Electrical Power Systems Flashcards

Question 1

What is electric current, and what unit measures it?

Accepted Answer

Electric current is the rate at which electric charge flows past a point in a circuit. It is measured in amperes, where one ampere equals one coulomb of charge passing a point each second. Current flows only when a closed path exists, so a break in the circuit stops it.

Question 2

How does voltage differ from current in an electrical circuit?

Accepted Answer

Voltage is the electric potential difference between two points, the push that drives charge through a circuit. Current is the actual flow of charge that results. Voltage is measured in volts, while current is measured in amperes. A battery supplies voltage, and the load resistance then determines how much current flows.

Question 3

What does Ohm's law state about voltage, current, and resistance?

Accepted Answer

Ohm's law states that voltage equals current multiplied by resistance, written as V equals I times R. For a fixed resistance, doubling the voltage doubles the current. Resistance is measured in ohms, and the law holds for many common conductors at constant temperature. Some devices, such as diodes, do not follow it.

Question 4

How do resistors combine when they are connected in series and in parallel?

Accepted Answer

In a series circuit, resistances add directly, so the total resistance is the sum of each resistor. In a parallel circuit, the reciprocal of total resistance equals the sum of the reciprocals of each branch. Parallel combinations always give a total resistance smaller than the smallest individual resistor.

Question 5

What is the difference between direct current and alternating current?

Accepted Answer

Direct current flows steadily in one direction, as supplied by batteries and many electronic circuits. Alternating current periodically reverses direction, and the standard household supply is alternating. Alternating voltage can be stepped up or down efficiently with transformers, which is a key reason it dominates power distribution.

Question 6

What is electrical power, and how is it calculated from voltage and current?

Accepted Answer

Electrical power is the rate at which electrical energy is converted into another form, such as heat, light, or motion. For a simple resistive load, power equals voltage multiplied by current, written as P equals V times I. Power is measured in watts, where one watt equals one joule per second.

Question 7

What is the difference between real power, reactive power, and apparent power?

Accepted Answer

Real power is the portion of power that performs useful work and is measured in watts. Reactive power is stored and released by inductors and capacitors, does no net work, and is measured in volt-amperes reactive. Apparent power is the product of voltage and current magnitudes and is measured in volt-amperes.

Question 8

What is energy, and how do utilities usually bill customers for electricity?

Accepted Answer

Energy is the capacity to do work, and electrical energy is power sustained over time. Utilities usually bill customers in kilowatt-hours, which is one kilowatt of power used for one hour. Multiplying a load's wattage by its hours of operation and dividing by one thousand gives the kilowatt-hours used.

Question 9

What is power factor, and why does a low value matter to utilities?

Accepted Answer

Power factor is the ratio of real power to apparent power, a number between zero and one. A low power factor means more current must flow to deliver the same useful work, which increases losses in conductors and equipment. Utilities often encourage customers to correct low power factor with capacitors or other compensation.

Question 10

What is a three-phase power system, and why is it widely used?

Accepted Answer

A three-phase system uses three alternating voltages that are offset by one third of a cycle, or 120 degrees, from each other. Three-phase power delivers steadier total power than single-phase power and requires less conductor material for the same load. It is widely used for industrial motors and for transmission of bulk power.

Question 11

What is the relationship between line voltage and phase voltage in a balanced wye connection?

Accepted Answer

In a balanced wye connection, line voltage equals the square root of three times the phase voltage, which is about 1.73 times larger. Line current equals phase current in a wye connection. This relationship lets a single system supply both three-phase equipment and single-phase loads at the lower phase voltage.

Question 12

What is the difference between a delta connection and a wye connection?

Accepted Answer

A wye connection joins one end of each winding to a common neutral point, while a delta connection joins the windings end to end in a closed loop. In a delta connection, line voltage equals phase voltage, but line current is the square root of three times phase current.

Question 13

Why does a three-phase motor produce a rotating magnetic field?

Accepted Answer

A three-phase motor produces a rotating magnetic field because its three stator windings are spaced around the core and carry currents that are offset in time. As each current peaks in turn, the peak magnetic field shifts around the stator, creating a field that rotates at synchronous speed.

Question 14

How does a power factor correction capacitor bank improve a facility's electrical system?

Accepted Answer

A capacitor bank supplies reactive power locally, so the inductive loads of motors draw less reactive current from the utility. This raises the power factor closer to one, reducing the current flowing through feeders and transformers. Lower current means less heating and voltage drop, and it can free capacity in existing equipment.

Question 15

What is harmonic distortion, and what problems can it cause?

Accepted Answer

Harmonic distortion is the presence of current or voltage components at multiples of the fundamental frequency, which makes the waveform deviate from a smooth sine wave. Nonlinear loads such as variable-speed drives and rectifiers are common sources. Harmonics can overheat transformers and neutral conductors, cause nuisance tripping, and interfere with sensitive electronics.

Question 16

What is a per-unit system, and why do power engineers use it?

Accepted Answer

A per-unit system expresses voltages, currents, and impedances as fractions of chosen base values rather than in absolute units. Engineers use it because quantities referred across transformers with different voltage ratings become directly comparable, and calculations simplify considerably. A value of 1.0 per unit means the quantity equals its base.

Question 17

How does a synchronous generator produce alternating voltage?

Accepted Answer

A synchronous generator produces alternating voltage by rotating a magnetic field past stationary conductors. Faraday's law states that a changing magnetic flux linkage induces voltage in a coil. As the rotor turns, the induced voltage follows a sine wave, and the frequency matches rotor speed times the number of pole pairs.

Question 18

What are the main types of power plants used to generate electricity?

Accepted Answer

Power plants are commonly grouped by their energy source. Thermal plants, including coal, natural gas, and nuclear, heat water to produce steam that drives a turbine. Hydroelectric plants use falling water to turn turbines directly. Wind and solar plants convert moving air or sunlight into electricity.

Question 19

What is the role of a turbine in a conventional power plant?

Accepted Answer

A turbine converts the kinetic and thermal energy of a moving fluid into mechanical rotation. In a steam plant, high-pressure steam expands across blades, spinning a shaft that is coupled to a generator. The turbine's mechanical output is then converted into electrical energy by the generator.

Question 20

Why is high-voltage transmission used for long distance power lines?

Accepted Answer

High-voltage transmission reduces current for a given power level, because power equals voltage times current. Lower current produces less resistive loss in the conductors, since losses scale with the square of current. This allows power to travel long distances with much less energy wasted as heat.

Question 21

What is the role of a substation in an electrical grid?

Accepted Answer

A substation is a facility where voltage is transformed, switched, and monitored as power moves between parts of the grid. Transmission substations step voltage down from long-distance levels to subtransmission levels. Distribution substations further reduce voltage for local feeders serving neighborhoods and businesses. Substations also contain circuit breakers that isolate faults.

Question 22

What is the difference between a transmission network and a distribution network?

Accepted Answer

A transmission network carries bulk power over long distances at high voltage, linking generating plants to load centers. A distribution network carries power the final stretch at lower voltage to individual customers. Transmission lines are typically meshed for redundancy, while distribution feeders are usually radial, meaning power flows from a single source outward.

Question 23

What is grid frequency, and why must generation and load stay balanced?

Accepted Answer

Grid frequency is the rate at which alternating current cycles per second, nominally 50 or 60 hertz depending on region. Generation and load must stay balanced because a surplus of generation raises frequency while a deficit lowers it. Sustained imbalance can cause equipment damage or cascading outages.

Question 24

How do renewable sources such as wind and solar affect grid operation?

Accepted Answer

Wind and solar output varies with weather and time of day, so it is called variable generation. This variability requires grid operators to forecast output and keep flexible resources such as storage and gas turbines available. Because inverter-based sources provide less inherent inertia than spinning generators, grid operators must manage frequency more carefully.

Question 25

How does a transformer change voltage using electromagnetic induction?

Accepted Answer

A transformer changes voltage by using two coils wound on a shared magnetic core. Alternating current in the primary coil creates a changing magnetic flux, which induces voltage in the secondary coil. The voltage ratio equals the turns ratio, so more secondary turns produce a higher voltage.

Question 26

What is the ideal transformer equation relating voltage, current, and turns?

Accepted Answer

For an ideal transformer, voltage is proportional to the number of turns, while current is inversely proportional. So if the secondary voltage is double the primary voltage, the secondary current is half the primary current. Power in equals power out, which is why stepping voltage up reduces current.

Question 27

Where do step-up and step-down transformers sit along the path from generator to customer?

Accepted Answer

Step-up transformers sit at generating plants and raise voltage for efficient long-distance transmission, reducing current and resistive losses. Step-down transformers then lower voltage in stages, at substations and along distribution lines, until it reaches the level used in buildings. Each stage matches voltage to the needs of the next part of the system.

Question 28

What are the main losses in a transformer, and how are they classified?

Accepted Answer

Transformer losses fall into two main groups: no-load losses and load losses. No-load losses, often called core losses, occur whenever the transformer is energized and include hysteresis and eddy current losses in the magnetic core. Load losses, or copper losses, arise from resistance in the windings and rise with the square of current.

Question 29

What is transformer efficiency, and how is it calculated?

Accepted Answer

Transformer efficiency is the ratio of output power to input power, usually expressed as a percentage. It equals output power divided by the sum of output power and total losses. Large power transformers often reach efficiencies above ninety-five percent at rated load, though exact values depend on design and loading.

Question 30

What is a distribution transformer, and where is it typically located?

Accepted Answer

A distribution transformer steps voltage down from primary feeder levels to the utilization voltage used by homes and small businesses. It is typically mounted on a utility pole, set on a concrete pad, or placed in a pad-mounted enclosure near the customers it serves.

Question 31

How does a three-phase distribution feeder differ from a single-phase lateral?

Accepted Answer

A three-phase feeder is the main trunk line that carries power from the substation through a service area, with all three phases present. A single-phase lateral is a smaller branch tapped from that trunk to serve a few customers. Laterals carry only one phase and one conductor plus a neutral.

Question 32

Why is voltage drop an important consideration in distribution design?

Accepted Answer

Voltage drop is the reduction in voltage along a conductor caused by its impedance as current flows. Excessive voltage drop can cause lights to dim, motors to run hot, and electronic equipment to malfunction. Designers select conductor size, feeder length, and regulating devices to keep voltage within acceptable limits at every customer.

Question 33

What is the purpose of a circuit breaker in a power system?

Accepted Answer

A circuit breaker automatically interrupts current flow when it detects a fault such as an overload or short circuit. Its purpose is to protect equipment and people by isolating the faulted section quickly. Unlike a fuse, a breaker can be reset after it trips, though some utility breakers are designed for repeated operation.

Question 34

How do fuses and circuit breakers differ in how they protect a circuit?

Accepted Answer

A fuse contains a metal element that melts when current exceeds its rating, so it must be replaced after operating. A circuit breaker uses a mechanism that opens contacts when current or fault conditions exceed a set level, and it can be reset.

Question 35

What is a protective relay, and how does it decide to trip a breaker?

Accepted Answer

A protective relay is a device that monitors measured currents and voltages and sends a trip signal when conditions indicate a fault. It compares measurements against settings for pickup level and time delay, distinguishing faults from normal events. Relays are coordinated so the nearest breaker to a fault operates first, limiting the outage.

Question 36

What is a ground fault, and why is grounding important for electrical safety?

Accepted Answer

A ground fault occurs when an energized conductor unintentionally contacts earth or a grounded part. Grounding connects equipment and system neutrals to earth through low-impedance paths so that fault current flows and protective devices operate quickly. Without a reliable ground path, metal enclosures can become energized, creating shock hazards.

Question 37

What is a residual current device, and what hazard does it protect against?

Accepted Answer

A residual current device compares the current flowing out on the energized conductor with the current returning on the neutral. When the difference exceeds a set threshold, it quickly disconnects the circuit. It is designed to protect people from electric shock by detecting leakage current, including leakage through a person's body to ground.

Question 38

What is power quality, and which disturbances most often affect it?

Accepted Answer

Power quality describes how closely the voltage and frequency of supplied power match their ideal values. Common disturbances include voltage sags and swells, short interruptions, transients, harmonic distortion, and frequency deviations. Each can cause equipment malfunction, data loss, or premature failure of sensitive loads.

Question 39

What is the difference between a voltage sag and a voltage swell?

Accepted Answer

A voltage sag is a brief decrease in RMS voltage below nominal, lasting from a fraction of a second to a few seconds. A voltage swell is a brief increase in RMS voltage above nominal over a similar short duration. Sags are usually caused by faults or large motor starts.

Question 40

How can engineers reduce the impact of voltage sags on sensitive equipment?

Accepted Answer

Engineers reduce sag impact by improving ride-through capability at the equipment and by limiting the sag reaching it. Common measures include uninterruptible power supplies for critical loads, dynamic voltage restorers, and soft starters for motors. Reducing inrush current during motor starts also lessens the sags they cause.

Electrical Power Systems Flashcards

1Core quantities and circuit laws

2Power factor and three-phase systems

3Generation and transmission

4Transformers and distribution

5Protection, safety, and power quality

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