cee lab manual

Experiment No: 10 Date:

AIM : To measure distortion in current transformer due to saturation using MATLAB

Specific Objective:

After performing this experiment one should be able to:

measure distortion in current transformer due to saturation

Theory:

A current transformer (CT) is used to measure current in a shunt inductor connected on a 120 kV network. The CT is rated 2000 A / 5 A, 5 VA. The primary winding which consists of a single turn passing through the CT toroidal core is connected in series with the shunt inductor rated 69.3 Mvar, 69.3 kV (120kV/sqrt(3)), 1 kA rms. The secondary winding consisting of 1*2000/5 = 400 turns is short circuited through a 1 ohm load resistance. A voltage sensor connected at the secondary reads a voltage which should be proportional to the primary current. In steady state, the current flowing in the secondary is 1000*5/2000 = 2.5 A (2.5 V rms or 3.54 V peak read by the voltage measurement block V2).

Open the CT dialog box and observe how the CT parameters are specified. The CT is assumed to saturate at 10 pu and a simple 2 segment saturation characteristic is used.

The primary current reflected on the secondary and the voltage developed across the 1 ohm resistance are sent to trace 1 of the Scope block. The CT flux , measured by the Multimeter block is converted in pu and sent to trace 2.

(1 pu flux = 0.0125 V *sqrt(2)/ (2*pi*50) = 5.63e-5 V.s)

The switch connected in series with the CT secondary is normally closed.This switch will be used later to illustrate overvoltage produced when CT secondary is left open.

Sankalchand Patel College of Engg. VIII Electrical Commissioning of Electrical Equipments

Demonstration

1. Normal operationIn this test , the breaker is closed at a peak of source voltage (t = 1.25 cycle). This switching

produces no current asymmetry. Start the simulation and observe the CT primary current and secondary voltage (first trace of Scope block).As expected the the CT current and voltage are sinusoidal and the measurement error due to CT resistance and leakage reactance is not significant. The flux contains a DC component but it stays lower than the 10 pu saturation value.

2. CT saturation due to current asymmetryNow, change the breaker closing time in order to close at a voltage zero crossing. Use t = 1/50

s. This switching instant will now produce full current asymmetry in the shunt reactor. Restart the simulation. Observe that for the first 3 cycles, the flux stays lower than the saturation knee point (10 pu). The CT voltage output V2 then follows the primary current. However, after 3 cycles, the flux asymmetry produced by the primary current causes CT saturation, thus producing large distortion of CT secondary voltage.

3. Overvoltage due to CT secondary opening

Reprogram the primary breaker closing time at t = 1.25/50 s (no flux asymmetry) and change the secondary switch opening time to t = 0.1 s. Restart the simulation and observe the large overvoltage produced when the CT secondary is opened. The flux has a square wave shape chopped at +10 and -10 pu. Large dphi/dt produced at flux inversion generates high voltage spikes (250 V).

Conclusion:


Experiment No: 5Date:

AIM: To find out phase shift of Δ/Y transformer.

Apparatus:

Δ/Y Connected 3- ø transformer. Ammeter (0-10A) Voltmeter (0-500V) Wattmeter (0-1200 W)

Rationale:

The parallel operation of transformer is largely dependent upon polarity, phase sequence and phase–displacement of both the transformers. The correct selection and connecting up of transformers, which are going to operate in parallel can best be studied and to be checked with the use of vector diagrams. Vector diagrams are used to represent induced emf in transformer phase.

In 3- ø transformer, there is inherent angular displacement between DELTA voltage vector of the primary side and STAR induced voltage vector of secondary winding. In figure, where a Δ/Y transformer is shown and standard supply voltage having counter clockwise revolving vector is applied to primary. It is to be noted from vector diagram that secondary STAR voltage has 300 displacements in clockwise or negative direction w.r.t. DELTA vectors of applied voltage.

For 3- ø transformer having similarity in connections of primary and secondary windings like Y-Y or Δ/Δ, a reversal of polarity on either side of one transformer winding will make paralleling without an alteration of connections.

However, the transformers which are having different connections of primary and secondary windings like -Y, Y- transformers, the change of external connections to primary side affects both phase sequence and polarity of induced secondary vectors. Although it should be noted that a reversal of internal winding connections on either side i.e. on primary or on secondary side, produces a reversal of polarity without altering the phase sequence.

The property of -Y, Y- transformer also helps to connect the effect of wrong polarity resulting from a reversal of internal connections on primary and secondary side by changing in external connections and this make parallel operation without alternating to the internal connection. In the -Y, Y- the internal connections on the sides are such, which secondary vectors of the induced terminal voltage are displaced 300 in forward direction. This is not due to reversal of polarity but to a re-arrangement of winging so that in case primary side is -Δ, transformer A2-C1, B2-A1 and C2-B1 are joined, to form the terminal points instead of A2-B1,B2-C1 and C2-A1.

Figure shows a total 600 phase displacement between secondary induced terminal voltages of transformer. Make the connections as per circuit dia. Take the set of readings of ammeter, voltmeter, and wattmeter. Then find out PF and phase shift of any two phases.


Procedure:

To find out phase shift of the transformer winding, connect the ckt., as shown in fig. Now to find out PF of the ckt. We measure Voltage, Current and Power of the ckt. And then we can have cosø = W/ (V*I). Same way here phase displacement is nothing but angular difference in between the voltages of primary side and secondary side. So to measure that phase displacement we will take the voltage reference of primary side and current reference of secondary side and then we will measure the power. And putting the values in the equation we would have phase displacement,

Phase displacement = W2 / (V1*I2)

Where, W2 = Power consumption with connection as shown in fig.

V1 = Primary side voltage.

I2 = Secondary side current.

Circuit Diagram:

Observation:

Sr. No. PowerW2

Watts

VoltageV1

Volt

CurrentI2

Amp.

PhaseDisplacementW2 / (V1*I2)


Calculation:

Conclusion



AIM : To measure the dielectric strength of oil by 0-60kV oil test kit

Apparatus:

30kv oil testing set

THEORY:

The liquid tested is transformer oil. It should have a dielectric strength of at least 30 kV (rms) with a gap of 2.5mm. If this not obtained then the oil should be filtered and again tested filtration of oil is done either

(1) At 900 C at vacuum 2.67 kN/m2 (2) At room temperature at vacuum 0.07 kN/m2

Generally a sample of 300-400ml of transformer oil is taken. The electrode may be of brass, copper, bronze and stainless steel. Diameter should be 12.5 to 15mm. These should be immersed in oil to test at 40mm oil depth.

The voltage rise of the apparatus is 2kV/sec. the auto trip relay acts in 0.02 sec. after break down. Generally 6 reading are taken at 1 mm interval and the average of 6 is taken to give the dielectric strength of liquid.

The final report of the test must contain the following:1. Breakdown voltage obtained 2. Type of electrode 3. Frequency of test voltage 4. Oil temperature

Oil:

Petroleum based mineral insulating oil is used in transformer as coolant and as a dielectric medium. Periodic testing of transformer oil is necessary to ensure safe, economical, trouble free and undistributed power supply.

Deterioration of oil begins from the moment it is fill in the transformer due to again and oxidation. The oil produces undesirable products like acids, sledges, moisture etc.

Oil Deterioration:

The transformer oil is liable to deteriorate under normal operating conditions. In some application oil is in contact with air. It’s hence prone to oxidize accelerated by the presence of catalysts consequently the oil darken in colour and the acid in it begins to increase thereby increasing sludge and consequently causing other electrical properties such as dissipation factor tanδ to increase ultimately hindering the life of transformer.


Field Test:

Primary information about quality of insulation oil can be gathered on site as follows:

1. By conducting electric strength and acidity test kits are available commercially which are useful in evaluating the above said tests on site.

2. By visual examination of oil. i. Cloudiness in oil may be due to suspended particle, sediments of moisture.

ii. Dark brown color oil may indicate the presence of dissolve impurities. iii. Green color indicates the presence of copper compounds and rapid deterioration

of oil may be expected. iv. Acric acid smell indicates the presence of volatile acids, which may corrosion

information thus obtained, is recorded.

Procedure:

As per specification, handle the oil and place the test sample bowl, place it in the equipment.

Adjust the gap between electrodes to required value. Apply the high voltage across the electrodes and increase till break down occurs. The auto trip relay automatically trip supply with in 0.02 sec. of break down. Take 6 readings at interval of 10 min. each. In case bubble is formed, wait for 5 min

to take the reading. Take the average of all the reading to get the average break down voltage.

Observation:

1. Liquid material used: - _________________________________

2. Break down voltage: - ___________________ kV _________ mm gap of.

Conclusion:

Sankalchand Patel College of Engg. VIII Electrical Commissioning of Electrical

Equipments


Aim: To study method of measuring earthing resistance.

Specific Objective:After performing this experiment one should be able to: Know different methods of measuring earthing resistance.

Rationale:

Earthing is very important for electrical apparatus and the person, working with electricity, as far as safety is concerned. If an apparatus of some installation is not earthed and if a person happens to touch the metal case, he is likely to get an electrical shock, if the insulation between line path and the apparatus is not provided healthy. Hence in order to avoid such accidents it is always desirable to earth the body or non conducting metallic parts of electrical apparatus in use.

Direct measurement of earth resistance

The meggar earth tester is essentially a direct reading ohm meter and it is having a hand driven generator which supplies the testing current. The ohm meter consists of two coil (Current coil and potential coil) mounted at a fix angle to each other on a common axle. The current coil carries current proportional to current flowing in test ckt. Whereas potential coil is having potential proportional to the needle is proportional to the ratio of current in two coils. It gives resistance directly (Because potential coil acts as voltmeter and current coil acts as an ammeter). So D.C. is converted into A.C. R=V/I the hand operated generator produces D.C. current but to eliminate the effect of electrolytic, it is necessary to pass A.C. through the coil. So D.C. is converted into A.C. A synchronous rotary rectifier is also mounted on the same shaft of generator because ohm-meter is moving coil type instrument working on D.C. alone.

Method of use :

For measurement of earth resistance two different electrodes are inserted into ground at a distance of 25m and 12.5 minimum from earth electrode which is under test. Now the farthest end terminal is connected to C2 terminal of Earth Tester and nearer terminal is connected with the P2 terminal of Earth tester. Terminals P1 and C1 of the Earth tester are shorted together and are connected to earth electrode, which is under test.

The megger should be placed on a horizontal stand and should be free from the surrounding magnetic field. The range switch is set at the required position. The handle is then turned at a slightly higher than the rated speed and reading of the needle deflection is noted.

Two or three readings are taken by placing the electrode at other positions keeping the distance same as for the first reading. The average of this reading is the earth resistance which is found as ___________ ohm.

After having no of measurements, earthing resistance v/s distance graph is plotted as shown in fig (2). This graph shows the characteristic of soil or formation of the soil. As per the graphs, initially the earthing resistance raises with the distance because comparatively

Sankalchand PatelCollege of Engg. VIII Electrical Commissioning of Electrical Equipment

higher resistance is offered by nearby soil and the contact resistances of the electrodes. After that it slightly rises because long distance between the two electrodes and high soil resistance of long path.

Two point method:

This method is useful when another electrode is of known resistance. For measurement a known value, current I is allowed to flow and the potential between the electrodes measured. The I will give the combine resistance of two electrodes A and B. The value of earth resistance must be less than the prescribed value. Thus resistance of electrode is negligible then resistance of A= measured resistance.

Three point method:

In this method the two auxiliary electrodes B and C are used and it is necessary that their resistance be known. Three electrodes are placed at the corner of the triangle. The resistance between each other pair of electrode A and B is R1, R2 is that of between B and C and R3 is between A and C.

Resistance of electrode = (R1+R2+R3)/2

Fall of potential method:

In this method again the auxiliary electrodes are used, which are not placed at the corner of the triangle but in straight line. An alternating current I and C passed through A and C and the potential between A and B is measured. Thus the resistance of electrode A is R = E/I.

The resistance of (3 M) B and C does not affect the result. The electrode C must be placed at a sufficient distance from A so that resistance are A and C are quite independent. In order to know whether the electrode has been so placed or not the electrode placed at any distance and at earth resistance is measured by placing B at different points between A and C and curve of resistance along Y-axis and distance along X-axis is obtained. If the curve is similar to that of obtained it shows that area of A and C are not independent. If independent curve P is obtained earth resistance is determined by placing electrode B anywhere between neutral line X and Y for horizontal position.


Equipments

Fig-1

Fig-2



AIM : To find out number of poles of induction motor without energizing the motor.

Specific Objective:

After performing this experiment one should be able to: Find number of poles of induction motor without energizing the motor.

Apparatus :

Centre tap Galvanometer(0-3V) Connecting wires.

Rationale:

If 3-phase supply is given to induction motor it produces rotating magnetic field which rotates at synchronous speed. Rotor of this induction motor also rotates at a speed near to synchronous speed and motor gets magnetized. When supply is switched off, the residual magnetism remains present in the motor ckts. The no of poles of this no of poles for stator winding we can find by manually sweeping the stator winding. The oscillation of emf induced is equal to the no of pole pairs sweeping the stator winding. Because when the rotor completes one revolution as it is hand rotated, one complete a.c. cycle completes. So we get one revolution of galvanometer pointer.

Procedure:

The circuit is connected as shown in figure. The shaft is rotated at approximately 10 revolutions per minute by hand and counting the no. of oscillation made by galvanometer pointer. The no of pole pair is then given by

No. of pole pairs = No. of oscillationsNo. of revolutions

Multiplying the no. of pole pair by two will give the no of poles of induction motor.

Theory:

If 3-phase supply is given to induction motor , it produce rotating magnetic field which rotates at a speed near to synchronous speed & motor gets magnetized when supply is switch off. And the residual magnetism remains present in motor circuit. The no. of poles of this residual magnetism is equal to no of poles of rotating field. Now this no. of poles we can found by manually sweeping the stator winding. The oscillation of emf induced is equal to the no. of pole pairs sweeping the stator winding. Because when the rotor completes one revolution, one complete a.c cycle completes. So we get one revolution of galvanometer pointer.


Circuit Diagram:

Observation:

No. of oscillations No. of revolutions No. of pole pairs No. of pole

Calculation:

No. of pole pairs = No. of oscillations No. of revolutions

Conclusion :


Equipment

cee lab manual

Documents