From the perspective of the whole power system, microgrid is an independent adaptive intelligent unit. Through the Mosaic cooperation between the power network architecture layer and the integrated management architecture layer, the goal of intra-domain autonomy and power balance is achieved. The most typical energy operation blocks in microgrids have two types: micropower and local load. The two match each other through energy storage, exchange, and regulation systems. Under the grid-connected condition, there is electric energy exchange between the whole microgrid system and the main network. All parts of the above power operation belong to the power network architecture layer of the microgrid. Microgrid can be divided into two types: isolated island type and grid-connected type. Isolated island microgrid does not generate any form of energy exchange with the large network, and the internal energy supply of the system is self-sufficient. The access and cutting of the micro power supply and the integration and removal of the load in the domain are the sources of complex control strategies for the microgrid system. The part that promotes the efficient use of energy and realizes the effective control and protection of the system belongs to the integrated management architecture layer of microgrid. The main factors to be considered in the design of microgrid system include the following aspects: power quality optimization of microgrid system; Maximum utilization of renewable energy; Reduce system construction cost and operation cost. The above three requirements are difficult to achieve absolute unity in a specific project, usually find a compromise design scheme according to local conditions. In the practice of microgrid projects, according to different infrastructure environments, grid-connected microgrid is connected to the large network, and the main network and microgrid can exchange energy under specific conditions. According to the different control strategy, its operating state can be divided into island mode and grid-connected mode. Of course, even under grid-connected conditions, it is necessary to reduce the dependence of the system on the large grid as much as possible. This is the basic purpose of microgrid system control strategy. Figure 3 is a single bus multi-loop photovoltaic hydropower integrated grid-connected microgrid system.
Design of photovoltaic module for microgrid system
The micro-power module in the design of an ideal microgrid system should have the functions of instant access and instant exit, and the corresponding cracking and merging actions should be smooth and controllable, so as to meet the three requirements mentioned above. For each specific photovoltaic power generation module, its own unique interface, control, and protection are closely related to the operating state of the microgrid system. The photovoltaic power generation module consists of module system, monitoring system, inverter system, control system and management system. The equipment of photovoltaic branch includes: photovoltaic module group, confluence distribution equipment, direct AC voltage regulation equipment, inverter equipment, isolation switch equipment, access interface device, and monitoring, control and protection device. The photovoltaic module design of the microgrid system generally follows the process shown in Figure 4 to achieve the rationality and functional completeness of the system design. There is no denying that photovoltaic power generation has many shortcomings compared with traditional power generation. First of all, the output power of photovoltaic power generation is closely related to the solar radiation and solar spectral characteristics, which has natural defects of instability and difficult to smooth adjustment; Secondly, the power factor of the inverter group is as high as 0.99, so there is almost no adjustable space for reactive power when the active power is unchanged. Finally, during the inverter converter operation, the harmonics generated by the power electronic devices are injected into the power grid, which seriously affects the power quality.
Photovoltaic module configuration improvement exploration
Based on the above shortcomings, the photovoltaic module configuration in the microgrid system needs to be improved accordingly.
- The cost of photovoltaic branch equipment is reduced. The important share of the construction cost of microgrid engineering mainly falls on the photovoltaic branch road. The cost control of the equipment in each link of the photovoltaic branch is still an important topic for future microgrid projects.
- Solar cell conversion efficiency is improved. The value of improving the conversion efficiency of industrial solar cells is self-evident. The improvement of device efficiency under the premise of stable cost will produce a series of serial gains.
- The short wave of the solar cell is correspondingly enhanced. Compared with large-scale photovoltaic power stations, the design environment of microgrid photovoltaic branches is much more complex. Good shortwave response can effectively improve the overall energy utilization efficiency of microgrid system in the living environment with closely related load.
- Array mismatch improvement. Although the production of solar cells adopts a relatively sophisticated microelectronics manufacturing process, the state of the final formed components is not consistent. In the same inverter group, the operation mismatch of the uneven PV array will occur. Although the causes of mismatching are many, but from the production process research and improvement is the most fundamental method. In addition, single-component MPPT control is also a feasible solution to array mismatch.
- Intelligent monitoring system. In addition to the output of the array itself, the monitoring target of the photovoltaic branch should also take into account the temperature, solar radiation intensity, solar radiation amount, solar spectral characteristics and other related quantities. The amount of environmental data collected and photovoltaic output, through a special algorithm, can effectively support the intelligent photovoltaic power generation branch.
- Power quality improvement. Power quality control is the soft underbelly of photovoltaic modules, which is the biggest bottleneck that photovoltaic power generation can not be transformed from auxiliary energy to alternative energy. The power quality problems in photovoltaic branch operation mainly include: DC component infiltration, harmonic pollution caused by power electronic device operation, voltage fluctuation flicker, waveform distortion and three-phase unbalance. The comprehensive treatment based on energy storage equipment and power quality management equipment is the most effective solution at present.
- Equipment standardization. The standardization of photovoltaic branch related equipment will help simplify the work of the control system and is conducive to the realization of intelligent operation of the microgrid. Microgrid fusion technology will put forward higher requirements for the standardization of photovoltaic equipment.
- Improve extensibility. Due to the needs of load variation, the expansion and extension capacity of microgrid photovoltaic branch must be fully considered in the planning and layout. In the early stages of the microgrid project, power and communication interfaces must be fully reserved. The design of the monitoring platform should also consider the possible extension range of the system.
- Power regulation and power prediction. Power regulation and prediction based on power detection is the core technology of PV module control and protection. In addition, the grid-connected microgrid must have both island protection and low voltage crossing capability.
