Comparison of AC and DC Transmission
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Comparison of AC and DC transmission 1. Current limit Long overhead line -other limits are reached first. Long distance underground cable transmission is difficult due to excessive line charging, leaving little margin for the normal load current. 2. Voltage limit With DC, switching surges are lower. Corona under foul weather conditions is a limiting factor in AC transmission. Losses and radio interference associated with corona are a matter of concern. In DC foul weather reduces RI associated with corona 3. Rea cti ve p ower & voltage regulat ion Excessive reactive power consumption and drop in voltage is associated with long distance AC transmission when loading is above he surge impedance loading, (Economic loading on overhead lines is always above SIL). Ferranti effect is another problem associated with long AC lines. Hence, long distance AC transmission is feasible only with the use of series & shunt compensation. 4. Stability This is the most serious limitation of long distance Ac transmission. Serie s compensation (35- 50%) is always necessary. Even then transmission distances are limited. No such problem with Dc since it is an asynchronous link. 5. Short ci rc ui t currents AC interconnections result in tremendous increase in the fault level. DC links do not increase the fault level. Fault currents on DC line are restricted due to fast thyristor control. 6. Powe r per conductor per circui t 3-phase, 3-conductor Ac line carries the same power as a bipolar DC link9for the same current and insulation level0. Reliabili ty of the DCline is equal tothat of double circuit AC line. DC line is simpler, cheaper, and more efficient and requires lesser right of way. With underground cables, the ratio of DC to AC power per cable is 5-10. 7. Ground return This is feasible with DC. It is not feasible with Ac due to higher impedance and interference with communication circuits
Transcript of Comparison of AC and DC Transmission
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Comparison of AC and DC transmission
1. Current limit
Long overhead line -other limits are reached first.
Long distance underground cable transmission is difficult due to excessive line charging, leaving
little margin for the normal load current.
2. Voltage limit
With DC, switching surges are lower. Corona under foul weather conditions is a limiting factor in
AC transmission. Losses and radio interference associated with corona are a matter of concern. In
DC foul weather reduces RI associated with corona
3. Reactive power & voltage regulation
Excessive reactive power consumption and drop in voltage is associated with long distance AC
transmission when loading is above he surge impedance loading, (Economic loading on overheadlines is always above SIL). Ferranti effect is another problem associated with long AC lines.
Hence, long distance AC transmission is feasible only with the use of series & shuntcompensation.
4. Stability
This is the most serious limitation of long distance Ac transmission. Series compensation (35-
50%) is always necessary. Even then transmission distances are limited. No such problem with Dcsince it is an asynchronous link.
5. Short circuit currents
AC interconnections result in tremendous increase in the fault level. DC links do not increase the
fault level. Fault currents on DC line are restricted due to fast thyristor control.
6. Power per conductor per circuit
3-phase, 3-conductor Ac line carries the same power as a bipolar DC link9for the same current and
insulation level0. Reliability of the DCline is equal tothat of double circuit AC line. DC line issimpler, cheaper, and more efficient and requires lesser right of way. With underground cables,
the ratio of DC to AC power per cable is 5-10.
7. Ground return
This is feasible with DC. It is not feasible with Ac due to higher impedance and interference with
communication circuits
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8. Harmonics
Converters in Dc link generate harmonics on AC and DC sides. Elaborate filtering is required.
9. Circuit breakers
These are not required with two-terminal Dc links. Fast current control limits the fault current.
However multi-terminal links require breakers. The problem with DC breakers is the absence of
current zero. Currently, high voltage DC breakers are available.
10. Control of line power
This is easy because very simple and fast control of DC links is possible through communications
and firing angle control. In AC, it is difficult.
11. Pollution
This affects DC link more than AC link.
Applications of DC link
• Submarine crossings
• Interconnection of systems with different frequency
• Long distance overhead lines
• Underground transmission
• Back to back ties for interconnection without affecting the fault level and independence of
operation
