CN113054658A - Multi-port low-voltage power distribution network seamless loop closing transfer device and method thereof - Google Patents

Multi-port low-voltage power distribution network seamless loop closing transfer device and method thereof Download PDF

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Publication number
CN113054658A
CN113054658A CN202110274297.0A CN202110274297A CN113054658A CN 113054658 A CN113054658 A CN 113054658A CN 202110274297 A CN202110274297 A CN 202110274297A CN 113054658 A CN113054658 A CN 113054658A
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voltage
current
loop
loop closing
transformer
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CN113054658B (en
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王晓聪
李俊林
韩捷
刘晓
张晏玉
张旭
张迎晓
林劝立
段宜廷
刘智超
官志涛
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Guangzhou Power Supply Bureau of Guangdong Power Grid Co Ltd
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Guangzhou Power Supply Bureau of Guangdong Power Grid Co Ltd
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    • 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
    • H02J3/007—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources
    • H02J3/0073—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources for providing alternative feeding paths between load and source when the main path fails, e.g. transformers, busbars
    • 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • H02J3/1807—Arrangements for adjusting, eliminating or compensating reactive power in networks using series compensators
    • H02J3/1814—Arrangements for adjusting, eliminating or compensating reactive power in networks using series compensators wherein al least one reactive element is actively controlled by a bridge converter, e.g. unified power flow controllers [UPFC]
    • 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

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Abstract

The invention provides a multi-port low-voltage distribution network seamless loop closing transfer device and a method thereof, belonging to the technical field of low-voltage distribution network loop closing transfer, wherein the device specifically comprises the following steps: the loop closing switch is arranged among the feeders; the converter is connected with the series coupling transformer and is connected into a feeder line in a series mode, the converter connected with the feeder line with the largest power flow margin is used for stabilizing the voltage of a direct current bus, and the rest converters are used for providing series voltage compensation of closed loop and power transfer; the direct current side of each converter is connected in parallel with two ends of a direct current side voltage stabilizing capacitor; the energy storage power supply is used for stabilizing the voltage of the direct current side; the side bypass switch of the series coupling transformer and the secondary side bypass switch of the series coupling transformer are jointly used for connecting and disconnecting the converter; when the series voltage of the converter is converted, the voltage on two sides of the loop closing point is equal to the voltage of the outgoing line of the maintenance line transformer; and when the loop is closed, the voltages on the two sides of the loop closing switch are equal. The invention realizes seamless loop closing, has no impact power conversion and reduces the interference to a relay protection system.

Description

Multi-port low-voltage power distribution network seamless loop closing transfer device and method thereof
Technical Field
The invention belongs to the technical field of low-voltage distribution network loop closing and power transferring, and particularly relates to a seamless loop closing and power transferring device and method for a multi-port low-voltage distribution network.
Background
With the continuous development of the technology, the requirements of users on the continuity and reliability of power supply are higher and higher, and the power supply is very sensitive to power failure. Therefore, how to reduce the power failure time and reduce the influence of power failure on the society caused by newly increased capacity expansion, sudden accidents and regular maintenance of the power grid becomes one of the most concerned problems of power supply enterprises. The 380V low-voltage distribution network is used as the last link for distributing electric energy to users, various users are connected with the power grid, and a power failure switching method is often adopted when the low-voltage distribution network is maintained in equipment maintenance, so that power failure of the users for a certain time and times is caused, and the power supply reliability is reduced, so that seamless loop-closing power transfer of the 380V low-voltage distribution network is an important measure for improving the satisfaction degree of the users.
The 380V low-voltage distribution network is wide in distribution, multiple in branches and complex in closed-loop control, and researches on 380V low-voltage distribution network closed-loop operation and devices for directly distributing electric energy to transformer areas and low-voltage lines of mass residential users are few. The voltage of buses at two ends of the loop closing switch has a difference value, so that a circulating current can appear in a distribution network in the loop closing operation process, overcurrent protection and quick-break protection misreading can be caused by overlarge current, tripping can occur, and loop closing is unsuccessful.
The existing power conversion mode mostly adopts a single power supply to share the power of the original power supply, the risks of power overload of a power conversion line and insufficient power conversion capacity easily occur under the condition of a load peak or large power conversion flow, and the traditional loop closing operation decision of the low-voltage power distribution network is generally directly analyzed according to experience or offline and is not suitable for loop closing of a complex power grid.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a multi-port low-voltage distribution network seamless loop closing transfer device and a method thereof, and aims to solve the problem that loop closing current generated by the conventional low-voltage distribution network loop closing transfer method has large interference on relay protection of a distribution network.
In order to achieve the aim, the invention provides a multi-port low-voltage distribution network seamless loop-closing transfer device which comprises a transformer, a series coupling transformer, a primary side bypass switch of the series coupling transformer, a secondary side bypass switch of the series coupling transformer, a current transformer, a loop-closing switch, a direct-current side voltage-stabilizing capacitor and an energy storage power supply;
the loop closing switch is arranged among the feeders and is used for closing loops among the feeders; the feeder line is connected with the direct current side through a converter; the converter is connected with the series coupling transformer and is connected into a feeder line in a series mode, the converter corresponding to the feeder line with the largest power flow margin is selected for stabilizing the voltage of the direct current bus, and the rest converters are used for providing series voltage compensation during loop closing and power transfer; the direct current side of each converter is connected in parallel with two ends of a direct current side voltage-stabilizing capacitor and used for realizing energy exchange between the converter and the voltage-stabilizing capacitor; the energy storage power supply is used for stabilizing the voltage of the direct current side when the voltage stabilizing capability of the system converter is insufficient; the primary side bypass switch of the series coupling transformer is arranged between the series coupling transformer and the converter; the secondary side bypass switch of the series coupling transformer is arranged on a feeder line connected with the series flow coupling transformer; the series coupling transformer side bypass switch and the series coupling transformer secondary side bypass switch are jointly used for connecting and cutting the converter on the feeder line;
the series voltage of the converter enables the voltages on two sides of the loop closing switch to be equal during loop closing, and enables the voltages on two sides of the loop closing point to be equal to the voltage of the outgoing line of the maintenance line transformer during power conversion.
On the other hand, the invention provides a corresponding loop closing method based on a multi-port low-voltage distribution network seamless loop closing transfer device, which comprises the following steps: (1) reading real-time tide data of a power grid, and setting a voltage difference threshold;
(2) screening an optimal loop closing mode based on the real-time power flow data;
(3) after the phase sequences of three-phase voltages on two sides of a loop closing point are adjusted to be consistent, detecting whether the voltage difference on the two sides of the loop closing point is less than or equal to a voltage difference threshold value in real time; if yes, directly closing the ring; otherwise, turning to the step (4);
(4) judging whether the circuit with the largest power flow margin can support the voltage of the direct current side voltage-stabilizing capacitor, if so, turning to the step (5), otherwise, accessing an energy storage power supply to support the voltage of the direct current side voltage-stabilizing capacitor, and turning to the step (5);
(5) calculating reference voltage values of a plurality of converters according to the voltage difference of two sides of the current loop closing point;
(6) based on the reference voltage value, the plurality of converters perform voltage compensation on the maintenance line through current transformation; and (4) turning to the step (3).
Preferably, the step (2) specifically comprises the following steps:
(2.1) calculating the voltage difference of two sides of the closed-loop switch based on real-time power flow data, network topology analysis and state estimation, and constructing a power grid node admittance matrix;
(2.2) modifying the current injection quantity of a node voltage matrix equation according to the power grid node admittance matrix, and calculating steady-state load flow data after loop closing;
(2.3) carrying out real-time online comprehensive evaluation on the basis of steady-state power flow data after loop closing;
and (2.4) according to the comprehensive evaluation result, carrying out retrieval and verification on the closed-loop path, and selecting the optimal closed-loop topology and closed-loop mode.
Preferably, the online comprehensive evaluation comprises safety stability evaluation, power quality evaluation and economy evaluation.
Preferably, a safety stability index, a power quality index and a network loss index are established based on online comprehensive evaluation; the safety stability indexes comprise loop closing impact current verification, branch thermal stability verification, static safety analysis verification and short circuit current level verification.
Preferably, the voltage difference between the two sides of the loop closing point is less than or equal to the voltage difference threshold in the form of:
Figure BDA0002975938880000031
Figure BDA0002975938880000032
wherein, ViAnd VjThe voltage on both sides of the loop closing point; delta U is the amplitude value representation of the voltage difference threshold value on the two sides of the closed-loop switch; and delta theta is represented by the phase of the voltage difference threshold value on two sides of the loop closing switch.
On the other hand, the invention provides a corresponding power transfer method based on a multi-port low-voltage distribution network seamless loop-closing power transfer device, which comprises the following steps:
(1) reading real-time tide data and setting a maximum impact current value;
(2) calculating a reference voltage value of a sending end converter and a change step length of the reference voltage value based on the real-time load flow data;
(3) according to the current reference voltage value, the current of the sending end converter is changed, and the compensation voltage of the series point is changed, so that the voltages on the two sides of the loop closing point are gradually equal to the voltage of the outgoing line of the maintenance line transformer;
(4) judging whether the difference value of the loop closing current and the overhauling line current is smaller than the maximum impact current threshold value or not, and judging whether the current of the transformer outlet end of the overhauling line end is smaller than the switchable current threshold value or not, if so, cutting off the transformer at the overhauling line end to finish the electricity conversion process; otherwise, updating the current reference voltage value according to the change step length, and turning to the step (3).
Preferably, the condition that the transformer outlet end current of the service line end meets the switchable current threshold is as follows:
Figure BDA0002975938880000041
Figure BDA0002975938880000042
wherein, the delta I is a current threshold value of a wire outlet end of the cut-off transformer; delta beta is the current angle threshold of the outlet end of the cut transformer; i isr1The current of the transformer outlet end of the line end is overhauled before the power is converted.
Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
the existing low-voltage distribution network loop closing power transfer operation generally does not need a compensation device, the impact current generated at the moment of loop closing has certain requirements on the parameters of a distribution network device, and the generated loop closing current has a large influence on the relay protection of the distribution network; the traditional loop closing operation is generally judged according to experience or analyzed offline, the operation is required when the load of a ring network is small and the voltage difference between two sides is small, the success rate cannot reach 100%, and the loop closing operation is suitable for a specific operation mode or a power grid with a small scale and is not suitable for increasingly complex power grid loop closing risk analysis.
According to the method, a multi-port topological result is set, on-line loop closing safety and stability evaluation, electric energy quality evaluation and economy evaluation are carried out through pre-calculation of the power flow after loop closing, and an optimal loop closing route is selected; the converter provided by the invention can be used for carrying out series compensation on voltages on two sides of the contact switch, realizes seamless loop closing, has no impact power conversion, and can reduce interference on a relay protection system. Compared with a device for connecting compensation voltage in series on a feeder line side and connecting the compensation voltage in parallel on a direct current side to stabilize voltage, the device does not need to additionally arrange a parallel port for taking electricity and stabilizing voltage from the feeder line, a circuit with larger current margin is selected, the voltage stabilization function can be realized by converting current of a current transformer, an energy storage power supply can also be used as backup selection of voltage stabilization, a compensation point is flexibly selected, and the possibility of a loop-closing power conversion process can be improved.
Drawings
Fig. 1 is a general structure diagram of a seamless loop closing transfer device for a multiport low-voltage distribution network, provided by the invention;
FIG. 2 is a topology diagram of a converter provided by an embodiment of the present invention;
FIG. 3 is a flow chart of a loop closing method according to an embodiment of the present invention;
FIG. 4 is a flow chart of a power conversion method provided by an embodiment of the invention;
description of the labeling:
11-a first substation transformer; 12-a second substation transformer; 1 n-nth substation transformer Dn; 21-a first substation transformer outgoing switch; 22-a second substation transformer outgoing switch; 2 n-nth transformer substation transformer outgoing line switch; 31-a first busbar; 32-a second bus bar; 3 n-nth bus; 41-a first series-coupled transformer; 42-a second series-coupled transformer; 4 n-nth series coupling transformer; 51-a first series-coupled transformer primary side bypass switch; 52-a second series-coupled transformer primary side bypass switch; 5 n-nth series coupling transformer primary side bypass switch; 61-a first series coupled transformer secondary bypass switch; 62-a second series coupled transformer secondary bypass switch; 6 n-nth series coupling transformer secondary side bypass switch; 71-a first current transformer; 72-a second current transformer; 7 n-nth current transformer; 81-a first closing ring switch; 82-a second loop closing switch; 8 n-nth closing ring switch; 91-first load; 92-a second load; 9 n-nth load; 10-direct current side voltage stabilizing capacitor.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention provides a seamless loop-closing power transfer device for a multi-port low-voltage power distribution network, and relates to a flexible seamless loop-closing power transfer method suitable for a 380v power distribution network.
As shown in fig. 1, the topology adopted by the present invention is: loop closing switches (a first loop closing switch 81, a second loop closing switch 82, … … and an nth loop closing switch) are arranged between feeders (a first bus bar 31, a second bus bar 32, … … and an nth bus bar) capable of closing loops in a power distribution system; the feeder is connected with the direct current side through a current transformer (a first current transformer 71, a second current transformer 72, … … and an nth current transformer, the specific structure is shown in figure 2); the converter connecting series coupling transformers (a first series coupling transformer 41, a second series coupling transformer 42, … …, an nth series coupling transformer 4n) are connected into a 380v distribution network line in a series mode to provide series voltage compensation; wherein, the series coupling transformer is provided with a short-circuit switch (corresponding to a first series coupling transformer secondary side bypass switch 61, a second series coupling transformer secondary side bypass switch 62, … … and an nth series coupling transformer secondary side bypass switch 6n) on a feeder side, and is provided with a short-circuit bypass switch (corresponding to a first series coupling transformer primary side bypass switch 51, a second series coupling transformer primary side bypass switch 52, … … and an nth series coupling transformer primary side bypass switch 5n) on a converter side; the direct current side of each converter is connected with a direct current side voltage stabilizing capacitor 10 for energy exchange; and an energy storage power supply is simultaneously configured on the direct current side, so that a backup voltage stabilization function is realized.
The device takes multiple ports as characteristic values, can be matched with a dispatching power grid system, and selects an optimal closed-loop switching path and a method thereof. Firstly, data of an EMS system (energy management system) are collected, on the basis of carrying out bad data identification, network topology analysis and state estimation on a power grid, voltage difference at two sides of a closed loop switch is read, and a power grid node admittance matrix is constructed; modifying the current injection quantity of a node voltage matrix equation according to the power grid node admittance matrix, and calculating the steady-state load flow after loop closing; and performing real-time online comprehensive evaluation according to the steady-state tide after loop closing. The online comprehensive evaluation comprises safety stability evaluation, electric energy quality evaluation and economy evaluation; establishing a safety stability index, an electric energy quality index and a network loss index, wherein the safety stability index comprises loop closing impact current verification, branch thermal stability verification, static safety analysis verification and short circuit current level verification; the loop closing path of the multi-port low-voltage distribution network seamless loop closing transfer device is searched and checked, the optimal loop closing topology and the optimal loop closing operation mode are selected, the limitation of the traditional loop closing analysis decision can be broken through, and the loop closing risk and the economic analysis level can be improved.
And the method is divided into an emergency mode and a normal mode according to whether a direct current power supply needs to be added to maintain the voltage stability of the direct current side.
In the normal mode, a line with larger current margin is required to be selected as an auxiliary control line, and a current transformer of the auxiliary control line is used for converting current to realize capacitance voltage stabilization of a common direct current bus; and on the premise of meeting the active power balance constraint, the rest converters inject voltage into the power grid system, and change the line voltage at two ends of the loop closing switch, so that the seamless loop closing power conversion operation of the real-time line is realized.
In an emergency mode (namely, when the power flow margin of the circuit is insufficient), not only a circuit with a large power flow margin needs to be selected as an auxiliary control circuit, but also an energy storage power supply needs to be connected to a direct current side, the voltage of a common direct current bus is stabilized together with a current transformer of the auxiliary control circuit, and the voltage of the circuit at two ends of a loop closing switch is changed by other converters, so that seamless loop closing power switching is realized.
More specifically, the invention provides a multi-port low-voltage distribution network seamless loop-closing transfer device which comprises a transformer, a series coupling transformer, a primary side bypass switch of the series coupling transformer, a secondary side bypass switch of the series coupling transformer, a current transformer, a loop-closing switch, a direct-current side voltage-stabilizing capacitor and an energy storage power supply;
the loop closing switch is arranged among the feeders and is used for closing loops among the feeders; the feeder line is connected with the direct current side through a converter; the converter is connected with the series coupling transformer and is connected into a feeder line in a series mode, the converter corresponding to the feeder line with the largest power flow margin is selected for stabilizing the voltage of the direct current bus, and the rest converters are used for providing series voltage compensation during loop closing and power transfer; the direct current side of each converter is connected in parallel with two ends of a direct current side voltage-stabilizing capacitor and used for realizing energy exchange between the converter and the voltage-stabilizing capacitor; the energy storage power supply is used for stabilizing the voltage of the direct current side when the voltage stabilizing capability of the system converter is insufficient; the primary side bypass switch of the series coupling transformer is arranged between the series coupling transformer and the converter; the secondary side bypass switch of the series coupling transformer is arranged on a feeder line connected with the series flow coupling transformer; the series coupling transformer side bypass switch and the series coupling transformer secondary side bypass switch are jointly used for connecting and cutting the converter on the feeder line;
when the series voltage of the converter is converted, the voltage on two sides of the loop closing point is equal to the voltage of the outgoing line of the maintenance line transformer; and when the loop is closed, the voltages on the two sides of the loop closing switch are consistent.
On the other hand, as shown in fig. 3, the present invention provides a corresponding loop closing method based on a multi-port low-voltage distribution network seamless loop closing transfer device, which includes the following steps:
(1) reading real-time tide data of a power grid, and setting a voltage difference threshold;
(2) screening an optimal loop closing mode based on the real-time power flow data;
(3) after the phase sequences of three-phase voltages on two sides of a loop closing point are adjusted to be consistent, detecting whether the voltage difference on the two sides of the loop closing point is less than or equal to a voltage difference threshold value in real time; if yes, directly closing the ring; otherwise, turning to the step (4);
(4) judging whether the circuit with the largest power flow margin can support the voltage of the direct current side voltage-stabilizing capacitor, if so, turning to the step (5), otherwise, accessing an energy storage power supply to support the voltage of the direct current side voltage-stabilizing capacitor, and turning to the step (5);
(5) calculating reference voltage values of a plurality of converters according to the voltage difference of two sides of the current loop closing point;
(6) based on the reference voltage value, the plurality of converters perform voltage compensation on the maintenance line through current transformation; and (4) turning to the step (3).
Preferably, the step (2) specifically comprises the following steps:
(2.1) calculating the voltage difference of two sides of the closed-loop switch based on real-time power flow data, network topology analysis and state estimation, and constructing a power grid node admittance matrix;
(2.2) modifying the current injection quantity of a node voltage matrix equation according to the power grid node admittance matrix, and calculating steady-state load flow data after loop closing;
(2.3) carrying out real-time online comprehensive evaluation on the basis of steady-state power flow data after loop closing;
and (2.4) according to the comprehensive evaluation result, carrying out retrieval and verification on the closed-loop path, and selecting the optimal closed-loop topology and closed-loop mode.
Preferably, the online comprehensive evaluation comprises safety stability evaluation, power quality evaluation and economy evaluation.
Preferably, a safety stability index, a power quality index and a network loss index are established based on online comprehensive evaluation; the safety stability indexes comprise loop closing impact current verification, branch thermal stability verification, static safety analysis verification and short circuit current level verification.
Preferably, the voltage difference between the two sides of the loop closing point is less than or equal to the voltage difference threshold in the form of:
Figure BDA0002975938880000081
Figure BDA0002975938880000082
wherein, ViAnd VjThe voltage on both sides of the loop closing point; delta U is the amplitude value representation of the voltage difference threshold value on the two sides of the closed-loop switch; and delta theta is represented by the phase of the voltage difference threshold value on two sides of the loop closing switch.
On the other hand, as shown in fig. 4, the present invention provides a corresponding power transfer method based on a seamless loop-closing power transfer device for a multi-port low-voltage distribution network, which includes the following steps:
(1) reading real-time tide data and setting a maximum impact current value;
(2) calculating a reference voltage value of a sending end converter and a change step length of the reference voltage value based on the real-time load flow data;
(3) according to the current reference voltage value, the current of the sending end converter is changed, and the compensation voltage of the series point is changed, so that the voltages on the two sides of the loop closing point are gradually equal to the voltage of the outgoing line of the maintenance line transformer;
(4) judging whether the difference value of the loop closing current and the overhauling line current is smaller than the maximum impact current threshold value or not, and judging whether the current of the transformer outlet end of the overhauling line end is smaller than the switchable current threshold value or not, if so, cutting off the transformer at the overhauling line end; otherwise, updating the current reference voltage value according to the change step length, and turning to the step (3).
Preferably, the condition that the transformer outlet end current of the service line end meets the switchable current threshold is as follows:
Figure BDA0002975938880000091
Figure BDA0002975938880000092
wherein, the delta I is a current threshold value of a wire outlet end of the cut-off transformer; delta beta is the current angle threshold of the outlet end of the cut transformer; i isr1The current of the transformer outlet end of the line end is overhauled before the power is converted.
Examples
Fig. 1 is a general structure diagram of a seamless loop closing transfer device for a multiport low-voltage distribution network, provided by the invention;
FIG. 2 is a schematic diagram of a current transformer;
taking a multi-port low-voltage distribution network seamless loop-closing switching device comprising two groups of converters as an example, the delta U is a voltage difference threshold value of voltage differences on two sides of a loop-closing switch; delta alpha is the voltage angle threshold of the voltage difference at the two sides of the loop closing switch; delta I is the current threshold of the outgoing line end of the cut-off transformer; and delta beta is the current angle threshold of the outgoing line end of the cut-off transformer.
The goals of converter regulation are: in the aspect of realizing seamless loop closing, the instantaneous impact current of the loop closing is eliminated, and the voltages on the two sides of the loop closing switch are required to be consistent. In the aspect of realizing the power conversion without impulse current, the current required to flow through the loop closing switch is gradually equal to the initial current of the outlet end of the transformer in the receiving end maintenance line.
And the line with larger power flow margin can be used as an auxiliary line, and a normal mode is selected for loop closing power conversion. The main line serves as a power conversion line, and the auxiliary control line serves as a line for maintaining system power balance. If no optional auxiliary line exists, for example, the power flows of other lines do not have proper margin for flexible transfer, the emergency mode is selected for loop closing transfer.
As shown in fig. 3, in the normal mode, the loop closing method includes:
(1) reading system data of a power grid, wherein the system data comprises real-time tide data and a voltage difference threshold;
(2) screening an optimal loop closing path based on the real-time power flow data and the voltage difference threshold value, and further determining a loop closing point;
(3) checking whether the phase sequence of the A, B phases at the two sides of the ring closing point is consistent with the phase sequence of the three-phase voltage C, and if not, adjusting the phase sequence;
(4) when the phase sequences are consistent, detecting whether the voltage difference between two sides of the loop closing point is less than or equal to a preset voltage difference threshold value in real time; the voltage difference threshold comprises a magnitude and a phase angle of the voltage difference; if yes, directly closing the ring; otherwise; turning to the step (5);
(5) calculating reference voltage values of a plurality of converters according to the voltage difference of two sides of the current loop closing point;
(6) carrying out voltage compensation on the maintenance line by connecting a plurality of converters in series; and (4) turning to the step (3).
As shown in fig. 4, in the normal mode, the power conversion method includes:
(1) reading system data of a power grid; the system data comprises real-time tide data and a maximum impact current value;
(2) calculating a reference voltage value of a sending end converter and a change step length of the reference voltage value based on the real-time load flow data;
(3) according to the current reference voltage value, a sending end converter is adopted for converting current, and the compensation voltage of the series point is changed, so that the voltages on the two sides of the loop closing point are gradually equal to the voltage of the outgoing line of the maintenance line transformer;
(4) judging whether the difference value of the loop closing current and the overhauling line current is smaller than the maximum impact current threshold value or not, judging whether the current of the transformer outlet end of the overhauling line end is smaller than the switchable current threshold value or not, and if so, switching off the transformer outlet end; otherwise, updating the current reference voltage value according to the change step length, and turning to the step (3).
In an emergency mode, an energy storage power supply is required to be put into use to keep the voltage of the direct current side stable and ensure the normal power exchange of the direct current side; therefore, the voltage at two ends of the direct current side voltage stabilizing capacitor is detected as a control signal of the output voltage of the energy storage power supply, and then the loop closing method in the normal mode is carried out.
Namely: when the circuit with larger tidal current margin is not enough to support the voltage of the direct current side voltage-stabilizing capacitor, the voltage at the two ends of the direct current side voltage-stabilizing capacitor acts on tidal current data, and then the completion of loop closing is supported.
Taking the 10kV substation transformer 11 needing to be overhauled as an example, assume that through calculation, other lines are selected as auxiliary control lines to maintain the voltage stability of the dc side voltage stabilizing capacitor. The second bus bar 32 is selected as a power transfer end, and the first converter 71 connected with the first bus bar 31 and the second converter 72 connected with the second bus bar 32 are respectively selected to carry out series voltage compensation on the maintenance line, and two compensation modes are available.
In the loop closing power conversion process, selection can be carried out according to the voltage difference of two sides of the loop closing switch; if the voltage difference is small, the compensation degree is small, and a compensation mode can be selected optionally; if the voltage difference is larger and the compensation degree is larger, two compensation modes can be selected at the same time for compensation.
Compensation method 1: and selecting a converter of the power conversion receiving end bus for compensation. Firstly, the voltages at two sides of the first loop closing switch 81 are detected, referring to fig. 2, the voltage value of the outlet end of the transformer of the first substation is set as V1As a receiving end line (i.e., the first bus bar 31); the voltage value of the outlet end of the transformer of the second substation is V2As the send-end line (i.e., second bus bar 32); will ViAnd VjAs a reference voltage signal of the first current transformer 71; the first converter 71 changes the voltage V between a closed loop switch point connected into the first bus 31 and the outlet end of the first substation transformer 11 needing to be subjected to power failure maintenance by 10kV/400Ve1So that the voltage difference between both sides of the loop closing switch 81The following relationship is satisfied:
Figure BDA0002975938880000111
Figure BDA0002975938880000112
the seamless loop closing condition that the voltage difference between the two ends of the first loop closing switch 81 between the first bus 31 and the second bus 32 is less than the voltage threshold is satisfied, and at the moment, the current I flowing through the first loop closing switch 8112Approximately equals to 0, and the seamless loop closing operation is realized.
The transfer operation needs to transfer the load on the first bus bar 31 to the second bus bar 32, the first loop closing switch 81 is used as a power transfer path, and when the voltages at the two ends of the first loop closing switch 81 are not consistent, the voltage difference generates power flow.
Firstly, the current I of the outlet end of the first substation transformer 11 before power conversion is detectedr10And is combined with I12Performing difference operation; calculating corresponding V by combining virtual impedanceiThe reference voltage signal of the first converter 71 is obtained. In order to avoid the impact on the grid system caused by the excessive instantaneous converted electric power, the first converter 71 gradually converts the current by calculating the step length of the reference voltage value until the actual current I at the outlet end of the first substation transformer 71r1The condition for cutting off the first substation transformer 71 is met (namely the current is less than the current threshold value for cutting off the main transformer); the concrete form is as follows:
Figure BDA0002975938880000113
Figure BDA0002975938880000114
at the same time, the current flowing through the first loop closing switch 81 also satisfies:
Figure BDA0002975938880000115
Figure BDA0002975938880000116
the non-impact power conversion process is completed, and the main transformer cutting operation can be carried out.
Compensation mode 2: and selecting a converter of the power transfer terminal bus for compensation. First the voltage across the loop closing switch is detected, see fig. 2, by calculating ViAnd VjAs a reference voltage signal of the second converter 72, the second converter 72 changes the voltage V between the closed loop switch point connected to the second bus 32 and the outlet terminal of the second substation transformer 12 requiring 10kV/400V blackout for maintenancee2So that the voltages satisfy the following relationship:
Figure BDA0002975938880000121
Figure BDA0002975938880000122
the seamless loop closing condition that the voltages at two ends of the first loop closing switch 81 between the first bus bar 31 and the second bus bar 32 are equal is met; at this time, the current I flowing through the first loop closing switch 8112And is also less than the maximum inrush current value.
The transfer operation requires the load on the first bus bar 31 to be transferred to the second bus bar 32, and the first loop closing switch 81 serves as a power transfer path.
Firstly, the current I of the outlet end of the first substation transformer 11 before power conversion is detectedr10And is combined with I12Performing difference operation; calculating corresponding V by combining virtual impedanceiTo obtain a reference voltage signal of the second converter 72. In order to avoid the impact on the power grid system caused by excessive instantaneous converted electric power, the second converter 72 compensates the gradual conversion by calculating the reference voltage value until the first converterActual current I of 11 leading-out terminals of transformer substation12The condition for cutting off the first substation transformer 11 (i.e. the actual current I at the outlet end of the first substation transformer 11) is satisfied12Less than the current threshold for cutting off the main transformer); the concrete form is as follows:
Figure BDA0002975938880000123
Figure BDA0002975938880000124
at the same time, the current flowing through the first loop closing switch 81 also satisfies:
Figure BDA0002975938880000125
Figure BDA0002975938880000126
the non-impact power conversion process is completed, and the main transformer cutting operation can be carried out. After the main transformer is cut off, the output voltage of each converter is adjusted, the second converter 72 is withdrawn, the second primary side bypass switch 52 is closed, the second secondary side bypass switch 62 is closed, the second series coupling transformer 42 is withdrawn, and the whole loop closing power conversion process is finished.
In summary, the existing low-voltage distribution network loop closing power transfer operation generally does not need a compensation device, the impact current generated at the moment of loop closing has certain requirements on the parameters of the distribution network device, and the generated loop closing current has a great influence on the relay protection of the distribution network; the traditional loop closing operation is generally judged according to experience or analyzed offline, the operation is required when the load of a ring network is small and the voltage difference between two sides is small, the success rate cannot reach 100%, and the loop closing operation is suitable for a specific operation mode or a power grid with a small scale and is not suitable for increasingly complex power grid loop closing risk analysis.
According to the method, a multi-port topological result is set, on-line loop closing safety and stability evaluation, electric energy quality evaluation and economy evaluation are carried out through pre-calculation of the power flow after loop closing, and an optimal loop closing route is selected; the converter provided by the invention can be used for carrying out series compensation on voltages on two sides of the contact switch, realizes seamless loop closing, has no impact power conversion, and can reduce interference on a relay protection system. Compared with a device for connecting compensation voltage in series on a feeder line side and connecting the compensation voltage in parallel on a direct current side to stabilize voltage, the device does not need to additionally arrange a parallel port for taking electricity and stabilizing voltage from the feeder line, a circuit with larger current margin is selected, the voltage stabilization function can be realized by converting current of a current transformer, an energy storage power supply can also be used as backup selection of voltage stabilization, a compensation point is flexibly selected, and the possibility of a loop-closing power conversion process can be improved.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (8)

1. A multi-port low-voltage distribution network seamless loop-closing transfer device comprises a transformer, a series coupling transformer, a primary side bypass switch of the series coupling transformer, a secondary side bypass switch of the series coupling transformer, a converter, a loop-closing switch, a direct-current side voltage-stabilizing capacitor and an energy storage power supply;
the loop closing switch is arranged among the feeders; the current transformer is connected with the series coupling transformer and is connected into a feeder line in a series connection mode, wherein the current transformer connected with the feeder line with the largest power flow margin is used for stabilizing the voltage of a direct current bus, and the rest current transformers are used for providing series voltage compensation of closed loop and power transfer; the direct current side of each converter is connected in parallel with two ends of the direct current side voltage-stabilizing capacitor; the energy storage power supply is used for stabilizing the voltage of the direct current side when the voltage stabilizing capability of the system converter is insufficient; the primary side bypass switch of the series coupling transformer is arranged between the series coupling transformer and the converter; the secondary side bypass switch of the series coupling transformer is arranged on a feeder line connected with the series coupling transformer; the series coupling transformer side bypass switch and the series coupling transformer secondary side bypass switch are jointly used for switching in and switching out the converter;
when the loop closing is carried out, the voltages on two sides of the loop closing switch are equal by the series voltage of the current transformer; and when the power is switched, the voltage on the two sides of the loop closing point is equal to the voltage of the outgoing line of the maintenance line transformer.
2. The loop closing method of the seamless loop closing transfer device for the multiport low-voltage distribution network according to claim 1 is characterized by comprising the following steps:
(1) reading real-time tide data of a power grid, and setting a voltage difference threshold;
(2) screening an optimal loop closing mode based on the real-time power flow data;
(3) after the phase sequences of three-phase voltages on two sides of a loop closing point are adjusted to be consistent, detecting whether the voltage difference on the two sides of the loop closing point is less than or equal to a voltage difference threshold value in real time; if yes, directly closing the ring; otherwise, turning to the step (4);
(4) judging whether the circuit with the largest power flow margin can support the voltage of the direct current side voltage-stabilizing capacitor, if so, turning to the step (5), otherwise, accessing an energy storage power supply to support the voltage of the direct current side voltage-stabilizing capacitor, and turning to the step (5);
(5) calculating reference voltage values of a plurality of converters according to the voltage difference of two sides of the current loop closing point;
(6) based on the reference voltage value, the plurality of converters perform voltage compensation on the maintenance line through current transformation; and (4) turning to the step (3).
3. The loop closing method according to claim 2, characterized in that said step (2) comprises in particular the steps of:
(2.1) calculating the voltage difference of two sides of the closed-loop switch based on real-time power flow data, network topology analysis and state estimation, and constructing a power grid node admittance matrix;
(2.2) modifying the current injection quantity of a node voltage matrix equation according to the power grid node admittance matrix, and calculating steady-state load flow data after loop closing;
(2.3) carrying out real-time online comprehensive evaluation on the basis of steady-state power flow data after loop closing;
and (2.4) according to the comprehensive evaluation result, carrying out retrieval and verification on the closed-loop path, and selecting the optimal closed-loop topology and closed-loop mode.
4. The loop closing method according to claim 3, wherein the online comprehensive evaluation comprises safety stability evaluation, power quality evaluation and economy evaluation.
5. The loop closing method according to claim 3 or 4, wherein the comprehensive evaluation result is a safety stability index, a power quality index and a network loss index established based on-line comprehensive evaluation; the safety stability indexes comprise loop closing impact current verification, branch thermal stability verification, static safety analysis verification and short circuit current level verification.
6. The looping method according to any one of claims 2 to 4, characterized in that the voltage difference across the looping point is equal to or less than a voltage difference threshold in the form of:
Figure FDA0002975938870000021
Figure FDA0002975938870000022
wherein, ViAnd VjThe voltage on both sides of the loop closing point; delta U is the amplitude value representation of the voltage difference threshold value on the two sides of the closed-loop switch; and delta theta is represented by the phase of the voltage difference threshold value on two sides of the loop closing switch.
7. The power transfer method of the seamless loop-closing power transfer device for the multiport low-voltage power distribution network according to claim 1 is characterized by comprising the following steps:
(1) reading real-time tide data and setting a maximum impact current value;
(2) calculating a reference voltage value of a sending end converter and a change step length of the reference voltage value based on the real-time load flow data;
(3) according to the current reference voltage value, the current of the sending end converter is changed, and the compensation voltage of the series point is changed, so that the voltages on the two sides of the loop closing point are gradually equal to the voltage of the outgoing line of the maintenance line transformer;
(4) judging whether the difference value of the loop closing current and the overhauling line current is smaller than the maximum impact current threshold value or not, and judging whether the current of the transformer outlet end of the overhauling line end is smaller than the switchable current threshold value or not, if so, cutting off the transformer at the overhauling line end; otherwise, updating the current reference voltage value according to the change step length, and turning to the step (3).
8. The method of converting electricity according to claim 7, wherein the condition that the service line side transformer outlet current meets the cuttable current threshold is:
Figure FDA0002975938870000031
Figure FDA0002975938870000032
wherein, the delta I is a current threshold value of a wire outlet end of the cut-off transformer; delta beta is the current angle threshold of the outlet end of the cut transformer; i isr1The current of the transformer outlet end of the line end is overhauled before the power is converted.
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