MERLIN GERIN KNOW HOW. THM filtering and the management of harmonics upstream of UPS

Transcription

1 MERLIN GERIN KNOW HOW THM filtering and the management of harmonics upstream of UPS

2 THM filtering and the control of harmonics upstream of UPSs Authors : S. BERNARD - J.N. FIORINA - B GROS - G. TROCHAIN Contents abstract Introduction. UPS and harmonics...3 background information on UPS Harmonics upstream of UPS...4 origin of harmonics and theoretical analysis...4 actual observations...6 conclusions Controlling harmonics upstream of UPS...8 preventing the effects of harmonics...8 meeting user needs and compliance with standards Solutions for controlling upstream harmonics...10 possible solutions...10 choke in the rectifier input circuit...10 LC passive filters pulse...13 THM active filtering Comparison and conclusions...16 THM approach...16 comparison of solutions...16 conclusions...17 MGE UPS SYSTEMS MGE0246UKI 02/2000 1

3 1. Abstract Authors: S. BERNARD J.N FIORINA B. GROS G. TROCHAIN THM filtering and the control of harmonics upstream of UPS UPS (Uninterruptible Power Supplies) produce harmonic currents on the upstream power system. The reason for this is that they draw their input power from the upstream source via a rectifier-charger. These harmonic currents distort the current (THDI) and voltage (THDU), increase the rms value of the current and reduce the power factor. Harmonic currents must therefore be controlled to avoid the negative effects of distortion, such as the malfunctioning of sensitive electronic devices, premature aging of equipment, flicker and the derating of sources. Users must also control harmonics to obtain "clean" electrical installations complying with the IEC /EN guide for devices with input currents exceeding 16 A/ph. Four filtering solutions used with UPSs are presented and the advantages and drawbacks of each discussed. The solutions fall into two categories: low-cost solutions based on standard passive filters and high-performance solutions based on a THM active filter. The THM active filter from MGE UPS SYSTEMS proves to offer the highest performance and is the only solution to comply with the IEC /EN guide. This type of filter is in fact a simplified version of the SineWave active harmonic conditioner, optimized for use with a UPS. A comparison of the different solutions reveals that the 12-pulse filter is today obsolete. MGE UPS SYSTEMS MGE0246UKI 02/2000 2

4 1. Introduction. UPSs and harmonics background information on UPSs UPS systems (uninterruptible power supplies) are power electronics interfaces installed between incoming utility power and loads that are highly sensitive to the quality and continuity of the supply of electrical power. UPSs supply dependable power, i.e. without interruptions and with a consistently high level of quality, compatible with the operation of sensitive loads. Double-conversion UPS (also known as on-line systems) are used extensively for medium to high-power applications ( 10 kva). They are made up of (see figure 1): rectifier/charger at the input that draws power from the utility and rectifies it to charge or float charge a battery and support the inverter; battery that provides backup power in the event of a power failure; inverter that supplies power with a consistently high level of quality (frequency, voltage, etc., within tight tolerances), to the load static switch used, when necessary, to transfer the load without interruption to the utility via a bypass (thus enabling downgraded supply; manual bypass used for maintenance operations. This type of UPS can operate in the following modes: normal mode: inverter supplied via the rectifier charger; backup mode: inverter supplied by the battery; bypass mode: load supplied directly by the utility, via the bypass. Fig. 1. Simplified diagram of a high-power double-conversion (on-line) UPS (bridge rectifier on the input). Given its role as an interface between the utility and sensitive loads, and its component parts, the UPS may be considered as: a non-linear load, i.e. a source of current harmonics for the upstream distribution system, because it is connected to the upstream system via the input rectifier/charger; a power source for the downstream loads that are, generally speaking, also nonlinear loads that cause harmonics. These two conditions make it necessary to take the following into account in UPS operation, particularly for compliance with standards: control of harmonics reinjected upstream; immunity from downstream harmonics. In this document, we will examine UPS operation mainly concerning the upstream side, with respect to the problems caused by harmonics and we will look at the solutions proposed by manufacturers to control upstream harmonic pollution and ensure "clean" UPS operation. MGE UPS SYSTEMS MGE0246UKI 02/2000 3

5 2. Harmonics upstream of UPS origin of harmonics and theoretical analysis the Graëtz bridge rectifier The presence of harmonics upstream of a UPS is due to its basic design, i.e. the fact that it draws power from the upstream AC system via a rectifier/charger. The function of the rectifier/charger is to charge or float charge the battery at a constant voltage and to supply the power demanded by the inverter. The most common type of rectifier/charger is based on a Graëtz bridge with SCRs (see figure 2), combined with a very high-impedance smoothing inductor (or choke) such that the output current Id is perfectly smooth, DC. waveform of the input current The input currents of each phase, supplied by the source, are made up of rectangular waveforms (I1, I2, I3) in figure 3, containing harmonics. Each SCR conducts the current for one third of a cycle. The line currents I1, I2 and I3 each stay on, in turn and for one third of the cycle, the value and waveform of the rectified current Id, which is a direct current if the smoothing impedance is considered infinite. If, at the same time, the source impedance Zs is assumed to be equal to zero, the current immediately reaches the value Id when an SCR starts to conduct. Fig. 2. Diagram of the rectifier/charger. Fig. 3. Theoretical currents upstream of the rectifier/charger with infinite downstream smoothing impedance and zero source impedance. theoretical spectrum of harmonic currents These square-wave currents are the sum of the harmonic currents according to the Fourier series expansion. This type of bridge, referred to as a six-pulse bridge (because the rectifier has six legs), in fact generates harmonics of the following orders only: n = 6 k ± 1, where k has the values 1, 2, 3, etc. (whole numbers). The rms value of each harmonic current depends on that of the fundamental H1 (for which the value is approximately 0.78 Id), i.e.: H1 Hn =. n The amplitude of the harmonics decreases as their harmonic order n increases. The spectrum of the harmonic currents is shown in the following table T1 and figure 4. MGE UPS SYSTEMS MGE0246UKI 02/2000 4

6 2. Harmonics upstream of UPS (cont.) Table T1. Theoretical spectrum of harmonic currents. Harmonic % of H1 H5 20% (1/5) H7 14% (1/7) H11 9% (1/11) H13 8% (1/13) H17 6% (1/17) H19 5% (1/19) Fig. 4. Theoretical harmonic spectrum. rms value of the current Irms= ( I ) + ( I ) + ( I ) + ( I ) i.e. I rms = 1.05 I1. power factor Because the rectifier/charger at the UPS input is a non-linear load, the power factor λ is defined by the general relation: P λ = S P (kw) is the active power of the UPS. S (kva) is the apparent power of the UPS. Because of the harmonics: S = P + Q + D Q (kvar) is the reactive power. D (kva) is the value of the distorting power caused by the harmonics. Consequently, the harmonics represent a loss equal to the distorting power they consume, which reduces the input power factor. current and voltage distortion The total harmonic distortion of the current (abbreviated to THDI), measures the relative value of the harmonics with respect to the fundamental. The spectrum presented above corresponds to a level of distortion equal to: THDI = ( I ) + ( I ) + ( I ) i.e. approximately 30%. 5 7 I In this theoretical case under consideration, the voltage distortion (THDU) is equal to zero because the source is assumed to have zero impedance (infinite power). MGE UPS SYSTEMS MGE0246UKI 02/2000 5

7 2. Harmonics upstream of UPS (cont.) actual observations influence of the source impedance and switching In fact, a number of factors modify these theoretical values and result in voltage distortion. Current variations are not instantaneous and an overlap (two SCRs conducting simultaneously) occurs during switching. The line current does not have a perfectly rectangular waveform and its harmonic content is slightly modified. The smoothing impedance is not infinite and so current Id does not have negligible ripple content. The true waveform of the input and output currents is shown in figure 5. Fig. 5. Real rectifier input and output currents. With this current waveform, the true spectrum, for example for a Galaxy PW UPS at full load with a power factor of 0.8, is: Table T2. Example of the true spectrum of harmonic currents. Harmonic % of H1 H3 1% H5 32% H7 5% H9 1% H11 8% H13 4% H15 1% H17 3% Note the increase in harmonic order H5 and the decrease in H7, H13 and H17 with respect to the theoretical values. Note also the low levels of H3, H9 and H15. The corresponding THDI is 33%, which is close to the theoretical value of 30%. The THDU at the rectifier input depends on the total source impedance and the SCR firing angle delay. The THDU rises rapidly to significant levels, even for low source-impedance values (see figure 6). Fig. 6. Voltage distortion as a function of the source impedance (expressed as the short-circuit voltage) for different values of the SCR delay angle α. MGE UPS SYSTEMS MGE0246UKI 02/2000 6

8 2. Harmonics upstream of UPS (cont.) influence of partial load on THDI The results presented assume a 100% load on the UPS. In fact, the THDI increases with decreasing load. This phenomenon is due to the fact that the harmonic content of the current decreases less quickly than the fundamental. As switching is faster for lower currents, the resulting waveform is squarer and the currents have a higher harmonic content. conclusions UPSs cause harmonic currents due to the Graëtz bridge of the rectifier at the input, notably harmonic current H5 and to a lesser degree H7 and H11. In particular, these harmonics produce: current distortion (THDI approximately 30%) and voltage distortion (THDU). The higher the source impedance, the higher the THDU; an increase in the rms value of the current; a reduction in the power factor. What is more, the THDI increases as the UPS load drops, i.e. as the UPS is supporting lower than rated output. MGE UPS SYSTEMS MGE0246UKI 02/2000 7

9 3. Controlling harmonics upstream of UPS preventing the effects of harmonics Upstream harmonics must be controlled to avoid the following negative effects on the power-distribution network: increase in the rms current Harmonics increase the rms current (calculated as 5% above, see the I rms calculation), which results in: increased losses; reduced efficiency; overloads and temperature rise; risk of damage to other loads. increase in current and voltage distortion current distortion (THDI) Total harmonic distortion, or THDI, of the input current typically around 30%. This value demonstrates the significant level of distortion in the current (see figure 5) must be considerably reduced to limit the effects of the harmonic currents. voltage distortion (THDU) The flow of harmonic currents in the source impedance results in distortion of the input voltage as well. Total harmonic distortion, or THDU, of the input voltage must remain within limits (generally 5%) to avoid excessive disturbances to other loads connected to the same set of busbars. As the THDU depends on the source impedance (see figure 6), it is necessary, when a diesel generator set is installed, to respect this limit both for sources with high impedance (generator) and with a lower impedance (transformer). It follows that to reduce the THDU, it is necessary to reduce the harmonic currents, if possible with a solution that is independent of the source. effect of distortion The increase in distortion (current and voltage) can cause the following: damage to or malfunctioning of sensitive electronic equipment; premature aging; flicker (lighting systems, screens); source derating. To avoid these problems, it is necessary to limit the harmonic content and consequently the THDU and THDI. decrease in the power factor Harmonics decrease the power factor. This may result in: drawing an excessive level of apparent power. A reduction in harmonics is the means to achieve savings on the corresponding costs. Note also that an excessively low value of cos ϕ1(where ϕ1 is the phase displacement between the fundamental current and voltage) can also lead to the billing of penalties the utility. It is therefore important to raise both the power factor λ and at the same time, wherever possible, the displacement power factor cos ϕ1 compliance with standards to meet customer needs UPSs have become indispensable. However, if the user is to benefit fully from the advantages offered by a UPS, it is necessary to effectively deal with the problems related to upstream harmonics. Clearly, users cannot accept that the solution for electrical-power problems for critical loads result in further problems on their electrical-distribution network. MGE UPS SYSTEMS MGE0246UKI 02/2000 8

10 3. Controlling harmonics upstream of UPS (cont.) Control of upstream harmonics is indispensable to ensure that users: have a "clean" electrical-distribution by reducing THDI through limiting or elimination of harmonics; obtain THDU values compatible with all their loads, by respecting the 5% limit; obtain acceptable power-factor and phase-displacement values ( 0,94); have an installation that complies with current and future standards on harmonic disturbances: standards on harmonic emissions: - IEC / EN for devices with an input current 16 A/ph; - IEC / EN for devices with an input current > 16 A/ph; (see table T3); standards and recommendations on the quality of distribution systems, which are coming into force in a number of countries, notably: - IEC / EN ; - EN (Europe); - IEEE (United States); - ASE 3600 (Switzerland); - G5/4 (UK), etc.; can easily combine a UPS and a diesel generator set, without risking an increase in harmonics when the load is transferred to the diesel generator set; obtain a reduction in demand for kva and avoid to oversize the power sources. Table T3. Example of limited harmonic currents in compliance with IEC / EN guide for devices with an input current > 16 A/ph (stage 1, connection without restrictions). Harmonic % of H1 H3 21.6% H5 10.7% H7 7.2% H9 3.8% H11 3.1% H13 2.0% H15 0.7% H17 1.2% H19 1.1% H21 0.6% H23 0.9% H25 0.8% H27 0.6% H29 0.7% H31 0.7% H33 0.6% H even orders 0.6% or 8/n (n even order) MGE UPS SYSTEMS MGE0246UKI 02/2000 9

11 4. Solutions for controlling upstream harmonics possible solutions Reductions in THDI and THDU are linked and depend on the reduction of the most dominant harmonic currents. To that end, MGE UPS SYSTEMS, in the framework of THM (Total Harmonic Management), proposes a number of solutions that may be integrated in the UPS: installation of a choke in the rectifier input to attenuate the amplitude of the harmonics (in particular high-order harmonics); installation of an LC passive harmonic filter in the input of the rectifier bridge, tuned to the harmonic order to be eliminated; use of a number of rectifiers supplied with voltages that are slightly shifted in phase to eliminate the most disturbing harmonics, through current recombination. This solution, known as double-bridge or 12-pulse filtering, is in fact a form of harmonic recombination; use a THM active filter, derived from active harmonic conditioners such as SineWave, that compensates for harmonics in real time to eliminate distortion. We are going to study the characteristics as well as the advantages and disadvantages of each of the above solutions : rectifier with inline choke operating principle This is simply a palliative solution that attenuates all harmonic orders. It consists in installing an LF smoothing choke upstream of the UPS rectifier, thus adding to the total inductance LS of the source (generator and cables) (see figure 7). This is a means to reduce distortion by a factor of: Ls, thus: Ls + LF Ls THDU(A) = THDU(B) x Ls + LF The THDU at point A is commonly only half that at point B. This choke is fitted as standard in all MGE UPS SYSTEMS units. Fig. 7. Installation of an additional LF inductor upstream of the UPS attenuates the effects of harmonics. MGE UPS SYSTEMS MGE0246UKI 02/

12 4. Solutions for controlling upstream harmonics (cont.) advantages simple solution, reliable and relatively inexpensive. the choke can be used with all types of sources. disadvantages limited effectiveness. large dimensions. line drop. LC passive filter operating principle This solution (see figure 8) consists in installing, in addition to an LF smoothing choke, an LC filter in parallel with the rectifier/charger input. The LP inductor and the CP capacitor are selected such that the filter impedance is zero for the fifth harmonic current (the most significant) and low for the seventh harmonic current. To that end, values are selected such that Lp.Cp.ω 2 = 1 for H5, i.e. f = 5 x 50 Hz = 250 Hz. Thus for: the fifth harmonic order (250 Hz), the parallel impedance is equal to zero. The entire fifth harmonic current flows through the parallel circuit and no longer affects the other loads; the seventh harmonic order (350 Hz), given the slight shift in tuning, the parallel impedance is still low and a part of the H7 current is also eliminated; the harmonics with higher orders, the parallel impedance of the filter is very close to that of its LP inductor and it operates as a voltage divider. Fig. 8. A tuned LC filter. MGE UPS SYSTEMS MGE0246UKI 02/

13 4. Solutions for controlling upstream harmonics (cont.) different versions MGE UPS SYSTEMS offers the following three types of LC filters: non-compensated harmonic filter This is the filter described above (see figure 8). compensated harmonic filter This filter (see figure 9) is ideal for installations with a diesel generator set. The inductive load (additional compared to the above filter) enables a reduction in the capacitive energy that must be supplied by the diesel generator set at start-up and under steady-state conditions. non-compensated harmonic filter with contactor This filter (see figure 10) is ideal for installations with a diesel generator set, where the margin is small between the power rating of the engine generator set and the power it must supply. The LC circuit starts operating only when the contactor closes. Closing is automatic and depends on a pre-set value that corresponds to a UPS percent load that is acceptable for the engine generator set. Fig. 9. Compensated harmonic filter. Fig. 10. Non-compensated harmonic filter with contactor. advantages simple solution and reliable. the choke can be added to the installation at any time. satisfactory performance, notably for the tuned frequency: THDI 5%. increase in the input power factor. The presence of the parallel filter circuit tuned to the fifth harmonic causes a capacitive current to appear at the fundamental frequency. This capacitive current may, to some extent, compensate the lag of the rectifier current and thus improve the power factor of the rectifier. disadvantages limited spectrum. This solution is effective only for frequencies close to the tuned frequency. It eliminates the fifth harmonic current, a part of H7, but very little of H11 or H13. poorly suited for partial loads. Effectiveness is cut in half when the UPS operates at 50% load (the THDI rises from 5% to 10%). poorly suited to parallel UPS systems, because a filter is required for each rectifier/charger. dependence on the source. If the installation includes a diesel generator set, the basic filter (non compensated) may not be used if the diesel generator set cannot accept a capacitive current equal to 30% of the rectifier current. In this case, a compensated filter is required, or a more expensive filter with a contactor. MGE UPS SYSTEMS MGE0246UKI 02/

14 4. Solutions for controlling upstream harmonics (cont.) 12-pulse filter operating principle This solution, for which MGE UPS SYSTEMS was one of the precursors back in 1975, may be used for both single and parallel UPS systems. It uses (see figure 11) a transformer with two secondary windings to supply voltages with a 30% phase shift, each of the secondaries supplying a Graëtz-bridge rectifier. The result is 12-pulse rectification because the double bridge has twelve legs. The rectifiers must supply identical DC currents to ensure that the AC currents drawn on the transformer secondaries have the same values. In this manner, the harmonic currents generated by each of the rectifiers on the primary of the transformer are recombined. The selected phase shift eliminates harmonic orders 6 k ± 1 (where k is a whole number) and only orders 12 k ± 1 remain. Harmonic orders H5, H7, H17 and H19 are thus eliminated. Harmonic orders H11, H13, H23 and H25 remain. The resulting current has a waveform that better resembles a sinusoidal current (see figure 12) than that obtained by a single rectifier. Fig pulse filter. Fig. 12. Resulting current waveform (input current of the 12-pulse filter).. different versions double bridge and transformer with two secondaries. double bridge and autotransformer. series or parallel connection of the rectifiers. advantages acceptable performance, but less than that of passive filters: THDI 10%. complete galvanic isolation at the UPS input for versions with transformers such as those proposed by MGE UPS SYSYTEMS. disadvantages must be planned at the installation design stage. complex (balanced voltages, short-circuit currents, rectifier currents). expensive (double rectifier, transformer with double secondary or autotransformer). poorly suited to changes in the load level. Effectiveness drops with the percent load (the THDI rises from 10% at full load to 15% at 50% load). not compliant with guide IEC (see example in Table T5, page 15, for harmonics H11 and H13). MGE UPS SYSTEMS MGE0246UKI 02/

15 4. Solutions for controlling upstream harmonics (cont.) THM active filter operating principle MGE UPS SYSTEMS offers the only UPS on the market equipped upstream with built-in THM (Total Harmonic Management) filters. The operating principle of this type of filter is based on that of the SineWave active harmonic conditioner. The filter is a simplified version that has been optimized for the Galaxy family of UPSs. THM filters react in real time to the harmonics detected and eliminate them using the method shown in figure 13. the current transformers measure the rectifier input current harmonics. the electronic circuitry of the THM filter continuously analyzes the harmonic spectrum up to harmonic order 25 and determines the instantaneous value of H5, H7, H11, H13, H17, H19, H23 and H25 harmonics. based on the values determined, the filter continuously injects the required harmonic currents upstream that are then combined with the fundamental current provided by the source to supply the rectifier with the input current waveform it needs to operate. To sum up, the THM filter continuously supplies the harmonics required by the rectifier/charger and consequently the source needs only to supply a sinusoidal fundamental current. Fig. 13. Simplified diagram of the THM active filter. In addition, the standard THM active filter offers the possibility of improving the displacement power factor (cos ϕ1) at the same time as the power factor (λ), the latter reaching a value of up to MGE UPS SYSTEMS MGE0246UKI 02/

16 4. Solutions for controlling upstream harmonics (cont.) advantages the most effective filter. It can totally eliminate all harmonics up to order 25, as a result a reduction in the THDI by a factor of almost ten: THDI 4%. constant performance at all load levels. THDI 4 % even at half-load. flexibility. This type of filter can be easily installed as a retrofit, even for old UPSs. adapts to the load. Filter adapts in real time to the harmonics measured results in a virtually constant level of performance that does not depend on the load percentage of the UPS. This is an important advantage given that: - most single UPSs are oversized with excess load capacity and operate at approximately 75% of their rated output; - redundant parallel UPS systems often operate at 50% load. not dependent on the source. Operation is identical and performance levels are maintained, whether the source is a transformer or generator set. considerable increase in the power factor l. By eliminating the harmonics, the power factor is increased to at least 0.95 and up to 0.98 and, at the same, the displacement power factor (cos ϕ1) is increased by reactive energy compensation. highly cost effective for parallel UPS configurations because a single filter, connected in parallel, can condition the entire UPS system. simple to select. The range of THM filters from MGE UPS SYSTEMS comprises five filters covering the entire range of power ratings: THM-S (small) up to 30 kva; THM-M (medium) up to 60 kva; THM-L (large) up to 200 kva; THM-XL (extra large) up to 300 kva. isolation is possible for maintenance (filter is parallel connected). disadvantages The only potential disadvantage of this type of filter is the initial investment, due to the power electronics. However, technological progress, the drop in the price of components and the optimization carried out for Galaxy UPSs have made THM filters, at equal power ratings, much less expensive than a 12-pulse filter, even though they offer much higher performance and greater possibilities. THM filters are nevertheless more expensive than passive filters, but offer much better and broader performance. MGE UPS SYSTEMS MGE0246UKI 02/

17 5. Comparison and conclusions THM approach MGE UPS SYSTEMS proposes an overall approach to controlling harmonics in the framework of THM (Total Harmonic Management). This approach starts with an on-site audit of the customer's installation followed by proposed solutions precisely suited to the customer's operating goals. For UPSs, this approach makes it possible either to guarantee a desired level of harmonic limitation or to completely eliminate harmonics. To that end and depending on the needs of customers, MGE UPS SYSTEMS can propose the entire range of solutions presented in the preceding pages of this document. comparison of solutions A comparison between all the above solutions is a very helpful means to identify the position of the various solutions compared to each other and depending on the specific application at hand. Table T4 below is a comparison that sums up the merits and demerits of the various solutions. Table T5 provides an example of performance in relation to the recommendations of the IEC guide. See also figure 14 which compares the performance of the different solutions depending on the percent load of the UPS. The graph makes very clear one of the major advantages of the built-in THM solution, i.e. constant performance that does not depend on the load percentage of the UPS. Table T4. Comparison of solutions with respect to customer s procurement-decision criteria. Criterion choke LC filter 12-pulse filter Built-in THM Reduction in distortion * ** ** *** (THDI, THDU) Harmonics concerned and effectiveness of reduction Adaptability (performance depending on the load level) see figure 14 ** All orders (attenuation) ** H5, H7 * H5, H7, H17, H19 *** H2 to H25 ** * *** Power factor improvement ** * ** *** Independent of source *** ** ** *** impedence effect Efficiency *** *** * ** Reliability *** *** ** ** Cost *** *** ** ** Maintenance * * * *** (possible isolation) Parallel UPS systems * one filter/ups Compliant with no IEC guide(seet5) *** Excellent ** Good * Average MGE UPS SYSTEMS MGE0246UKI 02/ * one filter/ups No * one filter/ups No *** only one filter Yes Table T5: Example of typical performance for Galaxy UPSs (at full load) based on the latest certifications of our most recent products. Hk % H1 Limits of Without filter LC filter 12 pulse filter Built-in THM IEC H % H % 32 % 2.4 % 2.8 % 2.5 % H7 7.2 % 3.5 % 1.6 % 1.5 % 1.5 % H9 3.8 % H % 7 % 3.4 % (1) 9.1 % (1) 2 % H % 2.7 % 1.5 % 4.7 % (1) 0.5 % H % 2.5 % 1 % 1 % 1.1 % H % 2.1 % 0.9 % 0.7 % 0.9 % THDI 35 % 5 % 10 % 4 % (1) Values not respecting the limits of IEC guide

18 5. Comparison and conclusions (cont.) Figure 14. Comparison of the various upstream filtering solutions depending on the percent load on the UPS. conclusions The above comparisons are translated in graphic form in figure 15, according to: the overall performance levels of the solutions (on the basis of the advantages and disadvantages presented above and summed up in table 4); their ratio performance/price. Figure 15. Comparison of the solutions depending on the overall performance level with respect to the cost. Clearly, two types of solutions stand out. The low-cost solutions offering average performance. These solutions implement a standard passive filter. It is possible to reinforce the solution, depending on the installation (compensated filters or non-compensated filters with contactor). The high-performance solutions, which are also more expensive, compliant with the recommendations of the IEC guide. These solutions implement a THM active filter. The many advantages provided by THM solutions at a relatively low cost make 12-pulse solutions (for which MGE UPS was one of the pioneers back in 1975) virtually obsolete. Moreover these solutions do not meet the recommendations of the IEC guide (see Table T5). MGE UPS SYSTEMS MGE0246UKI 08/99 17

19 MGE UPS SYSTEMS 140, avenue Jean Kuntzmann Zirst Montbonnot Saint Martin SAINT ISMIER Cedex France Tel : 33 (0) MGE0246UKI As standards, specifications and designs change from time to time, please ask confirmation of the information given in this publication. Product names mentioned herein may be trademarks and/or registered trademarks of their respective companies. Published by: MGE UPS SYSTEMS - 02/2000 Designed by: P. Andreani - INTERDOC