Relay contact systems

       Relay contact systems

 a.     Self-reset.

The contacts remain operated only while the controlling quantity is applied, returning to their original condition when it is removed.

 

 b.     Hand or electrical reset.

These contacts remain in the operated position after the controlling quantity is removed. They can be reset either by hand or by an auxiliary electromagnetic element.

 The majority of protective relay elements have self-reset contact systems, which, if it is so desired, can be made 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 a lock-out condition. Contacts are shown on diagrams in the position corresponding to the un-operated or de-energized condition regardless of the continuous service condition of the equipment. For example, a voltage supervising relay, which is continually picked-up, would still be shown in the de-energized condition.

 A 'make' contact is one that closes when the relay picks up, whereas a 'break' contact is one that is closed when the relay is un-energized and opens when the relay picks up. Examples of these conventions and variations are shown in Figure 6.
 


 

Figure 6 indications of contacts on diagrams.

 

A protective relay is usually required to trip a circuit breaker, the tripping mechanism of which may be a solenoid with a plunger acting directly on the mechanism latch or, in the case of air-blast or pneumatically operated breakers, an electrically operated valve. The relay may energize the tripping coil directly, or, according to the coil rating, and the number of circuits to be energized, may do so through the agency of another multi-channel auxiliary relay.

The power required by the trip coil of the 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, being made up of a hand-trip control switch and the contacts of the protective relays in parallel to energize the trip coil from a battery, through a normally open auxiliary switch operated by the circuit breaker. This auxiliary switch is needed to open the trip circuit when the circuit breaker opens, since the protective relay contacts will usually be quite incapable of performing the interrupting duty. The auxiliary switch will be adjusted to close as early as possible in the closing stroke, to make the protection effective in case the breaker is being closed on to a fault.

 

Protective relays are precise measuring devices, the contacts of which should not be expected to perform large making and breaking duties. Attracted armature relays, which combine many of the characteristics of measuring devices and contactors,

Occupy an intermediate position and according to their design and consequent closeness to one or other category, may have an appreciable contact capacity.

Most other types of relay develop an effort which is independent of the position of the moving system.

At setting, the electromechanical effort is absorbed by the controlling force, the margin for operating the contacts being negligibly small. Not only does this limit the 'making' capacity of the contacts, but if more than one contact pair is fitted any slight misalignment may result in only one contact being closed at the minimum operating value, there being insufficient force to compress the spring of the first contact to make, by the small amount required to permit closure of the second.

For this reason, the provision of multiple contacts on such elements is undesirable. Although two contacts can be fitted, care must be taken in their alignment, and a small tolerance in the closing value of operating current may have to be allowed between them. These effects can be reduced by providing a small amount of 'run-in' to contact make in the relay behavior, by special shaping of the active parts.

For the above reasons it is often better to use inter-posing contactor type elements which do not have the same limitations, although some measuring relay elements are capable of tripping the smaller types of circuit breaker directly. These may be small attracted armature type elements fitted in the same case as the measuring relay.

In general, static relays have discrete measuring and tripping circuits, or modules. The functioning of the measuring modules will not react on the tripping modules. Such a relay is equivalent to a sensitive electromechanical relay with a tripping contactor, so that the number or rating of outputs has no more significance than the fact that they have been provided.

For larger switchgear installations the tripping power requirement of each circuit breaker is considerable, and, further, two or more breakers 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), and other control functions to be performed. These various operations are carried out by multi-contact tripping relays, which are energized by the protection relays and provide the necessary number of adequately rated output contacts.

      Operation indicators.

As a guide for power system operation staff, protective systems are invariably provided with indicating devices. In British practice these are called 'flags', whereas in America they are known as 'targets'. Not every component relay will have one, as indicators are arranged to operate only 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 relay movement to expose the indicator pattern, which, on GEC Measurements relays, consists of a red diagonal stripe on a white background.

Electrical indicators may be simple attracted armature elements either with or without contacts. Operation of the armature releases a shutter to expose an indicator as above.

An alternative type consists of a small cylindrical permanent magnet magnetized across a diameter, and lying between the poles of an electromagnet. The magnet, which is free to rotate, lines up its magnetic axis with the electromagnet poles, but can be made to reverse its orientation by the application of a field. The edge of the magnet is colored to give the indication.

 

     Relay tripping circuits.

Auxiliary contactors can be used to supplement protective relays in a number of ways:

   a.       Series sealing.

   b.       Shunt reinforcing.

   c.        Shunt reinforcement with sealing. These are illustrated   in
          Figure 7.

When such auxiliary elements are fitted, they can conveniently carry the operation indicator, avoiding the need for indicators on the measuring elements.

Electrically operated indicators avoid imposing an additional friction load on the measuring element, which would be a serious handicap for certain types. Another advantage is that the indicator can operate only after the main contacts have closed.
 


Figure 7 Typical relay tripping circuits.

                                                                                      

 With indicators operated directly by the measuring elements, care must be taken to line up their operation with the closure of the main 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 tripping operation has not been completed.
 
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a.     Series sealing.

The coil of the series contactor carries the trip current initiated by the protective relay, and the contactor closes a contact in parallel with the   protective relay contact.

This closure relieves the protective relay contact of further duty and keeps the tripping circuit securely closed, even if chatter occurs at the main contact. Nothing is added to the total tripping time, and the indicator does not operate until current is actually flowing through the trip coil.

The main disadvantage of this method is that such series elements must have their coils matched with the trip circuit with which they are 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 usually interrupt their own coil current, the auxiliary elements must be fast enough to operate and release the flag before their coil current is cut off.

This may pose a problem in design if a variable number of auxiliary elements (for different phases and so on) may be required to operate in parallel to energize a common tripping relay.
 

 b.     Shunt reinforcing.

Here the sensitive contacts are arranged to trip the circuit breaker and simultaneously to energize the auxiliary unit, which then reinforces the contact which is energizing the trip coil.

It should be noted that two contacts are required on the protective relay, since it is not permissible to energize the trip coil and the reinforcing contactor in parallel. If this were done, and more than one protective relay were connected to trip the same circuit breaker, all the auxiliary relays would be energized in parallel for each relay operation and the indication would be confused. The duplicate main contacts are frequently provided

As a three point arrangement to reduce the number of contact fingers.
 


Figure 8 Examples of trip circuit supervision.

 

 c.      Shunt reinforcement with sealing.

This is a development of the shunt reinforcing circuit to make it applicable to relays with low torque movements or where there is a possibility of contact bounce for any other reason.

Using the shunt reinforcing system under these circumstances would result in chattering on the auxiliary unit, and the possible burning out of the contacts not only of the sensitive element but also of the auxiliary unit. The chattering would only end when the circuit breaker had finally tripped.

It will be seen that the effect of bounce is countered by means of a further contact on the auxiliary unit connected as a retaining contact.

This means that provision must be made for releasing the sealing circuit when tripping is complete; this is a disadvantage, because it is sometimes in-convenient to find a suitable contact to use for this purpose.

 

     Supervision of  trip circuits.

The trip circuit extends beyond the relay enclosure and passes through more components, such as fuses, links, relay contacts, auxiliary switch contacts and so on, and in some cases through a considerable amount of circuit wiring with intermediate terminal boards.

These complications, coupled with the importance of the circuit, have directed attention to its supervision.

The simplest arrangement contains a healthy trip lamp, as shown in Figure 8(a).

The resistance in series with the lamp prevents the breaker being tripped by an internal short circuit caused by failure of the lamp. This provides super-vision while the circuit breaker is closed; a simple extension gives pre-closing supervision.

Figure 1.8(b) shows how, by the addition of a normally closed auxiliary switch and a resistance unit, supervision can be obtained while the breaker is both open and closed.

I n either case, the addition of a normally open push-button contact in series with the lamp will make the supervision indication available only when required.

Schemes using a lamp to indicate continuity are suitable for locally controlled installations, but when control is exercised from a distance it is necessary to use a relay system. Figure 8(c) illustrates such a scheme, which is applicable wherever a remote signal is required.

With the circuit healthy either or both of relays A and B are operated and energize relay C. Both A and B must reset to allow C to drop-off. Relays A and C are time-delayed by copper slugs to prevent spurious alarms during tripping or closing operations. The resistors 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 take place.

The alarm supply should be independent of the tripping supply so that indication will be obtained in the event of the failure of the tripping battery.

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