Advantages of HVDC transmission

What is HVDC Transmission?

• The electricity is converted from alternating current (AC) to direct current (DC) at the sending end of a high-voltage direct current (HVDC) system.
• The power is then transmitted using DC
• Finally converted back from DC to AC at the receiving end, and then delivered to the AC grid at the receiving end.

The use of high-voltage direct current (HVDC) technology is evolving not only in the transfer of enormous amounts of power across great distances but also in the interconnection of various forms of renewable energy.

Advantages of HVDC

1). Low Transmission Cost

The cost of transmission is determined by a variety of factors, including the cost of equipment used for voltage conversion at terminal stations, the number and size of cables used, transmission tower size, and power losses in transmission, among others.

The HVAC voltage conversion equipment used at the AC terminal station is primarily a transformer, which is simpler and less expensive than HVDC’s thyristor-based converters. It is the only HVAC transmission feature that out performs HVDC in terms of least cost requirement.

HVAC transmission requires a minimum of three conductors for three-phase power transmission, but HVDC transmission, which can use the ground as the return path, can employ only one conductor for monopolar transmission or two conductors for bipolar transmission.

It significantly reduces the overall cost of transmission. However, the three conductors used for three-phase electricity can be used for HVDC transmission, with the possibility to send twice as much power via a double bipolar link.

HVAC transmission lines necessitate a greater distance between phase to ground and phase to phase conductors. To ensure such separation, the HVAC transmission tower must be taller and broader than HVDC.

When compared to HVAC towers, using an HVDC transmission tower reduces installation costs.

HVDC transmission has much lower power losses than HVAC. HVDC is thus a more efficient mode of transmission than HVAC.

The whole transmission cost is divided into two major categories:

1. Terminal station cost and
2. Transmission line cost.

The former is a constant figure that is independent of transmission distance, but the latter is dependent on transmission line distance.

AC terminal costs are quite modest, however HVDC terminal costs are extremely costly. However, the cost of an HVAC transmission line per 100 kilometres is significantly higher than that of an HVDC transmission line.

As a result, the entire cost graph for HVAC and HVDC intersects at a point known as the break-even distance.

The transmission distance at which the overall total investment cost for HVAC starts to increase above that of HVDC is referred to as the Break-even Distance. This distance is determined by the mode of transmission.

The break-even distance for overhead transmission is projected to be 400-500 miles (600-800 kilometres), while underwater transmission is 20-50 kilometres and underground transmission is 50-100 kilometres.

As a result, HVDC is a significantly more efficient and cost-effective option for power transmission over the break-even distance.

2). Reduced Power Losses

When compared to HVAC, HVDC transmission has very low losses. Here are some of the losses that are eliminated or significantly reduced with HVDC.

Absence of Reactive Power Loss

In the function of HVAC, the transmission line experiences reactive power loss that is directly proportional to the line length, frequency, and inductive load at the receiving end. It decreases power transmission efficiency and wastes energy.

It is for this reason that the length of HVAC transmission lines is kept below a certain limit to allow for effective power transfer. As a result, the HVAC employs series and shunt compensation to lower line VARs and provide system stability.

There is no frequency or charging current in the case of HVDC. As a result, HVDC is devoid of reactive power losses and does not require such compensations as HVAC.

Reduced Corona Losses

When the transmission voltage exceeds a particular limit known as the corona threshold, the air molecules surrounding the conductor begin to ionise and emit sparks that waste energy; this is known as corona discharge.

Corona discharge losses are affected by both voltage and frequency, and because DC has no frequency, the corona loss in HVDC is nearly three times lower than in HVAC.

Absence of Skin Effect

The skin effect causes the alternating current to reside more on the conductor’s surface, leaving the core unoccupied.

As a result, in HVAC transmission, the current density is greatest on the surface and lowest near the conductor core.

Because the conductor’s cross-sectional area at the core is inefficient and resistance is inversely proportional to cross-sectional area, the total resistance of the conductor increases.

As a result, I²R losses in the transmission line increase.

HVAC verse HVDC Skin Effect

DC, on the other end, is evenly distributed in the conductor. As a result, there is no such factor as skin effect and the losses generated by it in HVDC transmission.

No Radiation Losses & Induction Losses

Because of its constantly changing magnetic field, HVAC transmission lines suffer from radiation and induction losses.

The former is because long transmission lines begin to serve as line antennas, emitting energy that does not return.

The later is caused by current produced in neighbouring conductors.

The magnetic field is uniform in the case of direct current. As a result, no radiation or induction losses occur in HVDC transmission lines.

Charging Current

Underground and underwater power transmission cables have parasitic capacitance. It does not provide electricity till completely charged.

As a result, the wires require additional charging current. The capacitance of the cable increases with length, and so the charging current does as well.

In AC, the cable charges and discharges numerous times per second, and the cables demand more current from the charging station. This current raises the cable’s I²R losses.

In the case of direct current, the cable is charged only once, at the time of switching. As a result, there are no charging current losses in HVDC transmission.

Due to dielectric losses, there is no heating

The alternating electric field produced by AC has an effect on the insulating material within transmission lines.

This insulation material absorbs alternating electric field energy and transforms it to heat. It also shortens the insulation’s lifespan.

The electric field in HVDC is uniform. As a result,

1. There are no such dielectric losses and
2. No insulation heating problem.

3). Thinner conductor

The skin effect causes the HVAC current to stay closer to the conductor’s surface, resulting in HVAC requires a bigger diameter conductor to increase the surface area and supply more current.

There is no skin effect with HVDC, and the current is equally distributed inside the conductor. As a result, it may use a thinner conductor to supply the same amount of current.

4). Line Length Limitations

The reactive power loss in HVAC lines is linearly proportional to the line length.

As a result, there is a definite length limit to the HVAC line after which the reactive power losses become extremely significant and the system becomes unstable.

In overhead power transmission, the distance is normally around 500 kilometres.

HVDC transmission has no line length restrictions.

5). Reduced Current and Voltage Ratings of Cable

A cable’s voltage and current ratings are the maximum allowed limits that it can withstand. The peak voltage and current of the AC are 1.4 times greater than the average (the actual average power delivered).

However, in DC, the peak and average numbers are the same.

The conductor utilised, however, must be rated for the peak values. As a result, when compared to HVAC transmission, HVDC can carry the same amount of power with a lower grade cable. Indeed, the HVAC consumes nearly 30% of the conductor’s carrying capacity.

6). Right-of-Way

The right-of-way is the right to occupy land from one piece of land to another. HVDC transmission has a narrower right-of-way because it can use a smaller transmission tower with fewer cables.

Transmission towers in HVAC are taller and require substantial support to sustain the mechanical stress of many conductors.

Also, because HVAC insulators are larger and rated for peak voltage, they must be larger than HVDC to transport the same amount of power.

The cost of materials and construction requirements for the transmission system are influenced by the right-of-way. Based on its right-of-way, HVDC is better to HVAC transmission.

7). Cable Power Transmission

Multiple conductors are insulated in cables. Since the conductors are adjacent, they have parasitic capacitance. These conductors store charge in the electric field between them when energised.

The cable doesn’t work till completely charged. Changing the voltage charges and discharges the cable. It increases line I²R losses and current flow. These charging currents grow with line length.

The cable charges and discharges almost 50/60 times a second because AC voltage varies constantly, whereas DC voltage only varies during switching.

Thus, cable capacitance restricts cable length and only affects HVAC. In HVDC, there is no limit to power loss.

They use cables to deliver power offshore via submarine or subsurface power transmission. To avoid losses and save money, HVDC power transmission is employed instead of HVAC.

8). Bipolar Transmission (Bipolar Communication)

The HVDC provides many modes of power transmission, one of which is bipolar transmission, which has two conductors running in parallel with opposite polarity and a voltage balanced with regard to earth.

In the event of a power failure or line breakage, the system restarts operating in Monopolar mode, with the other line supplying current and the earth (ground) serving as the current return path.

9). Power Flow Controllability

The solid-state HVDC converter provides complete control over power flow distribution in an AC power transmission network. These converters may fast turn on and off numerous times in a single cycle.

It increases the harmonic performance of the system while also dampening power swings. It also contributes to the network’s power supply capabilities.

10). Quick (Fast) Fault Clearance

Fault current is the flow of aberrant current through any unintentional path. These fault-currents exist as a result of any inadvertent defects in the electrical system and are quite enormous in magnitude.

It can affect the whole transmission system, transmitting and receiving stations, power generation unit, and even the load in an HVAC system.

These fault currents are lower in HVDC, which considerably lowers the damage caused by it and confines it to a specific portion. Because of its fast switching action, it can respond swiftly in the event of an electrical fault.

11). Asynchronous AC Grid Interconnection

HVDC enables asynchronous connecting of two grids with completely different electrical properties such as frequency, phase, and so on.

As all know, many regions use different frequencies, such as 60 Hz in the United States and 50 Hz in Europe. Aside from frequency, the phase (time shift) of the systems may differ from one another.

Asynchronous systems are two such power grids that cannot be connected using a standard AC link.

HVDC, on the other end, fully eliminates such factors, i.e. there is no frequency or phase. As a result, the HVDC may readily connect asynchronous systems that are fully independent of one another.

12). Smart Grid

A smart grid allows various small generation units (solar, wind, and nuclear power plants, for example) to send power to a common system while intelligently controlling the power flow.

Such a system is only viable with HVDC since it allows for asynchronous connectivity between generation units and full control over power flow distribution.

13). Low Noise Interference 

In comparison to HVAC, HVDC produces extremely low noise interference in adjoining communication cables.

The HVAC system produces audible noise as well as radio and television interference. Below the HVAC overhead transmission lines, a simple buzzing sound can be heard. The frequency of the created interference waves determines their strength.

Because DC has no frequency, the noise produced is relatively low in intensity.

Another aspect is that when there is severe weather, the noise intensity in the HVAC system increases, whereas it decreases in the HVDC system.

14). Maintaining a good voltage Regulation

In DC lines, voltage drop does not occur due to inductive reactance. In the case of HVDC transmission, voltage regulation will be improved.

Other Advantages of HVDC

• Over a longer distance than the break-even point, the HVDC has a lower cost of transmission than the HVAC.
• When compared conventional HVAC transmission lines, HVDC lines incur significantly lower levels of power loss.
• In high-voltage direct current, neither radiation nor induction losses occur.
• In HVDC, there are no dielectric losses, which contributes to an increase in the lifetime of the conductor.
• When compared to HVAC, the noise interference generated by the HVDC line is of an extremely low intensity.
• In HVDC, the amount of noise that is produced is reduced, however in HVAC, the amount of noise that is produced is increased.
• When carrying the same amount of power as HVAC, HVDC can get away with using conductors of a smaller diameter.
• The HVDC makes use of the conductor’s full capability, both in terms of its voltage and current ratings.
• The right-of-way for HVDC is considerably more constrained than that of HVAC.
• The transmission towers for HVDC are much more compact than those for HVAC.
• In comparison, the HVDC transmission line does not have any length restrictions, however the HVAC transmission line does have length restrictions that are particular to the mode of transmission.
• In HVDC, there are no complications with the charging current.
• As an alternative current return channel, the HVDC may potentially use the earth or the ocean.
• The high-voltage direct current (HVDC) system offers a dependable bipolar transmission that enables electricity to be sent even in the event that power is lost on one line when utilising monopolar transmission.
• The solid-state converter in the HVDC system has the potential of quickly clearing faults and lowers the fault current.
• The high-voltage direct current (HVDC) provides complete control over the flow of power in any AC power network.
• It makes it possible for two grids to be interconnected asynchronously despite the fact that their electrical properties are completely different from one another.
• The implementation of smart grids, which enable full control over the electricity and connections between generation units, can make use of HVDC as one of the technologies.
• Underground and underwater power transmission can be accomplished with the help of HVDC.

Why is HVDC better than HVAC?

When compared to their equivalent high voltage alternating current (HVAC) transmission systems, high voltage direct current (HVDC) transmission lines are more efficient for transporting electricity over long distances because they experience less power loss.

What is the essential difference between HVDC and HVAC?

HVAC requires only a simple voltage transformation. In the case of HVDC, the voltage transformation is complicated.

HVAC transmission systems are appropriate for distances of less than 600 kilometres.

HVDC transmission is ideal for high-power transmission across long distances, often greater than 600 kilometres.