PARTNER PROGRESS REPORT

Improvement of low voltage vacuum circuit breaker on the basis of vacuum switching electric arc investigation - LOVARC

Funding: NATO – SfP 974083

Coordinator: University “Politehnica” Bucharest, Romania, Ro Project Co-manager; Prof. Dan Pavelescu

Partners:

INOE 2000, Romania, RO Project co-manager: Dr. Viorel Braic

ICPE SA, Bucharest, Romania, Partner project manager: Eng. Gheorghe Dumitrescu

GREMY University of Orleans, France, Project partner manager: Dr. Claude FLEURIER – Professor

Technische Universitat Braunschweig, Germany, Project manager: Dr. Manfred LINDMAYER – Professor N.P.D.

Electrotehnichal Institute, Warsaw, Poland, Project manager:  Dr. Albert Gmitrzak

End-users:

• Eng. Bogdan NISIPEANU – Manager Electroaparataj S.A.; Bucharest; ROMANIA

• Eng. Tadeusz LEWANDOWSKI – Manager Zakad Aparatury Laczeniowej; Warsaw; POLAND

• Eng. Wojciech SLIWINSKI – Manager Workshop & Switchgear Department of Electrotechnical Institute; Warsaw; POLAND

OBJECTIVE

Circuit breakers are used in large quantities in the knots of electric power systems. Their task is to securely switch off high short-circuit current and to protect cables and electrical equipment from damage.

The introduction in manufacturing of a vacuum low voltage circuit breaker represents a modern solution that will have important positive effects. It means the increase of the exploitation safety for energetic installations, ecological protection, and a very good electromagnetic compatibility behaviour.

The VLVCB is a new type of switch power apparatus which was offered on the market, after year 1992. Until now R&D worldwide has mainly been concentrated on VCB for medium voltage (12 kV to 36 kV).

There are specific features in LV circuit breakers, which have not been investigated thoroughly:

- higher current breaking capability (100 kA against typical values of 30-40 kA) hence higher magnetic forces, higher thermal stress on the contacts;

- shorter gap distance;

- simpler design (e.g. vapor shield connected to one of the contacts instead of the insulated arrangement).

Although the VLVCB has well known incontestable advantages compared to the classical solution, it has also some weak points among which we mention:

- Breaking capability at short-circuit is less than that of the classical circuit breaker, for the same rating parameters (about 50 kA, in comparison with the 100 kA required at least);

- Very high contact pressure force, a fact that requires a drive mechanism of special type, increasing the erosion process of contacts and limiting the lifetime.

Taking into account these elements, we consider that for the development of switching models and VLVCB prototype with high switching performances, scientific and technological research activities have to be carried out in the following fields:

a) Very high current breaking capability

b) UHV very high current contact material

c) UHV quenching chamber components and sub-units

d) VLVCB driving mechanism

Methodology

Taking into account the magnitude of the proposed R&D topics, the LOVARC project will be composed of a number of three sub-projects:

LOVARC I. Study of the high power LV vacuum switching arc

This sub-project has as a main objective the study of the vacuum LVVA in switching regimes, that are characteristic to the operating of VLVCB. This study will be carried out along two directions:

a) The study of LVVA electrical and thermal characteristics

b) The investigation of the discharge plasma characteristics

b1) Investigation of the arc plasma with simultaneous space, time and spectral resolution;

b2) Investigation of the PA region of the LVVA.

System for explosive detection following exposure to a pulsed neutron beam via gamma radiation signature of constituent elements – Pulsed Neutron Chemical Analysis Sensor – PNCAS.

LOVARC II. Quenching chamber of the VLVCB (design, materials and specific technologies)

a) Materials and specific technologies of U.H.V. quenching chamber

a1) Experimental researches on the quality of ceramics (Al₂O₃) produced in the partner countries.

a2) Metal ceramic joints technology

a3) Vacuum brazing of different materials relevant to the quenching chamber manufacturing.

- Joint activity with partner bellows producer (Helium Leak Detector will be provided by INOE).

- Vacuum brazing tests for metallic parts (using mainly Cu - Ag eutectic alloy and Nikel - Bor based alloy).

a4) Design and testing of preliminarily models for the quenching chamber.

a5) Tests of the final version of VLVCB quenching chamber.

b) Study of the influence of contacts composition and geometry.

The activity in this respect, will have the aim to establish the VLVCB quenching chamber composition of contact pieces and their geometry.

Initially, the “classical” composition for this type of contacts, based on Cu - Cr, will be used. After this, other combinations (for example Cu - W) will be analyzed. The influence of various technological parameters (for the elaboration of contact material) will be studied. From these parameters we quote:

• Granulometric distribution of powders;

• Additives influence: Bi, Sb, Te, Zr;

• Temperature and atmosphere of sintering;

• Cooling velocity.

The contact pieces will be characterized according to the following material parameters:

• density;

• hardness;

• electric resistivity;

• gases content;

• microstructures aspect.

The checking of the adopted technological solutions will be carried out through operational tests related to:

• Welding resistance;

• Chopping current;

• Rating current and breaking capability;

• Erosion rate of contacts.

c) Quenching chamber prototype optimization

The VLVCB quenching chamber prototype will be technically and economically optimized. In this field of activity the collaboration between R&D partners and end-users is very important.

c1) Experiments on post-arc current.

As it is shown in literature, in the case of LV circuit breakers, the PA current has a specific evolution, being strongly dependent on the configuration of the switching system, and time evolution of TRV. In the case in which the quenching chamber shield is connected to one of the electrodes, interesting polarity effects are produced, which can influence breaking capability of the respective configuration. As it was mentioned before, the research team from Romania, has a measurement modern system of post-arc current, based on coaxial shunts with active resistance, which allows the detection of some PA currents with a peak value of 10 mA. Accordingly, the study of PA current in the case of various VLVCB models of quenching chamber, can offer an important criterion of improving its performances.

c2) Experiments on the influence of axial and radial magnetic field.

For a certain value of the breaking current, this study will be carried out taking into account the presence of an axial or radial magnetic field. The magnetic field will be produced by the breaking current by an internal or external “coil effect” in the case of the axial magnetic field, or by a loop effect in the case of the radial magnetic field.

The study of quenching chamber models will be the base to elaborate quenching chamber optimized prototype, fundamental element in the structure of the VLVCB. The prototypes will be elaborated and tested jointly in the partner countries, thus a complete objectivity in the interpretation of the results will be assured.

d) Checks and specific tests on UHV quenching chamber

- Tightness tests will be carried out as well as vacuum level measuring, at various time intervals, to determine the “losing rate of vacuum”. Depending on these results, conclusions concerning quenching chamber reliability, will be made.

- The contact voltage drop will be measured, in the nominal regime, and its dependence on contact pressure force, will be determined.

- Checking tests for electric parts heating will be carried out, depending on contact pressure force.

- Specific dielectric tests will be performed, according to the international standards.

- Tests of mechanical lifetime will be done, at admissible maximum stroke of contacts, mainly to check the quenching chamber bellows.

One of the basic tests refers to the checking of quenching chamber breaking capability, in the short-circuit operating regime.

II. Contribution of INOE 2000

Vacuum brazing of quenching chamber for low voltage high current interrupters

High-vacuum as brazing atmosphere for alumina – copper (oxygen free) parts prevents metal oxidation and provides a good permanent bond between the ceramic and the metal.

The vacuum brazing method is compulsory because at the end a chamber with a clean high vacuum inside (approx. 10^-4 Pa) is obtained.

Quenching Chamber Production