Three Phase Two Leg Neutral Point Clamped Converter with output DC Voltage Regulation and Input Power Factor Correction

Received Jan 15 , 2012 Revised Apr 17, 2012 Accepted Apr 27 , 2012 A three-phase neutral point clamped (NPC) converter is p esented for power factor correction and dc-link voltage regulation. T he adopted converter has simpler circuit configuration compared to three lev el PWM converters A simplified space vector pulse width modulation sche me (SVPWM) is adopted to track line current commands. Using a sim plified SVPWM algorithm, the calculate time for the time duration f voltage vector is reduced. The adopted NPC converter has less power s witches compared with the conventional three-level NPC converter. Only ei ght power switches and four clamping diodes with voltage stress of half th e dc bus voltage are used in the circuit configuration. Based on the proposed control algorithm, a reference voltage vector is generated on the ac ter minal for drawing the sinusoidal line currents with unity power factor. Co mputer simulation results based on MATLAB/SIMULINK are presented to verify the validity and effectiveness of the proposed control strategy . Keyword:


INTRODUCTION
Diode or phase-controlled rectifiers are widely utilized in the front-end converter for the uncontrollable or controllable DC-bus voltage in industrial and commercial applications. However, low power factor and non sinusoidal line currents are drawn from the AC source owing to a large electrolytic capacitor used on the DC link. Power pollutants such as reactive power and current harmonics result in line-voltage distortion, heating of the transformer core and electrical machines, and increased losses in the transmission and distribution line. To meet the relevant standards in Europe and America, several current wave-shaping solutions have been proposed to achieve power factor correction and current-harmonic reduction. Multilevel rectifiers and inverters have been proposed for high power and medium-voltage applications because they provide advantages such as the low voltage rating of power semiconductors and low voltage harmonics. However, the disadvantages of the multilevel rectifiers are the large number of power semiconductors in the circuit, a complex control scheme and the neutral-point voltage balance problem. In industrial applications with a unidirectional power flow, conventional multilevel converters are too expensive and complicated to implement. A three-phase three-level AC/DC converter with fewer power switches is presented to achieve almost unity power factor, to regulate the DC-link voltage and to achieve fast dynamic response. The circuit topologies of multilevel inverters can be classified into neutral point diode clamped (NPC) inverters [12][13], flying capacitor clamped inverters [14][15], and cascade full bridge inverters. Three-phase three-level NPC converters were proposed in [18][19][20] to draw the sinusoidal line currents in phase with mains voltages. The input power factor is close to unity. However, twelve power switches and six clamping diodes are used in the circuit configuration.  Fig.1shows the circuit configuration of the adopted three-phase NPC converter. Two NPC legs are used in the converter. Each NPC leg has four power semiconductor switches and two clamping diodes. Each power semiconductor has a voltage rating of half the dc bus voltage. Three boost inductors are connected to converter on the ac terminal. These inductors are used to achieve boost operation and to achieve line current filtering. The phase C is directly connected to the midpoint of the split dc capacitors. Three valid voltage levels are generated on the ac terminal voltages Vac and Vbc. Five voltage levels are generated on the voltage V ab based on the proper operation of two NPC legs. A dc bus voltage controller is used to balance the power demand between the ac source and dc load. A current controller is used to obtain the reference control voltage vector of the converter. The simplified space vector modulation is adopted to obtain the time duration of the selected voltage vectors. The time intervals of power switches corresponding to the selected voltage vectors are calculated to drive the converter. The control goals of the proposed converter are to obtain a constant dc-link voltage, to draw the sinusoidal line currents, and to achieve unity input power factor.

PROPOSED EIGHT SWITCH NPC CONVERTER
Where Vsm and Ism are the peak source and peak current of three-phase ac source, respectively, ϕ is the phase angle between the source voltage and line current. From the voltage equations on the ac terminal of the converter shown in Fig. 1 In the steady state, the reference control voltages in the stationary reference frame can be calculated from (2)-(4) for the given source voltages and the obtained line currents from the output of the control loop. The differential equation of (4) can be rewritten in the orthogonal coordinate and expressed as For a unity power factor operation, the phase angle α between the line current and phase voltage is zero. Fig. 2 shows the phasor diagram of the converter for unity power factor operation. For the rectifier operation, the line current is in phase with phase voltage. There is a phase lag between the source voltage Vs and reference control voltage V. The magnitude and phase angle of the reference control voltage V can be calculated from (6) and expressed as With the generated voltage vector in (7) on the ac terminal, the line currents are controlled to be sinusoidal waves with nearly unity power factor However, the calculation time of trigonometric function and square root operation is too long. The high performance of digital controller is needed to obtain the reference voltage vector.
For the adopted three-phase NPC converter, eight power switches, and four clamping diodes are used.
The power switches Sxy and Sxy 1 (x = a,b ; y = 1,2) are complementary to each other to avoid the short circuit in each converter leg. Three valid switching states are available in each converter leg to achieve three voltage levels vdc/2, 0, -vdc/2 on the ac terminal. For example of converter leg a, the upper two power switches S a1 and S a2 are closed to achieve voltage V a0 ,=V ac =V dc /2. In this operation state, line current i sa is decreasing because v sac = v sa -v sc <V dc /2. If the power switches S a1 1 and S a2 are closed to turn on, the ac side voltage v ac equals -v dc =2. For this operation state, line current isa is increasing because v sac = v sa -v sc > -V dc /2. A zero voltage level is generated on the voltage vac if the middle two switches S a1 1 and S a2 are closed. For this operation condition, the phase current i sa is increasing (or decreasing) if the line voltage v sac is positive (or negative). Based on the above descriptions of three operation states in each converter leg, there are nine possible switching combinations in the adopted converter to control the line currents.
where v oN is the voltage between the dc bus midpoint o and the point N on the ac source, and f a and f b are defined as  (11) Applying (6) into (8)-(10), the ac terminal voltages in the orthogonal coordinate system are given by Based on the combinations of the switching states of switches, there are nine valid voltage vectors. The switching combinations can be represented by the order sets [S a1 , S a2 , S b1 , S b2 ], where S a1 = 1 denotes that power switch S a1 is closed, and S a1 = 0 denotes that power switch Sa1 is open. The same notation applied to power switches S a2 , S b1 , and S b2 . The reference control voltages V α and V β in the orthogonal coordinate can be obtained from the ac terminal voltages v ac and v bc in the abc coordinate system by applying (12). These voltage vectors are given by

Calculation of switching time
The space vector modulation and the sequencing of the selected switching vectors have been proposed in [9]- [11]. other two vectors V4 and V8 are opposite in direction, but their vector length is V ctors divide the whole circle into four sectors (sectors 1, 2, 5, and 6) with 60 four sectors (sectors 3, 4, 7 and 8) with 30 0 angle duration. Fig. 3 and Table I show the definition of these

Calculation of switching times for Reference Vector
The space vector modulation and the sequencing of the selected switching vectors have been [11]. Based on the nine possible voltage vectors generated by the converter, there are eight system. The reference control voltage vector V * can be synthesized within one cycle time of length T. The reference control voltage vector V * can be expressed as and V5, V2 and V6, V3 and V7 are √6 on the orthogonal coordinate plane. other two vectors V4 and V8 are opposite in direction, but their vector length is V dc /√2. These nine ctors divide the whole circle into four sectors (sectors 1, 2, 5, and 6) with 60 0 angle duration and angle duration. Fig. 3 and Table I show the definition of these The space vector modulation and the sequencing of the selected switching vectors have been [11]. Based on the nine possible voltage vectors generated by the converter, there are eight can be synthesized within one (13) anded voltage. Based on the can be expressed by two adjacent voltage (14) Where Tx and Ty are time duration of voltage vectors Vx and Vy, respectively, in each sector. The (15) . The relationships between each sector and the reference voltage vector as given in Table 2 By projecting the control voltage vector and two adjacent vectors onto the real and imaginary parts in the αβ coordinate system, one can obtain the following relations The time durations of the selected switching vectors V 1 , V 2 , and V 0 are easily calculated and expressed as Sequencing of power switches within one switching period T Table2 give the sequencing of the power switches in sectors 2-8, respectively. The reference voltage vector V * can be composed to control the line currents with unity power factor. In the conventional space vector modulation, the magnitude and phase angle of the reference voltage vector must be given to determine the sector location and to calculate the time duration of adjacent vectors. This control algorithm is so complicated that it requires a long computation time. To reduce the computation time of trigonometric function and square root operation in (7), four lines are defined to divide the whole circle into eight sectors. These four lines are 3 : , 3 : Therefore, any point of reference voltage vector V * in the orthogonal coordinate system can determine its location through these four equations in (18). Table 3and Fig. 4 give the relationships between the sector where the reference voltage vector V* lies in and the values of variables P1-P4.  P1 (1,0,1,1) (1,1,1,1) (1,1,0,1) (1,1,0,0) (0,1,0,0) (0,0,0,0) (0,0,1,0) Now we can use variables P1-P4 to determine the sector where reference vector V* lies in and to calculate the time duration of power switches. No calculation of trigonometric function and square root operation is needed in the adopted algorithm.  5 give the control block of the proposed control algorithm. A dc bus voltage controller is used to balance the power demand between the ac source and dc load. A current controller is used to obtain the reference control voltage vector of the converter. The simplified space vector modulation is adopted to obtain the time duration of the selected voltage vectors. The time intervals of power switches corresponding to the selected voltage vectors are calculated to drive the converter. The control goals of the proposed converter are to obtain a constant dc-link voltage, to draw the sinusoidal line currents, and to achieve unity input power factor.

Proposed Controller Control Block
The algorithm of the closed-loop compensators in the synchronous reference frame is used in the control scheme. A voltage controller is adopted to obtain the line current command i sd * . For a unity power factor operation, the line current command i sq * is set to zero. The reference voltage vector V e in the synchronous reference frame can be obtained by the transformation from α-β coordinate system to synchronous reference frame  If the voltage drop on the resistor r can be neglected, then the reference voltage on the ac terminal in the synchronous reference frame is expressed as Three-phase line currents are measured and transformed into the synchronous reference frame. Based on (22) and (23), the voltage command on the ac terminal in the synchronous reference frame are calculated. Using the coordinate transformation from the synchronous reference frame into the stationary reference frame, the voltage commands  Table 2 is exported to the gate drive circuits. The reference voltage vector V * can be composed by the properly two voltage vectors in each sector

Simulation Parameters
Some simulation is presented to confirm the validity and effectiveness of the proposed control scheme. Simulation was performed in the SIMULINK toolbox of MATLAB. The power stage parameters of the three-phase converter are Dc link capacitance: 2200 µF, Power switches: IGBT IRG4PC40W, Boost inductor: 5 mH, Switching frequency: 20 kHz, The line voltage is :110 V, source frequency : 60 Hz, The dc link voltage equals: 400 V, Resistive Load R :20 ohms, R-L Load, R=15 ohms, L=5mH

Simulation Outputs
For R Load, simulated results of power factor with controller are shown in Fig 8 and without controller is shown in Fig. 8 Simulated results of AC terminal voltages for V 0 =400V are shown in Fig 9. Simulated results of capacitor voltages for V 0 =400, Vo=500 and V 0 =600 are shown in

CONCLUSIONS
A simple PWM scheme based on space vector modulation for an eight-switch three-phase NPC converter is presented to save the calculation time, to obtain the time duration of power switches, and to control the line currents with unity input power factor. The circuit configuration is simple compared with the conventional twelve-switch three-phase NPC converter. Only eight power switches and four clamping diodes are used in the converter.
In the proposed converter, the dc bus voltage is greater than two times of line voltage (V dc >2V line ). The size of capacitor in the proposed converter is twice larger than that in the conventional twelve switches NPC converter to maintain the same voltage ripple. With the proposed control scheme, the sinusoidal line currents with unity power factor drawn from the ac source and constant dc-link voltage are achieved. Computer simulation results are presented to demonstrate the validity and effectiveness of the proposed control scheme.