The Research of Three-phase Boost / Buck-boost DC-AC Inverter

This paper presents a new inverter based on three-phase Boost/Buck-boost single-stage inverter. The basic configuration of the new topology and their fundamental principle are firstly introduced, the method of design double-loop controller and sliding mode controller are clarified, analyzed and compared in the following. Finally the validity and feasibility of the new topology are tested by simulation. The results indicate that regulation of the voltage transfer ratio and output frequency can be realized optionally by the new converter, furthermore the harmonic distortion of waveform is low. So the inherent drawback of low voltage transfer ratio of traditional converter is effectively settled. This study may provide inspiration for further engineering application.


Introduction
As growing in the field of new energy development and utilization, high power density, high reliability, pollution-free and high performance of a new generation of high frequency transformation technology research has important theoretical value and engineering value.Recently, based on the new concept of novel topologies of the DC-AC inverter has attracted more and more attention.Ramón O. Cáceres referred to a boost DC-AC inverter [1].The new inverter generates an output voltage larger than the dc input one depending on the instantaneous duty cycle.So this property is not found in the traditional VSI, which produces an ac output instantaneous voltage always lower than the dc input.Through the two groups of converter are cooperate, the load voltage can be sine waveform.The topology can reduce the DC source and the inverter needed DC-DC link.So it can reduce the volume, reduce costs and improve efficiency.
The paper present three-phase Boost, Buck-boost single-stage converter [2].A control strategy for the threephase boost inverter which each Boost is controlled by means of a double-loop regulation scheme that consists of a new inductor current control inner loop and an also new output voltage control outer loop is applied.A control strategy for the three-phase buck-boost inverter in which each buck-boost is controlled by means of a sliding mode controller is applied.

Introduction of the Three-phase
Boost/Buck-boost Single-stage Inverter

The Topology of Three-phase Boost Single-Stage Inverter
The three-phase boost single-stage inverter is shown in Figure 1.In this topology, three boost inverter which driven by three 120°phase-shift DC-biased sinusoidal reference make the output capacitor voltage changes over the reference voltage to adjust the output voltage of the boost and output voltage is an AC output voltage [3].The boost DC-AC inverter exhibits several advantages, the most important of which is that it can naturally generate an AC output voltage from a lower DC input voltage in a single-stage power stage.
Control of the three-phase boost inverter can be achieved by controlling each boost separately.So, analysis one boost inverter can be an example to explain the principle and working process.Using the average concepts, the following voltage relationship for the continuous conduction mode is given by where is the duty cycle.[4], the single-stage-phase ac output voltage of the boost converter can be compared with the dc input voltage gain is: where is maximum voltage gain ( is the output voltage peak-peak of single-stage boost converter.

op V
According to equation (2), the topology of the ac output can be higher or below the intermediate dc voltage.
Three-phase boost single-stage inverter output current of inverter side is positive and negative alternating.The current bidirectional boost dc-dc converter is shown in Figure 2.
The fundamental principle is: 1 and 2 are driven by two Complementary signal differ 180°phase-shifted, 1 and 2 are the diode parallel.If 1 on, 2 off, the inductor current direction is positive, the current will be flow through the 1 ; If the inductor current is negative, the current will be flow through the 1 .If 1 off, 2 on, the inductor current direction is positive, the current will be flow through the ; otherwise the current will be flow through the .

2
A three-phase buck-b ilar to a three-phase boost .

The Topology of Three-phase Buck-Boost Single-stage Inverter
oost is sim single-stage inverter.A three-phase buck-boost singlestage inverter is shown in Figure 3.Each of three-phase buck-boost single-stage inverter ac output voltage of the boost converter can be compared with the dc input voltage gain is: where the is maximum voltage gain (  he inductor current direction is positive, the cur-2.3.The Compare of the Two Topology logy and

he Three
A doub control method [5][6][7][8] for the three-phase ied.The mode eq 2 S on t obtained, in which internal resistance have been neglected: rent will be flow through the 2 D ; otherwise the current will be flow through the 2 S .
The equation (8) shows that boost converter has nonlinear relationship between different variables.So designing consistency to the requirements of the boost inverter accurate, stable and suitable for various complex conditions of the controller is very difficult.A control strategy for the three-phase boost inverter in which each boost is controlled by means of a double-loop regulation scheme is shown Figure 5.
Although the two converter have different topo fundamental principle, but from the practical analysis is similar: 1) Three-phase boost and buck-boost singlestage inverter have same number of switches, the complexity of the circuit is similar; 2) Under the condition of the same input voltage, these topology can be through the duty ratios to adjusted the output voltage, to achieve the goal of change the voltage transfer ratio; 3) For the single-stage boost and buck-boost converter, the voltage stress depends on the maximum gain and output voltage and current stress depends on the required current.
The current control loop is defined by equation ( 4) and equation (6).If choosing the duty cycle to the controller output, the controller is variable and the plant seen by the controller would exhibit a variable gain caused by the variable output voltage 01 .Therefore, the control strategy of the inductor current feedback value compared with the reference value to control can replace equation ( 6) with PI controller.The duty cycle 1 is then obtained by means of the following expression, in which is the controller output boost is introduced.For simplify the system analysis, only for a boost converter to introduce.
Firstly, the current inner loop is stud The cancellation of the input voltage influence acts as a feed-forward control.This cancellation would not be required if the current loop bandwidth is much faster than the input voltage.uations particularized for the boost converter are described as follows: Concerning the output voltage loop which is introduced in Figure 6, is now defined by equation ( 5) equation (7).The design of the control structure for the output voltage is based on the same philosophy as the current loop.If the control variable are now the current reference ( Lref i ) for the inner loop, the plant seen by the controller would show again a variable gain caused by the term 1 1 d  .Therefore, the capacitor current ( c i ) is now proposed to be the control variable, replace equation (7)with PI controller from the equation ( 5), the calculation of the current reference from the use of duty cycle ( 1 d ),which appears inside the term 1 as shown in equation ( 5), However, the calculation of duty cycle ( 1 ) is provided by the inner current loop, and its use in the current reference calculation would cause a coupling between both inner and outer control loops that could and are the values for the in-( 4)-( 7), the following expression can be 1 c r uc ductanc ca ci ce, ind tor and capacitor equivalent series resistance.
From equation e, pa . Proposed output voltage control loop.
ake the system unstable.Using the average mode, m boost converter is 1 01 Therefore, the proposed output voltage control loop can also be seen as the result of compensating the plant variable gain (defined by ) with V instead, the V current reference is then given by the following expression

Sliding Mode Controller Analysis e the sliding ck-boost inverter dy ee-phase buck-boost single-stage inve ter output vo
As for the three-phase buck-boost to introduc control method.For simplify the analysis, only for a buck-boost converter to introduce [9].
For the purpose of optimizing the bu namics, while ensuring correct operation in any working condition, a sliding mode control [10,11] is more feasible approach.
Control of the thr rter by controlling each boost converter.The fundamental principle is shown in Figure 7.
The sliding mode controller make the conver ltage track the reference signal as precisely as possible.For the current bidirectional dc-dc converter, the statespace molding of the equivalent circuit with state variables 1 L i and 1 V is given by where and are the inductor and capacitance; is a loa  is t status of the switches, defined When good transient response of the output voltage is needed, a sliding surface equation in the state space, expressed by a linear combination of the feedback current error, and is the feedback voltage error, can be given atus of the switches with the value of ( ) . Sliding mode controller scheme.

EPE
If the sliding mode exists, the system behavior is completely determined by coefficients 1 K and 2 K .It is determines the system respon The sliding mode function is applied to a hysteresis comparator se, stability and robustness.i and make so as to realize the accurate trac g of o h rence signal.In addition, the inductor current is related to the load, the reference value is difficult to determine.Therefore, in practical applications, the inductor current feedback by a high-pass filter, take its high-frequency component to replace the inductor current error, so only need to control the high frequency component of the inductor current.

The Analysis of Simulation
, the output frequency is 50Hz, sliding mode controller coefficients The contrast analyses are threeloop control and sliding mode contr The contrast analyses are the three-phase boost double-loop control and three-phase buck-boost sliding mode control of capacitance voltage: The contrast analyses are the three-phase Boost double-loop control and three-phase buck-boost sliding mode control of inductor curre Three-phase buck-boost sliding surface of the sliding mode control: Three-phase boost single-stage inverter load sharp reduction in 0.05s, the simulation waveform graph is shown below:

The Simulation Results Analysis
1) Three-phase boost/buck-boost single-stage inverter indicates that regulation of the voltage transfer ratio can be realized optionally and the output voltage can be accurately tracked according to the reference given by the change; 2) As can be seen from the Figure 8 and Figure 9 the harmonic for three-phase symmetrical sinusoidal wa From Figure 8 can be seen that the double ithout overshoot system into a steady state quickly.From Figure 9 can be seen sliding mode control of the load voltage startup performance is poorer, before into the sliding mode surface ,the system has overshoot, there is a lot of impulse voltage, and power is not enough; 4) The double-loop control is the direct control of the current and it has strong robustness to external interference.The buck-boost inverter capacitor voltage stress is lower than the boost inverter, so the switching loss of the former is smaller than the latter, which adopts the doubleloop control of output harmonic is smaller than using sliding mode control of output harmonic, as shown in Figure 10 the harmonic is 0.6%, and in Figure 11, the harmonic is 2.05%; 5) Compared with double-loop control, sliding mode control has better tracking.Sliding mode control of the feedback voltage is almost consistent and a given voltage (155.5 V) for 155.4 V, and the feedback voltage is 156 V in the double-loop control; 6) Fig- ure 12 and Figure 13 shows the output capacitor voltage has dc-biased.

Conclusions
The paper deals with the topology and comparison of three-phase boost, buck-boost single-stage inverter.A control strategy for the three-phase buck-boost inverter has been proposed in this project in which buck-boost converters of the buck-boost inverter are controlled by means of a sliding mode control, while the three-phase boost inverter in which each boost is controlled are by means of a double-loop control scheme that consists of a new inductor current control inner loop and a new output voltage control outer loop.These loops include several compensations that mak e boost converters.In addition, some feed-forward regulations are also designed to make the system highly robust for both of input voltage and output current disturbances.The simulation result shows that regulation of the voltage transfer ratio and output frequency can be realized optionally by the new converter, furthermore the harmonic distortions of waveform is low and have the advantage, such as robustness, good tracking performance.These new inverter is intended to be used in uninterruptible power supply (UPS) and ac driver systems design whenever an ac voltage larger than the dc link voltage is needed, with no need of a second power conversion stage.

Figure 2 .
Figure 2. The current bidirectional boost dc-dc converter.
are e inductor voltage and current, 1 o i is output rrent, and 1 d is the duty cycle.The inductor and capacitor different equation th the cu s are: loop control scheme for each Boost are a current loop bandwidth close to 2KHz and a voltage loop bandwidth of about 800Hz,both phase margins of 50°.The contrast analysis diagrams are the three-phase boo 3 0 L L st double-loop control and three-phase buck-boost sliding phase boost doubleol of load FFT: nt: distortion of the output voltage waveform ve is low; 3) -loop control, w mode control of load voltage:

Figure 8 .Figrue 9 .
Figure 8. Three-phase boost double-loop control of the load.

Figure 10 .
Figure 10.Three-phase boost double-loop control of the load FFT.

ments [ 2
] Z. X. Yan, J. X.Li, W. Zhang, Q. Zhang, Y. N. Zheng and W. Y. Wu, "Topology Family and the Simulation of " BOOK" Differential Single-stage Stage Inverter," IEEE, 2010.doi:10.1109/63.737601e possible an accurate control of th6.AcknowledgeThis work was supported by key programs of NSFC(50837003), and by the Program of the Science and Technology Foundation of Hebei province of China (11213943),and also be supported by the Doctor Research Fund of Yanshan University(No.B549).