A device for detecting and locating the intrusion of alternating current into a DC system of a power plant

Abstract

The present invention discloses a device for detecting the intrusion of alternating current into a DC system in a power plant, which includes a resonant filtering module and a tuned clamp ammeter. The resonant filtering module includes a power frequency filtering branch, a variable frequency filtering branch, a capacitance discrimination capacitor, a switch and an NTC thermistor. The tuned clamp ammeter includes a clamp ammeter and a tuning circuit provided on the clamp current transformer. The tuning circuit includes a third winding wound around the clamp current transformer and an LC resonant circuit connected in series with the third winding. The tuned clamp ammeter sequentially detects each possible branch in the DC system, and finds the faulty branch by judging whether there is an alternating current component. The present invention can find the DC branch where the alternating current intrudes in a short time, and can accurately judge whether the incoming alternating current is directly connected resistively, or intrudes through electromagnetic coupling or capacitance coupling. It also has the ability to quickly find the problem of alternating current intrusion into the DC system caused by frequency converters. In addition, it can also eliminate the hidden danger of alternating current intrusion into DC.

Description

Technical Field

[0001] The invention relates to electric power instruments and meters, and in particular to an AC intrusion positioning and detection device for a DC system of a power plant. Background Art

[0002] AC entering the DC system is a common electromagnetic interference in the secondary system of a power plant. There are many reasons for this. The most common ones are miswiring by secondary staff, damage to the AC / DC isolation board, poor insulation of the protection device plug-in, unreasonable cable shielding grounding, water seepage in outdoor equipment, etc., which cause the AC power to enter the DC system in the form of direct or electromagnetic coupling or electric field coupling. Since the 110 / 220V DC in the power station is an ungrounded system, once the grounded AC component enters the DC system for some reason, a large AC voltage to the ground will be superimposed on the DC bus. For a 110V DC system, the normal voltage to the ground is 50-60V. Once AC enters, the voltage to the ground may rise to about 220V AC. The AC component entering the DC system will be applied to some intermediate relays through the cable distributed capacitance of each DC circuit and the busbar to ground capacitance, causing serious accidents of switch malfunction. In addition, since the AC component may be at the same frequency as the current and voltage of the transformer, it may also interfere with some protection devices and automation devices, causing device damage and switch malfunction. In addition, due to the interference of AC intrusion, DC grounding detectors often cannot give correct alarms, which brings inconvenience to fault detection.

[0003] The secondary DC system plays a pivotal role in the auxiliary equipment of power stations. In recent years, AC intrusion into the DC system of power stations has caused many serious accidents. AC intrusion into the DC system will cause the voltage of the entire system to fluctuate distortedly, and its peak value can reach several times the normal voltage of the DC system to the ground, and the duration is long. This overvoltage is not only easy to break down the primary and secondary of the secondary DC power supply board, but also causes great accident hazards to the secondary system of the plant through cable capacitance induction, parasitic coupling, etc. In recent years, false tripping accidents caused by AC inrush into the DC system have occurred one after another. However, the AC inrush detection device in the prior art can only detect the AC component of the DC bus voltage to the ground, and does not have the function of locating the source of the AC inrush, nor does it have the properties of the AC source, whether it is resistive direct access, electromagnetic coupling inrush, or capacitive coupling inrush. In addition, with the popularization of high and low voltage inverters in power plants, AC inrush is likely to be caused by the low-frequency AC of the inverter. Due to the low frequency, the existing DC grounding detection device that relies on low-frequency signal injection is completely powerless. For the above problems, the existing technology has no solution capability. For the sudden AC inrush into the DC system, it is completely incapable of rapid qualitative positioning, resulting in the hidden dangers of AC inrush into the DC system of the power plant remaining unresolved. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a device for detecting AC intrusion in a DC system of a power plant.

[0005] The technical solution of the present invention is:

[0006] A device for detecting AC intrusion in a DC system of a power plant comprises a resonant filter module and a tuned clamp ammeter. The resonant filter module comprises a power frequency filter branch, a variable frequency filter branch, a capacitance-sensing capacitor, a switch and an NTC thermistor. The power frequency filter branch and the variable frequency filter branch are connected in parallel with each other, one end of the parallel connection is connected to a signal input end of the resonant filter module through the NTC thermistor, and the other end of the parallel connection is connected to another signal input end of the resonant filter module through the capacitance-sensing capacitor. The switch is connected in parallel with the capacitance-sensing capacitor. The two signal input ends of the resonant filter module are respectively connected to the positive or negative busbar of the DC system and the ground. The tuned clamp ammeter detects each possible branch on the DC system in turn, and finds the faulty branch by determining whether there is an AC current component.

[0007] Preferably, the industrial frequency filter branch includes an industrial frequency resonant capacitor, an industrial frequency resonant inductor and an industrial frequency ammeter, and the two ends of the industrial frequency resonant capacitor are respectively connected in series with the industrial frequency resonant inductor and the industrial frequency ammeter; the variable frequency filter branch includes a variable frequency resonant capacitor, a variable frequency resonant inductor, an autocoupler and a variable frequency ammeter, and the two ends of the variable frequency resonant capacitor are respectively connected in series with the variable frequency ammeter and the primary side of the autocoupler, and the variable frequency resonant inductor is connected in parallel with the secondary side of the autocoupler.

[0008] Preferably, the tuned clamp ammeter consists of a clamp ammeter and a tuning circuit arranged on a clamp transformer of the clamp ammeter, and the tuning circuit consists of a third winding wound on the clamp transformer and a resonant capacitor and a variable inductor connected in series with the third winding.

[0009] Preferably, the industrial frequency resonant capacitor and the variable frequency resonant capacitor are film capacitors with a capacity between 2 and 20 microfarads.

[0010] Preferably, the industrial frequency filter branch is used to filter out 50Hz industrial frequency AC signals; the variable frequency filter branch realizes continuous variable resonant frequency by changing the winding ratio of the secondary side inductance element of the autovoltage regulator, and is used to filter out variable frequency AC signals and detect the output signal of the inverter.

[0011] Preferably, the inductive-capacitive discrimination capacitor realizes the detection of AC intrusion properties. When the resonant filter branch is tuned to resonance, the switch is disconnected and the inductive-capacitive discrimination capacitor is connected in series. At this time, by observing the change in the resonant current of the ammeter, it can be distinguished whether the incoming AC is through the capacitor or the inductor.

[0012] The advantages of the present invention are:

[0013] 1. The present invention overcomes the problem that the existing technology cannot detect the source of AC intrusion into the DC system. The attached clamp ammeter can be used for non-destructive detection to find the DC branch where AC intrusion occurs in a short time, and then quickly find the fault location;

[0014] 2. The present invention can effectively detect the properties of AC intrusion, and can accurately determine whether the incoming AC is directly connected by resistance, electromagnetic coupling intrusion, or capacitive coupling intrusion, thereby providing more powerful information for finding the fault point, which can speed up the search for the fault point and clarify the cause of the fault, whether it is caused by poor insulation or electromagnetic or electric field coupling;

[0015] 3. The present invention can effectively detect the problem of AC output signal of the inverter entering the DC system. With the popularization of high and low voltage inverters in power plants, AC entering is likely to be caused by the low-frequency AC of the inverter. Due to the low frequency, the existing DC grounding detection device that relies on low-frequency signal injection is completely powerless. In addition, the low-frequency AC signal output by the inverter is a PWM modulated wave, and its rich harmonics will act as a carrier and enter the DC system through various coupling forms. The invention uses a continuously adjustable resonant filter circuit to quickly find the problem of AC entering the DC system caused by the inverter;

[0016] 4. The resonant filter module of the present invention can form a base impedance to the AC component that penetrates into the DC system, so that the invading AC component is filtered out and the invading AC component can no longer be transmitted to other circuits through the DC bus, thereby reducing the overvoltage and possible hidden dangers caused by AC penetrating into DC to a minimum, playing a role in eliminating the hidden dangers of AC penetrating into DC. It can be seen that the present invention can fundamentally solve the deficiencies of the prior art and effectively overcome a series of major hidden dangers caused by AC penetrating into the DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0018] Figure 1 is a circuit diagram of a resonant filter module of the present invention;

[0019] Figure 2 It is a circuit structure diagram of the tuning clamp ammeter of the present invention;

[0020] Figure 3 It is a schematic diagram of finding AC leakage into DC according to the present invention;

[0021] Figure 4 It is the equivalent circuit of AC intrusion into the DC system of a power plant;

[0022] Figure 5 It is an equivalent circuit of a resonant filter branch set up after AC enters the DC system of a power plant. DETAILED DESCRIPTION

[0023] The present invention discloses a device for locating and detecting AC inrush in a DC system of a power plant, comprising a resonant filter module 1 and a tuned clamp ammeter 12, wherein two signal input terminals of the resonant filter module 1 are respectively connected to the positive or negative busbar of the DC system and the ground; the tuned clamp ammeter 12 detects each possible branch on the DC system in turn, and finds the faulty branch by determining whether there is an AC current component. Figure 3 It is a schematic diagram of the present invention for finding AC leakage into DC.

[0024] like Figure 1 As shown, the resonant filter module 1 includes a power frequency resonant capacitor 2, a power frequency resonant inductor 3, a variable frequency resonant capacitor 4, a variable frequency resonant inductor 5, an auto-voltage regulator 6, a power frequency ammeter 7, a variable frequency ammeter 8, a capacitance-sensing capacitor 9, and a switch 10. The power frequency resonant capacitor 2 is connected in series with the power frequency resonant inductor 3 and the power frequency ammeter 7, and one end is connected to a signal input end of the resonant filter module 1, and the other end is connected to one end of the capacitance-sensing capacitor 9; the variable frequency resonant capacitor 4 is connected in series with the primary side of the auto-voltage regulator 6, and then connected in series with the variable frequency ammeter 8, and one end is connected to a signal input end of the resonant filter module 1, and the other end is connected to one end of the capacitance-sensing capacitor 9, and the other end of the capacitance-sensing capacitor 9 is connected to another signal input end of the resonant filter module 1, and the variable frequency resonant inductor 5 is connected in parallel with the secondary side of the auto-voltage regulator 6; the switch 10 is connected in parallel with the capacitance-sensing capacitor 9.

[0025] See attached Figure 2 The detection clamp meter of the present invention is a tuned clamp meter. Through an additional tuning circuit, the clamp meter can be made sensitive to a signal of a specific frequency. In this way, the source of AC intrusion can be conveniently detected in a DC system with variable frequency or industrial frequency interference. It is only necessary to use the clamp meter to clamp and measure the load current of each branch of the DC split screen. The branch where AC current is detected is the fault branch, thereby realizing the positioning detection of AC intrusion.

[0026] The equivalent circuit of AC inrush into the DC system of a power plant is as follows Figure 4 As shown, U1 is the DC power supply voltage of the DC system, points A and B represent the positive and negative busbars, R1 and C1 are the DC system's distributed capacitance to the ground and grounding resistance, and U2 is the incoming AC power supply. The AC power supply is added between the DC system's ground and the positive or negative through impedance R2 or capacitive reactance C2 (cable induction method), forming AC intrusion. Assuming U2=sinwt, according to the circuit principle, the DC system's positive and negative busbar voltages to the ground are:

[0027]

[0028] It can be seen that after AC intrusion, the magnitude of the overvoltage of the DC system to the ground is related to the impedance parameters of the system. The greater the insulation resistance of the system to the ground and the smaller the distributed capacitance, the greater the overvoltage amplitude; at the same time, the smaller the transition impedance of the AC intrusion point, the greater the overvoltage amplitude. To reduce the overvoltage amplitude, it is necessary to reduce the insulation of the DC system to the ground and increase the distributed capacitance to the ground. However, these parameters of the DC system cannot be increased at will. If a resonant filter branch is set on the DC bus, such as Figure 5 As shown, the filter branch is tuned to the power frequency resonance, that is, wL=1 / wC3. Ignoring the resistance of the filter branch, substituting into (1) and (2), we get:

[0029]

[0030] It can be seen from this that the incoming AC is completely absorbed by the resonant circuit, and the DC system voltage to ground is only related to the system resistance component. When the AC system has no transition impedance and metallic properties invade the DC system, the DC system only exhibits the voltage distribution to ground during general DC grounding, and the incoming AC overvoltage component is eliminated.

[0031] The present invention sets a resonant filter device 1, whose signal input terminal is connected between the positive and negative busbars of the DC system and the earth. Through the resonant filtering effect of the LC filter branch, on the one hand, the incoming AC component is eliminated, and on the other hand, the AC intrusion component flowing into the DC busbar is increased, while the AC intrusion component of the normal DC branch is reduced to near zero. In this way, through a clamp ammeter, each possible branch on the DC system is detected in turn, and by judging whether there is an AC current component, the fault branch can be found relatively quickly. The present invention is provided with two groups of different resonant filter branches, one group is a power frequency filter branch, which is used to filter out 50Hz power frequency AC signals, and the other group is a variable frequency filter branch. The winding ratio of the secondary side inductor element is changed by a voltage regulator to achieve continuous variable resonant frequency, which is used to filter out variable frequency AC signals, so that the output signal of the frequency converter can be effectively detected. The present invention detects the properties of AC intrusion by distinguishing capacitors by inductive capacitance. When the resonant filter branch is tuned to resonance, the switch 10 is disconnected and the capacitor is connected in series. At this time, by observing the change in the resonant current of the ammeter, it is possible to distinguish whether the incoming AC is through the capacitor or the inductor, thereby helping to clarify the properties of the AC component that is connected. The power frequency resonant capacitor 2 and the variable frequency resonant capacitor 4 are film capacitors with a capacity between 2-20 microfarads, which can further improve the frequency stability of the resonant circuit and will not add excessive capacitance to the ground to the DC system of the power plant. The NTC thermistor is used to detect the moment when the circuit is connected to the DC system and slow down the charging impact current of the capacitor.

[0032] The present invention overcomes the problem that the prior art cannot detect the source of AC intrusion into a DC system, and can utilize the accompanying clamp ammeter for non-destructive detection to find the DC branch where AC intrusion occurs in a short time, thereby quickly finding the fault location.

[0033] The present invention can effectively detect the properties of AC intrusion, and can accurately determine whether the incoming AC is directly connected by resistive connection, electromagnetic coupling intrusion, or capacitive coupling intrusion, thereby providing more powerful information for finding the fault point, which can speed up the finding of the fault point and clarify the cause of the fault, whether it is caused by poor insulation or electromagnetic or electric field coupling.

[0034] The present invention can effectively detect the problem of AC output signal of the inverter entering the DC system. With the popularization of high and low voltage inverters in power plants, AC intrusion is likely to be caused by the low-frequency AC of the inverter. Due to the low frequency, the existing DC grounding detection device that relies on low-frequency signal injection is completely powerless. In addition, the low-frequency AC signal output by the inverter is a PWM modulated wave, and its rich harmonics will act as a carrier and enter the DC system through various coupling forms. The invention uses a continuously adjustable resonant filter circuit to quickly find the problem of AC intrusion into the DC system caused by the inverter.

[0035] The resonant filter module of the present invention can form a base impedance to the AC component that penetrates into the DC system, so that the invading AC component is filtered out and the invading AC component can no longer be transmitted to other circuits through the DC bus, thereby reducing the overvoltage and possible hidden dangers caused by the AC penetrating into the DC to a minimum, and playing a role in eliminating the hidden dangers of AC penetrating into the DC. It can be seen that the present invention can fundamentally solve the deficiencies of the prior art and effectively overcome a series of major hidden dangers caused by the AC penetrating into the DC system.

[0036] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any modifications made according to the spirit of the main technical solution of the present invention should be included in the protection scope of the present invention.

Claims

1. A device for detecting AC intrusion in a power plant DC system. It is characterized in that The invention comprises a resonant filter module (1) and a tuning clamp ammeter (12), wherein the resonant filter module (1) comprises an industrial frequency filter branch, a variable frequency filter branch, a capacitance-sensing capacitor (9), a switch (10) and an NTC thermistor (17), wherein the industrial frequency filter branch and the variable frequency filter branch are connected in parallel with each other, one end of the parallel connection is connected to a signal input end of the resonant filter module (1) via the NTC thermistor (17), and the other end of the parallel connection is connected to another signal input end of the resonant filter module (1) via the capacitance-sensing capacitor (9), and the switch (10) is connected in parallel with the capacitance-sensing capacitor (9); The two signal input ends of the resonant filter module (1) are respectively connected to the positive or negative busbar of the DC system and the ground; the tuned clamp ammeter (12) detects each possible branch on the DC system in turn, and finds the fault branch by determining whether there is an AC current component; The power frequency filter branch comprises a power frequency resonant capacitor (2), a power frequency resonant inductor (3) and a power frequency ammeter (7), and the two ends of the power frequency resonant capacitor (2) are respectively connected in series with the power frequency resonant inductor (3) and the power frequency ammeter (7); the variable frequency filter branch comprises a variable frequency resonant capacitor (4), a variable frequency resonant inductor (5), an auto-coupling voltage regulator (6) and a variable frequency ammeter (8), and the two ends of the variable frequency resonant capacitor (4) are respectively connected in series with the variable frequency ammeter (8) and the primary side of the auto-coupling voltage regulator (6), and the variable frequency resonant inductor (5) is connected in parallel with the secondary side of the auto-coupling voltage regulator (6).

2. The AC intrusion detection device for the DC system of a power plant according to claim 1, It is characterized in that The tuning clamp ammeter (12) is composed of a clamp ammeter (16) and a tuning circuit provided on a clamp transformer of the clamp ammeter, wherein the tuning circuit is composed of a third winding (13) wound on the clamp transformer and a resonant capacitor (14) and a variable inductor (15) connected in series with the third winding (13).

3. The AC intrusion detection device for a power plant DC system according to claim 1, It is characterized in that The industrial frequency resonant capacitor (2) and the variable frequency resonant capacitor (4) are film capacitors with a capacity between 2 and 20 microfarads.

4. The AC intrusion detection device for a power plant DC system according to claim 1, It is characterized in that The industrial frequency filter branch is used to filter out 50 Hz industrial frequency AC signals; the variable frequency filter branch realizes continuous variable resonance frequency by changing the winding ratio of the secondary side inductor element of the autovoltage regulator (6), and is used to filter out variable frequency AC signals and detect the output signal of the frequency converter.

5. The AC intrusion detection device for the DC system of a power plant according to claim 4, It is characterized in that The inductive-capacitive discrimination capacitor (9) detects the properties of AC intrusion. When the resonant filter branch is tuned to resonance, the switch (10) is disconnected and the inductive-capacitive discrimination capacitor (9) is connected in series. At this time, by observing the change in the resonant current of the ammeter, it can be distinguished whether the incoming AC is entering through the capacitor or the inductor.

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