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Fault Detection System for High Voltage Frequency Converter Based on the Fault Tree Analysis Method

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2017 3rd International Conference on Computer Science and Mechanical Automation (CSMA 2017) ISBN: 978-1-60595-506-3

Fault Detection System for High Voltage Frequency Converter

Based on the Fault Tree Analysis Method

Ran CHEN

1,a

,

Yu-Yuan DU

1,b

and

Hui-Ming LI

2,c

1

College of Information Science and Engineering, Northeastern University, Shenyang 110819

2

Shenyang Ligong University, Shenyang 110168

a18742122266@163.com, bduyuyuan@ise.neu.edu.cn, c2043953265@qq.com

*Ran CHEN

Keywords: Unit series multi-level voltage source type, High voltage frequency converter, Fault tree, Fault detection.

Abstract. High voltage frequency converter plays a necessary role in saving energy and reducing consumption of modern industrial enterprises. When the high voltage frequency converter breaks down, it needs to be located fault components as soon as possible and taken effective measures. Because the high voltage frequency converter is various and complicated, it is hard to complete the task of an intelligent fault detection system by using the traditional Fourier transform. The fault detection system for unit series multi-level voltage source type high voltage frequency converter based on the fault tree analysis method is put forward in this paper. The fault tree for a frequency converter is constructed based on the analysis of frequency converter's configuration, the function of each part and the failure mode.

Introduction

With the development of modern science and technology and the development of production, the production of modern enterprises change to the large-scale, complex, sophisticated and automated direction. At present, in the metallurgical, steel, petroleum, chemical, water treatment and other industries, high-voltage frequency converter is widely used to drive fans, pumps, compressors and various large-scale machinery, has become one of the main means to promote technological progress of energy-saving, transformation of traditional industries and process automation level.[1, 2] High voltage frequency converter running in fault, ranging from the speed control system cannot work properly to the equipment is leading to damage, which result in a damage impact on the production system and will bring huge economic losses.

In the process of fault diagnosis, there are the following problems. [3, 4] First, the number of experts in the field is short, the technical force is relatively weak; second, the diagnosis process is generally random and blind; third, the fault knowledge management is not standard. Therefore, the establishment of efficient and accurate high voltage frequency converter fault detection system is particularly important. When the fault occurs, the system can quickly find out the reasons and countermeasures, as soon as possible to eliminate the fault and resume production. Above of all, how to balance the scouting speed and accuracy is crucial for high voltage frequency converter fault detection. In this paper, we focus on the fault of unit series multi-level voltage type high voltage frequency converter. The objective is to reduce the number of blind finding and improve the fault detection accuracy.

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Basic Aspects of Fault Tree and High Voltage Frequency Converter

Fault Tree Analysis Introduction

Fault tree analysis is a graphical interpretation from the results to the cause analysis method. It is used by the method of analysis that fault tree analysis refine the dendritic reasons for the formation of system fault.[5, 6] Fault tree model is a behavioral model based on functional characteristics and the diagnosis object structure, and it is a stereotyped causal model that expressed an inverted tree structure link between the use of things logic gate, which cause the system undesirable event to be the top event and other events may leading to top event to be the middle events and bottom events.

[image:2.612.137.471.262.343.2]

Under the clear fault tree graphics, it shows the system fault of the internal connection, and reflects the logical relationship between the components faults and the system faults, so as to analyze some specific fault modes. The steps of fault tree analysis are different according to the object of evaluation, the purpose of analysis, and the degree of thickness, but can be carried out as shown in Fig.1.

Figure 1. The steps of fault tree diagnosis.

Fault tree analysis is an important method for analyzing the system reliability and stability. The key point is to make a qualitative and quantitative analysis of the fault tree. The fault tree analysis method can be used to analyze the reason for system fault caused by the calculation of each unit of the system reliability and the influence to the system, so as to search for weak links and take corresponding measures in the optimization design. Recently, the research based on fault tree model for fault source search attracted people's attention. It owns both advantages based on rules and quantitative diagnosis model and provides an effective way to find faults in complex systems.

Unit Series Multi-level Voltage Source Type High Voltage Frequency Converter Introduction

At the present stage in high voltage 6~10KV application widely as "High-Low" scheme of high voltage frequency converter, this scheme is to supply voltage through the input transformer (input rectifier transformer) step-down, and then through the low voltage frequency converter frequency conversion, directly supply low-voltage motors. [7]The scheme is only suitable for small and medium capacity motors, and the input transformers and converters are not too far from the motor.

Unit series multi-level voltage source type high voltage frequency converter is adopted that the power range from 3MW to 220MW and the maximum output voltage is 10KV. It has the advantages of low harmonic output, harmonic content is less than 0.3% without an external output filter; the motor torque ripple and noise are lower than the other structures of the high voltage frequency converter; power factor up to 0.95; nondestructive damage to the motor insulation.[8]The disadvantages are the follows: only works in one quadrant; cannot bypass switching; cannot be achieved without fuse design; structure of unit series multi-level voltage source type high voltage frequency converter decide that its volume is big and heavy; the various use of components decrease the reliability and stability; the massive capacitor using this type of high voltage frequency converter lead to more leakage phenomenon.

Fault Type of High Voltage Frequency Converter

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For high voltage frequency converters, there is a normal operating range that is more than the peak voltage of three-phase AC input line. When the input voltage exceeds the rated voltage, the high voltage frequency converter power devices present damage effect. Similar to the over-voltage fault, the under-voltage fault of the frequency converter is defined as the failure of the voltage at which the voltage of each power unit continues for a certain low voltage. When the under-voltage fault occurs in the high voltage frequency converter, the under-voltage protection circuit will turn on. Common faults in the use of high voltage drives except the failures listed what is said above, there are other faults, such as over-current faults, over-load faults, and parameter setting faults.

Fault Tree Establishment and Simplification

The establishment of fault tree is the key of fault tree analysis, and the improvement of fault tree establishment will directly affect the accuracy of qualitative analysis and quantitative calculation.

Fault Tree Establishment

[image:3.612.110.506.321.422.2]

In this paper, according to unit series multi-level voltage source type high voltage frequency converter fault sampling and the analysis, the most commonly used deduction method to establish the fault tree of unit series multi-level voltage source type high voltage frequency converter as shown in Fig.2.

Figure 2. The fault tree of unit series multi-level voltage source type high voltage frequency converter.

Among them, the bottom events are follows: A, oscillation circuit fault; B, regulator circuit fault; C, protection circuit fault; D, load circuit fault; E, PLC no response; F, the controller no response; G, negative cut off the negative control loop; H, the number of units do not match; I, thyristor short circuit; J, thyristor open circuit; K, leakage phenomenon; L, Qncut off; M, Qnswitch fault; N, voltage

overshoot; O, short voltage amplitude; P, energy storage capacitor fault; Q, charge resistor open circuit; R, over-current; S, short circuit; T, over-voltage; U, under-voltage; V, phase loss; W, overheating fault.

Fault Tree Simplification and Equivalence

[image:3.612.147.472.607.670.2]

According to the simplification and equivalence principle of the fault tree, the simplified tree of fault can be obtained from the qualitative analysis result as shown in Fig.3.

Figure 3. The simplified fault tree of power unit fault.

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The Advantages of Fault Tree

Based on the above understanding of the basic concepts of the fault tree and the establishment of the fault tree of the unit series multi-level voltage source type high voltage frequency converter, it can be seen that the process of establishing the fault tree is a process of detailed analysis of the frequency converter fault. This fault detection system has the following two advantages.

Firstly, in the fault tree, the logical relationship between events is clear, which basically contains the common fault of the unit series multi-level voltage source type high voltage frequency converter. Secondly, high voltage frequency converter component fault can constitute system fault, this attribute determines the logical relationship in the tree to improve the accuracy of fault detection.

Fault Tree Analysis

Fault tree can be used for qualitative analysis of system reliability and quantitative assessment. Qualitative analysis is dedicated to the key factors of top event or critical event analysis, and quantitative assessment is mainly used to assess the probability of occurrence of top event.

Fault Tree Qualitative Analysis

[image:4.612.147.471.410.688.2]

The main task of the qualitative analysis of the fault tree is to find the full fault modes that lead to the system faults. The minimal cut-set method can be used for qualitative analysis of fault tree. The minimal cut-set is the smallest event combination that causes the top event to occur. The minimal cut-sets of the fault tree can be obtained by the Fussel algorithm. The basic principle is as follows: the fault tree OR gate increase the number of cut-sets, AND gate increase capacity of the cut-sets (cut-sets contain the number of bottom events). Minimal cut-sets identify the weak links of the system and the key components that affect its reliability. Through the qualitative analysis of the fault tree, the minimal cut-set of the fault tree is shown in Table 1.

Table 1. Fault tree Qualitative analysis results.

Number Minimal Cut-set

1 A, oscillation circuit fault 2 B, regulator circuit fault 3 C, protection circuit fault 4 D, load circuit fault 5 E, PLC no response

6 F, the controller no response

7 G, negative cut off the negative control loop 8 H, the number of units do not match

9 I, thyristor short circuit 10 J, thyristor open circuit 11 K, leakage phenomenon

12 L, Qn cut off; M, Qn switch fault

13 N, voltage overshoot 14 O, short voltage amplitude 15 P, energy storage capacitor fault 16 Q, charge resistor open circuit 17 R, over-current; S, short circuit

18 T, over-voltage; U, under-voltage; V, phase loss 19 W, overheating fault

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the relevant subsystem diagnosis mode. The fault will be positioned to the corresponding cause, and ultimately give the diagnosis conclusions, countermeasures and diagnostic recommendations.

Fault Tree Quantitative Analysis

The purpose of quantitative analysis is to determine the probability of occurrence of the top event under the condition that the probability of the bottom event is given. Due to the variety of high voltage frequency converter fault, a fault may correspond to a variety of fault reasons. In order to avoid the blindness of diagnosis and achieve the purpose of conflict resolution, this paper introduces the critical importance calculation in the quantitative analysis of fault tree.

The critical importance represents the size of the impact of each minimal cut-set (bottom event). The greater the critical importance, the greater the impact of the minimal cut set on the top event. Because the critical importance can measure the important degree of the basic event from the increase of the probability of the basic event to the sensitivity of the probability of the top event occurrence and the probability size of its own occurrence. Therefore, critical importance of the reaction of the bottom event on the high voltage frequency converter fault would be more comprehensive.

The importance of the basic event is expressed by the ratio of the relative change in the probability of occurrence of the basic event and the probability of occurrence of the top event. The critical importance means that it is measured by the importance of each basic event from the dual point of view of sensitivity and probability of occurrence. It is defined as equation 1.

( ) ln ( ) / ln [ ( ) / ] *[ / ( )]

c

g i i i

I i = ∂ P Tq = ∂P Tq q P T

(1)

( ) / i

P T q

∂ ∂ is the probability of the top event occurring when and only when the basic event i occurs,

[∂P T( ) /∂qi]*qi is the probability of generating a top event from the bottom event i.

The larger the value, the greater the likelihood that the top event will occur from the basic event i trigger. For example, the fault cell of the power unit fault tree-inverter fault tree shown in Figure 3, assume that: the probability of Qn cut off qL =0.00012; the probability of Qn switch fault qM

=0.00015; the probability of voltage overshoot qN = 0.00061; the probability of short voltage

amplitude qO =0.00041.The specific calculation process is as follows: firstly, alculate the probability

of occurrence of the top event with the second layer node as the top event and the critical importance of the bottom event, for node IGBT fault (node M9):

9

( M ) 1 (1 L) * (1 M) 0.00027

P T = − −qq = (2)

( )

(

)

(

)

9

1 * / 0.444

c

g M L M

I L = −q q P T =

(3)

(

)

(

)

(

)

9

1 * / 0.555

c

g L M M

I M = −q q P T =

(4)

The critical importance of M is higher than the critical importance of L, therefore the priority of Qn

switch fault is 1, and the priority of Qn cut off is 2. Then, calculate the probability of top event

occurrence and the critical importance of each second layer node:

( )

(

(

)

)

(

) (

)

9

1 1 M * 1 N * 1 O 0.00129

P T = − −P Tqq =

(5)

(

) (

)

(

)

( )

9 9

( ) 1 * 1 * / 0.209

c

g M N O M

I T = −qq P T P T =

(6)

(

)

(

(

)

)

( )

4

( ) 1 * 1 * / 0.473

c

g O M N

I N = −qP T q P T =

(7)

(

)

(

(

4

)

)

( )

( ) 1 * 1 * / 0.317

c

g N M O

I O = −qP T q P T =

(8)

(6)
[image:6.612.185.429.81.170.2]

Table 2. Rules Table.

Sequence Fault Priority

L Qn cut off 1

M Qn switch fault 2

N voltage overshoot 1

O short voltage amplitude 2

M4 IGBT fault 3

In this way, the high voltage frequency converter fault detection system will first verify the voltage overshoot fault when detecting the cause of the fault, thus reducing the number of blind finding and improving the fault detection accuracy. As can be seen from the above table, the priority of the fault is not unique, there can be many faults have the same priority, which does not prevent the normal operation of the fault detection system, because these faults belong to the framework is different, in other words the minimal cut-set corresponding fault tree is different.

Conclusion

In this paper, after analyzing the typical fault of the unit series multi-level voltage source type high voltage frequency converter, combined with the advanced fault diagnosis technology, the fault tree analysis method is applied to the fault diagnosis of the unit series multi-level voltage source type high voltage frequency converter. For the domestic market application of high voltage frequency converter structure principle is basically same, this method of fault detection can be used for reference that it has a certain value in the modern enterprise.

References

[1] Fu-Rong Xu, Hui-Ming Chen, Application Status and Development Trend of High Voltage Variable Frequency Speed Regulation Technology [J], Frequency Converter World, 2007 (4): 61-67. In Chinese

[2] Xuan-Zheng Zhang, Hong-Bing Gu, Application of Medium and High Voltage Frequency Converter [M], Beijing: Industrial Electronics Press, 2007, 6: 1-12. In Chinese

[3] Tian-Bing Ma, Fei Du, Frequency diagnosis technology of frequency converter and its development trend [J], Mining Industry Automation, 2007, 6 (3): 32-34. In Chinese

[4] Yong Huang, Fuji inverter fault diagnosis and countermeasures research [D], Shanghai Jiaotong University, 2005.In Chinese

[5] Vesely W E, Dugan J, Fragola J, et al, Fault tree handbook with aerospace applications[S], National Aeronautics and Space Administration, NASA, 2002.

[6] Uolkanovski A, Cepin M, Mavko B. Application of the fault tree analysis for assessment of power system reliability [J], Reliability Engineering and System Safety, 2009, 94(6):1116-1127.

[7] Jing-Chun Zhang, Xia-Nai Gu, Hui-Fang Mo. Motor Fault Diagnosis Based on Wavelet Energy Spectrum Analysis [J], Journal of Electric Power Systems and Automation, 2006 43(03): SS-5. In Chinese

Figure

Fig.1.
Fig.2.
Table 1. Fault tree Qualitative analysis results.
Table 2. Rules Table.

References

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