Implementation of motor protection algorithm based on MSP430F149

目前，我国高压电动机的保护主要有机电式和集成电路两种，但都存在着诸多弊病，不能很好地保护电动机。其中，异步电动机的机电式保护，主要以电流增大作为判据，保护原理粗略，对断相等严重不对称故障，由于一般不出现显著的电流增大，从而使保护装置难以及时动作，造成事故扩大；集成电路式保护虽在保护原理上有所改善，但其保护特性一般无法与电动机热曲线实现较好配合，常发生拒动或误动，严重的甚至烧毁电动机。因此，研究电机保护算法具有重要的意义。

1　电动机故障分析
电动机的故障形式可分为对称和不对称两类。
对称故障包括过载、堵转和三相短路等，这类故障对电动机的损坏主要是热效应和机械应力，使绕组发热甚至损坏。其主要特征是三相仍基本对称但同时出现过电流，故障严重程度基本反映在过电流的程度，因此仍然以过流程度作为这类故障的判据。对于严重的三相短路的保护应采用速断跳闸；堵转故障的保护应采用短时限跳闸；而对于对称过载应采用及时限跳闸，反时限特性应与电动机的温升指数特性相配合。
不对称故障又可进一步分为非接地性和接地性两类。
非接地性不对称故障，主要包括断相、相间短路、匝间短路及不平衡运行等。这类故障会引起三相电流不对称。由于我国电动机的中心点不接地，故定子电流可分解为正序和负序分量(零序分量为0)。而电动机在正常运行时负序电流分量基本没有，所以采用负序电流分量作为这类故障的判据。这类故障对电动机的损害主要是负序电流引起的负序效应，可能造成电动机端部发热、转子振动及起动力矩降低等一系列问题，如果有过电流出现，还会使绕组发热。这类故障的保护应采用短时限跳闸或速断。
接地性不对称故障，包括单相接地短路和两相接地短路。由于电动机外壳必须安全接地，因此绕组端部碰壳、绝缘破坏等都可能导致接地故障，特别是处在尘埃重或湿度大的环境下，故障率就更高。发生接地性不对称故障时，会出现零序电流分量，这是区别其他任何非接地性故障的根本特征，可作为接地性故障的主要判据。这类故障应采用速断或短时限跳闸加以保护。
由以上分析和实验表明，过电流、负序和零序3个分量的不同分布组合与各故障类型之间具有很好的对应关系，表1列出了这样的对应关系。

2. Proposal of motor protection algorithm
The protection algorithm uses mathematical operation methods to measure, analyze and judge the fault quantity. The basic problem is to find an appropriate discrete operation method so that the accuracy of the operation result can meet the requirements and the calculation time is as short as possible. The
Fourier algorithm assumes that the sampled analog signal is a periodic time function. In addition to the fundamental wave, it also contains an unattenuated DC component and harmonics. When this algorithm is implemented on a DSP microcontroller, it actually calculates discrete sampling values. First, calculate the real part Irn and the imaginary part Iin of In (where n = a, b, c): Where: N is the number of samples in a cycle T. The larger N is, the higher the accuracy. When N = 12 points, the maximum amplitude error: ΔImax/Im = 3.41% When performing real-time calculations on a microcontroller, it must be calculated once after each new sample value appears, and the N sample values ​​before this new sample value (including the new one) should be calculated at the same time. During the calculation, the N sample values ​​are multiplied by different coefficients and then summed. After calculating the real part Irn and the imaginary part Iin of In, the amplitude In and the phase angle θ can be obtained: In this way, the amplitude and phase of any harmonic can be obtained. The phase-to-phase short circuit of the stator winding is a serious fault for the asynchronous motor. It not only causes the insulation damage of the winding and the burning of the core, but also significantly reduces the voltage of the power supply network, destroying the normal operation of other equipment. Therefore, protection against phase-to-phase short circuit faults should be carried out. Of course, it also belongs to overcurrent protection. This fault will produce an attenuated non-periodic component (including an attenuated DC component). Similarly, the inrush current generated when the motor starts will also produce this attenuated non-periodic component. Although the Fourier algorithm has a certain inhibitory effect on this attenuated non-periodic component, experiments have shown that if no measures are taken, in the most serious case, the calculation error of the Fourier algorithm caused by the non-periodic component may exceed 10%. Such a large error must not be ignored for motor protection and must be compensated. The simplest way is to first perform a subtraction filter on the input signal and then perform the Fourier calculation, so that the non-periodic component can be weakened and the calculation error of the Fourier algorithm can be reduced. [page] For motor protection, only the fundamental (50Hz) current signal needs to be calculated. Other interference signals (such as harmonics and various noises) must be filtered out. A passive ALF (analog low-pass filter) is used before A/D to filter out higher harmonics above the 9th order. After discretization by A/D or VFC, the Fourier algorithm is used. The algorithm itself has a strong filtering effect (it can filter out harmonics of various orders). If a subtraction filter is used before the Fourier algorithm to filter out the attenuated non-periodic components, there is no need to use another digital filtering algorithm. 3 Hardware implementation block diagram The hardware implementation block diagram is shown in Figure 1. The three-phase current is converted into a suitable level through the current-voltage converter and the amplification part, and then the high-frequency component is filtered out by the analog low-pass filter (ALF). ALF can use a passive low-pass filter, which is used to eliminate the frequency aliasing problem, that is, in order to meet the sampling theorem, limit the highest frequency of the input signal, and is designed here to eliminate harmonics above the 7th or 9th order. Then, the sampling and holding circuit plays such a role: when multiple analog quantities need to be detected at the same time, there must be a circuit in the input channel of the detection system to sample and hold the analog signal at this moment when the A/D is converted, so that the A/D converter can obtain a stable conversion value. The signal enters the microcontroller chip for comprehensive processing (MSP430F149 has a built-in 12-bit A/D converter, and no external A/D converter is required). The output circuit is the execution circuit of the protection device, which consists of a tripping circuit and an audible and visual alarm indication circuit, and has functions such as tripping, alarming and indicating fault types. The external bus is connected to an external expansion SRAM, E2PROM and a keyboard display dedicated interface chip 8729. The setting value is entered through the keyboard, and the display unit is composed of an LCD module, which can display real-time data, alarm information, etc.

4 Software Design Flowchart
First, initialize the system. A self-test program performs a simple interactive 0/1 test on the RAM. By writing an AAH, 55H model 66 into the entire RAM, writing the inverted RAM value into the RAM and rechecking it, and then clearing the entire RAM. If an error is found, the reset is set. Then, determine whether the motor is powered on and running. If so, perform data acquisition and processing and automatically detect the motor operation status. If the motor is found to be operating abnormally, immediately identify the fault type, perform fault processing, display the fault information, and then execute the corresponding tripping link.

5 Conclusion
Based on the detailed analysis of the fault characteristics of high-voltage asynchronous motors, the overcurrent protection is decomposed into three categories: overcurrent, negative sequence and zero sequence protection. The comprehensive protection formed by this can basically cover all common fault types of motors. A digital comprehensive protection algorithm is proposed, which uses an ultra-low power single-chip microcomputer to monitor motor faults. It has the characteristics of low cost, high cost performance, strong reliability and good adaptability, and has broad application prospects.

参考文献

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［2］沈标正.电机故障诊断技术［M］.北京：机械工业出版社，2001.