Advances in Transient Simulation Techniques for Modern ...

NSERC Industrial Research Chair in Power Systems Simulation

Advances in Transient Simulation Techniques for Modern Power Systems

Aniruddha M. Gole

Electrical and Computer Engineering Department University of Manitoba


Types of Simulation Studies

Why is Elec tromagnetic Transients Simulation Important in Modern Power Systems?

How can Simulation help in design and Decision Making?

What are the emerging tools for the above?

How can very Large Systems be accurately modelledwith the least computational effort?

Outline


Load Flow: Static Solution of the NetworkApplicability: Line Loading Determination, Dispatch

Transient Stability: Solution of the Electromechanical Interactions (rotor angle and frequency swings)

Electromagnetic Transients: Full Model of system including differential equations of network

Switching and Lightning Transients, ProtectionPower Electronic Systems: HVDC and FACTS

Other Methods:Small Signal Analysis (eigenvalue analysis) Helpful in

understanding interactions.Voltage Stability etc.

Range of Simulation Studies


Simulation Techniques:

Loadflow & Short Circuit50/60 Hz only

Transient Stability

~1 Hz to 50/60 Hz

Electromagnetic Transients - EMTP/EMTDC/ATP

0 Hz to 5-10 kHz

Special Models

Region often neglected by non-real timeelectromagnetic transient simulations

(short duration simulations)

Frequency

Real Time Electromagnetic Transients - RTDS

0 Hz to 2 - 3 kHz

Continuous real time simulationscover the entire frequency range

Digital Tools for System Simulation : Range of Applicibility


• Features– Very accurate time-domain models of a broad spectrum of systems components (machines, t- lines,FACTS, Custom Power etc.)

– Accurate models for switching and nonlinear components

–-Repeated Automatic runs useful in DESIGN

Purpose: System Level modelling of Large Power System

• Overvoltage and Harmonics in the Network

• Stability and Control of the Network

Electromagnetic Transients Simulation


Why is Electromagnetic Transients Simulation Increasingly Important in Modern Power Systems?


Traditional Power Network

• 3-phase Ac Generators• Transmission Lines and

Cables• Induction motors and other

loads• Protection Equipment (non-

electronic)• Integrated and Regulated


Emerging Power Networks

More deregulated/ interest in markets Require Advance Protection and Control MethodsIncreasing inclusion of renewable energy sources (wind)Require More Precise control of Power Flow-a move towards the pipeline model through the use of Power Electronics

HVDC and FACTS Controllers


Traditional Power Network

• Power Flow dictated by voltage profile

• Pipeline: Flow is locally controllable

V1

V3

V2

V5

V6

V4

Evolution of the Energy Supply System

Emerging Power Network


Emerging Power Networks Use of Power Electronics: HVDC

Systems

• Large Power Electronic Systems: Gigawatt range HVDC Transmission

dc LineSE Ac System

Dc

Filte

r

Ac

Filte

rsZsys

RE Ac System

Zsys

Electrode line impedance

Completely decoupled. Any desired level of power flow can be established


HVDC +/- 500 kV

Manitoba Hydro’s Nelson River HVDC Transmission System 4 GW over 950 km


Emerging Power Networks and PE: HVDC Systems

Many technology revisions


Back-to-Back Dc in N.America


How Can Transients Simulation be used for getting Design

Decision Support Information?


Evolution of Simulation Based DesignTraditionally, a human carried out a number

of studies for different operating conditions with varying component or parameter values to get the best design -Time-consuming

use of automated multiple-runs. parameters are varied sequentially or randomly with human inspection of results

Emerging software tools include non-linear optimization wrappers that conduct multiple simulations and minimize a certain objective function


Optimization-Enabled Transient Simulation

• A mathematical optimization algorithmstrategically selects the trial points

• Result- orders of magnitude less runs than with brute force approach

Optimization Tool (PSCAD/EMTDC V4.1)

Initialization

Select newcandidate

point x

Converged?

End

Yes

No

OBJECTIVEFUNCTION

EVALUATION USINGTRANSIENT

SIMULATIONf( x)

NonlinearOptimization

Supervisory Process


Ensuring Robustness• Partial objectives

function evaluate the performance regarding various aspects of the design,

• The aggregate objective function is a weighted sum of all partial OFs.

Initialization

Select newCandidate

point x

Converged?

End

Yes

No

Simulate state 1

Simulatestate 2

Simulatestate N

1( )of x 2 ( )of x ( )Nof x

1

( ) ( )N

i ii

OF w of=

= ⋅∑x x

NonlinearOptimization

x = (x1,x2,…,xd)T

d = number of designparameters


HVDC Controller Optimization (200 MW B-B Scheme)

12 PulseConverter

12 PulseConverter

Inverter RectifierTr

Tr

Tr

Tr

Var Comp.Filters:11, 13, HP(4x26.25 Mvar)

Tline:160 km,230 kV

Var Comp. Filters:11 (26.25), 13 (26.25),HP(2x26.25 Mvar)

Tline:200 km,345 kV

Errorprocessing/

scaling

Voltage control loop

Current control loop

Extinction anglecontrol loop

MinPI

Controller αError

processing/scaling

Errorprocessing/

scaling

DividePower order

DC voltage

Current order

Kp+1/sTi

Design Objective: Immunity against ac voltage magnitude and phase changes.


−Δφ +Δφ −Δ|V| +Δ|V| −ΔP +ΔP

3 3.5 4 4.5 5 5.5 6 6.5 70

0.5

1

1.5

2

DC

Cur

rent

[pu]

3 3.5 4 4.5 5 5.5 6 6.5 7-1.2

-1

-0.8

-0.6

-0.4

-0.2

Time [sec]

DC

Vol

tage

[pu]

(a)

(b)

3 3.5 4 4.5 5 5.5 6 6.5 70

0.5

1

1.5

2

DC

Cur

rent

[pu]

3 3.5 4 4.5 5 5.5 6 6.5 7-1.5

-1

-0.5

0

Time [sec]

DC

Vol

tage

[pu]

Pre- and post- optimization HVDC Response (strong ac network ESCR=3)


OE-EMT with RTDSReal Time Optimization Platform development

TMS320C6713 DSK bearing rectifier controller


Extension to Multi-Objective Optimization (Pareto Optimal)

PSCAD/EMTDC Electromagnetic

Transient Simulation Program

Initialization

Optimization

Is Pareto Frontier

Complete?

No

End

Yes

Selecting a New Objective Function

( ) ( ) ( )xxx 2211 fkfkfov +=

The Optimization Toolwith Pareto Optimality Enabled


Automatically Generated Pareto Frontier:Capacitor size v/s low ripple tradeoff in Statcom

The Pareto FrontierThe capacitor size and the objective function used for optimization of the transient performance.

0 0.5 1 1.5 2 2.550

100

150

200

Capacitor Size [pu]

Perfo

rman

ce

A B

After this point any reduction in the capacitor size results in huge degradation of the system performance


Generalization: Simulation Based Decision Support Tools

Decision Support ToolAny supervisory algorithm for conducting multiple runs that automates the design process and provides decision making informationExamples:

Optimization Tool- judiciously selects trial parameter values in subsequent runs to obtain the best ones

Sensitivity Analysis Tool- Provides sensitivity information for design, i.e. how do changes in parameters affect performance?

Minimizes human interaction and number of simulation runs

PSCAD/EMTDC Electromagnetic Transient

Simulation Program

Supervisory Algorithm to -Guide the Simulation

Simulation Parameter Values

Simulation Results


Generation of Surrogate Models from Sensitivity Information)

Multiple-run simulations are conducted to obtain a second-order polynomial surrogate model of the performance measures.

Further decisions regarding parameter selection can be made with the use of the resulting simplified surrogate model

[ ] [ ]

( ) ( ) ( )

( ) ( )( ) L

L

LL

+ΔΔ∂∂

∂+Δ

∂∂

+Δ∂∂

+≅Δ+

ΔΔ=Δ=

2121

212

1

2

11

00

10100

21

,,,,,

xxxx

fxx

f

xxfff

xxxx Tn

Tn

xxx

xx


Example: Determiniation of Required Resolution in Switching angles for Selective Harmonic Elimination (SHE)

Proper Choice of the Switching Angles

Vdc

Vdc

S1

S4

D1

D4

S3

S6

D3

D6

S5

S2

D5

D2

abc

Elimination of the Certain Harmonics


50.8°46.3°29.9°23.3°11.0°

α5α4α3α2α1

SHE Switching AnglesVdc = ±12 kV , V1 = 8 kV

Selective Harmonic EliminationHaving five chops in a quarter-cycle allows to adjust the fundamental component and to eliminate four harmonics (5,7,11,13).

The operating point:

Switching Angles for Selective Harmonic Elimination (SHE)


Sensitivity Matrix for 5th harmonic variation with angles

1α∂∂

2.608-2.165-1.3800.072-2.263

-2.1652.2120.123-1.1790.168

-1.3800.1231.076-0.7520.130

0.072-1.179-0.7521.357-1.233

-2.2630.1680.130-1.2332.445

0.031-0.038-0.0360.011-0.0331

|V5| 2α∂∂

3α∂∂

4α∂∂

5α∂∂

1α∂∂

2α∂∂

3α∂∂

4α∂∂

5α∂∂


Surrogate Model Relating Harmonic Distortion to Switching Angle Jitter

( )( )( ) ( )( )( )( )

( )

1 1 2 2 3 3

1 2 1 3

2 3

1 1 2 2 3 3 1 2 1 3 2 3

257

2

D D D D

D D

D

D D D D D D

α α α α α α

α α α α

α α

α α α α α α α α α α α α

α

α α α α

α α

α

Δ = + + Δ

+ ±Δ ±Δ + ±Δ ±Δ

+ ±Δ ±Δ

= + + ± ± ± Δ

( )1 1 2 2 2 3 1 2 1 3 2 3

max57

maxmax

DD D D D D Dα α α α α α α α α α α α

αΔ ≤+ + ± ± ±

max 0.125αΔ ≤ °

.

Surrogate Model

Which gives

max57 2% requires:D <


Towards model based specifications(an emerging trend in HVDC/FACTS

procurement)


The Approach of Model Based Specification

• Suppliers are provided with a model platform of the system into which the proposed equipment is to be installed

•The specification is stated in terms of a desired performance requirement for the overall power network

•This is an emerging trend in procurement of large Power Electronic Applications in Power Transmission Systems


Advantages of MBSPowerful automated decision support tools can substitute many human-in-loop tasks

–Multiple-runs–Non-linear Optimization–Multi-objective Pareto analysis and sensitivity analysis

Initialization

Select new candidate

point x

Converged?

End

Yes

No

EMT SIMULATOR

Design compatibility

index= of(x)

Nonlinear Optimization


Advantages of MBSConfidentiality and Security

–Portions of the electrical Power Network can be compiled and/or encrypted

CMP

KB

BP

P

By-PassBreaker

DCV

GMD

DCV

DC

FLT

IFLTDC

TimedFaultLogic

TFDC

GM

SG

MD

A B C

VA

CMP

AC SYSTEM

VA

NA

VB

NB

VC

NC

IA

TAP

HVDC Back to Back200 MW, 83.3 kV, 2.4 kA

F = 660 [Hz]

F = 1440 [Hz]

DCVW

AOW

AM

SAM

D

A

B

C

AM

GM

KB

ComBus

1 3 5

4 6 2

AO1.0E

6 [ohm]

A

B

C

A

B

C

A B C

240 [MVA]

230.0 [kV] 35.2 [kV] 35.2 [kV]

#1 #3

Tap

MinD

E

F = 1440 [Hz]HIGH PASS FILTERS26.25

VRMS

CMPW

Vac230

V230rms

C230L

+

C230L

F = 660 [Hz]

F = 780 [Hz]

C230L

+

F = 660 [Hz]

F = 780 [Hz]

C230L

+

F = 780 [Hz]

C

B

A

B230F

F = 660 [Hz]

F = 660 [Hz]

F = 780 [Hz]

F = 660 [Hz]

F = 780 [Hz]

F = 780 [Hz]

11th & 13th FILTERS26.25 MVAR

11th &13 th FILTERS26.25 MVAR

B230HP

B230F

*0.0501

RE

SCR=1.43 @ 75 Deg60.0 Hz 230 kV

Vrms

Sw

Sw2

DPL

FilterSwitch

&230 KV Fault

Detectora230F

3 PhaseRMS VRMS

A

B

C

AM

GM

KB

ComBus

1 3 5

4 6 2

AO

AC Network

Receiving End AC Network

RELine

1

RELine

1

CPanelV230rms

#NaN

DC Voltage

#NaN

DC Current

#NaN

Pre230

#NaN

Qre230

#NaN

PL230B

#NaN

QL230B

#NaN

Bypass

1

BP OPEN


The Approach of Model Based Specification: An Example

• 200 MW back-back HVDC Scheme connecting two weak ac networks (based on an actual scheme)

System Description is provided as a model

•Heirarchical application of tools: -> load flow/stability; harmonic analysis, etc. used to first select the dc converter and filter component values: Performance requirement Load rejection overvoltage magnitude and harmonic distortion on ac bus

200 MW

TOV to be within a specified envelope


MBS Example…contd.Second stage: Design (and supply) the controller

Controls:


Advantage of MBS: Automation of the Design Process

The simulation is driven by a non-linear optimization Outer Loop

Initialization

Select new Candidate point x

Converged?

End

Yes

No

SimulateCond. 1

Simulate Cond. 2

Simulate Cond. N

1( )of x 2 ( )of x ( )Nof x

1

( ) ( )N

i ii

OF w of=

= ⋅∑x x

Nonlinear Optimization

x = (x1,x2,…,xd)T

d = nu mber of design parameters

Cond.1:

•Strong System (ESCR =3)

Cond 2:

•Weak System (ESCR =2)


−Δφ +Δφ −Δ|V| +Δ|V| −ΔP +ΔP

3 3.5 4 4.5 5 5.5 6 6.5 70

0.5

1

1.5

2

DC

Cur

rent

[pu]

3 3.5 4 4.5 5 5.5 6 6.5 7-1.2

-1

-0.8

-0.6

-0.4

-0.2

Time [sec]

DC

Vol

tage

[pu]

(a)

(b)

3 3.5 4 4.5 5 5.5 6 6.5 70

0.5

1

1.5

2

DC

Cur

rent

[pu]

3 3.5 4 4.5 5 5.5 6 6.5 7-1.5

-1

-0.5

0

Time [sec]

DC

Vol

tage

[pu]

Pre- and post- optimization HVDC Response (strong system)

1.1 1.65 4.02 2 2

0 1.1 1.65

( ) 2( ) ( )dref d dref d dref dOF I I dt I I dt I I dt= − + − + −∫ ∫ ∫


Pre- and post- optimization HVDC Response (weak system)

3 3.5 4 4.5 5 5.5 6 6.5 70

0.5

1

1.5

2

DC

Cur

rent

[pu]

3 3.5 4 4.5 5 5.5 6 6.5 7-1.5

-1

-0.5

0

Time [sec]

DC

Vol

tage

[pu]

(a)

(b)

3 3.5 4 4.5 5 5.5 6 6.5 70

0.5

1

1.5

2

DC

Cur

rent

[pu]

3 3.5 4 4.5 5 5.5 6 6.5 7-1.5

-1

-0.5

0

Time [sec]

DC

Vol

tage

[pu]


Other AdvantagesThe model is evolutionary.

By following this process, the utility always maintains an up-to-date model of its system

• The model is always available for future procurements

• Useful for system maintenance and modifications

• Invaluable for Operator Training

• Unexpected advantage: A repository of the accumulated expertise of previous engineers


Summary of MBS

• The MBS approach is effective in that the specification can be in the form of a performance measure

•Efficient means of communicating information between parties

•Enables automated design procedures

•Issues can be identified and resolved on a continuous basis

•Confidentiality of portions of the model can be maintained

•The model has other uses such as training of new personnel

•Careful maintenance of the model is necessary


• Emerging Tools and Methods to Facilitate Simulation tools for Decision Support Systems

• 1) Methods for accurately representing very large systems as wide-band dynamic Equivalents

• 2) New Computer Platforms for Optimization and Sensitivity Analysis


The nonlinear TSA simulation block reproduces the low frequency electromechanical oscillations of the internal system

Proposed system equivalent, a multi-port Frequency Dependent Network Equivalent (FDNE) and a real-time Transient Stability Analysis (TSA) type solution block.

The linear FDNE model reproduces the high frequency electromagnetic transient of the external network

Wideband Multi-port Eqvts. For RTDS

X.Lin, Y.Ming, Y. Liang and A.M. Gole



X.Lin, Y.Ming, Y. Liang and A.M. Gole

0.6 0.8 1 1.2 1.4 1.6 1.8 2

0

100

200

300

400

500

600DC LINK 2 Inverter DC voltage

Time (Second)

DC

Vol

tage

(kV

)

RTDS FULL MODELRTDS+FDNE+TSARTDS+TSARTDS+FDNE

0.31 0.32 0.33 0.34 0.35-400

-200

0

200

400Bus #3 phase A Voltage

Time (Second)

Pha

se V

olta

ge (k

V)

RTDS FULL MODELRTDS+FDNE+TSARTDS+TSA

Faults in Multi-infeed HVDC System

•Both EMT and Electromechanical transients are important

•Interface is posible on Converter bus- no boundary bus required.

Largest System Tested so far: 470 bus

•Int. system 62 bus, 17 gen.

•11 equivalents with 408 buses.

Racks reqd: 6 instead of 20



Steps in Constructing the FDNE

Acquire the frequency domain response of the external systemTune the coefficients of a s-domain rational function,.

Make its frequency domain response similar to the frequency domain response of the external systemModel the s-domain function in the EMT time-domain

simulation•It is difficult to acquire multi-port frequency response data for a large network, especially in cases. Solution: A method for estimating the frequency domain characteristics by analyzing commonly available power-flow data •Coefficients of the s-domain function are tuned using a Vector Fitting technique

• S-domain function mplemented in the EMT simulation in classical history current term/ conductance formPassivity must be ensured.•Application of repeated curve fitting in problem areas ensures FDNE passivity (which prevents the simulation from blowing up) and accuracy

0

0.1

0.2

0.3

0.4

MAGNITUDE

|Y(f)

| (S

)

1000 2000 3000-180

0

180PHASE

Frequency (Hz)

angl

e(Y(

f)) (

deg)

Original MagnitudeFitted Magnitude

Original PhaseFitted Phase

2000 4000 6000

0

Minimum Eigenvalue of the Conductance Matrix

Frequency (Hz)

First FitSecond FitThird Fit


Parallel Computers for Simulation

Sensitivity Analysis and

Many Optimization Methods are highly Parralelizable

Can be implemented on Cluster Computers

Challenge: Maximum utilization of computing resources

40 CPU cluster at U of Manitoba


Concluding Remarks:

• Transient Simulation is playing an increasing role in the design of Modern Power Systems

•The trend is towards conducting automated runs that conduct design and yield other decision support information

•The approach permits power-hardware/control co-design, including simultaneous tuning of multiple-controllers

•Such tools can help in the procurement, training and knowledge maintenance process in utlitities

•This trend is expected to drive improvements in the simulation itself by introducing new techniques and platforms.


References1. Gole, A.M., Filizadeh,S., Menzies, R.W. and Wilson, P.L, “Optimization-Enabled

Electromagnetic Transient Simulation”IEEE Transactions on Power Delivery, v 20, Issue 1, January 2005, pp 286-293

2. Filizadeh, S. and Gole, A.M., “Inclusion of Robustness into Design Using Optimization-Enabled Transient Simulation”, IEEE Trans. on Power Delivery, v 20, Issue 3, July 2005, pp 1991-97

3. Filizadeh, S.; Gole, A.M.; Woodford, D.A. and Irwin, G.D.; “An Optimization-Enabled Electromagnetic Transient Simulation-Based Methodology for HVDC Controller Design”, IEEE Trans. Power Delivery, Vol. 22, Issue 4 , October 2007, pp 2559 – 2566

4. Lin, Xi, Gole, A. M. and Yu, Ming ; “A Wide-band Multi-port System Equivalent for Real Time Digital Power System Simulators”, to be published in the IEEE Trans. Power Systems

5. Heidari, M.; Filizadeh, S.; and Gole, A.M.; “Support Tools for Simulation-Based Optimal Design of Power Networks With Embedded Power Electronics”, IEEE Trans. Power Delivery, Vol. 23, Issue 3, July 2008, pp 1561- 1570

6. Gole, A.M., Woodford, D.A., Filizadeh, S. and Irwin GD, “ Use of Models in the Specification and Procurement of Power Electronic Equipment in Power Systems” , GCMS’0 8, Edinburgh, Scotland, June 16-19, 2008

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