A hybrid MMC control method and system

Abstract

The application discloses a hybrid MMC control method and system, which comprises the following steps: under the working conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through, a control pulse is formulated according to a converter voltage reference value formulated by a sub-module capacitor voltage of an MMC converter valve, an arm circulating current formulated by a circulating current injection voltage value and a DC voltage given value formulated by a DC current given value, and the hybrid MMC is controlled to be put into operation. According to the converter voltage reference value formulated by the sub-module capacitor voltage of the MMC converter valve, the hybrid MMC can ensure that the MMC arm current zero-crossing point is guaranteed, capacitor voltage balance is realized, and system collapse caused by the capacitor voltage separation from the full-bridge sub-module capacitor voltage and the continuous charging of the capacitor voltage when the arm current is not in the zero-crossing point is avoided. The circulating current injection voltage value formulated by the MMC converter valve arm circulating current is injected into a circulating current injection link, so that the arm current always has a zero-crossing point, and the safe operation of the MMC arm is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current transmission, in particular to a hybrid MMC control method and system. BACKGROUND

[0002] In the face of the severe challenges of global energy security, environmental pollution and climate change, the country vigorously develops and utilizes renewable clean energy and optimizes energy structure. The line commutated converter (LCC) of the power grid is mature, reliable and efficient for long-distance and large-capacity power transmission, but it has a high risk of commutation failure under a weak AC system. The voltage source converter (VSC) has the advantages of independent regulation of active and reactive power, weak grid access, no commutation failure risk and no need for synchronous networking, and is a friendly technical means for building a multi-terminal DC network, realizing long-distance power transmission and connecting renewable energy to the grid. Direct current transmission is developing towards higher voltage, larger transmission capacity and longer distance, and VSC-LCC hybrid multi-terminal direct current transmission is the future development trend. The under-construction Wudongde project uses two VSCs in cascade and then parallel to an LCC to form a parallel multi-terminal power grid.

[0003] The half-bridge topology direct current converter does not have the ability to clear the direct current fault, and a direct current circuit breaker needs to be added to isolate the fault current, which increases the cost of equipment and the complexity of control. The half-bridge / full-bridge sub-module hybrid MMC topology uses the full-bridge characteristic to block the direct current fault current, but the proportion of full-bridge sub-modules is at least 50%. The smaller the proportion of full-bridge sub-modules, the fewer IGBT power elements, and the lower the cost of equipment. If the hybrid MMC has the functions of step-down / semi-pressure operation, online switching of single valve groups and direct current fault ride-through, and if the proportion of full-bridge sub-modules is 50%, the half-bridge sub-module capacitor voltage will continue to charge when there is no zero point in the bridge arm current, and it will be separated from the full-bridge sub-module capacitor voltage, leading to system collapse. Therefore, the hybrid MMC lacks an effective control method system under the conditions of step-down / semi-pressure operation, online switching of single valve groups and direct current fault ride-through in the converter station. SUMMARY

[0004] In order to solve the problem of the lack of effective control method system of the hybrid MMC under the conditions of step-down / semi-pressure operation, online switching of single valve groups and direct current fault ride-through in the converter station in the prior art, the present application provides a hybrid MMC control method and system.

[0005] The technical scheme provided by the present application is as follows:

[0006] A hybrid MMC control method, comprising:

[0007] Under normal conditions, control pulse signals are formulated according to the converter voltage reference value formulated by the reactive power and the active power, the preset circulating current suppression voltage and the direct current voltage given value, and the hybrid MMC is controlled to be put into operation.

[0008] When in the voltage reduction / half voltage operation, single valve group on-line switching and DC fault ride through working condition of the converter station, according to the converter voltage reference value formulated by the capacitor voltage of the sub-module of the MMC converter valve, the circulating current injection voltage value formulated by the circulating current of the bridge arm of the MMC converter valve and the DC voltage given value formulated by the given value of the DC current, the control pulse is formulated, and the input of the hybrid MMC is controlled.

[0009] Preferably, the converter voltage reference value formulated by the capacitor voltage of the sub-module of the MMC converter valve is formulated by the following steps:

[0010] The average measured value of the capacitor voltage of the sub-module of the MMC converter valve is subtracted from the set value, and the d component of the current reference value is obtained through outer loop PI regulation;

[0011] The d component of the current reference value is regulated through inner loop PI, and the d component of the converter voltage reference value is obtained;

[0012] The measured value of the reactive power at the point of common connection of the MMC converter valve is subtracted from the set value, and the q component of the current reference value is obtained through outer loop PI regulation;

[0013] The q component of the current reference value is regulated through inner loop PI, and the q component of the converter voltage reference value is obtained.

[0014] Preferably, the formulation of the d component and the q component of the current reference value is as follows:

[0015]

[0016]

[0017] Wherein, i dref is the d component of the current reference value, U c-ref is the average measured value of the capacitor voltage of the sub-module of the MMC converter valve, U c-avr is the set value of the capacitor voltage of the sub-module of the MMC converter valve, k pd1 is the proportional control coefficient of the d component inner loop PI regulation, k id1 is the integral control coefficient of the d component inner loop PI control, i qref is the q component of the current reference value, Q ref is the measured value of the reactive power, Q s is the set value of the reactive power, k pq1 is the proportional control coefficient of the q component inner loop PI regulation, k iq1 is the integral control coefficient of the q component inner loop PI control.

[0018] Preferably, the d component and the q component of the converter voltage reference value are formulated as follows, respectively:

[0019]

[0020]

[0021] wherein e sd is the d component of the converter voltage reference value, u sd is the d component of the MMC converter valve side voltage, ω is the angular frequency, i dref is the d component of the current reference value, i d is the d component of the MMC converter valve side current, k pd2 is the d component outer loop PI control proportional control coefficient, k id2 is the d component outer loop PI control integral control coefficient, e sq is the q component of the converter voltage reference value, u sq is the q component of the MMC converter valve side voltage, i qref is the q component of the current reference value, i q is the q component of the MMC converter valve side current, k pq2 is the q component outer loop PI control proportional control coefficient, k iq2 is the q component outer loop PI control integral control coefficient.

[0022] wherein,

[0023] L = L t + 0.5Larm

[0024] wherein L t is the transformer leakage reactance value, and Larm is the bridge arm reactor value.

[0025] Preferably, the circulating current injection voltage value formulated by the MMC converter valve bridge arm circulating current is formulated by the following steps:

[0026] The grid current is subjected to park transformation and angle transformation to obtain a phase angle value of circulating current injection;

[0027] According to the phase angle value and a circulating current injection amplitude, a circulating current injection expected value having d-axis and q-axis components is formulated by trigonometric function transformation;

[0028] According to the bridge arm circulating current, the phase angle value and the circulating current injection expected value, a circulating current injection voltage value is calculated;

[0029] wherein the circulating current injection amplitude is a preset multiple of the DC component of the bridge arm current.

[0030] Preferably, the three-phase circulating current voltage reference value is calculated according to the bridge arm circulating current, phase angle value and circulating current injection desired value, as the circulating current injection voltage value, comprising:

[0031] The bridge arm circulating current is converted by dq transformation to obtain dq bridge arm circulating current and voltage feed-forward value;

[0032] The dq bridge arm circulating current and circulating current injection desired value are subjected to PI regulation to obtain initial circulating current injection voltage value;

[0033] The initial circulating current injection voltage value is subtracted from the voltage feed-forward value and subjected to park inverse transformation to obtain the circulating current injection voltage value.

[0034] Preferably, the DC voltage given value formulated from the DC current given value is formulated by the following steps:

[0035] The DC current measured value is subtracted from the given value and subjected to PI regulation to obtain the DC voltage given value.

[0036] Preferably, under normal operating conditions, the control pulse signal is formulated according to the converter voltage reference value formulated from the reactive power and active power, the preset circulating current suppression voltage and the DC voltage given value, and the mixed MMC is controlled to be put into operation, comprising:

[0037] Based on the reactive power and active power at the MMC converter valve common connection point, the current reference value is obtained by inner loop PI regulation, and the converter voltage reference value is obtained by outer loop PI regulation;

[0038] The circulating current suppression voltage of the MMC converter valve is set to 0;

[0039] The control pulse signal is formulated according to the voltage reference value, the circulating current suppression voltage and the DC voltage given value, and the mixed MMC is controlled to be put into operation.

[0040] A mixed MMC control system, the system comprising:

[0041] a converter, a circulating current suppression controller, a mode selection controller and a control pulse generator;

[0042] The converter is used to formulate the converter voltage reference value;

[0043] The circulating current suppression controller is used to formulate the circulating current suppression voltage value under normal operating conditions and converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through operating conditions;

[0044] The mode selection controller is used to formulate the DC voltage given value under normal operating conditions and converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through operating conditions.

[0045] The pulse control is generated for formulating control pulses in normal operation and in sub-voltage / semi-voltage operation of the converter station, on-line switching of single valve group, and DC fault ride-through operation, and for controlling the input of the hybrid MMC.

[0046] Preferably, the converter comprises:

[0047] an outer loop power controller and an inner loop power controller;

[0048] The outer loop power controller is configured to formulate a current reference value in normal operation by outer loop PI regulation according to the reactive power and active power at the common connection point of the MMC converter valve, and to formulate a current reference value in special operation by outer loop PI regulation according to the capacitor voltage of the sub-module in the MMC converter valve and the reactive power reference value in sub-voltage / semi-voltage operation of the converter station, on-line switching of single valve group, and DC fault ride-through operation.

[0049] The inner loop power controller is configured to formulate a converter voltage reference value in normal operation by inner loop PI regulation according to the current reference value in normal operation, and to formulate a converter voltage reference value in special operation by inner loop PI regulation according to the current reference value in special operation.

[0050] Preferably, the circulating current suppression controller sets the circulating current suppression voltage of the MMC converter valve to 0 in normal operation.

[0051] The circulating current suppression controller formulates a circulating current injection voltage value according to the grid current in sub-voltage / semi-voltage operation of the converter station, on-line switching of single valve group, and DC fault ride-through operation.

[0052] Preferably, the circulating current suppression controller comprises:

[0053] a phase angle control module, a circulating current injection setting module, and a circulating current injection control module.

[0054] The phase angle control module performs park transformation and angle transformation according to the grid current to obtain a phase angle value of the circulating current injection.

[0055] The circulating current injection setting module formulates a circulating current injection expected value having d-axis and q-axis components by trigonometric function transformation according to the phase angle value and a circulating current injection amplitude.

[0056] The circulating current injection control module calculates a circulating current injection voltage value according to the bridge arm circulating current, the phase angle value, and the circulating current injection expected value.

[0057] The circulating current injection amplitude is a preset multiple of the DC component of the bridge arm current.

[0058] Preferably, the mode selection controller sets a fixed amplitude DC voltage given value under normal operating conditions.

[0059] The mode selection controller subtracts the DC current measurement value from the given value and obtains a DC voltage given value through PI regulation under the operating conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through.

[0060] Compared with the prior art, the present application has the following beneficial effects:

[0061] The technical scheme provided by the present application comprises: under normal operating conditions, a control pulse signal is formulated according to a converter voltage reference value formulated by reactive power and active power, a preset circulating current suppression voltage and a DC voltage given value, and the hybrid MMC is controlled to be put into operation; under the operating conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through, a control pulse is formulated according to a converter voltage reference value formulated by the capacitor voltage of the sub-module of the MMC converter valve, a circulating current injection voltage value formulated by the circulating current of the MMC converter valve bridge arm and a DC voltage given value formulated by the DC current given value, and the hybrid MMC is controlled to be put into operation. The method provided by the present scheme can ensure the zero-crossing point of the MMC bridge arm current and realize capacitor voltage balance under the operating conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through, thereby avoiding the system collapse caused by the separation of the capacitor voltage of the full-bridge sub-module and the bridge arm current not being at the zero-crossing point. BRIEF DESCRIPTION OF DRAWINGS

[0062] Figure 1 It is a hybrid MMC control block diagram of the present application.

[0063] Figure 2 It is a conventional MMC overall control block diagram in the embodiment of the present application.

[0064] Figure 3 It is a circulating current injection control block diagram of the present application.

[0065] Figure 4 It is a fundamental frequency separation phase-locked angle control block diagram of the present application.

[0066] Figure 5 It is a circulating current injection given value block diagram of the present application. DETAILED DESCRIPTION

[0067] For better understanding of the present application, the contents of the present application are further explained below in conjunction with the accompanying drawings and examples.

[0068] Embodiment 1:

[0069] The embodiment provides a hybrid MMC control method, comprising:

[0070] The traditional MMC overall control block diagram is shown in Figure 2 . P ref , Q ref are active and reactive power instruction values of an input outer loop power controller, i d_ref , i q_ref are dq-axis current reference values output by the outer loop power controller. After adding a circulating current suppression additional control signal u cirj_ref , the control system finally outputs upper and lower bridge arm voltage reference signals u pj_ref and u nj_ref , which are converter voltage reference values, and then generates corresponding trigger pulses through a multi-level carrier phase shift modulation algorithm, thereby controlling the turn-on and turn-off of each sub-module.

[0071] When in the voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through working conditions of the converter station, the hybrid MMC control block diagram is shown in Figure 1 , wherein U c_avr is the difference between the average average value of the sub-module capacitor voltage and a given value preset in advance, and then the d component of the current reference value is obtained through PI regulation of the outer loop; the q component of the current reference value is obtained in the same way as the traditional way, that is, the difference between the measured value of the reactive power at the point of common coupling of the MMC converter valve and the set value preset in advance is obtained, and then the q component of the current reference value is obtained through PI regulation of the outer loop, and the dq components of the current reference value are obtained through PI regulation of the inner loop to obtain the converter voltage reference value.

[0072] The d component and the q component of the current reference value are respectively as follows:

[0073]

[0074]

[0075] , wherein i dref is the d component of the current reference value, U c-ref is the average measured value of the sub-module capacitor voltage of the MMC converter valve, U c-avr is the set value of the sub-module capacitor voltage of the MMC converter valve, k pd1 is the proportional control coefficient of the d component inner loop PI regulation, k id1 is the integral control coefficient of the d component inner loop PI control, i qrefQ is the q component of the current reference value ref Q is the measured value of the reactive power s k is the set value of the reactive power pq1 k is the proportional control coefficient of the q component inner loop PI regulation iq1 is the integral control coefficient of the q component inner loop PI control.

[0076] The formulation of the d component and the q component of the converter voltage reference value is as follows respectively:

[0077]

[0078]

[0079] wherein e sd u is the d component of the converter voltage reference value sd ω is the angular frequency, i dref i is the d component of the current reference value d k is the d component of the MMC converter valve side current pd2 k is the proportional control coefficient of the d component outer loop PI control id2 e is the integral control coefficient of the d component outer loop PI control sq u is the q component of the converter voltage reference value sq k is the q component of the MMC converter valve side current qref i is the q component of the current reference value q k is the q component of the MMC converter valve side current pq2 k is the proportional control coefficient of the q component outer loop PI control iq2 is the integral control coefficient of the q component outer loop PI control;

[0080] In the formula,

[0081] L = L t + 0.5Larm

[0082] wherein L t Larm is the bridge arm reactor value.

[0083] The control system is divided into outer loop control and inner loop control. The inner loop control is basically the same as the circulating current suppression and the half-bridge MMC. The difference is that the d-axis current reference value Idref is directly obtained by the sub-module capacitor voltage controller, instead of being obtained by the DC voltage controller. The DC voltage reference value U dcref is given and the control mode is related. When the station is in the voltage control mode, U dcref is directly set to U dcref_setIn current mode, the current is provided by the DC current controller. The mode selection logic switches between the DC current controller and the voltage controller.

[0084] Under normal operating conditions, the active power outer loop control of the hybrid MMC adopts constant DC voltage or constant active power control, and the reactive power outer loop control adopts constant AC voltage or reactive power control, with circulating current suppression at 0. Under the conditions of converter station step-down / half-voltage operation, single valve group online commissioning / discontinuation, and DC fault ride-through, the active power outer loop control uses constant DC current control for the stator module capacitor voltage of the high valve group and constant DC voltage setpoint for the low valve group, while constant DC voltage or constant active power control is used for the low valve group. The reactive power outer loop control adopts constant AC voltage or reactive power control. When the circulating current switches to circulating current injection and the system power voltage returns to normal, the control mode switches back to normal operating conditions and exits the circulating current injection mechanism.

[0085] Circulation injection control block diagram, as follows Figure 3 As shown. Bridge arm circulation i cirj (j=a,b,c) After the abc / dq coordinate transformation, the bridge arm current i in the dq coordinate is obtained. 2fd and i 2fq Compare them with the reference value i of the circulating dq axis components. 2fd_ref and i 2fq_ref After subtraction, the voltage feedforward quantity 2ω0L0i is introduced after passing through a proportional-integral (PI) regulator. 2fq and 2ω0L0i 2fd By eliminating the dq-axis coupling portion, the dq-axis reference value u of the internal unbalanced voltage drop can be obtained. cird_ref and u cirq_ref Finally, the three-phase circulating current voltage reference value u is obtained through inverse dq / abc coordinate transformation. cirj_ref This is the circulating current injection voltage value. 2fd_ref and i 2fq_ref The desired circulating current injection size is determined, and the size of the circulating current injection is a fixed value, approximately 0.4 to 0.6 times the DC current. The phase of the circulating current injection is either in phase (Idc<0) or out of phase (Idc>0) with the real-time reactive component of the bridge arm.

[0086] Block diagram of grid voltage phase-locked angle control, as shown Figure 4 As shown, the grid voltage E sa E sb E sc The grid voltage angle (angle) is obtained through PLL phase-locked loop (PLL) for coordinate transformation. Grid current (i) sa i sb i sc The alternating current i in the dq coordinate system is obtained after the abc / dq transformation. sd i sqAfter arctan transformation, the angle thetai is obtained, and the angle thetai plus or minus 90 degrees is used for the coordinate transformation in the circulating current injection and the phase of the circulating current injection.

[0087] The circulating current injection value control block diagram is shown in Figure 2. Figure 5 As shown in Figure 2, the angle thetai obtained by separating the fundamental frequency is subjected to cos and sin operations to obtain i 2fd_ref and i 2fq_ref .

[0088] When in normal operation, the mode selection is preset with a direct current voltage given value; when in converter station voltage reduction / half voltage operation, single valve group online switching and direct current fault ride-through operation, the direct current current measurement value is subtracted from the given value, and a PI regulation is performed to obtain the direct current voltage given value.

[0089] Finally, in normal operation, a control pulse signal is formulated according to the converter voltage reference value formulated by the reactive power and the active power, the preset circulating current suppression voltage and the direct current voltage given value, and the mixed MMC is controlled to be put into operation;

[0090] When in converter station voltage reduction / half voltage operation, single valve group online switching and direct current fault ride-through operation, a control pulse is formulated according to the converter voltage reference value formulated by the capacitor voltage of the sub-module in the MMC converter valve, the circulating current injection voltage value formulated by the bridge arm circulating current of the MMC converter valve and the direct current voltage given value formulated by the direct current current given value, and the mixed MMC is controlled to be put into operation.

[0091] Embodiment 2

[0092] The embodiment provides a mixed MMC control method system, which comprises:

[0093] a converter, a circulating current suppression controller, a mode selection controller and a control pulse generator;

[0094] The converter is configured to formulate a converter voltage reference value.

[0095] The circulating current suppression controller is configured to formulate a circulating current suppression voltage value in normal operation and in converter station voltage reduction / half voltage operation, single valve group online switching and direct current fault ride-through operation.

[0096] The mode selection controller is configured to formulate a direct current voltage given value in normal operation and in converter station voltage reduction / half voltage operation, single valve group online switching and direct current fault ride-through operation.

[0097] The pulse control is generated for formulating control pulses in normal operation and in sub-voltage / half-voltage operation of the converter station, on-line switching of single valve group, and DC fault ride-through operation, and controls the input of the hybrid MMC.

[0098] The converter comprises:

[0099] The outer loop power controller and the inner loop power controller;

[0100] The outer loop power controller is configured to formulate a current reference value in normal operation by outer loop PI regulation according to reactive power and active power at a common connection point of the MMC converter valve, and formulate a current reference value in special operation by outer loop PI regulation according to capacitor voltage of a sub-module in the MMC converter valve and a reactive power reference value in sub-voltage / half-voltage operation of the converter station, on-line switching of single valve group, and DC fault ride-through operation.

[0101] The inner loop power controller is configured to formulate a converter voltage reference value in normal operation by inner loop PI regulation according to the current reference value in normal operation, and formulate a converter voltage reference value in special operation by inner loop PI regulation according to the current reference value in special operation.

[0102] The circulating current suppression controller sets a circulating current suppression voltage of the MMC converter valve to 0 in normal operation.

[0103] The circulating current suppression controller formulates a circulating current injection voltage value according to grid current in sub-voltage / half-voltage operation of the converter station, on-line switching of single valve group, and DC fault ride-through operation.

[0104] The circulating current suppression controller comprises:

[0105] The phase angle control module, the circulating current injection setting module, and the circulating current injection control module.

[0106] The phase angle control module performs park transformation and angle transformation according to grid current to obtain a phase angle value of circulating current injection.

[0107] The circulating current injection setting module formulates a circulating current injection expected value having d-axis and q-axis components by trigonometric function transformation according to the phase angle value and a circulating current injection amplitude.

[0108] The circulating current injection control module calculates a circulating current injection voltage value according to the bridge arm circulating current, the phase angle value, and the circulating current injection expected value.

[0109] The circulating current injection amplitude is a preset multiple of a DC component of the bridge arm current.

[0110] The mode selection controller sets a fixed-amplitude DC voltage given value in normal operation.

[0111] The mode selection controller subtracts the DC current measurement value from the given value and obtains a DC voltage given value through PI regulation under the commutation station voltage reduction / half voltage operation, single valve group on-line switching and DC fault ride-through working conditions.

[0112] Obviously, the described embodiments are part but not all of the embodiments of the present application. Had the person of ordinary skill in the art obtained all other embodiments based on the embodiments in the present application without creative work, they would have belonged to the scope of protection of the present application.

[0113] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROMs, optical storage devices, etc.) containing computer usable program code.

[0114] The embodiments of the present application are described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing apparatus to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing apparatus generate a means for implementing the functions specified in the flowcharts and / or block diagrams. Figure 1 one or more flows and / or blocks Figure 1 means for carrying out the functions specified in the flowcharts and / or block diagrams.

[0115] These computer program instructions can also be stored in a computer-readable memory that can direct the computer or other programmable data processing apparatus to work in a specific manner, so that the instructions stored in the computer-readable memory produce a manufactured product including instruction means, which implements the functions specified in the flowcharts and / or block diagrams. Figure 1 one or more flows and / or blocks Figure 1 means for carrying out the functions specified in the flowcharts and / or block diagrams.

[0116] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus, so that a series of operation steps are performed on the computer or other programmable data processing apparatus to produce a computer-implemented process, so that the instructions executed on the computer or other programmable data processing apparatus provide a means for implementing the functions specified in the flowcharts and / or block diagrams. Figure 1one or more processes and / or blocks Figure 1 the steps of a function specified in one or more blocks.

[0117] The above merely provides an embodiment of the present application, but is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall fall within the scope of claims of the present application.

Claims

1. A hybrid MMC control method, characterized by, The application relates to a control method for a hybrid MMC (Modular Multilevel Converter) in a converter station. In normal operation, control pulse signals are generated according to a converter voltage reference value formulated by reactive power and active power, a preset circulating current suppression voltage and a DC voltage given value, and the hybrid MMC is controlled to be put into operation. In voltage reduction / half voltage operation, single valve group on-line switching and DC fault ride-through operation, control pulses are generated according to a converter voltage reference value formulated by capacitor voltage of a sub-module in the MMC converter valve, a circulating current injection voltage value formulated by circulating current of a bridge arm of the MMC converter valve and a DC voltage given value formulated by a DC current given value, and the hybrid MMC is controlled to be put into operation. The circulating current injection voltage value formulated by the circulating current of the bridge arm of the MMC converter valve is formulated through the following steps: The grid current is subjected to park transformation and angle transformation to obtain a phase angle value of the circulating current injection; The phase angle value and a circulating current injection amplitude are subjected to trigonometric function transformation to formulate a circulating current injection expected value with d-axis and q-axis components; The circulating current injection voltage value is calculated according to the bridge arm circulating current and the circulating current injection expected value; The circulating current injection amplitude is a preset multiple of a DC component of the bridge arm current; The circulating current injection voltage value calculated according to the bridge arm circulating current and the circulating current injection expected value comprises the following steps: The bridge arm circulating current is subjected to dq transformation to obtain dq bridge arm circulating current and voltage pre-feeding amount; The dq bridge arm circulating current and the circulating current injection expected value are subjected to PI regulation to obtain an initial circulating current injection voltage value; The initial circulating current injection voltage value is subtracted from the voltage pre-feeding amount, and park inverse transformation is performed to obtain the circulating current injection voltage value.

2. The method of claim 1, wherein, The converter voltage reference value formulated by the capacitor voltage of the sub-module in the MMC converter valve is formulated through the following steps: The average measurement value of the capacitor voltage of the sub-module in the MMC converter valve is subtracted from a set value, and the d component of the current reference value is obtained through outer loop PI regulation; The d component of the current reference value is subjected to inner loop PI regulation to obtain the d component of the converter voltage reference value; The measurement value of the reactive power at the common connection point of the MMC converter valve is subtracted from a set value, and the q component of the current reference value is obtained through outer loop PI regulation; The q component of the current reference value is subjected to inner loop PI regulation to obtain the q component of the converter voltage reference value.

3. The method of claim 2, wherein, The d component and the q component of the current reference value are respectively as shown in the following formulae: wherein, i dref is the d-component of the current reference value, U c-ref is the average measured value of the MMC converter sub-module capacitor voltage, U c-avr is the set value of the average capacitor voltage of the MMC converter sub-module, k pd1 is the proportional control coefficient of the d-component inner loop PI regulation, k id1 is the integral control coefficient of the d-component inner loop PI control, i qref is the q-component of the current reference value, Q ref is the measured value of the reactive power, Q s is the set value of the reactive power, k pq1 is the proportional control coefficient of the q-component inner loop PI regulation, k iq1 is the integral control coefficient of the q-component inner loop PI control.

4. The method of claim 2, wherein, The d component and the q component of the converter voltage reference value are respectively as shown in the following formulae: wherein e sd is the d component of the MMC converter voltage reference value, u sd is the d component of the MMC converter valve side voltage, ω is the angular frequency, i dref is the d component of the current reference value, i d is the d component of the MMC converter valve side current, k pd2 is the d component outer loop PI control proportional control coefficient, k id2 is the d component outer loop PI control integral control coefficient, e sq is the q component of the converter voltage reference value, u sq is the q component of the MMC converter valve side voltage, i qref is the q component of the current reference value, i q is the q component of the MMC converter valve side current, k pq2 is the q component outer loop PI control proportional control coefficient, k iq2 is the q component outer loop PI control integral control coefficient; In the formulae, L = L t + 0.5Larm where L t is the transformer leakage reactance value, Larmis the bridge reactor value.

5. The method of claim 1, wherein, The DC voltage given value formulated by the DC current given value is formulated through the following steps: The DC current measurement value is subtracted from the given value, and the DC voltage given value is obtained through PI regulation.

6. The method of claim 1, wherein, In normal operation, control pulse signals are generated according to a converter voltage reference value formulated by reactive power and active power, a preset circulating current suppression voltage and a DC voltage given value, and the hybrid MMC is controlled to be put into operation, which comprises the following steps: The current reference value is obtained through inner loop PI regulation based on the reactive power and the active power at the common connection point of the MMC converter valve, and the converter voltage reference value is obtained through outer loop PI regulation based on the current reference value. Setting the circulating current suppression voltage of the MMC converter valve to 0; According to the voltage reference value, the circulating current suppression voltage and the DC voltage given value, a control pulse signal is formulated, and the mixed MMC is controlled to be put into operation.

7. A hybrid MMC control system characterized by, The system comprises: a converter, a circulating current suppression controller, a mode selection controller and a control pulse generator; the converter is configured to formulate a converter voltage reference value; the circulating current suppression controller is configured to formulate a circulating current suppression voltage value in normal operation and in sub-pressure / half-pressure operation of the converter station, in on-line switching of a single valve group and in DC fault ride-through operation; the mode selection controller is configured to formulate a DC voltage given value in normal operation and in sub-pressure / half-pressure operation of the converter station, in on-line switching of a single valve group and in DC fault ride-through operation; the pulse control generator is configured to formulate a control pulse in normal operation and in sub-pressure / half-pressure operation of the converter station, in on-line switching of a single valve group and in DC fault ride-through operation, and to control the mixed MMC to be put into operation; the circulating current suppression controller comprises: a phase angle control module, a circulating current injection given module and a circulating current injection control module; the phase angle control module is configured to perform park transformation and angle transformation according to grid current to obtain a phase angle value of circulating current injection; the circulating current injection given module is configured to formulate a circulating current injection expected value with d-axis and q-axis components by trigonometric function transformation according to the phase angle value and a circulating current injection amplitude; the circulating current injection control module is configured to calculate a circulating current injection voltage value according to the bridge arm circulating current and the circulating current injection expected value; the circulating current injection amplitude is a preset multiple of a DC component of the bridge arm current; the calculation of the circulating current injection voltage value according to the bridge arm circulating current and the circulating current injection expected value comprises: the bridge arm circulating current is subjected to dq transformation to obtain dq bridge arm circulating current and voltage pre-feeding amount; the dq bridge arm circulating current and the circulating current injection expected value are subjected to PI regulation to obtain an initial circulating current injection voltage value; the initial circulating current injection voltage value is subtracted from the voltage pre-feeding amount, and park inverse transformation is performed to obtain the circulating current injection voltage value.

8. The system of claim 7, wherein, The converter comprises: an outer loop power controller and an inner loop power controller; the outer loop power controller is configured to formulate a current reference value in normal operation by outer loop PI regulation according to reactive power and active power at the point of common coupling of the MMC converter valve, and to formulate a current reference value in special operation by outer loop PI regulation according to sub-module capacitor voltage and reactive power reference value in sub-pressure / half-pressure operation of the converter station, in on-line switching of a single valve group and in DC fault ride-through operation; the inner loop power controller is configured to formulate a converter voltage reference value in normal operation by inner loop PI regulation according to the current reference value in normal operation, and to formulate a converter voltage reference value in special operation by inner loop PI regulation according to the current reference value in special operation.

9. The system of claim 7, wherein, The circulating current suppression controller sets the circulating current suppression voltage of the MMC converter valve to 0 in normal operation. The circulating current suppression controller formulates a circulating current injection voltage value according to grid current under the conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through.

10. The system of claim 7, wherein, The mode selection controller sets a fixed amplitude DC voltage given value under normal conditions. The mode selection controller subtracts the DC current measurement value from the given value and obtains the DC voltage given value through PI regulation under the conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through.

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