الجمعة، 8 أبريل 2011

تعاريف مصطلحات الكهرباء Electricity and power definitions

Auxiliary power/energy
All electricity consumed internally within the boundary of a power station or cogeneration plant to run the plant.

Aeroderivative gas turbine
Aeroderivative gas turbines originate from the aviation industry. They are lightweight. They have higher thermal efficiency and capital cost than industrial gas turbines and their maintenance costs can also be higher. Exhaust gas temperatures are generally lower than industrial gas turbines. Performance decreases dramatically at high ambient temperatures.

Auxiliary boiler
Fuel fired boiler used to raise the site steam and/or hot water requirements.

Availability (%)
Availability is an indicator of the maximum amount of electrical energy that a unit is able to generate during a period, after making allowances for outages due to all causes
(Total installed capacity (MW) * period hours - MWh losses due
to all outages during the period) * 100 divided by:
Total installed capacity (MW) * period hours

Back pressure steam turbine
A simple non-condensing steam turbine. High-pressure steam is expanded through the backpressure steam turbine to generate electricity, and is exhausted at the required steam conditions for use in a process or for direct heating.

Base load
Operating regime in which the generator operates at its full capacity (or close to it) at all times.

Biomass
Is material produced by photosynthesis or is an organic by-product of a waste stream suited as a feedstock for electrical and/or thermal energy or the manufacture of fuels and substitutes for petrochemicals and other energy intensive products. Includes a wide variety of renewable organic materials (eg sugar cane pulp, food processing waste, animal manures and sewerage, and organic municipal solid waste). Energy conversion paths include combustion and gasification for heat and power (cogeneration), pyrolysis for liquid and gaseous fuels and chemicals, and anaerobic digestion and fermentation. Biomass fuels tend to be renewable.

Boiler blowdown
Stream of water which is bled from the boiler drum or steam supply system to control the concentration of total solids and other contaminants in the boiler water.

Boiler drum
Generally the steam drum of a boiler where the steam generated is separated from the circulating boiler water.

Bottoming cycle
Heat is recovered from an industrial process (eg a furnace) and then used to generate steam through a boiler, which is then used to generate electricity. Lower pressure steam can also be used for process.

Capacity
The rated continuous load carrying ability expressed in megawatts, of generation equipment; sometimes referred to as maximum continuous rating (MCR).

Capacity factor (%)
Total energy produced for a specified period relative to the total possible amount of energy that could have been produced for the same period.
Total period energy generated (MWh) * 100 divided by:
Total installed capacity (MW) *period hours

Centralised Generation

Electricity production based on large-scale power stations which are usually remote from electricity consumers, and which require long-distance, high-voltage transmission networks.

Circuit Breakers
A type of switch used to control the flow of power in an electrical circuit. Special forms of these are used in switchyards and substations for the control of power between power (transmission and distribution) lines.

Clean-as-new
New unit, no degradation.

Cogeneration
Involves the generation of two products from a single fuel. e.g. useful combined heat and power. It is typically, two to three times more efficient than major conventional, predominantly coal-fired, centralised power stations. Production efficiency is achieved by harnessing heat that would otherwise be wasted. The heat and power can also be converted into many applications such as cooling.

Condensate
Steam which has been condensed for return to the boiler.

Creep
The plastic (i.e. inelastic) movement of a metal or other solid. Temperature and stress on the material directly influence the rate of creep. The composition of the material also has a bearing on its rate of creep.

Consideration of the material's creep properties is particularly impotant in the selection of materials for components subject to high temperature and high stresses, such as those in boilers, turbines and superheaters.

Deaerator
Used to preheat feedwater before entering the waste heat boiler, and to drive off non-condensable and potentially corrosive dissolved gases. Requires steam to provide the energy for heating. Not relevant to hot water case.

Deep Connection
Transmission assets and services provided by the Transmission Network Operator which are:
initially for one participant, but where the assets may eventually become part of the shared network; and
immediately part of the shared network, but are assets which would not need to exist if not for connection to one participant.
Charges and risks associated with Deep Connection services may eventually be shared with other participants, when arrangements change.

DegradationDemand

Deterioration in power output and/or heat rate of an engine or turbine under operating conditions due to, for example, inlet air contaminants, fuel contaminants and thermal stress. Continuous process occurring between overhauls.

Demand
Demand is usually taken to mean the electricity requirements of the end consumer. However, in the NEM, the term "demand" includes consumption and losses in the distribution and transmission networks and in the power stations as well as end user consumption. Extreme care should therefore be taken in the use of published "demand" values.

The demand requirements are usually in two forms:
Instantaneous Demand - the size of the demand at any instance in time and is measured in MW. It is important in determining the generating capacity required to on-line, particularly at peak usage times. It also is important in determining if the transmission and distribution systems can cope with the flow of electricity during peak demand times; and
Period Demand - the amount of electrical energy used by the end use consumers over a period of time (measured in MWh or GWh). This measure is used to estimate how hard the power stations need to operate during that period, the amount of fuel consumed in the generation of that energy and the potential income for electricity retailers and generators.

Demineralised water

Pure water produced by removing mineral salts, usually by an ion exchange process.

Distributed Generation
Is power generation generally located close to where it is consumed, for example, supplying electricity on-site or over-the-fence. Distributed generators can also export electricity into the local grid. Also referred to as decentralised, embedded or localised. Includes cogeneration and other types of generation such as fuel cells and photovoltaic. This can be as small as a 3 kWe micro-cogeneration plant, or as large as a 450 MW industrial on-site system.

Distribution
Electrical cabling system which transfers power, usually over long distances, to the consumers. Distribution systems are usually operated at medium to low voltages (eg 110kV, 66kV, 33kV, 11kV, 6.6kV & 415V).

Dump condenser
Excess steam from the waste heat boiler bypasses the steam turbine and goes directly to the condenser. Used to balance site load with steam generated.

Dump stack & damper
Used to control flow and temperature of exhaust gas to bypass the waste heat boiler. Can also be used to isolate equipment in the exhaust gas stream when the equipment is out of service or requires maintenance. Used to balance steam generated with site load.

DUOS
Distribution use of system charge. In the National Electricity Code, the charge for using the electricity network on or below 66 kV voltage level.

Economiser
A counterflow heat exchanger for recovering energy from the exhaust gas. It increases the temperature of the water entering the boiler drum using otherwise wasted exhaust heat and hence increasing steam-raising ability. Economisers are assumed on WHBs in this analysis.

Efficiency
Thermodynamically, the ratio of useful energy output to energy input into a process. Has many specific definitions and care needs to be taken that the meaning is clear. See also Heat Rate.

Efficiency (Generated)%
Total energy generated (kWh) * 3600 * 100 divided by:
Quantity fuel (kg) * higher heating value of fuel consumed (kJ/kg)
Efficiency (gen) is related to Heat Rate (gen) by:
Efficiency (gen) (%) = 3600 * 100 .
Heat Rate (gen) (kJ/kWh)

Efficiency (Sent Out)%
(Total energy generated (kWh) - total auxiliary energy (kWh) * 3600 * 100 divided by / Quantity fuel (kg) * higher heating value of fuel consumed (kJ/kg)

Efficiency (s/o) is related to Heat Rate (s/o) by:
Efficiency (s/o) (%) = 3600 * 100 / Heat Rate (s/o) (kJ/kWh)

Electrical efficiency (Cogen) NFT)
[Cogen gross elec output] + [imported electricity] - [parasitic electricity] dividede by:
[Fuel to cogen unit] + [fuel used for imported electricity]

Embedded Generation

Smaller-scale generators that are connected to electricity distribution networks. By nature of where they connect to the grid, they are distributed generators. Are in contrast to large-scale coal-fired generators that are connected to very high voltage electricity transmission networks.

Enthalpy
Measure of the heat content of a substance.

Entropy
A measure of unavailable energy in a thermodynamic system.

Feedwater
Total flow supplied to the boiler, sum of condensate and makeup.

Forced outage rate (%)
Forced outages require the removal of a generating unit from service for repairs that have not been planned.

Energy lost due to forced outages (MWh) * 100 divide by:
Total installed capacity (MW) * period hours

Fossil Fuel
Derived from hydrocarbon and includes coal, natural gas, coal seam methane, oil and liquid petroleum gas.

Gas turbine

An engine operating on what is known as the Brayton cycle with continuous steady flow compression of air, constant pressure combustion and expansion of the compressed heated gases through an expansion turbine. Working fluid is usually air, fuels can be gaseous or premium liquid fuels such as distillate.

Generated Energy (MWh)
Electrical energy generated by the power plant.

Greenhouse Gases (GHG)
Those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and re-emit infrared radiation.

Greenhouse Intensity (GI)

Measure of Greenhouse efficiency as the emission rate of greenhouse gases from fuel burning expressed in kg CO2 (equiv.) per MWh sent out. For cogeneration, this is discounted for steam/heat production.

Heat rate
A form of expressing efficiency of an engine or turbine. The fuel heating value consumed per unit of useful output (usually electrical output). Common unit is kJ/kWh. To convert to efficiency divide by 3600 and invert.

Heat Rate (Generated) (kJ/kWh)
Quantity fuel (kg) * higher heating value of fuel consumed (kJ/kg) divided by:
Total energy generated (kWh)

Heat Rate (gen) is related to Efficiency (gen) by:
Heat Rate (gen) (kJ/kWh) = 3600 * 100 divided by:/ Efficiency (gen) (%)

Heat Rate (Sent Out) (kJ/kWh)
Quantity fuel (kg) * higher heating value of fuel consumed (kJ/kg) divided by:/ Total energy generated (kWh) - Total auxiliary energy (kWh)

Heat Rate (s/o) is related to Efficiency (s/o) by
Heat Rate (s/o) (kJ/kWh) = 3600 * 100 ./ Efficiency (s/o) (%)

Heat Recovery Steam Generator
A boiler that uses waste heat (such as gas turbine or reciprocating engine exhaust gas) to produce steam or hot water.

Higher Heating Value (HHV)

The amount of heat recovered when the products of complete combustion of a unit quantity of a fuel are cooled to the initial temperature of the air and fuel (kJ/kg).

For Natural Gas, the relationship between Higher Heating Value (HHV) and Lower Heating Value (LHV) is (approximately):
HHV = 1.1 * LHV

Industrial gas turbine
Heavier, more robust, and cheaper than aeroderivative gas turbines. Generally have lower thermal efficiency and higher exhaust gas temperatures than aeroderivative gas turbines.

Isentropic
Constant entropy, or effectively zero loss. Used in the context of steam turbine ability to convert steam energy into mechanical energy, as opposed to leaving the turbine as exhaust steam energy at the exhaust steam pressure.

ISO conditions
Ambient conditions stated in the ISO Standard 2314 = 15°C, 0 metres ASL, (i.e. at average sea level) and 60% Relative Humidity (RH).

Load factor (%)
(Also called Output Factor) - total energy produced for a specified period relative to the total possible amount of energy that could have been produced for the operating hours during the same period. It can be thought of as the load (% MCR) that would have produced the same total energy over the same operating hours.

Total energy generated during the period (MWH) * 100
Total installed capacity (MW) * operating hours

Lower Heating Value (LHV)
The amount of heat recovered when the products of complete combustion of a unit quantity of a fuel are cooled to just above the dew point of the water vapour it contains (kJ/kg).

For Natural Gas, the relationship between Lower Heating Value (LHV) and Higher Heating Value (HHV) is (approximately):
LHV = HHV/1.1

Makeup

Treated raw water, which is added to the system to replace steam and water lost to site requirements, blowdown, evaporation, sampling or venting.

Network Losses
Amount of energy lost when electricity flows in the power lines of a transmission and/or distribution network. The electrical resistance of the power lines is the main contributor to these losses.

Operating Hours

Total number of hours during which a unit generates electricity over a period of time.

Parasitics
The plant's internal power consumption and losses.

Peaking

Operating regime in which the generator operates at its full capacity (or close to it) only for short periods at times of high demand. The plant is shut down for the remainder of the time.

Planned outage rate (%)
Planned outages are due to overhaul (or other) work which is planned well in advance, usually by more than one year.

Energy lost due to planned outages (MWh) * 100
Total installed capacity (MW) * period hours

Pool Price
Price of electricity available for supply in the National Electricity Market. Each generator provides prices at which it is willing to supply designated levels of MW. The grouping of these prices within the NEM is known as the pool.

Process steam
Steam used within an industrial plant, usually for heating.

Pulveriser
A machine that grinds coal into a fine powder for injection into the furnace of a pulverised coal fired boiler.

Reciprocating engine
An engine characterised by the movement of the pistons in the cylinders back and forth in a straight line driving a crankshaft to convert the work into rotary shaft work. Typical vehicle engine.

Renewable Generation
Produces no net greenhouse emissions. Includes power generated from non-hydrocarbon, natural resources such as biomass, hydro, wind, solar (photovoltaic) and tidal. It also includes power generated using waste products. Tend to be distributed and connected to local distribution networks.

Reserve Plant Margin
Reserve Plant Margin is a well used (but generally misunderstood) simplistic measure which, in theory, is meant to give an indication of the ability of the electricity system's generation plant to cope with the estimated peak (instantaneous) demands on the system.

It is measured by:
1. Summing the supply capacities of all the power stations able to operate at any time during a particular interval of time(usually the three winter months or the three summer months);
2. Dividing by the total power station sent out supply required to meet the maximum peak (instantaneous) demand estimated for that interval of time;
3. Taking away 1.

A value of (say) 0.3 means that there is a 30% "excess" in sent out supply capacity available to meet the maximum peak demand (i.e. the Reserve Plant Margin - 30%).
Note: The reserve plant margin calculation may not take into account:
The generating capacity kept in reserve for system stability reasons;
The unavailability of power stations due to scheduled and unscheduled outages;
The short term overload capabilities of some of the power stations;
The energy limits of the northern hydros and some peaking plants, such as Wivenhoe pumped storage hydro power station;
The ability of demand side management techniques (such as hot water switching and short term reduction in industrial loads) to reduce peak loads;
The interconnectors being able to supply some of the required capacity; or
The load limits on the interconnectors and inter-zonal transmission lines.
For these, and other, reasons, the reserve plant is compared to a "guesstimated" optimum value that is supposed to take some account of these factors. Historically, values between about 20% to 25% have been used in Queensland. A reserve plant margin less than this meant that there was a risk of not having sufficient plant to meet the demand peaks, while more than this indicated an over-capacity in generating plant to meet the peak demands and that some plant would be under-utilised.

The main problem with using the reserve plant measure is that most people are not be aware of the way in which it is calculated and so would assume that there was always a large excess in generating capacity and that all peak loads would be met. With the advent of the NEM, the importance of the Reserve Plant Margin measure has, thankfully, declined in importance, usefulness and usage.

Retrofit
Any improvement activity on an existing power plant that generally involves fitting new equipment to an existing plant.

Sent Out Energy
Electrical energy leaving the power plant. This is the generated energy minus the auxiliary energy used in the plant.

Energy generated (MWh) minus auxiliary energy (MWh).

Shallow Connection
Transmission assets and services provided for one participant where the assets will never be part of the shared network.

Steam turbine
An engine in which a vaned wheel is made to revolve by the impingement of steam. Converts steam energy to mechanical energy.

Substation

Similar to a "switchyard" but is usually associated with lower voltage distribution lines. Most of the equipment is enclosed within a building.

Superheater
A heat exchanger part of a boiler for increasing the temperature of saturated steam to superheated steam. Generally steam admitted into a steam turbine must be superheated (that is, above the saturation temperature at that pressure).

Supplementary fired
Additional gas firing into a waste heat boiler when the unfired exhaust gas is not sufficiently hot for raising the site steam requirements (temperature or steam mass flow). Can generally be done on gas turbine exhausts because sufficient oxygen remains in the exhaust. Not generally done on reciprocating engine exhausts due to lower excess oxygen in exhaust.

Supply
Supply has many meanings depending on the point of view of the organisation providing the data. The main definitions can be described by working back from the electricity end user to the power station:

Supply from the distribution system - this is equal to the end user consumption;
Supply from the transmission system - this is equal to the end user demand plus the demand from any large user being supplied directly from the transmission system plus the losses in the distribution system;
Supply sent out from the power stations - this is equal to the supply from the transmission system plus transmission system losses; and
Supply generated - this is the electricity generated by the power stations and is equal to the supply sent out from the power stations plus losses within the power stations that are associated with the generation of the electricity.
Note: For each of these, the "instantaneous" and "period" values of the electricity supply are required.

Switchyard
Fenced area containing electrical equipment used to control the transfer of power from one set of power lines to another set of power lines. It is usually associated with high voltage transmission lines, but is also used in association with lower voltage distribution lines.

Thermal efficiency (Cogen) (overall)
[Cogen gross elec output] + [imported electricity] + [site steam] divided by:
[fuel to cogen unit] + [fuel used for imported electricity] + [fuel to aux boiler]

Where [site steam] includes hot water as applicable.

Topping cycle
High-pressure steam is raised in an auxiliary boiler and expanded through a backpressure steam turbine to the required site steam conditions.

Total installed capacity
Sum of the capacity of each unit making up the power plant.

Transmission
Electrical cabling system which transfers large amounts of power, usually over long distances. Transmission systems are usually operated at extra high voltages (eg 132kV, 275kV & 330kV).

TUOS
Transmission use of system charge. In the National Electricity Code, the charge for using the electricity network above 66 kV voltage level

Turnkey
Installation to the point of readiness for operation, generally a single design and construct contract. Some owner's costs, such as owner's engineering, spares, owner's start up labour and fuel, may be excluded and need to be considered in the indirect costs or elsewhere.

Waste

Waste products used in power generation include cane residue (bagasse) in the sugar industry; sludge gas from sewerage treatment plants; and methane from landfill sites.

Waste heat boiler
A boiler that uses waste heat (such as gas turbine or reciprocating engine exhaust gas) to produce steam or hot water

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