Mapua Institute of Technology

SURGE and LINE PROTECTION OF POWER SYSTEMS

by

Engr. Michael C. Pacis

What is a Power System?- a power system is one of the tools in converting and transporting electrical energy

- the conveyance of electric power from a power station to consumers premises. MCPACIS 2005

Single Line Diagramsprovides a compact way to represent a great deal of additional information for various devices in the power system.in a architect if theres a building layout, in a power systems engineer there is a one line diagram.usually expressed in line to neutral basis. MCPACIS 2005

POWER QUALITY:TRANSMISSION & DISTRIBUTIONCauses:Transmission line failureFlash-over from lightningHigh voltage switches re-closingDisturbances caused by wind generatorsCable failuresEtc..

Its a long way from power producer to end-user

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Different types of Protection in a Power System1. Fuse2. Circuit Breakers3. Relays4. Surge Arresters

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What is a fuse?is a short piece of metal, inserted in the circuit, which melts when excessive current flows though it and thus, it breaks the circuit.invented in 1890 by Thomas Alva Edison.the fuse element is gradually made of materials having low melting point, high conductivity and least deterioration due to oxidation.it has a temperature below its melting point. the greater the excessive current, the smaller is the time taken by the fuse to blow out.MCPACIS 2005

Time Vs Current Curve Figure 1. time vs current curve for a 100 A fuseIt may take 10 minutes or more to blow a fuse at 25% over its rated current. MCPACIS 2005

Examples of FusesBlade FuseANL FuseGlass FuseMEGA FuseMCPACIS 2005

Fuse and Switch ApplicationsMCPACIS 2005

Important Terms1. Current Rating is the current which the fuse element can normally carry without overheating or melting.

2. Fusing Current it is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it.- to compute for the fusing current I for a round wire, we use the formula:

I = kd3/2

I = fusing current, Ad = diameter of the wire, cmK = fuse constantMCPACIS 2005

Table 1. Fuse constants for various elements3. Fusing Factor the ratio of minimum fusing current to the rating of the fuse element.- the value is always more than one, the smaller the fusing factor, the greater the difficulty in avoiding deterioration due to overheating and oxidation at rated carrying current.MCPACIS 2005

4. Breaking Capacity it is the rms value of a.c. component of maximum prospective current that a fuse can deal with at rated service voltage.Figure 2. how a.c. current is cutoff by a fuseMCPACIS 2005

Advantages of a fuse1. It is the cheapest form of protection available.2. It requires no maintenance.3. Its operation is inherently completely automatic4. It can break heavy short circuit currents without noise or smoke.5. The smaller sizes of fuse element impose a current limiting effect under short circuit conditions.6. The inverse time-current characteristic of a fuse makes it suitable for overcurrent protection.7. The minimum time of operation can be made such shorterthan with the circuit breakers.

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Disadvantages of a fuse1. Considerable time is lost in rewiring or replacing a fuse after operation.2. On heavy short-circuits, discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned.3. The current-time characteristic of a fuse cannot always be co related with that of the protected apparatus.MCPACIS 2005

Identification of FusesFigure 3. How to identify a fuseMCPACIS 2005

Desirable Characteristic of a fuse element1. Low melting point2. High conductivity3. Free from deterioration4. Low costMCPACIS 2005

Fuse Element Materials1. Lead2. Tin3. Copper4. Zinc5. SilverMCPACIS 2005

Types of fuses used in a Power SystemI . Low Voltage Fuse made up to 500A rated current, the breaking capacity is about 4000 A.1. Semi- enclosed rewirable fuse also known as kit-kat type, is used where low values of fault current are to be interrupted. It consists of a base and a fuse carrier. Figure 4. Semi enclosed rewirable fuseMCPACIS 2005

2. High Rupturing Capacity ( H.R.C. ) Cartridge Fuse- the primary objection of low and uncertain breaking capacity of semi enclosed rewirable fuses is overcome in H.R.C.

60 A HRC FuseMCPACIS 2005

II. High Voltage Fuse are used up to 33 KV with breaking capacity of 8700 A at that voltage.1. Cartridge Type this is similar in general construction to the low voltage cartridge type except that special design features are incorporated.2. Liquid Type these fuses are filled with carbon tetrachloride and have the widest range of applications to high voltage systems.- they may be used for circuits up to about 100 A rated current on systems up to 132KV and have a breaking capacity of 6100 A.3. Metal Clad Fuses metal clad oil-immersed fuses have been developed with the object of providing a substitute for the oil circuit breaker.MCPACIS 2005

CIRCUIT BREAKERSIn the earlier days, the function of the switching on and off of various circuits is performed by a switch and a fuse placed in series with the circuit.

Disadvantages of this set upTime of Replacement.A fuse cannot successfully interrupt heavy fault currents that results from faults on modern high-voltage and large capacity circuit. Applications of this set-upOnly use for low voltage and small capacity circuit.For switching and protection of distribution transformers, lighting circuits and branch circuits of distribution lines MCPACIS 2005

What is a Circuit Breaker?Break a circuit automatically under fault condition.Make a circuit either manually or by remote control under fault condition.Make a circuit either manually or by remote control under normal condition.

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Operating PrinciplesA circuit breaker essentially consists of stationary and moving contacts called electrodesUnder normal conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. These contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the breaker become energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit. When the contacts of the circuit breaker are separated under fault conditions, an arc is struck between them MCPACIS 2005

Components and Parts of a Circuit BreakerFigure 5. Side View of a typical Circuit BreakerMCPACIS 2005

Figure 6. Assembly of a typical Circuit BreakerMCPACIS 2005

Important Terms1. Arc Voltage It is the voltage that appears across the contacts of the circuit breaker during the arcing period.2. Restriking Voltage It is the transient voltage that appears across the contacts at or nears current zero during arcing period.3. Recovery Voltage It is the normal frequency (50 hz) r.m.s. voltage that appears across the contacts of the circuit breaker after the final arc extinction. It is approximately equal to the system voltage.

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How current was interrupted by a CB? Figure 7. Interruption of A.C. current in a CBMCPACIS 2005

Methods of Arc Extinction

1. High Resistance Method In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc.The resistance of the arc maybe increased by: a. Lengthening the arcb. Cooling the arcc. Reducing cross-section of the arcd. Splitting the arc MCPACIS 2005

2. Low Resistance or Current Zero Method In this method, arc resistance are kept low until current zero where the arc extinguishes naturally and is prevented from restriking in spite of the rising voltage across the contacts.

The de-ionization of the medium can be achieved by:a. Lengthening the gapb. High pressurec. Coolingd. blast effectMCPACIS 2005

Classification of Circuit BreakersOil Circuit Breakers which employ some insulating oil for arc extinction.Air Blast Circuit Breaker in which high pressure air-blast is used for extinguishing the arc.Sulfur Hexa Fluoride Circuit Breaker in which sulfur hexe fluoride (SF6) gas is used for arc extinction.Vacuum Circuit Breaker in which vacuum is used for arc extinction.MCPACIS 2005

Oil Circuit BreakerIn such circuit breaker, some insulating (e.g. transformer oil) is used as an arc quenching medium. The contacts are opened under oil and is struck between them.The heat of the arc evaporates the surrounding oil and dissociates into a substantial volume of gaseous hydrogen at high pressure.The hydrogen gas occupies a volume of about 1000 times that of the oil decomposed. Application is limited only to 11 kV.Figure 8. OCB operationMCPACIS 2005

AdvantagesIt absorbs the arc energy to decompose the oil into gases which have excellent cooling properties.It act as an insulator and permits smaller clearance between live conductors and earthed components.The surrounding oil presents cooling surface in close proximity to the arc.DisadvantagesIt is flammable and there is a risk of fire.It may form an explosive mixture with air.MCPACIS 2005

AIR-BLAST CIRCUIT BREAKER-These breakers employ a high pressure air blast as an arc quenching medium. The contacts are opened in a flow of air blast established by the opening of the blast valve. The air-blast cools the arc and sweeps away the arcing products to the atmosphere, this rapidly increases the dielectric strength of the medium between contacts and prevents from re-establishing the arc. Application ranging 110 kV for this type.Figure 9. Types of ACB operationMCPACIS 2005

AdvantagesRisk of fire is eliminatedArc produced are completely removed by the blast whereas the oil deteriorates with successive operations.The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small.Due to lesser arc energy, air-blast circuit breakers are very suitable for conditions where frequent operation is required.DisadvantagesThe air has relatively inferior arc extinguishing properties.The air-blast circuit breaker are very sensitive to the variations in the rate of rise of restriking voltage.Considerable maintenance is required for the compressor plant which supplies the air-blast.MCPACIS 2005

SULFUR HEXA FLUORIDE (SF6) CIRCUIT BREAKERSulfur Hexa Fluoride gas is used as the arc quenching medium. The SF6 is an electro-negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6 gas and an arc is struck with them. The conducting free electrons in the arc rapidly capture by the gas to form relatively immobile negative ions, this loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. Application range from 115 kV to 830 kV.MCPACIS 2005

CONSTRUCTION OF SF6 CBFigure 10. Operation of SF6 Circuit BreakerMCPACIS 2005

AdvantagesDue to superior arc quenching property of SF6, such circuit breakers are very short arcing time.Since the dielectric strength of SF6 gas is 2 to 3 times that of the air, such breakers can interrupt much larger current.The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to the atmosphere unlike the air blast circuit breaker.The closed gas enclosure keeps the interior dry so that there is no moi9sture problem.There is no risk of fire in such breakers because SF6 gas is non-flammable.There are no carbon deposits so that tracking and insulation problems are eliminated.Since SF6 breakers are totally enclosed and sealed from atmosphere, they are particularly suitable where explosion hazards exist.DisadvantagesSF6 breakers are costly Since Sf6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.MCPACIS 2005

VACUUM CIRCUIT BREAKERS-In such breakers, vacuum (degree o9f vacuum in the range of 10-7 to 10-5 torr), is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far arc quenching properties than any other medium. For example, when contacts of a breaker are opened in a vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than that obtained with other circuit breaker. Application ranging form 22 kV to 66 kV and any 60 to 100 MVA capacity.MCPACIS 2005

CONSTRUCTION OF A VACUUM CBFigure 11. Operation of Vacuum Circuit BreakerMCPACIS 2005

AdvantagesThey are compact, reliable and have longer life.There are no fire hazards.There are no generation of gas during and after operation.They can interrupt any fault current.They require little maintenance and are quiet in operation.They can successfully withstand lightning surges.They have low arc energy.They have low inertia and hence require smaller power for control mechanism.MCPACIS 2005

SELECTION OF CIRCUIT BREAKERSInterrupting Capacity The current in which the circuit breaker must be able to interrupt at the time contacts apart.KAIC = Factor x Isc

For Circuit Breaker above 600 VFactors:1.0---------------8 cycle duty1.1---------------5 cycle duty1.3---------------3 cycle duty1.2---------------2 cycle duty1.4---------------15 cycle dutyFor CB below 600 V

Interrupting capacity = 1.0 IscMCPACIS 2005

Momentary Current Rating the current which a breaker carry immediately after the fault occurs.MCR = factor x IscFor Circuit Breaker above 600 VFactors:1.6----------for HV above 5 kV1.5----------directly feed from generator(remote from the station)(600 to 5001 V)For CB below 600 V

MCR = 1.25 Isc SELECTION OF CIRCUIT BREAKERSMCPACIS 2005

SELECTION OF HIGH VOLTAGE FUSESThree Phase KVA Interrupting Duty ( KVAID )KVAID = KVAsc (For Current Limiting Fuses above 600 V)Kilo Ampere Interrupting Capacity or Duty (KAIC)

KAIC = Factor x Isc

For Current Limiting Fuses above 600VFactor:1.6 ------------------ anywhere in the system

For Low voltage Fuses below 600VFactor:1.25 ---------------- anywhere in the system

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SAMPLE PROBLEMS5. Calculate the MCR and KAIC ratings of the circuit breaker and the available fuse ratings to limit the fault current at point F. Use base values of MVAbase = 10 and KVbase = 6.24.M1M2M3M430 MVA%X= 1513.8 KV30 MVA%X= 1013.8 KV/6.24KVeach motor has10 MVA%X= 20FMCPACIS 2005

SAMPLE PROBLEMSLet: KVAbase = 10,000 and KVb = 6.24Computing for Isc3phase = 29607.706 ASolving for MCR and KAIC of Circuit BreakerSince above 5 KVKAIC = (1.1) x (29,607.706) = 32.768 KAMCR = (1.6) x (29,607.706) = 47.372 KASolving for KVAID and KAIC of the required fuseKVAsc = KVAbase x Xth = 10,000 x (100 / 3.125)KVAsc = 320,000KAIC = (1.6) x (29,607.706) = 47.372 KA

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PROTECTIVE RELAYa device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

PARTS1. Primary winding of a current transformer2. Secondary winding of current transformer.3. Tripping circuitFigure 12. Relay operationMCPACIS 2005

THREE PHASE RELAY OPERATIONMCPACIS 2005

Fundamental requirements of Protective RelayingI. Selectivity the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system.MCPACIS 2005

PROTECTIVE ZONES1. generators2. low-tension switchgear3. transformers4. high-tension switchgear5. Transmission linesII. Speed the relay should disconnect the faulty part as soon as possible.III. Sensitivity it is the ability of the relay system to operate with low value of actuating quantity.- the smaller VA input required to the system, the more sensitive the relay.MCPACIS 2005

IV. Reliability it is the ability of the relay system to operate under the pre-determined conditions.

V. Simplicity the relaying system should be simple so that it can be easily maintained. The simpler the protection scheme, the greater will be its reliability.

VI. Economy the most important aspect of a particular protection scheme.- the protective gear should cost 5% of the total cost.MCPACIS 2005

BASIC RELAYSI. Electromagnetic Attraction Relays- operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid.Types:1. Attracted Armature type relay Figure 14. Construction of an armature type relayMCPACIS 2005

2. Solenoid type relay consists of a movable iron plunger.

3. Balanced Beam Relay consists of an iron armature fastened to a balanced beam.

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II. Electromagnetic Induction Relays- operates on the principle of induction motor and are widely used for protecting relaying purposes involving a.c. quantities.- consists of a pivoted aluminum disc placed in two alternating magnetic fields of the same frequency but displaced in time and spaced.Figure 15. elementary arrangement of induction relaysMCPACIS 2005

Types of Induction Relays1. Shaded-pole Structure consists of a pivoted aluminum disc to rotate in the air gap of an electromagnet. One half of each pole of the magnet is surrounded by a copper band known as shaded ring.

2. Watthour-meter Structure it consists of a pivoted aluminum disc arranged to rotate freely between the poles of two electromagnets. This relay is inoperative by opening the secondary winding.MCPACIS 2005

Surge Arresters- used to protect the transformer from lightning surgesMCPACIS 2005

CREDITSPower System Analysis and Design by Mulukutha SarmaPower System Analysis by StevensonElements of Power System Analysis by MethaMCPACIS 2005

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