AC power | EPFL Graph Search

In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the direction of energy flow. Its SI unit is the watt. The portion of instantaneous power that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as instantaneous active power, and its time average is known as active power or real power. The portion of instantaneous power that results in no net transfer of energy but instead oscillates between the source and load in each cycle due to stored energy, is known as instantaneous reactive power, and its amplitude is the absolute value of reactive power. Active, reactive, apparent, and complex power in sinusoidal steady-state In a simple alternating current (AC) circuit consisting of a source and a linear time-

Convertisseur de fréquence indirect à rapport de tension fixe

Aziza Benaboud

Many electric power generators use gas turbines as power sources. Typically they are connected to the turbines through a mechanical gearbox in order to adapt their synchronous speed to the optimal rotation speed of the turbine, which is very often much higher than the synchronous speed. However, due to direct network connection, the generator speed cannot be variable : it is imposed by the network and constant. To overcome this problem, we propose to replace the mechanical gearbox by a flexible electronic solution which offers a high efficiency. Using this approach, the turbine is directly connected to the synchronous generator, which is connected to the grid through an indirect static frequency converter with an intermediary DC circuit. However, this type of converter is not common in this application because of very high switching losses due to the high frequency of the PWM technique normally used for its control. In this dissertation, a new control strategy is proposed for the three level Neutral Point Clamped converter, characterized by its high efficiency due to the use of square-wave operation mode. The main advantage of this mode is the quasi absence of switching losses. In this mode, only the frequency can be varied between the input and the output voltage, but their magnitudes are not freely controllable. A voltage magnitude adaptation can be done by the generator's excitation. The produced active and reactive power can be controlled by the generator excitation as well as both the angle shift between the generator and rectifier voltages and between the inverter and network voltages. The capacitive intermediary circuit brings the advantage of decoupling of harmonics between the generator and the network currents. A control method is also proposed to resolve some problems incurred by using square wave operation mode, in particular to reduce the harmonics distortion of the output inverter voltage and current. As second important contribution, this thesis proposes a new fast-ramping DC-component elimination strategy for AC currents. In comparison to the usually slow transient that characterizes a DC component free current transient, we achieve that much faster transients also without DC-component, simply by choosing a well defined transition period. Simulation and experimental results for different operating points and transitions between them highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality without continuous component and less harmonics.

EPFL2007

A VSC-Based BESS Model for Multi-Objective OPF Using Mixed Integer SOCP

Xiaoying Tang, Tong Wu

This paper presents a new model of voltage source converter (VSC)-based battery energy storage systems (BESSs) that interface with power grids. A VSC-based BESS is made up of a series connection of a VSC, its connecting transformer and a BESS. The VSC allows a BESS to generate both active and reactive powers in all four quadrants. The proposed model captures the coupling between active power, reactive power and the voltage of a BESS. In addition, the proposed model explicitly describes the relationship between the control configuration of Pulse Width Modulation (PWM)-VSC and the power output of the BESS. Therefore, the proposed model possesses unparalleled control capabilities in the operational parameters of both the AC and DC sides of the converter. By incorporating such a model into the active- reactive optimal power flow ( A-R-OPF) formulation, we can not only optimize the active and reactive power output of the BESS in power systems, but also understand how the optimal powers are generated by setting the operational parameters of both the BESS and PWM-VSC. To solve the A-R-OPF problem with the proposed model, we propose a sequence of strong relaxations to transform the problem into a mixed-integer second order cone programming (SOCP) problem. Such a formulation is amendable for efficient solutions using off-the-shelf solvers. Case studies on the IEEE benchmark systems show that more than 17.73% of power losses in transmission lines and more than 0.961% of interface losses can be reduced by using the proposed model in comparison to the traditional BESS model.

IEEE2020

Principe d'alimentation par convertisseurs multiniveaux à stockage intégré

Claude Fahrni

Facilities operating in cycles, such as particles accelerators, require pulsed power. Directly drawing this pulsed power from the industrial network is generally not acceptable. A system with a local energy storage system lessens the impact on the network. In this work, a power supply is proposed that allows pulsed operation of a proton synchrotron without disturbing the industrial network with large variations of active power. The context of this study is the search of a completely static replace ment of an existing inertial energy storage device. The application is the proton synchrotron of the CERN. Several solutions to store the energy needed by the proton synchrotron are studied, they include inertial energy storage, capacitive energy storage and magnetic energy storage. On this basis, the solution based on capacitive energy storage with interleaved multilevel converters is retained. The sizing of the storage elements and the filter is carried out. The control strategies are studied and designed. They are composed of a current balancing control for the parallel connected converters and a voltage balance techniques for the DC buses of the series connected converters. Based on the pseudo-continuous modeling and controller sizing techniques, an advanced model for interleaved multilevel converters is established. It allows to have a better definition of the step response of the system. It is possible to use a PID controller instead of a PI controller to obtain of better step response. Numerical simulations are carried out to verify the control strategies of the system. A system with a multilevel converter is simulated with all control strategies and with the losses compensation rules. A low power system is built to study and verify the control algorithms of the power supply.

EPFL2008

Convertisseur de fréquence indirect à rapport de tension fixe

Aziza Benaboud

EPFL2007