Prime-mover dynamo plants – Electric control – Fluid-current motors
Reexamination Certificate
2001-05-23
2003-05-20
Ponomarenko, Nicholas (Department: 2834)
Prime-mover dynamo plants
Electric control
Fluid-current motors
C290S055000, C363S013000
Reexamination Certificate
active
06566764
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to supplying a utility with power from a variable speed wind turbine, and, more particularly, to converting a variable frequency output from a generator directly into constant frequency using a matrix converter.
BACKGROUND OF THE INVENTION
Wind speed fluctuates over time. Some wind turbines are not able to track these fluctuations and rotate only at a single speed (frequency). A way of operating at a fixed speed despite variations in wind speed is to use a synchronous generator or a directly connected induction (asynchronous) generator.
Since the maximum power available from the wind is a function of wind speed, and since the power captured by a propeller of a wind turbine is a function by rotor speed and wind speed, fixed speed wind turbines fail to recover this maximum power. Fixed speed turbines also suffer from noise, reliability problems and high stresses on the utility grid. Furthermore the lagging power factor of a grid-connected asynchronous generator demands a large capacitor battery to compensate for the lagging power factor. Accordingly, variable speed implementations have been proposed to recover the maximum power of the wind and better address these other problems of fixed speed turbines. Examples of these variable speed wind turbines are described in U.S. Pat. Nos. 5,083,039 and 5,225,712, and PCT Application U.S. Ser. No. 99/07996, each of which is incorporated by reference herein in its entirety.
A variable speed wind turbine
100
is also shown in FIG.
1
. One or more wind turbine blades (not shown) drives rotor shaft
111
of asynchronous doubly-fed induction generator
110
. Turbine
100
supplies power from rotor
112
and stator
113
of generator
110
when shaft
111
is rotating above synchronous speed. At speeds above synchronous speed, excitation power may be supplied to rotor
112
from rotor inverter
151
in order to achieve unity power factor at the stator side. At shaft speeds lower than synchronous speed, power is supplied from stator
113
and slip power along with the excitation power is supplied to rotor
112
from rotor inverter
151
.
To supply power from stator
113
, Y/&Dgr;-contacter
130
shifts the three stator windings selectively into a Y-connection or a &Dgr;-connection.
FIG. 1A
shows the Y-connection and
FIG. 1
B shows the &Dgr;-connection of the stator windings. The purpose of Y/&Dgr;-switch
130
is to achieve a higher operational speed range and to reduce iron losses in the stator. Iron loss is a loss mechanism similar to the ohmic losses of a resistor. (In a generator, the ohmic losses are called copper losses). The iron loss originates both from eddy currents and hysteresis losses. Eddy currents are currents induced in the iron of the generator while hysteresis loss occurs when magnetic energy is stored and removed from the generator iron. The magnitude of the iron losses depends on the voltage across the windings, and since the voltage across the stator windings in a Y-connection is decreased by a factor of {square root over (3)}, the iron losses will decrease. Specifically, for a given stator and rotor voltage, the speed range in Y-connection is increased by a factor of {square root over (3)} compared to the speed range in &Dgr;-connection. For example, if the speed range in &Dgr;-connection is ±36% around synchronous speed, the speed range is extended to ±52% around synchronous speed when connecting the generator in Y-connection. This increased speed and frequency range is derived from analysis of the following relationship between the rotor voltages and the stator voltages:
u
r
=|s|·u
s
·n
(1)
where u
s
is the voltage across the stator winding, u
r
is the voltage across the rotor winding, n is the winding ratio between rotor and stator, and s is the slip.
The output voltage and current from the stator are fed into a medium voltage transformer. The transformer may be located in the top of the turbine or elsewhere. When a transformer is located in the top of a turbine, the transformer can be constructed in at least two ways. The first way is with a primary winding (10 kV) and a secondary winding (690V) and a special tap on the secondary winding (480V). The second way is with a primary winding (10 kV) and a secondary winding (690V) and a tertiary winding (480V). When the medium voltage transformer is not in the top of the turbine, there is still a need for the converter voltage level (480V), and that can also be implemented in several ways, such as either having a transformer with primary winding (690V) and secondary winding (480V) or having a autotransformer with one active winding (690V) but a secondary tap (480V). The medium voltage transformer steps up the voltage to an amount, for example 10 kV at the primary side, required for a power supply, such as a utility grid. The contactor
113
, however, is only exemplary and the stator windings can be directly connected to transformer
170
in either Y-connection or &Dgr;-connection. Further, the output from stator
113
can be connected directly to the utility grid or to a separate transformer, instead of transformer
170
.
To supply power to/from rotor
112
, current induced in rotor
112
is passed through an output filter
140
, which is designed to prevent large voltage changes across the generator windings and thereby increase the lifetime of the winding insulation, and then is passed to a back-to-back indirect power converter
150
. Power converter
150
includes a converter stage
151
, which converts the variable frequency output of generator
110
to a DC voltage, a DC link
152
, including an electrolytic capacitance
153
, and a converter stage
154
, which converts the DC link voltage into a fixed frequency output. The output of converter
154
is fed to a filter
160
, which smoothes the current to be supplied and boosts the DC-link voltage. To reduce the voltage ratings of the switches included in converters
151
and
154
, the filtered fixed-frequency output is applied to the low-voltage, tertiary windings of transformer
170
, for example 480 V.
In accordance with FIG.
1
and assuming ideal components:
P
m
=
P
r
+
P
s
=
sP
s
+
P
s
where
s
=
ω
r
-
ω
s
ω
s
(
2
)
P
r
=
sP
m
1
+
s
(
3
)
where P
m
is the mechanical input power from the wind, P
r
is the power supplied from the rotor circuit, P
s
is the power supplied from the stator, and &ohgr;
r
and &ohgr;
s
are the angular frequency of the rotor shaft and the stator field, respectively.
The configuration of
FIG. 1
, which uses doubly-fed induction generator
110
and indirect power conversion circuit
150
, has certain disadvantages. In the turbine of
FIG. 1
, the switches in the rotor inverter
151
have to be designed to withstand the full load conditions at synchronous speed. At synchronous speed or near synchronous speed, high thermal stress on the switches in the rotor inverter occur because the load on the switches is unequally distributed. As an example, a generator may be running at synchronous speed and delivering a maximum power P
m
of 2 MW. At synchronous speed the rotor current Ir is direct current with a frequency of 0 Hz·I
r
is calculated as:
I
r
=
I
s
n
·
cos
(
φ
n
)
(
4
)
where n is the winding ratio between rotor and stator, I
s
is the stator current, and cos(&phgr;
n
) is the nominal displacement angle of the generator when the rotor is short circuited. The maximum stator current I
s
at synchronous speed is given by:
I
s
=
P
max
U
s
·
3
(
5
)
where U
s
is the line-line stator voltage. A typical stator voltage for a wind turbine that produces 2 MW is 690V. Using equation 4 and equation 5, the rotor current is 707 A, assuming a ratio n=2.63 and cos((&phgr;
n
)=0.9. At synchronous speed, the currents in the rotor windings have DC-values, and the current in a specific winding can assume any arbitrary DC-value between zero and 707·{square root over (2)}. In a worst case scen
Helle Lars
Rebsdorf Anders V.
Finnegan Henderson Farabow Garrett & Dunner LLP
Ponomarenko Nicholas
Vestas Wind Systems A/S, R&D
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