DEPARTMENT OF ELECTRICAL

ENGINEERING

POWER SYSTEMS LAB

Lab Manual

EEP 303

Dr. A.R. Abhyankar

(Course Coordinator)

Experiment No. 1

Determination of Positive, Negative and Zero-Sequence

Reactances of Synchronous Machine

MOTIVATION:

In balanced three-phase network complete symmetry or balance is maintained. The load

(or fault) impendances are the same on all three phases and the voltages, emfs and

currents are characterized by complete three phase symmetry, i.e. they are of equal

magnitude in each phase and displaced 120 degree in time. The networks can be analyzed

on single-phase basis. In the unsymmetrically faulted or loaded system, neither the

currents nor the voltages will possess three-phase symmetry. It is no longer possible to

limit the analysis to one phase, because coupling exists between the three phases. The

complexity of analysis can be offset to a considerable extent by interconnection of

sequence networks having positive-,negetive-, and zero-sequence reactances of the

machine.

OBJECTIVES:

To determine experimentally positive-, negative-, and zero-sequence reactances of a

synchronous machine.

THEORY:

Symmetrical Components

The basic feature of this method, as the name implies, is the resolution of the

unsymmetrical phase currents and voltages into a set of components that possess certain

symmetrical features. The positive-sequence components possess certain symmetrical;

features. The positive-sequence components possess three-phase symmetry having the

phase sequence RYB; the negative-sequence components possess three-phase symmetry

having the phase sequence RBY; and the zero-sequence are all of equal magnitude and

phase. The impedances offered to these sequence components are called positive-,

negative-, and zero-sequence impedances. As a consequence of symmetry between three

phases of a synchronous machine, no coupling exists between the positive-, negative-,

and zero sequence networks. An unbalanced three-phase network can be analyzed by

appropriate interconnection of the sequence networks.

Positive Sequence Reactance X1:

A system component operating under balanced condition of current and voltage is in

effect in a positive sequence mode. The positive sequence reactance of a synchronous Machine under steady state condition is the direct axis synchronous reactance Xd of the

machine.

The positive-sequence impedance can also be defined as the impedance offered by

the machine to the flow of positive-sequence currents. A set of positive-sequence currents

in the armature winding creates a magnetic field that rotates in the normal direction of the

air gap.

Determination of positive-sequence reactance with the help of open circuit and short

circuit characteristics:

The synchronous machine is run at synchronous speed with the help of a prime mover

(d.c. motor).a curve between voltage and excitation is drawn on no load. The curve thus

obtained is the no-load or open circuit characteristics. At synchronous speed, a curve

between armature current and excitation current is drawn when the armature terminals are

short circuited. The curve thus obtained is the three-phase short circuit characteristics.

The saturated value of synchronous impedance at an excitation current ei is given by

SCI

E where E is the rated value of induced emf at an excitation current of and ISC is the

three-phase short circuit current at the same excitation current. (If the positive-sequence

resistance is neglected the positive-sequence reactance is same as the positive-sequence

impedance).

Determination of negative-sequence-reactance X2 The negetive-sequence impedance of a synchronous machine is the impedance offered by

that machine to the flow of negetive-sequence current. A set of negative-sequence

currents in the armature creates in the air gap a magnetic field that rotates at synchronous

speed in the direction opposite to the normal direction of rotation. Thus the negetive

sequence magnetic field rotates with twice the normal speed with respect to the rotor.

Double frequency currents are established in the shorted rotor field winding and the

damper winding thus keeping the flux linkage of these windings almost constant at their

initial zero value.(The flux due to the armature current is forced into paths of high

reluctance which do not link any rotor circuits.) these paths are same as those of sub-

transient reactance. The armature flux linkage per armature ampere under this condition

is the negetive-sequence inductance L2. The negetive sequence is 22 LX .( see ref. 4.)

Since the mmf wave rotates at twice the synchronous speed with respect to the

rotor, it alternately meets reluctance of the two rotor axis (d- and q- axis), corresponding

to sub transient reactance ''dX and''

qX . For all practical purposes it is usually taken as the

arithmetical mean:

2

''''

2

qd XXX

Test for determining X2 (Method I)

The synchronous machine is run at synchronous speed with the help of a prime mover.

The machine is unexcited and connected to a voltage source, which is gradually increased

till full load current flows. Power input is measured by a wattmeter. The terminals are so

connected that the direction of rotating field produced by the armature current is opposite

to the direction of rotation of the pole structure. It is important to keep the field winding

short circuited during the test. The negative-sequence impedance Z2 is

2

22

I

VZ Where,

V2 = Applied terminal voltage per phase

P = Power input per phase

I2 = Current flowing per phase

X2 = sin2Z where 22

21cosIV

P

Test for determining X2 (Method II)

The negative sequence reactance X2 can also be obtained by driving the machine at rated

speed with low excitation and with a sustained 2-phase short circuit between two of the

line terminals. Let the voltage between the open phase and any one of the short-circuited

phase be Vos and the short-circuited current be ISC. If the wattmeter is connected with its

current coil excited by it measures the negative-sequence power loss ( 22 Ri loss). The

negative-sequence impedance Z2 is

SCos

SC

os

IVP

ZX

I

VZ

1

22

2

cos

sin

3

Where,

Zero-Sequence Reactance X0 If zero sequence currents are applied to the armature, there is no space fundamental

mmf. Hence the reactance is small and is hardly affected by the motion of the rotor.

There is, in general, a third space harmonic of air gap mmf, which is stationary but

pulsating. This mmf is opposed by currents induced in the rotor circuits; therefore not

much air gap flux is produced. The zero-sequence currents produce leakage fluxes (slot-

leakage end-winding-leakage and differential leakage). Altogether very little flux is set

up, and the zero-sequence reactance is the lowest of the synchronous machine

reactances.(Ref.4.)

The actual value of X0 varies through a wider range than that of the other reactances

and depends upon the pitch of the armature coils. The reactance is the least for a pitch of

two-thirds because then each slot has two coil sides carrying equal and opposite currents.

Test for determination of X0

The machine is driven at rated speed with field winding short

circuited. All phases are connected in series and a single phase voltage is applied across

them. It may be sometimes more convenient to connect these phases in parallel. The

series connection is however preferred as the currents of the same magnitude and phase

angle are flowing through all the three phases, a condition which must be positively

fulfilled while determining X0. For series connection

0

00

3I

VZ Where,

V0 = Applied voltage across the three phase windings in series.

I0 = currents flowing i9n the three phase windings in series.

(If the zero sequence resistance is neglected then the zero-sequence reactance X0 equals

the zero sequence impedance Z0).

PRE-EXPERIMENTAL QUIZ

1. What do you understand by the term phase sequence?

2. What is rotating field?

3. What is pulsating field?

4. Can zero-sequence currents produce rotating field?

5. What field can zero-sequence currents produce?

6. Explain how double frequency currents are produced in the rotor field when

negative-sequence currents are impressed on the armature.

7. If rotor is at stand still, what will be the frequency of rotor field currents

when (a) positive-sequence currents are impressed (b) negative-sequence currents are

impressed?

8. Explain why X1 and X2 are different in a synchronous machine where as

they are equal in transformers.

9. Explain why X0 is quite small in a synchronous machine.

10. Explain how X2 is the arithmetic mean of ''

dX and''

qX .

11. Define positive-, negative-, and zero-sequence components of a set of

unbalanced three phasors.

EQUIPMENTS AND COMPONETS

1. A three-phase alternator coupled to a d.c. Motor.

2. a.c. voltmeters.

3. a.c./d.c. Ammeters.

4. Wattmeter.

5. Tachometer

6. Rheostat.

7. Three-phase variac.

8. Single-phase variac.

PROCEDURE, CONNECTION DIAGRAM AND EXPERIMENTATION

Determination of X1 (a) Open Circuit Test:

(i) Run the synchronous machine at rated speed.

(ii) Connect a voltmeter across the machine terminals and measure the voltage

corresponding to field excitation given by ammeter reading. (Fig. 1 a)

(iii) Repeat step (ii) for different exciting currents and plot the open circuit

characteristics.

(b) Short Circuit Test:

(i) Run the machine at rated speed.

(ii) Apply low voltage to the field circuit so that exciting current is small. Alternately

connect a high resistance rheostat in the field circuit with full field voltage applied

(Fig. b). Connect an ammeter in the field circuit.

(iii) Apply three-phase short circuit at the synchronous machine terminals, with ammeter

connected in any phase.

(iv) Measure the short circuit current corresponding to the field current given by the

ammeter reading.

(v) The short circuit characteristics is given by joining the origin with the point in step 4.

Determination of X2..(Method I)

(i) Short circuit the field winding.

(ii) Run the machine at rated speed.

(iii) Apply low voltage from a three-phase variac to the armature terminals with phase

sequence of the machine and supply reversed with respect to each other (Fig 2 a). The

armature current given by the ammeter reading should not exceed the rated value.

(iv) Measure the applied voltage, current and power inputs.

Determination of X2..(Method II)

(i) Run the machine at rated speed.

(ii) Short circuit two phases of the alternator through an ammeter as shown in Fig 2b

(iii) Connect the voltage coil of the wattmeter and a voltmeter between the open phase

and any short circuited phase.

(iv) Gradually increase the excitation such that the short circuit current does not exceed

full load value.

Determination of X0:

(i) Connect the armature winding in parallel with proper polarity (Fig 3).

(ii) Short-circuit the field winding.

(iii) Run the machine at rated speed.

(iv) Apply low voltage from a variac and measure both voltage and current taken by

the armature winding.

DATA SHEET

Name plate details of the machine.

Name of the manufacturer:

Rated output:

Voltage rating:

Current rating:

No. of phases:

Frequency:

TABULATIONS:

Determination of X1

O.C.C Table 1

S.No. Excitation Ie Open Circuit Voltage

S.C.C: Excitation Ie , Short Circuit Current ISC

Determination of X2 (Method I)

Table 2

S.No. V2 I2

Determination of X2 (Method II)

Table 3

S.No. Vos ISC

Determination of X0

Table 4

S.No. V0 I0

DATA PROCESSING AND ANALYSIS:

(a)Determination of X1

Corresponding to rated voltage on open circuit the excitation (field current)

Ie = from O.C.C

Corresponding to Ie = E = per phase

From S.C.C the positive sequence reactance is

X1= SCI

E =

(b)Determination of X2 The negative-sequence impedance and reactance are

(Method I) 2

22

I

VZ ,

(Method II) SC

os

I

VZ

32

(c)Determination of X0

The zero sequence reactance

3/0

0

0I

VX (Parallel connection)

POST-EXPERIMENTAL QUIZ:

1. Explain why resistances are neglected in fault studies. 2. Does the positive-sequence impedance determined from O.C.C and S.C.C apply

to the steady state condition or transient condition?

3. The negative-sequence resistance is quite large compared to the positive-, or zero sequence resistance. Explain.

4. If the armature windings of synchronous machine are connected in delta, how would you determine the zero sequence reactance?

5. For the zero-sequence reactance if the windings are connected in parallel, will the results so obtained be correct?

6. Do X2 and X0 vary in relation to saturation? 7. How would you connect the sequence network for a single line to ground fault on

the armature?

8. If the three-phase generated emfs are balanced, will you obtain negative sequence voltage in the network under unsymmetrical fault condition?

9. What is the relationship between positive-, negative-, and zero-sequence currents in case of a single line to ground fault for an unloaded alternator solidly grounded.

10. For three-phase fault on the synchronous machine armature, will there be many negative-, and zero-sequence currents?

SUGGESTION FOR FURTHER EXPERIMENTATION

1. The value of positive-sequence reactance X1 obtained from O.C.C. and S.C.C in this experiment should be compared with the value of Xd obtained from slip-test

2. The negative-sequence reactance can also be obtained from the following tests. a) Open circuit characteristic

b) Short circuit characteristic in three-phase short circuit condition.

c) A sustained short circuit with two lines short circuited, and plotting the short

circuit characteristic.

3. The zero-sequence reactance can be obtained by plotting the short circuit

characteristic with a single line to neutral sustained short circuit on the graph for

item no. 2.

L F A

F FF

M

R

YB

AlternatorV

DC motor Starter

220 V

D.C

+

220 V

D.C

D.C.

Motor

Open-circuit test

R

YB

AlternatorA

Short-circuit test

Low

excitation

Field

-

+

-

L F A

F FF

M

DC motor Starter

220 V

D.C

+

D.C.

Motor

-

A

220 V

D.C

Field

-

+

A

Fig. 1 O.C. and S.C. tests on alternator to determine X1

Field

R

YB

Alternator

Determination of X2 by application of negative

voltage to alternator

R

Y

B

L F A

F FF

M

DC motor Starter

220 V

D.C

+

D.C.

Motor

-

3 Phase

Variac

V

Fig 2.a Determination of X2 (Method 1)

R

YB

AlternatorV

220 V

D.C

Field

-

+

A

L F A

F FF

M

DC motor Starter

220 V

D.C

+

D.C.

Motor

-

Determination of X2 by two phase short circuit

test

Fig. 2.b: Determination of X2 (Method 2)

R

YB

Alternator

Field

A

1 Phase

Variac

230 V

A.C

L F A

F FF

M

DC motor Starter

220 V

D.C

+

D.C.

Motor

-

Fig. 3: Determination of X0

EXPERIMENT NO. 2

MEASUREMENT OF POSITIVE AND NEGATIVE SEQUENCE IMPEDANCE OF

THREE PHASE THREE WINDING AND ZERO SEQUENCE IMPEDANCE OF

THREE PHASE TWO WINDING TRANSFORMERS

MOTIVATION:

The voltage drop caused by current of a certain sequence in any part of a balanced

circuit depends on the impedance of that part of the circuit to current of that sequence

.The impedances of a circuit to positive sequence, negative sequence and zero sequence currents are referred to as positive-sequence, negative-sequence and zero-

sequence impedances respectively.

The analysis of an unsymmetrical fault on an otherwise symmetrical system can

be carried out by use of symmetrical components. The component currents of one phase

sequence cause voltage drops of like sequence only and are independent of currents of

other sequences. Thus in a balanced system, currents of any one sequence may be

considered to flow in an independent network composed of the impedances to the current

of that sequence only. The single phase equivalent circuit composed of the impedances to currents of any one sequence only is called the sequence network for that particular

sequence.

The motivation of obtaining the values of the sequence impedances of a power

system is to enable us to construct the sequence networks for the complete system. This

allows us to study and examine the effect of unsymmetrical fault on the using the tool of

symmetrical components. In this experiment we will only learn about the measurement of

sequence impedances of three phase two winding and three phase winding transformers

experimentally.

OBJECTIVES:

(i ) To measure the positive and negative sequence impedance of a three phase three

winding transformer.

( ii ) To measure the zero sequence impedance of a three phase two winding transformer.

THEORY:

1). Three phase two winding transformer

The positive sequence impedance of a transformer equals, of course, leakage

impedance. It is obtained by a short circuit test similar to as is performed in a single

phase transformer. Since the transformer is a static device, the leakage impedance

will not change if the phase sequence is altered from RYB to RBY. Thus positive

and negative sequence impedances are identical.

Z+ = Z- = Z leakage

Measurement of Z+ and Z - in p.u.

To measure this impedance, the low voltage side is short circuited and the

high voltage side is fed from a source at nominal frequency but reduced voltage in order

to circulate a 100% short circuit current in the windings. Under these conditions measure

the current drawn by and the power consumed by the transformer. From these data

determine the transformer leakage impedance which equals to positive and negative

sequence impedances.

V ph* = phase voltage in p.u. to circulate 1 p.u. rated current under short

circuit.

Z+* = Z -

*= V ph*/ 1 p.u = V ph* = Z*L

R*

L (1.00)2 = power consumed (3-phase)/ rated KVA of the transformer

Hence X*

L = [(Z*

L)2 (R*L)

2 ]

1/2

Z+* = Z -

* = RL +j XL

*denotes per unit

Measurement of zero sequence impedance : (zo)

The zero sequence impedance of a transformer depends greatly on the winding

type (or Y) and also upon whether or not the neutrals are grounded.

Fig. 2.a depicts the test set up for finding Zo experimentally. Obviously Zo

=E/Io, which depends in general upon the side of the transformer to which the transformer

to which the voltage source is connected i.e. the side from which the measurement is

made.

Case A: Transformer Y-Y connected (both neutral solidly grounded) as in fig.2.4. in this

case Zo measured from any side will be the same.

Case B: Transformer Y- with neutral grounded, as in fig.2.1 the zero sequence impedance measured from the Y side equals the leakage impedance. As the

secondary currents circulate in the delta, no zero sequence current will leave

the delta terminals.

If the transformer is excited from the side, no zero sequence current can flow as no return path exists. Thus the zero sequence impedance as seen from

the side is infinite.

Case C: Transformer Y-Y, one neutral grounded as in fig.2.2. Although a path is

provided to ground for the zero sequence currents in the primaries, no such

path exists in the secondary. Consequently, from the zero sequence point of

view, the secondary will act as if open-circuited. This means that from the

primary side the zero seq. currents will meet a total interruption as return path

for these currents does not exist.

Since the magnetizing impedance is very large, for all practical purposes we

assume zero sequence impedance to be infinite from both terminals.

2) Three phase three winding transformer:

While both the primary and secondary winding of a two winding

transformer have the same KVA rating, all the three windings of a three

winding. Transformer may have different KVA ratings. The p.u. impedance in

the impedance diagram should therefore be expressed on a common KVA basis.

Three impedances are measured by standard short circuit test. These are:

Zps = leakage impedance measured in primary with secondary short

circuited and tertiary open.

Zpt =leakage impedance measured in primary with tertiary short circuited

and secondary open.

Zst = leakage impedance measured in secondary with tertiary short

circuited and primary open.

Zps, Zpt, Zst are evaluated as referred to the primary side.

Let Zp, Zs, and Zt be the impedance of the primary, secondary and tertiary

windings referred to primary circuit. Then from transformer theory we have

Zps = Zp+ Zs

Zpt = Zp+ Zt

Zst = Zs+ Zt

Solving the above three equations simultaneously, we can write

Zp =( Zps +Zpt- Zst )/2

Zs =( Zps +Zst- Zpt )/2

Zt =( Zpt +Zst- Zps )/2

Zp, Zs, and Zt are the positive and negative sequence impedances of the three

windings respectively as shown in fig.2.5.

From the theory of the zero sequence impedance measurement for a three phase two

winding transformer for different connections and the consequent resulting zero

sequence impedance networks, one can readily obtain the zero sequence networks

for a three phase three winding transformer for different connections. The zero

sequence impedances from the positive or negative sequence and neutrals solidly

grounded is given in figure 2.6 (b).

PRE-EXPERIMENTAL QUIZ:

1. Define symmetrical components. 2. What if the importance of sequence impedances? 3. The internal voltages of a three phase synchronous generator corresponds to

(a) Positive sequence. (b) Negative sequence.

(c) Zero sequence.

4. The impedances of rotating machines to currents of the three sequence will generally be

(a) Same for each sequence (b) Different for each sequence

5. What is the utility of a three phase three winding transformer? 6. Why is tertiary connected in ? 7. What are the representative p.u. value for the sequence impedances for three

phase two winding and three phase three winding transformer ?

MATERIALS AND EQUIPMENTS:

Three phase two winding transformer (or a blank of three single phase

two winding transformers), three phase three winding transformer and voltmeter,

ammeter and wattmeter of suitable ratings.

Procedure, connection diagrams, experimentation and precaution:

(A) Three-phase two-winding Transformer:

Measurement of zero sequence impedance.

These are measured for four different connections as shown in Fig.2.1 to Fig. 2.4

Use a single phase supply. Do not exceed the current loading of each winding beyond the

rating. Use in the circuit connection voltmeter and ammeter of proper ratings. Find the

zero sequence impedance from the relation

Zo =3E/I

Compare the value of this zero sequence impedance with the previously calculated

positive and negative sequence impedance.

(B) Three phase three winding transformer :

Measurement of positive and negative sequence impedances.(Fig. 2.5)

These are obtained experimentally from three independent short circuit measurements.

The positive and negative sequence impedances are equal to the leakage impedances.

Experimental procedure for the measurement of leakage impedances for a three phase

three winding transformer with primary connected in Y, secondary in Y and tertiary in is given below. All impedances are referred to same KVA base and to a same voltage

base generally referred to primary circuit. Before performing the experiment obtain a

table as follows.

-------------------------------------------------------------------------------------------------

Winding Rated voltage Rated MVA

(Line to line KV) (Three phase)

--------------------------------------------------------------------------------------------- Primary

Secondary

Tertiary

Measurement 1: Obtain leakage impedance Zps in p.u. measured in primary with secondary

short circuited and tertiary open.

Measurement 2: Obtain leakage impedance Z pt in p. u. measured in primary with tertiary

short circuited and secondary open

Measurement 3: Obtain leakage impedance Z st in p. u. measured in secondary with

tertiary short circuited and primary open

Measurement impedance impedance winding winding impedance

Measured Symbol short open in p.u

in winding circuited circuited

1. Primary Zps Secondary Tertiary

2. Primary Zpt Tertiary Secondary

3. Secondary Zst Tertiary Primary

Z p = (Z p s +Z pt- Z s t)/2

Zs = (Z p s +Z s t- Z pt)/2

Z t = (Z pt +Z st- Z p s)/2

Where Z p, Z s and Z t are the leakage impedances of the primary, secondary tertiary windings

referred to primary circuit as shown in the equivalent circuit of fig.2.6 (a).

After knowing the values of the leakage impedances for the primary, secondary and

tertiary, the zero sequence impedance networks can be established with out any

additional experimentation. If for the Y-Y- connection of the three phase three winding transformer the neutrals are solidly grounded then the zero sequence

network is that given in fig.2.6 (b) .

For all the practical purposes the resistive parts of all impedances can be neglected

DATA PROCESSING AND ANALYSIS :

Show detailed computations for obtaining the sequence impedances in p.u. through sample

calculations.

POST EXPERIMENTAL QUESTIONS:

1. Do the positive and negative sequence impedances depending upon the transformer connection?

2. Why is the magnetizing impedance not considered while determining sequence impedances?

3. Draw the zero sequence networks for the following transformer connections

(i) . / Y

( ii) / (iii) / (iv) Y/

(v) / (v)Y //

4. A transformer is Y/Y connected with neutrals grounded through Zl. Draw the zero sequence

networks .

5. For transformer with / connection, are the zero sequence impedances as viewed from primary and secondary the same ?

6. Compare for different connections the zero sequence impedance values with those of positive

and negative sequence impedances ?

ADDITIONAL QUIZ:

Draw the one line diagram for the zero sequence networks power systems given.

REFFERENCES :

1. Elements of power system analysis, William D.strevenson.

2. Electric energy systems theory an introduction, O.I. Elgerd, McGraw Hill.

3. Symmetrical components, Wagner and Evans.

4. Modern power system analysis, I.J.Nagarath and D.P.Kothari, Tata McGraw Hill.

Connection diagrams for the measurement of zeros sequence impedance

of three phase two winding transformer (Fig. 2.1 to Fig. 2.4).

IO3IO

E

Fig. 2 (a)

A

E

IO

I O

IOI=3IO

Fig. 2.1

EI=3IO

IO

IO

IO

Fig. 2.2

E

I=3IO

IO

IO

IO

Fig 2.3

E

I=3IO

IO

IO

IO

Fig. 2.4

Open

Supply

TP S

Fig. 2.5 THREE WINDING TRANSFORMER: +Ve & -Ve Seq. Impedance

Measurement

S.C. TEST-1: Excite P, Short S, Open T (Gives ZPS)

S.C. TEST-2: Excite P, Short T, Open S (Gives ZPT)

S

Open

S.C. TEST-3: Excite S, Short T, Open P (Gives ZST)

Open

P

Supply

A

PS

Short

T

A

P

Short

T

A

S

T

Short

Zs

Zt

Zp

Primary

Secondary

Tertiary

P

S

T

Fig. 2.6 (b)

Experiment No. 3

i) IDMT CHARACTERISTICS OF AN OVERCURRENT RELAY

MOTIVATION:

Protective relays and relaying systems detect abnormal conditions like

faults in electrical circuits and operate automatic switchgear to isolate faulty equipment

from the system as quickly as possible. This limits the damage at the fault location and

prevents the effects of the fault spreading into the system. As a rule, short circuit

conditions in power systems are accompanied by large increase of the currents. The

protective relaying which responds to rises in current flowing through the protected

element over a predetermined value is called Over Current Protection and the relays used for this purpose are known as over current relays. Earth fault protection can be

provided with normal over current relays if the minimum earth fault current is sufficient

in magnitude. The design of a comprehensive protection scheme in a power system

requires the detailed study of time-current characteristics of the various relays used in the

scheme. Thus, it is necessary to obtain the operating time-current characteristics of these

relays.

OBJECTIVE:

1. To obtain the IDMT characteristics of the Induction type over current relay THEORY:

The over current relay works on the induction principle. The moving system

consists of an aluminum disc fixed on a vertical shaft and rotating on two jeweled

bearings, between the poles of an electromagnet and a damping magnet. The winding of

the electromagnet is provided with generally seven taps, which are brought on to the front

panel, and the required tap is selected by a push in type plug. The pick up current setting

can thus be varied by the use of such plug multiplier setting. The pick up current values

of earth fault relays are normally quite low.

The operating time of all over current relays tends to become asymptotic to a definite

minimum value with increase in the value of current. This is an inherent property of the

electromagnetic relays due to saturation of the magnetic circuit. By varying the point of

saturation, different characteristics can be obtained and these are:

1. Definite time 2. Inverse definite minimum time (IDMT) 3. very inverse 4. extremely inverse

The torque of these relays is proportional to 1 2sin , where 1 and 2

are the two fluxes and is the angle between them. Both fluxes are produced by the same quantity in the case of single quantity relays (current or voltage operated). The

torque is proportional to I2 (or V

2) if the coil current is below saturation or KI

2 if the core

is made to saturate at a very early stage. The result is that, by increasing I, K decreases so

that the time of the operation remains the same over the working range. The time

characteristics so obtained is known as definite time characteristic.

If the core is made to saturate at a later stage, the characteristic obtained is known

as IDMT. The time-current characteristic is inverse over some range and after saturation

assumes the definite time form. In order to ensure selectivity, it is essential that the time

of operation of the relays should be dependent on the severity of the fault in such a way

that more severe the fault, the less is the time to operate, this being called the inverse time

characteristic. This will also ensure that a relay shall not operate under normal overload

conditions of short duration.

It is also essential that there shall be a definite minimum time of operation, which

can be adjusted to suit the requirements of the particular installation. At low values of

operating current the shape of the curve is determined by the effect of the restraining

force of the control spring, while at high values the effect of saturation predominates.

Different time settings can be obtained by moving a knurled clamping screw along a

calibrated scale graduated from 0.1 to 1.0 in steps of 0.05. This arrangement is called

time multiplier setting and will vary the travel of the disc required to close the contacts.

This will shift the time-current characteristic of the relay parallel to itself.

By delaying the saturation to a further point, the time current characteristics called

very-inverse and extremely-inverse can be obtained. In the laboratory, the operation of

the circuit breaker is simulated by a three phase contactor. The closure of the relay

contacts short-circuits the no volt coil of the contactor, which in turn disconnects the faulty system.

PRE-EXPERIMENTAL QUIZ

1. What do you understand by (a) primary relay (b) secondary relay (c) auxiliary relay?

2. What do you understand by (a) single quantity relays (b) double or more quantity relays?

3. What are (a) direct acting (b) indirect acting relays? 4. What are the types of the over current and earth fault relays? 5. What is the purpose of shaded pole structure? 6. Can the earth fault and over current relays be actuated by both AC and DC? 7. What is meant by (a) pick up current (b) drop out current (c) drop out ratio 8. What sequence filter can be used for ground fault protection? 9. In systems with low ground fault current, is there a necessity to provide an earth

fault relay?

10. Where do you connect (a) over-current (b) earth fault relay in power system? 11. What is meant by (a) sensitivity (b) selectivity of a relay?

MATERIALS AND EQUIPMENT:

CDG 11 over-current relay, current transformer, 3 phase contactor, 1 phase load,

digital timer and ammeter.

PROCEDURE:

1. OVER-CURRENT RELAY a. Study the construction of the relay and identify the various parts. b. Connect as shown in the figure. c. Set the pick up value of the current at 100% Full Load current by inserting

the plug in the groove.

d. Set the time multiplier setting initially at 1.0 e. Adjust the load current to about 1.3 times the full load current by shorting

the switch K. open the switch K to permit the adjusted current to flow

through the relay and record the time taken for this overload condition.

f. Vary the value f the load current in steps and record the time taken for the operation of the relay in each case with the help of the timer.

g. Repeat the steps (e) and (f) for TMS values of 0.2, 0.4, 0.6 and 0.8 h. Repeat the above experiment with different pick up current values using

the plug setting bridge.

DATA PROCESSING AND ANALYSIS

1. Plot operating time versus the multiples of plug setting value for different time multiplier settings on the same graph for over current relay.

2. plot on a log-log sheet operating time versus the multiples of the plug setting value for different time multiplier settings on the same graph sheet for the over

current relay. Plot the curves for earth fault relay as in (a) and (b)

3. observe the inverse nature of the characteristics as well as the definite minimum time required for the operation of the relay in either case if possible.

POST EXPERIMENTAL QUIZ

a. When do you like to add a direction feature for over-current protection? b. How can you make out whether the overcurrent or earh fault relay is sensitive to

direction?

c. Can the over current and earth fault relays be made to operate instantaneously and how?

d. What do you understand by time and current grading of over current relays? e. When do you use negative phase sequence filters? f. Why is the relay disc normally not circular? g. What are the equations of IDMT, very inverse and extremely inverse over-

current relay?

DATA SHEET

Type of the relay:

Pick up current (plug setting multiplier) = A

Serial

number

Current in

amperes

Current in

times the

plug setting

multiplier

Operating time in seconds for time multiplier

setting of

0.2 0.4 0.6 0.8 1.0

REFERENCES:

1. A.R. van C Warrington, Protective relays, their theory and Practice Volume I and II, Chapman & Hall 1969

2. B. Ravindranath and M. Chander, Power System protection and switchgear, Wiley Eastern Limited, 1976

3. V.A. Slabikov, Generation, protection and switchgear, Coimbatore Institute of technology, Coimbatore, 1967

4. H> Cotton, The transmission and distribution of electrical energy, The FLBS and The English university press, London, 1962

5. C.R Mason,The art and science of protective relays.

A

+

- 10 V

ACTUATOR 1 4

2 7

4

3

9 10 OVER CURRENT

RELAY

C.T. 5/5 Amp.

4

8

CONTACTOR SINGLE PHASE AUTO TRANSFORMER

RHEOSTAT 12 Ohm,12Amp

230 V

OVER CURRENT RELAY

EXPERIMENT NO. 4

PERCENTAGE DIFFERENTIAL RELAY

MOTIVATION:

The principle of Unit protection systems was first established by Merz & Price; their fundamental differential systems have formed the basis of many highly

developed protective arrangements for feeders, generators, transformers and bus bars.

The relays compare the currents entering & leaving the circuit element to be protected,

which should be the same under normal conditions and during an external fault. Any

difference current must be flowing into a fault within protected circuit. When this system

is applied to electrical equipment it is called differential current protection. When it is

applied to lines or cables it is called pilot differential protection, because pilot wires or an

equivalent link or channel is required to bring the current to the relay from the remote end

of the line. The study of construction and operating characteristics of a percentage

differential relay is extremely important.

OBJECTIVE:

(1) To study and determine the operating characteristics of a percentage differential relay

for 15% and 30% bias setting.

(2) Use the transformer differential relay for single phase transformer protection.

(3) Study the harmonic restrained feature of transformer differential protection.

THEORY:

The differential relay is one that operates when the vector difference of two

or more similar electrical quantities exceeds a predetermined value. This means for a

differential relay, it should have: (1) two or more similar electrical quantities, and (2)

these quantities should have phase displacement (Normally approx. 180o), for the

operation of the relay. The name is not due to a particular construction of the relay but is

due to the way in which the relay is connected in the circuit.

The simple differential protection scheme also known as Merz-Price

protection scheme assumes that the two CTs used were identical. But in practice it is not

true. The CTs of the type normally used do not transform their currents so accurately

under transient conditions especially. This is true because the short circuit current is

offset. i.e. it contains d.c. components. Suppose the two CTs under normal conditions

differ in their magnetic properties slightly in terms of different amounts of residual

magnetism or in terms of unequal burden on the two CTs, one of the CT will saturate

earlier during short circuit currents (offset current) and thus the two CTs will transform

their primary currents differentially even for a through fault condition. This effect is more

pronounced especially when the scheme is used for the protection of power transformers.

To accommodate these features, Merz-Price protection is modified by biasing the relay.

This is commonly known as biased differential protection or a percentage differential

protection and is shown in fig. 1.

The relay consists of an operating coil and a restraining coil. The operating coil is

connected to the mid-point of the restraining coil. The operating current is a variable

quantity because of the restraining coil. Normally, no current flows through the operating

coil under through fault conditions, but owing to the dissimilarities in CTs, the

differential current through the coil is ( i1- i2) and the equivalent current in the restraining

coil is ( i1+ i2)/2. The torque developed by the operating coil is proportional to the

ampere-turns. i.e.

To (i1- i2) no

Where no is the no. of turns in the operating coil. The torque due to restraining coil is:

T (i1+i2) nr/2

Where nr is the no.of turns in the restraining coil. At balance,

( i1- i2) no = ( i1+ i2) nr/2

or

( i1- i2) nr

_________ = _____

( i1+ i2)/2 no

The operating characteristic is shown in fig. 2. It is clear from the characteristic that

except for the affect of the control spring at low currents, the ratio of the differential

operating currents to the average restraining current is a fixed percentage. This is why it

is known as percentage differential relay.

PRE-EXPERIMENTAL QUIZ:

1. Differentiate between Merz-Price protection and percentage bias protection schemes.

2. Why is a biased differential protection known as percentage bias protection scheme?

3. Why is it claimed that current balance differential protection scheme is highly dependable for the protection of electrical apparatus?

4. What problem can be caused due to non-identical characteristics of CTs? 5. Why is it necessary to provide percentage bias feature to differential relays? 6. What is the difference between transverse and longitudinal differential protection

schemes?

APPARATUS:

1. Ammeters 15A -- 2 nos.

2. Ammeter 5A -- 1 no.

3. Autotransformers -- 2 nos.

4. Rheostats 100 , 5A -- 2nos.

5. Current transformers -- 2 nos.

6. Percentage differential relay -- 1 no.

PROCEDURE:

(1) To study and determine the operating characteristics of a percentage differential

relay

Make the connections as shown in the Fig.4.1. To ascertain differential connections

of the CTs, the variacs are kept at minimum voltage position and the rheostats at the

maximum resistance position. The voltage of variac A is increased slightly so that there is some current through the ammeter connected in the operating coil circuit. Now with

the increase in voltage of variac B if the current in the same ammeter starts decreasing, the connections are proper i.e. differential, and if, the current increases, the connections

are cumulative & hence improper. Then any two terminals of one of the CTs should be

interchanged after switching off the power supply.

Once the polarities of the CTs for differential connections are ascertained,

variac A is selected as the operating variac and B as the resetting variac. Voltage of variac A is increased till the relay operates which is indicated by the falling of the flag. Note down the currents i1, i2 and (i1 i2). Repeat the procedure at least for 6 to 7 operating points.

In order to obtain the operating characteristics for a different bias setting,

the taps (three in case of English Electric Electromechanical relay) corresponding to the

desired setting are selected. The experiment is repeated as for the 1st setting. The

observations are tabulated in the following format.

(2) Transformer differential relay for single phase transformer protection.

i) Make connections as shown in Fig. 4.2

ii) Initially keep the switch (SW1) open. That means fault resistance is not connected.

iii) Slowly increase the voltage of variac. Relay will not operate.

iv) Now fault is simulated by adding a fault resistance (close SW1). The relay will trip

due to unbalanced current through CT1 and CT2.

(3) Harmonic restrained feature of transformer differential protection.

i) Make connections as per circuit diagram given in Fig. 4.3.

ii) Initially, keep the switch (SW2) open.

iii) Increase the current till the relay operates. Note down the current value.

iv) Close the switch (SW2).

v) Again, increase the current till the relay operates. Note down the current value.

The current at which relay operates now is more than the previous value because of

harmonic restrained feature of the relay.

DATA SHEET:

S.NO. i1 i2 i1 i2 (i1+i2)/2

1.

2.

3.

4.

5.

6.

7.

Plot (i1 i2) Vs (i1+i2)/2. The curves for the different tap settings must be drawn on the same axis in order to have the relative idea of slopes or bias of the

characteristics.

The slopes of the two characteristics are calculated from the characteristics

drawn & the results should be compared with those of the bias selected on the relay.

Discrepancy, if any should be explained.

POST EXPERIMENTAL QUIZ:

1. Explain how the bias settings are adjusted in a relay. What changes are introduced in the relay to obtain a particular setting?

2. What are the percentage biases usually provided for a (1) Generator (2) Transformer and why?

3. Give some applications of percentage differential relay. 4. Draw a suitable diagram for differential protection of a delta star connected 3-

phase transformer.

5. Why is percentage bias setting for transformer differential protection is high as compared to that for the percentage differential protection of a generator?

6. What do you understand by magnetizing inrush current of a transformer? How would you prevent mal-operation of a percentage differential relay due to this

phenomenon?

i1 i2

i1-i2

i2i1

i1

Fig. 4.1: Percentage Differential Protection

OPERATING

ZONE

i1-i2

(i1+i2)/2

NON-OPERATING

ZONE

5DIFF.

RELAY

A A

A

30V

610

7 8

S1

S2

S1

S2P2

P1 P1

P2

C.T. C.T.

Single

Phase

Variac

Single

Phase AC

Supply

Single

Phase AC

Supply

Single

Phase

Variac

Fig 4.2. Circuit for determination of relay characteristics

Single

Phase AC

Supply

CT 1 CT 2P1 P2 P1 P2

S1 S2 S2S1

Load

Resistor

Fault

Resistor

1:1

Transformer

Fig 4.3. Circuit for single phase transformer protection

Single

Phase AC

Supply

A

A

DC

AC

7

10

RELAY

Fig 4.4. Circuit for demonstration of harmonic restraint feature

LOAD FLOW STUDY

For a given IEEE 14 bus test system use Newton Raphson Load flow program to study

the following. Detailed procedure is given in additional sheets.

1. Load ability Margin Determination

2. Line Over Load Alleviation using Generator Rescheduling

3. Reactive Rescheduling to Improve Voltage Profile

4. Line Over Load Alleviation using Series Compensation

5. Ferranti Effect

LOAD MODEL:

The loads in a power system are modeled as ZIP model or exponential model to

incorporate the load changes with system voltage (voltage dependent loads). Here we are

using ZIP model in which load modeling is done as

PLi = PLoi * (ai+biV+ciV2).

QLi = QLo *(ci+diV+eiV2).

where PLoi and QLoi are nominal loads.

Where a, b, c and d, e, f are coefficients for constant power, constant current and constant

impendence components respectively.

BUS INJECTION MULTIPLIER ():

In order to incorporate change in loads/generations, load multiplier is used. The new load at bus i is given by

PLoi = PLoi * (1+ ) QLoi = QLoi * (1+ )

and the corresponding change in power generated.

Pgoi=Pgoi * (1+ )

where PLo, QLo, Pgo are base case variables in a given loading interval.

GENERATOR REPRESENTATION FOR PV BUS:

Real power generation and voltage magnitude are specified at a PV bus. The voltage at

such a bus can be maintained as long as reactive generations are within limits.

INPUT FILE:

User can create his/her own input file name it as per his/her wish. . The master data file is

(inputfile.m) and none of the students should alter this file. The input file has following

data

Bus data: Bus name/Bus no., status of bus(whether PV=1,PQ=2 or slack=0), initial

voltage magnitude, nominal voltage, initial angle, total power generation(Pg + jQg), total

power demand(Pd + jQd), Qmax, Qmin (reactive power limits),bus shunt (mho),a, b, c

(ZIP coefficients). For load bus initial voltage and angle can be taken as flat start 1 p.u. at

an angle 0 degrees. Real and reactive loads are the nominal values.

[Always a+b+c=1 and d+e+f=1] condition should be satisfied.

Linedata: starting bus name/bus no., ending bus name/bus no., line impedance(R + jX),

half line charging impedance (B/2*j), tap ratio (real tap+ imaginary tap*j].

SYSTEM STUDIES:

For performing system studies copy master input file to your own file in which you will

be making changes for different system studies. Do not alter master file under any

circumstances.

The executable file is NRLF

During execution of program you will be asked to specify load multiplier . Zero value for indicates base case load/generations as given in data file. Following are the hints for doing the system studies.

First take a printout of base LF input (only once) and output file (=0). This will be useful in other studies.

LOADABILITY LIMIT DETERMINATION:

This study should be done in the following stages.

Stage1: Consider only constant P, Q type of load, selecting load model appropriately.

Keep reactive limits Qmax, Qmin very high (say +100 and -100). Run LF with different

load multiplier and stop when LF diverges (does not provide solution). This loading is

generally the approximate loadability limit given by maximum power theorem for the

network. Note down critical. (1+ critical) is the actual scaling of base case load.

Stage 2: With reactive limits still same as in stage1, observe the effect of voltage

dependent load on loadability limit. This can be done by selecting load coefficient

approximately. In this case is a load unit multiplier and not load power multiplier. You can see that critical is higher then that in stage1.

Stage3: Again consider only constant P, Q type of load and have normal limits as given

in master data file. As you increase during the study for loadability limit determination, you will see critical to be much lower than that in stage1.

Note: Use Constant PQ loads and very high Q limits in all the remaining studies.

LINE OVER LOAD ALLEVIATION USING GENERATION RESCHEDULING:

Study the line flow in the base case LF results. Identify one of the lines with

relatively heavy flow as overloaded by arbitrarily choosing its limit to be 0.9 to 0.95

times base case flow. The objective is now to reschedule real power generations at all PV buses to alleviate this overload. (please note that in spite of the fact that out of total 4

PV buses only one has real generation in original data file, others being synchronous

condenser buses we consider all such buses to have real power generation capability for

this study). First obtain sensitivity of these generator buses to the flow on the line

designated as over loaded. This is done by running repeated load flows with each

generation perturbed by a few percentage (say .02 p u), one at time and observing the

change in line flow. The ratio of flow change to the generation change is the desired

sensitivity. Rank them in order of sensitivity. Normally an optimal power flow utilizes

this information to provide new generation schedule to achieve the objective. A crude

way is as follows:

Staring from top ranked generator, vary its generations to remove the overload.

The generation change limit being +/-0.1p.u. If this generator change cant completely alleviate overload, try next generation in order of ranking. Simple a priori calculation

using sensitivity information will tell us how much approximate change and at how

generators will be required to achieve the objective. Take a final printout of the output

file reflecting these results.

LINE OVERLOAD ALLEVIATION USING SERIES COMPENSATION:

Series capacitive compensation is used to increase the loadability limit of a

transmission line. For a given loading and topology any such change causes re-routing of

power flows in the system. Following the procedure similar to that in line overload

alleviation using generation rescheduling, we can identify the line and amount of

compensation required to achieve the objective with minimum cost. Take a final printout

of output file.

VOLTAGE CONTROL:

Run a load flow with higher loading (say =0.3, i.e. 1.3 times base case loading. Keep Q limits very high for this exercise). Study the bus voltages in output file (take

printout). Identify the lowest bus voltage as violating the minimum limit, say 1.0 p. u.

There could be more than one bus with voltages less than 1.0 p. u. however; we focus

only on worst bus voltage.

Voltages can be controlled by P-V bus voltage setting, transformer tapes and bus

shunts. In this exercise we will consider only P-V voltage settings as controls. The

sensitivities of load bus voltages to P-V bus controls are obtained using perturbation

technique similar in order of their sensitivities. Staring from the top ranked control see

how the designated bus voltages can be brought within limits (how many controls and

how much change). Assume maximum limit of P-V bus voltage to be 1.1 p. u. (workout

control margin available accordingly). Take a printout showing how the objective is

fulfilled.

FEERRANTY EFFECT:

By making negative (-1 to 0) in a load flow run, we can set up a light load condition case. You can observe from output file load bus voltages can be higher than P-

V bus and slack bus voltages. Another indication of Ferranti effect is the reactive power

absorption by the synchronous machines. Take a final printout of output file.