A modified sequence domain impedance definition and its equivalence to the dq-domain impedance definition for the stability analysis of AC power electronic systems

Representations of AC power systems by frequency dependent impedance equivalents is an emerging technique in the dynamic analysis of power systems including power electronic converters. The technique has been applied for decades in DC-power systems, and it was recently adopted to map the impedances in AC systems. Most of the work on AC systems can be categorized in two approaches. One is the analysis of the system in the \textit{dq}-domain, whereas the other applies harmonic linearization in the phase domain through symmetric components. Impedance models based on analytical calculations, numerical simulation and experimental studies have been previously developed and verified in both domains independently. The authors of previous studies discuss the advantages and disadvantages of each domain separately, but neither a rigorous comparison nor an attempt to bridge them has been conducted. The present paper attempts to close this gap by deriving the mathematical formulation that shows the equivalence between the \textit{dq}-domain and the sequence domain impedances. A modified form of the sequence domain impedance matrix is proposed, and with this definition the stability estimates obtained with the Generalized Nyquist Criterion (GNC) become equivalent in both domains. The second contribution of the paper is the definition of a \textit{Mirror Frequency Decoupled} (MFD) system. The analysis of MFD systems is less complex than that of non-MFD systems because the positive and negative sequences are decoupled. This paper shows that if a system is incorrectly assumed to be MFD, this will lead to an erroneous or ambiguous estimation of the equivalent impedance

Double-Frame Current Control with a Multivariable PI Controller and Power Compensation forWeak Unbalanced Networks

The handling of weak networks with asymmetric loads and disturbances implies the accurate handling of the second-harmonic component that appears in an unbalanced network. This paper proposes a classic vector control approach using a PI-based controller with superior decoupling capabilities for operation in weak networks with unbalanced phase voltages. A synchronization method for weak unbalanced networks is detailed, with dedicated dimensioning rules. The use of a double-frame controller allows a current symmetry or controlled imbalance to be forced for compensation of power oscillations by controlling the negative current sequence. This paper also serves as a useful reminder of the proper way to cancel the inherent coupling effect due to the transformation to the synchronous rotating reference frame, and of basic considerations of the relationship between switching frequency and control bandwidth.Comment: 17 pages, contribution to the 2014 CAS - CERN Accelerator School: Power Converters, Baden, Switzerland, 7-14 May 201

Microwave apparatus for gravitational waves observation

In this report the theoretical and experimental activities for the development of superconducting microwave cavities for the detection of gravitational waves are presented.Comment: 42 pages, 28 figure

Fully Decou pled Controller Models for Voltage Source Converter based High Voltage DC Transmission

VSC-HVDC has two distinct advantages over its earlier generation thyristor based High Voltage DC transmission. Synchronous voltage source is not required to commutate against, for its operation and it does not suffer from commutation failures under adverse conditions in interfacing ac system. These two properties make it amenable to wider application areas. To make it adapt to operational conditions imposed on it in various applications, its controller parameters need to be assessed and tuned through extensive simulation studies. To facilitate this, two alternative controllers viz. a fully decoupled controller model and also an instantaneous theory based fully decoupled hybrid controller model are developed in the thesis. The decoupling is achieved by exploiting similarity transformation in both the controllers. In the first controller model, the Park's currents and voltages are directly obtained from the measured network variables and the reference park's currents for the inner current loop are obtained from the instantaneous measured power. In the second one, both the feedback as well as reference Park's currents for the inner current loop are obtained from the Clarke's variables. An AC system interfacing electronics based power transmission or distribution network experiences non-sinusoidal voltage and current waveforms. Instantaneous power theory being suitable for steady as well as transient states, is used for handling measured inputs. The performance of the models is assessed through SIMULINK Power system Blockset aided simulations on a VSC-HVDC link interfacing an ac system, having normal fault level, low fault level and also witnessing a single line to ground fault on its rectifier transformer primary side