High Voltage Distribution & Equipment

Question 1

High Voltage can have three systems of distribution, what are they?

Answer 1

1. Radial
2. Ring or Loop
3. Network or Grid

Question 2

What are the pros (3) and cons (1) of a radial system of High Voltage system distribution?

Answer 2

A radial system is the simplest and cheapest form of HV supply, and the easiest to protect against overload and short-circuits. But it is the least reliable because there is only one overhead line or cable supplying the plant or locality being served.

Question 3

What is the difference between a ring/loop system of High Voltage distribution vs a radial system?

Answer 3

The ring/loop supplies a locality from two directions so that, if one HV feeder goes down, there is an alternative supply thus is more reliable.

Question 4

What is the difference between a network/grid system of High Voltage distribution vs a ring/loop system?

Answer 4

A network system is essentially a loop system with additional interconnecting ties. In a network system, power is delivered by three feeders. It is the most efficient system but also the most expensive, and the protective relay system that goes with it becomes fairly sophisticated. Critical loads, found in major airports and hospitals, are often on a network system.

Question 5

In general, High Voltage service equipment may be located in what two areas?

Answer 5

1. in an outdoor switching compound, in the case of large industrial consumers using the higher levels of distribution voltage
2. within a building on the premises, in the case of consumers using 13.8 kV or less.

Question 6

A substation has traditionally meant a compound that contains what kind of High Voltage equipment? (8)

Answer 6

1. line terminals
2. disconnects
3. circuit-breakers
4. power transformers
5. lightning arresters
6. instrument transformers
7. metering equipment
8. control equipment.

Question 7

Describe a unit substation (6)

Answer 7

1. The unit substation is a factory-made metal enclosure made up of individual compartments.
2. The modular design allows for flexibility, and the units are assembled on the job site.
3. The equipment housed in the enclosure is completely enclosed by metal and no live parts are exposed.
4. (type metal enclosed) A small Plexiglas viewing window is provided in compartments that house equipment such as load-break switches to provide visible information on the status of the switch position.
5. (type metal-clad) The circuit breaker can be rolled out from the front and can be done safely by a robot
6. Access to equipment within the enclosure compartments is at the front, through the interlocked doors

Question 8

An electrical equipment vault is a room in a building specially constructed to house electrical service equipment, including transformers and are required when liquid filled equipment is used. What 5 features must they have according to the National Building Code?

Answer 8

1. The vault must be built of concrete and located so that at least one wall is an outside wall. This permits direct ventilation of the vault without the need for ducting systems.
2. Doors must be of steel (or fire-proof materials) and be equipped with padlocks to prevent unauthorized access. Door openings must have a step or sill.
3. Floors must provide for drainage where there is a liquid-filled transformer used, and the floor must slope towards the drainage area.
4. Vaults must be large enough to accommodate all equipment, with provisions for minimum clearances. 5. Vaults should not be located close to where gas, water, sewer or other such services enter or leave a building. This is because of the dangers of leakage or seepage into the electrical equipment which can cause problems or even explosions

Question 9

Lightning arresters are also called surge arresters, and are mainly installed on overhead lines. List 3 features of their installation.

Answer 9

1. installed in parallel to HV equipment
2. They are single-pole devices
3. One is required for each phase.

Question 10

Current-limiting series reactors are used to limit fault currents in the system. What form are they made in and how are they connected?

Answer 10

1. made in the form of air-cored coils of heavy copper wire wound with relatively few turns.
2. They are series-connected with feeder cables, busbars, and generators and offer low impedance at normal frequency.

Question 11

What main reason are current-limited series reactors installed?

Answer 11

Reactors permit the installation of circuit-breakers having lower interrupting ampacity than might otherwise be required. This means a cost saving on circuit-breaker investment.

Question 12

What are instrument transformers and why are they used?

Answer 12

PTs (VTs) and CTs are used for reducing current and voltage levels for metering and protective relays, while at the same time isolating these devices from the HV system

Question 13

PTs (VTs) step down voltage to what level?

Answer 13

120v

Question 14

CTs step down primary current to what level?

Answer 14

5 A