Page Contents

I. Preliminary Theoretical Calculations

An in-line 3-phase, 480 V input, 480 V output, 60 Hz, 100 kVA, 0.9 pf lagging, Uninterruptible Power Supply (UPS), shown in Figure 11, is made up of the following components:

• an IGBT 6-switch 2-level input rectifier, connected to the ac grid through a reactance (5 % on the UPS base), with a nominal input voltage of 480 V, and allowable variations from 430 to 530 V, feeding the intermediate DC bus

• an IGBT 6-switch 2-level output inverter, with a voltage regulated at 480 V, whatever the operating mode, connected to the load through a reactance (5 % on the UPS base)

All IGBT switches operate at 25 kHz. The 6-switch IGBT converters use a Sine Pulse Width Modulation (SPWM) modulation pattern.

Modes of operation considered:

• Case A – normal operation: the rectifier is fed from the AC supply at rated voltage at a set power factor and regulates the DC link at 810 V; the output inverter supplies rated load.

• Case B – UPS mode: the AC supply is disconnected; the battery DC-DC converter regulates the DC bus voltage to 700 V; the output inverter supplies rated load.

Open

Figure 11: Single line diagram of a UPS system containing a IGBT switch based input rectifier and output inverter

Steady State Rated Operation of the Output Inverter – Case A

1. Draw the output voltage (line-line) for square wave operation, derived from the line to capacitor midpoint voltages. Show the gating pattern of the switches in phases A and B. Compute the rms line-line output voltage and the fundamental component. Draw the harmonic spectrum of the output voltage showing the first 5 harmonics.

2. Draw the waveforms for a SPWM pattern generator for 50 % output voltage in the linear mode. Define the modulation index M in terms of the control signal and the amplitude of the carrier. Draw the inverter transfer characteristics, line-line output voltage (fundamental) versus modulation index, in the linear and over-modulation regions.

3. For the rated output voltage, compute the modulation index. Draw the approximate harmonic spectrum of the output voltage, indicating the first 3 harmonic component groups. Draw the corresponding line current harmonic spectrum.

4. Compute the average DC bus current for rated load. Draw the approximate capacitor current waveform and give the harmonic content.

5. Estimate the dominant harmonic currents of the inverter output line current, taking into account the link reactance.

Steady State Rated Operation of the Input Rectifier – Case A

1. Compute the modulation index for rated input voltage, and minimum and maximum values. Draw the transfer characteristics for the rectifier, AC voltage as a function of the modulation index, showing the 3 operating points. Neglect the voltage drop in the line reactance.

2. Compute the line current (fundamental), and real and reactive power supplied to the rectifier, for all 3 cases. Assume a unity power factor operation.

3. Explain, by means of a phasor diagram, how the input power factor can be varied, from 0.75 leading to 0.75 lagging. Indicate the impact on the real and reactive power of the rectifier.

Steady State Rated Operation of the Output Inverter – Case B

1. Draw the inverter transfer characteristics, line-line output voltage (fundamental) versus modulation index, in the linear and over-modulation regions. Draw the corresponding characteristic for Case A. Compute the modulation index.

2. Indicate the changes in harmonic content of the inverter output voltage compared to Case A. similarly for the DC bus current and the inverter current.

II. Simulation Procedure

Steady State Rated Operation of the Output Inverter – Case A

After selecting the Part 1 tab and clicking on the Part 1 button to activate it and based on the values calculated in Section I, proceed with the questions below. Note that the ac side PWM rectifier, DC-DC battery converter and DC-link of the UPS system have been modelled as a stiff DC input voltage source for simplicity when analyzing the 2-level output inverter operation.

1. Plot the output voltage (line-line) for square wave operation (over-modulation), derived from the line to capacitor midpoint voltages. Plot the gating pattern of the switches in phases A and B. Indicate the rms line-line output voltage and the fundamental component. Plot the harmonic spectrum of the output voltage showing the first 5 harmonics.

2. Set the manual switch to position 2. Plot the waveforms for a PWM pattern generator for 50 % output voltage in the linear mode. Plot and illustrate the modulation index M in terms of the control signal and the amplitude of the carrier. Record and plot the inverter transfer characteristics, line-line output voltage (fundamental) versus modulation index, in the linear and over-modulation regions.

Table 5: Part 1 Question 2 recorded values

3. Using the modulation index computed for the rated output voltage in section I, plot the harmonic spectrum of the output voltage, indicating the first 3 harmonic component groups. Plot the corresponding line current harmonic spectrum. (Note: refer to Harmonics analysis using ScopeView for more details on how to plot the harmonic spectrum)

4. Plot and record the average DC bus current for rated load. Plot the harmonic content.

Steady State Rated Operation of the Input Rectifier – Case A

After selecting the Part 2 tab and clicking on the Part 2 button to activate it and based on the values calculated in Section I, proceed with the questions below. Note DC-link of the UPS system only contains a DC capacitor and that the inverter load has been added to the DC-link. 

1. Adjusting the AC side voltage to the rated input voltage, minimum and maximum values and measure the modulation index. The closed control system will automatically adjust the modulation index to reach the reference DC-link reference voltage of 810 V. 

Table 6: Part 2 Question 1 recorded values

2. Record the line current (fundamental), and real and reactive power supplied to the rectifier, for all 3 cases by adjusting the ac side voltage to the rated input voltage, minimum and maximum values. Set the power factor reference to unity.

Table 7: Part 2 Question 2 recorded values

3. For the rated voltage input, vary the input power factor from 0.75 leading to 0.75 lagging and record the values in Table 11.

Table 8: Part 2 Question 3 recorded values

Steady State Rated Operation of the Output Inverter – Case B

After selecting the Part 3 tab and clicking on the Part 3 button to activate it and based on the values calculated in Section I, proceed with the questions below.

1. Record and plot the inverter transfer characteristics, line-line output voltage (fundamental) versus modulation index, in the linear and over-modulation regions. Record the modulation index required to provide 480V given a 700V DC link.

Table 9: Part 3 Question 1 recorded values

2. Plot harmonic content of the inverter output voltage, the DC bus current and the inverter current for the rate 480V AC output voltage. (Note: refer to  Harmonics analysis using ScopeView for more details on how to plot the harmonic spectrum)

III. Questions

1. Tabulate the theoretical results calculated in Section I and the simulated results obtained in Section II for the operation of the 2-level converter in rectification and inversion modes.

2. Draw the phasor diagrams using the data collected in Part 2 Question 3 for leading, lagging, and unity power factor operation. Explain the impact of the inverter limits on the operating power factor.

3. Explain an alternate power converter configuration for the 2-level rectifier, which can provide the DC-link power and draw power at unity power factor.

4. Explain the closed loop control configuration and operational limits for both rectification and inversion modes.