Extra High Voltage Direct Current Transmission System
Question
Q) What are the advantages and disadvantages of HVDC transmission?
Cheaper in cost : Bipolar H.V.D.C. transmission line require two conductors only while in AC system it requires 3 conductors to carry 3 phase power. Thus there is a saving in conductor as well as insulator material.
No skin effect : Due to absence of line inductance, current flows through entire cross section of conductor, thus there is full utilization of line conductor in case of dc transmission system.
Better transmission efficiency : DC transmission system requires only two conductors, also the effective resistance of line is less than AC transmission systems (absence of skin effect), the line losses are less than AC transmission system thus have better transmission efficiency.
Lower voltage regulation : Lower voltage regulation due to absence of inductance and hence inductive reactance, voltage drop in DC transmission line is less than AC transmission line.
Line loading : Permissible loading on EHV AC line is limited by transient stability limit and line reactance to almost one third of thermal rating of conductors. No such limit exists in case of H.V.D.C. lines.
Surge impedance loading : Long EHV AC lines are loaded to less than 80 percent of natural load. No such condition is applicable to H.V.D.C. lines.
Less dielectric power loss : By using dc cables have lesser dielectric power loss in comparison with AC This facilitates higher current carrying capacity.
Absence in charging current and limitations of cable lengths : Use of EHV AC for under ground cable has restriction of long distance because of large charging current. But there is no such restriction in DC system since charging current is absent.
Lesser corona loss : Since the corona loss is directly proportional to ( f + 25 ) where f is supply frequency, so corona loss in DC system is less than AC system for same conductor diameter and operating voltage,
Greater reliability : A two conductor bipolar dc line is more reliable than a 3 wire 3 phase AC line because the dc line may be operated in monopolar mode with ground return when the other line develops a fault.
Limitation of High Voltage Direct Current (HVDC) Transmission System
Question
Q) State Limitation of High Voltage Direct Current Transmission System.
Answer
- A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power. The above figure shows the simple block diagram of HVDC system.
- Generated voltage is stepped up with the help of step up transformer and then converted in to DC with the help of three phase converter unit.
- This high voltage DC supply is transmitted over a long distance and at the end of transmission system it is again converted (inverted) in to the AC supply.
- This voltage is then stepped down to the suitable value with the help of step down transformer.
Question
Q) What are the advantages and disadvantages of HVDC transmission?
Answer
HVDC transmission system has many advantages and disadvantages over AC transmission system, some of these are disadvantages on given below.
Cheaper in cost : Bipolar H.V.D.C. transmission line require two conductors only while in AC system it requires 3 conductors to carry 3 phase power. Thus there is a saving in conductor as well as insulator material.
No skin effect : Due to absence of line inductance, current flows through entire cross section of conductor, thus there is full utilization of line conductor in case of dc transmission system.
Better transmission efficiency : DC transmission system requires only two conductors, also the effective resistance of line is less than AC transmission systems (absence of skin effect), the line losses are less than AC transmission system thus have better transmission efficiency.
Lower voltage regulation : Lower voltage regulation due to absence of inductance and hence inductive reactance, voltage drop in DC transmission line is less than AC transmission line.
Line loading : Permissible loading on EHV AC line is limited by transient stability limit and line reactance to almost one third of thermal rating of conductors. No such limit exists in case of H.V.D.C. lines.
Surge impedance loading : Long EHV AC lines are loaded to less than 80 percent of natural load. No such condition is applicable to H.V.D.C. lines.
Less dielectric power loss : By using dc cables have lesser dielectric power loss in comparison with AC This facilitates higher current carrying capacity.
Absence in charging current and limitations of cable lengths : Use of EHV AC for under ground cable has restriction of long distance because of large charging current. But there is no such restriction in DC system since charging current is absent.
Lesser corona loss : Since the corona loss is directly proportional to ( f + 25 ) where f is supply frequency, so corona loss in DC system is less than AC system for same conductor diameter and operating voltage,
Greater reliability : A two conductor bipolar dc line is more reliable than a 3 wire 3 phase AC line because the dc line may be operated in monopolar mode with ground return when the other line develops a fault.
Limitation of High Voltage Direct Current (HVDC) Transmission System
Question
Q) State Limitation of High Voltage Direct Current Transmission System.
Answer
- Power cannot be generated at HVDC level.
- Switches and circuit breaker design is complicated in case of HVDC system.
- Rectifier circuit produces non linear current from AC side . Cost of DC transmission system is less than AC transmission system above 500km only.
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Electrical