Electrical power stations use large steam turbines, up to 2,000,000 hp (1,500 MW),driving electric generators to produce the electricity. Steam is generated by boiling water using heat from burning fossil fuels, Geothermal heat or Nuclear energy. The turbines used for electric power generation are normally directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3,000 r/min for 50 Hz systems, and 3,600 r/min for 60 Hz systems.
The generation of alternating current electricity requires precise speed control. To adapt to the changing demand of electricity, the turbine controller has to control the turbine speed by controlling the steam flow into the turbine. The steam flow is controlled by hydraulically operated steam control valves.
Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the steam control valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart. As steam turbines are very expensive and a break causes severe damage, any uncontrolled situation has to be avoided. A key safety feature is the design of the hydraulic actuation system of the steam control valves.
Current Steam Valve Actuation Systems
Today’s steam valve actuation systems are position-controlled cylinders using proportional valves with external analog electronics. The actuator works against the integrated failsafe spring, which is able to close the steam valve without any external energy, when the control port “A” of the cylinder is connected to tank.
Some disadvantages of the existing system are:
- Commissioning: 6 to 7 potentiometers have to be adjusted at commissioning of the system, which is very time consuming and costly as it requires a skilled person to tune a closed-loop system.
- Replacement: Replacement of a valve or an electronics card is difficult, requiring skilled staff to adjust several potentiometers
- Diagnostics: Troubleshooting is limited to the cylinder position signal
New Generation of Electro-Hydraulic Steam Valve Actuation Solution:
For the new generation of electro-hydraulic steam valve actuation systems, the following specification was given:
- After replacing a valve, no new control loop, valve tuning or signal scaling is needed
- Easy signal scaling after replacement of the actuator position sensor
- Ability to deal with existing analog command signals for turbine upgrades
- Addition of monitoring signals for proactive maintenance and fault identification
- Pressure transducer to provide an additional pressure signal for diagnosis
- Integration of the proven spring failsafe solution
When analyzing the specification, it is clear that digital electronics is required. The ideal solution is a combination of digital control electronics with a proportional valve, the Moog Axis Control Valve (ACV). See picture.
The ACV customized for the steam valve actuation application, provides the following features:
- Easy replacement for existing solutions for turbine upgrades
- Position controller of the actuator integrated in the valve (ACV)
- No new control loop tuning and no signal scaling needed after valve replacement as the stored digital parameters are recalled and easily loaded into the new ACV valve
- Easy signal scaling after replacement of position sensor as the ACV has an integrated semi automatic sensor calibration procedure.
- The ACV can handle existing analog command signals for turbine upgrades
- FIELD-bus interface to monitor signals for proactive maintenance is available
- FIELD-bus interface for remote maintenance is available
- No electric cabinet for control electronics needed
- Integrated pressure transducer in “A” for system diagnostics
- Spool design maintains existing spring failsafe solution
A customized ACV provides all specified features, simplifies the system (no electric cabinet needed), offers comprehensive diagnostics features such as remote maintenance and supports proactive maintenance through an impressive variety of important signals. The new solution is suited for retrofits and turbine upgrades as well as for new turbines.
Analog control and valve electronics can only monitor the deviation between the command signal and the actual position signal. When the deviation exceeds a defined level, the failsafe function is triggered and the actuator has to be stopped or moved into a defined end position for safety reasons. This is interpreted by the host controller of the machine and the hydraulic actuator system as an unknown defect, which has to be analyzed by the maintenance staff after the emergency stop, thereby resulting in troubleshooting downtime.
By contrast, a modern electro-hydraulic actuation system using a digital Axis Control Valve is able to control the valve itself, in addition to the actuator position. When using a proactive maintenance approach, currently being demanded in more and more factories, it is mandatory to obtain significantly more information about the actual status and wear of the electro-hydraulic actuation system and its components. For example it is highly valuable to monitor, relative to defined tolerances, the static and dynamic behavior of the Servo-Proportional Valve, temperature of the integrated valve/axis electronics, sensor signals, leakage (wear of seals) of the actuator, and process data. The Digital Valve's axis control electronics can make all relevant internal control data available for continuous process monitoring. To be able to transmit continuously the large amount of status information available per axis to a host controller, a FIELD-bus interface is essential for the Axis Control Valve.
With the available data, it is now possible to monitor the wear of the electro-hydraulic actuator, allowing for proactive maintenance at the next planned machine service. The available data provides information about the required activities and enables spare parts to be available for the planned machine service, reducing the down time required. If an Axis Control Valve has to be replaced, no new tuning and adjustments are required as all control parameters are simply copied to the new valve, further reducing down time. Contrast this to the emergency stop scenario with the analog valve. Time is money and digital diagnostics saves both.
Bernhard Zervas is currently the Systems Engineering Manager for Moog's Industrial operations in Germany. He has over 30 years experience in the international hydraulic industry, with a focus on industrial electro-hydraulic closed-loop, electro mechanical and hybrid applications.