Reliability , stability both improve.

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How??

SIMULTANEOUS EHV AC-DC POWER TRANSMISSION

Long EHV ac lines cannot be loaded to their thermal limits in order to keep sufficient

margin against transient instability. The transmission angle between sending end and

receiving end in long EHV ac lines seldom exceeds 30°. In these conditions ac current in

the transmission line conductor is much below its thermal current carrying capacity. But

optimum use of transmission lines requires loading of EHV ac lines close to their thenr^al

limits. Flexible AC Transmission System (FACTS) components are used to achieve this.

In this thesis it has been shown to achieve the same goal by simultaneous ac-dc power

transmission in which the conductors are allowed to carry superimposed dc current along

with ac current. Zig-zag connected transformers at both end in simultaneous ac-dc power

transmission are used to avoid saturation of core due to dc current injection. Laboratory

modelled experimental verification as well as simulation results establish the feasibility of

simultaneous ac-dc power transmission.

The present research explores the feasibility of converting a double circuit EHV ac line

to simultaneous ac-dc power flow line to get substantial power upgradation of existing

transmission line. In the proposed study of conversion of existing EHV ac transmission

line to simultaneous ac-dc transmission line, no alterations of conductors, insulator strings

and towers structures of the original EHV ac line are required. Substantial gain in the

loadability of the line is obtained. Major advantage of the composite ac-dc line is that the

transmission angle between two ends of line in steady state may go much beyond 30°-45°

values. This is because of inherently fast controllability of dc link embedded in the

composite ac-dc line. Such a large transmission angle is impossible to achieve in pure

EHV ac transmission line

HVDC transmission lines in parallel with EHV ac lines are recommended to improve

transient and dynamic stability as well as to damp out oscillations in power system. In the

present work it has been demonstrated to get advantage of HVDC in parallel with EHV ac

lines by simultaneous ac-dc power transmission. A single machine infinite bus connected

by a double circuit ac line, which is converted for simultaneous ac-dc power transmission

line has been taken up for, detailed study. For the purpose of comparison, the same double

circuit line with pure ac transmission imder same disturbances is also studied. Various

short circuit faults at number of locations of the transmission line have been created and

simulated responses have been studied. A novel approach to solve the first swing stability

problem by simultaneous ac-dc power transmission has been presented. It has been

demonstrated that the stability of the system can be effectively improved by simultaneous

ac-dc power transmission with fast dc power modulation. AC and DC powers flow

independently and added dc power flow does not cause any transient instability.

It has also been shown that it is possible to tap small amount of power, (up to 10% of

total transfer capability of the simultaneous ac-dc transmission system), to feed remotely

located communities, in the same simple way as tapping in case of EHV ac line. It is

economical as compared to complicated methods of tapping from HVDC line. The results

clearly indicate that the lapping of small amount of ac power from the simultaneous ac-dc

transmission line has negligible impact on the performance of composite ac-dc power

flow.