MANITOBA HVDC RESEARCH CENTRE, a Division of Manitoba … · 2016-10-15 · Title Sub Synchronous...
Transcript of MANITOBA HVDC RESEARCH CENTRE, a Division of Manitoba … · 2016-10-15 · Title Sub Synchronous...
Title
Sub Synchronous Oscillations – An Introduction
October 2016
MANITOBA HVDC RESEARCH CENTRE, a Division of Manitoba Hydro International Ltd.
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International Ltd.
Presented by: Dharshana Muthumuni
Title Presentation Outline
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
• 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
Title
Introduction / Background
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Title Introduction / Background
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Transients in the Electric Network
• Transients following a system disturbance should damp out in a reasonable timeframe.
Title Introduction / Background
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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
Title Introduction / Background
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Network Characteristics
System Impedance Vs Frequency Plots
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
Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ...
...
...
0.0
0.2k
0.4k
0.6k
0.8k
1.0k
1.2k
1.4k
Z+(o
hm
s)
case0 case1
A Typical frequency characteristic
Title Introduction / Background
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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).
Main : Graphs
0.170 0.190 0.210 0.230 0.250 0.270 ...
...
...
-300
-200
-100
0
100
200
300
y
Eb
Base Case - Case1
Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ...
...
...
0.0
0.2k
0.4k
0.6k
0.8k
1.0k
1.2k
1.4k
Z+(ohm
s)
case0 case1
Title Mechanical Oscillations
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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
• fm Hz mechanical oscillations => (60-fm) Hz and (60 + fm) Hz
Gen
Speed - W
Electric network
Title Mechanical shaft-mass system
© 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.
Speed - Wk
LP HP G IP
δ
Title Mechanical shaft-mass system
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
• 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.
Title Mechanical Resonance Frequencies
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Mechanical modes can be calculated from shaft data:
Inertias of individual masses (J)
Shaft Spring Constant (K)
Typical resonant frequencies of Thermal Generator Turbine units are in the range of 15-25 Hz.
Speed - Wk
LP HP G IP
δ
fi=1
2𝜋√𝐾𝑖.𝑊𝑏
2𝐻𝑖
60Hz - 25 Hz => 35 Hz
Title
Series compensation and impact on SSR
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Title Series compensation and impact on SSR
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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
Hz 0.0 0.2k 0.4k 0.6k 0.8k 1.0k 1.2k ...
...
...
0.0
0.2k
0.4k
0.6k
0.8k
1.0k
1.2k
1.4k
Z+(o
hm
s)
case0 case1
Title Series compensation and impact on SSR
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Higher the level of series compensation, the resonant point moves toward the system frequency
Frequency Scan - Generator Terminals
0
0.2
0.4
0.6
0.8
1
1.2
1.4
5 9
13
17
21
25
29
33
37
41
45
49
53
57
Frequency (Hz)
|Z+
| (o
hm
s) 45% compensation
55% compensation
60% compensation
75% compensation
𝑓𝑒 = 𝑓0√𝑋𝑐
𝑋𝑙
Title
Types / Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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.
Gens
1.00 2.00 3.00 4.00 ...
...
...
0.700
0.750
0.800
0.850
0.900
0.950
1.000
1.050
1.100
Machine Output Volts p.u.
-2.00
-1.50
-1.00
-0.50
0.00
0.50
1.00
1.50
2.00
2.50 Capacitor Volts, A
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0 Machine Current, A,out+
Generator Series cap.
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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
1X
1V
1I 1R
mX
'
2X'
2I
s
R '
2agP
Electrical resonant frequency
System nominal frequency
Negative Slip (S)
S s
R '
2Higher effective NEGATIVE resistance)
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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.
)]')[(1(
')()'(2
0
00
do
d
b
sTww
Tww
w
xxwR
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
2. Torsional Interaction
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0 10 20 30 40 50 60 70 80
Electrical damping from the machine/electric-network
mechanical shaft oscillations – derived from EMT simulations
(damping analysis using EMT simulations).
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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)
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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).
Source: EPRI report: “Torsional Interaction Between
Electrical Network Phenomena and Turbine-Generator
Shafts”
1
2
3
103 105 107
Shaft Torque (PU)
Cycles to failure)
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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.
Rectifier
Rbus
Gens
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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”
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
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
Title Types/Classification of SSO
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
5. SSCI involving T3 Wind Units
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
0 5 10 15 20 25 30 35
Dynamic frequency scan to estimate damping provided by the
DFIG to sub synchronous frequency transients
Title
Do I need to perform studies?
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Title Performing studies
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International
Identifying potential risks
WGR Location
Series Cap Locations
Title
Thank you
© Manitoba HVDC Research Centre | a division of Manitoba Hydro International