Generating Electrical Power: HYDROPOWER GENERATING STATIONS

Hydropower generating stations convert the energy of moving water into electrical energy by means of a hydraulic turbine coupled to a synchro nous generator.

Available hydro power

The power that can be extracted from a waterfall depends upon its height and rate of flow. The size and physical location of a hydropower station depends, therefore, on these two factors. The available hydro power can be calculated by the equation…

P=9.8 qh

where

P available water power [k W] q = water rate of flow [m^3/s] h = head of water [m] 9.8 coefficient to take care of units Owing to friction losses in the water conduits, turbine casing, and the turbine itself, the mechanical power output of the turbine is somewhat less than that calculated. However, the efficiency of large hydraulic turbines is between 90 and 94%. The generator efficiency is even higher, ranging from 97 to 99%, depending on the size of the generator.

Example 1: A large hydropower station has a head of 324 m and an average flow of 1370 m^3/s. The reservoir of water behind the dams and dikes is composed of a series of lakes covering an area of 6400 km^2

Calculate:

a. The available hydraulic power

b. The number of days this power could be sustained if the level of the impounded water were allowed to drop by I m (assume no precipitation or evaporation and neglect water brought in by surrounding rivers and streams)

Solution:

a. The available hydropower is P=9.8 qh

= 9.8 X 1370 x 324

= 4350000 kW = 4350 MW

b. A drop of I m in the water level corresponds to 6400 x 10^6 m^3 of water. Because the flow is 1370 m^3 /s, the time for all this water to flow through the turbines is t = 6400 X 10^6 = 11370 4.67 X 10^6 s 1298 h 54 days

As a matter of interest, a flow of 1370 m^3/s is about 10 times the amount of water used by the city of New York and its suburbs.

Types of hydropower stations

Hydropower stations are divided into three groups depending on the head of water:

• 1. High-head development
• 2. Medium-head development
• 3. Low-head development

High-head developments have heads in excess of 300 m, and high-speed Pelton turbines are used.

Such generating stations are found in the Alps and other mountainous regions. The amount of impounded water is usually small.

Medium-head developments have heads between 30 m and 300 m, and medium-speed Francis turbines are used. The generating station is fed by a huge reservoir of water retained by dikes and a dam.

The dam is usually built across a river bed in a relatively mountainous region. A great deal of water is impounded behind the dam. Low-head developments have heads under 30 m, and low-speed Kaplan or Francis turbines are used.

These generating stations often extract the energy from flowing rivers. The turbines are designed to handle large volumes of water at low pressure. No reservoir is provided.

---8 Grand Coulee Dam on the Columbia River in the state of Washington is 108 m high and 1270 m wide. It’s the largest hydropower plant in the world, having 18 generating units of 125 MW each and 12 generating units of 600 MW each, for a total of 9450 MW of installed capacity. The spillway can be seen in the middle of the dam.

--- This generating station on the contains 26 3-phase alternators rated 50 MVA, 13.2 kV, 75 r/min, 60 Hz at a power factor of 0.8 lagging. An additional 10 units rated 65 MVA, 95.7 r/min make up the complete installation. The output ranges between 1000 MW and 1575 MW depending upon the seasonal water flow.

--- Cross-section view of a medium-head hydropower plant.

Makeup of a hydropower plant

A hydropower installation consists of dams, water ways, and conduits that form a reservoir and channel the water toward the turbines. These, and other items described, enable us to understand some of the basic features and components of a hydropower plant.

1. Dams. Dams made of earth or concrete are built across river beds to create storage reservoirs.

Reservoirs can compensate for the reduced precipitation during dry seasons and for the abnormal flows that accompany heavy rains and melting snow. Dams permit us to regulate the water flow throughout the year, so that the powerhouse may run at close to full capacity.

Spillways adjacent to the dam are provided to discharge water whenever the reservoir level is too high.

We have seen that the demand for electricity varies considerably throughout the day, and from season to season. Consequently, the available water cannot al ways be used to supply energy to the system. If the water reservoir is small or almost nonexistent (such as in run-of.-river stations), we unfortunately have to let the water through the spillway without using it.

Dams often serve a dual purpose, providing irrigation and navigation facilities, in addition to their power-generating role. The integrated system of the Tennessee Valley Authority is a good example.

2. Conduits, Penstocks, and Scroll-Case. In large installations, conduits lead the water from the dam site to the generating plant. They may be open canals or tunnels carved through rock. The conduits feed one or more penstocks (huge steel pipes), which bring the water to the individual turbines. Enormous valves, sometimes several meters in diameter, enable the water supply to be shut off in the conduits.

The penstocks channel the water into a scroll-case that surrounds the runner (turbine) so that water is evenly distributed around its circumference. Guide vanes and wicket gates control the water so that it flows smoothly into the runner blades. The wicket gates open and close in response to a powerful hydraulic mechanism that is controlled by the respective turbine governors.

3. Draft Tube and Tailrace. Water that has passed through the runner moves next through a carefully designed vertical channel. called draft tube. The draft tube improves the hydraulic efficiency of the turbine. It leads out to the tailrace, which channels the water into the downstream river bed.

4. Powerhouse. The powerhouse contains the synchronous generators, transformers, circuit breakers, etc., and associated control apparatus.

Instruments, relays, and meters are contained in a central room where the entire station can be monitored and controlled. Finally, many other devices (too numerous to mention here) make up the complete hydropower station.

---11 Spiral case feeds water around the circumference of a 483 MW turbine.

---.12 Inside the spiral case, a set of adjustable wicket gates control the amount of water flowing into the turbine.

--- Runner of a Francis-type turbine being lowered into position at the Grand Coulee Dam. The turbine is rated at 620 MW, 72 rpm and operates on a nominal head of 87 m. Other details: runner diameter: 10 m; runner mass: 500 t; maximum head: 108 m; minimum head: 67 m; turbine efficiency: 93%; number of wicket gates: 32; mass per wicket gate: 6.3 t; turbine shaft length: 6.7 m; mass of shaft: 175 t.

Pumped-storage installations

We have already seen that peak-power stations are needed to meet the variable system demand. To understand the different types of peaking systems used, consider a network (electric system) in which the daily demand varies between 100 MW and 160 MW. One obvious solution to this variable demand is to install a 100 MW base-power station and a peak-power unit of 60 MW, driven intermittently by a gas turbine.

However, another solution is to install a larger base-power unit of 130 MW and a smaller peaking station of 30 MW. The peaking station must be able to both deliver and absorb 30 MW of electric power.

During lightly loaded periods (indicated by a minus sign), the peaking station receives and stores energy provided by the base-power generating plant. Then, during periods of heavy demand (shown by a plus), the peaking station returns the energy it had .previously stored.

This second solution has two advantages:

1. The base-power station is larger and, consequently, more efficient.

2. The peak-power station is much smaller and, therefore, less costly.

Large blocks of energy can only be stored mechanically. and that is why we often resort to a hydraulic pumped-storage station. Such a peak-power generating station consists of an upper and a lower reservoir of water connected by a penstock and an associated generating/pumping unit During system peaks the station acts like an ordinary hydropower generating station. delivering electrical energy as water flows from the upper to the lower reservoir.

However, during light load periods the process is reversed. The generator then operates as a synchronous motor, driving the turbine as an enormous pump. Water now flows from the lower to the upper reservoir, thereby storing energy in preparation for the next system peak. The generating/pumping cycle is repeated once or twice per day, depending on the nature of the system load. Peak-power generators have ratings between 50 MW and 500 MW. They are reversible because the direction of rotation has to be changed when the turbine operates as a pump.

Starting such big synchronous motors puts a heavy load on the transmission line. and special methods must be used to bring them up to speed.

Pony motors are often used, but static electronic frequency converters are also gaining ground. (A pony motor is a machine that brings a much larger machine up to speed.) Pumped-storage installations operating in con junction with nuclear plants make a very attractive combination because nuclear plants give best efficiency when operating at constant load.

--- 14 A 100 MW base power station and a 60 MW peak power station can supply the network demand.

--- 15. A 130 MW base power station and a 30 MW pumped storage unit can also supply the network demand.

--- 16 This pumped storage station in Tennessee pumps water from Lake Nickajack to the top of Raccoon Mountain, where it’s stored in a 2 km^2 (=500 acres) reservoir, giving a 316 m head. The four alternator/pump units can each deliver 425 MVA during the system peaks. The units can be changed over from generators to pumps in a few minutes.