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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
Common solutions for swells are
- Power Line conditioners - Uninterrupted Power Systems (UPS) - Ferroresonant “control” transformers
26
What is harmonic distortion?
corruption of the fundamental sine wave at frequencies that are multiples of the fundamental
27
What are some indications that harmonics may exist in a system?
-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
28
Why has harmonic distortion been a significant problem with IT equipment?
Because of “switch-mode power supplies” (AC power is converted to pulsating DC)
29
When harmonics exist in a three-phase four-wire circuit, what may happen to the neutral currents?
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)
30
What are some of the methods used to deal with problems associated with harmonics?
Installation of K-rated transformers Over-sizing neutral conductors Harmonic filters
31
What is a voltage imbalance?
supplied voltages not equal
32
What is the maximum voltage imbalance allowed between phases?
4%
33
In the example below, are the voltage readings acceptable? Example: L1 – L2 = 204 volts L2 – L3 = 208 volts L1 – L3 = 210 volts
Voltage difference = 6 volts Maximum Voltage imbalance – lowest voltage X 4 % 204V X .04 = 8.16V Yes, voltage difference is acceptable
34
Harmonics are produced by what? On Single phase and 3 phase what are main causes?
Nonlinear loads that draw current in pulses rather than in continuous manner 1 phase- computer supplies 3 phase- VFD, Electronic drives
35
What does a filter capacitor do?
Smooths pulsations when converting AC to DC
36
What meter can you use to check for harmonics in a single phase system on transformer currents?
True RMS ammeter for current
37
Most accurate method for testing harmonics is to use a?
Harmonic analyzer
38
Harmonics occur in which 3 phase system generally?
4w Wye
39
How do you fix a 3 phase, 4w system with harmonics?
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
What do you do if triplen harmonics are present on the neutral conductor?
Add harmonic filters at the load
41
Harmonics problems generally involve heating of the?
Transformer
42
Transformer Harmonic derating factor is:
THDF=(1.412) x (RMS phase-current) / Instantaneous peak phase current
43
A positive sequence has what rotation and what effect?
Forward, Heating
44
A negative sequence has what rotation and what effect?
Reverse, Heating and motor problems
45
A zero sequence has what rotation and effect?
No rotation Heating, adds in neutral of 3 phase 4w system
46
What are harmonics?
Voltage and currents that operate at a frequency that is a multiple of the fundamental power frequency
47
Fundamental frequency is? 2nd Harmonic? 3rd Harmonic? 5th Harmonic?
60hz 120hz 180hz 300hz
48
Harmonics are produced by what loads?
Non-linear
49
How are harmonics classified? 3 classifications
Name, frequency, and sequence
50
How would a harmonic operating at 300Hz be classified?
Fifth harmonic, operating at 300 Hz with a negative sequence
51
Negative sequence harmonics cause overheating of conductors, transformers and circuit breakers but also cause additional problems with what type of loads?
Motors
52
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.
same, opposite
53
What could be a possible result when 120V non-linear loads are connected to a three-phase four-wire system:
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
Triplens harmonics commonly cause damage in what type of system?
Three-phase, four-wire systems.
55
What is an indication that harmonics may be present in panelboards and bus ducts?
The panelboards and bus ducts produce a buzzing sound.
56
What are some possible solutions if it’s been determined that harmonics exist?
- 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
What is power quality?
a measure of how well an organization’s electrical system supports the reliable operation of its loads.
58
Power experts agree at least ?/3 of all power problems originate within the facility.
2/3
59
Power disturbances are defined by
magnitude and direction
60
Voltage dips (Sags) are the most common, accounting for up to ?% of power disturbances.
90%
61
Typically, a voltage unbalance of more than ?% can cause motors and other three phase loads to overheat or fail prematurely.
2%
62
Do harmonics have any useful purpose?
No
63
What size are the neutral buses rated at in K-rated transformers?
200%
64
K-Factors values range from?
1 to 50
65
1.0 K rating is for what loads only?
Linear
66
50 k rated is for what condition?
harshest harmonic condition
67
3 reasons a transformer could have reduced life expectancy
Non-linear loads Overheating 10 degrees C higher than rating
68
What is K factor?
transformers ability to serve varying degrees of non-linear loads without exceeding the rated temperature rise limits
69
Standard transformers are designed for linear loads only. True or false
True
70
How much larger is the neutral in a K rated transformer than a standard transformer ?
x2
71
Why are line and load reactors used?
To filter the waveform and reduce electrical noise and transients in the system
72
2 advantages of using line reactors?
-Protects the drive from damage and tripping out -Minimizes harmonics of the equipment within the system
73
2 advantages of load reactors
Amplifies the voltage peaks at high distances Absorbs current surges(overloads) created by the motor
74
What are transients?
Sharp but brief distubances in voltage lasting 1000th of a second
75
What are transients cause by?
Turning large equipment off or on Capacitor switching Lightning strikes
76
What is a recloser?
automatic, high voltage electric switch
77
What is an interruption?
Complete loss of supply voltage or load current
78
5 Primary types of waveform Distortion
DC offset Harmonics Interharmonics Notching Noise
79
Poor power quality results from power disturbances generated by ?
Internal and external circuits

Level 4 Electrical (6 decks)

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