1

Voltage stability assessment and coordinated control utilizing PMUs

Dinh Thuc Duong

Kjetil Uhlen

2

Outline

• Background: From VIP to NiP to STRONgrid

• Overall aim: Coordination and optimal utilization of

FACTS (SVCs) using PMUs – Primary and secondary voltage control

– Wide area power oscillation damping

• Voltage stability assessment with PMUs and partial

state estimation

Acknowledgement: The work presented on

coordinated control has been a collaboration

between ABB, Statnett and Sintef.

SINTEF Energy Research

The Voltage Instability Predictor (VIP)

Z app Maximum power transfer

<=> ZNET = (Zapp)*

ZNET

V

Zapp

network equiv. load

E

Relay tracks system strength

based on local V & I

22

11

IZVE

IZVE

NET

NET

Circle of

radius | Z NET |

SINTEF Energy Research 4

NiP – Vision and time schedule

Control Signals

Decision making

for on-line Control

Monitoring

based on VIP & PMU

J

Hasle

Device

level

VIP box

WEB

HMI

Measurements

Phase 0 - 2000/1 NIP Phase 1 - 2001/2

Processed Data

NIP Phase 2 - 2003/4

Close - Loop

PMU box

SINTEF Energy Research 5

Integration with SCADA/EMS Simple and understandable indicators are needed

VIP example

0.5 1 1.5 2 2.5 0

0.2

0.4

0.6

0.8

1

1.2

PLoad (pu)

Vo

lta

ge

(p

u)

Nose Curve

Worst Contingency

Nose Curve

Normal Operation

Actual Operation

Point

Pmargin

SINTEF Energy Research 6

Pilot node voltages

SVC :

+/- 160 Mvar

SVC :

+/- 250 Mvar

SVC :

360 Mvar

SVR

control

Set point

controls

Voltage setpoint

Coordinated Secondary Voltage control

SINTEF Energy Research 7

Coordinated Secondary Voltage control

(SVR scheme)

Overall objective of the SVR scheme is:

To maintain a sufficient and balanced reactive reserve in SVC units and rotating condensers during normal operation.

To keep the pilot node voltages within specified limits.

Benefits:

Desired voltage profiles are maintained automatically.

By having automatic and coordinated control of the SVCs, the operators can concentrate on maintaining the necessary reactive reserves (e.g. by connecting/disconnecting capacitor banks).

Motivation: Transfer limits are normally determined from the ”N-1” security criterion. Sufficient on-line reactive reserves are important in order to reduce the consequences of critical contingencies.

SINTEF Energy Research 8

Operation: Principles and characteristic

Voltage [kV] as function of sum reactive power [MVar]

360

380

400

420

440

-1000 -800 -600 -400 -200 0 200 400 600 800 1000

Voltage [kV]

ReserveNormal operating range

Gain: K = 50 Mvar/kV

[Mvar]

SINTEF Energy Research 9

SVC + SC :

-250/320 Mvar

SVC :

+/- 250 Mvar TCR :

-360 Mvar

SC :

-90/250 Mvar

SINTEF Energy Research 10

First day

operational results

Trend curves of pilot

node voltage (420 kV

Frogner) and total Mvar

setpoint.

14 hours of operation

FROGNER420BU_P

400

405

410

415

420

425

430

00:00:00 02:00:00 04:00:00 06:00:00 08:00:00 10:00:00 12:00:00 14:00:00

Time of day (2003-12-05)

Pilo

t n

od

e v

olt

ag

e [

kV

]

DIV_RCCSSVCBEREGNET_Q

-500

-400

-300

-200

-100

0

100

00:00:00 02:00:00 04:00:00 06:00:00 08:00:00 10:00:00 12:00:00 14:00:00

Time of day (2003-12-05)

Re

ac

tiv

e p

ow

er

se

tpo

int

[Mv

ar]

SINTEF Energy Research 11

410

415

420

425

430

-500 -400 -300 -200 -100 0 100

Pilot node voltage [kV]

Su

m r

ea

cti

ve

po

we

r [M

va

r]

First day operational results

Measured pilot node voltage versus reactive power setpoint

Gain:

K = 33 Mvar/kV

After increasing the

gain to: K = 50 Mvar/kV

SINTEF Energy Research 12

Testing dynamic

response

Step change in voltage

setpoint: 415 420 kV

Gain: K = 50 Mvar / kV

Response time (time

constant): ~1-2 minutes

«step-wise» response

due to SCADA

implementation

Pilot node voltage [kV]

412

414

416

418

420

422

13:15:00 13:20:00 13:25:00 13:30:00 13:35:00 13:40:00 13:45:00

Time (2003-12-05)Sum reactive power [Mvar]

-400

-300

-200

-100

0

100

13:15:00 13:20:00 13:25:00 13:30:00 13:35:00 13:40:00 13:45:00

Time (2003-12-05)

SINTEF Energy Research

Use phasor measurements from several pilot nodes

Optimize the use of the SVCs and SCs

Coordinate with stabilizing controls

13

Pilot nodes

Voltage phasors

SVC :

+/- 160 Mvar

SVC :

+/- 250 Mvar SVC :

360 Mvar

SVR

control Set point

controls

Voltage setpoint

Further developments

14

“Wide-area POD” design

To improve damping of low frequency inter-area modes:

Targeting more than one mode: Two main inter-area modes of interest

(approximately at 0.3 Hz and 0.5 Hz)

Observability: New measurements available by utilizing the PMUs

Controllability: Several SVCs available (see map)

Robust design: Ability to improve damping in a largest possible

range of operating conditions.

Coordinated design: Avoid adverse interactions

Between different SVCs

Between other control objectives (e.g. voltage control)

Field test of WA-POD in Hasle

Field test in Hasle substation 8 November 2011-11-15.

Purpose: Test and verification of Wide Area Power

Oscillation Damper (WA-POD) on a 180 Mvar TCR

Static Var Compensation unit in Hasle.

System damping measured and analysed during

disconnection and re-connection of the 420 kV line

Hasle-Tegneby.

Performance of WA-POD was tested and compared

against ABB’s state of the art Phasor-POD and the case

without any power oscillation damper.

Wide area measurements for

POD Control

PMUs streaming phasors from:

Nedre Røssåga

Kristiansand

SVC Located at Hasle

PDC receiving voltage phasors

Extracts voltage phasor angle

ABB Mach2 Controller

Local control

WAPOD Control

Switch-over logic

Hasle

Nedre

Røssåga

Kristiansand

Hasle SVC (4x90 Mvar TCR)

SVC and WA-POD implementation

Voltage Regulator

POD Control

Switch-Over Logic

WAPODControl

( )V t

( )V t

refV

Voltage Setpoint

( )V t

Slope

Voltage Meas.

From PDC

( )ijP t

From Adjacent Line

( )ttV

( )shB t

( )refTOT

tV

( )SVCtQ

1 1( ), ( )

L Lt Q tP

Line used for Testing

From Norway (Tegneby S/S)

From Norway (Tveiten S/S)

From Sweden (Borgvik S/S)

From Sweden (Halden S/S)

2 2( ), ( )

L Lt Q tP

( )t 1V

2V

2 × 90 Mvar TCR

Test: Switching of 420kV Hasle-Tegneby

20 30 40 50 60 70 80

-55

-50

-45

-40

Time (sec)

Vo

ltag

e A

ng

le D

iffe

ren

ces

(deg

)

No POD, 11:49:30.000

WAPOD, 12:39:30.000

Local POD, 13:04:30.000

20 30 40 50 60 70 80

0.6

0.7

0.8

0.9

1

1.1

1.2

Time (sec)

Act

ive

Po

wer

(p

u)

No POD, 11:49:30.000

WAPOD, 12:39:30.000

Local POD, 13:04:30.000

Disconnection of 420 kV

Hasle-Tegneby

0 5 10 15 20 25 30-50

-48

-46

-44

-42

-40

-38

Time (sec.)

Vo

lta

ge

an

gle

fiff. (

alig

ne

d)

No POD

WAPOD

Local POD

Smart Transmission Grids Operation and Control

A project funded by

– And co-funded by Nordic TSOs and DSOs

• Objectives:

– Address the challenges that the secure and reliable operation of the

power grids will face in the future.

– Support the development of better tools for planning, operation and

control of power grids

– We seek to establish an interdisciplinary theoretical and

experimental foundation for research and development

Project partners

Institute of Physical

Energetics

Ind

ust

ry

Aca

de

mia

Stage 1

• PMUs are connected at the LV networks

LV PMU Network in STRONg2rid

On-line PDC

Planned PMU

On-line PMU

NI ABB SEL

SEL PDC Open PDC

PMUs

PDCs

PRL

Research platform

PMU

PDC-KTH

PMU

PDC-NTNU

PMU PMU

PMU

PDC-DTU

PMU

PDC-Aalto

PMU PMU

KTH NTNU

AaltoDTU

Each University (PMUs, PDC)

• Output stream for each other university (only

their PMUs)

Laboratory activities: – Wind power conversion systems and grid connection – Converter controls for Multi-Terminal HVDC – PMU placement

Renewable Energy / Smart Grids Laboratory

26

Statnett PRL

Synchrophasor Software Development Kit • Real-Time Data Mediator for LabView

Sample Application WAMS Visualization Tool - Mobile

• Portable Monitoring Applications

”On-line voltage stability assessment”

28

PMU

PMU

«VIP» PMU

Objective:

To develop a robust

and simple voltage

stability assessment

tool for on-line use

Topology

Power

flow data

SCADA/EMS

State estimator

29

Boundary selection and system reduction

B1

study area

B2

B3B4 B5

G1 G2

B1

study area

B2

G1 G2

S1S2

study area

Sub 1

G1

Sub 2

132kV

400kVPMU

#1

Example of subsystem boundary in sub-transmission system

B1 and B2 are strong buses

30

1. Voltage stability assessment based on impedance matching

ZTh

ZL

IL

VL

ETh

𝑚𝑎𝑟𝑔𝑖𝑛 =𝑆𝑚𝑎𝑥 − 𝑆𝐿

𝑆𝐿× 100% 𝐸𝑇ℎ = 𝑉𝐿 + 𝐼𝐿𝑍𝑇ℎ

𝑍𝐿 = 𝑍𝑇ℎ

𝑆𝑚𝑎𝑥 =|𝐸𝑇ℎ

2 | |𝑍𝑇ℎ| − 𝑖𝑚𝑎𝑔 𝑍𝑇ℎ sin𝛿 + 𝑟𝑒𝑎𝑙 𝑍𝑇ℎ cos𝛿

2 𝑖𝑚𝑎𝑔 𝑍𝑇ℎ cos𝛿 − 𝑟𝑒𝑎𝑙 𝑍𝑇ℎ sin𝛿 2

Smax is prefered as main indicator because of small error

𝐼𝑆𝐼 =𝑍𝑇ℎ𝑍𝐿

31

2. Estimation of Thevenin equivalent

L2

Line 1

G1L1

T1 B2B1

ZL1

ZL

ZL2

ZT

EThEG

B1 B2

Fig. 1: A simple two-node grid.

Fig. 2: Equivalent circuit of the two-node grid

32

2. Estimation of Thevenin equivalence (cont.)

• Thevenin impedance

• Admittance matrix

• Modified admittance matrix, including load L1 but

excluding critical load, L2

𝑌 =

1

𝑍𝐿+

1

𝑍𝑇−

1

𝑍𝐿

−1

𝑍𝐿

1

𝑍𝐿

𝑌𝑒𝑞 =

1

𝑍𝐿+

1

𝑍𝑇+

1

𝑍𝐿1−1

𝑍𝐿

−1

𝑍𝐿

1

𝑍𝐿

𝑍𝑇ℎ = 𝑍𝐿 +𝑍𝐿1𝑍𝑇

𝑍𝐿1 + 𝑍𝑇

33

2. Estimation of Thevenin equivalence (cont.)

• “Full” admittance matrix, including critical load L2

Z’Th

B2

E’Th

B1ZTh

ZL

IL

VL

ETh

1 2

34

3. Effect of limited active power on Thevenin impedance

ZLoad

ZL2ZL1

V1 V2

Simulation:

• V1: unlimited source.

• V2:

- constant terminal voltage

- P = 10 MW (constant)

Thevenin impedance:

𝑍𝑇ℎ = 𝑍𝐿1//𝑍𝐿2

Not equal

0 10 20 30 40 5040

60

80

100

120

140

160

Impedance (

Ohm

)

Time (s)

0 10 20 30 40 500

50

100

150

200

250

300

Load p

ow

er

MV

A

Load pow er

Load impedance

Thevenin impedance

35

3. Effect of limited active power on Thevenin impedance (cont.)

#1

#2 #3 #4 #5

#6

#7#8

G1 G2

#1

#2

#8

G1

G2

Z18

Z28 Z88

#1 #8

G1

Z18

Z88 Z82new

#1

#2

#8

G1

G2

Z18

Z28new

Z88

36

3. Effect of limited active power on Thevenin impedance (cont.)

0 10 20 30 40 5010

20

30

40

50

60

Time (s)

Appare

nt

pow

er

(MV

A)

With modification

Load

Unmodified Thevenin equivalence

37

Application example: Analysis of voltage stability in the Lofoten peninsula

https://maps.google.no/maps?ll=68.532256,15.320435&spn=1.536177,5.559082&t=m&z=8

38

Boundary selection and system reduction

B1

study area

B2

B3B4 B5

G1 G2

B1

study area

B2

G1 G2

S1S2

study area

Sub 1

G1

Sub 2

132kV

400kVPMU

#1

Example of subsystem boundary in sub-transmission system

B1 and B2 are strong buses

39

Grid model in Lofoten area

Kvandal-Sør

58043

Skjomen

58113

Kanstadbotn

58173

Kvitfors

58183Fygle

58187

Kvitnes

58293

Hinnøy

58213

Melbu

58193

Stokmarknes

58168

Sortland

58203

Risøyhamn

58223

Kilbotn

58253

Medkila

58333

Gråheia

58233

Gåsvatn

58283

Heggen

58263Møkkland

58243

Ma

in tra

nsm

issio

n g

rid

40

Results from simulation on Norwegian transmission system

0 10 20 30 40 500

5

10

15

20

25

Time (s)

Impedance (

pu)

Thevenin impedance

Load impedance

0 10 20 30 40 500.04

0.05

0.06

0.07

0.08

Time (s)

Appare

nt pow

er

(pu)

Estimated max loadability

Load

Increase all loads in study area

0 5 10 150

5

10

15

20

25

Time (s)

Impedance (

pu)

Thevenin impedance

Load impedance

0 5 10 150.04

0.05

0.06

0.07

0.08

Time (s)

Appare

nt pow

er

(pu)

Estimated max loadability

Load

• Boundary: one 420/132kV substation and one large power plant.

• Voltage: 132kV with 30 buses.

• Only 30 bus are taken into the algorithm. Simulation is run on the whole

system.

Increase the weakest load in study area

41

Results from simulation on Norwegian transmission system

Trip of one critical line and Voltage collapse due to load restoration

0 1 2 3 4 50

5

10

15

20

25

Time (s)

Impedance (

pu)

Thevenin impedance

Load impedance

0 1 2 3 4 50

0.02

0.04

0.06

0.08

0.1

Time (s)

Appare

nt pow

er

(pu)

Estimated max loadability

Load

42

Resume

• Background: From VIP to NiP to STRONgrid

• Overall aim: Coordination and optimal utilization of

FACTS (SVCs) using PMUs – Coordinated secondary voltage control

– Wide area power oscillation damping

• Voltage stability assessment with PMUs and partial

state estimation – focusing on the area of concern and using all available information

• Focus on practical applications and tool

43

3. Effect of limited active power on Thevenin impedance (cont.)

2

4

Th

Th

n

ER

P

RTh

RL

IL

VL

G1

ETh

- Fictitious impedance

Modelling limited power source:

- Rated active power of G1: Pn

2

4

Th

Th

n

EZ jX

P

0 5 10 15 20 25 30 35 40 45 5040

60

80

100

120

140

160

Impedance (

Ohm

)

Time (s)

0 5 10 15 20 25 30 35 40 45 500

50

100

150

200

250

300

Load p

ow

er

MV

A

Load pow er

Load impedance

Thevenin impedance

0 10 20 30 40 5040

60

80

100

120

140

160

Time (s)

Pow

er

(MV

A)

Estimated

Load

j

44

3. Effect of limited active power on Thevenin impedance (cont.)

An example of a small meshed system:

#1

#2 #3 #4 #5

#6

#7#8

G1 G2

small

E EB

eq

EB I

Y YY

Y Y

• There is no direct connection from

generators to considered load.

• Keep all generators and the

considered load. Simplify the rest by

Gaussian elimination technique

IeqYeqY