TechRef_Load[1].pdf

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D I g S I L E N T T e c h n i c a lD o c u m e n t a t i o n

General Load Model


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G e n e r a l L o a d M o d e l

DIgSILENT GmbH

Heinrich-Hertz-Strasse 9

D-72810 Gomaringen

Tel.: +49 7072 9168 - 0

Fax: +49 7072 9168- 88

http://www.digsilent.de

e-mail: [email protected]

General Load Model

Published by

DIgSILENT GmbH, Germany

Copyright 2008. All rights

reserved. Unauthorised copying

or publishing of this or any part

of this document is prohibited.

TechRef ElmLod V4

Build 507 30.10.2008


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T a b l e o f C o n t e n t s


Table of Contents

1 General Description........................................................................................................................... 4

1.1 Load-Flow Analysis ................................................................................................................................... 41.1.1 Balanced Load-Flow.............................................................................................................................51.1.2 Unbalanced Load-Flow .........................................................................................................................51.1.2.1 3-phase loads .................................................................................................................................61.1.2.2 2-Phase Loads ................................................................................................................................71.1.2.3 1-phase loads .................................................................................................................................8

1.1.3 DC loads .............................................................................................................................................91.1.4 Voltage dependency........................................................................................................................... 101.1.5 Load Scaling factors........................................................................................................................... 12

1.2 Short-Circuit Analysis .............................................................................................................................. 14

1.3 Harmonic Analysis................................................................................................................................... 151.3.1 Passive Load...................................................................................................................................... 151.3.2 Harmonic Current Injections............................................................................................................... 17

1.4 RMS Simulation (Transient Stability) ........................................................................................................ 18

1.5 EMT Simulation....................................................................................................................................... 21

2 Input/Output Definition of the Dynamic Model.............................................................................. 22

2.1 Three-Phase Load................................................................................................................................... 222.1.1 RMS-Simulation ................................................................................................................................. 222.1.2 EMT-Simulation ................................................................................................................................. 23

2.2 Single-, Two-Phase Load and DC-Load..................................................................................................... 23

3 Parameter Definitions ..................................................................................................................... 24

3.1 General Load Type (TypLod) ................................................................................................................... 24

3.2 General Load Element (ElmLod)............................................................................................................... 25


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1General Description

In power systems, electrical load consists of various different types of electrical devices, from incandescent lamps

and heaters to large arc furnaces and motors. It is often very difficult to identify the exact composition of static

and dynamic loads in the network. This load composition can also vary depending on factors such as the season,

time of day etc.

Additionally, the term loadcan be used for entire MV-feeders in case of an HV-system or LV-feeders if an MV-

system is in the centre of interest.

The PowerFactorymodel General Load can therefore represent:

A complete feeder

A combination of dynamic and static loads

The general load model diagram is shown in Figure 1 below.

Figure 1: DIgSILENT General Load Model

1.1

Load-Flow Analysis

In the element window for the load, the user is free to choose whether the load is balanced or unbalanced.

Furthermore the user can specify the input parameters for the load using the Input Modedrop down menu asshown in Figure 2 below. Based on the available data, the user can select the relevant combination of parameters

from S (apparent power), P (real power), Q (reactive power), cos(phi) (power factor) and I (current).

For load-flow analysis, it suffices to only specify the load's electrical consumption.

Other data characterizing a load, such as the number of phases or voltage dependency factors (see also 1.1.3)

are defined in the Load Type. If no load type is specified, a balanced, three-phase load is assumed having default

parameters for voltage dependency kpu=1.6 and kqu=1.8 (see also section 1.1.4).


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Figure 2: Specifying the input parameters for the load model

1.1.1Balanced Load-Flow

Depending on the selected input mode, the user needs to specify two input parameters. Figure 3 shows the load

model used for balanced load-flow analysis, where only P0and Q0are specified.

All loads specified as 2-phase and/or 1-phase loads are only considered in unbalancedload flow calculations.

They are ignored when a balancedload-flow is performed.

Figure 3: Load model used for balanced load-flows.

1.1.2Unbalanced Load-Flow

When running a load flow with a complete ABC-network representation, network unbalances resulting from either

unbalanced loads or unbalanced branch elements can be considered.

Additional data for unbalanced load flow analysis is:

Technology

Figure 4 shows the input dialog window for specifying the type data for General loads.


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Figure 4: General load type data input window

1.1.2.13-phase loads (3PH-D, 3PH PH-E, 3PH-YN)

The actual load per phase is entered in the input dialogue-box of the load element. The user has the choice

between:

Balanced load, only specifying the sum of all phases. In this case, it is assumed that the load is shared

equally amongst the phases.

Unbalanced load, specifying the individual phase loads.

Figure 5: 3-phase, Technology 3PH D load model diagram


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Figure 6: 3-phase, Technology 3PH PH-E load model diagram

Figure 7: 3-phase, Technology 3PH YN load model diagram

1.1.2.22-Phase Loads (2PH PH-E, 2PH-YN)

Such a load type can be used for modelling loads in two-phase or bi-phase systems.


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Figure 9: 2-phase, Technology 2PH-YN load model diagram

1.1.2.31-phase loads (1PH PH-PH, 1PH PH-N, 1PH PH-E)

The 1PH PH-PH load model can be used for representing loads connected betweentwo phases, which is in fact

a single-phase load between two phases (see Figure 10).

Figure 10: 1-phase, Technology 1PH PH-PH load model diagram

The 1PH PH-N load model can be used for a load connected between one phase and the neutral phase (see

Figure 11).


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Figure 11: 1-phase, Technology 1PH PH-N load model diagram

The 1PH PH-E load model can be used for a load connected between one phase and earth (see Figure 12).


1.1.3DC loads

DC-loads are always single-phase, as shown in Figure 10. For load flow analysis, the load is characterized by

the active power flow P. Inductive effects are only considered in transient simulations.

Figure 13: 1-phase DC load model diagram


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1.1.4Voltage dependency

Voltage dependency of loads can be modelled using a potential approach, as shown in (1) and (2). In theseequations, the subscript 0 indicates the initial operating condition as defined in the input dialogue box of the Load

Element.

( )

+

+

=

cPebPeaPe

v

vbPaP

v

vbP

v

vaPPP

_

0

_

0

_

0

0 1 (1)

where

cPbPaP =1

( )

+

+

=

cQebQeaQe

v

vbQaQ

v

vbQ

v

vaQQQ

_

0

_

0

_

0

0 1 (2)

where

cQbQaQ =1

By specifying the respective exponents (e_aP/e_bP/e_cPand e_aQ/e_bQ/e_cQ) the inherent load behaviour can

be modelled. Table 1 provides the value for the exponents required to achieve constant power, current and

impedance behaviour. However, the relative proportion of each coefficient can be freely defined using the

coefficients aP, bP, cP and aQ, bQ, cQ. See Figure 11.

Table 1: Selection of exponentvalue for different load model behaviour

Exponent Constant

0 power

1 current

2 impedance


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Figure 14: Specification of the voltage dependency factors

NOTE: These factors are only considered if the Consider Voltage Dependency of Loads is checked

in the Load-flow Calculation window, as shown in Figure 15 below.

Figure 15: Load-flow Calculation window indicating the use of voltage dependency terms for load-

flow calculations


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Figure 16: Scaling factor specification

Figure 17: Diagram indicating load scaling (adjustment) in order to maintain the feeder settingsspecified in the Feeder Definition


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1.2

Short-Circuit Analysis

Short circuit calculations according to IEC 60909, VDE102/103 or ANSI C37 generally neglect loads. Only motor

contributions are considered here.

The COMPLETE short circuit method utilises constant impedance (Z) or constant current I0models. Z and I0are

calculated from a preceding load-flow analysis. Figure 18 shows the type data window used to select whether the

load is constant current or constant impedance. See notes on Harmonic Analysis for further explanation of the

models.

Figure 18: Selection of load type of either constant current or constant impedance for consideration

in short circuit calculations using the Complete Method.

Figure 19 shows a 3-phase representation of a Y connected constant impedance and Figure 20 shows the 3-

phase representation of the D connected constant current model.

Figure 19: 3-phase constant impedance model, in Y and D connection, used for short circuit

calculations using the Complete method.


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Figure 20: 3-phase constant current load, in Y and D connection, used load for short circuit

calculations utilising the Complete method

1.3

Harmonic Analysis

In the type data of the general load model, the harmonic load model can be specified as either constant

impedance or current source. Figure 21 shows the selection of the harmonic load model type.

Figure 21: Selection of the load model type for harmonic analysis

1.3.1Passive Load

Passive loads can be represented as purely inductive or capacitive or a mixture of inductive and capacitive. Figure

22 shows the single-phase representation of a purely inductive and capacitive load.


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Figure 22: Purely inductive/capacitive load models used for harmonic analysis

The parameters R,L (G,C) are calculated from a preceding load-flow.

Figure 23 shows the single-phase load model for mixed inductive/capacitive load models (e.g. cables), used

during harmonic analysis.

Figure 23: The single phase equivalent for mixed inductive/capacitive load model

NOTE: Figure 22 and Figure 23 are single phase representations of the passive load. The 3-phase presentation is

similar to that shown in Figure 6 and Figure 5, and with either Y or D connections.

The inductive/capacitive portion of the load is specified by (7) and (8);

1

1

+

=

+

=

C

LCL

Cc

Q

QQQ

Q

Q

Q (7)

=

C

L

n

Q

Q

yC

1

(8)


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1.3.2Harmonic Current Injections

Nonlinear loads are described by their harmonic current spectrum. A requirement for modelling current injectingloads is to set the corresponding parameter on the harmonics page to current source.

For balanced loads, only characteristic harmonics can be specified. Figure 24 shows the specification of the

characteristic harmonic spectrum in the element data window of the load.

The angles of harmonic currents are defined with reference to the fundamental frequency phase angle (cosine

reference). This way of entering phase angles allows defining the current waveform independent from the power

factor at fundamental frequency.

Figure 24 shows the current spectrum of an ideal 6-pulse rectifier. Absolute current angles will be adjusted by

PowerFactoryaccording to (10).

Figure 24: Specification of the harmonic current spectrum for balanced loads

The harmonic currents are defined by (9) and the phase angle of the harmonic currents is defined by (10).

( ) ( ) ( )nfff IkI .= (9)


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( ) 1n

ff

f+= (10)

where fnis the nominal frequency and 1is phase angle of the fundamental current.

In case of unbalanced loads, frequency, phase angle and magnitude of harmonic currents can be entered

individually for each phase. Figure 25 shows the input window for entering such data.

Figure 25: Specification of the harmonic current spectrum per phase for unbalanced loads

1.4

RMS Simulation (Transient Stability)For RMS simulations a three-phase load can be modelled as a percentage of static and dynamic load. The static

portion is modelled as constant impedance whereas the dynamic load can be modelled as either a linear load or a

non-linear load.

Two-phase, single-phase- and DC loads are generally modelled as constant impedance.


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Figure 26: Diagram indicating the mixture of static and dynamic loads used for stability studies.

The background of the dynamic, voltage- and frequency dependent load model according to Figure 24 and Figure

25 is a motor-load in parallel to a static, non-linear load. The parameters of the block diagrams in Figure 24 and

Figure 25 can either be calculated from such a configuration, but they are usually identified from load

measurements.

The model according to Figure 24 is a small signal approximation of the model according to Figure 25.Parameters with equal names correspond to each other.

Figure 27: Model used to approximate the behaviour of the linear dynamic load.


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Figure 28: Model used to approximate the behaviour of the non-linear dynamic load.

Since the block diagrams according to Figure 27 and Figure 28 represent small signal models, they are only valid

in a limited voltage range. This voltage range is defined by the variables uminand umax. Outside this voltage range,

the power is adjusted according to Figure 29.

Figure 29: Low/High voltage approximations used in the non-linear dynamic load model.

With reference to the outputs of the block diagrams according to Figure 24 and Figure 25, the load equations

representing the full voltage range can be expressed as follows:


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( ) max2

max

minmin

2

min

min

min

2

min

2

maxmin

:1

2:21

20:

2

:1

.

.

uuuuk

uuu

u

uuk

uu

u

uk

uuuk

QkQ

PkP

out

out

>+=


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2Input/Output Definition of the Dynamic Model

2.1

Three-Phase Load

2.1.1RMS-Simulation

Figure 30: Input/Output Definition of General Load Model (RMS-Simulation)

Table 2: Input Variables (RMS-Simulation)

Parameter Description Unit

Pext Active Power Input MW

Qext Reactive Power Input Mvar

Table 3: State Variables (RMS-Simulation)


xu Delayed Voltage (Time constant T1) p.u.

xf Delayed Frequency (Time constant T1) p.u

cosphiu cosine of voltage angle

sinphiu sine of voltage angle

Table 4: Additional Parameters and Signals (RMS-Simulation)


fe Electrical Frequency p.u.

scale Scaling Factor

Pext

Qext


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2.1.2EMT-Simulation

Table 5: State Variables (EMT-Simulation)Parameter Description Unit

curLA Inductive Current, Phase A p.u.

curLB Inductive Current, Phase B p.u

curLC Inductive Current, Phase C p.u.

2.2

Single-, Two-Phase Load and DC-Load

Constant impedance load models. No input or output variables.


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3Parameter Definitions

3.1

General Load Type (TypLod)

Table 6: Input parameters of Load - Type


loc_name Name

systp System Type

nlnph Phases

cnm Connection

kpu Static Voltage Dependence: Volt. Dependence on P

kqu Static Voltage Dependence: Volt. Dependence on Q

i_csrc Current Source/Impedance

i_pure Load model

pgrd Capacitive/Inductive Reactive Power: QL/QC %

qcq Capacitive/Inductive Reactive Power: QC/Q %

xt Transformer Short Circuit Reactance %

Prp Power of parallel Resistance/Total Active Power %

lodst Percentage: Static (const Z) %

loddy Percentage: Dynamic %

i_nln Percentage: Nonlinear Model

t1 Dynamic Load Time Constant skpf Dynamic Active Load: Frequ. Dependence on P

kpu Static Voltage Dependence: Volt. Dependence on P

tpf Dynamic Active Load: Transient Frequency

Dependence

s

tpu Dynamic Active Load: Transient Voltage

Dependence

s

kqf Dynamic Reactive Load: Frequ. Dependence on Q

kqu Static Voltage Dependence: Volt. Dependence on Q

tqf Dynamic Reactive Load: Transient Frequency

Dependence

s

tqu Dynamic Reactive Load: Transient VoltageDependence

s

udmax Voltage Limits: Upper Voltage Limit p.u.

udmin Voltage Limits: Lower Voltage Limit p.u.

i_nln Percentage: Nonlinear Model


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i_prty Load shedding/transfer: Load priority

shed Load shedding/transfer: Shedding steps

trans Load shedding/transfer: Transferable %

pTrans Load shedding/transfer: Node to Transfer to

(ElmTerm,StaBar)

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