Analysis of Winding Deformation Diagnosing of Power Transformer

1. Introduction

The method of frequency response approach (FRA) is extensively used to check whether the winding deforms in the power transformer after short circuit impact. Since 1996, Guangdong Electric Test Institute conducted the winding deformation test for 110kV and above power transformer. In 1999, SGEG set up a project to further study how to determine the winding deforms. By the end of 2002, the total number of all measured 110kV and above power transformers has already exceeded 2000. Much experience are gained and Guide for Detecting and Diagnosing Winding Deformation of Power Transformer (frequency response approach) was complied for the first time in China. At present, this guide has become a standard for electric companies in Guangdong province. Based on that, Guangdong Electric Test Institute cooperates with SGEG to set this guide as a electric industry standard (DL/T911-2004). The method proposed in the guide is used to diagnose that 47 power transformers have severe winding deformation in Guangdong province. All winding deformations are confirmed through cover inspection. Then broken power transformers are maintained at once so that the fault of power transformer never arise during the operation.

Main methods to diagnose power transformer winding deformation presented in the Guide for Detecting and Diagnosing Winding Deformation of Power Transformer (frequency response approach) include relevant coefficient comparison, longitudinal comparison and horizontal comparison method. The paper introduces horizontal comparison method through instances.

2. Basic Principle of FRA

Affected by high-frequency voltage, each winding of power transformer can be regarded as passive linear two-port network composed of distributive parameters such as linear resistance, inductance and capacitance. The internal features can be described through transfer function H (jω), which is shown in Fig.1. If the winding deforms, internal distributive parameters are bound to change, leading to the change of zero and pole point of transfer function. Finally, the features of frequency response change.

Fig.1 Circuit of frequency response analysis method

L--distributive inductance

K--distributive capacitance

C--to-ground distributive capacitance

V1--excitation terminal voltage of equivalent network

V2--reponse terminal voltage

VS--the voltage of sine wave exciting signal source

RS--output impedance of signal source

R--match resistance

How to use the frequency response approach is as follows. First of all, amplitude-frequency response features of all windings are detected; secondly, detection results are compared in a longitudinal or horizontal manner; then the difference of amplitude-frequency response features is relied on to determine whether the winding of power transformer deforms.

The amplitude-frequency response features of transformer windings are obtained through frequency sweeping method shown in Fig.1. Change the frequency represented by the letter “f” of sine wave excitation source “Vs”; measure the signal amplitude ratio of response terminal voltage V2 and excitation terminal voltageV1under different frequencies; the amplitude-frequency response curve is obtained.

The measured amplitude-frequency response curve is represented by the logarithm, that is:

Where:

H(f)--the module 丨H (jω)丨 of transfer function

V2(f)--peak value or effective value 丨 V2(jω)丨of response terminal voltage

V1(f)--peak value or effective value 丨 V1(jω)丨of excitation terminal voltage

3. Diagnosis Analysis Method

The main principle of frequency response approach is based on the comparison of winding frequency response characteristic change. To be specific, the consistency of curves is determined through number of resonant points, position, amplitude and trend of different curves. The relevant coefficient R is introduced to represent the consistency. The greater the R is, the better the consistency is.

The relevant coefficient R can be calculated according to the following formula.

1)  Standard deviations of two sequences are calculated: 

2) The covariances of two two sequences are calculated:

3) The normalization covariance coefficients of two sequences are calculated:

4) The relevant coefficient Rxy is calculated according to the following formula:

Through plenty of tests and analysis, the relation between relevant coefficient and curve similarity is as follows: generally speaking, if R＞1.0,the curve similarity is good; if R＜1.0, the curve difference is great; if 0.6＜R＜1.0, the curves are slightly different.

The two-time test graphs for the same phase of same power transformer are compared, especially compared with the feature graph of new measured power transformer before the operation to determine whether power transformer has winding deformation, which is called longitudinal comparison method; while the horizontal method is to compare phase-A, phase-B and phase-C graph of the same power transformer or the same graph of same-model power transformer. Actually, the horizontal method is more extensively used than the other one.

As for power transformers with good production processes, the difference in the graph of phase-A, phase-B and phase-C is used to determine whether the winding deforms because the graphs of three phases are consistent and the structure of three-phase winding is basically consistent. The level of design and production is relatively fixed if two power transformers are produced in the same factory and belong to the same model. Therefore, the winding structure is similar and feature graph is consistent. It is likely that several power transformers used in the substation belong to the same model and manufacturer, so times of wrong judgment can be reduced through the measurement and comparison of the graphs of these power transformers.

4. Instances

4.1 Statistic data

It is found that the difference of three phases’ graph of several SFZ8-40000/110 power transformers is great when inspecting them in some electric company. The windings are likely to deform. Since the batch of products have been put into use in 1993, most of them are subject to short circuit impact with varying degrees. The accumulative effect may exist in the internal windings of some power transformers. The deformation can not only reduce the mechanical intensity of windings, but also cause inter-turn short circuit or partial electric field concentration. In order to correctly determine whether the winding deforms, 14 measured power transformers are compared. Results are shown in Tab.1.

Notes:

Con: consistency

G: good

B: bad

R12: Relevant coefficient of curve 1 and 2

R23: Relevant coefficient of curve 2 and 3

R31: Relevant coefficient of curve 3 and 1

As for high voltage, 1 is phase OA; 2 is phase OB; 3 is phase OC

As for low voltage, 1 is phase AB; 2 is phase BC; 3 is phase CA.  

4.2 Result analysis

The horizontal comparison method is used. The measured results are compared according to the graphs of three phases. If the consistency of three phases is very good and relevant coefficient R ＞1, we determine that windings do not deform; if the consistency of three phases is not good and relevant coefficient R＜1, the graphs of power transformer belonging to the same model and manufacturer have to be compared for further confirmation.

Based on measurement results, relevant coefficient R of high-voltage winding three phases of all 14 tested power transformers is more than 1 and the waveform consistency is very good. Hence, the horizontal comparison shows that high-voltage winding of all power transformers do not deform. As for low-voltage winding, the consistency of first eight power transformers is good and relevant coefficient R is are more than 1; the consistency of remaining six power transformers is bad and relevant coefficient R is less than 1. Among them, four power transformers R is less than 0.6.

From the waveform analysis, 14 same-model power transformers are similar in the aspect of the number of resonant peaks and valleys, position, amplitude and trend. However, features of No.1 to No.8 power transformers are superior to No.9 to No.14 transformers. The good consistency of eight transformers indicates that it is not the structure of power transformer that affects the graph difference. Furthermore, six power transformers (No.9 to No.14), the consistency of which is bad and relevant coefficient R＜1, may have winding deformation. The power transformers (No.11 to No.14) deforms worst while No.10 and No.9 transformers are likely to slightly deform. Fig.2 shows the graph of No.8 power transformer. The graphs of No.13 and No.14 power transformers with bad consistency of three phases are respectively shown in Fig.3 and 4. The rest of graphs are omitted. As for the frequency ranging from 100kHz to 300kHz, there is great discrepancy among three graphs

Fig.2 Frequency response graphs of low-voltage side of No.8 transformer

Fig.3 Frequency response graphs of low-voltage side of No.14 power transformer

Fig.4 Frequency response graphs of low-voltage side of No.13 power transformer

In order to obtain correct result, the same phase of same-model power transformer can be compared horizontally. As shown in Fig.5, low-voltage phase BC curves of six transformers are compared. The curve 1 to 5 correspond to No.3 to No.7 transformer. The attribute of these power transformers have been proved good; the curve 6 corresponds to No.13 power transformer which is likely to have winding deformation. Based on the curves, the first five curves present good consistency; as for the curve 6, the fourth resonant peak is around 159kHz when the frequency is 200kHz. Therefore, it is determined that No.13 power transformer has serious winding deformation. This method can be applied to other power transformers.

Fig.5 Frequency response curves of low-voltage side phase BC of six transformers

1: Low-voltage phase BC of No.3 transformer

2: Low-voltage phase BC of No.4 transformer

3: Low-voltage phase BC of No.5 transformer

4: Low-voltage phase BC of No.6 transformer

5: Low-voltage phase BC of No.7 transformer

6:Low-voltage phase BC of No.8 transformer

5. Hanging the Cover and Inspection

Based on the conclusion of winding deformation diagnosis, covers of No.11 to No.14 power transformers, which deform worst, are hanged and inspected. The results verify the conclusion. Inspection details are as follows: ① The regulating voltage coils, high-voltage coils and leads are basically well, no deformation and displacement; ② The low-voltage windings deform seriously, such as phase A, B and C of No.11 transformer, low-voltage phase B of No.12 transformer, low-voltage phase B and C of No.13 transformer, phase B and C of No.14 transformer. All windings have crosswise deformation and bump. Owing to severe deformation, stray turns have appeared at some places. 

6. Precautions

The winding deformation test is a sensitive test and is inclined to be affected by various accidental factors. In order to ensure reliable results, please keep the following tips in mind during the test:

1) The input (excitation) terminal and measuring (response) terminal should be selected according to the mode shown in Fig.6 for facilitating the standardization management of results.

Fig.6 General test methods of winding connection

2)  Before the detection, all leads connected to transformer bushings should be removed. In addition, removed leads have to be kept far away from the bushings. As for the power transformers with bushing leads, the bushing tap can be used as response terminal for detecting. But the comparison should be done to results obtained under same condition.

3) The amplitude-frequency response attribute is related to the position of tap switch. It is better to detect at the top tap or ensure the tap switch at the same position each time.

4) Because the detection signal is weak, all connection wires should be stable and reliable.

5) The earthing wires of two signal detection terminals should be reliably connected to obvious earthing terminal at transformer shell (such as iron core earthing terminal); earthing wire should be short as much as possible.

7. Conclusions

The winding deformation measurement is a new method to diagnose power transformer condition. In recent years, remarkable achievements have been made. The Guangdong Electric Test Institute has complied Guide for Detecting and Diagnosing Winding Deformation of Power Transformer (frequency response approach) for the first time in China. Based on that, electric industry standard DL/T911-2004 Frequency Response Analysis on Winding Deformation of Power Transformerhas been worked out. The paper uses horizontal comparison method to introduce how to apply this standard to diagnose the test graph and correctly determine that several power transformers deform. Practice proves that the analysis method can act as a main method to determine whether the winding deform.