Energy & Power Generation Consulting,
Engineering & Management Services



Solar Centaur 40 Gas Turbine

PROJECT: Gas Turbine Refurbishment & Turbine/HRSG Controls Upgrade Project

This State Prison was a facility that had a single 3.0 MW Centaur 40 Solar Turbine exhausting heat into its own Heat Recovery Steam Generator (HRSG). Steam produced from the HRSG was used for facility heating, laundry cleaning, kitchen dish washing, and food preparation. The facility has a central power house that also serves as the electrical distribution hub for switchgear control between the gas turbine/generator, the local utility power, and additional high voltage load feeder breakers.

The original system was installed in the mid 1980's and ran almost continuously until the year 2005 at which time the turbine had in excess of 50,000 operating hours. Solar Turbine requires a complete turbine overhaul when the operating hours exceed this standard operating limit. The gas turbine/HRSG system was abandoned and steam was produced out of a single standby package boiler. Due to problems with this package boiler, that at times left the facility without heating steam, The State issued an emergency contract to refurbish the gas turbine, upgrade the turbine controls from the old hardwired relay logic to a PLC controlled system, and upgrade the HRSG controls from the old Bailey single loop controllers. Engineering services were provided to manage the turbine controls subcontractor and to be the primary control system design engineer for the HRSG controls upgrade.

Some of the key features used within the turbine controls were the standard start/stop logic but of special interest was the automated generation load control and the Wet-Nox controls. The engineering service was responsible to coordinate the turbine controls subcontractor on these key features since they did not have much experience with this type of generation in this west coast state. The load controls were design around the strategy of maintaining the paralleled utility power at a set point of approximately 100 kW. Therefore, as the prison facility's load would change, the gas turbine/generator would swing to meet the facility's load and maintain the utility power near it's operator set point.

The State air emission's requirements for this unit specified that for every pound mass of fuel burned an equivalent pound mass of water and was to be injected into the combustion process to maintain exhaust Nox levels within source tested standards. Therefore, engineering service created the control loop design using new process parameters to improve stability within the control loop. The previous Solar Turbine design required constant online operator interaction to maintain the one-to-one ratio of water to fuel. The new Nox controls have achieved zero operator interaction because of this improved design.

The Heat Recovery Steam Generator (HRSG) control system upgrade portion of this project provided an opportunity to install advanced energy anticipation control philosophy for a much needed requirement of stable steam generation. The previous control system was so unstable, the alarm acknowledge push button had been worn off the old controller faceplate! This HRSG configuration also employed the use of a natural gas fired duct burner to supplement the heat energy of the gas turbine exhaust for steam production. The new control system design utilized a similar mode of operation as the original design and was called "single element" control where this "element" was the steam pressure process variable. In addition, the new control logic design utilized a much more anticipatory control philosophy using four elements of process variables to correctly modify the energy demand signal to the diverter valve/duct burner combination in anticipation of process disturbances. The logic allowed the operator to conduct a bumpless "flying swap" between either of these modes. The improved stability was the first this site had seen since the original commissioning of the unit!

Lastly, the engineering service was the control system engineer that correctly identified the requirement to upgrade the existing gas turbine hydraulic starter system to an Allen-Bradley PowerFlex Variable Frequency Drive(VFD), provided the installation design sketches, coordinated the installation construction, determined and programmed the VFD configuration parameters, and successfully started and commissioned this device.


• Completed an assessment of the existing hardware logic to determine the scope of the control system upgrade to PLC control of the gas turbine speed governor, generator synchronization, and load sharing control.

• Reversed engineered the logic design for the HRSG diverter valve/duct burner controller and drum level controller using a handheld programmer to obtain the source code and then drawing up the logic in a functional logic drawing.

• Reverse engineered the application of the existing PLC ladder logic for the Solar Turbine hydraulic starter system and created modified logic design to correctly apply the start, stop, and speed set points for the new Variable Frequency Drive (VFD) unit.

PROJECT: State Prison Solar Turbines (2) Maintenance Services

This State Prison was a facility that has two 3.0 MW Centaur 40 Solar Turbines with each turbine exhausting heat into its own Heat Recovery Steam Generator (HRSG). Steam produced from the HRSG was used for facility heating, laundry cleaning, kitchen dish washing, and food preparation. The facility has a central power house that also serves as the electrical distribution hub for switchgear control between the two gas turbine/generators, local utility power, and four additional high voltage load feeder breakers.

Three years prior (at the end of 2004), The State completed a major turbine, turbine controls, and facility control system upgrade. The goal of the 2004 project was to fully automate the dispatching of the two gas turbine/generators based on facility loads such that the local utility power consumption was held at a fixed operator set point of approximately 300 kW. The operator would select the lead/lag assignment between the two turbines and the automatic controls would start/stop the lag turbine based on the facilities change in load demand always attempting to maintain 300 kW of "import" power from the local utility power source.

Engineering services were required to resolve instabilities resulting from the automatic controls not correctly controlling the gas turbines. The State was unsuccessful at forcing the original contractor to resolve final commissioning instabilities in the 2004 upgrade project.

The primary load and switchgear breaker control system logic was performed in PLC style controllers supplied by ENCORP. The Solar Turbine logic was resident in Allen-Bradley PLC-5's and obtained direct on/off digital commands and an analog kW set point from the ENCORP PLC's.

The engineering services successfully diagnosed numerous tedious logic errors, incorrect tuning values, and design flaws that all contributed to, at times, very severe load instabilities. Prior to these improvements, the units always ran in manual with the operator performing all load changes through 15 minute periodic inspections of the facility load requirements. Now the units run in full automatic and the site operators have stated they have never seen this good a performance in the machines since their original commissioning.


• Completed a full assessment of the multiple control systems used to attempt to automate load dispatch of gas turbin/generators, facility loads, and utility supplemental power. Completed work through use of breaking down site documentation.

• Reversed engineered the Allen-Bradley PLC-5 ladder logic used to convert the kW set point to the eventual fuel valve demand signal to the gas turbine into simple block diagram format. Includes speed control, over-rides, and closed loop feedback.