Thermal Impact Analysis of Circulating Current in High Power Modular Online Uninterruptible Power Supplies Application

Overview

The study asks whether paralleling modules in a modular UPS has thermal and control side effects beyond simple N+1 redundancy arithmetic. On a laboratory set-up with three inverter modules, plug-and-play module rotation (normal and emergency cycles) was demonstrated and the output voltage transients checked against IEC 62040-3 limits. The module model is 200 kW nominal, 5 kHz switching, 700 V DC side, 230 V bus voltage, 290 A bus current, a three-phase H-bridge with LC filter (1.8 mH, 27 µF).

Transient time requirements are tabulated for linear and nonlinear loads: 14% voltage variation allowed for 20-40 ms, 12% for 40-60 ms, 11% for 60-100 ms and 10% for 100-1000 ms. The authors recommend keeping overshoot or sag on module switching within 10% of nominal critical-bus voltage. During module rotation the measured overshoot was 8.77% or 8.92%, with a transient time of about three and a half utility periods (70 ms).

In the thermal analysis, unsynchronised PWM and module-to-module differences in filters and control parameters cause zero-sequence circulating current that unbalances losses and junction temperatures between semiconductor devices. Citing prior work, the authors note DC/AC modules are normally designed to work at 50% to 75% load, so the thermal study spans 50-100% load. A 30% filter-inductor mismatch between modules plus unsynchronised PWM produced an obvious circulating current with a DC component and unequal device losses and temperatures.

To suppress the circulating current, synchronisation and zero-vector time regulation were applied, but this raised the temperature and losses of T2 (T2 higher than T1, and D1 higher than D2). Limits: thermal results rest on PLECS simulation, and experimental validation is limited to the module-rotation transient.

Key points

1. Modelled module rating: 200 kW nominal, 5 kHz switching, 700 V DC side, 230 V bus voltage, 290 A bus current, three-phase H-bridge with LC filter (1.8 mH, 27 µF).
2. Transient time requirements for linear and nonlinear loads: 14% voltage variation allowed for 20-40 ms, 12% for 40-60 ms, 11% for 60-100 ms and 10% for 100-1000 ms.
3. The authors recommend keeping overshoot or sag on module switching within 10% of nominal critical-bus voltage.
4. During module rotation the measured overshoot was 8.77% or 8.92%, with a transient time of about three and a half utility periods (70 ms).
5. Citing prior work, DC/AC modules are normally designed to work at 50% to 75% load, so the thermal study spans 50-100% load.
6. A 30% filter-inductor mismatch between modules plus unsynchronised PWM produced an obvious circulating current with a DC component and unequal device losses and temperatures.

Using it in design

• For paralleled modules in a modular UPS, do not look only at the N+1 redundancy count; add to the design check that module-to-module filter and control parameter differences cause circulating current and thermal imbalance.
• Verify output voltage overshoot at module commissioning and rotation against IEC 62040-3 transient limits (for example 14% for 20-40 ms) and a 10% target.
• Before choosing a circulating-current suppression method, check the thermal balance of modules in the typical 50%-75% load range and the way PWM synchronisation redistributes losses (higher T2 and D1 temperatures).

Who should read it

Power-electronics and UPS engineers working on modular UPS paralleling, control and thermal design.

Document contents

1. 1. Introduction
2. 2. System Control Mechanism (2.1 Single Module Control, 2.2 Power Balance among Modules, 2.3 Voltage Amplitude and Phase Synchronized, 2.4 Cycle and Plug'n'Play Control)
3. 3. Experimental Results (3.1 Modules, 3.2 Cycle Control Transient Performance)
4. 4. Thermal Analysis Results (4.1 Single DC/AC Module, 4.2 DC/AC Modules with Zero Sequence Circulating Current, 4.3 DC/AC Modules with Suppressed Zero Sequence Circulating Current)
5. 5. Conclusions

Related standards

• IEC 62040-3

Read the document

Publication details

Authors

Chi Zhang; Josep Guerrero; Juan Vasquez

Publisher

Energies (MDPI AG)

Year

2017

Volume

10

Issue

1

Pages

50

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.3390/en10010050

Licence

CC BY 4.0 — share with attribution

Licence evidence

www.mdpi.com

Citation