Page Contents

I. Preliminary Theoretical Calculations

A battery energy storage system is connected to the DC bus of an in-line 3-phase, 480 V, 60 Hz, Uninterruptible Power Supply (UPS). It uses 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 22 kHz. The UPS DC link operates at 700 V.

Battery modes of operation considered, associated with the UPS operating modes:

• Case A – UPS normal mode, battery charging: the battery is fully discharged (400 V) and is being recharged at rated charging current (56 A).

• Case B – UPS emergency mode, battery discharging: no power is provided from the AC supply (open circuit), the load (90 kW) is fed from the fully charged battery (545 V, no load) through the output inverter of the UPS.

Open

Figure 17: Single line diagram of a UPS system containing a two-quadrant DC-DC Converter.

Operation of the DC-DC converter in battery charging mode- Case A

1. Draw the transfer characteristic of the converter, average output voltage, battery voltage, as a function of the duty cycle. Assume a continuous output current. Indicate the operating points for rated battery voltage and for minimum and maximum battery voltage.

2. Draw the waveform of the converter output voltage, indicating average, and maximum and minimum values. Draw the filter inductor current waveform.

3. Compute the current drawn from the DC bus for minimum and maximum battery voltage in the charging mode. Compute the power drawn from the DC bus in both cases.

4. Show the harmonic spectrum of the output voltage at minimum battery voltage. Estimate the value of the filter reactor to maintain the battery ripple current at its nominal value.

Operation of the DC-DC converter in battery discharging (UPS) mode- Case B

1. Draw the transfer characteristic of the converter, output DC-link voltage as a function of the duty cycle. Assume a continuous output current. Indicate the operating points for rated battery voltage and for minimum and maximum battery voltage.

2. Compute the power that is supplied by the battery to the DC bus. Compute the current supplied by the battery for minimum and maximum battery voltage.

3. Draw the waveform of the converter output DC-link voltage and the voltage across the inductor, indicating average, and maximum and minimum values. Draw the battery current waveform. Estimate the ripple current.

II. Simulation Procedure

Operation of the DC-DC converter in battery charging mode- Case A

Use the HYPERSIM circuit and the Dashboards panel of the two-quadrant DC-DC converter in buck mode, and based on the values calculated in Section I, proceed with the question below. Note that the ac side rectifier and DC-link of the UPS system have been modelled as a stiff DC input voltage source for simplicity when the two-quadrant converter is operating in buck mode. 

1. Record and plot the transfer characteristic of the converter, average output voltage as a function of the duty cycle for the rated, minimum, and maximum battery voltage. Indicate the operating points for rated battery voltage and for minimum and maximum battery voltage. Refer to Table 13.

2. Plot the waveforms of the converter output voltage and the voltage across the inductor, indicating average, and maximum and minimum values for the minimum battery voltage charging at the rated current. Using the computed value of the filter reactor obtained in section III, plot the filter inductor current waveform indicating average, and maximum and minimum values. Indicate the ripple current. Plot all waveforms once the current has reached its steady-state value.

3. Record and plot the current drawn from the DC bus for minimum and maximum battery voltage in the charging mode. Record and plot the power drawn from the DC bus in both cases.

4. Plot the harmonic spectrum of the output voltage at the minimum battery voltage once the steady-state current in reached.

5. Plot the PWM carrier waveform, the reference duty cycle and the pulses of IGBT-1 and IGBT-2 for the rated charging current.  

6. Set the switching frequency of 10kHz, repeat questions (2), (4), and (5).

7. Set the switching frequency back to 22kHz and the DC-DC converter control mode to closed loop position. Activate the trigger, and plot the step response showing the reference current, the measured battery current and the reference duty cycle.

Table 13: Recorded values for question 1-Case A.

Operation of the DC-DC converter in battery discharging (UPS) mode - Case B

Use the HYPERSIM circuit and the Dashboards panel of the two-quadrant DC-DC converter in boost mode, and based on the values calculated in Section I, proceed with the question below. Note that the DC-link of the UPS system only contains a DC capacitor, and that the inverter load has been added to the DC-link. 

1. Plot the transfer characteristic of the converter, output DC-link voltage as a function of the duty cycle. Indicate the operating points for rated battery voltage and for minimum and maximum battery voltage. Refer to Table 14. (Hint: Please note that the minimum battery voltage to maintain a 700V DC link is greater than 400V).

2. Plot the waveform of the converter output DC-link voltage and the voltage across the inductor, indicating the average, and maximum and minimum values for the maximum battery voltage. Plot the inductor current waveform, indicating average, and maximum and minimum values. Indicate the magnitude of ripple current. Plot all waveforms once the current has reached its steady-state value.

3. Record and plot the power supplied by the battery to the DC bus. Record and plot the current supplied by the battery for minimum and maximum battery voltage. Plot all waveforms once the current has reached its steady-state value.

4. Set the DC-DC converter control mode to closed loop position and the battery to its maximum voltage. Activate the trigger, and plot the step response showing the reference voltage, measured voltage, reference current, the measured battery current, and the reference duty cycle.

Table 14: Recorded values for question 1-Case B.

III. Questions

1. Tabulate the theoretical results calculated in Section I and the simulated results obtained in Section II for the operation of the DC-DC converter in battery charging and discharging modes.

2. Explain the impact and tradeoffs of the DC-DC converter switching frequency on the system’s passive components required to achieve the specified battery and DC-link ripple requirements.

3. Explain why the minimum battery voltage of 400V cannot supply the rated load when operating in UPS mode.

4. Explain the closed loop control configuration and operational limits for both charging and discharging modes.