World's biggest battery helps to stabilise Alaska

5 July 2002

A battery energy storage system is under construction to provide voltage compensation in support of Alaska's Northern Intertie. Tim DeVries, Golden Valley Electric Association, Fairbanks, Alaska, USA

Because of its isolation and limited back-up resources, black-outs are a fact of life in Alaska, which, electrically speaking is an island. Golden Valley Electric Association is an Alaskan rural utility co-op, serving 90 000 residents spread over 2200 square miles, with 2200 miles of distribution line, 336 miles of transmission, 25 MW of coal-fired generation and 200 MW of diesel and oil fired gas turbine generation.

In 1985, the 186 mile 138 kV Intertie went into operation, linking the Anchorage utilities, in the south, with the GVEA system, at Healy. This gave GVEA access to low cost natural gas and hydro, but also increased its exposure to generation outages and transmission line disturbances in the central and southern load centres. The Intertie can handle 77 MW securely.

Recognising that the State of Alaska needed to help the utilities with electrical infrastructure, the 1993 Alaska State Legislature approved a grant for a second transmission line between Healy and Fairbanks, called the Northern Intertie, with a capacity of 140 MW. This would be in addition to the existing Healy-Fairbanks trsansmission line, which has a capacity of 100 MW.

For the Northern Intertie project, an Intertie Participants Group was formed that included Homer Electric Association, Matanuska Electric Association, Anchorage Municipal Light and Power, Fairbanks Municipal Utilities Services, Chugach Electric Association, the City of Seward and GVEA.

A study undertaken for this project on the best way to provide voltage compensation looked at more than a dozen options before selecting a battery energy storage system (BESS) as the preferred option.

Life cycle cost analysis suggested that a BESS would provide the greatest benefits. Three primary benefit areas were identified: T&D (voltage regulation, first swing stability, loss reduction); generation (spinning reserves, ramp-rate constraint relief, load following, black start, load levelling and reduced or deferred turbine starts); and strategic (improved power quality, reduced demand peaks, and enhanced service reliability through reduced power supply generated outages).

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

Sizing the BESS

In the early 1990s, the State of Alaska was involved in a new DOE clean coal technology power plant, HCCP, to be built next to GVEA's existing coal fired power plant in Healy. This power plant, when combined with the Northern Intertie, would allow the secure import of 140 MW into the Fairbanks area. If the new Northern Intertie tripped, the existing transmission line could handle 100 MW securely and the BESS would be expected to pick up the rest, ie 40 MW.

Combustion turbines can then be brought on line within 15 minutes, so the amount of time that the BESS would have to supply energy was determined to be 15 minutes. At the end of 15 minutes the BESS would be backed off by the generation that was now on line. A conservative 4 MW/minute ramp down rate was selected for the BESS to give adequate time for the turbines to respond.

GVEA was extremely interested in the 40 MW size due to the number of power supply outages it is exposed to. Since GVEA imports a large portion of its power from Anchorage over the Intertie, any loss of generation in Anchorage or faults on the Intertie constitute power supply outages. By sizing the BESS at 40 MW it was estimated that about three quarters of the power supply related outages that GVEA typically experienced would be eliminated.

Specification and evaluation

A technical specification for the BESS was drawn up, starting from an old EPRI spec, modified in the light of experience with the battery system owned by Puerto Rico Electric Power Authority (PREPA), input from Abbas Akhil with Sandia National Laboratories, Phil Symons with Symons/EECI, Black & Veatch and GVEA.

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 document was written as a performance based specification. The vendor would be responsible for finding the right partners for the various subsystems, finding a battery manufacturer, co-ordinating building demolition and construction, and guaranteeing that the installation would work.

The specification required that the vendor guarantee for twenty 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 (ie 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 an automatic mode, as GVEA does not plan to man the facility.

The BESS is required to provide rated output during the following, normal, system conditions: nominal voltage = 138kV (1.0 pu); normal sustained voltage = 0.90 pu (min) and 1.1 pu (max); normal frequency = 60 Hz with normal deviation of ± 0.1 Hz; sustained frequency range = 59.0 Hz (min) and 60.5 Hz (max).

GVEA also requires the BESS to operate, without damage, under the following emergency conditions: emergency 5 minute voltage swings = 0.60 pu (min) and 1.15 pu (max); switching transient = 1.4 pu for 1 cycle; emergency frequency swings = 50 Hz (min) and 65 Hz (max).

GVEA asked that the BESS be able to operate in seven distinct modes: VAR support; spinning reserve; power system stabiliser;

automatic scheduling; scheduled load increases; automatic generation control; and charging.

The specification required the vendors to supply information on: battery replacement; maintenance hours; total energy in; self-discharge losses; charging efficiency; reduced capacity time; efficient use of building space; and anticipated cycling loads.

The specification was issued as a request for proposals in November of 1999, with the goal of having the BESS construction completed at about the same time as the Northern Intertie transmission line. Proposals were received in February of 2000.

The evaluation process was complex, involving GVEA people from purchasing, operations, power supply, dispatch, engineering, accounting, as well as a consultant. The major areas evaluated were: life cycle; warranties and guarantees; operational considerations; overall design; risk; commercial and contractual issues; flexibility of the proposed design; corporate pro-activeness; and exit strategy. These major areas were weighted and assigned points, with the points adding to 100.

During the evaluation process, however, it became clear that the Northern Intertie would be delayed over environmental permitting issues. GVEA decided to postpone the BESS selection process until it became clear when the permitting issues would be resolved.

When the Northern Intertie environmental hurdles were cleared, in March 2001, ABB, with their partners ABB Industrie (Switzerland) and Saft (France), were notified that they were the vendor of choice. After four months of negotiation the contract was signed on October 2001

BESS design

The BESS is comprised of the power converter system (PCS), the battery itself, and converter transformers. It is being installed in an existing 500ftx120ft building. The PCS interfaces the battery to the AC system using standard ABB medium voltage three level 2-phase modules using integrated gate-commutated thyristors (IGCTs). Maximum sustained output out of the PCS is 46 MVA. The PCS is water cooled, using a SwedeWater fine water/raw water system.

The fine water system flow is over 400 gal/min of 100 per cent dimineralised water. A heat exchanger is used in the raw water (glycol) side to dissipate heat, while the raw water side has external cooling towers for heat dissipation. The primary control of the PCS will be handled by an ABB programmable high-speed controller (PHSC) with local system control and interface to GVEA's SCADA system handled by redundant ABB micro SCADA systems.

The battery consists of four strings (expandable to eight) connected in parallel with each string containing 3 440 cells arranged in 10 cell modules. Each cell is a Saft type SBH920 liquid filled Ni-Cad. This is a high performance, 920 amp-hour, pocket plate Ni-Cad cell that can with stand repeated deep discharges with little effect on battery life. Each cell is 16in high x 21in long x 8in wide and weighs 159 pounds. There will be a total of 13 760 cells installed and 10 spare modules (100 cells) on site for maintenance purposes.

The strings are connected to a +/- 2500 volt bus. This bus is sized to handle the 12 000 amps output from the battery as the cell voltage approaches 1 V at the end of a 40 MW discharge. Nominal float voltage for the battery is 1.4 V/cell or 4816 V at the buss. The recharging voltage is limited to 1.45V/cell or 4 988 V at the buss.

Each module will be tied to the battery monitoring system (BMS). At each module a Philadelphia Scientific Sentry unit will monitor the module voltage, electrolyte level in one cell, temperature in one cell and the presence of any liquids in the bottom of the module. Each Sentry Unit will relay its information to a single Sergeant Module dedicated to each string. The Sergeant module will collect data from each Sentry Unit and then send the information to BMS supervisory computer. The Sergeant module will also monitor string current and ambient temperature.

As GVEA entered into contract negotiations with ABB it became clear, from a system standpoint, that 40 MW would not be needed from the BESS. The new 40 MW number had been developed assuming that the new power plant, HCCP, would be on line by the time the BESS and the second transmission line were constructed.

However, with no clear indication as to when HCCP would be brought on line, GVEA decided to delay purchasing two battery strings. This reduced the guaranteed output of the BESS to 26.67 MW. This is a significant reduction but GVEA should still be able to eliminate 68 per cent of the power supply outages experienced by our members.
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