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Posts Tagged ‘FACTS’

Thyristor-Controlled Series Compensation (TCSC) is used in power systems to dynamically control the reactance of a transmission line in order to provide sufficient load compensation.  The benefits of TCSC are seen in its ability to control the amount of compensation of a transmission line, and in its ability to operate in different modes. These traits are very desirable  since loads are constantly changing and cannot always be predicted.

TCSC designs operate in the same way as Fixed Series Compensation, but provide variable control of the reactance absorbed by the capacitor device. The basic structure of a TCSC can be seen below:

tcsc-compensation

A thyristor-controlled series compensator is composed of a series capacitance which has a parallel branch including a thyristor-controlled reactor.

TCSC operates in different modes depending on when the thyristors for the inductive branch are triggered.  The modes of operation are as listed:

  • Blocking mode: Thyristor valve is always  off, opening inductive branch, and effectively causing the TCSC to operate as FSC
  • Bypass mode:  Thyristor valve is always on, causing TCSC to operate as capacitor and inductor in parallel, reducing current through TCSC
  • Capacitive boost mode: Forward voltage thyristor valve is triggered slightly before capacitor voltage crosses zero to allow current to flow through inductive branch, adding to capacitive current. This effectively increases the observed capacitance of the TCSC without requiring a larger capacitor within the TCSC.

Because of TCSC allowing different operating modes depending on system requirements, TCSC is desired for several reasons. In addition to all of the benefits of FSC, TCSC allows for increased compensation simply by using a different mode of operation, as well as limitation of line current in the event of a fault. A benefit of using TCSC is the damping of sub synchronous resonance
caused by torsional oscillations and inter-area oscillations.  The ability to dampen these oscillations is due to the control system controlling the compensator. This results in the ability to transfer more power, and the possibility of connecting the power systems of several areas over
long distances.

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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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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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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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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As energy demand continues to rise, cost-effective delivery of electric power becomes a daunting task.  Many jurisdictions responded by introducing legislation to privatize the power generation industry, so that large networks of systems with multiple different owners could share the load.  As remote power systems add more interconnections, new challenges are emerging related to overall power system stability, particularly in relation to distributed generation as with renewable power sources in the home.

Traditionally, engineers integrated power systems under the assumption that power consumption increases gradually such that operators can simply add generation capacity to meet demand and one can consider the system relatively unchanging with time.  Similarly, operators could compensate for changes in the overall power profile by adding inductors or capacitors to substations depending on the typical load.  For example, a substation supplying power to an industrial mill would consume reactive volt-amperes (VARs) so the local utility would add a capacitor to compensate for the inductive load, in order to preserve voltage regulation.  However, once the mill is no longer operating (for example, at night), the resulting reduction of load causes a rise in the supply voltage, which can be well above the desired voltage.

Flexible AC Transmission Systems (FACTS) are different because, as the name implies, they are flexible: designed to be dynamically adjusting to the power demand and other conditions of power quality.  A basic installation might consist of an operator- or microprocessor-controlled bank of capacitors can consume reactive power when necessary.  The state of the art is to provide continuous switching using power electronic devices, which have a much faster response time than a human operator or even a microprocessor-based control system.  Novel devices even filter harmonic oscillations, which can significantly reduce power flow and cause stability issues.

Optimum usage of current transmission line assets is the most cost effective option available and FACTS devices allow utilities to provide greater power delivery with better system stability and power quality.  It is often prohibitively expensive to build new power lines, so these devices provide a stopgap measure capable of delivering increased capacity while also reducing transmission losses.

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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After being introduced to the problem of power quality issues, one might wonder what the real implications are, particularly for residential consumers. This article explains what voltage fluctuations, harmonic oscillations and transients really are, and why they are important considerations for any Electrical Engineer.

These are closely related to a forthcoming article on Flexible AC Transmission Systems.

Voltage Fluctuations

When the receiving terminal of a transmission line operates a high-power load drawing a large amount of current, the system voltage tends to drop, leading to an undervoltage condition colloquially known as voltage sag.  This can have undesirable effects for consumers, since devices may malfunction and particularly sensitive equipment such as electronics may not work at all.  Factories and other industrial plants often consume large amounts of power, so they can cause frequent and prolonged periods of undervoltage if left unchecked.

Conversely, when the receiving end has a lower load than expected (known as load rejection), the voltage can exceed the nominal voltage by a significant margin.  This can happen when a large load is suddenly disconnected from the grid, such as when a factory’s circuit breaker trips.  During periods of low load on the transmission line, the line voltage increases along the length of the line due to the line charging capacitance.

Whatever the cause, installation of FACTS devices can correct voltage fluctuations without re-quiring manual intervention by agents of the power utility.  In fact, this is the primary function and advantage conferred by parallel (shunt) compensation devices and will be the topic of a future post.

Harmonics and Transients

Some loads, such as rectifiers, are non-linear in nature and can result in a distortion of the ideal sinusoidal waveform shape.  In other instances, disturbances such as lightning hitting power transmission lines or a sudden transient fault like a power line swaying in the wind and hitting a tree can interrupt power delivery if not detected and counteracted.  The problem of trees in transmission line paths is a particular concern because impacts with them can cause blackouts and complete system failures, as happened during the Northeast Blackout of 2003.

Harmonics in the power waveform can cause equipment damage or malfunction, and more importantly, can cause the power system to become unstable.  Thus, to raise the dynamic stability limit of the power system, devices must be in place to handle harmonic and transient disturbances. Luckily, craft engineers and scientists have developed a FACTS configuration useful for controlling these problems as well.

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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