Transformers: Chapters 1 (Identification), 2 (Anatomy), & 3 (Theory) - Possible Test Questions - WEEK 11 Flashcards
1
Q
A transformer is:
A
- an electrical device that converts voltage from either high voltage to low voltage, or low voltage to high voltage. This process is called either stepping-down the voltage, or stepping-up the voltage, respectively.
2
Q
A transformer will work only on:
A
- alternating current systems and will not work on direct current systems.
3
Q
A transformer has a source side known as:
A
- the primary side. This is where the transformer connects to the source.
4
Q
The secondary side can be considered:
A
- the load side or the customer’s side.
5
Q
Transformers are either:
A
- pole-mounted (overhead)
- pad-mounted (underground)
6
Q
A pole-mounted transformer is:
A
- physically attached with bolts known as hanger bolts. This type of transformer has an overhead conductor for its primary source of feed (energization).
7
Q
A pad-mounted transformer is:
A
- placed on either a concrete or fiberglass pad on the ground. Its primary source of feed is an underground conductor.
8
Q
On an overhead transformer, there are points of:
A
- termination where the conductors are attached.
9
Q
The H1 bushing is always the:
A
- left bushing when facing the transformer, and the H2 is always on the right.
10
Q
For an overhead transformer, on the secondary side there are low-voltage bushings designated as the:
A
- X1, X2, and X3. The labeling of these bushing changes based upon the polarity.
11
Q
The high-voltage bushings have more insulation than:
A
- the low-voltage bushings.
12
Q
For an overhead transformer, the high-voltage primary bushings are located:
A
- on the top of the transformer.
13
Q
For an overhead transformer, the low-voltage secondary bushings are located:
A
- on the front of the transformer.
14
Q
On a pad-mounted transformer, the primary side is on:
A
- the left side, and the secondary is on the right side.
15
Q
For underground transformers, the primary bushings are designated:
A
- H1A
- H1B
16
Q
For underground transformers, the H1A bushing is always on the:
A
- left when facing the transformer, and the H1B is on the right.
17
Q
For underground transformers, the secondary bushings are designated as the:
A
- X1, X2, and X3. As with the overhead transformers, the primary bushings have more insulation than the secondary bushings.
18
Q
Pad-mounted transformers are equipped with a:
A
- “bayonet” fuse located above the primary bushings.
19
Q
Bayonet fuses are used in pad-mount transformers for both:
A
- energizing and de-energizing the transformer.
20
Q
The kVA (kilovolt-amperes) rating can usually be found:
A
- stenciled on the front of the transformer.
- This rating is also called the kW (kilo-watt) rating.
21
Q
As a general rule, an average size home will draw:
A
- 6,000 watts at peak load, and a large size home will draw 15,000 watts at peak load.
22
Q
There are two main types of overhead transformers:
A
- Conventional Transformers
- CSP (Completely Self Protected)
23
Q
A conventional transformer:
A
- needs an external protective fused device, known as a cutout or switch, to protect it from fault current. The cutout switch is also used to energize and de-energize the transformer.
24
Q
A CSP transformer
A
- is internally protected by a current limiting fuse, and is also protected by a circuit breaker that also energizes and de-energizes the secondary side of the transformer. Externally, there is also a lightning arrestor.
25
Transformers are machines with no moving parts. Internally, they are made up of three main components:
- the core
- the windings
- the insulating fluid.
- Transformers also come equipped with pressure relief valves. CSP transformers are equipped with internal fuses.
26
The transformer core is one of the key components as it provides:
- A path for the magnetic coupling of the two windings. The core concentrates the magnet flux lines, which substantially improves its efficiency.
27
The core is made of thin laminations of grain-oriented silicon steel rolled into layers, with each side coated with an insulating material. The construction of the core can have a significant impact on the watt losses of the transformer. The transformer core is normally constructed in one of two configurations:
- Wound Type Core
| - Shell Type Core
28
Copper is the most popular material for the windings because of its low:
resistance and superb conductivity.
29
Convection:
The insulating fluid plays a critical role in cooling and insulating the core and coil assembly. It facilitates the cooling by transferring the heat generated in the coil and core assembly to the tank or radiators, where it will be dissipated to the outside atmosphere. As the core heats the oil, the oil rises. The oil is then forced against the case of the transformer where it radiates heat into the atmosphere and cools. As it cools, it circulates down to the bottom of the core. This circulation of the oil by heat is called convection, and is similar to hot wax circulating near the burning wick of a candle.
30
The term PCB (Polychlorinated Biphenyls):
- relates to a substance that can be a part of an insulating fluid typically known as "askarel." PCB has been determined to be harmful to the environment and is prohibited in new transformers.
31
It is extremely important to understand that pressure in the form of gas can build up inside a transformer:
- This is normally a result of a short in the windings, which breaks down the impurities in the oil creating gas.
32
The valve will vent when the internal pressure reaches approximately:
- 10 lbs per square inch. It can also be operated manually by pulling on the ring on the outside of the tank.
33
CSP transformers have internal fuses that:
- isolate and protect the transformer from internal faults. The transformer must be de-energized and the lid removed to replace this fuse. This task is normally performed in the transformer shop.
34
The CSP also has an internal secondary breaker, which is:
- intended to disconnect the hot legs in the event of an overload or a short circuit on the secondary side.
35
Pad-mount transformers have fuses that are located internally, but are operated:
- externally. These fuses are located in the primary compartment and are under the oil level.
36
The amount of induced voltage can be transformed by changing the:
- number of turns of wire in the secondary coil. If 100 volts is applied to a primary coil wound 100 times, and a secondary coil of only 10 turns is near it, 10 volts is induced. Being able transform a voltage is not possible with direct current circuits and is why alternating current is predominant worldwide.
- Taken further, turns ratio is a comparison of the number of turns of wire in each coil. If the primary coil has 100 turns and the secondary coil has 10 turns, the ratio is 10 to 1 (10:1).
37
Polarity:
- Polarity is the direction of current flow through the two coils. Polarity is set during the manufacturing process and cannot be changed in the field. Polarity is determined by the direction the secondary coil is wrapped in relation to the direction the primary coil is wrapped.
38
The polarity of a transformer can be either:
- additive or subtractive. If it is additive, the x1 bushing will be on the right-hand side (while facing the transformer). If it is subtractive, the x1 will be on the left.
39
Polarity is very important to know when banking and paralleling transformers:
- By industry standards, transformers 200 kVA and smaller, with a primary rating of 8660 volts and below, will be rated additive. All other transformers will be subtractive.
40
The two main losses of the transformer are called:
- iron and copper losses.
41
Iron losses:
- or no-load losses, are created by the oscillation of the magnetic field in respect to the core.
42
Copper losses:
- are caused when a current flows in a conductor (caused by load). This flow generates heat in this conductor, and as the load increases, the heat increases. This heat limits current flow, and is also referred to as the I7R loss in engineering circles. Without these two losses, the transformer would be 100% efficient.
