Method and apparatus for eliminating voltage distortion in electricity distribution network

Abstract

The invention relates to a method for active measurement and filtering of voltage distortion caused by harmonic currents drawn by nonlinear loads (L) in an electrical distribution network (AC). The measurement and filtering of the voltage distortion in nonlinear loads (L) is performed by voltage measurement: - by measuring the voltage (V) condition from the mains (AC) phase-by-phase and removing the undistorted, mains frequency reference value of the voltage, such as the fundamental component (Vref); and - filtering the resulting voltage distortion into one or more frequency-band specific components, such as odd harmonics or similar, wherein one or more components of the voltage difference are amplified (Afl, Afl) per frequency band using a separate adjustable gain (Gfl, Gfn) for each frequency band, inverted and summed to obtain a current reference ( iref ) to an adjustable power supply (I) which supplies the current reference (iref) to the mains (AC).

Description

[0001] Method and apparatus for eliminating voltage distortion in electricity distribution network

[0002] The invention relates to a method and apparatus for eliminating voltage distortion in an electricity distribution network according to the preambles of the independent claims directed thereto.

[0003] AC power distribution in the electricity grid is generally based on a fundamental frequency of sine waveform, most commonly 50Hz or 60Hz. Electrical loads that draw sinusoidal current (power) at this same frequency are linear loads. Linear loads can be resistive, such as heating resistors used in direct electric heating. On the other hand, electrical loads that draw sinusoidal current (power) at the same fundamental frequency but have phase shift with the voltage are inductive (inductors) or capacitive (capacitors) linear loads.

[0004] Further, among linear loads, the most common electrical loads are electric motors. Motors connected directly to the mains network without speed control are linear loads, which are partially inductive with a power factor cos cp between 0.7 and 0.9. Such unregulated linear loads do not degrade the voltage quality of the electricity network. However, inductive and capacitive loads cause reactive power and degrade the efficiency of the electricity distribution network, while limiting the maximum of the electric power transmission. Over the past decades, adjustable motor drives and other adjustable electrical devices have become more common on the electricity network to improve the efficiency of the use of electrical energy, for example in the form of inverters, rectifiers, computers and televisions. Such adjustable electric drives draw from the network not only the fundamental frequency, but also other frequencies that differ from the grid waveform and are called non linear loads. Currents that deviate from the fundamental frequency of the network are called harmonic currents, which are caused by harmonic components from non-linear loads. Harmonics cause a deviation in the fundamental sinusoidal voltage of the electrical energy distribution, i.e. a voltage distortion, wherein high-power electrical loads drawing harmonic current from the network cause a significant change in the local distribution waveform of the voltage. As a result, the quality of the electricity network no longer meets the requirements. The increasing number of non-linear loads and harmonic currents in the electricity network causes local problems, first of all by causing local distortion of the electricity distribution voltage near non-linear loads, and by overloading the electricity distribution network due to harmonic currents. Both of the above problems eventually cause malfunctions and equipment damage to electrical equipment.

[0005] Problems caused by harmonic currents can be compensated by using harmonic filters. When the spectrum of harmonic frequencies at the problem site is invariant, filtering the highest power harmonic voltages and / or harmonic currents with frequency-specific passive filters works well. Further, if the frequency spectrum and amplitude of the harmonics at the problem site vary, an Active Harmonics Filter (AHF) is commonly used as a solution to the problems caused by the harmonics.

[0006] A conventional active harmonics filter (AHF) works on the principle of measuring and controlling current. A current-controlled AHF measures the harmonic current caused by a non-linear load on the electricity network and generates a current that is in opposite phase compared to the harmonic current and feeding it into the electricity network. This reduces the sum of the harmonic currents at the connection point of the AHF and the resulting voltage distortion is reduced. To work effectively, the AHF must be adjusted and set (parameter setting) to the electrical environment of the application. Without expert commissioning setup, a conventional AHF can amplify certain frequency harmonics rather than attenuate them. Furthermore, if there are substantial changes in the local electricity network after commissioning, the current-controlled AHF must be reset to reflect the changed conditions. For example, the addition of a high capacitance near the connection point of the current-controlled AHF will cause a resonant oscillation between the AHF and the capacitance, in which case the solution is to reset the parameters to produce less or no resonant frequency. Some current-controlled active harmonic filters have been developed with self-control features that allow the device itself to analyze the state of the local electricity network and, if necessary, reset itself accordingly. In some cases, changes in the electrical environment of the application can also be responded by measuring of the electricity network and setting the parameters of the AHF through a remote connection. Regardless of the method, the installation of current sensors for a conventional current-controlled AHF, the parameter setting based on measurements of the electricity network and the potential resetting is an availability-critical task requiring expertise and quality, which substantially increases the cost of network disturbances caused by the harmonics.

[0007] Patent US 6320392 describes a solution to a problem when using a conventional active harmonic filter, where the response of the compensation current cannot be properly determined due to the constantly changing topology of the network. The solution describes a filter apparatus where the network response to harmonic currents is measured by feeding frequencies that are different from the harmonic frequency into the network and measuring the response caused by these frequencies and thereby determining the compensation response computationally. Further in the paper "A Novel Voltage Control for Active Shunt Power Filters" (Zanchetta et al., Industrial Electronics, 2002), a single harmonic compensation using voltage measurement is described. The paper describes voltage measurement from the network and filtering with a band-pass filter to obtain the 3rd harmonic voltage and further to determine the amplitude and phase of the compensation current to be fed to the grid. In this context, the paper also describes the determination of the voltage distortion for the 3rd harmonic by removing the fundamental voltage from the computed harmonic voltage signal.

[0008] The method and apparatus of the present invention for eliminating voltage distortion in an electricity distribution network are to provide a decisive improvement on the problems described above, and thereby to raise essentially the available prior art. In order to attain this objective, the method and apparatus of the invention are principally characterized by what is presented in the characterizing part of the independent claims directed thereto.

[0009] The basic idea of the invention is, first of all, to eliminate voltage distortion in the electricity distribution network caused by harmonic currents drawn by non-linear loads by means of an active harmonic filter that is based on a voltage measurement instead of a current measurement. In this case, no current measurement from the network is required, but the method and the device operate by measuring only the voltage at its connection point.

[0010] The measurement of the voltage distortion in the electricity network is first of all carried out by the reference method, wherein, for example, the ideal sine- wave reference signal at network frequency is subtracted from the actual voltage value measured by-phase from the electricity network. The voltage distortion in the power grid is then obtained as the difference between the actual reference value and the ideal reference value.

[0011] If an ideal sinusoidal reference value is not available or cannot be used, a high-pass filter, an ideal band-pass filter, or a similar signal processing method may be used to help to measure the voltage distortion. When using one or more high-pass filters, the actual voltage value is measured from the electricity network, e.g. phase by phase, and then filtered by a high-pass filter that removes the lower fundamental frequency of the network signal and does not cause a phase shift in the remaining harmonic frequencies. The remaining signal after the high-pass filtering is the voltage distortion of the electricity network. In contrast, with ideal band-pass filters, the fundamental frequency of the network is attenuated to such an extent that no fundamental frequency component disturbing the control circuit is remained after summation of the parallel band-pass outputs. If there is still a residual fundamental frequency left after the band-pass filtering and summing, the fundamental frequency can also be compensated by arranging the adjustable power supply downstream in the control circuit so that it does not supply this current with fundamental frequency to the electricity network.

[0012] The resulting voltage distortion in the electricity network is further divided by band-pass filters into frequency-band specific components, amplified and inverted. Inversion can be performed at any point along the control path, e.g. after the voltage measurement, after the band-pass filtering, after the adjustable gain, or after the signal summation. Further, the inversion may be performed before the voltage measurement, in which case the voltage to be measured is already inverted. As a result of the above, a signal equivalent to the voltage distortion, but inversely biased, is obtained, which provides the internal current reference of the active harmonic filter of the invention. This internal current reference is fed to an adjustable power supply in the AHF device, preferably implemented by power electronics, which supplies the current corresponding to the provided current reference to the electricity network. The current supplied to the network by the power supply is in the opposite phase compared to the voltage distortion, thus regulating the voltage distortion to a minimum.

[0013] The main advantage of the method and apparatus according to the invention is, first of all, the simplicity of the voltage measurements of the method, which enables carrying out the measurements using conventional electrical measurement methods directly at the connection point.

[0014] A further advantage of the method and apparatus of the invention is its adaptability to the variability of the electricity network at the connection point of the active harmonic filter. For example, in terms of control response, the network in the vicinity of the connection point may have local power generation, the electricity network at the connection point may be low or high impedance relative to the main grid, may contain a large number of parallel capacitive, inductive or non-linear load, or may contain loads down- or upstream the electricity network. Adaptability prevents instability and resonant oscillation of the control system if loads or characteristics of the electricity network change, amplifying some frequencies. Adaptivity is implemented by means of the said band-pass filters and adjustable amplifiers. If the control system becomes unstable, the amplitude of the voltage oscillation increases at the resonant frequency. The method and apparatus of the invention allow the system to be stabilized by adjusting the gain of the frequency-specific amplifier.

[0015] Other preferred embodiments of the method and apparatus of the invention are presented in the dependent claims directed thereto.

[0016] In the subsequent description, the invention will be illustrated in detail while referring to the accompanying drawings, in which: Fig. la shows the general principle of a method and apparatus of the invention,

[0017] Fig. lb shows a complementary general principle of a method and apparatus according to a preferred embodiment of the invention.

[0018] The invention relates first of all to a method for eliminating voltage distortion in an electricity distribution network, caused by the harmonic currents generated by nonlinear loads L, based on measuring and filtering the voltage distortion for example, phase by phase from an AC network by means of an active harmonic filter. The removal of voltage distortion is first of all based on the voltage measurement of the alternating current network AC with the voltage measurement scheme V as shown in Figure la, whereby the measurement results in the real actual value of the voltage of the electricity distribution network. From the measured actual value, the ideal sinusoidal control voltage signal Vrefis subtracted, leaving after the voltage measurement arrangement a voltage distortion signal which is fed to at least one band-pass filter BPfi. Each band-pass filter BPf1... BPfnhas a specific frequency pass-band, i.e. for example the band-pass filter BPfionly allows the signal in the frequency band fi to pass through. After band-pass filtering, the signal of the specified frequency band is fed to amplifier Afi, whose gain Gfican be adjusted by a control. After amplification, the filtered and amplified signal is inverted by an inverter Yfi. For each desired frequency band fn, band-pass filtering is carried out, respectively, by a band-pass filter PBfn, amplified by the amplifier Afnwith an adjustable gain Gfnand inverted by the inverter Yfn. All filtered, amplified and inverted signals are combined in the adder Sfinto a single sum signal, resulting in an internal current reference irefof the active harmonic filter. The current reference irefis fed to an adjustable power supply I, preferably implemented with power electronics, which supplies the current corresponding to the received current reference without phase shift to the electricity network AC. In a preferred embodiment of the method of the invention, referring in particular to Figure lb, the actual voltage value of the AC network is measured by a voltage measurement arrangement V, and the fundamental frequency component is removed from the actual voltage value, for example by a high-pass filter HPfor a similar method of filtering the fundamental frequency of the electricity network. In a further preferred embodiment of the method of the invention, harmonic distortions in the electricity network are eliminated by selecting filter-specifically the harmonic frequency bands of the electricity network, such as odd (3rd, 5th, etc.) harmonics, as the frequency bands of the band-pass filters, and by setting a separate, adjustable gain for each frequency.

[0019] In a further preferred embodiment of the method of the invention, the resonant frequency oscillation of the control system is prevented by adjusting the gain Gfi, Gfnof the frequency band specific control amplifier Afi, Afn. If the amplitude of the resonant frequency signal fi, fnis found to decrease as the gain Gfi, Gfnof the control amplifier Afl, Afnis increased, the gain Gfi, Gfnis further increased. On the other hand, if the amplitude of the resonant frequency signal fi, fnalso increases when the gain Gfi, Gfnis increased, the gain Gfi, Gfnof the control amplifier Afi, Afnis decreased correspondingly. In a further preferred embodiment of the method of the invention, the gain Gfi, Gfnof a frequency specific gain control amplifier Afi, Afnis automatically controlled based on amplitude measurement data when the control values reach their lower or upper limit.

[0020] In a further preferred embodiment of the method of the invention, the inversion of the signal is carried out by an inverter Yfnat an arbitrary point of the control circuit, for example before the voltage measurement arrangement V, Vref; before the band-pass filter PBfi, PBfn; after the band-pass filter PBfi, PBfn; before the adjustable amplifier Afi, Afn; after the amplifier Afi, Afn; or after the adder Sf.

[0021] On the other hand, the invention relates to an apparatus for eliminating voltage distortion in an electricity distribution network, based on measuring and filtering voltage distortions, for example phase-by-phase, in an alternating current network. The apparatus for measuring and filtering voltage distortions comprises:

[0022] - a voltage measurement device V, Vrefsubstantially connected to the AC electricity distribution network for measuring the real actual value of the voltage and the voltage distortion;

[0023] - at least one frequency specific band-pass filter BPfito filter the frequency specific voltage distortion signal caused by a harmonic;

[0024] - an amplifier Afi, Afnwith adjustable gain Gfi, G;nfor each separate band-pass filtered frequency to increase or decrease the signal amplitude to compensate the changes caused by the variations in the electricity network at the connection point;

[0025] - an inverter Yfi, Yfnto invert the voltage signal; - an adder Sfto sum the inverted signals f ... fninto a single sum signal and to further obtain the current reference iref; and

[0026] - an adjustable power supply I to supply the current according to the current reference irefto the AC electricity network without substantial phase shift.

[0027] In a preferred embodiment of the apparatus of the invention, a high-pass filter HPfis connected to the network and arranged substantially in the connection with the voltage measurement arrangement.

[0028] As a further preferred embodiment of the apparatus of the invention, the adjustable power supply I does not produce the basic frequency of the electrical network for the current supplied to the network according to the current reference iref¬it is obvious that the invention is not limited to the embodiments described or explained above, but can be modified within the basic idea of the invention, depending on the circumstances, by using in the method the most appropriate type of filter for filtering the fundamental frequency of the network, such as an RC, LC, LCL, Chebyshev, Butterworth, Bessel or Cauer filter or a similar signal processing filter. In the design of the electronic circuitry implementing the basic idea of the method and the apparatus, the implementation may be modified by component choices to increase cost- effectiveness and to emphasize and modify the required characteristics. It is clear that digital or analogue components can be used to measure and filter the voltage distortion, and automatic computer-controlled control solutions based on sensors or the like can be used.

Claims

Claims1. A method for active measurement and filtering of voltage distortion caused by harmonic currents drawn by nonlinear loads (L) in an electricity distribution network (AC), wherein the voltage distortion measurement and filtering of the nonlinear loads (L) is performed by voltage measurement (V): - by measuring the actual voltage value by-phase from the electricity distribution network (AC) and removing the undistorted, network frequency reference value of the voltage, such as the fundamental frequency component Vref; and- by filtering the voltage distortion obtained into one or more frequency-band specific components (fi...fn), such as odd harmonics or the like; characterized in that one or more components of the voltage distortion (fi, fn)are amplified (Afi, Afn)on a frequency-band basis using a separately adjustable gain (Gfi, Gfn)for each frequency band, inverted and summed to obtain a current reference (iref)for an adjustable power supply (I), which supplies the current corresponding to the current reference (iref)to the electrical distribution network (AC).

2. The method according to claim 1, characterized in that the fundamental component of the voltage is removed from the actual value of the voltage by a high-pass filter (HPf), a band-pass filter (PBfi, PBfn), or the like.

3. The method according to claim 1 or 2, characterized in that the gain ( Gfi, Gfn) of each freguency-band specific component (fi, fn)of the voltage distortion is adjusted to a higher value, if increasing the gain ( Gfi, Gfn) decreases the amplitude of the frequency-band component, and to a lower value if increasing the gain ( Gfi, Gfn) increases the amplitude of the voltage distortion component (fi, fn).

4. The method of claim 3, characterized in that the gain of each frequency band specific component ( Gfi, Gfn) is automatically adjusted.

5. The method according to any of the preceding claims1 to 4, characterized in that the voltage distortion measured from the electricity network is amplified and inverted to obtain a current reference (iref)that is passed to a power supply (I) which supplies a current corresponding to the current reference (iref)to the electricity network (AC) without substantial phase shift.

6. The method according to claim 5, characterized in that the power supply is most preferably implemented by power electronics.

7. An apparatus for the active measurement and filtering of voltage distortion caused by harmonic currents drawn by non-linear loads in an electrical distribution network, characterized in that the apparatus comprises means for measuring and filtering the harmonics by voltage measurement on a phase-by-phase basis,comprising at least a voltage measurement arrangement (V, Vref), at least one frequency specific band-pass and / or band-stop filter (PBfi, PBfn), a frequency band specific amplifier (Afi, Afn)with adjustable gain (Gfi, Gfn), a frequency band specific signal inverter (Yfn), a signal adder (Sf)and an adjustable power supply (I).

8. The apparatus according to claim 7, characterized in that a high-pass filter (HPf)is arranged in connection with the voltage measurement arrangement (V) to filter out the fundamental frequency of the electricity network (AC).

9. The apparatus according to claim 7 or 8, characterized in that it comprises means for detecting amplitude changes of the frequency band specific signals (f1 ...fn)to determine the frequency band specific adjustable gain (Gfi, Gfn).

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