Hello guys, welcome back to my blog. In this article, I will discuss what is a DC microgrid, significance, need to control dc microgrid, types of controlling techniques on DC microgrid, etc.
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DC Microgrid
What is DC Microgrid?
A Microgrid is a localized electrical network consisting of distributed energy sources (DES), interconnected loads and communication systems. It is a small scale power grid. It can operate either, independently or with the help of main electrical grid. During periods of heavy demand it can act as back-up power for main electric grid.
If the DC terminals from DC bus (feeds DC loads connected to it) are connected to electronic loads, electric vehicles, and batteries then it forms a DC Microgrid. DC microgrids are highly efficient, reliable, easy to control and economic because power quality issues such as reactive power and skin effect are not present. The main advantage of DC microgrid is limited number of power conversion stages and no reactive current circulation.
Significance of DC microgrid
At the point of use, DC systems make sense because many Distributed Energy Resource (DER) systems such as photovoltaic (PV), fuel cells, battery energy storage and low-power rating electronic devices such as laptops, cell phone, wireless phones, DVD players and also Electric Vehicles and light-emitting diodes (LED) are natively DC powered. By using DC microgrid systems, renewable resources can be interconnected very easily.
DC microgrid consists of converter based parallel DERs, battery energy storage systems(BESS) and each converter has current, voltage and droop control. By choosing a suitable voltage level, the energy sources and power electronic loads can be supplied more effectively and thereby avoiding a few conversion stages as shown in Fig 1. The Energy Storage System (ESS) can be directly connected to the main DC bus or through a DC-DC converter.
Flexibility in conversion and power level is given by integrating DERs to a common ac or dc grid through power electronic interface. One of the main advantage of microgrid is that, it can be operated in grid-connected mode or islanded mode. To allow microgrid to activate in both grid-connected or island form it can attach and cut off from the grid.
In the first mode, the microgrid can deliver or supply power from or to the utility grid according to its generation and load profiles. Generally, in this mode the voltage and frequency are maintained within the range of grid value. The microgrid should manage the power flow depending on the power generation and the loads. In this case, microgrid control system is used in order to distribute the active and reactive power among the DGs (Distributed Generators) and to manage the power interchange between the microgrid and the utility grid.
When the Microgrid is in grid-joint approach, the electrical energy and regularity of micro-grid are prohibited by using smart grid while the distributed generators provide constant active power and reactive power.
In islanded method, the voltage and frequency of micro-grid are used to by a single dominant distributed generation (DG) in micro-grid. Distributed energy comprises solar (photovoltaic), wind and micro resources such as micro turbines and fuel cells. The power-sharing approach of the DG systems in the islanded microgrid is capable of changing from completely centralized to totally decentralize. In this mode, the flexible operation of the microgrid depends on the reliable and perfect control system.
In a DC microgrid, DC-DC converter is the most important part of a DC distribution system. Electric power supply of separate systems are the most common applications of the DC microgrids such as electric vehicles (EV), spacecraft, data centers, telecommunication systems (Telecom) and also rural areas.
The power in DC grids can be transmitted using two configuration system namely: two-wire (unipolar) and three wire (bipolar) systems. The number of available voltage levels is the difference between these two DC grid configurations.
In a unipolar DC system, as shown in Fig 2a, loads and sources are connected between the positive and the negative pole of the DC bus. Selection of DC bus voltage level is a key factor in this system because the energy is transmitted over the DC bus at one voltage level.
The bipolar system also known as three-wire DC bus system, which consists of +Vdc, −Vdc and a neutral line is shown in Fig 2b. Here customers have option to choose three different voltage levels: +Vdc, −Vdc and 2Vdc. If fault condition takes place in any one of the DC poles, the power can still be supplied by the other two wires (bipolar) and an auxiliary converter. Therefore, the reliability, availability and power quality of the system are increased during fault conditions.
As Bipolar DC system is capable of transmitting power more efficiently that a unipolar system, it is most often used.
Need to control DC microgrid
As depletion of fossil energy and environmental pollution are increasing, exploiting the clean renewable energy resources has become the urgency. But increased penetration of distributed energy resources into conventional power systems has increased its control challenges. Whenever fault happens DGs must disconnect with the main grid which causes a great loss to renewable energy.
As a result, the concept of microgrid is introduced. Thus, controlling of microgrids has become a great issue due to the uncertainty of micro generators and the low level power capacity of microgrid system. This is the reason why we need to control DC microgrid.
Types of controlling techniques on DC microgrid
Control strategies in DC micro grids are divided into three main types: constant-current control, constant-voltage control and droop control.
01. Constant-current control: As the name itself says, Constant-current control aims to keep the output current of microgrid terminal to be a constant. The output current of micro terminal is compared with reference current. The error is imported into PI controller, then the control signal is generated using PWM control. It is usually applied in some special cases. For example, when the required power is too much or too less, there may be great fluctuation in DC bus voltage.
The energy storage system should work under constant-current control to absurd or release a maximum current to help recover bus voltage at a short time.
02. Constant-voltage control: Constant-voltage control is a control strategy to keep the bus voltage constant. The error of the DC bus voltage is given to PI controller. The control signal is generated through PWM control.
03. Droop control: When there is more than one micro terminal to stabilize the bus voltage, the load sharing problem occurs. The most common solution is droop control. Fig .3 represents V-I characteristic line. The load change will bring change in the output current of converters, which further leads to the drop in bus voltage. The relationship of current and voltage is complies with the V-I characteristics line. The control strategy of droop control is the same as that of constant-voltage control except that the reference voltage needs to be replaced by droop voltage.
I hope this article “What is a DC microgrid” may help you all a lot. Thank you for reading.
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FAQs
According to the communication method, there are three basic control strategies which are distributed control, centralized control, and decentralized control. These three control methods have their own features respectively. Their common purpose is to make the dc microgrids can be better and stable operation.
What is the need for DC microgrid? ›
A DC microgrid provides continuous power supply to the load during power outages and grid disturbances using ESSs. (5) The ESS can be used for backup power supply, e.g., hybrid electric vehicles (EVs) can be powered when the vehicles are not in use.
What is DC microgrid and control system? ›
Abstract: A microgrid can be defined as power cluster of distributed generation, load, and energy storage device accumulated together in the vicinity to each other. It gives opportunity to utilize renewable energy sources for green and clean environment.
What are the modes of control operation of microgrid? ›
The microgrid control consists of: (a) micro source and load controllers, (b) microgrid system central controller, and (c) distribution management system. The function of microgrid control is of three sections: (a) the upstream network interface, (b) microgrid control, and (c) protection, local control.
What are the control strategies used in DC to DC converters? ›
The currently prevailing control methods implemented in DC-DC converters are voltage-mode, current-mode and hysteretic control.
What is DC control system? ›
Description. A charger and DC/DC step-up system controls the AC input from the household power supply and boosts the voltage to that required by the battery using a DC/DC circuit. The MCU performs both Power Factor Correction (PFC) control and control of the DC/DC step-up circuit.
What are the needs of DC power supply? ›
The main purpose of DC DC power supplies is to produce a regulated voltage output for electronic and electric devices. Unlike AC voltage, DC voltage can't be stepped up or stepped down using a transformer. Instead, a DC DC converter is utilized for this purpose.
What are the main issues of microgrid? ›
Stability, reliability, and protection are the key issues of microgrids due to reverse power flows of distributed generation units, local oscillations, transient modes of microgrid, severe frequency deviations in islanded mode operation, and economical and supply‐demand uncertainties of microgrid.
What is an example of DC microgrid? ›
An example of a DC-microgrid is the International Space Station. The International Space Station has several solar arrays and batteries to supply power while operation sys- tem and other devices consume power. When the solar arrays have access to the light, it will provide power for the load and charging the batteries.
What are the control layers of microgrid? ›
2.4, hierarchical control has four levels, that is, the local (primary), secondary, central/emergency, and the global control. Figure 2.4. Microgrid control layers.
Microgrid Controller optimizes sites and systems that operate entirely or partially off-grid, supporting storage, load, solar, other generators and islanding control.
What is droop control in DC microgrid? ›
Droop control is a popular technique in dc microgrid to equalise current sharing among converters like reactive power sharing in the ac microgrid. Conventional droop control works on adding virtual resistance in line to equalise current sharing.
What are the four modes of control? ›
The four most popular control modes are on/off, proportional, integral and derivative. On /off control activates an output until the measured value reaches the reference value. A common example is the household thermostat.
What are the top microgrid controllers? ›
Schneider Electric (France), General Electric (GE) (US), Siemens (Germany), Eaton Corporation (US), ABB (Switzerland), and Schweitzer Engineering Laboratories (US) are among the major players in the microgrid controller market.
What are the two components of microgrid? ›
A microgrid is a local energy system which incorporates three key components; Generation, Storage and Demand all within a bounded and controlled network.
What do you mean by control strategies? ›
Control strategies are specific plans for what to do when your process shows the presence of special causes. This plan describes the out-of-control situation, possible causes, how to check each cause and the result of your check. All control charts in use should have a control strategy.
What are the types of control strategies in a DC chopper? ›
There are mainly two techniques or methods to control the output voltage of a chopper ie Time Ratio Control Method and Current Limit Control. The output voltage of chopper depends on the duty cycle. By changing the duty cycle, the output voltage can be varied.
What are the different types of control strategies in a DC chopper? ›
There are two kinds of control strategies used in DC choppers namely time ratio control and current limit control.
What type of control can be used for a DC motor? ›
DC Motor Speed Control
There are three main ways to achieve speed regulation in series DC motors–flux control, voltage control, and armature resistance control. In the flux control method, a rheostat (a type of variable resistor) is connected in series with the field windings.
What are two methods of DC motor control? ›
Speed control of DC motor can be achieved by flux control, armature control, or voltage control. Voltage control is the most widely used method.
The main unit houses the command circuit, the error detector, the gain controls of the forward path and tachogenerator channels, the power stage and the waveform capture/display unit.
What are the 3 types of power supply? ›
There are three major kinds of power supplies: unregulated (also called brute force), linear regulated, and switching.
What are the 4 types of power supply? ›
Some types of supplies are separate pieces of loads, whereas others are fabricated into the appliances that they control.
- Power Supply Circuit. ...
- 1) SMPS- Switched Mode Power Supply. ...
- 2) Uninterruptible Power Supply. ...
- 3) AC Power Supply. ...
- 4) DC Power Supply. ...
- 5) Regulated Power Supply. ...
- 6) Programmable Power Supply.
DC Circuit Types
Every electrical circuit can be classified into three groups – series, series-parallel and parallel. Thus, DC circuits can be divided into three categories: series DC circuit, series and parallel DC circuit, and parallel DC circuit.
What are the protection challenges in microgrid? ›
The main protection challenges in the microgrid are the bi-directional power flow, protection blinding, sympathetic tripping, change in short-circuit level due to different modes of operation, and limited fault current contribution by converter- interfaced sources.
What problems do microgrids solve? ›
More companies and governments are considering microgrids as the solution to a variety of challenges. There's evidence that microgrids could reduce safety concerns, propel the use of electric vehicles, eliminate transmission losses, and even enable a more robust sharing economy.
What is the disadvantages of microgrid? ›
Disadvantages of a microgrid
Electrical energy needs to be stored in batteries which require space and maintenance. Re-synchronization with the main grid can be a problem. Issues such as standby charges as well as net metering are obstacles for microgrids.
What are the types of DC microgrid? ›
The distribution network of a dc microgrid can be one of three types: monopolar; bipolar; or hom*opolar. The main difference between ac and dc microgrids is presented in Table 6. Table 6. Comparison of ac and dc microgrids.
What are the main types of sources in microgrid? ›
A microgrid utilizes renewable energy sources such as solar panels, wind turbines, battery storage, diesel gensets and combined heat and power (CHP) modules–operating separately or in parallel. Diesel or gas generator sets may also be included, along with battery banks to store electricity and deliver it when needed.
What is centralized and decentralized control of microgrids? ›
The microgrid central controller assigns the setpoints to the loads, distributed generation and storage units, in order to attain the microgrid's goal. In decentralized control, the decisions are taken locally and negotiations can take place, or may take place, as the different actors usually have different goals.
Synopsis: There are four control layers in a building envelope that must be managed to build a durable and comfortable home: water, air, vapor, and temperature.
How much is a microgrid controller? ›
Controller costs range from $6,200 to $470,000 per megawatt (MW). Infrastructure (distribution system, information technology communications upgrades, metering) can range from 1 to 38 percent of total microgrid costs.
Which is used to control the speed of a DC drive? ›
Speed control of DC motor can be achieved by flux control, armature control, or voltage control. Voltage control is the most widely used method.
What are the challenges in microgrid protection? ›
The main protection challenges in the microgrid are the bi-directional power flow, protection blinding, sympathetic tripping, change in short-circuit level due to different modes of operation, and limited fault current contribution by converter- interfaced sources.
What is protection What are the issues of protection in microgrid systems? ›
Local control and protection level encompasses primary voltage and frequency regulation, primary real and reactive power control for each local generation and energy storage unit. To a large extent, the control of microgrids relies on information and communication technology (ICT).
