FR2779583A1 - ANTI-POLLUTION SYSTEM FOR ELECTRICAL NETWORK - Google Patents

Abstract

The invention concerns an antipollution system for an electric network wherein the alternating generation is ensured, in normal operating conditions, by at least one alternating current generator which supplies the network three-phase electrical energy, the latter being transported by means of a three-phase power cable, said cable output power, called "point of regulation", which is adjusted, for supplying at least one load, wherein the alternating grid is a grid with fixed or variable frequency, said system comprising at least one non-linear load supplied by said grid and at least a device for parallel compensating of the current harmonics generated by said non-linear load(s), said device absorbing the harmonic currents in phase opposition with the polluted current harmonics of the load(s).

Description

ANTI-POLLUTION SYSTEM FOR ELECTRICAL NETWORK

DESCRIPTION

  Technical Field The present invention relates to a system

  antipollution for electrical network.

  BACKGROUND OF THE PRIOR ART Principle of electrical generation - a known aircraft network On board certain aircraft, for example of the AIRBUS type, there are currently two types of electrical networks: - an alternative network;

- a continuous network.

  The alternative generation is ensured, in normal mode, by alternators (IDG or "Integrated Drive Generator") coupled to the reactors and whose driving speed is regulated via a

  hydraulic system (CSD or "Constant Speed Drive").

  These supply the three-phase electrical energy of the network under (115/200 V, 400 Hz) and have a nominal power of 90 kVA or 115 kVA. In case of failure of one or more generators, a turbine (APU or Auxiliary Power Unit), located at the rear of the device, drives a generator of 115 kVA. If none of these sources are available, there is, as a last resort, a small alternator, with a power of the order of 5 kVA, driven with a retractable wind turbine. For each alternator, the electrical energy is transported by means of three-phase power cables ("Feeders") to the electrical cores, located under the cockpit and from which fuel (ovens, fans, etc.) are fed. ..). The supply voltage, at the output of the three-phase power cable, is regulated to be maintained at 115 V 2 V. It is commonly

  called "p.o.r." ("Point Of Regulation") voltage.

  Continuous generation, in normal operating mode, is obtained by conversion of

  alternative energy using transformers-

  rectifiers (TR). The continuous is mainly used to power calculators and relay logic. If no alternative source is available, two batteries can support this generation,

  with an autonomy of about thirty minutes.

  The future alternative network We are now moving towards variable frequency reciprocating networks, to overcome the use of the CSD hydraulic system, which is a complex and expensive system regulating the drive speed of the alternators so that those These provide electrical quantities of fixed frequency whatever the engine speed. By then directly coupling the generators to the reactors, the frequency of the network varies according to the different

  flight phases, in a range from 380 to 800 Hz.

  Until now, most of the equipment connected to the AC network was linear (asynchronous motors, ...) and did not pose any problems with the quality of the currents and

network voltages.

  As part of the plane "more electric" appear new users of the network: electric actuators, autopiloted motors, ... These new consumers have the distinction of using static converters, which absorb non-sinusoidal currents (so-called loads "nonlinear") and thus alter the quality of the network. In addition, on a variable frequency network, the number of non-linear loads will necessarily increase: in fact, some devices do not accept to operate with a variable power frequency. Therefore, in order to supply them at constant frequency, it is necessary to consider straightening the network voltage and then waving it at the desired frequency, hence an inexorable increase in the number of

  static converters connected to the network.

  Influence on the network of harmonics of current consumed by non-linear loads Each phase of the network can be modeled by a source of sinusoidal voltage E, of pulsation

  o, in series with the internal impedance Zr of the network.

  Thus, the harmonic currents absorbed by a non-linear load 10 have the effect of deforming the voltage at the control point (por) via the impedance Zr, as illustrated in FIG. 1: V. = E-Zr ( f) * (ifond + Iharm) with: - E perfect sinusoidal voltage; - Zr (f) the impedance of the network at the frequency f; - ifond + Iharm the current absorbed by the load

non-linear 10.

  In order to characterize the deformation of the voltage wave V ", the voltage harmonic distortion ratio, denoted THDv and defined by: VH2 THDv h = 2 Vf is used with: Vf effective value of the fundamental of V; Vh effective value of the harmonic of rank h

of Vs.

  The current harmonic distortion rate is likewise defined by: I: h2 THDi = h = 2 If Due to the presence of equipment sensitive to electrical disturbances, there are standards (ABD0013 and DO 160) which set the limits of harmonic content of the current and especially the network voltage. These current limits are: - THDv 5 with (Vh / V1) <4% Vh

- THDi 30%.

  Example of a studied case On a variable frequency network, many devices must be connected to the network via a rectifier. The most widespread and easiest structure to implement is a diode bridge followed by an inductor and a

  capacitor, as shown in FIG.

  The load 10 supplied by the rectifier stage 13 is simply modeled by a resistor. The value of this one is fixed by the power of the equipment which one wishes to represent (4 kW for example for a fan, typical case of load which one will consider). The value of the capacitor is even higher than the power of the load is high. In a modeling of the overall system, the network, seen from the control point, consists of the IDG alternator in series with the three-phase power cable. An external three-phase power cable may have a direct impedance comparable to a 40 mΩ resistor in series with an inductance of

pH approx.

  The voltage at the terminals of the network is written: V, (t) = E (t) - Zri. he (t) - Zrh. ih (t) Vl (t) h = 2 Vh (t)

  (the index 1 represents the fundamental magnitudes).

  E represents the f.e.m. of the machine.

  OnIa Zrl = Zd (feeder) + Zl (machine) a: LZrh = Zd (feeder) + Zh (machine) Z1 is the synchronous impedance of the machine (1 to 2 ohms in reduced unit) and Zh its impedance called "subtransitory "(0.1 to 0.15 ohm in reduced unit,

for an alternator of 90 kVA).

  For the study of network voltage deformations due to nonlinear charges, the source can be modeled by a perfect voltage source in series with the impedance Zrh (see Figure 2). In the case of a single non-linear load, connected to the control point via a given three-phase impedance wiring, the modeling of the studied case is illustrated in the figure

  3, the network being referenced 11 and line 12.

  Simulation software, for example the SABER software as described in documents [1],

  [2] and [3] at the end of the description, allows to simulate

  physical systems of various kinds and particularly electrical systems. The programming is done by wired diagrams integrating pre-existing components and user-created elements, described by programs whose syntax uses a behavioral language, for example the MAST language as described in FIG.

document [4].

  The calculation itself is done with a variable step. Depending on the calculated values, the simulator validates them or goes back

modifying the calculation step.

  For different load values, this may possibly represent several "small" loads connected to the control point via a diode bridge (the overall load current being the same in both cases), and for = 400 and 800 Hz, the following harmonic distortion rates are measured at the mains and the voltage at the control point: f 400 Hz 800 Hz 4 KW THDv: 3.8% THDv 4.3% THDi: 59% THDi: 32.% 12 KW THDv: 5.5% THDv: 8.6% THDi: 29% THDi: 24% KW THDv: 10% THDv: 15% THDi: 23% THDi: 19% Solutions considered to reduce disturbances It can thus be seen that as soon as the power absorbed by the non-linear load (s) exceeds about ten kW, the distortion rates take unacceptable values. We can consider two solutions to reduce these rates of harmonic distortion: * A "sinus absorption" rectifier: the diode bridge of each nonlinear load is replaced by a converter controlled so that the currents it absorbs are sinusoidal (or at least as much as possible), in phase with the voltage at

regulation point.

  * A parallel compensator of harmonics: one places at the point of regulation a device which compensates the harmonics of current rejected by the

charge.

  Sinusoidal Current Converter The purpose of such a converter is to provide rectification of the AC mains voltage while absorbing currents with the lowest possible distortion, in phase with the current.

network voltage.

  As illustrated in FIGS. 4 and 5, the rectifier 20 consists of six switches 21, 22, 23, 24, 25, 26 formed by semiconductors SC1, SC2 with closed and controlled opening, associated

to diodes D1 and D2.

  This rectifier ensures the exchange of energy between the network, considered as an alternating voltage source 115 V, and a capacitor 30 considered as a source of continuous voltage. It is preceded by three inductors 27, 28, 29 (one per phase), in order to be driven in three-phase current source by

report to the network.

  The rectifier is controlled so that it absorbs currents faithfully following three-phase sinusoidal currents of reference isrefl (t),

isref2 (t), isref3 (t).

  The creation of a three-phase sinusoidal current reference Isref is obtained by multiplying a three-phase sinusoidal unit function Stu, in phase with the mains voltages, by the amplitude Ire

desired current.

  The amplitude Isref is obtained at the output of a regulator circuit 31 of Vc (PI = propotional and integral). The operating principle of this rectifier is illustrated in FIG.

  used, being illustrated in Figure 6.

  To maintain the voltage V 0 at a constant value, regulation is used, as illustrated in FIG. 7, where p, (t) being the instantaneous active power on the mains side and Isref the amplitude of the desired current. The integration constant Ti is chosen

  equal to the time constant t of the system Ti = t = (RC) / 2.

  To achieve the synthesis of a unitary three-phase sinusoidal signal in phase with the mains voltage, the three voltages at the control point are first filtered in order to extract the fundamentals. The filter used is a Butterworth filter of order 4, cutoff frequency

  fc = lkHz and slope -80 dB / decade.

  We then obtain, for each phase, a sinusoidal signal at the fundamental frequency of the voltage at the control point (variable between 380 and 800 Hz), but out of phase with the latter. To "recalibrate" this signal in phase with the voltage, it is projected in the fixed two-phase reference (a, A) and the angle of the rotary vector obtained (with respect to the axis c) is calculated. This angle is then expanded to compensate for the phase shift introduced by the Butterworth filter.

  phase shift being variable with the frequency of the network.

  The angle 0 obtained is that of the fundamental of the voltage

at the point of regulation.

  Its successive passages by 0 make it possible to know continuously the frequency of the network and thus to calculate, at the end of each period, the phase shift at

  introduce on y during the next period.

  The unitary three-phase sinusoid (sl (t), s2 (t), s3 (t)) is then obtained in phase with the network voltage by:

  (sl (t), s2 (t), s3 (t)) = (cos, cos (O-2ir / 3), cos (O-47c / 3)).

  FIG. 8 illustrates the synthesis of a unitary three-phase sinusoid in phase with the network voltage. The rectifier can take eight different states, summarized in the following table: STATUS t1 t2 t3 VcSl Vcó2 Vcs 3

0 0 0 0 0 0 0

  1 1 OO 2Vc / 3 -Vc / 3 -Vc / 3 2 0 1 O -Vc / 3 2Vc / 3 -Vc / 3 3 1 1 O Vc / 3 Vc / 3 -2Vc / 3 4 OO 1 -Vc / 3 -Vc / 3 2Vc / 3 1 0 1 Vc / 3 -2 Vc / 3 Vc / 3 6 O 1 1 -2 Vc / 3 Vc / 3 Vc / 3

7 1 1 1 O O O

  The matrix that makes it possible to express the - + coordinates of a three-phase vector X in the two-phase fixed reference frame (a, a) is the following: n = 2 2]

P (2,3) 2V 2

2 2

  Thus, for each state of the rectifier, it is possible to represent the voltages V 0, V 2, V 0, 3 by a vector in the reference (a, 3). The following table and FIG. 9 recall what are the possible values of the module of the argument of this vector as a function of

the state n of the rectifier.

STATE O 1 2 3 4 5 6 7

  The equation which at any moment governs the evolution of the current absorbed by the converter is: d Is ((cn) Vc Vc V Vc A4V '- No 0Nn 27 3 2n 3 No t) --- '- + L ds () - Vs (t) - Vcs (t) (1) dt The system of the simple voltages of the network is written: Vsi (t) = Vs-, cos (, ot) Vs2 (t) = Vs / - cos (<ot -2) vs3 (t) = Vs 2 cos (cot -3 In the frame (a, A), - this system gives a rotating vector: Vs (t) [Vs (t) Vs3eicOt v5p (t) j Graphically, equation (1) is thus represented, at a moment and for any state of the rectifier (state 3 for example), by the vectors of the

figure 9.

  If we consider a time interval [to, tj] such that the variation of the current in this interval can be considered as linear (t1 - to "-): 2 = t -> Is Is (t) = Is (to) + At. Dt to 't We suppose IS (to) Isref (t0) There exists a unique vector which, applied at the limits of L, makes it possible to have: IS (ti) = Isref (tl) We note: SL ds] dt this vector, we also note:

P .... P7

the seven vectors ^ L dI4s tdtJ

possible at t = to.

  We control the value of V, at 300 or 400 V, according to the dynamics (that is to say the variations of

  current possible) required for the system.

  Real-time vector control of the rectifier In order to optimize the switching frequency of the switches and thus to reduce the switching losses of the converter, vector control can be used as described in FIG.

referenced document [5].

  This command works here in real time with a decision frequency fe = 100 kHz: all

the 10 gs.

  A referenced document [6] describes such a sinus absorption converter under vector control. We denote by L s the vector which, applied to the dst terminals of L during te, allows to have between two instants to = kte and t, = to + te the relation:

-4 - +

  I s (t1) = Isref (to) (2) is: Is (to) + te L dtI: sref (to) (3) 10d To vector L dJ corresponds to a single input vector of the rectifier, denoted Vcs * ( t0) and given by dt Vs (to) - Vcs * (to). According to equation (3), this vector is thus determined by:

  Vcs * (to) = Vcs (to) - (L / te). I sref (to) - Is (to).

  By applying this vector during te to the rectifier input, the relation (2) is thus realized

-4 -4 -4

  Is (ti) = Isref (to). Thus, among the seven vectors V cs possible, we choose to apply a linear temporal combination of three vectors, the two vectors adjacent to Vcs (to) Vsal (to) and Vsa2 (to) in the frame (ac, To) and the null vector, so as to realize the

relation (2).

  The application times of the different vectors are limited, in minimum value, to 1 gs, in order to have conduction times of the controlled switches sufficiently high and compatible with the

existing technologies.

  The operating principle of this

  control is shown in Figure 11.

  In the vector representation example

--4 -4 - + ->

  of Figure 12: Vcsal (to) = Vcs (3) and Vcsa2 (to) = Vcs (2).

  The simulation parameters are: C = 1000 gF, L = 200 AH and Vcref = 300 V. Without filtering, the HF harmonics due to

  this time switches are centered on 100 kHz.

  A simple unamortized LC filter, of resonance frequency f = 15 kHz (1 = 35 aH, C = 3.2 gF),

  suffices to eliminate sufficiently these HF harmonics.

  The following distortion rates are found: <400 Hz 800 Hz Pch 4 KW THDv; 1% THDv 0.9% THDi 3.8% THDi 3.3% 12 KW THDv 1% THDv = 1.2% THDi 1.1% THDi 1.4% The disadvantages of current sinusoidal absorption converters, controlled in real time using the vector control, are its weight and cost, all non-linear loads must then be replaced by these converters. The subject of the invention is an electrical generation system using a parallel harmonic compensator which alone makes it possible to reduce the disturbances produced by a set of non-linear loads and therefore not to present the

  disadvantages of the converter described above.

  DESCRIPTION OF THE INVENTION The present invention relates to an antipollution system for an electrical network in which the alternative generation is ensured, under normal conditions, by at least one alternator which supplies the three-phase electrical energy of the network, the latter being transported to the using a three-phase power cable, the output power of this cable, called "control point", which is regulated, to supply at least one load, the alternating network being a fixed or variable frequency network, said system comprising at least one non-linear load fed by this network, characterized in that said system comprises at least one device for parallel compensation of the current harmonics generated by at least this (or these) non-linear load (s) , said device absorbing the harmonic currents Ic, (t) in phase opposition with the harmonics of the currents

  polluted Ich (t) of the (or) charge (s).

  Advantageously, the parallel compensation device comprises at least one rectifier driven in

  current associated with a capacitor.

  This rectifier may consist of six switches formed by controlled closing and opening semiconductors, associated with diodes, preceded by three inductors: one per phase, this rectifier ensuring the exchange of energy between the

network and the capacitor.

  Advantageously, this rectifier is controlled by a vector control, the three-phase reference that follows the current absorbed by the compensation device corresponding to the anti-harmonics of the

charging current.

  In one embodiment, the system of the invention comprises time extraction means

real load current.

  Advantageously, a Fourier calculation module is activated at the end of each period of the network. Advantageously, each rectifier driven in

  current compensates for at least one harmonic.

  The Fourier calculation module may comprise: means for detecting zero crossing of the angle θ of the network; means for calculating the Fourier coefficients of ich: ah and bh;

  means of putting into form Ih cos (hot -

4h);

- Phase advance means.

  Advantageously, the system of the invention may comprise three compensators. For example, it is possible to implement a system comprising: a compensator for harmonics of ranks 5, 7, 11 and 13; a compensator for harmonics of ranks 17 and 19; a compensator for harmonics of ranks 23 and 25; in order to compensate for all the first harmonics of

charging current.

  In an exemplary embodiment, said system comprises: a DSP module responsible for generating, all gs, the three reference currents of the compensator which, based on load currents and network voltages sampled at a frequency fe = 100 kHz, ensures the regulation of the DC voltage, the digital filtering of the network voltages and the calculation of the harmonics absorbed by the load at the end of each period of the network; an FPGA module responsible for ensuring the vector control of the rectifier, the acquisition of the inputs of this module being synchronized with the generation of the

DSP module outputs.

  The system of the invention does not have the disadvantage of a "sinus absorption" rectifier, since it alone makes it possible to reduce the disturbances produced by a set of non-linear loads. Using the optimized vector control and using the real-time extraction method of the harmonics of the load current, the performances obtained are satisfactory over the entire range of load power and frequency considered (0 -> 70 kW, 400 -> 800 Hz). This solution is compatible with currently available technologies. The system of the invention is not limited to the electrical networks installed on board an aircraft. It is also applicable in many other fields where alternative electrical energy is used to feed polluting loads (consumer electrical distribution, ships, trains ...)

Brief description of the figures

  FIG. 1 illustrates the modeling of a phase of the network on which a non-linear load is connected; FIG. 2 illustrates the modeling of the considered network as seen from the control point; FIG. 3 illustrates the modeling of a given system; - Figures 4 and 5 respectively illustrate a bridge arm and the operating principle of a sinusoidal absorption rectifier; FIG. 6 illustrates the electrical quantities used; FIG. 7 illustrates the DC voltage regulation; FIG. 8 illustrates the synthesis of a unitary three-phase sinusoid in phase with the network voltage; FIG. 9 illustrates the vector Vcs (n) and the representation of equation (1) in reference system (a, 3); FIG. 10 illustrates a dual damped RC filter; - Figure 11 illustrates the principle of a vector control; FIG. 12 illustrates an example of a vector representation; FIG. 13 illustrates the system of the invention; FIG. 14 illustrates a regulation loop of the voltage Vc; FIG. 15 illustrates the compensation of the reactive power consumed by the load; FIG. 16 illustrates a first embodiment of the system of the invention; FIG. 17 illustrates a regulation loop of V0; FIG. 18 illustrates a second embodiment of the system of the invention; - Figure 19 illustrates the calculation of

harmonics to compensate; -

  FIG. 20 illustrates an exemplary practical embodiment of the system of the invention; DETAILED DESCRIPTION OF EMBODIMENTS The invention relates to an antipollution system

for electrical network.

  The object of the invention is to compensate for the harmonics generated by one (or more) non-linear load (s), as previously described (diode bridge + capacitance + inductance +

  resistance) or any other type of polluting load.

  In order to make the currents of the sinusoidal network, the system of the invention comprises at least one parallel compensator which absorbs harmonic currents ice (t) in opposition to the harmonics of the polluted currents ich (t) of the load. The schematic diagram of such an arrangement is illustrated in FIG. 13. As shown in this figure, the receiver convention for the load and the compensator and the generator convention for the network is taken. A first embodiment of the system of the invention is illustrated in FIG. 16, using the same references as those of FIG. 3. The structure of the compensator is similar to that of the sinusoidal current absorption converter: a rectifier is used current-driven 40, associated with a capacitor C. To drive it, using an optimized vector control 41. But the three-phase reference Isref, which follows the current absorbed by the compensator, is no longer sinusoidal; it corresponds to the "antiharmonics" of the charging current. In this embodiment the switches are considered perfect and bicommandables, but we take into account the overall losses of the compensator, placing a resistor Rpe in parallel with the capacitor C. The losses of the

  compensator are estimated at approximately 300 W.

  The capacity of capacitor C on the continuous side of the compensator is, for example, equal to 1000 gF, in order to

  to have a good quality of continuous voltage.

  Compensation by indirect servo-control of the mains currents to a reference three-phase sinusoid In this first embodiment, the mains currents are slaved to a three-phase reference sinusoid Isref (t), in phase with the voltage at the control point. The three-phase reference of the current absorbed by the current-driven rectifier is then: Icsref (t) = Isref (t) - Ich (t) The reference of the network current Isref (t), as for the sinus absorption, is obtained by regulating (42) the DC voltage Vc of the rectifier, necessary for its proper operation. This regulation is again based on an instantaneous power balance between the input and the output of the rectifier 40, which gives the following relation: Vc2 (P) = Rpe Pcs RpeC + 2 p o Rpe is the resistance corresponding to the losses of the compensator. Assuming the perfectly sinusoidal network currents and merged with their three-phase reference I sref (t), we always have ps (t) = 3V. By noting Pch the instantaneous active power consumed by the load 10, we now have p (t) = p (t) + pch (t). The regulation loop is

illustrated in Figure 14.

  The power pch appears as a

disturbance.

  As before, the three-phase sinusoidal reference Isref of the grid current is obtained by multiplying (44) Isref by a sinusoidal three-phase unit function Stu (43) in phase with the grid voltage. The latter is then synthesized. The compensator does not only absorb the "harmonic" charge current, since it puts the current Is in phase with the voltage Vs. It also compensates for the reactive power consumed by the load. As illustrated in FIG. 15, the vector i comp (background) ("background" for fundamental component) makes it possible to go from a vector ih (background) to a vector

  network in phase with the voltage p.o.r.

  The compensator consumes a weak active power, corresponding to its losses: Pcomp = 3.Vp.o.r. Icomp (background) .sin (6), of the order of 300 W; and provides a reactive power opposite to that consumed by the load:

  15. Qcomp = 3.Vp.o.r. Icomp (background) .sin (6) = 3.Vp.o.r. Ich (background) .sin (,).

  Several non-linear loads can be connected to the control point and can operate at different times. The regulation of V, must. therefore be robust vis-a-vis variations in the active power Pch consumed by all loads. The instantaneous active power consumed by the load is written in the form Pch (t) = Pch + Pch (t), Pch (t) being the fluctuating power absorbed by the load and resulting from the composition of the harmonics of ranks

  different from the voltage and charging current.

  RC * If one chooses xi = T = -, one dimensionizes the regulator so that the answer of Vó with a step of Vcref, outside any variation of the perturbation Pch, is fast:

(p) =, with - 2ms.

c2ref 1+ -p a a

  During a Pch step, the error s = Vc2 - VC2ref then responds to Pch according to the transfer function: R (T) p H (p) = E

  (p) = a Pch (1 + 'rP 1 ± pa P with: - C = 1000 gF, Vcref = 400 V, - Rpe = 500 Q to have losses 300 W. we have t = 250 ms, and : FI (a) Pch (1 + Tp). At a Pch step, s tends to 0 with a time constant of 250 ms, which is too slow for the application in question (the compensation is

  "lost" for several hundred ms).

  RC * If one chooses -i T = -, one has: Rti p Ho (p (P) = C ai Pch 1 + 'rip + RC T 2 2 a More Ti decreases and more an error tends quickly towards 0. Indeed by decreasing Ti, the integral gain is increased, thus the error decreases more rapidly, choosing ci = 0.015, which allows the error to cancel in approximately 40 ms, without oscillations, during a step of charge power

12 kW.

  The response of Vc to a step of Vcref is, then, more satisfactory (overshoot, high response time, ..). This is not a problem because, in steady state, Vcref is fixed and only the robustness of the regulation with respect to load variations matters. Moreover, the tensioning of the compensator is not realized with a Vcref step but with an appropriate signal allowing to bring, without problems,

V, at its reference value.

  Consider the case of a single pollutant load connected to the control point whose power consumption varies and can therefore represent a set of identical loads of powers more

low.

  We choose Vcref = 400 V and L = 80 gH, which allows the compensator to have a satisfactory dynamic for all the powers considered. As for the sinusoidal absorption, a passive filter is placed at the control point which allows to eliminate on the current the HF harmonics due to the switching of the rectifier. The chosen structure is that of Figure 10, with the same values of

components.

  It can be seen that the first embodiment of the system of the invention makes it possible to obtain satisfactory results up to a load power of approximately 20 kW. In fact: 400 Hz 800 Hz Pch-KW THDv: 2% THDv: 4.5% THDi: 6.5% THDi: 10% KW THDv: 3.2% THDv: 5.8% THDi: 5, 6% THDi: 9% KW THDv: 4.5% THDv: 8.5% THDi: 5.2% THDi: 9.5% A problem of this first embodiment lies in the 10 As of delay introduced by the command of the rectifier. The reference being followed with 10 ace of delay, each harmonic generated by the compensator is out of phase with its reference, this phase shift increasing with the rank of the considered harmonic. This first embodiment therefore has the following advantages: * It is easy to implement; the

Calculations made are simple.

  * The switches switch at 65 kHz, on average, while the frequency of calculating the

control is 100 kHz.

  On the other hand: * We compensate obligatorily all the harmonics: we can not choose to not compensate

  than the overriding harmonics.

  * The harmonics reinjected are out of phase with the harmonics of the reference (they are late), which makes the compensation

  partial, or even ineffective for HF harmonics.

  * Reactive power compensation implies oversized current

components of the compensator.

  * It is not possible to introduce a limitation on the current absorbed by the compensator, under penalty of losing the control of the compensation

during this limitation.

  * The regulation of the DC voltage of the

  compensator is sensitive to load variations.

  In order to no longer be confronted with these various problems, a second embodiment of the system of the invention has been developed, based on the real-time extraction of the harmonics of the load current. It is thus possible to choose the harmonics to be compensated, and in particular to adjust the phase of the reference of each harmonic to be reinjected, in order to take into account the 10 gs of delay introduced by the command

of the rectifier.

  Compensation by real-time calculation of the harmonics of the charging current In this second embodiment, illustrated in FIG. 18, in order to perform the parallel harmonic compensation, one or more current-driven rectifiers 40 are used, each enabling

  compensate for one or more harmonics.

  For simplicity, consider the case of a rectifier 40 compensating for a harmonic charge current of given rank h: the three-phase reference of the current it must absorb is then: I csref (t) = I x (t) - Ih (t) o: * Ih (t) corresponds to the three-phase harmonic current of rank h of the load current; * Ix (t) is the fundamental component of the current absorbed by the rectifier, corresponding to its losses (no more compensation is

reactive power).

  To enslave the current absorbed by the rectifier to this reference, an optimized vector control 41 is used, with a calculation frequency

fcalc = 100 kHz.

  The fundamental component Ix that the rectifier must absorb is obtained by regulating (42) its DC voltage Vc illustrated on

Figure 17.

  Ph represents the fluctuating power term absorbed by the load resulting from the composition of the network voltage (assumed here purely

  fundamental) with the harmonic h of the charging current.

  For a harmonic ih (ch) of the charge current of rank h = 6 k 1, and assuming that we can assimilate the network voltage to its fundamental only, of amplitude V ,, the instantaneous power resulting from the composition of these two terms are written (it is a fluctuating power): h (t) = 3. Vs Ih (ch) co (6knt + ak)

  O xk is the phase of ih (ch) relative to the reference.

  In steady state, this term causes a ripple (very low, of a few volts at most) on the DC voltage Vc. During a load variation, that is to say during a variation of Ih (ch), the disturbance introduced on the regulation of Vó remains negligible and the regulation is therefore robust vis-à-vis

of this disturbance.

  By neglecting the term Ph in the synthesis of the regulator and then compensating for the pole of the open-loop transfer function (Ti = =, (712 we obtain between the error s = Vc2 - Vc2ref and P the following relation: H ( p) = p (p) R = T) Pch a + Tp + - paa being the quantity defined previously calculated so that the response of Vc to a Vcref step is fast in

closed loop (TBF = 2 ms).

  We consider the case of the preponderant harmonic of the charging current (of rank h = 5), that is to say the one which is likely to cause the most disturbances in the control loop. A 20 to 40 kW (large variation but one that can be envisaged) active load power step at 800 Hz causes I5 to change from 15 A (eff) to about 25 A. Taking Ik = 0, we simulate with the simulation program the theoretical response of V to this variation, corresponding to the function of

H transfer (pl above.

  The voltage Vc stabilizes in about 5 ms, the ripple from 1 V to about 2 V. The disturbance introduced by the load variation is therefore negligible: the pole of the open loop transfer function is compensated; the validity of this choice is checked during simulations carried out on the

complete system.

  In FIG. 18, a "Fourier calculation" module 45 is placed between a sensor 46 situated between the regulation point and the line 12 leading to the diode bridge and a comparator 47 connected to the control 41. This module is activated at the end of each period of the network and is carried out in several successive steps: 1) The end of each period T of the network is detected by zero crossing of the angle 0 of the network, which is synthesized. We then calculate the Fourier coefficients ah and bh of the three load currents on

this period.

  2) For a given phase, and assuming that the harmonic content does not change from one period to another (ah and bh are assumed to be unchanged), the harmonic of rank h of the load current over the next period s' then write, with this = 0 (t): ih (<ch) (t) = ah.cos (hot) + bh.sin (hot), which we put in the form ih (ch) (t) = Ih.cos (hot-ih) A harmonic content variation between two periods can be due to: - a frequency variation of the network. However, the maximum frequency variation of the network between two successive periods is 0.05%, therefore the frequency variation of each compensated harmonic is also 0.05% maximum, which proves to be negligible vis-à- compensation screw; a variation of the power absorbed by the load; the compensation is then partial during

one to two periods of the network.

  3) In order to obtain the reference of the compensator over the next period, the phase of this signal is advanced by an angle θ, so as to take into account the 10 gs of delay introduced by the optimized vector control: ih (t) = ih (ch) (t + Ph / ho) that is: ih (t) = Ih.cos (h) t-h + h), with

Oh = (2r * h * 10.10-6) / T.

  If necessary for technological reasons, the value of Ih can be limited. The totality of the harmonic h is not then compensated, but the paralleling of a second compensator makes it possible to easily compensate, if necessary, the remaining part. 4) We then apply to the compensator (-ih (t) + ix (t)) as reference current, until

following passage of 0 by 0.

  For each of the phases, the computation of the harmonic h to be compensated is thus performed as

illustrated in Figure 19.

  The object of the invention is to maintain the voltage harmonic distortion rate THDv at a value of less than 5% if possible, throughout the

  frequency range of the considered network (380 Hz -

  > 800 Hz) and for loads that can consume a total of about 70 kW. The considered alternator having a rated power of 90 kVA, this represents a maximum nonlinear charge rate of almost 80%, which is considered possible on a variable frequency network. That's why we compensate all the first

  harmonics of the charging current up to rank 25.

  This one always has a weak amplitude, but corresponds to a high frequency. It can therefore have a significant repercussion on the voltage at

  Regulatory point, especially at high power.

  For this we use three compensators: - one for harmonics of ranks 5, 7, 11 and 13 (noted Cl); - one for harmonics of ranks 17 and 19

(C2);

  one for harmonics of ranks 23 and 25 (C3). It is difficult and not optimal to use a single structure to compensate for all these harmonics, because the reference of the compensator then obtained has extremely steep edges, which we do not know how to follow in the 10 outs with Reasonable inductance (with too low values, the RF ripple generated is too much

  important on the "flat" parts of the reference).

  For the three compensators, Vcref = 400 V and L = 80 gH for C1, L = 160 pH for C2,

L = 200 gH for C3.

  The filter placed at the control point for eliminating the current HF harmonics (100 kHz) due to the breakdown of the rectifier, has the structure of Figure 10 with here: If = 20 pH, Cf = 4 AF, Rf = 3 Q This filter has a cut-off frequency of approximately 15 kHz and makes it possible to obtain a compromise

  filtering / dissipated power satisfactory.

  The following results are then obtained: 400 Hz 800 Hz Pch KW THDv: 3% THDv: 1.9% THDi: 2.3% THDi: 1.2% KW THDv: 3.9% THDv: 3.8% THDi: 1.8% THDi: 1.8% KW THDv: 5.5% THDv: 5.5% THDi: 1.7% THDi: 2-% KW THDv: 6.2% THDv: 6.1% THDi: 1 , 6% THDi: 1.7% Harmonic compensation is satisfactory, the harmonic distortion rate current THDi being maintained at values around 2%. The "quality" of the compensation is the same regardless of the load power considered, but the residual current harmonics have amplitudes even greater than the load power is high. The harmonic distortion rate in voltage THDv is therefore even higher than the charge power is important, but remains less than or equal to

About 6%.

  This second embodiment has the following advantages: The number of adjustment parameters of the system is high, which makes it "flexible" of use: one can choose to compensate only the overriding harmonics, possibly limiting (for technological reasons ) the level of each

harmonic to compensate.

  * The 10 gs of delay introduced by the command on each harmonic are "compensated" and the performances remain- therefore satisfactory for

  frequencies and high load powers.

  * The reactive power consumed by the load is no longer compensated, each compensator absorbs only a very low fundamental current, corresponding to its losses (cos4 still remains between 0.95 and 1). As a result, the instantaneous maximum value of the current absorbed by each compensator also remains lower than in the

* first embodiment.

  * The regulation of the DC voltage of the compensators is much more robust vis-à-vis the variations of load, so the control of the system is

  very well preserved during these variations.

  On the other hand: * The computing power needed is more

  higher than for the first embodiment.

  Practical embodiment example In an exemplary practical embodiment of the second embodiment, the following control elements are used for each compensator: * A DSP module (digital signal processor) which is responsible for generating, all

  As, the three reference currents of the compensator.

  For this, from load currents and network voltages sampled at a frequency fe = 100 kHz, it provides the regulation of the DC voltage, the digital filtering of the network voltages and the calculation of the harmonics absorbed by the load at the end of each period of the network. The time required to acquire the data and to calculate the references,

  supposedly known and fixed, must be less than 10 gs.

  In order not to generate errors on the compensation, harmonic phase is introduced to compensate

  an advance corresponding to this calculation time.

  The fastest current DSPs have a cycle time of 25 ns: this allows at least 400 elementary instructions to be executed in 10 gs (which is sufficient for the application in question), since several instructions can be processed in the same cycle. . An example of DSP-having such performance and adapted to this command is the TMS320 C50 from Texas Instruments A sampling frequency of 100 kHz amounts to taking only five points per period on the 25th harmonic of charging currents at 800 Hz , which is small but sufficient to preserve the essentials of

  information contained in these signals.

  FIG. 20 presents the complete synoptic of the compensation thus made for a harmonic of

  rank h and a phase (phase a for example) given.

  * An FPGA module, which is a "network of

  user programmable logic gates ".

  It is responsible for ensuring the vector control of the rectifier, that is to say, to calculate all the gs, the vectors to be applied as well as their application times. These calculations are simply based on

  comparisons and elementary operations.

  The acquisition of the inputs of this module is synchronized

  with the output generation of the DSP module.

REFERENCES

  [1] "Saber Simulator, Introduction To The Saber

  Simulator "(Analogy, Inc., June 1991).

  [2] "Welcome To The Saber / DesignStar Introductory

  Race! "(Analogy, Inc., 11/11/93).

  [3] "Simulation For Circuits Through SABER Systems"

(Analogy, Inc., 1993).

  [4] MAST Modeling Class, Modeling Language For The

  SaberSSimulator "(Analogy, Inc. 1/3/94).

  [5] Joachin Holtz's "Pulsewidth Modulation - A Survey" (IEEE Transactions on Industrial Electronics,

  volume 39, number 5, December 1992).

  [6] "Indirect Current Control Of A Unity Power Factor Sinusoidal Current Boost Type Three-Phase Rectifier" by Juan W. Dixon and Boon-Teck Ooi (IEEE Transactions On Industrial Electronics, Vol.

number 4, November 1988).

Claims (11)

  1. Antipollution system for the electrical network in which the alternative generation is ensured, under normal conditions, by at least one alternator which supplies the three-phase electrical energy of the network, the latter being transported by means of a three-phase power cable , the output power of this cable, called "control point", which is regulated, for supplying at least one load, the alternating network being a fixed or variable frequency network, said system comprising at least one load ( 10) non-linear powered by this network, characterized in that said system comprises at least one device for parallel compensation current harmonics generated by this (or these) load (s) non-linear (s), said device absorbing the harmonic currents Ic5 (t) in phase opposition with the harmonics of   polluted currents Ich (t) of the charge (s).   2. System according to claim 1, wherein the parallel compensation device comprises at least one current-driven rectifier (40) associated with a capacitor (C).   3. System according to claim 2, wherein the rectifier (40) consists of six switches formed by controlled closing and opening semiconductors, associated with diodes, preceded by three inductors: one per phase, this rectifier ensuring exchange of energy between the network and the capacitor.   4. System according to claim 3, wherein the rectifier is controlled by a vector control (41), the three-phase reference that follows the current absorbed by the compensation device corresponding to the harmonic of the charging current. 5. System according to any one of   claims 1 or 2 comprising means   extraction of the charging current in real time.   The system of claim 5 including a Fourier calculation module (45) activated at   the end of each period of the network.   The system of claim 6, wherein each current-controlled rectifier compensates for least one harmonic.   8. System according to claim 6, wherein the Fourier calculation module comprises: zero crossing detection means of the angle 0 of the network; - means for calculating the coefficients Fourier de ich: ah and bh; -   means of putting into form Ih.cos (hwt -% h); - Phase advance means.   9. System according to claim 5 comprising at least one compensator so as to compensate for all the first harmonics of the current of charge.   10. System according to claim 5 comprising calculating means for generating the three reference currents of the compensator and responsible for the control. vector of the rectifier.   11. System according to claim 10, wherein the calculation means are a DSP module for the generation of the three reference currents and a FGPA module for the vector control of the rectifier. 5