Problem Statement:

Arizona Public Service has requested a feasibility study to determine if it would be cost effective to replace the excitation systems on generators 1, 2, and 3 at the Cholla Power Plant with a static type.

A newer system may offer economic benefits through improved reliability and availability.  Also newer excitation/regulation systems offer enhanced performance and features that may lead to cost savings.

Objective:

Perform quantitative analysis to determine the feasibility and benefits of replacing the excitation systems on generator units 1, 2, and 3. This analysis includes but is not limited to:

• Current system operation costs
• Cost of new static excitation systems
• Predicted operating costs of new systems
• Determine the payback time for upgrading

Requirements and Specifications:

Mechanical
The wiring and interconnections to the system need to be safe and reliable.  The interconnections need to be capable of handling sufficient voltages and currents.  The excitation system should be mounted within an appropriate enclosure for safety.  Units 2 and 3 will require room for the installation of collector rings.  Other mechanical requirements may include semiconductor cooling and vibration damping.

Electrical
The excitation system needs to be capable of accurately controlling the rotor excitation current to regulate the generator’s complex output power.  The power bridge and control unit should have sufficient redundancy to accommodate online repairs.  The control system should have an external interface to monitor and tune operations.
 
UNIT 1

• 13,800 VAC RMS input
• 2183-2418 VDC at 375 VDC output

UNIT 2/3

• 22,000 VAC RMS input
• 3176 VDC at 425 VDC output

Environmental
The static excitation system should be robust to changes in environmental conditions: dust, humidity, vibrations, and temperature.  For reliability purposes the system should also be immune to EMI.

Documentation
Cost of installing the new rotor excitation system will be fully analyzed.  Vendor quotes, installation cost and predicted operation cost estimates will be compared with the existing cost model to determine feasibility. 

• Component engineering estimates
• Installation costs
• Operation costs
• Payback time

General
The costs analysis needs to clearly indicate payback time.  The new rotor excitation system for units 1, 2 and 3 need to be reliable to minimize down time and maintenance costs.  The reduced maintenance costs should pay for the upgrade within 8 years. 

System Diagram: Unit 1 Excitation System (Westinghouse 1961)

System Diagram: Unit’s 2 and 3 Excitation Systems (Siemens 1978, 1980)