Filtering Mains Power

Since the most common type of filter used by system designers is the mains filter, this section discusses these components in some depth.

--- Filter wiring

The operation of standard mains filters

A typical filter includes components to block both common mode and differential mode components. The common mode choke L consists of two identical windings on a single high permeability, usually toroidal, core, configured so that differential (line-to-neutral) currents cancel each other. This allows high inductance values, typically 1-10mH, in a small volume without fear of choke saturation caused by the mains frequency supply current. The full inductance of each winding is available to attenuate common mode currents with respect to ground/earth, but only the leakage inductance will attenuate differential mode interference.

Capacitors Cx1 and Cx2 attenuate differential mode only but can have fairly high values, 0.1 to 1~tF being typical of smaller units. Either may be omitted depending on the detailed performance required, remembering that the differential source or load impedances may be too low for the capacitor to be useful. Capacitors Cy1 and Cy2 attenuate common mode interference and if Cx2 is large, have no significant effect on differential mode.

--- The basic mains filter

Mains filters do not have a specific "input" side and an "output" side, although it is sometimes convenient to think of them in those terms. The filter is a passive device and will offer the same attenuation in both directions in a given circuit. Nevertheless, it is normal for filter terminals to be dedicated either to the mains port or to the equipment port. This is because of the likely impedances to be met at each side; the general rule is that capacitors should face a high impedance and inductors should face a low impedance.

--- Variations on the theme of single-stage mains filters

Extending the performance of standard filters

Greater differential mode filtering

---A typical 2-stage filter for a switched-mode power converter.

Filters which have to cope with high levels of low-frequency differential mode noise (e.g. from switch-mode converters, phase angle power controllers, motor drives, and the like) often need more differential mode attenuation than can be achieved by X capacitors alone, and may need to use differential mode chokes. These may be two separate chokes, or can be two windings on the same core with their senses reversed (in contrast to the similar-looking common mode choke). It is difficult to get much differential mode inductance into a small package, because of core saturation, so these filters tend to be larger and more expensive.

Adding further stages to the simple single-stage filter is usually the most cost-effective way of improving a filter's performance. Parasitic reactances limit the attenuation capability of a single component. Also, as already mentioned a multi- stage filter is inherently less affected by extremes of source or load impedance.

Ground/earth leakage current

The Y capacitors being connected to ground/earth, result in a power frequency current flow through the ground/earth terminal. This can be calculated from

I_l kg = 2/l:F -- 1.1 V 1.2 C microamps

where V is the voltage across the Y capacitor(s) - e.g., 230V for the L-E connection

F is the power frequency, typically 50Hz

C is the total Y capacitance for a given connection, in microfarads

The factors 1.1 and 1.2 allow for maximum voltage (+10%) and capacitor (+20%) tolerances, respectively

Safety requirements may place limits on allowable ground/earth leakage, particularly for portable or pluggable apparatus. This in turn limits the total value of the Y capacitors. For medical apparatus, ground/earth-leakage currents may be limited to such low levels that the use of any reasonable size of Y capacitors is impossible. Such filters need to use better inductors and/or more stages to improve common mode attenuation, and tend to be larger and more expensive for a given performance.

In large systems the ground/earth leakages from many small Y capacitors can create significant ground/earth currents. As well as the safety implications, these can cause ground/earth voltage differences which impose hum and high levels of transients on cables between different equipments. Modern best-EMC-practices require equipotential three- dimensional meshed ground/earth bonding (MESH-BN), but many older installations do not achieve this so apparatus intended for use in such installations may benefit from the use of filters with small or even non-existent Y capacitors.

It is always best to use supply filters for which third-party safety approval certificates have been obtained and checked for authenticity, filter model and variant, temperature range, voltage and current ratings, and the application of the correct safety standard.

Filters sold for use on 50/60Hz may generally be used on power circuits ranging from DC to 400Hz with the same performance, but it is best to check with the manufacturers beforehand, and be aware of the implications for ground/earth leakage and dissipation in the filter components.

Operating current

The current rating of a mains filter is principally determined by its inductor(s). These carry the peak current, an excess of which can cause three principal effects:

++ overheating of the choke and the whole filter, with consequences for reliability and possibly safety

++ saturation of the choke core, which causes an instantaneous loss of inductance and therefore of attenuation

++ voltage drop across the filter, resulting in a lower supply voltage available to the connected equipment.

Clearly you need to choose a properly rated filter. This is not always simple: only a resistive load takes a purely sinusoidal current, whose peak and RMS values can be easily determined. Electronic equipment, low-energy lighting and motor drive inverters (for example) take a particularly non-sinusoidal waveform- see the discussion on mains harmonics. This can have a peak current very much higher (sometimes 5-10 times) than the rated RMS value. If there is any question of this peak current exceeding the filter's capability, then consult the filter' s manufacturer to check the consequences. It may be necessary to measure or calculate the waveform of the current that will be drawn under worst case conditions, to gain enough information.

The filter manufacturer may publish a derating curve which applies to the RMS current and is relevant for overheating. An inductance saturation curve may also be given; this section loss of inductance versus current and should be applied to the peak current, to check how effective the filter attenuation will remain. The same issues occur when using a filter on DC power, in which case the maximum DC current is applicable both for heating effect and saturation.

Overvoltages

--- Application of varistors external to a filter

Mains filter components are of course rated to withstand the nominal operating voltage, plus some safety factor. But the mains supply frequently carries transients which cause a brief overvoltage of several times the nominal. Section 9 looks at surge suppression in more detail. Before selecting a filter, you should consider the likely transient environment and decide whether the anticipated overvoltages will cause the filter too much stress. Filter capacitors are normally specified to IEC 384-14 or its European derivative EN 132400. This requires a pulsed overvoltage stress performance for X class capacitors depending on the sub-class (X1, X2 or X3) that is quoted, and a similar performance for Y capacitors. The construction of filter chokes is more variable; filter manufacturers will normally provide a maximum overvoltage figure for line-to-line and line-to- ground/earth for a two second application.

A severe transient environment combined with a filter of low specification may prompt the need for external surge suppression. A metal-oxide varistor (MOV) network across the filter input terminals will usually give more than adequate protection, although the varistor(s) will need to be sized for the maximum anticipated surge energy.

The alternative placement of the varistor(s) on the equipment side of the filter protects the equipment and allows the varistor to be sized for a much lower surge level, since the filter attenuates the surge and provides a higher source impedance. On the other hand, the filter components remain unprotected. To make the best design decision, the expected transient environment, the filter capabilities and the cost and performance of various surge suppressors must all be considered.

Filters and varistors between phase and ground/earth have implications for safety hi-pot testing. It is unrealistic to remove a filter for this sort of test and so it must be specified to survive the intended applied voltage; capacitors to EN 132400 are designed and rated with this in mind. Varistors will intentionally break down when this is applied, and must be temporarily removed from circuit to allow proper testing. Filters with large values of capacitance may be fitted with high-value "bleed" resistors to discharge these capacitors when power is removed to help protect personnel from shocks. These resistors can suffer damage or give false readings on high-pot tests. Some filter manufacturers provide identical filters with the bleed resistors removed- solely for use in safety testing. Also, remember that both varistors and filters have potentially hazardous failure modes, and should themselves be protected by a suitable fuse.

Varistors to ground/earth are usually regarded with disfavor when it comes to safety assessment anyway, because of their effect of increased ground/earth leakage as they wear out.

It should normally only be necessary to use them in severe transient environments on permanently-wired equipment, in which case the ground/earth leakage rules are relaxed.

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