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 voltage reduction / half voltage operation of a converter station, on-line switching of a single valve group and DC fault ride-through, a control pulse is formulated according to a converter voltage reference value formulated by capacitor voltage of a sub-module in an MMC converter valve, an arm current loop injection voltage value formulated by a time-varying value of an arm current loop 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 capacitor voltage of the sub-module in 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 separation of capacitor voltage of a full-bridge sub-module and the capacitor voltage of the sub-module is avoided under special working conditions. The time-varying value formulated by the arm current loop of the MMC converter valve obtains an expected value of the arm current loop injection, and the arm current always has a zero-crossing point through an arm current loop injection link, so that 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 / step-half voltage operation, online switching of single valve groups and direct current fault ride-through, and 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 / step-half voltage 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 an effective control method system for the hybrid MMC under the conditions of step-down / step-half voltage 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 in the MMC converter valve, the circulating current injection voltage value and the DC voltage given value formulated by the DC current given value, the control pulse is formulated, and the input of the hybrid MMC is controlled;

[0009] Wherein, the circulating current injection voltage value is determined by the grid current and the time-varying variable of the MMC converter valve bridge arm circulating current.

[0010] Preferably, the converter voltage reference value formulated by the capacitor voltage of the sub-module in the MMC converter valve is formulated by the following steps:

[0011] The average measured value of the capacitor voltage of the sub-module in 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;

[0012] 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;

[0013] 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;

[0014] 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.

[0015] Preferably, the formulation of the d component and the q component of the current reference value is as follows:

[0016]

[0017]

[0018] 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 in the MMC converter valve, U c-avr is the set value of the capacitor voltage of the sub-module in 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.

[0019] Preferably, the d component and the q component of the converter voltage reference value are determined as follows, respectively:

[0020]

[0021]

[0022] 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.

[0023] wherein,

[0024] L = L t + 0.5Larm

[0025] wherein L t is the transformer leakage reactance value, and Larm is the bridge arm reactor value.

[0026] Preferably, the determination of the circulating current injection voltage value comprises:

[0027] performing park transformation and angle transformation on the grid current to obtain a phase angle value of the circulating current injection;

[0028] obtaining a circulating current injection amplitude value according to the MMC converter valve bridge arm circulating current time variable;

[0029] determining a circulating current injection expected value having d-axis and q-axis components through trigonometric function transformation according to the phase angle value and the circulating current injection amplitude value;

[0030] calculating the circulating current injection voltage value according to the bridge arm circulating current time variable, the phase angle value, and the circulating current injection expected value;

[0031] Wherein, the sum of the circulating current variable of the MMC converter valve bridge arm and the circulating current injection amplitude is a constant value, and the constant value is a preset multiple of the DC component of the bridge arm current.

[0032] Preferably, the amplitude of the circulating injection is determined by the following formula:

[0033] A 环流注入 =A 定 -ampl_s1

[0034] Among them, A 环流注入 A represents the amplitude of the circulating injection. 定 The amplitude is a fixed value, and ampl_s1 is a variable when the MMC converter valve arm is circulating.

[0035] The amplitude setting value A 定 It is determined by the following formula:

[0036] A 定 =nI dc

[0037] Where n is a preset multiple, taking a value of 0.4-0.6, I dc This represents the DC component of the bridge arm current.

[0038] Preferably, the step of calculating the three-phase circulating current voltage reference value as the circulating current injection voltage value based on the bridge arm circulating current, phase angle value, and expected circulating current injection value includes:

[0039] The bridge arm circulating current is transformed by dq to obtain the dq bridge arm circulating current and the voltage feedforward.

[0040] The circulating current and the desired value of the circulating current injection in the dq bridge arm are adjusted by PI to obtain the initial circulating current injection voltage value.

[0041] The initial circulating current injection voltage value is subtracted from the voltage feedforward value, and then an inverse Park transformation is performed to obtain the circulating current injection voltage value.

[0042] Preferably, the DC voltage setpoint determined by the DC current setpoint is determined through the following steps:

[0043] The difference between the measured DC current value and the given value is calculated, and the DC voltage given value is obtained by adjusting the PI controller.

[0044] Preferably, under normal operating conditions, a control pulse signal is generated based on the converter voltage reference value determined by reactive power and active power, a preset circulating current suppression voltage, and a DC voltage setpoint, and the activation of the hybrid MMC is controlled, including:

[0045] The current reference value is obtained by inner loop PI regulation based on the reactive power and active power at the common connection point of the MMC converter valve, and the converter voltage reference value is obtained by outer loop PI regulation of the current reference value;

[0046] The circulating current suppression voltage of the MMC converter valve is set to 0;

[0047] The control pulse signal is formulated according to the voltage reference value, the circulating current suppression voltage and the given DC voltage value, and the mixed MMC is controlled to be put into operation.

[0048] A mixed MMC control system, the system comprising:

[0049] a converter, a circulating current suppression controller, a mode selection controller and a control pulse generator;

[0050] The converter is configured to formulate a converter voltage reference value;

[0051] The circulating current suppression controller is configured to formulate a circulating current suppression voltage value in normal working condition and a circulating current injection voltage value in converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through working condition;

[0052] The mode selection controller is configured to formulate a given DC voltage value in normal working condition and in converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through working condition;

[0053] The pulse control generator is configured to formulate a control pulse in normal working condition and in converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through working condition, and control the mixed MMC to be put into operation.

[0054] Preferably, the converter comprises:

[0055] an outer loop power controller and an inner loop power controller;

[0056] The outer loop power controller is configured to formulate a current reference value in normal working condition by outer loop PI regulation based on the reactive power and active power at the common connection point of the MMC converter valve, and formulate a current reference value in special working condition by outer loop PI regulation based on the sub-module capacitor voltage and reactive power reference value in MMC converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through working condition;

[0057] The inner loop power controller is configured to formulate a converter voltage reference value in normal working condition by inner loop PI regulation based on the current reference value in normal working condition, and formulate a converter voltage reference value in special working condition by inner loop PI regulation based on the current reference value in special working condition.

[0058] Preferably, the circulating current suppression controller sets the circulating current suppression voltage of the MMC converter valve to 0 under normal operating conditions.

[0059] The circulating current suppression controller determines the circulating current injection voltage value according to the grid current and the MMC converter valve bridge arm circulating current time-varying variable under the operating conditions of voltage reduction / half voltage operation of the converter station, online switching of a single valve group, and DC fault ride-through.

[0060] Preferably, the circulating current suppression controller comprises:

[0061] a phase angle control module, a circulating current injection setting module, and a circulating current injection control module.

[0062] 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.

[0063] The circulating current injection setting module determines a circulating current injection amplitude value according to the MMC converter valve bridge arm circulating current time-varying variable, and determines a circulating current injection expected value with d-axis and q-axis components through trigonometric function transformation according to the phase angle value and the circulating current injection amplitude value.

[0064] 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.

[0065] The sum of the MMC converter valve bridge arm circulating current time-varying variable and the circulating current injection amplitude value is a constant value, and the constant value is a preset multiple of a DC component of the bridge arm current.

[0066] Preferably, the circulating current injection setting module determines the circulating current injection amplitude value through the following formula:

[0067] A 环流注入 = A 定 -ampl_s1

[0068] wherein A 环流注入 is the amplitude value of the circulating current injection, A 定 is the amplitude constant value, and ampl_s1 is the MMC converter valve bridge arm circulating current time-varying variable.

[0069] The amplitude constant value A 定 is determined through the following formula:

[0070] A 定 = nI dc

[0071] wherein n is a preset multiple and takes a value of 0.4-0.6, and I dc is the DC component of the bridge arm current.

[0072] Preferably, the mode selection controller sets a fixed amplitude DC voltage given value under normal operating conditions;

[0073] The mode selection controller subtracts the DC current measurement value from the given value under the operating conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through, and obtains a DC voltage given value through PI regulation.

[0074] Compared with the prior art, the present application has the following beneficial effects:

[0075] 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 mixed 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 a sub-module capacitor voltage of the MMC converter valve, a circulating current injection voltage value and a DC voltage given value formulated by a DC current given value, and the mixed MMC is controlled to be put into operation; wherein, the circulating current injection voltage value is determined by a grid current and a time-varying variable of the MMC converter valve bridge arm circulating current. The method provided by the present application can ensure the zero-crossing point of the MMC bridge arm current under the operating conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through according to the converter voltage reference value formulated by the sub-module capacitor voltage of the MMC converter valve, realize capacitor voltage balance, and avoid system collapse caused by the separation of the capacitor voltage from the full-bridge sub-module capacitor voltage when the bridge arm current is not at the zero-crossing point. In the present application, the expected value of the circulating current injection is obtained from the time-varying variable of the MMC converter valve bridge arm circulating current, and the circulating current injection link is passed through, so that the bridge arm current always has a zero-crossing point, and the safe operation of the MMC bridge arm is ensured. BRIEF DESCRIPTION OF DRAWINGS

[0076] Figure 1 A mixed MMC control block diagram of the present application;

[0077] Figure 2 A conventional MMC overall control block diagram in the embodiment of the present application;

[0078] Figure 3 A circulating current injection control block diagram of the present application;

[0079] Figure 4 A fundamental frequency separation phase-locked angle control block diagram of the present application;

[0080] Figure 5 A circulating current injection given value block diagram of the present application. DETAILED DESCRIPTION

[0081] 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.

[0082] Embodiment 1:

[0083] The embodiment provides a hybrid MMC control method, comprising:

[0084] 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.

[0085] 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 the given value preset in advance, and then the d component of the current reference value is obtained through the 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 the PI regulation of the outer loop; the dq components of the current reference value are obtained through the PI regulation of the inner loop, and the converter voltage reference value is obtained.

[0086] The d component and the q component of the current reference value are respectively shown in the following formula:

[0087]

[0088]

[0089] , 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.

[0090] The formulation of the d component and the q component of the converter voltage reference value is as follows respectively:

[0091]

[0092]

[0093] wherein e sd u is the d component of the converter voltage reference value 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 d is the d component of the MMC converter valve side current, k pd2 k is the proportional control coefficient of the d component outer loop PI control id2 is the integral control coefficient of the d component outer loop PI control, e sq u is the q component of the converter voltage reference value sq is the q component of the MMC converter valve side voltage, i qref is the q component of the current reference value q is the q component of the MMC converter valve side current, k 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;

[0094] In the formula,

[0095] L = L t + 0.5Larm

[0096] wherein L t is the transformer leakage reactance value, and Larm is the bridge arm reactor value.

[0097] 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 in relation to the control mode. When the station is in the voltage control mode, U dcref is directly set to U dcref _ set ​; the current mode is given by a DC current controller. The mode selection logic enables the switching between the DC current controller and the voltage controller.

[0098] Under normal operating conditions, the outer loop control of the hybrid MMC active power adopts constant DC voltage or constant active power control, and the outer loop control of the reactive power adopts constant AC voltage or constant reactive power control, and the circulating current suppression is 0; under the operating conditions of converter station voltage reduction / half voltage operation, single valve group online switching and DC fault ride-through, the outer loop control of the active power adopts high valve group stator module capacitor voltage, the constant DC voltage is given by constant DC current control, the low valve group adopts constant DC voltage or constant active power, the outer loop control of the reactive power adopts constant AC voltage or constant reactive power control, the circulating current is switched to circulating current injection, and when the system power voltage returns to normal, the control mode switches back to the normal operating condition and the circulating current injection mechanism is exited.

[0099] The circulating current injection control block diagram is shown in Figure 3 The bridge arm circulating current i cirj (j=a, b, c) is obtained after abc / dq coordinate transformation to get the dq coordinate bridge arm current i 2fd and i 2fq , which are subtracted from the reference values of the circulating current dq axis components i 2fd_ref and i 2fq_ref , and then pass through proportional integral (PI) regulators, and then introduce the voltage feedforward quantities 2ω0L0i 2fq and 2ω0L0i 2fd to eliminate the dq axis coupling part, so as to obtain the dq axis reference values of the internal unbalanced voltage drop u cird_ref and u cirq_ref . Finally, the dq / abc coordinate inverse transformation is carried out to obtain the three-phase circulating current voltage reference values u cirj_ref , which are the circulating current injection voltage values. 2fd_ref and i 2fq_ref take the expected circulating current injection size, and the circulating current injection amplitude is calculated according to the MMC converter valve bridge arm circulating current time variable; the sum of the MMC converter valve bridge arm circulating current time variable and the circulating current injection amplitude is a constant value, and the constant value is a preset multiple of the DC component of the bridge arm current, which is about 0.4 to 0.6 times the DC current. In this embodiment, the amplitude and phase of the circulating current injection are determined according to the MMC converter valve bridge arm circulating current time variable. Since the sum of the amplitude of the circulating current injection and the amplitude of the MMC converter valve bridge arm circulating current time variable is a constant value, the amplitude of the circulating current injection is determined according to the MMC converter valve bridge arm circulating current time variable, which ensures that the bridge arm current can also pass through the zero point under the condition of MMC converter valve bridge arm circulating current variation, avoiding the continuous charging of the plate bridge sub-module.

[0100] The phase of the circulating current injection is in phase (Idc<0) or out of phase (Idc>0) with the real-time bridge arm reactive component.

[0101] The amplitude of the circulating injection is determined by the following formula:

[0102] A 环流注入 =A 定 -ampl_s1

[0103] Among them, A 环流注入 A represents the amplitude of the circulating injection. 定 The amplitude is a fixed value, and ampl_s1 is a variable when the MMC converter valve arm is circulating.

[0104] A 定 =nI dc

[0105] Where n is a preset multiple, taking a value of 0.4-0.6, I dc This represents the DC component of the bridge arm current.

[0106] The block diagram of the base frequency separation phase-locked angle control is as follows: 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 sq Then, after an arctan transformation, the angle theata1 is obtained. The angle theata1 is obtained by adding 90° or subtracting 90°, which is used for coordinate system transformation and phase of the circulation injection.

[0107] Circulation injection setpoint control block diagram, as shown Figure 5 As shown, the angle theata obtained by separating the fundamental frequency is used to obtain i through cosine and sinine operations respectively. 2fd_ref and i 2fq_ref .

[0108] Under normal operating conditions, the mode selection is based on the preset DC voltage setpoint. Under conditions such as reduced / half-voltage operation of the converter station, online activation / deactivation of a single valve group, and DC fault ride-through, the difference between the measured DC current value and the setpoint is calculated and adjusted using a PI controller to obtain the DC voltage setpoint.

[0109] Finally, under normal operating conditions, based on the converter voltage reference value determined by reactive power and active power, the preset circulating current suppression voltage and DC voltage setpoint, a control pulse signal is formulated to control the activation of the hybrid MMC.

[0110] The converter, the circulating current suppression controller, the mode selection controller and the control pulse generator are used to formulate the control pulse and control the input of the hybrid MMC 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 under the normal working condition, the converter station voltage reduction / half voltage operation, the online switching of the single valve group and the DC fault ride-through working condition.

[0111] Embodiment 2

[0112] The embodiment provides a hybrid MMC control method system, which comprises:

[0113] The converter, the circulating current suppression controller, the mode selection controller and the control pulse generator are used to formulate the control pulse and control the input of the hybrid MMC 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 under the normal working condition, the converter station voltage reduction / half voltage operation, the online switching of the single valve group and the DC fault ride-through working condition.

[0114] The converter is used to formulate the converter voltage reference value.

[0115] The circulating current suppression controller is used to formulate the circulating current suppression voltage value under the normal working condition and the converter station voltage reduction / half voltage operation, the online switching of the single valve group and the DC fault ride-through working condition.

[0116] The mode selection controller is used to formulate the DC voltage given value under the normal working condition and the converter station voltage reduction / half voltage operation, the online switching of the single valve group and the DC fault ride-through working condition.

[0117] The pulse control generator is used to formulate the control pulse under the normal working condition and the converter station voltage reduction / half voltage operation, the online switching of the single valve group and the DC fault ride-through working condition, and control the input of the hybrid MMC.

[0118] The converter comprises:

[0119] The outer loop power controller and the inner loop power controller.

[0120] The outer loop power controller is used to formulate the current reference value under the normal working condition by outer loop PI regulation according to the reactive power and the active power at the point of common coupling of the MMC converter valve under the normal working condition, and formulate the current reference value under the special working condition by outer loop PI regulation according to the capacitor voltage of the sub-module of the MMC converter valve and the reactive power reference value under the converter station voltage reduction / half voltage operation, the online switching of the single valve group and the DC fault ride-through working condition.

[0121] The inner loop power controller is used to formulate the converter voltage reference value under the normal working condition by inner loop PI regulation according to the current reference value under the normal working condition, and formulate the converter voltage reference value under the special working condition by inner loop PI regulation according to the current reference value under the special working condition.

[0122] The circulating current suppression controller sets the circulating current suppression voltage of the MMC converter valve to 0 under the normal working condition.

[0123] The circulating current suppression controller formulates the circulating current injection voltage value according to the grid current under the conditions of the voltage reduction / half voltage operation of the converter station, the on-line switching of the single valve group, and the DC fault ride-through.

[0124] The circulating current suppression controller comprises:

[0125] The phase angle control module, the circulating current injection given module, and the circulating current injection control module;

[0126] 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;

[0127] The circulating current injection given module formulates a circulating current injection expected value with d-axis and q-axis components through trigonometric function transformation according to the phase angle value and a circulating current injection amplitude;

[0128] 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;

[0129] The circulating current injection amplitude is calculated according to the MMC converter valve bridge arm circulating current time-varying variable;

[0130] The sum of the MMC converter valve bridge arm circulating current time-varying variable and the circulating current injection amplitude is a constant value, and the constant value is a preset multiple of the DC component of the bridge arm current.

[0131] The mode selection controller sets a fixed-amplitude DC voltage given value under normal conditions;

[0132] 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 conditions of the voltage reduction / half voltage operation of the converter station, the on-line switching of the single valve group, and the DC fault ride-through.

[0133] Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0134] 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 adopt a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer usable storage media containing computer usable program codes (including but not limited to disk storage, CD-ROM, optical storage, etc.).

[0135] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flowcharts and / or blocks Figure 1 means for functionally implementing the steps in one or more flowcharts and / or blocks

[0136] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks. Figure 1 one or more flowcharts and / or blocks Figure 1 means for functionally implementing the steps in one or more flowcharts and / or blocks

[0137] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flowcharts and / or blocks Figure 1 means for functionally implementing the steps in one or more flowcharts and / or blocks

[0138] The above merely provides an embodiment of the present application, but is not intended to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall fall within the scope of the claims of the present application.

Claims

1. A hybrid MMC control method, characterized by, include: Under normal operating conditions, based on the converter voltage reference value determined by reactive power and active power, the preset circulating current suppression voltage and DC voltage setpoint, a control pulse signal is generated to control the activation of the hybrid MMC. When the converter station is operating at reduced / half voltage, single valve group is online in operation or out of operation, and DC fault ride-through conditions, control pulses are generated based on the converter voltage reference value determined by the capacitor voltage of the submodule in the MMC converter valve, the circulating current injection voltage value, and the DC voltage setpoint determined by the DC current setpoint value, and the activation of the hybrid MMC is controlled. The circulating current injection voltage value is determined by the grid current and the variables during circulating current in the MMC converter valve bridge arm; The determination of the circulating current injection voltage value includes: The grid current is subjected to Park transformation and angle transformation to obtain the phase angle value of the circulating current injection; The amplitude of the circulating injection is obtained based on the circulating flow variables of the MMC converter valve bridge arm. Based on the phase angle value and the circulation injection amplitude, a desired circulation injection value with d-axis and q-axis components is determined through trigonometric function transformation. The circulating current injection voltage value is calculated based on the bridge arm circulating current and the expected value of circulating current injection. Wherein, the sum of the circulating current variable of the MMC converter valve bridge arm and the circulating current injection amplitude is a constant value, and the constant value is a preset multiple of the DC component of the bridge arm current; The amplitude of the circulating injection is determined by the following formula: A 环流注入 = A 定 -ampl_s1 Wherein, A 环流注入 is the amplitude of the injected current, A 定 is the amplitude value, and ampl_s1 is a time-varying variable of the MMC converter valve bridge arm circulating current; said amplitude value A 定 is determined by the equation: A 定 = n dc Wherein, n is a preset multiple, and the value is 0.4-0.6, I dc is the DC component of the bridge arm current; Based on the bridge arm circulating current and the expected value of circulating current injection, the reference value of the three-phase circulating current voltage is calculated as the circulating current injection voltage value, including: The dq bridge arm circulating current is obtained by dq transformation, which yields the dq bridge arm circulating current and the voltage feedforward. The circulating current and the desired value of the circulating current injection in the dq bridge arm are adjusted by PI to obtain the initial circulating current injection voltage value; The initial circulating current injection voltage value is subtracted from the voltage feedforward value, and then Park transform is performed to obtain the circulating current injection voltage value.

2. The method of claim 1, wherein, The converter voltage reference value, determined by the capacitor voltage of the submodule in the MMC converter valve, is established through the following steps: The average measured value of the capacitor voltage of the submodule in the MMC converter valve is subtracted from the set value, and the d component of the current reference value is obtained by adjusting the outer loop PI. The d-component of the current reference value is obtained by adjusting the inner loop PI. The difference between the measured value and the set value of the reactive power at the common connection point of the MMC converter valve is obtained by adjusting the outer loop PI to obtain the q component of the current reference value. The q-component of the current reference value is obtained by adjusting the inner loop PI to obtain the q-component of the converter voltage reference value.

3. The method of claim 2, wherein, The d-component and q-component of the current reference value are defined as follows: 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 MMC converter valve sub-module, U c-avr is the set value of the capacitor voltage of the MMC converter valve 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 q-component of the converter voltage reference value are defined as follows: wherein e sd is the d component of the MMC converter voltage reference, 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, i d is the d component of the MMC converter valve side current, k pd2 is the proportional control coefficient of the d component outer loop PI control, k id2 is the integral control coefficient of the d component outer loop PI control, e sq is the q component of the converter voltage reference, u sq is the q component of the MMC converter valve side voltage, i qref is the q component of the current reference, i q is the q component of the MMC converter valve side current, k pq2 is the proportional control coefficient of the q component outer loop PI control, k iq2 is the integral control coefficient of the q component outer loop PI control; In the formula, L = L t + 0.5Larm where L t is the transformer leakage reactance value, Larmis the bridge arm reactor value.

5. The method of claim 1, wherein, The DC voltage setpoint, determined by the DC current setpoint, is determined through the following steps: The difference between the measured DC current value and the setpoint is calculated, and the DC voltage setpoint is obtained through PI regulation.

6. The method of claim 1, wherein, Under normal operating conditions, based on the converter voltage reference value determined by reactive power and active power, the preset circulating current suppression voltage, and the DC voltage setpoint, a control pulse signal is generated to control the activation of the hybrid MMC, including: The current reference value is obtained by inner ring PI regulation based on the reactive power and active power at the common connection point of the MMC converter valve, and the converter voltage reference value is obtained by outer ring PI regulation based on the current reference value; The circulating current suppression voltage of the MMC converter valve is set to 0; According to the voltage reference value, the circulating current suppression voltage and the given value of the DC voltage, 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 a normal working condition and a circulating current injection voltage value in a converter station voltage reduction / half voltage operation, a single valve group online switching and a DC fault ride-through working condition; the mode selection controller is configured to formulate a given value of the DC voltage in the normal working condition and the converter station voltage reduction / half voltage operation, the single valve group online switching and the DC fault ride-through working condition; the pulse control generator is configured to formulate a control pulse in the normal working condition and the converter station voltage reduction / half voltage operation, the single valve group online switching and the DC fault ride-through working condition, and control the mixed MMC to be put into operation.

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

9. The system of claim 7, wherein, The circulating current suppression controller is configured to set the circulating current suppression voltage of the MMC converter valve to 0 in the normal working condition; The circulating current suppression controller is configured to formulate a circulating current injection voltage value based on the grid current and the MMC converter valve bridge arm circulating current time-varying variable in the converter station voltage reduction / half voltage operation, the single valve group online switching and the DC fault ride-through working condition.

10. The system of claim 9, wherein, 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 based on the grid current to obtain a phase angle value of the circulating current injection; the circulating current injection given module is configured to determine a circulating current injection amplitude based on the MMC converter valve bridge arm circulating current time-varying variable, and formulate a circulating current injection expected value having d-axis and q-axis components by trigonometric function transformation based on the phase angle value and the circulating current injection amplitude; the circulating current injection control module is configured to calculate a circulating current injection voltage value based on the bridge arm circulating current, the phase angle value and the circulating current injection expected value. The sum of the time-varying variable of the MMC converter valve bridge arm circulating current and the circulating current injection amplitude is a constant value, and the constant value is a preset multiple of the DC component of the bridge arm current.

11. The system of claim 10, wherein, The circulating current injection given module determines the circulating current injection amplitude by the following formula: A 环流注入 = A 定 -ampl_s1 Wherein, A 环流注入 is the amplitude of the injected current, A 定 is the amplitude value, and ampl_s1 is a time-varying variable of the MMC converter valve bridge arm circulating current; said amplitude value A 定 is determined by the formula: A 定 = nI dc Wherein, n is a preset multiple, and the value is 0.4-0.6, I dc is the DC component of the bridge arm current.

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

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