Ground detection circuit and method for two-phase three-wire power grid

Abstract

The application provides a ground wire detection circuit and method for a two-phase three-wire power grid, which comprises a filter circuit, an isolation capacitor, a voltage dividing circuit, a filter capacitor and a sampling point. The input end of the filter circuit is connected with three output ends of a power grid transformer, i.e. L1, L2 and a center point N of power grid output. The output end of the filter circuit is connected with the isolation capacitor and the voltage dividing circuit in sequence. The voltage dividing end point of the voltage dividing circuit is connected with an ADC module of a single-chip microcomputer, and a signal filter capacitor is connected in parallel with the two ends of a resistor in the voltage dividing circuit which is far away from the isolation capacitor. The filter circuit comprises a filter capacitor CY1 and a filter capacitor CY2, and the capacitances of CY1 and CY2 are equal. The application can adapt to the ground wire detection of a two-phase three-wire power grid with a voltage phase of 180° between phase lines in Japan, North America and other regions, and can adapt to the promotion needs of new energy automobile charging equipment in foreign countries.

Description

Technical Field

[0001] This invention relates to the field of charging detection technology, and in particular to a ground wire detection circuit and detection method for a two-phase three-wire power grid. Background Technology

[0002] In the field of new energy vehicle charging, to ensure the safety of vehicles and people during charging, charging equipment needs to detect the input grounding impedance to confirm the ground wire connection. Two common solutions are used for different power grid configurations: Solution 1: For single-phase three-wire power grids (domestic and international), with the neutral (N) wire grounded, the voltages of L and PE are isolated, divided, and sent to the MCU for AD sampling. The result is compared with a sampling threshold to determine if the ground wire is connected. Solution 2: For two-phase three-wire power grids (domestic and international), where the voltage phase between phases is 120°, L1 and L2 are isolated and then combined. The combined signal is divided and sent to the MCU for AD sampling. The result is compared with a sampling threshold to determine if the ground wire is connected. Unlike Solution 2, power grids in Japan and North America are also two-phase three-wire systems, but the voltage phase between phases is 180°. In this case, since the voltages of L1 and L2 are equal in magnitude but opposite in direction, existing solutions cannot perform ground wire detection. Summary of the Invention

[0003] The purpose of this invention is to provide a ground wire detection circuit and method for a two-phase three-wire power grid, which can be adapted to the ground wire detection of two-phase three-wire power grids in regions such as Japan and North America where the voltage phase between phase lines is 180°, and is suitable for the promotion of new energy vehicle charging equipment abroad.

[0004] To achieve the above objectives, the technical solution adopted by this invention is: a ground wire detection circuit for a two-phase three-wire power grid, comprising a filter circuit, an isolation capacitor, a voltage divider circuit, a filter capacitor, and sampling points. The input terminal of the filter circuit is connected to the three output terminals of the power grid transformer, namely L1, L2, and the center point N of the power grid output. The output terminal of the filter circuit is connected in series with the isolation capacitor and the voltage divider circuit. The voltage divider terminals of the voltage divider circuit are the sampling points connected to the ADC module of the microcontroller. A signal filter capacitor is connected in parallel across the resistor in the voltage divider circuit furthest from the isolation capacitor. The filter circuit includes filter capacitors CY1 and CY2, with equal capacitance.

[0005] Preferably, the detection method of this detection circuit includes the following steps: Step 1, the power grid transformer is connected to two-phase three-wire power, and the detection begins when the power supply is normal;

[0006] Step 2: The microcontroller continuously detects the grounding impedance detection signal at the sampling point;

[0007] Step 3: When fully grounded, the grounding impedance detection signal value is the voltage division value after the equivalent resistance of the isolation capacitor is connected in series with the voltage divider circuit, denoted as Vadc1;

[0008] Step 4: When the grounding is not completely grounded, the grounding impedance detection signal value is Vadc2. Since there is a voltage drop across the grounding impedance, Vadc2 < Vadc1.

[0009] Step 5: When there is no grounding, that is, the grounding impedance approaches ∞, the capacitances of CY1 and CY2 are equal, and the grounding impedance detection signal value is Vadc3, Vadc3 < Vadc1.

[0010] Step 6: After the microcontroller detects an abnormal grounding impedance fault, the charging equipment indicates a fault.

[0011] Beneficial effects: The local grounding detection circuit and method use fewer components, occupy a small circuit board area, are highly accurate, low in cost, safe, reliable, and practical. It can be adapted to the grounding detection of two-phase three-wire power grids with a voltage phase of 180° between phase lines in regions such as Japan and North America. It also meets the needs of promoting new energy vehicle charging equipment abroad, thereby enhancing the competitiveness of domestically produced new energy vehicles in overseas markets. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the present invention. Detailed Implementation

[0013] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0014] This invention discloses a ground wire detection circuit and detection method for a two-phase three-wire power grid, such as... Figure 1 As shown, on the grid transformer side, T_L1 represents the grid output L1; T_L2 represents the grid output L2; T_N represents the grid transformer center point; T_PE represents the grid grounding point; the voltages of T_L1 to T_N and T_L2 to T_N are equal in magnitude but opposite in direction. Rpe represents the impedance from the grid transformer center point T_N to the grounding point T_PE, i.e., the grounding impedance.

[0015] On the PCB side, CY1 and CY2 are the input filter capacitors of the PCB board, CX1 is the isolation capacitor for the sampling circuit input, R1 and R2 form a voltage divider circuit, and C1 is the signal filter capacitor. The grounding impedance Rpe is detected by acquiring the voltage value at the front end of R2.

[0016] The specific implementation process is as follows: Step 1: Connect the power grid transformer to two-phase three-wire power, ensure normal power supply, and begin testing;

[0017] Step 2: The microcontroller continuously detects the voltage value at the front end of R2, i.e., the grounding impedance detection signal;

[0018] Step 3: When fully grounded, i.e., Rpe=0, the voltage of T_PE is equal to the voltage of T_N, and the voltage of PCB_L1 to PCB_PE is equal to the voltage of T_L1 to T_N, both of which are the phase line voltages of the power grid. At this time, the ADC signal value is the voltage division value after the equivalent resistance of CX1 is connected in series with R1 and R2, denoted as Vadc1;

[0019] Step 4: When the ground is not completely grounded, i.e. Rpe≠0, since the equivalent resistance of CX1 is connected in parallel with the series resistance network of R1 and R2 and the equivalent resistance of the filter capacitor CY1, and then connected in series with Rpe, the phase voltage is connected. The sampling voltage entering the microcontroller at this time is denoted as Vadc2. Since there is a voltage drop across the grounding impedance Rpe, Vadc2 < Vadc1.

[0020] Step 5: When there is no grounding, i.e., Rpe=∞; the PCB board is not connected to the center point of the power grid. The equivalent resistance of CY2, the equivalent resistance of CY1, the equivalent resistance of CX1, and the voltage divider formed by the resistor network of R1 and R2; since CY1 and CY2 are equal, the voltage of PCB_L1 to PCB_PE is less than the phase line voltage. At this time, the sampling voltage Vadc3 entering the microcontroller is less than Vadc1.

[0021] Step 6: After the microcontroller detects an abnormal grounding impedance fault, the charging equipment indicates a fault.

[0022] In practical work, appropriate CY1 and CY2 are selected to modulate the voltage entering the ADC so that it meets the input voltage range of the MCU, and the software parameters are determined according to the required grounding impedance threshold.

[0023] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the specific implementation of the present invention with reference to the above embodiments. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the protection scope of the claims of the present invention.

Claims

1. A method for detecting the ground wire of a two-phase three-wire power grid, characterized in that: The detection circuit for the ground wire of a two-phase three-wire power grid is implemented. It comprises a filter circuit, an isolation capacitor, a voltage divider circuit, a filter capacitor, and sampling points. The input of the filter circuit is connected to the three output terminals of the power grid transformer: L1, L2, and the center point N. The output of the filter circuit is connected in series with the isolation capacitor and the voltage divider circuit. The voltage divider terminals of the voltage divider circuit are the sampling points connected to the ADC module of the microcontroller. A signal filter capacitor is connected in parallel across the resistor furthest from the isolation capacitor in the voltage divider circuit. The filter circuit includes filter capacitors CY1 and CY2, with equal capacitance. Includes the following steps: Step 1: Connect the power grid transformer to two-phase three-wire power. If the power supply is normal, begin testing. Step 2: The microcontroller continuously detects the grounding impedance detection signal at the sampling point; Step 3: When fully grounded, the grounding impedance detection signal value is the voltage division value after the equivalent resistance of the isolation capacitor is connected in series with the voltage divider circuit, denoted as Vadc1; Step 4: When the grounding is not completely grounded, the grounding impedance detection signal value is Vadc2. Since there is a voltage drop across the grounding impedance, Vadc2 < Vadc1. Step 5: When there is no grounding, that is, the grounding impedance approaches ∞, the capacitances of CY1 and CY2 are equal, and the grounding impedance detection signal value is Vadc3, Vadc3 < Vadc1. Step 6: After the microcontroller detects an abnormal grounding impedance fault, the charging equipment indicates a fault.

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