A novel frequency computation technique suitable for single or three phase voltage signals is proposed. The method is based on a generalization of the zero crossing detection to a level crossing detection. This yields several estimates of the frequency within one cycle. A composite "best estimatet" is obtained by an appropriate weighted average of these estimates. The method is insensitive to amplitude variations, unbalance and is capable of tracking small frequency deviations in as short a duration as a fraction of a cycle. The measurement scheme is particularily suitable for integration with the digital schemes for real-time monitoring control and protection of power systems.

A New Technique for Rapid Tracking of Frequency Deviations Based on Level Crossings

This paper presents a new nonlinear observer (extended linear observer) which estimates the transient state of power system. The transient state of the power system is represented as a continuous time nonlinear differential equation and observation of output is obtained as a discretized value at every cycle of system frequency by using the mean value detecting circuit device. The nonlinear differential equation of state is discretized at each observation instant by Taylor expansion, and then the theory of discrete linear observers is applied to the discretized system. Digital computer simulation confirms that the observer estimates the state successfully not only in the case of hunting but also in step-out transient state of a synchronous machine.

On the estimation of transient state of power system by discrete nonlinear observer

A circuit for fast amplitude detection of constant period sinusoidal signals is proposed in this paper. Amplitude information is taken by several sample pulses in each half period and the sampled voltage level is held and equalized to signal amplitude by a weighting function amplifier.

Fast amplitude detection for constant period sinusoidal signals

This paper presents a second-order observer which estimates the states of a nonlinear plant based on discrete deterministic measurements. The observer is obtained by applying the time-varying linear observer theory to the augmented linearized model which is obtained by replacing each quadratic term in the original system with new state variables. The gain of the observer is evaluated sequentially through a procedure similar to that in the Kalman filter. The comparison between the extended linear observer and this one is made through the computer simulation for two model systems of nonlinear type wherein one of them is an electric power system model. These simulation results indicate that the use of second-order observer leads to improved estimation performance.

https://ieeexplore.ieee.org/iel5/9/24158/01101411.pdf

Second-order observer for nonlinear systems from discrete noiseless measurements

This paper presents some distinctive features of a power system which is composed of a generator and a large power system connected to it through relatively long tie-line. The generator is closely connected to an induction machine which is regarded as a dynamic load. The phenomena-affected with dynamic load are explored by extending the approach developed by deMello and Concordia [1], and the laboratory field tests. The features are compared with the case of impedance load to which many studies have already been conducted.

Effects of Induction Machine Load on Power System

A fast response detecting method of mean value of power system quantities such as, voltage, current, power, torque angle and shaft speed deviation is proposed. The detected value, which is linearly proportional to the input signal, represents the mean value of each half cycle of the input signal that is held as a dc voltage as soon as that half cycle is over. The detection is stable against the random noise included in the signal, and does not have drooping characteristic.

Fast response detection of mean value of power system quantities

To suppress the hunting of synchronous machine caused by stepwise load change, a dual mode field excitation control was investigated for conventional (one-axis) machine and 2-axis machine. For one-axis machine, a dual mode is applied successively, the first one being a bang-bang suboptimal control mode decided by use of energy function instead of Pontryagin's switching function, this mode is applicable to dynamic equation described in nonlinear form. The second one is a linear feedback control mode composed of a linear combination of state variables, and it will be adopted to the range near the steady state operating point where the equation will be linearized. For 2-axis machine at the instant of load change the original excitation introduced by mixing these two controls is efficiently applied. From the results of the investigation it is recognized that the employment of 2-axis synchronous machine is a potential means for the improvement of system stability.

S. Takata

Papers

Fast response detection of mean value of power system quantities

A Dual Mode Excitation Control of Synchronous Machine for Suppressing its Hunting