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ETAP Workshop Notes © 1996-2009 Operation Technology, Inc.
Load Flow Analysis
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 2
System Concepts
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 3
jQP
IV
SS
IVS
LL
LN
*
13
*
1
3
3
Lagging Power Factor Leading Power Factor
Inductive loads have lagging Power Factors.
Capacitive loads have leading Power Factors.
Current and Voltage
Power in Balanced 3-Phase
Systems
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 4
Leading
Power
Factor
Lagging
Power
Factor
ETAP displays lagging Power Factors as positive and leading Power Factors
as negative. The Power Factor is displayed in percent.
jQ P
Leading & Lagging Power
Factors
P - jQ P + jQ
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 5
B
2
BB
B
BB
MVA
)kV(Z
kV3
kVAI
B
actualpu
B
actualpu
Z
ZZ
I
II
B
actualpu
B
actualpu
S
SS
V
VV
B
2
BB
B
BB
S
VZ
V3
SI
ZI3V
VI3S If you have two bases:
Then you may calculate the other two
by using the relationships enclosed in
brackets. The different bases are:
•IB (Base Current)
•ZB (Base Impedance)
•VB (Base Voltage)
•SB (Base Power)
ETAP selects for LF:
•100 MVA for SB which is fixed for the
entire system.
•The kV rating of reference point is
used along with the transformer turn
ratios are applied to determine the
base voltage for different parts of the
system.
3-Phase Per Unit System
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 6
Example 1: The diagram shows a simple radial system. ETAP converts the branch
impedance values to the correct base for Load Flow calculations. The LF reports show
the branch impedance values in percent. The transformer turn ratio (N1/N2) is 3.31
and the X/R = 12.14
2
B
1
B kV2N
1NkV
Transformer Turn Ratio: The transformer turn ratio is
used by ETAP to determine the base voltage for different
parts of the system. Different turn ratios are applied starting
from the utility kV rating.
To determine base voltage use:
2
pu
pu
R
X1
R
XZ
X
Transformer T7: The following equations are used to find
the impedance of transformer T7 in 100 MVA base.
R
X
xR
pu
pu
1
BkV
2
BkV
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 7
Impedance Z1: The base voltage is determined by using the transformer turn ratio. The base
impedance for Z1 is determined using the base voltage at Bus5 and the MVA base.
06478.0)14.12(1
)14.12(065.0X
2pu 005336.0
14.12
06478.0R pu
The transformer impedance must be converted to 100 MVA base and therefore the
following relation must be used, where “n” stands for new and “o” stands for old.
)3538.1j1115.0(5
100
5.13
8.13)06478.0j1033.5(
S
S
V
VZZ
2
3
o
B
n
B
2
n
B
o
Bo
pu
n
pu
38.135j15.11Z100Z% pu
0695.431.3
5.13
2N
1N
kVV
utility
B165608.0
100
)0695.4(
MVA
VZ
22
BB
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 8
8.603j38.60Z100Z% pu
)0382.6j6038.0(1656.0
)1j1.0(
Z
ZZ
B
actualpu
The per-unit value of the impedance may be determined as soon as the base
impedance is known. The per-unit value is multiplied by one hundred to obtain
the percent impedance. This value will be the value displayed on the LF report.
The LF report generated by ETAP displays the following percent impedance values
in 100 MVA base
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 9
Load Flow Analysis
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 10
Load Flow Problem
• Given
– Load Power Consumption at all buses
– Configuration
– Power Production at each generator
• Basic Requirement
– Power Flow in each line and transformer
– Voltage Magnitude and Phase Angle at each bus
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 11
Load Flow Studies
• Determine Steady State Operating Conditions
– Voltage Profile
– Power Flows
– Current Flows
– Power Factors
– Transformer LTC Settings
– Voltage Drops
– Generator’s Mvar Demand (Qmax & Qmin)
– Total Generation & Power Demand
– Steady State Stability Limits
– MW & Mvar Losses
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 12
Size & Determine System
Equipment & Parameters• Cable / Feeder Capacity
• Capacitor Size
• Transformer MVA & kV Ratings (Turn Ratios)
• Transformer Impedance & Tap Setting
• Current Limiting Reactor Rating & Imp.
• MCC & Switchgear Current Ratings
• Generator Operating Mode (Isochronous / Droop)
• Generator’s Mvar Demand
• Transmission, Distribution & Utilization kV
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 13
Optimize Operating
Conditions
• Bus Voltages are Within Acceptable Limits
• Voltages are Within Rated Insulation Limits
of Equipment
• Power & Current Flows Do Not Exceed the
Maximum Ratings
• System MW & Mvar Losses are Determined
• Circulating Mvar Flows are Eliminated
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 14
Assume VR
Calc: I = Sload / VR
Calc: Vd = I * Z
Re-Calc VR = Vs - Vd
Calculation Process
• Non-Linear System
• Calculated Iteratively
– Assume the LoadVoltage (Initial Conditions)
– Calculate the Current I
– Based on the Current,Calculate Voltage Drop Vd
– Re-Calculate Load Voltage VR
– Re-use Load Voltage as initial condition until the results are within the specified precision.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 15
1. Accelerated Gauss-Seidel Method
• Low Requirements on initial values,
but slow in speed.
2. Newton-Raphson Method
• Fast in speed, but high requirement on
initial values.
• First order derivative is used to speed up
calculation.
3. Fast-Decoupled Method
• Two sets of iteration equations: real
power – voltage angle,
reactive power – voltage magnitude.
• Fast in speed, but low in solution
precision.
• Better for radial systems and
systems with long lines.
Load Flow Calculation
Methods
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 16
kV
kVAFLA
kV
kVAFLA
EffPF
HP
EffPF
kWkVA
Rated
Rated
RatedRated
1
33
7457.0
Where PF and Efficiency are taken at 100 %
loading conditionskV
kVA1000I
)kV3(
kVA1000I
kVA
kWPF
)kVar()kW(kVA
1
3
22
Load Nameplate Data
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 17
Constant Power Loads
• In Load Flow calculations induction, synchronous and lump loads are treated as constant power loads.
• The power output remains constant even if the input voltage changes (constant kVA).
• The lump load power output behaves like a constant power load for the specified % motor load.
• In Load Flow calculations Static Loads, Lump Loads
(% static), Capacitors and Harmonic Filters and Motor
Operated Valves are treated as Constant Impedance
Loads.
• The Input Power increases proportionally to the
square of the Input Voltage.
• In Load Flow Harmonic Filters may be used as
capacitive loads for Power Factor Correction.
• MOVs are modeled as constant impedance loads
because of their operating characteristics.
Constant Impedance Loads
© 1996-2008 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 18
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 19
• The current remains constant even if the
voltage changes.
• DC Constant current loads are used to test
Battery discharge capacity.
• AC constant current loads may be used to test
UPS systems performance.
• DC Constant Current Loads may be defined in
ETAP by defining Load Duty Cycles used for
Battery Sizing & Discharge purposes.
Constant Current Loads
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 20
Constant Current Loads
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 21
Exponential Load
Polynomial Load
Comprehensive
Load
Generic Loads
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 22
Feedback Voltage
•AVR: Automatic Voltage
Regulation
•Fixed: Fixed Excitation
(no AVR action)
Generator Operation Modes
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 23
Governor Operating Modes
• Isochronous: This governor setting allows the
generator’s power output to be adjusted based on
the system demand.
• Droop: This governor setting allows the generator
to be Base Loaded, meaning that the MW output is
fixed.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 24
Isochronous Mode
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 25
Droop Mode
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 26
Droop Mode
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 27
Droop Mode
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 28
Adjusting Steam Flow
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 29
Adjusting Excitation
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 30
Swing Mode
•Governor is operating in
Isochronous mode
•Automatic Voltage Regulator
Voltage Control
•Governor is operating in
Droop Mode
•Automatic Voltage Regulator
Mvar Control
•Governor is operating in
Droop Mode
•Fixed Field Excitation (no AVR
action)
PF Control
•Governor is operating in
Droop Mode
•AVR Adjusts to Power Factor
Setting
In ETAP Generators and Power Grids have four operating
modes that are used in Load Flow calculations.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 31
• If in Voltage Control Mode, the limits of P & Q are reached, the
model is changed to a Load Model (P & Q are kept fixed)
• In the Swing Mode, the voltage is kept fixed. P & Q can vary
based on the Power Demand
• In the Voltage Control Mode, P & V are kept fixed while Q &
are varied
• In the Mvar Control Mode, P and Q are kept fixed while V &
are varied
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 32
Generator Capability Curve
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 33
Generator Capability Curve
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 34
Generator Capability Curve
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 35
Field Winding Heating Limit
Armature Winding Heating Limit
Machine Rating (Power Factor Point)
Steady State Stability Curve
Maximum & Minimum
Reactive Power
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 36
Field Winding
Heating LimitMachine Rating
(Power Factor
Point)
Steady State Stability Curve
Generator Capability Curve
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 37
Load Flow Loading Page
Generator/Power Grid Rating Page
10 Different Generation
Categories for Every
Generator or Power Grid
in the System
Generation Categories
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 38
X
V)*COS(
X
*VVQ
)(*SINX
*VVP
X
V)(*COS
X
*VVj)(*SIN
X
*VV
jQPI*VS
2
221
21
2121
2
221
2121
21
222
111
VV
VV
Power Flow
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 39
Example: Two voltage sources designated as V1 and V2 are
connected as shown. If V1= 100 /0 , V2 = 100 /30 and X = 0 +j5
determine the power flow in the system.
I
var536535.10X|I|
268j1000)68.2j10)(50j6.86(IV
268j1000)68.2j10(100IV
68.2j10I
5j
)50j6.86(0j100
X
VVI
22
*
2
*
1
21
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 40
2
1
0
1
Real Power Flow
Reactive Power Flow
Power Flow1
2
V E( )
Xsin
V E( )
Xcos
V2
X
0
The following graph shows the power flow from Machine M2. This
machine behaves as a generator supplying real power and
absorbing reactive power from machine M1.
S
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 41
ETAP displays bus voltage values in two ways
•kV value
•Percent of Nominal Bus kV
%83.97100%
5.13
min alNo
Calculated
Calculated
kV
kVV
kV 8.13min alNokV
%85.96100%
03.4
min alNo
Calculated
Calculated
kV
kVV
kV 16.4min alNokV
For Bus4:
For Bus5:
Bus Voltage
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 42
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 43
Lump Load Negative
Loading
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 44
Load Flow Adjustments
• Transformer Impedance
– Adjust transformer impedance based on possible length variation
tolerance
• Reactor Impedance
– Adjust reactor impedance based on specified tolerance
• Overload Heater
– Adjust Overload Heater resistance based on specified tolerance
• Transmission Line Length
– Adjust Transmission Line Impedance based on possible length
variation tolerance
• Cable Length
– Adjust Cable Impedance based on possible length variation tolerance
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 45
Adjustments applied
•Individual
•Global
Temperature Correction
• Cable Resistance
• Transmission Line
Resistance
Load Flow Study Case
Adjustment Page
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 46
Allowable Voltage Drop
NEC and ANSI C84.1
Load Flow Example 1
Part 1
© 1996-2009 Operation Technology, Inc. - Workshop Notes: Load Flow AnalysisSlide 47
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 48
Load Flow Example 1
Part 2
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 49
Load Flow Alerts
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 50
Bus Alerts Monitor Continuous Amps
Cable Monitor Continuous Amps
Reactor Monitor Continuous Amps
Line Monitor Line Ampacity
Transformer Monitor Maximum MVA Output
UPS/Panel Monitor Panel Continuous Amps
Generator Monitor Generator Rated MW
Equipment Overload Alerts
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 51
Protective Devices Monitored parameters % Condition reported
Low Voltage Circuit Breaker Continuous rated Current OverLoad
High Voltage Circuit Breaker Continuous rated Current OverLoad
Fuses Rated Current OverLoad
Contactors Continuous rated Current OverLoad
SPDT / SPST switches Continuous rated Current OverLoad
Protective Device Alerts
If the Auto Display
feature is active, the
Alert View Window will
appear as soon as the
Load Flow calculation
has finished.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 52
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 53
Advanced LF Topics
Load Flow Convergence
Voltage Control
Mvar Control
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 54
Load Flow Convergence
• Negative Impedance
• Zero or Very Small Impedance
• Widely Different Branch Impedance Values
• Long Radial System Configurations
• Bad Bus Voltage Initial Values
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 55
Voltage Control
• Under/Over Voltage Conditions must be
fixed for proper equipment operation and
insulation ratings be met.
• Methods of Improving Voltage Conditions:
– Transformer Replacement
– Capacitor Addition
– Transformer Tap Adjustment
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 56
Under-Voltage Example
• Create Under Voltage
Condition
– Change Syn2 Quantity to 6.
(Info Page, Quantity Field)
– Run LF
– Bus8 Turns Magenta (Under
Voltage Condition)
• Method 1 - Change Xfmr
– Change T4 from 3 MVA to 8
MVA, will notice slight
improvement on the Bus8 kV
– Too Expensive and time
consuming
• Method 2 - Shunt Capacitor
– Add Shunt Capacitor to Bus8
– 300 kvar 3 Banks
– Voltage is improved
• Method 3 - Change Tap
– Place LTC on Primary of T6
– Select Bus8 for Control Bus
– Select Update LTC in the Study Case
– Run LF
– Bus Voltage Comes within specified limits
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 57
Mvar Control
• Vars from Utility
– Add Switch to CAP1
– Open Switch
– Run LF
• Method 1 – Generator
– Change Generator from Voltage Control to Mvar Control
– Set Mvar Design Setting to 5 Mvars
• Method 2 – Add Capacitor
– Close Switch
– Run Load Flow
– Var Contribution from the
Utility reduces
• Method 3 – Xfmr MVA
– Change T1 Mva to 40 MVA
– Will notice decrease in the
contribution from the Utility
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 58
Panel Systems
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 59
Panel Boards
• They are a collection of branch circuits
feeding system loads
• Panel System is used for representing power
and lighting panels in electrical systems
Click to drop once on OLV
Double-Click to drop multiple panels
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 60
A panel branch circuit load can be modeled as
an internal or external load
Advantages:
1. Easier Data Entry
2. Concise System
Representation
Representation
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 61
Pin 0 is the top pin of the panel
ETAP allows up to 24 external load connections
Pin Assignment
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 62
Assumptions
• Vrated (internal load) = Vrated (Panel Voltage)
• Note that if a 1-Phase load is connected to a 3-
Phase panel circuit, the rated voltage of the panel
circuit is (1/√3) times the rated panel voltage
• The voltage of L1 or L2 phase in a 1-Phase 3-Wire
panel is (1/2) times the rated voltage of the panel
• There are no losses in the feeders connecting a
load to the panel
• Static loads are calculated based on their rated
voltage
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 63
Line-Line Connections
Load Connected Between Two Phases of a
3-Phase System
A
B
C
Load
IBCIC = -IBC
A
B
C
LoadB
IB = IBC
Angle by which load current IBC lags the load voltage = θ
Therefore, for load connected between phases B and C:
SBC = VBC.IBC
PBC = VBC.IBC.cos θ
QBC = VBC.IBC.sin θ
For load connected to phase B
SB = VB.IB
PB = VB.IB.cos (θ - 30)
QB = VB.IB.sin (θ - 30)
And, for load connected to phase C
SC = VC.IC
PC = VC.IC.cos (θ + 30)
QC = VC.IC.sin (θ + 30)
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 64
3-Phase 4-Wire Panel
3-Phase 3-Wire Panel
1-Phase 3-Wire Panel
1-Phase 2-Wire Panel
NEC Selection
A, B, C from top to bottom or
left to right from the front of
the panel
Phase B shall be the highest
voltage (LG) on a 3-phase, 4-
wire delta connected system
(midpoint grounded)
Info Page
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 65
Intelligent kV Calculation
If a 1-Phase panel is connected to a 3-Phase bus
having a nominal voltage equal to 0.48 kV, the
default rated kV of the panel is set to (0.48/1.732
=) 0.277 kV
For IEC, Enclosure Type
is Ingress Protection
(IPxy), where IP00 means
no protection or shielding
on the panel
Select ANSI or IEC
Breakers or Fuses from
Main Device Library
Rating Page
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 66
Schedule Page
Circuit Numbers with
Column Layout
Circuit Numbers with
Standard Layout
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 67
Description TabFirst 14 load items in the list are based on NEC 1999
Last 10 load types in the Panel Code Factor Table are user-defined
Load Type is used to determine the Code Factors used in calculating the total
panel load
External loads are classified as motor load or static load according to the
element type
For External links the load status is determined from the connected load’s
demand factor status
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 68
Rating Tab
Enter per phase VA, W, or
Amperes for this load.
For example, if total Watts
for a 3-phase load are
1200, enter W as 400
(=1200/3)
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 69
Loading Tab
For internal loads, enter the % loading for the selected loading category
For both internal and external loads, Amp values are
calculated based on terminal bus nominal kV
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 70
Protective Device Tab
Library Quick Pick -
LV Circuit Breaker
(Molded Case, with
Thermal Magnetic Trip
Device) or
Library Quick Pick –
Fuse will appear
depending on the
Type of protective
device selected.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 71
Feeder Tab
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 72
Action Buttons
Copy the content of the selected
row to clipboard. Circuit number,
Phase, Pole, Load Name, Link
and State are not copied.
Paste the entire content (of the
copied row) in the selected row.
This will work when the Link
Type is other than space or
unusable, and only for fields
which are not blocked.
Blank out the contents of the entire
selected row.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 73
Summary Page
Continuous Load – Per Phase and Total
Non-Continuous Load – Per Phase and Total
Connected Load – Per Phase and Total (Continuous + Non-Continuous Load)
Code Demand – Per Phase and Total
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 74
Output Report
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 75
Panel Code Factors
Code demand load depends on Panel Code Factors
The first fourteen have fixed formats per NEC 1999
Code demand load calculation for internal loads are done
for each types of load separately and then summed up