Transformador Aislamiento Con Drivers

8/13/2019 Transformador Aislamiento Con Drivers

1/6

MEDIUM VOLTAGE DRIVES: ARE ISOLATION TRANSFORMERS REQUIRED?Copyright Material IEEEPaper No. PCIC-2002-23

Frank DeWinter Navid Zargari Steven Rizzo Yuan XiaoRockwell Automation

135 Dundas St. Cambridge, ONCanada N1R-5x1

Abstract: Traditionally isolation transformers are usedon Medium Voltage Drives. Although this provides ameans for reducing harmonics. reducing voltage stresson motors or provided a means to achieve a MediumVoltage drive utilizing Low Voltage cells, it increases the

experimental results on a 4160V. 600hp ac drive areincluded to verify the system operation.2. HARMONICS MITIGATION

size, st. complexity and losses of the drive system.With the implementation of alternative topologies andnew Pulse Width Modulation switching patterns, thesegoals can be achieved without the use of an isolationtransformer. The paper presents the topology and thetechniques that can be utilized and proposes an overallac drive system without the need for the transformer.Experimental results on a 600hp, 4160V drive are alsoincluded.

1. INTRODUCTIONAdjustable Speed Drives (ASD) are widely used inindustrial applications. Nowadays, this has found itsway into Medium Voltage applications where typicallythe system voltage is between 2.4kV to 7.2kV [I 1.Unlike low voltage applications where the topology ofchoice is a conventional Voltage Source Inverter WSI ,in MV applications there is no dominant topology 13.41.One thing that most MV drives have in commonhowever,. is an isolation transformer. The isolationtransformer is used to perform a variety of functions.

The more common reasons to use an isolationtransformer are as follows:t Voltage Step Down: Needed when the supply busvoltage is higher than the motor voltage.t Harmonic Cancellation: Many AC drive topologiesuse a multi-winding isolation transformer to cancelline current harmonics with the appropriate winding

Commercially available ac drives today use variousmethodologies to improve the quality of the input linecurrents in order to meet harmonic guidelines such asIEEE-519. These are categorized below.2 1 Using mulfi-winding ransformer

Using an input transformer to eliminate unwantedharmonics has become the norm of the industry. Thesecondary windings of the transformer are phaseshifted to eliminate the harmonics. For example atransformer with two secondary windings that areshifted by.30 degrees (using a Y and a A winding) cansignificantly reduce the 5 and 7 input currentharmonics. Such a transformer is used in three-levelVSI based ac drives as shown in Fig. 1151 Anotherexample is the use of a multi-winding transformer inconjunction with low-voltage ac drive cells to build aMedium Vqltage ac drive [SI This topology, shown inFig.2. however, suffers from complexity and a very highcomponent count as compared with other topologies.

phase shift to cancel unwanted harmonics.t Isolation: By isolating the secondary of thetransformer. a new ground reference can be

Fig. 1 A 3-level VSI based ac drive usinga 3 windingisolation transformer for harmonic mitigation.established that reduces the motor line and neutralto ground voltage. Doing this, a motor withstandard insulation design can be used.This paper proposes an alternative method other thanisolation transformer. First an Active Front End PWMRectifier is proposed to achieve harmonic mitigation

and to meet the IEEE-519 harmonics guidelines. Thenthe voltage stress across the motor windings for variousMedium Voltage Drives are studied. Finally an overallsystem solution is proposed that is smaller and morecost effective than any other MV AC drive. The

0 7803 7384 71021317.00@ 2 2 IEEE 191 02CH37328


2/6

Fig. 2 A series connection of H bridge invertersthrough a multi-winding ransformer to achieve aMedium Voltage drive and harmonic mitigation.2.2 UsingPulse Wdth Modulated convertersIn recent years power semiconductors capable ofhigh switching frequency have become available. Theadvance in semiconductor technology and availability ofPulse Width Modulation switching patterns offer analternative method in harmonic mitigation through theuse of PWM converters and selectively eliminatingharmonics (known as Selective Harmonic Elimination,r SHE). These converters are also known as ActiveFront Ends in the industry. Althou,gh Active Front Endrectifiers are applicable to both current source andvoltage source ac drives, the 3-level VSI active frontend rectifier may require an input. transformer forvoltage level adjusting due to the control aspects of therectifier. Hence, the only topology that can achieveharmonic mitigation without the need for an inputtransformer is the so-called PWM Rectifier used as theinput stage of Current Source based ac drives. The

high switching frequency operation is achieved by usingthe newly developed Symmetric Gate CommutatedThyristors (SGCT) m These devices have theadvantage of low conduction losses (same as GTOs orthyristors) and fast switching times and minimalsnubber requirements (same as transistors). A PWMCSI AC drive with PWM Rectifier (Active Front End) isshown in Fig. 3[SI. An SGCT is shown in Fig. 4. Itcan be seen that the SGCT is an integration of thePower Semiconductor and its gate driver. The close

proximity of the gate driver lo the Power Semiconductorgives it a superior switching capability and minimizesthe snubber requirement. The leg of a 2400V drive isshown in Fig. 4. It consists of a very small capacitorand a resistor to limit the discharge current.e J

Z

TTT T ZrFig. A PWM CSI drive with Active Front End

The input line current and voltage are shown in Fig.5. The Total Harmonic Distortion F H D of the input linecurrent is within the IEEE-519 harmonic guidelines.These limits and the typical THD characteristics of adrive with PWM rectifier are shown in Fig.. 6.

to dt + bus

ro Phase A

Fig. 4 (lefl) An SGCT, (right) the components ofone leg of a 2400V ac drive

192


3/6

(I,THD=4.3%, V,THD=2.3%). . . . .. . .. I

Fig. 6 The line current harmonics and the IEEE-519limits.

The motor voltage and current waveforms are shownin Fig. 7. It can be seen that both current and voltagewaveforms are near sinusoidal resulting in no voltagestress on the motor windings even if the motor isconnected through long cables.The above discussion indicates that usingtransformers for the purposes of harmonic mitigationalone is not justified, since alternative methods andtopologies are available that fulfil the harmonicrequirements. The remainder of this paper focuses onthe use of an input transformer and an alternativemethodology for mitigating the motor neutral to groundvoltage.3. NEUTRAL TO GROUND VOLTAGE CREATED

BY AC DRIVESAc drive systems are always connected to the main

grid through a front-end rectifier. There is usually a dclink buffer stage between the rectifier and the inverter.The dc link can be voltage source, using capacitors orcurrent source using inductors , shown in Fig. 8. Themid point neutral voltage of the dc link can beobtained from:

Lwhere, v is the mmmon mode voltage, vd s thepositive side voltage of the dc link and v s thenegative 'side voltage of the dc link. The voltageacross the dc link is defined as:

where vdfl esignates a differential voltage which is infact the dc voltage. The motor neutral-to-ground andline-to-ground voltages can be calculated from:

v. = VO., , + V, ,v = v v ,where, v is the motor neutral to ground voltage.v .s the DC link line side neutral to ground voltage,and v is the line to neutral voltage of the motor.

The above equations prove that the motor neutral-to-ground and line-to-ground voltages are directly relatedto the neutral point of the dc link, or common modevoltage. It should also be pointed out that without anisolation transformer, the neutral voltage of ac drivesystems are identical, i.e. the topology of the drive doesnot affect the common mode voltage if no isolationtransformer is present.

-193-


4/6

GNDrTi

Fig. 8 A typical MV drive comprisedi rectifier, aninverter and a dc link buffer stage.-Fig. 9. Eliminating wmmon mode voltage using acommon mode choke.

3.1. Common mode voltage in ac drivesd e oltage discussed above.

There are a few methods to eliminate the wmmon

3.1.l. sing isolation transformersThe wmmon method of mitigating wmmon modevoltage is by using an isolation transformer. Since the

secondary of the isolation transformer is floating, thenema\ point of the dc link can be grounded withoutresulting in a ground current flow. This approach isused by neutral point damped ac drives. The otherapproach is to ground the neutral of the motor or wyepoint of the output filter capacitors through a groundingnetwork, thus, limiting the ground current. Thisapproach is used by most Current Source based acdrives. By doing this, the motor neutral voltage will notbe subjected to any excessive voltage.

3.1.2. Using active switching methodIn recent years, researchers have been investigatinga more advanced method of eliminating wmmon modevoltage. These are based on'using. switching patternsto eliminate the wmmon mode voltage. However, theycompromise the inverter performance andlor increasethe switching loss [9]. Furthermore, they are at aresearch level and are yet to be implemented in realcommercial ac drives. Another possibility along thesame lines is a 4-leg inverter bridge, which also suffersfrom similar drawbacks plus the additional ws t of afourth leg of the inverter [lo]

3.1.3. Usinga common mode chokeA wmmon mode choke e n be used to block thewmmon mode voltage and achieve zero voltage on the

motor neutral. This method is feasible and particularlyattractive to current source based -drives since thistopology already uses a dc link inductor and the impacton the cost and size of the system can be minimized.One such design that integrates he common mode andthe dc link inductor into one is shown in Fig. 9. Thereactor is a %leg design shown in Fig. 10 that isoptimized to obtain the highest common modeinductance and at the same time ensure a smoothcurrent necessary for operation of CSI drives 11I] hisinductor also provides an inherent limit to the shortcircuit current of the drive.

Fig. I O . An integratedcommonand differentialdc link reactor.4. THE PROPOSED SYSTEM SOLUTION AND

EXPERIMENTAL RESULTSThe proposed topology is based on the current-source inverter topology with a PWM Rectifier front endfor harmonic elimination, Fig. 3, but with the commonmode choke shown in Fig. 10. The PWM rectifier wasdiscussed in Section 2.2. The regular dc link reactorand the common mode choke are integrated into one

reactor with a three-legged wr e to optimize the Sizeand cast of the drive system. Both the inverter and therectifier use SGCTs and both operate below 500 Hzswitching frequency to minimize the switching losses.Considering that there is no isolation transformer, thissystem offers significant advantages over other ac drivetopologies.

4 1 Results without the common mode chokeThe tests were first conducted. on a system withoutthe wmmon mode choke. The test circuit is shown in

Fig. 11. The motor line-to-ground and neutral-to-ground voltages were measured. These results areshown in Fig. 12. It can be seen that without theisolation transformer and wmmon mode choke, themotor neutral to ground voltage is present.

-194-


5/6


6/6

[I E. Wu, S Riuo. N.Zargari, Y. Xiao. An lntegrateddc Link Choke for Eliminationof Motor Common-Mode Voltage in Medium Voltage Drives', Conf.Rec. IEEE IA. 2001.

BIOGRAPHYF. Dewinter:Frank A. Dewinter received his B.Sc. in electricalengineering from the University of Alberta in 1980. Heis the business unit manager for medium voltage drivesand was previously the director of research anddevelopment of medium voltage drives. Both of thesewere at Rockwell Automation in Cambridge, Ontario.He has co-authored over 30 papers on harmonics.application and the design of Medium Voltage Drives.He is a senior member of IEEE, a member of the PClCexecutive committee and was the conference chairmanof the 2001 PClC conference in Toronto. He is aregistered professional engineer in the province ofOntario.N. Zargari received his E.Eng. degree from TehranUniversity, Iran. and MASc. and Ph.D. degrees fromConcordia University, Montreal, PQ. Canada, in 1987,1991. and 1995, respectively. Since November of1994, he has been with Rockwell Automation and ispresently the Manager of the Medium Voltage Electricaland Power Electronic Development Department. He isinvolved with simulation and design of converters formedium voltage applications. His research interestsinclude power converter topologies. their control andelectrical ac drives.S. Riuo received his E.Eng. degree from McMasterUniversity, Hamilton, ON anada and Masters Degreefrom Teesside University, Middlesbrough, UK in 1986and 1998 respectively. Since September of 1992, hehas been with Rockwell Automation and is presently theManager of the Medium Voltage Research andDevelopment Department, His research interests are inthe application of high power devices in medium.voltageapplications.Y. Xiao received his E.Sc. degree and M.Eng. degreefrom Xi'an Jiaotong University, Xi'an. China and Ph.D.from Teesside University, Middlesbrough, UK in 1982,1985 and 1998 respectively. Since December of 1996he has been with Rockwell Automation and is presentlya Senior Designer in the Medium Voltage Electrical andPower Electronic Development Department. Hisresearch interests are on high power'mnverters for acdrives. His areas of interest include modelling andanalysis of converters for medium voltage applications.

196

Transformador Aislamiento Con Drivers

Documents

Transcript of Transformador Aislamiento Con Drivers