Design of intelligent power load control and monitoring system

0 Introduction

Through the intelligent power load control and monitoring system, the digital information of the intelligent distribution equipment can be collected and processed, so that the distribution station can be operated with few or no personnel, providing a decision-making basis for the planning and design of the power system and the power production plan. This paper focuses on giving a new design method for power load control and monitoring system. Based on reliable performance, high precision, low power consumption and small size, this method can better adapt to the functional needs of load management, power analysis, power collection, multiple control methods, optimized power analysis, etc., and can meet various application requirements of power load side management.

System working principle

Figure 1 shows the working principle block diagram of this system. This system uses LPC2132FBD64, which is suitable for embedded system design, as the main processor; the interface circuit mainly completes the acquisition of pulse quantity, switch quantity and analog quantity, and the main processing circuit is responsible for calculating them to form various real-time data such as power, electricity, meter readings, demand, current, voltage, etc. Then the main processing circuit performs closed-loop management based on these real-time data and the working parameters issued by the master station, and at the same time sends these real-time data to the display drive circuit through the I2C bus, and then displays them through the LCD liquid crystal module. When the master station calls for measurement information, the system sends these data back to the master station through GPRS. At the same time, the main processing board controls the corresponding relays to achieve the purpose of closed-loop control and remote control. This system highlights modular design, the functions of each part are relatively independent, and the cross-connection between the programs of each module is avoided as much as possible. Therefore, it has good scalability, which can reduce the workload of modifying and maintaining programs in the future, and at the same time improve the readability, reliability and stability of the program, making remote upgrade and maintenance possible.

2 Main module functions

2.1 Data Collection

The data collection of this system is first the collection of state quantities, that is, real-time collection of position status and other status information, recording them in memory when position changes occur, and setting them to two modes as needed: waiting for the master station to query or actively reporting to the master station.

The second is the data collection of electric energy meters. Usually, the terminal can collect and store the data of electric energy meters through the RS-485 communication interface according to the set terminal meter reading day. Each power management terminal can read multiple electric energy meters at the same time. The data storage can store the electricity of two meter reading cycles (data at the end of the month or at zero o'clock on the meter reading day), and can be sent to the main station when the main station calls for testing, or it can be sent to the main station at a fixed time.

AC analog quantity acquisition refers to the terminal using the ATT7022B metering chip to measure the instantaneous values ​​of three-phase voltage, current, active power, reactive power, power factor, active power, reactive power, etc. on the user side in real time, with an accuracy of usually 0.5%.

2.2 Negative control function

(1) Power constant value control

Power set value control can be divided into four control modes, with their priorities from low to high: time period control, plant rest control, business stop control, and current power down-floating control. The terminal can modify the corresponding control mode and control parameters according to the power control time period, power set value, set value floating coefficient, alarm time, control round and other instructions issued by the master station, and notify the customer through the sound alarm. During the automatic execution of the terminal, the set value, control object, execution result, etc. can be displayed on the LCD.

(2) Electric energy constant value control

The electric energy constant value closed-loop control can be divided into three control types: monthly electric energy control, purchased electric energy (fee) control, and fee reminder alarm.

(3) Remote control function

After receiving the tripping control command from the master station, the terminal will control the controlled load switch according to the set alarm delay time and power-limiting time; at the same time, the terminal should have an audible alarm to notify the customer, and record the tripping time, tripping round, power before tripping, power 2 minutes after tripping, etc. At the same time, the terminal display screen should display the execution result. When the terminal receives the control command to allow closing from the master station, the audible alarm will notify the customer and allow the customer to close the switch. When the terminal receives the command to allow/prohibit calls and allow/prohibit active reporting from the master station, the terminal will control the call and actively report to the system according to the command of the master station.

2.3 Wireless communication function

This system uses built-in GSM/GPRS module communication (external radio communication can also be used, users can choose). As a new communication method, GPRS has many advantages in distribution automation applications. The integrated management system of distribution transformers based on GPRS communication is a promising implementation solution.

2.4 Harmonic analysis and alarm function

The terminal can analyze the voltage harmonics and current harmonics of the monitoring point according to the voltage and current harmonic thresholds, and record the daily maximum and occurrence time of the 2nd to 19th harmonic voltage content rate and total distortion rate of each phase, and count the harmonic over-limit data of each phase. The data includes the daily cumulative time of the over-limit of the total distorted voltage content rate of each phase, the daily cumulative time of the over-limit of the 2nd to 19th harmonic voltage content rate of each phase, the daily cumulative time of the over-limit of the total distorted current of each phase, and the daily cumulative time of the over-limit of the 2nd to 19th harmonic current of each phase. When the user's harmonic exceeds the standard, the system will automatically alarm.

2.5 Event recording and important event reporting function

The terminal can record the time, status, data, event type and related conditions of power-on, power-off, programming, time calibration, demand clearing, phase failure, overvoltage, voltage loss, current loss, reverse phase sequence, overload, three-phase current imbalance, CT primary short circuit, CT secondary short circuit, CT secondary open circuit, meter box door opening, etc. It can also record them in two queues: general events and important events, in the order of occurrence. The maximum length of each queue record is 255, and the queue is refreshed in a first-in-first-out manner.

For important events set by the master station (such as power control tripping, electric control tripping, remote control tripping, important parameter changes, etc.), when the event occurs, the terminal can refresh the contents of the important event counter in real time, make records, and directly and actively report the event records through ACD settings.

2.6 Storage and reporting of load curve and historical data

Daily load curve data is collected every 15 minutes, 96 data per day, usually saving the data of the last 10 days (including active power, reactive power, voltage, current, etc.); and can be reported to the master station according to the settings. The system terminal has an active reporting function, which can report data or passively report (wait for the master station to test the data), and can also realize the query function of historical data.

2.7 Meter reading test function

In order to facilitate users to know whether the electric energy meter connected to the terminal can communicate successfully, different meter addresses and communication rates can also be set through the LCD screen and buttons to perform manual meter reading.

3. Software and Hardware Implementation of AC Analog Quantity Acquisition Module

3.1 Hardware circuit of AC analog quantity acquisition module

After the system passes through the voltage transformer and current transformer, it can process the three-phase AC voltage and three-phase AC current values ​​of the electrical equipment, and then send them to the ATT7022B chip for measurement and calculation. Figure 2 shows the voltage and current acquisition circuit of this system.

ATT7022B can measure active power, reactive power, apparent power, active energy and reactive energy of each phase and combined phase, and can also measure parameters such as current, voltage RMS, power factor, phase angle, frequency, etc. of each phase. The peripheral circuit of ATT7022B chip is shown in Figure 3. ATT7022B integrates 7 16-bit ADCs and adopts double-ended differential signal input. The maximum input voltage is 1.5 V, and the maximum effective value of the sinusoidal signal that can be input is 1V. In practical applications, the voltage channel Un can be selected to correspond to the ADC input at around 0.5 V, while the ADC input of the current channel Ib can be selected at around 0.1 V. The metering module mainly performs high-pass filtering and phase shift filtering or phase correction on the data collected by the voltage and current sampling channels to obtain the required parameter quantities (including voltage, current RMS, power, frequency, phase angle and other three-phase parameters), and stores these parameters in the corresponding registers, and then transmits the parameters to MC-U through the SPI port.

3.2 Data transmission software flow between ATT7022B and microprocessor

Figure 4 shows the SPI communication process based on ATT7022B. This program mainly completes the initialization of ATT7022B. It sends data to it or modifies the parameters in the ATT7022B register according to the query command of LPC2132FBD64. If an abnormal situation occurs, the terminal automatically sends an alarm message to the monitoring center.

The data transmission between ATT7022B and microprocessor is initiated by LPC2132FBD64 sending 8-bit command word to DIN terminal of SPI interface. Commands are divided into two categories: read command and write command, which can be distinguished by the highest bit of command word. If the highest bit is 1, the command is a write command, if the highest bit is 0, it is a read command.

By reading commands, LPC2132FBD64 can read the active power, reactive power, apparent power, active energy and reactive energy of each phase and the combined phase in ATT7022B, and can also obtain the effective value of each phase current and voltage, as well as parameters such as power factor, phase angle, frequency, etc.

The write command is the corresponding program of the calibration command, which mainly completes the parameter accuracy correction of ATT7022B. ATT7022B must be corrected for parameter accuracy to ensure that the acquisition results meet the accuracy requirements of the system.

4 System Performance Test

This product has passed the field test and expert appraisal, and all performances are up to standard. The error between the test results of the data acquisition function and the test results of the measurement center is within 0.2%. The negative control function has been tested 500 times, and no misoperation has occurred. In terms of reliability and stability, except for the operating temperature of the liquid crystal at -20 to 70 degrees, the operating temperature of all other components is -40 to 85 degrees, which can cope with a variety of harsh environments.

5 Conclusion

This system can measure the power quality data information of the power grid in real time and accurately, obtain real-time and detailed power quality monitoring reports at any time, and directly manage and control the power settings. Therefore, it can effectively improve the curve shape of the power load, make the load curve flatter, reduce the peak-to-valley difference, and achieve the best distribution of power load in a certain time and space. At the same time, it can also increase the load rate of users and power grids, thereby improving the utilization rate of power generation, supply, and power consumption equipment, and achieving safe and economical operation of the power grid. Since the widespread application of load control and monitoring systems is the development trend of automation technology in power companies, this system is of great significance to the current development of power resources of power companies, as well as the economy of power companies and the adjustment of power supply and consumption order.