Energy Storage Converter PWM Phase Software Synchronization Method

Abstract

The present invention belongs to the field of energy storage technology and relates to a software synchronization method for PWM phase of an energy storage converter. The method samples a discrete voltage signal of a common grid connection point (PCC) on the AC side, extracts the fundamental component, decomposes the symmetrical components to extract the positive sequence component of the fundamental component, locks the frequency and phase, and adjusts the sampling frequency according to the frequency output to ensure that the number of sampling points in each voltage cycle remains unchanged and each sampling cycle corresponds to a fixed voltage phase difference. A fixed relationship between the voltage phase and the sampling moment is established, and a fixed PWM carrier phase is configured to trigger sampling, thereby establishing a fixed relationship between the voltage phase and the PWM carrier phase. This method enables each bridge arm / power module unit to achieve PWM phase synchronization while ensuring that each bridge arm / power module unit operates independently.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy storage, and in particular relates to a PWM phase software synchronization method for an energy storage converter. Background Art

[0002] As the proportion of renewable energy generation continues to increase, grid-connected battery energy storage systems (BESS) are rapidly developing to address the randomness, volatility, and intermittency of renewable energy output. A grid-connected BESS consists of a battery pack, a combiner box, and a power conversion system (PCS). The PCS is a power electronic converter that enables flexible active and reactive power control. The PCS typically uses a DC / AC conversion topology, with both two-level and three-level structures being common. The DC side of the converter is directly connected to the battery, while the AC side is connected to the grid via a filter unit.

[0003] Industrial and commercial energy storage capacity configurations are increasingly flexible, but the capacity of a single IGBT and the number of parallel IGBTs are limited. The industry addresses this issue through centralized high-power bridge arm paralleling and small and medium-power modules paralleling. Because the DC side is connected to the battery, meaning the DC / AC converter shares a common bus, the differential-mode component of the output voltage of the same-phase inverter bridge arm will cause differential-mode currents between the bridge arms / power modules. This differential-mode current not only increases system losses but also affects current ripple, which in turn affects control feedback, ultimately limiting the number of parallel bridge arms / modules.

[0004] Grid-connected BESSs are primarily concerned with exchanging active and reactive power with the grid. PCSs have no special functional requirements and all utilize a three-phase, three-wire topology, achieving active and reactive power control by controlling the interphase differential-mode output. Low-frequency components in the differential-mode signal affect the system's active and reactive power output, while high-frequency components affect the inverter current ripple in each leg. Phases A, B, and C have different modulation signals, resulting in different duty cycles at the same time, meaning that the switching frequency components vary. Therefore, PWM phase synchronization can reduce the switching frequency component in the interphase differential-mode signal. If the modulation signals of each leg / power module in the same phase (e.g., phase A) are nearly identical, meaning the duty cycle differences are minimal, PWM phase synchronization can fully align the switching frequency components of the interphase differential-mode signal. Therefore, whether a single leg is connected in parallel or modules are connected in parallel, as long as all PWM phases are synchronized and the switching frequency vectors are aligned, the DC / AC converter can minimize the negative impact of the differential-mode signal after paralleling.

[0005] Traditional solutions all address the PWM phase synchronization problem by adding hardware devices: The first solution is to adopt a DC / DC-DC / AC two-stage converter structure and use DC / DC isolation to solve the DC / AC common bus problem. This solution increases system losses and reduces the system's economic efficiency - that is, it increases cost. The second solution is to have a processor transmit the PWM drive signal to each bridge arm / power module via optical fiber. This solution also increases cost and has high requirements for processor performance. At the same time, each bridge arm / power module unit cannot operate independently. The third solution is to synchronize the PWM carrier signal of each bridge arm / power module through an external synchronization signal. This solution is the lowest cost of the three solutions, but the synchronization signal is susceptible to interference, which affects system safety. Currently, no relevant researchers have found a method to achieve PWM phase synchronization relying solely on software control without adding hardware devices. Summary of the Invention

[0006] In response to the above-mentioned problems of the prior art in synchronizing PWM phases, such as high cost and poor susceptibility of synchronization signals, the present invention provides a software synchronization method for PWM phases of energy storage converters. The method can achieve PWM phase synchronization of each bridge arm / power module unit while ensuring that each bridge arm / power module unit can operate independently without the need for additional hardware devices. The method is low in cost, has few unreliable factors, and is safe and stable.

[0007] In order to achieve the above object, the present invention provides a method for PWM phase software synchronization of an energy storage converter, the specific steps of which are as follows:

[0008] S1. Configure each bridge arm / power module unit in the PWM carrier to increase and decrease counting mode, trigger grid-side voltage sampling at the same position of the carrier, obtain the discrete voltage signal of the AC side common grid connection point PCC, and set the number of sampling points N for each sampling cycle;

[0009] S2. Extract the fundamental component of the PCC voltage discrete signal and obtain the current value and translation of the fundamental component;

[0010] S3. Extract the positive sequence component of the fundamental component and perform Park transformation to obtain the voltage reactive axis component Uq. Input Uq into the PI controller of the phase-locked loop (PLL). Calculate the sampling period according to formula (1) based on the PI controller output and the number of sampling points. Calculate the PWM carrier period value according to formula (2) based on the sampling period. Formulas (1) and (2) are expressed as:

[0011]

[0012]

[0013] Where Δt is the sampling period, f Gridis the output voltage frequency of the PI controller, PRD is the PWM period value, f MCU_PWM PWM clock counting frequency of the main control chip;

[0014] S4. Modify the PWM cycle value of each bridge arm / power module unit according to step S3. The phase-locked loop PLL tracks the grid phase under the action of the PI controller so that the PWM cycle value of each bridge arm / power module unit is the same after stabilization, thereby achieving PWM carrier synchronization;

[0015] S5. Configure the same PWM action for each bridge arm / power module unit and start the operation. The AC side output PWM switching signal frequency and phase of each bridge arm / power module unit are the same, thus achieving PWM phase synchronization.

[0016] Preferably, in step S1 , the number of sampling points N in each sampling period is ≥ 20, and each sampling period corresponds to a fixed grid phase difference Δθ = 2π / N.

[0017] Preferably, in step S2, when extracting the fundamental component, when the voltage harmonic THDr ≥ 15%, a discrete sliding Fourier transform SDFT is used to extract the fundamental component of the PCC voltage discrete signal; when the voltage harmonic THDr < 15%, a second-order generalized integrator SOGI is used to extract the fundamental component of the PCC voltage discrete signal.

[0018] Preferably, the energy storage converter includes a PCS, which includes multiple parallel bridge arms / power module units, each bridge arm / power module unit is connected to a battery pack on the DC side and to the low-voltage side of an isolation transformer on the AC side, and the PCS is connected to the DC bus through a soft start circuit.

[0019] Specifically, before configuring each bridge arm / power module unit, the following steps are also included:

[0020] The PCS charges the DC bus through a soft start circuit, and then closes the AC and DC circuit breakers respectively, blocking the IGBTs of each bridge arm / power module unit and entering standby mode.

[0021] Compared with the prior art, the advantages and positive effects of the present invention are:

[0022] (1) The present invention provides a method for PWM phase software synchronization of an energy storage converter. By tracking the phase of an AC reference signal, a fixed correspondence between the PWM carrier phase and the reference signal phase is achieved. The reference signal uses the positive sequence component of the fundamental component of the PCC voltage discrete signal as the synchronization reference. The sampling period value is adjusted to achieve the same PWM carrier period value. Voltage sampling is triggered at the same PWM carrier fixed phase. The sampling moment is stabilized in a fixed phase sequence to achieve PWM carrier phase synchronization of each bridge arm / power module unit, achieve PWM carrier synchronization, and achieve PWM phase synchronization through PWM carrier synchronization. No additional hardware devices are required. PWM phase synchronization is achieved through software methods, which reduces the cost of the energy storage converter system and reduces unreliable factors.

[0023] (2) In the energy storage converter PWM phase software synchronization method of the present invention, each bridge arm / power module unit operates independently and is decoupled from each other. Each module unit is flexibly configured and does not require additional software for unified modulation or scheduling. At the same time, the phase-locked loop (PLL) output mode is fine-tuned on the classic dual-loop control system without changing the algorithm architecture, thus saving costs.

[0024] (3) The energy storage converter PWM phase software synchronization method of the present invention is adaptive to the PCC voltage frequency change. When the frequency changes, the SDFT automatically adjusts to the full cycle filtering, and the SOGI center frequency is adjusted to the PCC voltage frequency. It adapts to the grid frequency change without increasing any calculation amount, which not only ensures the filtering effect, but also reduces the impact of voltage fluctuation time harmonics and subharmonics. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a wiring diagram of the energy storage converter PCS according to an embodiment of the present invention;

[0026] Figure 2 This is a flow chart of the energy storage converter PWM software synchronization method according to an embodiment of the present invention;

[0027] Figure 3 A schematic diagram of the PWM phases of the embodiment of the present invention in which each bridge arm / power module unit samples a discrete voltage signal at the same position on the PWM carrier and is not synchronized;

[0028] Figure 4 This is a schematic diagram of the structure of SOGI according to an embodiment of the present invention;

[0029] Figure 5 This is a schematic diagram showing the principle of extracting the fundamental positive sequence component and phase-locked output according to an embodiment of the present invention;

[0030] Figure 6 Schematic diagram of the PWM carrier phase in steady state according to an embodiment of the present invention.

[0031] In the figure, A, the sampling value. DETAILED DESCRIPTION

[0032] The present invention is described in detail below by way of exemplary embodiments, but it should be understood that elements, structures, and features of one embodiment may be beneficially combined in other embodiments without further description.

[0033] The energy storage converter of the embodiment of the present invention includes a PCS, which includes multiple parallel bridge arms / power module units. The DC side of each bridge arm / power module unit is connected to a battery pack, and the AC side is connected to the low-voltage side of an isolation transformer. The PCS is connected to the DC bus through a soft start circuit. For PCS wiring, see Figure 1 .

[0034] See also Figure 2 For the above energy storage converter, this embodiment provides a method for PWM phase software synchronization of the energy storage converter, and the specific method is as follows:

[0035] S1. Configure each bridge arm / power module unit in the PWM carrier to increase or decrease counting mode, trigger grid-side voltage sampling at the same position of the carrier, obtain the discrete voltage signal of the AC side common grid connection point PCC, and set the number of sampling points N in each sampling cycle.

[0036] Specifically, the number of sampling points N in each sampling period is ≥ 20, and each sampling period corresponds to a fixed grid phase difference Δθ = 2π / N. In a specific embodiment, the number of sampling points is 30, and may also be 20 or 40, depending on actual needs.

[0037] It should be noted that the number of sampling points in each cycle is set to N ≥ 20 (for example, if the lower limit of the operating frequency is 45 Hz, the sampling rate must satisfy Fs ≥ 0.9 kHz). The sampling trigger positions of the discrete signals of the PCC voltage sampled by each bridge arm / power module unit are different. Since N ≥ 20 is guaranteed, the fundamental components extracted from the sampling signals of each bridge arm / power module unit are consistent.

[0038] S2. Extract the fundamental component of the PCC voltage discrete signal and obtain the current value and translation amount of the fundamental component.

[0039] Specifically, when extracting the fundamental component, the discrete sliding Fourier transform (SDFT) is used to extract the fundamental component of the PCC voltage discrete signal when the voltage harmonic THDr is ≥ 15%. When the voltage harmonic THDr is < 15%, the second-order generalized integrator (SOGI) is used to extract the fundamental component of the PCC voltage discrete signal.

[0040] In this embodiment, the specific implementation method of SDFT is expressed as follows:

[0041]

[0042] Where u(k) is the k-th voltage sampling value, and Mod represents the remainder;

[0043] In this embodiment, see Figure 4 , the second-order generalized integrator SOGI adopts a series-parallel two-stage structure. The transfer function of the second-order generalized integrator SOGI is expressed as: in, f0 is the rated value of PCC voltage frequency. The series-parallel structure can ensure that u v 、u qv The extraction is all bandpass characteristics.

[0044] It should be noted that the cascaded form of SDFT and SOGI can ensure the filtering effect while reducing the impact of voltage fluctuation time harmonics and subharmonics.

[0045] S3. Extract the positive sequence component of the fundamental component and perform Park transformation to obtain the voltage reactive axis component Uq. Input Uq into the PI controller of the phase-locked loop (PLL). Calculate the sampling period according to formula (1) based on the PI controller output and the number of sampling points. Calculate the PWM carrier period value according to formula (2) based on the sampling period. Formulas (1) and (2) are expressed as:

[0046]

[0047]

[0048] Where Δt is the sampling period, f Grid is the output voltage frequency of the PI controller, PRD is the PWM period value, f MCU_PWM The PWM clock counting frequency of the main control chip.

[0049] Specifically, the voltage reactive axis component Uq is calculated by the following formula:

[0050]

[0051] Where U α is the current instantaneous value of the voltage fundamental positive sequence component, U β is the instantaneous value of the shift of the voltage fundamental positive sequence component;

[0052] The phase used in the above formula is a fixed interval sequence. Uq is input into the PI controller of the phase-locked loop PLL. According to the phase-locked loop output f Grid , the sampling period is calculated according to a fixed phase shift.

[0053] It should be noted that the positive sequence component of the fundamental wave component is selected as the synchronization reference, and the synchronization reference can be any form of digital or analog AC signal.

[0054] S4. Modify the PWM cycle value of each bridge arm / power module unit according to step S3. The phase-locked loop PLL tracks the grid phase under the action of the PI controller so that the PWM cycle value of each bridge arm / power module unit is the same after stabilization (i.e., the phase-locked loop PLL input Uq of each bridge arm / power module unit is approximately zero), thereby achieving PWM carrier synchronization.

[0055] It should be noted that, since each bridge arm / power module unit samples the same AC side common grid connection point PCC voltage, the PWM period value of each bridge arm / power module unit is modified through step S3, and the PWM period value of each bridge arm / power module unit is stabilized; since each bridge arm / power module unit triggers voltage sampling at the same PWM phase in step S1, the sampling moment of each bridge arm / power module unit is stabilized at the PCC voltage fixed phase sequence, so that the PWM carrier phase of each bridge arm / power module unit is consistent, thus achieving the synchronization of the PWM carrier period and phase of each bridge arm / power module unit (see Figure 6 ).

[0056] S5. Configure the same PWM action for each bridge arm / power module unit and start the operation. The AC side output PWM switching signal frequency and phase of each bridge arm / power module unit are the same, thus achieving PWM phase synchronization.

[0057] Continue to see Figure 2 In one embodiment, before configuring each bridge arm / power module unit, the following steps are further included:

[0058] The PCS charges the DC bus through a soft start circuit, and then closes the AC and DC circuit breakers respectively, blocking the IGBTs of each bridge arm / power module unit and entering standby mode.

[0059] The above-mentioned energy storage converter PWM phase software synchronization method in this embodiment achieves a fixed correspondence between the PWM carrier phase and the reference signal phase by tracking the phase of the AC reference signal. The reference signal uses the positive sequence component of the fundamental component of the PCC voltage discrete signal as the synchronization reference. The sampling period value is adjusted to achieve the same PWM carrier period value. Voltage sampling is triggered at the same PWM carrier fixed phase. The sampling moment stabilizes in a fixed phase sequence, achieving PWM carrier phase synchronization for each bridge arm / power module unit, achieving PWM carrier synchronization, and achieving PWM phase synchronization through PWM carrier synchronization. PWM phase synchronization is achieved through PWM carrier synchronization without adding additional hardware devices, reducing the cost of the energy storage converter system and reducing unreliability factors.

[0060] In addition, it should be noted that the method of the present invention triggers sampling at a fixed phase of the PWM carrier to synchronize the carrier phase, but is not limited to PWM carrier phase synchronization. The interleaving of the PWM carrier can also be achieved by modifying the trigger phase.

[0061] The above embodiments are used to explain the present invention rather than to limit the present invention. Any modifications and changes made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for PWM phase software synchronization of an energy storage converter, characterized in that: The specific steps are: S1. Configure each bridge arm / power module unit in the PWM carrier to increase and decrease counting mode, trigger grid-side voltage sampling at the same position of the carrier, obtain the discrete voltage signal of the AC side common grid connection point PCC, and set the number of sampling points N for each sampling cycle; S2. Extract the fundamental component of the PCC voltage discrete signal and obtain the current value and translation of the fundamental component; S3. Extract the positive sequence component of the fundamental component and perform Park transformation to obtain the voltage reactive axis component Uq. Input Uq into the PI controller of the phase-locked loop (PLL). Calculate the sampling period according to formula (1) based on the PI controller output and the number of sampling points. Calculate the PWM carrier period value according to formula (2) based on the sampling period. Formulas (1) and (2) are expressed as: Where Δt is the sampling period, f Grid is the output voltage frequency of the PI controller, PRD is the PWM period value, f MCU_PWM PWM clock counting frequency of the main control chip; S4. Modify the PWM cycle value of each bridge arm / power module unit according to step S3. The phase-locked loop PLL tracks the grid phase under the action of the PI controller so that the PWM cycle value of each bridge arm / power module unit is the same after stabilization, thereby achieving PWM carrier synchronization; S5. Configure the same PWM action for each bridge arm / power module unit and start the operation. The AC side output PWM switching signal frequency and phase of each bridge arm / power module unit are the same, thus achieving PWM phase synchronization.

2. The energy storage converter PWM phase software synchronization method according to claim 1, characterized in that: In step S1 , the number of sampling points N in each sampling period is ≥ 20, and each sampling period corresponds to a fixed grid phase difference Δθ = 2π / N.

3. The energy storage converter PWM phase software synchronization method according to claim 1, characterized in that: In step S2, when extracting the fundamental component, a discrete sliding Fourier transform (SDFT) is used to extract the fundamental component of the PCC voltage discrete signal when the voltage harmonic THDr is ≥15%. When the voltage harmonic THDr is <15%, a second-order generalized integrator (SOGI) is used to extract the fundamental component of the PCC voltage discrete signal.

4. The energy storage converter PWM phase software synchronization method according to any one of claims 1 to 3, characterized in that: The energy storage converter includes a PCS, which includes multiple parallel bridge arms / power module units. The DC side of each bridge arm / power module unit is connected to a battery pack, and the AC side is connected to the low-voltage side of an isolation transformer. The PCS is connected to the DC bus through a soft start circuit.

5. The energy storage converter PWM phase software synchronization method according to claim 4, characterized in that: Before configuring each bridge arm / power module unit, the following steps are also included: The PCS charges the DC bus through a soft start circuit, and then closes the AC and DC circuit breakers respectively, blocking the IGBTs of each bridge arm / power module unit and entering standby mode.

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