Fault phase current acquisition circuit for a feeder terminal device
By using a combination of dual-range current transformers and analog-to-digital converters in the feeder terminal unit, along with overload protection devices, the problem of current measurement for high-impedance grounding faults in 10kV systems was solved, enabling accurate current measurement and fault diagnosis under both short-circuit and non-fault conditions.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2021-05-10
- Publication Date
- 2026-04-07
AI Technical Summary
In the existing technology, the change in the fault phase current of a 10kV ungrounded neutral line system during a high-impedance grounding fault cannot be accurately measured, making it impossible to determine whether a high-impedance grounding fault has occurred on the line.
The system employs a combination of dual-range current transformers and analog-to-digital converters, including one with a larger range for waveform recording and another with a smaller range and higher accuracy for measuring sudden current changes. Combined with overload protection devices, this ensures accurate measurement of current signals under both short-circuit and non-fault conditions.
It achieves the requirement of recording waveforms during short-circuit faults, while accurately measuring the sudden change signal of 350mA current under non-fault conditions, and accurately judging high-resistance grounding faults.
Smart Images
Figure CN113267670B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of feeder terminal equipment technology, and in particular to a fault phase current acquisition circuit for a feeder terminal equipment. Background Technology
[0002] The State Grid standardized electronic distribution switch monitoring terminal unit (Feeder Terminal Unit, FTU) uses a 600A:1V current transformer (CT) on the primary side to acquire and convert the current to obtain a small voltage signal. The function of the current transformer is to convert a large primary current into a smaller secondary current through a certain transformation ratio for protection, measurement, and other purposes. The circuit is required to meet the waveform recording requirement of 20 times overload during short-circuit faults. Generally, the FTU internally uses a 20V:3.25V secondary side small CT to convert this load current into a small signal, which is then fed into a 16-bit ADC (Analog-to-Digital Converter) for waveform recording. When testing a rated 600A load current signal using this transformation ratio, the device can generate an analog signal of ±229.775mV at the ADC input.
[0003] In existing technologies, in a 10kV neutral-point ungrounded line system, due to the presence of distributed capacitance and resistance, when a 2kΩ high-impedance ground fault occurs on any line, the minimum current surge after the fault phase grounds under the equivalent model is approximately 350mA. If a traditional single-range current transformer (CT) is used, because the secondary side CT has a large range (i.e., a large transformation ratio) to meet the waveform recording requirements, the 350mA fault phase current change can only generate a ±0.134035mV analog surge at the ADC input. Since the ADC is 16-bit and its measurement range is ±10V, meaning the minimum ADC scale 1LBS corresponds to 20V / 65535 = 0.305180mV, and the ADC itself has noise issues, it is impossible to accurately measure the 350mA current surge signal. Because the transient current surge signal cannot be accurately measured, it is impossible to determine whether a high-impedance ground fault has occurred on the line. Summary of the Invention
[0004] To address the aforementioned problems in the existing technology, a fault phase current acquisition circuit for a feeder terminal device is provided.
[0005] The specific technical solution is as follows:
[0006] This invention includes a fault phase current acquisition circuit for a feeder terminal device, comprising:
[0007] A first current transformer, having a first preset transformation ratio, is disposed on the secondary side of the feeder terminal device. The first input terminal of the primary side of the first current transformer is connected to the first output terminal of the primary side of the feeder terminal device, and the second input terminal of the primary side of the first current transformer is connected to the second output terminal of the primary side of the feeder terminal device.
[0008] A second current transformer, having a second preset ratio, is disposed on the secondary side of the feeder terminal device and connected in parallel with the first current transformer. The first input terminal of the primary side of the second current transformer is connected to the first output terminal of the primary side of the feeder terminal device, and the second input terminal of the primary side of the second current transformer is connected to the second output terminal of the primary side of the feeder terminal device.
[0009] An analog-to-digital converter (ADC) is provided, wherein a first input terminal of the ADC is connected to a first output terminal on the secondary side of a first current transformer, and a second input terminal of the ADC is connected to a second output terminal on the secondary side of the first current transformer. The first current transformer is used to provide waveform recording for the ADC in the event of a circuit fault.
[0010] The third input terminal of the analog-to-digital converter is connected to the first output terminal on the secondary side of the second current transformer, and the fourth input terminal of the analog-to-digital converter is connected to the second output terminal on the secondary side of the second current transformer. The second current transformer is used to measure the sudden current generated by high-impedance grounding.
[0011] Preferred options also include:
[0012] An overload protection device is installed between the first output terminal of the primary side of the feeder terminal device and the first input terminal of the primary side of the second current transformer. It is used to monitor the output signal of the primary side of the feeder terminal device. When the output signal exceeds a preset threshold, the overload protection device automatically disconnects the primary side of the second current transformer.
[0013] Preferably, the overload protection device specifically includes:
[0014] A rectifier bridge, wherein the first input terminal of the rectifier bridge is connected to the first output terminal of the primary side of the feeder terminal device, and the second input terminal of the rectifier bridge is connected to the second input terminal of the primary side of the second current transformer;
[0015] A relay, wherein the first output terminal and the second output terminal of the rectifier bridge are respectively connected to both sides of the coil of the relay;
[0016] The relay also includes a contact connected between the first input terminal on the primary side of the second current transformer and the first output terminal on the primary side of the feeder terminal device.
[0017] Preferably, the overload protection device further includes:
[0018] A switching transistor, the gate of which is connected to the first output terminal of the rectifier bridge through a first resistor, the source of which is grounded, and the drain of which is connected to the relay coil.
[0019] A first capacitor is connected between the first resistor and the second output terminal of the rectifier bridge;
[0020] A second resistor is connected between the first resistor and the second output terminal of the rectifier bridge, and is connected in parallel with the first capacitor;
[0021] A Zener diode is connected between the first resistor and the second output terminal of the rectifier bridge, and is connected in parallel with the first capacitor and the second resistor.
[0022] Preferably, the overload protection device further includes an isolated DC power supply connected to the relay coil to provide current to the relay coil.
[0023] Preferably, the first preset transformation ratio is 20V:3.25V.
[0024] Preferably, the second preset ratio is 1V:3.25V.
[0025] Preferably, it further includes a third current transformer with a third preset ratio, which is disposed on the primary side of the feeder terminal device. The first output terminal and the second output terminal on the secondary side of the third current transformer serve as the first output terminal and the second output terminal on the primary side of the feeder terminal device, respectively.
[0026] Preferably, the third preset ratio is 600A:1V.
[0027] Preferably, the first resistor is a current-limiting resistor.
[0028] The technical solution of the present invention has the following advantages or beneficial effects: it provides a fault phase current acquisition circuit of dual-range CT for feeder terminal device, which can not only meet the 20 times recording requirement when a short circuit fault occurs in the line, but also accurately measure the sudden change signal of 350mA current in non-line fault conditions, thereby accurately judging the high resistance grounding fault generated by any phase line of the 10KV system. Attached Figure Description
[0029] Embodiments of the invention will be described more fully with reference to the accompanying drawings. However, the drawings are for illustration and explanation only and do not constitute a limitation on the scope of the invention.
[0030] Figure 1This is a structural diagram of the fault phase current acquisition circuit of the feeder terminal device in the first embodiment of the present invention;
[0031] Figure 2 This is a structural diagram of the fault phase current acquisition circuit of the feeder terminal device in the second embodiment of the present invention. Detailed Implementation
[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0033] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.
[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.
[0035] This invention includes a fault phase current acquisition circuit for a feeder terminal device, such as... Figure 1 and Figure 2 As shown, it includes:
[0036] The first current transformer CT1 has a first preset transformation ratio and is located on the secondary side of the feeder terminal unit (FTU). The first input terminal of the primary side of the first current transformer CT1 is connected to the first output terminal of the primary side of the feeder terminal unit, and the second input terminal of the primary side of the first current transformer CT1 is connected to the second output terminal of the primary side of the feeder terminal unit.
[0037] The second current transformer CT2 has a second preset ratio, is located on the secondary side of the feeder terminal device, and is connected in parallel with the first current transformer CT1. The first input terminal of the primary side of the second current transformer CT2 is connected to the first output terminal of the primary side of the feeder terminal device, and the second input terminal of the primary side of the second current transformer CT2 is connected to the second output terminal of the primary side of the feeder terminal device.
[0038] The analog-to-digital converter (ADC) has its first input terminal connected to the first output terminal on the secondary side of the first current transformer CT1, and its second input terminal connected to the second output terminal on the secondary side of the first current transformer CT1. The first current transformer CT1 is used to provide waveform recording for the ADC in the event of a circuit fault.
[0039] The third input terminal of the analog-to-digital converter is connected to the first output terminal on the secondary side of the second current transformer CT2, and the fourth input terminal of the analog-to-digital converter is connected to the second output terminal on the secondary side of the second current transformer CT2. The second current transformer CT2 is used to measure the sudden current generated by high impedance grounding.
[0040] Specifically, in this embodiment, the secondary side of the FTU device includes two current transformers with different ranges connected in parallel. The first current transformer has a larger range and is used to meet the recording requirements of the ADC when a short-circuit fault occurs in the line. The second current transformer has a smaller range but higher accuracy. It can capture the 350mA sudden current generated by a 2KΩ high-impedance grounding under a rated load current of 600A, thereby accurately determining the high-impedance grounding fault generated by any phase line of the line.
[0041] In a preferred embodiment, such as Figure 1 and Figure 2 As shown, it also includes:
[0042] The overload protection device K is located between the first output terminal of the primary side of the feeder terminal device and the first input terminal of the primary side of the second current transformer CT2. It is used to monitor the output signal of the primary side of the feeder terminal device. When the output signal exceeds a preset threshold, the overload protection device automatically disconnects the primary side of the second current transformer.
[0043] Specifically, the overload protection device in this embodiment is used to monitor the output signal of the primary CT. When the output signal of the primary CT exceeds a preset threshold, the overload protection device automatically cuts off the small-range high-precision CT, that is, cuts off the input primary side of CT2, so as to protect the small-range high-precision CT.
[0044] In a preferred embodiment, such as Figure 2 As shown, the overload protection device specifically includes:
[0045] The rectifier bridge has its first input terminal connected to the first output terminal on the primary side of the feeder terminal device, and its second input terminal connected to the second input terminal on the primary side of the second current transformer CT2.
[0046] The first and second output terminals of the relay and rectifier bridge are respectively connected to the two sides of the relay coil K1A;
[0047] The relay also includes a contact K1B, which is connected between the first input terminal on the primary side of the second current transformer and the first output terminal on the primary side of the feeder terminal device.
[0048] Switch Q1, the gate of switch Q1 is connected to the first output terminal of rectifier bridge through a first resistor R1, the source of switch Q1 is grounded, and the drain of switch Q1 is connected to the coil K1A of relay.
[0049] The first capacitor C1 is connected between the first resistor R1 and the second output terminal of the rectifier bridge;
[0050] The second resistor R2 is connected between the first resistor R1 and the second output terminal of the rectifier bridge, and is connected in parallel with the first capacitor C1;
[0051] Zener diode D5 is connected between the first resistor R1 and the second output terminal of the rectifier bridge, and is connected in parallel with the first capacitor C1 and the second resistor R2.
[0052] Specifically, the rectifier bridge in this embodiment consists of four diodes ( Figure 2 The FTU device (D1-D4) connected end-to-end is used to convert the AC output of the primary CT into DC. In this embodiment, the FTU device uses two parallel small-range current transformers. The first current transformer CT1 is a wide-range small-signal transformer, and the second current transformer CT2 is a narrow-range high-precision small-signal transformer. The primary sides of both CT1 and CT2 are connected to the secondary side of CT3, and the secondary sides of both CT1 and CT2 are connected to the input of the analog-to-digital converter (ADC). Figure 2 (AD1P, AD1N, AD2P, AD2N are shown). To prevent overload damage to the second current transformer CT2 due to an input signal exceeding its range when the line load current is short-circuited, a relay K is added in this embodiment. The preferred relay model is G5V-2-H1-5VDC, and the relay coil K1A is powered by an isolated 5V power supply. When a short-circuit overload occurs on the primary side, the normally closed contact K1B of the relay is promptly disconnected, thus protecting the second current transformer CT2.
[0053] like Figure 2 As shown, when the primary current is less than the rated current, the input signal at the PN terminal is less than ±1V AC signal. The small AC voltage signal first passes through the rectifier bridge. The rectifier bridge D1 to D4 uses diodes of model 1N4448. The forward voltage drop of a single diode is 0.65V. Within the rated range, the diode is insufficient to generate enough forward voltage, so the protection circuit will not start working, the relay coil will not be energized, thus keeping the second current transformer CT2 in the signal input state and monitoring the high-resistance sudden current signal in real time.
[0054] When a short circuit fault occurs on the primary side, the primary side current increases several times more than 600A. In this case, the PN port will generate a small AC signal of ±4 to 20V. The signal first passes through the rectifier bridge D1 to D4 and the current-limiting resistor R1 to charge the first capacitor C1. When the voltage across C1 rises to 1.5V or higher, the switching transistor Q1 starts to conduct. Q1 is preferably an N-MOSFET, model SI2302. Subsequently, the relay coil K1A is energized, the relay contact K1B is opened, and the connection between the second current transformer CT2 and the PN port is disconnected, thus protecting the second current transformer CT2 from damage due to over-range input. If the short circuit current is too large, the voltage at the PN port will be too high. To prevent the gate of the switching transistor Q1 from breaking down, a Zener diode D5 and a current-limiting resistor R1 are used to clamp the gate voltage of the switching transistor Q1, always keeping the gate voltage of Q1 below 5.1V.
[0055] When the fault short-circuit current in the line disappears, the rectifier bridges D1-D4 return to the cutoff state. The residual charge in the first capacitor C1 discharges through the second resistor R2. After the discharge is complete, the gate voltage of the switching transistor Q1 is lower than the turn-on voltage, and Q1 enters the cutoff state and stops conducting. The relay coil K1A loses its excitation voltage, and the relay contact K1B returns to the normally closed state. The second current transformer CT2 reconnects to the PN signal port and continues to collect current readings within the rated range.
[0056] Through the above technical solution, the FTU fault phase current acquisition circuit in this embodiment can effectively acquire and record short-circuit currents up to 12kA. At the same time, it can accurately measure sudden current changes of 350mA when the rated load current is less than or equal to 600A. In addition, it can effectively perform automatic switching protection for small-range current transformers to prevent damage to the current acquisition circuit.
[0057] In a preferred embodiment, such as Figure 2 As shown, the overload protection device also includes an isolated DC power supply connected to the relay coil K1A to provide current to the relay coil K1A.
[0058] In a preferred embodiment, the first preset transformation ratio is 20V:3.25V. The first current transformer CT1 is integrated inside the FTU and has a large range, which can meet the recording requirements of 20 times the short-circuit current. At a current of 12KA, it can output an analog signal with an effective value of 3.25V, which can be used by the analog-to-digital converter (ADC) to record the current waveform when a short-circuit fault occurs in the line. The second preset transformation ratio is 1V:3.25V. The second current transformer CT2 is integrated inside the FTU and can collect the 350mA sudden current generated by a 2KΩ high-impedance grounding under a rated load current of 600A, thereby accurately determining the high-impedance grounding fault generated by any phase line of the 10KV system.
[0059] In a preferred embodiment, such as Figure 1 As shown, the FTU also includes a third current transformer CT3 with a third preset ratio of 600A:1V. CT3 is located on the primary side of the FTU, and the first and second output terminals on the secondary side of the third current transformer CT3 serve as the first and second output terminals on the primary side of the feeder terminal device, respectively.
[0060] The beneficial effects of this invention are as follows: It provides a fault phase current acquisition circuit for a dual-range CT for the feeder terminal device, which can not only meet the 20-fold recording requirement when a short-circuit fault occurs in the line, but also accurately measure the sudden change signal of 350mA current in non-line fault conditions, thereby accurately determining the high-resistance grounding fault generated by any phase line of the 10KV system.
[0061] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.
Claims
1. A fault phase current acquisition circuit for a feeder terminal device, characterized in that, include: A first current transformer, having a first preset transformation ratio, is disposed on the secondary side of the feeder terminal device. The first input terminal of the primary side of the first current transformer is connected to the first output terminal of the primary side of the feeder terminal device, and the second input terminal of the primary side of the first current transformer is connected to the second output terminal of the primary side of the feeder terminal device. A second current transformer, having a second preset ratio, is disposed on the secondary side of the feeder terminal device and connected in parallel with the first current transformer. The first input terminal of the primary side of the second current transformer is connected to the first output terminal of the primary side of the feeder terminal device, and the second input terminal of the primary side of the second current transformer is connected to the second output terminal of the primary side of the feeder terminal device. An analog-to-digital converter (ADC) is provided, wherein a first input terminal of the ADC is connected to a first output terminal on the secondary side of a first current transformer, and a second input terminal of the ADC is connected to a second output terminal on the secondary side of the first current transformer. The first current transformer is used to provide waveform recording for the ADC in the event of a circuit fault. The third input terminal of the analog-to-digital converter is connected to the first output terminal on the secondary side of the second current transformer, and the fourth input terminal of the analog-to-digital converter is connected to the second output terminal on the secondary side of the second current transformer. The second current transformer is used to measure the sudden current generated by high-impedance grounding.
2. The fault phase current acquisition circuit according to claim 1, characterized in that, Also includes: An overload protection device is installed between the first output terminal of the primary side of the feeder terminal device and the first input terminal of the primary side of the second current transformer. It is used to monitor the output signal of the primary side of the feeder terminal device. When the output signal exceeds a preset threshold, the overload protection device automatically disconnects the primary side of the second current transformer.
3. The fault phase current acquisition circuit according to claim 2, characterized in that, The overload protection device specifically includes: A rectifier bridge, wherein the first input terminal of the rectifier bridge is connected to the first output terminal of the primary side of the feeder terminal device, and the second input terminal of the rectifier bridge is connected to the second input terminal of the primary side of the second current transformer; A relay, wherein the first output terminal and the second output terminal of the rectifier bridge are respectively connected to both sides of the coil of the relay; The relay also includes a contact connected between the first input terminal on the primary side of the second current transformer and the first output terminal on the primary side of the feeder terminal device.
4. The fault phase current acquisition circuit according to claim 3, characterized in that, The overload protection device specifically includes: A switching transistor, the gate of which is connected to the first output terminal of the rectifier bridge through a first resistor, the source of which is grounded, and the drain of which is connected to the relay coil. A first capacitor is connected between the first resistor and the second output terminal of the rectifier bridge; A second resistor is connected between the first resistor and the second output terminal of the rectifier bridge, and is connected in parallel with the first capacitor; A Zener diode is connected between the first resistor and the second output terminal of the rectifier bridge, and is connected in parallel with the first capacitor and the second resistor.
5. The fault phase current acquisition circuit according to claim 3, characterized in that, The overload protection device further includes an isolated DC power supply connected to the relay coil to provide current to the relay coil.
6. The fault phase current acquisition circuit according to claim 1, characterized in that, The first preset ratio is 20A:3.25A.
7. The fault phase current acquisition circuit according to claim 1, characterized in that, The second preset ratio is 1A:3.25A.
8. The fault phase current acquisition circuit according to claim 1, characterized in that, It also includes a third current transformer with a third preset ratio, which is disposed on the primary side of the feeder terminal device. The first output terminal and the second output terminal on the secondary side of the third current transformer serve as the first output terminal and the second output terminal on the primary side of the feeder terminal device, respectively.
9. The fault phase current acquisition circuit according to claim 8, characterized in that, The third preset ratio is 600A:1A.
10. The fault phase current acquisition circuit according to claim 4, characterized in that, The first resistor is a current-limiting resistor.
Citation Information
Patent Citations
Fault phase current acquisition circuit of feeder terminal device
CN218512525U