HVDC Transmission-Advantages & Disadvantages

What is High Voltage DC Transmission?

Power is generated in a power station in the form of three-phase a. c. This voltage is stepped up by the step-up transformer. 

With the help of a static rectifier this high voltage a.c. is converted into high voltage d. c. Power is transmitted at high voltage d. c. through the high voltage d. c. transmission line. With the help of static inverters, the high voltage d.c. is converted into high voltage a. c. at the receiving end. 
Using the step-down transformer, the high voltage a. c. is converted into low voltage a. c. to the required value. From there the LT transmission lines are run in different areas.

HVDC Transmission Advantages:
The following are the advantages of high voltage d. c. transmission.
(1) Economical transmission of bulk power :
By the HVDCT system, the bulk power is transmitted to a distance more than the breakeven distance. At the breakeven distance, the cost of terminal equipment is balanced against the cost of conductors and poles. It becomes economical when the distance is more than this distance.

(2) Decrease in the number of lines :
For the transmission of bulk power, two three-phase circuits are required i.e. six conductors are required while in HVDCT only two conductors are required so the cost of the line decreases.

(3) Size of the tower :
Compared to HVACT, phase-to-phase clearance and phase-to-ground clearance required is comparatively less in HVDCT, and number of wires are only two. So the width and height of tower required is less. So the size of tower required is small. As a result, the cost of the tower is reduced.

(4) Reduction in numbers of intermediate substations :
In long-distance HVACT lines, intermediate substations are required to be installed at a distance of 300 km for the compensation of reactive power. This is not required in HVDC so the cost is reduced.

(5) Earth return :
Earth return can be used in HVDCT while it is not possible in HVACT as the line is always three-phase type.

(6) Skin effect:
There is no skin effect in d. c. so the current density in the conductor is uniform throughout the cross-section of the conductor so full cross-section of conductor is useful.

Moreover, there is a reduction in power loss as the effective resistance is less.

(7) Charging current :
There is an absence of charging current in HVDCT.

(8) Corona :
Effect of the corona is less in d. c. system than in a.c. a system so the corona loss is reduced in d. c. system and there is no disturbance to the nearby communication line.

(9) Underground cable :
Due to the absence of the charging current, power transmission can be done through the underground cable or marine cable. 

In this, there is no restriction to the length of the cable. While in a. c. system, there is a restriction to the length of the cable. For example, for 145kV the distance is 60 km, for 245 kV it is 40 km and for 400 kV it is 25 km.

(10) Reduction in line loss :
Due to the absence of reactive power, the line losses are reduced in HVDCT, so there is an increase in the transmission efficiency.

(11) Power factor:
There is no question of power factor in HVDCT so there is no requirement of power factor correction and no problems arise due to the reduced power factor.

(12) Reduction in the size of conductor : 
When equal power has to be transmitted for the same distance, with equal power loss volume of conductor material required in d. c. two-wire system is less than that required in a three-phase three-wire system.

(13) Voltage regulation :
In a long-distance HVAC line, the voltage changes with the distance of the line and there is a change in voltage due to the change in load. This difficulty does not arise in the HVDCT system as in this with the control of rectifier and inverter, the line can be operated for constant current or constant voltage regulation.

(14) Stability and line loading :
There is an asynchronous tie in the HVDCT system so there is no question of transient stability. Due to this, the line can be loaded up to its thermal limit or up to the thermal limit of the thyristor, whichever reaches first. While in an HVAC system due to the difficulty of stability, the line can be loaded up to 1/3 of the thermal rating of the conductor.

(15) Quick power transfer and control :
In an HVDC system, the magnitude and direction of power flow can be controlled by controlling the converters. Due to this, the limit of transient stability can be increased.

(16) Decrease in right of way :
The right of way is less in the HVDCT system compared to the HVACT system.

(17) Asynchronous tie :
Two a. c. systems having different frequencies can be interconnected through the HVDCT system. This is called an asynchronous tie. Disturbances of one system are not transferred to the other system so the total shut down and blackout can be prevented.

(18) Flexibility in operation :
In the HVDCT system when the bipolar system is used and if there is a permanent fault in one conductor, the earth can be used as the return conductor and the system can be converted into monopolar mode and continuity of power can be maintained. In a three-phase system, this is not possible.

(19) Short circuit level :
In the HVACT system parallel lines are used for the transmission of bulk power, so there is a decrease in the equivalent reactance. When two such systems are inter-connected, there is an increase in short circuit KVA of both systems. But if two systems are interconnected through the HVDC system, the fault level of each system remains the same.

Disadvantages/Limitation of HVDC Transmission System :
There are some limitations or disadvantages too of the HVDCT system along with its advantages.

(1) Cost of terminal equipment:
In the HVDCT system, the rectifiers are used at the sending end to convert high voltage a. c. into high voltage d. c., and inverters, filters are used at the receiving end to convert d. c. into a. c. The cost of such terminal equipment is much high.

(2) Additional equipment:
In the HVDCT system, some additional equipment like converter transformer, electrical and mechanical auxiliaries, valve control, pole control, etc. are used. All such equipment is of high technology and are to be imported.

(3)Transformation of d. c. voltage :
In a. c. system, voltages can be stepped-up or stepped-down easily with the help of transformers but in the d. c. system, this is not possible. Hence, this system cannot be used for low voltage transmission and distribution.

(4) D. C. circuit breaker :
D. C. circuit breakers are still in the development stage and its cost is more than the a. c. circuit breakers.

(5) System failure :
The possibility of system failure is more in the HVDCT system as there are some abnormal operating conditions.

(6) Complicated control :
Control of thyristor is complicated.

(7)Complicated cooling :
The cooling system of thyristors is also complicated.

(8) Harmonic filter:
Harmonics are produced in a. c. side in the HVDCT system. Large numbers of harmonic filters are required to suppress the harmonics, which increases the cost.

(9) Power loss :
Losses in thyristors and in converter transformers are additional, which nullify the reduced losses in the line.

(10) Multi-terminal network:
HVDC is not suitable for multi-terminal networks.

(11) Overload capacity :
The overload capacity of converters is not much, so the overload is not permissible.

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