Patent Application
20240396336
The invention relates to a method and system for stabilizing the electrical power in an electrical grid.
In a weak electrical grid or an island grid, a load rejection caused by a grid fault or a fault of a large consumer like an electrical arc furnace or a large drive may lead to the increase of the frequency. This can lead to a trip of the power generators supplying the electrical grid and/or tripping of consumers. In both cases, valuable production time may be lost and the restart of the overall system may be time and cost intense.
Additionally, when harmonic filters are connected to the electrical grid for compensation of reactive power from e.g. large drives, arc furnaces or rectifier systems and the returns after a grid fault, this may lead to an overswing of the grid voltage due to reduced active and reactive power loading of the grid. This also may lead to a trip (switch off) of large electrical consumers, if the voltage variation is large enough. Specially drives are usually not very tolerant to overvoltages. Restarting of generators is typically a time-consuming issue.
WO 2020/113 336 A1 describes a method and system for stabilizing electrical power for arc furnaces and their power supplies. The method comprises causing a load to absorb power in response to determining a loss of arc event of an arc furnace electrode.
It is an objective of the invention to reduce tripping of generators in an electrical grid and/or to reduce downtimes of consumers in the electrical grid.
These objectives are achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.
A first aspect of the invention relates to a method for stabilizing electrical power in an electrical grid. The electrical grid may be a low (up to 1 kV), medium (up to 50 kV) and high (more than 50 kV) voltage grid. It may be the grid of a production facility or may be a large scale grid for supplying a large amount of consumers and production facilities. The electrical grid may be a three-phase grid.
According to an embodiment of the invention, the method comprises: detecting a reduced power demand in the electrical grid and determining an active power and a reactive power to be compensated in the electrical grid. Voltage and current in the electrical grid may be measured and the power demand may be determined therefrom. For example, the power over time provided by the electrical grid may be calculated and, when the power over time reduces suddenly, a reduced power demand may be assumed. It also may be that the current power demand of consumers and/or loads is reported to a controller performing the method and that these controllers sums up the reported power demands to determine power demand over time.
From measured voltage and the measured current in the electrical grid, also the active power and the reactive power in the electrical grid may be determined. When the voltage and the current are considered as complex numbers, the complex power is the product of voltage and the complex conjugate of the current. The active power (or real power) is the real part of the complex power. The reactive power is the imaginary part of the complex power.
According to an embodiment of the invention, the method further comprises: compensating at least a part of the active power and at least a part of the reactive power by controlling a power compensating circuit connected to the electrical grid. With the power compensating circuit, at least a part of the missing active and reactive power can be generated. In such a way, the reduced power can be balanced and the amplitude and frequency of the voltage in the grid can be stabilized.
The power compensating circuit comprises an at least resistive load connectable to the electrical grid via semiconductor switches. The at least resistive load may provide a resistance and optionally a capacity and/or impedance. An active power and a reactive power may be generated by connecting and disconnecting the at least resistive load to the electrical grid. This may be done with the frequency of the electrical grid.
A compensated active power and a compensated reactive power is adjusted by setting switching angles of the semiconductor switches with respect to a phase angle of a grid voltage in the electrical grid. Thus, not only the reduced active power, which may be caused by a tripped consumer and/or load, may be at least partially compensated, but also a changed reactive power, which for example may be caused by harmonic filters subjected to a frequency change of the voltage, may be at least partially compensated.
According to an embodiment of the invention, the semiconductor switches are thyristors and the switching angles are firing angles of the thyristors. Such firing angles may have a value between 0° and 180°. In the case of other actively controllable semiconductor switches, turn-on switching angles and turn-off switching angles may be determined.
According to an embodiment of the invention, the reduced power demand is detected by measuring a voltage and a current in the electrical grid and by calculating an electrical power from the measured voltage and the measured current. A controller for determining the reduced power and controlling the power compensating circuit may receive signals from voltage and current signals in the electrical grid. As already mentioned, from these signals over time, the active and reactive power can be calculated.
According to an embodiment of the invention, the power compensating circuit comprises a pair of antiparallel connected semiconductor switches for connecting and disconnecting two phases of the electrical grid. The at least resistive load is connected in series with the pair of antiparallel connected semiconductor switches. All pairs of phases of the electrical grid may be connected by a pair of antiparallel connected semiconductor switches and an at least resistive load. The pairs of antiparallel connected semiconductor switches may be delta-connected between the phases.
It also may be that pairs of antiparallel connected semiconductor switches are star-connected between the phases. In this case, each phase may be connected to a star-point via a pair of antiparallel connected semiconductor switches.
According to an embodiment of the invention, the power compensating circuit comprises an active rectifier with a half-bridge for each phase of the electrical grid, wherein the at least resistive load is connected in parallel to the half-bridges. A further possibility is to rectify the voltage in the electrical grid with an active rectifier, which may comprise a half-bridge for each phase. On the DC side, the rectified voltage may be applied to the at least resistive load.
Each half-bridge may comprise an upper and a lower semiconductor switch, where a phase of the grid is connected between the switches. On their other ends, the half-bridges are connected in parallel and provide the DC outputs of the rectifier. The switching angles of the upper and a lower semiconductor switch may be selected, such that the power compensating circuit provides a desired active power and reactive power.
According to an embodiment of the invention, the power compensating circuit comprises a transformer connected between the electrical grid and the semiconductor switches, which transformer has an adjustable transformation ratio. The transformer may comprise a tap changer with which the winding number of a winding of the transformer can be changed. The transformation ratio may be adjusted with the tap changer. In such a way, the voltage applied to the at least resistive load may be adjusted, which also may be used for controlling the active power and reactive power generated by the at least resistive load.
The compensated active power and the compensated reactive power is adjusted by setting the adjustable transformation ratio.
According to an embodiment of the invention, the power compensating circuit comprises a first rectifier and a second rectifier connected to the electrical grid. Both rectifiers may be connected via a transformer or directly to the electrical grid. The compensated active power and the compensated reactive power is adjusted by setting first switching angles for the first rectifier and corresponding different second switching angles for the second rectifier. The rectifier may be designed equal and the corresponding first and second switching angles may relate to the same semiconductor switch of the first and second rectifier, respectively.
According to an embodiment of the invention, the switching angles of upper semiconductor switches of half-bridges of the first converter are different from the switching angles of lower semiconductor switches of the half-bridges of the first converter. The switching angles of upper semiconductor switches of half-bridges of the second converter are equal to the switching angles of the lower semiconductor switches of the half-bridges of the second converter. The switching angles of lower semiconductor switches of the half-bridges of the second converter are equal to the switching angles of the upper semiconductor switches of the half-bridges of the first converter. Such a switching scheme may be called “split-alpha”. The switching of the two rectifiers in this way results in rather low higher order harmonics, since the rectifiers are switched symmetrically with respect to negating the respective phase voltages. Furthermore, with this switching, scheme active power and reactive power may be controlled independently from each other.
According to an embodiment of the invention, the switching angles of upper semiconductor switches of half-bridges of the first converter are equal to the switching angles of lower semiconductor switches of the half-bridges of the first converter. The switching angles of upper semiconductor switches of half-bridges of the second converter are equal to the switching angles of lower semiconductor switches of the half-bridges of the second converter. The switching angles of the upper and lower semiconductor switches of the half-bridges of the first converter are different from the switching angles of the upper and lower semiconductor switches of the half-bridges of the second converter. In other words, the on-times of the semiconductor switches of the first rectifier may be different (i.e. different closing angle and/or different opening angle) from the on-times of the semiconductor switches of the second rectifier. Also with this switching scheme, active power and reactive power may be controlled independently from each other. It has to be noted that all these switching angles are provided with respect to a zero crossing from negative to positive of the respective phase voltage of the electrical grid.
According to an embodiment of the invention, the first rectifier and the second rectifier are connected in series at their DC outputs and the at least resistive load is connected in parallel to the series connected DC outputs. It also may be that the first rectifier, the second rectifier and the at least resistive load are connected in parallel to the DC outputs of the first rectifier and the second rectifier. It also may be possible that a separate at least resistive load is connected to each rectifier, i.e. their DC outputs.
According to an embodiment of the invention, the first rectifier and the second rectifier are connected to the electrical grid via a transformer with a secondary winding for each rectifier. In such a way, when the first and second rectifier are switched symmetrically, higher order harmonics caused by the switching may compensate in the transformer.
A further aspect of the invention relates to a system for stabilizing electrical power in an electrical grid. It has to be understood that features of the method as described in the above and in the following may be features of the system as described in the above and in the following, and vice versa.
According to an embodiment of the invention, the system comprises the power compensating circuit, such as described above and below, and a controller for controlling the power compensating circuit, such as described above and below. The system is adapted for performing the method as described herein.
According to an embodiment of the invention, a harmonic filter is connected to the electrical grid. The system furthermore may comprise a harmonic filter, which may be a passive filter for filtering higher order harmonics caused by loads connected to the grid. The reactive power generated by the harmonic filter, when the active power demand reduces, may be compensated by the method.
According to an embodiment of the invention, at least one load is connected to the electrical grid, which, when being disconnected from the electrical grid, causes the reduced power demand. The at least one load comprises at least one of an electrical drive and an electrical arc furnace. An electrical drive may comprise a converter and an electrical motor and/or generator. It has to be noted that such a load may have an active power demand of more than 1 MW.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.
The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.
In general, based on the received data (such as the voltage measurements 24 and current measurements 26), the controller 22 determines, whether there is a power demand drop in the electrical grid 18 and controls the power compensating circuit 12 to balance or at least reduce this power demand drop. For example, the active power and the reactive power can be compensated in case of a load rejection of one of the loads 16 or a grid fault of the electrical grid 18, in particular, when the electrical grid 18 is weak or is in an island operation. This will prevent the electrical grid 18 from overvoltage and increasing of the frequency.
The controller 22 may be adapted for detecting disturbances and/or faults from one or more of the loads 16, or from the electrical grid 18, which may comprise overhead lines. Further faults and/or a detected power demand may be determined from a loss of arc at an electrical arc furnace 16b and a restrike within 100 ms to 1000 ms, a trip of a large electrical drive 16 and in general a trip of a large electrical load 16c.
The power compensating circuit 12 and the controller 22 are designed to simultaneously generated active power and reactive power, in particular to prevent a voltage overswing on the electrical grid 18 and to simultaneously prevent an increase in frequency. The voltage overswing and the increase in frequency may be caused by the one or more generators 20 still producing a large amount of power, while the power demand has dropped. This will prevent the loads 16 and the generators 20 from tripping and thus after the event, the system 10, such as a plant, mine or remote industrial area, will be able to continue its operation without any disturbances. This may prevent the system 10 from a restart. A trip and a restart may lead to a loss of production time.
In the case of a determined reduced power demand, such as a load rejection or grid fault, the power compensating circuit 12 can compensate not only the active/real power but also the changed reactive power. The reactive power may be changed due to the harmonic filter 14, which may comprise filter capacitances and filter inductances, which in the case of a changing voltage may change the reactive power.
The following
Each semiconductor switch arrangement 30 comprises a load 34, or, as shown in
Each load 34 is an at least resistive (or ohmic) load and can comprise reactive, i.e. capacitive and/or inductive, parts. Each load 34 may be passive, i.e. may be composed of resistors, capacitors and/or inductors. For example, each load 34 may be adapted for dissipating at least 0.1 MW. The properties of the loads 34 described with respect to
In
In
In
The power compensating circuit 12 also may comprise a mechanical switch 35 for connecting the power compensating circuit 12 to the electrical grid 18 and for completely disconnecting it therefrom.
Again, the active power and reactive power of the power compensating circuit 12 can be controlled by controlling the switching angles of the semiconductor switches 32. In
An optional transformer 42 may be connected between the electrical grid 18 and the rectifier 36. The transformer 42 may have an adjustable transformation ratio, for example via a tap changer 44. By changing the transformation ratio, also the ratio between active power and reactive power provided by the power compensating circuit 12 may be changed.
With the tap changer 44, power load changes can be considered depending on the available taps. Depending on the transformer tap position of the tap changer 44 and the switching angles of the semiconductor switches 32, the active power and the reactive power drawn by the power compensating circuit 12 is controllable more independently from each other. This allows to adapt two various operating points and/or is more flexible.
The transformer 42 also may be provided in
In
In
As shown in
As shown in
It is also possible that the switching scheme of
In step S10, the controller 22 detects a reduced power demand in the electrical grid 18. As described above, this may be done by measuring a voltage 24 and a current 26 in the electrical grid 18 and by calculating an electrical power from the voltage 24 and the current 26. Additionally or alternatively, the controller 22 determines a grid fault or other faults by evaluating the measurement data of the voltage 24 and the current 26. Additionally or alternatively, the controller 22 receives data from the loads 16, 16a, 16b, 16c and/or from the harmonic filter 14 that indicates a reduced power demand. Such data may comprise information that one of the loads 16, 16a, 16b, 16c has a fault and/or has tripped.
In step S12, the controller 22 determines an active power and a reactive power to be compensated in the electrical grid 18. For example, the controller determines the active and reactive power that was drawn from the electrical grid 18 before the reduced power demand has appeared. The active power to be compensated may be the difference between the active power before the appearance of the reduced power demand and the active power after that. Analogously, the reactive power to be compensated may be the difference between the reactive power before the appearance of the reduced power demand and the reactive power after that. It also may be that, when one of the loads 16 trips, the reduced power demand in active and reactive power is already stored in the controller 22 and is then used as active power and a reactive power to be compensated. A further possibility is that the reactive power generated by the harmonic filter 14 with respect to a specific voltage change in the grid is stored and/or may be calculated by the controller 22 and is then used as reactive power to be compensated.
In step S14, the controller 22 controls the power compensating circuit 12 to compensate at least a part of the active power and at least a part of the reactive power. The power compensating circuit 12 is controlled, such that the at least resistive load 34 is connected to the electrical grid 18 and disconnected from the grid 18 via the semiconductor switches 32, such that it generates the active power and the reactive power to be compensated.
In particular, the power compensating circuit 12 is controlled to not only compensate the active power but also the reactive power, which for example is generated by the harmonic filter 14. When the voltage returns after a fault, this prevents the system 10 from an overswing of the voltage due to missing reactive and active power and thus prevents a tripping of the loads 16, which may save the time for restarting the overall system 10.
The compensated active power and a compensated reactive power are adjusted and/or generated by correspondingly setting the switching angles α1, α2 of the semiconductor switches 32 of the power compensating circuit 12. The switching angles α1, α2 are set with respect to a phase angle of the grid voltage 24 in the electrical grid 18. For example, the switching angle for a specific semiconductor switch may be set to a specific angle after the zero crossing of the respective phase voltage of the electrical grid.
Additionally, when the power compensating circuit 12 comprises a transformer 42, 42′ with an adjustable transformation ratio, the compensated active power and the compensated reactive power may be adjusted by setting the adjustable transformation ratio correspondingly.
When the power compensating circuit 12 comprises a rectifier 36, 36a, 36b, different switching schemes can be used to generate a desired active power and reactive power. For example, the rectifier 36 in
A first possibility is switching the rectifiers with the control scheme called “split-alpha”. The switching angles α1 of upper semiconductor switches 32a of the half-bridges 38 of the first converter 36a are selected to be different from the switching angles α2 of the lower semiconductor switches 32b of the half-bridges 38 of the first converter 36a. The switching angles α2 of the upper semiconductor switches 32a of the half-bridges 38 of the second converter 36b are set equal to the switching angles α2 of the lower semiconductor switches 32b of the half-bridges 38 of the second converter 36b. The switching angles α1 of the lower semiconductor switches 32 of the half-bridges 38 of the second converter 36b are set equal to the switching angles α1 of the upper semiconductor switches 32a of the half-bridges 38 of the first converter 36a.
A second possibility is to use the same switching angles α1, α2 for the upper and lower semiconductor switches 32a, 32b for each rectifier 36a, 36b, but to use different switching angles α1, α2 for the rectifier 36a, 36b. The switching angles α1 of the upper semiconductor switches 32a of the half-bridges 38 of the first converter 36a are selected to be equal to the switching angles α1 of the lower semiconductor switches 32b of the half-bridges 38 of the first converter 36a. The switching angles α2 of the upper semiconductor switches 32a of the half-bridges 38 of the second converter 36b are selected to be equal to the switching angles α2 of the lower semiconductor switches 32b of the half-bridges 38 of the second converter 36b. The switching angles α1 of the upper and lower semiconductor switches 32a, 32b of the half-bridges 38 of the first converter 36a are selected to be different from the switching angles α2 of the upper and lower semiconductor switches 32a, 32b of the half-bridges 38 of the second converter 36b.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/078998 | 10/19/2021 | WO |
