Basic of protection system *****part 4

· Relay contact systems
a. Self-reset.
The contacts remain operated only whilethe controlling quantity is applied, returning to their original condition whenit is removed.

b. Hand or electrical reset.
These contacts remain in the operatedposition after the controlling quantity is removed. They can be reset either byhand or by an auxiliary electro­magnetic element.
The majority of protective relayelements have self-reset contact systems, which, if it is so desired, can bemade to give hand reset output contacts by the use of auxiliary elements.
Hand or electrically reset relays are used when it is necessary to maintain a signal or alock-out condition. Contacts areshown on diagrams in the position corresponding to the un-operated orde-energized condition regardless of the continuous service condition of theequipment. For example, a voltage supervising relay, which is continuallypicked-up, would still be shown in the de-energized condition.
A 'make' contact is one that closes when the relaypicks up, whereas a 'break' contact is one that is closed when the relay isun-energized and opens when the relay picks up. Examples of these conven­tionsand variations are shown in Figure 6.


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


A protective relay is usually requiredto trip a circuit breaker, the tripping mechanism of which may be a solenoidwith a plunger acting directly on the mechanism latch or, in the case ofair-blast or pneumatically operated breakers, an electrically operated valve.The relay may energize the tripping coil directly, or, according to the coilrating, and the number of circuits to be energized, may do so through theagency of another multi-channel auxiliary relay.
The power required by the trip coil ofthe circuit breaker may range from up to 50 watts, for a small 'distribution'circuit breaker, to 3000 watts for a large extra-high-voltage circuit breaker.
The basic trip circuit is simple, beingmade up of a hand-trip control switch and the contacts of the protective relaysin parallel to energize the trip coil from a battery, through a normally openauxiliary switch operated by the circuit breaker. This auxiliary switch isneeded to open the trip circuit when the circuit breaker opens, since theprotective relay contacts will usually be quite incapable of performing theinterrupting duty. The auxiliary switch will be adjusted to close as early aspossible in the closing stroke, to make the protection effective in case thebreaker is being closed on to a fault.

Protective relays are precise measuringdevices, the contacts of which should not be expected to perform large makingand breaking duties. Attracted armature relays, which combine many of thecharacteristics of measuring devices and contactors,
Occupy an intermediate position and according to theirdesign and consequent closeness to one or other category, may have anappreciable contact capacity.
Most other types of relay develop aneffort which is independent of the position of the moving system.
At setting, the electromechanical effort is absorbed bythe controlling force, the margin for operating the contacts being negligiblysmall. Not only does this limit the 'making' capacity of the contacts, but ifmore than one contact pair is fitted any slight misalignment may result in onlyone contact being closed at the minimum operating value, there beinginsufficient force to compress the spring of the first contact to make, by thesmall amount required to permit closure of the second.
For this reason, the provision of multiple contacts onsuch elements is undesirable. Although two contacts can be fitted, care must betaken in their alignment, and a small tolerance in the closing value ofoperating current may have to be allowed between them. These effects can bereduced by providing a small amount of ^'run-in^' tocontact make in the relay behavior, by special shaping of the active parts.
For the above reasons it is often better to useinter-posing contactor type elements which do not have the same limitations,although some measuring relay elements are capable of tripping the smallertypes of circuit breaker directly. These may be small attracted armature typeelements fitted in the same case as the measuring relay.
In general, static relays have discrete measuring andtripping circuits, or modules. The functioning of the measuring modules willnot react on the tripping modules. Such a relay is equivalent to a sensitiveelectromechanical relay with a tripping contactor, so that the number or ratingof outputs has no more significance than the fact that they have been provided.
For larger switchgear installations the tripping powerrequirement of each circuit breaker is considerable, and, further, two or morebreakers may have to be tripped by one protective system.
There may also be remote signaling requirements,interlocking with other functions (for example auto-reclosing arrange­ments), andother control functions to be performed. These various operations are carriedout by multi-contact tripping relays, which are energized by the protectionrelays and provide the necessary number of adequately rated output contacts.
· Operation indicators.
As a guide for power system operation staff, protectivesystems are invariably provided with indicating devices. In British practicethese are called 'flags', whereas in America they are known as ^'targets'.Not every component relay will have one, as indicators are arranged to operateonly if a trip operation is initiated. Indicators, with very few exceptions,are bi-stable devices, and may be either mechanically or electrically operated.A mechanical indicator consists of a small shutter which is
Released by the protective relaymovement to expose the indicator pattern, which, on GEC Measurements relays,consists of a red diagonal stripe on a white background.
Electrical indicators may be simpleattracted arma­ture elements either with or without contacts. Operation of thearmature releases a shutter to expose an indicator as above.
An alternative type consists of a smallcylindrical permanent magnet magnetized across a diameter, and lying betweenthe poles of an electromagnet. The magnet, which is free to rotate, lines upits magnetic axis with the electromagnet poles, but can be made to reverse itsorientation by the application of a field. The edge of the magnet is colored togive the indication.




· Relay tripping circuits.
Auxiliary contactors can be used tosupplement protective relays in a number of ways:
a. Seriessealing.
b. Shuntreinforcing.
c. Shunt reinforcement withsealing. These are illustrated in
Figure 7.
When such auxiliary elements are fitted, they canconveniently carry the operation indicator, avoiding the need for indicators onthe measuring elements.
Electrically operated indicators avoid imposing anadditional friction load on the measuring element, which would be a serioushandicap for certain types. Another advantage is that the indicator can operateonly after the main contacts have closed.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


With indicators operated directly by the measuringelements, care must be taken to line up their operation with the closure of themain contacts. The indicator must have operated by the time the contacts make,but must not have done so more than marginally earlier.
This is to stop indication occurring when the trippingoperation has not been completed.

Ta. Series sealing.
The coil of the series contactor carries the trip currentinitiated by the protective relay, and the contactor closes a contact inparallel with the pro­tective relay contact.
This closure relieves the protective relay contact of further duty andkeeps the tripping circuit securely closed, even if chatter occurs at the maincontact. Nothing is added to the total tripping time, and the indicator doesnot operate until current is actually flowing through the trip coil.
The main disadvantage of this method is that such serieselements must have their coils matched with the trip circuit with which theyare associated.
The coils of these contactors must be of low impedance,with about
5 % of the trip supply voltage being dropped across them.
When used in association with high speed trip relays, which usuallyinterrupt their own coil current, the auxiliary elements must be fast enough tooperate and release the flag before their coilcurrent is cut off.
This may pose a problem in design if a variable number ofauxiliary elements (for different phases and so on) may be required to operatein parallel to energize a common tripping relay.

b. Shunt reinforcing.
Here the sensitive contacts are arranged to trip thecircuit breaker and simultaneously to energize the auxiliary unit, which then reinforcesthe contact which is energizing the trip coil.
It should be noted that two contacts are required on theprotective relay, since it is not permissible to energize the trip coil and thereinforcing contactor in parallel. If this were done, and more than oneprotective relay were connected to trip the same circuit breaker, all theauxiliary relays would be energized in parallel for each relay operation andthe indication would be confused. The duplicate main contacts are frequentlyprovided
As a three point arrangement to reduce the number of contact fingers.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


c. Shuntreinforcement with sealing.
This is a development of the shuntreinforcing circuit to make it applicable to relays with low torque move­mentsor where there is a possibility of contact bounce for any other reason.
Using the shunt reinforcing system underthese circumstances would result in chattering on the auxiliary unit, and thepossible burning out of the contacts not only of the sensitive element but alsoof the auxiliary unit. The chattering would only end when the circuit breakerhad finally tripped.
It will be seen that the effect ofbounce is countered by means of a further contact on the auxiliary unitconnected as a retaining contact.
This means that provision must be madefor releasing the sealingcircuit when tripping is complete; this is a disadvantage, because it issometimes in-convenient to find a suitable contact to use for this purpose.

· Supervision of tripcircuits.
The trip circuit extends beyond the relay enclosure andpasses through more components, such as fuses, links, relay contacts, auxiliaryswitch contacts and so on, andin some cases through a considerable amount of circuit wiring with intermediate terminalboards.
These complications, coupled with the importance of thecircuit, have directed attention to its supervision.
The simplest arrangement contains a healthy trip lamp, asshown in Figure 8(a).
The resistance in series with the lamp prevents thebreaker being tripped by an internal short circuit caused by failure of thelamp. This provides super-vision while the circuit breaker is closed; a simpleextension gives pre-closing supervision.
Figure 1.8(b) shows how, by the addition of a normallyclosed auxiliary switch and a resistance unit, supervision can be obtainedwhile the breaker is both open and closed.
I n either case, the addition of a normally openpush-button contact in series with the lamp will make the supervisionindication available only when required.
Schemes using a lamp to indicate continuity are suitable for locallycontrolled installations, but when control is exercised from a distance it isnecessary to use a relaysystem. Figure 8(c) illustrates such a scheme, which is applicable wherever a remote signal isrequired.
With the circuit healthy either or both of relays A and Bare operated and energize relay C.Both Aand B must reset toallow C to drop-off. Relays A and C are time-delayed by copperslugs to prevent spurious alarms during tripping or closing operations. Theresistors are mounted separately from the relays and their values are chosen such that if any one component is inadvertently short-circuited, a tripping operation will not takeplace.
The alarm supply should be independent of the tripping supply so thatindication will be obtained in the event of the failure of the trippingbattery.






Classification and function of relays
A protection relay is a device thatsenses any change in the signal which it is receiving, usually from a current and/or voltagesource. If the magnitude of the incoming signal is outside a preset range, therelay will operate, generally to close or open electrical contacts to initiatesome further operation, for example the tripping of a circuit breaker.
3.1 Classification:
Protectionrelays can be classified in accordance with the function which they carry out, theirconstruction, the incoming signal and the type of functioning.
3.1.1General function:
· Auxiliary.
· Protection.
· Monitoring.
· Control.
3.1.2 Construction:
· Electromagnetic.
· Solid state.
· Microprocessor.
· Computerized.
· Nonelectric (thermal, pressure ......etc.).

3.1.3 Incoming signal:
· Current.
· Voltage.
· Frequency.
· Temperature.
· Pressure.
· Velocity.
· Others.
3.1.4 Type of protection
· Over current.
· Directional over current.
· Distance.
· Over voltage.
· Differential.
· Reverse power.
· Other.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Figure 1 Armature-type relay

In some cases a letter is added to thenumber associated with the protection in order to specify its place oflocation, for example G for generator, Τ for transformeretc. Nonelectric relays are outside the scope of this book and therefore arenot referred to.
3.2 Electromagneticrelays
Electromagneticrelays are constructed with electrical, magnetic and mechanical components,have an operating coil and various contacts and are very robust and reliable. The constructioncharacteristics can be classifiedin three groups, as detailed below.
3.2.1 Attraction relays
Attraction relays can be supplied by ACor DC, and operate by the movement of a piece of metal when it is attracted by the magnetic field produced by a coil. There are two main types of relay inthis class.
The attractedarmature relay, which is shown in figure 1, consists of a bar or plate of metal which pivots when it is attracted towardsthe coil.
The armature carries the moving part of the contact, which isclosed or opened according to the design when the armature is attractedto the coil. The other type is the piston orsolenoid relay, illustrated in Figure 2,in which α bar or pistonis attracted axially within the field of thesolenoid. In this case, the piston also carries the operating contacts.
It can be shown that the force of attraction is equal to K₁I²- K₂, where Κ₁ depends upon the number of turns onthe operating solenoid, the air gap, the effective area and the reluctance ofthe magnetic circuit, among other factors. K2 is the restraining force, usuallyproduced by a spring. When the relay is balanced, the resultant forceis zero and therefore Κ₁1² = K2,

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]



In orderto control the value at which the relay starts to operate, the restrainingtension of the spring or theresistance of the solenoid circuit can be varied, thus modifying therestricting force. Attraction relays effectively have no time delay and, forthat reason, are widely used when instantaneous operations are required.
3.2.2 Relays with moveable coils
This type of relay consists of a rotating movement with asmall coil suspended or pivoted with the freedom to rotate between the poles ofa permanent magnet. The coil is restrained by two springs which also serve asconnections to carry the current to the coil.
The torque produced in the coil is given by:
T = B.l.a.N.i
Where:
T= torque
B = flux density
L =length of the coil
a = diameter of the coil
N = number of turns on the coil
i = current flowing through the coil


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Figure 2 Solenoid-type relay

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

figure 3 Inverse time characteristic

From theabove equation it will be noted that the torque developed is proportional tothe current. The speed of movement is controlled by the damping action, whichis proportional to the torque. It thus follows that the relay has an inversetime characteristic similar to that illustrated in Figure 3. The relay can bedesigned so that the coil makes a large angular movement, for example 80º.

3.2.3 Induction relays
An induction relay works only with alternating current.It consists of an electromagnetic system whichoperates on a moving conductor, generally in the form of a disc or cup, andfunctions through the interaction of electromagnetic fluxes with the parasiticFault currents which are induced in the rotor by these fluxes. These twofluxes, which are mutually displaced both in angle and in position, produce atorque that can be expressed by
T= Κ₁.Φ1.Φ₂ .sin θ,
Where Φ1 and Φ₂ are theinteracting fluxes and θis the phase angle between Φ1 and Φ₂. It should be noted that the torque is a maximum when thefluxes are out of phase by 90º, and zero when they are in phase.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

It can be shown that Φ₁= Φ₁sin ωt, and Φ₂= Φ₂sin (ωt+ θ), where θ is theangle by which Φ₂ leads Φ₁.Then:

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]




Figure 4 shows the interrelationshipbetween the currents and the opposing forces. Thus:
F=(F₁-F₂) α (Φ₂i_Φ1+Φ₁i_Φ2 )
F α Φ₂Φ₁sin θ α T
Induction relays can be grouped intothree classes as set out below.
· Shaded-polerelay
In this case a portion of theelectromagnetic section is short-circuited by means of a copper ring or coil.This creates a flux in the area influenced by the short circuited section (the so-calledshaded section) which lags the flux in the nonshaded section, see Figure 5.


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


In its more common form, this type ofrelay uses an arrangement of coils above and below the disc with the upper andlower coils fed by different values or, in some cases, with just one supply forthe top coil, which induces an out-of-phase flux in the lower coilbecause of the air gap. Figure 6 illustrates a typical arrangement.
· Cup-type relay
This type of relay has a cylindersimilar to a cu which can rotate in the annular air gap between the poles ofthe coils, and has a fixed central core, see Figure 7. The operation of thisrelay is very similar to that


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


Of an induction motor with salient poles for the windings ofthe stator. Configurations with four or eight poles spaced symmetrically aroundthe circumference of the cup are often used. The movement of the cylinder islimited to a small amount by the contact and the stops. Α specialspring provides the restraining torque.
The torque is a function of theproduct of the two currents through the coils and the cosine of the anglebetween them. The torque equation is
T= ( KI₁I₂cos (θ₁₂ – Φ) – K_s ),
Where K, .Κs and Φ are design constants, Ι₁ and I₂ arethe currents through the two coils and θ₁₂isthe angle between I₁ and I₂.
In the first two types of relaymentioned above, which are provided with a disc, the inertia of the discprovides the time-delay character­istic. The time delay can be increased by theaddition of a permanent magnet. The cup-type relay has a small inertia and istherefore principally used when high speed operation is required, for examplein instantaneous units.