CN106340854A - Device for inhibiting direct current of AC transformer neutral point - Google Patents

Abstract

The invention provides a device for suppressing the neutral point direct current of an AC transformer, which includes a DC blocking capacitor, a bypass protection circuit and a control circuit; one end of the DC blocking capacitor is connected to the neutral point of the AC transformer, and the other end of the DC blocking capacitor is grounded The bypass protection circuit includes n bypass protection units, which are sequentially connected in parallel at both ends of the DC blocking capacitor; the positive input terminal of the control circuit is connected to the neutral point of the AC transformer, the negative input terminal of the control circuit is grounded, and the The control signal output terminal is connected to the control terminal of the bypass protection circuit, and the output terminal of the control circuit is used to connect to an external AC system. Due to the long reclosing time of the mechanical switch in the prior art and the limited number of actions, the life of the whole device is short and the performance is poor; however, the device provided by the present invention eliminates the mechanical switch and improves the life and performance of the DC blocking device.

Description

A device for suppressing neutral point direct current of AC transformer

technical field

The invention belongs to the field of power systems, and in particular relates to a device for suppressing the neutral point direct current of an AC transformer. The device can inhibit the direct current from flowing into the grid through the neutral point of the AC transformer, thereby improving the safety and reliability of the AC transformer and the grid.

Background technique

With the vigorous development of high-voltage direct current transmission (HVDC), when the DC system operates in a single-pole ground loop or bipolar asymmetric loop mode, the DC current generated in the ground will flow into the neutral point of the AC transformer and cause the AC transformer to generate a DC bias. magnetic. The DC bias will cause the AC transformer to cause vibration, noise aggravation, temperature rise, reactive power consumption and harmonic increase and other hazards. At the same time, the increase in harmonics may cause the relay protection system of the power system to malfunction. Therefore, the research on the DC bias problem caused by the DC current flowing into the neutral point of the AC transformer has always been attached great importance. At present, the schemes for suppressing the DC bias of AC transformers are roughly divided into three categories. The first type is the reverse injection current method. The implementation scheme is to add a controllable voltage source or current source between the neutral point of the AC transformer and the earth, and provide reverse current in real time by dynamically measuring the DC current. However, due to the large project volume, the construction price and maintenance cost of this scheme are relatively high. The second type is the neutral point series resistance method. The implementation scheme is to add a setting resistor between the neutral point of the AC transformer and the earth. The principle of this scheme is simple and the cost is low. But there are also obvious disadvantages, for example, it will affect the relay protection system. When the grid structure changes, the resistance needs to be re-adjusted, and this solution can only reduce the DC current and cannot eliminate it. The third type is the neutral point series capacitance method. The implementation plan is to add a capacitor (referred to as a DC blocking capacitor) between the neutral point of the AC transformer and the earth. When the system fails, a huge short-circuit current flows through the neutral point, and a high transient voltage is generated on the DC blocking capacitor, which will damage the DC blocking capacitor. At the same time, raising the neutral point of the AC transformer may damage the AC transformer.

The three methods of suppressing DC current have their own advantages and disadvantages. Considering factors such as suppression effect, cost and maintenance comprehensively, the third category is that the neutral point series capacitance method has a wide range of applications.

HVDC single-pole operation generally occurs only during system commissioning, initial operation, or fault conditions, so the frequency of occurrence is not very high, so the DC blocking capacitor of the DC blocking device may not be used under normal circumstances. When it is detected that the neutral point of the AC transformer contains DC current, the DC blocking capacitor is automatically switched on. After switching on, if an asymmetric fault occurs in the AC system at the same time, the capacitor protection device in the DC blocking device will operate to isolate the DC blocking capacitor. After the fault current disappears, the bypass protection device exits, and the DC blocking capacitor is put into operation again. For fast protection, high-voltage and high-power power electronic switching devices are generally used as the first bypass element, while mechanical switches are used as the main and subsequent bypass elements, which is low in cost and high in reliability.

In the current large and medium-sized cities in China, rail transit such as subways has been vigorously developed due to the advantages of strong transportation capacity, fast and punctual, and most of the current subway traffic uses DC traction bilateral power supply. During the operation of the subway, the traction substation of the subway transports electric energy to the subway along the line through the overhead catenary, and the locomotive obtains electric energy through the sliding contact of the pantograph (collector shoe) and the overhead catenary (contact rail), and then contacts with the locomotive The running track returns to the subway traction substation. Since it is difficult to achieve complete insulation between the track and the ground, the traction load current does not return to the subway traction substation along the running track of the current locomotive, but a part of the current must leak into the ground at the place where the rail and the ground are poorly insulated, forming stray current. With the continuous increase of rail traffic in the city, the stray current will continue to become larger and more complex. When this part of the stray current flows into the neutral point of the AC transformer, it will cause frequent action of the DC blocking device, especially when the DC blocking device contains a mechanical switch, which will greatly reduce the life and performance of the mechanical switch.

Contents of the invention

Aiming at the defects of the prior art, the object of the present invention is to provide a device for suppressing the neutral point DC current of the AC transformer, which aims to solve the problem of short life of the entire device due to the long reclosing time of the mechanical switch itself and the limited number of actions in the prior art. The problem of poor performance.

The invention provides a device for suppressing the DC current at the neutral point of an AC transformer, comprising: a DC blocking capacitor, a bypass protection circuit and a control circuit; one end of the DC blocking capacitor is connected to the neutral point of the AC transformer, and the DC blocking capacitor The other end of the capacitor is grounded; the bypass protection circuit includes n bypass protection units, which are sequentially connected in parallel at both ends of the DC blocking capacitor; the positive input terminal of the control circuit is connected to the neutral point of the AC transformer, The negative input terminal of the control circuit is grounded, the control signal output terminal of the control circuit is connected to the control terminal of the bypass protection circuit, and the output terminal of the control circuit is used to connect to an external AC system, where n is A positive integer greater than or equal to 1.

Wherein, the AC transformer is grounded through the DC blocking capacitor, which can effectively suppress the DC current flowing into the AC transformer through the neutral point.

Furthermore, the bypass protection unit includes: a first thyristor SCR ₁₁ , a second thyristor SCR ₁₂ and a current-limiting reactance; one end of the current-limiting reactance is connected to the neutral point of the AC transformer, and the current-limiting reactance The other end of the thyristor is connected to the anode of the first thyristor SCR ₁₁ and the cathode of the second thyristor SCR ₁₂ ; the cathode of the first thyristor SCR ₁₁ and the anode of the second thyristor SCR ₁₂ are both grounded; the first thyristor SCR 12 is grounded; The control pole of a thyristor SCR ₁₁ and the control pole of the second thyristor SCR ₁₂ are both connected to the control signal output terminal of the control circuit.

Furthermore, the control circuit includes: a shutdown circuit and an opening circuit; the shutdown circuit is used to control the bypass protection unit to terminate the protection action, and the opening circuit is used to control the bypass protection unit to start the protection operation.

Furthermore, the bypass protection unit includes: a first transistor IGBT ₁₁ , a second transistor IGBT ₁₂ , a current-limiting reactance and a discharge resistor; one end of the current-limiting reactance is connected to the neutral point of the AC transformer, and the The other end of the current limiting reactance is connected to the drain of the first transistor IGBT ₁₁ , the source of the first transistor IGBT ₁₁ is connected to the source of the second transistor IGBT ₁₂ , and the source of the second transistor IGBT ₁₂ The drain is grounded, the gate of the first transistor IGBT ₁₁ and the gate of the second transistor IGBT ₁₂ are connected to an external control signal; one end of the discharge resistor is connected to the drain of the first transistor IGBT ₁₁ , the other end of the discharge resistor is connected to the drain of the second transistor IGBT ₁₂ .

Furthermore, the discharge resistor is a non-linear resistor, and the material of the discharge resistor is ZnO or SiC.

Through the above technical scheme conceived by the present invention, compared with the prior art, since the device of the present invention saves the mechanical switch (the mechanical switch itself has a long reclosing time and a limited number of actions), it can achieve the beneficial effects of improving the life and performance of the DC blocking device. effect; at the same time, it can also solve the DC bias problem of transformer neutral point caused by HVDC single pole operation.

Description of drawings

Fig. 1 is a structural schematic diagram of a device for suppressing neutral point DC current of an AC transformer provided by the first embodiment of the present invention;

2 is a schematic structural diagram of a control circuit of a device for suppressing neutral point DC current of an AC transformer provided in the first embodiment of the present invention;

Fig. 3 is a schematic structural diagram of the device for suppressing the neutral point direct current of the AC transformer provided by the second embodiment of the present invention.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

The present invention proposes a new AC transformer neutral point DC blocking scheme, which can not only adapt to the HVDC single-pole operation condition, but also adapt to the new situation, and can protect the AC transformer more effectively. The present invention proposes a set of novel isolating devices with higher feasibility for urban areas with a large number of rail traffic (more stray current) such as subways (resulting in frequent actions of the DC isolating device), thereby reducing the neutral point of the AC transformer due to DC bias. Hazards caused by magnetism. The device is composed of a DC blocking capacitor, a bypass protection unit and a control circuit, and two different topological structures are proposed for the capacitor bypass protection unit. The two structures have their own advantages and disadvantages, and a more suitable topology should be selected on different occasions according to the actual situation.

The present invention is mainly aimed at a new type of AC transformer neutral point DC current suppression device designed and proposed in cities where there are a large number of rail transits (more stray currents) such as subways, which can suppress the flow into the neutral point of the AC transformer generated by the DC system. DC current at the point, thereby eliminating the DC bias damage to the AC transformer. It is especially suitable for areas where the AC transformer neutral point blocking device needs frequent and fast actions due to large and complex stray current values, such as areas with developed rail transit. The device is also applicable to AC transformer neutral point isolation due to DC current problems caused by high-voltage DC transmission and geomagnetism.

The scheme adopted in the present invention to suppress the DC bias at the neutral point of the AC transformer is the neutral point series capacitance method, and a bypass protection circuit is added to protect the DC blocking capacitor from being broken down by the transient high voltage and avoid the neutral point potential of the AC transformer. Threat to its safe operation after elevated. The bypass protection device chooses the following two structures.

The first structure: consists of a bidirectional thyristor solid-state switch and a control system (for triggering and turning off the thyristor) composed of a current-limiting reactance and a reverse resistance thyristor (SCR). The shutdown circuit can be a simple AC electronic switch. When the main circuit returns to the normal state and the SCR needs to be shut down to block the DC current state, since the DC current may exist in the main circuit, the SCR cannot be automatically shut down, so an auxiliary shutdown circuit is required. When the control system sends a signal to the shutdown circuit to close the AC switch and remove the SCR trigger pulse at the same time, the superposition of the AC current and the possible DC current in the main circuit will generate a zero-crossing current. When the current crosses zero, the SCR can be automatically turned off. After the SCR is turned off, the control system sends a signal to the shutdown circuit to disconnect the AC switch, and the SCR shutdown process is completed. The shutdown circuit requires small capacity, low cost, simple and reliable, long service life and can operate frequently.

The second structure: a switch and control circuit (triggering and turning off the IGBT) composed of a current-limiting reactance, a discharge resistor and an insulated gate bipolar transistor (IGBT). Since the IGBT is a fully-controlled power electronic device, it is only necessary to compare the DC current value flowing into the neutral point of the AC transformer with the DC current setting value to control the IGBT gate voltage to be the turn-on voltage or the turn-off voltage, thereby controlling the effective IGBT. off and on.

In comparison, when the first structure is adopted, the conduction capability of the bypass protection circuit is stronger and the cost is lower, but since the SCR cannot be directly turned off, an additional control shutdown circuit is required; when the second structure is adopted In the structure, the DC blocking device is more controllable, but due to the characteristics of the IGBT, the conduction current capability of the bypass protection circuit is not as good as the first structure, and the cost is higher than the first structure.

The present invention will be described in further detail below in conjunction with the accompanying drawings. The present invention proposes a novel AC transformer neutral point direct current suppressing device, and the specific topological structures are shown in Fig. 1 and Fig. 3 .

In Fig. 1 (the first structure), the switch K1 is an AC transformer neutral point directly grounded knife switch, the DC blocking device 1 is composed of a DC blocking capacitor C1, and n groups of bypass protection units connected in parallel (the bypass protection unit is composed of a bidirectional thyristor SCR ₁₁ , SCR ₁₂ in parallel and current-limiting reactance, when the alternating current is on the positive half axis, the alternating current flows into the earth through SCR ₁₁ ... SCR _n1 , when the alternating current is on the negative half axis, the alternating current flows into the earth through SCR ₁₂ ... SCR _n2 ) and control system. The shutdown circuit of the control system is externally connected with a current-limiting reactance and connected to the AC system. The current-limiting reactance has two functions: one is to limit the rapid rise and fall of the current. When the bypass protection unit is turned on, the current-limiting reactance limits the di/dt, thereby protecting the circuit devices from being damaged or reducing their life due to instantaneous over-current; When the current value in the circuit is too large, the current-limiting reactance in the bypass protection unit can act as a current equalizer to protect the circuit components. The bypass protection unit (the control system provides its turn-on and turn-off signals) mainly provides protection for the DC blocking capacitor C1 when it is overcurrent or overvoltage, and prevents the DC blocking capacitor from being damaged due to transient high voltage.

The parallel connection of n groups of bypass protection units can effectively reduce the current flowing through each protection unit, thereby reducing the requirements for protection devices, effectively reducing device costs and improving feasibility.

Figure 2 shows the structure of the control system, including two parts, the opening signal and the closing circuit. In the figure, port 1 and port 2 are connected to both ends of the DC blocking capacitor, and port 3 is connected to the gate (G) poles of all SCRs. The shut-off circuit is connected to the AC system through the current-limiting reactance, so as to shut down the SCR after the protection ends.

In order to further illustrate the bypass protection unit in the embodiment of the present invention, its working principle is described in detail in conjunction with Fig. 1 and Fig. 2 as follows: When the bypass protection unit starts the protection action, the opening signal in the control system outputs a trigger pulse to the SCR _11. SCR ₁₂ ... SCR _n1 and SCR _n2 gates make all SCRs conduct, and the fault current flows into the ground through the bypass protection unit, thereby protecting the DC blocking capacitor. When the bypass protection unit stops the protection action, the shut-off circuit in the control system sends two kinds of signals to complete the shut-off action. The first signal makes the opening signal interrupt its output trigger pulse; the second signal makes the AC switch close, so that the AC system is incorporated into the protection device, and then all SCRs are quickly turned off (that is, the bypass protection unit stops the protection action).

In Fig. 3 (the second structure), the switch K1 is an AC transformer neutral point directly grounded knife switch, and the DC blocking device 2 consists of a DC blocking capacitor C1 and n groups of bypass protection units connected in parallel (the bypass protection unit consists of a discharge resistor and a positive Insulated gate bipolar transistors IGBT ₁₁ , IGBT ₁₂ connected in reverse phase and current-limiting reactance are connected in series, and IGBT ₁₁ , IGBT ₁₂ connected in positive and negative phases can realize complete controllability of current and voltage flow, improving the reliability and reliability of the device. safety) and control system composition. The functions of the bypass protection unit and the current-limiting reactance are the same as those of the first structure. The discharge resistor should be a non-linear resistor, and the material is generally ZnO or SiC. The main function is to prevent the IGBT from being damaged due to the overvoltage generated at the moment the IGBT is turned on or off, that is, to control the voltage at both ends of the IGBT not to exceed the maximum voltage it can withstand. value. Since the IGBT is a fully-controlled power electronic device, the control circuit can directly control its effective turn-on and turn-off, so there is no need to separately design its turn-off circuit.

In order to further illustrate the bypass protection unit in the embodiment of the present invention, its working principle is described in detail in conjunction with FIG. 3 as follows: when the bypass protection unit starts a protection action, the control signal outputs a turn-on signal (high level) to the IGBT ₁₁ , IGBT ₁₂ ... IGBT _n1 , IGBT _n2 gates make all IGBTs turn on, and the fault current flows into the ground through the bypass protection unit, thereby protecting the DC blocking capacitor. When the bypass protection unit stops the protection action, the control signal outputs a shutdown signal (low level) to all IGBT gates to turn off the IGBTs (that is, the bypass protection unit stops the protection action).

In the embodiment of the present invention, there are four working states of the device for suppressing the neutral point DC current of the AC transformer: (1) no fault state (2) no fault state but the DC current exceeds the set value state (3) fault state (4) ) overhaul status. Specifically include:

(1) When the AC transformer is in a fault-free state, the switch K1 is closed, the blocking capacitor C1 and the bypass protection unit are in a non-operating state, the neutral point of the AC transformer is directly grounded, and the AC transformer and the power system operate normally.

(2) When the AC transformer is in the state of no fault but the DC current exceeds the set value, the switch K1 is turned off, the DC blocking capacitor is put into operation, the control circuit controls the bypass protection unit to be in the non-operating state, and the neutral point of the AC transformer passes through the DC blocking capacitor Ground to suppress DC current flowing into the AC transformer through the neutral point.

(3) When the AC transformer is in a fault state, the protection system detects overcurrent or overvoltage, the switch K1 is turned off, and the control system controls the bypass protection unit to be in the running state. At this time, the DC blocking capacitor is short-circuited, and the fault current passes The circuit protection unit flows into the ground, thereby protecting the DC blocking capacitor. After the fault current disappears, supply power through the control system to stop the protection action of the bypass protection unit, and then adjust the operating state of the device to state (1) or state (2) according to the real-time detection results.

(4) When the AC transformer is in the maintenance state, the switch K1 is closed, and the neutral point of the AC transformer is directly grounded. The DC blocking capacitor and the bypass protection unit are in the non-operating state. After the DC blocking capacitor is discharged, the device should be overhauled.

The invention proposes a new topological structure aiming at the DC bias problem of the neutral point of the AC transformer. Because the device omits a mechanical switch, the DC blocking device can operate frequently and rapidly, and at the same time, the life and performance of the DC blocking device can be improved.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (5)

1. a kind of device of suppression AC transformer neutral point direct current is it is characterised in that include: block capacitor, bypass Protection circuit and control circuit；

One end of described block capacitor connects to AC transformer neutral point, the other end ground connection of described block capacitor；

Described bypass protection circuit includes n bypass protection unit, is connected in parallel on the two ends of described block capacitor successively；

The positive input terminal of described control circuit connects to described AC transformer neutral point, the negative input termination of described control circuit Ground, the control signal outfan of described control circuit connects to the control end of described bypass protection circuit, described control circuit Outfan is used for connecting the AC system of outside.

2. device as claimed in claim 1 is it is characterised in that described bypass protection unit includes: the first IGCT scr₁₁, the Two IGCT scr₁₂And current-limiting reactor；

One end of described current-limiting reactor connects to described AC transformer neutral point, and the other end of described current-limiting reactor connects to institute State the first IGCT scr₁₁Anode and described second IGCT scr₁₂Negative electrode；Described first IGCT scr₁₁Negative electrode and Described second IGCT scr₁₂Anode be all grounded；Described first IGCT scr₁₁Control pole and described second IGCT scr₁₂Control pole be connected to the control signal outfan of described control circuit.

3. device as claimed in claim 1 or 2 is it is characterised in that described control circuit includes: breaking circuit and open electricity Road；Described breaking circuit is used for controlling bypass protection unit to terminate protection act, and described circuit of opening is used for controlling bypass protection Unit starts protection act.

4. device as claimed in claim 1 is it is characterised in that described bypass protection unit includes: the first transistor igbt₁₁、 Transistor seconds igbt₁₂, current-limiting reactor and discharge resistance；

One end of described current-limiting reactor connects to described AC transformer neutral point, and the other end of described current-limiting reactor connects to institute State the first transistor igbt₁₁Drain electrode, described the first transistor igbt₁₁Source electrode connect to described transistor seconds igbt₁₂'s Source electrode, described transistor seconds igbt₁₂Grounded drain, described the first transistor igbt₁₁Grid and described transistor seconds igbt₁₂Grid be all connected with outside control signal；

One end of described discharge resistance connects to described the first transistor igbt₁₁Drain electrode, the other end of described discharge resistance is even It is connected to described transistor seconds igbt₁₂Drain electrode.

5. device as claimed in claim 4 it is characterised in that described discharge resistance be nonlinear resistance, described discharge resistance Material be zno or sic.