Voltage balancing control method for modular multilevel converter during pole-to-ground fault ride-through

Abstract

The present invention discloses a method for voltage balancing control during pole-to-ground fault crossing of a modular multi-level converter. First, the electrical quantity on the AC and DC sides of the MMC is detected in real time to determine whether a DC pole-to-ground fault occurs; when a DC pole-to-ground fault is detected, the MMC immediately switches to the pole-to-ground fault crossing control strategy and superimposes the baseband feedforward compensation term FD on the circulating current suppressor control instruction; according to the circulating current suppressor control instruction after superimposing the baseband feedforward compensation term, the capacitor voltage is balanced. The present invention does not require additional complex additional control, and the capacitor voltage of the upper and lower bridge arm submodules can be balanced only by superimposing the baseband circulating current feedforward term on the existing circulating current suppressor control instruction; the present invention can not only effectively maintain the capacitor voltage balance of the upper and lower bridge arm submodules of the MMC during the pole-to-ground fault crossing, but also minimize the circulating current, thereby reducing the amplitude of the bridge arm current and reducing the operating loss of the MMC.

Description

Technical Field

[0001] The present invention belongs to the field of flexible direct current transmission system control and protection, and more specifically, relates to a submodule capacitor voltage balancing control method of a modular multilevel converter (MMC) during pole-to-ground fault riding. Background Art

[0002] In recent years, flexible DC transmission technology based on modular multilevel converters (MMCs) has developed rapidly. However, when DC transmission lines use overhead transmission lines, pole-to-ground faults occur frequently. This fault will cause huge voltage stress on the healthy poles and neutral points on the AC side of the MMC, seriously endangering system operation.

[0003] To solve the above problems, the academic community has proposed a pole-to-ground fault ride-through control method. By adjusting the reference voltage of the fault pole bridge arm, the voltage stress on the AC and DC sides can be effectively eliminated; by making full use of the sound pole bridge arm, the MMC can continue to operate and transmit half of the rated active power during the pole-to-ground fault. This control strategy has been adopted by the Wudongde DC project of the Southern Power Grid Company. However, under the above fault ride-through control method, there will be a problem of unbalanced submodule capacitor voltage between the upper and lower bridge arms of the MMC. The reason is that the active power on the AC side is usually evenly divided between the upper and lower bridge arms of the MMC, and during the pole-to-ground fault ride-through, only the sound DC bus can transmit active power, and the faulty DC bus cannot transmit active power. According to relevant literature, the imbalance is closely related to the bridge arm resistance, inductance and operating conditions of the MMC. If the unbalanced problem of the submodule capacitor voltage is not handled, the operating performance of the MMC will deteriorate, leading to serious consequences.

[0004] There are two main existing solutions to the problem of unbalanced submodule capacitor voltage during pole-to-ground fault ride-through:

[0005] (1) During the pole-to-ground fault ride-through, a closed-loop feedback control of the difference in the capacitor voltage of the upper and lower bridge arm submodules is constructed. Among them, the reference command is zero, and the controller output is used as the phase shift angle of the AC component in the upper and lower bridge arm reference voltage. This control method can effectively eliminate the imbalance phenomenon of MMC during the pole-to-ground fault ride-through. However, this method will cause a large fundamental frequency circulating current, which will increase the fluctuation of the submodule capacitor voltage and threaten the safety during the period; on the other hand, it will increase the amplitude of the bridge arm current and increase the operating loss of the converter.

[0006] (2) Based on the indirect modulation method, a dual closed-loop feedback control is constructed. Among them, the difference control of the upper and lower bridge arm submodule capacitor energy is used as the outer closed-loop part of the overall strategy, and the base frequency circulating current control is used as the inner closed-loop part of the overall strategy. The outer loop reference command is zero, and the outer loop output is used as the command for the inner loop base frequency circulating current amplitude. This method can not only effectively eliminate the imbalance phenomenon of MMC during the pole-to-ground fault crossing, but also does not have the problem of increased submodule capacitor voltage fluctuation and increased operating loss. However, this method is based on the indirect modulation method and is not commonly used in actual engineering.

[0007] In summary, the above two solutions cannot effectively solve the capacitor voltage imbalance problem between MMCs during pole-to-ground fault crossing. Summary of the invention

[0008] In view of the problems existing in the prior art, this application mainly provides a voltage balancing control method based on base frequency feedforward compensation for MMCs with fault ride-through capability. On the basis of the existing pole-to-ground ride-through control strategy, a base frequency feedforward compensation term is additionally added to the instruction of the circulating current suppressor to achieve the balance of the submodule capacitor voltage during the pole-to-ground fault ride-through. The base frequency is the same frequency as the AC power grid frequency.

[0009] A method for controlling voltage sharing during pole-to-ground fault ride-through of a modular multilevel converter comprises the following steps:

[0010] Step (1), real-time detection of AC and DC side electrical quantities to determine whether a DC pole-to-ground fault occurs. If yes, then sequentially execute step (2), otherwise continue detection;

[0011] Step (2), the modular multilevel converter MMC switches to the pole-to-ground fault ride-through control strategy, and simultaneously executes step (3);

[0012] Step (3), superimposing the baseband feedforward compensation term FD on the circulating current suppressor control instruction.

[0013] Step (4), according to the circulating current suppressor control instruction after superimposing the baseband feedforward compensation term, the capacitor voltage is balanced.

[0014] Preferably, the fundamental frequency feedforward compensation term FD is calculated by the following formula:

[0015] FD=Asin(ωt+φ)

[0016] Wherein, A is the amplitude of the fundamental frequency feedforward compensation term, φ is the phase of the fundamental frequency feedforward compensation term, and ω is the frequency of the AC power grid. The amplitude A and phase φ of the fundamental frequency feedforward compensation term can be determined by the following formula:

[0017]

[0018]

[0019] Where m is the AC voltage modulation degree, is the AC active current during fault ride-through, is the AC reactive current during fault ride-through, L is the bridge arm inductance, R is the bridge arm resistance, K p is the proportional parameter of the circulating current suppressor. a is an additionally defined variable, which can be calculated by the following formula:

[0020]

[0021] Wherein, N is the number of MMC bridge arm submodules, and C is the submodule capacitance.

[0022] In general, the above technical concept of the present invention has the following technical advantages compared with the prior art:

[0023] 1. This technology does not require additional complex control. It can achieve the balance of the capacitor voltage of the upper and lower bridge arm submodules by simply superimposing the base frequency feedforward compensation term on the existing circulating current suppressor control instructions;

[0024] 2. This technology can not only effectively maintain the balance of capacitor voltages of the upper and lower bridge arm submodules of the MMC during pole-to-ground fault ride-through, but also minimize the circulating current, thereby reducing the amplitude of the bridge arm current and reducing the operating loss of the MMC. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the pressure equalization control structure of the present invention;

[0026] Figure 2 It is a flow chart of the implementation of voltage sharing control based on fundamental frequency feedforward compensation of the present invention;

[0027] Figure 3 It is a schematic diagram of the MMC structure of the present invention;

[0028] Figure 4 This is an example simulation diagram of the present invention. Among them, (a) is the positive and negative DC bus voltage; (b) is the active and reactive power of the AC port; (c) is the change diagram of the capacitor voltage of the upper and lower bridge arm submodules of phase A over time; (d) is the change diagram of the three-phase circulating current over time.

[0029] Figure 5This is the effect diagram of other voltage-sharing control methods that have been made public in academia (J.Hu, K.Xu, L.Lin and R.Zeng.Analysis and enhanced control of hybrid-MMC-based HVDC systems duringasymmetrical DC voltage faults[J].IEEE Trans.Power Del., vol.32, no.3, pp.1394–1403, 2017). Among them, (a) is the change of the capacitor voltage of the upper and lower bridge arm submodules of phase A over time, and (d) is the change of the three-phase circulating current over time. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0031] The MMC with fault ride-through capability involved in the present invention specifically includes three phases a, b, and c, each phase is composed of two upper and lower bridge arms, and each bridge arm has a negative level output capability greater than half of the rated DC voltage. The converter topologies that currently meet the requirements include full-bridge MMC, half-bridge and full-bridge sub-module hybrid MMC (the ratio of full-bridge to half-bridge sub-modules is greater than or equal to 1:1), and half-bridge and other sub-modules with negative level output capability. The voltage-sharing control structure based on baseband feedforward compensation is as follows Figure 1 shown. Figure 2 The implementation process of the pressure equalization control specifically includes the following steps:

[0032] Step (1), real-time detection of AC and DC side electrical quantities, and judging whether a DC pole-to-ground fault occurs according to the mature DC pole-to-ground fault judgment criteria of existing projects. If yes, then sequentially execute step (2), otherwise continue to detect;

[0033] Step (2), the modular multilevel converter MMC switches to the pole-to-ground fault ride-through control strategy, and simultaneously executes step (3);

[0034] Step (3), superimposing the baseband feedforward compensation term FD on the circulating current suppressor control instruction.

[0035] Step (4), according to the circulating current suppressor control instruction after superimposing the baseband feedforward compensation term, the capacitor voltage is balanced.

[0036] In step (1), the electrical quantities detected in real time mainly include AC terminal voltage, AC line current, positive and negative DC bus voltages, positive and negative DC bus currents, bridge arm currents and submodule capacitor voltages.

[0037] In step (2), the pole-to-ground fault ride-through strategy is a control method that has been described in detail in existing literature, specifically including: active power control, reactive power control, AC current control, phase-locked control, and circulating current suppression control. In addition, the active power control instruction is half of the normal operating rated value of the MMC, the reactive power control instruction is the normal operating rated value of the MMC, the common mode voltage reference instruction of the faulty pole bridge arm is zero, and the common mode voltage reference instruction of the healthy pole bridge arm is the normal operating rated value.

[0038] In step (3), in order to achieve the balance of the capacitor voltages of the upper and lower bridge arms, it is necessary to construct an active power interaction channel between the upper and lower bridge arms. Considering that the output voltages of all bridge arms during fault ride-through contain fundamental frequency components. Therefore, the active power interaction channel between the upper and lower bridge arms can be achieved through fundamental frequency circulation. In order to stimulate the fundamental frequency circulation, the present invention superimposes a fundamental frequency feedforward compensation term FD on the instruction of the circulation suppressor. Specifically, the fundamental frequency feedforward compensation term FD can be calculated by the following formula:

[0039] FD=Asin(ωt+φ)

[0040] Wherein, A is the amplitude of the fundamental frequency feedforward compensation term, φ is the phase of the fundamental frequency feedforward compensation term, and ω is the frequency of the AC power grid. The amplitude A and phase φ of the fundamental frequency feedforward compensation term can be determined by the following formula:

[0041]

[0042]

[0043] Where m is the AC voltage modulation degree, is the AC active current during fault ride-through, is the AC reactive current during fault ride-through, L is the bridge arm inductance, R is the bridge arm resistance, K p is the proportional parameter of the circulating current suppressor. a is an additionally defined variable, which can be calculated by the following formula:

[0044]

[0045] Wherein, N is the number of MMC bridge arm submodules, and C is the submodule capacitance.

[0046] In order to enable those skilled in the art to better understand the present invention, the voltage sharing control strategy based on fundamental frequency feedforward compensation of the present invention is described in detail below in conjunction with specific embodiments.

[0047] In the following example, the MMC with negative level output capability takes the full-bridge submodule MMC as an example, and the topology results are as follows: Figure 3 As shown. Among them, FBSM represents a full-bridge submodule, and the subscript i represents the submodule serial number. The effective value of the AC side phase voltage is 260kV, the DC bus voltage is 500kV, and each bridge arm contains 200 full-bridge submodules. The submodule capacitance is 12mF, the bridge arm inductance is 40mH, and the bridge arm inductance equivalent resistance is 0.2Ω. The grounding electrode inductance is 1H, and the grounding electrode resistance is 1kΩ. The active power, reactive power and other positive directions of the variables transmitted by the converter are indicated in the figure. The proportional control parameter in the circulating current suppressor is 15, and the resonance control parameter is 2000. Assume that a negative pole-to-ground fault occurs at 1s, a pole-to-ground fault is crossed at 1.0005, and the voltage equalization control strategy proposed in the present invention is put into use at 2.5s. After calculation, the amplitude and phase of the proposed baseband feedforward compensation term are 1.005kA and 11.4°, respectively.

[0048] During the fault ride-through period, the positive and negative DC bus voltages change with time as shown in Figure 4 As shown in (a), the results show that the pole-to-ground fault ride-through strategy can quickly and effectively eliminate the fault voltage stress of the positive DC bus; the changes of active and reactive power on the AC side over time are shown in Figure 4 (b) shows that the MMC can still maintain continuous operation during the fault ride-through period and can simultaneously transmit half of the rated active power and rated reactive power; the change of the capacitor voltage of the upper and lower bridge arm submodules of phase A over time is shown in Figure 4 As shown in (c), the results show that during the pole-to-ground fault ride-through, the upper and lower bridge arms of the MMC will indeed be unbalanced. The voltage balancing strategy proposed in the present invention can quickly and effectively maintain the balanced operation of the upper and lower bridge arms, and the fluctuation of the submodule capacitor voltage during the fault ride-through is basically consistent with that during normal operation, and will not increase significantly; the change of the interphase circulating current over time is shown in Figure 4 As shown in (d), the results show that during steady-state operation, the circulating current only contains DC components, while during the pole-to-ground fault ride-through, a certain component of fundamental frequency appears in the circulating current, and this fundamental frequency will not increase significantly after the voltage balancing control strategy is put into use.

[0049] If the voltage-sharing control method published in academia is adopted (J.Hu, K.Xu, L.Lin and R.Zeng.Analysis and enhanced control of hybrid-MMC-based HVDC systems during asymmetrical DC voltage faults[J].IEEE Trans.Power Del., vol.32, no.3, pp.1394–1403, 2017), the simulation results are as follows: Figure 5For comparison purposes, it is also assumed that a negative pole-to-ground fault occurs at 1 second, a pole-to-ground fault ride-through occurs at 1.0005 seconds, and the voltage balancing control strategy is activated at 2.5 seconds. Figure 5 (a) is the voltage fluctuation of the upper and lower bridge arm submodule capacitors. The results show that when the voltage balancing control method disclosed by the academic community is adopted, the average value of the voltage of the upper and lower bridge arm submodule capacitors can effectively maintain balance, but compared with the voltage balancing control method proposed in the present invention, the voltage fluctuation of the submodule capacitors under this method will be significantly increased. Figure 5 (b) is the interphase circulating current. The results show that when the voltage-sharing control method disclosed by the academic community is adopted, a certain amount of fundamental frequency components will also appear in the interphase circulating current. However, the fundamental frequency circulating current is significantly larger than the method proposed by the present invention. This will increase the amplitude of the bridge arm current, thereby increasing the operating loss.

[0050] The coordination and cooperation scheme is applicable to high, medium and low voltage flexible direct current transmission fields.

[0051] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A voltage sharing control method for a modular multilevel converter during pole-to-ground fault ride-through, characterized in that: The following steps are involved: Step (1), real-time detection of AC and DC side electrical quantities to determine whether a DC pole-to-ground fault occurs; if yes, then sequentially execute step (2); otherwise, continue detection; Step (2), the modular multilevel converter MMC switches to the pole-to-ground fault ride-through control strategy, and simultaneously executes step (3); Step (3), superimposing the baseband feedforward compensation term FD on the circulating current suppressor control instruction; Step (4), completing the capacitor voltage balance according to the circulating current suppressor control instruction after superimposing the baseband feedforward compensation term; The fundamental frequency feedforward compensation term FD is calculated by the following formula: FD=Asin(ωt+φ) Wherein, A is the amplitude of the fundamental frequency feedforward compensation term, φ is the phase of the fundamental frequency feedforward compensation term, and ω is the frequency of the AC power grid; the amplitude A and phase φ of the fundamental frequency feedforward compensation term can be determined by the following formula: Where m is the AC voltage modulation degree, is the AC active current during fault ride-through, is the AC reactive current during fault ride-through, L is the bridge arm inductance, R is the bridge arm resistance, K p is the proportional parameter of the circulating current suppressor; a is an additionally defined variable, which can be calculated by the following formula: Wherein, N is the number of MMC bridge arm submodules, and C is the submodule capacitance.

2. The method for controlling voltage sharing during pole-to-ground fault ride-through of a modular multilevel converter according to claim 1, characterized in that: In step (1), the electrical quantities detected in real time include AC terminal voltage, AC line current, positive and negative DC bus voltages, positive and negative DC bus currents, bridge arm currents and submodule capacitor voltages.

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