If multiple register transmission (function code 16) is selected, then the P, Q, and Droop set points are sent.
If single register transmission (function code 06) is selected, the droop set point is only sent if droop is enabled (Enabled selected in parameter 2801). The P and Q set points are sent if the P and Q reference parameters are enabled (parameters 2781/2782).
P/Q mode (grid-tie)
When the microgrid is connected to the mains, the controller calculates the references for active and reactive power and transmits them to the BESS. The references are dynamic and change as the loads in the microgrid change. The references will therefore be transmitted continuously from the controller at the speed of Modbus.
The BCU or PCS should be able to change to P/Q mode by self-detection, command or breaker feedback.
V/f mode (island/off-grid/grid forming)
If the microgrid is disconnected from the mains, the BESS must change to island operation and do V/f regulation. This shift is normally done by the BCU/PCS, using breaker feedback or its own grid measurements and protection.
Island mode works best in BESS systems that include droop for paralleling with gensets and/or other sources.
It is preferred that the BESS keeps V/f control. The controller then sends the active and reactive power references (like in P/Q mode). The BCU/PCS then adjusts the frequency and voltage according to the droop curves of the BESS system.
Some BESS cannot do fully automatic droop control. For these, the controller can operate with its own droop curves, so that the power (as a function of frequency) and reactive power (as a function of voltage) are adjusted.
Droop mode (V/f)
When droop mode is enabled, the controller transmits the frequency and voltage offsets. These are added to the nominal frequency and voltage, which correspond to the frequency at 0 kW and voltage at 0 kvar.