KR20220170645A - Bi-directional power conversion system with wide DC bus control range - Google Patents

Abstract

The present invention relates to a bidirectional power conversion system and, more specifically, to a bidirectional power conversion system for enabling a bidirectional power flow by using a center-tapped transformer and a full-bridge circuit and capable of controlling a DC bus voltage over a wide range. To this end, the present invention provides the bidirectional power conversion system which comprises: a DC source; an AC grid or load; a center-tapped transformer connected to the AC grid or load; and a power conversion device connected between the center-tapped transformer and the DC source and performing conversion based on AC/DC.

Description

Bi-directional power conversion system with wide DC bus control range

The present invention relates to a bidirectional power conversion system, and more particularly, to a bidirectional power conversion system capable of enabling bidirectional power flow and controlling a DC bus voltage in a wide range using a center tapped transformer and a full-bridge circuit.

The importance of renewable energy sources such as photovoltaic power generation, wind power generation, and fuel cells and various distributed power generation systems using them is being emphasized day by day worldwide. Along with this trend, a system for stable and efficient power conversion is required, and a lot of research and development are being conducted accordingly.

In particular, as the functional requirements of a power conversion system coupled to a DC-based energy source with variable voltage and an energy storage system requiring bi-directional power conversion increase, multi-purpose bi-directional power conversion technology is in the limelight.

The overall configuration of the energy storage system (ESS: Energy Storage System) is a power conversion device PCS (Power Conversion System), a battery management system BMS (Battery Management System) and ESS control system EMS (Energy Management System) include The PCS is a device that converts power supplied from various energy sources into commercial AC power or converts power suitable for storage in a battery cell. At this time, energy conversion is required in both directions between the voltage of the DC link and the battery cell, and a power conversion device that plays this role is called a bidirectional power converter system.

1 is a diagram illustrating a topology of a conventional bidirectional power conversion system, and generally, a structure in which an isolated AC-DC converter and a DC-AC inverter of FIG. 1 are combined is widely applied in the corresponding application field.

Referring to FIG. 1, the conventional bidirectional power conversion system converts DC power from a DC source into AC power, converts it to high frequency, converts it back to DC power, and then converts it back to AC and transmits it to a load. Then, the AC power from the grid is converted into DC power, converted back into AC power, subjected to high-frequency transformation, and then converted into DC power and delivered to a DC source such as a battery.

The high-frequency transformer enables more power transmission than a low-frequency transformer based on the same size (volume and weight), and the DC current of the source is first converted into AC for stable input power. Such a conventional bi-directional power conversion system not only has minimal isolation, but also has a problem that two power converters are exposed to a fault current when an error occurs, which causes damage to an expensive power converter.

In order to strengthen the active response to the increasing energy crisis and to increase the energy consumption efficiency of outdated domestic buildings, the development of a bi-directional power conversion system for ESS is required.

In particular, there is a demand for development of a bidirectional power conversion system capable of reducing control complexity and cost by reducing the number of power converters.

In addition, there is an urgent need to develop a technology for ensuring stability of the system by providing appropriate isolation for the power converter when the fault current increases.

Republic of Korea Patent Application No. 10-2017-0050151 Republic of Korea Patent Application No. 10-2014-0120651 Republic of Korea Patent Application No. 10-2017-0097885

The present invention reflects the above-described needs, and an object of the present invention is to provide a bidirectional power conversion system that enables bidirectional power flow using a center tapped transformer and a full-bridge circuit and can control a DC bus voltage in a wide range.

Another object of the present invention is to provide a bi-directional power conversion system that protects the power conversion device by appropriately isolating the power conversion device when the fault current increases to ensure the safety of the system.

To this end, the bidirectional power conversion system according to the present invention includes a DC source; AC grid or load; a center tapped transformer connected to the AC grid or load; and a power converter connected between the center tap transformer and the DC source to perform AC/DC conversion.

In addition, the power converter may correspond to any one of a DC-AC inverter, an AC-DC converter, a DC-DC converter, and an AC-AC converter.

In addition, the power conversion device is a DC-AC inverter,

The center tap transformer receives AC power from the grid, transforms it, and transfers it to the inverter or transforms AC power from the inverter and transfers it to the grid or load;

The inverter converts AC power from the center-tap transformer into DC power and transfers it to the DC source side, or converts DC power from the DC source side into AC power and transfers it to the center-tap transformer.

In addition, the inverter is characterized in that it is composed of one or more full-bridge modules.

In addition, the full-bridge module is characterized in that it is composed of a capacitor and a bridge circuit connected between the DC source and the center tap transformer.

In addition, the DC source is a variable DC (Variable DC) source,

The DC-AC converter is composed of first and second full-bridge modules,

The first full-bridge module is connected to one end of the variable DC source and a first end of the center tapped transformer;

The other end of the variable DC source is connected to the center tap of the center tap transformer;

The first full-bridge module is composed of a first bridge circuit and a first capacitor, and charges the first capacitor with power from the variable DC source, and the energy charged in the first capacitor exceeds a first level. It is characterized in that when AC power is supplied to the center tap transformer through the first stage of the center tap transformer with the charged energy.

In addition, the second full-bridge module is connected to one end of the variable DC source and a second end of the center tapped transformer, and is composed of a second bridge circuit and a second capacitor to supply power from the variable DC source to the second capacitor. In addition, when the energy charged in the second capacitor reaches a second level or higher, the charged energy is provided as AC power to the center tap transformer through the second terminal of the center tap transformer.

In addition, the AC power provided by the center tap transformer through the first stage of the center tap transformer is rectified through the first full-bridge circuit and transferred to the variable DC source side;

AC power provided by the center-tap transformer through a second stage of the center-tap transformer may be rectified through the second full-bridge circuit and transferred to the variable DC source side.

The present invention enables bi-directional power flow and wide-range DC bus voltage control using a center-tapped transformer and a full-bridge circuit.

In addition, it is possible to provide a single power converter that performs a single power conversion rather than a multi-stage power conversion combining an existing AC-DC converter and a DC-AC inverter.

In addition, since all bridge circuits can be connected in series in the single power conversion device, multiple voltage levels can be synthesized through series connection of all bridge circuits to improve output quality.

In addition, when the fault current increases, the power conversion device is properly separated to protect the power conversion device and ensure the safety of the system.

1 is a diagram showing the topology of a conventional bi-directional power conversion system;
2 is a functional conceptual diagram of a bi-directional power conversion system according to a preferred embodiment of the present invention;
3 is a diagram showing the topology of a bi-directional power conversion system 100 according to a preferred embodiment of the present invention;
4 is an overall circuit diagram of the bidirectional power conversion system 100 of FIG. 3;
5 to 7 are diagrams showing the operating principle of the bi-directional power conversion system 100 according to the present invention of FIGS. 2 to 4 as an equivalent circuit;
8 is a graph showing simulation results for (a) input dc current, (b) capacitor voltage, (c) grid current and voltage when received from the system, and (d) grid current and voltage when transmitted to the system, respectively;
9 shows (a) reference and grid current of the current controller, (b) reference and grid current expanded to a certain period in a steady-state, (c) between the grid and the DC side. Reference and grid current expanded to a certain period in a transient-state performing power transmission and reception change, (d) a graph showing simulation results for the Fourier spectrum of ac voltage and current in steady state,
10 is a diagram showing an example of an experimental set configured to experimentally verify the effectiveness of a circuit proposed in a bidirectional power conversion system according to the present invention;
11 is a graph showing waveforms as a result of an experiment with a DC link voltage of 0V and a rated voltage of 100V in order to simulate variable DC link voltage synthesis.

The present invention provides a bi-directional power conversion system coupled to a DC-based energy source having a variable voltage and an energy storage system requiring bi-directional power conversion.

Hereinafter, the configuration and operation of a bi-directional power conversion system according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

<Function and composition of bi-directional power conversion system>

Figure 2 shows a functional conceptual diagram of a bi-directional power conversion system according to a preferred embodiment of the present invention, Figure 3 shows a topology of a bi-directional power conversion system 100 according to a preferred embodiment of the present invention, Figure 4 shows an overall circuit diagram of the bidirectional power conversion system 100 of FIG. 3 .

As shown in FIG. 2, the bi-directional power conversion system 100 receives AC power from the grid 102, converts it into DC power, stores it in the battery 104, and simultaneously supplies power from the battery 104. DC power is converted into AC power, and bi-directional conversion is performed to supply it to the camping car 106, building 108, or factory 110. At this time, the bidirectional converter performs communication with an EMS (Energy Management System), which is a system that controls an ESS (Energy Storage System).

In addition, although the bidirectional power conversion device 100 in FIG. 2 is mainly described with the battery 104 of the ESS and bidirectional conversion, it is not limited thereto, and may be combined with various types of DC-based energy sources having variable voltages. For example, it is possible to perform two-way conversion with energy sources such as solar power generation, wind power generation, and fuel cells.

In general, in a narrow sense, a converter means a device that converts alternating current into direct current, and an inverter means a device that converts direct current into alternating current. It is a broad concept meaning a conversion device between the liver.

At this time, the bidirectional power converter 100 or the converter in the power field according to the present invention is not limited to a DC-AC inverter, but is a common AC such as an AC-DC converter, a DC-DC converter, and an AC-AC converter. / It is possible to expand the application by using DC-based power converters.

Hereinafter, the bidirectional power converter 100 according to the present invention will be described in more detail with a focus on the DC-AC inverter.

<Configuration of bi-directional power conversion system>

Referring to FIG. 3, the bidirectional power conversion system 100 according to the present invention includes a DC source, a DC-AC inverter 200, a center-tapped transformer 202, and an AC end grid or AC load (Grid or Load).

The DC-AC inverter 200 converts DC power from a DC source such as a battery into AC power and transmits it to the center tap transformer 202, or converts AC power from the center tap transformer 202 into DC power. and provide it as a DC source such as a battery.

The center tap transformer 202 receives AC power from the DC-AC inverter 200 and converts it to a first level and provides it as a load, or receives AC power from a grid and converts it to a second level. The voltage is converted to and provided to the DC-AC inverter 200.

Here, the bidirectional power conversion system 100 according to the present invention has one power conversion step, unlike the conventional bidirectional power conversion system of FIG. 1 . That is, the conventional two-way power conversion system of FIG. 1 includes several power conversion steps and can reduce the overall system size by using a high-frequency transformer, but the ripple effect due to an AC grid accident connected to the grid directly affects the inverter. affect.

On the other hand, the bidirectional power conversion system 100 according to the present invention has a structure in which the center tap commercial AC frequency transformer 202 is connected to the grid (AC link or load), and the size of the system is somewhat increased, but the ripple effect due to AC link failure is reduced to the inverter. It has a small effect on the inverter, thereby strengthening the inverter protection function and simultaneously controlling both the AC and DC components independently in the event of an accident.

<Circuit configuration of bi-directional power conversion system 100>

FIG. 4 shows an overall circuit diagram of the bidirectional power conversion system 100 of FIG. 3 . The bidirectional power conversion system 100 includes two full bridge modules (Full Bridge Modules, 302 and 304), a variable DC voltage source 310, a center tap transformer 300 and an AC stage system or an AC load (AC grid or load).

The first full-bridge module 302 connected to one end of the variable DC voltage source and the first end of the center tapped transformer 300 is composed of a bridge circuit and a capacitor to supply power from the DC voltage source to the capacitor. In addition, when the energy charged in the capacitor exceeds a predetermined level, the charged energy is supplied to the center tap transformer 300 as AC power through the first terminal of the center tap transformer 300.

The second full-bridge module 304 connected to one end of the variable DC voltage source and the second end of the center tapped transformer 300 is composed of a bridge circuit and a capacitor, and charges the capacitor with power from the DC source. When the energy charged in the capacitor exceeds a predetermined level, the charged energy is supplied to the center tapped transformer 300 as AC power through the second terminal of the center tapped transformer 300.

The other end of the variable DC voltage source 310 is connected to the center-tapped center of the center-tapped transformer 300 .

Further, the AC power provided by the center tap transformer 300 through the first stage of the center tap transformer 300 is rectified through the first full bridge circuit 302 and transferred to the variable DC voltage source 310 side. .

Further, the AC power provided by the center tap transformer 300 through the second stage of the center tap transformer 300 is rectified through the second full bridge circuit 304 and delivered to the variable DC source side.

The center tap transformer 300 is composed of first to third coils, the first and second coils are connected in series, and one end of the first coil is the first end of the center tap transformer 300 and is connected to the first full bridge. circuit 302, and the other end of the first coil is connected to the second coil and the negative electrode of the variable DC source 310. One end of the second coil is connected to the first coil and the negative electrode of the variable DC source 310, and the other end of the second coil is connected to the second full-bridge circuit 304 as a second end of the center tapped transformer 300. . The third coil is located close to the first and second coils to generate and output energy according to the energy induced from the first and second coils. Energy excited by the third coil is transferred to the load LOAD.

And, in the center tap transformer 300, AC power provided from the grid is provided to the third coil, and the first and second coils are located close to the third coil to generate energy according to the energy induced from the third coil. and transferred to the first to second full-bridge circuits 302 and 304.

In addition, line impedances 306 and 308 located between the center tap transformer 300 and the first and second full-bridge circuits 302 and 304 limit error current from being supplied to the converter. At this time, the line impedances 306 and 308 are composed of a reactor L and a resistor R connected to the reactor in series to suppress a DC bias.

Here, in the bidirectional power conversion system 100 according to the present invention, as shown in FIG. 4, only one full-bridge module is connected to both ends of the center-tap transformer, but the AC power input to the center-tap transformer is exemplified. It is also possible to connect multiple full-bridge circuits to modify the waveform, which is apparent to those skilled in the art by virtue of the present invention.

In addition, the bidirectional power conversion system 100 can reduce the number of inverters required, as well as reduce the complexity of control and reduce costs. In addition, when the fault current increases, the inverter is properly disconnected to protect the inverter.

The operating principle of the bidirectional power conversion system 100 according to the present invention of FIGS. 2 to 4 described above with reference to FIGS. 5 to 7 will be described based on an equivalent circuit, and the DC link voltage and current control and full bridge Capacitor energy balancing control is described.

As shown in FIG. 5A, since the full-bridge module can synthesize both DC and AC voltages, the current flowing through the arm has both DC and AC current components. In addition, since the DC current component flows in from both sides of the secondary terminal of the transformer, the DC components of the magnetic flux induced in the transformer core cancel each other out. These DC current components return to the DC link along the middle tap connection of the transformer. Therefore, only the AC magnetic flux is transferred to the first block through the transformer, and power is transmitted to the AC grid and the load of the AC link. DC and AC can be independently controlled through the characteristics of these circuits. As shown in FIG. 5A, the voltages synthesized by each full-bridge circuit combine DC and AC voltages of the same magnitude, and the AC voltages have a phase difference of 180 degrees from each other. The winding ratio of the transformer can be changed according to the converter output design, and in the present invention, for simplicity of discussion, N ₁ = 2N ₂ was assumed.

Referring to FIG. 5A, the relationship between voltage and current using KVL in an equivalent circuit is as shown in Equation 1 below, and can be expressed as Equation 2 assuming that v* _DC1 =v* _DC2 =v* _DC .

[Equation 1]

[Equation 2]

Accordingly, the AC equivalent circuit according to Equation 2 is shown in FIG. 5B. Here, Z _EqAC =2Z _arm .

Referring to FIG. 5B and Equation 2, the DC component does not appear in the AC link. On the other hand, if the equivalent circuit of FIG. 5A is expressed using KVL based on the DC link, the relationship between voltage and current is as shown in Equation 3, which The DC equivalent circuit of FIG. 5c can be obtained using Here, Zeqdc = Zarm/2.

[Equation 3]

Accordingly, the DC link and the AC link can be independently controlled without saturation of the transformer core by synthesizing the full bridge output voltages (V* _FB1 , V* _FB2 ) as shown in Equation 4 in the full bridge circuit.

[Equation 4]

Here, V* _DC and V* _ac are output control commands of the DC line voltage controller and the AC line voltage controller, respectively.

<Capacitor Energy Control of Full-Bridge Circuit>

The bidirectional power conversion circuit according to the present invention includes a full-bridge circuit, and the full-bridge circuit is composed of a capacitor as a finite energy source and a power semiconductor switch (bridge circuit). Therefore, balance control of the capacitor energy in this system is essential. Capacitor energy control consists of total energy control and energy equalization control between individual capacitors.

5C shows a DC equivalent circuit for controlling the DC stage input/output and controlling the total energy of the capacitor. In order to keep the total energy of the capacitor constant, it is necessary to adjust additional power to maintain the total energy in addition to the input/output power in the DC or AC circuit. In the present invention, assuming that the DC link is a robust power source, the power for maintaining the total energy of the capacitor is adjusted in the DC link. First, a DC link current controller is configured based on the equivalent circuit of FIG. 5C, as shown in FIG. The command (i* _DC ) of the DC link current controller is composed of the sum of the direct current command for active power control in EMS (Energy Management System) and the direct current command for capacitor total energy control. The capacitor total energy controller is configured as shown in FIG. 7A. It is assumed that the capacitor energy used in the controller is calculated as in Equation 5, and the capacitance of the capacitors of the full-bridge circuit is the same.

[Equation 5]

[Equation 6]

Next, the all-bridge cell balancing controller of FIG. 7B may be configured based on the equivalent circuit of FIG. 5D in order to balance the capacitor energy between all bridge circuits.

Referring to FIG. 7B, the command of the balancing controller is 0, and control is performed to make Equation 6, which is the difference between the capacitor energies of the two bridges, become 0.

As in the equivalent circuit of FIG. 5D, a current (i _bal ) for balancing is generated by generating a difference (v* _bal ) between the output voltages of the two bridges. In fact, v* _bal has a very small value compared to the synthesized voltage for DC link and AC link input/output power control, and when an imbalance of capacitor energy occurs, the input/output power is relatively large, so the current (i _bal ) for balancing The value also has a very small value compared to direct current and alternating current. At this time, when the capacitor energy control of the full-bridge circuit described above is normally performed, the present system can operate stably in the entire output range.

<Simulation and experiment results>

In order to verify the bidirectional power conversion circuit and control algorithm according to the present invention, simulations and experiments were performed for a 7kW inverter. Table 1 shows the integers used in the PLECS-based simulations and experiments.

Parameter Value Rated Output Power 7[kW] Capacitor voltage 300[V] DC bus voltage 100[V] AC side voltage (line-to-line rms) 220[V]/60[Hz] Switching frequency (fsw) 5kHz

In this system, the DC link voltage is rated at 100V, and the voltage is variable. In order for the full-bridge circuit to independently control the DC link and the AC link, the capacitor voltage was balanced so that it was maintained at 300V.

8 is a graph showing simulation results for (a) input dc current, (b) capacitor voltage, (c) grid current and voltage when received from the system, and (d) grid current and voltage when transmitted to the system.

9 shows (a) reference and grid current of the current controller, (b) reference and grid current expanded to a certain period in a steady-state, (c) between the grid and the DC side. It is a graph showing the simulation results for the reference and grid current expanded to a certain period in the transient-state performing power transmission and reception change, (d) the Fourier spectrum of the ac voltage and current in the steady state.

As shown in FIGS. 8 and 9 , in order to simulate bi-directional power transfer, power was transferred from the DC link to the AC link in the initial stage of the simulation, and power was transferred from the AC link to the DC link in the latter half. In order to check the DC link voltage control performance, a voltage change of 20 V was simulated for a short time at 20 and 50 seconds, and it was confirmed that it was well controlled without capacitor energy imbalance or system dissipation.

To experimentally verify the effectiveness of the proposed circuit, an experimental set of FIG. 10 was constructed. FIG. 11 is a result waveform obtained by experimenting with a DC link voltage of 0V and a rated voltage of 100V in order to simulate variable DC link voltage synthesis. After connecting the DC link of the full-bridge circuit to the DC voltage source simulator, an open-loop test was performed.

From the experimental results of FIG. 11, it can be seen that the bidirectional power conversion circuit according to the present invention is independently well controlled, as shown in the voltage and current of the DC link and the voltage and current waveforms of the load resistance of the first cutoff of the transformer.

In the present invention, a center-tapped transformer-based bidirectional inverter for a wide range of DC link control performance and independent AC link control is proposed. The functions and operating principles of this circuit were described based on the equivalent circuit, and the system energy balance control was discussed. In order to verify the stable operation and effectiveness of this power conversion system, PLECS simulations and experiments were performed. From the simulation results, the bidirectional power transfer operation of this system and the control algorithm for variable DC voltage were verified, and the validity of the operation of the proposed circuit was confirmed through experiments.

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.

100: bi-directional power conversion system
102: grid
104: battery

Claims (8)

DC source;
AC grid or load;
a center tapped transformer connected to the AC grid or load; and
A bi-directional power conversion system comprising a; power conversion device connected between the center tap transformer and the DC source and converting based on AC/DC. According to claim 1,
The power converter is a directional power conversion system, characterized in that any one of a DC-AC inverter, AC-DC converter, DC-DC converter, AC-AC converter. According to claim 2
The power converter is a DC-AC inverter,
The center tap transformer receives AC power from the grid, transforms it, and transfers it to the inverter or transforms AC power from the inverter and transfers it to the grid or load;
The inverter converts AC power from the center tap transformer into DC power and transfers it to the DC source side, or converts DC power from the DC source side into AC power and transfers it to the center tap transformer. conversion system. According to claim 3,
The inverter is a bi-directional power conversion system, characterized in that consisting of one or more full-bridge modules. According to claim 4,
The full-bridge module is a bidirectional power conversion system, characterized in that composed of a capacitor and a bridge circuit connected between the DC source and the center tap transformer. According to claim 3,
The DC source is a variable DC (Variable DC) source,
The DC-AC converter is composed of first and second full-bridge modules,
The first full-bridge module is connected to one end of the variable DC source and a first end of the center tapped transformer;
The other end of the variable DC source is connected to the center tap of the center tap transformer;
The first full-bridge module is composed of a first bridge circuit and a first capacitor, and charges the first capacitor with power from the variable DC source, and the energy charged in the first capacitor exceeds a first level. The bidirectional power conversion system, characterized in that for providing AC power to the center tap transformer through the first stage of the center tap transformer with the charged energy when the According to claim 6,
The second full-bridge module is connected to one end of the variable DC source and a second end of the center tapped transformer, and is composed of a second bridge circuit and a second capacitor to charge power from the variable DC source to the second capacitor. In addition, when the energy charged in the second capacitor reaches a second level or higher, the charged energy is supplied to the center tap transformer as AC power through a second stage of the center tap transformer. . According to claim 7,
AC power provided by the center-tap transformer through a first stage of the center-tap transformer is rectified through the first full-bridge circuit and transmitted to the variable DC source side;
The bi-directional power conversion system, characterized in that the AC power provided by the center-tap transformer through the second stage of the center-tap transformer is rectified through the second full-bridge circuit and delivered to the variable DC source side.

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