نویزهای مد مشترک، اغتشاشاتی هستند که به یک اندازه عناصر فعال مدار را تحت تأثیر قرار می دهند. برای مثال در تقویت کننده های تفاضلی، نویزهای مشابه اعمالی به هر دو ورودی، در خروجی تقویت کننده ظاهر نمی شوند زیرا ضریب تقویت ذاتی این نوع نوع تقویت کننده برای سیگنالهای ورودی غیر هم فاز مقداری زیاد و برای سیگنالهای ورودی هم فاز و هم اندازه ( مفهوم مشترک) در حدود صفر است. همین مضمون در ترانسهای ایزوله مجهیز به شیلد زمین شده مابین اولیه و ثانویه نیز قابل اطلاق است. در واقع در این نوع ترانسها نویزها فرکانس بالای اعمالی به دو سمت ترانس از طریق خازنهای پراکنده شیلد به زمین اتصال کوتاه و از فرآیند انتقال ترانس حذف می شوند. در این ارتباط می توانید به IEEE 1100 مراجعه کنید.
22.214.171.124 Inter winding electrostatic shielding (transformers)
A solidly grounded bypass capacitor that creates a capacitive voltage divider and current shunt can be introduced into the inter winding capacitance between the primary and secondary in a transformer by adding a metal foil between the windings, and then by suitably bonding it in low-inductance fashion to equipment ground within the transformer (see Figure 4-38 and Lewis [B38>). This has three major effects, as follows:
a) Inter winding short circuits are largely prevented due to the introduction of a solidly grounded fault current path as provided by the electrostatic shield (see Figure 4-40).
b) High-frequency currents in the common mode are capacitively shunted into the grounding system in bidirectional fashion from either the primary or the secondary circuits (see Figure 4-39).
c) The capacitive voltage divider action reduces the available noise voltage to be coupled capacitively between the two windings (see Figure 4-39).
The benefits from effect a) are obvious, but the conditions in effects b) and c) produce mixed results. For example, the capacitive shunting action beneficially reduces the amount of common-mode current coupled across the transformer from either direction, but also increases the common-mode current flow in the grounding system the transformer and its shield are referenced to. With a suitably designed signal reference structure (SRS) grounding system, per Chapter 8, this is not normally a problem. However, if nonrecommended grounding system designs are employed, this can be a significant problem—especially SPG designs and most variations of them (see Chapter 8).
Also, if the shield’s grounding/bonding conductor is not installed as a low-inductance pathway, then per
Figure 4-39 it can be seen that it will act to defeat the shunt and voltage divider action provided by the electrostatic shield, since it is an inductance added in conjugate (vectorially, with XL 180° from XC) with the capacitance provided between the electrostatic shield and the associated faces of the windings. Bypass capacitors must be grounded via low-inductance means if they are to be fully effective and if the exhibition of unwanted resonances is to be avoided.
Electrostatic shielding can produce practical reductions in common-mode noise transfer across the transformer in ranges from approximately –20 dB to –40 dB and sometimes to –60 dB across some reasonably defined range of frequencies. This will be strongly influenced by specific product design, number of phases, input and output voltage, kVA rating, and the physical size of the transformer involved.
Practical attenuation values above this are generally not realizable in real-world installations of the transformer—particularly when the installation conforms to the requirements of the NEC. Performance attenuation tests that involve factory-specified and artificial capacitive voltage divider actions are generally not a valid means of determining the performance of the electrostatic shielding system in practical cases (see
Adding more (ungrounded) shields to the primary and secondary windings and operating them at their associated winding’s line-voltage potential permits a beneficial reduction in common-mode to transverse mod noise conversion across the transformer. Several tens of decibels of attenuation across a wide range of frequencies can be realized by this simple method of additional shielding.
At higher frequencies, where wave and transmission line theory must be used, the inter winding shield appears as a point of impedance mismatch from which transient currents (and voltages) can be reflected and re-reflected. This produces attenuation on the downstream side of the point of impedance mismatch. Also, reflections initiated by traveling waves on the ac power wiring to and from the shield are also found on the grounding conductor(s) and grounding system to which the shield has been connected for reference purposes. This latter point is very important and underscores the reason that specialized broadband SRS grounding techniques, as discussed in Chapter 8, must be used when avoiding noise problems in the grounding system, as opposed to SPG and related hybrid designs.