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Sunday, 13 March 2011 19:09

Hydroelectric Power Generation

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Human beings learned to harness the energy of running water many millennia ago. For more than a century, electricity has been generated using water power. Most people associate the use of water power with the damming of rivers, but hydroelectric energy can also be generated by the harnessing of the tides.

Hydroelectric generation operations span a vast terrain and many climates, ranging from the Arctic permafrost to equatorial rainforest. The geographic location of the generating plant will affect the hazardous conditions that may be present, since occupational hazards such as aggressive insects and animals, or even poisonous plants, will vary from location to location.

A hydrogenerating station generally consists of a dam that traps a large quantity of water, a spillway that releases surplus water in controlled fashion and a powerhouse. Dykes and other water containment and control structures may also be part of the hydroelectric power station, although they are not directly involved in generating electricity. The powerhouse contains conducting channels that guide water through turbines that convert the linear flow of the water into a rotating flow. Water will either fall through the blades of the turbine or else flow horizontally through them. The turbine and generator are connected to each other. Thus, rotation of the turbine causes rotation of the rotor of the generator.

The electric power potential from water flow is the product of the mass of the water, the height through which it falls and gravitational acceleration. The mass is a function of the amount of water that is available and its rate of flow. The design of the power station will determine the height of the water. Most designs draw in water from near the top of the dam and then discharge it at the bottom into an existing downstream riverbed. This optimizes height while maintaining reasonable and controllable flow.

In most modern hydroelectric generating stations, the turbogenerators are oriented vertically. These are the familiar structures that protrude above the main floor in these stations. However, almost all of the structure is located below what is visible at main-floor level. This includes the generator pit, and below that the turbine pit and intake and discharge tube. These structures and the water-guiding channels are entered on occasion.

In stations of older vintage, the turbogenerator is oriented horizontally. The shaft from the turbine protrudes from a wall into the powerhouse, where it connects to the generator. The generator resembles a very large, old-style, open-case electric motor. In testimony to the design and quality of construction of this equipment, some turn-of-the-century facilities still are operating. Some present-day stations incorporate updated versions of the designs of the older stations. In such stations, the water channel completely surrounds the turbogenerator and entry is gained through a tubular casing that passes through the water channel.

A magnetic field is maintained in the windings of the rotor in the generator. The power for this field is provided by banks of lead-acid or caustic-filled nickel cadmium batteries. The motion of the rotor and the magnetic field that is present in its windings induce an electromagnetic field in the windings of the stator. The induced electromagnetic field provides the electrical energy which is supplied to the power grid. Electric voltage is the electrical pressure that arises from the flowing water. In order to maintain the electrical pressure—that is, the voltage—at a constant level requires changing the flow of water across the turbine. This will be done as demand or conditions change.

The flow of electricity can lead to electrical arcing, as for example, in the exciter assembly in the rotor. Electrical arcing can generate ozone, which, even at low levels can adversely affect the rubber in fire hose and other materials.

Hydroelectric power generators produce very high currents and high voltages. Conductors from the generators connect to a unit transformer and from this to a power transformer. The power transformer boosts the voltage and reduces the current for transmission over long distances. Low current minimizes energy loss due to heating during transmission. Some systems use sulphur hexafluoride gas in place of conventional oils as an insulator. Electrical arcing can produce breakdown products which can be significantly more hazardous than sulphur hexafluoride.

The electric circuits include breakers that can rapidly and unpredictably cut out the generator from the power grid. Some units utilize a blast of compressed air to break the connection. When such a unit kicks in, it will produce an extremely high level of impulsive noise.

Administration and Station Operations

Most people are familiar with the administration and station operations aspects of hydro generation, which generally create the public profile of the organization. The power plant administration seeks to ensure that the plant provide reliable service. Administration includes office personnel involved in business and technical functions, and management. Station operations personnel include plant managers and supervisors, and process operators.

Hydrogeneration is a process operation but unlike other process operations, such as those in the chemical industry, many hydrogenerating stations have no operating staff. The generating equipment is operated by remote control, sometimes from long distances. Almost all work activity occurs during maintenance, repair, modification and upgrading of plant and equipment. This mode of operation demands effective systems which can transfer control away from energy production to maintenance to prevent unexpected startup.

Hazards and the management structure

Electrical utilities are traditionally managed as “bottom-up” organizations. That is, the organizational structure has traditionally provided a path of upward mobility that begins with entry-level positions and leads to senior management. Relatively few individuals enter the organization laterally. This means that the supervision and the management in a power utility will likely have experienced the same working conditions as the individuals who presently occupy entry-level positions. Such an organizational structure can have implications with respect to potential worker exposure to hazardous agents, especially those which have chronic cumulative effects. For example, consider noise. Employees who currently serve in management positions could themselves have sustained serious hearing loss when they were employed in jobs that had occupational noise exposures. Their hearing loss could go undetected in company audiometric testing programmes, since such programmes generally include only those employees who are currently exposed to high levels of noise at work.

Maintenance of Generating Equipment

Maintenance of generating equipment subdivides into two main types of activity: electrical maintenance and mechanical maintenance. While both types of work may occur simultaneously and side by side, the skills and work needed to perform these are completely different.

Maintenance could necessitate shutting down and dismantling a unit. Water flow at the intake is controlled by headgates. Headgates are steel structures that are lowered into the intake channel to block the flow of water. Blocking the flow permits water to drain from the interior channels. The quiescent water level in the outlet from the turbine (draught tube) is below the level of the scroll case and blades of the turbine runner. This permits access to these structures. The scroll case is a tapered, spiral-shaped structure that directs the flow of water around the turbine runner in a uniform manner. Water passes from the scroll case through guide vanes that direct flow, and movable vanes (wicket gates) that control the volume.

When needed, the generator and turbine can be removed from their normal locations and placed onto the main floor of the powerhouse. Removal may be necessary for repainting or degreasing and repair and replacement of windings, bearings, brakes or hydraulic systems.

Sometimes the blades of the runner, as well as wicket gates, the guide vanes and the water-conducting structures in the scroll case and draught tube, sustain damage from cavitation. Cavitation occurs when the pressure in the water falls below its vapour pressure. When this happens, gas bubbles form and the turbulence that is caused by these bubbles erodes the materials which the water touches. It may be necessary to repair the damaged materials by welding, or by repairing and recoating the steel and concrete surfaces.

Steel structures may also require repair and recoating if they have become corroded.

Hazards

There are a variety of hazards associated with the generation of hydroelectric power. Some of these hazards are shared by all the employees who work in the industry, while others are restricted to those involved in either electrical or mechanical maintenance activities. Most of the hazards which can arise are summarized in table 1 and table 2, which also summarize precautions.

Table 1. Controlling exposures to selected chemical and biological hazards in hydroelectric power generation

Exposure

Where it can be found

Affected workers

Approaches to control

Abrasive dusts
(blasting)

Dust can contain blast material and paint dust. Paint applied prior to 1971 may contain PCBs.

Mechanical
maintenance
workers

-Dust control system
-Personal protective equipment
-Respiratory protection
-Personal hygiene measures
-Medical surveillance (depends on circumstances)

Asbestos

Asbestos may be present in generator brakes, pipe and electrical insulation, spray-on coatings, asbestos cement and other products; exposure depends on friability and proximity to source.

Electrical maintenance
workers, mechanical
maintenance
workers

-Adopt current best practices for work involving asbestos-
containing products.
-Personal protective equipment
-Respiratory protection
-Personal hygiene measures
-Medical surveillance (depends on circumstances)

Battery
explosion
products

Short circuit across terminals in banks of batteries could cause explosion and fire and exposure to liquid and aerosols of the electrolyte.

Electrical maintenance
workers

-Shielding of battery terminals and noninsulated conductors
-Practices and procedures to ensure safe conditions of work around this equipment

Coating
decomposition
products

Emissions can include: carbon monoxide, inorganic pigments containing lead and other chromates and decomposition products from paint resins. PCBs may have been used as plasticizers prior to 1971. PCBs can form furans and dioxins, when heated.

Mechanical
maintenance
workers

-Local exhaust ventilation
-Respiratory protection
-Personal hygiene measures
-Medical surveillance (depends on composition of the coating)

Chlorine

Chlorine exposure can occur during connection/disconnection of chlorine cylinders in water and wastewater treatment systems.

Operators

-Follow chlorine industry guidelines when working with chlorine cylinders
-Escape respirator

Degreasing
solvents

Degreasing of electrical equipment requires solvents with specific properties of inflammability, solvation and rapid evaporation without leaving a residue; solvents meeting these characteristics are volatile and can pose inhalation hazards.

Electrical maintenance
workers

-Local exhaust ventilation
-Personal protective equipment
-Respiratory protection

Diesel
exhaust emissions

Emissions primarily include nitrogen dioxide, nitric oxide, carbon monoxide, carbon dioxide, sulphur dioxide and particulates containing polycyclic aromatic hydrocarbons (PAHs) from vehicles or engines operated in the powerhouse.

All workers

-Prohibit operation of automobiles and trucks in buildings.
-Local exhaust system to collect exhaust at source
-Catalytic converters on exhaust systems

Insect remains

Some insects breed in the fast waters around the station; following mating, the adults die and the carcasses decay and dry; some individuals develop allergic respiratory
sensitization to substances in the dust.

 

 

Following draining, insect larvae living in the water channels may attempt to lower their bodies into remaining water by production of thread-like ropes; some individuals may develop allergic respiratory sensitivity to dust resulting from drying out of these materials.

All workers



 

 

 

 


Maintenance workers

-Insects that spend part of their lives in fast-running waters lose habitat as a result of construction of a
hydrogenerating station. These organisms may use the water channels of the station as surrogate habitat. Dust from dried remains can cause allergic sensitization.

-Control measures include:
Lighting that does not attract flying insects
Screens on windows, doors and openings in the building envelope.
Vacuum cleaning to remove carcasses

Oils and lubricants

Oils and hydraulic fluids coat windings of the rotor and stator; decomposition of hydrocarbons in contact with hot surfaces can produce polycyclic aromatic hydrocarbons (PAHs). Exposure can occur by inhalation and skin contact. Skin contact can cause dermatitis.

Electrical maintenance
workers, mechanical
maintenance
workers

-Personal protective equipment (depends on circumstances)

Ozone

Ozone generated by arcing in the rotor and other electrical equipment could pose an exposure problem, depending on proximity to the source.

All workers

-Maintain electrical equipment to prevent arcing

Paint fumes

Paint aerosols contain sprayed paint and diluent; solvent in droplets and vapour can form flammable mixture; resin system can include isocyanates, epoxies, amines, peroxides and other reactive intermediates.

Solvent vapours can be present in paint storage and mixing areas, and paint booth; flammable mixtures can develop inside confined spaces during spraying.

Bystanders, painters

-Paint spray booth
-Personal protective equipment
-Respiratory protection
-Personal hygiene measures
-Medical surveillance (depends on circumstances)

Polychlorinated
biphenyls (PCBs)

PCBs were used in electrical insulating fluids until the early 1970s; original fluids or residuals may still be present in cables, capacitors, transformers or other equipment; exposure can occur by inhalation or skin contact. Fire or extreme heating during service can convert PCBs into furans and dioxins.

Electrical maintenance
workers

-Personal protective equipment
-Respiratory protection
-Medical surveillance (depends on circumstances)

Sulphur hexafluoride
and breakdown
products

Electrical arc breakdown of sulphur hexafluoride produces gaseous and solid substances of considerably greater toxicity.

Release of large quantities of sulphur hexafluoride into subgrade spaces can create oxygen deficiency by displacing the atmosphere.

Electrical maintenance
workers

-Local exhaust ventilation
-Personal protective equipment
-Respiratory protection
-Medical surveillance (depends on circumstances)

Welding and brazing
fumes

Cadmium, lead, silver in solder




Work primarily involves carbon and stainless steels; aluminium welding may occur. Build-up welding is required to repair erosion due to cavitation.
Emissions include: shield gases and fluxes, metal fumes, ozone, nitrogen dioxide, visible and ultraviolet energy.

Electrical
maintenance
workers

 

 

Mechanical
maintenance
workers

-Local exhaust    ventilation
-Personal protective equipment
-Respiratory protection
-Personal hygiene measures

-Medical surveillance (depends on composition of base metal and metal in wire or rod)

 

Table 2. Controlling exposures to selected chemical and biological hazards in hydroelectric power generation

Exposure

Where it can be found

Affected workers

Approaches to control

Awkward working
postures

Prolonged work in awkward posture can lead to musculoskeletal injury.
Fall hazard exists around pits and openings in structures.

All workers

-Equipment designed to reflect ergonomic principles
-Training in muscle conditioning, lifting and back care
-Work practices chosen to minimize occurrence of musculoskeletal injury

Confined spaces

The dam, control structures, control gates, water-conducting channels, generator and turbine machinery contain many pits, sumps, tanks and other enclosed and partially enclosed spaces that can become oxygen deficient, can confine hazardous atmospheres, or can contain other hazardous conditions.

All workers

-Air testing devices
-Portable ventilation systems
-Personal protective equipment
-Respiratory protection

Drowning

Drowning can occur following a fall into fast-moving water in the forebay (intake zone) or tailrace (discharge zone) or other area. Extremely cold water is present in higher latitudes during spring, fall and winter months.

All workers

-Personnel containment barriers
-Fall-arrest systems
-Life jackets

Electrocution

Areas in the station contain energized, unshielded conductors; equipment containing shielded conductors can become live following removal of the shielding. Electrocution risk results from deliberate entry into unauthorized areas or from accidental failure of protection systems.

All workers

-Establish practices and procedures to ensure safe conditions of work with electrical equipment.

Electromagnetic
fields (including
radiofrequency)

Generating and other electrical equipment produces DC and 60 Hz (and higher) AC fields; exposure depends on proximity to source and shielding offered by structures. Magnetic fields are especially difficult to attenuate by shielding. Significance of exposure has yet to be established.

Radio frequency: Effects on humans not fully established.

All workers

-Hazard not established below present limits

Heat

Generators develop considerable heat; generators and heat exchangers may discharge heated air into the powerhouse; powerhouse structure can absorb and radiate solar energy into the building; heat injury can occur during warmer months, depending on climate and level of exertion.

Indoor workers

-Deflecting heated air towards the roof, shielding, engineering controls
-Electrolyte replacement drinks
-Personal protective equipment

Noise

Steady-state noise from generators and other sources and tasks could exceed regulated limits; air blast breakers produce very high levels of impact noise; these could discharge at any time.

All workers

-Apply noise control technology.
-Personal hearing protection

Shiftwork

Shift operations can produce physiological and psychosocial stresses; psychosocial stresses can be especially serious for the small numbers involved in small and isolated communities where these operations tend to be located.

Operators

-Adopt work schedules that reflect current knowledge about circadian rhythms.

Vibration, hand-arm

Vibration produced by powered hand tools and hand-held equipment is transmitted through hand grips.

Electrical maintenance
workers, mechanical
maintenance
workers

-Utilize tools meeting current standards for hand-arm vibration.
-Vibration-absorbing gloves

Vibration, whole-body

Structure-borne vibration originating from the rotational motion of generators and turbulence of water flows is transmitted through floors and walls.

All workers

-Monitor and service rotating equipment to minimize vibration.

Visual display units

Effective use of computerized workstations depends on application of visual and office ergonomic principles.

Office workers
(management,
administrative and technical staff)

-Apply office ergonomic principles to selection and utilization of video displays

Weather-related
problems

Ultraviolet energy can cause sunburn, skin cancer and cataracts.

Cold can cause cold stress and frostbite.
Heat can cause heat stress.

Outdoor workers

-Work clothing that protects against cold
-Work clothing that shields against solar radiation
-Eye protection that provides protection against solar radiation
-Sunscreens (seek medical advice for prolonged use)

 

Environmental Effects

Hydroelectric generation of power has been promoted as being environmentally friendly. Of course, it does provide tremendous benefit to society through the provision of energy and the stabilization of the flow of water. But such generation of energy does not come without an environmental cost, which has in recent years received more and more public recognition and attention. For example, it is now known that flooding large areas of the earth and of rock by acidic water leads to the leaching of metals from these materials. Bioaccumulation of mercury has been found in fish that have been caught in the water from such flooded areas.

Flooding also changes the turbulence patterns in the water as well as the level of oxygenation. Both of these can have serious ecological effects. For example, salmon runs have disappeared on dammed rivers. This disappearance has occurred, in part, because the fish either cannot locate or traverse a path to the higher water level. In addition, the water has come to resemble a lake more than a river, and the still water of a lake is not compatible with salmon runs.

Flooding also destroys fish habitat and can destroy the breeding areas for insects, upon which fish and other organisms depend for nourishment. In some cases, flooding has destroyed productive agricultural and forest lands. Flooding of large areas has also raised concern about climatic change and other changes in the ecological balance. The holdback of fresh water that had been destined to flow into a body of salt water has also raised concern about changes in salinity.

 

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Read 8216 times Last modified on Friday, 16 September 2011 14:06

Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides