A solution for stability

Tim De Vries, Golden Valley Electric Association, Jim McDowall, Saft, Niklaus Umbricht, ABB and Gerhard Linhofer, ABB

The recent blackout in the northeastern US and Canada has shown the importance of a reliable power supply. On August 26, 2003, the world’s largest battery energy storage system (BESS) was inaugurated in Fairbanks, Alaska. The BESS will stabilize the grid and reduce the outages Golden Valley’s customers experience by 65 per cent.

Golden Valley Electric Association (GVEA) is a rural electric cooperative based in Fairbanks, Alaska, serving

Figure 1. Battery module, containing 10 battery cells
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90 000 residents spread over 2200 square miles (5700 km2). Back-up power is essential to the local population due to the extremely low temperatures, which in winter can fall to -51à‚°C. A dwelling without power under these conditions will begin to experience frozen pipes in just a few hours. Traditional solutions for producing reserve power require building and maintaining transmission and generation capacity well in excess of normal demand, so a battery energy storage system (BESS) supplied and installed by an ABB-led consortium represented a cost-effective and efficient alternative for GVEA.

At the heart of the world’s most powerful storage battery system are two core components. The converter designed and supplied by ABB and the Nickel-Cadmium (Ni-Cd) batteries, developed by Saft. The converter changes the batteries’ DC power into AC power ready for use in GVEA’s transmission system. The batteries are the energy storage medium.

In operation, it will produce up to 27 MW of power for 15 minutes, which allows the utility enough time to bring back-up generation on line. While the BESS is capable of producing up to 46 MW for a short time, the client’s primary need for the system is to cover the 15-minute period resulting from sudden loss of generation and start-up of back-up generation.

Although the BESS is initially configured with four battery strings, it can readily be expanded to six strings to provide a full 40 MW for 15 minutes. The facility can ultimately accommodate up to eight battery strings, giving considerable flexibility to boost output or prolong the useful life of the system beyond the planned operation for 20 years.

Project requirements

The final specification required that the vendor provide a turnkey BESS. GVEA chose not to specify a particular battery type or dictate the type of power conversion equipment that could be proposed. The specification was written as a performance-based specification, with the vendor responsible for finding the right partners for the various subsystems, finding a battery manufacturer, coordinating building demolition and construction, and guaranteeing that the installation would work. The specification required that the vendor guarantee for 20 years that the BESS could supply 40 MW for 15 minutes, with a 4 MW/min ramp down after the 15-minute mark. The BESS is required to be capable of operating in all four quadrants (i.e. full power circle) and to provide continuous, infinitely adjustable, control of real and reactive power over the entire operating range. The specification also required that the BESS be able to operate in automatic mode, as GVEA does not plan to man the facility.

GVEA required the BESS to provide rated output for the following power system characteristics:

  • Nominal voltage: 138 kV (1.0 p.u.)
  • Normal sustained voltage: 0.90 p.u. (min) and 1.1 p.u. (max)
  • Normal frequency: 60 Hz with normal deviation of +/- 0.1 Hz
  • Sustained frequency range: 59.0 Hz (min) and 60.5 Hz (max).

The BESS is able to operate in seven distinct modes, detailed below.

Figure 2. Principle diagram of the BESS electric system
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VAR support: This mode requires that the BESS provide voltage support for the power system under steady state and emergency operating conditions.

Spinning reserve: This mode requires that the BESS respond to remote generation trips in the Railbelt system. Spinning reserve mode will initiate at a system frequency of 59.8 Hz with the BESS loading to full output at 59.4 Hz if system frequency continues to drop. Spinning reserve has the highest priority of all modes.

Power system stabilizer: The control for the BESS includes a power system stabilizer to damp power system oscillations.

Automatic scheduling: This mode is used to provide instantaneous system support in the event of a breaker trip on either a transmission line or a local generator. The BESS has 30 independently triggered inputs, which will be tied remotely to the trip circuits of breakers.

Scheduled load increases: This mode is initiated and terminated by SCADA and puts the BESS in a frequency and voltage regulation mode to allow it to respond to the addition of large motor loads.

Automatic generation control: This mode requires the BESS to be capable of operating by Automatic Generation Control (AGC) similar to that of rotating machinery.

Charging: This mode allows the SCADA dispatcher to control how fast the BESS will be charged and when the charging will start after a BESS discharge event.

Since the BESS is critical to the stable and economic operation of the system, it must be available for use to the maximum extent possible. An 18-month availability guarantee period was implemented where the ownership and operation transfer to Golden Valley, but the maintenance responsibility stays with the vendor. If the BESS fails to meet the required availability, the 18-month period is extended until 18 consecutive months at the required availability is achieved. Each BESS proposal was evaluated over a 20-year life span. To accomplish this it was necessary to determine how many, if any, complete change-outs of the battery would be required. Each vendor was required to provide battery life information based on the operating descriptions and outage data contained in the specification and to guarantee that.

The two partners, Saft and ABB, provided an overall optimized turnkey solution. The two key components were the battery from Saft and the power electronics system from ABB. The battery is the energy storage medium and the power electronics system is the intelligent interface between the battery and the electric grid operated by GVEA.

The battery

The battery, which is manufactured from recycled cadmium is safe, reliable and will be recycled again at the end of its 20-year life cycle.

The Alaskan BESS battery comprises a total of 13 760 Saft SBH 920 high-performance rechargeable nickel-cadmium cells arranged in four parallel strings to provide a nominal DC link voltage of 5000 V and a storage capacity of 3680 Ah. The cells are built into ten-cell modules that are contained in a drive-in racking system. Rack sections are arranged in a narrow-aisle configuration, with installation and service access via an operator-up swing-arm fork truck.

The complete battery weighs about 1300 tonnes and the size of the battery room is 120 m x 26 m. The initial battery configuration comprises four individual strings operating in parallel, with expansion capability to eight strings. Each string has 3440 cells, type SBH920, connected in series. The basic unit of the battery is the SBH920 cell, a high performance, pocket plate nickel-cadmium cell from Saft’s standard production range.

The battery uses reliable pocket plate construction, with thin, high performance plates. This design allows the full 20-25 year life to be attained without any loss of the beneficial characteristics of Ni-Cd batteries. The Saft SBH cell type can deliver 80 per cent of its rated capacity in 20 minutes. Ni-Cd pocket plate cells can withstand repeated deep discharges with little effect on battery life.

To provide adequate insulation for this high voltage system, a PE liner at the inside of the module case (between battery cells and metal tray) is used. Each module is fitted with a self-contained single-point filling system, allowing all ten cells to be topped up in a single operation without removing the module from the racking. Connections between modules are made using large nickel-plated copper bus bars. The open cell terminals for each module are fitted with adapters to facilitate these connections.

The battery monitoring system was supplied by Philadelphia Scientific Inc. The system measures, records and reports on module voltage, string current, cell electrolyte level (one cell per module) and cell internal temperature (also one cell per module).

The electrical system

Four battery strings are presently installed. All preparations have been done already to allow an extension to up to eight battery strings at a later stage. Every string (and sub-string) can be switched off and isolated by DC switches. Two switches per sub-string, one at each end, to permit complete isolation from the rest of the system. Two disconnectors allow separation of the battery and the DC link of the converter in case of maintenance work on the batteries. The converter can stay in operation and provide reactive power to the grid for voltage control. Filter circuits in the DC link eliminate the potential danger of resonances at higher frequencies should any harmonics be generated in the grid by non-linear loads.

Figure 3. Cycling characteristics of the battery. Number of charge-discharge cycles vs. depth of discharge
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The voltage source converter, which is the heart of the electrical system, is built up from standardized PEBBs (Power Electronic Building Blocks). One double-stack PEBB in NPC connection (Neutral Point Clamped) forms a single-phase H-bridge. Four H-bridges per phase are installed, altogether 12 single bridges. The stacks are cooled by a closed loop cooling circuit with de-ionized water. Every bridge is connected to its dedicated transformer winding. In the transformer the voltage contribution of the bridges are added. This results in an excellent voltage wave shape, similar to the voltage quality one could expect from rotating machines. Voltage limiter units ensure that the DC link is protected against any overvoltages, which might appear due to sudden load rejections, or any disturbances in the electric grid.

The main components, converter and transformer, are already designed and built for handling the total power when the battery gets extended from four to eight strings.

The active switching elements used in the converter are Integrated Gate Commutated Thyristors (IGCTs). These are advanced Gate Turn Off Thyristors (GTOs). Compared to other elements with switch-off capability, IGCTs have the advantage of lower conduction and switching losses, and in addition to that allow a snubber-less converter design because of their superior switch-off characteristics.

This converter design offers the following advantages:

  • Development time is greatly reduced by implementing well-known techniques.
  • The three level medium voltage modules have proven to be highly reliable products. Its application leads to low FIT values.
  • The use of the double stack modules allows short distances between the power semiconductors, which offers low stray inductances, and reduces the space requirement for the complete converter.
  • The clamp diodes and capacitors are integrated in the semiconductor stack. In this way the stray inductances in the clamp circuit are also minimized, allowing use of higher IGCT switch-off currents.
  • One single clamp for two phases reduces the need for bulky and costly clamp inductors and resistors.
  • Ease of serviceability is a primary consideration during mechanical design of the converter. To guarantee an individual semiconductor replacement, all power semiconductors in the stack are readily accessible.

The system is designed for four-quadrant operation. It can charge as well as discharge the battery (absorb power from and supply power to the grid) and it can absorb reactive power from or supply reactive power to the grid. It is designed to be able to do that with the DC link voltage varying according to the charging condition of the battery.

The control system

Local system control is provided by an ABB Spider MicroSCADA human machine interface (HMI) based on the Microsoft Windows operating system. The system is operated by the use of pictures, windows and function keys with the aid of a mouse and a keyboard. The MicroSCADA system is used in applications around the world for substations, large rectifiers, and static frequency converters.

Sequence and closed loop control, and complete protection are done by ABB’s programmable high-speed controller (PHSC). The PHSC system is freely

programmable with the graphic function plan program FUPLA. The PHSC system fulfills the requirements for high availability and reliability. PHSC reliability has

been demonstrated as suitable for both system control and protection in numerous applications currently in operation.

Figure 4. A battery module on the move
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World record

During commissioning tests the ABB power conversion system and the Saft battery set an unofficial world record by achieving a peak discharge of 26.7 MW with just two strings operational, making use of the short time overload capability of the battery modules. This makes the Alaskan BESS over 27 per cent more powerful than the previous record holder ” a 21 MW BESS commissioned by PREPA (the Puerto Rico Power Authority) at Sabana Llana, Puerto Rico in 1994.

Three primary areas of benefit have been identified for the BESS system:

Figure 5. BESS control and protection system overview
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  • T&D: voltage regulation, first swing stability, loss reduction.
  • Generation: spinning reserves, ramp-rate constraint relief, load following, black start, load leveling, reduced or deferred turbine starts.
  • Strategic: improved power quality, reduced demand peaks, enhanced service reliability through reduced power supply generated outages.

The primary benefit was the ability of the BESS to instantly contribute to system stability following the loss of a major transmission line or generator. The BESS does also provide spinning reserves that will, potentially, allow generation units to be run at lower levels or shut down entirely, resulting in significant savings.

Almost instantaneous active power is available. There are cases where the BESS has to ramp up prior that the impact of a generator loss can be seen at the PCC (Point of Common Coupling).

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