CN2847637Y - Device for suppressing transformer neutral point DC current by capacitance method - Google Patents

Abstract

The utility model discloses a device for suppressing direct current at the neutral point of a transformer by capacitance method, which comprises a capacitor circuit, a state conversion switch circuit and an overvoltage bypass circuit, and the three circuits are connected in parallel and connected in series at the neutral point of the transformer and the grounding between. The utility model has the advantages of simple structure, safety and reliability, and can completely restrain the DC current at the neutral point of the transformer.

Description

A device for suppressing DC current at neutral point of transformer by capacitance method

technical field

The utility model relates to a device for suppressing the direct current at the neutral point of a transformer in the process of high-voltage power transmission and transformation.

Background technique

With the construction of my country's power grid, more and more direct current transmission lines have been put into operation in areas with relatively tight power loads. When the DC transmission system operates with a single pole, the ground pole current flows into the AC system through the neutral point of the transformer of the AC system, which has different degrees of influence on the AC transmission system, especially the magnetic bias generated by the DC component superimposed on the neutral point of the AC transformer. It causes magnetic saturation, and generates harmonics, oscillation and noise in the AC system, which affects the safe and stable operation of the AC system. By 2010, my country plans to build a number of DC transmission lines, especially ±800KV UHV DC transmission lines. Therefore, the impact of DC power transmission on AC power transmission will become more and more obvious, and the research on the DC current of the neutral point of the transformer will also arise from this.

At present, the main methods for suppressing the DC current at the neutral point of the transformer are: 1. The current is suppressed by connecting a resistor in series at the neutral point of the transformer. This method is simple and easy, but it cannot achieve the purpose of completely suppressing the DC current; The counter electromotive force applied and generated around the neutral point DC suppresses the neutral point DC. This method is complex, consumes more power, and cannot completely suppress the DC current.

Publication No. CN1625010A describes a "method for limiting the DC current at the neutral point of a transformer by the reverse current method" in the Chinese invention patent application specification. The technical solution is mainly: (1) select an independent Compensation grounding pole; (2) Connect the "ground" terminal of the DC generating device to an independent compensation grounding pole, and connect the "output" terminal of the DC generating device to the neutral point of the transformer ground; (3) Transformer neutral point DC current The detection device measures the DC current at the neutral point of the transformer to obtain the value and direction of the DC current at the neutral point; (4) The controller of the DC generator automatically activates the DC generator to inject a current opposite to the DC bias to the neutral point of the transformer. Although this method does not change the original wiring of the transformer, the effect of offsetting the DC bias is obvious, but the device is complicated, the power consumption is large, and the complete suppression of the DC current cannot be realized.

Contents of the invention

In order to overcome the above-mentioned defects in the prior art, the utility model provides a device which is simple in structure, safe and reliable, and can completely suppress the DC current at the neutral point of the transformer.

The technical scheme of the utility model is achieved in that:

A device for suppressing DC current at the neutral point of a transformer by capacitance method, comprising a capacitor loop, a state transfer switch loop and an overvoltage bypass loop, the three loops are connected in parallel and connected in series between the neutral point of the transformer and the grounding pole.

The capacitor circuit includes a capacitor, the state transfer switch circuit includes a state transfer switch, the bypass circuit includes a rectifier bridge, a thyristor, an overvoltage trigger unit, an inductor and a freewheeling diode, and the two arms of the rectifier bridge and the capacitor, The state transfer switch is connected in parallel, the rectifier bridge is connected in series with the thyristor and the inductor, the anode and gate of the thyristor are connected in parallel with an overvoltage trigger unit, and the two ends of the inductor are connected in parallel with a freewheeling diode.

The inductance is a switch closing inductance and a damping inductance connected in series.

An overvoltage gap discharge device is connected in parallel at both ends of the capacitor.

A zinc oxide overvoltage breakdown discharge circuit is connected in parallel at both ends of the capacitor.

The thyristor is two or more thyristors running in parallel.

The thyristor overvoltage trigger unit is two or more sets of trigger units connected in parallel.

The state transfer switch is a double switch or a single switch with multiple contacts.

The beneficial effects of the utility model are as follows:

1. Since the utility model uses a capacitor connected in series at the neutral point of the transformer with direct current to block the direct current of the neutral point, and is grounded with a small capacitive reactance for the alternating current, the device can completely suppress the direct current of the neutral point of the transformer and ensure Other equipment does not need to adjust parameters to cooperate with it, especially all relay protection settings do not need to be reset.

2. Since the utility model also utilizes the bypass thyristor and the state transfer switch to cooperate with the capacitor, it can quickly realize safe and reliable metallic grounding when the neutral point overvoltage occurs.

3. Since the utility model can add an overvoltage gap discharge device or a zinc oxide overvoltage breakdown discharge circuit at both ends of the capacitor, the reliability of the device is high.

4. Since the utility model adopts two sets of thyristor trigger circuits, the reliability of overvoltage bypass startup is improved.

5. Since the state transfer switch of the present invention can adopt double switches or single switch with multiple contacts, the closing fault current capability of the device can be improved.

Description of drawings

Fig. 1 is a block diagram of the utility model;

Fig. 2 is the circuit structure diagram of the utility model embodiment one;

Fig. 3 is the circuit structure diagram of the second embodiment of the utility model;

Fig. 4 is the circuit structure diagram of the utility model embodiment three;

Fig. 5 is a circuit structure diagram of the fourth embodiment of the utility model;

Fig. 6 is a circuit structure diagram of Embodiment 5 of the present utility model.

In the figure: 1-transformer neutral point 2-capacitor circuit 3-state transfer switch circuit 4-overvoltage fast switch circuit 5-DC blocking capacitor 6, 61, 62-transfer switch 7-rectifier bridge 8, 81, 82-thyristor 9, 91, 92-overvoltage trigger unit 10-inductance 101-switch closing inductance 102-damping inductance 11-freewheeling diode 12-overvoltage gap discharge device 13-varistor

Detailed ways

As shown in Figure 1, the capacitance method suppresses the neutral point DC device of the transformer, which includes three basic circuits: the capacitor circuit 2, the state conversion switch circuit 3, and the overvoltage fast bypass circuit 4. These three circuits are connected in parallel and one end is connected in series to the neutral point of the transformer point 1, and the other end is grounded. When the state transfer switch is in the breaking state, the neutral point of the transformer is grounded through the capacitor loop 2, thereby blocking the current of the neutral point of the transformer. The state transfer switch circuit 3 is composed of a switch that can be controlled to close/break, and its state determines the operating state of the device; when the switch is closed, all devices of the device are bypassed, and the neutral point of the transformer is grounded metallically through the switch contact. In the case of capacitor grounding, since the DC current at the neutral point 1 of the transformer is blocked by the capacitor, a voltage is formed across the capacitor. When the voltage across the capacitor is higher than a certain voltage limit—the voltage threshold, the overvoltage fast bypass circuit 4 is turned on, and at the same time, the state transfer switch is driven to close by the conduction current—entering the direct grounding state. Since the voltage threshold is much higher than the highest voltage formed across the capacitor in the DC system, only when a three-phase unbalanced fault occurs in the AC system, the voltage across the capacitor can exceed the voltage threshold.

As shown in Figure 2, it is the embodiment 1 of the device for suppressing the DC current at the neutral point of the transformer by the capacitance method. In the figure, the bypass circuit is composed of a rectifier bridge 7, a thyristor 8, an overvoltage trigger unit 9, an inductor 10 and a freewheeling diode 11. 8. The inductor 10 is connected in series, the two ends of the thyristor 8 are connected in parallel with an overvoltage trigger unit 9 , and the two ends of the inductor 10 are connected in parallel with a freewheeling diode 11 .

The bypass circuit 4 is connected in parallel with the DC blocking capacitor 5 and the state changeover switch 6 and then connected in series to the neutral point of the transformer. Wherein, the DC blocking capacitor 5 plays a role of suppressing the DC neutral point of the transformer, and at the same time grounds the AC neutral point with low impedance. When the state changeover switch 6 is in the closed state, it is in the directly grounded operating state; when the switch 6 is in the open state, it is in the capacitive grounded operating state. The rectifier bridge 7 plays the role of voltage polarity conversion. Regardless of the voltage polarity on both sides of the capacitor 5 and the state changeover switch 6, the rectifier bridge 7 ensures that the voltage applied to the thyristor 8 is connected positively with the anode and negatively connected with the cathode; when applied to the thyristor When the voltage on 8 is higher than the trigger threshold, the overvoltage trigger unit 9 is turned on and triggers the thyristor 8 to be turned on; when the thyristor 8 is turned on, the conduction current excites the inductor 10 and drives the state changeover switch 6 to trip and close; when the thyristor 8. When the current drops, the freewheeling diode 7 quickly releases the current of the inductor 10.

The inductor 10 in FIG. 2 may be a switch closing inductor 101 and a damping inductor 102 connected in series, as shown in FIGS. The switch 6 is closed; the damping inductor 102 is used to suppress the rising rate of the conduction current of the thyristor 8 and protect the thyristor 8 .

As shown in Fig. 3, it is the embodiment 2 of the device for suppressing the neutral point DC current of the transformer by the capacitance method. On the basis that other structures are the same as in Embodiment 1, in order to increase the reliability of the device, an overvoltage gap discharge device 12 or a zinc oxide overvoltage breakdown discharge circuit can be added at both ends of the capacitor 5, such as at both ends of the capacitor 5 A varistor 13 is connected in parallel.

As shown in Fig. 4, it is the embodiment 3 of the device for suppressing the neutral point DC current of the transformer by the capacitance method. When the device is applied to a system with a large neutral point fault current, on the basis that other structures are the same as in Embodiment 1, two or more thyristors 8 can be operated in parallel to improve the current capacity of the device. As shown in Fig. 4, it is the device for suppressing the neutral point DC current of the transformer when two thyristors 81 and 82 operate in parallel.

As shown in Fig. 5, it is the embodiment 4 of the device for suppressing the neutral point DC current of the transformer by the capacitance method. On the basis of other structures being the same as in Embodiment 1, a double set of thyristor trigger circuit is used to improve the reliability of overvoltage bypass startup.

As shown in Fig. 6, it is Embodiment 7 of the device for suppressing the neutral point DC current of the transformer by the capacitance method. On the basis of other structures being the same as those in Embodiment 1, the state transfer switch can adopt double switches or single switches with multiple contacts, so as to improve the ability of the device to close the fault current. As shown in Figure 6, it is the device for suppressing the neutral point DC current of the transformer when double switches are used.

Claims (8)

1, a kind of capacitance method suppresses the device of transformer neutral point direct current, is characterized in that, comprises capacitance circuit (2), state transfer switch circuit (3) and overvoltage bypass circuit (4), and described three circuits are connected in parallel Afterwards, it is connected in series between the neutral point (1) of the transformer and the grounding pole. 2. The device for suppressing the DC current at the neutral point of a transformer by the capacitance method according to claim 1, characterized in that, the capacitance loop (2) includes a capacitor (5), and the state transition switch loop (3) includes a state transition switch (6), the bypass loop (4) includes a rectifier bridge (7), a thyristor (8), an overvoltage trigger unit (9), an inductor (10) and a freewheeling diode (11), and the rectifier bridge (7) The two arms of the capacitor (5) and the state transfer switch (6) are connected in parallel, the rectifier bridge (7) is connected in series with the thyristor (8) and the inductor (10), and the anode and gate of the thyristor (8) are connected in parallel with an overvoltage trigger unit (9), freewheeling diodes (11) are connected in parallel at both ends of the inductance (10). 3. The device for suppressing the neutral point DC current of a transformer by capacitance method according to claim 2, characterized in that the inductance (10) is a switch closing inductance (101) and a damping inductance (102) connected in series. 4. The device for suppressing the DC current at the neutral point of a transformer by capacitance method according to claim 2, characterized in that an overvoltage gap discharge device (12) is connected in parallel at both ends of the capacitor (5). 5. The device for suppressing the DC current at the neutral point of a transformer by capacitance method according to claim 2 or 4, characterized in that a zinc oxide overvoltage breakdown discharge circuit is connected in parallel at both ends of the capacitor (5). 6. The device for suppressing DC current at the neutral point of a transformer by capacitance method according to claim 2, characterized in that the thyristor (8) is two or more thyristors operating in parallel. 7. The device for suppressing DC current at the neutral point of a transformer by capacitance method according to claim 2, characterized in that the thyristor overvoltage trigger unit (9) is two or more parallel trigger units (91, 92 ). 8. The device for suppressing the DC current at the neutral point of a transformer according to claim 2, characterized in that the state change switch (6) is a double switch or a single switch with multiple contacts.

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