LI Ning, WANG Yue, JIANG Yingwei and WANG Zhaoan
Xi’an Jiaotong University, China xjtulining1983@ Abstract-- A fast algorithm for three-level space vector pulse wide modulation (SVPWM) in neutral point clamped (NPC) inverters based on traditional three-level sinusoidal pulse wide modulation (SPWM) is proposed. The acting time relation of each state in each phase is got by deeply researching the similarities and differences of three-level SPWM and SVPWM. This algorithm can realize all kinds of state sequences easily, it does not need to do trigonometric function, irrational operation and coordinate transformation, it just needs ordinary arithmetic, therefore the calculation is very simple and the result is much more accurate. The fast algorithm can be easily implemented on microprocessor and the executing speed is faster than the conventional threelevel SVPWM algorithm. Index Terms-- fast algorithm, three-level SVPWM, acting time relation, state sequences.
Fig.1 Schematic diagram of three-level SPWM
B. Conventional three-level SVPWM
020
120
220 Vref
220 Vref 4 210
Sec. 1
021
121 010
221 110
221 110 2 222 111 000 1 211 100
I. INTRODUCTION Since the development of power electronics, many kinds of PWM technologies have been used in power electronic circuits, SPWM and SVPWM are two mostly used schemes of them [1]. SPWM uses triangular carrier to compare with sinusoidal wave to get pulse signals, it realizes easily, yet the voltage utilization is low. SVPWM can obviously reduce the current harmonic component and raise the utilization ratio of the power, however, in traditional SVPWM, nonlinear operation is needed to be carried out, such as the sine and arctan function etc, these operations will influence the control speed of the system. In threelevel NPC inverters, the simplified or fast algorithm of three-level SVPWM is attracting more and more attention, due to the complexity of three-level SVPWM[2][3][4]. Some simplifications to SVPWM algorithms have already been proposed in literatures[4][5][6][7][8][9][10], yet some of these algorithms still have to use look-up tables to pick out the switching state vectors which compose the reference vector, some of them can not change the switching state sequences easily. In this paper, a fast algorithm for three-level SVPWM in NPC inverters is discussed, it is very fast and simple to implement, further more, it can realize different state sequences easily, therefore it can be used in many different situations.
978-1-4244-5670-3/10/$26.00 ©2010 IEEE
53
reference vector Vref, secondly, it identifies the sector and triangle in which the reference vector falls, as shown in Fig. 2. After the triangle is identified, a look-up table with as many entries as the number of triangles is used to find the vectors to be used. Then the SVPWM algorithm carries several calculations to obtain the duty ratios of those vectors, at last, it decides the switching state sequences and transforms switching state into gate pulses. Fig.3 shows some commonly used state sequences of three-level SVPWM. In the six sequences, SVPWM1 is an asymmetrical sequence, it uses two zero switching states, there are six communications per switching cycle. SVPWM2 uses symmetrical sequence to get low THD, there are also two zero switching states, therefore the
2010 2nd IEEE International Symposium on Power Electronics for Distributed Generation Systems
A Fast Algorithm for Three-level SVPWM in NPC Inverters Based on Traditional Three-level SPWM
210 3 200
Hale Waihona Puke 022122 011222 111 000 112 001 212 101
211 100
200
012
201
Tri.1: 000-100-110-111-211-221-222 Tri.2: 100-110-210-211-221 Tri.3: 100-110-210-211 Tri.4: 110-210-211-221
II. FAST ALGORITHM FOR THREE-LEVEL SVPWM A. Traditional three-level SPWM Fig.1 shows the schematic diagram of three-level SPWM in regular sampling method. There are two carriers in three-level SPWM, in one switch period, if the reference voltage is larger than zero, the reference compares with the upper carrier, otherwise, it compares with the lower carrier. From Fig.1, it is clear that in traditional three-level SPWM, there are two states in each phase and the wave form of each phase is symmetrical per switch cycle, so, totally, in a three-phase system, there are six commutations per switch cycle.