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 15, 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).

• A battery energy storage system is connected to a bi-directional IGBT DC-DC converter to charge and discharge the battery. The battery nominal voltage is 480 V, varying from 400 V (full discharge) to 545 V (floating charge voltage), with a charging current set to 56 A and a peak-to-peak ripple current of 5 %. The battery internal resistance is assumed to be 0.5 Ω.

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

The UPS DC link operates at 700 V.

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

For the theoretical calculations, please refer to Lab 3 and Lab 4. This lab serves as a continuation of those two and utilizes the same DC-DC converter studied in Lab 3, as well as the IGBT rectifier and inverter systems examined in Lab 4.

II. Simulation Procedure

Normal Operation: AC Source is Active - Case A

Start by activating the AC source in the Dashboards panel. In this scenario, the AC source supplies power to the rectifier, which then sends power to both the inverter connected to the load and the DC-DC converter to charge the battery.

1. Adjust 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 without load. 

Table 4: Case A 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 5: Case A 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 8.

Table 6: Case A Question 3 recorded values

4. 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 7: Case A Question 4 recorded values

5. Using the modulation index computed for the rated output voltage in section I Lab 4, 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)

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

UPS Operation: AC Source is Inactive - Case B

Deactivate the AC source in the Dashboards panel. In this scenario, the battery supplies power to the load through the inverter, simulating a power outage condition.

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 8: Case B 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)

Transition Operation: Switching between AC Source to Battery Power

To capture the transition between the AC source and battery power, select Display 2 and activate the Trigger Switch. Ensure the LED light is green before capturing results.

1. Starting with an Active AC Source:

   • If the AC source is not active, switch it on while the Trigger Switch is off, and allow the system to reach a steady state.

   • Once the system is stable, turn on the Trigger Switch and then deactivate the AC source.

   • Observe and analyze the variations in current and voltage displayed during the transition phase.

2. Starting with a Deactivated AC Source:

   • Deactivate the AC source, ensuring the system reaches a steady state.

   • Activate the Trigger Switch, then switch the AC source to active.

   • Observe and analyze the variations in current and voltage displayed during the transition phase.

3. Data Analysis:

   • Retrieve the saved OPREC file to review the results more clearly and conduct a detailed analysis.

4. Further Exercises:

   • Repeat the above exercises with different power factor references.

   • Perform the same transitions at rated input voltage, as well as at minimum and maximum voltage values.