CN111864762A - Reactive power coordination control method for hybrid multi-feed-in direct current system for reducing switching of filter - Google Patents

Reactive power coordination control method for hybrid multi-feed-in direct current system for reducing switching of filter Download PDF

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CN111864762A
CN111864762A CN202010715751.7A CN202010715751A CN111864762A CN 111864762 A CN111864762 A CN 111864762A CN 202010715751 A CN202010715751 A CN 202010715751A CN 111864762 A CN111864762 A CN 111864762A
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reactive power
hvdc
filter
vsc
reactive
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CN111864762B (en
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姚伟
李佳
严才
夏勇军
陈堃
张侃君
张隆恩
文劲宇
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Huazhong University of Science and Technology
Electric Power Research Institute of State Grid Hubei Electric Power Co Ltd
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Huazhong University of Science and Technology
Electric Power Research Institute of State Grid Hubei Electric Power 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
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • 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
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention discloses a reactive power coordination control method for a hybrid multi-feed-in direct current system for reducing switching of a filter, and belongs to the field of reactive power control of hybrid multi-feed-in direct current transmission systems. Before the reactive power injected into the LCC-HVDC rectifier station by the alternating current power grid exceeds the reactive power regulation interval of the VSC-HVDC receiving end, the VSC-HVDC system is preferentially utilized to regulate the reactive power, the switching of the filter is reduced, the frequent switching problem of the filter is relieved, the service life of primary equipment is prolonged, and the loss is reduced; after the reactive power injected into the LCC-HVDC rectifier station by the AC power grid exceeds the reactive power regulation interval of the VSC-HVDC receiving end, reactive power is immediately compensated, reactive response in a short time is improved, transient low voltage and transient overvoltage of a converter bus of the LCC-HVDC system are effectively inhibited, and transient stability of the power grid is improved. The VSC-HVDC system reactive power regulation method makes full use of the advantage of high reactive power regulation speed of the VSC-HVDC system, assists the filter to carry out switching, inhibits reactive power sudden change at the switching moment and slows down voltage fluctuation.

Description

Reactive power coordination control method for hybrid multi-feed-in direct current system for reducing switching of filter
Technical Field
The invention belongs to the field of reactive power control of hybrid multi-feed-in direct-current power transmission systems, and particularly relates to a reactive power coordination control method of a hybrid multi-feed-in direct-current system for reducing switching of a filter.
Background
And after the VSC-HVDC and the LCC-HVDC are connected into the same alternating current grid, a hybrid multi-feed direct current transmission system (HMIDC) is formed. In a hybrid multi-feed Direct Current system, a Line commuttedconverter based High Voltage Direct Current (LCC-HVDC) and a Voltage source Converter based High Voltage Direct Current (VSC-HVDC) based on a grid commutation Converter generally run independently, and no coordination exists between the LCC-HVDC and the LCC-HVDC, so that the reactive power of the VSC-HVDC system cannot be utilized, and the advantage of dynamic reactive power regulation of the VSC-HVDC system is exerted. And the LCC-HVDC system has frequent filter switching, large equipment loss and short service life during operation.
How to organically combine the technical characteristics of two direct current transmission systems and fully exert the operation advantages of a hybrid system structure becomes an urgent research topic. The prior research is related to the coupling characteristics and coordination control of LCC-HVDC and VSC-HVDC in a hybrid multi-feed system. In the aspect of reactive power coordination control, research and exploration are carried out in some documents, and most of the documents focus on improving LCC-HVDC operation conditions by utilizing the advantages of high reactive power regulation speed and dynamic reactive power regulation of a VSC-HVDC system. Guojia provides reactive control based on a transient voltage deviation amount and transient reactive control based on a gamma angle in the 'additional reactive power control of a hybrid multi-feed-in direct current transmission system', and regulates VSC-HVDC outer loop power control through an additional control link on the premise of not changing the structure of an original control system, so that reactive compensation is rapidly and accurately promoted. However, the above existing researches do not alleviate the problem of frequent switching of filters in the LCC-HVDC system, and since the VSC-HVDC system adopts constant ac voltage control, only when the voltage of the converter bus deviates from the setting value, the VSC-HVDC system compensates the reactive power, and the response speed of the VSC-HVDC system compensates the reactive power is slow.
Disclosure of Invention
Aiming at the defects and the improvement requirements of the prior art, the invention provides a reactive power coordination control method of a hybrid multi-feed-in direct current system for reducing filter switching, and aims to reduce filter switching.
To achieve the above object, according to one aspect of the present invention, there is provided a hybrid multi-feed dc system reactive power coordination control method for reducing filter switching, the method comprising the steps of:
s1, when reactive power injected into an LCC-HVDC rectifier station by an alternating current power grid exceeds a reactive power regulation interval of a VSC-HVDC receiving end, converting an offset value of the injected reactive power into a reactive compensation value, adding the reactive compensation value into an outer ring fixed reactive power control link of the VSC-HVDC receiving end, and regulating the VSC-HVDC receiving end to absorb or send the reactive power, wherein a reactive control strategy of the LCC-HVDC is constant reactive power control of an alternating current filter or constant alternating current voltage control of the alternating current filter;
s2, when the reactive power control strategy of the LCC-HVDC is constant reactive power control of the alternating current filter, judging whether conditions (i-c) are met simultaneously:
firstly, the reactive power of VSC-HVDC receiving end compensation reaches the maximum value of reactive power regulation capacity;
reactive power injected into the LCC-HVDC rectifier station by the alternating current power grid exceeds a control dead zone of constant and reactive power control of the filter;
the two conditions are still met after one switching period is delayed;
if yes, inputting a filter, otherwise, judging whether conditions i-iii are met simultaneously:
the reactive power absorbed by the VSC-HVDC receiving end reaches the minimum value of the reactive power regulation capacity;
injecting reactive power into the LCC-HVDC rectifier station by the alternating current power grid to exceed a control dead zone of constant and reactive power control of the filter;
iii, the two conditions are still met after one switching period is delayed;
if so, cutting off the filter, otherwise, not switching the filter;
when the reactive power control strategy of the LCC-HVDC is constant alternating voltage control of an alternating current filter, judging whether conditions (a) to (c) are met simultaneously:
(a) reactive power compensated by a VSC-HVDC receiving end reaches the maximum value of the reactive power regulation capacity;
(b) LCC-HVDC converter bus voltage exceeding filter constant alternating current voltage control dead zone
(c) The two conditions are still met after one switching period is delayed;
if yes, putting a filter, otherwise, judging whether the conditions A-C are met simultaneously:
the reactive power absorbed by the receiving end of the A, VSC-HVDC reaches the minimum value of the reactive power regulation capacity;
the voltage of the second LCC-HVDC converter bus exceeds the fixed alternating voltage control dead zone of the filter;
third, the two conditions are still met after one switching period is delayed;
if so, cutting off the filter, otherwise, not switching the filter.
Preferably, in step S1, when the reactive power injected by the system is greater than the upper limit value of reactive power compensation of the VSC-HVDC receiving end, the injected reactive power differs from the upper limit value and increases with 0, a positive reactive power compensation value is calculated by the PI controller and added to an outer-loop fixed reactive power control link of the VSC-HVDC receiving end, and the VSC-HVDC receiving end sends out the reactive power; when the reactive power injected by the system is smaller than the lower limit value of reactive compensation of the VSC-HVDC receiving end, the difference between the injected reactive power and the lower limit value is smaller than 0, a negative reactive compensation value is calculated through the PI controller and added to an outer ring fixed reactive power control link of the VSC-HVDC receiving end, and the VSC-HVDC receiving end absorbs the reactive power.
Preferably, when the reactive control strategy of the LCC-HVDC is constant-reactive control of the alternating current filters, the size of a control dead zone is matched with the reactive power of each group of filters and the response characteristic of the alternating current system; when the reactive control strategy of the LCC-HVDC is that the alternating current filter controls the fixed alternating current voltage, the size of the control dead zone is set according to the transient low voltage and the transient high voltage allowed to operate by the LCC-HVDC rectifier station.
Preferably, in step S2, the duration of each stage of the filter switching is required to ensure that the reactive power injected into the ac system by the LCC is equivalently replaced by the ramp-type reactive power.
Preferably, the maximum reactive regulation capacity is QIs free of-QMarginMinimum value of reactive regulation capacity ═ QMargin-QIs free ofWherein the VSC-HVDC receiving end operates to transmit reactive power
Figure BDA0002597214250000041
QIs full ofRated full load power, Q, for VSC-HVDC systemsIs provided withAnd active power is transmitted for VSC-HVDC receiving end operation.
Preferably, the method further comprises the step S3 of evaluating the effectiveness of the reactive power coordination control method under different transmission powers of the LCC-HVDC.
Preferably, the effectiveness evaluation is performed through the switching number of the filter and the maximum voltage deviation rate η:
Figure BDA0002597214250000042
wherein: u shapeac_min(max)The voltage is the lowest value or the highest value of the transient voltage of a converter bus of the LCC-HVDC rectifier station; u shapeac_NAnd the rated voltage is the converter bus of the LCC-HVDC rectifier station.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) before the reactive power injected into the LCC-HVDC rectifier station by the alternating current power grid exceeds the reactive power regulation interval of the VSC-HVDC receiving end, the VSC-HVDC system is preferentially utilized to regulate the reactive power, the switching of the filter is reduced, the frequent switching problem of the filter is relieved, the service life of primary equipment is prolonged, and the loss is reduced; after the reactive power injected into the LCC-HVDC rectifier station by the AC power grid exceeds the reactive power regulation interval of the VSC-HVDC receiving end, reactive power is immediately compensated, reactive response in a short time is improved, transient low voltage and transient overvoltage of a converter bus of the LCC-HVDC system are effectively inhibited, and transient stability of the power grid is improved. Because the invention adopts the reactive power criterion, when the reactive power exchanged by the AC/DC system exceeds a certain range of VSC-HVDC, the reactive power is immediately compensated, and the reactive power is not compensated until the voltage of the converter bus of the LCC-HVDC rectifier station changes, therefore, the response speed is high.
(2) The VSC-HVDC system reactive power regulation method makes full use of the advantage of high reactive power regulation speed of the VSC-HVDC system, assists the filter to carry out switching, inhibits reactive power sudden change at the switching moment and slows down voltage fluctuation.
Drawings
Fig. 1 is a schematic diagram of reactive power coordination control process of a smoothing and filtering device suitable for a hybrid multi-feed dc system according to the present invention;
fig. 2 is a schematic diagram of the yu hubei back-to-back networking engineering provided by the embodiment of the invention;
FIG. 3 is a QPC functional diagram provided by the present invention;
FIG. 4 is a schematic view of Gamma-kick function provided by the present invention;
FIG. 5 is a control logic block diagram of the VSC-HVDC system provided by the present invention;
FIG. 6 is a comparison graph of the characteristics of the present invention under an overload condition;
FIG. 7 is a comparison graph of the characteristics of the present invention under low power conditions.
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. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
As shown in fig. 1, the present invention provides a reactive power coordination control method for a smoothing and filtering system suitable for a hybrid multi-feed dc system, which includes the following steps:
step S1, when reactive power injected into an LCC-HVDC rectifier station by an alternating current power grid exceeds a reactive power regulation interval of a VSC-HVDC receiving end, converting an offset value of the injected reactive power into a reactive compensation value, adding the reactive compensation value into an outer ring fixed reactive power control link of the VSC-HVDC receiving end, and regulating the VSC-HVDC receiving end to absorb or send the reactive power, wherein a reactive control strategy of the LCC-HVDC is constant reactive power control of an alternating current filter or constant alternating current voltage control of the alternating current filter.
Measuring reactive power Q injected into LCC-HVDC rectifier station in AC networkEx_mIn time, the communication delay between the VSC-HVDC receiving end and the LCC-HVDC rectifier station is simulated through a first-order inertia link, where G is a proportionality constant and T is a time constant, and in this embodiment, G is set to 1, and T is set to 0.02 s.
Considering that the reactive power injected into the LCC-HVDC rectifier station by the AC power grid in actual operation is not fixed and has certain fluctuation, a reactive power regulation interval [ Q ] of the VSC-HVDC receiving end is setEX_ref_L，QEX_ref_H]And only when the interval is exceeded, the reactive compensation is carried out. In this embodiment, since the reactive power injected into the LCC-HVDC rectifier station by the ac grid under the rated working condition is about 10Mvar, taking the reactive fluctuation of ± 20Mvar as an example, the upper limit Q is finally setEX_ref_H30Mvar, lower limit QEX_ref_L＝-10Mvar。
When the reactive power injected by the system is greater than the upper limit value of reactive compensation of the VSC-HVDC receiving end, the injected reactive power is different from the upper limit value and is larger than 0, a positive reactive compensation value is calculated through a PI controller and is added to an outer ring fixed reactive power control link of the VSC-HVDC receiving end, and the VSC-HVDC receiving end sends out reactive power; when the reactive power injected by the system is smaller than the lower limit value of reactive compensation of the VSC-HVDC receiving end, the difference between the injected reactive power and the lower limit value is smaller than 0, a negative reactive compensation value is calculated through the PI controller and added to an outer ring fixed reactive power control link of the VSC-HVDC receiving end, and the VSC-HVDC receiving end absorbs the reactive power.
S2, when the reactive power control strategy of the LCC-HVDC is constant reactive power control of the alternating current filter, judging whether conditions (i-c) are met simultaneously:
firstly, the reactive power of VSC-HVDC receiving end compensation reaches the maximum value of reactive power regulation capacity;
reactive power injected into the LCC-HVDC rectifier station by the alternating current power grid exceeds a control dead zone of constant and reactive power control of the filter;
the two conditions are still met after one switching period is delayed;
if yes, inputting a filter, otherwise, judging whether conditions i-iii are met simultaneously:
the reactive power absorbed by the VSC-HVDC receiving end reaches the minimum value of the reactive power regulation capacity;
injecting reactive power into the LCC-HVDC rectifier station by the alternating current power grid to exceed a control dead zone of constant and reactive power control of the filter;
iii, the two conditions are still met after one switching period is delayed;
if so, cutting off the filter, otherwise, not switching the filter;
when the reactive power control strategy of the LCC-HVDC is the constant alternating voltage control of the alternating current filter, whether the conditions of (i) - (iii):
firstly, the reactive power of VSC-HVDC receiving end compensation reaches the maximum value of reactive power regulation capacity;
LCC-HVDC converter bus voltage exceeding filter constant AC voltage control dead zone
The two conditions are still met after one switching period is delayed;
if yes, inputting a filter, otherwise, judging whether conditions i-iii are met simultaneously:
the reactive power absorbed by the VSC-HVDC receiving end reaches the minimum value of the reactive power regulation capacity;
the voltage of the LCC-HVDC converter bus exceeds a fixed alternating voltage control dead zone of the filter;
iii, the two conditions are still met after one switching period is delayed;
if so, cutting off the filter, otherwise, not switching the filter.
VSC-HVDC system is limited by its own element, and the reactive power which can be provided under the condition of constant transmission active power is limited, so that the reactive power regulation capacity [ Q ] is setVSC_MIN，QVSC_MAX]. The VSC-HVDC receiving end reactive power regulation capacity is calculated according to full-load power, in the embodiment, rated full-load power of a VSC-HVDC system is 1250MW, active power transmitted by the VSC-HVDC receiving end in operation is 1000MW, the maximum reactive power regulation capacity of the VSC-HVDC obtained through calculation is 750Mvar, certain safety margin is considered, and Q is setVSC_MAX＝600Mvar，QVSC_MIN＝-600Mvar。
When the reactive power control strategy of the LCC-HVDC is constant reactive power control of the alternating current filter, in order to prevent frequent switching of the filter, the constant reactive power control of the alternating current filter is provided with a control dead zone [ Q ]MIN，QMAX]In this embodiment, because the reactive power injected into the LCC-HVDC rectifier station by the ac grid under the rated condition is about 10Mvar, the reactive capacity of the filter of the LCC-HVDC rectifier station is 170 Mvar/group, the dead zone setting value is generally slightly larger than half of the capacity of a group of filters, is set to ± 95Mvar, and finally, Q is setMIN＝-85Mvar，QMAX＝105Mvar。
When the reactive power control strategy of the LCC-HVDC is constant alternating voltage control of an alternating current filter, a dead zone [ U ] is controlledMIN，UMAX]The size of the dead zone is set according to the transient low voltage and the transient high voltage that the LCC-HVDC rectifier station allows to operate.
The response of a switch and a filter of the circuit breaker needs a certain time, so that the switching instruction of the filter is sent for the next switching instruction after delaying for 500ms, that is, when the reactive power of VSC-HVDC receiving end compensation reaches the maximum reactive power regulation capacity and the reactive power injected by the system exceeds the control dead zone of the fixed reactive power control of the alternating current filter, and if the two conditions are met after delaying for 500ms, the switching instruction of the filter is sent.
Control dead band [ QMIN，QMAX]，QMAXGreater than half of the capacity of a bank of filters, | QMINI is greater than half the capacity of a bank of filters.
The filter switches the duration of each stage, and reactive power injected into an alternating current system by the LCC needs to be equivalently replaced by oblique reactive power, so that the aims of reducing reactive impact and slowing down voltage fluctuation are fulfilled.
In this embodiment, the switching of the VSC-HVDC receiving-end auxiliary filter is divided into two cases:
(1) when the filter is put into use, the reactive power sent by the VSC-HVDC receiving end is rapidly reduced to a reactive capacity value equal to that of the filter to be put into use within 30ms, and then the VSC-HVDC receiving end is restored to the original state within 470 ms.
(2) When the filter is cut off, the reactive power absorbed by the VSC-HVDC receiving end is rapidly reduced by a reactive capacity value equal to that of the filter to be put into the VSC-HVDC receiving end within 30ms, and then the VSC-HVDC receiving end is restored to the original state within 470 ms.
Maximum value of reactive power regulating capacity (Q)Is free of-QMarginMinimum value of reactive regulation capacity ═ QMargin-QIs free ofWherein the VSC-HVDC receiving end operates to transmit reactive power
Figure BDA0002597214250000081
QIs full ofRated full load power, Q, for VSC-HVDC systemsIs provided withAnd active power is transmitted for VSC-HVDC receiving end operation.
The method further comprises the step S3 of evaluating the effectiveness of the reactive power coordination control method under different transmission powers of the LCC-HVDC.
And (3) carrying out effectiveness evaluation through the switching number of the filters and the maximum voltage deviation rate eta:
Figure BDA0002597214250000082
wherein: u shapeac_min(max)For the lowest or the highest transient voltage of the converter busbar of the LCC-HVDC rectifier stationA high value; u shapeac_NAnd the rated voltage is the converter bus of the LCC-HVDC rectifier station.
In the embodiment, Yubei back-to-back networking engineering is taken as a research object, and reactive power control strategies and control characteristics of LCC-HVDC and VSC-HVDC are firstly analyzed; based on the control characteristic analysis, the method is provided; and finally, building a simulation model of the hybrid multi-feed direct current system in the PSCAD/EMTDC, and performing simulation verification on the effectiveness of the method.
The simulation model of this embodiment adopts the hybrid multi-feed dc system as shown in fig. 2, the sending end of the flexible dc back-to-back system is located in the nine disks of the southwest power grid, and the receiving end is located in the longquan of the chinese power grid, so as to realize asynchronous interconnection of the southwest power grid and the chinese power grid, and the electric power of the three gorges hydropower station is transmitted to the load center of the east china power grid through the longzheng dc. Obviously, the construction of the project enables the dragon spring to form a hybrid multi-feed direct current system. Because the electrical distance between the receiving end of the flexible direct current back-to-back system and the rectification side of the Longzheng direct current is short, the reactive power requirement change of the rectification side of the Longzheng direct current can be met by adjusting the reactive power sent by the receiving end of the flexible direct current back-to-back system.
The reactive control analysis of LCC-HVDC is as follows:
the basic reactive power control strategy of LCC-HVDC mainly comprises absolute minimum filter bank control, limit voltage control, maximum reactive power limitation and constant reactive power or constant voltage control of an alternating current filter bank. The absolute minimum filter bank control, the limit voltage control and the minimum reactive power limitation are limiting conditions mainly set for normal operation of the alternating-current and direct-current power transmission system. The priority of the control strategy is arranged from high to low as follows:
1) absolute minimum filterbank control: the safe operation of the filter is protected, and the overload operation is prevented.
2) Limiting voltage control: and ensuring that the voltage of the commutation bus does not exceed the set maximum limit value and minimum limit value.
3) Maximum reactive power limit: overvoltage is prevented from occurring when the AC/DC system exchanges reactive power.
4) Minimum filterbank control function: and under the condition of meeting the AC/DC harmonic characteristics, the filter groups with the minimum input quantity and the proper models are controlled.
5) And (3) constant-reactive power or constant-voltage control of the alternating current filter: the control comprises two control targets, wherein one control target is constant reactive power control for controlling the reactive power exchanged by an alternating current and direct current system in operation within a specified range, and the other control target is constant alternating current voltage control for controlling the voltage of a current converting bus within a specified range. The two are selected from one another, and can not be started simultaneously.
In addition, in order to meet certain specific working conditions, a reactive power control (QPC) function and a Gamma-kick function are also provided, and schematic diagrams thereof are respectively shown in FIG. 3 and FIG. 4. The effect of the QPC function is to consume overcompensated reactive power by increasing the converter firing or extinction angle when for some reason the filter cannot be cut off. The Gamma-kick function is used for adjusting reactive power consumed by the current converter by quickly changing the arc extinguishing angle so as to counteract reactive impact brought by the switching moment of the filter. But both functions require a change in the LCC-HVDC system operating firing angle at the expense of the economy of the conventional dc unit, so there is further room for optimization in a hybrid multi-feed dc system.
When the alternating current filter adopts constant reactive power control, the LCC-HVDC system switches the filter according to the reactive power exchanged by the alternating current-direct current system, so that reactive local balance is realized. In order to avoid frequent switching of the filter, the fixed reactive power is provided with a dead zone [ Q ]MIN,QMAX]Wherein Q isMAXUpper limit value for reactive power exchange, QMINIs the lower limit value of reactive power exchange. The switch and filter response of the circuit breaker needs a certain time, so that the reactive control of the LCC-HVDC sends a switching instruction for a period of time and then sends a next switching instruction.
Because of the dead zone and the time delay, the reactive power adjusting response speed by switching the filter is slow, and the reactive power capacity of the filter is a fixed value, the reactive power can not be continuously adjusted, and only the filter is in a step shape, and reactive power mutation and large voltage fluctuation can be caused at the switching moment. In addition, when the power is operated at 0.1 times of low power, the voltage of a converter bus is overhigh due to reactive overcompensation and limitation of the control function of the minimum filter bank.
Reactive analysis of VSC-HVDC is as follows:
the VSC-HVDC system control strategy block diagram is shown in FIG. 5, and adopts double closed-loop current vector control, and mainly comprises an outer loop controller, an inner loop controller, a phase-locked synchronizer and a trigger pulse generator. The power or voltage instruction value is transmitted to the inner loop controller through the outer loop controller to obtain an active current setting value and a reactive current setting value, a d-axis component and a q-axis component of the outlet voltage of the converter are obtained through coupling calculation, and finally a sinusoidal modulation wave based on an abc coordinate system is obtained through coordinate system change.
Double closed-loop current vector control for realizing active current idAnd a reactive current iqDecoupled control of, actual value of reactive power QsFast following reactive setting value QsrefTherefore, the reactive power regulation speed of the VSC-HVDC system is high, and continuous regulation can be realized. However, due to the limitation of the elements of the VSC-HVDC system, the reactive power which can be provided without changing the transmission active power is limited, so that the reactive power regulation capacity of the VSC-HVDC system is small.
According to the analysis, the adjustable reactive capacity of the switching filter is large, but the response speed is low, and continuous adjustment cannot be realized; the VSC-HVDC system has fast reactive power regulation speed and can realize continuous regulation, but has limited reactive power regulation capability. Based on this, a reactive coordination control module is arranged at the receiving end of the VSC-HVDC system, and particularly as shown in fig. 1, the reactive coordination control is carried out by fully utilizing the advantages of filter switching and reactive regulation of the VSC-HVDC system. Aiming at the problem of frequent switching of the filter in actual operation, the VSC-HVDC system is preferentially utilized to adjust the reactive power, the switching of the filter is reduced, in addition, the advantage of high reactive power adjusting speed of the VSC-HVDC system is fully utilized, the switching of the filter is assisted, the reactive power mutation is inhibited, and the voltage fluctuation is slowed down.
In order to verify the effectiveness of the method, a simulation model of a hybrid multi-feed direct current system shown in fig. 2 is built in PSCAD/EMTDC, wherein a receiving end of a VSC-HVDC system is connected with a rectifying station of an LCC-HVDC system through a connecting line with the length of 20 km.
The main parameters of the VSC-HVDC and LCC-HVDC systems are shown in Table 1.
Figure BDA0002597214250000111
TABLE 1
The method of the invention has the following function and effect analysis:
a) overload condition
When the direct current instruction value of LCC-HVDC is linearly increased from 1.0 to 1.1 in 2s, the influences of the fixed reactive power control, the fixed alternating voltage control and the reactive coordination control of the invention on the filter number of the rectification side of the LCC-HVDC system, the reactive power sent by the receiving end of the VSC-HVDC system, the reactive power injected into a rectification station by an alternating current power grid at the rectification side of the LCC-HVDC system and the voltage of a current conversion bus at the rectification side of the LCC-HVDC system are analyzed, and the simulation result is shown in fig. 6.
As shown in fig. 6, when the power transmitted by the LCC-HVDC system increases, the reactive power is under-compensated, and the reactive power injected by the grid into the rectifier station increases rapidly, and the voltage drops. When the constant reactive power control is adopted, due to the fact that delay exists in the switching of the filters, when the reactive power injected into the rectifying station by the power grid exceeds the dead zone range, timing is started, the reactive power injected into the rectifying station still exceeds the dead zone range after a period of time, 1 group of filters are put into the rectifying station, and the lowest transient voltage of a converter bus is 0.9776 p.u.; when the constant alternating voltage control is adopted, the voltage is lower than a setting value, the receiving end of the VSC-HVDC system sends out reactive power for compensation, the reactive power injected into a rectifying station by a power grid is reduced, the voltage drop is restrained, and the transient voltage of a converter bus is 0.9821p.u. the lowest; when the reactive coordination control is adopted, the reactive power is immediately compensated when the reactive power exceeds a certain range, the reactive response is fast, the voltage drop is better inhibited, and the minimum transient voltage of the current conversion bus is 0.9865 p.u..
b) Low power operating conditions
When the direct current instruction value of LCC-HVDC is changed from 1.0 to 0.5 at 2s, the influence of the fixed reactive power control, the fixed alternating voltage control and the reactive coordination control of the invention on the filter number at the rectification side of the LCC-HVDC system, the reactive power sent by the receiving end of the VSC-HVDC system, the reactive power injected into the rectification station by the alternating current power grid at the rectification side of the LCC-HVDC system and the voltage of the current conversion bus at the rectification side of the LCC-HVDC system when the receiving end of the VSC-HVDC system respectively adopts the fixed reactive power control, the fixed alternating voltage control and the reactive coordination control is analyzed.
As shown in fig. 7, when the power transmitted by the LCC-HVDC system is reduced, the reactive power is overcompensated, the rectifying station reversely injects reactive power into the grid, and the voltage rises. When the constant reactive power control is adopted, 4 groups of filters need to be cut off to reduce reactive output, and because delay and dead zones exist in the switching of the filters, the reactive power cannot be immediately regulated at first, so that a current conversion bus is rapidly increased, the maximum transient voltage is 1.1081p.u., and the reactive power cannot be continuously regulated, so that reactive power mutation is caused at the cutting-off moment, and the voltage fluctuation is large; when the constant alternating voltage control is adopted, the voltage is higher than a setting value, the receiving end of the VSC-HVDC system absorbs reactive power, the reactive power injected into a power grid by a finishing station is reduced, the voltage rise is restrained, the transient voltage of a current conversion bus is 1.0782p.u. at most, 1 group of filter balance reactive power needs to be cut off when the absorbed reactive power reaches the maximum value, and reactive power mutation and voltage fluctuation also exist at the cutting-off moment; when reactive coordination control is adopted, reactive power is immediately compensated when the reactive power exceeds a certain range, reactive response is fast, voltage drop is better inhibited, the transient voltage of a current conversion bus is 1.0620p.u., 1 group of filters also needs to be cut off, but at the moment of cutting off the reactive power compensator, the reactive power absorbed by the receiving end of the VSC-HVDC system is reduced by a certain value within a short time, and then the VSC-HVDC system is restored to the original state for a long time, so that reactive sudden change at the moment of cutting off is inhibited, and reactive power change and voltage fluctuation are slowed down.
c) Validity verification of LCC-HVDC at different transmission powers
When LCC-HVDC is operated with overload (low power), reactive power consumed by the converter is increased (reduced), reactive under compensation (overcompensation) is needed, a reactive compensator needs to be put into (cut off), and reduction (increase) of the voltage of a converter bus is restrained. In order to evaluate the effectiveness of the reactive coordination control strategy of the invention under different transmission powers of the LCC-HVDC system, the maximum voltage deviation rate eta is defined as shown in formula (1) except for comparing the switching number of the filters, and the smaller the value, the better the control performance is.
Figure BDA0002597214250000131
In the formula: u shapeac_min(max)The minimum value or the maximum value of transient voltage of a converter bus of the LCC-HVDC system rectifier station is obtained; u shapeac_NAnd the rated voltage is provided for a converter bus of the LCC-HVDC system rectifier station.
The power transmitted by the LCC-HVDC system is changed, the switching number of the filters and the maximum voltage deviation rate eta are used as evaluation indexes, and effectiveness simulation results under different transmission powers are shown in a table 2, wherein "+" and "-" respectively represent the switching-in and the cutting-out of the filters.
Figure BDA0002597214250000132
Figure BDA0002597214250000141
TABLE 2
Simulation results show that compared with constant reactive power control, the reactive coordination control of the invention can reduce filter switching, inhibit transient low voltage and transient high voltage, and simultaneously inhibit reactive sudden change and slow down voltage fluctuation. Compared with the fixed alternating voltage, the maximum voltage deviation rate of the reactive coordination control of the invention is slightly larger than the fixed alternating voltage when the load is 0.9 times and 0.1 time, but other working conditions are obviously smaller, and the reactive coordination control is slightly superior to the fixed alternating voltage control in the aspects of reactive response and suppression of transient low voltage and transient high voltage; the reactive coordination control of the invention is obviously superior to the constant alternating voltage control in the aspects of suppressing reactive sudden change and slowing down voltage fluctuation.
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 (7)

1. A reactive power coordination control method for a hybrid multi-feed-in direct current system for reducing switching of a filter is characterized by comprising the following steps:
s1, when reactive power injected into an LCC-HVDC rectifier station by an alternating current power grid exceeds a reactive power regulation interval of a VSC-HVDC receiving end, converting an offset value of the injected reactive power into a reactive compensation value, adding the reactive compensation value into an outer ring fixed reactive power control link of the VSC-HVDC receiving end, and regulating the VSC-HVDC receiving end to absorb or send the reactive power, wherein a reactive control strategy of the LCC-HVDC is constant reactive power control of an alternating current filter or constant alternating current voltage control of the alternating current filter;
s2, when the reactive power control strategy of the LCC-HVDC is constant reactive power control of the alternating current filter, judging whether conditions (i-c) are met simultaneously:
firstly, the reactive power of VSC-HVDC receiving end compensation reaches the maximum value of reactive power regulation capacity;
reactive power injected into the LCC-HVDC rectifier station by the alternating current power grid exceeds a control dead zone of constant and reactive power control of the filter;
the two conditions are still met after one switching period is delayed;
if yes, inputting a filter, otherwise, judging whether conditions i-iii are met simultaneously:
the reactive power absorbed by the VSC-HVDC receiving end reaches the minimum value of the reactive power regulation capacity;
injecting reactive power into the LCC-HVDC rectifier station by the alternating current power grid to exceed a control dead zone of constant and reactive power control of the filter;
iii, the two conditions are still met after one switching period is delayed;
if so, cutting off the filter, otherwise, not switching the filter;
when the reactive power control strategy of the LCC-HVDC is constant alternating voltage control of an alternating current filter, judging whether conditions (a) to (c) are met simultaneously:
(a) reactive power compensated by a VSC-HVDC receiving end reaches the maximum value of the reactive power regulation capacity;
(b) LCC-HVDC converter bus voltage exceeding filter constant alternating current voltage control dead zone
(c) The two conditions are still met after one switching period is delayed;
if yes, putting a filter, otherwise, judging whether the conditions A-C are met simultaneously:
the reactive power absorbed by the receiving end of the A, VSC-HVDC reaches the minimum value of the reactive power regulation capacity;
the voltage of the second LCC-HVDC converter bus exceeds the fixed alternating voltage control dead zone of the filter;
third, the two conditions are still met after one switching period is delayed;
if so, cutting off the filter, otherwise, not switching the filter.
2. The method according to claim 1, wherein in step S1, when the reactive power injected by the system is greater than the upper limit value of reactive power compensation of the VSC-HVDC receiving end, the injected reactive power is different from the upper limit value and is greater than 0, a positive reactive power compensation value is calculated through the PI controller, and is added to the outer fixed reactive power control link of the VSC-HVDC receiving end, so that the VSC-HVDC receiving end sends out the reactive power; when the reactive power injected by the system is smaller than the lower limit value of reactive compensation of the VSC-HVDC receiving end, the difference between the injected reactive power and the lower limit value is smaller than 0, a negative reactive compensation value is calculated through the PI controller and added to an outer ring fixed reactive power control link of the VSC-HVDC receiving end, and the VSC-HVDC receiving end absorbs the reactive power.
3. The method according to claim 1 or 2, characterized in that when the reactive control strategy of the LCC-HVDC is ac filter constant reactive control, the size of the control dead zone is matched to the magnitude of the reactive power of each set of filters and the response characteristics of the ac system; when the reactive control strategy of the LCC-HVDC is that the alternating current filter controls the fixed alternating current voltage, the size of the control dead zone is set according to the transient low voltage and the transient high voltage allowed to operate by the LCC-HVDC rectifier station.
4. A method according to any one of claims 1 to 3, wherein in step S2, the duration of the filter switching stages is such that the reactive power injected into the ac system by the LCC is replaced by a ramp-type reactive power.
5. Method according to any of claims 1 to 4, characterized in that the maximum reactive regulation capacity is QIs free of-QMarginMinimum value of reactive regulation capacity ═ QMargin-QIs free ofWherein the VSC-HVDC receiving end operates to transmit reactive power
Figure FDA0002597214240000021
QIs full ofRated full load power, Q, for VSC-HVDC systemsIs provided withAnd active power is transmitted for VSC-HVDC receiving end operation.
6. The method according to any of the claims 1 to 5, characterized in that the method further comprises a step S3. evaluation of the effectiveness of the reactive power coordination control method for LCC-HVDC different transmission powers.
7. The method according to claim 6, characterized in that the validity assessment is performed by the number of switches of the filter and the maximum voltage excursion ratio η:
Figure FDA0002597214240000031
wherein: u shapeac_min(max)The voltage is the lowest value or the highest value of the transient voltage of a converter bus of the LCC-HVDC rectifier station; u shapeac_NAnd the rated voltage is the converter bus of the LCC-HVDC rectifier station.
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