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

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Matching Batteries for Power and Energy Loads
Problem Description
The need to protect substation equipment during momentary power disturbances and
longer-term outages depends on the equipment sensitivity, function, and exposure to
power disturbance events at the location. Currently, most substations have control and
protection equipment that are identified as critical to station functions. By far the most
common practice to protect this equipment is the installation of 48-, 125- and 250-Vdc
stationary batteries sized for several days of outage protection.
These station batteries require a significant amount of real estate often in the control
house of modern transmission and distribution stations. Sixty to 120 cells of large 100-
to 400-AH batteries on several battery racks are not uncommon. These typically require
environmental space conditioning and periodic maintenance. In some cases, without
space conditioning, the expected battery life and capacity are less. Reducing the size of
this battery and increasing the ambient temperature range for effective operation,
allowing it to be installed outdoors, is of interest to many utility planners and substation
engineers.
The size of this critical load and the period that protection is required varies with the
substation design and function. Most substations have requirements from a few hundred
watts to several kilowatts, which is well suited for the station battery. Most of the
systems installed today tend to be oversized for longer duration protection up 10s of
hours. Also contributing to large size is the difficulty to match load power and energy
requirement to the battery capabilities. This is because some of the loads are relatively
low power for the full duration of the outage, and other loads, such as breaker trip coils,
are relatively high power and high inrush, for a very short time. The station battery is
effectively oversized to meet both power and energy requirements at the rated voltage.
There is an opportunity to better optimize this system with a hybrid energy storage
design. The idea is to match high-powered short-term energy storage with those high
inrush loads and to match the longer-term battery storage for the average load. Relieving
the station battery of these high inrush loads will allow a significant reduction in size and
may extend the life or reduce the cost of the smaller battery because its characteristics
can be better matched to the duty.
This application entails segregation or buffering of high-inrush, low-energy loads
effectively removing them from the station battery-sizing requirement. While the
traditional lead-acid battery is capable of handling these momentary loads it must be
sized to do so. The high power energy storage device that will support these high inrush
loads will be electrochemical capacitors. Removal of high inrush requirements and
added redundancy provided by the capacitor string is expected to allow reduction of the
station battery by up to 50%. The technical criteria for this application will be substation
specific but likely will fall within these parameters:
• Application – High-inrush load support for a 48-kWatt-Hour station battery
requirement (3 kW x 8 hours x 2)
• Voltage Rating – 120 Volts dc
Electrochemical Capacitors 48