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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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