One of the enticing benefits of installing a battery storage system is the potential to have an Emergency Power Supply (EPS) in the event that the grid goes down. Some of the coupling systems provide this (with or without extra components), some don’t. Table 3 summarises the possibilities of the different systems that we have looked at.
Note that having an Emergency Power Supply is not the same as having a system that allows a grid-connected PV inverter to function when the grid goes down. Without battery storage, grid connected PV systems do not generate electricity when the grid goes down, to avoid exporting into the grid whilst repairs are being made. However with batteries, it is possible in some cases for generation to continue as long as there is capacity in the batteries and / or a suitable load to utilise the generation.
In order for a PV array to function when the grid goes down, there has to be a safety mechanism in place to prevent the PV system from exporting to the grid. This tends to limit the ratio of the size of the PV array connected to the battery system to the rate at which the batteries can be charged (in other words the battery system must be able to dissipate all of the PV generation in the event of a power-cut). With the SMA system this ratio is 2: 1; with other systems this ratio tends to be lower, pretty much 1:1.
In a power cut, storage system can either provide DC power direct from the battery, or they can provide AC power via the inverter. Most customers want the ability to run AC loads.
There are two ways of servicing AC loads in a power cut:
If the inverter has an AC socket, an emergency load can be plugged in and out as required.
The system can be set up with a changeover relay, where AC loads can be run from the grid or battery. The schematic below shows the typical set up for an AC coupled system:
Note that during a power cut, an isolation relay (or manual changeover switch) is operated so that loads are isolated from the grid, and instead run from the inverter.
These systems require either an inverter package that can be switched between on- and off-grid modes, or an installation that includes separate grid-connected and standalone inverters.
The speed of switching between normal (grid connected) and off-grid (power cut) modes will affect how the system is used. Most systems will not operate in the same way as a UPS (uninterruptable power supply) – in other words, the delay in switching will cause loads to switch off on the transition to and from on-grid to off-grid mode. If this is an issue, a separate UPS may be required to ride through the switch cycle.
The storage capacity (usable capacity and maximum depth of discharge) and the charger size will limit the available power (kW) and consumption (kWh) in a power cut. Both the power requirement (kW) and the expected kWh consumption (i.e. back up duration) need to be taken into account in designing any grid back up.
Where a system is designed to have a grid back-up, some re-configuration of the load via a secondary consumer unit may be advisable to ensure that only the loads required in a power cut will draw off the battery in the power cut.
For example, with a DC coupled system, the schematic would look like this:
Battery inverter / chargers are generally single phase. Thus if a battery system needs to be connected to more than one phase of a 3 phase connection, three chargers are needed, along with a battery fuse. One charger is connected to each phase.
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