The document discusses load flow analysis calculations and transformer parameters. It explains how to calculate the X/R ratio of a transformer using nameplate data like impedance and losses. It also describes how to size transformers based on standards by considering cooling type, altitude, temperature, load variation, and short-circuit requirements. The document shows the load flow calculation process using vector diagrams and equations, comparing hand calculations to results from the ETAP software.

1. Chapter 2- Part two
Load flow analysis
Computer Application in Power Systems
Chapter Two :- Load flow Analysis
Golis University
Abdiaziz Hussein

2. Transformer X/R Ratio Calculation

3. Transformer X/R Ratio Calculation
 X/R ratio is not available in transformer data sheet ??
 Calculate using Copper losses / Full load losses provided in data sheet
 Transformer Datasheet:
 MVA Rating = 25 MVA (shall be assumed as Base MVA)
 % Impedance (Z) = 8%
 Copper / Full load losses = 100 kW (0.1 MW)
 % Full load losses = (Full load losses (MW) / Base MVA) * 100 %
= (0.1 / 25)*100
% Full load losses = 0.4 % = % R
 % X = Sqrt (Z2 – R2)
= Sqrt (82 – 0.42)
% X = 7.9899 %
 X/R = (7.9899) / 0.4 = 19.974

4. Transformer MVA Sizing

5. Transformer MVA Sizing
 Based on ANSI C57 and IEC 60076 standards
 Considers multiple cooling stages and transformer types
(3-phase & 1-phase)
 Considers altitude correction factors
 Considers temperature correction factors
 Load variation
 Considers short-circuit impedance requirements
 Use connected load or operating load

6. Transformer MVA Page in ETAP

7. Temperature Correction
 Ambient temperature in degree Celsius
 For air cooled transformers use the air ambient temperature
 For water cooled transformers use the water temperature
 Based on C57.92-1981 and IEC 76-2

8. Altitude Correction
 Derate the transformer capacity based on altitude of the installation
 Derating starts above 1000 m or 3300 ft
 Based on C57.92-1981 and IEC 76-2

9. Load Variation
 Growth Factor
Allowance for future load growth
 Load Factor
The ratio of the average load to the peak load over a
designated period of time
 Load Factor = 100 x ( kWi x Ti ) / ( kWp x Tt )%
 Load Factor = 100 x E / ( kWi x Tt )%

10. Short Circuit Impedance

11. Connected / Operating Load
 Connected Load
 Determine the radial load connected to the transformer secondary
 Can include spare loads for conservative results
 Operating Load
 PowerStation’s load flow program determines the power flow through
the transformer
 Automatically updated in the transformer required MVAfield (load
flow study case info page option)
 Can determine loading in radial or looped systems

12. Sizing Results & Automatic Update
 Sizing results include MVAand % Z for
 Required Size (Actual calculated Size)
 Next Standard Larger Size
 Next Standard Smaller Size
 Automatically Update the calculated rated MVA, Max MVA and % Z into the
Rating page of the transformer

13. Generator load Sharing

14. Generator load Sharing

16. Load Flow Analysis Calculation
(Comparing Hand Calculations with ETAP Results)

17. How Load Flow Calculation Works?
O
VS
I
VR
IR
IX
δ
Ф Ф
A
D
C
E
∆V
δV
IRCosФB
IRSinФ
IXSinФ (BE = BC)
IXCosФ
L
VS VR
Utility
∆V = Voltage Drop
Load

18. O
VS
I
VR
IR
IX
δ
Ф Ф
A
How Load Flow Calculation Works?
D
C
E
∆V
δV
IRCosФB
OD = VS, OA = VR
OD2 = OC2 + CD2
OD2 = (OA+AB+BC)2 + (DE-CE)2
AB = IR COSФ, BC = IX SINФ
DE = IX COSФ, CE = IR SINФ
VS2 = (VR + IR COSФ + IXSINФ)2 + (IX COSФ – IR SINФ)2
= (VR + (R*P + X*Q)/VR)2 + (X*P – R*Q)/VR)2
= √{(VR + (R*P + X*Q)/VR)2 + (X*P – R*Q)/VR)2}
= CD / OC = (DE – CE) / (OA + AB + BC)
= (IX COSФ - IR SINФ) / (VR + IR COSФ + IX SINФ)
= ((X*P – R*Q)/VR) / (VR + (R*P + X*Q)/VR)
= ATAN [((X*P – R*Q)/VR) / (VR + (R*P + X*Q)/VR)] Radian
VS
T
ANδ
δ
IRSinФ
IXSinФ (BE = BC)
IXCosФ

19. How Load Flow Calculation Works?
Grid Data
Grid Voltage = 33kV
Bus Data
Bus Voltage = 33kV
Transformer Data
Voltage Rating – 33/11.55 kV
MVA Rating – 15 MVA
% Impedance – 10%
X/R Ratio – 18.6
Load Data
Voltage Rating – 11 kV
, MVA Rating – 12 MVA
Motor Load - 80%, Static Load – 20%
Power Factor – 80%