Comparison of micro-grid and off-grid photovoltaic power generation system

Polaris solar PV net news: first, the micro-grid

Micro-grid system is a new network architecture, is composed of distributed generation, load, storage system and control device of the system unit, is to achieve a self control, protection, and management of autonomous systems, parallel operation can be either external, can also be run in isolation.

Micro-grid system is compared with the traditional grid system concept refers to multiple, distributed power and its associated load according to certain network topologies, and through the switch connected to the conventional power grid.

Core equipment of micro-grid system that distributed generation and energy storage systems, including PV inverters, wind turbines, diesel engines, PCS as well as BMS of the inverter.

In recent years, with the rapid development of micro-grid system, grid-connected inverter has been given a new definition: distributed solar power. It can be used as a micro-grid system of mains power, become one of the core technology of micro-grid system as a whole, due to the special nature of his power (current source), it can be used as a supplementary power supply micro-grid system.

PCS: energy conversion systems, in recent years is the rise of a new type of distributed application is very flexible and very powerful power supply and an energy storage system. As a four-quadrant converter, it can realize the function grid-connected inverter, it can also access grid-connected rectifier and charging function, you can also achieve independent off-grid inverter function.

Micro-grid system, distributed generation capacity is a certain principle, otherwise will cause excess power or lack of power.

Capacity of the inverter should be less than or equal to the total load capacity to close to the load capacity for the best, not too small, nor too high, if too high, excess power could not be absorbed, could easily lead to system crashes. In recent years research results showed that, dang grid inverse variable device capacity is greater than load total capacity Shi, if at has PCS as support source, grid inverse variable device of extra power will feed into PCS, makes PCS in “away from network charging” State, this State is not too stable of, currently also no real effective of control means, so recommends grid inverse variable device of capacity is less than is equal to total load capacity.

PCS capacity total load capacity 1.5 times should be set, but the battery capacity needs in accordance with the system can handle up to 3 to 5 rainy days of capacity to configure. Can also provide power for the load of the inverter, but micro-grid system is run at night, most of the time, grid-connected inverter not working at this time, so most of the energy micro-grid system is provided by PCS, PCS capacity needs to take into account for a rainy day and night conditions.

Micro-grid system, a typical main circuit topology

As shown in Figure 1 for the micro-grid system architecture diagram, together with the PCS of the inverter and connected to the system bus, while load hanging on the bus.

(1) when the power did not come off when it does, if it is during the day, run in grid-connected inverter based distributed power system, PCS in a storage battery charging status to serve as standby power supplies. If it is in the evening, still powered by the grid load, PCS as a backup power supply for battery charging.

(2) when power off, if it is during the day, systems running on PCS to support status of the source, as an auxiliary of the inverter power supply; if it’s at night, only the PCS as the primary power source to power systems, and PCS as well as capacity to meet the load requirements of long-term supply.

Second, off-grid systems

Off-grid photovoltaic power generation system, also known as stand-alone photovoltaic power systems, mainly by the PV module, DC/DC charge controller for off-grid inverters, load composition, with an independent power supply and energy storage function independently. Off-grid PV system applied to the region away from the main power grid, such as desert, beaches, Islands and other regions.

Off grid inverters are not active-phase commutating device, can only run the inverter status, rectification is not yet running. Main circuit structure and grid-connected inverters are exactly the same, just different control method and the input and output side of the connection object. Off grid inverters (three-phase) the AC output should be 380V/Hz.

In addition, the solar cell is different from a regular battery, it is a battery that can be regarded as a constant current source, the output power under the influence of light, temperature and other factors.

Typical off-grid photovoltaic power generation system topology

As shown in Figure 2 main circuit for off-grid photovoltaic power generation system, PV modules, controllers, and off-grid inverter AC load, if you have special needs can also be added on the controller side DC load.

Off-grid PV systems are passive generation systems, does not require the participation of large power, flexibility and mobility more superior, and more stringent requirements for such systems on DC/DC controller, must be able to effectively control the charging status and ensure that PV components operating at maximum power point.

Off-grid PV system capacity mainly in the off-grid inverter and battery configurations, off-grid inverter capacity equal to 1.5 times the load capacity is required, battery capacity needs to be able to take 3 to 5 consecutive rainy days of power supply, DC/DC controllers need to be larger than the capacity of PV components of capacity is slightly redundant.

Third, micro-grid and off-grid PV power system comparison

(A) common

Comes with a separate power supply characteristics;

Need energy storage systems;

Are needed for the batteries matching the best capacity.

(B) the difference between

Micro-grid system is an active system can be connected with the power grid, off-grid photovoltaic power generation system is a passive system, and cannot be connected with the power grid;

Micro-grid system is more complex, you need to configure the distributed power more, off-grid PV systems only require the controller and off-grid inverters can be;

Micro-grid energy storage system in the system for four-quadrant converter, can achieve bidirectional energy flow, energy storage systems in off-grid photovoltaic power generation system for single-phase inverters, bidirectional energy flow cannot be achieved.

(C) the practical application of principles

If applications for desert, beaches and other more remote, and no artificial occasions of frequent maintenance, off-grid PV system, so the system does not need to be manned, can run independently, and only need regular maintenance.

If applications as close to the power grid, and need frequent maintenance and artificial occasions to make strict scheduling requirements, with micro-grid system is appropriate, because the system of distributed power source switching and scheduling operation requires human intervention, and some even need to monitor the system.

Original title: comparison of micro-grid and off-grid photovoltaic power generation system

Posted in News.