Investigation on the Performance of a Modular Multi-level Converter (MMC) based UPS System

Examensarbete för masterexamen
Master Thesis
Electric power engineering (MPEPO), MSc
Kigozi, John Walusimbi
An Uninterruptible Power Supply (UPS) is a custom power device commonly used in Low voltage networks to protect sensitive loads by mitigating power quality problems like voltage dips, interruptions and improvement of power factor. Sudden power outages, that are unplanned and power disturbances in unprotected low and medium voltage networks with sensitive load of a duration ranging from a hundreds of milliseconds to several hours could eventuate into adverse financial losses. In this thesis, a study is performed on the state of the art developments in UPS designs and available topologies commonly used in the industry with static converters. A Modular Multi-level Converter (MMC) based UPS system is proposed and simulated in Matlab/SIMULINK with PLECS blockset under different operating conditions such as load unbalance, voltage dips and operation with both resistive-inductive and non-linear loads with a focus on converter performance. The UPS system is compared the Two-level Voltage Source Converter (VSC) based UPS topology. The results showed that the output voltage wave forms of the Medium Voltage (MV) MMC have a more sinusoidal shape compared with the Two-level converter that has a square output voltage, hence less requirements on the filters for the MMC. In the thesis an MMC UPS was simulated with a switching frequency of 1000Hz which was significantly lower than the 2000Hz for Two-level converter based UPS system, resulting in lower losses when using the MMC based UPS system. The structure and operation of the MMC was described and it showed that series modules could be added in the arms of the MMC to alter its operating voltage and power requirement, offering system scalability. With the structure of MMCs, it’s possible to perform cell hot swaps and this increases the reliability of the MMC UPS as modules can be replaced while UPS still remains in operation without necessarily invoking the maintenance by-pass mode. In the MMC, the failed module can also be by-passed, thus offering very high redundancy and possibility to use low cost valves in the modules. The risks of operating the MV MMC and Transformerless UPS systems are also discussed and suggestions are made on how to manage the risks. Finally practical scenarios are identified where the proposed MMC based UPS systems can be applied in industry and how they can improve on the power quality and reduce on the operational losses.
Elkraftteknik , Electric power engineering
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