1. Optimized load sharing control by means of thermal reliability management
Carsten Nesgaard
Department of Electric Power Engineering
Technical University of Denmark
Henrik Møller

2. Power system configuration
Parallel-connected units:
Parallel-operation of multiple converters is employed when specifications require:
Fault tolerance
Reliability enhancement
Redundancy
Modular system structure.

3. Converter
Schematic overview of key converter elements:

4. Reliability Annual ‘down-time’:
A test system comprised of two parallel-connected converters utilizing current sharing has an annual ‘down-time’ of 10 minutes and 14 seconds.
Tav = 95,7C
10 min. 14 sec. / year
Tav = 77,3C
Different Tav due to:
Parameter/component variations
Working environment conditions

5. Current sharing technique (“Prior art”)
Converter and current sharing structure:

6. Current sharing technique (“Prior art”)
Measurements:
The following two graphs show actual measurements of a two-converter power system.
Current division in a two-converter system:
Efficiency in a two-converter system:
Converter 1
Converter 2
NO sharing efficiency
Current sharing efficiency

7. Thermal reliability management (Invention)
Principle of the invention:
Thermal reliability management implementation is simple and cost-effective.
The temperature sensing device is simply mounted at the most critical location within the converter.
Temperature sensing signal replaces current sensing signal:

8. Thermal reliability management (Invention)
Measurements:
The following two graphs show actual measurements of a two-converter power system utilizing thermal reliability management.
Current division in a two-converter system:
Efficiency in a two-converter system:
Converter 1
Converter 2
NO sharing efficiency
Current sharing efficiency
Thermal rel. man. efficiency

9. Thermal reliability management (Invention)
Dynamic change in power throughput:

10. Reliability Annual ‘down-time’:
Applying thermal reliability management to the previously shown two-converter power system results in an annual ‘down-time’ of 6 minutes and 11 seconds.
Total annual ‘down-time’ is reduced by almost 40%.
6 min. 11 sec. / year
Tav = 78,5C

11. Summary
Advantages:
Optimized reliability  fundamentally different load sharing control
Minimized losses  increased overall efficiency
Simple implementation
Ready for immediate implementation in both new and existing designs
Disadvantages:
Individual converter failure rate might increase slightly. However, it is aaby far compensated through a much lower average temperature.