Posted: September 23rd, 2015

Key skills to a successful engineering career.

Mixed Session: System Modeling – Model
Refinement
ENGG300 – Engineering Project Practice
Abstract— In this mixed session, you will develop one of the
models from the last session into a much more detailed model. In
engineering, often times ”back of the envelope” calculations can
go a very long way to aiding in the development of a project
plan, a technical approach, or the solution to the entire problem.
Your ability to quickly assess things like ”ROM” (rough order of
magnitude) estimates or feasibility studies are key skills to your
successful engineering career.
I. INTRODUCTION
The start of most engineering projects is really coming to
terms with the fundamental questions, ”Can we do this?” All
the detailed design, precision manufacturing, and painstaking
testing are for naught if the approach is fundamentally
unsound. But how do we keep from going down the wrong
path? How do we eliminate the blind alleyways? And, how do
we do this before wasting too much time and resource on it.
The answer is system modeling and refinement. In the
previous section, you saw that it is a relative simple matter
(you did it in under an hour) to use a spreadsheet to determine
whether a solution to a problem can be modeled as a linear
(aka proportional), exponential, or geometric system. Some
systems behave one way under one condition and another way
under other conditions–like the DRAM; for small DRAMs,
the decoding logic which scales proportionally dominates,
while for large ones, the RAM cells dominate geometrically.
In this mixed session, you will look at adding details to your
models. This is called model refinement, or just refinement.
The idea is that once you take a first look at a system, you can
consider it in ever deeper levels of detail. You need to get to a
level of detail that reflects where you are in the engineering
process. For requirements, this is answering the question
(with minimal risk!), ”Can we do this?”. For project planning,
you want to answer the question from the perspective of a
WBS or GANTT chart. For initial design, you will obviously
need to go deeper. At some point, this will mean moving
away from a spreadsheet to appropriate design tools.
II. BACKGROUND
In a typical town of 30,000 people, you will have a variety
of housing, business, and industry. Assume that you have one
”Westfields” similar to what is at the Macquarie Centre
serving this community. Also assume that you will have a
combination of houses and units providing living space. Also
assume, for the time being, that there is no industry here; it is
a ”bedroom” community and everyone works elsewhere.
You are to plan the electrical distribution system for this
time in terms of the number of poles, the km of wire, and the
number of transformers. Power for the town is distributed by
a substation at the edge of the town. You will need to use your
utility pole spacing from last time, but now you will need to
augment this with the following assumptions:
1. The Westfields is in the geographic town centre.
2. The town is a similar mix of housing that around the
Uni.
3. Three-phase power is routed directly from the
substation over 3 parallel wires and is then
”‘transformed”’ to two-phase power at a transformer
on a utility pole. The three-phase and two-phase can
be on the same pole, but must be on different wires.
4. Two-phase power can only be carried over a
maximum distance of 250m before it degrades.
5. Two-phase power can only deliver 2000 amps
6. Three-phase power has no limitation for your
purpose in terms of distance or amps delivered.
III. QUESTIONS
Answer each of these. A diagram, discussion, and
spreadsheet model together may be helpful for each answer.
1. How is your town arranged?
2. What sorts of electric power users/entities are you
going to consider in your model and how much
current does each entity in your town draw? Justify
this or provide a reference.
3. How do you plan on balancing two-phase vs. threephase
requirements?
4. Do you have enough information or what additional
assumptions are you going to make to determine
your wire, pole, and transformer needs?
5. Quantify your need for utility poles, wire, and
transformers based on your model.
6. How could your model be improved.
7. Discuss the impact on your system if you could
double the two-phase amperage delivery assumption,
but not the two-phase distance assumption. What
opportunities and limitations does this uncover?

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