IMPLEMENTATION OF THREE PHASE-DISCONTINUOUS SPACE VECTOR MODULATION USING SINGLE DSC-PWM MODULE IMPLEMENTATION OF THREE PHASE-DISCONTINUOUS SPACE VECTOR MODULATION USING SINGLE DSC-PWM MODULE

The paper presents an implementation of a discontinuous space vector modulation of three phase inverter. The modulation is implemented in a DSC (Digital Signal Controller) using only one PWM module. Phases of the inverter work in a complementary mode. This implementation gives an assumption to use one DSC for simultaneous controlling of several three phase inverters. Furthermore, a simulation comparison of discontinuous and continuous space vector modulation is made.


Introduction
Thanks to its simplicity and ability to supply three phase load by a voltage with variable frequency and magnitude, three phase voltage inverters have a wide use. Control of these inverters plays a big role. The simplest and the most widely used control method is the carrier-based sine-triangle PWM. Its drawback is low use of DC-bus voltage. Next method with very good preferences is a Space Vector Modulation (SVM). It has also a lot of variants and improvements. Discontinuous Space Vector Modulation (marked as DPWM) is one of them. It suffers from worst properties of voltage and current waveforms, but also it allows the control to be simpler and also leads to less switching losses.

Space Vector Modulation
We can look at the Three-phase voltage, created by inverter like at a single voltage vector in a vector space. It is rotating by an angular speed of electric field and is stated by its amplitude. The idea of SVM is to create this vector using switching elements of the inverter. The inverter can create six voltage vectors (U 0 , U 60 , U 120 , U 180 , U 240 , U 300 ) and two zero vectors (0000, 0111), which set in by switching on all of the top or bottom transistors.
It is obvious from Fig. 1

The paper presents an implementation of a discontinuous space vector modulation of three phase inverter. The modulation is implemented in a DSC (Digital Signal Controller) using only one PWM module. Phases of the inverter work in a complementary mode. This implementation gives an assumption to use one DSC for simultaneous controlling of several three phase inverters. Furthermore, a simulation comparison of discontinuous and continuous space vector modulation is made.
Keywords: Three-phase inverter, space vector modulation, discontinuous space vector modulation, pulse width modulation.

IMPLEMENTATION OF THREE PHASE-DISCONTINUOUS SPACE VECTOR MODULATION USING SINGLE DSC-PWM MODULE
Stepan Janous -Jozef Sedlak -Michal Prazenica -Jozef Kuchta * Fig. 1 Voltage vectors of the inverter [1] Equation 1 doesn't deal with zero vectors. Therefore, it is valid only if sum of U x and U xϮ60 gives a vector with the same amplitude as vector Uout has. If we assume that the change of Uout within one period is small, we can rewrite Eq. 1 as: ( 2) And if we take zero vectors into account, we get: transistors in terms of every PWM period. Composition of the switching states is different for every sector. In Fig. 2 we have an example for 1st sector. a, b and c are the respective branches.
Thanks to appropriate arrangement of the voltage vectors it is achieved that every transistor is once switched on and once switched off in every PWM period. It is obvious from Fig. 2 that a switchover in c branch is made only because of change from one zero vector to another zero vector. It leads us to one possible simplification of the switching scheme. This is in literature called Discontinuous Space Vector Modulation. Its switching scheme is in Fig. 3 [2], [3], [1].

Implementation of Discontinuous Space Vector Modulation in a Digital Signal Processor
Exactly, the fact that in the terms of every PWM period transistors in one branch don't change their state gives an assumption to drive the whole three-phase inverter with only one PWM module. Digital Signal Controller by Texas Instruments TMS320F28335 has six PWM modules. Every module has its own register to set up a PWM period and also has two compare registers, as we can see in Fig.4. If we are going to use a DPWM, we can use these two registers for controlling two branches.
In Fig. 5 we have a reference waveform for one branch of the inverter calculated by an algorithm of DPWM in Code Composer Studio v4. TBPRD is a register that defines the length of the PWM  [4] period. TBCTR is a counter that is incrementing until it reach the TBPRD value and then starts to decrement until zero. We can set up some events during PWM period with CMPA and CMPB registers. Mainly switching of a PWM output.
It is necessary to switchover a dispatching of the compare registers to the inverter branches due to an actual sector of the reference vector [4].

Simulation comparison of continuous and discontinuous SVM
We used MatLab-Simulink software in order to compare these two types of control with regard to quality of output parameters. The proposed simulated layout is in Fig. 6. In Fig. 7 reference waveforms for continuous SVM and in Fig. 8 for DPWM are shown.
Currents were simulated with the use of mathematical model of an ideal three-phase inverter, which supplied RL load. Waveforms of currents were shown in Figs. 9 and 10 respectively. Finally, a comparison of FFT analysis and currents THD is made [5,6,7,8]. Amplitude spectrums of harmonic components for respective PWM methods are shown in Figs. 11 and 12.

Conclusion
The principle of space vector modulation was introduced in this paper. Then we dealt with discontinuous space vector modu-lation. We showed that DPWM can be implemented in DSP using only one PWM module. It means a great opportunity to save control accessories in some three-phase power systems, such as motor drives. However, a quality of current waveforms is worst in case of DPWM.