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Publication Title | Real Time Simulation of an LC Output Filter Grid-Connected Gas Turbine under Voltage Sag Operation

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Real Time Simulation of an LC Output Filter Grid-Connected Gas Turbine under Voltage Sag Operation

Fouad SALHA (*), Frédéric COLAS (*), Xavier Guillaud (**)

fouad.salha@ec-lille.fr , frederic.colas@lille.ensam.fr , xavier.guillaud@ec-lille.fr (*) L2EP, ENSAM CER de Lille Tel : 0033 3-20-62-29-43 Fax : 0033 3-20-62-27-50 8, bd Louis XIV 59046 Lille CEDEX France

(**) L2EP, Ecole Centrale de Lille, France, Tel : 0033 3-20-33-53-87

Cité Scientifique, BP 48 59651 Villeneuve d’Ascq France

Abstract— The objective of this work is to analyze the dynamic behavior of a distributed generator, based on a gas microturbine GMT, using a grid-connected inverter with an output LC filter. In this paper, we focus on what occurs in the case of a disturbance on a network when a Voltage Sag is induced on the output LC filter terminal. The proposed control strategy depends on the control of the output LC filter voltage with a resonant controller. We use a state feedback structure and pole assignment method to design the controller. Dynamic analysis was conducted in case of a Voltage Sag operation caused by a three-phase balanced fault. The proposed control strategy is based on modifying the overall control system from a voltage control to a current control during a voltage sag occurrence. Real time simulation results are discussed in the last part of this paper.

Keywords — Gas Microturbine, Distributed Generation, Voltage Source Inverter (VSI), LC Filter, Voltage Sag, Current Limiter, Resonant Controller, Pole Assignment.

I. INTRODUCTION

In recent years, electricity markets have undergone significant liberalization. This transformation has coincided with the rapid growth and adoption of Distributed Generation technologies. This is due to the trend towards large-scale integration of renewable energy sources with existing electrical power systems. This, in turn, has created a greater need for highly reliable, high quality electrical power generation. In Distributed Generation-based power systems, low voltage distributed energy sources, such as gas microturbines, photovoltaic arrays and wind turbines, and storage devices, such as flywheels, supercapacitors, batteries and controllable load form an energy system structure commonly known as a microgrid. Thus, a microgrid can operate either in parallel with the utility grid or in a stand-alone mode creating an islanded system. In fact, in standalone mode operation, the loads in the microgrid can receive power from local microsources, depending on the customer’s situation [1]. Typically, a microgrid has at least one voltage generator that is responsible for regulating microgrid voltage and frequency during stand-alone operation mode. Considering that the primary energy introduced to the gas microturbine is controllable, this allows the microturbine

to operate as a voltage generator, while other sources in the microgrid operate as generator current. In this paper, the GMT has been adapted to be a voltage source by using a three-phase voltage inverter equipped with an output LC filter. An inverter voltage control system has been established by using a resonant controller. The reason for this is that it proposes a new stationary reference frame control method that introduces a cosine transfer function with a specified resonant frequency into the voltage compensator. Hence, this controller is generally composed of a proportional and a resonance term, which contains two imaginary poles that are intended to obtain an infinite gain at the resonance frequency. One of the most critical and frequently occurring problems impacting transmission and distribution systems and overall system stability is Voltage Sag. This problem occurs for many reasons including short circuits, transformer energizing and capacitor bank charging. During a Voltage Sag event, the amplitude of effective load voltage decreases [2] in a very short time, from 0.9 of the nominal load voltage to 0.1. This paper focuses on the dynamic behavior of a voltage generator-based gas microturbine in a microgrid connected to distributed network, in the event of a Voltage Sag of 80% of the nominal value, resulting from a balanced three-phase short circuit.

This paper is divided into four main sections. Section II describes the voltage source distributed generator- based gas microturbine with attached elements that are used for connection to the distribution network. In section III, the proposed control strategy will be discussed in detail. The state feedback structure is first applied to a single phase system, and then a three phase LC filter. In section VI, analysis of system behavior in the case of Voltage Sag caused by a three-phase balanced short circuit will be discussed. Finally, system implementation on a real time simulator will be presented, and results will be demonstrated.

II. VOLTAGE SOURCE DISTRIBUTED GENERA TOR-BASED GAS MICROTURBINE

Generally, this model of generator consists of three main parts; a Gas Micro Turbine (GMT), Permanent

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