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Level 4 Electrical > Power Quality > Flashcards

Power Quality Flashcards

1
Q

5 Environmental causes why a commercial power grid might experience power problems

A

-Hurricanes
-Lightning Storms
-Snow
-Ice
-Flooding

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2
Q

3 Non-Environmental causes why a commercial grid might experience power problems

A

-Equipment failure
-Traffic accidents
-Major switching operations

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3
Q

4 causes why a facility might experience power problems

A

-Load construction
-Heavy start up loads
-Faulty distribution components
-Background electrical noise

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4
Q

How is a power quality disturbance generally defined?

A

Any change in power that interferes with the normal operation of electrical equipment

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5
Q

What device is commonly used to view AC power?

A

Oscilloscope

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6
Q

What are changes in a sine wave that indicate power quality disturbance
(8 answers)

A

Size
Shape
Symmetry
Frequency
Notches
Impulses
Ringing
Drops to zero

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7
Q

7 categories of power quality disturbances
(T.I.S.S.W.V.F)

A

Transients
Interruptions
Sag/Under voltage
Swell/Overvoltage
Waveform Distortion
Voltage fluctuations
Frequency Variations

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8
Q

Potentially, what is the most damaging type of power quality disturbance?

A

Transients

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9
Q

2 types of transients are?

A

Impulsive
Oscillatory

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10
Q

How long may impulsive transients last?

A

5-50 nanoseconds

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11
Q

What does ESD stand for?

A

Electrostatic discharge

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12
Q

What is created when a lightning strike occurs that can cause damage by inducing current onto nearby conductive structures?

A

Electromagnetic fields

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13
Q

What are two surge suppression devices used to try to eliminate ESD?

A

TVSS- Transient voltage surge suppressors
SPD- surge protection devices (same thing)

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14
Q

What electronic device is used to help protect against impulsive transients, swells and other high voltage condition?

A

MOV (Metal Oxide Varistor)

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15
Q

What is the most effective method of protection against power disturbances for electronic equipment?

A

Cascading SPD and UPS devices

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16
Q

What is the most recognized problem with an oscillatory transient?

A

Tripping of adjustable speed drives (indication of overvoltage on drive)

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17
Q

What devices can be installed ahead of an adjustable speed drive (ASD) to help prevent the drive from tripping due to an overvoltage?

A

Reactors or Chokes

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18
Q

What are the four types of interruptions and their time frames?
IMTS

A

Instantaneous- 0.5 – 30 cycles
Momentary- 30 cycles –to two seconds
Temporary- two seconds to two minutes
Sustained- greater than two minutes

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19
Q

One of the most common examples of what can cause an interruption in commercial power systems are

A

utility protective devices (circuit reclosures)

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20
Q

What does Sag mean and how long does it last?

A

Reduction in AC voltage at a given frequency
0.5 cycles to 1 minute

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21
Q

Common causes of sag

A
  • Starting of large loads
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22
Q

Instead of using “Across The Line” motor starters, what are some alternate motor starting methods to reduce the load on the rest of the electrical infrastructure?

A
  • Autotransformer starters
  • Start-delta starters
  • Solid state or soft starters
  • Adjustable speed drives (ASD) or Variable speed drives (VFD)
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23
Q

What does Swell mean and how long does it last?

A

Increase in AC voltage at a given frequency
0.5 cycles to 1 minute

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24
Q

Common causes of swell

A
  • High impedance neutral connections
  • Sudden large load reductions
  • A single phase fault on a three phase system
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25
Q

Common solutions for swells are

A
  • Power Line conditioners
  • Uninterrupted Power Systems (UPS)
  • Ferroresonant “control” transformers
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26
Q

What is harmonic distortion?

A

corruption of the fundamental sine wave at frequencies that are multiples of the fundamental

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27
Q

What are some indications that harmonics may exist in a system?

A

-Overheated transformers, neutral conductors & electrical distribution equipment
-Tripping of circuit breakers
-Loss of synchronization of timing circuits that are dependent upon a clean sine wave trigger at the zero crossover point

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28
Q

Why has harmonic distortion been a significant problem with IT equipment?

A

Because of “switch-mode power supplies”
(AC power is converted to pulsating DC)

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29
Q

When harmonics exist in a three-phase four-wire circuit, what may happen to the neutral currents?

A

The current on the neutral conductors do not cancel each other out, instead they add together, the neutral current could be 1.73 times the phase current. (Damage to the insulation)

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30
Q

What are some of the methods used to deal with problems associated with harmonics?

A

Installation of K-rated transformers
Over-sizing neutral conductors
Harmonic filters

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31
Q

What is a voltage imbalance?

A

supplied voltages not equal

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32
Q

What is the maximum voltage imbalance allowed between phases?

A

4%

33
Q

In the example below, are the voltage readings acceptable?
Example:
L1 – L2 = 204 volts
L2 – L3 = 208 volts
L1 – L3 = 210 volts

A

Voltage difference = 6 volts
Maximum Voltage imbalance – lowest voltage X 4 %
204V X .04 = 8.16V
Yes, voltage difference is acceptable

34
Q

Harmonics are produced by what?
On Single phase and 3 phase what are main causes?

A

Nonlinear loads that draw current in pulses rather than in continuous manner
1 phase- computer supplies
3 phase- VFD, Electronic drives

35
Q

What does a filter capacitor do?

A

Smooths pulsations when converting AC to DC

36
Q

What meter can you use to check for harmonics in a single phase system on transformer currents?

A

True RMS ammeter for current

37
Q

Most accurate method for testing harmonics is to use a?

A

Harmonic analyzer

38
Q

Harmonics occur in which 3 phase system generally?

A

4w Wye

39
Q

How do you fix a 3 phase, 4w system with harmonics?

A

Reducing the frequency of the neutral current by balancing the current on the phase conductors. If all phases have equal current the neutral should have no current

40
Q

What do you do if triplen harmonics are present on the neutral conductor?

A

Add harmonic filters at the load

41
Q

Harmonics problems generally involve heating of the?

A

Transformer

42
Q

Transformer Harmonic derating factor is:

A

THDF=(1.412) x (RMS phase-current) / Instantaneous peak phase current

43
Q

A positive sequence has what rotation and what effect?

A

Forward, Heating

44
Q

A negative sequence has what rotation and what effect?

A

Reverse, Heating and motor problems

45
Q

A zero sequence has what rotation and effect?

A

No rotation
Heating, adds in neutral of 3 phase 4w system

46
Q

What are harmonics?

A

Voltage and currents that operate at a frequency that is a multiple of the fundamental power frequency

47
Q

Fundamental frequency is?
2nd Harmonic?
3rd Harmonic?
5th Harmonic?

A

60hz
120hz
180hz
300hz

48
Q

Harmonics are produced by what loads?

A

Non-linear

49
Q

How are harmonics classified?
3 classifications

A

Name, frequency, and sequence

50
Q

How would a harmonic operating at 300Hz be classified?

A

Fifth harmonic, operating at 300 Hz with a negative sequence

51
Q

Negative sequence harmonics cause overheating of conductors, transformers and circuit breakers but also cause additional problems with what type of loads?

A

Motors

52
Q

When referring to an induction motor, a positive sequence harmonic rotates in the ? direction as the fundamental frequency, and a negative sequence harmonic rotates in the ? direction as the fundamental frequency.

A

same, opposite

53
Q

What could be a possible result when 120V non-linear loads are connected to a three-phase four-wire system:

A

The currents on neutral conductors do not cancel out; instead the currents add together causing excessive current to flow on the neutral conductor. (Neutral conductors are unfused)

54
Q

Triplens harmonics commonly cause damage in what type of system?

A

Three-phase, four-wire systems.

55
Q

What is an indication that harmonics may be present in panelboards and bus ducts?

A

The panelboards and bus ducts produce a buzzing sound.

56
Q

What are some possible solutions if it’s been determined that harmonics exist?

A
  • A harmonic filters at the load
  • Pull extra neutrals, a separate neutral for each phase conductor
  • Install a larger common neutral conductor if not practical to use a separate neutral for each phase conductor
  • Reduce the amount of load on the transformer
  • Use a K-rated transformer depending on the load(s)
57
Q

What is power quality?

A

a measure of how well an organization’s electrical system
supports the reliable operation of its loads.

58
Q

Power experts agree at least ?/3 of all power problems originate within the facility.

A

2/3

59
Q

Power disturbances are defined by

A

magnitude and direction

60
Q

Voltage dips (Sags) are the most common, accounting for up to ?% of power disturbances.

A

90%

61
Q

Typically, a voltage unbalance of more than ?% can cause motors and other three phase loads to overheat or fail prematurely.

A

2%

62
Q

Do harmonics have any useful purpose?

A

No

63
Q

What size are the neutral buses rated at in K-rated transformers?

A

200%

64
Q

K-Factors values range from?

A

1 to 50

65
Q

1.0 K rating is for what loads only?

A

Linear

66
Q

50 k rated is for what condition?

A

harshest harmonic condition

67
Q

3 reasons a transformer could have reduced life expectancy

A

Non-linear loads
Overheating
10 degrees C higher than rating

68
Q

What is K factor?

A

transformers ability to serve varying degrees of non-linear loads without exceeding the rated temperature rise limits

69
Q

Standard transformers are designed for linear loads only. True or false

A

True

70
Q

How much larger is the neutral in a K rated transformer than a standard transformer ?

A

x2

71
Q

Why are line and load reactors used?

A

To filter the waveform and reduce electrical noise and transients in the system

72
Q

2 advantages of using line reactors?

A

-Protects the drive from damage and tripping out
-Minimizes harmonics of the equipment within the system

73
Q

2 advantages of load reactors

A

Amplifies the voltage peaks at high distances
Absorbs current surges(overloads) created by the motor

74
Q

What are transients?

A

Sharp but brief distubances in voltage lasting 1000th of a second

75
Q

What are transients cause by?

A

Turning large equipment off or on
Capacitor switching
Lightning strikes

76
Q

What is a recloser?

A

automatic, high voltage electric switch

77
Q

What is an interruption?

A

Complete loss of supply voltage or load current

78
Q

5 Primary types of waveform Distortion

A

DC offset
Harmonics
Interharmonics
Notching
Noise

79
Q

Poor power quality results from power disturbances generated by ?

A

Internal and external circuits

Level 4 Electrical (6 decks)

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