Design and analysis of single-phase five-level inverter based on integrated energy system

New energy power generation, mainly photovoltaic power generation and wind power generation, is one of the key research topics today. It is found that the grid-connected phase of power transmission is very important in the transmission process after power generation through these two ways. This paper introduces in detail the various stages of power transmission under the background of integrated energy system, mainly photovoltaic power generation and wind power generation as the research object. This paper presents a new grid-connected symmetrical cascade half-bridge multilevel inverter (SCHB) under the background of comprehensive energy and presents a Pulse Width Modulation (PWM) control scheme which can make the inverter output stable multilevel voltage. By this scheme, the voltage of the shunt capacitor is basically unchanged and the multistage voltage output of SCHB inverter is guaranteed. The advantages of the designed inverter are that the whole inverter can be powered by a single power supply and a single inductance, and the number of voltage levels can be controlled because of the use of a small amount of reactive power components. This paper describes the topological results of the inverter and the PWM control scheme of stable output multistage voltage.


Introduction
Nowadays, new energy sources, mainly photovoltaic power generation and wind power generation, play an important role in the integrated energy system.In the process of electric energy transportation, the energy storage device is an important part to ensure the adjustable transportation of electric energy.Therefore, the final stage of the grid connected plays a decisive role.At present, integrated energy systems require high and stable output voltage, so multilevel inverters (MLI) are becoming more and more popular [1].Kiaer's paper proposes an integrated inverter based on series of multiple photovoltaic power sources, which can achieve higher efficiency and lower cost in grid-connected system [2].It is proposed in Zhang's article that a DC-DC module can be added to the front end of each module.This is used to solve the problem of overmodulation due to unbalanced voltage, resulting in the final waveform quality reduction [3].Chen's research shows that single-stage MLI can output step voltage through a small number of switches, but the increase in the number of reactive elements will produce current spikes, which will lead to the decline of current quality [4].This paper innovatively applies the symmetrical cascade plate bridge inverter to the integrated energy system and explores its advantages in improving the utilization rate of renewable energy.The definition, topological structure, and operating principal analysis of integrated energy systems, mainly wind power and photovoltaic power generation are illustrated in Section II.The circuit structure, operating principle and theoretical analysis of symmetrical cascade half-bridge inverters are presented in Section III.Simulation results and analysis are described in Section IV.The conclusion is going to be drawn in Section V.

Definition
Integrated energy system refers to the effective integration of traditional energy sources such as coal, oil and natural gas with new energy sources such as nuclear power, solar power, wind power, biomass energy and geothermal energy in each area with advanced physical infrastructure and innovative management methods.Integrated energy systems improve energy efficiency to a large extent.The system uses the adjustment between different energies to complement each other reasonably for maximizing the overall efficiency of the system [5].However, at present, with the large-scale development of non-renewable energy, the world's fossil fuel supply is facing a serious shortage of crisis.Similarly, the excessive use of coal, oil, natural gas, and other energy sources also brings more and more crises to the environment of human existence [6].The growing recognition of these disasters has created an urgent need to build new clean, low-carbon, safe and efficient energy systems around the world [7].Countries are seeking new sources of clean energy.Solar energy and wind energy are widely used in clean energy.This kind of new energy is a good way for the energy industry to generate electricity in a low-carbon renewable energy, high economic efficiency, safety, and environmental protection way.

The structure
At present, photovoltaic power generation by solar energy and wind power generation by wind power play a pivotal role in the world's power generation industry.The application of integrated energy system is illustrated in Figure 1 below.

Operation analysis
2.3.1.Photovoltaic power generation.Photovoltaic power generation can be defined as a system that utilizes the photovoltaic effect of semiconductor materials to transform solar energy into electrical energy.Since the power grid is generally AC power grid, direct current can't be directly connected to the grid.Therefore, the main role of photovoltaic grid-connected inverter is to convert the DC energy generated by solar photovoltaic modules into sinusoidal alternating current with the same frequency and phase as the grid.Power lines eventually feed this alternating current into the grid [8].

Wind power generation.
Wind power generation is based on the principle of converting the kinetic energy of wind into mechanical energy by the rotation of a turbine.The generator rotates to generate electricity under the drive of the wind wheel shaft, which produces alternating current.Although the power grid is an AC network, the power generated by generators cannot be directly incorporated into the grid due to its unstable quality.Therefore, it is necessary to do AC/DC conversion work to improve this problem [9].
The main function of wind power grid-connected inverter is to convert DC into AC and then into the power grid.Its main purpose is to improve power quality.Factors such as wind speed and equipment in the environment will directly affect the rotation of the generator, so the voltage and current fluctuations are unstable.The method of rectifier before inverter is helpful to improve such instability.

Storge.
Figure 1 shows the operation model of new energy power system with photovoltaic and wind power generation and energy storage.In the process of electric energy transportation, there will be peaks and troughs of users' demand for electricity.When scheduling is required during troughs, some energy storage devices such as batteries will participate in the scheduling for energy storage during this period.Energy storage devices can store excess wind and electricity during troughs.During peak hours, the battery can supply the power grid for normal use and prevent the power grid from collapsing due to low frequency [10].
The advantages of energy storage on the grid side are as follows: (1) Energy storage can not only supply power to the grid, but also provide load consumption for the grid.( 2) Power supply quality and reliability.(3) When the load peaks, the transmission pressure of the transmission and distribution equipment would be relieved; When the output spikes, the transmission pressure of its transmission and distribution would also be relieved 2.3.4.Grid-connected inverter.The principle of the two power generation systems has been described above.Photovoltaic power generation generates direct current and wind power generation generates alternating current.The power is converted into direct current by devices such as boosts and rectifiers.Energy storage equipment is suitable for direct current environment, so it is necessary to use the inverter to integrate the energy in the energy storage equipment into the AC grid.
In photovoltaic power generation (PV) and some new energy generation applications, voltagesource multilevel inverter (MLI) is widely used.This is because the voltage-source multilevel inverter has the characteristics of high-quality output voltage and low voltage conversion rate.This can be used as an output filter and play the role of smoothing the output voltage waveform [11].MLI is commonly used for buck voltage conversion, powered by high voltage combined with multiple PV power supplies, PV string mismatch occured.Cascaded full-bridge inverters could be used for multi-level operation of low-voltage power supply.Although DC isolated power supply is affected by partial unbalanced current, resulting in reduced waveform quality [12].Therefore, a kind of inverter called symmetrical cascade bridge inverter is proposed in this paper.It can realize five voltage level operation, and ensure the waveform output stability

Circuit structure
The energy storage segment of the integrated energy system based on wind power and photoelectric mainly relies on voltage-source multilevel inverters.In this context, the paper proposes a single-phase five-level inverter, named symmetrical cascade half-bridge inverter (SCHB) for its topological 1.The DC link is generated by the connection of a single power supply and an inductor works for the entire inverter circuit.This power supply branch is relatively simple.
2. The input current is generated continuously, and the output voltage increases in multiple.The specific value of the voltage boost multiple will be explained in detail later.
3. Only one inductor is needed to cascade with multiple symmetrical half-bridges to expand higher voltage levels, which is more practical and greatly optimized in terms of power loss due to reduced reactive components.
4. In the working process of the circuit, the charging and discharging process of HB capacitors alternates.So the voltage of the capacitor can realize self-balance through the self-adaptation of the working state, which is very suitable for the working scene requiring high-quality output stable voltage.

Operating principles 3.2.1. Operating status.
The working state of the SCHB inverter needs to be matched with appropriate PWM scheme and control strategy to make the charging and discharging states of capacitor Ca and Cb alternate.Thus, the capacitor voltage (VC) can achieve self-balance and maintain at the voltage level of VC continuously.The following will describe the working state of the SCHB inverter in detail.
Figure 2. shows the working state of SCHB inverter in positive half cycle (PHC).Since the capacitor can realize voltage self-balancing, it can be assumed that the capacitor voltage is continuously maintained at the voltage level of VC.And the five working states of positive half cycle (PHC) of SCHB inverter is shown in Figure 3 [13].
Figure 3. (a) shows the initial state of SCHB inverter, whose output voltage Vab=0, because this output terminal ab is in a short-circuit state.At this time, the capacitor is in the state of charging, and will supply power to the capacitor according to the voltage value of the two capacitors in the circuit.If the voltage of the two capacitors is not equal, the priority is to supply power to the capacitor with lower voltage content.If the capacitor voltages are equal, the input current will flow through the diode and form a path through which the power supply and inductive link together charge the capacitor.The capacitor can alternate through the above states of charge and discharge, to achieve voltage selfbalance.As shown in Figure 3. (c), the output AC path only flows through the capacitor element.And since the capacitor voltage is basically unchanged, the output voltage is also basically maintained at the voltage level of VC.As the voltage of the inductor is equivalent to the DC power supply Vin in the shoot-through state in Figure 3. (b), the power supply and inductor together supply power to the capacitor in Figure 3. (c).So, the capacitance voltage is VC=2Vin.During this process, the inductor is discharging while the capacitor is charging, as illustrated in Figure 3. (a), where a capacitor with a lower voltage has a higher priority for charging.
From Figure 3. (c) to Figure 3. (d), the inverter enters the shoot-through state.In this state, the voltage of capacitor Cb remains at VC, so the output voltage continues to be VC, as shown in Figure 3. (d).In this case, the DC power supply is charging the inductor.
Figure 3. (e) shows the voltage level of output voltage Vab=2VC.Since the output AC path flows through the power supply, inductor and capacitor, and the power supply and inductor in the previous state jointly supply power to the capacitor, the output voltage in this state should be 2V.
The detail data of working state in positive half cycle and negative half cycle is shown in Table 1  and 2. 1/0 in the table represents the state of switch on/off.Positive half cycle is the output AC path flowing through the capacitor Cb, so the output voltage is the voltage VC of Cb.The negative half cycle is the voltage VC flowing through the capacitor Ca so the output voltage is Ca.Since the above two kinds of current are perceptual and continuous, there will be no voltage peak in the conversion process of each state which will be verified by subsequent experiments.There are two modulation signals respectively Vac and Vdc.Shoot-through state occurs when carrier T1 is greater than Vdc while Vac operating at low level (Vac lower than 0.5) and carrier T2 is greater than Vdc while Vac operating at high level (Vac higher than 0.5).The corresponding control signals of the switches are shown in Figure 5. Switch S1a turn on while PHC, but S1b only operating under high switching frequency when Vac > 0.5.During NFC, switch S1b is kept on and switch S2a and S2b are charged to the inductor with high switching frequency create the shoot-through state.According to this control strategy, the output end can generate five output voltage levels.

Theoretical analysis
According to the previous analysis in this the inductor would be charged in shoot-through state of FB module but discharged in non-shoot-through state.analysis of the inverter is out below.
The duty ratio of shoot-through state can be expressed as

2(1 )
st dc dM =− (1) Applying volt-second equilibrium to the inductor, the following equation can be written.
(2) By combining formula (1) and ( 2), the average voltage (VC) at both ends of the capacitor can be found and expressed as Therefore, the peak value of the fundamental component of the output voltage can be expressed as ab,pk c ac ac dc dc The voltage of the capacitor is coupled with the ripple of low frequency voltage, which is caused by the pulsed output current.The input inductors design should consider the requirements of current ripple.The ripple of high frequency current (ΔIL) is caused by the charge and discharge of the inductor during each carrier cycle ( s 1 f ), which can be expressed as According to the theoretical analysis, by changing Mac and Mdc, the duty ratio can be adjusted to adjust the voltage at both ends of the capacitor and indirectly change the output boost voltage.

Inverter cascading expansion
The previous section of this article describes how to increase the capacitor voltage to increase the voltage of a single voltage level, but this is still in the realm of five levels.A higher voltage level can be achieved by using cascaded inverters.
This section describes how to expand the output voltage to a higher voltage level by cascade.There are two cascades: horizontal, shown in Figure 6.(a) and vertical, shown in Figure 6.(b).
As shown in Figure 6, each cascade of output voltage of the inverter will generate two more voltage levels.Therefore, if the inverter is cascaded in the way shown in Figure 6 output voltage levels will be obtained.However, different cascades use different components.Therefore, it can be concluded that if 2 (n+1) voltage levels are to be obtained, 2 (n-1) HB modules and diodes are required.
Through the cascaded structure, the peak output voltage can be adjusted based on the number of cascaded branches, and could be expressed as follows: Both cascades can achieve the effect of expanding voltage level.However, compared with longitudinal cascades, horizontal cascades require more components.Each cascade requires one more power supply and inductance which obviously violates the characteristics of SCHB inverter described at the beginning.Therefore, the method as shown in Figure 6 (b) is adopted for cascaded expansion of multilevel level, that is, single inductance and single power supply are maintained and the power supply branch is simple.

Simulation analysis
The simulation software used in the experimental simulation part of this paper is PLCES.The simulated variables are for the modulation wave Vdc and Mac=0.73 for the modulation Vac.simulation input voltage source Vin is 100V, and the load is connected to the RL branch.The output active power is set to 1kw, and the load resistance is 50  .According to the previous analysis, the voltage of capacitors Ca and Cb should be stable at the voltage level of 2Vin=200V.However, due to the change of working state mentioned above, the single-phase pulsating output will cause the voltage of the two capacitors to fluctuate slightly.The simulation results are shown in Figure 7 below.
As shown in Figure 7, the voltage of capacitor Ca and Cb is consistent with the theoretical analysis, maintaining around 200V with small fluctuations.However, the amplitude of fluctuations is relatively small, indicating convergence.That is to say, the capacitor can realize self-equilibrium.The voltage level can be obtained by formula (3).The output voltage is five level levels, and the size of each voltage level is equal to VC.The voltage level can also be modified by changing the modulation ratio of PWM scheme of the control strategy.

Conclusion
Under the background of the new energy generation system which mainly consists of wind power and photovoltaic power generation, a new symmetrical cascade half-bridge five-level inverter in the stage of integrating electric energy into the power grid is proposed in this paper.It is powered by a DC link of single power supply and single inductance, and outputs a boost voltage, which innovatively reduces the number of reactive components.The corresponding PWM control strategy is designed to keep the boost ratio and stabilize the output voltage.Then the feasible expansion strategy is proposed by comparison to ensure the stability of multistage output voltage.The inverter feasibility is verified by experimental simulation, which also show that the SCHB inverter could realize DC-AC and output the boosting voltage stably.At present, this paper is based on single-phase five-level inverter to do research.Since three-phase inverter is widely used in the power grid currently, the future research may try to do three-phase exploration.

Figure 1 .
Figure 1.Application diagram of integrated energy system.

Figure 2 .
Figure 2. Topology of SCHB inverter.SCHB inverter has many key features: 1.The DC link is generated by the connection of a single power supply and an inductor works for the entire inverter circuit.This power supply branch is relatively simple.2.The input current is generated continuously, and the output voltage increases in multiple.The specific value of the voltage boost multiple will be explained in detail later.3.Only one inductor is needed to cascade with multiple symmetrical half-bridges to expand higher voltage levels, which is more practical and greatly optimized in terms of power loss due to reduced reactive components.4.In the working process of the circuit, the charging and discharging process of HB capacitors alternates.So the voltage of the capacitor can realize self-balance through the self-adaptation of the working state, which is very suitable for the working scene requiring high-quality output stable voltage.

Figure 3 .
Figure 3. Five working states of positive half cycle (PHC) of SCHB inverter Transition from the state in Figure 3. (a) to the state called shoot-through, that is, the state shown in Figure 3. (b).Currently, in the FB module, S1a and S2a with the same bridge arm are closed, and S1b and S2b with the same bridge arm are also closed, which aims to charge the inductor.Because the output terminal ab is still in a short-circuit state, the output voltage Vab is still 0V.The internal loop is DC power to charge the inductor.As shown in Figure3.(c), the output AC path only flows through the capacitor element.And since the capacitor voltage is basically unchanged, the output voltage is also basically maintained at the voltage level of VC.As the voltage of the inductor is equivalent to the DC power supply Vin in the shoot-through state in Figure3.(b), the power supply and inductor together supply power to the capacitor in Figure3.(c).So, the capacitance voltage is VC=2Vin.During this process, the inductor is discharging while the capacitor is charging, as illustrated in Figure3.(a), where a capacitor with a lower voltage has a higher priority for charging.From Figure3.(c) to Figure3.(d), the inverter enters the shoot-through state.In this state, the voltage of capacitor Cb remains at VC, so the output voltage continues to be VC, as shown in Figure3.(d).In this case, the DC power supply is charging the inductor.Figure3.(e) shows the voltage level of output voltage Vab=2VC.Since the output AC path flows through the power supply, inductor and capacitor, and the power supply and inductor in the previous state jointly supply power to the capacitor, the output voltage in this state should be 2V.The detail data of working state in positive half cycle and negative half cycle is shown in Table1 and 2.1/0 in the table represents the state of switch on/off.Positive half cycle is the output AC path flowing through the capacitor Cb, so the output voltage is the voltage VC of Cb.The negative half cycle is the voltage VC flowing through the capacitor Ca so the output voltage is Ca.Since the above two kinds of current are perceptual and continuous, there will be no voltage peak in the conversion process of each state which will be verified by subsequent experiments.

Figure 4 .
Figure 4. Simulation diagram of PWM control strategy scheme.The PWM scheme waveform of SCHB inverter is shown in Figure5.There are two modulation signals respectively Vac and Vdc.Shoot-through state occurs when carrier T1 is greater than Vdc while Vac operating at low level (Vac lower than 0.5) and carrier T2 is greater than Vdc while Vac operating at high level (Vac higher than 0.5).The corresponding control signals of the switches are shown in Figure5.Switch S1a turn on while PHC, but S1b only operating under high switching frequency when Vac > 0.5.During NFC, switch S1b is kept on and switch S2a and S2b are charged to the inductor with high switching frequency create the shoot-through state.According to this control strategy, the output end can generate five output voltage levels.

Figure 5 .
Figure 5. Waveform diagram of PWM control strategy scheme.
to minimize high frequency current ripple, e.g.you can use the

Table 1 .
Five working states of SCHB inverter in Positive Half Cycle (PHC).

Table 2 .
Five working states of SCHB inverter in Negative Half Cycle (NHC).