high voltage direct current transmission · pdf filehigh voltage direct current transmission...
TRANSCRIPT
HIGH VOLTAGE DIRECT CURRENT TRANSMISSION
BULK TRANSMISSION OF ELECTRICAL POWER
www.eeecube.com
www.eeecube.com
History
First commercial application of HVDC between Swedish mainland and theisland of Gotland in 1954.
Underwater link of 90 km and 20 MW.
After the advent of thyristor convertor,
New Brunswick and Quebec 320 MW back-to-back DC interconnection commissioned in1972.
With reduced size, cost and improved reliability of power electronic converters,has made HVDC transmission more widespread.
In North America, total HVDC transmission capacity in 1987 was 14,000 MW.
www.eeecube.com
www.eeecube.com
Advantages In a number of applications HVDC is more effective than AC transmission.
Examples include
Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 kmBaltic Cable between Sweden and Germany)
Long power transmission without intermediate taps.
Example, in remote areas
Power transmission and stabilization between unsynchronized AC distribution systems
Connecting a remote generating plant to the distribution grid
Reducing line cost: 1) fewer conductors 2) thinner conductors since HVDC does notsuffer from the skin effect
Facilitate power transmission between different countries that use AC at differingvoltages and frequencies
Synchronize AC produced by renewable energy sourceswww.eeecube.com
www.eeecube.com
Disadvantages
The disadvantages of HVDC are in conversion, switching and control.
Expensive inverters with limited overload capacity
Higher losses in static inverters at smaller transmission distances
The cost of the inverters may not be offset by reductions in line construction costand lower line loss.
High voltage DC circuit breakers are difficult to build because some mechanismmust be included in the circuit breaker to force current to zero, otherwise arcing andcontact wear would be too great to allow reliable switching.
www.eeecube.com
www.eeecube.com
UNIT I
INTRODUCTION
Book Ref :
1. Padiyar , K. R., “HVDC power transmission system
2. Edward Wilson Kimbark, “Direct Current Transmission
www.eeecube.com
www.eeecube.com
INTRODUCTION
• Modern DC power transmission is relatively a new technology which made amodest beginning in the year 1954.
• The advent of thyristor valve is the improvements over last 18 years has beenresponsible for the acceleration of the growth of HVDC technology.
• Improving reliability and reducing cost of converter stations.
• The latest development of multi-terminal system operation has increased the scopeof application of HVDC systems
• While it is true that the HVDC systems are quite reliable and converter controlallows flexibility in the system operation.
www.eeecube.com
www.eeecube.com
COMPARISON OF AC AND DC TRANSMISSION
The relative merits of two modes of transmission (ac & dc) whichneed to be considered by a system planner are based on thefollowing factors:
• Economics of transmission
• Technical performance
• Reliability
www.eeecube.com
www.eeecube.com
• Cost of transmission
• Insulators
• Line compensation
• Cost of Compensation
• Converters and Filters.
ECONOMICS OF POWER TRANSMISSION
www.eeecube.com
www.eeecube.com
ECONOMICS OF POWER TRANSMISSION
• The cost of transmission line includes the investment and operational costs.
• The investment includes costs of Right of Way (ROW), transmission towers,conductors, insulators and terminal equipment. The operational costs includemainly the cost of losses.
• The characteristics of the insulators vary the type of voltage applied.
• For simplicity, if it is assumed that the insulator characteristics are similar forac & dc and depend on the peak level of the voltage applied with the respect tothe ground.
• Then it can be shown that for lines designed with the same insulation level, a dcline carry as much power with two conductors (with positive and negativepolarities with respect to ground) as an ac line with three conductors for the samesize.
• This implies that for a given power level dc line requires less ROW, simpler andcheaper towers and reduced conductor and insulation costs.
www.eeecube.com
www.eeecube.com
• The power losses are also reduced with dc as there are only two conductors.
• The absence of skin effect with dc is also beneficial in reducing power lossesmarginally.
• The dielectric losses in case of power cables is also very less for dctransmission.
• The corona effects tend to be less significant on dc conductors than for ac and thisalso leads to the choice of economic size of the conductors with dc transmission.
• The other factors that influence the line cost are the cost of compensation andterminal equipment.
• Dc lines do not require compensation but the terminal equipment costs areincreased due to the presence of the converter and filters.
www.eeecube.com
www.eeecube.com
• Ac tends to be more economical than dc for distance less than break evendistance and costlier for longer distances.
• The break even distance can vary from 500 to 800 km in overhead lines dependingon the per unit line costs.
www.eeecube.com
www.eeecube.com
• The DC transmission has some positive feature which are lacking in ACtransmission. These are mainly due to the fast controllability of power in DC linesthrough converter control.
• The following are the advantages:
a. Full control over power transmitted.
b. The ability to enhance transient and dynamic stability in association AC networks.
c. Fast control to limit fault currents in DC lines. These make it feasible to avoid DC breakersin two terminal DC links.
TECHNICAL PERFORMANCE
www.eeecube.com
www.eeecube.com
In addition, the DC transmission overcomes some of the problems of AC transmission. These are described further:
• Stability Limits
• Voltage Control
www.eeecube.com
www.eeecube.com
• The power transfer in AC lines is dependenton the angle difference between the voltagephasors at the two ends.
• For a given power level, these angle increaseswith distance.
• The maximum power transfer is limited bythe considerations of steady state and transientstability..
• The Figure shows the power capability of theDC lines which is unaffected by the distanceof transmission, and only its limited by thecurrent carrying capacity of the conductors(Thermal Limit )
STABILITY LIMITS
www.eeecube.com
www.eeecube.com
VOLTAGE CONTROL
• The voltage control in AC lines is complicated by the line charging and inductivevoltage drops.
• The voltage profile in an AC line is relatively flat only for the fixed level of powertransfer corresponding to surge impedance loading (SIL).
• The voltage profile varies with the line loading.
• For the constant voltage at the line terminals, the mid point voltages reduced forthe line loading higher then SIL and increase for loading less than SIL.
• This is shown in figure followed:
www.eeecube.com
www.eeecube.com
• The maintenance of constant voltages at the two ends requires reactive powercontrol from inductive to capacitive as the line loading is increased.
• The reactive power requirements increase with the increase in the line lengths.
• Although dc converter stations require reactive power related to the line loadings,the line itself does not require reactive power.
• The steady state charging currents in ac lines pose serious problems in cables thisputs the break even distance for the cable transmission around 40 km.
Fig - Variation of voltage along the line.
www.eeecube.com
www.eeecube.com
RELIABILITY
• The reliability of dc transmission systems is quite good and comparable to that ofAc systems.
• An exhaustive record of existing HVDC links in the world is available from whichthe reliability statistics can be computed.
• It must be remembered that the performance of the thyristor valves is much morereliable than mercury arc valves and further development in devices control andprotection is likely to improve the reliability level
• For example the development of direct light triggered (LTT) is expected toimprove reliability because of the elimination of the high voltage pulsetransformers and auxiliary supplies for turning on the device.
• Both energy availability and transient reliability of existing dc systems withthyristor valves is 95% or more.
www.eeecube.com
www.eeecube.com
APPLICATIONS OF DC TRANSMISSION
The detailed comparison of ac & dc transmission in terms of economics andtechnical performance leads to the following areas of application for dctransmission.
Long distance bulk power transmission.
Underground or underwater cables.
Asynchronous interconnections of ac systems operating at different frequencies or where independent control of systems is desired.
Control and stabilization of power flows in ac ties in an integrated power system.
www.eeecube.com
www.eeecube.com
DISADVANTAGES OF DC TRANSMISSION
• The scope of application of DC transmission is limited by the followingfactors:
• The difficulty of breaking dc currents which results in high cost of dc breakers.
• Inability to use transformers to change the voltage levels.
• High cost of conversion equipment.
• Generation of harmonics which require ac & dc filters, adding to the cost of converter stations.
• Complexity of control.
www.eeecube.com
www.eeecube.com
DESCRIPTION OF DC TRANSMISSION SYSTEM
• Types of DC Links
• Converter Station
• Converter Unit
• Converter Transformer
• Filters
• Reactive Power Source
• Smoothing Reactor
• DC Switchgear
www.eeecube.com
www.eeecube.com
Types of DC link
HVDC links can be broadly classified into:
Monopolar links
Bipolar links
Homopolar links
Back-to-back links
Multiterminal links
www.eeecube.com
www.eeecube.com
MONOPOLAR LINKS
• It uses one conductor
• The return path is provided by ground or water
• Use of this system is mainly due to cost considerations
• A metallic return may be used where earth resistivity is too high
www.eeecube.com
www.eeecube.com
BIPOLAR LINKS• It uses two conductors, one positive and the other negative
• Each terminal has two converters of equal rated voltage, connected in series on the DC side
• The junctions between the converters is grounded
• Currents in the two poles are equal and there is no ground current
• If one pole is isolated due to fault, the other pole can operate with ground and carry half the rated load (or more using overload capabilities of its converter line)
www.eeecube.com
www.eeecube.com
HOMOPOLAR LINKS
• It has two or more conductors all having the same polarity, usually negative
• Since the corona effect in DC transmission lines is less for negative polarity, homopolar link is usually operated with negative polarity
• The return path for such a system is through ground
www.eeecube.com
www.eeecube.com
Components of HVDC Transmission Systems
1. Converters
2. Smoothing reactors
3. Filters
4. Reactive power supplies
5. Electrodes
6. DC lines
7. AC circuit breakers
www.eeecube.com
www.eeecube.com
Converters
• They perform AC/DC and DC/AC conversion
• They consist of valve bridges and transformers
• Valve bridge consists of high voltage valves connected in a 6-pulse or 12-pulse arrangement
• The transformers are ungrounded such that the DC system will be able to establish its own reference to ground
Smoothing reactors
• They are high reactors with inductance as high as 1 H in series with each pole
• They serve the following:
• They decrease harmonics in voltages and currents in DC lines
• They prevent commutation failures in inverters
• Prevent current from being discontinuous for light loads
Harmonic filters
• Converters generate harmonics in voltages and currents. These harmonics may cause overheating of capacitors and nearby generators and interference with telecommunication systems
• Harmonic filters are used to mitigate these harmonics
www.eeecube.com
www.eeecube.com
Reactive power supplies
• Under steady state condition conditions, the reactive power consumed by the converter is about 50% of the active power transferred
• Under transient conditions it could be much higher
• Reactive power is, therefore, provided near the converters
• For a strong AC power system, this reactive power is provided by a shunt capacitor
Electrodes
• Electrodes are conductors that provide connection to the earth for neutral. They have large surface to minimize current densities and surface voltage gradients
DC lines
• They may be overhead lines or cables
• DC lines are very similar to AC lines
AC circuit breakers
• They used to clear faults in the transformer and for taking the DC link out of service
• They are not used for clearing DC faults
• DC faults are cleared by converter control more rapidly
www.eeecube.com
www.eeecube.com
PLANNING FOR HVDC TRANSMISSION
• The system planner should consider for perfect HVDC transmission
• Some of the factors should be consider:
• Cost
• Technical Performance
• Reliability
• Consideration in the planning for DC depends on the applications
• Long Distance Bulk Power Transmission
• Interconnection between two adjacent systems
www.eeecube.com
www.eeecube.com
1st Application
• DC and AC Alternatives for the same level of system security and reliability are likely tohave the same power carrying capability.
2nd Applications
• DC and AC Alternatives for the same level of system security and AC interconnectionwill be more than that for DC.
• The choice for DC interconnection will be based on the following consideration.
• Small fluctuation in the voltage and frequency do not affect the power flow which can be set at any desired value.
• The system security can be enhanced by fast control of DC power.
www.eeecube.com
www.eeecube.com
DC link interconnection , there are three possibleconfiguration for interconnection. These are:
• A two terminal transmission where each terminal islocated at suitable place somewhere within the networkand connected by a DC overhead line or cable.
• A back to back HVDC station located somewhere withinone of the network and an AC line from the othernetwork to the common station.
• A back to back station located close to the borderbetween the two systems.
www.eeecube.com
www.eeecube.com
CHOICE OF VOLTAGE LEVEL
• For a long distance bulk power transmission, the voltage level is chosen to minimize the total costs for a given power level (P).
• The total costs include investment (C1) and cost of losses (C2).
• The investment cost per unit length are modeled as:
C1 = A0 + A1nV + A2nq ---------------------(1)
Where V is the voltage level with respect to ground
• N is the number of conductors
• Q is the total cross section of each conductor
• A0 , A1 and A2 are constants
• The cost of losses per unit length is given by
𝐶2 =𝑛[
𝑝
𝑛𝑉
2𝜌𝑇𝐿𝑝]
𝑞-------------------------- (2)
Where
𝜌 = Conductor resistivity
T = total operation time in a year
L = Loss load factor
www.eeecube.com
www.eeecube.com
• Equation 2 can be simplified as;
𝐶2 =[ 𝐴3
𝑝
𝑣
2𝜌]
𝑛𝑞-------------------(3)
Where 𝐴3 = TLp
• By minimizing the sum of C2 and the third temt in C‘, we have.
nq= (A3𝜌/A2).(P/V)
J = P/(nqV) = A2/(A3𝜌)
Where J is the current density.
Total Costs
C = C1 + C2
www.eeecube.com
www.eeecube.com
MODERN TRENDS IN DC TRANSMISSION
• The continuing technological developments in the areas of power semiconductordevices, digital electronics, adaptive control, DC protection equipment haveincreased the pace of application of DC transmission.
• The major contribution of these developments is to reduce the cost of converterstations while improving the reliability and performance.
Power Semiconductor and valves
Converter Control
DC Breakers
Conversion of existing AC lines
Operation with weak AC system
Active DC filter
Capacitor Commutated Converter (CCC)
UHV DC Transmissionwww.eeecube.com
www.eeecube.com
POWER SEMICONDUCTORS AND VALVES
• The cost of the converters can come down if the number of devices to beconnected in series and parallel can be brought down.
• The size of the devices has gone up to 100 mm (in diameters) and there is no needfor parallel connection.
• The development of light triggered thyristors should also improve the reliability ofconverter operation.
• The cost of the valves is also reduced by the application of zinc oxide gaplessarresters and protective firing methods.
• The power rating of thyristors is increased by better cooling methods. Deionizedwater cooling has now become a standard and results in reduced losses in cooling.
www.eeecube.com
www.eeecube.com
CONVERTER CONTROL
• The development of micro-computer based converter control equipment has nowmade it possible to design systems with completely redundant converter controlwith automatic transfer between systems in the case of a malfunction.
• Not only is the forced outage rate of control equipment reduced but it is alsopossible to perform scheduled preventive maintenance on the stand-by systemwhen the converter is in operation.
• The use of a mini-simulator will make it feasible to check vital control andprotection functions.
• The micro-computer based control also has the flexibility to try adaptive controlalgorithms or even the use of expert systems for fault diagnosis and protection.
www.eeecube.com
www.eeecube.com
DC BREAKERS
• With the development and testing of prototype DC breakers, it will be possible togoing for tapping an existing DC link or the development of new MTDC systems.
• The DC breaker ratings are not likely to exceed the full load ratings as the controlintervention is expected to limit the fault current.
• The control and protection of MTDC systems is not a straight forward extensionof that used in the two-terminal DC systems.
• The possibility of decentralized control necessitated by communication failure, thecoordination of control and protection are some of the issues currently beingstudied.
www.eeecube.com
www.eeecube.com
CONVERSION OF EXISTING AC LINES
• The constraints on Row are forcing some utilities to look into the option ofconverting existing AC circuits to DC in order to increase the power transfer limit.
• There could be some operational problems due to electromagnetic induction fromAC circuits operating in the same Row.
• An experimental project of converting a single circuit of a double circuit 220 KVline is currently under commissioning stage in India.
www.eeecube.com
www.eeecube.com
OPERATION WITH WEAK AC SYSTEMS
• The strength of AC systems connected to the terminals of a DC link is measured in terms of short circuit ratio (SCR) which is defined as
SCR=𝑆ℎ𝑜𝑟𝑡 𝑐𝑖𝑟𝑐𝑢𝑖𝑡 𝑙𝑒𝑣𝑒𝑙 𝑎𝑡 𝑡ℎ𝑒 𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑟 𝑏𝑢𝑠
𝑅𝑎𝑡𝑒𝑑 𝐷𝐶 𝑝𝑜𝑤𝑒𝑟
www.eeecube.com
www.eeecube.com