PLANNING FOR HVDC TRANSMISSION

PLANNING FOR HVDC TRANSMISSION : 

                      The system planner must consider DC alternative in transmission expansion. The factors to be considered are (i) cost (ii)technical performance, and (iii) reliability.

                      Generally, the last two factors are considered as constraints to be met and the minimum cost option is selected among various alternatives that meet the specification on technical performance and reliability.

                     For submarine, cable transmission and interconnecting two systems of different nominal frequencies, the choice of DC is obvious. In other cases, the choice is to based on detailed techno-economic comparison.

                     The consideration in the planning for DC depends on the application. Two applications can be considered as representative.

These are 

1.  Long distance bulk power transmission 
2. Interconnection between two adjacent systems 

      In the first application, the DC and Ac alternatives for the same level of system security and reliability are likely to have the same power carrying capability. Thus the cost comparison would form the basis for the selection of the DC (or AC) alternative, if the requirements regarding technical performance are not critical. 

            In the second Application, Ac interconnection poses several problems in certain cases.For the same level of systems security(and reliability), the required capacity of AC interconnection will be much more than that for DC (even ignoring the beneficial aspects of DC power modulation).Thus the choice for DC interconnection will be based on the following considerations.

1. Small fluctuations in the voltage and frequency do not affect the power flow which can be set at any desired value.
2. The system security can be enhanced by fast control of DC power.

       Having settled on the DC link for interconnection, there are three possible configurations for interconnection. These are :

1. A two terminal transmission where each terminal is located at a suitable place  some where within the network and connected by a DC overhead line or cable.
2. A back to back HVDC station (also called HVDC coupling station)located some where within one of the network and an AC line from the other network to the common station.
3. A back to back station located close to the border between the two systems. This is a special case of the above.

In The choice between the first and second configuration, it is to be noted that converter costs are less for the common coupling station and the AC line costs are greater than the DC line costs. If the distance involved are less than 200km, the second configuration is to be preferred. If the short circuit ratio (SCR) is acceptable, then the third alternative will be the most economic. 

         The specification and design of DC systems require an understanding of the various interactions between the DC and AC systems.The interruption (or reduction) of power in a DC link can occur due to (i) DC line faults (ii)AC system faults.
       The speed of recovery from transient DC lines faults is of concern in maintaining the integrity of the overall system. The power flow and stability studies are used in this context. The recovery of DC link from AC system faults is more complex. The depression of AC voltage at the inverter bus can lead to communication failure and loss of DC power.The DC power output can lead to the reduction of AC voltage and failure of communication (due to corresponding increase in the var demand). An optimum rate of increase in DC power can be determined from stability study. This is influenced by control strategy and system characteristics.

 The following aspects also require a detailed study of the system interactions.

• Var requirements of converter stations 

•  Dynamic overvoltages 

•  Harmonic generation and design of filters 

• Damping of low frequency and subsynchronous torsional osillations 

•  Carrier frequency interference caused by spiky currents in valve (at the beginning of conduction) due to the discharge of stray capacitances and snubber circuits.

      The converter control plays a major role in these interactions and the control strategy should be such as to improve the overall system performance.Digital simulation and HVDC simulators are used for planning and design studies.