Patent Application
20160146192
This application refers to a wind turbine frequency converter with a multilevel topology which allows black start.
Present day trend is to increase the wind turbine capacity and have larger turbines that have higher power capacities. However, irrespective of the wind turbine capacity, all these turbines generate power at low voltage, e.g. 575V/690V. This effectively means that large magnitudes of current, e.g. kilo Amps, have to be transferred, which leads to prohibitively large power losses. The current solution used to address this problem is to use AC transformers to step-up the voltage to medium voltage, e.g. 10s of kV, and thereby reduce the current magnitude.
Most of the current wind turbines use a power conversion system to convert the variable frequency output from the generator of the wind turbine to almost constant grid voltage and frequency. Additionally, most of the power conversion systems used in the wind turbines is designed to follow the grid voltage and frequency signal.
Applicant's wind converters today use a pad mount transformer to connect to the MV grid.
US 2013/0221678 A1 discloses a multilevel converter with at least one phase each including several cascaded base units each including a first switching element, a second switching element, and a separate and electrically isolated DC source. Cascaded elements are used to achieve voltage scaling. The cascaded H-bridge topology however requires isolated DC source and supply.
US 2013/0249322 A1 discloses a multilevel converter which includes at least one phase, whereby each phase of the multilevel converter includes a direct current link, a first circuit, a second circuit, and a phase capacitor. The DC link is electrically coupled to the DC-DC converter of the wind power generation system.
US 2012/0163044 A1 discloses a multilevel voltage arrangement having a transformer and a power converter or inverter comprising one or more common DC links connectable to a power source, said arrangement comprising one first set of H-bridges being connected to said common DC link. This topology comprises an open winding generator to achieve medium voltage. This topology also requires isolated DC source.
U.S. Pat. No. 7,638,983 B2 discloses a controller of a grid coupled type doubly-fed induction generator having a multi-level converter topology, which can control the doubly-fed induction generator having a high voltage specification and can perform a fault ride-through function, an anti-islanding function and a grid voltage synchronization function required for a dispersed power generation facility. A H-bridge multilevel for a DFIG wind turbine type is proposed. A boost converter composed of a 3-leg IGBT and a boost inductor is used to generate a direct current voltage source required for the H-bridge multilevel converter.
U.S. Pat. No. 5,642,275 discloses a multilevel cascade voltage source inverter having separate DC sources, whereby this inverter is applicable to high voltage, high power applications such as flexible AC transmission systems including static VAR generation, power line conditioning, series compensation, phase shifting and voltage balancing and fuel cell and photovoltaic utility interface systems. A cascaded H-bridge topology is proposed for full converter.
Wind turbines usually generate power at low voltage at large magnitudes of current to be transferred, which leads to prohibitively large power losses. Conventionally to address this problem AC (alternating current) transformers to step-up the low voltage to medium voltage, e.g. at around 46 KV, are used to reduce the current magnitude. Conventional power conversion systems use semiconductor devices, e.g. IGBTs (insulated gate bipolar transistors), which can be operated merely at low voltage but not at medium voltage.
Additionally, most of the power conversion systems used in the wind turbines is designed to follow the grid voltage and frequency signal.
It is an object of the invention to provide a power conversion system for converting a variable frequency output of a wind turbine generator, to a constant grid voltage and frequency. For example generated power at low voltage, e.g. at around 575V to 690V, and large magnitudes of current, e.g. KAs, should be converted to a constant grid voltage and frequency. Large power losses resulting from large magnitudes of current must be avoided. Moreover a black start of the wind power station should be possible. Conventional wind converters which use a pad mount transformer to connect to the medium voltage grid should be replaced.
The object of the present invention is solved by a multilevel topology power converter according to the features of the main claim and a method for controlling a multilevel topology power converter according to the auxiliary claim.
According to a first aspect a multilevel topology power converter for electrical adaption of a low voltage alternating current of an electrical wind power generator and a medium voltage transmission level for a transmission link to each other is proposed, whereby the power converter comprises for each phase a switching unit for adapting the low voltage and the medium voltage to each other, the switching unit being controlled by a controller.
According to a second aspect of the present invention a method for controlling a multilevel topology power converter is suggested, whereby the controller of the converter controls in a dual mode operating by load angle control in a stand-alone mode and by current control in a grid-connected mode of the electrical wind power generator.
A generic frame work for multilevel types of converter topology is proposed, which not only eliminates the bulky transformer but also provides smoother AC voltage wave form and less susceptible to dv/dt stresses. The proposed topology in addition to the capability of operating in stand-alone mode is unique. Also, we suggest how our method could be used to develop a MVDC grid solution for medium capacity wind farms, for which today's HVDC solutions are prohibitively expensive and AC solutions may not be feasible, e.g. at offshore wind farms.
Advantageously this topology can potentially reduce the weight and the footprint of wind electrical conversion systems by reducing filter size and by eliminating conventional AC transformer. Also the prices of semiconductor devices are decreasing, so replacing copper/iron, especially within transformers, by solid-state devices could potentially lead to lower cost. Also, in future, this approach can benefit substantially from the enormous improvements in semiconductor device technology like, increased liability, smaller footprint, higher efficiency, increased power rating and so on. Also, the proposed converter control allows the wind turbines to black start and even for an AC grid when needed. The converter can operate in both grid connected and stand-alone mode. These features are not available in conventional wind turbine systems.
The inventive power conversion system uses semiconductor devices, e.g. IGBTs, that can be operated at low voltages and also medium voltages.
It was discovered that a multilevel topology power converter for electrical adaption of a low voltage alternating current (LVAC) of a near electrical wind power generator can be used to provide a medium voltage (MV) transmission level close at the generator, whereby a longer transmission link from the converter towards a HVAC station or a HVDC station can be provided and power losses resulting from too high current can be avoided. This adaption is performed without a transformer but with a controlled switching unit. Basically the inventive converter can operate into two directions, which is for generating electrical power from the generator or which is for synchronizing the generator to the grid.
A black start in particular is the process of restoring a power station to operation without relying on the external electric power transmission network. According to this application the wording “without external electric power transmission network” means being in the mode of stand-alone and not in the mode of grid-connected.
A pad mount transformer in particular is a ground mounted electric power distribution transformer in a locked steel cabinet mounted on a concrete pad. According to this application this transformer should become obsolete.
AC is alternating current. DC is direct current. LV means low voltage. MV means medium voltage. HV means high voltage. The ranges for the voltage levels are well known in the state of the art.
Load angle is the angle with which a magnet wheel of a synchronous machine runs at a higher or lower speed than a synchronous rotating field.
Further embodiments are discussed by the following subclaims.
According to an advantageous embodiment the multilevel topology power converter can comprise for each phase a current converter for providing a direct current; a plurality of n>1 capacitors C1 . . . Cn electrically connected in series; the control switching unit for separately switching n+1 terminal potentials along terminals of the series of the n>1 capacitors C1 . . . Cn. “n” is an element of the natural numbers.
According to a further embodiment the multilevel topology power converter can comprise for each phase the current converter for providing by two terminals a low voltage direct current from the low voltage alternating current of the electrical power generator; the plurality of n>1 capacitors C1 . . . Cn electrically connected in series between the two terminals of the low voltage direct current; the control switching unit for separately switching n+1 terminal potentials along terminals of the series of the n>1 capacitors C1 . . . Cn to provide a medium voltage alternating current for the transmission link.
According to a further advantageous embodiment the multilevel topology power converter can comprise for each phase the electrical power generator generating the low voltage alternating current; the control switching unit for separately switching n+1 terminal potentials from the electrical generator along terminals of a series of n>1 capacitors C1 . . . Cn electrically connected in series between two main terminals to provide a first medium voltage direct current by the two main terminals; the current converter for adapting the first medium voltage direct current into a second medium voltage direct current for the transmission link.
According to a further advantageous embodiment the multilevel topology power converter with the controlled switching unit comprises semiconductor devices, in particular present day IGBT-devices, that can enable connection to MVAC voltages of 10 kV or higher. IGBT is Insulated Gate Bipolar Transistor.
According to a further advantageous embodiment a plurality of electrical power generator assembly levels can be electrical connected in parallel to each other and their current converters can adapt their second medium voltage direct currents to each other.
According to a further advantageous embodiment the plurality of the electrical power generator assembly levels can be electrically connected in parallel to a common current converter which increases the voltage of the second medium voltage direct current to a higher voltage of the second medium voltage direct current for the transmission link.
According to a further advantageous embodiment a DC/AC converter changes the higher voltage of the second medium voltage direct current into a high voltage alternating current.
According to a further advantageous embodiment an AC/AC converter changes the medium voltage alternating current into a high voltage alternating current.
According to a further advantageous embodiment the controller of the converter can control in a dual mode operating by load angle control in a stand-alone mode and by current control in a grid-connected mode of the electrical wind power generator.
According to a further advantageous embodiment the controller of the converter can switch from the standard current control of the grid-connected mode to the load angle control of the stand-alone mode of the electrical wind power generator and vice versa.
According to a further advantageous embodiment the controller of the converter can generate an own reference signal while operating in the stand-alone mode.
According to a further advantageous embodiment in the stand-alone mode the controller of the converter can control the active power output of the converter by the load angle control, if the converter regulates to a constant output voltage of the converter.
According to a further advantageous embodiment in the stand-alone mode the controller of the converter can regulate to the constant output voltage thus the active power output of the converter is proportional to the load angle.
According to a further advantageous embodiment for the stand-alone mode within the converter the capacities of the capacitors C1 . . . Cn can be increased to provide a stiffer series of the capacitors C1 . . . Cn.
According to a further advantageous embodiment for the stand-alone mode the converter can comprise a weak voltage regulation of the voltages at the capacitors C1 . . . Cn.
The invention will be closer discussed by embodiments in relation to the drawings. It shows:
According to this embodiment the converter I comprises for each phase an electrical power generator I generating the low voltage alternating current LVAC; a control switching unit 9 for separately switching n+1 terminal potentials from the electrical generator 1 along terminals of a series of n>1 capacitors C1 . . . Cn 7 electrically connected in series between two main terminals that provide a first medium voltage direct current MVDC by the two main terminals; a current converter 5 for adapting the first medium voltage direct current MVDC into a second medium voltage direct current MVDC for the transmission link. The first and the second medium voltage direct current can be on the level of 10 kV. The control switching unit 9 comprises semiconductor devices, in particular IGBTs which can be used for up to AC line voltages of 10 kV.
If the number n of levels or switches is reduced a switching at higher frequency without substantially compromising any of the benefits can be performed. The advantages which remain can be lower harmonics and lower electromagnetic impact EMI. This would require an adapted device technology like the wide band gap devices (for example on silicon-carbide basis). However, the wideband gap device technology has to be found to withstand high voltages so generate medium voltage on the AC side, which especially can be more than 15 kV.
For the stand-alone mode it is proposed to use angle Θ control that is based on the following logic. Basing on formular 1:
P=V·V
PCC·Sin Θ/Zth (1)
whereby
P is the active power of the converter I output.
V is the generator voltage.
VPCC is the output voltage of the converter I.
angle Θ is the load angle of the generator 1. Zth is the Thevinin impedance, whereby resistances are neglected. Thevinin impedance is the equivalent impedance obtained at terminals A-B of any linear electric network with all its independent current sources open circuited and all its independent voltage sources short circuited.
If VPCC is held constant the active power output P is proportional to the load angle Θ.
If the converter regulates voltage instead of reactive power then, the active power output p of the wind converter can be controlled by controlling the load angle Θ instead of current.
However, in practice there is a need for stiffer DC link, which could be provided by the multilevel topology. The DC link is the arrangement of the capacities C1 . . . Cn connected in series. This need is because in any operating mode, the power must be balanced, which means mechanical power output of the turbine should be equal to the electrical power output if losses are neglected. Since angle Θ control is slow, the input power to the converter I which is the output power from the electrical generator I is not equal to the converter I output power. This delay in power transfer causes the DC current through the DC link to be non-zero and this can be overcome either by performing a weak voltage regulation at the DC link or by making the DC link stiff. It is suggested that the multilevel topology with a stiff DC link is a better choice as it provides better stability.
The invention refers to a multilevel topology power converter I for electrical adaption of a low voltage alternating current LVAC of an electrical wind power generator 1 and a medium voltage MV transmission level for a transmission link to each other, the power converter I comprising for each phase:
a switching unit 9 for adapting the low voltage LV and the medium voltage MV to each other, the switching unit 9 being controlled by a controller 15. For each phase a current converter 5 provides a direct current DC towards or from a plurality of n>1 capacitors C1 . . . Cn 7 electrically connected in series, whereby the controlled switching unit 9 switches n+1 terminal potentials along terminals of the series of the n>1 capacitors C1 . . . Cn 7 which can be called “DC link”.
The invention refers to a multilevel topology power converter I for electrical adaption of a low voltage alternating current LVAC of an electrical wind power generator 1 to a medium voltage MV transmission level for a transmission link, the power converter I comprising for each phase:
