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    Transformers, protection of and temperature rise time

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    1. (a) What is a Vector grouping?
    (b) How does the knowledge of vector grouping assist in connecting transformers to operate in parallel?
    (c) What additional information is needed to ensure that transformers are capable of parallel operation?
    (d) What are the only phase displacements in degrees with which it is possible to parallel the secondaries of two transformers?
    (e) Draw the phase relationships and connections for
    (i) Dd0 connection
    (ii) Yy6 connection.

    2. (a) Name the types of on-load tap changer.
    (b) Which is the most commonlly used and why?

    3. (a) What is the advantage of monitoring winding temperature as a method of overload protection?
    (b) Name two other types of overcurrent protection.

    4. (a) What is a conservator?
    (b) How does this device protect against pollution of the transformer oil?

    5. List the symbols, and their meaning, used to classify the methods of cooling a transformer and state the types of cooling of an ONAF transformer.

    6. Describe, with the aid of a diagram, the process involved in tap changing using a centre-tapped reactor.

    7. (a) Describe, with the aid of a diagram, the method of operation of a Buchholz relay.
    (b) State which faults can be detected by this relay and in what way they activate the relay.
    (c) What is the major advantage of this relay over other forms of transformer protection?

    8. An oil-filled transformer has a c.m.r. of 500 kVA which allows it to run continuously in an ambient of 35 degrees C with an oil temperature rise of 50 degrees C. The transformer has a time constant of 1.5 hours and the ratio of copper losses to iron losses at c.m.r. is 1.5:1.

    After switch-on, the transformer supplies a load of 600 kVA for one hour and the load then rises to 750 kVA. Calculate the period of time that the transformer can supply this load without exceeding its rated maximum temperature. Sketch the heating curves for the two loads.

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    Solution Preview

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    1 (a)
    A short hand nomenclature is used to describe the physical and electrical properties of transformers. More specifically these describe the physical layout of the transformer by a letter so that
    the high voltage coil is identified by a capital letter, (Y, D, Z) as either Star connected, Delta connected or Zig-Zag connected, possibly followed by a N designation to identify if any external neutral connection is present
    the low voltage coil is identified by a lower case letter, (y, d, z) as either Star connected, Delta connected or Zig-Zag connected, possibly followed by a n designation to identify if any external neutral connection is present
    In addition the relative phases between high voltage and low voltage windings are also identified by something called Vector Groups, through a naming scheme appropriately known as Vector Grouping. Group numbers, as assigned for 3 phase transformers, exist for 10 of these groups ranging from Group 0 through to Group 11 (with the omission of Groups 3 & 9 since 〖90〗^0 & 〖270〗^0 relative phase between windings are not physically realisable). Each of these Group number increments represent a change in phase lag of 〖30〗^0 so that
    Group 0: The low voltage winding is in phase with corresponding high voltage winding, 0^0 phase
    Group 1: The low voltage winding lags the high voltage winding by 〖30〗^0
    Group 2: The low voltage winding lags the high voltage winding by 〖60〗^0
    Group 4: The low voltage winding lags the high voltage winding by 〖120〗^0
    Group 5: The low voltage winding lags the high voltage winding by 〖150〗^0
    Group 6: The low voltage winding lags the high voltage winding by 〖180〗^0
    Group 7: The low voltage winding lags the high voltage winding by 〖210〗^0
    Group 8: The low voltage winding lags the high voltage winding by 〖240〗^0
    Group 10: The low voltage winding lags the high voltage winding by 〖300〗^0
    Group 11: The low voltage winding lags the high voltage winding by 〖330〗^0

    1 (b)
    One of the requirements to be met for connecting transformers in parallel is that the transformers must have the same phase displacement, meaning that the Vector Group assignment number must match between any parallel connected transformers.
    1 (c)
    Additionally to connect transformers in parallel the following criteria must also be met
    Any parallel connected transformers must present the same voltage ratio
    Any parallel connected transformers must have the same impedance drop, voltage regulation.
    Any parallel connected transformers must have the same (or very near identical) winding resistances and reactances (similar frequency characteristics)

    1 (d)
    It is only possible to parallel the secondary's of transformers if the phase relationships between windings are either in phase, 〖120〗^0 or 〖240〗^0
    1 (e)
    Dd0 transformer topology (Delta high voltage winding, Delta low voltage winding, zero phase lag), is shown below

    Yy6 transformer topology (Star high voltage winding, Star low voltage winding, 〖180〗^0 phase lag), is shown below

    Reference

    2 (a)
    'On Load Tap' changing involves altering the transformer winding ratio without interruption to the supply. Three main methods are employed
    Parallel winding tap changing
    Centre tapped reactor tap changing
    Resistance tap changing
    2 (b)
    Resistance tap changing is the most common methodology employed for 'On Load Tap' changing because it is more rapid the Centre tapped methodology and less expensive than the Parallel winding methodology.

    3 (a)
    Oil temperature monitoring is an indirect method employed to monitor stress on a transformer. It is the easiest and cheapest method employed for such purposes and can be very versatile. It can be ...

    Solution Summary

    Several problems and solutions describing transformers, transformer protection including Buchholz relay, Conservators and a calculation involving transformer temperature heating curves for different loads to determine time before transformer temperature rating exceeded for adding increased load.

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