Technical Questions

Question 1

What are the 3 major components of a power system?

Answer 1

Generation, transmission, & distribution

Question 2

How does electrical power get from the plant to the home?

Answer 2

Power plants are connected to each other through the electrical system (sometimes called the “power grid”). If one power plant can’t produce enough electricity another power plant can send somewhere it’s needed.
1) Electricity is made at a power plant by huge generators. Most power plants use coal, but some use natural gas, water, or wind.
2) The current is sent through transformers to increase the voltage to push the power long distances.
3) The electrical charge goes through high-voltage transmission lines that stretch across the country.
4) It reaches a substation, where the voltage is lowered using step-down transformers so it can be sent on smaller power lines.
5) It travels through distribution lines to neighborhoods, where smaller pole-top transformers reduce the voltage again to make the power safe to use in homes.
6) It connects to your house through the service drop and passes through a meter that measures how much power the home uses.
7) The electricity goes to the service panel in your basement or garage, where breakers or fuses protect the wires inside your house from being overloaded.
8) The electricity travels through wires inside the walls to the outlets and switches all over the house.

Question 3

What are the voltages associated with each phase of the delivery of electricity?

Answer 3

Power Plant Generators - 10-20 kV
Transmission Lines - 200-700 KV
Substation - 34.5 kV
Distribution Lines - 4.8 kV
Service meter - 240/120 V

Question 4

What do you need for system protection?

Answer 4

Need CTs (current transformers) and PTs (power transformers) to measure the current and voltages. Then they go into a relay to give it a logic function to see if the currents and voltages are on reasonable levels or above what they need to be. If above, circuit trips, which isolates that part of the system from the others.

Question 5

Explain three phase power. How does it relate to single phase power?

Answer 5

Three-phase power is more efficient than single-phase. The power never falls to 0. Each phase is 120 degrees out of phase with each other. In three-phase systems, the transmitted instantaneous power is constant.

Question 6

What is a bushing and why is it used?

Answer 6

A bushing is an insulated device that allows an electrical conductor to pass safely through a conducting barrier such as the wall of a transformer or circuit breaker.

Question 7

What is a transformer? How does it work and what is it composed of?

Answer 7

A transformer is composed of a core and windings around the core. The core is usually made of silicon steel. Faraday’s law states that the EMF is also given by the rate of change of the magnetic flux.

Question 8

What are the components of a transformer?

Answer 8

a. Core: made up of thin laminations of high-grade electrical sheet steel. They are stacked up until height is reached
b. Windings: Insulated copper or aluminum wire wound around the core. One for primary, another for secondary.
c. Bushings: porcelain or rubber
Insulated, allows an electrical conductor to pass safely through the wall of the transformer
d. Transformer oil: provides cooling, protects core and coil from chemical attacks, prevents buildup of sludge in the transformer
e. Tap changer: used to regulate the output voltage to required levels.
Using a tap changes the voltage ratio of a transformer so that its secondary voltage
stays at nominal
f. Cooling fans

Question 9

You have a transformer. One the primary side, if the voltage increases, does the current increase or decrease?

Answer 9

In case of transformer, when voltage increases then current decrease because power remains constant i.e. both side power is P = VI.

Question 10

What Electrical devices are used to step up voltage, and how does it affect the current?

Answer 10

Transformers are used to step up or down voltage. The current has an inverse relationship with the voltage (V = IR or P=VI)

Question 11

What factors affect the efficiency of a transformer?

Answer 11

a) Loop area
b) Resistance of primary and secondary coil
c) Flux coupling
d) Hysteresis loss
e) Eddy-Current Loss/Insulation and the use of f) ferromagnetic sheets to make a core vs one big block.

Question 12

What is power factor?

Answer 12

This applies to AC circuits. Power factor is the ratio of the real power that is used to do work and the apparent power that is supplied by the circuit. (Watts/VA) [Cos(theta)].
Inductive loads tend to have a power factor closer to 0.
Resistive loads tend to have a power factor closer to 1.
A capacitor induces a LEADING power factor
An inductor induces a LAGGING power factor

Question 13

How do you improve power factor?

Answer 13

a) Minimize operation of idling or lightly-loaded motors.
b) Avoid operation of equipment above its rated voltage.
c) Replace standard motors as they burn out with energy-efficient motors.
d) Install capacitors in your AC circuit.

Question 14

What is the difference between Watts, VA, and VAR?

Answer 14

Watts measures the real power used to do work. VA measures the “apparent power” supplied by the circuit. VAR measures the reactive power. These three measurements make up the power triangle. X-axis is Watts, Y-axis is VAR, hypotenuse is VA

Question 15

What are the causes of power loss across transmission lines?

Answer 15

a) Resistivity of wire (I2R loss)
b) Loss due to heat
c) Corona Loss
d) Increased loss due to the spiraling of conductors (ACSR)
e) Skin effect

Question 16

How do you size wires and circuit breakers?

Answer 16

Wires and circuit breakers are sized by dividing the KVA rating by the RMS voltage in order to get the RMS current.

Question 17

What is fault current?

Answer 17

In an electrical power system, fault current is any abnormal electrical current. Need to have relay protection to be able to isolate the fault. (A tree falls on a power line, etc.)

Question 18

What device converts mechanical energy into electrical energy and visa versa?

Answer 18

An electric generator driven by a turbine is used to convert mechanical energy to electrical. A motor converts electrical energy to mechanical energy. synchronous motor.

Question 19

What are the four parameters that affect an electric transmission line’s ability to fulfill its function as part of a power system?

Answer 19

a) resistance
b) inductance
c) capacitance
d) conductance

Question 20

What are the voltages of Delta and Wye three phase circuits?

Answer 20

Delta:
-Line to Line Voltage = 480V
- 3 times the power of Wye connection
Wye:
- Line to Line Voltage = 208V
- Line to Neutral Voltage = 120V
-one third times the power of Delta connection

Question 21

What is per unit and why is it useful?

Answer 21

A per unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per unit do not change when they are referred from one side of a transformer to the other.

Question 22

What is overcurrent protection and why is it used?

Answer 22

Overcurrent occurs when the current in amperes is greater than the rated current of the equipment or conductors. An overcurrent protection device protects the circuit by opening the device when the current reaches a value that will cause an excessive or dangerous temperature rise in conductors. This is in result of an overload, short circuit or ground fault.

Question 23

Referring impedance in a transformer?

Answer 23

Multiply the turns ratio squared (secondary to primary). Divide when primary to secondary.

Question 24

What is Kirchhoff’s voltage law?

Answer 24

Voltage drop around a closed loop is zero

Question 25

What is Kirchhoff’s current law?

Answer 25

Sum of the incoming and outgoing currents at a node is zero

Question 26

For a series RC (RL) circuit, what is the voltage across the resistor just after the switch is closed?

Answer 26

For an inductor, the instantaneous voltage is defined as v = L di/dt. Because voltage across an inductor is proportional to the rate of change of the current, current cannot change instantaneously through the inductor. Just after the switch closes, there is no current passing through the circuit, since current cannot change instantaneously, and the voltage drop across the resistor is 0, since v = i*r and i = 0. The entire voltage drop happens across the inductor.

For a capacitor, the instantaneous current is defined as i = C dv/dt. Because current across a capacitor is proportional to the rate of change of the voltage, voltage cannot change instantaneously across the capacitor. Just after the the switch closes, there is no voltage across the capacitor, since the voltage cannot change instantaneously, and the entire voltage drop happens across the resistor.

TL;DR: At t = 0, a capacitor acts like a short and an inductor acts like an open.

Question 27

For a series RC (RL) circuit, what is the voltage across the resistor a long time after the switch is closed.

Answer 27

TL;DR: At t = 0, a capacitor acts like an open and an inductor acts like a short.

Question 28

How do you convert AC voltage to DC?

Answer 28

Need to use a full wave rectifier. The full wave rectifier converts both halves of the AC sine wave to positive voltage direct current.

Question 29

How do you convert DC voltage to AC?

Answer 29

A power inverter

Question 30

What is a voltage divider? What does it do? How do you calculate the Voltages?

Answer 30

A voltage divider is a simple circuit that turns a large voltage into a smaller one using two series resistors and an input voltage. Calculate the voltage by: Vs*(R1/R1+R2)

Question 31

How do you measure capacitance?

Answer 31

The charge on the conductor per unit of potential difference between them. C = q/v

Question 32

When building a circuit, what factors do you consider when choosing the parts?

Answer 32

a) Number of parts needed
b) materials for the parts
c) ratings for the parts (how accurate)
d) lifetime of the parts

Question 33

Describe different sources of generation:

Answer 33

(Examples of type, examples of REAL LADWP sites)
a) Coal (Intermountain Power Plant in Utah)
b) Nuclear (Palo Verde)
c) Gas (Scattergood; Haynes)
d) Hydro (Castaic)
e) Steam
f) Solar (Beacon)
g) Wind (Pine Tree; Oregon)
h) Geothermal (Nevada)

Question 34

What steps does electric power go through to be delivered to the customer?

Answer 34

Generation > Step up > Transmission > Step Down > Distribution > Step Down > Customer

Question 35

What is power factor and how do you correct it?

Answer 35

a) Power factor is the ratio of “Real Power” flowing to the load to “Apparent power”. It describes how much of the generated power is actually doing useful work.
b) The goal for a Utility is a power factor of 1. Otherwise, greater current has to be supplied for a given amount of power use.
c) Capacitive loads induce a leading power factor vs inductive loads, which induce lagging power factor
d) Power systems usually encounter lagging power factor, because transformers and motors are all inductive. To correct this, capacitors need to be added to the system to counteract.

Question 36

What are the components of a transformer? What does a transformer do?

Answer 36

a) Core: made up of thin laminations of high-grade electrical sheet steel. They are stacked up until height is reached
b) Windings: Insulated copper or aluminum wire wound around the core. One for primary, another for secondary.
c) Bushings: porcelain or rubber
Insulated, allows an electrical conductor to pass safely through the wall of the transformer
d) Transformer oil: provides cooling, protects core and coil from chemical attacks, prevents build up of sludge in the transformer
e) Tap changer: used to regulate the output voltage to required levels.
Using a tap changes the voltage ratio of a transformer so that its secondary voltage stays at nominal
f) Cooling fans

Question 37

Types of Transformer losses:

Answer 37

a) Hysteresis losses: directly proportional to the volume of the lamination
b) Eddy current losses: directly proportional to the thickness of the lamination
c) Copper losses from the windings, since all wires have resistances

Question 38

Parts of a Transmission Line:

Answer 38

Transmission Lines are made up of various components, namely poles, lattice structures, conductors, cables, insulators, and foundations.

Question 39

What is the method of symmetrical components?

Answer 39

It is a method of solving unbalanced three-phase circuit problems.