Difference between HVDC & EHV transmission System

Difference between HVDC Transmission System and EHV transmission System:

While comparing these two transmission lines we should consider the following.

  • Economics aspects
  • Technical Performance


S.No HVDC Transmission Lines HVAC Transmission Lines
1 For lines designed for same insulation It can carry as much power with level, it can carry more power with two conductors of the same size. conductors (positive and negative). It can carry as much power with three number of conductors
2 For a given power level, it requires less right of way, cheaper towers, reduced conductor costs. For given Power level, the cost of the towers and conductors are high
3 Cable insulation required is less Cable Insulation strength is required more. Hence it consume high cost on the insulation factor.
4 The net power losses are reduced as the decreased the number of comductor usage The power losses is very high as it has three number of conductors.
5 HVDC does not have skin effect hence the net operation cost get reduced. Due to the skin effect, power losses increases, therefore the operation cost very increases as compared with HVDC transmission lines.
6 Dielectric losses is less Dielectric losses is high
7 Corona loss and radio interference is less compared to that of A.C. This leads to the choice of economic size of conductors for D.C Corona loss and radio interference is high and which causes high Operational cost
8 DC does not require shunt compensation. So In DC we do not want to install shunt compensation devices. It is requires shunt compensation devices.
9 Terminal equipment such as converters inverters, filters cost is high Terminal equipment cost is low
10 HVDC requires High maintenance cost. I.e failure of IGBTs leads to high cost. A 400 Volts FF300 model IGBTs are cost about $2000, So think what about 500kV igbt cost. In AC No maintenance cost. Even We do not want to touch our system for a year.
11 DC Tranmission systems are more economical, if the transmitting distance is more than breake even point. The breaken even point will be 500 to 600 KM depending on the PU Units For transferring power to Short distance, AC Tranmission power has more advantages.
1 Full Control on Power Transmitted: DC does not affect by the distance of the transmission line. In DC we can send constant power over long distance. But In AC transmission, the tranmitting power is depending on the voltage ange. Hence power carrying capatity of the transmission line decreases after some distance. The power transfer is limited by the consideration of transient and steady state stability.
2 Voltage control: DC Converter station requires reactive power related to the line loading, the line itself does not require reactive power control Voltage control is very compicated in AC transmission system due to line charging and inductive voltage drops. Voltage profile is flat due to surge impedance loading. AC transmission line needs reactive power control to maintain the same voltage at receiving end. The reactive power componsation is directly propotional to the length of the transmission line, hence the long distance transmission lines needs high reactive power.
3 Fault current: DC line allows the fault current upto maimum rating of the line. The fault current increases as the number of interconnection of AC system increases.
4 DC Does not require any line compensation devices because of the Inductor and capacitor charecteristics on DC. Inductor act as short circuit and capacitor acts as open circuit on DC AC System Needs reactive power compensation componts (shunt reactor )in long transmission lines to overcome line charging  and stability limitations.
5 Interconnection problem: In DC Two systems are interconnected with different frequencies. Synchronization of DC tie lines, no need of co-ordinated control for inter connection. Two system can not be interconnected with different frequencies.
6 Ground Impedance: DC does not have any ground impedance, hence the ground can be taken as current return path. But In AC system, the ground offers high magnitude of impedance, hence the ground current can not permitted for steady state operation.
7 Fast control to limit the fault current in DC: DC breakers in Two terminal DC links are avoided. At the same time the cost of DC circuit breakers are very high as compared with AC circuit breaker. Circuit Breaker is compulsary.
8 Transformer can not be used to reduce the voltage level Transformer is the heart of the AC power system. Which can be used to reduce the voltage level.
Learn More:   What is Fault in Electrical, Types, Symmetrical & Unsymmetrical Fault



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