Changes are happening very quickly in the electric power industry, flexibility, efficiency and predictive maintenance are keys to increasing reliability
Transformation… a thorough or dramatic change in form. This is precisely what is happening to the energy industry, often taking place way too fast for operators to keep up. The uncertainties keep growing, and the ramifications are challenging. Here are some of the trends that are challenging utilities in the transformation of the grid:
Turndown Ratio and Optimizing Combustion Dynamics
Wind and solar generation output varies significantly over hours to days. This intermittency and variability of wind and solar power generation presents challenges for grid operators to maintain stable and reliable grid operation, especially where renewable power is given dispatch priority (example: Western states, notably California), requiring flexibility and efficiency in operating gas turbines so that the system can respond quickly to fluctuations, outages, and grid support obligations.
Meeting this challenge is achieved by operating a central power plant so that they maintain their connection to the grid but run at part-load, enabling them the ability to rapidly respond to the rapidly changing demands on the system network. Thus, the optimization of the turndown ratio has become a trend among operators. Optimizing the combustion system allows the gas turbine to run at a reduced load (as low as 25 percent) without exceeding emission limits. Optimizing the combustion system is a valuable feature because it means the plant can remain online during times of high output from wind farms and solar arrays.
Turbine Technology Services offers DynaFlex Performance™ -Extended Load Turndown in Mode 6, providing more operational flexibility, less unit cycling.
Many 7FA DLN-2.6 turbines are commissioned with only 60 percent load turndown. We maximize turndown while also maintaining emissions compliance. Besides increasing operational flexibility, combined-cycle plants can avoid overnight shutdowns, reducing start/stop cycles and extending hardware life.
7FA gas turbines in 2-on-1 combined-cycle configurations incur increased life cycle costs if subject to load demand that requires one unit to be cycled offline frequently. One way to reduce the need for unit cycling is to enable the individual gas turbines —and, in turn, the combined cycle block as a whole —to turndown to a lower load.
Earlier vintage DLN-2.6 combustion systems were designed to operate with a “lean-PM1” split schedule in Mode 6, the low emissions (NOx and CO) operating mode. With the lean-PM1 operation, load turndown in Mode 6 is typically in the range of 50-60%, depending on ambient temperature, part-load control curve configuration and the thoroughness of DLN tuning at the low end of Mode 6. Below this load range,combustors with lean-PM1 split schedules will start to generate increased CO emissions, pushing them out of emissions compliance.
More recently, GE developed a new operational configuration employing “rich-PM1” split schedules. In Mode 6, rich-PM1 operation enables the combustion system to turndown to lower loads than the lean-PM1 system without experiencing increases in CO emissions. Typically, units employing rich-PM1 operation can achieve nominally 40% load turndown before CO emissions begin to increase.
Older units with lean-PM1 split schedules can be modified to operate with rich-PM1 split schedules and thereby achieve increased load turndown. This operational modification involves making changes to control logic but does not include any hardware adjustments. A complete retuning of the combustion system throughout the Mode 6 load range must also be performed.
As mentioned, the energy industry is increasingly embracing renewables; one very impactful trend is emerging: gas turbine operators are having to multi-start machines, sometimes as often as twice a day. Multi-starting has become the new norm with 10-minute ramp-up periods. It is not uncommon to hear that some operators are having over 500 starts per year.
Vintage gas turbines were designed for operating with minimal starts. But with the grid transformation, multi-starting is putting high-integrity components through multiple strain cycles. Making assumptions that components can withstand these requirements based on the generalization that the increased number of starts is equivalent to a defined number of operating hours is at best optimistic, and at worst potentially dangerous for parts management.
In traditional use, the dominant failure process for hot components was “creep.” Therefore, they were designed using creep-resistant alloys. In this new era of multi-starts, the components experience high thermal transients during startup, and together with ever-shortening ramp-up times and increasing operating cycles, many operators have seen significant detrimental effects on the hot component’s integrity. For example, one operator we talked with recently related an incident during an inspection where a fuel nozzle that passed inspection just eight months earlier, he could now put his index finger into the opening.
Keeping track of as many as 500 starts per year and the fired factored hours has become a nightmare. And, no amount of skills in managing Excel spreadsheets will be adequate in suppressing downtime and keeping the equipment reliable.
If either of these trends are affecting you and the management of your gas turbines, contact an expert at TTS…we can help.
Turbine Technology Services offers Lodestar Turbine Parts Management™ -Predictive Maintenance Software Designed for Component Life & Outage Tracking of Gas Turbines Parts
Lodestar automates the capture and presentation of historical data and calculation of remaining critical parts life into a user-friendly interface. These features allow users to:
- Significantly improve critical part management.
- Increase maintenance personnel productivity.
- Reduce overall maintenance costs.
- Minimize outage downtime and delays.
- Share unit information with key service personnel.
Lodestar provides service and maintenance managers, procurement managers, site personnel and fleet owners a very clear picture of what parts are driving upcoming outage decisions and the full range of parts repair/replace options currently available within the fleet.
The Lodestar scenario builder tool allows users to simulate different unit operating scenarios, including stop/start cycles to predict the impact on parts life and to optimize outage planning decisions.