In 2007 Diakont was contracted to upgrade legacy steam turbine mechanical-hydraulic control systems (HCS) with Electro-Hydraulic Turbine Control Systems (EHCS) at several European nuclear power plants. As part of the contract, the utility set forth six goals for the upgrades:
This editorial will present an overview of Diakont’s approach to complete these project goals, and the resulting effect on the turbomachinery’s performance and reliability.
As operating nuclear power plants increase in age, operators are confronted with the challenge of repairing and/or retrofitting legacy control systems. At many plants built in the 1970s and earlier, steam throttling is achieved using a mechanical fly-ball governor coupled to a hydraulic main valve actuation system. This type of legacy system lacks the functionality, reliability, and maintainability of a modern control system. And more importantly, BWRs, VVERs, and in load-following mode also PWRs, have a reduced plant electrical output condition as a result of a larger-than-necessary backpressure operating margin. Because of this, plant owners globally are investing in turbine control system upgrades to capture increased performance and system reliability.
Electro-Hydraulic Control System Upgrade
To achieve the goals established by the utility, Diakont evaluated the critical components and sub-systems of the turbomachinery, and performed an upgrade of the legacy mechanical-hydraulic turbine control system with an electro-hydraulic system. Fundamentally, this replaced the mechanical speed control system with a dual-train PLC-based electronic system using 12 independent speed sensors, and a dual-redundant electromechanical actuator to drive the hydraulic control of the main steam throttle valve. A separate independent triple-redundant overspeed trip system provided a final safety function. Figure 1 shows a diagram of the EHCS. All work was classified as Nuclear Safety Related.
Figure 1: Diagram of Electro-Hydraulic Control System
Although the EHCS served an identical function to the HCS, the overall performance was drastically increased due to the reduction in speed overshoot. This was accomplished through the use of a patented Diakont algorithm that “learns” a specific turbine’s response to load and power changes, as well as the response characteristics of the hydraulic system that drives the steam valve. The system was thereby able to foresee potential overshoot and make prior adjustments so that it does not occur.
The replacement of the HCS’ hydraulic servo with an electromechanical actuator provided significant reduction in mechanical hysteresis. Electromechanical actuators are high-accuracy linear drives; the unit was specifically designed by Diakont for this system and had a specified repeatable resolution of 0.004”, while able to traverse its entire linear travel in under 50ms.
Figure 2: Diakont EMA Installed on PWR-Reactor High-Pressure Turbine
The increased control resulting from the system replacement reduced the fluctuations in backpressure applied to the nuclear steam supply system (NSSS). At the plant discussed in this paper, the typical full-load backpressure operating range with the legacy control system was 8.5 PSI wide (±4.25 PSI of variance from a setpoint). Following the upgrade to Diakont’s EHCS, this operating range was reduced to 1.0 PSI (±0.5 PSI of variance). By reducing this variance, the plant was able to increase their steam pressure setpoint by 4.8 PSI, which resulted in an increase of plant power output by 2.0 MWe, with no change to the reactor’s licensed thermal output. Figure 3 graphically illustrates the increase in operating pressure possible due to the reduction in pressure variation.
Figure 3: Turbine Pressure with Legacy HCS Compared with EHCS Upgrade
The increased throttle control afforded by the EHCS upgrade also provided decreased vibration and turbine bearing wear during operation and during spin-up. There are specific rotational speeds for each turbine that create extremely high resonant vibration levels, and it is critical to minimize time spent in these operating bands. Legacy hydraulic control systems made throttle changes slowly, which exposed the turbine to these high levels of vibration for an unnecessary amount of time. Diakont’s EHCS system provided precision control that allows the turbine to accelerate quickly through these critical bands, resulting in significantly lower turbine vibration levels.
The entire EHCS was designed with built-in redundancy for unsurpassed reliability. Each sensor, I/O system, control system, and actuator was paired with at least a single backup system that would take control within milliseconds in the event of an error or failure. The EHCS was equipped with two independent control channels in which all hardware was duplicated. While the active channel controls the system, the inactive channel performed the same actions as the active channel without issuing the final EMA driving signals. This was the basis for a “hot standby” function that would automatically transition from one channel to the other in less than 10 ms in a possible failure event.
Safety critical parameters were determined based on measurements from multiple sensors. In particular, pressure in the steam supply was measured using 4 sensors and the data was interpreted by “3 out of 4” logic. If any of these sensors were to fail, a non-fatal alarm would be generated that would alert the operator, and the faulty sensor would be automatically identified by comparing its readings to those of the other sensors. The functionality of the system would be unimpeded and the turbine would not trip.
The EHCS system was also equipped with an independent overspeed protection system that provided greatly increased performance compared to the legacy mechanical trip device. In the event of a load rejection combined with a control system failure that did not trip the turbine, the overspeed trip protection of a steam turbine would prevent a “runaway” condition, where at the increased rotational speed individual blade sections could break free of the rotor, and in a worst case scenario, penetrate the turbine casing. If a trip signal were generated from the independent overspeed protection system, all hydraulic pressure would be purged from the stop valve, and the steam valve would be shut by the closure spring within milliseconds.
A patented probabilistic safety assessment (PSA) method was utilized during the design process. Diakont’s PSA tool captured and analyzed all conditions, threats, and outcomes that could affect critical turbine activities. The methodology considered all interlocks, safety barriers, and also human factors. Diakont’s unique PSA tool considered dynamic activities and process transients in the creation of a probabilistic model of the process. Following the model build using software, the PSA tool revealed the weak points in the equipment or process which have the most critical effects on safety. This analysis was run repeatedly as the system and software were developed, allowing Diakont engineers to make iterative adjustments to the system’s logic, redefining responses to various sensors and operating scenarios.
Once the system design was completed, a third party certified laboratory tested the hardware for environmental compliance. The equipment was subjected to a wide range of air temperatures, air pressures and humidity, and was tested for electromagnetic compatibility, resistance to mechanical vibration, enclosure protection against the invasion of solid foreign bodies, dust and water, breakdown strength of insulation, stability to drifting power sources, and other criteria.
Functional testing was performed on the EHCS to validate the system performance under an excessive set of transient and steady state operating conditions. Initially, a mathematical model was used to simulate the equipment operation. After completing acceptance testing, the EHCS documentation and test results were presented to the plants. Once the plant approved the documentation, the hardware was delivered to the site for installation.
Installation and Commissioning
Under Diakont’s supervision, an external organization was contracted to decommission the old plant equipment and install the EHCS. The system installation was to be completed during an outage which already had planned turbine maintenance. In order to meet the required short installation timeframe, Diakont installed as much of the EHCS hardware as possible prior to the planned maintenance. Each electrical cabinet in the EHCS incorporated an electrical wiring kit that was installed prior to the maintenance, with pre-wired interconnections for the other parts of the system.
During the turbine maintenance, the installation was finalized and the first stage of functional testing was initiated. At the same time, portions of the mechanical-hydraulic control system were removed, and the riding cut-off valves were modified to enable a single point of actuation for both control and intercept valves. Thanks to the early integration of control cabinet equipment, Diakont completed the installation and commissioning within the planned maintenance schedule.
Due to their age, design, and maintenance challenges, steam turbines using legacy hydraulic control systems operate less efficiently and with poorer performance than similar sized turbines operating with modern control systems. Outdated control systems have sluggish response times and can cause turbines to drift, generally reducing efficiency and increasing operation and maintenance costs.
Operating successfully for over six years, Diakont’s first turbine control system upgrades have significantly improved plant performance with faster responsiveness, more precise control, fewer mechanical components, redundant electronic controls, flexible control schemes, and improved fire safety. The increased turbine control lowered the pressure variance allowing the turbine to run at an increased operating pressure, resulting in an electric power output increase of 2 MW, which “paid for” the upgrade within 1 year. With these inherent benefits, the electro-hydraulic turbine control system project was considered a wholesale success, and has resulted in the plant operator deciding to roll-out the upgrade fleetwide.