The Research Method of Prevention Transformer Differential Protection Misoperation Caused by the Sym

The Research Method of Prevention Transformer Differential Protection Misoperation Caused by the Sympathetic Inrush Jingbin Yang1, Dalong Fu2, Youwen Tian*3, Yuwei Li4.

Shenyang Agricultural University, Shenyang City, Liaoning Province, 120 Dongling Road, China *youwen_tian10@163.com Abstract Differential protection has been the main protection of transformer for a long time. The key and difficult problem is how to prevent the differential protection misoperation caused by inrush current. In recent years, there has been a lot of sympathetic inrush accidents caused by misoperation of transformer differential protection, affecting the safe and stable operation of power system. This paper established the simulation model of inrush current and, analysis of sympathetic inrush mechanism. At the same time, the fundamental reasons of differential protection misoperation caused by the sympathetic inrush were found out. And relevant methods and measures of preventing the sympathetic inrush misoperation in the transformer differential protection are put forward.

Keywords Transformer; Differential; Prodection; Synpathetic Inrush Current; Measures

Introduction Power transformer is the most important equipment in power system, which has extremely important significance for reliable transportation, flexible distribution and safe use of electricity. Its running safety is directly related to the ability of the power system’s continuous, reliable and stable working. If power transformer happens fault, it will not only make the system power supply interruption, but also influence the industrial and agricultural production and peopleʹs normal life (HAYWARD. 1941). Even it can endanger the personal safety and equipment.(SUN Hu et al. 2003) Especially because the transformer is expensive and complex structure, once it is damaged due to fault, the maintenance difficulty is more serious and the maintenance time is longer, which will bring enormous loss to national economy.

For a long time, differential protection has always been one of the main protection schemes of the internal fault of transformer. The reasons of longitudinal differential protection misoperation are in many aspects. When transformer closes in the removal of no‐load or voltage recovers after external fault, the inrush current of transformer core saturation and produce is the most serious problem faced by the transformer differential protection. (Xuesong Zhang 2006 , Daqiang Bi , 2007) In recent years, there are many maloperation of adjacent parallel transformer differential protection when a no‐load transformer is closed. This brings a lot of harm to the normal operation of the main equipment. This kind of phenomenon has something with sympathetic inrush current (Hongchun Shu et al.

2006).

Theoretical Analysis and Simulation Verification of Produce Mechanism of Transformer and Sympathetic Inrush Current To research produce mechanism of sympathetic inrush current, we use parallel two transformers operation as an example. The equivalent circuit model is shown in Figure 1. The parallel operation of transformers can produces sympathetic inrush current. When the T1 is in normal operation, T2 is no‐load. If the inrush current in T2 is large, T1 will appear sympathetic inrush current corresponding to inrush current. And great changes will take place in the line current waveforms.

In Figure 1, the system voltage is indicated by US. A resistance is RS. A inductance is LS. Equivalent

12 International Journal of Engineering Practical Research, Vol. 4 No. 1‐April 2015 2326‐5914/15/01 012‐06 © 2015 DEStech Publications, Inc. doi: 10.12783/ijepr.2015.0401.03

The Research Method of Prevention Transformer Differential Protection Misoperation Caused by the Sympathetic Inrush 13

resistor of the primary side of the transformer T1 is R1.

The inductance is L1. Equivalent resistance of the primary side of the transformer T2 is R2. Inductance is L2. B is a common point. When the switch S closes, transformer T2 will produce inrush current i2, which is completely inclined to one side of the timeline.

There are a lot of non‐periodic components. Non‐ periodic components of the current generate voltage drop, which make bus voltage of T1 and T2 occur DC excursion through the system resistance. Since the magnetic flux in the transformer is the integral of the voltage. Magnetic flux of the transformer T1 will be inclined to the side as the new bus voltage shift, the superimposition of the incremental offsets magnetic flux and components of the cycle magnetic flux ,which will make the core of T1 saturated and occur sympathetic inrush current. Set iS, i1,respectively,as system current and the current that flows through T1, iS = i1 + i2. The whole process can be described as follows.

FIGURE 1. TWO PARALLEL RUNNING TRANSFORMER EQUIVALENT CIRCUIT DIAGRAM.

The Occurrence of Sympathetic Inrush Current As shown in Figure 1, before T2 is closed, T1 is on operation normally, the transformer T1 core is not saturated. So i1 equals 0. After T2 is closed, T1 has not yet saturated in time, i1 is still 0. At the moment, iS equals i2, the voltage balance equation of transformer T1 is as follow: d 1 di  us  Ls 2  Rs i2 (1) dt dt

integration is zero. i2 is as inrush current. According to the waveform of inrush current, we know when θ equals 0 and θ equals 2π, i2 (0) ≈i2 (2π) ≈0.By above formulas, we can get:  1 (2 )   1 (0)  Rs 02 i2 ( )d (3) According to formula (3), it can be analyzed that the changes of the T1 flux are determined mainly by periodic integration of the i2 in T2 . Each cycle increment of the flux linkage of T1 is:  1   Rs 02 i2 ( )d (4) To consider it easily, let’s assume that the magnetizing inrush current i2 of T2 is inclined to the time‐axis positive side .(Similarly ,it does not affect the results if assuming it is inclined to the time‐axis negative side) From the formula (4), it can be seen that as Δψ1 is negative, flux of T1 is increased in the opposite direction, gradually reaching saturation point. Before reaching saturation point, i1 can be substantially considered to be zero. Because of the role of the AC component of the flux ψ1, before the non‐periodic component of flux Ψ1 does not fully reach the saturation point, part flux of each cycle may exceed the saturation point at some point. And therefore, flow will generate at the peak of the flux, with intermittent angle. With T1 flux increasing in the opposite direction and inrush current i1 growing, intermittent angle decreases. Due to flux of T1 increases in the negative direction, flux is not saturated in T1, when there is a positive saturation of flux in T2. So negative saturation only occurs in T1.That is, i1 and i2 are in contrary direction and staggered in time.

Development Stage of the Sympathetic Inrush Current When sympathetic inrush current appears in T1, change of flux ψ1 is formed jointly by i1 and i2, then:  1     2 [( Rs  R1 )i1  Rs i2 ]d (5)

After finishing:  1     2 [ Rs (i1  i2 )  R1i1 ]d (6)

(2)

Because intermittent angle exists in sympathetic inrush current i1 and inrush current i2, i1 is inclined to one side of the timeline, both of i1 and i2 contain non‐ periodic components. Assuming i1.f or i2.f are non‐ periodic component of i1 or i2and isf is non‐periodic component , we can know:

The us (θ) is a sinusoidal voltage source, a cycle of

isf  i1 f  i2 f (7)

Considering a cycle, the result of integral of both sides is as follow:

1 (2 )  1 (0)  02 us ( )d  Ls [i2 (2 )  i2 (0)]  Rs 02 i2 ( )d

14 Jingbin Yang, Dalong Fu, Youwen Tian, Yuwei Li

FIGURE 2. A PARALLEL TRANSFORMER SIMULATION MODEL.

After sympathetic inrush current occurring, the variation of flux in transformer T1 is:  1  2 [ Rs (i1 f  i2 f )  R1i1 f ] (8) As i1 and i2 are in the opposite direction, we assume that the amplitude of i2 is positive and the magnitude i1 is negative. With appearance of i1, the flux ψ 1 of T1 increases slowly in reverse direction. As the flux of T1 cumulates negatively and constantly, sympathetic inrush current i1 increases rapidly. We can conclude the following equation: i1 f  

Rs Rs i2 f i1 f   i2 f (9) Rs  R1 Rs  R1

Changes of T1 flux in each cycle is as follows:  1     2 R1i1d (10)

Since i1 is negative, Δψ1 changes in sign, indicating that the flux of transformer T1 decreases gradually shifts. At this point, there is a positive and negative half‐cycle symmetry current. The magnetic chain ψ1 of T1 increases to the maximum in the opposite direction, while sympathetic inrush current i1 increases to the maximum, the absolute value of the magnetic chain in T1 decreases, then, i1 begins to decrease slowly (Yanyun Zou 2009, Wancai Wu 2011, Hongchun Shu 2006, Gong Wei et al 2012, Daqiang Bi et al 2005, Xiaofan Shen et al 2011, Xinchen Li 2014) .

Simulation Establishing simulation shown based on Figure 2 in Matlab, main parameters of the transformer model

are: the winding wiring, voltage, leakage inductance in resistance, saturation characteristic in core, hysteresis loop, etc. The transformer characteristics can be easily set. In the study, we use trial and error methods in the hysteresis loop to determine the hysteresis loop by comparing simulation waveforms, the theoretical waveforms and actual waveforms.

Parameters of transformer T1 are: capacity 150MVA, frequency 50Hz, rated voltage (500/ 3 ) kV / (220 3 ) kV. High‐voltage and low‐voltage winding resistance and leakage inductance are the same, which are respectively 0.004 and 0.08. Excitation resistor is 450 (per unit), with two linear imitating saturation characteristics i ‐ φ: 0,0; 0,1.2; 1.0,1.45. Parameters of transformer T2 are consistent with the T1’s.

After setting parameters, the above theoretical analysis are simulated.Simulation results are shown below.

FIGURE 3. WAVEFORM OF SYMPATHETIC INRUSH I2 THROUGH THE TRANSFORMER T2.

The figures show that the transformer produces inrush current rapidly, after the unloaded transformer is closed.Inrush current decreases with the passage of time.After Inrush current appears, the running transformer will generate sympathetic inrush current.

The Research Method of Prevention Transformer Differential Protection Misoperation Caused by the Sympathetic Inrush 15

And first sympathetic inrush current increases and then decreases. Simulation results coincide with the theoretical analysis.

FIGURE 6. PHI 2 (0) =1.2 SYMPATHETIC INRUSH WAVEFORM.

FIGURE 4. WAVEFORM OF SYMPATHETIC INRUSH I1THROUGH THE TRANSFORMER T1.

Factors that Affect Sympathetic Inrush After analyzing the generation mechanism of sympathetic inrush , this study further analyzes how the residual magnetism of no‐load switching transformer and system impedance parameters influence on amplitude of sympathetic inrush current and saturation velocity by numerical simulation. We can sum up characteristics of the parallel transformer sympathetic inrush.

FIGURE 7. THE SYSTEM RESISTANCE IS 6 OHM SYMPATHETIC INRUSH WAVEFORM.

The Effect of Sympathetic Inrush from T2 Different Residual Magnetism Fix closed time of T2 and change the initial value of magnetic chain ψ2(0). As the remanence ψ2 (0) increases gradually, ψ2 (t) also increases, sympathetic inrush current generates rapidly and amplitude increases in negative direction. In other words, when closing angle must be fixed, the greater the remanent magnetization of the transformer, larger sympathetic inrush,the speed of reaching the maximum is faster.

FIGURE 8. THE SYSTEM RESISTANCE IS 10 OHM SYMPATHETIC INRUSH WAVEFORM.

Figure 5 for φ2(0) = 0.8 sympathetic inrush waveform, Figure 6 for φ2 (0) = 1.2 sympathetic inrush waveform. Compared with φ2 (0) = 0.8, φ2 (0) = 1.2, the remanence of T2 transformer is bigger, sympathetic inrush also is bigger, and the speed of reaching the maximum is faster.

FIGURE 9. SYSTEM REACTANCE 0.2H FOR THE SYMPATHETIC INRUSH WAVEFORM.

The Influence of the System Resistance on Sympathetic Inrush

FIGURE 5. PHI 2 (0) =0.8 SYMPATHETIC INRUSH WAVEFORM.

System resistance can affect the amplitude and attenuation of sympathetic inrush. The depth of influence depends on its proportion in the whole circuit. The production process of sympathetic inrush

16 Jingbin Yang, Dalong Fu, Youwen Tian, Yuwei Li

actual is the process that the switching transformer transient flux is redistributed to other transformer, in which system resistance plays an important role. The size of system resistance determines the speed of redistribution. The greater the System resistance is, the faster the T1 saturates, and the occurrence of sympathetic inrush and the saturated velocity are also faster.

Figure 7 is sympathetic inrush waveform with system resistance of 6 Ω. Figure 8 is sympathetic inrush waveform with the system resistance of 10 Ω.

Compared with the system resistance for 6 Ω, the system resistance of 10 Ω accounts for a larger proportion in the whole circuit. And the faster sympathetic inrush occurs and saturats , the bigger sympathetic inrush current will be.

The Effects of the Reactance of the System on Sympathetic Inrush The greater the system reactance, the slower sympathetic inrush saturats and the peak of sympathetic inrush is smaller. But the system reactance is smaller than transformer short‐circuit reactance, therefore, it is limited to increase the system reactance for reducing peak of inrush current.

Similarly, it is also limited to reduce the attenuation velocity of inrush current.

Figure 9 is inrush current waveform with the system reactance for 0.2 H. Figure 10 is inrush current waveform with the system reactance for 0.15 H. Compared to the system reactance for 0.2H ,when the system reactance is 0.15H, the faster inrush current generates and saturates, the larger the peak of inrush current is.

FIGURE 10 SYSTEM REACTANCE 0.15H FOR THE SYMPATHETIC INRUSH WAVEFORM.

Measures for Preventing the Sympathetic Inrush of Transformer Misoperation Through the above analysis, we can see that the main

cause of the differential protection misoperation are that local transient saturation of current transformer is caused by aperiodic components of sympathetic inrush and second harmonic content in the differential current becomes low. Therefore, we shall take corresponding measuresto prevent the phenomenon.

In order to prevent transient saturation of current transformer, if condition permits, we can replace the P TA with TP TA. To meet the sensitivity requirements, we increase the fixed value of the differential protection of generator and transformer appropriately.

Before closing no‐load Transformer, all capacitors should be exited. We also should close the no‐load transformer at the peak period of load. These measures can reduce sympathetic inrush efficiently.

Before closing the switch of no‐load transformer, neutral point should not ground.which makes it generate excitation inrush current and do not produce sympathetic inrush. It should be noted that, if the switch transformer neutral point ungrounded, to prevent overvoltage problem when closing the no‐ load transformer, clearance protection should be installed in the transformer neutral point, such as gapless metal oxide arrester: the special protection of transformer neutral point ,or rod gaps parallel arrester protection. This can prevent impulsive overvoltage from damaging transformer core and insulation.

Conclusions Formation of sympathetic inrush is related to many factors. Its essential reason is that voltage fluctuations are caused by excitation inrush current of switching transformer, which flows through the system resistance. On the basis of summarizing predecessorsʹ work, we set up simulation models of parallel transformers, and analyze the generation mechanism of sympathetic inrush, influence of the parameters in the model and the causes of the differential protection misoperation in this paper. Finally, related methods and measures of preventing the transformer differential protection misoperation are put forward, hoping that they will be helpful to the reliable operation of the transformer protection.

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