Power Struggle - Part 2
A Short Primer on "Load-frequency Control"
If you ever wanted to know how to operate a central station turbine generator, but were afraid to ask, now's your chance.
Here's your first question. How do you supply the right amount of electric energy from a generator when it is to serve a varying load as people, located far from the central station and out of sight are turning their electric switches on and off?
The answer: you can do it by regulating the flow of steam energy. Just open or close a valve between a steam boiler drum and a steam turbine that permits steam flow into the central station turbine generator. Turning a water valve regulating flow of water energy from a penstock into a hydroelectric turbine generator will do the same thing.
But how do you know how much steam from a boiler or water from a reservoir, to feed into the turbine generator? Easy. Watch a frequency meter.
The turbine is quite a massive piece of machinery connected to the generator rotor that is also a pretty heavy thing, connected by a steel shaft perhaps six inches to a foot thick. It acts as the "flywheel" of the system. It stores kinetic energy just as the flywheel of an un-interruptible power system ("UPS") stores energy to maintain power for your computer during an unexpected power outage or the flywheel of your car stores energy it gives up as needed between power strokes from the cylinders or when you start up hill before you step on the gas. (It only has so much so you better push in on the clutch as the engine slows down too much or it will stall.) If the steam energy or water energy going into the turbine is less than the electrical energy needs of the load, or if one of your turbine generators breaks down, the turbine generators still on line will give up some of the rotational energy stored in their flywheels to make up the difference and fall below 60 cycles per second. The turbines and generators rotating on the same shaft, or directly coupled, will slow down and frequency will fall. If customers turn off their switches your turbine generators will receive too much steam energy – more than the customers are using -- the surplus goes to the stored energy and the frequency goes up above 60 cycles per second or 60 Hz. When frequency is right on 60 Hz, no corrective action is needed.
If you have a scientific bent, the amount of energy stored is equal to the mass of the flywheel, in this case the mass of the turbine, generator, and connecting shaft, times the square of its rotational velocity or MV squared. As it gives up its stored energy it will slow down. The mass is unchanged so frequency drops. (Better shed some of your load before it slows down too much or it won't be able to come back up again. Those customers will be annoyed if you shed them for a few minutes but they'll really be irate if the system goes down completely; you have many generators, and it will take hours to rebuild your system carefully keeping everything in balance all the while. What a hard decision for a poor working stiff! Wrong decision and it's off with his head. )
You can shed load from your control center where you have remote control over circuit breakers at distribution substations. Maybe a plan of automatic load shedding keyed to low frequency will help as it frees the system dispatcher from responsibility for the decision.
Frequency is available all over interconnected electric systems no matter how large the interconnection – even when it extends from the Rockies to the Atlantic -- so it is a handy signal on which to base the control of system operation. In these days of large scale interconnection it doesn't vary very much since it is determined by the sum total of energy stored in all the generators operating in parallel if the system stays together. That's a pretty big mass for all the generators from the Atlantic Ocean to the Rocky Mountains that are now operating normally in parallel. Plug a frequency meter into your wall socket and you will see it vary, usually only from 59.97 to 60.03. If a big unit trips off line somewhere, it might go down to 59.94 for a few minutes. If it goes down much below that, the trip of lines may have put you in an island of generation and transmission lines, separated from the remainder of the interconnection, with not enough generation to supply the load and not enough stored energy to give your system operator much time to react.
For normal operation, to avoid having an operator stand there all day with his hand on the turbine valve, a fly-ball governor-- just like the one regulating the speed of your grandmother's Victorola -- can maintain a constant frequency by opening and closing turbine valves. Modern day systems with several generators will have a computer to allocate load among generators so that the least cost combination is supplying energy.
1890s Solution -
In 1886 a single phase alternating current system constructed at Great Barrington, Massachusetts is quickly followed by another single phase system in Buffalo, NY, distributing electric energy for light and heat. Distribution lines from a central station extend to customers throughout the municipalities, no longer limited to 1/2 mile from the generating station. At the Telluride silver mine in Colorado, poly-phase AC invented by Nikola Tesla, an eccentric Serbian immigrant, is used to transmit electric power from a small hydroelectric development a few miles away to power electric motors in a mineshaft that is located on the face of a cliff. The mine owner used the new technology because there was just no place to locate a fossil fuel DC generator near the mineshaft.
In Germany a high voltage transmission line 130 miles long transmits poly-phase alternating current from Laufen to Frankfurt. In 1896 the poly-phase AC service invented by Nikola Tesla that is now being promoted by George Westinghouse is deployed at Niarara Falls, permitting transmission of some of the hydropower to Buffalo, 18 miles away.
Why sell power to Buffalo? So that sufficient power could be sold from the project to satisfy the minimum revenue required for an economically feasible Niagara development. With poly-phase AC you can start and run electric motors as well as lighting light bulbs and energizing heating elements. The use of poly-phase AC makes it possible to build larger AC central stations to serve several load centers many miles away from each other to serve light, heat, and power loads.
War of the currents. Edison fights for the survival of the DC power system by staging electrocutions of cats, dogs, horses, and elephants using AC. He wires up the first death chair in Auburn Prison in 1888 with AC. He is very good at public relations.
Nonetheless, the great economies of scale of large generating units and ability to vary voltages easily wins the day for AC because power from large generating units could be distributed in a single system to industrial, commercial and residential loads over a broad area. That would be impossible with DC.
In 1889 Edison's electric power distributing company had merged with other Edison companies to form Edison General Electric. 1892 Edison General Electric merges with the Thomson-Houston company and changes its name to General Electric.
1910s - Entrepreneurs build larger generating stations serving two or three load centers using AC. They use high voltage primary distribution lines within the load centers and even higher voltage transmission lines between load centers. In fact two or more load centers, each with their own generator can operate "normally-in-parallel" connected by transmission.
[ A short course on parallel operation. If two AC generators are supplying energy on the same circuit or transmission network, they are said to be operating "in-parallel". They will be electromagnetically interlocked. The heavier the transmission lines between them, the more rigid the interlock. If the lines are light, the generator phase angles can vary somewhat but the interlock will be maintained by energy from the leading generator automatically moving swiftly from the leading to the lagging generator (or motor) to restore the synchronism if the lines between them are heavy enough. If the synchronizing energy overloads the line, circuit breakers will trip it off before it the overload becomes so great that the line will be permanently damaged.
Wall Street commences forming Public Utility Holding Companies to concentrate their control over small operating electric utilities, making it possible to integrate their load and create regional "superpower" systems. These regional electric power systems were proposed by former President Herbert Hoover (an engineer before he became a politician), W.S. Murray, and other engineers.
The Eastern Seaboard was a favored location. These could use much larger coal fired steam turbines central generating units to serve regional load by integrating the load centers with high voltage transmission. The savings in power supply costs from savings in fuel and generator construction pay for the premium necessary to gain control. They could gain additional savings from an "economic dispatch" of the generators so that as load varied, power would come from the least cost combination of generators on line.
Each electric utility or holding company system builds strong "backbone" transmission to connect its major power stations and its largest load centers. Under pressure from conservationist Pennsylvania Governor Gifford Pinchot, electric utilities in Pennsylvania interconnect and engage in central economic dispatch. They don't organize as "control areas". They bill one another for power transfers after the fact, having used central economic dispatch.
Ultimately this will become the "PJM" or Pennsylvania, New Jersey and Maryland interconnection, now including Delaware as well. 1923 - Congress enacts regulations of hydroelectric power development and creates the Federal Power Commission (FPC).
1930s - The "War of the Currents" is over. AC has won. By this time, most DC systems are gone but Edison's name lives on as "the father of the electric power industry" even though he did his best to kill the AC power system which prevailed. His prowess at public relations is noted in some but not all biographies. Who ever heard of Nikola Tesla?
In 1935 Congress enacts legislation to limit the economic power of electric utility holding companies and to regulate growing interstate commerce in electric power that the Supreme Court had held in the "Attleboro" case couldn't be regulated by the states even in the absence of conflicting federal legislation. The "death sentence" of Section 11 of the Public Utility Holding Company Act requires divestiture of all electric operating properties that can't be integrated but permits the Holding Company keep properties in a single integrated system that by integrating loads over large areas makes it feasible to install larger scale more economical base load generating units. The 1935 Federal Power Act lets businessmen build those divested isolated operating properties back up into integrated systems by acquisition of assets or merger under the regulation of the Federal Power Commission.
Federal Power Commission orders interconnection of several power systems to improve bulk power supply in aid of war effort. FPC also plans a strong national "grid", a real "grid" but is persuaded after the war ends, by Philip Sporn, the CEO of the largest US electric utility, that the plan should be classified as a "military secret" and it becomes unavailable as a post-war political issue. A "grid" would have the capacity to meet most all loads supplied from most any combination of generators. We now start calling it a "grid" but calling it a grid doesn't make it one. The interconnection that was in place had a heavy backbone for each utility but the interconnections among utilities were of much lower capacity.
In the 1950s many more electric utilities interconnect with adjacent utilities for frequency support of the larger generating units they are installing and develop methods of "area control", first with "flat tie-line control" used to regulate electric energy flows among "control areas", and when that proves inadequate for large interconnections, "tie-line-bias control" is developed and proves satisfactory. "Interconnected Systems Group" (ISG) is organized. Four large groups of control areas interconnect and commence operating normally-in-parallel. Why interconnect so widely? It is because generating units are getting larger. When one of them trips off line, you want as much rotating mass operating in parallel as possible so frequency won't decay quickly. You want to give the system dispatcher as much time as possible to restore the balance between generation and load, without having to cut off service to some of your customers. They need at least 15 minutes to reach other dispatchers at other control centers over the telephone.
[Area control for the grid-maven. Generators operating normally-in-parallel are electromagnetically interlocked and power will flow to where it is needed to maintain frequency. If two electric utilities want to operate normally-in-parallel, they must find some way to allocate generation responsibilities to supply only their own customers. If they are operating normally-in-parallel they can no longer use frequency control since the frequency on the combined system is like the balance in a joint bank account – the balance will vary with your deposits and withdrawals, but the balance will also reflect what your wife deposits and withdrawals also. So to make sure you are not burning coal to supply your neighbor's customers, you change the control of the valves on your generators from frequency control to control by signals from a meter on the transmission tie line between the two electric utilities. Its signal is telemetered back to your control center.
This is "flat tie-line control". If energy is flowing out of your "control area" into another "control area", it means you have an area control error and you must cut down on the flow of steam to your turbine or turbines. Conversely, when the meter shows an inward flow of energy, a flow into your control area, you are not doing your share and you must increase the supply of energy from your generators by permitting more steam to flow into the turbine or turbines to bring your tie line back to zero.
Computers are helpful in all this. First analog, then digital computers are used. They can send raise or lower signals over telephone lines to your generators from your control center driven by your area control error. The system lambda is its incremental cost. If generation must be raised, raise it at the generator with the lowest incremental cost. Conversely if generation is to be lower, lower it at the one with the highest incremental cost. If you have more than one transmission boundary tie to other control areas, use the algebraic sum of flows to get the "Area Control Error" (ACE). But you still need to control frequency on the entire interconnection.
To control frequency on the entire interconnection, you ask one of the utilities in the interconnection to continue to regulate on frequency.
This flat tie-line control works OK on small interconnections but as interconnections get larger and larger it imposes too heavy a burden on the utility assigned to maintain frequency on the whole interconnection. Big swings in loading are imposed on his generators and his economics are bad. If you want to spread the burden of frequency control on the entire interconnection, flat tie-line control is counterproductive. If someone else's generator trips off line, your tie line meters reflect energy flowing out which is a signal to cut down your generation. But if the obligation to help the interconnection remain at 60 cycles per second is to be shared by all, you should be generating a little more until the utility suffering an outage can press more resources into its supply. The answer is "tie-line-bias control" in which a combination of tie-line readings and frequency are used by each control area and the obligation to maintain frequency on the interconnection is shared by all.
When frequency is exactly 60 Hz, tie-line readings are used exclusively for system control. When frequency varies up or down from 60 Hz, utilities bias their control; they supply a little less or a little more than their individual responsibilities under flat tie-line control, to help maintain frequency at 60 Hz on the entire interconnection. Usually the "little more" or "little less" is based on the "natural frequency response characteristic" of the system which is its change in output resulting from governor response to the change in frequency. In this way the governors and the controller are in harmony.
Frequency can fall off quite a bit when somebody's big 1,200,000 kW central station trips off line with a big hole in a boiler tube. (If it is a small hole, they can continue to operate it until the weekend. Then they can take if off line, cool down the boiler and plug the ends of the tube and wait for repair during the four weeks of boiler maintenance at low load time in the Spring or Fall. If it is a big generator, the rerouting of flows when it trips off line can cause protective relays on transmission lines to trip if those lines in the path of least resistance (really least impedance for AC lines) are not heavy enough to carry the flows.) A cascading outage can occur if the tripping on one line will reroute flows to other lines inadequate to hand the flow of power --as in fact it did in 1965. But the system operators should avoid a scenario where the trip of a single unit, or of a single transmission line, will cause a cascading outage.]
The newly formed utility groups refer to themselves as the NE, NW, SE, and SW regions of the ISG. ISG publishes Operating Guides for its members. One of the ISG guides on transfers between control areas limits inter "control area" transfers to those which will not result in a cascading outage in the event of a outage of a single generator or a single transmission line.
PART THREE CONTINUES NEXT WEEK...
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