Design principle and application of ACR230ELH power quality analyzer

Abstract: This paper introduces the design principle of ACR230ELH power quality analyzer and its practical application in the power distribution system of the central venue of the 2010 Shanghai World Expo.
With the vigorous development of power electronics technology, a large number of nonlinear loads have been added to the power supply system. The widespread application of low-voltage and small-capacity household appliances to large-capacity industrial AC converters has caused the distortion of the voltage and current waveforms of the power grid, threatening the safe, stable and economic operation of the power system. In the construction of some important national projects, the monitoring of power grid quality is particularly important. As a 96-type power quality analyzer with complete functions and small size, it plays an important guiding role in power quality detection and solving the problem of harmonic generation.
Advanced technology applied:
In order to measure, evaluate and assess the harmonic hazards of the above-mentioned power grid, Shanghai Ankerui Electric Co., Ltd. has developed a high-end power quality analysis product ACR230ELH based on the DSP+ARM7 modular design solution.
This product adopts the high-end power standard design solution popular in the world today: the implementation method of DSP+MCU, combining the high-speed digital signal processing function of DSP with the perfect management, communication and rich interface functions of high-end MCU (ARM). The basic working principle is as follows:

Figure 1 Hardware design principle block diagram
The mutual inductor sends the voltage and current signals from the power grid to the 16-bit high-speed Sigma-DelTa AD in the meter in real time for synchronous sampling of 6-channel power signals. After the AD conversion is completed, the data is transmitted to the DSP for various complex numerical operations. Through the corresponding mathematical operations, the DSP completes the calculation function of the entire meter, including the measurement of all power parameters, active, reactive, and apparent power measurement. After completion, the DSP exchanges the corresponding calculated data with ARM7. ARM7 mainly completes LCD display, data statistics storage, external communication, menu keyboard operation, DI/DO input control, etc.
The A/D uses the AD73360L chip launched by ADI, which has a 16-bit serial analog-to-digital converter with 6 independent channels. It adopts the Sigma-Delta conversion principle and has good anti-aliasing performance. The data conversion output interface of AD73360 is a synchronous serial port (SPORT port). The DSP uses the ADSP 219x chip, which also has the same SPORT interface. It is very convenient to use and eliminates a series of troubles caused by IO port simulation.
ARM7 uses NXP's LPC2138, which is a powerful chip with rich peripheral port resources. The internal RAM can reach up to 32K and the running speed can reach 60M. It has a multi-channel 32-bit timer, multiple PWM output resources, multiple SCI, SPI, and IIC interfaces; the IO port can withstand 5V voltage input; and it also has multiple operating power consumption modes inside.

The software of this table is divided into two parts. The first part is the relevant program flow code of DSP. This part of the code is mainly divided into the following parts according to its functions: AD sampling control, serial port data transmission, a large number of complex numerical calculations (including Fourier transform), energy accumulation, power quality analysis, power pulse output and other parts; the second part is the relevant program flow in ARM7, which mainly includes: LCD display, key processing, digital communication, realization of switch input and output functions and realization of some event recording functions; in the selection of programming language, the DSP part adopts C language and assembly language mixed programming. In order to ensure the good real-time performance of the system, assembly language is mainly used, and C language is used for the whole program framework process scheduling, which not only ensures the readability of the program, but also takes into account the good real-time performance of the system. The program of ARM7 is developed based on the uc/OS-II operating system platform. The program is simple and easy to read, with good portability, which is convenient for the subsequent upgrade of the product. The software of the whole system roughly completes three parts of work: system initialization code, uc/OS-II operating system transplantation, and application task writing.
Algorithm description
Based on the above modular design concept, the powerful numerical calculation function of the digital signal processor is fully considered to enable it to complete all calculation and transformation functions; specifically including voltage, current effective value, active, reactive power, apparent power, power factor, accumulation of active, reactive, and apparent electric energy, and calculation of power quality indicators. Considering that the calculation formulas of conventional electrical parameters are relatively common, I will not go into details here, and focus on describing the calculation method of power quality indicators: Calculations of power quality include: 2-31 harmonic analysis, voltage crest factor, telephone waveform factor, current K factor, three-phase voltage and current imbalance, voltage and current positive sequence, negative sequence, and zero sequence analysis, voltage deviation, and frequency deviation.
The relevant indicators of power quality are as follows:
A. Harmonics (GB/T 14549);
The 2nd to 31st harmonic analysis of voltage is mainly realized by using DSP FFT algorithm, and the fundamental wave component and harmonic component obtained are U1, U2…Uh respectively; The calculation formula of each harmonic content rate is as follows:
; ;
The 2nd to 31st harmonic analysis of current is similar;

Figure 2 Harmonic waveform and harmonic bar graph of power grid
B. Unbalance (GB/T 15543)
Voltage unbalance includes the amplitude of each phase signal and the angular unbalance between the three phases. The specific calculation refers to the following formula:
The calculation method of current unbalance is the same as that of current.

Figure 3 Three-phase unbalanced waveform