Handbook of Energy Storage for Transmission or ... - W2agz.com

EPRI Proprietary Licensed Material
The battery is typically connected to the DC bus that feeds the PCS. In this
configuration, the PCS would be designed to operate within the voltage window of the
cell stack or series of cell stacks. An alternative configuration is to insert a DC/DC
chopper circuit between the battery and the DC bus so that the PCS operates at a voltage
independent of the battery state of charge. This configuration can be used to optimize the
PCS according to the switching device characteristics and the AC voltage and to share
multiple devices on the DC bus, such as fuel cells, flywheels, or photovoltaic systems as
illustrated in Figure 5. The enhanced flexibility is offset by the cost of the chopper and
the DC/DC conversion efficiency penalty.
Figure 5
Possible Hybrid Configuration with DC Bus Isolation
AC Electrical Interconnection
Most VRB applications require a PCS to convert the DC energy of the battery into usable
AC electric power. Modern power conversion technology provides for bi-directional
power flow, so the same equipment can be used for both charging and discharging the
battery.
A wide range of PCS configuration options are possible. These include off-grid systems,
such as would be required for remote renewable energy applications that provide constant
AC voltages to the load. Grid-connected systems, such as would be used for utility and
industrial applications, are connected at a fixed voltage and vary current to and from the
grid. PCSs are available for 50 or 60 Hz networks.
The systems are designed to meet all utility interconnection requirements, such as
• Over/under voltage protection
• Overcurrent protection
Vanadium Redox Battery 14