A control method for island operation of a flexible direct current power transmission system

Abstract

The application provides a control method for island operation of a flexible direct current power transmission system. The flexible direct current power transmission system is connected to an alternating current system, and the flexible direct current power transmission system comprises a converter. The method comprises: when the flexible direct current power transmission system is in island operation, starting and steady state operation are realized in an open loop control mode in a starting and non-fault state; and when a fault of the alternating current system is detected, the open loop control mode is switched to a double closed loop control mode to limit fault current and suppress interference.

Description

Technical Field

[0001] This application relates to the field of flexible DC transmission technology in power systems, specifically to a control method for islanded operation of a flexible DC transmission system. Background Technology

[0002] To address the growing energy shortage and environmental degradation, the development and utilization of new energy sources such as wind power and photovoltaics have rapidly progressed. Currently, most new energy power generation bases are located far from load centers, where local AC power grids are relatively weak. Large-scale integration of new energy sources may lead to reduced power quality, decreased system stability, and altered transient characteristics in the regional power grid. Flexible DC transmission systems, employing turn-off power electronic devices, feature flexible control, rapid dynamic response, and low harmonic content. They can quickly and independently regulate active and reactive power, possessing flexible control and coordination capabilities. Widely considered a reliable and effective means of integrating and utilizing new energy sources, they have broad application prospects.

[0003] When a flexible DC converter station is connected to an AC grid area without AC power or connected to a new energy system, the flexible DC converter station needs to provide a stable AC voltage to the passive system or new energy source in islanded control mode. Considering that the flexible DC converter station is prone to high-frequency oscillations when starting in islanded mode, the islanded control mode adopts open-loop control to directly output a constant AC voltage to avoid oscillations during startup. Since open-loop control has no feedback loop and cannot suppress interference, when a fault occurs in the AC system and current limiting is required, the control mode should be switched to closed-loop control mode. By setting a current limiting loop in the controller to control the fault current from exceeding the limit, tripping of the flexible DC converter station due to fault overcurrent can be avoided.

[0004] Existing literature on islanded operation control strategies for flexible DC transmission systems is mostly limited to specific open-loop and closed-loop control, with few studies exploring control strategies that flexibly combine the advantages of both modes. For example, "Research on Black-Start Control Capability of Flexible DC Transmission Systems" (Zeng Dan & Yao Jianguo, Modern Power, 2012) proposed an open-loop constant voltage and constant frequency control strategy, but did not address dual closed-loop control and switching. "Direct Voltage Control of Passive Network Power Supply for MMC-type Flexible DC Transmission Systems" (Guan Minyuan & Xu Zheng, Electric Power Automation, 2012) proposed a dual closed-loop control strategy for passive systems, achieving current suppression under fault conditions. However, it did not address open-loop control and switching.

[0005] The combination of open-loop and closed-loop control also has the problem of system instability after switching. During startup and stable operation, the system uses open-loop control, while the integrator of the closed-loop control may saturate. During the switching process, the integrator undergoes a desaturation process, during which the closed-loop control fails, leading to system instability. Summary of the Invention

[0006] This application provides a control method for islanded operation of a flexible DC transmission system. The flexible DC transmission system includes an AC system and a converter. The method includes: when the flexible DC transmission system is in islanded operation, the converter is started and operated in a steady-state manner using an open-loop control method in both start-up and non-fault states; when a fault is detected in the AC system, the converter switches from the open-loop control method to a dual closed-loop control method to limit the fault current and suppress interference.

[0007] According to some embodiments, the open-loop control mode includes: using a preset converter grid-side AC voltage and a preset frequency reference value as inputs to the open-loop controller, and outputting a constant converter AC voltage reference wave after adjustment by the open-loop controller, thereby realizing the control of the AC side current waveform and phase of the converter.

[0008] According to some embodiments, the dual closed-loop control method includes outer loop control and inner loop control. The outer loop control inputs the AC voltage and frequency of the converter grid side, passes through a proportional-integral controller, and outputs inner loop active current reference values ​​and inner loop reactive current reference values. The inner loop control receives the inner loop active current reference values ​​and inner loop reactive current reference values ​​from the outer loop control, and quickly tracks the inner loop active current reference values ​​and inner loop reactive current reference values ​​to achieve direct control of the AC side current waveform and phase of the converter.

[0009] According to some embodiments, the dual closed-loop control method further includes a current limiting element, which controls the fault current of the AC system to prevent it from exceeding the limit.

[0010] According to some embodiments, when switching from the open-loop control mode to the dual closed-loop control mode, an initial value is assigned to the input of the integrator in the outer loop control. This avoids control failure and system instability caused by the integrator desaturation process.

[0011] According to some embodiments, the detection of the AC system fault includes the following monitoring method: based on a voltage criterion, when the system voltage U... s System voltage preset low value U s_setL or U s System voltage preset high value U s_setH Or when DC voltage U DC DC voltage preset low value U DC_setL or U DC DC voltage preset high value U DC_setH Or when the zero-sequence voltage U0 on the AC valve side > the zero-sequence voltage threshold U on the AC valve side 0_set At this time, the system switches from open-loop control to dual closed-loop control; according to the current criterion, when the grid-side current I... s >Grid-side current threshold I s_setOr bridge arm current I b >Bridge arm current threshold I b_set At this time, the system switches from open-loop control to dual closed-loop control; according to the power criterion, when the active power P < active power threshold P _set When the system switches from open-loop control to dual closed-loop control, based on the frequency criterion, when the system frequency f < the preset low value f... _setL Or f>system frequency preset high value f _setH At this time, the system switches from open-loop control to dual closed-loop control.

[0012] According to some embodiments, the system voltage preset low value U s_setL ≤1pu, system voltage preset high value U s_setH ≥1pu; DC voltage preset low value U DC_setL ≤1pu, DC voltage preset high value U DC_setH ≥1pu; AC valve-side zero-sequence voltage threshold U 0_set ≥0pu, grid-side current threshold I s_set ≥1pu, bridge arm current threshold I b_set ≥1pu, active power threshold P _set ≤1 pu, system frequency preset low value f _setL <50Hz, system frequency preset high value f _setH >50Hz, where pu is a per-unit value, and the per-unit value here is the relative value between the actual value and the rated value.

[0013] According to some embodiments, the flexible DC transmission system can operate with or without load when in islanded operation.

[0014] The technical solution provided in this application avoids high-frequency oscillations caused by connecting transformers or short unloaded lines in the islanded open-loop mode. In the event of a fault, the system switches from open-loop mode to dual closed-loop control mode, effectively suppressing fault overcurrent. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of a flexible DC transmission system according to an embodiment of this application.

[0017] Figure 2 This is a schematic flowchart of a control method for islanded operation of a flexible DC transmission system according to an embodiment of this application.

[0018] Figure 3 This is a schematic diagram of the open-loop control mode of an islanded operation control method according to an embodiment of this application.

[0019] Figure 4 This is a schematic diagram of a dual-closed-loop control mode of an islanded operation control method according to an embodiment of this application.

[0020] Figure 5 This is a schematic diagram of another islanded operation control method with dual closed-loop control, as described in an embodiment of this application. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] It should be understood that the terms "comprising" and "including" used in the specification and claims of this application indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0023] Figure 1 This is a schematic diagram of a flexible DC transmission system according to an embodiment of this application.

[0024] like Figure 1 As shown, the flexible DC transmission system includes an AC system, a converter, a connecting transformer 3, a new energy source 1, and DC field equipment 4.

[0025] The primary side of the connecting transformer 3 is connected to the new energy source 1 through the incoming line switch 2. The secondary side of the connecting transformer 3 is connected to the converter and DC field equipment 4. The converter and DC field equipment 4 is connected to the converter and DC field equipment 6 of the opposite station through the DC line 5. The converter and DC field equipment 6 of the opposite station is connected to the AC power grid or power source 9 through the connecting transformer 7 of the opposite station and the primary side switch 8 of the opposite station.

[0026] Figure 2 This is a schematic flowchart of a control method for islanded operation of a flexible DC transmission system according to an embodiment of this application.

[0027] In S10, when the flexible DC transmission system is in islanded operation, the converter achieves startup and steady-state operation in both startup and non-faulty states using open-loop control.

[0028] When the flexible DC transmission system is in islanded operation, it can operate with or without load.

[0029] After the flexible DC transmission system is unlocked in islanded mode, it is started using an open-loop control method, such as... Figure 3 As shown, the preset d-axis component U of the converter grid-side AC voltage d * Preset converter grid-side AC voltage d-axis component U q * and preset frequency reference value f ref * As the input to the open-loop controller, after PI regulation by the open-loop controller, the output is a constant AC voltage reference wave for the converter, vd = U. d * vq = 0, which enables control of the AC side current waveform and phase of the converter.

[0030] During islanded startup of the flexible DC transmission system, an open-loop control mode is adopted, with the voltage reference wave starting from 0kV and rising to the rated voltage U at a rate of αkV / s. N After establishing a stable AC voltage of kV, the flexible DC transmission system maintains open-loop control operation to provide stable voltage amplitude and frequency for new energy systems or passive AC systems.

[0031] This startup method avoids large disturbances and impacts during islanded startup, suppresses high-frequency components and distortions in output voltage and current, and maintains stable system operation.

[0032] In S20, when an AC system fault is detected in the flexible DC transmission system, the converter switches from open-loop control to dual closed-loop control to limit the fault current and suppress interference.

[0033] An AC system fault was detected, and the detection methods included voltage criteria, current criteria, power criteria, and frequency criteria.

[0034] According to the voltage criterion, when the system voltage U s System voltage preset low value U s_setL or U s System voltage preset high value U s_setH Or when DC voltage U DC DC voltage preset low value U DC_setL or U DC DC voltage preset high value U DC_setH Or when the zero-sequence voltage U0 on the AC valve side > the zero-sequence voltage threshold U on the AC valve side 0_set At this time, the system switches from open-loop control to dual closed-loop control.

[0035] Among them, the system voltage preset low value U s_setL ≤1pu, system voltage preset high value U s_setH ≥1pu; DC voltage preset low value U DC_setL≤1pu, DC voltage preset high value U DC_setH ≥1pu; AC valve-side zero-sequence voltage threshold U 0_se t ≥0pu. Where pu is a per-unit value, and the per-unit value here is the relative value between the actual value and the rated value.

[0036] According to the current criterion, when the grid-side current I s >Grid-side current threshold I s_set Or bridge arm current I b >Bridge arm current threshold I b_set At this time, the system switches from open-loop control to dual closed-loop control.

[0037] Among them, the grid-side current threshold I s_set ≥1pu, bridge arm current threshold I b_set ≥1pu.

[0038] According to the power criterion, when the active power P < the active power threshold P _set At this time, the system switches from open-loop control to dual closed-loop control.

[0039] Among them, the active power threshold P _set ≤1pu.

[0040] According to the frequency criterion, when the system frequency f < the preset low value of the system frequency f _setL Or f>system frequency preset high value f _setH At this time, the system switches from open-loop control to dual closed-loop control.

[0041] System frequency preset low value f _setL <50Hz, system frequency preset high value f _setH >50Hz.

[0042] Dual closed-loop control includes outer loop control and inner loop control, such as Figure 4 As shown.

[0043] In the outer loop control, the input converter grid-side AC voltage U d And the preset converter grid-side AC voltage d-axis component U d * The differential input is given to the proportional-integral (PI) controller, which outputs the inner-loop active current reference value. The input is the AC voltage U on the grid side of the converter. q And the preset converter grid-side AC voltage q-axis component U q * The difference is input to the proportional-integral controller (PI), and after passing through the PI controller, the inner loop reactive current reference value is output.

[0044] In the inner loop control, the system receives the inner loop active current reference value and the inner loop reactive current reference value from the outer loop control, and quickly tracks the inner loop active current reference value, the inner loop reactive current reference value, and the preset frequency reference value f. ref * This enables direct control of the AC side current waveform and phase of the converter.

[0045] In the dual closed-loop control mode, the outer loop controls the output voltage amplitude and frequency, while the inner loop controls the current.

[0046] In the control mode selection, setting it to A is the open-loop control mode, and setting it to B is the dual closed-loop control mode.

[0047] Optionally, to meet the requirement that the current does not exceed the limit under fault conditions, the dual closed-loop control method also includes a current-limiting circuit Idmax to control the fault current of the AC system to not exceed the limit, such as... Figure 5 As shown. The current limiting circuit is set between the outer loop control and the inner loop control channels to prevent the flexible DC transmission system from tripping due to overcurrent in the event of a fault.

[0048] Optionally, when switching from open-loop control to dual closed-loop control, an initial value is assigned to the input of the integrator in the outer loop control. This initial value is between -2 and 2 times the integral value of the integrator during normal operation before the fault, but not limited to this value, so that the integrator immediately exits the saturation state. This is to avoid control failure and system instability caused by the desaturation process of the integrator in the closed-loop control.

[0049] The technical solution provided in this application avoids high-frequency oscillations caused by connecting transformers or short unloaded lines in islanded open-loop operation. In case of a fault, the system switches from open-loop mode to dual closed-loop control mode, effectively suppressing fault overcurrent. During control switching, by assigning an initial value to the input of the integrator in the outer loop control, control failure and system instability caused by the integrator desaturation process in the closed-loop control are avoided.

[0050] The above embodiments are only for illustrating the technical concept of this application and should not be used to limit the scope of protection of this application. Any modifications made to the technical solution based on the technical concept proposed in this application shall fall within the scope of protection of this application.

Claims

1. A control method for islanded operation of a flexible DC transmission system, wherein the flexible DC transmission system is connected to an AC system, the flexible DC transmission system includes a converter, and the method includes: When the flexible DC transmission system is in islanded operation, the converter is started up and operated in steady state using an open-loop control method in both startup and non-fault states. When a fault is detected in the AC system, the converter switches from the open-loop control mode to a dual closed-loop control mode to limit the fault current and suppress interference. The detection of a fault in the AC system includes the following monitoring methods: According to the voltage criterion, when the system voltage U s System voltage preset low value U s_setL or U s System voltage preset high value U s_setH Or when DC voltage U DC DC voltage preset low value U DC_setL or U DC DC voltage preset high value U DC_setH Or when the zero-sequence voltage on the AC valve side U 0> AC valve side zero-sequence voltage threshold U 0_set At this time, the system switches from open-loop control to dual closed-loop control; According to the current criterion, when the grid-side current... I s >Grid-side current threshold I s_set or bridge arm current I b > Bridge arm current threshold I b_set At this time, the system switches from open-loop control to dual closed-loop control; According to the power criterion, when the active power P Active power threshold P _set At this time, the system switches from open-loop control to dual closed-loop control; According to the frequency criterion, when the system frequency f System frequency preset low value f _setL or f System frequency preset high value f _setH At this time, the system switches from open-loop control to dual closed-loop control; The dual closed-loop control method includes: The outer loop control takes the AC voltage and frequency input to the grid side of the converter as input, and outputs the inner loop active current reference value and the inner loop reactive current reference value after passing through the proportional-integral controller. The inner loop control receives the inner loop active current reference value and the inner loop reactive current reference value from the outer loop control output, and quickly tracks the inner loop active current reference value and the inner loop reactive current reference value to realize direct control of the AC side current waveform and phase of the converter. When switching from the open-loop control mode to the dual closed-loop control mode, an initial value is assigned to the input of the integrator of the outer loop control.

2. The control method as described in claim 1, wherein, The open-loop control mode includes: The preset AC voltage and preset frequency reference value of the converter grid side are used as inputs to the open-loop controller. After adjustment by the open-loop controller, a constant AC voltage reference wave of the converter is output, thereby realizing the control of the AC current waveform and phase of the converter.

3. The control method as described in claim 1, wherein, The dual closed-loop control method also includes: The current limiting mechanism controls the fault current of the AC system to prevent it from exceeding the limit.

4. The control method as described in claim 1, wherein, The system voltage is preset to a low value. U s_setL ≤ 1 pu, system voltage preset high value U s_setH ≥1 pu; DC voltage preset low value U DC_setL ≤ 1 pu, DC voltage preset high value U DC_setH ≥ 1 pu; AC valve-side zero-sequence voltage threshold U 0_se t ≥ 0 pu, grid-side current threshold I s_set ≥ 1 pu, bridge arm current threshold I b_set ≥ 1 pu, active power threshold P _set ≤ 1 pu, system frequency preset low value f _setL <50Hz, system frequency preset high value f _setH > 50Hz, where pu is a per-unit value, which is the relative value between the actual value and the rated value.

5. The control method as described in claim 1, wherein, The flexible DC transmission system can operate with or without load when in islanded mode.

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