By Bob Fesmire, ABB Inc.
In theory, a transmission system can carry power up to its thermal loading limit, but in practice a number of other constraints come into play before that limit is reached. System stability, voltage fluctuations and loop flows all conspire to keep a transmission line from performing at its best. Meanwhile, load characteristics are changing too, becoming more dynamic and producing greater voltage fluctuations (in part because of the proliferation of small AC motors like those used in air conditioners). Couple these physical limitations with the much discussed lack of investment in transmission and it’s easy to see how congestion and reliability concerns have reached the point where they are now.
The term Flexible AC Transmission Systems (FACTS) refers to a collection of technologies that have been in use serving various purposes for decades. Collectively, FACTS devices offer a fix to many of the issues transmission operators face today.
Series Capacitors-the Original FACTS Device
The oldest FACTS technology is the series capacitor, which has been used in various forms since the 1950s, primarily on the long AC transmission lines of the WECC. That’s because “series caps” provide a self-regulating source of reactive power that increases proportionally to the loading of the system, allowing those long transmission lines to carry more real power from plants serving load centers hundreds of miles away. Series capacitors (SCs) also provide voltage stability and damping of power oscillations, and they are used to control power flows to optimize the use of parallel paths.
There are relatively few SC installations in the East, primarily because the very characteristics that make them so useful in the West can become an economic liability when applied to a more meshed system of shorter lines. Generally speaking, when one transmission line is outfitted with SCs, all parallel paths need to be series compensated as well. Without this equal compensation, power will flow to the compensated line to the point of overloading it while the parallel paths remain underutilized. Historically, the cost associated with an effective series compensation effort in the East just didn’t make sense.
Series capacitor technology has advanced, however, and today the devices are being used all over the world. Thyristor-controlled SCs provide improved stability and damping capabilities and offer variable compensation that wasn’t possible with older designs. Still, in the developed grids of North America, most FACTS installations today utilize newer technologies. The two most common are static VAr compensators (SVCs) and voltage source-based static compensators (STATCOMs), both of which are being deployed to solve a wide range of problems.
SVC and STATCOM Basics
SVCs work as a shunt capacitor, typically using a transformer with the SVC operating at medium voltage. They can provide inductive or capacitive reactive power in equal or asymmetric amounts. Typically, SVCs are rated such that they are able to vary the system voltage by at least ±5 percent, which equates to a dynamic operating range of about 10 percent to 20 percent of short-circuit power at the point of common connection.
STATCOMs act as a constant current source and utilize voltage source converter technology together with a coupling transformer and controls. The voltage source converter is the key component: It converts DC voltage into a three-phase set of output voltages that have the desired amplitude, frequency and phase. Like SVCs, STATCOMs can produce or absorb reactive power, but unlike their FACTS family cousins they are inherently symmetrical in ratings for inductive and capacitive reactive power. To achieve an asymmetrical rating, STATCOMs must be augmented with an additional source of VArs such as mechanically switched shunt capacitors.
Both SVCs and STATCOMs respond rapidly and continuously to changes in power system operating conditions, but they each have their particular advantages. For example, STATCOMs contribute to voltage regulation more effectively than SVCs during undervoltage situations, while SVCs contribute more effectively during overvoltage situations. The STATCOM stability loop is also more robust than that of an SVC with respect to the variation of network capacity. Both technologies generate harmonics, so they both require the use of harmonic filters. The thyristor-controlled reactor (TCR) in an SVC is a source of harmonic current, mostly in the lower frequencies, and filter ratings are usually in the range of 25 percent to 50 percent of the TCR size. STATCOMs, meanwhile, are a source of harmonic voltage. Most of the harmonics are in higher frequencies, but the devices can generate harmonics in the same range as SVCs, depending on the particular design.
SVCs used to be more homogeneous, with a given design used for multiple applications. Today there is a greater degree of customization. There is also a good deal of variation when it comes to configuring an SVC or STATCOM for a particular application, and this creates a certain amount of ambiguity when comparing the two technologies head to head. For example, STATCOMs are generally thought of as being smaller than SVCs, and often they are. However, the electrical losses in STATCOMs are higher than those in SVCs. As a result, they require cooling equipment. At the same time, space-saving design tactics like stacking components and using vertical busbars can minimize an SVC installation’s footprint. A STATCOM also does not have short time overload capacity unless its power rating is de-rated initially.
So, it’s worth noting that there is some overlap in the applicability of these two devices. Obviously, the demands of a particular project will set the criteria for choosing one or the other.
In terms of cost, an SVC is presently less expensive than a comparable STATCOM, all things being equal, but again the particular requirements of a given project may indicate a preference for a STATCOM. Losses are not generally a major cost concern, provided the unit is operated near zero MVAr. In other cases, both SVCs and STATCOMs will incur losses. Both devices are considered essentially maintenance-free, requiring just one or two man-days of attention per year. Maintenance cost (or the lack thereof) has been a compelling consideration particularly for utilities faced with the prospect of operating older synchronous condensers, as we’ll see in a real-world example below.
In the field, SVCs and STATCOMs are being used today in a wide range of applications. These include:
- Dynamic voltage stabilization, where the device’s fast response smoothes out voltage variations and increases power transfer capacity;
- Steady-state voltage support, where bulk reactive power is supplied via mechanically switched capacitor banks or reactors that are controlled by an SVC;
- Synchronous stability improvements, where transient stability is increased along with improved power system damping;
- Dynamic load balancing; and,
- Power quality enhancement.
In addition, SVC characteristics at depressed voltage can be efficiently improved by adding an extra thyristor-switched capacitor that operates only during undervoltage conditions. The add-on TSC is typically rated between 50 percent and 100 percent of the SVC rating, and can be implemented without incurring additional costs in other parts of the system.
SVCs and STATCOMs are typically installed close to major load centers to mitigate grid disturbances for sensitive loads, or in critical substations to provide voltage regulation and dampening during the daily load cycle as well as during power swings or a major contingency. They may also be used at the infeeds to large industrial or traction loads, though these applications are not always undertaken by the utility.
As mentioned earlier, an increasingly common application for FACTS devices is the replacement of aging synchronous machines in growing urban areas. These projects often represent a confluence of operational, economic and environmental objectives and make a strong case for wider FACTS adoption.
The experience of one major West Coast utility is typical. One major urban center where the company operates is generation-poor and contains a significant amount of dynamic load. Synchronous condensers were used historically to provide the VArs needed for successful voltage recovery following faults, but these units were now more than 50 years old in some cases and were unreliable. Not surprisingly, maintenance costs were high and getting higher. Faced with growing load and a corresponding need for stronger voltage support, the company decided to replace the synchronous condensers with an SVC.
The synchronous condensers could have been upgraded, but the cost for this would have been comparable to that of a FACTS installation. Even with the upgrade, at least two of the six units would have had only 10 years of remaining life. However, there were even larger cost considerations in operations and maintenance. The synchronous condensers incurred losses five to 10 times that of the SVC, and they used a lot of water for evaporative cooling. That, combined with the chemicals used to clean the cooling system, made the units an environmental liability. The SVC, by comparison, uses a closed-loop cooling system that requires less than 10 gallons of make-up water per year.
Maintenance on the synchronous condensers would also have continued even after the upgrade. The ongoing costs were estimated at more than 10 times that of the SVC on an annual basis, and this did not include the collateral equipment such as breakers, oil pumps and controls that were similarly outdated and difficult to maintain. From a cost perspective, the SVC was the clear choice.
A similar rationale was used by a Western municipal utility when it faced a decision on what to do with an old generation plant that was now being used solely for dynamic voltage support. Such urban generators are typically less efficient and more expensive to run than their modern equivalents-especially in light of recent oil and gas prices-but they are kept going under reliability-must-run arrangements. Because of their location, the units are often considered a source of sight and noise pollution. To make matters worse, they must often run on the hottest days, thus exacerbating poor air quality on the worst days of the year. Given, too, the value of the land such generators occupy, it’s not surprising that utilities in urban areas have come under pressure to retire their older city-center plants. The problem then becomes one of how to meet the reliability needs of the local power system once the generator is removed.
Space was very limited at the muni’s site, and its proximity to an adjacent park made ambient noise a major concern as well. Also, magnetic fields created by conventional SVCs could potentially contribute to destructive forces in the rebar and oil tank beneath the site. For these reasons, the utility opted for a +/-100 MVAR STATCOM, along with three 31.2 MVAr capacitor banks controlled by the STATCOM.
In this case, the STATCOM option offered some key advantages over the SVC in terms of space, primarily because the main reactive elements are housed inside a two-story building. Ambient noise is reduced by the enclosure’s acoustic noise screening and the encapsulation of the step-up transformers with still more sound insulation.
As technology continues to advance, so, too, will FACTS devices continue to evolve. High-power thyristors and capacitors with greater power density, for example, will serve to reduce the size of SVCs. Already a new generation of FACTS, dubbed SVC Light, is pushing the technological boundaries. These devices are STATCOMs that use high-tech devices like insulated gate bipolar transistor (IGBT) valves and high-performance computer systems in place of conventional equipment. They are largely enclosed with only heat exchangers, commutation reactors and the power transformer located outdoors. The result is an even smaller footprint, extremely fast response time (governed only by the speed of the data processing), and no need for harmonic filters. The units even have the capability to mitigate harmonics on the system.
As greater demands are placed on the transmission system in terms of transfer capacity, and simultaneously greater expectations are imposed in terms of reliability, FACTS devices like SVCs and STATCOMs will play an important role. Their numerous applications, not to mention increasingly attractive economics, indicate even wider adoption of these technologies in the near future.
Bob Fesmire is a communications manager in ABB’s power technologies division and writes regularly on transmission and distribution, IT systems and other industry topics. The opinions expressed here are his own and do not necessarily represent those of ABB.