Method and apparatus for computer-implemented controlling of a doubly-fed electric machine

FIELD OF INVENTION

The invention relates to a method and an apparatus for com puter-implemented controlling of a doubly-fed electric ma- chine, where stator windings of a stator are directly con nected to an electrical grid and where rotor windings of a rotor are connected to the electrical grid via a power con version system, comprising an AC-to-DC converter and a DC-to- AC converter and being adapted to control a rotor current.

BACKGROUND

Generally, doubly-fed electric machines also called doubly- fed induction generator (DFIG) are electric motors or elec- trie generators, where both the field windings and armature windings are separately connected to equipment outside the machine. By feeding adjustable frequency AC power to the field windings, the magnetic field can be made to rotate, al lowing variation in motor or generator speed. This is useful, for instance, for generators used in wind turbines. DFIG- based wind turbines are advantageous in wind turbine technol ogy because of their flexibility and ability to control ac tive and reactive power. The principle of the DFIG is that stator windings are con nected to the grid and rotor windings are connected to the converter via slip rings and back-to-back voltage source con verter that controls both rotor and the grid currents. By us ing the converter to control the rotor currents, it is possi- ble to adjust the active and reactive power fed to the grid from the stator independently of the generator's rotational speed. As the rotor circuit is controlled by a power convert er, the induction generator is able to both import and export reactive power. This allows the DFIG to support the grid dur ing severe voltage disturbances. Further, the control of the rotor voltages and currents enables the induction machine to remain synchronized with the grid while the wind turbine speed varies.

When used with a wind power generation system, an operation at the maximum power point (MPP) is targeted. The MPP depends on the aerodynamic part of the turbine. Generally, rated pow er can be kept from a mechanical point of view when operating the wind power generation system at synchronous speed. Howev er, when operating the DFIG around synchronous speed, there is a thermal limitation which requires operation of the wind power generation system with lower than rated power. At this condition, the rotor frequency is close to 0 Hz (DC) so that a load sharing between inverter switches of the power con verter is drastically reduced because of high asymmetrical rotor currents. As a result, the rotor current asymmetry pro duces an overload of one of the branches of the power conver sion system. This overload leads to an overheating of one of the inverter switching, damage and/or life span reduction when the current level and/or time limit is exceeded. On the other hand, rotor current asymmetry is required to create ro tor magnetic poles on an airgap of the electrical machine.

Up to now, when a DFIG operates around its synchronous speed, two different control strategies may be applied. According to a first control strategy, an aerodynamic load is controlled such to avoid and/or limit the synchronous speed operation. According to a control second strategy, a drastic reduction of active/reactive power is applied to minimize the rotor current.

US 10742 149 B1 discloses a method for computer-implemented controlling a DFIG, whose rotor windings are connected to an electrical grid via a power conversion system. The power con version system comprises an AC-to-DC converter and a DC-to-AC converter and is adapted to control a rotor current. For con- trolling the DFIG, a rotational speed of the machine is ob tained, and it is determined, whether it is within a prede termined operational speed range around the synchronous speed. If the obtained rotational speed is within the prede termined operational speed range, the AC-to-DC converter of the power conversion system is controlled to force injection of a stator reactive power to create a harmonic at a frequen cy different than a rated frequency of the machine. The DC- to-AC converter is controlled to compensate the created sta tor reactive power and the harmonic.

US 8853 876 B1 discloses a method for operating a power gen eration system that supplies power for application to a load. The method includes receiving, at a power converter, an al ternating current power generated by a generator operating at a speed that is substantially equal to its synchronous speed and converting, with the power converter, the alternating current power to an output power, wherein the power converter includes at least one switching element. In addition, the method includes receiving a control command to control a switching frequency of the at least one switching element and adjusting the switching frequency to an adjusted switching frequency that is substantially equal to a fundamental fre quency of the load.

It is an object of the present invention to provide a method and an apparatus for computer-implemented controlling a dou bly-fed electric machine which enlarge the limits of time and loads supported by a power conversion system when operating at synchronous speed.

SUMMARY

This object is solved by a method according to the features of claim 1 and an apparatus according to the features of claim 8. Preferred embodiments are set out in the dependent claims. The invention provides a method for computer-implemented con trolling a doubly-fed electric machine, where stator windings of a stator are directly connected to an electrical grid and where rotor windings of a rotor are connected to the electric grid via a power conversion system. The power conversion sys tem comprises an AC-to-DC converter and a DC-to-AC converter. The power conversion system is adapted to control a rotor current.

The method comprises the following steps during the operation of the doubly-fed electric machine, also referred to as a doubly-fed induction generator (DFIG).

In step a), a rotational speed of the machine is obtained.

The term "obtaining a rotational speed" means that the rota tional speed of the rotor of the electric machine is received by a processor implementing the method of the invention. The rotational speed refers to a digital representation of the rotational speed of the machine. The rotational speed may be determined by a respective sensor of the machine.

In step b), it is determined whether the obtained rotational speed is within a predetermined operational speed range around the synchronous speed. The synchronous speed is the rotational speed of the machine where the rated power can be kept from a mechanical point of view. It is a function of control parameters of the doubly-fed electric machine and ex ternal environmental parameters, such as wind speed, wind di rection and so on in case the doubly-fed electric machine is used in a wind power generator system.

If it is determined that the obtained rotational speed is within the predetermined operational speed range, the follow ing steps are performed.

In step c), the AC-to-DC converter of the power conversion system is controlled to force injection of a stator reactive power to create a harmonic at a frequency different than a rated frequency of the machine.

In step d), the DC-to-AC converter of the power conversion system is controlled to compensate the created stator reac tive power and the thus created harmonic.

According to the invention, step c) comprises controlling the AC-to-DC converter such that the forced injection of the sta tor reactive power is maximized at the synchronous speed and decreasing to the borders of the predetermined operational speed range.

The present invention provides an easy and straightforward method for controlling a doubly-fed electric machine when op erating around rotational speed. To do so, the power conver sion system of the doubly-fed electric machine is controlled such that injection of stator reactive power at a frequency different than rated is forced. The stator reactive power must be enough to briefly change the current direction, thus reducing the duty cycle asymmetry. This portion of reactive power at a different frequency than rated is compensated by the grid-side converter (DC-to-AC converter) before entering the grid. Thus, no perturbance is seen by the external elec trical grid.

The method as described above enables an aerodynamic perfor mance optimization as consequence of relaxed electrical limi tations at synchronous speed. Furthermore, a reduction of thermal damage of generator-side converter power electronic, i.e., the AC-to-DC converter is achieved. The efficiency of the doubly-fed electric machine is increased as to avoidance of speed exclusion zones around the synchronous speed. Fur thermore, unexpected turbine stops can be avoided because of component overheating. In an example, the predetermined operational speed range may be within +1 % of slip, preferably +0.5 % of slip, around the synchronous speed.

In particular, the decrease of the stator reactive power may be linear.

In an example, step c) may comprise controlling the AC-to-DC converter such that an AC voltage is imposed in the rotor windings of the doubly-fed electric machine. In particular, the AC voltage may be imposed with a frequency depending on the rotational speed and grid frequency.

In an example, step c) may comprise controlling the AC-to-DC converter such that the harmonic is absorbed by an impedance of the DC-to-AC converter.

In a further aspect, an apparatus for computer-implemented controlling a doubly-fed electric machine, where the appa ratus is configured to perform the method according to the invention or one or more preferred embodiments of the method according to the invention, is provided.

In a further aspect, a computer program product with a pro gram code which is stored on a non-transitory machine- readable carrier, for carrying out the method according to the invention or one or more preferred embodiments thereof when the program code is executed on a computer, is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in detail with respect to the accompanying drawings.

Fig. 1 is a schematic illustration of a doubly-fed elec tric machine connected to an electrical grid, ac cording to an example; Fig. 2 illustrates a schematic illustration of a power conversion system used to control the doubly-fed electric machine, according to an example; Fig. 3 shows the currents in the power conversion system in case the doubly-fed electric machine is operated around synchronous speed, according to an example; and

Fig. 4 illustrates a chart used for controlling the power conversion system according to an example.

DETAILED DESCRIPTION

Fig. 1 shows a block diagram of a doubly-fed electric machine 10 which may be part of a not shown wind power generator sys tem. Although the description below is in a context of a wind power generation system, the description is equally applica ble to a variable speed drive system that uses the doubly-fed electric machine.

The machine 10 may comprise a stator 12 with stator windings (not shown) and a rotor 14 with rotor windings (not shown). The stator 12 of the electric machine may be connected to an electrical grid 18 via a transformer 16. The rotor 14 may be connected to the electrical grid 18 via a power conversion system 20. The power conversion system 20 may comprise an AC- to-DC converter 22 (also called generator-side converter) and a DC-to-AC converter 24 (also called grid-side converter). The power conversion system may be controlled via a processor 26 implementing or configured to implement a method according to any of the disclosed examples, from the rotor side of the electric machine by using the variable voltage and variable frequency machine-side converter 22. The generator-side con verter 22 and the grid-side converter 24 are coupled via a capacitor 27 representing a DC bus. In the normal operation, the rotor 14 of the electric machine 10 may be rotating due to wind energy within a certain speed range. To achieve maximum power production (MPP) the genera tor-side converter 22 may be used to control the electric ma chine 10 and its active and reactive power production Pn and Qn. However, production of stator active power Pn and stator reactive power Qn is thermally limited when operating around synchronous speed n _sync At this condition, the rotor frequen cy is close to or substantially 0 Hz (DC), resulting in a re duced load sharing between inverter switches of each phase. This is illustrated by reference to Figs. 2 and 3.

Fig. 2 illustrates the inverter switches of the generator- side converter 22 and the grid-side converter 24 according to an example. As known for a skilled person, each phase may comprise two inverter switches connected in series to the ca pacitor 27. A node between the two inverter switches may be coupled to a respective phase. Hence, each of the generator- side converter 22 and the grid-side converter 24 may comprise six inverter switches "-1", "-2", "-3", "-4", "-5", and "-6", where the inverter switches with the suffixes "-1" and "-2" relate to a first phase, where the inverter switches with the suffixes "-3" and "-4" relate to the second phase and where the inverter switches with the suffixes "-5" and "-6" relate to the third phase.

If the rotor frequency is close to 0 Hz (DC), a rotor current (DC) asymmetry which is usually required to create rotor mag netic poles on an airgap between stator 12 and rotor 14, pro duce an overload of the branches of the generator-side con verter 22. For example, around synchronous speed, current Ii is flowing through inverter switch 22-1 of the first branch and inverter switches 22-3 and 22-5 of the second and third branch.

It can be seen from Fig. 3, in this situation the current Ii equals the sum of currents I2 and I3, i.e. Ii = -(I2 + I3).

This overload leads to overheating in the inverter switch 22- 1 and may damage and/or reduce life span when a certain cur rent level and/or time limit is exceeded. In this situation, the inverter switches of each of the group are always con ducting, resulting in a lack of load sharing. Hence, high asymmetrical rotor current leads to overload of the inverter switch branches and causes overheating, reduction of life span of components.

To enable operation at synchronous speed, for example when avoidance is not possible and to avoid drastic reduction of load and operation time, the processor 26 may be configured to perform the following steps of the invention.

The rotational speed n of the electric machine 10 may be ob tained during operation of the electric machine 10. It may be then determined by the processor 26, whether the obtained ro tational speed n is within a predetermined operational speed range around the synchronous speed n _sy nc This may be done by obtaining the slip s during operation of the electric machine 10. For example, if the predetermined operational speed range is within +1 % of slip or preferably +0.5 % of slip around the synchronous speed n _sy nc, the following steps may be per formed by the processor 26.

The generator-side converter 22 may be controlled to force injection of a stator reactive power Qharm to create a harmon ic at a frequency different than the rated frequency of the machine 10. At the same time, the grid-side converter 24 of the power conversion system 20 may be controlled such to com pensate the created stator creative power Qharm and the har monic.

By controlling the power conversion system 20 in such a way, the current direction can be changed briefly from a DC cur rent to an AC current. This reduces the duty cycle asymmetry. As the generated portion of stator reactive power Qharm at different frequency than the rated frequency is compensated back by the grid-side converter 24 before entering the elec- trical grid 18, no perturbance is seen by the electrical grid 18.

To assure a good transmission of this control strategy at synchronous speed n _S ync, a hysteresis loop as illustrated in the diagram of Fig. 4 may be used. The graph of the example of Fig. 4 is symmetrical with respect to the slip s shown on x-axis. If slip s = 0, the electrical machine 10 is operating at synchronous speed n _S ync. If the slip s is within a prede- termined operational range, for example within +0.5 %, around the synchronous speed (s = 0), the amount of the created sta tor reactive power Qharm at a frequency different than the rated frequency for positive and negative slip values may be defined. The closer rotational speed is to the synchronous speed (s = 0), the bigger the forced injection of stator re active power Qharm is. If the rotational speed corresponds to a slip s at the borders of the predetermined range (i.e., s = +0.5 %), the injected stator reactive power Qharm at the dif ferent frequency than rated is decreased to 0.

The flow of the different powers can be seen from Fig. 1 where Pn illustrates the stator reactive power, Qn stator reactive power, Qharm the stator reactive power at different frequency than rated and Q24 the grid-side converter 24 reac- tive power.