Application of digital power quality online monitoring system in power system

The online power quality monitoring equipment is the most basic and the most important equipment in the power quality supervision and detection network of the power grid. At present, the monitoring equipment of 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 of Hebei Province began to install harmonic online monitoring devices one after another. 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 operation experience and experience, a digital power quality online monitoring terminal was developed.
The new 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 the DSP digital signal processor and high-speed multi-channel AD simultaneous sampling technology equivalent to the latest foreign products, and uses PC104 industrial control computer for data processing and display storage. It has powerful functions and is easy to operate and upgrade software. 1.
Function and composition
The power quality monitoring system of the power grid consists 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, 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 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) and three-phase current (5A or 1A).
B. With a public telephone modem interface, you can dial and connect to receive data conveniently at the central station.
C. Large screen (320×240) backlit LCD graphic display.
D. Chinese graphics (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 data packets of 3min or 5min.
f. It has a LAN connection interface, and a laptop can be used to record data on site.
g. Comprehensive measurement of multiple parameters, real-time fixed-point alarm, parameter value and parameter alarm status can be set.
h. Harmonic voltage, current, negative sequence voltage, and 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 diagrams, waveform diagrams, curve diagrams, vector diagrams, etc.). It can manage multiple power quality monitoring terminals, analyze and process the collected data, analyze and form reports for the power quality of a certain period or a certain event process period, automatically form daily, monthly and annual reports, automatically find out the period and line with excessive harmonic content, 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. ［page]

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: 47～53Hz, accuracy is ±0.01Hz(50Hz),
Signal conversion accuracy: 14bit,
Sampling frequency: 8khz/channel.
3. Software/hardware composition of power quality monitoring terminal.
The hardware of power quality monitoring terminal consists of TA/TV and signal preprocessing, DSP processor, PC104 industrial computer, PC104 and DSP parallel communication ISA bus parallel expansion, modem, LCD display (VGA monochrome with backlight), network adapter, power supply, etc.
The software of power quality monitoring terminal consists of DSP software and PC104 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, 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 2FFT transformation calculation of 1024 points and transmit them to the PC104 processing unit, which requires a faster clock frequency. In this device, the internal clock of the DSP is nearly 40MHz. The
fast 14-bit high-precision AD converter is expanded in the DSP processing part. The AD converter can perform 6-channel simultaneous sampling, which provides a guarantee for the accurate calculation of active power, reactive power, and 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 the AD converter. "
b. Data processing, convert the format of the collected data.
c. FFT conversion 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-way high-speed AD converter), after FFT conversion, calculate the RMS value, and upload the data. According to the needs, only 31 or 61 harmonics or higher harmonics are transmitted during data transmission.
Perform FFT operation, take 31 (or 61) harmonics every 0.5s, and take 6 times every 3s to calculate the RMS value. The formula is:

Where uhk is the root mean square value of the hth 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, data is uploaded 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, which has a floppy disk and hard disk interface, can directly drive a 320×240 LCD monochrome display, 2-way RS232C serial interface, 1-way 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 15g. For ease of use and reliability, the hard disk uses an electronic disk or a laptop hard disk.
The industrial control board has a watch-dog function, which automatically resets when it does not work properly.
The PC104 board is responsible for data processing, storage, and display, as well as the communication connection and data transmission between the power quality monitoring terminal and the central station, and forms a substation report. Send a 0.5s pulse to the DSP to collect DSP data.
3.2.1 The software composition of PC104
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 the voltage and current fundamental wave and each harmonic, the vector diagram of voltage and current, and the voltage and current waveform on the 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, and over-limit setting of harmonic content rate, 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 harmonic components (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/t14549-1993 "Harmonics of Public Power Grid for Power Quality" and gb/t15543-1995 "Allowable Unbalance of Three-Phase Voltage for Power Quality". The specific formula is as follows. [page]

Where uh is the hth harmonic voltage (RMS value);
u1 is the fundamental voltage (RMS value).
b. Hth harmonic current content rate

Where ih is the hth harmonic current (RMS value);
i1 is the fundamental current (RMS value).
c. Harmonic voltage content

f. Total harmonic distortion of current

g. hth harmonic power and phase
3.2.3.2 Calculation of harmonic maximum value and probability value
a. Calculation of harmonic maximum value (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 and the value of the largest phase during the measurement period, and store them.
3.2.3.3 Harmonic over-limit alarm
Compare the measured value with the allowed 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 once), 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 allowed 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 and calculate once)
Calculate the rate of exceeding the upper limit and the rate of exceeding the lower limit, and count the cumulative time of exceeding the upper limit and the cumulative 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 the 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.
The monitored voltage is displayed in real time, and the refresh cycle is 2 seconds.
3.2.4.2 Calculate the voltage qualification rate 

3.2.5 Frequency 
Using zero-crossing detection circuit and dsp capture function, the width of the whole cycle is accurately measured, 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, voltage/current fundamental and harmonic amplitudes and phase angles. The amplitudes and phase angles of each harmonic are divided into digital display and bar graph plus angle pointer display.
3.3 ISA parallel expansion unit for PC104 and dsp communication 
In order to facilitate the 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 communication (interrupt) communication.
3.4 Modem and LAN communication management 
The modem is connected to the rs232c serial interface, and several control lines are extended separately 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 communication in lan network mode. 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 standard for power quality.
c. The Chinese and graphic display interface of the power quality monitoring terminal make it more convenient and intuitive for users to use.
d. The power quality monitoring terminal adopts the design of DSP and PC104 industrial control board, 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.