Power quality in supplying welding networks in the automotive industry

Transcription

1 Power quality in supplying weling networs in the automotive inustry Dietrich Stae, Helge Schäfer, Peter Ulrich Preface In many inustrial sectors, electric resistance weling is a stanar prouction process. The short pulses of current on which the weling process epens cause huge, rapi variations in the apparent power ( ). The uration (T) of a weling pulse will be between 60 an 600 ms, an these pulses are repeate 10 to 150 times a minute, which is the repetition rate (r). These successive suen variations in apparent power bring about voltage changes, eviations an flicer which may cause unacceptable or troublesome interference in the power networ. It often goes unnotice that the variation in the apparent power (cause by loa variation) brought about by weling machine usage is manifol (as much as three-, four-, or fivefol) in relation to the nominal capacity (rate power) when switche on for 50 % of the time. Liewise, it may be as much as 80 % of the short-circuit power. Hence remeial treatment is necessary. The consequences (such as flicer) of unue voltage changes an eviations may be: an operating ban for the machine/station (for breach of compliance) reuce prouctivity of the machine reuce prouct quality (e.g. inaequate strength of wele joint) costly remeies of electric installation in terms of time an money belate installation of compensatory equipment lea to a rise of unplanne investment costs isruptive effects on en-user installations across ownstream, upstream networs or on the same level as each other a isruptive influence on information an automation equipment fluctuations in the luminance emitte by luminaires (flicer). There are istinctions to be mae between public utility gris an inustrial networs. The amount of interference permitte to be cause by voltage changes in public utility gris is limite by their reference curve (Fig. 1), where: meium voltage (MV): Grenz = 2 %; low voltage (LV): Grenz = 3 % (for r < 0,01 min -1 : MV: max 3 %; LV: max 6 %) To prevent a public low voltage gri exceeing a total flicer level of P st = 1, iniviual networ users must ahere to the following values: short-term flicer value (10-minute value) P st = 0.8 an long-term flicer value (120-minute value) P lt =

2 Fig. 1: Reference curve ref (r) for P ref = 1 an regular, rectangular voltage changes [1] In an inustrial power system it is useful to follow the above limits at all IPCs, while, at the same time an at all costs, taing note at the planning stage of all prescribe operating features an operational requirements. Principles of calculation The voltage change ( U) which is prouce by variations in apparent power ( ) is, as a goo approximation, the variation in the longituinal voltage. Using the equations = P 2 + Q 2 an Q ϕ = arc tan P the figure for relative voltage change when the loa is symmetrical an three-phase comes out as: U = 100% = U S S v : ψ : v cos ( ψ ϕ ) 100% minimum short-circuit value at point of loa variation impeance angle an for single-phase loas with a connection at L1 an L2 for the external wire to neutral wire voltage: L1 N L2 N L3 N S S 0 v v 3 cos 3 cos ( ψ ϕ + 30 ) ( ψ ϕ 30 ) 100% 100% 2

3 For rectangular (equiistant) voltage changes the permissible voltage change ( zul ) for an iniviual networ user with a nown repetition rate (r) is easy to etermine. Using the equation: zul ( r) () r Grenz P= 1 in per units (p.u.) the answer is: ref ( r) ( r) = 0,8 ( r) Grenz ref to enable the short-term flicer value to be maintaine as (P st =0.8) an to enable the longterm flicer value to be ept at (P lt = 0.5). zul ( r) ( r) = 0,5 ( r) Grenz ref If the reference curve is approximate in accorance with Fig. 1 (P ref = 1) in the area covere by r = 1 min -1 up to r = 1000 min -1, for square, equiistant voltage changes (F = 1), using mathematical analysis, then: It follows that: r Pref. = ( ref F) = 1 F: form factor (here F = 1) 27 P an thus that: = ( F) r 27 P = 0.36 F r in %; r in min -1 The form factor can be looe up in Fig. 2 for the rectangular pulses prouce, for example by weling machines, by referring to the pulse uration (T). If the voltage changes of iniviual flicer sources (i) are low in probability of coincience, then the sum of the flicer effect will be: P = P i 3, 2 Σ. Fig. 2: Form factors for rectangular pulses [2] 3

4 Flicer isturbances in a 20-V public gri resulting from an unregulate weling machine an how to cure them n unregulate pressure weling machine with phase C current operating in a company s low-voltage networ may cause an unacceptable long-term flicer value in a public 20-V gri when it is switche on. Fluctuations in the luminance (or flicering of the light), P st > 1, will result. Fig. 3 is a highly simplifie iagram of the gri showing the most important gri etails for the 20-V gri an the figures for the pressure weling machine (PWM). Fig. 3 simplifie iagram of the gri Flicer calculation (by mathematical analysis) leas to the following algorithm: 2.22 Relative voltage change: = cos % = 1.4 % 143 Form factor (Fig. 2): T = 240 ms F = 1.3 Long-term flicer: P = = Result: = 1.4 % < Grenz = 2 % is o, but flicer level is very high: P lt = 1.56 > P lt Grenz = 0.5 lt Here, the limit for human recognition of a flicer is significantly exceee. Reuction of the flicer level by means of a compensatory system is preferable to costly changes to the gri (for example, raising of the short-circuit power S ). Flicer compensation If a compensation conenser is switche on at the same time as the weling pulse starts, it will ieally be possible to eliminate the relative voltage change. The compensatory figure for = 0 will be QC = P ( cos ψ + tanϕ) = 2. 06MVr. 4

5 It must be sai that staggere switching of the compensatory equipment either on or off (with a elay of T V ) will give two pulses. The repetition rate r will be ouble an the permissible range of the interference pulses will be etermine by the length of the elay. s the T V is reuce, the flicer will be brought own to the permissible level. P lt P Grenz = 0.5 requires a form factor of: 0.5 F = 0.34 an this is reache at T 3,2 V 7 ms (see Fig. 2) The effect is achieve with a compensation system with thyristors as this is able to cope with fast an frequent switching. The moment for switching the thyristors is ecie by the current measurement or by irect control from an external trigger, e.g. on the weling machine. How to ensure voltage quality in the case of a number of unregulate weling machines In a nown case, when the weling shop of an automotive manufacturer which containe a number of weling machines with various power usage was relocate, there was a reuction of the short-circuit power in the 20-V networ supply own to approx. 60 % of the original. The power supply to the weling shop is now provie by a 20-V networ internal to the company supplying three equal 525-V low voltage networs with rate power of 7.5 MV which are exclusively use for the operation of unregulate weling machines with threephase C current (see Fig. 4). It was necessary to select a suitable compensation plan an to organise the relevant equipment. Fig. 4: Highly simplifie gri iagram for the networ supplying power to the weling machines In all three weling networs (Fig. 4) there are two heavy-uty pressure weling machines which may affect voltage by as much as 8 % when weling at high power. In aition, there are about twenty small an meium-size pressure weling machines in each of the networ sections. The 20-V networ is also use to supply other epartments within the company. mong other things, a number of lights are connecte to the three networ elements. 5

6 The following picture, ifferent from that in the first case shown above, has evelope in respect of maintenance of voltage quality: - The in-company electricity supply is couple to the public gri on the 110-V sie (as a PCC). This is the point at which the requirements liste above ( 2 %; P lt 0.5) have be guarantee, quite simply in this case because of the high shortcircuit power in the 110-V gri. - It is aequate for the in-company 20-V networ (at IPC1) to limit the voltage changes to 2 %, thus avoiing interference with prouction equipment. figure of 1.5 p.u. for the P lt can be tolerate, as this oes not excee the recognisability level in the ownstream low-voltage networs with neon striplights use for illumination. - Maintenance of the flicer level is irrelevant for the 525-V weling networs themselves (at IPC2). The size of the permissible ip in the voltage epens on the associate lowering of the weling power for the unregulate weling machines so there is no loss of wel quality. In the case cite, it was foun that the permissible voltage rop was = 6 % (i.e. 94 % of the agree supply voltage is maintaine). Iniviual compensation was not use in any of the weling machines, even in the largest, because, as shown in the first exemplary case, it is not possible to limit the voltage rop in this situation. Remey n intelligent limiting system for the gri or networ which gives the signal for a weling machine or cascae of weling machines to start up will ensure that the voltage rop in the low-voltage networ affecte is partially reuce in such a way that there will always be power at least 94 % of the contractual voltage serving each weling operation. This limiting metho must be use in conjunction with a rapi-response reactive power compensation system, in the form of an IGBT-converter for the output current with DC lin. In the present case, the compensatory power was set to enable maximum prouctivity in the weling complex if the limiting value of = - 6 % is applie. The alternative, unacceptable networ restrictions which woul have impacte on prouction were thus not necessary (see Fig. 5). 6

7 Fig. 5: Structure of system, simplifie The metho reuces the compensatory power require to about 25 % of that which woul be neee for iniviual compensation of weling machines. s such, it offers significant savings on investment costs while proviing full assurance that prouctivity requirements are met. In the same case, it was also necessary to chec the flicer situation in the 20-V networ. For this purpose, a worst case scenario was taen in which the two largest weling machines in each element of the LV networ, which normally wor asynchronously, were to cause a voltage rop in each part of the LV networ of at most 6 % when woring in conjunction with the compensatory equipment. The calculations an values applie to this case for woring out P lt were : T = 500 ms (F = 1.3 for rectangular pulses as in Fig. 2) NS = 6 % r = 8 The flicer value which results from the aggregate flicer of the three networs for the meium-voltage networ (at IPC1) is thus: 95.4 P lt ( IPC1) = = There is no isturbing effect for the wors in general from a long-term flicer value of P lt = 1.5 in the lighting (fluorescent strips). Bibliography [1] DIN EN (VDE 0846 Part 0): ; Berlin-Offenbach [2] Grunsätze für ie Beurteilung von Netzrücwirungen 3. revise eition 1992, publishe by the Vereinigung Deutscher Eletrizitätswere VDEW e. V. 7