The electrical power generated in the form of AC power and most of the power utilized in the form of AC. Because it is difficult to generate bulk DC power. This power transmitted for long distances by the HVAC transmission system. But there are some disadvantages to a conventional transmission system. To overcome these limitations HVDC transmission system used.
Advantages of the HVDC transmission system
1) Economical transmission of the bulk power
In a conventional transmission line, the distance cannot be more than the breakeven distance. But in the HVDC transmission line, the distance can be more than the breakeven distance.
Although, this system is more economical when the distance is more than the breakeven distance. Because at this distance, the cost of conductors and poles balanced.
2) Decrease in the number of conductors
In the HVAC system, the power transmitted in the form of three-phase AC power. Therefore, three or four conductors need as per the type of transmission line.
But in the case of HVDC transmission lines, only two conductors required. Hence, the cost of the conductor decreased.
Corona effect appears in both HVAC and HVDC systems. But, in an HVDC system, the effect of the corona is very less compared to the HVAC system. And there is no disturbance to the nearby communication line.
4) Size of tower
In the HVDC transmission line, phase-phase and phase-ground clearance required is less compared to the HVAC line. Therefore, the height and width of the tower required is less.
The number of conductors required in this system is less. So, the size of the tower is less which results in less cost of the tower.
5) Earth return
For the monopolar HVDC transmission system, earth return can be used. That means, only one conductor required to transmit the power. This is not possible in the HVAC transmission line.
6) Charging current
In the DC transmission line, the capacitance is not produced between two phases or between the phase and ground. Therefore, the charging current is absent in the HVDC system.
7) Skin effect
The current density is uniform throughout the line. Hence, there is no skin effect in the HVDC system.
And it utilizes an entire cross-section area of the conductor. So, the resistance of the line is not increasing and the power loss is less.
8) Reduction in line loss
The line loss reduced due to the absence of the reactive power in the HVDC transmission line. This increases the efficiency of the system.
9) Reduction in size of the conductor
When equal power transmitted for the same distance, less volume of conductor required for the HVDC two-wire system compared to the HVAC three-phase three-wire system.
The underground system can be established for the HVDC system because of the absence of the charging current.
In the HVAC system, the distance of underground cables is a constraint. For example, 145 kV line the distance is 60 km, for 245 kV it is 40 km and for 400 kV it is 25 km.
This constraint is not affected in the HVDC underground system. So, it is possible to establish more distance underground and marine lines for the HVDC system.
11) Reduction in the number of intermediate substations
For compensation of reactive power, intermediate substations required to be installed at 300 km in HVAC lines. Because of the absence of reactive power, this is not required in the HVDC line. Hence, the cost is reduced.
12) Power factor
Generally, we are not considered a power factor in the case of DC. Similarly, for the HVDC system also, the power factor is not considering.
13) Stability and line loading
The line can be loaded up to its thermal limit or the thermal limit of thyristors because of the absence of the transient.
While in the case of the AC transmission line, a transient is present. Therefore, the line can be loaded up to one-third of the thermal rating of the conductor.
14) Flexibility in operation
When a fault occurs in a bipolar HVDC system, the earth can be used as a return path. So, the system will continue in operation in case of a fault in one conductor.
But this is not possible in case of a three-phase AC transmission line. Once fault occurred, the line will be going in maintenance.
15) Quick power transfer and control
The magnitude and direction of power flow in the transmission line controlled by the converters. Due to this the limit of transient stability can be increased.
16) Short circuit level
Parallel lines used to transmit the bulk power in an HVAC system. When this system interconnected, there is an increase in short circuit kVA of both the systems.
But if two systems interconnected with HVDC lines, the fault level of each system remains the same.
17) Voltage regulation
Due to the change in load, the voltage of the AC transmission line is changing. And for the long-distance line, the voltage is change with the distance. This difficulty does not arise in the HVDC transmission system with the control of the rectifier and inverter.
Hence, the line will operate on constant current and constant voltage regulation.
18) Asynchronous tie
The frequency is the most important quantity in the case of the AC fundamental. For the interconnection of the tie line, it is necessary to match the frequency of both tie lines.
If the frequency is not matched, this is known as an asynchronous tie. This cannot interconnect directly.
But this is possible in the case of the HVDC transmission line. And the disturbance of one system is not transferred to another system. Hence, the total shutdown and blackout can be prevented.
1) Cost of terminal equipment
In the HVDC transmission line, the rectifier used at the sending end and the inverter used at the receiving end. The smoothing filters need at receiving end. The cost of this equipment is very high.
2) DC circuit breaker
The DC circuit breaker is still under development and the cost is high compared to the AC circuit breaker.
3) Additional equipment
This system needs some additional equipment like converter transformer, electrical and mechanical auxiliaries, pole control, valve control, and many more. All this equipment is of high technology and the cost of this equipment is high.
4) Complicated control
The converter used to control the transmission line. But it is difficult to control the converter under certain abnormal conditions.
5) Change the voltage level
In the AC system, with the help of a transformer, the voltage can be easily stepped up and stepped down. Therefore, this system cannot use for low voltage transmission.
6) System failure
There is some abnormal operating condition in which the system may fail to operate.
7) Harmonic filter
In the input side, the AC supply is given to the rectifiers. To mitigate these harmonics, a large amount of filter required. And the cost of this equipment is high.
8) Complicated cooling
The converter used power electronics switches. When this is in operation, a very high amount of heat produced in the thyristor.
9) Overload capacity
The converters cannot operate on overload conditions. Therefore, it is not permissible.
10) Multi-terminal network
HVDC transmission line is not suitable for a multi-terminal network.
11) Power loss
The losses occur in the converters and other auxiliaries, which nullify the reduced loss in the line.