DC Micro-grid Common DC Bus Technology and Application

1. Changes and trends in DC applications

One hundred years ago Edison DC and Nicolas Tesla's technical line struggled with AC power. Tesla's AC power accounted for most of the market share for a hundred years and was almost synonymous with power transmission. However, the facts have subverted everyone's intuitive feeling. Most of the final links in practical electrical equipment are DC, because DC power systems have high reliability, stable voltage, and good anti-interference performance. DC switching power supplies are also more efficient than AC UPS. Low DC voltage makes it easy to achieve longer battery backup time and easier uninterruptible power supply.

The limited technical conditions of the year led to the rapid development of AC transformers. However, DC transformers have not yet been developed. Without DC transformers, it is impossible to increase voltage, reduce current and losses, and deliver electricity to households.

Nowadays, with the continuous development of power electronics technology, the key issues in the development of DC power are solved. Along with the outbreak of new energy, DC has obvious advantages and a better development path. Can be viewed from the following three aspects:

1 At the load end, the digital society, such as Tencent, an innovative enterprise, when 1 million servers are in operation, its energy consumption has accounted for the largest proportion of all expenditures, its data center's load is DC, and the second is the electrification of traffic. Regardless of whether the vehicles flying in the sky, on the road, or in the water, have electrification trends, such as the US fuel cell aircraft and Tesla/Weilai electric vehicles, which are more typical electrified vehicles, the compound is also DC. of. These two are the largest and most mainstream changes on the load side.

2 The main energy source of the power generation end is fossil energy. The energy supply situation can no longer meet the demand, and a new energy structure must be sought. The vast majority of new forms of power generation, such as photovoltaic power generation, fuel cells, and biomass, are mostly direct current.

3 The energy storage end Because the new energy form of the power generation end is mostly unstable, energy storage needs to be configured, and most of the energy storage is direct current. For example, the energy storage devices such as lithium batteries, lead carbon batteries, and super capacitors are DC.

From the load end, power generation end, energy storage end can be seen that the DC status is more and more significant.

2. Advantages of DC

Technically speaking, AC power has amplitude, frequency, and phase, which need to be controlled. DC power, however, only needs to control the voltage amplitude. It is more simple and stable, and the actual link of the actual power equipment is mostly DC. DC comes with keywords: simple, green, and efficient.

Specifically, DC microgrids have the following advantages:

1, there is no problem of reactive power

There is no reactive current component in the DC system. In the case of providing the same active power, the DC system current amplitude and corresponding loss are smaller than the AC system. When the DC microgrid voltage is twice the AC line voltage, the DC line The active loss is only 15%-50% of the AC line. The absence of reactive power also makes the voltage distribution independent of line inductance and capacitance parameters, which is more conducive to voltage control.

2, no phase problem

The switching of equipment (mainly power supply) in the AC grid is closely related to the characteristics of the AC, such as phase, phase sequence, and frequency. Devices on the DC micro-grid do not need to consider the phase problem, switching is easier, and the stability is also enhanced.

3, simple structure

Since most of the new energy generation is direct current, if it is incorporated into the DC bus, only one level conversion is needed, many converters are omitted, losses are reduced, costs are saved, and reliability is improved;

4, small size

DC/DC converters often use high switching frequency, so the device power density is much larger than that of industrial frequency converters, and the volume is smaller.

5, high quality power supply

Since most of the energy storage is direct current, the DC micro-grid is easier to access the energy storage device, thus providing high quality power supply;

6, green, environmental protection

Direct current does not generate alternating electromagnetic fields, so electromagnetic radiation is small and more environmentally friendly.

Of course, there are currently some technical issues in the DC microgrid that need to be solved urgently. For example, DC circuit breakers and other current implementations are complex and costly.

3. Common DC bus technology in DC microgrids

DC is an organizational method that can couple various new energy generation, energy storage, and loads through DC buses. This is the concept of so-called DC microgrids. DC micro-grid is a micro-grid made of direct current, and it is an important part of the future smart power distribution system. It is of great significance to promoting energy conservation and emission reduction and achieving sustainable energy development. Compared with AC microgrids, DC microgrids can more efficiently and reliably accept distributed renewable energy power generation systems such as wind and light, energy storage units, electric vehicles, and other DC power loads.

The AC microgrid needs to control the frequency, amplitude, and phase provided, and the key issue of DC microgrids is the DC bus's amplitude, which requires a common DC bus technology in the DC microgrid.

Definition: Connect all micro power supplies through a common DC bus. The entire DC bus connects to the AC power grid through a bidirectional DC/AC. This is called the technical route of the common DC bus.

The common DC bus has been around for decades, and it is a very old technology line. The earliest time was to exchange large power grids to provide direct current through rectification. There is only one way, and the load is simple, but it is very inflexible.

Today's power generation methods, energy storage methods, and load types are all more complicated than before. All these factors affect the DC bus. Therefore, the current DC bus technology is more flexible and complicated at the same time. To a certain extent, it needs an upper system for effective energy management.

Three elements of the common DC bus technology: bidirectional DC/AC, bidirectional DC/DC, and energy management.

Bidirectional AC/DC

Because the early loads are exchanges, the earliest problem to be solved is DCAC, which is now being done by many companies. No details are given here.

Bidirectional DC/DC

If DC bus is used to supply DC load, DCDC must be needed. With the increase of DC load, the scene and voltage level of DCDC also need to be more and more abundant, and the price needs to be lower and lower.