Software and hardware design of intelligent release

binghuoshaonian

Software and hardware design of intelligent release [Copy link]

Abstract: Firstly, the hardware and software design of intelligent release and its key technologies are introduced, then a new data processing method is proposed, and finally some anti-interference measures are summarized.

Keywords: intelligent trip device; sampling; fast Fourier transform; wavelet transform; anti-interference

　

0 Introduction

Intelligent circuit breakers refer to circuit breakers that use intelligent trip devices. Intelligent trip devices enable circuit breakers to achieve functions such as telemetry, remote control, remote signaling and remote adjustment. Nowadays, intelligent trip devices use microprocessors such as single-chip microcomputers and DSP as the basis for logic processing. The development trend is that there are more and more functions. In addition to the traditional trip function, there are also pre-trip alarm functions, line parameter detection functions and test functions; another trend is to use fieldbus technology and take the networking of equipment as the goal.

This article mainly introduces the issues that need to be paid attention to in the hardware and software aspects and the corresponding treatment methods in the process of developing intelligent releases.

1 Hardware design of intelligent release

According to the functions to be achieved by the intelligent release, the hardware can be divided into several parts, such as central processing unit (microprocessor and its peripheral circuits), sampling circuit, key display circuit, communication circuit, actuator, etc.

1.1 Sampling Circuit

The function of the sampling circuit is to send the external current and voltage signals to the microprocessor A/D sampling channel port after passing through the mutual inductor, filtering, and amplitude adjustment links. In these links, the following issues should be paid attention to.

1) Selection of current transformers The function of current transformers is to linearly convert large-amplitude electrical signals in the line into processable electrical signals. The linearity and accuracy of the conversion will directly affect the credibility of key data, which is the basis for the operation of intelligent releases. Commonly used current transformers are iron core and hollow core. The iron core type has good linearity when processing small currents, but the iron core is easily saturated when the current is large, resulting in linear distortion and a small measurement range; the hollow type has good linearity when processing large currents and a wide measurement range, but it is easily disturbed when the current is small, and linear distortion will also occur, resulting in large measurement errors. However, the current measurement range of the intelligent release is from hundreds of A to tens of kA, with a wide range of variation. In order to maintain linearity within the entire measurement range, it is best to use a combination of two types of current transformers.

2) Amplitude adjustment link Since the current measurement range is very large, and the microprocessor A/D conversion reference voltage is generally very small, this project uses CYGNAL's C8051 chip as the CPU chip, and its A/D conversion reference voltage range is 0~3.3V. If the input electrical signal amplitude exceeds 3.3V for a certain period of time, the C8051 chip will be damaged. If all electrical signal amplitudes are reduced to below 3.3V, the accuracy of A/D conversion will be greatly reduced, which will cause great trouble for subsequent data processing. This design adopts a multi-range conversion method. The maximum amplitude of the signal sent to the A/D conversion port in each range is slightly less than 3.3V. Different transmission channels are used in hardware according to the signal amplitude. Of course, the realization of this function also requires software judgment.

1.2 Central Processing Unit

The CPU chip in this design uses CYGNAL's C8051, a new high-speed integrated chip that integrates 8 A/D conversion channels, temperature sensor, 32K FLASH memory, WATCHDOG monitor, communication interface and standard JTAG program burning port in a thumb-sized volume. This reduces the number of peripheral components of the control system and simplifies the circuit, thereby improving stability and anti-interference capabilities.

1.3 Keyboard display circuit

The keyboard display circuit uses a serial interface 7281 chip, which can control up to 16-bit digital tubes or 128 independent LEDs through an external shift register 74HC164. Its drive output polarity and output timing are software controllable, so it can be used with various drive circuits. At the same time, the 7281 chip can not only control the flickering properties and flickering frequency of each display bit, but also can connect a keyboard matrix of up to 64 keys. The keyboard is interlocked and has an internal de-jitter function. In addition, the 7281 chip uses a high-speed two-wire interface to communicate with the CPU, occupying only a small amount of I/O ports and CPU time.

1.4 Execution Unit

The execution unit uses an electromagnet with a permanent magnet. When working normally, it remains in the attracted state under the action of the permanent magnet. When the execution circuit receives the pulse control signal from the CPU, the Darlington tube is triggered to pass current through the coil to generate reverse magnetic flux. Under the action of the reaction spring, the iron core opens, driving the circuit breaker to disconnect.

1.5 Problems that are easily overlooked in hardware devices

The CYGNAL51 chip has its own internal reset and simple external reset circuits, which are not easily overlooked. However, in actual operation, since the keyboard and display are controlled by the management chip 7281, when the program runs away, the C8051 chip can be reset and run again through the external or internal reset circuit, but the reset of the C8051 chip cannot be transmitted to the 7281 chip, and the display on the display board will not be refreshed. Therefore, when resetting the C8051 chip, the 7281 chip must also be reset. A feasible solution is to let the C8051 chip and the 7281 chip share the same reset source, so that once the program dies, the two chips will be reset at the same time.

2. Software Design of Intelligent Release

The software design is mainly divided into two parts, the main program and the interrupt program. The main program includes subroutines such as fault processing, keyboard processing, display processing, and communication processing; the interrupt program includes timer interrupt, keyboard interrupt, communication interrupt, etc.

The single-chip microcomputer samples the power frequency current signal, and 32 points can be collected in each cycle (20ms). In this way, the system sampling frequency of 6MHz will not cause distortion. Since the delay protection requires high precision, the effective value of the current must be calculated first. There are many methods to calculate the effective value of the current. The following introduces a more reliable algorithm. Since the actual signal is superimposed with high-frequency signals and non-periodic signals, in order to truly and effectively reflect the nature of the measured signal, someone proposed to use the FFT algorithm to calculate the fundamental wave parameters and higher harmonic parameters of the line signal from the measured data. Since the fundamental wave component of the signal accounts for more than 95% of the total signal, the calculated data can be used as the basis for various protection algorithms. The premise of current protection is whether the moment of fault occurrence can be judged in time and correctly. The FFT algorithm determines the fault condition by calculating whether the effective value of the fundamental wave component in a cycle is greater than the threshold value. This can complete the judgment task, but the real-time performance is not high. An improved algorithm based on wavelet analysis and FFT is proposed below. After the wavelet algorithm detects a suspicious signal point during the sampling process, the FFT algorithm performs a valid value judgment. If it does not exceed the threshold value, the suspicious signal point is invalid, and the wavelet algorithm is returned to continue to search for the sampling suspicious point; if the valid value exceeds the threshold value, the suspicious point is considered valid, and the corresponding signal is output according to the protection condition. The flowchart of the algorithm is shown in Figure 1.

Figure 1 Data processing flow chart

Instantaneous protection is a special protection method. It does not need to compare the effective value, but adopts the method of sampling and comparing immediately. Once a sampling point is found to exceed the specified threshold, the single-chip microcomputer system will immediately send a trip signal. However, due to the uncertain working conditions of the release, it is inevitable to be affected by external interference sources, resulting in malfunction of the release. In order to reduce the possibility of malfunction, it can be determined whether several consecutive sampling points exceed the threshold value. If they exceed, it is determined that a fault has occurred, otherwise it is determined to be external interference. This method improves reliability but reduces timeliness. As for the number of consecutive sampling points to be determined, it is determined according to actual needs.

3 Anti-interference measures

The interference sources that affect the intelligent release include surge currents from electrical equipment, radio frequency radiation from walkie-talkies and mobile phones, and the internal switching power supply and chopper release circuit of the intelligent release. The existence of these interference sources causes the program to crash, or inaccurate display of parameters such as current and voltage leads to incorrect fault status judgment, which in turn causes the release to malfunction. In order to reduce the impact of interference, corresponding measures need to be taken in hardware and software.

3.1 Hardware Anti-interference

The measures taken include:

1) Reasonable wiring makes the common ground of the digital circuit and the analog circuit work in a suspended mode, that is, the reference potential of each circuit of the system is connected to each other instead of being connected to the ground, so that the system has a strong anti-interference ability;

2) The analog circuit ground and the digital circuit ground are grounded separately and then merged at one point. This is because the chopper discharge circuit will have very high transient interference after starting up. After separating the logic ground (host) and the analog ground (A/D), this interference is reduced to a very low level.

3) Spraying an insulating layer on the surface of circuit boards and components is necessary for moisture-proof and insulation, and also plays an important role in preventing electromagnetic interference; applying a metal shielding layer inside the casing to form an equipotential shielding also has a great shielding effect on electromagnetic interference;

4) Installing a filter circuit behind the voltage-stabilized power supply and isolation transformer can attenuate the interference current in the live wire and the neutral wire;

5) When choosing chips, try to use chip packages with small size. Since SMD packages are smaller and have stronger anti-interference capabilities than DIP packages, SMD packages are chosen;

6) Minimize peripheral circuits to simplify circuit board wiring;

7) The hardware circuit should use multi-stage followers and high-frequency filtering circuits to ensure that the signal is not distorted.

3.2 Software anti-interference

There are several ways to resist interference in software.

1) In order to prevent the device from being disturbed and entering a "freeze" state, some monitoring measures are added to the program. The watchdog is used to detect deadlocks in the program and automatically reset it when necessary. Software traps are set in unused interrupt vector areas and blank program areas to force the program to return to normal track after running away. Redundant instructions are written where necessary to adjust the instruction length and prevent program confusion.

2) Digital filtering of the sampled signal First, each sampling point is judged by comparing it with the adjacent values, the previous value and the maximum value, and judging whether it is an interference signal based on the symmetry detection method and the limit detection method; the data obtained by FFT calculation of the most recent sampling point is averaged with the data of the previous few times, and the "outliers" are discarded.

3) When the software "WATCHDOG" technology system is disturbed by external factors during operation, the program will deviate from the normal track, causing a series of problems, an infinite loop, and the system cannot return to normal. The characteristics of software watchdog technology are: in the normal operation of the system, the "WATCHDOG" must be "fed" at fixed intervals. If the "fed" operation is detected within the specified time, it indicates that the system is running normally, otherwise it is considered that the system is wrong and a reset signal is automatically issued. The CYGNAL51 chip used in this project has a built-in watchdog, which reduces the workload of project developers.

In addition, improvements in data processing algorithms can also greatly improve the system's anti-interference ability. However, this is often at the expense of code length. The trade-off depends on the actual project requirements.

4 Conclusion

With the continuous emergence of high-performance, low-cost chips, while retaining the advantages of traditional equipment, intelligent releases have made great improvements in protection diversity, judgment accuracy and anti-interference, self-diagnosis protection, real-time communication and display, which greatly facilitates users and has broad market prospects.

　

Author Bio

Mo Yingtao (1981-), male, master student (currently studying), his research interests are the application of power electronics in power systems and distributed power generation.

Wu Weilin (1944-), male, is a professor and doctoral supervisor at the School of Electrical Engineering, Zhejiang University. His research interests include reactive power compensation, power quality, and distributed generation.