Design of low voltage reactive power compensation controller based on LPC2220

The reactive power compensation controller designed in this paper strives to be simple and practical in hardware, reduce costs and improve product reliability, give full play to the advantages of software design, and add practical functions.

Controller Working Principle

There are a large number of inductive loads in the power grid. By connecting the capacitive load in parallel, the energy exchange between the two loads can be realized. The reactive power required by the inductive load is compensated by the reactive power output by the capacitive load. Installing parallel capacitors improves the voltage quality of the power grid. The basic framework of the reactive compensation controller designed in this paper is shown in Figure 1. The three-phase voltage and current transformer converts the higher voltage and current signals into low voltage signals. The LPC2220 is used for A/D conversion and data analysis and calculates the voltage/current, and then calculates the power factor, reactive power and other parameters. The composite switch is used to switch the capacitor to achieve zero-crossing switching. The capacitor switching adopts a combination of three-phase co-compensation and three-phase split compensation. The controller also has the function of recording the historical data of voltage, current, power factor, reactive power and capacitor switching, as well as querying the historical data. The operation mode is divided into automatic and manual. Manual switching is suitable for debugging. Automatic operation automatically exits when power is off and can automatically resume after power is supplied. Users can change the control parameters of automatic capacitor switching on site according to the actual application environment. Real-time data and capacitor switching conditions can be queried at any time through the on-site LCD display or remote PC.

Figure 1 Reactive power compensation controller structure diagram

Hardware Design

Voltage and current transformer selection

LPC2220 has eight A/D conversion interfaces, which can convert 0V-VREF (typically 3V, maximum not exceeding VDD) analog voltage into a 10-bit digital signal. The voltage/current transformers all use the TR series voltage output converter for detection of Tianrui Electronics. The voltage transformer uses the voltage output voltage converter TRl102-IC for detection, with specifications of 380V/3.53V, nonlinearity ratio difference <±0.1%, angle difference <=±5 minutes. The current transformer uses the voltage output current converter TR0102-2C for detection, with specifications of 5A/2.88V, nonlinearity ratio difference (±0.1%, angle difference <=±5 minutes.

Capacitor switching switch

The composite switch combines the advantages of mechanical contactors and electronic contactless thyristor capacitor switching switches, such as voltage zero-crossing input, low inrush current, current zero-crossing cut-off, no overvoltage, long life, low power consumption, and low temperature rise. There are many mature products on the market. It can realize three-phase co-compensation and three-phase split compensation. The schematic diagram of the composite switch is shown in Figure 2. During the entire control process, when the capacitor needs to be put into operation, the reactive power compensation controller only needs to send a voltage signal to the composite switch, and the composite switch will automatically complete the capacitor input and cut-off process.

Figure 2 Basic principle block diagram of compound switch

Main circuit design

LPC2220 is a microprocessor based on 16/32-bit ARMTT TDMI-S CPU that supports real-time simulation and tracing. Applications that have strict control over code size can use 16-bit Thumb mode to reduce code size by more than 30% with little performance loss. The on-chip 128-width memory interface and unique acceleration mechanism enable 32-bit code to run at the maximum clock rate. At the same time, due to the 144-pin package, extremely low power consumption, multiple 32-bit timers, 8-way 10-bit ADO, PWM output and up to 9 external interrupts of LPC2220, they are particularly suitable for industrial control. By configuring the bus, LPC222O can provide up to 76 GPIOs. Multiple serial interfaces include two 16C550 industrial standard UARTs, a high-speed I2C interface (400kbit/s) and two SPI interfaces, 8-channel 10-bit A/D, converter conversion time as low as 2.44ms, CPU operating voltage range 1.65~1.95V (1.8V±8.3%), I/O operating voltage range 3.0-3.6V (3.3V±10%).

The storage of historical data can self-monitor the operation status of the controller. After background analysis, it can be used to determine the performance of the reactive compensation device, analyze the actual load of the power grid in the area and the load change curve, which has a very important reference value for future power grid maintenance and transformation. The controller needs to store the hourly data, switching data and alarm data within three months of operation. The amount of data is large and the types are also more. When LPC2220 accesses the external memory, it must go through its external memory controller (FMC). FMC is a slave module on the AMBA-AHB bus. It provides an interface for the AMBA AHB system bus and external memory. The module can support up to 4 groups of independently configured external memories at the same time. Each group supports RAM, ROM, Flash (flash memory), BurstROM, etc. The maximum storage capacity is 16MB, and the data bus width can be configured to 8, 16, and 21 bits through programming. SST39LF/VFl60 is a LM×t6 CMOS multi-function parallel FLASH device that can perform fast erasure (sector, block, chip) and word programming. It has software and hardware write protection functions, and the power-off data retention time is greater than 100 years. Therefore, this chip is often used in large-capacity data storage, especially for applications that require programs, configurations or data storage to be updated easily and at low cost. The specific wiring method is to connect CS0 of LPC2220 to CE of SST39VF160. Pin90 of LPC2220 is connected to the read signal OE; WE (Pin29) of LPC2220 is connected to the WE of SST39VF160; 16-bit data bus [DO~D15] is connected to [DO~D15] of LPC2220; the pin address output line [AI~A20] of LPC2220 external memory is connected to [A0~A19] of SST39VF160 chip.

The human-machine interface unit is responsible for the information exchange between the device and the operator. A friendly human-machine interface is very important for the use and maintenance of the device. The LCD display part can use a segmented LCD screen, and the commonly used 256-segment (32×8) LCD screen control chip HTl622 only needs 4 lines to communicate with the main controller, and the interface is very convenient.

The low-voltage reactive power compensation controller works on the secondary side (low-voltage side) of the transformer. 220V voltage is the most easily available power supply for the controller. A DC power supply of 12V or above is input from the power adapter, and a stable 12V DC voltage can be obtained through devices such as 7912, which is used for the control signal of the composite bypass. Considering the high-efficiency and energy-saving characteristics of the switching power supply, and the internal circuit working in a high-frequency switching state, the energy consumed by itself is very low, and the power efficiency can reach about 80%, which is nearly doubled that of ordinary linear regulated power supplies. Each module of the controller requires 5V, 3.3V, and 12V.

MC34063 itself contains the main functions required by DC/DC converters, and is cheap. It consists of a reference voltage generator with automatic temperature compensation function, a comparator, an oscillator with controllable duty cycle, an RS trigger and a large current output switch circuit, etc., and can output a switching current of 1.5A. It can use the least external components to form a switching boost converter, a buck converter and a power inverter. The voltage is reduced to 5V by MC3406318, part of which is provided to peripherals, and the voltage can be reduced to 3.3V and 1.8V by REG 1117.

The remote communication function between the controller and the host computer can be achieved by using the UART expansion LQ-8100 GPRS transmission module. This module has an RS-232 data interface, which can realize transparent wireless transmission of the serial port, and is stable, reliable, high-speed and simple to configure. The GPRS technology used by LQ-8100 realizes data packet sending and receiving. Users are always online and charged according to traffic, which quickly reduces service costs. The wiring of LQ-8100 and the terminal is shown in Figure 3.

Figure 3 LQ-8100 and terminal wiring

Software Design of Reactive Power Compensation Controller

Software design must be carried out on the basis of hardware and software functional division. The controller is a multi-tasking system with high requirements for real-time performance and reliability. As an embedded real-time operating system, uC/OS II has the characteristics of open source code, portability, curability, scalability, multi-tasking, task stack, system service, interrupt management, etc. uC/OS-II real-time operating system is embedded on LPC2220. (When uC/OS-It performs task scheduling, it will store the CPU register of the current task in the task stack. Then, when exiting from another task, the stack restores the original working register and continues to run the original task. The controller software architecture block diagram is shown in Figure 4. The bottom driver needs to complete the bottom functions such as keyboard reading, LCD display, and interface reading and writing, and encapsulate the code into functions for the upper layer to call. The operating system layer can divide multiple "simultaneous" events into relatively independent "tasks" to ensure that the events are handled in a timely manner. User tasks are modularly written according to the tasks that the system needs to manage. According to the system function, it can be divided into acquisition module, calculation module, switching control module, data storage module, communication module, etc. The working process of the whole system is that the system boots up data initialization, reads the power grid parameters, performs corresponding calculations to determine whether to switch the capacitor, and outputs the control signal. When an interrupt occurs during the whole process, such as modifying the set parameters, recording historical data, etc., the uC/OS-II operating system can respond to the interrupt in a timely manner and execute the corresponding task.

Figure 4 System design hierarchy diagram

The selection of reactive power compensation control quantity is directly related to the effect of reactive power compensation. Using power factor as control quantity is one of the traditional methods of reactive power compensation. However, using power factor as switching criterion alone cannot directly reflect the size of reactive power shortage. It may happen that the actual total amount of reactive power is already large but the power factor is still within the "reasonable" range. Therefore, the reactive power compensation effect of the automatic switching device composed of power factor as switching criterion is poor, and even there is a defect of frequent misoperation under certain load conditions. If reactive power is used as the switching criterion, since the detection quantity is consistent with the control target, it can truly achieve the purpose of making up for the lack of reactive power and cutting for the excess reactive power. It can avoid switching oscillation and realize the one-time switching of capacitor banks, avoiding the impact of repeated trial switching on the power grid and capacitors. Combining the advantages and disadvantages of the two control quantities, and the different applicable environments in practice, the software method can be used to set different control quantities or compound control when designing reactive power compensation controllers. The capacitor switching adopts a combination of three-phase co-compensation and three-phase split compensation. First, the minimum amount of three-phase reactive power is compared, and then the three-phase co-compensation is used to compensate, and then the remaining reactive power is compensated by the three-phase split compensation method. The control scheme software block diagram is shown in Figure 5.

Figure 5 Control scheme software block diagram

The LPC2220 external memory controller EMC contains four independent 32-bit configuration registers (BCFG0~BCFG3) with the same mapping meaning, and corresponds to four address spaces (Bank0~Bank3) respectively. The configuration register BCFG0 of the controller's external FLASH memory SST39VFI60 can be configured to 0X1000FFE8.

Conclusion

This paper designs a controller based on LPC2220 ARM7 microprocessor and uC/OS-II real-time embedded operating system, and uses a composite switch as the capacitor switching controller. The hardware design has the characteristics of high reliability and low cost. Software design Embedding a real-time multi-task operating system in the control system can improve the efficiency of the system, shorten the development cycle, and facilitate program maintenance and upgrades. At the same time, users can use it more flexibly and conveniently. The development of control and signal processing theory and technology, as well as the development of sensor and computer technology, will promote the improvement of the automation level of the power system by applying some new achievements of electronic information to power reactive compensation devices.