WO2016165739A1

WO2016165739A1 - Renewable energy system, renewable energy park, method for operating a renewable energy system, and method for operating a renewable energy park

Info

Publication number: WO2016165739A1
Application number: PCT/EP2015/058008
Authority: WO
Inventors: Thomas Smolka; Manuel SOJER
Original assignee: Maschinenfabrik Reinhausen Gmbh
Priority date: 2015-04-14
Filing date: 2015-04-14
Publication date: 2016-10-20

Abstract

A renewable energy system (10) comprises: a power generating unit (11), which can generate electric alternating current; a system transformer (12) with adjustable transmission ratio, the primary side of which system transformer (12) is connected to the power generating unit (11) and the secondary side of which can be connected to a power network (20); a system control apparatus (13), which is connected to the system transformer (12) and is constructed such that it can adjust the transmission ratio of the system transformer (12) in dependence on a system reactive power and/or on a system reactive power requirement and/or on a primary side system voltage and/or on a primary side system voltage requirement and/or on a secondary side system voltage and/or on a secondary side system voltage requirement and/or on an operating point of the power generating unit (11) and/or on an operating point of the system (10).

Description

RENEWABLE ENERGY SYSTEM, RENEWABLE ENERGY PARK, METHOD FOR OPERATING A RENEWABLE ENERGY SYSTEM, AND METHOD FOR OPERATING A

RENEWABLE ENERGY PARK The invention relates to a renewable energy system, in particular a wind energy turbine or a photovoltaic system; to a renewable energy park, in particular a wind farm and/or a solar farm; to a method for operating a renewable energy system; and to a method for operating a renewable energy park.

DE 10 2008 018 748 A1 describes a wind energy turbine with a rotor, a generator driven thereby with a converter for generating electrical power for supply to a network via a transformer for which voltage monitoring is provided, and a control apparatus comprising a converter controller, wherein the control apparatus applies a control signal for a reactive component to the converter, wherein a voltage measuring device is disposed on the transformer, the voltage signal of said voltage measuring device being applied to an input of a condition-based setpoint slider, the output signal of said slider being applied to a limiting module for the reactive component acting on the converter.

This document additionally describes a method for operating the wind energy turbine by setting a control signal for a reactive component of the output power, measuring a voltage at the transformer, determining a correction signal for the reactive power degree by means of a condition-based set point slider, and using this shifted signal for limiting the control signal of the reactive component.

DE 10 2008 048 258 A1 describes a wind farm with a transmission station, which transmits electrical power generated at the wind farm from a wind farm's internal network to an external network, a tap changer for setting the transmission ratio between the volt- age in the wind farm's internal network and the voltage in the external network, and a wind farm regulator, which specifies a transmission ratio to the tap changer, wherein the wind farm regulator specifies the transmission ratio to the tap changer in dependence on the electrical load of a wind energy turbine of the wind farm. A U-Q family of characteristics is stored in a family-of-characteristic-curves memory. A wind energy turbine of the wind farm comprises a logic module, which compares the voltage U and the reactive component Q of the electrical power output by the wind energy turbine to the U-Q family of characteristics. The wind farm regulator comprises a logic module, which compares the voltage U and the reactive component Q of the wind farm's internal network to the U-Q family of characteristics. The wind farm regulator comprises an assessment module, which deter- mines whether the tap changer will be switched based on the present active power output and/or based on a voltage average value and/or based on a condition quantity of the tap changer. The tap changer is designed to carry out a change over several switching steps in one switching process.

This document additionally describes a method for operating a wind farm, in which the electrical power generated in the wind farm is transported via a wind farm's internal network to a transmission station and in which the electrical power, prior to being transmitted to an external network, is transformed to a voltage which is higher by a selectable transmission ratio than the voltage in the wind farm's internal network, comprising the steps: a) determining the electrical load of a wind energy turbine of the wind farm; and b) setting the transmission ratio in dependence on the electrical load of the wind energy tur- bine. The electrical load of the wind energy turbine is determined by comparing the voltage U and a reactive component Q of the electrical power generated by the wind energy turbine to a U-Q family of characteristics. A reactive power reserve is determined by means of the U-Q family of characteristics. When the transmission ratio is changed, new setpoints are specified for the voltage and/or the reactive component to the wind energy turbines of the wind farm.

This document describes that the electrical load of the wind energy turbine is determined by comparing the voltage U and a reactive component Q of the electrical power generated in the wind energy turbine to a U-Q family of characteristics, wherein "reactive component" is to be understood as a generic term for a variety of presentation options, such as reactive current, reactive power, phase angle, etc. "U-Q family of characteristics" is intended to be understood as follows. The generator of a wind energy turbine generates the electrical power at small voltages, which are mostly between 400 V and 6 kV. Prior to transmission to the wind farm's internal network, the electrical power is converted to a medium voltage of, for instance, 20 kV by means of a transformer belonging to the wind energy turbine. Both the active component and the reactive component of the electrical power are transmitted via the transformer of the wind energy turbine. It is known that the transformer's ability to transmit the reactive component depends on the voltage at which the transformer is being operated. At one end of the permissible voltage range, the wind energy turbine can generate the capacitive reactive component without limitation (over- excited operation), while it reaches its system limits quicker in the transmission of an inductive reactive component (under-excited operation). At the other end of the permissible voltage range, the wind energy turbine can generate an inductive reactive component very well, while it reaches its system limits quicker in the transmission of a capacitive reactive component. DE 10 2008 018 748 gives a detailed description of this interrelationship. A U- Q family of characteristics that is based on this understanding lacks two corners in comparison to the rectangular shape that one would expect if the ability to transmit the reac- tive component would be the same across the entire permissible voltage range. The shape of the U-Q family of characteristics thus approximates a rhombus. With regard to the U-Q family of characteristics on the side of the wind farm's internal network, the ability to feed in a capacitive reactive component is reduced at a high voltage and the ability to feed in an inductive reactive component is reduced at a low voltage. This is reversed on the side of the generator. The voltage U is preferably regarded on the side of the wind farm's internal network, but it is also possible to regard it on the generator side. The U-Q family of characteristics changes in dependence on the amount of power output by the wind energy turbine. It is thus possible to feed in a higher proportion of reactive power when the active power is lower. The shape of the U-Q family of characteristics remains rhombus-like, however. If the operating point of the wind energy turbine as defined by the voltage U and the reactive component Q is compared to the U-Q family of characteristics, a lower electrical load of the wind energy turbine can be assumed when the operating point is located at a far distance from all limits of the U-Q family of characteristics. The electrical load becomes greater with the operating point approximating a limit of the U-Q family of characteristics. When the operating point approximates a limit of the U-Q family of characteristics where a corner is missing as compared to the rectangle, the typical reaction would be to reduce the reactive component Q and accept that the wind energy turbine is no longer able to meet the requirement for the reactive power. In the context of the invention as known from this document, it is instead possible to change the voltage in the wind farm's internal network by changing the transmission ratio. The operating point of the wind energy turbine is thereby distanced from the limits of the U-Q family of characteristics and the electrical load of the wind energy turbine is reduced. The additional scope created in this way can be used for increasing the reactive component Q so that the re- quirement for the reactive power can be met again.

This known wind farm comprises a plurality of wind energy turbines. Each wind energy turbine comprises a rotor, the rotation of which is converted to electrical power by a generator. Already in the wind energy turbine is the electrical power, which is generated by the generator at a voltage of, for instance, 690 V, transformed to a medium-voltage level of 20 kV. The electrical power is fed into a wind farm's internal network at the voltage of 20 kV and conducted to the transmission station via the wind farm's internal network. The sections of the wind farm's internal network each also have the effect of an inductance, a capacitance, and a resistance. Provided in the transmission station is a transformer with a tap changer, by means of which the electrical power is converted from the 20 kV medium voltage to a high voltage of 1 10 kV. The electrical power is transmitted to an external network in the transmission station. The external network is normally a public power network, via which electrical power is supplied to the consumers. The wind farm regulator is provided with various data on the condition of the wind farm. The data comprise the actual voltage and the actual current in the wind farm's internal network, requirements from outside for the voltage and the reactive power at which the electrical power is supposed to be transmitted to the external network, and data on the condition of the wind energy turbines. The wind farm regulator processes these data and uses them to calculate specifications, which are transmitted to the components of the wind farm. In this way, the tap changer receives a specification for the transmission ratio. The wind energy turbines are given specifications for the voltage and the reactive power.

As rendered in FIG. 6 of the present patent application, this document shows a U-Q family of characteristics, which indicates the operating range within which a wind energy turbine can feed electrical power into the wind farm's internal network. In the U-Q family of characteristics, the voltage U is indicated in normalized units on one axis with a value of 1.00 corresponding to the nominal voltage. The reactive component Q is plotted, likewise in normalized units, on the other axis, with the current and voltage being in phase when the value of Q is equal to 0.0. Only active power is therefore transmitted when Q is equal to 0.0, and the reactive component is 0. A capacitive reactive power is fed in when the values of Q are positive, and an inductive reactive power is fed in when the values of Q are negative. If the wind energy turbine were able to provide reactive power to the same extent over the entire permissible voltage range between 0.95 and 1 .06, then the U-Q family of characteristics would have the shape of a rectangle. However, as explained above, the ability to feed in an inductive reactive power is reduced under low voltages. In contrast, when the voltages are at the upper end of the permissible range, the ability to feed in a capacitive reactive power is reduced. In comparison to a rectangle, the bottom- left corner and the top-right corner are therefore missing, and the family of characteristics diagram approximates to the shape of a rhombus.

If the voltage U and the reactive component Q, with which the wind energy turbine feeds electrical power into the wind farm's internal network, are determined, then it is possible to plot the operating point, at which the wind energy turbine is operating, into the U-Q family of characteristics of FIG. 6. An operating point in the center of the U-Q family of characteristics in general indicates a rather low electrical load level of the wind energy turbine. An operating point close to the limit of the U-Q family of characteristics is an indication of a high electrical load level of the wind energy turbine. The figure shown applies to a high or regular active power output. A different limit function, in which the operating range of the U-Q diagram is enlarged to the left and right, for instance, would be used for the low active power output range. One possible operating point 1 16 of a wind energy turbine is plotted into the U-Q family of characteristics in FIG. 6. The voltage U is 1 .04 of the nominal voltage, and a sufficient amount of capacitive reactive power is fed in so that the operating point is directly adjacent to the limit of the U-Q family of characteristics. The operating point 1 16 can be maintained provided that the wind energy turbine can supply just as much reactive power as is required. In another instance, the wind energy turbine would actually have to supply more reactive power in order to meet the requirement for the reactive component, but this is not possible due to the high electrical load level. An increase in the reactive power component would result in an operating point outside the U-Q family of characteristics, and therefore outside the permissible range. In this instance, the wind energy turbine signals its high electrical load level to the wind farm regulator. On the basis of the method according to the invention, the wind farm regulator can react by switching the tap changer by one step so that the voltage in the wind farm's internal network falls. The operating point 1 16 changes to the operating point 1 16a. The wind energy turbine regulator detects the decrease in the electrical load of the wind energy turbine at the operating point 1 16a in comparison to the operating point 1 16. If the capacitive reactive power at the operating point 1 16a is insufficient, the wind energy turbine regulator will set the operating point 124. At the operating point 124, and in contrast to the operating point 1 16, the wind energy turbine is therefore able to meet a requirement for increased capacitive reactive power. At the same time, the electrical load at the operating point 124 is lower than at the operating point 1 16, because the operating point 124 is still at some distance from the limits of the U-Q family of characteristics. Actuating the tap changer thus results in the operating point of the wind energy turbine first being shifted so as to reduce the electrical load level. Part of the additional scope created in this way is used for feeding in more capacitive reactive power.

The requirements for operating wind energy turbines and wind farms and, in general, other types of renewable energy systems and renewable energy parks, are defined by the network operators in network connection codes and they are continuously being tightened. The guidelines „Erzeugungsanlagen am Mittelspannungsnetz" (MSR 2008) published in June 2008 by the "BDEW Bundesverband der Energie- und Wasserwirtschaft e.V.", or the "Vierte Erganzung" of January 2013 (4^th Supplement to MSR 2008), for instance, contain such network connection codes. The said guidelines apply to planning, construction, operation and modification of generating plants which are connected to a network operator's medium-voltage network and operated in parallel with this network. They also apply if the network connection point of the generating plant is located in the low-voltage network, while the junction point with the public network is located in the medium-voltage network. This refers, for instance, to generating plants connected to a low- voltage network that is linked with the network operator's medium-voltage network through a separate customer transformer, and which no customers of public supply are connected to. For generating plants with the network connection point located in the medium-voltage network but the junction point located in the high- or extra-high-voltage network, the rele- vant technical connection rules shall be applied. For generating plants having both their network connection point and their junction point in the low-voltage network, the guidelines apply that were published by the VDEW on generating plants connected to the low- voltage network "Eigenerzeugungsanlagen am Niederspannungsnetz". Generating plants in the sense of these guidelines are, for instance, wind energy turbines, hydro power plants, cogeneration units (e.g. biomass, biogas or natural gas-fired power stations), and photovoltaic plants. A generating plant may be composed of a single generator or of several generating units (e.g. wind farm). The electrical energy can be generated by synchronous or asynchronous generators with or without converters or by direct current generators (e.g. solar cells of photovoltaic plants) with power inverters.

According to the currently applicable network connection codes, which have been tightened by the above-mentioned 4^th Supplement to MSR 2008 as of January 2013, the known wind energy turbines, wind farms, photovoltaic systems, and solar farms cannot supply the required reactive power in the instance of undervoltage in the network, or they can do so only with effort and input, for example by reducing active power, or they even have to be disconnected from the network. This applies in particular to wind energy turbines without full-scale converter, e.g. with a double-fed asynchronous machine. The known wind energy turbines with full-scale converters can accomplish this by having additional power electronics cabinets integrated into the wind energy turbine; such full-scale converters and additional power electronics cabinets are, however, very expensive.

Against this background, the invention suggest the subject-matters of the independent claims. Advantageous developments and embodiments of the invention are described in the dependent claims.

According to a first aspect, the invention proposes a renewable energy system, in particular a wind energy turbine or a photovoltaic system, comprising or having

- a power generating unit, which can generate electric alternating current, in particular three-phase current;

a system transformer with adjustable transmission ratio, the primary side of which system transformer is connected to the power generating unit and the secondary side of which can be connected to a power network;

- a system control apparatus, which is connected to the system transformer and is constructed such that it can adjust the transmission ratio of the system transformer in de- pendence on a system reactive power or on a present reactive power supplied by the system, and/or on a system reactive power requirement or on a predetermined reactive power that the system is supposed to supply to the power network, in particular according to a requirement from a network operator, and/or on a primary side system voltage or on a present voltage being applied to the primary side of the system transformer, and/or on a primary side system voltage requirement or on a predetermined voltage that is supposed to be applied to the primary side of the system transformer; and/or on a secondary side system voltage or on a present voltage being applied to the secondary side of the system transformer, and/or on a secondary side system voltage requirement or on a predetermined voltage that is supposed to be applied to the secondary side of the system transformer, in particular according to a requirement from a network operator, and/or on a working or operating point of the power generating unit, and/or on a working or operating point of the system.

The system as proposed according to the first aspect enables compliance with stricter network connection codes, in particular with reactive power requirements.

The system control apparatus is preferably constructed such that it can adjust this transmission ratio in dependence on the working or operating point of the power generating unit, which working or operating point is defined, for instance, by the present voltage of the power generating unit, its present active power, and its present reactive power; and/or in dependence on the working or operating point of the system, which working or operating point is defined, for instance, by the present voltage of the system, its present reactive power, and its present active power. Each of these operating points can also be defined differently, as required, for instance by the corresponding present voltage, the corresponding present active power, and the corresponding present phase shift; or by the corre- sponding present voltage, the corresponding present current, and the corresponding present phase shift; or by another combination of current and/or voltage and/or phase shift and/or active power and/or reactive power and/or apparent power.

The system transformer can be constructed in an optional manner, as required, for instance as a dry-type transformer or as an oil transformer. In this instance, the trans- former's primary side is also referred to as lower voltage side or generator side, and its secondary side is also referred to as higher voltage side or network side.

The power network can be constructed in an optional manner, as required, for instance as a park-internal power network, which several systems are connected to, and which typically has a nominal voltage in the medium-voltage range of 12 kV or 24 kV or 36 kV, for instance, or as a consumer power network or a distributor network or an integrated network, which typically has a nominal voltage in the medium-voltage range of 12 kV or 24 kV or 36 kV, for instance, or in the high-voltage range of 1 10 kV or 120 kV, for instance. Preferably, the power network is the park-internal power network of a park as proposed according to the second aspect.

The system control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust this transmission ratio by being able to actuate the system transformer in dependence on the system reactive power requirement in such a manner that the system transformer has a transmission ratio that allows the system to provide and/or supply to the power network a reactive power corresponding to the system reactive power requirement.

It can be provided that each of the proposed systems is a wind energy turbine wherein the power generating unit comprises

a rotor;

a generator, which is coupled to the rotor, and which can generate the electric alternating current.

The generator can be constructed in an optional manner, as required, for instance as a one-phase or a multi-phase or a three-phase generator or as a rotary current generator and/or such that it is coupled via a transmission or directly to the rotor.

It can be provided that each of the proposed systems comprises a power converter, namely a converter, which is connected to the generator and the primary side.

Since the system voltage can be changed as required by means of the system transformer, the power converter can be designed for a lower nominal power and thus be more cost-efficient in comparison to known systems.

The power converter can be constructed in an optional manner, as required, and for instance have a direct voltage intermediate circuit or a direct current intermediate circuit or no intermediate circuit, or it can be a direct inverter or a matrix inverter.

It can be provided that the generator is a synchronous generator.

The primary side is thus connected to the generator via the power converter.

This generator typically generates a voltage in the low-voltage range of 690 V, for instance.

It can be provided that

the generator is a double-fed asynchronous generator with a stator and a rotor, in particular a slip ring rotor;

the stator is connected to the primary side;

the rotor is connected to the power converter.

The primary side is thus connected to the generator via the power converter.

At the stator and at the rotor, this generator typically generates a voltage in the low- voltage range of 690 V, for instance, or at the stator a voltage in the low medium-voltage range of 6 kV, for instance, and at the rotor a voltage in the low-voltage range of 690 V, for instance.

It can be provided that the system transformer has a primary winding or has a primary winding for each phase, with the primary winding being connected to the stator and to the power converter.

The primary winding is thus connected to the rotor via the power converter.

It can be provided that the system transformer has a first primary winding or a first primary winding for each phase, which primary winding is connected to the stator, and a second primary winding or a second primary winding for each phase, which primary winding is connected to the power converter.

The second primary winding is thus connected to the rotor via the power converter.

It can be provided that each of the proposed systems is a photovoltaic system wherein the power generating unit comprises

- a solar module;

a power converter, namely a power inverter, which is connected to the solar module, and which can generate the electric alternating current.

The power converter can be constructed in any manner as required, for instance as a one-phase or multi-phase or three-phase power converter. This power converter typically generates a voltage in the low-voltage range of 230 V or 690 V, for instance.

The power generating unit can be constructed in an optional manner, as required, and for instance comprise or have at least one additional or further solar module and/or at least one additional or further power converter.

It can be provided that each of the proposed systems comprises

a system reactive power sensor, which can detect the system reactive power and can produce a corresponding system reactive power signal, and which is in particular connected to the system control apparatus;

wherein the system control apparatus is constructed such that it can adjust the transmission ratio of the system transformer in dependence on the system reactive power signal.

The system reactive power sensor can be constructed in an optional manner, as required, and for instance be connected to the primary side and/or the secondary side of the system transformer and/or comprise sensors for the system's voltage, current, and phase shift and produce corresponding measurement signals, from which the system reactive power sensor or the system control apparatus can determine the system reactive power and, in particular, the system active power as well.

The system control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust this transmission ratio in dependence on the system reactive power signal by being able to adjust this transmission ratio in dependence on a comparison of the system reactive power signal to the system reactive power requirement or by being able to adjust this transmission ratio by such a comparison.

It can be provided that each of the proposed systems comprises

a system voltage sensor, which can detect the primary side system voltage and can produce a corresponding system voltage signal, and which is in particular connected to the system control apparatus; and/or

a system voltage sensor, which can detect the secondary side system voltage and can produce a corresponding system voltage signal, and which is in particular connected to the system control apparatus;

wherein the system control apparatus is constructed such that it can adjust the transmission ratio of the system transformer in dependence on at least one of these system voltage signals.

The system control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust this transmission ratio in dependence on at least one of these system voltage signals by being able to adjust this transmission ratio in dependence on a comparison of the primary side system voltage signal to the primary side system voltage requirement or by being able to adjust this transmission ratio by such a comparison and/or by being able to adjust this transmission ratio in dependence on a comparison of the secondary side system voltage signal to the secondary side system voltage re- quirement or by being able to adjust this transmission ratio by such a comparison.

It can be provided that the system control apparatus can be connected to a remote control system producing remote control signals and it can be constructed such that it can adjust the transmission ratio of the system transformer in dependence on the remote control signals.

The remote control system can be constructed in an optional manner, as required, and, for instance, comprise the park control apparatus of one of the parks proposed according to the second aspect.

It can be provided that each of the proposed systems comprises an on-load tap changer for adjusting the transmission ratio of the system transformer.

This on-load tap changer can be constructed in an optional manner, as required, and for instance mounted to a housing of the system transformer, which is constructed, in par- ticular, as dry-type transformer, or arranged in a housing of the system transformer, which is constructed, in particular, as oil transformer. The on-load tap changer is preferably designed for a transformer performance at which the system can be connected to a medium- voltage network, for instance for an apparent power of the system transformer of at least 300 kVA and/or not more than 6 MVA, or of at least 6 MVA and/or for a mains voltage of 12 kV or 24 kV or 36 kV and/or for a continuous current or a maximum rated current of at least 30 A or at least 100 A per phase.

It can be provided that this on-load tap changer comprises at least one switch, which serves for switching between windings or taps of the system transformer, and comprises at least one vacuum switching tube and/or at least one power semiconductor.

It can be provided that

the power converter is a power converter with adjustable phase shift between the current supplied by the power converter and the voltage supplied by the power converter;

- the system control apparatus is connected to the power converter and is constructed such that it can adjust the phase shift of the power converter in dependence on the system reactive power and/or on the system reactive power requirement and/or on the primary side system voltage and/or on the primary side system voltage requirement and/or on the secondary side system voltage and/or on the secondary side sys- tern voltage requirement and/or on the operating point of the power generating unit and/or on the operating point of the system.

The system control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust the phase shift by being able to actuate the power converter in dependence on the system reactive power requirement in such a manner that the power converter has a phase shift that allows the system to provide and/or supply to the power network a reactive power corresponding to the system reactive power requirement.

According to a second aspect, the invention proposes a renewable energy park, in particular a wind farm or a solar farm, or a wind and solar farm, comprising or having

a park-internal power network;

- at least two renewable energy systems, each of which is constructed like one of the systems as proposed according to the first aspect and is connected with the secondary side of its system transformer to the park-internal power network;

a park transformer with adjustable transmission ratio, the primary side of which park transformer is connected to the park-internal power network and the secondary side of which can be connected to a consumer power network;

a park control apparatus, which is connected to the park transformer and is con- structed such that it can adjust the transmission ratio of the park transformer in dependence on a park reactive power or a present reactive power being supplied by the park, and/or on a park reactive power requirement or a predetermined reactive power that the park is supposed to supply to the consumer power network, in particular ac- cording to a requirement from a network operator, and/or on a primary side park voltage or on a present voltage being applied to the primary side of the park transformer, and/or on a primary side park voltage requirement or on a predetermined voltage that is supposed to be applied to the primary side of the park transformer, in particular according to a requirement from a network operator, and/or on a secondary side park voltage or on a present voltage being applied to the secondary side of the park transformer, and/or on a secondary side park voltage requirement or on a predetermined voltage that is supposed to be applied to the secondary side of the park transformer, in particular according to a requirement from a network operator, and/or on a working or operating point of the park.

The park as proposed according to the second aspect enables compliance with stricter network connection codes, in particular with reactive power requirements.

The park control apparatus is preferably constructed such that it can adjust this transmission ratio in dependence on the park's working or operating point, which is defined, for instance, by the park's present voltage, its present reactive power, and its pre- sent active power. This operating point can also be defined differently, as required, for instance by its present voltage, its present active power, and its present phase shift; or by its present voltage, its present current, and its present phase shift; or by another combination of current and/or voltage and/or phase shift and/or active power and/or reactive power and/or apparent power.

The park as proposed according to the second aspect can be constructed in an optional manner, as required, and for instance comprise or have at least one additional or further park transformer and/or at least one additional or further park control apparatus.

The park transformer can be constructed in an optional manner, as required, for instance as a dry-type transformer or as an oil transformer. In this instance, the transform- er's primary side is also referred to as lower voltage side or generator side, and its secondary side is also referred to as higher voltage side or network side.

The consumer power network can be constructed in an optional manner, as required, for instance as a distributor network or an integrated network, which typically has a nominal voltage in the medium-voltage range of 12 kV or 24 kV or 36 kV, for instance, or in the high-voltage range of 1 10 kV, for instance. Preferably, the park-internal power network is the power network of the systems as proposed according to the first aspect. The park control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust this transmission ratio by being able to actuate the park transformer in dependence on the park reactive power requirement in such a manner that the park transformer has a transmission ratio that allows the park to provide and/or supply to the consumer power network a reactive power corresponding to the park reactive power requirement.

It can be provided that the park control apparatus is connected to the system control apparatuses and is constructed such that it can adjust the transmission ratio of each system transformer in dependence on the park reactive power and/or on the park reactive power requirement and/or on the primary side park voltage and/or on the primary side park voltage requirement and/or on the secondary side park voltage and/or on the secondary side park voltage requirement and/or on the operating point of the park.

The park control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust the transmission ratio of each system transformer by being able to actuate the system transformers in dependence on the park reactive power requirement and, in particular, in dependence on the corresponding system reactive power requirements in such a manner that the system transformers each have a transmission ratio that allows the systems to jointly provide and/or supply to the consumer power network a reactive power corresponding to the park reactive power requirement.

It can be provided that each of the proposed parks comprises a park reactive power sensor, which can detect the park reactive power and can produce a corresponding park reactive power signal, and which is in particular connected to the park control apparatus; wherein the park control apparatus is constructed such that it can adjust the transmission ratio of the park transformer in dependence on the park reactive power signal.

The park reactive power sensor can be constructed in an optional manner, as required, and for instance be connected to the primary side and/or the secondary side of the park transformer and/or comprise sensors for the park's voltage, current, and phase shift and produce corresponding measurement signals, from which the park reactive power sensor or the park control apparatus can determine the park reactive power and, in partic- ular, the park active power as well.

The park control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust this transmission ratio in dependence on the park reactive power signal by being able to adjust this transmission ratio in dependence on a comparison of the park reactive power signal to the park reactive power requirement or by being able to adjust this transmission ratio by such a comparison.

It can be provided that each of the proposed parks comprises a park voltage sensor, which can detect the primary side park voltage and can produce a corresponding park voltage signal, and which is in particular connected to the park control apparatus; and/or

a park voltage sensor, which can detect the secondary side park voltage and can produce a corresponding park voltage signal, and which is in particular connected to the park control apparatus;

wherein the park control apparatus is constructed such that it can adjust the transmission ratio of the park transformer in dependence on at least one of these the park voltage signals.

The park control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust this transmission ratio in dependence on at least one of these park voltage signals by being able to adjust this transmission ratio in dependence on a comparison of the primary side park voltage signal to the primary side park voltage requirement or by being able to adjust this transmission ratio by such a comparison and/or by being able to adjust this transmission ratio in dependence on a comparison of the secondary side park voltage signal to the secondary side park voltage requirement or by being able to adjust this transmission ratio by such a comparison.

It can be provided that the park control apparatus can be connected to a remote control system producing remote control signals and it can be constructed such that it can adjust the transmission ratio of the park transformer in dependence on the remote control signals.

It can be provided that each of the proposed parks comprises an on-load tap changer for adjusting the transmission ratio of the park transformer.

This on-load tap changer can be constructed in an optional manner, as required, and for instance mounted to a housing of the park transformer, which is constructed, in particular, as dry-type transformer, or arranged in a housing of the park transformer, which is constructed, in particular, as oil transformer.

It can be provided that this on-load tap changer comprises at least one switch, which serves for switching between windings or taps of the park transformer, and comprises at least one vacuum switching tube and/or at least one power semiconductor.

It can be provided that

each power converter is a power converter with adjustable phase shift between the current supplied by the power converter and the voltage supplied by the power converter;

- the park control apparatus is connected to the power converters and is constructed such that it can adjust the phase shift of each power converter in dependence on the park reactive power and/or on the park reactive power requirement and/or on the primary side park voltage and/or on the primary side park voltage requirement and/or on the secondary side park voltage and/or on the secondary side park voltage requirement and/or on the operating point of the park.

The park control apparatus can be constructed in an optional manner, as required, for instance such that it can adjust the phase shift of each power converter by being able to actuate the power converters in dependence on the park reactive power requirement and, in particular, in dependence on the corresponding system reactive power requirements in such a manner that the power converters each have a phase shift that allows the systems to jointly provide and/or supply to the consumer power network a reactive power corresponding to the park reactive power requirement.

According to a third aspect, the invention proposes a method for operating a renewable energy system, wherein

the system comprises

· a power generating unit, which can generate electric alternating current;

• a system transformer with adjustable transmission ratio, the primary side of which system transformer is connected to the power generating unit and the secondary side of which can be connected to a power network;

the transmission ratio of the system transformer is adjusted in dependence on a sys- tern reactive power and/or on a system reactive power requirement and/or on a primary side system voltage and/or on a primary side system voltage requirement and/or on a secondary side system voltage and/or on a secondary side system voltage requirement and/or on a working or operating point of the power generating unit and/or on a working or operating point of the system.

The method as proposed according to the third aspect enables compliance with stricter network connection codes, in particular with reactive power requirements.

The adjustment of this transmission ratio is preferably carried out in dependence on the working or operating point of the power generating unit, which working or operating point is defined, for instance, by the present voltage of the power generating unit, its pre- sent active power, and its present reactive power; and/or in dependence on the working or operating point of the system, which working or operating point is defined, for instance, by the present voltage of the system, its present reactive power, and its present active power. Each of these operating points can also be defined differently, as required, for instance by the corresponding present voltage, the corresponding present active power, and the corresponding present phase shift; or by the corresponding present voltage, the corresponding present current, and the corresponding present phase shift; or by another com- bination of current and/or voltage and/or phase shift and/or active power and/or reactive power and/or apparent power.

The adjustment of this transmission ratio can be carried out in an optional manner, as required, for instance by the system transformer being actuated in dependence on the system reactive power requirement in such a manner that the system transformer has a transmission ratio that allows the system to provide and/or supply to the power network a reactive power corresponding to the system reactive power requirement.

The system can be constructed in an optional manner, as required, for instance like one of the systems proposed according to the first aspect.

It can be provided that

the phase shift of the power converter is adjusted in dependence on the system reac- tive power and/or on the system reactive power requirement and/or on the primary side system voltage and/or on the primary side system voltage requirement and/or on the secondary side system voltage and/or on the secondary side system voltage requirement and/or on the operating point of the power generating unit and/or on the operating point of the system.

The adjustment of the phase shift can be carried out in an optional manner, as required, for instance by the power converter being actuated in dependence on the system reactive power requirement in such a manner that the power converter has a phase shift that allows the system to provide and/or supply to the power network a reactive power corresponding to the system reactive power requirement.

According to a fourth aspect, the invention proposes a method for operating a renewable energy park, wherein

the park comprises

• a park-internal power network;

• at least two renewable energy systems, each of which is constructed like one of the systems as proposed according to the first aspect and is connected with the secondary side of its system transformer to the park-internal power network;

• a park transformer with adjustable transmission ratio, the primary side of which park transformer is connected to the park-internal power network and the secondary side of which can be connected to a consumer power network;

- the transmission ratio of the park transformer is adjusted in dependence on a park reactive power and/or on a park reactive power requirement and/or on a primary side park voltage and/or on a primary side park voltage requirement and/or on a secondary side park voltage and/or on a secondary side park voltage requirement and/or on a working or operating point of the park.

The method as proposed according to the fourth aspect enables compliance with stricter network connection codes, in particular with reactive power requirements.

The adjustment of this transmission ratio is preferably carried out in dependence on the park's working or operating point, which is defined, for instance, by the park's present voltage, its present reactive power, and its present active power. This operating point can also be defined differently, as required, for instance by its present voltage, its present ac- tive power, and its present phase shift; or by its present voltage, its present current, and its present phase shift; or by another combination of current and/or voltage and/or phase shift and/or active power and/or reactive power and/or apparent power.

The adjustment of this transmission ratio can be carried out in an optional manner, as required, for instance by the park transformer being actuated in dependence on the park reactive power requirement in such a manner that the park transformer has a transmission ratio that allows the park to provide and/or supply to the consumer power network a park reactive power corresponding to the park reactive power requirement.

The park can be constructed in an optional manner, as required, for instance like one of the parks proposed according to the second aspect.

Each system can be constructed in an optional manner, as required, and for instance be operated or be able to be operated by one of the methods for operating a renewable energy system as proposed according to the third aspect.

It can be provided that the transmission ratio of each system transformer is adjusted in dependence on the park reactive power and/or on the park reactive power requirement and/or on the primary side park voltage and/or on the primary side park voltage requirement and/or on the secondary side park voltage and/or on the secondary side park voltage requirement and/or on the operating point of the park.

The adjustment of the transmission ratio of each system transformer can be carried out in an optional manner, as required, for instance by the system transformers being ac- tuated in dependence on the park reactive power requirement and, in particular, in dependence on the corresponding system reactive power requirements in such a manner that the system transformers each have a transmission ratio that allows the systems to jointly provide and/or supply to the consumer power network a reactive power corresponding to the park reactive power requirement.

It can be provided that

the phase shift of each power converter is adjusted in dependence on the park reactive power and/or on the park reactive power requirement and/or on the primary side park voltage and/or on the primary side park voltage requirement and/or on the secondary side park voltage and/or on the secondary side park voltage requirement and/or on the operating point of the park.

The adjustment of the phase shift of each power converter can be carried out in an optional manner, as required, for instance by the power converters being actuated in de- pendence on the park reactive power requirement and, in particular, in dependence on the corresponding system reactive power requirements in such a manner that the power converters each have a phase shift that allows the system to provide and/or supply to the power network a reactive power corresponding to the park reactive power requirement.

Each of the proposed systems and parks can be used to carry out, for instance, one of the proposed methods.

It can be provided that each of the proposed systems and parks is constructed such that it carries out and/or is able to carry out one of the proposed methods and/or that it serves for and/or that it is suited for carrying out and/or being able to carry out one of the proposed methods.

The explanations and exemplifications regarding one of the aspects of the invention, in particular regarding individual features of this aspect, also apply correspondingly for the other aspects of the invention.

In the following, embodiments of the invention are explained in detail by means of the attached drawings. The individual features thereof are, however, not limited to the individ- ual embodiments but can be connected and/or combined with individual features described further above and/or with individual features of other embodiments. Each example in the illustrations is provided by way of explanation, not limitation of the invention. The reference characters included in the claims are by no means intended to limit the scope of protection, but rather merely refer to the embodiments shown in the figures, in which: FIG. 1 shows a first embodiment of a renewable energy system, namely a wind energy turbine with a synchronous generator and a full-scale converter;

FIG. 2 shows a second embodiment of a renewable energy system, namely a wind energy turbine with an asynchronous generator, a partial converter, and a system transformer having one primary winding;

FIG. 3 shows a third embodiment of a renewable energy system, namely a wind energy turbine with an asynchronous generator, a partial converter, and a system trans- former having two primary windings;

FIG. 4 shows a fourth embodiment of a renewable energy system, namely a photovoltaic system with a power inverter.

FIG. 5 shows a preferred embodiment of a renewable energy park;

FIG. 6 shows a U-Q family of characteristics of a wind energy turbine for an average active power.

FIG. 1 schematically presents a first embodiment of a renewable energy system 10, here exemplarily illustrated by a wind energy turbine.

This system 10 comprises a power generating unit 1 1 , which can generate electric al- ternating current; a system transformer 12 with adjustable transmission ratio; a system control apparatus 13; a power converter 14; a system reactive power sensor 15; and an on-load tap changer 16 for adjusting the transmission ratio of the system transformer 12, which on-load tap changer 16 is integrated into the system transformer 12.

In this embodiment, the power generating unit 1 1 comprises a wind-drivable rotor 17 and a generator 18, which is coupled to the rotor 17, and which can generate the electric alternating current. In this instance, the generator 18 is a three-phase synchronous generator coupled directly, that is without transmission, to the rotor 17.

In this embodiment, the power converter 14 is a converter, which is connected to the generator 18 with its input and to the primary side of the system transformer 12 with its output, which primary side is also referred to as lower voltage side or generator side.

In this embodiment, the system transformer 12 has a primary winding 19 for each phase, which primary winding 19 is connected to the output of the power converter 14. The primary side is therefore connected to the power generating unit 1 1 via the power converter 14. The system transformer 12 can be connected to a power network, which is exemplified by a park-internal power network 20 (FIG. 5) of a wind farm 25 (FIG. 5), with the system transformer's 12 secondary side, which is also referred to as higher voltage side or network side.

In this embodiment, the system control apparatus 13 is connected to the on-load tap changer 16, and thus to the system transformer 12, as well as to the power converter 14 and the system reactive power sensor 15.

In this embodiment, the system reactive power sensor 15 comprises sensors, which are not illustrated, for the voltage, the current, and the phase shift of system 10 on the primary side of the system transformer 12, which sensors can produce corresponding measurement signals, from which the system reactive power sensor 15 can determine the system reactive power Q_WEA and produce a corresponding system reactive power signal and transmit it together with the measurement signals to the system control apparatus 13. This voltage is also referred to as primary side system voltage.

In this embodiment, the system control apparatus 13 is constructed such that it can adjust the transmission ratio of the system transformer 12 in dependence on the system reactive power signal and thus on the system reactive power and in dependence on a system reactive power requirement. The system reactive power requirement is stored as a characteristic curve in a memory of the system control apparatus 13. In the instance of the system 10 not being able to meet the system reactive power requirement, the system control apparatus 13 can, for instance, adjust the transmission ratio such that the primary side system voltage falls to a value at which the system 10 can supply sufficient system reac- tive power.

FIG. 2 schematically illustrates a second embodiment of a renewable energy system 10. This second embodiment resembles the first embodiment so that primarily the differences will be explained below.

In this embodiment, the generator 18 is exemplified by a double-fed three-phase asynchronous generator with a stator 21 and a rotor 22, which in this instance is a slip ring rotor. The generator 18 is coupled to the rotor 17 via a transmission 23. The stator 21 is connected to the primary side and the rotor 22 to the power converter 14. The primary side is thus connected to the generator 18 via the power converter 14.

On the one hand, the primary winding 19 is directly connected to the stator 21 , and on the other hand, it is connected to the output of the power converter 14 and thus to the rotor 22 via the power converter 14.

In this embodiment, the system reactive power sensor 15 comprises sensors, which are not illustrated, for the voltage, the current, and the phase shift of system 10 on the secondary side of the system transformer 12, which sensors can produce corresponding measurement signals, from which the system reactive power sensor 15 can determine the system reactive power Q_WEA and produce a corresponding system reactive power signal and transmit it together with the measurement signals to the system control apparatus 13. This voltage is also referred to here as secondary side system voltage.

FIG. 3 schematically illustrates a third embodiment of a renewable energy system 10. This third embodiment resembles the second embodiment so that primarily the differences will be explained below.

In this embodiment, the system transformer 12 has a first primary winding 19' and a second primary winding 19" per phase. The first primary winding 19' is directly connected to the stator 21 . The second primary winding 19" is connected to the output of the power converter 14 and thus to the rotor 22 via the power converter 14.

FIG. 4 schematically presents a fourth embodiment of a renewable energy system 10, here exemplarily illustrated by a photovoltaic system. This fourth embodiment resembles the first embodiment so that primarily the differences will be explained below.

In this embodiment, the power generating unit 1 1 comprises a solar module 24, and the power converter 14 is a power inverter, which is connected to the solar module 24, and which can generate the electric alternating current.

FIG. 5 schematically presents a preferred embodiment of a renewable energy park 25, here exemplarily illustrated by a wind farm.

In this embodiment, the park 25 comprises a park-internal power network 20, two renewable energy systems 10, a park transformer 26 with adjustable transmission ratio, a park control apparatus 27, a park reactive power sensor 28, and an on-load tap changer 29 for adjusting the transmission ratio of the park transformer 26, which on-load tap changer 29 is integrated into the park transformer 26.

The systems 10 are constructed according to the first embodiment and are connected to the park-internal power network 20 with the secondary side of their system transformer 12. The park transformer 26 is connected to the park-internal power network 20 with its primary side, which is also referred to here as lower voltage side or generator side, and it can be connected to a consumer power network 30 with its secondary side, which is also referred to here as higher voltage side or network side.

In this embodiment, the park control apparatus 27 is connected to the on-load tap changer 29, and thus to the park transformer 26, as well as to the system control apparatuses 13 of the systems 10 and to the park reactive power sensor 28.

In this embodiment, the park reactive power sensor 28 is constructed similar to the system reactive power sensor 15 and comprises sensors, which are not illustrated, for the voltage, the current, and the phase shift of the park 25 on the secondary side of the park transformer 26, which sensors can produce corresponding measurement signals, from which the park reactive power sensor 28 can determine the park reactive power Q_WP and produce a corresponding park reactive power signal and transmit it together with the measurement signals to the park reactive power sensor 28. This voltage is also referred to here as secondary side park voltage.

In this embodiment, the park control apparatus 27 is constructed such that it can adjust the transmission ratio of the park transformer 26 in dependence on the park reactive power signal and thus on the park reactive power and in dependence on a park reactive power requirement. The park reactive power requirement is stored as a characteristic curve in a memory of the park control apparatus 27. In the instance of the park 25 not being able to meet the park reactive power requirement, the park control apparatus 27 can, for instance, adjust the transmission ratio of the park transformer 26 such that the secondary side system voltage falls to a value at which the park 25 can supply sufficient system reactive power.

In this embodiment, the park control apparatus 27 is constructed such that it can adjust the transmission ratio of each system transformer 12 in dependence on the park reac- tive power signal and thus on the park reactive power requirement. The park control apparatus 27 adjusts the transmission ratio of each system transformer 12 by actuating the system transformers 12 by means of the assigned system control apparatuses 13 in dependence on the park reactive power requirement in such a manner that the system transformers 12 each have a transmission ratio at which the systems 10 can jointly provide and supply the reactive power requirement to the consumer power network 30.

FIG. 6 exemplarily illustrates a U-Q family of characteristics 31 for a system 10, referred to with index i, of the park 25, in which U-Q family of characteristics 31 the secondary side system voltage UWEAI for an average system active power P_WEAi of this system 10 is plotted over the system reactive power QWEA_I- The assigned system control apparatus 13 can, independently and/or on command by the park control apparatus 27, adjust the transmission ratio of the respective system transformer 12 by corresponding actuation of the on-load tap changer 16 such that the system voltage UWEAI falls and the present operating point 1 16 of this system 10 is shifted to 1 16a. As a greater capacitive system reactive power QwEAi is possible at this new operating point 1 16a, the system control appa- ratus 13 can now actuate the power converter 14 such that it supplies a greater reactive power.

The control of the system transformers 12 by the system control apparatus 13 and the park control apparatus 27 can be carried out in analogy to the control of the park transformer 26 as described in DE 10 2008 048 258 A1 , for instance.

A similar U-Q family of characteristics can also be provided for a park 25 and, for instance, be provided or stored in the park control apparatus 27 of said park 25.

REFERENCE SIGNS

10 System

11 Power generating unit

12 System transformer

13 System control apparatus

14 Power converter

15 System reactive power sensor

16 On-load tap changer of 12

17 Rotor

18 Generator

19 Primary winding

20 Power network, park-internal power network

21 Stator

22 Rotor

23 Transmission

24 Solar module

25 Park

26 Park transformer

27 Park control apparatus

28 Park reactive power sensor

29 On-load tap changer of 26

30 Consumer power network

Claims

1. A renewable energy system (10), comprising

a power generating unit (1 1 ), which can generate electric alternating current;

a system transformer (12) with adjustable transmission ratio, the primary side of which system transformer (12) is connected to the power generating unit (1 1 ) and the secondary side of which can be connected to a power network (20);

a system control apparatus (13), which is connected to the system transformer (12) and is constructed such that it can adjust the transmission ratio of the system transformer (12) in dependence on a system reactive power and/or on a system reactive power requirement and/or on a primary side system voltage and/or on a primary side system voltage requirement and/or on a secondary side system voltage and/or on a secondary side system voltage requirement and/or on an operating point of the power generating unit (1 1 ) and/or on an operating point of the system (10).

2. The system (10) according to claim 1 , namely wind energy turbine, wherein the power generating unit (1 1 ) comprises

a rotor (17);

a generator (18), which is coupled to the rotor (17), and which can generate the electric alternating current.

3. The system (10) according to one of the previous claims, comprising a power converter (14), namely a converter, which is connected to the generator (18) and the primary side.

4. The system (10) according to claim 1 , namely photovoltaic system, wherein the power generating unit (1 1 ) comprises

a solar module (24);

a power converter (14), namely a power inverter, which is connected to the solar module (24), and which can generate the electric alternating current.

5. The system (10) according to one of the previous claims, comprising

a system reactive power sensor (15), which can detect the system reactive power and can produce a corresponding system reactive power signal;

wherein the system control apparatus (13) is constructed such that it can adjust the transmission ratio of the system transformer (12) in dependence on the system reactive power signal.

6. The system (10) according to the previous claim, comprising

a system voltage sensor, which can detect the primary side system voltage and can produce a corresponding system voltage signal; and/or

a system voltage sensor, which can detect the secondary side system voltage and can produce a corresponding system voltage signal;

wherein the system control apparatus (13) is constructed such that it can adjust the transmission ratio of the system transformer (12) in dependence on at least one of these system voltage signals.

7. The system (10) according to one of the previous claims wherein the system control apparatus (13) can be connected to a remote control system (27) producing remote control signals and it can be constructed such that it can adjust the transmission ratio of the system transformer (12) in dependence on the remote control signals.

8. The system (10) according to one of the previous claims, comprising an on-load tap changer (16) for adjusting the transmission ratio of the system transformer (12). 9. The system (10) according to one of the previous claims wherein

the power converter (14) is a power converter with adjustable phase shift between the current supplied by the power converter (14) and the voltage supplied by the power converter (14);

the system control apparatus (13) is connected to the power converter (14) and is constructed such that it can adjust the phase shift of the power converter (14) in dependence on the system reactive power and/or on the system reactive power requirement and/or on the primary side system voltage and/or on the primary side system voltage requirement and/or on the secondary side system voltage and/or on the secondary side system voltage requirement and/or on the operating point of the pow- er generating unit (1 1 ) and/or on the operating point of the system (10).

10. A renewable energy park (25), comprising

a park-internal power network (20);

at least two renewable energy systems (10), each of which is constructed according to one of the previous claims and is connected with the secondary side of its system transformer (12) to the park-internal power network (20);

a park transformer (26) with adjustable transmission ratio, the primary side of which park transformer (26) is connected to the park-internal power network (20) and the secondary side of which can be connected to a consumer power network (30); a park control apparatus (27), which is connected to the park transformer (26) and is constructed such that it can adjust the transmission ratio of the park transformer (26) in dependence on a park reactive power and/or on a park reactive power requirement and/or on a primary side park voltage and/or on a primary side park voltage requirement and/or on a secondary side park voltage and/or on a secondary side park voltage requirement and/or on an operating point of the park (25).

1 1 . The park (25) according to the previous claim wherein the park control apparatus (27) is connected to the system control apparatuses (13) and is constructed such that it can adjust the transmission ratio of each system transformer (12) in dependence on the park reactive power and/or on the park reactive power requirement and/or on the primary side park voltage and/or on the primary side park voltage requirement and/or on the secondary side park voltage and/or on the secondary side park voltage requirement and/or on the operating point of the park (25).

12. The park (25) according to one of the previous claims wherein the park control apparatus (27) can be connected to a remote control system producing remote control signals and it can be constructed such that it can adjust the transmission ratio of the park transformer (26) in dependence on the remote control signals.

13. The park (25) according to one of the previous claims wherein

each power converter (14) is a power converter with adjustable phase shift between the current supplied by the power converter (14) and the voltage supplied by the power converter (14);

the park control apparatus (27) is connected to the power converters (14) and is constructed such that it can adjust the phase shift of each power converter (14) in dependence on the park reactive power and/or on the park reactive power requirement and/or on the primary side park voltage and/or on the primary side park voltage requirement and/or on the secondary side park voltage and/or on the secondary side park voltage requirement and/or on the operating point of the park (25).

14. A method for operating a renewable energy system (10) wherein

the system (10) comprises

• a power generating unit (1 1 ), which can generate electric alternating current;

• a system transformer (12) with adjustable transmission ratio, the primary side of which system transformer (12) is connected to the power generating unit (1 1 ) and the secondary side of which can be connected to a power network (20); the transmission ratio of the system transformer (12) is adjusted in dependence on a system reactive power and/or on a system reactive power requirement and/or on a primary side system voltage and/or on a primary side system voltage requirement and/or on a secondary side system voltage and/or on a secondary side system voltage requirement and/or on an operating point of the power generating unit (1 1 ) and/or on an operating point of the system (10).

5. A method for operating a renewable energy park (25) wherein

the park (25) comprises

• a park-internal power network (20);

• at least two renewable energy systems (10), each of which is constructed according to one of the previous claims and is connected with the secondary side of its system transformer (12) to the park-internal power network (20), and each of which is, in particular, operated by a method for operating a renewable energy system (10) according to one of the previous claims;

• a park transformer (26) with adjustable transmission ratio, the primary side of which park transformer (26) is connected to the park-internal power network (20) and the secondary side of which can be connected to a consumer power network (30);

the transmission ratio of the park transformer (26) is adjusted in dependence on a park reactive power and/or on a park reactive power requirement and/or on a primary side park voltage and/or on a primary side park voltage requirement and/or on a secondary side park voltage and/or on a secondary side park voltage requirement and/or on an operating point of the park (25).