15-06-2012, 04:11 PM
Improvement of Power System Stability by Simultaneous AC-DC Power Transmission
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INTRODUCTION
HVDC transmission lines in parallel with EHV ac lines are recommended to improve transient and dynamic stability as well as to damp out oscilla-
tions in power system. Long EHV ac lines can not be loaded to its thermal limit to keep sufficient margin against transient instability. But for optimum use of transmission lines here is a need to load EHV ac lines close to their thermal limits by using flexible ac trans- mission system (FACTS) components .Very fast control of SCRs in FACTS devices like state VAR system (SVS), controlled series capacitor (CSC), static phase shiftier (SPS) and controlled braking resistors oscillations as well as to control the voltage profile of the line by con- trolling the total reactive power flow. Only the basic idea is proposed along with the feasibility study using elementary laboratory model. The main object is to emphasize the possibility of simultaneous ac-dc transmission with its inherent advantage of power flow control improves stability and damps out oscilla- tions in power system.
SELECTION OF TRANSMISSION VOLTAGE
The instantaneous value of each conductor voltage with respect to ground becomes more in case of simul- taneous ac-dc transmission system by the amount of the dc voltage superimposed on ac and more discs are to be added in each string insulator to withstand this increased dc voltage. However, there is no change re- quired in the conductor separation distance, as the line-to-line voltage remains unaltered. Therefore, tower structure does not need any modification if same conductor is used.Another possibility could be that the original ac voltage of the transmission be reduced as dc voltage is added such that peak voltage with re- spect to ground remain unchanged. Therefore, there would be no need to modify the towers and insulator strings.
PROPOSED APPLICATIONS
1.Long EHV ac lines can not be loaded to their ther- mal limit to keep sufficient margin against transient instability and to keep voltage regulation within al- lowable limit, the simultaneous power flow does not imposed any extra burden on stability of the system, rather it improves the stability. The resistive drop due to dc current being very small in comparison to im- pedance drop due to ac current, there is also no appre- ciable change in voltage regulation due to superim- posed dc current.
2. Therefore one possible application of simultaneous ac-dc transmission is to load the line close to its ther- mal limit by transmitting additional dc power. Figure3 shows the variation of Pt/Pac for changing values of k and x at unity power factor. However, it is to be noted that additional conductor insulation is to be provided due to insertion of dc.
CONCLUSION
A simple scheme of simultaneous EHV ac-dc power transmission through the same transmission line has been presented. Expressions of active and reactive powers associated with ac and dc, conductor voltage level and total power have been obtained for
steady state normal operating condition. The possible applications of the proposed scheme may be listed as: loading a line close to its thermal limit, improvement of transient and dynamic stability and damping of os- cillations. In LV and MV distribution system the pro- posed scheme may be applied in a workplace having high ambient temperature or fed with high frequency supply or with PV solar cells. Only the basic scheme has been presented with qualitative assessment for its implementation. Details of practical adaptation are beyond the scope of the present work.