trfo voltage drop

An example of calculating transformer size and voltage drop due to starting of large motorExampleLets calculate voltage drop in transformer 1000KVA, 11/0.480 kV, impedance 5.75% due to starting of 300 kW, 460V, 0.8 power factor, motor code D (kva/hp). Motor starts 2 times per hour and the allowable voltage drop at transformer secondary terminal is 10%.Calculation can be checked by using this MS Excel Spreadsheet dedicated especially to this kind of problem.Ok, lets get into the calculationsMotor current / TorqueMotor full load current = (Kw x 1000) / (1.732 x Volt (L-L) x P.F Motor full load current = 300 1000 / 1.732 x 460 x 0.8 = 471 Amp. Motor locked rotor current = Multiplier x Motor full load current

Locked rotor current (Kva/Hp)Motor CodeMinMax

A3.15

B3.163.55

C3.564

D4.14.5

E4.65

F5.15.6

G5.76.3

H6.47.1

J7.28

K8.19

L9.110

M10.111.2

N11.312.5

P12.614

R14.116

S16.118

T18.120

U20.122.4

V22.5

Min. motor locked rotor current (L1) = 4.10 471 = 1930 Amp Max. motor locked rotor current (L2) = 4.50 471 = 2118 Amp Motor inrush Kva at Starting (Irsm) = Volt x locked rotor current x Full load current x 1.732 / 1000 Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 471 x 1.732 / 1000 = 1688 kVA

Transformer Transformer full load current = kVA / (1.732 x Volt) Transformer full load current = 1000 / (1.73 2 480) = 1203 Amp. Short circuit current at TC secondary (Isc) = Transformer full load current / Impedance Short circuit current at TC secondary = 1203 / 5.75 = 20919 Amp Maximum kVA of TC at rated Short circuit current (Q1) = (Volt x Isc x 1.732) / 1000 Maximum kVA of TC at rated Short circuit current (Q1) = 480 x 20919 x 1.732 / 1000 = 17391 kVA Voltage drop at transformer secondary due to Motor Inrush (Vd) = (Irsm) / Q1 Voltage drop at transformer secondary due to Motor inrush (Vd) = 1688 / 17391 = 10% Voltage drop at Transformer secondary is 10% which is within permissible limit. Motor full load current 65% of Transformer full load current 471 Amp 65% x 1203 Amp = 471 Amp 781 AmpHere voltage drop is within limit and Motor full load current TC full load current.Size of Transformer is Adequate.Recommended EE articles // Comparison of Motor Speed Control MethodsMarch 13, 2015 Should We Blame Supplier For Voltage Dips and Transients?March 11, 2015 4 Practical Approaches To Minimize Voltage Drop ProblemsFebruary 25, 2015 Basic Transformer Routine Test Measurement of Winding ResistancesFebruary 20, 2015Share with engineers //Article Tags //large motor, motor, motor current, motor full load current, motor torque, transformer size, voltage drop, Filed Under Category //Maintenance Transmission and DistributionAbout Author //

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.RSS Feed for Comments15 Comments1. bhavin mistryJan 28, 2015Dear sir, i want know about maximum secondry connectable load as per transfor mer rating(reply)2. Djarot PrasetyoNov 15, 2014Hi! Id like to know, what standard did you use for the locked rotor current?Thanks!(reply)3. Benn RicheySep 02, 2014Theres an error in the calculations above: Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 471 x 1.732 / 1000 = 1688 kVAIt should be: Motor inrush Kva at Starting (Irsm) = 460 x 4.5 x 471 x 1.732 / 1000 = 1688 kVA(reply)4. Mujeeb RazaAug 28, 2014Hi All,I want to get this into detail, if the output of this transformer is connected to load of small industries (assume same data), What factors are to look into while selecting cable for the secondary of transformer up to the LV panel.(reply) Older CommentsRSS Feed for CommentsLeave a CommentClick here to cancel reply.Top of FormTell us what you're thinking... we care about your opinion!and oh, not to forget - if you want a picture to show with your comment, go get a free Gravatar!Name *Email *Website

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Environment:Applies to Low Voltage Transformers by SquareD/Schneider Electric

Cause:Transformers are often required to power motor loads

Resolution:

This is provided on page 39 of the Low Voltage Transformers Selection Guide, document # 7400CT9601. Please see attachment below. When selecting a Transformer to feed a motor, it is important to note that the starting current of a motor can 6 to 7 times the full-load running current, or even higher if it is a high efficiency motor. This initial high current can cause excessive voltage drop because of regulation through the Transformer. Reduced voltage could cause the motor to fail to start and remain in a stalled condition, or it could cause the starter coil to release or ``chatter``. A typical desirable voltage drop is to allow 10-12% voltage drop at motor start. The voltage decrease during motor starting can be estimated as follows:

Voltage Drop (%) = (Motor Locked Rotor Current / Transformer Secondary Full Load Rating) * Transformer Impedance (%)

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NotesLoading, please wait.Cables for MV Power Distribution - Earthed versus Unearthed Systems By Jeson Pitt on August 12th, 2013

Power cables can basically be classified into earthed and unearthed cables, where earthed and unearthed refer to the application for which the cable is used. Earthed system refers to a three phase system whose star point is grounded directly and the voltage between the healthy phases and the ground will be - 11kV/1.732 or 6.6/1.732. In the case of unearthed cable, ground voltage is equal to phase to phase voltage.Earthed CablesUnearthed Cables

Medium Voltage (MV) voltage power distribution system cables can be both earthed and unearthed. If the system is earthed, then we use earthed rated cable for manufacturing; and if the system is unearthed, we use an unearthed rated cable for manufacturing. Compared with the earthed cable as per the manufacturer's specifications, the unearthed cable needs higher insulation levels.The greatest difference arises in the voltage grade (Uo/U), which is:Earthed SystemUnearthed System

1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV3.3/3.3 kV and 11/11 kV

In an exception to the abover, the cables of 6.35/11kV for an earthed system can also be used in the place of 6.6/6.6 kV for an unearthed system. This is because each core of the cable has the insulation level to withstand 6.6kV due to which between core to core insulation level will be 6.6kV+6.6kV = 11kV.Contents [hide]The Difference in OriginInsulation StrengthCable RequirementsPreferable Cable for MV TransmissionThe Difference in OriginThe first generators and transformers had small capacities in which the fault current was less and the star point was solidly grounded due to which they were called earthed system. Generators that are now available have 500MVA capacity and higher fault levels. So, if there is an earth fault, a heavy current flows into the fault, which leads to the damage of the generators and transformers. In such a scenario, to reduce the fault current, the star point is connected to the earth through a resistance. In case of an earth fault in one phase, the voltage of the faulty phase with respect to the earth appears across the resistance. Due to this, the voltage of the remaining two healthy phases with respect to the earth rises by 1.7 times. If the insulation system is not designed to sustain these increased voltages, they are likely to develop earth faults.In case of earthed cable, three phase cables are earthed to a ground and each of the phase system is grounded to the earth. While the unearthed system (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage; in such situations the insulation level of the conductor to the armor should be equal to the insulation level of conductor to conductor. In the three phase earthed systems, phase to earth voltage is 1.732 times less than phase to phase voltage. Thus, the voltage stress on the cable to armor is 1.732 times less than the voltage stress between conductor to conductor.Insulation StrengthUnearthed cable requires more insulation strength than earthed cable. If a fault occurs in the phase to ground voltage is 3 time the normal phase to ground voltage. So, if an earthed rated cable is used in an unearthed system, it may result in an insulation puncture. Hence, it is essential to use unearthed rated cable in such situations, especially in the case of 6.6kV systems where resistance type earthing is used. Cable RequirementsCarrying forward the above point, 11kV earthed cable can be used in place of 6.6kV unearthed system since the cable manufacturing process is the same. The size of the cable will depend on the current rating and voltage level. So, Voltage grade (Uo/U) where Uo is phase to earth voltage and U is phase to phase voltageEarthed system has an insulation grade of kV/1.75x kV For earthed system (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV Unearthed system has insulation grade kV/kV 3 phase 3 wires system generally comes with unearthed grade cable and 3 phase 4 wire systems can be used as earthed grade cables Preferable Cable for MV TransmissionFor MV transmission, earthed cable will be more economical, but unearthed cable offers more insulation. This is because, if an earth fault occurs in the underground system, the voltage between the healthy phases and the ground will be equal to phase to phase voltage - 11kV or 6.6kV and higher insulation level is required. The voltage of the healthy phases rises by nearly 1.7 times resulting in an earth fault since the insulation of these phases is not designed for increases voltage. It is advisable to opt for an unearthed cable so that the core insulation has enough strength. Earthing, Cables, High Voltage, Distribution, Medium Voltage

More interesting Notes:Possibly related posts:Laplace TransformPower Transformers - An IntroductionPhotovoltaic (PV) Panel - Performance ModellingPower FactorIntroduction to Current Transformers

Jeson PittJeson works as a Sales Representative for D&F Liquidators, a leading supplier of electrical products. He is passionate about everything electrical.About the author

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Differences Between Earthed and Unearthed CablesIntroductionIn HT electrical distribution, the system can be earthed or unearthed.The selection of unearthed or earthed cable depends on distribution system. If such system is earthed, then we have to use cable which is manufactured for earthed system. (which the specifies the manufacturer). If the system is unearthed then we need to use cable which is manufactured for unearthed system.The unearthed system requires high insulation level compared to earthed system.For earthed and unearthed XLPE cables, the IS 7098 part2 1985 does not give any difference in specification. The insulation level for cable for unearthed system has to be more.

Earthed SystemEarlier the generators and transformers were of small capacities and hence the fault current was less. The star point was solidly grounded. This is called earthed system.In three phases earthed system, phase to earth voltage is 1.732 times less than phase to phase voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress between conductors to conductor.Where in unearthed system, (if system neutral is not grounded) phase to ground voltage can be equal to phase to phase voltage. In such case the insulation level of conductor to armor should be equal to insulation level of conductor to conductor.In an earthed cable, the three phase of cable are earthed to a ground. Each of the phases of system is grounded to earth.Example: 1.9/3.3 KV, 3.8/6.6 KV system

Unearthed SystemToday generators of 500MVA capacities are used and therefore the fault level has increased. In case of an earth fault, heavy current flows into the fault and this lead to damage of generators and transformers. To reduce the fault current, the star point is connected to earth through a resistance. If an earth fault occurs on one phase, the voltage of the faulty phase with respect to earth appears across the resistance.Therefore, the voltage of the other two healthy phases with respect to earth rises by 1.7 times.If the insulation of these phases is not designed for these increased voltages, they may develop earth fault. This is called unearthed system.In an unearth system, the phases are not grounded to earth .As a result of which there are chances of getting shock by personnel who are operating it.Example: 6.6/6.6 KV, 3.3/3.3 KV system.Unearthed cable has more insulation strength as compared to earthed cable. When fault occur phase to ground voltage is 3 time the normal phase to ground voltage. So if we used earthed cable in unearthed System, It may be chances of insulation puncture.So unearthed cable are used. Such type of cable is used in 6.6 KV systems where resistance type earthing is used.NomenclatureIn simple logic the 11 KV earthed cable is suitable for use in 6.6 KV unearthed system. The process of manufacture of cable is same.The size of cable will depend on current rating and voltage level. Voltage Grade (Uo/U) where Uo is Phase to Earth Voltage & U is Phase to Phase Voltage. Earthed system has insulation grade of KV / 1.75 x KV. For Earthed System (Uo/U): 1.9/3.3 kV, 3.8/6.6 kV, 6.35/11 kV, 12.7/22 kV and 19/33 kV. Unearthed system has insulation grade of KV / KV. For Unearthed System (Uo/U): 3.3/3.3 kV and 11/11 kV. 3 phase 3 wire system has normally Unearthed grade cables and 3 phase 4 wire systems can be used earthed grade cables, insulation used is less, and cost is less.Thumb RuleAs a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.Because each core of cable have the insulation level to withstand 6.6kv so between core to core insulation level will be 6.6kV+6.6kV = 11kVFor transmission of HT, earthed cable will be more economical due to low cost where as unearthed cables are not economical but insulation will be good.Generally 6.6 kV and 11kV systems are earthed through a neutral grounding resistor and the shield and armor are also earthed, especially in industrial power distribution applications. Such a case is similar to an unearthed application but with earthed shield (sometimes called solid bonding).In such cases, unearthed cables may be used so that the core insulation will have enough strength but current rating is de-rated to the value of earthed cables.But it is always better to mention the type of system earthing in the cable specification when ordering the cables so that the cable manufacturer will take care of insulation strength and de rating.Recommended EE articles // 6 Transformer Types You Can See In Commercial InstallationsMarch 16, 2015 Degradation of Insulation in Switchgear (Whats Really Happening)March 4, 2015 Test On 110kV Power Cable After Installation (2)December 26, 2014 Test On 110kV Power Cable After Installation (1)December 22, 2014Share with engineers //Article Tags //cable, distribution, earth, earth fault, earthed cables, insulation, unearthed cables, Filed Under Category //Cables High Voltage Low Voltage Medium Voltage Transmission and DistributionAbout Author //

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.RSS Feed for Comments14 Comments1. General Guidelines for Online Partial Discharge Testing of Power Cables | EEPMar 30, 2015[] Cable earth shield coming out of the cable (whether single core or three core) must not be touching any earthed metal except at terminating point to Substation Earth. Earth shield at cable termination should be insulated using shrink tubes in order to get access to individual cable earths. [](reply)2. DK SharmaMar 24, 2015We use 11 KV cable to connect our Distribution transformer which has its primary or HV winding in Delta connection and Secondary or LV winding in Star connection. Since Delta connection has no earth, we always need 11 KV Unearthed (UE) cable.So we should always ask for 11 KV UE cable.(reply)3. Manuel BolotinhaMar 15, 2015This article about earthed and unearthed cables is very basic and with poor technical content.(reply)4. AMEER AHMADMar 15, 2015I am not agree with As a thumb rule we can say that 6.6KV unearthed cable is equal to 11k earthed cable i.e 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.Reason: For 6.6/11kV Earthed cable, the system nominal voltage is 11kV. If we use 6.6/6.6kV Unearthed cable for 6.6/11kV Earthed system with 11kV nominal voltage, the phase to earth voltage remain 6.6kV for Earthed system(Agreed), but phase to phase voltage remain 11kV for Earthed system and this 6.6/6.6kV Unearthed cable can not meet the 11kV nominal system phase to phase voltage insulation.(reply)5. vikas j trivediNov 02, 2014How to calculate feeder htvr through metlab or etap(reply)6. JAYESH R MANKADJul 25, 2014It is good and interesting web. Informative topics are covered(reply)7. noureddineJan 26, 2014Mr. Peteryou must check set time for overload in your breakermay be the setting is wrong.thanks.(reply)8. guestJan 03, 2014Hows 6.6kV+6.6kV = 11kV? As far as I know 6.6+6.6=13.2(reply) ParagMay 08, 2014It means it is suitable for 11KV insulation requirement (since 13.2KV>11KV). Standard cable insulation ratings are as 1.1KV, 3.3KV, 11KV, 33KV)(reply)9. BalachandarDec 10, 2013I can not agree this statement ie., 6.6/6.6kv Unearthed cable can be used for 6.6/11kv earthed system.but we can use 6.6/11kv cable for 6.6/6.6kv system.(reply)10. Peter AbbeyJan 21, 2013I have a question and it goes thus:A motor Filter Fans RPM on motor data plate is 3545 RPM, Relay overload set is 2.9A. During solo run, motor runs at 1.6A (starting current) at 3615 RPM. The LV motor tripped at 4.9A with enclosure door closed during load run. What are the possible causes of the motor trip? What can be done to avoid such trippings? (What are your recommendations?)(the motor specification are: Voltage = 460 V, Power = 1.5kW (2HP), No. of Phase = 3,Frequency = 60Hz,No. of poles = 2)Your prompt response will highly be apprexciated.Kind Regards,Peter.(reply) RagnarokMar 04, 2013Hi Peter, the starting current of 3 ph induction motor has nothing to do with the load. (Load affect the run up time and thermal withstand)The starting current for induction motors can as high as 7-8x the rated full load current. By your description of 1.5kW 460 3ph motors, I would think 4.9A sounds like a reasonable starting current. (not taking PF/eff into account, the rated full load current is at least 1.89A)(reply) TeguhMar 08, 2013Dear Mr Peter,if the motor trip at load run, means we have to check the load first. is it suitable for your motor or not, you can ask the mechanical engineer for that. well, for the motor itself, you can do normal check such as its impedance, megger, bearing etc.(reply) AKReddyMay 13, 2013I too experienced similar problem, in my case motor fan belt coupled, we loosen the belt and it worked for us(reply)RSS Feed for CommentsLeave a CommentClick here to cancel reply.Top of FormTell us what you're thinking... we care about your opinion!and oh, not to forget - if you want a picture to show with your comment, go get a free Gravatar!Name *Email *Website

We believe you're an engineer, but nevertheless ... *+ 6 = eightSign me up for the newsletter

Bottom of FormCONNECT //Subscribe to Technical Articles //Receive free alert to our HV/MV/LV Technical Articles. Start sharpening your technical skills!SubscribeLatest Technical Articles Good Voltage Regulation and Justified Power Factor CorrectionApril 1, 2015 General Guidelines for Online Partial Discharge Testing of Power CablesMarch 30, 2015 6 Energy-Efficiency Improvement Opportunities In Fan SystemsMarch 27, 2015

Electrical Engineering Daily DoseDaily dose of knowledge and news fromElectrical Engineering WorldScience at GlanceOur mission is to be the leading provider of scientific information in the field of power and engineering in general. We publish, we share and we spread the knowledge.You're welcome to read, write and contribute to EEP in any way!Energy EfficencyWe see the results of power usage all around us. Every day, the generation and usage of energy produces more pollution than any other single industry.It's our time to use energy on smart and more efficent way. EEP is on your side!Electrical SoftwareElectrical design without appropriate software and tools is impossible to imagine. This section covers professional engineering software for design, MV and LV network and photometric calculations, voltage drop and short circuit calculations etc. 2015 EEP - Electrical Engineering Portal. All Rights Reserved | Privacy Policy | 42 queries in 0.218 seconds.Powered by CsanyiGroup SHARE TOPGetPDF

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