School of Electrical Engineering
and Telecommunications
Term 2, 2022
ELEC 3105 - Electrical Energy
Assignment
INTRODUCTION
Assignments must be submitted as a single (either .doc, .docx or .pdf file) in a properly formatted document.
An appropriately prepared document (using Word, LaTeX etc) is strongly suggested. Handwritten assign-
ments in a digital device are also acceptable but will have to be legible. Scanned copies or photographs
of A4 pages of handwritten solutions will not be accepted or marked.
This assignment contributes 13% toward your final marks.
The last day of submitting the assignment is 11.59pm, Sunday, 17th July 2022 via the Turnitin Assignment
submission link in Moodle:
https://moodle.telt.unsw.edu.au/mod/turnitintooltwo/view.php?id=3916487.
Late submission will not be permitted under any circumstances. Submission via email attachment will also
not be considered.
Since this assignment counts towards your formal assessment for this subject, you are required to sign a
declaration and attach it as a cover sheet to the front of your submission. Without this signed declaration,
the assignment will not be considered for assessment. A fillable pdf file for signed declaration can be found
in Moodle.
Note that plagiarism (i.e. copying of assignment or pay someone to do your assignment) is an academic
offence. If suspected of plagiarism, you may be referred to disciplinary board and will be subjected to heavy
penalties.
Detailed information can be found in https://student.unsw.edu.au/plagiarism
LEARNING OUTCOMES AND GRADUATE ATTRIBUTES
This assignment covers the first three topics of ELEC3105; Energy generation, Three-phase systems and
Transformers.
The key learning outcomes of the activity are:
• Develop the capacity to select, design and justify important power engineering devices for real-life
applications,
• Be able to apply underlying theories and concepts in the design process.
The key graduate attributes linked to the activity are:
• Analytical skills, critical thinking and creative problem solving.
• Demonstration of the understanding of principles.
• Effective use and communication of relevant information.
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ASSIGNMENT TASKS [TOTAL OF 100 MARKS + 11 BONUS MARKS]
It’s January 2023.
The crypto-crash of mid-2022, followed by the crypto-crash of late 2022 and the great iceberg lettuce
shortage means that previously "profitable mining and donkey NFT businesses" have now pitched to also
cultivate leafy green vegetables in greenhouses heated by the stacks of graphics cards.
You have been assigned by your electrical engineering services company to investigate the power supply
issues at one such facility, known as "Sneed’s Fine Mine, Dine & Wine" (formerly Chuck’s).
Part 1 [30 marks]:
The three-phase supply to the facility includes a transformer with the following information in its nameplate.
TRANSFORMER
Primary Phases 3
Secondary Phases 3
Rated Power 6 MVA
Primary Voltage 33 kV
Secondary Voltage 11 kV
Frequency 50 Hz
Impedance (%) XTx%
Vector Symbol Dyn-1
Oil Specs IEC-60296
Type of Cooling ONAF
Further to the nameplate, you have also been provided with the testing report of the transformer where
following parameters have been provided:
Series Resistance: RTx%; Magnetizing Impedance: Xm %; No-load losses: PNoLoad kW.
1. [3 marks] Calculate the parameters of the equivalent transformer circuit in Ω. Explain the steps in your
calculation.
2. [3 marks] Draw the equivalent per-phase circuit of the transformer in per-unit and in Ω referred to the
secondary.
3. [6 marks] Calculate the efficiency of the transformer when it supplies 60% of its rated power at unity
power factor.
4. [15 marks] Plot the efficiency of the transformer (using MATLAB, python/Jupyter, Excel etc), as a
function of the total load through the transformer from 5% to 110%∗ rated power (in MVA) at unity power
factor (in steps of 5%). In the same graph, plot the efficiency of the transformer at 0.6 lagging power
factor and at 0.6 leading power factor for the same power range. Compare the three cases and comment
on the results.
5. [3 marks] What is the meaning of the vector symbol Dyn-1 in the transformer? Draw the winding
connections of the transformer clearly showing the phase and line voltages of the primary and secondary.
What is the turns ratio relation for this transformer? What other phase-shift angles between primary and
secondary can this connection provide?
Your code for the calculation of Question 4 should be included in the Appendix of the report and submitted
as a separate file in Moodle.
∗ A transformer can operate at powers above its rated power for short time intervals. We call this "overloading"
of a transformer and how long we can operate the transformer at this condition depends on the previous load
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and the corresponding windings or oil temperature before overloading. Manufacturers will typically provide
instructions and information for the duration of overloading a transformer can handle.
Part 2 [37 marks]:
"Sneed’s Fine Mine, Dine & Wine" consists of three separate buildings which are all fed by the transformer
of part 1. The 33 kV supply substation connects to the primary of the transformer through a line with an
impedance of R1+ jX1 Ω and the 11 kV secondary of the transformer feeds all the loads through a cable
with an impedance of R2+ jX2 Ω.
The loads at the facility include:
1. Load 1: The mining graphics cards / heating elements: S1 MVA at a p f1 lagging power factor.
2. Load 2: Compressors and other electric machines: P2+ jQ2.
3. Load 3: Auxiliary loads of 500 kW at 0.85 lagging power factor.
1. [6 marks] Draw the equivalent circuit in per-unit. You can assume a simplified transformer equivalent
circuit. You may use per-unit impedances for the lines / transformers and per-unit power values for the
loads (AKA no need to calculate load impedance).
2. [9 marks] The rated voltage at the loads is 11 kV with a tolerance of ± 6%. Calculate the minimum and
maximum supply voltage from the 33 kV substation when all the loads are connected.
3. [6 marks] Draw the phasor diagrams for the minimum and maximum voltages from Question 2 (2 phasor
diagrams). Clearly define all voltages and currents based on the diagram of Question 1.
4. [9 marks] If the supply voltage is 33 kV, what is the voltage at the loads when :
1. All three loads are connected.
2. Loads 1 and 3 are connected.
3. Only Load 1 is connected.
For each case draw the corresponding phasor diagrams. Are the supply voltage limits of ±6% satisfied
at all conditions?
5. [7 marks] What is the value of the capacitors that need to be connected in each phase at the load side
to make the total power factor when all loads are connected equal to unity? What are the differences
between the possible connections of the capacitors?
Part 3 [33 marks]:
"Sneed’s Fine Mine, Dine & Wine" is part of "Umbrella Corporation", a global conglomerate with a substantial
carbon footprint. Local billionaire Mike Nikkon-Brookes has threatened with a hostile takeover unless 50% of
the energy consumed by each facility on a daily basis is generated by renewable energy resources (on a
nice sunny day at least).
1. [3 marks] Calculate the total energy consumption of your facility over one day and the amount of energy
that needs to be generated by renewables.
2. [6 marks] Explain why the five wind-turbines that your friend from high-school, Bruce, wants to sell you
for "mates rates", would not be sufficient, especially as "you can carry one of them in the back of his
ute".
3. [21 marks] Calculate the number of solar panels that you would require to satisfy the renewable energy
requirements of your facility. The PV panels will be connected to the grid through inverters which are
95% efficient, transformers which are 98% efficient and cables that have 1% losses.
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You can use any of the solar panels described in the following datasheet.
1. Solar Panel 1: https://moodle.telt.unsw.edu.au/mod/resource/view.php?id=4779571
Assume that the power output of each solar panel is directly proportional to irradiance and that the solar
irradiance in the location where you plan to install your solar farm is given in the following table (e.g.
from 7:30 am to 8:30 am the irradiance averages 200 W/m2):
Table 1 Daily Solar Irradiance
Time Irradiance (W/m2)
8 am, 5 pm 200
9 am, 4 pm 400
10 am, 3 pm 600
11 am, 2 pm 800
12 pm, 1 pm 1000
Explain all steps in your calculations, appropriately.
4. [3 marks] Draw a single-line diagram of "Sneed’s Fine Mine, Dine & Wine" including all components in
the system from Part 1 to Part 3.
Bonus Questions:
1. [4 marks] How can overloading of a transformer be useful? Are there any potential drawbacks?
2. [4 marks] Calculate the voltage regulation (up and down) of the transformer for an operating condition
of your choice.
3. [3 marks] Is ELEC3105 memeable? Have a go, class of 2022! (3 marks because most of you chose 3
at the random number generator...)
Your parameters for the assignment can be found in the Moodle quiz question here:
https://moodle.telt.unsw.edu.au/mod/quiz/view.php?id=4779588
In the end, the "power supply issues" were just possums that ate the insulation of the cables and caused a
short-circuit...
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