MMC (Modular Multilevel Converter) control technology under unbalanced network voltage scene of offshore wind power flexible direct current transmission

Abstract

The invention provides an MMC (Modular Multilevel Converter) control technology under an unbalanced network voltage scene of offshore wind power flexible direct current power transmission, which comprises the following steps of: injecting reverse circulation control voltage u 'Cj into upper and lower bridge arm modulation voltage reference values upjref and unjref, namely superposing the circulation control voltage u' Cj with the upper bridge arm modulation voltage reference value upjref and the lower bridge arm modulation voltage reference value unjref respectively, and obtaining new modulating wave voltage reference values u'pjref and u'njref. Aiming at the MMC system under the unbalanced network voltage of the network side caused by the AC short circuit of the flexible DC system of the offshore wind power, the MMC control strategy under the unbalanced network voltage of the secondary circulating current embedded third harmonic cooperative injection is researched, so that the submodule capacitor voltage fluctuation and circulating current of the MMC system can be reduced, and the effective reactive compensation can be realized to carry out fault ride-through.

Description

Technical Field

[0001] The present invention belongs to the field of MMC control in an unbalanced grid voltage scenario of offshore wind power flexible direct current transmission, and in particular, relates to an MMC control technology in an unbalanced grid voltage scenario of offshore wind power flexible direct current transmission. Background Art

[0002] In recent years, China's flexible DC transmission projects have entered a period of rapid development. The Chongqing-Hubei back-to-back flexible DC project, the Zhangbei four-terminal flexible DC grid, and the Wudongde UHV multi-terminal DC project have been put into operation one after another. Some UHV DC projects using flexible DC technology such as Baihetan-Jiangsu have also been put into operation. However, the short-circuit current of offshore wind power connected through flexible DC transmission has an impact on the unbalanced grid voltage of the receiving grid that cannot be ignored, making it difficult to directly apply the conventional MMC control strategy, because it not only requires the MMC to effectively achieve the fault crossing of the receiving grid, but also effectively suppresses the voltage fluctuation and circulating current of the MMC converter submodule capacitor. Therefore, it is very important to study the control strategy of modular multilevel converters under unbalanced grid voltage scenarios under flexible DC short-circuit scenarios, especially the voltage fluctuation of submodule capacitors, for the stability of the offshore wind power transmission system through MMC.

[0003] At present, the MMC control strategy mainly explores the submodule capacitor voltage, circulating current suppression, etc., but its control premise is the balance of the three-phase power grid, and it cannot be directly used in the scenario of unbalanced grid voltage caused by short-circuit current. For the unbalanced grid voltage scenario, although some studies have adjusted the MMC control strategy by introducing the unbalanced degree of grid voltage, the above studies will produce different degrees of double frequency pulsation when performing reactive support for fault crossing, causing the submodule capacitor voltage and bridge arm current to fluctuate, increasing the risk of MMC damage, and excessively limiting the circulating current will cause the grid fault to recover too slowly, resulting in the failure of fault crossing. Summary of the invention

[0004] The purpose of the present invention is to provide an MMC control technology under the scenario of unbalanced grid voltage of offshore wind power flexible direct current transmission. For the MMC system under unbalanced grid voltage caused by AC short circuit of offshore wind power through flexible direct current system, the MMC control strategy under unbalanced grid voltage with secondary circulating current embedded in third harmonic coordinated injection is studied, which is not only conducive to reducing the capacitor voltage fluctuation and circulating current of the MMC system submodule, but also can realize effective reactive power compensation to perform fault crossing. The adopted technical scheme is as follows:

[0005] An MMC control technology for offshore wind power flexible direct current transmission in an unbalanced grid voltage scenario includes the following steps:

[0006] The third harmonic content u of the submodule capacitor voltage suppression component caused by the unbalanced grid voltage injected into the three-phase bridge arm in real time 3s Then, a third harmonic content u is injected into it. 3sThe circulating current control voltage u' that causes an increase in the arm circulating current Cj ;

[0007] The secondary circulating current value u' Cj is respectively superimposed with the reference value u of the modulation voltage of the upper arm pjref , and the reference value u of the modulation voltage of the lower arm njref to obtain new reference values u' pjref and u' njref .

[0008] Preferably, the calculation process of the circulating current control voltage u' Cj includes:

[0009] First, calculate the calculated value i of the secondary circulating current Rj ;

[0010] After that, the calculated value i of the secondary circulating current Rj subtracts half of the sum of the currents of the upper arm and the lower arm of the MMC device, and the output result of the proportional resonance controller PR is used to obtain the circulating current control voltage u' Cj .

[0011] Preferably, the specific process of calculating the calculated value i of the secondary circulating current Rj includes:

[0012]

[0013] where j = a, b, c

[0014] I DC - DC bus current of the MMC device;

[0015] i j - Three-phase output AC current of the MMC device;

[0016] - Three-phase arm circulating current of the MMC device;

[0017] is the power factor angle.

[0018] Preferably, the optimal injection phase z and the optimal injection amplitude y of the calculated value i of the secondary circulating current Rj are obtained by using the double-layer particle swarm optimization algorithm;

[0019] The input of the bottom-layer particle swarm optimization algorithm is the superposition amount T of the amplitudes of the second, third, and fourth harmonic voltage fluctuations, the grid voltage unbalance degree ε, and the power factor angle and the output is the fault ride-through control parameter k;

[0020] The upper-layer particle swarm optimization parameters are the fault ride-through control parameter k and the grid voltage unbalance degree ε, and the output of the upper-layer particle swarm optimization parameters is the secondary circulation i that suppresses the maximum fluctuation reduction rate of the three-phase grid-side voltage. Rj The optimal injection phase z and the optimal injection amplitude y.

[0021] Preferably, the optimal injection phase z is:

[0022]

[0023] Preferably, the optimal injection amplitude is:

[0024]

[0025] Preferably, the reference value u of the modulation voltage of the upper bridge arm pjref , and the reference value u of the modulation voltage of the lower bridge arm njref The calculation steps specifically include:

[0026] Obtain the third harmonic content u to be injected 3s And the three-phase positive-sequence voltage reference value Three-phase negative-sequence voltage reference value According to the corresponding modulation method, obtain the reference value u of the modulation voltage of the upper bridge arm after injecting the third harmonic pjref , and the reference value u of the modulation voltage of the lower bridge arm njref .

[0027] Preferably, the reference value u of the modulation voltage of the upper bridge arm pjref , and the reference value u of the modulation voltage of the lower bridge arm njref Specifically:

[0028]

[0029] Preferably, the obtaining steps of the third harmonic content u 3s Specifically are:

[0030] Use the active power, reactive power and their third harmonic generation links to obtain the third harmonic content u that needs to be reversely injected to suppress the capacitor voltage fluctuation value of the sub-module caused by the unbalanced network voltage 3s .

[0031] Preferably, the obtaining steps of the three-phase positive-sequence voltage reference value Three-phase negative-sequence voltage reference value Specifically are: Obtain by using the active power, reactive power and their positive and negative sequence controllers.

[0032] Compared with the prior art, the advantages of the present invention are:

[0033] According to the MMC control strategy for the scenario of unbalanced grid-side voltage caused by AC short circuit in the offshore wind power through the flexible DC system, that is, on the premise that the flexible DC system can effectively ride through faults, the second harmonic circulation is embedded with the third harmonic, and the correlation setting value of the second harmonic circulation embedded with the third harmonic is given. In this way, it will not overly limit the circulating current, ensure the speed of grid fault recovery, cause the failure of fault ride-through, and can effectively suppress the circulating current flowing through the bridge arm of the MMC system and the voltage fluctuation of the sub-module capacitor, realizing the system function and the reliability of the valve-level module MMC topology power device in the scenario of AC short circuit in the offshore wind power through the flexible DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 FIG. Figure 1 Topological structure diagram of offshore wind power flexible DC transmission;

[0035] Figure 2 FIG. is the MMC circuit topology diagram;

[0036] Figure 3 FIG. is the control block diagram of the MMC system with the second harmonic circulation embedded with the third harmonic under unbalanced grid voltage;

[0037] FIG. 4(a) is the design block diagram of the cooperative injection controller with the second harmonic circulation embedded with the third harmonic;

[0038] FIG. 4(b) is the design block diagram of the circulating current injection controller;

[0039] Figure 5 FIG. is the optimization result diagram. Diagram (a) is the three-dimensional change relationship diagram of the maximum reduction rate of the three-phase grid-side voltage with and z; Diagram (b) is the three-dimensional change relationship diagram of the maximum reduction rate of the three-phase module voltage with the power factor angle and the injection amplitude y of the second harmonic circulation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] The MMC control technology for the unbalanced grid voltage scenario of the offshore wind power flexible DC transmission of the present invention will be described in more detail below with reference to the schematic diagrams, which show the preferred embodiments of the present invention. It should be understood that those skilled in the art can modify the present invention described herein while still achieving the advantageous effects of the present invention. Therefore, the following description should be understood as a broad guidance for those skilled in the art and not as a limitation to the present invention.

[0041] The offshore wind power flexible direct current transmission project consists of an offshore wind farm, an offshore booster station, an offshore converter station, a submarine cable, an onshore cable and an onshore converter station, etc. Most of them adopt a symmetrical monopole topology structure, that is, the offshore wind farm is usually connected to the offshore booster station through a submarine cable at an AC voltage level of 33kV or 66kV. Multiple offshore booster stations are then connected to the AC field of the offshore converter station through submarine cables. After being connected in parallel through double-connected transformers, they are connected to the offshore converter station MMC to complete AC / DC. The DC is sent to the onshore converter station MMC through submarine cables and land cables, and is connected to the onshore AC power grid after completing DC / AC. Figure 1 .

[0042] Figure 2 is the topological structure of the converter station MMC, Figure 2 (a) It can be seen that the MMC has a total of 6 bridge arms, each of which consists of N identical sub-modules (SM) and a bridge arm choke inductor L r Made up of Figure 2 (b) It can be seen that each half-bridge SM (HB-SM) includes two insulated gate transistors (IGBT) T1 and T2, two anti-parallel diodes D1 and D2 and a DC capacitor C.

[0043] Figure 2 Middle,U dc - DC voltage, I dc - DC current;

[0044] u j - three-phase AC voltage, i j - three-phase alternating current; j = a, b, c;

[0045] u pj -Upper bridge arm voltage, u nj -Lower bridge arm voltage, i pj - Upper arm current, i nj -Lower arm current;

[0046] i Cj -Three-phase bridge arm circulation.

[0047] When a phase or the negative pole of the DC side of the onshore converter station MMC is short-circuited to the ground, a short-circuit current will be generated, causing an imbalance in the phase voltage on the grid side, which in turn causes the MMC bridge arm circulating current and the submodule capacitor voltage fluctuation to increase. At this time, in order to ensure system stability and provide reactive power support to the grid to achieve fault ride-through, the MMC control system needs a reasonable and effective control strategy to smoothly achieve fault ride-through while ensuring rapid grid fault recovery, reasonably suppress the MMC bridge arm circulating current and submodule capacitor voltage fluctuations, and ensure equipment safety and stability.

[0048] To quickly suppress the short-circuit current, it is necessary to increase the grid-side voltage, that is, to improve the voltage utilization rate of the MMC system. However, this will cause fluctuations in the capacitor voltage of the sub-module in the bridge arm. At this time, the common method is to inject a third-harmonic component into its control voltage. However, another drawback is generated, that is, the circulating current in the bridge arm is increased, which is harmful to the stable operation of the MMC system.

[0049] In this embodiment, a mathematical model of the secondary circulating current embedding third-harmonic collaborative injection method is established for the MMC system under unbalanced grid voltage caused by AC short circuit of offshore wind power through a flexible DC system, and the correlation setting value of the secondary circulating current embedding third-harmonic is given. On the basis of quantitatively solving the injection amplitude and phase angle of the third harmonic, the MMC control strategy under unbalanced grid voltage with secondary circulating current embedding third-harmonic collaborative injection is studied. It is not only beneficial to reduce the capacitor voltage fluctuation and circulating current of the sub-module in the MMC system, but also can realize effective reactive power compensation for fault ride-through.

[0050] In this embodiment, a secondary circulating current & third-harmonic injection strategy is proposed, that is, first inject a third-harmonic content u 3s into the three-phase bridge arm in real time to mitigate the capacitor voltage fluctuation of the sub-module caused by the unbalanced grid voltage, and then inject a circulating current control voltage u′ Cj into the three-phase bridge arm in real time to cancel the increment of the circulating current caused by the third-harmonic injection, reduce the operation risk of the MMC system, and achieve the effect of further reducing the capacitor voltage fluctuation and circulating current of the sub-module, so as to realize effective reactive power compensation for fault ride-through.

[0051] The control block diagram of the MMC system is as shown in Figure 3 and is mainly composed of an offshore wind power flexible DC input system, a receiving-end power grid, a signal detection unit, an isolation drive unit, and a control unit. The design of its secondary circulating current embedding third-harmonic collaborative injection controller adopts a modular design idea, that is, first divide the whole controller into two major modules: a secondary circulating current controller and a third-harmonic controller, and then design their respective internal structures respectively, as shown in Figures 4(a) to 4(b) .

[0052] As can be seen from Figures 4(a) to 4(b) :

[0053] First of all, through the active power and reactive power, a third-harmonic generation link and positive and negative sequence controllers are formed to obtain the required injected third-harmonic content u 3s and the three-phase positive and negative sequence voltage reference values

[0054] Then, according to the corresponding modulation method in the existing technology, the upper and lower bridge arm modulation voltage reference values after the third-harmonic injection are u pjref and u njref .

[0055] Where:

[0056]

[0057] That is, by detecting the output voltage and current u of the MMC device j , i j the corresponding active power and reactive power are obtained;

[0058] Then, the third - harmonic content u to be injected in the opposite direction required to suppress the sub - module capacitor voltage fluctuation caused by the unbalanced grid voltage is obtained by using the active power, reactive power and their third - harmonic generation links 3s .

[0059] Similarly, by using the active power, reactive power and their positive and negative sequence controllers, the positive and negative sequence reference values of the circulating current fluctuation caused by the unbalanced grid voltage are formed

[0060] Then, the third - harmonic content u 3s subtracts the positive and negative sequence reference values of the circulating current fluctuation caused by the unbalanced grid voltage and then adds it to the DC bus voltage of the MMC device to obtain the upper and lower arm modulation voltage reference values u pjref , u njref .

[0061] Although this reference value can limit the suppression of the sub - module capacitor voltage fluctuation caused by the unbalanced grid voltage by injecting the third - harmonic content, but also because injecting the third - harmonic content will increase the arm circulating current, it is necessary to inject the reverse circulating current control voltage u' pjref , u njref into the upper and lower arm modulation voltage reference values u Cj .

[0062] Subsequently, it enters the circulating current injection link, and the circulating current control voltage u' obtained by calculating the reverse circulating current is injected into the three - phase arms respectively Cj , to cancel the circulating current increment caused by the third - harmonic injection in real time. The value of the circulating current control voltage u' Cj is related to the upper and lower arm currents and the secondary circulating current i Rj . Its correlation relationship is that the result of the difference between the two passes through the proportional - resonant controller PR outputs u' Cj . The circulating current control voltage u' Cj is superimposed on the arm modulation voltage values u pjref , u njref to obtain the new modulation wave voltage reference values u' pjref , u' njref , completing the control of the circulating current increment.

[0063] u' pjref = upjref -u' Cj ;

[0064] u' njref = u njref -u' Cj ;

[0065] u' Cj is the circulating current control voltage;

[0066] Finally, the control quantity is converted into a PWM signal through the modulation strategy and the corresponding SM voltage sharing strategy, and is transmitted to each SM to control the on-off of each IGBT, so as to realize the overall control of the MMC system of this patent.

[0067] Such as Figures 4(a) to 4(b) in, the calculation process of the secondary circulating current calculated value i Rj is described as follows:

[0068]

[0069] where j = a, b, c

[0070] I DC - DC bus current of the MMC device; that is Figure 2 the I in dc .

[0071] i j - Three-phase output AC current of the MMC device;

[0072] - Three-phase bridge arm circulating current of the MMC device;

[0073] is the power factor angle.

[0074] When a short-circuit current occurs and fault ride-through is required, the circulating current control voltage u' Cj is injected into the bridge arm, and different degrees of second, third, and fourth harmonic voltage fluctuations occur in the output voltage of the three-phase bridge arm.

[0075] The voltage fluctuation frequency multiplication component will be affected by the grid voltage unbalance degree ε, the fault ride-through control parameter k, the power factor angle and the amplitude y and phase angle z of the secondary circulating current injection amount.

[0076] If the amplitude y and phase angle z of the secondary circulating current calculated value i Rj can be controlled, the secondary circulating current injection amount can generate an additional fluctuation amount in the opposite direction to the voltage fluctuation caused by the third harmonic injection, thereby further reducing the sub-module capacitor voltage fluctuation and achieving the purpose of stable system operation.

[0077] Therefore, in this embodiment, under the condition of unbalanced grid voltage of short-circuit current, the problem of minimizing the capacitor voltage fluctuation value of the sub-module during the injection of the third harmonic is optimized with the amplitude y and phase z of the secondary circulating current injection as independent variables. The optimal solution is obtained by using a double-layer particle swarm optimization algorithm.

[0078] Among them, the input of the underlying particle swarm optimization algorithm is the superposition quantity T of the voltage fluctuation amplitudes of the second, third, and fourth frequencies, the grid voltage unbalance degree ε, and the power factor angle The output is the fault ride-through control parameter k, which is positively correlated with the fault ride-through speed time t and the fault ride-through relay protection threshold S, that is, the larger t and S are, the larger the fault ride-through control parameter k is, and vice versa.

[0079] The upper-layer particle swarm optimization parameters are the fault ride-through control parameter k and the grid voltage unbalance degree ε, and the output of the upper-layer particle swarm optimization is the secondary circulating current i that suppresses the maximum voltage fluctuation reduction rate of the three-phase grid side Rj The optimal injection phase z and the optimal injection amplitude y, and the optimization results are as Figure 5 shown.

[0080] Figure 5 Figure (a) in it shows the three-dimensional variation relationship of the maximum voltage fluctuation reduction rate of the three-phase grid side with and z.

[0081] Analyzing this figure, it can be seen that when and z change simultaneously, the optimal injection phase can be approximated as:

[0082]

[0083] Figure 5 Figure (b) in it shows the three-dimensional variation relationship of the maximum voltage fluctuation reduction rate of the three-phase module with the power factor angle and the secondary circulating current injection amplitude y on the premise of keeping the quantitative relationship of Equation (1) unchanged when the multi-variable control parameter, that is, the fault ride-through control parameter k ∈ [-1, 1].

[0084] Analyzing this figure, it can be seen that when and y change simultaneously, the optimal injection amplitude can be approximated as:

[0085]

[0086] The above is only the preferred embodiment of the present invention and does not impose any limitation on the present invention. Any person skilled in the art, without departing from the technical solution of the present invention, makes any form of equivalent replacement or modification and other changes to the technical solution and technical content disclosed by the present invention, which are all within the content of the technical solution of the present invention and still belong to the protection scope of the present invention.

Claims

1. A MMC control technology under the unbalanced grid voltage scenario of flexible HVDC transmission for offshore wind power, characterized in that including the following steps: Inject the third harmonic content u of the sub-module capacitor voltage suppression component caused by the unbalanced grid voltage into the three-phase bridge arm in real time 3s , and then inject the circulating current control voltage u' that can suppress the increase in the bridge arm circulating current caused by the third harmonic content u 3s ; Cj ; The secondary circulation value u′ Cj is respectively superimposed with the modulation voltage reference value u of the upper bridge arm pjref , and the modulation voltage reference value u of the lower bridge arm njref to obtain new modulation wave voltage reference values u′ pjref and u′ njref .

2. The MMC control technology under the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 1, wherein The circulating current control voltage u′ Cj The calculation process includes: First, calculate the calculated value i of the secondary circulation Rj ; After that, the calculated value i of the secondary circulation Rj subtracts half of the sum of the upper and lower arm currents of the MMC device, and the proportional-resonant controller PR outputs the result to obtain the circulating current control voltage u′ Cj .

3. The MMC control technology under the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 2, characterized in that, Calculating the calculated value i of the secondary circulation Rj The specific process includes: where j = a, b, c I DC -DC bus current of the MMC device; i j -Three-phase output AC current of the MMC device; - Circulating current in the three-phase bridge arms of the MMC device; is the power factor angle.

4. The MMC control technology under the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 2, wherein The calculated value i of the secondary circulation Rj The optimal injection phase z and the optimal injection amplitude y are obtained by using the double-layer particle swarm optimization algorithm; The input quantities of the underlying particle swarm optimization algorithm are the superimposed quantity T of the amplitudes of the second, third, and fourth harmonic voltage fluctuations, the grid voltage unbalance degree ε, and the power factor angle The output quantity is the fault ride-through control parameter k; The upper-layer particle swarm optimization parameters are the fault ride-through control parameter k and the grid voltage unbalance degree ε, and the output of the upper-layer particle swarm optimization parameters is the secondary circulation i that suppresses the maximum reduction rate of the three-phase grid-side voltage fluctuation. Rj The optimal injection phase z and the optimal injection amplitude y.

5. The MMC control technology under the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 4, characterized in that the optimal injection phase z is:

6. The MMC control technology in the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 4, characterized in that, the optimal injection amplitude is:

7. The MMC control technology under the unbalanced grid voltage scenario of flexible HVDC transmission for offshore wind power according to claim 1, wherein Upper bridge arm modulation voltage reference value u pjref and lower bridge arm modulation voltage reference value u njref The calculation steps are specifically as follows: Obtain the third harmonic content u to be injected 3s and the positive sequence voltage reference values of the three phases Negative sequence voltage reference values of the three phases Obtain the upper bridge arm modulation voltage reference value u after third harmonic injection according to the corresponding modulation method pjref and the lower bridge arm modulation voltage reference value u njref .

8. The MMC control technology in the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 7, characterized in that, Upper bridge arm modulation voltage reference value u pjref and lower bridge arm modulation voltage reference value u njref Specifically:

9. The MMC control technology under the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 7, wherein Obtaining steps of the third harmonic content u 3s are specifically as follows: The third-harmonic content u to be injected in the reverse direction required to suppress the sub-module capacitor voltage fluctuation value caused by the unbalanced grid voltage is obtained by using the active power, reactive power, and their third-harmonic generation links 3s 。 10. The MMC control technology under the unbalanced network voltage scenario of flexible HVDC transmission for offshore wind power according to claim 7, characterized in that, Three-phase positive-sequence voltage reference value Three-phase negative-sequence voltage reference value The obtaining steps are specifically as follows: Obtained by using active power, reactive power and their positive and negative sequence controllers.

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