Generation System: Hydroelectric Energy

WEEK 02: GENERATION: FOSSIL-FUEL / HYDRO PLANTS

Sections: Introduction | Turbines | Power Plant Design | Performance

B. Hydroelectric Power Plant converts the inherent energy of water under pressure into electrical energy. The size, location and type of power plant depend upon the topography, the geological conditions and the amount of water and head (low, medium, or high) available. Its main elements are:

1. Principal Elements
(a) Reservoir usually formed by building a dam across a river. Dams can be of two types: (1) impounding, or non-overflow, usually provided with a means to release excess flow, by a separate spillway action, by regulating gates, or by large spillway gates. Earth dams, rock-fill dams, and high-reservoir concrete-arch dams are examples of this type. (2) Spillway, or overflow dams are always concrete, and for low-head installations, the powerhouse usually forms part of the dam.
(b) Intakes consists of canals, flumes, or concrete passageways to carry the water directly to low-head turbines or to the pressure conduits used for medium- and high-head turbines.
(c) Pressure Conduits consists of concrete or rock tunnel, steel pipelines, steel penstocks or a combination thereof. They also connect the upper reservoir to the surge tank or penstock.
(d) Surge Tank, to prevent excessive pressure rises and drops during sudden load changes, installed somewhere along the pressure conduit when the latter is quite long.
(e) Trash Racks are provided at the inlet to the intake or the conduit to protect the turbine against floating or other material. Cleaning devices such as manual or motor-operated rakes are provided to remove debris from the racks.
(f) Head Gates or Stop Logs, usually made of steel, are provided at the inlet to the intake or conduit and at the outlet of the draft tube for shutting off the flow to the turbine for safety and for ease of maintenance.
(g) Penstocks are closed conduits connecting the upper reservoir, tunnel, or surge tank with the turbine casing. In medium-head installations, each turbine usually has its own penstock. In case of high heads, a single, branched, penstock is provided to supply two or more turbines.
(h) Hydraulic Turbine consisting primarily of a runner, connected to a shaft, for producing prime motive power from the inherent energy of water under pressure. a mechanism for controlling the quantity of water flowing to the runner, and water passages leading to and away from the runner. Penstock valves, located at the intake of the turbine spiral case, are provided when the conduit is of considerable length, thus providing means of shutting off the flow for safety, maintenance and to reduce leakage during long turbine shutdowns.
(i) Governor for operating the hydraulic turbine control mechanism.
(j) Generator connected to the hydraulic-turbine shaft to convert the prime motive power of the turbine to electric power.
(k) Pressure Regulator, sometimes used instead of a surge tank, to prevent excessive pressure rises and drops during sudden load changes in plants with long pressure conduits.
(l) Draft Tube usually part of the powerhouse structure to carry water away from the turbine runner.
(m) Tailrace used to carry water away from the draft tube to the tailrace reservoir.
(n) Tailrace Reservoir receives the water discharged from the draft tube or tailrace and is usually part of the original river at an elevation lower than the upper reservoir.

2. Powerhouse Structure to enclose and support the hydraulic turbine, generator, governor, pressure regulator, water passages including draft tube, basements, passageways for access to the turbine casing and draft tube and auxiliaries, including the foundation and the superstructure. Transformers and oil circuit breakers are located with the superstructure, on the roof or on a deck built over the draft-tube extension. The transformers and switchgear are usually located outdoors adjacent to the powerhouse and are not integral part of it. Cranes are provided in the powerhouse to handle the heaviest machinery pieces.

3. Powerhouse Auxiliaries include special apparatus consisting of:
(a) Service Units a small hydraulic turbine and generator used for supplying power for internal plant use and as as source of independent power supply in case the power plant is electrically separated from the main system.
(b) Casing drain valves for draining the turbines.
(c) Strainers, or filters for bearing and cooling-water supply.
(d) Air Compressors for charging governor oil systems, generator brakes, tail-water depression systems
(e) Carbon Dioxide System for fire protection.
(f) DC Service for emergency power supply.

Sections: Introduction | Turbines | Power Plant Design | Performance

Hydraulic Turbines

Waterwheels are generally grouped into (1) reaction, where the water enters the turbine with high potential energy in the form of pressure and a lesser amount of kinetic energy in the form of velocity, and (2) impulse, when the water enters the turbine with high kinetic energy and a relatively low value of potential energy.

Classification of Turbines
(1) Reaction Turbines The water enters the guide case of the turbine with high potential energy and relatively low kinetic energy. The potential energy, which is a function of the pressure difference between the runner inlet and exit, causes the fluid to flow through the runner buckets. As the fluid flows over the curved surface of the runner buckets, the fluid velocity on one side of the bucket is higher than on the opposite side. This difference in velocity on the surfaces of the bucket causes a pressure differential across the bucket which exerts a force on the bucket. This force at its respective radius in the runner, the revolving part, then causes the runner to restore and impart mechanical energy to the turbine shaft.
(1.1) Francis reaction turbine. Water enters the spiral case from intake passages or penstocks, passes through the stay ring, guided by the stationary stay-ring vanes, then through the movable wicket gates through the runner and into the draft tube, through which it flows into the tailrace or the tail-water reservoir. They are normally used for head ranging from 100 to 1500 ft (30.5 to 457m). Specific speeds (n_s) vary from 15 to 100 (57 to 382)
(1.2) Propeller Turbines, also of reaction type, the runner has has unshrouded blades (no crown or band). The blades, 3 to 10, are either fixed or adjustable. Usually used for heads from 10 ft (3.05 m) up to 120 ft (36.5 m) but heads up to 200 ft (61 m) are feasible. Specific speeds (n_s) vary from 80 to 250 (305 to 954).
1.2a Fixed-Blade Propeller Turbines, the runner blades are in a permanent fixed position. The blade angle is usually set between 20 to 28, where maximum efficiency occurs.
Kaplan 1.2b Adjustable-Blade Propeller Turbines The blade angle may vary from -10° minimum to 40° maximum. The blades maybe adjusted by hand, by electric motor through a train of gears or by oil-pressure-operated blades. The latter more commonly known as a Kaplan Turbine
1.2c Diagonal-Flow Propeller Turbines in which the axis of the blades is at approximately 45° with the main shaft. The blades may either be fixed or adjustable. Some turbines are designed that the blades can be closed against one another to shut off the flow of water through the runner.
1.2d Axial-Flow Propeller Turbines is characterized by the straight through water passageway from intake to discharge. The turbine shaft is either horizontal, vertical or inclined. More recently, horizontal axial-flow turbines have been installed to harness tidal power. There are four types under this classification. The rim type has the generator rotor mounted around the periphery of the turbine runner. The pit and bulb type locates the generator in series with the turbine runner at a submerged elevation. The bulb type has the generator enclosed in a streamline, watertight housing located in the water passageway on either the upstream or the downstream side of the runner. The tube type has the generator located outside of the water passages, where the shaft may be inclined, thereby raising the generator above tailwater elevation.

Impulse (2) Impulse Turbines consist of one or more free jets of water discharging into an aerated space and impinging on a set of buckets attached around the periphery of a disk. Generally, the buckets are bowl-shaped and have a central dividing wall, or splitter, extending radially outward from the shaft. The splitter divides the stream, and the bowl-shape portions of the bucket turn the water back, imparting the full effect of the jet to the runner. The free jet is formed by water passing through the nozzle pipe, the needle nozzle, and then through the nozzle tip. These turbines are used when the head is too high for Francis turbines, which is normally a head exceeding 1600 ft (500 m) or below this number where excessive erosion due to foreign materials in the water presents a problem. THe runaway speed for these turbines ranges from 160 to 190 percent of normal speed, depending upon the specific speed of the runner. THe higher the specific speed, the higher the runaway speed.

Sections: Introduction | Turbines | Power Plant Design | Performance

Power Plant Design

Plant Arrangement The setting or arrangement of hydraulic turbines in a power plant varies with the type of turbine, the head, and the type of dam and intake.

Head range, ft	Type of Turbine	General Arrangement
Up to 120	Fixed-blade propeller	Vertical with concrete semispiral or plate-steel spiral case.
Up to 200	Conventional adjustable-blade propeller or Deriaz.	Vertical with concrete semispiral or plate-steel spiral case.
Up to 90	Pit, bulb, or tube	Horizontal or inclined, concrete and/or plate-steel intake.
100 to 1500	Francis	Vertical or horizontal plate-steel spiral case.
1000 to 5800	Impulse	Vertical or horizontal plate-steel spiral case.

Plant Design Factors
A. Specific Speed, n_s, is the relationship between the speed of the runner at the point of the highest efficiency and the maximum power output at this speed. Since both power and speed vary with head, specific speed is also the relationship between speed n₁and speed P₁at 1 ft (m) head. Subscript 1 denotes that the value is reduced by the similarity law to a 1-ft (m) head basis. Thus, specific speed is equal to:

n(P) / H^5/4

where n, is the best efficiency speed; P, is the maximum power output at this speed; and H, the runner head where n and P operates. Impulse turbines' specific speed, n_s, can be improved by increasing the number of jets used on a single runner or by increasing the number of runners per unit.

B. Speed Hydraulic turbines are usually connected to ac generators. THe turbine speed must agree with one of the synchronous speeds required for the system frequency. Synchronous speeds are determined by the formula n = 120 × frequency / number of poles in a generator. The number of poles should be even. The speed should be as high as practicable, since the higher the speed the less expensive will be the turbine and generator and the more efficient will be the generator.

C. Number of Units should be kept to a minimum, thus reducing the number of auxiliaries and the amount of associated equipment and also reducing initial and maintenance costs for the entire plant. The larger the unit, the higher the efficiency and generally the lower the cost per unit output. However, other considerations, such as flexibility of operation, higher efficiency operation during low-load demands, and minimum loss of capacity during shutdown for repair and maintenance, might dictate the use of multiple units.

D. Weight of Runner The approximate weight of any Francis runner is 0.030D³, where D is the diameter of the runner in inches at the centerline of the distributor. For fixed-type propeller runner, the weight is taken to be 0.009D³, and for Kaplan-type runners as 0.014D³.

E. Turbine Thrust The hydraulic thrust on Francis runners varies with type, design, specific speed, the pressure between the movable wicket gates and runner, seal design and clearance, and the method of venting. It is approximately between 25 and 45% of the weight of the full head of water acting on the discharge diameter, D_d, of the runner. The higher the specific speed, the greater is the thrust. With propeller-type runners, the thrust is nearly equal to the weight of water on the full area. Impulse turbines have no hydraulic thrust of any consequence.

F. WR² of turbine runners, which is their weight times the square of the radius of gyration, varies widely with the type of runner and its design, thus it must be calculated for each specific design, based upon its configuration and distribution of weight.

G. Runaway Speed is the value of overspeed if the turbine runner is allowed to revolve freely without load and with the wicket gates wide open. The runaway speed, at normal head, varies with the specific speed and for Francis turbines ranges from 170% (normal speed = 100%) at low specific speed [n_s= 20 (76)] to approximately 195% at high specific speed [n_s= 100 (381)]. For propeller turbines, the runaway speed varies with blade angle -- the steeper the blade angle, the lower the runaway speed. For fixed-blade set at 16 to 28º, where maximum efficiency is usually obtained, the runaway speed ranges from 225 to 180%, respectively. For adjustable-blade, where the minimum blade angle at 6 to 16º, to obtain efficiency at part load, the maximum possible runaway speed is about 290 to 270%, respectively. For all turbines, if the maximum head is higher than the normal head, the runaway speed will be increased in proportion to the square root of the head. Therefore, runaway speeds should be based on the maximum operating head rather than the normal head. Any runaway speed above 180% increases the cost of the generator.

Sections: Introduction | Turbines | Power Plant Design | Performance

Hydraulic Performance

Hydraulic Performance. Hydraulic turbines derive power from the pressure or force exerted by water falling through a given distance (the head).

Turbine Characteristics The theoretical power usually expressed in horsepower, Pt = HQw / 550 = HQ / 8.82, where H = the head in feet, Q = flow of water in cubic feet per second, and w = weight of water in pounds per cubic foot. The head is established by the topography of the country and the location of the dam, intake works, powerhouse, and tailrace or tailwater reservoir. An analysis of the river-flow records, type of turbine, and type of load (whether base or peak) will fix the maximum and mean value of flow to be used in the design. The actual horsepower P = Pt × h, where h is the turbine efficiency. For general purposes, a mean efficiency of 90% is assumed. Generator efficiency h_granges from 94 to 98%, The combined efficiency of both turbine and generator is about 85 to 93%.

Proportionality Laws The law of proportionality (the variation of power, speed, and discharge with runner size and head) for turbines of varying size, but with same basic dimensional relationship in water passageway design (also called homologous turbines) are shown below:

For Constant
Runner Diameter
For Constant Head
For Variable diameter
and head
P µ H^3/2
P µ D²
P µ H^3/2D²
n µ H^1/2
n µ 1/D
n µ H^1/2/D
Q µ H^1/2
Q µ D²
Q µ H^1/2/D²
where D is the nominal diameter of the turbine runner.

Efficiency of the head contemplated, will affect the maximum efficiency obtainable, as well as the percentage of full load where this maximum occurs and the efficiencies of part loads. As the specific speed increases, the percentage of full load at which the maximum efficiency occurs increases and part-load efficiencies drop.

Cavitation occurs when the pressure at any point in the flowing water drops below the vapor pressure of water. The relationship which produces cavitation is between vapor pressure, barometric pressure, setting of the runner with respect to tailwater, and net effective head on the turbine and is expressed by the Thoma cavitation coefficient, s = (H_b- H_v- H_s)/ H, where H_b= barometric head, ft (m) of water; H_v= vapor pressure of water, abs; H_s= elevation, ft (m) of the runner above tailwater, measured at the centerline of the distributor of a Francis turbine and at the centerline of the blades of a propeller runner (if the runner is submerged, this quantity becomes negative); and H = total of net effective head, ft (m), on the turbine. In absence of cavitation test, the value of s should not be lower than n_s^3/2/ 2000 or (n_s^3/2/ 15,000) for Francis and propeller runners and n_s² / 25,000 or (n_s² / 350,000 ) for adjustable blade propeller. The value of s at which a plant operates, depends upon the setting of the runner with respect to tailwater, is called the plant s. To avoid excessive cavitation, the plant s should exceed the critical s. The greater this margin, the less possibility of cavitation during operation.

Speed Regulation is accomplished by changing the flow of water to the turbine. The flow is controlled by the wicket gates of reaction turbines and by the needle vale or jet deflector of impulse turbines. The turbine governor moves the gates or needle in response to speed changes resulting from load or head changes.

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