Voltage support method for three-phase four-wire cascaded multilevel system considering power balance

Abstract

The present invention discloses a voltage support method for a three-phase four-wire cascade multi-level system considering power balance, step one, calculating the following parameters in the three-phase four-wire cascade multi-level energy storage system: the positive-sequence, negative-sequence, and zero-sequence voltage amplitude, initial phase angle, and voltage imbalance of the PCC point fault, separating the three-phase current output by the system into positive, negative, and zero sequences, and obtaining the voltage amplitude expression of the positive, negative, and zero sequences; step two, achieving power balance in the three-phase four-wire cascade multi-level energy storage system by adjusting the ratio of the injected negative-sequence current and zero-sequence current to the positive-sequence current, and obtaining the conditions for the system to meet power balance; step three, calculating the optimal voltage support point under the condition that the grid imbalance is minimal and the grid currents of each phase and the neutral current meet the current limit. After applying the voltage support method proposed by the present invention, the PCC point imbalance is significantly reduced, and the power balance of the cascade multi-level energy storage system is achieved at the same time.

Description

Technical Field

[0001] The present invention relates to a multi-level system voltage support method, in particular to a method for calculating an optimal voltage support point of a three-phase four-wire cascade multi-level energy storage system considering power balance, and belongs to the technical field of energy storage inverter control. Background Art

[0002] Grid voltage drop is one of the most common faults in power systems, usually caused by accidental contact at the far end, lightning strikes, and short circuits caused by equipment failures. In order to ensure that independent microgrids can maintain reliable power supply and operation capabilities during short-term recoverable faults, it is of great significance to study the fault ride-through control of independent microgrids. Low voltage ride-through capability is an important indicator for measuring the stability of modern power systems. In practical applications, modern power systems require equipment to have a certain ability to cope with grid voltage drops, and the next generation of grid-connected standards will put forward higher requirements for voltage support in low voltage ride-through. In addition, in the three-phase four-wire system, the existence of the zero-sequence current channel makes the zero-sequence current at the PCC point controllable, so that the zero-sequence component can be used in the voltage support control strategy based on the three-phase four-wire system to optimize the voltage support effect.

[0003] The interphase balancing control of cascaded multilevel systems is mainly divided into zero-sequence voltage method and negative-sequence current method. The document "Investigation of Negative-Sequence Injection Capability of Cascaded H-Bridge Converters in Star and Delta Configuration" (E. Behrouzian, M. Bongiorno, IEEE Transactions on Power Electronics, 2016, 32 (2): 1675-1683) published on April 14, 2016, analyzes the relationship between zero-sequence voltage and active power in the abc coordinate system, and realizes interphase average active power control, DC voltage control and reference current tracking control in an unbalanced power grid by injecting appropriate phase voltage. The document "Clustered Voltage Balancing Mechanism and Its Control Strategy for Star-Connected Cascaded H-Bridge STATCOM" (D. Lu, J. Wang, J. Yao, et al, IEEE Transactions on Industrial Electronics, 2017, 64 (10): 7623-7633) published on April 14, 2017 controls the negative-sequence modulation index based on unbalanced power information without using any controller, which results in a steady-state error in the DC voltage within the phase. In addition, estimating the negative-sequence current component based on the positive-sequence transformation and directly adding the feedforward voltage of each phase to the resulting modulation index will result in poor dynamic response. When the voltage imbalance is small, the zero-sequence voltage injection method can effectively achieve phase-to-phase power balance and ensure safe operation of the equipment. However, when the voltage imbalance is large, the zero-sequence voltage injection method fails. At this time, negative-sequence current injection is required as a voltage crossing strategy. However, both negative sequence current and zero sequence voltage will have an adverse effect on the power imbalance of the system when dealing with the power balance of the three-phase four-wire cascaded multi-level system under unbalanced power grid. Therefore, the power balance problem of the three-phase four-wire cascaded multi-level system needs further research.

[0004] As for the research on low voltage ride-through, the document "Flexible positive and negative sequence current control method and equivalent sequence network model for inverter-type distributed power sources" (Chen Qilei, Fan Xinrong, Zhang Peichao, et al., Power System Protection and Control, 2018, 46(14): 57-62) published on July 16, 2018 proposed a positive and negative sequence independent control strategy, which injects positive and negative sequence reactive current into the power grid when an asymmetric fault occurs in the power grid. However, the current limitation is not considered in dealing with the problem of excessive current during the fault ride-through process. The control strategy mentioned in the document "Control Strategy for Distribution Generation Inverters to Maximize the Voltage Support in the Lowest Phase During Voltage Sags" (M. Castilla, IEEE Transactions on Industrial Electronics, vol. 65, no. 3, pp. 2346-2355) with a publication date of August 2017 aims to maximize the voltage of the lowest phase. However, in this strategy, only the positive sequence current is injected, the injection of the negative sequence current is ignored, no measures are taken for the negative sequence voltage, and no goal of reducing the imbalance of the PCC point is proposed. The document "An Advanced Voltage Support Scheme Considering the Impact of Zero-Sequence Voltage under Microgrid Faults Using Model Predictive Control" (SR Mohapatra, V. Agarwal, IEEE Transactions on Industrial Electronics, 2020, 67 (10): 8957-8968) published on February 13, 2020 proposed two control strategies for three-phase four-wire grid-connected inverters: one is to use positive and negative sequence grid currents to compensate for zero-sequence voltage, and the other is to use zero-sequence current to compensate for zero-sequence voltage. The parameters "k" and "k1" are introduced to determine whether the PCC point (point-to-point connection point) voltage requires zero-sequence current to remain within the limit value.

[0005] In summary, although there have been many studies on voltage support, most of the existing literature focuses on the traditional three-phase two-level inverter. In the control objectives achieved, the problem of active power balance is not taken into account, so it cannot be directly applied to the cascade system, while the cascade multi-level system has been widely used in the field of new energy. In view of the contradiction between power balance and voltage support in the cascade multi-level system in some cases, the three-phase four-wire system introduces zero-sequence current as a new controllable degree of freedom of the system, which effectively balances voltage support and power balance. Therefore, the study of the voltage support strategy considering power balance for the three-phase four-wire cascade multi-level energy storage system is necessary and of great significance. Summary of the invention

[0006] In order to solve the shortcomings of the above-mentioned technology, the present invention provides a three-phase four-wire cascaded multi-level system voltage support method considering power balance, so as to solve the problem that the injection of zero-sequence voltage or negative-sequence current in the three-phase four-wire is not conducive to the voltage support under the unbalanced power grid. It is necessary to introduce a new method to solve the problem that power balance and voltage support cannot be achieved simultaneously in the three-phase four-wire cascaded multi-level energy storage system.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a voltage support method for a three-phase four-wire cascaded multi-level system considering power balance, comprising the following steps:

[0008] Step 1: Calculate the following parameters in the three-phase four-wire cascade multi-level energy storage system: the positive-sequence, negative-sequence, and zero-sequence voltage amplitudes, initial phase angles, and voltage imbalance of the PCC point fault, separate the positive-negative-zero sequence of the three-phase current output by the system, and obtain the voltage amplitude expressions of the positive-sequence, negative-sequence, and zero-sequence;

[0009] Step 2: Power balance in the three-phase four-wire cascaded multi-level energy storage system is achieved by adjusting the ratio of the injected negative sequence current and zero sequence current to the positive sequence current, and the conditions for the system to meet power balance are obtained;

[0010] Step 3: Under the condition that the grid imbalance is minimal and the grid currents of each phase and the neutral line current meet the current limit, the optimal voltage support point is calculated.

[0011] Preferably, in step 1, the three-phase PCC voltage is decomposed, and after Clarke transformation, the component v of the PCC phase voltage on the αβ0 axis is α , v β , v 0 It is expressed as:

[0012]

[0013] Among them, v + α 、v +β , v - α 、v - β They are v α , v β The positive and negative sequence components of V + is the positive sequence voltage amplitude, V - is the negative sequence voltage amplitude, V 0 is the zero-sequence voltage amplitude; is the initial phase of the positive sequence voltage, is the initial phase of negative sequence voltage, is the initial phase of zero-sequence voltage; ω is the fundamental angular frequency of the power grid;

[0014] According to the stationary coordinate system expression, the expressions of the positive-sequence and negative-sequence voltage amplitudes and the positive-sequence and negative-sequence voltage phase differences are derived as follows:

[0015]

[0016]

[0017]

[0018]

[0019]

[0020] Among them, atan2 is the inverse tangent function.

[0021] Preferably, in step 1, when the power grid drops, the three-phase current expression output by the system is subjected to Clarke transformation to separate the positive and negative zero sequences:

[0022]

[0023] Among them, I + , I - , I 0 are the current amplitudes of positive sequence, negative sequence and zero sequence respectively; φ + ,φ - ,φ 0 are respectively the initial phases of the grid current of positive sequence, negative sequence and zero sequence, then:

[0024]

[0025] Among them, I p + , I p - , I p 0 are respectively the amplitudes of the active components of the positive and negative sequence currents, I q+ , I q - , I q 0 are the reactive component amplitudes of the positive and negative sequence currents respectively.

[0026] Preferably, in step 1, the injected current vector i is decomposed into the sum of four vectors, namely, the active component and reactive component of the positive sequence and the negative sequence, to obtain I p + ,I p - , I p , I q + , I q - , I q 0 :

[0027]

[0028] Among them, R g and L g is the grid-side impedance, and the relationship between the instantaneous expression of each phase current, the current amplitude expression and the positive and negative zero-sequence current is listed, that is:

[0029]

[0030]

[0031] Preferably, in step 1, the positive and negative sequence voltage and current expressions can be obtained as follows:

[0032]

[0033] Among them, V g + is the positive sequence grid voltage amplitude, V g - is the negative sequence grid voltage amplitude, V g 0 is the zero-sequence grid voltage amplitude; is the initial phase of the positive sequence grid voltage, is the initial phase of negative sequence grid voltage, is the initial phase of zero-sequence grid voltage;

[0034] After positive and negative sequence separation, we get:

[0035]

[0036]

[0037] Square both sides of formula (13) and (14) and add them together to derive the positive sequence voltage amplitude expression:

[0038]

[0039] Similarly, the negative sequence and zero sequence voltage amplitude expressions are:

[0040]

[0041]

[0042] Substituting formula (9) into formulas (15), (16), and (17), the voltage amplitude expressions of the positive sequence, negative sequence, and zero sequence are obtained after processing:

[0043]

[0044] Preferably, in step 2, according to formulas (10), (11), (15), (16), and (17), the mathematical expression for the three-phase active power balance of the three-phase four-wire cascaded multi-level energy storage system under the unbalanced power grid is written as:

[0045]

[0046] Among them, V ga is the voltage amplitude of phase a grid, V gb is the voltage amplitude of phase b grid, V gc is the voltage amplitude of phase c grid; is the initial phase of the a-phase grid voltage, is the initial phase of the b-phase grid voltage, is the initial phase of the c-phase grid voltage.

[0047] Preferably, in step 2, k is introduced 1 and k 2 , define k 1 is the ratio of negative sequence current to positive sequence current, that is, I - =k 1 I + ; define k 2 is the ratio of zero-sequence current to positive-sequence current, that is, I 0 =k 1 I + ; Select the positive sequence current amplitude I + Set as a variable, and solve formula (19) according to the power balance condition to obtain I + , I - , I 0 The ratio between them is finally solved as:

[0048]

[0049] in:

[0050]

[0051] When k 1 , k 2 When the expression of formula (20) is satisfied, the system meets the power balance condition.

[0052] Preferably, in step 3, the minimum grid imbalance is expressed as a sum, that is:

[0053]

[0054] By determining the power balance condition, I + , I - , I 0 After the proportional relationship between them, other parameters of the power grid can be expressed by I + Therefore, the voltage support problem is equivalent to a problem with a unique variable I + The mathematical model is:

[0055]

[0056] It can be seen from observation that the minimum value must be taken at the turning point or boundary condition. Since the function is a linear function, the structure is relatively simple. Therefore, through simple mathematical analysis, it can be seen that the minimum value must be taken at the turning point or boundary condition.

[0057] Preferably, in step 3, when the conditions in step 1 and step 2 are met and the minimum value of the function in formula (23) is obtained, the corresponding calculation is as follows:

[0058]

[0059] at this time:

[0060]

[0061] When the positive sequence current amplitude is I y When the corresponding maximum value of the three-phase current and the neutral current amplitude is M y Similarly, let the positive sequence current amplitude be I z When the corresponding maximum value of the three-phase current and the neutral current amplitude is M z ;

[0062] When M y With M z When there is a value that satisfies the current limiting condition, that is, M y / z ≤I max , and its corresponding I y / z with I x By comparison and contrasting the corresponding grid imbalance, the optimal positive sequence current amplitude injected into the grid is:

[0063]

[0064] Preferably, in step 3, when M y With M z When both meet the current limiting condition, that is, M y ≤I max ,M z ≤I max , will I y and I z分别 with I x By comparison and contrasting the corresponding grid imbalance, the optimal positive sequence current amplitude injected into the grid is:

[0065]

[0066] When M y With M z When the current limit condition is not met, that is, M y >I max ,M z >I max , the optimal positive sequence current amplitude injected into the power grid is:

[0067] I + =I x (28).

[0068] The present invention aims at the voltage support problem of a three-phase four-wire cascaded multi-level energy storage system under an unbalanced power grid, and proposes a three-phase four-wire cascaded multi-level energy storage system optimization voltage support control method considering power balance. The zero-sequence channel under the three-phase four-wire topology is used to provide a new degree of freedom for the control system to solve the problem that power balance and voltage support of the three-phase four-wire cascaded multi-level energy storage system are difficult to achieve simultaneously under an unbalanced power grid. Power balance is achieved by adjusting the ratio of the injected negative sequence current and the zero sequence current to the positive sequence current, and the optimal voltage support point is calculated under the condition that the imbalance of the power grid is minimized and the current of each phase of the power grid and the neutral line current meet the current limit, and the accurate value of the injected positive sequence current amplitude and phase is given. After applying the voltage support method proposed by the present invention, the PCC point imbalance is significantly reduced, and the power balance of the cascaded multi-level energy storage system is achieved at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 This is a typical topological structure diagram of a three-phase four-wire cascade multi-level energy storage system according to an embodiment of the present invention;

[0070] Figure 2 This is a voltage support control block diagram of a three-phase four-wire cascaded multi-level energy storage system according to an embodiment of the present invention;

[0071] Figure 3This is a three-phase grid current experimental diagram of an embodiment of the present invention;

[0072] Figure 4 This is a neutral current experimental diagram of an embodiment of the present invention;

[0073] Figure 5 3 is a three-phase PCC voltage test diagram of an embodiment of the present invention.

[0074] Figure 6 It is a three-phase active power experimental diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0075] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0076] The method for calculating the optimal voltage support reference current of the four-bridge-arm inverter according to the embodiment of the present invention includes the following contents:

[0077] 1. Calculate the voltage amplitude and initial phase angle of the positive, negative and zero sequence of the PCC point fault

[0078] The topology of the three-phase four-wire cascade multi-level energy storage system is as follows: Figure 1 The system adopts star connection, and the three phases are connected in series by the same H-bridge inverter, where V dc It is a DC voltage measurement. Through the LC filter (L, C f ) to filter out harmonics and use active damping to effectively increase system damping and improve system operation stability. g and L g is the equivalent impedance of the neutral line. gx (x=a,b,c) is the grid-side current;

[0079] i x (x=a,b,c) is the current on the cascade H-bridge side; i n is the neutral current; V gx (x=a,b,c) is the grid voltage.

[0080] When a fault occurs in the power grid and the grid voltage drops, negative-sequence voltage components and zero-sequence voltage components will be generated. In a three-phase three-wire system, since there is no zero-sequence current path, the injection of positive and negative sequence currents will not cause changes in the zero-sequence voltage. However, in a three-phase four-wire system, the zero-sequence component must be strictly considered. The three-phase PCC voltage is decomposed below. After Clarke transformation, the component v of the PCC phase voltage on the αβ0 axis is α , v β , v 0 It can be expressed as:

[0081]

[0082] where v + α 、v + β 、v - α 、v - β They are v α , v β The positive and negative sequence components of V + is the positive sequence voltage amplitude, V - is the negative sequence voltage amplitude, V 0 is the zero-sequence voltage amplitude; is the initial phase of the positive sequence voltage, is the initial phase of negative sequence voltage, is the initial phase of the zero-sequence voltage; ω is the fundamental angular frequency of the power grid. According to the stationary coordinate system expression, the expressions of the positive and negative sequence voltage amplitudes and the positive and negative sequence voltage phase difference can be derived as follows:

[0083]

[0084]

[0085]

[0086]

[0087]

[0088] Among them, atan2 is the inverse tangent function. Similarly, under the condition of power grid drop, the three-phase current expression of the system output is separated into positive and negative zero sequence by Clarke transformation:

[0089]

[0090] And I + , I - , I 0 are respectively the positive and negative zero-sequence current amplitudes; φ + ,φ - ,φ 0 are respectively the initial phases of the positive and negative zero-sequence grid currents, then:

[0091]

[0092] Among them I p + , I p - , I p 0 are the amplitudes of the positive and negative zero-sequence current active components, respectively. q +, I q - , I q 0 are the amplitudes of the reactive components of the positive and negative zero-sequence currents, respectively. When a voltage drop occurs, the grid fault only causes the voltage amplitude of the PCC point to drop without a phase angle jump. The injected current vector i is decomposed into the sum of four vectors, namely the active and reactive components of the positive and negative sequences, and I p + ,I p - , I p 0 , I q + , I q - , I q 0 :

[0093]

[0094] Since the current needs to be limited in the control, it is necessary to list the relationship between the instantaneous expression of each phase current and the current amplitude expression and the positive and negative zero-sequence current, that is:

[0095]

[0096]

[0097] From the above positive and negative sequence voltage and current expressions, we can get:

[0098]

[0099] Among them, V g + is the positive sequence grid voltage amplitude, V g - is the negative sequence grid voltage amplitude, V g 0 is the zero-sequence grid voltage amplitude; is the initial phase of the positive sequence grid voltage, is the initial phase of negative sequence grid voltage, is the initial phase of the zero-sequence grid voltage.

[0100] After positive and negative sequence separation, we can get:

[0101]

[0102]

[0103] By squaring both sides of formulas (13) and (14) and then adding them together, we can derive the expression for the positive sequence voltage amplitude:

[0104]

[0105] Similarly, the negative-sequence and zero-sequence voltage amplitude expressions are:

[0106]

[0107]

[0108] Substituting formula (9) into formula (15), formula (16), and formula (17) respectively, the positive and negative zero-sequence voltage amplitude expressions can be obtained after processing:

[0109]

[0110] 2. Power balancing in a three-phase four-wire cascaded multi-level energy storage system is achieved by adjusting the ratio of injected negative sequence current and zero sequence current to positive sequence current;

[0111] The greatest innovation of the present invention is to achieve power balancing by adjusting the ratio of the injected negative sequence current and zero sequence current to the positive sequence current while reducing the voltage imbalance at the PCC point to a minimum.

[0112] According to formulas (10), (11), (15), (16), and (17), the mathematical expression for the three-phase active power balance of the three-phase four-wire cascaded multi-level energy storage system under unbalanced power grid can be written as:

[0113]

[0114] Among them, V ga is the voltage amplitude of phase a grid, V gb is the voltage amplitude of phase b grid, V gc is the voltage amplitude of phase c grid; is the initial phase of the a-phase grid voltage, is the initial phase of the b-phase grid voltage, is the initial phase of the c-phase grid voltage.

[0115] From formula (19), we can establish an equation with three unknowns and two equations. The three unknowns are the positive and negative zero-sequence current amplitudes I + , I - , I 0 , and the rest can be regarded as known quantities in the derivation of the formula. Therefore, from formula (19), we can get I + , I - , I 0 So we introduce k 1 and k 2 , define k 1 is the ratio of negative sequence current to positive sequence current, that is, I- =k 1 I + ; define k 2 is the ratio of zero-sequence current to positive-sequence current, that is, I 0 =k 1 I + . Select the positive sequence current amplitude I + Set as a variable. Then, by using the power balance condition, we can solve formula (19) to obtain I + , I - , I 0 The ratio between them. Finally, we can solve:

[0116]

[0117] in:

[0118]

[0119] In the above formula, k 1 , k 2 It is only related to the unbalance of the grid voltage. This means that if k 1 , k 2 When the expression in formula (20) is satisfied, the system meets the power balance condition, and the power balance problem in the three-phase four-wire cascade system will be solved. When the cascade system faces an unbalanced power grid, in determining k 1 , k 2 After the value is calculated, the power balance can be guaranteed. At the same time, under this condition, other unknown quantities of the power grid can be obtained by I + This greatly simplifies the analysis of the problem. Therefore, the power balance control method is the basis of the proposed three-phase four-wire cascaded multi-level energy storage system optimized voltage support control method considering power balance, and is conducive to the subsequent derivation of voltage support.

[0120] 3. Under the condition that the grid imbalance is minimal and the grid currents of each phase and the neutral line current meet the current limit, the optimal voltage support point is calculated:

[0121] The grid imbalance (negative sequence imbalance and zero sequence imbalance) can be expressed as the ratio of the negative sequence current amplitude and the zero sequence voltage amplitude to the positive sequence voltage amplitude. The minimum grid imbalance can be expressed as the sum, that is:

[0122]

[0123] The above formula is the core of the voltage support strategy. At the same time, from formulas (10), (11), (15), (16), (17), and (19), it can be seen that when I is determined by power balancing conditions, + , I - , I0 After the proportional relationship between them is obtained, other parameters of the power grid can be used + Therefore, the voltage support problem of this model can be equivalent to a problem with a unique variable I + The mathematical model is:

[0124]

[0125] It can be observed that the minimum value must be obtained at the turning point or boundary condition. When the above conditions are met and the minimum value of the function in formula (23) is obtained, the corresponding calculation is as follows:

[0126]

[0127] at this time:

[0128]

[0129] When the positive sequence current amplitude is I y When the corresponding maximum value of the three-phase current and the neutral current amplitude is M y Similarly, let the positive sequence current amplitude be I z When the corresponding maximum value of the three-phase current and the neutral current amplitude is M z .

[0130] When M y With M z When there is a value that satisfies the current limiting condition, that is, M y / z ≤I max , and its corresponding I y / z with I x By comparison and contrasting the corresponding grid imbalance, the optimal positive sequence current amplitude injected into the grid is:

[0131]

[0132] When M y With M z When both meet the current limiting condition, that is, M y ≤I max ,M z ≤I max , will I y and I z with I x By comparison and contrasting the corresponding grid imbalance, the optimal positive sequence current amplitude injected into the grid is:

[0133]

[0134] When M y With M zWhen the current limit condition is not met, M y >I max ,M z >I max , the optimal positive sequence current amplitude injected into the power grid is:

[0135] I + =I x (28)

[0136] In the optimized voltage support control method of a power-balanced three-phase four-wire cascaded multi-level energy storage system, the amplitude ratio k of the negative sequence current and the zero sequence current to the positive sequence current is determined. 1 , k 2 The power balance of the cascade system can be guaranteed. At this time, by determining the value of the injected positive sequence current, the value and phase of the injected negative sequence current and positive sequence current can be derived. max It is the maximum current value to ensure safe operation of the system. + That is, the variable in the formula is only related to the voltage imbalance in the determined system. By setting the appropriate injected positive and negative zero current reference values, the constraints on the amplitude of each phase current and the neutral current and the demand for power balance can be met while achieving the optimal support for the grid voltage.

[0137] Through the analysis in the previous article, the control block diagram of the optimized voltage support control method of the three-phase four-wire cascaded multi-level energy storage system considering power balance is as follows: Figure 2 shown.

[0138] Test verification

[0139] The method of the present invention was verified by experiments, and the specific experimental parameters are shown in Table 1 below.

[0140] Table 1 Experimental parameters of three-phase four-wire cascaded multi-level energy storage system

[0141]

[0142] In the two-phase drop experiment in this experiment, it is assumed that the voltage of phases b and c drops and the voltage of phases b and c are set to drop to 0.5pu, and phase a remains unchanged.

[0143] Different control strategies are adopted in different time periods in the experiment, specifically: 1 At this time, the grid is balanced and balanced current control is adopted; at t 1 The voltage drops at time t 1 ~t 2 The current control adopted when the voltage support control strategy is not implemented and the balanced power grid is still maintained; t 2The voltage support control strategy for the three-phase four-wire cascade multi-level energy storage system under the complete unbalanced power grid proposed by the present invention is adopted.

[0144] At 0~t 1 At this time, the three-phase power grid is balanced, the system outputs three-phase balanced current, the grid-connected power is 6.3kW, and the current flowing through the neutral line is 0.

[0145] In t 1 ~t 2 , at this time, the grid is unbalanced in three phases. Since no balancing control is applied, the system still outputs three-phase balanced current, and the three-phase grid-connected power is 6.3kW, 3.1kW and 3.1kW respectively, and the current flowing through the neutral line is 0. At this time, the grid imbalance at the PCC is 0.36.

[0146] From 2 The proposed three-phase four-wire cascaded multi-level energy storage system optimization voltage support control method considering power balance was applied. At this time, the ratio of the negative sequence and zero sequence current amplitude to the positive sequence current amplitude was 0.2 and 0.2 respectively, and the optimal value of the injected positive sequence current amplitude was 56.66A. The specific experimental results are shown in Figure 3-Figure 6 shown.

[0147] Figure 3 and Figure 4 This indicates that the three-phase grid current and neutral line current before and after voltage support are controlled within the safety current threshold I max The constraints on current limiting are met, which is the same as the simulation results. Figure 5 The PCC voltage changes during the entire control process. After control, the PCC voltage achieves the optimal voltage support effect, and the imbalance is significantly improved. After control, the imbalance of the power grid is reduced from 0.36 to 0.22. Figure 6 The effect of the control method on the power balance of the three-phase four-wire cascaded multi-level energy storage system under unbalanced power grid was verified, and the three-phase active power was maintained at about 4.8kW. The experimental results show that by optimizing the voltage support control method of the three-phase four-wire cascaded multi-level energy storage system considering power balance, the PCC point imbalance is significantly reduced, and the cascaded multi-level energy storage system maintains power balance, and power balance and voltage support are achieved, which proves the effectiveness of the voltage support control scheme of the present invention.

[0148] The present invention aims at the voltage support problem of a three-phase four-wire cascaded multi-level energy storage system under an unbalanced power grid, and proposes a method for optimizing voltage support control of a four-wire cascaded multi-level energy storage system taking power balance into consideration. The zero-sequence channel under the three-phase four-wire topology is used to provide a new degree of freedom for the control system, so as to solve the problem that power balance and voltage support of a three-phase four-wire cascaded multi-level energy storage system under an unbalanced power grid are difficult to achieve simultaneously. Power balance is achieved by adjusting the ratio of the injected negative-sequence current and the zero-sequence current to the positive-sequence current, and the optimal voltage support point is calculated under the condition that the imbalance of the power grid is minimal and the current of each phase of the power grid and the neutral current meet the current limit, and the accurate value of the injected positive-sequence current amplitude and phase is given. Experimental comparison was carried out on a hardware-in-the-loop experimental platform to verify the optimality and effectiveness of the voltage support method of the present invention.

[0149] The above implementation modes are not limitations of the present invention, and the present invention is not limited to the above examples. Any changes, modifications, additions or substitutions made by technicians in this technical field within the scope of the technical solution of the present invention also belong to the protection scope of the present invention.

Claims

1. A voltage support method for a three-phase four-wire cascaded multi-level system considering power balance, characterized in that: The following steps are involved: Step 1: Calculate the following parameters in the three-phase four-wire cascade multi-level energy storage system: the positive-sequence, negative-sequence, and zero-sequence voltage amplitudes, initial phase angles, and voltage imbalance of the PCC point fault, separate the positive-negative-zero sequence of the three-phase current output by the system, and obtain the voltage amplitude expressions of the positive-sequence, negative-sequence, and zero-sequence; Step 2: Power balance in the three-phase four-wire cascaded multi-level energy storage system is achieved by adjusting the ratio of the injected negative sequence current and zero sequence current to the positive sequence current, and the conditions for the system to meet power balance are obtained; Step 3: Under the condition that the grid imbalance is minimal and the grid currents of each phase and the neutral line current meet the current limit, the optimal voltage support point is calculated; In step 3, the minimum grid imbalance is expressed as the sum, that is: By determining the power balance condition, I + , I - , I 0 After the proportional relationship between them, other parameters of the power grid can be expressed by I + Therefore, the voltage support problem is equivalent to a problem with a unique variable I + The mathematical model is: Among them, V + is the positive sequence voltage amplitude, V - is the negative sequence voltage amplitude, V 0 is the zero-sequence voltage amplitude; I + is the positive sequence current amplitude, I - is the negative sequence current amplitude, I 0 is the zero-sequence current amplitude; V g + is the positive sequence grid voltage amplitude, V g - is the negative sequence grid voltage amplitude, V g 0 is the zero-sequence grid voltage amplitude; ω is the fundamental angular frequency of the power grid, R g and L g is the grid side impedance, I a ,I b ,I c is the current amplitude of each phase; k1 is the ratio of negative sequence current to positive sequence current, that is, I - =k1I + ; k2 is the ratio of zero sequence current to positive sequence current, that is, I 0 =k2I + ; It can be observed that the minimum value must be taken at the turning point or boundary condition; In step 3, when the conditions in step 1 and step 2 are met and the minimum value of the function in formula (23) is obtained, the corresponding calculation is as follows: at this time: Among them, Φ + , Φ - , Φ 0 They are respectively the initial phases of the grid current of positive sequence, negative sequence and zero sequence; Let the positive sequence current amplitude be I y When the corresponding maximum value of the three-phase current and the neutral current amplitude is M y Similarly, let the positive sequence current amplitude be I z When the corresponding maximum value of the three-phase current and the neutral current amplitude is M z ; When M y With M z When there is a value that satisfies the current limiting condition, that is, M y / z ≤I max , and its corresponding I y / z with I x By comparison and contrasting the corresponding grid imbalance, the optimal positive sequence current amplitude injected into the grid is: When M y With M z When both meet the current limiting condition, that is, M y ≤I max ,M z ≤I max , will I y and I z Respectively with I x By comparison and contrasting the corresponding grid imbalance, the optimal positive sequence current amplitude injected into the grid is: When M y With M z When the current limit condition is not met, that is, M y >I max , M z >I max , the optimal positive sequence current amplitude injected into the power grid is: I + =I x (28)。 2. The voltage support method for a three-phase four-wire cascaded multi-level system considering power balance according to claim 1, characterized in that: In step 1, the three-phase PCC voltage is decomposed, and after Clarke transformation, the component v of the PCC phase voltage on the αβ0 axis is α 、v β , v0 is expressed as: Among them, v + α 、v + β , v - α 、v - β They are v α , v β The positive and negative sequence components of is the initial phase of the positive sequence voltage, is the initial phase of negative sequence voltage, is the initial phase of zero-sequence voltage; According to the stationary coordinate system expression, the expressions of the positive-sequence and negative-sequence voltage amplitudes and the positive-sequence and negative-sequence voltage phase differences are derived as follows: Among them, atan2 is the inverse tangent function.

3. The voltage support method for a three-phase four-wire cascaded multi-level system considering power balance according to claim 2, characterized in that: In the step 1, when the power grid drops, the three-phase current expression output by the system is subjected to Clarke transformation to separate the positive and negative zero sequences: Then we have: Among them, I p + , I p - , I p 0 are respectively the amplitudes of the active components of the positive sequence, negative sequence and zero sequence currents, I q + , I q - , I q 0 They are the amplitudes of the reactive components of the positive, negative and zero sequence currents respectively.

4. The voltage support method for a three-phase four-wire cascaded multi-level system considering power balance according to claim 3, characterized in that: In step 1, the injected current vector i is decomposed into the sum of four vectors, namely, the active component and reactive component of the positive sequence and negative sequence, and I is obtained. p + ,I p - ,I p 0 ,I q + ,I q - ,I q 0 : Among them, the relationship between the instantaneous expression of each phase current, the current amplitude expression and the positive and negative zero-sequence current is listed, that is:

5. The voltage support method for a three-phase four-wire cascaded multi-level system considering power balance according to claim 4, characterized in that: In step 1, the positive and negative sequence voltage and current expressions can be obtained as follows: in, is the initial phase of the positive sequence grid voltage, is the initial phase of negative sequence grid voltage, is the initial phase of zero-sequence grid voltage; After positive and negative sequence separation, we get: Square both sides of formula (13) and (14) and add them together to derive the positive sequence voltage amplitude expression: Similarly, the negative sequence and zero sequence voltage amplitude expressions are: Substituting formula (9) into formulas (15), (16), and (17), the voltage amplitude expressions of the positive sequence, negative sequence, and zero sequence are obtained after processing:

6. The voltage support method for a three-phase four-wire cascaded multi-level system considering power balance according to claim 5, characterized in that: In step 2, according to formulas (10), (11), (15), (16), and (17), the mathematical expression for the three-phase active power balance of the three-phase four-wire cascaded multi-level energy storage system under an unbalanced power grid is written as: Among them, V ga is the voltage amplitude of phase a grid, V gb is the voltage amplitude of phase b grid, V gc is the voltage amplitude of phase c grid; is the initial phase of the a-phase grid voltage, is the initial phase of the b-phase grid voltage, is the initial phase of the c-phase grid voltage.

7. The voltage support method for a three-phase four-wire cascaded multi-level system considering power balance according to claim 6, characterized in that: Select the positive sequence current amplitude I + Set as a variable, and solve formula (19) according to the power balance condition to obtain I + , I - , I 0 The ratio between them is finally solved as: in: When k1 and k2 satisfy the expression of formula (20), the system meets the power balance condition.

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