Load Flow Study Flashcards
If x >R ,then PF is
Zero
If X
1
Objectives of load flow study
1) Analyze power system under varying opearting condition
2) find power system behaviour
3) Effects(study)of change in V,Load,Q and P
Results of load flow
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
What is Ferranti effect?
.
Importance of load flow study?
1)How will system perform if add future generators or loads
2)to find initial rotor angle for stability studies(generator)
See screen shot
If under voltage what var you use?
Capacitive var
If over voltage,which var you use
Inductive var
Per unit value formula
Actual value /Base value
Charging current is high or less for. Short transmission line?
Less
What is acceleration factor?
M
What is slack bus?
.
What is swing bus?
.
What is convergence?
.
In case of more than one bus what we should mention?
Slack bus
When ferranti effect occurs?
It occurs when there is no load or light load
What is the reason for ferranti effect?
Shunt capacitance of transmission line.so flow of charging current
What is the effect of ferranti effect in DC system?
In DC system there is no capacitance and inductance.so no ferranti effect.
In which ferranti effect is higher,cable or transmission line?
Cable because capacitance of cable is 8 to 10 times higher than the transmission line
Reactive power flow direction
Lower voltage value to higher voltage value (verify)
Real power flows from which direction?
Real power flows from higher angle to lower angle(verify)
HV to LV
P depend on what?
Angle
Q depends on what?
Voltage
How ferranti effect can be reduced?
Reduced by adding shunt reactor or adding load in the receiving end
What is surge impedance loading?
Nautical or characteristic impedance
Reason for SIL?
To get maximum capacity of transmission line
Surge impedance(Zs) formula
Zs= sqr root of (L/C)
What happens to resistance for surge impedance loading?
Neglected
Load=SIL
As
Load
As
Load > SIL
Da
Constant power load formula,eg,k value?
Load takes constant power irrespective of voltage change
P=v^2/z
Eg;Motor
K=0
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
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
For polynomial load what is p1 ,p2 and p3
P1= constant z load P2= constant current load P3 = constant p load
Kpf and Kqf?
..
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
Transformer types
As
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
What happens if you connect 60hz transformer in 50HZ supply?
X L decrease = z decrease = I increases = Flux increases
Why transformer rated in MVA?
1) it is inducing real and reactive power flow that is bi directional
What parameter copper loss depends on ?
Current
What parameter iron loss depends on?
Voltage
Can Trafo provide reactive power?
Yes
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
If the load variation is slow?what is the economical solution?
Variable shunt reactor
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
Standard for reactors
IEC 60076-6
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
No of tap calculation?
No of taps =
1+ {(%max tap-%min tap)/%step)}
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 %
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
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
X/R ratio is high ? What happens to efficiency and power flow?
Efficiency and power flow will be high
R is high ? What happens to efficiency ?
Efficiency is high(verify)
What happens if transformer has low impedance?
Delivere more power and load more
For what reactive power is used?
Used for magnetising (field or current)
Reactive power depends on what?
Voltage
If power demand is more than supplied by Transmission line?
Voltage at receiving end decreases causing tripping of Generator or motor
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
Which power can be sent for long distances?
Real power
Can reactive power can be sent long distances?
No due to high line reactance(resulting voltage gradient)
Can transformer can be used as reactive power control devices?
Yes
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
Effects of reactive power?
Impact the PF Losses increases Current increases Power dissipation increases Occupy equipment loading Uneconomic operation of ps Reduce plant life
Reactive compensation type
Load compensation
Line compensation
Series compensation
Shunt compensation
Where shunt reactor is used?
In long transmission line
Between line and neutral
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
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
Where series compensation is used?
EHV and UHV lines
Uses of series compensation
Improves transfer capability and reduces line reactance
Power transfer equation
(Vs*Vr/X L )sin del
4 modes of operation in Generator?
PF mode
Swing mode
Voltage control mode
Mvar control mode
Uses of VFD
1) To reduce a reactive power requirement from grid by load
2) as standard speed controller
VFD modeling conditions
VFD output > VFD Input
VFD PF < Load PF
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
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
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
