Distribution network system of Bipolar Low Voltage Direct Current and controlling method supporting the same

Abstract

The present invention discloses a bipolar LVDC distribution network system and a control method thereof, comprising: a power source supplying power having a first magnitude voltage; a first winding transformer and a second winding transformer disposed adjacent to wiring to which the power is applied; a first AC / DC converter connected to the first winding transformer to convert the power obtained by the first winding transformer into DC; a second AC / DC converter connected to the second winding transformer to convert the power obtained by the second winding transformer into DC and connected in a cascade manner in series with the first AC / DC; and a buck converter to which the outputs of the first AC / DC converter and the second AC / DC converter are connected.

Description

Technology Field

[0001] The present invention relates to a bipolar LVDC distribution network, and more specifically, to a bipolar LVDC distribution network system and a control method thereof that provides simplified and stable distribution performance. Background Technology

[0002] Low Voltage Direct Current (LVDC) distribution networks are composed of direct current voltage and can transmit and distribute more power over the same lines when using the same voltage compared to alternating current (AC). In addition, they offer the advantage of higher system efficiency because the integration of renewable energy and distributed power sources can reduce the conversion stages of power converters compared to alternating current (AC) distribution networks.

[0003] In conventional LVDC distribution network configurations, power supply converters are designed using a 3-level AC / DC converter in a bipolar mode (±750Vdc). In a bipolar LVDC distribution network, power is supplied to loads using 750Vdc by utilizing an upper +750Vdc and a lower -750Vdc. The 3-level AC / DC converter is configured as a 500kW class unit, and a 250kW class unit stack for the AC / DC converter adopts a 2-parallel structure. The 3-level AC / DC converter with a 2-parallel structure receives commercial power of 380Vac as input, steps it up to 1,500Vdc, and outputs bipolar ±750Vdc through neutral point voltage control. In a structure where loads are connected to a bipolar 750Vdc distribution line, if load imbalance occurs, there is inevitably a limit to the neutral point voltage control of the 3-level inverter. Therefore, there is a problem requiring the addition of a bidirectional DC / DC converter capable of controlling load imbalance in hardware. Furthermore, when commercial power is rectified and stepped up to 1,500 Vdc, the step-up ratio appears to be approximately three times, resulting in losses in terms of efficiency for power supply converters that must operate continuously. Additionally, regarding the control aspects of a 3-level AC / DC converter with a 2-parallel structure, zero-current control via Pulse Width Modulation (PWM) synchronization and neutral point voltage control for load imbalance must be performed internally; however, this requires high performance or complexity of the controller, which can compromise system stability or lead to a complex system configuration. The problem to be solved

[0004] The present invention addresses the aforementioned conventional problems, such as providing a configuration and control technology for a bipolar LVDC distribution network using a 3-level AC / DC converter to improve the efficiency and stability of the LVDC distribution network. means of solving the problem

[0005] A bipolar LVDC distribution network system according to an embodiment of the present invention is characterized by comprising: a power source supplying power having a first magnitude voltage; a first winding transformer and a second winding transformer arranged adjacent to a wiring to which the power is applied; a first AC / DC converter connected to the first winding transformer to convert the power obtained by the first winding transformer into DC; a second AC / DC converter connected to the second winding transformer to convert the power obtained by the second winding transformer into DC and connected in a cascade manner in series with the first AC / DC; and a buck converter to which the outputs of the first AC / DC converter and the second AC / DC converter are connected.

[0006] Here, the first winding transformer and the second winding transformer are each characterized by converting the power supplied from the power source into 380Vac power and supplying it to the first AC / DC converter and the second AC / DC converter, respectively.

[0007] Additionally, the buck converter is characterized by including a first input wiring connected to the first AC / DC converter, a second input wiring commonly connected to the first AC / DC converter and the second AC / DC converter, a third input wiring connected to the second AC / DC converter, a first switch and a second switch connected in parallel between the first input wiring and the second input wiring, a third switch and a fourth switch connected in parallel between the second input wiring and the third input wiring, a first output wiring connected to a first node between the first switch and the second switch, a second output wiring connected to a second node between the second switch and the third switch, and a third output wiring connected to a third node between the third switch and the fourth switch.

[0008] Here, the distribution network system further includes a control circuit for controlling the buck converter, and the control circuit can control the second switch to change from a turn-off state to a turn-on state when a short circuit occurs in the first output wiring.

[0009] Alternatively, the control circuit controls the fourth switch to change from a turn-off state to a turn-on state when a short circuit occurs in the third output wiring.

[0010] Alternatively, the control circuit controls the second switch and the fourth switch to change from a turn-off state to a turn-on state when a short circuit occurs in the first output wiring and the second output wiring.

[0011] In relation to a bipolar LVDC distribution network control method according to an embodiment of the present invention, the system comprises two AC / DC converters connected in a series cascade manner, each receiving power from a power source converted through a respective winding transformer, and a buck converter connected to the AC / DC converters. The bipolar LVDC distribution network control method is characterized by including the steps of: collecting a signal related to a short circuit from a short circuit detection module connected to the output wiring of the distribution network; identifying which wiring has a short circuit when the signal related to the short circuit is collected; and controlling a switch of the buck converter connected to the wiring where the short circuit occurred.

[0012] Here, the step of controlling the switch may include the step of controlling the second switch, which is connected to the first output wiring and is in a turn-off state among the first switch and the second switch included in the buck converter, to change to a turn-on state when a short circuit occurs in the first output wiring.

[0013] Alternatively, the step of controlling the switch may include the step of controlling the fourth switch, which is connected to the third output wiring and is in a turn-off state among the third switch and the fourth switch included in the buck converter, to change to a turn-on state when a short circuit occurs in the third output wiring.

[0014] Alternatively, the step of controlling the switch may include, when a short circuit occurs in the output wirings, the first output wiring and the third output wiring, controlling the second switch, which is in a turn-off state among the first switch and the second switch included in the buck converter connected to the first output wiring, to turn on, and controlling the fourth switch, which is in a turn-off state among the third switch and the fourth switch included in the buck converter connected to the third output wiring, to turn on. Effects of the invention

[0015] According to the bipolar LVDC distribution network system and the control method thereof of the present invention, the present invention provides a more simplified structure compared to the conventional 2-parallel structure of a unit stack, and can provide the effect of improving problems regarding the efficiency and heat dissipation of the entire system. Brief explanation of the drawing

[0016] FIG. 1 is a diagram showing an example of a bipolar LVDC distribution network system configuration according to an embodiment of the present invention. FIG. 2 is a diagram showing the connection state of a buck converter and AC / DC converters according to an embodiment of the present invention. FIG. 3 is a diagram showing the switch control state included in a buck converter according to an embodiment of the present invention. FIG. 4 is a diagram showing an example of a bipolar LVDC distribution network control method according to an embodiment of the present invention. Specific details for implementing the invention

[0017] It should be noted that in the following description, only the parts necessary for understanding the embodiments of the present invention are described, and the description of other parts will be omitted to the extent that it does not detract from the gist of the present invention.

[0018] The terms and words used in the specification and claims described below should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention. Accordingly, the embodiments described in this specification and the configurations illustrated in the drawings are merely preferred embodiments of the invention and do not represent all aspects of the technical spirit of the invention; therefore, it should be understood that various equivalents and modifications capable of replacing them may exist at the time of filing this application.

[0019] The present invention, described below, applies a series cascade method of a 3-level AC / DC converter using a 3-winding transformer, thereby eliminating the need for DC voltage balancing control for load imbalance in an LVDC distribution network and supporting highly simplified control by arranging a Buck converter for fault current limiting for short-circuit protection. In addition, the step-up ratio of the AC / DC converter is reduced from approximately 3 times to 1.4 times compared to a conventional system, thereby improving efficiency and providing effects such as reduced switching noise and reduced heat generation.

[0020] Hereinafter, embodiments of the present invention will be described in more detail with reference to the attached drawings.

[0021] FIG. 1 is a diagram showing an example of a bipolar LVDC distribution network system configuration according to an embodiment of the present invention.

[0022] Referring to FIG. 1, a bipolar LVDC distribution network system (10) according to an embodiment of the present invention may include a power source (110) (or commercial power source), a first winding transformer (121), a second winding transformer (122), a first AC / DC converter (131), a second AC / DC converter (132), and a buck converter (140).

[0023] The above power source (110) is a commercial power source or power supply device and can supply power of a certain voltage through a transmission and distribution network. For example, the above power source (110) can supply 22.9 kVac power. The above power source (110) can supply power through first to third phase wiring (R, S, T). The above power source (110) may include an Energy Storage System (ESS) device.

[0024] The first winding transformer (121) may be positioned between the power source (110) and the first AC / DC converter (131). That is, the primary winding of the first winding transformer (121) may be positioned toward the three-phase power of the power source (110), and the secondary winding of the first winding transformer (121) may be positioned toward the first AC / DC converter (131). Thus, the first winding transformer (121) can induce the three-phase power of the power source (110) and transmit it to the first AC / DC converter (131). In this process, the first winding transformer (121) can transform 22.9kV AC power into a 380Vac voltage and transmit it to the first AC / DC converter (131).

[0025] The first AC / DC converter (131) is a 3-level AC / DC converter and is connected to the first winding transformer (121) to convert the AC power of 380 Vac delivered by the first winding transformer (121) into DC and then deliver it to the buck converter (140). At this time, the first AC / DC converter (131) can convert the 380 Vac into 750 Vdc power and then deliver it to the buck converter (140). In this regard, the first AC / DC converter (131) is connected to the first winding transformer (121) at the 3-level and can be connected to the buck converter (140) via P'-O' wiring. The first AC / DC converter (131) can perform DC conversion and voltage control independently of the second AC / DC converter (132). The first AC / DC converter (131) can be connected in a cascade format with the second AC / DC converter (132).

[0026] The second winding transformer (122) has substantially the same or similar form as the first winding transformer (121) and can perform the same or similar function. For example, the second winding transformer (122) can be placed between the power source (110) and the second AC / DC converter (132). That is, the primary winding of the second winding transformer (122) can be placed toward the three-phase power of the power source (110), and the secondary winding of the second winding transformer (122) can be placed toward the second AC / DC converter (132). Thus, the second winding transformer (122) can induce the three-phase power of the power source (110) and transmit it to the second AC / DC converter (132). In this process, the second winding transformer (122) can transform 22.9kv AC power into 380Vac voltage in the same or similar way as the first winding transformer (121) and deliver it to the second AC / DC converter (132).

[0027] The second AC / DC converter (132) may be a 3-level AC / DC converter, identical or similar to the first AC / DC converter (131). The second AC / DC converter (132) is connected to the second winding transformer (122) to convert the AC power 380Vac delivered by the second winding transformer (122) into 750V DC and then deliver it to the buck converter (140). In the same way as the connection between the first AC / DC converter (131) and the first winding transformer (121), the second AC / DC converter (132) may be connected to the second winding transformer (122) at 3 levels and connected to the buck converter (140) via N'-O' wiring. One output wiring (e.g., third phase wiring) of the first AC / DC converter (131) and one output wiring (e.g., third phase wiring) of the second AC / DC converter (132) can be connected by a common wiring (e.g., O' wiring). The second AC / DC converter (132) can perform DC conversion and voltage control independently of the first AC / DC converter (131). As previously described, the second AC / DC converter (132) can be connected in a cascade manner with the first AC / DC converter (131).

[0028] One side of the buck converter (140) may be connected to the first AC / DC converter (131), and the other side may be connected to the second AC / DC converter (132). Alternatively, the buck converter (140) may include a first contact to which the output of the first AC / DC converter (131) is connected, and a second contact to which the output of the second AC / DC converter (132) is connected. Here, the O' wiring of the first AC / DC converter (131) and the O' wiring of the second AC / DC converter (132) may be connected in common.

[0029] FIG. 2 is a diagram showing the connection state of a buck converter and AC / DC converters according to an embodiment of the present invention.

[0030] Referring to FIG. 2, a buck converter (140) according to an embodiment of the present invention may include four switches (S1, S2, S3, S4), three inductors (Lp, Lo, Ln), and four capacitors (Ci_1, Ci_2, Co_1, Co_2). For example, the buck converter (140) may be connected as inputs to a first input wiring (P') and a second input wiring (O') connected to a first AC / DC converter (131), a second input wiring (O') and a third input wiring (N') connected to a second AC / DC converter (132). The buck converter (140) may include first to third output wirings (P, O, N). Loads (e.g., DC Loads) may be connected to each of at least two of the first to third output wires (P, O, N) (e.g., the first output wire (P) and the second output wire (O) or the second output wire (O) and the third output wire (N)).

[0031] The switches (S1, S2, S3, S4) may include, for example, first to fourth switches (S1, S2, S3, S4). The first switch (S1) and the second switch (S2) may be connected in series between the first input wire (P') and the second input wire (O'). Additionally, a first input capacitor (Ci_1) is disposed between the first input wire (P') and the second input wire (O'), and the first input capacitor (Ci_1) may be disposed in parallel with the first switch (S1) and the second switch (S2).

[0032] The third switch (S3) and the fourth switch (S4) may be connected in series between the second input wire (O') and the third input wire (N'). Additionally, a second input capacitor (Ci_2) may be placed between the third input wire (N') and the second input wire (O'), and the second input capacitor (Ci_2) may be placed in parallel with the third switch (S3) and the fourth switch (S4).

[0033] A first switch node (N1) may be formed between the first switch (S1) and the second switch (S2), a second switch node (N2) may be formed between the second switch (S2) and the third switch (S3), and a third switch node (N3) may be formed between the third switch (S3) and the fourth switch (S4). One side of the first output wiring (P) may be connected to the first switch node (N1). The second input wiring (O') and the second output wiring (O) may be commonly connected to the second switch node (N2). One side of the third output wiring (N) may be connected to the third switch node (N3). A first inductor (Lp) is disposed between the first switch node (N1) and the other side of the first output wiring (P), and a second inductor (Lo) may be disposed between the second switch node (N2) and the input node (Ni). One side of the third output wiring (N) is connected to the third switch node (N3), and a third inductor (Ln) may be disposed between the third switch node (N3) and the third output wiring (N).

[0034] A first output capacitor (Co_1) and a second switch (S2) may be arranged in parallel between the first output wire (P) and the second output wire (O). A second output capacitor (Co_2) and a third switch (S3) may be arranged in parallel between the second output wire (O) and the third output wire (N).

[0035] The buck converter (140) described above may, for example, form a first input closed loop (Lpi_1) comprising both ends of a first input capacitor (Ci_1), a first switch (S1), a second switch (S2), and a second inductor (Lp, Lo, Ln) in response to the control of a first switch (S1) and a second switch (S2) (e.g., in response to both of the first switch (S1) and the second switch (S2) being turned on), and form a second input closed loop (Lpi_2) comprising both ends of a second input capacitor (Ci_2), a third switch (S3), a fourth switch (S4), and a second inductor (Lp, Lo, Ln) in response to the control of a third switch (S3) and a fourth switch (S4) (e.g., in response to both of the third switch (S3) and the fourth switch (S4) being turned on).

[0036] FIG. 3 is a diagram showing the switch control state included in a buck converter according to an embodiment of the present invention.

[0037] Referring to FIG. 3, the buck converter (140) operates to protect AC / DC converters (131, 132) and coordinate control of the grid network in the event of a short circuit in the busbar, and in a normal state, the first switch (S1) and the third switch (S3) are always turned on and conduct. If a short circuit occurs in the PO busbar (or between the first output wire (P) and the second output wire (O)), the second switch (S2) can be switched from the turn-off state to the turn-on state to limit the current. If a short circuit occurs in the ON busbar (or between the second output wire (O) and the third output wire (N)), the fourth switch (S4) can be switched from the turn-off state to the turn-on state to limit the current, thereby enabling stable operation of the AC / DC converter (e.g., the second AC / DC converter (132)) and the distribution network.

[0038] As another example, if a short circuit occurs in two busbars (e.g., the first output wire (P) and the third output wire (N)) within a bipolar LVDC distribution network, the second switch (S2) and the fourth switch (S4) can both be turned on from the off state to reduce the short-circuit current.

[0039] In the illustrated drawing, "0" means the switch is turned off, "1" means the switch is turned on, and "Switching" may mean the state of transitioning from the off state to the turned-on state.

[0040] FIG. 4 is a diagram showing an example of a bipolar LVDC distribution network control method according to an embodiment of the present invention.

[0041] Referring to FIG. 4, according to the switch control state of FIG. 3 described above, the bipolar LVDC distribution network system of the present invention may include a first detection module capable of detecting a PO bus short circuit and a second detection module capable of detecting an ON bus short circuit, and a control circuit capable of controlling the switch states of the first to fourth switches (S1, S2, S3, S4) according to signals received from the first detection module and the second detection module. Based on this, in relation to the bipolar LVDC distribution network control method of the present invention, the control circuit may perform a step 401 of collecting short-circuit related detection signals from the detection modules, a step 403 of confirming which bus has a short circuit by checking which detection module the collected signal was received from, and a step 405 of controlling switches (S1, S2, S3, S4) connected to the bus that has a short circuit. In this process, the control circuit can control the second switch (S2) to switch from the turn-off state to the turn-on state (the first switch (S1) remains in the turn-on state) when a short circuit occurs between the PO bus (or the first output wire (P) and the second output wire (O)), and the fourth switch (S4) to switch from the turn-off state to the turn-on state (the third switch (S3) remains in the turn-on state) when a short circuit occurs between the ON bus (or the second output wire (O) and the third output wire (N)).

[0042] As described above, the present invention relates to the configuration of a 3-level AC / DC converter using a 3-winding transformer and control technology for a Buck converter for the efficiency and system stability of a bipolar LVDC distribution network. The AC / DC converters are connected in a series cascade manner, and the 3-level AC / DC converter for each unit stack connected to the 3-winding transformer is operated by setting the output voltage to 750Vdc, and a Buck converter is additionally configured at the output terminal to limit the fault current of the busbar. In the present invention described above, the Buck converter (140) has a current limiting function to prevent a short-circuit accident of the power supply device, is always turned on in a normal state, and performs the current limiting function within 1 second in the event of a short-circuit accident. The present invention forms DC 1,500V by connecting two AC / DC 3-level converters in series, thereby enabling high efficiency and improved switching noise with a lower step-up ratio compared to existing LVDC distribution networks (e.g., a method of stepping up AC 380V to 1,500V). In addition, regarding load imbalance at the PO bus and ON bus, the present invention supports stable voltage balancing even when an unbalanced load is applied by configuring AC / DC converter stacks, each controlled independently, to handle the load imbalance.

[0043] In contrast to the conventional method of using a 3-level AC / DC converter with a parallel structure of unit stacks, which controls the output voltage to 1,500 Vdc to form a bipolar 750 Vdc, resulting in low efficiency due to a high step-up ratio and heat generation problems, the present invention described above uses a 3-level AC / DC converter with a series structure using a 3-winding transformer to control the output voltage to 750 Vdc in a unit stack, thereby increasing efficiency by about 1.5% and improving the efficiency of the LVDC distribution network system.

[0044] In addition, conventional parallel 3-level converters perform voltage and current control through Master / Slave concept control via communication, and zero-current control must be performed via PWM synchronization through direct connection between stacks to reduce circulating current. However, the structure of the serial converter of the present invention does not require Master / Slave concept control, and therefore, all unit stacks perform only voltage control routines. By adopting a serial converter structure, there is no need to add functions for PWM synchronization and zero-current control through a structure connected via a 3-winding isolated transformer, thereby providing the advantage of simplifying the 3-level AC / DC converter controller.

[0045] Additionally, in the case of conventional bipolar LVDC distribution networks, unbalanced voltage control must be additionally configured through an external bidirectional DC / DC converter as well as an internal neutral point voltage controller to compensate for load imbalance; however, in the case of the present invention, when a short-circuit fault occurs on a busbar, control for limiting the fault current is performed only on the faulted busbar through a Buck converter, thereby supporting the stable operation of the LVDC distribution network through the advantage of having almost no effect on other buses.

[0046] Meanwhile, the embodiments disclosed in this specification and drawings are merely specific examples provided to aid understanding and are not intended to limit the scope of the invention. It is obvious to those skilled in the art that other variations based on the technical concept of the invention are possible in addition to the embodiments disclosed herein. Explanation of the symbols

[0047] 10: Bipolar LVDC Distribution Network System 110: Power 121, 122: Wind-wound transformers 131, 132: AC / DC Converter 140: Buck Converter

Claims

Claim 1 A bipolar LVDC distribution network system comprising: a power source supplying power having a first magnitude voltage; a first winding transformer and a second winding transformer disposed adjacent to the wiring to which the power is applied; a first AC / DC converter connected to the first winding transformer to convert the power obtained by the first winding transformer into DC; a second AC / DC converter connected to the second winding transformer to convert the power obtained by the second winding transformer into DC and connected in a series cascade manner with the first AC / DC; a buck converter to which the outputs of the first AC / DC converter and the second AC / DC converter are connected; and a control circuit for controlling the buck converter; wherein the control circuit collects a signal related to a short circuit from a short circuit detection module connected to the output wiring of the distribution network, and upon collecting the signal related to the short circuit, identifies which wiring has a short circuit and controls the switch of the buck converter connected to the wiring where the short circuit occurred. Claim 2 A bipolar LVDC distribution network system according to claim 1, wherein the first winding transformer and the second winding transformer each convert power supplied from the power source into 380Vac power and supply it to the first AC / DC converter and the second AC / DC converter, respectively. Claim 3 A bipolar LVDC distribution network system according to claim 1, wherein the buck converter comprises: a first input wiring connected to the first AC / DC converter; a second input wiring commonly connected to the first AC / DC converter and the second AC / DC converter; a third input wiring connected to the second AC / DC converter; a first switch and a second switch connected in parallel between the first input wiring and the second input wiring; a third switch and a fourth switch connected in parallel between the second input wiring and the third input wiring; a first output wiring connected to a first node between the first switch and the second switch; a second output wiring connected to a second node between the second switch and the third switch; and a third output wiring connected to a third node between the third switch and the fourth switch. Claim 4 In paragraph 3, the control circuit controls the second switch to change from a turn-off state to a turn-on state when a short circuit occurs in the first output wiring, in a bipolar LVDC distribution network system. Claim 5 In paragraph 3, the control circuit controls the fourth switch to change from a turn-off state to a turn-on state when a short circuit occurs in the third output wiring, in a bipolar LVDC distribution network system. Claim 6 In paragraph 3, the control circuit controls the second switch and the fourth switch to change from a turn-off state to a turn-on state when a short circuit occurs in the first output wiring and the second output wiring, in a bipolar LVDC distribution network system. Claim 7 A bipolar LVDC distribution network control method comprising two AC / DC converters connected in a series cascade manner, each receiving power from a power source converted through a respective winding transformer, and a buck converter connected to said AC / DC converters, the method comprising: a step of collecting a signal related to a short circuit from a short circuit detection module connected to output wirings of said distribution network; a step of determining which wiring has a short circuit when the signal related to the short circuit is collected; and a step of controlling a switch of said buck converter connected to the wiring where the short circuit occurred. Claim 8 A bipolar LVDC distribution network control method according to claim 7, wherein the step of controlling the switch comprises the step of controlling the second switch, which is in a turn-off state among the first switch and the second switch included in the buck converter and connected to the first output wiring, to change to a turn-on state when a short circuit occurs in the first output wiring. Claim 9 A bipolar LVDC distribution network control method according to claim 7, wherein the step of controlling the switch comprises the step of controlling the fourth switch, which is in a turn-off state among the third switch and the fourth switch included in the buck converter and connected to the third output wiring, to change to a turn-on state when a short circuit occurs in the third output wiring. Claim 10 In claim 7, the step of controlling the switch comprises: a step of, when a short circuit occurs in the output wires, the first output wire and the third output wire, controlling the second switch, which is in a turn-off state among the first switch and the second switch included in the buck converter connected to the first output wire, to change to a turn-on state, and controlling the fourth switch, which is in a turn-off state among the third switch and the fourth switch included in the buck converter connected to the third output wire, to a turn-on state; a bipolar LVDC distribution network control method.

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