Australian Paper Mill - No.8 STG Upgrade Project

Following on from the successes of upgrading of No.7 Steam Turbine Generator (STG) in 2008 and No.6 STG in March 2010, Energy Power Consulting Engineers was awarded the contract by Australian Paper Mills to upgrade No.8 STG at Maryvale Mill in late 2010.

While No.8 STG is also a backpressure Stal Laval contra-rotating radial-flow turbine, unlike No.6 and No.7 STG’s, it is also an extraction machine allowing for steam to be extracted to an intermediate steam pressure range.

No.8 Steam Turbine Generator (STG) also has a three-phase synchronous generator connected at each end of the turbine. The generator rotors are directly and solidly coupled to the turbine. The two generators are electrically identical and connected in parallel. Since they have the same phase sequence and rotate at the same speed, the generators have the same characteristics as a single generator. The excitation for the generators is obtained from an AC exciter, connected to one of the generator rotors.

As a Turnkey Contract, Energy Power Consulting Engineers provided all the design, engineering, installation and Basler Electric DECS 200N Automatic Voltage Regulator (AVR) system, ABB S800 I/O, uncontrolled bridge rectifier (for the excitation) and the redundant 24Vdc battery charger system.

The design and engineering services included:

• Detailed Arrangement and Layout Drawings

• Detailed Schematics

• DCS I/O drawings

• Control System Functional Description and Operating

• Manuals

• Protection co-ordination study

Programming of Generator Protection Relays and program scheme logic to ensure compliance in accordance with the relevant standards. The new control system comprises ABB S800 remote I/O modules connected on an H-Net fibre network to an existing Bailey Distributed Control System (DCS) platform.

No.8 STG is designed such that the primary method of control is from the existing central control room Human Man Interface (HMI) located in the Powerhouse. The HMI provides the Operator with an overview of all No.8 STG systems. In addition, No.8 STG can also be controlled locally from a control console located adjacent to the machine.

A significant improvement with respect to the turbine control has been gained with the installation of an electrically controlled actuator. To facilitate this improvement a number of the existing control oil solenoid valves were removed from service while others were replaced with new ISO solenoid valves.

Another significant improvement has been the removal of the uncontrolled rectifier bridge previously located below the rotating exciter and the installation of new uncontrolled bridge in a floor standing control panel located in an air-conditioned room. Previously, carbon dust falling from the brushes on the rotating exciter would lead to significant outages either as a direct consequence of a short circuit on the uncontrolled bridge or for maintenances reasons while excess carbon dust was removed from the uncontrolled bridge.

The complete upgrade of the governing and excitation systems and the hydraulic, lube and control oil systems allowed the STG to be started, operated and stopped from a single switch. Unlike No.6 and No.7 STG’s, the existing hydraulic servo-motors were much larger than those installed on No.6 and No.7 STG’s and it was deemed too expensive to replace the servo-valves with new hydraulic amplifiers. Consequently, the servo-motors were modified to allow the fitment of electric actuators to replace the hydraulically actuated servo-valves. No.8 STG is able to operate in base load mode, load share mode (parallel to the Grid or isochronously islanded from the Grid), back pressure control mode (maintaining a preset exhaust steam pressure) or in extraction mode (maintaining a preset extraction steam pressure).

The upgrade of No.8 STG was successfully completed and commissioned.