Quick answer
A third-party EMS needs to read key battery and site data, such as state of charge, available power, faults and whole-site import and export. It also needs limited permission to tell the inverter how much to charge or discharge. The battery, inverter and protection system must still control safety and grid limits.
A third-party energy management system, or EMS, needs access to read:
- The amount of charge in the battery
- The charging and discharging power available
- The inverter’s operating status
- Active alarms and faults
- The amount of electricity the client imports from and exports to the grid
The EMS also needs permission to send a limited control instruction to the inverter. This instruction tells the inverter how many kilowatts, or kW, to charge or discharge.
The inverter, battery management system, or BMS, and protection equipment must continue to control the battery’s safety limits, protection settings, and grid-code settings. The EMS should not have permission to change them.
What readings does an EMS need?
The EMS needs to read the following information before it sends the next instruction to the inverter:
- Battery state of charge: How much energy the battery has available before the EMS sends the next instruction
- Current charging or discharging power: What the battery is already doing
- Available charging and discharging power: How many kW the battery can still charge or discharge
- Inverter operating status: Whether the inverter is operating, has stopped, or has reported a fault
- Active alarms and faults: Whether the EMS should stop sending instructions because the inverter has reported a fault or the battery has reached a charging, temperature, or state-of-charge limit
- Delivered inverter power: Whether the inverter carried out the previous instruction
- Whole-site import and export: Whether the client is taking electricity from the grid or sending electricity back to it
A grid meter and current transformer, or CT, clamps at the site connection point usually measure the client’s imports and exports.
On many commercial battery projects, the inverter already receives these readings from its own grid meter. The EMS may be able to read the same value through the inverter, so the project may not need another meter.
The EMS supplier still needs to check the exact inverter model and firmware version. Many inverters can share monitoring data, but not every inverter gives the EMS all the readings it needs.
The EMS does not normally need every reading from the BMS. For example, it usually does not need the voltage of each individual battery cell. The BMS continues to monitor and protect the cells.
What control permission does the EMS need?
For normal battery optimisation, the EMS needs permission to tell the inverter how many kW to charge or discharge. The technical term for this instruction is an active-power setpoint.
Depending on the equipment, the EMS may also need permission to:
- Allow or stop charging
- Allow or stop discharging
- Put the inverter into external-control mode
- Set how long the inverter should wait for the next EMS instruction
- Set how the inverter should respond if the instructions stop arriving
The inverter should accept power instructions from the EMS without giving the EMS permission to change the battery’s safety limits, protection settings, or grid-code settings.
The EMS should not override:
- BMS charging, temperature, or state-of-charge limits
- An inverter shutdown after a fault
- Anti-islanding protection, which prevents the inverter from feeding electricity into the grid when the grid supply fails
- Approved import or export limits
- Permanent inverter settings that the EMS does not need for normal control
Only give the third party remote access to the controller, inverter, and meter that the EMS needs. Limit that access to the named equipment and keep a record of what the third party does.
The key point is to limit how much control the EMS has. The EMS can ask the inverter to charge or discharge, but the inverter, BMS, and protection equipment must remain in control of the battery safety limits, protection settings, and approved import or export limits.
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