If the two samples are taken at different instants, the pulse width representing the one taken first in time sequence is delayed by the time difference between the two sampling instants, before feeding to the AND gate. It simply means that one or both signals can be sampled.įor the same reactance relay of the previous case, for operation, we haveįigure (11.10) shows the voltage and current waveforms.Ĭomparison Of two instantaneous magnitudes here is made by converting the magnitudes into proportional pulse widths and then comparing with the help of an AND gate. It is also possible to compare the instantaneous magnitude of one signal at a certain moment with the instantaneous magnitude of the second signal at that very moment or at certain other moment. The block schematic diagram is shown in Fig. The reactance relay operates for X < K, i.e. If current I lags behind voltage V by an angle Φ then the value of voltage at current zero is V m sin Φ. Reactance characteristic is one such case where the instantaneous value of the voltage at the moment of current zero is compared against the rectified and smoothed value of current. Sometimes it is convenient to get the required characteristics by comparing the magnitude of one input signal at a certain point on its wave against the rectified and smoothed value of the second signal. The operating time here is determined by the time constant of the slowest arm of the phase-splitting circuit and by the speed of the output device. Here the input is split into six components 60° apart, so that, it is smoothed within 5%. A faster method is phase splitting before rectification as shown in Fig, (11.8). Since the above method involves smoothing the operation is slow. The tripping signal is provided if the operating signal exceeds the level of restraint. This is then compared with the peak value of the operating signal, which may or may not be rectified, but is not smoothed. In the averaging type instantaneous amplitude comparator the restraining signal is rectified and smoothed completely in order to provide a level of restraint. Instantaneous or direct amplitude comparators can be of two types: averaging type and phase splitting type. Also the rectifiers are not protected at higher currents Instantaneous Comparators: In this case the limiting action is the wrong way, as the rectifiers have higher resistance at lower voltages. The operation in this case depends on the average of the difference of the rectified voltages (v 1-v 2). (11.5) works with voltage input signals derived from PTs. The opposed voltage type comparator shown in Fig. The tripping occurs when the capacitor voltage reaches the setting value of the level detector and triggers a thyristor. The output is obtained only if the averaged value is positive.Īn integrator circuit is shown in Fig. The two currents i 1 and i 2 are rectified and their difference (i 1 – i 2) is averaged. Instead of the polarized relay a static integrator can be used consisting of an averaging circuit and the polarity detector circuit as shown in Fig. The relay voltage will never exceed twice the forward voltage drop of the rectifiers, and typically will be of the order of 1 volt. This arrangement provides a sensitive relay whose voltage may be ideally represented by Fig. The polarized relay operates when S 1>S 2, where S 1=K 1i 1 and S 2=K 2i 2. Rectifier bridge comparator can either be of circulating current type or opposed voltage type.īasic circuit for the circulating current type of Static Amplitude Comparator is shown in Fig. It is possible to arrange rectifier bridge networks as amplitude comparators.
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