Eelectrical transformers

Question 1

polyphase systems

Answer 1

Three phase systems apply to both the generation and transmission of electrical energy

Question 2

six - phase power

Answer 2

is usually applied to power recifiers

Question 3

Transmission using transformers

Answer 3

Three - phase transmission saves materials, installation time and maintenance costs.

Question 4

Measurement of three phase power

Answer 4

Three phase power may be measured using either the two - wattmeter method or polyphase wattmeter method

Question 5

What is a transformer?

Answer 5

An electrical device that transfers electrical energy from one circuit to another through the process of electromagnetic induction.

Question 6

Transformer formula

Answer 6

VP/VS = NP/NS = IS/IP

Question 7

Types of transformers

Answer 7

1) Step up and step down Transformer

2) Power Transformer

3) Distribution Transformer

4) Three phase Transformer

5) Auto transformer

Question 8

Types of losses in a transformer

Answer 8

Iron losses
Copper losses
stray losses
dielectric loss

Question 9

How are iron losses caused?

Answer 9

by alternating flux in the core of
the transformer

Question 10

How are coper losses found

Answer 10

Electrical energy that is lost as heat in a conductor due to resistance

Question 11

Iron lost is divided into ?

Answer 11

hyteresis and eddy current loss

Question 12

How to minimize iron losses

Answer 12

Using silicon steel material for the construction of the core of the transformer

Question 13

The eddy current loss is minimized how?

Answer 13

By making the core with thin laminations

Question 14

What is cooling of transformers

Answer 14

Cooling of the Transformer is the process in which the heat generated in the transformer is dissipated or treated to a safe value

Question 15

Types of cooling methods are

Answer 15

1) Air dry transformers are cooled with air

2) synthetic oil

3)Mineral

Question 16

Types of test done on a transformer

Answer 16

1 Open circuit test
is used to determine no load loss and no load current

2 Short circuit test

is used to determine copper losses in the transformer at full load

Question 17

Five commonly used methods of connecting single-phase transformers to form Three-Phase Transformers banks are?

Answer 17

1 delta-delta
2 Open-delta
3 wye-wye
4 delta-wye
5 wye-delta

Question 18

What is the most common type of step-down transformer?

Answer 18

A transformer bank connected to a wye-delta

Question 19

A transformer bank connected to a wye-delta is used to??

Answer 19

Step down relatively high transmission line voltages

example ( 60,900 volts or more)

at the consumer’s location

Question 20

Two reasons for selecting this type of transformer bank are?

Answer 20

The three-phase voltages are decreased by the transformer ratio multiplied by the factor of 1.73

2) The insulation requirements for the high-voltage primary windings are reduced.

Question 21

Three phase transformers

Advantages of a three-phase transformer

Answer 21

The efficiency of a three-phase transformer is higher than the overall efficiency of three single-phase transformers connected in a three-phase bank.

Question 22

Disadvantages of three-phase transformers

Answer 22

If one of the phase windings becomes defective the whole three-phase unit must be taken out of service.

whereas

If a single-phase transformer in a three-phase bank becomes defective it can easily be replaced

Question 23

Note :

Answer 23

To get the greatest step up, the transformation ratio is the best if the primary I connected delta and the secondary is connected wye. Likewise, to get the largest decrease in voltage, the ratio of transformation is the greatest if the primary is connected wye and the secondary is connected deltaa

Question 24

What are instrument transformers?

Answer 24

Instrument transformers are used to measure and control AC circuits

Question 25

Note :

Answer 25

Direct measurement of high voltage or heavy currents involves large and expensive instruments, relays and other circuit components of many designs.

Question 26

uses of the transformer

Answer 26

1Safety 2Accuracy 3Convenience

Question 27

What can Instrument transformers protect?

Answer 27

It can protect the - The operator The measuring devices Control the equiqment

Question 28

What are the two distinct classes of instrument transformers?

Answer 28

1] The instrument's potential transformer 2] The instrument current transformer. note.,. (The word “instrument” is usually omitted for brevity.)

Question 29

Potential meter =

Answer 29

voltage meter

Question 30

What are three-phase transformers designed to do?

Answer 30

Three-phase transformer units are designed to be installed as a complete unit.

Question 31

Advantage of three-phase transformers

Answer 31

Instead of installing three individual transformers and field connecting them into the desired pattern, a transformer (a unit) is used.

Question 32

How are transformers' windings assembled?

Answer 32

They are assembled on a common core and appropriate leads are brought out.

Question 33

High voltage leads

Answer 33

(3) Marked H1 H2 H3

Question 34

Secondary leads

Answer 34

Marked X1 X2 X3

Question 35

NOTE

Answer 35

In three-phase transformers, the phase rotation or phase sequence between the primary and the secondary is critical

Question 36

Displacement transformers

Answer 36

If the pattern is a delta-to-delta pattern, the displacement maybe 0 degrees or 180 degrees. If the secondary pattern is a wye, the angular displacement is typically 30 degrees

Question 37

Advantages of three-phase transformers

Answer 37

1) It has a high efficiency 2)Less flux leakage than three single-phase units 3) The cost is less for a three-phase unit compared to three single-phase units. 4) Three phase transformers are easier to install all the internal connections are done

Question 38

Disadvantages of Three-phase transformers

Answer 38

1) If one of the three-phase coils fails, the entire transformer must be replaced rather than one transformer phase.

Question 39

What is power factor Power factor

Answer 39

The cosine of the angle between voltage and current in an a.c circuit

Question 40

Advantages of power factor correction

Answer 40

Increase effiency less energy used and reduces voltage drop which can lead to damage of equipment

Question 41

Disadvantages of power factor correction

Answer 41

Maintenance requirements for synchrous condensers Costly installation for advanced systems Over-correction risk can lead to instability

Question 42

How to improve power factor connection

Answer 42

A device taking leading power should be connected in parallel with the load example capacitors The capacitor draws in a leading current and neutralizes the reactive component

Question 43

Application of different Transformer Connections - Star Delta

Answer 43

Star Delta Three phase power supply from a power plant or substation to a commercial or residential grid.

Question 44

Delta

Answer 44

Utilized various industrial and commercial applications due to its ability to handle high power and provide stability in three-phase systems.

Question 45

Delta fauilt tolarance

Answer 45

Fault Tolerance: Delta connections can provide some level of fault tolerance. If one phase fails, the other two can still function, allowing for continued operation of critical equipment.

Question 46

Delta-Delta:

Answer 46

Delta-Delta: Used for both balanced and unbalanced loads. Utilized in systems that use lose voltages that convey big currents.

Question 47

Delta:

Answer 47

Delta: (VL) Line Voltage = (Vph)Phase Voltage (IL) Line Current= √3 x Phase Current (Iph) Phase Current= (VL/Z) Line Voltage ÷ Impedance

Question 48

Star Delta:

Answer 48

Star Delta: Power (P)= √3 x (VL) Line Voltage x (IL) Line Current x Cos ø To find: Cos ø= R/Z Resistance ÷ Impedance (P.F)Power Factor= (P)Power /(V)Voltage ÷ (I)Current

Question 49

Star:

Answer 49

Star: (Vph) Phase Voltage= Supply Voltage x √3 (VL) Line Voltage= √ 3 x Phase Voltage