s.k.b.p.polytechnic kopargaon department of … · web viewthe analysis of the power system calls...

S.K.B.P.POLYTECHNIC KOPARGAON DEPARTMENT OF ELECTRICAL ENGINEERING

TOPIC 1:FUNDAMENTAL 4(10)

Contents:

1. Switchgear equipments - Symbols and functions

Isolator :-

To disconnect the part of the system for maintenance from live circuit under no current condition.

Circuit breaker:

To make or break acircuit manually or remotely under normal condition and to break circuit automatically under fault condition.

Earthing switch:

To discharge voltage on the line (due to charges of line capacitance) to earth after disconnecting line from live section.

Lightning arrester:

To divert the high voltage surges towards the earth, due to lightning or switching.

2. Functions of protective system.3. Normal & abnormal conditions.

4. Causes of Fault Occurrence in Power System:

1) Failure of insulation of components and equipment parts

2) Mechanical failure

3) Breaking of conductors.

4) Over temperature.

5) Lightning & switching surges.

6) Lines affected by birds ,objects , falling of trees

7) Harmonics produced in rotating machines.

8) Drawbacks in system design.

9) Lack of selection of good quality material.

10) Accidents

11) Excessive internal and external stresses.

12) Unbalanced currents.

PREPARED BY B.B.KADAM SAP(17508)


Explain the terms base kVA, base kV and percentage reactance. State the relationship between them.

Ans:

The analysis of the power system calls for calculations combining the different voltage levels and VA ratings of the components such as generators, transformers, motors etc. Transforming these calculations to individual voltage and current ratings of these components becomes cumbersome and time consuming. Hence the calculations are done on a common platform of these quantities. The values of these specified at the platform are called as base values. Then these quantities for individual components are expressed as fractions or percentages of the base values assumed/ specified. Hence base values of voltage and current assumed lead to base VA (base voltage x base current), base impedance [(base voltage)/(base current)]. Thus the calculations are simplified.



5. Use of current limiting reactors & their arrangements.



TOPIC 2: CIRCUIT INTERRUPTING DEVICES 8(16)

1. Construction, characteristics of HRC Fuse

HRC fuse mainly consists of heat resisting ceramic body. The current carrying element is compactly surrounded by the filling powder. Filling material acts as an arc quenching and cooling medium when the fuse element blows off due to excessive heat .Under normal conditions, the fuse element is at a temperature below its melting point. Therefore, it carries the normal current without overheating. When a fault occurs, the current increases and the heat produced is sufficient to melt these elements. Fuse element melts before the fault current reaches its first peak value. Vaporized metal /fuse element chemically reacts with filling powder and results in the formation of high resistance substance that helps in quenching the arc.

2. Isolators-



Definition of IsolatorCircuit breaker always trip the circuit but open contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off the circuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works. So definition of isolator can be rewritten as Isolator is a manually operated mechanical switch which separates a part of the electrical power. Isolators are used to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Isolators are generally used on both ends of the breaker in order that repair or replacement of circuit breaker can be done without and danger.

Types of Electrical IsolatorsThere are different types of isolators available depending upon system requirement such as

1. Double Break Isolator2. Single Break Isolator3. Pantograph type Isolator.Depending upon the position in power system, the isolators can be categorized as

1. Bus side isolator – the isolator is directly connected with main bus2. Line side isolator – the isolator is situated at line side of any feeder3. Transfer bus side isolator – the isolator is directly connected with transfer bus.

Constructional Features of Double Break IsolatorsLets have a discussion on constructional features of Double Break Isolators. These have three stacks of post insulators as shown in the figure. The central post insulator carries a tubular or flat male contact which can be rotated horizontally with rotation of central post insulator.

This rod type contact is also called moving contact. The female type contacts are fixed on the top of the other post insulators which fitted at both sides of the central post insulator. The


https://www.electrical4u.com/electric-power-single-and-three-phase/

https://www.electrical4u.com/electric-circuit-and-electrical-circuit-element/

https://www.electrical4u.com/electrical-circuit-breaker-operation-and-types-of-circuit-breaker/


female contacts are generally in the form of spring loaded figure contacts. The rotational movement of male contact causes to come itself into female contacts and isolators becomes closed. The rotation of male contact in opposite direction make to it out from female contacts and isolators becomes open. Rotation of the central post insulator is done by a driving lever mechanism at the base of the post insulator and it connected to operating handle (in case of hand operation) or motor (in case of motorized operation) of the isolator through a mechanical tierod.

Constructional features of Single Break IsolatorsThe contact arm is divided into two parts one carries male contact and other female contact. The contact arm moves due to rotation of the post insulator upon which the contact arms are fitted. Rotation of both post insulators stacks in opposite to each other causes to close the isolator by closing the contact arm. Counter rotation of both post insulators stacks open the contact arm and isolator becomes in off condition. This motorized form of this type of isolators is generally used but emergency hand driven mechanism is also provided.

3. Arc formation process, methods of arc extinction – High resistance method, Low resistance or current zero method

Two main methods of arc extinction: i) High resistance method ii) Zero current extinction

A. High resistance method: Arc path resistance is increased to reduce the current to low values while interrupting the arc. Arc resistance = varc/iarc. The arc resistance mainly increased by three methods given below: i) Lengthening of the arc by arc runners ii) Splitting the arc by arc splitters iii) Arc cooling

B. Current zero or Low Resistance Method:

This method is employed in a.c. circuit breakers since the ac passes through zero 100 times /second in 50 cycle current wave. When current wave passes through every zero, the arc vanishes for a brief moment. However the arc restrikes again with the rising current waves. In this method, at current zero instant, fresh unionized medium is introduced between the space in between the contacts. Due to this medium deionization effect takes place. The dielectric strength of the contact space increases to such an extent that the arc does not continue after current zero

4. Definition: Arc voltage, Recovery voltage, Restriking voltage, RRRV Circuit breakers- Concept, Classification, Working principle, Construction, Specification & Applications of:



A) ARC VOLTAGE : -

It is the voltage that appears across the contacts of circuit breaker during the arcing period.

B) RECOVERY VOLTAGE :

It is the normal frequency (50 Hz) r.m.s. voltage that appears across the contact of the circuit breaker after final arc extinction. It is approximately equal to the system voltage.

C) RESTRIKING VOLTAGE : -

It is the transient voltage that appears across the contacts at or near current zero during arcing period.

D) RRRV –

It is defined as the rate of increase of restriking voltage and is abbrevinated by R.R,R.V. usually, the restriking voltage is in kv and time in microseconds so that RRRV is in kv/usec.

E) Rated current: The rated current of a circuit breaker is the r.m.s. value of current which the circuit breaker can carry continuously and with the temperature rise of various parts within specified limits.

F) Breaking capacity: It is the highest value of short circuit current (r.m.s. current) which a circuit breaker is capable of breaking under specified conditions of recovery voltage and power frequency voltage. Breaking MVA=3 kA×kV where, kA is rated breaking current and kV is rated voltage.

G) Making capacity: The peak value of current (including d.c. components) during the first cycle of current wave after the closure of circuit breaker is known as Making capacity. Generally, Making capacity=2.55× Breaking capacity

H) Short time current rating: It is the period for which the circuit breaker is able to carry fault current while remaining closed. The circuit breaker should be able to carry high current safely for some specified period while remaining closed i.e. they should have proven short time rating



5. L.T.- Air circuit breakers (ACB), Miniature circuit breakers ( M C B ) Moulded case circuit breaker ( M C C B ),



Minimum oil circuit breaker:

EARTH LEAKAGE CIRCUIT BREAKER(ELCB) (https://www.youtube.com/watch?v=gA1iyIO6I70)



Earth leakage circuit breaker is a safety device used in electrical installations with high earth impedance to prevent shocks and disconnect power under earth fault conditions. It works on principle of relaying when the current in the earth path exceeds a set value. ELCB is used for protection against electric leakage in the circuit of 50 Hz or 60 Hz, rated voltage single phase: 240 V, 3-ph: 440V. Rated current up to 60 Amp. When the earth fault occurs, the ELCB cuts off the power within the time of 0.1 sec. automatically to protect personnel. Under normal conditions (IL–IN) = If is very low or nearly zero. The CT surrounding the phase and neutral senses the differential current under earth fault and actuates the CB to operate (open). The difference current If through fault path resistance Re is the leakage to earth. If this value exceeds a preset value, then the CB opens. Normally it is around 35 mA for tripping in domestic installations with tripping time being as low as 25 msec



H.T – AIR BLAST CIRCUIT BREAKER

(www.youtube.com/watch?v=ku5Xz_SWY4g)



VACUUM CIRCUIT BREAKER. (www.youtube.com/watch?v=vvhG38dqXsQ)

Process of arc development and its extinction in Vacuum CB : During the operation of the breaker, the moving contact separates from the fixed contact resulting in arc between them. The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts. The arc gets extinguished quickly because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by surfaces of moving and fixed members and shields. Since vacuum has good recovery of dielectric strength the arc extinction occurs with a short contact separation. Comparison of fuse & MCCB

CHARACTERISTICS OF SF6 GAS:



1) Stable at high temperature around 500

2) Inert;

3) Electronegative;

4) Non-reactive with structured material upto 500

5) Low arc time constant;

6) Five times heavier compared to air;

7) Very much better dielectric properties compared to air and oil.

8) Higher rate of rise of dielectric strength.

9) The products of decomposed gas at hig

form the original gas.

10) For equal pressure the heat transfer capacity is more than twice of air.

may reduce the life.

SF6 CIRCUIT BREAKER(www.youtube.com/watch?v=6GSuzpHTzbc)


http://www.youtube.com/watch?v=6GSuzpHTzbc


TYPES (www.youtube.com/watch?v=ngbMkLKpgTw)

FAULTS OCCUR IN 3-Q INDUCTION MOTOR

1) Prolonged overloading



2) Single phasing 3) Stalling 4) Phase to phase faults / phase fault 5) Inter- turn faults 6) Earth faults 7) Reversal of phases 8) Failure of bearings/ Rotor Jam 9) Supply under-voltage 10) Unbalanced Supply Voltage 11) Faults in stator or associated circuit 12) Faults in rotor or associated circuit

FACTORS TO BE CONSIDERED WHILE SELECTING MCCB FOR MOTOR OPERATION:

i) Specifications of motor ( Rated voltage & HP capacity )ii) Type of motor.iii) Cost of motoriv) Simplicity of protectionv) Starting currents & Permissible overloadsvi) Cost of protection schemes.vii) Breaking timeviii) Ambient conditions.

TOPIC3 : PROTECTIVE RELAYING 9(20)



Contents:

QUALITY REQUIREMENTS OF RELAY SYSTEM:

Essential features of protective relaying

A) SELECTIVITY: -

It is the ability of protective system to select correctly that part of system in trouble and disconnect the faulty part without disturbing the rest of the system.

B) SPEED:

The relay system should disconnect the faulty section as fast as possible to prevent the electrical apparatus from damage and for system stability.

C) SENSITIVITY: -

It is the ability of the relay system to operate with low value of actuating quantity.

D) RELIABILITY: -

It is the ability of the relay system to operate under predetermined conditions.

E) SIMPLICITY: -

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

F) ECONOMY: -

The most important factor in the choice of particular protection scheme is the economic aspect. The protective gear should not cost more than 5% of the total cost of equipment to be protected.

BASIC RELAY TERMINOLOGY -

a) Pickup current:

the threshold value of operating current above which the relay operates.

b) Relay time:

time interval between occurrence of fault and closure of relay contacts.

c) Plug setting multiplier:

PSM = ( Fault current in relay coil) / ( Pickup current)

d) Reset current:



The value of current below which the relay resets and comes back to its original position is called as reset current or dropout.

e) TSM: the adjustment arrangement provided for setting the operation time of the induction relay is known as TSM (Time Setting Multiplier). TSM dial is calibrated from 0 to 1.

f) PSM: Plug setting multiplier: PSM = ( Fault current in relay coil) / ( Pickup current)

Plug setting multiplier(PSM): It is related to the current setting of the overcurrent relays. The plug setting multiplier is defined as the ratio of the ‘transformed fault current on the relay side’ to the ‘relay pickup current’.

PSM = 𝑡𝑟𝑎𝑛𝑠𝑓𝑜𝑟𝑚𝑒𝑑𝑓𝑎𝑢𝑙𝑡𝑐𝑢𝑟𝑟𝑒𝑛𝑡𝑜𝑛𝐶𝑇𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦𝑠𝑖𝑑𝑒 /𝑟𝑒𝑙𝑎𝑦𝑐𝑢𝑟𝑟𝑒𝑛𝑡𝑠𝑒𝑡𝑡𝑖𝑛𝑔= 𝑓𝑎𝑢𝑙𝑡𝑐𝑢𝑟𝑟𝑒𝑛𝑡𝑜𝑛𝐶𝑇𝑝𝑟𝑖𝑚𝑎𝑟𝑦𝑠𝑖𝑑𝑒 (𝑖.𝑒𝑙𝑖𝑛𝑒𝑓𝑎𝑢𝑙𝑡𝑐𝑢𝑟𝑟𝑒𝑛𝑡)/𝑟𝑒𝑙𝑎𝑦𝑐𝑢𝑟𝑟𝑒𝑛𝑡𝑠𝑒𝑡𝑡𝑖𝑛𝑔 x 𝐶𝑇𝑟𝑎𝑡𝑖𝑜Time setting multiplier(TSM): It is related to the operating time of the relay during faults. The angular distance through which the relay disc travels during faults before the trip contacts close is varied to get different times of operation of the relay. This is time setting. These times of motion of the disc are set in 10 steps starting with maximum time setting of 1 (when disc is set farthest) to minimum (when disc is nearest) of 0 (instantaneous). For example if the angular travel from trip contacts is set to maximum then TSM =1, then for a particular PSM if the operating time of relay is 1 second then for the same PSM if the TSM is set to 0.4 then the time of operation will be (1 second x TSM)

= 1 x 0.4 = 0.4 seconds. Thus TSM is used to define the steps for time setting of relay operation.

g) Reset current:The value of current below which the relay resets and comes back to its original state is called as reset current or dropout

NUMERICALS ON PSM &TMS

1. Classification Electromagnetic relay –



OPERATION OF ATTRACTED ARMATURE TYPE, SOLENOID TYPE AND TYPE RELAYS.

ATTRACTED ARMATURE TYPE RELAY: It consists of a laminated electromagnet M carrying a coil C and a pivoted laminated armature. The armature is balanced by a counterweight and carries a pair of contacts at its end. Under normal operating conditions, the current through the relay coil C, is such that counterweight holds the armature in the position shown. However when a short circuit occurs, the current through relay coil increases sufficiently and armature is attracted upwards which shorts the pair of contacts and completes the trip circuit

Merits:

1. Simple construction

2. Reliable operation.

3. Unaffected by temperature changes.

4. Long life.



5. Robust construction.

Demerits-

1. Somewhat less accurate than static/ microprocessor based relay.

2. Require maintenance.

3. Wrongly operated sometimes because of vibrations.

4. Counter weight has to be adjusted for every setting.

ATTRACTED ARMATURE TYPE RELAY

Balanced beam



ELECTRO MAGNETIC INDUCTION TYPE - OPERATION OF SHADED POLE TYPE AND WATT HOUR METER TYPE RELAYS.

BLOCK DIAGRAM, OPERATION, ADVANTAGES & DISADVANTAGES OF STATIC

Advantages static relays. 1) Low power required, hence less burden. 2) No motional parts hence bouncing, friction, erosion, arcing etc eliminated. 3) Not affected by gravity, may be used in any position. 4) Improved selectivity as resetting and over shoot times are reduced. 5) Lower operating times.

C.T. Burden: It is defined as load connected across C. T. secondary. It is specified in VA ( Volt –Ampere)

If C.T. secondary is kept open, the secondary current becomes zero, the secondary mmf also becomes zero. The working flux therefore increases and core gets saturated. Due to this, voltage induced in secondary rises to very high value. Also the primary gets over heated. The peak value of secondary emf may be several times the rms value. This may cause danger to person working on secondary side of CT.

CT and PT as Protective transformers. -Safety precautions while using C.T. and P.T. Circuit Diagram with Relay

Safety precautions while using CT & PT:

1. CT secondary terminals should never be kept open. CTs they must be energized only after connecting the burden across them.

2. PT secondary should never be shorted as they are designed for high impe burdens (extremely low currents).

3. To be used as per the specified rating of voltage, current & burdens only.4. burdens should never be exceeded when multiple ones are connected across one

instrument transformer. burdens only, else for lower present and compensation is needed.

5. CTs for measurement must not be interchanged with those for protection and vice versa.

6. PTs for measurement must not be interchanged with those for protection and vice versa.

OVER CURRENT RELAY-TIME CURRENT CHARACTERISTICS.

OPERATION OF STATIC OVER CURRENT RELAY WITH BLOCK DIAGRAM



OPERATION OF ΜP BASED OVER CURRENT RELAY WITH BLOCK DIAGRAM

Microprocessor based over current relay: The ac voltage proportional to the load current is converted in to dc through a precision rectifier. Thus the microprocessor accepts d. c. voltage proportional to the load current. The schematic diagram is shown in the figure. The output of rectifier is fed to the multiplexer. The output of multiplexer is fed to the A/D converter to obtain the signal in digital form. The A/D converter ADC 0800 has been used for this purpose. The microprocessor sends signal to the ADC for starting the conversion. The microprocessor reads the end of conversion signal to examine whether the conversion is over or not. As soon as conversion is over, the microprocessor reads the current signal in digital form and then compares it with the pickup value. The microprocessor first determines the magnitude of the fault current and then selects the corresponding time of operation from the look up table. Then it goes in delay subroutine and sends a trip signal to the circuit breaker after the predetermined time delay.



2. DISTANCE RELAYING- PRINCIPLE, OPERATION OF – DEFINITE DISTANCE RELAY, TIME DISTANCE RELAY AND MHO RELAY

Time graded protection of feeders using IDMT over-current relays: Figure shows time-graded over current protection of radial feeder using IDMT over-current relays. Here the operating time is inversely proportional to the fault current and finally becomes definite for particular current. With this arrangement, the farther the circuit-breaker from the generating station, the shorter is its relay operating time. The line or feeder is divided into number of sections. Over-current relays are provided for each section. On occurrence of fault in any section, all the relays towards generating station are initiated to operate but the nearest relay operates first and trips the respective CB. If this relay fails, the next relay towards generating station operates and so on. The relays towards generating station are set for higher currents and they operate with time delays according to their inverse definite minimum time characteristics.



DIRECTIONAL RELAY- THE NEED OF DIRECTIONAL RELAY, CONSTRUCTION, OPERATION OF INDUCTION TYPE DIRECTIONAL OVER CURRENT RELAY

Here the upper electromagnet has a primary and a secondary winding. The primary is connected to the secondary of a CT in the line to be protected and is tapped at intervals. The tapings are connected to plug setting bridge by which the number of active turns on the relay operating coil can be varied there by giving the desired current setting



DISTANCE PROTECTION: Action of relay depends on impedance (distance) up to fault point. At fault point the ratio of V/I (=Z) at the relay falls below preset value due to which the relay operates to trip the circuit breaker. ‘V’ is the restraining quantity while ‘I’ is the operating quantity. Advantages:

1. System is economical 2. High speed of interruption 3. Suitable for very long and high voltage transmission lines. 4. No problem of pilot wires.

DIFFERENTIAL RELAY- OPERATION OF CURRENT DIFFERENTIAL RELAY & VOLTAGE DIFFERENTIAL RELAY.

DIFFERENCE THE BETWEEN DEFINITE CHARACTERISTICS AND INVERSE CHARACTERISTICS OF RELAYS. The difference between the definite and inverse characteristics in respect is relays is that the Definite time relays operate after a predetermined time when the current exceeds the pickup value irrespective of the current magnitude whereas the Inverse time relays operate in similar manner but the operating time depends on the current magnitude. Higher the magnitude lower will be the operating time.

DIFFERENTIAL RELAYING When the differential relaying is used for protection, the CTs must be identical in design, otherwise if the error is excessive, it will cause a wrong operation of relay. To safe guard against such operations, biased current protection is used. Such a protection provides a biasing feature which automatically increases the relay setting in proportion to the load or through fault current i.e. the relay is set to operate not at definite current, but at a certain percentage of through current. By suitably proportionating the ratio of restraining coil turns to operating coil turns, any amount of biasing can be achieved and compensation for unwanted operations due to momentary high currents can be provided.

PERCENTAGE BIAS:



The ratio of differential operating current to average restraining current, expressed in percentage, is called percentage bias.

TOPIC 4 PROTECTION OF ALTERNATOR

6(12)



Contents:

1. ABNORMALITIES & FAULTS

INTER -TURN FAULT

Figure shows scheme for one phase only. Under normal working conditions the two currents in the stator wind S1 and S2 are identical and by virtue of the cross connected CT secondaries the relay current is zero, hence no relay operation. But when one of the windings is faulty



(inter turn fault) its current differs and hence the two CT secondary curre difference current is diverted through the relay coil to operate it leading to isolation of the alternator from the power system. It is identical for other phases. winding sections S currents are different, due which the Else proportion

RESTRICTED EARTH FAULT PROTECTION:

Referring to Figure, the star connected neutral earthed side is protected by restricted earth fault protection. An earth fault F1 beyond the transformer causes the currents I2 and I1 to flow in CT secondaries. Therefore, the resultant current in earth fault relay is negligible and relay does not operate. For earth fault within the transformer star connected winding F2 only I2 flows and I1 is negligible. So earth fault relay operates. When fault occurs very near to neutral point, the voltage available for driving earth

fault relay is very small. Hence the practice is to set the relay such that it operates for earth fault current of the order of 15% of rated current. Such setting protects restricted portion of winding, hence the name is restricted earth fault protection.

DIFFICULTIES IN DIFFERENTIAL PROTECTION SCHEMEi) Difference in length of pilot wires on either side of relay: The difficulty is overcome by connecting adjustable resistors to pilot wires

ii) Difference in CT ratio: This difficulty is overcome by using biased differential relay.

iii) Magnetizing current in rush: This difficulty is overcome by providing time lag of 0.2 Sec. in the relay, by this time the inrush current will vanish and relay does not trip unnecessarily. This problem can be overcome by using second harmonic restraint relay.

iv) Phase difference between primary and secondary currents: This difficulty is overcome by proper connections of CTs on both sides. e.g for delta-star connected transformer, CTs are connected in star-delta fashion.

v) Tap changing affects the ratio of transformer: This problem is overcome by adjusting the turns ratio of CT accordingly.



REVERSE POWER PROTECTION:

In the reverse power protection scheme which is used for alternators the directional relays are used to sense the reverse power (current) flow and trip the relevant CBs. generators they operate when the machine enters the motoring mode that is the machine gets supply form the power system instead of supplying power.

MERZ PRICE PROTECTION OF ALTERNATOR:

The arrangement is as shown in the figure. The scheme is used to sense phase to phase and earth faults in alternator windings. The scheme does not respond to through faults (external faults) or overloads as the CT currents on the two



sides of generator winding are identical. The relay operating coils produce the required motion of the relay disc for trip signal to CB when the phasor difference of the CT secondary currents on the two sides of the alternator windings for any phase is more than the set value (relay pickup value). It is essential that due importance be given to the polarities of CT connections such that any OC will get the difference current for the two side CT secondaries of respective phase

NEGATIVE PHASE SEQUENCE PROTECTION OF ALTERNATOR



TOPIC 5 : PROTECTION OF TRANSFORMER 8(14)

Contents :-

1. Abnormalities & faults.

2. Faults likely to occur in a power transformer: 1. Earth fault 2. Overloads and overheating 3. Incipient faults below oil level resulting into decomposition of oil 4. Through faults 5. High voltage surges due to lightning or switching 6. Tap changer faults 7. Phase-to-phase, Phase-to-ground faults 8. Saturation of magnetic core 9. Inter-turn and winding faults

Incipient faults: - It means slow developing faults.



Through faults: - It means faults beyond protection zone but fed through the protected zone. Differential, Biased differential protectionLimitations of differential protection of transformer,Problems in applying biased differential protection:

BUCHHOLZ RELAY

Fluid actuated relay placed between the conservator and the tank containing the components

FUNCTIONS

- to be protected such as the windings of transformers

- where abnormal arcing occurs such as in tap changing chambers (studs/contacts etc.)

Normally for transformers of capacities 500 kVA or more

Applications of Buchholz’s relay:-

Detect incipient faults (minor faults leading to decomposition of oil leading to gas formation) (occurring below oil level in oil immersed transformers) such as , phase-core and give the alarm signals so that preventive action is taken before the condition leads to a major fault



- Detect sudden heavy oil movements due to severely violent faults in the tanks and give the trip signals

TOPIC 6: PROTECTION OF MOTOR 3(06)

Contents:

1. Abnormalities & faults. 2. Short circuit protection, Overload protection, Single phase preventer- (circuit diagram,

operation)



TOPIC 7 : PROTECTION OF BUSBAR & TRANSMISSION LINE 4(10)

Contents :

1. Abnormalities & faults. 2. Bus Bar Protection – Operation of Differential Protection and Fault bus protection schemes.

3. Transmission line, over current, distance protection.

Requirements of transmission line protection: i) Faults on lines should be quickly detected to initiate actions to maintain system stability. ii) For very long lines the protection system must be capable of identifying the fault location. iii) In the event of short circuit fault on the line, the circuit breaker nearest to it must operateto open the line, while the other circuit breakers remain closed. iv) Adjacent circuit breakers should provide immediate backup protection in the event of failure of circuit breaker (nearest to fault) to operate.

Abnormalities in transmission lines: Transmission lines are being exposed to atmospheric conditions, the chances of fault occurring are due to: i) Storms, ii) Falling of external objects on lines iii) Flash over resulting from dirt deposit on insulators etc

Harmful effects of Travelling Waves: i) Insulation of windings may be damaged due to internal flash over by high peak voltage of the voltage surge. ii) Inter- turn insulation of transformer may be damaged due to steep front of surge wave. iii) Insulators of the terminal equipment may be damaged due to external flash over. iv) Resonance and high voltage causes oscillations in the electrical equipment

Pilot wire protection



Here two wires called as pilot wires are used to carry the information signals of relaying from one end of protected line to other end. These can be buried cables or auxiliary overhead lines other than the power lines. As pilot wires are expensive the three phase quantities are converted to equivalent single phase ones and relayed through one pair of pilot wires & not three wires. These are used for short lines with the break even distance being 15 km to 30 km in terms of cost The pilot wire schemes can be implemented by two principles namely circulating current & balanced voltage as shown above.



TOPIC 8: NEUTRALEARTHING 2(04)



1. Introduction & importance. 2. Types of earthing: diagram, procedure

3. Substation earthing: diagram, procedure4. Difference between Equipment earthing and Neutral earthing

Advantages of grounding the neutral of a system: 1. Reduces voltage stresses/ (transients effects) due to switching operations. 2. Reduces voltage stresses/ (transients effects) due to lightning surges. 3. Limits fault currents. 4. Sensing faults through appropriate relay systems possible and thus protects the system. 5. Safety of personnel, equipment. 6. Discharging paths for stored charges on system before maintenance provides protection.

TOPIC 9: OVERVOLTAGE PROTECTION 4(08)

Contents :



1. Causes of over voltages.

2. Causes of over voltages: a) Internal causes: i) Switching surges ii) Arcing ground iii) Insulation failures iv) Resonance b) External causes: i) Direct Lightning strokes, ii) Lightning discharge near the line iii) Voltage induced due to change in atmospheric condition iv) Voltage induced due to frictional effects of small particles such as dirt, dust snow.

3. Lightning phenomena, over voltage due to lightning, typical waveform of lightning surge

4. Protection of transmission line & substation from direct stroke. 5. Types of lightning arresters - Rod gap, Horn gap, Expulsion and Thyrite type, their construction

& principle of operation.. Protection against traveling waves.

The principle of lightning arrester is as follows:

Break down of series spark gaps and non characteristics of normal behavior (open circuit) at normal system voltages & breaking down to conduct when the voltage across them (to earth) rises due to lightning. On break down the lightning surge is conducted regained the arrestor



stops conduction to become normal. Not sealed, is in contact with the air outside. Slower Not as accurate. After every operation Bigger Low Lesser Deteriorate with time as not fully sealed.

Under normal working voltage the cylinder (zinc made) B is at earth potential, hence series gaps remain open. When over voltage occurs which is sufficient to produce arc between gaps A and B; heavy current will flow to earth through shunted gap B- C & series resistance, instead of shunt resistance. When surge is over the arc B to C gets extinguished & normal condition is restored. The equipment is protected form voltage surges.

SURGE ABSORBER - DEFINITION & WORKING WITH NEAT DIAGRAM

6. Necessity of Insulation co-ordination



Insulation Co-ordination : It is the correlation of the insulation of electrical equipment and the lines with the characteristics of protective devices such that the insulation of the whole power system is protected from the excessive over voltages. Importance : The insulation strength of various equipments like transformers , circuit breakers etc. should be higher than that of lightning arresters and other surge protective devices. The insulation Co-ordination is thus the matching of the volt time flash over and break down characteristcs of equipment and protective devices in order to obtain maximum protective margin at a reasonable cost. Basic Insulation Level (BIL): Basic Impulse Insulation Level (BIL) is the reference level expressed in impulse crest voltage with a standard wave not longer than a 1.2/50μsec wave according to IS. Significance of BIL in Insulation coordination: Insulation coordination is the co-relation of the insulation of electric equipment and lines with the characteristics of protective devices such that the insulation of the whole power system is protected from excessive over voltages. In order to protect the equipment of power system from over-voltages of very high magnitude, it is necessary to fix an insulation level for the system to see that any insulation in the system does not breakdown or flash over below BIL. Curve A is volt time curve of protective device and curve B is that of equipment (apparatus) to be protected.

List of Practicals:1. Survey of different switchgear equipment used in electrical power system and study of their

technical specifications. (Market survey/ web based search/ visit) 2. Demonstration of working of MCB, MCCB and identification of different parts and their

function3. Plot current (i) Vs. time (t) characteristics of a fuse (Kitkat/HRC) 4. Performance test of an electromechanical IDMT over current relay.5. Study and understand the function and operation of microprocessor based over current relay.6. Demonstrate operation of a protection system used for a three phase induction motor.7. Collect data for protection system used in a typical HT substation (Transformer and Busbar).8. Collect data for different types and specifications of lightening arrestor. 9. Collect data about a typical HT/LT substation earthing scheme.


s.k.b.p.polytechnic kopargaon department of … · web viewthe analysis of the power system calls...

Documents