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Jawaharlal Nehru Engineering College Electrical Engineering Department
Laboratory Manual POWER SYSTEM PROTECTION
For Final Year (EEP) Students

© Author JNEC, Aurangabad. FOREWORD
It is my great pleasure to present this laboratory manual for final year ELECTRICAL ELECTRONIC & POWER engineering students for the subject of Power System Protection. Keeping in view the vast coverage required for visualization of concepts of Power System Protection with simple language. As a student, many of you may be wondering with some of the questions in your mind regarding the subject and exactly what has been tried is to answer through this manual. Faculty members are also advised that covering these aspects in initial stage itself, will greatly relive them in future as much of the load will be taken care by the enthusiasm energies of the students once they are conceptually clear.
H.O.D. (EEP)

LABORATORY MANUAL CONTENTS
This manual is intended for the final year students of ELECTRICAL ELECTRONIC & POWER engineering branch in the subject of Power System Protection. This manual typically contains practical/Lab Sessions related Power System Protection covering various aspects related to the subject to enhance understanding.
Although, as per the syllabus, only descriptive treatment is prescribed, we have made the efforts to cover various aspects of Power System Protection subject covering types of different protective schemes, their operating principals, their characteristics and Applications will be complete in itself to make it meaningful, elaborative understandable concepts and conceptual visualization.
Students are advised to thoroughly go through this manual rather than only topics mentioned in the syllabus as practical aspects are the key to understanding and conceptual visualization of theoretical aspects covered in the books.
Good Luck for your Enjoyable Laboratory Sessions
Prof. P.V.Dhote

SUBJECT INDEX

1. Do’s and Don’ts

2. Lab exercise:

EXPERIMENT. NO. 1
2 3 4
5 6 7 8

TITLE
To realize the various Time-current characteristics using over-current relay and Earth fault relay. To study Buchholz Relay To study Parallel feeder protection. Application of differential protection scheme for transformer protection. To study Radial feeder protection. To study Air Circuit Breaker To study Distance Relays Introduction to Static relays

1. DOs and DON’Ts: DO’s in Laboratory:
1. Understand the equipment to be tested and apparatus to be used . 2. Select proper type (i.e. A. C. or D. C.) and range of meters. 3. Do not touch the live terminals. 4. Use suitable wires (type and size). 5. All the connections should be tight.
DONT’s in Laboratory: 1. Do not leave loose wires (i.e. wires not connected). 2. Get the connection checked before switching ‘ON’ the supply. 3. Never exceed the permissible values of current, voltage, and / or speed of any machine, apparatus, wire, load, etc. 4. Switch ON or OFF the load gradually and not suddenly. 5. Strictly observe the instructions given by the teacher/Lab Instructor

Instructions for Laboratory Teachers:
1. Submission related to whatever lab work has been completed should be done during the next lab session. The immediate arrangements for printouts related to submission on the day of practical assignments.
2. Students should be taught for taking the observations /readings of different measuring instruments under the able observation of lab teacher.
3. The promptness of submission should be encouraged by way of marking and evaluation patterns that will benefit the sincere students.

EXPERIMENT NO: 1

Date:

AIM: To realize the various Time-current characteristics using over-current relay and Earth fault relay.



Introduction:

A protective relay, which operates when the load current exceeds a preset value, is called

an over-current relay. The value of the present current above which the relay operates is known as it

pickup value. An over current relay is used for the protection of distribution lines, large motors, power

equipment etc. A scheme which incorporates over-current relays for the protection of an element of a

power system, is known as an over current scheme or over current protection. An over current scheme

may include one or more over current relays.



Time – Current Characteristics :

A wide variety of time-current characteristics is available for over current relays.

(i) Definite – time over current relay A definite-time over current relay operates after a predetermined time when the current exceeds its pick-up value. The operating time is constant, irrespective of the magnitude of the current above the pick-up value. The desired definite operating time can be set with the help of an intentional time – delay mechanism provided in the relaying unit. Curve (a) of Fig. 1 shows the time-current characteristic for this type of relay.
(ii) Instantaneous over current relay An instantaneous relay operates in a definite time when the current exceeds its pick-up value. The operating time is constant, irrespective of the magnitude of the current, as shown by the curve (a) of Fig. 1 There is no intentional time – delay. It operates in 0.1s or less. Sometimes the term like “high set” or “high speed” is used for very fast relays having operating times less than 0.1s.
(iii) Inverse-time over current relay An inverse-time over current relay operates when the current exceeds its pick up value. The operating time decreases as the current increases. Curve (b) of Fig. 1 shows the inverse time-current characteristic of this type of relays.
(iv) Inverse definite minimum time over current (I.D.M.T.) relay This type of relay gives an inverse-time current characteristic at lower values of the fault current and definite-time characteristic at higher values of the fault current. Generally, an inverse-time characteristic is obtained if the value of the plug setting multiplier is below 10. for values of plug setting multiplier between 10 and 20, the characteristic tends to become a straight line, i.e. towards the definite time characteristic. Fig. 2 shows the characteristic of an I.D.M.T. relay along with other characteristics. I.D.M.T. relays are widely used for the protection of distribution lines.
(v) Very inverse – time over current relay A very inverse-time over current relay gives more inverse characteristic than that of a plain inverse relay or the I.D.M.T. relays. Its time-current characteristic lies between an I.D.M.T. characteristic and extremely inverse characteristic, as shown in Fig. 2 The very inverse characteristic gives better selectivity than the I.D.M.T. Characteristic. Hence, it can be sued where an I.D.M.T. relays fails to achieve good selectivity.

(vi)Extremely inverse – time over current relay An extremely inverse time over current relay gives a time-current characteristic more inverse than that of the very inverse and I.D.M.T. relays, as shown in Fig. 2 When I.D.M.T. relays and very inverse relays fail in selectivity, extremely inverse relays are employed



Method of defining shape of Time-current characteristics

The general expression for time-current characteristics is given by

t= K I n −1

The approximate expression is

t= K In

For definite-time characteristic, the value of n is equal to 0. According to the British Standard, the

following are the important characteristics of over current relays.

(i) I.D.M.T. : (ii) Very Inverse: (iii) Extremely Inverse:

t = 0.14 I0.02−1
t = 13.5 I −1
t = 80 I 2 −1

• (i)
(ii)

Settings of over current relays: Current Setting : The current above which a over current relay should operate can be set .If time-current curves are drawn, taking current in Amps on the X-axis, there will be one graph for each setting of the relay. To avoid this complex situation, the plug setting multipliers (PSM) are taken on the X-axis.
The actual r.m.s. current flowing in the relay expressed as a multiple of the setting current (pick up current) is known as the plug setting multiplier (PSM)
Suppose, the rating of the relay is 5A and it is set at 200% i.e. at 10A.If the current flowing through the relay is 100A, then the PSM will be 10.
Hence, PSM can be expressed as
Secou nrdra er nytc
PSM = Rle aycsuertrte in ng t

P im r n a tr u d , i y . e . f l u c a tr u n u in r t lt r g c e f u a rre

=

R la es ye c  C . T t . u r ta i rn r ti g e o nt

If P.S.M. is taken on the X-axis, there will only one curve for all the settings of the relay. The curve is generally plotted on log/log graph. Time Setting : the operating time of the relay can be set at a desired value. There are 10 steps in which time can be set. The term time multiplier setting (TMS) is used for these steps of time settings. The values of TMS are 0.1, 0.2……0.9,1.
Suppose that at a particular value of current or PSM, the operating time is 4s with TMS = 1. The operating time for the same current with TMS = 0.5 will be 4X0.5 = 2s The operating time with TMS = 0.2 will be 4X0.2 = 0.8s



Observation Table:

(1) For TMS = 1

Sr. No. 1 2 3 4 5 6 7 8 9 10

PSM (Plug Setting Multiplier)

CONCLUSION :

Time in seconds

QUIZ:
1. What are the various types of over current relays? 2. What is PSM and TSM?

Earth Fault Protection A fault which involves ground is called an earth fault. Examples are – single line to ground (L-G) fault and double line to ground (2L-G) fault. Faults which do not involve ground are called phase faults. The protective scheme used for the protection of an element of a power system against earth faults is known as earth fault protection.
Earth Fault Relay and Over-current Relay Relay which are used for the protection of a section (or an element) of the power system against earth faults are called earth fault relays. Similarly, relays used for the protection of a section of the power system against phase faults are called phase fault relays or over-current relays. The operating principles and constructional features of earth fault relays and phase fault relays are the same. They differ only in the current levels of their operation. The plug setting for earth fault relays varies from 20% to 80% of the C.T. secondary rating in steps of 10%. Earth fault relays are more sensitive than the relays used for phase faults. The plug settings for phase fault relays varies from 50% to 200% of the C.T. secondary rating in steps of 25%. The name phase fault relay or phase relays is not common. The common name for such relays is over-current relay. One should not confuse this term with the general meaning of over-current relay. In a general sense, a relay which operates when the current exceeds its pick-up value is called an over-current relay. But in the context under consideration, i.e. phase fault protection and earth fault protection, the relays which are used for the protection of the system against phase faults are called overcurrent relays.
Earth Fault Protective Schemes An earth fault relay may be a residual current as shown in fig. (a) ia, ib and ic are currents in the secondary of C.T.s of different phase. The same ( ia + ib + ic ) is called residual current. Under normal conditions the residual current is zero. When an earth fault occurs, the residual current is non-zero. When it exceeds pick-up value, the earth fault relay operates. In this scheme, the relay operates only for earth faults. During balanced load conditions, the earth fault relay carries no current; hence theoretically its current setting may be any value greater than zero. But in practice, it is not true as ideal conditions do not exist in the system. Usually, the minimum plug setting is made at 20% or 30%. The manufacturer provides a range of plug settings for earth fault relay from 20% to 80% of the C.T. secondary rating in steps of 10%.
The magnitude of the earth fault current depends on the fault impedance. In case of an earth fault, the fault impedance depends on the system parameter and also on the type of neutral Earthing. The neutral may be solidly grounded, grounded through resistance or reactance. The fault impedance for earth faults is much higher than that for phase faults. Hence, the earth fault current is low compared to the phase fault currents. An earth fault relay is set independent of load current. Its setting is below normal load current. When an earth fault relay is set at lower values, its ohmic impedance is high, resulting in a high C.T. burden.
Figure (b) and (c) show an earth fault relay used for the protection of transformer and an alternator, respectively. When an earth fault occurs, zero sequence current flows through the neutral. It actuates earth fault relay.
Figure (d) shoes the connection of an earth fault relay using a special type of C.T. known as a corebalance C.T., which encircles the three-phase conductors.
Combined Earth Fault and Phase Fault Protective Scheme Figure 3.16 shows two over-current relays (phase to phase fault relays) and one earth fault relay. When an earth fault occurs, the burden on the active C.T. is that of an over-current relay (phase fault relay) and the earth fault relay in series. Thus, the C.T. burden becomes high and may cause saturation.

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Jawaharlal Nehru Engineering College Electrical Engineering