WO2022270818A1 - Bipolar power conversion device using zig-zag transformer - Google Patents

Abstract

The present invention relates to a bipolar power conversion device using a zig-zag transformer with respect to a cathode of a DC power source provided from a direct current distribution system, so as to enable independent power control of respective poles and load-side control dynamic characteristics to be improved. To this end, the bipolar power conversion device according to the present invention comprises: power sources of the cathode and an anode; first and second power conversion devices, which perform conversion on the basis of AC/DC from the power sources of the cathode and the anode, so as to output as a grid or a load; and a zig-zag transformer connected between the first and second power conversion devices and the grid or the load, wherein the zig-zag transformer is connected to a point of common coupling (PCC) end of the first and second power conversion devices, and is wired such that the neutral point of the zig-zag transformer coincides with the neutral point of the cathode or anode power source.

Description

Bipolar power converter using zigzag transformer

The present invention relates to a DC power distribution system, and more particularly, to a bipolar power converter capable of independent power control of each pole and improvement of load-side control dynamic characteristics by using a zigzag transformer for the poles of DC power supplied from the DC distribution system. It is about.

As the issue of carbon dioxide (greenhouse gas) emissions from the combustion of fossil fuels such as coal and oil has become serious, the international community is strengthening decarbonization policies by presenting carbon neutrality and reduction targets for each country in accordance with climate change.

In accordance with these greenhouse gas emission regulations and the seriousness of air pollution, the amount of power generation and power generation facilities using renewable energy are expanding.

With the spread of renewable energy sources, a plan to apply DC technology to DC-based power converters and distribution systems is emerging, and demand for LVDC (Low Voltage Direct Current) and MVDC (Medium Voltage Direct Current), which are next-generation power distribution methods, is increasing. It is a trend that

As a prior art, there is Korean Patent Publication No. 1020200113870, which has been published on the name of a DC/DC converter applied to an LVDC low voltage power distribution system. This is an LVDC low voltage including a battery unit that stores power, a PCS (Power Conditioning System) that converts the battery unit into a voltage corresponding to the load and supplies energy to the load, a control unit, and a supercapacitor unit in which supercapacitors are connected in series and parallel. In a DC/DC converter installed in a wiring system, the DC/DC converter is composed of a symmetrical schematic circuit in which power is transmitted in both directions, and two switches are connected in series, one of which conducts in the ground direction, A circuit in which a smoothing capacitor is connected in parallel to a circuit to which two switches are connected, an inductor is connected between the intermediate contacts of the two switches, and a ground contact is connected to each other in the connection of the two switches. The applied DC/DC converter is disclosed.

And, Korean Patent Publication No. 1020200053144 converts a DC input voltage into a first AC voltage, and includes an input unit including a plurality of connection terminals for outputting the first AC voltage; and a plurality of output units connected to the connection terminals and transforming the first AC voltage into a second AC voltage and converting the second AC voltage into a DC output voltage.

This DC distribution method can provide higher efficiency and flexibility than the conventional AC distribution method, and the DC distribution method is divided into a monopole method and a bipolar method.

In the unipolar structure, an anode and a cathode supply one common DC voltage, and in a bipolar structure, a neutral point is shared between an anode and a cathode, so that each independent DC voltage can be supplied.

Here, the unipolar structure having one polarity has the advantage of being easy to control due to its simple structure, but has a disadvantage of low power supply reliability because the entire power is cut off in the event of a DC link fault. On the other hand, the bipolar structure is capable of independent operation of the two poles and has the advantage of being able to deal stably in the event of a DC link accident, so research and development on this are required.

The present invention reflects the above-described needs, and provides a bipolar power converter capable of independently controlling power of each pole and improving load-side control dynamic characteristics by using a zigzag transformer for the poles of DC power provided from a DC power distribution system. aims to do

To this end, the bipolar power converter according to the present invention converts the positive and negative power sources and the positive and negative power sources on an AC/DC basis and outputs the first and second power to a grid or a load. A conversion device and a zigzag transformer connected between the first and second power conversion devices and the grid or load,

The zigzag transformer is connected to PCC (Point of Common Coupling) terminals of the first and second power converters, and the neutral point of the zigzag transformer is connected to coincide with the neutral point of the positive and negative power sources. Characterized in that.

In addition, the first and second power converters may correspond to any one of a 2-level DC-AC inverter and a 3-level or higher multi-level DC-AC inverter.

In addition, the first and second power converters are DC-AC inverters, and the first and second inverters are connected in parallel based on AC PCC, and the PCC terminals of the first and second inverters connected in parallel The connected zigzag transformer is characterized in that it is connected in parallel to the grid or load of the AC system.

In addition, the first and second power converters are DC-AC inverters, and the first and second inverters are connected in series based on AC PCC, and the PCC terminals of the first and second inverters connected in series The connected zigzag transformer is characterized in that it is connected in series with a grid or load of an AC system in a delta or wye connection.

In addition, the control unit for driving the first and second inverters is characterized in that it is composed of a DC link voltage control unit and a DC link current control unit for DC offset injection of the DC link.

In addition, a protection circuit may be further included between the zigzag transformer and the first and second power converters.

The present invention enables independent power control of each pole and improvement of load-side control dynamic characteristics by using a zigzag transformer for the poles of DC power supplied from a DC power distribution system.

In addition, it is possible to cope with different loads for each pole of the DC power provided from the DC power distribution system, as well as to actively and stably cope with rapid current control and DC link accidents.

1 and 2 show a circuit diagram of a bipolar power converter using a zigzag transformer according to a preferred embodiment of the present invention, and FIG. 1 is a diagram illustrating a state in which zigzag transformers 200 are connected in parallel, FIG. 2 is a diagram illustrating a state in which zigzag transformers 300 are connected in series,

3 is a diagram illustrating the structure of zigzag transformers 200 and 300 in the circuit diagram of the bipolar power converter of FIGS. 1 and 2;

4 is a diagram showing an average equivalent circuit model of a 3-phase 2-level inverter in the circuit diagram of the bipolar power converter of FIG. 2;

5 is a diagram showing an entire equivalent model of a proposed bipolar DC power distribution inverter circuit using the average equivalent circuit model of FIG. 4;

6 is a block diagram for controlling V _PG in the bipolar DC power distribution inverter circuit of FIG. 5;

7 is a diagram showing the hardware configuration of a bipolar power converter according to a preferred embodiment of the present invention of FIGS. 1 and 2 using a 10 kVA class prototype converter;

8 and 9 are graphs showing simulation results and experimental results through the experimental apparatus of FIG. 7 .

The present invention can improve the reliability of the system by independently converting the bipolar structure of the DC power provided from the DC power distribution system into AC and then transforming it.

That is, the present invention enables not only to cope with different loads for each pole of DC power supplied from the DC power distribution system, but also to actively and stably cope with rapid flow control and DC linkage accidents.

The configuration and operation of a bipolar power converter using a zigzag transformer according to a preferred embodiment of the present invention will be described in detail below with reference to the drawings.

1 and 2 are circuit diagrams of a power converter using a zigzag transformer according to a preferred embodiment of the present invention, and FIG. 1 is a diagram illustrating a state in which zigzag transformers 200 are connected in parallel, and FIG. 2 is a delta zigzag It is a diagram illustrating a state in which the transformers 300 are connected in series.

The zigzag transformers 200 and 300 according to the present invention described above may be connected in series or parallel to the output terminals of the first and second inverters 100 and 102 . 1 is a diagram illustrating a state in which zigzag transformers 200 are connected in parallel according to the present invention, and FIG. 2 is a diagram illustrating a state in which delta zigzag transformers 300 are connected in series.

Referring to FIG. 1, a bipolar power converter using a zigzag transformer according to the present invention uses an AC PCC (Point of Connect in parallel based on Common Coupling, and connect so that the neutral point of the Zigzag Transformer (200) connected to the PCC terminal of the parallel inverter coincides with the neutral point (G) of the DC power supply.

As shown in FIG. 1, the positive node (P node) of the DC link voltage of the upper inverter 100 provides the positive pole of the entire bipole DC link, and the lower inverter 102 The negative node (N node) of the DC link voltage of N provides the negative pole of the entire bipole DC link. By providing the neutral point (G) of the zigzag transformer 200 having a low impedance to the AC zero component, the power converter can provide a complete bipolar operating structure, and each inverter uses a sinusoidal pulse width modulation method (Sinusoidal Pulse Width Modulation : SPWM). At this time, the circuit construction method in the bipolar power converter according to the present invention is not limited to a 2-level DC-AC inverter, but uses general AC/DC-based power converters such as a 3-level or higher multi-level DC-AC inverter. Therefore, extended application is possible.

Hereinafter, the bipolar power converter according to the present invention will be described in more detail, focusing on a two-level DC-AC inverter as a circuit configuration method.

Referring to FIG. 1, a first inverter 100 receives positive DC power and converts it into first AC power and outputs it, and a second inverter 102 receives negative DC power and converts it into second AC power and outputs it. ), and providing a neutral pole to the first and second inverters 100 and 102, and receiving the first and second AC power from the first and second converters 100 and 102 to output a predetermined output It consists of a zigzag transformer 200 that transforms and outputs AC power.

The first inverter 100 is composed of a capacitor and a bridge circuit, receives positive DC power from a DC link, converts it into first AC power, and outputs it.

The second inverter 102 is composed of a capacitor and a bridge circuit, receives negative DC power from the DC link, converts it into second AC power, and outputs it.

The zigzag transformer 300 provides a neutral point for the positive DC power and the negative DC power input to the first inverter 100 and the second inverter 102, and the converter and the second inverter 100, 102 ) receives the first and second AC power output and transforms and outputs them.

In addition, a protection circuit composed of inductance and resistance may be further provided between the zigzag transformer 200 and the first and second inverters 100 and 102, which is obvious to those skilled in the art according to the present invention.

Referring to FIGS. 1 and 2, the zigzag transformers 200 and 300 of the bipolar power converter according to the present invention provide low impedance on the ground side by connecting A, B, and C in a zigzag manner in the order of A, B, and C, respectively. to form a neutral point.

In addition, the positive electrode of the first inverter 100 and the negative electrode of the second inverter 102 are connected in series or parallel to the neutral point formed by the zigzag transformers 200 and 300, and the DC terminals of the first and second inverters 100 and 102 are connected. A neutral terminal is provided to provide a bipolar structure.

In addition, it is possible to control the current for the DC link neutral point by adjusting the zero-sequence reference, so that the DC link voltage (V _PG ,V _GN ) for the anode is also adjustable, and in FIGS. 4 to 6 described later Explain in detail.

The above zigzag transformers 200 and 300 will be described in more detail. 3 illustrates the structure of the zigzag transformers 200 and 300 described above.

3 (a) and (b) illustrate the structure of a grounding transformer. The grounding transformer is a transformer for creating a neutral point in the power system and detects voltage and current, which are elements necessary for system protection. In particular, a relatively low impedance path is provided for the neutral wire connected to the neutral point, and temporary overvoltage is limited when a ground fault occurs. These grounding transformers use zigzag and delta or wye connection transformers.

3(c) and (d) illustrate the structure of the zigzag transformer 300, the primary sides of the zigzag transformers 200 and 300 are connected with one winding on each of u, v, and w, and 2 The secondary has a zigzag shape with two windings connected on each phase, and the zero sequence current in the two windings on the same limb cancels the ampere turns.

These zigzag transformers 200 and 300 are mainly used when a neutral point cannot be obtained in a power system and is also referred to as a grounding transformer.

Windings a ₁ and a ₂ constituting the zigzag transformers 200 and 300 are on the same limb and are wound in opposite directions at the same turn ratio. All currents are the same (i _a1 =i _a2 =i _b1 =i _b2 =i _c1 =i _c2 ) to maintain the MMF (Magnetic Motive Force) balance of each leg. These zigzag transformers (200, 300) have the advantage of being able to remove the harmonic current of the neutral zero sequence in a three-phase, four-wire system.

In particular, the zigzag transformers 200 and 300 according to the present invention provide a neutral pole for DC power input to the first and second inverters 100 and 102 to form a bipolar structure.

And Figure 3 (e) shows the equivalent circuit of the zigzag transformer (200, 300). The zigzag transformers 200 and 300 according to the present invention set the Z _G value to 0 in order to flow the neutral point current components generated during parallel operation of the first and second inverters 100 and 102 to the neutral point of the zigzag transformers 200 and 300 and simulated. In this way, the present invention can improve DC-side characteristics of the first and second inverters 100 and 102 by providing a neutral point through the zigzag transformers 200 and 300 .

<Analysis of zero-segment offset voltage using average model>

FIG. 4 is a diagram showing the average equivalent circuit model of the 3-phase 2-level inverter in the circuit diagram of the bipolar power converter of FIG. 2, and FIG. 5 is a bipolar DC power distribution inverter proposed using the average equivalent circuit model of FIG. It is a diagram showing the entire equivalent model of the circuit.

Referring to FIGS. 4 and 5 , V _PG and V _GN represent voltages between the neutral point of the zigzag transformer and the positive and negative poles of the DC link, and V _PN represents the total voltage between the positive and negative poles.

In FIG. 4, d _a V _dc of the a phase means V _a and has the relationship of Equation 1 below. (However, dχ is the duty cycle of the x phase, and x is one of a, b, and c phases.)

Assuming that the grid voltage is three-phase balanced, the sum of the phase voltages of the upper and lower inverters is zero. Based on this, the Kirchhoff voltage equation for the DC equivalent circuit of FIG. 3 is as follows. In addition, V _OG = 0 is assumed by reflecting the characteristics of the zigzag transformer described later.

That is, the DC link voltage is expressed as the sum of the half of the DC link voltage of each inverter and the offset voltage. The DC link voltage and the offset voltage component of each pole have relational expressions such as Equations 2 to 4, and it can be seen that the DC link voltage can be controlled according to the injection of the zero-phase component of each pole.

<Average model based offset voltage injection control method>

FIG. 6 is a block diagram for controlling the positive pole V _PG in the bipolar DC power distribution inverter circuit of FIG. 5 .

Referring to FIG. 6, the control configuration for driving the bipolar DC power distribution inverter circuit of FIG. 5 consists of DC link voltage control of the upper inverter and DC link current control for DC offset injection of the DC link. Also, the same control can be applied to the bottom inverter to control V _GN .

At this time, the offset voltage, which is an output of the DC link current controller, is added to the inverter pole voltage command.

Here, Equations 5 and 6 represent the offset voltage of each pole, and the DC link voltage can be controlled by injecting the offset voltage of each pole through an average equivalent model analysis.

<DC link dynamic response using zigzag transformer characteristics>

In FIG. 2, the transformer has a delta connection on the grid side and provides a circulation path for third harmonic current due to core nonlinearity. As shown in FIG. 2, the inverter stage of the transformer applied in the present invention consists of a zigzag connection. The zigzag transformer is mainly used for the purpose of providing a neutral point of a power system and grounding for protection.

At this time, due to the principle and structural characteristics of the zigzag transformer, the zero-segment impedance is very small, about the size of the leakage impedance, because the flux due to the zero-sequence component of the transformer winding current cancels each other. Therefore, when the neutral point of the zigzag transformer is connected to the neutral point of the DC link bipole, a very fast DC link dynamic response can be obtained.

<Simulation and experiment results>

Simulations and experiments were performed to verify the validity of the DC link dynamic characteristics improvement technique of the proposed bipolar power converter.

7 is a diagram showing the hardware configuration of a bipolar power converter according to a preferred embodiment of the present invention of FIGS. 1 and 2 using a 10 kVA class prototype converter, and a table of conditions used in simulation and experiment Same as 1.

Parameter	Value
DC voltage ( V dc-link )	400V
Grid Inductance (L g )	4mH
switching frequency (f s )	5kHz
Interface inductance (L sh )	2mH
DC side load resistance ( R load )	60Ω

In order to verify the independent control of the anode and the improvement of the dynamic characteristics, the output response characteristics according to the DC link offset voltage injection were simulated. The experiment was performed based on the simulation results, and FIGS. 8 and 9 are graphs showing the simulation results and the experimental results. .

8A shows DC link voltages (V _PG , V _GN ) when no offset voltage is injected into each pole.

As shown in Equations 3 and 4 analyzed through the average equivalent model, it can be seen that half of the converter DC link voltage appears.

In FIG. 8B , while initially operating without applying an offset voltage, the anode voltage (V _PG ) is increased by 40V by injecting the offset voltage to the anode at 1 second, and the DC link voltage (V _PN ) is also increased by 40V. At this time, it took 0.01 second to reach the steady state after the injection of the offset voltage.

FIG. 8c shows that while operating in the same initial state as FIG. 8b, a negative offset is injected into the positive electrode and a positive offset is injected into the negative electrode for 1 second to increase the DC link voltages (V _PG , V _GN ) by 40V, respectively, and return to normal after 0.01 second. It has a status response.

In FIG. 9a, as in FIG. 8b, an offset of 25V was injected into only the anode, and a steady-state response was obtained after 0.005 seconds.

In FIG. 9b, as shown in FIG. 8c, an offset of 25V was injected into the positive and negative electrodes, and it was confirmed that a steady state was reached after 0.005 seconds.

Through the above simulation and experimental results, it was verified that independent voltage control and dynamic characteristics improvement of each pole according to offset voltage injection are possible.

Therefore, the present invention proposes an improved bipolar structure using a zigzag transformer to ensure reliability and flexibility of a DC distribution system. At this time, the zero-sequence offset voltage component of each pole was analyzed through the average model of the proposed circuit, and it was confirmed that the converter DC link voltage control is possible according to the zero-sequence injection of each pole. In addition, it was confirmed that the independent control of each pole can improve the dynamic characteristics of the control bandwidth and secure the load stability in the event of a DC link fault.

In addition, the proposed circuit and dynamic response characteristics were validated through simulation and a 10kVA-class prototype hardware device.

In the above description, the present invention has been shown and described in relation to specific embodiments, but it is common knowledge in the art that various modifications and changes are possible without departing from the spirit and scope of the invention indicated by the claims. Anyone who has it will be able to easily understand.

[Description of code]

100: first inverter

102: second inverter

200, 300: zigzag transformer

The present invention enables independent power control of each pole and improvement of load-side control dynamic characteristics by using a zigzag transformer for the poles of DC power supplied from a DC power distribution system.

In addition, it is possible to cope with different loads for each pole of the DC power provided from the DC power distribution system, as well as to actively and stably cope with rapid current control and DC link accidents.

Through this DC-based power converter and distribution system, it can be applied to power generation and power generation facilities using renewable energy.

Claims (6)

1. The positive and negative power sources,

   First and second power converters for converting AC/DC from the positive and negative power sources and outputting them to a grid or a load;

   And a zigzag transformer connected between the first and second power converters and the grid or load,

   The zigzag transformer is connected to PCC (Point of Common Coupling) terminals of the first and second power converters, and the neutral point of the zigzag transformer is connected to match the neutral point of the positive and negative power sources Bipolar power converter.
2. According to claim 1,

   The first and second power converters,

   A bipolar power converter, characterized in that any one of a 2-level DC-AC inverter, a 3-level or higher multi-level DC-AC inverter.
3. According to claim 2,

   The first and second power converters are DC-AC inverters,

   The first and second inverters are connected in parallel based on AC PCC,

   The zigzag transformers connected to the PCC terminals of the first and second inverters connected in parallel are connected in parallel to a grid or load of an AC system.
4. According to claim 2,

   The first and second power converters are DC-AC inverters,

   The first and second inverters are connected in series based on AC PCC,

   The zigzag transformer connected to the PCC terminals of the first and second inverters connected in series is a bipolar power converter, characterized in that connected in series to a grid or load of an AC system in a delta or wye connection.
5. According to claim 3 or 4,

   The control unit for driving the first and second inverters,

   A bipolar power converter characterized in that it consists of a DC link voltage control unit and a DC link current control unit for DC offset injection of the DC link.
6. According to claim 1,

   A bipolar power converter, characterized in that a protection circuit is further included between the zigzag transformer and the first and second power converters.

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