7 Topic: Operating Voltages (30m)
Instructions:
- Cover the following content as a group (either reading out loud or independently) then give an opportunity to answer any questions.
- Have students do the review questions independently, then take up answers.
The electricity in any utility goes through many voltage changes. First, voltage is stepped up after generation to transmission voltages. After being transported to a more central location, the voltage is stepped down to distribution levels. Finally, through the use of distribution transformers, the voltage levels are further reduced to supply low voltages for customer use.
In addition to the various voltages, we will look at some terms which tend to be confusing. First, we will explain some common terms, then identify some of the common voltages.
Terms
Some terms that are used have very similar meanings: phase and line are an example of this. We have all heard of a three-phase or single-phase power line.
Phase voltage
Is the voltage across one coil in a generator.
Phase to Phase Voltage
Is the voltage between any two lines in a three-phase system that produced three distinct and separate voltages (three-phase generator.)
Line voltage
Is also referred to as “line to line voltage” and is used to describe the voltage a power line can produce between two phases. Since a line with two conductors has the same line to line voltage as a line with three conductors, they can both be classified as having the same voltage. For example, a two- or three- phase lines can both be called 25kV lines.
Line to Ground Voltage
Describe the voltage a single-phase or single line conductor has to ground. For example, a single-phase rural line is rated at 14.4 kV to ground. Also, one phase of a 25kV line to ground can give 14.4 kV
Voltages Through a Utility
Previously, we have learned that voltage is raised to higher levels for transporting. These high voltages exist on transmission lines and are always interconnected in a “transmission grid system”.
The three major transmission voltages used at this utility are:
- 230,000 volts
- 138,000 volts
- 72,000 volts
Three major distribution voltages are:
- 25,000 volts
- 14,400 volts
- 4160 volts
- 2400 volts
Distribution and transmission lines are always designated according to their line-to-line voltage (voltage between phases or line values).
Note: The only exception to this rule is the 14.4 kV rural line. It taps off the 25 kV line but can only supply 14.4 kV because it has no other phase to find reference with. The only other reference available is ground, so that is used.
Once power is delivered to a customer’s site, it is lowered to the supply voltage the customer desires. This is done through a distribution transformer and delivered in “secondary” lines or services.
Five Major Loads
The loads which utilities supply are divided into five major types:
- Farms
- Residential – housing in towns and cities
- Commercial – small business
- Industrial – can be supplied by high voltage lines to their own substations
- Streetlights
Utilities make every effort to comply with the customer’s request, but if the transformers and metering needed to supply a given voltage do not exist, a different voltage will be supplied. The new standard is to avoid three-phase, three wire services in a favour of a three-phase, four-wire.
Review Questions: Operating Voltages (30m)
Line to line volts can refer to the voltage:
- Between phases
- Phase to ground
- Phase to neutral
- None of the above
( True / False ) Line to line voltage is a phase-to-phase voltage.
A single-phase line can only supply:
- Line to line voltage
- Phase to phase voltage
- Line to ground voltage
- None of the above
( True / False ) A two-phase line has less supply voltage than a three-phase line.
List the three transmission voltages
List three voltages commonly used in distribution
Transmission and distribution voltages are mainly classified by:
- Line to ground volts
- Line to line volts
- Phase to ground volts
- None of the above
( True / False ) A 14.4 kV line is rated by a phase to ground voltage
Secondary lines are rated to their:
- Line to ground voltage
- Line to line voltage
- Supply voltages
- All of the above
What are the five major types of load?
Answer Key
1. a, 2. T, 3. c, 4. F, 5. 230 kV, 138 kV, 72kV, 6. 25kV, 4.16kV, 2.4kV, 7. b, 8. T, 9. a, 10. Farms, Residential, Commercial, Industrial, Streetlights
