Wind power converter parallel system based on DPWM strategy and its zero-sequence circulating current suppression method

Abstract

The present invention discloses a wind power converter parallel system based on a DPWM strategy and a method for suppressing zero-sequence circulating current therefrom. The zero-sequence circulating current suppression method employs a hybrid modulation strategy of a first DPWM modulation strategy and a second DPWM modulation strategy for a first converter, wherein the two DPWM modulation strategies complement each other and are switched using hysteresis control. The first DPWM modulation strategy is employed for the second converter. The present invention controls the zero-sequence circulating current of a converter and employs hysteresis control to switch between the complementary hybrid DPWM modulation strategies. This effectively suppresses zero-sequence circulating current while reducing switching losses. This solves the problem of existing circulating current suppression methods significantly deteriorating current harmonic performance and circulating current suppression effectiveness at the lower switching frequencies required by medium-voltage, high-power wind power converters.

Description

Technical Field

[0001] The present invention relates to a wind power converter, in particular to a wind power converter parallel system based on a DPWM strategy and a zero-sequence circulating current suppression method thereof. Background Art

[0002] The continuous increase in the capacity of individual wind turbines is a key development trend. As the energy conversion interface between wind turbines and the power grid, the capacity of wind power converters also needs to be increased accordingly. In high-power wind power converter applications, three-level NPC converter topologies are often used in parallel to increase converter capacity. In practical applications, due to inconsistent hardware parameters or output voltages of parallel converters, zero-sequence circulating currents can occur between parallel systems. This can increase converter operating losses, reduce converter efficiency, generate current distortion, and affect the service life of switching components. In severe cases, it can even compromise system safety and operator safety. Therefore, research on circulating current suppression methods for parallel systems is necessary. In medium-voltage, high-power applications, switching component losses are a significant component of system losses. To improve system efficiency, it is often necessary to reduce switching losses by reducing the switching frequency. However, simply reducing the switching frequency can result in poor output current harmonic performance and even render the system inoperable. Therefore, it is necessary to study modulation strategies that reduce switching losses and propose corresponding circulating current suppression strategies.

[0003] Current circulating current suppression methods for parallel systems are primarily categorized into two main types: SPWM-based and SVPWM-based modulation strategies. SPWM-based methods employ a PIR controller to adjust the zero-sequence voltage injected into the modulation wave, thereby altering the converter's zero-sequence duty cycle to control zero-sequence circulating current. SVPWM-based methods, on the other hand, employ a PI controller or deadbeat controller to achieve circulating current suppression in parallel systems by adjusting the duration of redundant small vectors without changing the output voltage.

[0004] The two aforementioned methods for suppressing zero-sequence circulating current in parallel wind turbine converter systems based on SPWM and SVPWM modulation strategies are essentially the same. To ensure harmonic performance of the grid-connected current, a high switching frequency is often required. However, for medium-voltage, high-power wind turbine converters, switching losses dominate system losses. Limited by device switching losses, switching frequencies are typically low, typically less than 1 kHz. Simply reducing the switching frequency of existing modulation strategies can lead to excessive harmonic content in the output current waveform, significantly reducing the effectiveness of suppressing zero-sequence circulating current, and even preventing the converter from operating properly. Summary of the Invention

[0005] The object of the present invention is to provide a wind power converter parallel system based on a DPWM strategy and a zero-sequence circulating current suppression method thereof, which can reduce switching losses and effectively suppress zero-sequence circulating current.

[0006] To achieve the above objectives, the present invention first provides a zero-sequence circulating current suppression method for a wind power converter parallel system based on a DPWM strategy. The wind power converter parallel system includes two NPC converters connected in parallel, namely a first converter and a second converter; the first converter adopts a hybrid modulation strategy of a first DPWM modulation strategy and a second DPWM modulation strategy, the two DPWM modulation strategies are complementary, and switching is performed using a hysteresis control method; the second converter adopts the first DPWM modulation strategy.

[0007] Preferably, the specific steps of the hysteresis control are: obtaining the zero-sequence circulating current i of the first converter according to the three-phase current sampling of the converter z1 ; With the zero-sequence circulating current i of the first converter z1 As the control object, a hysteresis controller is used to suppress zero-sequence circulating current. For two parallel converters, the zero-sequence circulating currents are equal in magnitude but opposite in direction. Suppressing the zero-sequence circulating current of the first converter will naturally suppress the zero-sequence circulating current of the second converter, meaning that circulating current control only needs to be implemented on the first converter.

[0008] Furthermore, when the zero-sequence circulating current i z1 Reaching the upper threshold i zth When the zero-sequence circulating current i z1 Reaching the lower threshold-i zth When , the modulation strategy of the first converter is switched from the second DPWM modulation strategy to the first DPWM modulation strategy, thereby limiting the size of the zero-sequence circulating current within a certain range.

[0009] Furthermore, the upper threshold value i zth and lower threshold-i zth The selection should be based on the current harmonic content THD being within 5%.

[0010] Furthermore, by adjusting the zero-sequence duty cycle d of the first converter z1 Realize the control of zero-sequence circulating current.

[0011] Furthermore, in the hysteresis control, the zero-sequence duty cycle d of the first converter is controlled by switching between the two DPWM modulation strategies of the first converter. z1 The adjustment is performed to reduce the difference in zero-sequence duty cycle between the first converter and the second converter, thereby achieving the purpose of suppressing circulating current.

[0012] Preferably, the first DPWM modulation strategy and the second DPWM modulation strategy are two complementary modulation strategies among DPWMMAX, DPWMMIN, DPWM0, DPWM1, DPWM2, and DPWM3.

[0013] Furthermore, the first DPWM modulation strategy is DPWM0 or DPWM2, and the second DPWM modulation strategy is DPWM2 or DPWM0.

[0014] The present invention then provides a wind power converter parallel system based on a DPWM strategy, comprising a first converter and a second converter connected in parallel, wherein the two NPC converters are configured to execute the zero-sequence circulating current suppression method as described above.

[0015] Preferably, the two converters adopt a series inductor parallel structure, that is, the bridge arms of the parallel modules are connected in series with the inductors before being connected in parallel, which can effectively improve the circulation and current sharing problems.

[0016] Compared with the prior art, the present invention has the following beneficial effects: the three-level converter modulation strategy adopted by the present invention uses DPWM modulation, which can effectively reduce the switching loss of the system by about 1 / 3; at the same time, the zero-sequence circulating current of one of the converters is used as the control object, and the complementary hybrid DPWM modulation strategy is switched using hysteresis control, which can reduce the switching loss while effectively suppressing the zero-sequence circulating current; thereby solving the problem that the current harmonic performance and the suppression effect of the circulating current of the existing circulating current suppression method are significantly deteriorated at the lower switching frequency required in medium-voltage high-power wind power conversion sites. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the basic three-level space vector diagram provided by the present invention. In the figure, p, o, and n represent the three power levels that each phase of the three-level converter can output, and xyz (x, y, z = p, o, n) represents the 27 basic switching state combinations of the three-level converter. I-VI represent the six large sectors divided by the regular hexagonal space vector, and 1-6 represent the six small sectors divided into each large sector.

[0018] Figure 2 It is a seven-segment unified pulse modulation model of the three-level SVPWM modulation strategy provided by the present invention, in which k (-1≤k≤1) is a redundant small vector allocation factor;

[0019] Figure 3 This is a topology diagram of a three-level wind power converter parallel system provided by Example 1 of the present invention, in which e a 、e b 、e c Three-phase grid voltage, i a 、i b 、ic is the grid-connected current of the converter, V dc is the DC bus voltage, i ax 、i bx 、i cx (x=1,2) is the phase current of converter x, L x is the filter inductance of converter x, L t is the common inductor of the two converters, O1 and O2 are the midpoints of the DC side of the converters;

[0020] Figure 4 This is the hysteresis switching principle of the DPWM modulation strategy of the first converter in Example 1 of the present invention. zth and -i zth are the upper and lower thresholds of the zero-sequence circulating current.

[0021] Among them: a first converter 100 and a second converter 200. DETAILED DESCRIPTION

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Example 1

[0024] like Figures 1 to 4 As shown, this embodiment provides a wind power converter parallel system based on a DPWM strategy, including a first converter 100 and a second converter 200. The two converters adopt a series-inductor parallel structure, that is, the bridge arms of each parallel module output inductors (L1, L2) in series before being connected in parallel. The two NPC converters are configured to execute a zero-sequence circulating current suppression method based on the DPWM strategy, with the specific steps as follows:

[0025] Step 1: Implement DPWM modulation of the converter. The second converter 200 applies the first DPWM modulation strategy (DPWM0), and the first converter 100 applies a hybrid modulation strategy of the first DPWM modulation strategy and the second DPWM modulation strategy (DPWM2). This modulation strategy can effectively reduce the switching loss of the system by approximately 1 / 3. This embodiment uses DPWM0 and DPWM2 as examples. In actual applications, other complementary DPWM modulation strategies (such as DPWM1 and DPWM3, DPWM5 and DPWM6) can also be used.

[0026] Step 2: Collect the three-phase current i output by the two converters x1 ,i x2 (x=a,b,c), the zero-sequence circulating current i of the two converters is obtained according to the three-phase current sampling of the converter z1 ,i z2 ;

[0027] Step 3: Take the zero-sequence circulating current i of the first converter 100 z1 As the control object, a hysteresis controller is used to suppress the zero-sequence circulating current. z1 The absolute value of exceeds the threshold value i of the hysteresis controller zth The DPWM modulation strategy of the first converter 100 is switched in time to limit the zero-sequence circulating current of the converter parallel system to an acceptable range.

[0028] The following is a detailed analysis of each of the above steps.

[0029] In step 1, each phase of the three-level converter can output V dc / 2, 0, and –V dc / 2 three level states, so the three-phase can output 27 switching states, Figure 1 A three-level basic space vector diagram consisting of 27 switching states. Based on the length of the vector, it can be divided into four categories: large vectors (six such as pnn), medium vectors (six such as pon), small vectors (12 such as poo and onn), and zero vectors (ooo, nnn, ppp). Large and medium vectors have no redundant states, small vectors have one redundant state, and zero vectors have two redundant states.

[0030] The SVPWM modulation algorithm of the three-level converter is based on the idea of vector synthesis. After the steps of sector judgment, vector selection, basic action time calculation, time state allocation, etc., the following can be obtained: Figure 2 The three-level SVPWM seven-segment unified modulation model shown (using the I-1 region as an example) has the only control degree of freedom in the redundant vector allocation factor k. In conventional SVPWM modulation algorithms, k is always zero, resulting in a continuous pulse sequence. Consequently, each of the three phases switches once during each switching cycle. When k = 1 or -1, one phase is clamped at a certain level during each switching cycle, meaning that the switching device of one phase does not operate during each switching cycle. This reduces switching losses by approximately one-third, and the modulation strategy in this case is DPWM modulation.

[0031] Depending on the k-value clamping method, various DPWM modulation strategies suitable for three-level converters can be obtained, such as DPWMMAX, DPWMMIN, DPWM0, DPWM1, DPWM2, and DPWM3. This paper analyzes the three-level DPWM0 modulation strategy and its complementary DPWM2 modulation strategy as examples. The clamping conditions of DPWM0 and DPWM2 in each sector are shown in Table 1 below. It can be seen that the k values selected for the two modulation strategies are complementary in each sector.

[0032] Table 1 Modulation strategy k value table for each sector in Example 1

[0033]

[0034] In step 2, collect Figure 3 The three-phase current i of the first converter 100 and the second converter 200 x1 and i x2 (x=a, b, c), for a parallel system of two converters, the zero-sequence circulating currents of the first converter 100 and the second converter 200 are equal in magnitude and opposite in direction. The zero-sequence circulating current can be defined as:

[0035]

[0036] In step three, Figure 3 The topology diagram of the three-level converter parallel system is used to establish Kirchhoff's voltage equation:

[0037]

[0038] Among them, u Ai 、u Bi 、u Ci (i=1,2) is the three-phase output voltage of converter i, u oin It is the voltage between the midpoint of the DC side of converter i and the neutral point n of the three-phase power supply.

[0039] Let x in equation (2) be 1 and 2 respectively, and we can get two equations by combining and simplifying them:

[0040]

[0041] The output phase voltage u of converter i xi Expressed in duty cycle:

[0042]

[0043] Among them, d xi is the duty cycle of the xth phase of converter i, and n is defined as xi As the benchmark function, Figure 2 The unified modulation model of phase x is n when the initial state is n xi =-1, when the initial state of phase x is o, n xi =0.

[0044] The zero-sequence voltage is defined as:

[0045]

[0046] Among them, d zi (i=1,2) is the zero-sequence duty cycle of the two converters. When the reference vector is located in cells 1, 3, 5 of sectors I, III, and V and cells 2, 4, and 6 of sectors II, IV, and VI, n i=-1(i=1,2), when the reference vector is located in other areas n i =1.

[0047] According to equations (3) and (5), the mathematical model of the zero-sequence circulating current of the first converter 100 can be obtained as follows:

[0048]

[0049] For two converters connected in parallel, the zero-sequence circulating currents are equal in magnitude and opposite in direction. It is only necessary to suppress the zero-sequence circulating current of the first converter 100, and the zero-sequence circulating current of the second converter 200 will be suppressed naturally. That is, it is only necessary to implement circulating current control on the first converter 100. When controlling the circulating current, the output voltage should not be changed. Therefore, the area where the reference vector is located does not change, that is, n1, n2 and d z2 will not change, so according to formula (6), we can adjust d z1 Realize the control of zero-sequence circulating current.

[0050] exist Figure 2 In the unified modulation model, the zero-sequence duty cycle is maximum when k = 1 and minimum when k = -1. The distribution of the redundant small vector allocation factors k for DPWM0 and DPWM2 in each sector (Table 1) shows that the k values selected for the two DPWM modulation strategies in each region are always complementary. Therefore, at any time during the entire three-level vector modulation, the DPWM0 and DPWM2 modulation strategies always have opposite effects on the zero-sequence circulating current.

[0051] Therefore, for the zero-sequence circulating current suppression strategy of the first converter 100, a mixed modulation strategy of DPWM0 and DPWM2 can be adopted. Figure 4 As shown, the modulation strategy switching adopts the hysteresis control mode, by detecting the zero sequence circulating current i of the first converter 100 z1 , when the zero-sequence circulating current i z1 Reaching the upper threshold i zth When the modulation strategy is switched from DPWM0 to DPWM2, the zero-sequence circulating current is reduced to equal to or close to 0 (it may be difficult to reach 0 due to the progress of hysteresis control); when the zero-sequence circulating current i z1 Reaching the lower threshold-i zth When the modulation strategy is switched from DPWM2 to DPWM0, the zero-sequence circulating current is reduced to equal to or close to 0 again; generally, i zth and -i zth Control the zero-sequence circulating current so that the current harmonic content THD is within 5%, thereby limiting the size of the zero-sequence circulating current to a certain range (-i zth ~i zth ) within.

[0052] When switching modulation strategies, it is important to ensure that at the beginning or end of each switching cycle, the additional switching actions caused by the modulation strategy switching can be reduced. Because DPWM0 and DPWM2 can ensure that one phase of the switching device is inactive during each switching cycle, the switching losses of the entire system can be reduced by about 1 / 3.

[0053] Example 2

[0054] As described in Example 1, DPWM has different modulation modes such as DPWMMAX, DPWMMIN, DPWM0, DPWM1, DPWM2, and DPWM3. In actual applications, DPWMMAX and DPWMMIN are generally not used due to considerations such as harmonics and common-mode voltage.

[0055] This embodiment uses a mixed modulation strategy of DPWM1 and DPWM3 based on the system in Example 1, and can also achieve a similar circulating current control effect as Example 1. The two NPC converters are configured to execute a zero-sequence circulating current suppression method based on the DPWM strategy, and the specific steps are as follows:

[0056] Step 1: Implement DPWM modulation of the converter. The second converter 200 applies the first DPWM modulation strategy (DPWM1), and the first converter 100 applies a hybrid modulation strategy of the first DPWM modulation strategy and the second DPWM modulation strategy (DPWM3). This modulation strategy can effectively reduce the switching loss of the system by about 1 / 3.

[0057] Step 2: Collect the three-phase current i output by the two converters x1 ,i x2 (x=a,b,c), the zero-sequence circulating current i of the two converters is obtained according to the three-phase current sampling of the converter z1 ,i z2 ;

[0058] Step 3: Take the zero-sequence circulating current i of the first converter 100 z1 As the control object, a hysteresis controller is used to suppress the zero-sequence circulating current. z1 The absolute value of exceeds the threshold value i of the hysteresis controller zth The DPWM modulation strategy of the first converter 100 is switched in time to limit the zero-sequence circulating current of the converter parallel system to an acceptable range.

[0059] The differences between the above steps and Example 1 are analyzed in detail below.

[0060] In step 1, the difference from embodiment 1 is the k value clamping method. The clamping conditions of DPWM1 and DPWM3 in each sector are shown in Table 2 below. It can be seen that the selection of k values of the two modulation strategies in each sector are complementary.

[0061] Table 2 Modulation strategy k value table for each sector in Example 2

[0062]

[0063] In step three, Figure 2 In the unified modulation model, the zero-sequence duty cycle is maximum when k = 1 and minimum when k = -1. The distribution of the redundant small vector allocation factors k for DPWM1 and DPWM3 in each sector (Table 1) shows that the k values selected for the two DPWM modulation strategies in each region are always complementary. Therefore, at any time during the entire three-level vector modulation, the DPWM1 and DPWM3 modulation strategies always have opposite effects on the zero-sequence circulating current.

[0064] Therefore, for the zero-sequence circulating current suppression strategy of the first converter 100, a hybrid modulation strategy of DPWM1 and DPWM3 can be adopted. The modulation strategy switching adopts a hysteresis control method, by detecting the zero-sequence circulating current i z1 , when the zero-sequence circulating current i z1 Reaching the upper threshold i zth When the modulation strategy is switched from DPWM1 to DPWM3, the zero-sequence circulating current is reduced to or close to 0; when the zero-sequence circulating current i z1 Reaching the lower threshold-i zth When the modulation strategy is switched from DPWM3 to DPWM1, the zero-sequence circulating current is reduced to or close to 0 again; generally, i zth and -i zth The zero-sequence circulating current is controlled so that the current harmonic content THD is within 5%, thereby limiting the size of the zero-sequence circulating current within a certain range.

[0065] When switching modulation strategies, it is important to ensure that at the beginning or end of each switching cycle, the additional switching action caused by the modulation strategy switching can be reduced. Because DPWM1 and DPWM3 can ensure that one phase of the switching device is inactive during each switching cycle, the switching losses of the entire system can be reduced by about 1 / 3.

[0066] Explanation of related terms

[0067] DPWM: (Discontinuous Pulse Width Modulation) discontinuous pulse width modulation;

[0068] NPC converter: (Neutral-Point Clamped) neutral point clamped converter;

[0069] SPWM (Sinusoidal Pulse Width Modulation): sinusoidal pulse width modulation;

[0070] SVPWM (Space Vector Pulse Width Modulation): Space Vector Pulse Width Modulation;

[0071] PI / PIR controller: Proportional-Integral / Proportional-Integral-Resonant controller.

[0072] The above detailed description of the specific embodiments of the present application is intended only as an example, and the present application is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications or substitutions made to the present application are also within the scope of the present application. Therefore, equivalent changes, modifications, and improvements made without departing from the spirit and scope of the present application should be included within the scope of the present application.

Claims

1. A method for suppressing zero-sequence circulating current in a wind power converter parallel system based on a DPWM strategy, characterized by: The wind power converter parallel system comprises two NPC converters connected in parallel, namely a first converter (100) and a second converter (200); the first converter (100) adopts a mixed modulation strategy of a first DPWM modulation strategy DPWM1 and a second DPWM modulation strategy DPWM3, the two DPWM modulation strategies complement each other and are switched using a hysteresis control method; the second converter (200) adopts the first DPWM modulation strategy; The zero-sequence circulating current i of the first converter (100) is obtained based on the three-phase current sampling of the converter. z1 ; With the zero-sequence circulating current i of the first converter (100) z1 As the control object, a hysteresis controller is used to suppress the zero-sequence circulating current; For the zero-sequence circulating current suppression strategy of the first converter (100), a hybrid modulation strategy of DPWM1 and DPWM3 is adopted; The modulation strategy switching adopts a hysteresis control mode, by detecting the zero-sequence circulating current i of the first converter (100) z1 , when the zero-sequence circulating current i z1 Reaching the upper threshold i zth When , the modulation strategy is switched from DPWM1 to DPWM3, so that the zero-sequence circulating current is reduced to 0; When the zero-sequence circulating current i z1 Reaching the lower threshold-i zth When , the modulation strategy is switched from DPWM3 to DPWM1, and the zero-sequence circulating current is reduced to 0 again; Select i zth and -i zth The zero-sequence circulating current is controlled so that the current harmonic content THD is within 5%, and the magnitude of the zero-sequence circulating current is limited within a preset range.

2. The method for suppressing zero-sequence circulating current according to claim 1, wherein: In the hysteresis control, the zero-sequence duty cycle d of the first converter is controlled by switching between the two DPWM modulation strategies of the first converter. z1 to reduce the difference in zero-sequence duty cycle between the first converter and the second converter.

3. The method for suppressing zero-sequence circulating current according to any one of claims 1 to 2, characterized in that: The first DPWM modulation strategy and the second DPWM modulation strategy are two complementary modulation strategies among DPWMMAX, DPWMMIN, DPWM0, DPWM1, DPWM2, and DPWM3.

4. The method for suppressing zero-sequence circulating current according to claim 3, wherein: The first DPWM modulation strategy is DPWM0 or DPWM2, and the second DPWM modulation strategy is DPWM2 or DPWM0.

5. A wind power converter parallel system based on a DPWM strategy, comprising a first converter (100) and a second converter (200) connected in parallel, characterized in that: The two NPC converters are configured to execute the zero-sequence circulating current suppression method according to any one of claims 1 to 4.

6. The wind power converter parallel system according to claim 5, characterized in that: The two NPC converters adopt a series inductor parallel structure.

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