WO2018009992A1

WO2018009992A1 - Method for protecting transformers against inner faults

Info

Publication number: WO2018009992A1
Application number: PCT/BR2017/000071
Authority: WO
Inventors: Fabiano Gustavo Silveira MAGRIN; Maria Cristina Dias Tavares
Original assignee: Universidade Estadual De Campinas - Unicamp
Priority date: 2016-07-12
Filing date: 2017-07-07
Publication date: 2018-01-18
Also published as: BR102016016171A2

Abstract

The present invention relates to a method for protecting transformers against inner faults. In short, the method disclosed in the present invention is aimed at increasing the detection sensitivity of inner faults in devices, mainly power transformers, but is also useful for detecting inner faults in other devices, such as motors, generators, shunt reactors, capacitor banks, transmission lines and even a combination of multiple devices in the protection zone. The present method increases the sensitivity to inner faults, thereby reducing operation times and damages to devices protected by this method.

Description

FIELD OF INTERNAL FAULT TRANSFORMER PROTECTION FIELD OF THE INVENTION:

1001] The present invention fits into the field of fault detection methodologies for a power transformer, wherein said methodology is applied with high sensitivity for transformer internal faults, between turns and ground, enabling the removal of the power transformer. service power quickly and reliably. In addition, the present invention can be applied to fault detection in other equipment such as motors, shunt bumpers, capacitor banks and transmission lines, with transformers being considered the most critical case.

BACKGROUND OF THE INVENTION;

The reference of. Quality of life is directly linked to the supply of electricity, and energy demand tends to increase steadily, especially in developing countries. To meet this growing demand, there is an unremitting pursuit of greater efficiency in the power generation, transmission and distribution system. To increase this energy efficiency it is necessary that the electrical system is safe and reliable, leading to a high quality of electric power.

[0.03] Usually, power quality ratings in distribution and the quality of power transmission are monitored by evaluative and comparative standards to desirable standards.

[Q04] One of the equipment that has primary function in. Maintaining high levels of power quality are the power transformers. They are the main equipment, and also the most expensive, of a power substation. SE, transmission or distribution. In generation, transformers are not the most expensive, but this does not reduce their importance.

Thus, transformers must be serviced frequently, but even so, they are subject to short circuits, which is considered serious as there is always a risk of fire. When faults occur inside the transformer, unlike overhead transmission lines, damage to the equipment occurs that needs to be repaired. However, it is in reducing the extent of damage that protective equipment works. The faster the abnormality is detected, the less damage to the equipment.

According to studies in the field of this invention, 12% of electrical system faults occur in transformers. In the literature, transformer failure statistics are presented and, although there is a percentage variation in relation to the fault locations, it is possible to verify that most of the faults occur in the transformer windings, followed by the tap-changer and thirdly. by the bushings _*

Analyzing the faults in the windings, it appears that a fault between few turns does not cause large variation of current at the input and output of the transformer, but internally the leakage current, ie, fault, is high and causes great heating. location on the transformer. This heating generates gases that are detected, for example, by the Buchholz relay. This process is time consuming, increasing the damage to the transformer. There may also be an evolution of the fault, thus being detectable by the protection relay, but again o ^: Damage dealt will be high.

[008] Ground faults in a transformer's delta winding can also go unnoticed by conventional differential protection and the greater the damage to the transformer, the longer the maintenance time, leading to unavailability of power supply and increased power costs. repair.

Technique Est:

[009] Document CN101702512 presents a proposal directed only to generators. First a calculation of. minimum current of I.2 .. When it is above 10% of the load, the zero sequence impedance angle is compared to certain limits. If between Degrees and 90 degrees the fault is external, and if between -90 degrees and -180 degrees, the fault is internal. However, the possibilities offered by this invention are only restricted to generators, unlike the present invention, which. extends its field of application through inventive and differentiated technology, which will be detailed below.

[010] US5514978 is based on the negative sequence impedance calculation. The method makes several accounts, compares 22 with a limit, but. do inverse accounts, read 12, and, after multiplying by Z2, compare. With a threshold of V2 „Do the opposite too, reads V2 θ compares with a 12 after calculation with a standard impedance 22. However, the technology disclosed in US5514978 is exclusively motor-related, while the present invention broadens its scope by of inventive and differentiated technology.

[011] In US781.2615, the operating principle distinguished from the present invention. It is mentioned that US7812615 technology can identify 2¾ faults between turns. However, some technical problems can be identified, such as: having a low current sensor. negative sequence; use a minimum negative sequence differential current and use the angle to discriminate the fault, it is not clear how to use the method for more than two currents. In this regard, it is noteworthy that, in a differential manner, the present invention has a very accurate sensitivity, less than 1% for faults between turns.

In the document CN100550557, it initially already has a different operating principle of the present invention, besides presenting other additional distinctions such as: making use of a negative sequence current differential, having as limitations a minimum operating value and the differential itself that has an intrinsic desensitization to the method. Again, the present invention, in a differential manner, proposes a very accurate sensitivity, which solves a latent deficiency of the state of the art.

[013] US79O3381 works very similarly to CN100550557, and the same considerations can be verified.

[014:] US8823307 restricts its specification to generators only, being based on comparison of magnitude and angle of various quantities such as voltage and sequence current (positive, negative and zero}. While its application is specific to generators and considerably complex, the present invention proposes simplicity and broad application. £ 015] Document CN1015050: S1 is also intended for generators. Based on the negative sequence voltage distribution on the generator., It detects whether the fault is internal or external to the generator based on the negative sequence voltage profile on the generator output and after the elevator bus. However, unlike the present invention, its application is exclusively restricted to generators.

[016] US517Q308 discloses a technology based on the calculation of parallel transformer admittance. It applies to the detection of internal faults during power-up, which is not proposed by the present invention.

In US4204237, a traditional transformer differential solid state relay technology is described. Said subject matter acts on the main protection of the traffic, while in a differential manner, the present invention aims to complement the main protection.

[018] Document BR8606027 reveals a study of the traditional transformer differential. In a similar way to US4204.237, its subject matter acts in the main protection of the traffic, whereas in a different way, the present invention complements a. main protection.

[019] _. There is also the article "Analysis on the Negative Sequence Impedance Directional Protection for Internal Stator Fault of Turbo Generator" which is based on negative sequence impedance but does not have the same expected front and back impedances. It is therefore restricted, unlike the present invention, which has no specificity only for generators and also contemplates the performance of equipment with more than two inputs / outputs.

[020] Document CN10: 4991160 describes a technology for detecting transformer internal faults. For its operation, a capacitive coupling sensor must be installed. This equipment injects high frequency which is measured and treated for detection. The present invention differs from CN1Q4991160 mainly in that it uses a different operating principle based on negative sequence impedances. In addition, the CN104991I60 requires the installation of extra equipment in the transformer, unlike the present invention, which still has the advantage of using only existing equipment for measurement, characterizing its adaptability and portability.

[021] In document BR _. 200900722, the technology described in the document relates to detection of faults internal to the power transformer. Said technology uses the negative sequence differential principle. However, the invention disclosed herein has a proposal that differs from BR2009007.22, mainly in that it uses a different principle of operation based on negative sequence impedances.

Advantages of the Invention;

[022] The present invention allows to present greater and improved sensitivity in detecting internal faults in power transformers. Said invention is not subject to the transformer energizing current or even the over-excitation of the power transformer. State of the art methods based on negative sequence current apply a minimum current comparator for executing an algorithm and the present invention does not, contributing to the increased sensitivity of the element. Another differential is that in no other work is it mentioned that after an external fault is output, there is a transient in the negative sequence diagram and that this transient resembles an internal fault, requiring a block or appropriate desensitization of the algorithm. A further advantage of the present invention is still the ease of expansion of the method for equipment with more than two inputs / outputs.

BRIEF DESCRIPTION OF THE INVENTION;

[023] The present invention relates to a transformer protection method for internal faults.

In short, the method disclosed in the present invention aims to increase the sensitivity of detection of internal faults in equipment, mainly power transformers, but may also be employed in the detection of internal faults in other equipment such as motors, generators, shunt reactors, capacitor bank, transmission lines and even for the combination of more than one equipment in the protection zone.

The present method acts by increasing a. sensitivity to internal faults, thereby shortening operating times and promoting a reduction in equipment damage under the protection it provides.

BRIEF DESCRIPTION OF THE FIGURES:

[026] To obtain full and complete visualization of the object of this invention, reference figures are given as follows.

[027] FIGURE 1 shows the example power system. [028] FIGURE 2 graphically shows the negative sequence impedances seen by the delta side:

FIGURE 3 graphically shows the negative sequence impedances seen by the star side.

FIGURE 4 graphically shows the negative sequence impedances seen by the delta side.

[031] FIGURE 5. graphically shows the negative sequence impedances seen by the star side.

[032] FIGURE € graphically shows negative sequence impedances., Delta side,

[033] FIGURE 7 graphically shows the negative sequence impedances, debut side.

FIGURE 8 shows the sequence diagram for single-phase fault in bar R.

[03.5] FIGURE 9 presents a sequence diagram for single-phase fault in S-bar.

[036] FIGURE 10 represents a. impedance view seen from both sides of the trafo.

[037] FIGURE. 11 is a graphical presentation of the action plan referenced in the present invention.

[038] FIGURE 12 is a graphical presentation of misconduct for external fault.

[13] FIGURE 13 is a graphical presentation of the new plan of action.

[040] FIGURE 1.4 is a sequence diagram for external two-phase fault.

[041] FIGURE 15 shows the negative sequence impedance excursion.

[042} Figure 16 shows the currents and actuation of the element during internal transformer fault. DETAILED DESCRIPTION OF THE INVENTION:

[043] The present invention relates to a transformer protection method for internal faults which comprises the following steps and substeps:

a> Acquisition of network data;

a.l) Monitors all current and voltage signals of the protected equipment (three-phase currents and voltages of each terminal);

a.2} Eliminates frequency overlap; a.3) Converts all signals from analog to digital;

a.4} Extracts the phasors of the fundamental component; b} Signal treatment;

b.1) Conditions the signals to values per unit, pu;

Calculates the negative sequence currents and voltages of each terminal;

b.3) Calculates the negative sequence impedances of each terminal;

c) Identification of the need for unlocking;

cl) Calculates the derivatives of the voltages of each terminal in relation to the past (from 1 to 3 cycles); c2) Compare sub-step (cl) with threshold L1 (from 0.01 to 5);

c 3) Compare sub-step (cl) with threshold L2 (from 0.5 to 10);

c.4) Compare sub-step (cl) with threshold L3 (from 10 to 120);

c.5) Declares external disturbance, does not remove blockage if one of the derivatives of sub-step (c4) is true for time tl (from 0 to 3 cycles);

c.6) Declares internal disturbance if one of the derivatives of sub-step (c.2) is true and sub-step (c.4) is false by time t2 (from 0 to 3 cycles); c7) Declare internal disturbance if more than one derivative of sub-step (c.3) is true and (c.4) is false by time t3 (from 0 to .3 cycles);

c, 8) Locks again if the unlock remains for time t4 (from 100 to 500 ms); d) Identification of the fault;

d. (l) Compare the results of sub-step (b.3) with the acting characteristic '(In this step there may be different types of characteristics, eg single line, double line, circular among others); d.2) Declares internal fault if sub-step (d.l) is true for all terminals;

d.3). Declares external fault if a sub-step terminal (d.l) is. false;

and). Three phase fault identification;

and. l) Calculates the positive sequence currents of each. terminal;

e.2) Calculates operating and differential currents;

e.3) Calculates the variations of the sub-step currents (e .2);

e.4) Compares against the L4 threshold. (from 0.5 to 2.0);

e.5) Declare blocking if sub-step (e.4) is true.

(0441 In summary, the method disclosed in the present invention aims to increase fault detection sensitivity internal equipment, mainly in power transformers, but can also be used to detect internal faults in other equipment such as motors, generators, shunt reactors, capacitor banks, transmission lines and even for the combination of more than one equipment. in the protection zone.

[045] In general all works based on negative sequence fail to show how the method behaves with regard to sensitivity with and without load, presence of source on only one side of the transformer, but mainly in two other situations of paramount importance: 1) How the element behaves during system imbalances of power and 2) How the element behaves after an external fault is extinguished.

[046] The present invention has been developed by contemplating these situations.

[047] The present method is not intended to replace the transformer's main protection scheme, but to supplement it by increasing the sensitivity to internal faults, thereby reducing actuation times and promoting a reduction in damage to equipment under protection, particularly in the case of of transformers ..

[048] This method measures all: currents entering or leaving the equipment under protection. Also measures all voltages around the protected equipment. These currents and voltages are filtered through analog and digital filters in order to extract the fundamental components. Having the fundamental component factors, negative sequence currents, voltages, and impedances are calculated. Negative sequence impedances are compared to an impedance plane to detect if the fault is internal to the transformer. When all impedances fall within the specified plane, the fault is internal. In case of external fault, and after its elimination, there is a transient in the negative sequence diagram that can cause negative sequence dependent elements to act. To avoid this kind of misconduct, this kind of phenomenon should be identified and the blocking or desensitization of the method execution should be promoted.

Method Development Study

[049] The rationale of the method, the presentation of the terms and the performance of the developmental tests are described below for a better understanding of the invention:

Fault Analysis

Fault analysis comprises the study and recognition of how electrical quantities behave for transformer internal faults. External faults are also analyzed to find out what differentiates them from internal faults, enabling the development of the method of the present invention which is disclosed herein.

The analyzes discussed below refer to a transformer. DY and source only on one side, delta side, as shown in Figure 1.

Spire-Ground Internal Faults

[052] It is possible to verify that the sequence impedances have a well-defined behavior for all faults in front of. Delta CT, as shown in figure 2, and that. for reverse lack the behavior is quite different. The same occurs when analyzing faults from the point of view of star side, as shown in Figure 3. For both sides of the transformer, the negative pre-sequence impedance is random because the voltage and currents are close to zero.

Internal Faults between Sponges

[053] Figures 4 and 5 show the negative sequence impedance behavior for inter-loop faults. It is observed that the behavior of impedances is the same as in the case of ground faults.

[054] These analyzes show that the symmetrical components for transformer internal faults have a well-defined behavior showing that the negative sequence element is promising for the development of the transformer protection method disclosed herein.

(055] It is clear from the analysis that effective transformer protection cannot occur only by analyzing one side of the transformer, so the current most commonly used function is the percent differential, but negative sequence differentials already exist commercially. still can't be highly sensitive, leaving part of the winding uncovered »

[056] _. Therefore, it is important to analyze the. previous situations in relation to _. load variation, and source internal impedance. Thus, some analyzes will be presented below considering the infinite source and the practically zero load.

Strong Source and No Charge

Spire-Ground Fouls

[057] As currents tend to grow a lot, impedances tend to origin according to Figure 6. Figure 7 shows that the negative sequence impedance tends to a specific point which is related to the pre-fault impedance.

The Elaboration of the Method

[056] The above tests and concepts justify the development of the transformer protection method for internal faults as disclosed in the present invention.

In view of the conclusion that negative sequence currents and voltages have a well-defined behavior, the sequence diagrams for the fault types to which the method of the present invention applies is presented here.

Single Phase Faults

[060] Figure 8 shows the sequence diagram for a single phase fault on the R bar, ie on the star side of the transformer. It can be observed that the positive sequence and negative sequence current of source S are the same transformer currents, both on the delta side and the star side, only with the reversal of direction. For zero sequence current, there is no current on the delta side and there is current on the transformer neutral and the star side.

[061] According to Figure 8, the equivalent sequence impedances are;

[062] It is known that:

[063] The current divider will have>:

1064} Consequently the currents on both sides of the transformer are:

[065] Writing the negative sequence impedance equations for each side of the transformer has:

[066] As it is not possible to measure, the current but only

current 12s, the above equation should be rewritten as:

[067] Note that the negative sequence impedance measured by the delta side for an external fault on the R bar will have the value of the source impedance S with a negative value. The impedance measured by the star side for the same fault will have the value of the sum of the impedances of the source S and the transformer, with positive value.

[068] A more detailed analysis can be done and it is found that for all delta right CT faults, regardless of whether the fault is internal to the transformer or even external even within the equivalent system of the source. R, the impedance seen by the transformer delta side will always be the impedance of the source S with negative value, equation (D ·

[069] A. The same analysis can be done for the star side, ie for all star side CT right faults, the measured impedance will always be in accordance with (2). Figure 9 shows the diagram for a fault also external to the protection section. S-bar There is no zero sequence current circulation, but there is still negative sequence current circulation, but as. currents measured by the delta or star side are now the current coming from the. equivalent R.

With this we can rewrite the voltage equations by current on each side of the transformer.

1071J It is found that the behavior is reversed in relation to the case of the R-bar. The delta side will always see a positive impedance according to equation (3) for all delta left CT faults. The impedance seen by the star side will always be in accordance with equation (4) for all left faults of the star side CT, even for internal faults.

[072] Therefore Figure 10 summarizes the negative sequence impedance view for both sides. It turns out that only for. transformer internal faults both sides may see a negative impedance.

[073] Impedances on both sides determine whether the sources are strong or weak, so these impedances can range from zero to infinity. Thus it is determined that zero and minus infinity are points of action and, consequently, the region of operation, as shown in Figure 11.

[074] The above analyzes already rule out the need for an analysis of faults internal to the protection section or even the internal ground-faults ...

External Single Phase Fault with Load

[0.75] The external fault situation with source on one side only, ie radial system needs to be analyzed.

[076] Looking back at Figures 2 and 3, it can be seen that for a fault on S-bar, the negative sequence impedance seen by the delta side is equal to the positive pre-fault sequence impedance, ie basically the impedance. of cargo.

[077] To. On the star side, the negative sequence impedance seen is precisely the load impedance, but in the opposite quadrant. If these points are placed in the action plan we could have the protection element misbehaving for an external fault, as shown in Figure 12.

[078] Based on this problem, the element's acting region is modified as shown in Figure 13.

[079] This element is referred to here as negative impedance for internal faults or negative impedance for internal fault, Z2IF.

Blasphemous Fouls

[080] In the previous sections we analyzed the single-phase external and internal transformer faults for ring and radial systems. In this section, we analyze whether the Z2IF calculation can also be applied to biphasic faults.

[0811 Figure 14 shows the sequence diagram for. the two-phase transformer external fault on the R-bar and rewriting the negative sequence impedance equations for each side of the transformer have:

[082] These are exactly the same equations (1) and (2) for a single-phase fault in bar B.

[083] _. If a fault is applied to the S bar, the impedance result will be the same as that. Single-phase fault on the R-bar. The same analysis can be made for transformer-internal faults, so it follows that the element behaves the same regardless of the type of fault and excluding the three-phase fault.

[084] In order to demonstrate the operation of the method described herein, Figure 15 shows the method response in the presence of an internal loop fault with 1% of the shorted star winding. It is possible to verify the negative sequence impedance excursion to the permanent state of the impedances when the fault is present.

[085] Figure 16 complements the analysis showing the currents of. both sides of the transformer and the operation bit of the method described herein.

Claims

1- Transformer protection method for internal faults, characterized by the following steps:

(a) acquisition of network data;

a.2) Eliminates frequency overlap; a.3) Converts all signals from analog to digital;

a.4) JSxtracts the phasors of the fundamental component; b) Signal treatment;

B. l) Condition the signals to values per unit, pu;

b.2) _: Calculates the negative sequence currents and voltages of each terminal;

b.3) Calculates the negative sequence impedances of each terminal;

c) Identification of the need for unlocking;

ç. l) Calculates the derivatives of the voltages of each terminal in relation to the past (from 1 to 3 cycles); c.2) Compares sub-step (c.1) with threshold LI (from 0.01 to 5);

c.3) Compares the sub-step (c.l) with the L2 threshold (from 0.5 to 10);

c.4) Compare sub-step (cl) with threshold L3 (from 10 to 120);

c.5) Declares external disturbance, does not remove block, if one of the derivatives of sub-step (c.4) is true for time tl (from 0: to 3 cycles);

c.6.) Declares internal disturbance if one of the derivatives of sub-step (c.2) is true and sub-step (c.4) is false by time t2 (from 0 to 3 cycles); c.7) Declare internal disturbance if more than one derivative of sub-step (c.3) is true and (c.4) is false by time t3 (0 to 3 cycles) _/

0.8) Locks again if the unlock remains for time t4 (from 100 to 500 ms);

d) Identification of the fault;

d. l) Compare the results of sub-step (b.3) with the actuation characteristic. (In this step there may be different types of characteristics, for example, single line, double line, circular among others); d.2) Declares internal fault if sub-step (d.l) is true for all. terminals;

d.3) Declares external fault if a sub-step terminal (d.l) is false;

e) Three-phase fault identification;

and. l) Calculates the positive sequence currents of each terminal;

e.2) Calculates operating and differential currents;

e.3) Calculates the variations of the sub-step currents (e.2);

e.4) Compares against the L4 threshold (from 0.5 to 2.0);

e.5) Declare blocking if sub-step (e.4) is true.