Development of a Simulator for Steam Turbine Generator Protection System Based on a Distributed Control System

Most manufacturing facilities have shifted to industrial automation systems in the last decades [1-3]. New technologies and architectural designs have come up with the expansion of industrial plants' organizational structures. Industrial machine systems must be highly reconfigurable to accept changes in processes and products as the global marketplace becomes more and more customer-driven. Rigid, static automation systems will eventually give way to more flexible varieties. Inherently flexible control systems are the focus of achieving such flexibility. There has been a noticeable shift in industrial control systems in recent years from centralized control to a much greater usage of distributed intelligence.

One of the machines primarily found in large industries is the steam turbine generator, which is used in the power generation industry, oil and gas processing, chemical industry, paper pulp industry, and many more. Steam turbine generators are very complex machines and very dangerous because they have the potential to explode. Therefore, designing a proper operating system and procedure is very important. Moreover, the facility must be operated by trained operators for safety reasons.

Steam turbine generators need to use a protection system to increase operation safety. The protection system aims to detect abnormal conditions and protect the steam turbine from potentially dangerous conditions. The protection system is also designed to take preventive action or shut down if there is an emergency condition. Many studies have been carried out to improve safety in the operation of steam turbine generators [4-13]. Wang and Liu [4] investigated the effect of adding filters on steam turbine control valves. These valves are usually between the boiler and the intermediate-pressure turbine in thermal power plants. They used the ANSYS CFX software package for this numerical simulation. Bolin and Engeda [7] studied various mechanisms of valve instability. Pondini et al. [8, 9] used Matlab/Simulink software to present a dynamic model for the control valve and steam turbine actuation system operating between the boiler and the medium-pressure turbine in a thermal power plant.

As automation technology develops, control technology is also used to operate various production facilities. It is aimed to improve safety and simplify complex processing processes. Distributed control systems (DCS) are one of the solutions to overcome the complexity of control systems of the steam turbine generator. Much research has been conducted to implement DCS in power-generating and distributing systems [14-19]. Yulianto et al. [14] use risk-based maintenance strategies for DCS as power plant asset management. Implementing DCS in steam turbine generator operations is very helpful in increasing security and reducing complexity. DCS consists of various hardware and software connected by a communication network. DCS provides the ability to control and pivot processes concisely remotely. Besides a reliable control system, a safe process also needs an operator who understands the principles of the control system and is an expert in operating a steam turbine generator.

Several studies related to simulators in power plants were carried out [20-25]. Ulfiana et al. [20] developed a gas-fired power plant operation simulator using LabView as a simulation platform. They designed a simulation of the operational system for a gas-fired power plant but without implementing a protection system. Zabre et al. [21] developed a Combined Cycle Power Plant simulator for operator training. They also designed an operational system simulator where the operator can automatically monitor and control the system. However, the system did not implement a protection system. Agha et al. [22] researched resetting parameters of the unit 2 turbine protection system at Star Energy Geothermal Ltd. This research was focused on identifying the vibration protection system for the turbine. The model developed did not consider protection systems for temperature, pressure, and speed. Abu Bakar et al. performed another research on energy system simulators [23]. They built a Savonius turbine for educational purposes. Two models of the drag-driven vertical axis, 2-bladed and 3-bladed Savonius turbines, were constructed to generate electricity and integrated with a model house.

Many studies were carried out to develop simulators to aid the operator in operating power plant systems, including identification of turbine vibration and protection systems. However, the protection system of steam turbine generators is more complex than all the systems that have been developed. Due to its complexity, a high-skill operator is required to operate the facility. However, it is not easy to train the operator using the actual facilities because they are continuously used in production. Hence, a simulator that can demonstrate the actual condition of the operation process is needed.

Therefore, in this research, a simulator of a protection system of a steam turbine generator based on a distributed control system based on DCS ABB 800xA was developed. The system was developed by considering many parameters to ensure the safety of the operation, such as turbine speed, inlet temperature, vibration, turbine shaft position, steam drum tank level, and lubricating oil pressure. The system also provides a manual emergency push button to anticipate an uncontrollable condition. The developed simulator is expected to help the operators learn and practice the protection system of steam turbine generators.

There are several aspects should be considered in developing the user interface in the operator workplace, such as:

·The HMI display should be user-friendly to ensure that the critical data is displayed clearly and easily understood.

·Appropriate colours, graphics, and icons should enhance clarity and attractiveness.

·The layout of interface elements should be arranged logically and orderly to make the system easy to use.

By considering those aspects, the display of the operator workplace was developed, as shown in Figure 5.

Several tests have been performed to ensure the simulator works appropriately and protects the steam turbine generator from abnormal conditions. Tests were also performed to check interlocking responses caused by a single variable. All the variables in this study have been tested. Every variable was tested several times for various cases. Another test was performed to check the simulator's ability to detect and respond to abnormal conditions. 

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Figure 5. HMI design of steam turbine generator

3.1 Testing on manual emergency push button

Even though the operation was developed by implementing automatic systems, manual procedures for emergency cases should be provided. The test ensures that the emergency push button has been performed appropriately. The operator should press the emergency push button manually. Before the push button is pressed, the system is operated in normal condition, indicated by the green color on the notification bar of all variables, as shown in Figure 6(a). After the emergency is pressed, the colored notification bar changes to red, followed by the alarm blinking. In this case, the Interlocking is active. The test on this feature has been performed many times, and the system works correctly for all the tests.

3.2 Testing on the interlocking caused by steam turbine temperature

This test was performed to check the function of the interlocking system due to the steam turbine temperature. The interlock system is active when the steam temperature enters the inlet is less than 380℃, and this condition occurred minimum at two of three inlets. Eight temperature variations have been tested; the result is shown in Table 1.

Table 1 shows that the alarm is inactive, and the indicator is green when there is only one inlet where the temperature is less than 380℃, as shown by tests 1 and 5. The same condition also occurred when the temperature at all the inlets was more than 380℃, as shown by test no. 4. On the contrary, the locking system is active, which is indicated by the red color notification bar and alarm blinking when the temperature of the steam at two inlets is less than 380℃, as shown by tests no. 2, 3, and 4. The notification bar on the operator workplace is presented in Figure 6(b). The same system response was shown by tests 7 and 8 when the temperature at all the inlets is less than 380℃.

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Figure 6. Notification bar, (a) normal condition, (b) turbine trip caused by steam tempeature, and (c) turbine trip caused by turbine shaft position and vibration

Table 1. Data of interlocking test on steam turbine temperature

3.3 Testing on the interlocking caused by pressure switch lube oil

This test was performed to ensure that the interlock caused by lubricating oil pressure works correctly. The interlock occurs when at least two out of three of the lubricating oil pressures are in a lower condition. Then, the interlock system activates and triggers the turbine trip, indicated by the red alarm blinking on the operator's workplace display. Eight pressure variations for all three switches have been tested, and the results are presented in Table 2.

Table 2. Interlocking test result caused by pressure switch lube oil

Table 2 shows that the alarm is inactive, and the indicator is green when there is only one or no switch oil in lower low pressure, as shown by tests 1, 2, 4, and 7. On the contrary, the interlocking system is active, indicated by a red notification bar and alarm blinking when there are two or more pressure switches lube oil in lower low pressure, as shown in tests 3, 5, 6, and 8.

3.4 Testing on interlocking caused by turbine shaft position

This test was carried out to ensure that the interlock system caused by the turbine shaft position follows the design. The main purpose of controlling this variable is to protect the turbine from damage that may occur when the turbine shaft is in an unsafe position. The interlock system is active and alarm-blinking when the shaft is in a higher-high or lower-low position.

Table 3 shows that the alarm is inactive, and the indicator is green when there is only one or no turbine shaft in higher-high/lower-low conditions, as shown by tests 1, 2, and 4. On the contrary, the interlocking system is active, indicated by a red notification bar and alarm blinking when there are two or more turbine shafts in higher-high/lower-low positions, as shown in tests 3, 5, and 6.

Table 3. Interlocking test result caused by turbine shaft position

3.5 Testing on interlocking caused by overspeed

The test was performed to check the system in monitoring the turbine speed and conducting interlocking when the speed is over the specification (≥ 3300). Six tests were carried out, and the results are presented in Table 4. From Table 4, when the turbine speed is below 3300 rpm, as shown in test no. 1, 2. 3, and 4, then the notification bar is green without blinking. On the other hand, when the speed is more than 3300 rpm, the alarm blinks, and the notification is red color. It indicates that the system is in abnormal condition.

Table 4. Interlocking test result caused by turbine speed

3.6 Testing on the interlocking caused by steam drum tank level

This test was aimed to ensure the interlocking system worked well when monitoring the steam drum tank level. The Interlocking is active when the steam drum tank level is higher-high, which is ≥ 99%. Six tests were carried out, and the results are presented in Table 5. The test showed that the alarm was not blinking, and the notification bar color was red when the tank level was below 99%, as shown in tests 1 to 4. However, when the tank level reaches 99% or more, as can be seen in test 5 and test 6, then the alarm blinks with the red color notification bar. In this case, the operation is abnormal, and the turbine could be tripped.

Table 5. Interlocking test result caused by turbine speed

3.7 Testing on the interlocking caused by bearing vibration

This test was performed to check the ability of the simulation system to detect abnormal conditions due to vibration on the turbine and generator bearings. Two conditions cause the Interlocking to be active. The first is that at least two bearings in the turbine and generator are in an off condition. An off condition means that the bearing is working improperly. The second is the off condition, which occurred in the X and Y axes. Interlocking is active when the two conditions are fulfilled at the same time.

Four tests were performed, and the results are provided in Table 6. In test 1, there are two Off conditions on the X-axis and Y-axis of the turbine bearing. In this case, the Interlocking is inactive because abnormal conditions only occur in the turbine. The first condition to activate interlocking still needs to be fulfilled. The Interlocking is also inactive in test 2 because the Off conditions are only on the X-axis of the turbine bearing and generator bearing. In this case, the second condition to activate the interlocking system still needs to be fulfilled. Test 3 and test 4 fulfil the two conditions of the interlocking system. The Off conditions were found on the turbine and generator; moreover, the Off conditions were found on the X-axis and Y-axis.

Table 6. Interlocking test result caused by turbine speed

3.8 Testing on the interlocking caused by all variables

Complete and comprehensive tests were performed to check the effect of all the variables. Three tests were conducted, and the results are provided in Table 7. From this table, it can be seen that test 1 produces regular operation; meanwhile, test 2 and test 3 produce interlocking and turbine trips. Two factors caused the Interlocking on test 2; the first is two of the three turbine shaft positions in higher high or lower low (HH/LL) conditions. The second is two bearing vibrations, rear bearing vibration X and front bearing Vibration Y. The notification bar on the operator workplace is shown in Figure 6.

Meanwhile, five factors caused by test 3 are pressure Switch Lube Oil, Turbine Back Steam Pressure, Overspeed, Steam Drum Level Tank, and turbine bearing vibration. Several tests have been performed to ensure the simulator works appropriately and protects the steam turbine generator from abnormal conditions. Tests were also performed to check interlocking responses caused by a single variable. All the variables in this study have been tested. Every variable was tested several times for various cases. Another test was performed to check the simulator's ability to detect and respond to abnormal conditions. 

The series of tests demonstrate that the developed system could operate well as a simulator of an automatic turbine protection system. The developed simulator system is very comprehensive in detecting the potential of turbine trips. This system considered all the variables that were highly reported as the factors causing the turbine malfunction. It is the main advantage of the proposed system over other simulators. Dulai and Bica [27] developed a system for steam turbines only to control the turbine speed. Meanwhile, Hirkude et al. [28] modelled and simulated on the load-governing system of a steam turbine.

The proposed simulator is easily operated. The system could be connected to real steam power generating facilities to obtain the real data, or the operator could input and modify the data and the threshold of all variables for training purposes. The developed simulator could be implemented in various steam power generating facilities, as long as the real facilities provide the data for all the variables.

Table 7. Interlocking test results caused by all variables