Power System Oscillations Overvie · •Inter-Area Oscillations can be devastating. Forced Oscillations are problematic… •August 10, 1996 –WECC blackout caused by oscillations

Power System Oscillations

Overview

Mani V. Venkatasubramanian

Washington State University

Pullman WA

Stability Mechanisms

• Voltage Stability

– Sufficient reactive power reserves?

– Voltage collapse

• Small-Signal Stability

– Can damp out small disturbances?

– Oscillatory instability

• Transient Stability

– Can withstand large disturbances?

– Loss of synchronism and islanding

• Operational Reliability

© Washington State University

Real-time security monitors @ WSU

Oscillation

Monitoring

System

Voltage

Stability

Monitor

Angle

Stability

Monitor

PMU

Real-time

data

PMUs

&

PDC

System

Security

Status

Real-time

Display &

Control

TVA, Entergy, Peak RC

Southern CO

Entergy, Southern CO

4

Inter-Area Oscillations

Powerworld Animation © Washington State University

Small-signal Instability in WECC

Northwest

California

COI

lines

August 10, 1996 WECC blackout

Negative damping

Growing oscillations

Unstable 0.25 Hz

Inter-area mode 5


August 10, 1996 western event

• Three 500 kV lines out

• 14.01 to 14.52: Two 500 kV lines tripped owing to faults. Damping drops from high to low

• 15.42: 500 kV Keeler-Alston line tripped: Damping drops to near 3%

• 15.47: 230 kV Ross-Lexington line trips. Poorly damped inter-area oscillations start. Additional trippings. Growing oscillations. Blackout.


6

August 10, 1996 Sequence of Events

• Heavy summer loading conditions

• Low voltages in Pacific Northwest

• 15.42 PDT: Trip Keeler 500 to Allston

500 (Case 0)

• 15.47 PDT: Trip a) Ross 230 to

Lexington 230, b) St.John 115 to View

Tap 115 and c) Swift generator (200

MW) (near Portland) (Case 1)



• McNary generation starts tripping from faulty field protection relay. Oscillations appear. (Case 2)

• 15.48 PDT: Nearly all McNary units trip out. AGC ramps up Coulee, John Day generation for load balancing. Oscillations grow. (Case 3)

• 200 MVAR Cap bank switched in at Malin 500. (Case 4). Little effect. COI lines tripped. Blackout.



Case

0

Case

1

Case

2

Case

3

Case

4


August 10, 1996 event

Analysis of Validated Model

Small-signal

Stable

Small-signal

Unstable

0.25 Hz inter-area mode changed from +2%

positive damping to -2% negative damping© Washington State University

Entergy, Southern COEntergy, Southern CO

Two Types of PMU Analysis Engines

Event Analysis

0.4 Hz at +10% damping. Inter-Area Mode.

Ambient Noise Analysis

0.4 Hz at +10% damping. Inter-Area Mode.


PMU Data Analysis of

Aug 10 1996 event

300 350 400 450 500 550 600 650 700 750 800

1100

1200

1300

1400

1500

1600

1700

Time (s)

M

W

Malin to Round_Mountain 1_Line MW flow vs time

Keeler-Allston line trips

Ross-Lexington line trips

Frequency

(Hz)

Damping

Ratio (%)

Prony 0.2651 3.17

Matrix

Pencil0.2664 2.41

HTLS 0.2664 2.40

20 s data of Malin- Round Mountain MW

after the first tripping


13

Before Keeler-Allston trip

150 200 250 300 350 4000

0.5

1

1.5

Time (s)

Fre

qu

en

cy (

Hz) Frequency Estimates

150 200 250 300 350 4000

2

4

6

8

Time (s)Da

mp

ing

Ra

tio (

%)

Damping Ratio Estimates

Mode Damping near 6% before Keeler-Alston trip


After Keeler-Alston trip

600 620 640 660 680 700 7200

0.5

1

1.5

Time (s)

Fre

qu

en

cy (

Hz) Frequency Estimates

600 620 640 660 680 700 7200

2

4

6

8

Time (s)Da

mp

ing

Ra

tio

(%

)

Damping Ratio Estimates


Mode Damping near 2% after Keeler-Alston trip

Oscillations

Natural Mode Oscillations, Forced Oscillations

Well-damped oscillations, Poorly damped oscillations, undamped oscillations, growing oscillations

Cause fatigue in rotor shafts, Reduces rotor life

Power quality concerns

Undesirable tripping of generation units or curtailment

Resonance effect with inter-area modes

Can lead to blackouts


16

Well-damped oscillations


17

Poorly Damped Oscillations


18

Local oscillations


June 17 2016 Mississippi event


19


Oscillation Monitoring

Oscillation Monitoring System for WECC and Entergy

Monitoring hundreds of PMUs simultaneously

System modes are changing – adaptive engines

Interactions with power electronics

Damping Monitor Engine – ambient data analysis

Event Analysis Engine – detection and analysis of

ringdowns and oscillations

Real-time engines and off-line engines


Oscillation Monitoring System

OpenPDC

OMS

OMS real-time version built into OpenPDC.

Off-line version available as stand-alone programs.

IEEE C37.118

BPA PDCstream

Real-time PMU

data streamOMS results

Oscillation

Monitor

openPDC

SQL server

Calculated

measurements


Fast Frequency Domain

Decomposition (FFDD)

– Collect and preprocess signals from PMUs

– Power spectrum estimation by FFT and Multi-

Taper Method

– Apply SVD on the power spectrum

• Approximate the largest singular value by the trace of

the power spectrum matrix (Fast FDD)

– Apply inverse FFT on largest singular values

– Extract pole frequency and damping ratio

from exponential form by ringdown analysis

– Can process 1000+ signals simultaneously.© Washington State University

Medium Resonance on November 29, 2005

• 20 MW 0.27 Hz Forced Oscillation in Alberta Canada.

• System mode 0.26 Hz at around 7% damping.

• 200 MW Oscillations on California-Oregon Inter-tie.

• Resonance Amplification Factor = 10.

• Recent IEEE Trans. paper

Thanks to Greg Stults (BPA) and Jim Burns (BPA)


June 17 2016 Mississippi event


24© Washington State University

Oscillations Summary• Inter-Area Oscillations can be devastating. Forced

Oscillations are problematic…

•August 10, 1996 – WECC blackout caused by oscillations

•Nov 29, 2005 Alberta event - resonance between forced oscillation and (0.25 Hz) inter-area mode.

• Sept 5, 2015 Nevada event –resonance between forced oscillation and (0.4 Hz) inter-area mode.

• June 17, 2016 oscillation event in the east.

• Inter-Area Resonance – potential risk for

operational reliability of the grid

•Urgent need to analyze oscillation sources

and work on damping controllers25

Power System Oscillations Overvie · •Inter-Area Oscillations can be devastating. Forced Oscillations are problematic… •August 10, 1996 –WECC blackout caused by oscillations

Documents