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

DC transformer with true bipolar off-grid operation capability and control method Download PDF

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Publication number
CN112421602B
CN112421602B CN202011251085.2A CN202011251085A CN112421602B CN 112421602 B CN112421602 B CN 112421602B CN 202011251085 A CN202011251085 A CN 202011251085A CN 112421602 B CN112421602 B CN 112421602B
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transformer
direct current
grid
current
controller
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CN112421602A (en
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张建文
孔心怡
施刚
周剑桥
蔡旭
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Shanghai Jiaotong University
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Shanghai Jiaotong University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/08—Three-wire systems; Systems having more than three wires
    • H02J1/082—Plural DC voltage, e.g. DC supply voltage with at least two different DC voltage levels
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/10—Parallel operation of dc sources
    • H02J1/102—Parallel operation of dc sources being switching converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/10—Parallel operation of dc sources
    • H02J1/106—Parallel operation of dc sources for load balancing, symmetrisation, or sharing
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/14—Balancing the load in a network
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Inverter Devices (AREA)

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

DC transformer with true bipolar off-grid operation capability and control method
Technical Field
The application relates to the technical field of direct current transformers, in particular to a direct current transformer with true bipolar off-grid operation capability and a control method.
Background
With the gradual deterioration of the environment and the increasing shortage of fossil energy in recent years, renewable energy has been developed in a long-standing manner in recent years. The penetration of the distributed power generation has important significance for reducing environmental protection pressure, saving power transmission and transformation investment and improving power supply reliability, and the direct current transmission and direct current distribution technology is one of effective technical means for solving the grid connection of the distributed power supply.
There are currently two dominant dc power distribution architectures, i.e., monopolar and bipolar. The unipolar dc network provides a single dc voltage level between the two wires, which is simple in structure, easier to control, but less reliable, and when the dc bus fails, all loads are at risk of losing power. The bipolar tributary network provides two dc voltage levels between the three-wire structure, namely a positive/negative ground voltage and a positive/negative voltage. The working mode of the pseudo bipolar structure is similar to that of a unipolar structure, and the operation of the positive and negative poles with asymmetric loads cannot be realized; the true bipolar dc network can also operate in a single pole fault, and can provide more options for loads and distributed power sources due to the two different levels. In addition, the true bipolar network has a certain similarity with the traditional three-phase four-wire system alternating current system, namely, the voltage between the positive electrode and the negative electrode is similar to the line voltage in the three-phase four-wire system, and the voltage of each electrode is similar to the phase voltage, so that the similarity with the alternating current system is beneficial to the system operation, improvement and control scheme design. The true bipolar structure has the most extensive applicability, and can ensure high power supply reliability and flexibility of direct current load access of the direct current distribution network by combining the double-bus power supply structure.
The DC transformer is a key device for realizing bus interconnection, voltage conversion, electric energy transmission and electric isolation of a DC power transmission and distribution system or connecting a DC source-load-storage and the like into a DC power grid. For Low Voltage DC (LVDC) ports, the true bipolar DC ports with unbalanced operation capability can meet the requirements of Low Voltage users on multiple Voltage levels, safe electricity utilization, reliable power supply and the like, while the conventional DC transformer can only provide a single-pole or pseudo-bipolar Low Voltage DC port, the prior art realizes the true bipolar operation of DC by adding a power balancing device on two sets of DC transformers or the true bipolar bus, and the method can increase the equipment investment cost and the operation reliability.
Disclosure of Invention
This section is intended to outline some aspects of embodiments of the application and to briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section as well as in the description of the application and in the title of the application, which may not be used to limit the scope of the application.
The present application has been made in view of the above-mentioned problems existing in the prior art of power balance control of the positive and negative poles of a transformer.
Therefore, the technical problems solved by the application are as follows: and the power balance control problem during bipolar asymmetric operation during off-grid ensures the safe and stable operation of a true bipolar direct current system.
In order to solve the technical problems, the application provides the following technical scheme: the direct current transformer comprises 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 and controls the DC current in the input DC transformer.
As a preferred embodiment of the dc transformer with true bipolar off-grid operation capability of the present application, wherein: the direct current transformer further comprises a grid-connected direct current transformer and an off-grid direct current transformer.
As a preferred embodiment of the dc transformer with true bipolar off-grid operation capability of the present application, wherein: the controller comprises a grid-connected controller and a grid-off controller, wherein the grid-connected controller is connected with the grid-connected direct current transformer, and the grid-off controller is connected with the grid-off direct current transformer.
As a preferred embodiment of the dc transformer with true bipolar off-grid operation capability of the present application, wherein: the grid-connected controller comprises a power outer ring controller and a current inner ring controller, wherein the power outer ring controller and the current inner ring controller are used for controlling direct current input into the grid-connected controller, and the current inner ring controller is provided with two current inner ring controllers 1 and 2 which are respectively connected with the input stage converter and the intermediate isolation transformer and used for controlling direct current injected into the input stage converter and the intermediate isolation transformer.
As a preferred embodiment of the dc transformer with true bipolar off-grid operation capability of the present application, wherein: the off-grid controller comprises a voltage controller and a current controller, and controls direct current input into the off-grid direct current transformer, wherein the direct current is injected into an output stage converter of the off-grid direct current transformer.
As a preferred embodiment of the dc transformer with true bipolar off-grid operation capability of the present application, wherein: the input stage converter comprises a full-bridge structure.
As a preferred embodiment of the dc transformer with true bipolar off-grid operation capability of the present application, wherein: the output stage converter comprises a three-level structure or a half-bridge structure.
As a preferred embodiment of the dc transformer with true bipolar off-grid operation capability of the present application, wherein: the working principle of the direct current transformer comprises that,
the DC transformer 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 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
as a preferred embodiment of the control method of a dc transformer with true bipolar off-grid operation capability according to the present application, the method comprises: 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, the power of the positive electrode and the negative electrode of the transformer is balanced through PWM modulation, and meanwhile, no direct current bias magnetic flux exists; and the controller is used for adjusting the reference value and the deviation value, so that the control of the transformer is realized.
As a preferred embodiment of the control method of a dc transformer with true bipolar off-grid operation capability according to the present application, the method comprises: 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 an I 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.
The application has the beneficial effects that: 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 grid-connected true bipolar direct current transformer is used for injecting direct current on the grid-connected side to provide bipolar unbalanced power, so that bipolar unbalanced stable operation can be realized; the off-grid type true bipolar direct current transformer controls the output voltage by adjusting the duty ratio, so that the stability of the output voltage is ensured; 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.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings that are needed in the description of the embodiments will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the present application, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art. Wherein:
fig. 1 is a schematic diagram of a dc transformer with true bipolar off-grid operation capability according to a first embodiment of the present application;
FIG. 2 is a block diagram of a dual closed loop controller for a DC transformer with true bipolar off-grid operation capability according to a first embodiment of the present application;
FIG. 3 is a block diagram of a grid-connected controller for a DC transformer with true bipolar off-grid operation capability according to a first embodiment of the present application;
fig. 4 is a block diagram of an off-grid controller for a dc transformer with true bipolar and off-grid operation capability according to a first embodiment of the present application;
fig. 5 is a waveform diagram illustrating operation of a dc transformer with true bipolar off-grid operation capability without dc injection according to a first embodiment of the present application;
fig. 6 is a waveform diagram illustrating operation of a dc transformer with true bipolar off-grid operation capability according to a first embodiment of the present application;
fig. 7 is a schematic flow chart of a dc transformer control method with true bipolar off-grid operation capability according to a first embodiment of the present application;
fig. 8 is a topology diagram of a grid-connected dc transformer in a scheme one of a dc transformer with true bipolar off-grid operation capability and a control method according to a fourth embodiment of the present application;
fig. 9 shows an unbalanced power P in a dc transformer with true bipolar off-grid operation capability and a control method according to a fourth embodiment of the present application u Outputting a graph;
fig. 10 shows a dc transformer with true bipolar off-grid operation capability and a control method according to a fourth embodiment of the present application o Positive electrode power P 1 And negative electrode power P 2 Outputting a graph;
fig. 11 shows a dc component I of an on-line inductor current in a scheme of a dc transformer with true bipolar off-grid operation capability and a control method according to a fourth embodiment of the present application dc1 And the direct current component I of the secondary inductor current dc2 A graph;
fig. 12 is a topology diagram of an off-grid dc transformer in a second embodiment of a dc transformer with true bipolar and off-grid operation capability and a control method according to a fourth embodiment of the present application;
fig. 13 shows a dc transformer and control with true bipolar off-grid operation capability according to a fourth embodiment of the present applicationScheme II of the method o Positive electrode output voltage v C1 And a negative electrode output voltage v C2 A graph;
fig. 14 shows a dc component I of an on-line inductor current in a second embodiment of a dc transformer with true bipolar off-grid operation capability and a control method according to a fourth embodiment of the present application dc1 And the direct current component I of the secondary inductor current dc2 A graph;
fig. 15 is a graph showing a difference between bipolar voltages in a dc transformer with true bipolar off-grid operation capability and a control method according to a fourth embodiment of the present application.
Detailed Description
So that the manner in which the above recited objects, features and advantages of the present application can be understood in detail, a more particular description of the application, briefly summarized above, may be had by reference to the embodiments, some of which are illustrated in the appended drawings. All other embodiments, which can be made by one of ordinary skill in the art based on the embodiments of the present application without making any inventive effort, shall fall within the scope of the present application.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application, but the present application may be practiced in other ways other than those described herein, and persons skilled in the art will readily appreciate that the present application is not limited to the specific embodiments disclosed below.
Further, reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic can be included in at least one implementation of the application. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.
While the embodiments of the present application have been illustrated and described in detail in the drawings, the cross-sectional view of the device structure is not to scale in the general sense for ease of illustration, and the drawings are merely exemplary and should not be construed as limiting the scope of the application. In addition, the three-dimensional dimensions of length, width and depth should be included in actual fabrication.
Also in the description of the present application, it should be noted that the orientation or positional relationship indicated by the terms "upper, lower, inner and outer", etc. are based on the orientation or positional relationship shown in the drawings, are merely for convenience of describing the present application and simplifying the description, and do not indicate or imply that the apparatus or elements referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the present application. Furthermore, the terms "first, second, or third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
The terms "mounted, connected, and coupled" should be construed broadly in this disclosure unless otherwise specifically indicated and defined, such as: can be fixed connection, detachable connection or integral connection; it may also be a mechanical connection, an electrical connection, or a direct connection, or may be indirectly connected through an intermediate medium, or may be a communication between two elements. The specific meaning of the above terms in the present application will be understood in specific cases by those of ordinary skill in the art.
Example 1
Referring to fig. 1-7, a first embodiment of the present application provides a dc transformer with true bipolar off-grid operation capability, comprising: the dc transformer 100 includes an input stage converter 101, an intermediate isolation transformer 102, and an output stage converter 103, and dc current is injected into the input stage converter 101 and the intermediate isolation transformer 102 and output from the output stage converter 103; the controller 200 is connected to the dc transformer 100 and controls the dc current input to the dc transformer 100.
Wherein the dc transformer 100 further comprises a grid-connected dc transformer 100a and an off-grid dc transformer 100b, the input stage converter 101 adopts a full-bridge structure, the output stage converter 103 adopts a three-level structure or a half-bridge structure to provide a true bipolar low-voltage dc port, and referring to fig. 1, two types of dc transformers are used, wherein v i Input for transformerSide ac square wave voltage v o For the output side thereof to be alternating square wave voltage, L 2 Leakage inductance equivalent to secondary side of transformer, i L2 Is the secondary side inductance current of the transformer, V 1 For input side DC voltage source, V 2 For output side bipolar low voltage DC bus voltage, V C1 And V C2 Capacitor voltages i of low-voltage DC positive electrode and negative electrode respectively C1 And i C2 Respectively positive electrode capacitance current and negative electrode capacitance current, R 1 And R is 2 Respectively a positive electrode load and a negative electrode load, I R1 And I R2 Respectively positive and negative load currents, V 2N A single-pole rated direct-current voltage which is a low-voltage direct-current port;
further, 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 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, which control the direct current input to the grid-connected controller 201, and the current inner loop controller 201b has two current inner loop controllers 1201b-1 and 2201b-2, respectively, and is connected to the input stage converter 101a and the output stage converter 103a, and controls the direct current input thereto, and specifically, refer to fig. 2 to 3.
Referring to fig. 4, the off-grid controller 202 includes a voltage controller 202a and a current controller 202b, and controls the dc current input to the off-grid dc transformer 100b, wherein the dc current is injected into the output stage converter 103a of the off-grid dc transformer 100b.
The working principle of the DC transformer 100 comprises that the DC transformer 100 is similar to DAB (Dual Active Bridge) and DAB modulation mode, adopts the traditional single phase shift modulation mode, and controls the voltage v of the primary and secondary side bridge arms 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 The dc transformer 100 operating without dc component is shown in fig. 5, where:
at [ t ] 2 ，t 3 ]Within a period of time, i L2 The expression is:
due toThen t 3 The inductor current is as follows:
in the same way, i can be obtained L2 At [ t ] 3 ，t 5 ]The expression within the period is:
due toThen t is obtained from the above 5 The secondary inductor current is as follows:
i L2 symmetrical over half a switching period, having i L2 (t 5 )＝-i L2 (t 2 ) According to formula t 3 The inductor current can be obtained at the moment:
by combining the formulas, i in half switching period during forward operation can be written L2 The expression of (2) is:
the average power output by the positive electrode is expressed as:
similarly, the expression of the average power output by the negative electrode 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 With reference to fig. 6 for operating conditions,
at this time, the expression of the average power output from the positive electrode is:
the expression of the average power output by the negative electrode is:
the average power output from the positive and negative electrodes is given by:
P 2 -P 1 ＝V 2 I dc2
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
example 2
Referring to fig. 7, a second embodiment of the present application, which is different from the first embodiment, provides a control method of a dc transformer with true bipolar and off-grid operation capability, including:
s1: 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;
s2: PI control is carried out on the deviation, a direct current injection voltage reference value is obtained, and the transformer is enabled to have no direct current bias magnetic flux through PWM modulation;
s3: and the controller is used for adjusting the reference value and the deviation value, so that the control of the transformer is realized.
Wherein: the transformer comprises a grid-connected type true bipolar direct current transformer based on direct current injection and a grid-off 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 by the unbalanced power outer ring controller, the unbalanced power and the controlled deviation value are regulated by the I regulator, and a direct current reference value is generated, so that the bipolar unbalanced power of the transformer is controlled.
Example 3
The third embodiment of the application analyzes the energy balance mechanism when the off-grid true bipolar direct current transformer is provided with an asymmetric load, and verifies the self-balancing capability of bipolar voltage when the off-grid true bipolar direct current transformer is provided with an asymmetric load through a countercheck method; 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.
Assuming that the low-voltage side positive DC voltage is relative to the unipolar rated DC voltage V 2N The offset of (a) is Δv, and the actual dc voltages of the low-voltage positive and negative electrodes are respectively:
in the positive half of the switching cycle, [ t ] 2 ，t 3 ]Within a period of time, i L2 The expression is:
due toThen t 3 The inductor current is as follows:
in the same way, i can be obtained L2 At [ t ] 3 ，t 5 ]The expression within the period is:
due toThen t is obtained from the above 5 The secondary inductor current is as follows:
similarly, i in the negative half of the switching period can be calculated L2 ，
From i in half switching cycles L2 It can be seen that the initial inductor current of the two switching cycles is not equal, i.e. the DC transformer cannot realize steady-state operation when the low-side bipolar voltage is unbalanced, when V C1 >V C2 When the inductance current increases in one switching period, as can be seen from fig. 1, the discharging current of the positive electrode capacitor gradually increases, and the charging current of the negative electrode capacitor gradually increases, so that the two electrode capacitors gradually return to the same voltage value, i.e. the direct current transformer has the self-balancing capability of the bipolar voltage when the low voltage side is provided with an asymmetric load;
then analyzing steady state operation condition of the true bipolar DC transformer with asymmetric load, assuming that the positive electrode is heavy load and the negative electrode is light load, i.e. I in FIG. 1 R1 >I R2 When the load is asymmetric, the low-voltage direct-current side bipolar voltage is self-balanced, so that the secondary side alternating-current voltage waveform is still a square wave without direct-current bias, and the asymmetry of the load is reflected on the direct-current bias of the secondary side inductive current;
as known from kirchhoff's current law, at any time, the secondary inductor current has the following expression:
i L2 (t)＝-[i o1 (t)+i o2 (t)]＝-[i c1 (t)+I R1 -i c2 (t)-I R2 ]
after one switching cycle is averaged, the secondary inductor current has only a dc component, while the capacitor current is zero after the switching cycle is averaged, so that after the switching cycle is averaged, the above equation is converted into:
I dc ＝-(I R1 -I R2 )＝ΔI R
when the output side bipolar belt of the direct current transformer is asymmetrically loaded, bipolar voltage has self-balancing capability and cannot be influenced, and the difference of bipolar current is reflected to secondary side inductor current, direct current bias is introduced to the secondary side inductor current, and the magnitude of the direct current bias is equal to the difference of bipolar load current.
Example 4
Referring to fig. 8 to 12, a 4 th embodiment of the present application is provided with different schemes for explaining the method of the present application to verify the true effect of the method.
Scheme one: based on the direct current transformer structure shown in fig. 1, a grid-connected NPC-DAB system is built by adopting MATLAB/Simulink software, and simulation verification is carried out on the topology by referring to fig. 8, wherein simulation parameters are shown in the following table 1;
table 1: grid-connected topology simulation parameters.
Parameters (parameters) Value of Parameters (parameters) Value of
High frequency transformer frequency 5000Hz Rated DC voltage of input side bus 50V
Leakage inductance of high-frequency transformer 40uH Rated DC voltage of output bus 50V、50V
High frequency transformer transformation ratio 1:1
Under the working conditions shown in the table, the unbalanced power P is given before the simulation time t=0.04 s u After a reference value of 0, t=0.04 s, the unbalanced power P is given u The reference value is 250W, and under the simulation parameters, the theoretical result is: secondary dc component I of transformer dc2 The simulation results are shown in fig. 9 to 11, with 0 before t=0.04 s and 5A after t=0.04 s.
FIG. 9 shows an unbalanced power P in accordance with an embodiment of the present application u An output curve, wherein: before the simulation time t=0.04 s, the unbalanced power is stabilized at about 0, and after t=0.04 s, the unbalanced power reaches about 250W at the steady state;
FIG. 10 shows the total power P of the output side in the first embodiment of the present application o Positive electrode power P 1 And negative electrode power P 2 An output curve, wherein: before the simulation time t=0.04 s, the total power of the output side is always stabilized at about 590W; before the simulation time t=0.04 s, the positive electrode power is stabilized at about 295W, and after t=0.04 s, the positive electrode power reaches about 170W at steady state; before the simulation time t=0.04 s, the negative electrode power is stabilized at about 295W, and after t=0.04 s, the negative electrode power reaches about 420W at steady state;
FIG. 11 shows a DC component I of the primary inductor current in accordance with an embodiment of the present application dc1 And the direct current component I of the secondary inductor current dc2 Curve, wherein: before the simulation time t=0.04 s, the primary side direct current component is stabilized at about 0, and after t=0.04 s, the primary side direct current component is at about-5A when reaching a steady state; before the simulation time t=0.04 s, the secondary sideThe direct current component is stabilized at about 0, and after t=0.04 s, the secondary direct current component is about 5A when reaching a steady state;
the simulation result of the drawing is consistent with the theoretical result, so that the primary side direct current injection and the secondary side direct current injection of the transformer and the rapid tracking of unbalanced power can be realized through the designed double closed loop controller.
Scheme II: based on the direct current transformer structure shown in fig. 1, an off-grid NPC-DAB system shown in fig. 12 is built by MATLAB/Simulink software, simulation verification is carried out on the topology, and simulation parameters are shown in the following table 2;
table 2: off-grid topology simulation parameters
Parameters (parameters) Value of Parameters (parameters) Value of
High frequency transformer frequency 5000Hz Rated DC voltage of input side bus 50V
Leakage inductance of high-frequency transformer 40uH Output-side bipolar rated DC voltage 50V、50V
High frequency transformer transformation ratio 1:1
Under the working conditions shown in the table, before the simulation time t=0.04 s, the given output voltage V reference value is 100V, and after t=0.08 s, the given output voltage V reference value is 105V; before the simulation time t=0.12 s, the positive electrode load R is given 1 After a reference value of 20Ω and t=0.12 s, the positive electrode load R is given 1 The reference value is 10Ω; before the simulation time t=0.12 s, the reference value of the given negative load R2 is 2000 Ω, and after t=0.12 s, the negative load R is given 2 The reference value is 1000Ω. Under the simulation parameters, the theoretical results are: secondary dc component I of transformer dc2 -2.475A before t=0.08 s, -2.59A at t=0.08 s-0.12 s, -5.19A after t=0.12 s, the simulation results are shown in fig. 13-15;
FIG. 13 shows the total voltage v at the output side in the second embodiment of the present application o Positive electrode output voltage v C1 And a negative electrode output voltage v C2 Curve, wherein: before the simulation time t=0.08 s, the total voltage of the output side is stabilized at about 100V, and after t=0.08 s, the total voltage of the output side reaches about 105V at the steady state; before the simulation time t=0.08 s, the positive output voltage is stabilized at about 50V, and after t=0.08 s, the positive output voltage reaches about 52.5V when reaching a steady state; before the simulation time t=0.08 s, the negative electrode output voltage is stabilized at about 50V, and after t=0.08 s, the negative electrode output voltage reaches about 52.5V when reaching a steady state;
FIG. 14 shows a DC component I of the primary inductor current in a second embodiment of the application dc1 And the direct current component I of the secondary inductor current dc2 Curve, wherein: before the simulation time t=0.12 s, the primary side direct current component is stabilized at about 2.5A, only the output voltage changes to be fluctuated when t=0.08 s, and after t=0.12 s, the primary side direct current component reaches about 5.2A when reaching a steady state; before the simulation time t=0.12 s, the secondary side direct current component is stabilized at about-2.5, only the output voltage changes to be fluctuated when t=0.08 s, and after t=0.12 s, the secondary side direct current component reaches about-5.2A when reaching the steady state；
Fig. 15 is a plot of the difference between bipolar voltages, with the difference between bipolar voltages being substantially 0, for a bipolar load change, and with a better self-balancing capability.
It should be noted that the above embodiments are only for illustrating the technical solution of the present application and not for limiting the same, and although the present application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solution of the present application may be modified or substituted without departing from the spirit and scope of the technical solution of the present application, which is intended to be covered in the scope of the claims of the present application.

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.
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CN110445389A (en) * 2019-07-16 2019-11-12 上海交通大学 The solid-state transformer device and its control method for having true bipolar low-pressure DC port
CN110995030A (en) * 2019-12-02 2020-04-10 上海交通大学 Solid-state transformer with medium-voltage direct current and true-dual ultra-low-voltage direct current ports
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