High-voltage direct current
Introduction:
AC VS DC:
Ø AC won DC because AC can be transmitted with lower losses at high voltages.
Ø DC flows limited by voltage and resistance
Ø AC flows limited by resistance but also by the work needed to establish electric and magnetic fields
– Electric fields charge and discharge the insulation media surrounding the electric conductor
Ø Current “leaking” through cable insulation limits the useful length of cables for ac power transmission
– Magnetic fields surround the electric conductors and can magnetize magnetic materials and induce currents into conductors
Ø Energy required to build up the magnetic fields limits the useful length of overhead lines for power transmission
Ø DC is difficult to transform from a low voltage to a high voltage and back.
ü In fact, it is not possible to do electrically.
ü The coupling between ac and dc systems still uses transformers.
Limitations on long distance power transmission are
At ź wave length the voltage goes to infinity and the current goes to zero.
Solution: Detune antenna or use HVdc.
Long distance HVDC lines carrying hydroelectricity from Canada's Nelson river to this station where it is converted to AC for use in Winnipeg's local grid
A high-voltage, direct current (The modern form of
High voltage transmission:
High voltage is used for transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, higher voltage reduces the transmission power loss. Power in a circuit is proportional to the current, but the power lost as heat in the wires is proportional to the square of the current. However, power is also proportional to voltage, so for a given power level, higher voltage can be traded off for lower current. Thus, the higher the voltage, the lower the power loss. Power loss can also be reduced by reducing resistance, commonly achieved by increasing the diameter of the conductor; but larger conductors are heavier and more expensive.High voltages cannot be easily used in lighting and motors, and so transmission-level voltage must be reduced to values compatible with end-use equipment. The transformer, which only works with alternating current, is an efficient way to change voltages. The competition between the DC of Thomas Edison and the AC of Nikola Tesla and George Westinghouse was known as the War of Currents, with AC emerging victorious. Practical manipulation of DC voltages only became possible with the development of high power electronic devices such as mercury arc valves and later semiconductor devices, such as thyristors, insulated-gate bipolar transistors (IGBTs), high power capable MOSFETs (power metal–oxide–semiconductor field-effect transistors) and gate turn-off thyristors (GTOs).
History of HVDC transmission
One conversion technique attempted for conversion of direct current from a high transmission voltage to lower utilization voltage was to charge series-connected batteries, then connect the batteries in parallel to serve distribution loads. While at least two commercial installations were tried around the turn of the 20th century, the technique was not generally useful owing to the limited capacity of batteries, difficulties in switching between series and parallel connections, and the inherent energy inefficiency of a battery charge/discharge cycle.
The grid controlled mercury arc valve became available for power transmission during the period 1920 to 1940. Starting in 1932, General Electric tested mercury-vapor valves and a 12 kV DC transmission line, which also served to convert 40 Hz generation to serve 60 Hz loads, at Mechanicville, New York. In 1941, a 60 MW, +/-200 kV, 115 km buried cable link was designed for the city of Berlin using mercury arc valves (Elbe-Project), but owing to the collapse of the German government in 1945 the project was never completed. The nominal justification for the project was that, during wartime, a buried cable would be less conspicuous as a bombing target. The equipment was moved to the Soviet Union and was put into service there
Introduction of the fully-static mercury arc valve to commercial service in 1954 marked the beginning of the modern era of
Advantages of HVDC over AC transmission
The advantage of The Advantages of state-on-the-art HVDCplus Technology
Ø No need for active power transmission
Ø No require for reactive power load
Ø Active and reactive power can be controlled separately
Ø Connection to a weak or isolated AC system
Ø Multi-terminal hvdc -System can configured much simpler
Ø Smaller Space and Mva requirement for filters
Ø Transmission to an Island
In a number of applications
Ø Undersea cables, where high capacitance causes additional AC losses. (e.g., 250 km Baltic Cable between Sweden and Germany] and the 600 km NorNed cable between Norway and the Netherlands)
Ø Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas
Ø Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install
Ø Power transmission and stabilization between unsynchronised AC distribution systems
Ø Connecting a remote generating plant to the distribution grid, for example Nelson River Bipole
Ø Stabilizing a predominantly AC power-grid, without increasing prospective short circuit current
Ø Reducing line cost. HVDC needs fewer conductors as there is no need to support multiple phases. Also, thinner conductors can be used since HVDC does not suffer from the skin effect
Ø Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies
Ø Synchronize AC produced by renewable energy sources
Long undersea high voltage cables have a high electrical capacitance, since the conductors are surrounded by a relatively thin layer of insulation and a metal sheath. The geometry is that of a long co-axial capacitor. Where alternating current is used for cable transmission, this capacitance appears in parallel with load. Additional current must flow in the cable to charge the cable capacitance, which generates additional losses in the conductors of the cable. Additionally, there is a dielectric loss component in the material of the cable insulation, which consumes power.When, however, direct current is used, the cable capacitance is only charged when the cable is first energized or when the voltage is changed; there is no steady-state additional current required. For a long AC undersea cable, the entire current-carrying capacity of the conductor could be used to supply the charging current alone. This limits the length of AC cables. DC cables have no such limitation. Although some DC leakage current continues to flow through the dielectric, this is very small compared to the cable rating.
Because
Disadvantages
The disadvantages of The required static inverters are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions in line construction cost and lower line loss. With two exceptions, all former mercury rectifiers worldwide have been dismantled or replaced by thyristor units. Pole 1 of the HVDC scheme between the North and South Islands of New Zealand still uses mercury arc rectifiers, as does Pole 1 of the Vancouver Island link in Canada.
In contrast to AC systems[citation needed], realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the inverter control system instead of the inherent properties of the transmission line. Multi-terminal lines are rare. One is in operation at the Hydro Québec - New England transmission from Radisson to Sandy Pond Another example is the Sardinia-mainland Italy link which was modified in 1989 to also provide power to the island of Corsica.
High voltage DC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force current to zero, otherwise arcing and contact wear would be too great to allow reliable switching.
Operating a
Costs of high voltage DC transmission
Normally manufacturers such as AREVA, Siemens and ABB do not state specific cost information of a particular project since this is a commercial matter between the manufacturer and the client.Costs vary widely depending on the specifics of the project such as power rating, circuit length, overhead vs. underwater route, land costs, and AC network improvements required at either terminal. A detailed evaluation of DC vs. AC cost may be required where there is no clear technical advantage to DC alone and only economics drives the selection.
However some practitioners have given out some information that can be reasonably well relied upon:
For an 8 GW 40 km link laid under the English Channel, the following are approximate primary equipment costs for a 2000 MW 500 kV bipolar conventional HVDC link (exclude way-leaving, on-shore reinforcement works, consenting, engineering, insurance, etc.)
Ø Converter stations ~£110M
Ø Subsea cable + installation ~£1M/km
So for an 8 GW capacity between England and France in four links, little is left over from £750M for the installed works. Add another £200–300M for the other works depending on additional onshore works required Rectifying and inverting
Ø Components
Two of three thyristor valve stacks used for long distance transmission of power from Manitoba Hydro dams
Early static systems used mercury arc rectifiers, which were unreliable. Two Because the voltages in
The low-voltage control circuits used to switch the thyristors on and off need to be isolated from the high voltages present on the transmission lines. This is usually done optically. In a hybrid control system, the low-voltage control electronics sends light pulses along optical fibres to the high-side control electronics. Another system, called direct light triggering, dispenses with the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors (LTTs).
A complete switching element is commonly referred to as a valve, irrespective of its construction.
Ø Rectifying and inverting systems
Rectification and inversion use essentially the same machinery. Many substations are set up in such a way that they can act as both rectifiers and inverters. At the AC end a set of transformers, often three physically separate single-phase transformers, isolate the station from the AC supply, to provide a local earth, and to ensure the correct eventual DC voltage. The output of these transformers is then connected to a bridge rectifier formed by a number of valves. The basic configuration uses six valves, connecting each of the three phases to each of the two DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails.An enhancement of this configuration uses 12 valves (often known as a twelve-pulse system). The AC is split into two separate three phase supplies before transformation. One of the sets of supplies is then configured to have a star (wye) secondary, the other a delta secondary, establishing a thirty degree phase difference between the two sets of three phases. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30 degrees, and harmonics are considerably reduced.
In addition to the conversion transformers and valve-sets, various passive resistive and reactive components help filter harmonics out of the DC rails.
Ø Configurations
A block diagram of a bipolar HVDC transmission system, between two stations designated A and B. AC - represents an alternating current network CON - represents a converter valve, either rectifier or inverter, TR represents a power transformer, DCTL is the direct-current transmission line conductor, DCL is a direct-current filter inductor, BP represents a bypass switch, and PM represent power factor correction and harmonic filter networks required at both ends of the link. The DC transmission line may be very short in a back-to-back link, or extend hundreds of miles (km) overhead, underground or underwater. One conductor of the DC line may be replaced by connections to earth ground.
Ø Monopole and earth return
In a common configuration, called monopole, one of the terminals of the rectifier is connected to earth ground. The other terminal, at a potential high above, or below, ground, is connected to a transmission line. The earthed terminal may or may not be connected to the corresponding connection at the inverting station by means of a second conductor.If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations. Therefore it is a type of single wire earth return. The issues surrounding earth-return current include:
ü Electrochemical corrosion of long buried metal objects such as pipelines
ü Underwater earth-return electrodes in seawater may produce chlorine or otherwise affect water chemistry.
ü An unbalanced current path may result in a net magnetic field, which can affect magnetic navigational compasses for ships passing over an underwater cable.
These effects can be eliminated with installation of a metallic return conductor between the two ends of the monopolar transmission line. Since one terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmission voltage which makes it less costly than the high-voltage conductor. Use of a metallic return conductor is decided based on economic, technical and environmental factorsModern monopolar systems for pure overhead lines carry typically 1,500 MW. If underground or underwater cables are used, the typical value is 600 MW.
Most monopolar systems are designed for future bipolar expansion. Transmission line towers may be designed to carry two conductors, even if only one is used initially for the monopole transmission system. The second conductor is either unused, used as electrode line or connected in parallel with the other (as in case of Baltic-Cable).
Bipolar
In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor. However, there are a number of advantages to bipolar transmission which can make it the attractive option.Ø Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return. This reduces earth return loss and environmental effects.
Ø When a fault develops in a line, with earth return electrodes installed at each end of the line, approximately half the rated power can continue to flow using the earth as a return path, operating in monopolar mode.
Ø Since for a given total power rating each conductor of a bipolar line carries only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating.
Ø In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged.
A bipolar system may also be installed with a metallic earth return conductor.Bipolar systems may carry as much as 3,200 MW at voltages of +/-600 kV. Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a bipole.
Back to back
A back-to-back station (or B2B for short) is a plant in which both static inverters and rectifiers are in the same area, usually in the same building. The length of the direct current line is kept as short as possible. - coupling of electricity mains of different frequency (as in Japan; and the GCC interconnection between UAE [50Hz] and Saudi Arabia [60Hz] under construction in ±2009-2011)
- coupling two networks of the same nominal frequency but no fixed phase relationship (as until 1995/96 in Etzenricht, Dürnrohr and Vienna).
- different frequency and phase number (for example, as a replacement for traction current converter plants)
Systems with transmission lines
The most common configuration of an Multi-terminal
Tripole: current-modulating control
A newly patented scheme (As of 2004Current modulation of direct current transmission lines) is intended for conversion of existing AC transmission lines to ) (Combined with the higher average power possible with a DC transmission line for the same line-to-ground voltage, a tripole conversion of an existing AC line could allow up to 80% more power to be transferred using the same transmission right-of-way, towers, and conductors. Some AC lines cannot be loaded to their thermal limit due to system stability, reliability, and reactive power concerns, which would not exist with an HVDC link.
The system would operate without earth-return current. Since a single failure of a pole converter or a conductor results in only a small loss of capacity and no earth-return current, reliability of this scheme would be high, with no time required for switching.
As of 2005India has been converted to bipole , no tri-pole conversions are in operation, although a transmission line in
Corona discharge
Corona discharge is the creation of ions in a fluid (such as air) by the presence of a strong electric field. Electrons are torn from neutral air, and either the positive ions or the electrons are attracted to the conductor, while the charged particles drift. This effect can cause considerable power loss, create audible and radio-frequency interference, generate toxic compounds such as oxides of nitrogen and ozone, and bring forth arcing.Both AC and DC transmission lines can generate coronas, in the former case in the form of oscillating particles, in the latter a constant wind. Due to the space charge formed around the conductors, an HVDC system may have about half the loss per unit length of a high voltage AC system carrying the same amount of power. With monopolar transmission the choice of polarity of the energized conductor leads to a degree of control over the corona discharge. In particular, the polarity of the ions emitted can be controlled, which may have an environmental impact on particulate condensation. (particles of different polarities have a different mean-free path.) Negative coronas generate considerably more ozone than positive coronas, and generate it further downwind of the power line, creating the potential for health effects. The use of a positive voltage will reduce the ozone impacts of monopole HVDC power lines.
Applications
Ø Overview
The controllability of current-flow through Ø AC network interconnections
AC transmission lines can only interconnect synchronized AC networks that oscillate at the same frequency and in phase. Many areas that wish to share power have unsynchronized networks. The power grids of the UK, Northern Europe and continental Europe are not united into a single synchronized network. Japan has 50 Hz and 60 Hz networks. Continental North America, while operating at 60 Hz throughout, is divided into regions which are unsynchronised: East, West, Texas, Quebec, and Alaska. Brazil and Paraguay, which share the enormous Itaipu hydroelectric plant, operate on 60 Hz and 50 Hz respectively. However, HVDC systems make it possible to interconnect unsynchronized AC networks, and also add the possibility of controlling AC voltage and reactive power flow.A generator connected to a long AC transmission line may become unstable and fall out of synchronization with a distant AC power system. An
In general, however, an
The conversion electronics also present an opportunity to effectively manage the power grid by means of controlling the magnitude and direction of power flow. An additional advantage of the existence of
Ø Renewable electricity superhighways
A number of studies have highlighted the potential benefits of very wide area super grids based on In January, the European Commission proposed €300 million to subsidize the development of
Ø Smaller scale use
The development of insulated gate bipolar transistors (IGBT) and gate turn-off thyristors (GTO) has made smaller Frequently asked questions:
ü Part of the Energy Solution?
HV is needed to transmit DC a long distance.
Semiconductor thyristors able to handle high currents (4,000 A) and block high voltages (up to 10 kV) were needed for the widespread adoption ofHVDC .
Newer semiconductor VSC (voltage source converters), with transistors that can rapidly switch between two voltages, has allowed lower power DC.
VSC converter stations also are smaller and can be constructed as self-contained modules, reducing construction times and costs.
HV is needed to transmit DC a long distance.
Semiconductor thyristors able to handle high currents (4,000 A) and block high voltages (up to 10 kV) were needed for the widespread adoption of
Newer semiconductor VSC (voltage source converters), with transistors that can rapidly switch between two voltages, has allowed lower power DC.
VSC converter stations also are smaller and can be constructed as self-contained modules, reducing construction times and costs.
ü Why has HVDC taken off?
Long distance transmission increases competition in new wholesale electricity markets.
Long distance electricity trade, including across nations, allows arbitrage of price differences.
Contractual provision of transmission services demands more stable networks.
Bi-directional power transfers, often needed in new electricity markets, can be accommodated at lower cost usingHVDC
Long distance transmission increases competition in new wholesale electricity markets.
Long distance electricity trade, including across nations, allows arbitrage of price differences.
Contractual provision of transmission services demands more stable networks.
Bi-directional power transfers, often needed in new electricity markets, can be accommodated at lower cost using
ü Increased Benefits of Long-Distance Transmission
Electricity Costs and Prices Fluctuate Substantially
For equivalent transmission capacity, a DC line has lower construction costs than an AC line:
A double HVAC three-phase circuit with 6 conductors is needed to get the reliability of a two-pole DC link.
DC requires less insulation ceteris paribus.
For the same conductor, DC losses are less, so other costs, and generally final losses too, can be reduced.
An optimized DC link has smaller towers than an optimized AC link of equal capacity.
Electricity Costs and Prices Fluctuate Substantially
For equivalent transmission capacity, a DC line has lower construction costs than an AC line:
A double HVAC three-phase circuit with 6 conductors is needed to get the reliability of a two-pole DC link.
DC requires less insulation ceteris paribus.
For the same conductor, DC losses are less, so other costs, and generally final losses too, can be reduced.
An optimized DC link has smaller towers than an optimized AC link of equal capacity.
ü Relative Cost of AC versus DC
*Example Losses on Optimized Systems for 1200 MW
Typical tower structures and rights-of-way for alternative transmission systems of 2,000 MW capacity.
*Right-of-way for an AC Line designed to carry 2,000 MW is more than 70% wider than the right-of-way for a DC line of equivalent capacity.
*This is particularly important where land is expensive or permitting is a problem.
*HVDC “light” is now also transmitted via underground cable – the recently commissioned Murray-Link in Australia is 200 MW over 177 km.
*Can reduce land and environmental costs, but is more expensive per km than overhead line.
Case :1: costs per km basis
*Above costs are on a per km basis. The remaining costs also differ:
*The need to convert to and from AC implies the terminal stations for a DC line cost more.
*There are extra losses in DC/AC conversion relative to AC voltage transformation.
*Operation and maintenance costs are lower for an optimizedHVDC than for an equal capacity optimized AC system.
Case :2 :cost with length
*The cost advantage ofHVDC increases with the length, but decreases with the capacity, of a link.
*For both AC and DC, design characteristics trade-off fixed and variable costs, but losses are lower on the optimized DC link.
*The time profile of use of the link affects the cost of losses, since the MC of electricity fluctuates.
*Interest rates also affect the trade-off between capital and operating costs.
Case : 3:Typical Break-Even Distances
*HVDC is particularly suited to undersea transmission, where the losses from AC are large.
*First commercialHVDC link (Gotland 1 Sweden, in 1954) was an undersea one.
*Back-to-back converters are used to connect two AC systems with different frequencies – as inJapan – or two regions where AC is not synchronized – as in the US .
*Example Losses on Optimized Systems for 1200 MW
Typical tower structures and rights-of-way for alternative transmission systems of 2,000 MW capacity.
*Right-of-way for an AC Line designed to carry 2,000 MW is more than 70% wider than the right-of-way for a DC line of equivalent capacity.
*This is particularly important where land is expensive or permitting is a problem.
*
*Can reduce land and environmental costs, but is more expensive per km than overhead line.
Case :1: costs per km basis
*Above costs are on a per km basis. The remaining costs also differ:
*The need to convert to and from AC implies the terminal stations for a DC line cost more.
*There are extra losses in DC/AC conversion relative to AC voltage transformation.
*Operation and maintenance costs are lower for an optimized
Case :2 :cost with length
*The cost advantage of
*For both AC and DC, design characteristics trade-off fixed and variable costs, but losses are lower on the optimized DC link.
*The time profile of use of the link affects the cost of losses, since the MC of electricity fluctuates.
*Interest rates also affect the trade-off between capital and operating costs.
Case : 3:Typical Break-Even Distances
*
*First commercial
*Back-to-back converters are used to connect two AC systems with different frequencies – as in
ü Special Applications of HVDC
Four major independent a synchronous networks, tied together only by DC interconnections:
1. Eastern Interconnected Network – all regions east of theRockies except ERCOT and Quebec portion of the NPCC reliability council.
2.Quebec – part of the NPCC reliability council.
3.Texas – the ERCOT reliability council.
4. Western Interconnected Network – theWSCC reliability council.
*HVDC links can stabilize AC system frequencies and voltages, and help with unplanned outages.
*A DC link is asynchronous, and the conversion stations include frequency control functions.
*Changing DC power flow rapidly and independently of AC flows can help control reactive power.
*HVDC links designed to carry a maximum load cannot be overloaded by outage of parallel AC lines.
Four major independent a synchronous networks, tied together only by DC interconnections:
1. Eastern Interconnected Network – all regions east of the
2.
3.
4. Western Interconnected Network – the
*
*A DC link is asynchronous, and the conversion stations include frequency control functions.
*Changing DC power flow rapidly and independently of AC flows can help control reactive power.
*
ü Special Applications (continued)
*Most earlyHVDC links were submarine cables where the cost advantage of DC is greatest.
*Others involved hydroelectric resources, since there is no practical alternative to long distance high voltage transmission of hydroelectric energy.
*Pacific DC tie installed in 1970 parallel to 2 AC circuits – system stabilization was a major issue.
*SquareButte link in N. Dakota (750 km, 500 MW, 250 kV) displaced transporting coal, with system stabilization a major ancillary benefit.
*Most early
*Others involved hydroelectric resources, since there is no practical alternative to long distance high voltage transmission of hydroelectric energy.
*Pacific DC tie installed in 1970 parallel to 2 AC circuits – system stabilization was a major issue.
*Square
ü Some Early HVDC Projects
Itaipu , Brazil : 6,300 MW at ±600 kV DC.
*Two bipolar DC lines bring power generated at 50 Hz in the 12,600 MW Itaipu hydroelectric plant to the 60Hz network inSão Paulo .
*Leyte-Luzon , Philippines : 350 kV monopolar, 440MW, 430 km overhead, 21 km submarine.
*Takes geothermal energy fromLeyte to Luzon Assists with stabilizing the AC network.
*Two bipolar DC lines bring power generated at 50 Hz in the 12,600 MW Itaipu hydroelectric plant to the 60Hz network in
*
*Takes geothermal energy from
ü Selected Recent Projects
Rihand-Delhi , India : 1,500 MW at ±500 kV .Existing 400 kV AC lines parallel the link.Takes power 814 km from a 3,000 MW coal-based thermal power station to Delhi .
HVDC halved the right-of-way needs, lowered transmission losses and increased the stability and controllability of the system.
ü Selected Projects (continued)
*ProposedNeptune Project: 1,000 km 1,200 MW submarine cable from Nova Scotia to Boston , New York city and NJ.
*Take natural gas energy to NY with less visual impact, while avoiding a NIMBY problem in NY and allowing old oil-fired plant in NY to be retired.
*Help improve network stability and reliability.
*The southern end has a summer peak demand, the northern end a winter one, so a bi-directional link allows savings from electricity trade.
*Proposed
*Take natural gas energy to NY with less visual impact, while avoiding a NIMBY problem in NY and allowing old oil-fired plant in NY to be retired.
*Help improve network stability and reliability.
*The southern end has a summer peak demand, the northern end a winter one, so a bi-directional link allows savings from electricity trade.
ü Selected Projects (continued)
*Variable costs of an overheadHVDC link are less than the variable costs of pipeline gas.
*For 1,000–5,000 MW over 5,000 km pipeline gas is about 1.2–1.9 times more expensive (Arrillaga, 1998).
*Relative costs depend on the cost of land, and the price of gas among other factors.
*LNG also competes withHVDC for exploiting some gas reserves.
*Variable costs of an overhead
*For 1,000–5,000 MW over 5,000 km pipeline gas is about 1.2–1.9 times more expensive (Arrillaga, 1998).
*Relative costs depend on the cost of land, and the price of gas among other factors.
*LNG also competes with
ü HVDC versus Gas Pipeline
*HVDC seems particularly suited to many renewable energy sources:
*Sources of supply (hydro, geothermal, wind, tidal) are often distant from demand centers.
*Wind turbines operating at variable speed generate power at different frequencies, requiring conversions to and from DC.
*Large hydro projects, for example, also often supply multiple transmission systems.
*
*Sources of supply (hydro, geothermal, wind, tidal) are often distant from demand centers.
*Wind turbines operating at variable speed generate power at different frequencies, requiring conversions to and from DC.
*Large hydro projects, for example, also often supply multiple transmission systems.
ü Renewable Energy & HVDC
*HVDC would appear to be particularly relevant for developing large scale solar electrical power.
*Major sources are low latitude, and high altitude deserts, and these tend to be remote from major demand centers.
*Photovoltaic cells also produce electricity as DC, eliminating the need to convert at source.
*
*Major sources are low latitude, and high altitude deserts, and these tend to be remote from major demand centers.
*Photovoltaic cells also produce electricity as DC, eliminating the need to convert at source.
ü HVDC & Solar Power
Panels are assumed to have an efficiency of 14% at peak radiation and standard temperature reduced to approximately 13% efficiency due to system losses.
6 kWh/m2 light a day yields about 280 kWh/m2 of electricity a year for panels at 13% efficiency.
For average distances of 5,000 km,HVDC transmission losses would be about 25%.
About 20 panels each 30km×30km (18,000km2) would be needed to replace the 3,800 billion kWh of electricity produced in US in 2000.
First installed inJapan (Saijo) and USA (Hesperia) in the early 1980s.
Now more than 25 plants world-wide with peak power output from 300 kW to more than 3 MW
Most of the plants have fixed, tilted structures, without tracking.
These plants have proved easy to monitor and control and have achieved a 25% annual capacity factor even with modest downtime.
Panels are assumed to have an efficiency of 14% at peak radiation and standard temperature reduced to approximately 13% efficiency due to system losses.
6 kWh/m2 light a day yields about 280 kWh/m2 of electricity a year for panels at 13% efficiency.
For average distances of 5,000 km,
About 20 panels each 30km×30km (18,000km2) would be needed to replace the 3,800 billion kWh of electricity produced in US in 2000.
First installed in
Now more than 25 plants world-wide with peak power output from 300 kW to more than 3 MW
Most of the plants have fixed, tilted structures, without tracking.
These plants have proved easy to monitor and control and have achieved a 25% annual capacity factor even with modest downtime.
ü Grid-Connected PV Plants
Available sunlight does not vary greatly by season in the SW, while demand also peaks in summer.
Following map is Dec/July means over 10 years.
Available sunlight does not vary greatly by season in the SW, while demand also peaks in summer.
Following map is Dec/July means over 10 years.
ü Seasonal Fluctuations
Daily Fluctuations
Capacity is needed to meet unexpected falls in output or demand surges.
Balance of system capital costs depend on peak load net of solar output.
Solar output is less peaked when panels track the sun, but this raises costs.
For SW of US, power could be sent west in morning hours, east in the afternoons.
Spatial and Temporal Arbitrage
High capacityHVDC (bi-directional) links between time zones, or different climates, can flatten peaks in solar output and in demand.
Only excess demands are traded as geographical differences in prices are eliminated through arbitrage.
Hydroelectric capacity and pumped storage allow electricity prices to be arbitraged over time.
Hydrogen produced through electrolysis might be another cost-effective way to store electricity.
Siberia has large coal and gas reserves and could produce 450-600 billion kWh of hydroelectricity annually, 45% of Japanese output in 1995.
A 1,800 km 11,000MW HVDC link would enable electricity to be exported fromSiberia to Japan .
Siberia could also be linked to Alaska via HVDC .
Daily Fluctuations
Capacity is needed to meet unexpected falls in output or demand surges.
Balance of system capital costs depend on peak load net of solar output.
Solar output is less peaked when panels track the sun, but this raises costs.
For SW of US, power could be sent west in morning hours, east in the afternoons.
Spatial and Temporal Arbitrage
High capacity
Only excess demands are traded as geographical differences in prices are eliminated through arbitrage.
Hydroelectric capacity and pumped storage allow electricity prices to be arbitraged over time.
Hydrogen produced through electrolysis might be another cost-effective way to store electricity.
Siberia has large coal and gas reserves and could produce 450-600 billion kWh of hydroelectricity annually, 45% of Japanese output in 1995.
A 1,800 km 11,000MW HVDC link would enable electricity to be exported from
Ø Zaire could produce 250–500 billion kWh of hydroelectricity annually to send to Europe (5-6,000 km) on a 30-60,000 MW link.
Hydroelectric projects on a similar scale have been proposed forCanada , China and Brazil .
Hydroelectric projects on a similar scale have been proposed for
Ø Transcontinental Energy Bridges
For transfers of 5,000 MW over 4,000 km, the optimum voltage rises to 1,000–1,100 kV.
Technological developments in converter stations would be required to handle these voltages.
Lower line losses would reduce the optimum voltage.
However, environmentalist opposition and unstable international relations may be the biggest obstacle to such grandiose schemes.
For transfers of 5,000 MW over 4,000 km, the optimum voltage rises to 1,000–1,100 kV.
Technological developments in converter stations would be required to handle these voltages.
Lower line losses would reduce the optimum voltage.
However, environmentalist opposition and unstable international relations may be the biggest obstacle to such grandiose schemes.
NOTE:
These informations are collected from different websites.
These informations are collected from different websites.