750-at003A_2014-3_Application Guidelines for 12-Pulse Operation of PowerFlex

Application TechniqueApplication Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC Drives2Rockwell Automation Publication 750-AT003A-EN-P - March 2014Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesImportant User InformationRead this document and the documents listed in the additional resources section about installation, configuration, and operation of this equipment before you install, configure, operate, or maintain this product. Users are required tofamiliarize themselves with installation and wiring instructions in addition to requirements of all applicable codes, laws, and standards.Activities including installation, adjustments, putting into service, use, assembly, disassembly, and maintenance are required to be carried out by suitably trained personnel in accordance with applicable code of practice.If this equipment is used in a manner not specified by the manufacturer, the protection provided by the equipment may be impaired.In no event will Rockwell Automation, Inc. be responsible or liable for indirect or consequential damages resulting from the use or application of this equipment.The examples and diagrams in this manual are included solely for illustrative purposes. Because of the many variables and requirements associated with any particular installation, Rockwell Automation, Inc. cannot assume responsibility or liability for actual use based on the examples and diagrams.No patent liability is assumed by Rockwell Automation, Inc. with respect to use of information, circuits, equipment, or software described in this manual.Reproduction of the contents of this manual, in whole or in part, without written permission of Rockwell Automation, Inc., is prohibited.Throughout this manual, when necessary, we use notes to make you aware of safety considerations.Labels may also be on or inside the equipment to provide specific precautions.Allen-Bradley, PowerFlex, Rockwell Software, and Rockwell Automation are trademarks of Rockwell Automation, Inc.Trademarks not belonging to Rockwell Automation are property of their respective companies.WARNING: Identifies information about practices or circumstances that can cause an explosion in a hazardous environment,which may lead to personal injury or death, property damage, or economic loss.ATTENTION: Identifies information about practices or circumstances that can lead to personal injury or death, property damage, or economic loss. Attentions help you identify a hazard, avoid a hazard, and recognize the consequence.IMPORTANTIdentifies information that is critical for successful application and understanding of the product.SHOCK HAZARD: Labels may be on or inside the equipment, for example, a drive or motor, to alert people that dangerousvoltage may be present.BURN HAZARD: Labels may be on or inside the equipment, for example, a drive or motor, to alert people that surfaces mayreach dangerous temperatures.ARC FLASH HAZARD: Labels may be on or inside the equipment, for example, a motor control center, to alert people topotential Arc Flash. Arc Flash will cause severe injury or death. Wear proper Personal Protective Equipment (PPE). Follow ALLRegulatory requirements for safe work practices and for Personal Protective Equipment (PPE).Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC Drives Document Scope The scope of this document is to provide application guidelines for 12-pulseoperation of PowerFlex 750-Series, frames 6…10, 400V…690V AC input drives.While several different transformer configurations are possible, only thecommonly used delta-delta-wye transformer, which provides 30 degree phaseshift between the two sets of secondary windings is addressed. The guidelinesinclude sizing of the transformer and rectifier, grounding considerations, pre-charge, and circuit protection recommendations.Drive Harmonics and Impact on Supply Network Standard AC drive topologies consist of AC-DC-AC power conversion with a 3-phase, 6-pulse rectifier bridge consisting of diodes or SCRs. An AC drive is a non-linear load on the utility power supply; a buffered (with AC line reactor or DC link choke) 6-pulse bridge generates close to 40% current total harmonic distortion (THD). In addition, harmonic currents produced by a rectifier are the cause of voltage THD at the secondary of the transformer that feeds other loads as well – generally referred to as a point of common coupling (PCC). The extent of voltage distortion depends on the stiffness of the power supply (kVA rating and impedance of the transformer or generator). The stiffness of the power supply is measured by the short circuit ratio, which is the ratio of the available short-circuit current to the rated full load current.The one-line diagram in Figure1 illustrates a typical industrial plant with AC drive loads, and different PCC where harmonic distortion measurements could be made. The dominant harmonics for 6-pulse, 12-pulse and 18-pulse rectifiers are {5th, 7th}, {11th, 13th} and {17th, 19th} respectively.Figure 1 - Typical Power Distribution with Harmonic Distortion Measurement PointsRockwell Automation Publication 750-AT003A-EN-P - March 20143Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesFigure2 is the equivalent circuit for power distribution of Figure1. Non-linearloads are represented by current sources at multiple characteristic harmonicfrequencies.Figure 2 - Non-linear Load Current SourcesThe harmonic current magnitudes in Figure2 depend on the source stiffness andany internal or external buffering (DC link choke or AC line reactor) in the ACdrive. When the harmonics currents generated by non-linear loads flow throughthe system impedances, voltage distortion is created. This affects other loadsconnected at the PCC nodes. With a generator source that has high internalimpedance (typically 25%), significant voltage distortion could be created bynon-linear loads, which could adversely affect other loads at the PCC nodes.Background voltage distortion, such as pre-existing voltage harmonics or voltageunbalance, can also affect the current harmonic distortion of non-linear loads.This is particularly true for multi-pulse rectifiers that rely on special transformerconfigurations to generate phase-shifted voltages, as will be discussed later in thisdocument for 12-pulse drive systems.Harmonics standards, such as IEEE 519-1992, specify maximum current THDand limits for individual harmonic current magnitudes (Table 10.3 of IEEE519-1992), and voltage THD (Table 10.2 of IEEE 519-1992) at the PCC, as afunction of the short-circuit ratio. While PCC in this definition refers to PCC1shown in Figure1, customers may expect conformance to the specifications atPCC2 or PCC3 shown in Figure1, where linear and non-linear loads areconnected.The detrimental effects of harmonics include increased losses in powerdistribution and generators, de-rating of transformers due to harmonic currents,and susceptibility of loads to voltage distortion. T o determine transformer de-rating, a quantity called the K-factor, defined as follows, is used (I1 is thefundamental current and I n is the harmonic current of order n). The K-factor for6-pulse and 12-pulse drives is about 6 and 2 respectively, while it is nearly equal tothe ideal value of 1 for 18-pulse drives. Background voltage distortion andunbalance will affect this significantly, particularly for 12-pulse and 18-pulsedrives that require special phase-shifting transformer configurations.4Rockwell Automation Publication 750-AT003A-EN-P - March 2014Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesT ransformer de-rating as a function of K-factor is shown in Figure3.Figure 3 - Transformer De-rating for Non-linear LoadsRockwell Automation Web-Based Harmonics Simulation ToolsW eb-based harmonics simulation tools provided by Rockwell Automation are part of a suite of simulation tools hosted by T ransim® T echnology Corporation. Users are only required to have an internet connection and a web browser to run the simulation on a remote server. All Rockwell Automation employees and distributors have access to the tool and must connect from within the Rockwell Automation firewall.The web-based simulation tools can be accessed at:/rockwellCurrently, there are two harmonics simulation tools, which are intended for different applications as described below.Spreadsheet-Based Harmonics Calculator ToolThe harmonics calculator is a spreadsheet based tool, which uses look-up tables to estimate the voltage and current harmonics at different nodes in a network consisting of multiple linear and non-linear loads. Only numerical outputs are provided including checks for conformance to harmonics standards. This tool is more flexible than the harmonics simulator tool in terms of the load configurations that can be simulated.ܭ=Σ(݊I݊I1/)2ܰ2Σܰ2(I݊I1/)2Rockwell Automation Publication 750-AT003A-EN-P - March 201456Rockwell Automation Publication 750-AT003A-EN-P - March 2014Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesThe harmonics calculator doesn’t take into account the effects of pre-existing voltage distortion or unbalance. Interaction between loads is also not considered. The harmonic contributions of different loads arenumerically added, without considering phase shift. Hence, the results are conservative.The system configuration can be specified on the landing page for the harmonics calculator tool, as shown in Figure 4. The Notes tab has useful information on how to use the program, criteria for pass/fail and assumptions made to calculate harmonics. The Tutorial tab has an illustrative example with instructions on how to use system one-line diagrams to enter data into the harmonics calculator.Figure 4 - Web-based Harmonics Calculator Tool Data Input PageThe General Parameters section, bottom left in Figure 4, are default values and should work in most applications. The Notices section is active after a simulation run and includes design checks (for example, transformer rating exceeded) and useful information like Active Filter current rating required to meet targetcurrent THD.Rockwell Automation Publication 750-AT003A-EN-P - March 20147Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesDifferent topologies of 6-pulse, 12-pulse and 18-pulse drives can be specified, as shown in Figure 5. For the delta-delta-wye 12-pulse transformer, the ISO xfmr option can be used. This tool doesn’t consider the effect of transformerimpedance mismatch and circulating currents, which will be explained later in this document.Figure 5 - Web-based Harmonics Calculator Tool, 6-pulse, 12-pulse and 18-pulse Drive ConfigurationsApplication ExampleThis application example demonstrates the effect of multi-pulse rectifiers on voltage and current distortion.For a system consisting of a 2 MVA utility transformer, 1.5 MVA usertransformer and 700 Hp, 6-pulse load (buffered with DC link choke), the voltage and current THD at the 3 PCC points as identified in Figure 4, conformance to the IEEE 519 standard, and the active filter current rating required to meet target current THD of 5% at PCC1 and PCC2 are shown in Figure 6 on page 8.Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesThe Results (top) section in Figure6 shows voltage and current THD at 3 PCCnodes. The Intermediate calculations (bottom) section shows intermediatecalculations for harmonic currents, voltages and active filter current ratingrequired to meet 5% current THD.Figure 6 - Harmonics Calculator Simulation Results for 6-pulse Drive Load8Rockwell Automation Publication 750-AT003A-EN-P - March 2014Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC Drives The results of a 700 Hp drive with a 12-pulse rectifier (delta-delta-wye isolation transformer) are shown in Figure7. Note that the 12-pulse rectifier considerably reduces current and voltage distortion, and helps meet the IEEE 519 limits for current THD at PCC1 and PCC2 (limit is 8% based on the short-circuit ratio). The Results (top) section in Figure7 shows the voltage and current THD at 3 PCC nodes. The Intermediate Calculations section show the intermediate calculations for harmonic currents, voltages and active filter current rating required to meet 5% current THD.Figure 7 - Harmonics Calculator Simulation Results for 12-pulse Drive LoadRockwell Automation Publication 750-AT003A-EN-P - March 2014910Rockwell Automation Publication 750-AT003A-EN-P - March 2014Application Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesHarmonics Circuit Simulator ToolThe harmonics simulator is a circuit simulation tool with one or two load branches. Each of the loads can be set to be of a specific type. W aveforms of voltage and current at different points of common coupling aregenerated along with numerical values for voltage and current THD, and checks for conformance to harmonics standards.The one-line diagram of the system that is simulated is shown in Figure 8. On each branch, the load can only be of a certain type; currently only 6-pulse or 18-pulse drives are supported on each individual transformer. Pre-existing distortion at the output of the utility transformer, 5thharmonic and voltage unbalance, can be set. T ransformer B can be set to either delta-delta or delta-wye. T o simulate a 12-pulse load, 6-pulse loads must be added on each transformer, and T ransformer B must be set to delta-delta.Figure 8 - Web-based Harmonics Circuit Simulation Tool Data Input PageW aveforms of voltage and current at the output of T ransformer A, voltage and current THD are shown in Figure9.Figure 9 - Harmonics Simulator Results for 6-pulse, Single-branch Load12-pulse Drive Configurations There are two main configurations of 12-pulse systems; true 12-pulse, and quasi12-pulse systems, as shown in Figure 10. In the true 12-pulse system, 3-phasevoltages from the utility transformer are converted to two sets of 3-phase voltages with 30 degrees of phase shift between the sets. Each set of 3-phase voltages arefed to a full wave bridge rectifier, whose DC output has 12 pulses perfundamental line cycle. A common method to achieve the 30 degrees of phaseshift is to use a transformer with delta and wye secondary windings.In the quasi 12-pulse system, two separate 3-phase drive loads are fed by acommon transformer. The 3-phase voltages to one load are phase shifted by 30degrees from the other. For perfect harmonic cancellation and to achieve 12-pulse harmonic performance at the primary of the transformer, the loads mustbalanced.Figure 10 - 12-pulse System ConfigurationsTrue 12-pulse SystemQuasi 12-pulse SystemHarmonic Cancellation in 12-pulse Drive ConfigurationsThe predominant line current harmonics in a 3-phase, 6-pulse rectifier are the 5th and 7th harmonics of the fundamental. In a 12-pulse system, for both true12-pulse and balanced loads on quasi 12-pulse, the 5th and 7th harmonics are cancelled as demonstrated in the phasor diagram of Figure11. Then, the dominant harmonics are the 11th and 13th, which are significantly lower in amplitude than the 5th and 7th. However, pre-existing voltage distortion and unbalance will significantly affect the performance of any 12-pulse system, as will be shown later.Figure 11 - Cancellation of 5th and 7th Harmonics in a 12-pulse Drive(a)(b)12-pulse Configurations for PowerFlex 750-Series Drives, Frames 6 (10)For PowerFlex 750-Series drives, frames 6…10, several different 12-pulse configurations are possible, as shown in Figure12, and Figure13 and Figure14 on page 15.In the configuration shown in Figure12, which is also similar to the configuration shown in Figure13 and Figure14, using the delta-delta-wye transformer and two rectifier bridges, a 12-pulse DC bus supply is created. Although shown specifically for frames 6...8, this configuration can be used to achieve true 12-pulse input for any combination of frames 6...10 DC input drives on the DC bus. With a diode rectifier front-end, integral pre-charge is required in the DC input drives, which is present on PowerFlex 750-Series frames 6 (10)drives.Quasi 12-pulse operation with AC input frames 6…10 drives can be achieved by distributing the load evenly between the delta and wye secondary windings. Figure 12 - External Rectifier Bridges for PowerFlex 750-Series Frames 6…8 Drives with DC InputFigure 13 - external Rectifier Bridges for PowerFlex 750-Series Frame 9 Drive with DC InputFigure 14 - External Rectifier Bridges for PowerFlex 750-Series Frame 10 Drive with DC InputRecommendations for sizing the AC line reactor are provided in T ransformer,AC Line Reactor and Rectifier Sizing on page 17.For a true 12-pulse configuration (as shown in Figure 12…Figure 14) the system can be solidly grounded or high resistance grounded at the neutral point of the wye secondary winding as shown in Figure 15. Since the delta secondary winding is not galvanically isolated from the wye secondary winding because of theinterconnection at the DC bus, it should not be separately grounded using agrounding transformer, as would typically be done for a single transformer with delta secondary winding.With the wye secondary neutral solidly grounded, the system behaves like asolidly grounded system. If the circuit protection on the wye winding were toopen (blown fuse or tripped circuit breaker), the rectifier on the delta secondary winding must also be disconnected from its supply and/or the inverters on theDC bus must be disabled. A fault must be enunciated.The co-ordination of protection between the delta and wye secondary windings described above also applies to a high resistance grounded system. In this case, the system must be equipped with a suitable ground fault detection device to indicate the presence of a fault.Figure 15 - True 12-pulse Drive GroundingFor quasi 12-pulse configurations, the delta winding must be groundedseparately, either solidly grounded or high resistance grounded, using a grounding transformer.On high resistance grounded and ungrounded systems, common-modecapacitors and transient suppression device networks must not be connected to ground. This is covered in more detail in Fusing, T ransient Over-voltageProtection and Common-Mode Jumper Recommendations on page 20.Solid ground on wye secondary winding High resistance ground on wye secondary windingFor 12-pulse configurations that have diode bridge rectifiers, the DC input drives must have integral pre-charge. PowerFlex 750-Series, frames 6…10 drives with DC input all have integral pre-charge. On frames 6 and 7, the pre-charge circuit consists of a resistor in the +DC bus with SCR bypass and an anti-parallel diode for regenerative current flow. On frames 8…10, pre-charge resistors are placed on both +DC and –DC, with a Molded Case Switch (MCS) for bypass.If SCR bridge rectifiers that have phase angle controlled pre-charge are used, then they must have independent means to synchronize to the phase shifted voltages. PowerFlex SCR Bus Supplies have this feature when two master units are used. See the PowerFlex SCR Bus Supply User Manual, publication 20S-UM001, for further information and particular parameter settings required for 12-pulse operation.Transformer, AC Line Reactor and Rectifier Sizing In this section, guidelines for sizing the transformer, AC line reactor and rectifier are provided. While the guidelines are based on certain assumptions for background voltage distortion (3% 5th harmonic voltage and 1% line unbalance), more severe background voltage distortion can result in higher continuous rating requirements for these components.Rectifier and AC Line Reactor SizingFor the true 12-pulse configuration of Figure12, the causes of current unbalance between the two rectifier bridges are:a.Fundamental voltage mismatch between delta and wye windings. Theturns ratio between the delta and wye windings must ideally be .Since this is a non-integral ratio, the output voltages will not be perfectly balanced.b.Impedance mismatch between the delta and wye windings due todifferences in the number of turns and winding geometry. This asymmetry also presents different commutation inductances to the rectifier bridges.c.Pre-existing 5th harmonic voltage has a different phase relationship withrespect to the fundamental on the delta and wye secondaries.d.Source voltage unbalance affects the delta and wye secondaries differently,due to the phase shift between them.The extent of current unbalance can be reduced by adding an AC line reactor to each bridge. The impedance of the reactor minimizes the effect of impedance asymmetry between the delta and wye secondary windings, and helps to reduce the current unbalance caused by voltage unbalance or pre-existing voltage distortion. A line reactor sized at 3% impedance based on the rated voltage and current of one rectifier bridge is sufficient.3:1For rectifier sizing, the worst case pre-existing distortion assumed is 3% of 5th harmonic distortion and 1% voltage unbalance. This results in worst case current sharing of 65%:35%. For example, if the total RMS AC input current for the drive is 100 A with 6-pulse input, each rectifier bridge must be sized for 65 A. Current unbalance as a function of pre-existing voltage unbalance and distortion is shown in Figure 16 for the 12-pulse configuration of Frames 6…10 with DC input. The line reactor is required to ensure low ripple on the DC bus of the DC input drives and to balance the currents between the rectifier bridges when the input voltages are unbalanced or have significant harmonic distortion.If the drive’s RMS AC input current for 6-pulse operation is I line_6P (For drive input current ratings, see the PowerFlex 750-series AC Drives T echnical Data, publication 750-TD001), then the line reactor inductance (L AC ) required is determined as follows, where V LL is the AC RMS voltage of each secondary winding and f line is the line frequency:Figure 16 - Current Unbalance with AC Line Reactor Voltage distortion at the primary side of the transformer can vary fromapproximately 4% THD with utility source to about 10% THD with generator source (typical impedance of 25%). This must be considered for other loads connected at the point of common coupling (PCC1 in Figure 1 on page 3 and Figure 2 on page 4).C u r r e n t U n b a l a n c e (%)Transformer Sizing and SpecificationsT ransformer kVA rating is based on worst case current unbalance between the delta and wye secondary windings.Given that the drive’s input AC RMS current for 6-pulse input is I line_6P and with the worst case unbalance of 65:35 between the two rectifier bridges, the total kVA rating of the transformer is determined as follows. The AC RMS voltage of each secondary winding is V LL. Since each secondary winding carries 6-pulse currents, an appropriate de-rating factor is applied:Further, the open circuit voltage of the secondary windings must be matched. The impedance of the transformer must be between 4…6%. The K-factor of the transformer must be greater than 6 to handle the harmonic currents.The selection of PowerFlex SCR bus supplies for true 12-pulse operation of PowerFlex 750-Series, frames 6…10 drives, along with Bulletin 1321 line reactor catalog numbers and minimum transformer kVA is provided in PowerFlex SCR Bus Supply Selection Chart for 12-pulse Operation of PowerFlex 750-Series, Frames 6…10 Drives on page 21.Electrostatic Shielding:If the voltage on the primary side of the transformer is greater than twice the secondary voltage, an electrostatic shield between the primary and secondary windings is strongly recommended to prevent transients on the high voltage side from propagating to the secondary windings. An electrostatic shield reduces the capacitive coupling between the primary and secondary, and must be tied to ground. In addition, it is also recommended that the neutral of the wye secondary winding be solidly grounded. However, high resistance grounding with proper choice of resistance value to limit secondary transient voltage to ground is also acceptable. For quasi 12-pulse systems, the delta winding must be solidly grounded or high resistance grounded using a grounding transformer.Fusing, Transient Over-voltage Protection and Common-Mode Jumper Recommendations The recommended method for fusing and transient overvoltage protection is illustrated in Figure17. The AC line fuses, typically high speed semiconductor fuses, should be integral to the rectifier bridge and are intended for short circuit protection of the rectifier power device. PowerFlex SCR bus supplies have AC fuses (on the 400 A and 600 A units, the fuses are on the AC line; on the 1000 A unit, the fuses are in series with each SCR device). If the rectifier bridge includes an integral capacitor bank, DC bus fuses should be placed after the capacitor bank and before the interconnection to the system DC bus, to prevent faults from cascading. PowerFlex SCR bus supplies do not have integral capacitor banks; however, the units rated at 400 A and 600 A have integral DC fuses, whereas the 1000 A unit does not have DC fuses.Metal oxide varistors (MOV) are used for transient overvoltage protection and should be part of the rectifier power structure. PowerFlex SCR bus supplies have integral MOVs.Fuses or circuit breakers should be used for branch circuit protection, with ratings in accordance with the NEC in the United States or local electrical codes as applicable. This is to protect each transformer secondary winding and the cabling to the rectifier bridge from overloads and short-circuits. The current rating to use for selection of the branch circuit protective device is 0.65I line_6P, where a 65%:35% split is assumed for current sharing between the two rectifier bridges, and I line_6P is the drive’s rated input RMS current for 6-pulse operation. If the secondary wye winding of the transformer is not solidly grounded, then any common mode capacitor networks on the AC line or DC bus (on both the rectifier bridges and the drives on the DC bus), and the MOVs must be disconnected from ground. For instructions on how to disconnect the MOVs and common mode capacitors, see the PowerFlex SCR Bus Supply User Manual, publication 20S-UM001, and the PowerFlex 750-Series AC Drives Installation Instructions, publication 750-IN001.T o achieve compliance with the EMC directive of IEC, the AC system must be solidly grounded at the wye secondary and the common mode capacitor networks must be connected to ground.Figure 17 - 12-pulse Drive Branch Circuit Protection, Fusing and Transient Overvoltage ProtectionApplication Guidelines for 12-Pulse Operation of PowerFlex 750-Series AC DrivesPowerFlex SCR Bus Supply Selection Chart for 12-pulse Operation of PowerFlex 750-Series, Frames 6…10 DrivesTable1…Table4, beginning on page 22, provide the PowerFlex SCR bus supply and Bulletin 1321 line reactor catalog numbers, ratings for input branch circuit protection devices, and the kVA rating of the delta-delta-wye transformer for use with PowerFlex 750-Series, frame 6…10 drives.The ratings specified in the tables are based on maximum voltage unbalance of 1% and 5th harmonic distortion of 3%, which is a conservative estimate of worst case power quality. However, if the voltage unbalance or harmonic distortion exceeds these values, then the current unbalance between the rectifier bridges will exceed the assumptions made in Table1…Table4, requiring higher current rating for the rectifier, transformer and branch circuit protective device. Please contact Rockwell Automation T echnical Support for assistance in sizing the components.Rockwell Automation Publication 750-AT003A-EN-P - March 201421。