CN106452133A - Core converter for building bipolar DC microgrid and control method of core converter - Google Patents

Abstract

The invention discloses a core converter for constructing a bipolar DC microgrid, which is used as an interface converter between a high-voltage AC distribution network and a low-voltage bipolar DC microgrid. It consists of three parts including input filter, rectification stage and isolation stage; modular design. The rectification stage adopts H-bridge cascade structure, and the modular isolation stage adopts an improved three-port active full-bridge structure, and the corresponding phases of the output buses are connected in parallel, so as to realize the matching of the grid voltage and power on both sides of the converter. The invention also discloses the control method of the converter: the rectification stage adopts vector control of zero-sequence voltage injection, and the isolation stage adopts voltage and current double closed-loop decoupling phase-shifting control. The invention enables energy to be freely exchanged between two power grids and between two buses of a bipolar direct current microgrid. Realize the functional integration of traditional power frequency transformers, rectifiers, and voltage balancers for bipolar DC microgrid construction, thereby optimizing the construction method of bipolar DC microgrids and improving the power density of construction equipment.

Description

A core converter for constructing a bipolar DC microgrid and its control method

technical field

The invention relates to the technical field of power electronic converters, in particular to an interface converter for constructing a bipolar DC microgrid.

Background technique

The DC microgrid with bipolar power supply has advantages in many aspects, such as photovoltaic, fuel cell and other new energy access efficiency, data center and computer room power supply efficiency, load and micro-source access flexibility, power supply reliability, and system grounding. Show innate advantages. However, its construction has particularities compared with other types of DC microgrids, which are specifically manifested in the structure of the microgrid neutral line and the balance of the neutral line voltage. Due to the different access methods of loads and micro-sources in the bipolar DC microgrid, it will cause unbalanced bipolar mounted power, which will lead to unbalanced neutral line voltage. For example, one load is connected between the positive and negative busbars of the microgrid, and the other load is connected between the positive busbar and the neutral line of the microgrid.

In the construction of traditional bipolar DC microgrid, two-level or three-level rectifiers are usually used to form the DC bus; the capacitor banks are connected in series and then merged into the DC bus, and the midpoint of the capacitor series leads to the neutral line, finally forming a bipolar DC bus This construction method usually requires the use of a dedicated voltage equalizer to solve the power imbalance problem of bipolar loading; the use of industrial frequency transformers to achieve the isolation of bipolar DC micro-grid and AC distribution network and the voltage and power matching problem. It can be seen that the current construction scheme of bipolar DC microgrid has the following deficiencies: (1) Two-level or three-level rectifiers do not have the ability to balance the neutral line voltage, or the balance ability is weak; (2) It is necessary to set up an independent (3) The industrial frequency transformer occupies a large area, is bulky, has large loss and high noise, which has become a major obstacle to the realization of high power density and high efficiency power conversion system.

Contents of the invention

The problem to be solved:

In order to overcome the above-mentioned difficulties and shortcomings in the construction of bipolar DC microgrids, the present invention proposes a new converter topology and control method that integrates power frequency transformers, rectifiers, and voltage balancers. The core converter of the interconnection interface between the network and the low-voltage bipolar DC microgrid, thereby optimizing the construction method of the bipolar DC microgrid and improving the power density of the equipment.

Technical solutions:

A core converter for constructing a bipolar DC microgrid, characterized in that: ① its topology includes a three-part structure of an input filter, a rectification stage and an isolation stage; ② a modular design;

The input filter adopts an inductive filter. The input end of the input filter is connected to the medium-voltage AC distribution network, and the output end is connected to the AC side of the rectification stage;

The AC side of the rectification stage is connected to the output end of the input filter; the DC side of the rectification stage is connected to the primary side of the isolation stage; the secondary side of the isolation stage is connected to the low-voltage bipolar DC microgrid;

The rectification stage includes a rectifier and an output filter, and each phase of the rectifier adopts a cascaded structure of N H-bridges, where N is a natural number. The output filter adopts a capacitive filter and is connected in parallel on the DC side of each H-bridge;

The isolation stage is composed of 3N units with the same structure, and N is the same as the number of H-bridges in the rectification stage. Each unit consists of a primary structure, a high-frequency isolation transformer, and a secondary structure. The primary and secondary structures of the isolation level unit are three-port structures formed by three H-bridge topologies;

The primary side of the isolation level refers to the primary structure of all isolation level units; the secondary side of the isolation level refers to the secondary structure of all isolation level units;

The H-bridge DC side of the primary structure of the isolation level unit is connected in parallel with the DC side of the rectification level H-bridge, and the AC side is connected to the primary side of the high-frequency isolation transformer;

The H-bridge 1 AC side of the secondary structure of the isolation level unit is connected to the secondary winding 1 of the high-frequency isolation transformer, the positive pole of the H-bridge 1 DC side is the positive pole of the bipolar DC bus, and the negative pole is the neutral line of the bipolar DC bus; The H-bridge 2 AC side of the secondary structure of the isolation level unit is connected to the secondary winding 2 of the high-frequency isolation transformer, the positive pole of the H-bridge 2 DC side is the neutral line of the bipolar DC bus, and the negative pole is the negative pole of the bipolar DC bus. The H-bridge 1 and H-bridge 2 DC sides of the secondary structure of the isolation level unit are connected in parallel with DC voltage stabilizing capacitors, thus forming a bipolar DC bus structure;

The high-frequency isolation transformer has a single primary winding and double secondary winding structure;

The modular design refers to a module composed of one H-bridge of the rectification stage and one unit of the isolation stage. The AC side of the module is the AC side of the rectification-level H-bridge, and the DC side of the module is the positive pole, neutral line and negative pole of the bipolar DC bus formed by the secondary structure of the isolation level unit;

N modules are connected in series on the AC side, and N is a natural number to form a single-phase structure on the AC side. Three single-phase structures are connected in a star shape to form a three-phase structure and form the AC side of the converter; The neutral line and the negative pole are connected in parallel to form the DC side of the bipolar busbar structure of the converter. Thereby finally forming the core converter structure proposed by the present invention;

The AC side of the core converter is connected to a medium-voltage AC distribution network, and the DC side of the core converter is connected to a low-voltage bipolar DC microgrid;

The semiconductor switches in the rectification stage and the isolation stage circuit all adopt fully controlled devices such as IGBT or MOSFET.

Compared with the prior art, a core converter for constructing a bipolar DC microgrid provided by the present invention has the following beneficial technical effects:

(1) The new topology realizes the functional integration of power frequency transformers, two-level or three-level full-bridge topology rectifiers, and voltage balancers, which reduces the number of devices to build bipolar DC microgrids and simplifies system construction;

(2) High-frequency transformers are used instead of power-frequency transformers to realize the isolation of the AC distribution network and the bipolar DC microgrid. At the same time, the high frequency of the transformer reduces the volume of equipment for constructing the bipolar DC microgrid;

(3) The isolation-level module of the new topology adopts a three-port structure, which not only provides energy exchange paths for the power grids on both sides of the converter, but also provides energy exchange paths for the two-pole busbars in the DC microgrid;

(4) Modular design facilitates system maintenance and expansion.

Description of drawings

Fig. 1 is a three-phase topological diagram of a converter proposed by the present invention.

Fig. 2 is a topology diagram of a converter module proposed by the present invention.

Figure 3 is a topological diagram of the module isolation level.

Figure 4 is an H-bridge topology diagram.

Figure 5 is a waveform diagram of the primary and secondary voltages of the high-frequency transformer at the isolation level of the module.

Fig. 6 is a control block diagram of the rectification stage of the converter of the present invention.

Figure 7 is a block diagram of the module isolation level control.

detailed description

A converter provided by the invention is used to construct a bipolar DC microgrid, and serves as an interface converter between a high-voltage AC distribution network and a low-voltage bipolar DC microgrid. The rectification stage structure of the cascaded H bridge is adopted to match the high voltage of the AC distribution network; the high-frequency isolation stage with 3N three-port structure is used to realize the isolation of the power grid on both sides of the converter; the 3N three-port secondary structure of the isolation stage corresponds to The output phases are connected in parallel to realize the construction of the bipolar DC microgrid and the voltage and power matching of the bipolar DC microgrid. The present invention will be described in further detail below in conjunction with the accompanying drawings and specific methods.

Figure 1 shows the three-phase topology of the converter proposed by this method, including: input filter 2 and converter 3 . The input side of the input filter 2 is connected to the medium-voltage AC distribution network 1 , and the output side of the input filter 2 is connected to the input port of the converter 3 . The output end of the converter 3 forms a bipolar DC microgrid, which is connected to the bipolar DC microgrid 7 .

The input filter 2 adopts an inductance filter, and can use three single-phase inductors or one three-phase inductor, so the input filter 2 has three input ports and three output ports.

The converter 3 includes three identical single-phase structures: an A-phase structure 4 , a B-phase structure 5 , and a C-phase structure 6 . Each phase input port includes two input wires, and the output port includes three output wires. The input line 8 of the A-phase structure 4 is connected to the corresponding port on the output side of the input filter 2, the input line 9 of the B-phase structure 5 is connected to the corresponding port on the output side of the input filter 2, and the input line 10 of the C-phase structure 6 is connected to the input The output side of the filter 2 is connected to the corresponding port. The input line 11 of the A-phase structure 4 is connected with the input line 12 of the B-phase structure 5 and the input line 13 of the C-phase structure 6 . Thus, the star connection on the AC side of the converter 3 is formed. The three output lines of A-phase structure 4, B-phase structure 5, and C-phase structure 6 are connected in parallel in parallel to form a bipolar DC microgrid positive bus 14, a bipolar DC microgrid neutral line 15, and a bipolar DC microgrid Grid negative busbar 16.

The single-phase structure of the converter 3 is composed of N modules with the same structure, as shown in FIG. 2 . The module is composed of a module rectification stage 17 and a module isolation stage 18 . The module rectification stage 17 adopts a single-phase H-bridge structure, and a filter capacitor 24 is connected in parallel on the DC side. The AC side of the module rectification stage 17 forms a cascaded H-bridge structure with other modules, serving as the rectification stage of the converter 3 .

Taking a certain module in A-phase structure 4 as an example, if the module is the first module in A-phase structure 4, the input line 19 on the AC side of the module is the input line 8 of A-phase structure 4, and the AC side of the module The input line 20 is connected to the AC side input line 19 of the second module in the A-phase structure 4; The AC side input line 20 of the N-1th module is connected, and the AC side input line 20 of the Nth module in the A-phase structure 4 is the input line 11 of the A-phase structure 4; The kth module, k is an integer, and the value range is [2, N-1], then the AC side input line 19 of the kth module is connected to the AC side input line 20 of the k-1th module in the A-phase structure 4, and the The AC-side input line 20 of the k module is connected to the AC-side input line 19 of the k+1th module in the A-phase structure 4, thereby forming the AC-side structure of the converter.

The output DC side port of the module includes three output lines, which are positive pole 21 , neutral line 22 , and negative pole 23 . The positive poles 21 of the output lines of all the modules in the converter 3 are connected in parallel to form the positive pole bus 14 of the bipolar DC microgrid; the neutral lines 22 of the output lines of all the modules in the converter 3 are connected in parallel to form the bipolar DC microgrid The neutral line 15 and the negative poles 23 of the output lines of all the modules in the converter 3 are connected in parallel to form the negative busbar 16 of the bipolar DC microgrid.

The structure of the module isolation stage 18 is shown in FIG. 3 , which consists of a primary structure 25 of the module isolation stage 18 , a high frequency transformer 26 , a secondary structure 27 of the module isolation stage 18 and an output filter 30 . The primary structure 25 of the module isolation stage 18 adopts an H-bridge structure, the input terminal is connected to the output terminal of the module rectification stage 17, and the output terminal of the primary structure 25 of the module isolation stage 18 is connected to the primary winding of the high-frequency transformer 26; the high-frequency transformer 26 is a three-winding A transformer has one primary winding and two secondary windings. The secondary structure 27 of the module isolation stage 18 is composed of an H bridge 28 and an H bridge 29 . The AC side of the H-bridge 28 is connected to the secondary winding 1 of the high-frequency transformer 26 , and the AC side of the H-bridge 29 is connected to the secondary winding 2 of the high-frequency transformer 26 . The positive pole of the DC side of the H-bridge 28 is the positive pole 21 of the module output DC side port; the negative pole of the DC side of the H-bridge 28 is connected to the positive pole of the DC side of the H-bridge 29 to form the module center line 22; the negative pole of the DC side of the H-bridge 29 is the module output The negative pole 23 of the DC side port. The output filter 30 is composed of a capacitor connected in parallel between the positive pole 21 and the neutral line 22 on the output DC side of the module, and a capacitor connected in parallel between the negative pole 23 and the neutral line 22 on the output DC side of the module.

The H-bridge structure is shown in Figure 4. It consists of four fully-controlled power semiconductor devices S1, S2, S3, and S4. The fully-controlled power semiconductor devices can use IGBTs or MOSFETs. Take IGBTs as an example. The collectors of S1 and S3 are connected together, and the emitters of S2 and S4 are connected together to form the H-bridge DC side; the emitter of S1 is connected with the collector of S2, and the emitter of S3 is connected with the collector of S4. Together, they form the AC side of the H-bridge.

The stable operation and function realization of the converter of the present invention, the control needs to realize:

(1) The unit power factor rectification of the rectification stage of the converter completes the conversion from high-voltage AC to DC. At the same time, the voltages of the parallel filter capacitors 24 on the DC side of all module rectification stages 17 are required to be equal after rectification, and equal to the set value of the DC voltage of the module rectification stage 17, that is, U _{dc_ref} , as shown in formula (1-1):

(1-1)

_{Wherein Udc_a_1} , ..., Udc_a_N _is the first module of converter A phase, ..., the voltage of the parallel filter capacitor 24 of each module rectification stage 17 DC side in the _Nth module; _{Wherein Udc_b_1} , ..., Udc_b_ _N is the first module of the B phase of the converter, ..., the voltage of the parallel filter capacitor 24 of the rectification stage 17 DC side of each module in the Nth module; where Udc_c_1 , ..., _{Udc_c_N} is the first of the _C phase of the converter modules, ..., the voltage of the parallel filter capacitor 24 on the DC side of each module rectification stage 17 in the Nth module.

Define the first module of phase A, ..., the voltage sum of the parallel filter capacitor 24 of each module rectification stage 17 DC side in the _{Nth module is Udc_a_sum} ; the first module of B phase, ..., each module rectification stage 17 in the Nth module The sum of the voltages of the parallel filter capacitors 24 on the DC side is Udc_b_sum ; the _sum of the voltages of the rectifier stages 17 of the modules in the first module of the C phase, ..., the Nth module, the voltages of the parallel filter capacitors 24 on the DC side _{is Udc_c_sum} , and the relationship satisfies Formula (1-2):

(1-2)

The rectification stage control of the converter adopts vector control, the control block diagram is shown in Figure 6, and the control method is as follows:

The first step is to obtain the angle θ of the A-phase voltage in the medium-voltage AC distribution network 1

Sampling the three-phase voltages u _a , ub , uc in the _AC medium voltage distribution network 1 , through the coordinate transformation from three-phase static to two-phase rotation, the voltage reactive component u _q in the two-phase rotating coordinate system is _obtained , and the transformation The formula is shown in (1-3):

(1-3)

The calculated u _q is passed through the PI controller to obtain the angular velocity correction amount Δω, and the correction amount is added to 100π to obtain the current angular velocity ω _s . After integrating ω _s , take the modulo value with 2π (when the angle is greater than 2π, let It is 0, so as to ensure that the obtained angle information is in the range of 0~2π), and then feed back the angle θ to the angle used in the coordinate transformation calculation of formula (1-3), forming a closed control loop. The angle θ is the angle of the A-phase voltage in the current medium-voltage AC distribution network 1 .

The second step is to decouple the current input from the AC side of the converter

The current i _as of the input line 8 , the current i _bs of the input line 9 , and the current i _cs of the input line 10 of the converter are sampled. Through the coordinate transformation from three-phase stationary to two-phase rotating (3S/2R decoupling transformation), the three-phase current is transformed into active current _id and reactive current i _q in the rotating coordinate system . The transformation formula is shown in formula (1-4), and the angle in the formula adopts the value of θ obtained in the first step.

(1-4)

The third step is to control the current input on the AC side of the converter

Calculate A, B, C three-phase first module, ..., the average value U dc_ave_sum of the voltages U _{dc_a_sum} , U _{dc_b_sum} , U _{dc_c_sum} of the parallel filter capacitor 24 of the rectification stage 17 DC side of each module in the Nth module U _{dc_ave_sum} , N* After subtracting U _{dc_ave_sum} from U _{dc_ref and} passing through the PI controller, the given value id _{_ ref} of the input current active component of the rectifier stage of the converter is obtained. Subtract _{id _ ref} from the input active current id of the converter obtained in the second step, and get the active component v _d of the AC output voltage of the converter after passing through the PI controller _; subtract "0" from the obtained in the second step The input reactive current i _q of the converter is obtained through the PI controller, and the reactive component v _q of the AC output voltage of the converter is obtained. Through the coordinate transformation from two-phase rotation to three-phase stationary (3S/2R decoupling inverse transformation), the transformation formula is shown in formula (1-5). Thus the three-phase voltage modulation waves u _as , u _bs , u _cs required to be synthesized on the AC side of the converter are obtained.

(1-5)

The fourth step is to inject the zero-sequence voltage of the modulated wave, and balance the total voltage of the DC side of each phase of the converter A, B, and C

Subtract N* U _{dc_ref from} the first module of phase A, ..., the sum of the voltage U _{dc_a_sum} of the parallel filter capacitor 24 on the DC side of the rectification stage 17 of each module in the Nth module, after passing through the PI controller, the power compensation amount of phase A is obtained P _{a o} ; N* U _{dc_ref} minus the first module of phase B, ..., the sum of the voltages U _{dc_b_sum} of the parallel filter capacitor 24 on the DC side of the rectification stage 17 of each module in the Nth module, after passing through the PI controller, the B phase is obtained Power compensation amount P _{b o} . Using Equation (1-6), Equation (1-7) and Equation (1-8):

(1-6)

(1-7)

(1-8)

The amplitude U ₀ and angle θ ₀ of the zero-sequence voltage to be injected into the modulated wave can be obtained: thus the injected zero-sequence voltage u ₀ can be obtained. add u ₀ to the original modulation wave u _as , u _bs , u _cs respectively to obtain the modified waveform u _{as_c} , u _{bs_c} , u _{cs_c} of the three-phase voltage modulation wave to be synthesized on the ac side of the rectifier stage of the converter The AC side of the rectifier stage outputs such a voltage waveform, which can realize unity power factor rectification, and can also balance the total voltage of the DC side of each phase of the converter A, B, and C, and realize formula (1-2).

The fifth step, module rectification stage 17 DC side voltage balance control

In the A phase of the converter of the present invention, the voltage setting value U _{dc_ref} of the parallel filter capacitor 24 of the module rectification stage 17 is subtracted from the kth (k is an integer, the value range [1, N-1]) module rectification stage 17 The voltage u _a _ _{dc_k} of the parallel filter capacitor 24, after passing through the PI controller, obtains the coefficient ε _{a_k} of the output current modulation wave of the rectification stage 17 of the kth module, and multiplies this value by u _{as_c} /N to obtain the final converter The modulation wave u a_k_m output by the AC side of the rectifier stage 17 of the kth module of the A phase; the modulation wave u _{a_N_m} output by the AC side of the rectifier stage 17 of the _Nth module of the final converter A phase can be obtained by using formula (1-9 ) . Afterwards, the modulated wave is converted into a PWM signal to control the H-bridge semiconductor device of the rectification stage 17 of the module by means of SPWM, wherein the triangle carrier angles of the SPWM are different from each other by π/N.

(1-9)

Similarly, the modulated waves ub_x_m and _{uc_x_m} output by the AC side of the rectification stage 17 of each module of the B-phase and C-phase converters can be _obtained (here x is an integer, and the value range is [1, N]). Through this control, the DC voltage of the module rectification stage 17 can be equalized and equal to the set value U _{dc_ref} , which satisfies the control requirement of formula (1-1).

Through the above five steps, the control of the rectification stage of the converter can be finally realized.

(2) The isolation level of the converter is required to be realized through control: first, the energy is isolated and transmitted between the primary structure 25 of the module isolation level 18 and the secondary structure 27 of the module isolation level 18; second, the energy is transmitted in the secondary structure of the module isolation level 18 Three, the voltage U _po between the positive busbar 14 of the bipolar DC microgrid and the neutral line 15 of the bipolar DC microgrid, the positive neutral line 15 of the bipolar DC microgrid and the bipolar DC microgrid The voltage U _on between the negative bus bars 16 of the DC microgrid is equal and equal to the set value, that is, U _po = U _on = U _line _ _ref . Among them, U _line _ _ref is the setting value of the bipolar DC microgrid bus voltage. Thus, the converter of the present invention has the functions of a power frequency transformer and a voltage balancer.

As shown in FIG. 5 , the output waveforms of the H-bridge AC side of the primary structure of the module isolation stage 18, the AC side of the H-bridge 28 and the H-bridge 29 of the secondary structure are symmetrical AC square wave voltages with positive and negative half cycles each accounting for 50%. δ ₁ is the angle between the AC side voltage u ₁ of the primary structure H-bridge of the module isolation level 18 and the AC side voltage u ₂ of the secondary structure H-bridge 28 of the module isolation level 18 . δ ₂ is the angle between the AC side voltage u ₁ of the primary structure H-bridge of the module isolation level 18 and the AC side u ₃ of the secondary structure H-bridge 29 of the module isolation level 18 . Where δ ₁ , δ ₂ ∈ [-π/2, π/2]; module isolation level 18 high-frequency transformer secondary side 1 pair of primary winding turns ratio is n ₂ , module isolation level 18 high-frequency transformer secondary side 2 pairs The turn ratio of the primary winding is n ₃ respectively; u ₂ and u ₃ are the voltage u ₂ ' of the secondary side 1 of the high frequency transformer and the voltage u ₃ ' of the secondary side 2 of the high frequency transformer converted to the primary side, the conversion formula is as follows Formula (1-10) shows:

(1-10)

The control of the isolation stage of the converter adopts phase-shift control, double closed-loop voltage and current, and the trigger pulses of all isolation stage modules are the same. The control block diagram is shown in Figure 7, and the control method is as follows:

The first step, the voltage loop control

_After subtracting the DC side voltage U _po of the H-bridge 28 on the secondary side of the module isolation stage 18 from the bipolar DC micro - grid bus voltage set value U _{line_ref} , after passing through the PI controller, the output current of the H-bridge 28 DC side is obtained. Fixed value i _{dc_ 28 _ref} ; bipolar DC micro-grid bus voltage set value U _line _ _ref subtracts the DC side voltage Uon of the secondary side H bridge 29 of the module isolation level 18, and obtains the DC side of the H bridge 29 through the PI controller The given value of output current i _{dc_ 29 _ref} .

The second step, current loop control

i _{dc_ 28 _ref} subtracts the H-bridge 28 DC side output current idc_ ₂₈ , and the coupling phase shift angle Δδ ₁ of the H-bridge 28 is obtained through the PI controller; i _{dc_ 29 _ref} subtracts the H-bridge 29 DC side output current idc_ ₂₉ , The coupling phase shift angle Δδ ₂ of the H-bridge 29 is obtained through the PI controller.

The third step, decoupling control and phase shift angle acquisition

Δδ ₁ and Δδ ₂ are decoupled by formula (1-11), and the phase shift angles δ ₁ and δ ₂ of the AC side voltage waveforms of the module isolation level 18 secondary H bridge 28 and the module isolation level 18 secondary H bridge 29 are obtained, G ₁₁ , G ₁₂ , G ₂₁ , and G ₂₂ in the decoupling matrix can be obtained from formulas (1-12), (1-13), (1-14), and (1-15). Where f _s is the operating frequency of the high-frequency transformer with module isolation level 18; L ₁ is the leakage inductance of the primary winding of the high-frequency transformer, L ₂ is the leakage inductance of the secondary winding 1 of the high-frequency transformer, and L ₃ is the secondary winding 2 of the high-frequency transformer leakage inductance. L ₁₂ is the equivalent converted inductance between the primary winding of the high-frequency transformer and the secondary winding 1, L ₁₃ is the equivalent converted inductance between the primary winding of the high-frequency transformer and the secondary winding 2, and L ₂₃ is the high-frequency transformer The equivalent converted inductance between the secondary winding 1 and the secondary winding 2, the conversion formula is shown in formula (1-16).

(1-11)

(1-12)

(1-13)

(1-14)

(1-15)

(1-16)

According to the phase shift angles δ ₁ and δ ₂ , the voltages of the H-bridge 28 and the AC side of the H-bridge 29 on the secondary side of the isolation stage module are adjusted, so as to realize the energy transfer of the isolation stage and the stability of the bipolar DC microgrid bus voltage.

Claims (4)

1. A core converter for constructing a bipolar DC microgrid, which is used for bidirectional energy interconnection between a medium-voltage AC distribution network and a low-voltage bipolar DC microgrid, characterized in that: The converter includes three parts: input filter, rectification stage and isolation stage; adopts modular structure design; The input end of the input filter is connected to the medium-voltage AC distribution network, and the output end is connected to the AC side of the rectification stage; the DC side of the rectification stage is connected to the primary side of the isolation stage; the DC side of the rectification stage is connected to the primary side of the isolation stage; The secondary side of the isolation stage is connected to the low-voltage bipolar DC microgrid; The input filter adopts an inductive filter; The inductive filter can use three single-phase inductors or one three-phase inductor; The rectification stage is a three-phase structure, including a rectifier and an output filter; The AC side of the rectifier is the AC side of the rectification stage, which is connected in star form, and each phase adopts a cascade structure of N H bridges, where N is a natural number; the output filter adopts a capacitor filter, connected in parallel On the DC side of each H-bridge of the rectifier; The isolation stage is composed of 3N units of the same structure, and N is the same as the number of H bridges in the rectification stage; Each unit consists of a primary structure, a high-frequency isolation transformer, and a secondary structure. The primary and secondary structures of the isolation level unit are three-port structures formed by three H-bridge topologies; The primary side of the isolation level refers to the primary structure of all isolation level units; the secondary side of the isolation level refers to the secondary structure of all isolation level units; The H-bridge DC side of the primary structure of the isolation level unit is connected in parallel with the DC side of the rectification level H-bridge, and the AC side is connected to the primary side of the high-frequency isolation transformer; The H bridge 1 AC side of the secondary structure of the isolation level unit is connected to the secondary winding 1 of the high frequency isolation transformer, the positive pole of the H bridge 1 DC side of the secondary structure of the isolation level unit is the positive pole of the bipolar DC bus, and the negative pole The neutral line of the extremely bipolar DC bus; the AC side of the H bridge 2 of the secondary structure of the isolation level unit is connected to the secondary winding 2 of the high frequency isolation transformer, and the positive pole of the DC side of the H bridge 2 of the secondary structure of the isolation level unit is The neutral line of the bipolar DC bus, the negative pole of the negative pole of the bipolar DC bus; The H-bridge 1 and H-bridge 2 DC sides of the secondary structure of the isolation level unit are connected in parallel with DC voltage stabilizing capacitors, thus forming a bipolar DC bus structure; The high-frequency isolation transformer has a single primary winding and double secondary winding structure; The modular design refers to a module composed of an H-bridge of the rectification stage and a unit of the isolation stage; The AC side of the module is the AC side of the rectification-level H-bridge, and the DC side of the module is the positive pole, neutral line and negative pole of the bipolar DC bus formed by the secondary structure of the isolation level unit; The DC sides of all modules are connected in parallel according to the positive, neutral and negative poles to form the DC side of the bipolar busbar structure of the core converter; Thus, the core converter structure proposed by the present invention is finally formed. 2. The control method of the converter according to claim 1, characterized in that: The control target of the rectification stage of the converter: realize the rectification of the unit power factor of the AC side; the capacitor voltage of each H bridge DC side of the rectification _{stage is equal to the set value Udc_ref} ; The control target of the isolation level of the converter: realize the isolated transfer of energy between the primary structure of the module isolation level and the secondary structure of the module isolation level; realize the transfer of energy between the two H-bridges of the secondary structure of the module isolation level; bipolar The voltage U _po between the positive busbar of the DC microgrid and the neutral line of the bipolar DC microgrid, and the voltage U _on between the neutral line of the bipolar DC microgrid and the negative busbar of the bipolar DC microgrid are equal and equal to the set value, That is, U _po = U _on = U _{line_ref} ; Where U _{line_ref} is the set value of the bipolar DC microgrid bus voltage. 3. The control method of the converter according to claim 2, characterized in that: the control of the rectification stage adopts the following five steps: (1) Obtain the angle of phase A voltage in the medium voltage AC distribution network Sampling the three-phase voltages u _a , ub , uc in the AC medium voltage distribution _network , through the coordinate transformation from three-phase static to two-phase rotation, the voltage reactive component u _q in the two-phase rotating coordinate system is _obtained , the transformation formula As shown in (1-3): (1-3) The calculated u _q is passed through the PI controller to obtain the angular velocity correction amount Δω, and the correction amount is added to 100π to obtain the current angular velocity ω _s . After integrating ω _s , take the modulo value with 2π (when the angle is greater than 2π, let It is 0, so as to ensure that the obtained angle information is in the range of 0~2π), and then feed back the angle θ to the angle used in the coordinate transformation calculation of formula (1-3), forming a closed control loop; The angle θ is the angle of the A-phase voltage in the current medium-voltage AC distribution network 1; (2) Decoupling the AC side input current of the converter sampling the current i _as , i _bs , i _cs of the input line of the converter; By transforming the formula (1-4), the three-phase current is changed into active current i _d and reactive current i _q in the rotating coordinate system; The angle in the formula adopts the value of θ obtained in step (1); (1-4) (3) Control the current input on the AC side of the converter Calculate the sum of the voltages U _{dc_a_sum} , U _{dc_b_sum} , and U _{dc_c_sum} of the parallel filter capacitors on the rectification stage H-bridge DC side of each module in the first module to the Nth module of each phase of the converter A, B, and C respectively, and then calculate the three The average value Udc_ave_sum , N* Udc_ref minus _{Udc_ave_sum passes} through the PI controller to obtain the given value _{id_ref of the} active component of the input current of the rectifier stage of the converter ; Subtract the input active current id of the converter obtained by step (2) from _{id _ ref} , and get the active component v _d of the AC output voltage of the converter after passing through the PI controller _; subtract "0" by the step (2 ) to obtain the input reactive current i _q of the converter, and obtain the reactive component v _q of the AC output voltage of the converter after passing through the PI controller; By transforming the formula (1-5), the three-phase voltage modulation waves u _as , u _bs , u _cs required to be synthesized on the AC side of the converter are obtained; (1-5) (4) Balance the total voltage of the DC side of each phase A, B, and C in the rectifier stage of the converter to realize the unit power factor rectification of the AC side of the converter Subtract the sum U _{dc_a_sum} of the voltages U dc_a_sum of the parallel filter capacitors on the DC side of the rectification stage H-bridge of each module in the first module to the Nth module of phase A from N* U _{dc_ref} , and after passing through the PI controller, get the power compensation amount of phase A P _{a o} ; N* U _{dc_ref} minus the sum U _{dc_b_sum} of the voltages of the parallel filter capacitors on the DC side of the rectification stage H-bridge of each module in the first module to the Nth module of the B-phase, after passing through the PI controller, the B-phase power compensation amount P _{b o} ; Using Equation (1-6), Equation (1-7) and Equation (1-8): (1-6) (1-7) (1-8) Obtain the zero-sequence voltage amplitude U ₀ and angle θ ₀ that need to be injected into the modulated wave, so as to obtain the injected zero-sequence voltage u ₀ ; Add u ₀ and the modulation waves u _as , u _bs , u _cs obtained in step (3) respectively to obtain the modified waveforms u _{as_c} , u _{bs_c} of the three-phase voltage modulation waves that need to be synthesized on the ac side of the rectifier stage of the converter , u _{cs_c} ; The rectifier stage of the converter outputs the voltage waveforms of u _{as_c} , u _{bs_c} and u _{cs_c} for the three phases A, B and C respectively, which can realize unit power factor rectification; (5) Module rectification stage DC side voltage balance control In phase A of the converter, the voltage setting value Udc_ref of the parallel filter capacitor of the module rectification stage _is subtracted from the parallel filter filter of the kth (k is an integer, the value range [1, N-1]) module rectification stage The voltage u _a _ _{dc_k} of the capacitor, after passing through the PI controller, obtains the coefficient ε _{a_k} of the output current modulation wave of the rectifier stage of the kth module, and multiplies this value by u _{as_c} /N to obtain the kth of the A phase of the final converter The modulation wave u _{a_k_m} and u _{as_c} output by the AC side of the module rectification stage are obtained by step (4); the modulation wave u _{a_N_m} output by the AC side of the Nth module rectification stage of the final converter phase A can be obtained by using formula (1-9) ; Afterwards, the modulated wave is converted into a PWM signal by means of SPWM to control the H-bridge semiconductor device of the rectification stage of the module, wherein the triangular carrier angles of the H-bridges acting on the rectification stage of the A-phase module differ from each other by π/N; (1-9) Similarly, the modulated waves u _{b_x_m} and u _{c_x_m} output by the AC side of the rectification stage of each module of the phase B and phase C of the converter can be obtained (here x is an integer, and the value range is [1, N]); In this way, the DC voltages of the module rectification stages are equal and equal to the set value U _{dc_ref} , that is, the capacitor voltages on the DC side of each H-bridge are equal to the set value. 4. The control method of the converter according to claim 2, characterized in that: The output waveforms of the H-bridge AC side of the primary structure of the module isolation level, and the AC side of the H-bridge 1 and H-bridge 2 of the secondary structure are symmetrical AC square wave voltages with positive and negative half cycles accounting for 50% each; δ ₁ is the angle between the AC side voltage u ₁ of the H-bridge primary structure of the module isolation level and the AC side voltage u ₂ of the H-bridge 1 secondary structure of the module isolation level; δ ₂ is the angle between the AC side voltage u ₁ of the primary structure H bridge of the module isolation level and the AC side u ₃ of the secondary structure H bridge 2 of the module isolation level; Among them, δ ₁ , δ ₂ ∈[-π/2,π/2]; the turn ratio of the secondary side 1 of the module isolation high-frequency transformer to the primary winding is n ₂ , and the secondary side 2 of the module isolation high-frequency transformer is to the primary side The turn ratio of the winding is n ₃ respectively; u ₂ and u ₃ are the voltage u ₂ ' of the secondary side 1 of the high-frequency transformer and the voltage u ₃ ' of the secondary side 2 of the high-frequency transformer converted to the primary side, and the conversion formula is as follows: 1-10) as shown: (1-10) The control of the isolation stage of the converter adopts phase-shift control, voltage and current double closed-loop; The trigger pulse is the same for all isolation level modules; The isolation level control uses the following five steps: (1) Voltage loop control After subtracting the DC side voltage U _po of H-bridge 1 on the secondary side of the module isolation stage from the bipolar DC micro-grid bus voltage set value U _line _ _ref , after passing through the PI controller, the given output current of the H-bridge 1 DC side is obtained Value i _{dc_ 28 _ref} ; bipolar DC micro-grid bus voltage set value U _line _ _ref subtracts the DC side voltage U _on of the module isolation stage secondary side H-bridge 2, and obtains the H-bridge 2 DC side output through the PI controller Current given value i _{dc_ 29 _ref} ; (2) Current loop control i _{dc_ 28 _ref} subtracts the H-bridge 1 DC side output current idc_ ₂₈ , and the coupling phase shift angle Δδ ₁ of the H-bridge 1 is obtained through the PI controller; i _{dc_ 29 _ref} subtracts the H-bridge 2 DC side output current idc_ ₂₉ , Obtain the coupling phase shift angle Δδ ₂ of the H-bridge 1 through the PI controller; (3) Decoupling control and phase shift angle acquisition Δδ ₁ and Δδ ₂ are decoupled by formula (1-11), and the phase shift angles δ ₁ and δ ₂ of the voltage waveforms on the AC side of the module isolation level secondary side H-bridge 1 and module isolation level secondary side H-bridge 2 are obtained, decoupling G ₁₁ , G ₁₂ , G ₂₁ , and G ₂₂ in the matrix can be obtained from formulas (1-12), (1-13), (1-14), and (1-15); Where f _s is the working frequency of the module isolation high-frequency transformer; L ₁ is the leakage inductance of the primary winding of the high-frequency transformer, L ₂ is the leakage inductance of the secondary winding 1 of the high-frequency transformer, and L ₃ is the leakage inductance of the secondary winding 2 of the high-frequency transformer feel; L ₁₂ is the equivalent converted inductance between the primary winding of the high-frequency transformer and the secondary winding 1, L ₁₃ is the equivalent converted inductance between the primary winding of the high-frequency transformer and the secondary winding 2, and L ₂₃ is the high-frequency transformer The equivalent converted inductance between the secondary winding 1 and the secondary winding 2, the conversion formula is shown in formula (1-16); (1-11) (1-12) (1-13) (1-14) (1-15) (1-16) According to the phase shift angles δ ₁ and δ ₂ , the voltages on the AC side of H-bridge 1 and H-bridge 2 on the secondary side of the isolation-level module are adjusted, so as to realize the energy transfer of the isolation level and the stability of the bus voltage of the bipolar DC microgrid.