Inverter based generation for large industrial plants

Abstract

Reducing the cost and CO2 emission for the power generation for a liquefied natural gas (LNG) production plant by using inverter based generation is the main challenge posed in this thesis. Both factors can be decreased by using inverter based generation instead of the conventional gas powered power plant for generating the power for a islanded LNG plant. The overall objective is to develop and investigate the dynamic performance of these inverter based generation for usage in an industrial islanded plant. Within existing literature there are no examples of large scale (230MW) inverter based generation in islanded mode. This thesis provides insight on the economical cost and the impact during different operational scenarios of using the inverter based generation in large industrial islanded plants. The general methodological approach consist of 5 steps. Firstly the conventional generation is analysed, then the financial analysis is conducted. Two proposals for inverter based generation are introduced and financially analysed. The fourth step is to develop a Powerfactory model for the inverter based generation and finally several disturbances where applied to the new system to evaluate the performance during operational scenarios. The first step was to develop a model for the LNG plant and the power generation. I made a Powerfactory model with topology and parameters based on a recent study by Shell. The model was adjusted to resemble a real case study of a LNG plan located in Tanzania and generic controls from existing literature where implemented. The second step was a financial analysis of the conventional power plant. This lead to two proposals for implementing inverter based generation in a LNG plant. These where to replace the spinning reserve for a lithium ion battery and to replace a gas turbine by a combination of solar PV and a VRF battery. Both options where financially analysed. Replacing the spinning reserve with a lithium ion battery leads to a reduction of capital expenditure by 34M$ at expense of increasing the risk of load shedding. This can be mitigated by implementing a lithium ion battery as spinning reserve. The capital expenditure of the battery is 50M$. The maintenance of the gas powered power plant is reduced by 0.94M$ resulting in a return of investment within 16.6 years. The replacement of a gas turbine by a combination of solar PV and a VRF battery increases the capital expenditure by 222M$ but reduces the operational expenditure by 15 M$/year/turbine making for a return of investment within 13.1 years. The third step was to develop models for the two proposals. This was done using Powerfactory 2019. The lithium ion battery Powerfactory model is adapted for the operation in islanded mode. For the second proposal a technical model is developed. This model was implemented in the Powerfactory software. The fourth step was the case study of the system including the new proposal. Three studies where conducted: generation loss, 3 phase short circuit and load rejection. All of theses studies are compared to the conventional generation model. For the lithium ion battery the result was an improved reaction in the generation loss scenario. Because the ramp rate of the battery is much larger than the generator it is replacing. The control is able to detect a variation in frequency and starts to provide active power. In both the load rejection and the short circuit the contribution of the inverter is limited. The battery is unable to be absorb the excess energy since the battery is already full in the load rejection scenario. In the short circuit study case the maximum output current ofthe inverter is 20x lower than that of the generator it is replacing. For the solar PV and the VRF battery the load rejection study case shows improved reaction. The reaction is faster than that of the turbine it is replacing since the extra energy can be converted instantly so the result is less frequency deviation. In the generation loss study case and the short circuit the reaction of the inverter is limited. In the generation loss scenario the inverter is providing maximum output power to decrease the gas consumption of the remaining turbines and is unable to provide more. The inverter is also limited in the short circuit current during the short circuit study case. It is unable to providemore 1 pu current. Overall, the combination of implementing inverter based generation in to an islanded LNG plant a good solution to reduce the cost and emission of the power generation. Both solutions have a return of investment within 20 years and have positive effects in the study cases. A combination of the two proposals can mitigate the limited reaction of each of the individual proposals. For the short circuit scenario conventional protection will not be triggered.