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EPRI Proprietary Licensed Material application, providing uninterruptible power for substation critical loads, has two possible solutions. The first solution replaces the substation battery by providing short-term bridging energy to carry critical loads during transfer to an alternate source, or until the primary source returns to normal operation. The second solution supplements the substation battery by picking up high inrush loads, allowing reduction in battery size. These T&D energy storage applications are described below based on the generic need that they serve. Included in the descriptions are technical specifications such as response time, run time, duty cycle and power requirements. Voltage Stabilization Support to the Electric Grid Problem Description Need for system stability, in both central and distributed power systems, as well as the specific functions of reactive power supply and frequency regulation support, are considered here. The reactive power (VAR) control application solves the problem of maintaining power flow and voltage stability. The frequency regulation application solves the problem of controlled injection needed to regulate system frequency. The latter is particularly relevant for improving stability of relatively weak areas in the T&D system. These applications are cited as ancillary services in FERC order 888, 1996, and will eventually carry a location dependent market value in a restructured utility situation. The technical criteria for grid stabilization, identified here as a “mini-facts” application, will be somewhat site and utility system specific but likely will fall within these parameters: • Application – Reactive power supply and frequency regulation support of T&D • Power Rating – 2-40 MVA (power may be real, reactive, or both) • Energy Capacity – .5-10 kWh at MVA rating • Duration – Corrective action for cycles up to a few seconds • Response Time – 5 to 100 milliseconds • Duty Cycle – Variable depending on conditions, may be a continuous problem • Roundtrip Efficiency – 80-90% (assumes less than 10% duty cycle) • No load Losses – less than 3% • Plant Footprint – .05 MW/m 2 (assumes siting in low-density area) • Environmental Issues – EMI Other, non-energy storage, alternatives for solving this problem are overexcited synchronous motors and generators, switched capacitors, as well as fast acting static var compensators (SVCs). Also, utilities’ traditional options to improve voltage regulation and control frequency are upgrading transformer and feeder capacity, and cycling power plants. Electrochemical Capacitors 40
EPRI Proprietary Licensed Material Stored Energy for “Distributed Mini FACTS” Controllers This energy storage application is based on benefits of active power injection coupled with dynamic reactive power exchange for improved stability in the power system. The need for dynamic reactive power compensation (“fast VARS”) as opposed to fixed or mechanically switched capacitor banks have long been recognized as a way to improve T&D system stability and increase power transfer limits. This concept has been applied in large-scale inverter-based Flexible AC Transmission Systems (FACTS). These systems have the ability to affect changes of 10 to 100 MVAR and respond in less than one-quarter of a cycle and they have brought about a new way of thinking regarding active and reactive power. An example is the STATCOM, which outpaces switched passive capacitors, reactors, and LTC transformers in rapid voltage regulation. STATCOM responds even faster than conventional generators, SVCs, or synchronous condensers, which in the past were the main supplier of “fast VAR” to the electric systems. Also, this type of dynamic reactive compensation is better at supporting voltage during system contingencies than conventional capacitor banks that lose capacity when system voltage decreases, (See Figure 19). % Decrease in Capacitor VAR Output 40% 35% 30% 25% 20% 15% 10% 5% 0% 80% 85% 90% 95% 100% Line Voltage Figure 19 Loss of capacitor VAR output as a function of line voltage Combining energy storage with FACTS controllers offers three distinct advantages: 1. Energy storage devices can provide system damping while maintaining constant voltage following a system disturbance. 2. Energy storage increases the dynamic control range allowing the interchange of small amounts of real power with the system. 3. Distributed energy storage can maintain the speed of locally connected induction motors during a power system disturbance, thus helping to prevent a voltage collapse in areas where there is a large concentration of induction motors. An EPRI study [1] found that adding energy storage (in this case, SMES) to a FACTS device increased the control leverage of the reactive power modulation of a FACTS device by 33% (i.e., operating the FACTS + energy storage in four-quadrant, reactive plus real power mode provided 33% greater transmission enhancement). Figure 20 shows the results of a study conducted by Siemens on the effectiveness of short-term Electrochemical Capacitors 41
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