Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
1999-09-13
2001-11-20
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S076680, C307S105000, C363S040000
Reexamination Certificate
active
06320392
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an active filter device provided for compensating harmonic distortion in a supply line, said device comprising a signal processing unit having a first input for receiving an electric current and a voltage, applied on said supply line and being provided for determining, on the basis of said received current and voltage, a load current value for each harmonic frequency of a predetermined first set of harmonic frequency components of a load current on said supply line, said signal processing unit being further provided for generating for each of said harmonics, a control signal on the basis of said load current value, said device further comprising a power module, having a second input for receiving said control signal, and a power source for generating for each of said harmonics a compensation current.
Such an active filter device is known from the article “A new high performance active harmonic conditioner based on the current injection mode” of Serge Bernard and Gerard Trochain published in Power Quality Bremen—November 1995 proceedings p. 243-252. The known active filter device is connected in parallel to the supply line, which connects a power source to a load. The load generates harmonic currents, which are injected into the supply line and which will consequently disturb the power supplied by the source. The active filter device uses power electronics to generate harmonic components, in order to cancel the harmonic currents on the supply line. The signal processing device monitors the electric current and voltage, applied on the supply line, and determines for each harmonic frequency of the first set of harmonic frequencies, the load current on the supply line. On the basis of this load current a control signal is generated, which is supplied to the power module. The latter being provided for generating, under control of the control signal, the compensation current for each harmonic frequency of said first set. The compensation current is then injected into the supply line, in order to compensate the load current. In such a manner, the current on the supply line is “cleaned” from the harmonic currents, injected by the load.
A drawback of the known active filter device is, that the reaction of the supply line upon application of the compensation current, is not accurately determined because the topology of the supply line changes all the time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an active filter device enabling to adequately monitor the reaction of the supply line, when a compensation current is applied thereon.
An active filter device according to the present invention is therefore characterised in that said signal processing unit is provided for selecting among a second set of frequency values at least one frequency value (f) and for determining a first (f
1
) respectively a second (f
2
) frequency value by shifting said selected frequency value f over a first (g
1
≠0) resp. a second (g
2
≠0) value in such a manner that f
1
=f+g
1
and f
2
=f−g
2
, said first f
1
and second f
2
frequency being offset from the harmonic frequencies, said power module being provided for injecting on said supply line, a first respectively a second monitoring current at said first f
1
resp. said second f
2
frequency, said signal processing unit being provided for measuring from said received current and voltage, a response current generated on said supply line upon injection of said monitoring current and for determining from said response current a transfer function from a voltage of said power source to a line current on said supply line at the first and second frequency, and by determining from said transfer function a phase shift from said voltage to line current at said frequency value f, and for determining said control signal from said phase shift, said power module being provided for generating said compensation current under control of said control signal. By injecting a monitoring current at the first and second frequency, different from the harmonic frequency, a response current at those frequencies is obtained on the supply line, enabling in such a manner to monitor the response of the supply line. As the response current is on a frequency, different from the harmonic, the response current is not added to the load current at the considered harmonic frequencies. A reliable measurement of the response current is then obtained. From the response current a transfer function from a voltage of said power source to a line current at the first and second frequency is determined. This enables to determine the phase shift from said voltage to line current at the frequency f. This phase shift is then used for generating the control signal. A more accurate compensation current is thus generated by a measurement offset from the harmonic frequency.
A first preferred embodiment of an active filter device according to the present invention is characterised in that said second set of frequency values corresponds to said first set of harmonic frequency components. In such a manner the frequency value f is the considered harmonic, f
1
and f
2
being situated on both sides of the considered harmonic.
Preferably the absolute value of g
1
and g
2
is equal. This facilitates the transformation towards the harmonic upon determination of the phase shift, as an average value is determined.
Preferably said power module is provided for generating a voltage with a zero phase and for deriving said monitoring current from said voltage, said signal processing unit being provided for determining the phase of said response current. When the voltage applied for generating the monitoring current has a zero phase, the determination of the transfer function becomes easier whereas the phase of the response current indicates directly the phase shift of the transfer function at the considered harmonic frequency.
Preferably said signal processing unit is provided for generating said control signal as a pulse width modulated signal. A pulse width modulated signal is particularly suitable as a control signal for a power transistor, which equips the power module.
REFERENCES:
patent: 5397927 (1995-03-01), Suelzle et al.
patent: 5465203 (1995-11-01), Bhattacharya et al.
patent: 0 600 809 A1 (1994-06-01), None
patent: 0 666 632 (1995-08-01), None
Serge Bernard et al, “A New High Performance Active Harmonic Conditioner Based on the Current Injection Mode”, Sep. 22, 1995,Power Quality Bremen, Nov. 1995 proceedings, pp. 243-252.
Asea Brown Boveri Jumet S.A.
Brown Glenn W.
Kinberg Robert
Nguyen Vincent Q
Venable
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