
HKJC – BMS Proposal
Solar PV System Dry Contact to BMS: Mafoo Quarters A-D
Schematic Drawing

A programmable logic controller (PLC) is set up at the switch room to receive modbus output from
the three inverters through RS485.
The PLC is set to monitor the values of the two parameters (Inverter OFF and Inverter FAULT) sent out
by the inverters through RS485 to determine the inverters status.
The pre-meter and post-meter MCB/MCCB in each inverter circuit can also output a contactor relay signal
when there is a trip/ manual switching-off, so that the PLC can give fault status to BMS in case of
MCB/MCCB tripping.

The PLC receives the Modbus signal from the inverters and the relay signal from the
MCB/MCCBs and convert into dry contact output to BMS according to the following logic.
The signals are converted to dry contact output by the PLC. The dry contact table is shown below.
Inverter is
ON
Inverter is
FAULT
When
Inverter not
shutting
down
When
MCB/MCCB
normal
(no trip)
When
Inverter not
Fault status
Inverter is
not
FAULT
Inverter not FAULT,
otherwise FAULT happened
When
MCB/MCCB
normal
(no trip)
Inverter is
ON
Inverter is
ON
MCB/MCCB
Relay
Value=7: Inverter Modbus gives Fault status
Value=6: Inverter Modbus gives Shutting down status
Value>1:
Inverter is
not OFF