Comprehensive control method for reactive power support and fault ride-through during offshore wind power short-circuit fault

Abstract

The invention provides a comprehensive control method for reactive power support and fault ride-through in offshore wind power short-circuit fault, which comprises the following steps of: performing common-mode injection calculation: obtaining a circulating current reference value (izxref, respectively adding a PR controller in each phase to control feedback circulating current (izx to change along with the (izxref, and outputting a result Uzx to an MMC converter by the PR controller; and determining an optimal phase angle difference: introducing a phase angle control variable lambda, and searching the phase angle difference of alternating current components in the voltage of the upper bridge arm and the lower bridge arm, so that the non-fault pole bridge arm transmits half rated power, the fault pole bridge arm does not transmit active power, but reactive power can be provided for a power grid. According to the method, under the condition that the circulating current flowing through the MMC system bridge arms and the submodule capacitor voltage fluctuation can be effectively restrained during the offshore wind power short-circuit fault, the upper bridge arm and the lower bridge arm reach power balance through the phase angle control variable, and fault ride-through and reactive power support during the offshore wind power short-circuit fault are achieved.

Description

Technical Field

[0001] The present invention belongs to the field of MMC technology of offshore wind power flexible direct current transmission system, and in particular relates to a comprehensive control method for reactive power support and fault ride-through during offshore wind power short circuit fault. Background Art

[0002] With the rapid development of offshore wind power, modular multilevel converters (MMCs) are gaining popularity due to their ability to independently control active and reactive power. Within their capability curves, they regulate AC system voltage by varying reactive power at the output, implementing static VAR compensation (STATCOM) functionality. This improves the stability of interconnected AC grids and accelerates grid fault recovery. This is particularly true for large receiving-end grids like the Shanghai Grid, which suffers from a severe shortage of dynamic reactive power. A short-circuit fault on the grid-connected side significantly impacts voltage recovery. Consequently, the application of modular multilevel converter HVDC technology is attracting increasing attention.

[0003] Some literature has studied the control of submodule capacitor voltage fluctuations and bridge arm circulating current in MMC devices to improve device safety. However, excessively suppressing submodule capacitor voltage and bridge arm circulating current will significantly limit the short-circuit current in short-circuit faults, which will not only slow grid fault recovery but also hinder fault ride-through. To this end, other literature has begun to study various fault ride-through strategies, such as control methods that design inner loop limit values based on voltage drop, short-circuit current size, etc., to improve the success rate of fault ride-through. However, most of these control strategies come at the expense of active power at the sending end and cause busbar voltage fluctuations in the MMC DC system. This, in turn, affects the significant fluctuations in submodule capacitor voltage and increases the circulating current bridge arm, posing a safety hazard to the MMC DC transmission system. Not only can these strategies fail to provide power support to the AC grid, they may also deteriorate grid stability. Therefore, it is necessary to explore the fault ride-through control method of large-scale deep-sea wind power flexible direct current connected to large receiving power grids, optimize and form a flexible direct current short-circuit current composite control strategy suitable for large receiving power grids, transmit the active power of wind power end to the receiving end as much as possible, and provide STATCOM functions such as reactive power to the receiving power grid while effectively suppressing the capacitor voltage fluctuation of MMC system submodules. This is of great significance for improving the stable operation of the power grid and rapid fault ride-through. Summary of the Invention

[0004] The object of the present invention is to provide a comprehensive control method for reactive power support and fault ride-through during an offshore wind power short-circuit fault.

[0005] This invention takes the MMC of the offshore wind power flexible DC transmission system as the research object, comprehensively considers the suppression of sub-modules with common-mode voltage injection at the front end of the MMC and the function of the STATCOM for reactive voltage support during the back-end fault ride-through, and proposes a composite control strategy for reactive power support and fault ride-through with sub-module fluctuation suppression added.

[0006] This control strategy reasonably controls the capacitor voltage of the sub-module and the magnitude of the circulating current through a quasi-PR controller for injecting common-mode voltage and suppressing the circulating current.

[0007] Then, by introducing a new phase angle control variable λ, the power balance of both the upper and lower bridge arms is achieved, and then the reactive and active power control of the receiving-end power grid is realized, achieving the function of the static var compensator STATCOM, thereby improving the stability of the interconnected AC power grid, accelerating the recovery of the power grid from faults, realizing the reliability of the power devices of the valve-level module MMC topology, and the rapid recovery of the offshore wind power connected to the power grid under the short-circuit scenario of the flexible DC system. The technical solutions adopted are as follows:

[0008] An integrated control method for reactive power support and fault ride-through during offshore wind power short-circuit faults, including the following steps:

[0009] Perform common-mode injection calculation: Obtain the circulating current reference value i zx_ref , and add a PR controller to each phase to control the circulating current i zx to follow i zx_ref variation, and the output result U zx of the PR controller;

[0010] Determine the optimal phase angle difference: Introduce the phase angle control variable λ, and find the phase angle difference of the AC components in the voltages of the upper and lower bridge arms, so that the non-faulty pole bridge arm transmits half of the rated power, and the faulty pole bridge arm does not transmit active power but can provide reactive power to the power grid.

[0011] Preferably, the common-mode injection calculation specifically includes the following steps:

[0012] Step 1, calculate the circulating current reference value i zx_ref :

[0013] The three-phase output current i x of the MMC converter is transformed into dq-axis currents after three-phase rotation, and then the three-phase output AC voltage modulation wave is used to obtain the amplitudes U sd , U sq in the dq coordinate system after three-phase rotation;

[0014] The three-phase AC output voltage is based on the DC bus voltage m u of the MMC converter and the phase-locked angle of the MMC converter, and the circulating current reference value i zx_ref is obtained according to the power balance principle of the sub-module capacitor voltage of the MMC converter;

[0015] Step 2: Calculate the feedback circulating current i zx :

[0016] The currents i px and i nx of the upper and lower arms of the MMC converter are obtained through sampling analysis and calculation. Half of the sum of the two is the feedback circulating current i zx ;

[0017] Step 3: The resonant controller receives i zx_ref and i zx and outputs U zx .

[0018] Preferably, determining the optimal phase angle difference includes the following steps:

[0019] Step a: Obtain the amplitude and phase angle Mag(u diffj ) and Arg(u diffj ) of the common-mode voltage output by the MMC converter:

[0020] The amplitude and phase angle Mag(u diffj ) and Arg(u diffj ) of the common-mode voltage can be obtained through the phase discriminator and the amplitude calculation formula;

[0021] Step b: Calculate the energies ∑W p and ∑W n stored in the capacitors of the upper and lower arms of the MMC converter:

[0022]

[0023] W P = ∑W p , W N = ∑W n ;

[0024] where p pj represents the three-phase received power of the upper arm, and p nj represents the three-phase received power of the lower arm;

[0025] Larm represents the arm inductor;

[0026] ψ v is the phase angle;

[0027] U j is the peak value of the j-phase AC voltage, and E j is the peak value of the j-phase output voltage of the MMC arm;

[0028] W p is the cumulative energy of the upper arm; W n is the cumulative energy of the lower arm;

[0029] Step c, calculating the phase angle difference λ of the AC components in the upper and lower bridge arm voltages:

[0030]

[0031] where P' represents the difference between the cumulative energy W of the upper bridge arm and the cumulative energy W of the lower bridge arm. P and the cumulative energy W of the lower bridge arm N difference.

[0032] Preferably, after step 3, the following steps are further included:

[0033] Step 4, calculating the instantaneous values u of the three-phase modulation waves mx , where x = a, b, c;

[0034] Step 5, calculating the instantaneous values u', u', u' of the three-phase modulation waves of A, B, and C after injecting the common-mode voltage ma , u' mb , u' mc ;

[0035] Step 6, calculating the reference values v, v of the control voltages of the upper and lower bridge arms of the three-phase MMC control system px * , v nx * ;

[0036] Step 7, calculating the control values of the control voltages of the upper and lower bridge arms of the three-phase MMC control system after injecting the circulating current.

[0037] Preferably, step 4 specifically includes the following steps:

[0038] The reference value of the DC bus voltage of the MMC converter is subjected to current inner-loop decoupling control with the reactive and active currents after the three-phase output current of the MMC converter undergoes three-phase rotation transformation, and then undergoes rotation transformation to static transformation, so as to obtain the instantaneous values u of the three-phase modulation waves mx .

[0039] Preferably, in step 5:

[0040] The difference between the instantaneous value u of the three-phase modulation wave mx and the common-mode voltage u* of the three-phase MMC control system com is the instantaneous value u', u', u' of the three-phase modulation waves of A, B, and C after injecting the common-mode voltage ma , u' mb , u' mc .

[0041] Preferably, in step 5:

[0042]

[0043] Among them, θ is the common-mode voltage injection angle, and n is the output voltage frequency order.

[0044] Preferably, in step 6:

[0045] The instantaneous values of the three-phase modulation waves are superimposed on half of the actual value of the DC bus voltage of the three-phase MMC control system to obtain the reference control voltages v px * 、v nx * .

[0046] Preferably, in step 7:

[0047] The reference control voltages v pa * 、v na * of the upper and lower arms of the three-phase MMC control system are superimposed on the circulating current injection value U zx to obtain the control values of the control voltages of the upper and lower arms of the three-phase MMC control system.

[0048] Preferably, in determining the optimal phase angle difference, the upper arm is the non-faulty pole arm.

[0049] Compared with the prior art, the advantages of the present invention are as follows:

[0050] According to the recovery characteristics of the short-circuit fault of the offshore wind power HVDC system, while effectively suppressing the circulating current flowing through the MMC system arm and the voltage fluctuation of the sub-module capacitor, it can also provide the reactive and active support capabilities required for rapid recovery during grid faults as much as possible. And according to the short-circuit fault characteristics, by controlling the phase angle variable, the upper and lower arms can both achieve power balance, so that the system can provide corresponding reactive and active support to the connected grid as much as possible during system faults, and realize the fault ride-through of the offshore wind power through the HVDC system short circuit and the reactive power required for the rapid recovery of the connected grid. Description of the Drawings

[0051] Figure 1 is the topology diagram of the offshore wind power flexible DC transmission;

[0052] Figure 2 is the MMC circuit topology diagram;

[0053] Figure 3 is the schematic diagram of the comprehensive control method for reactive power support and fault ride-through during offshore wind power short-circuit faults;

[0054] Figure 4 is the equivalent circuit diagram after introducing the phase angle control variable λ;

[0055] Figure 5 is the system schematic diagram after introducing the phase angle control variable λ. DETAILED DESCRIPTION

[0056] The following, with reference to schematic diagrams, provides a more detailed description of the integrated control method for reactive power support and fault ride-through during an offshore wind power short-circuit fault. These schematic diagrams illustrate preferred embodiments of the present invention. It should be understood that those skilled in the art may modify the present invention described herein while still achieving the beneficial effects of the present invention. Therefore, the following description should be understood as generally known to those skilled in the art and is not intended to limit the present invention.

[0057] The offshore wind power flexible DC transmission project consists of an offshore wind farm, an offshore booster station, an offshore converter station, submarine cables, onshore cables, and an onshore converter station. Most of them adopt a symmetrical monopole topology. 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 the AC to DC conversion. The DC is transmitted to the onshore converter station MMC through submarine cables and onshore cables. After the DC to AC conversion is completed, it is connected to the onshore AC power grid. Figure 1 .

[0058] Figure 2 is the topology 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 in series. Figure 2 (b) It can be seen that each half-bridge SM (HB-SM) includes two insulated gate transistors (IGBTs) T1 and T2, two anti-parallel diodes D1 and D2, and a DC capacitor C.

[0059] Figure 2 Middle,U dc -DC voltage, I dc - DC current;

[0060] u x - three-phase AC voltage, i x - three-phase alternating current;

[0061] u px -Upper arm voltage, u nx -Low-side bridge voltage, i px - Upper arm current, i nx -lower arm current;

[0062] i Cx -Three-phase bridge arm circulation.

[0063] When a short circuit occurs between a certain phase of the onshore converter station MMC or the negative pole of the DC side and the ground, a short-circuit current will be generated, which will cause the grid-side phase voltage imbalance, and then cause the circulating current in the MMC arm and the voltage fluctuation of the sub-module capacitor to increase. At this time, in order to ensure the stability of the system and provide reactive power support to the power grid to achieve fault ride-through, the MMC control system requires a reasonable and effective control strategy to smoothly achieve fault ride-through during the rapid recovery of the grid fault, reasonably suppress the circulating current in the MMC arm and the voltage fluctuation of the sub-module capacitor, and ensure the safe and stable operation of the equipment.

[0064] In order to ensure that the HVDC transmission system of offshore wind power can achieve fault ride-through during short-circuit faults, provide reactive power support to the power grid, and reasonably suppress the circulating current in the MMC arm and the voltage fluctuation of the sub-module capacitor, the present invention proposes a composite overall control strategy, as Figure 3 shown.

[0065] First, the outer-loop PI controller is used to control the stability of the sub-module capacitor voltage of the MMC. The output of this outer-loop PI controller is the reference value of the active current in the d-q coordinate system and the reference value of the reactive current is directly given according to the actual needs of the rapid recovery of the power grid. This reference value can be calculated from the reactive power deficit of the system.

[0066] The three-phase output current i x of the MMC collected by the current sensor is transformed through the abc / dq transformation to obtain the dq components i sq and i sd .

[0067] Then, two inner-loop PI controllers are used to adjust the error between the feedback values (i sq and i sd ) of the output current in the d-q coordinate system and the command values ( and ) into intermediate variables. After that, the decoupling control and the grid voltage feed-forward method (current feed-forward decoupling control link) are used to obtain the modulation voltage command values ( and ) for reactive power support and fault ride-through.

[0068] Among them, u sd , u sq are the three-phase output voltages u x of the MMC obtained through the abc / dq transformation.

[0069] Then, the above voltage command values ( and ) are converted to the abc coordinate system to obtain the modulation voltage values (u ma , u mb and u mc ) of all phases of the MMC.

[0070] After that, a common-mode voltage is injected into the modulation voltage to obtain three-phase modulation values (u′ ma , u′ mb and u′ mc ) after the injection of the common-mode voltage.

[0071] In order to effectively suppress the excessive circulating current caused by the injected common-mode voltage, a double-frequency quasi-PR controller PR2 and a quadruple-frequency quasi-PR controller PR4 are respectively added to each phase to control the feedback circulating current i zx to follow the circulating current reference value i zx_ref . The output result U zx of the PR controller is superimposed on the arm modulation wave (MMC converter) to complete the control of the circulating current.

[0072] Among them, Figure 3 in, the "MMC-STATCOM control strategy" is the PR controller, which integrates the functions of the double-frequency quasi-PR controller PR2 and the quadruple-frequency quasi-PR controller PR4. In other embodiments, the PR controller also integrates the function of the triple-frequency quasi-PR controller PR3.

[0073] Finally, the control quantities (i px and i nx ) obtained by the sub-module voltage equalization strategy of the circulating current injection are superimposed on the reactive power support control quantity i zx_ref required for fault ride-through to implement the overall control strategy of reactive power support and fault ride-through of the present invention.

[0074] Among them, taking phase a as an example:

[0075] represents the upper arm control voltage reference value; represents the lower arm control voltage reference value.

[0076] represents half of the MMC DC bus voltage

[0077] The calculation process of

[0078] is as follows:

[0079] The input of the PR controller is i zx_ref -i zx , and the output is the other content obtained by filtering the double-frequency harmonic component and the quadruple-frequency harmonic component from the input value.

[0080] Combined with Figure 3 , the specific calculation process is as follows:

[0081] Step 1: Calculate the reference value of the circulating current \(i\). zx_ref :

[0082] The three-phase output current \(i\) of the MMC converter x is transformed into the \(dq\)-axis current after three-phase rotation transformation. Then, the three-phase output AC voltage modulation wave is used, and the amplitude \(U\) in the \(dq\) coordinate system is obtained after three-phase rotation transformation. sd , \(U\) sq ;

[0083] The three-phase AC output voltage is based on the DC bus voltage \(m\) of the MMC converter u and the phase-locked angle of the MMC converter, and the reference value of the circulating current \(i\) is obtained according to the principle of power balance of the sub-module capacitor voltage of the MMC converter. zx_ref ;

[0084] Step 2: Calculate the feedback circulating current \(i\). zx :

[0085] The currents \(i\) of the upper and lower arms of the MMC converter are obtained through sampling analysis and calculation. px and \(i\) nx , and half of the sum of the two is the feedback circulating current \(i\). zx ;

[0086] Step 3: Combine \(i\) zx_ref and \(i\) zx to calculate \(U\) zx , specifically:

[0087] Since the injection of the common-mode voltage by the MMC converter will cause the second, third, and fourth harmonic power fluctuations of the arm, the second, third, and fourth-order quasi-resonant controllers are used for reverse power suppression, so as to obtain the circulating current injection value \(U\). zx .

[0088] Step 4: Calculate the instantaneous value \(u\) of the three-phase modulation wave. mx (\(x = a, b, c\)), and the specific expression is:

[0089] The reference value \(U^{*}_{dc}\) of the DC bus voltage of the MMC converter and the feedback of the measured value \(U_{dc}\) of the DC bus voltage are controlled by the outer loop of the PI regulator to obtain the reference value of the inner loop current of the MMC converter. This reference value is then decoupled by the current inner loop with the reactive and active currents after the three-phase rotation transformation of the three-phase output current of the MMC converter, and then transformed from rotation to static transformation, so as to obtain the instantaneous value \(u\) of the three-phase modulation wave. mx (\(x = a, b, c\)).

[0090] Step 5: Calculate the instantaneous values \(u'\) of the three-phase modulation waves of phases A, B, and C after injecting the common-mode voltage. ma , \(u'\) mb , \(u'\) mc , and the specific expression is:

[0091] The instantaneous value of the three-phase modulation wave and the common-mode voltage u of the three-phase MMC control system com *The difference between them is the instantaneous value u' of the three-phase modulation waves of phases A, B, and C after injecting the common-mode voltage ma , u' mb , u' mc .

[0092] Among them, the common-mode voltage u com is

[0093]

[0094] Step 6: Calculate the reference values v px * , v nx * (x＝a、b、c) of the control voltages of the upper and lower arms of the three-phase MMC control system (here taking phase a v pa * , v na * as an example). The specific expression is:

[0095] Superimpose the instantaneous value u mx (x = a, b, c) of the three-phase modulation wave with half of the actual value of the DC bus voltage of the three-phase MMC control system to obtain the reference values v px * , v nx * (x = a, b, c) of the control voltages of the upper and lower arms of the three-phase MMC control system.

[0096] Step 7: Calculate the control values of the control voltages of the upper and lower arms of the three-phase MMC control system after injecting the circulating current (here taking phase a as an example). The specific expression is:

[0097] Superimpose the reference values v pa * , v na * of the control voltages of the upper and lower arms of the three-phase MMC control system with the circulating current injection value U zx to obtain the control values of the control voltages of the upper and lower arms of the three-phase MMC control system. These control values are converted into corresponding pulses by a digital processor to control the on and off of the three-phase MMC power devices.

[0098] It can be seen from Figure 3 that the overall control strategy for reactive power support and fault ride-through mainly consists of the sub-module suppression strategy for injecting the common-mode voltage at its front end and the STATCOM strategy for reactive power and voltage support during fault ride-through at its back end.

[0099] Among them, the sub-module suppression strategy belongs to device-level control and is mainly used to protect the safe and stable operation of the MMC device. It is completed by the Figure 3 "common-mode voltage injection" module, "circulating current injection" module, and "MMC converter" in

[0100] The reactive voltage support STATCOM strategy for fault ride-through belongs to system-level control. Its main function is to provide reactive power support when a short-circuit fault occurs in the transmission system and help the power grid recover quickly. It is completed by the Figure 3 "MMC-STATCOM control strategy" module and "MMC converter" in

[0101] In addition, Figure 3 in, u md = u sd ; u mq = u sq ."MMC converter" means MMC rectifier.

[0102] "z" represents alternating current.

[0103] The actual three-phase output current (i x , i px and i nx ) of the MMC converter is obtained by sampling through a current sensor, and the three-phase output voltage u x of the MMC converter is collected through a voltage sensor.

[0104] 1. Reliability protection strategy for common-mode voltage injection and secondary circulating current optimal value of MMC topology at valve level module

[0105] To reduce the severe amplitude of the sub-module capacitor voltage fluctuation caused by the short-circuit current impact, the present invention is achieved by injecting a common-mode voltage into the modulation wave, that is, by increasing the modulation degree to reduce the sub-module capacitor voltage fluctuation under the condition of constant system output power. The Fourier series of the common-mode voltage is expressed as Equation (1):

[0106]

[0107] In the formula, the term added to u com represents the common-mode voltage that needs to be injected in a symmetric system. U a0 is the effective value of the output phase voltage, θ is the common-mode voltage injection angle, and n is the output voltage frequency order.

[0108]

[0109] The output phase voltage after injecting the common-mode voltage can be expressed as follows:

[0110]

[0111] In the formula, ψv represents the initial phase of the phase voltage, ω is the power frequency rotation rate of phase a, and t is the power frequency rotation time of phase a.

[0112] It can be analyzed from Equation (1) that the common-mode voltage only contains odd harmonics with multiples of 3. The injection amount of the third harmonic is 30 times that of the ninth harmonic, and the higher the frequency, the less the harmonic content. The injection amplitude of the third harmonic is approximately

[0113] For simplicity of analysis, the influence of the introduction of other harmonics except the third harmonic can be ignored. At this time, Equation (2) can be expressed as:

[0114]

[0115] Considering the internal circulation of the current with multiples of 3, the output current can be expressed as follows:

[0116]

[0117] In Equation (4), I ao is the effective value of the output phase current, is the power factor angle.

[0118] Although the fluctuation of the capacitor voltage of the sub-module can be suppressed during the injection of the common-mode voltage, it may cause an increase in the circulating current. Therefore, a circulating current suppression strategy is also required during this method.

[0119] Taking phase a as an example, the powers of the upper and lower bridge arms are respectively:

[0120]

[0121]

[0122] In Equations (5) and (6), u pa , u na , i pa , i na represent the voltages and currents of the upper and lower bridge arms respectively, and U dc , I dc are the DC bus voltage and current.

[0123] Among them, m u , m i are respectively:

[0124]

[0125] Analysis of equations (5) and (6) shows that although the injection of the common-mode voltage will cause the double-frequency and quadruple-frequency power fluctuations of the arm circulating current, at this time, since the double-frequency and quadruple-frequency power fluctuations introduced after the injection of the common-mode voltage are always opposite to the original double-frequency and quadruple-frequency power fluctuations, the double-frequency power fluctuation of the arm is reduced, that is, injecting the common-mode voltage into the modulation wave can effectively suppress the arm circulating current while reducing the capacitor voltage of the sub-module.

[0126] 2. Fault Ride-Through Technology for Reactive Power Support in HVDC Light Transmission System for Offshore Wind Power

[0127] In the HVDC light transmission system for offshore wind power, since a fault often occurs in the arm of the MMC, a short circuit may occur between a certain pole and the ground (such as the lower arm being grounded). At this time, the upper arm can operate normally, and although the lower arm (the arm of the faulty pole) can receive the energy from the AC side, the energy cannot flow to the DC side. The accumulation of energy will cause the capacitor voltage of the sub-module to continuously increase, which will not only lead to the collapse of the converter, but also prevent it from providing reactive power support to the power grid, which is not conducive to the rapid recovery of the power grid.

[0128] To solve the above problems and ensure the maximum power transmission of the MMC during the pole-to-ground fault, the present invention introduces a new phase angle control variable λ to achieve power balance between the upper and lower arms, as Figure 4 shown.

[0129] By changing λ, the power flowing in from the AC side is redistributed between the upper and lower arms. Eventually, the non-faulty pole arm transmits half of the rated power, and the faulty pole arm does not transmit active power, but can provide reactive power to the power grid for reactive power support. The magnitude of the reactive power support is related to the phase angle control quantity. Taking phase a as an example.

[0130] After introducing the phase angle variable λ, the output voltages of the upper and lower arms are:

[0131]

[0132] where E a is the peak value of the output voltage of phase a of the MMC arm and is a function of time t;

[0133] ψ v is the phase angle.

[0134] Ignoring the self-loss of the MMC, the three-phase received powers of the upper and lower arms are:

[0135]

[0136] where U j is the peak value of the AC voltage of phase j, and E j is the peak value of the output voltage of phase j of the MMC arm;

[0137] p pjIndicates the three-phase received power of the upper bridge arm, p nj Indicates the three-phase received power of the lower bridge arm.

[0138] The difference between the received and output powers of the upper and lower bridge arms can be expressed as the energy fluctuation of the capacitor of the bridge arm sub-module, as shown in Equation (10):

[0139]

[0140] In Equation (10), W P , W N Indicates the cumulative energy of the upper and lower bridge arms;

[0141] W P = ∑W p , W N = ∑W n ; By monitoring the voltage and current of each sub-module, the energy W stored in each DC capacitor C is calculated p , and finally accumulated to calculate W P .

[0142] Larm represents the arm inductor.

[0143] Among them, the phase angle difference λ is calculated as follows:

[0144]

[0145] In Equation (11), P' represents the difference in cumulative energy between the upper and lower bridge arms, W P -W N = P’.

[0146] It can be seen from Equation (10) that by controlling the phase angle difference λ of the AC components in the upper and lower bridge arm voltages, the active power transmitted by the upper and lower bridge arms can be redistributed, so that the energy of the sub-module capacitor in the MMC is kept stable, and further the voltage fluctuation of the sub-module capacitor caused by the common-mode voltage injection is stabilized. At this time, the output voltage of the upper bridge arm still contains a DC component and can transmit active power to the DC side.

[0147] By adjusting λ, all the active power can be transmitted by the upper bridge arm (non-fault pole bridge arm), and the lower bridge arm (fault pole bridge arm) does not transmit active power, but can provide reactive power to the power grid.

[0148] So far, the power balance of the upper and lower bridge arms of the MMC has been achieved, and there is no energy accumulation in each cycle, and at the same time, the maximum power transmission is ensured.

[0149] In summary, determining the optimal phase angle difference includes the following steps:

[0150] Step a, the amplitude and phase angle Mag(u diffj ), Arg(u diffj) Obtained.

[0151] The common-mode voltage of the MMC converter output can be obtained through the decoupling control of the current inner loop (the above step 5 has elaborated on how to obtain this value), and then the amplitude and phase angle of the common-mode voltage Mag(u diffj ) and Arg(u diffj ) can be obtained through the phase discriminator and the amplitude calculation formula.

[0152] Step b: Calculate the energy ΣW p and ΣW n stored in the capacitors of the upper and lower arms of the MMC converter.

[0153] Since the MMC converter cannot stably transmit active power in the short-circuited arm, the arm close to the faulty pole will continuously accumulate the energy obtained from the AC side during this state. Its energy calculation is obtained by integrating the power flowing through the upper and lower arms of the converter within one cycle, that is, the sum of the voltages Σu cp and Σu cn of the sub-module capacitors inserted into the upper and lower arms and the integral accumulation of the arm current.

[0154] It can be seen from this that within one cycle, the energy accumulated in the upper arm (non-faulty pole arm) is zero, that is, the input and output powers are equal, so the upper arm can operate normally; however, the lower arm (faulty pole arm) can receive the energy from the AC side, but the energy cannot flow to the DC side. The accumulation of energy will cause the voltage of the sub-module capacitor to continuously increase, eventually causing the converter to collapse.

[0155] Step c: Calculate the phase angle difference λ of the AC components in the upper and lower arm voltages.

[0156] To solve the problem of converter collapse caused by the accumulated energy, the phase angle difference λ of the AC components in the upper and lower arm voltages can be changed, so that the power flowing in from the AC side is redistributed between the upper and lower arms. Eventually, the non-faulty pole arm transmits half of the rated power, and the faulty pole arm does not transmit active power, but can provide reactive power to the grid, so that both the upper and lower arms reach power balance. The phase angle difference λ is calculated as follows:

[0157] The difference in the accumulated energy between the upper and lower arms passes through a low-pass filter to remove the noise and the disturbance components of the short-circuit current impact, and then through the PI regulator operation, the phase angle difference λ can be obtained.

[0158] When operating in a steady state, λ can be expressed as

[0159]

[0160] Step d: Calculate the control voltage control values of the upper and lower arms of the three-phase MMC control system after adjusting the phase angle difference λ.

[0161] The common phase angle difference λ and the phase angle Arg(u diffj ) After reconstruction, new phase angles of the upper and lower arm control voltages are obtained. This value is then multiplied by the magnitude Mag(u diffj ) to obtain the reference values of the upper and lower arm control voltages of the three-phase MMC control system. This reference value is then superimposed on the actually detected upper and lower arm voltage values to obtain the control values of the upper and lower arm control voltages. This control value is converted into corresponding pulses by a digital processor to control the turning on and off of the three-phase MMC power devices.

[0162] The system-level control block diagram after introducing λ is as shown in Figure 5 Figure.

[0163] Mag(u diffj ) and Arg(u diffj ) respectively represent the magnitude and phase angle of the common-mode voltage output by the MMC converter.

[0164] ΣW p and ΣW n respectively represent the energies stored in the upper and lower arm capacitors.

[0165] Figure 5 Process description:

[0166] Step 1: The inner-loop current control of the MMC converter obtains the magnitude and phase angle Mag(u com *) and Arg(u diffj ) of the common-mode voltage u diffj .

[0167] Step 2: For the upper arm of the MMC converter, the sum of the common-mode voltage phase angle Arg(u diffj ) and the phase angle difference λ is subjected to a sine operation to obtain its angle transformation value; for the lower arm of the MMC converter, the difference between the common-mode voltage phase angle Mag(u diffj ) and the phase angle difference λ is subjected to a sine operation to obtain its angle transformation value.

[0168] Step 3: For the upper arm of the MMC converter, the product of the common-mode voltage magnitude Mag(u diffj ) and the sine operation angle is the upper arm reference voltage u pj ; for the lower arm of the MMC converter, the triple harmonic injection voltage u 3com minus the product of the common-mode voltage magnitude Mag(u diffj ) and the sine operation angle, and this value is used as the lower arm reference voltage u nj .

[0169] Step 4: For the upper arm reference voltage u pj , the lower arm reference voltage u njPerform sub-module capacitor voltage equalization and carrier phase shift control operations to generate pulses for the IGBTs, the power devices of each sub-module of the MMC converter, thereby controlling the turn-on and turn-off of the power devices.

[0170] Step 5: Under normal operating conditions, active and reactive power can be transmitted normally by controlling the turn-on and turn-off of the power devices of the MMC converter, and the accumulated energy of its arm is zero.

[0171] However, due to the short-circuit condition of the MMC converter, its short-circuit arm cannot stably transmit active power. In this state, the arm near the fault pole will continuously accumulate the energy obtained from the AC side. Its energy calculation is the integration of the power flowing through the upper and lower arms of the converter within one cycle, that is, the sum of the voltages of the sub-module capacitors Σu cp 、∑u cn and the integration accumulation of the arm current, so as to obtain the energy ∑W p 、∑W n stored in all the DC capacitors C of the upper and lower arms of the MMC converter, that is, W P ,W N represents the accumulated energy of the upper and lower arms, W P =∑W p ,W N =∑W n .

[0172] Step 6: After filtering out the interference signal through a low-pass filter for the difference between the accumulated energies of the upper and lower arms and then passing it through a PI controller, the phase angle difference λ can be obtained. The phase angle λ can be expressed by the formula:

[0173]

[0174] 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 within the technical field, without departing from the technical solution of the present invention, makes any form of equivalent substitution or modification and other changes to the technical solution and technical content disclosed by the present invention, all of which fall 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 comprehensive control method for reactive power support and fault ride-through during short-circuit faults in offshore wind power, characterized in that, Including the following steps: Perform common-mode injection calculation: obtain the circulating current reference value i zx_ref , add a PR controller to each phase respectively to control the circulating current i zx to follow i zx_ref as it changes, and the output result U of the PR controller zx ; Determine the optimal phase angle difference: Introduce the phase angle control variable λ, and find the phase angle difference of the AC components in the voltages of the upper and lower bridge arms, so that the non-faulty pole bridge arm transmits half of the rated power, and the faulty pole bridge arm does not transmit active power but can provide reactive power to the power grid.

2. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 1, wherein The calculation of common-mode injection specifically includes the following steps: Step 1, calculate the reference value i of the circulating current zx_ref : The three-phase output current i of the MMC converter x After three-phase rotation transformation, the dq-axis current is obtained. Then, using the three-phase output AC voltage modulation wave, the amplitude U in the dq coordinate system is obtained after three-phase rotation transformation sd , U sq ; The three-phase AC output voltage is based on the DC bus voltage m of the MMC converter u and the phase-locked angle of the MMC converter. The circulating current reference value i is obtained according to the principle of power balance of the sub-module capacitor voltage of the MMC converter zx_ref ; Step 2, calculate the feedback loop current i zx : The currents i of the upper and lower arms of the MMC converter are obtained through sampling analysis and calculation px and i nx , and half of the sum of the two is the feedback circulating current i zx ; Step 3, the resonance controller receives i zx_ref and i zx , and outputs u zx .

3. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 2, characterized in that, Determine the optimal phase angle difference, including the following steps: Step a, the amplitude and phase angle of the common-mode voltage output by the MMC converter Mag(u diffj ) and Arg(u diffj ) are obtained: The magnitude and phase angle of the common-mode voltage can be obtained through the phase discriminator and the magnitude calculation formula Mag(u diffj )、Arg(u diffj ); Step b, calculate the stored energy ∑W in the capacitors of the upper and lower arms of the MMC converter p , ∑W n Calculate: Among them, p pj represents the three-phase received power of the upper bridge arm, and p nj represents the three-phase received power of the lower bridge arm; Larm represents the bridge arm inductor; ψ v is the phase angle; U j is the peak value of the AC voltage of phase j, E j is the peak value of the output voltage of phase j of the MMC arm; W p Accumulate energy for the upper bridge arm; W n Accumulate energy for the lower bridge arm; Step c, control the calculation of the phase angle difference λ of the AC components in the voltages of the upper and lower bridge arms: where P' represents the difference between the cumulative energy W of the upper bridge arm P and the cumulative energy W of the lower bridge arm N .

4. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 2, wherein After step 3, the following steps are also included: Step 4, instantaneous value u of three-phase modulation wave mx , where x = a, b, c; Step 5: Calculate the instantaneous values u′, u′, u′ of the A, B, and C phase modulation waves after injecting the common-mode voltage ma , u′ mb , u′ mc ; Step 6, calculate the upper and lower arm control voltage reference values v px * , v nx * ; Step 7, calculate the control voltage control values of the upper and lower bridge arms of the three-phase MMC control system after circulating current injection.

5. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 4, wherein, Step 4 specifically includes the following steps: The DC bus voltage reference value of the MMC converter and the reactive and active currents of the three-phase output current of the MMC converter after three-phase rotation transformation are subjected to current inner-loop decoupling control, and then transformed from rotation to static transformation to obtain the instantaneous value u of the three-phase modulation wave mx 。 6. The integrated control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 4, characterized in that, In step 5: Instantaneous value u of three-phase modulation wave mx and the common-mode voltage u com * of the three-phase MMC control system. The difference between them is the instantaneous value u′ of the three-phase modulation waves of phases A, B, and C after injecting the common-mode voltage ma , u′ mb , u′ mc .

7. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 6, characterized in that In step 5: Where, θ is the common-mode voltage injection angle, and n is the output voltage frequency number.

8. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 4, characterized in that In step 6: The instantaneous values of the three-phase modulation waves are superimposed on half of the actual value of the DC bus voltage of the three-phase MMC control system to obtain the reference control voltages v px * and v nx * for the upper and lower arms of the three-phase MMC control system.

9. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 4, characterized in that In step 7: The upper and lower arm control voltage reference values v pa * and v na * are superimposed with the circulating current injection value U zx to obtain the upper and lower arm control voltage control values of the three-phase MMC control system.

10. The comprehensive control method for reactive power support and fault ride-through during short-circuit faults of offshore wind power according to claim 2, characterized in that In determining the optimal phase angle difference, the upper bridge arm is the non-faulty pole bridge arm.

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