Transformer

Question 0

Windings

Answer 0

The windings provide the magneto motive force. The winding that takes in the power is the input and the winding that delivers the power is output winding. High voltage winding. Low voltage winding.

Question 1

Core

Answer 1

The core carries the magnetic flux.

Question 2

Tappings

Answer 2

Tappings are additional connections to the windings for altering the voltage ratio.

Question 3

Tapping range

Answer 3

Tapping range is the difference between the rated voltage and the maximum or minimum adjustable voltage of the winding. Tapping range is quoted as positive and negative percentage of the rated voltage.

Question 4

Maximum voltage

Answer 4

The max voltage for equipment Um(subscript) is the maximum rms value of the phase-to-phase voltage for which the insulation of a transformer winding is designed.

The maximum voltage Um(subscript) is linked to values of rated power-frequency withstand voltage (AC, rms) and rated lightning impulse withstand voltage (LI, peak value, for max voltage greater than 1.1kV only) which identify the insulation level of the particular winding.

Question 5

Three phase electric power

Answer 5

Three phase electric power is a common method of alternating-current electric power generation, transmission and distribution. It is a type of polyphase system. Used by electrical grids and large motors or heavy loads.

But most households use single phase.

Question 6

Three phase power supply system

Answer 6

Three conductors each carry alternating current of the same frequency but the phases of the voltage on each conductor is displaced by 120 degrees.

The delay (one third and two thirds of the cycle respectively) gives constant power transfer over each cycle.

The potential of any phase is exactly equal to and opposite of the sum of the other two phases. Meaning if the load on the three phases is “balanced” - the return path for the current in any phase conductor is the other two phase conductors.

Question 7

Three phase with neutral

Answer 7

Three phase systems may also utilise a fourth wire, particularly in low voltage distribution less than 1000v. The neural wire allows three separate single phase supplies to be provided at a constant voltage and is commonly used for supplying groups of domestic properties which are each single-phase loads. The connections are arranged so, as far as possible, in each group equal power is drawn from each phase. In high voltage distribution the currents are usually well balanced and the neutral conductor can usually be omitted.

Question 8

Advantage of three phase versus single phase (quantity and cost of conductors)

Answer 8

While single phase AC power requires two conductors (go and return) a three phase supply can transmit three times the power using only one extra conductor (3 instead of 2). This means a 50% increase in transmission cost yields 200% increase in power transmitted.

Question 9

Advantage of three phase with neutral (size of conductors and neutral)

Answer 9

The phase currents tend to cancel each other out in the case of a linear balanced load making it possible to reduce the size of the neutral conductor because it carries little to no current.

All the phase conductors carry the same current so can be the same size for a balanced load.

Three phase systems are usually more economical than an equivalent single phase or two phase system because it uses less conductor material to transmit electrical power.

Question 10

Advantage of three phase supply (power transfer in from generator or motor)

Answer 10

Power transfer into a linear balanced load is constant which helps to reduce generator and motor vibrations.

Question 11

Advantage of three phase (rotating magnetic field)

Answer 11

Three phase systems can produce a rotating magnetic field with a specified direction and constant magnitude which simplifies the design of electric motors.

Connection of the phases in the right order is required to ensure the intended direction of rotation of the three phase motors.

Question 12

Electrical generator

Answer 12

At the power station an electrical generator converts mechanical power into set of three AC electric currents, one from each coil or winding of the generator.

The windings are arranged such that the currents vary sinusoidally at the same frequency but with the peaks and troughs of their wave forms offset to provide three complimentary currents with phase separation of one third cycle.

The generator frequency is typically 50 or 60Hz.

Question 13

What do transformers do at a power station?

Answer 13

At the power station transformers change the voltage from the generator to a level suitable for minimising transmission losses.

Question 14

Delta connected transformer

Answer 14

A delta connected transformer winding is connected between phases if a three phase system.

Question 15

A star transformer

Answer 15

A star or wye transformer connects each winding from a phase wire to a common neutral point.

Question 16

Open delta or V system

Answer 16

Only two sets of transformers are used. A closed delta system can operate as an open delta system if one of the transformers has failed or needs to be removed. In open delta, each transformer must carry current for its respective phase as well as current for the third phase, therefore capacity is reduced to 87%. With one of the three transformers missing and the remaining two at 87% the capacity is 58% = (⅔) x 87%

Question 17

Grounded delta fed system

Answer 17

Where a delta fed system must be grounded for protection from surge voltages, a grounding transformer usually a zigzag transformer may be connected to allow any ground fault currents to return from any phase to ground.

Another variation is a “corner grounded” delta system, which is a closed delta that is grounded at one of the junctions of the transformers.

Question 18

Delta configuration

Answer 18

Delta configuration only requires 3 wires for transmission

Question 19

Wye (star) configuration

Answer 19

Star (Wye) configuration requires four wires. The fourth wire is provided as a neutral and is normally grounded.

A four wire system with symmetrical voltages between phase and neutral is obtained when neutral is connected to the “common star point” of all supply windings. In such a system all three phases will have the same magnitude of voltage relative to neutral. Other non symmetrical systems have been used.

The four wired system is used when ground referenced voltages or the flexibility of more voltage selections are required. Faults on one phase to ground will cause