Year 2 Theory Book

Flashcards about NZ National Grid & Protection, Earthing, Circuit Protection and other related topics.

True or False: Hydroelectric stations generate 75% of all our electricity?

More generating stations are hydroelectric than any other generating systems.

True or False: Thermal electric generating stations obtain mechanical energy from a head of water.

Thermal electric generating stations obtain the mechanical energy from geothermal steam or the combustion of oil, gas or coal.

True or False: Thermal electric generating stations are cheaper to run than hydroelectric stations.

False. Thermal electric stations are more expensive because: they burn non-renewable fossil fuels such as coal, natural gas or oil and they are labour intensive to run; they use burner units and so are more expensive to maintain than simple mechanical parts of hydroelectric stations

True or False: The South Island has no thermal electric generating stations.

The thermal generating stations are all in the North Island.

True or False: Thermal electric generating stations take longer to start up than hydroelectric stations.

A thermal electric generating station could take up to 2 or 3 hours to start. A hydroelectric station takes only 2 or 3 minutes.

What generating system drives the turbines at Wairakei?

Natural steam

What generating system drives the turbines at Benmore?

Water

What generating system drives the turbines at Stratford?

Natural gas

What generating system drives the turbines at Manapouri?

Water

What generating system drives the turbines at Huntly?

Steam

What generating system drives the turbines at Wellington?

Wind

A domestic consumer at full load takes 15 kW from a 230 V system by a line that has a resistance of 0.2 Ω. Calculate the voltage at your consumer’s switchboard.

I = P / V, 1500 / 230, = 65.22A Voltage drop along the line = IR, =65.22 X 0.2, = 13V Voltage at consumer’s switchboard = 230 – 13, = 217V

A domestic consumer at full load takes 15 kW from a 230 V system by a line that has a resistance of 0.2 Ω. Suggest any alterations to the supply line that may be necessary.

The voltage at the consumer’s switchboard is very low so the cross-sectional area of the line should be increased. This increase in the conductor cross sections means that the line resistance will be lower. The voltage drop along the line will therefore be less.

Typical Voltage (a)(i)

11kV

Substation type (a)(ii)

Generator

Typical Voltage (b)(i)

220 or 110kV

Substation type (b)(ii)

Network

Typical Voltage (c)(i)

33kV

Substation type (c)(ii)

Zone

Typical Voltage (d)(i)

11kV

Substation type (d)(ii)

Berm

Typical Voltage (e)(i)

400 / 230V

If the line voltage is 415 volts, what would be the calculated phase voltage?

Vphase = VLINE / √3, = 415 / 1.732, = 240V(approx.)

What is the conductor used to earth the metallic pipes called?

Protective earthing conductor

The multiple earthed neutral system means a system of supply of electricity in which the neutral is connected to earth at:

i. the source of supply, ii. points on the supply system, iii. every electrical installation connected to that system

What is the measurement unit that the difference in electrical potential between two bodies is measured in?

Volts

In dry weather, a person may experience an electric spark by touching a car door. This is because the…

Car has picked up an electrostatic charge

The force that causes the electrons to flow is called:

Electromotive force

A conductor is a material that will:

Conduct the electrical energy to where it can be used

The source of electricity generated by electric conductors moving through a magnetic field is called an:

Electromagnetic source of EMF

What happens when light falls on the junction of a solar cell?

Light energy is converted directly into electrical energy

What is the reason for using high voltages for the long-distance transmission of power?

Reduce power transmission losses

With reference to the diagram, the item marked '3' is a:

Distribution substation

To maintain a continuous current flow in a circuit, a continuous source of electrical energy must be provided to:

Maintain the potential difference at the beginning of the circuit

When a "short-circuit" occurs in an electrical circuit:

Excessive current flows from the energy source

Electric current is the coordinated flow of electrons from:

A point of higher potential to a point of lower potential

In a circuit, conventional current flow is said to go from:

Positive to negative

How much current will flow in the circuit if the lamp has a resistance of 6 W, and the switch is closed?

2A

The power dissipated by a resistor can be determined using the formula:

P = I2 x R

Calculate the electrical energy consumed if an electric heater rated at 1200 W has been switched on for an average of 4 hours per day for 7 days.

33.6 kWh

Electrical power can be transmitted using low voltage and high currents. Higher currents result in:

Higher transmission losses

In economic terms, the higher the voltage used for power transmission systems, the:

Higher the cost of installing and maintaining the transmission lines

Transmission system voltages are far higher than the voltages required by the average consumer, therefore the voltage is:

Stepped down to a suitable value using transformers

The distribution system is a:

Single wire earth return distribution system

One advantage of this system is that:

Two voltages are available to the consumer

With reference to a conductor and a magnetic field, what is needed to induce an e.m.f?

Relative motion between the conductor and magnetic field

What is the more common name used for an a.c. generator?

An alternator

What is the fixed system of coils in an alternator called?

The stator

What is the rotating electromagnet in an alternator called?

The rotor

Explain the function of the so called “exciter” in an alternator.

The exciter provides D.C. current to the rotor to set up a magnetic field and thus a magnet in the rotor

Which rotor should be used in a high speed alternator?

Cylindrical rotor

Explain why there cannot be uneven numbers of poles in an alternator.

Because for every north pole there must be an associated south pole for a magnetic flux path to be complete

State 5 advantages of a three phase power system over a single phase power system.

For a given size, a 3phase generator or alternator has a greater output than a single phase generator or alternator. To transmit power at a given voltage over a given distance, a 3phase transmission line require less copper than a single phase system. Three phase motors are self starting, single phase motors need auxiliary windings or a commutator and brushes. Three phase motors have a more uniform running torque whereas single phase motors have a pulsating running torque. Three phase motors are smaller in construction for the same power output as single phase motors.

The frequency of the e.m.f. produced in an alternator is proportional to two factors. What are these?

The speed in revolutions per second. The number of pairs of poles

A two pole alternator that runs at a speed of 3000 revolutions per minute will produce what frequency?

f = Np / 60, = 3000 x 1 / 60, = 50Hz

Which of two alternators, one an 8 pole and the other a 2 pole, will have to rotate at a higher r.p.m to produce 50 Hz?

The 2 pole will have to rotate faster than the 8 pole

What is a “transient voltage”?

It is a short term fluctuation in the output voltage of an alternator due to accelerations and decelerations of the alternator rotor

If the maximum voltage that the alternator can generate is 110 Volts, state the values of the e.m.f.’s generated for the rotor positions as shown.

Position A►110 Volts Position B► 0 Volts Position C► -110 Volts Position D► 0 Volts