Patent Application
20230246451
The present invention relates to a generating unit, GU, with integrated power electronics to comply with feed-in requirements of public power grids to improve grid stability, as well as to a generating system, GS, with the GU.
If a machine converts non-electrical energy into electrical energy for it to be fed into the public power grid, certain requirements then apply for maintaining grid stability, such as the possibility of remote-controlled active power reduction, active power adjustment in the event of frequency deviations, the provision of reactive power for static voltage stability and faults ride-through, i.e. running through a strong voltage change with the highly dynamic grid support using reactive current. Depending on the national regulations, these requirements are defined in guidelines, rules of application, decrees or the like.
These requirements for a GU are typically implemented entirely or in part by using an externally excited synchronous generator or by using a frequency converter system between the generator and the power grid as implemented in
The current generated by the generator is first rectified using a rectifier/generator converter (131) and routed via the intermediate circuit capacitor (132), and then converted back by way of an inverter/grid converter (133) using modulation (e.g. pulse width modulation, PWM) into an approximately sinusoidal alternating current having a frequency equal to the grid voltage. Depending on the required reactive power, the current is there fed into the grid sooner or later in relation to the grid voltage. Theoretically, converter systems without an intermediate circuit or mixed forms are also conceivable.
The active power fed into the power grid (170) can also be controlled by energizing a brake resistance (140), the chopper: the brake resistance (140) is switched via an electronic switch (134), the so-called chopper, to be clocking parallel to the Intermediate circuit capacitor (132) so that excess energy in the intermediate circuit capacitor (132) is converted into thermal energy. As a rule, the chopper is activated automatically when a specific intermediate circuit voltage is exceeded, which is the result of the power limitation of the grid converter (133) and as a result of which more current is introduced into the intermediate circuit than can also be drawn.
As an alternative or in addition, the ultimately required power fed into the grid can be set by controlling the process input power (111) of a primary energy source which is converted using a primary energy converter (112) into mechanical power for driving the generator (120).
This configuration has the drawback that the entire active power provided by the generator (120) has to be routed through the lossy power electronics (130). Furthermore, the serial connection of the power electronics (130) requires two converters, a rectifier (131) and an inverter (133).
The invention is based on the object of providing a GU which overcomes the drawbacks mentioned.
This object is satisfied by the object of claim 1.
Advantageous embodiments are the object of the dependent claims.
A summary is provided below to present a representative selection of concepts in a simplified manner that shall be further discussed in the subsequent detailed description.
According to preferred configurations, a GU can be composed of an asynchronous generator with a direct connecting branch to a power grid, a measuring device for determining the profiles of a GU grid voltage and a GU grid current at the power grid, as well as power electronics.
The power electronics can there be connected in parallel with the asynchronous generator and the power grid in order to feed a current into the power grid that is shifted with respect to the grid voltage. The power electronics can preferably be connected at a node within the GU in parallel with the asynchronous generator and the power grid. The voltage present at the node can correspond to a low voltage in the range of approximately 0.4 kV.
The power electronics can comprise an inverter with an inverter output for direct connection to the generator power grid branch and an intermediate circuit energy storage device having an intermediate circuit voltage that is connected in parallel with the inverter on the DC side. The power electronics can also comprise a control unit. The intermediate circuit energy storage device there preferably corresponds to an intermediate circuit capacitor and the inverter to a two-level, three-level or multi-level inverter. The inverter can also comprise an output filter for smoothing the pulsing inverter output current, which can be connected in series between the output terminals of the inverter and the node.
Based on a GU grid current measured and a grid voltage measured, the control unit can determine a phase shift angle and the value of the GU grid current and, based on this, actuate or control the inverter such that the value and the phase shift angle of the output current of the power electronics assume values which, together with the current of the asynchronous generator, result in a target value and a target phase shift angle of the GU grid current. The inverter is preferably there actuated with a pulse width modulated signal. The actuation of the inverter can preferably have a tolerance band control or a vector control in a d/q coordinate system revolving at the grid frequency. In addition, the control unit can determine a value of the active grid current based on the grid current measured and the grid voltage measured.
The inverter can also be actuated by the control unit based on the intermediate circuit voltage of the intermediate circuit energy storage device such that the intermediate circuit energy is converted by pulse width modulation (PWM) into an approximately sinusoidal, three-phase alternating current with a frequency approximately the same as that of the grid, a variable amplitude, and a variable phase shift between the grid voltage and the inverter output current.
Furthermore, the GU can comprise a chopper with a serial brake resistance for controlling the active power fed into the grid. The series composed of the chopper and the brake resistance can be connected in parallel with the DC side of the inverter. The chopper can be actuated by a voltage comparison with a hysteresis such that the energy stored in the intermediate circuit energy storage device can be converted into thermal energy by switching on the brake resistance. The actuation of the chopper can also comprise tolerance band control. If the inverter is actuated such that a current flows from the node in the direction of the intermediate circuit and its voltage rises as a result, then the chopper can dissipate the excess energy to the brake resistance and enable the active power fed into the grid to be controlled.
In addition, the GU can comprise an Organic Rankine Cycle, ORC, unit/system with an expansion machine which can drive the asynchronous generator. The asynchronous generator can be integrated into the expansion machine, where the asynchronous generator preferably corresponds to an AC asynchronous machine with a short-circuit/squirrel cage rotor.
Furthermore, the GU mentioned can be connected in parallel with a plurality of other GUs on the low-voltage side of a transformer within an GS. The high-voltage side of the transformer can preferably be connected to a grid connection point of a medium-voltage grid.
Preferred embodiments of the invention shall be explained in more detail below with reference to the drawings, where:
Unless otherwise stated, complex numbers are used hereafter in the notation
X
=Re{X}+jIm{X}=Xr+jXq=Xejφ=X[sin(φ)+j cos(φ)].
Furthermore, unless otherwise stated, the term ‘phase shift angle φ’ is used hereafter for the phase shift between the voltage and the corresponding current. Furthermore, unless otherwise stated, the term ‘phase shift angle φ’ is used hereafter for the phase shift between the voltage and the corresponding current. Furthermore, unless otherwise stated, the term ‘phase shift angle φ’ is used hereafter for the phase shift between the voltage and the corresponding current.
Furthermore, unless otherwise stated, the term ‘value’ is used for the effective value of an AC magnitude.
The generator (220) can there be connected directly to the power grid (270) without the power electronics (130) known from prior art connected in series between the generator and the power grid, and can feed the active electrical power generated by the generator (220) into the power grid (270) directly and with almost no losses.
As shown in
According to one aspect, the GU (200) can comprise a measuring device (260) with a voltage measuring device (261) for measuring the grid voltage profile and a current measuring device (262) for measuring the grid current profile. The voltage (261) and current measuring devices (262) can there preferably each comprise three measuring sensors for phase-by-phase measurement of the voltage or current strength, respectively, in the power grid. As an alternative, the measurement can be conducted with two sensors (Aaron circuit). In addition, the measuring device (260) can comprise a power meter for direct power measurement.
Furthermore, the GU (200) can comprise an Organic Rankine Cycle, ORC, unit/system (210) shown schematically in
According to one aspect, the generator (220) can comprise an asynchronous generator which preferably corresponds to a AC asynchronous machine with a short-circuit or squirrel-cage rotor. In one aspect, the asynchronous generator can be integrated into the expansion machine (212).
As shown in
As shown in
The inverter can preferably comprise three half-bridges, each with two switching elements which can switch the intermediate circuit voltage Udc present at the intermediate circuit energy storage device (232) with positive or negative polarity to the respective bridge branch or respective inverter phase output (238-1, 238-2, 238-3). Switching elements T1 . . . T6 of the inverter (233) can be actuated with a PWM actuation signal generated by the control unit (236), so that three approximately sinusoidal AC voltages Ux1 . . . Ux3 offset by 120° can be generated. An inverter (233) of this type can be configured, for example, as a two-level inverter, three-level inverter, or multi-level inverter. The switching elements used in the half-bridge preferably comprise IGBTs with anti-parallel freewheeling diodes, where the freewheeling diodes can enable energy to flow from the power grid (270) into the inverter (233).
The control unit (236) preferably comprises an inlet for being able to receive the grid voltages and grid currents measured by the measuring device (260). The measurement results determined by the measuring device (260) can there be transmitted to the control unit (236) preferably via a hardwired electrical signal line or alternatively via a network interface or a wireless network interface, such as wireless LAN or Bluetooth.
According to a further aspect, the chopper (234), as shown in
According to one aspect, the power electronics (230) can comprise a serial output filter or output chokes (237) for smoothing the pulsing inverter output current. The output filter (237) can there be integrated into the inverter (233) or connected in series with the inverter output. The output filter (237) preferably comprises at least three output chokes which can be connected in series with the respective inverter phase output (238-1, 238-2, 238-3). In addition or as an alternative, the output filter (237) can comprise a three-phase transformer for grid connection. Unless otherwise specified, only the inductive part of phase impedances Z1 . . . Z3 of the output filter is examined for the sake of simplicity. Those skilled in the art understand that phase impedances Z1 . . . Z3 can also be composed of a transformer impedance and/or grid connection impedance.
The schematic structure of the GU (200), the GS (300), and the power electronics (233) has presently been described. The operating principle and the setting options for phase shift angle go with the power electronics (233) according to the embodiment of
Based on the intermediate circuit voltage UDC defined by the topology of the inverter (233) and the grid voltage, the inverter (233) can generate an approximately sinusoidal and symmetrical AC system at the output of the filter (237). Depending on the type of inverter (line-commutated or self-commutated), an external grid may be necessary for this. This AC system can be connected in parallel with the grid (270) and the generator (220).
The generated mth inverter phase output current ixm(t) with 1≤m≤3 can have a time profile according to
i
xm(t)=√{square root over (2)}ixm sin(ωt−φ)
where ω can correspond to the angular frequency of the voltage of the grid (270), Ixm to the magnitude or effective value of the mth inverter phase output current and go to the phase shift angle, i.e. the phase shift between ixm and the mth inverter phase output voltage uxm.
Based on the magnitude and the phase shift angle go of the respective inverter phase output current generated, reactive power can be exchanged between the power electronics (230) and the node (250), similar to a conventional phase shifter. For this purpose, the inverter phase output current is fed in or drawn offset to the grid voltage.
As shown in the corresponding vector diagram in
Active Power Control Through Active Power Drawn by the Power Electronics
In addition or as an alternative, active power can be transferred from the node (250) to the intermediate circuit energy storage device by appropriate PWM of the power electronics (230). As a result, the intermediate circuit voltage can rise and the automatically triggered chopper (234) can dissipate the excess energy to the brake resistance (240). In this way, the active power of the GU fed in can thus be controlled.
According to one aspect, as shown in Table 1, different phase shift angles can lead to different quadrants of performance. The resulting current from the power electronics and the effect on the grid feed-in are considered there. The initial situation of an ORC process in operation is examined there, in which active power is fed into the grid and inductive reactive power is drawn. The signage corresponds to the consumer sign convention.
Actuating the inverter (233) and the chopper (234) with the control unit (236) for reactive and active power control with the power electronics (230) shall be discussed hereafter according to the embodiment of
According to one aspect, the value of the current in the power electronics Ix and the phase shift angle φxn between the current of the power electronics and the grid voltage can be set such that adding the generator current Ig results in a grid current In which at an agreed point exceeds the required target active power and can provide the required target reactive power. Accordingly, the power electronics currents Ix impressed or exchanged at the node (250) can exhibit a value Ix and a phase shift angle φxn in relation to the grid voltage according to
I
x∠φxn=Ix=In−Ig=In,r+jIn,g−(Ig,r+jIg,q).
As can be seen by a person skilled in the art, the magnitude of reactive current in the power electronics can decrease when the reactive current of the generator Ig,q and the required grid reactive current In,q cancel each other out in part or entirely. This could be the case when the generator draws inductive reactive power (in consumer sign convention, VZS, Qg>0) and grid-related inductive reactive power is required by the GU. When capacitive reactive power is drawn on the grid side, the values of reactive current of the generator (220) and of the power grid (270) can add up, which means that the value of reactive current of the power electronics (230) can increase.
According to one aspect, the regulation of the active power can be set remotely or in dependence of the frequency. The active power can be converted by adjusting the process entry power (the heat output (211) supplied to the ORC system (210)). In addition or as an alternative, the excess energy from the node (250) can be converted into thermal energy via the intermediate circuit energy storage device (232), the chopper (234) and the brake resistance (240) and thus dissipated from the GU (200).
According to one aspect, the provision of reactive power for static voltage stability and/or for dynamic grid support, fault ride-through, FRT, can be controlled by shifting the output current of the power electronics. In this case, controlling the static voltage stability and/or dynamic grid support can be optimized, where the control error in the static voltage stability is corrected with a greater control settling time than in the case of dynamic grid support.
According to one aspect, methods known to those skilled in the art such as tolerance band control or phased feedback of the grid current with the target value of the grid current via a P controller can be used to control the power electronics output current and/or the grid current. According to one aspect, the control can comprise vector control in a coordinate system (d/q coordinates) revolving at grid frequency.
Omitting the rectifier (131) in the embodiment according to
Another advantage can be the reduced winding stress for the generator (220): While the generators in the serial connection of the power electronics (130) need to be configured for chopped DC voltages of 600 . . . 800 V with high slew rates, the generator (220) can be configured according to
A further advantage can be that the GUs (200, 310 . . . 313) are built modularly and uniformly in the sense of a kit and, depending on the required feed-in requirements, can be optionally equipped or retrofitted with or without power electronics (230) for a certified feed-in.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20185775.2 | Jul 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/069422 | 7/13/2021 | WO |
