With the rapid development of power electronics technology, a large number of nonlinear loads have been added to power supply systems. The widespread application of AC converters, ranging from low-voltage, small-capacity household appliances to large-capacity industrial AC converters, has caused distortions in the voltage and current waveforms of the power grid, threatening the safe, stable, and economical operation of the power system. Monitoring power grid quality is particularly important in the construction of some key national projects. The Model 96 ACR230ELH power quality analyzer, with its comprehensive functions and compact size, plays a crucial guiding role in power quality detection and resolving harmonic generation issues.
Advanced technologies applied:
In order to measure, assess and evaluate the aforementioned harmonic hazards in the power grid, Shanghai Acrel Electric Co., Ltd. has developed the ACR230ELH, a high-end power quality analysis product based on a DSP+ARM7 modular design.
This product adopts a popular high-performance power design scheme: the DSP+MCU approach, combining the high-speed digital signal processing capabilities of the DSP with the comprehensive management, communication, and rich interface functions of a high-end MCU (ARM). The basic working principle is as follows:
Figure 1 Hardware Design Block Diagram
The current transformer sends the voltage and current signals from the power grid to the meter's 16-bit high-speed Sigma-Delta AD converter in real time for synchronous sampling of the power signals across six channels. After the AD conversion is completed, the data is transmitted to the DSP for various complex numerical calculations. Through the corresponding mathematical operations, the DSP completes the calculation functions of the entire meter, including the measurement of all power parameters, active power, reactive power, and apparent energy. After completion, the DSP exchanges the corresponding calculation data with the ARM7. The ARM7 mainly performs tasks such as LCD display, data statistics and storage, external communication, menu and keyboard operation, and input control of DI/DO quantities.
The A/D converter uses the AD73360L chip from Analog Devices (ADI), a 16-bit serial analog-to-digital converter with six independent channels. It employs the Sigma-Delta conversion principle and has excellent anti-aliasing performance. The AD73360's data conversion output interface is a synchronous serial port (SPORT port). The DSP used is the ADSP 219x chip, which also has the same SPORT interface, making it very convenient to use and eliminating the hassles associated with emulated I/O ports.
The ARM7 uses NXP's LPC2138, a powerful chip with abundant peripheral port resources. Its internal RAM can reach up to 32K, and its operating speed can reach 60M. It has a multi-channel 32-bit timer, multiple PWM output resources, and multiple SCI, SPI, and IIC interfaces. The I/O ports can withstand 5V voltage input. It also has multiple internal power consumption modes.
The software is divided into two parts. The first part consists of the DSP's relevant program flow code, which is mainly divided into the following functional sections: AD sampling control, serial data transmission, numerous complex numerical calculations (including Fourier transform), energy accumulation, power quality analysis, and power pulse output. The second part consists of the ARM7's relevant program flow, mainly including: LCD display, key processing, digital communication, implementation of switch input/output functions, and implementation of some event logging functions. Regarding the programming language selection, the DSP part uses a mixed programming approach of C and assembly language. To ensure good real-time performance, assembly language is used as the primary language, while C language is used for process scheduling of the entire program framework. This approach ensures both program readability and good real-time performance. The ARM7 program is developed based on the uc/OS-II operating system platform. The program is simple and easy to read, has good portability, and facilitates future product upgrades. The entire system software roughly completes three parts: system initialization code, uc/OS-II operating system porting, and application task writing.
Algorithm Description
Based on the aforementioned modular design concept, the powerful numerical calculation capabilities of the digital signal processor are fully considered, enabling it to complete all calculation and transformation functions. Specifically, this includes the calculation of RMS voltage and current, active and reactive power, apparent power, power factor, cumulative active, reactive, and apparent energy, and power quality indicators. Considering that the calculation formulas for conventional electrical parameters are relatively common, they will not be elaborated upon here. The focus will be on describing the calculation methods for power quality indicators: Power quality calculations specifically include: 2nd-31st harmonic analysis, voltage crest factor, telephone waveform factor, current K-coefficient, three-phase voltage and current imbalance, positive-sequence, negative-sequence, and zero-sequence analysis of voltage and current, voltage deviation, and frequency deviation.
The relevant indicators for power quality are as follows:
A. Harmonics (GB/T 14549);
The analysis of the 2nd to 31st harmonics of the voltage was mainly achieved using the FFT algorithm of the DSP. The fundamental and harmonic components obtained were U1, U2, ..., Uh, respectively. The formulas for calculating the content of each harmonic are as follows:
; ;
The analysis of the 2nd to 31st harmonics of the current is similar;
Figure 2. Power grid harmonic waveforms and harmonic bar diagrams
B. Imbalance (GB/T 15543)
Voltage imbalance includes the amplitude imbalance of the signal in each phase and the angular imbalance between the three phases. The specific calculation is based on the following formula:
The calculation method for current imbalance is the same as that for current.
Figure 3 Three-phase unbalance waveform
C. Frequency deviation (GB/T 15945)
Frequency deviation is the difference between the actual and nominal values of the system frequency. The allowable normal frequency deviation for a power system is ±0.2Hz. When the system capacity is small, the deviation can be relaxed to ±0.5Hz. The system frequency variation caused by user impulse loads must not exceed ±0.2Hz. The limit may be appropriately adjusted according to the nature and size of the impulse load and the system conditions, but the safety, stable operation, and normal power supply of the nearby power grid, generator units, and users must 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 permissible deviation of three-phase power supply voltage of 10kV and below is ±7% of the rated voltage. The permissible deviation of single-phase power supply voltage of 220V is +7% and -10% of the rated voltage.
Figure 4 Voltage Change
In addition, during the actual operation of the power system, there are some parameters that indirectly reflect the actual situation of the power grid waveform, such as voltage crest factor, voltage waveform factor and current K coefficient (the first two mainly measure the impact of voltage waveform distortion, and the third mainly measures the changes caused by current waveform distortion).
The voltage crest factor is calculated as follows:
Telephone waveform factor:
Current K coefficient:
Functionality and performance:
The ACR230ELH implements 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 energy (including absorbed active energy, released active energy, inductive reactive energy, and capacitive reactive energy) high-precision metering; can measure active energy for this month, last month, the month before last, and total energy in three rates for eight time periods;
3. Comprehensive measurement of 2nd to 31st harmonic components of voltage and current, as well as THD, current K coefficient, voltage crest factor, telephone waveform factor, and three-phase voltage and current imbalance, etc., allows users to easily analyze the power grid quality.
4. It has a communication interface with 4 digital inputs, 2 digital outputs, and 1 485 interface with Modbus protocol, 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 image below for detailed results.
Figure 5. Appearance of the exported OEM instrument
Application Cases
The Expo Center, the first permanent venue in the Shanghai World Expo Park, is located in the riverside green space of Area B of the Expo Park, east of the Lupu Bridge. It measures approximately 350 meters east to west and 140 meters north to south, with a total construction area of approximately 140,000 square meters. It has been completed and is now in trial operation. As one of the most important permanent venues of the Expo, the Expo Center will serve as the Expo's operation command center, celebration and conference center, press center, and forum and activity center during the Expo.
Figure 6. Exterior view of the Expo Center venue
Shanghai Acrel Electric Co., Ltd.'s ACR230ELH system handled all the power display instruments for the five substations in the central venue of the Expo Center project. Taking Substation #1, one of the five underground substations in the Expo Center, as an example: this is a 0.4kV low-voltage distribution substation system, consisting of 2 receiving cabinets, 2 emergency incoming line cabinets, 1 bus tie cabinet, 4 compensation cabinets, and 155 feeder cabinets. The feeder cabinet loads can be specifically divided into: exhibition power, lighting power, air conditioning fan power, power supply, and fire protection power. The power distribution system uses energy-saving light sources such as LEDs, high-power variable frequency central air conditioning, and a large number of non-linear electrical devices such as exhibition power supplies. Although this reduces actual power consumption to achieve energy-saving goals, these devices are major sources of harmonics in the power system. Even if these devices are supplied with ideal sinusoidal voltage, the current they draw is non-linear, meaning harmonic currents exist. The harmonic currents generated by these devices are injected into the power system, causing harmonic components in the voltage at various points in the system. These devices, though individually small in capacity, are numerous and scattered across various locations, making them difficult for the power sector to manage. If the current harmonic content of these devices is too high, it could severely impact the power system of the Expo Center venue.
Figure 7
After the ACR230ELH multi-functional power meter is used on site, users can not only understand all the conventional electrical parameters, but also understand the power quality level of the relevant systems before and after the commissioning of various power equipment at the World Expo Center venue and the impact of its changes on the relevant equipment. This facilitates the diagnosis of power quality faults and the measurement of the causes of anomalies, providing real-time data for the stable operation of large public buildings.
The following functions of this product were mainly used in this project: conventional electrical parameter measurement, multi-rate energy metering, power quality analysis, on/off status indication of disconnect switches, and 485 communication networking. The field system network structure (as shown in Figure 10) is: Field device layer – Network communication layer – Station control management layer. That is, the field intelligent devices are first connected to the local Modbus bus, then the Modbus bus is converted into TCP/IP Ethernet via a serial port network server to exchange data with the monitoring host. Finally, management personnel can easily understand and grasp the field conditions and perform real-time monitoring through the monitoring system software.
1. Real-time monitoring of electrical parameters required for on-site equipment layer:
The field equipment layer consists of data acquisition terminals, mainly composed of intelligent instruments (primarily the ACR230ELH power quality analyzer in this project), which upload stored parameters to the data center. In the Expo Center site management project, these parameters include, 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 tie circuit; the three-phase current and power of ordinary low-voltage outgoing line circuits; and the three-phase current, active and reactive power, voltage, and current of important low-voltage outgoing line circuits, as well as the 2nd to 31st harmonics and harmonic distortion rates (THDi, THDu), etc.
2. Data transmission is implemented at the network communication layer:
Electrical parameters collected from field devices (such as the ACR230ELH) are exchanged and shared via Ethernet to enable system information exchange, and the data is provided to the main transformer monitoring system. Simultaneously, various control commands from the host computer to the field devices are relayed.
3. The station control management layer reflects the various operational statuses on site in the most intuitive and convenient way:
The station control management layer, for the monitoring system administrators, is the direct human-computer interaction window and the top-level department of the system. The monitoring system software has a user-friendly interface, calculates, analyzes, and processes various types of data collected from the field, and intuitively presents various electrical parameters and valuable historical data since the system's operation in the form of reports and graphs, such as harmonic data, bar graphs and curves, daily, monthly, and yearly power consumption reports, log reports, etc.
Conclusion
The ACR230ELH from Shanghai Acrel Electric Co., Ltd. can monitor the load characteristics of outgoing loads in real time, monitor power quality parameters, and perform real-time analysis of parameters such as harmonics. This facilitates real-time monitoring of the power system's operating status and harmonic pollution level by the host computer system, providing reliable and accurate data to Expo operation and maintenance engineers so that they can take timely and appropriate countermeasures to ensure a green and environmentally friendly Expo.
Figure 8 System network topology
References
1) "Principles and Application Guide of Digital Instruments for Power Measurement", Ren Zhicheng, Zhou Zhong, China Electric Power Press
2) Application Guide to 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
