Question

QUESTION ONE a. Show that in a star connected generator, the line-to-line voltage is related to the generated phase voltage by $V_{LL} = \sqrt{3}V_{PH}\angle(\theta_{PH} + 30^\circ)$ b. An unbalanced three phase star connected load of $Z_A = (80 + j50)\Omega$, $Z_B = (20 + j30)\Omega$ and $Z_C = (80 - j40)\Omega$, is supplied from a star connected, 3 wire, 240V, 50 Hz per phase source. Calculate the following: i. the line currents taken by each load ii. the voltages across each of the loads iii. the displacement neutral voltage iv. The real and reactive power drawn from the source and the power factor. Draw a phasor diagram showing, the source phase voltages, load voltage drops, the displacement voltage and the line currents QUESTION TWO a. From first principles, show that the instantaneous power in a balanced three phase network is not pulsating. What is the significance of the power not being pulsating? b. A 480 V source supplies 4 parallel connected balanced three phase loads: Load 1: 150kVA @ 0.8 pf leading Load 2: 100kW @ unity pf Load 3: 200kVA @ 0.6 pf lagging Load 4: 80 kW and 95 kVAr (inductive) i) Calculate the overall power factor of the loads as seen by the source ii) Calculate the amount of reactive power required to bring the overall power factor to unity iii) Calculate the capacitance value of the delta connected capacitor bank connected in parallel to the loads to achieve unity power factor.

          QUESTION ONE
a. Show that in a star connected generator, the line-to-line voltage is related to the generated
phase voltage by $V_{LL} = \sqrt{3}V_{PH}\angle(\theta_{PH} + 30^\circ)$
b. An unbalanced three phase star connected load of $Z_A = (80 + j50)\Omega$, $Z_B = (20 + j30)\Omega$
and $Z_C = (80 - j40)\Omega$, is supplied from a star connected, 3 wire, 240V, 50 Hz per phase
source. Calculate the following:
i.
the line currents taken by each load
ii.
the voltages across each of the loads
iii.
the displacement neutral voltage
iv.
The real and reactive power drawn from the source and the power factor.
Draw a phasor diagram showing, the source phase voltages, load voltage drops, the
displacement voltage and the line currents
QUESTION TWO
a. From first principles, show that the instantaneous power in a balanced three phase
network is not pulsating. What is the significance of the power not being pulsating?
b. A 480 V source supplies 4 parallel connected balanced three phase loads:
Load 1: 150kVA @ 0.8 pf leading
Load 2: 100kW @ unity pf
Load 3: 200kVA @ 0.6 pf lagging
Load 4: 80 kW and 95 kVAr (inductive)
i)
Calculate the overall power factor of the loads as seen by the source
ii)
Calculate the amount of reactive power required to bring the overall power factor
to unity
iii)
Calculate the capacitance value of the delta connected capacitor bank connected
in parallel to the loads to achieve unity power factor.

QUESTION ONE
a. Show that in a star connected generator, the line-to-line voltage is related to the generated
phase voltage by VLL = √(3)VPH∠(θPH + 30^∘)
b. An unbalanced three phase star connected load of ZA = (80 + j50)Ω, ZB = (20 + j30)Ω
and ZC = (80 - j40)Ω, is supplied from a star connected, 3 wire, 240V, 50 Hz per phase
source. Calculate the following:
i.
the line currents taken by each load
ii.
the voltages across each of the loads
iii.
the displacement neutral voltage
iv.
The real and reactive power drawn from the source and the power factor.
Draw a phasor diagram showing, the source phase voltages, load voltage drops, the
displacement voltage and the line currents
QUESTION TWO
a. From first principles, show that the instantaneous power in a balanced three phase
network is not pulsating. What is the significance of the power not being pulsating?
b. A 480 V source supplies 4 parallel connected balanced three phase loads:
Load 1: 150kVA @ 0.8 pf leading
Load 2: 100kW @ unity pf
Load 3: 200kVA @ 0.6 pf lagging
Load 4: 80 kW and 95 kVAr (inductive)
i)
Calculate the overall power factor of the loads as seen by the source
ii)
Calculate the amount of reactive power required to bring the overall power factor
to unity
iii)
Calculate the capacitance value of the delta connected capacitor bank connected
in parallel to the loads to achieve unity power factor.

Added by Jonathan A.

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