EENG 6O46 : POWER SYSTEM PROTECTION AND CIRCUIT

BREAKERS

Course Description• CHAPTER – I: PROTECTION OF POWER APPARATUS &

TRANSMISSION SYSTEMS• General philosophy of protection – Characteristic function of

protective relays – basic relay elements and relay terminology – basic construction of static relays – non-critical switching circuits. Protection of generators stator phase fault protection –loss of excitation protection, generator off-line protection – Transformer protection – factors affecting differential protection – magnetizing inrush current – Application and connection of transformer differential relays – transformers over current protection – Example motor protection. Bus protection -line protection – classification of lines and feeders – Techniques applicable for line protection – distance protection for phase faults – Fault resistance and relaying – long line protection – Backup remote local and Breaker failure.

CHAPTER – II: PROTECTION OF REACTORS, BOOSTERS & CAPACITORS

• Placement of reactors in power system – Types of reactor –

reactor rating application and protection – booster in the power system – transformer tap changing – protection of boosters – capacitors in an interconnected power system – series – shunt – series shunt connections – protection of capacitors

DIGITAL PROTECTION• • Digital signal processing – Digital filtering in protection relays –

digital data transmission – Numeric relay hardware – relay algorithms – distance relays – direction comparison relays – differential relays – software considerations – numeric relay testing – concepts of modern coordinated control system

CHAPTER – III: SWITCH GEAR

• Insulation of switchgear – rated and tested voltage coordination between inner and external insulation. Insulation clearances in air, oil SF6 and vacuum, bushing insulation, solid insulating materials – dielectric and mechanical strength consideration. Switchgear terminology – Arc characteristics – direct and alternating current interruption – arc quenching phenomena – computer simulation of arc models – transient re-striking voltage – RRRV-recovery voltage-current chopping-capacitive current breaking-auto re-closing.

•

CHAPTER-IV: CIRCUIT BREAKERS

Types of faults in power systems-short circuit current and short circuit MVA calculations for different types of faults-rating of circuit breakers – symmetrical and asymmetrical ratings. Classification of circuit breakers-design, construction and operating principles of bulk oil, minimum oil, airblast, SF6 and vacuum circuit breakers – Comparison of different types of circuit breakers .Testing of circuit breakers.

Text Book and References

1. Chunikhin, A. and Zhavoronkov, M., “High Voltage Switchgear Analysis and Design”, MirPublishers, Moscow, 1989.

2. Kuffel, E., Zaengl, W.S., and Kuffel J., High Voltage Engineering Fundamentals,

3. Newness, Second Edition, Butterworth-Heinemann Publishers, New Delhi, 2000

4.Flursscheim, C.H. (Editor), Power circuit breaker-theory and design, IEEMonograph Series 17, Peter Peregrinus Ltd., Southgate House, Stevenage, Herts, SC1 1HQ, England,1977..

5.Ananthakrishnan S and Guruprasad K.P., Transient Recovery Voltage and Circuit Breakers, Tata McGraw-Hill Publishing Company Ltd., New Delhi, 1999.

CONTD…5.IEEE Standard Collection, Surge Protection C62, 1995

Editions, (Institute of Electrical and Electronics Engineers, Inc.), USA.

6. Funio Nakanishi, Switching Phenomena in high voltage circuit breakers, MarcelDekker Inc., New York, 1991.

7. Stanley H.Horowitz (Ed), “Protecting relaying for power systems”, IEEE Press, 1980.

8. Y.G. Paithankar and S.R Bhide, “Fundamentals of Power System Protection”, Prentice -Hall of India, 2003

9. Y.G. Paithankar, “Principles of Power System Protection”, Marcel Dekker Inc., 1998.

10. P.Kundur, “Power System Stability and Control”, McGraw-Hill, 1993.

11. Badri Ram and D.N. Vishwakarma, “Power System Protection and Switchgear”, TataMcGraw- Hill Publishing Company, 2002.

12. J.L.Blackburn, “Power System Protection: Principles and Applications”, Marcel Dekker,New York, 1998

Assessment and Evaluation:

1. Assignments : 10%

2. Term Paper : 10 %

3. Mid Semester Examination : 30%

4. Final Examination : 50

Introduction

• The protective system is very much essential for an Electrical Power System.

• It is used to isolate the faulty equipment from the system and protect the other equipments

as quickly as possible.o In case of Short circuit , the need for a

protective system is must, if not isolated it would totally damage the power system.

Protective system

• Includes circuit breakers and protective relays, to isolate the faulty section of the power system from the healthy ones.

PRINCIPLES OF THE PROTECTIVE SYSTEM

• The function of a protective relay is to sense the abnormal conditions in the power system and gives an alarm or isolates the part from the healthy system.

• It minimizes the damage to the equipment and interruptions to the service when electrical failure occurs.

• The relays are compact, self contained devices which respond to abnormal conditions . Whenever an fault occurs, the relay contacts get closed which in turn closes the trip circuit of a circuit breaker.

• The circuit breaker opens and the faulty part is disconnected from the supply.

• Thus the entire process includes the operations like occurrence of fault, operation of relay, opening of a circuit breaker and removal of faulty element . This entire process is automatic and fast, due to protective relaying scheme.

• The Protective relaying scheme includes protective current transformers, voltage transformers, protective relays, circuit breakers etc and normally about 5 percentage of the total cost of the project goes towards the protective relaying schemes.

• It should be noted that a relay does not prevent the appearance of faults, but its takes

action only after the fault has occurred.

BASIC REQUIREMENTS OF PROTECTIVERELAYING

• Speed• Selectivity• Sensitivity• Reliability• Simplicity• Economic

Speed

• The relay system should disconnect the faulty section as quickly as possible for the following reasons improves the stability of power system. decreases the amount of damage caused. decreases the possibility of development of one

type of fault into the other more serve type. Permits the use of rapid reclosure of circuit

breakers to restore service to consumers.

Selectivity

• It is the ability of the protective system to determine the point at which the fault occurs and disconnect the faulty part without disturbing the rest of the system.

• The relay should be able to detect the point at which the fault occurs and cause the opening of the circuit breakers closest to the fault minimum or no damage to the system.

Sensitivity• It is the ability of the relay system to operate with

low value of actuating quantity.• The relay should be sensitive to operate when the

fault current exceeds the pick up value and should not operate when the fault current is less than pick up value.

• Sensitivity of a relay is a function of the volt amperes input to the coil of the relay necessary to cause its operation.

• The smaller the volt ampere input required to cause relay operation , the more sensitive is the relay

Contd…• Mathematically it is expressed as sensitivity

factor Ks.

Ks = Is/ IoWhere Ks = sensitivity factor Is = minimum short circuit current in

the zone. Io = minimum operating current for

the protection.

RELIABILITY

• It is the ability of the relay system to operate under the pre determined conditions.

• The failure of the protective system may be due to the failure of any of the elements of the system viz, protective relay, circuit breaker, P.T, C.T Battery etc. In order to get high reliability.

• The reliability should be of the order of above 95 percentage.

Simplicity

• The relaying system should be simple so that it can be easily maintained.

• The simpler the protection scheme, the greater will be the reliability.

Economic

• The most important factor in the choice of a particular protection scheme is the economic aspect.

FUNCTIONS OF PROTECTIVE RELAY

• The prefect removal of the component which is behaving abnormally by closing the trip circuit of circuit breaker to sound an alarm.

• To disconnect the abnormally operating part as to avoid the damage to reset of the system.

• To prevent the subsequent faults by disconnecting the abnormally operating part.

• To disconnect the faulty part as quickly as possible so as to minimize the damage to the faulty part itself.

• The protective relaying plays an important role in sensing the faults, minimizing the effect of faults and minimizing the damage due to the faults.

NATURE AND CAUSES OF FAULTS

• The nature of a fault implies any abnormal condition which causes a reduction in the basic insulation strength between conductors or between phase conductors and earth.

• Faults are normally caused by breaking of conductors or due to insulation failure.

• The other reasons are mechanical failure, accidents, excessive internal and external stresses.

Causes of faults

• When a fault occurs on the system, the voltages of the three phases become un equal.

• As the fault current are large the apparatus may get damaged.

FAULT STASTISTICSS.NO EQUIPMENT % OF total faults

1. Over head lines 50

2. Switchgear 1 5

3. Transformer 12

4. Cables 10

5. Miscellaneous 8

6. Control equipment 37. CTs and PTs 2

Fault distribution in power system

Fault occurance

Line to Line to Ground 2 0r less

• In case of three phase system the breakdown of insulation between one of the phases and earth is known as Line to ground fault.(L - G)

• If there is insulation breakdown between either of the two phases it is known as Line to Line Fault. ( L - L).

• If there is insulation breakdown between two phases and earth it is known as Double Line ground Fault ( L - L - G).

• The breakdown of insulation between the phases is three phase fault (L - L - L)

Causes of a Fault or Short Circuit

• Over voltage due to switching• Over voltage due to the direct and indirect

lightning strokes.• Bridging of conductors by birds.• Breakdown of insulation due to less dielectric

strength.• Mechanical damage to the equipment.

ZONES OF PROTECTION

• In a protective relaying scheme , it is a usual practice to divide the entire system into several protection zones.

• When a fault occurs in a given zone, then only the circuit breakers within that zone will be opened.

• This will isolate only the faulty part, leaving the healthy circuit intact.

Various protective zones of power system

• The boundaries of protective zones are decided by the locations of the current transformer. In practice, various protective zones are overlapped.

• The overlapping of protective zones is done to ensure complete safety of each and every element of the system.

• The zone which is unprotected is called dead spot.

• The zones are overlapped and hence there is no chance of existence of a dead spot in a system.

For the failures within the region where two adjacent protective zones are overlapped, more circuit breakers get tripped than minimum necessary to disconnect the faulty element If there are no over laps, then dead spot may exist, means the circuit breakers lying within the zone may not trip even though the fault occurs.

This may cause damage to the healthy system. The extent of overlapping of protective zones is relatively small .

Types of Protection

(i) Primary Protection

(ii) Back up Protection

Primary Protection

• The primary protection is the first line of defense and is responsible to protect all the power system elements from all the types of faults. The backup protection comes into play only when the primary protection fails.

Back up ProtectionThe backup protection is provided as the main

protection can fail due to many reasons like, 1. Failure in circuit breaker 2. Failure in protective relay 3. Failure in tripping circuit 4. Failure in d.c tripping voltage 5. Loss of voltage or current supply to the relay. Thus it the backup protection is absent and the

main protection tails then there is a possibility of severe damage to the system.

Methods of Back up Protection

• Relay Back Up Protection : In this method, a single breaker is used by both primary and back up protection but the two protective systems are different.

• Breaker Back Up Protection: In this method, Separate breakers are provided for primary and back up protection. Both breakers are at the same station.

• Remote Back Up Protection : In this method, separate breakers are provided for primary and backup Protection. But the breakers are at different stations and are completely isolated.

• Centrally Co ordinated Back up Protection : In this method, primary protection is at various stations. There is a central control room and back up protection for all the stations is at central control room.

Neutral Earthing or Grounding

• It is nothing but, the neutral point of generator , transformer, system, Circuit , rotating machines etc; is connected to earth directly or through a reactance .

• The term earthing is used in U.K .• The term grounding is used in U.S.A.

Neutral grounding is very important

• The earth fault protection is based on the method of neutral earthing.

• The system voltage during earth fault depends on neutral earthing.

• It is provided basically for the purpose of protection against arcing grounds, unbalanced voltages with respect to earth, protection from lighting and for improvement of the system

Terminologies used in Protective Relay

• Protective Relay : It is an electrical relay used for protection of electrical devices. It closes its contacts, when operating quantity reaches certain pre determined value. It is used to initiate isolation of apart of circuit during abnormal conditions.

• Trip Circuit : The circuit which comprises of trip coil, relay contacts, auxiliary switch, battery supply ,seal in coil etc which controls the circuit breaker for opening operation.

• Relay Time: The time interval between occurrence of fault and the closure of relay contacts.

Breaker Time : The time interval between closure of trip circuit and final arc interruption.

Fault clearing time = Relay time + Breaker time.Pick Up : The operation of relay is called relay pick up.The pick up value is the minimum value of operating

quantity at which the relay operates and closes it contacts.

Dropout or Reset : The value of current/ voltage below which the relay resists and comes back to original position.

Burden : The power consumed by the relay circuits expressed in VA or W under certain specified conditions.

• Earth fault : a fault involving earth or ground eg L G FAULT L L G FAULT.

• Phase fault : a fault which does not involve earth. Eg L L fault.• Instantaneous Relay : It is a fast relay having relay time less than

o.2 sec and have no time lag.• Electromechanical Relay : It operates on the electromagnetic

principle in which the measurement is performed by movable parts.

• Static Relay : It is also called solid state relay in which relay measurement is performed by static circuit.

• Distance Relay : This relay measures the impedance or reactance at the relay location. The impedance of the line is proportional to length of the line and it is used for the protection of transmission line.

• Over current Relay : This relay operates when the actuating current exceeds a pick up value.

• Under Voltage Relay : This relay operates when the system voltages falls below the pick up value.

• Directional Relay: This relay is able to sense whether the faults lies in the forward direction or reverse direction with respect to the relay location.

• Differential Relay : This relay measures the difference of two actuating quantities and operates when this difference exceeds a pre set value.

• Auto reclosure : The process of automating reclosing of circuit breaker after its opening.

• Reach: It is the distance up to which the relay will cover the protection.

• Over Reach : If the relays operates for a fault beyond its preset reach, it is called over reach.

• Under Reach : If the relay fails to operate for a fault within its pre set value .

CLASSIFICATION OF RELAYS

• Most of the relays in service on electrical power system are of electro mechanical type.

They work on the following two main operating principles

(i) Electromagnetic Attraction Relays(ii) Electromagnetic Induction Relays

According to construction

(i) Electromagnetic Relays(ii) Induction Relay(iii) Electro thermal Relay(iv) Physio electric Relay(v) Electro dynamic Relay(vi) Static Relay(vii) Micro Processor Relay

Electromagnetic Relays

(a) Attracted armature type Relay(b) Solenoid type Relay(c) Balanced beam type Relay(d) Moving coil type Relay(e) Moving Iron type RelayThese relays are actuated by a.c or d.c

quantities.

Electromagnetic Induction or Induction relays

• Uses the principle of Induction motor.• Actuated by a.c. quantities only(a) Shaded pole structure (or) Induction disc

type Relay.(b) Watt hour meter (or) Double winding

structure type Relay.(c) Induction Cup type Relay.

(iii) Electrothermal Relays: Used for thermal overload protection using bimetallic strips.

(iv) Physico electric Relays: Bucholtz relay is under this kind, utilized in transformer Protection.

(V) Electrodynamic Relays : Operate on the same principles as the moving coil instruments.

(vi) Static Relays : Such relays employing thermionic valves, transistors or amplifiers to obtain the operating characteristics.

According to Application

(i) Falls below specified limit or value (a) Under voltage Relay (b) Under Current Relay (c) Under Power Relay (d) Under Frequency Relay

(ii)Exceeds specified value

(a) Over Voltage Relay(b) Over Current Relay(c) Over Power Relay(iii) Directional Reverse Current Relay : The

relay is actuated when the direction of current is reversed or the phase of the current becomes more than the pre determined value.

(iv) Directional Reverse power Relay : The relay is actuated when the phase

displacement between applied voltage and current attains a specified value.

Distance Type Relay

• These relay work on the principle of measurement of voltage to current ratio.

• In these type there are two coils.• One coil is energized by current while other by

voltage.• The torque produced is proportional to the

ratio of the two quantities. When the ratio reduces below a set value the relay operates.

Differential Type Relays

A differential relay operates when the vector difference of two (or) more electrical quantities in the circuit in which relay is connected, exceeds a set value

Electromagnetic Relay

In an electromagnetic relay the driving torque is created based on an electrical or electronic principle.

The restraining torque is generally provided with the help of springs.

The torques are mechanically compared and the relay operates when driving or operating torque is more than the restraining torque

• TR = Td - Tr

Where TR = Resultant torque Td = Driving torque or operating

torque. Tr = Restraining torque.The relay operates When the resultant torque is

positive.

Basic Trip Circuit Operation

Tripping Schemes in Circuit Breaker

• Two schemes are very popularly used for tripping in circuit breakers which are

(1) Relay With Make type contact (2) Relay With Break type contact

Relays With Make Type Contact

Relays With Brake Type Contact

Electromagnetic Attraction Relays• In these relays , there is a coil which energizes

an electromagnet.• When the operating current becomes large,

the magnetic field produced by an electromagnet is so high that it attracts the armature or plunger ,making contact with the trip circuit contacts.

• Types of electromagnetic attraction type are(i) Attracted armature relay (ii) Solenoid and

plunger type relay

Attracted Armature Type Relay

• There are two types of structure available for attracted armature type relay which are

(i) Hinged armature type(ii) Polarised moving iron type

Hinged Armature Type

Polarised moving iron type Relay

Solenoid and plunger type relay

Operating Principle of Electromagnetic Attraction Relays

• The electromagnetic force produced due to operating quantity which is exterted on armature, moving iron or plunger is the proportional to the square of the flux in the air gap.

• Thus neglecting the saturation effect, the force is proportional to the square of the operating current.

Induction Type Relay

• It is also called as magnitude relays.• It works on the principle of the induction

motor or energy meter.• The coils are energized with the help of

alternating current.• The torque is produced in these relays due to

the interaction of one alternating flux with eddy currents in the rotor by another alternating flux.

Types of Induction Relays

• Based on the construction they are(i) Shaded Pole type(ii) Watt hour meter type(iii) Induction Cup Type

Watthour Meter Type Induction Relay

Induction Cup Type Relay

Time Current Characteristics

• Time required to rotate the disc depends on a torque.

• The torque varies as current in primary circuit .

• More the torque, lesser is the time required hence relay has inverse time characteristics.

• Such characteristics are called Inverse Definite Minimum Type (I D M T)

STATIC RELAYS OR SOLID STATE RELAYS

• Static relay is an electrical relay in which the response (or) action is developed by electrical/magnetic or other components without mechanical motion of components.

• A static relay is defined as one in which there is no armature or other moving element, the desired response being developed by electronic solid state , magnetic or other components without mechanical motion.

Contd….

A relay using a combination of both static and electromagnetic units is also called Static relay.

A static relay employs semiconductor diodes, transistors, Zener diodes , SCR’s logic gates. etc., as its components. Nowadays I.Cs are used in place of transistors because of their reliability and compactness.

Basic Elements of Static Relay

Comparison of static Relays with Electromagnetic Relay

• The conventional electromagnetic relays use the moving parts such as an armature, disc etc.

• There are lot of manufacturing difficulties and problems related to mechanical stability associated with electromagnetic relays.

• The CT’S and PT’s are subjected to high burdens in case of electromagnetic relay

Contd…

• The static relays are commonly using the transistor circuits and called transistor relays.

• This because transistors can be used as an amplifying device as well as a switching device.

• Hence any functional characteristics as per the requirement can be obtained by the static relays.

• The transistor circuits can perform functions like summation, integration, comparison etc..

Advantages of Static Relays

• The moving parts are absent, the minimum maintenance is required . No bearing or friction troubles exist. The moving parts are present only in the actual tripping circuit and not in the control circuit.

• The burden on the CT’s gets considerably reduced thus smaller C.T.s can be used.

• The power consumption is very low as most of the circuits are electronic.

• The response is very quick.

Contd

• The resetting time can be reduced and overshoots can be reduced and overshoots can be reduced due to absence of mechanical inertia and thermal storage.

• The sensitivity is high as signal amplification can be achieved very easily.

• The low energy levels required in the measuring circuits make the relays smaller and compact in size.

• The testing and servicing is simplified.

Static current relay

Directional Static Over current Relay

Static Differential Relay

Generator Protection

• The generators used in the power system are the alternators which produce very high a.c. voltages. The protection of generators is very much complex due to the following reasons,

• They are very large machines producing very high voltages and are connected to busbars.

• Various other equipments are always associated with the generators. Such equipments are prime movers, excitation systems, voltage regulators, cooling systems etc.

Contd…

• The generators are very costly, expensive and very important factor in a power system. The Protection scheme must be such that it should not shut off the generators as far as possible.

• The shut off generators result in a power shortage.

Generator Faults

1.Stator Faults : The faults associated with the stator of the generator.

2. Rotor Faults : The faults associated with the rotor of the generator.

3. Abnormal running conditions

Stator faults

(i) Phase to Earth faults.(ii) Phase to phase faults.(iii) Stator Inter Turn faults.

Phase to Earth faults.

• Occurs in the armature slots.• Are dangerous and can cause severe damage to

the expensive machine.• The fault current less than 20 A cause negligible

burning of core if machine is tripped quickly.• To avoid the damage due to phase to earth faults a

separate ,sensitive earth fault protection is necessary for the generators along with the earthing resistance.

Phase to phase faults.

• Short circuit between two phase windings.• once phase to earth fault occurs, due to the over

heating phase to phase fault also may occur.• It is likely to occur at the end connections of the

armature winding which are overheating parts outside the slots.

• It causes severe arcing with very high temperatures. May lead to melting of copper and fire if the insulation is not fire resistant.

Stator Inter Turn faults

• Coils used in the alternators are generally multiturn coils. So short circuit between the turns of one coil may occur which is called an inter turn fault.

• occurs due to current surges with high value of voltage across the turns.

• But if the coils used are single turn then this fault can not occur.

Rotor Faults

(i) Field Over loading(ii) Field winding grounding (iii) Heating of rotor(iv) Rotor earth fault

Abnormal Running Conditions

• Overloading• Overspeeding• Unbalanced loading• Overvoltage• Failure of Prime mover• Loss of excitation (field failure)• Cooling system failure

Overloading

• Continuous overloading, overheating of the stator.

• Increases the winding temperature.• Temperature rise exceeds the certain limit the

insulation of the winding may get damaged.• The degree of overloading decides the effect

and temperature rise.• The over current protection is generally set to

very high value.

Overspeeding• In case of hydraulic generators a sudden loss

of load results in over speeding of the generator.

• Due to the water flow to the turbine cannot be stopped or reduced instantly.

Unbalanced Loading

• In the circulation of negative sequence currents, produce the rotating magnetic field.

• At the synchronous speed with respect to rotor.• The direction of rotation is opposite to that rotor.• The relative speed between the two is double the

synchronous speed, thus e.m.f get induced , having double the normal frequency in the rotor winding.

• The reasons are (i) occurrence of unsymmetrical fault near the generating station.(ii)failure of C.B.

Overvoltage

• Due to overspeeding of generators.• Faulty operation of voltage regulators.• Atmospheric surge voltages • Surge arresters and surge capacitors are often

used.

Failure of Prime mover

• Motoring operation of synchronous generator.• Draws active power from the network .• If it persists for more than twenty seconds • Serious overheating of the steam turbine

blades may result.• The reverse power protection achieved by

directional power relays is used.

Loss of Excitation

• Due to field failure.• Loss of synchronism within a second causes

the increase in speed of the generator.• Generator starts working as an induction

generator , drawing the reactive power from the bus.

• Leads to the overheating of the stator and rotor body,

• Disconnection of alternator should be taken immedately.

Cooling System Failure

• Severe overheating to rise the temperature• Insulation failure .• Thermocouples or resistance thermometers • Wrong synchronization , local overheating ,

leakage in hydrogen circuit , moisture in the generator winding, oxygen in pure water circuit , Vibrations.

Basic Differential Protection Scheme for Generators