TECHBriefs 2007 No. 3 6 Burns & McDonnell

bidding. After technical and commercial reviews of the bids, a contract was awarded to Ziegler Power Systems to supply six 2,100-kW Caterpillar diesel generators and associated equipment.

As the project continued, a number of challenges arose. The electrical and control systems had to be designed to allow automatic starting and loading of a diesel generator

By Mark Caldarello, PEIndustry deregulation, increasingly loaded transmission systems and power flows resulting from unplanned market mechanisms have increased the possibility of blackouts. Following a complete loss of system power, it is necessary for one or more power plants to restart in an island mode and provide power to neighboring power plants, allowing them to start up and complete system restoration. For these and other reasons, Alliant Energy equipped its new Emery Generating Station near Mason City, Iowa, with black start diesel generators.

The 550-MW plant includes two GE 7FA dual fuel combustion turbine generators, two Alstom heat recovery steam generators, and one GE D11 steam turbine generator. After some investigation, it was discovered that General Electric had no records showing that a 7FA gas turbine had ever been black started. There were cases where similar sized machines had been black started overseas using a small on-site battery-started turbine to provide starting power. There also were cases where large gas turbines equipped with starting motors had been black started using on-site diesel generators, but the 7FA equipped with a static starting system did not have any such record. The Emery Generating Station would be the first U.S. project to have that capability on a General Electric 7FA. (See Table 1.)

Design RequirementsThe electrical power required to start up a power station depends on the type and size of loads involved. For a combined-cycle plant such as Emery, the mechanical condition of the heat recovery steam generator at the time of the blackout determines which loads will be required. For design purposes, a load list was developed using worst-case conditions. In addition, loads were included that would allow the plant to start up on fuel oil in case the gas supply wasn’t available. (See Table 2.)

A specification was developed and sent to diesel generator suppliers for competitive

PowerIowa Black StartSuccessful Planning Prepares Plantto Recover from System Blackouts

Table 1: A profile of Emery Generating Station and its black start equipment.

Total Plant Black Start Equipment

Cost $400 million $5 million

Power Output 550 MW 12.6 MW

Plant Configuration 2 x 1 combined cycle 6 diesel generators

Machines GE7FA(CTG)GED11(STG)

Caterpillar 3516B

Fuel Natural gas and fuel oil Fuel oil

On-line Date April 28, 2004 Aug. 21, 2004

Table 2: Black start loads.

Load Description Load Type Load Size

Lighting and Heating Resistive 200 kW

CTG Lube Oil Pump Motor 100 hp

Air Compressor Motor 100 hp

Boiler Feed Pump Motor 3400 hp

Condensate Pump Motor 350 hp

Circulating Water Pump Motor 1600 hp

Closed Cooling Water Pumps Motor 400 hp

Fuel Oil Heaters Resistive 805 kW

Liquid Fuel Pump Motor 250 hp

Auxiliary Boiler Fan and Feed Pump Motor 90 hp

HRSG Recirculating Pumps Motor 50 hp

Atomizing Air Compressor Motor 500 hp

Load Commutated Inverter (LCI) 12 Pulse Inverter 3500 kW

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in an emergency. Harmonics issues had to be evaluated. GE turbine controls had to be changed. And a multi-disciplined project team had to plan, design and implement every detail.

Automatic LoadingAlthough the term “black start” headlines this project, the diesel generators also serve as standby generators that automatically start and provide power to essential plant equipment during temporary power outages. The automatic starting and loading of one diesel generator is the first step in the black start sequence, but since most power outages are not widespread blackouts, the sequence will usually end after normal power is restored, allowing a conventional plant startup to be completed. During power outages, the diesel generators provide power to essential loads that keep the turbines on turning gear, maintain lube oil pressure, and keep the generator seals pressurized to allow for an immediate restart once power is restored. Without power for these essential loads, the turbines would have to cool down for several days before the rotor bow could be worked out and the generators would have to be refilled with hydrogen before a restart. (See Table 3.)

Providing the ability to automatically serve essential loads from one of the diesel generators required planning from the initial stages. All

essential loads had to be segregated from non-essential loads. Every essential motor, charger, heater and lighting panel had to be supplied from one of four motor control centers, and the power distribution system had to be structured to allow the essential motor control centers to be automatically connected to a diesel generator in an emergency. To avoid overloading a diesel generator, a control system was required to automatically strip non-essential loads and then supply the essential motor control centers in 15 second intervals to prevent excessive starting current.

GE Controls ChangesSince the combustion turbines were not purchased with black start capabilities, some equipment and software changes were required. The combustion turbine and control system needed the ability to start up in an island mode, close the generator breaker to a dead bus, remain in island mode after the generator breaker closed, and continue in island mode while switchyard and transmission system breakers closed. To accomplish this, under-voltage relays were added in the GE generator control panel, additional control wiring was added between the turbine control system and the generator breaker, and software modifications were made that allowed an operator to select island mode start and enable dead bus closing from the operator station.

For more information, please e-mail: mcaldarello@burnsmcd.com

Mark Caldarello, PE, is a senior electrical engineer at Burns & McDonnell. He has 11 years of experience with power generating stations across the United States and is registered in Florida, Kansas, Minnesota, Texas and Utah. He has a bachelor’s degree in electrical engineering from the Georgia Institute of Technology.

Table 3: Essential loads.

Loads Purposes

Turbine Loads Lube oil pumpHydraulic pumpSeal oil pumpLube mist separatorTurning gearTurbine cooling fansBattery charger

• Maintain the turbines on turning gear with lube oil• Prevent a rotor bow• Keep the generator seals pressurized• Prevent a hydrogen purge from the generator• Allow for an immediate restart once startup power is

restored• Maintain DC power for control

Diesel Generator Loads

Battery chargersFuel oil systemSpace heatersBuilding lighting

• Maintain DC power for starting• Fuel oil heating and transfer• Preventcondensationinequipment• Safetyandoperations

Other Plant Loads

Battery chargerSelected HVACHeat tracingSpace heatersSelected lighting

• Maintain DC power and UPS power• Conditioned spaces for sensitive electronics• Freeze protection• Prevent condensation in equipment• Safety and operations

TECHBriefs 2007 No. 3 8 Burns & McDonnell

Detailed DesignNumerous equipment shop drawings were reviewed for technical compliance and then used to complete the final design of everything from equipment foundations to relay settings. Details were coordinated for the design of the fuel system, building layout, HVAC, underground conduit, circuit breaker controls and synchronizing schemes. Although the result was one of a kind, the formula for success wasn’t anything new. As always, it took detailed interface engineering, timely decisions, and change management as well as close coordination with the client, vendors and contractors.

Test RunA test run was conducted on Aug. 21, 2004, with harmonic analysis equipment in place. (See Table 4.) Switchyard breakers were opened to simulate a blackout and the first diesel generator went to work automatically providing power to essential loads. After an operator pressed the “black start” button, all six diesel generators went into action. The plant auxiliary equipment was started as normal and at 10:03 a.m., an operator gave a start command to

the combustion turbine. The LCI cranked the generator and 15 minutes later, the turbine purge cycle was complete. Seven minutes later, the turbine was at full speed, and eight minutes later, the generator breaker was closed onto a dead bus. The whole operation went flawlessly.

The black start demonstration was another accomplishment for a plant successful in schedule, budget and performance. Hopefully, the region will never experience a major blackout, but if it does, Emery will be ready.

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Table 4: Test run data.

Before Turbine Start During Turbine Start

Diesel generator load 4080 kW 8300 kW

Largest loads started by the diesel generators

Boiler feed pump (3400 hp)

LCI static starter (3500 kW)

Peak current 1919 A 1400 A

Minimum voltage 3260 V 3627 V

Voltage total harmonic distortion 0.9% 6%

Current total harmonic distortion 1.7% 3.7%