Abstract:

Teaching power system stability analysis and control in undergraduate Electrical Engineering programmes is a challenging task. One of the main difficulties is perhaps the fact that high-voltage transmission networks are complex systems that include several hundreds or even thousands of devices.

Before the diffusion of powerful workstations and personal computers, the simulation of large power systems was a cumbersome task mostly reserved to system operators. For this reason, since the beginning of ac power systems, i.e., in the early years of the twentieth century until recent years, researchers had the need to simplify (and often over-simplify) the models of power systems to be able to study the stability and control of power systems. For their simplicity, some of these models have also been and are still widely utilised not only in research papers but also in education.

This paper discusses the advantages and, more importantly, the drawbacks of using exclusively simplified model to explain power system dynamics, stability analysis and control. The advantages are clear: a simple model is easier to explain, include a small number of variables and, very often, stability criteria and control design can be solved analytically. Moreover, the small number of parameters and variables allows drawing simple diagrams and qualitative plots that help the students visualize and, hence, hopefully remember the concepts.

The simplifications that are introduced to reduce the equations to a minimum set are often over-simplifications which can lead to misinterpretations or, even, erroneous conclusions. A famous sentence often attributed to Albert Einstein is: “Everything should be made as simple as possible, but no simpler.” This concept applies very well to power system models. In particular, over-simplifications may prevent students to capture the idiosyncrasies of power system dynamic behavior. On the other hand, if complex models are utilised, the learning approach has to rely on software tools and computer-based simulations.

Three very common simplified models will be considered and the common misunderstandings that they originate will be duly discussed in the final paper, as follows:
(i) the single-machine infinite-bus model utilized to explain transient instability and the loss of synchronism of synchronous machines;
(ii) the single bus model utilized to explain primary frequency control; and
(iii) the DC power flow model, that is utilized in optimizations problems and, in particular, electricity markets.

Several examples based on the experience matured by the author in teaching the modules “Power System Dynamics and Control” and “Stability Analysis of Nonlinear Systems” at the University College Dublin will complete the paper. The example discuss the added value of utilising relatively complex models, e.g., the IEEE standard test systems for teaching and the issues encountered by the students when using computer tools for power system analysis.

Finally, the paper will also discuss the debate following of a panel session with title “Thinking Outside the "Black Box" – Analytical Foundations of Power System Research” hold at the 2019 IEEE PES General Meeting in Atlanta, that was aimed at discussing pros and cons of computer-based versus analytical approaches for the study of power systems. At this panel session, the author was the only supporter of the computer-based approach.

Keywords:

Power system analysis, power system stability analysis, power system control, modelling, electrical engineering.