C8051F021 single chip microcomputer realizes data acquisition system

The power angle measurement data acquisition card introduced is a PCI bus interface card implemented with the Cygnal C8051F021 single-chip microcomputer. The card uses two dual-port RAMs to exchange data with the GPS receiving board and the host computer (PC), thereby achieving high-speed and reliable data acquisition, processing and transmission. This data acquisition card uses an on-chip 12-bit high-speed ADC and an off-chip sampling and holding circuit to accurately and quickly sample voltage and current through a direct AC sampling method, ensuring the real-time measurement requirements of the power system.

1 Introduction

In the process of achieving automation, the first step is data acquisition. For this purpose, we have researched and developed a wide-area measurement and control system for power system power angle. The data acquisition card implemented with the C8051F021 single-chip microcomputer can not only accurately, efficiently and in real time collect AD data and exchange data with the host computer through the dual-port RAM and PCI interface, but also realize remote synchronization of collected data by receiving second pulses through communication with GPS receivers.

2 Power angle and its measurement

2.1 Generator power angle

The transmission system consisting of generators connected to the infinite capacity system bus through transformers, output lines, is shown in Figure 1.

If G in the figure is a hidden pole generator, the active power output of the generator is

The above formula is also called the power characteristic of the generator. In the formula, Xd∑=Xd+XTL, where Xd is the equivalent reactance of the generator d-axis, XTL is the connection reactance between the generator and the infinite system, Eq is the no-load potential of the generator, V and I are the system reference bus voltage and current, and ψ is the phase difference between the bus voltage and current. When the generator's potential Eq and the receiving terminal voltage V are constant, the transmitted power P is a sinusoidal function of the angle δ, and the angle δ is the phase difference between Eq and V. Because the power P is closely related to the angle δ, δ is called the "power angle" or "power angle".

2.2 Power Angle Measurement

For the non-salient pole generator and the salient pole generator, the voltage and current vector diagrams are shown in Figure 2(a) and (b). From the vector diagrams, we can know that the calculation formula of the power angle δ is

In the formula, Xq∑=Xq+XTL, where Xq is the generator transverse axis reactance. For a certain system, Xd∑ and Xq∑ are both constants. Therefore, when measuring with a computer, it is only necessary to measure the voltage V, current I and power factor angle at the system bus, and then the power angle can be calculated by equations (2) and (3). In addition, the electromechanical transient process describing the power system after a large disturbance is a set of nonlinear differential equations that cannot be linearized. Therefore, the time domain analysis method of numerical integration method (such as Euler method, Runge-Kutta method, implicit integration method) is generally used to plot the calculation results into a curve of operating parameters (such as power angle) against time to determine the transient stability of the power system. The above calculations and curve control are all based on the accurate acquisition of the AC signal of the power system.

This data acquisition card collects 3-phase voltage and current through 6 channels for computer processing through AC sampling. That is, the AC signal does not pass through the power converter, but directly converts the voltage and current measured by the transformer (PT/CT) into a small signal measurable by the computer (-2.5V~+2.5V for this data acquisition card) through a high-precision PT and CT. Then it is sent to the computer for processing after A/D conversion, and the electrical parameters such as the effective value of voltage and current and the power factor angle are calculated to obtain the power angle value and its change with time, which are provided to the power dispatching monitoring center for monitoring the stability of the power system.

3. Capture Card Hardware Design

This acquisition card is mainly composed of C8051F021, A/D filter sampling and holding circuit, dual-port RAM, GPS interface, PCI interface and other parts. The voltage and current from the on-site PT and CT are isolated and transformed by the isolation transformer and then input into the data acquisition card, and then pass through the second-order filter to the A/D converter front channel. The single-chip microcomputer obtains the precise second pulse and corresponding time (year, month, day, hour, minute, second) through the GPS interface and dual-port RAM and realizes the synchronous acquisition of the signal. The digital quantity obtained after conversion is sent to the host computer for processing by another dual-port RAM and PCI interface. The structural block diagram of the system is shown in Figure 3.

This acquisition card uses Cygnal's C8051F021 microcontroller, which is a fully integrated mixed-signal system-on-chip (System On Chip) with a high-speed CIP-51 core that is fully compatible with the MCS-51 instruction set; the peak speed can reach 25MI/S; almost all analog and digital peripherals and other functional components (including programmable gain amplifier PGA, ADC, DAC, voltage comparator, voltage reference, UART, timer, programmable counter/timer array PCA, etc.) required to form a single-chip data acquisition or control system are integrated in one chip. Its instruction cycle is 83ns (3.3V, 12MHz); it has a large-capacity FLASH memory that can be programmed in the system (ISP) and in the application (IAP); it supports JTAG debugging and boundary scanning that complies with the IEEE 1194.1 standard, and can perform non-intrusive, full-speed in-system debugging. Because the C8051F microcontroller has better performance than other 8-bit microcontrollers, it has become the preferred model for many measurement and control system designs as soon as it was released.

The ADC converter uses an on-chip 12-bit, 100ks/s ADC, and each conversion takes only 10μs, which fully meets the system's requirements for real-time and fast performance. The F021 microcontroller ADC has 8 external inputs, and the reference voltage can be provided internally or externally (2.5V). It can be programmed as a single-ended input or a differential input and has a programmable amplifier gain. This acquisition card uses 6 single-ended inputs to complete the synchronous acquisition of 3-phase voltage and current. Because the ADC can only perform AD conversion on 0-2.5V signals when using single-ended input, the -2.5V-+2.5V sinusoidal AC signal sent by PT and CT needs to be added with a DC bias voltage of 2.5V in the front channel of the A/D conversion, so that the input signal amplitude is 0V-5V. The signal is now attenuated to 0V-2.5V by the on-chip programmable gain amplifier, and then the A/D conversion is completed. During acquisition, a sample-and-hold command is issued through the acquisition program to enable the sample/hold device to hold the value at the moment of acquisition. The acquired instantaneous signal enters the attenuation circuit through the analog channel switch selection, and is then converted into 12-bit parallel data through the A/D converter and output to the dual-port RAM for reading and processing by the host computer.

The dual-port RAM uses a 32k, 8-bit high-speed IDT7007S. The MCU and the host computer can perform read and write operations simultaneously from both sides of the dual-port RAM (but data cannot be written to the same address unit at the same time), and the access operation time of the dual-port RAM is only 55ns at most. Therefore, the dual-port RAM greatly improves the speed of data exchange between the MCU and the host computer, thereby providing a guarantee for the application of the data acquisition card in the field of power system measurement and control.

The PCI interface is mainly completed by the PCI interface CH365. CH365 is a universal interface circuit connected to the PCI bus, supporting I/O port mapping, memory mapping, extended ROM and interrupts. This acquisition card mainly uses the memory mapping function of CH365 to convert the 32-bit high-speed PCI bus into an 8-bit data, 16-bit address active parallel interface. CH365 can exchange data with an external microcontroller or DSP through a dual-port memory, and then transmit the data to the host computer for further processing. If the width of the read and write strobe pulse of CH365 is set to 30ns, and double words are used for data exchange, the measured data transmission speed can reach 7M bytes per second. Compared with other mainstream buses, the PCI bus is faster, more real-time, and more controllable, so CH365 is particularly suitable for high-speed real-time I/O control cards, communication interface cards, and data acquisition cards.

In addition to the above-mentioned considerations on the speed, accuracy, and real-time transmission of data acquisition and processing, this acquisition card also takes into account the remote synchronous acquisition of various dependent channels. In the design, the GPS Receiver Board (GPS Receiver Board) GPS15L of GARMIN is used. Through the satellite precision timing function, the precise second pulse provided by the satellite realizes remote synchronous acquisition. The receiving board receives signals from at least 11 satellites. The dedicated integrated circuit and processing software can extract and output two time signals from the received information: one is the second pulse 1PPS, whose synchronization error with the coordinated universal time UTC (international standard time) does not exceed 1μs; the other is the standard time code (year, month, day, hour, minute, second) corresponding to the leading edge of the 1PPS pulse output through the serial port, that is, the "time mark" of 1PPS. The distribution of the sending and receiving ends within the power system is widely dispersed. If a GPS receiver is installed at each end, the globality and high precision of GPS can ensure that the relative error between the time signal in various places and UTC does not exceed 1μs. This high-precision time synchronization on a global scale has extremely high utilization value in power system detection and measurement. This data acquisition card uses the second pulse and time mark provided by the GPS receiving board to perform remote synchronous data acquisition. Practice has proved that its effect is ideal.

4. Capture Card Software Design

The software program of this acquisition card mainly includes the serial port communication program with the GPS receiving board, the PCI interface program for communicating with the host computer, and the data acquisition and processing program. The software flow chart is shown in Figure 4.

The acquisition card can communicate with the host computer through the dual-port RAM to change the number of sampling points and sampling frequency, divide a power frequency cycle into 40 to 256 points for sampling, and then perform numerical conversion on the sampling points and send them to the host computer for further processing, such as the amplitude of voltage, current and power angle, curve display monitoring, and timely taking corresponding control measures when a system failure occurs.

The features of this acquisition card are as follows:

·Due to the use of second-order filtering circuit, the acquisition card can effectively shield the influence of high-order harmonics on A/D conversion. C8051F021 is a high-speed mixed signal model. Under the external crystal oscillator of 12MHz, its clock period is 83ns, so the sampling frequency can be adjusted in a wide range. It can be widely used in various AC sampling and control devices with high sampling rate requirements.

·Can input 8 single input or 4 differential input PT/CT signals simultaneously for high-precision and high-speed sampling.

The chip uses a successive approximation A/D converter with 12-bit resolution and 100ks/s conversion speed and a peripheral filter sample-and-hold circuit to enable fast analog-to-digital conversion.

By receiving the second pulse and time mark of GPS, the synchronous collection of signals in different places can be guaranteed.

Dual-port RAM and dedicated PCI interface make communication and data exchange between upper and lower computers faster.

5 Conclusion

Practice has proved that the data acquisition system implemented with the C8051F021 microcontroller can not only meet the needs of power system measurement and control devices, but also has a good cost-effectiveness. It is very suitable for measurement parts of transmission system measurement and control devices and instruments of various voltage levels.