FACTS will boost transmission capacity and flexibility

Transmission system needs vary widely around the world, and new technology is providing the flexibility to meet those varying demands

Kevin Dodman

European Editor

Power transmission system capacity utilization and flexibility are of growing importance, particularly in the developed world, where environmental concerns are limiting the construction of new lines. In developing countries, where power demand is steadily rising, there is an increasing need to link grid systems and transmit power over long distances to enable limited generating capacity to be used more effectively.

In both areas, Flexible AC Transmission System (FACTS) technology, which encompasses a number of thyristor-based power control systems, is set to be used more widely because of its ability both to increase the capacity of power lines and to improve transmission system flexibility.

FACTS technology

FACTS technology is already being used in the United States, and initial results indicate that it has potential for widespread use in both mature and developing markets.

Thyristor switches operate at speeds far in excess of their mechanical counterparts, so they can be used to control impedance, voltage, current and phase angles in ways that are not possible with traditional switching methods.

This improves flexibility by enabling more power to be carried by lines where there are concerns over loop flow or instability. It has been estimated that it may be possible to double the capacity of some crucial transmission corridors.

A capacity increase from 300 to 400 MW has already been achieved on the 230-kV link between Shiprock, N.M., USA, and Glen Canyon, Ariz., USA. The link uses advanced series compensation (ASC), a FACTS device combining thyristor-based, high-power electronics with conventional series capacitors.

For several years the link had been a serious constraint for the Western Area Power Administration, and the installation of a parallel 500-kV line had been considered. Environmental protests made this impossible, however, prompting the installation of ASC on the existing line. The ASC system was installed midway along the link, at the Kayenta substation, by the Siemens Power Transmission and Distribution Group of Erlangen, Germany, at a cost of around (US)$7 million.

Western is now able to transmit some of the 600 to 800 MW of electrical power generated by excess coal-fired capacity located east of Shiprock to customers in Arizona, USA, and California, USA, during peak periods. As a result, it is estimated that the ASC system will achieve a pay-back period of less than four years.

The introduction of ASC means the regional ties between power utilities and consumers that have existed up until now can begin to loosen. Unlike the situation with interconnected networks where the power stations provide a uniform level of power in accordance with a previously agreed schedule, ASC technology allows “channels” of power transport to be established along which electricity can flow from power station to consumer even when the two are separated by other network operators` regions. In Europe this is a particularly interesting concept because future legislation will permit and encourage the transmission of electricity over long distances.

Consequently, it is possible to imagine the exchange of power on an international scale, such as from Scandinavia to central and southern Europe, or the supply of power across time zones, to follow the east/west movement of peak loads each day.

If there is a suitable interconnected network already in existence, these moving geographic peaks of demand from, for instance, the Commonwealth of Independent States to Western Europe could be handled without having to upgrade the local power generating capacity, using only the existing high-voltage transmission lines. With the ASC system it would even be possible to bypass fully loaded branch lines in a network and divert the flow of power along alternative routes.

The environment would also benefit from these new projects because more efficient use of the transmission routes that already exist would save electricity and reduce the need for new lines.

The new technology would also enable a reassessment of many renewable energy sources that were previously regarded as uneconomic because they were too far from centers of population.

As explained by Ramey, Nelson, Bian and Lemak in their paper “Use of FACTS Power Flow Controller to Enhance Transmission Transfer Limits,”1 there are typically three types of transfer limits:

– thermal limits, based upon the loadings and thermal ampacities of the lines

– voltage limits, based on the ability of the system to maintain adequate voltage and

– transient stability limits, based upon the ability of generators to stay in synchronism after clearing a fault.

In 1978, the Electric Power Research Institute (EPRI) of the United States first demonstrated thyristor-based systems for voltage control and damping.2 In 1985, a system was installed to damp subsynchronous oscillations on a 500-kV line owned by Southern California Edison, and in 1991, the American Electric Power Service Corp. tested a thyristor-based switch in part of a series capacitor bank installed on a 345-kV line near Charleston, W.Va., USA.

This was followed in 1992 by the system installed by the Western Area Power Administration at the Kayenta substation, which uses technology similar to the Charleston installation.

Inverter-based FACTS

Also under development are inverter-based FACTS devices, or power flow controllers, that would give stability benefits because of their speed and could increase thermal loadings and improve voltage regulation by virtue of their flexibility. Instead of using thyristors to enhance the speed of switching passive components such as capacitors and transformers, these devices use gate turn-off thyristors configured in a three-phase inverter to generate a voltage waveform which can be placed in series with the transmission line to control loading or in shunt to control voltage.

The first demonstration of a large inverter-based shunt device, a +/- 100-MVA static condenser, or STATCON, has been installed on the Tennessee Valley Authority (TVA) 161-kV transmission system to control local system voltage. It was developed as a result of research and development collaboration between EPRI, the Westinghouse Science and Technology Center and TVA.

The system will regulate voltage on a 500-kV line (which tends to rise when loads are light) and two 161-kV lines, where voltage tends to sag under peak loads. Without the new system, the TVA might have had to construct another 161-kV line to provide an adequate operating margin for this area of the power system.

Ramey, Nelson, Bian and Lemak also describe further possible developments of the inverter-based controller principle, including the series power flow controller (SPFC) and the most comprehensive version, the unified power flow controller (UPFC).

Power flow controllers

An SFPC is an inverter-based device connected to the transmission system via a series insertion transformer instead of a shunt transformer. In addition to providing capacitive series compensation, the SPFC can decrease power transfer by behaving inductively. It can also reverse power flow by inserting a series voltage.

If the SPFC is provided with a means to interchange real power, the constraint that the inserted voltage must be in quadrature with the line current is removed. This changes the characteristics of the device, and if the real power interchange can be affected both to and from the power system, the device is known as a UPFC.

A UPFC-equipped transmission line would be able to carry a maximum of real power and a minimum of reactive power. This form of FACTS technology would be useful for application to lines that need to achieve the maximum possible rates of utilization.

Ramey, Nelson, Bian and Lemak compare the performance of a thyristor-controlled series capacitor to that of a UPFC. Figure 1a shows a typical 200-mile-long, single-conductor 230-kV line with midpoint reactive compensation transmitting power between two unit-voltage infinite buses with a fixed 15 degree phase angle difference.

Figures 1b to 1e demonstrate the effect of X/R ratio as a function of percent reactive compensation. X/R ratios of eight, 10 and 12 are considered; and for reference, the power transfer for a lossless line is also shown.

As shown in Figure 1b, maximum power transmission occurs at around 90-percent compensation in all three cases, but at levels of compensation above 70 percent, the line does not reasonably approximate a lossless line. The reason for the relatively poor performance of the actual line compared to a lossless line is that actual lines must circulate large amounts of reactive power to maintain terminal voltage at the required level, to overcome the line voltage drop principally due to resistance.

At 80 percent to 90 percent reactive compensation, receiving-end reactive flows begin to exceed real power flows and a similar phenomenon occurs at the sending end. If the reactive capability is not available as controlled shunt sources at the line terminals, the reactive power must be circulated through other lines in the system, which is likely to lead to poor voltage regulation.

A UPFC-equipped line is shown in Figure 2. In contrast with the variable capacitors, the UPFC has the capability to compensate for voltage drop due to resistance and thus to provide real power compensation. The use of a UPFC does not necessitate the transmission of large amounts of reactive power. In fact, it could control the line to transmit no reactive power. If it was placed at the sending end, it could be used to “pre-compensate” the line for real and reactive voltage drops, so the entire line capacity could be used for real power transfer.

Referring to Figure 2 again, any surplus or shortage of reactive power could be compensated for by the series and shunt inverters, which could produce or absorb reactive power as needed. Consequently, the UPFC could be operated so that the system supplying the sending end of the line supplies only real power to the line and only real power is transmitted to the receiving end.

To illustrate how it could work in practice, Figure 3a shows the variable portion of the capacitor bank in Figure 1a replaced with a UPFC at the sending end. Figures 3b and 3c compare the MW transferred to the receiving end, to line losses and line ratings, for an X/R ratio of 10.

Although the UPFC-controlled line is not lossless, it shares the characteristic with the lossless line in that it can transfer as many MW as the line has MVA of thermal capacity.

Cost comparison

To compare the relative costs of the variable capacitor scheme and a UPFC, Ramey, Nelson, Bian and Lemak make the following cost estimates:

– variable series capacitors: $40/controlled kVAR

– UPFC: $45/controlled kVA

– line loss: $1,000/kW

– residual line capacity: -$80/kVA.

Figure 4 shows the available control range for the two alternatives at 15 degrees terminal phase difference, while Figure 5 shows the real power transfer control range as a function of phase angle difference across the line.

The total evaluated costs for the variable capacitor scheme are more than double those for the UPFC scheme. It is therefore important to consider overall system performance and actual and anticipated system operating costs when evaluating different FACTS devices.

In this example, the difference in initial costs, which seems considerable when examined in isolation, is more than offset when overall system costs are taken into account.

The future

On the Shiprock to Glen Canyon link, ASC, the basic form of FACTS technology, is demonstrating the value of thyristor switching. STATCON, the next step in FACTS development, is also expected to show significant benefits, which should allow development of SPFC and UPFC systems, enabling power engineers to achieve the greatest possible capacity from transmission systems, thereby minimizing operating costs, reducing the need for new power lines and maximizing system flexibility.

Regional transmission needs

According to UK-based power consulting company Mott Ewbank Preece (MEP), the key drivers affecting power transmission are:

1. The rate of demand growth for electricity

Y technological developments, including FACTS technology

Y environmental and associated supply considerations

Y developments in deregulation and commercialization of the power sector

Y national and regional power strategies and

Y procurement trends.

2. Four distinct geographic segments of the market

Y mature and wealthy areas, such as North America, Western Europe and Japan

Y mature and poor regions, such as Eastern Europe

Y developing countries with good economic prospects, such as China, India and the Middle East and

Y developing areas with poor economic prospects, such as many African countries.

Mature and wealthy

No major growth is forecast in national transmission systems in wealthy, mature market regions. These areas have relatively stable populations and are concentrating on improving the economic efficiency of power supply. This is likely to be reinforced by the continuing economic development of service industries and the decline of the more energy-intensive areas of manufacturing.

In MEP?s view, areas of significant activity in the wealthy and mature markets will be:

Y regional interconnections

Y upgrading and rehabilitation of existing transmission systems

Y replacement of supervisory control and data acquisition (SCADA) systems with new technology and

Y foreign equipment supply into the Japanese market.

Mature and poor

With relatively stable populations and falling energy consumption per dollar of gross domestic product, electricity consumption in mature, poor markets is likely to fall. However, this trend will not be reinforced by a move toward service industries, because manufacturing is likely to maintain its position.

It is likely that the areas of significant activity will be:

Y transmission systems connecting new generating capacity with centers of use

Y replacement of outdated transmission systems and

Y installation of improved network control systems.

Developing countries

In those developing countries that are regarded as having good economic prospects, there is expected to be rapid growth in power demand, which will lead to expansion of national and regional transmission systems. Economic growth in these countries will encourage private investment and the principal areas of activity are likely to be:

Y new long-distance transmission systems

Y submarine cable interconnections and

Y upgrading existing transmission systems.

Conversely, activity in the poorer developing countries will be severely limited, because even though there is a desperate need for improvements in electricity supplies, poor economic performance will limit the availability of investment funds.

In some developing countries, even funding agencies such as the World Bank are withholding finance, often because of government failure to introduce true market pricing for electricity.

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Thyristor-based power control devices can squeeze more out of existing transmission systems.

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1 D.G. Ramey, R.J. Nelson, J. Bian and T.A. Lemak, “Use of FACTS Power Flow Controller to Enhance Transmission Transfer Limits.” Paper presented at American Power Conference, Chicago, Ill., USA, April 26, 1994.

2 Narain G. Hingorani and Karl E. Stahlkopf, “High-Power Electronics,” Scientific American, November 1993.