Avoiding Grid Meltdown

It was the scapegoat for much of last year's energy crisis. The nation's electrical grid got a lot of the blame when California neighborhoods went dark and East Coast residents braced for similar experiences. There was enough electrical power to prevent these calamities, it seemed, but too far away to do any good. The electrical grid was overloaded to a point that prevented distant energy suppliers from getting power to where it was needed.

In the era of deregulated utilities, conventional wisdom is that there's more money in generating power than in owning transmission lines. That is one reason North America in 2011 is expected to have just 4% more high-voltage lines than exist today.

This economic reality is motivating utilities and grid operators to get more out of existing transmission lines. The key technology to do this is a new class of solid-state power controllers pioneered by the Electric Power Research Institute. Headquartered in Palo Alto, Calif., EPRI is considered the premier collaborative science and technology development organization for the power industry, working with firms in the U.S. and 40 other countries. Over the years EPRI and its collaborators have devised innovative technologies ranging from clean-burning gas generators to variable-speed wind turbines.

Work in solid-state power controllers was sponsored by EPRI in collaboration with Westinghouse Electric Corp. in Pittsburgh (now Siemens Corp.), General Electric, and ABB.

Power plays
During last summer's energy problems, much was made about the nation's energy grid consisting of three distinct sections. These sections connect together at only a handful of points. The relatively few connection points create difficulties when large amounts of power must go from one grid to another: Grid connections may become choke points during periods of peak demands.

There are historical and regulatory, as well as technological, reasons why the Eastern, Western, and Texas grids are largely selfcontained. Utilities initially built transmission lines from their generating plants to customers. As populations sprawled, transmission networks expanded as well. Eventually utilities added a few connection points between grids for the sake of reliability and to potentially lower operating costs.

However, stability becomes more of a problem as grids grow large and contain multiple generators. When there's a fault on one transmission line, the generator closest to it supplies most of the fault current. Others supply current inversely proportional to their distance from the problem.

The sudden load on the generators makes them slow down, but not equally. The nearest generator slows most, putting it out of sync with others on the grid. As it comes back up to speed, rotors on other generators slow as their load is reduced by the slowest generator picking up load. The result is a rocking effect between rotors. If the fault persists, generators acting to pick up the resulting load will fall out of sync in succession.

For such reasons, grids contain intermediate switching stations every 100 miles or so that can disconnect transmission lines in case of faults. These stations also generally contain banks of capacitors used to compensate for line inductance and to smooth out ripples caused by rocking. Even so, it becomes difficult to assure stability for heavily loaded grids. One result is that grid operators must keep grid loads below peak capacity with enough cushion to ensure they can recover from temporary overloads.

An EPRI program called Facts, for flexible ac transmission systems, among other things aims to increase the capacity of existing grid connections by lessening instabilities caused by faults. Facts devices accomplish this via solid-state switching circuits that are fast and able to work at high voltages.

"Facts technology effectively transforms the grid into an active element," explains Dr. Abdel-Aty Edris, EPRI technical leader for transmission and substation asset utilization.

One category of Facts devices in particular has received much attention with regard to boosting grid transmission capability. These devices combine a power converter circuit with a dc storage element (usually a capacitor). They inject either voltage or current into the transmission line to control power flows on transmission systems.

The point of this configuration is that it can be made to behave like a capacitor or inductor of varying values simply by controlling the firing angle of solid-state switches. One such device installed for the Bonneville Power Administration helps mitigate power oscillation problems caused by generator rocking.

"It can detect a major disturbance and act within a single ac cycle," says EPRI's Edris, who headed up the research institute's Facts program. "By compensating for line impedance in a continuous way rather than with discrete switching, it more quickly damps oscillations to let transmission lines run closer to their thermal limits and carry more power."

Estimates vary as to just how much more power Facts-equipped transmission systems can carry. But improvements in the range of 20 to 40% are possible from better control of transmission system dynamics and flow control, says Edris.

Today there are only a handful of Facts controllers installed in the U.S. grid. Indications are that widespread deployment would help solve a variety of energy delivery problems. But EPRI admits it's tough to even get a handle on how big the Facts market might be in the world of deregulated energy.

Get in sync
It is interesting to examine the operation of one Facts device, the SSSC or static synchronous series compensator, to see how Facts circuits manipulate line parameters.

The SSSC has at its heart a circuit called a solid-state synchronous voltage source. It is built with an array of gate turn-off solid-state switches arranged to form an inverter. This circuit produces a set of three nearly sinusoidal voltages at line frequency with controllable amplitude and phase angle. A connection to a power storage device also lets it exchange real and reactive power with the transmission system.

When connected to a capacitor, the SSSC effectively functions as either a capacitor or reactor in series with the transmission line. It is viewed as a means of decreasing or increasing the effective line impedance.

An analysis of phase-angle relationships on the circuit elements that characterize the transmission line reveals a key behavior. A compensating capacitance in series with the line increases the voltage across the impedance of the line. This boosts line current and transmitted power.

In a similar manner, the synchronous ac voltage source can boost line voltage by providing an output that precisely matches the voltage of a series capacitor. But where a capacitor can only increase transmittable power, the SSSC can also decrease it by adjusting the polarity of injected voltage. Moreover, controlling the angular position of injected voltage with respect to line current lets the SSSC exchange both real and reactive power with the ac system.

The ability to exchange real power lets the device simultaneously compensate both reactive and resistive components of the series line impedance. For example, by providing a component of voltage out of phase with that developed across the line resistance, the SSSC can counteract the effect of resistive voltage drop on the transmission line.

It is also interesting to compare the SSSC to conventional thyristor-switched devices, older technology normally used to provide series compensation. These devices attach directly to the transmission line. That means they must reside on a high-voltage platform, employ a cooling system, and use components with enough electrical insulation to withstand hundreds of kilovolts.

In contrast, an SSSC couples into the transmission line via a coupling transformer. The circuitry sits in a building at ground potential and works at less than 20 kV, considered low in the world of transmission lines. This lessens the need for electrical insulation. And the infrastructure between the power circuitry and controls is relatively inexpensive. Experts say the SSSC's less-complicated operating logistics make it cost competitive with older systems.

Load'em up
Where the SSSC injects voltage to adjust transmission line impedance, a related Facts device called a static synchronous compensator, or Statcom, uses a sophisticated voltage source to inject current into the transmission line. The injected current controls transmission line voltage. As with the SSSC, properly controlling firing angles of gate turn-off solidstate switches lets the device absorb or inject reactive power. The point is to compensate for voltage sags or peaks that would otherwise arise when big loads go on and off the line.

A third Facts device called a unified power flow controller is basically a Statcom/SSSC combo which can together control voltage, impedance, and phase angle selectively or concurrently. The beauty of a UPFC is that it lets operators control how much power flows through any specific transmission line. This, in turn, permits selectively unloading lines that are near their limits.

The device can also change from one compensation mode to another in real time as, for example, from series reactance to phase-angle control. Power engineers say this will be important in the future for handling system and equipment changes within interconnected power systems. Finally, UPFCs will help operators expand power systems without altering hardware.

A review of deregulation economics and basic physics of power transmission helps explain why the UPFC is important.

Power flows from where it is produced to its destination along the path of least impedance. In other words, if two lines carry power from a given generator, Kirchhoff's Laws prevail. Power will divide between the two lines in inverse proportion to their electrical impedances. If the two lines are not identical, more power flows over the one with the least impedance.

This basic physical law becomes more important as power travels to loads ever farther away. Without a UPFC, line impedances dictate the route taken; it may end up being quite indirect. One result of this behavior is that the capacity of the route with the least impedance determines how much power can flow between two points.

Specifically, the thermal capacity of the low-impedance route is the limiting factor. This is the point where the wire overheats and begins to sag, perhaps shorting out into trees or other obstacles.

Grid owners monitor the amount of power flowing through each of their lines and transformers, calculating real and reactive components along with voltages at each node (bus voltages). When asked to provide additional power, they weigh requests in light of existing load factors. They request load relief from the relevant authority if they detect a potential overload.

Not surprisingly, line capacity impacts prices for electricity. Most markets are organized such that spot prices can be set at every node in the system. In the absence of overloads, spot prices are roughly equal throughout a given market. But without a UPFC managing things, an overloaded line or transformer can force a move from the most economical generator to a more expensive one somewhere else on the grid, simply to relieve an overstressed link.

Similarly, UPFCs promise to prevent a few overloaded lines from separating markets. Otherwise chokepoints of this nature introduce a potential for abusive power pricing, depending on how jurisdictions allow prices to be set.

Power play
One other interesting development created under the auspices of EPRI is new technology for back-to-back converters. It is a means of connecting two different grids that otherwise are incompatible with each other — as, for example, grids operating asynchronously where one has stability problems that must be isolated from the second. The basic idea is to convert power from one grid to produce high-voltage dc, pump the dc to the other grid, then invert back to high-voltage sine waves via sophisticated inverters.

For years, back-to-back converters have relied on ordinary thyristors as the key switching elements in rectifiers and inverters used for this translation. But there are operating qualities associated with thyristors that limit under what conditions such conversion can take place.

One dictate is that both grids must constantly have enough power available to support thyristor commutation. (As a quick review, thyristors begin conduction in response to a pulse on their gate. Once they start conducting current, they can only be turned off by reducing the principal current to near zero, below a holdingcurrent level.)

Commutation problems also can arise if, among other things, one grid has stability problems. This is more likely in grids experiencing super-heavy demand.

Solid-state switches created via EPRI programs help address commutation problems. The solid-state switches that came out of this work are used not only in back-to-back converters but also in other Facts power control systems.

Though gate-turn-off switches were first developed in the 1980s, it was only through Facts that devices able to withstand tens of kilovolts became practical. The fact that GTOs can be switched off as well as on via application of a gate signal eliminates commutation difficulties that accompany ordinary thyristors.

EPRI foresees the day when power systems are characterized by numerous Facts controllers. Researchers there are working on a hierarchical control scheme that essentially ensures numerous controllers don't inadvertently work against each other as they route power and manipulate line conditions.

"Flow controllers are one of the tools in the system that this hierarchical controller would orchestrate," says EPRI's Edris. "We are defining the concept now. The aim is to maximize power transfer throughout the network by watching phasor measurements, voltages, and current flows to make sure there aren't any controllers working at cross purposes."