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NOW that we are familiar with the principal machines, transformers, and other power devices, we are in a position to see how they are used in a large electrical system. Such a system comprises all the apparatus used in the generation, transmission, and distribution of electric energy, starting from the generating station and ending up in the most remote summer home in the country. This section and the next three sections are, therefore, devoted to the following major topics:
Demand of an electrical system
The total power drawn by the 'customers of a large utility system fluctuates between wide limits. depending on the seasons and time of day. ++ ++ 1 shows how the system demand (power) varies during a typical day in the summer and a typical day in the winter. The pattern of the daily demand is remarkably similar for the two seasons. During the winter the peak demand of 15 GW 15000 MW) is higher than the summer peak of 10 GW. Nevertheless, both peaks occur about 17:00 ( 5 P.M.) because increased domestic activity at this time coincides with industrial and commercial centers that are still operating at full capacity.
--- Demand curve of a large electric utility system during one year.
--- Load duration curve of a large electric utility system.
The load curve shows the seasonal variations for the same system. Note that the peak demand during the winter (15 GW) is more than twice the minimum demand during the summer (6 GW). In examining the curve, we note that the demand throughout the year never falls below 6 GW. This is the base load of the system. We also see that the annual peak load is 15 GW. The base load has to be fed 100% of the time, but the peak load may occur for only 0.1% of the time. Between these two extremes, we have intermediate loads that have to be fed for less than 100% of the time.
If we plot the duration of each demand on an annual base, we obtain the load duration curve. For example, the curve shows that a demand of 9 GW lasts 70% of the time, while a demand of 12 GW lasts for only 15% of the time. The graph is divided into base, intermediate, and peak load sections. The peak-load portion usually includes demands that last for less than 15% of the time. On this basis the system has to deliver 6 GW of base power, another 6 GW of intermediate power, and 3 GW of peak power.
These power blocks give rise to three types of generating stations:
Consequently, they are equipped with prime movers such as diesel engines, gas turbines, compressed-air motors, or pumped-storage turbines that can be started up in a few minutes. In this regard, it’s worth mentioning that thermal generating stations using gas or coal take from 4 to 8 hours to start up, while nuclear stations may take several days. Obviously, such generating stations cannot be used to supply short term peak power.
Returning, it so happens that the areas of the dotted and cross-hatched parts are proportional to the relative amount of energy (kW·h) associated with the base, intermediate, and peak loads. Thus, the base-power stations supply 58% of the total annual energy requirements, while the peak-load stations contribute only 1.3%.
The peak-load stations are in service for an average of only 1 hour per day. Consequently, peak power is very expensive because the stations that produce it are idle most of the time.
Location of the generating station
In planning an electric utility system, the physical location of the generating station, transmission lines, and substations must be carefully planned to arrive at an acceptable, economic solution. We can sometimes locate a generating station next to the primary source of energy (such as a coal mine) and use transmission lines to carry the electrical energy to where it’s needed. When this is neither practical or economical, we have to transport the primary energy (coal, gas, oil by ship, train, or pipeline to the generating station). The generating station may, therefore, be near to, or far away from, the ultimate user of the electrical energy. --- also shows some of the obstacles that prevent transmission lines from following the shortest route. Due to these obstacles, both physical and legal, transmission lines often follow a path between the generating station and the ultimate user.
Types of generating stations
There are three main types of generating stations:
Thermal generating stations produce most of the electrical energy in the United States. Nevertheless, important hydropower stations and nuclear generating stations produce about 20% of the total requirements.
Some of the largest hydropower stations are located in Quebec and British Columbia, in Canada.
Although we can harness the wind, tides, and solar energy, these energy sources represent a tiny part of the total energy we need.
---4 Extracting, hauling, and transforming the primary sources of energy is done in different ways. The dotted transmission lines connecting the generating stations G with the consumers must go around various obstacles. thermal station; GH : hydro station; GN: nuclear station.
Controlling the power balance between generator and load
The electrical energy consumed by the thousands of customers must immediately be supplied by the ac generators because electrical energy cannot be stored. How do we maintain this almost instantaneous balance between customer requirements and generated power? To answer the question, let us consider a single hydropower station supplying a regional load R_l. Water behind the dam flows through the turbine, causing the turbine and generator to rotate.
The mechanical power P_T developed by the turbine depends exclusively on the opening of the wicket gates that control the water flow. The greater the opening, the more water is admitted to the turbine and the increased power is immediately trans mitted to the generator.
On the other hand, the electric power P_L. drawn from the generator depends exclusively on the load.
When the mechanical power supplied to the rotor is equal to the electrical power P_L. consumed by the load, the generator is in dynamic equilibrium and its speed remains constant. The electrical system is said to be stable.
However, we have just seen that the system demand fluctuates continually. so P_L. is sometimes greater and sometimes less than P_T. If P_L. is greater than P_T , the generating unit (turbine and generator) begins to slow down. Conversely, if P_L. is less than P_T , the generating unit speeds up.
The speed variation of the generator is, therefore, an excellent indicator of the state of equilibrium between PL and PT and, hence, of the stability of the system. If the speed falls the wicket gates must open, and if it rises they must close so as to maintain a continuous state of equilibrium between PT and PL' Although we could adjust the gates manually by observing the speed, an automatic speed regulator is always used.
Speed regulators, or governors, are extremely sensitive devices. They can detect speed changes as small as 0.02%. Thus, if the speed of a generator increases from 1800 rpm to 1800.36 r/min, the governor begins to act on the wicket gate mechanism. If the load should suddenly increase, the speed will drop momentarily, but the governor will quickly bring it back to rated speed. The same corrective action takes place when the load is suddenly removed.
Clearly, any speed change produces a corresponding change in the system frequency. The frequency is therefore an excellent indicator of the stability of a system. The system is stable so long as the frequency is constant.
The governors of thermal and nuclear stations operate the same way, except that they regulate the steam valves, allowing more or less steam to flow through the turbines. The resulting change in steam flow has to be accompanied by a change in the rate of combustion. Thus, in the case of a coal-burning boiler, we have to reduce combustion as soon as the valves are closed off. otherwise the boiler pressure will quickly exceed the safety limits.
Advantage of interconnected systems
---.5 Power supplied to three independent regions, burner; steam boiler steam boiler; hydraulic; turbine synchronous; generator; steam; turbine; turbine; synchronous
---.6 Three networks connected by four tie-lines.
Consider the three generating stations, connected to their respective regional loads R_L , R2, and R3. Because the three systems are not connected, each can operate at its own frequency, and a disturbance on one does not affect the others. However, it’s preferable to interconnect the systems because ( 1) it improves the overall stability, (2) it provides better continuity of service. and (3) it’s more economical.
--- shows four interconnecting transmission lines, tying together both the generating stations and the regions being serviced. High-speed circuit breakers d , to d/D are installed to automatically interrupt power in case of a fault and to reroute the flow of electric power.'" We now discuss the advantages of such a network.
1. Stability. Systems that are interconnected have greater reserve power than a system working alone. In effect, a large system is better able to withstand a large disturbance and, consequently, it’s inherently more stable. For example, if the load suddenly increases in region R_L energy immediately flows from stations and G3 and over the interconnecting tie-lines. The heavy load is, therefore, shared by all three stations in stead of being carried by one alone.
2. Continuity of Service. If a generating station should break down. or if it has to be shut down for annual inspection and repair, the customers it serves can temporarily be supplied by the two remaining stations. Energy flowing over the tie lines is automatically metered and credited to the station that supplies it, less any wheeling charges. A wheeling charge is the amount paid to another electric utility when its transmission lines are used to deliver power to a third party.
3. Economy. When several regions are interconnected, the load can be shared among the various generating stations so that the overall operating cost is minimized. For example, instead of operating all three stations at reduced capacity during the night when demand is low, we can shut down one station completely and let the others carry the load. In this way we greatly reduce the operating cost of one station while improving the efficiency of the other stations, because they now run closer to their rated capacity.
Electric utility companies are, therefore. interested in grouping their resources by a grid of inter connecting transmission lines. A central dispatching office (control center) distributes the load among the various companies and generating stations so as to minimize the costs. Due to the complexity of some systems, control decisions are invariably made with the aid of a computer. The dispatching office also has to predict daily and seasonal load changes and to direct the start-up and shut-down of generating units so as to maintain good stability of the immense and complicated network.
For example, the New England Power Exchange (NEPEX) coordinates the resources of 13 electrical utility companies serving Connecticut, Rhode Island, Maine, and New Hampshire. It also super vises power flow between this huge network and the state of New York and Canada.
Although such interconnected systems must necessarily operate at the same frequency, the load can still be allocated among the individual generating units, according to a specific program. Thus, if a generating unit has to deliver more power, its governor setting is changed slightly so that more power is delivered to the generator. The increased electrical output from this unit produces a corresponding decrease in the total power supplied by all the other generating units of the interconnected system.
Conditions during an outage
--- 7 Technicians in the control rooms of two generating stations communicate with each other, or with a central dispatching office, while supervising the operation of their respective generating units.
A major disturbance on a system creates a state of emergency and immediate steps must be taken to prevent it from spreading to other regions. The sudden loss of an important load or a permanent short-circuit on a transmission line constitutes a major contingency.
If a big load is suddenly lost, all the turbines begin to speed up and the frequency increases everywhere on the system. On the other hand, if a generator is disconnected, the speed of the remaining generators de creases because they suddenly have to carry the entire load. The frequency starts to decrease--sometimes at the rate of 5 Hz per second. Under these conditions, no time must be lost and, if conventional methods are unable to bring the frequency back to normal, one or more loads must be dropped. Such load shedding is done by frequency-sensitive relays that open selected circuit breakers as the frequency falls. For example on a 60 Hz system the relays may be set to shed 15% of the system load when the frequency reaches 59.3 Hz, another 15% when it reaches 58.9 Hz, and a final 30% when the frequency is 58 Hz. Load shedding must be done in less than one second to save the loads judged to be of prime importance. As far as the disconnected customers are connected, such an outage creates serious problems.
Elevators stop between floors, arc furnaces start to cool down, paper tears as it moves through a paper mill, traffic lights stop functioning. and so forth.
Clearly, it’s in everyone's interest to provide uninterrupted service.
Experience over many years has shown that most system short-circuits are very brief. They may be caused by lightning, by polluted insulators. by falling trees, or by overvoltages created when circuit breakers open and close. Such disturbances usually produce a short-circuit between two phases or between one phase and ground. Three-phase short-circuits are very rare.
Because line short-circuits are, in general. very brief, a major outage can usually be prevented by simply opening a short-circuited line and reclosing it very quickly. Naturally, such fast switching of circuit breakers is done automatically because it all happens in a matter of a few cycles.
Frequency and electric clocks
The frequency of a system fluctuates as the load varies, but the turbine governors always bring it back to 60 Hz. Owing to these fluctuations, the system gains or loses a few cycles throughout the day. When the accumulated loss or gain is about 180 cycles, the error is corrected by making all the generators turn either faster or slower for a brief period. The frequency correction is affected ac cording to instructions from the dispatching center. In this way a 60 Hz network generates exactly 5 184000 cycles in a 24-hour period. Electric clocks connected to the network indicate the correct time to within 3 seconds, because the position of the second hand is directly related to the number of elapsed cycles.
Thursday, March 17, 2016 10:10