Week 1

Question 1

What are 5 reasons we transformers are used?

Answer 1

1. Transmission
2. Isolation
3. Metering/Relaying and instrumentation
4. System Matching
5. HV DC

Question 2

What are 2 reasons we step up voltage from the generator?

Answer 2

1. Reduce the power loss
2. Reduce the conductor size

Question 3

Transformers move power by stepping ______ or stepping ________

Answer 3

Up/Down

Question 4

Power transmitted by the transformer is a function of :

Answer 4

Voltage x Current

Question 5

S = VI where V is the:

Answer 5

System Voltage

Question 6

S = VI where I is the:

Answer 6

Current determined by the load

Question 7

If we were to double the transmission voltage but maintain the same power flow, what would happen to I
and PLoss assuming the line impedance stayed the same?

Answer 7

Current decreases and Ploss decreases

Question 8

Using the equations below, explain how power losses decreases when voltage is increased:

Inital: S1 = V1I1, Ploss1 = I1^2R
New: S2 = V2I2, Ploss2 = I2^2R

Answer 8

Keeping the power the same and doubling the voltage, the formula for power can be rearranged into I2 = S/2V1. This is where I2 is equal to 1/2I1. Power loss is therefore reduced by 1/4.

Question 9

Increasing the voltage of a transformer decreases the Ploss exponentially, linearly, or non-linearly?

Answer 9

Exponentially

Question 10

Isolation prevents high or low voltage from coming through?

Answer 10

High

Question 11

Isolation can filter high frequency, voltage or current? How does it manage to do this?

Answer 11

1. High frequency
2. With an electrostatic shield

Question 12

Isolation transformers have at least _____ windings that are electrically insulated from each other.

Answer 12

2

Question 13

Isolation transformers help limit the amount of _________ available

Answer 13

Fault current

Question 14

A transformer has a primary winding of 13800V and a secondary winding of 600V. The secondary is connected to a load. Half of the windings are non-insulated. If the circuit opens, what will happen to the load?

Answer 14

The windings would become shorted and the load would be damaged.

Question 15

PT (VT) stands for:

Answer 15

Potential transformer/Voltage transformer

Question 16

CT stands for:

Answer 16

Current transformer

Question 17

What is the purpose of a voltage transformer?

Answer 17

Bring voltage down toa safe value for the use of meters or protective relays

Question 18

What is the purpose of a current transformer?

Answer 18

Bring current down to a safe level so that it can be used for meters or protective relays.

Question 19

CT transformers are made of a _______ core.

Answer 19

Torroid

Question 20

The primary winding of the PT is connected in parallel or series with the line?

Answer 20

Parallel

Question 21

THe primary winding of the CT is connected in parallel or series with the line?

Answer 21

In series

Question 22

What type of transformer is used to transmit power from Canada to the US?

Answer 22

A phase shifting transformer

Question 23

What is the purpose of a phase shifting transformer?

Answer 23

To match the voltage of 2 different systems

Question 24

How does a phase shifting transformer control power flow?

Answer 24

By shifting the phase angle of the voltage

Question 25

A phase shifting transformer must do 4 things. What are these 4 things?

Answer 25

1. Match the amplitude 2. Match the phase shift 3. Match the frequency 4. Match the phase sequence

Question 26

Lenz's Law

Answer 26

An induced voltage will cause a current to flow in a closed circuit in such a direction that the magnetic field which is caused by the current will oppose the change that produced the current.

Question 27

At no load, the induced voltage in a transformer is caused by:

Answer 27

Self inductance

Question 28

When a voltage is applied to the primary winding at no load, is the generated voltage the same or opposite of the source voltage? What is the law that determines this?

Answer 28

Opposite Lenz's Law

Question 29

At no load the net voltage across the input is:

Answer 29

Vp - V1

Question 30

Explain the operation of a transformer at No Load

Answer 30

When a voltage is supplied to the primary winding, an exciting current is produced in the winding. The exciting current generates an exiting flux in the core. Since the flux is common to both windings, the voltage induced on the secondary is by mutual inductance.

Question 31

The assumptions for an ideal transformer are:

Answer 31

1. Winding resistance is 0 2. Winding reactance is 0 3. Power losses in the core are 0 4. Reactive losses in the core are 0

Question 32

At load, exciting flux induces a voltage in the primary and secondary winding. Is this true or false?

Answer 32

True

Question 33

At load, V2 will lag exciting flux by _______. Why is this?

Answer 33

1. 90 degrees 2. Inductance and V1 and V2 are in phase because of the same flux.

Question 34

Why are V2 and I2 in phase for a transformer at load?

Answer 34

Rload is purely resistive

Question 35

V2 is directly proportional or inversely proportional to V1 at load? Why?

Answer 35

1. Directly proportional 2. Due to turns ratio

Question 36

At load, I2 is determined by ______

Answer 36

The load

Question 37

At load, I1 is determined by ______

Answer 37

The load in the secondary

Question 38

At load, when I2 increases what happens to I1?

Answer 38

I1 increases