NAS battery technology may offer utilities a viable solution to the challenges of load-levelling and power quality through energy storage. US Utility AEP is conducting the first USA-based grid-connected demonstration.

Dave Nichols, American Electric Power Service Corporation, Ohio, USA

Following the well documented problems of recent years, US energy providers are currently focused on controlling costs and conserving capital. At the same time power suppliers need to be meeting customer service and reliability performance expectations. Increasingly, both the public and private sectors recognise the value that Energy Storage (ES) can provide to meet customer energy needs in a cost effective manner.

One high value ES application is for load levelling, or peak shaving (PS), which is accomplished by using existing generation, transmission and distribution assets to charge storage devices when load is low and discharge them during heavily loaded periods. This process enables the efficient operation of generation facilities and maximizes the transmission and distribution infrastructure use.

Power Quality (PQ) problems are caused by brief momentary power variations lasting less than seconds, and long-term outage. Instantaneous response from an ES device can provide sufficient energy to ‘ride through’ short-term power delivery anomalies and provide bridge power to standby power generation.

Furthermore, when properly located, energy storage can be used to offset or delay new line construction and equipment upgrade. The total value of an energy storage application is the aggregated worth that accrues in the wholesale or retail market, for the electric power system (EPS) and for locational value.

Sodium sulphur batteries

The sodium sulphur (NAS) battery system was initially designed in Japan as an energy management device for load levelling and peak shaving applications. Today’s NAS batteries are multiple function storage devices that can satisfy a variety of energy and power requirements from load levelling that spans over several hours, to bridging applications that need energy for minutes, to uninterruptible power supply (UPS) applications that require the energy in a fraction of a second.

The NAS battery possesses excellent energy and power density, high electrical efficiency, long life, a small footprint, pulse power capability, instantaneous response, and reliable operation. These characteristics make it an excellent candidate for EPS applications. Modules have a pulse power capability up to five times their continuous rating (for 30 seconds) that is limited by the cell temperature rise, internal resistance, and depth of discharge. The duration of peak power is limited by the internal temperature rise of the battery. With appropriate thermal management, the NAS battery can deliver pulse power for durations of tens of minutes with trade-offs on the number peak shaving cycles.

NAS battery system

The system uses two high density NAS battery modules as the DC energy source. The NAS batteries are manufactured and supplied by Japan’s NGK Insulators (NGK). Each module has the capacity to provide 375 kWh of DC energy at 52.6 kW rated power for 2500 full charge/discharge cycles. NGK, Tokyo Electric Power Corporation (TEPCO) and American Electric Power (AEP) have developed various discharge profiles that successfully combine the peak shaving and power quality applications. The choice of profile for an application is made based on the desired level of PQ protection, peak shaving energy, and the desired lifetime of the batteries.

The Power Conversion System (PCS) is required to convert the battery DC voltage to an AC voltage required by the grid. The PCS uses two insulated gate bi-polar transistor (IGBT)-based high frequency bi-directional converters to process energy in each direction and uses pulse-width modulation (PWM) techniques to regulate a voltage or a current depending on the operating mode. A low pass three-phase filter placed at the output of each converter is used to eliminate the high switching frequency noise and unwanted harmonics from going into the power system lines. The output of the filter goes into a step-up transformer. The transformers provide galvanic isolation between the power grid and the power electronics, a delta-wye transformation and regulation of 480 V AC at the grid side of the system. In this application, the transformer is made with high leakage inductance to further improve the filtering efficiency of the LC filters.

Figure 1. The power supply profile shown has one-hour ramp up, five hours flat and three hours ramp down regions and provides 375 kWh of DC energy under normal operating conditions
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A silicon controlled rectifier (SCR)-based static switch (SS), which is surrounded by appropriate circuit breakers that provide different operational schemes, is used to achieve fast transfer of the sources during a PQ event. The SS is designed to provide transfer times less than a quarter cycle. The circuit breaker CB3 is used to provide bypassing of the NAS battery system for maintenance or when a problem is detected.

Demonstration project

In collaboration with the Electric Power Research Institute (EPRI), and under the sponsorship of the Department of Energy, AEP is currently conducting the first US grid-connected demonstration of the sodium sulphur technology at its office in Gahanna, Ohio, USA. The system has been operating as a combined peak shaving/power quality mode since its startup and acceptance testing in September 2002.

The objective of the demonstration is to validate NAS PQ/PS operating characteristics, gain familiarity with the technology and develop needed economic models. Other AEP partners in the project include NGK, TEPCO and ABB

Figure 2. The system response to a PQ event test during a peak shaving cycle
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Acceptance test overview

Acceptance tests were done on the battery modules in Japan prior to delivery and the entire system was tested at ABB’s facility prior to shipment to Gahanna. Final tests were conducted to validate compliance with standards and specification.

A monitoring programme is underway that will be utilized to assess PQ/PS performance and to develop economic models for energy storage. The monitoring package includes advanced communications between internal NGK and ABB controllers and a central computer located at the NAS Center at AEP’s Dolan Technology Center to provide real time and historical data on performance.

Peak shaving and charging

The daily operation pattern of the NAS battery system is shown in Figure 1. The system starts charging everyday at 2000 hours, which is considered the start of off-peak hours, and depending on the selected peak shaving profile, takes around nine to ten hours to complete full charging. The last one-hour of the charging cycle is the supplementary charging state done to optimize pulse power capability.

After charging is completed, the system waits in standby state until the time set for the peak shaving action is reached. In this example the peak shaving is started at 0700 hours. The timing of these activities can be reprogrammed by the user to match the load demands in different applications and installation sites.

Power quality event

Figure 2 shows the response of the system to a power quality event test. Only two phases, a and b, were plotted to provide visibility of the dynamics. Phase a of the source voltage was sagged below 80 per cent of nominal (480 V) and initiated the PQ event. The system responded to the disturbance in 2 milliseconds and picked up the load. The load voltages (second trace) and the load currents (bottom trace) show a smooth transition and continuous power flow to the load. Comparison of the load and source currents indicate peak shaving prior to the PQ event. During this test, the load was set at 300 kW and the system was peak shaving at 100 kW correspondingly the power coming from the source is 200 kW.

The performance analysis will quantify overall system efficiency, power quality events, and daily cycling. It is also planned to calculate various energy quantities, such as on-peak and off-peak energy consumption and peak demand, as inputs to the economic analysis.

Building loads will be analysed to derive peak shaving benefits to the customer using tariff schedules at three AEP subsidiary companies. These will be combined with the benefits associated with enhanced power quality provided by the system to characterise the overall economic benefits under various commercial environments.

Two-year summary

With the end of the two-year demonstration period approaching, it is possible to make some observations about system performance. The NAS battery system has been performing peak shaving operations five days a week for the project duration. Those operations have been flawless. In addition, AEP has experienced several PQ operations that enabled it to assess the PQ functionality. Combined PS/PQ operation is a challenging task for the PCS. Analysis of those PQ events confirms the functionality of the hardware and provides insight to improvements to the control software.

Remaining project work includes analysis of the economics associated with combined PS/PQ operation and an estimate of life expectancy, this should be complete by early 2005. Additionally, AEP is looking for extra power system applications for the technology. Applications to be evaluated include transmission and distribution deferral, spinning reserve, and voltage support. In all cases, the benefits will derive from actual AEP avoided costs.