Amplitude comparator compares the amplitudes of two (or more) input quantities. The phase angle between the quantities under comparison (inputs) is not recognized or noticed by the amplitude comparator.
If the two input signals are S1 and S2 (say S1 the operating and S2 restraining), the amplitude comparator gives positive output only if –
S2/S1 < K.
The function is represented by a circle in the complex plane with its centre at the origin. This defines the boundary of the marginal operation.
The main purpose of amplitude comparators is to provide direction and distance protection.
Mainly there are three types of amplitude comparators viz.:
1. Integrating Comparators,
2. Instantaneous Comparators, and
3. Sampling Comparators.
These are discussed in brief as follows:
1. Integrating Comparators:
The input quantities to static relays are either in the form of sinusoidal current derived from main CT or sinusoidal voltage derived from PT. CT or PT provides analogous output, faithful to the main circuit quantities. It is possible to arrange rectifier bridge networks as amplitude comparators. Rectifier bridge comparator can either be of circulating current type or opposed voltage type. The former is more efficient, because the nonlinear resistance characteristics of the rectifiers provide a limiting action so that all the difference current goes through the relay at low currents and only a fraction of it does so at high currents.
The basic circuit for the comparator is shown in Fig. 3.7. Current input signals are used in this comparator. Two full-wave rectifier bridges are connected in opposition and the output relay is connected in parallel to the two rectifier bridges. The output of the rectifier bridge comparator is received by the output stage continuously. The output device should be an integrating device operating on the average value of the difference between i0 and ir.
This could be a polarized relay or an integrating circuit followed by a level detector. The operation of the polarised relay (PR) occurs when S1 > S2 where S1 = Ki0 and S2 = Kir. The output voltage characteristics across the relay are shown in Fig. 3.7 (b). The voltage across the relay will never exceed twice the forward voltage drop of one of the rectifiers and this will normally be of the order of IV. The comparatively low voltage level required for the operation enables reasonable accuracy in impedance measurement to be achieved even for line or source impedance ratios of the order of 30.
A static integrating circuit consisting of an averaging circuit and polarity detecting circuit is shown in Fig. 3.8. The two currents i0 and ir are rectified and their difference (i0 – ir) is averaged. The output is obtained only if the averaged value is positive.
An integrator circuit is shown in Fig. 3.9. The tripping occurs when the capacitor voltage attains the setting value of the level detector and triggers a thyristor.
The opposed voltage type rectifier bridge comparator is shown in Fig. 3.10. The bridge action can be seen from the diagram. The operation of the relay depends on the average of the difference between v0 and vr derived from PTs. The bridge is less sensitive at low voltage inputs and the comparator has no limiting action on both voltage and current in the output device.
2. Instantaneous Amplitude Comparators:
Instantaneous amplitude comparators, also called the direct amplitude comparators, are of two types viz., averaging type and phase splitting type.
In the averaging type instantaneous amplitude comparator the restraining signal is rectified and smoothened completely in order to provide a 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 smoothened. The peak of the operating input signal should exceed the level of restraint for the tripping output. Smoothening is done with the help of a capacitor; so there is a delay in operation.
Phase-splitting type instantaneous amplitude comparator is faster in operation. Here phase splitting is done before rectification, i.e., the input is split into six components 60° apart, so that it is smoothened within 5%. In this both operating and restraining input signals are smoothened out before being compared so that a continuous output signal is obtained. The averaging circuit can be eliminated. The operating time is determined by the time constant of the slowest arm of the phase-splitting circuit and by the speed to the output device.
3. Sampling Comparators:
Sometimes it is convenient to obtain the required characteristics by comparing the amplitude of one input signal at a certain point on its wave with the rectified and smoothened value of the second signal. In case of a reactance relay the instantaneous value of the voltage at the instant of current zero is compared with the rectified and smoothened value of current. Let the phase angle of the circuit be ɸ lagging, (i.e., current lagging behind voltage by ɸ. The value of voltage at current zero will be V sin ɸ. The reactance relay operates when X < K where X is the reactance seen by the relay and K is design reactance of the system.
Since X = Z sin ɸ
so Z sin ɸ < K
or V/I sin ɸ < K
or Vmax/√2 sin ɸ K Iav x 1.11
or Vmax sin ɸ < K’ Iav
It is also possible to compare the instantaneous magnitude of one signal at a certain instant with the instantaneous magnitude of the second signal at that very instant or at certain other instant. It simply means that one or both signals can be sampled. Voltage is sampled again when the current is passing through zero value and current is sampled after a delay of say α.
For the same reactance relay, for operation we have –
V/I sin ɸ < K or Vmax sin ɸ < K’ Imax sin α
The amplitudes of these two signals are converted into proportional pulse widths and these pulses are compared in an AND gate. 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.
By using sampling techniques, the phase shifting and mixing circuits are eliminated resulting in saving in space and cost but the sampling techniques require a high degree of sophistication in the relay circuitry. The measuring circuits are isolated from the transformer secondaries, except during 50 µs sampling period in each half cycle, and the possibility of maloperation due to spikes on the input waveforms is remote.