lower the rated primary current the more difficult it is (and the moreexpensive it is) to achieve a given accuracy.

Considering a core of certain fixed dimensions and magnetic materials with a secondary winding of say 200 turns (current ratio 200/1 turns ratio 1/200) and say it takes 2 amperes of the 200A primary current to magnetize the core, the error is therefore only 1% approximately. However considering a 50/1 CT with 50 secondary turns on the same core it still takes 2 amperes to magnetize to core. The error is then 4% approximately. To obtain a 1% accuracy on the 50/1 ring CT a much larger core and/or expensive core material is required.

3.1.6 SATURATION

Magnetic materials are such that when the magnetic flux reaches a certain value the core will saturate. At this point a large proportion of the primary current is required to magnetize the core increasing the primary current in the saturation region will therefore cause only a marginal increase in secondary current.

This phenomenon can be used to protect an instrument against damage due to heavy over current and a Saturation Factor is sometimes specified. For example, if a Instrument Sensitivity Factor (Fs) of less than 5 is specified, the CT must be designed to ensure that, at the rated burden, the core is well into the saturation region (defined point) at 51 times the rated primary current.

3.1.7 TAPED RING CT'S (R RANGE)

Suitable for Indoor use, are probably the most common type of CT Installed in LV switchgear and control gear Circular cores wound in clock spring fashion provide a near ideal magnetic circuit, free of air gaps and having very low leakage or stray fields. After applying suitable robust insulation to these cores, the windings are applied evenly around the cores by tropical winding machines. Taped ring CTs are so used extensively in HV switchgear and power transformers where the insulation is provided by the HV bushings. Where CTs are to be installed under hot oil, insulation materials have to be selected to avoid pollution of the oil.

3.1.7 OPEN CIRCUIT CURRENT TRANSFORMERS

It is important to ensure that the secondary of any CT is not left disconnected while the primary supply is on. In this condition, high voltage spikes are produced in the transformer secondary, often thousands of volts, sufficient to break down the transformer insulation.

3.1.9 DUAL OR MULTI-RATIO TRANSFORMERS

Frequently when a new plant is commissioned, it is planned for further extension and consequent increase in power consumption. In this event, it may be advisable to install dual ratio CTs with a tapped secondary to allow alterations to the metering without the expense and disruption of replacing the CTs. In this case, the unused terminal should be left open-circuit.

3.1.10 CONSTRUCTION OF MEASURING CURRENT TRANSFORMERS

Measuring current transformers are available in a variety of different forms and terminations to meet the requirements of the particular installation.

Model R25

Model R40

Model R80

Model R102

Model R125

Model M102

Model M10235

3.1.11 RESIN CAST CT'S

Produced by casting the transformer in liquid resin which is then cured to the solid state. These transformers are expensive, but they are robust and immune to difficult climatic conditions. Special resins are now available to make the transformers suitable for outdoor use. Resin Cast CTs are commonly used at high voltages, up to 33kV. In these applications, highly controlled casting techniques are required to avoid air voids where corona discharge could affect the quality of insulation.

3.1.12 ENCLOSED MOLDED CASE CT'S (M RANGE)

Similar to the ring CTs but enclosed in tough injection molded shells to provide a clean and uniform appearance. Mounting brackets and facilities for bus bar clamps and other accessories can be incorporated in the mould design thereby resulting in a lower cost unit. The speed and simplicity with which molded case CTs can be clamped to bus bars is an important feature of the unit.

Model MT0

Model MWBD

Model M15

Model M1530

Model MT21

Model M25

Model M2530

Model MT33

Model M40

Model M4032

Model M4085

Model MT53

Model MT61

Model M64

Model M6431

3.2 CT's USING AS PROTECTION SCHEEM

Protective Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit. These replica currents are used as inputs to protective relays which will automatically isolate part of a power circuit In the event of an abnormal or fault condition therein, yet permit other parts of the plant to continue in operation.

Figure No.3.3

Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high over currents, requiring the protective current transformer to be linear, and therefore below magnetic saturation. at values up to perhaps 30 times full load current .This wide operating range means that protective current transformers require to be constructed with larger cross-sections resulting in heavier cores than equivalent current transformers used for measuring duties only. For space and economy reasons, equipment designers should however avoid over specifying protective current transformers ITL technical staff are always prepared to assist in specifying protective CT's but require some or all of the following information

Protected equipment and type of protection.

Maximum fault level for stability.

Sensitivity required.

Type of relay and likely setting.

Pilot wire resistance, or length of run and pilot wire used.

Primary conductor diameter or bus bar dimensions.

System voltage level.

Accuracy classes

Accuracy classes are defined as 5P or 10P with limits according to the following table No.5.1extracted from I EC 60044-1

3.2.1 CAUTION

RELAY MANUFACTURER'S RECOMMENDATIONS SHOULD ALWAYS BE FOLLOWED

i. IEC SPECIFICATION

According to 60044-1 protective current transformers are specified as follows:

Rated Output:

The burden including relay and pilot wires Standard burdens are 2.5,5,75,10, 15 and 30VA.

Figure No.3.4

Table No.3.2

Accuracy Class

Current error at rated primary current

Phase displacement at rated primary current

Composite error at rated accuracy limit primary current

%

min

centiradians

%

5P

"1

"60

"1.8

5

10P

"3

10

ii. ACCURACY LIMIT FACTOR

Accuracy limit Factor is defined as the multiple of rated primary current up to which the transformer will comply with the requirements of 'Composite Error'. Composite Error is the deviation from an ideal CT (as in Current Error), but takes account of harmonics in the secondary current caused by non-linear magnetic conditions through the cycle at higher flux densities.

Standard Accuracy Limit Factors are 5, 10, 15, 20 and 30. The electrical requirements of a protection current transformer can therefore be defined as:

iii. RATIO/VA BURDEN/ACCURACY CLASS/ACCURACY LIMIT FACTOR

For example:

1600/5, 15VA 5P10 Selection of Accuracy Class & Limit Factor

3.2.2 GENERAL

Class 5P and 10P protective current transformers are generally used in over current and unrestricted earth leakage protection. With the exception of simple trip relays, the protective device usually has an intentional time delay, thereby ensuring that the severe effect of transients has passed before the relay is called to operate. Protection Current Transformers used for such applications are normally working under steady state conditions Three examples of such protection is shown. In some systems, it may be sufficient to simply detect a fault and isolate that circuit.

Figure No. 3.4

3.2.3 PHASE FAULT STABILITY

Current transformers which are well matched and operating below saturation, will deliver no current to the earth fault relay, If however, the transformers are badly matched, a spill current will arise which will trip the relay. Similarly, current transformers must operate below the saturation region, since, in a 3 phase system, third harmonics in the secondary are additive through the relay thereby creating instability and erroneously tripping the earth fault relay.

3.2.4 TIME GRADING

Time lags on relays are set in such a way that a fault in a subsection will isolate that section of the distribution only. Accurate time grading can be adversely affected by inaccuracy or saturation in the associated current transformer. The following table is intended to show typical examples of CT applications However; in all cases manufacturer's recommendations must be followed. Table No. 3.3

Table No. 3.3

Protective System

CT Secondary

VA

Class

Current for phase & earth fault

1A

5A

2.5

7.5

10P20 or 5P20

10P20 or 5P20

Unrestricted earth fault

1A

5A

2.5

7.5

10P20 or 5P20

Sensitive earth fault

1A or 5A

Class PX use relay manufacturers formulae

Distance protection

1A or 5A

Class PX use relay manufacturers formulae

Differential protection

1A or 5A

Class PX use relay manufacturers formulae

High impedance differential impedance

1A or 5A

Class PX use relay manufacturers formulae

High speed feeder protection

1A or 5A

Class PX use relay manufacturers formulae

Motor protection

1A or 5A

5

5P10

3.2.5 BALANCED FORMS OF PROTECTION

In balanced systems of protection, electrical power is monitored by the protective CTs at two points in the system as shown in Figure No. (3.5).

Figure No.3.5

The protected zone is between the two CTs If the power out differs from the power in, then a fault has developed within the protected zone and the protection relay will operate. A 'Through Fault' is one outside the protected zone Should such a fault occur, the relay protecting the protected zone will not trip, since the power out will still equal the power in. Numerous different types of balanced systems exist and advice may often have to be obtained from the relay manufacturer. However, in all cases Sensitivity and Stability must be considered.

3.2.6 SENSITIVITY

Sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. To provide fast operation on an in zone fault, the current transformer should have a 'Knee Point Voltage' at least twice the setting voltage of the relay.

The 'Knee Point Voltage' (Vkp) is defined as the secondary voltage at which an increase of 10% produces an increase in magnetizing current of 50%. It is the secondary voltage above which the CT is near magnetic saturation.

Differential relays may be set to a required sensitivity but will operate at some higher value depending on the magnetizing currents of the CTs, for example.

The diagram shows a restricted earth fault system with the relay fed from 400/5 CTs. The relay may be set at 10%, but it requires more than 40A to operate the relay since the CT in the faulty phase has to deliver its own magnetizing current and that of the other CTs in addition to the relay operating current.

Figure No.3.6

3.2.7 STABILITY

That quality whereby a protective system remains inoperative under all conditions other than those for which it is designed to operate, i.e. an in-zone fault Stability is defined as the ratio of the maximum through fault current at which the system is stable to nominal full load current. Good quality current transformers will produce linear output to the defined knee point voltage (Vkp).

Typically,

Vkp = 2If (Rs+Rp) for stability, where

If = max through fault secondary current at stability limit

Rs = CT secondary winding resistance

Rp = loop lead resistance from CT to relay

3.2.8 TRANSIENT EFFECTS

Balanced protective systems may use time lag or high speed armature relays. Where high speed relays are used, operation of the relay occurs in the transient region of fault current, which includes the dc a symmetrical component. The buildup of magnetic flux may therefore be high enough to preclude the possibility of avoiding the saturation region.

The resulting transient instability can fortunately be overcome using some of the following techniques.

a)Relays incorporating capacitors to block the dc asymmetrical component.b) Biased relays, where dc asymmetrical currents are compensated by anti phase coils.

c) Stabilizing resistors in series with current operated relays or in parallel with voltage operated relays. These limit the spill current (or voltage) to a maximum value below the setting value.

For series resistors in current operated armature relays.

Rs = (Vkp/2) - (VA/Ir),where:

Rs = value of stabilizing resistor in ohms

Vkp = CT knee point voltage

VA = relay burden (typically 3VA)

Ir = relay setting current

Note:

The value of Rs varies with each fault setting. An adjustable resistor is therefore required for optimum results. Often a fixed resistor suitable for mid-setting will suffice.

3.2.9 CLASS PX PROTECTION CT'S

Class 5P protection current transformers may be adequate for some balanced systems, however more commonly; the designer will specify a special 'Class PX' CT giving the following information.

(a) Turns ratio.(b) Knee point voltage Vkp.(c) Maximum exciting current al Vkp.(d) CT secondary resistance.

TABLE NO 3.4

Apparatus

Protective System

Min. Stability Limit x Rated Current

Generator & Synchronous Motors

Differential Earth Fault

Longitudinal Differential

12.5

12.5

Transformers

Differential Earth Fault

Longitudinal Differential

16

16

Induction Motors / Busbars Feeders

Differential Earth Fault

Longitudinal Differential

1.25x Starting Current, 1x Switchgear short-circuit rating, short circuit rating, 30

3.2.10 PILOT WIRE BURDEN FOR CLASS PX CTS

Figure No.3.7

For 'Class X' current transformers, the cross section and length of pilot wires can have a significant effect on the required Vkp and therefore the size and cost of the CT. When the relay is located some distance from the CT, the burden is increased by the resistance of the pilot wires.

The graph shows the additional burden of pilot leads of various diameters. It should be noted that, by using a 1 amp instrument and CT, the VA burden imposed by the pilot wires is reduced by a factor of 25

CHAPTER NO. 4

CIRCUIT BREAKERS

A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Figure No.4.1

A 2 pole miniature circuit breaker

4.1 PRINCIPLES

4.1.1 CLOSING

In dc circuits the current reaches about 95% of its final steady state value after time 3, where is the time constant. Similarly, when an alternating current is closed, the current reaches a steady value after a transient process. The time depends upon the resistive, inductive and capacitive elements of the circuit. The highest switching current is achieved if switching is effected at zero voltage (very high peak currents can develop if the switch is closed on short circuit conditions).

4.1.2 OPENING

Below a threshold voltage, any circuit can be opened without any arc formation. In practice, however, the commonly used switches do produce an arc while interrupting the current. The arc must be either kept limited or extinguished at the earliest in order not to damage the contacts.

4.1.3 CONTACTS

Copper is by far the most widely used contact material. But since non-conducting layers are formed on copper contacts as a result of switching, a wiping action is provided while designing copper contacts. These are also plated with a layer of silver in many applications. In low voltage circuits, silver is also in use as contact material.

4.1.4 ARC EXTINCTION

Since switching almost invariably gives rise to arcing, extinguishing such arcs assumes vital importance to prolong contact life.

The following methods are employed:

1. Lengthening of the arc till it extinguishes

2. Intensive cooling (in jet chambers)

3. Division into partial arcs

4. Zero point quenching

5. Connecting capacitors in parallel with contacts in dc circuits f) Use of vacuum g) Use of air h) Use of oil

4.1.5 INSULATION

The contacts need to be kept properly insulated from other metal parts including the body. Different insulating materials are in use. The most commonly used material is cast epoxy. Besides, PVC, polystyrene, polycarbonate and ceramics are also in use.

4.2 OPERATION

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Large high-voltage circuit breakers have separate devices to sense an over current or other faults. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically stored energy within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. When a current is interrupted, an arc is generated - this arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Finally, once the fault condition has been cleared, the contacts must again be reclosed to restore power to the interrupted circuit.

4.2.1 MAGNETIC CIRCUIT BREAKER

Magnetic circuit breakers use a solenoid (electromagnet) that's pulling force increases with the current. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.

4.2.2 THERMAL CIRCUIT BREAKER

Thermal breakers use a bimetallic strip, which heats and bend with increased current, and is similarly arranged to release the latch. This type is commonly used with motor control circuits. Thermal breakers often have a compensation element to reduce the effect of ambient temperature on the device rating.

4.2.3 THERMOMAGNETIC CIRCUIT BREAKER

Thermo magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over current conditions.

4.2.3 HIGH VOLTAGE CIRCUIT BREAKERS

Circuit breakers for larger currents are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.

4.3 ARC INTERRUPTION

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings will have metal plates or non-metallic arc chutes to divide and cool the arc.

Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.

Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (