WO2010060903A1

WO2010060903A1 - A method and device for facilitating the localisation of a fault in a grid

Info

Publication number: WO2010060903A1
Application number: PCT/EP2009/065725
Authority: WO
Inventors: Jorge Martinez Garcia
Original assignee: Vestas Wind Systems A/S
Priority date: 2008-11-28
Filing date: 2009-11-24
Publication date: 2010-06-03

Abstract

It is presented a method for facilitating the localization of faults in an electrical grid (1). When a fault condition in the grid (1) is detected at a power plant (2), the power converters (9) of the distributed power generators (4, 4-1, 4-2) of the power plant (2) are utilized to provide a power injection into the grid (1 ). The magnitude of the power injection is dependent on the fault condition. A power plant (2) is also presented.

Description

A METHOD AND DEVICE FOR FACILITATING THE LOCALISATION OF A

FAULT iN A GRID

Technical Field

The technical field of the present invention is power systems. More specifically, the present invention relates to a method and device for facilitating the localization of a fault in an electrical grid.

Technical Background

The field of clean technology is becoming more and more important as environmental issues are becoming an evident problem. Wind power technology is therefore an important field in that it is an environmental friendly and alternative way of generating and providing power to customers. For several reasons, such as the quality of power supplied to customers and to keep the power equipment of the grid intact, it is important to keep the grid free from faults to as large a degree as possible. It is, of course, not possible to keep the grid free from faults, and it is thereby very important for the grid operator to be able to localize a fault condition in the grid and, if necessary, disconnect a (preferably minimal) part of the grid subject to the fault condition.

Normally, when there is a fault condition to the grid involving a voltage dip, power generators will, due to the inherent properties of the generators, try to compensate for this voltage dip by injecting more (reactive) current into the grid. Thereby, the power generators in question contribute to the fault condition with fault currents, making it easier for the grid operator to detect where the fault is located. However, for variable-speed wind turbine generators, which is the dominating wind turbine technology in modern wind farms, the generator may not be directly coupled to the grid, as at least the rotor of the generator may be coupled to a power converter which in turn is connected to the grid. Moreover, wind farms utilizing HVDC connection, e.g. for connecting offshore wind farms to a (long distance) point of common coupling, are also coupled to the grid via converters.

In comparison to a generator directly coupled to the grid, a power converter has small transient response to a fault condition on the grid, i.e. the transient in this kind of Wind Turbine Generators (abbreviated WTG) with power converters will normally be shorter and with much less current into the grid. Therefore the WTG will not react in the same way as other type of WTGs, i.e. as directly coupled generators, do. European laid open patent application EP1855367 discloses a method and device for injecting reactive current during a mains supply voltage dip, particularly for wind farms. The magnitude and phase of the mains supply voltage is permanently monitored, and upon a voltage dip, a reactive current is injected in the affected mains supply phase or phases in a way that can vary over the duration of the voltage dip, at short intervals. The current is proportional to the magnitude of the dip. The method and device disclosed in EP1855367 injects current to follow the grid codes, i.e. to compensate for the voltage dip.

Summary

It is with respect to the above considerations and others that the present invention has been made.

In view of the above, it would therefore be desirable to achieve an improved method for e.g. power plants in an electrical grid. In particular, it would be advantageous to achieve a method for facilitating fault localization in a grid.

To better address one or more of these concerns, in a first aspect of the present invention there is provided a method for facilitating the localization of a fault condition in a grid, said method comprising: detecting, at a power plant comprising distributed power generators connected to said grid by means of power converters, the presence of the fault condition, determining a magnitude of at least one electrical parameter related to said fault condition, providing power to the grid by means of said power converters during a predetermined period of time, wherein said predetermined period of time and a magnitude of said power is based on the at least one electrical parameter. A grid fault is to be construed as any kind of fault in the grid involving e.g. short circuit in power lines, transformer faults, generator faults etc., giving rise to changes in the grid voltage, which changes are below or above an accepted, by the grid operator, predetermined threshold. Moreover, in general, it is to be understood that the grid fault may have arisen in another part of the grid than in the wind farm of the present method. In this context, a distributed power generator is defined as a power generator in renewable energy; in particular wind turbine generators of a wind farm, solar power generators, wave power generators or the like.

Beneficially, the present method may be used to provide power, e.g. fault currents to the fault, and thereby facilitating the localization of the fault condition in the grid.

In one embodiment, the grid may comprise a plurality of grid nodes, said plurality of grid nodes being interconnected by means of grid protectors.

Grid protectors are in this context to be construed as relays and switches, which may disconnect parts of the grid e.g. by disconnecting a grid node.

The present method may thus facilitate for a grid operator and grid protectors to localize the fault condition, i.e. the grid node(s) that are affected by the fault condition may be localized. The grid operator and/or grid protectors may, when having decided the severity of the grid fault, make a decision whether to disconnect a (preferably) minimal part of the grid, i.e. the affected grid node(s).

One embodiment may comprise detecting, by at least one grid protector of said grid protectors, an increase in power consumption in a faulty node connected to said grid protector, said detecting an increase in power consumption being in response to said providing power to the grid. In one embodiment the detecting may comprises: determining a voltage level value outside a predetermined accepted voltage interval. wherein said determining a magnitude is based on said impedance value and said voltage value.

In one embodiment said determining at least one electrical parameter may further comprise: determining a grid impedance value associated with said voltage dip.

In one embodiment, the providing may further comprise providing power from at least one central power converter of the power plant.

If the power converters are unable to deliver the magnitude of power based on the magnitude of the at least one electrical parameter, the central power converter of e.g. a substation of the power plant may provide additional power by means of e.g. storage devices such as batteries, traditional capacitors or super capacitors.

The power converters may comprise full-power converters. Full-power converters may be utilized in variable-speed wind turbine generators, thereby becoming 'invisible' to the grid in the sense that their fast response to e.g. voltage dips in the grid provides for less fault currents to the grid during these voltage dips. A full-power converter is to be construed as a converter being able to deliver the full power of a distributed power generator to the grid, i.e. the converter couples the full available power of the generator to the grid, in contrast to e.g. a Doubly Fed Induction Generator (abbreviated DFIG) WTG, where the stator of the generator is coupled to the grid directly, whereas the rotor is coupled via a power converter.

The present method may facilitate the localization of a grid fault by 'emulating' a generator being directly coupled to the grid, i.e. without being coupled via a power converter. According to the inventive method, fault currents may be provided to the grid by means of power injections of the power converters connecting the distributed power generators of the power plant, thereby facilitating the fault localization in the grid.

In one embodiment, the providing may comprise providing said power at the beginning of the fault condition. Thereby, the fault currents due to the power provided to the grid may be localized faster and a decision of e.g. disconnecting the faulty node(s) may be taken in a shorter time after the fault condition has arisen. Thereby, damages to e.g. transformers and other power equipment in the grid may be reduced.

According to a second aspect of the present invention, it is provided a power plant comprising: at least one distributed power generator comprising a power converter, at least one sensor for detecting a fault condition in a grid, and for determining a magnitude of at least one electrical parameter related to said fault condition, a processor for determining a magnitude of power to be provided to the grid, based on the magnitude of the at least one electrical parameter, wherein said power converter is arranged to provide the power into the grid in response to said fault condition being detected at the power plant.

Generally, this second aspect may exhibit the same advantages and features as the first aspect. Each power converter may comprise a full-power converter.

One embodiment may comprise a central power converter being arranged to provide additional power into said grid simultaneously with said power converter.

The determining the at least one electrical parameter may comprise determining a grid impedance.

Each of the at least one distributed power generators may be coupled to a point of common coupling of said power plant by means of HVDC coupling. When utilizing HVDC coupling, e.g. within a power plant, such as an offshore wind farm, to connect the power generators of the wind farm, e.g. connecting the WTGs to the point of common coupling, the converters used for the AC/DC conversion, may react in the same way as a full-power converter. The inventive method and power plant as disclosed herein may provide power to the grid in case of e.g. a voltage dip on the grid, and thereby facilitate the localization the fault condition in the grid. Additional possible features and preferred embodiments are set out in the dependent claims and disclosed in the following. Brief Description of the Drawings

Embodiments of the present invention will now be described in more detail, reference being made to the enclosed drawings, in which:

Fig. 1 shows a block diagram of a part of a grid comprising a wind farm according to an embodiment of the present invention.

Figs. 2a-b shows schematic views of variable-speed wind turbine generators.

Fig. 3 shows a graph of providing power into the grid when a fault condition occurs. Figs 4-5 are flowcharts illustrating methods for facilitating the localization of a fault condition in the grid according embodiments of the present invention.

Detailed Desciption of Embodiments In general, the present invention may relate to any kind of power plant utilizing power converters for grid connection. The distributed power generators can e.g. be wind turbine generators, solar power generators, wave power generators and the like, but by way of example, a wind farm comprising wind turbine generators will be described with reference to Figs 1- 5 below.

Fig. 1 shows a block diagram of a part of a grid 1 comprising a wind farm 2 according to an embodiment of the present invention. The part of the grid 1 in this illustrative example further comprises grid nodes N1 and N2 and grid protectors 3-1 , 3-2, and 3-3, which grid protectors may for instance be relays and switches, protecting the grid 1 as a whole by e.g. disconnecting a part of the grid 1 subject to a fault condition. By way of example, however not limiting the scope of the invention, the wind farm 2 comprises two wind turbine generators 4-1 and 4-2 connected to a substation 5, comprising a central power converter 5', by means of respective power converters (9 in Figs 2a-b), which in turn connects the wind farm 1 to the grid 1 by means of a point of common coupling (abbreviated PCC) 6. It is to be understood that the PCC 6 may be comprised within e.g. the substation 5 in some embodiments of the invention. In one embodiment, it is envisaged that the wind turbine generators 4-1 and 4-2 may be directly coupled to the central power converter 5', i.e. without coupling via power converters 9 between the central power converter 5' and the WTGs 4-1 and 4-2. In one embodiment, the PCC 6 may comprise a Static Synchronous

Compensator (abbreviated STATCOM).

As a skilled person readily would understand, a single wind turbine generator 4-1 in the grid 1 may also provide at least some of the advantages described below relating to the wind farm 2. At the PCC 6, there is a processor 7 and a sensor 8 operable coupled to each other. The sensor 8 can detect a fault condition in the grid 1. A grid fault can in this respect e.g. be a voltage dip in the grid 1 , or an overvoltage condition depending on e.g. short circuit between phases, or phase(s) to ground short circuits, or a generator fault at a power plant (not shown). In one embodiment, the sensor 8 may detect a fault condition of each phase separately, i.e. the sensor 8 may be able to detect asymmetrical faults of the three phases in the grid 1. The detection of the fault condition in the grid 1 may comprise detecting a change in grid characteristics, such as a voltage level change of the grid 1. Sensors as described above are well-known in the art and will therefore not be described in more detail herein.

The processor 7 may receive signals from the sensor 8, comprising information relating to detection of the grid fault. The processor 7 may then determine a magnitude of power, e.g. reactive current, to be provided to the grid 1. Control equipment (not shown) of the wind turbine generators 4-1 and 4-2 may then receive signals with instructions to provide an additional power injection into the grid 1 from the wind turbine generators 4-1 and 4-2. The respective power converters of the wind turbine generators may then provide a power injection to the grid 1 , for instance the injection of reactive current, with a magnitude determined by the processor 7. It is to be noted that the sensor 8 and the processor 7 may in an embodiment be situated in the wind turbine generators 4-1 and 4-2, as well as in the PCC 6. Alternatively, the determining a magnitude of power may be achieved individually at a wind turbine generator level, i.e. a processor of each wind turbine generator 4-1 and 4-2 may determine the magnitude of power to be provided to the grid 1 from each individual WTG 4-1 , 4-2. This inventive method may thus facilitate the localization of a grid fault in that the wind farm 2 is involved in providing a fault current to the grid 1 , instead of being 'invisible' to the grid 1 , behind power converters of the wind turbine generators 4-1 and 4-2. More specifically, this emulation of a generator directly connected to the grid facilitate the localization of the grid fault in the grid 1 by providing power to the faulty part of the grid, wherein e.g. the increased currents present in the faulty part of the grid facilitates for a grid operator to localize and make a decision whether to disconnect the faulty part of the grid 1 or not.

The invention will now be described in further detail with reference to Figs 2-6.

Figs. 2a-b shows schematic views of variable-speed wind turbine generators 4. The wind turbine generator 4 shown in Fig. 2a illustrates a variable speed wind turbine generator 4 comprising a power converter 9 of full power type, i.e. all of the nominal power of the generator is transmitted via the power converter 9 to the grid 1. The power converter 9 comprises a back-to- back converter: an AC/DC converter 9-1 , a DC/AC converter 9-2, and a DC bus capacitor 9-3. The AC/DC converter 9-1 and the DC/AC converter 9-2 may be of a standard voltage source converter type with transistors, such an IGBT, GTO or other type of electronics switch. The power electronics inside the power converter 9-1 inter alia controls the generator 10 speed. The generator can be of either synchronous or asynchronous (induction generator) type. Whereas the power electronics inside the power converter 9-2 inter alia controls the flow of power to the grid, both reactive and active power. A gear box 11 may be arranged between the generator and the shaft of the wind turbine but is however not necessary for the construction of the wind turbine generator 4, and may be omitted, as a skilled person will readily understand, e.g. depending on the number of poles if the generator 10 is of synchronous type.

The wind turbine generator 4 is preferably connected to the grid via transformer 12. The wind turbine generator 4 exemplified in Fig. 2a is a typical wind turbine generator that is 'invisible' to the grid 1 in that the generator 10 is not directly connected to the grid 1 in the sense that the (full) power converter 9 is the actual point of connection with the grid 1 (or generally, with substation 5). By providing a power injection to the grid 1 in case of a fault condition, such as a voltage dip, the localization of the fault condition may be facilitated and thereby damage to electrical equipment of the grid 1 , e.g. transformers, may be prevented. In general, the grid operator may ensure a higher quality of power to customers if grid faults can be detected and thereby a decision taken whether to disconnect the fault or not. The wind turbine generator 4 shown in Fig. 2b illustrates a wind turbine generator of doubly fed induction generator-type (DFIG-type). A difference with respect to the full power converter type of wind turbine generator as described above with reference to Fig. 2a, is that a stator of the generator 10 is connected directly to the grid 1 , wherein a rotor of the generator 10 is connected to the grid 1 via the power converter 9.

Wind turbine generators of DFIG-type have some well known advantages with respect to the above described full power converter type of wind turbine generators, such as reduced losses due to thermal heating of the power converter 9. However, DFIG-type wind turbine generators may still exhibit some of the disadvantages of the full converter type of wind turbine generators, relating to the contribution of fault currents when the grid 1 is subject to a fault condition. Therefore, it may preferable to be able to provide a power injection into the grid in case of a fault condition even for wind turbine generators of DFIG-type. An example of a situation with a fault condition in the grid 1 will now be described with reference to Figs 1 and 3. In this example, a fault condition - a generator fault in grid node N2 has occurred, giving rise to a voltage dip in the part of the grid 1 illustrated in Fig. 1. As the fault occurs, a voltage dip occurs in the grid 1 , and thereby the (automatic) response from generators of a power plant (not shown) in grid node N1 is to inject a reactive current into the grid 1 in order to regulate the voltage in the grid 1. Such action facilitates the localization of the fault in grid node N2, e.g. grid protectors 3-1 , 3-1 , and 3-3 may detect a raised current level and in case of a major fault, the grid protector 3-1 may disconnect grid node N2 from the rest of the grid 1 until the problem that has occurred in grid node N2 has been resolved.

Previously, the generators 10 of the wind farm 2 have been arranged behind fast responding (i.e. fast shut down) power converters and hence 'invisible' to the grid 1 , and therefore, a power injection, i.e. the provision of power would not have been possible. Thereby only low currents from the wind farm 2 may have been provided to the fault, and fault detection would have rendered more difficult. However, by the realization of the inventor that utilization of the inherent properties of the power converter 9, i.e. its fast response and capability to overload, emulation of a generator that is directly connected to the grid 1 is possible.

After the fault starts and has been detected (low voltage level detected by the sensor 8) at the wind farm 2, a transient will occur as a natural response from the power converter 9. A magnitude of reactive power, e.g. a current injection, to be provided to the grid is determined by the processor 7. As the power converter 9 resumes control, reactive power is provided to the grid 1. In this example, the reactive current is N times greater than a nominal current I_n. The injected current will decay following a programmed curve (e.g. linearly or exponentially, using typical synchronous time constants) as the power converter 9 normally will not be able to deliver the high current constantly.

When the initial reactive power has been provided to the grid 1 , the power converter 9 may in an embodiment keep providing power to the grid 1 according to a grid code that applies.

In any embodiment, the central power converter 5' located in the substation 5, may be utilized to provide additional power to the grid 1 , if the power converters 9 of the wind turbine generators 4-1 and 4-2 are unable to provide enough power to the fault in the grid 1.

The additional power provided to the grid 1 from the central converter 5' may be provided by means of e.g. storage devices such as batteries, traditional capacitors or super capacitors.

Alternatively, the STATCOM may be utilized to provide additional power to the grid 1. As the skilled person knows, for the STACOM, the maximal available reactive power decreases more slowly when there is a grid voltage dip than for some other type of compensators such as Static Var Compensators (abbreviated SVC). Thereby, a reactive power injection to the grid 1 via the STATCOM may be achievable during a voltage dip in the grid 1.

To determine the magnitude of power to be injected into the grid 1 , sensor 8 may, when a fault has been detected, e.g. when a voltage level falling below a predetermined threshold has been detected e.g. at the PCC 6, determine at least one electrical parameter related to the fault condition.

In general, a fault condition can be detected by determining a voltage level value, at e.g. the PCC 6, outside a predetermined accepted voltage interval.

The determining of the at least one electrical parameter may comprise guessing a grid impedance, and thereby determining the value of the peak current to be provided to the grid 1 according to the level of voltage at the PCC 6.

A ratio between the peak current to be provided to the grid 1 and the voltage level at the PCC 6 can e.g. be determined on basis of previous estimation values, knowing the structure of the grid where the system is connected, can be determined or simulated the distance to the fault when different voltages levels are present at the PCC 6.

In another embodiment, the power to be provided, i.e. the current to be injected into the grid 1 , can be determined as a function of the grid impedance to the fault.

In an embodiment, an equivalent impedance of the grid 1 may be determined using real time fault location information, utilizing techniques known in the art (e.g. by injecting a disturbance to the grid and analyzing the transient response) to determine the impedance of the grid in every moment.

In an embodiment, typical transient impedances of synchronous machines in combination with the grid impedance may be used to determine the power (by e.g. Ohm's law) to be provided to the grid 1.

Figs 4-5 are flowcharts illustrating methods for facilitating the localization of a fault condition in the grid according to embodiments of the present invention.

In a step S1 , the presence of the grid fault condition is detected. The detection may be at the wind farm 2. In this context the fault is to be construed to have arisen in another part of the grid 1 than the wind farm 2. The detection may be carried out by the sensor 8 as has been described in more detail above. The detection may comprise detecting changes, above or below a predetermined threshold of grid characteristics, and thereby alerting the processor 7 that a fault condition has been detected.

For instance, changes in grid characteristics which the sensor 8 may sense may be changes in the voltage level in the grid 1.

In particular, the detection can comprise determining a voltage level value outside a predetermined accepted voltage interval of the grid 1. In a step S2, a magnitude of at least one electrical parameter is determined. When the fault condition has been detected by detecting changes in the grid characteristics with a value below (or above) a predetermined threshold, a magnitude of reactive power (e.g. current) is determined based on the grid characteristics value. For example, when detecting a voltage dip, the electrical parameter to be determined may be grid impedance. Thereby, the magnitude of power to be provided to the grid 1 may be determined. This is illustrated in a step S2'.

Alternatively, when a voltage value of the grid 1 is determined, a power value associated with the voltage value can be determined by e.g. comparing in a table the voltage value with usual grid impedances for that voltage value.

Such a table may be based on e.g. the power converter 9 capabilities.

By way of example, the table may comprise injecting a maximum current (power) of the power converters 9 capacity (in one embodiment, the table may be adapted to take the central converters' 5' available power output into account); 100% of maximum current for zero voltage, 90% of maximum current injection for 10 % voltage, i.e. for a 90% voltage dip, etc.

In one embodiment, the determining of the at least one electrical parameter may comprise guessing a grid impedance, and thereby determining the value of the peak current to be provided to the grid 1 according to the level of voltage at the PCC 6.

In one embodiment, an equivalent impedance of the grid 1 may be determined using real time fault location information, utilizing techniques known in the art (e.g. by injecting a disturbance to the grid and analyzing the transient response) to determine the impedance of the grid in every moment. In an embodiment, typical transient impedances of synchronous machines in combination with the grid impedance may be used to determine the power (by e.g. Ohm's law) to be provided to the grid 1.

In a step S3, the power is provided to the grid 1. The power is provided by the wind turbine generators providing power to the power converter s 9, and if necessarry, overloading their power converters 9.

Alternatively, the central power converter 5' of the substation 5 of the wind farm 2 may provide additional power into the grid, if the power converters 9 of wind turbine generators 4-1 and 4-2 are not able to provide enough power into the grid 1. Alternatively, the STATCOM may provide additional power to the grid

1.

Thereby, grid protectors 3-1 , 3-2, and 3-3 may be able to detect the additional power (i.e. a reactive current injection) provided to the grid giving rise to fault currents detectable by the grid protectors 3-1 , 3-2 and 3-3. Hence a decision may be taken whether to disconnect nodes with fault in the grid 1.

The algorithms above disclose in detail how to determine the magnitude of power to be provided to the grid. The algorithms above may also include rules for setting the protection settings of the grid protectors. In one embodiment the algorithms comprises rules by which the grid protectors react to the additional power provided to the grid and disconnect the nodes with faults without receiving instructions from the PCC 6. Alternatively, the algorithms comprise rules based on the operational status of the grid protectors, which are supervised or monitored by the PCC 6.

In yet an alternative, a combination of the rules is provided by which the grid protectors normally operate via communication with the PCC 6 but may operate independently in case communication for some reason is lost with the PCC 6. 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 in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Furthermore, any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method for facilitating the localization of a fault condition in a grid, said method comprising: detecting (S1), at a power plant comprising distributed power generators (4, 4-1 , 4-2) connected to said grid by means of power converters (9), the presence of the fault condition, determining (S2) a magnitude of at least one electrical parameter related to said fault condition, providing power (S3) to the grid by means of said power converters (9) during a predetermined period of time, wherein said predetermined period of time and a magnitude of said power is based on the at least one electrical parameter.

2. The method as claimed in claim 1 , wherein said grid comprises a plurality of grid nodes, said plurality of grid nodes being interconnected by means of grid protectors.

3. The method as claimed in claim 2, further comprising: detecting, by at least one grid protector of said grid protectors, an increase in power consumption in a faulty node connected to said grid protector, said detecting an increase in power consumption being in response to said providing power to the grid.

4. The method as claimed in any one of the preceding claims, wherein said detecting comprises: determining a voltage level value outside a predetermined accepted voltage interval. wherein said determining a magnitude is based on said impedance value and said voltage value.

5. The method as claimed in claim 4, wherein said determining at least one electrical parameter further comprises: determining a grid impedance value associated with said voltage dip.

6. The method as claimed in any one of the preceding claims, wherein said providing further comprises providing power from at least one central power converter (5') of the power plant (2).

7. The method as claimed in any one of the preceding claims, wherein said power converters (5^J) comprise full-power converters.

8. The method as claimed in any one of the preceding claims, wherein said providing comprises providing said power at the beginning of the fault condition.

9. A power plant (2) comprising: at least one distributed power generator (4, 4-1 , 4-2) comprising a power converter (9), at least one sensor (8) for detecting a fault condition in a grid (1), and for determining a magnitude of at least one electrical parameter related to said fault condition, a processor (7) for determining a magnitude of power to be provided to the grid (1), based on the magnitude of the at least one electrical parameter, wherein said power converter (9) is arranged to provide the power into the grid (1) in response to said fault condition being detected at the power plant (2).

10. The power plant (2) as claimed in claim 9, wherein each power converter (9) comprises a full-power converter.

11. The power plant (2) as claimed in claim 9 or 10, wherein said power plant (2) comprises a central power converter (5') being arranged to provide additional power into said grid simultaneously with said power converter (9).

12. The power plant (2) as claimed in claims 9-11 , wherein determining said at least one electrical parameter comprises determining a grid impedance.

13. The power plant (2) as claimed in claims 9-12, wherein each of said at least one distributed power generators (4, 4-1 , 4-2) is coupled to a point of common coupling (6) of said power plant (2) by means of HVDC coupling.