Inverter power distribution control method based on virtual impedance of resistive-inductive network

Abstract

The invention relates to an inverter power distribution control method based on virtual impedance of a resistance-inductive network, which comprises the following steps of: according to the characteristics of the resistance-inductive network, constructing a virtual resistor and a virtual inductor by using information in two aspects of active power and reactive power output by each inverter in a micro-grid: defining the virtual resistor Rvi as Rvi = eta1ikRpPi + eta1ikRqQi; the virtual inductance Lvi is as follows: Lvi = eta1ikLpPi + eta1ikLqQi; wherein # imgabs0 # is marked as a power coupling coefficient; m1 is a droop coefficient of reactive power in a voltage control equation of droop control of the minimum-capacity inverter in the system; alpha 1 is the power coupling coefficient of the inverter with the minimum capacity in the system. According to the method, a virtual resistor of a resistive-inductive network is introduced, so that the output active power of the parallel inverters in a power grid is reasonably distributed, and the problem of circulating current between the parallel inverters is inhibited. The method is not only suitable for a low-voltage power grid, but also has good applicability to a high-voltage power grid, and the application range of the method is widened.

Description

Technical Field

[0001] The invention belongs to the technical field of microgrid inverter power control, and in particular relates to an inverter power distribution control method based on virtual impedance of a resistive-inductive network. Background Art

[0002] Microgrids have attracted extensive attention from researchers as an effective means of connecting renewable energy to the grid. Microgrids contain a large number of power electronic devices. Although microgrids are flexible to control and have fast response speeds, the lack of inertia and damping elements reduces the stability of microgrids.

[0003] In microgrids, inverters are more often connected to the grid in parallel. When the inverters in the system work in parallel, the equivalent impedance between each inverter and the common bus is not equal due to the different distribution of the inverters and the different distances to the common bus. Therefore, the output power of each inverter is no longer proportional to its rated capacity, resulting in a circulating current between the inverters, which is later referred to as a circulating current. The circulating current caused by the uneven power output of the inverter will seriously affect the service life of the power electronic switching devices, the stable operation of the converter, and the power quality of the system. Therefore, it is of great significance to study how to reduce the circulating current generated between inverters.

[0004] Researchers from various countries have proposed different methods to suppress the circulating current between inverters, mainly including open-loop control and closed-loop control. For open-loop or closed-loop control methods, they usually rely on system parameter detection or communication to achieve the equal distribution of inverter output power. Since it involves inverter communication and line impedance measurement, this will increase the cost and complexity of the system and is not suitable for complex microgrid configurations. In addition, in most previous studies, compensation is often performed while ignoring the resistance of the system, which is more suitable for high-voltage networks and is not applicable when the resistance of the low-voltage network accounts for a large proportion. Summary of the invention

[0005] In view of the deficiencies of the prior art, the technical problem that the present invention intends to solve is to provide an inverter power distribution control method based on virtual impedance of a resistive-inductive network. The method introduces the virtual resistance of the resistive-inductive network to reasonably distribute the active power output by the parallel inverters in the power grid, suppress the circulation problem between the parallel inverters, and realize adaptive power distribution without the need for a communication module. Since the resistive-inductive network takes into account both virtual reactance and virtual resistance, the method of the present application is not only applicable to low-voltage power grids, but also has good applicability to high-voltage power grids, thus broadening the scope of application of the method.

[0006] The technical solution adopted by the present invention to solve the technical problem is:

[0007] An inverter power distribution control method based on virtual impedance of a resistive-inductive network, the method comprising the following contents:

[0008] According to the characteristics of the resistive-inductive network, the virtual resistance and virtual inductance are constructed by using the information of active power and reactive power output by each inverter in the microgrid:

[0009] Define the virtual resistance R vi For: R vi =η 1i k Rp P i +η 1i k Rq Q i ;

[0010] The virtual inductance L vi For: L vi =η 1i k Lp P i +η 1i k Lq Q i ;

[0011] Among them, k Rp is the virtual active resistance adjustment coefficient; k Rq is the virtual reactive resistance adjustment coefficient; k Lp is the virtual active inductance adjustment coefficient; k Lq is the virtual reactive inductance adjustment coefficient; P i is the active power output by the i-th inverter; Q i is the reactive power output by the i-th inverter; Denoted as power coupling coefficient; m i is the droop coefficient of reactive power in the voltage control equation of the droop control of the i-th inverter, m1 is the droop coefficient of reactive power in the voltage control equation of the droop control of the minimum capacity inverter in the system; α1 is the power coupling coefficient of the minimum capacity inverter in the system;

[0012] After adding virtual impedance, the active power output by the inverter is:

[0013]

[0014] Among them, R t =R i +R vi ,X t =X i +X vi ;X vi represents virtual reactance, X vi =2Πf0L vi , f0 is the rated frequency of the system; ni is the droop coefficient of active power in the voltage control equation of droop control, and U0 is the common bus voltage.

[0015] Furthermore, the resistive-inductive droop control equation is:

[0016] ω i =ω ref +aP i +bQ i

[0017] U i =U ref +n i P i +m i Q i

[0018] Where: ref and U ref is the rated frequency and voltage reference value of the inverter; m i and n i is the droop coefficient of reactive power and active power in the voltage control equation, a and b are the droop control coefficients of active power and reactive power in the frequency control equation, P i and Q i is the active power and reactive power actually output by the inverter; ω i is the output frequency of the i-th inverter.

[0019] Furthermore, the microgrid is a low voltage grid, a high voltage grid or a medium voltage grid.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention is aimed at the existing adaptive virtual impedance method, which mostly only considers high-voltage microgrids and only uses adaptive virtual inductance to suppress circulating current, ignoring the influence of line resistance. Considering the influence of line resistance and line inductance, the present method is not only applicable to high-voltage microgrids, but also to low-voltage microgrids and medium-voltage microgrids, and has a wider application range. Therefore, the present invention can adapt to power grids of various voltage levels, and it is no longer necessary to ignore resistance or reactance according to the voltage level.

[0022] Since the capacity and spatial distribution of each inverter are random, the output impedance of the inverter does not match the line impedance, which will cause circulating currents between the inverters. Considering the influence of line resistance, the traditional dynamic virtual impedance control strategy can be enhanced. According to the power output of the inverter, the line impedance is adaptively compensated, which reduces the active power sharing error of the parallel system and suppresses the circulating current between the inverters.

[0023] The present invention aims to propose an inverter power adaptive distribution control based on virtual complex impedance of a resistive-inductive network, which only uses local information of the inverter, has a relatively simple structure, and only requires the output of the feedback power of the inverter. It does not require line impedance measurement or communication modules, reduces the dependence on communication between inverters, and has the advantages of high reliability, low cost, and "plug and play". BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a schematic diagram of the compensation principle of an adaptive virtual impedance of an inverter power distribution control method of an adaptive virtual impedance of a resistive-inductive network according to an embodiment of the present invention.

[0025] Figure 2 The control principle diagram in the dq coordinate system of an embodiment of an inverter power distribution control method with adaptive virtual impedance of a resistive-inductive network of the present invention.

[0026] Figure 3 The present invention is a virtual impedance method control block diagram of an embodiment of an inverter power distribution control method with adaptive virtual impedance of a resistive-inductive network.

[0027] Figure 4 The present invention is a block diagram of an inverter based on traditional droop control according to an embodiment of an inverter power distribution control method with adaptive virtual impedance of a resistive-inductive network.

[0028] Figure 5 A block diagram of droop control introducing virtual impedance in an embodiment of an inverter power distribution control method with adaptive virtual impedance of a resistive-inductive network of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further explained below in conjunction with the embodiments and drawings, but this is not intended to limit the scope of protection of the present application.

[0030] The present invention provides an inverter power distribution control method based on virtual impedance of a resistive-inductive network, the method comprising the following steps:

[0031] Based on the premise that the actual microgrid composed of inverters is resistive and inductive, the relationship between the actual output power and branch impedance of each inverter without adding virtual impedance is derived:

[0032]

[0033] Among them, U i is the output voltage of the i-th inverter, U0 is the common bus voltage, δ i is the relative angle between the ith inverter and the common bus voltage; X i represents the reactance between the ith inverter and the common bus voltage, R irepresents the resistance between the ith inverter and the common bus voltage; P i and Q i It is the active power and reactive power actually output by the inverter;

[0034] According to the characteristics of the resistive-inductive network, the droop control equation of the resistive-inductive network is combined with the loop resistance and reactance:

[0035] ω i =ω ref +aP i +bQ i

[0036] U i =U ref +n i P i +m i Q i

[0037] Where: ref and U ref is the rated frequency and voltage reference value of the inverter; m i and n i is the droop coefficient of reactive power and active power in the voltage control equation, a and b are the droop control coefficients of active power and reactive power in the frequency control equation, P i and Q i It is the active power and reactive power actually output by the inverter;

[0038] The expression of active power output by each inverter in the resistive-inductive network is derived:

[0039]

[0040] in: Denoted as power coupling coefficient;

[0041] According to the characteristics of the resistive-inductive network, the virtual resistance and virtual inductance are constructed using the output active power and reactive power information:

[0042] Define the virtual resistance R vi For: R vi =η 1i k Rp P i +η 1i k Rq Q i ;

[0043] The virtual inductance L vi For: L vi =η 1i k Lp P i+η 1i k Lq Q i ;

[0044] Among them, k Rp is the virtual active resistance adjustment coefficient; k Rq is the virtual reactive resistance adjustment coefficient; k Lp is the virtual active inductance adjustment coefficient; k Lq is the virtual reactive inductance adjustment coefficient; P i is the active power output by the i-th inverter; Q i is the reactive power output by the i-th inverter; m i is the droop coefficient of reactive power in the voltage control equation of the droop control of the i-th inverter, m1 is the droop coefficient of reactive power in the voltage control equation of the droop control of the minimum capacity inverter in the system;

[0045] After adding virtual impedance, the active power output by the inverter is:

[0046]

[0047] Among them, R t =R i +R vi ,X t =X i +X vi .

[0048] At the same time, the active power distribution coefficient is introduced to verify the rationality of the applicability of the constructed virtual dynamic impedance. The virtual dynamic impedance includes virtual resistance and virtual reactance.

[0049] The detailed derivation and verification process is given below:

[0050] 1. Generation of circulation

[0051] Under ideal power output, there is no circulating current between inverters, and the output current is proportional to the rated capacity of the inverter.

[0052] Right now:

[0053]

[0054] in: is the ideal output current of the i-th inverter; S i Define the circulating current I between inverters for the power of the i-th inverter Hi is the ideal output current of the inverter and the actual current I i difference,

[0055] Right now:

[0056] The output current of the i-th inverter is I i for:

[0057]

[0058] in: is the complex power, ^ is the conjugate symbol;

[0059] The existing circulation expression is:

[0060]

[0061] in:

[0062]

[0063] Substituting the current expression into the circulating current expression, we can get the circulating current expression related to active power:

[0064]

[0065] To eliminate the circulation, I H =0, then the denominator in the above formula is equal to 0, and we get:

[0066] S1:S2=P1:P2;

[0067] Therefore, when the output active power is proportional to the rated power, the influence of the circulating current can be eliminated. The circulating current between the inverters can be suppressed by adjusting the power relationship of the parallel inverter outputs.

[0068] 2. Relationship between output power and impedance

[0069] The change of loop impedance will cause the inverter output power and output voltage to change. Here, the active output power P of the i-th inverter is i for:

[0070]

[0071] Among them, U i is the output voltage of the i-th inverter, U0 is the common bus voltage, δ i is the relative angle between the ith inverter and the common bus voltage; X i represents the reactance between the ith inverter and the common bus voltage, R i represents the resistance between the i-th inverter and the common bus voltage;

[0072] Similarly, the reactive power output by the inverter can be obtained as:

[0073]

[0074] In practice, since the phase difference between the inverter power supply voltage and the common bus voltage is not large, it can be considered that sinδ≈δ; cosδ≈1. Substituting into equations (1) and (2), we can obtain:

[0075] The active power output by the inverter is:

[0076]

[0077] The reactive power output by the inverter is:

[0078]

[0079] The traditional droop control equation is:

[0080] ω i =ω ref +aP i +bQ i (5)

[0081] U i =U ref +n i P i +m i Q i (6)

[0082] Where: ref and U ref is the rated frequency and voltage reference value of the inverter; m i and n i is the droop coefficient of reactive power and active power, P i and Q i is the active power and reactive power actually output by the inverter; U i is the output voltage of the i-th inverter; ω i is the output frequency of the i-th inverter.

[0083] Substituting formula (6) into formula (3) and (4), we can obtain:

[0084]

[0085] in: Denoted as the power coupling coefficient, Z i is the impedance of the i-th inverter circuit.

[0086] In order to ensure that the active power output by each inverter can be distributed according to the rated active power ratio when inverters with different capacities share the load, the droop coefficient m i and n iIt should be inversely proportional to its rated power, so the active power of each inverter reference output must meet the following conditions:

[0087]

[0088] Where: P i * and is the active power expected to be output by the i-th inverter and the j-th inverter.

[0089] Taking two inverters as an example, when the active power output by the two inverters is output in proportion to the rated capacity, the conditions that should be met are:

[0090]

[0091] In fact:

[0092]

[0093] in,

[0094] Here, the active power distribution coefficient is recorded as:

[0095]

[0096] From (11), we can see that the closer β is to 1, the more reasonable the active power distribution coefficient is.

[0097] 3. Application of adaptive impedance

[0098] Without considering the addition of additional impedance, the active power distribution coefficient is:

[0099]

[0100] Define virtual resistance R vi for:

[0101] R vi =η 1i k Rp P i +η 1i k Rq Q i (13)

[0102] in:

[0103] k Rp is the virtual active resistance adjustment coefficient;

[0104] k Rq is the virtual reactive resistance adjustment coefficient;

[0105] P i is the active power output by the inverter;

[0106] Q i is the reactive power output by the inverter;

[0107]

[0108] Wherein, m1 is the droop coefficient of reactive power in the voltage control equation of the droop control of the minimum capacity inverter in the system; α1 is the power coupling coefficient of the minimum capacity inverter in the system;

[0109] Define the virtual inductance as:

[0110] L vi =η 1i k Lp P i +η 1i k Lq Q i (14)

[0111] in:

[0112] k Lp is the virtual active inductance adjustment coefficient;

[0113] k Lq is the virtual reactive inductance adjustment coefficient;

[0114] Then the dynamic virtual reactance X vi for:

[0115] X vi =2Πf0L vi (15)

[0116] Where: f0 is the rated frequency of the system, which is 50 Hz in this implementation.

[0117] Consider adding dynamic virtual reactance and dynamic virtual resistance to perform droop control:

[0118] The active power distribution coefficient with dynamic virtual impedance is:

[0119]

[0120] The virtual impedance method is a control algorithm that simulates resistors and inductors. It is equivalent to increasing the output filter impedance to change the impedance between the inverter and the common bus. Figure 1 As shown, two parallel inverter branches D G1 , D G2 , respectively introduce dynamic virtual reactance X vi and virtual resistor R vi , the two add up to the virtual impedance Z vIn these two branches, there are actual resistances R1 and R2 and actual inductances X1 and X2.

[0121] according to Figure 3 It can be seen that the virtual impedance has an impact on the reference voltage, which is:

[0122]

[0123] in:

[0124] E ref The voltage reference obtained for the inverter control algorithm

[0125] Z v =R v +jX v is the virtual impedance

[0126] I0 is the current output by the inverter

[0127] The reference voltage after adding virtual impedance

[0128] The expression converted into dq axis is:

[0129]

[0130]

[0131] in:

[0132] R v and X v are virtual resistance and virtual reactance;

[0133] E dref and E qref is the reference voltage of the outer voltage loop without virtual impedance;

[0134] and is the reference voltage of the outer voltage loop in the virtual impedance dq axis coordinate system;

[0135] The logic block diagram of dq control is as follows Figure 2 Shown

[0136] When the virtual impedance is not considered, the voltage at the common bus is:

[0137] U0=E refi -I0×Z i

[0138] When the virtual impedance is added, the common bus voltage is:

[0139] U0=E refi -I0×(Zi +Z v )

[0140] From the above, it can be seen that a virtual impedance is simulated to compensate the voltage reference value, which can achieve the same effect as connecting a real impedance in series in the line without generating additional losses.

[0141] Take two inverters as an example, assuming: P1<γ 12 P2; Q1<Q2

[0142] Then: β0<1

[0143] According to the set virtual impedance, we can get:

[0144] R v2 >R v1 ;X v2 >X v1

[0145] At this time: β0<β1<1

[0146] From the above, we can see that after adding dynamic virtual impedance, the active power distribution coefficient is closer to 1, which can play a role in suppressing circulating current. The principle diagram is as follows Figure 5 shown.

[0147] Any matters not described in the present invention are applicable to the prior art.

Claims

1. A method for controlling inverter power distribution based on virtual impedance of a resistive-inductive network, characterized in that: The method includes the following: According to the characteristics of the resistive-inductive network, the virtual resistance and virtual inductance are constructed by using the information of active power and reactive power output by each inverter in the microgrid: Define the virtual resistance R vi For: R vi =η 1i k Rp P i +η 1i k Rq Q i ; The virtual inductance L vi For: L vi =η 1i k Lp P i +η 1i k Lq Q i ; Among them, k Rp is the virtual active resistance adjustment coefficient; k Rq is the virtual reactive resistance adjustment coefficient; k Lp is the virtual active inductance adjustment coefficient; k Lq is the virtual reactive inductance adjustment coefficient; P i is the active power output by the i-th inverter; Q i is the reactive power output by the i-th inverter; Denoted as power coupling coefficient; m i is the droop coefficient of reactive power in the voltage control equation of the droop control of the i-th inverter, m1 is the droop coefficient of reactive power in the voltage control equation of the droop control of the minimum capacity inverter in the system; α1 is the power coupling coefficient of the minimum capacity inverter in the system; After adding virtual impedance, the active power output by the inverter is: Among them, R t =R i +R vi ,X t =X i +X vi ;X vi represents virtual reactance, X vi =2Πf0L vi , f0 is the rated frequency of the system; n i is the droop coefficient of active power in the voltage control equation of droop control, U0 is the common bus voltage, U ref is the voltage reference value of the inverter.

2. The method according to claim 1, characterized in that The resistive-inductive droop control equation is: oh i =ω ref +aP i +bQ i U i =U ref +n i P i +m i Q i Where: ref and U ref is the rated frequency and voltage reference value of the inverter; m i and n i is the droop coefficient of reactive power and active power in the voltage control equation, a and b are the droop control coefficients of active power and reactive power in the frequency control equation, P i and Q i is the active power and reactive power actually output by the inverter; ω i is the output frequency of the i-th inverter.

3. The method according to claim 1, characterized in that The microgrid is a low voltage grid, a high voltage grid or a medium voltage grid.

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