- Function of backup fuel generator set
The scale of renewable energy system can be designed to be 100% independent without backup generator set. However, because the available power provided by renewable energy power generation system and user load demand change from time to time, a system powered by renewable energy is large-scale and expensive. “Demand side” management is one of the ways to solve this “super scale” problem. For example, reduce power consumption behavior to reduce the total power load and match the power generated by renewable energy system from time to time. But this is usually not the first choice for consumers.
Note: standard as / nzs4509 2-2010 independent power supply system Part 2: system design guidelines stipulates that if the system is not equipped with backup fuel generator set, the scale of photovoltaic system needs to be increased to 130% ~ 200% (1.3 ~ 2.0 times). Renewable energy power generation varies greatly in different seasons. For example, in summer, there are more photovoltaic power generation, and the load also increases due to the large demand for air conditioning (peak in summer); In winter, due to the shorter sunshine time, the photovoltaic power generation is reduced, and the lighting load at night is increased (peak in winter).
Ideally, the system should achieve energy balance, that is, the balance between the electric energy output from renewable energy system, backup fuel generator set and battery and the electric energy required by the load. It should be remembered that the output power of renewable energy is related to local conditions, such as the overcast time of photovoltaic and the flat wind time of wind power.
- Use of backup fuel generator set in independent power supply system
The traditional fuel generator (generator set) is directly connected with the load and operates according to the load. Although this scheme is simple, its efficiency is not high, especially in the case of light load of motor. In addition, every time there is power demand, the motor has to run, which is very inconvenient for motor driving, reducing fuel efficiency and reducing the service life of the generator set. A more effective system mode is that the load is connected with the battery, and the backup unit charges the battery. In this way, the load rate of the generator set is higher, so the efficiency is higher; The generator set only operates intermittently to charge the battery and improve the efficiency. If the system suddenly increases a large load, the backup generator set will directly participate in the power supply to the load. This approach reduces the need to adjust other parts of the system (such as inverters) to meet these sudden increases in load.
The generator set can not only be directly connected with the battery, but also directly supply power to the load while charging the battery when necessary. The current generated by the generator set is rectified by the battery charger, which charges the battery. In the automation system, a sensor will be set. When the battery voltage drops to the preset value, the sensor will send a signal to the generator to start the generator.
The generator set can not only charge the battery, but also directly provide electric energy to users. In this case, the transfer switch can be set between the inverter and the generator set. This change-over switch can be a large relay with AC power supply of generator set or a manual change-over switch. When the AC generator set starts, the relay acts to switch the load of the inverter to the generator set for power supply. The opening and closing switch shall be designed with a delay of 10~30s, because it takes time for the generator to reach the normal speed. After reaching the normal speed, the change-over switch will connect the load with the AC generator set. The advantage of this transfer switch system is that no matter how large the load can be directly driven by the AC generator set, which will effectively reduce the investment cost of system components such as inverter.
Bidirectional inverter is a device that integrates inverter and battery charge discharge converter. Transfer switches (relays / Contactors) have a fixed current rating when applied. The power supply current of the generator set connected to it shall not exceed the rated current of the inverter change-over switch (relay / contactor). If the current of the generator set exceeds the rated value, an auxiliary relay / contactor shall be added to the bidirectional converter.
The generator set of series / switch system can be started and stopped in the following ways:
(2) Controlled by the system controller (e.g. solar controller / regulator).
(3) Controlled by frequency converter.
The inverter operates in the “interactive” mode, which is both an inverter and a charger. It is connected with the internal relay / contactor to switch the load between the inverter and the generator set.
The difference between the standard inverter and charger mentioned above is that the inverter operates in parallel with the generator set, which will have the following advantages:
(1) Switching between two AC power supplies will not cause interruption of load power supply.
(2) The output of the inverter and the output of the generator set can be added to meet the load greater than that supplied by the inverter or the generator set alone. The system installation / designer can select some parameters to affect the control of the interactive inverter on the start and stop of the generator set.
Read more: Electrical Protection of PV Modules