HVDC Transmission Advantage and Disadvantage
Advantage of HVDC Transmission
1. Reactive power Required
A HVDC line neither generate more preserver reactive power . the converter at the two end draw some reactive power from A.C system for their operation . This reactive power varies with the transmitted power but is that for independent of line length. It is found that for lines greater than 400KM length. A DC line requires less reactive
power than lines.
2. System stability
Two separate AC system interconnected by the AC line must operate with two separate AC system interconnected by a DC link may operate asynchronously may even operate at different frequency.
3. Independent control of AC system
4. Grater Reliability
A two conductor by polar DC line is more relible than a thrre conductor three phase line because in the event of a fault on one conductor , the other conductor of the DC line continue to operate with ground return where as three phase AC line can not operate on two phase.
5. Tower size and cost
The phase to phase clearness and phase to ground clearance are smaller for DC transmission as compared to equivalent AC transmission line for
same power and distance .
6. Power factor
Line power factor always unity .
7. Less cost
A basic DC line less costly compared with three phase AC line because only two conductor are needed in DC line com paired with in AC line.
8. Ground return
Ground can be used as a return conductor in case of DC system.
9. No skin effect
The skin effect is absent in DC line resulting in uniform current density in conductor and hence efficient utilization of conductor cross sectional area.
10. Lesser corona loss and radio interference
The corona loss and radio interference is less in DC line as compaired to AC line of same conductor diameter and voltage in bad weather .
11. No comparison required
No service or shunt compensation is required for long distance DC line.
12. Fast change of energy flow
The quick control of converter values permit fast changes in megnitude and direction of energy flow when required this result in increased in limits of transient stability cordination of system control at two ends.
13. No change in current
Continuous line charging current are absent in HVDC over head line and cables.
14. Longer life of cables
Dielectric fatigue is absent i DC so that insulation has longer life
Disadvantage of HVDC line
A part from many advantages of HVDC transmission there are some disadvanteges also which are is follows
1. Expensive converter
The converter required at both ends of a line have proved to be reliable but expesive. therefore HVDC can be used economically only for long distance transmission.
2. Reactive power requirement
The operation of converters at the ends of line required supply of reactive power from the AC system which may be as large as 75% of the active power.
3. Harmonics and filter
The converter produce lot of harmonics both in DC and AC sides which may cause interference with communication lines filters are required on the AC side of each convertor for reducing the magnitude of harmonics in the AC networks.
4. LESS over head capacity
the converter have very little over head load capacity and hence should be operated care fully with on the AC side.
5. complex control
control of HVDC sub station and transmission is very typical.
6. Losses and coiling system
HVDC substation have additional losses in converter values for which a suitable colling system necessary to dissipate the heat.