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DSP and CAN bus based on the design of RTU

In Electronic Infomation Category: D | on April 20,2011

Abstract: DSP and SST39VF080-70-4C-EI datasheet and the CAN field bus based on the distributed design of new substation RTU. The RTU is divided into traffic signal control module and SST39VF080-70-4C-EI price and control module, introduced the design of these two modules and SST39VF080-70-4C-EI suppliers and two modules in CPLD design. RTU in the design of software, the program uses a modular design.

Remote Terminal Unit (RTU) as the embodiment of "decentralized monitoring and control, management focus on" ideas of products since the 80s from the 20th century and quickly introduced into China has been widely used application of the RTU in the substation on the main electrical parameters to achieve the remote site acquisition and control command, remote release, and the information is assembled into packets or results, on the side to the control center or scheduling. Throughout the RTU products at home and abroad, and gradually from a centralized control structure to the modular, distributed, open system control structure development. The substation large volumes of data and information, real-time demanding, so will have a powerful and efficient computing power and rich DSP peripheral interface circuit used in the design of RTU; the same time the introduction of the open structure of the CAN field bus introduction substation communication used by field data which formed an open, reliable and real-time monitoring system.

1 System Architecture Design

RTU system uses a DSP-microprocessor, CAN bus communication for on-site distributed and modular structure. As easy to implement distributed functional decomposition modular structure, according to the need for centralized and decentralized installation of assembling, so the system has good flexibility. System structure shown in Figure 1.

Figure 1 System structure

RTU communication can be divided into the main control module and signal monitoring module. CAN bus for field data communication. Signal control module on the one hand to monitor remote substation circuit breaker (switch) position, knife position, OLTC transformer tap position, the accident changed signal, alarm signals (remote signal), while monitoring the main transformer, the line of the active power, reactive power, voltage, current, power factor, active power level, the degree of reactive power, frequency, etc. (telemetry) and converted into corresponding electrical parameters; the other hand, the host computer or control center issued the command through signal Control module control signal output port to control the circuit breaker points, co-location, OLTC transformer tap position (remote control and remote adjustment).

Taking into account the versatility and flexibility of RTU, communication control module has a variety of communication port mode:

(1) CAN bus signal control module and the next request for data associated with the order issued; (2) RS232 ports for communication with the PC, RTU processed data will be submitted for further processing with the PC, or by PC, remote operation and control; (3) remote communication port, to communicate with the remote control center; (4) RS485 port, used to implement the traditional RTU equipment or other under-intelligent instrument interfaces compatible.

2 communication control module of the hardware circuit design

With built-in CAN controller TMS320LF2407A (referred to as LF2407A) as the communication control module of the processing core, with the ALTERA company MAX7000S series EPM7128STC100-7 CPLD external digital processor integrated circuit, the system structure shown in Figure 2 instructions. Communication from the functional master module is divided into three parts: CAN bus interface part; and serial conversion part; keyboard and display part and power-down data storage section.

Figure 2 System diagram

The LF2407A CAN bus controller within the integrated, so do not have external CAN controller, CAN bus to realize the underlying protocol, simply CAN output terminal and the physical channel with interference between the light compartment and the CAN bus transceiver be. Light separated by TIs 8-pin dual-channel high-speed optical compartment HCPL2631, CAN transceiver with a variety of optional PHILIPS companys PCA82C250 protection and immunity as a differential drive, bus interface, in order to better solve the problem of radio interference, the communication medium used shielded cables, in order to overcome the long-term effects, reducing the communications medium signal reflection, the transmission line at both ends of two parallel 120 matching resistor.

The communication control module system has multiple serial ports for data communication with the outside world, real-time communication requirements are also higher, using common I / O port lines to form the serial port is not applicable here, the choice has four asynchronous TL16C554A chip communication unit and - string conversion. The PC, RS232 port remote communication ports are also to be able to 485 devices compatible with the traditional, so the use MAX232 chip, MAX 1482 chip will TL16C554A serial TTL level output signal into RS232, RS485 level.

Communication control module in order to maintain a certain independence of function, select the character comes with T6963C controller, LCD modules and 4 buttons together constitute the man-machine Interface. Monitor the communication status of the system to display, 4 function keys to complete the user settings, including the address of the module, various communication port baud rate and so on. X5045 to power down the system, some system parameters will be saved, re-starting when the system can be re-transferred to the data. High-speed static RAM expansion as an external data storage and debugging program memory space.

3 signal control module circuit design

Signal the main control module has a collection of objects: After the external voltage and current transformer transformed the standard three-phase 0 ~ 100 V AC voltage, and 0 ~ 5 A three-phase phase current; reflect the line state and the protection of substation switching operation volume; power meter pulse data. According to the classification of signals corresponding to analog, digital and pulse volume. Structural principle of the signal control module shown in Figure 3.

Figure 3, the structure of the signal control module diagram

Analog in order to ensure measurement accuracy and real-time, for analog sampling and measurement hardware circuitry using AC synchronous method. Hardware synchronization and frequency sampling circuit mainly by the hysteresis voltage comparator circuit, high-speed optical coupling, phase-locked frequency doubling circuit and pulse shaping circuit. Hysteresis comparator circuit the AC sine wave input signal into a 0 ~ 5 V square wave signal the same frequency, high-speed optocoupler 6N137 analog and digital parts of the isolated circuit, phase-locked PLL frequency circuit by the CD4046 and the three constitute a counter chip MC14522 programmable frequency multiplier 128, the output signal frequency sinusoidal input signal frequency of 128 times, and follow input sync changes. MC14522 synchronization signal output by the partial pressure after being sent to the capture module TMS320LF2407A CAP1, CAP2 for frequency measurements. CD4046 The synchronized output signals obtained by pulse shaping the appropriate pulse signal, then A / D converter ADS7864 of / HOLDA, / HOLDB, / HOLDC, select the input multiplexer and start the A / D conversion. ADS7864 is a high-speed (2 s), low power consumption (50 mW), single power supply (+5 V) 12-bit dual-working A / D converter. It to 500 kHz sampling rate for 6-channel signal sample at the same time, especially for power monitoring system. 6 ADS7864 input channels can be divided into three pairs, measuring three-phase power monitoring applications, and analog signals into digital signals required LF2407A, stored in on-chip FIFO register 6. To improve the efficiency of the system, the ADS7864s / / BUSY signal to the CPLD, produced by the three judge ADS7864 / BUSY signal to generate an interrupt to notify LF2407A take a one-time reading of the FIFO in the 6 ADS7864 good data processing conversion. In addition, ADS7864 with bipolar ( 5 V) input, the input AC voltage signal is 0 ~ 100 V, the current signal is 0 ~ 5 A, and therefore need to add signal conditioning and level conversion circuit.

And pulse switch input volume level is 12 V. HCPL2631 separated by high-speed optical isolation, to achieve immunity level matching and isolation, isolation and pulse switch after the amount of signal through the corresponding data buffer, respectively, the unit becomes LF2407A external I / O input signal, LF2407A through regular access to the corresponding The I / O ports on the switch to achieve the collection volume and pulse volume. When the signal control module detected a switch which changes the amount of bits, to produce a sequence of events information. Analysis of the reasons for the important role of grid failure.

Use CPLD integration and programmability of digital circuits will be integrated into the external processing on a single chip to achieve on other chips, liquid crystal display and keyboard address decoding, read and write, control, and signal buffering.

4 System Software

Hardware architecture based on the characteristics of the design during the system software can be the master and the RTU communications signal monitoring and control is divided into two separate modules, to separate the two considerations and design. Here, the use of modular programming approach to the design module of the overall software. Features from the software perspective, communication control module includes a variety of communication port communication program and human machine interface programming, signal control module software components include site acquisition and processing signals of various power parameters calculation and analysis, and communication with the host module procedures. In practice, communication and signal control module control module between multiple tasks is often overlapping, so that by the hardware interrupt request in response to different tasks and improve the processors real-time responsiveness.

5 Experimental results and analysis

Use of existing laboratory conditions, in order to verify the hardware A / D sampling system is good or bad, low-voltage 380 V/220 V power measurement, sampling 128 points compared with the oscilloscope waveform shown in Figure 4. Can be seen on analog sampling is accurate and feasible.

128 Figure 4 sampling points and oscilloscopes waveform comparison

Table 1 was 15 times the sampling operation harmonic peak

On this basis, for the harmonic analysis. This experiment uses the grid voltage transformed by the partial pressure of the rms voltage of about 1.7 V (normal multimeter test) test. Sign extended by sampling data directly FFT transform, the corresponding harmonic peak had 15 as shown in Table 1, the data can be seen from the table even harmonics is very small, almost zero; and gradually decreasing odd harmonics.

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