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GENERATION
energize - June 2006 - Page 51
A better understanding of the prob lems
associated with voltage dips at the terminals
of these generators is necessary to ensure
rotor side converters are adequately rated
and protected.
Loss of generation during a critical period
of a voltage dip can also introduce stability
problems on the network and thus it isimportant that this problem is addressed. The
response of the rotor circuit of the generator
to a voltage dip is presented. The influence
of trapped flux in the generator at the instant
of voltage recovery appears to generate
rotor currents that can be sufficient to
damage converter devices. Measurements
and simulations are presented to further
analyse the threat of voltage dips on rotor
circuit converters.
The growing demand for dist r ibuted
generation and renewable energy resources
has seen an increase in the popularity of wind
power. Wind turbines are quickly becoming
cost effective sources of generation as
their power ratings increase with advances
in materials, power electronics and
control technologies.
Wind ene rgy conver ters (WECs) can be
found in various forms, the most popular of
which is the doubly fed induction generator
(DFIG) which has several advantages over its
fixed frequency/speed counterparts [1]. The
DFIG has a wound rotor that is connected
to the grid through back-to-back voltagesource converters (Fig. 1). These converters
decouple the mechanical and electrical
rotor frequencies and hence supply power
at the grid voltage and frequency [2]. This
also allows for increased efficiency as the
turbine speed can be adjusted to maximise
the output power of the generator for a
particular wind speed. The converters can
also control the flux and hence the torque
of the generator. A reduction in torque
pulsations and oscillations that are common
with wind turbines is achieved resulting in
better power quality and a longer gearboxlifespan [3].
The DFIG will act as an asynchronous
generator only if the rotor circuit allows
bidirectional power flow at both sub-
The threat that voltage dips impose on
wind power generationby Simon Davies and Dr. John van Coller, University of the Witwatersrand.
synchronous and super-synchronous speeds
(typically ±30% of synchronous speed [2]).
Only the power in the rotor circuit needs to
be converted which reduces the ratings of
the converters to approximately 25% of the
total power. The converters can also supply or
absorb reactive power to and from the gridand hence maintain the terminal voltage at
the generator. The generation/absorption of
reactive power is limited by the rating of the
converters connected to the rotor circuit.
One drawback of using DFIGs is the
vulnerability of the rotor converters to supply
disturbances such as voltage dips. Converter
failure resulting in the loss of generation
during a critical period of a voltage dip can
introduce stability problems on the network.
It is thus very important to understand the
threat that voltage dips impose on the
generators and their associated converters.
A significant amount of research aimed at
improving the control of the converters to
handle system disturbances such as voltage
dips has been published [2-4]. However, very
little information exists as to what is happening
from the generator’s perspective during a
voltage dip. The aim of this paper is to identify
and explain the transients that can cause
problems to rotor side converters used in
DFIG schemes. Measurements conducted ona simplified LV DFIG system identify how the
response of the generator to a voltage dip
can develop significant electrical transients
in the rotor circuit. Simulations performed
using the Alternative Transients Program (ATP)
confirm these results and allow for a more
detailed understanding of how the generator
behaves during a voltage dip.
Voltage dips
Voltage dips are classif ied as a sudden
reduction in the RMS voltage for a period of
between 20 ms and 3 s of any or all phase
voltages in a single or poly-phase supply.
The duration of a voltage dip is the time
measured from the moment the RMS voltage
drops below 0,9 per unit of the declared
Fig. 1: Doubly fed induction generator.
Fig. 2: Simplified DFIG test circuit.
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GENERATION
energize - June 2006 - Page 52
voltage to when the voltage rises above 0,9 per unit of the declared voltage [5].
Voltage dips are generally caused by network faults. Large fault currents flowing
through the network result in large voltdrops across the network impedances
resulting in voltage reduction further down the network.
Doubly fed induction generator response to voltage dips
A voltage dip at the supply terminals of an induction generator will produce
transient currents and transient torques depending on the severity of the dip
and on the machine parameters [6]. The sudden reduction of voltage at the
supply terminals of the generator as a result of a three phase fault will have the
following consequences:
The generated real power is reduced [6].
There is still the ability for both rotor and stator currents to flow so torque can still be
produced. Large transient rotor and stator currents as a result of the voltage dip
will generate large transient torques [7]. Fatigue as a result of exposure to voltage
dips can reduce the lifetime of the gearbox connected to the turbine.
The theorem of constant flux linkage [6] states that the mutual flux linking the stator
and rotor windings in the machine cannot instantaneously change. At the instant
of a voltage dip, the stator voltage attempts to enforce a new flux condition inthe generator. This forces the rotor circuit to respond to maintain the mutual flux
at that instant. This effect results in significant transient currents developed in the
rotor circuit. Furthermore, because the rotor windings have rotational speed and
the flux linkage is not able to change instantaneously, the flux fixed to the stator
at this instant induces high voltages on the rotor side.
In a short-circuited squirrel cage generator these transient rotor currents and
voltages have little consequence; however in the case of DFIGs, with rotor
circuit converters, the resulting transient rotor voltages and currents can have
damaging effects if no protection exists (i.e. a crowbar) or if devices are not
rated sufficiently.
Doubly fed induction generator response to voltage recovery
The response of the generator to voltage recovery after a voltage dip can
generate even larger transients depending on the flux conditions within the
generator at the time of recovery. These can be identified as re-switching
transients similar to those developed when a machine changes from one steady
state operating condition to another (e.g. star-delta switching) [6]. Some of the
more important considerations due to voltage recovery are as follows:
The process of voltage recovery in the machine is complicated and depends
on the flux conditions within the machine. Effects such as skin effect in the rotor
windings, the reaction of the core due to the rapid rise in flux and mechanical
considerations inherent in the design of the shaft and windings complicate the
process [6, 8].
The theorem of constant flux linkage plays an important role in the understanding
of the transients generated during voltage recovery. At the instant of voltagerecovery the flux linkage must remain constant. The severity of the transients
generated is thus dependant on the flux conditions within the machine at that
instant.
It takes time for magnetic energy stored in a generator to dissipate. Trapped flux
will continue to induce both rotor and stator emfs dependant on the size and
parameters (time constants) of the generator. Larger generators tend to have
longer time constants, thus the effect of a voltage dip can be more severe.
Upon voltage recovery, phase differences may exist between the supply voltage
and induced emfs as a result of trapped flux. If phase opposition occurs, the
transient currents may be very severe (recorded currents similar to that of
direct-on-line start are not uncommon). Thus the amount of stored energy in
the generator and the position of the rotor which affects the flux linkage, willaffect the transients developed in both the stator and rotor circuits at the instant
of voltage recovery [6, 8].
Large transient torques are associated with the transient currents developed at
voltage recovery. These torques can be severe enough to damage mechanical
Fig. 3: 3 phase 0,4 p.u. voltage dip.
Fig. 4: Measured stator current
Fig. 5: Simulated stator current.
Fig. 6: Measured rotor current.
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GENERATION
energize - June 2006 - Page 53
components such as the gearbox connected
to the turbine.
Simplified test circuit
Rotor transients developed by the generator
during a voltage dip will be injected into the
DC bus via the rotor side converter. In this
case, the converter is simplified to a three
phase diode bridge rectifier (the anti-parallel
diodes of the rotor side converter) connected
to a DC bus capacitor and a load resistor
(Fig. 2). Converter control circuitry can limit the
conduction of the IGBTs; however it cannot
control the response of the anti-parallel
diodes to the electrical transients developed
by the rotor during a voltage dip.
Measurements and simulat ions were
conducted on a 19 kW 4 pole wound rotor
induction generator. Balanced three phase
and single phase dips were applied using
the Eskom/Wits voltage dip test bed. A varietyof dips were chosen to show the effects of
different voltage dip parameters on rotor
circuit transients.
Results
Simulations were performed using the
Alternat ive Transients Program (ATP) [9] .
A standard d-q model of a wound rotor
induction generator is used to predict the
transient performance during a voltage dip.
The test circuit described in Fig. 2 was used
in all simulations. Saturation and skin effect in
the windings were neglected.
Figs, 3 - 9 show both the measured and
simulated results for a balanced three
phase 0,4 p.u. voltage dip. The induction
generator is fully loaded with a constant
load torque and operates at 0,2 p.u. slip.
These waveforms are representative of all
tests performed and give valuable insight
into what is happening from the generator’s
perspective during a voltage dip.
The waveforms show that there are two
distinct transients – at the start of the dip and
at the instant the voltage recovers. Measured waveforms are on the left and simulated
waveforms are on the right.
There is reasonable agreement between
measured and simulated waveforms
although it is very difficult to
simulate the same conditions
in the generator at the instant
of the start of the voltage dip
and at the instant of voltage
recovery. Analysing the stator
current (Figs. 4, 5) it is evident
that at the start of the voltage
dip, the stator voltage attemptsto enforce a new flux condition
in the generator. The rotor
circuit is forced to compensate
and maintain the mutual flux
producing an equal and
opposite MMF to that of the
stator current (Figs. 6, 7). This
rotor current surge is injected
into the DC bus through the
rotor side converter and is the
cause of the DC bus voltage
transient at the start of the dip
(Figs. 8, 9). Phase opposition ofthe rotor flux in the generator is
believed to be the cause of the
transient at voltage recovery.
This is explained in more detail
in the next section.
Analysis
The most important factor
influencing rotor transients
during a voltage dip is the
dip magnitude. Larger dips
produce larger electrical and
mechanical transients. TheIGBTs used in the rotor converters
are typically rated at 2 p.u.
voltage and current. Table 1 shows typical
p.u. transients for a selection of voltage
dips. Single phase and three phase dips
exhibit very similar transients and have been
grouped together. It is clear that voltage dips
are cause of concern to converter devices as
over-currents and over-voltages may exceed
the ratings of the devices.
Perhaps the most interesting result is that
shorter dips produce larger transients thanlonger dips. This implies that the effect
of remnant flux in the machine plays an
important role in the rotor transients at the
instant of voltage recovery. Trapped flux
within the machine during a dip will induce
EMFs in both the stator and rotor windings.
If at the point of voltage recovery a phase
difference exists between the induced emfs
in the generator and the grid voltage, very
large currents will result. If phase opposition
occurs, currents may even approach that of
direct-on-line starting. This effect is well known
during supply switching or reconnection
(star-delta switching) and appears to occur
for both three phase and single phase dips.
Large generators will store large amounts ofenergy for longer periods of time as they have
longer time constants. The effect of trapped
flux in a larger machine can thus be more
significant. Preliminary simulations of a larger
generator confirm this fact.
Dip magnitude (p.u.) Dip duration (ms) DC bus voltage/current (p.u.)
0,8 100 2,0
0,6 100 2,7
0,4 100 3,5
0,8 50 2,6
0,6 50 3,1
0,4 50 3,6
Table 1: Selection of dip results.
Fig. 7: Simulated rotor current.
Fig. 8: Measured DC bus voltage.
Fig. 9: Simulated DC bus voltage.
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GENERATION
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The point on the 50 Hz cycle at the start
of the dip and at voltage recovery has an
effect on the rotor transients developed by
the generator. This is clearly evident for single
phase dips which may be very severe at
one point in the cycle, but quite insignificant
at another.
The size of the DC bus capacitance also
affects the transients in the rotor circuit during
a dip. A larger capacitance reduces the
rotor transient considerably. The tendency
to use smaller capacitance in the DC bus
of modern converters limits the effect of
the capacitance in the reduction of rotor
transients during a voltage dip.
The torque transients as a result of voltage
dips cannot be neglected. The interaction of
both large stator and rotor currents during a
voltage dip will produce significant transient
torques. Shorter dips appear to be more
severe in terms of the mechanical response
of the generator with torques approaching
direct-on-line start values. Gearbox failure is
a possibility for more severe dips if adequate
measures are not taken to limit over-current
transients in the generator.
Conclusions
This paper has identified the threat that
voltage dips impose on DFIGs. The response
of the induction generator to a dip has
been discussed with specific regard to the
rotor transients. The mechanism of constant
flux linkage plays an important role in the
magnitude of the currents and voltages
produced in the rotor circuit. Trapped flux
present in the generator during a voltage
dip appears to play a significant role in the
production of transient currents, especially
for shorter dips. The above analysis assumes
that the equivalent circuit of Fig. 2 is valid
during the dip. If the converters maintain
some control over the rotor current during the
dip then a more detailed investigation would
be required. Voltage dips are a common
occurrence on the utility networks. The effect
of these dips on wind power generators
such as DFIGs must be well understood to
ensure equipment is adequately rated and
protected.
Acknowledgement
This paper was first presented at Cigré’s
5th Southern Africa Regional Conference
in October 2005and is reproduced with
permission.
References
[1] V. T. Ranganathan, R. Datta, Variable-speed wind power generation usingdoubly fed wound rotor induction machine- A comparison with alternative schemes,IEEE Transactions on energy conversion, v17 n 3 Sept 2002 pp 414-421.
[2] R. Pena, J. C. Clare, G. M. Asher, Doublyfed induction generator using back-to-back PWM converters supplying anisolated load from a variable speed windturbine. IEE Proceedings: Electric Power Applications, v 143, n 5, Sep, 1996, p380-387.
[3] I. Cadirci, M. Ermis, Double-output induction
generator operating at subsynchronousand supersynchronous speeds: steady-state performance optimisation and wind-energy recovery, IEE Proceedings,Part B: Electric Power Applications, v 139,n 5, Sep, 1992, pp 429-442.
[4] J. B. Ekanayake, L. Holdsworth, X. Wu, N.Jenkins, Dynamic modeling of doubly fedinduction generator wind turbines, IEEETransactions on Power Systems, v18, n2,May 2003, pp803-809
[5] NRS. Minimum standards, NRS 048-2(Electricity Supply – Quality of Supply). NRS,2003.
[6] M. G. Say, Alternating Current Machines4th ed, Pitman, London, 1976.
[7] J. C. Das, Effects of momentary voltagedips on the operation of induction andsynchronous motors, IEEE Transactions onIndustry Applications, v 26, n 4, Jul-Aug,1990, pp 711-718.
[8] P. K. Kovacs, Transien t Phenomena inElectrical machines, Elsevier, New York,1984.
[9] Leuven EMTP Centre, Alternative TransientsProgram Rule Book, Leuven, Belgium,
1992.
Contact Simon Davies,