Large amount of such renewable energy generations as wind/photovoltaic generations directly connected to grid acting as distributed generations will cause control, protection, security, and safety problems. Microgrid, which has advantages in usage and control of distributed generations, is a promising approach to coordinate the conflict between distributed generations and the grid. Regarded as mobile power storages, batteries of electric vehicles can depress the fluctuation of power through the point of common coupling of microgrid. This paper presents a control strategy for microgrid containing renewable energy generations and electric vehicles. The control strategy uses current control for renewable energy generations under parallel-to-grid mode, and uses master-slave control under islanding mode. Simulations and laboratory experiments prove that the control strategy works well for microgrid containing renewable energy generations and electric vehicles and provides maximum power output of renewable energy and a stable and sustainable running under islanding mode.
As long as the growing demands for green, clean, and high-quality energy supplies, renewable energy generations such as solar and wind power acting as distributed generations (DGs) are gaining more and more attentions. Discussions about grid of the future on 2012 International Council on Large Electric Systems (CIGRE2012) pointed that new technologies, new participants, and new market environments are leading the traditional value chain of “fossil energy source-grid transmission end user” to a new value chain adopting renewable energy generations and distributed generations, power storages, and electric vehicles (EVs) [
Control strategies of microgrid containing control of grid and control of DGs must coordinate DGs with main grid under parallel-to-grid mode and coordinate different DGs with loads under islanding mode. Commonly used control methods of microgrid include peel-to-peel control, master-slave control, and multiagent control, while control methods of DGs include current control method, voltage control method, and droop control method. The main concern of control method under parallel-to-grid mode is how to depress the fluctuations of power outputs of DGs while using the maximum amount of DG energy such as wind power or solar power. The main concern of control method under islanding mode is how to coordinate power outputs of different DGs with loads to keep stable voltage and frequency level for constant running.
Electric vehicles regarded as a new traffic method have been paid more common attention. While commonly treated as loads, batteries of electric vehicles can provide power support when necessary—which is called vehicle-to-grid (V2G) mode [
To coordinate DGs and grid, considering the instability and unpredictability of DGs and EVs, a control strategy for microgrid containing renewable energy generations and electric vehicles was presented in this paper. Control methods of microgrid were analyzed and studied to propose a comprehensive control strategy for microgrid with DGs and EVs. The control strategy uses MPPT current control for renewable energy generations under parallel-to-grid mode and uses master-slave control which elects battery storages as master while other DGs and EVs as slaves under islanding mode. A common structure microgrid with DGs, battery storage, and EVs was built both in simulation and laboratory experiment. Simulations and laboratory experiments prove that the control strategy works well for microgrid containing renewable energy and EVs and provides maximum power output of renewable energy and a stable and sustainable running under islanding mode.
Generally, distributed generations (DGs) refer to such environment friendly or renewable power generation devices as photovoltaic, wind power, fuel cells or microgas turbine which are located near loads and capacity of tens of kilowatts to several megawatts. DGs can improve power quality and power reliability with the following characteristics [
Commonly adopted DGs are microgas turbine, fuel cell, wind power, photovoltaic, and power storage devices. Microgas turbine burns gas, methane, or gasoline with total efficiency up to 75% under thermoelectricity cogeneration mode which is a promising commercial DG [
Wind power generation uses wind power to produce electric power which has features of environment friendly and zero emission and now has mature technology of single generator capacity of 2500 kW. As shown in Figure
Wind power generation system structure.
Paralleled in through an inverter, wind power generation will consume reactive power while producing active power, and wind power generation is normally combined with power storage, photovoltaic generation, diesel generator, or reactive power generation devices. Operation modes of wind power generation are mainly two types: (1) stand alone operation mode, which uses small wind generators to charge battery storages and through an inverter to support end user or medium wind generators combined with diesel generators or photovoltaics to form hybrid power supply; (2) operation with grid mode, which uses large wind generators capacity range 200 kW to 2500 kW to form wind power farm as power source of grid.
Power output of wind power generation is closely related to the rotating speed of generator under certain wind speed. Figure
Relation of torque/speed and power/speed of wind generator at two wind speeds.
Photovoltaic generation (PV), based on the photovoltaic effect of photovoltaic cell to convert solar energy to DC energy, has features of environment friendly, easy maintenance, zero emission, and low cost. Operation modes of PV are mainly two types: (1) islanding mode, that power system with PV is not connected to public power grid, is commonly used for areas far from public power grid such as an island; (2) parallel-in mode, that PV is paralleled to public power grid. This mode can use photovoltaic generations as mass power production and is the most used mode in the world [
Output voltage and current of photovoltaic cell change with light intensity and temperature. Figure
Relation of
Relation of
Acting as a distributed generation, output power is affected by weather conditions (wind/light) wind power generation and photovoltaic generation are intermittent power supply which cannot meet the load demand full time. Thereby, power storage devices are widely used in distributed generation for continuous power supply. Effects of power storage devices in distributed generation are as follows [
As the development and spread of electric vehicles, influence of batteries especially large capacity batteries of EVs connected to grid is gaining more and more attention. Batteries of electric vehicles can provide power support when necessary—which is called vehicle-to-grid (V2G) mode [
Traditional power distribution grid distributes power to end user through a branchy, radialized network which transfers power in single direction. Single DG directly connected to power distribution grid will change the power flow direction and cause analysis, control, and protection problem of power distribution grid. Restrain and isolate methods are commonly adopted by a grid to decrease the impact of DGs. A concept of combined DGs with loads to form a so-called microgrid to coordinate grid and DGs was proposed [
Single EV has little effect on the utility grid even under V2G mode. But when a certain number of electric vehicles are paralleled in the grid, they will inevitably affect the utility grid. Charging station was adopted in China for individual and public EV charging. A microgrid scheme which combines photovoltaic, power storage, and EV charging station together was proposed by Beijing Jiaotong University (BJTU) and implemented in laboratory where the microgrid with a simplified structure can be regarded as a submicrogrid in the modified structure of CERTS microgrid as shown in Figure
Modified structure of CERTS microgrid.
The submicrogrid with EVs is a simplified microgrid with 1 concentrated busbar as shown in Figure
Submicrogrid with EV.
Control methods of microgrid consist of microsource control and the grid control. Most DGs are using MPPT control to reach their maximum power output and through a converter/inverter connected to microgrid busbar. As the system in Figure
Current control method of microgrid inverter uses the output current as control signal which produces current of the same frequency and same phase angle with the grid voltage to provide power. Under current control method micro sources can be regarded as controllable current source with large inner impedance. Basic control diagram of current control method is shown as Figure
Basic diagram of current control method.
There are two ways of current control method: indirect current control and direct current control. Indirect current control uses the setting current, voltage vector of PWM to form closed-loop control which through voltage control to achieve current control. This method is simple and is no need of current detection, but it has disadvantages of slow dynamic response and transient DC-drift and is affected by the quality of grid voltage.
As shown in Figure
Diagram of direct current control with voltage feed-forward.
Voltage control method of microgrid inverter uses the output voltage as the control signal which produces voltage of the same frequency and the same phase angle with the grid voltage to provide power. Under voltage control method, micro sources can be regarded as controllable voltage source with small inner impedance. Basic control diagram of voltage control method is shown as in Figure
Basic diagram of voltage control method.
Voltage control method as in Figure
Micro sources in parallel-to-grid mode microgrid usually work in current control mode, while another mode called droop control method is also used to make a microgrid work more like traditional power grid. Droop control method decouples power and voltage and frequency to form droop style
Control diagram of droop control method.
Under peer-to-peer control method, each micro source acts as a “peer” and equally in microgrid. Each micro source participates in the regulation of voltage and frequency according to their own inner control [
Under master-slave control method, one or more micro sources are elected as a master source of microgrid which provides reference voltage and frequency, other micro sources act as a slave source [
The master-slave grid control method of microgrid has the advantages of simple, good voltage, and frequency regulation character and dynamic response. But the performance of master-slave control method rely on master source: master source needs rapid change to voltage control mode when islanding happens to microgrid; voltage and frequency are regulated by master source where master source failure will cause whole microgrid failure; multimaster sources will cause current loop; fluctuations of power are balanced mainly by main source which require certain power reservation of main source.
Direct current control method with voltage feed-forward as in Figure
Simulations of microgrid with renewable energy and EV were carried out, and a laboratory platform of microgrid was established. Control methods in 3.3 were used both in simulation and laboratory platform. An islanding detection method of over/load voltage and over/low frequency was adopted for control mode change, and a load shedding method was adopted.
A simulation model as in Figure
Simulation mode of microgrid with PV and EV.
DG1 in Figure
Simulation model of PV (DG1).
Simulation model of battery (DG2).
Figure
Simulation model of EV (DG3).
Diagrams of current control method and voltage control method of the inverter adopted are shown in Figures
Diagram of current control in simulation.
Diagram of voltage control in simulation.
Figures
Time list of simulation events.
Time (s) | Event |
---|---|
1.5 | EV connected to grid as load |
4 | Utility grid failure |
5 | EV change to V2G mode |
12.5 | EV exit |
As shown in Figure
Output signal of microgrid island detection.
Voltage wave of microgrid busbar.
Current wave of DG1 (PV).
Active power output of DG1.
Current wave of DG2 (battery).
Active power output of DG2 (battery).
Current wave of DG3 (EV).
Active power output of DG3 (EV).
Real-time capacity of EV.
A laboratory platform of microgrid with 25 kW photovoltaic, 50 kWh/25 kW lithium battery packs, 15 kWh/18 kW EV, 25 kW load simulator and 24 kW lighting loads was formed in BJTU as shown in Figure
Laboratory platform of microgrid with photovoltaic and EV.
Structure of the Laboratory platform.
Experiments of parallel/islanding were carried out in different load status. Figures
Voltage/current wave of parallel/islanding transfer (CH1: phase A voltage, CH2: phase A current of
Parallel to islanding
Islanding to parallel
Frequency wave.
Voltage amplitude wave.
Power wave of converter 1.
Power wave of converter 2.
Power wave of converter 3.
Power wave of converter 4.
This paper presented a control strategy for microgrid containing renewable energy and electric vehicles. The strategy uses current control mode for renewable energy generations under parallel-to-grid mode and uses master-slave grid control which elects battery storages as main power sources DGs and EVs as slave power source under islanding mode. Compared with other strategies, this strategy does not need to change working mode of DGs while transferring between parallel-to-grid and islanding and can fully use the power of DGs, immune to output power fluctuation of DGs, and prolong the stable running time of microgrid under islanding mode. According to simulation results and laboratory experiments, the proposed control strategy works well and provides maximum power output of renewable energy and stable and sustainable microgrid running under islanding mode. We hope that the strategy will beadopted for industrial applications.
The authors gratefully acknowledge the support of the National Natural Science Foundation of China, for financial support under Grant no. 51277009.