EE 554 Power Electronics and Machines
Assignment 1
Submission date: 22nd June 2021
Chapter1: Power Electronics
1.
A six pulse fully controlled rectifier is fed from a 3-phase supply through finite
supply inductance.
(a)
Assume for this part of the question, that the circuit operates with a firing
delay angle () of 300, an overlay angle (μ) of 150 and that the output
current is smooth. On one of the waveform templates provided, and
assuming phase sequence A-B-C, label the supply phase voltages (VAN,
VBN, VCN). Draw the waveforms VXN, VYN and VXY and, using the thyristor
numbering T1, T3, T5 and T4, T6, T2 for the top and bottom devices
respectively, clearly indicate the conduction period for each thyristor.
[9 marks]
(b)
A 3-phase fully controlled bridge rectifier is fed from a 415 V, 50 Hz supply
through line reactors of 0.3 mH per-phase that have negligible resistance.
The rectifier feeds a load through a smoothing inductor that has negligible
resistance. At a particular operating condition, the rectifier delay angle is
450 and the load current is 300 A. Assume that the load voltage is smooth.
i) Determine the load voltage and the rectifier overlap angle.
[2 marks]
ii) On a second waveform template, draw the rectifier output voltage and
the load voltage and indicate the voltage time area appearing across
the smoothing inductor.
[4 marks]
iii) Determine the smoothing inductance required to limit the peak-topeak load current ripple to 20 A at this operating condition.
[5
marks]
You may use the following results, where the symbols have their normal meaning
without deriving them:
3√3𝐸
3𝐼 ℎ𝜔
𝑉̅ = 𝜋 cos(𝛼) − 𝑑𝜋 ,
2𝐼𝑑 ℎ =
√3𝐸
𝜔
{cos(𝛼) − cos(𝛼 + 𝜇)}
2.
(a)
Draw a diagram showing how an H-bridge can be arranged to give unity
displacement factor single phase AC-DC conversion with bidirectional
power flow capability. Draw a phasor diagram for the fundamental
frequency quantities illustrating how the circuit operates with both
directions of power flow. Illustrate how a control scheme that aims to keep
the dc link voltage constant, whilst maintaining unity displacement factor,
could be arranged based on these phasor diagrams.
[10 marks]
(b) A circuit such as that considered in part (a) is operated with a DC link
voltage of 500 V and fed from a 240 V, 50 Hz supply of negligible
impedance. The DC link power is 10 kW and a control loop ensures unity
displacement factor at the ac supply. A high frequency PWM scheme is used
to control the bridge and consequently the effects of switching harmonics
may be neglected. The modulation index of the PWM is 0.716. Determine:
i)
the rms ac supply current,
[1 mark]
ii)
the fundamental (50 Hz) magnitude of the H-bridge input voltage and
its phase shift with respect to the supply voltage,
[4 marks]
iii) the inductance of the ac side inductor, assuming its resistance is
negligible,
[2 marks]
iv) the value of DC side capacitance required to limit the 100 Hz
component of the ripple voltage on the DC side to 20 V peak to peak.
[3 marks]
3.
(a) With the aid of circuit diagrams, show how 3-phase multi-level inverters
capable of producing 9-level line-to-line voltage waveform at the load can
be arranged using the “diode clamped” topology and the “flying capacitor”
topology. For each circuit, label the devices to indicate which are operated
with complimentary ON/OFF signals (for example A and NOT A). It is only
necessary to draw one of the 3 inverter legs in detail for each circuit.
[8 marks]
(b) List the advantages and disadvantages of each circuit, taking into account
the problems of extending each circuit to produce more levels. [6 marks]
(c)
For the diode clamped topology considered in part (a), list the allowable
switching patterns (using the same device labelling as before), and indicate
the voltage produced between the leg output and the centre point of the
DC link for each switching pattern. For the switching pattern that produces
zero voltage between the leg output and the DC link centre point, illustrate
the current path taken through the converter for each direction of load
current (redraw the inverter leg if necessary, to make this clear). [6 marks]
4.
(a) A Unified Power Flow Controller of 1 kV maximum controllable voltage is
attached to the supply side of a lossless 33 kV 3-phase transmission line of
reactive impedance 120 ohms/phase and rating of 3 MVA. Describe the
maximum controllable real and reactive power for the transmission system
and draw the operating region about the maximum power point of an
uncompensated line.
[14 marks]
(b) What are the main advantages of a unified power flow controller compared
with a static synchronous series compensator or GTO Thyristor controlled
series compensator and a phase angle controller?
[6 marks]