low-leakage EM C filter

ReId of the invention

[0001] The present invention pertainsto a means for limiting earth leakage current produced by EM C Filters. This invention is particularly usef ul in power systems where one of the phase conductors istied to earth, but can be advantageous in most AC and DC power supply systems

Description of related art

[0002] Power line EM C filters include in most cases capacitors to earth, so called Y capacitance, together with an appropriate inductance, in order to achieve common mode attenuation. An unavoidable unwanted consequence of this isthat a current f lowsto earth through the Y capacitor - the so called Earth leakage Current (ELC). In figure 1 , which shows, in single-line diagram form, an example of a known EM C filter, the Y capacitance is indicated by reference number 30, 36 isthe corresponding inductance, and 35 isthe path of earth leakage current. As a rule, the higher the capacity of the Y capacitor and the voltage across it, the more intense isthe ELC.

[0003] At high level, this ELC is considered dangerousto personnel. Apart f rom the personal danger, excessive ELC can interfere with the reliable operation of an electrical system. In particular installations that include Residual Current Detection (RCD) will be interrupted due to tripping of the RCD device. Hence it is considered good design to minimize earth leakage current when designing EM C filtera

[0004] For power systemswith star grounded supply (i.e. a TN system in Europe) earth leakage can be a problem, although because the system is balanced around earth potential, the ELC is usually moderate, in normal conditions High ELC values may however arise in high-power f ilters or where strong common-mode attenuation is needed.

[0005] With the Japanese 230V delta power supply, one phase is grounded (so called " corner earth" ). In this case there is no cancellation of earth currents. A large ELCwill exist unless only small Y capacitors are used.

[0006] The use of RCD devices in Japan is prevalent. This poses a limit to the size of the Y capacitor that can be safely used, without risk of tripping the RDCdevice. There is therefore a need to find a method where large Y capacitors can be used whilst maintaining a low earth leakage current.

[0007] A similar problem existswith IT power systems, as employed, among others, in ships and factories Here the mains power is only loosely referenced to earth via high impedance. This is done so that in the event of one phase short circuiting to earth, the installation will continue to operate with relative safety. However in this shorted mode the power system is in effect " corner earthed" . If large Y capacitors are used then a high ELCwill exist.

[0008] There istherefore an increasing demand for noise suppression f ilterswith low ELC. Traditionally to limit ELC, such filters have been constructed with reduced capacitance in the earth path. However in order to maintain a suitable attenuation, the filter inductance must accordingly be increased to compensate for the reduced capacitance, which can make the f ilter larger and more expensive. Even increasing the inductance value is not a complete solution however and filterswith small Y capacitance are often less effective, for EM C noise reduction, than filterswith a higher Y capacitance. In addition, this increased inductance can lead to increased power loss, temperature rise and end-to-end voltage drop, all adverse conditions.

[0009] The module described here hasthe benef it that large Y capacitors can be used in the f ilter construction without the risk of tripping RCD devices Hence there is no need to increase the size of the inductor with its associated disadvantages The module will maintain low ELC at switch-on and can eliminate virtually all earth leakage current,

during any fault condition and during normal running conditions The invention can be equally applied to single and multistage f iltersfor single or multi-wire applications

Brief summary of the invention

[0010] According to the invention, these aims are achieved by means of the object of the appended claims.

Brief Description of the Drawings

[0011] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the f igures, in which :

Rg. 1 is a single-line diagram. It shows, in a simplified schematic way, an EM Cf ilter of known type and the path of the leakage current.

Figure 2 shows, in a simplified schematic way, a three-phase EM C f ilter of known type and the path of the leakage current.

Figure 3 shows, in a simplified schematic way, a three-phase EM C f ilter including the leakage suppression features of the present invention inserted in a " corner earth" three-phase system.

Figure 4 shows, in a simplified schematic way, an EM Cf ilter including the leakage suppression features of the present invention inserted in a single-phase system.

Figure 5 shows schematically a possible realization of a regulator according to one aspect of the present invention.

Figures θ and 7 show schematically other variants of realization of the regulator of the present invention.

Figures 8-11 illustrate alternative connections of the regulator in a series of EM Cf ilters according to the present invention.

Figure 12 is a schematic illustration of the principle of virtual shunt nodes introduced in the embodiment of f igure 1 1.

Figure 13 is a f urther variant of realization of a voltage regulator according to the invention.

Figure 14 illustrates a post-regulator circuit using only discrete component, according to an embodiment of the invention.

Figure 15 shows, in a simplified schematic way, another embodiment of the invention including a relay circuit.

Detailed Description of possible embodiments of the Invention

[0012] The problem and the causes of earth leakage in EξM C f ilter is illustrated in f igure 1. A power line 1 1 , which can be single or multi-phase, is connected to a device 15 by means of a filter 20, in order to suppress possible interference generated by device 15 and transmitted along the line 11. The filter, in its simplest realization includes the series inductances 36 and a " Y" capacitor 30 connected between phase conductors and the earth conductor. Any potential seen acrossthe " Y" capacitor 30 will contribute largely to the earth leakage current 36.

[0013] The same situation, but in the case of a three-phase power line, is represented in f igure 2. In this case know f ilters employ a bank of " X" capacitors 331 connected across phases L1 , L2, L3, which contribute to the reduction of differential mode noise. Common-mode noise, on the other hand, is suppressed by the capacitor 330 connected between the star point 90 and the ground. If , as in the represented corner-grounded example, the phase potential are not balanced with respect to the ground potential, the Y" capacitor 330 sees a large potential and letsflow a substantial earth leakage current 35.

[0014] Figure 3 shows, schematically, some features of an Electromagnetic Compatibility filter (EM C filter) according to an aspect of the present invention. Thisf ilter, intended for use in a three-phase power line, includes a virtual earth regulator 120 having a reference input (Ref) connected to earth potential or, equivalently, to a conductor essentially at earth potential. The regulator 120 stabilizesthe potential of its output (Out) node 100 to a level sensibly constant, forcing it to the earth potential present at the reference (Ref) input. Such regulation and forcing can be obtained by several known devices, all enclosed in the scope of the present invention, some of which will be better discussed in the following by way of example.

[0015] A noise shunt module is connected between the output 100 of the voltage regulator 120 and the earth reference potential, for absorbing an eventual noise transmitted on the phase conductors L1 , L2, L3. In the represented example the noise shunt module is a simple " Y" capacitor 330, but it isforeseen, in the scope of the present invention, that it may be replaced by a suitable capacitive network, of by a circuit, including passive and/or active elements, presenting a low impedance at the f requencies at which noise is expected.

[0016] The effect of the virtual earth regulator 120 isthat the " Y" capacitor 330 sees, instead of the f ull line potential, a much reduced potential which could be limited, according to the precision of the regulation achieved, to some Volts, or even lower. In thisway the leakage acrossthe " Y" capacitor 330 is reduced to an insignificant level. Correspondingly, the voltage drop acrossthe " X" capacitors 331 increases as a result of the introduction of the virtual earth regulator 120. The associated leak current 129, however, isdrawn by the output of the regulator 120 and returned to the phases L1 , L2, L3, via the regulator's supply lines, without contributing to the earth leakage current.

[0017] It should be appreciated that it is not essential to the f unctioning of the invention that the potential virtual ground node 100 should be forced to an absolutely constant value. Indeed it is sufficient that the

variation of this potential should be suff iciently reduced in amplitude and/or in speed, in order to limit the earth leakage current through the " Y" capacitor 330 to a harmless quantity. Also, it is not essential that the regulation should be strictly linear or effective at all f requencies

[0018] The virtual ground regulator needs not have a large bandwidth. In practical realizations it is suff icient that the regulator 120 should have low output impedance at low f requency, in order to stabilize the virtual ground potential of node 100 at the mainsf requency of 50 or 60 Hz, and up to some low harmonic f requency of the mainsf requency, while presenting a high output impedance at high f requency, far f rom the mainsf requency, where the noise is expected. Thus the virtual ground regulator does not necessarily contribute directly to noise suppression, the common-mode noise being shunted to ground via the capacitor 330, as is usual.

[0019] To simplify view, figure 3 and several subsequent f igures do not represent inductive elements of the EM Cf ilter. It is to be understood that these drawing are partial representations given here to describe the invention in itsvarious embodiments, and are not complete. The f ilter of the invention may also include other elements not represented, like inductors, common-mode inductors, current compensated coils, active elements, connector, fuses and other components; as suggested by circumstances and according to known practices in the art.

[0020] Figure 4 shows another example of realization of a f ilter having a virtual ground node according to the invention. In this case the filter, represented in simplified form without inductive elements, is inserted in a single-phase line having a live (L) and a neutral (N) conductor, aswell as an earth conductor and comprises a " δ" capacitor 250, " X" capacitors 231 and a " Y" capacitor 230. The virtual earth regulator 120 stabilizes the voltage of the virtual earth node 100 to a value close to earth potential. The earth leakage current through the " Y" capacitor 230 is in this way considerably reduced, while the leakage acrossthe V network " X" capacitors 231 is taken up by the regulator 120 and amountsto a

negligible electrical load across the live and the neutral conductors, without contributing to earth leakage.

[0021] Even if the represented examplesdescribe three-phase and single-phase applications, this is not a limitation of the present invention, which encloses also application to DC and AC power systems having any number of phases and any kind of earthing system.

[0022] Figure 5 shows one possible circuit realization of a Virtual Earth regulator 120a. This is called a " Coarse Regulator" asthe output voltage is only held near to earth potential (typically within +/-10V). Power is fed via f uses 122 to a bridge rectifier 123. The rectified voltage isfed across a complementary pair of FETs (QI and Q2). The FET gates are connected together and are referenced to the reference input (Ref), usually at earth potential, via a high impedance RC circuit 124. The regulator output isfed f rom FET sourcesvia source resistors (2x Rs) and a low impedance 129. Current limiting to protect the FETs is provided by Zener diodes (2x Dg) and source resistors (2x Rs). The output voltage can be controlled f rom the resistor star network (3x Rv) and f rom gate impedances (Rg // Cg). The output will be of a square wave when controlled more by Rg and Cb. On the other hand, when the output is controlled more by Rv the action of the regulator 120b is more linear and the output approaches a sine wave.

[0023] Note that output impedance 129 also create a high impedance at high f requency and function to block RF noise current f rom passing through the regulator. This ensuresthat most RF current passesf rom power line to earth via the capacitors " X" and " Y" .

[0024] It should be noted that, since the virtual earth regulator 120a is referenced to ground, its components, and particularly the RC circuit 124, ought to be dimensioned and approved for service at mains voltage. Also the impedance of the RC circuit 124 should be as high as possible, not to contribute to the earth leak current.

[0025] Figure 6 shows another possible circuit realization of a Virtual Earth Regulator (VER). This regulator 120b is a series connection of the coarse regulator 120a and a post-regulator 12Od. The post-regulator 12Od is formed by an operational amplifier 126 fed by an auxiliary power supply 128, which is referenced to the coarse regulator output. The operational amplifier is conf igured as a voltage follower, lts input is referenced to earth via high impedance 125. The output of the post- regulator isthus held very precisely at earth potential. The coarse regulator 120a avoidsthe use of a high-voltage operational amplifier, because the operational amplif ier 126 is protected f rom the mains voltage by the coarse regulator 120a. The supply voltage 128 must be large enough to accommodate the dynamic swing of the coarse regulator output (+/-10V). Impedance 125 should preferably be dimensioned and approved for mains voltage, in the eventuality of a failure of the coarse regulator 120a or of the auxiliary supply 128.

[0026] In the embodiment of figure 5, the voltage at the output of the coarse regulator 120a is not sinusoidal but approximately square, in reason of the dead zone of the FETs QI , and Q2. The " Y" capacitor 330 (see f igure 3) has a low impedance for fast -varying signals Thus, when the coarse regulator of f igure 5 is used, peaks of leakage current flow through the capacitor 330, in correspondence of the transitions in the output of the coarse regulator 120a, in which the slope dV/dt is high.

[0027] The coarse regulator 120a of figure 5 is a simpler solution that could provide acceptable results in many cases In particular, in a power system f itted with RDC devices, these often apply a certain level of integration and are relatively insensitive to short peaks of leakage current. Using the coarse regulator alone may be adequate in such cases

[0028] The post-regulator 12Od provides a low-amplitude and low-slope voltage at the output node 100, effectively minimizing leakage acrossthe shunt module 330. The same effect could be achieved replacing the voltage follower realized with the operational amplif ier 126 with a circuit made of discrete components, realizing the same f unction.

[0029] Figure 14 illustrates a possible realization of post-regulators 12Of , which could replace the block 12Od in figure 6. Components in common with the circuit of f igure 6 or of other figures are indicated by the same reference numbers. The regulator 12Of usestwo transistors Q.1 and Q2 and a separate floating power supply 128 connected by means of the terminals U and L2 to a suitable source of AC power.

[0030] The input of the post regulator istied close to ground potential by means of the high-impedance network 125 and, at least at mains f requency, the electric potential at the output terminal (Out) will be very low. Hence the leakage of the filter capacitor connected to the output (not represented on thisf igure) will be very low. The output impedance network 129, on the other hand, blocks RF noise current f rom passing through the regulator. This ensuresthat most RF current passesf rom power line to earth via the capacitors " X" and " Y" .

[0031] The discrete post-regulator of figure 14 is less expensive to realize than the operational amplif ier circuit of f igure 6, and its bandwith can be chosen in order to reduce the risk of parasitic resonances and instabilities.

[0032] Figure 7 shows another form of coarse regulator 120c using switching techniques. This circuit shares many componentswith the coarse regulator 120a of f igure 5, and these are indicated by the same reference numbers A notable difference isthat transistors QI and Q2 are here driven by the digital control circuit 729. This hasthe advantage of high efficiency.

[0033] Figure 8 shows an alternative connection of the virtual earth regulator 120, in which the virtual earth node is interposed between two " Y" capacitors 333 and 332. This is usef ul when large " X" capacitors are employed. The virtual earth regulator 120 needs only draw current f rom the smaller 333 capacitor, thus lower-rated components can be used for the regulator's output stage.

[0034] Figure 9 shows the virtual earth regulator 120 connected to an active shunt module 360, in lieu of a capacitor. The active shunt module 360 is an active impedance that performsthe same tasks asthe " Y" capacitor 330 of previous examples Thanksto the superior performance of the active shunt module 360, this circuit is suitable to use with one of the coarse regulators 120a or 120c of figures δ or 7.

[0035] Figure 10 showsthe concept of a Virtual Earth Bus 1 10. One VER module 120 can be used to limit leakage of multiple capacitor networks as used in a multi-stage f ilter. Optionally the virtual earth bus 1 10, in passing f rom one stage to the next, is passed through the common mode inductors 380, with same number and sense of turns as power conductors L1 -L3. A second stage of noise attenuation, comprising capacitors 341 and 340 is inserted. Snce the " Y" capacitor 340 is connected to the virtual earth bus 1 10, which is kept close to earth potential by the virtual earth regulator 120, it does not contribute appreciably to earth leaking current. It should be understood that one or both capacitors 330, 340 could be replaced, according to the necessity, by capacitive networks or other shunt means, including active shunt modules

[0036] Figure 1 1 shows the Virtual Earth Bus in a multi-stage filter employing the virtual shunt node technique asdescribed in European patent application EP1619768, which is hereby incorporated by reference. M ultiple common mode stages are formed while requiring only one VER module 120.

[0037] The virtual shunt node inductor 600 is realized in a manner to provide, on the virtual earth bus 1 10, a voltage drop equal or proportional to that present in the power conductors !./! , l_2, L3 when traversing the inductor 600. In thisway the circuit of f igure 1 1 is equivalent, for what noise f iltering is concerned, to a two-stage filter having two independent banks of " X" capacitors, as illustrated in figure 12.

[0038] Figure 13 shows an improved course regulator 12Oe. Components in common with the coarse regulator 120a of figure 5 are indicated by the same reference numbers. With thisvariant the FET gates are driven form the output of an operational amplif ier 226. The operational amplif ier is configured as a voltage follower such that the regulator output is maintained very near earth potential. The non-inverting input of the operational amplifier is referenced to earth via high impedance and this is compared with a feed-back voltage f rom the FET sources. The cross-over dead region of the FETs is, in effect, taken up by the gain of the operational amplifier. In thisway the arrangement of Figure 13 provides an output voltage which is much closer to earth potential than the arrangement of Figure 5. In an equivalent variant, the operational amplif ier 226 could be replaced by an amplifier stage with suitable forward gain, realized with discrete components.

[0039] The operational amplifier 226 requires a small auxiliary power supply 228 with voltage greater than FET dead region (approx. +/-5V). The power supply is referenced to the FET source output.

[0040] Note that regulator 120b of Figure 6 provides the most precise virtual earth regulation. The variant 12Oe of Figure 13, however, hasthe advantage of smaller power and dimensioning of the operational amplif ier 226 than that required in circuit 120b.

[0041] Figure 15 exemplif ies another embodiment of the invention, in which a relay circuit 700, comprising two discriminator blocks 720 and 730, a logic AND 710, a voltage regulator 740 and a relay device 750, is added to the EM C filter. The EM C filter also comprises a noise shunt module 850 and a X capacitor network 800 and a Y capacitor network 900 both being connected between the phases 910, 920 and 930 within the EM Cf ilter.

[0042] The X capacitor network 800 causes phases that are in an open circuit state (due to a fault) not to become completely dead. Under a double fault condition (2 phases open) both open phaseswill be energized to the voltage of the remaining phase. Under a single fault

(one phase open) the open phase will be energized to half the normal phase to earth voltage. In this latter case, the discriminator blocks 720 and 730 will discriminate between full and half voltage to determine if a fault has occurred. Note that other componentswithin the power supply will also influence the phase voltages during a fault.

[0043] The control of relay circuit 700 has no reference to earth, fault detection and relay actuation being entirely between phases and all relay control and actuation circuits are referenced in relation to the phase L1. Note that the use of identifiers L1 , L2 and L3 are only for technical description purposes and, in practice, any phase can be connected to any terminal (i.e., this system is not phase sensitive).

[0044] Discriminators 720 and 730 monitor the line voltages of phases L2 and L3 respectively relative to phase L1. If the voltage f rom either phase falls below a threshold voltage then the output flag is set low. The discriminator threshold voltage is set above half the highest peak operating voltage and below the lowest peak operating voltage. The relay block is controlled by the Logic AND output. The Logic AND provides a high output flag only when both discriminator f lags are high.

[0045] The relay coil 751 is buffered with a f ield effect transistor 752. Power is provided to the relay coil via the voltage regulator 740. This is advantageous because the relay coil voltage is lessthan the supply voltage and because the supply voltage will vary. The voltage regulator consists of a surge capacitor 741 , rectifier diode 742 and a varistor 743 to limit peak dc voltage. According to a non-represented variant, the relay device 750 may be replaced by any suitable switch device, like for example a solid- state relay, a transistor, and so on. An EM C filter comprising the above relay circuit allowsfor insulating the noise shunt module in the event of a fault.

[0046] At switch-on the voltage across the shunt module 850 can swing anywhere between zero and peak line voltage. Relay 750 isde-energized at this time so its normally-open contacts prevent any f low of current to

ground and inrush earth leakage is minimized. Under a single-mode fault or double-mode fault the relay 750 is also de-energized, and shunt module 850 is insulated f rom earth.

[0047] In the circuit of figure 15, one side of the shunt module 850 is referenced to ground by means of the relay device 750, the other terminal of the shunt module 850 is connected to the output of virtual earth regulator (VER) 12Og. That limits power f requency ELC resulting f rom power supply imbalance.

[0048] In normal operating condition, when the relay device 750 is closed, the Y Capacitor Network 900 is connected to the power lines L1 -L3 and the Z capacitor C ₂ Of the shunt module 850 is connected between the Y capacitor network star point 100 and earth. The series combination of these two modules providesthe earth path capacitance within the filter.

[0049] The output of the VER circuit 12Og is connected to the Y star point 100. At power f requenciesthe Y star point 100 is maintained at near earth potential by the VER module 12Og. Thus the Z capacitance sees near earth potential on both electrodes and hence near zero current at power f requencies. This avoids generating the power f requency component of

[0050] Power is provided to the VER module 12Og via the voltage supply 123a. This comprisesthe surge rated capacitors 860. These capacitors are dimensioned to limit power within the power dissipation capability of the Voltage Follower. Maximum power is reached when the load impedance equalsthe equivalent capacitor impedance. Hence the power supply is self limiting.

[0051] The X capacitor network provides differential mode attenuation in the usual way within the filter. It also provides, via capacitors 860 and varistors SI , S2 a stable reference point for the power supply 123a. Due to the large value of the " X" capacitors, the star point presents a very low impedance, and a very low ripple supply voltage can be obtained in this

way. In addition the X capacitor network provides a return path for the current drawn f rom the Y capacitor network, instead of passing current to earth, thus avoiding excessive EiLC.

[0052] The high passfilter 866 monitorsthe voltage V _y of Y star point 100 and providesthe control voltage Vj at the noninverting input voltage of operational amplifier 326, according to transfer f unction Vj/V _y = B(f). Qrcuit analysis of the f requency behavior shows that the VER circuit 12Og presents a very low output impedance to star point 100 at low f requency, thus limiting the voltage drop across shunt module 850, and VLC. At higher f requencies, defined by the transfer function B(f) of high-pass filter 866, the output impedance of VER circuit 12Og increases, and it can be completely disregarded for the noise, which is shunted to ground by the shunt module 850.

[0053] The cutoff f requency of high-passfilter 866 can be placed about 1 kHz, for example. In this way the VER module is effective in limiting the earth leakage current at 50-60 Hz and for the strongest harmonic components of the line voltage, yet the VER module 12Og does not have to process any high-f requency noise component. This allowsthe use of a simple VER module, having high stability and low power consumption. In the presented example, the output stage of the VER module 12Og is represented by a single Q4, to simplify the drawing, but would preferably be realized by a complementary pair of transistors as in f igure 13.

[0054] According to an independent aspect of the present invention, the VER module 12Og and the power supply unit 123a presented in figure 15 could be combined with one of the arrangement of the preceding embodiments, dispensing with the relay protection circuit 700.