By Terry Hunter, P.Eng.
Harmonics in commercial buildings have been a problem for number of years. One of thecontributing factors is the computer industry's use of switch mode power supplies. In some casesthese power supplies make up a major portion of commercial building loads. The switch modepower supply draws current in narrow pulses from the utility. These pulses are not sinusoidal andproduce odd order multiples of the base frequency (60 Hz). The produced harmonic currentstravel through cables, transformers, circuit breakers and produce voltages at the variousfrequencies. If these voltages become excessive, the equipment that produced them can shutdown, or potentially destroy itself.
The solution to harmonic problems require detailed knowledge of the distribution system and theconnected loads. Complexity is added in commercial buildings because most of the connectedload is random. Any potential solution must be evaluated based on the effect to the entire powersystem, rather than a particular load. For a solution to be acceptable it must reduce the levels ofharmonics for all, or at least most, load conditions and never create adverse conditions. At times acombination of solutions is required to adequately reduce the harmonics present in the system.
Case Study Background
The site of the study was B.C. Systems, which is a communications centre for most provincialgovernment operations. A major load is the 2 U.P.S.'s (Uninterruptible Power Supplies) thatsupply the computer equipment. A UPS is commonly used to supply critical loads such ascomputers. A characteristic of these devices is the ability to block disturbances. A disturbance ortransient on the line side of the UPS is generally not present at the load terminals. This effectivelycreates a "Clean" bus, a voltage supply that is free from disturbances. In addition, the UPSprovides ride through during power outages.
Sensitive loads connected to this "Clean" bus are usually considered protected from powerquality problems. However this is not always the case. Most loads connected to the "Clean" busutilize switched mode power supplies. These devices produce harmonic currents. The UPS alsocreates harmonics in the output inverter stage. Filtering is used in the UPS output stage to limitthe voltage distortion. But since this filter is designed without detailed load data, it isunreasonable to expect the filters to effectively limit the harmonic distortion in every situation. This case study illustrates such a situation.
The two UPSs supplied 10 Computer Power Centres (CPCs) and 3 M-G sets. All of the loadswere fed by step-down delta-wye transformers, 600 to 120/280 Volts. If the critical UPS bus ofB.C. Systems loses power, up to 46,000 computers in B.C. can lose information and/or becomeidle. To maintain power to the building there are two utility sources. Should both fail, there isover three megawatts of back up diesel power, and now there is 2.25 megawatts of redundantUPS. Other electrical specialty designs have been implemented to improve power quality to thisbuilding. A Scada System has been installed along with a BMI 8800 disturbance analyzer forafter the fact analysis. Automatic paralleling capabilities, with emergency diesel and utility, wasincorporated to increase system reliability during UPS maintenance periods, and a completesystem wrap-around bypass was installed to allow UPS maintenance.
B.C. Systems encountered problems when the existing computers and M-G sets were replaced.The M-G sets were used to generate 415 Hz voltage for IBM processors (model 3090). TheseIBM processors were working beyond their capability, and the IBM ES9000 model was beingstudied as a replacement . The ES9000 was designed to operate with 60 Hz input power,eliminating the need for the M-G sets. The new ES9000 processors developed 35% THD currentdistortion. This meant that 2 major sources of harmonics would be added to the UPS bus.
The ES9000 processors are very robust devices, they can handle +15% and -20% voltagevariations, can be single phased, and handle up to 10% THD in the voltage. These make themvery attractive from the power quality point of view. When one new processor was temporarilyinstalled on an isolation transformer fed by the UPS, the voltage distortion at the processor inputwas 9.7% THD. Too close for reliability.
A method to reduce the harmonic distortion was required in order to make installation of theES9000 feasible.
Harmonic Reduction Goals
The objectives that needed to be addressed in this survey were as follows:
The major harmonic in the system was the 5th. The largest loads were the 2 transformers feedingthe M-G sets. These 2, 450 kVA transformers fed 3 model 720 processors and 3 M-G sets. The720 processors were the source of the harmonics measured on these feeders. Once the M-G setswere replaced with the ES9000 processors, the harmonics on these transformers would increasesignificantly.
Other significant loads were the CPC panels, numbers 4 and 7. These loads were fed by 125 kVA,600V to 120/280V transformers. Panels 4 and 7 had the largest loading of the CPC panels withhigh levels of 5th harmonics.The load monitoring showed that the UPS load was almost constant. The load levels of the 450kVA transformers were almost constant as well.
Effect of Harmonics on Transformers
The initial concern was the high voltage distortion that would occur when installing the newES9000 processors. However the harmonic loading of the CPC transformers also requiredanalysis. All loads from the UPS distribution bus were supplied via step-down, delta-wyetransformers. The harmonics listed in the table would add to the heating of the transformer.
The Computer and Business Equipment Manufacturers Association (CBEMA) establishedtransformer derating factors to prevent the overloading of transformers, while the InternationalElectrical and Electronics Engineers Association (IEEE) established standards (C57.110) tooverrate transformers for appropriate harmonic loading. Derating a transformer is a safe practicefor transformers already installed, however, proper engineering design will overrate a transformerin the original design for the appropriate harmonic loading.
K-Factor is discussed in the IEEE Standard C57.110, and is a measure to rate harmonic loadingon a transformer. A purely linear load, with no Silicon Controlled Rectifiers for a load (SCRs)will have a K-Factor of one. A transformer used for 50% nonlinear, 50% linear loads, will have aK-Factor of four. For 100% nonlinear loads, the transformer K-Factor will equal thirteen. (Basedon IEEE 519 theoretical harmonic generation).
F-Factor is a means of rating a transformer in multiples of the eddy current losses versus the I(squared) R losses. The primary winding has to have sufficient conductor capability to withstandthe high third harmonic circulating currents, as well as the ninth and fifteenth harmonics. Thesecondary windings can use multiple, smaller, parallel conductors instead of single larger ones.This reduces the skin effect andAC resistance at frequencies greater than design. (60 Hz). Thetransformer's neutral, to the point of connection of the phase windings, has to be larger to sustainthe third zero sequence currents generated by the non linear devices. The reasons for specifying aK rated transformer versus derating an off the shelf transformer are:
A distribution transformer in a switch mode power supply environment has to be designed forharmonic loads and has several challenges. The third harmonic is a circulating, zero sequenceharmonic current. This harmonic order, along with all other zero sequence harmonics (3, 9, 15,21, etc.) will circulate in the delta portion of a transformer's windings. The secondary windingsmust be capable of withstanding the skin effect of higher frequency harmonics. The higher thefrequency, the greater the value of current travelling in the outside periphery of a cable. Thisaccelerates transformer winding insulation deterioration due to increased electrical and thermalstress.
Harmonics and Phase Shift Transformers
Phase shift transformers will magnetically cancel lower order harmonics such as the fifth andseventh. Phase shift transformers are usually either a two or three winding transformer with a 30degree phase shift in the windings. This phase shift is ideal for the fifth and seventh orderharmonics because, if equal in magnitude, they will cancel. Even if they are not equal in magnitude, the result is the difference in their magnitudes. The advantages of phase shifttransformers as compared to power line filters are:
The CPC's would also benefit from the installation of phase shift transformers. The majorharmonics, 5th and 7th, would be cancelled. This would reduce the voltage distortion at the 600Vbus and also at the 280V supply to the CPC's.
UPS Trap Filters
The UPS output filters were not effective in limiting the UPS distribution bus voltage distortion.The UPS's were 20 years old. At that time, most UPSs of this size would supply power to M-Gsets, which in turn would power the computers at 415Hz. Under these conditions the outputfilters would only filter the harmonics generated by the UPS. As the computer industry evolvedthe M-G sets became obsolete. The mainframe and all periphery devices are powered directly by60Hz. All of this equipment produces harmonics due to the switched mode power supplies. Theexisting filters in the UPS were not capable of controlling the harmonics produced by the presentloads.
There were a number of possible solutions to the existing harmonic problems:
Option 1: Change Upstrap Filter Reactors
This was only recommended to achieve a temporary reduction in THD until the UPS moduleswere replaced. (Replacement of the UPS's was being considered by the customer). Should any ofthe later choices be selected, this is not necessary.
Replace the reactors in the UPS modules with: three modules; three reactors per module, pertuned frequency (5th and 7th). Hence 9 reactors for the fifth and 9 reactors for the seventh.
Option 1b: Replace The UPS's With New Units Equipped With Larger Filters
This was recommended since UPS replacement was already being considered. Installation of newUPSs with filters designed for the present loads would reduce the voltage distortion.
Option 2.: 300 kVA Phase Shift Transformers For The Processors
The new IBM processors are by far the worst offender of harmonic currents and voltage drops.Hence, a solution to these harmonics will improve the entire system (THDV). The processorsrequire step down transformers from 600 to 120/280V. By installing phase shifting transformersthe 5th and 7th harmonics can be partially or totally cancelled in the transformer. The degree ofcancellation depends on the loading of each secondary winding. With identical loading, totalcancellation is achieved. The transformer size was selected at 300 kVA, allowing for futuregrowth to the IBM ES9000 complex. The transformer should have an extra quiet rating due tothe environment it is going into. The unit should be designed to suit a raised floor entry and exit.
Design Specifications for the Transformers
Recommend three be ordered and installed. Two for the planned ES9000 machines and one forthe future machine.
Transformer Specifications:
Option 4: Higher Order Harmonic Filtering
The purpose would be to design and install a filter to band reject harmonics in the frequency rangestarting at the eleventh order. If this option is chosen with #2 and #3, there should be no need tobe concerned with present or future harmonics. This filter would be small in physical size, due tothe magnitude of harmonics to be imported. Because we are on an isolated bus, power factorcorrection is not a concern, but reduction of THD and stiffness of the system are.
Selected Options
After reviewing the options, it was decided to install the phase shift transformers as detailed inoptions 2 and 3. Modification of the UPS filter was not selected since the UPSs were planned tobe replaced. The high order harmonic filtering was delayed until after the transformer installationwhen the need for it will be re-evaluated.
The phase shift transformers used in this situation are in a way unique. The concept of harmoniccancellation using a 30 degree phase shift is well known. This is usually implemented using 2transformers, one being delta-wye and the other delta-delta or wye-wye. Since there is an inherent30 degree phase shift in the delta-wye transformer, the 5th and 7th harmonics from the delta-wyewill cancel the 5th and 7th from the other transformer. However in this situation there weredisadvantages to this method of producing a 30 degree phase shift.The disadvantages were based on the use of a wye-wye or delta-delta transformer. The delta-deltatransformer secondary is by nature ungrounded, and thereby cannot supply single phase loads(based on the Canadian Electrical Code). The wye-wye transformer lacks a delta winding whichtraps the 3rd order harmonics. This application required both a primary delta winding and asecondary with a neutral point that could be grounded.
The solution was the use of a delta primary, dual secondary wye, with a 30 degree phase shiftbetween the two secondary windings. This was feasible since the load was easily split between the2 secondary windings. An added advantage of this design is that the cancellation of the harmonicsoccurs in the magnetic circuit of the transformers, rather than on the primary electrical circuit.
Results
The phase shift transformers worked very successfully, and all but eliminated 5th and 7th orderharmonic currents on the primary windings. Third and other triplen harmonics were trapped in thedelta windings. The before picture had voltage distortion as large as 9.7%, the after pictureharmonics were reduced to 2.3%. Three 300 kVA transformers were installed to handle the IBMcomputer loads, replacing the two 450 kVA units. Another three phase shift transformers wereinstalled to handle CPC panels 4, 6, and 7. The customer was so impressed with the long termbenefits of the solution that a fourth transformer was purchased to handle the data centre whichcontains hundreds of low power levels, high harmonic content modems.
The final phase of this project has yet to be implemented. This will be an evaluation of the needfor high frequency (11th order and higher) filtering, and also evaluating all local areas for the needto reduce neutral to ground voltages with zero sequence filtering.
Terry Hunter is with Elite Engineering located in Victoria, B.C.
