Power generated at the generating stations is transmitted to the end user through transmission and distribution lines. HVDC line refers high voltage direct current transmission whereas HVAC transfer high voltage AC power. The main objective during the power transmission is to reduce power losses.
The big advantage of using HVAC system is that voltage can be easily step up and step down. HVDC on the other hand becomes economical when a large amount of power needs to be transmitted over long distances. However, DC is not effective for general local distribution. In this article, we will discuss some of the factors which need to be considered while choosing the kind of transmission (HVAC vs HVDC ) system.
Comparison of HVAC and HVDC Transmission :
HVDC has comparatively less losses as compared to HVAC in long distance transmission. Since XL= 2πfL, DC Frequency is zero due to which there is no inductive reactance and hence no inductive losses. Also XC=1/ 2πfC where f=0 which means capacitive reactance is infinite. Due to infinite capacitive reactance, there is no leakage current and hence there are no capacitive losses in HVDC. Hence capacitive and inductive losses are absent in DC transmission line. Besides this, skin effect and proximity effects are also absent in case of dc transmission. Moreover, corona losses in dc power lines occurs at higher voltage as compared to HVAC.
On the other hand, HVAC line experiences proximity effect and skin effect losses. Inductive and capacitive losses also becomes significant in long AC transmission lines. Hence transmission losses in HVAC are far more than HVDC in long distance power transmission.
Generation and utilization of power is through AC lines. For HVDC transmission, generated AC power at the generating end is converted into DC with the help of rectifier. That power is then transmitted to a long distance at high voltage. After that, DC power at the distribution end is again converted into AC with the help of inverter. Hence high cost converter equipment’s are required in case of HVDC transmission. It is to be noted that it is necessary to convert DC power into AC for economical transmission and distribution.
Number of Conductors :
Number of conductors in DC transmission are less as compare to HVAC lines. In case of DC, generally 2 conductors are used for single line. HVAC transmission towers on the other hand carry three separate wires referring three distinct phases R, Y and B. Moreover, each tower supports six conductors in case of double circuit transmission line.
Besides this, skin effect causes the HVAC current to stay on the outer surface of conductor due to high resistance at the center of conductor. Hence thick conductors (more copper) are needed in case of HVAC in order to supply the required amount of current. On the other hand, there is no skin effect in HVDC and current is evenly distributed inside the conductor. Hence HVDC has thin conductors as compare to HVAC for carrying the same amount of current.
Overall cost of any transmission system (HVAC vs HVDC ) depends on the terminal cost and transmission line cost. Terminal cost remains constant while cost of transmission line varies with the distance. Hence overall transmission cost increases with the distance.
Transmission towers used in HVAC needs to be more stronger since they have to support the mechanical load of much more conductors as compared to HVDC. Hence larger and wider supporting towers are used in HVAC which increases the cost of AC transmission system.
DC transmission on the other hand requires less number of conductors and small towers for the same voltage level which results in low cost. However, the use of converters increases the cost of HVDC transmission lines. These converters also introduces harmonics in the power system due to which more equipment’s are required in DC substation to eliminate the produced harmonics. Thus, installation cost of HVDC is increased. In order to understand the overall cost comparison of both HVAC and HVDC transmission systems, let analyze the below graph.
In the above graph, HVAC vs HVDC terminal cost is plotted against transmission line distance in kilometers. Up to the break even point which is around 600 km, cost of HVDC transmission line is higher than that of HVAC. After 600 km, cost of HVDC transmission becomes less than that of HVAC. This simply means that HVDC transmission is cheaper than HVAC in a long distance transmission.
Below the break even distance, it is preferable to use HVAC. HVDC system on the other hand is preferable in long distance transmission. During short transmission, high cost converter equipment’s makes HVDC costly. However, DC is economical to transfer large amount of power over much shorter distance (above 40 km) in under ground cables.
Spacing between the conductors in HVAC lines are much more as compared to HVDC lines. The reason for keeping greater spacing between HVAC conductors is to avoid proximity effect and corona loss.
Asynchronous Interconnection :
It is impossible to interconnect two power grids working at different frequencies with HVAC power lines. However, DC power can be used in such cases in which there is a need to connect two power grids having different frequencies, phase etc.
Altering the Voltage Level :
It is always necessary to vary the power into different voltages and current ratios for economical transmission and distribution. One big disadvantage of using HVDC is that change in voltage level is not possible in this type of transmission. On the other hand, it is very easy to alter the voltage level in HVAC with the help of transformers.
Line Length :
Reactive losses in AC transmission are directly proportional to the length of transmission. That’s why HVAC transmission line length is maintained up to a certain limit so that the reactive power losses doesn’t becomes so high. The limit is usually around 500Km. HVDC on the other hand doesn’t have reactive power losses due to which there is no limitation of length in this type of transmission. Hence it is always economical to use HVDC in a long distance transmission.
Conductors used in transmission lines are rated (insulated) for peak voltages. Now, the actual average power delivered by the AC line is the RMS (root mean square) of the peak line voltage. On the other hand, power delivered by the DC line is determined by the peak voltage value which is root 2 times the rms voltage values.
Let assume that we have a AC transmission line of 500 KV and we convert the line into DC, then that power line can work at the peak voltage which is 707 KV (2 times the RMS ). This means that DC transmission for the same current carry almost 1.4 times more power as compare to the HVAC system with peak voltage capacity. Hence HVAC almost wastes 30% of the conductor power carrying capacity.
During AC transmission, some amount of power is consumed due to line inductance. An increase in inductance will result in the increase voltage drop between sending and receiving ends of the line which results in poor voltage regulation. HVDC on the other hand has better voltage regulation due to the absence of line inductance.
Interference with nearby Communication Lines:
Interference signals produced by transmission lines depend on their frequency. DC has zero frequency due to which dc power lines produces very low noise interference in the nearby communication lines. HVAC on the other hand generates audible noise as well as radio and TV interference.
A long HVAC transmission line consumes reactive power and will need series and shunt compensation in order to improve the stability of power line. HVDC on the other hand doesn’t requires reactive power due to which no stability problems arises in this type of transmission.
The big advantage of using HVAC is that it is easy to change the voltage level with the help of step up and step down transformers. Moreover, AC substations are less expensive and they have low maintenance cost as compare to DC substation. DC transmission system on the other hand is cheaper than HVAC in a long distance with much higher voltages and power levels. HVDC is also useful to transfer energy between grids having different frequencies.
That’s all. Hope this will helps You.
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