Method for determining whether differential current is abnormal and differential current protection device

Abstract

The invention relates to a method for determining whether a differential current is abnormal, in which a differential current protection device performs differential current protection on the basis of the differential current, the differential current protection device having at least two current detection units, the difference of the detected currents of the at least two current detection units forming the differential current; the method comprises the following steps: comparing the differential current with a preset differential current threshold value; under the condition that the differential current is larger than the differential current threshold value, the maximum detection current in the detection current is determined, the variable quantity of the maximum detection current is determined, and the variable quantity of the differential current is determined; if the ratio of the variable quantity of the maximum detection current to the variable quantity of the differential current is smaller than a proportion threshold value, it is determined that the differential current is abnormal. In addition, the invention also relates to a differential current protection device.

Description

Technical Field

[0001] The present disclosure relates to a method for determining whether a differential current is abnormal and a differential current protection device. Background Art

[0002] Differential current protection is a key protection method in electrical systems. It detects a current imbalance between the two ends of an electrical device to determine if it is faulty. When a fault occurs within the device, the current imbalance between the two ends creates a differential current. When the differential current exceeds a predetermined threshold, the differential current protection device triggers protection, tripping the corresponding circuit breaker.

[0003] Current detection across electrical equipment is typically achieved through current transformers. Current transformers can fail or become saturated. The resulting differential current can cause the differential current protection device to malfunction, leading to inadvertent disconnection of the electrical equipment. Summary of the Invention

[0004] The present disclosure provides a method for determining whether a differential current is abnormal. The method according to the present disclosure can improve the reliability of differential current protection.

[0005] The present disclosure proposes a method for determining whether a differential current is abnormal, wherein a differential current protection device performs differential current protection based on the differential current, and the differential current protection device has at least two current detection units, and the difference in detection currents of the at least two current detection units forms the differential current; wherein the method includes: comparing the differential current with a predetermined differential current threshold; when the differential current is greater than the differential current threshold, determining the maximum detection current among the detection currents, determining the change in the maximum detection current, and determining the change in the differential current; and if the ratio of the change in the maximum detection current to the change in the differential current is less than a proportional threshold, determining that the differential current is abnormal, wherein the proportional threshold is less than or equal to one-half.

[0006] In an embodiment of the present disclosure, the proportional threshold is equal to the inverse of the number of the current detection units.

[0007] In an embodiment according to the present disclosure, a maximum detection current change threshold is pre-given, and the ratio of the change in the maximum detection current to the change in the differential current is less than a proportional threshold, including: the ratio of the change in the maximum detection current to the change in the differential current is less than the proportional threshold and the change in the maximum detection current is less than the maximum detection current change threshold.

[0008] In an embodiment of the present disclosure, the maximum detection current change threshold is set to half of the differential current threshold.

[0009] In an embodiment according to the present disclosure, when it is determined that the differential current is abnormal, the differential current protection device is locked.

[0010] In an embodiment of the present disclosure, when it is determined that the differential current is abnormal, the differential current threshold is increased to prevent the differential current protection device from malfunctioning.

[0011] In an embodiment according to the present disclosure, when the differential current decreases to less than the differential current threshold, the differential current protection device restores the differential current protection function.

[0012] In an embodiment according to the present disclosure, the two current detection units are configured as current transformers.

[0013] The present disclosure also proposes a differential current protection device, which includes: a first current detection unit, a second current detection unit and a control unit; wherein the first and second current detection units are respectively arranged at the first end and the second end of the electrical equipment to be protected; the first and second current detection units respectively detect the currents at the first end and the second end and feed the first and second detection currents to the control unit; a differential current is formed according to the first and second detection currents; the control unit executes the method according to the above embodiment according to the first and second detection currents and the differential current; when the differential current is not abnormal, the control unit controls the circuit breaker according to the differential current to disconnect the power supply of the electrical equipment.

[0014] In an embodiment of the present disclosure, the differential current protection device further includes: a third current detection unit, which is arranged at the third end of the electrical equipment; the third current detection unit detects the current at the third end and feeds the third detection current to the control unit; a differential current is formed according to the first, second and third detection currents; and the control unit executes the method according to the above embodiment based on the first, second and third detection currents and the differential current.

[0015] In an embodiment according to the present disclosure, the electrical equipment includes a transformer, a power line, a generator, a motor, and the like.

[0016] For differential current anomalies caused by current transformer saturation, existing processing methods are mainly based on the assumption that current transformer saturation is caused by large current. Large current causes the current transformer to saturate quickly, that is, the saturation time is short, usually less than one current cycle. Other methods have been used to deal with the rapid saturation of the current transformer to ensure that the current differential protection does not malfunction. However, these processing methods are not applicable to small currents, such as excitation surge currents that cause the current transformer to saturate slowly, that is, the saturation time is long, usually greater than one current cycle. The method and differential current protection device according to the present disclosure can identify such abnormal differential current. The method and differential current protection device according to the present disclosure therefore improve the reliability of differential current protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] To more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some exemplary embodiments of the present disclosure, and those skilled in the art can derive other drawings based on these drawings without inventive effort.

[0018] Figure 1 shows a schematic circuit diagram of a three-phase differential current protection device according to an embodiment of the present disclosure,

[0019] Figure 2 shows a schematic circuit diagram of a differential current protection device according to an embodiment of the present disclosure,

[0020] Figure 3 shows a schematic circuit diagram of a differential current protection device according to another embodiment of the present disclosure,

[0021] Figure 4 shows a schematic flow chart of a method according to an embodiment of the present disclosure,

[0022] Figures 5 to 7 The graph shows the curves of the detection current and differential current of the current transformer changing with time. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the present disclosure more apparent, the exemplary embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present disclosure, rather than all the embodiments of the present disclosure, and it should be understood that the present disclosure is not limited to the exemplary embodiments described herein.

[0024] In this specification and the accompanying drawings, substantially identical or similar method steps and elements are represented by identical or similar reference numerals, and repeated descriptions of these method steps and elements will be omitted. At the same time, in the description of the present disclosure, the terms "first", "second", etc. are only used to distinguish the description and are not to be understood as indicating or implying relative importance or ranking. In the embodiments of the present disclosure, unless otherwise expressly stated, "connected" does not mean that "direct connection" or "direct contact" is required, but only electrical connectivity is required.

[0025] Figure 1 FIG1 shows a schematic circuit diagram of a three-phase differential current protection device 100 according to an embodiment of the present disclosure. As can be seen from FIG1 , the electrical device 120 is a three-phase electrical device. The three-phase differential current protection device 100 requires two current sensing units, such as current transformers, to be arranged in each phase. The first and second current sensing units 131 and 132 are arranged in the first phase line A, and the first and second current sensing units 131 and 132 are arranged at the first and second ends A1 and A2 of the electrical device 120, respectively. The control unit 110 is capable of determining a first differential current based on the difference between the corresponding first and second detection currents IA1 and IA2. The third and fourth current sensing units 133 and 134 are arranged in the second phase line B, and the third and fourth current sensing units 133 and 134 are arranged at the third and fourth ends B1 and B2 of the electrical device 220, respectively. The control unit 110 is capable of determining a second differential current based on the difference between the corresponding third and fourth detection currents IB1 and IB2. The fifth and sixth current detection units 135, 136 are arranged in the third phase line C and are respectively arranged at the fifth terminal C1 and the sixth terminal C2 of the electrical device 120. The control unit 210 can determine a third differential current according to the difference between the corresponding fifth and sixth detection currents IC1, IC2.

[0026] If the electrical device 120 itself fails, such as a short circuit or open circuit, a first, second, and / or third differential current is generated, or at least one of the first, second, and third differential currents is greater than a predetermined threshold. The control unit 110 can control the circuit breakers in each phase line to shut down based on the first, second, and / or third differential currents to disconnect the power supply to the electrical device 120. Furthermore, the control unit 110 can independently execute the method of the present disclosure (which will be described in detail below) based on the first and second detection currents IA1, IA2 and the first differential current, independently execute the method of the present disclosure (which will be described in detail below) based on the third and fourth detection currents IB1, IB2 and the second differential current, and independently execute the method of the present disclosure (which will be described in detail below) based on the fifth and sixth detection currents IC1, IC2 and the third differential current to determine whether the first, second, and third differential currents are abnormal. If the differential currents are abnormal, the control unit 110 locks, i.e., does not perform differential current protection, to prevent malfunction of the circuit breaker and power outage of the electrical device 120.

[0027] In an embodiment according to the present disclosure, the electrical device may be, for example, a transformer, an electrical line generator, or a motor.

[0028] Figure 2 2 shows a schematic circuit diagram of a differential current protection device 200 according to an embodiment of the present disclosure. Figure 2 The illustrated embodiment may be understood as Figure 1 The differential current protection device shown is implemented in one phase. The differential current protection device 200 includes a first current detection unit 231, a second current detection unit 232, and a control unit 210. In this embodiment, the first and second current detection units 231, 232 can be configured as current transformers, for example. The first and second current detection units 231, 232 are exemplarily arranged in the first phase line A, and the first and second current detection units 231, 232 are respectively arranged at the first end A1 and the second end A2 of the electrical device 220 to be protected. The first and second current detection units 231, 232 respectively detect the currents at the first end A1 and the second end A2 and feed the first and second detection currents IA1, IA2 to the control unit 210. The control unit 210 determines the magnitude of the differential current based on the first and second detection currents IA1, IA2, in particular the difference between the first and second detection currents IA1, IA2.

[0029] If electrical equipment 220 experiences a fault, such as a short circuit or open circuit, the first and second detection currents IA1 and IA2 may be unequal in magnitude, generating a differential current, or the differential current may be greater than a predetermined threshold. Control unit 210 controls circuit breakers CBA1 and CBA2 to shut down based on the differential current, thereby disconnecting power to electrical equipment 220. Furthermore, control unit 210 may also execute the method of the present disclosure (described in detail below) based on the first and second detection currents IA1 and IA2 and the differential current to determine whether the differential current is abnormal. If the differential current is abnormal, differential current protection device 200 is locked, meaning that differential current protection is not performed, to prevent malfunction of circuit breakers CBA1 and CBA2, which could cause power outages to electrical equipment 220.

[0030] Figure 3 FIG2 shows a schematic circuit diagram of a differential current protection device 300 according to another embodiment of the present disclosure. Figure 3 The differential current protection device 300 shown in FIG has three current detection units 331, 332, and 333, which are respectively used to detect the current at the three ports A1, A2, and A3 of the electrical device 320, where the three ports A1, A2, and A3 are located in the same phase. The three current detection units 331, 332, and 333 can be, for example, current transformers. In the event of a fault in the electrical device 320, a differential current is formed based on the first, second, and third detection currents IA1, IA2, and IA3. The control unit 310 can determine whether the electrical device 320 has a fault based on the differential current. In addition, the control unit 310 can also perform the method according to the present disclosure (which will be described in detail later) based on the first, second, and third detection currents IA1, IB1, and IC1 and the differential current to determine whether the differential current is abnormal. In the event of an abnormal differential current, the control unit 310 does not perform differential current protection to avoid malfunction of the circuit breaker and power outage of the transformer.

[0031] Figure 4 The schematic flow chart of the method 400 according to the embodiment of the present disclosure is shown. In step S401, the detection currents I1, I2, I3, etc. on each side detected by each current detection unit, especially the current transformer, and the differential current I formed by these detection currents are first provided. diff In this disclosure, the current I mentioned can be understood as the effective value of the current. In step S402, the differential current I diff and the predetermined differential current threshold I threshold In comparison, in the embodiment according to the present disclosure, the differential current threshold I thresholdFor example, it can be the threshold for starting differential protection. In the prior art, the differential current protection device can control the corresponding circuit breaker to disconnect to protect the electrical equipment when the differential current is greater than the threshold. diff Greater than the differential current threshold I threshold In the case of I diff >I threshold The method disclosed herein determines the differential current I according to the following steps: diff In step S403, the maximum detection current I among the detection currents I1, I2, I3... is determined. max , i.e. I max =Max[I1, I2, I3…], where the Max function represents the maximum value. Then, in step S404, the maximum detection current I is determined. max Change over time |ΔI max |, for example, ΔI max =I max (t)-I max (t-Tn), where Tn represents the period of the current. In step 405, the differential current I is determined diff Change in |ΔI diff |, for example, ΔI diff =I diff (t)-I diff (t-Tn), Tn represents the period of the current. In step S406, the maximum detection current change amount |ΔI max | and the change in differential current |ΔI diff Is the ratio of | less than the ratio threshold T p , that is, to judge |ΔI max | / |ΔI diff |<T p Is it true, where the ratio threshold T p Less than or equal to one half. At the ratio threshold T p When it is equal to one-half, it is judged that |ΔI max |<0.5*|ΔI diff In the embodiment of the present disclosure, the ratio threshold T p The size of is related to the number of ports of the electrical device or the number of current measuring units. For example, for an electrical device with two ports, the ratio threshold T p Can be equal to one-half; for electrical equipment with three ports, such as a three-winding transformer, the proportional threshold T p It can be equal to one third. In other words, for a differential current protection device with multiple current transformers, the effect of a change in a detection current on the change in differential current accounts for one-third of the number of current transformers. If the change in the maximum detection current |ΔImax | and the change in differential current |ΔI diff The ratio of | is less than the ratio threshold T p , then determine the differential current I diff Abnormal (step S407), and no differential protection is performed. On the contrary, it is considered that the differential current is normal and is caused by a fault in the electrical equipment, and the differential protection is started (step S408).

[0032] The reason for the abnormal differential current is that the current of one of the two or more detection currents drops. The reason for the drop in detection current may be a current transformer fault, such as current transformer saturation, current transformer disconnection or abnormal current loss. For the abnormal differential current caused by current transformer saturation, the existing processing method is mainly based on the following assumption, that is, the current transformer will not malfunction if it saturates quickly. For the rapid saturation of current transformers (saturation time is less than one current cycle), there are related methods to ensure differential protection. However, this processing method cannot be applied to the slow saturation of current transformers caused by excitation inrush current (saturation time is greater than one cycle). The excitation inrush current is smaller than the overcurrent caused by line faults, such as short-circuit current, and cannot instantly cause the current transformer to saturate. However, the large DC component contained in the excitation inrush current will cause the current transformer to saturate after several cycles.

[0033] In the present disclosure, when differential current protection is activated, the differential current protection device checks the changes in the sensed currents of all current transformers. If the change in the maximum sensed current is significantly smaller than the change in the differential current, this indicates that the differential current is caused by a current drop in one of the current transformers. This current transformer may be saturated or faulty. Such a differential current is abnormal, and the differential current protection must be locked out.

[0034] Figures 5 to 7 The graph shows the curves of the detection current and differential current of the current transformer changing with time, wherein the vertical axis is per unit value and the horizontal axis is time (unit: s (seconds)). Figure 5 The curve in Figure 1 shows the instantaneous value of the current changing with time. An overcurrent occurs at 0.1s, which is caused by, for example, a magnetizing inrush current. As time passes, the overcurrent decreases. Under the influence of the decaying DC component, one of the current transformers saturates at 0.3s. Figure 5 As can be seen in the diagram, before 0.3s, the currents PhA1 and PhA2 detected by the two current transformers are essentially the same. After 0.3s, the current detected by the unsaturated current transformer is larger (PhA1), while the current detected by the saturated current transformer is relatively smaller (PhA2). Therefore, before 0.3s, the differential current DiffA is very small, almost zero, and from 0.3s onwards, the differential current increases significantly.

[0035] Figure 6 FIG. 2 shows the change of the amplitude of the detection current PhA1 and PhA2 and the differential current DiffA with time. Figure 6 As can be seen in the figure, starting at 0.3s, the amplitude of the saturated current transformer's sensed current PhA2 suddenly decreases and changes little for a period of time (until 0.65s) due to saturation. In contrast, the amplitude of the unsaturated current transformer's sensed current PhA1 gradually decreases, which can reflect the gradual decrease in actual current. The differential current DiffA increases instantaneously at 0.3s and gradually decreases. By 0.65s, due to the current transformer no longer being saturated, the differential current DiffA is almost zero.

[0036] Figure 7 The change amount of the detection current PhA1 and PhA2 and the differential current DiffA, especially the change amount of the effective value, changes with time. Figure 7 As can be seen, after one of the current transformers saturates (at 0.3s), the differential current DiffA changes significantly, while the sense current of the unsaturated current transformer, i.e., the maximum sense current PhA1, changes much less. Therefore, the abnormal differential current can be determined based on the changes in the differential current DiffA and the maximum sense current PhA1. This differential current is caused by the slow saturation of the current transformer due to the magnetizing inrush current, not by a fault in the electrical equipment. Therefore, in this case, it is important to avoid accidental tripping of the circuit breaker, which could cause a power outage in the electrical equipment.

[0037] In the embodiment of the present disclosure, for example, a maximum detection current change threshold value I may be pre-set. set , and in step S406, the maximum detection current change amount |ΔI max | and the change in differential current |ΔI diff Is the ratio of | less than the ratio threshold T p And the maximum detection current change |ΔI max Is it less than the maximum detection current change threshold I set In the embodiment of the present disclosure, the maximum detection current change threshold value I set For example, it is set as the differential current threshold I threshold One half of I set =0.5*I threshold Therefore, the judgment condition in step S406 may be:

[0038] |ΔI max |<MIN[0.5*I threshold , T p |ΔI diff|]

[0039] Among them, the MIN function means taking the minimum value. Combined with the ratio threshold T p When it is equal to one-half, that is, to judge |ΔI max |<0.5*|ΔI diff Whether it is established, the judgment condition in step S406 can be:

[0040] |ΔI max |<MIN[0.5*I threshold , 0.5*|ΔI diff |]

[0041] Maximum detection current change threshold I set It can also be set based on experience. By setting the maximum detection current change threshold I set Whether the differential current is abnormal can be determined more quickly.

[0042] In an embodiment according to the present disclosure, when it is determined that the differential current is abnormal, the differential current protection device is locked.

[0043] In an embodiment according to the present disclosure, the differential current I diff Reduced to less than the differential current threshold I threshold In this case, the differential current protection device restores the differential current protection function.

[0044] In an embodiment according to the present disclosure, when determining the differential current I diff In the event of an abnormality, for example, the differential current threshold I threshold For example, the differential current threshold I threshold Transformed into a larger second differential current threshold I threshold2 By increasing the differential current threshold, the malfunction of the differential current protection device can be prevented.

[0045] The block diagrams of the circuits, units, devices, apparatuses, equipment, and systems involved in this disclosure are intended only as illustrative examples and are not intended to require or imply that they must be connected, arranged, or configured in the manner shown in the block diagrams. As will be appreciated by those skilled in the art, these circuits, units, devices, apparatuses, equipment, and systems may be connected, arranged, or configured in any manner as long as the desired purpose can be achieved. The circuits, units, devices, and apparatuses involved in this disclosure may be implemented in any suitable manner, such as using an application-specific integrated circuit, a field programmable gate array (FPGA), or the like, or may be implemented using a general-purpose processor in combination with a program.

[0046] Those skilled in the art should understand that the above-mentioned specific embodiments are merely examples and not limitations, and that various modifications, combinations, partial combinations and replacements may be made to the embodiments of the present disclosure according to design requirements and other factors. As long as they are within the scope of the attached claims or their equivalents, they fall within the scope of rights to be protected by the present disclosure.

Claims

1. A method for determining whether a differential current is abnormal, wherein: A differential current protection device performs differential current protection according to the differential current, the differential current protection device having at least two current detection units, and a difference between detection currents of the at least two current detection units forming the differential current; wherein the method includes: comparing the differential current with a predetermined differential current threshold; When the differential current is greater than the differential current threshold, determining a maximum detection current among the detection currents, Determine the change in the maximum detection current, determining an amount of change in the differential current; and If the ratio of the change in the maximum detection current to the change in the differential current is less than a proportional threshold, it is determined that the differential current is abnormal, wherein the proportional threshold is less than or equal to one half.

2. The method according to claim 1, wherein The proportional threshold is equal to the inverse of the number of the current detection units.

3. The method according to claim 1, wherein The maximum detection current change threshold is given in advance, and The ratio of the change in the maximum detection current to the change in the differential current is less than a ratio threshold, including: A ratio of a change in the maximum detection current to a change in the differential current is smaller than a ratio threshold, and the change in the maximum detection current is smaller than a maximum detection current change threshold.

4. The method according to claim 1, wherein The maximum detection current change threshold is set to half of the differential current threshold.

5. The method according to claim 1, wherein When it is determined that the differential current is abnormal, the differential current protection device is locked.

6. The method according to claim 1, wherein When it is determined that the differential current is abnormal, the differential current threshold is increased to prevent the differential current protection device from malfunctioning.

7. The method according to claim 5 or 6, wherein: When the differential current decreases to be less than the differential current threshold, the differential current protection device restores the differential current protection function.

8. The method according to claim 1, wherein The two current detection units are designed as current transformers.

9. A differential current protection device comprising: a first current detection unit, a second current detection unit, and a control unit; Wherein, the first and second current detection units are respectively arranged at a first end and a second end of the electrical equipment to be protected; The first and second current detection units detect currents at the first and second terminals, respectively, and feed the first and second detection currents to the control unit; forming a differential current based on the first and second detection currents; The control unit performs the method according to any one of claims 1 to 8 according to the first and second detection currents and the differential current; When the differential current is not abnormal, the control unit controls the circuit breaker according to the differential current to disconnect the power supply to the electrical device.

10. The differential current protection device according to claim 9, further comprising: a third current detection unit, the third current detection unit being arranged at a third end of the electrical device; The third current detection unit detects the current of the third terminal and feeds the third detection current to the control unit; forming a differential current based on the first, second and third detection currents; The control unit performs the method according to any one of claims 1 to 8 based on the first, second, and third detection currents and the differential current.

11. The differential current protection device according to claim 9, wherein: The electrical equipment includes transformers, power lines, generators and motors.

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