MODERN TRENDS IN DC TRANSMISSION

MODERN TRENDS IN DC TRANSMISSION :

                 The continuing technological developments in the areas of power semiconductor devices,digital electronics, adaptive control, 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 broughtdown. 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 on the increase. The development of light triggered thyristors should 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 ameans 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 at 2500 V and  2000A. 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 consists of an MOS integrated circuit created 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.

1. CoNvErTeR CoNtRoL :

          The development of micro-computer based converter control equipment has now made it 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 systems 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.

   2.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 system. The DC breaker ratings  as the control intervenction is expected to limit the fault current.

         The control and protection of MTDC systems is not a straightforward extenction 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.

    3.CONVERSION OF EXISTING AC LINES

            

               The constraints on RoW are forcing some utilities to look into the operation fo converting existing AC circuits to DC in order to increase the power transfer limit. There could be some operational problems due to electromagnetic induction from AC circuits operating in the same RoW.

   

              An experimental project of converting a single circuit of a double circuit 220kv line is currently under commissioning stage in india.

   4.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 

          If SCR is less than 3, the AC System is said to be weak.The conventional constant exinction angle control may not be satisfactory with weak AC system. The recovery of inverters following the clearing of fault in the connected AC system can also be problematic.

              Constant reactive current control or AC voltage control have been suggested to overcome some of the problems of weak AC systems. The use of fst reactive power control at the converter bus by applying static var systems is another alternative. Limiting dynamic overvoltages through  converter control during load rejection is becoming a standard practice.

                   The power modulation techniques used to improve dynamic stability of power systems will have to be modifited in the presence of weak AC systems. Coordinated reactive and active power modulation has been suggested to over come the problems of voltage variations that can limit the effective of power modulation.