Modularized parallel structure of unified power quality conditioner and control method thereof

Abstract

The invention relates to a modular parallel structure of a unified power quality conditioner (UPQC) and a control method of the modular parallel structure. The parallel structure comprises a plurality of UPQC modules which are connected in parallel, alternating-current series ports of the UPQC modules are connected in parallel, alternating-current parallel ports of the UPQC modules are connected in parallel, and direct-current ports of the UPQC modules are connected in parallel. The control method comprises a first droop control method for a direct current port, a second droop control method for an alternating current series port and a current sharing control method for an alternating current parallel port. Compared with the prior art, the droop control method of the direct current port and the serial port is constructed, each UPQC module can be independently controlled and does not depend on an inter-module communication network, so that effective capacity expansion is ensured, meanwhile, module power uniform distribution is achieved, and stable operation of the system is reliably guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a unified power quality conditioner modular parallel structure and a control method thereof. Background Art

[0002] Due to the widespread application of a large number of power electronic devices, the nonlinear loads in the power transmission and distribution links and power consumption terminals have increased significantly, which can easily cause the current and voltage waveforms of the power grid to change, causing harmonic pollution of the power grid and affecting the safe and stable operation of the power grid. Especially in low-voltage distribution networks in rural or urban areas, due to long lines and diversified loads, there are many power quality problems such as low power factor, three-phase imbalance, unqualified voltage, harmonics, etc., which will seriously affect the power supply quality. Unified Power Quality Conditioner (UPQC) is a power quality regulation device with comprehensive compensation function. It has the functions of both parallel active filters and dynamic voltage restorers. It can realize comprehensive management of multiple power qualities including current and voltage power quality. It is a customized power technology device that effectively manages power quality problems.

[0003] UPQC with modular design can also achieve rapid expansion of rated power, that is, expand its output power. Usually, its DC port and AC port are connected in parallel to increase its current output capacity while keeping the rated voltage unchanged. For the AC port on the parallel side, it mainly controls the output current, and conventional control methods can be used. For the DC port and the AC port on the series side, it is necessary to control the voltage, and a master-slave control method is usually used. However, master-slave control requires strong communication lines and requires higher reliability for the host. For example, most of the existing technologies use a central controller to control each parallel module, and multiple communication lines are required to connect these parallel modules and the central controller. Once the communication line is disconnected or the communication is not smooth, the system will not be able to operate normally, and it is difficult to ensure uniform power distribution between modules, which is not conducive to stable power expansion. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a unified power quality conditioner modular parallel structure and a control method thereof, which can ensure uniform power distribution of each parallel module while achieving power expansion and ensure stable operation of the system.

[0005] The object of the present invention can be achieved through the following technical scheme: a unified power quality conditioner modular parallel structure, including multiple parallel UPQC modules, the AC series ports of each UPQC module are connected in parallel with each other, the AC parallel ports of each UPQC module are connected in parallel with each other, and the DC ports of each UPQC module are connected in parallel with each other.

[0006] Furthermore, the UPQC module includes a series-side converter and a parallel-side converter, and the series-side converter and the parallel-side converter are connected via a DC port.

[0007] Furthermore, the AC output of the series-side converter is connected in series between the power grid and the load to adjust the load voltage amplitude and compensate for the power grid voltage harmonics, thereby ensuring that a stable three-phase sinusoidal voltage is provided to the load.

[0008] Furthermore, the AC output of the parallel-side converter is connected in parallel between the power grid and the load to compensate for the load harmonic current and ensure the stability of the intermediate DC bus voltage to ensure that the current flowing into the power grid is a balanced three-phase sinusoidal current.

[0009] A control method for a unified power quality conditioner modular parallel structure includes: a first droop control method for a DC port, a second droop control method for an AC series port, and a current equalization control method for an AC parallel port.

[0010] Furthermore, the specific process of the first droop control method is: according to the rated voltage droop curve, the DC port voltage setting instruction is calculated, and the error instruction is obtained by subtracting the voltage feedback value, and the active current setting of the AC parallel port is obtained through the voltage regulator.

[0011] Furthermore, the specific process of the current balancing control method is: according to the current reactive power, harmonic, and unbalance instructions, combined with the active current given by the AC parallel port, the AC parallel port current error instruction is obtained by subtracting the AC current feedback value, and the duty cycle of the parallel side converter is obtained through the current regulator, and the parallel side converter is driven in combination with the SPWM modulation strategy.

[0012] Furthermore, the DC port voltage setting instruction is:

[0013]

[0014] in, is the bus voltage rating, l q is the droop coefficient, P is the module active power;

[0015] Calculate the active current given value based on the DC bus voltage given value and voltage feedback value In addition, the grid current is collected, and the compensation current setting is calculated according to its reactive current and harmonic current. The active current setting and the reactive and harmonic compensation current are used together as the output current setting of the AC parallel port:

[0016]

[0017] in, is the active current given, is the compensation current given.

[0018] Furthermore, the specific process of the second droop control method is: according to the rated voltage amplitude and the frequency droop curve, the series side AC output voltage given instruction is calculated, and the error instruction is obtained by subtracting it from the voltage feedback value, and the duty cycle of the series side converter is obtained through the voltage and current dual closed-loop strategy, and the series side converter is driven in combination with the SPWM modulation strategy.

[0019] Furthermore, the series side AC output voltage setting instruction includes a phase setting θ ref and voltage amplitude given by v ref :

[0020]

[0021] θ ref =θ * -m p P

[0022] in, is the voltage to be compensated, θ * is the grid phase, n p is the voltage droop coefficient, m p is the frequency droop coefficient, Q is the reactive power of the AC port on the series side, and P is the active power of the AC port on the series side.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] The present invention proposes a unified power quality conditioner modular parallel structure, in which the AC series port, DC port and AC parallel port of each UPQC module are connected in parallel, thereby enabling each UPQC module to be easily controlled independently without relying on an inter-module communication network, thereby ensuring effective capacity expansion and achieving uniform distribution of module power, thereby ensuring stable operation of the system.

[0025] The present invention designs a corresponding control method for the proposed unified power quality conditioner modular parallel structure, adopts droop control for the DC port and the AC series port, and adopts current equalization control for the AC parallel port. On the one hand, according to the rated voltage droop curve, the DC port is drooped to obtain the active current setting of the AC parallel port, and then combined with the current reactive power, harmonics, and unbalanced instructions to obtain the parallel side converter duty cycle; on the other hand, according to the rated voltage amplitude and frequency droop curve, the AC series port is drooped to obtain the series side converter duty cycle. Thereby, a droop control process that is independent of the communication line is realized, which can more reliably ensure the stable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the modular parallel structure of the unified power quality conditioner in the present invention;

[0027] Figure 2 It is a control block diagram of the modular parallel structure of the unified power quality conditioner in the present invention. DETAILED DESCRIPTION

[0028] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Example

[0030] like Figure 1 As shown, a unified power quality conditioner modular parallel structure includes multiple parallel UPQC modules, the AC series ports of each UPQC module are connected in parallel with each other, the AC parallel ports of each UPQC module are connected in parallel with each other, and the DC ports of each UPQC module are connected in parallel with each other.

[0031] Each UPQC module includes a series-side converter and a parallel-side converter, which are connected via a DC port. The AC output of the series-side converter is connected in series between the grid and the load to adjust the load voltage amplitude and compensate for the grid voltage harmonics, ensuring that a stable three-phase sinusoidal voltage is provided to the load; the AC output of the parallel-side converter is connected in parallel between the grid and the load to compensate for the load harmonic current and ensure the stability of the intermediate DC bus voltage to ensure that the current flowing into the grid is a balanced three-phase sinusoidal current.

[0032] In view of the modular parallel structure of the unified power quality conditioner, this scheme proposes a corresponding control method, wherein the DC port and the AC series port adopt voltage droop control, and the AC parallel port adopts current sharing control. For the DC port, the DC port voltage setting instruction is calculated according to the rated voltage droop curve, and the error instruction is obtained by subtracting the voltage feedback value, and the AC parallel port active current setting is obtained through the voltage regulator.

[0033] For the AC parallel port, according to the current reactive power, harmonic and unbalance instructions, combined with the AC parallel port active current given, the parallel port current error instruction is obtained by subtracting the current feedback value, and the duty cycle of the parallel side converter is obtained through the current regulator, and the parallel side converter is driven in combination with the SPWM modulation strategy.

[0034] For the AC series port, the output voltage given instruction is calculated according to the rated voltage amplitude and frequency droop curve, and the error instruction is obtained by subtracting it from the voltage feedback value. The duty cycle of the series side converter is obtained through the voltage and current dual closed-loop strategy, and the series side converter is driven in combination with the SPWM modulation strategy.

[0035] In this embodiment, the control block diagram is as follows Figure 2 As shown, specifically, for the parallel-side converter: it is necessary to ensure that the DC bus voltage is stable, and at the same time it is necessary to ensure that the output current can compensate for the reactive current and harmonic current of the power grid.

[0036] The DC bus voltage adopts droop control mode, and its given value is

[0037]

[0038] in, is the bus voltage rating, l q is the droop coefficient, and P is the module active power.

[0039] The voltage regulator calculates the active current given value based on the DC bus voltage given value and the voltage feedback value. In addition, the system collects the grid current and calculates the compensation current given by its reactive current and harmonic current. The active current setting, reactive power and harmonic compensation current are used together as the output current setting of the AC parallel port:

[0040]

[0041] The output current is given by i ref The difference is made with the AC current feedback value, and the modulation wave is obtained through the current regulator. After SPWM adjustment, the PWM wave is output to drive the IGBT switching device in the parallel side converter.

[0042] For the series side converter: it is necessary to ensure that the output voltage can compensate for the grid voltage so that the compensated output voltage is the rated voltage of the grid.

[0043] The AC output voltage of the series side module adopts the droop control method, and its amplitude and frequency given values ​​are

[0044]

[0045] θ ref =θ * -m p P

[0046] in, is the voltage to be compensated, θ * is the grid phase, n p is the voltage droop coefficient, m p is the frequency droop coefficient, Q is the reactive power of the AC port on the series side, and P is the active power of the AC port on the series side.

[0047] The phase of the output voltage is given by θ ref , combined with the voltage amplitude given by v ref, and get the output voltage given value. The voltage regulator gets the AC current given value based on the voltage given value and voltage feedback Given the alternating current and AC current feedback value i sabc The difference is made, and the modulated wave is obtained through the current regulator. After SPWM adjustment, the PWM wave is output to drive the IGBT switching device in the series side converter.

Claims

1. A unified power quality conditioner modular parallel structure, characterized in that: It comprises a plurality of parallel UPQC modules, wherein the AC series ports of the UPQC modules are connected in parallel with each other, the AC parallel ports of the UPQC modules are connected in parallel with each other, and the DC ports of the UPQC modules are connected in parallel with each other.

2. The unified power quality conditioner modular parallel structure according to claim 1, characterized in that: The UPQC module includes a series-side converter and a parallel-side converter, and the series-side converter and the parallel-side converter are connected via a DC port.

3. The unified power quality conditioner modular parallel structure according to claim 2 is characterized in that: The AC output of the series-side converter is connected in series between the power grid and the load to adjust the load voltage amplitude and compensate for the power grid voltage harmonics, thereby ensuring that a stable three-phase sinusoidal voltage is provided to the load.

4. The unified power quality conditioner modular parallel structure according to claim 2, characterized in that: The AC output of the parallel-side converter is connected in parallel between the power grid and the load to compensate for the load harmonic current and ensure the stability of the intermediate DC bus voltage to ensure that the current flowing into the power grid is a balanced three-phase sinusoidal current.

5. A control method for a unified power quality conditioner modular parallel structure, applied to a unified power quality conditioner modular parallel structure as claimed in claim 1, characterized in that: include: A first droop control method for a DC port, a second droop control method for an AC series port, and a current sharing control method for an AC parallel port.

6. A control method for a unified power quality conditioner modular parallel structure according to claim 5, characterized in that: The specific process of the first droop control method is: according to the rated voltage droop curve, the DC port voltage setting instruction is calculated, and the error instruction is obtained by subtracting the voltage feedback value, and the active current setting of the AC parallel port is obtained through the voltage regulator.

7. A control method for a unified power quality conditioner modular parallel structure according to claim 6, characterized in that: The specific process of the current balancing control method is as follows: according to the current reactive power, harmonic and unbalance instructions, combined with the active current given by the AC parallel port, the AC parallel port current error instruction is obtained by subtracting the AC current feedback value, the parallel side converter duty cycle is obtained through the current regulator, and the parallel side converter is driven in combination with the SPWM modulation strategy.

8. A control method for a unified power quality conditioner modular parallel structure according to claim 7, characterized in that: The DC port voltage setting instruction is: in, is the bus voltage rating, l q is the droop coefficient, P is the module active power; Calculate the active current given value based on the DC bus voltage given value and voltage feedback value In addition, the grid current is collected, and the compensation current setting is calculated according to its reactive current and harmonic current. The active current setting and the reactive and harmonic compensation current are used together as the output current setting of the AC parallel port: in, is the active current given, is the compensation current given.

9. The control method of a unified power quality conditioner modular parallel structure according to claim 5, characterized in that: The specific process of the second droop control method is as follows: according to the rated voltage amplitude and the frequency droop curve, the series side AC output voltage given instruction is calculated, and the error instruction is obtained by subtracting the voltage feedback value, and the series side converter duty cycle is obtained through the voltage and current dual closed-loop strategy, and the series side converter is driven in combination with the SPWM modulation strategy.

10. A control method for a unified power quality conditioner modular parallel structure according to claim 9, characterized in that: The series side AC output voltage setting instruction includes a phase setting θ ref and voltage amplitude given by v ref : i ref =θ * -m p P in, is the voltage to be compensated, θ * is the grid phase, n p is the voltage droop coefficient, m p is the frequency droop coefficient, Q is the reactive power of the AC port on the series side, and P is the active power of the AC port on the series side.

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