DC microgrids
DC microgrids are a collection of distributed power resources that include generators and energy storage devices. DC microgrids provide an efficient way to use power from renewable energy sources to improve the environment through the production of clean energy. These microgrids are situated in close proximity to the loads for which the distributed energy resources are installed, and therefore, these loads are also included as a part of DC microgrids. A typical DC microgrid will power generation from various renewable energy sources, and battery banks, which are all connected to a DC bus. The power to DC and AC loads (after converting DC to AC) is then provided by DC bus. The DC bus also provides additional power to the AC grid or can take power from the grid if there is a shortage. A diagram representing a typical DC microgrid is shown in Fig. 1(a). There are many significant advantages of employing DC microgrids, which are mentioned below.
Advantages of DC microgrids
Efficient utilization and Clean energy
DC microgrids employ renewable energy sources for power generation and therefore provide clean energy. These various renewable sources from various sites are combined together to form DC microgrids which ensure efficient utilization of generated energy. If the power production yield of one site is low, it can take power from another site that is producing extra energy.
LOw losses
Most of the modern world runs on AC systems due to the revolutionary device known as Transformer. The transformers single-handedly turn the tide of DC or AC debate. Transformers step-up and step-down voltage with efficiency reaching 100% with minimal circuitry and no moving parts. This is the major reason for the widespread use of AC systems. However, due to recent advances, efficient boost and buck converters can be provided to DC microgrids to step up and step down the voltages, respectively.
The major benefit of DC microgrids is low losses associated with the presence of loads nearby generation and storage units, which avoids transmission losses. The locality of DC microgrids is the main reason for low losses. However, if the distance between load and microgrid is increased, much higher losses will occur in a DC system rather than an AC system.
No reactive power in DC system
Since DC microgrids essentially operate on DC, there is no reactive component in the system. To elaborate on this, we can look at the power triangle in Fig. 1(b). The AC system has an apparent power as it has a reactive component. The reactive component decreases the power output due to capacitance and inductance effects. However, the DC system has no reactive component, and therefore, the apparent power is equal to the system’s true power.
No frequency and no synchronization
The DC system has no frequency component which means that the DC microgrid will have no frequency component either as depicted in Fig. 1(c). This means that all sources can be combined without synchronization of frequency and only the voltage from all the sources needs to be regulated for the DC bus.
The major components of a DC microgrid, as represented by Fig. 1(a) are shown below.
Components of a DC microgrid
Generators
The generation unit of a DC microgrid involves generation of power through renewable energy sources that include PV systems, and wind turbines. These sources are particularly chosen because they can be installed at remote locations and do not partcularly require the on-grid solutions. The wind turbines do have area requirements but they can be installed on any location with availability of wind. Other renewable energy sources require stringent condition of locations but these two sources can be installed anywhere with potential.
Battery bank
The battery bank is used to store extra energy generated from our generation system. It can also store energy from the grid to provide energy to the loads when there is low availability of renewable energy resources.
DC and AC Loads
The DC microgrids are installed to provide power to the loads. If there are no loads, then there is no need for DC microgrids. These DC and AC loads can include laptops, fans, motors, air-conditioners, etc.
Grid voltage converter
The bi-directional grid voltage converter (GVC) is also the main component of DC microgrids as it supplies additional power from the DC bus to the AC grid or from the AC grid to the DC bus. The GVC is primarily responsible for marinating the voltage at the DC bus.