Frequency stability control method and system for grid-connected photovoltaic and energy storage parallel systems containing electric vehicle loads

Abstract

The present invention discloses a frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system containing electric vehicle loads and a system thereof, wherein the method comprises: generating reference power; obtaining the state of charge (SOC) of an energy storage battery, the instantaneous value #imgabs0# of a three-phase bus voltage, and the maximum power #imgabs1# of a photovoltaic panel to calculate and generate a reference active power #imgabs2# of a photovoltaic inverter and a reference active power #imgabs3# of an energy storage inverter; calculating instantaneous power; respectively calculating the instantaneous active power #imgabs4# and instantaneous reactive power #imgabs5# of the photovoltaic inverter, and the instantaneous active power #imgabs6# and instantaneous reactive power #imgabs7# of the energy storage inverter; photovoltaic reserved capacity VSG control; energy storage constant frequency VSG control; the system comprises: a reference power generation module, an instantaneous power calculation module, a photovoltaic reserved capacity VSG control module, an energy storage constant frequency VSG control module, and a PWM modulation module.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a frequency stabilization control method and system for a grid-connected photovoltaic and energy storage parallel system containing electric vehicle loads. Background Art

[0002] Against the backdrop of increasing renewable energy penetration, the power system is undergoing a profound transformation from a traditional synchronous motor-dominated model to a new "dual-high" architecture (a high proportion of renewable energy and power electronic equipment). This structural change significantly reduces system rotational inertia, leading to the gradual emergence of weak grid characteristics in the power grid. The resulting stability issues have become a key bottleneck hindering the large-scale development of renewable energy. Focusing on parallel photovoltaic and energy storage microgrids, the typical control architecture is as follows: the photovoltaic inverter adopts a grid-following control strategy, achieving stable power output based on upper-level current commands; the energy storage converter is often configured with a grid-forming control mode, assuming the core function of bus voltage and frequency support. However, due to the inherent capacity and insufficient system inertia of the microgrid, the integration of electric vehicle loads poses challenges to system stability. Unlike traditional steady-state loads, electric vehicle loads have high power demands and rapid charging and discharging. The addition of electric vehicle loads can cause drastic fluctuations in system load power, leading to significant frequency deviations in the system, which can trigger a chain reaction such as unit disconnection.

[0003] Some literature has studied the improvement of the stability of grid-connected solar-storage parallel systems:

[0004] The article, titled "Active Power Oscillation Suppression Strategy for Diesel-Storage Microgrids Based on Active Power Proportional Differential Feedforward (VSG)" (Renewable Energy, 2024), improves traditional grid-connected control technology based on the principle of damping compensation. This improves the system's damping characteristics, enhances the steady-state control accuracy of the output power of grid-connected PV and energy storage units, and reduces the system's energy storage reserve. Furthermore, the article utilizes frequency security warnings and controller dynamic performance constraints to quantitatively evaluate grid-connected controller parameters and proposes a frequency support control strategy for grid-connected PV and energy storage systems based on damping compensation. However, the proposed frequency stability control strategy has limited applicability in off-grid operation, particularly in applications where the capacity of the PV and energy storage parallel system is limited, and its control performance needs further optimization.

[0005] The article, titled "Transient Stability Analysis of a Grid-Connected Converter System with Improved VSG for Photovoltaic Energy Storage" (Control Engineering, 2025), studies a photovoltaic energy storage system connected to the grid via a VSG. An improved first-order inertia component based on a tanh function is introduced into the inertia to achieve smooth inertia adjustment during system adaptation. Furthermore, to address the potential for power angle instability in the system under large disturbances, the selection of parameter damping and inertia is optimized, and the impact of VSG external characteristic parameters on transient stability is analyzed. However, the proposed optimization method only improves the system's power angle stability, but has difficulty improving frequency stability. Furthermore, the proposed method does not consider the impact of the energy storage system's state of charge, which may lead to overcharging / overdischarging of the energy storage battery. Summary of the Invention

[0006] In view of this, the present invention provides a frequency stability control method and system for a grid-type photovoltaic and energy storage parallel system containing electric vehicle loads, which is used to at least solve the problem that traditional control methods in the prior art are difficult to improve frequency stability.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system including electric vehicle loads is characterized by comprising the following steps:

[0009] Generate reference power; obtain the energy storage battery state of charge SOC and the instantaneous value of the three-phase bus voltage and the maximum power of photovoltaic panels Calculate and generate the reference active power of the photovoltaic inverter and energy storage inverter reference active power ;

[0010] Calculate the instantaneous power; based on the instantaneous value of the three-phase output voltage of the photovoltaic inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the photovoltaic inverter respectively and instantaneous reactive power ;According to the instantaneous value of the three-phase output voltage of the energy storage inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the energy storage inverter respectively and instantaneous reactive power ;

[0011] Photovoltaic reserved capacity VSG control; based on the reference active power of the photovoltaic inverter , photovoltaic inverter instantaneous active power and instantaneous reactive power Calculate the potential phase inside the photovoltaic inverter and its amplitude , the potential phase in the photovoltaic inverter and amplitude Perform PWM modulation to generate photovoltaic inverter modulation wave ;

[0012] Energy storage constant frequency VSG control; based on the reference active power of the energy storage inverter , Instantaneous active power of energy storage inverter and instantaneous reactive power Calculate the potential phase in the energy storage inverter and its amplitude , the potential phase in the energy storage inverter and amplitude Perform PWM modulation to generate energy storage inverter modulation wave .

[0013] Preferably, the specific contents of generating the reference power include:

[0014] ;

[0015] in, is the reference power integral coefficient of the energy storage inverter; and are the proportional coefficient and integral coefficient of the bus voltage frequency-locked loop respectively; is the q-axis component of the bus voltage, which is the instantaneous value of the three-phase bus voltage Generated by dq transformation, is the system rated frequency; is the bus voltage angular frequency.

[0016] Preferably, calculate the potential phase in the photovoltaic inverter and its amplitude The specific contents include:

[0017] ;

[0018] in, and and are the virtual inertia, droop coefficient and instantaneous value of angular frequency of photovoltaic reserved capacity VSG control respectively; 、 and They are the excitation control coefficient, reactive droop coefficient and reference reactive power of photovoltaic reserved capacity VSG control respectively. is the system rated voltage, is the rated frequency of the system.

[0019] Preferably, calculate the potential phase in the energy storage inverter and its amplitude The specific contents include:

[0020] ;

[0021] in, and are the virtual inertia, droop coefficient and instantaneous value of angular frequency of energy storage constant frequency VSG control respectively; and They are the excitation control coefficient, reactive droop coefficient and reference reactive power of energy storage constant frequency VSG control respectively. is the system rated voltage, is the rated frequency of the system.

[0022] The frequency stability control system of a grid-connected photovoltaic and energy storage parallel system with electric vehicle loads includes: a reference power generation module, an instantaneous power calculation module, a photovoltaic reserved capacity VSG control module, an energy storage constant frequency VSG control module, and a PWM modulation module;

[0023] Reference power generation module, used to obtain the energy storage battery state of charge SOC and the instantaneous value of the three-phase bus voltage and the maximum power of photovoltaic panels Calculate and generate the reference active power of the photovoltaic inverter and energy storage inverter reference active power ;

[0024] The instantaneous power calculation module is used to calculate the instantaneous value of the three-phase output voltage of the photovoltaic inverter based on the sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the photovoltaic inverter respectively and instantaneous reactive power ;According to the instantaneous value of the three-phase output voltage of the energy storage inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the energy storage inverter respectively and instantaneous reactive power ;

[0025] Photovoltaic reserved capacity VSG control module, used to reference active power of photovoltaic inverter , photovoltaic inverter instantaneous active power and instantaneous reactive power Calculate the potential phase inside the photovoltaic inverter and its amplitude ;

[0026] Energy storage constant frequency VSG control module, used to reference active power of energy storage inverter , Instantaneous active power of energy storage inverter and instantaneous reactive power Calculate the potential phase in the energy storage inverter and its amplitude ;

[0027] PWM modulation module, used to adjust the potential phase of the photovoltaic inverter and amplitude Perform PWM modulation to generate photovoltaic inverter modulation wave , and the potential phase in the energy storage inverter and amplitude Perform PWM modulation to generate energy storage inverter modulation wave .

[0028] It can be seen from the above technical solution that, compared with the prior art, the present invention discloses a frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system including electric vehicle loads, which has the following beneficial effects:

[0029] The photovoltaic inverter in this invention adopts a reserved capacity control strategy, dynamically adjusting the power output margin to achieve coordinated operation with the energy storage system. When electric vehicle loads are connected to the microgrid, the photovoltaic inverter and the energy storage inverter can jointly support load power changes, significantly reducing the frequency drop caused by sudden load changes. The maximum system frequency deviation is significantly reduced compared to traditional control methods.

[0030] In terms of system frequency regulation, the energy storage inverter applies phase-locked loop-based constant frequency control technology. By real-time tracking of the grid phase and implementing active power compensation, the microgrid frequency fluctuation is strictly controlled near the rated frequency, effectively improving the frequency stability of the parallel photovoltaic and energy storage system.

[0031] The present invention effectively constructs a dynamic coupling mechanism between the SOC of the energy storage battery and the photovoltaic power generation power. The power reference value of the photovoltaic inverter is adaptively generated by the energy storage SOC. When the SOC increases, the photovoltaic input power is reduced, and when the SOC is too low, the photovoltaic array input power is increased. This coordinated control strategy based on energy state not only ensures the good photovoltaic absorption capacity, but also avoids system instability caused by excess energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 A structural diagram of the grid-connected solar-storage parallel system provided in this embodiment;

[0034] Figure 2 This is a control principle diagram of the frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system including electric vehicle loads provided by the present invention;

[0035] Figure 3 This is a control principle diagram of the reference power generation link in the frequency stabilization control method of the grid-connected photovoltaic and energy storage parallel system containing electric vehicle loads provided by the present invention;

[0036] Figure 4 This is a time-domain waveform diagram of the bus frequency when the solar-storage parallel system in this embodiment adopts traditional grid-following / grid-forming hybrid control;

[0037] Figure 5 : This is a time domain waveform diagram of the bus frequency when the photovoltaic energy storage parallel system in this embodiment adopts the control method proposed by the present invention. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0039] The present invention provides a frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system including electric vehicle loads, comprising the following steps:

[0040] Generate reference power; obtain the energy storage battery state of charge SOC and the instantaneous value of the three-phase bus voltage and the maximum power of photovoltaic panels Calculate and generate the reference active power of the photovoltaic inverter and energy storage inverter reference active power ;

[0041] Calculate the instantaneous power; based on the instantaneous value of the three-phase output voltage of the photovoltaic inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the photovoltaic inverter respectively and instantaneous reactive power ;According to the instantaneous value of the three-phase output voltage of the energy storage inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the energy storage inverter respectively and instantaneous reactive power ;

[0042] Photovoltaic reserved capacity VSG control; based on the reference active power of the photovoltaic inverter , photovoltaic inverter instantaneous active power and instantaneous reactive power Calculate the potential phase inside the photovoltaic inverter and its amplitude , the potential phase in the photovoltaic inverter and amplitude Perform PWM modulation to generate photovoltaic inverter modulation wave ;

[0043] Energy storage constant frequency VSG control; based on the reference active power of the energy storage inverter , Instantaneous active power of energy storage inverter and instantaneous reactive power Calculate the potential phase in the energy storage inverter and its amplitude , the potential phase in the energy storage inverter and amplitude Perform PWM modulation to generate energy storage inverter modulation wave .

[0044] It should be noted that:

[0045] Figure 1 This is the structure diagram of the grid-type solar-storage parallel system. In the figure, and They are the output filter inductors of the photovoltaic inverter and the energy storage inverter respectively; and They are the output filter capacitors of the photovoltaic inverter and the energy storage inverter respectively; and are the line impedances for the PV inverter and energy storage inverter connected to the AC bus; and are the instantaneous values of the output three-phase voltages of the photovoltaic inverter and the energy storage inverter respectively; is the instantaneous value of the three-phase voltage of the AC bus; and are the output three-phase currents of the photovoltaic inverter and the energy storage inverter respectively; and They are the three-phase modulation waves of the photovoltaic inverter and the energy storage inverter respectively. and are the internal potential amplitudes of the photovoltaic inverter and the energy storage inverter, and are the internal potential phases of the PV inverter and energy storage inverter, respectively. The PV inverter, energy storage inverter, and load are connected to the same AC bus. The load includes conventional steady-state load and electric vehicle load.

[0046] Figure 2This is a control principle diagram of the method proposed in the present invention; Figure 3 This is a control principle diagram of the reference power generation link in the method proposed in the present invention.

[0047] The photovoltaic inverter in the present invention adopts a reserved capacity control strategy and realizes coordinated operation with the energy storage system by dynamically adjusting the power output margin. When the electric vehicle load is connected to the microgrid, the photovoltaic inverter and the energy storage inverter can jointly support the load power changes, significantly reducing the frequency drop caused by the sudden change in load, and the maximum deviation of the system frequency is significantly reduced compared with the traditional control method. The energy storage inverter uses a constant frequency control technology based on a phase-locked loop. By tracking the grid phase in real time and implementing active power compensation, the frequency of the microgrid is strictly controlled near the rated frequency, effectively improving the frequency stability of the photovoltaic and storage parallel system. The present invention innovatively constructs a dynamic coupling mechanism between the SOC of the energy storage battery and the photovoltaic power generation power. The power reference value of the photovoltaic inverter is generated by the adaptive energy storage SOC. When the SOC increases, the photovoltaic input power is reduced, and when the SOC is too low, the photovoltaic array input power is increased. This coordinated control strategy based on energy state not only ensures the good photovoltaic absorption capacity, but also avoids system instability caused by excess energy.

[0048] To further implement the above technical solution, the specific contents of generating the reference power include:

[0049] ;

[0050] in, is the reference power integral coefficient of the energy storage inverter; and are the proportional coefficient and integral coefficient of the bus voltage frequency-locked loop respectively; is the q-axis component of the bus voltage, which is the instantaneous value of the three-phase bus voltage Generated by dq transformation, is the system rated frequency; is the bus voltage angular frequency.

[0051] In order to further implement the above technical solution, calculate the potential phase of the photovoltaic inverter and its amplitude The specific contents include:

[0052] ;

[0053] in, and and are the virtual inertia, droop coefficient and instantaneous value of angular frequency of photovoltaic reserved capacity VSG control respectively; and They are the excitation control coefficient, reactive droop coefficient and reference reactive power of photovoltaic reserved capacity VSG control respectively. is the system rated voltage, is the rated frequency of the system.

[0054] In order to further implement the above technical solution, calculate the potential phase in the energy storage inverter and its amplitude The specific contents include:

[0055] ;

[0056] in, and are the virtual inertia, droop coefficient and instantaneous value of angular frequency of energy storage constant frequency VSG control respectively; and They are the excitation control coefficient, reactive droop coefficient and reference reactive power of energy storage constant frequency VSG control respectively. is the system rated voltage, is the rated frequency of the system.

[0057] The frequency stability control system of a grid-connected photovoltaic and energy storage parallel system with electric vehicle loads includes: a reference power generation module, an instantaneous power calculation module, a photovoltaic reserved capacity VSG control module, an energy storage constant frequency VSG control module, and a PWM modulation module;

[0058] Reference power generation module, used to obtain the energy storage battery state of charge SOC and the instantaneous value of the three-phase bus voltage and the maximum power of photovoltaic panels Calculate and generate the reference active power of the photovoltaic inverter and energy storage inverter reference active power ;

[0059] Instantaneous power calculation module, used to calculate the instantaneous active power of photovoltaic inverters and instantaneous reactive power , and the instantaneous active power of the energy storage inverter and instantaneous reactive power ;

[0060] Photovoltaic reserved capacity VSG control module, used to reference active power of photovoltaic inverter , photovoltaic inverter instantaneous active power and instantaneous reactive power Calculate the potential phase inside the photovoltaic inverter and its amplitude ;

[0061] Energy storage constant frequency VSG control module, used to reference active power of energy storage inverter , Instantaneous active power of energy storage inverter and instantaneous reactive power Calculate the potential phase in the energy storage inverter and its amplitude ;

[0062] PWM modulation module, used to adjust the potential phase of the photovoltaic inverter and amplitude Perform PWM modulation to generate photovoltaic inverter modulation wave , and the potential phase in the energy storage inverter and amplitude Perform PWM modulation to generate energy storage inverter modulation wave .

[0063] The present invention will be further described below through specific experiments:

[0064] Figure 4 This is the bus frequency time domain waveform diagram of the photovoltaic and energy storage parallel system using traditional grid-following / grid-forming hybrid control. When the electric vehicle load is put into operation and removed, there is a large time domain offset in the system frequency, with the frequency offsets being 2.42 rad / s and 2.06 rad / s respectively. After the electric vehicle load is put into operation, the system frequency is difficult to recover to the rated frequency (314.16 rad / s).

[0065] Figure 5 This is a time-domain waveform of the bus frequency in a parallel PV-storage system using the control method proposed in this invention. When electric vehicle loads are added and removed, system frequency deviations are significantly limited, with the deviations being 0.33 rad / s and 0.35 rad / s, respectively. After the electric vehicle loads are added, the system frequency quickly recovers to near the rated frequency (314.16 rad / s), unaffected by load changes. Compared to traditional control methods, the control method proposed in this invention can provide a grid-type parallel PV-storage system with greater frequency stability.

[0066] In this embodiment, it is assumed that the power of the conventional steady-state load remains unchanged, and the power of the electric vehicle load will undergo a step change at a certain moment, thereby causing fluctuations in the load power.

[0067] Maximum photovoltaic power The measurement methods of SOC, as well as the parameter settings of the frequency-locked loop, photovoltaic VSG and energy storage VSG are relatively mature, so there is no need to introduce them in detail.

[0068] parameter The setting determines the speed at which the reference power of the energy storage converter follows the frequency change. The larger the value, the faster the reference power of the energy storage converter changes. Too large may cause the energy storage VSG control to become unstable. Therefore, in this embodiment, set =100 / S n , S n It is the total rated capacity of the grid-type photovoltaic storage parallel system.

[0069] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system containing electric vehicle loads, characterized in that: The following steps are involved: Generate reference power; obtain the energy storage battery state of charge SOC and the instantaneous value of the three-phase bus voltage and the maximum power of photovoltaic panels Calculate and generate the reference active power of the photovoltaic inverter and energy storage inverter reference active power ; Specific content includes: ； in, is the reference power integral coefficient of the energy storage inverter; and are the proportional coefficient and integral coefficient of the bus voltage frequency-locked loop respectively; is the q-axis component of the bus voltage, which is the instantaneous value of the three-phase bus voltage Generated by dq transformation, is the system rated frequency; is the bus voltage angular frequency; Calculate the instantaneous power; based on the instantaneous value of the three-phase output voltage of the photovoltaic inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the photovoltaic inverter respectively and instantaneous reactive power ;According to the instantaneous value of the three-phase output voltage of the energy storage inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the energy storage inverter respectively and instantaneous reactive power ; Photovoltaic reserved capacity VSG control; based on the reference active power of the photovoltaic inverter , photovoltaic inverter instantaneous active power and instantaneous reactive power Calculate the potential phase inside the photovoltaic inverter and its amplitude , the potential phase in the photovoltaic inverter and amplitude Perform PWM modulation to generate photovoltaic inverter modulation wave ; Energy storage constant frequency VSG control; based on the reference active power of the energy storage inverter , Instantaneous active power of energy storage inverter and instantaneous reactive power Calculate the potential phase in the energy storage inverter and its amplitude , the potential phase in the energy storage inverter and amplitude Perform PWM modulation to generate energy storage inverter modulation wave .

2. The frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system containing electric vehicle loads according to claim 1 is characterized in that: Calculate the potential phase in photovoltaic inverters and its amplitude The specific contents include: ； in, and and are the virtual inertia, droop coefficient and instantaneous value of angular frequency of photovoltaic reserved capacity VSG control respectively; 、 and They are the excitation control coefficient, reactive droop coefficient and reference reactive power of photovoltaic reserved capacity VSG control respectively. is the system rated voltage, is the rated frequency of the system.

3. The frequency stabilization control method for a grid-connected photovoltaic and energy storage parallel system containing electric vehicle loads according to claim 1 is characterized in that: Calculate the potential phase in the energy storage inverter and its amplitude The specific contents include: ； in, and are the virtual inertia, droop coefficient and instantaneous value of angular frequency of energy storage constant frequency VSG control respectively; and They are the excitation control coefficient, reactive droop coefficient and reference reactive power of energy storage constant frequency VSG control respectively. is the system rated voltage, is the rated frequency of the system.

4. A frequency stabilization control system for a grid-connected photovoltaic and energy storage system with electric vehicle loads, based on the frequency stabilization control method for a grid-connected photovoltaic and energy storage system with electric vehicle loads according to any one of claims 1 to 3, characterized in that: include: Reference power generation module, instantaneous power calculation module, photovoltaic reserved capacity VSG control module, energy storage constant frequency VSG control module and PWM modulation module; Reference power generation module, used to obtain the energy storage battery state of charge SOC and the instantaneous value of the three-phase bus voltage and the maximum power of photovoltaic panels Calculate and generate the reference active power of the photovoltaic inverter and energy storage inverter reference active power ; The instantaneous power calculation module is used to calculate the instantaneous value of the three-phase output voltage of the photovoltaic inverter based on the sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the photovoltaic inverter respectively and instantaneous reactive power ;According to the instantaneous value of the three-phase output voltage of the energy storage inverter obtained by sampling And the instantaneous value of three-phase output current , calculate the instantaneous active power of the energy storage inverter respectively and instantaneous reactive power ; Photovoltaic reserved capacity VSG control module, used to reference active power of photovoltaic inverter , photovoltaic inverter instantaneous active power and instantaneous reactive power Calculate the potential phase inside the photovoltaic inverter and its amplitude ; Energy storage constant frequency VSG control module, used to reference active power of energy storage inverter , Instantaneous active power of energy storage inverter and instantaneous reactive power Calculate the potential phase in the energy storage inverter and its amplitude ; PWM modulation module, used to adjust the potential phase of the photovoltaic inverter and amplitude Perform PWM modulation to generate photovoltaic inverter modulation wave , and the potential phase in the energy storage inverter and amplitude Perform PWM modulation to generate energy storage inverter modulation wave .

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