MODERN TRENDS IN DC TRANSMISSION

      The continuing technological developments in the areas of power semiconductor devices,digita electronics,adaptive control,Dc protection equipment have increased the pace of applications of Dc transmission.The major contribution of these developments is to reduce the cost of converter stations while improving the reliability and performance.

POWER SEMICONDUCTORS AND VALVES

    The cost of the converters can come down if the number of devices to be connected 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 need for parallel connection.The increase in the current rating of the devices has made it possible to provide higher overload capability at reasonable costs and reduce the lower limits on transformer leakage impedance thereby improving the power factor.The voltage ratings are also improve the reliability of converter operation.the cost of the valves is also reduced by the application of zinc oxide gapless arresters and protective firing methods.
   The power rating of thyristors is increased by better cooling methods.Deionized water cooling has now become a standard and results in reduced losses in cooling.Two phase flow using forced vaporization is also being investigated as a means of reducing thermal resistance between the heat sink and the ambient.
   As forced commutated converters operating at high voltages are uneconomic,the development of devices that can be turned off by application of a gate signal would be desirable.Gate turn off (GTO) thyristors are already available 2500 V and 2000 A.However,the main disadvantage of GTo's is the large gate current needed to turn them off.MOS (metal oxide semiconductor) controlled thyristor or MCT appears to be a promising technology.An MCT would consist of an MOS integrated circuit created on the top surface of a high power thyristor.In this device,a very large line current can be switched off by a small gate current.The turn - off time of MCT is also less than one third that of GTOs.however,MCTs are still in the early stages of development.

    The cost of silicon used in the manufacture of power semiconductor devices can be brought down (by 15 to 20 percent) from the use of magnetic CZ (Czochralski) method,instead of the conventional FZ (float zone) method.Research is also underway in reducing this packaging cost of a device.

CONVERTER CONTROL

    The development of micro-computer based converter control equipment has now made in possible to design systems with completely redundant converter control with automatic transfer between systems in the case of a malfunction.not only is the forced outage rate of control equipment reduced but it is also possible to perform scheduled preventive maintenance on the stand - by system when the converter is in operation.The use of a mini - simulator will make it feasible to check vital control and protection functions.
    The micro - computer based control also has the flexibility to try adaptive control algorithms or even the use of expert systems for fault diagnosis and protection.

DC BREAKERS

   With the development and testing of prototype DC breakers,it will be possible to go in for tapping an existing DC link or the development of new MTDC systems.Parallel,rather than series operation of converters is likely as it allows certain flexibility in the planned growth of a system.The DC breaker ratings are not likely to exceed the full load ratings as the control intervention is expected to limit the fault current.
     The control and protection of MTDC systems is not a straightforward extension of that used in the two - terminal DC systems.The possibility of decentralized control necessitated by communication failure,the coordination of control and protection are some of the issues currently being studied.