Purpose of electrostatic shield
What is the electrostatic shield and why is it actually used? This article briefly explains the purpose of electrostatic shield in VFD input transformers [1].
First note that there are several terms being used for what we called “electrostatic shield” in the title of this article:
– Electrostatic screen
– Electrostatic shield
– EMC shield
– Faraday shield
– High-frequency screen
All of them refer to a shield that is placed in between the high voltage (HV) and low voltage (LV) windings. There may be another shield between the core and the windings to homogenize the magnetic field. However, that is not the scope of this post.
Purpose of electrostatic shield
Purpose of the electrostatic shield is to minimize the (natural) capacitive coupling between HV and LV windings. Why is it actually important? In normal steady state operation there is practically no difference whether the transformer has such a shield or not. Yet, during transient phenomena the situation looks different. Fast transients, for example switching overvoltages, are characterized by oscillations of very high frequency. Such signals would find a low impedance capacitive path to transfer from HV side to LV. It could lead to a dangerous situation with severe overvoltage on the LV side as the capacitive coupling “bypasses” the inductive transformer turn ratio. Hence we want to set a barrier for this parallel path and minimize this kind of voltage transfer. The other reason for such a shield is to reduce the electromagnetic noise generated during VFD operation to propagate into the utility or industrial grid.
Design of the shield
In principle, the electrostatic shield is a special grounded single turn. Several designs of the shield exist and each manufacturer uses his own experience and best practice. Important is to minimize additional losses inside the screen due to magnetic flux components and eddy currents. Few more points are really essential to respect when designing an electrostatic shield for a multi-winding converter transformer.
The shield is grounded, i.e. connected to a defined earth potential. As mentioned, the functionality of the shield is crucial during fast transients. Therefore, the grounding connection shall be properly designed for those high frequencies (hundreds of kHz or even higher).
Shield – needed or not?
Some VFD manufacturers specify the electrostatic screen while other don’t. Obviously, the shield has certain cost impact on the transformer. On the other hand the cost adder is rather low. It may be worth to spend little extra money and increase the robustness of the system. Exceptions may be cases where the transformer turn ratio is limited and consequently the technical risk is generally small.
Summary
Electrostatic shield in transformers installed between primary and secondary windings serves to minimize the capacitive coupling. While it has practically no impact at power frequency the shield becomes really valuable in high frequency range, e.g. during very fast voltage transients. Multi-winding VFD transformers deserve special care related to suitable design of electrostatic screen.
References
[1] VFD transformers – series of articles, https://mb-drive-services.com/category/vfd_transformers/
[2] Transformers – Hitachi ABB Power Grids, https://www.hitachiabb-powergrids.com/offering/product-and-system/transformers
[3] Trasfor – Customized dry type transformers, http://trasfor.com/
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