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Power System Studies > Load Flow Study > Flashcards

Load Flow Study Flashcards

1
Q

If x >R ,then PF is

A

Zero

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2
Q

If X

A

1

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3
Q

Objectives of load flow study

A

1) Analyze power system under varying opearting condition
2) find power system behaviour
3) Effects(study)of change in V,Load,Q and P

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4
Q

Results of load flow

A

1) real and reactive power flow
2) power flow in each branch circuit
3) Source loading
4) Voltage magnitude
5) voltage drop
6) Phase angles
7) Transformer Tap setting
8) power loss
9) Generator exciter voltage set points
10) Generator regulator voltage set points
11) under and over voltage condition

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5
Q

What is Ferranti effect?

A

.

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6
Q

Importance of load flow study?

A

1)How will system perform if add future generators or loads
2)to find initial rotor angle for stability studies(generator)
See screen shot

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7
Q

If under voltage what var you use?

A

Capacitive var

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8
Q

If over voltage,which var you use

A

Inductive var

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9
Q

Per unit value formula

A

Actual value /Base value

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10
Q

Charging current is high or less for. Short transmission line?

A

Less

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11
Q

What is acceleration factor?

A

M

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12
Q

What is slack bus?

A

.

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13
Q

What is swing bus?

A

.

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14
Q

What is convergence?

A

.

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15
Q

In case of more than one bus what we should mention?

A

Slack bus

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16
Q

When ferranti effect occurs?

A

It occurs when there is no load or light load

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17
Q

What is the reason for ferranti effect?

A

Shunt capacitance of transmission line.so flow of charging current

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18
Q

What is the effect of ferranti effect in DC system?

A

In DC system there is no capacitance and inductance.so no ferranti effect.

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19
Q

In which ferranti effect is higher,cable or transmission line?

A

Cable because capacitance of cable is 8 to 10 times higher than the transmission line

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20
Q

Reactive power flow direction

A

Lower voltage value to higher voltage value (verify)

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21
Q

Real power flows from which direction?

A

Real power flows from higher angle to lower angle(verify)

HV to LV

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22
Q

P depend on what?

A

Angle

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23
Q

Q depends on what?

A

Voltage

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24
Q

How ferranti effect can be reduced?

A

Reduced by adding shunt reactor or adding load in the receiving end

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25
Q

What is surge impedance loading?

A

Nautical or characteristic impedance

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26
Q

Reason for SIL?

A

To get maximum capacity of transmission line

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27
Q

Surge impedance(Zs) formula

A

Zs= sqr root of (L/C)

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28
Q

What happens to resistance for surge impedance loading?

A

Neglected

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29
Q

Load=SIL

A

As

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30
Q

Load

A

As

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31
Q

Load > SIL

A

Da

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32
Q

Constant power load formula,eg,k value?

A

Load takes constant power irrespective of voltage change

P=v^2/z

Eg;Motor

K=0

33
Q

Constant impedance load formula,eg,k value?

A

It will maintain constant impedance of load with respective of change in squares of voltage

I=p/V

I.e : heating and lighting

K=1

34
Q

Constant current load formula,eg,k value?

A

It will maintain constant current with respective of change in voltage

P=VI

I.e:UPS,VFD,Rectifier

35
Q

For polynomial load what is p1 ,p2 and p3

A 
P1= constant z load
P2= constant current load
P3 = constant p load
36
Q

Kpf and Kqf?

A

..

37
Q

Uses of transformer?

A

Isolation

Impedance matching - amplifiers

Grounding/earthing- zigzag TRF

To reduce short circuit- lighting TFR

VFD -12 p need 30 def phase shift by phase shifting transformer

Convert

38
Q

Transformer types

A

As

39
Q

If you connect 50HZ TRF in 60hz supply?what will happen?

A

Transformer (core)losses increases

X L ^ = f L ^
X L ^ = f L ^

Current decreases so power losses increases

Voltage and power rating decrease

40
Q

What happens if you connect 60hz transformer in 50HZ supply?

A

X L decrease = z decrease = I increases = Flux increases

41
Q

Why transformer rated in MVA?

A

1) it is inducing real and reactive power flow that is bi directional

42
Q

What parameter copper loss depends on ?

A

Current

43
Q

What parameter iron loss depends on?

A

Voltage

44
Q

Can Trafo provide reactive power?

A

Yes

45
Q

What should you do if connect 60 Hz TRF in 50 hz supply?

A

B=E/F

1) do , B must be constant.it can be done by changing E
2) decrease voltage and increase frequency and power
3) We cannot maintain v/f constant ,but you can use by changing voltage proportional to frequency

46
Q

If the load variation is slow?what is the economical solution?

A

Variable shunt reactor

47
Q

Surge impedance?

A

An impedance which renders the line as infinite line is known as surge impedance.

For OH Line it is 400ohms
For underground cables 40ohms

48
Q

Standard for reactors

A

IEC 60076-6

49
Q

In order for the LTC to work properly .

A

The sum of the upper band and lower bands to be larger than it or equal to OLTC step

50
Q

No of tap calculation?

A

No of taps =

1+ {(%max tap-%min tap)/%step)}

51
Q

After load flow operating voltage at bus 2 is 110.7%.what is max tap setting?

A

Max tolerance voltage at bus 2 is 105%

So max tap is = 110.7-105= 5.7 %

52
Q

For transformer x/r calculation what formula are used?

A

R=
copper loss or load loss/TRF mva

X = sqr root of (z^2 -R^2)

X/R = value

53
Q

Conditions for paralleling of transformer?

A

Paralleled TRF must have Similar

1) vector group**(phase shift)
2) voltage rating **
3) Impedance(loads shared)
4) Taps same
5) X/R ratio same
6) Power rating

54
Q

X/R ratio is high ? What happens to efficiency and power flow?

A

Efficiency and power flow will be high

55
Q

R is high ? What happens to efficiency ?

A

Efficiency is high(verify)

56
Q

What happens if transformer has low impedance?

A

Delivere more power and load more

57
Q

For what reactive power is used?

A

Used for magnetising (field or current)

58
Q

Reactive power depends on what?

A

Voltage

59
Q

If power demand is more than supplied by Transmission line?

A

Voltage at receiving end decreases causing tripping of Generator or motor

60
Q

If power demand is less than reactive power supplied by Transmission line?

A

Receiving end voltage increases causing tripping of transmission line equipment,leading PF,insulation failure of winding and cables

61
Q

Which power can be sent for long distances?

A

Real power

62
Q

Can reactive power can be sent long distances?

A

No due to high line reactance(resulting voltage gradient)

63
Q

Can transformer can be used as reactive power control devices?

A

Yes

64
Q

Reactive power source?

A

Dynamic(high cost and rotating part)

1) synchronous generator
2) synchronous condenser
3) SSD/converters/abc/Facts

Static(low cost and no rotating part)

1) capacitive and inductive compensator
2) ug and oh lines
3) PC system

65
Q

Effects of reactive power?

A 
Impact the PF
Losses increases 
Current increases 
Power dissipation increases
Occupy equipment loading
Uneconomic operation of ps
Reduce plant life
66
Q

Reactive compensation type

A

Load compensation
Line compensation
Series compensation
Shunt compensation

67
Q

Where shunt reactor is used?

A

In long transmission line

Between line and neutral

68
Q

Advantage and disadvantage of shunt compensation(cap bank)

A

Advantages

1) static equipment
2) less loss

Disadvantage

1) reactive power is not controllable
2) not ideal device to regulate voltages

69
Q

Advantages and disadvantages of synchronous condenser?

A

Advantages
1)q is controllable
2)v^ ,q absorbed
V decreases , q supplied

Disadvantage

1) rotating part
2) high losses
3) maintenance requires

70
Q

Where series compensation is used?

A

EHV and UHV lines

71
Q

Uses of series compensation

A

Improves transfer capability and reduces line reactance

72
Q

Power transfer equation

A

(Vs*Vr/X L )sin del

73
Q

4 modes of operation in Generator?

A

PF mode
Swing mode
Voltage control mode
Mvar control mode

74
Q

Uses of VFD

A

1) To reduce a reactive power requirement from grid by load

2) as standard speed controller

75
Q

VFD modeling conditions

A

VFD output > VFD Input

VFD PF < Load PF

76
Q

What is k rated transformers?

A

The k rated transformers are the transformers designed to withstand the effects of harmonic currents without exceeding the temperature rating of insulation system

77
Q

Why is lv 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

78
Q

Why don’t we go for resistance earthing for generator in lv system?

A

Because when SLG fault occurs fault current value will be very little and it cannot be sensed or detected by LV system protection devices

Power System Studies (10 decks)

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