CN112421602B - DC transformer with true bipolar off-grid operation capability and control method - Google Patents

Abstract

The application discloses a direct current transformer with true bipolar off-grid operation capability and a control method thereof, wherein the direct current transformer comprises an input stage converter, an intermediate isolation transformer and an output stage converter; the controller is connected to the DC transformer and controls the DC current input into the DC transformer. The topology of the direct current transformer can lead out a low-voltage true bipolar direct current port, can supply power for a plurality of direct current loads with different voltage levels, and improves the reliability of a power grid and the flexibility of access of power equipment; different from the existing mode of adopting two sets of direct current transformers to run in series and parallel or adopting a true bipolar running balancer, the low-voltage direct current true bipolar parallel off-grid running can be realized by adopting a single direct current transformer; the two types of systems are injected with direct current at the source side, so that the direct current magnetic flux bias of the medium-high frequency alternating current coupling transformer is eliminated, the influence of direct current injection on the alternating current transformer under the power balance control is avoided, and the feasibility and the effectiveness of the equipment are improved.

Description

A DC transformer with true bipolar and off-grid operation capability and its control method

technical field

The technical field that the present invention relates to, in particular, relates to a DC transformer with true bipolar and off-grid operation capability and a control method.

Background technique

In recent years, with the gradual deterioration of the environment and the increasing scarcity of fossil energy, renewable energy has made great progress in recent years. The penetration of distributed power generation is of great significance to reduce the pressure on environmental protection, save investment in power transmission and transformation, and improve the reliability of power supply. DC transmission and DC power distribution technology is one of the effective technical means to solve the grid connection of distributed power generation.

There are currently two mainstream DC power distribution architectures, unipolar and bipolar. The unipolar DC network provides a single DC voltage level between the two lines. Its structure is simple and easier to control, but its reliability is poor. When the DC bus fails, all loads will face the risk of losing power. Bipolar tributary networks provide two DC voltage levels between the three-wire configuration, positive/negative to ground and positive to negative. Among them, the pseudo-bipolar structure works similar to the unipolar structure, and it is also impossible to realize the positive and negative poles with asymmetrical load operation; while the true bipolar DC network can also operate under unipolar faults, and because it has two different levels , can provide more choices for load and distributed power supply. In addition, the true bipolar network has some similarities with the traditional three-phase four-wire AC system, that is, the voltage between the positive and negative poles is analogous to the line voltage in the three-phase four-wire system, and the voltage of each pole is Analogous to phase voltages, this similarity to AC systems facilitates system operation, improvement, and control scheme design. The true bipolar structure has the widest applicability. Combined with the double-bus power supply structure, it can ensure the high power supply reliability of the DC distribution network and the flexibility of DC load access.

DC transformer is the key equipment to realize DC transmission and distribution system busbar interconnection, voltage conversion, power transmission and electrical isolation, or DC source-load-storage, etc. to connect to the DC grid. For low voltage DC (Low Voltage DC, LVDC) ports, true bipolar DC ports with unbalanced operation capability can meet the needs of low voltage users for multi-voltage levels, power safety and reliable power supply, while conventional DC transformers can only provide single Pole or pseudo-bipolar low-voltage DC ports. The current technology realizes true bipolar operation of DC through two sets of DC transformers or a power balance device on the true bipolar bus. This method will increase equipment investment costs and operational reliability. The present invention provides a low-cost, high-efficiency, high-reliability realization scheme of a direct current transformer with true bipolar and off-grid operation capability through topological structure design and control on a single set of direct current transformer.

Contents of the invention

The purpose of this section is to outline some aspects of embodiments of the invention and briefly describe some preferred embodiments. Some simplifications or omissions may be made in this section, as well as in the abstract and titles of this application, to avoid obscuring the purpose of this section, abstract and titles, and such simplifications or omissions should not be used to limit the scope of the invention.

In view of the above existing problems in the control of the positive and negative power balance of the transformer, the present invention is proposed.

Therefore, the technical problem solved by the present invention is: the power balance control problem when the bipolar asymmetric operation is performed when the grid is connected and off-grid, and the safe and stable operation of the true bipolar DC system is ensured.

In order to solve the above-mentioned technical problems, the present invention provides the following technical solutions: the DC transformer includes an input-stage converter, an intermediate isolation transformer and an output-stage converter, and direct current is injected into the input-stage converter and the output-stage converter; the controller is connected to The DC transformer controls the DC current in the input DC transformer.

As a preferred solution of the DC transformer with true bipolar and off-grid operation capability described in the present invention, wherein: the DC transformer also includes, the DC transformer is divided into a grid-connected DC transformer and an off-grid DC transformer device.

As a preferred solution of the DC transformer with true bipolar and off-grid operation capability described in the present invention, wherein: the controller includes, the controller includes a grid-connected controller and an off-grid controller, wherein The grid-connected controller is connected to the grid-connected DC transformer, and the off-grid controller is connected to the off-grid DC transformer.

As a preferred solution of the DC transformer with true bipolar and off-grid operation capability described in the present invention, wherein: the grid-connected controller includes a power outer loop controller and a current inner loop controller, and the control input The DC current of the grid-connected controller, the current inner-loop controller has two current inner-loop controllers 1 and 2, respectively, and is connected with the input stage converter and the intermediate isolation transformer , to control the direct current injected into it.

As a preferred solution of the DC transformer with true bipolar and off-grid operation capability described in the present invention, wherein: the off-grid controller includes a voltage controller and a current controller, and controls input to the off-grid DC current of the DC transformer, wherein the DC current is injected at the output stage converter of the off-grid DC transformer.

As a preferred solution of the DC transformer with true bipolar and off-grid operation capability according to the present invention, wherein: the input-stage converter includes a full-bridge structure.

As a preferred solution of the DC transformer with true bipolar and off-grid operation capability according to the present invention, wherein: the output stage converter includes, the output stage converter adopts a three-level structure or a half-bridge structure.

As a preferred solution of the DC transformer with true bipolar and off-grid operation capability described in the present invention, the working principle of the DC transformer includes:

The DC transformer adopts the traditional single-phase-shift modulation method, and the transmission power is controlled by controlling the phase-shift angle Φ between the primary and secondary bridge arm voltages v _i and v _o , and the ratio of the phase-shift angle to π is set as the phase-shift ratio The empty ratio is D, that is Set the primary and secondary turn ratio of the transformer as N, and V ₁ /N=V ₂ ; in the case of true bipolar symmetrical operation, that is, the DC transformer when the secondary inductor current i _L2 does not contain a DC component, under this working condition , calculate the size of the inductor current in the time period [t ₂ , t ₃ ], and obtain the expression of i _L2 in half a switching period when working in the forward direction, which is:

By injecting DC into the secondary side of the transformer, the positive and negative energy interactions can be realized, thereby balancing the bipolar power and realizing the stable true bipolar asymmetric operation of the DC transformer. When the DC component _Idc2 is injected into the inductor current of the secondary side, its The expression is i′ _L2 (t)=i _L2 (t)+I _dc2 ,

From the average power output by the positive and negative poles:

P ₂ −P ₁ =V _{2 Idc2}

Among them: P ₁ is the average power output by the positive pole, and P ₂ is the average power output by the negative pole; P _u is used to represent the unbalanced power of the two poles, that is, P _u = P ₂ -P ₁ , the above formula can be rewritten as:

P _u = V ₂ I _dc2

As a preferred solution of the control method of the DC transformer with true bipolar and off-grid operation capability described in the present invention, wherein: the current reference value of the primary and secondary DC injection of the transformer is set and compared with the DC component of the actual inductor current Make a comparison to obtain the current deviation; perform PI control on the deviation to obtain the reference value of the DC injection voltage and make the positive and negative power balance of the transformer through PWM modulation, and at the same time there is no DC bias flux; use the controller to determine the reference value and deviation The adjustment of the value realizes the control of the transformer.

As a preferred scheme of the control method of the DC transformer with true bipolar and off-grid operation capability according to the present invention, wherein: realizing the control of the transformer includes: the transformer includes a grid-connected true dual-polarity transformer based on DC injection. pole DC transformer and off-grid true bipolar DC transformer, wherein controlling the grid-connected true bipolar DC transformer based on DC injection includes: controlling the unbalanced power of the transformer through an unbalanced power outer loop controller, Use the I regulator to adjust the unbalanced power and the controlled deviation value to generate a DC reference value to realize the control of the bipolar unbalanced power of the transformer; controlling the off-grid true bipolar DC transformer includes: using current The controller adjusts the DC injection of the primary side of the transformer to achieve the balance of the DC components of the primary and secondary inductor currents, thereby eliminating the DC bias of the transformer, using the deviation between the voltage reference value and the actual value of the voltage controller, and then using the PI regulator to generate a shift The phase shift duty cycle under phase modulation realizes the control of the output voltage of the transformer.

Beneficial effects of the present invention: the DC transformer topology can lead to a low-voltage true bipolar DC port, which can supply power to multiple DC loads of different voltage levels, improving the reliability of the power grid and the flexibility of power equipment access; different from the existing Using two sets of DC transformers in series or using a true bipolar operating balancer, a single DC transformer can be used to realize low-voltage DC true bipolar and off-grid operation; the grid-connected true bipolar DC transformer injects DC, providing bipolar unbalanced power, which can realize bipolar unbalanced and stable operation; the off-grid true bipolar DC transformer controls the output voltage by adjusting the duty cycle, ensuring the stability of the output voltage; both types of systems are Injecting DC at the source side eliminates the DC flux bias of the medium and high frequency AC coupling transformer, avoids the influence of DC injection on the AC transformer due to power balance control, and improves the feasibility and effectiveness of the equipment.

Description of drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the accompanying drawings that need to be used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For Those of ordinary skill in the art can also obtain other drawings based on these drawings without any creative effort. in:

Fig. 1 is a schematic structural diagram of a DC transformer with true bipolar and off-grid operation capability described in the first embodiment of the present invention;

Fig. 2 is a block diagram of a dual closed-loop controller of a DC transformer with true bipolar and off-grid operation capability described in the first embodiment of the present invention;

Fig. 3 is a block diagram of a grid-connected controller of a DC transformer with true bipolar and off-grid operation capability described in the first embodiment of the present invention;

Fig. 4 is a block diagram of an off-grid controller of a DC transformer with true bipolar and off-grid operation capability described in the first embodiment of the present invention;

5 is a working waveform diagram of a DC transformer with true bipolar and off-grid operation capability without DC injection according to the first embodiment of the present invention;

Fig. 6 is a working waveform diagram of a DC transformer with true bipolar and off-grid operation capability with DC injection according to the first embodiment of the present invention;

7 is a schematic flowchart of a DC transformer control method with true bipolar and off-grid operation capability described in the first embodiment of the present invention;

Fig. 8 is a topological structure diagram of a grid-connected DC transformer in Scheme 1 of a DC transformer with true bipolar and off-grid operation capability and a control method described in the fourth embodiment of the present invention;

9 is an output curve diagram of unbalanced power P _u in scheme 1 of a DC transformer with true bipolar and off-grid operation capability and a control method described in the fourth embodiment of the present invention;

Fig. 10 shows the total power P o of the output side, the positive power P ₁ and the negative power P ₂ in the scheme 1 of a DC transformer with true bipolar off-grid _operation capability and its control method described in the fourth embodiment of the present invention output graph;

Fig. 11 shows the DC component I _dc1 of the primary side inductor current and the DC component of the secondary side inductor current in Scheme 1 of a DC transformer with true bipolar and off-grid operation capability and its control method described in the fourth embodiment of the present invention I _dc2 curve graph;

Fig. 12 is a topological structure diagram of an off-grid DC transformer in Scheme 2 of a DC transformer with true bipolar and off-grid operation capability and a control method according to the fourth embodiment of the present invention;

Fig. 13 shows the total voltage v _o on the output side, the positive output voltage v _C1 and the negative output voltage in the scheme 2 of a DC transformer with true bipolar off-grid operation capability and its control method described in the fourth embodiment of the present invention v _C2 curve;

Fig. 14 shows the DC component I _dc1 of the primary side inductor current and the DC component of the secondary side inductor current in the scheme 2 of a DC transformer with true bipolar and off-grid operation capability and its control method described in the fourth embodiment of the present invention I _dc2 curve graph;

Fig. 15 is a curve diagram of bipolar voltage difference in Scheme 2 of a DC transformer with true bipolar off-grid operation capability and its control method according to the fourth embodiment of the present invention.

Detailed ways

In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation modes of the present invention will be described in detail below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Example. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative efforts shall fall within the protection scope of the present invention.

In the following description, a lot of specific details are set forth in order to fully understand the present invention, but the present invention can also be implemented in other ways different from those described here, and those skilled in the art can do it without departing from the meaning of the present invention. By analogy, the present invention is therefore not limited to the specific examples disclosed below.

Second, "one embodiment" or "an embodiment" referred to herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. "In one embodiment" appearing in different places in this specification does not all refer to the same embodiment, nor is it a separate or selective embodiment that is mutually exclusive with other embodiments.

The present invention is described in detail in conjunction with schematic diagrams. When describing the embodiments of the present invention in detail, for the convenience of explanation, the cross-sectional view showing the device structure will not be partially enlarged according to the general scale, and the schematic diagram is only an example, which should not limit the present invention. scope of protection. In addition, the three-dimensional space dimensions of length, width and depth should be included in actual production.

At the same time, in the description of the present invention, it should be noted that the orientation or positional relationship indicated by "upper, lower, inner and outer" in the terms is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention. The invention and the simplified description do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and thus should not be construed as limiting the present invention. In addition, the terms "first, second or third" are used for descriptive purposes only, and should not be construed as indicating or implying relative importance.

Unless otherwise specified and limited in the present invention, the term "installation, connection, connection" should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection or an integrated connection; it can also be a mechanical connection, an electrical connection or a direct connection. A connection can also be an indirect connection through an intermediary, or it can be an internal communication between two elements. Those of ordinary skill in the art can understand the specific meanings of the above terms in the present invention in specific situations.

Example 1

Referring to Figures 1 to 7, it is the first embodiment of the present invention, which provides a DC transformer with true bipolar and off-grid operation capability, including: a DC transformer 100 includes an input stage converter 101, an intermediate isolation Transformer 102 and output stage converter 103 inject DC current at input stage converter 101 and intermediate isolation transformer 102, and output by output stage converter 103; current.

The DC transformer 100 also includes a grid-connected DC transformer 100a and an off-grid DC transformer 100b, the input-stage converter 101 adopts a full-bridge structure, and the output-stage converter 103 adopts a three-level structure or a half-bridge structure to provide true bipolar Type low-voltage DC port, refer to Figure 1 for two types of DC transformers, where v _i is the AC square wave voltage on the input side of the transformer, v _o is the AC square wave voltage on the output side, and L ₂ is the leakage of the transformer equivalent to the secondary side Inductance, i _L2 is the transformer secondary inductor current, V ₁ is the DC voltage source on the input side, V ₂ is the bipolar low-voltage DC bus voltage on the output side, V _C1 and V _C2 are the capacitor voltages of the positive and negative poles of the low-voltage DC, respectively, and i _C1 and i _C2 are positive capacitor current and negative capacitor current respectively, _R1 and _R2 are positive load and negative load respectively, I _R1 and I _R2 are positive load current and negative load current respectively, V _2N is unipolar of low voltage DC port Rated DC voltage;

Further, the controller 200 includes, the controller 200 includes a grid-connected controller 201 and an off-grid controller 202, wherein the grid-connected controller 201 is connected to the grid-connected DC transformer 100a, and the off-grid controller 202 is connected to In the off-grid DC transformer 100b.

The grid-connected controller 201 includes a power outer-loop controller 201a and a current inner-loop controller 201b to control the DC current input to the grid-connected controller 201, and the current inner-loop controller 201b has two current inner-loop controllers, respectively The controller 1201b-1 and the current inner loop controller 2201b-2 are connected to the input-stage converter 101a and the output-stage converter 103a to control the DC current injected therein. Refer to FIGS. 2-3 for details.

Referring to FIG. 4, the off-grid controller 202 includes a voltage controller 202a and a current controller 202b to control the direct current input to the off-grid DC transformer 100b, wherein the DC current is transformed at the output stage of the off-grid DC transformer 100b device 103a injection.

The working principle of the DC transformer 100 includes that the DC transformer 100 is similar to DAB (Dual Active Bridge), and the DAB modulation method is similar. The traditional single-phase-shift modulation method is adopted, and the phase-shift angle between the primary and secondary bridge arm voltages v _i and v _o is controlled. The size of Φ is used to control the transmission power, and the ratio of the phase shift angle to π is set as the phase shift duty cycle as D, that is Set the primary and secondary turns ratio of the transformer as N, and V ₁ /N=V ₂ ; refer to Figure 5 for the working condition of the DC transformer 100 when the secondary side inductive current i _L2 does not contain a DC component during true bipolar symmetrical operation, In this case:

In the period [t ₂ , t ₃ ], the expression of i _L2 is:

because Then the inductor current at time _t3 is:

Similarly, the expression of i _L2 in [t ₃ , t ₅ ] period can be obtained as:

because Then from the above formula, the secondary inductor current at time _t5 can be obtained as:

i _L2 is symmetrical in positive and negative half switching periods, i _L2 (t ₅ )=-i _L2 (t ₂ ), according to the formula t ₃ , the inductor current can be obtained as follows:

Combining the above formulas, the expression of i _L2 in half a switching period during forward operation can be written as:

The expression of the average power of the positive output is:

Similarly, the expression of the average power output by the negative pole is:

By injecting DC into the secondary side of the transformer, the positive and negative energy interactions can be realized, thereby balancing the bipolar power and realizing the stable true bipolar asymmetric operation of the DC transformer. When the DC component _Idc2 is injected into the inductor current of the secondary side, its The expression is i′ _L2 (t)=i _L2 (t)+I _dc2 , and its working condition refers to Fig. 6,

At this time, the expression of the average power output by the positive electrode is:

The expression of the average power output by the negative pole is:

From the average power output by the positive and negative poles:

P ₂ −P ₁ =V _{2 Idc2}

Using P _u to represent the bipolar unbalanced power, that is, P _u = P ₂ -P ₁ , the above formula can be rewritten as:

P _u = V ₂ I _dc2

Example 2

Referring to FIG. 7, it is the second embodiment of the present invention. This embodiment is different from the first embodiment in that it provides a control method for a DC transformer with true bipolar and off-grid operation capability, including:

S1: Set the current reference value of the DC injection of the primary and secondary sides of the transformer and compare it with the DC component of the actual inductor current to obtain the current deviation;

S2: Perform PI control on the deviation, obtain the DC injection voltage reference value and perform PWM modulation so that there is no DC bias flux in the transformer;

S3: Use the controller to adjust the reference value and deviation value to realize the control of the transformer.

Among them: transformers include grid-connected true bipolar DC transformers based on DC injection and off-grid true bipolar DC transformers, wherein the control of grid-connected true bipolar DC transformers based on DC injection includes: through the unbalanced power outer loop The controller controls the unbalanced power of the transformer, uses the I regulator to adjust the unbalanced power and the controlled deviation value, generates a DC reference value, and realizes the control of the bipolar unbalanced power of the transformer.

Example 3

The third embodiment of the present invention analyzes the energy balance mechanism of the off-grid true bipolar DC transformer with an asymmetric load, and verifies that the bipolar voltage of the off-grid true bipolar DC transformer with an asymmetric load is self-balancing ability; the control of the off-grid true bipolar DC transformer includes: using the current controller to adjust the DC injection of the primary side of the transformer to achieve the balance of the DC components of the primary and secondary inductor currents, thereby eliminating the DC bias of the transformer, using The voltage controller uses the deviation between the voltage reference value and the actual value, and then uses the PI regulator to generate the phase-shift duty cycle under phase-shift modulation to realize the control of the output voltage of the transformer.

Assuming that the offset of the positive DC voltage on the low-voltage side relative to the unipolar rated DC voltage V _2N is ΔV, the actual DC voltages of the low-voltage positive and negative poles are:

In the positive half switching cycle, within [t ₂ , t ₃ ] period, the expression of i _L2 is:

because Then the inductor current at time _t3 is:

Similarly, the expression of i _L2 in [t ₃ , t ₅ ] period can be obtained as:

because Then from the above formula, the secondary inductor current at time _t5 can be obtained as:

Similarly, i _L2 in the negative half switching cycle can be calculated,

From i _L2 in half a switching cycle, it can be known that the initial inductor currents of two switching cycles are not equal, that is, the DC transformer cannot achieve steady-state operation when the bipolar voltage on the low-voltage side is unbalanced. When V _C1 >V _C2 , in The inductor current increases during a switching cycle. From Figure 1, it can be seen that the discharge current of the positive capacitor will gradually increase, while the charging current of the negative capacitor will gradually increase, so the two capacitors will gradually return to the same voltage value, that is, the DC transformer is on the low voltage side. With an asymmetrical load, it has bipolar voltage self-balancing capability;

Then analyze the steady-state operation of the true bipolar DC transformer with an asymmetric load, assuming that the positive pole is heavily loaded and the negative pole is lightly loaded, that is, I _R1 > I _R2 in Figure 1. When the load is asymmetrical, due to the low-voltage DC side bipolar The voltage is self-balanced, so the AC voltage waveform on the secondary side is still a square wave without DC bias, and the asymmetry of the load will be reflected in the DC bias of the secondary inductor current;

According to Kirchhoff's current law, at any time, the expression of the secondary inductor current is:

i _L2 (t)＝-[i _o1 (t)+i _o2 (t)]＝-[i _c1 (t)+I _R1 -i _c2 (t)-I _R2 ]

After a switching period is averaged, the secondary side inductor current only has a DC component, while the capacitor current is zero after the switching period is averaged, so after the switching period is averaged, the above formula is transformed into:

I _dc ＝-(I _R1 -I _R2 )＝ΔI _R

It can be obtained from the above formula that when the bipolar output side of the DC transformer has an asymmetrical load, the bipolar voltage has self-balancing capability and will not be affected, while the difference in the bipolar current will be reflected in the secondary inductor current, which will give The secondary inductor current introduces a DC bias equal to the difference between the bipolar load currents.

Example 4

Referring to Figures 8-12, it is the fourth embodiment of the present invention, which provides different schemes to illustrate the method of the present invention, so as to verify the real effect of the method.

Option 1: Based on the DC transformer structure shown in Figure 1, use MATLAB/Simulink software to build a grid-connected NPC-DAB system. Referring to Figure 8, simulate and verify this topology. The simulation parameters are shown in Table 1 below;

Table 1: Simulation parameters of grid-connected topology.

parameter value parameter value High Frequency Transformer Frequency 5000Hz Input side bus rated DC voltage 50V High Frequency Transformer Leakage Inductance 40uH Output side bus rated DC voltage 50V, 50V High Frequency Transformer Ratio 1:1

Under the working conditions shown in the above table, before the simulation time t=0.04s, the given reference value of unbalanced power P _u is 0, and after t=0.04s, the given reference value of unbalanced power P _u is 250W. Under the simulation parameters, the theoretical result is: the DC component I _dc2 of the secondary side of the transformer is 0 before t=0.04s, and is 5A after t=0.04s. The simulation results are shown in Figures 9-11.

Fig. 9 is the output curve of the unbalanced power P _u in Embodiment 1 of the present invention, wherein: before the simulation time t=0.04s, the unbalanced power is stable at about 0, and after t=0.04s, the unbalanced power is at 250W when reaching the steady state about;

Fig. 10 is the output curves of the total output power P _o , the positive power P ₁ and the negative power P ₂ in Embodiment 1 of the present invention, wherein: before the simulation time t=0.04s, the total power of the output side has been stable at about 590W; the simulation time Before t=0.04s, the positive power is stable at about 295W, after t=0.04s, the positive power is about 170W when reaching the steady state; before the simulation time t=0.04s, the negative power is stable at about 295W, after t=0.04s, When the steady state is reached, the negative power is about 420W;

Fig. 11 is the curve of the DC component _Idc1 of the primary side inductor current and the DC component _Idc2 of the secondary side inductor current in Embodiment 1 of the present invention, wherein: before the simulation time t=0.04s, the primary side DC component is stable at about 0, t= After 0.04s, the DC component of the primary side is around -5A when the steady state is reached; before the simulation time t=0.04s, the DC component of the secondary side is stable at around 0, and after t=0.04s, the DC component of the secondary side is at 5A when the steady state is reached about;

It can be seen that the simulation results of the accompanying drawings are consistent with the theoretical results, so the fast tracking of DC injection and unbalanced power in the primary and secondary sides of the transformer can be realized through the designed double closed-loop controller.

Scheme 2: Based on the DC transformer structure shown in Figure 1, use MATLAB/Simulink software to build an off-grid NPC-DAB system as shown in Figure 12, and perform simulation verification for this topology. The simulation parameters are shown in Table 2 below;

Table 2: Off-grid topology simulation parameters

parameter value parameter value High Frequency Transformer Frequency 5000Hz Input side bus rated DC voltage 50V High Frequency Transformer Leakage Inductance 40uH Output side bipolar rated DC voltage 50V, 50V High Frequency Transformer Ratio 1:1

Under the working conditions shown in the above table, before the simulation time t=0.04s, the given output voltage v reference value is 100V, after t=0.08s, the given output voltage v reference value is 105V; simulation time t=0.12s Before, the reference value of positive load _R1 is given as 20Ω, after t=0.12s, the reference value of positive load _R1 is given as 10Ω; before the simulation time t=0.12s, the reference value of negative load R2 is given as 2000Ω, t= After 0.12s, the reference value of the given negative load R ₂ is 1000Ω. Under the simulation parameters, the theoretical result is: the transformer secondary DC component I _dc2 is -2.475A before t=0.08s, -2.59A when t=0.08s～0.12s, and - after t=0.12s 5.19A, the simulation results are shown in Figures 13-15;

Figure 13 is the curves of the total output voltage v _o , the positive output voltage v _C1 and the negative output voltage v _C2 in Embodiment 2 of the present invention, wherein: before the simulation time t=0.08s, the total output voltage is stable at about 100V, and t= After 0.08s, the total voltage of the output side is about 105V when reaching the steady state; before the simulation time t=0.08s, the positive output voltage is stable at about 50V, and after t=0.08s, the positive output voltage is about 52.5V when reaching the steady state; Before the simulation time t=0.08s, the negative output voltage is stable at around 50V, and after t=0.08s, the negative output voltage is around 52.5V when reaching the steady state;

Fig. 14 is the curve of the DC component _Idc1 of the primary side inductor current and the DC component _Idc2 of the secondary side inductor current in Embodiment 2 of the present invention, wherein: before the simulation time t=0.12s, the primary side DC component is stable at about 2.5A, only At t=0.08s, the output voltage fluctuates. After t=0.12s, the DC component of the primary side is about 5.2A when reaching the steady state; before the simulation time t=0.12s, the DC component of the secondary side is stable at about -2.5 , only when t=0.08s fluctuates due to the output voltage change, after t=0.12s, the DC component of the secondary side is around -5.2A when reaching the steady state;

Fig. 15 is a curve of the difference between bipolar voltages in Embodiment 2 of the present invention. The difference between bipolar voltages is basically 0. When the bipolar load changes, the bipolar voltage has better self-balancing ability.

It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention without limitation, although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be carried out Modifications or equivalent replacements without departing from the spirit and scope of the technical solution of the present invention shall be covered by the claims of the present invention.

Claims (7)

1. A dc transformer having true bipolar off-grid operation capability, characterized by: comprising the steps of (a) a step of,

the direct current transformer (100) comprises an input stage converter (101), an intermediate isolation transformer (102) and an output stage converter (103), wherein direct current is injected into the input stage converter (101) and the output stage converter (103);

a controller (200) connected to the DC transformer (100) for controlling the DC current input to the DC transformer (100);

the direct current transformer (100) further comprises a grid-connected direct current transformer (100 a) and an off-grid direct current transformer (100 b) which are respectively arranged on the direct current transformer (100);

the working principle of the direct current transformer (100) comprises,

the DC transformer (100) adopts a traditional single phase shift modulation mode and controls the voltage v of a primary bridge arm and a secondary bridge arm _i And v _o The transmission power is controlled by the magnitude of the phase shift angle phi, the ratio of the phase shift angle phi to pi is set to be the phase shift duty ratio D, namelySetting the primary and secondary side turn ratio of the transformer as N and V ₁ /N＝V ₂ The method comprises the steps of carrying out a first treatment on the surface of the In true bipolar symmetric operation, i.e. secondary inductor current i _L2 DC transformer (100) without DC component, under this condition [ t ] is determined ₂ ，t ₃ ]The magnitude of the inductive current in the period of time is obtained in half a switching period i during the forward operation _L2 The expression of (2) is:

by injecting direct current into the secondary side of the transformer, the energy interaction of the positive electrode and the negative electrode can be realized, so that bipolar power is balanced, stable true bipolar asymmetric operation of the direct current transformer is realized, and when the inductance current of the secondary side is injected with a direct current component I _dc2 When the expression is i' _L2 (t)＝i _L2 (t)+I _dc2 ,

The average power output by the positive and negative electrodes is obtained by:

P ₂ -P ₁ ＝V ₂ I _dc2

wherein: p (P) ₁ Average power of positive electrode output, P ₂ Average power output for the negative electrode; by P _u Representing bipolar unbalanced power, i.e. P _u ＝P ₂ -P ₁ The above formula can be rewritten as:

P _u ＝V ₂ I _dc2 。

2. the dc transformer with true bipolar off-grid operation capability of claim 1, wherein: the controller (200) comprises a grid-connected controller (201) and an off-grid controller (202), wherein the grid-connected controller (201) is connected to the grid-connected direct current transformer (100 a), and the off-grid controller (202) is connected to the off-grid direct current transformer (100 b).

3. The dc transformer with true bipolar off-grid operation capability of claim 2, wherein: the grid-connected controller (201) comprises a power outer loop controller (201 a) and a current inner loop controller (201 b), wherein the direct current input into the grid-connected controller (201) is controlled, the current inner loop controller (201 b) comprises two current inner loop controllers (201 b-1) and 2 (201 b-2) which are respectively connected with the input stage converter (101 a) and the output stage converter (103 a), and the direct current input into the grid-connected controller is controlled.

4. A dc transformer with true bipolar off-grid operation capability as claimed in claim 2 or 3, characterized in that: the off-grid controller (202) comprises a voltage controller (202 a) and a current controller (202 b) and controls direct current input into the off-grid direct current transformer (100 b), wherein the direct current is injected into an output stage converter (103 a) of the off-grid direct current transformer (100 b).

5. The dc transformer with true bipolar off-grid operation capability of claim 1, wherein: the input stage converter (101) comprises a full-bridge structure adopted by the input stage converter (101).

6. The dc transformer with true bipolar off-grid operation capability of claim 5, wherein: the output stage converter (103) comprises a three-level structure or a half-bridge structure.

7. A control method of a DC transformer with true bipolar off-grid operation capability is characterized by comprising the following steps of,

setting current reference values of primary and secondary side direct current injection of the transformer and comparing the current reference values with direct current components of actual inductance current to obtain current deviation;

PI control is carried out on the deviation, a direct current injection voltage reference value is obtained, PWM modulation is carried out to balance the power of the positive electrode and the negative electrode of the output side of the transformer, and meanwhile, no direct current bias magnetic flux exists in the transformer;

the controller is utilized to adjust the reference value and the deviation value, so as to realize the control of the transformer;

the working principle of the direct current transformer (100) comprises,

the DC transformer (100) adopts a traditional single phase shift modulation mode and controls the voltage v of a primary bridge arm and a secondary bridge arm _i And v _o The transmission power is controlled by the magnitude of the phase shift angle phi, the ratio of the phase shift angle phi to pi is set to be the phase shift duty ratio D, namelySetting the primary and secondary side turn ratio of the transformer as N and V ₁ /N＝V ₂ The method comprises the steps of carrying out a first treatment on the surface of the In true bipolar symmetric operation, i.e. secondary inductor current i _L2 DC transformer (100) without DC component, under this condition [ t ] is determined ₂ ，t ₃ ]The magnitude of the inductive current in the period of time is obtained in half a switching period during the forward operation

i _L2 The expression of (2) is:

by injecting direct current into the secondary side of the transformer, the energy interaction of the positive electrode and the negative electrode can be realized, so that bipolar power is balanced, stable true bipolar asymmetric operation of the direct current transformer is realized, and when the inductance current of the secondary side is injected with a direct current component I _dc2 When the expression is i' _L2 (t)＝i _L2 (t)+I _dc2 The average power output by the positive and negative electrodes is obtained by:

P ₂ -P ₁ ＝V ₂ I _dc2

wherein: p (P) ₁ Average power of positive electrode output, P ₂ Average power output for the negative electrode; by P _u Representing bipolar unbalanced power, i.e. P _u ＝P ₂ -P ₁ The above formula can be rewritten as:

P _u ＝V ₂ I _dc2

the implementation of the control of the transformer comprises the following steps:

the transformer comprises a grid-connected type true bipolar direct current transformer based on direct current injection and a grid-separated type true bipolar direct current transformer, wherein the control of the grid-connected type true bipolar direct current transformer based on direct current injection comprises the following steps: the unbalanced power of the transformer is controlled through an unbalanced power outer ring controller, the unbalanced power and the controlled deviation value are regulated through a PI regulator, a direct current reference value is generated, and the bipolar unbalanced power of the transformer is controlled; the control of the off-grid true bipolar direct current transformer comprises the following steps: the DC injection of the primary side of the transformer is regulated by the current controller, so that the balance of the DC components of the primary side inductance current and the secondary side inductance current is realized, the DC magnetic bias of the transformer is eliminated, the deviation of the voltage reference value and the actual value is utilized by the voltage controller, and the phase-shifting duty ratio under the phase-shifting modulation is generated by the PI regulator, so that the control of the output voltage of the transformer is realized.