<p style="text-align:justify"><span style="font-size:14px">The following are the important requirements of a protective system:</span></p>

<p style="text-align:justify">&nbsp;</p>

<ol>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><u><strong>Reliability</strong>:</u> Reliability is a qualitative term. Less the probability of failure of protective system, better is its reliability. The advent of modern numerical relays (with their many new features like self-diagnosis of internal failure of part or full of its circuitry containing discrete components and microprocessor or digital signal processor, storage of pre-fault and post-fault data, circuit breaker failure protection, indication of location of fault in transmission line etc.) has made a protective system much more reliable than the earlier electromechanical or static relays. With these numerical relays, the power system is available for more numbers of hours in a year than earlier technology of protection. This increases the revenue earned by electricity company and industries and this modern technology adds to the comfort and convenience of consumers.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><u><strong>Selectivity</strong>:</u> Selectivity means isolation of minimum faulty section from the rest of the healthy system. This requirement demands that only faulty portion is isolated and healthy power system keeps on feeding normal power to consumers.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong><u>Speed:</u></strong> It is obvious that faster the protective system (protective relays and associated breakers) operates, less will be the damage to equipment. As such, the equipments are short-time (1 or 3 second) rated for high fault currents and therefore there will be practically no damage to the equipment if the relays and breakers operate fast. Breaker time as fast as 1 or 2 cycles is possible with the advent of SF<sub>6</sub> and vacuum breakers.</span></p>
	</li>
</ol>

<p style="text-align:justify">&nbsp;</p>

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Fast relay operation is possible but the requirement of selectivity (in case of remote back-up protection, the back-up relay has to wait until the primary relay operates as we shall study) and speed are contradictory. Such contradiction can be addressed by numerical relays having facility of reverse interlocking.</span></p>

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<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Another important reason why the protective system should be fast can be appreciated by consideration of power system stability. The relays and breakers should operate before transient stability limit of generators operating in the system is reached. That means the fault must be cleared before critical clearing time decided by stability considerations. If the relays operate after that time, the generator poles will slip, they will be disconnected from bus due to operation of pole slipping protection and this will further deteriorate system stability, threaten the remaining generators to go out of step and finally resulting in cascaded tripping in the worst case. This would lead to dark out in the state grid.</span></p>

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<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Lastly, the relays should not be made to operate faster than one cycle as otherwise they may unnecessarily operate due to transient conditions like lightning and switching surges.</span></p>

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<ol start="4">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong><u>Discrimination:</u></strong> A protective relay should be able to discriminate between a fault and an overload. It should be able to discriminate between a fault and load conditions even when the minimum fault current (under the condition of minimum number of generators connected to the power system) is less than maximum load current (with maximum number of generators connected).</span></p>
	</li>
</ol>

<p style="margin-left:72px; text-align:justify">&nbsp;</p>

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">When a power transformer is switched ON, it will take a magnetizing inrush current, which in the worst case can be 10 times its rated full load current. This inrush exponentially decays to normal value after 15 to 30 cycles depending upon X/R ratio. This is neither fault nor abnormality. This behaviour is part of normal operation of transformer. Unfortunately, the fault current for an internal fault within the transformer may be less than this inrush current.</span></p>

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<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">As the fault path is an arc within the oil filled transformer, it is highly hazardous because oil is inflammable. Such a fault has to be cleared within a couple of cycles. This is made possible by feature (of protective relay) known as second harmonic stabilization using which operation of relay is blocked in case of inrush function. But fault current, in certain cases contain second harmonics and hence this feature of harmonic restraint is to be unblocked.</span></p>

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<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">In modern interconnected systems, conditions like power swings can mislead a distance relay to mal-operate while protecting the transmission lines.</span></p>

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<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">In case of differential protection, the protective relay should operate only for internal fault very sensitively and should be stable for external faults. But this is never so simple as the non-identical C. T. saturation characteristics result in a spill current due to severe through (external) fault and differential relay may mal-operate. Also, the relay can never be connected at equipotential points in differential protection scheme because of geography of the location. This gives rise to unwanted relay operation. This requires discriminatory features in the protective scheme.</span></p>

<p style="margin-left:72px; text-align:justify">&nbsp;</p>

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">In case of generator protection, field failure relay should not operate for either pole-slipping conditions or due to power swings. Similarly, a reverse power relay for generator protection should operate for failure of prime-mover only and not for internal faults in the generator (in which case there is a current reversal from bus to generator which looks like reverse power).</span></p>

<p style="margin-left:72px; text-align:justify">&nbsp;</p>

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">No protective scheme should trip a breaker when a fuse placed in secondary of potential transformer fails.</span></p>

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<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Therefore, it seems that a protective relay should operate when current, voltage or power (or flux or frequency or angle between current and voltage) deviate from given limit. But relay operating criteria is not as simple as this. The criteria is to be so chosen that a relay should operate only under the conditions it is required to operate and should never operate under conditions against which the power system does not require interruption. Many of such conditions look like fault (or more severe than faulty conditions) where relay operation is not anticipated.</span></p>

<p style="margin-left:72px; text-align:justify">&nbsp;</p>

<ol start="5">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong><u>Stability:</u></strong> Stability is a quality required only for differential protection by virtue of which the protective scheme is sensitive for internal faults within the unit to be protected and stable for external faults out of the protection zone of the equipment.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong><u>Sensitivity:</u> </strong>Sensitivity refers to the minimum level of fault current at which a protective relay operates</span></p>
	</li>
</ol>