Low voltage dynamic reactive power compensation device based on TMS320LF2407A

1. Overview

In recent years, with the implementation and deepening of urban and rural power grid transformation, it is increasingly recognized that installing low-voltage dynamic reactive compensation devices on 0.4kV power grids can improve power supply quality, tap the potential of power supply equipment, and reduce line losses. Low-voltage dynamic reactive compensation devices are generally composed of microcontrollers, switches for switching capacitors, capacitor banks, air switches, fuses, stainless steel shells, etc. Its structure is simple, switching is convenient and flexible, and the energy-saving effect is significant. Therefore, more than 300 companies in the country produce reactive compensation devices. However, most of them have low technical levels, lack of relatively complete testing equipment, small production volume, and difficult to guarantee quality. In particular, the controller and the switch for switching capacitors, which are the key units of the reactive compensation device, are even more different. This article focuses on the analysis of some core units of the reactive compensation device, and records the data of the operation of a reactive compensation device produced by Senbao Electric Company in the Xi'an Power Supply Bureau. Information source: http://www.tede.cn

2. The significance and principle of reactive power compensation

Most of the loads in the distribution network, whether industrial or civil, are inductive loads. When they are running, they need to absorb a large amount of reactive power from the power grid, which reduces the power factor and power quality of the power grid and increases the "technical loss power" of the power grid. After installing a parallel capacitor compensation device in the power grid, it can reduce the reactive power transmitted from the power supply to the inductive load through the transmission line. Since the flow of reactive power in the power grid is reduced, the power loss caused by the transmission of reactive power in the transmission line and transformer can be reduced, thereby improving the power factor of the power grid, reducing line loss, and significantly improving the power quality.

The equivalent circuit of the inductive load in the power grid can be regarded as a circuit in which the resistor R and the inductor L are connected in series.

After connecting the R and L series circuit in parallel with the capacitor C, the circuit is shown in Figure 1-a. The current equation of the circuit is:

Figure 1 Circuit and vector diagram of parallel capacitor compensation for reactive power

a) Compensation circuit b) Phase diagram (under-compensation) c) Phase diagram (over-compensation)

It can be seen from the phasor diagram in Figure 1-b that after the capacitor is connected in parallel, the phase difference between the voltage and becomes smaller, that is, the power factor of the power supply circuit is improved. At this time, the phase of the supply current lags behind the voltage, which is called undercompensation; if the capacity of capacitor C is too large, the phase of the supply current leads the voltage, which is called overcompensation, and its vector diagram is shown in Figure 1-c. This will cause the voltage on the secondary side of the transformer to rise; the temperature rise of the capacitor will increase, the power loss of the capacitor itself will increase, and the service life of the capacitor will be shortened; the transmission of capacitive reactive power on the line will also increase the power loss. Therefore, this situation should be avoided.

3. Reactive power compensation device structure and main circuit

The compensation device is mainly composed of a cabinet, a controller, an air switch, a lightning arrester, a capacitor, a fuse and a compound switch. Its main circuit diagram is as follows (the inside of the box is the main circuit of the compensation device; the outside of the box is the low-voltage distribution network):

Figure 2 Main circuit of reactive power compensation device

4. Intelligent controller

At present, the controllers of most manufacturers mainly use 51 series single-chip microcomputers as the computing units, which have the following disadvantages: ① Limited hardware resources, weak instruction functions and computing power. It is difficult to complete signal sampling, power calculation, power grid harmonic analysis, capacitor switching, RS-485 remote communication and short-distance RS-232 wireless communication functions by 51 single-chip microcomputers; ② There are many external expansion chips, the overall structure of the controller is complex, and the reliability is reduced; ③ The control strategy is single and the remote signaling ability is weak: the controller control strategy is based on power factor or reactive power control. This simple control strategy is easy to cause the controller to malfunction when the power grid is lightly loaded, causing the line to be over-compensated; ④ The controller protection function is not perfect and does not meet the functions specified in the power industry standard DL/T597-1996 << Technical Conditions for Ordering Low-voltage Reactive Compensation Controllers >>. (That is, the controller should have overvoltage, undervoltage, switching delay protection functions, capacitor input and removal threshold functions, cyclic switching functions, and the panel should have hardware or software locking functions to prevent capacitor switching oscillation when the load is small, and anti-interference functions.)

In view of the current status of reactive power compensation devices, this paper develops an intelligent low-voltage reactive power compensation device controller based on a digital signal processor after comparing the advantages and disadvantages of various reactive power compensation devices.

The controller uses TMS320LF2407A chip, which is the most comprehensive and best-performing fixed-point 16-bit controller. ① It contains multiple processing units such as hardware multiplier, accumulator, arithmetic logic unit, auxiliary arithmetic unit, etc. These units can complete the planned tasks in parallel within one instruction cycle; ② It can independently access 64K bytes of program memory and data memory space; ③ The internal bus adopts a parallel architecture. The system is equipped with program read bus, program address bus, data read bus, and data write bus. Since the bus is independent, it can access the program and data memory at the same time, and provide data and instructions to the processor at the same time, thereby improving data throughput; ④ Special DSP instructions have single-cycle multiplication and addition operations; ⑤ FFT reverse order addressing capability and single-cycle instruction execution time of 25nS and rich integrated peripheral interfaces. These rich resources can well solve the problems that are difficult to solve for 51 series microcontrollers.

The controller collects three-phase voltage and current, and uses the FFT transformation algorithm to obtain the three-phase power factor, voltage, current, active power, reactive power, 2nd to 13th harmonic content, active electricity, reactive electricity, voltage, and current distortion rate of the power grid; it has the function of storing the power factor, voltage, current maximum value, power outage, call time and cumulative power outage time at the hour of each day; the hourly specified daily harmonic data; the data storage period is 2 months; the 128X64 large-screen LCD displays the full Chinese operation interface and data for easy user operation; it has RS-232 and RS-485 communication physical interfaces, which can realize remote signaling or short-distance wireless communication, and truly achieves the integration of reactive compensation automatic control and comprehensive measurement and control of distribution parameters, laying the software and hardware foundation for the distribution network automation, remote communication, and unmanned operation proposed by the urban network transformation. The hardware structure block diagram of the controller is shown in the figure:

Figure 3 Schematic diagram of controller hardware structure

The overall structure diagram of the software and the flow chart of some modules are as follows

Figure 4 Overall software structure diagram

Figure 5 Flowchart of the underlying driver module

5. Switches for parallel capacitors

In the 0.4kV distribution network, the current reactive compensation device capacitor switching switch is realized by contactor or thyristor. If the contactor is used to switch the capacitor, the disadvantages are: ① When the capacitor is put into operation, it is difficult to control the voltage to be put into operation when it passes through zero, so it is easy to generate inrush current, spark between contacts, and burn contacts; ② When the capacitor is cut off, it is not easy to control the current to pass through zero, so that the contacts stick and arc; ③ Excessive inrush current will also damage the capacitor and shorten the service life of the capacitor. If a thyristor (also known as a solid-state relay) is used to switch the capacitor, its advantages are that the voltage passes through zero to trigger the conduction of the main circuit, there is no arcing, the action response is fast, and the switching inrush current can be greatly limited, which is particularly suitable for frequent switching. Its disadvantages are: ① The power consumption is large, and it increases with the increase of the capacitor current; ② The thyristor circuit itself is a harmonic source, and a large number of uses are prone to harmonic pollution to the low-voltage power grid.

Based on the above situation, Xi'an Senbao Company has developed a new type of switch - compound switch. The compound switch mainly consists of a control board, a thyristor and a magnetic latching relay. The principle block diagram is shown in Figure 3-1:

Figure 6 Schematic diagram of compound switch module

The process of compound switch inputting capacitor: the power distribution integrated controller sends a trigger signal to the compound switch. After receiving the signal, the compound switch control board starts to detect the voltage zero point, and connects the thyristor to the main circuit when the voltage passes zero. After a certain delay, the magnetic latching relay is energized under equal potential. Since the thyristor is a contactless switch and is input at zero potential, after normal operation, since the relay contact resistance is much smaller than the thyristor resistance, the capacitive current enters the power grid through the magnetic latching relay contact. Therefore, inrush current, arcing, and sparking are avoided when the capacitor is input, and the working loss of the thyristor is reduced. Information from: Power Transmission and Distribution Equipment Network

The process of compound switch cutting off capacitor: after the trigger signal output by the controller disappears, the magnetic holding relay is disconnected first under the condition of equal potential, and the thyristor is disconnected after a delay when the current passes through zero.

This switching switch fully absorbs the advantages of contactless switch thyristor and relay, so the switch has a long service life and low power consumption. It is an ideal switch for switching capacitors.

6. Operation status

Using the above technologies, Xi'an Senbao Company has designed and produced more than 400 low-voltage dynamic reactive power compensation devices, which are put into operation in Xi'an, Xinjiang, Qinghai, Henan, Northeast China and other places. The products have been put into the market for two years, and the on-site operation is normal, which has been well received by users. The following is the operation record of a reactive power compensation device put into operation on the 10# pole of Xingfu Road by Xi'an Power Supply Bureau.

Table 1: Operation status of reactive power compensation device on Xingfu Road 10# pole