Research on the Steam Humidity Measurement and Control System in Steam Turbine

1 Introduction

With the continuous progress of industrial technology, steam turbine units are developing towards high-power nuclear energy. The last few stages of high-power steam turbines in thermal power plants and all stages of nuclear power units work in a wet steam state. The steam engine has an important impact on the working efficiency of the steam turbine and the safety of the blades.

Therefore, measuring the humidity of the flowing wet steam in the steam turbine and controlling the humidity is of great significance to ensure the economic, efficient, safe and reliable operation of the steam turbine. Based on this background, this paper introduces a steam humidity measurement and control system that uses a microwave resonant cavity as a humidity sensor and an MSP430F157 microcontroller as a controller.

The MSP430 series microcontroller is an ultra-low power Flash 16-bit reduced instruction microcontroller launched by Texas Instruments. It has a very high integration level. Its main features are: processor power consumption (1.8 V ~ 3.6 V, 0.1 μA ~ 400 μA, 250 μA / MIPS) and line output leakage current (maximum 50 nA), which are much lower than other series products. The traditional MCS51 microcontroller consumes 10 mA ~ 20 mA in active mode. Vector interrupts support more than 10 interrupt sources and can be nested arbitrarily. Fast execution speed, one clock cycle can execute one instruction, while the traditional MCS51 microcontroller needs 12 clock cycles to execute one instruction.

2 Working Principle

The humidity measurement system consists of microwave devices such as microwave signal source, directional coupler, circulator, resonant cavity, detector, as well as circuits such as DC amplifier, frequency measurement system, and digital signal processing system.

When the system is working, the resonant frequency of the microwave resonant cavity will produce frequency deviation with the different humidity of the steam flowing through. Once the resonant frequency of the resonant cavity changes, the frequency scanning tracking loop will be unlocked. The detector detects the unlocking signal in time, and after being amplified by the DC amplifier, it is sent to the frequency tracking system to obtain a control signal. The control signal controls the VCO to generate a series of scanning signals to scan the stabilized resonant frequency until the resonant frequency is scanned and locked. The signal generated by the VCO is coupled to the frequency generator through the directional coupler in the frequency measurement loop. The signal obtained by mixing the signal with the local oscillator signal is amplified by the intermediate frequency amplifier and output as an intermediate frequency signal, which is sent to the frequency measurement system for frequency measurement. The measured intermediate frequency is then converted by the data processing system to obtain the resonant frequency and frequency deviation. The resonant frequency will be transmitted to the single-chip microcomputer together with the steam humidity and pressure for data processing to obtain the steam humidity.

3 Hardware Design

The control system of humidity measurement is mainly completed by MSP430F157 microcontroller. In essence, the work of the microcontroller control system can be summarized into three steps: real-time data acquisition, real-time data processing and real-time control output. The hardware circuit design of this system is mainly composed of temperature measurement module, frequency tracking module, power supply circuit, reset circuit, frequency measurement module and other parts. MSP430F157 microcontroller contains two clocks of 32.768 kHz and 8 MHz; the internal 12-bit DAC completes the digital-to-analog conversion of the tracking module, and the conversion result is used to adjust the VCO output voltage to ensure the consistency of the VCO output frequency and the resonant frequency of the resonant cavity; the timer Timer_A inside the microcontroller is used to generate the sampling frequency of D/A conversion. The humidity measurement control system based on MSP430F157 is shown in Figure 1.

3.1 Main peripheral circuits inside the microcontroller

Crystal oscillator circuit: The most sensitive signals of the single-chip microcomputer system are the clock signal, reset signal and interrupt signal. If electromagnetic interference invades these signals, it is most likely to disturb the microcontroller and cause the system to malfunction. Especially after the noise interference is accumulated in the clock signal. It will change the clock frequency division signal, causing the single-chip microcomputer to work in a disordered sequence. This system provides high-speed and low-speed crystal oscillator circuits, which can enable different modules inside the single-chip microcomputer to output clocks of different frequencies. Users can use high-speed crystal oscillators to generate high-frequency MCLKs for the CPU to meet the needs of high-speed data operations, or turn off high-speed crystal oscillators when the CPU is not needed. For real-time clocks, low-speed crystal oscillators can be used to generate low-frequency ACLKs. Some small capacitors should be connected in parallel to input, output, power supply and other circuits to avoid noise interference.

12-bit D/A conversion module: Compared with the earlier models, the MSP430F15/16x series microcontrollers have added dual 12-bit D/A converters. The D/A converter is mainly used to convert the digital output of the microcontroller into actual analog quantity to control external devices. This D/A converter can be set to 8-bit or 12-bit conversion mode during use. Ideally, when the 8-bit conversion mode is selected, the resolution is 1/256; when the 12-bit mode is selected, the resolution is as high as 1/4 096. The D/A converter completes the output of the control signal with the cooperation of the timer Timer_A.

Timer_A module: The timer of the MSP430F157 microcontroller is a 16-bit counter with four working modes: stop mode, up-counting mode, continuous counting mode and up/down counting mode. One of the four modes can be selected by setting the corresponding timer A register. Moreover, Timer_A has two interrupt vectors, namely CCR0 interrupt vector and TAIV interrupt vector, of which CCR0 is a single-source interrupt vector with the highest priority; TAIV is a multi-source interrupt vector with a lower priority. In this system, the up-counting mode is selected, and CCR0 is used to set the interrupt frequency of the timer to control the sampling time.

Control principle: When the resonant cavity changes from the detuned state to the resonant state, the output power drops to zero, and the detuned zero signal is output. At this time, a falling edge from "1" to "0" will be obtained, so that the Timer_A of the microcontroller stops counting, and the output remains unchanged until the humidity changes, and the resonant cavity changes from the resonant state to the detuned state, generating a rising edge from "0" to "1", and then Timer_A starts counting. When the count reaches CCR0, an interrupt flag is sent to start D/A conversion. If new data is obtained, a flag is set to notify the main program, and then wait for the next signal to arrive.

3.2 Device Selection

The operating voltage of MSP430F157 is 3.3 V, which cannot be provided by the existing switching power supply. In addition, considering that the system requires the power supply to have voltage stabilization function, small ripple, and low power consumption of the hardware system, the power supply part adopts TI's TPS76033 or LM1117. Both devices can well meet the hardware system requirements. In addition, both have very small packages, which can effectively save PCB area.

In the single-chip microcomputer system, the single-chip microcomputer needs a reset circuit, which can be an RC circuit or a reset device. The RC reset circuit has low cost, but low reliability; while the reset device has high reliability. In order to ensure the reliability of the reset circuit, the system uses the reset device MAX809STR to implement the reset circuit.

The temperature measurement circuit is implemented by the XTR101 device. XTR101 is a precision, low drift two-wire transmitter that can amplify weak voltage signals and convert them into 4 mA to 20 mA current signals for long-distance transmission. It is composed of a precision 1 mA current source. The offset voltage of XTR101 is 30μV, the temperature drift is 0.75μV/℃, and the nonlinearity is 0.01%.

Frequency measurement circuit: In order to simplify the circuit and improve integration, the frequency measurement part of the system is implemented using CPLD.

4 Software Design

The code storage space of the MSP430 series microcontrollers ranges from 1 kB to 60 kB. When the program size is greater than 8 kB, the use of assembly language will greatly reduce the efficiency of software design work. Using C language to implement system application software development can greatly improve the efficiency of development and debugging; at the same time, the generated documentation is easy to understand and easy to transplant. The C language suitable for the MSP430 series is highly compatible with the standard C language. The integrated debugging environment IAR Embedded Workbench and C language C-SPY debugger provided by IAR Corporation of the United States are selected as the development platform. It is a flexible and convenient integrated environment for developing applications for different target processors. It provides a friendly user interface and a powerful debugging environment, which is convenient for developing applications for the MSP430 series microcontrollers. The MSP430 series microcontrollers can use Workbench to download directly to the on-chip Flash memory for offline operation. During the debugging process, the contents of each register can be seen in the upper-level software and modified online. It supports single-step operation and can observe the real-time values ​​of each defined variable online. The organization method of putting all related files into one project is adopted. When compiling and running, the software will automatically combine the files according to the internal connection and support C language programming. The system software design process is shown in Figure 2.

The main program of the system is:

5 Conclusion

Compared with the control system implemented by separate components in the past, the humidity measurement control system implemented by the MSP430F157 microcontroller introduced in this article has high accuracy, simple design, and few connections, avoiding electromagnetic interference and short circuits caused by the proximity of wires. The software design uses IAR Embedded Workbench and C language C-SPY debugger as the development platform, which is powerful and easy to operate.