The modified space vector PWM for three-phase voltage source inverter with AC decoupling circuit

This paper proposes the modified space vector pulse width modulation (SVPWM) control algorithm for the three-phase voltage source inverter (VSI), which consists of traditional six switches VSI and three bidirectional switches for creating the ac decoupling circuit. This topology has some advantages such as the ability to reduce the leakage ground current from PV panel and therefore improve the efficiency of photovoltaic (PV) energy conversion based on the principle of decoupling when the zero space vectors occur. Likewise, no current flows through six traditional switches in zero space vectors, thus they operate at lower average temperature. In this case, the conduction losses are reduced. For avoiding shoot-through states between six traditional switches and three bidirectional switches, a delay time has been added to switching time and managed carefully to guarantee THD of output voltage. Operating of the inverter with the modified SVPWM method is simulated by using Matlab/Simulink software and implemented in the experimental prototype by using FPGA Virtex 5 (Xilinx).


INTRODUCTION
Nowadays, conventional three-phase voltage source inverter (CVSI) is widely used in industrial and energy system, in order to convert electrical energy from DC to AC (Fig. 1).This scheme is particularly applied in the electric motor drive systems and renewable energy systems such as photovoltaic (PV), fuel-cell conversion systems.The recent study on improving the efficiency of three-phase inverters is based on the structural changes of the VSI, therefore the principal scheme and control method for generating pulse patterns become more complex and it is difficult to be implemented for the existing VSI [1,2].Recently, many research papers have been published for single-phase VSI scheme for transformerless grid-connected system.The efficiency improvement is based on the principle of AC or DC decoupling, when zero state occurs.In these works, some topologies, based on the H-bridge with an AC bypass circuit consisting of a bidirectional switch (HERIC topology) [3] or a diode rectifier and a switch with clamping to the DC midpoint (HB-ZVR topology) [4], are proposed.In case of DC decoupling circuit, some topologies are invented, which consists one extra switch (H5 topology) [5] or two extra switches (H6 topology) [6,7].High conversion efficiency in these schemes is obtained.However, this principle has not been analyzed and applied for three-phase inverters in more details.In order to increase the efficiency, the proposed schemes usually pay attention to the use of multilevel principles [8,9].This leads to a change in configuration and availability of PWM methods such as SVPWM of the welldeveloped three-phase two-level VSI.This paper analyses the topology of modified three-phase VSI (MVSI), which consists of traditional six switches voltage source inverter and three bidirectional free-wheeling switches for creating the AC decoupling circuit as shown in Figs.2a and 2b.

Figure 3. MVSI with an output LCL filter
This topology has some advantages such as the ability to improve the efficiency based on the principle of decoupling, the ability of applying to the CVSI scheme, the simplicity of the control algorithm (modified SVPWM) and can widely applied to many applications.This topology can be applied to: The application of electrical energy conversion from DC to AC: • For providing the AC voltage with fixed frequency and effective value to the load (such as RL load) with the output LCL filter for obtaining the true sinusoidal waveform (Fig. 3).
• For providing the AC voltage with variable frequency and variable effective value to the load (such as induction motor), without the output LCL filter (Fig. 2a, 2b).Semiconductor losses in VSI can be calculated by using the method in [10].The efficiency of the VSI and proposed MVSI is calculated as follow: where Pin is the input DC power, Pout is the ouput AC power according to 1 st voltage harmonic:

B. Modified Space Vector PWM algorithm for the MVSI
The SVPWM is based on the formation of three voltage vectors in sequence in one sampling interval Ts so that the average output voltage meets the requirement.In the SVPWM method for the VSI, there are six active vectors and two zero vectors.The reference output vector is synthesized by using two active vectors and one zero vector.The active and zero vector of each sector in conventional algorithm are shown in Fig. 5.  where: Txduration for vector active vector Vx, Tyduration for active vector Vy, Tzduration for zero vector Vz (included V0 and V7), Mmodulation index M = V * /V1sw (V *amplitude of the reference voltage vector, V1swpeak value of six-step voltage).
Based on the analysis of SVPWM, the modified space vector PWM is proposed in this paper.The difference of this modified SVPWM from the conventional one is when the zero space vector (ZV) occurs, in the proposed topology (MVSI), all switches SS1, SS2 and SS3 are turned on and all S1, S2, S3, S4, S5 and S6 are turned off.This way, using SS1 -SS3 as shown in Figs.2a and 2b, the zero-voltage space vector is realized by short-circuiting the output of the inverter, during which period the DC-link is separated from the load or the grid, because all switches S1 -S6 are turned off.Three voltage vectors of each sector in modified algorithm are shown in Fig. 6.When the zero space vector occurs, six main switches are turned off instantly and three bidirectional switches are turned on after a while.When the zero space vector finishes, three bidirectional switches are turned off instantly and six main switches are turned on after a while.This interval is called dead time.
The flowchart of this algorithm is shown in Fig.

7.
For the modified algorithm, there are two switches which are turned off completely in each sector which is shown in Table 1.However, in the dead-time interval where no switch is turned on, so the current will flow through the free-wheeling diode and the output voltage can not be controlled.This state is called off-state and can distort the output voltage and current, which effect to the power quality.The switching states in the sector 1 are shown in Fig. 8.When the zero space vector finishes, two switches which according to sector were turned on before a while to create five-switch state.

TABLE 1. INACTIVE SWITCHES OF EACH SECTOR
Then, three bidirectional switches are turned off to create two-switch state and the inverter is in the modulation state.
The inactive switches of each sector are similar to the one of the modified algorithm and the active switches which must be turned on for a while are shown in Table 2.The flowchart of the proposed algorithm is shown in Fig. 9 and the switching state of sector 1 is shown in Fig. 10.As shown in Fig. 10, the problem which is mentioned in the modified algorithm is solved; the off-state is removed completely.The proposed algorithm is more complex than the modified algorithm, so it is difficult to implement on DSP because it needs to be implemented on a fast microprocessor.

Figure 1 .
Figure 1.Conventional three-phase voltage source inverter a) Decoupling circuit with three bidirectional switches b) Decoupling circuit with three diode rectifier and switches

Figure 2 .
Figure 2. Proposed high efficient three-phase voltage source inverter with AC decoupling circuit (MVSI)

Fig. 4
Fig.4 shows the PV panels three-phase gridconnected applications with an isolation low voltage transformer.Almost the network configurations are divided into two groups: with galvanic isolated transformer and transformerless.The grid connected configuration with isolated transformer has advantages such as safety, no leakage current, no DC current injection.In contrast, transformerless topology has higher efficiency, but it has more complex control algorithms and some related issues such as leakage current and DC current injection into the grid.

Figure 5 .
Figure 5. Space vectors of CVSI.The calculations of the duration of two active and two zero vectors are as follows for a half of sampling period[11].

Figure 6 .
Figure 6.Space vectors of MVSI However, the dead-time interval (DTI) should be applied between switching states of the main switches (S1 to S6) and bidirectonal switches (SS1 to SS3) to avoid short-circuit state.
is very simple and easy to implement by using a FPGA controller for generating switching patterns.C.The proposed SVPWM and Dead-time Generation algorithms for the MVSIIn order to improve the quality of output voltage and current, the above algorithms needs to be changed.At any time, at least two switches must be turned on.This can be implemented in three states, so this algorithm can be called three-stage dead-time generation algorithm.If there are only two switches which are turned on, this is called two-switch state.If there are two above switches and three bidirectional switches which are turned on, this is called five-switch state.The last state is zero state or modulation state in which there are three switches or bidirectional switches turned on.

Figure 13 .
Figure 13.Leakage current responses It can be seen that the efficiency of MVSI is higher than CVSI.Fig. 15 shows the efficiency comparison of CVSI and MVSI according to the output power.Otherwise, the leakage current of MVSI is lower than CVSI.

Figure 14 .
Figure 14.The output/input power, the corresponding efficiency and the leakage current of the proposed converter.

•Figure 16 .Figure 17 .Figure 18 .Figure 19 .
Figure 16.Experimental setup of MVSIThe switching pattern of sector 1 for the CVSI and MVSI is shown in Fig.17.In CVSI, pulse SS1 is always turned off.In MVSI, the zero-state duration in which all three pulses of CVSI are 1 or 0, so the pulses of AC decoupling circuit will be turned on in this duration and all pulse of VSI will be turned off immediately.There are also switching steps between normal mode and high efficiency mode, which are mentioned in proposed SVPWM algorithm and are shown in Fig.18.

TABLE 3 .
SIMULATION RESULTS OF CVSI

TABLE 4 .
SIMULATION RESULTS OF MVSI

TABLE 5 .
EXPERIMENTAL RESULTS OF CVSI