Electrical Power

Question 1

What is electrical demand?

Answer 1

A measurement of power, in kW, based on a customer’s highest use within a given unit of time.

Question 2

What is contract demand?

Answer 2

The addition of all of a utilities customer demand so that they can estimate the maximum overall generating capacity they need to provide.

Question 3

Electric utilities sell two products:

Answer 3

Usage and demand

Question 4

What is Usage?

Answer 4

The overall flow of electricity over time, in kWh

Question 5

What is Demand?

Answer 5

The simultaneous use of power, in kW. Demand becomes the load the utilities must provide.

Question 6

What is peak demand?

Answer 6

The most power, in kW, used over a specific time period.

Question 7

T/F Most medium and large commercial customers pay for demand.

Answer 7

True

Question 8

What is Load Factor?

Answer 8

The relationship between kWh consumption and kW demand for the same billing period. It describes the profile of the electrical use of a facility.

Question 9

Load factor for a given time period =

Answer 9

Average kW in time period / Peak kW in time period

Question 10

Average kW =

Answer 10

Total kWh / # of hours in time period

Question 11

A high load factor means power use is relatively ______.

Answer 11

constant

Question 12

A low load factor means power use is ________.

Answer 12

inconsistent, with high and low peaks.

Question 13

For an industrial facility, a ____ load factor is ideal.

Answer 13

high

Question 14

What are baseload plants?

Answer 14

Power plants that operate constantly to provide the minimum amount of power needed around the clock.

Question 15

What are some typical baseload plants?

Answer 15

Coal, natural gas, nuclear, hydroelectric dams, geothermal, biomass

Question 16

What are intermediate plants?

Answer 16

Power plants that operate daily or seasonally to follow the large but predictable swings in demand. Dispatched during baseload outage or maintenance.

Question 17

What are some typical intermediate plants?

Answer 17

Natural gas, hydroelectric, solar thermal, solar VP, wind farms

Question 18

What are peaking plants?

Answer 18

Power plants that are used only sporadically and for short periods of time to accommodate peak demand that exceeds the intermediate plants.

Question 19

What’s the most expensive type of plant to run?

Answer 19

Peaking plants, because they sit idle most of the year.

Question 20

What are some typical peaking plants?

Answer 20

Natural gas, oil and diesel generators, hydroelectric dams

Question 21

How much excess capacity beyond peak needs are utilities required to have as a margin of safety?

Answer 21

10-20%

Question 22

What are Demand Side Management strategies?

Answer 22

Strategies offered or incentivized by utilities to reduce the power use of its customers during time of extreme demand, done cooperatively.

Question 23

What are some DSM strategies?

Answer 23

Large lighting, motor, or HVAC retrofits, rescheduling large loads to off-peak hours, interruptible loads, curtailable loads,

Question 24

What’s an interruptible load?

Answer 24

One that the utility can remotely turn off at certain times of the day or year during peak times. The customer is offered a financial incentive, like a yearly rebate.

Question 25

What's a curtailable load?

Answer 25

One that a customer is willing to turn off if given sufficient notice by the utility. They work best for manufacturing facilities with significant inventory.

Question 26

What is Alternating Current (AC)?

Answer 26

The electrons oscillate between positive and negative voltage, relative to ground. The higher the voltage, the higher the sine wave.

Question 27

What is three-phase voltage?

Answer 27

Three separate lines carry the electricity and the AC voltage in each line is offset 1/3 from each other, allowing for a more constant power generation.

Question 28

What percentage of all electricity produced is lost in transmission due to the resistance of the wires?

Answer 28

7%

Question 29

What are substations?

Answer 29

Stations where voltage is stepped down to lower levels that can be safely distributed by local distribution lines to each customer.

Question 30

Where are step-down transformers?

Answer 30

At or near each customer. They make the final voltage adjustment.

Question 31

For most residential buildings, electricity enters as _________________.

Answer 31

120/240 volt single-phase service (two hot wires + 1 neutral carrying wire)

Question 32

For larger commercial buildings and industries, electricity enters as _________________.

Answer 32

277/480 volt three-phase service (3 hot wires + 1 neutral carrying wire)

Question 33

What is Real Power?

Answer 33

Power that is put directly to work lighting, turning, or burning something in watts.

Question 34

What is reactive power (phantom power, negative power, wattless power)?

Answer 34

Occurs when certain types of induction loads such as transformers or motors induce a magnetic field to produce work. The fields cause the current to fall out of phase, and when multiplied with voltage it produces power that dos not show up on the power meters as positive watts. Measured in kilovars of kVAR.

Question 35

What is apparent power (total power)?

Answer 35

The total amount of power needed to provide both real power and reactive power. Computes as the vector sum (hypotenuse) of real and reactive power. Measured in volt-amps.

Question 36

Power Factor =

Answer 36

Real power (kw) / Apparent power (kVA)

Question 37

Why does the utility care about power factor?

Answer 37

Even though reactive power doesn't show up as watts, it still represents electrons that need to be generated by the utility. If a facility has a low power factor, the utility must supply more current than is necessary for the actual power.

Question 38

T/F The power factor of a facility should be as high as possible.

Answer 38

True

Question 39

When do utilities generally charge a customer for power factor?

Answer 39

When the customer's power factor falls below a certain percentage, such as 90%.

Question 40

What are some power factor correction strategies?

Answer 40

Eliminate the inductive loads that generate kVAR, replace inductive motors, install capacitors designed to correct power factor by temporarily providing the reactive power, real-time metering

Question 41

T/F A typical single facility meter can be used to determine overall power factor but it can't determine what?

Answer 41

The cause