03 - Load Flow and Panel

ETAP 5.0ETAP 5.0

Copyright 2003 Operation Technology, Inc.

Load Flow Analysis

Copyright 2004 Operation Technology, Inc. – Workshop Notes: Load Flow Analysis Slide 2

System ConceptsSystem Concepts


jQPIV

SS

IVS

LL

LN

+=×=

×=

=

*

13

*1

3

3 φφ

φ

Inductive loads have lagging Power Factors. Capacitive loads have leading Power Factors.

Power in Balanced 3-Phase Systems

Lagging Power Factor Leading Power Factor Current and Voltage


Leading & Lagging Power Factors

PowerStation displays lagging Power Factors as positive and leading Power Factors as negative. The Power Factor is displayed in percent.

Leading Power Factor

Lagging Power Factor

jQ P +jQ P +jQ P −


3-Phase Per Unit System

B

2B

B

B

BB

MVA)kV(Z

kV3kVAI

=

=

B

actualpu

B

actualpu

ZZZ

III

=

=

B

actualpu

B

actualpu

SSS

VVV

=

=

=

=

=

=

B

2B

B

B

BB

SVZ

V3SI

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)

PowerStation 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 turnratios are applied to determine the base voltage for different parts of the system.


Example 1: The diagram shows a simple radial system. PowerStation 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

2B

1B kV

2N1NkV =

2

pu

pu

RX1

RXZ

X

+

×

=

Transformer T7: The following equations are used to find the impedance of transformer T7 in 100 MVA base.

=

RX

xR pu

pu

1BkV

2BkV

Transformer Turn Ratio: The transformer turn ratio is used by PowerStation 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:


005336.014.12

06478.0R pu ==06478.0)14.12(1)14.12(065.0X2pu =

+=

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

1005.138.13)06478.0j1033.5(

SS

VVZZ

23

oB

nB

2

nB

oBo

punpu +=

+×=

= −

38.135j15.11Z100Z% pu +=×=

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.

165608.0100

)0695.4(MVA

VZ22

BB ===0695.4

31.35.13

2N1N

kVV utility

B ==

=


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.

)0382.6j6038.0(1656.0

)1j1.0(Z

ZZB

actualpu +=

+==

8.603j38.60Z100Z% pu +=×=

The LF report generated by PowerStation displays the following percent impedance values in 100 MVA base


Load Flow AnalysisLoad Flow Analysis


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


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


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


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


Calculation Process

Assume VR

Calc: I = Sload / VR

Calc: Vd = I * Z

Re-Calc VR = Vs - Vd

• 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.


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


Load Nameplate Data

kVkVAFLA

kVkVAFLA

EffPFHP

EffPFkWkVA

Rated

Rated

RatedRated

=

×=

××

=×

=

φ

φ

1

3 3

7457.0

kVkVA1000I

)kV3(kVA1000I

kVAkWPF

)kVar()kW(kVA

1

3

22

×=

××=

=

+=

φ

φ

Where PF and Efficiency are taken at 100 % loading conditions


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.

Constant Impedance Loads• 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.


• 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 PowerStation by defining Load Duty Cycles used for Battery Sizing & Discharge purposes.

Constant Current Loads


Constant Current Loads


Generic Loads

Exponential Load

Polynomial Load

Comprehensive Load


Generator Operation Modes

Feedback Voltage •AVR: Automatic Voltage Regulation•Fixed: Fixed Excitation (no AVR action)


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.


Isochronous Mode


Droop Mode


Adjusting Steam Flow


Adjusting Excitation


In PowerStation Generators and Power Grids have four operating modes that are used in Load Flow calculations.

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 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 & θare varied

• In the Mvar Control Mode, P and Q are kept fixed while V & θare varied

• If in Voltage Control Mode, the limits of P & Q are reached, themodel is changed to a Load Model (P & Q are kept fixed)


Generator Capability Curve


Field Winding Heating Limit

Armature Winding Heating Limit

Machine Rating (Power Factor Point)

Steady State Stability Curve

Maximum & Minimum Reactive Power



Field Winding Heating Limit Machine Rating

(Power Factor Point)

Steady State Stability Curve


Generation CategoriesGenerator/Power Grid Rating Page

Load Flow Loading Page

10 Different Generation Categories for Every Generator or Power Grid in the System


Power Flow

∠=

∠=

222

111

VV

VV

δ

δ

XV)*COS(

X*VVQ

)(*SINX*VVP

XV)(*COS

X*VVj)(*SIN

X*VV

jQPI*VS

22

2121

2121

22

2121

2121

−−=

−=

−−+−=

+==

δδ

δδ

δδδδ


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

68.2j10I5j

)50j6.86(0j100X

VVI

22

*2

*1

21

=×=

−−=+−+=

+−=+−=

−−=

+−+=

−=


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.

2

1

0

1

Real Power FlowReactive Power Flow

Power Flow1

2−

V E⋅( )X

sin δ ∆( )⋅

V E⋅( )X

cos δ ∆( )⋅V2

X−

π0 δ ∆

S


Bus VoltagePowerStation displays bus voltage values in two ways

•kV value

•Percent of Nominal Bus kV

%83.97100%

5.13

min

=×=

=

alNo

Calculated

Calculated

kVkVV

kV 8.13min =alNokVFor Bus4:

For Bus5:

%85.96100%

03.4

min

=×=

=

alNo

Calculated

Calculated

kVkVV

kV 16.4min =alNokV


Lump Load Negative Loading


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


Load Flow Study Case Adjustment Page

Adjustments applied

•Individual

•Global

Temperature Correction

• Cable Resistance

• Transmission LineResistance


Allowable Voltage DropNEC and ANSI C84.1

Power Grid1000 MVAscX/R = 22

Gen110 MWVoltage ControlDesign:%Pf = 85MW = 5 Max Q = 4Min Q = -1

ImpedanceZ113.8 kV 100MVA% Z = 0.01+j1

Load Flow Example 1 Part 1

TransformersT1 = 30 MVAT2 = 15 MVAT3 = 5 MVAT4 = 3 MVASelect typical %Z & X/R

Cable1ICEA 15kV 3/C CU, 100% Size= 250Length= 400 ft

Cable2KERITE 5kV 3/C CU, 100% Size= 500Length= 300 ft


Load Flow Example 1 Part 2

Cable3ICEA 5kV 3/C CU, 133% Size= 500Length= 100 ft

TransformerT5 = 5 MVASelect typical %Z & X/R


Load Flow Alerts


Equipment Overload Alerts

Bus Alerts Monitor Continuous Amps

Cable Monitor Continuous Amps

Reactor Monitor Continuous Amps

Line Monitor Line Ampacity

Transformer Monitor Maximum MVA Output

DC Link DC Link Loading Capability (Idc, Max. MVA)

Panel Monitor Panel Continuous Amps

Generator Monitor Generator Rated MW


Protective Device Alerts

Protective Devices Monitored parameters % Condition reported

Low Voltage Circuit Breaker Continuous rated Current OverLoadHigh Voltage Circuit Breaker Continuous rated Current OverLoad

Fuses Rated Current OverLoadContactors Continuous rated Current OverLoad

SPDT / SPST switches Continuous rated Current OverLoad

If the Auto Display feature is active, the Alert View Window will appear as soon as the Load Flow calculation has finished.


Advanced LF TopicsAdvanced LF TopicsLoad Flow Convergence

Voltage Control

Mvar Control


Load Flow Convergence

• Negative Impedance

• Zero or Very Small Impedance

• Widely Different Branch Impedance Values

• Long Radial System Configurations

• Bad Bus Voltage Initial Values


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


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


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


Panel SystemsPanel Systems


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 OLVDouble-Click to drop multiple panels


RepresentationA panel branch circuit load can be modeled as an internal or external load

Advantages:1. Easier Data Entry2. Concise System

Representation


Pin AssignmentPin 0 is the top pin of the panelETAP allows up to 24 external load connections


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


Line-Line ConnectionsLoad Connected Between Two Phases of a3-Phase System

A

BC

Load

IBCIC = -IBC

ABC

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.IBCPBC = VBC.IBC.cos θQBC = VBC.IBC.sin θ

For load connected to phase B

SB = VB.IBPB = VB.IB.cos (θ - 30)QB = VB.IB.sin (θ - 30)

And, for load connected to phase C

SC = VC.ICPC = VC.IC.cos (θ + 30)QC = VC.IC.sin (θ + 30)


Info Page

NEC SelectionA, 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)

3-Phase 4-Wire Panel3-Phase 3-Wire Panel1-Phase 3-Wire Panel1-Phase 2-Wire Panel


Rating Page

Intelligent kV CalculationIf 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


Schedule Page

Circuit Numbers with

Standard Layout

Circuit Numbers with Column Layout


Description TabFirst 14 load items in the list are based on NEC 1999Last 10 load types in the Panel Code Factor Table are user-definedLoad Type is used to determine the Code Factors used in calculating the total panel loadExternal loads are classified as motor load or static load according to the element typeFor External links the load status is determined from the connected load’s demand factor status


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)


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


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.


Feeder Tab


Action ButtonsCopy 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.


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


Output Report


Panel Code Factors

The first fourteen have fixed formats per NEC 1999

Code demand load depends on Panel Code Factors

Code demand load calculation for internal loads are done for each types of load separately and then summed up

03 - Load Flow and Panel

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