Off-grid inverter parallel control method and system

Abstract

The application discloses a kind of off-grid inverter parallel control method and system, suitable for three-phase four-wire system T type three-level inverter with independent middle line module.Collect output voltage and current and calculate active, reactive power, respectively into VSG active ring and reactive ring, get voltage given phase angle and amplitude, synthesis initial voltage given value.Three-phase current is multiplied by virtual resistance coefficient after low-pass filtering, and a low-pass type virtual resistive voltage drop is obtained, which is deducted from the initial voltage given to form the compensated voltage given.Voltage error is regulated by QPR to generate current given, and current error is regulated by QPR to obtain modulation voltage, which is generated by SPWM to generate PWM drive signal.This method does not require rotating coordinate transformation and positive and negative sequence separation, has simple structure, and is suitable for single machine and multi-machine off-grid parallel operation.It relieves three-phase current imbalance under unbalanced load, suppresses multi-machine parallel current, and improves power supply quality and operation stability.

Description

Technical Field

[0001] This invention relates to the field of power electronic conversion and control technology, specifically to a parallel control method and system for off-grid inverters based on a virtual synchronous generator (VSG) and low-pass virtual resistive equalization, which is particularly suitable for high-stability control of a three-phase four-wire T-type three-level inverter with an independent neutral module in off-grid parallel operation scenarios. Background Technology

[0002] With the rapid development of distributed generation and microgrid technologies, off-grid inverter systems are widely used in scenarios such as islanded power supply, emergency backup power, and grid-connected power generation of new energy sources. When multiple inverters operate in parallel in off-grid mode, the following key technical issues need to be addressed: First, traditional inverter control strategies lack inertia and damping characteristics, making them prone to severe frequency and voltage oscillations due to load fluctuations when multiple inverters are connected in parallel, thus affecting the stability of the power supply system. Virtual synchronous generator (VSG) technology, by simulating the rotor motion equations and excitation regulation characteristics of a synchronous generator, can introduce virtual inertia and damping into the inverter, effectively improving the dynamic response characteristics of parallel systems. However, existing VSG control schemes still need improvement in current balancing capabilities and output voltage waveform quality when dealing with unbalanced loads.

[0003] Secondly, when off-grid inverters operate with unbalanced loads, negative-sequence and zero-sequence components have adverse effects. The negative-sequence component generates a voltage drop at the inverter output, leading to three-phase output voltage asymmetry and severely impacting power quality. The zero-sequence current requires an effective path; otherwise, it will cause the DC-side midpoint potential to drift, jeopardizing the safe operation of power switching devices. In existing technologies, three-phase three-wire inverters cannot provide a path for zero-sequence current, making it difficult to adapt to unbalanced power supply requirements such as single-phase loads.

[0004] Third, in multi-inverter parallel systems, the output impedance characteristics of each inverter differ, causing circulating current to flow between the inverters, further exacerbating the imbalance of output current and deteriorating power quality. Traditional solutions for achieving accurate circulating current suppression typically require the introduction of positive and negative sequence separation algorithms and multi-rotating coordinate system PI controllers. These solutions involve complex control structures, high computational demands, and high computational requirements for digital controllers, hindering low-cost engineering implementation. Furthermore, traditional PI controllers exhibit steady-state errors when tracking AC sinusoidal given signals, making it difficult to achieve zero steady-state error control of AC quantities. While simple PR (proportional resonant) controllers can achieve high-gain amplification of AC signals at specific frequencies, they are sensitive to frequency shifts and lack robustness.

[0005] Fourth, compared with traditional two-level inverters, T-type three-level inverters have advantages such as lower output harmonics and lower device voltage stress. However, their inherent midpoint potential imbalance problem is particularly prominent when driving unbalanced loads, and effective modulation strategies are needed to suppress it.

[0006] To address the aforementioned issues, there is an urgent need to propose a control scheme that can comprehensively solve multiple technical challenges in off-grid inverter parallel systems, such as inertia and damping support, parallel circulating current suppression, zero-sequence path provision, output impedance matching, and midpoint potential balance, while also having a simple control structure and being easy to implement in engineering. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a parallel control method and system for off-grid inverters based on VSG and low-pass virtual resistive balancing. Under the premise that the control loop does not require rotational coordinate transformation or positive and negative sequence separation, it provides virtual inertia and damping support for off-grid parallel inverters, realizes current balancing and parallel circulating current suppression under unbalanced loads, and takes into account both output voltage regulation and system operation stability.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A parallel control method for off-grid inverters based on VSG and low-pass virtual resistive equalization is applied to a three-phase four-wire T-type three-level inverter system with an independent neutral module. The method includes the following steps: Step S1: Collect the three-phase voltages Ua, Ub, Uc and the three-phase currents Ia, Ib, Ic output by the inverter; Step S2: Using the power calculation module, the active power P and reactive power Q are calculated from the three-phase voltage and three-phase current; Step S3: The active power P is sent to the VSG active loop control module to calculate the angle θ of the three-phase voltage setpoint; the reactive power Q is sent to the VSG reactive loop control module to calculate the voltage amplitude Vref of the three-phase voltage setpoint. Step S4: Combine the angle θ and the voltage amplitude Vref into initial given values ​​Ua_ref, Ub_ref, and Uc_ref for the three-phase voltage; Step S5: Pass the three-phase currents Ia, Ib, and Ic through a low-pass filter LPF to obtain the low-frequency filtered values ​​Ilpfa, Ilpfb, and Ilpfc of the three-phase currents; the cutoff frequency of the low-pass filter is lower than the system fundamental frequency and is used to obtain the low-frequency components of the current. This filtering stage is independent of the filtering in the power calculation path. Step S6: Multiply the low-frequency filtered value of the three-phase current by the virtual resistance coefficient Kvir to obtain the low-pass virtual resistive voltage drop, and subtract it from the initial set value of the three-phase voltage to obtain the compensated three-phase voltage set values ​​Ucomp_a, Ucomp_b, and Ucomp_c; Step S7: Compare the compensated three-phase voltage setpoint with the actual three-phase output voltage to obtain the voltage error signal. Send the voltage error signal to the voltage loop proportional resonant QPR controller to obtain the three-phase current setpoints Iref_a, Iref_b, and Iref_c. Step S8: Compare the given three-phase current with the actual three-phase current to obtain the current error signal, and send the current error signal to the current loop proportional resonant QPR controller to obtain the three-phase modulation voltages Eoa, Eob, and Eoc. Step S9: The three-phase modulation voltages Eoa, Eob, and Eoc are sent to the SPWM module to generate PWM pulse signals, which are used to drive the power switching devices of the T-type three-level inverter.

[0009] Preferably, the active power P and reactive power Q in step S2 are calculated using three-phase instantaneous power theory; the calculated P and Q are then fed into the VSG active power loop and VSG reactive power loop control modules respectively after low-pass filtering to filter out the pulsating components in the instantaneous power.

[0010] Preferably, the VSG active power loop control module in step S3 simulates the rotor motion equation of the synchronous generator: J·dω / dt = Pm / ω - Pe / ω - D·(ω - ω0), dθ / dt = ω Where J is the virtual inertia coefficient, in kg·m²; D is the damping coefficient, in N·m·s / rad; Pm is the virtual mechanical power, determined by the frequency-active droop characteristic Pm = Pref + Kpf·(ω0- ω), where Pref is the active power reference value, Kpf is the active-frequency droop coefficient; Pe is the active power P; ω is the virtual rotor angular frequency; ω0 is the rated angular frequency. In engineering implementation, since the system frequency fluctuation is extremely small, the rated angular frequency ω0 can be used to approximate the instantaneous ω, simplifying the calculation and the accuracy loss is negligible.

[0011] The VSG reactive power loop control module simulates the excitation regulation characteristics of a synchronous generator: Vref = V0 + Kqv·(Qref - Q) Where V0 is the rated voltage amplitude, Kqv is the reactive power-voltage droop coefficient, and Qref is the reactive power reference value.

[0012] Preferably, the virtual resistance coefficient Kvir in step S6 ranges from 0.1Ω to 2.0Ω.

[0013] Preferably, the transfer function of the voltage loop QPR controller in step S7 is: Gv(s) = Kpv + 2·Krv·ωc·s / (s² + 2·ωc·s + ω0²) Where Kpv is the voltage loop proportional coefficient, Krv is the voltage loop resonant coefficient, ωc is the cutoff angular frequency, and ω0 is the fundamental angular frequency; The transfer function of the current loop QPR controller mentioned in step S8 is: Gi(s) = Kpi + 2·Kri·ωc·s / (s² + 2·ωc·s + ω0²) Where Kpi is the current loop proportional coefficient and Kri is the current loop resonance coefficient.

[0014] Preferably, the SPWM module in step S9 adopts a phase disposition (PD) PWM modulation strategy and suppresses the midpoint potential fluctuation of the T-type three-level inverter by injecting zero-sequence components into the three-phase modulation voltage.

[0015] Preferably, in the scenario of multi-inverter off-grid parallel operation, each inverter independently operates steps S1 to S9; the active power distribution among the inverters is achieved through the frequency-active power droop characteristic of the VSG active power loop, and the reactive power distribution is achieved through the voltage-reactive power droop characteristic of the VSG reactive power loop; the parallel circulating current among the inverters is suppressed through the low-pass virtual resistive balancing stage in step S6, so that the low-frequency output impedance characteristics of the parallel inverters tend to be consistent.

[0016] Preferably, the T-type three-level inverter adopts a three-phase four-wire topology, and provides a flow path for the zero-sequence current generated by the unbalanced load through an independent neutral module, thereby achieving stable operation with unbalanced load in conjunction with the three-phase four-wire structure.

[0017] Corresponding to the above method, the present invention also provides an off-grid inverter parallel control system based on VSG and low-pass virtual resistive equalization, applied to a three-phase four-wire T-type three-level inverter with an independent neutral module, the system comprising: The acquisition module is used to acquire the three-phase voltages Ua, Ub, Uc and the three-phase currents Ia, Ib, Ic output by the inverter; The power calculation module is used to calculate the active power P and reactive power Q based on the three-phase voltage and three-phase current. The VSG active loop control module is used to calculate the angle θ of the three-phase voltage setpoint based on the active power P. The VSG reactive power loop control module is used to calculate the voltage amplitude Vref of the three-phase voltage setpoint based on the reactive power Q. The voltage setpoint synthesis module is used to synthesize the angle θ and the voltage amplitude Vref into initial setpoint values ​​Ua_ref, Ub_ref, and Uc_ref for the three-phase voltage; The low-pass filter module is used to perform low-pass filtering on the three-phase currents Ia, Ib, and Ic to obtain the low-frequency filtered values ​​Ilpfa, Ilpfb, and Ilpfc of the three-phase currents. The virtual resistive voltage drop compensation module is used to multiply the low-frequency filtered value of the three-phase current by the virtual resistance coefficient Kvir to obtain a low-pass virtual resistive voltage drop, and subtract it from the initial set value of the three-phase voltage to obtain the compensated three-phase voltage set values ​​Ucomp_a, Ucomp_b, and Ucomp_c. The voltage loop QPR control module is used to compare the compensated three-phase voltage setpoint with the actual three-phase output voltage, and the obtained voltage error signal is adjusted by QPR to output three-phase current setpoints Iref_a, Iref_b, and Iref_c. The current loop QPR control module is used to compare the given three-phase current with the actual three-phase current, and the current error signal obtained is adjusted by QPR to output three-phase modulation voltages Eoa, Eob, and Eoc. The SPWM modulation module is used to modulate the three-phase modulation voltages Eoa, Eob, and Eoc to generate PWM pulse signals, which are used to drive the power switching devices of the T-type three-level inverter.

[0018] Preferably, when the system is applied to a multi-machine off-grid parallel operation scenario, the system corresponding to each inverter operates independently; the active power distribution among the systems is achieved through the frequency-active power droop characteristic of the VSG active power loop, and the reactive power distribution is achieved through the voltage-reactive power droop characteristic of the VSG reactive power loop; the parallel circulating current among the systems is suppressed through the virtual resistive voltage drop compensation module.

[0019] Beneficial effects Compared with the prior art, the present invention has the following significant advantages: (1) Unbalanced load adaptive balancing capability: Through the independent low-pass virtual resistive droop mechanism of each phase, a larger virtual voltage drop can be automatically applied to the phase with larger current to limit its output current, alleviate the three-phase current imbalance under unbalanced load, and simultaneously suppress the full-phase sequence circulating current including zero sequence component when multiple machines are connected in parallel; the voltage regulation rate and phase voltage deviation introduced thereby are automatically compensated by the VSG reactive power-voltage droop loop, and the voltage loop QPR keeps the compensated voltage setpoint tracking without static error, and with the three-phase four-wire neutral line path, ensures stable operation under unbalanced conditions such as single-phase load.

[0020] (2) Significant effect of circulating current suppression when multiple inverters are connected off-grid: The low-pass virtual resistive equalization circuit provides a unified and adjustable low-frequency equivalent output impedance for each inverter in parallel, which significantly reduces the difference in output impedance between inverters in parallel and effectively suppresses the full-phase sequence parallel circulating current caused by hardware parameter mismatch and different line impedances; at the same time, the active-frequency and reactive-voltage droop characteristics of VSG ensure the automatic equalization of active and reactive power among multiple inverters, further reducing the generation of circulating current.

[0021] (3) The QPR controller achieves zero steady-state error tracking of AC signals and has high control accuracy: both the voltage loop and the current loop adopt quasi-proportional resonant controllers, which have high gain at the fundamental frequency and achieve zero steady-state error tracking of AC sinusoidal signals; specific subharmonics can be suppressed by adding subharmonic resonant terms to improve the quality of output power, and it has stronger frequency robustness compared with pure PR controllers.

[0022] (4) The control structure is simple and easy to implement in engineering: The entire control strategy is completed in the stationary ABC coordinate system, without the need for Park rotation transformation. The power calculation only involves a simple transformation in the stationary coordinate system, with extremely low computational load. It does not require a positive and negative sequence separation algorithm, which greatly simplifies the control structure. The dual closed-loop QPR control of the voltage outer loop and the current inner loop has a clear hierarchy, and the parameters are easy to tune, making it convenient for engineering practice.

[0023] (5) Strong adaptability and applicable to various working conditions: The control method proposed in this invention can simultaneously meet the operating requirements of various working conditions such as single-machine off-grid with balanced load, single-machine off-grid with unbalanced load, multi-machine off-grid parallel with balanced load, and multi-machine off-grid parallel with unbalanced load. There is no need to switch control modes between different working conditions, and the system has strong robustness. Attached Figure Description

[0024] Figure 1 This is a block diagram of the overall structure of the off-grid inverter parallel control system based on VSG and low-pass virtual resistive equalization provided in an embodiment of the present invention. Detailed Implementation

[0025] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0026] Example 1: Overall System Structure like Figure 1 As shown, the off-grid inverter parallel control system based on VSG and low-pass virtual resistive equalization provided by this invention is applied to a three-phase four-wire T-type three-level inverter with an independent neutral module. The system mainly includes: a main circuit unit, a sampling unit, and a control unit.

[0027] The main circuit unit adopts a T-type three-level topology. The DC side is connected to DC bus capacitors C1 and C2, and the AC side is connected to the AC bus after forming an LC filter with filter inductor L and filter capacitor C. The system adopts a three-phase four-wire structure, and provides a flow path for the zero-sequence current generated by unbalanced load through an independent neutral module (neutral inductor Ln).

[0028] The sampling unit includes voltage and current sensors, which are used to collect the three-phase voltages Ua, Ub, and Uc and the three-phase currents Ia, Ib, and Ic output by the inverter, respectively. The sampled signals are sent to the control unit after passing through a signal conditioning circuit.

[0029] The control unit is implemented based on a digital signal processor (DSP) and integrates a power calculation module, a VSG active loop control module, a VSG reactive loop control module, a low-pass filter module, a virtual resistive voltage drop compensation module, a voltage loop QPR control module, a current loop QPR control module, and an SPWM modulation module.

[0030] Example 2: Control Method Flow The control method of the present invention specifically includes the following steps: Step S1: Signal Acquisition The inverter outputs three-phase voltages Ua, Ub, Uc and three-phase currents Ia, Ib, Ic in real time via voltage and current sensors. These signals are then converted into digital signals by an analog-to-digital converter (ADC) and sent to the control unit.

[0031] Step S2: Power Calculation The power calculation module uses three-phase instantaneous power theory to calculate active power P and reactive power Q according to the following formulas: P = Uα·Iα + Uβ·Iβ + U0·I0 Q = Uα·Iβ - Uβ·Iα Where Uα, Uβ, and U0 are the components of the three-phase voltage in the αβ0 coordinate system, and Iα, Iβ, and I0 are the components of the three-phase current in the αβ0 coordinate system. This calculation method is not limited by the sum of the three-phase currents being zero, and can accurately calculate the power of a three-phase four-wire system including zero-sequence components.

[0032] The calculated P and Q are filtered by a first-order low-pass filter (the filter cutoff frequency is usually set to less than 1 / 10 of the fundamental frequency) and then sent to the VSG active power loop control module and the VSG reactive power loop control module, respectively, to filter out the second harmonic pulsation component in the power.

[0033] In each control stage of this invention, the symbol ω0 represents the angular frequency value corresponding to the rated operating frequency of the system (for example, 100 for a 50Hz system). π (rad / s). It should be noted that this symbol independently represents different physical setpoints in different control modules: in the VSG active power loop, ω0 serves as a fixed reference point for the frequency-active power droop characteristic, used to calculate the frequency deviation; in the QPR resonant controller, ω0 serves as the set center frequency of the resonant peak, used to achieve zero steady-state error tracking of the fundamental AC signal. Both numerically correspond to the system's rated frequency, but they are assigned independently in the digital controller's algorithm implementation and do not affect each other.

[0034] Step S3: VSG Control The VSG active power loop control module simulates the rotor motion equation of a synchronous generator: J·dω / dt = Pm / ω - Pe / ω - D·(ω - ω0) dθ / dt = ω Among them, the virtual mechanical power Pm is determined by the frequency-active power droop characteristic: Pm = Pref + Kpf·(ω0- ω) In the above equation, J is the virtual inertia coefficient (typical value 0.5~5 kg·m²), D is the damping coefficient (typical value 5~50 N·m·s / rad), Pref is the active power reference value, Kpf is the active power-frequency droop coefficient, ω0 is the rated angular frequency (corresponding to a 50Hz or 60Hz system), and Pe is the active power P calculated in step S2.

[0035] The active power loop output is the phase angle θ of the three-phase voltage setpoint, which is obtained by integrating the virtual rotor angular frequency ω.

[0036] The VSG reactive power loop control module simulates the excitation regulation characteristics of a synchronous generator: Vref = V0 + Kqv·(Qref - Q) Where V0 is the rated voltage amplitude (corresponding to the rated phase voltage peak value), Kqv is the reactive power-voltage droop coefficient, Qref is the reactive power reference value, and Q is the reactive power calculated in step S2. The reactive power loop output is the voltage amplitude Vref of the three-phase voltage setpoint.

[0037] Step S4: Voltage Given Synthesis The angle θ and voltage amplitude Vref obtained in step S3 are combined into the initial given value of the three-phase voltage using the following formula: Ua_ref = Vref × sin(θ) Ub_ref ​​= Vref × sin(θ - 2π / 3) Uc_ref = Vref × sin(θ + 2π / 3) Step S5: Low-frequency component extraction The three-phase currents Ia, Ib, and Ic are fed into a first-order low-pass filter LPF to obtain the low-frequency filtered values ​​Ilpfa, Ilpfb, and Ilpfc of the three-phase currents. The transfer function of the first-order low-pass filter is G_LPF(s) = ωc / (s + ωc), and its cutoff angular frequency ωc is less than the system fundamental angular frequency ω0 (for a 50Hz system, the cutoff frequency fc is set to 5Hz~20Hz, preferably 10Hz).

[0038] The amplitude-frequency response of the low-pass filter is as follows: In the DC and very low frequency range, the gain approaches 1, and the virtual resistance exhibits the full value Kvir, which can effectively reflect the amplitude difference of the three-phase current and the low-frequency component of the load fluctuation. At the fundamental frequency and above, the gain decreases as the frequency increases, which greatly weakens the effect on the steady-state fundamental current, avoids excessive voltage drop of virtual resistance on normal power transmission, and takes into account the output voltage regulation rate.

[0039] Step S6: Low-pass virtual resistive equalization Multiply the low-frequency filtered value of the three-phase current obtained in step S5 by the virtual resistance coefficient Kvir to obtain the low-pass virtual resistive voltage drop: Vvir_a = Kvir × Ilpfa Vvir_b = Kvir × Ilpfb Vvir_c = Kvir × Ilpfc Subtract the virtual voltage drop from the initial three-phase voltage setpoint obtained in step S4 to obtain the compensated three-phase voltage setpoint: Ucomp_a = Ua_ref - Vvir_a Ucomp_b = Ub_ref ​​- Vvir_b Ucomp_c = Uc_ref - Vvir_c Working mechanism: This step is equivalent to connecting a virtual impedance with low-pass amplitude-frequency characteristics in series at the output of each phase of the inverter. It eliminates the need for coordinate transformation and positive / negative sequence separation, and directly achieves resistive droop balancing in the stationary ABC coordinate system. 1. Single unit with unbalanced load scenario: When the three-phase current amplitudes are not equal, a larger virtual voltage drop is applied to the heavily loaded phase to limit the output current of that phase and alleviate the phase imbalance; the phase voltage deviation introduced by this link is of the nature of series impedance voltage drop, and its average amplitude is automatically compensated by the VSG reactive power-voltage droop loop, and the voltage loop QPR maintains zero steady-state error tracking of the compensated setpoint; 2. Multi-inverter parallel scenario: Each inverter is configured with consistent low-pass filter parameters and virtual resistance coefficient, so that the low-frequency equivalent output impedance of all inverters is uniformly dominated by virtual resistance, which greatly reduces the output impedance dispersion caused by differences in hardware parameters and line impedance, thereby suppressing the full-phase sequence circulating current (including negative sequence circulating current and zero sequence circulating current flowing through the neutral line) in the parallel system and realizing automatic distribution of load current; 3. Steady-state symmetrical operating condition: The three-phase current amplitudes are equal, and the virtual voltage drop has a symmetrical effect on the three phases, only slightly reducing the steady-state voltage amplitude. This can be automatically compensated by the reactive power droop loop, without affecting the symmetrical operation performance.

[0040] The value of Kvir ranges from 0.1Ω to 2.0Ω, with 0.5Ω being the preferred value. A larger value results in stronger current sharing and circulating current suppression, but also a larger steady-state voltage regulation rate, requiring a trade-off adjustment based on system requirements.

[0041] Step S7: Voltage Loop QPR Control The compensated three-phase voltage setpoints Ucomp_a, Ucomp_b, and Ucomp_c are compared with the actual three-phase output voltages Ua, Ub, and Uc, respectively, to obtain the voltage error signal. ΔUa = Ucomp_a - Ua ΔUb = Ucomp_b - Ub ΔUc = Ucomp_c - Uc The voltage error signal is fed into a voltage loop proportional resonant QPR controller, whose transfer function is: Gv(s) = Kpv + 2·Krv·ωc·s / (s² + 2·ωc·s + ω0²) Where Kpv is the voltage loop proportional coefficient, Krv is the voltage loop resonant coefficient, ωc is the cutoff angular frequency (used to adjust the resonant bandwidth and enhance frequency robustness), and ω0 is the fundamental angular frequency. Multiple resonant terms can be set at the 3rd, 5th, and 7th harmonic frequencies to compensate for specific harmonic orders.

[0042] The voltage loop QPR controller outputs three-phase current setpoints Iref_a, Iref_b, and Iref_c.

[0043] Step S8: Current Loop QPR Control The three-phase current setpoints Iref_a, Iref_b, and Iref_c are compared with the actual three-phase currents Ia, Ib, and Ic, respectively, to obtain the current error signal: ΔIa = Iref_a - Ia ΔIb = Iref_b - Ib ΔIc = Iref_c - Ic The current error signal is fed into a current loop quasi-proportional resonant QPR controller, whose transfer function is: Gi(s) = Kpi + 2·Kri·ωc·s / (s² + 2·ωc·s + ω0²) Where Kpi is the current loop proportional coefficient, Kri is the current loop resonant coefficient, ωc is the cutoff angular frequency, and ω0 is the fundamental angular frequency.

[0044] The current loop QPR controller outputs three-phase modulated voltages Eoa, Eob, and Eoc.

[0045] Step S9: SPWM modulation Each phase of a T-type three-level inverter contains four power switching devices (such as IGBTs or MOSFETs). Two of these switching devices are connected in series to form the vertical arm of the T, while the other two form the horizontal arm. Combined with the DC side midpoint O, it can output three power levels: +Udc / 2, 0, and -Udc / 2.

[0046] The SPWM modulation module employs a phase displacement (PD) strategy, where all carriers (upstream and downstream) have the same phase and are stacked. The three-phase modulation voltages Eoa, Eob, and Eoc are compared with the carriers to generate PWM pulse signals that drive the power switching devices in the T-type three-level inverter.

[0047] To suppress fluctuations in the DC-side neutral point potential of a T-type three-level inverter, a zero-sequence component Ez is injected into the modulation wave. The zero-sequence component Ez is calculated in real time based on the voltage deviation between the upper and lower DC-side capacitors C1 and C2. After being superimposed on the three-phase modulation voltage, the neutral point current is balanced by adjusting the duration of each phase level, thereby maintaining the neutral point potential balance.

[0048] To address the neutral point potential shift caused by neutral current in a three-phase four-wire system, the zero-sequence component injection amount can be dynamically adjusted in conjunction with neutral point voltage PI closed-loop regulation, further improving the stability of the neutral point potential under unbalanced loads.

[0049] Example 3: Parallel Operation of Multiple Machines In microgrids or islanded power supply systems, multiple inverters are connected in parallel via an AC bus to supply power to the load. Each inverter operates independently using the control methods described in steps S1 to S9 above, without the need for high-speed communication interconnection between them, relying solely on droop characteristics to achieve power sharing.

[0050] Specifically, the VSG active power loop of each inverter achieves automatic distribution of active power through the frequency-active power droop characteristic (Kpf coefficient): when the output active power of a certain inverter is too high, its virtual frequency decreases accordingly, causing the power angle of that inverter to decrease and the output power to automatically drop; conversely, the same applies. The VSG reactive power loop of each inverter achieves automatic distribution of reactive power through the voltage-reactive power droop characteristic (Kqv coefficient): when the output reactive power of a certain inverter is too high, its virtual voltage amplitude decreases accordingly, causing the output reactive power to automatically drop.

[0051] The low-pass virtual resistive equalization circuit of each inverter uses the same or similar Kvir coefficients and low-pass filter parameters to make the low-frequency equivalent output impedance of each inverter more consistent, thereby effectively suppressing the flow of full-phase sequence parallel circulating current between inverters. This circulating current suppression mechanism is decoupled from the active / reactive droop control, and its parameters can be designed independently without affecting each other.

[0052] The independent virtual voltage drop of each phase acts on the zero-sequence component in the three-phase current with the same coefficient, making the zero-sequence output impedance of each machine tend to be consistent, thereby suppressing the zero-sequence circulating current flowing through the neutral line.

[0053] Experiments show that, using the control method of this invention, the imbalance of the three-phase output current can be significantly reduced under 100% unbalanced load (such as single-phase full load) conditions, the parallel circulating current suppression effect is significant, and the parallel system can operate stably and reliably.

[0054] Industrial applicability The off-grid inverter parallel control method and system based on VSG and low-pass virtual resistive equalization proposed in this invention can be widely applied in the following fields: New energy microgrid islanded operation system (off-grid power supply of distributed power sources such as photovoltaic and wind power). Emergency backup power systems (UPS, energy storage power stations) for critical loads. Independent power supply systems for remote areas, islands, and field operation bases; Multi-machine parallel redundant power supply system.

[0055] The control strategy of this invention is fully programmable and can be directly applied to existing inverter products by upgrading the control software without modifying the hardware, thus exhibiting good compatibility and cost-effectiveness.

[0056] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A parallel control method for off-grid inverters based on VSG and low-pass virtual resistive equalization, characterized in that, The method for a three-phase four-wire T-type three-level inverter system with an independent neutral module includes the following steps: Step S1: Collect the three-phase voltages Ua, Ub, Uc and the three-phase currents Ia, Ib, Ic output by the inverter; Step S2: Using the power calculation module, the active power P and reactive power Q are calculated from the three-phase voltage and three-phase current; Step S3: The active power P is sent to the VSG active loop control module to calculate the angle θ of the three-phase voltage setpoint; the reactive power Q is sent to the VSG reactive loop control module to calculate the voltage amplitude Vref of the three-phase voltage setpoint. Step S4: Combine the angle θ and the voltage amplitude Vref into initial given values ​​Ua_ref, Ub_ref, and Uc_ref for the three-phase voltage; Step S5: Pass the three-phase currents Ia, Ib, and Ic through a low-pass filter LPF to obtain the low-frequency filtered values ​​Ilpfa, Ilpfb, and Ilpfc of the three-phase currents; Step S6: Multiply the low-frequency filtered value of the three-phase current by the virtual resistance coefficient Kvir to obtain the low-pass virtual resistive voltage drop, and subtract it from the initial set value of the three-phase voltage to obtain the compensated three-phase voltage set values ​​Ucomp_a, Ucomp_b, and Ucomp_c; Step S7: Compare the compensated three-phase voltage setpoint with the actual three-phase output voltage to obtain the voltage error signal. Send the voltage error signal to the voltage loop proportional resonant QPR controller to obtain the three-phase current setpoints Iref_a, Iref_b, and Iref_c. Step S8: Compare the given three-phase current with the actual three-phase current to obtain the current error signal, and send the current error signal to the current loop proportional resonant QPR controller to obtain the three-phase modulation voltages Eoa, Eob, and Eoc. Step S9: The three-phase modulation voltages Eoa, Eob, and Eoc are sent to the SPWM module to generate PWM pulse signals, which are used to drive the power switching devices of the T-type three-level inverter.

2. The method according to claim 1, characterized in that, The active power P and reactive power Q mentioned in step S2 are calculated using three-phase instantaneous power theory; the calculated P and Q are then fed into the VSG active loop and VSG reactive loop control modules respectively after being low-pass filtered.

3. The method according to claim 1, characterized in that, In step S3, the VSG active power loop control module simulates the rotor motion equation of the synchronous generator: J·dω / dt = Pm / ω - Pe / ω - D·(ω - ω0), dθ / dt = ω; where J is the virtual inertia coefficient, D is the damping coefficient, Pm is the virtual mechanical power, determined by the frequency-active power droop characteristic Pm =Pref + Kpf·(ω0- ω), Pe is the active power P, ω is the virtual rotor angular frequency, ω0 is the rated angular frequency, and Kpf is the active power-frequency droop coefficient; the VSG reactive power loop control module simulates the excitation regulation characteristic of the synchronous generator: Vref =V0+ Kqv·(Qref - Q), where V0 is the rated voltage amplitude, Kqv is the reactive power-voltage droop coefficient, and Qref is the reactive power reference value.

4. The method according to claim 1, characterized in that, The virtual resistance coefficient Kvir mentioned in step S6 ranges from 0.1Ω to 2.0Ω.

5. The method according to claim 1, characterized in that, The transfer function of the voltage loop QPR controller in step S7 is Gv(s) = Kpv + 2·Krv·ωc·s / (s² + 2·ωc·s + ω0²), where Kpv is the voltage loop proportional coefficient, Krv is the voltage loop resonant coefficient, ωc is the cutoff angular frequency, and ω0 is the fundamental angular frequency; the transfer function of the current loop QPR controller in step S8 is Gi(s) = Kpi + 2·Kri·ωc·s / (s² + 2·ωc·s + ω0²), where Kpi is the current loop proportional coefficient and Kri is the current loop resonant coefficient.

6. The method according to claim 1, characterized in that, The SPWM module in step S9 adopts a phase disposition (PD) PWM modulation strategy and suppresses the midpoint potential fluctuation of the T-type three-level inverter by injecting zero-sequence components into the three-phase modulation voltage.

7. The method according to claim 1, characterized in that, In a multi-inverter off-grid parallel operation scenario, each inverter independently operates steps S1 to S9; the active power distribution among the inverters is achieved through the frequency-active power droop characteristic of the VSG active power loop, and the reactive power distribution is achieved through the voltage-reactive power droop characteristic of the VSG reactive power loop; the parallel circulating current among the inverters is suppressed through the low-pass virtual resistive balancing stage in step S6, so that the low-frequency output impedance characteristics of the parallel inverters tend to be consistent.

8. The method according to claim 1, characterized in that, The T-type three-level inverter adopts a three-phase four-wire topology. It provides a flow path for the zero-sequence current generated by the unbalanced load through an independent neutral module, and achieves stable operation with unbalanced load in conjunction with the three-phase four-wire structure.

9. A parallel control system for off-grid inverters based on VSG and low-pass virtual resistive balancing, applied to a three-phase four-wire T-type three-level inverter with an independent neutral module, characterized in that, The system includes: The acquisition module is used to acquire the three-phase voltages Ua, Ub, Uc and the three-phase currents Ia, Ib, Ic output by the inverter; The power calculation module is used to calculate the active power P and reactive power Q based on the three-phase voltage and three-phase current. The VSG active loop control module is used to calculate the angle θ of the three-phase voltage setpoint based on the active power P. The VSG reactive power loop control module is used to calculate the voltage amplitude Vref of the three-phase voltage setpoint based on the reactive power Q. The voltage setpoint synthesis module is used to synthesize the angle θ and the voltage amplitude Vref into initial setpoint values ​​Ua_ref, Ub_ref, and Uc_ref for the three-phase voltage; The low-pass filter module is used to perform low-pass filtering on the three-phase currents Ia, Ib, and Ic to obtain the low-frequency filtered values ​​Ilpfa, Ilpfb, and Ilpfc of the three-phase currents. The virtual resistive voltage drop compensation module is used to multiply the low-frequency filtered value of the three-phase current by the virtual resistance coefficient Kvir to obtain a low-pass virtual resistive voltage drop, and subtract it from the initial set value of the three-phase voltage to obtain the compensated three-phase voltage set values ​​Ucomp_a, Ucomp_b, and Ucomp_c. The voltage loop proportional resonant (QPR) control module is used to compare the compensated three-phase voltage setpoint with the actual three-phase output voltage. The voltage error signal obtained is adjusted by QPR and then outputs three-phase current setpoints Iref_a, Iref_b, and Iref_c. The current loop proportional resonant (QPR) control module is used to compare the given three-phase current with the actual three-phase current. The current error signal obtained is adjusted by QPR and then outputs three-phase modulation voltages Eoa, Eob, and Eoc. The SPWM modulation module is used to modulate the three-phase modulation voltages Eoa, Eob, and Eoc to generate PWM pulse signals, which are used to drive the power switching devices of the T-type three-level inverter.

10. The system according to claim 9, characterized in that, When the system is applied to a multi-machine off-grid parallel operation scenario, the system corresponding to each inverter operates independently; the active power distribution among the systems is achieved through the frequency-active power droop characteristic of the VSG active power loop, and the reactive power distribution is achieved through the voltage-reactive power droop characteristic of the VSG reactive power loop; the parallel circulating current among the systems is suppressed through the virtual resistive voltage drop compensation module.

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