Method for detecting power transformer magnetizing current surge intended for microprocessor-based relay protection device
Patent Information
- Authority / Receiving Office
- RU · RU
- Patent Type
- Patents
- Current Assignee / Owner
- AKTSIONERNOE OBSCHESTVO TEKONGRUP
- Filing Date
- 2025-12-25
- Publication Date
- 2026-07-01
Smart Images
Figure 00000011_ABST
Abstract
Description
[0001] Technical field
[0002] The invention relates to digital (microprocessor) relay protection of electric power systems, and is part of measures designed to adjust the protection of a power transformer (hereinafter referred to as a transformer) from a surge in magnetization current.
[0003] Technical innovations in the protection device - MP URZA TEKON 300 are described.
[0004] It is known [1, pp. 105-106] that a significant portion of the magnetizing current (Inam) flows in the transformer winding located on the side of the electrical power source. Under normal operating conditions, the magnetizing current does not exceed a few percent of the rated current (approximately 5%). A sharp increase in magnetizing current when the transformer is turned on without a load or due to other reasons is called a magnetizing current inrush (hereinafter, current inrush).
[0005] The following description of the operation of the power transformer is limited:
[0006] - normal operating mode;
[0007] - emergency short circuit mode (internal short circuit) that occurs in the windings and terminals of the power transformer, as well as on the busbars (hereinafter referred to as SC);
[0008] - electric surge.
[0009] In order to reduce the length of the description, the terms used in it within this description have the following meanings:
[0010] “Transformer” is a three-phase power transformer or autotransformer for which the protection detects an inrush current or short circuit.
[0011] “Primary winding of a transformer” is the winding of a transformer to which the energy of the converted alternating current is supplied (according to GOST 16110-82).
[0012] “Secondary winding of a transformer” is a transformer winding from which the energy of the converted alternating current is removed (according to GOST 16110-82).
[0013] "Inrush detection" is a synonym for the words "detection", "identification", "classification" of inrush current.
[0014] "Protection" - actions, operation, algorithm of the protection device.
[0015] "Protection trip" is a term used to describe the action of a protective device which causes a signal to be sent to a circuit breaker (hereinafter referred to as the circuit breaker) from the electrical power source, causing it to open its switching contacts, thereby disconnecting the transformer from the electrical power source.
[0016] "Blocking protection" is a term used in relation to the action of a protection device that results in blocking (stopping) the operation of the protection.
[0017] “Second harmonic content” - the value (in percent or relative units) of the ratio of the second harmonic (100±10 Hz) of the secondary current to the fundamental harmonic (50±5 Hz) of the secondary current.
[0018] “Aperiodic component content” is the value (in percent or relative units) of the ratio of the aperiodic component of the secondary current to the fundamental harmonic of the secondary current.
[0019] “Second harmonic content of differential current” - the value (in percent or relative units) of the ratio of the second harmonic (100±10 Hz) of the differential current to the fundamental harmonic (505 Hz) of the differential current.
[0020] Prior art - general information on transformer differential protection.
[0021] Longitudinal differential current protection is designed for fast-acting protection of a transformer from short circuit.
[0022] Fig. 1 shows a circuit diagram in which digital protection device 1 is used to protect transformer 2. Fig. 1 shows transformer 2 with two Y / Δ-11 (hereinafter Y / Δ-11) windings. Fig. 1 shows one phase x, where x is phase A, or phase B, or phase C. Transformer 2 has a primary winding on the side of the electric power source connected in a "star" and a secondary winding connected in a "delta". Fig. 1 shows the transformer in normal operation, when the contacts of switches 3 and 4 are closed. Measuring current transformers (hereinafter referred to as measuring CTs) 5 and 6 are installed at the terminals of the windings of transformer 2, designed to transmit the measuring information signal to the digital protection device 1. Alternating primary currents I flow through the windings of measuring CTs 5 and 6 Yx and I Δх - Primary currents are transformed by measuring CTs into alternating secondary currents i Yx and i Δх, which enter the digital protection device 1.
[0023] Traditionally for the positive direction of primary currents I Yx and I Δх The direction of currents is assumed to be towards the transformer. In normal operating mode of the transformer, the primary current I Yx from the primary winding side has a positive direction, and the primary current I Δх from the secondary winding side of the transformer it has a negative direction.
[0024] In differential protection, instantaneous values of secondary currents i can be compared Yx and i Δх (simultaneously the modules and phases of currents or only phases), as well as complex effective values of secondary currents And [1, p. 295].
[0025] Transformer protection 2 has its own unique features. For example, in real-world conditions, digital protection device 1 is designed to programmatically record:
[0026] - transformer transformation ratio;
[0027] - transformation ratios of measuring CTs;
[0028] - transformation ratios of auxiliary current transformers of the digital protection device itself, which transform the alternating secondary currents i received by the digital protection device Yx and i Δх level that is suitable for processing them in the ADC block and in the computing device;
[0029] - compensation of the angular shift of the primary currents of the transformer windings (phase compensation operation), which depends on the circuit and the group of connections of the transformer windings [2, pp. 106-109].
[0030] These aspects should be taken into account in real transformer protection conditions, however, in this application, this aspect is deliberately not taken into account in order to better convey the essence of the claimed solution.
[0031] The currents of all sides of the transformer are used to calculate the differential current.
[0032] The instantaneous differential current is obtained from the instantaneous values of the secondary currents (before filtering) taking into account the phase compensation operation. The instantaneous differential current in phase (x) for a Y / Δ-11 transformer is:
[0033]
[0034] i' Yx - the instantaneous value of the secondary current from the side of the transformer winding connected in a star, after the phase compensation operation in phase (x).
[0035] i' Δх instantaneous value of the secondary current from the delta-connected side of the transformer winding after the phase compensation operation in phase (x).
[0036] The operation of differential protection is based on the fact that in normal operating mode (Fig. 1), when the contacts of switches 3 and 4 are closed, the instantaneous differential current i .диф.х phase (x) is equal to zero (i .диф.х = 0), and the protection device 1 does not operate.
[0037] If a short circuit occurs (Fig. 2), the secondary current i Δх phase (x) of the secondary winding of the transformer will become equal to zero (i Δх = 0), and the instantaneous differential current of this phase will be determined only by the secondary current i Yx phases (x) of the primary winding of the transformer: i диф.x = i' Yx In this case, a sharp increase in the amplitude of the differential current of the fundamental harmonic will lead to the activation of digital protection device 1, as a result of which a signal will be sent to open the contacts of switches 3 and 4.
[0038] A current surge can be caused by various reasons, but a current surge caused by the no-load transformer connection is the most unfavorable for the protection operation. In the no-load transformer connection mode (Fig. 3), when the contacts of switch 3 are closed and the contacts of switch 4 are open, the secondary phase currents (x) on the secondary side of the transformer winding will be equal to zero (i Δх= 0). Instantaneous differential current i .диф.х phase (x) will be determined by the secondary current of phase (x) from the primary winding of the transformer: i .диф.х = i' Yx. The resulting inrush current will cause a sharp increase in the amplitude of the fundamental differential current, which the protection will interpret as a short circuit and erroneously trigger the protection, causing the contacts of switch 3 to open. This cannot be allowed to happen. Therefore, it is necessary to identify indicators other than the magnitude and phase of the secondary current that can distinguish an inrush current from a short circuit. This will allow the inrush protection to be configured so that it only trips in the event of a short circuit.
[0039] When the transformer is turned on at idle speed from the star-connected primary winding side, the entire primary current is the magnetizing current (I Yx = I нам ).
[0040] Next, we discuss the known signs of magnetizing current during inrush current, which are used to distinguish inrush current from short circuits. These signs are also used to detect inrush current, regardless of the methods used to detect short circuits.
[0041] Prior art - harmonic composition of magnetization currents during current surges until the moment of saturation of measuring CTs.
[0042] It is known [1, p. 442] (in the general case) that the magnetizing current during a current surge can contain a large aperiodic component (component), as well as a significant percentage of higher harmonics (primarily the second). This is the fundamental difference between the magnetizing current during a current surge and the short-circuit current. The short-circuit current at the moment of its occurrence contains only the aperiodic component.
[0043] It is known [3] (in general) that during a current surge, conditions may arise where the magnetizing current may contain an aperiodic component (unipolar or aperiodic current surge) and may not contain an aperiodic component (bipolar or periodic current surge). The amplitude of a unipolar current surge with an aperiodic component can reach 6-8 times the amplitude of the transformer's rated current. A bipolar current surge, however, can only be observed in one phase, and its amplitude can reach 1-2 times the amplitude of the rated current.
[0044] It is known [1, p. 105] (in the general case) that the initial value of the amplitude of the magnetization current during a current surge depends on the magnitude of the residual magnetic flux in the transformer winding and on the voltage angle of the electrical energy source at the moment the transformer is turned on (hereinafter the voltage angle).
[0045] It is known [4] that the type (unipolar or bipolar) and amplitude of the magnetizing current are highly dependent on the voltage angle. The maximum value of the magnetizing current subjected to phase compensation is observed at voltage angles of 0° and 180°, while bipolar magnetizing current is obtained at voltage angles of 90° and 270°.
[0046] All the above-mentioned known data regarding the harmonic composition of the magnetizing current during a current surge are given in general terms and do not detail the composition of the magnetizing currents by phase.
[0047] It is also known [3] that the inrush current curve without an aperiodic component is characterized by a second harmonic content of at least 40%. And the inrush current curve with an aperiodic component is also characterized by a second harmonic, but its relative content is significantly less and can be approximately 15%.
[0048] TekonGroup conducted more detailed research into:
[0049] - Analysis of secondary current curves i x during a current surge for each of the three phases (x) depending on the voltage angle.
[0050] - Analysis of differential current I curves диф . х during a current surge for each of the three phases (x) depending on the voltage angle.
[0051] It should be noted that the secondary current curve i x phase (x) will differ from the differential current curve i диф . х phase (x), which includes only the secondary current i x phases (x). This is due to the fact that when calculating the differential current, the values of the secondary current that have undergone the phase compensation operation will be used.
[0052] The study focused on the harmonic content of magnetizing currents during inrush current. Specifically, the second harmonic and aperiodic component content was analyzed in each of the three transformer phases for 0°, 90°, and other voltage angles. Contrary to existing data, it was found that at zero voltage angle in the phase with the maximum unipolar inrush current (i.e., with the maximum aperiodic component), the second harmonic content can be below 10%. In the other two phases, with moderate aperiodic components, the second harmonic content will exceed 10%.
[0053] In conclusion of the above, it can be concluded that during a current surge, regardless of the value of the voltage angle in at least two phases, the secondary current i x will contain a sufficient share of the second harmonic and a sufficient share of the aperiodic component (the values of the shares (contents) are indicated in the section “Essence of the invention”).
[0054] Prior art - saturation of measuring CTs during current surge or internal short circuit of the transformer.
[0055] Saturation is a common problem with electromagnetic measuring CTs. After an inrush current or short circuit, the measuring CT correctly transforms the primary current into secondary current only for a few time periods. The measuring CTs then saturate with the aperiodic component of the inrush current or the aperiodic component of the short circuit current, which is present at the time of their occurrence. After the measuring CTs saturate, the aperiodic component disappears in the magnetizing current, short circuit current, or short circuit current during the inrush current. At the same time, a second harmonic appears in the short circuit current, which is also characteristic of the magnetizing current. Consequently, after the measuring CTs saturate, there is no qualitative difference in the harmonic composition of the short circuit current and magnetizing current during the inrush current.The errors of measuring current transformers when saturated can be so large that they can cause a significant delay in the protection device's response during a short circuit (loss of response speed) or erroneous (unscheduled) operation of the protection device intended to disconnect the transformer from the power source during a current surge. Loss of response speed leads to failure of the power transformer. Erroneous disconnection of a transformer is undesirable for its long service life.
[0056] The proposed technical solution for detecting current surges is carried out during a time interval in which saturation of the measuring CTs does not occur, and an undistorted signal from the measuring CTs is sent to the computing processor.
[0057] Prior Art - Known Methods of Inrush Current Detection. Known methods for relay protection against inrush currents are based on electromechanical components and are not used in digital protection due to their low sensitivity to short circuits. These methods include:
[0058] - Use of differential current cutoff (SU 936164 A1). - Use of intermediate fast-saturating current transformers of the differential protection circuit (SU 660139 A2, SU 256032 Al, SU 545036 A1).
[0059] There are known methods for detecting inrush current, which were initially based on the electromechanical element base of the relay, and then their basic principles migrated to digital protection.
[0060] Such methods include:
[0061] - Method of analyzing the "curve shape" of secondary or differential current (first method).
[0062] - Method of "harmonic analysis" of secondary current or differential current (second method).
[0063] The first method of analyzing the current "waveform" uses the differences in the inrush current magnetization curve and the short-circuit current curve (SU 365766 A1, SU 535644 Al, SU 943970 Al, SU 170100 Al, SU 1775788 A1, EP 2891216 B1, EP 4415195 A1). The inrush current magnetization curve, unlike the short-circuit current curve, has a unipolar (non-sinusoidal) shape with current-free pauses. These features of the inrush current magnetization curve are used to detect the inrush current. However, it should be taken into account that at the moment of switching on (as already noted above), a bipolar inrush current may be observed in one of the phases, which the protection may mistake for a short circuit. To eliminate false triggering of the protection, the “curve shape” method is carried out simultaneously in at least two phases [4, pp. 200-202].
[0064] The second method of "harmonic analysis" of current evaluates the content of the second harmonic of the secondary current or the content of the second harmonic of the differential current. A well-known property of magnetizing current is used - the presence of the second harmonic in it. During a current inrush, the content of the second harmonic will be significant. The method of "harmonic analysis" for the content of the second harmonic in the differential current is used by almost all domestic and foreign manufacturers of protective devices, namely Siemens, ABB, General Electric, Alstom, Schneider Electric, RADIUS Avtomatika [2, pp. 78, 89, 194, 210] and TekonGroup, since this feature clearly distinguishes the magnetizing current during a current inrush from the short-circuit current, which practically does not contain higher harmonics (does not contain until the saturation of the measuring CTs). The generally accepted threshold value for the content of the second harmonic in the secondary current or the second harmonic in the differential current is 15%.If the calculated value exceeds the specified threshold, the condition is recognized as an inrush current, and protection operation is blocked for a period of time, which can range from 60 to 500 ms. A short blocking time does not ensure acceptable reliability for protection operation when the transformer is turned on without load, and a long blocking period may result in protection failure when a short circuit occurs due to an inrush current. Therefore, when blocking is enabled, monitoring of the detected inrush current continues using the same method used for inrush current detection, or another method. It is known that if inrush current monitoring is continued using the second harmonic content method of the secondary current [5], the second harmonic content of the decaying inrush current will increase. However, if the second harmonic content of the differential current begins to decrease, this will indicate the occurrence of a short circuit.
[0065] It should be noted that each of the above-described methods of detecting inrush currents has its own drawback, which prevents reliable detection. Therefore, to overcome the drawbacks of each of the above methods, manufacturers of protective devices employ two well-known methods in parallel [5]. For example, they simultaneously use the "harmonic analysis" method of current and the "waveform analysis" method. It is possible to temporarily use both known methods, and after the inrush current is detected, only one of the methods, for example, the "waveform analysis" method, is used to continue monitoring the already detected inrush current [2, p. 194].
[0066] A device for differential protection of a transformer converter unit (RU 2785823 C1) is known. It consists of three units, designed for each phase, to monitor for the presence of an aperiodic component in the magnetizing inrush current. These units can be implemented using microcontrollers. These same units are designed to detect short circuits, which, like inrush currents, contain an aperiodic component. The operation of the units is not described, and it is unclear how the unit distinguishes between inrush currents and short circuits, both of which are characterized by an aperiodic component at the moment of occurrence.
[0067] Description of analogues and prototype.
[0068] The "Device for detecting the mode of switching on a transformer or a short circuit in the primary circuit by the second harmonic of the current" (SU No. 534823 A1) is known. In the no-load switching mode (Fig. 4), when the contacts of switch 3 are closed and the contacts of switch 4 are open, the secondary currents i are analyzed in the device 7 on the electromechanical element base x The phases (x) of the primary winding are examined for the presence of the second harmonic. The presence of the second harmonic is characteristic of a current surge, while the second harmonic is practically absent in short-circuit current. It is also indicated that the claimed device 7 can be used in differential protection device 8.
[0069] The same analysis of currents of all phases of the primary winding is carried out in the well-known "Digital Protection Relay" (EP 3678270 A1). In the well-known digital relay, an analysis of secondary currents i is carried out to detect inrush currents. x phases (x) for the content of the second harmonic.
[0070] This solution is used for transformer turn-on mode.
[0071] As a prototype [5], a known method for detecting a surge in the magnetization current of a three-phase power transformer is adopted, which consists in the fact that in each phase with alternating current on the side of the electrical energy source, a measuring CT is located, the secondary current of which is fed to a digital protection device, in which the following actions are performed:
[0072] (a) Calculate the fundamental harmonic of the secondary current for each phase;
[0073] (b) Calculate the second harmonic of the secondary current for each phase;
[0074] (c) Calculate the value of the second harmonic content of the secondary current for each phase as the ratio of the second harmonic of the secondary current to the fundamental harmonic of the secondary current;
[0075] (d) Detects a magnetizing current inrush if the calculated value of the second harmonic content of the secondary current exceeds a set threshold value for at least one phase.
[0076] These analogs and prototypes are unreliable methods for detecting inrush currents, as modern transformers are made of improved magnetic materials, and their magnetization currents during inrush currents may contain second harmonics of less than 15%. Furthermore, reducing the generally accepted threshold value of 15% (the threshold value specified in EP 3678270 A1) may lead to a delay in protection response during severe short circuits, in which a second harmonic of less than 15% also appears in the current after the measuring CTs have saturated.
[0077] Our search did not reveal a single source of information that used the characteristics of a current surge, namely the aperiodic component and the second harmonic in the secondary phase currents, together to detect a current surge.
[0078] The essence of the invention.
[0079] The task is to improve the efficiency of the digital transformer protection device by increasing the stability of the protection failure during a current surge.
[0080] The technical result is achieved by proposing a reliable method for detecting inrush currents, which is used at the moment a potential inrush current occurs. Inrush current detection occurs within 40-60 milliseconds. During this time, the measuring CTs do not saturate, and an undistorted signal from the measuring CTs is sent to the computing processor. Faster saturation of the measuring CTs during a short circuit will not lead to false inrush current detection.
[0081] A method for detecting a surge current of a three-phase power transformer is proposed, which consists in the fact that in each phase with alternating current on the side of the electric power source there is a measuring current transformer, the secondary current of which is fed to a digital protection device, in which the following operations are carried out:
[0082] (a) Calculate the fundamental harmonic of the secondary current for each phase;
[0083] (b) Calculate the second harmonic of the secondary current for each phase;
[0084] (c) Calculate the value of the second harmonic content of the secondary current for each phase as the ratio of the second harmonic of the secondary current to the fundamental harmonic of the secondary current;
[0085] (d) selection of phases for which the calculated value of the second harmonic content of the secondary current is equal to or greater than the first set threshold value;
[0086] (e) calculation of the aperiodic component of the secondary current for each phase;
[0087] (f) calculating the value of the content of the aperiodic component of the secondary current for each phase as the ratio of the aperiodic component of the secondary current to the fundamental harmonic of the secondary current;
[0088] (g) selection of phases for which the calculated value of the secondary current aperiodic component content is equal to or greater than the second specified threshold value;
[0089] (h) detecting a magnetizing current inrush in the case where at least two selected phases for which the calculated value of the second harmonic content of the secondary current is equal to or exceeds the first preset threshold value coincide with at least two selected phases for which the calculated value of the aperiodic component content of the secondary current is equal to or exceeds the second preset threshold value.
[0090] The first threshold value can be set to 0.1 pu (10%), and the second threshold value can be set to 0.2 pu (20%).
[0091] List of figures.
[0092] Fig. 1 - a schematic diagram in which a protection device is applied to a transformer in normal operation mode (for the section “Background of the art - general information on differential protection of a transformer”).
[0093] Fig. 2 - a schematic diagram in which a protection device is used for a transformer during a short circuit (for the section “Prior art - general information on differential protection of a transformer”).
[0094] Fig. 3 - a schematic diagram in which the protection device is applied to a transformer in the no-load switching mode (for the section “Background of the art - general information on differential protection of a transformer”).
[0095] Fig. 4 - Schematic diagram using a surge or short circuit detection device (for the "Description of Analogues and Prototype" section).
[0096] Fig. 5 - a schematic diagram in which the protection device is applied to a transformer (for the section “Essence of the invention”).
[0097] Fig. 6 is a block diagram illustrating a first sequence of operations for detecting an inrush current.
[0098] Fig. 7 is a block diagram illustrating a second sequence of operations for detecting an inrush current.
[0099] Fig. 5 shows a three-phase transformer 2 Y / Δ-11 and phase x, where x is phases A, B and C. The proposed technical solution is not limited to the Y / Δ-11 transformer and can also be used in relation to transformers of other circuits and winding connection groups, including those with three windings. Further, the method is described using the example of a Y / Δ-11 transformer. Transformer 2 has a primary winding on the side of the electric power source, connected in a "star", and a secondary winding connected in a "delta". The electric power source is connected to the primary winding of the transformer. Between them, phase-by-phase measuring CTs 5 and switch 3 are installed. The secondary winding of the transformer is connected to the load (not shown). Between them, phase-by-phase measuring CTs 6 and switch 4 are installed. Alternating primary currents I flow through the primary windings of measuring CTs 5 and 6 Yx and I Δх , which are transformed by measuring CTs into alternating secondary currents i Yx and i Δх, which are fed to the input of the digital differential protection device 9 (hereinafter referred to as protection device 9).
[0100] Protection device 9 is designed to protect transformer 2 and includes:
[0101] - analog input block (not shown) with auxiliary current transformers designed to convert alternating secondary currents into analog signals, the level of which is suitable for processing in the ADC block and in the computing processor 10;
[0102] - an analog-to-digital conversion (ADC) block (not shown) configured to measure converted analog signals i Yx (t) and i Δх (t) at equally spaced discrete moments of time to convert them into instantaneous values of currents i Yx (nΔT) and i Δх (nΔT)
[0103] - computing processor (CPU) 10, designed for mathematical and logical operations described below;
[0104] - digital output block 11, designed to output a signal to open the contacts of the corresponding switch in accordance with the instruction received from CPU 10.
[0105] CPU 10, at least some of its functions, may be implemented as a microcontroller or an electronic circuit such as an application-specific integrated circuit (ASIC) or a programmable
[0106] FPGA, or can be realized by combining two or more of the specified devices.
[0107] CPU 10 is configured for computing and logical operations that are required for:
[0108] - Inrush current detection (software module 12 responsible for inrush current detection).
[0109] - Short circuit detection (software module 13 responsible for short circuit detection). Software method for detecting short circuits using
[0110] operation characteristics (brake characteristics) I диф = ƒ(I торм) is widely known and used by all manufacturers of digital security devices, so it is not described in this application.
[0111] - Issue of protection activation signal (operator "I" 14).
[0112] When a short circuit is detected, module 13 outputs a trip signal to the AND operator 14. The AND operator 14 calculates the logical AND between the protection trip signal output by module 13 and the blocking signal 15 output by module 12. If the blocking signal is present, the output of the AND operator 14 is inactive, and the protection trip output is blocked.
[0113] When the transformer 2 is turned on without a load (Fig. 5) (the contacts of the switch 3 are closed, the contacts of the switch 4 are open), alternating primary currents I flow through the windings of the measuring CTs 5 Yx . The currents in the phases (x) of the secondary winding are equal to zero (I Δx = 0).
[0114] It should be noted that the transformer can be connected to the power source from the side of its winding connected in a delta connection (switch 4 contacts are closed, and switch 3 contacts are open). The currents in the phases of the winding connected in a star connection, in this case, will be zero (I Yx =0). The winding connected in a "triangle" in this case becomes the primary winding.
[0115] Detection of current surge is carried out over an interval T0 of the order of two to three periods of industrial frequency ƒ0(50±5 Hz), i.e. for 256-384 measurements.
[0116] In order to detect the current surge, it is necessary to analyze the harmonic composition of the secondary current i x for each of the three phases, namely, to analyze the content of the second harmonic in the secondary current and the content of the aperiodic component in the secondary current for each phase.
[0117] The sequence of actions of the software module 12 for detecting a current surge is shown in Fig. 6 where:
[0118] i x (nΔT) - instantaneous value of secondary current for phase (x) from the side of the electrical energy source;
[0119] 16 - Digital Fourier Filter of Fundamental Harmonic A 1.х secondary current for phase (x);
[0120] 17 - Second Harmonic Fourier Digital Filter A 2.x secondary current for phase (x);
[0121] 18 - the ratio of the second harmonic of the secondary current to the fundamental harmonic of the secondary current (the content of the second harmonic A 2.отн.х secondary current) for phase (x);
[0122] 19 - moving average filter of aperiodic component I a.х secondary current for phase (x);
[0123] 20 - the ratio of the aperiodic component of the secondary current to the fundamental harmonic of the secondary current (the content of the aperiodic component I a.отн.х secondary current) for phase (x);
[0124] 21 - the logic module receives the calculated values of the second harmonic content for phases A, B and C 2.отн.хsecondary current. These calculated values are compared with the first setting of 0.1 p.u., and those phases for which condition A is true are selected. 2.отн.х ≥0.1 o.u.;
[0125] 22 - the calculated values for phases A, B and C of the content of the aperiodic component I are sent to the logic module a.отн.х secondary current. These calculated values are compared with the second setting of 0.2 p.u., and those phases for which condition I is true are selected. a.отн.х ≥0.2 o.u.
[0126] 23 - those phases (x) are selected for which two conditions are true: A 2.отн.х ≥0.1 o.u. and I a.отн.х ≥0.2 o.u.
[0127] 24 - checking the number of phases selected in the logic module 23 (x) for equality or exceeding the number "2" (x ≥ 2). This is due to the fact that during a current surge in the secondary current i x one of the three phases may have too little or no aperiodic component. And during a current surge in the secondary current i xone of the three phases, the content of the second harmonic may be below 10% (see the section of the description "Prior art - harmonic content of magnetizing currents during a current inrush before saturation of the measuring CTs"). To detect a current inrush (position 25), it is necessary that for at least two phases it be established that the secondary current i x each phase contains sufficient second harmonic and aperiodic component.
[0128] It is clear to a specialist that the order of actions of the software module 12 for detecting a current surge may be different from that shown in Fig. 6. Thus, Fig. 7 shows a sequence in which the phases (x) with an increased content of the second harmonic A are first selected 2.отн.х secondary current (position 21). There should be two or three such phases during the current surge. Then, the selected phases are checked for an increased content of the periodic component I a.отн.хsecondary current (position 26). If, for at least two phases, it is established that the secondary current i x each phase contains enough second harmonic and aperiodic component (position 24), then a current surge is detected (position 25).
[0129] Amplitude of the fundamental harmonic of the secondary current A 1.х , Ampere, for phase (x) is equal to:
[0130] Where
[0131] Re 1.х - the real component of the complex value of the fundamental harmonic of the signal;
[0132] Im 1.x - imaginary component of the complex value of the fundamental harmonic of the signal.
[0133] Where
[0134] i x (n⋅ΔT) - instantaneous value of secondary current for phase (x);
[0135] ΔT - signal sampling step, s;
[0136] n - number of the current sample of the secondary current signal;
[0137] N- number of secondary current signal samples;
[0138] ω0 - angular frequency of the fundamental harmonic, rad / s;
[0139] T0- interval, s;
[0140] ƒ0 - fundamental harmonic frequency, Hz.
[0141] Amplitude of the second harmonic of the secondary current A 2.x , Ampere, for phase (x) is equal to:
[0142] Where
[0143] Re 2.x - the real component of the complex value of the second harmonic of the secondary current signal;
[0144] Im 2.х - the imaginary component of the complex value of the second harmonic of the secondary current signal.
[0145]
[0146] Calculating the value of A 2.отн.х the content of the second harmonic of the secondary current as the ratio of the second harmonic of the secondary current to the fundamental harmonic of the secondary current for phase (x) is equal to:
[0147]
[0148] Sets the first threshold value (first setting) for the content of the second harmonic of the secondary current to 0.1 p.u.
[0149] If A 2.отн.х≥0.1 p.u., then a sufficient second harmonic content in the secondary current is detected for phase (x), and this phase is selected for further inrush detection operations. There should be two or three such phases during an inrush.
[0150] If A 2.отн.х < 0.1 p.u., then the absence of the second harmonic is recorded for phase (x).
[0151] Aperiodic component of secondary current I ax , in Amperes (A), for phase (x) is equal to:
[0152] Where
[0153] Calculate the value of the aperiodic component content of the secondary current I a.отн.х ratio of the aperiodic component of the secondary current I a.отн.х to the effective value of the fundamental harmonic of the secondary current, which is √2 times smaller than the amplitude of the fundamental harmonic A 1.х secondary current:
[0154]
[0155] Sets the second threshold value (second setting) for the aperiodic component of the secondary current I a.отн.х equal to 0.2 p.u.
[0156] If I a.отн.х ≥0.2 p.u., then a sufficient content of the secondary current's aperiodic component is recorded for phase (x), and this phase (x) is selected for further inrush current detection operations. There should be two or three such phases.
[0157] If I a.отн.х <0.2 p.u., then for phase (x) a low content or absence of an aperiodic component is recorded.
[0158] Inrush current detection occurs when at least two selected phases for which the calculated secondary current second harmonic content is equal to or greater than the first set threshold match two selected phases for which the calculated secondary current aperiodic component content is equal to or greater than the second set threshold. If an inrush current is detected, this is one of the protection blocking indicators for at least one phase.
[0159] It should be noted that the claimed inrush current detection method can be temporarily combined with an additional inrush current detection method, which is also implemented in inrush current detection module 12 (not shown in the figures). The additional method, similar to the claimed method, can also employ the "harmonic analysis" method of current. However, in this case, the additional method evaluates the second harmonic content in the differential current. The additional method has its own threshold (possibly the generally accepted 0.15 p.u.); exceeding this threshold leads to inrush current detection in the additional method, which is also one of the indicators for blocking the protection. Only combined inrush current detection in both the claimed and additional methods activates blocking signal 15.After this, monitoring of the detected inrush current over time continues, but only one of the methods is used, while the other method becomes inactive. The detected inrush current is monitored until it subsides or until a short circuit occurs due to the detected inrush current. In this case, blocking signal 15 is removed. However, these measures are the subject of a different technical solution.
[0160] Bibliography
[0161] 1. Fedoseyev AM, Relay protection of electric power systems: Textbook for universities. - 2nd ed., revised and enlarged. - M .: Energoatomizdat, 1990, pp. 105-106, 295, 442.
[0162] 2. Aleksandrov AM, Differential protection of transformers: a tutorial. - St. Petersburg: PEIPK, 2011., pp. 106-109, table 7-1 and table 7-2, pp. 78, 89, 194, 210.
[0163] 3. Golantzov E.B., Molchanov V.V., Differential protection of transformers with relays of the DZT-21 (DZT-23) type. - M.: Energoatomizdat, 1990, pp. 19-20.
[0164] 4. Electronic devices for relay protection and automation in traction power supply systems, Bykov V.A. and others, M., "Transport", 1974, Section 8 "Protection of traction substation transformers", §39. Adjustment of differential protection of transformers from magnetization currents when switched on in no-load mode, pp. 195-199, 200-203.
[0165] 5. A comprehensive model for studying the functioning of digital differential protection of a power transformer, Power Engineering. Publ. of Higher. Educational Institutions and Energy Associations of the CIS. Vol. 59, No. 3 (2016), pp. 218-222.
Claims
1. A method for detecting a surge in the magnetization current of a three-phase power transformer, which consists in the fact that in each phase with alternating current on the side of the electrical energy source there is a measuring current transformer, the secondary current of which is fed to a digital protection device, in which the following operations are carried out: (a) calculation of the fundamental harmonic of the secondary current for each phase; (b) calculation of the second harmonic of the secondary current for each phase; (c) calculating the value of the second harmonic content of the secondary current for each phase as the ratio of the second harmonic of the secondary current to the fundamental harmonic of the secondary current, characterized in that additional operations are carried out, namely: (d) selection of phases for which the calculated value of the second harmonic content of the secondary current is equal to or greater than the first specified threshold value; (e) calculation of the aperiodic component of the secondary current for each phase; (f) calculation of the value of the content of the aperiodic component of the secondary current for each phase as the ratio of the aperiodic component of the secondary current to the fundamental harmonic of the secondary current; (g) selection of phases for which the calculated value of the content of the aperiodic component of the secondary current is equal to or exceeds the second specified threshold value; (h) detecting a magnetising current inrush in the event that at least two selected phases for which the calculated value of the second harmonic content of the secondary current is equal to or exceeds a first predetermined threshold value coincide with at least two selected phases for which the calculated value of the aperiodic component content of the secondary current is equal to or exceeds a second predetermined threshold value.
2. The method according to paragraph 1, characterized in that the first threshold value is set to 0.1 relative units.
3. The method according to paragraph 1, characterized in that the second threshold value is set to 0.2 relative units.