Network construction type flexible direct current converter compatible with strong / weak power grid and control method and system thereof

Abstract

The invention discloses a network construction type flexible direct current converter compatible with a strong / weak power grid and a control method and system thereof, belongs to the technical field of flexible direct current transmission, and aims to solve the technical bottleneck that the prior art is difficult to adapt to power grid strength change and stably operate under a wide-range short circuit ratio. According to the method, overall energy balance of an upper bridge arm and a lower bridge arm of the flexible DC converter is taken as a starting point, a mapping relation between overall capacitance energy of the flexible DC converter and an alternating-current internal potential phase angle is established, and network construction control is performed on the alternating-current internal potential phase angle; and regulating and controlling the capacitance energy of each bridge arm sub-module to obtain the output reference voltage correction value of each phase of the upper and lower bridge arms, and finally independently correcting the output reference voltage of each phase of the upper and lower bridge arms. The output reference voltage of each phase of bridge arm is independently corrected according to the self capacitance energy of each phase of bridge arm essentially, the overall energy of the flexible DC converter and the energy of each bridge arm are always kept balanced when the strength of a power grid changes, and stable operation under a wide-range short-circuit ratio is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new power system construction and flexible direct current transmission technology, in particular to a network construction type flexible direct current converter compatible with strong / weak power grid and a control method and system thereof. BACKGROUND

[0002] The control means of the traditional flexible direct current converter adopts phase-locked loop-based grid-following control, which behaves as a current source in the alternating current system. With the gradual decline of the proportion of synchronous machines in the power grid, the strength of the power grid decreases. When the system is in a weak condition, the grid-following control converter is difficult to follow the grid voltage and is prone to instability. The network construction type flexible direct current converter realizes grid-connected operation through a power-frequency self-synchronization mechanism and behaves as a voltage source, which can actively support the power grid to enhance the stability and reliability of the power grid and effectively cope with the problem of decreasing power grid strength.

[0003] However, under strong system conditions, the network construction type flexible direct current converter has a risk of instability. In actual engineering, due to the randomness of new energy output in the alternating current power grid, load transfer and other conditions, the strength of the power grid has typical time-varying characteristics. Designing a flexible direct current converter for only a single strong / weak condition has been difficult to meet the complex operation requirements of new power systems. To solve the above problems, existing research mainly modifies the control loop or adopts hybrid control to expand the operating range. However, the former cannot finely adjust the energy fluctuations of the internal sub-module capacitors because the phases of the upper and lower bridge arms are still in a coupled control state, and the overall energy balance of the flexible direct current station is easily broken by rapid fluctuations in new energy output; the control mode switching process of the latter may cause transient impact current, thereby restricting its applicability in scenarios where the strength of the power grid changes continuously over a wide range. In summary, the current control method of the flexible direct current converter suitable for strong / weak power grids has limited performance, and there is an urgent need to research new methods of controlling flexible direct current converters that adapt to a wide range of short-circuit ratios.

[0004] Traditional network construction type flexible direct current converters usually rely on synchronous links and voltage and current double loops to construct alternating current internal potential phase angles and amplitudes, and the phases of the upper and lower bridge arms are in a coupled control state without considering independent adjustment of the energy of the sub-module capacitors of each bridge arm, which makes it difficult to achieve fast and accurate control of alternating current internal potential and stable transmission of power when the strength of the power grid changes. SUMMARY

[0005] It is necessary to solve the technical bottleneck of the network type flexible direct current converter in stable operation in strong / weak power grid, and the application provides a network type flexible direct current converter compatible with strong / weak power grid and a control method and system thereof. The mapping relationship between the overall capacitor energy of the flexible direct current converter and the phase angle of the AC internal potential is established based on the overall energy balance of the upper and lower bridge arms of the flexible direct current converter, the phase angle of the AC internal potential is controlled, the capacitor energy of each phase submodule of each bridge arm of the flexible direct current converter is regulated, and the correction value of the output reference voltage of each phase of the upper and lower bridge arms of the flexible direct current converter is obtained , and finally the output reference voltage of each phase of the upper and lower bridge arms is independently corrected. Essentially, the output reference voltage of each phase bridge arm is independently corrected according to the capacitor energy of each phase bridge arm. Therefore, the technical scheme of the application realizes the fast and independent regulation of the capacitor energy of the bridge arm of the flexible direct current converter by establishing and correcting the phase angle of the AC internal potential based on the capacitor energy of the flexible direct current converter and independently correcting the output reference voltage of each phase bridge arm according to the capacitor energy of each phase bridge arm. The overall energy and the energy of each bridge arm of the flexible direct current converter can always be balanced when the strength of the power grid changes, and the stable operation of the network type flexible direct current converter under a wide range of short circuit ratios is realized.

[0006] Therefore, the application provides the following technical scheme:

[0007] On the one hand, the application provides a network type flexible direct current converter compatible with strong / weak power grid, which comprises the following steps:

[0008] The mapping relationship between the overall capacitor energy of the flexible direct current converter and the phase angle of the AC internal potential is established, and the phase angle of the AC internal potential is controlled;

[0009] The control of the phase angle of the AC internal potential comprises: regulating the sum of the overall submodule capacitor energy of the upper and lower bridge arms of the flexible direct current converter, and calculating the reference value θ ref of the phase angle of the AC internal potential of the flexible direct current converter; and regulating the difference between the overall submodule capacitor energy of the upper and lower bridge arms of the flexible direct current converter, and calculating the phase angle deviation value of the AC internal potential of the upper and lower bridge arms of the flexible direct current converter;

[0010] The capacitor energy of each phase submodule of each bridge arm of the flexible direct current converter is regulated, and the correction value of the output reference voltage of each phase of the upper and lower bridge arms of the flexible direct current converter is obtained , j=a, b, c, k=p, n; j is a phase marker, a, b, and c correspond to three-phase A, B, and C, k is a bridge arm marker, p represents the upper bridge arm, and n represents the lower bridge arm;

[0011] Finally, the reference value θ ref of the phase angle of the AC internal potential is obtained , The output reference voltage of each phase of the upper and lower bridge arms is independently corrected, and then the actual output voltage of each phase of the upper and lower bridge arms of the flexible DC converter is obtained by modulating the upper and lower bridge arms of the flexible DC converter, thereby realizing the control of the flexible DC converter.

[0012] Optionally, the AC internal potential phase angle reference value θ of the flexible DC converter ref The acquisition process is as follows:

[0013] First, divide the sum of the overall submodule capacitor energy of the upper and lower arms of the flexible DC converter by the number of arms of the flexible DC converter to obtain the average value E of the flexible DC converter arm capacitor energy. avj ;

[0014] Secondly, regarding E avj With the corresponding bridge arm capacitor energy setting value E ref The difference E avj -E ref Adjustments were made to obtain the angular frequency deviation value of the flexible DC converter network. ;

[0015] Finally, calculate The sum of the power frequency and the angular frequency ω0 and to The reference value θ of the AC internal potential phase angle of the flexible DC converter is obtained by performing an integral operation. ref .

[0016] Optionally, the phase angle deviation of the AC internal potential of the upper and lower arms of the flexible DC converter... The acquisition process is as follows:

[0017] The deviation value of the overall submodule capacitor energy of the upper and lower bridge arms of the flexible DC converter is multiplied by the droop coefficient k2 to obtain the phase angle deviation value of the AC internal potential of the upper and lower bridge arms of the flexible DC converter. The droop coefficient k2 is a non-zero coefficient, and its specific value is an empirical value.

[0018] The control relationship between the deviation values ​​of capacitor energy in the upper and lower bridge arm submodules of the flexible DC converter and the deviation value of AC internal potential phase angle is similar to that of active power-frequency characteristics, exhibiting droop characteristics.

[0019] Optionally, the output reference voltage correction values ​​of each phase of the upper and lower arms of the flexible DC converter are... The acquisition process is as follows:

[0020] First, compare the actual energy values ​​of the capacitors in each phase of the upper and lower arms of the flexible DC converter with the corresponding set values ​​E. ref Subtracting these values ​​yields the energy deviation of the capacitors in each phase of each arm of the flexible DC converter.

[0021] Secondly, by multiplying the energy deviation of the capacitors in each phase of each arm of the flexible DC converter by the control coefficient, the output reference voltage correction values ​​for each phase of the upper and lower arms of the flexible DC converter are obtained. The control coefficient is a non-zero coefficient, and its specific value is an empirical value.

[0022] It should be understood that this step distinguishes the capacitance energy of each phase in each bridge arm, as mentioned above in the calculation of θ. ref , When using the overall capacitor energy of the upper and lower bridge arms, there is no need to distinguish between phases. Because... and θ ref The phase and phase correction of the AC internal potential of the upper and lower bridge arms are respectively used as reference values ​​for the direct calculation of the AC internal potential of each phase, while the output reference voltage correction values ​​of each phase of the upper and lower bridge arms are introduced. To enable independent adjustment of the reference voltage of each phase in the upper and lower bridge arms.

[0023] Optionally, based on θ ref , , The process of independently correcting the output reference voltage of each phase of the upper and lower bridge arms is as follows:

[0024] First, using θ ref and The phase angle of the AC internal potential reference value of the upper and lower bridge arms is corrected, thereby coordinating with the AC internal potential amplitude reference value U of the flexible DC converter. ref The corrected AC internal potential reference values ​​V for each phase of the upper and lower arms of the flexible DC converter are calculated. jkref j = a, b, c, k = p, n;

[0025] Then, based on the corrected AC internal potential reference values ​​V for each phase of the upper and lower bridge arms. jkref as well as Independently correct the output reference voltage of each phase of the upper and lower bridge arms:

[0026] ;

[0027]

[0028] In the formula, e jpref e is the reference voltage for each phase output of the upper bridge arm. jnref The reference voltage for each phase output of the lower bridge arm. These are the correction values ​​for the output reference voltage of each phase of the upper bridge arm. These are the correction values ​​for the output reference voltage of each phase of the lower bridge arm. This is the reference value for the internal potential of the positive terminal of the flexible DC converter. This is the reference value for the internal potential of the negative electrode of the flexible DC converter.

[0029] Optionally, the reference value of the positive electrode internal potential of the flexible DC converter. and the inner potential reference value of the negative pole of the HVDC converter is obtained by controlling the DC side of the HVDC converter, specifically:

[0030] Firstly, the actual value E of the outer potential of the DC side of the HVDC converter is calculated dc and the corresponding set value E dcref is calculated, and the difference E dc -E dcref is calculated, and PI control is performed on E dc -E dcref to obtain the DC current reference value i dcref of the positive and negative poles of the HVDC converter (the positive and negative values are the same);

[0031] Then, the difference between the DC current reference value i dcref of the positive and negative poles of the HVDC converter and the actual value of the corresponding positive and negative DC current is calculated, and the positive and negative DC reactor compensation voltages u Ln and u Ln are calculated based on the difference;

[0032] Finally, the sum of the outer potential V dcp of the DC side of the positive pole of the HVDC converter and the positive DC reactor compensation voltage u Lp is calculated to obtain , and the sum of the outer potential V dcn of the DC side of the negative pole of the HVDC converter and the negative DC reactor compensation voltage u Ln is calculated to obtain .

[0033] Optionally, the AC inner potential amplitude reference value U ref of the HVDC converter is obtained by controlling the AC side of the HVDC converter, specifically:

[0034] The three-phase AC voltage u abc of the AC outer potential of the HVDC converter on the valve side of the transformer is collected, and the AC outer potential voltage amplitude is obtained through Park transformation and vector synthesis;

[0035] According to the deviation of the AC outer potential voltage amplitude from the corresponding AC outer potential voltage amplitude set value, the PI control is performed to obtain the AC inner potential amplitude reference value U ref of the HVDC converter.

[0036] In the second aspect, the technical scheme of the present application further provides a control system based on the above method, comprising:

[0037] A potential phase angle control module is configured to establish a mapping relationship between the overall capacitance energy of the HVDC converter and the phase angle of the AC inner potential, and to perform network control on the phase angle of the AC inner potential;

[0038] The network construction control on the AC internal potential phase angle includes: regulating the sum of the capacitor energy of the upper and lower bridge arms of the VSC, and calculating the reference value θ of the AC internal potential phase angle of the VSC ref ; and regulating the difference of the capacitor energy of the upper and lower bridge arms of the VSC, and calculating the phase angle deviation value of the AC internal potential of the upper and lower bridge arms of the VSC ;

[0039] The output reference voltage correction module is configured to regulate the capacitor energy of each phase of the upper and lower bridge arms of the VSC, and obtain the correction value of the output reference voltage of each phase of the upper and lower bridge arms of the VSC , j=a, b, c, k=p, n; j is a phase marker, a, b and c correspond to three phases A, B and C, k is a bridge arm marker, p represents the upper bridge arm, and n represents the lower bridge arm; and the reference value θ ref , , independently correct the output reference voltage of each phase of the upper and lower bridge arms;

[0040] The actual output voltage modulation module modulates each phase of the upper and lower bridge arms of the VSC based on the corrected output reference voltage of each phase of the upper and lower bridge arms, and obtains the actual output voltage of each phase of the upper and lower bridge arms of the VSC, so as to realize the control of the VSC.

[0041] Optionally, the potential phase angle control module includes a reference value control module and a deviation value control module.

[0042] The reference value control module is configured to regulate the sum of the capacitor energy of the upper and lower bridge arms of the VSC, and calculate the reference value θ of the AC internal potential phase angle of the VSC ref .

[0043] The deviation value control module is configured to regulate the difference of the capacitor energy of the upper and lower bridge arms of the VSC, and calculate the phase angle deviation value of the AC internal potential of the upper and lower bridge arms of the VSC .

[0044] In the three aspects, the technical scheme of the present application further provides a VSC controlled by the above method.

[0045] Compared with the prior art, the present application has the following effects:

[0046] The control method for the network-constructing HVDC converter compatible with strong / weak power grids provided by the present application establishes and corrects the phase angle of the AC internal potential according to the overall capacitor energy of the HVDC converter and the capacitor energy of the upper and lower bridge arms, and finally realizes independent correction of the output reference voltage of each phase of the upper and lower bridge arms, that is, it is essentially to realize independent correction of the output reference voltage of each phase bridge arm according to the capacitor energy of each phase bridge arm, to ensure that the network-constructing HVDC converter under strong / weak power grids can quickly and independently regulate and control the bridge arm capacitor energy and keep it balanced, thereby effectively suppressing the risk of synchronous instability and realizing the stable establishment of the AC internal potential of the network-constructing HVDC converter under strong / weak power grids. The control method for the network-constructing HVDC converter compatible with strong / weak power grids is different from the existing correction control loop or the technical route of adopting hybrid control. It designs a separate bridge arm independent control method based on the energy balance of the HVDC converter, which not only can realize active regulation and control of the overall energy and the energy of each bridge arm, providing effective inertia support for the AC system, but also can solve the stable operation problem of the network-constructing HVDC converter under strong / weak power grids, and significantly improve the stable operation level of the flexible HVDC transmission system under high proportion of new energy access.

[0047] In one aspect, the present application quickly and independently regulates and controls the bridge arm capacitor energy of the HVDC converter through the establishment and correction of the phase angle of the AC internal potential by the capacitor energy of the HVDC converter and the independent correction of the output reference voltage of each phase bridge arm according to the capacitor energy of each phase bridge arm.

[0048] In the second aspect, the AC internal potential is controlled by the capacitor energy of the HVDC converter, and the independent regulation and control of the bridge arm capacitor energy of the HVDC converter ensures that the overall energy and the energy of each bridge arm of the HVDC converter always remain balanced when the strength of the power grid changes, thereby realizing the synchronous and stable operation of the network-constructing HVDC converter and the power grid under strong / weak power grids, and the stable establishment of the AC internal potential is the embodiment of the stable operation of the network-constructing HVDC converter.

[0049] In the third aspect, the present application is a new method for controlling the HVDC converter suitable for a wide range of short-circuit ratios. The existing technical solutions mainly focus on modifying the traditional control scheme and combining the grid-connection type and network-constructing type synchronous control to realize the synchronous operation of the HVDC converter and the power grid under strong / weak power grids. The present application designs a separate bridge arm independent control method based on the energy balance of the HVDC converter, so that the overall energy and the energy of each bridge arm of the HVDC converter always remain balanced, and the ability of the HVDC converter to stably operate under a wide range of short-circuit ratios is improved. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 It is a schematic diagram of the topology structure of the HVDC converter in one embodiment;

[0051] Figure 2 It is a flowchart of a control method for a network-constructing HVDC converter compatible with strong / weak power grids in one embodiment;

[0052] Figure 3 Flowchart for step 202 in one embodiment;

[0053] Figure 4 Flowchart for step 204 in one embodiment;

[0054] Figure 5 Flowchart for step 206 in one embodiment;

[0055] Figure 6 Flowchart for step 208 in one embodiment;

[0056] Figure 7 Control loop for obtaining the DC current reference value of the HVDC converter in one embodiment;

[0057] Figure 8 Flowchart for step 210 in one embodiment;

[0058] Figure 9 Flowchart for step 212 in one embodiment;

[0059] Figure 10 Flowchart for step 214 in one embodiment. DETAILED DESCRIPTION

[0060] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not intended to limit the present application. The technical features involved in each embodiment of the present application described below can be combined with each other as long as there is no conflict.

[0061] It should be noted that although the functional modules are divided in the device schematic diagram, and the logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in a different order than the module division in the device or the order in the flowchart. The terms "first", "second", etc. in the specification and claims and the above-described drawings are used to distinguish similar objects, and do not necessarily describe a specific order or sequence.

[0062] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terms used herein are only for the purpose of describing the embodiments of the present application and are not intended to limit the present application.

[0063] In order to enable those skilled in the art to better understand the technical solutions of the present application, the present application will be further described in detail below with reference to the accompanying drawings.

[0064] The VSC topology is shown as follows, u abc is an equivalent voltage source of the AC system; e jp is the output voltage of each phase of the upper bridge arm of the MMC, j is a, b, or c, corresponding to three phases A, B, and C; e jn is the output voltage of each phase of the lower bridge arm of the MMC; v jp is the AC internal potential of the upper bridge arm of the MMC, v jn is the AC internal potential of the lower bridge arm of the MMC; L0, L p , and L n are the inductance of the MMC bridge arm, the positive DC reactor reactance, and the negative DC reactor reactance, respectively; i dcp and i dcn are the positive and negative DC currents of the MMC, respectively; V dcp and V dcn are the external potentials of the positive and negative DC sides of the VSC, respectively. Based on the known equivalent AC source u abc and the external potentials of the positive and negative DC sides of the VSC V dcp , V dcn , a control method for a network-compatible VSC with strong / weak grid is provided, and the technical idea is as follows:

[0065] Step 1: Establish the mapping relationship between the overall capacitor energy of the VSC and the phase angle of the AC internal potential, and perform network control on the phase angle of the AC internal potential.

[0066] The network control on the phase angle of the AC internal potential includes: regulating the sum of the overall capacitor energies of the upper and lower bridge arms of the VSC, and calculating the reference value θ ref of the phase angle of the AC internal potential of the VSC; and regulating the difference between the overall capacitor energies of the upper and lower bridge arms of the VSC, and calculating the phase angle deviation value of the AC internal potential of the upper and lower bridge arms of the VSC.

[0067] It should be understood that the mapping relationship between the overall capacitor energy of the VSC and the phase angle of the AC internal potential is the basis for the network control on the phase angle of the AC internal potential, and specifically includes the relationship between the sum of the overall capacitor energies of the upper and lower bridge arms of the VSC and the reference value θ ref of the phase angle of the AC internal potential of the VSC, and the relationship between the difference between the overall capacitor energies of the upper and lower bridge arms of the VSC and the phase angle deviation value of the AC internal potential of the upper and lower bridge arms, which will be described in detail below.

[0068] Step 2: Regulate the capacitor energy of each phase of the upper and lower bridge arms of the VSC, and obtain the corrected output reference voltage value of each phase of the upper and lower bridge arms of the VSC j = a, b, c, k = p, n; j is the phase label, a, b, c correspond to the three phases A, B, C, k is the bridge arm label, p represents the upper bridge arm, and n represents the lower bridge arm.

[0069] Step 3: Based on θ ref , , The output reference voltage of each phase of the upper and lower bridge arms is independently corrected, and then the actual output voltage of each phase of the upper and lower bridge arms of the flexible DC converter is obtained by modulating the upper and lower bridge arms of the flexible DC converter, thereby realizing the control of the flexible DC converter.

[0070] It should be understood that the execution process of steps one and two is not specifically limited in the actual implementation. To more clearly illustrate the technical solution of the present invention, embodiments will be described in conjunction with the accompanying drawings.

[0071] In one embodiment, such as Figure 2 As shown in the figure, this application provides a control method for a grid-connected flexible DC converter that is compatible with both strong and weak power grids, including the following steps:

[0072] Step 202: Control the AC side of the flexible DC converter to obtain the reference value U of the AC internal potential amplitude of the flexible DC converter. ref .

[0073] Step 204: Adjust the sum of the capacitor energy of the upper and lower bridge arm sub-modules of the flexible DC converter to obtain the reference value θ of the AC internal potential phase angle of the flexible DC converter. ref .

[0074] Step 206: Adjust the energy difference of the overall sub-module capacitors of the upper and lower arms of the flexible DC converter to obtain the phase angle deviation value of the AC internal potential of the upper and lower arms of the flexible DC converter. .

[0075] Step 208: Adjust the DC side of the flexible DC converter to obtain the reference value V of the positive DC internal potential of the flexible DC converter. dcpref and the reference value of the negative DC internal potential V dcnref .

[0076] Step 210: Adjust the capacitor energy of each phase submodule of the upper and lower arms of the flexible DC converter to obtain the output reference voltage correction value of each arm of the flexible DC converter. (j=a, b, c, k=p, n).

[0077] Step 212, set the reference value U of the AC internal potential amplitude of the flexible DC converter. ref Reference value θ of AC internal potential phase angle of flexible DC converter ref Phase angle deviation of AC internal potential in the upper and lower arms of the flexible DC converter Reference value of DC internal potential at the positive terminal of flexible DC converter Vdcpref and the reference value of the negative DC internal potential V dcnref Correction values ​​of the output reference voltage of each phase of the upper and lower arms of the flexible DC converter Control is performed to obtain the output reference voltages of each phase of the upper arm and each phase of the lower arm of the flexible DC converter.

[0078] Step 214: Modulate the upper and lower upper arms of each phase of the flexible DC converter to obtain the actual output voltage of each phase of the flexible DC converter.

[0079] It should be understood that the execution order of steps 202-214 is not uniquely selectable. Unless there are significant logical requirements, the order of the steps can be interchanged. For example, the execution order of step 202 can be limited to before step 212. It should also be noted that the above embodiments are specific descriptions of the technical solution of this invention. For example, steps 204 and 206 correspond to the aforementioned first step; step 210 corresponds to the aforementioned second step; and steps 212 and 214 are an optional implementation of the aforementioned third step.

[0080] In one embodiment, such as Figure 3 As shown, in step 202, the AC side of the flexible DC converter is controlled to obtain the reference value U of the AC internal potential amplitude of the flexible DC converter. ref The process may include:

[0081] Step 302: Collect AC external potential data of the flexible DC converter on the transformer valve side to obtain the AC external potential voltage amplitude.

[0082] Specifically, in the embodiments of this application, the AC external potential and three-phase AC voltage u are... abc The d-axis component u is generated through the Park transform. d and q-axis component u q Calculate u d and u q The composite vector is used to obtain the amplitude of the AC external potential voltage.

[0083] Step 304: Based on the AC external potential voltage amplitude, adjust and generate the reference value U of the AC internal potential amplitude of the flexible DC converter. ref .

[0084] Specifically, the deviation between the AC external potential voltage amplitude and the set value of the AC external potential voltage amplitude is calculated, and the obtained deviation value is used to obtain the reference value U of the AC internal potential amplitude of the flexible DC converter through PI control. ref .

[0085] In one embodiment, as shown, in step 204, the sum of the capacitor energies of the upper and lower bridge arm submodules of the flexible DC converter is controlled to obtain the AC internal potential phase angle reference value θ of the flexible DC converter. refThe process (a type of mapping relationship) can include:

[0086] Step 402: Based on the sum of the capacitor energies of the upper and lower arm submodules of the flexible DC converter, calculate the average value E of the flexible DC converter arm capacitor energy. avj .

[0087] Specifically, the sum of the capacitor energies of the upper and lower bridge arm submodules of the flexible DC converter is divided by the number of bridge arms of the flexible DC converter to obtain the average value E of the flexible DC converter bridge arm capacitor energies. avj .

[0088] Step 404: Calculate the average energy E of the flexible DC converter arm capacitors. avj With the corresponding setting value E ref The difference, for E avj -E ref The difference is adjusted to obtain the angular frequency deviation value of the flexible DC converter network. .

[0089] Specifically, first, the average energy E of the flexible DC converter bridge arm capacitor is calculated. avj With the corresponding setting value E ref The difference is calculated by dividing the deviation value by the reference value of the flexible DC converter arm capacitor energy to obtain its per-unit value; then, the obtained per-unit value is multiplied by the droop coefficient k1 to obtain the flexible DC converter grid angular frequency deviation value. The droop coefficient k1 is a non-zero coefficient, and its specific value is an empirical value.

[0090] Step 406: Calculate the angular frequency deviation of the flexible DC converter network. The sum of the power frequency and the angular frequency ω0, for By performing integration, the reference value θ of the AC internal potential phase angle of the flexible DC converter is obtained. ref .

[0091] Specifically, the first step is to calculate the angular frequency deviation of the flexible DC converter network. The sum of the power frequency and the angular frequency ω0 Then, using formula (1), for By performing time integration, the reference value θ of the AC internal potential phase angle of the flexible DC converter is obtained. ref .

[0092] (1)

[0093] In one embodiment, such as Figure 5 As shown, in step 206, the difference in capacitor energy between the upper and lower arm submodules of the flexible DC converter is controlled to obtain the phase angle deviation value of the AC internal potential of the upper and lower arms of the flexible DC converter. The process (a type of mapping relationship) can include:

[0094] Step 502: Subtract the capacitor energy of the upper and lower bridge arm submodules of the flexible DC converter to obtain the capacitor energy deviation value of the upper and lower bridge arm submodules of the flexible DC converter.

[0095] Step 504: The energy deviation values ​​of the capacitors in the upper and lower arm submodules of the flexible DC converter are adjusted to obtain the phase angle deviation values ​​of the AC internal potential of the upper and lower arms of the flexible DC converter. .

[0096] Specifically, the energy deviation values ​​of the capacitors in the upper and lower arm submodules of the flexible DC converter are multiplied by the droop coefficient k2 to obtain the phase angle deviation values ​​of the AC internal potential of the upper and lower arms of the flexible DC converter. .

[0097] In one embodiment, such as Figure 6 As shown, in step 208, the DC side of the flexible DC converter is controlled to obtain the reference value V of the positive DC internal potential of the flexible DC converter. dcpref and the reference value of the negative DC internal potential V dcnref The process may include:

[0098] Step 602: Calculate the difference between the actual value of the external potential on the DC side of the flexible DC converter and the corresponding set value, and adjust the difference of the set value to obtain the reference values ​​i of the positive and negative DC currents of the flexible DC converter. dcref .

[0099] Specifically, first, calculate the actual value E of the external potential on the DC side of the flexible DC converter. dc With the corresponding setting value E dcref The difference E dc -E dcref Then for E dc -E dcref To implement PI regulation, such as Figure 7 As shown, the reference values ​​i of the positive and negative DC currents of the flexible DC converter are obtained. dcref .

[0100] Step 604: Calculate the reference value i of the positive DC current of the flexible DC converter. dcref With actual value i dcp The difference i dcref -i dcp The compensation voltage u of the positive DC reactor of the flexible DC converter is obtained. Lp .

[0101] Specifically, first, the reference value i of the positive DC current of the flexible DC converter is calculated. dcref With actual value i dcp The difference i dcref -i dcp As the deviation current on the positive DC reactor; then, through equation (2), the compensation voltage u of the positive DC reactor of the flexible DC converter is obtained.Lp .

[0102] (2)

[0103] Among them, L p This is the inductance value of the positive DC reactor. K is the sampling period, and k3 is the control coefficient.

[0104] Step 606: Calculate the reference value i of the negative DC current of the flexible DC converter. dcref With actual value i dcn The difference i dcref -i dcn The compensation voltage u of the negative DC reactor of the flexible DC converter is obtained. Ln .

[0105] Specifically, first, calculate the reference value i of the negative DC current of the flexible DC converter. dcref With actual value i dcn The difference i dcref -i dcn The deviation current on the negative DC reactor is used as the basis for determining the compensation voltage u of the negative DC reactor in the flexible DC converter. Ln .

[0106] (3)

[0107] Among them, L n This is the inductance value of the negative DC reactor. K is the sampling period, and k3 is the control coefficient.

[0108] Step 608, calculate the external potential V on the positive DC side of the flexible DC converter. dcp and positive DC reactor compensation voltage u Lp The sum of these values ​​yields the reference value V of the positive electrode internal potential of the flexible DC converter. dcpref .

[0109] Step 610, calculate the external potential V on the negative DC side of the flexible DC converter. dcn and the compensation voltage u of the negative DC reactor Ln The sum of these values ​​yields the reference value V of the internal potential at the negative electrode of the flexible DC converter. dcnref .

[0110] In one embodiment, such as Figure 8 As shown, in step 210, the capacitor energy of each phase submodule of each arm of the flexible DC converter is controlled to obtain the output reference voltage correction value of each arm of the flexible DC converter. The process (j=a, b, c, k=p, n) can include:

[0111] Step 802: Compare the actual energy values ​​of each phase capacitor in each arm of the flexible DC converter with the corresponding set value E. ref Subtracting these values ​​yields the energy deviation of the capacitors in each phase of each arm of the flexible DC converter.

[0112] Step 804: Multiply the energy deviation value of each phase capacitor of each arm of the flexible DC converter by the control coefficient to obtain the output reference voltage correction value of each arm of the flexible DC converter. .

[0113] In one embodiment, such as Figure 9 As shown, in step 212, the reference value U of the AC internal potential amplitude of the flexible DC converter is... ref Reference value θ of AC internal potential phase angle of flexible DC converter ref Phase angle deviation of AC internal potential in the upper and lower arms of the flexible DC converter Reference value of DC internal potential at the positive terminal of flexible DC converter V dcpref and the reference value of the negative DC internal potential V dcnref Correction values ​​of the output reference voltage of each arm of the flexible DC converter The process of controlling and obtaining the output reference voltages of each phase of the upper arm and each phase of the lower arm of the flexible DC converter can include:

[0114] Step 902, based on the reference value U of the AC internal potential amplitude of the flexible DC converter. ref Reference value θ of AC internal potential phase angle of flexible DC converter ref The reference values ​​of the AC internal potential of each arm of the flexible DC converter are obtained.

[0115] Specifically, based on the reference value U of the AC internal potential amplitude of the flexible DC converter ref Reference value θ of AC internal potential phase angle of flexible DC converter ref The reference values ​​of the AC internal potential of each arm of the flexible DC converter are obtained.

[0116] Step 904: Based on the phase angle deviation values ​​of the AC internal potential of the upper and lower arms of the flexible DC converter. The corrected AC internal potential reference values ​​for each phase of the upper arm of the flexible DC converter are obtained.

[0117] Specifically, based on the phase angle deviation of the AC internal potential of the upper and lower arms of the flexible DC converter. The phase of the AC internal potential reference value of each upper arm of the flexible DC converter is corrected. Combining steps 902 and 904, the corrected AC internal potential reference value of each phase of the upper arm of the flexible DC converter is obtained by formula (4).

[0118] (4)

[0119] Step 906: Calculate the reference value V of the positive electrode potential of the flexible DC converter.dcpref The difference between the AC internal potential reference value of each upper arm and the corrected value of the reference value of the corresponding upper arm is used to obtain the output reference voltage of each phase of the upper arm of the flexible DC converter.

[0120] Specifically, based on the reference value V of the positive electrode internal potential of the flexible DC converter dcpref Corrected reference values ​​of AC internal potential V for each phase of the upper bridge arm jpref Upper arm output reference voltage correction value The reference voltages of each phase of the upper arm of the flexible DC converter are obtained by formula (5).

[0121] (5)

[0122] Among them, e jpref This is the reference voltage for each phase output of the upper bridge arm.

[0123] Step 908: Calculate the AC internal potential reference value and the negative terminal internal potential reference value V of each lower arm of the flexible DC converter. dcpref The difference is calculated, and the sum of the difference and the corresponding output reference voltage correction value of each bridge arm is obtained to obtain the output reference voltage of each phase of the lower bridge arm of the flexible DC converter.

[0124] Specifically, based on the reference value V of the internal potential of the negative electrode of the flexible DC converter dcnref Reference value of AC internal potential V for each phase of the lower bridge arm jnref Lower bridge arm output reference voltage correction value The output reference voltage of each phase of the lower arm of the flexible DC converter can be obtained by formula (6).

[0125] (6)

[0126] Among them, e jnref This is the reference voltage for each phase output of the lower bridge arm.

[0127] In one embodiment, such as Figure 10 As shown, step 214, which modulates the upper and lower bridge arms of each phase of the flexible DC converter to obtain the actual output voltage of each phase of the flexible DC converter, includes:

[0128] Step 1002: Based on the output reference voltages of each phase of the upper arm and each phase of the lower arm of the flexible DC converter, the modulation waves of each phase of the upper and lower arms of the flexible DC converter are obtained using the conventional modulation method of the flexible DC converter.

[0129] Step 1004: Based on the modulation waves of the upper and lower arms of each phase of the flexible DC converter, drive the power electronic switching devices of the upper and lower arm submodules of each phase of the flexible DC converter to operate, and obtain the actual output voltage of the upper and lower arms of each phase of the flexible DC converter.

[0130] In some embodiments, the application provides a flexible direct current converter controlled by the above-mentioned network configuration type flexible direct current converter control method compatible with strong / weak power grids.

[0131] In some embodiments, the application also provides a control system based on the above-mentioned control system, comprising a potential phase angle control module, an output reference voltage correction module and an actual output voltage modulation module connected with each other or in sequence.

[0132] The potential phase angle control module is configured to establish a mapping relationship between the overall capacitor energy of the flexible direct current converter and the AC internal potential phase angle, and to control the AC internal potential phase angle.

[0133] Similarly, the network configuration control of the AC internal potential phase angle comprises: regulating the sum of the overall capacitor energy of the upper and lower bridge arms of the flexible direct current converter, and calculating the AC internal potential phase angle reference value θ ref of the flexible direct current converter; and regulating the difference between the overall capacitor energy of the upper and lower bridge arms of the flexible direct current converter, and calculating the AC internal potential phase angle deviation value of the upper and lower bridge arms of the flexible direct current converter.

[0134] The output reference voltage correction module is configured to regulate the capacitor energy of each phase submodule of each bridge arm of the flexible direct current converter, and obtain the output reference voltage correction value of each phase of the upper and lower bridge arms of the flexible direct current converter, j=a, b, c, k=p, n; j is a phase marker, a, b, and c correspond to three phases A, B, and C, k is a bridge arm marker, p represents the upper bridge arm, and n represents the lower bridge arm; and to correct the output reference voltage of each phase of the upper and lower bridge arms of the flexible direct current converter according to θ ref , , The output reference voltage of each phase of the upper and lower bridge arms is independently corrected.

[0135] The actual output voltage modulation module modulates each phase of the upper and lower bridge arms of the flexible direct current converter based on the corrected output reference voltage of each phase of the upper and lower bridge arms, to obtain the actual output voltage of each phase of the upper and lower bridge arms of the flexible direct current converter, and to realize the control of the flexible direct current converter.

[0136] The specific implementation process of each module is described above, and will not be repeated here. The division of the above-mentioned functional modules is only for illustration. In some embodiments, part of the functional modules can be combined, part of the functional modules can be split, and each functional module can be implemented in software, hardware or a combination of software and hardware. The software and hardware devices include but are not limited to general-purpose computer devices, programmable gate arrays, digital signal processors, microprocessors and their corresponding programming or burning software.

[0137] For example, in some embodiments, the above-mentioned potential phase angle control module comprises a potential phase angle reference value control module and a potential phase angle deviation value control module.

[0138] The potential phase angle reference value control module is configured to control the sum of the capacitor energy of the upper and lower bridge arms of the HVDC converter, and calculate the AC internal potential phase angle reference value θ of the HVDC converter ref ;

[0139] The potential phase angle deviation value control module is configured to control the difference between the capacitor energy of the upper and lower bridge arms of the HVDC converter, and calculate the AC internal potential phase angle deviation value of the upper and lower bridge arms of the HVDC converter .

[0140] In some embodiments, the control system further comprises: a potential amplitude reference value acquisition module and a potential reference value acquisition module.

[0141] The potential amplitude reference value acquisition module is configured to control the AC side of the HVDC converter, and obtain the AC internal potential amplitude reference value U of the HVDC converter ref .

[0142] The potential reference value acquisition module is configured to control the DC side of the HVDC converter, and obtain the positive DC internal potential reference value V of the HVDC converter dcpref and the negative DC internal potential reference value V dcnref .

[0143] Then, the output reference voltage correction module corrects the output reference voltage of each phase of the upper and lower bridge arms.

[0144] In summary, the technical scheme of the present application not only realizes active regulation and control of the overall energy and the energy of each bridge arm, but also provides active support for the AC system, and solves the problem of stable operation of the network type HVDC converter under strong / weak power grids, thereby significantly improving the stable operation level of the flexible HVDC power transmission system under high proportion of new energy access.

[0145] The technical features of the above embodiments can be combined in any manner. To make the description concise, not all possible combinations of the technical features in the above embodiments are described, but as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the present application.

[0146] The above-described embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the present application. It should be pointed out that for those skilled in the art, without departing from the concept of the present application, some modifications and improvements can be made, which are all within the scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A control method for a grid-connected flexible DC converter compatible with both strong and weak power grids, characterized in that: Includes the following steps: Establish a mapping relationship between the overall capacitor energy of the flexible DC converter and the phase angle of the AC internal potential, and implement grid control of the phase angle of the AC internal potential; The grid-based control of the AC internal potential phase angle includes: regulating the sum of the capacitor energy of the overall sub-modules of the upper and lower arms of the flexible DC converter, and calculating the reference value θ of the AC internal potential phase angle of the flexible DC converter. ref ; and adjust the energy difference of the overall sub-module capacitors of the upper and lower arms of the flexible DC converter, and calculate the phase angle deviation of the AC internal potential of the upper and lower arms of the flexible DC converter. ; By regulating the capacitor energy of each phase submodule in the upper and lower arms of the flexible DC converter, the correction values ​​of the output reference voltage for each phase in the upper and lower arms of the flexible DC converter are obtained. j = a, b, c, k = p, n; j is the phase marker, a, b, c correspond to the three phases A, B, C, k is the bridge arm marker, p represents the upper bridge arm, n represents the lower bridge arm; Finally, based on θ ref , , The output reference voltage of each phase of the upper and lower bridge arms is independently corrected, and then the actual output voltage of each phase of the upper and lower bridge arms of the flexible DC converter is obtained by modulating the upper and lower bridge arms of the flexible DC converter, thereby realizing the control of the flexible DC converter.

2. The method according to claim 1, characterized in that: The AC internal potential phase angle reference value θ of the flexible DC converter ref The acquisition process is as follows: First, divide the sum of the overall submodule capacitor energy of the upper and lower arms of the flexible DC converter by the number of arms of the flexible DC converter to obtain the average value E of the flexible DC converter arm capacitor energy. avj ; Secondly, regarding E avj With the corresponding bridge arm capacitor energy setting value E ref The difference E avj -E ref Adjustments were made to obtain the angular frequency deviation value of the flexible DC converter network. ; Finally, calculate The sum of the power frequency and the angular frequency ω0 and to The reference value θ of the AC internal potential phase angle of the flexible DC converter is obtained by performing an integral operation. ref .

3. The method according to claim 1, characterized in that: The phase angle deviation of the AC internal potential of the upper and lower arms of the flexible DC converter The acquisition process is as follows: The deviation value of the overall submodule capacitor energy of the upper and lower bridge arms of the flexible DC converter is multiplied by the droop coefficient k2 to obtain the phase angle deviation value of the AC internal potential of the upper and lower bridge arms of the flexible DC converter. The droop coefficient k2 is a non-zero coefficient.

4. The method according to claim 1, characterized in that: The correction values ​​of the output reference voltages of each phase of the upper and lower arms of the flexible DC converter The acquisition process is as follows: First, compare the actual energy values ​​of each phase capacitor in the upper and lower arms of the flexible DC converter with the corresponding setpoint E for the arm capacitor energy. ref Subtracting these values ​​yields the energy deviation of the capacitors in each phase of each arm of the flexible DC converter. Secondly, by multiplying the energy deviation of the capacitors in each phase of each arm of the flexible DC converter by the control coefficient, the output reference voltage correction values ​​for each phase of the upper and lower arms of the flexible DC converter are obtained. The control coefficient is a non-zero coefficient.

5. The method according to claim 1, characterized in that: Based on θ ref , , The process of independently correcting the output reference voltage of each phase of the upper and lower bridge arms is as follows: First, using θ ref and The phase angle of the AC internal potential reference value of the upper and lower bridge arms is corrected, thereby coordinating with the AC internal potential amplitude reference value U of the flexible DC converter. ref The corrected AC internal potential reference values ​​V for each phase of the upper and lower arms of the flexible DC converter are calculated. jkref j = a, b, c, k = p, n; Then, based on the corrected AC internal potential reference values ​​V of each phase of the upper and lower bridge arms. jkref as well as Independently correct the output reference voltage of each phase of the upper and lower bridge arms: ； ； In the formula, e jpref e is the reference voltage for each phase output of the upper bridge arm. jnref The reference voltage for each phase output of the lower bridge arm. These are the correction values ​​for the output reference voltage of each phase of the upper bridge arm. These are the correction values ​​for the output reference voltage of each phase of the lower bridge arm. This is the reference value for the internal potential of the positive terminal of the flexible DC converter. This is the reference value for the internal potential of the negative electrode of the flexible DC converter.

6. The method according to claim 5, characterized in that: Reference value of positive electrode internal potential of flexible DC converter Reference value of internal potential of negative electrode of flexible DC converter This is achieved by controlling the DC side of the flexible DC converter, specifically: First, calculate the actual value E of the DC side external potential of the flexible DC converter. dc With the corresponding setting value E dcref The difference E dc -E dcref , for E dc -E dcref PI control is performed to obtain the reference values ​​i for the positive and negative DC currents of the flexible DC converter. dcref ; Then, calculate the reference values ​​i for the positive and negative DC currents of the flexible DC converter respectively. dcref The difference between the actual values ​​of the positive and negative DC currents is used to calculate the compensation voltage u of the positive and negative DC reactors of the flexible DC converter. Lp u Ln ; Finally, calculate the external potential V on the positive DC side of the flexible DC converter. dcp and positive DC reactor compensation voltage u Lp The sum is obtained ; And calculate the external potential V on the negative DC side of the flexible DC converter. dcn and the compensation voltage u of the negative DC reactor Ln The sum is obtained .

7. The method according to claim 5, characterized in that: Reference value of AC internal potential amplitude U of flexible DC converter ref This is achieved by controlling the AC side of the flexible DC converter, specifically: The three-phase AC voltage u of the AC external potential of the flexible DC converter on the valve side of the transformer is collected. abc The AC external potential voltage amplitude is obtained through Park transformation and vector synthesis; Based on the deviation between the AC external potential voltage amplitude and the corresponding AC external potential voltage amplitude set value, the reference value U of the AC internal potential amplitude of the flexible DC converter is obtained through PI control. ref .

8. A control system based on the method of any one of claims 1-7, characterized in that: include: The potential phase angle control module is used to establish the mapping relationship between the overall capacitor energy of the flexible DC converter and the AC internal potential phase angle, and to perform grid control on the AC internal potential phase angle. The aforementioned grid-based control of the AC internal potential phase angle includes: regulating the sum of the capacitor energy of the overall sub-modules of the upper and lower arms of the flexible DC converter, and calculating the reference value θ of the AC internal potential phase angle of the flexible DC converter. ref ; and adjust the energy difference of the overall sub-module capacitors of the upper and lower arms of the flexible DC converter, and calculate the phase angle deviation of the AC internal potential of the upper and lower arms of the flexible DC converter. ; The output reference voltage correction module is used to regulate the capacitor energy of each phase submodule in the upper and lower arms of the flexible DC converter, thereby obtaining the output reference voltage correction value for each phase in the upper and lower arms of the flexible DC converter. j = a, b, c, k = p, n; j is the phase label, a, b, c correspond to phases A, B, and C, k is the bridge arm label, p represents the upper bridge arm, and n represents the lower bridge arm; and is used to determine the phase based on θ. ref , , Independently corrects the output reference voltage of each phase of the upper and lower bridge arms; The actual output voltage modulation module modulates the phases of the upper and lower bridge arms of the flexible DC converter based on the corrected reference voltages of each phase of the upper and lower bridge arms to obtain the actual output voltages of each phase of the upper and lower bridge arms, thereby realizing the control of the flexible DC converter.

9. The control system according to claim 8, characterized in that: The potential phase angle control module includes a potential phase angle reference value control module and a potential phase angle deviation value control module. The potential phase angle reference value control module is used to regulate the sum of the capacitor energy of the upper and lower bridge arm sub-modules of the flexible DC converter, and calculate the AC internal potential phase angle reference value θ of the flexible DC converter. ref ; The potential phase angle deviation control module is used to regulate the difference in capacitor energy between the upper and lower bridge arm sub-modules of the flexible DC converter, and calculates the AC internal potential phase angle deviation value of the upper and lower bridge arm of the flexible DC converter. .

10. A flexible DC converter, characterized in that: The flexible DC converter is controlled by the method described in any one of claims 1-7.

Images