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Electromagnetic compatibility, essential for fusion experiments
Fusion Engineering and Design 11 (1989) 189-195 North-Holland, Amsterdam
ELECTROMAGNETIC
COMPATIBILITY,
189
ESSENTIAL
FOR FUSION
EXPERIMENTS
P.C.T. v a n d e r L A A N
High-Voltage Group, Eindhoven University of Technology, Eindhoven, the Netherlands Submitted 6 October 1988; accepted 24 November 1988 Handling Editor: P. Komarek
Electromagnetic compatibility has been of crucial importance to fusion experiments, long before it was known by its present name, EMC. The huge amounts of pulsed power required to create and heat a plasma, and the stray fields produced in large volumes made it difficult, in cartier and in present experiments, to reach the ideal EMC-state where interference does not hinder (or impede) operation, measurements and data acquisition. The pressure to obtain results from the experiments has led to many ingenious solutions, but left usually tittle time for a more fundamental study. A basic problem is that the complex threa-dimensional (and often toroidal) circuitry of a fusion experiment cannot be correctly represented by network theory models. In particular extended leads no longer "transport" potentials or voltages, but of course continue to transport currents. We therefore concentrate on currents and on all closed current loops rather than on worrisome (but meaningless) "jumping potentials". With concepts developed along these lines we can design grounding systems which protect large interconnected electrical systems against interference. Such a grounding system may use wide strips, conduits or tubes to carry all interconnecting cables from subsystem to subsystem. Alternative descriptions of this grounding system may help to clarify its operation: a screen room reduced to its essential current-carrying elements, a minimization of transfer impedances or a structure to keep the common mode circuits closely under control. In this context ground loops can be quite useful, and we abandon the concept of sl/~gle-point grounding. Next to the protection by grounding and shielding, additional measures can be taken to carry signals safely into subsystems. Larger amplitude or differentiated signals are advantageous, provided the first attenuator or integrator section in the "receiver" uses only passive, linear components. EMC is essential for fusion experiments, as stated in the title. Techniques borrowed from fusion can however also be quite useful in other fields. Examples in power engineering measurements and in lightning studies are briefly mentioned.
1. Inm~duction, the E M C - p m b l e m in fusion Already in the first fusion experiments much effort was necessary to operate the various parts of the experiment in the correct timing sequence and to obtain measurement signals without (too much) interference. Ever since then it has remained difficult to obtain "ElectroMagnetic Compatibility" between the many subsystems of a fusion machine, and thus much E M C work has been done in fusion, even before the abbreviation E M C became familiar. In the battle against interference one generally distinguishes between sources and * A version of this paper was presented as an invited paper at the 15th Symposium on Fusion Technology, held in Utrecht, The Netherlands, September 19-23, 1988. 0920-3796/89/$03.50
victims of interference. Communication between source and victim takes place along an unwanted and often unexpected coupling-path. The sources of interference are easily found in fusion devices: to create and heat the plasma huge amounts of pulsed power are to be fed in. Diagnostic methods may also require intense power sources, for instance for laser or particle beams. Interference can also be caused by the strong B-fields produced in large volumes around the plasma. Victims in a fusion machine may be the control circuits of power sources; premature triggering as a result of interference is a well-known reason for malfunctioning. In addition interference can be very troublesome for the many measuring systems and the data acquisition and handling. This is particularly true,
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
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P.C.T. van der Laan / Electromagnetic compatibility
since often only weak signals are produced by measuring systems stretched to their technical limit. The EMC-problems in fusion machines are becoming more important with the increase in size and complexity of the machines. The slow rise of the main fields in a tokamak may,produce not much interference, but additional heating schemes (neutral or rf) imply more high-voltage supplies, flashover risks, high pulsed power levels and vulnerable control circuits for the timing of the heating pulses. The use of computers for control and data-acquisition greatly improves the data handling and storage. On the other hand the interlinking electrical networks for control and dataflow open up new possibilities for electrical interference. A shot, lost as a result of interference, is costly. Not only is experimental time lost, but also incorrectly timed shots may damage or cause extra aging of equipment.
2. Standard approach to solve EMC-problems, an art In practice the EMC-problems in fusion machines are solved b y hard work and by trial and error. Experience and intuition also play an important role. The solutions advocated still have a somewhat magic character, which is understandable since also the precise nature of the interference is not free of mysteries. It would be highly desirable to develop the above sketched art into a science, to save time in the construction and start up of new larger machines and also to be more confident that effective measures can be built in. As part of this development a critical look at the models used is necessary.
3. Are adequate models being used? Especially in fusion an interesting contrast exists between the models used inside and outside the plasma. The plasma - for the sake of the argument assumed to be inside a torus - is described with the full Maxwell equations, together with the appropriate plasma equations. Outside the torns network theory is used to design the electrical and electronic circuitry. This split into two separate "worlds" is of course artificial. On one hand the plasma behavior, its equilibrium and its stability may very well be influenced by the interaction with the external circuit. That interaction certainly influences the decay of the toroidal plasma current, see for instance Van der Laan et al. [1]. On the other hand we can expect problems when the three dimensional circuitry
outside the torus is described with network theory, which as a zero-dimensional model is only an approximation of the correct Maxwell equations. The fact that network theory is not adequate, cannot be overcome by the use of full Maxwell equations. The electrical circuitry is usually far too complicated to work out the boundary conditions for, let alone to solve the Maxwell equations. In fact we design electrical engineering such that we operate in regions where either the E- or the H-field are at their maximum. The maximum in the E-field then corresponds with a capacitor, whereas the maximum in H corresponds with an inductor. In network theory we hide these fields in the impedance symbol, where only the relation between current and voltage at the terminals is considered. The connecting wires in a network diagram are also quite abstract; they only symbolize that current and voltage are faithfully transported from A to B. Note that the wires have no length in network theory; all that remains are Kirchhoff's equations and V - i relations for the various interconnected network-elements. In reality deviations from this simple, but extremely useful model are to be expected for long connections. We discuss this for the case of grounding; obvious deviations from network theory are also found near the torus.
4. Grounding Grounding is a difficult subject, mainly because the generally accepted definition of the noun "ground" is incorrect. Most standard definitions contain two elements (a) A ground can absorb or supply current without any change in voltage; in other words the ground should be a perfect sink or source for current. (b) A ground is an equipotential point or plane which serves as a reference for the circuit considered. We discuss and criticize both these elements. 4.1. Ground, a sink or source?
To have a sink or source for currents we need a capacitor where the charges can accumulate. This capacitor cannot be provided by Mother Earth. As an isolated sphere with an average radius of 6367 km the Earth has a capacitance of 708/~F, as follows from the equation CA =
4~'~or.
(1)
P. C. T. van der Laan / Electromagnetic compatibifity
191
I (a)
(b)
Fig. 1. Incorrect (a) and correct Co) models of what a grounding system can do. The thickness of the lines indicates the magnitude of the current.
Our electrical engineering activity however, never influences the complete E-field around the Earth. At best we produce charge displacements in a small part of the Earth surface and therefore we cannot benefit from CA in our grounding. As argued in more detail by Van der Laan et al. [2,3], we must conclude that the first element of the definition of " g r o u n d " is unrealistic. Instead, we must go back to the continuity equation for charges, or as a consequence of that to Kirchhoff's current law (KCL): the sum of all currents, including the capacitive current, into any point is zero. It is also correct to state that any current - if we properly include the displacement current - must flow in a closed loop. This last statement is very obvious for the plasma currents in a torus or for the currents in the big toroidal coils. Unfortunately the statement is seldom taken seriously for grounding currents (see fig. 1). A grounding system never resembles a sewer system (fig. la), where more and more sewage pipes converge into one main pipe with " u n k n o w n " destination. Instead a grounding system consists of a number of interlinked loops (fig. lb). N o t e that fig. l b shows only the low-voltage parts of all circuits. Secondly, the connection to Earth in fig. l b is no longer unique but is only part of another current loop [4], which can be omitted for portable equipment.
axis
Fig. 2. A toroidal shell in a tokamak or in a toroidal pinch. The rising toroidal field causes voltages differences between points A and B on the shell. Voltmeter 1 on the outside sees a large voltage; voltmeter 2 on the left sees no voltage.
shell with a toroidal slit on the outside (see fig. 2). This metal shell is similar to a single-turn coil of a toroidal thetapinch. A varying toroidal flux O r , only present within the shell, induces interesting voltmeter readings. Voltmeter 1 shows a voltage ~ E . d/ffi - d O r / d t , also equal to the voltage across the slit. Voltmeter 2 however, show no voltage, since E in the shell is zero and no flux external to the shell is present. Clearly we cannot describe this as a potential difference between A and B. We have to accept that the non-conservative induction E-field causes V ^ - Vs to be dependent on the integration path r E . dl, that is dependent on the position of the voltmeter leads. As a second example we consider toroidal voltages measured between two points A and B on the stainless steel bellows of the vacuum vessel (fig. 3). A voltmeter 1 with its leads on the surface of the bellows, sees a resistive voltage proportional to the current in the bel-
2
4.2. Grouna[ a point of equipotential? This second element of the standard definition of ground implies that a connection to ground fLXes the potential of the connected point. That is true in electrostatics and in network theory. When, however, distribo uted time-varying magnetic fluxes are present, the situation changes. We demonstrate this for wiring near the plasma chamber. First we measure poloidal voltages between two points A and B of a conducting toroidal
Fig. 3. Toroidal voltage between two points A and B on the stainless steel bellows (without break) in a tokamak. Voltmeter 1 sees a resistive voltage; voltmeter 2 sees a lower voltage because its leads also enclose part of the return flux. If an iron return leg (R.L.) is present, the voltages depend on whether or not R.L. is enclosed.
192
P.C.T. van der Laan / Electromagnetic compatibility
lows. We discuss the case where we have no ceramic break in the bellows; obviously with a break current and voltage are zero. If we extend the measuring leads radially outward to voltmeter 2, also a part of the poloidai return flux is enclosed. If we move far enough out with voltmeter 2, no voltage is seen, given a situation with the flux returning through the air. With an iron return leg (R.L in fig. 3) present, the situation is again different; then it is important to know whether the loop formed by the measuring leads does or does not enclose the return leg. It is clear that the metal objects in these examples cannot be considered as equipotential surfaces. In fact the entire concept of potential loses its meaning in the presence of a time-varying magnetic flux where ~ E - d l differs from zero (see [5]). Such distributed magnetic fluxes which cause the failure of Kirchhoff's voltage law, (KVL), are quite common in large size electrical circuits. Grounding systems are in this category, in particular since the ground currents, often unknown in magnitude, flow in circuits with large areas.
5. New grounding rules The failure of the KVL, makes the concept of potentials "transported" by leads, meaningless. This is hard to accept, since network theory serves us usually so well. The advantage is however, that "jumping ground potentials" do not have to worry us anymore. In the analysis of a grounding system the current is much more meaningful than the voltage, with its dependence on the lay-out of the voltmeter leads. We therefore concentrate on the current. We redesign the dosed loops in which the currents flow to minimize magnetic fluxes. This requires a local and compact closing of current loops. By doing that we reduce self- and mutual inductances, and lower undesirable magnetic couping. We also obtain a more orderly design and move in the direction where network theory is again valid. The compact dosing solves the grounding problems locally. The connection with Mother Earth then only carries current in case of lightning or - depending on the regulations - for safety grounding.
6. Extended grounding structures In the description of extended grounding structures, for instance meant for a fusion experiment, we follow
~
c
Fig. 4. (a) A torus with subsystems mounted in a screen room and grounded on the floor panel. (b) The floor panel is retained as the most important part because it carries most of the current. The subsystems are shielded. (¢) The most important part of the floor is that below the leads. That part is retained, now formed in the shape of a conduit. Only one of the subsystems is shown.
three lines of thought, to emphasize different EMCaspects of grounding. (a) To reduce the distributed magnetic fluxes we might install a big screen room around the torus and its various subsystems (fig. 4a). Short grounding connections to the floor give low fluxes; the currents in the screen room walls limit the magnetic fluxes and do not produce additional flux. Since the grounding currents flow predominantly in the floor, we may omit the side walls and the ceiling. Metal shielding around the subsystems is more useful (fig. 4b). Finally we note that fluxes are minimized when the leads are mounted close to the metal floor. The currents in the floor are then localized under the leads. We again take out a large part of the floor, and leave only the metal under the leads, in the form of wide metal strips, metal conduits or metal tubes (fig. 4c). At the high frequencies often responsible for interference, the magnetic field lines have to go around these wide metal conductors; the weakened magnetic field produces then not much flux.
P.C.T. van der Laan / Electromagnetic compatibility
193
other pair of leads. Without the conduit the common mode circuits encircle large areas; in many cases the return path is left to chance because all design work concentrates on the differential mode circuits. Clearly the conduit, if continuous, makes the common mode circuit quite compact and keeps it well under control. One might argue that the conduit creates a new common mode circuit with wiring or soil at a larger distance. This is indeed true, but the separation between inside and outside, provided by the conduit, should give adequate protection.
7. Protection of measuring instruments Fig. 5. Interference currents I talon8 a length ! of the braid of a coaxial cable (a) or along the outside of a metal conduit, couple in small interference fluxes in the inside loop, which result in an interference voltage Vi given by I i I Z T, where Z T is the transfer impedance.
Moreover the shape of the conduit, or better still that of a tube, forces the field lines away from the leads, and reduces thereby the transfer impedance, as discussed in the next paragraphs. (b) The braid of a coaxial cable has the same favorable geometry as the tube. Nevertheless interference currents in the braid can couple in undesirable voltages via the so-called "transfer impedance" (fig. 5a). At high frequencies this transfer impedance increases linearly with frequency because the holes in the braid allow external magnetic field lines to penetrate. Clearly for a solid metal conduit this effect disappears (fig. 5b). A residual transfer impedance is here caused by magnetic flux which comes in from the top. A larger height to width ratio helps; for a tube the transfer impedance becomes negligibly small at high frequencies. Note that the interference current flows on the outside of the conduit, whereas the magnetic fields around the leads cause the flow of currents on.the inside of the conduit. Clearly we have a useful separation between these currents, especially for high frequencies. To have the benefits of this separation, we must have all conduits properly connected, between sections and to the subsystems where they end. Shielded cables inside the conduit should be grounded at both ends; the ground loop thus formed enclosed very little flux and is harmless. (c) This last point brings us to the third line of thought, where we consider common mode currents, such as the net current in a coaxial cable, or in any
At the boundaries of the extended grounding structure or at the shielded subsystems leads often have to come in from the outside. These "points of entry" require special attention. When for instance power leads come in, an appropriate filter should divert common mode currents to the grounding structure. Similarly coaxial cables may carry signals to sensitive measuring instruments. High-amplitude local Eand H-fields may cause interference signals in these instruments. Usually however, the measuring instrument proper is a rather small "receiving" antenna at quite a distance from the interference source. Much more interference is brought in by the leads, often in the form of common mode currents. Most instrument housings cannot safely carry such currents; interference cow pies in through a too large transfer impedance of the housing. The solution is to divert the common mode currents to an extra EMC-cabinet (fig. 6) before harm is done. At the point of entry the cable must be connected to the panel over 360 o. When all incoming leads, including the conduit are all attached to the rear panel, the front can often be left open. This situation is convenient for viewing, for instrument adjustments and for ventilation. Similarly in pinch experiments screen room doors could often be left open during shots, even though the many spark gaps produced a lot of interference. Also there a correct handling of the interference currents at the points where the leads came in, was more important than the current flow near the screen room door. The measuring instrument must also be protected against interference signals, which arrive as differential mode signals, indistinguishable from true signals. Such interference signals can couple in via the transfer impedance of the coaxial cable. Several solutions exist: a
194
P.C.T. van der Laan
/ Electromagnetic compatibility
I I
open front
coaxial cable
I ,
I
point of ~. en t(~... "., "~
I
//
I ~
~.~
- -
~
""
~..-~-~'~
~.c..o n .du i t. with
leads
Fig. 6. An EMC-eabinet can protect enclosed sensitive el~tro~e i ~ s ~ s . All ~ b l ~ ~ d ~he ~ndMt tr~sfer common m ~ e cu~ents to the outside of the ~binet. If ~ c o ~ tions are made to ~ e r ~ panel, the front c ~ often stay open.
larger signal amplitude, a better cable, copper tubing around the cable or the use of fiber optics. N o t e however, that many sensors, in use in fusion research, such as Rogowski coils, magnetic probes and some voltage sensors [6,7] also provide a solution with good E M C quality. Since differentiated signals are cartied in, the signal amplitude increases with frequency, just as most interference signals do. The signal to interference ratio can therefore remain high before and after the integration where the high frequencies are attenuated proportionally to 1/,o. The attenuation by the first part of the integrator, (or the first part of a regular attenuator, used when signals of large amplitude are carried in) is very important for the EMC-behavior. The components should be mounted in a shielded box close to the rear panel. Moreover passive linear elements ( R and C's) should be used to suppress interference bursts before they reach (essentially non-linear) electronics.
8. Comparison with single point grounding In single point grounding one attempts to avoid groundloops and one keeps the grounding currents separate, at least up to the "single point" (fig. 7). If all currents did flow in one direction, as in a sewer (fig. 7a) this would indeed reduce coupling, although beyond the
Fig. 7. Single-point grounding (a) is based on the sewer idea; all currents flow to the sewer. In fact currents can flow in opposite directions, for instance as a result of induction, when connecting cables c are present (b). A more compact grounding (c) is much better; the lower horizontal line can be a conduit.
single point things remain mysterious. If the currents circulate (fig. 7b), as they may do when connecting leads c are present, large loops are formed unnecessarily. A much better solution is to reduce the area of the loops (fig. 7c). When the lower line in that figure stands for a conduit, we are again in the situation of fig. 4c, recommended in this paper. N o t e that ground loops can be very useful, for instance in shielding or in the metal of the conduit discussed. Also a circulating current in a loop is less bothersome than a breakdown, with its new interference production, across a " b r e a k " in a loop. The ground loops formed by the braid of signal cables in a deep conduit are harmless, as has already been mentioned.
9. Fusion technology, useful in other fields We already mentioned in section 7 the differentiating sensors, c o m m o n in fusion research: Rogowski coils, magnetic probes and dV/dt sensors [5]. These types of sensors, with their good E M C properties have been successfully used in power engineering, a field in which these sensors are rather new. Measurements in highvoltage substations could be carried out with high band width and without interference problems [7]. A protecting cabinet, such as shown in fig. 6 is necessary; a stand-alone version with a power filter permits measurements in the field, even in high-interference surroundings. The Rogowsld coil, in a flexible version turns out to be very useful for the analysis of existing grounding systems. As advocated in this paper, the study of cur-
P. C. T. van der Laan / Electromagnetic compatibility
rents and the identification of circuits is the key for understanding and redesign. Pulsed power sources, borrowed from fusion technology are very useful to inject pulsed currents in grounding grids or in antenna towers. To simulate lightning strikes, current pulses were induced repetitively in a (guyed) antenna tower, by means of a toroidal B-coil mounted in two halves around the tower [8].
10. Conclusions In the analysis of EMC-problems network theory often turns out to be inadequate. This is especially true near the torus and for the grounding systems. We should forget the dangerous " j u m p i n g potentials", and instead concentrate on the currents. We redesign current loops to minimize fluxes and coupling; this means that loops should be made compact and should be closed as locally as possible. A grounding structure for a fusion machine can be built with metal conduits between the torus and its subsystems. This completely interconnected star provides local grounding and a safe pathway for all leads. Differentiating sensors combined with appropriate cabinets allow EMC-correct measurements in high-interference surroundings, in fusion, in power engineering and in lightning research.
Acknowledgements The author thanks his many colleagues, earlier in fusion research and more recently in power engineering and in EMC-research for many valuable discussions,
195
essential for the development of the ideas described in this paper. Of the Eindhoven group ir. M.A. van Houten and dr. A.P.J. van Denrsen and all the staff involved in measuring expeditions deserve special thanks.
References [1] P.C.T. van der Laan, W. Schuurman, J.W.A. Zwart and J.P. Goedbloed, On the decay of the longitudinal current in toroidal screw pinches, Proc. 4th Intern. Conf. on Plasma Physics and Contr. Nuclear Fusion Research, Madison, USA, IAEA-CN/28/B-4, Vol. I, 1971, p. 217-223. [2] P.C.T. van der Laan and M.A. van Houten, Design Philosophy for grounding, Proc. 5th Intern. Conf. on EMC, York, 1986, p. 267-272. [3] P.C.T. van der Laan, M.A. van Houten and A.P.J. van Deursen, Grounding philosophy, Proc. 7th. Intern. Symposium on Electromagnetic Compatibifity, Zfirich, 1987, 105Q3 p. 567-572. [4] H.W. Ott, Ground, a path for current flow, IEEE Intern. Symposium on EMC, 1979, p. 167-170. [5] P.C.T. van der Laan, Voltages in toroidal pinch experiments, Los Alamos Report LA-7335-MS (1978). [6] R. Keller, Wideband high voltage probe, Rev. Sci. Instrum. 35 (1964) 1057-1060. [7] E.J.M. van Heesch, J.N.A.M. van Rooij, R.G. Noij and P.C.T. van der Laan, A new current and voltage measuring system; Tests in a 150 kV and a ~ kV GIS., Proc. 5th Intern. Symposium on High Voltage Engineering, Braunschweig, 1987, Vol. 1, Paper 73.06, 4 p. [8] A.P.J. van Deursen, M.A. van Houten, E.W.L. van Engelen, P.F.M. Gulickx, P.C.T. van der Laan, E. Zwennes and A.J. van Dongen, Measurements of currents around and in large grounded structures, Proc. 19th Intern. Conf. on Lightning Protection, Graz, 1988, p. 143-148.
ELECTROMAGNETIC
COMPATIBILITY,
189
ESSENTIAL
FOR FUSION
EXPERIMENTS
P.C.T. v a n d e r L A A N
High-Voltage Group, Eindhoven University of Technology, Eindhoven, the Netherlands Submitted 6 October 1988; accepted 24 November 1988 Handling Editor: P. Komarek
Electromagnetic compatibility has been of crucial importance to fusion experiments, long before it was known by its present name, EMC. The huge amounts of pulsed power required to create and heat a plasma, and the stray fields produced in large volumes made it difficult, in cartier and in present experiments, to reach the ideal EMC-state where interference does not hinder (or impede) operation, measurements and data acquisition. The pressure to obtain results from the experiments has led to many ingenious solutions, but left usually tittle time for a more fundamental study. A basic problem is that the complex threa-dimensional (and often toroidal) circuitry of a fusion experiment cannot be correctly represented by network theory models. In particular extended leads no longer "transport" potentials or voltages, but of course continue to transport currents. We therefore concentrate on currents and on all closed current loops rather than on worrisome (but meaningless) "jumping potentials". With concepts developed along these lines we can design grounding systems which protect large interconnected electrical systems against interference. Such a grounding system may use wide strips, conduits or tubes to carry all interconnecting cables from subsystem to subsystem. Alternative descriptions of this grounding system may help to clarify its operation: a screen room reduced to its essential current-carrying elements, a minimization of transfer impedances or a structure to keep the common mode circuits closely under control. In this context ground loops can be quite useful, and we abandon the concept of sl/~gle-point grounding. Next to the protection by grounding and shielding, additional measures can be taken to carry signals safely into subsystems. Larger amplitude or differentiated signals are advantageous, provided the first attenuator or integrator section in the "receiver" uses only passive, linear components. EMC is essential for fusion experiments, as stated in the title. Techniques borrowed from fusion can however also be quite useful in other fields. Examples in power engineering measurements and in lightning studies are briefly mentioned.
1. Inm~duction, the E M C - p m b l e m in fusion Already in the first fusion experiments much effort was necessary to operate the various parts of the experiment in the correct timing sequence and to obtain measurement signals without (too much) interference. Ever since then it has remained difficult to obtain "ElectroMagnetic Compatibility" between the many subsystems of a fusion machine, and thus much E M C work has been done in fusion, even before the abbreviation E M C became familiar. In the battle against interference one generally distinguishes between sources and * A version of this paper was presented as an invited paper at the 15th Symposium on Fusion Technology, held in Utrecht, The Netherlands, September 19-23, 1988. 0920-3796/89/$03.50
victims of interference. Communication between source and victim takes place along an unwanted and often unexpected coupling-path. The sources of interference are easily found in fusion devices: to create and heat the plasma huge amounts of pulsed power are to be fed in. Diagnostic methods may also require intense power sources, for instance for laser or particle beams. Interference can also be caused by the strong B-fields produced in large volumes around the plasma. Victims in a fusion machine may be the control circuits of power sources; premature triggering as a result of interference is a well-known reason for malfunctioning. In addition interference can be very troublesome for the many measuring systems and the data acquisition and handling. This is particularly true,
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
190
P.C.T. van der Laan / Electromagnetic compatibility
since often only weak signals are produced by measuring systems stretched to their technical limit. The EMC-problems in fusion machines are becoming more important with the increase in size and complexity of the machines. The slow rise of the main fields in a tokamak may,produce not much interference, but additional heating schemes (neutral or rf) imply more high-voltage supplies, flashover risks, high pulsed power levels and vulnerable control circuits for the timing of the heating pulses. The use of computers for control and data-acquisition greatly improves the data handling and storage. On the other hand the interlinking electrical networks for control and dataflow open up new possibilities for electrical interference. A shot, lost as a result of interference, is costly. Not only is experimental time lost, but also incorrectly timed shots may damage or cause extra aging of equipment.
2. Standard approach to solve EMC-problems, an art In practice the EMC-problems in fusion machines are solved b y hard work and by trial and error. Experience and intuition also play an important role. The solutions advocated still have a somewhat magic character, which is understandable since also the precise nature of the interference is not free of mysteries. It would be highly desirable to develop the above sketched art into a science, to save time in the construction and start up of new larger machines and also to be more confident that effective measures can be built in. As part of this development a critical look at the models used is necessary.
3. Are adequate models being used? Especially in fusion an interesting contrast exists between the models used inside and outside the plasma. The plasma - for the sake of the argument assumed to be inside a torus - is described with the full Maxwell equations, together with the appropriate plasma equations. Outside the torns network theory is used to design the electrical and electronic circuitry. This split into two separate "worlds" is of course artificial. On one hand the plasma behavior, its equilibrium and its stability may very well be influenced by the interaction with the external circuit. That interaction certainly influences the decay of the toroidal plasma current, see for instance Van der Laan et al. [1]. On the other hand we can expect problems when the three dimensional circuitry
outside the torus is described with network theory, which as a zero-dimensional model is only an approximation of the correct Maxwell equations. The fact that network theory is not adequate, cannot be overcome by the use of full Maxwell equations. The electrical circuitry is usually far too complicated to work out the boundary conditions for, let alone to solve the Maxwell equations. In fact we design electrical engineering such that we operate in regions where either the E- or the H-field are at their maximum. The maximum in the E-field then corresponds with a capacitor, whereas the maximum in H corresponds with an inductor. In network theory we hide these fields in the impedance symbol, where only the relation between current and voltage at the terminals is considered. The connecting wires in a network diagram are also quite abstract; they only symbolize that current and voltage are faithfully transported from A to B. Note that the wires have no length in network theory; all that remains are Kirchhoff's equations and V - i relations for the various interconnected network-elements. In reality deviations from this simple, but extremely useful model are to be expected for long connections. We discuss this for the case of grounding; obvious deviations from network theory are also found near the torus.
4. Grounding Grounding is a difficult subject, mainly because the generally accepted definition of the noun "ground" is incorrect. Most standard definitions contain two elements (a) A ground can absorb or supply current without any change in voltage; in other words the ground should be a perfect sink or source for current. (b) A ground is an equipotential point or plane which serves as a reference for the circuit considered. We discuss and criticize both these elements. 4.1. Ground, a sink or source?
To have a sink or source for currents we need a capacitor where the charges can accumulate. This capacitor cannot be provided by Mother Earth. As an isolated sphere with an average radius of 6367 km the Earth has a capacitance of 708/~F, as follows from the equation CA =
4~'~or.
(1)
P. C. T. van der Laan / Electromagnetic compatibifity
191
I (a)
(b)
Fig. 1. Incorrect (a) and correct Co) models of what a grounding system can do. The thickness of the lines indicates the magnitude of the current.
Our electrical engineering activity however, never influences the complete E-field around the Earth. At best we produce charge displacements in a small part of the Earth surface and therefore we cannot benefit from CA in our grounding. As argued in more detail by Van der Laan et al. [2,3], we must conclude that the first element of the definition of " g r o u n d " is unrealistic. Instead, we must go back to the continuity equation for charges, or as a consequence of that to Kirchhoff's current law (KCL): the sum of all currents, including the capacitive current, into any point is zero. It is also correct to state that any current - if we properly include the displacement current - must flow in a closed loop. This last statement is very obvious for the plasma currents in a torus or for the currents in the big toroidal coils. Unfortunately the statement is seldom taken seriously for grounding currents (see fig. 1). A grounding system never resembles a sewer system (fig. la), where more and more sewage pipes converge into one main pipe with " u n k n o w n " destination. Instead a grounding system consists of a number of interlinked loops (fig. lb). N o t e that fig. l b shows only the low-voltage parts of all circuits. Secondly, the connection to Earth in fig. l b is no longer unique but is only part of another current loop [4], which can be omitted for portable equipment.
axis
Fig. 2. A toroidal shell in a tokamak or in a toroidal pinch. The rising toroidal field causes voltages differences between points A and B on the shell. Voltmeter 1 on the outside sees a large voltage; voltmeter 2 on the left sees no voltage.
shell with a toroidal slit on the outside (see fig. 2). This metal shell is similar to a single-turn coil of a toroidal thetapinch. A varying toroidal flux O r , only present within the shell, induces interesting voltmeter readings. Voltmeter 1 shows a voltage ~ E . d/ffi - d O r / d t , also equal to the voltage across the slit. Voltmeter 2 however, show no voltage, since E in the shell is zero and no flux external to the shell is present. Clearly we cannot describe this as a potential difference between A and B. We have to accept that the non-conservative induction E-field causes V ^ - Vs to be dependent on the integration path r E . dl, that is dependent on the position of the voltmeter leads. As a second example we consider toroidal voltages measured between two points A and B on the stainless steel bellows of the vacuum vessel (fig. 3). A voltmeter 1 with its leads on the surface of the bellows, sees a resistive voltage proportional to the current in the bel-
2
4.2. Grouna[ a point of equipotential? This second element of the standard definition of ground implies that a connection to ground fLXes the potential of the connected point. That is true in electrostatics and in network theory. When, however, distribo uted time-varying magnetic fluxes are present, the situation changes. We demonstrate this for wiring near the plasma chamber. First we measure poloidal voltages between two points A and B of a conducting toroidal
Fig. 3. Toroidal voltage between two points A and B on the stainless steel bellows (without break) in a tokamak. Voltmeter 1 sees a resistive voltage; voltmeter 2 sees a lower voltage because its leads also enclose part of the return flux. If an iron return leg (R.L.) is present, the voltages depend on whether or not R.L. is enclosed.
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P.C.T. van der Laan / Electromagnetic compatibility
lows. We discuss the case where we have no ceramic break in the bellows; obviously with a break current and voltage are zero. If we extend the measuring leads radially outward to voltmeter 2, also a part of the poloidai return flux is enclosed. If we move far enough out with voltmeter 2, no voltage is seen, given a situation with the flux returning through the air. With an iron return leg (R.L in fig. 3) present, the situation is again different; then it is important to know whether the loop formed by the measuring leads does or does not enclose the return leg. It is clear that the metal objects in these examples cannot be considered as equipotential surfaces. In fact the entire concept of potential loses its meaning in the presence of a time-varying magnetic flux where ~ E - d l differs from zero (see [5]). Such distributed magnetic fluxes which cause the failure of Kirchhoff's voltage law, (KVL), are quite common in large size electrical circuits. Grounding systems are in this category, in particular since the ground currents, often unknown in magnitude, flow in circuits with large areas.
5. New grounding rules The failure of the KVL, makes the concept of potentials "transported" by leads, meaningless. This is hard to accept, since network theory serves us usually so well. The advantage is however, that "jumping ground potentials" do not have to worry us anymore. In the analysis of a grounding system the current is much more meaningful than the voltage, with its dependence on the lay-out of the voltmeter leads. We therefore concentrate on the current. We redesign the dosed loops in which the currents flow to minimize magnetic fluxes. This requires a local and compact closing of current loops. By doing that we reduce self- and mutual inductances, and lower undesirable magnetic couping. We also obtain a more orderly design and move in the direction where network theory is again valid. The compact dosing solves the grounding problems locally. The connection with Mother Earth then only carries current in case of lightning or - depending on the regulations - for safety grounding.
6. Extended grounding structures In the description of extended grounding structures, for instance meant for a fusion experiment, we follow
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Fig. 4. (a) A torus with subsystems mounted in a screen room and grounded on the floor panel. (b) The floor panel is retained as the most important part because it carries most of the current. The subsystems are shielded. (¢) The most important part of the floor is that below the leads. That part is retained, now formed in the shape of a conduit. Only one of the subsystems is shown.
three lines of thought, to emphasize different EMCaspects of grounding. (a) To reduce the distributed magnetic fluxes we might install a big screen room around the torus and its various subsystems (fig. 4a). Short grounding connections to the floor give low fluxes; the currents in the screen room walls limit the magnetic fluxes and do not produce additional flux. Since the grounding currents flow predominantly in the floor, we may omit the side walls and the ceiling. Metal shielding around the subsystems is more useful (fig. 4b). Finally we note that fluxes are minimized when the leads are mounted close to the metal floor. The currents in the floor are then localized under the leads. We again take out a large part of the floor, and leave only the metal under the leads, in the form of wide metal strips, metal conduits or metal tubes (fig. 4c). At the high frequencies often responsible for interference, the magnetic field lines have to go around these wide metal conductors; the weakened magnetic field produces then not much flux.
P.C.T. van der Laan / Electromagnetic compatibility
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other pair of leads. Without the conduit the common mode circuits encircle large areas; in many cases the return path is left to chance because all design work concentrates on the differential mode circuits. Clearly the conduit, if continuous, makes the common mode circuit quite compact and keeps it well under control. One might argue that the conduit creates a new common mode circuit with wiring or soil at a larger distance. This is indeed true, but the separation between inside and outside, provided by the conduit, should give adequate protection.
7. Protection of measuring instruments Fig. 5. Interference currents I talon8 a length ! of the braid of a coaxial cable (a) or along the outside of a metal conduit, couple in small interference fluxes in the inside loop, which result in an interference voltage Vi given by I i I Z T, where Z T is the transfer impedance.
Moreover the shape of the conduit, or better still that of a tube, forces the field lines away from the leads, and reduces thereby the transfer impedance, as discussed in the next paragraphs. (b) The braid of a coaxial cable has the same favorable geometry as the tube. Nevertheless interference currents in the braid can couple in undesirable voltages via the so-called "transfer impedance" (fig. 5a). At high frequencies this transfer impedance increases linearly with frequency because the holes in the braid allow external magnetic field lines to penetrate. Clearly for a solid metal conduit this effect disappears (fig. 5b). A residual transfer impedance is here caused by magnetic flux which comes in from the top. A larger height to width ratio helps; for a tube the transfer impedance becomes negligibly small at high frequencies. Note that the interference current flows on the outside of the conduit, whereas the magnetic fields around the leads cause the flow of currents on.the inside of the conduit. Clearly we have a useful separation between these currents, especially for high frequencies. To have the benefits of this separation, we must have all conduits properly connected, between sections and to the subsystems where they end. Shielded cables inside the conduit should be grounded at both ends; the ground loop thus formed enclosed very little flux and is harmless. (c) This last point brings us to the third line of thought, where we consider common mode currents, such as the net current in a coaxial cable, or in any
At the boundaries of the extended grounding structure or at the shielded subsystems leads often have to come in from the outside. These "points of entry" require special attention. When for instance power leads come in, an appropriate filter should divert common mode currents to the grounding structure. Similarly coaxial cables may carry signals to sensitive measuring instruments. High-amplitude local Eand H-fields may cause interference signals in these instruments. Usually however, the measuring instrument proper is a rather small "receiving" antenna at quite a distance from the interference source. Much more interference is brought in by the leads, often in the form of common mode currents. Most instrument housings cannot safely carry such currents; interference cow pies in through a too large transfer impedance of the housing. The solution is to divert the common mode currents to an extra EMC-cabinet (fig. 6) before harm is done. At the point of entry the cable must be connected to the panel over 360 o. When all incoming leads, including the conduit are all attached to the rear panel, the front can often be left open. This situation is convenient for viewing, for instrument adjustments and for ventilation. Similarly in pinch experiments screen room doors could often be left open during shots, even though the many spark gaps produced a lot of interference. Also there a correct handling of the interference currents at the points where the leads came in, was more important than the current flow near the screen room door. The measuring instrument must also be protected against interference signals, which arrive as differential mode signals, indistinguishable from true signals. Such interference signals can couple in via the transfer impedance of the coaxial cable. Several solutions exist: a
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P.C.T. van der Laan
/ Electromagnetic compatibility
I I
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larger signal amplitude, a better cable, copper tubing around the cable or the use of fiber optics. N o t e however, that many sensors, in use in fusion research, such as Rogowski coils, magnetic probes and some voltage sensors [6,7] also provide a solution with good E M C quality. Since differentiated signals are cartied in, the signal amplitude increases with frequency, just as most interference signals do. The signal to interference ratio can therefore remain high before and after the integration where the high frequencies are attenuated proportionally to 1/,o. The attenuation by the first part of the integrator, (or the first part of a regular attenuator, used when signals of large amplitude are carried in) is very important for the EMC-behavior. The components should be mounted in a shielded box close to the rear panel. Moreover passive linear elements ( R and C's) should be used to suppress interference bursts before they reach (essentially non-linear) electronics.
8. Comparison with single point grounding In single point grounding one attempts to avoid groundloops and one keeps the grounding currents separate, at least up to the "single point" (fig. 7). If all currents did flow in one direction, as in a sewer (fig. 7a) this would indeed reduce coupling, although beyond the
Fig. 7. Single-point grounding (a) is based on the sewer idea; all currents flow to the sewer. In fact currents can flow in opposite directions, for instance as a result of induction, when connecting cables c are present (b). A more compact grounding (c) is much better; the lower horizontal line can be a conduit.
single point things remain mysterious. If the currents circulate (fig. 7b), as they may do when connecting leads c are present, large loops are formed unnecessarily. A much better solution is to reduce the area of the loops (fig. 7c). When the lower line in that figure stands for a conduit, we are again in the situation of fig. 4c, recommended in this paper. N o t e that ground loops can be very useful, for instance in shielding or in the metal of the conduit discussed. Also a circulating current in a loop is less bothersome than a breakdown, with its new interference production, across a " b r e a k " in a loop. The ground loops formed by the braid of signal cables in a deep conduit are harmless, as has already been mentioned.
9. Fusion technology, useful in other fields We already mentioned in section 7 the differentiating sensors, c o m m o n in fusion research: Rogowski coils, magnetic probes and dV/dt sensors [5]. These types of sensors, with their good E M C properties have been successfully used in power engineering, a field in which these sensors are rather new. Measurements in highvoltage substations could be carried out with high band width and without interference problems [7]. A protecting cabinet, such as shown in fig. 6 is necessary; a stand-alone version with a power filter permits measurements in the field, even in high-interference surroundings. The Rogowsld coil, in a flexible version turns out to be very useful for the analysis of existing grounding systems. As advocated in this paper, the study of cur-
P. C. T. van der Laan / Electromagnetic compatibility
rents and the identification of circuits is the key for understanding and redesign. Pulsed power sources, borrowed from fusion technology are very useful to inject pulsed currents in grounding grids or in antenna towers. To simulate lightning strikes, current pulses were induced repetitively in a (guyed) antenna tower, by means of a toroidal B-coil mounted in two halves around the tower [8].
10. Conclusions In the analysis of EMC-problems network theory often turns out to be inadequate. This is especially true near the torus and for the grounding systems. We should forget the dangerous " j u m p i n g potentials", and instead concentrate on the currents. We redesign current loops to minimize fluxes and coupling; this means that loops should be made compact and should be closed as locally as possible. A grounding structure for a fusion machine can be built with metal conduits between the torus and its subsystems. This completely interconnected star provides local grounding and a safe pathway for all leads. Differentiating sensors combined with appropriate cabinets allow EMC-correct measurements in high-interference surroundings, in fusion, in power engineering and in lightning research.
Acknowledgements The author thanks his many colleagues, earlier in fusion research and more recently in power engineering and in EMC-research for many valuable discussions,
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essential for the development of the ideas described in this paper. Of the Eindhoven group ir. M.A. van Houten and dr. A.P.J. van Denrsen and all the staff involved in measuring expeditions deserve special thanks.
References [1] P.C.T. van der Laan, W. Schuurman, J.W.A. Zwart and J.P. Goedbloed, On the decay of the longitudinal current in toroidal screw pinches, Proc. 4th Intern. Conf. on Plasma Physics and Contr. Nuclear Fusion Research, Madison, USA, IAEA-CN/28/B-4, Vol. I, 1971, p. 217-223. [2] P.C.T. van der Laan and M.A. van Houten, Design Philosophy for grounding, Proc. 5th Intern. Conf. on EMC, York, 1986, p. 267-272. [3] P.C.T. van der Laan, M.A. van Houten and A.P.J. van Deursen, Grounding philosophy, Proc. 7th. Intern. Symposium on Electromagnetic Compatibifity, Zfirich, 1987, 105Q3 p. 567-572. [4] H.W. Ott, Ground, a path for current flow, IEEE Intern. Symposium on EMC, 1979, p. 167-170. [5] P.C.T. van der Laan, Voltages in toroidal pinch experiments, Los Alamos Report LA-7335-MS (1978). [6] R. Keller, Wideband high voltage probe, Rev. Sci. Instrum. 35 (1964) 1057-1060. [7] E.J.M. van Heesch, J.N.A.M. van Rooij, R.G. Noij and P.C.T. van der Laan, A new current and voltage measuring system; Tests in a 150 kV and a ~ kV GIS., Proc. 5th Intern. Symposium on High Voltage Engineering, Braunschweig, 1987, Vol. 1, Paper 73.06, 4 p. [8] A.P.J. van Deursen, M.A. van Houten, E.W.L. van Engelen, P.F.M. Gulickx, P.C.T. van der Laan, E. Zwennes and A.J. van Dongen, Measurements of currents around and in large grounded structures, Proc. 19th Intern. Conf. on Lightning Protection, Graz, 1988, p. 143-148.