Accurate distribution control strategy for active power of micro-grid inverter

Abstract

The invention discloses a micro-grid inverter active power accurate distribution control strategy, and relates to the field of micro-grid inverters. The method is based on three micro-grid characteristics: firstly, the internal impedance of higher harmonics of a grid-connected inverter of the micro-grid presents an inductive characteristic, the active power injected into the higher harmonics is accurately equally divided according to a capacity proportion by adopting a P-omega droop control strategy, and the influence of impedance of a connecting line can be weakened by relatively large inductive reactance of the higher harmonics; secondly, since the injected harmonic load is a typical load, the injected higher harmonic active power and the injected fundamental active power have a fixed proportional relation; and thirdly, the injected fundamental wave active power and the actual output active power have the same droop control characteristic due to the adoption of the P-V droop control strategy. The method is not only suitable for active power accurate distribution based on P-V droop control and mainly based on resistance of the micro-grid inverter connecting line, but also suitable for reactive power accurate distribution based on Q-V droop control and mainly based on inductance of the micro-grid inverter connecting line.

Description

Technical Field

[0001] The present invention relates to the field of microgrid inverters, and in particular to a microgrid inverter active power precise distribution control strategy. Background Art

[0002] Ideally, multiple parallel-connected grid-connected inverters in a microgrid adopt a droop control strategy, and each inverter can automatically distribute load power according to capacity without communication. However, in actual projects, due to the inconsistent impedance of the outlet lines of each grid-connected inverter, each grid-connected inverter cannot accurately distribute power according to capacity ratio. Usually, the impedance of the low-voltage microgrid connection line is mainly resistive, so the PV (active power-voltage) and Q-ω droop control strategies are more conducive to inverter power distribution. Since frequency ω is a global variable, the frequency-related reactive power can be distributed according to capacity ratio to each inverter. However, for the inverter output active power related to the line voltage amplitude U, when the line impedance is unbalanced, due to the unequal voltage amplitude U at the connection point of each parallel inverter, the output active power will inevitably be unbalanced, and accurate distribution cannot be achieved according to capacity ratio. At the same time, uneven power distribution will also cause system circulation, reduce power transmission efficiency, cause overheating loss of power electronic equipment, shorten the life of the equipment and reduce its safety performance.

[0003] To address the above problems, the existing technologies mainly include:

[0004] One method is to use communication between inverters to achieve output power distribution of each inverter according to capacity. This method is not convenient for actual engineering applications for inverters that are distributed far away. In addition, the communication lines between inverters are easily affected by external interference, which reduces the reliability of microgrid operation.

[0005] The second method is to use virtual impedance (virtual resistance or virtual inductance) to strengthen the internal impedance of the inverter output and weaken the influence of line impedance. The main problem of this method is that the excessive internal impedance increases the inverter output voltage and power loss, causing the inverter output voltage to fluctuate, affecting the stable operation of the microgrid.

[0006] The third method is to take the feedback voltage of the inverter control system to the common connection point (PCC), thereby eliminating the output voltage deviation of each inverter and balancing the power output. However, in actual microgrids, due to the decentralized nature of inverter connections, it is difficult to find a suitable PCC point, and the measurement line is long, which will increase the measurement error and reduce the reliability of the system.

[0007] Therefore, the present invention proposes a microgrid inverter active power precise distribution control strategy based on high-order harmonic droop control to solve the problem that the active power cannot be accurately distributed according to the capacity ratio due to the unbalanced impedance of the inverter connection line in the low-voltage microgrid. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a precise distribution control strategy for active power of microgrid inverters in view of the deficiencies of the above-mentioned prior art, which mainly solves the problem that the active power between multiple inverters (based on PV (active power-voltage) droop control strategy) cannot be accurately distributed according to the capacity ratio due to the unbalanced impedance of the grid-connected inverter connection line in the low-voltage microgrid. At the same time, it can also solve the precise distribution of reactive power in the low-voltage microgrid inverter connection line that is mainly inductive (based on QV (reactive power-voltage) droop control strategy).

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0010] The present invention is not only applicable to precise active power distribution based on PV (active power-voltage) droop control when the microgrid inverter connection line is mainly resistive, but also applicable to precise reactive power distribution based on QV (reactive power-voltage) droop control when the microgrid inverter connection line is mainly inductive.

[0011] Typical harmonic loads such as three-phase uncontrolled rectification are injected to correct the output active power of multiple parallel inverters in the microgrid, so that each inverter can accurately distribute the output active power according to the capacity ratio; after the correction is completed, the harmonic load is disconnected from the grid.

[0012] The P-ω (active power-frequency) droop control strategy is adopted for injecting higher-order harmonics to realize accurate and equal distribution of the active power of higher-order injected harmonics output by each inverter according to the capacity ratio; the PV (active power-voltage) droop control strategy is adopted for both the injected fundamental active power distribution and the actual output active power to ensure the consistency of the control characteristics of the injected fundamental active power and the actual output active power.

[0013] The actual output active power of each inverter is corrected in real time according to a certain ratio based on the deviation between the standard value of the fundamental wave injected active power output by each inverter and the actual value of the fundamental wave injected active power.

[0014] A microgrid inverter active power precise distribution control strategy includes the following steps:

[0015] Step 1: Inject harmonic load at the common connection point of the microgrid grid-connected inverter;

[0016] Step 2: According to the impedance value of the inverter connection line, select the appropriate high-order harmonic order n; use P-ω and QV droop control strategy to achieve the inverter's n-th injection of high-order harmonic active power P n Output;

[0017] Step 3: Calculate the nth injected higher harmonic active power P output by the inverter n , calculate the standard value of injected fundamental wave active power P 1c ;

[0018] Step 4: Adopt PV and Q-ω droop control strategy to realize the output of injected fundamental active power P1 of the inverter;

[0019] Step 5: Calculate the injected fundamental wave active power P1 of the inverter output; Calculate the standard value of the injected fundamental wave active power P of the inverter output 1c The difference ΔP between the actual value P1 and 1c ;

[0020] Step 6: Calculate the actual active power deviation ΔP output by the inverter;

[0021] Step 7: According to the actual active power deviation ΔP, the power command value P of each inverter droop control is corrected in real time * ; Adopt PV and Q-ω droop control strategies to achieve the active power P output of each parallel inverter;

[0022] Step 8: According to the real-time corrected PV droop control strategy of step 7, the active power output of multiple parallel inverters in the microgrid is accurately distributed and controlled according to the capacity ratio;

[0023] Step 9: After the correction control is completed, the harmonic load is disconnected from the grid.

[0024] As a further preferred embodiment of the present invention, in step 2, the P-ω and QV droop control strategies are adopted, and the droop control equation of the nth-order high-order harmonic is:

[0025]

[0026] In the formula, k nω is the nth harmonic frequency droop coefficient, ω n ,ω n * are the nth order harmonic frequency and its command value output by the grid-connected inverter, P n , P n * are the corresponding nth injected harmonic active power and its command value respectively.

[0027] As a further preferred embodiment of the present invention, the injected fundamental wave active power standard value P output by the microgrid grid-connected inverter in step 3 is 1c and the nth harmonic power P n The ratio is the square of the harmonic order, that is:

[0028] P 1c =P n ×n 2 .

[0029] As a further preferred embodiment of the present invention, the droop control equation in step 4 is: .

[0030] As a further preferred embodiment of the present invention, ΔP in step 5 1c =P 1c -P1.

[0031] As a further preferred embodiment of the present invention, in step six

[0032]

[0033] As a further preferred embodiment of the present invention, the droop control equation in step seven is:

[0034]

[0035] As a further preferred embodiment of the present invention, the harmonic load is a harmonic load such as a three-phase uncontrolled rectifier.

[0036] The present invention has the following beneficial effects:

[0037] The microgrid inverter active power precise distribution control strategy based on high-order harmonic droop control described in the present invention can effectively solve the problem that the active power between multiple inverters (using PV droop control strategy) cannot be accurately distributed according to the capacity ratio due to the unbalanced impedance of the grid-connected inverter connection line in the low-voltage microgrid, and is easy to implement in engineering.

[0038] The present invention overcomes the shortcomings of complex control system, susceptibility to external interference, poor reliability, etc. caused by the use of communication connection between inverters in the microgrid and the use of virtual impedance in the inverter control system. It only needs to inject a certain amount of typical harmonic load into the microgrid to correct the uneven output active power distribution of multiple grid-connected inverters caused by unbalanced connection line impedance in real time. After the correction is completed, the injected harmonic load can be disconnected without affecting the normal operation of the microgrid.

[0039] The present invention is based on three microgrid characteristics: first, the high-order harmonic internal impedance of the microgrid grid-connected inverter presents an inductive characteristic, and the P-ω droop control strategy is adopted to realize the accurate equal distribution of the injected high-order harmonic active power according to the capacity ratio, and the larger high-order harmonic inductive reactance can weaken the influence of the connection line impedance; second, since the injected harmonic load is a typical load, there is a fixed proportional relationship between the injected high-order harmonic active power and the injected fundamental active power; third, since the injected fundamental active power and the actual output active power both adopt the PV droop control strategy, they have the same droop control characteristics.

[0040] The present invention is not only applicable to precise active power distribution based on PV (active power-voltage) droop control when the microgrid inverter connection line is mainly resistive, but also applicable to precise reactive power distribution based on QV (reactive power-voltage) droop control when the microgrid inverter connection line is mainly inductive. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is the microgrid inverter active power precise distribution control strategy based on high-order harmonic droop control described in the present invention. DETAILED DESCRIPTION

[0042] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific preferred embodiments.

[0043] In the description of the present invention, it should be understood that the terms "left side", "right side", "upper part", "lower part" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. "First", "second" and the like do not indicate the importance of the components, and therefore cannot be understood as limiting the present invention. The specific dimensions used in this embodiment are only for illustrating the technical solution, and do not limit the scope of protection of the present invention.

[0044] Ideally, the grid-connected inverters running in parallel in the microgrid adopt the PV (active power-voltage) droop control strategy, and each inverter can automatically distribute the load active power according to the capacity without communication. However, in actual projects, due to the inconsistent output line impedance of each grid-connected inverter, each grid-connected inverter cannot accurately distribute the active power according to the capacity ratio. Therefore, the present invention proposes a microgrid inverter active power precise distribution control strategy based on high-order harmonic droop control. The main control principles and schemes are as follows: Figure 1 shown.

[0045] 1) Traditional active power distribution principle based on PV droop control

[0046] For low-voltage microgrids, the impedance of the grid-connected inverter connection line usually exhibits a resistive characteristic, that is, the line resistive component R l >>Sensitive componentωL l Therefore, the line voltage amplitude plays a major role in the inverter output active power, and the output reactive power is mainly related to the line frequency. In this case, the PV (active power-voltage) and Q-ω (reactive power-frequency) droop control strategies are used, which is conducive to the proportional distribution of the output power of multiple parallel inverters. The characteristic equation is shown in formula (1):

[0047]

[0048] In the formula, k u is the voltage droop coefficient, U, U * are the output voltage and command value of the microgrid grid-connected inverter, P, P * are the corresponding output active power and its command value respectively; k ω is the frequency droop coefficient, ω, ω * are the grid-connected inverter output frequency and its command value, Q, Q * are the corresponding output reactive power and its command value respectively.

[0049] Analyzing equation (1), since frequency ω is a global variable, the reactive power related to frequency can be distributed to each inverter according to the capacity ratio. However, for the inverter output active power related to the line voltage amplitude U, when the line impedance is unbalanced, the voltage amplitude U of each parallel inverter connection point is not equal, which will inevitably cause the output active power to be unbalanced, that is, it is impossible to achieve accurate distribution according to the capacity ratio. Figure 1 ③.

[0050] 2) Active power precise distribution control principle based on high-order harmonic droop control

[0051] For grid-connected inverters, high-frequency filters (L-type, LC-type, and LCL-type filters) are usually set at the output end to filter out high-frequency switching components to suppress high-frequency switching harmonic components from flowing into the grid. Related research shows that the internal impedance of the inverter output is usually dominated by reactance (some scholars also use virtual reactance to enhance the inductive characteristics of the inverter output impedance). Here, the inverter output equivalent internal inductance is set to L e Obviously, as the harmonic frequency n increases, the harmonic inductive reactance nωL e At this time, if the harmonic order n is appropriately selected, the high-order harmonic inductive reactance value will be much larger than the line impedance, that is, nωL e >>Z l , so the influence of line impedance can be ignored, and the impedance of each inverter connection line is approximately balanced.

[0052] Based on this concept, the present invention proposes an active power precise distribution control strategy based on high-order harmonic droop control. The high-order harmonic source is injected into the system by typical harmonic loads such as three-phase uncontrolled rectifiers, and the following variable values ​​of the high-order harmonic loads such as voltage, frequency, and power are defined as injected variable values.

[0053] (1) Calculation of injected harmonic power

[0054] Since the inverter output impedance is mainly inductive under high-order harmonic conditions, the P-ω (active power-frequency) and QV (reactive power-voltage) droop control strategies are more conducive to high-order harmonic power distribution. The droop control equation for n-th order high-order harmonics is listed as follows:

[0055]

[0056] In the formula, k nω is the nth harmonic frequency droop coefficient, ω n ,ω n * are the nth order harmonic frequency and its command value output by the grid-connected inverter, P n , P n * are the corresponding nth injected harmonic active power and its command value respectively.

[0057] Analyzing formula (2), since the nth harmonic frequency is a global variable, the injected active power output by multiple parallel inverters in the microgrid can be accurately distributed according to capacity. Similarly, since the impedance of the lines connecting each inverter is approximately balanced, the injected reactive power output by each inverter can also be accurately distributed.

[0058] For typical harmonic loads such as three-phase uncontrolled rectifiers, there is a certain proportional relationship between the harmonic currents. Taking the three-phase uncontrolled rectifier load as an example, the ratio of the effective value of each harmonic current to the fundamental current is the reciprocal of the harmonic order. Therefore, the standard value of the injected fundamental active power output by the microgrid grid-connected inverter is P 1c and the nth harmonic power P n The ratio is the square of the harmonic order, that is:

[0059] P 1c =P n ×n 2 (3)

[0060] It should be noted that, according to formula (3), for each inverter, the output injected fundamental wave active power standard value P 1c Accurate allocation in proportion to capacity is achieved.

[0061] Similarly, refer to Figure 1①, the injected fundamental wave active power actually output by the inverter can be obtained. Because the inverter output inductive reactance is small under fundamental wave conditions, the inverter connection line is still connected with resistance R l The main factor is the PV and Q-ω droop control strategy, which is the same as formula (1):

[0062]

[0063] Analyzing formula (4), when the line impedance R l When unbalanced, the voltage amplitude U1 at the connection points of each parallel inverter is not equal, resulting in an imbalance in the output fundamental wave active power P1, that is, it is impossible to achieve accurate distribution according to the capacity ratio.

[0064] (2) PV droop control correction

[0065] Calculate the standard value P of the actual output injected fundamental active power of the inverter 1c The difference between the actual value P1 is defined as ΔP 1c :

[0066] ΔP 1c =P 1c -P1 (5)

[0067] According to the above analysis, since the injected fundamental wave active power and the actual output active power of the inverter both adopt the PV droop control strategy, the actual active power deviation ΔP is proportional to the injected fundamental wave active power deviation, which is defined as k c =P / P1, then:

[0068]

[0069] According to formula (6), the power command value P of the inverter PV droop control shown in formula (1) is updated in real time * , the inverter output active power deviation can be corrected in real time, and the active power of multiple parallel inverters in the microgrid can be accurately divided. Figure 1 ②At this time, the droop control equation of formula (1) is corrected to:

[0070]

[0071] The microgrid inverter active power precise distribution control strategy based on high-order harmonic droop control described in the present invention can effectively solve the problem that the active power between multiple inverters (using PV droop control strategy) cannot be accurately distributed according to the capacity ratio due to the unbalanced impedance of the grid-connected inverter connection line in the low-voltage microgrid, and is easy to implement in engineering.

[0072] The present invention overcomes the shortcomings of complex control system, susceptibility to external interference, poor reliability, etc. caused by the use of communication connection between inverters in the microgrid and the use of virtual impedance in the inverter control system. It only needs to inject a certain amount of typical harmonic load into the microgrid to correct the uneven output active power distribution of multiple grid-connected inverters caused by unbalanced connection line impedance in real time. After the correction is completed, the injected harmonic load can be disconnected without affecting the normal operation of the microgrid.

[0073] The present invention is based on three microgrid characteristics: first, the high-order harmonic internal impedance of the microgrid grid-connected inverter presents an inductive characteristic, and the P-ω droop control strategy is adopted to realize the accurate equal distribution of the injected high-order harmonic active power according to the capacity ratio, and the larger high-order harmonic inductive reactance can weaken the influence of the connection line impedance; second, since the injected harmonic load is a typical load, there is a fixed proportional relationship between the injected high-order harmonic active power and the injected fundamental active power; third, since the injected fundamental active power and the actual output active power both adopt the PV droop control strategy, they have the same droop control characteristics.

[0074] The present invention is not only applicable to precise active power distribution based on PV (active power-voltage) droop control when the microgrid inverter connection line is mainly resistive, but also applicable to precise reactive power distribution based on QV (reactive power-voltage) droop control when the microgrid inverter connection line is mainly inductive.

[0075] The preferred embodiments of the present invention are described in detail above; however, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all belong to the protection scope of the present invention.

Claims

1. A microgrid inverter active power precise distribution control strategy, characterized by: The following steps are involved: Step 1: Inject harmonic load at the common connection point of the microgrid grid-connected inverter; Step 2: According to the impedance value of the inverter connection line, select the appropriate high-order harmonic order n; use P-ω and QV droop control strategy to achieve the inverter's n-th injection of high-order harmonic active power P n Output; Step 3: Calculate the nth injected higher harmonic active power P output by the inverter n , calculate the standard value of injected fundamental wave active power P 1c ; Step 4: Adopt PV and Q-ω droop control strategy to realize the output of injected fundamental active power P1 of the inverter; Step 5: Calculate the injected fundamental wave active power P1 of the inverter output; Calculate the standard value of the injected fundamental wave active power P of the inverter output 1c The difference ΔP between the actual value P1 and 1c ; Step 6: Calculate the actual active power deviation ΔP output by the inverter; Step 7: According to the actual active power deviation ΔP, the power command value P of each inverter droop control is corrected in real time * ; Adopt PV and Q-ω droop control strategies to achieve the active power P output of each parallel inverter; Step 8: According to the real-time corrected PV droop control strategy of step 7, the active power output of multiple parallel inverters in the microgrid is accurately distributed and controlled according to the capacity ratio; Step 9: After the correction control is completed, the harmonic load is disconnected from the grid.

2. According to claim 1, a microgrid inverter active power precise distribution control strategy is characterized by: In step 2, the P-ω and QV droop control strategies are adopted, and the droop control equation of the nth-order high-order harmonic is: In the formula, k nω is the nth harmonic frequency droop coefficient, ω n ,ω n * are the nth order harmonic frequency and its command value output by the grid-connected inverter, P n , P n * are the corresponding nth order injected harmonic active power and its command value, Q, Q * are the corresponding output reactive power and its command value respectively.

3. According to claim 2, a microgrid inverter active power precise distribution control strategy is characterized by: The injected fundamental wave active power standard value P output by the microgrid grid-connected inverter in step 3 1c and the nth harmonic power P n The ratio is the square of the harmonic order, that is: P 1c =P n ×n 2 。 4. According to claim 3, a microgrid inverter active power precise distribution control strategy is characterized by: The droop control equation in step 4 is:

5. A microgrid inverter active power precise distribution control strategy according to claim 4, characterized in that: The ΔP in step 5 1c =P 1c -P1.

6. A microgrid inverter active power precise distribution control strategy according to claim 5, characterized in that: In step six 7. A microgrid inverter active power precise distribution control strategy according to claim 6, characterized in that: The droop control equation in step seven is:

8. The microgrid inverter active power precise distribution control strategy according to claim 1, characterized in that: The harmonic load is a harmonic load such as a three-phase uncontrolled rectifier.

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