CN111371093B

CN111371093B - Non-isolated hybrid flexible loop closing device and control method

Info

Publication number: CN111371093B
Application number: CN202010303221.1A
Authority: CN
Inventors: 葛雪峰; 袁宇波; 史明明; 周琦; 袁晓冬; 杨景刚; 陈舒; 陈久林
Original assignee: State Grid Corp of China SGCC; Electric Power Research Institute of State Grid Jiangsu Electric Power Co Ltd
Current assignee: State Grid Corp of China SGCC; Electric Power Research Institute of State Grid Jiangsu Electric Power Co Ltd
Priority date: 2020-04-17
Filing date: 2020-04-17
Publication date: 2022-08-19
Anticipated expiration: 2040-04-17
Also published as: CN111371093A

Abstract

The invention discloses a non-isolated hybrid flexible loop closing device and a control method thereof, wherein the non-isolated hybrid flexible loop closing device comprises a first voltage source type converter, a second voltage source type converter, a first switch, a second switch, a third switch, a first direct current link capacitor, a second direct current link capacitor, a super capacitor and a fourth switch; the first voltage source type converter is connected with the second voltage source type converter in parallel, and the output side of the first voltage source type converter is connected with the first circuit breaker and the second circuit breaker; the first switch and the second switch are connected in series between the first voltage source type converter and the second voltage source type converter; the third switch is connected with the super capacitor in series and then is connected with the direct current sides of the first voltage source type converter and the second voltage source type converter in parallel; the fourth switch is connected in parallel with the first voltage source type converter and the second voltage source type converter. The invention saves the isolation transformer of the hybrid flexible loop closing device, effectively improves the operation efficiency of the device and reduces the volume and the cost of equipment.

Description

Non-isolated hybrid flexible loop closing device and control method

Technical Field

The invention belongs to the technical field of power electronic automatic control, and particularly relates to a non-isolated hybrid flexible loop closing device and a control method.

Background

In recent years, the permeability of the distributed power supply in a distribution network is gradually improved, and great challenges are brought to the operation and control of the distribution network. Moreover, the requirements of users on power supply reliability and fault recovery time are more strict. The principle of 'closed-loop design and open-loop operation' is commonly adopted in distribution networks in China, and on the basis, load transfer under the conditions of faults and the like is realized by controlling distribution network switches, so that the reliable and economic operation of the distribution networks is maintained. But the action cost of the switch operation is large, the response time is slow, and the transient impact on the power grid is large. Therefore, researchers provide a flexible direct current loop closing technology based on a power electronic converter, impact caused by hard loop closing of a mechanical switch can be effectively solved, distribution network tide can be regulated and controlled flexibly, accurately and in real time, and power supply reliability, economy and power supply quality of a distribution network are comprehensively improved.

The flexible loop closing device is used as a full-power device, and the typical disadvantages of the flexible loop closing device are large size and low efficiency. Therefore, the chinese patent application with application number 201810635456.3 discloses a hybrid back-to-back intelligent soft switch, a control system and a control method, and proposes a hybrid structure of a back-to-back power frequency isolated converter and a mechanical switch, as shown in fig. 1, the hybrid structure is composed of a voltage source converter VSC1, a VSC2 and a mechanical switch, if the output of the VSC1 or the VSC2 does not include an isolated transformer, for the structure shown in the figure, if the mechanical switch is closed, and the switches S2A and S1A' are closed, the dc capacitor C is short-circuited, which affects the normal operation of the equipment. Therefore, in order to realize the normal operation of the VSC1 and the VSC2, an isolation transformer needs to be installed at the output of the VSC1 or the VSC 2. The installation of the isolation transformer increases the volume and the cost of the equipment and reduces the overall operation efficiency of the equipment.

Disclosure of Invention

Aiming at the problems, the invention provides a non-isolated hybrid flexible loop closing device and a control method, and by providing a novel topological structure and a novel control method, an isolation transformer of the hybrid flexible loop closing device is saved, the operation efficiency of equipment is effectively improved, and the volume and the cost of equipment are reduced.

In order to achieve the technical purpose and achieve the technical effects, the invention is realized by the following technical scheme:

in a first aspect, the present invention provides a non-isolated hybrid flexible ring closing device, comprising:

the direct current sides of the first voltage source type converter and the second voltage source type converter are connected in parallel, and the output sides of the first voltage source type converter and the second voltage source type converter are respectively connected with a first circuit breaker and a second circuit breaker;

a first switch and a second switch connected in series between the first voltage source type converter and the second voltage source type converter;

the first direct current link capacitor is connected in parallel to the direct current side of the first voltage source type converter;

the second direct current link capacitor is connected to the direct current side of the second voltage source type converter in parallel;

the third switch and the super capacitor are connected in series, then are connected with the direct current sides of the first voltage source type converter and the second voltage source type converter in parallel, and are positioned between the first switch and the second switch;

and two ends of the fourth switch are respectively connected with the output sides of the first voltage source type converter and the second voltage source type converter.

Optionally, when the non-isolated hybrid flexible loop closing device is in a normal operation state, the first switch, the second switch, the first circuit breaker and the second circuit breaker are closed, the third switch and the fourth switch are opened, intelligent exchange of power and energy between the first line and the second line connected with the first voltage source type converter and the second voltage source type converter is established through output power of the first voltage source type converter and the second voltage source type converter, the first voltage source type converter and the second voltage source type converter can flexibly output reactive power at the same time, and stability of alternating-current bus voltage is maintained.

Optionally, the first voltage source type converter adopts a voltage control mode; the second voltage source converter employs a PQ power control mode.

Optionally, the output end of the first voltage source type converter is connected to the first transformer through a first circuit breaker, a first line and a fifth switch which are connected in sequence, and the second voltage source type converter is connected to the second transformer through a second circuit breaker, a second line and a sixth switch which are connected in sequence;

when a certain transformer fails, the non-isolated hybrid flexible loop closing device is in a fault isolation state, a voltage source type converter connected with the transformer keeps non-off-line for a period of time, if the transformer has a permanent fault, the voltage source type converter connected with the transformer is locked, the connection between the first voltage source type converter and the second voltage source type converter is disconnected, and fault isolation is achieved.

Optionally, after fault isolation is completed, the non-isolated hybrid flexible loop closing device is in a power supply recovery state, a fifth switch or a sixth switch connected to the transformer is tripped, and the fourth switch is closed, so that power supply recovery is realized.

Optionally, when the first transformer fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

switching the first voltage source type converter to a rectification mode, regulating the voltage of a first direct current link capacitor positioned on the direct current side of the first voltage source type converter into the voltage of a super capacitor, and closing a third switch;

flexibly adjusting the output power of the first voltage source type converter until the current of the fourth switch is zero, switching off the fourth switch, and supplying power to the first line by the super capacitor through the first voltage source type converter;

adjusting the voltage of the second capacitor to enable the voltage of the second capacitor to be equal to that of the first capacitor, and closing the second switch after the fourth switch is switched off;

the first voltage source type converter is switched to a voltage control mode, the voltage amplitude and the phase of the two sides of the fifth switch are regulated to be approximately equal by regulating the outlet voltage of the first voltage source type converter, and the fifth switch is closed;

the method comprises the steps that a first voltage source type converter and a second voltage source type converter both operate in a PQ control mode, a super capacitor is charged by adjusting the power difference of the first voltage source type converter and the second voltage source type converter, when the charge state of the super capacitor reaches an upper limit value, the first voltage source type converter and the second voltage source type converter are controlled to meet the condition that the charging power of the super capacitor is zero, the first voltage source type converter is switched to the voltage control mode, a third switch is disconnected, and the normal operation mode is recovered;

when the second transformer fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

switching the second voltage source type converter to a rectification mode, regulating the voltage of a second direct current link capacitor positioned on the direct current side of the second voltage source type converter into the voltage of the super capacitor, and closing a third switch;

flexibly adjusting the output power of the second voltage source type converter until the current of the fourth switch is zero, switching off the fourth switch, and supplying power to the second circuit by the super capacitor through the second voltage source type converter;

adjusting the voltage of the first capacitor to enable the voltage of the first capacitor to be equal to that of the second capacitor, and closing the first switch after the fourth switch is switched off;

the second voltage source type converter is switched to a voltage control mode, the amplitude and the phase of the voltage on the two sides of the sixth switch are regulated to be approximately equal by regulating the outlet voltage of the second voltage source type converter, and the sixth switch is closed;

the first voltage source type converter and the second voltage source type converter both operate in a PQ control mode, the super capacitor is charged by adjusting the power difference of the first voltage source type converter and the second voltage source type converter, when the charge state of the super capacitor reaches an upper limit value, the first voltage source type converter and the second voltage source type converter are controlled to meet the condition that the charging power of the super capacitor is zero, the first voltage source type converter is switched to the voltage control mode, the third switch is switched off, and the normal operation mode is recovered.

Optionally, when the non-isolated hybrid flexible loop closing device is in an economic operation mode of a transformer and is independently powered by a second transformer, adjusting the voltage of the first direct-current link capacitor and the voltage of the second direct-current link capacitor to be equal to the voltage of the super capacitor, and closing the third switch;

when the active power output by the second voltage source type converter is adjusted to zero, the current flowing through the second switch is zero, and then the second switch is disconnected;

controlling the output power of the first voltage source type converter, and switching off the fifth switch when the output current of the first transformer is zero, so that the first voltage source type converter is switched to an off-grid operation mode;

the fourth switch is closed, the first voltage source type converter is adjusted to a grid-connected operation mode, the power of the first voltage source type converter is flexibly adjusted to charge the super capacitor, the third switch is opened after the charging meets the requirement, the first voltage source type converter and the second voltage source type converter are equivalent to two reactive power compensation devices, and the first voltage source type converter and the second voltage source type converter are switched to the second transformer to supply power independently;

or when the non-isolated hybrid flexible loop closing device is in the economic operation mode of the transformer and is independently powered by the first transformer, adjusting the voltage of the first direct-current link capacitor and the second direct-current link capacitor to be equal to the voltage of the super capacitor, and closing the third switch;

when the output active power of the first voltage source type converter is adjusted to zero, the current flowing through the first switch is zero, and then the first switch is disconnected;

controlling the output power of the second voltage source type converter, and switching off the sixth switch when the output current of the second transformer is zero, so that the second voltage source type converter is switched to an off-grid operation mode;

and closing the fourth switch, adjusting the second voltage source type converter to a grid-connected operation mode, flexibly adjusting the power of the second voltage source type converter, charging the super capacitor, and after the charging meets the requirement, opening the third switch, wherein the first voltage source type converter and the second voltage source type converter are equivalent to two reactive power compensation devices and are switched to the first transformer for independent power supply.

Optionally, when the non-isolated hybrid flexible loop closing device is in a transformer economic operation mode and is switched to be supplied with power by the second transformer independently and simultaneously by the first transformer and the second transformer, closing the third switch to regulate and control the current of the fourth switch to zero, and transferring the power flow from the fourth switch to the first voltage source type converter;

the fourth switch is opened, and the second switch is closed;

adjusting the output voltage of the first voltage source type converter, closing the fifth switch after meeting the voltage amplitude and the phase at two sides of the fifth switch, and putting the first transformer into operation;

adjusting the output power of the first voltage source type converter and the second voltage source type converter, and disconnecting the third switch when the charge state of the super capacitor reaches the upper limit;

or when the non-isolated hybrid flexible loop closing device is in the transformer economic operation mode and the first transformer supplies power independently and is switched to the first transformer and the second transformer to supply power simultaneously, the third switch is closed, the current of the fourth switch is regulated to zero, and the tide is transferred to the second voltage source type converter from the fourth switch;

opening the fourth switch and closing the first switch;

adjusting the output voltage of the second voltage source type converter, closing the sixth switch after meeting the voltage amplitude and the phase at two sides of the sixth switch, and putting the second transformer into operation;

and adjusting the output power of the first voltage source type converter and the second voltage source type converter, and disconnecting the third switch when the charge state of the super capacitor reaches the upper limit.

In a second aspect, the present invention provides a control method based on the non-isolated hybrid flexible loop closing device in the first aspect, including:

monitoring the running state of the non-isolated hybrid flexible loop closing device;

and controlling the non-isolated hybrid flexible loop closing device based on the running state of the non-isolated hybrid flexible loop closing device.

Optionally, when the operation state of the non-isolated hybrid flexible loop closing device is a normal operation state, the first switch, the second switch, the fifth switch and the sixth switch are closed, the third switch and the fourth switch are opened, intelligent exchange of power and energy between the first line and the second line respectively connected with the first voltage source type converter and the second voltage source type converter is established through the output power of the first voltage source type converter and the second voltage source type converter, the first voltage source type converter and the second voltage source type converter can flexibly output reactive power at the same time, and the stability of the alternating current bus voltage is maintained.

Optionally, the first voltage source converter adopts a voltage control mode; the second voltage source converter employs a PQ power control mode.

Optionally, the first voltage source type converter is connected to the first transformer through a first circuit breaker, a first line and a fifth switch which are connected in sequence, and the second voltage source type converter is connected to the second transformer through a second circuit breaker, a second line and a sixth switch which are connected in sequence;

Optionally, after fault isolation, the non-isolated hybrid flexible loop closing device is in a power supply recovery state, a fifth switch or a sixth switch connected to the transformer is tripped, and the fourth switch is closed, so that power supply recovery is realized.

Optionally, when the second transformer fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

adjusting the voltage of the first capacitor to enable the voltage of the first capacitor to be equal to that of the second capacitor, and after the fourth switch is switched off, switching on the first switch;

when the first transformer fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

when the active power output by the first voltage source type converter is adjusted to zero, the current flowing through the first switch is zero, and then the first switch is disconnected;

Optionally, when the non-isolated hybrid flexible loop closing device is in a transformer economic operation mode and the second transformer supplies power independently and is switched to the first transformer and the second transformer to supply power simultaneously, the third switch is closed, the current of the fourth switch is regulated to zero, and the power flow is transferred to the first voltage source type converter from the fourth switch;

opening the fourth switch and closing the second switch;

or when the non-isolated hybrid flexible loop closing device is in a transformer economic operation mode and the first transformer independently supplies power and is switched to the first transformer and the second transformer to supply power simultaneously, the third switch is closed, the current of the fourth switch is regulated to zero, and the tide is transferred to the second voltage source type converter from the fourth switch;

opening the fourth switch and closing the first switch;

Compared with the prior art, the invention has the beneficial effects that:

the invention provides a non-isolated hybrid flexible loop closing device and a control method, and by providing a novel topological structure and a novel control method, an isolation transformer of the hybrid flexible loop closing device is saved, the operation efficiency of equipment is effectively improved, and the volume and the cost of equipment are reduced.

Drawings

In order that the manner in which the present invention is more fully understood, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings, wherein:

fig. 1 is a schematic structural diagram of a hybrid back-to-back intelligent soft switch in the prior art;

FIG. 2 is a schematic structural diagram of a non-isolated hybrid flexible loop closing device according to the present invention;

fig. 3 is a control schematic diagram of the first voltage source type converter and the second voltage source type converter according to the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the scope of the invention.

The application of the principles of the present invention will now be described in detail with reference to the accompanying drawings.

Example 1

The invention provides a non-isolation type hybrid flexible loop closing device, which realizes the electrical isolation of a first voltage source type converter VSC1 and a second voltage source type converter VSC2 in any working mode by controlling the states of a first switch LC1 and a second switch LC2 on the side of a direct current bus, and particularly comprises the following components in percentage by weight as shown in FIG. 2:

the direct current sides of the first voltage source type converter VSC1 and the second voltage source type converter VSC2 are connected in parallel, and the output sides of the first voltage source type converter VSC1 and the second voltage source type converter VSC2 are connected with a first circuit breaker LV1 and a second circuit breaker LV2 respectively;

a first switch LC1 and a second switch LC2, which are connected in series between the first and second source converters VSC1, VSC 2;

a first dc link capacitor C1 connected in parallel to the dc side of the first source-side converter VSC 1;

a second dc link capacitor C2 connected in parallel to the dc side of the second source converter VSC 2;

a third switch LS and a super capacitor C3, which are connected in series and then connected in parallel with the dc sides of the first source converter VSC1 and the second source converter VSC2, and are located between the first switch LC1 and the second switch LC 2;

and a fourth switch LN, two ends of which are connected to the output sides of the first source-source converter VSC1 and the second source-source converter VSC2, respectively.

In a specific implementation manner of the embodiment of the present invention, when the non-isolated hybrid flexible loop closing device is in a normal operation state, the first switch LC1, the second switch LC2, the first circuit breaker LV1, and the second circuit breaker LV2 are closed, the third switch LS and the fourth switch LN are opened, and intelligent exchange of power and energy between a first line and a second line (i.e. different lines) connected to the first switch LS is established through output power of the first voltage source converter and the second voltage source converter LS, and the first voltage source converter VSC1 and the second voltage source converter VSC2 can flexibly output reactive power at the same time to maintain the stability of the ac bus voltage.

Fig. 3 is a block diagram of non-isolated hybrid flexible loop control in an embodiment of the present invention, in a normal operation stage, positive/negative controllable reactive power that can be injected to a grid side by the first source converter VSC1 and the second source converter VSC 2; both can provide reactive power in order to optimize the net loss to the electric wire netting, improve the electric energy quality, also can absorb reactive power and restrain node voltage and rise temporarily to the output reactive power control of first voltage source type converter VSC1, second voltage source type converter VSC2 is independent each other. Similarly, the active power flow of the non-isolated hybrid flexible loop closing device and the active power flow of the first line and the second line are also bidirectional; the first VSC1 adopts direct power control, and the second VSC2 controls the active exchange with the second line to maintain the DC link voltage V _dc The stability of (3). It should be noted that, in the structure shown in fig. 2, when the first switch LC1 and the second switch LC2 are closed, the voltage V of the first dc-link capacitor C1 is _dc1 A second direct currentVoltage V of chain capacitor C2 _dc2 Satisfy V _dc1 ＝V _dc2 ＝V _dc . Thus, in normal mode, one of the source converters is in voltage mode as shown, while the other converter is in PQ power control mode.

In a specific implementation of the embodiment of the present invention, the first source converter VSC1 is connected to the first transformer T1 via a first circuit breaker, line one and fifth switch LT1 connected in series, and the second source converter VSC2 is connected to the second transformer T2 via a second circuit breaker, line two and sixth switch LT2 connected in series;

when a certain transformer fails, the non-isolated hybrid flexible loop closing device is in a fault isolation state, the voltage source type converter connected with the transformer keeps non-offline for a period of time, if the transformer fails permanently, the voltage source type converter connected with the transformer is locked, the connection between the first voltage source type converter VSC1 and the second voltage source type converter VSC2 is disconnected, and fault isolation is achieved.

After fault isolation is completed, the non-isolated hybrid flexible loop closing device is in a power supply recovery state, a fifth switch LT1 or a sixth switch LT2 connected with the transformer is switched off, and a fourth switch LN is closed to realize power supply recovery.

When the first transformer T1 fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

the first source converter VSC1 is switched to a rectifying mode and the voltage of the first DC-link capacitor C1 on the DC side of the first source converter VSC1 is regulated to the voltage V of the super capacitor C3 _SC Closing the third switch LS;

flexibly adjusting the output power of the first voltage source converter VSC1 until the current of the fourth switch LN is zero, disconnecting the fourth switch LN, and supplying power to the second line through the first voltage source converter VSC1 by the super capacitor C3;

adjusting the voltage of the second capacitor to make the voltage of the second capacitor equal to that of the first capacitor, and after the fourth switch LN is opened, closing the second switch LC 2;

the first voltage source type converter VSC1 is switched to a voltage control mode, the amplitude and the phase of the voltage at the two sides of the fifth switch LT1 are regulated to be approximately equal by regulating the outlet voltage of the first voltage source type converter VSC1, and the fifth switch LT1 is closed;

the method comprises the steps that a first voltage source type converter VSC1 and a second voltage source type converter VSC2 both operate to a PQ control mode, a super capacitor C3 is charged by adjusting the power difference of the first voltage source type converter VSC1 and the second voltage source type converter VSC2, when the charge state of a super capacitor C3 reaches an upper limit value, a first voltage source type converter VSC1 and a second voltage source type converter VSC2 are controlled to meet the condition that the charging power of the super capacitor C3 is zero, the first voltage source type converter VSC1 is switched to a voltage control mode, a third switch LS is disconnected, and the normal operation mode is recovered;

when the second transformer T2 fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

switching the second voltage source converter VSC2 to rectifying mode and regulating the voltage of the second dc link capacitor C2 on the dc side of the second voltage source converter VSC2 to the voltage of the super capacitor C3, closing the third switch LS;

flexibly adjusting the output power of the second voltage source converter VSC2 until the current of the fourth switch LN is zero, disconnecting the fourth switch LN, and supplying power through the super capacitor C3 and the second voltage source converter VSC 2;

adjusting the voltage of the first capacitor to make the voltage of the first capacitor equal to that of the second capacitor, and closing the first switch LC1 after the fourth switch LN is opened;

the second voltage source type converter VSC2 is switched to a voltage control mode, the amplitude and the phase of the voltage on the two sides of the sixth switch LT2 are regulated to be approximately equal by regulating the outlet voltage of the second voltage source type converter VSC2, and the sixth switch LT2 is closed;

the first voltage source type converter VSC1 and the second voltage source type converter VSC2 both operate to a PQ control mode, the super capacitor C3 is charged by adjusting the power difference of the first voltage source type converter VSC1 and the second voltage source type converter VSC2, after the state of charge of the super capacitor C3 reaches the upper limit value, the first voltage source type converter VSC1 and the second voltage source type converter VSC2 are controlled, the charging power of the super capacitor C3 is zero, the first voltage source type converter VSC1 is switched to the voltage control mode, the third switch LS is disconnected, and the normal operation mode is recovered.

In a specific implementation manner of the embodiment of the present invention, when the non-isolated hybrid flexible loop closing device is in the transformer economic operation mode and is separately powered by the second transformer T2, the voltages of the first dc link capacitor C1 and the second dc link capacitor C2 are adjusted to be equal to the voltage of the super capacitor C3, and the third switch LS is closed;

when the active power output by the second voltage source converter VSC2 is adjusted to zero, the current flowing through the second switch LC2 is zero, and then the second switch LC2 is disconnected;

controlling the output power of the first voltage source type converter VSC1, and switching off the fifth switch LT1 when the output current of the first transformer T1 is zero, so that the first voltage source type converter VSC1 is switched to an off-grid operation mode;

the fourth switch LN is closed, the first voltage source type converter VSC1 is adjusted to be in a grid-connected operation mode, the power of the first voltage source type converter VSC1 is adjusted flexibly, the super capacitor C3 is charged, the third switch LS is opened after the charging meets the requirement, the first voltage source type converter VSC1 and the second voltage source type converter VSC2 are equivalent to two reactive power compensation devices, and the first voltage source type converter VSC1 and the second voltage source type converter VSC2 are switched to the second transformer T2 to supply power independently;

controlling the output power of the second voltage source type converter, and switching off a sixth switch LT2 when the output current of the second transformer is zero, so that the second voltage source type converter is switched to an off-grid operation mode;

In a specific implementation manner of the embodiment of the present invention, when the non-isolated hybrid flexible loop closing device is in the transformer economy running mode and the power supply of the second transformer T2 is switched to the power supply of the first transformer T1 and the power supply of the second transformer T2 simultaneously, the third switch LS is closed, the current of the fourth switch LN is regulated and controlled to be zero, and the power flow is transferred from the fourth switch LN to the first voltage source converter VSC 1;

opening the fourth switch LN and closing the second switch LC 2;

after the output voltage of the first voltage source type converter VSC1 is adjusted to meet the amplitude and the phase of the voltage on two sides of a fifth switch LT1, the fifth switch LT1 is closed, and a first transformer T1 is put into operation;

adjusting the power of the first voltage source type converter and the power of the second voltage source type converter, and disconnecting the third switch LS when the charge state of the super capacitor C3 reaches the upper limit;

opening the fourth switch and closing the first switch;

adjusting the output voltage of the second voltage source type converter, closing a sixth switch LT2 after the amplitude and the phase of the voltage on two sides of the sixth switch LT2 are met, and operating the second transformer;

and adjusting the power of the first voltage source type converter and the second voltage source type converter, and disconnecting the third switch when the charge state of the super capacitor reaches the upper limit.

Example 2

The embodiment of the invention provides a control method based on a non-isolated hybrid flexible loop closing device in embodiment 1, which comprises the following steps:

In a specific implementation manner of the embodiment of the present invention, when the operation state of the non-isolated hybrid flexible loop closing device is a normal operation state, the first switch LC1, the second switch LC2, the fifth switch LT1, and the sixth switch LT2 are closed, the third switch LS and the fourth switch LN are opened, and intelligent exchange of power and energy between the first line and the second line (i.e. different lines) connected to the first switch is established through the first voltage source converter and the second voltage source converter, and the first voltage source converter VSC1 and the second voltage source converter VSC2 can flexibly output reactive power at the same time, so that the voltage of the alternating current bus is maintained to be stable.

In a specific implementation manner of the embodiment of the present invention, the first source-side converter VSC1 adopts a voltage control mode; the second source converter VSC2 employs a PQ power control mode.

In a specific implementation of the embodiment of the invention, the first source converter VSC1 is connected to the first transformer T1 via a first circuit breaker LV1, a line one and a fifth switch LT1 connected in series, and the second source converter VSC2 is connected to the second transformer T2 via a second circuit breaker LV2, a line two and a sixth switch LT2 connected in series;

when a certain transformer fails, the non-isolated hybrid flexible loop closing device is in a fault isolation state, a voltage source type converter connected with the transformer keeps non-offline for a period of time, if the transformer is in a permanent fault, the voltage source type converter connected with the transformer is locked, the connection between the first voltage source type converter VSC1 and the second voltage source type converter VSC2 is disconnected, and fault isolation is achieved.

In a specific implementation manner of the embodiment of the present invention, after the fault is isolated, the non-isolated hybrid flexible loop closing device is in a power supply recovery state, and the fifth switch LT1 or the sixth switch LT2 connected to the transformer is tripped, and the fourth switch LN is closed, so that power supply recovery is achieved.

In a specific implementation manner of the embodiment of the present invention, when the second transformer T2 fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

flexibly adjusting the output power of the second VSC2 until the current of the fourth switch LN is zero, disconnecting the fourth switch LN, and supplying power to the second line through the second VSC2 by the super capacitor C3;

when the first transformer T1 fails and the failure is recovered, the non-isolated hybrid flexible loop-closing device is in a dual power supply state:

switching the first source converter VSC1 to a rectifying mode and regulating the voltage of the first dc-link capacitor C1 on the dc side of the first source converter VSC1 to the voltage of the super capacitor C3, closing the third switch LS;

flexibly adjusting the output power of the first VSC1 until the current of the fourth switch LN is zero, disconnecting the fourth switch LN, and supplying power to the first line through the first VSC1 by the super capacitor C3;

the first voltage source type converter VSC1 is switched to a voltage control mode, the amplitude and the phase of the voltage on the two sides of the fifth switch LT1 are regulated to be approximately equal by regulating the outlet voltage of the first voltage source type converter VSC1, and the fifth switch LT1 is closed;

In a specific implementation manner of the embodiment of the present invention, when the non-isolated hybrid flexible loop closing device is in the transformer economic operation mode and is separately powered by the second transformer T2, the voltages of the first dc-link capacitor C1 and the second dc-link capacitor C2 are adjusted to be equal to the voltage of the super capacitor C3, and the third switch LS is closed;

controlling the output power of the first voltage source type converter VSC1, and disconnecting the fifth switch LT1 when the output current of the first transformer T1 is zero, so that the first voltage source type converter VSC1 is switched to an off-grid operation mode;

the fourth switch LN is closed, the first voltage source type converter VSC1 is adjusted to be in a grid-connected operation mode, the power of the first voltage source type converter VSC1 is flexibly adjusted, the super capacitor C3 is charged, the third switch LS is opened after the charging meets the requirement, the first voltage source type converter VSC1 and the second voltage source type converter VSC2 are equivalent to two reactive power compensation devices, and the first voltage source type converter VSC1 and the second voltage source type converter VSC2 are switched to the second transformer T2 to supply power independently;

In a specific implementation manner of the embodiment of the present invention, when the non-isolated hybrid flexible loop closing device is in the transformer economic operation mode and the power supply of the non-isolated hybrid flexible loop closing device is switched from the second transformer T2 to the first transformer T1 and the second transformer T2 for simultaneous power supply, the third switch LS is closed, the current of the fourth switch LN is regulated to zero, and the power flow is transferred from the fourth switch LN to the first voltage source converter VSC 1;

opening the fourth switch LN and closing the second switch LC 2;

adjusting the output voltage of the first voltage source type converter VSC1, and closing a fifth switch LT1 and operating a first transformer T1 after the amplitude and the phase of the voltage on two sides of the fifth switch LT1 are met;

adjusting the power of the first voltage source type converter and the second voltage source type converter, and turning off the third switch LS when the charge state of the super capacitor C3 reaches the upper limit;

opening the fourth switch and closing the first switch;

adjusting the output voltage of the second voltage source type converter, closing a sixth switch LT2 after the amplitude and the phase of the voltage at two sides of the sixth switch LT2 are met, and commissioning the second transformer;

The foregoing shows and describes the general principles and broad features of the present invention and advantages thereof. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.

Claims

1. A non-isolated hybrid flexible ring closing device is characterized by comprising:

the direct current sides of the first voltage source type converter and the second voltage source type converter are connected in parallel, and the output sides of the first voltage source type converter and the second voltage source type converter are respectively connected with the first circuit breaker and the second circuit breaker;

a fourth switch, both ends of which are respectively connected with the output sides of the first voltage source type converter and the second voltage source type converter;

the output end of the first voltage source type converter is connected to the first transformer through a first breaker, a first line and a fifth switch which are connected in sequence, and the second voltage source type converter is connected to the second transformer through a second breaker, a second line and a sixth switch which are connected in sequence;

2. The non-isolated hybrid flexible ring closing device according to claim 1, wherein: when the non-isolated hybrid flexible loop closing device is in a normal operation state, the first switch, the second switch, the first circuit breaker and the second circuit breaker are closed, the third switch and the fourth switch are opened, intelligent exchange of power and energy between a first line and a second line which are connected with the first switch and the second switch is established through output power of the first voltage source type converter and the second voltage source type converter, the first voltage source type converter and the second voltage source type converter can flexibly output reactive power at the same time, and the stability of the voltage of an alternating current bus is maintained.

3. A non-isolated hybrid flexible ring closure device according to claim 2, wherein: the first voltage source type converter adopts a voltage control mode; the second voltage source converter employs a PQ power control mode.

4. The non-isolated hybrid flexible ring closing device according to claim 1, wherein: after fault isolation is completed, the non-isolated hybrid flexible loop closing device is in a power supply recovery state, a fifth switch or a sixth switch connected with the transformer is tripped, the fourth switch is closed, and power supply recovery is achieved.

5. A non-isolated hybrid flexible ring device according to claim 1 or 4, characterized in that:

flexibly adjusting the output power of the first voltage source type converter until the current of the fourth switch is zero, disconnecting the fourth switch, and supplying power to the first line by the super capacitor through the first voltage source type converter;

adjusting the voltage of the second capacitor to enable the voltage of the second capacitor to be equal to that of the first capacitor, and after the fourth switch is switched off, switching on the second switch;

the first voltage source type converter and the second voltage source type converter both operate in a PQ control mode, the super capacitor is charged by adjusting the power difference of the first voltage source type converter and the second voltage source type converter, when the charge state of the super capacitor reaches an upper limit value, the first voltage source type converter and the second voltage source type converter are controlled to meet the condition that the charging power of the super capacitor is zero, the first voltage source type converter is switched to the voltage control mode, the third switch is switched off, and the normal operation mode is recovered;

the second voltage source type converter is switched to a voltage control mode, the voltage amplitude and the phase of the two sides of the sixth switch are regulated to be approximately equal by regulating the outlet voltage of the second voltage source type converter, and the sixth switch is closed;

6. The non-isolated hybrid flexible ring closing device according to claim 1, wherein: when the non-isolated hybrid flexible loop closing device is in the transformer economic operation mode and is independently powered by a second transformer, adjusting the voltage of the first direct-current link capacitor and the second direct-current link capacitor to be equal to the voltage of the super capacitor, and closing a third switch;

when the output active power of the second voltage source type converter is adjusted to zero, the current flowing through the second switch is zero, and then the second switch is disconnected;

the fourth switch is closed, the first voltage source type converter is adjusted to a grid-connected operation mode, the power of the first voltage source type converter is flexibly adjusted, the super capacitor is charged, the third switch is opened after the charging meets the requirement, the first voltage source type converter and the second voltage source type converter are equivalent to two reactive power compensation devices, and the first voltage source type converter and the second voltage source type converter are switched to the second transformer to independently supply power;

the fourth switch is closed, the second voltage source type converter is adjusted to a grid-connected operation mode, the power of the second voltage source type converter is flexibly adjusted to charge the super capacitor, and after the charging meets the requirement, the third switch, the first voltage source type converter, the second voltage source type converter and the third voltage source type converter are disconnected,

The second voltage source type converter is equivalent to two reactive power compensation devices and is switched to the first transformer for independent power supply.

7. The non-isolated hybrid flexible ring closing device according to claim 1, wherein: when the non-isolated hybrid flexible loop closing device is in the transformer economic operation mode and the second transformer supplies power independently and is switched to the first transformer and the second transformer to supply power simultaneously, the third switch is closed, the current of the fourth switch is regulated to zero, and the tide is transferred to the first voltage source type converter from the fourth switch;

opening the fourth switch and closing the second switch;

opening the fourth switch and closing the first switch;

8. A control method of the non-isolated hybrid flexible loop closing device based on claim 1 is characterized by comprising the following steps:

9. The method for controlling a non-isolated hybrid flexible ring closing device according to claim 8, wherein: when the running state of the non-isolated hybrid flexible loop closing device is a normal running state, the first switch, the second switch, the fifth switch and the sixth switch are closed, the third switch and the fourth switch are disconnected, intelligent exchange of power and energy between the first line and the second line which are respectively connected with the first switch and the second switch is established through the output power of the first voltage source type converter and the second voltage source type converter, the first voltage source type converter and the second voltage source type converter can flexibly output reactive power at the same time, and the stability of the alternating-current bus voltage is maintained.

10. The control method of the non-isolated hybrid flexible loop closing device according to claim 9, wherein: the first voltage source type converter adopts a voltage control mode; the second voltage source converter employs a PQ power control mode.

11. The control method of the non-isolated hybrid flexible ring closing device according to claim 8, wherein: after fault isolation, the non-isolated hybrid flexible loop closing device is in a power supply recovery state, a fifth switch or a sixth switch connected with the transformer is switched off, and the fourth switch is closed, so that power supply recovery is realized.

12. The method for controlling a non-isolated hybrid flexible ring closing device according to claim 8, wherein: when the second transformer fails and the failure is recovered, the non-isolated hybrid flexible loop closing device is in a dual power supply state:

13. The control method of the non-isolated hybrid flexible ring closing device according to claim 8, wherein: when the non-isolated hybrid flexible loop closing device is in the transformer economic operation mode and is independently powered by a second transformer, adjusting the voltage of the first direct-current link capacitor and the second direct-current link capacitor to be equal to the voltage of the super capacitor, and closing the third switch; when the active power output by the second voltage source type converter is adjusted to zero, the current flowing through the second switch is zero, and then the second switch is disconnected;

the fourth switch is closed, the second voltage source type converter is adjusted to be in a grid-connected operation mode, the power of the second voltage source type converter is flexibly adjusted to charge the super capacitor, the third switch is opened after the charging meets the requirement, and the first voltage source type converter,

14. The method for controlling a non-isolated hybrid flexible ring closing device according to claim 8, wherein: when the non-isolated hybrid flexible loop closing device is in a transformer economic operation mode and the second transformer independently supplies power and switches to the first transformer and the second transformer to supply power simultaneously, the third switch is closed, the fourth switch current is regulated to zero, and the tide is transferred to the first voltage source type converter from the fourth switch;

opening the fourth switch and closing the second switch;

opening the fourth switch and closing the first switch;