CN106786470B - Flexible grounding system for neutral point of power grid - Google Patents

Flexible grounding system for neutral point of power grid Download PDF

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CN106786470B
CN106786470B CN201510809388.4A CN201510809388A CN106786470B CN 106786470 B CN106786470 B CN 106786470B CN 201510809388 A CN201510809388 A CN 201510809388A CN 106786470 B CN106786470 B CN 106786470B
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grounding
current
transformer
electronic device
power grid
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CN106786470A (en
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时振堂
杨明乾
黄长明
杜红勇
李琼
孙进
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China Petroleum and Chemical Corp
Sinopec Fushun Research Institute of Petroleum and Petrochemicals
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China Petroleum and Chemical Corp
Sinopec Fushun Research Institute of Petroleum and Petrochemicals
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/08—Limitation or suppression of earth fault currents, e.g. Petersen coil
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
    • H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
    • H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Emergency Protection Circuit Devices (AREA)

Abstract

The invention discloses a flexible grounding system for a neutral point of a power grid, which comprises: the power electronic device comprises a grounding transformer, a power electronic device, a controller and a rectifying circuit; the primary winding of the grounding transformer is connected to the power grid bus and the neutral point is grounded, the secondary winding of the grounding transformer is in an open triangular structure, the secondary winding is connected to the output end of the power electronic device, the controller is connected to the power grid and the power electronic device, the controller measures the voltage and the current of the power grid and the voltage and the current of the power electronic device and controls the power electronic device to input the current to the secondary winding of the grounding transformer, and the rectifying circuit provides direct current for the power electronic device. The system can realize various power grid neutral point grounding modes and can realize on-line switching, and the problem that power electronic devices bear high voltage is solved.

Description

Flexible grounding system for neutral point of power grid
Technical Field
The invention relates to the technical field of power systems, in particular to a power grid neutral point flexible grounding system.
Background
The electrical connection between the neutral point of the three-phase ac power system, i.e. the neutral points of the star-connected transformer and motor, and the ground is called as the neutral point grounding of the power grid. The neutral point grounding mode relates to the safety reliability and the economical efficiency of a power grid, and directly influences the selection of the insulation level of equipment, the overvoltage level of a system, the relay protection mode, communication interference and the like.
The 110kV and above power grid generally adopts a large-current grounding mode, namely a neutral point effective grounding mode, so that the potential of a neutral point is fixed at the ground potential, and when a single-phase grounding fault occurs, the non-fault phase voltage is increased and cannot exceed 1.4 times of the running phase voltage; the transient overvoltage level is also low; the fault current is very large, the relay protection can act on tripping rapidly to remove the fault, and the time for bearing the overvoltage by the system equipment is short. In actual operation, in order to reduce single-phase grounding current, a part of transformers adopt a non-grounding mode.
A3-66 kV power distribution network generally adopts a low-current grounding mode, a neutral point is not grounded or passes through a three-phase system of an arc suppression coil and a high-impedance grounding mode, when one phase has a grounding fault, the grounding fault current is usually much smaller than the load current due to the fact that a short circuit loop cannot be formed, and the neutral point is in a non-effective grounding mode. The power grid with the mode allows short-time operation with single-phase earth fault, and the power supply quality is good. Generally thought in China
Figure BDA0000852199270000011
The system belongs to a small-current grounding system power grid, and is generally held in the United states and Western Europe
Figure BDA0000852199270000012
The system of (2) is classified as a low current grounding system.
The neutral point low current grounding mode can be mainly divided into the following three modes: not grounded, grounded through an arc suppression coil and grounded through a resistor. If the single-phase grounding capacitance current of a 35kV power grid is larger than 10A and the grounding capacitance current of a 3 kV-10 kV power grid is larger than 30A, a neutral point is required to be grounded through an arc suppression coil. With the development of power supply networks, especially the increasing number of users adopting cable lines, the single-phase grounding capacitance current of the system is increased continuously, so that the possibility that a single-phase grounding fault in the power grid is developed into an accident is increased. In order to avoid the enlargement of single-phase earth faults, the number of lines with neutral points grounded through resistors and relay protection for rapidly removing faults is increased, the urban network planning and design guide (implementation) of China stipulates 35kV and 10kV in item 59, and when a cable line is long and the system capacitance current is large, a resistor mode can be adopted.
The neutral point is grounded through a resistor, a resistor is connected in series between the neutral point of a power grid and the ground, when a single-phase grounding fault occurs, a line grounding point generates large zero-sequence current, zero-sequence protection acts on tripping, a fault line and a fault point are cut off, and the normal operation state of the system is quickly recovered. The neutral point resistance grounding system generally adopts a grounding transformer with a star-shaped or zigzag star-shaped connection to be connected into a power grid, a resistor is connected into a neutral point of the grounding transformer, the resistor needs to bear voltage and current of a grounding fault for a short time and has corresponding capacity and a heat dissipation element, and a resistor box needs to be specially designed and manufactured. Because of the low probability of transient cable faults, power grids based on cable lines generally tend to employ a neutral point connected to ground via a resistor to quickly remove the fault. Because the fault resistance of a single-phase earth fault and the impedance of a line have large differences according to the fault property and the distance, the neutral point may have protection rejection through a resistance grounding mode.
The neutral point is connected to the electric network through the grounding transformer of star or zigzag star connection, and the arc suppression coil (i.e. variable single-phase inductor) is connected between the neutral point of the grounding transformer and the grounding network. The basic principle is as follows: when the power grid normally operates, the neutral point of the grounding transformer has zero ground voltage, and the current of the arc suppression coil is zero; when a single-phase earth fault occurs in the power grid, the non-grounded phase voltage rises, the neutral point of the grounding transformer rises to the ground voltage (the metallic grounding can rise to the phase voltage), and the arc suppression coil generates inductive current. The sum of the inductance current flowing through the arc suppression coil and the capacitance current distributed oppositely without the ground is the current flowing through the ground point, and the phase difference of the current on the inductance capacitor is 180 degrees, so that the current and the capacitance are mutually compensated. The proportion of transient faults of the overhead line is high, and the power grid mainly adopting the overhead line generally adopts a mode that a neutral point is not grounded or is grounded through an arc suppression coil. However, since both the arc suppression coil and the grounding transformer have active losses, it is practically impossible to completely compensate for the capacitive current. In order to avoid resonance, overcompensation is often used in power distribution networks.
Reactive compensation equipment is usually required to be configured in the distribution network to reduce the loss of the power network, and passive capacitor banks/static reactive compensation or active static var generators are mostly adopted. Harmonics in power distribution networks are typically filtered out using passive or active filters. Theoretically, reactive compensation, harmonic elimination and earth capacitance current compensation are reactive compensation technologies, and the existing power electronic technology can be realized and can be used for compensation according to reactive power uniformly.
Disclosure of Invention
Technical problem to be solved
The invention aims to solve the problems of neutral point flexible grounding and reactive compensation of a power grid of a low-current grounding system, realize a neutral point flexible grounding method which has multiple grounding modes and can be switched on line, and simultaneously solve the technical problem that a power electronic device controls a high-voltage power grid to bear high voltage through a low-voltage winding of a transformer.
(II) technical scheme
In order to solve the above technical problem, the present invention provides a flexible grounding system for a neutral point of an electric power grid, comprising: the power electronic device comprises a grounding transformer, a power electronic device, a controller and a rectifying circuit;
wherein a primary winding of the grounding transformer is connected to the power grid bus and a neutral point is grounded, a secondary winding of the grounding transformer is arranged in an open-delta structure, the secondary winding is connected to an output end of the power electronic device, the controller is connected to the power grid and the power electronic device, the controller measures voltage and current of the power grid and voltage and current of the power electronic device and controls the power electronic device to input current to the secondary winding of the grounding transformer; the input end of the rectification circuit is connected into the power grid bus, the output end of the rectification circuit is connected into the input end of the power electronic device, the rectification circuit is further connected to the controller, and the controller controls the rectification circuit to provide direct current for the power electronic device.
The iron core of the grounding transformer forms a zero-sequence magnetic flux closed loop, and any one of a three-phase five-column transformer, a shell type transformer or three single-phase transformer groups is adopted; the primary winding of the grounding transformer adopts a star connection method, the phase end is connected to the power grid bus and the neutral point for grounding, and the secondary winding of the grounding transformer adopts an open delta connection method and is connected to the output end of the power electronic device.
The system further comprises a rectifier transformer, a primary winding of the rectifier transformer is connected to the power grid bus, a secondary winding of the rectifier transformer is connected to the input end of the rectifier circuit, the controller is connected to the rectifier transformer, and the controller controls the rectifier transformer to provide alternating current for the rectifier circuit.
Wherein the power electronic device is an inverter, and the inverter converts the input direct current into alternating current under the control of the controller and inputs the alternating current into the grounding transformer.
The power electronic device generates alternating current under the control of the controller and inputs the alternating current into the grounding transformer, the primary winding of the grounding transformer injects current into a power grid and the ground, and the equivalent power grid is connected with a grounding system through a neutral point.
When a single-phase ground fault of the power grid occurs, the power electronic device collects the voltage and the current of the power grid for the controller, controls the power electronic device to generate alternating current to be input to the secondary winding of the grounding transformer, and injects capacitive current between a non-fault phase and the ground through the primary winding of the grounding transformer to equivalently compensate the capacitance current to the ground of the non-fault phase of the power grid, so that the ground current of a fault point is reduced, and the method is equivalent to a mode that a neutral point is grounded through an arc suppression coil.
When a single-phase grounding fault of a power grid occurs, the controller collects bus voltage of the power grid and current of a transformer branch circuit, controls the power electronic device to generate alternating current to be input into the secondary winding of the grounding transformer, and injects the alternating current into a fault phase circuit through the primary winding of the grounding transformer to form fault point grounding current, so that zero sequence current of a certain numerical value is generated, zero sequence protection action of a fault line is started, a fault line breaker trips to cut off the fault line, and an equivalent neutral point is grounded through a resistor.
When a single-phase grounding fault of a power grid occurs, the controller controls the power electronic device not to generate alternating current to be input into the grounding transformer, does not influence the running state of the power distribution network, and simulates a non-grounding mode of a neutral point.
A power grid neutral point flexible grounding system is provided, which is characterized by comprising: a grounding transformer, a power electronic device and a controller;
(III) advantageous effects
According to the power grid neutral point flexible grounding system provided by the invention, the power electronic device is connected to the secondary winding of the transformer in a triangular outlet mode, so that the power electronic device is isolated from primary equipment of a power grid system, the power grid short circuit caused by power electronic devices is avoided, and the number and cost of the devices are reduced; meanwhile, multiple secondary windings and power electronic devices can be used for realizing multiple functions, so that the power is improved; if an external direct-current power supply is arranged, the compensation effect is obviously improved, and the compensation error caused by active loss of a transformer and a power electronic device is overcome; in addition, the injected zero sequence current can be constant current in a certain range, the accuracy of protection action is improved, and protection configuration is facilitated.
Drawings
Fig. 1 is a circuit configuration diagram of a power grid neutral point flexible grounding system of the present invention;
FIG. 2 is a schematic current diagram of an embodiment of the power grid neutral point flexible grounding system of the present invention;
FIG. 3 is a graph of compensated capacitance-to-ground current phasors for an embodiment of the present invention;
FIG. 4 is a diagram of providing zero sequence current phasors according to an embodiment of the present invention;
FIG. 5 is a schematic circuit diagram of an external DC power supply according to another embodiment of the present invention;
FIG. 6 is a circuit diagram of an embodiment of a current compensation multiplexing arrangement of the present invention.
Detailed Description
The following detailed description of embodiments of the invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
Fig. 1 is a circuit configuration diagram of a power grid neutral point flexible grounding system of the present invention.
Referring to fig. 1, the flexible grounding compensation system for a neutral point of a power grid according to an embodiment of the present invention specifically includes: a grounding transformer 10, a power electronic device 20, a controller 30, and a rectifying circuit 40;
wherein, the primary winding of the grounding transformer 10 is respectively connected to each phase of the power grid bus 100, the secondary winding of the grounding transformer 10 is arranged in an open triangle structure, the outlet of the secondary winding is connected to the output end of the power electronic device 20, the controller 30 is connected to each phase of the power grid bus 100 and the power electronic device 20, the controller 30 measures the voltage and current of the power grid bus 100 and the voltage and current of each loop of the power electronic device 20, and controls the power electronic device 20 to input the current to the secondary winding of the grounding transformer 10. The input end of the rectifying circuit 40 is connected to the power grid bus 100, the output end is connected to the input end of the power electronic device 20, the rectifying circuit 40 is further connected to the controller 30, and the controller 30 controls the rectifying circuit to provide direct current for the power electronic device.
In this embodiment, the rectifier circuit 40 is used to convert ac power of the power grid bus into dc power to provide dc power to the power electronic device, and the controller adjusts the input of the rectifier circuit, thereby reducing the external power supply and improving the quality of the incoming current.
In the above embodiment, the grounding transformer 10 may adopt an iron core with a zero-sequence magnetic flux closed loop, and includes a transformer bank composed of a three-phase five-limb transformer, a shell type transformer or three single-phase transformers, a primary winding of the grounding transformer is star-connected and connected to the power grid bus, a neutral point is grounded, a secondary winding of the grounding transformer is open-delta-connected, and an outlet is connected to the output end of the power electronic device.
In addition, the system of the present invention further includes a dc bus 60, and the dc bus 60 provides a dc power supply for the power electronic device.
In addition, the power electronic device according to the present embodiment generates an alternating current under the control of the controller and inputs the alternating current to the grounding transformer.
In the embodiment of the invention, the voltage of a power grid bus is measured by using a potential transformer PT, the current of a transformer branch is measured by using a current transformer CT, and then a controller acquires the voltage value of the potential transformer and the current value of the current transformer to control the power electronic device and the rectifying circuit.
According to the invention, the power electronic device is connected to the power grid through the grounding transformer, so that the protection of the device is convenient to configure, and the reliability of the power grid is improved.
In addition, the power electronic device of the embodiment can optimally select power electronic devices according to the parameters of the secondary winding of the grounding transformer, reduce the number of the devices or the requirement on voltage resistance, improve the economy and reliability and reduce the cost.
In the above embodiments, the flexible grounding system for the power grid neutral point can inject controllable current into the power grid and the ground, which is equivalent to the power grid neutral point being operated through the arc suppression coil, through the resistor or in a non-grounding mode, and can be optionally operated in one mode or switched to be operated online. And the power electronic device injects controllable current to the secondary winding of the grounding transformer under the control of the controller, and the controllable current is injected into the power grid and the ground after being converted by the primary winding of the grounding transformer.
In addition, when the power grid normally operates, the controller controls the power electronic device to inject controllable reactive current or harmonic current into the secondary winding of the grounding transformer, and the controllable reactive current or the harmonic current is injected into the power grid through the grounding transformer, so that the function of reactive compensation or harmonic filtering is realized.
In one embodiment, when a single-phase ground fault of a power grid occurs, the power electronic device collects the bus voltage of the power grid and the current of the branch circuit of the transformer for the controller, controls the power electronic device to generate alternating current to be input into the grounding transformer, compensates the non-fault relative ground capacitance current of the power grid through the primary winding of the grounding transformer, reduces the fault point ground current, and simulates the grounding mode of a neutral point through an arc suppression coil.
In another embodiment, when a single-phase ground fault of the power grid occurs, the controller collects the voltage of a power grid bus and the current of the transformer branch, controls the power electronic device to generate alternating current to be input into the grounding transformer, injects the alternating current into a fault line through a primary winding of the grounding transformer, enables the ground current of a fault point to reach a certain value, starts a zero-sequence protection action of the fault line, trips a breaker of the fault line, cuts off the fault line, and simulates a neutral point to be grounded through a resistor.
In yet another embodiment, when a single-phase ground fault of the power grid occurs, the controller controls the power electronic device not to generate alternating current to be input to the grounding transformer, not to influence the operation state, and simulates a non-grounding mode of a neutral point.
In the above embodiment, as shown in fig. 2, when the a-phase is grounded due to a fault, the power electronic device injects a current into the secondary winding of the grounding transformer
Figure BDA0000852199270000081
Then three phases are induced in the primary winding
Figure BDA0000852199270000082
The zero sequence current is grounded through a neutral point of the grounding transformer to form a power grid ground loop, and the current injected into a fault point is
Figure BDA0000852199270000083
In the above embodiment, as shown in FIG. 3, if the current injected into the secondary winding is
Figure BDA0000852199270000084
Lags behind the phase voltage A by 90 degrees, has a phase difference of 180 degrees with the synthesized capacitance-to-ground current, and cancels the capacitance current of a fault point by adjusting the amplitude of the injected current, namely, the compensation capacitance electricity is realizedThe current effect is equivalent to that the neutral point operates in a mode of being grounded through an arc suppression coil. At this time, I should bea＝kIc0，Ic0Is a single-phase capacitance-to-ground current.
In the above embodiment, as shown in FIG. 4, if the current injected into the secondary winding is
Figure BDA0000852199270000086
The phase of the current is in phase with the A-phase voltage, the phase difference between the current and the ground capacitance is 90 degrees, the current of the ground capacitance is not counteracted, and the resistive current is injected into a fault point
Figure BDA0000852199270000085
The injected current is controlled, namely the zero sequence current passing through the fault line can be controlled, so that the zero sequence protection tripping of the fault line is started, the fault line is cut off, and the operation is equivalent to that the neutral point is grounded through a resistor. Different from the grounding of a neutral point resistor, the active injection current of the power electronic inversion unit can operate in a constant current source mode and is not limited by phase voltage. Meanwhile, the voltage drop is the sum of the impedance of a fault line and the voltage drop of the grounding resistance of a fault point, and the power consumption is far lower than that of a neutral point in a resistance grounding mode.
In addition, in another embodiment of externally connected dc power, as shown in fig. 5, the system of the present invention adds an external rectifier transformer 50 for regulating the input voltage of the rectifier circuit, so as to overcome the active loss of the transformer and the power electronic device and improve the quality of the injected current. The rectifier transformer 50 has a primary winding connected to the power grid bus 100, a secondary winding connected to an input of the rectifier circuit 40, and the controller 30 is connected to the rectifier transformer 50, and the controller 30 controls the rectifier transformer 50 to supply the rectifier circuit 40 with ac power. In the above embodiment, since the power electronic device consumes a small amount of energy, it can be realized by a device having a small capacity regardless of the dc power supply or the rectifier circuit.
In the multiple embodiment of the present invention, as shown in fig. 6, the grounding transformer is provided with a plurality of secondary windings and connected with a plurality of sets of power electronic devices, and shares the primary windings and the dc bus, so that the current is changed into multiple times, thereby realizing the multiple of the devices and conveniently realizing the capacity expansion.
The above is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the technical principle of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims (4)

1. A power grid neutral point flexible grounding system, comprising: the power electronic device comprises a grounding transformer, a power electronic device, a controller and a rectifying circuit;
wherein a primary winding of the grounding transformer is connected to the power grid bus and a neutral point is grounded, a secondary winding of the grounding transformer is arranged into an open triangular structure, the secondary winding is connected to an output end of the power electronic device, and the controller is connected to the power grid and the power electronic device; the input end of the rectification circuit is connected into the power grid bus, the output end of the rectification circuit is connected into the input end of the power electronic device, the rectification circuit is also connected to the controller, and the controller controls the rectification circuit to provide direct current for the power electronic device;
the controller measures the voltage and the current of the power grid and the voltage and the current of the power electronic device, and controls the power electronic device to input controllable current to the secondary winding of the grounding transformer, so that when a single-phase grounding fault of the power grid occurs, the neutral point flexible grounding mode is switched online, and the method comprises the following steps:
the equivalent neutral point is through arc suppression coil ground connection mode, includes: the controller adjusts the amplitude of alternating current input to the grounding transformer by the power electronic device, and compensates the non-fault relative ground capacitance current of the power grid through the primary winding of the grounding transformer, so that the fault point ground current is reduced;
the equivalent neutral point is grounded through a resistor, and the equivalent neutral point grounding method comprises the following steps: the controller controls the power electronic device to generate alternating current to be input into the grounding transformer, and the alternating current is injected into a fault line through a primary winding of the grounding transformer, so that the grounding current of a fault point reaches a preset value to start a zero sequence protection action of the fault line, and further, a breaker of the fault line is tripped to cut off the fault line; the active injection current of the power electronic device can run in a constant current source mode and is not limited by phase voltage;
the equivalent neutral point ungrounded mode comprises the following steps: the controller controls the power electronic device not to generate alternating current to be input into the grounding transformer, and the running state is not influenced;
when the power grid normally operates, the controller controls the power electronic device to inject controllable reactive current or harmonic current into the secondary winding of the grounding transformer, and the controllable reactive current or the harmonic current is injected into the power grid through the grounding transformer, so that the function of reactive compensation or harmonic filtering is realized.
2. The power grid neutral point flexible grounding system according to claim 1, wherein the iron core of the grounding transformer forms a zero sequence magnetic flux closed loop, and any one of a three-phase five-column transformer, a shell type transformer or three single-phase transformer groups is adopted; the primary winding of the grounding transformer adopts a star connection method, the phase end is connected to the power grid bus and the neutral point for grounding, and the secondary winding of the grounding transformer adopts an open delta connection method and is connected to the output end of the power electronic device.
3. The power grid neutral point flexible grounding system of claim 1, wherein the system further comprises a rectifier transformer, a primary winding of the rectifier transformer is connected to the power grid bus, a secondary winding is connected to an input of the rectifier circuit, and the controller is connected to the rectifier transformer, the controller controlling the rectifier transformer to provide alternating current to the rectifier circuit.
4. The power grid neutral point flexible grounding system of claim 1, wherein the power electronics device is an inverter that converts an input direct current to an alternating current input to the grounding transformer under control of the controller.
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