load flow analysis - etap.ir - load flow and panel.pdf · etap converts the branch impedance values...

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


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


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


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


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


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


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


Load 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


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.


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


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


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



• 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


Constant Current Loads


Exponential Load

Polynomial Load

Comprehensive

Load

Generic Loads


Feedback Voltage

•AVR: Automatic Voltage

Regulation

•Fixed: Fixed Excitation

(no AVR action)

Generator Operation Modes


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


Swing Mode

•Governor is operating in

Isochronous mode

•Automatic Voltage Regulator

Voltage Control


Droop Mode

•Automatic Voltage Regulator

Mvar Control


Droop Mode

•Fixed Field Excitation (no AVR

action)

PF Control


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.


• 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


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 LimitMachine Rating

(Power Factor

Point)

Steady State Stability Curve



Load Flow Loading Page

Generator/Power Grid Rating Page

10 Different Generation

Categories for Every

Generator or Power Grid

in the System

Generation Categories


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


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


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


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


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


Adjustments applied

•Individual

•Global

Temperature Correction

• Cable Resistance

• Transmission Line

Resistance

Load Flow Study Case

Adjustment Page


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


Load Flow Example 1

Part 2


Load Flow Alerts


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


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.



Advanced LF Topics

Load 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 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 OLV

Double-Click to drop multiple panels


A panel branch circuit load can be modeled as

an internal or external load

Advantages:

1. Easier Data Entry

2. Concise System

Representation

Representation


Pin 0 is the top pin of the panel

ETAP allows up to 24 external load connections

Pin Assignment


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 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)


3-Phase 4-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


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


Schedule Page

Circuit Numbers with

Column Layout

Circuit Numbers with

Standard Layout


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


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 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.


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

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

load flow analysis - etap.ir - load flow and panel.pdf · etap converts the branch impedance values...

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