COMPARATIVE HVDC – HVAC
Posted JUN 21 2015 by RENAN and JONATHAN
Nowadays, transporting great quantities of electrical energy is usually made by HVAC systems, as it is mostly viewed as the best solution, mainly because of its simplicity. However, trough the development of new HVDC technology, the HVAC role as the principal system of electrical transmission is being contested.
Technical specs
One of the HVDC advantages consists in its capacity of transmitting energy in greater distances then HVAC. The HVAC limitations in this aspect concerns mainly in two traits. Firstly, it is unable of using the totality of its cables because of two electromagnetic effects, the corona discharge and the skin effect. Those phenomenon are proportionally correspondent to the frequency and the magnetically characteristics of its conducers and are translated in energy loss. However, HVDC lines losses by the action of those effects are almost zero.
The other limitation possessed by HVAC lines is related with the existence of inductive and capacitive phenomenon, giving birth to new values of electrical potency. On regular grids, this potency is mostly of inductive nature (this can be corrected by batteries or condensators called FACTS). At longer grids, it would be necessary to build large and costly stations to compensate this change on the potency. On the other hand, HVDC power lines does not have this kind of problem.
The value of the electrical field at a power grid varies during the year, because of the humidity. The DC lines are less affected by this event. Another aspect related to the electrical field in HVAC is the existence of a phenomenon that may originate the creation of a current that may electrocute persons nearby. However, HVDC systems, even in high voltage levels, the intensity of the current is inferior.
There are situations other than great distance grids where HVDC may be advantageous. One of these situations is linked with the link of asynchronous lines. This situation is frequent in 50Hz to 60Hz power grids or even in grids that works in the same frequency but are not synchronous. Another situation is related with the capacity of HVDC power grids in connecting two asynchronous systems with the same frequency without been constituted by mash electrical network. This is not possible in HVAC systems.
Environmental specs
The corona discharge, mentioned before, is an electrical collapse of the air nearby a conducer, that occurs when the electrical field is big enough to make the electrons of air molecules be expelled. This phenomenon usually results in production of air ions that can be positives or negatives.
The corona discharge is practically eliminated when the electrical transmission is clean and regular. However, little debris in the grid may interfere with the electrical field making it increase the production of air ions. As the power grids are exposed in the nature, they are constantly harmed by debris, making the corona discharge a constant.
The positive polarity conducers produce positive ions while negative polarity conducers produce negative ions. Because of that, the air ions produced are thrown away from the conducers and are commonly attracted by the other conducer where they are neutralized or recombined with the reversal polarity ions. In HVAC grids, the ions produced move with high intensity because of the constant alteration in the conducer’s polarity. Differently, in a HVDC grid the ions do not move with high intensity because the conducers does not have its polarity changed.
The noise that can be heard, as well as radio interference, are also consequence of the corona discharge in power grids. In HVAC, this kind of noise can assume two distinct forms, random noise and buzz. In HVDC, only random noise can be noticed.
Results of a series of noises produced by corona discharge all over the conducer, the random noise reaches the observer within different moments.
The noise that can be heard is greater for HVAC grids under rain and great humidity. For HVDC power grids, the noise usually has its level diminished during this kind of condition, when the activities of air ions near the conducer are increased. This intense activity wrap the conducer with ions that diminish the electrical field in its surface, diminishing the corona discharge. As the noise is louder under good weather in HVDC grids and louder under bad weather in HVAC grids, it is plausible to compare both types of transmission taking in consideration the relative number of hours for bad weather noises (HVAC) and good weather noises (HVDC). The figure 1 shows this comparison.
Another important environmental aspect is the possibility of using the technology called HVDC-VCS (underground or underwater cables), which makes it possible to link the transmission lines coming from renewable sources with the AC power grid. Then, it is possible to transport energy to isolated islands, mining sites or offshore oil wells, eliminating then the need of fossil fuels, for example.
There are some points that can be highlighted in VCS, such as:
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Cables free of electrical discharges, storms, falling trees, etc.;
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There is no need of circuit breakers at both sides of the line, diminishing the installation costs;
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There is no limit for the length of the cables, turning intermediate stations unnecessary;
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The magnetic field is almost completely eliminated when adopting a bipolar system;
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There is only on polarity in DC, so there is no need of projecting cables with reversal polarity, simplifying the project.
DC transmission towers are not very different form AC, except because of its size. With the absence of skin effect, it is possible to use lighter and smaller conducers. We can also say that the stretch of land used by DC system is smaller than what is used by AC systems.
Regular transmission lines modify the landscape and its installation suffers from great resistance of the public that feels uncomfortable and afraid of possible health problems. It is also hard to obtain the necessary authorization of the authorities for its installation, turning the execution of the project slow and costly.
On the other hand, transmission lines with underground cables are much faster, easier and do not modify the landscape. Its installation does not suffer from restrain by the public and it is easier to obtain the authorization for its installation. It is also important to say that the magnetic field generated is rather low.
The renewable sources of energy are usually placed where the environment allows and, therefore, they are usually far away from consumers centers.
Generally, those renewable facilities are unable to achieve self-sustainability nor to supply any consumer center, as a city. In this case, a HVDC-VSC link is the perfect solution as it initially supplies the facility from an AC power grid. When the facility achieves electrical sustainability, the electrical flow is automatically inverted and it starts to supply consumer centers.
Economical specs
To make it possible to compare the costs between HVDC and HVAC power grids it is necessary to considerate every component of both systems. For DC alternative, it is necessary to evaluate the following costs:
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Converters;
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Equipment for the input and output of AC;
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Filters;
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Transmission lines;
The main equipment needed at a DC facilities is the converter, that may cost around 25% of the whole cost of the Project. For AC alternative, it is necessary to evaluate the following costs:
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Electrical transformers;
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Transmission lines;
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Charge compensators;
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Reactive compensators;
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Circuit breakers.
An AC power grid needs a great area for its construction and is more costly than a DC power grid with the same capacity. The converter stations are the key-point for making the economical comparison between both technologies. At an AC grid, the costs of transmission lines are higher while at a DC grid the costs of converter stations are higher.
It is best commented at the financial analysis section of this work, but it can be observed that the critical point between both technologies lies in 425 miles (680 Kilometers). This value may vary due to a variety of factors but it is usually better to use DC power grids in distances superior to 680 kilometers.