Electric Power Devices

Question 1

When transformers are connected delta-wye or wye-delta, which side has the extra square root of 3?

Answer 1

The WYE side has the extra square root of 3

Question 2

What does the F designation mean for a transformer?

Answer 2

F stands for forced
The fan blows air over the oil heat exchanges
NOT directly on the coils

Question 3

Transformer losses equation

Answer 3

Total losses (at any load) = ((load%)^2)(coil losses) + core losses

Coil losses = result of short circuit test
Core losses = no load losses

Question 4

Capacitor capacity when voltage varies

Answer 4

KVAR1 * (V2/V1)^2 = KVAR2

The voltages are squared

NOT the KVAR

Question 5

Transformer turns ratio

Answer 5

a = V pri (ph) / V sec (ph)

PHASE VOLTAGES

Question 6

Transformer ratings

Answer 6

Voltages given are LINE unless noted otherwise

Question 7

Transformer cooling classes

Answer 7

First letter - internal cooling medium in contact with windings
- O = mineral oils or other fluid with fire point <300 degrees Celsius
- K = insulating liquid with fire point >300 degrees Celsius
- L = insulating liquid with no measurable fire point

Second letter - circulating mechanism for internal cooling method
- N = natural convection flow through cooling equipment in windings
- F = forced circulation through cooling equipment in windings
- D = forced circulation through cooling equipment, directed from the cooling equipment into at least the main windings

Third letter - external cooling medium
- A = air
- W = water

Fourth letter - circulation mechanism for external cooling medium
- N = natural convection
- F = forced circulation (fan are air cooling or pumps are water cooling)

Question 8

Transformer losses naming

Answer 8

Pcu = transformer copper losses (winding losses)
Pcore = transformer iron losses (core losses)

Question 9

Equation for voltage rise due to addition of capacitor

Answer 9

% voltage rise = [ ((KVAR (capacitor)) * (transformer reactance) ]/ ((KVA (transformer))

Question 10

Line reactor

Answer 10

Located before the rectifier and operates on AC power

Question 11

DC Chopper

Answer 11

Used to convert a constant DC load to a variable DC load

It is not a reactor

It is an arrangement of switches

Question 12

DC Choke

Answer 12

Aka a load reactor is located between the AC-DC rectifier and the DC load

It filters high frequencies

Question 13

Inverter

Answer 13

Converts DC to AC

Consists of a series of switches

Not a reactor

Question 14

True or false

The apparent power output of a transformer operating at maximum efficient can be determined by multiplying the apparent power rating of the transformer by the square root of the ratio of te magnetizing core losses to the full load copper losses

Answer 14

True (equation is in handbook)

Question 15

True or false

Maximum efficiency of a transformer occurs when the transformer load is equal to the transformer rated current

Answer 15

False

Question 16

True or false

Maximum efficiency of a transformer occurs when the transformer load results in an equal amount of I^2*R losses in the transformer compared to the no load losses

Answer 16

True

Question 17

True or false

Transformer copper losses are directly proportional to changes in percent load

Answer 17

False

Question 18

True or false

As long as the percent load is constant, changes to the load power factor will not have an effect on transformer efficiency

Answer 18

False

Question 19

Transformer designations from highest output capacity to lowest

Answer 19

ODAF (directed)

OFAF

ONAF

ONON

Question 20

Appropriate application for reactors used in power systems

Answer 20

Transient current prevention

Harmonic reduction

Prevent damage to motors fed from variable-frequency drives

Question 21

Transformer short-circuit tests can be used to determine what?

Answer 21

Winding impedance

Percent impedance

Question 22

Transformer open circuit test can be used to determine what?

Answer 22

Core loss resistance

Magnetizing reactance