Electricity: electrical systems and devices – Safety and protection of systems and devices – With tuned circuit
Patent
1997-10-03
1999-10-19
Paladini, Albert W.
Electricity: electrical systems and devices
Safety and protection of systems and devices
With tuned circuit
361 45, H02H 333
Patent
active
059699302
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a protection device of an electrical installation in which a tripping relay is activated when a fault signal in a differential transformer exceeds a preset threshold.
BRIEF DESCRIPTION OF THE PRIOR ART
Trip devices by fault current have been used for many years for protection of machines and people. For protection of people, the tripping current can be about 30 mA, whereas it is in a range of about 300 to 500 mA for protection of machines.
However, during the last few years, more and more electronic devices with current rectifier effect have been incorporated in numerous electrical apparatuses. These rectifier effects may generate a DC component liable to influence the operation of the differential device. The increased use of electronic trip devices, notably in household appliances, also requires the latter to respond in complete safety not only to alternating currents but also to pulsed DC fault currents. The limit values defined for trip devices of this kind have been set by the standard VDE 0664. Trip devices of the previously mentioned kind are known, which meet this specific requirement, wherein the magnetic core of the transformer toroid is made of a crystalline material designed for this use. The main quantities characteristic of these materials are the induction amplitude B for a sinusoidal excitation current, the static induction elevation .DELTA.B stat for a half-wave rectified sinusoidal excitation current, and the dynamic induction elevation .DELTA.B dyn for a full-wave rectified sinusoidal excitation current.
In these trip devices, it is known to fit a capacitor between the transformer secondary winding and the relay tripping winding in order to increase the sensitivity of the trip device by increasing the power at the level of the relay. An oscillating circuit is thus formed by the secondary winding and the capacitor. The resonance frequency of this oscillating circuit then has to be tuned with the frequency of the voltage in the secondary winding due to the fault current. Tuning of this resonant circuit is performed by defining the number of turns of the secondary winding based on the capacitance values prescribed for the capacitor, and the tripping conditions of the trip device are determined. However, the number of turns finally set simply represents a compromise between the different forms of fault current.
European Patent application EP-0,563,606 describes a current transformer for a trip device, enabling safe interruption of a user circuit subjected to pulsed currents to be obtained, tripping being performed in a manner practically independent from the form of the fault current. These results are obtained due to the use of a magnetic core made of a nanocrystalline material presenting the following magnetic characteristics: Br/Bs<0.3, .DELTA.Bdyn>0.6T, for a field intensity amplitude of 100mA/cm, .DELTA.Bdyn max>0.7T and .DELTA.Bdyn/B>0.7, these magnetic cores having been achieved in two stages.
The magnetic quantities presented in this patent are in fact interesting for a trip device setup using a capacitor. The presence of this capacitor and the fact of obtaining .DELTA.Bdyn/B>0.7 enable a more symmetrical current form to be obtained on the secondary. In this case, the polarized electromagnetic trip device operates for two more symmetrical thresholds, which results in a greater ease of setting adjustment when manufacturing. However, although they are advantageous, these features are nevertheless constraining, as they require a particular treatment of the magnetic core materials.
Furthermore, in this patent, the trip device does not enable an efficient differential protection to be obtained for common applications such as graduators which can have very brief opening angles ranging from 135.degree. to 180.degree..
SUMMARY OF THE INVENTION
The present invention overcomes these problems and proposes a differential protection device whose sensitivity to pulsed currents is improved without constraining magnet
REFERENCES:
patent: 3614534 (1971-10-01), Gross
patent: 3657604 (1972-04-01), Willard
patent: 4366520 (1982-12-01), Finke et al.
patent: 4378579 (1983-03-01), Hudson, Jr.
Bonniau Michel
Paupert Marc
Huynh Kim
Paladini Albert W.
Schneider Electric Industries S.A.
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