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Title
MANITOBA HVDC RESEARCH CENTRE,
a Division of Manitoba Hydro International Ltd.
Sub Synchronous Oscillations –
An Introduction
October 2016
Presented by: Dharshana Muthumuni
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International Ltd.
Presentation Outline
Title
• General background
• AC Network characteristics
• Series compensation and impact on SSR
• Types/Classification of SSO
– SSR
• IG and SSTI
– Transient Torques
– HVDC – SSTI
– SSCI – Wind
• Simulation examples
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Title
Introduction / Background
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Introduction / Background
Title
Transients in the Electric Network
• Transients following a system disturbance should damp out
in a reasonable timeframe.
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Introduction / Background
Title
Transients in the Electric Network
Voltage and current transient frequencies
– High frequency transients
– ‘Traditional’ electromechanical oscillations
(close to system frequency on system
side, 0.1 – 5 Hz on mechanical side)
– Frequencies below system frequency (60
Hz)
• Sub Synchronous Oscillations
9.7 Hz
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Introduction / Background
Title
Network Characteristics
System Impedance Vs Frequency Plots
Base Case - Case1
1.4k
case0
case1
1.2k
Z+(ohms)
1.0k
0.8k
0.6k
0.4k
0.2k
0.0
Hz
0.0
0.2k
0.4k
0.6k
0.8k
1.0k
1.2k
A Typical frequency characteristic
Following a system disturbance currents (and voltages) corresponding to
network resonance frequencies can flow in the system (and hence into
generators stator windings).
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
...
...
...
Introduction / Background
Title
Network Characteristics
Following a system disturbance currents (and voltages)
corresponding to network resonance frequencies can flow in the
system (and hence into generators stator windings).
Base Case - Case1
Main : Graphs
Eb
300
1.4k
200
1.2k
case1
1.0k
Z+(ohms)
100
y
0
-100
0.8k
0.6k
0.4k
0.2k
-200
0.0
-300
0.170
case0
0.190
0.210
0.230
0.250
0.270
...
...
...
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Hz
0.0
0.2k
0.4k
0.6k
0.8k
1.0k
1.2k
...
...
...
Mechanical Oscillations
Title
Electric network
Gen
Speed - W
 Constant speed mechanical
rotation => 60 Hz on electrical
side
 1 Hz mechanical oscillations =>
59 Hz and 61 Hz
 8 Hz mechanical oscillations =>
52 Hz and 68 Hz
•
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
fm Hz mechanical oscillations =>
(60-fm) Hz and (60 + fm) Hz
Mechanical shaft-mass system
Title
δ
IP
HP
LP
•
G
•
Speed - Wk
•
•
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
In steady state (constant torque
transmission) each shaft rotates
at a constant speed and with a
constant ‘twist’.
Following a disturbance (network
or mechanical system side), the
shafts will go through a ‘transient’
period.
The oscillation frequencies will
correspond to ‘natural
frequencies’ of the mass-shaft
system.
Damping associated with the
mechanical system and the
damping provided by the
response of the electrical system
will determine the nature of the
oscillations.
Mechanical shaft-mass system
Title
• Interaction between the mechanical and electrical system:
– Damping associated with the mechanical system and the damping
provided by the response of the electrical system will determine
the nature of the oscillations.
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Mechanical Resonance Frequencies
Title
δ
IP
HP
LP
G
Mechanical modes can be
calculated from shaft data:
 Inertias of individual masses (J)
 Shaft Spring Constant (K)
Speed - Wk
fi=
60Hz - 25 Hz => 35 Hz
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
1 𝐾𝑖 .𝑊𝑏
√
2𝜋
2𝐻𝑖
Typical resonant frequencies of
Thermal Generator Turbine
units are in the range of 15-25
Hz.
Title
Series compensation and impact on SSR
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Series compensation and impact on SSR
Title
Impact on network characteristics (System Impedance Vs Frequency)
 Network resonance points can shift to the sub synchronous frequency
range (<60 Hz)
 Higher the level of series compensation, the resonant point moves toward
the system frequency
• Increases potential risk of SSO related issues
Base Case - Case1
1.4k
case0
case1
1.2k
Z+(ohms)
1.0k
0.8k
0.6k
0.4k
0.2k
0.0
Hz
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
0.0
0.2k
0.4k
0.6k
0.8k
1.0k
1.2k
..
..
..
Series compensation and impact on SSR
Title
Higher the level of series compensation, the resonant point moves toward
the system frequency
Frequency Scan - Generator Terminals
1.4
|Z+| (ohms)
1.2
1
45% compensation
0.8
55% compensation
0.6
60% compensation
0.4
75% compensation
0.2
Frequency (Hz)
𝑋𝑐
𝑓𝑒 = 𝑓0√
𝑋𝑙
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
57
53
49
45
41
37
33
29
25
17
21
13
9
5
0
Title
Types / Classification of SSO
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Types/Classification of SSO
Title
1. Self Excitation (Induction generator effect)
 Purely electrical phenomenon
 Self-excitation of Sub Synchronous Oscillations
 Generator ‘acts’ as a NEGATIVE resistance at the sub synchronous frequencies. If this
effective negative resistance is greater in magnitude compared to the positive resistance as
seen from the system, an unstable resonance condition can take place.
Machine Output Volts p.u.
1.100
1.050
1.000
0.950
0.900
0.850
0.800
0.750
0.700
Gens
Generator
Series cap.
2.50
2.00
1.50
1.00
0.50
0.00
-0.50
-1.00
-1.50
-2.00
4.0
3.0
2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
1.00
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Capacitor Volts, A
Machine Current, A,out+
2.00
3.00
4.00
...
...
...
Types/Classification of SSO
Title
1. Self Excitation (Induction generator effect)
 Synchronous generators as well as induction machines can contribute to
this condition
 Easy to explain (in a 20 min introduction) using an induction machine
example
R1
I1
I 2'
X1
V1
Pag
Xm
S
R2'
s
X 2'
System nominal frequency
R2'
s
Higher effective NEGATIVE resistance)
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Negative Slip (S)
Electrical resonant frequency
Types/Classification of SSO
Title
1. Self Excitation (Induction generator effect)
Synchronous generators
– Field (and damper) winding characteristics contribute to IG
– At frequencies (w) below the system frequency (wo), the
effective resistance is negative.
( w  w0 )T ' d 0
w( x  x' )
Rs  
wb
(1  [(w0  w)T ' do ]2 )
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Types/Classification of SSO
Title
2. Torsional Interaction
 Electrical resonance frequency (fe) of system caused by series
capacitors close to natural torsional resonance frequency (fm) of
mechanical system (turbine-generator unit)
Condition for SSR-TI Risk
=>: fe
fo - fm
 Due to interaction between the electrical (generator/line series capacitor…) and
mechanical systems (long shafts and masses of thermal units)
 Result in shaft fatigue & undesirable stress, damage
 Typical mechanical resonance frequencies (15 – 30 Hz)
 The risk is with higher Series Compensation (SC) levels
 This condition should be avoided through design considerations
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Types/Classification of SSO
Title
2. Torsional Interaction
Electrical
damping
from
the
machine/electric-network
mechanical shaft oscillations – derived from EMT simulations
(damping analysis using EMT simulations).
0.4
0.2
0
-0.2
0
10
20
30
40
50
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
60
70
80
Types/Classification of SSO
Title
3.Transient Torques
•
•
Shaft oscillations following disturbances will result in shortening of the shaft
life time due to material ‘fatigue’.
The critical items that determines the level of severity are:
•
Amplitude of the mechanical transient oscillations
•
Decay of the oscillations (damping)
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Types/Classification of SSO
Title
3.Transient Torques
Peak transient torque produced on each shaft segment can be used to determine the level
of risk.
Machines are designed to withstand for “terminal short circuit” and the peak torque
produced for this event can be used as a reference.


If the torque produced for an evet is larger than about 75% of the value for terminal short circuit, it is
considered as in the zone of vulnerability.
S-N curves (number of stress cycles as a function of stress amplitude before crack
initiation).

Shaft Torque (PU)
3
2
1
Source: EPRI report: “Torsional Interaction Between
103
105
107
© Manitoba HVDC Research Centre
|
Cycles to failure)
a division of Manitoba Hydro International
Electrical Network Phenomena and Turbine-Generator
Shafts”
Types/Classification of SSO
Title
4. Interaction with HVDC converter controls - SSTI
•
An issue associated with conventional (LCC) HVDC schemes
•
A parameter ‘Unit interaction Factor’ is used to determine the level of
risk (a screening level indicator)
– How close the generator is to the HVDC converter
– Other parallel AC connections (improved damping)
• Radial connection of a generator to the rectifier AC bus is the likely
worst case.
• Can be easily mitigated through proper design of HVDC control
schemes and settings.
Rbus
Gens
Rectifier
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Types/Classification of SSO
Title
4. Interaction with HVDC converter controls - SSTI
•
SCTOT – Short circuit level at the HVDC terminal
•
SCi – Short circuit level at the HVDC terminal with the generator under SSTI
investigation disconnected.
•
•
UIF > 0.1 is recommended for further analysis
UIF should be calculated for carefully selected (credible) N-x outage
conditions
Source: EPRI report: “Torsional Interaction Between Electrical Network Phenomena and Turbine-Generator Shafts”
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Types/Classification of SSO
Title
5. SSCI involving T3 Wind Units
•
Modern wind turbines use power
electronic converters (connected to
the generator) to improve
performance
•
Wind farms located far from the AC
grid has necessitated ‘Series
Compensated (SC)’ Transmission
lines
•
Problem: DFIG controls act to
‘amplify’ sub synchronous currents
entering the generator
–
Negative Damping
–
Rotor side converter plays the dominant
role
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Types/Classification of SSO
Title
5. SSCI involving T3 Wind
Units
Dynamic frequency scan to estimate damping provided by the
DFIG to sub synchronous frequency transients
10
0
-10 0
-20
-30
-40
-50
-60
-70
-80
-90
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
5
10
15
20
25
30
35
Title
Do I need to perform studies?
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Performing studies
Title
Identifying potential risks
Series Cap Locations
WGR Location
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International
Title
Thank you
© Manitoba HVDC Research Centre
|
a division of Manitoba Hydro International