voltage-sourced converter based facts controller...

Rensselaer Polytechnic InstituteElectrical, Computer, and Systems Engineering

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Voltage-Sourced Converter Based FACTS Controller Seminar

NSF US-Africa Research and Education Collaboration WorkshopDecember 15, 2004

Joe H. ChowElectrical, Computer, & Systems Engineering

Rensselaer Polytechnic InstituteTroy, New York, USA

www.ecse.rpi.edu/homepages/chowj

© 2004 by Joe H. Chow


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Outline• Research experience in Flexible AC

Transmission System (FACTS) controllers• Modeling of voltage-sourced converter

based FACTS controllers• Operating modes • Loadflow algorithm and sensitivity analysis• Dispatch strategies• Operator training simulator


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Presenter’s FACTS Research• GE

– EPRI project on SVC damping control (1985)– EPRI project on TCSC damping control (1993)

• RPI– EPRI/DoD project

• Multiple SVC/TCSC controllers, remote signals for damping (1998-2002)

– EPRI projects• Methodologies and Algorithms for Hierarchical Control

and Coordination of Multiple Voltage/VAR Reinforcement Devices in Power Transmission Systems, 2002-3 (NYPA)

• Web-based FACTS Controller Simulator, 2003• CSC Operator Training Simulator, 2004-5 (NYPA)

– NSF grant: basic FACTS controller research (2003-6)


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Presenter’s Selected Publications in FACTS • VSC-based FACTS Controller

– X. Wei, J. H. Chow, B. Fardanesh, and A.-A. Edris, “A Common Modeling Framework of Voltage-Sourced Converters for Loadflow, Sensitivity, and Dispatch Analysis,” IEEE T-PS, 19, 2004.

– X. Wei, J. H. Chow, B. Fardanesh, and A.-A. Edris, “Dispatchability of Voltage-Sourced Converter Based FACTS Controllers,” IX SEPOPE, Rio de Janeiro, May 2004.

– X. Wei, J. H. Chow, B. Fardanesh, and A.-A. Edris, “A Dispatch Strategy for a Unified Power Flow Controller to Maximize Voltage-Stability Limited Power Transfer,” to appear in IEEE T-PS.

– X. Wei, J. H. Chow, B. Fardanesh, and A.-A. Edris, “A Dispatch Startegy for Interline Power Flow Controller Operating at Rated Capacity,” presented at IEEE Power System Conf and Expos, 2004.


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Presenter’s Selected Publications in FACTS

• SVC, TCSC, and control– E. V. Larsen and J. H. Chow, “SVC Control Design Concepts for

System Dynamic Performance,” in IEEE Power Engineering Society Publication 87TH0187-5-PWR “Application of Static Var Systems for System Dynamic Performance.”

– E. V. Larsen, J. J. Sanchez-Gasca, and J. H. Chow, “Concepts for Design of FACTS Controllers to Damp Power Swings," IEEE T-PS, 10:948-956, 1995.

– G. N. Taranto and J. H. Chow, "A Robust Frequency Domain Optimization Technique for Tuning Series Compensation Damping Controller," IEEE T-PS, 10:1219-25, 1995.

– Jaewon Chang and J. H. Chow, “Time-Optimal Control of Power Systems Requiring Multiple Switchings of Series Capacitors,” IEEE T-PS, 13:367-73, 1998.

– J. H. Chow, J. J. Sanchez-Gasca, H. Ren, and S. Wang, “Power System Damping Controller Design using Multiple Input Signals,” IEEE Control Systems Magazine, 20:4:82-90, 2000.

– X. Wei, J. H. Chow, and J. J. Sanchez-Gasca, “On the Sensitivities of Network Variables for FACTS Device Damping Control,” Proc 2002 IEEE PES Winter Meeting, 2:1188-1193, 2002.


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Flexible AC Transmission System (FACTS) Controllers

• FACTS Controllers can be used to control flows and voltages in an AC transmission system

• Thyristor-controlled devices applied to capacitors and reactors: – SVC (Static Var Control)– TCSC (Thyristor-controlled series compensation)

These devices control voltages and flows by changing system impedances.


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Flexible AC Transmission System (FACTS) Controllers

• Voltage-sourced converters (VSC) using gate-turnoff (GTO) thyristors, require only a capacitor to hold DC voltage– Can operate in a stand-alone configuration– The DC capacitors of VSCs can be coupled to

circulate active powerThese devices control voltages and flows by inserting AC voltage sources that can lead or lag the AC current.


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VSC-based FACTS Controllers

SSSC

UPFCIPFC

STATCOM


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VSC-based FACTS Controllers

Back-to-Back (B2B) STATCOM

AEP Inez SubstationUPFC Equipment

Big Sandy Line

SeriesXfmer

SpareShuntXfmer

MainShuntXfmer

Shunt & SeriesIntermediate Xfmers

CoolingSystemHeatExchangers

UPFC Building(Inverters & Controls)

AEP Inez SubstationUPFC Inverter Poles

ACBus

Inverter Pole Assembly

DCCapacitors

DCClamp


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VSC Operation • In a voltage-sourced converter, the DC

voltage source does not change sign. • An AC current is injected into the VSC.• An AC voltage is obtained by switching on

and off gate-turnoff (GTO) thyristors; the switching is controlled to provide either a leading or lagging AC voltage.

• A capacitor is used to supply the DC voltage.


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VSC Operation

Single valve operation

vd

va

idDC Side AC Side

Active DCpower Active &

ReactiveAC Power

N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems, IEEE Press, 2000.


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VSC Operation

Svd

P

P&Qa

b

vabiab

id

1 1'

4

3 3'

4' 2 2' Single-phase, full-wave circuit


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VSC Operation

Operation Waveforms

+Vd

-Vd

1&2 on3&4 on

1&2 onVab

3&4 on

1,2

3,4

1,2

3,4

1',2'

3',4'

1',2'Iab

1,2 3,4 1,2 3,4

1',2' 3',4' 1',2'Id


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VSC Operation

Three-phase, full-wave circuit

vda

+vd /2

1

4

3 3'

4' 6 6'

N

1' 5 5'

2 2'

bc

or

-vd /2


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VSC Operation

Operation Waveforms

Va

Vb

Vc

14

1

63

6

25

+Vd/2

-Vd/2

-Vd/2

+Vd/2

+Vd/2

-Vd/2

Vn+Vd/6-Vd/6

Vab

+Vd

-Vd

1,6

3,4

1,61,3 4,6

+2Vd/3+Vd/3

-Vd/3-2Vd/3

Van


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VSC Operation

ia

4'

1 1'

4'A

1A 1'A

N

4

4A

D4

D1

+Vd /2

-Vd /2

a

6'

3 3'

6'A

3A 3'A

6

6A

D6

D3

b

2'

5'

2'A

5A 5'A

2

2A

D2

D5

c

5

Three-level Converter


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VSC Operation

+Vd /2

-Vd /2

Va+Vd /2

-Vd /2

1,1A

4,4A

1A,4A

Vb

3,3A

6,6A

3A,6A

Vab+Vd /2+Vd

-Vd /2

-Vd

σ

αο

180

α

Operation Waveforms


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Voltage/Current EquationsShunt VSC Series VSC

θ is bus angle relative to swing bus

α is Vm angle relative to swing bus


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FACTS Controller Operating Modes

• Shunt VSC1. Var reserve mode – voltage

regulation mode with droop and var reserve

2. Voltage regulation mode with droop

3. Var output setpoint control

ICmax ILmax

V1

V1=Vref

o

STATCOM control range

droop


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• Series VSC– Standalone1. Fixed line P flow setpoint2. Fixed series voltage injection magnitude (leading

or lagging)– Coupled1. Fixed line P,Q flow setpoints2. Fixed series voltage injection vd,vq, measured

with respect to the from (shunt) bus (Marcy) voltage

3. Fixed series voltage injection vd,vq, measured with respect to series line current (future implementation)


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STATCOM Model

Power circulation:

Voltage regulation:

(neglecting losses)


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STATCOM Model

Voltage-current diagram

ICmax ILmax

V1

V1=Vref

Ish

V1

Vm1

jXt1Ish

oo

~

~~

~

droop

STATCOM control range


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UPFC Model

UPFC

Power circulation:

Line flow:


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UPFC Model


0δδ >

2

~~

mse VI ⊥

No UPFC With UPFC, no power circulation

~Ise

V4~V1

~IseZ4~ V2

V4

Ise

V1

o

~

~ ~

~

Vm2~

IseZ4~

δ

δ0

o


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UPFC Model


oIsh

Vm1jXt1Ish V2

V4

Ise

V1

o

~

~

~ ~

~

~~

V1~

Shunt Series

Power circulation from shunt to series


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IPFC Model

Power circulation:

Line flow:

VSC 1

VSC 2


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IPFC Model

Voltage-current diagram – active power from VSC 2 to VSC 1

~Ise1

V4~V1

~

V6V8

Ise2

V5

o

~ ~~

~

Vm2~

o

Vm1V2~

PC>0


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IPFC Model

~Ise1

V4~V1

~ V6 V8

Ise2

V5

o

~~ ~

~

Vm2~

o

Vm1~

V2~

PC<0

Voltage-current diagram – active power from VSC 1 to VSC 2


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B2B STATCOM Model

Power circulation:

Shunt flow:


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B2B STATCOM Model


oIsh1

V1

Vm1

jXt1Ish1

oIsh2

V2

Vm2

jXt2Ish2

~

~

~

~

~

~

~~


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• Decoupled Loadflow Model– UPFC

• Power Injection Model (PIM)– UPFC

Loadflow Models using Bus Injections


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• PIM equations



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• For PIM, solution in regulation mode requires additional outer iteration loop

• Not applicable to fixed voltage injection mode or rated capacity mode– Equivalent injections are not known in advance– Not able to check and enforce actual limits during the

iterations• Not suitable for modeling IPFC and GUPFC

– Too many equivalent power injections, not enough information to specify all variables



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Voltage Source Model (VSM)• Use injected series voltage sources directly

– Require adding series voltage variables in loadflow and network equations – not in most power system software

• Capable of modeling all operating modes• Capable of modeling various FACTS configurations

including IPFC and GUPFC• Common framework – no change of variables from

EMTP to dynamic simulation to loadflow and state-estimators

• Advantages include – Fast Newton-Raphson algorithm convergence (UPFC

model by Fuerte-Esquivel and Acha)– Direct sensitivity analysis– Rated capacity dispatch


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VSM-based Loadflow Equations

Setpoints:

Power circulation:

UPFC


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Newton-Raphson Algorithm• Flow equations

• 2(N+M)-Ng-1 solution variables

• Update equation and Jacobian matrix

R is VSC equation


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Newton-Raphson Algorithm

• Given M VSCs:

Always keep the same 2M variablesfor VSCs

Flow equations: add 2M equations athe end

Jacobian: only need to change the last 2M rows

• Efficient coding


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Loadflow Convergence Example • 13-bus

test system– UPFC on

Line 4-6(A closed,B open,C closed)

• Large 1564-bus New York system


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NR Loadflow Convergence

• Convergence results for the large 1564-bus New York system are similar to those for the small test system


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Control Issues with FACTS Controllers

• What kind of control flexibility does a FACTS controller have?

• FACTS controller shift factors?• Setpoint control or other strategies?

– Steady state – Transient stability


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FACTS Control Variables and Dispatchability• Dispatchability

– Coupling VSCs improves dispatch flexibility• Independent and dependent control variables

2n + 1n + 1GUPFC32B2B STATCOM32UPFC11STATCOM

Controlled variables# of coupled VSCsFACTS Controller

Due to the power circulation constraint

α1Vm1, Vm2,…n, α2,…nGUPFCα1Vm1, Vm2, α2B2B STATCOMα1Vm1, Vm2, α2UPFCα1Vm1STATCOM

Dependent control variables

Independent control variables

FACTS Controller


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FACTS Control Variables and Dispatchability

UPFC shunt VSC power circulation Psh as a function of Vm1 and α1


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Sensitivity Analysis including FACTS Control Variables• How system flow and voltage variables

change when FACTS control variables change

• Using network equation and the VSC power balance equations, the sensitivity can be formulated with perturbation analysis

• The coefficients of S are real; similar to generator shift factors


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Generator Shift Factors• The shift factors show how one additional MW

from a generator serves one MW of load, at specific locations in a system, will be distributed between a number of transmission paths

• Generator shift factors are (mostly) determined from a DC loadflow

• These shift factors are used to determine how the generations should be adjusted in order to observe power transfer constraints


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Network Equation including FACTS Control Variables

Yik – components of admittance matrix YVg , Vb – generator and load bus voltages Ig , Ib– current injections at generator and load

buses

Need additional equations for active power circulation constraint.


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Perturbation Analysis

• Computation steps:1.Perturb each independent control variable

and solve for Vb from the network equations.

2.Solve dependent control variable from power balance equation based on Vb .

3.Update control voltages, repeat Steps 1 and 2 until convergence.

4.Repeat Steps 1 to 3 for each independent control variable separately.


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13-Bus Test System


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FACTS Controller Shift Factors• 13-bus system, UPFC on Line 4-6, System

base 100 MVA• (V4, Pd, Qd) = (1.027 pu, 940 MW, 140 Mvar)

0.0145-1.59826.6030P4-13 (pu)-0.485614.266610.2842P4-12 (pu)-0.0125-0.00170.5203V6 (pu)-0.04050.00430.7673V12 (pu)0.00070.01310.7842V4 (pu)

α2 (rad)Vm2 (pu)Vm1 (pu)


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Use of FACTS Shift Factors

• Voltage and power dispatch– From a base case, specify desired voltage or

power profile

– Increment setpoints


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UPFC Voltage Dispatch ExampleSeries VSCexceeds rating

Shunt VSCexceeds rating


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UPFC Voltage Dispatch Example• Sensitivity-based dispatch

– Base dispatch: Pd = 1000 MW, Qd = 150 MVar, V1d = 1.027 pu, yielding Vto= 1.02 pu.

– To achieve Vto=1.027 pu (same as from-bus), sensitivity analysis gives Qd = 159.8 MVar

– Loadflow solution gives the to-bus voltage at 1.0262 pu (off by 0.08%)


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Power Dispatch Example• 13-bus system – UPFC setpoints P = 960

MW, Q = 140 MVar. To increase other line power flow by 40 MW.

• Large system – similar properties as the small system


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Maximizing Power Transfer• Nonlinear programming formulation

where γ > 1 is the load factor• Linear programming formulation

Subject to: Nonlinear loadflow equations, and

Subject to: Linear sensitivity equations , and

Check MVA ratings after the solution

Voltage magnitude bounds

VSC rating limits


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Maximizing Power Transfer Example

• Voltage magnitudes are the binding constraints


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FACTS Controller Operating Constraints

• Max VSC voltage limit• Max VSC current limit• Max VSC MVA limit • Maximum power circulation between

coupled VSC• Maximum and minimum line side voltage

J. Bian, et al., “A Study of Equipment Sizes and Constraints for a Unified Power Flow Controller,” IEEE T-PD, 12:1385-91, 1997.


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FACTS Controllers Operating at Rated Capacity

• Maximum dispatch benefit often occurs when FACTS Controllers operate at rated capacity, as in post contingencies – not possible to maintain either voltage setpoint, or flow

setpoints, or both

• For STATCOM and SSSC, enforce the limit and maintain zero power generation

• For UPFC and IPFC, power circulation optimizes the coupled VSCs, and may become the deciding factor in optimal dispatch for maximum power transfer


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UPFC Operation at Rated Capacity

Regulation Mode

Series VSC reaches MVA limit

Shunt VSC reaches MVA limit

Both VSCs reach MVA limit


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4-bus System Example

• Voltage stability analysis – maximum power transfer with various levels of circulating active power between the shunt VSC and the series VSC


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4-bus System Example

V1d=1.03 pu, Pd=0.8Pload, both shunt and series VSCs MVA limit=50MVA


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4-bus System Example• In this system, by increasing Pc from shunt to

series, maximum power transfer is improved – At V3=0.95 pu, a 10 MW increase in Pc results in

20 MW increase in power transfer– If V3 is desired to be 1 pu, we can increase Pc as

Pload increases until it is no longer possible by the controller

• Coupling VSCs improves dispatch flexibility, and power circulation optimizes the coupled VSCs together

• Similar possibility with an IPFC: more complex – 2 active power paths


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Rated Capacity Solution Algorithm

• To generate PV curve, additional logic to switch from non-saturated to saturated operation

• MVA rated capacity operation code functional

• In development:– Fixed Vp,Vq with respect to from-bus

voltage phasor – Fixed Vm, variable power circulation


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• Regulation mode– Regulate voltage and line flow setpoints

• Fixed voltage injection mode for series element– Fix the magnitude and/or angle of the injected

voltage source

• Rated capacity operation mode


V1d, V2d, PdB2B STATCOMPd1, Qd1, Pd2IPFCV1d, Pd, QdUPFCV1dSTATCOM

345/138 kVTransformer

UPFC

SeriesReactor

SeriesReactor

LOGANINEZ

American Electric PowerInez Project


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• AEP Inez UPFC– Power control to limit steady-state and

post-disturbance flow on 138 kV system, thus eliminating thermal overload on the line and preventing low voltages on 138 kV transmission system.

– The overall project also includes mechanically-switched line reactors and shunt capacitors.

Series VSC Control

C. Schauder, et al., “AEP UPFC Project: Installation, Commissioning and Operation of the ±160 MVA STATCOM (Phase I),” IEEE T-PD, 13:1530-1535, 1998


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• NYPA Marcy CSC (Convertible Static Compensation)– NY Central East transfer is voltage limited at

Albany (before the Athans units came online)– In UPFC configuration, fixed (P,Q) setpoint control

to limit post-disturbance flow would not be optimal; prefer UPFC to carry more flow in post-disturbance than in pre-disturbance

– Fixed P setpoint control also reduces system synchronizing torque

– Use fixed inserted voltage control (vd,vq); in post-disturbance, this control will naturally accommodate additional flow; use studies to validate

Series VSC Control


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• NYPA Marcy CSC in UPFC mode– Use fixed inserted voltage control (vd,vq); in

post-disturbance, this control will naturally accommodate additional flow; use studies to validate

– Disturbance initiated control possibilities1. Ramp up P setpoint to some pre-determined

max value in post-disturbance 2. Use rated-capacity strategies as described

earlier to adjust inserted voltage magnitude and circulating power

Series VSC Control


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• Fixed Vm to allow unrestricted active power flow in post-disturbance condition, to be limited by rated current.

• In post disturbance – if Vm is not at limit, increase Vm to

maximum allowable, without violating current and MVA constraints.

– Adjust power circulation Pc (if available) to improve voltage profile.

A Series VSC Control Strategy


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• Synchronous post-contingency system– Constant power transfer– Voltage control with variable power transfer

Back-to-Back STATCOM

Z4V2Z3

V1V3 V4

jXt1

Ish1

+_Vm1

~

~ ~ ~ ~

Psh1

Ish2

jXt2

Vm2~+

_Psh2

System 1 System 2


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• Asynchronous post-contingency system– Constant power transfer– Variable power transfer for frequency

regulation, for loss of load or generation


Z4V2Z3

V1V3 V4

jXt1

Ish1

+_Vm1

~

~ ~ ~ ~

Psh1

Ish2

jXt2

Vm2~+

_Psh2

System 1 System 2


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• Frequency regulation in asynchronous post-contingency system– On-demand feedforward/adaptive +

feedback frequency control– Example: Bradley Lake Hydro control in

Alaska Kenai Peninsula


R. M Johnson, J. H. Chow, and M. V. Dillon, “Pelton Turbine Deflector OverspeedControl for a Small Power System,” IEEE T-PS, 19, 2004


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Bradley Lake Hydro Units• Two 65 MVA Pelton turbines with

deflectors for overspeed protection• Loss of Southern Tie isolates the Kenai

area and causes system oscillations

South North


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Pelton TurbinePower PlantMain Level

Dis

tribu

tor

Deflector Assembly

Turbine Pit


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Overspeed Deflector Control with Adaptive Feedforward Signal


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EPRI/NYPA FACTS Operator Training Simulator (2004-5)

• New York Power has installed a Convertible Static Compensator (CSC) as Marcy substation in Central New York State:– Two VSC modules of 100 MVA each– Two series transformers– One shunt transformer with two primary

windings


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Objectives• Development of an off-line general

purpose FACTS Operator Training Simulator, with customization to NYPA’sCSC (Convertible Static Compensation)

• Learning tool for operators to study the impact of CSC in the NY power system

• All 11 configurations of the CSC operation included in the simulator

S. Arabi, H. Hamadanizadeh, and B. Fardanesh, “Convertible Static Compensator Per-formance Studies on the NY State Transmission System,” IEEE T-PS, 17:701-6, 2002.


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CSC Configurations

Configuration Converter 1 Controlled Quantity

Converter 2 Controlled Quantity

One STATCOM VT – One STATCOM – VT

Two STATCOMs VT VT SSSC on line #1 P1 – SSSC on line #2 – P2

Two SSSCs P1 P2 IPFC P1 (or P1 & Q1) P2 & Q2 (or P2)

One STATCOM & SSSC on line #1

VT P1

One STATCOM & SSSC on line #2

VT P2

UPFC line #1 VT P1 & Q1 UPFC line #2 VT P2 & Q2

Training Simulator SystemMATLAB code

UPFCdispatch

SystemConfiguration

UPFCsettings System

Voltagesand Flows

UPFC control diagram Station one-line diagram

Excelfile

Excelfile

MATLABWorkspace


81Dual display monitors


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GENESIS

• FACTS control screens are identical to those supplied by Siemens for NYPA’s CSC using GENESIS 32 from Iconics(converted from version 3.54 to version 7).

• Simulation data are stored in GENESIS registers (Tags).

• Interface dialog boxes and data population (Input/output) are programmed using Visual Basic.


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Power system toolbox (MATLAB)

• Receive setpoints from an Excel file which contains the data from the GENESIS FACTS control screens.

• Loadflow computation with multiple FACTS dispatch modes.

• Plot one-line diagrams of different substations.

• Send result to an Excel file for use by GENESIS FACTS control screen.


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NYPA CSC Configuration Selection


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STATCOM operation screen


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Marcy CSC one-line diagram


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One-line diagrams of different substations

Substation access screen


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Marcy345 One-line diagram


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UPFC operation screen


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Acknowledgements• Research supported by

– NSF– EPRI: Dr. Aty Edris– NYPA: Drs. Bruce Fardanesh and Edvina

Uzunovic, and others• Students

– Xuan Wei, Xia Jiang• Computer code: Power System Toolbox

– Graham Rogers, Cherry Tree Scientific Software• Thank you for your attention

voltage-sourced converter based facts controller...

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