A device and method for locally suppressing DC bias of a transformer

Abstract

The present invention discloses a device and method for suppressing DC bias of a transformer. The device includes a bypass switch, a resistor, two coils, a DC-blocking unit, and a current monitoring and control unit; the bypass switch, coil L0, and resistor are connected in series between the neutral point of the transformer and the ground; the DC-blocking unit is connected in parallel across the bypass switch and includes two branches connected in reverse parallel and composed of a plurality of silicon carbide PiN diodes connected in series, and coil L1 is arranged on the branch with the anode connected to the neutral point; the current monitoring and control unit monitors the currents of the two coils and controls the closing of the bypass switch. The present invention utilizes the unidirectional conduction characteristic of the silicon carbide PiN diode to achieve in-situ suppression of the bias DC, and when a primary system fault occurs, the suppression device conducts instantaneously to maintain effective grounding of the neutral point, avoiding insulation breakdown of the transformer. Without the need to set up complex bypass control and protection branches, it can simultaneously achieve in-situ suppression during DC bias and instantaneously ground during a primary system fault, and has the advantages of safety, reliability, and economy.

Description

Technical Field

[0001] The present invention relates to measures for suppressing DC bias in transformers. More specifically, it relates to a device and method for suppressing DC bias in transformers based on silicon carbide PiN diodes. Background Art

[0002] With the continuous acceleration of urbanization construction and economic development, alleviating urban traffic pressure has become a hot issue concerning people's livelihood. Subways have become an important means of transportation for residents due to their safety, environmental friendliness, and convenience. However, subways will have an adverse impact on the urban power grid along the line during actual operation. Currently, DC power supply systems are generally adopted for subway power supply, with voltage systems of DC750V or DC1500V. A loop is formed among the positive busbar of the traction substation, the catenary, the running rail, and the negative busbar. Since the running rail cannot be completely insulated from the ground, part of the traction current will leak into the ground to form subway stray current. The stray current propagates in the ground, which will cause potential offsets between substations at different positions along the subway line, enabling DC current to be injected into the neutral point grounded main transformer of the substation, thereby triggering DC bias in the transformer. When DC bias occurs in the transformer, a DC component is generated in its winding current. Under the combined action of the AC magnetic flux and the DC magnetic flux, the iron core will enter the saturation state in advance, resulting in increased magnetostriction, and thus the vibration of the transformer will also increase, causing problems such as noise and temperature rise in the transformer, and even triggering a transformer fire accident, leading to a local power outage in the urban power grid.

[0003] Currently, the main measures for suppressing DC bias in transformers are: connecting a capacitor in series at the neutral point, connecting a small resistor in series at the neutral point, the reverse current injection method, and the potential compensation method. Due to the high project cost, operation and maintenance costs, and the possibility of overcompensation exacerbating the DC bias phenomenon, the reverse current injection method and the potential compensation method are not commonly used in actual projects. Instead, connecting a capacitor or a small resistor in series at the neutral point of the transformer is more commonly used to suppress DC bias. After connecting a resistor in series between the neutral point of the transformer and the ground, the total resistance of the ground branch increases, playing a role in shunting. Most of the DC current flows into the ground soil branch, and the DC bias current injected into the neutral point decreases, thus suppressing the DC bias of the transformer. In actual working conditions, when a primary system fault occurs, if the series resistance value is too large, it is easy to cause serious zero potential drift at the neutral point of the transformer, and the protection needs to be re-set, affecting the safe and stable operation of the power system. If the series resistance value is too small, the shunting effect weakens, and the bias DC current cannot be limited below the threshold value. Therefore, this method has certain limitations. After connecting a capacitor in series at the neutral point of the transformer, when there is DC current passing through the neutral point, the capacitor's DC isolation characteristic is utilized to achieve the effect of suppressing DC bias. However, in order to avoid damage to the suppression device under a primary system fault and to ensure the conduction of the fault current, a bypass protection system composed of power electronic devices such as a thyristor control circuit needs to be connected in parallel. The equipment structure is complex and the maintenance cost is high. Summary of the Invention

[0004] In view of the DC bias phenomenon of the transformer caused by the stray current generated during the operation of the urban subway, the present invention provides a device and method for suppressing the DC bias of the transformer based on a silicon carbide PiN diode, which can achieve in-situ suppression during DC bias and instantaneous grounding effect during a primary system fault, while suppressing the DC bias phenomenon of the transformer and protecting the suppression device from damage.

[0005] To achieve the above technical objectives, the present invention adopts the following technical solutions:

[0006] A device for suppressing the DC bias of a transformer based on a silicon carbide PiN diode, connected in series between the neutral point of the transformer and the ground, includes a bypass switch, a grounding resistor R, and an overcurrent operating coil L 0 、overcurrent operating coil L 1 、a DC isolation unit and a current monitoring and control unit; the bypass switch, coil L 0 and the grounding resistor R are connected in series between the neutral point of the transformer and the ground in sequence; the DC isolation unit is connected in parallel at both ends of the bypass switch and includes two diode branches connected in reverse parallel, and each diode branch is composed of several silicon carbide PiN diodes connected in series. Let the diode branch with the cathode of the diode connected to the neutral point of the transformer be diode branch A, and the other be diode branch B; coil L 1 is connected in series on diode branch B; the current monitoring and control unit is used to detect the current flowing through the two overcurrent operating coils and control the closing state of the bypass switch according to the current.

[0007] Further, the series position of coil L 1 on diode branch B is: one end is electrically connected to the neutral point of the transformer, and the other end is electrically connected to the anode end of the first silicon carbide PiN diode on diode branch B.

[0008] Further, the current monitoring and control unit: if the current of monitoring coil L 1 exceeds the fault current threshold, that is, when a fault current is detected, it is determined that a single-phase grounding fault has occurred in the primary system of the transformer, and the bypass switch is controlled to close, so that the neutral point of the transformer is effectively grounded through two paths of the bypass switch and the DC isolation unit; if the current of monitoring coil L 1 does not exceed the fault current threshold, that is, when no fault current is detected, it is determined that the primary system of the transformer is operating normally, and further the closing state of the bypass switch is controlled according to the current I 0 flowing through coil L 0 : if the current I 0 = 0, then the bypass switch is controlled to close to effectively ground the neutral point of the transformer, otherwise the bypass switch is controlled to open, and the DC isolation unit is not turned on, so that the bias current generated by the transformer is suppressed in-situ.

[0009] Furthermore, it is applied to suppressing the DC bias of transformers caused by stray currents generated during the operation of urban subways.

[0010] A method for suppressing the DC bias of a transformer based on a silicon carbide PiN diode, based on the transformer DC bias suppression device described in any of the above technical solutions, includes:

[0011] The current monitoring and control unit monitors the current flowing through coil L 1 and coil L 0 in real time;

[0012] If the current flowing through coil L 1 exceeds the fault current threshold, which is equivalent to detecting a fault current, it is determined that a single-phase grounding fault has occurred in the primary system of the transformer, and the bypass switch is controlled to close, so that the neutral point of the transformer is effectively grounded through two paths of the bypass switch and the DC-blocking unit;

[0013] If the current flowing through coil L 1 does not exceed the fault current threshold, which is equivalent to not detecting a fault current, it is determined that the primary system of the transformer is operating normally. Further, according to the current I 0 flowing through coil L 0 , the closing state of the bypass switch is controlled: if the current I 0 flowing through coil L 0 = 0, the bypass switch is controlled to close to effectively ground the neutral point of the transformer; otherwise, the bypass switch is controlled to open, and the DC-blocking unit functions to locally suppress the bias current generated by the transformer.

[0014] Beneficial effects

[0015] In the present invention, by connecting a DC-blocking part composed of silicon carbide PiN diodes in series at the neutral point of the transformer and controlling the bypass switch connected in parallel to the DC-blocking part by the current monitoring and control unit, there is no need to connect a complex bypass protection branch. Utilizing the good forward current conduction ability and unidirectional conduction characteristics of high-power diodes, it is possible to achieve local suppression during DC bias and instantaneous grounding effect during primary system faults, while suppressing the DC bias phenomenon of the transformer, protecting the suppression device from damage.

[0016] Moreover, in the present invention, due to the internal conductance modulation effect of the silicon carbide PiN diode, its specific on-resistance is reduced and the leakage current is small, having good forward conduction characteristics and being suitable for instantaneous conduction of fault currents. Due to the advantage of the short-time large-current operating characteristics of high-power diodes, the bias suppression device can operate for a long time. Description of the drawings

[0017] Figure 1 It is a schematic diagram of DC bias of a transformer caused by subway stray currents in the present invention

[0018] Figure 2 Structural diagram of the bias magnetic suppression device in the present invention

[0019] Figure 3 Equivalent circuit diagram of the suppression device in the present invention

[0020] Figure 4 Schematic diagram of the change in the hysteresis loop of the transformer core in the present invention

[0021] Figure 5 Schematic diagram of the change in the neutral point current during a primary system fault in the present invention

[0022] Figure 6 Workflow of the device described in the embodiments of the present application Specific implementation manners

[0023] The following details the embodiments of the present invention. Based on the technical solution of the present invention, the detailed implementation manners and specific operation processes are given, further explaining the technical solution of the present invention

[0024] During the actual operation of urban subways, since the running rails are not completely insulated from the ground, part of the traction current will leak into the ground to form subway stray current. The stray current propagates in the ground, causing potential offsets between substations at different positions along the subway line, resulting in direct current being injected into the neutral point grounded main transformer of the substation, thereby triggering DC bias magnetization of the transformer Figure 1 Schematic diagram of the DC bias magnetization of the transformer caused by subway stray current

[0025] The present invention provides a transformer DC bias magnetic suppression device and method. By connecting a DC bias magnetic suppression device based on a silicon carbide PiN diode in series at the neutral point of the transformer, in-situ suppression of the bias DC and instantaneous grounding effect during a primary system fault are achieved. The principle of in-situ suppression is as follows: when the primary system is operating normally, the neutral point voltage is zero or close to zero, the diode is not conducting, and at the same time, the unidirectional conductivity characteristic is used to achieve the DC isolation effect. The principle of the instantaneous grounding effect during a primary system fault is as follows: when a primary system fault occurs, the fault current increases sharply, the neutral point voltage of the transformer rises, causing the silicon carbide PiN diode of the DC isolation unit to conduct instantaneously, making the neutral point effectively grounded; the overcurrent action coil monitors the fault current and controls the bypass switch to close, making the neutral point effectively grounded, and double means ensure the conduction of the fault current

[0026] Specifically, the transformer DC bias magnetic suppression device based on a silicon carbide PiN diode in this embodiment is applied to suppress the DC bias magnetization of the transformer caused by the stray current generated during the operation of urban subways. Referring to Figure 2 as shown, it is connected in series between the neutral point of the transformer and the ground, and includes a bypass switch, a grounding resistor R, and an overcurrent action coil L 0, overcurrent operating coil L 1 , DC blocking unit and current monitoring and control unit.

[0027] Bypass switch, coil L 0 and grounding resistor R are connected in series between the neutral point of the transformer and the ground in sequence; the DC blocking unit is connected in parallel at both ends of the bypass switch and includes two reverse-parallel diode branches, each diode branch is composed of several silicon carbide PiN diodes connected in series. Let the diode branch with the cathode connected to the neutral point of the transformer be diode branch A, and the other be diode branch B; coil L 1 is connected in series on diode branch B, and one end is electrically connected to the neutral point of the transformer, and the other end is electrically connected to the anode end of the first silicon carbide PiN diode on diode branch B; the current monitoring and control unit is used to detect the current flowing through the two overcurrent operating coils and control the closing state of the bypass switch according to the current.

[0028] Based on the method for suppressing DC bias of the transformer by the above transformer DC bias suppression device, refer to Figure 6 as shown. First, the current monitoring and control unit monitors the current flowing through coil L 1 and coil L 0 in real time, and then controls the bypass switch in the following situations according to the monitored coil current:

[0029] 1. If the current I flowing through coil L 1 does not exceed the fault current threshold, that is, |I| ≤ |I K |, which is equivalent to not detecting a fault current, it is determined that the primary system of the transformer is operating normally. Further, according to the current I 0 flowing through coil L 0 controls the closing state of the bypass switch:

[0030] 1) If it is monitored that there is current injection into coil L 0 , which is equivalent to having DC input at the neutral point of the transformer, it is considered that the transformer has DC bias.

[0031] When there is DC injection at the neutral point, the transformer core is saturated, and the overcurrent operating coil L 0 controls the bypass switch to open and the bias suppression device is put into operation. Figure 2 is the structure diagram of the bias suppression device. The DC blocking part of the suppression device is composed of two reverse-parallel diode branches with several silicon carbide PiN diodes connected in series in the same direction. When the primary system is operating normally, the neutral point voltage of the transformer is zero or close to zero, and the silicon carbide PiN diode branch with the anode connected to the neutral point is not conducting, and the branch reverse-parallel to it also fails to conduct due to the unidirectional conductivity of the diode, thus forming the DC blocking part to achieve local suppression of the DC bias of the transformer. Figure 3 is the equivalent circuit diagram of the suppression device, using the adjustable DC voltage source US Simulate the potential difference generating the bias DC magnetic flux, considering the conduction voltage and dynamic resistance of the diode. According to Kirchhoff's voltage law:

[0032]

[0033] In the formula, I S is the reverse saturation current of the diode; R and R 0 are the small resistors for grounding the neutral point of the transformer; u D is the voltage applied across the diode; U T is the temperature voltage equivalent; V on is the threshold voltage of the diode; r D is the dynamic resistance of the diode.

[0034] The dynamic resistance equation of the diode is:

[0035]

[0036]

[0037] At room temperature, U T = 26 mV. Since the diode parameters V on , U T are constants and can be neglected compared with the value of the adjustable DC voltage source. Substituting the above equation into Kirchhoff's voltage equation, we get:

[0038]

[0039] When the primary system is operating normally, the voltage of the transformer neutral point is zero or close to zero, that is, when u D is 0, the silicon carbide PiN diode branch B connected in series between the anode and the neutral point is not conducting, and the diode branch A connected in antiparallel with it also fails to conduct due to the unidirectional conductivity of the diode, thus forming a DC blocking unit to achieve local suppression of the transformer bias DC magnetic flux.

[0040] Figure 4 is a schematic diagram of the change of the transformer iron core hysteresis loop. It can be seen from the figure that when the suppression device is put into operation during transformer DC bias, the distortion rate of the iron core hysteresis loop decreases and is approximately symmetric along the positive and negative half axes, and the suppression effect is better.

[0041] 2) If the current I 0 flowing through the coil L 0 = 0 or within an allowable range close to 0, it is equivalent to normal operation without bias current at the transformer neutral point, and the current monitoring and control unit controls the bypass switch to close, making the transformer neutral point effectively grounded.

[0042] 2. If it is monitored that the current I flowing through the coil L 1 exceeds the fault current threshold IK That is, |I| > |I K |, which is equivalent to detecting a fault current, and the current monitoring and control unit determines that a single-phase grounding fault has occurred in the primary system of the transformer.

[0043] When a fault occurs in the primary system, the fault current increases sharply, and the voltage of the transformer neutral point rises, causing the silicon carbide PiN diode on the diode branch B to conduct instantaneously, making the neutral point effectively grounded. Figure 5 It is a schematic diagram of the change in the neutral point current when a fault occurs in the primary system. The overcurrent operating coil monitors the fault current to control the closing of the bypass switch, making the neutral point effectively grounded. The dual means ensure the conduction of the fault current, enabling the timely conduction of the fault current and preventing the damage of the suppression device and the insulation breakdown of the transformer.

[0044] According to the working process of the above suppression device, the present invention connects several silicon carbide PiN diodes in series in the same direction at the transformer neutral point, and uses their unidirectional conduction characteristics to achieve in-situ suppression of DC bias magnetic field. When a fault occurs in the primary system, the suppression device conducts instantaneously to keep the neutral point effectively grounded, avoiding insulation breakdown of the transformer, and can simultaneously achieve in-situ suppression during DC bias magnetic field and instantaneous grounding effect during primary system fault without setting a complex bypass control protection branch.

[0045] For the built DC bias magnetic field suppression device, an adjustable DC voltage source U S is used to simulate the DC potential difference, and the potential difference is set to 20V, and the suppression device is put into operation. The simulation results show that after the suppression device is put into operation, the distortion rate of the core hysteresis loop decreases and is symmetric along the positive and negative half axes, which can effectively suppress the DC bias magnetic field of the transformer; when a single-phase grounding short circuit fault occurs in the primary system is set, the suppression device conducts instantaneously to make the neutral point effectively grounded.

[0046] The above embodiments are the preferred embodiments of the present application. Those of ordinary skill in the art can also make various transformations or improvements based on this. Without departing from the overall concept of the present application, these transformations or improvements should all fall within the scope protected by the present application.

Claims

1. A transformer DC bias suppression device based on a silicon carbide PiN diode, which is connected in series between the neutral point of the transformer and the ground. Characterized in that It includes a bypass switch, a grounding resistor R, and an overcurrent operating coil L 0 , and an overcurrent operating coil L 1 , a DC-blocking unit, and a current monitoring and control unit; the bypass switch, the coil L 0 and the grounding resistor R are sequentially connected in series between the neutral point of the transformer and the ground; the DC-blocking unit is connected in parallel across the two ends of the bypass switch and includes two diode branches connected in anti-parallel. Each diode branch is composed of a number of silicon carbide PiN diodes connected in series in the same direction. The diode branch with the cathode of the diode connected to the neutral point of the transformer is the diode branch A, and the other is the diode branch B; the coil L 1 is connected in series on the diode branch B; the current monitoring and control unit is used to detect the current flowing through the two overcurrent operating coils and control the closing state of the bypass switch according to the current.

2. The transformer DC bias suppression device according to claim 1, Characterized in that Coil L 1 The series connection position on the diode branch B is as follows: one end is electrically connected to the neutral point of the transformer, and the other end is electrically connected to the anode terminal of the first silicon carbide PiN diode on the diode branch B.

3. The transformer DC bias suppression device according to claim 1, Characterized in that The current monitoring and control unit is used for: When the current of coil L 1 exceeds the fault current threshold, that is, when a fault current is detected, it is determined that a single-phase grounding fault has occurred in the primary system of the transformer, and the bypass switch is controlled to close, so that the neutral point of the transformer is effectively grounded through two paths of the bypass switch and the DC-blocking unit; When the current in the monitoring coil L 1 does not exceed the fault current threshold, that is, when no fault current is detected, it is determined that the primary system of the transformer is operating normally. Further, according to the current I 0 flowing through the coil L 0 the closing state of the bypass switch is controlled: if the current I 0 is within the allowable range of 0 or close to 0, the bypass switch is controlled to close, so that the neutral point of the transformer is effectively grounded; otherwise, the bypass switch is controlled to open, and the DC-blocking unit is not turned on, so that the bias magnetic current generated by the transformer is suppressed locally.

4. The transformer DC bias suppression device according to claim 1, Characterized in that It is applied to suppress the DC bias of the transformer caused by the stray current generated during the operation of urban subways.

5. A method for suppressing the DC bias of a transformer based on a silicon carbide PiN diode, Characterized in that Based on the transformer DC bias suppression device according to any one of claims 1-4, including: The current monitoring and control unit monitors in real time the current flowing through coil L 1 and coil L 0 of the current; If the monitored current flowing through coil L 1 exceeds the fault current threshold, which is equivalent to detecting a fault current, it is determined that a single-phase ground fault has occurred in the primary system of the transformer, and the bypass switch is controlled to close, so that the neutral point of the transformer is effectively grounded through two paths of the bypass switch and the DC-blocking unit; If the monitored current flowing through coil L 1 does not exceed the fault current threshold, which is equivalent to not detecting a fault current, it is determined that the primary system of the transformer is operating normally. Further, the closing state of the bypass switch is controlled according to the current I0 flowing through coil L 0 : If the current I 0 flowing through coil L 0 = 0, the bypass switch is controlled to close, enabling the effective grounding of the transformer neutral point; otherwise, the bypass switch is controlled to open, and the DC blocking unit functions to locally suppress the bias current generated by the transformer.

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