Earthing and EARTHING AND PROTECTION System Protection While the use of electricity is a boon to us, its misuse, any fault in the system can be a curse, indeed, it could lead to complete damage of the equipment and subsequently can damage the entire system and can also cause major accidents, including fatal ones.
Hence, we have to cautions. In practical sense, we need to provide for safe use of electricity. As we shall learn, earthing is the most important factor in this regard. Of course, earthing may also serve other purposes in the electrical system. Protection of the electrical equipment and the network is another safety measure, as it helps save not only our investment in the electrical system, but also, it may help prevent accidents related to electricity. You shall be introduced to ‘Earthing and System Protection’ in this second part (Block 2) of this Course 1 : Electricity and Safety Measures as most useful and necessary and critical component of power system.
The first part introduces you to electrical Earthing. In this part, you will be acquainted with importance of earthing, earthing classification, line and pole earthing, measurement of earth resistance. It also touches upon the treatment for minimizing the earth resistance values, maintenance of earthing system, definitions of general earthing terms.
Thereafter, in the second part you will be introduced to the Electrical System Protection. In this part, you will learn about the objectives of protection, equipment for system protection, protective relays, functional requirements of relays, distribution system protection, substation protection where you will be introduced to principle of differential relay operation, transformer protection, bus protection. In the last section, you will learn about current and voltage transformers.
We hope that the concepts and information presented in this block would help you in improving your knowledge and performance of the power distribution system.
We wish you all the very best!
39
Contents
EARTHING AND PROTECTION Unit 4 Earthing 7 4.1 Introduction 9 4.2 Importance of Earthing 9 4.3 Classification of Earthing 10 Equipment Earthing 11 Discharge Earthing 13 System Earthing 13 4.4 Line and Pole Earthing 14 4.5 Points of Equipment to be Earthed and Purpose 17 4.6 Guidelines for Providing Earthing Arrangements 18 4.7 Measurement of Earth Resistance 19 4.8 Standard Earth Resistance Value 21 4.9 Treatment for Minimise Earth Resistance Values 21 4.10 Maintenance of Earthing System 22 4.11 Definitions of General Earthing Terms 23 4.12 Summary 24 4.13 Terminal Questions 25 4.14 Answers to Check Your Progress 25 4.15 Answers to Terminal Questions 26
Unit 5 Electrical System Protection 27
5.1 Introduction 29 5.2 Types and Objectives of Protection 29 5.3 Classification of Earthing 30 Protective Relays 31 5.4 Functional Requirements of the Relays 33 5.5 System Protection Concepts 33 5.6 Distribution System Protection 35 Overcurrent and Reclosing Relays 35 Underfrequency Relays 36 5.7 Substation Protection 37 Principle of Differential Relay Operation 37 Transformer Protection 37 Bus Protection 37 5.8 Instrument Transformers 38 Current Transformer (CT) 38 Voltage Transformer 40 5.9 Summary 40 5.10 Terminal Questions 41 5.11 Answers to Check Your Progress 41 5.12 Answers to Terminal Questions 42
Earthing 4.1 INTRODUCTION
God has given us earth soil in abundance, available throughout the world. This is a natural gift used in plenty of ways, i.e. planting, excavation for a building foundation, houses and roads, etc. Earthing soil has an unique electrical property in the form of conductivity that is put to practical use in everyday life in power plants and electrical utilities.
Earthing is of foremost importance for the safety of human beings, animals, consumer property and utilities’ equipment. In this unit, we shall learn the importance of earthing/requirement of good earthing.
Broadly speaking, earth resistance is the resistance of the soil to the passage of current, which may not be as low as that of a conductor like copper. But, since the cross-section area of the path for the current is very large, the actual resistance is normally, quite low and, hence, earth can be used as a reasonably efficient conductor.
Earthing is generally to be carried out in accordance with the requirements of Indian Electricity Rules, 1956 and IS : 3043-1987, at generating stations, transmission/distribution substations and consumers premises. Indeed, any equipment or device drawing or feeding power to the public power network, makes use of earth connectivity.
Objectives
After studying this unit, you should be able to
describe the importance of earthing,
list the various types of requirement of earthing,
explain different types of earthing,
explain different methods of earthing, and
list the purpose of earthing in different points of equipment.
4.2 IMPORTANCE OF EARTHING
Prime objective of earthing is to provide a zero potential surface in and around and under the area where the electrical equipment is installed or erected. In the case of shielded (enclosed) conductors, earthing of the shield isolates the conductor from external interference, and prevents the interference due to the current in the conductor from spreading outside the shield. 9
Earthing To achieve this objective, the normally non-current carrying (but conducting) part and Protection of the electrical equipment is connected to the general mass of the earth, which prevents the appearance of dangerous voltage on the enclosures and helps to provide safety to working staff and public who may come in contact with the equipment.
The basis of the use of earthing, as described here, is the fact that all the generating plants in a grid are connected through earthing. Under this system, earthing may be put to use as the return path for electric current, under abnormal conditions.
Check Your Progress 1
What is the use of earthing?
EARTHING REQUIREMENTS
Good earthing should have low resistance.
It should stabilize circuit potential with respect to ground and limit overall potential rise. It should prevent, or at least minimize, the damage to the equipment due to flow of eventual heavy fault currents.
It should protect men and material from injury or damage due to over-voltage.
It should improve the reliability of power supply.
It should provide low impedance path to fault currents to ensure prompt and consistent operation of protective relays, surge arrester, etc.
It should keep the maximum potential gradient along the surface of the sub-station within safe limits during ground fault.
4.3 CLASSIFICATION OF EARTHING
Earthing can be classified into the following categories based on the purpose for which the part of the equipment is connected to the general mass of earth. 10
CLASSIFICATION OF EARTHING Earthing Equipment Earthing Discharge Earthing System Earthing
4.3.1 Equipment Earthing
Earthing associated with non-current carrying parts of electrical equipment is called equipment earthing. Safety of the operator, the user and the safety of their property are the main reason for equipment earthing, e.g. outer metallic body of a transformer, metallic body of an electric motor.
As stated, for equipment earthing to be effective, it must first be done at the generating plant. Here the neutral of the star-connected three phases is grounded or earthed.
METHODS OF CONNECTING NEUTRAL OF THE ELECTRICAL EQUIPMENT TO GROUND
Four main methods are as follows :
Solid Earthing
Resistance Earthing
Reactance Earthing
Arc-suppression Coil (or) Peterson Coil Earthing
These methods of earthing system are shown in Figure 1.1, and are described below. These make use of the 3-phase, 4-wire system. In this system, three wires are provided for the phases and a fourth wire is provided for the neutral.
Generator Transformer N N
Solid Grounded Neutral Resistance Resistance Grounded
N N
Reactor Peterson Coil Arc Reactance Grounding Suppression Coil or Earth Fault Neutralizer
Resonant Grounding Figure 4.1 : Neutral Connection Methods 11
Earthing Solid Earthing and Protection
When the fault current is expected to be low and not likely to cause damage to plant, cables, and loss of stability of system, then earthing may be done directly through metallic conductor from system neutral to the main earthing ring without any impedance in the circuit. It should be ensured that the impedance between the ‘N’ & ‘E’ is so low, so that if an earth fault occurs in one phase of the system sufficient current will flow to operate the protective devices.
Resistance Earthing
Resistance earthing is generally used when the fault current is likely to be so high as to cause damage to the equipment, mainly transformers. If a resistance is inserted between the Neutral and Earth, quick acting protective devices are also used. The resistors shall comprise metallic resistance units, supported on insulators in a metal frame or shall be a liquid resistor of a weak aqueous solution, either of zinc chloride or sodium carbonate.
Metallic resistors have a constant resistance, which does not change with time, while liquid resistors have to be treated frequently specially after the clearance of a fault. Metallic resistors are slightly inductive and this is a disadvantage with overhead lines, since traveling waves and impulses are subject to positive reflection and this is likely to unduly stress the insulation of the equipment and may cause breakdown. Use of liquid resistors is recommended only at voltages above 6.6kV. All neutral earthing resistances are designed to carry their rated current for a short period, usually 30 seconds.
The earth resistance is of such a value that, should a fault occurs outside the equipment, the fault current will be restricted to the rated full load current of the equipment. If the earth resistance is too low, for any occurrence of the earth fault the equipment will be subjected to an electrical shock, due to the load resulting from the power loss in the resistor.
Reactance Earthing
When the zero sequence (a measure of the unbalanced current in a three-phase system) reactance of generators or transformers is so low as to cause excessive fault current, usually reactance earthing is used.
A single-phase reactor is inserted between the neutral and the earth to 12 limit fault current to the maximum of 3-phase short circuit current.
Earthing Here the current due to earth fault on one phase is limited to minimize damage to the equipment. Care should be taken to see that during system fault or switching operations, dangerously high transient voltages do not occur, due to the high value of reactance of the earthing reactor.
Arc-suppression Coil Earthing
In high voltage systems with isolated neutrals, overvoltages, caused by switching surges, or by lighting, may cause a line to flashover to earth. Considerable current will be drawn through the arc to charge the system capacitance to earth. The arc is quenched at zero voltage but may re-strike at a higher voltage. This successive re-striking of the arc often causes very high voltages to be built-up on the transmission lines, and is known as “arcing grounds”.
To avoid isolation of system under earth fault conditions, arc-suppression coils are sometimes used, as shown in Figure 4.1. An arc-suppression coil, also known as the Peterson coil, is a tuned earthing reactor. It is tuned to the system capacitance in such a way as to make the reactance of the zero sequence network practically infinite, so that no fault current flows to the earth and there is no tendency for arcing grounds to occur. With the use of Peterson coil, arc current is reduced to such a small value that it is usually self-extinguishing, which increases continuity in service.
4.3.2 Discharge Earthing
When the charged electrical equipment or system is isolated from the electrical power supply, the electrostatic charge still remains in the system. Before carrying out any work on this isolated electrical equipment, these electrostatic charges are discharged to earth through earth switches or earth rod. This method of earthing is called Discharge Earthing.
4.3.3 System Earthing
Earthing associated with current carrying parts of the equipment is called system earthing. The system security, reliability, performance, voltage stabilisation, all rely only on the system earthing, e.g. neutral of transformer and surge arrester earthing.
Check Your Progress 2
In the case of a 3-phase circuit or equipment, what part of the circuit is earthed?
13
Earthing and Protection 4.4 LINE AND POLE EARTHING
Here the reference is to a Pole-based aerial LT Distribution network. It is recommended that –
Every fifth metallic pole of LT line should be earthed.
Cross arms, top clip, insulator pins of P.S.C. pole should be earthed along with the pole.
Guarding at railway crossing, telephone crossing, road crossing should be earthed along with pole on both sides. If earth electrode is not available, an 8 S.W.G.G.I. wire coil of 25 mm diameter and 120 to 150 cm may be buried, to provide earthing. It is very necessary to earth the guarding. All the metal fittings of LT pole and stay should be earthed.
A separate earth electrode, to be used for earthing, and the earth wire should be routed through an Alkathine pipe without touching the pole.
MS Rod CI Cover Hinged to CI Frame Ground Level CI Frame
Copper Or GI Wire Bolt, Nut, Check Nut and d Washer to be of Copper for Copper Plate and GI for GI Plate
GI Pipe GI Plate or Copper Plate
Charcoal Enlarged Detail
Figure 4.2 : Earth Connection using a Metal Plate
The basic requirement to create earthing is to bury a conducting surface sufficiently deep in the ground, with connectivity being made available outside. 14
Earthing The buried conductor provides a conducting path to route the electrical current to earth and to provide a zero potential surface for connection.
The buried conductor is either a metal plate or a metallic pipe. Both these types are described below.
Plate Earthing
In major power stations and major sub-stations, 12 mm thick, 1200 mm long, 1200 mm wide cast iron plates are used.
For minor sub-stations, 18 mm broad, 50 x 50 cm G.I. plates are used. These plates are buried vertically in the pit. Coal, sand and salt are filled in the pit, each of 150 mm layer. The plate should be buried deep so that soil will be wet from all sides. In the case of multiple plate, the plates should be placed at a distance of 1200 cm from each other.
Pipe Earthing
For power stations and major sub-stations, 12 mm thick, 150 mm diameter, 300 cm, long G.I. pipes are used. A minimum distance of 1200 cm should be kept between earth electrodes in major sub-stations and 180 cm in case of minor sub-stations.
Lightning Arrester
A. B. SWITCH L. A. Earthing Lead Pole H.C. FUSE
Embedded Embedded Pole Earth Pole Earth
Transformer Pole N
L. T. Circuit Breaker Box A. B. Switch
Handle G L
Earthing Electrode
Figure 4.3 : Earthing Arrangement for Distribution Sub-station
15
Earthing At least one electrode should be used at every corner in the sub-stations. and Protection Each electrical equipment structure and the entire metal fitting should be earthed. Three earth electrodes are used for pole-mounted transformers.
The transformer neutral and body should be double earthed. A minimum 8 S.W.G.G.I. wire should be used for earthing. Separate earthing should be done for a distribution box. 7/10 S.W.G.G.I. wire should be used for tower earthing, and for S/s. gantries 3 mm thick, 50 mm x 50 mm M.S. plate is used. G.I. pipe of 25 mm thick, 1910 mm long is used for H.T. line, and 20 mm thick, 1720 mm long for L.T. lines.
The purpose of coal and salt is to keep the soil wet, permanently. The salt percolates and coal absorbs water keeping the soil wet. Care should always be taken by watering the earth pits in summer so that the pit soil will be wet. Coal is made of carbon which is good conductor minimizing the earth resistant. For sub-station earthing the provisions in IS : 3043 (1966) Section 1, 2 and 3 should be utilized.
Joints
Rivets are used for joining the earth system, nut bolts or welding may also be used, depending on the expected temperature rise of the system.
GI Bolts and Nuts
Equipment/Strip Tinned Copper Lug Cable Cable
Arrangement of Double Earth Connection to Equipment (Strip to Conductor Connection) Brazing for copper and welding for aluminum
Riveting
Aluminum Copper Over lap min 50 mm
Straight Joints (Strips) 16 ‘T’ Joint (Strips)
Earthing Figure 4.4 : Joints in Earth Connection
The joint to earth conductor in switchgear unit or cable sheaths is required to be separated frequently, hence nut bolts are used at joints. In the case of steel system, they should be joined together, by welding. Only the places where earth testing is carried should be nut-bolted.
All joints should be properly painted. Channel, supporting control boards and panels are used as earth electrodes. (This is possible only when they are connected to earth system at both ends.)
Check Your Progress 3
When should an earthing joint be secured with nut and bolt?
4.5 POINTS OF EQUIPMENT TO BE EARTHED AND PURPOSE
Different points of equipment to be earthed and the purpose are given below :
SL. NO. POINTS EARTHED PURPOSE OF EARTHING 1. Transformer neutral For holding neutral at zero potential 2. Generator neutral To prevent arching to ground on overhead lines 3. Star point of a load To discharge voltage surges 4. Neutral of a circuit To provide path for out of balance current 5. Start point of CT/PT Simpler earth fault protection secondary 6. Equipment earthing To hold non-current carrying metallic parts at zero potential (Body earthing). To hold the current carrying parts at zero potential for safety, even on earth fault 7. Reference earthing To provide a reference zero potential in the conductor circuit 8. Discharge earthing To discharge capacitive current charge through earth switch to earth 17
Earthing 9. Surge arrester To discharge the surge voltages and Protection 4.6 GUIDELINES FOR PROVIDING EARTHING ARRANGEMENTS
Some general guidelines for providing earthing are given below :
All medium voltage equipment shall be earthed by two separate and distinct connections with earth through an earth electrode. In the case of high and extra high voltages, the neutral points shall be earthed by not less than two separate and distinct connections with earth each having its own electrode at the generating station or sub-station and may be earthed at any other point, provided no interference is caused by such earthing. If necessary, the neutral may be earthed through suitable impedance.
In cases where direct earthing may prove harmful rather than provide safety (for example, high frequency and mains frequency coreless induction furnaces), relaxation may be obtained from the Competent Authority.
Earth electrode shall be provided at generating stations, sub-stations and consumer premises in accordance with the requirements.
As far as possible all earth connections shall be visible for inspection.
All connections shall be carefully made; if they are poorly made or inadequate for the purpose for which they are intended, loss of life or serious personal injury may result.
Each earth system shall be so devised that the testing of individual earth electrode is possible. It is recommended that the value of any earth system resistance shall not be more than 5 ohms, unless otherwise specified.
It is recommended that a drawing showing the main earth connection and earth electrodes be prepared for each installation.
No addition to the current-carrying system, either temporary or permanent, shall be made, which will increase the maximum available earth fault current or its duration, until it has been ascertained that the existing arrangement of earth electrodes, earth bus-bar, etc. are capable of carrying the new value of earth fault current which may be obtained by this addition.
No cut-out or link, other than a linked switch arranged to operate simultaneously on the earthed or earthed neutral conductor and the live conductors, shall be inserted on any supply system. This, however, does not include the case of a switch for use in controlling a generator or a transformer or a link for test purposes. 18
Earthing All materials, fittings, etc. used in earthing shall conform to Indian Standard specifications wherever these exist. In the case of materials for which Indian Standard specifications do not exist, the material shall be approved by the Competent Authority.
4.7 MEASUREMENT OF EARTH RESISTANCE
Since earthing is an important aspect of safety and operation of an electrical system, it is necessary to ensure that the earth connection is effective and sufficient. The most important parameter in the case of earthing is the earth resistance. A poor value of earth resistance at the site could render the earth connectivity ineffective. The condition of the equipment could be from dangerous to life-threatening.
For monitoring the healthiness of earth, the condition monitoring equipment used is “EARTH MEGGER”. The measurement of earth resistance is done using three-terminal earth megger or four-terminal earth megger.
Methods of Earth Resistance Measurements
Three-terminal
In this method, earth tester terminals C1 and P1 are shorted to each other and connected to the earth electrode (pipe) under test. Terminals P2 & C2 are connected to the two separate spikes driven in earth. These two spikes are kept in same line at the distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individual spikes. If we rotate generator handle with specific speed we get directly earth resistance on scale.
R
C1, C2
P1, P2
Electrode Under Test G.L.
25 M 25 M
Auxiliary Spike Figure 4.5 : Measuring Earth Resistance, 3-Terminal Method 19
Earthing Note : Spike length in the earth should not be more than and Protection th 1/20 distance between two spikes.
Four-terminal
Four electrodes are driven in earth along a straight line, at equal intervals, ‘a’ (= 20 x depth ‘c’, approximately). The depth of electrode in the ground (c) shall be of the order of 10 to 15 cm. The megger is placed on a steady and approximately level base. The two outer electrodes are connected to current terminals C1, C2 of earth meggar and the two inner electrodes to potential terminals P1, P2 of the megger.
Requirements : 1. Earth Megger
2. Earth Spikes l
P1 P2 C1 C2
a a a
Figure 4.6 : Measuring Earth Resistance, 4-Terminal method
Test Procedure
After proper connections are made and range appropriately selected, by cranking the megger at the prescribed speed (135 rev/min), a value ‘R’, in ohm, is obtained on the meter. Resistivity is calculated by substituting the value of ‘R’ thus obtained, in Eq. (1.1).
where = Resistivity of soil in Ohm-centimeter,
a = Distance between two successive electrodes in centimeters,
R = Ratio of voltage to current or electrode resistance in Ohm (meter reading on the megger),
c = Depth of burial of electrode in the ground (in cm), should be negligible compared to the spacing between the electrodes, then,
= 2 a R . . . (4.1) 20
Earthing Check Your Progress 4
What is the use of Megger?
4 .8 STANDARD EARTH RESISTANCE VALUE
Given here are the acceptable values of earth resistance at the named installation. Lower values are acceptable, but higher values would be point of concern.
ACCEPTABLE VALUES OF EARTH RESISTANCE (IN OHM) Major power station 0.5 Ω Major sub-stations 1.0 Ω Minor sub-station 2.0 Ω Neutral Bushing 2.0 Ω Service Connection 4.0 Ω L.T. Lightning Arrestor 4.0 Ω L.T. Pole 5.0 Ω H.T. Pole 10.0 Ω Tower 20.30 Ω
4.9 TREATMNT FOR MINIMISING EARTH RESISTANCE VALUES
If earth resistance is more than the acceptable value, the following Treatment may be used for minimizing resistance :
Oxidation on all intermediate joints should be removed and joints should be tightened.
Sufficient water should be poured in the earth electrode pit.
Earth electrode of bigger size, as far as possible, may be used.
Multiple electrodes may be connected in parallel.
Earth pit of more depth and width-breadth may be prepared.
Soil treatment may be undertaken. 21
Earthing and Protection 4.10 MAINTENANCE OF EARTHING SYSTEM
Checking and Testing
The earthing systems are to be inspected regularly. Regular checking of joints and/or broken connections, if any, and rectification of the same will prove to be of immense help in maintenance of earth grid and equipment. The condition of the electrodes and the joints is also to be checked. If the electrodes are corroded, immediate steps for replacement are to be taken. The earth resistance is to be measured periodically. The megger, or testers are to be used for this purpose.
As stated earlier, a low earth resistance path is a must for clearing the fault current instantaneously. For achieving low earth values, the following ways are adhered to :
A number of electrodes are connected in parallel thereby providing a low resistance.
The ground surrounding the electrodes is treated with common salt, which reduces the resistance by upto 80%. Calcium chloride and magnesium sulphate may also be used. But now-a-days Bentonite clay (absorbent aluminum phyllosilicate) is being used.
The following maintenance schedule is mandatory at each of the sub-stations :
SL. NO. ITEM PERIODICITY
1. Watering of earth pits Daily
2. Measurement of earth resistance of Half yearly@ individual earth pits
3. Measurement of combined earth Half yearly* resistance at all the pits
4. Checking of interconnections between Quarterly earth pits and tightness of bolts and nuts
@ Earth resistance of individual earth pits can be measured by disconnecting the earth connections to the electrode. This is possible if the connections are made to a common clamp which is in turn is fixed round the pipe.
* Combined earth resistance shall be the same at every earth pit unless it gets disconnected from the earth mat.
22
Earthing Check Your Progress 5
What is the aim of earthing maintenance?
4.11 DEFINITIONS OF GENERAL EARTHING
TERMS
Soil Resistivity
This is the resistivity of a typical sample of soil at the location.
Earth Surface Voltage
It is the voltage between a specified point on the ground around the rod and the reference earth.
Earth Electrode
These are conductors, which are in direct contact with the soil and provide the conductive part in electrical contact with earth. They can include plates, rods, tape, steel reinforcing bars.
Neutral Point
The common point of a star connected poly-phase (3-phase, in our case) system, or the earthed mid-point of a single-phase system.
Independent Earth Electrode
An earth electrode located at such a distance from the other electrodes that, its electrical potential is not significantly affected by the electric currents between earth and the other electrodes.
Exposed Conductive Part
Conductive part of equipment and which is not normally live, but which can become live when basic insulation fails, or a physical contact/short-circuit to a live part occurs. 23
Earthing Step Potential and Protection
It is the difference in surface potential experienced by a person bridging a distance of 1 m with his feet. It becomes important in the case of build-up of a significant potential gradient along the ground due to an earth fault in the equipment.
Touch Potential
Touch voltage is the potential difference between the GPR (Ground Potential Rise) and the earth surface potential at the point where a person is standing while his hands are in contact with a grounded structure like a transformer body. This becomes important in the case of a high fault current flowing into the earth, leading to a potential gradient away from the point of injection.
Figure 4.7 : Step and Touch Voltages
4.12 SUMMARY
In this unit, we have taken up many important aspects of earthing, in relation to electrical system. We note that earthing is necessary not just to provide a stable electrical network, from generation till distribution, but also, as an essential safety measure to protect the humans/animals and the electrical equipment. In fact, without proper earthing, the whole electrical system would be rendered unusable, or even, a hazard.
We learnt the requirements imposed on good and useful earthing. We also learnt how earthing is classified into equipment/discharge/system earthing. We noted that equipment earthing is the closest to us as users of electricity, for safety and proper functioning. The actual implementation, i.e. direct, resistive, 24 reactance or Peterson Coil based, is dependent on the expected fault currents.
Earthing In contrast, discharge earthing is needed to remove the residual charge on the conductors. Finally, the system earthing is mandatory for all the equipment, which forms the electrical network.
As next, we learnt about how the poles and the lines of the distribution system are earthed. This includes the line-transformers and the distribution boxes.
We learnt both the plate- and the pipe- earthing. We have also learnt the guidelines for providing earthing to various equipment.
Having understood the need for and the method of providing earthing, the next and the obvious learning is about how to ensure the proper functioning of the earthing connection. This includes the measurement of the earth resistivity, measures to reduce the resistivity and to maintain the earthing connection.
In this unit, we have understood and learnt the various dimensions of earthing as a very useful and necessary concept of electrical technology.
4.13 TERMINAL QUESTIONS ?
(a) Why do we need to measure the earth resistivity?
(b) What is the need for removing corrosion from the joints along the electrical path to earthing?
4.14 ANSWERS TO CHECK YOUR PROGRESS
Check Your Progress 1
Earthing provides a zero potential surface around any electrical equipment. In case of a fault, the unwanted current is diverted to earth, removing or reducing the danger to the users and the equipment.
Check Your Progress 2
In a 3-phase system, the neutral or the center of the star is earthed. This way, the unbalanced part of the current flows to ground.
Check Your Progress 3
Normally, any joint in an earthing path is to be avoided to keep the total resistance low. However, if the situation demands that the connectivity may need to be interrupted temporarily, for example, to introduce a test device, only in such cases should the joint be secured with nut and bolt. 25
Earthing Check Your Progress 4 and Protection The megger is used to measure the soil resistivity.
Check Your Progress 5
It is to ensure that the circuit resistance to ground is the lowest possible. This ensures, not only the safety of the people/equipment, but also quick reaction of the protection system.
4.15 ANSWERS TO TERMINAL QUESTIONS
(a) To be effective, the fault current must flow into earth with least possible hinderence. Any resistance along the path would lead to build up of voltage and hence could endanger anyone who may come in contact. Increased resistivity could also lead to high step and touch potential. Regular measurement will warn us of changing earth resistivity. This knowledge should help us take preventive steps.
(b) As stated already, the total path to earth connectivity must be of least possible resistance. Any and every joint along the path is a node or possible resistance, since the contact resistance between the two metal surfaces depends entirely on the cleanliness of the surfaces. With time, any surface corrodes. This could be due to the atmospheric conditions, the corrosive gases present in the area, or even the humidity. This could get worse due to the flow of current through the joint, which may cause heating. The corrosion, so produced, worsens the resistance across the joint. Hence, one should clean the joint regularly, to prevent a build up of corrosion.
26
Electrical System 5.1 INTRODUCTION Protection
In Unit 5 of this block, we have studied Earthing System. We have learnt that earthing is required for the safety of the personnel and the electrical equipment. It is this second aspect that concerns us in this unit on Protection. We are talking of protection of our investment in the electrical system. This has to be guarded against malfunction. In turn, such malfunction may affect the safety of the humans present in the vicinity, or even those using the distributed electrical power.
The protection needs in a Distribution System are much different from the schemes employed in Transmission Systems, where more complex and higher levels of redundancy are employed. In this unit explanation of system protection will start by first explaining the different types of protective relays and then proceed to explaining how distribution line are protected then substations. Objectives
After studying this unit, you should be able to define the purpose of protection devices, explain the difference between system Protection and personal protection, differentiate between eletromechanical and solid state protective relays, explain the concept of inverse current and time, discuss the main functions of various types of relays in protection system, and explain purpose of using current and voltage transformers.;
5.2 TYPES AND OBJECTIVES OF PROTECTION
There two types of protection which referred to in electric power systems are :
TYPES OF PROTECTION IN ELECTRIC POWER SYSTEMS
System Protection System protection which deals with protective relays, fault currents, circuit breakers, fuses, effective grounding, and so on.
Personal Protection Personal protection which deals with safety of the humans from any electrical fault, and to avoid this use rubber gloves, insulating blankets, grounding jumpers, switching platforms, tagging, and so on. 29
Earthing In this unit we will learn about – System Protection. and Protection
Before studying this let’s discuss the main purpose of protection devices
PURPOSE OF PROTECTION DEVICES
Among the multiple purposes, the protection devices :
Contribute to protecting people against electrical hazards,
Avoid damage to equipment,
Limit the thermal, dielectric and/ or mechanical stress on the equipment,
Maintain stability and service continuity in the power system, and
Protect adjacent installations (for example, by limiting the induced voltage in the adjacent circuits).
It is necessary to understand that system protection is for the protection of equipment; it is not intended for the protection of people.
The objective of system protection is to remove faulted equipment from the energized power system before it further damages other equipment or becomes harmful to the public or employees.
The explanation of system protection will start by first explaining the different types of protective relays and then proceed to explaining how distribution lines are protected, then transmission lines, then substations.
Check Your Progress 1
What is the need for protection systems?
5.3 EQUIPMENT FOR SYSTEM PROTECTION
In practice, common name used for System protection is protective relaying. This incorporate relay devices in substations that monitor the power system’s 30
Electrical System voltages and currents through the Current Transformers (CTs) and Potential Protection Transformer (PTs). These relays are programmed to initiate “trip” or “close” signals to circuit breakers under abnormal conditions (or if the thresholds are exceeded)
Protective relays initiate alarms to system control, notifying system control operators of changes or new conditions that have occurred in the system. By use of the system protection equipment the control operators react to these incoming alarms.
Proper Grounding is also another means for providing equipment protection. Effective or proper grounding provides additional safety for personnel and can minimize damage to equipment, cause protective relays to operate faster (i.e. open circuit breakers faster).
Power to relays, trip signals, circuit breaker control systems, and the system control equipment are usually given battery.
5.3.1 Protective Relays
Power system equipment protection is accomplished by protective relaying equipment.
A protective relay is a device that monitors system conditions (amps, volts, etc., using CTs and PTs) and reacts to the detection of abnormal conditions in the system by triping circuit breakers or other opening devices to protect the system.
As stated, for equipment earthing to be effective, it must first be done at the generating plant. Here the neutral of the star-connected three phases is grounded or earthed.
Let us discuss the sequences of relay operation for system protection.
The relay compares the real-time actual quantities against preset programmable threshold values and sends D.C. electrical control signals to trip circuit breakers or other opening devices (such as reclosers, motorized disconnect switches and self-contained protection devices etc.) for clearing faults or to equipment from abnormal conditions.
When system problems are detected and breakers are tripped, alarm indications are sent to system control and sometimes other protection operations are initiated. As a result, equipment may be deenergized, taken off line by tripping or opening devices and consumers will be out of power with minimal equipment damage. 31
Earthing Types of Protective Relays and Protection
We can broadly classify the protective relays as two types :
(a) Electromechanical, and
(b) Solid state.
PROTECTIVE RELAYS ARE MANUFACTURED AS TWO TYPES
Electromechanical Electromechanical relays are composed of coils of wire, magnets, spinning disks and moving electrical switch contacts, and these electromechanical relays are very mechanical in nature.
Solid State Solid-state relays are electronic relays and these relays have no moving parts. Now-a-days most of the utilities are installing the more modern solid-state relays.
Solid state has several advantages over the traditional electromechanical relay. The basic differences between solid state and electromagnetic relays are given below :
SOLID STATE ELECTROMECHANICAL RELAYS Advantages : Advantages : small space requirements, Multiple Usually self-powered, simple and functionality, easy to set up and test, single-function design. remote access capability, and they can provide fault location information self-testing. Disadvantages : Disadvantages : External power required, software can be complex. Normally one relay per phase, difficult to set up and adjust, and require more frequent testing
Check Your Progress 2
What is the need for protection system?
32
Electrical System
5.4 FUNCTIONAL REQUIREMENTS OF THE Protection RELAYS
Relays are used to quickly isolate/trip a faulty section, (if necessary, isolate from both ends) on the occurrence of a fault, without disturbing the healthy sections, so that the rest of the System can function satisfactorily.
Therefore, the protection system has to be reliable, selective and sensitive to operate at the required speed. The other main requirement of protection is that a healthy trip circuit is required to operate the protection and perhaps, clear the fault.
FUNCTIONAL REQUIREMENTS OF THE RELAYS Reliability The most important requisite of protective relay is reliability, since they supervise the circuit for a long time before a fault occurs, if and when a fault occurs, the relays must respond instantly and correctly. Selectivity The relay must be able to discriminate (select) between those conditions for which prompt operation is required and those for which no operation, or a time delayed operation is required. Sensitivity The relaying equipment must be sufficiently sensitive so that it operates correctly when required as soon as the minimum input conditions are met, under field conditions. Speed The relay must operate at the required speed. It should neither be too slow, which may result in damage to the equipment, nor should it be too fast, which may result in undesirable operation.
2.5 SYSTEM PROTECTION CONCEPTS
COMMON COMPONENTS COMPRISED BY POWER PROTECTION SYSTEMS Current and voltage to step down the high voltages and currents of transformer the electrical power system to convenient levels for the relays to deal with. Current and voltage to sense the fault and initiate a trip, or transformer relays disconnection. Circuit Breaker to open/close the system based on relay and autorecloser commands. Batteries to provide power in case of power disconnection in the system. Communication channels to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment. 33
Earthing Fuses are also capable of both sensing and disconnecting faults which are used and Protection in various parts of a distribution system
Concept of Inverse Current-Time
Usually, typical protective relays are designed to follow the inverse current-time curve. The time to trip a circuit breaker is inversely proportional to amount of fault current.
We can also explain, as the amount of fault current increases, the time to trip a circuit breaker decreases. Here we know that each circuit breaker has a fixed amount of time to trop or to open a circuit once it receives a trip signal from the relay.
In the inverse current-time curve the amount of current flowing in the line (e.g. Current Transformer) is shown along the vertical axis and the time to trip is shown along the horizontal axis.
When the current exceeds the instantaneous setting on the curve, the time to trip becomes as fast as possible and without any intentional time delay the relay gives a trip command to the breaker.
The coordination of all the protective relays system protection is a very special task which incorporate many important factors in the proper design and coordination of protective relaying.
Instantaneous Setting (Trip CB as fast as possible, if the current reaches or exceeds this value)
Minimum Pickup Setting
Current (never trip CB, if the current is at or below this value)
Time to Trip Figure 5.1 : Curve for Inverse Current-Time
Single Line Diagram
You must have seen or uses simplified drawings (one-line or single line diagrams) of electrical system. Electric utility personnel use one-line diagrams to understand, explain and perform their work activities on a daily basis.
Single-line diagram is a simplified drawing/representation of the electrical system or a portion of the system that shows the electrical placement of all major equipment used under that 34 system.
Electrical System Figure shows an example of a simple one-line diagram for a distribution Protection substation. Note the protective relay numbers in circles (here numbers shown in the example are not actual).
Fuse Ground Lightning Arrester Disconnect Switch
Transformer with LTC
Circuit Breaker 1 2 3 4
1 Instantaneous Overcurrent Relay Disconnect Switch 2 AC Time Overcurrent Relay 3 AC Reclosing Relay 4 Frequency Relay
Figure 5.2 : Single Line (One-line) Diagram
Check Your Progress 3
What is the speciality of a minimum Time Delay Relay?
5.6 DISTRIBUTION SYSTEM PROTECTION
In this section you will learn about the protection schemes which are generally used normally in power distribution system protection.
Distribution lines (i.e. feeders) are normally fed radially out of substations.
In general, the typical distribution line protection schemes used on radially fed distribution lines normally involve overcurrent protection with reclosing relays and, in several cases, under frequency load-shed relays.
5.6.1 Overcurrent and Reclosing Relays
In distribution feeder protection, a set of over current relays are used for each distribution feeder.
Normally, one relay is connected to each phase to protect from overcurrent and one for ground overcurrent or a total of four overcurrent relays. 35
Earthing The instantaneous and time delay capabilities are interconnected with the and Protection reclosing relay. This typical substation relay package must also coordinate with the downstream fuses that are located on the feeder itself. Current transformers (CTs) located on the circuit breaker bushings and the overcurrent relays are connected directly to current transformers. This arrangement enables the monitoring of actual current magnitudes flowing through the breaker in real time.
Normally there are four CTs used for each feeder breaker (one for each phase and one for the grounded neutral). Each overcurrent relay has both an instantaneous and a time delay overcurrent relay connected to the CTs.
These relays are looking for feeder faults that are phase to ground, phase to phase, two phases to ground or three phases.
For recommending the relay settings which are later programmed into the relays.
By analyzing the available fault current magnitudes for each circuit breaker connected to feeder the protection engineer recommends relay settings that are later programmed into the relays.
5.6.2 Underfrequency Relays
Underfrequency relays are also called as load-shed relays. We know that system frequency will drop if there is load-generation imbalance which occurs when the load connected to system increases from the generation in the system.
Underfreuency relays are used to shed load when the system frequency is dropping and this enable to stop or prevent a cascading outage disturbance.
We know that the standard frequency in the India is 50 hertz and the typical Underfrequency relay settings are chosen based on guidelines. While under frequency detection some remedial actions can also be applied to balance load demand and generation to stop the possibility of a cascading outage disturbance, some system start diesel engine generations and/or combustion turbines automatically upon Underfrequency detection to meet the load demand.
Transmission System Protection
Transmission protection is much different than distribution protection. Both fault current magnitude and direction are required for transmission line circuit breakers to trip.
To identify and trip the faulted transmission lines, application or concept of Zone relaying (sometimes called distance or impedance relaying) with directional overcurrent capability is normally used. 36
Electrical System
5.7 SUBSTATION PROTECTION Protection
Now, let us discuss about substation protection which is generally accomplished using differential relays. Differential relays are generally used to protect buses, transformers, and generators.
5.7.1 Principle of Differential Relay Operation
Differential relays operate on the principle that the current going into the protected device must be equal to the current leaving the device. A differential condition should be detected, and then all source breakers that can feed fault current on either side of the device are tripped to protect it from fault conditions.
5.7.2 Transformer Protection
A differential should be detected between the current entering the transformer and exiting the transformer after adjusting for small difference due to losses and magnetization and after that on the basis of significant differential the relay should trip the source breaker(s) so that Transformer can be de-energized immediately
Current transformers (CTs) on both the high voltage side and low voltage side of the transformer are connected to a transformer differential relay. Matching CTs are used to compensate for the transformer windings turns ratio.
5.7.3 Bus Protection
Bus Protection Using Differential Relays
To protect the bus in substation bus differential relays are used.
The current balance in the bus should be maintained, this means that current entering the bus (usually exiting the power transformer) and the current leaving the bus (usually the summation of the transmission or distribution lines) must be equal.
Any fault for example line-to-ground faults in the bus will cause change in the current balance in the differential relay and in this condition bus differential relays work and cause the relay contacts to close, thus initiating signals to circuit breakers to disconnect the system from the source. With the application of this protection scheme the system can be protected from the fault.
Overvoltage Relays and Undervoltage Relays
37
Earthing Change of voltages from nominal rated voltage can damage the system. and Protection It is necessary to monitor the high and low bus voltages to protect the system.
To meet this requirement. Protective relaying system is used. For example : (a) overvoltage relays are sometimes used to control (i.e. turn off) substation capacitor banks, whereas (b) undervoltages relays are sometimes used to switch on substation capacitor banks.
Over and Undervoltage relays are also used to trip breakers due to other abnormal conditions.
Check Your Progress 4
Why can’t we design a single protection device to be used in every case?
5.8 INSTRUMENT TRANSFORMERS
Instrument Transformers (Current and Voltage Transformers)
Current and Voltage transformers are the type of instrument transformers that are designed to isolate electrically the high voltage/current primary circuit from the low voltage/current secondary circuit and, thus, provide a safe means of supply for indicating instruments, meters and relays.
Current transformers are used in power installations for supplying the current in circuits of indicating instruments (ammeter, wattmeter, etc.), meters (energy meter, etc.) and protective relays.
5.8.1 Current Transformer (CT)
current transformer is a measurement device that is designed to provide a current in its secondary windings proportional to the current flowing in its primary windings.
38
Electrical System The current transformer isolates measurement and control circuitry from the high Protection voltages typically present on the circuit being measured. To facilitate the safe measurement of large currents, Current transformers are commonly used in metering and protective relaying.
Figure 5.3 : Current Transformers
Figure 5.4 : Single Core Ring Type Current Transformers
Current Transformer Connections
Figures 5.5 to 5.7 of current transformer connections for the line and transformer are shown below.
Line Line
Line Protection Line Protection Scheme Scheme
Figure 5.5 : Current Transformer (CT’s) Secondary Connections (For Line)
39
Earthing and Protection
Transformer Transformer Protection Protection Scheme Scheme
Figure 5.6 : Current Transformer (CT’s) Secondary Connections (For Transformer)
Figure 5.7 : Use of Current Transformers in High Current, Three-phase Supply
5.8.2 Voltage Transformer
Voltage transformers (VTs) also referred to as potential transformers (PTs).
Voltage transformers are commonly used for metering in high voltage circuit and also in protection in high-voltage circuits.
They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that
metering and protective relay equipment can be operated at a lower potential
These may be of single phase or three phase design and of the dry or oil immersed types.
A fundamental rating of the voltage transformer is its transformation ratio and burden, i.e. the total load presented by the instruments connected. All voltage transformers are designed for a standard secondary voltage say, 110 or 110/√ 3 V.
40
Electrical System Protection
Figure 5.8 : Typical 11 kV Potential Transformer and 33 kV Potential Transformer
5.9 SUMMARY
In this unit, we have learnt about protection systems as applicable to power distribution system. We understand that the protection system acts against any mal operation in the distribution system by limiting or isolating the faulty condition, while leaving the rest of the system in healthy condition.
We learn that a successful protection system is sensitive to catch the fault early, immune to false trigger, and fast in its operation, in order to limit any damage possibility. We know that the operation of any protection has to be repeatable, to ensure reliability of protection.
We have learnt that most protection schemes depend on one or more relays, which interpret the fault condition and initiate the protection action, e.g. breaking an abnormal current, or isolating a faulty circuit. The relays are dependent on various sensors to keep a check on the parameters needed to identify the fault status.
Two major input devices described here are the current transformer and the voltage transformer. In this unit, we have learnt the various types of CT and VT and their parameters.
We understand that in a distribution scheme, the one most important element is the supply transformer (as distinct from CT and VT). To keep the power distribution going, we need to protect the working of the supply or distribution transformers. The three problem issues, here, are Over-current, Over-voltage and Over-heating.
In this unit we have discussed some important features of Distribution Protection and Substation Protection.
We understand that the sometimes described schemes are not, in general, alternatives, but may need to be used in tandem to effect more complete protection. 41
Earthing Lastly, we learn that no protection scheme can be absolute, but we cannot afford and Protection to ignore its contribution to the reliability of power distribution and protection of our investment in the associated equipment.
5.10 TERMINAL QUESTIONS ?
(a) What is the role of a relay in a protection device?
(b) What is an Instrumentation Transformer?
(c) Which are the three main parameters of a power supply transformer, which should trigger protection?
(d) What is the role of a Buchholz Relay?
5.11 ANSWERS TO CHECK YOUR PROGRESS
Check Your Progress 1
Please see Section 2.2 of this unit for the answer.
Check Your Progress 2
A minimum time delay relay ensures a minimum time gap between the trigger event the operation of the relay. With the use of multiple such relays, we can ensure that the protection action in the different parts of a scheme happens in a defined and orderly manner, with essential time delays provided, when required
Check Your Progress 3
A protection relay depends on the input information for sensing the fault condition. Also the reaction of the device has to be tailored to the type of the fault. Since, it may not be possible to design a single universal sensor to sense every possible fault, it is not feasible to have a universal protection device. We need to match the protection to the type of fault.
5.12 ANSWERS TO TERMINAL QUESTIONS
42
Electrical System (a) The relay is the device, which initiates the actual protection action by Protection limiting the damaging current, or isolation the faulty section or equipment from normal operation, with the aim of containing the effect or the fault. It may be a mechanical device based on magnetic principle, or it may be an electronic switching device.
(b) Two major parameters of a power supply are Voltage and Current. In the case of power distribution, the relevant Voltage and Current may be too high to be interfaced to electronic devices, such as Protection Systems, which operate at a much lower power levels. In this case Instrumentation Transformers are used to step down the field parameters to manageable lower levels. The lower value is a faithful reproduction of the actual parameter. This process renders the signal safer, as well. As stated, Instrumentation Transformers come in two basic flavours, Voltage Transformers, VT (sometimes called Potential Transformer, PT), and Current Transformer, CT.
(c) The three main parameters are : Over-voltage, Over-current and Over-heat. All three (or any of the three) may be caused by a fault in the transformer or in a device or circuit around it. Any one or more of these conditions can disrupt the power supply, and could also cause major damage to the transformer. Hence protection scheme has to monitor these parameters.
(d) A Buchholz Relay is a specialized implementation to monitor and act on the thermal break-down of insulation in a transformer. When the transformer coils overheat, the surrounding insulation may be affected and result in discharge of gas, which travels upward to the coolant tank. The Bochholz relay has two floats, which move due to the presence of the gas. This results in the operation of the relay and points to the presence of ‘hot-spots’ in the transformer. This relay is specifically useful in case of slow built-up of a thermal fault.
43