For improving the system stability, what will happen if a HVDC line parallel to the EHV AC in installed.
Reliability , stability both improve.
read here
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.
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