HV AC Transformers Vs Power Electronics And DC/DC Converters | EEP

An old battle comes up again…

In the early years of electrical power distribution, Direct current (DC) was the standard current. However, due to flaws in Edison’s DC system, AC became the current standard due to its ability to step up low voltage into high voltage and to transmit it over long with the help of the transformer. The battle of the standard of current supremacy, which is referred to as “The war of current” began in the late 1880s.

Since then, AC electrical grid has well developed and proven concept due to its development since the 19th century. It is able to deliver electric power from power plant to household through substations. It also offers a simple and reliable principle that has become the standard over the last century due to, among the previously stated reasons, limited technological advancements.

Currently, not only is it being challenged by the aggressive introduction of distributed renewable energy generation but also under increasing pressure due to growing calls for simple, meshed, low, and medium voltage distribution grids as well as high voltage transmission networks.

Moreover, even based on the assumption that all parameters are equal, the very nature of most renewable energy sources (inherently DC) favors the DC grid over the ubiquitous AC grid on various accounts. The rising trend development of electric vehicle and the advances in power electronics and the role of DC/DC converters in an efficient voltage conversion brings about the re-evaluation of the AC grid.

Based on this project motivation, there are many questions behind this project work and some of them are stated below:

1. With the existing infrastructure of AC distribution, why should there be a shift to DC distribution?
2. Will the DC distribution grid be more energy-efficient, cost-effective, and reliable than the present AC distribution grid?
3. What are the current trends in the low voltage DC distribution?
4. What are the factors to be put into consideration for the adoption of DC and its transition?
5. Will the evolution and advancements in power electronics and DC/DC converters only be a game-changer for DC and bring a transition change to DC at low voltage distribution?
6. Even if the potential advantages of DC far outweighs AC, will that be a yardstick to adopt DC distribution?

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DC Versus AC based on present technology

DC vs AC power grid

Due to different architectures, voltage levels, and power electronics needed between AC and DC power grid, there are advantages and disadvantages of DC and AC power grid system. There are also challenges and opportunities within the development of the DC power grid system.

This paper will discuss the drawbacks, advantages, challenges, and opportunities of DC and AC power grid system development nowadays.

Merits of the DC system

The advantages of the DC systems over AC are presented below:

Advantage #1 – Due to the need for multiple levels of DC voltage, a DC system was deemed Superior to an AC system in terms of efficiency (i.e. higher efficiency gains means less energy wasted which means less money is wasted).

Even though the exact value of efficiency gain varies from application to application, the most common and very straightforward gain is in household electronics equipment. Most electronic equipment is DC and normally equipped with two conversion stages for AC power.

The two stages are AC to DC and DC to DC. Not only does the removal of the first AC to DC converter increase the efficiency, but also reduces the cost of subsequent current adaptors. other advantages of favoring the DC grid over the AC grid are the low copper cost, easy control, meshing simplicity, and obtaining different optimized architectures and topologies.

The decrease in the required amount of copper in the DC grid is mainly due to the use of higher RMS voltages, which is equivalent to the peak voltages in ac grids. A more simplified control is obtained from the fact that in a DC grid, only one control parameter is of significance: the voltage.

In contrast to AC electrical systems that impose the same frequency throughout the interconnected system, a DC grid does not have such constraints. This makes the meshing of DC grids relatively easier and more natural.

Advantage #2 – DC power deployment improves stability and increases the grid reliability

Advantage #3 – More efficient integration of renewable distributed generation.

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Advantage #4 – Simpler power electronic interfaces and fewer points of failure.

Advantage #5 – The total losses in DC distribution are less compare to AC. Using solar power to generating electricity, the losses incurred with converting to AC are avoided, with the fact that many household devices run on DC.

Advantage #6 – Electromagnetic interference (EMI) is lower in DC grids when compared to AC grids.

Advantage #7 – Fewer conductors are required, and there is the absence of skin effect (HVDC transmission in this case).

Advantage #8 – In today’s technology, DC is employed in high voltage long-distance transmission, which is better in cost when compared to the AC system.

Roberto Rudervall in his paper “High Voltage Direct Current (HVDC)Transmission Systems, Technology review paper,” investigated the cost of an HVDC system using life cycle cost analysis. In the study, a comparison between HVAC and HVDC systems was done using two case studies:

Study #1 – Between thyristor-based HVDC system and high voltage AC system – the overall cost of investment for HVDC converter station was higher than HVAC substations. However, the transmission medium (overhead lines and cables) cost and the operation and maintenance cost were cheaper in the HVDC system.

Figure 1 shows the losses and cost comparison between HVAC and HVDC systems. The breakeven distance in the figure depends on factors mentioned earlier.

Figure 1 – Cost and losses comparison of a thyristor-based HVDC system vs HVAC system

Study #2 – VSC-based HVDC system versus an HVAC system or load generation source – VSC-based HVDC systems are applicable to transmission capacity up to 200MW and short-distance transmissions.

Figure 2 shows a better advantage of the VSC-based system over the others in terms of cost considerations.

Figure 2 – Cost comparison with a VSC based HVDC system, HVAC system, and a local generation source (diesel source)

However, it was highlighted that the advancement in technology has led to the reduction of the HVDC systems cost while environmental impact consideration will account for the increase in costs of the HVAC systems. DC distribution system has a simple structure from which economical and low energy consumption is presented.

Nevertheless, the quality of DC power, and the transmission capacity, are much better compared to the AC technology.

Demerits of the DC system

Some of the disadvantages of the DC system are listed below:

Demerit #1

DC grid has major disadvantages concerning control and switching actions. The study of DC grid control has been limited to a traditional hierarchical control approach for both simple systems that are accurately modeled and more complex systems, approximated by simple models. These types of approaches lead to erroneous deductive and inductive reasoning.

The main reason for such an approach is mainly due to limited DC grid analyzing tools. Addressing the effect of system components on the dynamics of the whole system is the most common research focus for DC and AC grids.

Those components include the various types of sources with their respective converters, electrical loads (constant power loads), and cable parameters.

Demerit #2

The currently available and researched DC grids and microgrids have sufficient system capacitance. This capacitance is mainly from the terminal capacitors of system converters that imitate the voltage stiff AC grids. System control approach, system architecture, system stability analysis, and fault detection and isolation mechanisms are simply copied from the traditional AC system.

Also compared to AC, in DC grids, the amount of stored energy in DC grids is determined by the size of the passive components (capacitors and inductors) and storage devices (batteries or supercapacitors). Depending on the size of the system capacitance or storage, it can be defined as either a high or low-energy stored grid.

All currently available and researched DC microgrid applications have sufficiently high system capacitance or directly connected storage elements that ensure their voltage stiffness. Capacitance is mainly provided by terminals of system converters for both sources and loads.

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Demerit #3

The main challenge in developing DC grid distribution system technologies is not only the maturity of supporting equipment that available in the market but also the lack of study about it. In an AC grid, there is a power transformer that is able to convert the voltage magnitude within the grid system in order to make the grid system more flexible while delivering power to the household.

However, in a DC grid system, there has to certain power electronic technology to support DC grid systems such as DC-DC converters in order to either increase or decrease the amount of voltage within the grid. This implies that the development of DC grid distribution offers limitless possibilities for the development of electric power technologies itself.

Demerit #4

Limitation of DC switches and circuit breakers. One example of technology that might be developed in the future is the development of the DC distribution protection system. The common approach that can be taken in developing a DC distribution protection system is by employing the same characteristic as the AC distribution system and adjust it accordingly.

For example, AC distribution systems are traditionally voltage stiff due to the mechanical momentum of the generation mass. Therefore, fault currents are high and the protection system needs to react fast. In order to emulate the same behavior, the DC grid system needs to have a huge capacitance in order to have voltage stiff characteristics same as the AC distribution system.

Therefore, the adjustment for DC protection distribution technologies will be lessened. An example of DC protection technology is power semiconductor devices (PSD) which are used for solid-state circuit breakers (SSCB) which are arc-free.

This technology also can interrupt fault current fast and able to detect overcurrent or rate of current rise within DC distribution system.

Merits of the AC system

1. Inexpensive transmission is a huge advantage over DC. Since AC can easily and efficiently be converted to another voltage with a transformer.
2. In terms of cost-effectiveness and ease of maintainability, AC substations are easy to repair and maintain which makes them even less expensive than DC Substations.
3. Already developed technology and currently accepted technology worldwide.
4. Already developed protection technology compared to the DC distribution protection system.

Demerits of the AC system

1. In terms of cost, AC transmission lines cost more than DC transmission lines. 2. More inherent losses in AC system due to Skin effect compared to DC. 3. AC systems have problems in reactive power control

Title:	AC grid vs DC grid: AC transformers vs power electronics and DC/DC converters – Awoniyi Taiwo at UiT The Arctic University of Norway
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