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Turbine Generator Control Systems

Introduction

In the 1990's, co-generating plants in Pulp and Paper mills were expected to be key to greater profitability by allowing the mill to operate during a loss of power from the electric utility. Power loss at a pulp and paper mill results in a complete shutdown of the Kraft Recovery cycle due to the loss of the steam plant/powerhouse. Days of lost production resulting from a loss of power can have substantial financial impact.

The negative effect on the mill production cycle can be minimized with the use of appropriate control strategies and load shedding. Once service is restored, the mill can resynchronize with the electric utility and resume production. This strategy means that prime production losses can be measured in hours, not days.

Objectives

There are five primary objectives in the development of a strategy to manage the impact of a complete loss of power from the electric utility:

• Replace the existing turbine and generator controls with a modern, digital system using standard components;
• Develop a load shedding strategy that allows for the turbine and generator to survive a power loss and re-synchronize once the tie-in is re-established;
• Manage the VAR import within the specifications of the utility rate requirement while minimizing generator rotor heating
• dynamically share the load between multiple units for steam header and bus voltage control;
• Maximize utilization of the mill process heatsink to recover power while accurately controlling the header pressures.

To achieve the desired objectives, the project scope focuses on evaluation and upgrading or replacement of equipment in four major areas:

• Governor;
• Excitation;
• Generator protection;
• Operator controls.

Results

The results documented here are from the Northwood Pulp Mill. Under controlled conditions the integrated control system successfully passed all tests. Most importantly when an actual sudden loss of power occurred at a pulp and paper mill under normal operating conditions, the new controls maintained a power and steam balance and continued to operate. The integrated control system was successful in dynamically sharing the steam pressure and voltage control functions and the mill was able to fully realize the benefit of process heatsink. The mill was also able to automatically control circulating VARs and utilize the provisions of the utility contract.

Overall, the Northwood Pulp Mill increased generation capacity by 2-3 MW, resulting in a net savings of $500,000 per year.