EP2372725B1

EP2372725B1 - Production of electrical insulation and insulated products

Info

Publication number: EP2372725B1
Application number: EP10157948.0A
Authority: EP
Inventors: Ho Chau Hon; Cherif Ghoul; Lars E. Schmidt; Marco Schneider; Willi Gerig
Original assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Current assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Priority date: 2010-03-26
Filing date: 2010-03-26
Publication date: 2013-09-11
Anticipated expiration: 2030-03-26
Also published as: KR20110108279A; EP2372725A1; CN102254648B; KR101252593B1; CN102254648A

Description

TECHNICAL FIELD

The invention relates to electrical insulation products in general and to high or medium voltage products in particular. The present document concerns particularities in the manufacturing process of insulated products, such as barrier insulators, for example.
The present invention is based on a method for manufacturing an electrical insulator and to an electrical insulator manufactured by the method according to the independent claims.

BACKGROUND OF THE INVENTION

In the electro equipment industry thermosetting insulation material is used e.g. for producing dry type distribution transformers, medium voltage (MV) embedded poles and other insulation. The mixing of highly filled insulation material takes considerable time, often above 1 hour. The standard process includes mixing of resin with filler and hardener with filler separately in two pots equipped with an impeller mixer, in order to prevent premature reaction. After good filler dispersion is achieved in the two components, they are simultaneously forced through a so-called static mixer and injected into a mould.
The moulding principle is either traditional vacuum casting or automatic pressure gelation; a process which can also be vacuum assisted.
Several reliable mixing systems for producing the pourable compound are commercially available, e.g. from Hedrich Vacuum Systems and Huebers Verfahrenstechnik.

SUMMARY OF THE INVENTION

It may be seen as an object of the invention to provide an improved and efficient method for producing electrical insulators.
This object is achieved by accelerating the production process by employing a different mixing process for treating the insulation material. Instead of using a conventional system containing separate vessels for resin and hardener equipped with an impeller mixer, the inventive system is based on a fast rotating conveying screw.
According to an embodiment of the invention a method for manufacturing an electrical insulator for medium or high voltage equipment is provided, the method comprising the steps of forcing all liquid components of an insulating material of the electrical insulator through a static mixer, adding at least one filler to the components, forcing the components with the at least one filler through a tube comprising a fast-turning screw mixing device mixing the components with the at least one filler to a final mixture of the insulating material, leading the final mixture to a mould through an outlet of the mixing device that is fluidly connected to the mould, forming an electrical insulator by at least partially curing the final mixture, and moulding the electrical insulator.
According to another embodiment of the invention a method for manufacturing an electrical insulator for medium or high voltage equipment is provided, the method comprising the steps of forcing all liquid components of an insulating material of the electrical insulator through a static mixer, adding at least one filler to the components, forcing the components with the at least one filler through a tube comprising a fast-turning screw mixing device mixing the components with the at least one filler to a final mixture of the insulating material, leading the final mixture to a mould for vacuum casting, the mould being located in a vacuum chamber the inlet of the vacuum chamber connected to an outlet of the mixing device that is fluidly connected to the mould, forming an electrical insulator by at least partly curing the final mixture, and moulding the electrical insulator.
The curing described above may be part of the moulding According to another exemplary embodiment a method according to the above and below mentioned embodiments is provided, wherein at least two fillers are added to the components, and wherein a mixing proportion of the fillers of the insulating material follow a Fuller sieve curve with a Fuller distribution of $P = {(\frac{d}{D})}^{n},$

wherein d is the particle size of the fillers, D is the maximum particle size, being for example 300µm, P the ratio of particles smaller or equal to d, and n is the grading coefficient, being possibly 0,37 or 0,5.
The Fuller sieve curve describes the optimized filler composite of a mixture providing optimized characteristics of the mixture such as an optimized strength and porosity or cavity of the mixture.
By optimizing the composition of the two fillers according to the above mentioned Fuller distribution, the compactness of the manufactured insulator may be optimized providing an ideal relation between the fillers and the components/the matrix.
The components may comprise a hardener and a resin.
Instead of using a batch mixer, a continuous mixer with very high shear is used. The high shear reduces mixing time considerably compared to a conventional impeller mixer that mixes the dispersion of filler in resin and hardener typically with a low shear resulting in a rather lengthy procedure.
Insulating materials may be standard filled epoxy, for example with one filler size, typically silica, with a filler content of around 65 wt.-%, corresponding to around 44 vol.-%. The insulating material my comprise polymer concrete.
According to another embodiment of the invention at least two fillers may be used, with a filler content between 70 and 96 wt.-%, corresponding to around 49 to 91 vol.-%, and typically between 83 and 85 wt.-%, corresponding to around 60 to 70 vol.-%). Polymer concrete may be used for each of the fillers.
The time required for mixing a dispersion of filler in resin and hardener by applying a higher shear may be reduced by at least a factor 10 compared to prior art devices such as impeller mixers because the mixing efficiency is significantly increased. Filler, resin, and hardener may be seen as components of an insulating material of an electrical insulator.
According to an embodiment of the invention a method for manufacturing an insulator for medium or high voltage equipment is provided, wherein the material is forced through a tube, such as a cylindrical tube, with a fast turning screw inside, which causes high shear, resulting in a good dispersion of filler in a matrix of the material. The components pass the mixing screw in less than one minute, including a thin-film degassing unit. The high shear forces produced by the system allow for a higher filler content, which can be reached by applying more than one filler.
Moreover, the inventive method contributes to the following advantages:

significantly reduced mixing times and hence increased throughput in production
reduced risk of sedimentation of filler particles and better dispersion
higher filler content and hence reduced cost (up to 85 wt.- % of silica was experimentally verified, a higher solid content would be possible by using a larger number of different fillers and a larger maximum filler grain size)
lower material losses due to the relatively low volume of the confined mixing channel
possibly less abrasion: the mixing screw is made from a special, abrasion resistant alloy, so even abrasive fillers, such as alumina, can be processed with it

Further embodiments, advantages and applications of the invention will become apparent from the claims or claim combinations, from the following detailed description, and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings, which are schematically showed in

Fig. 1: optical micrograph of a first sample of a polymer concrete blended with a conventional impeller type mixer;
Fig. 2: optical micrograph of a second sample of a polymer concrete blended with a conventional impeller type mixer;
Fig. 3: overview of an optical micrograph of a sample of an inventive polymer concrete blended with a continuous screw mixer according to an exemplary embodiment of the invention (scanning electron micrograph);
Fig. 4: close-up view IV of the scanning electron micrograph shown in figure 3 according to an exemplary embodiment of the invention;
Fig. 5: a schematic diagram of a Fuller sieve curve with examples of different fillers;
Fig. 6A: a flow chart of a method for manufacturing an electrical insulator for medium or high voltage equipment according to an exemplary embodiment of the invention; and
Fig. 6B: a flow chart of a method for manufacturing an electrical insulator for medium or high voltage equipment according to another exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The difference of the technical effect of the present invention will become clear when comparing micrographs of the polymer concrete produced with the method according to the invention and micrographs of the polymer concrete produced with conventional production methods employing an impeller-type mixing device.
When focusing on figure 1 displaying an optical micrograph of a first sample of a polymer concrete blended with a conventional impeller mixer, essentially three distinguishable areas can be identified. Reference 1 denotes a large silica filler particle with a typical average diameter of 0.3 mm whereas 2 denotes a void and 3 denotes a an area with small silica filler particles with a typical average diameter of 0.016 mm, and an epoxy matrix. Especially the particles 1 and voids 2 have rather large dimensions.
The large void dimensions might lead to partial discharge if the insulation material is subjected to high electric fields which might cause degradation of the material during lifetime of the insulated products.
Most commonly, the insulation formulation consists of epoxy resin, hardener, catalyst, filler, and other liquid or solid additives. A typical filler content may be 65 wt.-%. Likewise the situation in figure 2 displaying an optical micrograph of a second sample of a polymer concrete produced by blending of the constituents by conventional production methods employing an impeller-type mixing device. The difference of the second sample compared to the first sample shown in figure 1 resides in that the void content could be greatly reduced by applying a combination of vacuum degassing and vibration. Again, essentially three distinguishable areas can be identified.
Reference 4 denotes a resin-rich area with very low filler content whereas 5 denotes a large filler particle (comparable to reference 1 in figure 1) and 6 denotes an area with small silica filler particles and epoxy matrix (comparable to reference 3 in figure 1). Especially the particles 4 and 5 have rather large dimensions. Again the inhomogeneous compound might undergo a long-term degradation if subjected to a high electric field.
A scanning electron micrograph of a first sample of a polymer concrete blended with the inventive production method according to the present invention is shown in figure 3. Although the scale of the micrograph is 500 µm (Micrometers) the comparatively large silica particles 1 are easily identified (comparable to reference 1 in figure 1). Reference 3 denotes an area with small silica filler particles and epoxy matrix (comparable to reference 3 in figure 1). Moreover a region pointed out by an arrow IV is contained in figure 3. Said region IV is a close-up of reference 3 and will be explained by figure 4.
As to the constituents required for producing this polymer concrete reference is made to example 5 discussed later in details. The step of forcing the mixture through a cylinder comprising a fast-turning screw mixing device to produce a final mixture causes a Shear modulus (Young's modulus) in flexion being in a range of about 20 to about 30 GPa and a Young's modulus in compression of about 30 to about 40 GPa.
The close-up of region IV shown in figure 4 proofs that region 3 comprises a comparatively large amount of small filler particles in the range of 0.004 mm diameter 7 surround larger particles in the range of 0.016 mm diameter 8 and particles in the range of 0.06 mm diameter 9. The high shear applied to the mixture during the mixing step causes the homogeneous distribution of particles. During this step the mixture is forced through a cylinder comprising a continuous fast-turning screw mixing device in order to produce a final mixture and a thin-film degassing unit leading to a material with very low void content. The dark portions 10 of figure 4 denote the epoxy matrix.
Fig. 5 schematically shows a Fuller sieve curve. The Fuller sieve curve of Fig. 5 describes the ratio of fillers of different sizes for obtaining a densely packed concrete as follows: $P = {(\frac{d}{D})}^{n}$

d is the particle size, D is the maximum particle size, being 300µm according to Fig. 5, P the ratio of particles smaller or equal to d, and n is the grading coefficient. For round particles n is usually set to 0.5 and for crushed particle to 0.37 (Fuller W.B., Thompson S.E., The laws of proportioning concrete, Transactions of the American Society of Civil Engineers. Paper no 1053, 1907, pp 67-143). 600 EST, W12 EST, W3, and Sihelco 30 are different fillers with different particle ratios.
The Fuller sieve curve describes the optimized filler composite of a mixture providing optimized characteristics of the mixture such as an optimized strength and porosity or cavity of the mixture.
Fig. 6A shows a flow chart of a method 100 for manufacturing an electrical insulator for medium or high voltage equipment comprising the steps of forcing all liquid components of an insulating material of the electrical insulator through a static mixer 101, adding at least one filler to the components 102, forcing the components with the at least one filler through a tube comprising a fast-turning screw mixing device mixing the components with the at least one filler to a final mixture of the insulating material 103, leading the final mixture to a mould through an outlet of the mixing device that is fluidly connected to the mould 104a, forming an electrical insulator by at least partially curing the final mixture 105, moulding the electrical insulator 106, degassing the final mixture by a degassing unit 107, and post-curing the final mixture at least partly outside the mould 108.
Fig. 6B shows a flow chart of a method 100 for manufacturing an electrical insulator for medium or high voltage equipment comprising the steps of forcing all liquid components of an insulating material of the electrical insulator through a static mixer 101, adding at least one filler to the components 102, forcing the components with the at least one filler through a tube comprising a fast-turning screw mixing device mixing the components with the at least one filler to a final mixture of the insulating material 103, leading the final mixture to a mould for vacuum casting, the mould being located in a vacuum chamber the inlet of the vacuum chamber connected to an outlet of the mixing device that is fluidly connected to the mould 104b, forming an electrical insulator by at least partly curing the final mixture 105, moulding the electrical insulator 106, degassing the final mixture by a degassing unit 107, and post-curing the final mixture at least partly outside the mould 108.

WAYS FOR CARRYING OUT THE INVENTION

Several ways of possible embodiments of the inventive method are disclosed and discussed hereinafter by means of examples.

Example 1 (Aromatic epoxy composition)

The epoxy resin compositions are made from components as given in Table 1. The components were mixed with different mixing devices (2 different production-size impeller mixers, and a production-size continuous screw mixer). The mixing times were recorded and about 4 mm thick quadratic plates (150 x 150 mm) were cast and cured for about 8 hours at about 80 °C and post-cured for about 4 h at about 140 °C. Samples for mechanical testing were machined out of these plates and tested according to the standards given in Table 2. Five samples were tested per listed property.
The production-size impeller mixers comprised two separate mixing containers equipped with an impeller for epoxy resin and anhydride hardener. In these containers the filler is dispersed into resin and hardener respectively. These two components are forced thereafter through a static mixer.
In the continuous screw mixer the epoxy composition is prepared by volumetric dosage of the liquid components through a static mixer. Thereafter the fillers are added and dispersed by forcing all components through a cylindrical tube equipped with a fast turning screw, also containing a thin-film degassing unit. Table 1 Raw material formulation for an aromatic epoxy composition (ingredients are given in phr: parts per hundred of epoxy resin)

Type Trade name Producer phr

Epoxy resin Epikote EPR 845 Hexion (DE) 100

Anhydride hardener Epikure EPH05389 Hexion (DE) 84

Silica fil-ler W12 Quarzwerke (DE) 320
Comparing impeller mixer A and B it was observed that impeller mixer A led to better results after about 240 min of mixing compared to impeller mixer B after about 480 min of mixing. A slight increase in mechanical properties with mixing time was observed for impeller mixer B.

As seen in Table 2, the continuous screw mixer leads to superior mechanical properties compared to both impeller mixers. Even with a very long mixing time of about 480 min, the mechanical properties of the continuous screw mixer can not be reached. Mechanical properties are a function of filler dispersion in the matrix material, which is again dependent on the mixing efficiency. The mixing efficiency depends on both mixing time and mixing geometry. The continuous screw mixer creates a very high shear, compared to the impeller mixers and hence, even the very short mixing time, which is in the range of about 1 minute, leads to good dispersion.

Table 2 Mechanical testing results of aromatic epoxy processed with different mixing devices:

Property	Standard	Impeller mixer A	Impeller mixer B	Impeller mixer B	Impeller mixer B	Continuous screw mixer
Mixing time (min)		240	15	30	480	1
Young's modulus in flexion (GPa)	ISO 178	10.9 ± 0.1	10.8 ± 0.1	11.1 ± 0.2	10.9 ± 0.2	11.4 ± 0.3
Flexural strength (MPa)	ISO 178	144 ± 4	122 ± 6	123 ± 9	127 ± 7	159 ± 3
Elongation at break (%)	ISO 178	1.57 ± 0.08	1.21 ± 0.07	1.20 ± 0.11	1.29 ± 0.11	1.70 ± 0.05

Examples 2 to 4 (Manufacturing of insulators from filled cycloaliphatic epoxy and polymer concrete)

The epoxy resin compositions (Table 3) are made from components as given in Table 4.

The epoxy composition is prepared by volumetric dosage of the liquid components through a static mixer. Thereafter the fillers are added and dispersed by forcing all components through a cylindrical tube containing a fast turning screw, also containing a thin-film degassing unit. The resting time of the material in the screw is in the range of minutes, typically below 1 minute. The exit of the mixing tube is directly connected by a hose to a heated steel mould mounted on a hot-press. The steel mould was for an medium voltage outdoor insulator. The mould on the hot-press is at a temperature of 125 °C. After injection and further two hours of curing the part is demoulded.

Table 3 Example compositions (ingredients are given in phr: parts per hundred of epoxy resin)

Ingredient	Example 2	Example 3	Example 4
Araldite CY184	100	100	100
Aradur HY1235	90	90	90
DY062	0.54	0.54	0.54
Silbond W12 EST	354	354	354
Sihelco 30		576	726
Filler content (wt.-%)	65	83	85

Table 4 Raw materials for cycloaliphatic epoxy compositions.

Type	Trade name	Producer
Cycloaliphatic epoxy resin	Araldite CY184	Huntsman Advanced Materials (CH)
Anhydride hardener	Aradur HY1235	Huntsman Advanced Materials (CH)
Accelerator	DY062	Huntsman Advanced Materials (CH)
Silica flour	Silbond W12 EST	Quarzwerke (DE)
Silica sand	Sihelco	30	Sihelco (CH)

Example 5 (Homogeneous mixture of highly filled polymer concrete)

The polymer concrete composition (Table 5) is made from components as given in Table 6. The four silica fillers of different sizes, were selected following a Fuller sieve curve to obtain dense filler packing. This leads to improved mechanical properties, reduced risk of sedimentation, reduced material cost, and increased thermal conductivity.

This example evaluates the filler dispersion for two dispersing methods: a lab-scale impeller mixer and a production-size continuous screw mixer. For the lab-scale impeller mixer, the components were mixed until a homogeneous mixture was obtained (typically 30 - 60 min). The mix was degassed thereafter at 5 mbar and cast into plate moulds giving 6 mm thick plates. The plates were cured for 2 h at 90 °C and 10 h at 140 °C. The mixing process for the continuous screw mixer was as described in examples 2-4. Small samples were cut out of the plates and prepared for microscopy. The samples were characterized with optical and scanning electron microscopy.

Table 5 Example composition (ingredients are given in phr: parts per hundred of epoxy resin)

Ingredient	Example 5
Araldite CY184	100
Aradur HY1102	90
DY070	0.3
Silbond 600 EST	66
Silbond W12 EST	131
Millisil W3	312
Sihelco 30	569
Filler content (wt.-%)	65

Table 6 Raw materials for cycloaliphatic epoxy compositions:

Type	Trade name	Producer
Cycloaliphatic epoxy resin	Araldite CY184	Huntsman Advanced Materials (CH)
Anhydride hardener	Aradur HY1135	Huntsman Advanced Materials (CH)
Accelerator	DY070	Huntsman Advanced Materials (CH)
Silica flour	Millisil W3	Quarzwerke (DE)
Silica flour	Silbond W12 EST	Quarzwerke (DE)
Silica flour	600 EST	Quarzwerke (DE)
Silica sand	Sihelco	30	Sihelco (CH)

Claims

Method (100) for manufacturing an electrical insulator for medium or high voltage equipment, the method (100) comprising the steps of:
forcing all liquid components of an insulating material of the electrical insulator through a static mixer (101);

adding at least one filler to the components (102);

forcing the components with the at least one filler through a tube comprising a fast-turning screw mixing device mixing the components with the at least one filler to a final mixture of the insulating material (103);

leading the final mixture to a mould through an outlet of the mixing device that is fluidly connected to the mould (104a);

forming an electrical insulator by at least partially curing the final mixture (105); and

moulding the electrical insulator (106).
Method (100) for manufacturing an electrical insulator for medium or high voltage equipment, the method (100) comprising the steps of:
forcing all liquid components of an insulating material of the electrical insulator through a static mixer (101);

adding at least one filler to the components (102);

forcing the components with the at least one filler through a tube comprising a fast-turning screw mixing device mixing the components with the at least one filler to a final mixture of the insulating material (103);

leading the final mixture to a mould for vacuum casting, the mould being located in a vacuum chamber the inlet of the vacuum chamber connected to an outlet of the mixing device that is fluidly connected to the mould (104b);

forming an electrical insulator by at least partly curing the final mixture (105); and

moulding the electrical insulator (106).
The method (100) according to claim 1, wherein the mould is part of a vacuum chamber.
The method (100) according to claim 1 or 2, further comprising the step of:
degassing the final mixture by a degassing unit (107);

wherein the fast-turning screw mixing device comprises the degassing unit.
The method (100) according to claim 1 or 2, wherein the curing is performed at least partly in the mould.
The method (100) according to claim 1 or 2, wherein the curing is performed at fully in the mould.
The method (100) according to anyone of claims 1 to 6, further comprising the step of:
post-curing the final mixture at least partly outside the mould (108), preferably in a forced convection oven.
The method (100) according to anyone of claims 1 to 7, wherein at least two fillers are added to the components; and
wherein the fillers are selected such that a mixing proportion of the fillers of the insulating material follows a Fuller sieve curve
with a Fuller distribution of $P = {(\frac{d}{D})}^{n}$

wherein d is the particle size of the fillers, D is the maximum particle size, P the ratio of particles smaller or equal to d, and n is the grading coefficient.
The method (100) according to anyone of claims 1 to 8, wherein the step of forcing the mixture through the tube comprises a fast-turning screw mixing device mixing the components with the at least one filler to a final mixture causing a Young's modulus in flexion being in a range of about 2 to about 40 GPa, preferably in a range of about 10 to about 30 GPa.
The method (100) of any one of claims 1 to 9, wherein the volume of the at least two fillers comprises at least 49 percent of the volume of the insulating material of the electrical insulator.
The method (100) according to any one of claims 8 and 9 to 10, as far as they depend on claim 8, wherein the proportion of the fillers is determined according to the grading coefficient n being 0,5.
The method (100) according to any one of claims 8 and 9 to 10, as far as they depend on claim 8, wherein the proportion of the fillers is determined according to the grading coefficient n being 0,37.