Advantages and Disadvantages of DC and AC Transmission

This article describes advantages and disadvantages of DC and AC transmission. Power transmission is of two types-AC transmission and DC transmission.

The bulk power is supplied from the generating station to consumers through a transmission and distribution system. The transmission line consists of electrical conductors that carry electric current, delivering power at the distribution substations.

In DC transmission, the AC voltage is first converted into DC using converters and then power is transmitted through a transmission line to the distribution center, where the DC voltage is converted into AC using inverters. In AC transmission, the voltage is stepped up and power is transmitted through the transmission line to the distribution center, where the voltage is stepped down to the utility level.

Thus, there are two types of power transmission namely AC and DC transmission. Both the transmission system- AC transmission( Alternating Voltage and current) and DC transmission system(DC voltage and current) have their own advantages and disadvantages.

We will first discuss what is the AC and DC transmission system and then discuss their advantages and disadvantages.

AC Transmission System

The infrastructure and network used for transmitting electrical power using an alternating current(AC) over long distances is called an AC transmission system. Key components of the AC transmission system are generating stations, step-up transformers, transmission lines, substations, step-down transformers, and distribution networks. The AC transmission system is globally used because of ease of voltage transformation, efficiency, and compatibility with existing infrastructure.

Advantages of AC Transmission

The followings are the advantages of the AC transmission system.

AC power can be easily increased or decreased using the step-up and step-down transformer. At generating satiation, the voltage is increased and power transmission is done at high voltage to reduce the line losses. At receiving end the voltage is again decreased for distribution and utilization.
The electric power can be generated at higher voltage like 3.3 kV,6.6 kV, and 11 kV.

The equipment used in AC transmission systems has a lower cost because they are produced in large quantities, therefore they are economical.
The arc quenching in the AC switchgear is easy because, at the natural zero-crossing of the AC waveform, the switchgear interrupts the circuit. Therefore, AC transmission is safer.
Most of the existing electrical transmission infrastructure is designed for AC and it is easy to add a new AC transmission system to the existing one.

The repair and maintenance of AC substation and transmission lines does not need much expertise and it is easy to maintain, and moreover, the maintenance is less expensive.
AC switchgear such as SF₆ circuit breakers is cheaper than DC switchgear.

Disadvantages of AC Transmission

The disadvantages of AC transmission systems are as follows.

The AC transmission line has three conductors and earth wire( Guard wire), leading to more weight and more components. Therefore, the construction of an AC transmission line is more intricate than the DC transmission line.
The AC transmission system needs three overhead conductors, whereas in monop o lar DC transmission. one conductor is required, and in bipolar DC transmission two conductors are required. Thus, AC transmission requires more conductor material.
The skin effect with AC transmission is more, and the skin effect cause increased conductor resistance. The increased resistance cause more power loss and also adversely affects the voltage regulation.

The capacitance forms between the line conductor in AC transmission, and capacitance draws continuous charging current even when the load current is zero, leading to power loss.
The corona loss can be decreased in the AC transmission system by increasing the spacing between the conductors, However, increased spacing cause an increase in the cost of towers.
The losses in AC transmission increase with the increase in the length of the transmission line because of increased resistance. For long-distance transmission, the transmission efficiency of DC transmission is better than that of AC transmission.

AC transmission generates electromagnetic fields that may links to nearby telecommunication lines and cause induction of RFI and EMI interference.
The reactive power compensation is required in AC transmission for ensuring the voltage regulation within the permissible limit.

DC Transmission System

The process of transmitting power over a long distance using direct current is called DC transmission. In DC transmission, the current flows in one direction from the source to the load. The AC power is first converted into DC power and then the power is transmitted through a DC transmission line. At the receiving end, the DC is converted into AC using inverters.

DC transmission system has its own advantages and disadvantages.

Advantages of DC Transmission

The high-voltage DC transmission system has the following advantages.

AC transmission system requires three wires whereas DC transmission requires two wires or one wire depending on the type of DC transmission technology. Therefore, DC transmission requires less conductor material compared to AC transmission.

The frequency of DC is zero, therefore there is no skin effect. The current flows through the entire cross-section area of the conductor. As a result uniform distribution of current, the effective resistance of the transmission line is low compared to AC transmission.
The DC transmission line has lower resistance compared to the AC transmission line and therefore, the resistive losses for the same amount of power transmitted over a given distance.

The capacitive effect in the DC transmission line is negligible and therefore no power loss occurs due to the line charging current.

The inductive effects in a DC transmission line are much lower in comparison to the inductance of AC transmission lines.
The phase displacement and surge problem do not exist in DC transmission.
The voltage drop in the DC transmission caused by resistive and inductive elements is very less than the AC transmission line for the same sending end voltage and load conditions.

The voltage regulation of DC transmission is better than AC transmission because of the lower voltage drop.
The insulation material requirement for a transmission line depends on the voltage stress on the insulating materials. If there is more voltage stress then we need more insulating material to sustain the potential stress. In the case of DC transmission, the potential stress is less compared to AC transmission because of the reduced corona effect, absence of charging current, and absence of frequency-dependent phenomena like skin and proximity effect. As a result, DC transmission requires fewer insulation materials.
The constant DC voltage does not create an alternating field that induces polarization and causes dielectric loss. Therefore, the dielectric loss is absent in DC transmission.

DC transmission does not suffer from stability and synchronizing problems.
In high-voltage DC transmission, there is no dielectric losses.
A DC transmission line produces less EMI and RFI interferences because of reduced corona loss, constant voltage, and reduced coupling effect.

Disadvantages of DC Transmission

The followings are the disadvantages of DC transmission.

It is not feasible to generate high DC voltage. For DC transmission, we need to generate high AC voltage and then need to covert it into DC using converters, which lead to increased cost.
The DC transmission requires converter and inverter at sensing and receiving end. The converters are costlier and complicated. Also, the converters capacities are limited and we need to connect several converters in the series and parallel combinations.

The equipment used for DC transmission like circuit breakers, switches and protective devices are limited and, therefore the cost of these equipment are more compared to the equipment used for AC transmission line.
In AC transmission voltage can be directly increased or decreased with use of transformer. However, in the case of DC transmission, additional electronic converters are required, leading to more cost and complexity.
Additional equipment like converter, inverter and synchronizing equipment are required for interconnecting DC systems with existing AC grids. As a result, the DC transmission becomes more complex and costlier.