Application of digital power quality online monitoring system in power system

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, imbalance, 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 my country'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 growing needs of power quality supervision, based on several years of operating 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 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 grid power quality monitoring system consists of power quality monitoring terminals, central stations and analysis software.

1.1 Power quality monitoring terminal

Input three-phase voltage 100V, three-phase current 5A or 1A for data signal processing, 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, and form substation reports.

The main functions of the power quality monitoring terminal are as follows.

a. Input signals are TV and 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 graphic (spectrum diagram, waveform diagram, curve diagram, vector diagram) operation interface.
e. The terminal can store data for more than one year, and the stored data is a group of data packets for 3min or 5min.
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 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 of time or a certain event process period, automatically form daily, monthly and annual reports, automatically find out the time periods and lines 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's 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: 47～53hz, 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 the 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 the power quality monitoring terminal consists of DSP software and PC104 software.

3.1dsp 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, the DSP part is expanded with high-speed SRAM and EEPROM. 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 it to the PC104 processing unit, which requires a faster clock frequency. In this device, the internal clock of the DSP is nearly 40MHz.

The DSP processing part is expanded with a fast 14-bit high-precision AD converter, which can perform 6-way simultaneous sampling, providing a guarantee for 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 channels of AD converter are sampled simultaneously.
b. Data processing, converting the format of the collected data.
c. FFT conversion calculation.
d. Data transmission, transferring DSP data to PC104.

3.1.3 Input and Operation

Input three-phase voltage and current, measure frequency, 1024 or 512-point AD conversion (AD uses dual 6-way high-speed AD converter), after FFT conversion, calculate the RMS value, and upload data. According to needs, only 31st or 61st harmonics or higher harmonics are transmitted during data transmission.

Perform FFT operation, take 31st (or 61st) harmonic every 0.5s, and calculate the RMS value 6 times every 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.

3.2pc104 part

The PC104 industrial control board uses a highly integrated PCM-3336 board, which has floppy disk and hard disk interfaces, can directly drive a 320×240 LCD monochrome display, 2 RS232C serial interfaces, 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 15G. 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 will automatically reset 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. It sends a 0.5s pulse to the DSP and collects 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 unbalance, voltage qualification rate, harmonic content rate, etc.
b. Graphically display the amplitude and phase angle of voltage and 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 rate, etc.

3.2.2 Receiving 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 harmonics and imbalance index

The calculation of harmonic and imbalance related indicators is based on the provisions of GB/T14549-1993 "Harmonics of public power grids for power quality" and GB/T15543-1995 "Allowable imbalance of three-phase voltage for power quality".

3.2.3.1 Harmonic calculation (calculated once for each set of data read)

a. The hth harmonic voltage content rate

Where uh is the hth harmonic voltage (root mean square value);

u1——fundamental voltage (root mean square value).

b. The hth harmonic current content

Where ih is the hth harmonic current (root mean square value);

i1——fundamental current (root mean square 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 and probability values

a. Calculation of maximum harmonic 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 during the measurement period and the value of the largest phase, and store them.

3.2.3.3 Harmonic over-limit alarm

The measured value is compared with the allowed value to determine whether it exceeds the limit. If so, an alarm is issued.

3.2.3.4 Voltage and current imbalance

Calculate the voltage and current unbalance (read a set of data every 3 seconds and calculate once), and calculate the 95% probability value of the voltage and current unbalance.

a. Take the maximum value of imbalance

b. 95 probability value. Calculate the 95 probability value within the measurement period (statistical period).

3.2.3.5 Unbalance Exceeding Limit Alarm

The measured value is compared with the allowed value to determine whether it exceeds the limit. If so, an alarm is issued.

3.2.4 Voltage qualification rate

3.2.4.1 Calculate voltage (read a set of data and calculate once every 3 seconds)

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, minimum value and 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.

The monitored voltage is displayed in real time with a refresh period of 2s.

3.2.4.2 Calculation of voltage qualification rate

3.2.5 Frequency

The zero-crossing detection circuit and DSP capture function are used to accurately measure the width of the whole cycle and thus calculate the frequency.

3.2.6 Display

The voltage/current waveform, voltage/current vector diagram, voltage/current fundamental wave and harmonic amplitude and phase angle are displayed in graphics and Chinese characters. 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 PC104 to communicate with DSP

In order to facilitate the communication between DSP and PC, a parallel interface with interrupt is extended, occupying the peripheral address and interrupt of PC104. The parallel communication is 8-bit bidirectional (interrupt) communication.

3.4 Modem and LAN communication management

The modem is connected to the rs232c serial interface, and several control lines are extended 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.