A battery energy storage system (BESS) is shut down from the top down: it is brought to zero power and stopped from the energy management system (EMS), the converter (PCS) is stopped, the AC breaker is opened and locked, and then the DC disconnectors or contactors of the racks are opened. Even so, the battery is never left without energy: its terminals keep their voltage, and the monitoring, cooling and fire detection systems must stay powered.
What has to be disconnected
A grid-connected BESS has several levels that are disconnected in order:
- Energy management system (EMS) and plant controller, which give the charge and discharge setpoints.
- Power conversion system (PCS), which converts the batteries’ DC into AC and controls active and reactive power.
- AC side: PCS breaker, transformer and medium-voltage switchgear.
- DC side: PCS DC bus, disconnectors or contactors of each rack and, inside each rack, the battery modules.
- Battery management system (BMS), which monitors voltage, temperature and state of charge of each module.
- Auxiliary services: cooling, fire detection and suppression, gas detection, ventilation.
What each part is and how it is commissioned is explained in BESS commissioning. Its parts, functions and standards are in BESS: parts, functions and standards.
Typical sequence, from the top down
- EMS to zero power. Zero setpoint for active and reactive power; notice to the control center
- PCS stop. Stop command from the EMS or the PCS panel; the converter stops switching
- Open the AC side. PCS breaker or medium-voltage switchgear, open and locked
- Open the DC side. Disconnectors or contactors of each rack, open and locked
- Wait and verify. Capacitor discharge per the manufacturer; absence of voltage at the work point
1. EMS to zero power. The EMS brings the setpoint to zero, so the PCS stops charging or discharging without an abrupt interruption. The control center is notified: the storage capacity goes out of service and the plant controller must no longer count on it.
2. PCS stop. With power at zero, the PCS receives the stop command and stops switching. Opening a breaker with the PCS running is possible as a protective action, but it is not a normal switching operation.
3. AC side. The PCS AC breaker is opened or, if the work is on the transformer or in medium voltage, the corresponding switchgear, and it is locked. In medium voltage, the switchgear is also earthed after verifying absence of voltage.
4. DC side. The rack disconnectors or contactors, which separate each bank from the PCS DC bus, are opened. In many systems they are contactors controlled by the BMS; the manual says whether opening them is enough or whether a padlocked manual disconnector must also be opened.
5. Wait and verify. The DC bus capacitors hold charge; the manufacturer’s time is respected and absence of voltage is verified at the work point, with a detector rated for the DC voltage of the system.
The battery is never left without energy
This is the basic difference from a substation or from a photovoltaic inverter at night. Opening everything above separates the battery from the grid and from the PCS, but each rack and each module keeps its voltage and its stored energy. What is left without voltage is whatever is downstream of the last open point.
| Section | State after the switching sequence |
|---|---|
| Grid and medium-voltage switchgear | Separated; earthed if the work requires it |
| PCS, AC side | No voltage, if the breaker is open and locked |
| PCS DC bus | No voltage after discharge, if the racks are open |
| Output terminals of each rack | Energized, upstream of its disconnector or contactor |
| Battery modules | Always energized, with stored energy |
Work inside a rack is not done “de-energized” in the substation sense. It is defined by the manufacturer, with its tools, its gloves and its authorized personnel.
Thermal risk
A battery cell can go into thermal runaway because of an internal fault, mechanical damage or out-of-range conditions, and the failure can propagate to neighboring cells. That risk does not disappear when the system is disconnected. That is why:
- The BMS stays powered to monitor voltages and temperatures.
- The cooling keeps running within the range the manufacturer requires.
- Fire detection and suppression are not disabled during the intervention, except as the manufacturer’s procedure authorizes for a specific job.
- BMS alarms are attended to before opening container or cabinet doors, following the site’s emergency response plan.
IEC 62933-5-2 covers the safety of grid-integrated electrochemical systems throughout their life cycle, including the interactions between subsystems. NFPA 855 covers, among other topics, the commissioning and decommissioning of stationary energy storage systems.
Golden rules applied to the BESS
The five safety rules of EN 50110-1 and those of the local electrical safety code apply on the AC side and in medium voltage as in any installation. On the DC side they are adapted:
- Disconnect: AC and DC, in the order of the manual.
- Secure against reconnection: padlock on the AC breaker and on the rack disconnectors; logic lockout in the EMS so it does not command a start.
- Verify: absence of voltage at the work point, on DC and on AC.
- Earth: in medium voltage and on AC according to the procedure; on DC, whatever the manufacturer defines.
- Protect against neighboring live parts: the racks remain energized.
The general principle of switching operations is in the guide on switching operations in substations, and the protection of the solar plant connection in solar plant protection: inverter and interconnection.
Returning to service
The order is reversed with the manufacturer’s checks: safety systems operational, BMS without alarms, racks connected to the DC bus, AC side closed and the PCS starting up and synchronizing with the grid. At the end the EMS resumes its setpoints. The protections at the point of connection must be set for a system that imports and exports power.
We leave your system ready to operate
Our team commissions the electrical part of grid-connected storage systems: substation, switchgear, transformer, protections at the point of connection, control and SCADA integration, in Colombia or elsewhere on request. See solar plant testing and commissioning or write to us through the contact page.
Related guides
- BESS: parts, functions and standards
- BESS commissioning: tests and requirements
- Switching operations in substations: opening, closing and safety
- Solar plant protection: inverter and interconnection
- Arc flash labels
References
- IEC 62933-5-2, Electrical energy storage (EES) systems – Part 5-2: Safety requirements for grid-integrated EES systems – Electrochemical-based systems.
- NFPA 855, Standard for the Installation of Stationary Energy Storage Systems.
- EN 50110-1, Operation of electrical installations – Part 1: General requirements.
Frequently asked questions
Does opening the AC breaker leave the BESS without voltage?
No. It separates the system from the grid, but the batteries remain charged and the PCS DC bus can keep voltage until the rack disconnectors or contactors are opened and the capacitors are discharged. Even with everything open, each module and each rack keeps its internal voltage.
Why are the auxiliary services of a disconnected BESS not switched off?
Because the battery management system, the cooling and the fire detection and suppression are what monitor and control the thermal risk of the cells. A battery disconnected from the grid still stores energy and can have an internal fault. What can be switched off and what cannot is defined by the manufacturer's manual.
Which standards cover BESS safety during operation?
IEC 62933-5-2 covers the safety requirements of grid-integrated electrochemical storage systems throughout their life cycle, considering the interactions between subsystems. NFPA 855 is the installation standard for stationary energy storage systems adopted in many US jurisdictions and cited in project specifications. The procedure for each system is its manufacturer's.