Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-06-09
2001-04-10
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
Reexamination Certificate
active
06215677
ABSTRACT:
TECHNICAL FIELD
The present invention relates to current sensing in an AC/DC converter provided with an artificial neutral point.
BACKGROUND OF THE INVENTION AND PRIOR ART
In power supply systems there is often a need to convert a multi-phase voltage from the mains into a unidirectional current for supplying different kinds of electronic equipment, such as telecommunication equipment.
In such multi-phase AC/DC converters there is usually provided a number of single phase converters or single phase modules (SPM) in order to output a rectified current. However in such AC/DC converters it is sometimes not possible to access the neutral wire of the mains, which can be required for control purposes of the AC/DC converter. Thus, in order to make a multi-phase AC/DC converter operate safely, having control of the input voltage of each SPM and at the same time generate a correct output signal, the control mechanism of the AC/DC converter requires access to a neutral point. Thus, in order to make a multi-phase AC/DCconverter having SPM:s connected in a star connection operate safely, the common point of the SPM:s have to be connected to a stable neutral point.
In the case when there is no access to a neutral point from the neutral wire of the mains, an artificial neutral point (ANP) can be created by means of using three small transformers having their secondary sides connected in a delta configuration.
Such a circuit is described in the international patent application PCT/SE99/00744, which is incorporated herein by reference.
A problem when using the circuit as described in the international patent application PCT/SE99/00744 is that if there is an asymmetry in the supply voltage from the mains, i.e. that the sum of the phase voltages in the mains does not become zero, there will be a current in the circuit forming the artificial neutral point, which in turn will increase the power loss in the artificial neutral point of the converter. Such a power loss is of course a disadvantage. A current in the artificial neutral point may also origin from different input power to different SPM:s, the SPMs may have different efficiency or there may be an asymmetry in the supply voltage.
A circuit can be provided in order to minimize the power loss. Such a circuit is described in the international patent application PCT/SE99/00743, which also is incorporated herein by reference. Thus, in order to reduce the current in the artificial point circuit, a control circuit which measures the current in the delta configured loop of the artificial neutral point (ANP) is provided.
In
FIG. 1
a diagram illustrating a 3-phase AC/DC converter as described in the international patent applications PCT/SE99/00743 and PCT/SE99/00744 is shown. Thus, the converter comprises a unit for deriving an artificial neutral point (ANP)
109
connected to the mains R,S,T, which supplies a signal indicative of asymmetries in the mains R,S,T to a control circuit
110
. The control circuit controls the single phase converters or single phase modules (SPM)
103
,
105
and
107
according to an algorithm designed to eliminate or minimize any current in the artificial neutral point.
However, in order to operate properly the control circuit
110
requires a signal indicative of the current on the secondary side of the ANP. A straight forward solution is to provide a current sensing unit directly in the neutral wire. Another possible solution is to detect the current in the loop on the secondary side in the ANP. Such solutions have however turned out to be costly and a simpler and cheaper method of providing a signal indicative of the current in the loop of the secondary side is therefore desired.
SUMMARY
It is an object of the present invention to provide a method and an apparatus by means of which a signal indicative of the artificial point current easily can be obtained.
This object is obtained by a voltage sensing circuit provided in connection with the artificial neutral point loop of the AC/DC converter. The voltage sensing circuit comprises a multitude , e.g. three, of small windings connected in a star configuration to a first common point and to three impedances.
In the circuit, the voltage between the first common point of the windings and a second common point of the impedances is measured. The voltage is indicative of the current flowing in the artificial neutral point using the resistance of the short-circuited secondary wire as a shunt.
By using such a circuit the current sensing in the neutral point can be simplified and the multi-phase AC/DC converter manufactured at a lower cost. Also the efficiency of the AC/DC converter will be improved.
REFERENCES:
patent: 3753070 (1973-08-01), Bunger et al.
patent: 3818307 (1974-06-01), Hamilton et al.
patent: 4206490 (1980-06-01), Parrier et al.
patent: 5191518 (1993-03-01), Recker et al.
patent: 5825162 (1998-10-01), Kida et al.
Karlsson Mats
Thoren Christer
Berhane Adolf Deneke
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
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