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Power System Studies > Harmonic Analysis 2 > Flashcards

Harmonic Analysis 2 Flashcards

1
Q

What happens to resonant frequency (increases or decreases) when utility fault MVA increases?

A

Resonant frequency increases as the utility fault MVA increases

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2
Q

Which grid (SC value max or min) are less likely to encounter problematic resonance conditions at low frequencies

A

Strong utility supplies(high fault MVA)

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3
Q

Series resonance results in low or high impedance?

A

Low impedance

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4
Q

Parallel Resonance results in high or low impedance

A

High impedance

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5
Q

Which resonance occurs more often ? (Parallel or series)

A

Parallel Resonance

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6
Q

Why can it matter of concern if the resonant frequency is close to one of the frequencies generated by harmonic sources in the system?

A

Because it can lead to resonance , which may cause excessive vibrations or other issues(such as over voltage) in the system

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7
Q

What is the result of series resonance?

A

Flow of unexpected amounts of harmonic currents through certain elements

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8
Q

What are the common manifestation of excessive harmonic current flow?

A

Inadvertent relay operation, Blown fuses and overheating of cables

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9
Q

What are the result of parallel resonance?

A

Presence of excess harmonic voltages across network elements

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10
Q

What are the common manifestation of excessive harmonic voltages?

A

Capacitor and insulation failure

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11
Q

When harmonics appear to be the cause of system problems, what should engineer do?

A

It is necessary to determine the resonant frequency at the problem sites

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12
Q

What is resonance?

A

Resonance is condition in which the inductive reactance is equal to capacitive reactance

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13
Q

With addition of each new capacitor banks, what happens to system’s resonant frequency?

A

System’s Resonant frequency is lowered

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14
Q

What is the natural frequency of a circuit and why is it typically not a problem in most power circuits?

A

The natural frequency of a circuit is the frequency at which the circuit is in resonance. (Any circuit has a resonant condition at some particular frequency). When there is no intentional capacitance added to the circuit. The natural frequency of most power circuits is in kilohertz range. Since there is normally no source of currents in this range , the natural frequency and resonance associated with it are typically not a problem

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15
Q

What problem can occur when capacitors for power factor (PF) improvement are applied to circuits with non linear loads that inject harmonic currents?

A

They may lower the resonant frequency of the circuit enough to create a resonant condition with the harmonic currents

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16
Q

What is the solution to the problem of resonant conditions created by capacitors for power factor improvement in circuits with non linear loads?

A

A solution is to detune the circuit by changing the point where the capacitor are connected to the circuit, adjusting the amount of applied capacitance or bu installing specially designed filter reactors

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17
Q

When does parallel resonance occurs?

A

It occurs when inductive reactance of inductor and capacitive reactance of a capacitor are equal in magnitude but opposite in phase

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18
Q

What does parallel resonance cause?

A

Leading to increase in the circuit impedance at particular frequency. It mainly cause voltage amplification

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19
Q

What is the formula for calculating parallel resonant frequency?

A

h= sqrt( capacitive MVar/ equivalent inductive MVA)

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20
Q

How do changes in capacitor bank size affects resonant frequency and peak resonant impedance in circuit?

A

The resonant frequency decreases as the capacitor bank size increases(therefore power factor increases) , second , the peak resonant impedance increases as the caoacitor bank size decrease(there fore power factor decreases)

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21
Q

How does resistive damping affect resonant impedance and circuit stability as frequency increases?

A

As the frequency increases, the resistive damping in the circuit Alisha increases, which helps to reduce the impact of increased resonant impedance . This makes the circuit more stable and less prone to problematic resonance(which reduces the sharpness or severity of the resonance peak)

22
Q

What is the effect of series resonance?

A

Current amplification

23
Q

What are the methods to mitigate harmonics?

A

1) Passive Filtering
2) Active Filtering
3) multiphase converters
4)Distributing loads to higher volatge levels

24
Q

What is the primary advantage of load connected harmonic filter I’m power systems?

A

They are more effective at mitigating harmonics

25
Q

WHy are bus connected harmonic filters typically preferred for improving overall plant power factor ?

A

It is better choice

26
Q

What common function do bis connected harmonic filter share with load connected harmonic filter?

A

Both provide reactive compensation,mitigate harmonics and are subject to overload

27
Q

What are the advantages of load connected harmonic filter?

A

1) need to oversize distribution equipment is eliminated
2) system losses are minimized
3) voltage distortion at the point of Utilization is minimized
4) The harmonic filter can be sized specifically for the load
5) harmonic filter can be switched on and off With load

28
Q

What is most cost effective solution for poor power factor, excessive volatge distortion and IEEE 519 violations

A

To install one or more large harmonic filter at a distribution busesp

29
Q

What is most cost effective solution for poor power factor, excessive volatge distortion and IEEE 519 violations

A

To install one or more large harmonic filter at a distribution busesp

30
Q

How harmonic filter used in arc furnace applications are connected?

A

Typically connected perpenantly to the furnace circuit. Some installation employ switching scheme when The harmonic filters will be switched based on the furnace loadings

31
Q

What type of Arc Furnaces large steel manufacturer uses?

A

AC Arc furnaces

32
Q

Arc furnaces can be further classified into ?

A

Scrap furnac3s and ladle furnaces

33
Q

Which type AC arc furnaces create significant harmonics?

A

The scrap furnaces generate more and these furnaces are generally large. Ladle furnaces generate harmonics less compared to scrap furnaces

34
Q

What other types furnaces steel manufacturer use?

A

DC arc furnaces

35
Q

What are the tyoical Pulse types of DC furnaces and how can they operate under abnormal condition?

A

DC furnaces are typically 12 Pulse types and can be operated as 6 Pulse systems. There are also higher Pulse order DC furnaces , such as 48 Pulse, which can operate as various lesser Pulse systems in abnormal conditions

36
Q

What are the principal harmonics by DC furnaces and how is the magnitude of each harmonics determined?

A

The principal harmonics generated by the DC furnaces are the typical h= P+-1 harmonics, where the magnitude of each harmonics is approximately the fundamental current divided by the order of the harmonics

37
Q

Does AC and DC harmonics create non charecteristic and interharmonics ?

A

Yes Some level

38
Q

How many times arc furnace transformer is energized per day?

A

50 to 100 times

39
Q

Does arc furnace transformer energuzation cause harmonics?

A

Yes

40
Q

What order harmonic filter does AC scrap furnaces uses?

A

2nd , 3rd, 4th and 5th harmonics

41
Q

Which order harmonic filter does DC arc furnaces uses?

A

5th, 7th and 11th harmonic filter to filter the harmonic generated by rectification. 2nd , 3rd and 4th harmonic filter is used for harmonic generated by arcing process

42
Q

What type of drive system does steel rolling mills use?

A

6 Pulse and 12 Pulse drive systems (which produce 1/h type harmonics) and some rolling mills utilize cycloconcertes(which also produce interharmoncs)

43
Q

What happens to harmonic currents distortion when we transfer loads to higher volatge?

A

Reduces the magnitude of harmonic currents, improved power quality and system can operate with lower harmonic distortion

44
Q

What is the phase angel retard method for mitigating harmonics?

A
45
Q

What happens to the magnitude of the harmonics if we increase the phase angel?

A

Magnitude of the harmonics increases

46
Q

Howard why can we mitigate harmonics by using iron core device with moderate flux densities?

A

By using iron core devices with moderate flux densities , harmonic effects can be reduced(i.e) , if transformers are used in ASD, one with low flux density iron core will improve the power quality of the drive. Because as the flux density increases, the magnitude of the harmonics produced increases.

47
Q

What happens to harmonics when we increase the flux density ?

A

As the flux density increases, harmonic produced also increases

48
Q

What happens to the amplitude of harmonics when the frequency (or the harmonic order ) increases?

A

Amplitude of the harmonics increases

49
Q

What happens to volatge quality when the impedance is smaller?

A

The smaller the impedance, better the volatage quality because voltage distortion at a given bus is dependant on the equivalent source impedance

50
Q

How resistive load affects the harmonic currents?

A

The resistive load reduce the magnification pf harmonic currents and thus attenuated, harmonic current levels near parallel resonance frequencies

51
Q

How motor loads and harmonic loads affects harmonic s and resonance?

A

Motor load and other dynamic loads that contribute to the short circuit capacity of the system can shift the frequency at which system resonance occur.motor load do not provide significant damping of resonant peaks

52
Q

How motor loads and harmonic loads affects harmonic s and resonance?

A

Motor load and other dynamic loads that contribute to the short circuit capacity of the system can shift the frequency at which system resonance occur.motor load do not provide significant damping of resonant peaks

Power System Studies (11 decks)

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