Energy storage type MMC topology and control method thereof

Abstract

The invention relates to an energy storage type MMC topology and a control method thereof. The MMC is composed of three phases and six bridge arms, each bridge arm is formed by connecting a bridge arm inductor and N identical sub-modules in series, and each sub-module comprises IGBTT1-T4, diodes D1-D4, a battery, a sub-module capacitor and a decoupling inductor. T1 and D1, T2 and D2, T3 and D3, and T4 and D4 are all of anti-parallel structures, and the anode of D1 and the cathode of the diode D2 are connected to the input end of the sub-module; the cathode of D3 is connected to the cathode of D1, the anode of D4 is connected to the anode of diode D2, and the anode of D3 and the cathode of D4 are connected to the output end of the sub-module; the anode of the battery is connected to the cathode of the D3, the cathode of the battery and the anode of the sub-module capacitor are connected to one end of the decoupling inductor, the other end of the decoupling inductor is connected to the output end of the sub-module, and the cathode of the sub-module capacitor is connected to the anode of the D4. According to the invention, the energy storage battery is used for balancing the DC power difference between the input side and the output side of the MMC, and the sub-module capacitor bears the ripple power of the bridge arm, thereby maintaining the stability of the output power of the MMC, and improving the quality of the output power of the MMC.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic multilevel converters, and particularly relates to a energy storage type MMC topology and a power decoupling method when an energy storage unit functions. Background Art

[0002] With the development of power electronic technology, flexible DC systems based on MMC have received extensive attention in the industry. Distributed energy storage type MMC plays an important role in energy transformation and energy consumption due to its strong energy storage capacity, convenient change of voltage and power, high output waveform quality, low switching frequency and voltage stress of power switching devices.

[0003] Affected by environmental factors, the output of renewable energy has certain volatility and intermittency. The grid connection of large-scale renewable energy affects the grid voltage distribution. The output power fluctuation of its units will destroy the active power balance and reactive power balance of the system, thus causing the frequency fluctuation of the grid and seriously affecting the voltage stability of the grid. New energy power sources are usually connected to the grid through power electronic converters, so the converter control has a profound impact on the stability of the power system. Compared with traditional converters such as two-level and three-level converters, MMC has fewer AC side harmonics, higher modularity and stronger fault tolerance ability, and is an ideal power converter topology.

[0004] Adopting traditional non-isolated DC-DC parallel energy storage units in MMC will greatly increase the cost and volume of sub-modules. Therefore, designing an energy storage type MMC with power device reuse and power decoupling ability is of great significance for suppressing frequency and voltage fluctuations, improving the stability and topological compactness of the power transmission system. Summary of the Invention

[0005] The object of the present invention is to solve the problem that the power generation of new energy is unstable affected by the environment and affects the grid voltage stability. The present invention selects the full-bridge sub-module as the basic topology, and provides an energy storage type MMC topology and its control strategy. This MMC can decouple active power and reactive power. Among them, the energy storage unit can effectively balance the DC power difference between the input side and the output side of the MMC, and maintain the power stability of the output side of the MMC. The proposed topology adopts a structure with power device reuse, improving the topological compactness.

[0006] To solve the above technical problems, the specific technical solutions adopted by the present invention are as follows:

[0007] A kind of energy storage type MMC topology and its control method. The MMC is composed of upper and lower arms. Each arm is composed of an arm inductor and N sub-modules with the same structure connected in series. One end of the arm inductor is connected to the bus, and the other end is connected to the input end of the first sub-module. The output end of the first sub-module is connected to the input end of the second sub-module, and so on. The output end of the (N - 1)th sub-module is connected to the input end of the Nth sub-module. The output end of the Nth sub-module is connected to the output side of the MMC. The upper and lower arms have symmetric structures. The output end of the first sub-module of the lower arm is connected to the input end of the second sub-module, and so on. The output end of the (N - 1)th sub-module of the lower arm is connected to the input end of the Nth sub-module. The output end of the Nth sub-module is connected to one end of the arm inductor of the lower arm. The other end of the arm inductor of the lower arm is connected to the negative pole of the DC bus. The sub-module includes a first series arm, a second series arm and an energy storage unit.

[0008] The first series arm includes an insulated gate bipolar transistor T1 and an insulated gate bipolar transistor T2, a diode D1 and a diode D2. The insulated gate bipolar transistor T1 and the diode D1 are anti-parallel connected. The emitter of the insulated gate bipolar transistor T1 and the anode of the diode D1 are connected. The insulated gate bipolar transistor T2 and the diode D2 are anti-parallel connected. The emitter of the insulated gate bipolar transistor T2 and the anode of the diode D2 are connected. The anode of the diode D1 and the cathode of the diode D2 are commonly connected to the input end of the sub-module. The second series arm includes an insulated gate bipolar transistor T3 and an insulated gate bipolar transistor T4, a diode D3 and a diode D4. The insulated gate bipolar transistor T3 and the diode D3 are anti-parallel connected. The collector of the insulated gate bipolar transistor T3 and the cathode of the diode D3 are commonly connected to the cathode of the diode D1. The insulated gate bipolar transistor T4 and the diode D4 are anti-parallel connected. The emitter of the insulated gate bipolar transistor T4 and the anode of the diode D4 are commonly connected to the anode of the diode D2. The anode of the diode D3 and the cathode of the diode D4 are commonly connected to the output end of the sub-module. The input end and the output end of the sub-module form the AC port of the sub-module. The energy storage unit includes a battery E, a sub-module capacitor C and a decoupling inductor L. The positive pole of the battery E is connected to the cathode of the diode D3. The negative pole of the battery E and the positive pole of the sub-module capacitor C are commonly connected to one end of the decoupling inductor L. The other end of the decoupling inductor L is connected to the output end of the sub-module. The negative pole of the sub-module capacitor C is connected to the anode of the diode D4. The two ends of the battery E form the first DC port, and the two ends of the sub-module capacitor C form the second DC port.

[0009] Furthermore, a single closed-loop control strategy for the arm current is adopted to control the arm current, and then the input power of the sub-module is controlled; a double closed-loop control strategy for the sub-module voltage and current is adopted to control the currents on the battery E and the sub-module capacitor C, so as to decouple the DC component and the AC component in the input power of the sub-module. The battery E, as the first DC port, can absorb the excess DC power provided by the input side of the MMC or make up for the DC power gap on the input side of the MMC; the sub-module capacitor C, as the second DC port, absorbs the ripple power in the MMC arm.

[0010] Ignoring the influence of the arm inductance and the sub-module decoupling inductance on the power, the power relationship in the energy storage type MMC is as follows

[0011]

[0012] Among them, the subscript x = a, b, c represents three phases of a, b, and c, and P Px and P Nx are the instantaneous powers of the upper and lower arms respectively, which are evenly distributed to the sub-modules in the arm. u Px and i Px represent the voltage and current of the upper arm respectively, u Nx and i Nx represent the voltage and current of the lower arm respectively. I dc is the DC current of the arm, U dc is the DC voltage on the input side of the MMC, E x and I x are the amplitudes of the output voltage and output current respectively. ω, θ x and are the angular frequency, initial phase angle, and power factor angle of the output voltage respectively. P sm is the input power of the sub-module AC port, P E and P C are the powers of the battery E and the sub-module capacitor C respectively.

[0013] The voltage fluctuation of the sub-module capacitor C is relatively small compared with the voltage mean value and can be ignored. Therefore, the power P E of the battery E and the power P C of the sub-module capacitor C are respectively

[0014]

[0015] It is stipulated that the direction from the collector node of the insulated gate bipolar transistor T1 and the insulated gate bipolar transistor T3 to the positive electrode of the battery E is the positive direction of the current i E on the battery E, the direction from the negative electrode of the battery E to the sub-module capacitor C is the positive direction of the current i C on the sub-module capacitor C, and the direction from the negative electrode of the battery E to the decoupling inductor L is the positive direction of the current iL In the positive direction, according to Kirchhoff's current law, i E , i C and i L have the mathematical relationship of

[0016] i L = i E - i C (3)

[0017] Combining formulas (1), (2) and (3), the currents flowing through the battery E and the sub-module capacitor C can be obtained as

[0018]

[0019] Combining equation (4), by decoupling the current i L in the inductance L, the currents on the battery E and the sub-module capacitor C are controlled, and then the powers on the battery E and the sub-module capacitor C are controlled. Taking the upper bridge arm as an example, the current i L of the decoupling inductance L is specifically calculated as

[0020]

[0021] where N is the number of sub-modules in a single bridge arm. The DC part of the bridge arm power is balanced by the battery E, and the ripple part is absorbed by the sub-module capacitor C, thus realizing power decoupling control.

[0022] Furthermore, the double closed-loop control strategy of the sub-module voltage and current consists of two control loops. The outer loop is the control of the sub-module capacitor voltage. The deviation between the reference value and the actual measured value of the sub-module capacitor voltage is used as the input of the voltage outer loop to keep the average voltage of the sub-module capacitor constant. The input of the voltage outer loop passes through the PI link to obtain the DC bias in the reference value of the current inner loop input. Adding it to the calculated current ripple amount gives the current reference value on the decoupling inductance L. After subtracting the actual measured value from the current reference value, it passes through the PR link to obtain the modulation wave of the insulated gate bipolar transistor T3. After comparing the modulation wave with the triangular carrier wave, the switching signal of the insulated gate bipolar transistor T3 is obtained. The switching signal of the insulated gate bipolar transistor T4 is opposite to that of the insulated gate bipolar transistor T3. The switching signals of the two together constitute the control signal of the second series bridge arm.

[0023] Furthermore, the single closed-loop control strategy of the bridge arm current controls the bridge arm power by controlling the bridge arm current, and further obtains the voltage and input power of the sub-module AC port to realize the control of the energy storage type MMC. The specific process is as follows:

[0024] The current deviation value is obtained by subtracting the actual measured value from the bridge arm current reference value; the bridge arm current reference value is

[0025]

[0026] The current on the output side of the MMC determines the fundamental frequency component in the arm current. Combining with Equation (3), the DC power difference between the input side and the output side of the MMC determines the DC component in the arm current. PR control is performed according to the arm current deviation value to obtain the modulation wave of the sub-module output voltage. While the second series arm participates in the control of the energy storage unit, it also participates in the arm power control. The modulation wave of its insulated gate bipolar transistor T3 is added to the modulation wave of the sub-module output voltage to obtain the modulation wave of the insulated gate bipolar transistor T1. The switching signal of the insulated gate bipolar transistor T2 is opposite to that of the insulated gate bipolar transistor T1, and the switching signals of the two together constitute the control signal of the first series arm.

[0027] The control strategy for the sub-module of the present invention includes three operating states, which are specifically as follows:

[0028] The first operating state: The insulated gate bipolar transistor T1 and the insulated gate bipolar transistor T4 are turned on, and the insulated gate bipolar transistor T2 and the insulated gate bipolar transistor T3 are turned off. At this time, the sub-module is in the input mode, the AC port transmits power to the two DC ports, the battery E balances the DC power, the sub-module capacitor C absorbs the ripple power, and the current of the decoupling inductor L rises in the positive current direction.

[0029] The second operating state: The insulated gate bipolar transistor T1 and the insulated gate bipolar transistor T3 are turned on, and the insulated gate bipolar transistor T2 and the insulated gate bipolar transistor T4 are turned off. At this time, the sub-module is in the bypass mode, the current of the decoupling inductor L charges the battery E, and there is no power change in the sub-module capacitor C.

[0030] The third operating state: The insulated gate bipolar transistor T2 and the insulated gate bipolar transistor T4 are turned on, and the insulated gate bipolar transistor T1 and the insulated gate bipolar transistor T3 are turned off. At this time, the sub-module is in the bypass mode, the battery E is in an open circuit state, and the sub-module capacitor C charges the decoupling inductor L.

[0031] The sub-module operates in the order of the first operating state, the second operating state, the first operating state, and the third operating state.

[0032] For the energy storage type MMC topology and its control method proposed by the present invention, on the one hand, an energy storage unit is added to the traditional full-bridge sub-module, which has a power decoupling function. The energy storage element can balance the DC power difference between the input side and the output side of the MMC, protect the active and reactive power balance of the system, and improve the power quality. On the other hand, the topology using the reuse of power devices saves the hardware cost and improves the topology compactness. Description of the Drawings

[0033] Figure 1 It is the structural topology diagram of the modular multilevel converter.

[0034] Figure 2 It is the power distribution block diagram of the MMC.

[0035] Figure 3 It is the double closed-loop control block diagram of the sub-module voltage and current.

[0036] Figure 4 It is the closed-loop control block diagram of the arm current.

[0037] Figure 5 It is the operation diagram of the sub-module working mode.

[0038] Figure 6 It is the waveform diagram of the inverter output voltage.

[0039] Figure 7 It is the diagram of the sub-module battery current.

[0040] Figure 8 It is the FFT analysis diagram of the sub-module battery power. Specific implementation manners

[0041] The implementation method of the present invention will be described in detail below in combination with the embodiments and the drawings:

[0042] The main circuit topology diagram of the energy storage type MMC of the present invention is as Figure 1 shown, which is composed of upper and lower two arms. Each arm is composed of an arm inductor and N sub-modules with the same structure connected in series. One end of the arm inductor is connected to the bus, and the other end is connected to the input end of the first sub-module. The output end of the first sub-module is connected to the input end of the second sub-module, and so on. The output end of the (N - 1)th sub-module is connected to the input end of the Nth sub-module, and the output end of the Nth sub-module is connected to the output side of the MMC. The upper and lower two arms are symmetric in structure. The output end of the first sub-module of the lower arm is connected to the input end of the second sub-module, and so on. The output end of the (N - 1)th sub-module of the lower arm is connected to the input end of the Nth sub-module, and the output end of the Nth sub-module is connected to one end of the arm inductor of the lower arm. The other end of the arm inductor of the lower arm is connected to the negative pole of the DC bus. The sub-module includes a first series arm, a second series arm and an energy storage unit.

[0043] The first series bridge arm includes an insulated gate bipolar transistor T1 and an insulated gate bipolar transistor T2, a diode D1 and a diode D2. The insulated gate bipolar transistor T1 and the diode D1 are anti-parallel connected, the emitter of the insulated gate bipolar transistor T1 is connected to the anode of the diode D1. The insulated gate bipolar transistor T2 and the diode D2 are anti-parallel connected, the emitter of the insulated gate bipolar transistor T2 is connected to the anode of the diode D2. The anode of the diode D1 and the cathode of the diode D2 are commonly connected to the input terminal of the sub-module. The second series bridge arm includes an insulated gate bipolar transistor T3 and an insulated gate bipolar transistor T4, a diode D3 and a diode D4. The insulated gate bipolar transistor T3 and the diode D3 are anti-parallel connected, the collector of the insulated gate bipolar transistor T3 and the cathode of the diode D3 are commonly connected to the cathode of the diode D1. The insulated gate bipolar transistor T4 and the diode D4 are anti-parallel connected, the emitter of the insulated gate bipolar transistor T4 and the anode of the diode D4 are commonly connected to the anode of the diode D2. The anode of the diode D3 and the cathode of the diode D4 are commonly connected to the output terminal of the sub-module. The input terminal and the output terminal of the sub-module form the AC port of the sub-module. The energy storage unit includes a battery E, a sub-module capacitor C and a decoupling inductor L. The positive pole of the battery E is connected to the cathode of the diode D3, the negative pole of the battery E and the positive pole of the sub-module capacitor C are commonly connected to one end of the decoupling inductor L. The other end of the decoupling inductor L is connected to the output terminal of the sub-module. The negative pole of the sub-module capacitor C is connected to the anode of the diode D4. The two ends of the battery E form the first DC port, and the two ends of the sub-module capacitor C form the second DC port.

[0044] Figure 2 It is the power distribution block diagram of the MMC. The energy storage type MMC sub-module has two DC ports. When the output of the new energy is affected by the environment and the DC input power is inconsistent with the power required by the AC side, the input power of the sub-module is controlled by controlling the current flowing through the arm inductor, and the current flowing through the battery E and the sub-module capacitor C is controlled by controlling the current flowing through the decoupling inductor L, so as to decouple the DC component and the AC component in the input power of the sub-module. The battery E as the first DC port can absorb the excess DC power provided by the input side of the MMC, or make up the DC power gap of the input side of the MMC; the sub-module capacitor C as the second DC port absorbs the ripple power in the MMC arm. Ignoring the influence of the arm inductor and the sub-module decoupling inductor on the power, the power relationship in the energy storage type MMC is

[0045]

[0046] Among them, the subscript x = a, b, c represents three phases of a, b and c, P Px and P Nx are the instantaneous powers of the upper and lower bridge arms respectively, which are evenly distributed to the sub-modules in the arm. u Px and i Px represent the voltage and current of the upper bridge arm respectively, u Nxand i Nx respectively represent the voltage and current of the upper bridge arm, I dc is the DC current of the bridge arm, U dc is the DC voltage at the input side of the MMC, E x and I x are the amplitudes of the output voltage and output current respectively. ω, θ x and are the angular frequency, initial phase angle and power factor angle of the output voltage respectively. P sm is the input power of the sub-module AC port, P E and P C are the powers of the battery E and the sub-module capacitor C respectively.

[0047] The voltage fluctuation of the sub-module capacitor C is relatively small compared to the voltage mean value and can be ignored. Therefore, the power P E of the battery E and the power P C of the sub-module capacitor C are respectively

[0048]

[0049] It is stipulated that the direction from the collector node of the insulated gate bipolar transistor T1 and the insulated gate bipolar transistor T3 to the positive electrode of the battery E is the positive direction of the current i E on the battery E, the direction from the negative electrode of the battery E to the sub-module capacitor C is the positive direction of the current i C on the sub-module capacitor C, and the direction from the negative electrode of the battery E to the decoupling inductor L is the positive direction of the current i L on the decoupling inductor L. According to Kirchhoff's current law, i E , i C and i L have the mathematical relationship

[0050] i L =i E -i C (3)

[0051] Combining formulas (1), (2) and (3), the currents flowing through the battery E and the sub-module capacitor C can be obtained as

[0052]

[0053] Combining equation (4), by controlling the current i L in the decoupling inductor L, the currents on the battery E and the sub-module capacitor C are controlled, and thus the powers on the battery E and the sub-module capacitor C are controlled. Taking the upper bridge arm as an example, the current i L of the decoupling inductor L is specifically calculated as

[0054]

[0055] Among them, N is the number of single-arm sub-modules. The DC part of the arm power is balanced by the battery E, and the ripple part is absorbed by the sub-module capacitor C, thereby realizing power decoupling control.

[0056] The control block diagram based on the sub-module capacitor voltage and the decoupling inductor current is as Figure 3 shown. The outer loop is the sub-module capacitor voltage control. The deviation between the sub-module capacitor voltage reference value and the actual measured value is used as the input of the voltage outer loop to keep the average voltage of the sub-module capacitor constant. The input of the voltage outer loop passes through a PI link to obtain the DC bias in the reference value of the current inner loop input. After adding it to the calculated current ripple, the reference current value on the decoupling inductor L is obtained. After subtracting the actual measured value from this current reference value, it passes through a PR link to obtain the modulation wave of the insulated gate bipolar transistor T3. After comparing the modulation wave with the triangular carrier wave, the switching signal of the insulated gate bipolar transistor T3 is obtained. The switching signal of the insulated gate bipolar transistor T4 is opposite to that of the insulated gate bipolar transistor T3. The switching signals of both together constitute the control signal of the second series arm.

[0057] The control block diagram of the energy storage type MMC arm current is as Figure 4 shown. According to the arm current reference value, the sub-module output voltage modulation wave is obtained, and combined with the energy storage unit modulation wave, the switching signal of the full-bridge MMC is generated to realize the control of the energy storage type MMC. The specific process is as follows:

[0058] The current deviation value is obtained by subtracting the actual measured value from the arm current reference value. The arm current reference value is

[0059]

[0060] The MMC output side current determines the fundamental frequency component in the arm current. Combining with Equation (3), the DC power difference between the MMC input side and the output side determines the DC component in the arm current. According to the arm current deviation value, PR control is performed to obtain the sub-module output voltage modulation wave. While the second series arm participates in the energy storage unit control, it also participates in the arm power control. The modulation wave of its insulated gate bipolar transistor T3 is added to the sub-module output voltage modulation wave to obtain the modulation wave of the insulated gate bipolar transistor T1. The switching signal of the insulated gate bipolar transistor T2 is opposite to that of the insulated gate bipolar transistor T1. The switching signals of both together constitute the control signal of the first series arm.

[0061] The working mode of the energy storage type MMC sub-module is as Figure 5 shown, including the following steps:

[0062] Step 1: Insulated gate bipolar transistor T1 and insulated gate bipolar transistor T4 are turned on, and insulated gate bipolar transistor T2 and insulated gate bipolar transistor T3 are turned off. At this time, the sub-module is in the first working state and in the input mode. The AC port transmits power to the two DC ports. The battery E balances the DC power, the sub-module capacitor C absorbs the ripple power, and the current of the decoupling inductor L rises in the positive current direction.

[0063] Step 2: Insulated gate bipolar transistor T1 and insulated gate bipolar transistor T3 are turned on, and insulated gate bipolar transistor T2 and insulated gate bipolar transistor T4 are turned off. At this time, the sub-module is in the second working state and in the bypass mode. The current of the decoupling inductor L charges the battery E, and there is no power change in the sub-module capacitor C.

[0064] Step 3: The sub-module is in the first working state, and the AC port continues to transmit power to the two DC ports. The current of the decoupling inductor L rises in the positive current direction.

[0065] Step 4: Insulated gate bipolar transistor T2 and insulated gate bipolar transistor T4 are turned on, and insulated gate bipolar transistor T1 and insulated gate bipolar transistor T3 are turned off. At this time, the sub-module is in the third working state and in the bypass mode. The battery E is in an open circuit state, and the sub-module capacitor C charges the decoupling inductor L.

[0066] The voltage waveform of the output side of the MMC is as Figure 6 shown. The single closed-loop control of the MMC arm current indirectly controls the output voltage of the AC side through the arm current, which reflects the effectiveness of the MMC arm current control strategy.

[0067] The battery current of the sub-module is as Figure 7 shown. The power of the input side of the MMC is greater than the power required by the output side. Therefore, the battery current is positive, and the battery E absorbs the excess active power. The FFT analysis of the sub-module battery power is as Figure 8 shown. The battery E mainly absorbs the excess active power, and the ripple power is borne by the sub-module capacitor, which reflects the effectiveness of the double closed-loop control of the sub-module voltage and inductor current and the power decoupling effect.

[0068] Matters not described in this invention are applicable to the prior art.

Claims

1. A storage-type MMC topology and its control method. The MMC is composed of three-phase six arms. Each phase consists of an upper and a lower arm. Each arm is composed of an arm inductor and N sub-modules with the same structure connected in series. One end of the arm inductor of the upper arm is connected to the positive pole of the DC bus, and the other end is connected to the input end of the first sub-module. The output end of the first sub-module is connected to the input end of the second sub-module, and so on. The output end of the (N - 1)th sub-module is connected to the input end of the Nth sub-module. The output end of the Nth sub-module of the upper arm and the input end of the first sub-module of the lower arm are connected to the output side of the MMC. The output end of the first sub-module of the lower arm is connected to the input end of the second sub-module, and so on. The output end of the (N - 1)th sub-module of the lower arm is connected to the input end of the Nth sub-module. The output end of the Nth sub-module is connected to one end of the arm inductor of the lower arm, and the other end of the arm inductor of the lower arm is connected to the negative pole of the DC bus. The sub-module includes a first series arm, a second series arm, and an energy storage unit, characterized in that, The first series arm includes an insulated gate bipolar transistor T1, an insulated gate bipolar transistor T2, a diode D1, and a diode D2. The insulated gate bipolar transistor T1 and the diode D1 are anti-parallel connected. The emitter of the insulated gate bipolar transistor T1 and the anode of the diode D1 are connected. The insulated gate bipolar transistor T2 and the diode D2 are anti-parallel connected. The emitter of the insulated gate bipolar transistor T2 and the anode of the diode D2 are connected. The anode of the diode D1 and the cathode of the diode D2 are commonly connected to the input end of the sub-module. The second series arm includes an insulated gate bipolar transistor T3, an insulated gate bipolar transistor T4, a diode D3, and a diode D4. The insulated gate bipolar transistor T3 and the diode D3 are anti-parallel connected. The collector of the insulated gate bipolar transistor T3 and the cathode of the diode D3 are commonly connected to the cathode of the diode D1. The insulated gate bipolar transistor T4 and the diode D4 are anti-parallel connected. The emitter of the insulated gate bipolar transistor T4 and the anode of the diode D4 are commonly connected to the anode of the diode D2. The anode of the diode D3 and the cathode of the diode D4 are commonly connected to the output end of the sub-module. The input end and the output end of the sub-module form the AC port of the sub-module. The energy storage unit includes a battery E, a sub-module capacitor C, and a decoupling inductor L. The positive pole of the battery E is connected to the cathode of the diode D3. The negative pole of the battery E and the positive pole of the sub-module capacitor C are commonly connected to one end of the decoupling inductor L. The other end of the decoupling inductor L is connected to the output end of the sub-module. The negative pole of the sub-module capacitor C is connected to the anode of the diode D4. The two ends of the battery E form the first DC port, and the two ends of the sub-module capacitor C form the second DC port.

2. A storage-type MMC as described in claim 1, characterized in that, Adopt the single closed-loop control strategy for the arm current to control the arm current, and then control the input power of the sub-module; adopt the double closed-loop control strategy for the sub-module voltage and current to control the currents on the battery E and the sub-module capacitor C, so as to decouple the DC component and the AC component in the input power of the sub-module. The battery E, as the first DC port, can absorb the excess DC power provided by the input side of the MMC or make up for the DC power gap on the input side of the MMC; the sub-module capacitor C, as the second DC port, absorbs the ripple power in the MMC arm. Ignoring the influence of the arm inductor and the sub-module decoupling inductor on the power, the power relationship in the energy storage type MMC is Among them, the subscript x = a, b, c represents three phases a, b, and c, and P Px and P Nx are the instantaneous powers of the upper and lower bridge arms respectively, which are evenly distributed to the sub-modules of the bridge arm. u Px and i Px represent the voltage and current of the upper bridge arm respectively. u Nx and i Nx represent the voltage and current of the upper bridge arm respectively. I dc is the DC current of the bridge arm, and U dc is the DC voltage at the input side of the MMC. E x and I x are the amplitudes of the output voltage and output current respectively. ω, θ x and are the angular frequency, initial phase angle, and power factor angle of the output voltage respectively. P sm is the input power of the AC port of the sub-module, and P E and P C are the powers of the battery E and the sub-module capacitor C respectively. The voltage fluctuation of the sub-module capacitor C is relatively small compared to the voltage mean value and can be ignored. Therefore, the power P of the battery E E and the power P of the sub-module capacitor C C are respectively It is stipulated that the direction from the collector nodes of the insulated gate bipolar transistor T1 and the insulated gate bipolar transistor T3 to the positive electrode of the battery E is the positive direction of the current i on the battery E E ; the direction from the negative electrode of the battery E to the sub-module capacitor C is the positive direction of the current i C on the sub-module capacitor C; the direction from the negative electrode of the battery E to the decoupling inductor L is the positive direction of the current i L on the decoupling inductor L. According to Kirchhoff's current law, i E , i C and i L have the following mathematical relationship i L = i E -i C (3) Combining formulas (1), (2) and (3), the currents flowing through the battery E and the sub-module capacitor C can be obtained as Combined with Equation (4), by decoupling the current \(i\) in the inductor \(L\) L , the currents on the battery \(E\) and the sub-module capacitor \(C\) are controlled, and then the powers on the battery \(E\) and the sub-module capacitor \(C\) are controlled. Taking the upper bridge arm as an example, the current \(i\) of the decoupling inductor \(L\) L The specific calculation is as follows where N is the number of sub-modules in a single arm. The DC part of the arm power is balanced by the battery E, and the ripple part is absorbed by the sub-module capacitor C, thus realizing the power decoupling control.

3. A sub-module voltage and current double closed-loop control strategy as described in claim 2, characterized in that, This strategy consists of two control loops. The outer loop is the sub-module capacitor voltage control. The deviation between the sub-module capacitor voltage reference value and the actual measured value is used as the input of the voltage outer loop to keep the average voltage of the sub-module capacitor constant; the input of the voltage outer loop passes through the PI link to obtain the DC bias in the input reference value of the current inner loop. Adding it to the calculated current ripple amount to obtain the current reference value on the decoupling inductor L. After subtracting the actual measured value from this current reference value and passing through the PR link, the modulation wave of the insulated gate bipolar transistor T3 is obtained. After comparing the modulation wave with the triangular carrier wave, the switching signal of the insulated gate bipolar transistor T3 is obtained. The switching signal of the insulated gate bipolar transistor T4 is opposite to that of the insulated gate bipolar transistor T3, and the switching signals of the two together constitute the control signal of the second series arm.

4. A single closed-loop control strategy for the arm current as described in claim 2, characterized in that, Control the arm power by controlling the arm current, and further obtain the voltage and input power of the sub-module AC port to realize the control of the energy storage type MMC. The specific process is as follows: Obtain the current deviation value by subtracting the actual measured value from the arm current reference value; the arm current reference value is The current on the output side of the MMC determines the fundamental frequency component in the arm current. Combining formula (3), the DC power difference between the input side and the output side of the MMC determines the DC component in the arm current. Perform PR control according to the arm current deviation value to obtain the sub-module output voltage modulation wave. While the second series arm participates in the control of the energy storage unit, it also participates in the arm power control. The modulation wave of its insulated gate bipolar transistor T3 is added to the sub-module output voltage modulation wave to obtain the modulation wave of the insulated gate bipolar transistor T1. The switching signal of the insulated gate bipolar transistor T2 is opposite to that of the insulated gate bipolar transistor T1, and the switching signals of the two together constitute the control signal of the first series arm.

5. The energy storage type MMC sub-module according to claim 1, characterized in that The control method includes the following steps: Step 1: Insulated gate bipolar transistor T1 and insulated gate bipolar transistor T4 are turned on, and insulated gate bipolar transistor T2 and insulated gate bipolar transistor T3 are turned off. At this time, the sub-module is in the first working state and in the input mode. The AC port transmits power to the two DC ports. The battery E balances the DC power, the sub-module capacitor C absorbs the ripple power, and the current of the decoupling inductor L rises in the positive current direction. Step 2: Insulated gate bipolar transistor T1 and insulated gate bipolar transistor T3 are turned on, and insulated gate bipolar transistor T2 and insulated gate bipolar transistor T4 are turned off. At this time, the sub-module is in the second working state and in the bypass mode. The current of the decoupling inductor L charges the battery E, and there is no power change in the sub-module capacitor C. Step 3: The sub-module is in the first working state, and the AC port continues to transmit power to the two DC ports. The current of the decoupling inductor L rises in the positive current direction. Step 4: Insulated gate bipolar transistor T2 and insulated gate bipolar transistor T4 are turned on, and insulated gate bipolar transistor T1 and insulated gate bipolar transistor T3 are turned off. At this time, the sub-module is in the third working state and in the bypass mode. The battery E is in an open circuit state, and the sub-module capacitor C charges the decoupling inductor L. The sub-module operates in the order of the first working state, the second working state, the first working state, and the third working state.

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