Design challenge – Small volume vs. voltage withstand in small separation transformers
They provide inductive coupling, i.e. signal or power transmission between different voltage levels, providing the capability to withstand high voltages between the levels. Separation transformers are normally needed in applications such as medical devices or rail.
Generally, the design of a separation transformer consists of a soft magnetic transformer core, at least two windings, housings, and - relevant for these parts - connecting/terminal elements and potting. Efficient transmission requires high coupling factors which causes a close arrangement with short distances between the windings and the core. Components that can withstand high voltages usually require greater distances between conducting parts, i.e. wire to wire and wire to core. This creates an obvious challenge for the designer to balance both requirements.
In terms of design and production processes, we have to separate them into three single tasks:
- a) The body of the component, where different elements made of different materials are assembled and insulated;
- b) The space outside (in the air) of the body including terminals and fixing elements of the component;
- c) The surface of the body which is in contact with the environment.
a) The component’s body – focus on consistent insulation
When designing a separation transformer, you need to allow for a sufficient distance between conducting parts and the right insulation material filling this distance. High voltages create a high electric field in the insulating material. Above a certain field strength, a current will flow through paths of the material which becomes conducting due to chemical deterioration processes. This will cause the component to fail. Hence the insulation layer must be thick enough to keep the field strength well below the threshold field strength under all circumstances. Local field strength, however, is not homogeneous and depends also on the shape of the conducting elements which act as electrodes. For example, the field strength is higher at the sharp edges of conductors.
Any air inclusions in the potting mass or between potting and conducting elements are a risk. Even if the distance between the conduction parts with different voltage levels is filled with sufficient insulation material to withstand the voltage, the field strength within the air inclusions exceeds easily the threshold field strength of air. The air becomes ionised, glowing starts and destroys the potting mass until breakthrough.
There are three potential risks for such air inclusions:
- Air bubbles in the potting mass. Vacuum potting is often mandatory to meet industry standards and certifications. However, the process is inconsistent across the industry. It’s not enough to apply a vacuum after filling in the potting mass. To eliminate air inclusions the potting mass must be filled in under vacuum conditions, and normal or even overpressure applied before the potting mass hardens.
- Remaining voids even when vacuum potting is applied accurately. They appear mostly in close structures, where air can’t be removed completely by vacuum, or vacuum voids are stabilised by the construction. The latter can fill with any gas over time. Therefore, the construction should allow easy removal of air and accessibility of potting mass into each area.
- Losing of the bond between potting mass and construction elements during operation and creation of air layers in between. Potting mass or any glues must be compatible with the other materials in the component, and this materials combination must match the operational conditions like temperature changes, shocks or mechanical load, and vibrations.
The engineer should utilise partial discharge tests as part of the development process to detect glowing as often required as standard tests, in addition to normal high voltage breakthrough tests. This also must be done after the simulation of operational conditions.
Norms define minimum air distances and corresponding test voltages depending on the application, nominal voltage, and safety requirements.
b) Space outside – air distance
At least the connecting elements (terminals) will have contact with air. The threshold field strength of air is much lower than that of insulating material – a big distance (so-called air distance) between the terminals is required. Norms define minimum air distances and corresponding test voltages depending on the application, nominal voltage, and safety requirements. In the end, the air distance gives the minimum dimension of a component for a given application and voltage to separate.
c) The surfaces – creep distance
Component’s surface between contacts may be contaminated with dust, condensed liquids, etc which reduces the threshold voltage below that of air. Therefore, the minimum distance of the shortest path along surfaces between contacts to be separated (so-called creep distance) must be longer than the air distance.
Again, norms define the required creep distance depending on the application and environmental conditions including pollution class and nominal voltage. In terms of component design, the path along the component’s surface between electrical contacts must be maximised, which can be achieved by adding fins. It’s the challenge for the designer to arrange the position of terminals and if necessary, fins or similar in a space-efficient way, fitting the spatial and connecting requirements of the transformer at the same time.
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