Siemens’ contract scope for Patriot covers the supply of single-shaft  power plant blocks, including two SGT6- 8000H gas turbines, two SST6-5000 steam turbines, two hydrogen-cooled SGen6-2000H generators, and two heat recovery steam generators, along with the SPPA-T3000 control system. The gas turbines and generators were manu- factured at Siemens’ Charlotte Energy Hub, the global base for the company’s 60 Hz power generating equipment.


Siemens will also provide mainte- nance and service for the main compo- nents associated with the gas turbine under a long-term service program. Parts, inspections, and scheduled ser- vice/maintenance, along with Siemens’ Power Diagnostics remote monitoring, are included in the service agreement.


Suited to the market

The decision to use an H-class gas tur- bine in a single-shaft configuration was largely down to the high base load effi- ciency. According to Siemens the design is capable of achieving 60% base load efficiency at ISO conditions.


Porter explained the attraction of the machines. “The way the  units  are  set up is to run every available hour of the day.  During  the  time  of  selection, the


Siemens H machines were the most ef- ficient out there, certainly in combined cycle mode. We expect the units to run base load in the most efficient manner.”


Having two combined cycle blocks, each with a single-shaft power train,  also gives Panda operational flexibility should it be required. Essentially, the configuration means the plant can run   at 50% load without losing efficiency. At the same time, each combined cycle block can be run up and down quickly – an operation that is supported by the inclusion of a synchronous self-shifting (SSS) clutch between the generator and the steam turbine.


The SSS clutch enables the gas tur- bine to start-up and heat the HRSG inde- pendently of the steam turbine, making faster starts possible. When the steam turbine reaches synchronization speed, the SSS clutch is engaged and transfers the steam turbine’s torque to the genera- tor so the plant can reach full load.


Having this flexibility not only means that Panda can respond to the needs of the market but also allows it to contin- ue operating during times of scheduled maintenance outages.


“During   these   times,   we   can still operate half the plant,” said Porter. “So far the plant has operated at base load and in the foreseeable future, I don’t an- ticipate a situation where we will have  to operate at a reduced load because of market conditions. But you can’t always predict the future.”


Jacqueline Engel, Product Manager for Siemens Energy Americas, added that a single-shaft set-up also has eco- nomic benefits.


“If you look down a single-shaft power train, there’s just one generator unlike a multi-shaft configuration where you could potentially have two. Siemens has a proven track record of single-shaft designs around the world and the US, so commissioning this configuration in the US is really pulling from that successful track record,” she said.


The SGT6-8000H has a design output of up to 310 MW and is capable of deliv- ering an efficiency of more than 60 per cent electrical efficiency in combined cycle. Since the start up of the first com- mercial H-class machine at Irsching in Germany, Siemens’ H-class units have accumulated more than 350 000 operat- ing hours globally.


Porter noted: “Having this experience and comfort level was important to us, and we plan to use this technology in an upcoming power project.”

Panda Liberty combined cycle plant is a sister plant to Panda Patriot and was the first specifically sited to utilize Marcellus Shale.

H-class design

The SGT6-8000H portion of the  fleet has accrued more operating hours than its 50Hz counterpart but as it follows a direct scale design methodology, users can have the same level of confidence   in the design and performance of both. The main difference between the 50 Hz and 60 Hz machines is the scaling fac- tor which means the 50 Hz units have a higher power output.


The SCC6-8000H is a single-shaft machine designed to deliver about 440 MW in combined cycle operation. It has a common design with the 50 Hz ver- sion, using a single tie-bolt rotor design adopted from previous F-class gas tur- bine models.


However, compared to F technol- ogy, firing temperature is increased by about 200°C. The turbine therefore has  a firing temperature in the 1500°C class, which contributes to the higher turbine efficiency.


Engel said: “The turbine allows the plant design to achieve these high levels of output and efficiency by managing airflow, as opposed to relying solely on higher operating temperatures.”


The turbine includes: a disc-type ro- tor with central tie-bolt and radial ser- rations; two journal bearings and one thrust  bearing;  generator  drive  at  the compressor intake end; and axial ex- haust diffuser.


Designed to combine high efficiency with operational flexibility, the turbine rotor is internally air-cooled. Unlike with steam cooling, this means that cooling is decoupled from any water/steam cycle or any other external cooling system.


It has a 13-stage axial compressor operating at a pressure ratio of 20:1 at ISO conditions. It has four variable in- let guide vanes as well as cantilevered vanes, which further increases its op- erational flexibility. The variable guide vanes allow adjustment of the mass flow to help maintain efficiency at low turn- down if necessary.


The power turbine section of the ma- chine has four stages, with air-cooled blades and vanes on the first three stages and thermal barrier coatings on the first and second stages. Stage-four is not cooled. The internally air-cooled blades in the first three rows have fine holes to allow air to escape and flow across the blades for film-cooling.


Their shape has been designed so that the gap between the blade-tips and tur- bine casing is as small as possible. This ensures the maximum amount of gas passes through the blade area. The size of this gap is kept to a minimum during operation by Siemens patented Hydraulic Clearance Optimization (HCO) system originally introduced in the SGT-4000F gas turbine series.


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