An In-depth Comparison of HVDC and HVAC

When planning a transmission system, choosing the mode of transmission is crucial.

High-voltage direct current (HVDC) and high-voltage alternating current (HVAC) transmission systems are designed to address the same purpose–transmitting power over long distances with minimal losses. Factors considered are transmission economics, system reliability, technical performance, and environmental effects.

 

A diagram of typical HVDC systems’ components. Image used courtesy of Ahmad Ezzeddine

 

A diagram of typical HVAC systems’ components. Image used courtesy of Ahmad Ezzeddine

 

The Economics of AC vs. DC

Economics consists of two things, system component costs and cost of losses. The two cables used in DC lines can have the same power rating as three transmission lines used in three-phase AC transmission. For the same reason, in the case of overhead transmission, transmission towers are simpler in design in HVDC.

Furthermore, the corona effect is relatively negligible in DC lines compared to AC lines. Therefore, the size of conductors for a DC line will be smaller than the size of a conductor of an AC line at the same current carrying capacity. AC systems also need power compensation and a lot of terminal equipment.

On the other hand, reaching high voltage levels in AC is much cheaper than in DC. The costs in DC systems are in the converter stations. The components used in conversion and filtration are costly.

DC cables are less affected by skin effects than AC and have negligible dielectric losses. Therefore, power losses in DC transmission lines are much less than in AC. Considering both sets, a cost-per-distance curve will have a break-even distance at the intersection point between AC and DC transmission costs.

The cost factors should also include an economic study of the present worth and annual interest over an estimated lifetime of the project.

 

A chart comparing the initial investment of HVAC with HVDC systems and the cost change of both systems over distances. The break-even distance is the point of intersection between the AC curve and the DC curve. Image used courtesy of Ahmad Ezzeddine

 

HVAC vs. HVDC Technical Performance

The technical performance of HVDC transmission has many advantages over HVAC. Since HVDC is a converter-based technology, the transmitted power is fully controlled. HVDC links can also help control transients and stability in HVAC networks.

In three-phase HVAC networks, the power transfer depends on an angle difference between each phase. The angle increases at high distances, which creates instability in the system. HVDC transmissions have positive polarity and negative polarity lines, therefore, no phasor angles, which means no stability constraints.

HVAC lines require reactive power in transmission. This causes voltage drops and requires compensation. Surge impedance loading and shunt compensation are solutions for this problem.

HVDC lines do not need reactive power, therefore, the DC system would not need capacitor banks or SIL. This creates a solution for underwater cable transmission.

Another major advantage of HVDC transmission is the ability to use the ground conductor as a return conductor only in a few cases. Its benefits rely on squishing transmission losses and lowering the initial installation cost. In some cases, it could be used as a substitute return line during faults. Earth lines are unused in transmission in the AC system due to high inductive effects.

 

A chart comparing the power transfer capability of HVDC and HVAC systems. Over long distances, HVAC systems' power transfer and stability of the systems decrease, whereas typical HVDC systems have a higher potential over long distances. Image used courtesy of Ahmad Ezzeddine

 

Transmission System Reliability

The reliability of HVDC systems has improved with the development of light-activated thyristors. Improvements in protection and control made HVDC systems highly reliable. To calculate the reliability of a system, two measures must be taken into consideration: energy availability and transient reliability.

Energy availability is a percentage of outage time per total time operated by the system.

\[Energy\,Availability=100(\frac{1-Outage\,Time}{Total\,Time})\%\]

Energy Availability=100 (( 1-Outage Time)/(Total Time))%

Transient reliability is a factor that specifies the performance of a DC system during a fault in an associated AC system.

\[Transient\,Reliability=\frac{100-Number\,of\,normal\,HVDC\,link\,performance\,during\,fault}{Number\,of\,recordable\,AC\,faults}\]

Transient Reliability = (100-number of normal HVDC link performance during fault)/(Number of recordable AC faults)

To check the reliability of HVDC systems, the mean time to failure (MTTF) and mean time to repair (MTTR) are two important factors determined by statistics from previous outages in HVDC systems.

MTTF is the time a system holds between two separate outages. MTTR is the time used to repair the outage. In HVDC systems, especially after advancements in the last two decades, MTTF is high in most cases for most components of an HVDC system, and the MTTR is low for most failures.

 

A chart showing the difference between the mean time to repair and the mean time to failure, The mean time to repair starts with a system outage and ends when the system is back online. The mean time to failure is the time between the installation of a system and the first outage or the time between two outages. Image used courtesy of Ahmad Ezzeddine

 

With all the statistics, HVDC links are more reliable than HVAC transmission systems.

 

Environmental Effects of Transmission Lines

In recent years, the environmental effects of transmission lines have come to light. Radio and television signal interference, audible noise, visual impact, and electric fields became the responsibility of grid operators and researchers to find solutions to these problems.

 

Electric Field

Two major sources of an electric field are the conductor corona effect that causes space charge clouds and the potential difference between the conductor and earth. Weather and humidity also strengthen electric fields. When a high electric field is anticipated, the area surrounding it should be fenced and marked as hazardous to prevent shock currents on terrestrials.

DC systems have no displacement currents at steady state operation; therefore, the electric field in HVDC systems is less severe than in HVAC systems.

\[I_{d}=E_{0}\frac{d\Phi E}{dt}\]

This equation calculates the displacement current where Id is the displacement current.E0 is the surrounding's permittivity surrounding dΦE is the electric flux variation, and dt is the change in time.

 

Radiated Interference

HVDC systems cause disturbances in high-frequency waves from the kilohertz region up to the megahertz region. The reason is the operation of the converter transformers and the high-frequency switching. Electromagnetic shielding is used to control this problem, and reinforced concrete covered with wire meshes is built around the converter station.

The corona effect on the surface of transmission lines is a major cause of radiated noise; it is highly observable on HVAC transmission lines and highly dependent on the electric field strength and surface diameter of the transmission line.

Radio interference is observed in HVDC transmission lines but at a lower level than in HVAC transmission lines.

Typical shielding effectiveness of valve hall shielding. Image used courtesy of Ahmad Ezzeddine

 

Acoustic Noise

The sound generated from converting stations, power plants, and substations is called noise. The noise permitted for a plant is dependent on local standards. Examples of these standards are BS 4142:2014 and IEC TS 61973:2012/AMD1:2019.

Noise sources in HVDC stations are converter transformers, reactors, and filter capacitors. Transformers produce noise from the flux density in the iron core. Standard transformers create an audible noise at no load that could be 20 dB higher at full load. In converter transformers, at full load, the noise is around 10 dB higher than at no load.

Reactors radiate noise as a function of AC harmonics, and capacitors generate noise due to multiple individual point sources.

There are several ways to reduce noise from their sources. The reason for noise reduction is to apply to the standards set by the local authorities.

1. Purchase quiet equipment. While this solution is the best, it may not be the most feasible or cost-effective.

2. Enclose equipment for noise attenuation, such as building walls and noise dampers around equipment. This is a good solution for long-distance noise reduction but very expensive for short distances.

3. Separate noisy equipment. The alternative to this is shielding other equipment in a reflected noise path.

 

Visual Impact

The visual impact addresses mainly overhead transmission lines and tower structures. The three-phase AC lines need bigger towers and spacing between them. Therefore, HVDC lines have lower visual and space impact than HVAC transmission.

Transmission companies are converting from HVAC to HVDC. Of course, it depends on all the above factors after planning and calculating all requirements. The factors should all be considered, whether the economics, performance, reliability, or environmental impact.

 

Featured image used courtesy of Adobe Stock