Application of Power Quality Online Monitoring Equipment Based on DSP in Power Grid

The power quality online monitoring equipment is the most basic and most important equipment for the power quality monitoring and inspection network of the power grid. Most of the monitoring equipments for power quality indicators (such as harmonics, unbalance, etc.) that are currently sold and used on the market. Can not fully adapt to the actual needs of China's power grid quality supervision and management. In southern Jiangxi Province, the harmonic grid online monitoring device was installed in 1996. The initial devices have many problems in data storage, data transmission and background statistical analysis. In order to meet the growing needs of power quality supervision, based on years of operational experience and experience, Baoding Guodian Zhongke Electric Co., Ltd. developed the GDDN-500 series of digital power quality online monitoring terminals.
The new GDDN-500 series digital power quality online monitoring device has the functions of collecting power quality parameters according to national standards, high reliability of long-term online operation, convenient and practical on-site operation, and communication with the central station; It is convenient to record, store data and read data. The device adopts the same DSP digital signal processor and high-speed multi-channel AD same mining technology as the latest foreign products, and adopts dual DSP structure design in data processing and display storage, which is powerful, easy to operate and software upgrade. ?
1 Function and composition?
The power quality monitoring system of the power grid is composed of a power quality monitoring terminal, a central station and analysis software. ?
1.1 Power quality monitoring terminal?
Input three-phase voltage 100V, three-phase current 5A or 1A for data signal processing, using FFT to calculate the amplitude and phase angle of each harmonic voltage and current. Calculate unbalanced voltage and current, and calculate technical data such as three-phase voltage, current, voltage pass rate, frequency, active power, reactive power and power factor. Responsible for data processing, storage and communication and data transmission with the central station to form a substation report.
The main functions of the power quality monitoring terminal are as follows. ?
a. The input signal is TV, TA secondary side three-phase voltage (100V), three-phase current (5A or 1A).
b. With the public MODEM interface, you can conveniently receive data at the central station.
c. Large screen (320 & TImes; 240) backlit LCD graphic display.
d. Chinese graphics (spectrum, waveform, graph, vector) operation interface.
e. The terminal can store data for more than one year, and the stored data is a group of data packets of 3 min or 5 min.
f. With a LAN connection interface, you can use the laptop to copy data on the spot.
g. Multi-parameter comprehensive measurement, real-time fixed-point alarm, parameter value and parameter alarm status can be set.
h. Harmonic voltage, current, negative sequence voltage, current over limit alarm exit relay. ?
1.2 The central station and the analysis software central station accept the data of the processor through a modem or network, perform statistical analysis, form files, reports and curves, and display data and graphics (such as spectrograms, waveforms, graphs, vector diagrams, etc.) . It can manage multiple power quality monitoring terminals, analyze and process the collected data, analyze and form reports on power quality during a certain period of time or an event period, and automatically form daily, monthly and annual reports, automatically Find the time period and line where the harmonic content rate exceeds the standard, and calculate the voltage qualification rate and power supply reliability.
The central station is a client-server mode, and the data is stored in the database of the server, which can be easily called and queried.
2 Main technical indicators?
2.1 Measurement items The device uses (220±15%) Vac or [(220 +10%)～(220-15%)] Vdc power supply. The measurable items include: voltage, current, frequency, voltage pass rate, active power. , reactive power, apparent power, power factor, voltage imbalance, current imbalance, harmonic voltage, harmonic current (to 31/50 times or higher), harmonic phase, harmonic power, distortion rate Wait. ?
2.2 Measurement accuracy
Voltage measurement: ±0.2%?
Current measurement: ±0.2%?
Voltage imbalance measurement error: ≤0.2%?
Current imbalance measurement error: ≤ 1%?
Frequency measurement: 47 to 53 Hz, accuracy is ±0.01 Hz (50 Hz)?
Signal conversion accuracy: 16bit?
Sampling frequency: 12.8kHz / channel?
3 Power quality monitoring terminal software/hardware structure?
The hardware of the power quality monitoring terminal is composed of TA/TV and signal preprocessing, DSP processor, LCD display (VGA monochrome with backlight), network adapter, power supply and the like.
The software of the power quality monitoring terminal is composed of DSP software. ?
3.1 DSP software?
3.1.1 DSP principle?
The monitoring terminal adopts TI's 320C2XX series TMS320F240 chip. Considering the limited internal storage capacity of the chip, the high-speed SRAM and EEPROM are expanded in the DSP part. The final design of the system needs to collect 1024 points (6 channels) in each power frequency cycle, and it needs to perform 1 024 6-base 2 FFT conversion calculation, which requires a faster clock frequency. In this device, the DSP The internal clock is nearly 40MHz. ?
A fast 16-bit high-precision AD converter is extended in the DSP processing section. The AD converter can perform 6-channel simultaneous sampling, which provides a guarantee for accurate calculation of active, reactive power, and positive/negative sequences. ?
3.1.2 The composition and function of DSP?
a. Data acquisition part, including sampling and calculation of frequency, 6 channels of AD converter are simultaneously sampled. "
b. Data processing, transforming the acquired data into a format.
c. FFT transformation calculation.
d. Data transfer, transferring the data of the DSP to the PC 104. ?
3.1.3 Input and operation Input three-phase voltage, current, measurement frequency, 1024 or 512-point AD conversion (where AD adopts double 6-channel high-speed AD converter), and after FFT transformation, calculate the root mean square value and upload the data. Only 31 or 61 harmonics or higher harmonics are transmitted during data transfer as needed.
Perform FFT operation, take 31 (or 61) harmonics every 0.5s, and calculate the rms value every 6s for 6s. The formula is:

Where Uhk is the square root mean square of the hth harmonic measured at kth time in 3s. ?
3.1.4 Data Transfer The data is uploaded every 3s according to the pulse given by the 0.5s host computer. Taking the 31st harmonic as an example, each set of data is as follows.
a. Frequency f.
Each harmonic is divided into a real part and an imaginary part, and the phase of U?a is used as a reference phase.
3.2 PC104 part?
The PC104 industrial control board adopts a highly integrated PCM-3336 board with a floppy disk and hard disk interface, which can directly drive 320 & TImes; 240 LCD monochrome display, 2 RS232C serial interface, 1 way printer parallel interface, can Direct keyboard and normal display. The board's BIO design can be connected to hard drives up to 15 G. For ease of use and reliability, the hard drive uses an electronic or notebook hard drive.
The industrial control board has the WATCH-DOG function, and it automatically resets when the work is not working properly.
Responsible for the processing, storage and display of data, communication connection and data transmission between the power quality monitoring terminal and the central station, forming a substation report. A 0.5 s pulse is sent to the DSP to collect DSP data. ?
3.2.1 Software composition of PC104?
a. Calculate and process various data, including voltage, current, active, reactive, positive and negative sequence, voltage imbalance, voltage yield, harmonic content, and so on. ?
b. Graphically display the voltage, current fundamental wave and the amplitude, phase angle, voltage and current vector, voltage and current waveform of each harmonic on the LCD.
c. Communication transmission function, including communication with DSP, communication with MODEM and network communication.
d. Parameter input, including voltage and current ratio, voltage upper and lower limits, and harmonic content exceeding the limit setting.
3.2.2 Receiving DSP data The data received from the DSP is temporary data, with frequency, three-phase voltage, three-phase current and corresponding positive and negative zero-sequence components and harmonic components (divided real and imaginary parts, total 2 & TImes ; 3 × 64 data). ?
3.2.3 Calculation of harmonics and imbalance indicators?
The calculation of the correlation index of harmonics and unbalance is based on GB/T 14549-1993 "Power Quality Harmony of Public Grid" and GB/T 15543-1995 "Power Quality Three-Phase Voltage Allowable Unbalance". The specific formula is as follows.
3.2.3.1 Harmonic calculation (calculated once per reading of a set of data)
a. The hth harmonic voltage content rate?

Where Uh is the hth harmonic voltage (square root mean square value);
U1 - the fundamental voltage (square root mean value).
b. hth harmonic current content rate
Where Ih - the hth harmonic current (square root mean value);
I1 - fundamental current (square root mean value).
c. Harmonic voltage content?

f. Current total harmonic distortion rate?

g. The hth harmonic power and phase?
3.2.3.2 Calculation of harmonic maximum and probability values ​​a. Calculation of harmonic maximum value (each time value and total distortion rate)
b. Calculation of 95% probability value Calculate the 95% probability value of each phase measured value and the value of the largest one phase in the measurement period, and store it.
3.2.3.3 Harmonic over-limit alarm The measured value is compared with the allowable value to determine whether it exceeds the limit. If the limit is exceeded, an alarm is issued. ?
3.2.3.4 Voltage and current unbalance The voltage and current imbalance are calculated (calculated once every 3 s to read a set of data), and the 95% probability value of voltage and current unbalance is calculated.
a. Take the maximum imbalance value b. 95% probability value. Calculate the 95% probability value within the measurement period (statistical period).
3.2.3.5 Unbalanced over-limit alarm The measured value is compared with the allowable value to determine whether it is over-limit. If the limit is exceeded, an alarm will be issued.
3.2.4 voltage pass rate
3.2.4.1 Calculate the voltage (calculate once per 3s reading a set of data)
Calculate the over-limit rate, the over-limit rate, the statistical over-limit accumulating time, and the lower-limit accumulating time; calculate the voltage pass rate; store the recorded data of the previous month and the current month, the previous day and the current day; record the maximum value, the minimum value, and the average value.
The setpoint and limit of the monitored voltage can be set. The voltage quality monitoring time is in min, and the average value of 1 min is taken as a statistical unit.
The monitored voltage is displayed in real time with a refresh period of 2s.
3.2.4.2 Calculating the voltage pass rate??

3.2.5 Frequency?
The zero-crossing detection circuit and DSP capture function are used to accurately measure the width of the entire cycle to calculate the frequency.
3.2.6 Display graphic and Chinese character mode display voltage/current waveform, voltage/current vector diagram, voltage/current fundamental wave and harmonic amplitude and phase angle. The amplitude and phase angle of each harmonic are divided into digital display and The bar graph plus the angle pointer is displayed. ?
3.3 ISA parallel expansion unit for PC104 and DSP communication?
For convenient communication between DSP and PC, the parallel interface with interrupt is extended, occupying the peripheral address and interrupt of PC104. The parallel communication is 8-bit bidirectional contactable (interrupt) communication. ?
3.4 MODEM and LAN communication management?
MODEM is connected to the RS232C serial interface, and several control lines are extended separately. The MODEM real-time monitoring and control is ensured that the MODEM communicates for a long time. ?
The extended network card allows LAN network communication. ?
4 Conclusion?
a. The power quality monitoring terminal can accurately monitor the power supply and power consumption status of the power grid in real time, in particular, it can grasp the over-standard condition of harmonics at any time, master the situation of asymmetry and voltage qualification rate, and provide power supply and power enterprises. Convenient monitoring equipment. ?
b. The power quality monitoring terminal has the characteristics of high sampling frequency, accurate measurement and fast calculation speed, and its measurement index meets the requirements of national standards for power quality. ?
c. The Chinese and graphic display interface of the power quality monitoring terminal makes the user more convenient and intuitive to use. ?
d. The power quality monitoring terminal adopts DSP and PC104 industrial control board design, with advanced technology and high accuracy, which can conveniently perform software maintenance and upgrade for DSP and PC104. ?
e. The power quality monitoring terminal can form a power quality monitoring network in the regional power grid and the provincial network or the combined power grid, and realize the statistical analysis of a large amount of historical data through a dedicated central station software, form various statistical reports, and draw a harmonic spectrum map. And the distribution map of various indicators provides an advanced means for the supervision of power quality.