F-class Best Practices: MONITORING AND DIAGNOSTICS

Monitoring of transformer oil, fault gases helps promote high reliability

Oil monitoring is key element of Colusa Generating Station’s transformer predictive maintenance program. Typically, this is done by periodic analysis of oil samples. Also recommended by experts is use of an online DGA monitor for immediate detection and measurement of fault gases found in transformer insulating oil. The ability to detect and correct small issues minimizes the possibility of failures conducive to long outages.

Plant personnel installed Serveron (a Qualitrol company) TM8™ online DGA monitors on all five 230-kV transformers at Colusa. The analyzers communicate via Modbus. By daisy-chaining the communications, a single cable is used to connect the TM8 to the plant distributed control system. This allows plant staff to monitor the transformers in real time and collect data historically.

Continuous trending and alarm management of key fault gases gives early and immediate notification of incipient faults that can lead to transformer failure. The gas chromatograph designed to measure the fault gases H2, O2, CO2, CO, methane, ethylene, ethane, and acetylene, plus nitrogen, oil temperature, ambient temperature, transformer load, and moisture-in-oil. Changes in the combustible-gas levels are addressed promptly and all of the history is properly documented in the work management system.

Startup emissions calculator

Colusa Generating Station’s PSD permit specified emissions limits during startup that went beyond what was typical in the industry at the time they were issued. It identified three startup categories—cold, warm, and hot—each having emissions limits and time constraints. Compounding startup complexity was that each criteria pollutant had pound-per-hour as well as pound-per-event limits. The latter generally was the sole permit-limit standard for the industry when Colusa went through its PSD review.

The complexity of the permit requirements, along with the relatively restrictive emissions limits, created challenges during GT starts. Initially, operators put a significant focus on air emissions and on preventing an exceedance, which meant less time was available to deal with other startup-related challenges.

Experienced operators know that the sensitivity of a gas turbine during startup is impacted by specific ambient conditions, load levels, and actual startup processes. Example: Ramping the unit during the transition from startup to normal operation, can cause emissions to spike quickly, risking an excess-emissions violation.

To mitigate startup challenges, plant personnel developed an “emissions calculator,” allowing them to focus more attention on plant systems. The control/monitoring tool set up to track NOx and CO emissions uses an existing board screen through the DCS which controls and monitors ammonia injection for NOx control in the HRSGs. The tool works in the following manner:

      • When a startup is initiated, the emissions calculator illuminates the startup category and associated permit limits.

      • The calculator monitors the time in startup, time remaining in the startup period, and emissions—including the cumulative lb/hr and lb/event totals for NOx and CO, as well as the current emissions rates based on data being fed to the DCS from the CEMS data acquisition and handling system. Notes: (1) The CEMS must be in operation during each startup and shutdown, (2) only one GT is allowed to start at any time, (3) cold starts are limited to 270 minutes, warm starts 180, and hot starts 90, (4) shutdowns must be completed in 30 minutes.

      • The rate limit of startup time remaining also is available to tell the CRO the highest emissions rate he or she can operate at and still be in compliance at the end of the hour or event.

      • Projected NOx is calculated as well to predict for the operator the emissions expected at the end of the event based on the emissions accumulated to that point in the monitored time period and the current emissions rate.

      • Time-to-exceed limit tells the operator how long he or she has until the emissions limit is exceeded. This helps determine if there’s time to correct the issue or if a shutdown should be initiated to avoid a violation.

      • The emissions calculator also tracks quarterly and annual cumulative emissions, and it sounds an alarm if the startup is predicted to exceed an emissions limit.

Experience: The emissions calculator is a valuable predictive tool that allows operators to make real-time decisions to avoid out-of-compliance conditions. There have been no exceedances to date.

Continuous monitoring system improves battery safety, maintenance efficiency (link)

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