CN112003291A - Method for improving sag reactive compensation performance of micro-grid - Google Patents

Abstract

The present invention is a method for improving the sag reactive power compensation performance of a microgrid, comprising: 1) establishing a mathematical model of the microgrid inverter outputting active and reactive power; 2) outputting the active and reactive power to the inverter in step 1). Simplify the mathematical model; 3) Establish the output reactive power-voltage droop control equation; 4) According to the voltage drop equation caused by the impedance of the transmission line in the microgrid, propose an improved voltage compensation phase; 5) Step 3) Output reactive power - The droop coefficient in the voltage droop control equation is modified to a new adaptive droop coefficient; 6) The improved voltage compensation phase in step 4) and the new adaptive droop coefficient in step 5) are introduced into step 2) Inverter output reactive power mathematical model In this paper, a new control equation is obtained to replace the original microgrid droop reactive power compensation control equation to improve the microgrid droop reactive power compensation performance. The present invention increases the voltage distribution accuracy and reduces the reactive power compensation range by introducing a voltage compensation phase and an adaptive droop coefficient.

Description

A method for improving the sag reactive power compensation performance of microgrid

technical field

The invention relates to a method for improving the droop reactive power compensation performance of a microgrid, in particular to a method of introducing an improved voltage compensation phase and an adaptive reactive power droop coefficient into a droop reactive power compensation control system to eliminate the voltage steady state deviation of the microgrid and reduce the reactive power compensation range.

Background technique

With the continuous increase of power grid capacity and the complexity of regional power grid structure, in order to improve the support for renewable energy power generation and realize the large-scale application of renewable energy, it is necessary to vigorously develop microgrids. The microgrid operates in parallel with the public grid through the Public Connection Point (PCC), and the voltage is restrained by the public grid. In the microgrid inverter control system, PQ droop control and constant voltage and constant frequency (V-f) control are mostly used. The traditional droop control strategy simulates the droop characteristics of the traditional synchronous generator, and independently decouples control of the output active power-frequency and reactive power-voltage of the inverter device. However, in the actual operation of the microgrid, there are problems such as uneven distribution of line impedance and nonlinear output voltage drop, which will lead to problems such as power distribution errors and voltage stability performance degradation.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for improving the droop reactive power compensation performance of a microgrid. The method uses a dual power point equivalent model to establish a mathematical expression of the power of the microgrid, and obtains the linear droop control equation of the grid-connected inverter. In the process of voltage regulation, there is a distribution error and the reactive power compensation range is too large. This method increases the voltage distribution accuracy and reduces the reactive power compensation range by introducing a voltage compensation phase and an adaptive droop coefficient.

The present invention adopts following technical scheme to realize:

A method for improving the sag reactive power compensation performance of a microgrid, comprising the following steps:

1) Establish a mathematical model of the microgrid inverter output active and reactive power;

2) Simplify the mathematical model of the inverter output active and reactive power in step 1);

3) According to step 2) mathematical model of inverter output reactive power, establish output reactive power-voltage droop control equation;

4) According to the voltage drop equation caused by the impedance of the transmission line in the microgrid, an improved voltage compensation phase is proposed;

5) Modify the droop coefficient in the output reactive power-voltage droop control equation in step 3) to a new adaptive droop coefficient;

6) Introduce the improved voltage compensation phase in step 4) and the new adaptive droop coefficient in step 5) into the mathematical model of inverter output reactive power in step 2) to obtain a new control equation to replace the original microgrid droop reactive power compensation Control equations to improve microgrid droop reactive power compensation performance.

A further improvement of the present invention is that step 1) establishes a mathematical model of the output active and reactive power of the microgrid inverter:

Among them: _{Pi is the active power output by the inverter; Q i is the reactive power output by the inverter; U i is the} output voltage of the inverter; U ₀ is the voltage across the load impedance; δ _i is the power angle; Z _i = R _i +JX _i is the line equivalent impedance; R _i is the line resistance; X _i is the line inductive reactance.

A further improvement of the present invention is that, in step 2), according to the microgrid, R _i >>X _i , Ri _{≈Z i ,} Xi ≈0 _, the impedance of the transmission line is resistive, and the inverter outputs active power in step 1). , Simplify the mathematical model of reactive power:

A further improvement of the present invention is that in step 3): according to the mathematical model of the reactive power output of the inverter in step 2), the output active power of a single DG inverter in the microgrid is related to the power angle, and the output reactive power is related to the voltage; The reactive power-voltage droop control equation is: U _i =U ₀ -kQ _i ;

Among them: U _i is the output voltage amplitude of the controlled inverter; U ₀ is the no-load output voltage amplitude reference value; _k is the reactive power droop coefficient; Qi is the reactive power distributed by the load.

A further improvement of the present invention is, step 4): according to the voltage drop equation caused by the impedance of the transmission line in the microgrid:

Among them: ΔU is the voltage drop caused by the line impedance; the scheme of compensating the voltage drop caused by the impedance difference between the DG transmission lines, so that the impedance voltage drop of each DG transmission line is consistent; the improved voltage compensation phase is:

Among them: ΔU _i is the voltage drop that needs to be compensated for DG _i ; ΔR _i is the resistance difference between the i-th line and the reference line; ΔX _i is the reactance difference; Negative, otherwise positive; m _i , _ni are the first-order function phase of DG _i active and reactive power regulation, expressed as: m _i =-a _i1 ΔP _i , _ni =-a _i2 ΔQ _i , a _i1 , a _i2 are the correlation coefficients of active and reactive power of DG _i respectively; ΔP _i and ΔQ _i are the changes of active and reactive power, respectively.

A further improvement of the present invention is that step 5): modify the droop coefficient in the output reactive power-voltage droop control equation in step 3) to a new adaptive droop coefficient:

Among them: _ki is the new adaptive droop reactive power compensation coefficient; U _max and U _min are the upper and lower thresholds of the voltage amplitude; when UU ₀ > 0, that is, when the adjustment voltage is positive, the numerator coefficient selects U _max -U ₀ ; When UU ₀ ≤ 0, that is, when the regulation voltage is negative, select U _min -U ₀ .

A further improvement of the present invention is that step 6): introducing the new adaptive droop coefficient _ki obtained in step 4) with the improved voltage compensation phase ΔU _i and step 5) into the mathematical model of inverter output reactive power in step 2), A new control equation is obtained: U _i =U ₀ -ki Q _i +ΔU _{i ,} which replaces the original microgrid droop reactive power compensation control equations and improves the microgrid droop reactive power compensation performance.

Compared with the prior art, the present invention at least has the following beneficial technical effects:

1. The present invention analyzes the voltage drop caused by the impedance of the transmission line at the distributed power point in the microgrid, and proposes a method for compensating for the voltage drop caused by the impedance difference existing between the transmission lines. Make the voltage drop caused by the impedance of each transmission line consistent, introduce the improved voltage compensation phase into the droop reactive power compensation control, improve the droop reactive power compensation control effect, and realize the rational distribution of reactive power between power points. The simulation and test results show that compared with the traditional droop reactive power compensation control, the reactive power regulation deviation accuracy of the droop reactive power compensation control system with improved voltage compensation phase is improved by as much as 10 times, which ensures the power supply reliability of the microgrid and has a higher performance. To a certain extent, using its own capacity, there will be more reactive power in a short period of time to coordinate the power imbalance of the adjustment system. The test results show that the improved droop reactive power compensation control proposed by the present invention has obvious effect of suppressing the output circulating current, and can achieve the goal of accurate reactive power distribution.

2. The present invention proposes an adaptive droop reactive power compensation control scheme in view of the fact that some electrical equipment is more sensitive to voltage fluctuations in the actual system of the microgrid. The fixed droop coefficient is improved to a new type of adaptive droop reactive power compensation coefficient, which changes in real time with the difference between the current voltage and the target voltage. Faced with the same voltage regulation target, reduce the reactive power compensation range and reduce the impact on the system. The simulation results show that: using adaptive droop reactive power compensation control, the steady-state voltage deviation is small and the voltage fluctuation is small. The adaptive algorithm reduces the voltage regulation amount, and the voltage regulation can be maintained within the set small offset range, eliminating the The steady-state voltage deviation is improved, and the utilization rate of distributed power points in the microgrid is improved.

Description of drawings

Figure 1 is a Thevenin equivalent circuit diagram of a microgrid containing two groups of DGs;

Figure 2 is a schematic diagram of droop reactive power compensation;

Figure 3 is a comparison diagram of the droop control curve;

Figure 4 is a microgrid simulation model containing two distributed power points;

Fig. 5 is the simulation waveform of reactive power distribution of system reactive power increase and traditional droop reactive power compensation control;

Fig. 6 is the simulation waveform of reactive power distribution of system reactive power increase and adaptive droop reactive power compensation control;

Figure 7 is the simulation waveform of the voltage change of the traditional drooping bus with the increase of system reactive power;

Figure 8 is the simulation waveform of the system reactive power increase and the adaptive drooping bus voltage change;

Figure 9 is the output circulating current test waveform between two grid-connected inverters obtained by using traditional droop reactive power compensation control;

Figure 10 shows the output circulating current test waveform between two grid-connected inverters obtained by adaptive droop reactive power compensation control.

Detailed ways

The technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings.

As shown in Figure 1, the Thevenin equivalent circuit diagram of the microgrid including two groups of DGs: U ₁ ∠δ ₁ and U ₂ ∠δ ₂ are the output voltages of the first and second grid-connected inverters; U ₀ ∠0 is the load The voltage across the impedance; Z ₁ =R ₁ +JX ₁ , Z ₂ =R ₂ +JX ₂ , are the equivalent impedances of the first and second transmission lines. Z _load is the equivalent impedance of the load.

The output active and reactive power of a single DG _i can be expressed as:

In the microgrid, the transmission line impedance is resistive (R _i ＞＞X _i , Ri ≈Z _i , Xi ≈0, power angle δ _i →0), that is, _{sinδ i ≈δ i , cosδ i} ≈1 _, The above formula can be simplified to:

In the actual microgrid system, the inverter parameters of each DG are not the same, the impedance of the transmission line has parameter drift, and there is an error in the acquisition, which will lead to the fact that the power transmitted to the load cannot be accurately matched according to the actual capacity.

As shown in Figure 2, the droop reactive power compensation control of the microgrid is realized by simulating the external characteristics of the droop of the synchronous generator. The traditional droop control is a differential regulation. It can be seen from equation (2) that a single DG inverter in the microgrid outputs active power. The power is related to the power angle, and the output reactive power is related to the voltage. In order to achieve grid-connected inverter voltage regulation, the control equation is:

U _i =U ₀ -kQ _i (3)

In formula (3): U _i is the output voltage amplitude of the controlled inverter; U ₀ is the no-load output voltage amplitude reference value; k is the reactive power droop coefficient; Q _i is the reactive power distributed by the load.

As shown in Figure 3, in order to improve the droop reactive power compensation control effect and realize the rational distribution of reactive power between DGs, the present invention introduces the voltage compensation phase in the droop control, and the voltage drop generated by the DG transmission line in the microgrid can be expressed as:

In formula (4): ΔU is the voltage drop caused by the line impedance. The invention proposes a solution for compensating the voltage drop caused by the impedance difference existing between the DG transmission lines, so that the voltage drop caused by the impedance of each DG transmission line is consistent. The improved voltage compensation can be expressed as:

In formula (5): ΔU _i is the voltage drop that needs to be compensated for DG _i ; ΔR _i is the resistance difference between the ith line and the reference line; ΔX _i is the reactance difference; the line impedance modulo value is compared with the reference line impedance modulo value Hour, the compensation pressure drop is negative, otherwise it is positive. m _i and _ni are the first-order function phases of the active and reactive power adjustment quantities of DG _i , which can be expressed as: m _i =-a _i1 ΔP _i , _ni =-a _i2 ΔQ _i . a _i1 and a _i2 are the correlation coefficients of active and reactive power of DG _i respectively; ΔP _i and ΔQ _i are the changes of active and reactive power, respectively.

In the traditional droop reactive power compensation control, the droop coefficient k is a fixed value, and the reactive power compensation amount has a linear relationship with the voltage regulation amount. However, in the actual microgrid system, some electrical equipment is more sensitive to voltage fluctuations. When the voltage is adjusted in a large range , it is very easy to cause the equipment to be disconnected from the network. The present invention proposes an adaptive droop control scheme, which automatically adjusts the droop coefficient to reduce the reactive power compensation range. The new adaptive droop coefficient can be expressed as:

In formula (6): k _i is the new adaptive droop reactive power compensation coefficient; U _max and U _min are the upper and lower thresholds of the voltage amplitude; when UU ₀ > 0, that is, when the adjustment voltage is positive, the numerator coefficient selects U _max -U ₀ ; when UU ₀ ≤ 0, that is, when the regulation voltage is negative, select U _min -U ₀ .

When facing the voltage regulation target of U ₁ →U ₂ , the traditional droop reactive power compensation control, because the droop coefficient is fixed, the reactive power regulation amount is ΔQ ₁ . With the novel adaptive droop reactive power control proposed in the present invention, the adaptive droop reactive power compensation coefficient _ki changes in real time with the difference between the current voltage and the target voltage. Facing the same voltage regulation target, the reactive power regulation amount is ΔQ ₂ (ΔQ ₂ <ΔQ ₁ ), the range of reactive power compensation is reduced, and the impact on the system is less.

The improved voltage compensation phase ΔU _i and the adaptive droop coefficient k _i are brought into the voltage droop reactive power compensation control, and the new control equation is obtained as:

U _i =U ₀ -ki Q _i +ΔU _{i (} 7)

As shown in Figure 4, a microgrid simulation model with two DGs is built under Mantlab/Simulink. The AC side voltage of DG ₁ and DG ₂ is U _ac = 0.4kV, which is connected to a double-winding split transformer with a capacity of 1000kVA, and the voltage is boosted. After reaching 10kV, it will be connected to the power grid. The RLC filtering parameters of the two transmission lines are the same, namely: L _f1 =L _f2 =4.7mH, R _f1 =R _f2 =5Ω, C _f1 =C _f2 =490μF; line impedance R ₀₁ =0.6+j0.15Ω, R ₀₂ = 0.3+j0.15Ω. The system control parameters are: active droop correlation coefficient a ₁ =1.5×10 ^-5 , reactive power droop correlation coefficient a ₂ =1.2×10 ^-5 ; adaptive reactive power droop coefficient k=3×10 ^-5 .

In order to verify the proposed improved droop reactive power compensation control performance, the capacity ratio of DG ₁ and DG ₂ inverters is set as 2:1, the simulation time is 1.2s, and the PCC increases the load condition at 0.6s. The reactive power change of the increased load is 10.5kVar.

As shown in Figure 5, within 0-0.6s, the output reactive power of DG ₁ is 9.85kVar, with a relative deviation of 1.5%; the output reactive power of DG ₂ is 7.52kVar, with a relative deviation of 50.4%. When the load condition of reactive power increment of 10.5kVar occurs at 0.6s, the output reactive power of DG ₁ increases to 14.21kVar with an increment of 4.36kVar; the output reactive power of DG ₂ increases to 10.42kVar with an increment of 2.90kVar. The total added load reactive power is 7.26kVar, and the deviation reaches 30.8%.

As shown in Figure 6, within 0-0.6s, the output reactive power of DG ₁ is 10.05kVar, with a relative deviation of 0.5%; the output reactive power of DG ₂ is 5.03kVar, with a relative deviation of 0.3%. When the load condition of reactive power increment of 10.5kVar occurs at 0.6s, the output reactive power of DG ₁ increases to 17.26kVar with an increment of 7.26kVar; the output reactive power of DG ₂ increases to 8.61kVar with an increment of 3.58kVar. The total added load reactive power is 10.84kVar, and the deviation is 3.2%. Compared with the traditional droop reactive power compensation control, the distribution deviation accuracy of the reactive power is improved by as much as 10 times, which ensures the reliability of the power supply of the microgrid and can utilize its own power to a greater extent. capacity, there is more reactive power in a short period of time to coordinate the power imbalance of the adjustment system.

As shown in Figure 7, at 0.6s, the PCC has an increased reactive power condition. After 0.07s, the bus voltage drops from 10kV to 8.3kV. The steady-state voltage deviation is large and the voltage fluctuation is obvious. The minimum drop is 6.7kV, which is very easy to cause Some electrical equipment stops running due to under-voltage protection action.

As shown in Figure 8, in the same reactive power increase condition on the 0.6s plane, the bus voltage recovers from 10kV to 9.9kV after 0.04s, the steady-state voltage deviation is small and the voltage fluctuation is small, and the minimum fluctuation drops to 8.8kV, which is Since the adaptive algorithm reduces the amount of voltage regulation, the voltage regulation can be maintained within the set small offset range, eliminating the steady-state voltage deviation and improving the utilization of distributed power points in the microgrid.

As shown in Figure 9, in order to verify the dynamic control performance of the control scheme proposed in the present invention, an experimental platform containing two grid-connected inverters was built. The hardware of the experimental platform is as follows: DSP selects TMS320F28335 of TI Company, and IGBT selects Infineon Company's TMS320F28335. K40T120, oscilloscope choose MDO4104B-3 oscilloscope of Tektronix Company. The experimental parameters are similar to the simulation parameters. The output circulating current can characterize whether the reactive power of the grid-connected inverter is accurately distributed. Using traditional droop reactive power compensation to control the output circulating current is relatively large, and the peak value reaches 1.8A.

As shown in FIG. 10 , the improved droop reactive power compensation proposed in the present invention controls the output circulating current, the peak value is 0.19A, the circulating current suppression effect is obvious, and the goal of accurate reactive power distribution can be achieved.

The above are only preferred embodiments of the present invention and do not limit the present invention. Any simple modifications, changes and equivalent structural changes made to the above embodiments according to the technical essence of the present invention still belong to the technology of the present invention. within the scope of the program.

Claims (7)

1. a method for improving microgrid sag reactive power compensation performance, is characterized in that, comprises the following steps: 1) Establish a mathematical model of the microgrid inverter output active and reactive power; 2) Simplify the mathematical model of the inverter output active and reactive power in step 1); 3) According to step 2) mathematical model of inverter output reactive power, establish output reactive power-voltage droop control equation; 4) According to the voltage drop equation caused by the impedance of the transmission line in the microgrid, an improved voltage compensation phase is proposed; 5) Modify the droop coefficient in the output reactive power-voltage droop control equation in step 3) to a new adaptive droop coefficient; 6) Introduce the improved voltage compensation phase in step 4) and the new adaptive droop coefficient in step 5) into the mathematical model of inverter output reactive power in step 2) to obtain a new control equation to replace the original microgrid droop reactive power compensation Control equations to improve microgrid droop reactive power compensation performance. 2. a kind of method that improves microgrid droop reactive power compensation performance according to claim 1, is characterized in that, step 1) establishes microgrid inverter output active, reactive power mathematical model:

Among them: _{Pi is the active power output by the inverter; Q i is the reactive power output by the inverter; U i is the} output voltage of the inverter; U ₀ is the voltage across the load impedance; δ _i is the power angle; Z _i = R _i +JX _i is the line equivalent impedance; R _i is the line resistance; X _i is the line inductive reactance. 3 . The method for improving the sag reactive power compensation performance of a microgrid according to claim 2 , wherein in step 2) according to the microgrid, Ri ＞X _{i ,} Ri _{≈Z i ,} and Xi ≈ 0, the impedance of the transmission line is resistive, and the mathematical model of the active and reactive power output of the inverter in step 1) is simplified:

4. a kind of method that improves microgrid droop reactive power compensation performance according to claim 3, is characterized in that, step 3): according to step 2) inverter output reactive power mathematical model, microgrid single DG inverter The output active power of the device is related to the power angle, and the output reactive power is related to the voltage; the establishment of the output reactive power-voltage droop control equation is: U _i =U ₀ -kQ _i ; Among them: U _i is the output voltage amplitude of the controlled inverter; U ₀ is the no-load output voltage amplitude reference value; _k is the reactive power droop coefficient; Qi is the reactive power distributed by the load. 5. a kind of method that improves microgrid droop reactive power compensation performance according to claim 4, is characterized in that, step 4): according to the voltage drop equation caused by transmission line impedance in microgrid:

Among them: ΔU is the voltage drop caused by the line impedance; the scheme of compensating the voltage drop caused by the impedance difference between the DG transmission lines, so that the impedance voltage drop of each DG transmission line is consistent; the improved voltage compensation phase is:

Among them: ΔU _i is the voltage drop that needs to be compensated for DG _i ; ΔR _i is the resistance difference between the i-th line and the reference line; ΔX _i is the reactance difference; Negative, otherwise positive; m _i , _ni are the first-order function phase of DG _i active and reactive power regulation, expressed as: m _i =-a _i1 ΔP _i , _ni =-a _i2 ΔQ _i , a _i1 , a _i2 are the correlation coefficients of active and reactive power of DG _i respectively; ΔP _i and ΔQ _i are the changes of active and reactive power, respectively. 6. a kind of method that improves microgrid droop reactive power compensation performance according to claim 5, is characterized in that, step 5): droop coefficient in step 3) output reactive power-voltage droop control equation is modified to novel adaptive Sag factor:

Among them: _ki is the new adaptive droop reactive power compensation coefficient; U _max and U _min are the upper and lower thresholds of the voltage amplitude; when UU ₀ > 0, that is, when the adjustment voltage is positive, the numerator coefficient selects U _max -U ₀ ; When UU ₀ ≤ 0, that is, when the regulation voltage is negative, select U _min -U ₀ . 7. a kind of method that improves microgrid droop reactive power compensation performance according to claim 6, it is characterized in that, step 6): obtain the novel self-adaptation that the improved voltage compensation phase ΔU _i and step 5) obtain in step 4) The droop coefficient k _i is introduced into the mathematical model of the inverter output reactive power in step 2, and the new control equation is obtained as: U _i =U ₀ -ki Q _i +ΔU _{i ,} which replaces the original microgrid droop reactive power compensation control equation , to improve the sag reactive power compensation performance of the microgrid.

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