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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
What is surge impedance loading?
Nautical or characteristic impedance
26
Reason for SIL?
To get maximum capacity of transmission line
27
Surge impedance(Zs) formula
Zs= sqr root of (L/C)
28
What happens to resistance for surge impedance loading?
Neglected
29
Load=SIL
As
30
Load
As
31
Load > SIL
Da
32
Constant power load formula,eg,k value?
Load takes constant power irrespective of voltage change P=v^2/z Eg;Motor K=0
33
Constant impedance load formula,eg,k value?
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
Constant current load formula,eg,k value?
It will maintain constant current with respective of change in voltage P=VI I.e:UPS,VFD,Rectifier
35
For polynomial load what is p1 ,p2 and p3
``` P1= constant z load P2= constant current load P3 = constant p load ```
36
Kpf and Kqf?
..
37
Uses of transformer?
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
Transformer types
As
39
If you connect 50HZ TRF in 60hz supply?what will happen?
Transformer (core)losses increases X L ^ = f L ^ X L ^ = f L ^ Current decreases so power losses increases Voltage and power rating decrease
40
What happens if you connect 60hz transformer in 50HZ supply?
X L decrease = z decrease = I increases = Flux increases
41
Why transformer rated in MVA?
1) it is inducing real and reactive power flow that is bi directional
42
What parameter copper loss depends on ?
Current
43
What parameter iron loss depends on?
Voltage
44
Can Trafo provide reactive power?
Yes
45
What should you do if connect 60 Hz TRF in 50 hz supply?
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
If the load variation is slow?what is the economical solution?
Variable shunt reactor
47
Surge impedance?
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
Standard for reactors
IEC 60076-6
49
In order for the LTC to work properly .
The sum of the upper band and lower bands to be larger than it or equal to OLTC step
50
No of tap calculation?
No of taps = | 1+ {(%max tap-%min tap)/%step)}
51
After load flow operating voltage at bus 2 is 110.7%.what is max tap setting?
Max tolerance voltage at bus 2 is 105% So max tap is = 110.7-105= 5.7 %
52
For transformer x/r calculation what formula are used?
R= copper loss or load loss/TRF mva X = sqr root of (z^2 -R^2) X/R = value
53
Conditions for paralleling of transformer?
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
X/R ratio is high ? What happens to efficiency and power flow?
Efficiency and power flow will be high
55
R is high ? What happens to efficiency ?
Efficiency is high(verify)
56
What happens if transformer has low impedance?
Delivere more power and load more
57
For what reactive power is used?
Used for magnetising (field or current)
58
Reactive power depends on what?
Voltage
59
If power demand is more than supplied by Transmission line?
Voltage at receiving end decreases causing tripping of Generator or motor
60
If power demand is less than reactive power supplied by Transmission line?
Receiving end voltage increases causing tripping of transmission line equipment,leading PF,insulation failure of winding and cables
61
Which power can be sent for long distances?
Real power
62
Can reactive power can be sent long distances?
No due to high line reactance(resulting voltage gradient)
63
Can transformer can be used as reactive power control devices?
Yes
64
Reactive power source?
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
Effects of reactive power?
``` Impact the PF Losses increases Current increases Power dissipation increases Occupy equipment loading Uneconomic operation of ps Reduce plant life ```
66
Reactive compensation type
Load compensation Line compensation Series compensation Shunt compensation
67
Where shunt reactor is used?
In long transmission line | Between line and neutral
68
Advantage and disadvantage of shunt compensation(cap bank)
Advantages 1) static equipment 2) less loss Disadvantage 1) reactive power is not controllable 2) not ideal device to regulate voltages
69
Advantages and disadvantages of synchronous condenser?
Advantages 1)q is controllable 2)v^ ,q absorbed V decreases , q supplied Disadvantage 1) rotating part 2) high losses 3) maintenance requires
70
Where series compensation is used?
EHV and UHV lines
71
Uses of series compensation
Improves transfer capability and reduces line reactance
72
Power transfer equation
(Vs*Vr/X L )sin del
73
4 modes of operation in Generator?
PF mode Swing mode Voltage control mode Mvar control mode
74
Uses of VFD
1) To reduce a reactive power requirement from grid by load | 2) as standard speed controller
75
VFD modeling conditions
VFD output > VFD Input | VFD PF < Load PF
76
What is k rated transformers?
The k rated transformers are the transformers designed to withstand the effects of harmonic currents without exceeding the temperature rating of insulation system
77
Why is lv side generators are solidly earthed?
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
Why don’t we go for resistance earthing for generator in lv system?
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 (11 decks)

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