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					Q1: Discribe The principle of HVDC system opertaion .

PRINCIPLE OF HVDC SYSTEM OPERATION

A typical HVDC transmission system is shown fig. At sending end there is one
rectifier unit whereas one inverter unit at the receiving end. The two ends ar
interconnected by a DC transmission line. The A.C produced by genrating station
after stepping up is converted to D.C by rectifier whereas th inverter conversts D.C
to A.C.




The converter makes use of thyristor for controlled operation. Thus by varying the
finger angle of thyristor, the dc output voltage magnitude is controlled. The firing
angle is 0 degree and 90 degree in rectifier while inverter the firing angle is 90 deg
and 180 degree. The converter and inverter station in HVDC use three phase
controlled bridge converter
From the figure Id is given by


VR= D.C output voltage at rectifier side
VI= D.C input voltage at inverter side
The power transfer I given by,
Q:2 discuss technical and economical advantage of D.C system over A.C
system.

Economical advantages:
  (i)   The HVDC transmission lines require only one conductor when compare
        to Ac transmission system which requires several conductors so the cost
        of HVDC is less when compare to the AC transmission system.
  (ii) The supporting structure required for an HVDC transmission system is
        narrow, whereas an AC transmission requires a lattice structure. So, the
        cost of supporting tower is less when compare to that of A.C system.
  (iii) Line losses in HVDC transmission are less when compare to the AC
        transmission for the same power transfer capacity so the energy cost in
        HVDC transmission is less when compare to A.C transmission line.
  (iv) The HVDC lines can be built in two stages. The second stage can be built
        whenever the extra power transfer capability is needed. The first stage
        can be operated as a monopolar line with two conductors without ground
        return. The investment on the second stage can, thus, be postponed.


Technical advantages:
  (i)   Reactive power requirement: When the load impedance is equal to
        surge impedance of line, the reactive power generated by the line
        capacitance equals the reactive power absorbed by the line inductance.
        The line cannot always be operated at its natural load since the loads
        varies with time. In D.C transmission, no reactive power is transmitted
        over the line the reactive power is independent of line length but varies
        with transmitted power.
  (ii)   System stability: In order to maintain stability, the length of an
        uncompensated AC’ line must be less than 500 km. whereas, when series
        compensation is used, the length may be longer than 500 km. A DC
        transmission system has no such stability problems.
  (iii) Short-circuit current: If two AC systems are interconnected by an AC
        line. Then the short circuit current in the system increases. Therefore.
        The existing circuit breakers (CBS) have to be replaced with new CBs of
        high ratings. However, in DC lines, the contribution of short-circuit
        current is the same as the rated current-carrying capacity of a dc line.
(iv)  Independent control of AC systems: The AU systems which arc
      interconnected by a DC line can be controlled independently. They are
      independent with reference to frequency, system control. Short- circuit
      rating, future extension etc.
(v) Fast change of energy flow: The transmitted power is proportional to
      the difference in terminal voltage. So, the direction of energy flow can he
      changed by changing the values of DC voltages.
(vi) Less corona loss and radio interference: The corona loss of a
      transmission system is proportional to (f+ 25), the frequency of a DC
      system is zero. So. Corona losses in a DC system are less when compared
      to an AC system of the same conductor diameter and voltage.
(vii) Greater reliability: If a fault occurs in one conductor of a bipolar
      system, the other conductor continues to operate s with ground return.
      So,. a two-conductor bipolar DC line is more reliable than a three-
      conductor three-phase AC line.


       Q3: Point out the Limitations and difficulties of HVDC Transmission
       System
       The lack of HVDC circuit breakers is regarded as a limitation to HVDC
       transmission. In ac circuits, circuits breakers take advantage of the
       current zeros occurring twice per cycle. The arc does not restrike between
       contacts because the design is such that the breakdown strength of the arc
       path between contacts is increased rapidly as to enable extinction. Grid
       control is converter valves on radial lines are used to block the dc
       temporarily. The realization of multi-terminal systems requires the use of
       HVDC circuit breakers. A number of breaker concepts have been
       described, and several laboratory prototypes have been developed. With
       the availability of these commercially, utility planners can proceed with
       serious consideration of multi-terminal HVDC systems.
       The reliability and maintenance of converters have been a major problem
       for dc systems with mercury-arc converters. This difficulty has been
       resolved in projects using thyristor valves. These valves have a little
       overload capacity, which can present a problem when one bipolar line is
       involved. In this case it is not possible to meet the requirement of 100
       percent half-hour overload capability to take care of a pole outage.
       The production of harmonics due to converter operation leads to audio
       frequency telephone line interference. Filter on both sides of the dc
       system are required to suppress these harmonics
 Q:4 what is DC link. Classify the type of HVDC links. Discuss the application of
 each of these links.

 A DC link is the DC power transmission network which consists of transformers,
 converter units, and conductors.
 The classification of HVDC systems depends upon the arrangement of the pole and the
 earth return.
 They are classifying as follows:
 (1) Monopolar link
 (ii) Bipolar link
 (iii) Homopolar link

 1. Monopolar link
 Monopolar HVDC transmission system is represented in the Fig. This system has only
 one pole and the return path is provided by permanent earth or sea. The pole generally
 has negative polarity with respect to earth.
 Full power and current is transmitted through a line conductor with earth or sea as a
 return conductor. The earth electrodes are designed for continuous full current operation.
 The sea or ground return is permanent and of continuous rating.




2. Bipolar link
This system has two poles, one positive and one negative pole with respect to
earth. During fault on one pole the bipolar system is changed to monopolar
mode. The system is represented in the Fig.
    This system is more commonly used for transmission of power over long
    distance. The mid points of convertors at each terminal are earthed through
    electrode line arid earth electrode. Power rating of one pole is about half of
    bipolar power rating. The earth carries only small out of balance current
    during normal operation.
    The normal bipolar HVDC system consists of two separate monopolar
    systems with a common earth. The two poles can operate independently.
    Normally they are operated with equal currents and hence ground carries no
    current.
    3. Homopolar link
    This system consists of two poles of same polarity and the return is through
    permanent earth. It is shown in the Fig.




Q:5 Describe the application of HVDC transmission system
        HVDC transmission is advantageous in the following areas of
        application:
           (i)    For long underwater cable crossings (wider than 32 km). In six
                  of the first seven commercial schemes. submarine cables are
                  the medium of power transfer. The success of the Gotland
                  scheme justified later underwater connections as mentioned
                  earlier. A 25-kin submarine cable between New Brunswick and
                  Prince Edward Island was completed in 1977. The initial
                  operation was at 138 kV ac. The design is such that the forecast
        increase in transmission capacity will be met by HVDC
        operation at 1200 kV.
(ii)     For long-distance, bulk-power transmission by overhead lines,
        when the savings in cost of a dc line would more than
        compensate for the cost of converter stations. For the same
        power capability, the cost per unit length of a dc line is lower
        than that of an ac line. In Figure 7-1., we show the comparative
        costs of ac and dc overhead lines versus distance of
        transmission. The break-even distance is the abscissa of the
        intersection of the dc transmission cost with the ac
        transmission cost. If the transmission distance is longer than
        the break-even distance, then dc is cheaper than ac. The break-
        even distance vanes with the power transmitted, the
        transmission voltage, the type of terrain, the cost of equipment,
        and other factors. This particular aspect will be treated Later on
        in the chapter. Thus dc transmission plays a significant role in
        situations where it is more economical to generate power at the
        mine mouth, hydrosite, or gas well and to transmit it
        electrically to the load center.

(iii)   The dc systems have an inherent short-time overload capacity
        that can be used for damping system oscillations. Two systems
        when interconnected by ac lines sustain instability. A dc link
        interconnecting the two would overcome this difficulty. The
        Eel River tie, Canada, has operated in this mode for the past
        several years. The Stegall project in Nebraska was constructed
        to connect east-west
  systems in the United Stat at a point along what might be termed the
“electric continental divide.”

   (iv)    A requirement to provide an intertie between two systems
          without raising the short-circuit level appreciably can be met
          by using a HVDC link.
   (v)     Two systems having different frequencies may be tied together
          through a dc interconnection.
   (vi)    For transmission in underground metropolitan cable systems
          where long distance are involved.

				
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posted:10/13/2012
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