FI98009C - Interference suppression method for the electricity transmission network and connection in the electricity transmission network - Google Patents

Description

98009 Interference suppression method for electrical energy transmission network and connection in electrical energy transmission network

The invention relates to an interference cancellation method according to the preamble of claim 1.

The invention also relates to a connection in an electrical energy transmission network.

10 Nonlinear loads that do not draw current sinusoidally but in the form of a sharp-edged pulse cause harmonics in the network. Such loads are, for example, compact fluorescent lamps with an electronic ballast which, at the half-wave peak of a voltage of 50 Hz, receive a current pulse lasting only 1.5 ms. In a modern office environment, all workloads may be of the same poor quality: computers, copiers, printers and air conditioners with electronic speed control. Em. the largest harmonic component-20 ti due to the load, the third harmonic is added almost arithmetically to the neutral conductor of the electrical energy transmission network. In practice, almost twice the currents have been found in the neutral conductor compared to the phase current. The neutral conductor does not have overload protection and as the load increases, the danger situation is obvious. In addition, 25 third harmonics have been found to interfere with computer equipment and data connections.

The object of the present invention is to obviate the shortcomings of the above-described technique and to provide a completely new type of interference cancellation method for an electric power transmission network as well as a connection in an electric power transmission network.

The invention is based on placing a band-stop filter with a center frequency equal to the third harmonic between the ground point of the load and the star point of the main transformer in the neutral conductor.

More specifically, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1 98009 2.

The connection according to the invention, in turn, is characterized by what is set forth in the characterizing part-5 of claim 5.

The invention provides considerable advantages.

With the solution according to the invention, the overload of the neutral conductor is prevented and at the same time the magnetic fields caused by the currents of the neutral conductor are substantially reduced. The power losses caused by the third harmonic interference component are reduced. Interference with telecommunication and computer equipment is also reduced. The impedance of the filter used is very small at the short-circuit current due to the saturation of the iron circuit.

The invention will now be examined in more detail with the aid of exemplary embodiments according to the accompanying figures.

Figure 1 schematically shows one electrical connection according to the invention.

Figure 2 shows a circuit diagram of one filter solution available in the invention.

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Figure 3 shows a side view of one alternative to implement the inductance of Figure 2.

Figure 4 shows graphically the frequency dependence of the resistance of one of the filters of Figure 2.

According to Figure 1, the TN-S electrical energy transmission network comprises a main transformer 2 having a star point 12, phase conductors L1, L2 and L3, a neutral conductor N and a protective earth PE. In the electricity grid 35, electrical energy is transmitted on the frequency f0, which is in Europe

typically 50 Hz and 60 Hz on the American continent. Loads 5 and 5 'are connected between the phase conductors L1 to L3 and the neutral conductor N. Loads 5 and 5 'are connected to neutral N

3 98009 from connection points 4 and 4 '. The neutral line N, in turn, is connected to the actual ground potential at ground point 3. According to the invention, the third harmonic (3 * f0) filterable band-stop filter 1, 1 'or 1 ·' is connected to the neutral line 5 N of the load 5 (or 5 ') zero point 4 (or 4') and the transformer 2 star points 12 or alternatively between the actual earth potential 3.

As shown in Figure 2, the filter is typically a series-parallel circuit tuned to the frequency of the third 10 harmonics, in which the series connection of the inductance L and the resistor R is connected in parallel with the capacitor C. The resonant frequency is thus 3 * f 0, in Europe 150 Hz and in America 180 Hz, respectively. When R is small, approximately 1-G) 2LC = 0 applies to the resonance of the circuit, where in the case of 50 Hz ω = 2π * ΐ50 1 / s. According to the invention, the inductance can be either an air, iron or ferrite core. The air gap of the iron or ferrite core is dimensioned so that the core material is not saturated with a current flowing in the neutral conductor that is twice the nominal phase current. The inductance L is preferably formed from a plurality of small iron core chokes connected in parallel. In this case, the sum of the conductor cross-sections of the coils connected in parallel must be at least equal to the cross-sectional area of the neutral conductor. The filter impedance Z is according to the following formula: R · (1 ~ <ice2LC) + J0) L (1 + -) Z = -T- ± -, R ~ (jH2LCRs, ,, a 1-0> 2LC + -S + jor (l + ic + - + - = rf t rf where

Rt t k = -, a .-—, T-RC, TL = -, RF represents peripheral core losses R RF R $ 4 98009

The eddy current losses of the choke core are proportional to the square of the frequency. In order for the choke Q value to be good enough, either a ferrite material or a thin, particularly low-loss "electric plate" must be used as the core material.

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Figure 3 shows one example for implementing the coil L of Figure 2. The cross-sectional area of the neutral conductor is assumed to be 35 mm2 for a nominal current In = 125 A. The capacitance of capacitor C is chosen to be 2.64 mF. The resonance of the filter at 150 10 Hz is then obtained by a coil L with an inductance of 0.426 mH. The first approximation of the inductance of one coil of a filter can be represented by the following formula when the width 10 of the air gap 7 and the cross-sectional area AFe of the iron circuit center column 10 are: • o 15 where N is the coil speed and μ0 is the permeability (1.256 μΗ / m).

In this case, a value of 7.6 mm is obtained for the air gap 10 when a value of 116 is used as the number of revolutions N and a value of 10.9 cm 2 is used as the cross-sectional area of the center column 10.

The E-shaped Fe circuits 8 are ground so that the air column has an air gap of 1 ^ / 2 = 3.8 mm. When two identical irons 8 are placed opposite each other, the desired width of the total air gap 7 becomes 7.6 mm.

A thread of 3.0 mm diameter is wound 116 turns around the center post 10. In order for the conductor cross-sectional area to be at least 35 mm2 of the neutral conductor cross-sectional area, for example, 10 coils are required in parallel. The coil body 9 is pressed between the connected irons 8.

According to Figure 4, the impedance of the filter reaches its maximum value of 10 ohms at 150 Hz and the impedance drops below

II

5 98009 0.15 ohms at 50 Hz.

In the following, it is considered whether the Fe circuit is saturated with a current flowing at 250 A / 50 Hz in the neutral conductor N. Then the current of one coil 5 is I = 1/10 * 250 A = 25 A

The peak value of the magnetic field is given by LI = m = N - A ..

P. 4.26-25 ^ 10-3 ^ 116-10.34-10 "4m2 10 When the Fe circuit of the filter 1 becomes saturated, harmonics are formed in the network.

In the following, the saturation of the magnetic circuit is evaluated by computer loads, the interference voltage caused by which can reach 15 values of 40 V with the third harmonic component (150 Hz).

U = N (ice - AFe v / 2 £ (150Hz) = -40v / 2V- = Q _ 5Qr 116 · 300π - -10.34-10-½2 s The inductions caused by the 50 Hz and 150 Hz current components are summed arithmetically and thus affecting the saturation of the iron core.

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The short-circuit current from the saturation of the Fe circuit has the advantage that the reactance of the filter decreases to a fraction corresponding to the air-core coil. The fuse operates quickly in the event of a short circuit, so there is no fault in the neutral conductor N of the harmonics acting for less than one second.

98009 6 The passage of an asymmetrical current of 50 Hz through a third harmonic filterable filter according to the invention causes a phase shift of the phase voltages. Thus, the filter should be dimensioned so that this displacement is less than 5 8% of the phase voltage. In other words, the inductance of the filter should be dimensioned at 50 Hz below a certain value. This condition can be represented by the formula: 0.08 * | c / n ^ "v / I-100 π-J *

In addition, the filter of the third harmonic interference component according to the invention must be dimensioned so that the input of the current flowing in the neutral conductor and the impedance of the filter at 150 Hz is at least equal to the interference voltage at 150 Hz.

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Claims (7)

A method for eliminating interference in an energy transmission network comprising a main transformer (2) having a star point (12), phase conductors (LI, L2, L3) and a zero conductor (N) connected at one end (3) the ground potential and the star point (12) of the transformer (2) and at its other end (4) to loads (5), in which energy transmission networks are transmitted electric power at a frequency f, characterized in that - a band interlock (1, 1 ', 1' ') with an average frequency of 3 * f is connected between the star point (12) of the transformer (2) and the connection points (4, 4 ') of the loads (5, 5') - 2. A method according to claim 1, characterized in that a band barrier filter (1) with an average frequency of 3 * f is connected between the ground potential (3) of the zero conductor (N) and the connection points (4) of the loads (5). 3. A method according to claim 1 or 2, characterized in that a series parallel resonant circuit is used as a band-bar filter (1). 25 4. A method according to claim 3, characterized in that, as an inductance (L) in the series resonant circuit, a coil equipped with an iron or ferrite core is used, the air gap of which is dimensioned such that the iron circuit (8) is not saturated with double phase mark current (50 Hz) when interference component (40 V) of the third harmonic simultaneously overlies the filter. 5. An electrical power transmission network comprising 5. a main transformer (2) having a star point (12), phase conductor (LI, L2, L3) connected to the main transformer (2), - a neutral conductor (N) connected to the earth potential (3 ) And the star point (12) of the main transformer (2), and - loads (5) coupled between the phase conductors (L1, L2, L3) and the neutral conductor (N) connected to the neutral conductor (N) connection points (4), characterized in that a Oxygen (1, 1 ', 11') having an average frequency of 3 * f is connected between the star point (12) of the main transformer (2) and the connection points (4, 4 ') of the loads (5, 5'). 6. A coupling according to claim 5, characterized in that a band-locking flutter (1) having an average frequency of 3 * f is connected between the ground potential (3) of the neutral conductor (N) and the connection points (4) of the loads (5). 7. A coupling according to claim 5 or 6, characterized in that the band-bar filter (1) is a series parallel resonant circuit. 1 Coupling according to claim 7, characterized in that the inductance (L) of the resonant circuit is a coil equipped with an iron or ferrite core, whose air gap is so dimensioned that the iron circuit (8) is not saturated with double phase mark current (50 Hz), when a disturbance component (40 V) of the third harmonic simultaneously overlies the filter.