Measurement of the saturation characteristics in the quadrature axis of synchronous machines

For the accurate prediction of the performances of saturated synchronous machines, the saturation characteristics in both the direct and quadrature axes are needed. The d-axis saturation characteristic of a synchronous machine can be measured easily by the open-circuit test with the machine excited from its field winding. On the other hand, the q-axis saturation characteristic of a synchronous machine cannot be measured applying a similar easy, simple method and, as a result, these characteristics are usually not experimentally measured and are not available. In this paper, three possible experimental methods for determining the q-axis saturation characteristics of both cylindrical-rotor and salient-pole synchronous machines are presented. The merits and demerits of these experimental methods are discussed from the point of view of their complexity and their accuracy. Comparisons between the measured q-axis saturation characteristics obtained by these experimental methods are made for a cylindrical-rotor machine and two salient-pole machines. Moreover, the various sources of errors, which may affect the accuracy of determining the q-axis saturation characteristics by these methods, are investigated.     

Determination of the parameters representing the cross-magnetizingeffect in saturated synchronous machines

This paper introduces the concept of the cross-magnetizing phenomenon in saturated synchronous machines (the magnetic coupling between the direct and quadrature-axis). The parameters which represent this cross-magnetizing effect are defined and techniques for determining these parameters are proposed. In these techniques, there is no need for an auxiliary winding in the q-axis of the machine and, thus, they can be applied to conventional synchronous machines. Experimental results which verify the accuracy of these techniques are presented     

Methods for determining the q-axis saturation characteristics of salient-pole synchronous machines from the measured d-axis characteristics

For the accurate analysis of salient-pole synchronous machines using the two-axis frame models, the direct- (d-) and quadrature-axis (q-axis) saturation characteristics are needed. Usually, the d-axis saturation characteristics can be obtained easily by the conventional open-circuit test with the machines excited from their field winding. On the other hand, the q-axis saturation characteristics of synchronous machines cannot be measured applying simple, conventional methods, and thus, they are usually not available. In this paper, four different methods for calculating the q-axis saturation characteristics of salient-pole synchronous machines from the measured d-axis saturation characteristics are explored. In these methods, the q-axis saturation characteristics can be calculated from the readily available test data, namely the d-axis saturation characteristics and the d- and q-axis unsaturated magnetizing reactances. Finally, a comparison between these methods is made.     

Effect of sequential reclosure of multi-phase system faults onturbine-generator shaft torsional torques

The authors present a method of reducing the high torsional torques induced in turbine-generator shafts during high-speed reclosure of multiphase system faults. The method is based on the sequential reclosing of the faulted circuit. In this context, investigations have been conducted on two turbine-generator units of different designs for various multiphase system faults. The results of both the three-phase-to-ground and double-line-to-ground faults are reported, and the effectiveness of the method is illustrated through parametric studies and simulation results. The effects of the reclosing time and the fault incidence instant on the effectiveness of the method have also been investigated     

A new method for determining the armature leakage reactance ofsynchronous machines

A method for determining the armature leakage reactance of synchronous machines is presented. In this method, the terminal voltage/armature current characteristic with the machine unloaded and unexcited is needed, together with the open-circuit characteristic of the machine. The armature leakage reactance is derived from these two curves using an iterative technique. The armature leakage reactances of five synchronous machines of different designs are obtained using this method. Their values are compared with those of the Potier reactance of these machines measured at various terminal voltages     

On the use of a digital computer as a two-phase variable-amplitudevariable-frequency oscillator

A feasibility study on the use of a digital computer as a variable-amplitude variable-frequency oscillator which generates two-phase sinusoidal excitation control signals for a dual-excited synchronous generator is described. The oscillator uses two input signals. One input signal is used to control the frequency of the oscillator outputs, and the other is used to control their amplitudes. By a single variable, the software can be configured to vary the oscillator frequency range and to greatly reduce (almost eliminate) the harmonic distortion of the output signals. The harmonic distortion is constant and independent of the output frequency for each possible oscillator frequency range. The software-based oscillator design is flexible and can be used to generate different types of multiphase signal waveforms     

Effect of field discharge resistors on turbine-generator shafttorsional torques

The possibility of reducing the torsional torques of turbine-generator shafts during disturbances by interrupting the generator excitation and switching in a field discharge resistor during the fault duration is discussed. The results of these investigations show that the use of such field discharge resistors reduces the turbine-generator torsional torques after the fault clearance. These results have been obtained through extensive parametric studies and are supported by simulation results     

Effect of saturation on the steady-state stability of a synchronousmachine connected to an infinite bus system

A synchronous machine model that includes the effect of the saturated reactances the cross-magnetizing phenomenon is presented. This model is used to study the steady-state stability of a synchronous machine connected to an infinite bus system. These studies show that the effect of saturation is noticeably large and depends mainly on the cross-magnetizing effect. The results obtained by this model are found to be in good agreement with those obtained from the experimental results     

Effect of controlling the sequential interruption of a system faulton turbine-generator shaft torsional torques

A new method is presented to reduce the high torsional torques induced in turbine-generator shafts during the clearing of multiphase faults. This method is based on controlling the sequential interruption of such faults. In this context, investigations have been conducted on two turbine-generator units of different designs for both three-phase and double line-to-ground faults. The effectiveness of the proposed method is illustrated through parametric studies and simulation results. The effects of the fault clearing time, the strength of the system to which the unit is connected, and the fault incidence instant on the effectiveness of the method have also been investigated     

Contribution of Power System Stabilizers to the Damping of Torsional Oscillations of Large Turbo Generators

ABSTRACT

In this paper, the damping torque contributions of power and speed based stabilizers and the effects of these damping torques on the torsional oscillations of large turbo-generators have been investigated under various operating conditions. The results of these investigations show that, in the case of uncompensated power systems, the power based stabilizers introduce almost no damping in the torsional frequency range while the speed based stabilizers contribute large negative damping torques. However, In the case of series capacitor compensated power systems, both power and speed based stabilizers may contribute negative damping in the frequency range of the torsional oscillations of the turbo-generators under certain operating conditions. However, under different operating conditions, power based stabilizers may also contribute relatively large positive damping in the torsional frequency range. The contribution of positive or negative damping by these stabilizers depends on the series capacitor compensation level, the value of the resistance of the transmission network and on the loading conditions of the system.