16-01-2013, 10:43 AM
Novel Scheme to Connect Wind Turbines to the Power Grid
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Abstract
In this paper, a new scheme is proposed to connect
wind turbines to the power grid. This scheme helps in limiting
the fault currents as well as in voltage unbalance compensation.
Voltage is injected in series with the transmission line to limit the
fault currents as well as to balance the voltages. This scheme allows
wind turbines to remain synchronized to the grid during faults or
during low voltages and is useful for doubly fed as well as squirrel
cage induction generators.
INTRODUCTION
WIND IS THE fastest growing energy source in the world.
In the past 5–6 years, wind energy use has grown 28%
annually, becoming the most developed technology among various
renewable energy technologies. Today, wind power capacity
of the world is approximately 50 GW and is expected to reach
160 GW by 2012 [1].Wind turbines of approximately 4500 kW
and rotor diameters of more than 100 m are now available [2].
However, the interconnection of wind turbines to the power
grid poses many problems. Reactive power compensation, lowvoltage
ride-through, and the grid voltage unbalance are some
of these issues.
Wind turbine systems are classified into two types: fixedspeed
wind turbines and variable-speed wind turbines. Fixedspeed
wind turbines are directly connected to the grid. Though
these are the cheapest wind turbines, they have a number of
drawbacks. In these turbines, reactive power cannot be controlled;
furthermore, any turbulence of wind results in power
variations, thus affecting the power quality of the grid [3].
In variable-speed wind turbines, power fluctuations caused by
wind variations can be absorbed by changing the rotor speed.
These turbines produce 8–15% more energy output, as compared
to fixed-speed wind turbines; however, they necessitate
power electronic converters.
NEW SCHEME FOR DFIG
The proposed new scheme for DFIG is shown in Fig. 3. In
a conventional scheme, the grid-side converter generally functions
as a STATCOM. In our scheme, the grid-side converter
is connected in series instead of in parallel. Also, instead of 1
three-phase converter, 3 single-phase converters are used.
The function of the rotor-side three-phase converter is similar
to that in a conventional scheme of a DFIG, i.e., for reactive
power control and speed control. Vector control as described in
the previous section is used for this purpose.
The grid-side converters have three objectives:
1) to maintain the dc link voltage;
2) to compensate for unbalanced voltages;
3) to limit the fault current/low-voltage ride-through.
The single-phase converter in phase A acts to compensate
for the voltage unbalance, and either the phase B or phase C
converter can operate to maintain the dc link voltage. If voltage
unbalance compensation is the only aim, then two converters
are sufficient. However, for a low-voltage ride-through, these
converters will add voltage in series of the faulty phase, such
that the voltage seen by the induction generator will be higher
than the voltage of the faulty phase. In the following analysis, the
function of all converters is interchangeable. This is required,
as the faults can occur on any phase.
Converter A Control
Converter A is used for voltage unbalance compensation.
Voltage unbalance compensation is achieved similarly to the
method and the remaining part of this system will be given
in [9]. However, in that scheme, a rectifier is used for the dc
link, while in this scheme, a series converter is used to maintain
a dc link voltage, as shown in Fig. 3.
As per the International Electrotechnical Commission (IEC)
definition, voltage unbalance is given as the ratio of the negative
sequence voltage to the positive sequence voltage.
Fault Current Limitation
The single line to a ground fault case was considered, as
this is the most common fault. This fault is detected by voltage
magnitude. The moment that voltage magnitude goes below a
certain limit, the converter in that phasewill inject some voltage,
such that the voltage seen by the generator will be higher than
the fault voltage, and the fault currents will be limited. Here, it
can be noted that the grid voltage is still the fault voltage, but by
injecting voltage in series with the line, the voltage seen by the
generator is changed. This way the generator operating point on
the curve shown in Fig. 1 can be moved upwards and can achieve
fault ride through. The same principle can be applied in the case
of low-voltage ride-through. During a fault, power in the faulty
phase will be transferred to other phases, as one converter is
still operating to maintain the dc link voltage. It should also be
noted that one converter is still operating for balancing voltages,
which will further help in stabilizing the system.
CONCLUSION
In this paper, a new scheme is presented to connect wind
turbines to the power grid. With this scheme, it is possible to
keep wind turbines synchronized to the grid, even in the case of
faults or low grid voltages. Voltage unbalance can be completely
removed by using this scheme. The scheme is applicable for
doubly fed as well as SCIGs. Simulation results are presented
for all the cases.