Lab 4 - Introduction

Three-phase voltage source converters are used to supply three-phase AC loads in applications such as uninterruptible power supplies, renewable energy interfaces, electric vehicle traction drives and AC motor drives. A voltage source inverter is shown below in figure 1 feeding a three-phase load. The 2-level inverter comprises of a switch network of 6 devices, a DC-link and branch inductors interfacing the AC system. The upper and lower switches on the same phase leg have opposite states. The 2-level voltage source converter features four quadrant operation on the voltage-current plane, and hence it can operate both as an inverter and rectifier as shown in figure 1 and figure 2. 

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Figure 1: Three-phase 2-level inverter

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Figure 2: Three-phase 2-level PWM rectifier

Pulse-width modulation (PWM) is used in three-phase voltage source inverters to shape a constant DC input voltage V_d into a three-phase AC output voltage, and to control the magnitude, frequency and phase of the output. To do this, three sinusoidal control voltages that are 120^o out of phase are compared with a triangular voltage waveform,. This PWM control signals are shown in figure 3, accompanied by pulse patterns for switches S1 and S2, and the associated time domain waveforms. The frequency modulation ratio m_f is defined as the ratio of the switching frequency f_s of the inverter and the frequency of the control voltage, f₁. Typically, the switching frequency is adjusted to optimize the converter switching loss performance and meet the system power quality requirements. The fundamental rms line voltage V_L1(rms) and DC voltage V_d ratio for the inverter as a function of m_a is shown in figure 4. For values of m_a ≤ 1.0, the fundamental-frequency voltage magnitude increases proportionally with the modulation index. However, at values of m_a ≥ 1.0, V_L1(rms) does not increase linearly. At these values of m_a, the inverter is operating in the over-modulation region, when the peak of the control voltages is allowed to exceed the peak value of the triangular waveform. At sufficiently large values of m_a (m_a ≥ 15), the PWM degenerates into a square-wave inverter waveform. This results in a maximum value of V_L1 = 0.78V_d.