Pulse-controlled inverter with variable operating sequence and wind power plant having such an inverter

A pulse inverter with variable pulse frequency for producing a sinusoidal alternating current is provided, wherein the pulse frequency variation is dependent on the configuration of the alternating current to be produced, the pulse frequency at the passage-through-zero of the alternating current to be produced is a multiple greater than in the region of the maximum amplitude of the alternating, current and the lowest pulse frequency in the region of the maximum amplitude of the alternating current is at least 100 Hz. In a preferred embodiment, the pulse frequency in the region of the passage-through-zero of the alternating current to be produced is in the range of about 14-18 kHz, and in the region of the maximum amplitudes of the current it is about 500 Hz to 2 kHz. A wind power installation may be provided with a pulse inverter as described above, or alternatively, a plurality of wind power installations provided with pulse inverters as described above are connected in parallel relationship. The switching frequency of the pulse inverter is variable, dependent on the alternating current to be produced. In that respect, in the region of the passage-through-zero of the alternating current produced, the switching frequency is at a maximum and the pulse duty cycle is at a minimum. In the region of the maximum amplitudes of the alternating current, the switching frequency is at a minimum, and the pulse duty cycle is at a maximum.

BACKGROUND OF THE INVENTION
 It is known in relation to wind power installations for them to be equipped
 with a synchronous generator and to provide an intermediate dc voltage
 circuit and a pulse inverter connected on the output side thereof as a
 frequency converter, for the variable-speed operation of the synchronous
 generator.
 FIG. 4 is a circuit diagram illustrating the principle of such a wind power
 installation, wherein a variable-speed synchronous generator directly
 driven by the rotor is provided with a frequency converter connected on
 the output side thereof. In the intermediate dc circuit, firstly the
 variable-frequency current generated by the generator is rectified and
 then it is fed into the main network by way of the frequency converter.
 The design configuration permits a wide range of speeds of rotation as the
 intermediate dc circuit provides for complete decoupling of the generator
 and therewith the rotor speed, from the mains frequency. The wide speed
 range permits effective wind-controlled operation of the rotor so that,
 when the design configuration is appropriate, it is possible to achieve a
 perceptible increase in its aerodynamically governed supply of power. It
 is almost self-evident that this design totally eliminates the unpleasant
 dynamic properties that the synchronous generator has in the event of
 direct connection to the mains network.
 Up to a few years ago, a serious objection to the `synchronous generator
 with intermediate dc circuit` system was the high level of costs and the
 poor overall level of electrical efficiency. Because all the electrical
 output flows by way of the converter, the level of efficiency in the case
 of old installations was basically lower than with the variable-speed
 generator arrangements which use the converter only in the rotor circuit
 current of an asynchronous generator. Modern converter technology however
 has made that objection substantially irrelevant. Nowadays rectifiers and
 converters are designed whose losses are extremely low so that the overall
 level of efficiency of that generator system is as in the case of
 double-feed asynchronous generators.
 The variable-speed synchronous generator with intermediate dc circuit is
 therefore nowadays very widespread in wind power installation technology.
 In particular modern inverters have made a significant contribution in
 that respect. In that connection, troublesome harmonics are substantially
 eliminated with so-called `pulse width-modulated (pwm) inverters`. Known
 pwm-inverters have a constant switching frequency or pulse duty cycle
 (also referred to as pulse frequency or pulse repetition rate) and the
 desired sinusoidal form of the alternating current to be fed in is formed
 by way of the ratio of the switch-on and switch-off times of two switches
 S1 and S2. The pulse duty cycle within which the switches S1 and S2 are
 switched on and off respectively is constant, as mentioned, and limited by
 the power loss of the inverter. In known inverters, the losses can be up
 to 2% or more of the total electrical output power generated, and that can
 be considerable in the light of the high level of costs of a wind power
 installation.
 If the switching frequency is reduced, the power loss can admittedly be
 minimised but that causes an increase in the content of troublesome
 harmonics. If the switching frequency is increased, the power loss rises,
 as mentioned, but then the harmonics are very substantially eliminated.
 DE 32 04 266 discloses a process and an apparatus for the operation of a
 pulse inverter in which an ac voltage which is synchronous with the
 desired inverter output voltage is compared to a delta voltage and when
 the two voltages are identical a change-over switching signal for the
 inverter switches is produced. To increase the output voltage amplitude
 the ratio of the control voltage amplitude and the delta voltage amplitude
 is raised to an over-proportional value.
 DE 32 07 440 discloses a process for optimising the voltage control of
 three-phase pulse inverters, in which a constant dc voltage is supplied,
 in particular by an intermediate circuit. To optimise the voltage control
 of the three-phase pulse inverter, that process provides for the
 production of switching patterns which permit continuous adjustment of the
 fundamental oscillation voltage with the minimum possible harmonics
 effect.
 Finally, DE 32 30 055 discloses a control assembly for a pulse inverter for
 producing an output ac voltage with a reference frequency which is
 predetermined by a frequency control, and a reference amplitude which is
 predetermined by an amplitude control voltage. The control assembly makes
 it possible in a simple manner to predetermine for an inverter, an output
 voltage which is optimised in regard to voltage utilisation and harmonics
 content.
 Therefore the object of the invention is to provide a pulse inverter for a
 wind power installation, which avoids the above-mentioned disadvantages
 and overall reduces the power loss with a minimum content of harmonics.
 BRIEF SUMMARY OF THE INVENTION
 In accordance with the invention, a pulse inverter with variable pulse
 frequency for producing a sinusoidal alternating current is provided,
 characterized in that the pulse frequency variation is dependent on the
 configuration of the alternating current to be produced. The pulse
 frequency at the passage-through-zero of the alternating current to be
 produced is a multiple greater than in the region of the maximum amplitude
 of the alternating current, and the lowest pulse frequency in the region
 of the maximum amplitude of the alternating current is at least 100 Hz. In
 a preferred embodiment, the pulse inverter is further characterized in
 that the pulse frequency in the region in the passage-through-zero of the
 alternating current to be produced is in the range of about 14-18 kHz, and
 in the region of the maximum amplitudes of the current it is about 500 Hz
 to 2 kHz. In a preferred embodiment, a wind power installation is provided
 with a pulse inverter as described above. Alternatively, a plurality of
 such wind power installations as described above are connected in mutually
 parallel relationship.
 The invention is based on the idea of moving completely away from a pulse
 inverter with a static switching frequency or pulse duty cycle, as is
 known from the state of the art and from FIG. 2, and making the switching
 frequency variable, more specifically in dependence on the alternating
 current to be generated. In that respect, the switching frequency is at a
 maximum, that is to say the pulse duty cycle is at a minimum, in the
 region of the passage-through-zero of the alternating current produced;
 the switching frequency is at a minimum, that is to say the pulse duty
 cycle is at a maximum, in the region of the maximum amplitudes of the
 alternating current.
 It was possible to find that, with such a pulse inverter, the switching
 losses of the power semiconductors can be minimised, which results in a
 drastic reduction in the power loss, and that the current which is to be
 fed in has a very high fundamental oscillation content without troublesome
 harmonics. In addition, as there is not a pronounced fixed switching
 frequency, no troublesome resonance phenomena occur when a plurality of
 wind power installations are switched in parallel relationship, which
 results in a further relative improvement in the fundamental oscillation
 content. While, with previous pulse inverters, a static switching
 frequency was accepted and attempts were made to optimise matters in the
 region of the switching times of the switches S1 and S2 in order to reduce
 the power loss and to minimise the harmonics content, the invention also
 proposes optimising the switching frequency of the pulse inverter, in
 which case the switching frequency changes in dependence on the sinusoidal
 current which is to be fed in. The configuration of the variable switching
 frequency is shown in simplified form in FIG. 3b.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 shows a switch S1 an da switch S2 and an inductor L connected on the
 output side thereof. The switch S1 is connected to the positive terminal
 of the dc voltage supplied and the switch S2 is connected to the negative
 terminal.
 FIG. 2 shows in a) the result of pulse inversion in the case of a known
 pulse inverter as shown in FIG. 1. In this case, the switching frequency
 f.sub.s or the inverse of the switching frequency, the pulse duty cycle T,
 as shown in FIG. 2b), is constant. Within a cycle, one switch S1 is
 switched on for a period t1 and one switch S2 is switched on for a period
 t2. By suitable presettings of and variations in the switching durations
 t1 and t2 or the corresponding switch-off times of the switches S1 and S2,
 sinusoidal alternating current--see FIG. 2a)--can be generated from the
 direct current supplied. The sinusoidal configuration can be optimised by
 optimising the switching times t1 to t2 within the switching period T. The
 switch-on and switch-off configuration shown in FIG. 2 is only shown in
 greatly simplified form, for reasons of clarity of the drawing. The
 switching frequency is however limited by the power loss P.sub.v of the
 pulse inverter. The power loss P.sub.v increases with an increasing
 switching frequency. The power loss P.sub.v admittedly decreases with
 decreasing switching frequency, but then the content of harmonics
 increases, which can result in mains incompatibilities.
 It will be seen from FIG. 3 at 3b) that the switching frequency for the
 current i which is to be fed in, in FIG. 3a), is adapted to be variable,
 and that the switching frequency is at a maximum in the region of the
 passages-through-zero of the alternating current i to be produced and at a
 minimum in the region of the maximum amplitudes of the alternating current
 i to be produced. In the region of the maximum amplitudes of the
 alternating current i to be produced the switching frequency f.sub.s is
 about 16 kHz at the maximum and about 1 kHz at the minimum. The
 variability of the switching frequency provides that in the region of the
 passages-through-zero, the alternating current to be produced is produced
 in virtually coincident relationship with the ideal sinusoidal curve and
 that in the region of the maximum amplitudes, the alternating current
 produced has a greater harmonics component than in the region of the
 passages-through-zero. Overall however the content of harmonics is at a
 minimum and is practically zero in the region of the
 passages-through-zero.
 If now a plurality of wind power installations with a synchronous generator
 and a corresponding pulse inverter with a control as shown in FIG. 3b) are
 connected in parallel, this does not involve a pronounced fixed switching
 frequency which causes problems--as hitherto--, and the variable switching
 frequency provides that there are no troublesome resonance phenomena
 between the individual wind power installations so that the fundamental
 oscillation content is overall significantly improved in a parallel
 connection of a plurality of wind power installations.
 FIG. 4 is a circuit diagram illustrating the principle of a variable-speed
 synchronous generator SG driven by a rotor R, with an output-side
 rectifier G and a pulse inverter PWR--see FIG. 5--as is known for example
 in the wind power installation ENERCON of type `E-40`. The synchronous
 machine in the case of the generator developed for the type `E-4` is an
 electrically excited synchronous machine with 84 poles. The diameter is
 about 4.8 m.
 The total losses with the frequency inverter with an actuating
 configuration as shown in FIG. 2 are still about 2.5% of the total
 electrically generated power, in the case of the known `E-40` wind power
 installation. Those losses can be considerably reduced by over 30% or more
 by means of the invention, while the mains feed can still be practically
 oscillation-free.
 From the foregoing, it will be appreciated that although embodiments of the
 invention have been illustrated and described, various modifications can
 be made without departing from the spirit of the invention. Thus, the
 present invention is not limited to the embodiments described herein, but
 rather is defined by the claims which followed.