The modern trends have developed Insulated Gate Bipolar Transistors (IGBT) and gate turn-off thyristors, which made the HVDC system very economical and easy to maintain. By controlling the firing pulses of thyristors, we can control the magnitude and direction of power flow effectively.
Since the permissible loading of an HVAC line is limited by transient stability limit and line reactance to almost one-third of thermal rating of conductors and absence of such limit in case of HVDC lines. An HVDC system of power transmission can be effectively used in a power system. Let us see the differences between high voltage direct current (HVDC) and high voltage alternating current (HVAC) transmission systems.
Comparison Between HVDC and HVAC System :
| HVDC Transmission System | HVAC Transmission System |
|---|---|
| It is economical for transmission of power above break-even point i.e., for long distances. | It is economical for transmission of power below break-even point i.e., for small distances. |
| The number of conductors required for transmitting power is less. | The number of conductors required for transmitting power is more. |
| Does not require any intermediate substations for reactive power compensation. | Requires intermediate substations for compensation. |
| Very fast and accurate power flow control is possible. | Power flow control is slow and is very difficult. |
| Skin effect is absent resulting in uniform distribution of current density across the cross-section of the conductor. | Skin effect is present due to which current density is non-uniformly distributed across the cross-section f the conductor. |
| Corona loss and radio interferences are absent resulting in less insulation level required for the transmission line. | Corona loss and radio interferences are more due to which high insulation level is required for the transmission line. |
| Voltage in the line does not fluctuate with the load. | Voltage in the line fluctuates with the load. |
| Does not require a double circuit, in this systems earth return is used. | Always requires a double circuit. |
| Transmission through underground or marine is also possible. | Limit is imposed on the length of the cable. |
| Transmission losses are less due to the absence of flow of reactive power. | Transmission losses are more due to the flow of reactive power. |
| The fault levels of the two networks are unaffected and remain unchanged when interconnected. | Fault levels of two networks get added up and are increased after the interconnection. |
| The cost of right of way is less and the cost of supporting towers is less, as this system requires narrow towers. | The cost of right of way is more and the cost of supporting tower is more as this system requires lattice-shaped towers. |
| DC breakers used in this system are of high cost, as it is difficult to break dc currents. | The circuit breakers used in this system are of low cost when compared to dc breakers. |
Advantages of HVDC Transmission Over HVAC Transmission :
Nowadays HVDC transmission systems are preferred over HVAC transmission systems due to their distinct superiority. The various advantages of HVDC transmission systems over HVAC systems on different basics are,
Economics :
- The cost of towers (pylons) is greatly reduced with less Right of Way (ROW) cost and a simpler structure.
- The same amount of bulk power can be transmitted using only two conductors.
- Losses are reduced to two-third of the HVAC link with the use of only two conductors. Hence, the efficiency of transmission lines is high.
- There is no skin effect and there is less corona loss due to the absence of frequency, since corona loss ∝ (f + 25).
- No intermediate substations are required between the two ends of the HVDC link.
Technical Performance :
- The use of an HVDC link improves the dynamic and transient stabilities in the ac networks with which it is connected or associated.
- The power transmitted between the ends of the link can be fully controlled.
- The voltage profile remains flat which implies that the voltage remains unchanged with the distance.
- HVDC link proves to be an ideal means by which asynchronous operating ac networks can be easily interconnected.
- An HVDC transmission line can transmit the power over long distances.
Reliability :
- HVDC link is more reliable than the HVAC link.
- This is because the fault level in HVDC link is less than an HVAC link.
- The incorporation of light-activated thyristors in the converter units has proved to increase the reliability of dc links.
Power Loss :
In the case of a 3-phase ac line, the power is given by,Pac = 3 Irms2 R ...(1)In the case of the bipolar HVDC line, the power loss is given by,Pdc = 2 Id2 R ...(2)Dividing equation 2 by 1, we get,
Assuming that the current is limited by temperature rise in each case, then Id = Irms.
Therefore, the losses in the case of the HVDC system reduced by one-third when compared with a 3-phase 3-wire ac system.
Overvoltages :
Overvoltages are primarily due to switching operations and lightning surges. The switching operations may involve, load switching, transformer tap changing, capacitor banks switching, etc. In the case of 3-phase ac lines, the switching surges are limited to a peak value which is about 2 or 3 times the normal operating peak voltage. But in the case of HVDC, the switching surges are about 1.7 times the normal operating peak voltages.
Investment Cost :
In the above figure, Bd is the break-even distance. It can be observed that the cost incurred on a 3-phase 3-wire transmission system appears to be economical below the break-even distance Bd (which is around 500km for overhead lines and for submarine lines it is around 50 km).
While for the transmission lines above Bd, an HVDC transmission system is very economical. Below the break-even distance, the HVDC line is not preferred as it raises the cost of the line compared to HVAC.