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EPRI Proprietary Licensed Material 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
EPRI Proprietary Licensed Material • Power Rating – 36 kW (assumes 1.5 kW load x 12 for inrush) • Energy Storage Capacity – 60 Wh • Duration – 6 seconds • Response Time – 5 milliseconds • Duty Cycle – Infrequent, e.g. 10 times per month, 120 per year • Roundtrip Efficiency – 70% (lower efficiency at maximum power) • No load Losses – less than 2% • Plant Footprint – .5 m 2 • Environmental Issues – same as lead-acid batteries The only alternative to energy storage devices is other available power sources or spinning reserves. This is usually in the form or a second independent feeder that is accessed within the required response time via an electronic switch. Availability of such independent feeders is rare. High-Inrush Load Support for Substation Batteries Electrochemical capacitors do not compete well with batteries when a long-duration energy supply is needed. However, they excel when the requirement is high power for a short-duration, such as for starting inrush, as required to operate a trip coil or motor operated switch. The fact that a station battery has to serve auxiliary loads for a long duration, and then at the end of this time period still have the necessary pulse power to operate high inrush loads, leads to over sizing station batteries relative to the average loads served. Another factor contributing to desire for redundancy. The proposed solution is to add the electrochemical capacitors in parallel on the battery buss via a small dc-to-dc converter to allow voltage matching during both charge and discharge. The conceptual layout is shown in Figure 27. Also required will be a small series impedance (not shown) to assure that the converter and capacitor combination show lower impedance for inrush circuit requirements. Some classification and segregation of load into high power and high energy may help with design and application of this solution. Electrochemical Capacitors 49
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