Power transmission grid reactive power, voltage automatic control and harmonic damping integrated device and method

Abstract

This invention relates to a control method for an integrated device for reactive power and voltage automatic control and harmonic damping in a power transmission network, comprising the steps of: the control system sampling the three-phase voltage of the high-voltage bus and the three-phase output current I of the medium-voltage full-bridge MMC converter. measure And the voltage U of the medium-voltage full-bridge MMC converter MMC The invention includes a phase-locked loop, a power module average DC voltage control loop, a bus voltage control loop, and a reference harmonic current generation loop. The advantages of this invention are: it proposes an integrated reactive power and voltage control and harmonic damping technology; it simultaneously achieves automatic reactive power and voltage control and harmonic damping functions for the power grid through a multi-level power electronic converter (MMC) and corresponding control strategies; compared with existing technologies, the device of this invention has a smaller footprint and lower cost; and the harmonic damping suppression function of the integrated automatic reactive power and voltage control and harmonic damping device for transmission networks proposed in this invention is unaffected by changes in the power grid structure.

Description

Technical Field

[0001] This invention belongs to the field of power transmission technology, and in particular relates to an integrated device and method for automatic control of reactive power and voltage and harmonic damping in power transmission networks. Background Technology

[0002] According to industry experts, to achieve the goal of low-carbon and low-carbon development, the installed capacity of new energy power generation, mainly wind power and photovoltaic power, should account for more than 70% of the power grid.

[0003] Offshore wind farms have advantages such as high power generation utilization hours and minimal impact on the residential environment, leading to their vigorous development along my country's coast. Currently, the vast majority of offshore wind farms in my country are connected to the main power grid via AC submarine cables. With the increase in the number of offshore wind farms, the total length of submarine cables has also increased accordingly. Due to the capacitive characteristics of the submarine cable impedance and the inductive characteristics of the power grid system impedance, the following problems arise: when the power output of the offshore wind farm is low, the reactive power fed back from the wind farm to the system causes the voltage at the wind power connection point to rise beyond the rated range; system background harmonics are amplified, and in severe cases, this can cause harmonic resonance overvoltage, damaging power grid equipment.

[0004] In response to the problem of wind farms feeding reactive power back into the system, causing the bus voltage at the wind power connection point to rise beyond the rated range, the state has formulated relevant standards requiring all wind and photovoltaic power plants to be equipped with fast reactive power regulation devices of a certain capacity, such as SVC and SVG; and in order to offset the capacitive reactive power of submarine cables, high-voltage parallel reactors must also be installed at the grid connection point of offshore wind farms.

[0005] To address the aforementioned problem of amplified background harmonics in the system, the usual solution is to install an LC-type passive filter. However, in order to offset the capacitive reactive power of the LC-type passive filter at the fundamental frequency, an additional high-voltage shunt reactor is required, which increases the cost and floor space required.

[0006] The patent CN106295231B, "A Method for Suppressing Harmonic Resonance in Offshore Wind Farms Based on Modal Calculation," the article "Analysis of Harmonic Resonance Problems in Offshore Wind Farms with Flexible DC Transmission" (Guangdong Electric Power, July 2020, Vol. 33, No. 7), and the article "Research on Harmonic Resonance in Offshore Wind Farms Considering the Network Structure" (Power System Protection and Control, November 2016, Vol. 44, No. 22) propose the identification of harmonic resonance frequencies (modes) in offshore wind farms and propose methods for determining the optimal installation point of the filter device; however, none of them involve specific harmonic damping suppression techniques, let alone the techniques and methods for combining reactive power voltage control functions and harmonic damping functions. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide an integrated device and control method for automatic reactive voltage control and harmonic damping of power transmission networks.

[0008] This integrated device for automatic reactive power and voltage control and harmonic damping of power transmission networks includes: a step-up transformer, a medium-voltage full-bridge MMC converter, and a control system;

[0009] The medium-voltage full-bridge MMC converter includes three-phase outputs. Each phase output of the medium-voltage full-bridge MMC converter is composed of several full-bridge power sub-modules connected in series. The three-phase outputs of the medium-voltage full-bridge MMC converter are connected in a star configuration to the medium-voltage side of the step-up transformer, the neutral point of the medium-voltage full-bridge MMC converter itself, and the neutral point of the step-up transformer, respectively.

[0010] The high-voltage side of the step-up transformer is connected to the three-phase high-voltage bus at the high-voltage transmission network access point; the control system is electrically connected to the high-voltage bus and the medium-voltage full-bridge MMC converter; the control system and the medium-voltage full-bridge MMC converter are connected through feedback signal optical fiber and control signal optical fiber.

[0011] As a preferred option, the step-up transformer adopts either the Yg / Yg-12 connection group or the Δ / Yg-12 connection group.

[0012] The control method for this integrated device for reactive power and voltage control and harmonic damping in power transmission networks includes the following steps:

[0013] Step 1: The control system samples the three-phase voltage of the high-voltage bus and the three-phase output current I of the medium-voltage full-bridge MMC converter. measure And the voltage U of the medium-voltage full-bridge MMC converter MMC The control system receives the DC capacitor voltages of all full-bridge power submodules in the medium-voltage full-bridge MMC converter via feedback signal optical fiber.

[0014] Step 2: Perform phase-locked loop, power module average DC voltage control, bus voltage control, and reference harmonic current generation; obtain the active current reference value I. P_Ref Reference value of fundamental reactive current I q_Ref Reference value I of harmonic current h_Ref ;

[0015] Step 3: Use the active current reference value I obtained in Step 2. P_Ref Reference value of fundamental reactive current I q_Ref Reference value I of harmonic current h_Ref Adding them together, we obtain the reference current I of the medium-voltage full-bridge MMC converter. Ref ; will I Ref Input current follower; sets the three-phase output current I of the medium-voltage full-bridge MMC converter. measureTracking the reference current I of the medium-voltage full-bridge MMC converter Ref ; Obtain the output signal of the current follower;

[0016] Step 4: Add the output signal of the current follower in Step 3 to the fundamental component of the high-voltage bus extracted in the bus voltage control stage in Step 2, and use it as the reference wave signal input of the PWM pulse generator. The PWM pulse generator triggers the PWM drive pulse signal.

[0017] Step 5: The control system transmits the PWM drive pulse signal to all full-bridge power sub-modules of the medium-voltage full-bridge MMC converter through the control signal fiber.

[0018] As a preferred option, step 2 specifically involves:

[0019] Phase-locked loop (PLL): The three-phase voltage of the high-voltage bus is input into the PLL. The PLL outputs a signal cos(wt) with the same frequency and phase as the three-phase voltage of the high-voltage bus, and also outputs a signal sin(wt) with the same frequency and phase difference of 90° as the high-voltage bus voltage. w is the grid angular frequency and t represents time.

[0020] The average DC voltage control loop for power modules: The difference between the average DC voltage of all full-bridge power sub-modules in the medium-voltage full-bridge MMC converter and the DC voltage setpoint is output through a proportional-integral controller; the output of the proportional-integral controller is multiplied by the signal cos(wt) obtained from the phase-locked loop to obtain the active current reference value I. P_Ref ;

[0021] Bus voltage control loop: The fundamental component of the high-voltage bus is extracted and RMS calculated. The result of the RMS calculation is used as the input of the reactive voltage slope controller. The reactive voltage slope controller outputs the reference reactive current RMS value of the medium-voltage full-bridge MMC converter. The reference reactive current RMS value of the medium-voltage full-bridge MMC converter is multiplied by the signal sin(wt) obtained from the phase-locked loop to obtain the fundamental reactive current reference value I. q_Ref ;

[0022] The reference harmonic current generation stage involves using the three-phase voltage of the high-voltage bus as input. Harmonic components h1 to hN are extracted from these components, with N ranging from the 3rd to the 23rd harmonic. Each of these components is then multiplied by a corresponding constant Uh1 to UhN, and phase-shifted using a delay stage. The shifted values ​​are then summed to obtain the reference value I for the harmonic current. h_Ref .

[0023] Preferably, the current follower in step 2 is a repetitive controller or a proportional resonant controller.

[0024] The beneficial effects of this invention are: this invention proposes an integrated reactive power and voltage control and harmonic damping technology; this invention simultaneously realizes the automatic reactive power and voltage control and harmonic damping functions of the power grid through a multi-level power electronic converter (MMC) and corresponding control strategies; compared with the prior art, the device of this invention has a small footprint and low cost; the harmonic damping suppression function of the integrated automatic reactive power and voltage control and harmonic damping device of the power grid proposed in this invention is not affected by changes in the power grid structure. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the circuit of the integrated device for automatic control of reactive power and voltage and harmonic damping in the power transmission network of the present invention;

[0026] Figure 2 This is the circuit diagram of the full-bridge power submodule unit in a medium-voltage full-bridge MMC converter;

[0027] Figure 3 This is a block diagram of the control strategy for an integrated device for reactive power and voltage automatic control and harmonic damping in power transmission networks.

[0028] Figure 4 This is the wiring diagram of the MATLAB simulation model of Embodiment 3 of the present invention;

[0029] Figure 5-1 This is a schematic diagram of the simulation results of the three-phase bus voltage waveform when the integrated device for reactive power, automatic voltage control and harmonic damping of the power transmission network is not in operation. Figure 5-2 This is a schematic diagram of the simulation results of the three-phase system current waveform when the integrated device for reactive power and voltage automatic control and harmonic damping of the power transmission network is not in operation.

[0030] Figure 6-1 This is a schematic diagram of the simulation results of the three-phase bus voltage waveform after the integrated device for reactive power and voltage automatic control and harmonic damping of the power transmission network is put into operation. Figure 6-2 This is a schematic diagram of the simulation results of the three-phase system current waveform after the integrated device for reactive power and voltage automatic control and harmonic damping of the power transmission network is put into operation.

[0031] Figure labeling: 1. Step-up transformer; 2. Medium-voltage full-bridge MMC converter; 3. Control system; 4. High-voltage busbar. Detailed Implementation

[0032] The present invention will be further described below with reference to embodiments. The description of the embodiments below is only for the purpose of helping to understand the present invention. It should be noted that those skilled in the art can make several modifications to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0033] Example 1

[0034] Embodiment 1 of this application provides a method such as Figure 1 The integrated device for reactive power and voltage automatic control and harmonic damping of the power transmission network shown includes: a step-up transformer 1, a medium-voltage full-bridge MMC converter 2, and a control system 3; the medium-voltage full-bridge MMC converter 2 has three-phase output, and each phase output of the medium-voltage full-bridge MMC converter 2 consists of several... Figure 2 The full-bridge power submodule shown is composed of connected units. Figure 2 In this circuit, T1, T2, T3, and T4 are IGBT components; D1, D2, D3, and D4 are power diodes. The three-phase output of the medium-voltage full-bridge MMC converter 2 is connected in a star configuration to the medium-voltage side of the step-up transformer 1, the neutral point of the medium-voltage full-bridge MMC converter 2 itself, and the neutral point of the step-up transformer 1. The high-voltage side of the step-up transformer 1 is connected to the three-phase high-voltage bus 4 at the high-voltage transmission network access point. The control system 3 is electrically connected to the high-voltage bus 4 and the medium-voltage full-bridge MMC converter 2. The control system 3 and the medium-voltage full-bridge MMC converter 2 are connected through feedback signal optical fiber and control signal optical fiber.

[0035] Example 2

[0036] Based on Embodiment 1, Embodiment 2 of this application provides as follows: Figure 3 The control method of the integrated device for automatic reactive power and voltage control and harmonic damping of the power transmission network in Embodiment 1 shown:

[0037] Step 1: The control system 3 samples the three-phase voltage of the high-voltage bus 4 and the three-phase output current I of the medium-voltage full-bridge MMC converter 2. measure The voltage U of the medium-voltage full-bridge MMC converter 2 MMC The control system 3 receives the DC capacitor voltages of all full-bridge power sub-modules in the medium-voltage full-bridge MMC converter 2 via a feedback signal optical fiber.

[0038] Step 2: Perform phase-locked loop, power module average DC voltage control, bus voltage control, and reference harmonic current generation; obtain the active current reference value I. P_Ref Reference value of fundamental reactive current I q_Ref Reference value I of harmonic current h_Ref ;

[0039] Phase-locked loop (PLL): The three-phase voltage of high-voltage bus 4 is input into the phase-locked loop (PLL). The PLL outputs a signal cos(wt) with the same frequency and phase as the three-phase voltage of high-voltage bus 4, and also outputs a signal sin(wt) with the same frequency and phase difference of 90° as the voltage of high-voltage bus 4. w is the grid angular frequency and t represents time.

[0040] The average DC voltage control loop for the power modules: The difference between the average DC voltage of all full-bridge power sub-modules in the medium-voltage full-bridge MMC converter 2 and the DC voltage setpoint is output through a proportional-integral (PI) controller; the output of the PI controller is multiplied by the signal cos(wt) obtained from the phase-locked loop to obtain the active current reference value I. P_Ref The purpose is to control the average DC voltage of all full-bridge power sub-modules in the medium-voltage full-bridge MMC converter 2 to be close to its set value.

[0041] Bus voltage control circuit: Extract the fundamental component of high-voltage bus 4 and perform RMS calculation on the fundamental component; use the result of the RMS calculation as the input of the reactive voltage slope controller, the reactive voltage slope controller outputs the reference reactive current effective value of medium-voltage full-bridge MMC converter 2, and multiply the reference reactive current effective value of medium-voltage full-bridge MMC converter 2 with the signal sin(wt) obtained from the phase-locked loop to obtain the fundamental reactive current reference value I. q_Ref ;

[0042] The reference harmonic current generation process involves using the three-phase voltage of high-voltage bus 4 as input. Harmonic components h1 to hN are extracted from the three-phase voltage of high-voltage bus 4, where N ranges from 3 to 23. Each of these components is then multiplied by a corresponding constant Uh1 to UhN, and phase-shifted using a delay circuit. The shifted values ​​are then summed to obtain the reference value I for the harmonic current. h_Ref ;

[0043] The current follower is either a repetitive controller or a proportional resonant controller (PR);

[0044] Step 3: Use the active current reference value I obtained in Step 2. P_Ref Reference value of fundamental reactive current I q_Ref Reference value I of harmonic current h_Ref Adding them together, we obtain the reference current I of the medium-voltage full-bridge MMC converter 2. Ref ; will I Ref Input current follower; sets the three-phase output current I of medium-voltage full-bridge MMC converter 2. measure The reference current I of the medium-voltage full-bridge MMC converter 2 is tracked. Ref ; Obtain the output signal of the current follower;

[0045] Step 4: Add the output signal of the current follower in Step 3 to the fundamental component of the high-voltage bus 4 extracted in the bus voltage control stage in Step 2, and use it as the reference wave signal input of the PWM pulse generator. The PWM pulse generator triggers the PWM drive pulse signal.

[0046] Step 5: The control system 3 transmits the PWM drive pulse signal to all full-bridge power sub-module units of the medium-voltage full-bridge MMC converter 2 through the control signal optical fiber.

[0047] Example 3

[0048] Based on Embodiment 2, Embodiment 3 of this application provides the application of the control method of Embodiment 2 in a simulation circuit:

[0049] The circuit of Example 1 was built in MATLAB and simulated using a simulation model. The specific parameters of the simulation circuit are as follows:

[0050] 1) Control system 3 rated voltage: 220kV;

[0051] 2) Control system 3 Rated frequency: 50Hz;

[0052] 3) Number of full-bridge power sub-modules cascaded per phase in medium-voltage full-bridge MMC converter 2: 40 (of which 4 are redundant modules);

[0053] 4) DC capacitor capacitance of the full-bridge power submodule unit: 7.6mF;

[0054] 5) DC side rated voltage setting of the full-bridge power submodule unit: 900V;

[0055] 6) Step-up transformer 1's turns ratio: 220kV / 30kV; rated capacity: 30MVA; short-circuit impedance: 10%;

[0056] 7) 220kV system background harmonic voltage (5th order): 457V; background harmonic current (5th order): 12A;

[0057] 8) Equivalent reactance of the 220kV system: 22mH;

[0058] 9) Total equivalent capacitance of 220kV submarine cable: 12.7uF;

[0059] 10) Total generating capacity of the two offshore wind farms: 190MW;

[0060] MATLAB simulation models such as Figure 4 As shown. Some simulation results (bus voltage and system current) are as follows. Figure 5-1 , Figure 5-2 , Figure 6-1 and Figure 6-2 As shown. Figure 5-1 This is a waveform diagram of the three-phase voltage of a 220kV bus. Figure 5-2 The waveform diagram of the three-phase current flowing into the control system; Figure 6-1The image shows the three-phase voltage waveforms of the 220kV bus after the integrated power transmission network reactive power and voltage automatic control and harmonic damping device of the present invention has been put into operation. Figure 6-2 The diagram shows the three-phase current waveforms flowing into the control system after the integrated automatic control and harmonic damping device for power grid reactive power and voltage of the present invention is put into operation.

[0061] from Figure 5-1 and Figure 5-2 It can be seen that when the integrated reactive power and voltage control and harmonic damping devices of the power transmission network are not in operation, the current waveform distortion is relatively severe; the bus voltage also exhibits some distortion. From Figure 6-1 and Figure 6-2 As can be seen, after the integrated device for automatic reactive power and voltage control and harmonic damping of the power transmission network is put into operation, the bus voltage has virtually no distortion; and the distortion length of the system current waveform is also very slight. This fully demonstrates the effectiveness of the integrated device and control method for automatic reactive power and voltage control and harmonic damping of the power transmission network in terms of the automatic reactive power and voltage control and harmonic damping functions of the power grid.

Claims

1. An integrated device for automatic reactive power and voltage control and harmonic damping in power transmission networks, characterized in that, include: Step-up transformer (1), medium-voltage full-bridge MMC converter (2) and control system (3); The medium-voltage full-bridge MMC converter (2) includes three-phase output. Each phase output of the medium-voltage full-bridge MMC converter (2) is composed of several full-bridge power sub-module units connected in series. The three-phase output of the medium-voltage full-bridge MMC converter (2) is connected to the medium-voltage side of the step-up transformer (1) in a star connection manner. The neutral point of the medium-voltage full-bridge MMC converter (2) is connected to the neutral point of the step-up transformer (1). The high-voltage side of the step-up transformer (1) is connected to the three-phase high-voltage bus (4) at the high-voltage power transmission network access point; the control system (3) is electrically connected to the high-voltage bus (4) and the medium-voltage full-bridge MMC converter (2); the control system (3) and the medium-voltage full-bridge MMC converter (2) are connected through the feedback signal fiber and the control signal fiber. The control method for an integrated device for reactive power and voltage automatic control and harmonic damping in power transmission networks includes the following steps: Step 1: The control system (3) samples the three-phase voltage of the high-voltage bus (4) and the three-phase output current I of the medium-voltage full-bridge MMC converter (2). measure The voltage U of the medium-voltage full-bridge MMC converter (2) MMC The control system (3) receives the DC capacitor voltage of all full-bridge power sub-module units in the medium-voltage full-bridge MMC converter (2) through the feedback signal optical fiber; Step 2: Perform phase-locked loop, power module average DC voltage control, bus voltage control, and reference harmonic current generation; obtain the active current reference value I. P_Ref Reference value of fundamental reactive current I q_Ref Reference value I of harmonic current h_Ref ; Step 3: Use the active current reference value I obtained in Step 2. P_Ref Reference value of fundamental reactive current I q_Ref Reference value I of harmonic current h_Ref By adding them together, we obtain the reference current I of the medium-voltage full-bridge MMC converter (2). Ref ; will I Ref Input current follower; make the three-phase output current I of the medium-voltage full-bridge MMC converter (2) measure The reference current I of the medium-voltage full-bridge MMC converter (2) is tracked. Ref ; Obtain the output signal of the current follower; Step 4: Add the output signal of the current follower in step 3 to the fundamental component of the high voltage bus (4) extracted in the bus voltage control loop in step 2, and use it as the reference wave signal input of the PWM pulse generator. The PWM pulse generator triggers the PWM drive pulse signal. Step 5: The control system (3) transmits the PWM drive pulse signal to all full-bridge power sub-module units of the medium-voltage full-bridge MMC converter (2) through the control signal fiber. Step 2 is as follows: Phase-locked loop: The three-phase voltage of the high-voltage bus (4) is input into the phase-locked loop. The phase-locked loop outputs a signal cos(wt) with the same frequency and phase as the three-phase voltage of the high-voltage bus (4), and also outputs a signal sin(wt) with the same frequency and phase difference of 90° as the voltage of the high-voltage bus (4). w is the grid angular frequency and t represents time. The average DC voltage control loop of the power module: The difference between the average DC voltage of all full-bridge power sub-modules in the medium-voltage full-bridge MMC converter (2) and the DC voltage setpoint is output through the proportional-integral controller; the output of the proportional-integral controller is multiplied by the signal cos(wt) obtained from the phase-locked loop to obtain the active current reference value I. P_Ref ; Bus voltage control loop: Extract the fundamental component of the high-voltage bus (4) and perform RMS calculation on the fundamental component; use the result of the RMS calculation as the input of the reactive voltage slope controller, the reactive voltage slope controller outputs the reference reactive current effective value of the medium-voltage full-bridge MMC converter (2), and multiplies the reference reactive current effective value of the medium-voltage full-bridge MMC converter (2) with the signal sin(wt) obtained from the phase-locked loop to obtain the fundamental reactive current reference value I. q_Ref ; Reference harmonic current generation stage: The three-phase voltage of the high-voltage bus (4) is used as input. The h1th to hNth harmonic components are extracted from the three-phase voltage of the high-voltage bus (4). The value of N ranges from 3 to 23rd. The h1th to hNth harmonic components are multiplied by the corresponding constants Uh1 to UhN respectively, and phase shifting is performed through a delay stage. The shifted values ​​are added together and used as the reference value I of the harmonic current. h_Ref .

2. The integrated device for automatic reactive power and voltage control and harmonic damping of power transmission networks according to claim 1, characterized in that: The step-up transformer (1) adopts the Yg / Yg-12 connection group or the ∆ / Yg-12 connection group.

3. The integrated device for automatic reactive power and voltage control and harmonic damping of power transmission networks according to claim 2, characterized in that: In step 2, the current follower is either a repetitive controller or a proportional resonant controller.

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