Light modular multilevel converter topology with high-power load networking capability and control strategy

Abstract

The invention provides a light modular multilevel converter topology with high-power-load network construction capability and a control strategy, and belongs to the field of flexible direct current power transmission and distribution. A conventional modularized multi-level current converter is subjected to double limitation of the overcurrent capability and the energy storage capacity of a semiconductor device in a network construction scene, so that the capacity expansion and transformation cost is high; the invention discloses a light modular multi-level converter topology with high-power load network construction capability and a control strategy, the voltage modulation degree of the modular multi-level converter is improved to reduce the bridge arm current of the converter during high-power load network construction support, the reactive support capability is enhanced, and a super-capacitance energy storage unit is configured to realize the high-power load network construction capability. Compared with the existing topology with high modulation degree operation capability, the topology provided by the invention effectively realizes the light-weight target, reduces the number of capacitors and switching devices, and reduces the network construction capacity expansion reconstruction cost.

Description

Technical Field

[0001] The present invention relates to the field of flexible direct current (DC) power transmission and distribution, and in particular to a lightweight modular multi-level converter topology and control strategy with high-load networking capability. Background Art

[0002] As the penetration of power electronics in power grids increases significantly, the system's voltage and inertia support capabilities are insufficient, and the stability foundation of traditional synchronous generators is declining. This places higher demands on the grid-building capabilities of power electronics equipment. Flexible DC, a key technology for renewable energy access and flexible interconnection of regional power grids, offers advantages such as high voltage levels and large capacity. However, its application in grid support and regulation is currently limited. One limiting factor is the insufficient overload capacity of modular multilevel converters (MMCs), which pose drawbacks such as high cost, bulk, and weight when expanding capacity. Therefore, there is an urgent need to lightweight MMCs for grid-building applications.

[0003] At present, scholars have conducted a lot of research on MMC lightweighting from the two aspects of improving control strategies and circuit topology. The improved control strategy mainly focuses on capacity reduction control. The use of second harmonic current injection, third harmonic voltage injection, overmodulation and other methods can reduce capacitor voltage ripple and reduce the requirements for sub-module capacitance value; the circuit topology includes the proposed bridge arm alternating topology, the lightweight topology of bridge arm unit connected to two-level converter and current-type MMC topology. Although the above methods have good lightweight effects, they do not improve the high-load overcurrent capability of the converter. In addition, the current active support capacity of MMC is insufficient. The energy of the sub-module capacitor itself cannot meet the grid inertia and damping support requirements. Therefore, upgrading the energy storage capacity is also one of the focuses of network construction and transformation. Summary of the Invention

[0004] In response to the shortcomings and problems of the existing technology, the present invention proposes a lightweight modular multi-level converter topology and control strategy with high-load grid-building capability. In terms of topology, the converter includes two parts: a phase unit and a DC side supercapacitor energy storage unit; the phase unit includes three phases (a, b, and c), each phase consists of an upper, middle, and lower bridge arm and two switching switches, wherein the upper and lower bridge arms are cascaded by full-bridge sub-modules, the middle bridge arm is cascaded by half-bridge sub-modules, and the switching switch is composed of switching devices connected in reverse series, wherein the upper switching switch is connected in parallel to the bottom of the upper bridge arm and the AC side outlet, and the lower switching switch is connected in parallel to the top of the lower bridge arm and the AC side outlet, and the AC side outlet of the converter is connected to the power grid through a step-down transformer; the DC side supercapacitor energy storage part is composed of multiple bidirectional Buck-boost converters connected in cascade with supercapacitor units. In terms of control strategy, it includes phase unit modulation strategy, switching switch state switching control strategy, DC side energy storage unit control strategy and DC fault blocking control strategy.

[0005] The control strategy of the lightweight modular multilevel converter described in the present invention is as follows: during the positive half-cycle of the AC voltage, the upper switch is turned on, the lower switch is turned off, the middle bridge arm is divided into the lower equivalent bridge arm for modulation, and the upper bridge arm is modulated independently; during the negative half-cycle of the AC voltage, the upper switch is turned off, the lower switch is turned on, the middle bridge arm is divided into the upper equivalent bridge arm for modulation, and the lower bridge arm is modulated independently; because the voltage modulation index of the converter is greater than 1, when the bridge arm modulation voltage is negative, some submodules of the upper / lower bridge arms are placed in a negative input state when necessary; taking phase a as an example, the phase unit bridge arm expression is:

[0006]

[0007] Where u pa * with u na * Indicates the equivalent upper and lower bridge arm voltages, u pa 、u ma with u na Represent the upper, middle and lower bridge arm voltages respectively, U dc is the DC voltage, u a is the AC output voltage u for phase a a =U m sinωt, ω is the AC side voltage AC angular frequency, U m is the AC side phase voltage amplitude; in the positive half cycle of the AC voltage, the switching of the upper bridge arm submodule is determined according to the voltage sorting result and the charging and discharging condition, the lower bridge arm and the middle bridge arm are sorted as a whole, and the submodule switching condition is determined according to the voltage sorting result and the charging and discharging condition; in the negative half cycle of the AC voltage, the upper bridge arm submodule and the middle bridge arm submodule are sorted as a whole, and the switching object is determined according to the bridge arm current direction, the lower bridge arm submodule is sorted separately, and the switching condition is determined according to the bridge arm current direction.

[0008] The switch state switching control strategy of the lightweight modular multilevel converter described in the present invention: To prevent overvoltage problems during switch operation, it is necessary to ensure that the number of submodules in the middle bridge arm in the operating state is zero at the switching time. Therefore, taking phase a as an example, the upper switch control is: by determining whether the number of submodules required to be activated in the equivalent upper bridge arm exceeds the total number of upper bridge arm submodules when the absolute value of the phase a AC voltage reference value is within 0.25 times the rated value of the submodule capacitor voltage. If not, the upper bridge arm is normally activated. If exceeded, all the upper bridge arm submodules are activated and all the middle bridge arm submodules are bypassed. The lower switch control is: by determining whether the number of submodules required to be activated in the equivalent lower bridge arm exceeds the total number of lower bridge arm submodules when the absolute value of the phase a AC voltage reference value is within 0.25 times the rated value of the submodule capacitor voltage. If not, the lower bridge arm is normally activated. If exceeded, all the lower bridge arm submodules are activated and all the middle bridge arm submodules are bypassed.

[0009] The DC-side energy storage unit control strategy of the lightweight modular multilevel converter described in the present invention is as follows: the active power set value of the energy storage unit is obtained by subtracting the grid-side frequency set value from the measured value, and then multiplying the difference by the active-frequency droop coefficient. The active power set value is compared with the actual value and subjected to proportional-integral control to obtain the duty cycle. The duty cycle is compared with the carrier with a phase difference of 2π / B on the B channel, and the sum is calculated to obtain the number of excess energy storage units required to be put into operation, where B is the number of excess energy storage units on the DC side. The on and off signals of the excess energy storage units on the DC side are further obtained through a sorting and voltage balancing method.

[0010] The lightweight modular multilevel converter described in the present invention uses full-bridge submodules and a switch composed of reverse series devices, and has a DC fault blocking capability. When the system protection detects a DC short circuit fault, all submodules and the switch are locked to achieve fault current blocking.

[0011] The present invention can achieve the following beneficial effects:

[0012] 1. This invention proposes to increase the voltage modulation of a modular multilevel converter to reduce the converter's bridge arm current when supporting a high-load grid, thereby enhancing reactive power support capabilities. Furthermore, by configuring a supercapacitor energy storage unit, this improves active power support capabilities, enabling the system to cope with sudden frequency drops on the grid side.

[0013] 2. Compared with existing topologies capable of high-speed operation, the topology proposed in this invention effectively achieves the goal of lightweighting, reduces the number of capacitors and switching devices, and reduces the cost of network construction, expansion and transformation;

[0014] 3. The present invention has a DC fault blocking capability by using reverse series devices in the switching switch, thereby reducing the number of configurations of full-bridge submodules. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The topological structure diagram of a lightweight modular multi-level converter with high load-bearing network construction capability;

[0016] Figure 2 Schematic diagram of the phase unit operation mode of a lightweight modular multi-level converter;

[0017] Figure 3 Schematic diagram of voltage equalization modulation of the equivalent upper bridge arm of phase a;

[0018] Figure 4 The switching control strategy for the switching state of the switch on phase a;

[0019] Figure 5 This is the control strategy for the DC side overcapacity energy storage unit. DETAILED DESCRIPTION

[0020] To make the basic principles, technical solutions, and advantages of the present invention more clearly understood, the following detailed description of a lightweight modular multilevel converter topology and control strategy with high-load networking capabilities, described in conjunction with the accompanying drawings and specific embodiments, is provided. It should be understood that the following description is merely illustrative and is not intended to limit the scope of protection and application of the present invention.

[0021] Figure 1 This is a topology diagram of a lightweight modular multilevel converter with high load-carrying network capability. Each phase consists of three bridge arms: upper, middle, and lower, a switch, and a bridge arm reactor L0. The upper and lower bridge arms are composed of F full-bridge submodules and 1 L0 in series, the middle bridge arm is composed of H half-bridge submodules in series, and the switch is composed of X+Y fully controlled devices in reverse series. The DC side consists of B energy storage submodules in cascade to form the energy storage part, where L f Filter inductor, C sc It is a supercapacitor, and the AC side is connected to the grid through a step-down transformer T. Figure 1 Middle,U dc and I dc Indicates the DC side voltage and current, I e is the DC current of the supercapacitor energy storage part, u j and i j (j=a, b, c) represents the AC side voltage and current, u pj 、u mj and u nj The voltage of the upper, middle and lower bridge arms of phase j, i pj 、i mj and i nj The upper, middle and lower bridge arm currents of phase j are respectively, S1 and S2 are bridge arm switching switches, X g is the grid impedance, u sj is the grid voltage.

[0022] Figure 2 The diagram below shows the operating mode of the phase unit of the lightweight modular multilevel converter. The switches in each phase are alternately turned on in a single cycle, making the bridge arm of the lightweight modular multilevel converter equivalent to the traditional MMC structure in the half-wave cycle. In the positive half-cycle of the AC voltage, S1 is turned on and S2 is turned off. The middle bridge arm is divided into the lower equivalent bridge arm to participate in the modulation, and the upper bridge arm is modulated separately, which is called mode I. Figure 2 (a) As shown; in the negative half cycle of the AC voltage, S1 is turned off and S2 is turned on. The middle bridge arm is divided into the upper equivalent bridge arm to participate in the modulation, and the lower bridge arm is modulated separately, which is called mode II. Figure 2 (b) shown.

[0023] Since the voltage modulation index of the converter is greater than 1, when the bridge arm modulation voltage is negative, some submodules of the upper / lower bridge arm are in the negative input state when necessary; taking phase a as an example, the phase unit bridge arm expression is:

[0024]

[0025] Where u pa * with u na * Indicates the equivalent upper and lower bridge arm voltages, u pa 、u ma with u na Represent the upper, middle and lower bridge arm voltages respectively, U dc is the DC voltage, u a is the AC output voltage of phase a, u a =U m sinωt, ω is the AC side voltage AC angular frequency, U m is the AC side phase voltage amplitude. Taking the equivalent upper bridge arm of phase a as an example, the equivalent upper bridge arm modulation voltage sharing diagram is as follows Figure 3 As shown. In the positive half cycle of the AC voltage, the upper bridge arm submodule is switched according to the voltage sorting result and the charge and discharge situation. The lower bridge arm and the middle bridge arm are sorted as a whole, and the submodule switching situation is determined according to the voltage sorting result and the charge and discharge situation. In the negative half cycle of the AC voltage, the upper bridge arm submodule and the middle bridge arm submodule are sorted as a whole, and the switching object is determined according to the bridge arm current direction. The lower bridge arm submodule is sorted separately and the switching direction is determined according to the bridge arm current i pa The direction determines the switching situation.

[0026] Due to the overall sorting and voltage balancing strategy of the two bridge arms, if there are multiple submodules in the middle bridge arm in the on-state at the switching moment, there is a risk of overvoltage during the on-off process of the switching switch. Therefore, to ensure the safety of the switching operation, it is necessary to ensure that the number of submodules in the running state of the middle bridge arm is 0 at the switching moment. Therefore, the submodules in the upper and lower bridge arms are preferentially turned on before and after the switching moment. Taking the switching strategy of phase a as an example, the state switching control strategy is as follows Figure 4 As shown, by judging the a-phase AC voltage reference value u a * The absolute value is 0.25 times the rated voltage of the submodule capacitor U c The number of submodules N that need to be opened in the equivalent upper bridge arm pa * Whether the total number of upper bridge arm sub-modules F is exceeded, if not, the upper bridge arm is put into operation normally, if exceeded, all upper bridge arm sub-modules are put into operation, and all middle bridge arm sub-modules are bypassed; lower switching switch control: by judging whether the number of sub-modules that need to be opened in the equivalent lower bridge arm exceeds the total number of lower bridge arm sub-modules when the absolute value of the a-phase AC voltage reference value is within 0.25 times the rated value of the sub-module capacitor voltage, if not exceeded, the lower bridge arm is put into operation normally, if exceeded, all lower bridge arm sub-modules are put into operation, and all middle bridge arm sub-modules are bypassed.

[0027] Figure 5 The control strategy of the DC side overcapacity energy storage unit is as follows: the active power given value of the energy storage unit is the difference between the grid side frequency given value ω0 and the measured value ω, and then the difference between the active power and frequency droop coefficient k ω The active power given value is compared with the actual value and the duty cycle D is obtained through proportional integral control. The duty cycle is compared with the carrier with a phase difference of 2π / B on the B path and the sum is calculated to obtain the number of super-capacity energy storage units that need to be put into use, where B is the number of super-capacity energy storage units on the DC side. The super-capacity voltage U is further collected. sci , the on and off signals of the DC side super-capacity energy storage unit are obtained through the sorting and voltage balancing method.

[0028] The lightweight modular multilevel converter uses full-bridge sub-modules and switching switches composed of reverse series devices, and has DC fault blocking capability. When the system protection detects a DC short-circuit fault, all sub-modules and switching switches are locked to achieve fault current blocking.

[0029] The advantages of the present invention are:

[0030] 1. This invention proposes to reduce the bridge arm current of the converter when supporting high-load grids by increasing the voltage modulation index of the modular multi-level converter. Furthermore, it configures a super-capacitive energy storage unit to improve the active power support capability and cope with sudden frequency drops on the grid side.

[0031] 2. Compared with existing topologies capable of high-speed operation, the topology proposed in this invention effectively achieves the goal of lightweighting, reduces the number of capacitors and switching devices, and reduces the cost of network construction, expansion and transformation;

[0032] 3. The present invention has a DC fault blocking capability by using reverse series devices in the switching switch, thereby reducing the number of configurations of full-bridge submodules.

[0033] The above is a specific embodiment of the present invention and its advantages, but the scope of protection of the present invention is not limited thereto. Those skilled in the art may make changes and modifications to the above embodiment without departing from the technical spirit and principles described in the present invention, and such changes and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A lightweight modular multilevel converter topology and control strategy with high load-carrying network capability, characterized by: In terms of topology, the converter consists of two parts: a phase unit and a DC side supercapacitor energy storage unit. The phase unit contains three phases a, b, and c. Each phase consists of upper, middle, and lower bridge arms and two switching switches. The upper and lower bridge arms are cascaded by full-bridge sub-modules, the middle bridge arm is cascaded by half-bridge sub-modules, and the switching switches are reversely connected in series by fully controlled switching devices. The upper switching switch is connected in parallel to the bottom of the upper bridge arm and the AC side outlet, and the lower switching switch is connected in parallel to the top of the lower bridge arm and the AC side outlet. The AC side outlet of the converter is connected to the power grid through a step-down transformer. The DC side supercapacitor energy storage part is composed of multiple bidirectional Buck-boost converters connected to supercapacitor units in cascade; in terms of control strategy, it includes phase unit modulation strategy, switching switch state switching control strategy and DC fault blocking control strategy.

2. The phase unit modulation strategy of the lightweight modular multilevel converter according to claim 1, characterized in that: During the positive half cycle of the AC phase voltage, the upper switch is turned on and the lower switch is turned off. The middle bridge arm is divided into the lower equivalent bridge arm to participate in the modulation, and the upper bridge arm is modulated separately. During the negative half cycle of the AC phase voltage, the upper switch is turned off and the lower switch is turned on. The middle bridge arm is divided into the upper equivalent bridge arm to participate in the modulation, and the lower bridge arm is modulated separately. Since the voltage modulation index of the converter is greater than 1, when the bridge arm modulation voltage is negative, some submodules of the upper / lower bridge arm are in the negative input state when necessary. Taking phase a as an example, the phase unit bridge arm expression is: Where u pa * with u na * Indicates the equivalent upper and lower bridge arm voltages, u pa 、u ma with u na Represent the upper, middle and lower bridge arm voltages respectively, U dc is the DC voltage, u a is the AC phase voltage of phase a, u a =U m sinωt, ω is the AC voltage AC angular frequency, U m is the AC phase voltage amplitude; in the positive half cycle of the AC phase voltage, the switching of the upper bridge arm submodule is determined according to the voltage sorting result and the charging and discharging condition, the lower bridge arm and the middle bridge arm are sorted as a whole, and the submodule switching condition is determined according to the voltage sorting result and the charging and discharging condition; in the negative half cycle of the AC phase voltage, the upper bridge arm submodule and the middle bridge arm submodule are sorted as a whole, and the switching object is determined according to the bridge arm current direction, the lower bridge arm submodule is sorted separately, and the switching condition is determined according to the bridge arm current direction.

3. The lightweight modular multi-level converter switch state switching control strategy according to claim 1, characterized in that: To prevent overvoltage problems when the switching switch is actuated, it is necessary to ensure that the number of sub-modules in the middle bridge arm in operation at the switching moment is 0. Therefore, taking phase a as an example, the upper switching switch control is: by judging whether the number of sub-modules that need to be turned on in the equivalent upper bridge arm exceeds the total number of sub-modules in the upper bridge arm when the absolute value of the phase a AC voltage reference value is within 0.25 times the rated value of the sub-module capacitor voltage, if not, the upper bridge arm is normally put into operation; if exceeded, all the upper bridge arm sub-modules are put into operation and all the middle bridge arm sub-modules are bypassed; the lower switching switch control is: by judging whether the number of sub-modules that need to be turned on in the equivalent lower bridge arm exceeds the total number of sub-modules in the lower bridge arm when the absolute value of the phase a AC voltage reference value is within 0.25 times the rated value of the sub-module capacitor voltage, if not, the lower bridge arm is normally put into operation; if exceeded, all the lower bridge arm sub-modules are put into operation and all the middle bridge arm sub-modules are bypassed.

4. The DC fault blocking control strategy for a lightweight modular multilevel converter according to claim 1, characterized in that: The lightweight modular multilevel converter uses full-bridge sub-modules and switching switches composed of reverse series devices, and has DC fault blocking capability. When the system protection detects a DC short-circuit fault, all sub-modules and switching switches are locked to achieve fault current blocking.

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