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Protection and coordination of transformer

  • Protection and coordination of transformer

    Generally speaking, it is possible to obtain better transformer protection and better coordination with other protective devices by this means, because both the tripping current and tripping time for relays are much more accurate and consistent than the fuses. Therefore, when the primary breakers are available, time-delay overcurrent relays can be used for transformer protection.


    Any protective scheme utilizing only current for selection is inherently handicapped in endeavoring to distinguish between small fault currents and large load currents* A differential protection scheme gives a marked improvement in protective ability. Originally, a single overcurrent relay was used as shown in Figure 9 below. However, it may trip on heavy external faults because of ratio or phase angle errors in the current transformer.



    Consequently, it has been largely superseded by the ratio differential scheme as shown in the figure below.



    Restraining windings are added to the relay which function in a manner to give a tripping characteristic depending upon the ratio of the unbalanced current to the total current. Thus, the relay retains its sensitivity for low internal fault currents, but is restrained from tripping due to minor unbalances when a heavy external fault occurs.


    The transformer protective scheme to be used will vary widely with particular installations. For the average, small or moderate-sized installation, primary fuses are quite satisfactory. Where primary breakers are available, some sort of relay protection is advisable as better protection and coordination can be affected. The type to be used--simple overcurrent, straight differential or ratio differential— will depend upon the size and value of the equipment to be protected. In addition to the cost factor, consideration should be given to hazards to personnel and property, and continuity of service.


    In any differential scheme. It Is well to use current transformers with matched characteristics under short-circuit conditions. This will give greater assurance of correct operation under fault conditions.


    Bus protection. Consideration should be given not only to protection of the transformer from faults, but also to the possibility of faults on the low-voltage bus. Phase-to-phase faults would probably result In sufficient current to blow the primary fuses. With a line-to-ground fault, if the secondary of the transformer bank is star connected and grounded, satisfactory protection would probably be obtainable.


    Available short-circuit currents. The available short-circuit current must be known In order that proper equipment and relay settings may be selected.


    At higher voltages, the size of wire for a feeder may be based, not on the ampere-load it Is expected to 26 carry, but on the short-time fault current which it must carry without excessive heating.

    Reactors. Reactors are primarily current limiting devices and may be used to reduce breaker size. Because the fault current is reduced, wire and equipment of lower ratings can be used safely. Use of reactors may necessitate use of voltage regulators also.


    Codes to be followed. Electrical construction should conform to the latest revision of the National Electrical Safety Code as a minimum. However, many states and communities have specific code requirements to be met.


    Safety items. The design of the station should provide safe working conditions for the personnel. Some of the items to be considered are:


    1. It is always well to have interrupting devices which are adequate to meet their maximum duties.
    2. The surest way to avoid contact with highvoltage conductors is to have them in an inaccessible position— high enough above the ground, in metal enclosures, or protected in some other manner.
    3. A disconnect switch should not be operated when carrying current. Unless the phases are widely separated, adequately designed barriers should be considered for confining accidental arcs.
    4. Interlocks may be installed between the disconnects and the load breaking device or breaker, so that the breaker in series must be open before the disconnects can be operated.
    5. Rubber gloves and other protective equipment, if not checked regularly, may deteriorate and become more of a hazard than a safety device, because they give the operator a false sense of security.
    6. The exits from any room containing electrical equipment should be so placed that anyone can reach safety in the case of failure of any piece of equipment in that room. A regular inspection should be made to see that these exits are not obstructed.
    7. Sufficient unobstructed room for the operator to perform the necessary operation should be provided.
    8. All circuits should be marked in the switching station. Cables should be identified with suitable tags at both ends.

    Fire protection.

    Although fires with modern oilfilled transformers and oil circuit breakers are rare, the possibility of an explosion and fire must be considered. Space separation between adjacent equipment will help to localize fires. Fires in high-voltage equipment are complicated by the possible presence of oil, insulation and energized equipment, and the possible effects of an electric arc on fire-fighting chemicals. Also, certain solutions may be conducting. For this reason, men should be trained in the proper procedures in case of electrical fires. This involves a knowledge of characteristics of fires, characteristics of fire-fighting equipment, and a knowledge of when the fire may get beyond their control and outside help will be necessary.


    Reference: Sadhu, P. H., Das, S. 2016. Elements of power system. Taylor & Franci Group.

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