What are the Advantages of HVDC over HVAC?
The electricity we consume travels very long distances before reaching us. The power generating stations, often in remote areas supply power that travels through several hundred miles & through multiple substations. The power transmission is done using very high voltage in order to reduce the line losses. But the power can be in either AC or DC form & both modes of power transmission are being used. We are more accustomed to AC transmission because of the AC utility poles outside our homes & the available AC supply outlets in our home but HVDC plays a vital role in power transmission with various advantages over HVAC.
The main objective in power transmission is to reduce the transmission power losses & supply power economically at minimum possible investment. Both types of transmission are affected by several factors but the HVDC transmission has more of the advantages than its disadvantages. In this article, we are going to discuss several such advantages of HVDC over HVAC transmission.
1) Low Cost of Transmission
The cost of transmission depends on various factors like the cost of equipment used for voltage conversion at the terminal stations, the number & size of conductors being used, transmission tower size and the power losses in the transmission, etc.
The equipment used for HVAC voltage conversion at the AC terminal station is mainly transformer which is simpler & cheaper than HVDC’s thyristors based converters. It is the only feature of HVAC transmission that surpasses the HVDC in terms of minimum cost requirement.
The HVAC transmission requires a minimum of 3 conductors for 3-phase power transmission while in case of HVDC that can utilize the earth as the return path can use only 1 conductor for mono-polar transmission or 2 conductors for a bipolar transmission. It substantially decreases the overall cost of the transmission. Even so, the 3 conductors used for 3-phase supply can be used for HVDC transmission with the capability to transmit double the same amount of power using a double bipolar link.
The HVAC transmission lines require relatively larger spacing between the phase to ground & phase to phase conductors. In order to maintain such spacing, the transmission tower used for HVAC needs to be taller & wider than HVDC. Using the HVDC transmission tower reduces the installation cost as compared to HVAC towers.
The power losses in the HVDC transmission is significantly lower than HVAC. Therefore, HVDC is a more efficient form of transmission then HVAC.
The overall transmission cost can be broken down into two main categories i.e. Terminal station cost & transmission line cost. The former is a constant figure that does not depend on the distance of transmission while the latter depends on the distance of the transmission line. The terminal cost for AC is quite low while HVDC is very high. But the transmission line cost per 100 Km for HVAC is far larger than that of the HVDC transmission line. Therefore, the overall cost graph for HVAC & HVDC meets at a point called break-even distance.
The transmission distance at which the overall investment cost for HVAC start increasing than HVDC is called Break-even Distance. This distance depends on the type of transmission. The break-even distance for overhead transmission is estimated at around 400 – 500 miles (600- 800 in kilometers) while the underwater transmission is 20-50 Km & underground is 50-100 km. Therefore, the HVDC is a far more efficient & economically cheaper choice for power transmission over the break-even distance.
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2) Reduced Power Losses
The HVDC transmission experiences very low losses as compared to HVAC. Here are some of the losses that are either completely eliminated or reduced significantly in HVDC.
Absence of Reactive Power Loss
The transmission line experiences reactive power loss in case of HVAC that is directly proportional to the line length, frequency as well as the inductive load at the receiving end. It reduces the effective power transmission & wastes energy. It is why HVAC transmission lines length is maintained below a specific point to allow efficient power transfer. For such reason, the HVAC uses series & shunt compensation to reduce the line VARs & offer stability in the system.
In the case of HVDC, there are is no frequency or charging current. Therefore the HVDC is free from reactive power losses & there is no need for such compensations as in HVAC.
Reduced Corona Losses
When the transmission voltage increases above a certain limit called the corona threshold, the air molecules surrounding the conductor start ionizing & generates sparks that waste the energy, this is called corona discharge. The losses due to corona discharge depend on the voltage level as well as its frequency & since DC has zero frequency, the corona loss in HVDC is almost 3 times lower than in HVAC.
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Absence of Skin Effect
The skin effect forces the AC current to reside more on the surface of the conductor & leave its core empty. Therefore in HVAC transmission, the current density is maximum on the surface & lowest at the core of conductor. Since the cross-sectional area at the core of the conductor is ineffective & the resistance is inversely proportional to the cross-sectional area, the total resistance of the conductor increases. Resulting an increase in I2R losses in the transmission line.
On the other hand, DC is uniformly distributed in a conductor. Therefore, there is no phenomenon as such skin effect & the losses caused by it in HVDC transmission.
No Radiation & Induction Losses
The HVAC transmission lines are also affected by radiation & induction losses due to its continuously varying magnetic field. The former is due to the fact that long transmission lines start to act line antenna & radiates energy that does not come back. While the latter is due to the current induced in the nearby conductors.
In the case of DC, the magnetic field is uniform. Therefore, there are no radiation or induction losses in HVDC transmission lines.
The Cables used for underground & underwater power transmission have parasitic capacitance. It does not supply power unless it is fully charged. Therefore, the cables require extra charging current. The capacitance increases with the length of the cable, thus the charging current also increases with the length.
In AC, the cable is charging & discharging multiple times in a second & the cables draw more current from the station required for charging. This current increases the I2R losses in the cable.
In the case of DC, the cable charge only once & at the time of switching. therefore, there are no losses due to charging current in HVDC transmission.
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Absence of Heating due to Dielectric Losses
The alternating electric field generated by AC affects the insulating material inside the transmission lines. This insulation material absorbs the energy due to alternating electric field & converts it into heat. It also reduces the lifetime of the insulation.
In HVDC, the electric field is uniform. Thus there are no such dielectric losses & there is no insulation heating problem.
3) Thinner Conductor
The skin effect causes the HVAC current to stay more on the surface of the conductor, which is why HVAC requires a thicker diameter conductor to increase the surface area to supply an increased amount of current.
In the case of HVDC, there is no skin effect & the current is evenly distributed inside the conductor. Therefore, it can use a relatively thinner conductor for the same amount of current supply.
4) Line Length Limitation
The HVAC lines experience reactive power loss that is directly proportional to the line length. Therefore, there is a certain limit to the HVAC line’s length after which the reactive power losses become severely high & the system becomes unstable. It is usually around 500Km in overhead power transmission
There are no line length limitations in HVDC transmission.
5) Reduced Current & Voltage Ratings Requirement of Cable
The voltage & current ratings of a cable are the maximum allowable limit that it can tolerate. The AC has a peak voltage & current that is actually 1.4 times larger than its average (the actual average power delivered). But in DC the peak & average values are the same.
However, the conductor used must be rated for the peak values. Therefore, the HVDC can carry the same amount of power using a lower rating cable as opposed to HVAC transmission. In fact, the HVAC wastes almost 30% of the conductor’s carrying capacity.
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The Right-of-way is the right to occupy land to & from another piece of land. In the case of HVDC transmission, it has a narrower right-of-way because it can use a smaller transmission tower with less number of conductors. Whereas in HVAC, the transmission towers are taller & require strong support to carry the mechanical load of many conductors. Also, the insulators used for HVAC are bigger & rated for peak voltage it means it must be larger than HVDC to carry the same amount of power.
The right-of-way affects the cost of materials, & construction requirements for the said transmission system. We can say HVDC is better than HVAC transmission based on its right-of-way.
7) Power Transmission using Cables
Cables are made of multiple conductors separated by insulating material between them. Since the conductors are close to each other, there exists parasitic or stray capacitance between them. When these conductors are energized, the electric field between them stores charge on them. The cable does not supply power until it’s fully charged. Increasing & decreasing the voltage supplied charges & discharges the cable. It results in drawing more current & causes an increase in I2R losses in the line. Also, these charging currents increase with the length of the line.
Since the AC voltage is always varying, the cable charges & discharges almost 50/60 times in a second while DC varies only once & only during switching. Thus the cable capacitance only affects the HVAC & it also limits the length of the cable. After a certain length for a specific voltage, the power loss increases significantly while in HVDC there is no such limit.
In order to transmit power offshores using submarine or underwater & underground power transmission, we use cables. In such cases, HVDC power transmission is used instead of HVAC to eliminate such losses & be more economical.
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8) Bipolar Transmission
The HVDC offers multiple modes of power transmission one of which is commonly used & very economical is bipolar transmission; it has two conductors running in parallel with opposite polarities having voltage balanced with respect to earth.
In case of power failure in one line or line breakage, the system resumes operation in Monopolar mode; where the other line starts supplying current while the earth (ground) is used as the return path for current.
9) Controllability of Power Flow
The HVDC converter based on solid-state electronics offers full control over the power flow distribution in an AC power transmission network. These converters can quickly switch ON & OFF multiple times in a cycle. It improves the harmonic performance of the system as well as dampens the power swings in the system. It also helps in the power supply capabilities of the network.
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10) Fast Fault Clearance
The flow of abnormal current through any unintended path is called fault current. These fault-current exists due to the presence of any accidental faults in the electrical system & they are quite large in amount. In an HVAC system, it can cause damage to the overall transmission system, transmitting & receiving stations as well as the power generation unit & even the load. In HVDC, these fault currents are lower which significantly reduces the damage caused by it & contains it to the specific section. Its fast switching operation allows it to respond quickly in case of any such electrical fault.
11) Asynchronous Interconnection between AC Girds
The HVDC allows asynchronous interconnection between two grids that have completely different electrical parameters such as frequency, phase, etc.
As we know, different regions operate on different frequencies such as 50 Hz in Europe & 60 Hz in the US. Apart from frequency, the phase (shift in time) of the systems may also vary from each other. Two such power grids are called asynchronous systems & they cannot be connected together using a normal AC link.
Whereas, HVDC completely eliminates such parameters i.e. there is no frequency & phase. Therefore, the HVDC can easily connect asynchronous systems together that are completely independent of each other.
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12) Smart Grid
The idea of a smart grid is to allow multiple small generation units (solar, wind & nuclear power plants, etc.) to supply power to a common grid & intelligently to control the power flow.
Such type of system is only possible using HVDC as it provides asynchronous interconnection between the generation units & offers full control over the power flow distribution.
13) Low Noise Interference
The HVDC generates very low noise interference in the nearby communication lines as compared to HVAC.
The HVAC generates audible noise, radio & TV interference. A simple buzzing sound can be heard below HVAC overhead transmission lines. These interference signals generated depends on its frequency. Since DC has 0 frequency the noise generated is very low in intensity.
Another point, in case of any bad weather, the Noise intensity in HVAC system increases while in the HVDC system the noise intensity reduces
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Main Advantages of HVDC over HVAC Transmission
- The HVDC has a lower cost of transmission than HVAC above the Break-even distance.
- The HVDC transmission lines experience lower power losses as opposed to HVAC.
- There are no radiation & induction losses in HVDC.
- There are no dielectric losses in HVDC which increases the lifetime of the conductor.
- The HVDC line generates very low-intensity noise interference as compared to HVAC.
- In HVDC, the noise intensity decreases in bad weather as opposed to HVAC where it increases.
- HVDC can utilize thinner diameter conductor than HVAC to carry the same amount of power.
- The HVDC utilizes the full capacity of the conductor voltage & Current ratings.
- The HVDC has narrower right-of-way than HVAC.
- The HVDC transmission towers are smaller than HVAC.
- The HVDC transmission line does not have length limitation whereas HVAC has a specific length for a specific mode of transmission.
- There are no charging current issues in HVDC.
- The HVDC can also use earth or the sea as the current return path.
- The HVDC provides reliable bipolar transmission that allows power transmission even in case of power failure in one line, using the monopolar transmission.
- The solid-state converter in HVDC allows fast fault clearance capability & reduces the fault current.
- The HVDC allows full control over the power flow in any AC power network.
- It allows asynchronous interconnection between two grids that operates at completely different electrical parameters.
- HVDC can be used to implement smart grids to allow full control over the power & interconnection between generation units.
- The HVDC is used for power transmission underground & underwater.
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