Dual-port networking control method for modular multilevel converter

Abstract

The invention discloses a dual-port networking control method for a modular multilevel converter, and belongs to the technical field of converter control. Comprising the following steps: acquiring an MMC alternating current output voltage additional quantity delta uac, d / q, and further calculating to obtain an MMC alternating current side output voltage reference value uac, ref, d / q; the MMC direct current output voltage additional quantity delta udc is obtained, and then MMC direct current side output voltage reference values udc, ref are obtained through calculation; uac, ref, d / q and udc, ref are respectively sent to AC side and DC side voltage and current double closed-loop control links, and MMC modulation signal differential moduli udiff, d / q and common moduli usum, j are respectively obtained at the output ends of the two links; and according to the udiff, d / q and usum, j, an output voltage expected value of the MMC bridge arm capacitor module is obtained by using a bridge arm output voltage reference value calculation method, and then trigger pulse signals of all switching devices in the MMC are generated through a modulation part. The method is simple in operation, low in requirement for computing resources, high in implementation of a control algorithm and high in engineering applicability.

Description

Technical Field

[0001] The invention relates to the technical field of converter control, and in particular to a modular multi-level converter dual-port networking control method. Background Art

[0002] With the continuous growth of my country's renewable energy installed capacity, the demand for the application of flexible DC transmission technology in large-capacity power transmission is increasing. Among them, modular multilevel converter (MMC) has become the mainstream choice due to its flexible structure and superior performance. In a flexible DC transmission system, the AC / DC system usually presents a segmented structure, that is, multiple AC subnets are interconnected through DC links. Therefore, it is necessary to use MMC converter control methods to ensure the safe and stable operation of the system.

[0003] The existing control methods of MMC can be roughly divided into active-reactive (PQ) control, DC voltage-reactive (Udc-Q) control and voltage-frequency (Vf) control. However, no matter which control method is adopted, the networking capabilities of the AC side and the DC side are incompatible, that is, when PQ control is adopted on the AC side, the MMC DC voltage needs to be provided by the external DC network; when Udc-Q control is adopted, the AC side of the converter essentially operates in the grid-following mode, and the AC network needs to provide a stable common coupling point voltage; when Vf control is adopted, the converter can provide voltage support on the AC side, but the DC side does not have the ability to operate in the grid.

[0004] The current single-port grid-forming control method of the converter has put forward a prerequisite for the operation properties of the deployed power grid, that is, the non-grid-forming port of the converter must be connected to an equivalent voltage source with voltage support capability. In fact, this requires the control modes between single-end grid-forming converters to cooperate with each other. For example, in a flexible DC transmission system, if the AC side of the sending end operates under Vf control, the receiving-end converter must be responsible for stabilizing the DC bus voltage so that it presents voltage source characteristics to the sending-end system. When the network operation properties need to change, such as from a voltage source to a current source, the MMC converter must change the operation control mode and parameters through the upper-level dispatch to ensure the stability of the system operation, which causes the current converter control to be inflexible and poor adaptability to the operation of AC and DC grids. At present, a very small number of MMC dual-port grid-forming operation control methods have been studied, but they do not have the ability to align the dq rotating coordinate system, and cannot achieve the decoupling control of active and reactive power without a phase-locked loop; they do not have the ability to control the energy of the MMC bridge arm capacitor, and cannot ensure the stable operation of the system under the switching of three operating conditions. Therefore, a modular multi-level converter dual-port networking control method is needed to solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a modular multi-level converter dual-port networking control method, comprising the following steps:

[0006] The MMC bridge arm capacitor AC side energy control method and reactive power control method are used to obtain the MMC AC output voltage additional amount Δu ac,d / q , and then calculate the MMC AC side output voltage reference value u ac,ref,d / q ;

[0007] Using the MMC bridge arm capacitor DC side energy control method to obtain the MMC DC output voltage addition Δu dc , and then calculate the MMC DC side output voltage reference value u dc,ref ;

[0008] will u ac,ref,d / q and u dc,ref The voltage and current double closed-loop control links on the AC side and the DC side are sent to the two links respectively, and the MMC modulation signal differential modulus u is obtained at the output ends of the two links. diff,d / q and common mode u sum,j ;

[0009] According to u diff,d / q and u sum,j , the expected value of the output voltage of the MMC bridge arm capacitor module is obtained by using the bridge arm output voltage reference value calculation method, and then the trigger pulse signal of all switching devices in the MMC is generated through the modulation part.

[0010] Furthermore, the MMC AC side output voltage reference value u ac,ref,d / q The calculation formula is:

[0011] u ac,ref,d / q =u ac,base,d / q +Δu ac,d / q

[0012] Among them, u ac,base,d / q It is the MMC AC voltage reference value output by the AC side synchronous control link.

[0013] Furthermore, the MMC DC side output voltage reference value u dc,ref The calculation formula is:

[0014] u dc,ref =u dc,base +Δu dc

[0015] Among them, u dc,base It is the MMC DC voltage reference value output by the DC side synchronous control link.

[0016] Furthermore, the energy control method of the AC side of the MMC bridge arm capacitor is expressed as:

[0017] Δu ac,d =G ac,w (s)(W m -Wref )

[0018]

[0019] Among them, Δu ac,d is the d-axis MMC AC output voltage addition, G ac,w (s) represents the PI controller transfer function in the AC side energy control method, W m is the bridge arm capacitance energy measurement, W ref is the reference value of bridge arm capacitance energy, u p,j and u n,j are the total voltage values ​​of the upper and lower bridge arm capacitors of phase j respectively; C eq It is the total equivalent capacitance value of N modules on the bridge arm.

[0020] Furthermore, when the bridge arm capacitance energy measurement value W m and the bridge arm capacitance energy reference value W ref When there is an error, the MMC converter will automatically adjust the active power output on the AC side to achieve stability of the bridge arm capacitor energy.

[0021] Furthermore, the calculation method of the bridge arm output voltage reference value is expressed as:

[0022]

[0023] Among them, u diff,j Indicates u diff,d / q The differential modulus of the MMC modulation signal in the three-phase coordinate system obtained after the inverse park transformation, u j,p and u j,n They represent the output voltage reference values ​​of the upper and lower bridge arms in phase j respectively.

[0024] The beneficial effects of the present invention are:

[0025] 1. The MMC dual-terminal networking control method proposed in the present invention can realize the voltage support capability of both the AC side and the DC side, avoiding the need for the voltage of the opposite port to be provided by an external power supply in the traditional single-port control, and also avoiding the problem of the need for pre-allocation of the converter operation mode. Even if the subnet on either side loses the external power supply support, the proposed MMC control can ensure the stable operation of the system, significantly improving the interconnection flexibility of the AC and DC subnets; at the same time, it can also realize the decoupling control of active and reactive power without a phase-locked loop.

[0026] 2. The MMC two-terminal networking control method proposed in the present invention has the ability to independently control the MMC capacitor energy, that is, it can directly control the voltage amplitude of the bridge arm capacitor module, significantly improving the safety of MMC operation to avoid exceeding the device withstand voltage limit. At the same time, the expanded control freedom also significantly improves the MMC control flexibility.

[0027] 3. The MMC dual-end networking control method proposed in the present invention completely relies on the electrical measurement quantity required by the traditional single-end control, without the need to add any additional measuring elements and devices, and has a high cost advantage.

[0028] 4. The MMC dual-end networking control method proposed in the present invention is simple and efficient. Compared with the traditional single-end control, it only adds a few PI controllers. It has simple calculation and low requirements on computing resources. The control algorithm is highly feasible and has strong engineering applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flow chart of the dual-port networking control method of the modular multi-level converter of the present invention;

[0030] Figure 2 It is the topology diagram of MMC converter;

[0031] Figure 3 This is the double-end network control block diagram of the MMC converter;

[0032] Figure 4 It is a schematic diagram of the phase alignment method of the dq rotating coordinate system;

[0033] Figure 5 It is a control block diagram of the MMC AC side energy control method and reactive power control method;

[0034] Figure 6 is a voltage vector diagram;

[0035] Figure 7 It is the control block diagram of the MMC DC side energy control method;

[0036] Figure 8 Three operating conditions for MMC double-ended grid converter;

[0037] Fig. 9 The waveform is when the AC side is connected to a voltage source and the DC side switches from being connected to a current source to being connected to a voltage source at 5s;

[0038] Fig.10 The DC side is connected to a voltage source, and the AC side switches from connecting to a current source to connecting to a voltage source at 5s. DETAILED DESCRIPTION

[0039] The present invention provides a modular multi-level converter dual-port networking control method, which is further described below in conjunction with the accompanying drawings and specific embodiments.

[0040] Figure 1 Flow chart of the dual-port networking control method of the modular multi-level converter of the present invention; Figure 2 As shown, the MMC converter topology is given. Figure 3The control block diagram of the double-terminal network of the MMC converter is given. g,d / q and i g,d / q is the voltage measurement value u at the MMC common coupling point (PCC) g,abc and the current measurement value i g,abc The dq axis voltage and current components in the dq rotating coordinate system; θ is the phase quantity during the transformation between the abc coordinate system and the dq rotating coordinate system; u dc is the measured value of the DC port voltage of the converter; ω0 is the reference angular frequency output by the synchronous control link on the AC side; i sum,j is the DC current of the MMC bridge arm, and the calculation method is:

[0041]

[0042] In the formula, and They are the current measurement quantities of the upper and lower bridge arms of one phase of MMC respectively.

[0043] Traditional AC side grid control methods include droop control, virtual synchronous control, etc. These AC side synchronous control links output the converter PCC point voltage reference value u ac,base The component u on the dq axis ac,base,d and u ac,base,q ,like Figure 4 As shown, where V is u ac,base Three-phase synthetic vector. In traditional VF control, the AC side voltage and current double closed-loop control link tracks u ac,base,d and u ac,base,q component, thereby achieving the MMC output voltage u g,d / q However, since there is no phase-locked loop, V cannot be guaranteed to coincide with the d-axis, i.e., u g,q ≠0. In the dq rotating coordinate system phase alignment method, the reference rotating phase θ is generated by ω0. base Based on this, the PI controller can output an additional phase Δθ, which can make θ=θ base +Δθ corresponds to the new rotation coordinate system d'q' that satisfies u g,q = 0. Then, according to the output power calculation formula in the rotating coordinate system:

[0044]

[0045] It can be seen that at this time, the AC side current control link can independently control the MMC output active power and reactive power by tracking the d-axis and q-axis current reference values.

[0046] The control method of the present invention is specifically implemented as follows:

[0047] Step 1: Use the MMC bridge arm capacitor AC side energy control method and reactive power control method to obtain the MMC AC output voltage addition Δu ac,d / q , and then calculate the MMC AC side output voltage reference value u ac,ref,d / q .

[0048] Figure 5 It is the control block diagram of the MMC AC side energy control method and reactive power control method. m and W ref They are the total energy measurement value and reference value of the MMC bridge arm capacitor; Q m and Q ref They are respectively the measured value and reference value of reactive power on the AC side of MMC; PI stands for proportional integral controller.

[0049] On this basis, the AC side energy control method can be expressed as:

[0050] Δu ac,d =G ac,w (s)(W m -W ref )

[0051] Among them, G ac,w (s) represents the PI controller transfer function in the AC side energy control method. m The calculation method is:

[0052]

[0053] Among them, u p,j and u n,j are the total voltage values ​​of the upper and lower bridge arm capacitors of one phase respectively; C eq It is the total equivalent capacitance value of N modules on the bridge arm.

[0054] MMC AC side output voltage reference value u ac,ref,d / q The calculation formula is:

[0055] u ac,ref,d / q =u ac,base,d / q +Δu ac,d / q

[0056] Among them, u ac,base,d / q It is the MMC AC voltage reference value output by the AC side synchronous control link.

[0057] This method will generate an additional voltage component Δu on the d-axis. ac,d To correct the reference value u input to the AC side voltage and current double closed loop control link ac,ref,d ,like Figure 6 As shown (Note: When the dq phase of the coordinate system in the figure is aligned, u g,q =0), when the bridge arm capacitance energy measurement value Wm With reference value W ref When there is an error, the MMC converter will automatically adjust the active power output on the AC side to achieve stability of the bridge arm capacitor energy.

[0058] Step 2: Use the MMC bridge arm capacitor DC side energy control method to obtain the MMC DC output voltage addition Δu dc , and then calculate the MMC DC side output voltage reference value u dc,ref .

[0059] Figure 7 The control block diagram of the MMC DC side energy control method is shown in Figure 2. The DC side energy control part is similar to the AC side energy control principle. According to the bridge arm capacitor energy measurement value W m With reference value W ref The error between the two generates an additional voltage Δu dc , and then combined with the DC side synchronous control link to output the DC voltage reference value u dc,base Calculate the DC side output voltage reference value u dc,ref =u dc,base +Δu dc .

[0060] Step 3: ac,ref,d / q and u dc,ref The voltage and current double closed-loop control links on the AC side and the DC side are sent to the two links respectively, and the MMC modulation signal differential modulus u is obtained at the output ends of the two links. diff,d / q and common mode u sum,j ,like Figure 3 shown.

[0061] In the AC side voltage and current double closed-loop control link, the MMC modulation signal differential modulus u is output after decoupling. diff,d / q .

[0062] In the DC side voltage and current double closed-loop control link, firstly according to the reference value u dc,ref With the measured value u dc The error between the two generates a DC current increment Δi through the PI link dc , and then divided by 3 to get the current increment of each bridge arm, and compared with the bridge arm current i sum,j Generate the bridge arm current reference value, and finally generate the common mode u of the three bridge arms of MMC according to the current control link sum,j .

[0063] Step 4: According to u diff,d / q and u sum,j, the expected value of the output voltage of the MMC bridge arm capacitor module is obtained by using the bridge arm output voltage reference value calculation method, and then the trigger pulse signal of all switch devices in the MMC is generated through the modulation part. Among them, the bridge arm output voltage reference value calculation method can be expressed as:

[0064]

[0065] where u diff,j Indicates u diff,d / q The differential modulus of the MMC modulation signal in the three-phase coordinate system obtained after the inverse park transformation. j,p and u j,n Respectively represent the output voltage reference values ​​of the upper and lower bridge arms in one phase.

[0066] From the above principle description, it can be seen that both the AC side and the DC side have synchronous control links, which can generate AC and DC side voltage reference values, which ensures that even the MMC AC and DC ports have voltage construction capabilities at the same time. And on this basis, the AC and DC side energy control parts will add additional adjustment amounts to the reference values, and dynamically adjust the output active power of the MMC AC and DC ports to achieve the energy stability of the capacitor module at the set energy reference value, ensuring the stability of the MMC operation. Obviously, the proposed two-terminal network control method can enable the MMC converter to operate stably in three situations: one side is connected to a voltage source, one side is connected to a current source, and both sides are connected to a voltage source at the same time, without changing any control parameters and operating modes. Figure 8 shown.

[0067] like Fig. 9 , Fig.10 As shown, the simulation verification results of the MMC dual-terminal networking control method are given. Fig. 9 The AC side of the MMC is connected to a voltage source, and after 5 seconds the DC side switches from being connected to a current source to being connected to a voltage source. Fig.10 The DC side of the MMC is connected to a voltage source, and the AC side switches from connecting to a current source to connecting to a voltage source after 5s. The control topology and parameter settings of the MMC converter remain completely consistent in the two experiments, and only the external power supply is adjusted. From the simulation results, it can be seen that the control method proposed in the present invention is Figure 8 The converter can operate stably under the three working conditions shown. MMC always keeps the AC side voltage and DC side voltage stable, and the energy of MMC capacitor module is always stable at 1p.u., and the AC and DC power change synchronously. Obviously, the two experiments show that the method proposed in the present invention can realize a set of control methods and enable MMC to have the ability to operate on the AC side and DC side, verifying the correctness and effectiveness of the theoretical analysis.

[0068] The MMC two-terminal networking control method proposed in the present invention has the ability to independently control the MMC capacitor energy, that is, it can directly control the voltage amplitude of the bridge arm capacitor module, significantly improve the MMC operation safety to avoid exceeding the device withstand voltage limit, and at the same time, the expanded control freedom also significantly improves the MMC control flexibility.

Claims

1. A modular multi-level converter dual-port networking control method, characterized in that: The following steps are involved: The MMC bridge arm capacitor AC side energy control method and reactive power control method are used to obtain the MMC AC output voltage additional amount Δu ac,d / q , and then calculate the MMC AC side output voltage reference value u ac,ref,d / q ; Using the MMC bridge arm capacitor DC side energy control method to obtain the MMC DC output voltage addition Δu dc , and then calculate the MMC DC side output voltage reference value u dc,ref ; will u ac,ref,d / q and u dc,ref The MMC modulation signal differential modulus u is obtained at the output ends of the two links respectively. diff,d / q and common mode u sum,j ; According to u diff,d / q and u sum,j , the expected value of the output voltage of the MMC bridge arm capacitor module is obtained by using the bridge arm output voltage reference value calculation method, and then the trigger pulse signal of all switching devices in the MMC is generated through the modulation part.

2. The modular multi-level converter dual-port networking control method according to claim 1, characterized in that: MMC AC side output voltage reference value u ac,ref,d / q The calculation formula is: u ac,ref,d / q =u ac,base,d / q +Δu ac,d / q Among them, u ac,base,d / q It is the MMC AC voltage reference value output by the AC side synchronous control link.

3. The modular multi-level converter dual-port networking control method according to claim 1, characterized in that: MMC DC side output voltage reference value u dc,ref The calculation formula is: u dc,ref =u dc,base +Δu dc Among them, u dc,base It is the MMC DC voltage reference value output by the DC side synchronous control link.

4. The modular multi-level converter dual-port networking control method according to claim 1, characterized in that: The energy control method of the AC side of the MMC bridge arm capacitor is expressed as: Δu ac,d =G ac,w (s)(In m -IN ref ) Among them, Δu ac,d is the d-axis MMC AC output voltage addition, G ac,w (s) represents the PI controller transfer function in the AC side energy control method, W m is the bridge arm capacitance energy measurement, W ref is the reference value of bridge arm capacitance energy, u p,j and u n,j are the total voltage values ​​of the upper and lower bridge arm capacitors of phase j respectively; C eq It is the total equivalent capacitance value of N modules on the bridge arm.

5. The modular multi-level converter dual-port networking control method according to claim 4, characterized in that: When the bridge arm capacitance energy measurement value W m and the bridge arm capacitance energy reference value W ref When there is an error, the MMC converter will automatically adjust the active power output on the AC side to achieve stability of the bridge arm capacitor energy.

6. The modular multi-level converter dual-port networking control method according to claim 1, characterized in that: The calculation method of the bridge arm output voltage reference value is expressed as: Among them, u diff,j Indicates u diff,d / q The differential modulus of the MMC modulation signal in the three-phase coordinate system obtained after the inverse park transformation, u j,p and u j,n They represent the output voltage reference values ​​of the upper and lower bridge arms in phase j respectively.

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