03 - Load Flow and Panel

(1)

ETAP 5.0

(2)

System Concepts

(3)

jQ

P

I

V

S

I

V

S

LL

LN

+

=

×

=

×

=

*

1

3 *

1

3

3 φ

φ

Inductive loads have lagging Power Factors.

Capacitive loads have leading Power Factors.

Power in Balanced 3-Phase

Systems

(4)

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

₊

P

j Q

P

+

j Q

P

−

(5)

3-Phase Per Unit System

B

2 B

B

MVA

)

kV

(

Z

kV

3 kVA

I

=

B

actual

pu

B

actual

pu

Z

I

=

B

actual

pu

B

actual

pu

S

V

=





















=

















=

B

2 B

B

S

V

Z

V

3 S

I

ZI

3 V

VI

3 S

If you have two bases:

Then you may calculate the other two

by using the relationships enclosed in

brackets. The different bases are:

•I

_B

(Base Current)

•Z

_B

(Base Impedance)

•V

_B

(Base Voltage)

•S

_B

(Base Power)

PowerStation selects for LF:

•100 MVA for S

_B

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.

(6)

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

2 B

1 B

kV

2 N

1 N

kV

=

2 pu

pu

R

X

1 R

X

Z

X









+









×

=

Transformer T7: The following equations are used to find

the impedance of transformer T7 in 100 MVA base.









=

R

X

x

R

_pu

pu

1 B

kV

2 B

kV

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.

(7)

005336

.

0

14 .

12 06478

.

0 R

_pu

=

06478

.

0 )

14 .

12 (

1 )

14 .

12 (

065 .

0 X

2 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

.

1 j

1115

.

0 (

5

100

5 .

13

8 .

13 )

06478

.

0 j

10

33 .

5 (

S

V

Z

2

3 o

B

n

B

2 n

B

o

B

o

pu

n

pu



=

+























+

×

=

























=

−

38 .

135 j

15 .

11 Z

100 Z

%

=

×

_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

.

0

100 )

0695

.

4 (

MVA

V

Z

2

2 B

B

=

0695

.

4

31 .

3

5 .

13

2 N

1 N

kV

V

_B

utility

=









=

(8)

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

.

6 j

6038

.

0 (

1656

.

0 )

1 j

1 .

0 (

Z

B

actual

pu

=

+

=

8 .

603 j

38 .

60 Z

100 Z

%

=

×

_pu

=

+

The LF report generated by PowerStation displays the following percent impedance

values in 100 MVA base

(9)

Load Flow Analysis

(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

(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

(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

(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

(14)

Calculation Process

Assume V

_R

Calc: I = S

_load

/ V

_R

Calc: Vd = I * Z

Re-Calc V

_R

= Vs - Vd

• Non-Linear System

• Calculated Iteratively

– Assume the Load

Voltage (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.

(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

(16)

Load Nameplate Data

kV

kVA

FLA

kV

kVA

FLA

Eff

PF

HP

Eff

PF

kW

kVA

Rated

=

×

=

×

=

×

=

φ

1

3

3 7457

.

0 kV

kVA

1000

I

)

kV

3 (

kVA

1000

I

kVA

kW

PF

)

kVar

(

)

kW

(

kVA

1

3

2

2 ×

=

×

=

+

=

φ

Where PF and Efficiency are taken at 100 %

loading conditions

(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.

(18)

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

(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

PowerStation by defining Load Duty Cycles

used for Battery Sizing & Discharge purposes.

(20)

(21)

Generic Loads

Exponential Load

Polynomial Load

Comprehensive

Load

(22)

Generator Operation Modes

Feedback Voltage

•AVR: Automatic Voltage

Regulation

•Fixed: Fixed Excitation

(no AVR action)

(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.

(24)

(25)

(26)

(27)

(28)

(29)

(30)

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

(31)

• 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 & θ

are varied

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

are varied

• If in Voltage Control Mode, the limits of P & Q are reached, the

(32)

(33)

(34)

(35)

Field Winding Heating Limit

Armature Winding Heating Limit

Machine Rating (Power Factor Point)

Steady State Stability Curve

Maximum & Minimum

Reactive Power

(36)

Generator Capability Curve

Field Winding

Heating Limit

Machine Rating

(Power Factor

Point)

(37)

Generation Categories

Generator/Power Grid Rating Page

Load Flow Loading Page

10 Different Generation

Categories for Every

Generator or Power Grid

in the System

(38)

Power Flow

















∠

=

∠

=

2

1

1 V

V

δ

X

V

)

*COS(

X

*V

V

Q

)

(

*SIN

X

*V

V

P

X

V

)

(

*COS

X

*V

V

j

)

(

*SIN

X

*V

V

jQ

P

I

*

V

S

2

1

2

1

2

1

2

1

2

1

2

1

2

1

2

1 −

−

=

−

=













−

+

−

=

+

=

δ

(39)

Example: Two voltage sources designated as V1 and V2 are

connected as shown. If V

₁

= 100 /0° , V

₂

= 100 /30° and X = 0 +j5

determine the power flow in the system.

I

var

536

5

35 .

10 X

|

I

|

268 j

1000

)

68 .

2 j

10 )(

50 j

6 .

86 (

I

V

268 j

1000

)

68 .

2 j

10 (

100 I

V

68 .

2 j

10 I

5 j

)

50 j

6 .

86 (

0 j

100 X

V

I

2

2 *

1

2

1 =

×

=

−

=

+

−

+

=

+

−

=

+

−

=

−

=

+

−

+

=

−

=

(40)

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 Flow

Reactive Power Flow

Power Flow

1

2 −

V E

⋅

(

)

X

⋅

sin

( )

δ

∆

V E

⋅

(

)

X

⋅

cos

( )

δ

∆

V

2 X

−

π

0 δ

_∆

S

(41)

Bus Voltage

PowerStation displays bus voltage values in two ways

•kV value

•Percent of Nominal Bus kV

%

83 .

97

100 %

5 .

13 min

=

×

=

al

No

Calculated

kV

V

kV

_No

_min

_al

=

13 .

8 For Bus4:

For Bus5:

%

85 .

96

100 %

03 .

4 min

=

×

=

al

No

Calculated

kV

V

kV

_No

_min

_al

=

4 .

16

(42)

(43)

Lump Load Negative

Loading

(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

(45)

Load Flow Study Case

Adjustment Page

Adjustments applied

•Individual

•Global

Temperature Correction

• Cable Resistance

• Transmission Line

Resistance

(46)

Allowable Voltage Drop

NEC and ANSI C84.1

(47)

Power Grid

1000 MVAsc

X/R = 22

Gen1

10 MW

Voltage Control

Design:

%Pf = 85

MW = 5

Max Q = 4

Min Q = -1

Impedance

Z1

13.8 kV

100MVA

% Z = 0.01+j1

Load Flow Example 1

Part 1 Transformers

T1 = 30 MVA

T2 = 15 MVA

T3 = 5 MVA

T4 = 3 MVA

Select typical %Z &

X/R

Cable1

ICEA 15kV 3/C CU,

100%

Size= 250

Length= 400 ft

Cable2

KERITE 5kV 3/C

CU, 100%

Size= 500

Length= 300 ft

(48)

Load Flow Example 1

Part 2 Cable3

ICEA 5kV 3/C

CU, 133%

Size= 500

Length= 100 ft

Transformer

T5 = 5 MVA

Select typical %Z

& X/R

(49)

(50)

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

(51)

Protective Device 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

(52)

If the Auto Display

feature is active, the

Alert View Window

will appear as soon as

the Load Flow

calculation has

finished.

(53)

Advanced LF Topics

Load Flow Convergence

Voltage Control

(54)

Load Flow Convergence

• Negative Impedance

• Zero or Very Small Impedance

• Widely Different Branch Impedance Values

• Long Radial System Configurations

(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

(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

(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

(58)

Panel Systems

(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

(60)

Representation

A panel branch circuit load can be modeled as

an internal or external load

Advantages:

1. Easier Data Entry

2. Concise System

(61)

Pin Assignment

Pin 0 is the top pin of the panel

(62)

Assumptions

• V

_rated

(internal load) = V

_rated

(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

(63)

Line-Line Connections

Load Connected Between Two Phases of a

3-Phase System

A

B

C

Load

I

_BC

I

C

= -I

BC

A

B

C

LoadB

I

_B

= I

_BC

Angle by which load current I

_BC

lags the load voltage = θ°

Therefore, for load connected between phases B and C:

S

_BC

= V

_BC

.I

_BC

P

_BC

= V

_BC

.I

_BC

.cos θ

Q

_BC

= V

_BC

.I

_BC

.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)

(64)

Info Page

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)

3-Phase 4-Wire Panel

3-Phase 3-Wire Panel

1-Phase 3-Wire Panel

1-Phase 2-Wire Panel

(65)

Rating 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

(66)

Schedule Page

Circuit Numbers with

Standard Layout

Circuit Numbers with

Column Layout

(67)

Description Tab

First 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

(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)

(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

(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.

(71)

(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.

(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)

(74)

(75)

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