Operation modes of DC microgrids
The major component of the DC microgrid’s operation is the grid voltage converter (GVC), which is responsible for maintaining a constant voltage at the DC bus. There are three different operation modes of DC microgrids. These operation modes of DC microgrids are written below.
1. Uncontrolled grid-connected mode
In this mode, the AC grid balances the power in the DC microgrid. In this operation mode, the surplus power from the DC microgrid (DCM) is stored in batteries (battery bank) and supplied to the AC grid. If a deficit power occurs within DCM, power from the AC grid is taken by GVC unit and supplied to DCM. In this mode, a constant voltage is maintained in DCM and it is ensured that power handled by GVC unit does not exceed its capacity. The uncontrolled grid-connected DC microgrid mode of operation is shown in Fig. 1(a). The power supplied by GVC in this mode can then be calculated from the equations below.
Power supplied by GVC in uncontrolled grid-connected mode, without considering power losses is given by Eq. 1.
Where PGVC is the power supplied by GVC unit, PLoad is the power consumed by the load, PWT is power supplied by a wind turbine, PPV is power supplied by the solar system, and PBB is the power supplied by the battery bank.
Similarly, power supplied by GVC in uncontrolled grid-connected mode, considering power losses is given by Eq. 2, where PLoss represents power loss.
2. Controlled grid-connected mode
In this mode, the GVC connected between the AC grid and DCM does not take part in regulating the voltage in DCM. This mode limits the power exchanged between the AC grid and DCM. This mode occurs when either the power flow from the AC grid to DCM is greater than the power limit of GVC or there is a low power flow from the AC grid that causes the voltage drop. Under such circumstances, the battery bank discharges to provides power to the DC bus in order to regulate the voltage. If the battery bank is unable to regulate voltage (maybe due to low charging or climatic conditions that hamper the output of the wind turbine or PV system), load shedding may be required to keep voltage stable. The controlled grid-connected DC microgrid mode of operation is shown in Fig. 1(b). The power supplied by the battery bank without considering power losses is given by Eq. 3.
The power supplied by the battery bank considering power losses for controlled grid-connected mode is given by Eq. 4.
3. Islanding mode
In islanding mode, there is no provision of AC grid and only the battery bank is responsible for regulating the voltage, rendering DCM into islanding mode. The battery bank charges up for surplus power and discharges when there is low output from the wind turbine system or PV system. The controlled grid-connected DC microgrid mode of operation is shown in Fig. 1(c). Under such conditions, the power supplied by the battery bank without considering power losses is given by Eq. 5.
The power supplied by the battery bank considering power losses for islanding mode is given by Eq. 6.
When the climate conditions are not favorable for generation of power through renewable energy sources such as wind turbines and PV systems, the battery bank discharges and regulates the voltage. It can also charge when there is surplus power generation from turbines and PV systems. However, if there is a prolonged discharge from battery bank unit, the power generation and voltage regulation factors of battery bank may decrease over the time. Under such circumstances, the critical loads will be supplied by battery bank, but for non-essential loads, load shedding might be necessary.
If the reverse conditions occur, where generated power increases the rating of the battery bank, the voltage of the battery bank will increase, which is also not favorable. To counter this increase, the output from wind turbines and PV systems can be decreased by changing the pitch of the wind turbines and disconnecting PV panels in PV systems.