ELECTRICAL POWER ASSIGNMENT- 1

ELECTRICAL POWER ASSIGNMENT- 1
BTEC HIGHER NATIONAL DIPLOMA SELVARAJAN PALANIAPPAN

UNIT TITLE ELECTRICAL POWER
SUB – TITLE Three phase systems
ASSIGNMENT – 1

Question – 1 (outcome 1.1, M1)
An unbalanced star-connected load shown in Figure-1 has branch impedances of
ZR = 10∠30oΩ, Zy = 20 ∠45°Ω, and ZB = 20 ∠60°Ω and is connected across a balanced 3-
phase, 3-wire supply of 440V. Find the following parameters:
(i) Line currents, and
(ii) Voltage across each impedance.
i.
R
ZR
ZB
ZY
B Y
Figure – 1
ELECTRICAL POWER ASSIGNMENT- 1
BTEC HIGHER NATIONAL DIPLOMA SELVARAJAN PALANIAPPAN
Question – 2 (outcome 1.2, D1)
a) Calculate the readings of the two wattmeter’s with current coil in line R and B
Connected to measure the total power for a balanced delta-connected load, fed from
a three phase, 200 V balanced supply with phase sequence as R-Y-B. The load
impedance per phase is (14-j14) Ω. Also draw the circuit.
b) The readings of the two wattmeter’s connected to measure a 3-Phase power is 8KW, each when the
power factor is factor is unity. What will be the reading of the wattmeter’s if the power factor falls to (i)
0.7 lagging (ii) 0.5 lagging and (iii) 0.4 lagging, the total 3 Phase power remaining unaltered.
ELECTRICAL POWER ASSIGNMENT- 1
BTEC HIGHER NATIONAL DIPLOMA SELVARAJAN PALANIAPPAN
Question – 3 (outcome 1.3, M2)
a) In a 3-phase 4-wire system, the currents in R, Y and B lines under abnormal
conditions of loading are as under :IR = 150∠30° A, IY = 250∠150° A, and IB =150∠30°
A. Calculate the zero, positive and negative phase sequence currents in the R line and
return current in the neutral connection.
b) A 50 MVA, 11KV, 3-phase alternator was subjected to different types of faults. The
fault currents were: 3-Phase fault = 2000A, Line -to-Line fault 2600 A and Line-toground fault 4200A. The generator neutral is solidly grounded. Find the values of three
sequence reactances of the alternator. Ignore resistances.
ELECTRICAL POWER ASSIGNMENT- 1
BTEC HIGHER NATIONAL DIPLOMA SELVARAJAN PALANIAPPAN
Question – 4 (outcome 1.4, D3)
a) Describe the constructional details of the three phase core and shell type
transformers.
b) What are the various methods of there phase transformer connections? Describe
any three types of connections in detail with diagrams, and state their advantages,
disadvantages and applications.
ELECTRICAL POWER ASSIGNMENT- 1
BTEC HIGHER NATIONAL DIPLOMA SELVARAJAN PALANIAPPAN
Question – 5 (outcome 1.5, D2)
Sketch and explain the typical torque-slip characteristics of a there phase induction
motor and explain the effect of change in the rotor resistance on the torque-slip
characteristics.
ELECTRICAL POWER ASSIGNMENT- 1
BTEC HIGHER NATIONAL DIPLOMA SELVARAJAN PALANIAPPAN
Question – 6 (outcome 1.5, D2)
Explain the process of crawling in three phase induction motor.

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