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Power System Studies > Harmonic Analysis > Flashcards

Harmonic Analysis Flashcards

1
Q

When harmonics are involved what percentage of Breaker settings is advisable for Other load breakers and capacitor bank breaker?

A

As per IEEE standard

Considering harmonics :
Capacitor bank breakers pick up= 1.4 times *full load amps

And add additional 20 % margin which is

1.6 times *full load amps

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2
Q

Capacitor bank voltage should be equal to

A

Bus voltage it is connected ,otherwise kvar value varies

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3
Q

What equipment is needed for 12 pulse converter?

A

Transformer

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4
Q

In this circuit or device current leads voltage by 90 degree and power factor is zero

A

Pure capacitor because true power equal to zero

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5
Q

In this circuit or device current lags voltage by 90 degree and power factor is zero

A

Pure inductive circuit because true power qual to zero and no dissipitative resistive component in circuit and circuit is reactive

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6
Q

In this circuit current and voltage are in phase and Power factor is one

A

Resistive circuit because reactive power equals to zero

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7
Q

Average power is zero in

A

Inductive load which leads to current is flowing without no work done by inductive load but current circulates in line thereby increasing losses and voltage

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8
Q

Average power is not zero in

A

Resistive load

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9
Q

Frequency of power is double of its voltage frequency in which load?

A

Pure inductive load

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10
Q

Real power is positive or unidirectional in which load ?

A

Resistive load

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11
Q

Power is negative for small duration and power is positive for longer duration for which power factor?

A

Cos(0.8)

Power factor=0.80

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12
Q

True power factor formula

A

Cos pi* sqr root of 1/1+(thd)^2

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13
Q

When try power factor and thd are used

A

When non linear load are present

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14
Q

Which is voltage harmonic devices?

A

Arc furnace and all other devices are current harmonic device

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15
Q

What are harmonic sources?

A

A devices which changes the nature of voltage and current waveforms

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16
Q

If inductive reactance increases?frequency?

A

Frequency increases

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17
Q

When capacitive reactance increases?frequency?

A

Frequency decreases

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18
Q

Which equipment provides low impedance path for high frequency harmonics?

A

Shunt capacitance so harmonic current goes to capacitance instead of source

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19
Q

Which equipment opposes high frequency harmonics?

A

Inductor

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20
Q

Formula for peak value

A

Square root of 2 * rms value

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21
Q

If source impedance of grid is increased what happens to source short circuit current and voltage thd? And current THD?

A

Source short circuit current decrease which results in increase in voltage THD

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22
Q

If source impedance of grid is decreased what happens to source short circuit current and voltage thd?

A

Source short circuit current increases which results in decrease in voltage THD

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23
Q

If x/R ratio of source increases?

A

THD increase

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24
Q

If x/R ratio of source decreases?

A

THD decreases

R value remains same while X value varies

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25
Q

If Transformer impedance is increased

A

THD below Transformer increases and THD above Transformer decreases

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26
Q

If transformer impedance is decreased

A

THD below TRANSFORMER decreases and THD above transformer increase

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27
Q

When connecting capacitor bank in Down stream system filled with harmonic source?

A

Smart thing is add reactor with capacitor bank because reactor limits harmonic current

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28
Q

cable selections is depends on what factors?

A

1) voltage drop
2) short circuit withstand capacity
3) current carrying capacity

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29
Q

what is formula for MIN cross sectional area of cable

A

A= SHORT CIRCUIT CUURENT * (SQRT ROOT OF t)/K

t= FAULT CLEARING TIME OR DURATION OF SHORT CIRCUIT
k= S.C TEMP RISE CONSTANT
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30
Q

WHAT IS K VALUE FOR XLPE AL CABLE AND XLPE CU CABLE?

A

K=0.094(FOR XLPE AL)

K=0.143(FOR XLPE CU)

31
Q

FLA FORMULA?

A

KVA/1.732*V

32
Q

VOLTAGE DROP FORMULA?

A

IRCOS(PI)+ jXSIN (PI)

33
Q

WE NEED GRID DATA FOR HARMONIC STUDY OR NOT?IF WE DON’T HAVE GRID DATA?

A

YES WE NEED GRID DATA.IF WE DONT HAVE GRID WE SHOULD SEE SWITCH GEAR DATA OR SLD

34
Q

THE WORST CASE SCENERIO FOR HARMONIC ANALYSIS OCCURS DURING

A

MIN SHORT CIRCUIT CURRENT

35
Q

HARMONIC ANALYSIS SHOULD BE DONE AT WHAT SC CURRENT?

A

MIN SHORT CIRCUIT CURRENT

36
Q

AC RESISTANCE OF CONDUCTOR SHOUL BE CONSIDERED AT WHAT TEMPERATURE?

A

1) AC RESISTANCE OF CONDUCTOR SHOULD BE CONSIDERED AT MINIMUM TEMP FOR SHORT CIRCUIT STUDY
2) AC RESISTANCE OF CONDUCTOR SHOULD BE CONSIDERED AT MAXIMUM TEMP FOR LOAD FLOW STUDY AND HARMONIC STUDY AND TRANSIENT STABILITY

37
Q

IF TEMPERATURE RISES WHAT HAPPENS TO RESISTANCE

A

RESISTANCE INCREASES

38
Q

R t FORMULA

A

R t = R o(1+ ALPHA*(t - t o)

AL[PHA FOR CU CABLE

ALPHA FOR AL CABLE

39
Q

YOU HAVE TO GET DC RESISTANCE OF CONDUCTOR AT WHAT TEMP

A

AT ALL TEMP

40
Q

KVA FORMULA

A

KVA= 1.732 VI

41
Q

FOR HARMONICS WHAT THREE LIMITS OR THINGS COME INTO PICTURE?

A

1) VOLTAGE DROP LIMIT
2) STABILITY LIMIT
3) THERMAL LIMIT

42
Q

WHY WE CANT GET PARAMETER SUCH AS LINE REACTANCE AND CAPACITANCE?

A

BECAUSE LINE REACTANCE AND CAPACITANCE DEPENDS UPON

1) TOWER STRUCTURE AND HEIGHT
2) SPACING BETWEEN CONDUCTORS
3) HOW MANY GROUND WIRES
4) SPACING BETWEEN GROUND WIRES

43
Q

IF WANT TO CONSIDER RLC PARAMETERS FOR BUSDUCT MODELLING.WHAT YOU HAVE TO DO

A

MODEL BUSDUCT AS AN IMPEDENCE(MODEL) IN ETAP

44
Q

HOW TO FIND CABLE TYPE WHENT IS MENTIONED?

A

YOU CAN FIND CABLE TYPE BY SEEING MIN CROSS SECTIONAL AREA OF CONDUCTOR
(A= I sh * SQRT OF t/K)

45
Q

(REG 5 WINDING INTO 3 WINDING TRAFO)

WHAT IS IMPEDANCE VOLTAGE (U k) OF WINDING UNDER TEST(SC TEST)?

A

PRIMARY WINDING VOLTAGE AT WHICH YOU GET RATED CURRENT AT SHORTED SECONDERY WINDING

46
Q

PER UNIT IMPEDANCE FORMULA

A

IMPEDANCE VOLTAGE/RATED VOLTGE (IF NECESSCARY MULTIPLY TIS VALUE BY 100%)

47
Q

Z P.U NEW FORMULA

A

Z p.u OLD * (KVA OLD/KVA NEW)^2 *

MVA NEW/MVA OLD

48
Q

X/R RATIO FORMULA

A

LOAD LOSS or Cu loss/BASE MVA

49
Q

HOW WILL DETERMINE MAX LOAD VALUE (i.e of trafo)IF IT IS NOT KNOWN

A

ASSUME LOAD VALUE AS 80% OF TRAFO MVA

50
Q

HOW PF OR KVAR CAN BE IMPROVED AT SOURCE(BUS)?

A

IMPROVED BY ADJUSTING GENERATING VOLTAGE(AVR) OR ADJUSTING OLTC

51
Q

WHAT IS A STANDARD FOR HARMONIC ANALYSIS?

A

IEEE 519 2014

52
Q

WHAT IS TDD

A

TOTAL DEMAND DISTORTION

53
Q

AS WHICH BUS GRID WILL ACT AS ALWAYS

A

GRID WILL ALWAYS BE SWING BUS

WHATEVER MISMATCH HAPPENS THAT WILL BE HANDLED SWING BUS

54
Q

WHEN GRID(SYSTEM )CONSIDERED WEAK.

A

WHEN IT HAS

1) LESSER S.C CURRENT
2) HIGH SOURCE IMPEDANCE
3) HARMONICS IS HIGH

55
Q

WHAT IS Q VALUE

A

QUALITY FACTOR

56
Q

WHAT IS FORMULA FOR h?

A

h = n*P *(+ OR - 1)
n- INTEGER
P- NO OF PULSE (i.e 6 PULSE CONVERTOR )

57
Q

WHY WE AVOID ADD HARMONIC FILTER TO HV AND EHV LINES

A

BECAUSE IT IS EXTREMELY EXPENSIVE

58
Q

SHOULD WE PERFORM HARMONIC ANALYSIS IF TOTAL THD VALUE IS WITHIN LIMIT AND ONE PARTICAL HARMONICS(i.e ; 5th HARMONIC) THD VALUE EXCEEDS LIMIT?

A

WE SHOULD PERFORM HARMONIC ANALYSIS

59
Q

IN HARMONIC SITUATION WHAT YOU ARE NEEDED TO FIND OUT?

A

1) WHERE THIS HARMONICS COMES FROM

2) WHAT ORDER HARMONICS

60
Q

PURPOSE OF IMPEDANCE SCANNING

A

TO FIND OUT WHERE YOUR IMPEDANCE IS MAXIMUM AND AT THIS(IMP MAX POINT) POINT WHICH HARMONIC IS PREDOMINANT (IN GRAPH)

61
Q

HOW TO MODEL CABLE LINE FOR HARMONIC ANALYSIS IF IT IS LESS THAN ONE KM?

A

SHOULD BE MODEL AS DISTRIBUTED PARAMETER OR FREQUENCY DEPENDANT MODEL FOR HARMONIC ANALYSIS

62
Q

HOW TO MODEL CABLE LINE FOR HARMONIC ANALYSIS IF IT IS MORE THAN ONE KM?

A

MODEL IT AS A DISTRBUTED PARAMETER .BECAUSE NOMINAL PI METHOD WONT WORK IF CABLE IS MORE THAN ONE KM AND FREQUENCY OF HARMONICS AND RESONANCE FREQUENCY OF NETWORK ALMOST COINCIDES(VERIFY)

63
Q

IF SYSTEM IS NOT GRID CONNECTED SHOULD WE PERFORM HARMONIC ANALYSIS?

A

HARMONIC ANALYSIS IS NECCESSARY

TO LOOK AT HARMONIC DISTORTION,RESONANCE POINT AND IMPACT OF ELEMENT

64
Q

WHEN WE SHOULD SUGGEST INSTALLING FILTER AT PCC?

A

1) AFTER MEASURING HARMONICS AT SIT

2) DURING DESIGN ANALYSIS PROCESS

65
Q

OTHER PURPOSE OF HARMONIC ANALYSIS

A

TO FIND WHERE RESONANCE FREQUENCY OCCURS

66
Q

RESONANT FREQUENCY

A

?`

67
Q

WHERE SHOULD BE HARMONIC FILTER PLACED

A

FILTERSHOUL BE PLACED NEAR TO HARMONIC SOURCE

68
Q

DOES SATURATION OF TRAFO CAUSE HARMONICS

A

YES BUT FOR SHORT PERIOD OF TIME

69
Q

DO WE NEED TO PERFORM HARMONIC ANALYSIS FOR ISLANDED SYSTEM?WHY?

A

YES BECAUSE SHORT CIRCUIT CURRENT AT ISLANDED SYSTEM IS LOW WHICH LEADS INCREASE IN HARMONICS.ALSO YOU NEED TO PERFORM HARMONIC ANALSIS TO CHECK WHETER DEISEL GENERATOR ARE CAPABLE OF WITHSTANDING HARMONICS

70
Q

HARMONICS ORDER TYPE AND ITS RESULTING CURRENT

A 
1ST ORDER-     +VE SEQUENCE CURRENT
2ND ORDER-   -VE SEQUENCE CURRENT
3RD ORDER-     ZERO SEQUENCE CURRENT
4TH ORDER-     +VE SEQUENCE CURRENT
5TH ORDER-      -VE SEQUENCE CURRENT
6TH ORDER-      ZERO SEQUENCE CURRENT
ETC
71
Q

WHAT IS EFFECT OF NEGATIVE SEQUENCE CURRENT IN GENERATORS?

A

GENERATOR ROTATES IN OPPOSITE DIRECUTION .SO TORQUE(BREAKING) IS GENERATOR WHICH CAUSES THE HEAT TO INCREASE

72
Q

What can harmonics do to transformers?

A

Harmonics generate additional heat in transformer thus reducing the life expectancy of trafo

73
Q

Why is low voltage side generators are solidly earthed?

A

We are going for solidly earthed because fault current will be higher and it could be easily detectable by current principle by over system protection devices.

Lv side has ACB ,MCCB,MCB.now except electronic devices all other like thermal magnetic or mcb cannot detect earth fault

Power System Studies (10 decks)

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