Application of DSP-based online power quality monitoring equipment in power grid

The online power quality monitoring equipment is the most basic and most important equipment in the power quality supervision and detection network of the power grid. At present, most of the monitoring equipment for some power quality indicators (such as harmonics, unbalance, etc.) produced at home and abroad and sold and used in the market cannot fully meet the actual needs of power quality supervision and management in China's power grid. In 1996, the southern power grid in Hebei Province began to install harmonic online monitoring devices. The initial devices had many problems in data storage, data transmission and background statistical analysis. In order to meet the needs of the growing development of power quality supervision, based on several years of operating experience and experience, Baoding Guodian Zhongke Electric Co., Ltd. developed and manufactured the GDDN-500 series digital power quality online monitoring terminal.
The new GDDN-500 series digital power quality online monitoring device has the functions of collecting various 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; at the same time, it can also record and store data for a long time and read data conveniently. The device adopts DSP digital signal processor and high-speed multi-channel AD simultaneous sampling technology equivalent to the latest foreign products, and adopts dual DSP structure design in data processing and display storage, which is powerful, easy to operate and upgrade software.
1 Function and composition The
power quality monitoring system of the power grid consists of power quality monitoring terminal, 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, and use FFT to calculate the amplitude and phase angle of each harmonic voltage and current. Calculate unbalanced voltage and current, calculate and display technical data such as three-phase voltage, current, voltage qualification rate, frequency, active power, reactive power and power factor. Responsible for data processing, storage, communication connection and data transmission between the central station, and 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) and three-phase current (5A or 1A).
b. With a public telephone MODEM interface, you can dial up and receive data conveniently at the central station.
c. Large screen (320×240) backlit LCD graphic display.
d. Chinese graphic (spectrum, waveform, curve, vector) operation interface.
e. The terminal can store data for more than one year, and the stored data is a group of 3min or 5min data packets.
f. With a LAN connection interface, you can use a laptop to copy data on site.
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 output relay.
1.2 Central station and analysis software
The central station receives data from the processor through a modem or network, performs statistical analysis, forms files, reports and curves, and can display data and graphics (such as spectrum, waveform, curve, vector, etc.). It can manage multiple power quality monitoring terminals, analyze and process the collected data, analyze and generate reports for the power quality of a certain period or a certain event process period, automatically generate daily, monthly and annual reports, automatically find the 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 server database, 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 qualification rate, active power, reactive power, apparent power, power factor, voltage unbalance, current unbalance, harmonic voltage, harmonic current (up to 31/50 or higher), harmonic phase, harmonic power, distortion rate, etc. 2.2
Measurement accuracy

Voltage measurement: ±0.2%
Current measurement: ±0.2%
Voltage unbalance measurement error: ≤0.2% Current
unbalance measurement error: ≤1%
Frequency measurement: 47-53 Hz, accuracy ±0.01Hz (50Hz)
Signal conversion accuracy: 16bit
Sampling frequency: 12.8kHz/channel
3 Software/hardware composition of power quality monitoring terminal
The hardware of the power quality monitoring terminal consists of TA/TV and signal preprocessing, DSP processor, LCD display (VGA monochrome with backlight), network adapter, power supply, etc. The software
of the power quality monitoring terminal consists of DSP software.
3.1 DSP software
3.1.1 DSP principle The
monitoring terminal uses the TMS320F240 chip of TI's 320C2XX series. Considering the limited internal storage capacity of the chip, high-speed SRAM and EEPROM are expanded in the DSP part. The final design of the system needs to collect 1024 points (6 channels at the same time) in each power frequency cycle, and needs to perform 6-channel radix 2 FFT transformation calculation of 1024 points, which requires a faster clock frequency. In this device, the internal clock of DSP is nearly 40MHz.
In the DSP processing part, a fast 16-bit high-precision AD converter is expanded. This AD converter can perform 6-channel simultaneous sampling, which provides a guarantee for accurate calculation of active power, reactive power, positive/negative sequence.
3.1.2 Composition and function of DSP
a. Data acquisition part, including frequency sampling and calculation, 6-channel simultaneous sampling of AD converter. "
b. Data processing, convert the format of the collected data.
c. FFT transformation calculation.
d. Data transmission, transfer the data of DSP to PC104.
3.1.3 Input and operation
Input three-phase voltage and current, measure frequency, 1024 or 512-point AD conversion (where AD uses dual 6-channel high-speed AD converters), after FFT transformation, calculate the root mean square value, and upload the data. According to needs, when transmitting data, only 31 or 61 harmonics or higher harmonics are transmitted.
Perform FFT operation, take 31 (or 61) harmonics every 0.5s, and take 6 times every 3s to calculate the root mean square value. The formula is: Where

Uhk is the root mean square value of the h-th harmonic measured at the kth time within 3s.
3.1.4 Data transmission
According to the pulse given by the host computer every 0.5s, upload data every 3s. Taking the 31st harmonic as an example, each group of data is as follows.
a. Frequency f.
Each harmonic is divided into real part and imaginary part, with the phase of Ua as the reference phase.
3.2 PC104 part
The PC104 industrial control board uses a highly integrated PCM-3336 board. The board has floppy disk and hard disk interfaces, can directly drive a 320×240 LCD monochrome display, 2 RS232C serial interfaces, and 1 printer parallel interface, and can directly connect a keyboard and a normal display. The BIO design of the board can connect a hard disk up to 15 G. For ease of use and reliability, the hard disk uses an electronic disk or a notebook hard disk.
The industrial control board has a WATCH-DOG function, which automatically resets when it does not work properly.
It is responsible for data processing, storage, and display, communication connection and data transmission between the power quality monitoring terminal and the central station, and forms a substation report. Send 0.5 s pulses to the DSP to collect DSP data.
3.2.1 PC104 software composition
a. Calculate and process various data, including voltage, current, active power, reactive power, positive and negative sequence, voltage imbalance, voltage qualification rate, harmonic content rate, etc. b
. Graphically display the amplitude and phase angle of voltage, current fundamental wave and each harmonic, vector diagram of voltage and current, and voltage and current waveform on LCD.
c. Communication transmission function, including communication with DSP, communication with MODEM and network communication.
d. Parameter input, including voltage and current transformation ratio, voltage upper and lower limits, over-limit setting of harmonic content, etc.
3.2.2 Receive DSP data
The data received from DSP is temporary data, including frequency, three-phase voltage, three-phase current and corresponding positive and negative zero-sequence components and each harmonic component (real part and imaginary part, a total of 2×3×64 data).
3.2.3 Calculation of harmonic and unbalance index
The calculation of harmonic and unbalance related indexes is based on the provisions of GB/T 14549-1993 "Harmonics of Public Power Grid for Power Quality" and GB/T 15543-1995 "Allowable Unbalance of Three-Phase Voltage for Power Quality". The specific formula is as follows.
3.2.3.1 Harmonic calculation (calculated once for each set of data read)
a. The content rate of the hth harmonic voltage, where

Uh is the hth harmonic voltage (RMS value);
U1 is the fundamental voltage (RMS value).
b. The content rate of the hth harmonic current

Where Ih is the hth harmonic current (RMS value);
I1 is the fundamental current (RMS value).
c. Harmonic voltage content,

f. Total harmonic distortion rate of current,

g. Power and phase of the hth harmonic
3.2.3.2 Calculation of maximum and probability values ​​of harmonics
a. Calculation of maximum harmonic values ​​(values ​​of each order and total distortion rate) b. Calculation of

95% probability value
Calculate the 95% probability value of the measured value of each phase during the measurement period and the value of the largest phase, and store them.
3.2.3.3 Harmonic over-limit alarm
Compare the measured value with the allowable value to determine whether it exceeds the limit. If it exceeds the limit, an alarm will be issued. 3.2.3.4
Voltage and current imbalance
Calculate the voltage and current imbalance (read a set of data every 3 seconds) and calculate the 95% probability value of the voltage and current imbalance.
a. Take the maximum value of the imbalance
b. 95% probability value. Calculate the 95% probability value within the measurement period (statistical period).
3.2.3.5 Unbalance over-limit alarm
Compare the measured value with the allowable value to determine whether it exceeds the limit. If it exceeds the limit, an alarm will be issued.
3.2.4 Voltage qualification rate
3.2.4.1 Calculate the voltage (read a set of data every 3 seconds)
Calculate the upper limit rate and the lower limit rate, and count the accumulated time of exceeding the upper limit and the accumulated time of exceeding the lower limit; calculate the voltage qualification 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 rated value and limit value of the monitored voltage can be set. The voltage quality monitoring statistical time is in minutes, and the voltage average value of 1 minute is taken as a statistical unit.
Real-time display of the monitored voltage, the refresh cycle is 2 seconds.
3.2.4.2 Calculation of voltage qualification rate

3.2.5 Frequency
Zero-crossing detection circuit and DSP capture function are used to accurately measure the width of the whole cycle, so as to calculate the frequency.
3.2.6 Display
Graphics and Chinese characters are used to display voltage/current waveforms, voltage/current vector diagrams, amplitude and phase angle of voltage/current fundamental waves and harmonics. The amplitude and phase angle of each harmonic are divided into digital display and bar graph plus angle pointer display.
3.3 ISA parallel expansion unit for communication
between PC104 and DSP In order to facilitate the communication between DSP and PC, the parallel interface with interrupt is expanded, occupying the peripheral address and interrupt of PC104. The parallel communication is 8-bit bidirectional communication (interrupt) communication.
3.4 MODEM and LAN communication management The
MODEM is connected to the RS232C serial interface, and several control lines are expanded to monitor and control the MODEM in real time to ensure that the MODEM communicates normally for a long time.
The extended network card allows LAN network communication. 4
Conclusion
a. The power quality monitoring terminal can monitor the power supply and power consumption of the power grid in real time and accurately, especially the harmonic exceeding the standard, the asymmetry and the voltage qualification rate at any time, and provide convenient monitoring equipment for power supply and power consumption enterprises.
b. The power quality monitoring terminal has the characteristics of high sampling frequency, accurate measurement and fast operation speed, and its measurement indicators meet the requirements of the national power quality standards.
c. The Chinese and graphic display interface of the power quality monitoring terminal makes it more convenient and intuitive for users to use.
d. The power quality monitoring terminal adopts DSP and PC104 industrial control board design, which is advanced in technology and high in accuracy. It can easily maintain and upgrade the software of DSP and PC104.
e. The power quality monitoring terminal can form a power quality monitoring network in the regional power grid, provincial power grid or joint power grid, and realize the statistical analysis of a large amount of historical data through the dedicated central station software, form various statistical reports, draw harmonic spectrum diagrams and distribution diagrams of various indicators, and provide advanced means for the supervision of power quality.