Archive for October, 2009

The main purpose of series compensation in power systems is to decrease the reactive impedance of the transmission line to reduce voltage drop over long distances and to reduce the Ferranti effect.  By adding series capacitors to the line, engineers can compensate for the physical inductance inherent in the transmission line.  The voltage drop across the line is reduced with more compensation, allowing more power to be received by the load for any given sending power.  Two main types of series compensation are fixed series compensation, and thyristor controlled series compensation, each with their own advantages.

Fixed Series Compensation

Fixed series compensation (FSC) of a line is desirable for power transmission due to the effects of line reactance modification.  By adding series capacitance, the reactive impedance of the line decreases, thus lowering the voltage drop across the transmission line.  This effect can be seen through the simplified power flow equation (see the post about Power Transfer) obtained by neglecting line resistance and line charging capacitance.

Line reactance is counteracted by a series capacitance, resulting in overall lower line impedance and a lower voltage drop across the line.

Simple Series Compensation Diagram

By adding the series capacitance, it can be seen that the receiving line end voltage will be closer to the sending line end voltage.  This decrease in voltage drop across the line allows more power to be transferred over the line for any given sending line end voltage.

The advantage to using FSC compared to thyristor controlled series compensation is price.  Usually FSC allows for a majority of compensation for a lower cost when compared to thyristor controlled series compensation. The following phasor diagram demonstrates the effect of series compensation:

Phasor Diagram of Fixed Series Compensation

This article was taken from the introduction of a report which was written by a partner and I, submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario.

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Recently, I completed my first “lab” for ECE4464: Power Systems II. Like ECE3333 (Power Systems I), this course is being taught by one of the most inspiring professors I have ever had, Prof. Rajiv Varma, Ph.D.

Using PowerWorld‘s Simulator software, we repeated one of our basic labs from ECE3333 as an introduction to computerized modelling of power systems. We connected a single synchronous machine to an infinite bus across a 600km, 1000MW-SIL power line.

It is simplest for me to just lift the objectives from my lab report:

In this lab, our objective is to simulate a simple single machine infinite-bus configuration using the PowerWorld Simulator software.  We design a local generator system (a synchronous generator) having a nominal generation capacity of 500MW and with no predefined peak generation (that is, the generator is modelled as having infinite generation capability).

In this manner, we can explore various phenomena like power transfer, power system stability and the effect of shunt compensation on the midline.  We model a 600km span of transmission line with a shunt compensation device installed at the midline (300km from both ends) and determine the stability limit with and without this compensation device enabled.

Please see the following images, which show the simulation being run in PowerWorld:

500 MegaWatt generation, no compensation

500 MegaWatt generation, no compensation

500 MegaWatt generation with Synchronous Condenser Compensation

500 MegaWatt generation with Synchronous Condenser Compensation

For my full report, see: Power Systems 4464 Lab 1 (PDF). Note that the synchronous condenser installed at the midline is a Switched Shunt Compensation unit. I thought the standard inductor/capacitor schematic symbol looked a little boring, so I overlaid a synchronous condenser on top of it.

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The development of high-power thyristors and Insulated Gate Bipolar Transistors (IGBT) enabled the cost-effective provision of FACTS devices.  The actual behaviour of these devices is beyond the scope of this article, but on a basic conceptual level, they are simply fast acting switches, controlled by some external means (a trigger).  Triggers can be either electric (a voltage applied at the gate terminal) or photonic (light), the latter of which is useful to isolate the control system electrically.

Thyristors can be conceptualized (and indeed, are drawn on schematic diagrams) as diodes with a switch (the gate voltage or photoelectric stimulus). In FACTS correction systems, whereby a thyristor should act as essentially a fast-acting switch, power needs to be transferred in both directions. Thyristors are also used in high-power rectifier circuits as well, particularly for High-Voltage Direct Current (HVDC) transmission.

How much faster is thyristor-based switching compared to mechanically-switched circuit breakers? Because thyristors are semiconductor devices, they can switch on the order of milliseconds. Conventional circuit breakers, on the other hand, take much longer to switch. They can switch in one or two cycles (of the 50-60Hz mains frequency), though for power system protection purposes, this is considered a rather slow switching speed.

The graph below illustrates the difference:

Thyristor Switching vs Mechanical Switching

Additionally, mechanically-switched capacitors do not have sufficient switching speed to support extremely rapid switching nor can they be realistically switched more than a few times per day.

We can see that advanced communications and control systems play an important role in flexible transmission and distribution systems.

This article was taken from the introduction of a report which was written by a partner and I, submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario.

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One of the most often overlooked–yet arguably most important–issues in software development is copyright and licensing of works. In particular, I will discuss how this affects the open source software community with relevance to the Debian project.

As with any artistic or creative works, software is protected by copyright and its use is often governed by some sort of license. Please note that I am not a lawyer and I am not qualified to give legal advice, so take my suggestions with a grain of salt and please do leave a comment if you know something that I don’t.

A license is a legal contract that permits end users use of software under agreed-upon guidelines. In the open source community, licenses protect the integrity of free software by ensuring that they continue to remain freely available. For example, the GNU General Public License (GPL) stipulates that any derivative works of GPL-licensed code must distribute source code back to the community, which enables a two-way sharing of information between the originating software developers and the others who benefit from their work. Other licenses, such as the BSD License, are more liberal and do not have this restriction, but do have a disclaimer of warranties which shields authors from unintended legal consequences of their work.

Though licensing is probably the most important document detailing the relationship of the supplier (software developer or team) and other users, it cannot mean anything without copyright. In general, it is most useful to provide a copyright statement somewhere in resulting packages. A copyright statement is what allows authors to assert a particular license in the first place.

Moreover, license terms can only be changed when all copyright holders agree to the change. Unless you are explicit in your copyright conditions in the beginning, this can lock your project in to an undesirable license.

To make matters even more complicated, the Berne Convention for the Protection of Literary and Artistic Works (or simply Berne Convention as it’s most often called) describes a mechanism by which copyright is automatically in force upon creation of a work, even if the author does not explicitly assert it. For software, this effectively means that anyone who contributes any code is automatically the copyright holder on their contribution, which means that things quickly get complicated when there are many authors and contributors involved.

In Debian, we cannot and do not distribute software without knowing copyright information (including years of copyright, names, e-mail addresses where people can be reached, or a web site in the case of an incorporated entity). This is pursuant to the Debian Free Software Guidelines (DFSG), which require that we distribute only “free” software in our main repository–it’s part of our Social Contract.

In this regard, I would make the following recommendations:

  1. When beginning any project (open source or not), include a copyright statement immediately. It will eventually become a force of habit; this is a very good thing, and will pay dividends in the future.
  2. Establish a policy whereby contributors are asked to assign you copyright of their work; make a note of this somewhere in your documentation. Better yet, if you are part of an incorporated entity, assign copyright to that entity.
  3. Be explicit about your licensing terms and make sure to include copies of the license with your software. This helps to resolve ambiguities where there are several derivatives of a license (occasionally, developers license software under the BSD License without specifying which version they mean)
  4. Be wary of the “Public Domain” — this is an even more contentious issue than choosing an appropriate license. It is probably preferable to use a non-restrictive license such as the aforementioned BSD License (and its variants) or the MIT/X11 license, which is even more permissive.

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The power flowing across an arbitrary power transmission line depends on the sending end voltage (V1), receiving end voltage (V2), line impedance (Z = R + jX) and the phase difference between the sending and receiving terminals.  Since the line resistance (R) is usually very small compared to its reactance (X), that term is negligible in the Power Transfer Equation (Equation 1, below).

Basic Power System Equations

The second equation indicates that the power factor (the cosine of the angle between current and voltage) has a direct impact on total power transfer. Power factor is somewhat like a measure of efficiency: it’s the ratio of real power delivered to your circuit compared to “apparent power”, which is temporarily absorbed by reactive elements like capacitors and inductors (also called reactors) and then returned to the generator.  This is because these are energy storage devices.

Lastly, equation 3 indicates the amount of thermal power lost dissipated to the resistance of the power line. It is converted to heat at a rate proportional to the apparent power flow, which is why a poor power factor is particularly bad for the grid — it means more power is lost due to heat in the transmission line. Since current results in heating of the line, excessive current will cause the line to physically droop. It becomes dangerous because it increases the likelihood that the line will contact another phase (line-to-line fault) or ground (via a tree or house, for example).

As a result, power factor (and subsequently, apparent power flow) affect the power system’s economic viability and profitability.

This article was taken from a report which I co-authored. It was submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario in Spring 2009.

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The acronym “FACTS” is a blanket term used to describe a family of power electronic devices that can improve power quality and transmission capability of existing power transmission systems, without significant investment in infrastructure.  Many of the most important issues in power engineering are simple to understand but complex to address, particularly since the system operates on a much larger scale than the simplified models studied in the classroom.

While initial investment in FACTS devices can number in the millions of US dollars due to their sheer size, the payback time is usually short due to the cost savings they can provide.  For the utility, installation of FACTS devices to increase utilization of existing transmission line assets is preferable to the planning and deployment of additional lines, since the cost of building a new transmission line can be in the range of millions of dollars per kilometre and take several years to complete.  Power utilities and consumers can realize the benefits of FACTS much more quickly, since planning through to deployment and testing only takes about a year.

In essence, FACTS devices can increase the efficiency of transmission lines up to their thermal limit, which can increase maximum power transfer from 50% to 100%.  Evidently, the time and cost benefits are substantial, however, once transmission lines reach their thermal limit, new lines must be constructed.  FACTS devices cannot offer any benefit once a line is operating at maximum efficiency and has reached the thermal limit.

This article was taken from a report which I co-authored. It was submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario in Spring 2009.

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