Showing papers by "Neville R. Watson published in 2003"

Measurement placement method for power system state estimation: part I

Abstract: This paper deals with a simple technique for measurement placement method of power system state estimation The minimum condition number of the measurement matrix is used as the criteria in conjunction with sequential elimination to generalize the measurement placement The singular value decomposition (SVD) approach will be used to solve the state estimation The simulation study is performed on the IEEE 14-bus test system by linear weighted least square (WLS) It is found that, this algorithm can give a solution of measurement placement of injection current and voltage that make the power system observable

A new concept for the control of the harmonic content of voltage source converters

Abstract: A new voltage source converter is proposed, based on the reinjection of the dc voltage via the neutral point of the double bridge converter It differs from the conventional in that the voltage applied to the bridges varies, even though the dc voltage remains constant To this effect a periodically varying waveform is supplied to the bridge and used to shape the ac side voltage into the specified waveform The ideal reinjection waveform required for a complete elimination of the harmonic content is derived and then a practical approximation to the ideal suitable for the practical implementation is proposed

z-domain frequency-dependent AC-system equivalent for electromagnetic transient simulation

Abstract: Modern power systems are very complex and to model them completely is impractical for electromagnetic transient studies. Therefore, areas outside the immediate area of interest must be represented by some form of frequency-dependent network equivalent (FDNE). One of the motivations for investigating z-domain fitting is that it can be directly implemented in a digital-simulation program without any loss of accuracy as it is already a discrete formulation. Fitting in the s-domain always requires 'discretising' a continuous system and the inherent approximations. The formulation for developing 1- and 2-port frequency-dependent equivalents of the AC system using z-domain rational-function fitting of the frequency response is presented and its use illustrated. This 1- and 2-port FDNE have been applied to the New Zealand lower South Island AC power system. The electromagnetic transient package PSCAD/EMTDC is used to assess the transient response of the 1- and 2-port FDNE developed under different conditions (linear load, fault and nonlinear loading). The study results have indicated the robustness and accuracy of land 2-port FDNE for electromagnetic transient simulation.

EMTDC Assessment of a New Type of VSC for Back to Back HVdc Interconnections

Abstract: A new type of voltage source converter is described based on the reinjection of dc voltage pulses into the main series connected bridges via the dc side neutral point This configuration produces a 36-pulse voltage waveform at the converter side of the interface transformers The paper analy- ses the steady state and dynamic performance of the scheme when applied to a back to back HVdc interconnection

Harmonic assessment using electromagnetic transient simulation and frequency-dependent network equivalents

Abstract: Electromagnetic transient simulation can be used to model complex non-linearities that are very difficult to represent adequately in the frequency domain. After reaching the steady state the voltage and current waveforms, represented by sets of discrete values at equally spaced intervals (corresponding with the integration steps), are subjected to fast Fourier transform processing to derive the harmonic spectra. However, the accuracy of the EMTP approach is limited by the restricted frequency dependence representation of the AC system components. It is shown that this problem is greatly reduced with the use of frequency-dependent network equivalents for the linear part of the system.

Transmission lines and cables

Abstract: In this chapter, the authors discusses the travelling wave transmission line models. The frequency dependent transmission line model was presented. Details of transmission line geometry and conductor data are required in order to calculate accurately the frequency-dependent electrical parameters of the line. The simulation time step was based on the shortest response time of the line. The chapter also presents the phase-domain models which is the most accurate and robust for detailed transmission line representation. Calculation of electrical parameters for overhead power transmission lines and underground power cables were also demonstrated.

Control and protection

Abstract: The control equations are solved separately from the power system equations though still using the EMTP philosophy, thereby maintaining the symmetry of the conductance matrix. The main facilities developed to segment the control, as well as devices or phenomena which cannot be directly modelled by the basic network components, are TACS and MODELS (in the original EMTP package) and a CMSF library (in the PSCAD/EMTDC package). The separate solution of control and power system introduces a time-step delay, however with the sample and hold used in digital control this is becoming less of an issue. Modern digital controls, with multiple time steps, are more the norm and can be adequately represented in EMT programs. The use of a modular approach to build up a control system, although it gives greater flexibility, introduces time-step delays in data paths, which can have a detrimental effect on the simulation results. The use of the z-domain for analysing the difference equations either generated using NIS, with and without time-step delay, or the root-matching technique, has been demonstrated. Interpolation is important for modelling controls as well as for the non-linear surge arrester, if numerical errors and possible instabilities are to be avoided. A description of the present state of protective system implementation has been given, indicating the difficulty of modelling individual devices in detail. Instead, the emphasis is on the use of real-time digital simulators interfaced with the actual protection hardware via digital-to-analogue conversion.

Numerical integrator substitution

Abstract: A continuous function can be simulated by substituting a numerical integration formula into the differential equation and rearranging the function into an appropriate form. Among the factors to be taken into account in the selection of the numerical integrator are the error due to truncated terms, its properties as a differentiator, error propagation and frequency response. Numerical integration substitution (NIS) constitutes the basis of Dommel's EMTP , which, as explained in the introductory chapter, is now the most generally accepted method for the solution of electromagnetic transients.

Frequency dependent network equivalents

Abstract: Frequency dependent network equivalents are important for modelling modern power systems due to their size and complexity. The first stage is to determine the response of the portion of the network to be replaced by an equivalent, as seen from its boundary busbar(s). This is most efficiently performed using frequency domain techniques to perform a frequency scan. Once determined, a rational function which is easily implemented can be fitted to match this response.

Appendix C: Numerical integration

Abstract: A numerical integration algorithm is either explicit or implicit. There are various ways of developing numerical integration algorithms, such as manipulation of Taylor series expansions or the use of numerical solution by polynomial approximation. Among the wealth of material from the literature, only a few of the classical numerical integration algorithms have been selected for presentation.

Analysis of continuous and discrete systems

Abstract: With the exceptions of a few auxiliary components, the electrical power system is a continuous system, which can be represented mathematically by a system of differential and algebraic equations. A convenient form of these equations is the state variable formulation, in which a system of n first-order linear differential equations results from an n order system. The state variable formulation is not unique and depends on the choice of state variables. The following state variable realisations have been described in this chapter: successive differentiation, controller canonical, observer canonical and diagonal canonical. Digital simulation is by nature a discrete time process and can only provide solutions for the differential and algebraic equations at discrete points in time, hence this requires the formulation of discrete systems. The discrete representation can always be expressed as a difference equation, where the output at a new time point is calculated from the output at previous time points and the inputs at the present and previous time points.