Novel intermediate frequency isolation UPQC system for power distribution system

Abstract

The invention discloses a novel intermediate frequency isolation UPQC system for a power distribution system. The novel intermediate frequency isolation UPQC system comprises three paths of single-phase voltage source type converters, a three-phase voltage source type inverter and a half-bridge CLLC converter, the three paths of single-phase voltage source type converters serve as series compensators; the half-bridge CLLC converter comprises a four-winding transformer, three half-bridge uncontrollable inverter circuits, an LLC filter circuit, an LC filter circuit and a half-bridge controllable rectifier circuit. The three half-bridge uncontrollable inverter circuits are connected with the three direct-current energy storage capacitors CA, CB and CC, and the half-bridge controllable rectifier circuit is connected with the energy storage capacitor CD and the direct-current side of the three-phase voltage source type inverter; and the three-phase voltage source type inverter is used as a parallel compensator. The novel intermediate-frequency isolation UPQC system for the power distribution system and the control method thereof have the advantages that isolation is achieved under the condition that a low-frequency transformer is omitted, the number of active switches is reduced, and the reliability of the UPQC system is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a power distribution network power quality control technology, in particular to a novel medium-frequency isolation UPQC system for a power distribution system. BACKGROUND

[0002] UPQC (Unified Power Quality Conditioner) is one of the electrical equipment for improving power quality.

[0003] UPQC alleviates grid and load disturbances through series and parallel, and the series compensator is responsible for alleviating disturbances related to grid voltage, such as voltage sag / variation and harmonics. On the other hand, the parallel compensator can alleviate disturbances related to the load, such as power factor correction and current harmonics.

[0004] UPQC combines series and parallel active filters, so that the advantages of series and parallel active filters in power system applications can be fully utilized, and comprehensive power quality regulation functions are achieved.

[0005] In traditional UPQC, all structures need a low-frequency transformer (LFT) as an isolation stage between the power line and the UPQC. Although the low-frequency transformer is robust and durable, it has the disadvantages of large size and heavy weight, and requires a lot of space for installation, which reduces the power density of the UPQC. In some applications of the power supply system, the UPQC may also be connected to a photovoltaic or energy storage system. The UPQC adopts a back-to-back structure with a common DC link, which requires the DC link voltage to be higher than the line-to-line voltage. Although the use of multi-level or modular converters can overcome this problem. However, since the series compensator of each phase of the UPQC is coupled, if a switch in one phase fails, it may cause the entire UPQC to malfunction. SUMMARY

[0006] The present application is to avoid the shortcomings of the prior art, and provides a novel medium-frequency isolation UPQC system for a power distribution system to eliminate the low-frequency transformer, reduce the number of active switches, and improve the reliability of the UPQC system.

[0007] The present application adopts the following technical solutions to solve the technical problems.

[0008] The novel medium-frequency isolation UPQC system for a power distribution system of the present application comprises a three-path single-phase voltage source type inverter, a three-phase voltage source type inverter, and a half-bridge CLLC converter.

[0009] The three-way single-phase voltage source type converter is connected with three DC energy storage capacitors C A 、 B 、 C on the DC side.

[0010] The half-bridge LLC converter is connected with three DC energy storage capacitors C A 、 B 、 C on the DC side. D and the DC side of the three-phase voltage source type inverter.

[0011] The three-phase voltage source type inverter is connected with the energy storage capacitor C D and the half-bridge controllable rectifier circuit of the half-bridge LLC converter on the DC side.

[0012] The structure of the novel medium-frequency isolation UPQC system for the power distribution system also has the following characteristics:

[0013] Further, the three-way single-phase voltage source type converter comprises three single-phase voltage source type converters.

[0014] Further, the single-phase voltage source type converter comprises four switch tube modules; the switch tube module comprises a fully controlled switch tube freewheeling diode; and the fully controlled switch tube freewheeling diodes are connected in parallel with each other.

[0015] Further, the four-winding transformer comprises a primary side and a secondary side; the primary side comprises three windings, and the secondary side comprises two windings.

[0016] Further, in the three-way half-bridge uncontrollable inversion circuit, each half-bridge uncontrollable inversion circuit comprises two voltage dividing capacitors and two diodes.

[0017] Further, the LLC filter circuit comprises two inductors and one capacitor.

[0018] Further, the LC filter circuit comprises an inductor L rd and a capacitor C rd .

[0019] Further, the half-bridge controllable rectifier circuit comprises two switch tube modules and two voltage dividing capacitors.

[0020] The application further discloses a control method of the novel medium-frequency isolation UPQC system for a power distribution system.

[0021] The three-path single-phase voltage source type converter is used as a series compensator; and a compensation voltage is injected into a three-phase power grid in an SPWM modulation mode, so that the voltage of the three-phase power grid is restored to a normal amplitude and phase level.

[0022] The half-bridge CLLC converter is used as medium-frequency isolation between the series compensator and the parallel compensator.

[0023] The three-phase voltage source type inverter is used as the parallel compensator.

[0024] Further, when any single-phase voltage source type converter of the three-path single-phase voltage source type converter fails, the single-phase voltage source type converter is cut off from the three-phase power grid by closing a bypass circuit breaker.

[0025] Compared with the prior art, the application has the following beneficial effects:

[0026] The application discloses a novel medium-frequency isolation UPQC system for a power distribution system, which comprises a three-path single-phase voltage source type converter, a three-phase voltage source type inverter and a half-bridge CLLC converter. A B C The half-bridge CLLC converter is used as medium-frequency isolation between the series compensator and the parallel compensator, and comprises a four-winding transformer, a three-path half-bridge uncontrolled inverter circuit, an LLC filter circuit, an LC filter circuit and a half-bridge controllable rectifier circuit. A B C The half-bridge controllable rectifier circuit is connected with the energy storage capacitor C D and a DC side of the three-phase voltage source type inverter. D The three-phase voltage source type inverter is used as the parallel compensator, and is connected with the three-phase power grid in series after a filter inductor.

[0027] The novel medium-frequency isolation UPQC system for a power distribution system and the control method thereof have the advantages of realizing isolation without a low-frequency transformer, reducing the number of active switches and improving the reliability of the UPQC system. BRIEF DESCRIPTION OF DRAWINGS​​​​

[0028] Figure 1 is the topology of the new medium frequency isolation UPQC system for power distribution system of the present application;

[0029] Figure 2 is the main waveform diagram of the Q-CLLC converter of the present application;

[0030] Figure 3 is the equivalent circuit diagram of the Q-CLLC converter of the present application.

[0031] The present application is further described below by specific embodiments, and in conjunction with the accompanying drawings. EMBODIMENTS

[0032] Referring to Figures 1-3 , the new medium frequency isolation UPQC system for power distribution system of the present application comprises a three-way single-phase voltage source type inverter, a three-phase voltage source type inverter and a half-bridge CLLC converter;

[0033] The three-way single-phase voltage source type inverter is used as a series compensator, and is connected to the three-phase power grid in a series manner at the AC side, and is connected with three DC energy storage capacitors C A , C B , C C at the DC side.

[0034] The half-bridge CLLC converter is used as the medium frequency isolation between the series compensator and the parallel compensator, and comprises a four-winding transformer, a three-way half-bridge uncontrolled inverter circuit, an LLC filter circuit, an LC filter circuit and a half-bridge controllable rectifier circuit; the three-way half-bridge uncontrolled inverter circuit is connected with the three DC energy storage capacitors C A , C B , C C , and the half-bridge controllable rectifier circuit is connected with the energy storage capacitor C D and the DC side of the three-phase voltage source type inverter.

[0035] The three-phase voltage source type inverter is used as a parallel compensator, and is connected to the three-phase power grid after a series filter inductor at the AC side, and is connected with the energy storage capacitor C D and the half-bridge controllable rectifier circuit of the half-bridge CLLC converter at the DC side.

[0036] As Figure 1As shown, this invention discloses a novel intermediate frequency isolation UPQC system for power distribution systems, comprising three subsystems: a three-phase single-phase voltage source inverter (1P-VSI) acting as a series compensator, a three-phase voltage source inverter (3P-VSI) acting as a parallel compensator, and a half-bridge CLLC converter (HBQ-CLLC) acting as intermediate frequency isolation. Using this novel intermediate frequency isolation UPQC topology, the 1P-VSI series compensation structure decouples phase-to-phase operation. Even if a 1P-VSI module fails, the UPQC can still operate normally to compensate for the grid voltage. Simultaneously, intermediate frequency isolation is provided through the half-bridge four-channel CLLC converter, eliminating the need for a large and heavy low-frequency transformer, thus significantly reducing the weight and overall cost of the UPQC.

[0037] In specific implementation, the three-way single-phase voltage source converter includes three single-phase voltage source converters.

[0038] In a specific implementation, the single-phase voltage source converter includes four switching transistor modules; each switching transistor module includes a fully controlled switching transistor freewheeling diode; the fully controlled switching transistor freewheeling diodes are connected in parallel with each other.

[0039] Each of the three single-phase voltage source converters (1P-VSI) consists of a single-phase voltage source converter. Each single-phase voltage source converter includes four switching modules; each switching module consists of a fully controlled switching transistor and a freewheeling diode connected in parallel; every two switching modules form a bridge arm; the two bridge arms are combined in parallel; therefore, the four switching modules form an H-bridge structure, thus constituting a single-phase voltage source converter.

[0040] The H-bridge structure of the phase voltage source converter connects the AC side to the three-phase power grid in series via an LC filter, and is equipped with a bypass switch to disconnect the faulty module in case of a fault; the DC side is connected to a DC energy storage capacitor C. x Where x = A, B, C, the DC energy storage capacitor Cx is connected in parallel with the voltage divider capacitor of the half-bridge uncontrolled inverter circuit of the half-bridge CLLC converter.

[0041] In specific implementation, the four-winding transformer includes a primary side and a secondary side; the primary side includes three windings and the secondary side includes two windings.

[0042] The turns ratio of the four-winding transformer is N. a :N b :N c :N d The aforementioned Figure 1 In the middle, the three windings on the primary side are L ma L mb and Lmc Secondary winding L md It includes two windings.

[0043] In specific implementation, each of the three-way uncontrollable half-bridge inverter circuits includes two voltage-dividing capacitors and two diodes.

[0044] The aforementioned three-phase half-bridge uncontrolled inverter circuit includes an A-phase half-bridge uncontrolled inverter circuit, a B-phase half-bridge uncontrolled inverter circuit, and a C-phase half-bridge uncontrolled inverter circuit. Each half-bridge uncontrolled inverter circuit includes two voltage-dividing capacitors C. h and two diodes (D) a D b and D c Two voltage divider capacitors are connected in series, and two diodes are also connected in series. The two series bridge circuits are connected in parallel to form a half-bridge uncontrolled inverter circuit. The midpoint of the diode branch is connected to the positive terminal of the output, and the midpoint of the voltage divider capacitor branch is connected to the negative terminal of the output.

[0045] In practice, the LLC filter circuit includes two inductors and one capacitor.

[0046] The LLC filter circuit also includes three circuits, corresponding to the three half-bridge uncontrollable inverter circuits, namely, phase A LLC filter circuit, phase B LLC filter circuit, and phase C LLC filter circuit. For example... Figure 1 Taking the A-phase LLC filter circuit as an example, it includes inductor L ra Capacitor C ra and inductor L ma Inductance L ra Capacitor C ra The inductor L is connected in series with the circuit. ra Connect the diode branch to the midpoint, capacitor C ra Connect to the midpoint of the voltage divider capacitor branch, inductor L ma It is connected in parallel across the primary winding of the transformer.

[0047] In specific implementation, the LC filter circuit includes an inductor L rd and capacitor C rd .

[0048] like Figure 1 Inductor L rd and capacitor C rd It is connected in series to the secondary side of the four-winding transformer.

[0049] In practice, the half-bridge controllable rectifier circuit includes two switching transistor modules and two voltage dividing capacitors.

[0050] Each switch tube module of the half-bridge controllable rectifier circuit is also composed of one fully controllable switch tube and one freewheeling diode connected in parallel with each other, same as the switch tube module of the single-phase voltage source type converter. h The two switch tube modules are connected in series to form a switch tube module branch. rd The midpoint of the switch tube module branch of the half-bridge controllable rectifier circuit is connected to an inductor L rd The midpoint of the voltage dividing capacitor branch is connected to a capacitor C D The two output sections of the half-bridge controllable rectifier circuit are connected to the two ends of a DC energy storage capacitor C

[0051] The three-phase voltage source type inverter (3P-VSI) adopts a typical three-phase two-level inverter structure. Each single-phase voltage source type inverter includes two switch tube modules connected in series with each other. Each switch tube module of the half-bridge controllable rectifier circuit is also composed of one fully controllable switch tube and one freewheeling diode connected in parallel with each other, same as the switch tube module of the single-phase voltage source type converter. The DC side of the three-phase voltage source type inverter (3P-VSI) is connected to an energy storage capacitor C D The three AC output sides of the three-phase voltage source type inverter (3P-VSI) are respectively connected to a filter inductor L sh and then connected to a large power grid.

[0052] The application further discloses a control method of the novel medium-frequency isolation UPQC system for a power distribution system.

[0053] The three single-phase voltage source type converters serve as series compensators. The three-phase power grid is injected with compensation voltage in an SPWM modulation mode, and the voltage of the three-phase power grid is restored to a normal amplitude and phase level.

[0054] The half-bridge CLLC converter serves as a medium-frequency isolation between the series compensator and the parallel compensator.

[0055] The three-phase voltage source type inverter serves as a parallel compensator.

[0056] In specific implementation, when any single-phase voltage source type converter of the three single-phase voltage source type converters fails, the single-phase voltage source type converter is cut off from the three-phase power grid by closing a bypass circuit breaker.

[0057] The control process of the parallel compensator adopts a traditional SPWM modulation method; and the half-bridge CLLC converter does not need a closed-loop voltage controller because the resonant loop naturally regulates the output voltage.

[0058] The closed-loop control method of the series compensator includes the steps of grid voltage information sampling, voltage and current double closed-loop control, proportional resonant control and SPWM modulation.

[0059] In the control process of the three-way single-phase voltage source converter as the series compensator, when a 1P-VSI module of one phase or one phase is detected to be faulty, the bypass circuit breaker of the phase is controlled to be closed to cut off the faulty 1P-VSI module from the three-phase grid; the remaining 1P-VSI modules of the other phases are normal, so that the UPQC system of the application can still operate normally to compensate the three-phase grid voltage; when the 1P-VSI module is restored to normal, the bypass circuit breaker is opened again to connect the restored 1P-VSI module to the three-phase grid.

[0060] In the voltage compensation process of the series compensator, when the three-phase grid voltage decreases, the voltage waveform of the three-phase grid is detected in real time by a voltage sensor, and the voltage (V La , V Lb and V Lc ) quality at the load end is monitored;

[0061] The control system of the series compensator calculates the required compensation voltage by analyzing the difference between the three-phase grid voltage and the load demand in real time, controls three 1P-VSI modules to generate compensation voltage through SPWM modulation, and the 1P-VSI modules can adjust the amplitude and phase of the compensated three-phase grid voltage. Through the 1P-VSI module, the series compensator injects compensation voltage into the grid end, and the grid voltage is restored to the normal amplitude and phase level.

[0062] It is assumed that the three-phase grid to be connected to the UPQC system of the application is a strong grid, and the UPQC system is installed near the distribution transformer, so the influence of the three-phase grid inductance can be ignored. The three-way single-phase voltage source converter as the series compensator compensates for the interference of the three-phase grid voltage, such as voltage sag / undulation and harmonic interference.

[0063] As shown in Figure 1 , the series compensation subsystem includes a three-way single-phase voltage source converter, three series inductors L serx and three series capacitors C serx . The four switch modules of the three-way single-phase voltage source converter form an H-bridge structure, the AC side is connected to the three-phase grid through the series inductor L serx and the series capacitor C serx , and the DC side is connected to the DC voltage V xConnecting DC energy storage capacitor C x wherein x represents the corresponding phases A, B and C.

[0064] wherein x represents the corresponding phases A, B and C. The system operates similarly to the basic VSI, when the grid is disturbed, the VSI controls the series compensation current i serx and the series capacitor voltage v serx The mathematical model of the series compensator can be expressed as the following formula (1).

[0065]

[0066] In formula (1), v oserx is the output voltage of the 1P-VSI module, i gx is the three-phase grid current.

[0067] As Figure 1 The parallel compensation subsystem includes a three-phase voltage source inverter 3P-VSI, three parallel inductors L shx and a DC capacitor C D . Among them, the three-phase voltage source inverter 3P-VSI acts as a parallel compensator, which is used to compensate for load disturbances such as current harmonics and unity power factor, which is independent of the load current i Lx When the load disturbance occurs, the parallel compensation subsystem must control the DC voltage V x and the parallel compensation current i shx The mathematical model of the parallel compensator can be expressed as the following formula (2).

[0068]

[0069] In formula (2), v osehx is the output voltage of the 3P-VSI module, V Lx is the load voltage.

[0070] As Figure 1 The half-bridge LLC converter (HB Q-CLLC) acts as a medium-frequency isolation subsystem, which is used to realize the medium-frequency isolation between the series compensator and the parallel compensator; in addition, it can also act as a DC transformer D cx , which adjusts the DC link voltage V x of the series compensator without any closed-loop voltage controller. C h is a half-bridge capacitor, L mx is the magnetizing inductance of the MFT, and the turns ratio is Na: Nb: Nc: Nd. In the present application, a one-way structure is selected, and diodes are used on the ports a, b and c. This topology reduces the number of active switches and is conducive to the application of modular structure.

[0071] The switches (Sd+ and Sd-) on the half-bridge controlled rectifier circuit are controlled at the resonant frequency to control the V x bridge voltage V bx of the half-bridge uncontrolled inverter circuit of the half-bridge CLLC converter can be expressed as the following equation (3).

[0072]

[0073] In the equation (3), V bd is the voltage between the midpoint of the switch module branch of the half-bridge controlled rectifier circuit and the midpoint of the voltage dividing capacitor branch, V d is the voltage across the DC energy storage capacitor C D .

[0074] Figure 2 The key waveforms of the half-bridge CLLC converter are shown to illustrate the basic principle of the Q-CLLC converter. S d+ and S d- are controlled by complementary gate signals with a duty cycle of 0.5. During the positive half cycle, S d+ is in the on state, v bd and i Lrd become positive, causing D x+ diodes to naturally conduct. Since it operates at the resonant frequency, S d+ turns on and off at zero voltage and zero current switching (ZVZCS), which greatly reduces switching loss. During this period, v bx also becomes positive. In the remaining negative half cycle, S d- is in the on state, and the operation is the same as the positive half cycle.

[0075] In the present application, the equivalent circuit considering the first harmonic approximation (FHA) is shown in FIG. Figure 3 where C rd and L rd refer to the primary side capacitance and inductance, and are expressed as the following equation (4).

[0076]

[0077] In the equation (4), n=N a / N d =N b / N d =N c / N d . Where x represents the corresponding phases A, B and C. C′ rd is the equivalent capacitance of the filter capacitor C rd transformed to the primary side by the four-winding transformer, and L′ rd is the equivalent inductance of the filter inductance L rdEquivalent inductance transformed to primary side by four-winding transformer.

[0078] As Figure 3 The mathematical expression of the voltage gain of the Q-CLLC converter in Laplace domain is as follows Formula (5).

[0079]

[0080] In Formula (5), R ac is the load resistance, P x is the input active power of the x phase, Z rx is the total impedance of the LC filter circuit of the x phase, Z' rx is the equivalent total impedance of the secondary side LC filter circuit transformed to the primary side by the four-winding transformer. s is the Laplace operator; wherein x represents the corresponding phase A, B and C.

[0081] The novel medium-frequency isolation UPQC system for power distribution system and the control method thereof of the present application can provide isolation in the medium-frequency range through the Q-CLLC converter, so that the LFT is not needed, the number of active switches is reduced in the unidirectional structure with constant switching frequency in resonance, and the need for voltage control is eliminated.

[0082] It is apparent for those skilled in the art that the present application is not limited to the details of the above exemplary embodiments, but can be implemented in other concrete forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all aspects as illustrative and not restrictive, and the scope of the present application is defined by the appended claims rather than the above description, and it is intended to encompass all changes falling within the meaning and range of equivalents of the claims. Any reference signs in the claims should not be considered as limiting the claims involved.

[0083] In addition, it should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be properly combined to form other embodiments that those skilled in the art can understand.

Claims

1. A novel medium-frequency isolation UPQC system for power distribution systems, characterized in that, It includes a three-phase single-phase voltage source converter, a three-phase voltage source inverter, and a half-bridge CLLC converter; The three single-phase voltage source converters serve as series compensators, with the AC side connected to the three-phase power grid in series and the DC side connected to three DC energy storage capacitors C. A C B C C Connected; The half-bridge CLLC converter serves as intermediate frequency isolation between the series and parallel compensators, and includes a four-winding transformer, three uncontrolled half-bridge inverter circuits, an LLC filter circuit, an LC filter circuit, and one controlled half-bridge rectifier circuit; the three uncontrolled half-bridge inverter circuits are connected to three DC energy storage capacitors C A C B C C The half-bridge controllable rectifier circuit is connected to the energy storage capacitor C. D and the DC side phase connection of the three-phase voltage source inverter; The three-phase voltage source inverter acts as a parallel compensator, with a series filter inductor on the AC side connected to the three-phase power grid, and the DC side connected to the energy storage capacitor C. D It is connected to the half-bridge controlled rectifier circuit of the half-bridge CLLC converter.

2. A novel medium-frequency isolation UPQC system for power distribution systems according to claim 1, characterized in that, The three-channel single-phase voltage source converter includes three single-phase voltage source converters.

3. A novel medium-frequency isolation UPQC system for power distribution systems according to claim 2, characterized in that, The single-phase voltage source converter includes four switching transistor modules; each switching transistor module includes a fully controlled switching transistor freewheeling diode; the fully controlled switching transistor freewheeling diodes are connected in parallel with each other.

4. A novel medium-frequency isolation UPQC system for power distribution systems according to claim 1, characterized in that, The four-winding transformer includes a primary side and a secondary side; the primary side includes three windings and the secondary side includes two windings.

5. A novel medium-frequency isolation UPQC system for power distribution systems according to claim 1, characterized in that, In the three-way uncontrolled half-bridge inverter circuit, each half-bridge uncontrolled inverter circuit includes two voltage dividing capacitors and two diodes.

6. A novel medium-frequency isolation UPQC system for power distribution systems according to claim 1, characterized in that, The LLC filter circuit includes two inductors and one capacitor.

7. A novel medium-frequency isolation UPQC system for power distribution systems according to claim 1, characterized in that, The LC filter circuit includes an inductor L rd and capacitor C rd .

8. A novel medium-frequency isolation UPQC system for power distribution systems according to claim 1, characterized in that, The half-bridge controllable rectifier circuit includes two switching transistor modules and two voltage dividing capacitors.

9. A control method for a novel medium-frequency isolated UPQC system for power distribution systems, characterized in that, The novel medium-frequency isolated UPQC system for power distribution systems includes three single-phase voltage source converters, a three-phase voltage source inverter, and a half-bridge CLLC converter. The three single-phase voltage source converters are used as series compensators; the SPWM modulation method is used to inject compensation voltage into the three-phase power grid, and the voltage of the three-phase power grid is restored to the normal amplitude and phase level. The half-bridge CLLC converter serves as intermediate frequency isolation between the series compensator and the parallel compensator; The three-phase voltage source inverter serves as a parallel compensator.

10. A control method for a novel medium-frequency isolated UPQC system for a power distribution system according to claim 9, characterized in that, When any one of the three single-phase voltage source converters fails, the single-phase voltage source converter is disconnected from the three-phase power grid by closing the bypass circuit breaker.

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