In a typical thermal power plant, monitoring steam turbo-generators is usually simple and trouble-free, but Huntly faced unique difficulties. It was necessary to meet extremely strict monitoring standards when it came time to replace the historical monitoring system.
Genesis Power Ltd., one of the four state-owned companies that comprised the previous New Zealand Electricity Department, owns and operates Huntly Power Station, the country’s largest thermal power plant. Initially, Huntly was designed to run four 250 MW units, which were put into service between 1981 and 1985. (Figure 1). A 385 MW combined cycle plant, Unit 5, and a 50 MW peaking gas turbine, Unit 6, have been installed at the power plant since 2004.
A 250 MW turbo-generator, a steam turbine (single reheat cycle), and a steam-driven main boiler feed pump are the components of each of the original units, which are numbered 1 through 4. These coal and gas-fired units have the unusual feature of being built atop a steel support structure to provide protection in the case of an earthquake (Figure 2).
Since then, Unit 3 has been retired from operation, and Units 1 and 4 have been running in accordance with system demands, with Unit 2 typically being dry-stored. But because New Zealand’s hydropower storage capacity is significantly below average for this time of year and wind generation is lower, this has only lately changed. Therefore, in order to conserve water for this year’s anticipated dry winter, all three units (1, 2, and 4) are currently in operation.
One Problem Solved, Another Created: In terms of safety, the steel foundations serve a crucial purpose, but due to their structural flexibility, they proved to be problematic. Due to a 100 Hz resonance in the flexible cross beam to which the bearing pedestal was fastened and a lack of horizontal stiffness in the bearing pedestal, the inboard bearing of the generators (bearing #7) has historically been subjected to significant vibration at running speed. Another resonance that was created by the two-pole generator shaft’s second critical speed slightly below running speed during run-up added to this. The crucial speed vibration would be considered during run-up if there was a pair imbalance.
The structure could have been stiffened in a number of ways, but the only one that was reasonably priced was to install dynamic vibration absorbers on the bearing pedestals. As shown in Figure 3, a dynamic vibration absorber is a small auxiliary weight coupled to the structure by a flexible steel shaft that serves as a spring. The shaft’s length and weight’s mass are calibrated together to have the auxiliary weight resonate at the same frequency as the original issue, which in this case was 100 Hz.
The absorber’s addition caused the original resonance to divide into two resonances, one at a slightly higher frequency and the other at a slightly lower frequency. Moving the resonance away from the rotational speed reduced the vibration at operating speed because the operational speed is constant, but it also added resonance to the machine start-up. Due to the fact that it was above the operational speed, the upper-frequency resonance did not pose any issues.
The overall vibration was still significant at 24 millimeters per second (mm/s) at 2,750 revolutions per minute (rpm) during the run-up, even with a lowered 100 Hz horizontal resonance and a pretty good state of balance. This was due to the fact that the total vibration, which cannot be lessened, is the vector sum of the horizontal resonance and the 1X running speed vibration that arises from being so close to the generator rotor’s second critical speed at 2,800 rpm (Figure 4). If there is unbalance, the overall vibrations during run-up at machine speeds between 2,750 and 2,850 rpm may be noticeably higher.
Issues with Monitoring Systems: All four 250 MW turbo-generators had the B&K Vibro Compass Classic monitoring system installed by Genesis Energy in 1994. For each unit’s steam turbine, generator, and feed pump, there was condition monitoring and protection, which comprised:
- Position and relative shaft vibration for each of the eight major bearings.
- The three turbine shafts’ eccentricity.
- The three turbine shafts’ differential expansion.
- The shaft’s axial position in relation to the thrust bearing.
- The vibration of each of the machine’s 12 bearings on a pedestal.
- The feed pump’s vibration and thrust position in relation to the steam turbine.
The monitoring systems on Units 2 and 4 were modified in 2010 as part of a control system upgrade to the B&K Vibro VC-6000 Compass 6000 system. Both historical systems were distinctive in that they could track the particular generating rotor dynamics (although modifications were made to the Compass 6000 system to accommodate this). This was particularly crucial during the period leading up to operational speed, when it was possible to track the essential speeds, resonances, and other vibration peaks to specific alarm thresholds, even those that were rather close to the operating speed. For both condition monitoring and protection, the alert limits were established for each peak at a certain speed band. The Compass Classic system’s implementation of this is seen in Figure 5.
Only Units 1 and 4 had been active up until this point, and in 2019 Unit 1 was scheduled to have a three-month maintenance outage, making it the perfect time to set up a new monitoring system. The 25-year-old Compass Classic monitoring system, which was installed in Unit 1, was already out-of-date and had no spare parts. There had to be a successor.
An analysis was conducted in 2018 to identify the functionally superior and most economical replacement for the current system. Transient speed monitoring needs for condition monitoring and protection posed one of the most challenging monitoring problems for turbo-generator equipment. There were additional criteria that had to be satisfied in addition to this, such as:
- Data archiving with the PI data historian.
- System configuration and remote monitoring.
Replaced Monitoring System: There weren’t many systems on the market that could meet all the monitoring requirements when Genesis Energy looked at potential options to upgrade the Unit 1 monitoring system. The old Compass Classic system was eventually replaced with the B&K Vibro VC-8000 Setpoint system. In June 2019, Setpoint was installed and subsequently launched (Figure 6). The Setpoint system has been offering the features and advantages listed below ever since it was put into service.
Data Historian for PI. Since Setpoint already had a PI data interface, installing a proprietary condition monitoring server wasn’t necessary, making the physical installation both affordable and quick. In PI, all data might be kept. The corporate network server and the local PI database server at the power plant make up the current Genesis Energy PI system. To reduce network traffic and hard drive space, Setpoint uploads both single-point data (static data) and time waveforms (dynamic data) onto the local PI server, while only static data is being communicated to the corporate network server.
The process data, which includes temperature, condenser vacuum, active load, reactive load, etc., is correlated with and trended with the static data, which includes vibration amplitude and phase measurements (overall, running speed, and running speed harmonics), axial displacement, eccentricity, differential expansion, etc. Although the dynamic data is kept in the local PI server, all diagnostic analysis, including orbit plots, shaft centerline plots, Bode plots, etc., is currently done in the Setpoint system rather than in PI.
Thermal expansion differs. The steam turbine rotor’s differential expansion in reference to the casing during initiation is closely monitored by setpoint. This is crucial for the high-pressure part, because loading occasionally needs to be delayed until the expansion is even. Setpoint has the requisite resolution to precisely track this circumstance and can turn on relays as needed. If the differential expansion limits surpass the alert alarm limits, alert alarm relays are set in the distributed control system (DCS) to halt initial loading. The unit trips if the danger alarm thresholds are exceeded.
This is crucial for the high-pressure part because loading occasionally needs to be delayed until the expansion is even. Setpoint has the requisite resolution to precisely track this circumstance and can turn on relays as needed. If the differential expansion limits surpass the alert alarm limits, alert alarm relays are set in the distributed control system (DCS) to halt initial loading. The unit trips if the danger alarm thresholds are exceeded.
Efficient and secure operation: For more than 40 years, Huntly Power Station has provided dependable energy, and during the past 25 years, the legacy monitoring system has been crucial in keeping these aging units operating safely with little downtime. Given the need to individually monitor the high transient speed vibration peaks in order to assure a successful run-up, this has not been a simple process.
In addition to improved transient speed monitoring, specialized balancing capability, and differential thermal expansion monitoring tied to the DCS, the new system installed to replace the outdated system on Unit 1 also offered these features. Additionally, PI Data Historian interface, remote monitoring access, and system configuration were available. As Unit 5 (the 385 MW combined cycle unit) transitions from baseload to two-shifting operation, plans are currently being made to examine the viability of installing Setpoint on this unit.
