Three-phase four-wire cascade STATCOM voltage support control method considering power constraint

Abstract

The invention discloses a four-wire cascade STATCOM optimized voltage support control method considering active power constraint, which comprises the following steps of: 1, calculating positive and negative zero sequence voltage amplitudes, an initial phase angle and a voltage unbalance degree of a PCC point fault in a three-phase four-wire cascade multi-level energy storage system, carrying out positive and negative zero sequence separation on three-phase current output by the system, and calculating a positive and negative zero sequence voltage amplitude value of the PCC point fault in the three-phase four-wire cascade multi-level energy storage system; obtaining a positive and negative zero sequence voltage amplitude expression; step 2, deriving according to voltage support under active power constraint and current amplitude limiting to obtain the amplitude and phase of the optimal reactive component current injected into the power grid; step 3, obtaining an active component of the reference current needing to be injected through DC capacitor voltage closed-loop control; a switching tube signal of an inverter is generated through a current control loop and a PS-SPWM modulation technology, and the voltage support control method is implemented. After the voltage supporting method provided by the invention is applied, the effect of reducing the degree of unbalance of the PCC point is obvious, and meanwhile, the power balance of the cascaded STATCOM is realized.

Description

Technical Field

[0001] The present invention relates to a voltage support control method, in particular to a method for calculating an optimal voltage support point of a three-phase four-wire cascaded STATCOM considering power constraints, 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 circuit 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] At present, the phase-to-phase 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" published on April 14, 2016 analyzed the relationship between zero-sequence voltage and active power in the abc coordinate system, and realized phase-to-phase average active power control, DC voltage control and reference current tracking control under 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" published on April 14, 2017 controls the negative-sequence modulation index based on unbalanced power information without using any controller, which leads to steady-state errors 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 lead to 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 the 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 system's grid imbalance when dealing with the power balance of the three-phase four-wire cascaded multi-level system under an 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" 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, but does not consider current limiting when dealing with the problem of excessive current during fault ride-through. 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" published on August 2017 aims to maximize the voltage of the lowest phase. However, in this strategy, only positive sequence current is injected, and the injection of negative sequence current is ignored. No measures are taken for negative sequence voltage, and no goal of reducing the imbalance of 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" 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.

[0005] In summary, although there have been many studies on voltage support, most of the existing technologies are aimed at traditional three-phase two-level inverters. 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. At the same time, in the three-phase four-wire cascade STATCOM, due to the existence of the zero-sequence channel, the injection of zero-sequence voltage and negative-sequence current will aggravate the imbalance of the power grid, which is not conducive to the low-voltage ride-through of the system. At the same time, most of the existing technologies are aimed at the traditional three-phase three-wire system, so they cannot be directly applied to the three-phase four-wire cascade STATCOM. Summary of the invention

[0006] In order to solve the deficiencies of the above-mentioned technology, the present invention provides a three-phase four-wire cascade STATCOM voltage support control method considering power constraints.

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

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

[0009] Separate the three-phase current output by the multi-level system into positive sequence, negative sequence and zero sequence to obtain the positive sequence, negative sequence and zero sequence current expressions;

[0010] Step 2: deriving the voltage support under active power constraint and current limitation, obtaining the amplitude and phase of the optimal reactive component current injected into the power grid;

[0011] Step three: obtain the active component of the required reference current injection through closed-loop control of the DC capacitor voltage; finally, generate the switching tube signal of the inverter through the current control loop and PS-SPWM modulation technology to implement the proposed voltage support control method.

[0012] Preferably, in step 1, the three-phase PCC voltage is decomposed, and after Clarke transformation, the component v of the three-phase PCC voltage on the αβ0 axis is α , v β , v0 is represented by:

[0013]

[0014] 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; ωt represents the electrical angle of the current changing with time t;

[0015] 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:

[0016]

[0017] Among them, atan2 is the inverse tangent function, is the initial phase of the voltage at PCC.

[0018] Preferably, in step 1, under the condition of grid drop, the three-phase current expression output by the multi-level system is separated into positive sequence, negative sequence and zero sequence by Clarke transformation:

[0019]

[0020] Among them, i α ,i β , i0 are the components of the three-phase current on the α, β, and 0 axes respectively; i α + 、i α - ,i β + 、i β - i α ,i β The positive and negative sequence components of I + , I - , I 0 are the current amplitudes of positive sequence, negative sequence and zero sequence respectively; φ + ,φ - ,φ 0 The initial phases of the grid current are positive sequence, negative sequence, and zero sequence respectively, then:

[0021]

[0022] Among them, I p + , I p - , I p 0 are the amplitudes of the active components of the positive, negative and zero sequence currents respectively; I q + , I q - , I q 0 are the amplitudes of the reactive components of the positive, negative and zero sequence currents respectively; δ + , δ - , δ 0 are the impedance angles of positive-sequence, negative-sequence and zero-sequence currents respectively.

[0023] Preferably, in step 1, the injected current vector is decomposed into the sum of four vectors, namely, the active component and reactive component of the positive sequence and the negative sequence, and I p + , I p- , I p 0 , I q + , I q - , I q 0 :

[0024]

[0025] Among them, R g and L g is the grid-side impedance, and the instantaneous current i of each phase is listed. a ,i b ,i c , current amplitude I a ,

[0026] I b , I c and the positive and negative zero-sequence current amplitude I + , I - , I 0 The relationship is:

[0027]

[0028] Preferably, in step 2, the three-phase currents in the three-phase four-wire cascade multi-level energy storage system are listed:

[0029]

[0030] in, 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; θ A ,θ B ,θ C is the phase difference between the grid voltage and grid current of each phase; i n , v n are the neutral current and neutral voltage respectively; the grid side impedance Z, the impedance angle θ L Respectively expressed as:

[0031]

[0032] Use R to replace the grid side resistor R g , replace the grid-side inductor L with l g , that is,

[0033]

[0034] The zero-sequence current under unbalanced power grid can be expressed as:

[0035]

[0036] As a preferred embodiment, in step 2, in order to suppress the power flow between phases, offset the unbalanced power flow caused by the power grid fault and ensure that the output active power of each phase is zero, the active power of the three phases is analyzed, and the following is obtained:

[0037]

[0038] At the same time, let θ A =θ B =θ C =θ, we can get:

[0039]

[0040] Where V ga 、V gb 、V gc are the voltage amplitudes of the three-phase power grid respectively;

[0041] After simplification, the three-phase current amplitude I can be obtained a , I b , I c It can be expressed as:

[0042]

[0043] Preferably, in step 2, when the three-phase current amplitude of the three-phase four-wire cascade STATCOM satisfies the expression in (16), the active power constraint condition is achieved;

[0044] At this time, the three-phase current I a , I b , I c All are related to the single variable θ, and each component is represented by θ. To mathematically model the system, the voltage support problem of the model is equivalent to:

[0045]

[0046] st can be expressed as the condition that needs to be satisfied for the inequality to hold under this condition, where I max It is the maximum current amplitude allowed to pass after three-phase current limiting.

[0047] Preferably, in step 2, based on (19), since:

[0048]

[0049] Simplifying, we get:

[0050]

[0051] Where V ga 、V gb 、V gc are the three-phase grid voltage amplitudes respectively. Similarly, the positive and negative zero-sequence current amplitude expressions can be obtained:

[0052]

[0053] It can be deduced that the positive and negative zero-sequence voltage amplitudes can be expressed as:

[0054]

[0055] T A0 , T A1 , T A2 , T B0 , T B1 , T B2 , T c0 , T c1 , T c2 They are process quantities, have no actual physical meaning and are only for the convenience of calculation.

[0056] Preferably, in step 2, for f(θ) in equation (19), which is a function with a period of 2π, the function value change is analyzed in the interval [0, 2π] to determine that the minimum value of the function is always at π / 2+θ L and 3π / 2+θ L Get it from

[0057] Then if θ=3π / 2+θ L When the current limiting condition is met, the optimal value is 3π / 2+θ L ;

[0058] And when θ=3π / 2+θ L The current limiting condition is not met when θ is in the range of [3π / 2,3π / 2+θ L ], so in [3π / 2,3π / 2+θ L ] must exist θ=θ x The maximum amplitude of each phase current is I max , and the unbalance degree of the power grid is the smallest at this point, that is:

[0059] max{I a ,I b ,I c =I max (26)

[0060] Simplify formula (18) and let:

[0061]

[0062] Then at this time:

[0063]

[0064] After determining the value of θ at the optimal support, the injected current i can be determined by (27): a ,i b ,i c The amplitude and phase of the active power are: a ,i b ,i c Only the voltage v at PCC pcc Reactive power is emitted, so the i obtained at this time a ,i b ,i c That is, the reactive current i required to be injected for voltage support at the PCC a q ,i b q ,i c q .

[0065] As a preferred embodiment, in step 3, in order to balance the switching loss and restore the capacitor voltage drop caused by the unbalanced power grid, a phase perpendicular to i is injected into each phase. a q ,i b q ,i c q The active current i a p ,i b p ,i c p , implemented by adding a DC capacitor voltage loop.

[0066] The present invention discloses a voltage support control method for a three-phase four-wire cascaded STATCOM considering power constraints. According to the characteristic that STATCOM cannot provide active power, the zero-sequence component in the three-phase four-wire system is introduced as a new controllable degree of freedom of the system, thereby effectively balancing the active power constraint and the voltage support. The voltage support control method for the three-phase four-wire cascaded STATCOM is reasonably designed under the conditions of current limitation and active power constraints to improve the low voltage ride-through capability of the system. After applying the voltage support method proposed by the present invention, the unbalance degree of the PCC point is significantly reduced, and the power balance of the cascaded STATCOM is achieved at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 is a function of the unbalance degree of the power grid in the interval [0,2π] according to the present invention;

[0068] Figure 2 It is a function of the unbalance degree of the power grid in the interval [3π / 2,5π / 2] implemented by the present invention;

[0069] Figure 3 It is a typical topological structure diagram of the three-phase four-wire cascaded STATCOM implemented in the present invention;

[0070] Figure 4 This is a block diagram of the DC capacitor voltage closed-loop control implemented in the present invention;

[0071] Figure 5 This is a voltage support control block diagram of a three-phase four-wire cascaded STATCOM according to an embodiment of the present invention;

[0072] Figure 6 This is a grid current experimental diagram of an embodiment of the present invention;

[0073] Figure 7 It is a neutral line current experimental diagram of an embodiment of the present invention.

[0074] Figure 8 It is a PCC voltage test diagram of an embodiment of the present invention.

[0075] Fig. 9 1 is a graph showing a capacitor voltage experiment according to an embodiment of the present invention.

[0076] Fig.10 It is a PCC point active power experimental diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0077] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0078] The present invention aims at the problem that active power constraint and voltage support in three-phase four-wire cascade STATCOM are contradictory in some cases, and proposes a three-phase four-wire cascade STATCOM voltage support control method considering power constraint, specifically an optimized voltage support control method. According to the characteristic that STATCOM cannot provide active power, by introducing the zero-sequence component in the three-phase four-wire system as a new controllable degree of freedom of the system, the active power constraint and voltage support are effectively balanced. And under the conditions of current limitation and active power constraint, the voltage support control method of the three-phase four-wire cascade STATCOM is reasonably designed to improve the low voltage ride-through capability of the system. The research on the voltage support strategy considering power balance for the three-phase four-wire cascade multi-level energy storage system is of great significance.

[0079] The method for calculating the optimal voltage support reference current of a three-phase four-wire cascaded STATCOM according to an embodiment of the present invention includes the following contents:

[0080] 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 three-phase current output by the system into positive-sequence, negative-sequence, and zero-sequence, and obtain the positive-sequence, negative-sequence, and zero-sequence current expressions;

[0081] The topology of three-phase four-wire cascade STATCOM is as follows: Figure 3 As shown in the figure, the system adopts a star connection mode, and the three phases are connected in series by the H-bridge inverter of the box, and C is the DC side capacitor. f ), filter out harmonics, and use active damping to effectively increase system damping and improve system operation stability. g and L g is the grid side impedance. gx (x=a,b,c) is the grid-side current; i x (x=a,b,c) is the current on the cascade H-bridge side; i n is the current flowing through the neutral line; V gx (x=a,b,c) is the grid voltage.

[0082] The three-phase PCC voltage is decomposed and the component v of the PCC phase voltage on the αβ0 axis is obtained by Clarke transformation. α , v β , v0 is represented by:

[0083]

[0084] 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 the zero-sequence voltage; ω is the fundamental angular frequency of the power grid; ωt represents the electrical angle of the current changing with time t.

[0085] 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:

[0086]

[0087] Among them, atan2 is the inverse tangent function, is the initial phase of the voltage at PCC.

[0088] When the power grid drops, the three-phase current expression of the system output is separated into positive and negative zero sequences by Clarke transformation:

[0089]

[0090] Among them, i α ,i β , i0 are the components of the three-phase current on the α, β, and 0 axes respectively; i α + 、i α - ,i β + 、i β - i α ,i β The positive and negative sequence components of I + , I - , I 0 are the current amplitudes of positive sequence, negative sequence and zero sequence respectively; φ + ,φ - ,φ 0 The initial phases of the grid current are positive sequence, negative sequence, and zero sequence respectively, then:

[0091]

[0092] Among them, I p + , I p - , I p 0 are the amplitudes of the active components of the positive, negative and zero sequence currents, I q + , I q - , I q 0 are the amplitudes of the reactive components of the positive, negative and zero sequence currents respectively; δ + , δ - , δ 0 are the impedance angles of positive-sequence, negative-sequence and zero-sequence currents respectively.

[0093] Decompose the injected current vector i into the sum of four vectors, namely the positive sequence and negative sequence active work and reactive work, and obtain I p + , I p - , I p0 , I q + , I q - , I q 0 :

[0094]

[0095] Among them, R g and L g is the grid-side impedance, and the instantaneous current i of each phase is listed. a ,i b ,i c , current amplitude I a ,

[0096] I b , I c and the positive and negative zero-sequence current amplitude I + ,I-,I 0 The relationship is:

[0097]

[0098] Step 2:

[0099] Based on the derivation of voltage support under active power constraint and current limitation, the amplitude and phase of the optimal reactive component current injected into the grid are obtained;

[0100] First, the three-phase current of the three-phase four-wire cascade STATCOM system is listed:

[0101]

[0102] in, 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; θ A ,θ B ,θ C is the phase difference between the grid voltage and grid current of each phase; i n , v n are the neutral current and neutral voltage respectively;

[0103] Grid side impedance Z, impedance angle θ L Respectively expressed as:

[0104]

[0105] To facilitate the writing of formulas below, R is used to replace the grid-side resistor R. g , replace the grid-side inductor L with l g , that is,

[0106]

[0107] The zero-sequence current under unbalanced power grid can be expressed as:

[0108]

[0109] In order to suppress the power flow between phases, offset the unbalanced power flow caused by the grid fault and ensure that the output active power of each phase is zero, the active power of the three phases is analyzed as follows:

[0110]

[0111] At the same time, let θ A =θ B =θ C =θ, we can get:

[0112]

[0113] Where V ga 、V gb 、V gc are the three-phase grid voltage amplitudes respectively. After simplification, the three-phase current amplitude I a , I b , I c It can be expressed as:

[0114]

[0115] It can be seen that when the three-phase current amplitude of the three-phase four-wire cascade STATCOM satisfies the expression in (16), the active power constraint condition is achieved. a , I b , I c are only related to the single variable θ. Therefore, each component of the model can also be represented by θ, so the system is mathematically modeled. Therefore, the voltage support problem of this model can be equivalent to:

[0116]

[0117] st can be expressed as the condition that needs to be satisfied for the inequality to hold under this condition, where I max It is the maximum current amplitude allowed to pass after three-phase current limiting.

[0118] And because:

[0119]

[0120] Simplifying, we get:

[0121]

[0122] Where V ga 、V gb 、V gc are the three-phase grid voltage amplitudes respectively. Similarly, the positive and negative zero-sequence current amplitude expressions can be obtained:

[0123]

[0124] It can be deduced that the positive and negative zero-sequence voltage amplitudes can be expressed as:

[0125]

[0126] T A0 , T A1 , T A2 , T B0 , T B1 , T B2 , T c0 , T c1 , T c2 are process quantities, which have no actual physical meaning and are only convenient for calculation. We analyze f(θ) in equation (19). Since this equation is a function with a period of 2π, we analyze its function value change in [0,2π], as follows: Figure 1 shown.

[0127] The function value changes in [0,2π] as follows Figure 1 As shown, its minimum value is always at π / 2+θ L and 3π / 2+θ L In order to make the calculated results of the current amplitude of each phase in the above formula positive, the interval [3π / 2,5π / 2] is taken for discussion for the convenience of calculation, such as Figure 2 shown.

[0128] Then if θ=3π / 2+θ L When the current limiting condition is met, the optimal value is 3π / 2+θ L .

[0129] And when θ=3π / 2+θ L The current limiting condition is not satisfied when θ is in the range of [3π / 2,3π / 2+θ L ] is monotonically increasing in this interval, so in [3π / 2,3π / 2+θ L ] must exist θ=θ x The maximum amplitude of each phase current is I max , and the unbalance degree of the power grid is the smallest at this point, that is:

[0130] max{I a ,I b,I c =I max (26)

[0131] Simplify formula (18) and let:

[0132]

[0133] Then at this time:

[0134]

[0135] After determining the value of θ at the optimal support, the injected current i can be determined by (27): a ,i b ,i c The magnitude and phase of the active power. At the same time, due to the active power constraint, the i a ,i b ,i c Only the voltage v at PCC pcc Reactive power is emitted, so the i obtained at this time a ,i b ,i c That is, the reactive current i required to be injected for voltage support at the PCC a q ,i b q ,i c q .

[0136] Step 3:

[0137] The active component of the required reference current injection is obtained through the DC capacitor voltage closed-loop control. Finally, the switch signal of the inverter is generated through the current control loop and PS-SPWM modulation technology to realize the proposed three-phase four-wire cascaded STATCOM optimized voltage support control method considering active power constraints.

[0138] In order to balance the switching loss and restore the capacitor voltage drop caused by the unbalanced grid, it is still necessary to inject a phase perpendicular to i into each phase. a q ,i b q ,i c q The active current i a p ,i b p ,i c p. Therefore, it is necessary to add a DC capacitor voltage loop to achieve capacitor voltage stabilization. Since the current is small, it is only used to restore the capacitor voltage and compensate for the switching loss of the circuit, so the chance will not affect the result. However, when the zero-sequence current is small, it may still have a slight impact on the waveform, but it will not affect the voltage support effect and current limiting conditions of the system. Therefore, an optimized voltage support control process of a three-phase four-wire cascaded STATCOM considering active power constraints can be established. At this time, the active power constraint is strictly guaranteed, and the current limiting conditions are also met. On this basis, this control method can achieve optimal support for the three-phase four-wire cascaded STATCOM. The block diagram of the DC capacitor voltage closed-loop control is shown in Figure 4 shown.

[0139] 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 5 shown.

[0140] Test verification,

[0141] Furthermore, the three-phase four-wire cascaded STATCOM voltage support method considering power constraints of this embodiment is verified through experiments, and the specific experimental parameters are shown in Table 1 below.

[0142] Table 1 Experimental parameters of three-phase four-wire cascaded multi-level energy storage system

[0143]

[0144] In the two-phase drop experiment, it is assumed that the voltage of phases b and c drops and the voltage of phases b and c is set to drop to 0.6pu, while phase a remains unchanged.

[0145] Different control strategies are adopted in different time periods in the simulation, specifically: the grid is balanced from 0 to t1, and balanced current control is adopted at this time; voltage drop occurs at t1, but no voltage support control strategy is implemented from t1 to t2, and current control is still adopted when the grid is balanced; starting from t2, the complete three-phase four-wire cascade STATCOM optimized voltage support control method considering active power constraints mentioned above is adopted.

[0146] From 0 to t1, the three-phase power grid is balanced, the system outputs three-phase balanced current, the capacitor voltage is balanced, the three-phase PCC reactive power is 0, and the current flowing through the neutral line is 0.

[0147] At t1-t2, the three-phase grid is unbalanced. Since no balancing control is applied, the system still outputs three-phase balanced current, the capacitor voltage begins to drop, and a small amount of current flows through the neutral line. The three-phase PCC point begins to generate active power. At this time, the grid imbalance at the PCC is 0.37.

[0148] Starting from t2, the proposed three-phase four-wire cascade STATCOM optimized voltage support control method considering active power constraints is applied. Figure 6 and Figure 7 As shown in the figure, the three-phase grid current and neutral line current before and after the 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 8 As shown in the figure, the PCC voltage support effect is obvious at this time. After control, the unbalance degree of the power grid is reduced from 0.45 to 0.36, and the unbalance degree of the power grid is significantly improved. Due to the limitation of the oscilloscope, Figure 8 It is the result of subtracting the initial voltage of 600V from the capacitor voltage. Fig. 9 and Fig.10 It shows that the capacitor has achieved voltage stabilization and met the conditions of active power constraint, which is consistent with the simulation results. At the same time, the active power at the PCC is controlled to be 0, which meets the requirements of the active power constraint of the cascaded STATCOM. The experimental results prove the effectiveness of the four-wire cascaded STATCOM optimized voltage support control method considering active power constraint. It shows that the four-wire cascaded STATCOM optimized voltage support control method considering active power constraint can effectively support the PCC voltage under unbalanced power grid, while meeting the power constraint and current limit conditions to ensure the normal operation of the system.

[0149] In summary, the voltage support control method of the three-phase four-wire cascade STATCOM considering power constraints proposed by the present invention analyzes the voltage support problem of the three-phase four-wire cascade STATCOM according to the characteristic that the output active power and the zero of the STATCOM. The mathematical model of the voltage support of the three-phase four-wire cascade STATCOM under the unbalanced power grid is constructed by the conditions of current limitation and active power constraint, and the optimal value of the reactive current injected into each phase required for the optimal voltage support under the active power constraint and current limitation is obtained. At the same time, the capacitor voltage stabilization is realized by the DC capacitor voltage closed-loop control. Under this strategy, a small amount of active power will be absorbed in the system, but the overall absorbed active power is almost 0. This control method significantly improves the imbalance of the power grid while stabilizing the DC capacitor voltage. Then the overall control method of the four-wire cascade STATCOM optimized voltage support control method considering active power constraints is designed. Finally, the voltage support control strategy proposed is verified by hardware-in-the-loop experiment, which verifies the effectiveness and feasibility of the method.

[0150] The above implementation modes are not limitations of the present invention, and the present invention is not limited to the above examples. 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 control method for a three-phase four-wire cascaded STATCOM considering power constraints, 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: positive-sequence, negative-sequence, and zero-sequence voltage amplitudes, initial phase angles, and voltage unbalance of the PCC point fault; Separate the three-phase current output by the multi-level system into positive sequence, negative sequence and zero sequence to obtain the positive sequence, negative sequence and zero sequence current expressions; Step 2: deriving the voltage support under active power constraint and current limitation, obtaining the amplitude and phase of the optimal reactive component current injected into the power grid; Step three: obtain the active component of the required reference current injection through closed-loop control of the DC capacitor voltage; finally, generate the switching tube signal of the inverter through the current control loop and PS-SPWM modulation technology to implement the proposed voltage support control method.

2. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 1 is characterized in that: In step 1, the three-phase PCC voltage is decomposed and the components v of the three-phase PCC voltage on the αβ0 axis are obtained by Clarke transformation. α , v β , v0 is represented by: 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; ωt represents the electrical angle of the current changing with time t; 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, is the initial phase of the voltage at PCC.

3. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 2 is characterized in that: In the step 1, under the condition of grid drop, the three-phase current expression output by the multi-level system is separated into positive sequence, negative sequence and zero sequence by Clarke transformation: Among them, i α ,i β , i0 are the components of the three-phase current on the α, β, and 0 axes respectively; i α + 、i α - ,i β + 、i β - i α ,i β The positive and negative sequence components of I + , I - , I 0 are the current amplitudes of positive sequence, negative sequence and zero sequence respectively; φ + ,φ - ,φ 0 The initial phases of the grid current are positive sequence, negative sequence, and zero sequence respectively, then: Among them, I p + , I p - , I p 0 are the amplitudes of the active components of the positive, negative and zero sequence currents respectively; I q + , I q - , I q 0 are the amplitudes of the reactive components of the positive, negative and zero sequence currents respectively; δ + , δ - , δ 0 are the impedance angles of positive-sequence, negative-sequence and zero-sequence currents respectively.

4. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 3 is characterized in that: In the step 1, the injected current vector is decomposed into the sum of four vectors, namely, the positive sequence, negative sequence active component and reactive component, and I p + , I p - , I p 0 , I q + , I q - , I q 0 : Among them, R g and L g is the grid-side impedance, and the instantaneous current i of each phase is listed. a ,i b ,i c , current amplitude I a , I b , I c and the positive and negative zero-sequence current amplitude I + , I - , I 0 The relationship is:

5. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 3 is characterized in that: In step 2, the three-phase currents in the three-phase four-wire cascade multi-level energy storage system are listed: in, 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; θ A ,θ B ,θ C is the phase difference between the grid voltage and grid current of each phase; i n , v n are the neutral current and neutral voltage respectively; the grid side impedance Z, the impedance angle θ L Respectively expressed as: Use R to replace the grid side resistor R g , replace the grid-side inductor L with l g , that is, The zero-sequence current under unbalanced power grid can be expressed as:

6. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 5, characterized in that: In step 2, in order to suppress the power flow between phases and offset the unbalanced power flow caused by the power grid fault while ensuring that the output active power of each phase is zero, the active power of the three phases is analyzed, and the following is obtained: At the same time, let θ A =θ B =θ C =θ, we can get: Where V ga 、V gb 、V gc are the voltage amplitudes of the three-phase power grid respectively; After simplification, the three-phase current amplitude I can be obtained a , I b , I c It can be expressed as:

7. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 6, characterized in that: In the step 2, when the three-phase current amplitude of the three-phase four-wire cascade STATCOM satisfies the expression in (16), the active power constraint condition is achieved; At this time, the three-phase current I a , I b , I c All are related to the single variable θ, and each component is represented by θ. To mathematically model the system, the voltage support problem of the model is equivalent to: st can be expressed as the condition that needs to be satisfied for the inequality to hold under this condition, where I max It is the maximum current amplitude allowed to pass after three-phase current limiting.

8. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 7, characterized in that: In step 2, based on (19), since: Simplifying, we get: Where V ga 、V gb 、V gc are the three-phase grid voltage amplitudes respectively. Similarly, the positive and negative zero-sequence current amplitude expressions can be obtained: It can be deduced that the positive and negative zero-sequence voltage amplitudes can be expressed as: T A0 , T A1 , T A2 , T B0 , T B1 , T B2 , T c0 , T c1 , T c2 They are process quantities, have no actual physical meaning and are only for the convenience of calculation.

9. The voltage support control method of a three-phase four-wire cascaded STATCOM considering power constraints according to claim 7, characterized in that: In step 2, for f(θ) in equation (19), which is a function with a period of 2π, the function value change is analyzed in the interval [0, 2π] to determine that the minimum value of the function is always at π / 2+θ L and 3π / 2+θ L Get it from Then if θ=3π / 2+θ L When the current limiting condition is met, the optimal value is 3π / 2+θ L ; And when θ=3π / 2+θ L The current limiting condition is not met when θ is in the range of [3π / 2,3π / 2+θ L ], so in [3π / 2,3π / 2+θ L ] must exist θ=θ x The maximum amplitude of each phase current is I max , and the unbalance degree of the power grid is the smallest at this point, that is: max{I a ,I b ,I c }=I max (26) Simplify formula (18) and let: Then at this time: After determining the value of θ at the optimal support, the injected current i can be determined by (27): a ,i b ,i c The amplitude and phase of the active power are: a ,i b ,i c Only the voltage v at PCC pcc Reactive power is emitted, so the i obtained at this time a ,i b ,i c That is, the reactive current i required to be injected for voltage support at the PCC a q ,i b q ,i c q .

10. The voltage support control method of three-phase four-wire cascaded STATCOM considering power constraints according to claim 7, characterized in that: In step 3, in order to balance the switching loss and restore the capacitor voltage drop caused by the unbalanced power grid, a phase perpendicular to i is injected into each phase. a q ,i b q ,i c q The active current i a p ,i b p ,i c p , implemented by adding a DC capacitor voltage loop.

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