The invention relates to a medium voltage switchgear with frame, wherein a three phase arrangement of pole parts with vacuum interrupters inside circuit breakers are fixed at one end on a support or a frame element, which is arranged in a switchgear panel, wherein the open ends of the pole parts, opposite to the fixed ends of the pole parts fixed on the support or frame, are among themselves mechanically joined or connected by an additional joining element, and near to the aforesaid open ends of the pole parts, one electrical terminal per pole part is placed, according to the preamble of claim 1 .

The invention is based on medium voltage vacuum circuit breakers for switchgears and vacuum circuit breaker "stand-alone". In general such medium voltage vacuum circuit breakers mainly consist of a drive mechanism and electric poles. Vacuum interrupters are installed within the poles or pole parts. The drive is connected to the vacuum interrupters via pushrods which drive the mechanical movement of the switching contacts inside the vacuum interrupters. The poles provide the mechanical support to the vacuum interrupters. The poles are fixed to the circuit breaker structure and therewith to the gas-insulated switchgear panel or inside air-insulated environment.

An assembled pole parts are disclosed in EP 2 720 245 A1 . This document discloses a vacuum interrupter which is arranged or mounted between two halfshells made of insulating material. In three phase arrangements, three pole parts are arranged in parallel. Short circuits cause a high mechanical impact to the positioned pole parts, so that joining elements are used to mechanically connect such pole parts in such arrangements.

Additionally to this mechanical requirement, the pole-design has to withstand also the dielectric and thermal stress during service and testing conditions.

In respect of dielectric stress the insulation parts of electric poles must provide sufficient electric creepage distance on electrically stressed paths and sufficient high electric resistivity. Furthermore the design should avoid thin gas gaps between insulating and electrically stressed parts where an accumulation of the electric field appears.

In respect of thermal stress the insulating parts of electric poles must withstand the ambient temperature in the circuit breaker compartment and the

temperature of conducting parts with which they are in contact. Mechanical and dielectric properties of the insulating parts of the poles must not change inappropriately.

In respect of mechanical stress the pole design and especially the mechanically supporting parts of the poles must withstand the mechanical stress during switching of the circuit breaker and the electromagnetic forces during short circuit current application like short time current or short circuit current interruption operation.

Mainly two different embodiments of electric poles for medium-voltage vacuum circuit breakers are known. I.e. embedded pole parts, the vacuum interrupter and connecting parts are embedded in insulating material like thermosets, bulk moulded compounds (BMC) and thermoplastic material, and discrete structures, assembled pole parts in which mechanically supporting elements, electrically connecting elements, insulating elements and the vacuum interrupter are glued, screwed or snapped together. In assembled poles the mechanical support for the vacuum interrupter is known to be made from insulating threaded rods, insulating plates or insulating half-shells. In order to assure additional mechanical stability between the poles simple mechanical cross beams are known to be fixed across the poles.

As a result from well known pole designs, disadvantages for such known arrangements are at first the mechanical instability of electric poles during short circuit current, which can cause further damage to the switchgear.

Furthermore flashovers can occur between electric poles at narrow pole- distance.

Furthermore, known constructions for preventing the aforesaid negative consequences result in constructive big structures for high power ratings.

According to that, it is the object of the invention, to prevent the aforesaid functionally bad consequences with constructionally compact elements, in such, that the function of mechanical reinforcement is coupled with the enhancement of dielectric withstand, with easy structural features.

This is solved by the features of claim 1 .

Further advantageous embodiments of the invention is mentioned in the depending claims.

The invention proposes, that the joining element is made of insulating material and tightly fixed among the open ends of the pole parts in such, that they mechanically interconnect the pole parts and additionally increases the dielectrical withstand between the terminals of the pole parts from each other. Thermal conductivity can be provided to enable the part as a heat sink.

So as a result, the joining element made of insulating material, which is fixed tightly over the open ends of the pole parts, reinforces the mechanical stiffness of the pole part arrangement, and simultaneously results in an increase of the dielectrical withstand between the electrical terminals of the pole parts in this region.

So, by using this joining element, positioned at the defined place, mechanical reinforcement as well as increase of dielectrical withstand is enhanced by only one element.

In a special embodiement the joining element is at least partly covered by a conductive surface deposition in order to compensate electrical surface charging.

In a further advantageous embodiment, the joining element is a plate.

Alternatively and higly advantageous the joining element is reinforced at least at one surface by a crossbeam structure.

In a further advantageous embodiment, the joining element is provided with sealing strips or paths or regions between each pole part, at that side of the joining element, which is directly fixed on the pole parts.

In a further advantageous embodiment, the joining element is fixed commonly at the open ends of the pole parts via insulating screws, screwed into female threads, which are integrated or implemented in the insulating half shells or embedding cover of the pole parts.

In a further advantageous embodiment, the insulating half shells or the insulating embedding cover of the vacuum interrupters are made of

thermoplastic, BMC or duroplastic material.

In a further advantageous embodiment, the joining element is fixed commonly at the open ends of the pole parts via high strength insulating or metallic screws, at least at the resulting four corners of the joining element, screwed into female screw threads, which are integrated or implemented in the insulating half shells or resin of the pole parts.

In a further advantageous and final embodiment, the insulating half shells or the insulating material for embedding the vacuum interrupters are made of thermoplastic material.

The invention can be realized in an advantageous way in so called assembled pole parts, in which the insulation cover of the vacuum interrupters consists of assembled half shells, like is is shown in figure 1 .

But this is not the only possible embodiment for the invention. It is also applicable for pole parts partly or fully embedded vacuum interrupters.

So an electrical and mechanical joining and therefore reinforceing element 1 is for the medium voltage switchgear the basical part of the invention. This joining element is in one embodiment designed as or provided with a cross beam structure 3 on the opposite surface of the plate fixed tightly among the electric poles or pole parts 2. It is fixed to the ends of the poles opposite to the drive.

It significantly decreases the mechanical and electrical stress on and between the poles.

The joining element 1 is provided with openings 3, in which counter- or fixation- discs or -plates 4 are introduced, in order to fix the joining element with each of the pole parts 2, like shown in figure 3.

The complete reinforcement element including the mechanically reinforcing cross beam structure is made from thermoplastic material and is produced in a single injection molding process. The joining element is designed to be fixed on poles which mechanical support structure is realized by thermoplastic half- shells. The fixation of the joining element to the end of the pole half-shells is done by e.g. high-strength, insulating screws and/or some overlapping areas to fix or take the forces from the half shell to the support structure. The transition between the end-faces of the pole half-shells and the joining element is sealed by an elastic two compound sealing material. This sealing is already fixed to the joining element during its molding process. It is important, that the sealing is tight, in dielectric sense.

Above mentioned design of the reinforcement element prevents from flash- overs between conducting parts of the poles at high voltage levels and narrow pole distances.

Secondly it improves the mechanical stability during fault current by coupling the ends of the poles mechanically and reducing the risk of cracking of the pole supports or of the poles themselves.

A further alternative embodiment of the joining element may be provided with a low conductive suface finish in order to allow the equalization of accumulated electric charges between the insulating pole supports.

A further embodiment of the reinforcement element may be fixed to the pole half-shells by self-tapping screws.

Figure 2 shows the joining element 1 from the side, which comes into tight mechanical contact to the ends of the pole parts 2.

The joining element is provided at that side with sealing strips 4, in order to result in good dieletric withstand. Therefore sealing elements were extruded on the joining elements surface. So far, also sealing elements can be used which are positioned in grooves on the surface of the joining element. Furthermore glueing is possible.