Determination of transient shaft torques in turbine generators by means of the electrical — Mechanical analogy

A brief description is given of the mechanical-transients analyzer and its application to the determination of the shaft torques in turbine generators during transient disturbances, such as electric short circuits and synchronizing out of phase. This device employs the principle of the electrical-mechanical analogy with analogous electric circuits to represent a given mechanical system and special electric circuits for producing the desired transient excitation functions. With the analyzer a detailed study has been made of the shaft torques that can occur in turbogenerators with mechanical systems having both one and two modes of vibration. The relative severity of various types of faults has been determined together with the effect of external system or fault resistance and generator loading. Short circuits produce rotor air-gap torques containing damped unidirectional, fundamental electrical frequency, and second-harmonic frequency components. In the practical range of machine design, the fundamental-frequency component has the dominant effect on the resulting transient shaft torques for the more severe types of disturbances. The types of short circuits studied include three-phase, line-to-line, single-line-to-ground, and double-line-to-ground faults from no load and full load and with varying amounts, of external resistance. It was found that for short circuits at no load with no external resistance a line-to-line fault results in the most severe shaft torques. For faults with low external resistance (of the order of one per cent per phase), a double-line-to-ground fault produces the highest shaft torques which for mechanical systems of low natural frequency may exceed the torques produced by a line-to-line fault with zero resistance. However, for values of external resistance as high as the machine subtransient reactance, the shaft torques are lower than with zero resistance. The most severe fault torques are produced by line-to-line short circuits from full load with no external impedance. Synchronization at angles greater than 30 degrees can produce transient shaft torques which in some cases are more severe than those resulting from short circuits.