3.1 Converter Topology

The Single-Phase Two-Level converter is shown in figure 2 below:

Figure 2: Single-Phase Two-Level Inverter

The single-phase two-level inverter has four IGBT/diode switches (named as S₁ to S₄) with two switches per arm.
The inverter has two inputs: (i) positive and (ii) negative terminals of the DC voltage source.
It also has two outputs  and  which feed the load.

3.2 Control of the Switches and PWM Generation

The control of the switches of the first arm, that is displayed in figure 3, is explained below.
The same control is to be applied for arm two with a phase-shift of +180^o.

The PWM train is generated using the method of intersection between the reference signal that is a sine-wave signal oscillating at either 60 Hz or 50 Hz, and one triangular carrier oscillating at the switching frequency.
The reason behind having one carrier (instead of two) is the fact that each arm contains two IGBTs (instead of four IGBTs, as is the case in a three-level inverter).
The comparison between the reference signal for the first arm and the carrier is illustrated in figure 4.

The switching frequency is a user-controlled parameter, varying between 900 Hz and 3000 Hz.
It is up to the user to select switching frequency values that are multiples of 60 or 50 depending on whether 60 Hz or 50 Hz is chosen as a reference frequency.

Figure 3: First Arm of the Single-Phase Two-Level Inverter

Figure 4: PWM Generation for the First Arm of the Single-Phase Two-Level Inverter

From figure 4, the student can see that switches S1 and S2 are complementary, which means that both of the switches in the same leg cannot be turned ON at the same time,
as the input DC voltage would be in short-circuit, thus violating the Kirchhoff’s voltage law (KVL).
Therefore, we have:

Table 1: The Switching States in the Single-Phase Two-Level Inverter

Table 1 above shows the switching state of the chosen PWM generation and the corresponding voltage levels.
It can be observed from the table that when the two top or the two bottom devices are turned on, the output voltage is zero.

3.3 Load and filter

The load and filter are show in figure 5 below.

Figure 5: Load and Filter in the Single-Phase Two-Level Inverter

3.2.1 Load

The load is composed of the combination in series of (i) a constant resistance, (ii) a constant inductance and (iii) a controllable AC-source.
Therefore, the student controls the three parameters of the AC-source, namely, the amplitude, frequency, and phase-shift.
In section 5, exercises showing the impact of changing these parameters will be covered thoroughly.

3.3.2 Filter

The filter is composed of inductance in series with the load, and the capacitance.
The filter inductance value corresponds to 10% of the load inductance.

3.4 Inverter Parameters and Nameplate

The three-phase two-level parameters and its nameplate are shown in table 2 below.

Table 2: Single-Phase Two-Level Inverter Parameters and Nameplate Ratings

3.5 Measurements

Three spots were considered for current, voltage and power measurements, as shown in figure 6 below.

• The DC bus at the input of the converter bridge
• The AC bus at the output of the converter bridge
• The input of the load

Figure 6: Measurements in the Single-Phase Two-Level Inverter