Basics of power systems operation and planning

Question 1

Electric systems have two unique physical operating constraints

Answer 1

 Electric energy is not commercially stored86 like natural gas and water. Production and
consumption (generation and load) must be balanced in near real-time. This requires
continuous monitoring of load, generation, and the voltages and energy flows throughout the
power system, as well as adjusting generation output to match consumption.
 The transmission and distribution network is primarily passive, with few “control valves” or
“booster pumps” to regulate electrical energy flows on individual lines. Flow-control actions
are limited primarily to adjusting generation output and to opening and closing switches to
add, remove, or reroute transmission and distribution lines and equipment from service

Question 2

what is load following?

Answer 2

Load following resources are dispatched to follow within-the-hour load changes in the load (or
net load when including variable generation) consistent with the economic dispatch cycle (5 to
10 minutes per cycle). These are therefore used to follow variability in the load over periods of 5
to 20 minutes; sufficient load-following is procured in the economic dispatch stage to meet
variations in load.

Question 3

what is an ideal voltage source?

Answer 3

the one that provides a given, known voltage, doesn’t have to be constant, but the voltage is not a function of the amount of current drawn

A real battery approximates the ideal source, but as current increases, the output drops somewhat

Question 4

What is voltage

Answer 4

electrons wont flow through a circuit unless they are given some energy to help send them on their way. That “push” is measured in volts. Voltage is defined to be the amount of energy given to a unit of charge.

voltage describes the potential for charge to do work

voltage is always measured with respect to something

Question 5

what is voltage drop?

Answer 5

an electric space heater may have a resistance of ten ohms, and the wires which supply it may have a resistance of 0.2 ohms, about 2% of the total circuit resistance. This means that approximately 2% of the supplied voltage is lost in the wire itself. Excessive voltage drop may result in unsatisfactory operation of, and damage to, electrical and electronic equipments.

Question 6

what is Ohm’s law

Answer 6

I=V/R

Question 7

what is power factor?

Answer 7

PF=Real Power/Apparent Power

it’s the cosine of the angle between the voltage and the current, ideal PF is 1, SO REAL POWER EQUALS APPARENT POWER

Question 8

what is the fundamental definition of capacitance

Answer 8

capacitance is equal to the amount of charge required to create a 1-V potential difference between the plates

capacitance is a parameter in electrical circuits that describes the ability of a circuit component to store energy in an electrical field

Question 9

what happens to capacitors in DC condition?

Answer 9

the capacitors appears to be an open circuit, through which no current flows

Question 10

what does capacitors do to voltage

Question 11

what is inductors?

Answer 11

inductance is a mirror image of capacitance, while capacitors storage energy in an electric field, inductors store energy in a magnetic filed. while capacitors prevent voltage from changing instantaneously, inductors prevent current from changing instantaneously

Question 12

what does transformer do?

Answer 12

modern systems generate voltages in the range of 12 to 25kV. Transformers boost that voltage to hundres of thousands of volts for long-distance transmission. At the receiving end, transformers drop the transmission line voltage to perhaps 4 to 25kV at electrical substations for local distribution. Other transformers then drop the voltage to safe levels for homes.

Question 13

what influences transmission line power losses?

Answer 13

transmission line power losses are proportional to the square of current and are inversely proportional to the square of voltage. Raising voltages by a factor of 10 lowers line losses by a factor of 100.

Question 14

three important conclusions about power factor

Answer 14

1. currents flowing through any of these components will have the same ac frequency as the source of the voltage that drives the current
2. there can be phase shift between current and voltage
3. resistive elements are the only components that dissipate any net energy

Question 15

elements of power triangle?

Answer 15

1. apparent power, the hypotenuse, is the product of Volt-amps, it has unit volt-amps (VA)
2. horizontal component P=VIcos(a), which is real power in watts
3. vertical side of the triangle, Q=VIsin(a), is reactive power and incapable of doing any work (VAR)

Question 16

why do utilities hate reactive power?

Answer 16

reactive power increases line losses for the utility, while doesn’t result in any more kilowatt-hours of energy sales to the customer

Question 17

what are transformers rated in?

Answer 17

KVA

Question 18

What causes lagging power factor?

Answer 18

electric motors or anything else that’s highly inductive

Question 19

what does the VAR rating of a capacitor depend on?

Answer 19

The square of the voltage. The VAR rating itself is meaningless without knowing the voltage at which the capacitor will be used.