CN104836455B - A kind of power distribution network electric power electric transformer and its control method - Google Patents

Abstract

The present invention provides a distribution network power electronic transformer and its control method. The distribution network power electronic transformer includes an input stage, an isolation conversion stage and an output stage, and the output stage is provided with a supercapacitor; the output stage A voltage source type four-bridge inverter topology is adopted, the positive pole of the supercapacitor is connected to the midpoint of the fourth bridge arm, and the negative pole is connected to the negative terminal of the DC bus. The distribution network power electronic transformer described in the present invention utilizes the fourth bridge arm which serves as the neutral line inside and simultaneously serves as the charging and discharging interface of the supercapacitor energy storage system, seamlessly connects the supercapacitor, has fast response speed, and does not need additional power conversion circuits and controls system to achieve flexible energy storage. The distribution network power electronic transformer described in the present invention has short-term uninterrupted power supply capability and ultra-high power fluctuation tolerance, and can realize stable operation under sharp load increase.

Description

A distribution network power electronic transformer and its control method

technical field

The invention relates to the field of power electronics, in particular to a distribution network power electronic transformer and a control method thereof.

Background technique

In recent years, power electronic conversion technology has developed rapidly. Its latest application in power systems—Power Electronic Transformer (PET), combined with high-frequency transformers, can realize voltage conversion and energy transfer in power systems. It has been hot researched by scholars at home and abroad. The outstanding feature of PET applied to the distribution network is the real-time control of the voltage amplitude and phase of the AC side, which can realize the flexible adjustment of the voltage, current and power of the primary and secondary sides of the transformer, so it has the potential to solve many new issues in the power system. The application prospect broad.

However, the Distribution Power Electronic Transformer (DPET) operates at the end of the power grid. The operating environment is complex, the voltage on the high-voltage side is unstable, and deep voltage drops and short-term power outages caused by faults occur from time to time. It is a widely used measure to configure power electronic equipment equipped with super capacitor (Super Capacitor, SC) energy storage in the line to perform voltage compensation when the power supply voltage drops. In the prior art, it is generally chosen to add an additional SC energy storage system on the DPET low-voltage DC bus. Although this can achieve the compensation effect, this method adds an additional DC/DC power conversion circuit, increases the system cost, and makes the compensation response Slow down.

Contents of the invention

Aiming at the defects in the prior art, the present invention provides a distribution network power electronic transformer and its control method to solve the system cost increase and compensation caused by additional SC energy storage system on the DPET low-voltage DC bus in the prior art The problem of slow response.

In order to solve the above problems, the present invention provides the following solutions:

In a first aspect, the present invention provides a distribution network power electronic transformer, including an input stage, an isolation conversion stage, and an output stage, and the output stage is provided with a supercapacitor;

The output stage adopts a voltage source type four-leg inverter topology, the positive pole of the supercapacitor is connected to the midpoint of the fourth bridge arm, and the negative pole is connected to the negative terminal of the DC bus.

Wherein, the power electronic transformer of the distribution network charges and discharges the supercapacitor through a bridge arm power device.

Wherein, the input stage adopts a cascaded H-bridge multi-level topology, and each bridge arm is composed of cascaded HM link modules with a number of n. The main circuit of the HM link module is an H-bridge structure, and the three bridge arms pass through the Y Type connection, transforming high-voltage power-frequency AC into 3n floating DC.

Wherein, each unit of the isolation conversion stage adopts a dual active bridge topology DAB, and the total number of units is 3n. One side of each DAB is connected to the HM link, and the other side is connected in parallel to form a DC bus.

Wherein, the supercapacitor is connected in series with an inductor to reduce current ripple.

In the second aspect, the present invention also provides a method for controlling a distribution network power electronic transformer, wherein the distribution network power electronic transformer includes an input stage, an isolation conversion stage, and an output stage, and the output stage is provided with a supercapacitor ;

The output stage adopts a voltage source type four-leg inverter topology, the positive pole of the supercapacitor is connected to the midpoint of the fourth bridge arm, and the negative pole is connected to the negative terminal of the DC bus;

The control methods include:

The fourth bridge arm adopts a current-type control in which instructions can be superimposed, and performs compound control on the neutral line current and the charging and discharging current of the supercapacitor, wherein the current control instruction of the fourth bridge arm is Specifically, when the supercapacitor is discharged, the control objective is to keep the DC bus voltage constant, and the current control command is obtained equal to discharge current When the supercapacitor is charging, the control goal is to keep the voltage at the supercapacitor terminal constant and obtain the current control command Equal to the charge current command Among them, the neutral line current control command is obtained by adding the three-phase currents, i _n = _{ia +i b + ic} , and controls the output current of the fourth bridge arm _-in to offset the neutral line current in the system due to the three-phase imbalance .

Wherein, the input stage adopts a cascaded H-bridge multi-level topology, and each bridge arm is composed of cascaded HM link modules with a number of n. The main circuit of the HM link module is an H-bridge structure, and the three bridge arms pass through the Y Type connection, transforming high-voltage power-frequency AC into 3n floating DC.

The bridge arms of the three phases of the input stage are symmetrical, and the control strategy of each phase is the same; wherein the control strategy of each phase adopts the double closed-loop control of voltage and current based on the synchronously rotating d-q coordinate system, combined with the carrier shifting CPS-SPWM modulation strategy.

Wherein, each unit of the isolation conversion stage adopts a dual active bridge topology DAB, and the total number of units is 3n. One side of each DAB is connected to the HM link, and the other side is connected in parallel to form a DC bus.

The isolation conversion stage transmits energy bidirectionally through 3n DABs to jointly realize the constant DC bus voltage. For a single DAB, the driving signals of the two active bridges are complementary trigger pulses with a duty cycle of 50%. The two bridges There is a phase shift angle corresponding to the conduction of the switch tube when When is positive, power flows in the forward direction, when When it is a negative value, the power flows in the opposite direction; adding average power control in each DAB, the difference between the different power of each DAB and the command power will produce different phase shift angle variables Balance the power flow of each DAB by adjusting the phase shift angle.

Wherein, the first three bridge arms and the fourth bridge arm of the voltage source type four-leg inverter adopted in the output stage are independent control parts, wherein the first three bridge arms control the positive sequence and negative sequence components of the output voltage, Guarantee three-phase output constant voltage and constant frequency.

Wherein, the supercapacitor is connected in series with an inductor to reduce current ripple.

It can be seen from the above technical solution that the power electronic transformer for distribution network according to the present invention uses the fourth bridge arm which serves as the neutral line inside and simultaneously serves as the charging and discharging interface of the supercapacitor energy storage system, seamlessly connects the supercapacitor, and has a fast response speed and does not need Additional power conversion circuits and control systems are added to achieve flexible energy storage.

The control method of the distribution network power electronic transformer provided by the present invention does not affect the control of the neutral line current by the fourth bridge arm, has short-term uninterrupted power supply capability, and has ultra-high power fluctuation tolerance, and can realize rapid load increase. Stable operation.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are For some embodiments of the present invention, those skilled in the art can also obtain other drawings based on these drawings without creative work.

Fig. 1 is a schematic diagram of the circuit structure of a distribution network power electronic transformer provided by an embodiment of the present invention;

FIG. 2 is a schematic diagram of a control strategy of a fourth bridge arm in an embodiment of the present invention;

Fig. 3 is a schematic diagram of an input stage control strategy in an embodiment of the present invention;

Fig. 4 is a schematic diagram of a dual active bridge DAB moving direction control strategy in an embodiment of the present invention;

Fig. 5 is a schematic diagram of an output stage VSI control strategy in an embodiment of the present invention.

detailed description

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments It is a part of embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

Figure 1 shows a schematic diagram of the circuit structure of a distribution network power electronic transformer provided by an embodiment of the present invention. The distribution network power electronic transformer provided in this embodiment is a distribution network power electronic transformer that flexibly interacts with a supercapacitor ( Super Capacitor based Distribution Power Electronic Transformer, referred to as SCDPET). The SCDPET provided in this embodiment utilizes the fourth bridge arm inside the DPET as the neutral line and also serves as the charging and discharging interface of the supercapacitor energy storage system, seamlessly connects the supercapacitor, and realizes flexible energy storage without additional power conversion circuits and control systems. As shown in Figure 1, the SCDPET provided by this embodiment includes: an input stage, an isolation conversion stage, and an output stage, and the output stage is provided with a supercapacitor, that is, the output stage integrates a supercapacitor energy storage system, as shown in Figure 1 .

The output stage adopts a voltage source type four-leg inverter topology, the positive pole of the supercapacitor is connected to the midpoint of the fourth bridge arm, and the negative pole is connected to the negative terminal of the DC bus.

Wherein, the power electronic transformer of the distribution network charges and discharges the supercapacitor through a bridge arm power device.

Wherein, the input stage adopts a Cascade H-bridge Multi-level (Cascade H-bridge Multi-level, CHM) topology, each bridge arm is composed of cascaded HM chain link modules with a number of n, and the HM chain link module main circuit It is an H-bridge structure, and the three bridge arms are composed of Y-shaped connections, which convert the high-voltage power-frequency AC into 3n suspended DC. The cascaded multi-level structure enables the input-level power devices to operate at a lower voltage stress and switching frequency. Applied to high voltage and high power.

Wherein, each unit of the isolation conversion stage adopts a dual active bridge topology (Dual Active Bridge, DAB) to control the two-way flow of energy; the total number of units is 3n, one side of each DAB is connected to the HM link, and the other side is connected in parallel to form a DC bus .

In a preferred embodiment of the present invention, the supercapacitor is connected in series with an inductor, and the series inductor L reduces current ripple.

The output stage adopts a voltage source type four-leg voltage source inverter (4-leg Voltage Source Inverter, VSI) topology, which can directly control the neutral current, and has the advantages of flexible control, no need for large DC link capacitors and high DC voltage utilization Advantages, suitable for distribution network applications. The positive pole of the supercapacitor is connected to the midpoint of the fourth bridge arm, and the negative pole is connected to the negative terminal of the DC bus (optionally, the inductance L is connected in series to reduce the current ripple). Interact with DC bus energy.

The SCDPET provided by the embodiment of the present invention utilizes the fourth bridge arm which serves as the neutral line and acts as the charging and discharging interface of the supercapacitor energy storage system at the same time, seamlessly connects the supercapacitor, has fast response speed, and does not need additional power conversion circuits and control systems to realize Flexible energy storage. The distribution network power electronic transformer provided by the embodiment of the present invention does not affect the control of the neutral line current by the fourth bridge arm, has short-term uninterrupted power supply capability, and has ultra-high power fluctuation tolerance, and can realize stability under sudden load increase run.

An embodiment of the present invention provides a method for controlling a distribution network power electronic transformer, wherein the distribution network power electronic transformer is a distribution network power electronic transformer (SuperCapacitor based Distribution Power Electronic Transformer) that flexibly interacts with a supercapacitor. Transformer, referred to as SCDPET). The SCDPET includes an input stage, an isolation conversion stage and an output stage, and the output stage is provided with a supercapacitor;

The output stage adopts a voltage source type four-leg inverter topology, the positive pole of the supercapacitor is connected to the midpoint of the fourth bridge arm, and the negative pole is connected to the negative terminal of the DC bus;

The control method of described SCDPET comprises:

The fourth bridge arm adopts a current-type control in which instructions can be superimposed, and performs compound control on the neutral line current and the charging and discharging current of the supercapacitor, wherein the current control instruction of the fourth bridge arm is Specifically, when the supercapacitor is discharged, the control objective is to keep the DC bus voltage constant, and the current control command is obtained equal to discharge current When the supercapacitor is charging, the control goal is to keep the voltage at the supercapacitor terminal constant and obtain the current control command Equal to the charge current command Among them, the neutral line current control command is obtained by adding the three-phase currents, i _n = _{ia +i b + ic} , and controls the output current of the fourth bridge arm _-in to offset the neutral line current in the system due to the three-phase imbalance .

SCDPET operates at the end of the power grid. Due to faults and other reasons, the voltage on the high-voltage side drops deeply, or even short-term power outages, and the load on the low-voltage side fluctuates frequently. These situations can be regarded as short-term power shortages from the perspective of power. Therefore, it is difficult for SCDPET to rely on the stored energy of the DC side capacitor to maintain the output side voltage. At this time, with the short-term high-power charging and discharging of the super capacitor, and the energy interaction with the DC side capacitor, the SCDPET can run smoothly and maintain a stable output.

Among them, the supercapacitor mainly operates in three states:

(1) When the supercapacitor has sufficient energy storage and the SCDPET is operating normally, no energy interaction is required, and the fourth bridge arm maintains a constant voltage at the supercapacitor terminal, which is in an energy storage state and remains in standby.

(2) When there is a power shortage, the DC bus voltage will show signs of drop. At this time, the energy of the super capacitor flows into the DC bus through the fourth bridge arm to supplement the power required by the load to achieve the purpose of stabilizing the DC bus voltage. The super capacitor works in the discharge state.

(3) When the power grid returns to normal, the dual active bridge DAB maintains a constant DC bus voltage, and the fourth bridge arm controls the supercapacitor to work in a constant DC current charging state. Once the supercapacitor voltage is charged to the expected value, it will return to the energy storage state.

The supercapacitor continuously cycles between the three states and is flexibly controlled by the fourth bridge arm to ensure fast power supply.

While the supercapacitor is being charged and discharged, it cannot affect the control of the neutral current. Therefore, the fourth bridge arm adopts a current-type control in which instructions can be superimposed to control the neutral current and the charging and discharging current of the supercapacitor. The control strategy of the fourth bridge arm is shown in Figure 2. . The switch is the supercapacitor control mode selection switch. When discharging, the control target is to keep the DC bus voltage constant and obtain the current control command equal to discharge current When charging, the control goal is to keep the voltage at the supercapacitor terminal constant and obtain the current control command Equal to the charge current command The neutral line current control command is obtained by adding the three-phase currents, i _n = _{ia +i b + ic} , and controlling the output current of the fourth bridge arm _-in will offset the neutral line current in the system due to three-phase imbalance. The final current control command of the fourth bridge arm is Current tracking adopts simple and easy-to-implement proportional-integral (PI) control.

In one embodiment of the present invention, the input stage adopts a cascaded H-bridge multi-level topology, each bridge arm is composed of cascaded HM chain link modules with a number of n, and the main circuit of the HM chain link module is an H bridge structure , the three bridge arms are composed of Y-shaped connections, which convert high-voltage power-frequency AC into suspended DC with a quantity of 3n.

The bridge arms of the three phases of the input stage are symmetrical, and the control strategy of each phase is the same; the control strategy of each phase is shown in Figure 3, and the double closed-loop control of voltage and current based on the synchronous rotating dq coordinate system is adopted, and the carrier is shifted to CPS-SPWM modulation strategy. u _sd , u _sq are the rotating components of the grid voltage U _s in the dq coordinate system, and the phase signal θ output by the phase-locked link PLL is used to provide the reference phase required for voltage vector directional control and modulation. The outer loop voltage control is based on the measured capacitor voltages u _d1 , u _d2 and u _dn etc. The total DC voltage of the stable bridge arm capacitors is equal to the reference value U _dref , For the obtained active reference current command, specify the reactive reference current to zero. The inner loop current controller realizes the direct control of the current waveform of the AC side of the CHM rectifier, so that the output current i _s quickly tracks the reference current, which is reflected in the dq coordinate system as the active current i _sd , and the reactive current i _sq tracks their reference command v _dref and v _qref are the d and q components of the modulated wave obtained by the inner loop current control. The voltage equalization control makes the capacitor voltages of each HM module equal, and the obtained adjustment component Δd is superimposed on the modulation wave d component of each level. The triangular carrier waves of each HM module are staggered by 2π/N, and the triangular carrier waves between the bridge arms inside the modules are staggered by π, so as to realize CPS-SPWM modulation and obtain multi-level output waveforms.

In one embodiment of the present invention, each unit of the isolation conversion stage adopts a dual active bridge topology (DualActive Bridge, DAB) to control the bidirectional flow of energy; the total number of units is 3n, and one side of each DAB is connected to the HM chain link, and the other One side is connected in parallel to form a DC bus.

The isolation conversion stage transmits energy bidirectionally through 3n DABs to jointly realize the constant DC bus voltage. Taking a single DAB as an example, the driving signals of the two active bridges of the DAB are complementary trigger pulses with a duty cycle of 50%, and there is a phase shift angle for the conduction of the switch tubes of the two bridges. when When is positive, power flows in the forward direction, when When negative, power flows in reverse.

Due to the possible imbalance between the high-voltage DC side voltages and the inconsistency of the parameters of each DAB, the power flow between the DABs will be unbalanced in the steady state, and in extreme cases it may cause voltage and current stress on the device. is too big. Therefore, the average power control is added to each DAB, and the difference between the different power of each DAB and the command power will produce different phase shift angle adjustments. Thereby adjusting the phase shift angle to balance the power flow of each DAB.

DAB phase shift angle control is shown in Figure 4, is the reference value of the DC bus voltage, P ^* and i ^* are the power command and current command obtained through the PI regulator, and control the shifting angle The phase shift angle adjustment amount obtained by superimposing the power adjustment Get 50% duty cycle, phase difference square wave.

In one embodiment of the present invention, the first three bridge arms and the fourth bridge arm of the voltage source four-leg inverter adopted in the output stage are independent control parts. For the distribution network, it is expected that the output of SCDPET will not be affected by the high-voltage power grid, and will not be affected by violent fluctuations and imbalances in the load. This requires the front three bridge arms of the VSI to control the positive sequence and negative sequence components of the output voltage to ensure that the three Phase output constant voltage constant frequency. The fourth bridge arm controls the neutral current to eliminate zero-sequence voltage distortion; and controls the charging and discharging of the supercapacitor to supplement the power deficit. Wherein the first three bridge arms and the fourth bridge arm are two independent control parts.

The control strategy of the three-phase bridge arm is shown in Figure 5. The voltage outer loop is used to achieve constant control of the output voltage, and the current inner loop is used to achieve fast tracking of the output current. Both the voltage and current closed loops use decoupling control. u _a ^ref , u _b ^ref , u _c ^ref represent the reference signal of three-phase voltage output, u _od ^* , u _oq ^* represent the voltage reference signal under the dq axis, u _od , u _oq represent the actual value of the output voltage, and the voltage outer loop solution After coupling control, the output current reference values i _d ^* and i _q ^* under the dq axis are obtained. i _d and i _q represent the actual value of the output current. After the current inner loop decoupling control, the voltage modulation signals u _d and u under the dq axis are obtained _q , get the three-phase voltage modulation signals u _a ^* , u _b ^* , u _c ^* after coordinate transformation, and then drive VSI output after PWM modulation.

Specifically, an inductor is connected in series with the supercapacitor to reduce current ripple.

To sum up, SCDPET controls the input stage CHM rectifier to change the high-voltage AC into 3N suspended DC through the HM chain link module. The DC voltages of all levels are constant and equal, and the power factor of the high-voltage AC side is guaranteed; DC/AC conversion, the DC is modulated into high-frequency AC, coupled by a high-frequency transformer, the secondary side performs AC/DC conversion, and the high-frequency AC is restored to DC; the output stage inverts the DC to achieve constant voltage and constant frequency output; The fourth bridge arm regulates the neutral current and controls the charging and discharging of the supercapacitor at the same time.

The distribution network power electronic transformer (namely SCDPET) provided in this embodiment uses the fourth bridge arm as the charging and discharging interface of the supercapacitor energy storage system at the same time, without additional power conversion circuit and control system, which reduces the system cost.

The distribution network power electronic transformer provided in this embodiment has short-term uninterrupted power supply capability, and has ultra-high power fluctuation tolerance, and can realize stable operation under sudden load increase

The power electronic transformer for distribution network provided in this embodiment is seamlessly connected with supercapacitors, has fast response speed, and realizes flexible energy storage.

In the method for controlling the power electronic transformer of the distribution network provided in this embodiment, the fourth bridge arm adopts current composite control with superimposed commands, and the supercapacitor is charged and discharged without affecting the neutral current control.

The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: it can still be described in the foregoing embodiments Modifications are made to the recorded technical solutions, or equivalent replacements are made to some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims (6)

1. a kind of power distribution network electric power electric transformer, it is characterised in that described including input stage, isolated variable level and output stage Super capacitor is provided with output stage；

The output stage uses voltage-source type four bridge legs inversion topological, and the positive pole of the super capacitor is connected in four bridge legs Point, negative pole connection dc bus negative terminal；

Wherein, the power distribution network electric power electric transformer carries out discharge and recharge by bridge arm power device to the super capacitor；

The input stage is made up of using many level topologys of cascaded H-bridges, each bridge arm quantity for n HM chain link module-cascades, HM Chain link module main circuit is H bridge constructions, and three bridge arms are connected by Y types and constituted, and is 3n's by high pressure industrial frequency AC formation number The direct current of suspension；

The isolated variable level each unit is using double active bridge topology DAB, and total unit number is 3n, each DAB sides and HM chain link phases Even, opposite side composes in parallel dc bus；

The super capacitor is in series with inductance, to reduce current ripple.

2. a kind of power distribution network electric power electric transformer control method, it is characterised in that

The power distribution network electric power electric transformer includes being provided with input stage, isolated variable level and output stage, the output stage Super capacitor；

The output stage uses voltage-source type four bridge legs inversion topological, and the positive pole of the super capacitor is connected in four bridge legs Point, negative pole connection dc bus negative terminal；

The control method includes：

The four bridge legs are using stackable current-mode control is instructed, and centering line current and super capacitor charging and discharging currents are carried out Complex controll, wherein the instruction of four bridge legs current control isSpecifically, when super capacitor discharges, control targe is Keep DC bus-bar voltage constant, obtain current control instructionEqual to discharge currentWhen super capacitor charges, mesh is controlled It is designated as keeping super capacitor terminal voltage constant, obtains current control instructionEqual to charging current instructionWherein, center line electricity Flow control instructions are added by three-phase current and obtained, i_n=i_a+i_b+i_c, control four bridge legs output current-i_n, with bucking-out system The current in middle wire produced due to three-phase imbalance.

3. power distribution network electric power electric transformer control method according to claim 2, it is characterised in that the input stage is adopted With many level topologys of cascaded H-bridges, each bridge arm is made up of quantity for n HM chain link module-cascades, and HM chain link modules main circuit is H Bridge construction, three bridge arms are connected by Y types and constituted, by direct current of the high pressure industrial frequency AC formation number for 3n suspension；

Each bridge arm of input stage three-phase is symmetrical, and each phase control strategy is identical；Wherein each phase control strategy is used based on synchronous rotation Turn voltage, the current double closed-loop control of d-q coordinate systems, CPS-SPWM modulation strategies are shifted to reference to carrier wave.

4. power distribution network electric power electric transformer control method according to claim 3, it is characterised in that the isolated variable Level each unit is 3n using double active bridge topology DAB, total unit number, and each DAB sides are connected with HM chain links, and opposite side is composed in parallel Dc bus；

The isolated variable level realizes that DC bus-bar voltage is constant jointly by the DAB transmitted in both directions energy that quantity is 3n, wherein For single DAB, the drive signal of two active bridges is the complementary trigger pulse that dutycycle is 50%, and two bridges are to inductive switch There is a phase shift angle in pipe conductingWhenFor on the occasion of when, power forward flow, whenDuring for negative value, power reverse flow； Equal Power Control is added in each DAB, power different each DAB can produce different phase shifting angles from the difference of command power Spend regulated quantityTo adjust the flow of power that phase shifting angle balances each DAB.

5. power distribution network electric power electric transformer control method according to claim 4, it is characterised in that the output stage is adopted First three bridge arm and four bridge legs of voltage-source type four-leg inverter are independent control part, and wherein first three bridge arm is to output The positive sequence and negative sequence component of voltage are controlled, it is ensured that three-phase exports constant voltage constant frequency.

6. according to any described power distribution network electric power electric transformer control methods of claim 2-5, it is characterised in that described super Level capacitances in series has inductance, to reduce current ripple.