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Electrical Grid 2 > Chapter 2 > Flashcards

Chapter 2 Flashcards

Three-Phase Power

1
Q

What’s the main difference between a Three-Phase System and and Single-Phase System?

A

Three currents are used to supply the power and energy, unlike the single-phase system, which only uses one current. This allows the delivered power to be more evenly distributed.

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2
Q

What is essential when it comes to distributing or generating large amounts of power?

A

A more even distribution of power becomes important in terms of both efficiency and the size of the equipment needed.

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3
Q

What are the three advantages of three-phase systems over single-phase systems include:

A
  1. ) A 73% increase in power capability for 50% increase in conductors
  2. ) Less pulsating force applied to rotating generators and motors.
  3. ) Generators, transformers, and motors are smaller and less expensive then single-phase equipment of the same rating.
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4
Q

What is another difference in generating Three-Phase Power to Single-Phase Power?

A

One coil of wire Vs. three coils of wire.

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5
Q

How are the three coils spaced in a Three-Phase generator?

A

Equally 120 Degrees apart from one another.

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6
Q

What does the even spacing provide?

A

A Constant degree of separation, and three voltages and currents induced will maintain in a constant order of rotation.

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7
Q

How is the Three-Phase Sine Wave produced?

A
  • Each coil of wire will produce its own voltage and current.
  • The voltage and current on each coil will be 120 Degrees out of phase with the others.
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8
Q

In a single phase motor, how many times does the motor receive no power?

A

180x per second. (3zeros per cycle X 60 Cycles per second.)

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9
Q

In a single phase motor, because there is no power produced 180x/Sec what must the motor rely on to keep it spinning?

A

Its own inertia.

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10
Q

What did a careful study of the three-phase sine wave reveal?

A

That a potential difference exists between any two of the three phases at any time throughout the cycle.

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11
Q

When one phase is at Zero, what are the other two phases doing?

A

They’re providing power to the circuit.

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12
Q

What are the results of Three-Phase power?

A

Much smoother and more efficient operation of equipment

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13
Q

Because Three-Phase power delivered to a load is constant, the Three-Phase load can be delivered how?

A

Much smaller than a Single-Phase load while providing the same output.

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14
Q

In actual practice, how are the coils and magnetic field portrayed?

A

The coils are stationary and the magnetic field is rotated. This allows for a much better connection to the coil and allows for efficient transfer of power to the system.

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15
Q

How can the three coils of a Three-Phase Generator be connected?

A

DELTA or WYE

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16
Q

What is the difference in the coils between a DELTA or WYE?

A

Although they are both spaced 120 Degrees from each other the connection between the coils is different.

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17
Q

In a WYE System how is the voltage different?

A

The System Voltage is higher than the Phase Voltage by a value of 1.732.

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18
Q

In a WYE System how is the current different?

A

Coil Current and Phase Current are the same.

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19
Q

In a DELTA System how is the voltage different?

A

The system voltage and is the same as the coil voltage.

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20
Q

In a DELTA System how is the current different?

A

The Phase Current of a DELTA is higher than the Coil Current by a value of 1.732.

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21
Q

What is the predominant choice of the utility companies?

A

The WYE System

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22
Q

Why is the WYE System the popular choice?

A

Because the Phase and Line Current are equal. Therefore, can serve a given load with less current than DELTA Systems; thus, increasing the efficiency of the system.

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23
Q

Early on, which system was believed to be safer for lineman to work on energized? and why?

A

DELTA, because it has no neutral, the only means of a short circuit is between the phases.

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24
Q

What is a predominant rule?

A

ALWAYS work on one phase at a time.

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25
Q

What was the problem with working on DELTA energized?

A

If a tree limb was rubbing one of the other phases, or another lineman was working on different phase of the same circuit, a short circuit could occur.

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26
Q

When did lineman begin to use live-line tools? and which hat were they?

A

In the early 1920’s, Rubber Gloves and crude forms of Hot Sticks.

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27
Q

As live-line tools and procedures improved, ________ Systems began to fade away.

A

DELTA.

Today they only exist in isolated areas.

28
Q

Some utilities have what kind of systems?

A

WYE w/o a Neutral. Three-Wire WYE

29
Q

How can Three-Wire WYE be compared to Four-Wire WYE?

A

Rare, but more reliable.

30
Q

What can Four-Wire WYE experience?

A

More electrical interruptions because the neutral is in close proximity to the phases.

31
Q

Which two systems are identical in appearance?

A

Three-Wire WYE and DELTA

32
Q

What is the only way to determine the true identity of the system?

A

Confirm the secondary connections of the substation transformer bank.

33
Q

Which system is much more reliable in regards to fault protection? and why?

A

Four-Wire WYE. Because the neutral is extended throughout the circuit, faults have an immediate and reliable path to the source (Substation), which in turn causes protective equipment to operate.

34
Q

What happens if the soil conditions are highly resistive and prevent fault current from returning to the source in a Three-Wire WYE System?

A

The broken phase will remain on the ground energized.

35
Q

How else might fault currents return to the source?

A

Via Steel pipes or any conductive object that is buried in the ground.

36
Q

How is electricity produced?

A

Generators rotate a magnet near coils of conductor

37
Q

How is the voltage of each phase determined?

A

Dependent upon the degree of rotation at any given time.

38
Q

In the power delivery system, how are many generators connected?

A

The Transmission Network.

39
Q

How do you connect these generating facilities together, (Bring them Online)?

A

The angle of rotation of the generators must be exactly the same. Unison.

40
Q

Before closing the switches, what must you do when a new generation unit, or an existing unit, is placed online?

A

Operators must first get the generators rotating in unison.

41
Q

How are all generators connected to a regional transmission network connected?

A

They are all at the same angle of rotation at any given time.

42
Q

What happens if a unit experiences mechanical problem, or begins to lag or lead in rotation?

A

Protective equipment just outside the generating station automatically opens. This can also be done manually.

43
Q

What is the direction that a motor spins dependent on?

A

The direction that the AC Generators spin at the engineering facility and how electricians connect the motor to the AC System.

44
Q

What happens of the Phase Sequence is changed?

A

It will affect the direction that the motor(s) rotate.

45
Q

What happens if Phase Sequence is changed at the substation?

A

All Three-Phase motors connected to the distribution circuits exiting that substation would rotate in the opposite direction.

46
Q

Who usually causes rotation errors in isolated locations?

A

Lineworkers.

47
Q

How can you prevent a rotation problem?

A

The jumper connections should be drawn on the job print, or street beneath.

48
Q

What happens if any two phases are reversed?

A

Rotation will change.

49
Q

What tool can be used to verify both before & after working on jumper connections?

A

Rotation Meter

50
Q

What needs to be done when maintenance work is performed at a junction of the mainline feeder?

A

The phase sequence needs to be proven correct before leaving the work site and closing out the job.

51
Q

What tool is used to prove the sequence has not changed?

A

Phasing Meter

52
Q

What is a Phasing Meter?

A

A high voltage voltmeter used for proving phase sequence. The meter consists of a phasing probe and a meter probe, which attaches to hot sticks. An insulated jumper connects two probes.

53
Q

What steps are taken to prove phasing?

A

1.) Verify the meter is working properly.
2.) Verify the correct voltage on both sides of the open point.
3.) Confirm proper phase sequence between the open points of the same phase. To ensure that the meter is working properly a cross-phase check of the meter should follow any zero reading.
Once the line is phased, and the proper sequence over the open point is proven, the results are usually reported to the system operator.

54
Q

What does is mean if voltage is read across two corresponding conductors of an open point?

A

The phase sequence has been changed and the troubleshooting process must begin.

55
Q

How is the direction of rotation determined in Three-Phase motors?

A

By electricians and is dependent upon the equipment that the motor is connected to.

56
Q

What is the lineworker’s role when work is performed on Three-Phase Service, Transformer Bank, or Primary Distribution System feeding the Three-Phase Motors.

A

To ensure that rotation is not changed.

57
Q

What is done to maintain rotation?

A

Extra care is taken, using rotation meters and recording jumper connections BEFORE work commences.

58
Q

What is the most common electrical motor? And why?

A

Three-Phase AC Induction Motor, because it is rugged, reliable, and efficient.

59
Q

What parts do Three-Phase Motors consist of?

A

Stator & Rotor

60
Q

How is the Rotor made up

A

Copper Rods connected at each end by rings. The Rotor is then attached to a shaft that protrudes from the body of the motor.

61
Q

How does the Stator work?

A

Stator poles are connected to each phase of the Three-Phase Service. Each phase is wrapped around two stator poles 180 Degrees apart.

62
Q

How is the phase wrapped around the Stator Poles?

A

In a manner that causes them to develop opposite magnetic polarities when current is applied.

63
Q

What happens as each phase peaks?

A

As each phase peaks in the sine wave, the magnetic field rotates.

64
Q

What happens as the magnetic field peaks between the stator poles?

A

Current is induced in the rods of the rotor.

65
Q

What is the magnetic field of the rotor attracted to?

A

Rotating magnetic field of the stator.

66
Q

As the magnetic field of the _____ rotates, the ______ follows in the same direction.

A

Stator, Rotor

Electrical Grid 2 (3 decks)

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