Application of MAX125 in Power Quality Monitoring System

1 Introduction

The real-time performance of power quality monitoring of circuit systems is high, including not only frequency, voltage, current, active power, reactive power, harmonic components, sequence components, etc., but also the frequency of some collected feature quantities changes quickly and complexly, such as transient mutations. High-frequency fault traveling waves, etc. It is difficult or even difficult to implement multi-channel sampling calculation by ordinary acquisition and processing methods. In order to overcome the above deficiencies and meet the requirements of modern power systems, advanced digital signal processing technology is applied to power systems. Give full play to its fast and powerful computing and parallel operation processing capabilities, meet the requirements of power system monitoring real-time and processing algorithm complexity, and lay a technical foundation for the development of new theories and new algorithms applied to power system practice.

The TMS320LF2407A DSP is used as the control core. The power quality monitoring system with fast response, small delay and no phase offset is designed. In the whole control system, the conversion accuracy of the signal acquisition unit determines the performance of the whole control system. The TMS320LF2407 has a 10-bit A/D converter with built-in sample and hold circuit, but can only receive 0V-3.3V unipolar signals. For AC signals, an additional limiting boost circuit is required, and each channel in the same sequencer Crosstalk is severe, so the 10-bit conversion accuracy is difficult to meet the requirements of high-performance systems. Since this application requires high acquisition accuracy and requires a 14-bit A/D converter, Maxim's MAX125 converter is used as the signal acquisition unit. core.

2 MAX125 features and working principle

The MAX125 is an 8-channel high-speed 14-bit A/D converter with a sample/hold (T/H) and a single conversion time of 3μs. By writing a control word to the MAX125, it can be operated in a single-channel conversion. It can be operated in multi-channel continuous conversion, and its conversion data has buffer function. The DSP can be easily read. The MAX125 is powered by bipolar power, the input voltage range is ±5V, and the converter's maximum input overvoltage is up to ±17V. And the damage of a channel will not affect the normal operation of the whole circuit. Table 1 shows the working mode of MAX125, MAX125 better meets the accuracy requirements. Through reasonable control circuit design, the data acquisition channel can better meet the requirements. The real-time requirements of the system, Figure 1 shows the internal structure of the MAX125.

Figure 2 shows the operation timing of the MAX125. Each CONVST pulse starts 1-4 T/H conversions. After the MAX125 is initialized, the operating mode is set to 0X0002. The system uses two MAX125 CH1A, CH2A, and CH3A channels to respectively compare the three-phase voltage and The three-phase current is converted, that is, only three channels per MAX125 need to be converted, the conversion time of each channel is 3μs, the conversion of three channels requires 9μs, and the sampling rate is up to 100kS/s.

The MAX125 conversion is started by the CONVST pulse. After about 9μs, the three channels are converted, and one interrupt signal is generated. The interrupt signal is sent to the programmable logic controller (GAL), and the GAL notifies the DSP to read the sample data, and the GAL gives The MAX125's RD pin applies a read pulse, and three consecutive read operations can read three consecutive data. Figure 3 shows the block diagram of the acquisition channel.

3 acquisition system design
The MAX125 has A and B2 sets of signal inputs, each set of 4 input channels, and built-in 4 sample-and-holders. In practical applications, the load current, compensation current, power supply current, and capacitor voltage on the DC side of the inverter are separately collected. The MAX125 can work in parallel with the DSP to reduce the workload of the DSP. The output data of the MAX125 can be level-converted before being sent to the data line of the TMS320LF2407. If the output of the MAX125 is directly sent to the data input pin of the TMS320LF2407, It is possible to exceed the withstand voltage value (3.6V) of the TMS320LF2407 pin. This system uses 74AHCT245 as the level shifter. The MAX125 output DOD13 is connected to the 74AHCT245 latch for level conversion, and then enters the TMS320LF2407 data bus, 74AHCT245. The operating voltage is 3.3V, the output signal level is 3.3V, and the input signal of 5.5V can be accepted. In this design, two 74AHCT245 are used as the level conversion unit, the input data is 14-bit data from MAX125, and the redundant two inputs. The pin is grounded, the output signal is connected to the DO-D15 pin of the TMS320LF2407, and the two 74AHCT245 control signals are sent by the GAL. The circuit principle is shown in Figure 4.

The main instructions of the control system are all completed in the timer. Figure 5 shows the flow of the timer period interrupt service routine. When the value of the timer counter is equal to the value of the timer period register, a periodic interrupt request is generated. After the interrupt is responded, the interrupt service routine is entered. After confirming that the interrupt source is correct, the external A/D converter is first started to collect the load current. Compensate the current and the capacitor voltage, and then shift the conversion result to obtain the necessary processing such as the complement code.

4 Conclusion

The design of this power system power quality monitoring system is based on the TSM320LF2407A DSP, which is powerful and expandable. The A/D conversion uses a 14-bit MAX125 converter, which can be simultaneously collected by multiple channels. The conversion accuracy is high. The experimental verification signal acquisition module works stably and reliably, and has been put into mass production.

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