Energy and Power Engineering, 2013, 5, 454-457
doi:10.4236/epe.2013.54B087 Published Online July 2013 (http://www.scirp.org/journal/epe)
Overview of Sub-synchronous Oscillation in
Wind Power System
Han Chen, Chunlin Guo, Jianting Xu, Pengxin Hou
State Key Laboratory for Alternat e El ectrical Power Sy stem with Renewable E n ergy Sources,
North China Electric Power University, Beijing, China
Received April, 2013
ABSTRACT
Nowadays with the improvement in the degree of emphasis on new energy, the wind power system has developed more
and more rapidly over the world. Usually the wind plants are located in the remote areas which are far from the load
centers. Generally series compensated AC transmission and high voltage DC transmission are made use of to improve
the transmission capacity as two main effective ways which can solve the problem of large scale wind power transmis-
sion. The paper describes the three kinds of impact varieties and impact mechanisms in the su b-synchronous oscillation
phenomena of wind power system based on doubly fed induction generator (DFIG) wind generators. At last, we point
out the important problem that should be stressed in the wind power system.
Keywords: Sub-synchronous Oscillation (SSO); Sub-synchronous Resonance(SSR); Sub-Synchronous Torsional
Interactions (SSTI); Sub-Synchronous Contro l Interactions (SSCI); Wind Turb ines; Doubly-fed Inductio n
Generator
1. Introduction
Sub-synchronous oscillation phenomenon in the thermal
power system is in an abnormal state that the steam tur-
bine unit is being under special operation state after dis-
turbance, on the running condition, electrical system and
steam turbine generator set are having significant energy
exchange at one or more the synchronous frequency. The
year 1970 and 1971, there is a severe large-scale turbo-
generator rotor shaft damage caused by transmission se-
ries compensation capacitor[1]. After this accident, peo-
ple in academic and industry began to study sub-syn-
chronous oscillation problem hotly, and achieved re-
markable results in mechanism analysis method and the
restraining method.
Nowadays the wind power system has developed more
and more rapidly which has become the most important
part of the energy strategic planning.
However as the wind power plant is far from the load
center, series compensated AC transmission and high
voltage DC transmission are used widely to settle the
large scale wind power transmission problem. Series
compensation may induce sub-synchronous oscillation
problem of wind turbine, affect the safe and stable opera-
tion of wind farms as well as the transmission system
[2,3]. Sub-synchronous oscillation in a wind farm in the
form of three kinds of mechanism is discussed based on
the current mainstream models doubly-fed induction ge-
nerator (DFIG), and we pointed out that the current
problems faced now.
2. The Simple Analysis of Sub-synchronous
Oscillation Problem in Wind Power
System
2.1. Introduction of Doubly-fed induction
Generator (DFIG)
The working principle of doubly-fed induction generator
is that the frequency converter generates a low-speed
rotating magnetic field in the rotor winding. The speed of
rotating magnetic field n1 is added up with the rotor me-
chanical speed n2 driven by wind and it forms a rotating
magnetic field in the rotor winding of which the speed is
n, that means：n1 + n2 = n. When the wind speed changes,
changes along, the CPU calculate the inverter output
frequency of the rotor current according to expression .
By changing the frequency n1 of the rotor currents to
adjust the rotational speed of the rotating magnetic field,
it is possible to compensate for the changes in speed of
the generator rotor, thereby maintaining the constant
output freq uen cy of the gri d [ 4] .
As the double-fed asynchronous generator only flow
through the slip power inverter, inverter capacity is rela-
tively small, usually about 10% to 30% of the rated gen-
erator power. The loss and the investment are relatively

H. CHEN ET AL. 455
low, the technology is mature, so it’s the most main-
stream fan [3]
The current amplitude, phase, frequency of doubly fed
induction motor rotor excitation can be adjusted; the ac-
tive power, reactive power can be adjusted too; excellent
network characteristics; therefore, doubly-fed induction
motors currently are the most promising wind turbine.
Due to the different mechanism of sub-synchronous
oscillation, there are different kinds of math model,
analysis method and solutions. The structural character-
istics of the wind turbine have decided that the SSO
problem caused by series compensated AC transmission
and high voltage DC transmission is not all the same. In
addition to possible sub-synchronous resonance (SSR)
and sub-synchronous torsional interactions (SSTI), wind
turbines converter internal control may also cause sub-
synchronous oscillation problem (SSCI). [4]
2.2. The Sub-synchronous Resonance in Wind
Power System
According to IEEE working group, the definition of sub-
synchronous resonance (SSR) refers to electro-me-
chanical oscillatory behavior caused by the coupling be-
tween Turbine and transmission system(Figure 1) with
the series capacitor compensation. Because the rendered
weak damping of the oscillations, un-damping, even
negative damping characteristics, the trend of the ampli-
tude of this oscillation is gradually increasing.
Electrical oscillation frequency as shown in type (1),
0"C er
E
T
X
ff
X
XX

 (1)
where, 0
f
is the synchronous frequency, the impedance
X
is based on the electrical frequency corresponding to
average rotor speed 0. Under ideal conditions, f0
f
is
equal to synchronous frequen cy.
When there is the current whose frequency is the re-
sonant frequency in the stator winding, the rotor wind-
ings will produce the rotor current of which the fre-
quency is r. To three-phase armature current, it will
produce the rotating magnetic field of the positive and
negative sequence in the synchronous generator. In the
joint action of each phase current time distribution and
spatial distribution of each armature winding, the fre-
quency of electrical angular velocity of the magnetic
field rotation is
f
2er f

. The rotor frequency of the cur-
rent caused by rotating magnetic field is determined by
the relative speed of the rotating magnetic field and the
rotor. Thus, the positive sequence component of the sta-
tor current will produce rotor current component
(0er
). If the generator shafting torsional vibration
frequency in natural is at the frequency of r, that is to
say, the system electrical resonant frequency and gen-
erator shafting natural torsional vibration frequency are
complementary. Then the energy exchange happened.
Between generator and series compensation system
through continuous weakly damped oscillation and tran-
sient effect. Oscillation caused by the mutual encour-
agement may maintain or may lead to instability.
=
r
ffff
Double-fed induction wind power unit shaft system
has the characteristics of low natural frequency of tor-
sional vibration. This means that if sub-synchronous re-
sonance is to occur, if the system should have a high
electrical resonant frequency ,meaning a high degree of
Line Series Compensation. For wind turbine, it’s more
difficult to have the SSR problem compared to the ther-
mal power units. Paper [5] Unified Power Flow Control-
ler (UPFC) is used to enhance the wind farm system sta-
bility, at the same time, it can increase sub-synchronous
damping of system to suppress sub-synchronous reso-
nance of wind turbines. It can produce corresponding
sub-synchronous damping torque by adding additional
control and improving UPFC control strategy in the pitch
angle control to achieve the purpose of sub-synchronous
resonance suppression. Paper [6] established the imped-
ance model of the wind turbine and the electrical system
and analyzed the SSR stability between the doubly-fed
induction wind turbine and series compensation network
using Nyquist and Bode diagram. Finally it concluded
that low wind speed are more damaging than high wind
speed for SSR, and RSC current controller is not very
good to reduce SSR.
The document [7,8] designed SSR damping controller
for the doubly fed induction wind turbine and verified the
validity through a numerical example simulation. These
data models are most used in single - infinity system
equivalent model and the series compensation degree is
generally higher than real wind power system, SSR
causes cannot be explained using the time domain model
from the aspects of mechanism.
2.3. Sub-synchronous Torsional Interactions
(SSTI)
HVDC and SVC, PSS device -- quick power regulation
devices are likely to stimulate torsional vibration. The
oscillation is called “devices caused sub-synchronous
oscillation (device dependent sub-synchronous oscilla-
tion)”. With further study of the SSO problems, re-
searchers this time called the synchronous oscillation
Figure 1. Simple wind power system model.

H. CHEN ET AL.
456
problem as sub-synchronous torsional vibration interac-
tion (SSTI), defined as the interaction at sub-synchro-
nous frequency occurring between the generator and the
shaft power control equipment. Here it mainly refers to
the interaction between the wind generator con troller and
unit shaft system, this phenomenon occurs in comple-
mentary electrical frequency with shafting mode state
frequency, if the system having negative damping. In a
test of the Square Butte HVDC converter station in
America, there is the 30.5 Hz sub-synchronous current
component happening in the DC transmission system,
followed by the induced serious shaft torsional vibration
near turbo-generator. This is the first report about the
HVDC induced generator torsional vibration and it has
attracted close attention of scholars all over the world.
The analysis after the accident showed that the occur-
rence of sub-synchronous oscillation is related to the way
of the dynamic characteristics of the HVDC transmission
system regulation [1].
The paper [9] described the SSTI specific behavior by
PSCAD / EMTDC time domain simulation of the induc-
tion fan speed. It conducted the disturbance analysis of
different rotational component at zhe modulation fre-
quency and analysed the reason of sub-synchronous os-
cillation in the wind power system with HVDC transmis-
sion lines. The paper [10] also summarized the sub-syn-
chronous oscillation reason of different types of wind
turbines, analyzed the double-fed induction motor, and
found that SSTI occurred usually in a lower degree of
series compensation system through simulation. The SSTI
problem is very complicated and we need consider the
electromechanical transient process and electromagnetic
transient process of the network and the system control-
ler at the sub-synchronous frequency [11]. The wind
power system started up later and most of the studies are
about the HVDC in traditional power system. And the
study of SSTI in wind power system is less and less. This
phenomenon may be solved by adding a special damping
controlle r [12,13].
2.4. Sub-synchronous Oscillation Problem
Caused by Wind Turbines Converter
Internal (SSCI)
This SSCI problem is mainly the interaction of the wind
turbine contro ller and series compensation system, which
is the form of wind turbine different from the thermal
power units. Actually, SSCI is not only limited to the
wind turbine and other FACTS Controller (SVC, STA-
TCOM, etc.) with series compensation system interac-
tions are likely to induce SSCI [10]. SSCI phenomenon
leads to negative damping of the system with the fast
direct current control of the wind turbine. SSCI is com-
pletely unrelated with the wind turbine shafting unlike
SSR and SSTI. SSCI is only the interaction between the
generator control system with fixed series compensation,
the oscillation frequency is determined by the generator
control system and transmission line parameters. In addi-
tion, compared with the SSTI and SSR, due to SSCI
having no participation function with the mechanical
system, the oscillation damping effect of the system is
smaller, so oscillation caused by SSCI spreads faster.
In September 2009, a wind farm in Texas of the Unit-
ed States a SSCI accident happened, which caused a
large number of wind turbines fault and internal lever
circuit damage, then record waveforms are shown in
Figure 2 below [11].
It is currently released SSCI accident for the first time.
Accident happened because that the line from wind farm
had a ground fault and broken then, the system connec-
tion mode changed and at last formed the radial power
connection mode at which wind farm and the system
fixed by a single line of series compensation (75%). Af-
ter the accident, a sustained increase of oscillation phe-
nomenon happened between the wind turbine control
system and the fixed series compensation, and there is
serious distortion of voltage and current. Approximately
0.25 s later, the current increased to about 300% of the
wind turbine rating, the voltag e increased to ab out 150%,
which had great damage to the wind turbine. After 3s,
fixed series compensation protection device made the
fixed series compensation bypassed, the line oscillation
was gradually suppressed, SSCI disappeared. This phe-
nomenon has attracted extensive attention of scholars at
home and abroad, and they have been studying the me-
chanism and its suppression measures.
Sub-synchronous oscillation caused by the wind tur-
bine controller is caused by the resonant current of the
electrical resonant circuit. To the wind power system
using series compensation, harmonic current induced the
corresponding sub-synchronous frequency current in the
rotor of the generator [12-14].
The current will cause the rotor curren t waveform dis-
tortion and phase offset, the converter controller will
adjust the inverter output voltage after the change, then
changes the actual current in the rotor and so forms a
closed loop system. If the output voltage helps increase
Figure 2. Texas wind farm SSCI wave record chart.

H. CHEN ET AL.
457
the rotor current, the harmonic current oscillation will b e
unstable, resulting in the instability of the whole system
oscillation.
The literature [15] outlined the SSCI problem about
the doubly-fed induction generator, and that achieved
great SSCI suppression effect by designing the damping
controller. But the damping controller is designed for
specific control effect and does not have the universal
applicability .Paper [16] overviewed the reason of SSCI
phenomenon in wind power system through comprehen-
sive simulation based on ERCOT models and specific
models and told us that the Type 4 wind turbine don’t
have the SSCI proble m.
3. Conclusions
Along with the rapid development of wind power, it’s
more possible to send the power of great capacity. How-
ever, as two ways to send the wind power, series capaci-
tor compensation and HVDC may bring sub-synchronous
oscillation problems. At present, there are few studies on
sub-synchronous problem caused by wind turbine. This
paper discusses three kind of action mechanism of sub-
synchronous oscillations in the wind farm based on the
doubly fed induction generator (DFIG) and introduces
the foreign research directions and conclusions. For the
synchronous oscillation problem triggered by wind tur-
bine, the grid connection ways of the wind farm is dif-
ferent from that in traditional thermal power generation
due to the structure of the wind power system. Regard-
less of the mechanism or the modeling, or suppression
measures, there are many questions to be solved.
4. Acknowledgements
The paper is supported by the Key Project of the Na-
tional Research Program of China(2011BAA01B02) and
the National High Technology R&D Program of
China(863 Program)(2011AA05A 109).
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