What is AC Transmission Line?
AC transmission lines are commonly used for transmitting electrical power from the point of generation (such as a power plant) to the point of consumption (such as a residential or industrial area). The power is transmitted over long distances through transmission lines, which are typically made up of high-voltage overhead wires or underground cables.
To transmit the power over long distances, it is necessary to step up the voltage at the generating end using a step-up transformer. This increases the efficiency of the transmission by reducing the losses due to resistance. At the consumer end, the voltage is then stepped down using a step-down transformer to a level that is suitable for use.
What is DC Transmission Line?
In a DC transmission system, the power is typically generated as alternating current (AC) and then converted into direct current (DC) using a mercury arc rectifier. The DC power is then transmitted over long distances using DC transmission lines, which can be either overhead lines or underground cables.
At the consumer end, the DC power is then converted back into AC using a thyratron (a type of gas-filled tube that can be used as a switch or amplifier). The AC power can then be used by the consumer.
Advantages of DC Transmission:
- DC transmission systems only require two conductors for power transmission, which can reduce the cost of conductor materials.
- DC transmission systems have lower potential stresses on the insulation, so they require less insulation.
- In a DC transmission system, there is no effect of inductance or capacitance, which can cause problems like power factor and surge.
- There is no skin effect in a DC transmission system, so the full cross-section of the conductor is utilized.
- DC transmission systems have a unity power factor and low voltage drop over long distances, which means they don't need stabilizers like AC transmission systems do.
- DC transmission systems can use earth as a negative conductor, which reduces the effect of corona.
- DC transmission lines are easier to construct than AC transmission lines.
- Underground cables can be effectively used in DC transmission because of their low potential stress and negligible dielectric loss.
- DC transmission systems have better voltage regulation compared to AC transmission systems because they have a lower voltage drop for a given load and sending end voltage.
- DC transmission systems have less interference with communication lines.
- DC transmission systems are more efficient at transmitting power over long distances because they have less power loss due to resistance.
- DC transmission systems can easily transmit power through submarine cables, which can be useful for offshore wind farms or interconnecting islands.
- DC transmission systems are more reliable because they are less susceptible to electrical noise and interference.
- DC transmission systems can be used to transmit power at high voltage levels, which can help reduce transmission losses and increase transmission capacity.
- DC transmission systems are easier to control and regulate compared to AC systems because they don't have the complex reactance and phase angle considerations. This makes them suitable for use in large power grids or for interconnecting different power systems.
Disadvantages of DC Transmission System:
- It is difficult to generate electrical power at high voltage levels using a DC transmission system because of problems with commutation (the process of switching current from one conductor to another).
- It is difficult to step up or step down the voltage in a DC transmission system.
- DC transmission systems require costly switchgear because low-resistance interruption methods cannot be used for circuit breakers.
- The cost of converting and inverting stations (equipment used to change AC to DC or DC to AC) is very high.
- DC transmission systems require converting equipment at the sending end and inverting equipment at the receiving end substations, which adds to the cost and complexity of the system.
- DC transmission systems are not as widely used as AC transmission systems, so there may be a lack of standardization and availability of equipment and components.
- DC transmission systems require special control equipment, which can be complex and expensive to maintain.
- DC transmission systems are not suitable for transmission of power over very long distances (hundreds of kilometers or more) because of the high cost of converting and inverting equipment.
- DC transmission systems are not as efficient at transmitting power over short distances (less than a few hundred kilometers) because of the loss of power in the converting and inverting equipment.
- DC transmission systems are not suitable for transmitting power to multiple locations or for distributing power to smaller loads, as the voltage levels are typically too high for direct use. AC transmission systems are generally preferred for these types of applications.
Advantages of AC Transmission System:
- It is easy to generate electricity at high voltage levels (such as 3.3 kV, 6.6 kV, and 11 kV) using an AC transmission system.
- The voltage generated by an AC transmission system can be easily increased or decreased using transformers.
- AC switchgear (equipment used to switch AC electrical circuits) is less expensive than DC switchgear.
- The maintenance of AC substations (a facility where electrical power is transformed from one voltage level to another) is simpler than the maintenance of DC substations.
- AC transmission systems are more efficient than DC systems because they use transformers instead of Motor-Generator sets to change the voltage level.
Disadvantages of AC Transmission System :
- AC transmission lines require larger conductors than DC transmission lines.
- The cost of AC transmission lines is generally higher than the cost of DC transmission lines.
- AC transmission lines have more losses due to the skin effect, which is the tendency of AC currents to flow only on the surface of conductors.
- AC transmission lines have continuous power losses due to the capacitance of the lines, even when there is no load or the line is open.
- AC transmission lines have additional losses due to inductance.
- AC transmission lines require more insulation than DC transmission lines.
- AC transmission lines can experience corona losses, which are caused by the ionization of the air around the conductor.
- AC transmission lines can interfere with other communication lines.
- AC transmission systems can have stability and synchronizing problems.
- AC transmission systems are less efficient than DC transmission systems.
- It can be difficult to control reactive power (power that is returned to the source after being transmitted) in an AC transmission system.
- Alternators (machines that generate AC electrical power) must be synchronized before they can be connected in parallel. This involves matching the phase and frequency of the alternating current they produce.
- In cables, the alternating current can cause sheath loss, which is the loss of power due to the resistance of the outer layer of the cable (the sheath). This can be a problem in the long-distance transmission where the loss of power due to sheath resistance can be significant.