4 The distortions produced by the Variable Speed Drive can be classified in the following way: Variable Speed Drives DISTORTIONS DISTORTIONS PROVOKED BY VARIABLE SPEED DRIVES rectifier bridge 1.Harmonic distortion: Produced by the rectifier bridge. inverter bridge 2.Electric noise: Produced by the inverter bridge. We analyze what do they mean and the different ways to reduce them, specially thanks to the use of different filter types.
6 What are “the harmonics”? Variable Speed Drives DISTORTIONS HARMONIC DISTORTION It can be demonstrate that any periodic waveform (squared, triangular, …) can be represented as the sum of several sinusoidal waves with different frequencies and phases. All those waves constitute the harmonic spectrum of the wave. » Harmonic content of a squared wave
7 Which are the effects of the input harmonic currents? Variable Speed Drives DISTORTIONS HARMONIC DISTORTION The increment of the RMS value of the current through the conductors and transformers. This “extra” current is not real (it will not produce work) but nevertheless due to this current, transformers and wires must be over- dimensioned to prevent over-heating effects. Distortion of the input voltage wave. That could implicate important effects: It could affect to the correct functioning of those equipments with single-phase power supply such us computers (PCs) and other devices of low consumption. Non desired resonance effects can be produced affecting to the distribution system.
8 Which are the effects of the input harmonic currents? Variable Speed Drives DISTORTIONS HARMONIC DISTORTION Power factor degradation. Overload of transformer, wiring, components, … Overload of power factor correction capacitors.
9 How compensate the input harmonic distortion? Variable Speed Drives DISTORTIONS HARMONIC DISTORTION 1.Incorporating inputs coils, as well called choke inductances. These passive filters can be placed in the rectifier bridge input, realizing a double mission: On the one hand, they protect to the rectifier from voltage variation of the main power supply. On the other hand, they filter the produced harmonics making softer the sinusoidal wave of current. They can also be placed in the DC bus. The rectifier bridge will not be as protected as in the previous configuration, but this is always a low cost option. 2.Using Active filters of harmonics. This system allows analyzing the harmonics generated by the drive and they can be annulled by mean of the generation of the opposite harmonics.
10 How compensate the input harmonic distortion? Variable Speed Drives DISTORTIONS HARMONIC DISTORTION 3.Installing a 12 pulses distribution system. If we divide the rectifying input of the inverter bridge into two rectifier bridges and the input voltage of each rectifier bridge is 30º diphase each other, in theory the harmonic smaller than 11 order can be suppressed. To do that, a transformer with double secondary is required. One secondary in star connection and the other secondary in delta connection. Half a load will be supplied by the first secondary and the rest will be supplied by the second one, getting their currents with 30º diphase. It is possible to demonstrate then that harmonic 5 th and 7 th has been erased.
12 12 pulses rectifying system. Variable Speed Drives DISTORTIONS HARMONIC DISTORTION Moment where the thyristors of the rectifier bridge are in conduction: The input voltage is higher than the Bus voltage. The input inductances avoid the complete discharge and make the waveform softer, getting it similar to a square wave.
13 12 pulses rectifying bridge. Variable Speed Drives DISTORTIONS HARMONIC DISTORTION We can suppose a quasi ideal system applied to the rectifier, considering squared wave signal as reference. Having into consideration that the double secondary, explained before, enters a 30º diphase in the currents applied to each rectifier, the result is a waveform much more sinusoidal in the inverter bridge:
14 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) 3
15 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) ELECTRIC NOISE inverter bridge The electric noise is produced by the inverter bridge. It is due to the interruption of the current signal when the thyristors commutate their status (switch over ON and OFF and vice versa). For this reason the electric noise is a high frequency current signal which is coupled to the current that is flowing in the drive to the motor and that additionally can be emitted.. It is necessary to deal with two kind of emissions: Conduced Radiated
16 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) RADIATED EMI RADIATED The RADIATED electric noise will be attenuated considering: The use of metallic conductions. The use of shielded wires. The own metallic cabinet of the drive will help to minimize this effect. » Power wires screened
17 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) CONDUCED EMI CONDUCED The CONDUCED electric noise can be attenuated in the following ways: RFI Input Filters EMC Filters Recommended use of RFI Input Filters (Radio Frequency Interferences) as well called EMC Filters (Electro-Magnetic Compatibility). Regulation which controls the selection of these filters is UNE-EN 61800-3/A11:2002 UNE-EN 61800-3/A11:2002. See details on Annex I. Noise coupled to the input signal of the drive.
18 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) CONDUCED EMI CONDUCED The CONDUCED electric noise can be attenuated in the following ways : Output Common Mode Ferrites Using Output Common Mode Ferrites, in case of the couple noise is common mode noise, that means, noise signal coupled to the capacitances existing between the phases and the earth and also between the motor windings and the earth. This noise is the responsible of the bearing damages. dV/dt Output Filters Using dV/dt Output Filters, in case of the coupled noise is differential noise, that means, noise signal coupled to the capacitances existing between phases. This noise will produce isolation drillings and additionally will increase the dV/dt factor. These filters can be: Output Inductance Output Inductance (output coils in series, one per phase) Iron Dust Toroids Iron Dust Toroids, in all output phases. Noise coupled in the current flowing to the motor.
19 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) FUNCTION OF THE dV/dt FILTER It is possible to observe that the output waveform of the drive is as follow: This is the result of the inverter bridge action. Show film
20 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) FUNCTION OF THE dV/dt FILTER If the waveform is amplified, it is possible to observe that the angle is not 90º exactly: DrivedV/dtLosses 132kW800V/µs1380W 132kW4000V/µs1100W
21 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) FUNCTION OF THE dV/dt FILTER By incrementing the dV/dt ramp is possible to reduce the drive losses, that allows to the drives to dissipate less power and consequently they can be smaller. DrivedV/dtLosses 132kW800V/µs1380W 132kW4000V/µs1100W disadvantage The main disadvantage of this method is the appearance of brusque over-impulses in the drive output which will be higher at motor input. It is possible to check this in the measurement realized using a competitor drive: » Actual measurement on competitor drive of 200A on load
22 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) FUNCTION OF THE dV/dt FILTER Power Electronics To solve this problem, Power Electronics works over the gate resistor of the IGBTs, guaranteeing that those over-impulses do not overcome a concrete value. » Actual measurement on a drive of 200A on load: Competitor Rg is the gate resistor and it controls the load of the “capacitor” which conform the IGBT. POWER ELECTRONICS » Actual measurement on a drive of 200A on load: POWER ELECTRONICS
23 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) FUNCTION OF THE dV/dt FILTER Power Electronics, dV/dt As a complement of that design, the drives of Power Electronics, integrate additionally the dV/dt output filters. These filters, as has already been explained, consist of coils placed in serial connection with the output, which complete the attenuation of those non-desirable effects before exposed. The result: POWER ELECTRONICS » Actual measurement on a drive of 200A on load: POWER ELECTRONICS Power Supply dV/dt Filter RFI Filters Inductances 1 st Level: Electronic 2 nd Level: Cooling 3 rd Level: Filters
24 Variable Speed Drives ELECTRIC INTERFERENCES (NOISE) CONDUCED EMI Sinusoidal Output Filter Finally, there is a filter type which deal with the problem by converting the chopped signal into a sinusoidal signal: this is the Sinusoidal Output Filter (LC circuit in the drive output). The noise depend on the capacitance and on the dV/dt factor. The capacitance is variable on each installation. The dV/dt depends on the drive. If the dV/dt factor is reduced up to non-considering values, the noise will almost disappear. The result:
26 Annex I ELECTRO-MAGNETIC COMPATIBILITY NORM UNE-EN 61800-3/A11:2002 Limits application in the measurement of the Conduced Perturbations Nowadays exist several norms about EMC according to the 89/336/EEC directive that refer to the variable speed drive or to those equipments incorporating drives. Specific norms for drives UNE-EN 61800-3/A11:2002: UNE-EN 61800-3/A11:2002: Adjustable speed electrical power drive systems. Part 3: EMC product standard including specific test methods.
27 Annex I ELECTRO-MAGNETIC COMPATIBILITY NORM UNE-EN 61800-3/A11:2002 Norm Classifying CriteriaApplication Limit First environmentNon restricted distributionLimit 1 Restricted distributionLimit 2 Second environment Input current 100A Limit 3 Input current > 100ALimit 4 Where: First Environment: Includes domestic or residential use. It includes also, places directly connected, without intermediate transformers, to a power supply distribution system of low energy which additionally gives supply to buildings used for domestic uses (cinemas, theatres, shopping centres, hospitals,…). Second Environment: (Named also industrial). It includes all places different from those which are directly connected to a power supply distribution system of low energy which additionally gives supply to buildings used for domestic uses (factories and facilities supplied with transformer of medium voltage to low voltage).
28 Annex I ELECTRO-MAGNETIC COMPATIBILITY NORM UNE-EN 61800-3/A11:2002 Norm Where: Non-Restricted Distribution: Marketing modality where the power supply of the drive does not depend on the customer or user regarding to EMC issues for the application of operation. Restricted Distribution: Marketing modality where the manufacturer limits the supplying of the drive to those customers or users which, in an independent or together way, have technical competence on the EMC requirements for the application of operation. Classifying CriteriaApplication Limit First environmentNon restricted distributionLimit 1 Restricted distributionLimit 2 Second environment Input current 100A Limit 3 Input current > 100ALimit 4
30 Annex II FILTERS SUMMARY FILTER TYPES Summarizing: Harmonic Distortion Passive filter: Input coils (Choke Inductances) DC Bus Inductance Three-phase Input Inductance Active filters Electric Noise Radiated Conduced Metallic conductions Screened cables Metallic cabinets EMC Input filter (or RFI filter). For noise coupled to the inputs signal Output Common Mode Ferrites. For noise coupled in the signal flowing to the motor. dV/dt Output Filter (differential mode noise) Output Inductances in series (1 per phase) Iron Dust Toroids (1 per phase) Sinusoidal Output Filter (LC circuit)