Design and application of ACR230ELH power quality analyzer

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 economical operation of the power system. In the construction of some important national projects, the monitoring of power grid quality is particularly important. As the current fully functional and small-sized 96-type ACR230ELH power quality analyzer, 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, which combines 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 block diagram

The transformer 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 energy. 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 powerful and has rich peripheral port resources. The internal RAM can reach up to 32K, the running speed can reach 60M, and it has multi-channel 32-bit timers, multi-channel 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.

The table software 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 the function: 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, etc.; 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 ARM7 program 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 subsequent product upgrades. The software of the entire system roughly completes three parts of work: system initialization code, uc/OS-II operating system porting, and application task writing. The

algorithm description

is based on the above modular design concept, fully considering the powerful numerical calculation function of the digital signal processor, allowing it to complete all calculation transformation functions; specifically including voltage, current effective value, active, reactive power, apparent power, power factor, active, reactive, apparent energy accumulation, and power quality index calculation. 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: Power quality calculations, specifically including: 2-31 harmonic analysis, voltage crest factor, electrical waveform factor, current K factor, three-phase voltage, current imbalance, voltage, current positive sequence, negative sequence, zero sequence analysis, voltage deviation, 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 the FFT algorithm of DSP. The fundamental wave component and harmonic wave components obtained are U1, U2…Uh respectively; the calculation formula of each harmonic content is as follows:

The analysis of the 2nd to 31st harmonics of current is similar;

Figure 2 Grid harmonic waveform and harmonic bar graph

B. Unbalance (GB/T 15543)

Voltage unbalance includes the amplitude of each phase signal and the angle unbalance between the three phases. The specific calculation refers to the following formula:

The current unbalance is calculated in the same way as the current. [page]

Figure 3 Three-phase unbalanced waveform

C. Frequency Deviation (GB/T 15945)

Frequency deviation is the difference between the actual value and the nominal value of the system frequency. The allowable value of normal frequency deviation of the power system is ±0.2Hz. When the system capacity is small, the deviation value can be relaxed to ±0.5Hz. The system frequency change caused by the user's impact load shall not exceed ±0.2Hz. The limit value may also be appropriately changed according to the nature and size of the impact load and the conditions of the system, but the safety, stable operation and normal power supply of the nearby power grid, generator sets and users should be guaranteed.

D. Voltage Deviation (GB/T 12325)

The sum of the absolute values ​​of the positive and negative deviations of the power supply voltage of 35KV and above shall not exceed 10% of the rated voltage.

The allowable deviation of the three-phase power supply voltage of 10KV and below is ±7% of the rated voltage. The allowable deviation of the 220V single-phase power supply voltage is +7%, -10% of the rated voltage.

Figure 4 Voltage change

In addition, during the actual operation of the power system, there are some parameters that will indirectly reflect the actual situation of the grid waveform, such as voltage crest factor, voltage waveform factor and current K factor (the first two are mainly to measure the impact of voltage waveform distortion, and the third is mainly to measure the changes caused by current waveform distortion).

The calculation of voltage crest factor is as follows:

Electrical waveform factor:

Current K factor:

Functions and performance:

ACR230ELH realizes the following functions:

1. High-precision measurement of conventional three-phase AC electrical quantities, such as three-phase phase voltage, line voltage, current, active power, reactive power, apparent power, power factor, etc.;

2. Professional four-quadrant electric energy (including absorbed active energy, released active energy, inductive reactive energy, and capacitive reactive energy) with high precision measurement; it can measure the active energy of this month, last month, the month before the month before, and the total active energy in 8 time periods and three rates;

3. Comprehensive measurement of 2nd to 31st harmonic components of voltage and current and THD, current K factor, voltage crest factor, electrical waveform factor, and three-phase voltage and current imbalance, so that users can easily analyze the quality of the power supply network;

4. It has 4 switch inputs, 2 switch outputs, and a 485 interface Modbus protocol communication interface, which fully meets the needs of power automation remote control and telemetry.

This product has passed CE certification and has been successfully exported to the UK and Spain. See the figure below for detailed effects.

Figure 5 Appearance of export OEM instrument

Application Cases

The Expo Center is the first permanent venue in the Shanghai World Expo Park. It is located in the riverside green space of Zone B of the Expo Park on the east side of the Lupu Bridge. It is about 350 meters long from east to west and about 140 meters wide from north to south. The total construction area is about 140,000 square meters. It has been completed and put into trial operation. As one of the most important venues in the permanent venues of the World Expo, the Expo Center will assume the functions of the World Expo operation command center, celebration conference center, news center, forum activity center, etc. during the World Expo. [page]

Figure 6: Exterior view of the Expo Center

In this project, ACR230ELH of Shanghai Ankerui Electric Co., Ltd. has taken over all the power data display instruments of the five substations in the central venue. Take the 1# substation among the five underground substations of the Expo Center as an example: this is a 0.4kV low-voltage distribution station system, which consists of 2 power receiving cabinets, 2 emergency incoming line cabinets, 1 busbar cabinet, 4 compensation cabinets, and 155 feeder cabinets. The load of the feeder cabinet can be specifically divided into: exhibition power, lighting power, air conditioning fan power, power power and fire protection power. The power distribution system uses a large number of nonlinear power-consuming devices such as LED and other energy-saving light sources, high-power variable frequency central air conditioners, and exhibition power. Although the actual power consumption is reduced to achieve the goal of energy saving, these devices are the main sources of harmonics in the power system; even if these devices are supplied with ideal sinusoidal voltage, the current they use is nonlinear, that is, there are harmonic currents. The harmonic currents generated by these devices are injected into the power system, causing harmonic components to be generated in the voltages of various parts of the system. The individual capacity of these devices is not large, but the base number is large and scattered everywhere, making it difficult for the power department to manage. If the current harmonic content of these devices is too large, it will have a serious impact on the power system of the Expo Center.

Figure 7

After the ACR230ELH multifunctional power meter is used on site, users can not only understand all conventional electrical parameters through the meter, but also understand the power quality level of related systems before and after the commissioning of various power equipment on the site of the Expo Center and the impact of its changes on related equipment, which is convenient for power quality fault diagnosis and abnormal cause measurement, and provides real-time data for the stable operation of large public buildings.

In this project, the following functions of the product are mainly used: conventional power parameter measurement, multi-rate power metering, power quality analysis, disconnection and closing status indication of disconnecting switches, and 485 communication networking. The network structure of the field system (as shown in Figure 10): field device layer - network communication layer - station control management layer. That is: first connect the field intelligent equipment to the local Modbus bus, then convert the Modbus bus through the serial port device networking server to TCP/IP Ethernet to exchange data with the monitoring host, and finally facilitate the management personnel to understand and grasp the field conditions through the monitoring system software and conduct real-time monitoring.

1. The field equipment layer collects the electrical parameters required for real-time monitoring:

The field equipment layer is a data acquisition terminal, which is mainly composed of intelligent instruments (mainly ACR230ELH power quality analyzer in this project), and uploads the stored parameters to the data center. In the Expo Center field management project, for example, the three-phase current, three-phase voltage, active/reactive power, power factor and frequency of the main incoming line circuit; the power factor, active and reactive power of the capacitor compensation circuit; the three-phase voltage and current of the contact circuit; the three-phase current and power of the ordinary low-voltage outgoing line circuit; the three-phase current, active and reactive power, 2-31 harmonics of voltage and current, harmonic distortion rate (THDi, THDu) and other electrical parameters of the important low-voltage outgoing line circuit;

2. The network communication layer realizes data transmission:

the electrical parameters collected in the field equipment (such as ACR230ELH) realize the exchange and sharing of system information through Ethernet, and provide the data to the total transformer monitoring system. At the same time, convey various control commands of the host computer to the field equipment.

3. The station control management layer reflects the various operating states of the site in the most intuitive and convenient form:
The station control management layer is the direct window for human-computer interaction for the management personnel of the monitoring system, and is also the top-level department of the system. The monitoring system software has a good human-computer interaction interface, calculates, analyzes and processes various types of data information collected on site, and intuitively reflects various electrical parameter information and historical data with reference value since the system was put into operation in the form of reports and graphics, such as harmonic data, bar graphs and curves, daily, monthly and annual reports of power consumption, log reports, etc.

Figure 8 System network topology

Conclusion

The ACR230ELH of Shanghai Ankerui Electric Co., Ltd. can monitor the load characteristics of the feed-in load in real time, monitor the power quality parameters, and perform real-time analysis on harmonics and other parameters, so that the host computer system can monitor the power system operation status and harmonic pollution level in real time, and provide reliable and real data to the Expo operation and maintenance engineers so that they can take reasonable countermeasures in time to ensure the realization of a green and environmentally friendly Expo.

References
1) "Principles and Application Guide of Digital Power Meters", Ren Zhicheng, Zhou Zhong, China Electric Power Press
2) "Application Guide of National Standards for Power Quality", China Standards Press
3) "Principles of Digital Signal Processing and Its MatLab Implementation", Electronic Industry Press
4) "Industrial and Civil Power Distribution Design Manual", Ren Yuanhui, China Electric Power Press