Wind energy system and method for identifying low-frequency oscillations in an electrical supply network

Patent metadata

Field	Value
Publication number	US-11658490-B2
Application number	US-201917258130-A
Country	US
Kind code	B2
Filing date	Jul 5, 2019
Priority date	Jul 6, 2018
Publication date	May 23, 2023
Grant date	May 23, 2023

Abstract

Provided is a method for identifying low-frequency oscillations, in particular subsynchronous resonances, in an electrical supply network, wherein the electrical supply network has a line voltage with a nominal line frequency, comprising the steps: detecting at least one electrical signal of the electrical supply network as at least one test signal and filtering and/or transforming the at least one detected test signal into at least one check signal, temporal derivation of the at least one check signal or difference formation of temporally separated values of the check signal, in order to obtain a gradient signal in each case, identifying the presence of a low-frequency oscillation if the gradient signal or at least one of the gradient signals meets a predetermined check criterion, in particular at least one predetermined check limit is exceeded.

First claim

The invention claimed is:

1. A method for identifying low-frequency oscillations in an electrical supply network having a line voltage associated with a nominal line frequency, the method comprising:
detecting at least one electrical signal of the electrical supply network as at least one test signal;
filtering and/or transforming the at least one test signal into at least one check signal;
obtaining a gradient signal by:
temporally differentiating the at least one check signal, or
obtaining a difference of temporally separated values of the at least one check signal;

determining whether the gradient signal meets a predetermined criterion; and
in response to determining that the gradient signal meets the predetermined criterion, identifying a presence of a low-frequency oscillation, wherein:
transforming the at least one test signal into the at least one check signal includes transforming the line voltages of the electrical supply network in three phases and current fed into the electrical supply network in three phases into a voltage signal, an active power signal and a reactive power signal as a voltage check signal, an active power check signal or a reactive power check signal,
temporally differentiating the at least one check signal includes temporally differentiating the voltage check signal, the active power check signal and the reactive power check signal to obtain a voltage gradient signal, an active power gradient signal and a reactive power gradient signal, respectively, or obtaining the difference of the temporally separated values of the at least one check signal includes obtaining a difference of temporally separated values of the voltage check signal, the active power check signal and the reactive power check signal to obtain the voltage gradient signal, the active power gradient signal and the reactive power gradient signal, respectively,
the voltage gradient signal, the active power gradient signal and the reactive power gradient signal are checked for the presence of the low-frequency oscillation, and
the presence of the low-frequency oscillation is identified in response to the low-frequency oscillation being found at least in the voltage gradient signal and the active power gradient signal or in the voltage gradient signal and the reactive power gradient signal.

2. The method as claimed in claim 1 , wherein:
the predetermined criterion is met in response to the gradient signal exceeding a gradient maximum value, and
determining whether the gradient signal meets the predetermined criterion includes:
determining whether the gradient signal exceeds the gradient maximum value at least once, or
determining whether the gradient signal exceeds the gradient maximum value at least for a predetermined minimum period.

3. The method as claimed in claim 1 , wherein obtaining the difference of temporally separated values of the at least one check signal includes obtaining a difference between a maximum and minimum value of the at least one check signal in a check period.

4. The method as claimed in claim 1 , wherein:
the at least one test signal is the line voltages of the electrical supply network in three phases,
the at least one test signal is the current fed into the electrical supply network in three phases, or
the at least one test signal is the line voltages of the electrical supply network in three phases and the current fed into the electrical supply network in three phases.

5. The method as claimed in claim 1 , wherein obtaining the difference of the temporally separated values of the at least one check signal yields a plurality of gradient signals including the gradient signal.

6. The method as claimed in claim 1 , comprising:
detecting a line frequency of the electrical supply network as a further test signal or transforming the further test signal into a frequency signal as a frequency check signal;
temporally differentiating or obtaining a difference of the frequency check signal to obtain a frequency gradient signal;
checking the frequency gradient signal and at least one further gradient signal for the presence of the low-frequency oscillation, the at least one further gradient signal being one of: the voltage gradient signal, the active power gradient signal or the reactive power gradient signal; and
identifying the presence of the low-frequency oscillation in response to the low-frequency oscillation being identified in the frequency gradient signal and in the at least one further gradient signal.

7. The method as claimed in claim 1 , wherein detecting the at least one electrical signal of the electrical supply network includes at least one of:
detecting a three-phase line voltage; and transforming the three-phase line voltage to produce a direct variable that is a space vector variable of the three-phase line voltage, transforming the three-phase line voltage including determining a positive sequence voltage according to the method of the symmetrical components to form a check signal; or
detecting a three-phase feed current; and transforming the three-phase feed current to produce a direct variable that is a space vector variable of the three-phase line voltage, transforming the three-phase feed current including determining a positive sequence current according to the method of the symmetrical components to form the check signal.

8. The method as claimed in claim 7 , wherein the direct variable of the three-phase line voltage or the positive sequence voltage and the direct variable of the three-phase feed current or the positive sequence current are transformed into a voltage signal, an active power signal and a reactive power signal as a voltage check signal, active power check signal or reactive power check signal, respectively.

9. The method as claimed in claim 1 , comprising:
feeding electrical power into the electrical supply network by a wind power installation or wind farm; and
identifying the presence of the low-frequency by the wind power installation or wind farm.

10. The method as claimed in claim 1 , comprising:
identifying the low-frequency oscillation in the voltage gradient signal and the active power gradient signal or in the voltage gradient signal and the reactive power gradient signal;
identifying that the low-frequency oscillation in the voltage gradient signal and the active power gradient signal or in the voltage gradient signal and the reactive power gradient signal has the same oscillation frequency or oscillates with the same frequency as the line frequency; and
identifying that the low-frequency oscillation is caused in the electrical supply network.

11. The method as claimed in claim 10 , comprising:
determining the voltage gradient signal, active power gradient signal and reactive power gradient signal as a difference between a maximum value and minimum value in a check period of a corresponding check signal; and
determining the same oscillation frequency in the check period or determining that respective time intervals between the maximum value and the minimum value of the corresponding check signal are the same for the voltage gradient signal, the active power gradient signal or the reactive power gradient signal.

12. The method as claimed in claim 1 , comprising:
determining a gradient quotient as a quotient of two gradient signals; and
identifying that the low-frequency oscillation is caused in the electrical supply network depending on the gradient quotient, wherein the gradient quotient is one of:
a voltage and active power quotient formed as a quotient between the voltage gradient signal and the active power gradient signal, or
a voltage and reactive power quotient formed

Assignees

• Wobben Properties Gmbh

Inventors

• Busker Kai
• Mendonca Angelo
• Schwanka Trevisan Aramis

Classifications

• H02J2101/28

  Wind energy · CPC title

• H02J3/00142

  Oscillations concerning frequency · CPC title

• H02J3/381 Primary

  Dispersed generators · CPC title

• H02J3/46

  Controlling the sharing of generated power between the generators, sources or networks · CPC title

• G01R19/2513

  Arrangements for monitoring electric power systems, e.g. power lines or loads; Logging · CPC title