finalhvlab

1 Relay & HV Lab Manual

CYCLE – I

EXPERIMENT NO – 01

IDMT CHARACTERISTICS OF ELECTRO – MECHANICAL TYPEOVER – VOLTAGE RELAY

AIM : To study of the operation of electromechanical type over – voltage relay and hence the obtain it’s inverse time/voltage characteristics.

APPARATUS REQUIRED : One over – voltage relay, Two auto – transformers, one 110V/440V transformer, one (0-600)V a.c. voltmeter, one stop watch, one two – way switch, one SPST switch and connecting wires.

CIRCUIT DIAGRAM :

3

+ Aux unit

220V DC

_ Lamp

4

8

a

A A 9 b

RE

(0-600)V230V AC

N Operating coil

10

Auto Tr-1 Auto Tr-2 110/440V Tr

Department of EEE RVCE

DISC

V


DETAILS OF THE RELAY :

1. Voltage Rating – 100 V

2. Setting range – 110 – 170% adjustable in seven equal steps of 10%

3. Resetting voltage – The disc will completely reset at 90% of more of the voltage setting.

4. Pick up voltage – Equal to the set tap voltage with a maximum error of ± 5%

5. Thermal rating – The relay will withstand the voltage setting continuously for 60° C rise in coil

temperature.

6. Auxiliary unit and Operation indicator – Auxiliary volts = 220V DC.

7. Resetting time – with the T.M.S. = 1.0 the relay resetting time 10 sec.

8. Accuracy – the operating value confirms to error class index 5.0 as per – S3231/1965 at the

voltage setting.

9. Applications – The over – voltage relay is used for the protection of AC circuits, static

capacitors and machine such as generators and synchronous motors.

PROCEDURE :

1. The connections are made as shown in the diagram.2. Using a screw driver and the link provided on the link board the relay, a plug setting of 121

volts (say) is selected.3. A.T.M.S of 1.0 is set by rotating the knurled bakelite disc mounted at the top of the relay

frame.4. With the SP switch ‘S’ open, the two – way switch on position ‘a’ and the auto – transformers

at minimum positions, both the supply switches are closed.5. Using the auto – transformer – 1, the out put voltage is made equal to the rated voltage of 110

volts.6. The two way switch is now put on position ‘b’ and using the auto-transformer-2, the output

voltage made equal to 1.3 times the plug setting voltage i,e., 157.3 volts.7. The two-way switch is now put back to position ‘a’ and switch ‘s’ is closed.8. The two-way switch is then put on position ‘b’ and simultaneously the stop watch is started.

The disc starts moving in the contact closing direction (initially the trip contacts will be open) and comes to rest when the contacts are shorted.

9. The operating time of the relay is found by stopping the stop watch when the relay trips.This is indicated by glowing of the lamp and the operation indicator flag provided at the top right hand corner of the relay.

10. The switch ‘S’ is then opened and the indicated flag is reset manually.11. Using the auto – transformer -2, the output voltage is made equal to 1.5 times the plug Setting

voltage i.e., 181.5 volts.12. The procedure explained in steps 7 to 10 is now repeated.13. The experiment is repeated as explained in steps 11 and 12 for the remaining applied

voltages given in the tabular column.



TABULAR COLUMN:

PLUG SETTING VOLTAGE = 110%=121 VOLTS

Slno TMS=1.0 TMS= 0.8 TMS=0.6Applied Voltage Volts

Operating Time Secs

Operating Time Secs

Operating Time Secs

1. 121 x 1.3 = 157.32. 121 x 1.5 = 181.53. 121 x 2 = 242.04. 121 x 2.5 = 302.55. 121 x 3 = 363.0

PLUG SETTING VOLTAGE =120%= 132 VOLTS

1. 132 x 1.3 = 171.62. 132 x 1.5 = 198.03. 132 x 2 = 264.04. 132 x 2.5 = 330.05. 132 x 3 = 396.0

PLUG SETTING VOLTAGE =130%= 143 VOLTS

1. 143 x 1.3 = 185.92. 143 x 1.5 = 214.53. 143 x 2 = 286.04. 143 x 2.5 = 357.55. 143 x 3 = 429.0

14) The procedure explained in steps 6 to13 is carried out again for the plug setting voltages of 132 and 143 volts.15) A.T.M.S Of 0.8 is now set and the experiment is repeated again as explained in steps

6 to 14.16) A.T.M.S OF 0.6 is then set and the experiment is repeated again as explain in steps 6 to 14.

The operating time of the relay for various plug settings and time multiplier settings are entered in the tabular column as shown.

NOTE:- As the applied voltages are set with switch ‘s’ open, they will decrease when the switch ‘s’ is closed. Care should be taken to see that the applied voltages given in the Tabular column are the voltage when the switch ‘s’ is closed.

GRAPHS: A graph of operating time V/S applied voltage for any one plug setting (say 132 V) is drawn as shown in fig. (a) below. A common graph of operating time V/S multiples of plug setting is drawn as shown in fig(b) below. It can be seen that for a given T.M.S the operating time V/S multiples of Plug Setting voltage characteristics is same irrespective of the plug setting.



Plug setting 14 Voltage-132V 14 12 12 10 Operating 10 Operating time in time in 8 secs 8 secs 6 6 4 TMS=1.0

4 TMS=1.0 TMS=0.8 TMS=0.8 2 TMS=0.6

2 TMS=0.6

0 150 200 300 400 0 1 2 3

Applied voltage Multiples of plug setting voltage.

Fig (a) Fig (b)


CURRENT – TIME CHARACTERISTICS OF FUSES

AIM : To obtain the characteristics of the fuse wires and hence to determine the fusing factor and fuse constants.

APPARATUS REQUIRED : One 15 amps auto transformer, one (0 – 10/20)A a.c ammeter, two (4x4) Ohms rheostats, one stop watch, one fuse mounting board, one fuse enclosure, one SPST switch, fuse wires of different capacities and connecting wires.

CIRCUIT DIAGRAM: SPST

(0-10/20)A SPST

FUSEWIRE

R (4X4) Ω (4X4) Ω 230 V A.C CI CI N


A


PROCEDURE:

1. The connections are made as shown in figure. A fuse wire of current rating 3 amps is fixed onto the fuse board. While fixing the fuse wire, the terminals are so selected that the length of the fuse wire is 6 cms. The fuse wire is then covered using the enclosure.

2. With the SPST switch closed and (4x4) Ohms rheostats in cut-in position the supply switch is closed.

3. By using the auto- transformer and (4x4) Ohms rheostats, a current of about 10% more than the current rating of the fuse wire is passed through the circuit.

4. Now, the SPST switch is opened and at the same instant, the stopwatch is started. The time taken for the fuse to blowout is noted. If the fuse wire blows, then the above procedure is repeated with a slightly lesser current. If the fuse does not blowout, the above procedure is repeated with a slightly higher current. This way, the minimum fusing current required to cause blowing of the fuse and also the corresponding melting time are found. This is entered as the first reading in the tabular column.

5. The fusing current is now increased, in steps using the auto-transformer, (4x4) Ohms rheostats and the SPST switch. At each step, the melting time is found using the stopwatch and the SPST switch.

6. The procedure given in steps 3, 4, & 5, ar now repeated for the same fuse wire but, of length of 9 cms.

7. The procedure given in steps 3, 4, 5 & 6 are now repeated for fuse wires of ratings 5 amps and 10 amps respectively. While finding the melting time for different fusing currents in the case of 10 Amps fuse wire, care should be taken to see that the (4x4)Ohms rheostats do not carry currents more than their rated value (12 Amps)

The readings are entered in the tabular column as shown.

TABULAR COLUMN :

Fuse Rating Sl.No.

6 Cms 9 CmsF. Current Amps M. Time Secs. F. Current Amps M. Time Secs

3 A

12345

5 A

12345

10 A

1234



5

NOTE :

1. The fuse wire should always be covered using the enclosure before finding the melting times.

2. Whenever the SPST switch is opened to pass the current through the fuse wire, the current through the fuse wire will be less than the set current due to increase in the resistance of the path as the temperature of the wire increases considerably. Therefore, the fusing current is the current that would be passing through the fuse wire just before it blows and not the set current.

TO FIND THE CONSTANT ‘K’

From the above tabular column, another tabular column which gives the minimum fusing current for different fuse wires is prepared as given.

Sl.No

Fuse Rating SWG

Fuse Diameter (d) m.m

Fuse Length Cms.

Min. F Current (I)

Amps

Melting Time Secs. Log

e dLog

e I1.2.3.

3510

383529

0.15240.21330.3454

999

While preparing the above table, care should be taken to see that the minimum fusing currents correspond to the same length fuse wire (say 9 cms) and have approximately the same melting time (say 30 secs). The minimum fusing current according to Schwartz and James is “The minimum current to fuse the wire in such a time interval as shall be necessary for the wire to have attained its steady temperature”.

For a round wire, the approximate value of fusing current is given by I = Kdn where ‘k’ is a constant depending upon the metal of the fuse wire, ‘d’ is diameter of the wire in m.m and ‘n’ is called preece’s constant (=3/2). Taking logarithms on both the sides of the above equation.

Log e I = Log e K + n Log e d.

Therefore, if a graph of Log e I V/S Log e d is drawn as shown, the interception on the Y axis will give “Log e K” from which the constant K can be found. As the preece’s constant is known and is approximately equal to 3/2, the slope tan = n = 3/2. Hence the scales on X and Y axis should be so chose that the slope is –3/2 i.e., = tan –1 3/2 = 630.

GRAPHS :

A graph of fusing current V/S fuse diameter and another graph of melting time V/S fusing current is drawn as shown. It can be seen that the characteristics are inverse type i.e. time of operation is inversely proportional to the fusing current.



3A 5A 10A Fusing Melting Current Time Amps I=Kdn in

Log e I Log e K secs

θ 0 0 Fuse dia in m.m Loged 0 5 10 15

Fusing Current in Amps

NOTE : All the three inverse characteristics refer to the fuse wires of same length (say 9 cms).

FUSING FACTOR : 60A 100A 160A 300A 430A 600A

Fusing factor of a fuse can be determined fromthe time-current characteristics of the fuse. This Meltingcharacteristics defines the operating time as a Time function of the fusing current. Fig. (c) shows (sec)such characteristics for a fuse of different current ratings. It may be seen that the characteristics are inverse type, i.e the time of operation is inversely proportional to the fusing current. The minimum fusing current is the asymptotic value of the time current curve. For example the minimum fusing current for the 60A fuseis 100A and therefore the fusing factor is 0 Fusing Current 100/60 i.e 1.66.

. Minimum fusing current . . Fusing factor = --------------------------------------

Rated current

The fusing factor for the given fuse wires of 3A, 5A and 10A rating are calculated for the same length of 9 cms (say).

RESULT:

‘K’ for the given fuse wire = __________________

Fusing Factor for

1. 3A fuse wire = ______________________

2. 5A Fuse wire = _______________________



3. 10A Fuse wire = _______________________


BREAK DOWN STRENGTH OF TRANSFORMER OIL USING OIL TESTING UNIT

AIM : To determine the breakdown strength of transformer oil as per Indian Standards Specifications.

APPARATUS OF THE TEST SET: One transformer oil kit, one oil cell, two 500 ml glass containers, one glass rod, about 500 ml of transformer oil.

DESCRIPTION OF THE TEST SET: The test set, which operates on 230V, 50Hz supply has mainly two transformers. One is a toroidal wound auto transformer used to apply steeples continuously variable voltage to the other HT transformer. The HT transformer operates as low flux density ensuring distortion free output voltage. It is a 60 KV, 0.5 KVA capacity transformer. It is so designed that the short circuit current of the secondary is more than 20 MA at all voltages about 10 KV. The max. short circuit current does not exceed 200 MA thus preventing the unnecessary pitting of the electrodes. The test set consists of an over-load relay which trips and disconnects the HT transformer when the breakdown occurs across the gaps. The oil or the insulating material to be tested has to be put in the cells only after removing a plastic enclosure provided. When the enclosure is removed, it actuates a micro-switch shutting off the supply to the unit. A zero return interlock arrangement makes it obligatory to bring the HT voltage to zero after every breakdown test. The panel board of the test set consists of a voltage control knob, a moving iron volt meter to indicated the voltage applied in KV, three indicator lamps to indicate mains ‘ON’, HT. ‘ON’ and HT ‘OFF’. It also consists of mains switch, a HT ‘ON’ switch and a HT ‘OFF switch.

TEST CELL FOR TRANSFORMER OIL :

As per IS 6792, the test cell and the electrodes should be as follows.

(1) The cell, mode of glass or plastic,shall be transparent and non-absorbant,It shall have an effective volume between300 and 500 ml. It should preferable be closed.

100 10 10

(2) The copper, brass or bronze orstainless steel polished electrodesshall be spherical surfaced of the shape and dimension as given 10in fig. The electrodes shall be mounded on a horizontal axis 70and shall be 2.5 m.m a part.The axis of the electrodes shall 10be immersed to a depth of 40mm.



PREPARATION OF THE SAMPLES : The IS6792/1972 states as under.

1) The sampling vessel containing the test oil shall be gently agitated and turned over several time in such a way as, to ensure as far as possible, a homogenous distribution of the impurities contained in the oil without causing the formation of air bubbles.

2) Immediately before use, the cell shall be cleaned by rinsing with the test oil before proceeding to final filling.

3) Immediately after this, the sample should be poured down into the test cell, slowly in order to avoid air bubbles forming (a clean glass rod may be used).

4) The oil temp. at the time of test shall be same as that of the ambient air, preferable in the neighborhood of 270 C and noted.

PRECAUTION DURING SAMPLING : BS – 148/1972 suggests the following precautions necessary for sampling.

1) The utmost care should be taken to avoid contamination of the samples with traces of external impurities such as dust and moisture.

2) The hands of the samples should not come into contact with the sample.3) Test should not be carried out on the sample until it is at least as warm as the surrounding air.4) Only glass sample containers should be used.5) Cotton waste or other fibrous material should not be used to wipe containers or test cell.

TESTING OF TRANSFORMER OIL :

CIRCUIT DIAGRAM :

R 2.5mm 230V AC

N

60KV 0.5 KVA

PROCEDURE:

1. The gap between the two spheres in the oil test cell is adjusted to 2.5 mm using ‘GO’ (Green Coloured) and ‘NO GO’ (Red Coloured) gauges.

2. The test cell is cleaned and the transformer oil to be tested is poured inside the cell taking all the precautions outline earlier.

3. The plastic enclosure is now removed and the oil test cell is placed inside the fiber glass chamber.


V


4. The leads from the HT transformer are connected to the electrodes and the plastic chamber is replaced.

5. The mains cord is then connected to the 230V AC supply and the toggle switch on the panel is put in ‘ON’ position. The ‘MAINS ON’ as well as ‘HT OFF’ indicator lamps light up.

6. The ‘HT START’ Button is then pressed and the voltage control knob is turned in full anti-clock-wise direction. This will result in the excitation of the primary of the HT transformer. Make sure that ‘HT OFF’ lamp goes off and ‘HT ON’ lam lights up.

7. The voltage knob is then advanced in clockwise direction slowly, the rate of increase of the voltage being uniform and equal to approximately 2 KV/sec. This corresponds to an approximate speed of half minute for the full turn.

8. The voltage knob is advanced, till the breakdown occurs, watching the voltmeter throughout. Immediately after the breakdown, the ‘HT ON’ lamp goes off and ‘HT OFF’ lamp lights up. Do not advance the voltage control any further. The breakdown voltage is the voltage reached during the test at the time of breakdown and is noted down. If the first spark is an established one (not transient) the ‘HT ON’ lamp goes off and ‘HT OFF’ lamp light up. Breakdown voltage is the voltage reached during the test at the time of the first spark over occurs, whether it be transient or established.

9. The above test is carried out again five times on the same cell filling.The first application of the voltage is made as quickly as possible after the cell has been filled provided there are no longer any air bubbles in the oil. After each breakdown, the oil is gently stirred between the electrodes by means of a clean dry glass rod, avoiding as far as possible the production of air bubble. For the subsequent five tests, it is necessary to wait for five minutes before a new breakdown test is started. The electric strength of the oil is in the arithmetic mean of the six results which have been obtained and tabulated as shown.

TABULAR COLUMN :

TEST NO. 1 2 3 4 5 6

Break down Voltage

Break down Voltage (KV) Break down strength = --------------------------------------- = ------------------------ kV/Cm. Gap distance (cms)


MEASUREMENT OF H.V.A.C & H.V.D.C USING STANDARD SPHERES

AIM : To obtain the sphere-gap calibration curve and hence to determine the given unknown H.V.A.C & H.V.D.C voltages.

APPARATUS REQUIRED : The given H.V test set consisting of 230V/30KV and 10 mA transformer, rectifier unit, control panel for test set, a sphere gap (5 cms Diameter) arrangement with



horizontal axis, a current limiting resistor, earthing rod, barometer, thermometer and IS 1876-1961 book.

A) MEASUREMENT OF H.V.A.C VOLTAGES

CIRCUIT DIAGRAM : EARTHED

RESISTOR HV SPHERE SPHERE

R

230V 1Φ AC

N

Front Panel Details of Control Unit

PROCEDURE :


+ -

POLARITY MAINS IND HV OFF IND

HT ON IND FAULT FAULT RESET MEMORY PUSH

HT ON BUTTON FUSE

OFF MAINS

ON

HVAC VOLTMETER HVDC VOLTMETER

INPUT AC VOLTMETER

CONTROLCIRCUIT

H V

TRANSFORMER

V


1. The connections are made as shown in the circuit diagram by connecting the H.V terminal of the transformer to the H.V sphere through a current limiting resistor to avoid pitting of the sphere. This is prepared by putting water into the tube such that a resistance of about 1 Ohm/volt (about 30 K Ohms) is obtained which is verified using a multimeter.

2. The surface of both spheres are cleaned using a cloth.3. The borometeric pressure in m.m of Hg, room temperature are noted.4. Using the operating gear, the gap between the two spheres (distance between arcing points) is

adjusted to 6 m.m., taking care to see that the axes of the two sphere lie in a same horizontal plane.

5. The mains switch in the control unit is switched “ON”.6. With the dimmerstat at zero position, the “H.T.ON” button is then pressed. Observe the

“H.T.ON” indicator lamp glow.7. By turning the knob of the dimmerstat slowly, the A.C voltage applied to the H.V terminal of

the sphere gap is increased till the spark over occurs. In the event of spark over, the supply to the sphere in cut off. The fault indicator lamp will glow and all the meters indicate zero values.

8. The ‘memory’ push – button is than pressed and the A.C spark over voltage “V ind” (R.M.S) shown by the H.V.A.C voltmeter, connected on the L.V side of the Tr. but calibrated for secondary voltage, is noted.

9. The dimmerstat is brought to zero position and the ‘fault reset’ button is pressed.10. The mains switch is then switched ‘OFF’ and all the H.V points in the circuit is grounded

using the earthing rod.11. The procedure given in steps 5 to 10 is now repeated four more times and the spark over

voltage is noted in each case. The average value of the five readings Vind (R.M.S) in entered in the tabular column.

12. The procedure given in steps 4 to 11 is then repeated for sphere gap spacings of 8,10,12, and 14 mms.The readings are entered in the tabular column as shown.

TABULAR COLUMN :

Date : …………….. Time : ………………

Borometric Pressure : ……………. m.m of Hg. Room Temperature : …………… 0C.

Sl. No

Gap spacing

m.m

Vind (R.M.S) in KV

1 2 3 4 5 MEAN

Vind (Peak)

KV

Vact at S.T.P

KV

Vact (Peak) at R.T.P

KV1.2.3.4.5.

68101214

19.926.031.737.442.9

CALCULATIONS :

Measurement of H.V.A.C Voltage : For a given spacing between the spheres, the Voltages Vact

(Peak) at the standard temperature and pressure of 200 C and 1013 millibars is found from IS 1876-1961. The actual voltage in KV at room temperature and pressure is given K x Vact (STP)by Vact – actual peak over voltage at RTP = --------------------- KV

h



Where K is the air density correction factor and h is the humidity correction factor. 0.386 pTo find K, the relative air density‘d’ is found using the equation d = -----------------

(273 + t)Where p is the barometric pressure in m.m of Hg and ‘t’ is the room temperature in 0C. The value of ‘k’ for the above value of ‘d’ is found from the table of the values of ‘k’ for different values of’d’ given in IS 1876-1961. The value of h is taken as 1.

k 0.72 0.77 0.82 0.86 0.91 0.95 1.0 1.05 1.09 1.12

d 0.70 0.75 0.80 0.85 0.90 0.95 1.0 1.05 1.10 1.15

The calibration curve for thesphere-gap is then drawn as Vact (Peak)shown in figure (a). Using in KV at RTPthe calibration curve an unknown H.V.A.C voltagecan be found as explainedbelow. Let a test object, connected 0 across the H.V terminal of Vind (Peak) in KV at RTP the transformer and the ground Fig (a)

flash over at a voltage Vind(RMS) at RTP which is indicated by the voltmeter connected on the L.V side of the transformer. The peak value of this voltage is given by Vind (Peak) = 2 x Vind (RMS).

For example, let the voltmeter indicate 25 KV when a test object flashes over. Therefore Vind (RMS) = 25 KV.

Vind (peak) at RTP = 2 x 25 = 35.35 KV.

From the calibration curve the actual flash over voltage of the test object at RTP = ----------------- KV.

EXAMPLE NO – 02

For spheres of 5 cms diameter and a breakdown voltage of 28.9 KV (say) at STP the gap distance is 9 mm as given in IS 1876-1961. The breakdown voltage at RTP can be found as explained below.

Adjust the gap distance to 9 mm and then the procedure given in steps 5 to 11 is repeated.

Vind (RMS) at RTP = ……….. …………………….. KV.

Vind (Peak) at RTP = 2 x Vind (RMS) = …………………..KV.



From calibration curve, break down voltage at RTP Vact (peak) at RTP] = .………….. KV (Peak)

B) MEASUREMENT OF H.V.D.C VOLTAGES

CIRCUIT DIAGRAM :

SMOOTHING CAPACITOR

HV EARTHED

RECTIFIER RESISTOR SPHERE SPHERE

R

230 V RESISTANCE DIVIDER 1Φ AC +

N

Vind -

PROCEDURE 1) The connections are made as shown in the diagram taking care to see that the rectifier unit is

connected as shown to get +ve poliarty D.C voltage. The output from H.V rectifier is connected to the H.V sphere through a current limiting resistor to avoid pitting of the spheres. This is prepared by putting water into the tube such that a resistance of about 1 ohm/volt (about 40 K ohm) is obtained which is verified using a multimeter.

2) Using the operating gear, the gap between the two sphere is adjusted to 6 m.m taking care to see that the axis of the sphere lie in the same horizontal plane.

3) With the polarity switch in the control unit on +ve side, the main switch in the control unit is switched ON.

4) With the dimmerstat at zero position, the ‘H.T. ON’ botton is then pressed. Observe the ‘H.T.ON’ indicator glow.

5) By turning the knob of the dimmerstat slowly, the d.c voltage applied to the sphere gap is increased till the spark over occurs. The spark over voltage (Vind) shown by the H.V.D.C voltmeter, connected in the resistance divider circuit, is noted. Care should be taken while noting the voltmeter reading as, in the event of spark over, the supply to the H.V transformer is cut-off and all the meters indicate zero values and the fault indicator lamp will glow.

6) The dimmerstat is brought to zero position and the ‘fault reset’ button is pressed.

7) The mains switch is then switched ‘off and all the H.V points in the circuit is grounded using the earthing rod. (Touching of the H.V terminals is avoided as there may be residual charges due to smoothing capacitor).

8) The procedure given the steps 3 to 7 is repeated four more times and the spark over voltage is noted in each case. The mean value is the 5 readings of entered as V ind in the tabular column shown.


HV

TRANSFORMER

V

CONTROLUNIT V


9) The procedure in steps 2 to 8 is then repeated for sphere gap spacings as given in the tabular column. The readings are entered in the tabular column shown.

TABULAR COLUMN :

Date : ………………………. Time : ……………………….

Barometric Pressure : ………………mm of Hg. Room temperature : …………… 0C.

Sl. No

Gap spacing

m.m

Vind (KV)

1 2 3 4 5 MEAN

Vact at S.T.P KV

Vact at R.T.PKV

1.2.3.4.5.

68101214

19.926.031.737.442.9

CALCULATIONS :

MEASUREMENT OF H.V.D.C VOLTAGE :

For a given spacing between the spheres, the voltage Vact at STP of 200 C and 1013 millibars is found from IS 1876-1961. The actual voltage in KV at room temperature and pressure is given by

K x Vact (STP)Vact = Actual spark over voltage at RTP = --------------------------- KV

hWhere K is the air density correction factor and h is the humidity correction factor. 0.386pTo find K, the relative air density‘d’ is found using the equation, d = ----------------- (273 + t)Where p is the borometric pressure in m.m of Hg and ‘t’ is the room temperature in 0C.

The value of ‘k’ for the above value of ‘d’ is found from the table of the values of ‘k’ for different values of ‘d’ given in IS 1876-1961. The value of h is taken as 1.The calibration curve of the sphere-gap is then drawn as shown in figure. Using the calibration curve an unknown H.V.D.C voltage can be found as explained below.Let a test object, connected across the H.V terminal of the rectifier and the ground flashover at a voltage Vind (RTP) which is indicated by the voltmeter of the resistance divider circuit. The actual flashover voltage at RTP can be obtained from the calibration curve.

For example, let the voltmeter indicate 25KV when a test object flashes over.

Vind (RTP) = 25 KV

From the calibration curve, the actual flash over voltage of the test object at RTP = …….… KV.

EXAMPLE NO – 02

For spheres of 5 cms diameter and a breakdown voltage of 28.9 KV (say) at STP the gap distance is 9 mm as given in IS 1876-1961. The breakdown voltage at RTP can be found as explained below.



Vact

applied at RTP

0 Vind in KV at RTP

Sphere Gap Calibration Curve

Adjust the gap distance to 9 mm and then procedure given in steps 5 to 11 is repeated to find Vind (RTP) which is indicated by the voltmeter of the resistance divider circuit. The actual flashover voltage at RTP can be obtained from the calibration curve.

From the calibration curve, break down voltage at RTP [Vact (peak) at RTP] = ………..KV.

CYCLE – II


OPERATING CHARACTERISTICS OF MICRO – PROCESSORBASED (numeric) OVER-CURRENTRELAY

AIM: To study the operation of the given numerical over current and earth fault relay and to obtain its IDMT characteristics for different plug setting and time multiplier settings.

APPARATUS REQUIRED : The given numerical current and earth fault relay fixed on the relay test panel consisting of 100A current source, auxiliary power supply, current transformers, digital time, Digital ammeter (0-20)A.

RELAY DETAILS:

I) TECHNICAL DATA :

1. Type – PNA 442 (Three pole over current + One earth fault relay with instantaneous element)2. Rated AC current input – 1 amp.3. Frequency – 50/60 Hz.4. Auxiliary power supply – (85 – 275) volts AC/DC or (18 – 85) volts AC/DC.5. Rated AC Burden - <0.25V A at upf and at rated current on any setting.6. Rated DC burden - <5 watts during non-operated condition and <6 watts during operated

condition at rated current on any setting.7. % pick up range – 105 % to 120% of setting value.8. % drop off - > 90 % of setting value.9. Applications – Used for short circuit and earth fault protection of radial feeders, transformers,

back-up to differential relay and distance relays.



II. RELAY OUTLINE DIAGRAM :

NUMERIC OVER CURRENT & EARTH FAULT RELAY

PROK DV’sPNA 442

ON L1 L2 L3 e I> /Ie> I>> / le>>

> SET – FRST <

START

PD i 5

Prok dv’s

III. NOMENCLATURE AND DEFINITIONS OF KEYPADS, LED’S AND SYMBOLS :1. POWER ON LED indicates the power – on status.

2. L1 LED glowing continuously indicates the trip status in R phase, blinking of L1 LED indicates Pick-Up or Drop-Off status in IDMT curves.

3. L2 LED glowing continuously indicates the trip status in Y phase, blinking of L2 LED indicates Pick-Up or Drop-Off status in IDMT curves.

4. L3 LED glowing continuously indicates the trip status in “B” phase, blinking of L3 LED indicates Pick-Up or Drop-Off status in IDMT curves.

5. e LED glowing continuously indicates the Trip status in ‘e’ Earth, blinking ofe LED indicates Pick-Up or Drop-Off status in IDMT curves.

6. I > Ie > LED indicates the trip status of IDMT curve, I > refers the phase fault low – set and le > refers to earth fault low – set.

7. I >> / Ie >> LED indicates the trip status of High – set or instantaneous element, I>> refers to phase fault high – set and Ie > > refers to earth high – set.

8. > It is a time tested reliable front panel key switch, the forward symbol refers to increment of setting values like time and current range selection of curves and password.

9. < It is a time tested reliable front panel key switch, the reverse symbol refers to


2


decrement of setting value like time and current range selection of curves and pass word.

10. SET It is a time tested Reliable key pad switch the function key pad is used to ---------- reset the relay during post fault and set function of the variable.

F-RST

11. START It is a time tested reliable key pad switch. The function of START key pad switch is to start or begin the set of procedure.

IV) I.D.M.T RELAY SETTING RANGE :

A) LOW – SET SETTING RANGE :

Low-SetCurrent

Setting Range

In steps of

Time Setting Range

In steps of

I > Phase

0.5 to 2.0 x In

0.05x In

t > 0.1 – 1.6 0.1

Ie >Earth

0.1 to 0.8 x In

0.05x In

Te > 0.1 – 1.6 0.1

B) HIGH – SET SETTING RANGE :

High-SetCurrent

Setting Range

In steps off

InstantaneousTime

Setting Time

In steps of

I > >Phase

2.0 to 30.0 x In

0.1 x In t > > 0.0 to 1.6 sec

0.1 sec

I e > >Earth

0.5 to 16.0x In

0.5 x In te > >Earth

0.0 to 1.6 sec

0.1 sec

NOTE : I > : Phase low set fault currentIe > : Earth low set fault currentIn : Normal rated currentt > : T.M.S for phase low sette > : T.M.S for earth low seti > > : Phase High – set fault currentt > > : Inst. Time for phase in secondste > > : Inst. Time for earth in seconds.

V) COMMISIONING INSTRUCTIONS :

1. The relay unit is power by connecting auxiliary input power supply of 110 volts D.C Now, LCD in the front facia displays

PROK DV’s ensuring the user that the relay is ready for use. PNA 442

2. Press and hold the START and SET F-RST key pads. Now, release the



START key first and one or two seconds later release the SET F-RST key. Now, LCD displays

PASSWORD CHANGE? YES [>] / NO [<]

If the user wishes to retain the factory set pass word i.e, 1000/ old pass word, select the symbol decrement key by pressing <. Now the LCD displays.

ENTER PASS WORD 00 00

3. (a) Enter factory set password i.e, 1000/old pass word by pressing > increment or < decrement keys after the selection of the first digit [1]. Store the setting of the first digit by pressing SET F-RST key. Now the cursor points to the second digit. Select the second digit by pressing decrement or increment key and store by pressing SET F-RST key. the cursor points to the third digit and the fourth digit, the procedure is followed as per the setting of first and second digit. In case the user enter the four digit password other than the factory set/old pass word and returns to the main loop showing for a short duration ‘Access Denied’ . The LCD displays

ACCESS DENIED PROK DV’sPNA 442

(b). If the user enter the correct password by using < and > keys, for a short duration the LCD displays

Please waitsetting mode ……

4. (a) Then LCD displays the SET parameter

Phase CurrentI > = 0.00 (.5 – 2)

User can select phase low – set current (I>) any value between the range (0.5 to 2) which also means 50% - 200% in steps of 0.05 i.e, 5%.

NOTE XY :

The cursor points selection of 1st digit. Select appropriate values by using decrement [<] or increment [>] key pads and press the SET F-RST key pad to accept the set value. Now, the cursor points to next digit. Repeat the procedure till the last digit value. If the range defined inside the bracket is not selected properly, it gives a buzz sound saying out of range. If the entered data is within the setting range it accepts the entered values and goes to next setp 4b.

4. (b) The LCD display next parameter.

Curve 1



Normal 1NV I

The relay is provided with seven pre-defined IDMT curves, one user defined and one definite time. By pressing > or < keys the user can select any one curve type by pressing SET F-RST

4.(c) The LCD displays next parameter

Time multiplier TMS = 0.0 [.1 – 1.6]

The user can select TMS of any value ranging between (0.1 – 1.6) in steps of 0.1 and the selection is made as explained under note XY.

4.(d) The LCD display the next parameter

Phase High SetI > > = 00.0 [2 – 30]

The selection is made as explained under note XY.4.(e) The LCD display the next parameter

Inst. Time Phaset > > = 0.0 (0 – 1.6)

The selection is made as explained under note XY.

4.(f) The LCD display the next parameter Earth current

Ie > = 0.00 (0.1 – 0.8)

The user can select the earth current of any value between (0.1 – 0.8) which is 10% to 80% of rated current of 1A in steps of 0.05 i.e., 5%. The selection is made as explained under note XY.

4.(g) The LCD display the next parameter i.e, curve type in earth

Curve 1Normal I NV

The relay is provided with seven predetermined IDMT curves, one user defined and defined time. Selection is made as explained under 4b.

4.(h) The LCD display the next parameter

Time multiplierTMS = 0.0 (0.1 – 1.6)

The selection is made as explained under note XY.

4.(i) The LCD display the next parameter

Earth High setI e > > = 00.0 (0.5 – 1.6)



The selection is made as explained under note XY.4.(j) The LCD display the next parameter

Inst. Time Eartht >> = 0.0 (0 – 1.6)

The selection is made as explained under note XY.The entire setting procedure for phase and earth is now completed. The LCD now displays

Prok DV’s PNA – 442

In order to ensure the setting values repeat the set-up procedure from 1 to 4(j)

CIRCUIT DIAGRAM :

PROCEDURE: 1. The connections are made as shown in the diagram by connecting any one of the relay coils (S 1

and S2 in line L1 or L2 or L3) to 100 A current source through the 20A digital ammeter provided on the test panel. The auxiliary supply available on test panel is connected to terminals marked auxiliary supply of the relay. The two N.C contacts of the trip / timer circuit are connected in parallel.


+ -

AUXILIARY SUPPLY

S S1 S2

L1

S1 S2

L2

S1 S2

L3

S1 S2

C

C N/C

C N/C

(0-100A) SOURCE

NUMERIC O.C & Earth Fault Relay

O.C

O.C

O.C

LCD

< SET-FRST >

START

FT RST


2. Keeping the 1 phase auto-transformer of the 100A source at minimum position, the 3 phase supply switch to the relay test panel is closed.

3. Press and Hold the and key pads.

4. Release the key first and one or two seconds later release the

key.

5. Enter the factory set pass word of 1000.

6. The following parameters (say) are then selected in the order mentioned

1. Phase current 6. Earth currentI > = 1.0 Ie > = 0.1

2. Curve – 6 7. Curve - 61.0 Sec 3.0 sec

3. Time Multiplier 8. Time Multiplier T M S = 1.0 T M S = 1.0

4. Phase High Set 9. Earth High SetI > > = 20 Ie > > = 1.0

5. Inst. Time Phase 10. Inst. Time Earth t > > = 0.0 t > > 0.0

After all the above selections are made, the relay displays PNA 442.

7. Now, press the FT (Fault Through) button on the test panel and using the auto- transformer adjust the relay coil current to 1.2 amp. Press RST (Reset) button on the test panel and Set F – RST button on the relay. The timer will indicate 0000.

8. Press the FT button and observe the timer. Note down the time of operation of the relay after the relay trips. Press RST button on test panel and F-RST button on the relay.

9. Now, press the F.T button and adjust the relay current to 2.0A. Press the reset button on the test panel and F-RST button on the relay. The timer indicates 0000.

10. Press the FT button on the test panel and note down the relay operating time.11. The above procedure is repeated for the relay currents given in the tabular column.12. The TMS is now charged from 1.0 to 0.813. The procedure given in steps 7 to 11 is now repeated.14. The plug setting is now changed from 1.0 to 1.5 amp.15. The procedure given in steps 7 to 13 is now repeated.16. The 3 phase supply to the relay test panel is switched off.


START SET F-RST

START

SET F-RST


The relay operating times for different plug settings and TMS are entered in the tabular column.

TABULAR COLUMN:

PLUG SETTING = 100% = 1.0 AMPSl.No.

Relay Current Amps

Operating Time in SecsT.M.S = 1.0 T.M.S = 0.8

123456

I.0 X 1.2 = 1.2I.0 X 2 = 2.01.0 X 4 = 4.01.0 X 6 = 6.01.0 X 8 = 8.01.0 x 10 = 10.0

PLUG SETTING = 150% = 1.5 AMP.123456

1.5 X 1.2 = 1.81.5 X 2 = 3.01.5 X 4 = 6.01.5 X 6 = 9.01.5 x 8 = 12.01.5 x 10 = 15.00

GRAPHS :The I.D.M.T characteristics of the relay i.e, operating time V/S relay current for the two selected plug settings are drawn as shown.

plug setting plug setting = 1.0 A = 1.5 AOperating Operatingtime timein secs in secs

TMS = 1.0 TMS = 1.0 TMS = 0.8 TMS = 0.8

0 Relay current 0 Relay current in Amps in Amps


OPERATING CHARCTERISTICS OF MICROPROCESSOR BASED (NUMERIC) OVER/UNDER VOLTAGE RELAY

AIM: To study the operation of the given numerical over/under voltage relay and to obtain its IDMT characteristics for different voltage settings and time multiplier settings.

APPARATUS REQUIRED: The given numerical over voltage and under voltage relay fixed on the relay test panel consisting of three auto-transformers, three voltmeters, digital timer, fault through (F.T) button, Reset (RST) button and connecting wires.



RELAY DETAILS:

I. TEACHNICAL DATA:

1. Type – PNV (3phase 4 wire system)2. Frequency – 47 to 53 Hz.3. Auxiliary power supply – 85V to 275 Volts, A.C or D.C .4. % Pick up – Over voltage : 101% of the set value

under voltage : 99% of the set value5. % Drop out – Over voltage: 99% below pick up value.

under voltage : 101% above pick up value.6. Applications – Used for protection of power plants, feeders, Motors, Generators,

Transformers against voltage variations.

II. RELAY OUTLINE DIAGRAM :

LCD Display

LEDs

Increment Key Decrement Key

II. SETTING RANGE:

a) Over Voltage (OV) – 1) (0.01 to 1.3) Un in steps of 0.01 for IDMT curve where Un = system voltage.

2) T.M.S – (0.1 to 1.00) in steps of 0.13) Definate time – (0 to 300) Secs in steps of 1.0 sec for Definate time curve.

b) High Speed (OV) – 1) (1.01 to 1.3) Un in steps of 0.01 2) Time (0-5.0)Secs in steps of 0.1 sec.

c) Under Voltage – 1) (0.99 to 0.5)Un in steps of 0.01 for IDMT curve. (UV) 2) T.M.S. – (0.1 to 1.0) in steps of 0.1

3) Definite time – (0 to 300) Secs in steps of 1.0 Sec for Definate time curve.


ON L1 L2 L3 UV OV

>SET

F-RST <

START


d) High speed (UV) – 1) (0.99 to 0.5) Un in steps of 0.01 2) Time (0 to 5.0) Secs in steps of 0.1 sec. CIRCUIT DIAGRAM :

PROCEDURE:A) SETTING OF THE RELAY :

1. The connections are made as shown in the diagram by connecting the 3 phase 4wires supply to input terminals of three 1phase auto-transformers which are connected in star. The output terminals of the auto transformers are connected to the three voltmeters on the relay test panel.

2. With the auto- transformers in minimum position, the 3 phase supply switch is closed.Using the three 1 phase transformer, the phase voltages VRN, VYN and VBN are all made equal to 240 V which correspond to system line voltages of 415V. The Fault Through (FT) button on the relay test panel is now closed. The LCD display shows,

3. For accessing the relay, Press SET/F-RST and START keys simultaneously and then release SET/F-RST Key first and then release START Key. The display shows,

Previous or Factory Set Value

4. Select a system voltage of Un = 415V (say) by operating Increment / Decrement key. Press SET/F-RST Key to register the selected value. Selection of Un = 415V corresponds to 415/3 = 240V across the relay coils.

5. After the selection of system voltage the display shows.


PNVUn 380 3P4W

Rated Un 380V

Over voltage (0 >)1.20 (1.01 – 1.30)

R

Y

415 V 3Φ ACSUPPLYB

N

NUMERICAL OV/UV RELAY

L1

L2

L3

V

V

V

L.C.D

> <SET-FRST

START


Previous or Range of Factory value Lowset 6. Using the increment / Decrement keys select the over voltage limit of 1.05 (say) which

correspond to voltage setting of 240 x 1.05 = 252 volts.

7. After the selection of over voltage limit, press SET/F-RST Key to register the selected value. The user will now be given option to select either the

OR

8. Select IDMT characteristic using Increment / Decrement key. When the display shows IDMT option, press SET/F-RST key to register the selection. The display now shows,

Previous or factory set value.

9. Select T.M.S = 1.0 (say) and press SET/F-RST to enter the value. The display changes to


10. Select a High-Set value of 1.20 (say) which correspond to high-set voltage of 240 x 1.2 = 288 volts. Press SET/F-RST key to register the selection. The display changes to


11. The time for high set operation is selected as 2.00 Secs (say) and SET/F-RST key is pressed to register the selection. The display now changes to


12. Using the increment / Decrement keys select the under voltage limit of 0.95 (say) which correspond to voltage setting of 240 x 0.95 = 228 Volts. Press SET/F-RST key to register the selection. The display now shows the option for selection of

OR


Curve 1Definite time

2 IDMT Curve

Time Multiplier1.0 (0.1 – 1.0)

High Set 0 >>1.20 (1.01 – 1.30)

Inst. Time (t >>)1.0 (0.0 – 5.0)

Under voltage (u<)0.90 (0.99 – 0.50)

Curve 1Definite time

2 IDMT Curve


13. Select IDMT Characteristic using Increment /Decrement keys. When the display shows IDMT option, press SET/F-RST key to register the selection. The display now shows,


14. Select TMS = 1.0 (say) and press SET/F-RST to enter the value. The display changes to


15. Select a high set value of 0.70 (say) which set correspond to high set voltage of 240 X 0.7 = 168 Volts. Press SET/F-RST key to register the selection. The display now changes to


16. The time for high set operation is selected as 2.0 seconds (say) and SET/F-RST in pressed to register the selection. This completes the procedure for setting the relay. After the momentary display of updating the data, the display will show

B) TO OBTAIN I.D.M.T CHARACTERISTICS OF OVER-VOLTAGE RELAY :

1. Using the three 1 phase auto-transformer, the phase voltages VRN, VYN and VBN applied

across the relay coils are all made equal to 240Volts (if required ) and the RST (Reset) button

on the test panel is pressed.

2. Now, one of the phase voltages (say VRN) is increased to 260V using the corresponding auto-

transformer.

3. Press the FT button on the test panel and observe the timer. Note down the time of operation

after the relay trips. Press the RST button on the test panel and F- RST button on the relay.

4. Using the auto-transformer, the voltage VRN is increased to 265V.

5. Press FT button on the test panel and note down the relay operating time.


Time Multiplier1.0 (0.1 – 1.0)

High set (u<<)0.5 (0.99 – 0.50)

Inst. Time (t >>)1.0 (0.0 – 5.0)

PNVUn 415 3P4W


6. The above procedure is repeated for the voltages given in the tabular column.

7. The T.M.S is now changed from 1.0 to 0.5 by running the programme again using SET/F-RST

key and Increment / Decrement keys.

8. The procedure given in steps 1 to 6 is now repeated.

9. The over voltage limit is now increased to 1.1 (corresponding to voltage setting of 1.1 x 240 =

264V) and T.M.S is changed to 1.0 by running the programme again and using the required

keys.

10. Now, the procedure given in steps 1 to 8 is repeated for the voltages given in the tabular

column.

C. TO OBTAIN IDMT CHARECTORISTICS OF UNDER VOLTAGE RELAY :

1. After making sure that VRN = VYN = VBN = 240V, the RST button on the relay test panel is

pressed.

2. Now, one of the phase voltage (say VRN) is decreased to 215V using the corresponding auto-

transformer.

3. Press FT button and note down the time of operation of the relay. Press the RST button on the

test panel and F-RST button on the relay.

4. Using the auto-transformer, the voltage VRN is decreased to 210V.

5. Press the FT button on the test panel and note down the relay operating time.

6. The above procedure is repeated for the voltages given in the tabular column.

7. The TMS is now changes from 1.0 to 0.5 by running the programme again using SET/F-RST

key and Increment / Decrement keys.

8. The procedure given in steps 1 to 6 is now repeated.

9. The under voltage limit is now increased to 0.9 (corresponding to voltage setting of 0.9 x 240 =

216V) and TMS is changes to 1.0 by running the programme again and using the required

keys.

10. Now, the procedure given in steps 1 to 8 is repeated for the voltage given in the tabular

column.

NOTE :

1) The phase voltages VYN and VBN are both maintained constant at 240V throughout the experiment.

2) If IDMT characteristics of only over voltage relay is required, setting for over voltage relay only is made and for the under-voltage relay the previous or factory set values will remain.



TABULAR COLUMN :

OVER-VOLTAGE RELAY UNDER – VOLTAGE RELAY :

Voltage setting : VRN = VYN = VBN = 252V Voltage setting : VRN = VYN = VBN = 227VSl.No.

VRN Operating time in secs Sl.No.

VRN Operating time in secsT.M.S = 1.0 T.M.S = 0.5 T.M.S = 1.0 T.M.S = 0.5

1

2

3

4

5

6

7

8

260

265

270

275

280

285

290

295

1

2

3

4

5

6

7

8

215

210

205

200

195

190

185

180Voltage setting : VRN = VYN = VBN = 264V Voltage setting : VRN = VYN = VBN = 216V

1

2

3

4

5

6

7

8

270

275

280

285

290

295

300

305

1

2

3

4

5

6

7

8

205

200

195

190

185

180

175

170

GRAPHS: The IDMT characteristic (operating time v/s relay voltage VRN) of both the over – voltage and under-voltage relays are drawn as shown for one T.M.S value (say T.M.S =1.0)



(O>1.05) (O>1.1)

EXPERIMENT NO-7

SPARK OVER CHARACTERISTICS OF PLANE – PLANE ELECTRODES &POINT – PLANE ELECTRODES SUBJECTED TO HVAC AND HVDC IN AIR

AIM : To study the behavior of plane-plane and point-plane electrodes for measurement of H.V.A.C voltages.

APPARATUS REQUIRED : The given H.V test set consisting of 1 , 230V/ 30KV and 10 mA transformer, control panel for test set, a sphere-gap (15cms) diameter) arrangement with Horizontal axis, a current limiting resistor, plane-plane and point-plane electrodes, earthing rod, guages of different thickness, borometer, thermometer, one multimeter and IS 1876-1961 book.

CIRCUIT DIAGRAM :

Current Limiting Resistor

R

HV EARTHED 230 V SPHERE SPHERE

AC


(U< 0.9) (U<0.95)

Operating Time (Seconds)

170 180 190 200 210 220 230 240 250 260 270 280 290 300 310RELAY VOLTAGE, Volts RELAY VOLTAGE, Volts

80

70

60

50

40

30

20

10

Under Voltage Relay Over Voltage Relay

CONTROL UNIT

H V

T R A N S F O R M E R

V


N

PROCEDURE :

A) SPHERE GAP CALIBRATION CURVE :

1. The connections are made as shown in the circuit diagram by connecting the HV terminal of the transformer to the HV sphere through a current limiting resistor to avoid pitting of the spheres. This is prepared by putting water into the tube such that a resistance of about 1/volt (about 40 K) is obtained which is verified using a multi meter.

2. The surface of both the spheres are cleaned using a cloth.3. The borometric pressure in m.m of Hg , room temperature are noted.4. Using the operating gear and graduated scale provided on the supporting frame, the gap

between the two spheres (distance between arcing points) is adjusted to 6 m.m., taking care to see that the axes of the two spheres lie in a same horizontal plane.

5. The mains switch in the control unit is switched ‘ON’.6. With the dimmerstat at zero position, the ‘H.T. ON’ button is then pressed. Observe the ‘H.T.

ON’ indicator lamp glow.7. By turning the knob of the dimmerstat slowly, the A.C voltage applied to the H.V terminal of

the sphere gap is increased till the spark over occurs. In the event of spark over, the supply to the sphere in cut off. The fault indicator lamp will glow and all the meters indicate zero values.

8. The ‘memory’ push – button is then pressed and the A.C spark over voltage ‘V ind’ (R.M.S) shown by the H.V A.C voltmeter, connected on the L.V side of the Tr. but calibrated for secondary voltage, is noted.

9. The dimmerstat is brought to zero position and the ‘fault reset’ button is pressed.10. The mains switch is then switched “OFF” and all the H.V points in the circuit is grounded using the earthing rod.11. The procedure given in steps 5 to 10 is then repeated and the spark over voltage Vind (RMS) is

noted in each case. The average value of the five readings V ind (R.M.S) is entered in the tabular column.

12. The procedure given in steps 4 to 11 is then repeated for sphere gap spacings of 8, 10, 12 and 14 mm.The readings are entered in the tabular column – 1 as shown.

TABULAR COLUMN - 1 :

Date : …………….. Time : ……………Borometric Pressure : ……………. m.m of Hg. Room Temperature : ……….0C.

Sl. No

Gap spacing

m.m

Vind (R.M.S) in KV Vind

(Peck)KV

Vact at S.T.PKV

Vact (Peak) at R.T.P

KV

Voltage Gradient KV/CM1 2 3 4 5 MEAN

1

2

3

4

6

8

10

12

19.9

26.0

31.7

37.4



5 14 42.9

B) TO STUDY THE BEHAVIOR OF DIFFERENT ELECTRODE CONFIGURATIONS:

I. PLANE-PLANE ELECTRODES :

1. The H.V terminal of the transformer which was connected to the H.V sphere is now connected to one of the two plane electrodes. The other plane electrode is grounded.

2. Using the operating gear and the graduated scale provided on the supporting frame, a gap distance of 20mm is set between the flat surfaces of the plane-plane electrodes taking care to see that axis of both electrodes are same.

3. The procedure given in steps 5 to 11 above is now repeated.

4. The above procedure is repeated for the gap spacings given in the tabular column – 2.

II. POINT – PLANE ELECTRODES .

The ungrounded plane electrode (connected to the H.V side of the transformer) is now replaced by a point electrode and the experiment as explained for plane-plane electrodes is repeated for gap spacings given in the tabular column – 2.

TABULAR COLUMN – 2:

PLANE – PLANE ELECTRODESSl.No

GapDistance

m.m

Vind (R.M.S) in KV Vind (Peak)KV

Vact (Peak) at RTPKV

Voltage GradientKV/cm

1 2 3 4 5 MEAN

1

2

3

4

5

10

15

20POINT - PLANE ELECTRODES

1

2

3

4

5

25

30

35

40

45



CALCULATIONS :

MEASUREMENT OF H.V.A.C VOLTAGES :

For a given spacing between the sphere, the voltage Vact (Peak) at the standard temperature and pressure of 200 C and 1013 millibars respectively is found from IS 1876 – 1961. The actual voltage in KV at the room temperature and pressure is given by Vact (STP) x K

Actual peak spark over voltage at RTP = ------------------------ KV h

Where K and h are air density correction factor and humidity correction factor respectively. 0.386 p

To find K, the relative air density ‘d’ is found using the equation d = ----------- 273 + tWhere p is barometric pressure in m.m of Hg and t is the room temperature in 0C. The value of K for the above value of d is found from the table of the values of ‘K’ for different values of ‘d’ given in IS 1876 – 1961. The value of h is taken as 1.

The calibration curve for the sphere- gap is then drawn as shown in the figure below. The sphere gap calibration curve may now be used for measuring breakdown voltages of different electrode configurations. Voltages Vind (peak) for various gap settings (both for plane-plane and point-plane electrodes) at RTP is found using the equation Vind (peak) at RTP = 2 x Vind (RMS) at RTP. From the sphere gap calibration curve, the actual peak break down voltage at RTP for various gap setting given in the tabular column is read.

The voltage gradient (Electric stress) in KV/cm is calculated for sphere-sphere, plane-plane and point-plane electrodes and entered in the corresponding tabular column.

Vact

(Peak) RTP

0 Vind (peak) in KV Sphere gap calibration curve



GRAPHS :

7. A graph of break down voltages [Vact (peak) at R.T.P] V/s gap distance in m.m, which shows that the sphere-sphere electrodes are best suited for measurement of high voltages, is drawn as shown in fig (a).

8. Another graph of voltage gradient in KV/Cm V/s Gap distance in m.m. which shows that the voltage gradient required between the spheres is almost constant irrespective of the gap length, is drawn as shown in the next page fig (b).

Vact Voltage(Peak) gradient Sphere-Sphere Sphere-Sphere kV/Cm kV

Plane-Plane Point-Plane

Point-Plane Plane-Plane

0 5 10 15 20 25 30 35 0 10 20 30 40

Gap spacing in mm Gap spacing in mm

Fig. (a) Fig. (b)

CYCLE – III

EXPERIMENT NO. 08

GENERATOR PROTECTION - MERZ PRIZE PROTECTION SCHEME

AIM : To study the operation of the Merz-Prize protection scheme for generator protection under different internal and external fault (Through Fault) conditions.

APPARATUS REQUIRED : The given numeric over current relay fixed on the relay test panel consisting of 3 phase, 20 amps current source, 110V DC auxiliary supply, current transformers, digital timer, (0-20)A digital ammeter, 3 phase generator model and 3 phase loading rheostats.



RELAY TEST PANEL LAYOUT:


S1 S2 S1 S2

S1 S2 S1 S2

R Y B N S1 S2 S1 S2

3 Phase, 20 A Source CT1 CT2

L4 S1 S2

L3 S1 S2

L2 S1 S2

L1 S1 S2

Aux. Power Supply

+ -

GENERATOR MODELNUMERIC O.C & E.F RELAY

+ -

OC

Ov OC

OC

E/F

L.C.D

> <F-RST

START

5Ω

5Ω

5Ω

F.T RST


CIRCUIT DIAGRAM

PROCEDURE :

1. The connections are made as shown in the diagram by connecting 3 phase, 20A current source to the terminals marked RYB and N of the generator model through the (0-20) A digital ammeter provided on the test panel. The output terminals of the generator model are connected to the 3 phase resistance load of 5 ohms/phase connected in Y. The 110V DC auxiliary supply available on the test panel is connected to terminals marked Aux. supply of the relay. The two sets of CTs, CT1 and CT2 are both connected in Y on either side of the generator model and the three relay coils are connected in the residual circuit as shown to obtain differential protection scheme. The earth fault element of the relay is left unconnected. Care also is taken to connect all the neutral points to the neutral of the supply which is grounded.


GENERATOR MODEL CT1 CT2 5Ω

R R S1 S2 S1 S2

5Ω Y Y

S1 S2 S1 S2

B B 5Ω

S1 S2 S1 S2

N N

TRIP

CIRCUIT

N

S1 S1 S1

S2 S2 S2


2. Keeping the 3 phase auto-transformer of the 3 phase, 20A source at minimum position, the 3 phase supply switch to the relay test panel is closed.

3. Press and Hold the START and SET F-RST key pads.

4. Release the START key first and one or two seconds later, release the SET F-RST key.

5. Enter the factory set pass word o f 1000.

6. The following parameters (say) are then selected.

1. Phase Current 6. Earth currentI > = 1.0 I e > =

2. Curve – 6 7. Curve 3.0 Sec

3. Time Multiplier 8. Time Multiplier T M S = 1.0 T M S =

4. Phase High Set 9. Earth High SetI > > = 20 I > > =

5. Inst. Time Phase 10. Inst. Time Earth t > > = 0.0 t > >

As the earth fault element of the relay is not used in the scheme, parameters need not be chosen for this relay element. Hence, after the parameters for the phase current (differential current through the over current element) are selected, the SET F – RST key is pressed continuously till the LCD displays PNA 442.

7. Now, press the FT (Fault Through) button on the test panel and using the auto-transformer adjust the current through the 3 phase load resistance to 5 amps (say). Press the reset (RST) button on the test panel. The timer will indicate 0000.

8. An internal fault of SLG fault is now created by connecting one of the three series connected coils of R phase (say) of the generator model to neutral (ground).

9. Press the FT button on the test panel and observe the operation of the relay. The tripping of the relay is indicated by the continuous glowing of L1 and I> LEDS. The timer also stops. An YES is entered in the last column of the tabular column if the relay trip.

10. Press reset button on test panel and F-RST button on the relay. The timer indicates 0000 and the relay is now ready for next operation.

11. The procedure given in steps 8 to 10 is now repeated for different types of internal faults shown in tabular column.



12. An external fault of SLG fault is now created by shorting Y phase (say) of the load to the ground (neutral)

13. Press the FT button on the test panel and observe the operation of the relay. It can be seen that the relay does not trip and the timer does not stop. After about 30 secs, the reset button on the test panel is pressed and the timer indicates 0000. A ‘NO’ is entered in the last column of the tabular column.

14. The procedure given in steps 12 and 13 is now repeated for different types of external fault shown in the tabular column.

TABULAR COLUMN :

I N T E R N A L F A U L T SSL.NO.

TYPE OF FAULT CONNECTION RELAY TRIPPING YES/NO

1.

2.

3.

4.

5.

L – G Fault

L – L Fault

L – L – G Fault

Symmetrical fault with ground

Symmetrical fault without ground

R phase and Ground (Neutral) shorted

R and Y phase shorted

R and Y phase connected to ground (Neutral)

R, Y and B phase connected to ground (Neutral)

R, Y and B phase shorted

E X T E R N A L F A U L T S1.

2.

3.

4.

5.

L – G Fault

L – L Fault

L – L – G Fault

Symmetrical fault with ground

Symmetrical fault without ground

Y phase and ground (Neutral) shorted

Y and B phase shorted

Y and B phase connected to ground (Neutral)

R, Y and B phase connected to ground

R, Y and B phases shorted.

INFERENCE :

It is can be infered that the ‘Merz-Price protection scheme for generator protection protects the generator against internal faults and remains inoperative for external faults.



EXPERIMENT NO. 09

FIELD MAPPING USING ELECTROLYTIC TANK FOR CO-AXIAL CABLE AND PARALLEL PLATE CAPACITOR

AIM : 1. To plot the equipotential points and hence field lines for the given co-axial cable and parallel

plate model2. To determine the capacitance of the given co-axial cable and parallel plate model.3. To determine the voltage gradient.

APPARATUS REQUIRED: One electrolytic tank with pantograph arrangement, one isolating transformer, one Digital voltmeter, one 1 auto-transformer, one drawing sheet, pencil and eraser and the given co-axial cable and parallel plate model.

INTRODUCTION : A measure of the electric stress of a dielectric is the electric field strength, the determination of which is therefore an important task in high voltage technology. A mathematical solution is possible only in cases of simple arrangements, but for more complicated arrangements met with in practice, a solution can be obtained by plotting the potential distribution in an electrolytic tank. Conduction of current in an electrolytic tank is used as an analog in high voltage dielectrics. The electrolytic tank is used for plotting the equipotential lines and from these electric strength at any point can be determined.

For equipotential plots, the model is so constructed that they have the same shape and position as those in the original structure. The most convenient electrolyte used in the electrolytic tank is the ordinary tap water for most of the problems. For the two-dimensional field model (one in which the conductor configuration can be shown by a single cross section, all cross sections parallel to it being same) with more than one dielectric, shallow tanks are used where different dielectric constants are simulated by different heights of electrolyte. Three dimensional fields with circular symmetry can be readily simulated in wedge shape tank (sloping bottom). To obtain equipotent lines a low ac voltage (about 10V) at 50 Hz is applied across the electrodes. D.C voltage gives rise to much polarizing at the electrodes, which is a source of error. Polarization is greatly reduced when a.c is used.

While plotting the field lines, the following points must be remembered.

1. The field lines leave and enter the electrodes at right angles, because there can not be voltage drop and therefore no current flow, along an electrode. Near a corner in an electrode surface, a field line bisects the included angle.

2. Field lines are perpendicular to the equipotential lines (because there is no voltage drop along an equipotential and therefore there can be no component of field line along it).

3. In an uniform filed, the potential varies linearly with distance.

4. All meshes (called a field cell) formed by two field lines and two equipotentials have the same shape or ratio of length to width equal to the mesh factor ‘a’ and are all squares or curvilinear squares (Refer fig. A). By curvilinear square is meant an area that tends to yield true squares as it is subdivided into smaller and smaller areas by successive halving of the equipotential internal and the flux per tube.



a b a b

l f

c d c d l e

Field Cell Equipotential Lines Flux tube with Electrode 4 cells in series

Fig. A : Cross section of two sheet conductors with completed map. Inset shows three-dimensional view of a field cell.

5. The field cells are obtained in the uniform field region.

6. It can be shown that, for a two dimensional field with one dielectric, if,

lf = distance along field linesle = distance along equiptential linesnf = number of flux tubes corresponding to cells in parallelne = number of equipotential spaces corresponding to cells in series.co = capacitance per unit depth of a field celld = depth (into the page) of the field cell i.e., the depth of the electrodes.a = mesh factor (lf/le)

The total capacitance per unit length (into the plane of the diagram) is given by

nf o r nf le nf

C = Co x ---- = --------- ---- = o r ---- ---- Farads/meter ne a ne lf ne

7. The capacitance of any field cell is same.

CIRCUIT DIAGRAM :

R DVM 230VACN

Auto trans Isolating Transformer



PROCEDUR E : a) Co-axial cable model1. The given co-axial cable model / mounted on the non-conducting base is placed inside the

electrolytic tank taking care to see that the model as well as the surface inside the tank are clean and free of dust. If the water in the tank is not clean, then this water is drained out using the tap provided in tank and the inside surface of the tank is cleaned.

2. Clean water is then added into the tank upto the tip of the given co-axial cable model.3. Using the pantograph arrangement, the probe is moved along the surface of the water to ensure

that the water surface is perfectly horizontal. The levelling screws at the base of the tank can be used to obtain perfectly horizontal position of the tank.

4. The drawing sheet is now placed on the glass plate of the tank. Care must be taken in doing this and ensuring that the sheet is well anchored. A small undetected slide of the sheet during the plot will given an incorrect plot.

5. The trace of the electrodes ‘X’ and ‘Y’ (show in fig. b) is then obtained on the sheet, placed above the glass plate, by moving the probe along the electrode surfaces.

6. The electrical connections are then made as shown in the circuit diagram and a small voltage of 10 volts (with the probe touching the electrode ‘X’) is applied to the electrodes using the auto-transformer.

7. Guiding the probe along a circle corresponding to 2 volts from the reference electrode ’Y’ and using the DVM, the equipotential line corresponding to 2 volts is obtained. Alternatively, various points, which are at 2 volts w.r.t the reference electrode, ‘Y’ are first marked using the probe and the DVM. The equipotential line is then obtained by joining smoothly all these points.

8. The equipotential lines corresponding to voltages of 4V, 6V and 8V w.r.t the referance electrode ‘Y’ are similarly plotted.

b) Parallel plate capacitor model :

The equipotential lines corresponding to potential of 1V, 2V, 3V ------- 10V with respect to the reference electrode ‘Y’ (shown in the fig. c) is obtained in the same way as explained above.

CALCULATIONS :Details of the model: A single core co-axial cable with inner conductor having a diagram of 3.5 cms, the outer conductor of inside diagram of 19 cms. The insulation is paper of permitively r = 3. 10V

8V

4V

2V

0V a b l e

c d l e

Y

Fig. b. Field lines and equipotential lines.


X

d


After obtaining a family of equipotential lines on the drawing sheet, the electrodes X and Y are divided into a few parts (say 12) and the field lines are drawn as shown. Alternatively, filed lines are drawn by drawing straight lines passing through the center in such a way that the angle subtended by the two neighboring field lines is 300. As this is the case of cylindrical geometry having an axis of symmetry, the equipotential lines and field lines drawn will be such that the mesh factor ‘a’ = l f/le

will be same for all field cells. Measuring the lengths le and lf of any of the field cells, the total capacitance per meter length of the cable is calculated using the equation, le nf

C = o r ---- ---- F/m where nf = 12, ne = 5 and r = 3. lf ne

The capacitance calculated above can be verified using the well known equation for the capacitance of co-axial cable given by 24. 2 r

C = ------------- pF/m where b = 9.5 cm and a = 1.75 cms. log b/a

ELECTRIC STRESS : It can be seen from the field map that the voltage gradient is not uniform and is max. near the conductor. The max. stress can be found by considering the filed cell close to the conductor. dv 2 Max. stress Emax. = ----- = ----- v/m.m where lf in m.m is measured dx lf

between 10V and 8V lines. From this, the max. stress under normal working voltage can be found. Also, the max. operating voltage can be found for a given dielectric whose dielectric strength is known.

B) PARALLEL PLATE CAPACITOR MODEL:

CALCULATIONS: Details of the capacitor: length of the plates-15 cms, depth of the plate-5 cms, Distance between the plates-10 cms, dielectric air. FIELD LINES

A X 2cm E 1cm B 8V 6V 10V a b

a b lf

c d c le d

C Y D 0V 2V 4V Fig. c Parallel plate capacitor



After obtaining a family of equipotential lines on the drawing sheet,the boundaries AC and BD in the uniform field is marked.The length AB and CD of the electrodes is then divided into a few parts in such a way that the field cells thus obtained are as nearly square as possible. The field lines(shown as dotted lines in fig)are then drawn in such a way that they are perpendicular to the equipotential lines.It can be seen that in the present case of parallel plate capacitor,the distance between equipotentials is 2 cms and hence the electrodes AB and CD which are 15 cms long are divided into seven equal parts of 2 cms and one part of 1 cm as shown.All the cells in the region AE and CF have the same potential difference of 2 v across them and hence are called ‘cells of the same kind’.The area between EF and BD consists of a fractional flux tube which is also divided into cells(to make l f/le=1)as shown.Each of these cells has 1V across it.

The total capacitance of the given parallel plate capacitor can be found by any of the following two methods.

The capacitane of each cell= cod=EoErd/a FaradsHere a= lf/le=1,d=0.05 meters and Er=1(air)Therefore capacitance of each cell=8.854 x 10-12 x 0.05 = 0.4427pF.The capacitance between the ends of each flux tube with 5 cells in series=0.4427/5=0.08854pF The capacitance between the ends of the remainder flux tube with 10 cells in series =0.4427/10=0.0442pF.

There are seven 5 cell tubes and one remainder(10 ecll) tube.Hence the total capacitance ‘c’ between AC and BD is the sum of the capacitances of all the flux tubes(they are in parallel)Therefore C=(7 x 0.08854)+0.0442=0.664pF.

The total capacitance can also be found using the general equation C=Eo Er(le/lf)(nf/ne) Farads/meter. Wherenf = number of cells in parallelne=number of cells in series and where all cells are of the same kind.Hence, counting in terms of 2V cells, we haveC = 8.854 x 10-12 x 1x 1x 7.5/5=13.28pF/meter =13.28 x 0.05=0.664pF.

ELECTRIC STRESS:It can be seen from the field map that the voltage gradient is uniform between the parallel plates.Hence,the stress can be found by considering any cell between the plates and is given byE=dv/dx=2/lf V/cm where lf in cms is measured between any two neibouring equipotentials.From this the stress under normal working voltage can be found. Also ,the maximum opertating voltage can be found for a given dielectric strength is known.



NOTE: 1) It can be seen that the mesh factor a= lf/le need not be always choosen as equal to unity.but,it is important to see that the ratio lf/le is same for all the cells. Depending upon the value of’a’ selected,the ratio nf/ne gets altered in such a way that the total capacitance works out same.For example, let the length AB of the electrode be divided into 6 equal parts which makes le= 2.5 cms.The no. of flux tubes ‘nf’ will then be 6.Hence,C = Eo2r=(le/lf)(nf/ne)=8.854 x 10-2 x 2.5/2 x 6/5=13.28pF/meter=0.664pF.

2)The capacitance calculated above can be verified using the well known equation for capacitance of a parallel capacitor given by=AEoEr/d=15x5x10-4x 8.854x10-12x1/10x10-2 =0.664pF

EXPERIMENT NO. 10

A) GENERATION OF STANDARD IMPULSE VOLTAGE AND TO DETERMINE EFFICIENCY AND ENERGY OF IMPULSE GENERATOR

AIM :- To operate the impulse generator and generate standard lightning impulses and record the same on an oscilloscope. To determine energy & efficiency of impulse generator.

APPARATUS REQUIRED :- Impulse generator (5 stages, 30kv/stage, 45 Joules/stage), point- plane electrodes and plane-plane electrodes.

INTRODUCTION : - The impulse generator consists of several stages (N). Each stage is a hv capacitor chargeable up to a maximum of Vs. The capacitors are charged in parallel using a common DC source through series resistances. At a chosen instant, all the stage capacitors are made to be connected in series, giving rise to a step function of a nominal peak of (Vc x N), Vc being the actual charging voltage. This step function is applied to a wave shaping circuit, which converts the step function into a double exponential impulse wave of approximately 150 kV peak. The circuit constants determine the wave shape (front and tail times) and the charging voltage determines the impulse peak. The generated impulse is attenuated by a known fraction, using an impulse potential divider, the output of which is connected to a single shot CRO. If an impulse of a lesser or higher peak is needed, the charging voltage can be correspondingly changed. The actual peak of the impulse generated is the generator efficiency times the normal peak.

IMPULSE WAVEFORM AND ITS DEFINITION:

Transient over-voltage due to lighting and switching operation cause traveling waves of steep wave-fronts on transmission lines and other electrical apparatus, causing intense electric stresses leading to possible insulation failure. Such over-voltages are called impulses. Impulse wave-shapes are arbitrary, but, in general, the impulse voltage typically rise to a peak in about a microsecond and decay about 40 to 50 times slower. The wave-shape is defined in terms of its peak value, and the front and tail durations. For the purposes of testing power apparatus however, the shape of a lightning impulse wave has been standardized internationally.An impulse wave is formed by the addition of two exponentially decaying waves of equal peek magnitudes and of opposite polarities, one having a much larger time constant than the other.The general wave form as shown below in fig 1.



1.0 C V1=-t/T

1

V= V1+V2 = (-t/T1 - -t/T

2 )

C '

P

D'

D

Q 0 A t0 A' time

B'

B V2=- t /T2 -1.0 Fig: 1

Fig.1 shows two exponentials V1 &V2 of opposite polarities, with time constants T1 and T2, T1 >> T2. It also shows V, equal to V1+ V2. This sum is a wave that rises to its peak at t0 and decays slowly. V is constructed such that CD=AB,C1D1= A1B1; i.e., AD & A1D1 are the values of V at A & A1

respectively. It is seen that the rising portion OP (called wave front) of V is governed by T2 of V2 and the falling portion PQ (Called wave tail) of V is governed by T1 of V1.

In practice, an impulse voltage is generated by charging a capacitor though a small resistor (front resistance) and, when the peak is reached, discharging the same through a large resistor (tail resistance) Voltage

P' 100% 90% D'

C' E'

50%

B' 30%

10% A'

O O A B C D P E time(s) Fig:2

Basically, the impulse wave is defined by its front time and its tail time, In the Fig: 2, the front and tail times are, tf = OP and tt = OE, the time taken by the impulse to reach 100% peak on the wave



front and the total time to reach 50% peak on the wave tail respectively. However, the time regime OA usually contain oscillations and in the regime DP, the wave is flattened. Due to these, it is difficult to locate points. O and P1 accurately and the measurements of tf and tt becoming uncertain. To obviate these, we follow the internationally standardized method of measurement of t f & tt. The method is emobodied in IS of the bureau of Indian Standards, described below:

Step1: Mark points A1&D1 ( 10% & 90%) points on the front.

Step2: Mark point E1 ( 50% point on the tail)

Step3: Join D1 to A1 and produce to meet time axis at O1, called the virtual origin.

Step4: Mark points A,D & E

Step5: Front time = tf = 1.25(OD-OA)

Tail time = tt = OE.

In case oscillations are present around A1, one can use

Front time = tf = 1.667(OD-OB)

Tail time = tt = O1E.

(NOTE: As per IS, The tolerances on tf & tt are, +30% & +20% respectively)

SPECIFICATIONS : -

Type of the circuit : Marx

AC Input : 0-230 V

DC Input to impulse generator : 0-30KV

Wave front resistor : 45Ω per stage

No. of stages : 5

Wave tail resistance : 800Ω

Energy / Stage : 45 Joules

Resistance Divider : 1000: 1(kV to volt)

Capacitance / stage : 0.1μF

Wave shape : 1.2/50 μs

Loading capacitor : 2000pf

(Note: each stage consists of 2 capacitors of 0.2 μF each in series.)

CIRCUIT DIAREAM:



Grounding system SG1 SG2 SG3 SG4 SG5

Dischar resister

wave tail wave frontBleeding resistor

30-KV From Dimmer TRIGGER-GENERATOR

R0=20 KV (1mA)

PROCEDURE :

1. Check all the connections and grounding properly.2. Open the emergency switch and switch ON the control panel with the help of the Mains ON

push button.3. Then bring the dimmer to AERO position.4. Bring the sphere gap to zero position and set the gap of sphere with the help of the “sphere gap

increase” and “sphere gap decrease” push button.5. Open the earth with the help of “earth open” push button till the earth open.6. Select the polarity with the help “polarity” selector switch(say +ve polarity)7. Switch on the CRO and connect it to the socket provided8. Press the HT ON switch. As a result “HT ON” indicator will glow.9. Charge the capacitor little less than required level by increasing the DIMMER manually.10. Trigger the generator with the help of TRIGGER switch.11. After testing press the mains OFF switch, press the emergency button and ground all the

capacitors with the help of grounding rod.

RESULTS TO BE GENERATED: 1. Generate standard impulses with 10kv, 20kv and 30kv charging voltages2. Record the generated impulse on the CRO.3. Determine the peak of impulse voltage and impulse efficiency in each case.4. Repeat steps 1 to 3 for negative impulses.

PRECAUTIONS : 1. Charging voltage should never exceed 30kv.2. Open earthing rod before putting HT ON.3. Make sure the micro – switch of the door is properly closed. Other/ wise the HT will not work.4. After completion of experiments input power supply is switched off. All earthing connections

are to be checked. All capacitors are earthed using grounding rod.

TABULAR COLUMN:


K

V1mA

CRO

k1kmA

CRO

k1kmA


Sl.No

Impulse voltage(kV)

Efficiency(%)

Energy(kJ)

1 102 153 204 25

CALCULATIONS:

The efficiency of the impulse generator for each of the voltages generated is calculated using the equation,

Peak of impulse generated % Impulse efficiency = X 100 5 x dc charging voltage

Energy of the impulse = ½ CV2 = (1/2)(stage cap) (1/no.of stages used)(actual peak obtained)2

B) TO DETERMINE 50% PROBABILITY FLASH OVER VOLTAGE FOR AIR INSULATION SUBJECTED TO IMPULSE VOLTAGE.

AIM: - To determine the 50 % impulse breakdown voltage of point-plane and plane-plane gaps

APPARATUS: - Impulse generator (5 stages, 30kv/stage, 45 Joules/stage), sphear gaps and CRO.

INTRODUCTION : Breakdown under impulses is statistical in nature. That is, an impulse applied to an insulation “might or might not” cause breakdown. Thus impulse breakdown is associated with a probability. The probability is zero for all voltages below a certain value called the maximum voltage causing 0% breakdown. Likewise, the probability is 100 % for all voltages above a certain value called the minimum voltage causing 100% breakdown. The value of the voltage causing breakdown with a probability of 50% is called the 50 % impulse breakdown voltage. Unless otherwise stated, impulse bdv means 50 % bdv. The 50 % bdv is significant because the value is nearly equal to the ac breakdown voltage of a uniform field gap. It is used extensively in high voltage design work. A point plain gap is used because it represents a typical non-uniform gap occurring in engineering practice.

PROCEDURE :

1. Generate a standard impulse with 50% charging voltage, say, 15 kV and record the waveform on a CRO. Disconnect the CRO after recording

2. Connect the impulse generator to a point plane gap.3. Adjust the gap to 20mm. Set polarity to Positive.4. Apply 10 shots at an interval of 1 minute between shots of standard impulse generated by a

charging voltage Vc = 12kV = (2cm x 30kV)/5(no. of stages). This is based on the fact that the 50% impulse breakdown is approximately 30 kV/cm and that there are five stages.

5. If less than 5 shots causes breakdown of the gap in Step 4, apply 10 shots each with a charging voltage starting from 9 kV in steps of 1 kV up to the value required to cause breakdown of all the 10 shots.

6. If 6 or more shots cause breakdown in Step 4, apply 10 shots each with charging voltages from 7 kV in steps of 1 kV up to the value required to cause breakdown of all 10 shots.



7. Repeat steps 4 to 6 for -ve impulses. (polarity of the impulse is changed to –ve using reversal switch on DC supply.)

8. Repeat steps 4 to 6 for both +ve and -ve impulses for a 30mm gap, with the initial charging voltage of Vc = 18 kV = (3cm x 30 kV) / 5. In case of a 30 mm gap, the step 5 should be started with 13 kV and the step 6 with 10 kV.

TABULAR COLUMN:

DC ChargingVoltage (kV)

PeakImpulse

Voltage (kV)

Polarity IMPLSE NO %Breakdown1 2 3 4 5 6 7 8 9 10

V1

.

V2

Vn

Note: Indicate ‘X’ for chopped wave and ‘’ for full wave in the tabular column.

CALCULATIONS:

1. Draw graphs of Number of breakdowns for 10 shots (Y-axis) against impulse peak applied (X- axis) for all cases. These curves are called OGIVE curves.

2. Determine the 50% impulse breakdown for all the cases using the OGIVE curves as shown below.

100

% Breakdown 50

0 VA V50 VB PEAK OF IMPULSE APPLIED

VA = Max voltage causing 0% bdv. V50 = Min voltage causing 100% bdv VB = 50% bdv

(a) Determine V50, the charging voltage which causes an average of 5 breakdowns per set of 10 impulses applied consecutively. (b) Impulse breakdown Voltage =V50 x n x ηi Where, ηi = Impulse efficency = 0.95 (assumed)

n = No. of stages used = 5

QUESTION BANK



1. Conduct a suitable experiment to draw IDMT characteristics of the given non directional electromechanical over current relay choosing PSM = % and TSM= .

2. Obtain current- time characteristics for the given fuse wire of ratings 3A, 5A and 10A.

3. You are given fuse wires of ratings 3A, 5A and 10A. Conduct suitable experiments to determine their fusing factor and constant ، K’.

4. Obtain IDMT characteristics for the given microprocessor based over current relay choosing a plug setting of % and T.M.S of .

5. Operate the given microprocessor based over/under voltage relay as both over voltage and under-voltage relay to obtain their IDMT characteristics. Select suitable values for voltage settings and TMS.

6. Operate the given microprocessor based over/under voltage relay as both over voltage and under voltage relay to obtain their IDMT characteristics under the following settings.a) A voltage setting of 105% and TMS=1.0.b) A voltage setting of 110% and TMS=1.0.

7. Operate the given microprocessor based over/under voltage relay as both over voltage and under voltage relay to obtain their IDMT characteristics under the following settings.a) A voltage setting of 95% and TMS=0.8.b) A voltage setting of 90% and TMS=0.8.

8. Demonstrate the operation of differential relay for Merz-Prize protection scheme of the given transformer/ generator for the following internal faults.a) L-G fault b) L-L fault c) Symmetrical 3Φ fault.Also demonstrate that the scheme will remain inoperative for external faults given above.

9. Demonstrate the operation of differential relay for Merz Price protection scheme of the given transformer/ generator for the following internal faults.a) L-G fault b) L-L-G fault c) Symmetrical 3Φ fault.Also demonstrate that the scheme will remain inoperative for external faults given above.

10. Obtain the field plot of the given concentric U.G.cable model using the electrolytic tank. Using this calculate the capacitance and find the error in this value compared to the analytical value.

11. Determine the breakdown voltage of the voltage of the given transformer oil by conducting suitable experiment as per BIS 335 and hence find the breakdown strength of that oil.

12. Obtain the breakdown voltage v/s gap distance characteristics in air under the application of HVAC for (a) plane-plane electrode configuration, (b) point-plane electrode configuration.

13. Obtain the electric stress versus gap distance characteristics in air under the application of HVAC for plane-plane electrode configuration (neglecting fringing effects).

14. Obtain the electric stress versus gap distance characteristics in air under the application of HVDC for +ve polarity for plane-plane electrode configuration.



15. Obtain the electric stress versus gap distance characteristics in air under the application of HVDC for +ve polarity for (a) plane-plane electrode configuration, (b) point-plane electrode configuration.

16. Compare the breakdown voltages for a given gap distance under the application of +ve polarity of HVDC for (a) plane-plane electrode configuration, (b) point-plane electrode configuration (with plane being grounded).

17. Compare the breakdown voltages of (a) plane-plane and (b) point-plane gaps (with plane being grounded) under the application of HVAC for a given gap distance.

18. For a given 5 cm diameter sphere and a voltage of 28.9 kV at STP, find the gap distance from the relevant standard and using this gap distance conduct suitable experiment to find the breakdown voltage at RTP for HVAC.

19. For a given 5 cm diameter sphere and a voltage of 28.9 kV at STP, find the gap distance from the relevant standard and using this gap distance conduct suitable experiment to find the breakdown voltage at RTP for HVDC (+ve polarity).

20. Set up the impulse generator to develop a standard lightning impulse voltage. Draw the graph of the peak voltage generated v/s charging voltage in the range 10 to 24KV in steps of 2KV for positive / negative polarity.

21. You are given a parallel plate capacitor model and co-axial cable model. How do you processed to obtain the field lines and hence calculate the capacitances using electrolytic tank?.

22. Determine the 50% impulse breakdown voltage for a point plane gap set at 15mm, with polarity positive/ negative. Conduct breakdown with ac and compare the ac and impulse breakdown values.

23. Obtain the field plot of the given parallel plate capacitor model using the electrolytic tank. Using this calculate the capacitance and find the error in this value compared to the analytical value.

24. Conduct a suitable experiment using an electrolytic tank to obtain the field lines in the following model

a) Parallel plate capacitor b) Co-axial cable also, calculate the capacitance of the co-axial cable and identify the region of max. stress in the armature pole model.

VIVA QUESTIONS

1. Define fuse and explain its purpose in electric circuits.



2. Define (a) Rated carrying current (b) Fusing current (c) Fusing factor of fuse.3. Explain prospective current and cut-off current w. r. t. fuse.4. Show the typical characteristic of a 10A fuse and explain how the fusing factor can be obtained

from the same.5. Mention the materials of the fuse and their melting point.6. Mention the different types of fuses and their applications.7. Mention the advantages of fuse over circuit breakers.8. Mention the disadvantages of fuse.9. What are the basic requirements of protective relaying.10. Mention the classification of relays on the basis of

(a) Their applications (b) Principle of operation (c) Operating time.11. What are the main parts of electromechanical over current relay (induction type).12. What are the three different structures of induction type electromechanical relay.13. Define pickup and reset value of relay operating quality.14. How current transformer and potential transformer play their role in protection scheme.15. How the torque is developed on the disc in electromechanical induction type relay.16. How the differential over current relay differs in performance and construction with that of

non-directional one.17. Mention the application of directional and non-directional over current relays.18. What is meant by IDMT characteristic? Explain the same w.r.t. over current relays.19. Explain the typical characteristics of (a) Inverse (b) Very Inverse (c) Inverse Definite

Minimum Time (IDMT) (d) Definite Minimum Time (DMT).20. What is Merz Price differential protection? Mention its applicatons.21. What is percentage biased differential protection?22. Mention the advantages of static relays over the conventional electromechanical type.23. What are the advantages of numerical relay (Digital Relays) over the electromechanical relays.24. How the numerical relays differ in performance with that of electromechanical type?25. Explain PSM (Plug Setting Multiplier) and TMS (Time Multiplier Setting) with respect to

protective relaying.26. Mention different protection schemes for generator protection.27. What are the different settings that should be done for numerical over current and over voltage

relay?28. Mention the applications of under voltage relays.29. How do you classify high voltage with respect to tension on the line?30. In case of electrolytic tank how do you measure the capacitance of the given media?31. What do you understand by equipotential line?32. Why we cannot adopt the system with impulse applications in the laboratory?33. How does the breakdown occurs in the liquid? What conditions are adopted?34. Sphere gap alignment is done both in vertical as well as horizontal way? Which way is most

preferred? Why?35. What are the main conditions to be followed in the oil testing?36. When spark over characteristics has to be drawn we refer it w.r.t STP parameters? Why?37. What do you understanding by pitting? How it affects in case of sphere gap working?38. In case of breakdown in liquid, how does it occur?39. What are the requirements to be considered for the sphere gap activation?



40. What do you understand by stressed oil volume theory?41. What is bubble theory? How breakdown field strength does vary with the initial radius of the

bubble?42. What is Paschen’s law? How does breakdown in gas detines the relationship of V and PD?43. When do you say breakdown is related to corona discharges?44. Where exactly do you observe the condition of streamer breakdown?45. What do you understand by treeing and tracking?46. What is the capacity of steady potential at Van-de-Graff generator compared with the cascaded

transformer?47. Practically is it possible to develop the condition of high frequency with normal supply?48. In case of generation of high voltages wave tail and wave fronts are controlled by which

parameters present in the circuit?49. When and for what ratios of R1 & R2 switching surges occurs?50. CVT in a circuit where exactly we have to connect? Why it has to be done in that way?51. What is the other name of sphere gap?52. How does humidity effect the measurement of high voltage in the sphere gap?53. Why in a resistance divider circuit delay cable is been used?54. In case of impulse measurement in practical purpose resistance potential divider is used. Why?55. Where exactly Hall effect is adopted? What are the points to be remembered in calculating

“Hall-Coefficient”?56. In case of breakdown in solid for some reason thickness of solid insulation is increased. Why?57. On what frequencies thermal breakdown is very serious? Why?58. Gap length plays major role in breakdown of liquids. Explain.59. Define a standard lightning impulse60. Why are tolerances given for wavefront and wavetail61. How is an impulse formed from a step function62. What is a switching surges64. What is the significances of a step function65. What is 50% breakdown voltage66. Why is generally 50% breakdown is higher than ac breakdown67. What are the basic features of the impulse generator you have used68. Define impulse efficiency69. Always actual peak generated is less than nominal peak. Why?70. Explain the principle of operation of trigatron gap71. Explain the principle of operation of 3 electrode gap72. How are sphere gap used to calibrate an impulse generator73. Essential features of the CRO used to record impulses74. Impulse potential divider operating principles and compensation.


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