MX2011013494A - Process for obtaining the blocking of the differential protection against inrush starting currents. - Google Patents

Abstract

The present invention refers to a differential protection process by Lipschitz exponent blocking, which is calculated from the Wavelet Transform Modulus Maxima (WTMM), which is used for differentiating and quantifying the abrupt changes in the edges or discontinuities that a current signal has, due to that it is possible to identify and distinguish a power electric current from an overexciting electric current from a short circuit fault electric current. This process has a wide range of applications in the schemes for protecting electric power transformers, avoiding any type of unanticipated faults, providing more safety.

Description

PROCESS THROUGH WHICH THE BLOCKING OF THE DIFFERENTIAL PROTECTION IS OBTAINED BEFORE INRUSH STARTING CURRENTS.

DESCRIPTION OBJECT OF THE INVENTION The present innovation is applied in transformer protection schemes and is related to the development of a process by which the starting current (Inrush current) of a transformer can be blocked. The process is based on blocking using the Lipschitz Exponent, calculated from the Wavelet Modulus Maximum Transform (WTMM), which is used to identify and quantify singularities (abrupt changes in the signal under study). Through the Wavelet Transform, the Maximum Modulus (MM) is obtained, which is required to estimate the Lipschitz Exponent. The Lipschitz Exponent, in addition to being used to detect and identify a sfic signal, quantifies the singularities and irregular structures present in the edges or discontinuities of the signals. The developed process, unlike the proposed methods that use the Wavelet Transform, quantifies the singular change that the energization current presents, in addition to the overexcitation currents and the short circuit faults; in this way, it characterizes the events that cause false operations by measuring the change they present, obtaining an index with physical representation capable of discriminating said events.

BACKGROUND The power transformers are part of the primary equipment of the Power Electric System (SEP), being indispensable to transmit power blocks across the great distances that separate the generation centers and the consumers. The quality and continuity of electric power depends to a great extent on the good condition of these equipments; Although the transformers are very reliable elements, they are exposed to short circuit faults that can become very severe, to the point of completely destroying the transformer.

The 10MVA Transformers and those of higher capacities are currently protected against faults using the differential percentage relay, which the Electrical Protection Engineers must adjust according to the characteristics of the transformer and the system; In addition, appropriate current transformers (TC's) must be selected.

Initially, it was common for electric protection relay manufacturers to use the harmonic content method to discriminate between faults and energizing conditions. Currently, digital relays allow complement this method with others based on the recognition of the waveform. However, both methods have come to operate incorrectly by disconnecting the transformer unnecessarily, damaging the consumer and causing voltage instability, in the worst case inhibiting the operation of the relay in the presence of a fault. On the other hand, the use of new materials for the development of magnetic cores is investigated, using for example materials with better magnetic characteristics. These materials reduce the content harmonious, needing new methods of discrimination that do not use the harmonic content as a means of discrimination. Additionally, waveform methods have disadvantages, esally when the waveform of the differential current is symmetric. For all the above, new methods of discrimination have emerged. Among them we see the use of Analysis of the Main and Curvilinear Component (ACP and ACC resively), Vector Support Machine (MSV), Artificial Neural Networks (RNA), Fuzzy Logic and the Wavelet Transform (WT). The WT has been used to obtain characteristics of the correlation, between the signal under study (energization current and internal faults in transformers) and the mother Wavelet function to perform a discrimination; these characteristics have no physical representation and are dependent on the conditions under which they were evaluated; therefore, they are not currently in practice.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows the block diagram of the proposed process if the input signal corresponds to the database.

In figure 2, the input signal is integrated.

In Figure 3, the blocking logic is indicated by Lipschitz.

Figure 4 shows the print result mode.

Figure 5 shows the locking operation scheme by Lipschitz Exponent.

DETAILED DESCRIPTION OF THE INVENTION The proposed process corresponds to a scheme by means of which, from an input signal, it is possible to detect the presence of energization current in a transformer. That is, from the calculation of the Lipschitz Exponent, it is able to correctly identify if the input signal corresponds to an energizing current, it can even detect if the current corresponds to an overexcitation or to a signal given by internal faults in the transformer, contrary to any other type of signal. It should be noted that all the operations required by the process are carried out with the instantaneous current signal coming from the secondary of the CTs (/ si and IS2) and does not require voltage signals for its operation; In addition, it is proposed to make the model of the relay for a single-phase transformer with two windings that facilitates its application but that does not represent any inconvenience of taking it to three-phase transformers.

In general, digital processing is divided into the following 3 stages: 1. Entrance sign 2. Blockade by Lipschitz. 3. Results Figure 1 shows the block diagram of the proposed process. The process is intended to provide accurate and rapid discrimination between transformer energization, overexcitation and internal faults; the primary characteristic is its exception to the calculation of the 2nd harmonic component. In addition, blocking by Exponent Lipschitz operates in parallel to the 87T differential percentage element. The stages of the process are: a) Input signal.

The input signal is taken from event records, which correspond to digital files, from digital simulators or real records stored by the relay located in the substation. Figure 2 shows the obtaining of the input signal if it is a real-time processing; for this case, the digital signal processor performs the analog / digital conversion of the input signal corresponding to the instantaneous current value from the CTs, here the signal is sampled at 3.840 Hz, equivalent to 64m / c for a frequency fundamental 60Hz, this sampling frequency being commonly used by manufacturers of transformer differential protection relays. If the event records are digital files, being the case analyzed in this invention, each record has 2 vectors that correspond to the secondary current of the TC's (/ if and ½ sampled at 64m / c). The process automatically addresses the database in an internal structure and calls file by file for processing.

First / if e / s2 are scaled by TAP1 and TAP2 respectively to handle pu values according to: TAP \ = UVA · 1000 · Cl (5.3.1) V3 · V \ | CTR1 V3 · V2 · CT R2 Where MVA is the capacity of the transformer, Cl and C2 are defined by the transformer and the connections of the TC, VI and V2 is the nominal voltage in kV of the power transformer in windings 1 and 2 respectively; CT Rl and CT R2 are the transformation ratio of TC \ and TC2 respectively. Once the signals have been normalized, the one that calculates the ID and IRET enters the next block, to begin with the starting criterion, described below.

The differential current ID is obtained as the sum of the instantaneous currents that enter the differential protection element according to: / 0 [n] = isi [n] + iS2 [n] (5.3.3) Where \ \ [n] and iS2 [n] correspond to the current of TC1 and TC2 respectively, the instantaneous value of the current from the CTs contains magnitude and phase information.

For two holding elements the IRET holding current can be obtained as: ½7- [n] = (is, [n] - iS2 [n]) * (5.3.4) Where k is a compensation factor, usually k = 0.5. After calculating IQ and IRET the next step is to determine if it is an energizing current, an internal fault, an external fault or an overexcitation condition. The blocking logic discriminates between said events. b) Locking logic by Lipschitz Exponent The differential percentage relay is used to determine if it is an event within the operating area of the relay. The blockade by Lipschitz Exponent operates in parallel so as not to affect the operation of the 87T, in such a way that the failure release time is determined by the 87T, where it generally operates after 1 cycle of the failure due to the delay presented by the digital signal processing in the digital filters used.

In the process, the two singular changes of the current of energization, overexcitation and failure are identified and quantified, by means of the Discrete Wavelet Transform with a mother Wavelet function type Daubechies with a moment of cancellation (dbl).

In a fault, the estimate of the second Lipschitz Exponent (a2) is approximately 1; this is because there is no singularity after the first singularity and a mother Wavelet function with a moment of nullification (1 derivative) tends to 1 (0.968 <a.2 = 0.96), whereas in an energizing current the Lipschitz exponent should be around 0.7 (0.5745 <a2 <0.6964) and for overexcitation the a2 is around 0.65 (0.4163 a2 <0.8628). In this way an adequate discrimination criterion is that the operation of the relay is blocked if: a2 < 0.94 (5.3.5) The blocking logic of the proposed transformer differential protection acts as a comparator as shown in Figure 3.

Finally, the blocking logic delivers one of the following three results; NO OP., FAILURE and INRUSH; the non-operation is determined by the differential element, to identify a fault it is necessary that the 87T & a2 > 0.94 through the logic and shown in Figure 1, finally the process presented manages to inhibit the operation of 87T under conditions of energization and overexcitation if a2 < 0.94. c) Results The print result mode is shown in Figure 4, these results are presented in a .txt file where it is printed: The name of the file evaluated, the value of ai and a2 (LEI and LE2 respectively) and finally the event occurred; if it is an internal fault or an energizing current in the process, the result is FAILURE or INRUSH, respectively. On the other hand, if the relay did not operate, it means that an anomaly occurred outside the differential zone or a low energization current occurred ( such is the case of energizing the transformer in 90 °) that did not require the operation of the differential percentage relay (these events are identified as NO OP); if an overexcitation occurs, it is identified as INRUSH and blocks the differential relay.

Figure 5 indicates the operation scheme of the process for an energization current. The instantaneous differential current is stored in a data window of 64 samples in a virtual memory (buffer), this window is traversed each sample and discards the last of each new sample, in each window the calculation of equations is made 5.3.3 , 5.3.4 and by processing with the Wavelet Transform the two Lipschitz Exponents are obtained through the Discrete Wavelet Transform and the Local Maxima of the Wavelet Transform. If the second exponent < x2 meets condition 5.3.5 the relay blocks the operation of the differential percentage relay of the power transformer.

Claims (1)

CLAIMS Having sufficiently described my invention, I consider as a novelty and therefore claim as my exclusive property, what is contained in the following clauses:

1. Process by means of which the blocking of the differential protection against Inrush starting currents is obtained, from the calculation of the Lipschitz Exponent characterized by the following procedure: a) The digital signal processor performs the analog / digital conversion of the input signal corresponding to the instantaneous current value from the CTs, here the signal is sampled at 3840Hz, equivalent to 64m / c for a fundamental frequency of 60Hz, This sampling frequency is commonly used by manufacturers of transformer differential protection relays. b) The blockade by Lipschitz operates in parallel so as not to affect the operation of the 87T, in such a way that the failure release time is determined by the 87T where it generally operates after 1 cycle of the failure due to the delay presented by the digital signal processing in the digital filters used. c) In the process, the two singular changes of the energization current, overexcitation and failure are identified and quantified, by means of the Discrete Wavelet Transform with a mother Wavelet function type Daubechies with a moment of cancellation (dbl).