T.T. Nguyen

Effects of machine inertia constants on system transient stability

Abstract With the introduction of open transmission access and independent power producers, the present trend is that power systems will have an increasing range of generators which have widely-different inertia constants. This has raised concerns relating to system transient stability, particularly when some generators in the system have low inertias. The paper is devoted to the investigation of this aspect, and reports its findings. The findings of the paper reveal and clarify important points relating to the effects of machine inertia constants on system transient stability. The effects of inertia constants are strongly influenced by other factors relating to pre-fault and fault operating conditions and system configurations. The outcome of the paper brings out key factors that interact with machine inertias in determining system transient stability. These interactions lead to a general conclusion that, for a given system, there is no optimal inertia constant for a particular generator or for a particular group of generators in a system that gives the best system dynamic responses in all of the credible operating conditions and system configurations.

Impulse sampling sequences in time-convolution electromagnetic transient analysis in power systems

Abstract The paper refers specifically to the process by which numerical solutions for transmission line step responses and indicial-impedance functions, as formed from inverse Fourier transform evaluations, are used in time-convolution electromagnetic transient analysis in power systems. Previously, curve-fitting and linear-segment matching procedures have been adopted, whereas the present paper develops a form of representation based on sequences of impulse samples. By avoiding curve-fitting altogether, the proposals further advance time-convolution methods in power systems, and improve the procedures required in applying them in practice. Typical solutions in the paper confirm the solution accuracy to which the proposals can lead, together with their computing time requirements.

PROLOG network-graph generation in system surveillance

Abstract Based on the PROLOG programming language, the paper develops methods by which the configuration of an interconnected power transmission network may be determined automatically from telemetered status data for circuit breakers and isolators throughout the network in the first stage of state estimation for network surveillance purposes. A structure is first given for the data base from which logic processing begins and which includes the identification of substations at the first level, defined zones within substations at the second level, and individual switching elements within defined zones at the third. For all but the simplest networks, the data base can be a particularly extensive one. Recursive search procedures are then derived for determining closed paths through bus bar complexes of any given configuration. The searches lead to a definition of network nodes, and then to the interconnections between nodes and network branches. A branch incidence matrix in compacted list form gives the complete network graph. Data validation procedures are developed by which the consistency of related data items is checked, and inconsistencies where they arise are resolved, before data are used in graph generation procedures. An application of the methods is given for a section of a network which includes 24 network branches, 42 circuit breakers, and 106 isolators. The automatic network-graph procedures of the paper are validated for numerous combinations of circuit breaker and isolator open/closed positions in this small network section.

Network equivalents in power system electromagnetic transient analysis

Abstract The paper develops an analysis basis and procedure by which parts of a complete network may be represented in equivalent form in evaluations of electromagnetic transient propagation in power systems. In particular, the equivalents are based on impulse driving-point and transfer impedance functions for networks of multi-conductor power transmission lines. The paper shows how these functions may first be defined numerically as discrete time series and then used in over-voltage studies of line energisation within a network, the analysis of the electromagnetic transients of short-circuit fault conditions, and the evaluation of circuit-breaker transient recovery voltages in fault current interruption.

Discriminative response indices for numeric distance protection

Abstract This paper investigates the discriminative properties which numeric distance protection can achieve. As this form of protection begins to find widening application, there is an important need to develop means of expressing its discriminative response and performance in a form which finds wide agreement. The proposals of the paper seek to contribute to this need. They are in terms of response indices for each earth and phase fault measurement in a complete distance protection system. The indices derive directly from the internal processing modes and protection algorithms of the systems of distance protection to which they relate. Response indices are evaluated for a range of forward and reverse fault conditions for a representative distance protection application by examining the form they take in a particular case.

z-Plane load modelling with particular reference to subharmonic mode responses

Abstract Whilst there are often ample grounds on which to justify the use of simplified, linear transient representations for system loads in evaluating electromagnetic transients in power networks, it seems unlikely that typical practical requirements can be fulfilled from this basis in the particular context of subharmonic modes of system oscillatory response, for then the negative resistance characteristics of induction motors at subsynchronous frequencies can have a direct bearing on the system response and especially on subharmonic mode damping. The developments in z-plane electromagnetic transient analysis of the present paper seek to provide means by which this response interaction can be investigated. Representative examples of the methods in application included in the paper have been taken from an extensive series of studies relating to a very long distance transmission interconnection which supplies a load area with a high proportion of induction motor load. Saturated reactor compensators providing voltage-control functions in the primary transmission interconnection give rise to subharmonic modes. Using the modelling methods developed, the contribution of the response of dynamic loads to system subharmonic modes is investigated. The paper also shows the very effective counter to negative damping which the bypass circuits of saturated reactor compensators can provide.