BVRIT HYDERABAD College of Engineering for Women Department of Electrical & Electronics Engineering

Hand Out

Subject Name: Switch Gear & Protection Prepared by (Faculty(s) Name): Ms.K.Bhavya, Assistant Professor, EEE Year and Sem, Department: III Year- II Sem, EEE

Unit – I: Circuit Breakers -I

Important Points / Definitions: In an electric power system, switchgear is the combination of electrical disconnects switches,

fuses or circuit breakers used to control, protect and isolate electrical equipment. Circuit Breaker is an automatic device for stopping the flow of current in an electric circuit as a

safety measure. Relay is an electrical device, typically incorporating an electromagnet, which is activated by a

current or signal in one circuit to open or close another circuit. Re-striking Voltage is the transient voltage that appears across the contacts at or near current

zero during arcing period. If dielectric strength rise is greater than the rise of re-striking voltage then the arc will not re-

strike. Recovery voltage is the normal frequency rms voltage that appears across the contacts of the

circuit breaker after complete interruption of the arc (i.e. after the transient oscillation dies out). Re-striking voltage is the voltage that appears across the contacts at the instant of arc extinction.

The rate of rise of re-striking voltage (RRRV) is defined as the slope of the steepest tangent to the re-striking voltage curve or it is the slope of a line from voltage zero to the first peak of the wave. It is expressed in Volts/ps.

The rating of a circuit breaker includes, a. Rated short circuit breaking current. b. Rated short circuit making current. c. Rated operating sequence of circuit breaker. d. Rated short time current.

There are two methods by which interruption is done. a. High resistance method

b. Low resistance method or current zero interruption method. There are two theories which explain the phenomenon of arc extinction:

a. Energy balance theory,

b. Slepain’s Theory

The different types of high voltage circuit breakers which includes the following a. Air Circuit Breaker

b. SF6 Circuit Breaker c. Vacuum Circuit Breaker d. Oil Circuit Breaker e. Air Circuit Breaker

Different types of Oil Circuit Breaker

a. Bulk oil circuit breaker b. Minimum oil circuit breaker

Application and Uses of Air Circuit Breaker a. It is used for protection of plants, electrical machines, transformers, capacitors and generators b. Air circuit breaker is also used in the Electricity sharing system and GND about 15Kv c. Also used in Low as well as High Currents and voltage applications.

Questions:

1. Calculate the RRRV of a 220kV circuit breaker with earthed neutral. The short circuit test

data obtained is as follows: The current breaker is symmetrical and the re-striking voltage

has an oscillatory frequency of 15 kHz. The power factor of the fault is 0.2. Assume the short

circuit to be an earthed fault.

2. Compare the arc rupture in oil and air blast circuit breakers and summarize the relative

advantages and disadvantages of these types of switch gear. Explain the operating duty of a

circuit breaker.

3. For a 32 kV System reactance and capacitance up to the location of the circuit breaker is 3

ohms and 0.015 microfarad, respectively. Calculate the following

(a) The frequency of transients oscillation

(b) The maximum value of restriking voltage across the contacts of the circuit

breaker

(c) The maximum value of RRRV.

4. What are the various types of SF6 circuit breakers. Explain them in detail with a neat sketch.

5. Explain the following in detail:

(a) Symmetrical breaking capacity (b) Asymmetrical breaking capacity

(c) Rating of circuit breakers. 6. Explain the Phenomenon of current chopping in a circuit breaker. What measures are taken

to reduce it. 7. Distinguish between Air Blast circuit breaker and oil circuit breakers? 8. Explain resistance switching in detail with relevant diagrams and derive the expression of

damped oscillation. Fill in the blanks / choose the Best:

1. Circuit breakers usually operate under [b] a. Steady short circuit current b. Sub transient state of short circuit current c. Transient state of short circuit current d. None of these

2. The making and breaking currents of a 3 phase ac circuit breakers in power systems are respectively in ________form. [d]

a. rms value, rms value b. instantaneous value, rms value c. rms value, instantaneous value d. instantaneous value, instantaneous value

3. SF6 is which type of gas? [b] a. Electro positive b. Electro negative c. Both (a) and (b) d. None of these

4. A three phase circuit breaker is rated 2000 MVA, 33 kV. What will be its making current? [d] a. 35 kA b. 49 kA c. 70 kA d. 89 kA

5. What is switchgear? [a] a. An apparatus used for switching, controlling and protecting the electrical circuits and equipments. b. It detects the faults only. c. It corrects the faults only. d. All of the above.

6. What is the actuating quantity for the relays? [d] a. Magnitude b. Frequency c. Phase angle d. All of these

7. What is / are the main disadvantage / s of using oil as the quenching medium in the circuit breakers? [d] a. Need periodical replacement. b. Risk of formation of explosive mixture with air. c. Possibility of causing fire hazards. d. All of the above.

8. For voltages beyond 66 kV, switchgear equipment is installed outdoor. 9. A fuse performs both detection and Interruption functions. 10. A three phase, 33 kV oil circuit breaker is rated 1200 A, 2000 MVA, 3s. What is its

symmetrical breaking current? [C] a. 1200 A b. 3600 A c. 35 kA d. 104.8 kA

11. What is the major cause of the failure of the circuit breaker? [d] a. Trip circuit open b. Trip latch defective c. Spring defective d. All of these

12. The isolators used in the transmission lines are capable of breaking [C] a. Fault current b. No current c. Charging current d. Load current

13. Why are the isolators used? [c] a. Break abnormal current b. Making under fault conditions c. Breaking the circuit under no load condition d. None of the above

Unit – II: Electromagnetic and Static relays

Important Points / Definitions:

A relay is a switching device as it works to isolate or change the state of an electric circuit from one state to another.

Depending on the operating principle and structural features relays are of different types such as electromagnetic relays, thermal relays, power varied relays, multi-dimensional relays, and so on, with varied ratings, sizes and applications.

Overvoltage, over current, and Overpower Relay – The relay operates when the voltage, current or power arises above a specified value.

Under voltage, Undercurrent, and under power Relay – The relays operate when the voltage, current or power falls below a specified value.

Directional or Reverse Current Relay – The relay operates when the applied current assumes a specified phase shift on the supply voltage and the relay is compensated for the fall in voltage.

Directional or Reverse Power Relay – The relay operates when the applied voltage and current assumed specified space displacement and no compensation is allowed for fall in voltage.

Differential Relay – The relays operate when some specified phase or magnitude difference between two or more electrical quantities occurs.

Distance Relay – In this relay operation depends on the ratio of the voltage to the current.

According to timing characteristic, the relays can be divided into the following classes.

a. Instantaneous Relays – These relays employed after a small time duration from the incidence of the current or other quantity resulting in operation. The times require for the operation of such relays is less than 0.2 seconds.

b. Definite Time Lag Relays – In these relays, the time of operation is sensibly independent of the magnitude of the current or other quantity causing operation.

c. Inverse Time Lag Relays – In this relay, the magnitude of the current or other quantity causing operation is inversely equal to the time requires for the operation.

d. Inverse Definite Minimum Time Lag Relays – In these relays the time of operation is approximately equal to the lesser values of current or other quantity like voltage, frequency, etc., causing operation and tends to be a specific minimum time as the value rises without limit.

The relay which does not contain any moving parts is known as the static relay. Questions:

1. With the help of neat sketch explain the principle of Directional Over-current protection. 2. An over current relay of current rating 5A and setting 150% is connected to the secondary

of a CT of ratio 300:5. Calculate the current in lines for which the relay pick up. 3. Explain the merits and demerits of static relays. 4. Discuss how an amplitude comparator can be converted into a phase comparator and vice

versa. 5. Discuss the principle of operation of an Induction disc relay with relevant diagrams. 6. What are the advantages of Induction cup relays over Induction disc relays?

7. What is the purpose of shading in an Induction disc relay? 8. Where are the relays having extremely inverse and very inverse characteristics used? What

types of characteristics are used for protecting rectifiers, and for replacement of fuses? 9. Explain how the mho characteristic realized using a sampling comparator? 10. Explain in detail the role of protective relays in power systems. 11. Discuss in detail the causes and types and frequency of faults encountered in 12. Explain the merits and demerits of static relays. 13. Discuss how an amplitude comparator can be converted into a phase comparator and vice

versa. 14. What is meant by directional feature of a directional over current relay? Describe the

construction, principle of operation and application of a directional over current relay. 15. What is the difference between a polarized mho and simple mho relay. What are self-

polarized and cross-polarized mho relays? 16. (a) Describe briefly some important types of electromagnetic attraction relays.

(b) Describe the various steps for calculating the actual relay operating time. Fill in the blanks / choose the Best:

1. What does protective relay provide? [c] a. Provide additional safety to the circuit breaker in its operation. b. Close the contacts when the actuating quantity attains a certain predetermined value. c. Limit the arcing current during the circuit breaker operation. d. Earth or ground any stray voltage.

2. Time classification of relays includes [d] a) Instantaneous relays

b) Definite time lag

c) Inverse time lag

d) All of these

3. Directional relays respond to [a]

a) Power

b) Voltage

c) Current

d) Reactance

4. In carrier current protection, wave trap is used is for trapping [a] a) High frequency waves entering in generating units b) Power frequency waves c) Both (a) and (b) d) None of these

5. Operating current in relay is [c] a) A.C. only b) D.C. only c) Both (a) and (b) d) None of these

6. For motor protection, which relay is used? [d] a)Thermocouple type relays b) Bimetallic relays c) Electronic relays d) All of these

7. Pilot wire protection is for [a]

a) Overhead lines b) Transformer c) Motors d) Cables

8. Under voltage relays are used for [d] a) Motors b) Alternators c) Bus bars d) All of these

9. An impedance relay is used for [c] a) Earth faults b) Interphase faults c) Both (a) and (b) d) None of these

10. Relay gets its operating energy from [d] a) Transformer b) Alternator c) Overhead lines d) C.T., P.T.

11. Good relay should possess [d] a) Speed & reliability b) Aped & sensitivity c) Adequateness & selectivity d) All of these

12. Split-phase relay responds to [c] a) Over load faults b) Over voltage c) Inter turn faults d) All of these

UNIT III: Generator & Transformer Protection

Important Points / Definitions:

Fault Classification on Generator

o Internal faults

o Abnormal Service conditions

o External Faults (as back up protection)

o The hazards and problems considered

o Phase to phase faults in winding

o Phase-ground faults in winding

o Ground faults in the rotor

Protection Schemes for generators

o Over voltage protection

o Over fluxing protection

o Low forward power and reverse power protection

o Dead machine protection

o Stator over current protection

o Rotor Over current protection

o Generator Loss of excitation protection

o Generator negative phase sequence protection

o Back up distance protection

o Shaft damage protection

o Under frequency protection

There are mainly two types of differential relay depending upon the principle of operation.

1. Current Balance Differential Relay

2. Voltage Balance Differential Relay

Most faults in a generator are a consequence of insulation failure. They may lead to turn – to –

turn faults and ground faults. Hence ground fault protection is very essential for generators.

Failures in transformers can be classified into

o winding failures due to short circuits (turn-turn faults, phase-phase faults, phase-

ground, open winding)

o core faults (core insulation failure, shorted laminations)

o terminal failures (open leads, loose connections, short circuits)

o on-load tap changer failures (mechanical, electrical, short circuit, overheating)

Questions:

1. Enumerate the relaying schemes which are employed for the protection of modern

alternator.

2. Describe with a neat, the percentage differential protection of a modern alternator. 3. Explain briefly with schematic diagram, the protective gear for alternators connected to

grid against

i. fault between phases and

ii. fault between turns in one of the phase windings

4. Discuss the protection employed against the loss of excitation of an alternator.

5. What is restricted earth fault protection for alternators? Explain the difference between

primary protection and back up protection.

6. For protecting a 132/33kV, 50 MVA power transformers from internal faults, what are the

different protection schemes normally used? Discuss one of the them in brief, with sketch.

7. Three phase 33/6.6 kV transformer is connected star- delta and current transformers on the

low voltage side have ratio 300: 5. What will be the ratio of C T on the high voltage side of

Merz Price protection is to be adopted.

8. Discuss the percentage differential protection scheme of a transformer.

Fill in the blanks / choose the Best:

1. Various power system faults in increasing order of severity are [a]

a)LG,LL,LLG,LLLG

b)LLLG,LLG,LG,LL

c)LLG,LLLG,LL,LG

d) LL,LG,LLLG,LLG

2. Which of the following method of protection is used to achieve earth fault operation?

[a] a)Core balance method b) Relay connected with neutral to ground c) Frame leakage method d) None of these

3. When a line-to-line fault occurs, the short circuit current of an alternator depends upon its

[c] a) Sub transient reactance b) Transient reactance c) Synchronous reactance d) Short circuit reactance

4. Generator internal fault protection is usually based the principle of [a] a) Differential protection b) Cross-differential protection c) Negative sequence protection d) All of these

5. Pilot wire protection is basically used for the protection of [d] a) Transmission lines b) Alternators c) Switch gears d) Transformers

6. Per cent bias for a generator protection lies between [a] a) 5 to 40 b) 40 to 45 c) 45 to 20 d) None of the above

7. Carrier current protection scheme is normally used for [a] a) HV transmission lines only. b) HV cables only. c) HV transmission and cables. d) None of the above.

8. Number of pilot wires required for protecting 3-phase transmission lines using translay system

of protection is [d] a) 6 b) 4 c) 3 d) 2

9. The most commonly used system for protection of alternator is Circulating Current system.

10. Buchholz relay is gas actuated relay.

11. Over load Protection is generally provided for Alternators.

12. Earth relays have Lower Current Setting.

13. Buchholz relay is placed in between main tank and conservator.

14. The chief cause of over speed in alternator is sudden loss of load.

UNIT –IV Feeder and Bus-Bar Protection & Grounding

Important Points / Definitions:

Most commonly used schemes for bus zone protection are o Back up protection

o Circulating current protection

o Frame leakage protection

Types of protections for feeder are

o Over load protection

o Over current and earth fault protection

o Time graded protection

o Current graded protection

o Current balance differential protection

o Merz price voltage balance system

o Translay protection system

o Carrier current protection

The ungrounded system is one that has no intentional connection between the neutral or any

phase and ground

There are basically six (6) grounding systems in use. The six systems are the

o Equipment Grounds

o Static Grounds

o Systems Grounds

o Maintenance Grounds

o Electronic Grounds

o Lightning Grounds.

Several factors should be considered in the initial design of the grounding system.

o The area available for installation of the grounding system. This could lead

to the requirement and utilization of chemical rods, or wells.

o Water table and seasonal changes to it.

o Soil condition and resistivity, Please see chart of typical results. Also

elevation above sea level and hard rocky soil are concerns that would need

to be addressed.

o Available fault currents (i.e., three (3) phase, line-to-ground, and line-to-line-

to ground, etc.).

o NEC and ANSI/IEEE requirements. Also include here the requirements of the

process equipment to be installed.

o Consideration to the number of lightning strikes and thunder storm days per

year.

o Utility ties and/or service entrance voltage levels.

o Utilization of area were ground system is to be installed, (i.e., do not install

under paved parking lot).

Questions:

1. Discuss with a neat sketch the general principle of operation of a distance protection scheme. 2. Explain in detail the primary and back-up protection. What are the advantages of zonal protection? 3. (a) With a neat sketch, discuss the differential scheme for bus zone protection. (b) Discuss the working principle of frame leakage protection 4. Write short note on the following: i. Bus Fault protection ii. Merz-price voltage balance system for protection of feeders. 5. Give various schemes of protection for feeders. 6. Discuss the merits and demerits of earthing it solidly, through a resistance and through

reactance.

7. Distinguish between neutral earthing and equipment earthing.

8. Explain clearly the meaning of resonant grounding. What are the requirements of the

reactor in neutral connections of such grounding? Draw the connection of arc suppression coil

Fill in the blanks / choose the Best:

1. The size of the earth or ground wire is based on the: [d]

a) maximum fault current carrying through the ground wire

b) rated current carrying capacity of the service line

c) depends on the soil resistance

d) both (a) and (c)

2. Earth wire or ground wire is made of: [d]

a) copper

b) aluminium

c) iron

d) galvanized steel

3. Average resistance of human body is: [b]

a) 500 ohms

b) 1000 ohms

c) 1500 ohms

d) 2000 ohms

4. Generally grounding is provided for: [c]

a) only for the safety of the equipment

b) only for the safety of the operating personnel

c) both (a) and (b)

d) none of the above

5. Ground resistance should be designed such that: [a]

a) grounding resistance should be as low as possible

b) grounding resistance should be as high as possible

c) grounding resistance should be always zero

d) none of the above

6. When earth fault occurs: [a]

a) voltage potential at the earth mat increases due to grounding

b) voltage potential at the earth mat decreases due to grounding

c) voltage potential at the earth mat remains zero irrespective of fault

d) none of the above

7. The objective of earthing or grounding is: [a]

a) to provide as low resistance possible to the ground

b) to provide as high resistance possible to the ground

c) to provide flow of positive, negative and zero sequence currents

d) none of the above

8. Factors on which soil resistance depends: [d]

a) depth of the electrode

b) moisture

c) Nacl

d) all the above

9. Busbars operates at constant Voltage.

10. If a fault occurs on the bus itself in a single bus-bar system, then there is complete

shutdown.

11. The ground wire is colored in green.

12. In Peterson coil grounding, the inductance of the coil is Variable.

UNIT – V Protection against over voltages

Important Points / Definitions: The overvoltage in the power station can be caused either by the internal disturbance or by

the atmospheric interruption. On the basis of the generation of over voltages the voltage surge are classified into two

categories. These are o Internal Overvoltage o External Overvoltage

Types of lightning strokes o Direct Stroke o Indirect stroke

Protection against lightning o Earthing screens o Overhead ground wires o Lightning Arresters or Surge Diverters

Types of lightning arresters Rod gap arrester Horn gap arrester Multi gap arrester Expulsion type lightning arrester Valve type lightning arrester

A surge arrester is a device to protect electrical equipment from over-voltage transients caused by external (lightning) or internal (switching) events.

Switching impulse wave form has steep front and long damped oscillating tale. Short duration power frequency withstand voltage is the prescribed rms value of sinusoidal

power frequency voltage that the electrical equipment shall withstand for a specific period of time normally 60 seconds.

Questions: 1. What are various methods of over voltage protection of overhead transmission lines.

2. Define the terms:-

(a) Rated voltage of lightning arrester

(b) Normal discharge current

(c) Spark over voltage-power frequency impulse

(d) Non-linear resistance

(e) Protective ratio

(f) Residual voltage.

3. What are the causes of over voltages arising on a power system? Why is it necessary to

protect the line and other equipment of the power system against over voltages?

4. Describe the construction and principal of operation of (i) Expulsion type lighting arrestor,

and (ii) valve type lighting arrestor?

5. Describe the construction and working principle of a zinc oxide gapless arrester with a neat

sketch

6. Discuss the relative merits and demerits of

i. rod gap

ii. Expulsion arresters

iii. Valve arresters

Fill in the blanks / choose the Best:

1. Which device is used to protect the power system against travelling waves? [c] (a) capacitor. (b) Earth wire. (c) Surge diverter. (d) Earthing screen.

2. In the electric system, Resonance may result in [b] (a) zero voltage. (b) High voltage. (c) Low voltage. (d) High current.

3. A device which is used to protect an electric circuit from damage caused by over load or short circuit is [c] (a) Wave trapper (b) CVT (c) circuit breaker (d) Lighting arrester

4. In overhead system, the transmission voltage can be controlled by [c] (a) Excitation control (b) Using induction regulator (c) Reactive VAR injection method (d) Any of the above

5. An lighting arrester behave like a [a] (a) non-linear resistance. (b) Linear resistance (c) Variable resistance (d) None of the above

6. The over-voltage surges in power systems may be caused by [d] (A) Lightning (B) Switching (C) Resonance (D) Any of the above.

7. The protection against over-voltage due to lightening is provided by [d] (A) Use of surge diverters (B) Low tower footing resistance (C) Use of overhead ground wires (D) Any of the above

8. Insulators operate under No Load conditions. 9. In horn gaps the arc is produced at the bottom during high voltage surge. 10. Secondary type lightning arresters are used for the protection of distribution transformers. 11. The non-linear resistor in lightning arrester offers low resistance to the flow of discharge

currents and high resistance to power frequency voltage. 12. The surge current which flows through the lightning arrester after the spark over is

expressed in terms of peak value. 13. The most severe cause of switching over-voltages is Closing Charged Line.