Power frequency phase angle source based on single chip microcomputer

0Overview

Two columns of sinusoidal signals with a certain phase difference (phase angle source for short) are widely used in the fields of automatic calibration of various metering instruments in power systems and test power supply of relays used in relay protection. For this reason, we designed an industrial frequency phase angle source. The design method of this phase angle source is also a new technology that is being actively promoted by power systems and other industries at home and abroad. It uses a single-chip microcomputer as the control core, changes the frequency of the signal through an indirect frequency synthesizer (IS), generates two columns of signals through a digital waveform generator and realizes digital angle adjustment, and realizes amplitude adjustment through a programmable attenuator. The overall block diagram is shown in Figure 1. The achievable technical indicators are as follows: the frequency range is 1Hz~1kHz, the minimum step size is 1Hz, the phase angle range is 0°~360°, the minimum step size is 0.5°, the voltage effective value range is 0~10V, and the minimum adjustment step size is 0.01V. 

1Indirect frequency synthesizer (IS)

In the phase source system, the stability of the clock signal fCLK is very important. Both the phase angle accuracy and the frequency stability of the signal are related to fCLK. In order to meet the requirements, we selected a 32768Hz crystal oscillator to generate the reference signal, and used the timer/counter 8253 and the CD4046 phase-locked loop to form an indirect frequency synthesizer. The circuit diagram is shown in Figure 2. T1 of 8253 is used as a fixed 2048 divider, then fr=32768÷2048=16Hz, T2 is used as a programmable divider, and the division coefficient M=N×45 (N=1,2,3,…,1000) So, fCLK=Mfr=N×45×16=N×720Hz is obtained. 

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2Digital waveform generator

This part of the circuit consists of two preset 720-base counters, two EPROMs, two DAC08320s and op amps. The circuit is shown in Figure 3. A period of sinusoidal signal is divided into 720 points at every 0.5°. The corresponding quantized data are stored in two EPROM2716s. Two preset 720-base counting outputs are connected to the address lines of the two EPROMs. The values ​​of the two EPROMs are read out simultaneously with the same clock fCLK and sent to two D/A converters to generate two columns of sinusoidal waves. By presetting different initial values ​​for the two address counters, the required phase angle is generated. The frequency corresponding to the two columns of sinusoidal signals is the N value in the above-mentioned frequency divider. The set terminals of the first counter are all grounded. Assuming the preset initial value of the second counter is Z, the phase angle between v′01 and v′02 is △φ=Z×0.5°. The two counters are implemented with a GAL device. Since the decimal number 720 is converted to binary with 10 digits, and the microcontroller used is 8-bit, we use 74LS377, 74LS74, etc. to realize the assembly of 8-bit to 10-bit data. 0832 is connected in a straight-through type, the first-stage op amp realizes the conversion from current to voltage, and the output voltage is 0~-5V. The second-stage op amp converts the unipolar 0~-5V into a bipolar -10.242V~10.242V, and the effective value is 10.24V. 

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3Programmable attenuator

This part of the circuit is shown in Figure 4, where only the first path is drawn. The second path is different from the first path only in the chip select signal of the programmable attenuator. In practical applications, the effective value of the output signal voltage is often required to be adjustable. For this reason, we use AD7520 to design a programmable attenuator so that the effective value of the output signal voltage can be adjusted from 0 to 10V with a preset step size of 0.01V. Since = 0.01V, the accuracy requirement of 0.01V step size can be achieved. Interface conversion is required between the 8-bit microcontroller and the 10-bit AD7520. Please refer to the conversion circuit of 8-bit to 10-bit data assembly in Figure 3. 

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The function of the low-pass filter in the figure is to filter out the small steps in the waveform, and the function of the power amplifier is to improve the load carrying capacity of the signal source. There are many circuits available for power amplifiers, so I will not go into details here.

4. MCU minimum system and software flow

The minimum system of single-chip microcomputer is as follows: 89C51 single-chip microcomputer, keyboard display dedicated chip 8279, decoder 74LS154. The software flow is shown in Figure 5. Due to space limitations, it will not be introduced in detail here. It should be noted that we only discussed two signal channels in the above design. The same circuit as the second channel can be used to expand the third channel to meet the instrumentation needs of the three-phase AC circuit.

References

1. Song Dinghua. Principles of Single Chip Microcomputer and Interface Technology. Beijing: Electronic Industry Press, 2001.4
2. Jiang Dachuan. Functional Automatic Test System of Multifunctional Watt-hour Meter. Electrical Measurement and Instrumentation, 2001.1