Transformers, protection of and temperature rise time

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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 ...