Electricity: power supply or regulation systems – Including a transformer or an inductor – Polyphase or plural transformers
Patent
1997-12-18
1999-01-12
Nguyen, Matthew V.
Electricity: power supply or regulation systems
Including a transformer or an inductor
Polyphase or plural transformers
H01F 3012
Patent
active
058595295
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a voltage transformer having increased dielectric strength for use in a low-voltage power switch.
Current transformers that provide the input signals for an overcurrent trip are standard low-voltage power switch equipment. They may, however, have the additional task of measuring the voltage at the location of the power switch, for example for performing a power measurement, or having the capability to display the line voltage.
In general, this task can be performed with the aid of known inductive voltage transformers. The requirement exists, however, that, given the stipulated high-voltage check of a low-voltage power switch with, for example, 3.5 kV, voltage transformers must also be included if the transformers are connected in operation. Inductive voltage transformers dimensioned for this type of high testing voltage, however, represent a costly special design that does not possess the desired small size and low price.
Typical transformers, as used, for example, for supplying current to undervoltage trips (German book `Niederspannungs-Leistungsschalter`, Springer, 1970, pp. 135 to 137) are unsuitable for the present application because of their properties and their size.
A capacitive voltage transformer for a metal-clad high-voltage system is already known; this transformer comprises a capacitive divider and an amplifier circuit having operational amplifiers. This measuring principle cannot be applied in a low-voltage power switch, because no usable measured signal can be obtained here with a capacitive divider.
It is the object of the invention to provide a voltage transformer that has an increased dielectric strength, is small in size and can be produced economically.
In accordance with the invention, this objective is accomplished in that an operational amplifier is disposed downstream of a resistance divider comprising ohmic resistors having the required dielectric strength, with the primary winding of an inductive transformer being connected to the output of the operational amplifier and the secondary winding of the transformer forming the undervoltage side of the voltage transformer. The advantage of this type of voltage transformer over the known inductive voltage transformers is that resistors having the required dielectric strength can be obtained at low cost. The provided inductive transformers are also inexpensive components because they are allocated to the undervoltage side of the voltage transformer and only perform the task of effecting a galvanic separation between the voltage divider and the output side.
Although a voltage transformer of the invention is suitable in principle for all forms of power supply systems, the measurement can be impeded by scattered-in interference voltages if the voltage transformer is operated in a power supply system without neutral conductors and the input circuit formed by the resistors is very highly resistive. In accordance with a further development of the invention, such interferences ("humming") can be rendered ineffective in that, to form an artificial star, the one ends of the secondary windings of the inductive transformers are directly connected to one another, and the other secondary winding ends are connected to one another in each case by way of a resistor, and that an impedance converter is switched in each case between the secondary windings and a measuring device, the artificial star point being connected as a reference potential to the measuring device.
The invention is described in detail below with the aid of the specific embodiment illustrated in the FIGURE.
An ohmic voltage divider comprising a first resistor R1 and a second resistor R2 is connected to each phase conductor of the network L1, L2 and L3. Resistors R1 can have a resistance value of 4 M.OMEGA., for example, while resistors R2 have a resistance value of 40 k.OMEGA.. At the connecting point of resistors R1 and R2, the applied network voltage is thus divided in a ratio of 1:100. Resistors R1 possess a sufficient dielectric strength; a series connection
REFERENCES:
patent: 3840720 (1974-10-01), Wolf
patent: 4980811 (1990-12-01), Suzuji et al.
patent: 5663636 (1997-09-01), Falldin et al.
patent: 5729121 (1998-03-01), Coenders et al.
Baumgartl Ulrich
Rohl Wolfgang
Nguyen Matthew V.
Siemens Aktiengesellschaft
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