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PSERC

Mani V. Venkatasubramanian

Washington State University

Oscillation Monitoring System

PSERC S29 Project Tele-seminar

October 28, 2008

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PSERC

Overview of S29 project

• S19 project from June 2002 to May 2005

• S29 project from June 2006 to July 2008

• Detection, Prevention and Mitigation of Cascading Events

• Three tasks:

• Task I: Detection: Mladen, Texas A&M

� Advanced warning

• Task II: Prevention: Mani, Wash. State

� Wide-area monitoring and controls

• Task III: Mitigation: Vijay, Iowa State/ASU

� Adaptive islanding

• S29 Focus on Prototype Implementations

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PSERC

Task II: Mitigation

• Implementation of Wide-area Small-signal Stability controller, Wash. State University

• Two subtasks:

� Reliable Oscillation detection:• Multi-input Prony, Matrix Pencil and HTLS algorithms

• Rules for real-time analysis of data

• Noise? Linear versus nonlinear? Switching events?

� Real-time design of damping controls:• Which control to trigger? What design?

• Not part of the prototype testing

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PSERC

Problem Overview

• Low frequency electromechanical oscillations

� Local or inter-area oscillations

� Insufficient damping

� Example - Aug 10, 1996 WECC blackout

• Detection and control of small signal stability problem in power systems

• Research supported by PSERC, TVA, Entergy, BPA and EPG (CERTS)

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PSERC

Task II Project Team• WSU:

• Guoping Liu, Qiang Zhang, Jaime Quintero, Mani V. Venkatasubramanian

• TVA:

� Ritchie Carroll, Gary Kobet, Lisa Beard• Entergy:

� Floyd Galvan, Sujit Mandal, Sharma Kolluri• BPA:

� Bill Mittelstadt, Dmitry Kosterev

• EPG: � Manu Parashar

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PSERC

Oscillation Monitoring System (OMS)

• Goal of Oscillation Monitoring System (OMS)

� Early detection of poorly damped oscillations as they appear

� Trigger warning or control signals

• OMS is made possible by Wide Area PMU Measurements

� Growing numbers of PMUs across the power grid

� Fast algorithms available for online measurements

� Rule based automatic analysis of PMU measurements

� Prototype implementation at TVA

SVC SVC

SVC

LOAD

LOAD

LOAD

LOAD

PDC

P

PP

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PSERC

OMS Flowchart

Event

Analysis Engine

Damping

Monitor Engine

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PSERC

TVA Cumberland event

Four minutes of oscillations

Source: Gary Kobet/TVA

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PSERC

TVA Cumberland Event

• Recent oscillatory event at TVA:� Oscillations at Cumberland plant 9/18/2006� PMU recordings enabled the analysis� Local 1.2 Hz mode changed from +1.5% damping

to –0.2% damping and back to +1.5% damping during the event

� PSS installed at the plant subsequently� PMU based real-time alarm coded by TVA into TVA

PDC as back-up measure – uses standard deviation thresholds – plant operators to reduce MW output when alarm received.

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PSERC

Oscillation Monitoring System

• Software Engines built into TVA PDC

• Real-time streaming data input to the engines

• Fast detection of poorly damped oscillatory modes: mode frequency, damping and mode shape

• Multiple algorithms integrated by expert system like rules

• Focus on Redundancy and Reliability

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PSERC

OMS Engines

• Event Analysis Engine

� Automated Prony type analysis of oscillatory ringdown responses

� Five seconds of PMU data analyzed every one

second

• Damping Monitor Engine

� Automated analysis of ambient noise data

� Three minutes of PMU data analyzed every ten

seconds

� Provisional Patent application filed by WSU

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PSERC

Results from Two Engines

820 840 860 880 900 920 940

5.09

5.1

5.11

5.12

5.13

5.14

5.15

x 105

Time (s)

Bus Voltage Magnitude at Cumberland

Event

Analysis

1.2 Hz at +1.5% damping. Local Mode.

Ambient Noise Analysis

1.2 Hz at +1.8% damping. Local Mode.

Nov. 29th 2007 TVA event

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PSERC

Mode Shape – Local Mode

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Real Part

Imagin

ary

Part

Mode Shape Identified by FDD at 1.224 Hz

TVA_COLN_MAIN

TVA_CUMB_BUS1

TVA_CUMB_BUS2

TVA_HEND_BUS1

TVA_LOWN_161K

TVA_LOWN_500K

TVA_MARS_CUMB

TVA_RIDG_BUS1

TVA_RIDG_BUS2

CumberlandOther PMUs

Cumberland oscillating against rest of system

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PSERC

Basics of Prony Analysis

uDxCy

uBxAx

+=

+=&

• Assumptions: Linear Time-Invariant System, Distinct Eigenvalues, Step changes

in input, …

• Any output is a linear combination of fundamental modal responses

• Well-suited for Prony type curve fitting methods. Estimate oscillatory frequency,

damping ratio and mode shape.

• Estimates should be consistent:

• Moving time-windows (Linearity of responses)

• Different groupings of outputs (Superposition)

ta

j

ijjdj

t

j

iji

jnjj ectecty−−

∑∑ +−= )cos()( ϕωωζ

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PSERC

Power System Prony Analysis

• Nonlinear Large Scale System

• In theory, Prony Analysis works well for analyzing “Small-disturbance responses”

• Nonlinearity dominant just after large disturbances

• Switching of lines and cap banks in the middle of analysis windows

• Noise effect on results if disturbance “fades away”

• How to get reliable estimation automatically?

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PSERC

Rules for Real-time Prony Analysis

Three types of Consistency Crosscheck rules

� Different Curve-fitting Methods (Redundancy)

� Different Signal Groups (Superposition)

� Moving Window Analysis (Linearity of Reponses)

60 80 100 120 140 160 180

534

536

538

540

542

544

546

548

550

Time (s)

MALN_Malin

_Bus_Voltage_VMag

MALN_Malin_Bus_Voltage_VMag vs time

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Event Analysis Engine

Local

PMU

Analysis

Inter-area

Mode

Analysis

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Local PMU Analysis

• Signals from one PMU used at a time

• Parallel implementation of multiple PMU analysis

• Parallel implementation for multiple algorithms

• Check for consistency using rules:

� Crosscheck results from Prony, Matrix Pencil and HTLS

� Crosscheck results among moving time-windows

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PSERC

Interarea Mode Analysis

• Identify interarea modes and related PMUs from local analysis

• Grouping of signals from relevant multiple PMUs

• Check for consistency using three sets of rules:

� Crosscheck results from Prony, Matrix Pencil and HTLS

� Crosscheck results among moving time-windows

� Crosscheck results from different groupings

• Parallel implementation for different interareamodes

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PSERC

Event Analysis – Inter-area Example

60 80 100 120 140 160 180

534

536

538

540

542

544

546

548

550

Time (s)

MA

LN

_M

alin

_B

us_V

oltage_V

Mag

MALN_Malin_Bus_Voltage_VMag vs time

• WECC Aug. 4, 2000

• Alberta system separated at 19:56 GMT

• 0.27 Hz oscillation is poorly damped

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PSERC

Case Study 1 – Local PMU Analysis

60 70 80 90 100 110 120 130

0.1

0.2

0.3

0.4

0.5

Time (s)

Fre

qu

en

cy

(H

z)

Frequency estimates

Prony

MatrixPencil

HTLS

60 70 80 90 100 110 120 130

-0.2

-0.1

0

0.1

0.2

Time (s)

Da

mp

ing

ra

tio

Damping ratio estimates

Prony

MatrixPencil

HTLS

60 70 80 90 100 110 120 130

0.1

0.2

0.3

0.4

0.5

Time (s)

Fre

qu

en

cy

(H

z)

Frequency estimates

Prony

MatrixPencil

HTLS

60 70 80 90 100 110 120 130

-0.2

-0.1

0

0.1

0.2

Time (s)

Da

mp

ing

ra

tio

Damping ratio estimates

Prony

MatrixPencil

HTLS

Grand Coulee Malin

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PSERC

Inter-Area Oscillation Mode

60 70 80 90 100 110 120 130

1

2

3

4

5

6

7

8

Time (s)

Consistent local estimates vs time

BE50

COLS

GC50

MALN

JDAY

KEEL

VNCT

DVER

Consistent estimate

at 106 sec

Oscillation frequency =

0.286 Hz

Mean damping ratio =

+2.77%

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PSERC

Event Monitor – Local Mode

• Consistent estimate at +9 sec

• Frequency = 1.1785 Hz. Damping at 0.04%

320 325 330 335 340 345 350 355 360 365

2.945

2.95

2.955

2.96

2.965

2.97

2.975

x 105

Time (s)

Vo

lta

ge

at

PM

U 2

(V

)

PMU 2

335 340 345 350 355 360

1

2

3

4

5

6

7

8

Time (s)

Consistent local estimates vs time

PMU

PMU

PMU

PMU

PMU

PMU

PMU

PMU

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Case Study 3 – Growing Oscillations

• Consistent estimate at 809 sec; Local mode.

• Frequency = 0.6930 Hz. Damping ratio = -0. 12%

720 740 760 780 800 820 840 860 880

5.225

5.23

5.235

5.24

5.245

5.25

5.255

5.26

5.265

5.27

5.275

x 105

Time (s)

Voltage a

t P

MU

1 (

V)

PMU 1

740 750 760 770 780 790 800 810 820

0.5

0.6

0.7

0.8

0.9

Time (s)

Fre

quency (

Hz)

Frequency estimates

Prony

MatrixPencil

HTLS

740 750 760 770 780 790 800 810 820

-0.2

0

0.2

0.4

Time (s)

Dam

pin

g r

atio

Damping ratio estimates

Prony

MatrixPencil

HTLS

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PSERC

Damping Monitor Engine

• Frequency Domain Decomposition (FDD) algorithm proposed for off-line analysis in other areas

• Extended for real-time PMU analysis by Guoping

• Can detect damping ratio as well as mode shape of poorly damped oscillatory modes using short spans of ambient PMU data

• Mode shape information critical for correctly identifying problematic mode towards control actions

• Excellent results for modes with damping ratio up to +10%

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PSERC

FDD for Ambient Analysis

• FFT and Power Spectra from multiple signals

• Clean up spectra using Singular Value Decomposition procedure near dominant modes

• Prony type damping analysis after Inverse FFT around dominant modes

• ISCAS 2008 paper

• Simultaneous extraction of mode damping and mode shape from ambient data

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PSERC

Example of Damping Monitor Engine

300 400 500 600 700 8001000

1100

1200

1300

1400

1500

1600

1700

Time (s)

Activ

e P

ow

er (M

W)

Active Power from Malin to Round Mountain #1

Keeler-Allston Line trips

Ross-Lexington Line trips

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PSERC

Before Keeler-Allston Trip

150 200 250 300 350 4000

0.5

1

1.5

Time (s)

Fre

quency (H

z) Frequency Estimates

150 200 250 300 350 4000

2

4

6

8

Time (s)Dam

pin

g R

atio

(%

)

Damping Ratio Estimates

Dominant mode is McNary local mode

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PSERC

Before Keeler-Allston trip

150 200 250 300 350 4000

0.5

1

1.5

Time (s)

Fre

quency (H

z) Frequency Estimates

150 200 250 300 350 4000

2

4

6

8

Time (s)Dam

pin

g R

atio

(%

)

Damping Ratio Estimates

Second Dominant mode is COI interarea mode

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PSERC

After Allston-Keeler Trip

600 620 640 660 680 700 7200

0.5

1

1.5

Time (s)

Fre

qu

ency (

Hz) Frequency Estimates

600 620 640 660 680 700 7200

2

4

6

8

Time (s)Dam

pin

g R

atio

(%

)

Damping Ratio Estimates

Dominant mode is COI interarea mode

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PSERC

After Keeler-Allston Trip

600 620 640 660 680 700 7200

0.5

1

1.5

Time (s)

Fre

quency (H

z) Frequency Estimates

600 620 640 660 680 700 7200

2

4

6

8

Time (s)Dam

pin

g R

atio

(%

)

Damping Ratio Estimates

Second Dominant mode is McNary local mode

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PSERC

Complementary Engines

• Event Analysis Engine

� Three algorithms: Prony, Matrix Pencil and HankelTotal Least Square. Crosscheck Rules.

� Aimed at events resulting in sudden changes in damping

• Damping Monitor Engine

� Ambient noise based. Continuous.

� Frequency Domain Decomposition Algorithm. (Prony). Crosscheck Rules.

� Provides early warning on poorly damped modes

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PSERC

Example of results for TVA

+3.0%

+1.8%

No data

+1.7%

Damping Monitor

PSS offline (1U)+4.0%Feb. 5, 2008

PSS offline (2U)+1.5%Nov. 29, 2007

PSS installed (1U)+7.2%Dec. 16, 2006

No PSS (2U)+1.7%Sept. 18, 2006

PSS StatusEvent Analysis

Damping history of 1.2 Hz mode

PSS status and effectiveness from the damping level of the local mode.

PSS not effective for two units in service. PSS hardware problem

detected and fixed (June 2008).

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PSERC

Eastern System Interarea Mode• Interarea mode frequency varies between 0.4 Hz to 0.5 Hz

depending on season.

• Damping Monitor (ambient noise) showed the mode to be

poorly damped around +3% to +5% seasonally.

• 0.47 Hz Interarea mode clearly visible in Event Analysis of

Feb. 26th 2008 Florida blackout event.

• Mode involves many eastern control areas

• Frequency ~ 0.47 Hz, damping ~ +7%, on Feb. 26th 2008

• Likely not related to the blackout. Mode damping at +7% is

comparable to the interarea modes in the western system.

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PSERC

Possible Control Actions

• Operator Actions

� Reduce Critical Tie-line Transfers

� Switch Damping Enhancement Controls at Specific Thyristor Devices – SVC, HVDC

• Automatic Control Actions

� Switching of Damping Controls: Series Capacitors, Shunt Capacitors, Thyristor Devices

� Generation Rescheduling

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PSERC

SVC Damping Control

• Stressed Operating Condition

• Near-by tie line active power-flow used as control input

• Sending end => Phase Lag Compensator

• Receiving end => Phase Lead Compensator

SVCSVC

P

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PSERC

HVDC Damping Control

• Stressed Operating Condition

• Phase Angle Difference used as Control Input

• Phase Lead Compensator for HVDC Modulation

• Effective Improvement in Damping of InterareaMode for Diverse Levels of AC Power Transfer

PAC

PDC

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PSERC

• Successful implementation of real-time code into TVA PDC

• Advanced signal processing algorithms for oscillation analysis of events and ambient noise

• Automatic detection of poorly damped electromechanical modes and their mode shape

• Operator alerts, Operator alarms, Control actions, …

• Provides early warning on emerging oscillatory problems

• Can validate effectiveness and status of PSS at generators when PMU near generator

OMS Summary

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PSERC

Future Work

• Testing and tuning at TVA

• Conversion of OMS code to 64 bit architecture

• New dedicated eight processor machine with 32 GB dynamic memory at TVA

• OMS Engines for Eastern Grid?

• Operator Alerts and Alarms? Operator Actions?

• Implementation and testing at BPA and California ISO in collaboration with EPG, Operator Actions?

• Implementation at Entergy