Apr 10, 2012 - converters as well as bidirectional power flow is possible in. HVDC link. 3 ... The earth carries only a small out-of-balance current during the ...
Maharaja.K, AP/EEE, Sri Ramakrishna Institute of Technology 04.10.12 HVDC TRANSMISSION SYSTEM
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PRINCIPLES OF HVDC SYSTEM OPERATION A typical transmission consists of one rectifier station at the
sending end and one inverter station at the receiving end.
The two stations are interconnected by a DC transmission
line. The rectifier station converts AC to DC while the inverter station converts DC to AC.
By varying the firing angle of the thyrister in the converter,
the DC output magnitude is controlled. HVDC TRANSMISSION SYSTEM
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Contd… In rectifier the firing angle is 0 < α < 90 and in inverter the firing angle is 90> α >180. As the DC output voltage is a function of cosine of firing angle in converter hence the converter voltage becomes negative when α >90. This makes converter to operate as an inverter. In practical HVDC converter stations three phase bridge
converters are used both in rectifier and inverter side. By controlling the firing angle reversible operation of
converters as well as bidirectional power flow is possible in HVDC link. HVDC TRANSMISSION SYSTEM
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TYPES OF DC LINKS Monopolar DC link Bipolar DC link Homopolar DC link
HVDC TRANSMISSION SYSTEM
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Monopolar DC Link
As the name suggests, monopolar link has only one
conductor and return path is provided by permanent earth or sea. The line usually operates with negative polarity with respect to ground so as to reduce corona loss and radio interference. HVDC TRANSMISSION SYSTEM
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Contd… The ground return path has a low resistance and,
therefore, low power loss in comparison with a metallic line conductor of economical size and equal length provided the ground electrodes are of proper design.
HVDC TRANSMISSION SYSTEM
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Bipolar DC Link
This is most widely used dc link for overhead long distance
HVDC transmission systems and also for back-to-back HVDC system. This link has two conductors one operating with positive polarity and the other with negative polarity with respect to the earthed tower structure. There are two converters of equal voltage rating and connected in series at each end of the DC line. HVDC TRANSMISSION SYSTEM
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Contd… The neutral points i.e. the junction between converters
may grounded at one end or at both the ends. If it is grounded at both the ends each pole can operate independently. The earth carries only a small out-of-balance current during the normal operation. When the currents in the two conductors are equal, the ground current is zero. During fault or trouble on one of the lines, the other line along with the ground return can supply half of the rated load. Thus continuity of supply is maintained. HVDC TRANSMISSION SYSTEM
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Homopolar DC Link
A homopolar link has two or more conductors having
the same polarity, usually negative, and always operates with ground as the return conductor.
HVDC TRANSMISSION SYSTEM
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Contd… In case of a fault in any one of the conductors, the
converter equipment can be reconnected so that the healthy conductor can supply power. Such a scheme is very complicated and is preferred to a bipolar link provided continuous ground return does not pose additional problems.
HVDC TRANSMISSION SYSTEM
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MERITS OF HVDC TRANSMISSION SYSTEM Cheaper in cost: Bipolar HVDC transmission lines require two-pole conductors
while AC system requires 3 conductors to carry power. Potential stress on the insulation in case of DC system is
(1/2)^(0.5) times of that in AC system for same operating voltage. Hence for the same operating voltage less insulation required. EHV-AC transmission needs intermediate substations at an
interval of 300 km for compensation but HVDC transmission system does not require any intermediate compensation. Above breakeven distance
HVDC links becomes economical
over AC transmission lines. HVDC TRANSMISSION SYSTEM
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Contd… An HVDC line can be built in stages: The DC line can be built as a monopalar line with
ground return in the initial stage and may be converted into a bipolar line on a later date when the load requirement increases.
No Skin Effect: There is no skin effect in DC, so there is a uniform distribution of current over the cross section of the conductor. Low Transmission Loss: HVDC transmission needs only two conductor, no skin effect hence the transmission loss is very less. Voltage Regulation: There is no inductance, hence the voltage drop due to inductive reactance does not exist in DC line. HVDC TRANSMISSION SYSTEM 12
Contd… Line Loading: The permissible loading on an EHV-AC lines is limited due
by transient stability limit and line reactance to almost one-third of thermal rating of conductors. No such limits exist in case of HVDC links. Surge Impedance Loading: Long EHV-AC lines are loaded to less than 80 percent of natural load. No such condition is applicable to HVDC lines. Greater Reliability: A two-conductor bipolar DC line is more reliable than a 3wire 3-phase AC line because the DC line may be operated in a mono-polar mode with ground return when the other develops a fault. HVDC TRANSMISSION SYSTEM
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Contd… Rapid Change of Energy Flow: Control of
converter valves permit rapid change in magnitude and the direction of power flow when the AC system inter connected by a DC line.
Independent Control: The ac systems interconnected by a DC line can be controlled
independently. they can be completely independent as regards frequency, system control, short circuit rating, future extension etc.. Can achieve independence of frequencies of sending end(say 50Hz) and receiving end (60Hz) networks.
Lesser Dielectric Losses and Higher current carrying
capability: HVDC TRANSMISSION SYSTEM
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Contd… Absence of charging current and Limitation of cable length: There is no effect of capacitance in HVDC.
Lesser Corona and Radio Interference: Corona and radio interference directly proportional to frequency. In DC system no frequency. Higher Operating Voltage: The same size of the conductors and insulators can be employed for higher voltages in case of DC as compared to AC Reactive power compensation: No need of any compensation for HVDC line. Use as a Asynchronous link between two AC system: Can interconnect two AC system with different voltage level, different frequency and different phase angle. HVDC TRANSMISSION SYSTEM
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LIMITATIONS OF HVDC LINES Costly Terminal Equipment: Converters Reactive Power generators Filtering and smoothing equipment because of harmonics Complex cooling system HVDC circuit breakers is costly More Maintenance of line Insulators: Circuit breaking in multi-terminal DC system is
difficult: Voltage Transformation: HVDC TRANSMISSION SYSTEM