Cascaded H-bridge grid-connected converter voltage balancing method based on reactive current injection

Abstract

The invention provides a cascaded H-bridge grid-connected converter voltage balancing method based on reactive current injection, and belongs to the technical field of power electronics. The voltage balancing method comprises the steps of sampling, reactive current injection control, global voltage control, current inner loop control and active vector superposition control. According to the method, reactive current decoupled from global voltage control is injected in a self-adaptive manner through closed-loop control, the active regulation quantity acting on each H-bridge unit by active vector superposition control under different voltage deviations is increased, and it is ensured that the direct-current side capacitor voltage of each H-bridge unit can be stabilized within an allowable voltage deviation range; and meanwhile, active vector superposition is utilized to control the output increase of the integrator, and the modulation signal correction amount is increased as much as possible on the premise of satisfying linear modulation, so that the demand on reactive current is reduced, the influence of the reactive current on a power grid is reduced, and the method has certain feasibility.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics, and in particular relates to a voltage balancing method for a cascaded H-bridge converter based on reactive current injection. Background Art

[0002] The cascaded H-bridge converter has a modular structure, is easy to expand, and has good redundancy. At the same time, compared with the two-level converter, the cascaded H-bridge converter has a lower harmonic content in the output voltage waveform, and can use a lower switching frequency and a smaller filter inductor, saving the cost and volume of the filter. Therefore, it is widely used in medium-voltage direct-mounted power quality management devices, such as static VAR compensators (STATCOMs), active power filters (APFs) and other systems.

[0003] The voltage balance of the DC side capacitors of each H-bridge unit of the cascaded H-bridge grid-connected converter is the basis for supporting the stable operation of the system. However, the DC side capacitors of each H-bridge unit are independent of each other, and the active power loss of each H-bridge unit is different, so additional software and hardware measures are required to maintain the balance of the DC side capacitor voltage. Most hardware voltage balancing measures are achieved by adding an external DC power supply or connecting a large resistor in parallel at both ends of the DC side capacitor, which will greatly increase the equipment cost. The mainstream software voltage balancing method is achieved through multi-layer voltage balancing control including global voltage control and active vector superposition control.

[0004] However, under no-load conditions, due to the small amplitude of the output current, the active power regulation amount that can be achieved by the phase voltage control based on active vector superposition is difficult to compensate for the difference in active power loss between the H-bridge units, resulting in the gradual divergence of the DC side capacitor voltage of each H-bridge unit, causing the H-bridge unit to be over-voltage and damaged and the system to become unstable. At present, there are existing literatures that study the DC voltage balancing problem of cascaded H-bridge grid-connected converters under no-load conditions, such as:

[0005] In the literature “Z. Yang et al, Individual DC voltage balancing method at zero current mode for cascaded H-bridge based static synchronous compensator, 2017 IEEE Applied Power Electronics Conference and Exposition (APEC), Tampa, FL, USA, 2017, pp. 1511-1517, doi: 10.1109 / APEC.2017.7930899”, a DC voltage balancing module is designed in the control driving circuit of the cascaded H-bridge unit to adaptively adjust the power loss of each H-bridge unit. However, this voltage balancing method requires the addition of additional hardware circuits, which increases the equipment cost. Moreover, for the built cascade H-bridge grid-connected converter equipment, hardware circuit transformation will increase more costs.

[0006] The literature "Wu Liran, Wu Mingli, Analysis of DC voltage balancing control principle of cascaded H-bridge converter based on modulation wave reconstruction [J]. Transactions of China Electrotechnical Society, 2017, 32(22): 1-12. DOI: 10.19595 / j.cnki.1000-6753.tces.160495" proposed to construct the modulation signal correction value with the unitized converter output current under no-load conditions. However, under no-load conditions, the amplitude of the converter output current is small, the detection accuracy is limited, and it is easily interfered by sampling noise, making it difficult to quantify the amplitude.

[0007] In summary, the existing solutions still have the following disadvantages:

[0008] 1) In order to achieve voltage balance of the cascaded H-bridge converter under no-load conditions, the control drive circuit needs to be modified, which increases hardware cost and complexity;

[0009] 2) Under no-load conditions, the output current amplitude of the cascaded H-bridge converter is small and easily interfered by sampling noise, making it difficult to quantify the current amplitude to achieve unitization of the AC quantity. Summary of the invention

[0010] The technical problem to be solved by the present invention is that the DC side capacitor voltage diverges due to the fact that the output current amplitude is too small and the active power adjustment amount of the control output is difficult to make up for the difference in active power loss between the cascaded H-bridge units under no-load conditions in the traditional double-layer voltage balancing method. The purpose is to provide a voltage balancing method for a cascaded H-bridge grid-connected converter based on reactive current injection, which generates a reactive current command value by using a closed-loop proportional-integral control, increases the active power adjustment amount that can be acted on by the active vector superposition control, and enables the active power adjustment amount under no-load conditions to make up for the difference in active power loss between the cascaded H-bridge units, thereby meeting the need to maintain the voltage balance of the DC side capacitors between the cascaded H-bridge units. At the same time, by appropriately increasing the integral coefficient of the active vector control, increasing the integrator output under the same voltage deviation, and limiting and latching the proportional-integral regulator output of the active vector control, the modulation signal adjustment amount output by the active vector control is maintained at the maximum value set to ensure linear modulation, thereby reducing the reactive current injected into the power grid.

[0011] In order to solve the technical problem of the present invention, the present invention provides a voltage equalization method of a cascaded H-bridge grid-connected converter based on reactive current injection, wherein the circuit topology structure involved in the voltage equalization method includes a cascaded H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase grid-connected circuit breaker KM1 and a three-phase power grid; the cascaded H-bridge grid-connected converter is a three-phase cascaded H-bridge grid-connected converter connected in a star shape, wherein each phase of the cascaded H-bridge grid-connected converter includes N cascaded H-bridge units, and a capacitor is connected in parallel to the front end of each H-bridge unit, and the front end of each capacitor is respectively connected in parallel to a parallel loss equivalent resistor; the output end of the three-phase cascaded H-bridge grid-connected converter is sequentially connected in series with the three-phase filter inductor L and the three-phase grid-connected circuit breaker KM1 and then connected to the three-phase power grid;

[0012] The voltage balancing method can achieve intra-phase voltage balancing of a cascaded H-bridge grid-connected converter under no-load conditions, and comprises the following steps:

[0013] Step 1: Sampling and calculation of capacitor voltage

[0014] The capacitor voltages of the 3N H-bridge units are sampled in real time, and the sampled data is recorded as a DC voltage u dcx_k , where x is the phase sequence, x=a, b, c; k is the sequence number of each H-bridge unit in each phase cascaded H-bridge grid-connected converter, k=1, 2, ..., N;

[0015] Calculation of N DC voltages u in an x-phase cascaded H-bridge grid-connected converter dcx_k The sum is recorded as the total DC voltage u of phase x dcx_all , and then calculate the N DC voltages u dcx_k The average value is recorded as the average value of the x-phase DC voltage u dcx_av , 3N DC voltages u are calculateddcx_k The sum of the squares of the DC voltage is u dc_s ;

[0016] Step 2: White adaptive reactive current injection control

[0017] At the moment when the cascaded H-bridge grid-connected converter enters the no-load condition, each DC voltage u dcx_k The average value of the DC voltage u of phase x dcx_av The calculation of the deviation between the two phases gives the x-phase DC voltage dispersion δU dcx Then the three x-phase DC voltage discrete degrees δU dcx Take the average value to get the total DC voltage dispersion δU dc ;

[0018] The total DC voltage dispersion δU dc With the given reference value The error is input to the proportional regulator of the adaptive reactive current injection control, and then the lower limit is 0 and the upper limit is 0.1I N The limiter output is the q-axis reactive current reference value i qref , Among them, I N is the x-phase rated current of the cascaded H-bridge grid-connected converter, k pv0 A proportionality factor for a proportional regulator of an adaptive reactive current injection control;

[0019] Step 3: Global Voltage Control

[0020] The sum of the square of the DC voltage u dc_s With the given reference value After the difference is made and normalized, it is input to the proportional-integral regulator of the global voltage control, and the output is the d-axis active current reference value i dref ;

[0021] Step 4: Current inner loop control

[0022] First, the current of the three-phase filter inductor L is sampled in real time to obtain the three-phase output current i a ,i b ,i c And the output current dq axis component i in the dq coordinate system is obtained by coordinate transformation d ,i q ; Real-time sampling of the three-phase grid-connected voltage is performed to obtain the three-phase grid-connected voltage u sa ,u sb ,u sc And the grid voltage dq axis component u in the dq coordinate system is obtained by coordinate transformation sd ,u sq ;

[0023] Secondly, the q-axis reactive current reference value i qref , d-axis active current reference value i dref , output current dq axis component i d ,i q The input is sent to the proportional-integral regulator of the current inner loop, and the dq-axis component u of the grid-connected voltage is sd ,u sq Perform dq axis decoupling to obtain dq axis modulation signal m d ,m q ;

[0024] The dq axis modulation signal m d ,m q After coordinate inverse transformation and normalization, the initial modulation signal m of each H-bridge unit in the three-phase stationary coordinate system is obtained. x_k0 ;

[0025] Step 5: Active vector superposition control,

[0026] Step 5.1, record any H-bridge unit from the 1st to the N-1th H-bridge unit in phase x as H-bridge unit k1, and record the DC voltage u corresponding to the 1st to the N-1th H-bridge unit in phase x dcx_k Renamed as H-bridge unit k1 corresponds to DC voltage u dcx_k1 , k1=1, 2, ..., N-1; calculate the DC voltage u corresponding to N-1 H-bridge units k1 respectively dcx_k1 The average value of the DC voltage u of phase x dcx_av The deviation is input into the proportional-integral regulator of active vector superposition control;

[0027] When the no-load state is reached, the integral coefficient k of the proportional integral regulator of the active vector superposition control is iv2 From the initial value k iv2_0 Linear growth to the set value k iv2_ref , the growth amount of each control step is Δk iv2 , Δk iv2 =(k iv2_ref -k iv2_0 ) / (T ramp / T s ), T ramp is the growth time of the proportional coefficient of the proportional-integral regulator of the active vector superposition control, T s To control the cycle;

[0028] According to the output of the proportional-integral regulator of the active vector superposition control, the modulation signal correction amplitude ΔM of the N-1 H-bridge units k1 in the x phase is obtained. x_k1 , then for N-1 ΔM x_k1The sum is negative to obtain the modulation signal correction amplitude ΔM of the Nth H-bridge unit in phase x. x_N ;

[0029] The amplitude of the modulation signal correction of any H-bridge unit in phase x is recorded as ΔM x_k ;

[0030] Step 5.2: Correct the amplitude of the modulation signal of the N H-bridge units in phase x by ΔM x_k Respectively with the set value ΔM x_max For comparison: When the amplitude of the modulation signal correction value ΔM of any H-bridge unit x_k Equal to the set value ΔM x_max When the output of the proportional integral regulator of the active vector superposition control of the first N-1 H-bridge units is latched; when the amplitude of the modulation signal correction value ΔM of the N H-bridge units in the x phase x_k Both are less than the set value ΔM x_max When , release the latch;

[0031] The modulation signal correction amount of any H-bridge unit in phase x is recorded as Δm x_k , is the q-axis reactive current reference value i qref Unit alternating current with the same phase, Where t is any time in the operation of the cascaded H-bridge, ω1 is the fundamental frequency of the grid voltage;

[0032] The final modulation signal of any H-bridge unit in phase x is denoted as m x_k , m x_k =Δm x_k +m x_k0 .

[0033] Preferably, the total DC voltage u of the x phase in step 1 is dcx_all , the x-phase DC voltage average value u dcx_av and the DC voltage square sum u dc_s The calculation formulas are:

[0034]

[0035] Preferably, the x-phase DC voltage dispersion degree δU in step 2 is dcx and the total DC voltage dispersion δU dc The expressions are:

[0036]

[0037] Preferably, the d-axis active current reference value i in step 3 is dref The expression is:

[0038]

[0039] Among them, k pv1 and k iv1 are the proportional coefficient and integral coefficient of the proportional-integral regulator of the global voltage control, and s is the Laplace operator.

[0040] Preferably, the dq axis modulation signal m in step 4 d , m q The decoupling formula is:

[0041]

[0042] Among them, k pc k is the proportional coefficient of the proportional-integral regulator of the current inner loop, ic is the integral coefficient of the proportional-integral regulator of the inner current loop, s is the Laplace operator, and Lf is the inductance value of the three-phase filter inductor L;

[0043] The initial modulation signal m x_k0 The calculation formula is:

[0044]

[0045] Preferably, the amplitude of the modulation signal correction value ΔM of the N-1 H-bridge units k1 in step 5 is x_k1 and the modulation signal correction amplitude ΔM of the Nth H-bridge unit in phase x x_N The calculation formulas are:

[0046]

[0047] Among them, k pv2 is the proportional coefficient of the proportional-integral regulator of the active vector superposition control, k iv2 is the integral coefficient of the proportional-integral regulator of the active vector superposition control, and s is the Laplace operator.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] 1. The present invention does not require the addition of additional hardware equipment or modification of hardware circuits, which greatly saves system costs;

[0050] 2. The present invention can directly obtain the unit AC quantity in active vector control according to the phase of the injected reactive current, without detecting and calculating the output current amplitude, thus avoiding the interference of low-order harmonics in the output current on the modulation system, thereby effectively expanding the effective parameter domain of the proportional-integral regulator in active vector control.

[0051] 3. The present invention adaptively injects reactive current according to the discrete degree of the DC side capacitor voltage, which can effectively increase the active power adjustment amount of each H-bridge unit under different voltage discrete degrees, and ensure that the DC side capacitor voltage can be stabilized within the allowable voltage deviation range.

[0052] 4. The present invention increases the integral coefficient in the active vector superposition control under no-load conditions and utilizes the limiting function of the limiter, so that the modulation signal correction amount output by the active vector control can be accelerated to reach and stabilize at the expected value of the modulation signal correction amount set by the linear modulation, thereby reducing the reactive current required to maintain voltage stability under the same voltage deviation, thereby reducing the impact of the reactive current on the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 4 is a circuit topology diagram of a cascaded H-bridge grid-connected converter in an embodiment of the present invention.

[0054] Figure 2 4 is a control block diagram of a cascaded H-bridge grid-connected converter in an embodiment of the present invention.

[0055] Figure 3 The DC voltage waveform of each H-bridge unit of phase A using the traditional double-layer voltage balancing method.

[0056] Figure 4 The figure is a DC voltage waveform diagram of each H-bridge unit of phase A using the voltage balancing method of the present invention.

[0057] Figure 5 The waveform diagram is of the modulation signal correction amount of each H-bridge unit of phase A, the q-axis reactive current reference value and the q-axis component of the output current using the voltage balancing method of the present invention.

[0058] Figure 6 The figure is a flow chart of the voltage balancing method of the present invention. DETAILED DESCRIPTION

[0059] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0060] Figure 1 FIG. 1 is a circuit topology diagram of a cascaded H-bridge grid-connected converter in an embodiment of the present invention. Figure 1It can be seen that the circuit topology involved in the voltage equalization method includes a cascaded H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase grid-connected circuit breaker KM1 and a three-phase power grid; the cascaded H-bridge grid-connected converter is a three-phase cascaded H-bridge grid-connected converter connected in a star shape, wherein each phase of the cascaded H-bridge grid-connected converter includes N cascaded H-bridge units, and a capacitor is connected in parallel to the front end of each H-bridge unit, and the front end of each capacitor is respectively connected in parallel to a parallel loss equivalent resistor; the output end of the three-phase cascaded H-bridge grid-connected converter is connected in series with the three-phase filter inductor L and the three-phase grid-connected circuit breaker KM1 in sequence and then connected to the three-phase power grid.

[0061] exist Figure 1 Upper, C x_k is the capacitance, R px_k is the parallel loss equivalent resistance.

[0062] In this embodiment, N=6, that is, the cascaded H-bridge grid-connected converter is a three-phase grid-connected converter, and each phase includes 6 H-bridge units; C x_k =0.002F, that is, each H-bridge unit of the cascaded H-bridge grid-connected converter is connected in parallel with a capacitor with a capacitance of 0.002F. px_1 =11000Ω, other R px_k =22000Ω, that is, the DC side capacitor of the first H-bridge unit of each phase of the cascaded H-bridge converter is connected in parallel with a resistor with a resistance of 11000Ω, and the parallel resistance serves as the parallel loss equivalent resistance of the H-bridge unit. The parallel loss equivalent resistance of the DC side capacitors of the remaining H-bridge units of each phase of the cascaded H-bridge converter is 22000Ω; the difference in active loss between different H-bridge units is simulated by the difference in parallel resistance of the DC side capacitors, that is, the active loss of the first H-bridge unit of each phase is greater than that of other H-bridge units of the phase.

[0063] Figure 6 is a flow chart of the voltage balancing method of the present invention, Figure 2 The control block diagram of the cascaded H-bridge grid-connected converter in the embodiment of the present invention is shown in the two figures. It can be seen from the two figures that the voltage balancing method of the present invention includes four parts: reactive current injection control, global voltage control, current inner loop control, and active vector superposition control. Specifically, the present invention provides a cascaded H-bridge grid-connected converter voltage balancing method based on reactive current injection, and the voltage balancing method can achieve intra-phase voltage balancing of the cascaded H-bridge grid-connected converter under no-load conditions, and includes the following steps:

[0064] Step 1: Sampling and calculation of capacitor voltage

[0065] The capacitor voltages of the 3N H-bridge units are sampled in real time, and the sampled data is recorded as a DC voltage u dcx_k, where x is the phase sequence, x=a, b, c; k is the sequence number of each H-bridge unit in each phase cascaded H-bridge grid-connected converter, k=1, 2, ..., N;

[0066] Calculation of N DC voltages u in an x-phase cascaded H-bridge grid-connected converter dcx_k The sum is recorded as the total DC voltage u of phase x dcx_all , and then calculate the N DC voltages u dcx_k The average value is recorded as the average value of the x-phase DC voltage u dcx_av , 3N DC voltages u are calculated dcx_k The sum of the squares of the DC voltage is u dc_s .

[0067] In this embodiment, the x-phase total DC voltage u dcx_all , the x-phase DC voltage average value u dcx_av and the DC voltage square sum u dc_s The calculation formulas are:

[0068]

[0069] Step 2: White adaptive reactive current injection control

[0070] At the moment when the cascaded H-bridge grid-connected converter enters the no-load condition, each DC voltage u dcx_k The average value of the DC voltage u of phase x dcx_av The calculation of the deviation between the two phases gives the x-phase DC voltage dispersion δU dcx Then the three x-phase DC voltage discrete degrees δU dcx Take the average value to get the total DC voltage dispersion δU dc ;

[0071] The total DC voltage dispersion δU dc With the given reference value The error is input to the proportional regulator of the adaptive reactive current injection control, and then the lower limit is 0 and the upper limit is 0.1I N The limiter output is the q-axis reactive current reference value i qref , Where, IN is the x-phase rated current of the cascaded H-bridge grid-connected converter, k pv0 Scaling factor for the proportional regulator of the adaptive reactive current injection control.

[0072] In this embodiment, the x-phase DC voltage dispersion degree δU dcx and the total DC voltage dispersion δU dc The expressions are:

[0073]

[0074] Step 3: Global Voltage Control

[0075] The sum of the square of the DC voltage u dc_s With the given reference value After the difference is made and normalized, it is input to the proportional-integral regulator of the global voltage control, and the output is the d-axis active current reference value i dref ;

[0076] In this embodiment, the d-axis active current reference value i in step 3 is dref The expression is:

[0077]

[0078] Among them, k pv1 and k iv1 are the proportional coefficient and integral coefficient of the proportional-integral regulator of the global voltage control, and s is the Laplace operator.

[0079] Step 4: Current inner loop control

[0080] First, the current of the three-phase filter inductor L is sampled in real time to obtain the three-phase output current i a ,i b ,i c And the output current dq axis component i in the dq coordinate system is obtained by coordinate transformation d ,i q ; Real-time sampling of the three-phase grid-connected voltage is performed to obtain the three-phase grid-connected voltage u sa ,u sb ,u sc And the grid voltage dq axis component u in the dq coordinate system is obtained by coordinate transformation sd ,u sq ;

[0081] Secondly, the q-axis reactive current reference value i qref , d-axis active current reference value i dref , output current dq axis component i d ,i q The input is sent to the proportional-integral regulator of the current inner loop, and the dq-axis component u of the grid-connected voltage is sd ,u sq Perform dq axis decoupling to obtain dq axis modulation signal m d , m q .

[0082] The dq axis modulation signal m d , m q After coordinate inverse transformation and normalization, the initial modulation signal m of each H-bridge unit in the three-phase stationary coordinate system is obtained.x_k0 .

[0083] In this embodiment, the dq axis modulation signal m in step 4 d ,m q The decoupling formula is:

[0084]

[0085] Among them, k pc k is the proportional coefficient of the proportional-integral regulator of the current inner loop, ic is the integral coefficient of the proportional-integral regulator of the inner current loop, s is the Laplace operator, and Lf is the inductance value of the three-phase filter inductor L;

[0086] The initial modulation signal m x_k0 The calculation formula is:

[0087]

[0088] Step 5: Active power vector superposition control

[0089] Step 5.1, record any H-bridge unit from the 1st to the N-1th H-bridge unit in phase x as H-bridge unit k1, and record the DC voltage u corresponding to the 1st to the N-1th H-bridge unit in phase x dcx_k Renamed as H-bridge unit k1 corresponds to DC voltage u dcx_k1 , k1=1, 2, ..., N-1; calculate the DC voltage u corresponding to N-1 H-bridge units k1 respectively dcx_k1 The average value of the DC voltage u of phase x dcx_av The deviation is input into the proportional-integral regulator of active vector superposition control;

[0090] When the no-load state is reached, the integral coefficient k of the proportional integral regulator of the active vector superposition control is iv2 From the initial value k iv2_0 Linear growth to the set value k iv2_ref , the growth amount of each control step is Δk iv2 , Δk iv2 =(k iv2_ref -k iv2_0 ) / (T ramp / T s ), T ramp is the growth time of the proportional coefficient of the proportional-integral regulator of the active vector superposition control, T s To control the cycle;

[0091] According to the output of the proportional-integral regulator of the active vector superposition control, the modulation signal correction amplitude ΔM of the N-1 H-bridge units k1 in the x phase is obtained. x_k1 , then for N-1 ΔMx_k1 The sum is negative to obtain the modulation signal correction amplitude ΔM of the Nth H-bridge unit in phase x. x_N ;

[0092] The amplitude of the modulation signal correction of any H-bridge unit in phase x is recorded as ΔM x_k ;

[0093] Step 5.2: Correct the amplitude of the modulation signal of the N H-bridge units in phase x by ΔM x_k Respectively with the set value ΔM x_max For comparison: When the amplitude of the modulation signal correction value ΔM of any H-bridge unit x_k Equal to the set value ΔM x_max When the output of the proportional integral regulator of the active vector superposition control of the first N-1 H-bridge units is latched; when the amplitude of the modulation signal correction value ΔM of the N H-bridge units in the x phase x_k Both are less than the set value ΔM x_max When , release the latch;

[0094] The modulation signal correction amount of any H-bridge unit in phase x is recorded as Δm x_k , is the q-axis reactive current reference value i qref Unit alternating current with the same phase, Where t is any time in the operation of the cascaded H-bridge, ω1 is the fundamental frequency of the grid voltage;

[0095] The final modulation signal of any H-bridge unit in phase x is denoted as m x_k , m x_k =Δm x_k +m x_k0 .

[0096] In this embodiment, the modulation signal correction amplitude ΔM of the N-1 H-bridge units k1 in step 5 is x_k1 and the modulation signal correction amplitude vM of the Nth H-bridge unit in phase x x_N The calculation formulas are:

[0097]

[0098] Among them, k pv2 is the proportional coefficient of the proportional-integral regulator of the active vector superposition control, k iv2 is the integral coefficient of the proportional-integral regulator of the active vector superposition control, and s is the Laplace operator.

[0099] In this example, k pv0 =3000,I N =80A,u dc_ref=1500V, k pv1 =3693.7, k iv1 =87.06, k pc =125.65, k ic =3307.2, L f =0.02H,ki v2_0 =4, k iv2_ref =12, T ramp =10, T s =0.0001, k pv2 =6,ΔM x_max =0.15,ω1=100π.

[0100] It can be seen from the above steps that the voltage balancing method adaptively injects reactive current decoupled from the global voltage control through closed-loop control, thereby increasing the active regulation amount of active vector superposition control acting on each H-bridge unit under different voltage deviations, and ensuring that the DC side capacitor voltage of each H-bridge unit can be stabilized within the allowable voltage deviation range.

[0101] In order to demonstrate the beneficial effects of the present invention, simulation was performed.

[0102] Figure 3 The DC voltage waveform of each H-bridge unit of phase A using the traditional double-layer voltage balancing method is shown in Figure 2. Figure 4 The DC voltage waveform of each H-bridge unit of phase A using the voltage balancing method of the present invention is: Figure 5 The present invention provides a waveform of a modulation signal correction amount of each H-bridge unit of phase A, a waveform of a q-axis reactive current reference value, and a waveform of a q-axis component of an output current using the voltage balancing method of the present invention.

[0103] At 0.27 seconds, the startup process of the cascaded H-bridge grid-connected converter is completed, the DC side capacitor voltage of each H-bridge unit is pre-charged to a reference value of 1500V, and begins to enter a no-load state.

[0104] Depend on Figure 3 It can be seen that the A phase DC voltage u using the traditional double-layer voltage balancing method dca_k After entering the no-load state at 0.27 seconds, it begins to diverge. After 3.32 seconds, the DC voltage u of the first H-bridge unit of phase A is dca_1 The deviation from the reference value of 1500V exceeds 5% and continues to drop; the DC voltage u of the 5th and 6th H-bridge units of phase A dca_5 、u dca_6 Continue to rise.

[0105] Depend on Figure 4 It can be seen that the reactive current injection control is started at the same time when the no-load state is entered at 0.27 seconds, and the DC voltage u of each H-bridge unit of phase A is dca_kThe divergence trend in the no-load state is quickly suppressed, gradually converges, and finally stabilizes near the reference value of 1500V. The maximum voltage deviation in each H-bridge unit is only about 1V.

[0106] Depend on Figure 5 It can be seen that with the active vector superposition control output A phase modulation signal correction Δm a_k The increase of the q-axis reactive current reference value i required to maintain the DC voltage balance qref Decrease; at about 17 seconds, the modulation signal correction value Δm of the first H-bridge unit of phase A a_1 Reach the set limit value Δm a_max When the modulation signal correction of each H-bridge unit maintains the output of this beat, the q-axis reactive current reference value i qref It enters the steady state and reaches the minimum value, stabilizing at about 0.75A; at the same time, the output current q-axis component i q Always track the upper q-axis reactive current reference value i qref .

[0107] In summary, the present invention adaptively injects q-axis reactive current decoupled from the global voltage control through closed-loop control, thereby increasing the active regulation amount of active vector superposition control on each H-bridge unit under different voltage deviations, and ensuring that the DC side capacitor voltage of each H-bridge unit can be stabilized within the allowable voltage deviation range; at the same time, the growth of the active vector superposition control integrator output is used to increase the modulation signal correction amount as much as possible under the premise of satisfying linear modulation, thereby reducing the demand for reactive current and reducing the impact of reactive current on the power grid, which has certain feasibility.

Claims

1. A voltage equalization method for a cascaded H-bridge grid-connected converter based on reactive current injection, wherein the circuit topology structure involved in the voltage equalization method includes a cascaded H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase grid-connected circuit breaker KM1 and a three-phase power grid; the cascaded H-bridge grid-connected converter is a three-phase cascaded H-bridge grid-connected converter connected in star shape, wherein: Each phase cascaded H-bridge grid-connected converter includes N cascaded H-bridge units, and a capacitor is connected in parallel to the front end of each H-bridge unit, and the front end of each capacitor is connected in parallel to a parallel loss equivalent resistor; the output end of the three-phase cascaded H-bridge grid-connected converter is connected in series with the three-phase filter inductor L and the three-phase grid-connected circuit breaker KM1 in sequence and then connected to the three-phase power grid; The voltage balancing method is characterized in that the voltage balancing method can achieve intra-phase voltage balancing of the cascaded H-bridge grid-connected converter under no-load conditions, and comprises the following steps: Step 1: Sampling and calculation of capacitor voltage The capacitor voltages of the 3N H-bridge units are sampled in real time, and the sampled data is recorded as a DC voltage u dcx_k , where x is the phase sequence, x=a, b, c; k is the sequence number of each H-bridge unit in each phase cascaded H-bridge grid-connected converter, k=1, 2, ..., N; Calculation of N DC voltages u in an x-phase cascaded H-bridge grid-connected converter dcx_k The sum is recorded as the total DC voltage u of phase x dcx_all , and then calculate the N DC voltages u dcx_k The average value is recorded as the average value of the x-phase DC voltage u dcx_av , 3N DC voltages u are calculated dcx_k The sum of the squares of the DC voltage is u dc_s ; Step 2: Adaptive reactive current injection control At the moment when the cascaded H-bridge grid-connected converter enters the no-load condition, each DC voltage u dcx_k The average value of the DC voltage u of phase x dcx_av The calculation of the deviation between the two phases gives the x-phase DC voltage dispersion δU dcx Then the three x-phase DC voltage discrete degrees δU dcx Take the average value to get the total DC voltage dispersion δU dc ; The total DC voltage dispersion δU dc With the given reference value The error is input to the proportional regulator of the adaptive reactive current injection control, and then the lower limit is 0 and the upper limit is 0.1I N The limiter output is the q-axis reactive current reference value i qref , Among them, I N is the x-phase rated current of the cascaded H-bridge grid-connected converter, k pv0 A proportionality factor for a proportional regulator of an adaptive reactive current injection control; Step 3: Global voltage control The square sum of the DC voltage u dc_s With the given reference value After the difference is made and normalized, it is input to the proportional-integral regulator of the global voltage control, and the output is the d-axis active current reference value i dref ; Step 4: Current inner loop control First, the current of the three-phase filter inductor L is sampled in real time to obtain the three-phase output current i a ,i b ,i c And the output current dq axis component i in the dq coordinate system is obtained by coordinate transformation d ,i q ; Real-time sampling of the three-phase grid-connected voltage is performed to obtain the three-phase grid-connected voltage u sa ,u sb ,u sc And the grid voltage dq axis component u in the dq coordinate system is obtained by coordinate transformation sd ,u sq ; Secondly, the q-axis reactive current reference value i qref , d-axis active current reference value i dref , output current dq axis component i d ,i q The input is sent to the proportional-integral regulator of the current inner loop, and the dq-axis component u of the grid-connected voltage is sd ,u sq Perform dq axis decoupling to obtain dq axis modulation signal m d ,m q ; The dq axis modulation signal m d , m q After coordinate inverse transformation and normalization, the initial modulation signal m of each H-bridge unit in the three-phase stationary coordinate system is obtained. x_k0 ; Step 5: Active vector superposition control, Step 5.1, record any H-bridge unit from the 1st to the N-1th H-bridge unit in phase x as H-bridge unit k1, and record the DC voltage u corresponding to the 1st to the N-1th H-bridge unit in phase x dcx_k Renamed as H-bridge unit k1 corresponds to DC voltage u dcx_k1 , k1=1, 2, ..., N-1; calculate the DC voltage u corresponding to N-1 H-bridge units k1 respectively dcx_k1 The average value of the DC voltage u of phase x dcx_av The deviation is input into the proportional-integral regulator of active vector superposition control; When the no-load state is reached, the integral coefficient k of the proportional integral regulator of the active vector superposition control is iv2 From the initial value k iv2_0 Linear growth to the set value k iv2_ref , the growth amount of each control step is Δk iv2 , Δk iv2 =(k iv2_ref -k iv2_0 ) / (T ramp / T s ), T ramp is the growth time of the proportional coefficient of the proportional-integral regulator of the active vector superposition control, T s To control the cycle; According to the output of the proportional-integral regulator of the active vector superposition control, the modulation signal correction amplitude ΔM of the N-1 H-bridge units k1 in the x phase is obtained. x_k1 , then for N-1 ΔM x_k1 The sum is negative to obtain the modulation signal correction amplitude ΔM of the Nth H-bridge unit in phase x. x_N ; The amplitude of the modulation signal correction of any H-bridge unit in phase x is recorded as ΔM x_k ; Step 5.2: Correct the amplitude of the modulation signal of the N H-bridge units in phase x by ΔM x_k Respectively with the set value ΔM x_max For comparison: When the amplitude of the modulation signal correction value ΔM of any H-bridge unit x_k Equal to the set value ΔM x_max When the output of the proportional integral regulator of the active vector superposition control of the first N-1 H-bridge units is latched; when the amplitude of the modulation signal correction value ΔM of the N H-bridge units in the x phase x_k Both are less than the set value ΔM x_max When , release the latch; The modulation signal correction amount of any H-bridge unit in phase x is recorded as Δm x_k , is the q-axis reactive current reference value i qref Unit alternating current with the same phase, Where t is any time in the operation of the cascaded H-bridge, ω1 is the fundamental frequency of the grid voltage; The final modulation signal of any H-bridge unit in phase x is denoted as m x_k , m x_k =Δm x_k +m x_k0 .

2. The voltage balancing method of cascaded H-bridge grid-connected converter based on reactive current injection according to claim 1, characterized in that: The total DC voltage u of phase x in step 1 dcx_all , the x-phase DC voltage average value u dcx_av and the DC voltage square sum u dc_s The calculation formulas are:

3. The voltage balancing method of cascaded H-bridge grid-connected converter based on reactive current injection according to claim 1, characterized in that: Step 2: The x-phase DC voltage dispersion δU dcx and the total DC voltage dispersion δU dc The expressions are:

4. The voltage balancing method of cascaded H-bridge grid-connected converter based on reactive current injection according to claim 1, characterized in that: Step 3: The d-axis active current reference value i dref The expression is: Among them, K pv1 and k iv1 are the proportional coefficient and integral coefficient of the proportional-integral regulator of the global voltage control, and s is the Laplace operator.

5. The voltage balancing method of cascaded H-bridge grid-connected converter based on reactive current injection according to claim 1, characterized in that: Step 4: The dq axis modulation signal m d , m q The decoupling formula is: Among them, k pc k is the proportional coefficient of the proportional-integral regulator of the current inner loop, ic is the integral coefficient of the proportional-integral regulator of the current inner loop, s is the Laplace operator, L f is the inductance value of the three-phase filter inductor L; The initial modulation signal m x_k0 The calculation formula is:

6. The voltage balancing method of cascaded H-bridge grid-connected converter based on reactive current injection according to claim 1, characterized in that: Step 5: The modulated signal correction amplitude ΔM of the N-1 H-bridge units k1 x_k1 and the modulation signal correction amplitude ΔM of the Nth H-bridge unit in phase x x_N The calculation formulas are: Among them, k pv2 is the proportional coefficient of the proportional-integral regulator of the active vector superposition control, k iv2 is the integral coefficient of the proportional-integral regulator of the active vector superposition control, and s is the Laplace operator.

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