Simple cycle strikes gold

The Western 102 power plant is part of a new era of modern genset power plants growing in popularity in the USA. As the largest power plant of its type in the country, Western 102 will demonstrate the flexibility, efficiency and reliability of gas engine technology.

Dennis Finn, Wärtsilä North America

nother large natural gas fueled reciprocating engine power plant is rising in the western United States. This 115.6 MW facility, located near Reno, Nevada, is owned by Barrick Goldstrike Mines Inc. It will join the Plains End plant (111 MW) and the Red Bluff plant (50 MW), as part of the new era of large modern era genset power plants supplying electricity to the US transmission grid.

The Western 102 power project was contracted in late October of 2004 and is scheduled to start commercial operation in the fourth quarter of 2005. Western 102 consists of 14 gensets, each with an output of 8.439 MW at site conditions of 35à‚ºC and 1324 m above sea level. Even under these stringent site conditions, the plant output to the grid is still guaranteed to be 115.6 MW.

Figure 1. One of the 14 gensets is off-loaded at the port of entry, Houston, TX, USA
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This genset project takes advantage of Nevada Assembly Bill 661, which allows industrial customers of Sierra Pacific Resources, the local utility, to leave the grid if they meet several requirements. Among the more important of these is to have a hard generating asset located within Nevada to provide the industrial user with electricity. This asset must meet largest single component failure contingency criteria established by the utility. To meet these redundancy requirements the facility will have four step-up transformers, therefore, if any one transformer or the buss elements fail, the load will be transferred to the other three transformers resulting in no loss in the plant’s net output. The largest single component failure criteria also required that a complete redundant buss be provided, even though buss failure is an extremely rare occurrence.

Plant make-up

The Western 102 facility uses 14 Wärtsilä 20V34SG gensets, each rated at 8.439 MW at site conditions. The gensets are modern natural gas fuelled spark ignition machines with pre-combustion chamber (PCC) technology and they operate with lean burn fuel gas/air ratio to provide very low heat rate in conjunction with very low uncontrolled emissions rates. In addition, the units are equipped with selective catalytic reduction (SCR) systems for additional NOx control and Oxidation Catalyst for additional carbon monoxide (CO) and Volatile Organic Compound (VOC) control.

The genset original equipment manufacturer (OEM) provided plant-wide and engine controls as an integral part of their design. To facilitate the ten-minute start-up from all gensets shutdown in warm standby, each genset has the capability of synchronizing with the grid rather than having shared synchronizers for a block of gensets, which was the design on earlier projects.

The largest single shipping module consists of the genset skid, with the permanent base plate, engine, and generator shipped as a single unit, weighing approximately 130 t. The gensets are generally shippable as a single unit, via ocean freight from the OEM factories in either Finland or Italy. They are then transported by railroad from the port before being loaded on to a heavy transporter and driven from the railroad siding to the power plant building. The other significant shipping module is the piping module, which contains auxiliary pumps, heat exchangers, and heaters. The modules are installed closely coupled to the engines via piping flanges on one side and connected on the other side via piping flanges to the cooling water runs to/from the radiators. Larger loose shipments include the medium voltage (MV) switchgear, SCR systems, the step-up transformers, and the radiators.

The power plant building is arranged with two engine halls, with seven gensets in each, separated by a central section that contains the control room, MV switchgear area, motor control center (MCC) area, maintenance area, and personnel facilities. This building is a typical prefabricated type building, but includes sound insulation for noise control. Silencers in the charge air piping to the engine turbochargers and the large vertical silencer in the exhaust flue/exhaust gas stack provide additional noise control.

Figure 2. One of the genset skids being placed on its foundation pad
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Since this is a simple cycle installation engine, heat is rejected to the atmosphere via a closed loop cooling system using radiators, which allows the gensets to meet their various guarantees while using no process water for engine cooling.

To meet the utility imposed largest component failure criteria, the plant is equipped with a redundant buss bar system and has redundant step-up transformers and breakers to allow loss of one transformer without affecting plant output. The facility is designed for remote start, stop, and regulation by the utility automatic generation control (AGC) signal.

Technology benefits

This genset technology offers several benefits to the plant owner that prompted it to select this set-up over the various different technologies available. The benefits include better ancillary service, a lower cost to meet the largest single component failure criteria, lower water requirements, guaranteed heat rate, greater plant flexibility and the following:

  • The ability to start up from shutdown conditions and reach full plant output in less than ten minutes. Preferably these assets would be inputting power into the grid almost immediately after the AGC signal requests power (non-spinning reserve).
  • The ability to operate at a low minimum plant output reliably and then modulate between this minimum load and full rated plant output in a stable manner with repeated cycles over this load range (spinning reserve).
  • The ability to modulate from minimum to maximum load with a rapid rate of change, to provide system frequency control on demand surges (up regulation/frequency response)
  • The ability to modulate from high plant output to minimum load with a rapid rate of change, to provide system frequency control on sudden demand reductions (down regulation/frequency response).
  • The ability to provide voltage support, by being located at the end of transmission systems and by having quicker response time than other technologies (voltage control).
  • The ability to be located closer to industrial loads and by having lower rotational speed of the generation equipment than other technologies (reactive power supply).
  • The ability of the generation assets to start up if the grid fails, synchronize with the grid, and provide power to restore the grid, with a minimal starting power requirement (black start).

The genset technology was judged by the plant owner to be able to meet these ancillary service requirements to a greater degree than competing technologies.

Since the net plant output is coming from a group of 14 gensets, the failure of one unit represents only a seven per cent loss of full load rated output. Again, this proved to be a significant benefit over competing technologies that featured from one to three components generating the full load net plant output.

Modern genset technology does not require use of water to meet heat rate, output, and emissions guarantees even at the high ambient air temperature and the elevation of the Western 102 power plant. The only process water consumption is a small amount used to wash the engine turbochargers and a small amount used to keep the closed circuit cooling loop water expansion tanks at set point. This consumption amounts to about 7.5 litres per genset per week.

In areas of the country where there is a high cost for use of water, and certainly Nevada and much of the western US are such areas, this characteristic of the genset technology provides a significant advantage over other technologies.

Plant flexibility

Another major benefit provided by the genset technology is the net plant heat rate under the 35à‚ºC, 1324 m above sea level site conditions. This heat rate for the Western 102 project was guaranteed by the OEM at 9179 kJ/kWh at site conditions, which is a clear benefit over any other commercial simple cycle technology.

If gensets are removed from service to match demand reductions, the technology provides great operating flexibility, including an ability to operate down to seven per cent of rated output without loss of heat rate. Furthermore, an excellent heat rate is maintained and having all gensets in operation at minimum load maximizes the spinning reserve. The heat rate of the individual gensets at 50 per cent of rated load is approximately 90 per cent of full load heat rate, in contrast with the significant loss in heat rate of a simple cycle gas turbine.

Figure 3. Overall plant view, showing transformers in place on left side, building with two engine bays and center control/maintenance space, and exhaust stack bundles and radiators to the right
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Another example of the technology’s flexibility is that is has the ability to carry full plant output with only 4.5 bar fuel gas pressure to the individual gensets.

In addition to the ten-minute shutdown to full plant output capability this technology provides the further benefit of having power flowing into the grid in two to three minutes from the AGC signal asking for the plant start-up from the warm shutdown condition.

Flexibility is demonstrated with the plant’s ability to cycle from shutdown to full load and back to shutdown conditions several times each day without affecting the equipment or adding to the maintenance hours. Maintenance hours are the actual operated hours without adjustments for items such as number of starts or rapidity of the start/stop cycles.

Wärtsilä, the genset supplier for the Western 102 project is also the project engineering, procurement and construction contractor, providing a single point of responsibility for schedule and performance. The Western 102 project is scheduled to start providing the plant owner with operational characteristics when it enters commercial operation in late 2005.

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