Dynamic Braking System In Large Hydro Electric Power Plant

Figure 4: Velocity vs. pipe diameter

If the pipe is undersized, this friction loss can be substantial, and the available pressure is reduced. Another way of expressing potential hydraulic energy is by using the term "head". Static head is the difference in elevation between the reservoir and the turbine. Friction losses cause loss of head over a length of pipe, making it appear that the reservoir is lower than it really is when the water is flowing, as shown in Figure 5.

Figure 5: Loss of head (pressure) due to flow of water

Pressure head and velocity head

Besides the fluid energy due to pressure, water has energy due to its velocity, or kinetic energy. This is expressed in Bernoulli's equation, which is studied in the field of fluid dynamics. The point we need to make here is that there is water energy due to both pressure and flow. It is convenient to express the velocity energy in feet of head, which is called velocity head. Mathematically, the velocity head is: V2/64.4, where V is the velocity of the water in cfs. One type of turbine works better on high pressure, and one is better for higher flow applications, as is discussed below.

Two types of turbines — impulse (Pelton) and reaction (Francis)

There are two classes of turbines: impulse and reaction. A Pelton wheel, shown in Figure 6 is an example of an impulse turbine. This type of turbine is useful in applications where there is high pressure and relatively low flow. The water flow is controlled by one or more needle valves, which direct the water into buckets on a wheel or runner. As the water strikes the buckets, all of the head is converted to velocity head, and the water velocity is reduced almost to zero, which spins the runner. The water falls out of the buckets and through an air gap into a tailrace, where it flows from there by gravity.

Figure 6: Typical Pelton wheel turbine

The Pelton turbine diagram also shows a deflector. It has two purposes. The first one is to deflect water away from the runner buckets during an emergency shutdown. Such a condition might arise when the electrical generator trips off-line due to a power outage. This trip removes the load or restraint on the turbinegenerator, causing it to overspeed. We want to limit this overspeed since it stresses the equipment. However, if the needle valves are closed too quickly, pressure can increase to unacceptable levels upstream due to changing the momentum of the water in the pipeline. In this case, the deflector moves between the needle and the runner, deflecting the water into the tailrace as the needles close at a rate slow enough to avoid water hammer. The second use for the deflector is to match the generator speed and phase to the utility system before the circuit breaker is closed to connect the generator to the utility. This process is called synchronization.

A Francis turbine, shown cross-section in Figure 7 is an example of a reaction turbine. The water passes through a snail-shaped scroll case, through wicket gates that control the amount of water, and into the runner. The runner, which is totally submerged, changes the momentum of the water, which produces a reaction in the turbine.

Figure 7: Cross-sectional view of a horizontal Francis turbine.

With a Francis turbine, downstream pressure can be above zero. Precautions must be taken against water hammer with this type of turbine. Under the emergency stop, the turbine overspeeds. One would think that more water is going through the turbine than before the trip occurred since the turbine is spinning faster. However, the turbine has been designed to work efficiently at the design speed, so less water actually flows through the turbine during overspeed. Pressure relief valves are added to prevent water hammer due to the abrupt change of flow. Besides limiting pressure rise, the pressure relief valve prevents the water hammer from stirring up sediment in the pipes.

Electrical generation basics

The way we generate electricity is to spin a magnet inside a coil of wire such that the magnetic lines of force are cut by the coil. Magnetic theory teaches us that a voltage is induced into the stationary coil (stator). We can make the magnet in two different ways. In a synchronous generator, some of the generator's output power is fed into the rotating coil (rotor) via slip rings to make an electromagnet which can be precisely controlled. An induction generator is just an induction motor, where the magnet is induced into the rotor from the stator. Normally, an induction motor runs slower than the electrical system speed and absorbs power. If we drive the induction machine faster than the system speed with our turbine, power flows into the system.

With either generator type, alternating current (ac) voltage is produced. When the rotor poles are adjacent to the stator coil, maximum voltage is induced since the most magnetic lines of force are being cut by the stator coil. When the rotor is perpendicular to the stator, no magnetic lines of force are being cut, and voltage is zero at that instant. As the rotor continues to revolve, the north and south poles of the rotor are reversed from the previous condition, and maximum voltage is induced in the opposite direction.

Large power projects generate/distribute three phase power. There are usually three stator coils spaced 120 mechanical degrees apart. This produces three singlephase voltage waveforms which are 120 electrical degrees apart. This is called three phase power.

Synchronous generators are more complex, costly and harder to synchronize, but are more efficient, produce a better quality power, and are used for larger units. Induction generators are simpler, less expensive, and are used for smaller units.

Efficiency

In a turbine generator, there are mechanical losses in the turbine, and mechanical and electrical losses in the generator. The generator losses include copper losses due to the heating of the wire, power required to form the rotor field, and mechanical losses such as friction and windage (some force is required to move cooling air through the generator). If we compare the hydraulic energy into the turbine with the electrical power produced, we can expect a water-to-wire efficiency of about 50% to 90% depending on turbine size.

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John Cowdrey, 3746 El Dorado Springs Dr., Boulder, CO 80303 • 303-441-3245

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