Olof Helgadottir

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    Dynamic Behavior of Geothermal Power Plants Located ata Weak Point of a Transmission System

    THESIS

    Submitted in Partial Fulfillment

    of the REQUIREMENTS for theDegree of

    MASTER OF SCIENCE (Electrical Engineering)

    at the

    POLYTECHNIC UNIVERSITY

    By

    lf Helgadttir

    June 2008

    ___________________________Advisor

    ___________________________Date

    ___________________________Department Head

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    ii

    I was born in Reykjavik Iceland in 1980.

    I received my B.S. degree in electrical engineering from the University of Iceland in 2004 and an M.S. degree in

    electrical engineering in June 2008 from Polytechnic University, New York.

    During and after pursuing my B.S. degree I worked for the Icelandic transmission company, Landsnet, where I

    first worked on substation maintenance and then system planning and research.

    This thesis is submitted to fulfill the requirements for the M.S. degree. The preparation began in the fall of 2007

    and the simulations were done in spring 2008. All the data was provided by Landsnet, the Icelandic transmission

    company. The advisor, Professor de Len of Polytechnic University, was immensely supportive throughout the

    project and was vital in providing knowledge and focus on the most important theoretical and engineering issues.

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    AN ABSTRACT

    Dynamic Behavior of Geothermal Plants Located ata Weak Point of a Transmission System

    by

    lf Helgadttir

    Advisor: Prof. Francisco de Len

    Submitted in Partial Fulfillment of the Requirements

    for the Degree of Master of Science (Electrical Engineering)

    June 2008

    In recent years, predictions of increasing global climate change have sparked interest in the use of geothermalenergy. In addition to limiting countries fossil fuel dependence, studies have shown up to 35-fold reduction ofCO2 emissions when compared to the use of coal. In Iceland, 77% of power generation in 2008 is hydropowerand the rest comes from geothermal energy. Icelands power companies plan to build numerous geothermal

    plants in coming years thus increasing the proportion of geothermal energy of the total generated energy.The objective of this thesis is to give answers to the following questions:

    What will happen when a group of geothermal plants are connected to the transmission grid at a weak pointof the system?

    What is the difference in the power system stability when a large power demand is met with hydro powerplants vs. geothermal plants?

    When planning a new aluminum smelter in northern Iceland, looking at the voltage stability, with the N-1criteria in mind, which reinforcements are needed to connect the load and the new plants to the transmission

    system?To answer the questions three studies were carried out: (1) Transient stability, where the critical clearing time ofan important line is compared when the nearby generation is hydro versus geothermal. (2) Voltage stability forthe N-1 contingency was used to determine the transmission expansion. (3) A switching transient consisting of asingle pole opening and reclosing to determine the over-currents and over-voltages. While for the first twostudies the entire system was considered, for the latter the systems was reduced to the terminals of the line.

    The answewrs to the questions found durig the development of the thesis are:

    The geothermal power plants to be installed have larger inertia than the power hydro plants they substitute.Therefore, the system is more likely to maintain stability under transient condition when the load is suppliedfrom geothermal plants.

    When a large power demand is met with only geothermal power plants the system will be more stable than ifit is met with hydro plants.

    When planning a new 250 MW aluminum smelter in northern Iceland where the power demand is met withboth geothermal and hydro power plants we used a voltage stability study to determine the reinforcementsneeded to connect the load and the new plants to the transmission system.

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    Table of Contents1 Introduction....................................................................................................................................................... 1

    1.1 Motivation ............................................................................................................................................. 11.2 Objectives............................................................................................................................................. 11.3 Scopes and Limitations......................................................................................................................... 11.4 Literature Review.................................................................................................................................. 1

    1.4.1 Papers.............................................................................................................................................. 21.4.2 Books............................................................................................................................................... 2

    2 Difference between Generation Technologies.................................................................................................. 32.1 Thermal Power Plants........................................................................................................................... 3

    2.1.1 Steam Turbines................................................................................................................................ 32.1.2 Gas Turbines.................................................................................................................................... 32.1.3 Combined Cycle Power Generation................................................................................................. 3

    2.1.4 Cooling Towers ................................................................................................................................ 42.2 Hydro Power Plants.............................................................................................................................. 5

    2.2.1 Impulse Turbine ............................................................................................................................... 52.2.2 Reaction Turbine.............................................................................................................................. 6

    2.3 Geothermal Power Generation ............................................................................................................. 72.3.1 Dry Steam Power Plants.................................................................................................................. 72.3.2 Flash Steam Power Plants............................................................................................................... 72.3.3 Binary-Cycle Power Plants............................................................................................................... 8

    2.4 Alternative Generation Methods ........................................................................................................... 82.4.1 Solar Generation.............................................................................................................................. 8

    2.4.2 Wind Generation............................................................................................................................ 102.4.3 Nuclear Generation........................................................................................................................ 113 Power System Stability ................................................................................................................................... 12

    3.1 Rotor Angle Stability ........................................................................................................................... 123.1.1 Transient Stability........................................................................................................................... 123.1.2 Small-Signal Stability ..................................................................................................................... 15

    3.2 Voltage Stability .................................................................................................................................. 153.2.1 PV Curves...................................................................................................................................... 153.2.2 QV Curves ..................................................................................................................................... 16

    3.3 Mid-Term & Long-Term Stability ......................................................................................................... 173.3.1 Long-Term Stability........................................................................................................................ 173.3.2 Mid-Term Stability .......................................................................................................................... 18

    3.4 Switching Transients........................................................................................................................... 183.4.1 Voltage Transients on Closing and Reclosing Lines ...................................................................... 18

    4 Transient Stability Study......................................................................................................