ELECTRIC POWER QUALITY

Transcription

1 ELECTRC POWER QUALTY Dr. L.N Tripthy Associte Professor Electricl Engineering Deprtment GT, Srng, Odish Pge of 58/L N Tripthy

2 ELECTRC POWER QUALTY. Wht is Power Qulity? Any power problem mnifested in the vritions of (i) oltge (ii) Current (iii) Frequency in Power System results in filure or misopertion of customer equipment 3 Customer... Customer 3 Best Def.: PQ is the provision of voltges, system design so tht the user of Electricl Power cn utilize electricl energy from the distribution system successfully. Brod issues of Power Qulity (i) System Relibility (ii) Dielectric Selection in Equipments nd Conductors (iii) Long term System interruptions (Outges) (iv) oltge Unblnces in 3-phse Systems (v) Power Electronics & their interfce with electric power supply (vi) Wveform Distortions & Hrmonics Pge of 58/L N Tripthy

3 Four Mjor Resons for Growing Concern in PQ (i) Lod Equipment is more Sensitive to PQ vritions Power Electronics Devices Microprocessor Control (ii) High Efficiency Adjustble Speed Drives in Mnufcturing (iii) Shunt Cpcitors for Power Fctor Corrections Distribution System ~ ~ ASD Lod Bus C (iv) ndependent Service Providers known s Deregulted Power Mrkets Qulity of Customer Service is better for prticulr provider. Distribution compnies provide competition s regrds to qulity of Electricity Supply. Power Qulity = oltge Qulity P = Cos = Averge Power = R.M.S. oltge = R.M.S. Current ~ ~ M Lod Pge 3 of 58/L N Tripthy

4 Cos = Power Fctor To define Qulity of P is difficult, the Power Supply System cn only control the Qulity of oltge not Current (Lods might drw) Distribution System oltge = 3 volts in Asi & Europe =. per unit /phse n U.S.A., Cnd. pu = olts Frequency = 5 / 6 Hz Thus the stndrds in the PQ re re devoted to mintin Supply oltge within certin Limits Any devition of Supply voltge from 3 volts or /phse or Frequency from 5/6 Hz is PQ Problem. Concerns bout Power Qulity. Direct Economic mpct on mny industril customers.. Electroniclly controlled Energy efficient Motors re more sensitive to voltge vritions. 3. Electricl clocks sensitive to Frequency vritions 4. Outges cn cuse Production Losses 5. Computers re subject to PQ vritions cuse dt loss Loss in Automtion Function Simple Formuls for highlighting the effect of vritions in voltge & Frequency = f Lod = R + jl f = frequency R L oltge Source ~ L R cos R R L Pge 4 of 58/L N Tripthy

5 Pge 5 of 58/L N Tripthy cos P, m m cos L R R P m m Suppose voltge chnges by smll mount. A smll chnge in Power consumed by the Lod. [ R ) ( L R L R R P P + is smll nd hence is neglected.. P P P P P P or chnge in voltge by 5% increses the Power drwn by the Lod by.5% nd vice vers. -.5% P P -5%, Frequency chnge The Active Power P drwn by the Lod is, f L R R P For smll chnge in system frequency (f or )

6 Pge 6 of 58/L N Tripthy. L R L P L R R P or. L R L P P f Frequency f decreses the Power consumed by the lod increses nd vice vers. oltge nd Current Reltionships Consider Distribution System Schemtic Single-Phse Equivlent Circuit Digrm. f Current drwn by the Lods increses then s decreses or sgs. s = E s Z s. f decreses by cutting of one of the lods L, L, L 3 then s increses from its nominl vlue. Lods ~ s E s Z s Bus oltge

7 short-circuit L ~ Z s L L 3 increses considerble. Hence s flls down significntly.. Lightening cn strike the Distribution System inducing high impulse voltges on the Line: Cloud Cloud ~ Such High voltge cn cuse insultion filure of the line (distribution feeders) 3. Distorted Currents from Hrmonic Producing Lods distort the voltge s, s they pss through Z s. Pge 7 of 58/L N Tripthy

8 Suppose s is sinusoidl voltge initilly s s sin t, f rms E s E f the Current wveform is sinusoidl then s will be sinusoidl s sm sin t However if is nonsinusoidl sin t 3 sin 3t (t) Nonsinusoidl Current t then s (t) = E m sint Z s.[ sint + 3 sin3t] Hence s will be non-sinusoidl. Hence Power Qulity Problems essentilly belongs to ddress oltge Concerns nd their vritions, tht cn be controlled. Pge 8 of 58/L N Tripthy

9 TERMS AND DEFNATONS OF POWER QUALTY PROBLEMS The term Power Qulity is pplied to wide vriety of Electromgnetic phenomen on the phenomen on the Power System. Generl Clsses of Power Qulity Problems mpulsive. Trnsients 5ns 5ms Oscilltory durtion. Short durtion ritions Sg (Dips).5 cycle to min. durtion Swell nterruptions 3. Long durtion vritions Sustined > min. Under oltge Over oltge 4. oltge Unblnces Stedy stte phenomen 5. Wveform Distortion] (Stedy Stte) Hrmonics DC Offset Notching nter hrmonics 6. Frequency ritions Hz (6 Hz or 5 Hz) Pge 9 of 58/L N Tripthy

10 . TRANSENTS Brodly cn be clssified into two ctegories, which re monetry in nture. These two ctegories re : mpulsive & Oscilltory A. mpulsive Trnsients t is chrcterized by i) Sudden rise or fll of or or both ii) Non-power frequency chnge iii) Unidirectionl polrity ( Positive or negtive) iv) Rise or decy times t occurs mostly due to lightening lightening ~ Gen. trnsmission line lod ~ M lod volts.s 5s Time Pge of 58/L N Tripthy

11 This is. 5 s impulsive trnsients (previous figure) This rises to pek vlue of.k in. s nd decys to hlf the pek vlue in 5s time. KA Time mpulsive trnsients ffect trnsformers nd other power equipments, brek down their insultions, nd lso excite Oscilltory trnsients (nturl freq.) s Lightening Stroke Current (impulsive trnsient) B. Oscilltory Trnsients Any Oscilltory trnsient is sudden, non-power frequency chnge (cn be either positive or negtive polrity vlues) in stedy stte oltge or Current). Oscilltory Trnsient High Medium Low Freq. > 5 khz Freq. 5 5 khz < 5kHz durtion 5s s.3 5 ms mgnitude 4 p.u. 8 p.u. 4 p.u. High frequency trnsients occur mostly due to lightening or switching surges. Pge of 58/L N Tripthy

12 Bck to bck cpcitors energiztion cuses Medium Frequency trnsients (Cble switching lso cuses this trnsient) Low frequency trnsients occur due to i) Cpcitor switching in sub-trnsmission or Distribution K/4K 66K ~ Sub-trnsmission C K Dist. (i) Low frequency oscilltory trnsients occur due to cpcitor Bnk Switching frequency 3 to 9 Hz Pek mgnitude. p.u. (.3.5 p.u.) durtion.5 to 3 cycles (ii) Oscilltory trnsients with frequency < 3 Hz occur due to trnsformer energiztion nd Ferro resonnce ~ trnsformer C Rect. Lod Lod Pge of 58/L N Tripthy

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14 Series cpcitors in Trnsmission system lso produce oscilltory trnsients ~ Gen. series cpcitor trnsmission Dist. Ferro resonnce occurs when the trnsformer inrush current contining second nd 3 rd Hrmonic resonte with series cpcitor.. Long Durtion oltge ritions: R.M.S. oltge or Current t Power frequencies devites from their norml vlues for more thn min (> min.) These vritions re clssified into (i) Over voltge (ii) Under oltge Pge 4 of 58/L N Tripthy

15 (i) Over oltge R.M.S. A.C. oltge > % for time > min. durtion (ii) Under oltge R.M.S. A.C. oltge < 9% for time > min. durtion Over oltges result due to Lod switching opertion (switching off lrge lod or energizing Cpcitor bnk) ~ s R C M L R R s increses if M or ny lod is switched off ncorrect tp settings of the Trnsformer cn lso result in Over oltges. Under oltge decrese in R.M.S. vlue of the AC voltge to less thn 9% t the power frequency. Durtion > min. ~ Z s s L E s C Pge 5 of 58/L N Tripthy

16 Resons L is switched on or C is switched off Over loded circuit cn result in under voltge s = E s Z s s if s increses s flls down Brownout Sustined under voltge Sustined interruption f the supply voltge is zero for time > min., it is considered s sustined interruption. t is termed s outge (due to fult) 3. SHORT DURATON OLTAGE ARATONS They re cused by (i) (ii) (iii) Fult conditions on Distribution Circuit Energiztion of Lrge Lods (Requiring lrge strting currents) ntermittnt loose connections in the power wiring. They re clssified into (i) (ii) (iii) Sgs oltge dips Swells oltge swell nterruptions complete Loss of voltge Pge 6 of 58/L N Tripthy

17 . nterruption An interruption occurs when the supply voltge or Lod current decreses to less thn % (. per unit) for period of time not exceeding min. F ~ s L L +3. nterruption time 3 5 RMS oltge.5 Time (secs). 4. Momentry interruption Pge 7 of 58/L N Tripthy

18 (iii) oltge Sg (Dip) Decrese to % nd 9% (. nd.9 p.u.) in RMS voltge or current mgnitude Durtions:.5 cycle ( cycle of 5 Hz voltge wveform is ms to min). Associted (cuses) with () System Fults, mostly L-G fult (b) Strting of Lrge Motors (c) Energiztion of Hevy Lods For System fults typicl fult clering time vries from 3 to 3 cycles. Pge 8 of 58/L N Tripthy

19 oltge sg cused by single line-to-ground (SLG) fult Pge 9 of 58/L N Tripthy

20 Phse (RMS) Time (ms) oltge Sg Fig. 6 oltge Sg Fig. 6 shows typicl voltge sg ssocited with SLG fult (Single line to ground) F M Motor oltge Source Rectifier Lod Fig. 7 Single line to ground fult Fig.7 shows Lrge Motor Strting, M (induction motor) drws 6 to times its full lod current during strting. The voltge sgs immeditely to 8% of its initil vlue nd grdully returns to norml in bout 3 secs. (iv) Swells (oltge swells) increse in voltge or current between % nd 8% (. to.8 pu) for durtions between.5 cycle to min. Pge of 58/L N Tripthy

21 Wveform representtion Swell or Time (ms) Fig. 8 As with sg, swells re ssocited with system fult conditions but they re not s common s Sgs. During SLG fult cse the fulted phse will hve low voltge <. pu but the unfulted phse voltge might rise by % to 8%. Switching off Lrge lods cn lso cuse swell in voltge Fig.8 shows voltge swell due to SLG fult. A 5% voltge swell is quite common in US utility feeders. oltge Unblnces f phse voltges, b, c re not sme in mgnitude or they do not hve phse ngle difference, they constitute n unblnced set. They occur due to Pge of 58/L N Tripthy

22 (i) Fuse burning in one of the phses. n 3-phse system b c Blnced Set b c 4 Unblnced Set Pge of 58/L N Tripthy

23 Symmetricl Component nlysis is used to express the unblnced system in terms of their symmetricl components (positive, negtive or zero sequence). Single phse Lod in 3-phse circuit cn cuse voltge imblnce in less thn percent (% to 5% tolerted). t cn lso occur due to blown fuse in one phse of 3-phse cpcitor bnk. oltge Unblnce U nb = Unblnce 3 oltge Unb.% M T W T F S S M Symmetricl component nlysis is used to define voltge or current unblnce. oltge Unb.% M / T W Th F S S M Fig..9 oltge imblnce is defined s mximum devition from the verge of 3-phse voltge / 3-phse voltge verge. Similrly current unblnce is defined. Also mb. = Rtio of Zero-Seq. or ve Seq. Component / +ve Sequence component. Pge 3 of 58/L N Tripthy

24 SYMMETRCAL COMPONENTS OF AN UNBALANCED OLTAGE SET c b c c b c + + b Positive Sequence (bc) b Negtive Sequence (cb) Zero Sequence b c.5 j j,, hve different mgnitudes 3 b, c b, c Pge 4 of 58/L N Tripthy

25 c b 4.5 j j Conversely the sequence components re ( 3 ( 3 ( 3 b b b c ) c c ) ) Obtin Symmetricl components of set of unblnced currents.65, b.8, c.9 3 c c b = b Positive Seq. + Negtive Sequence + Zero Seq. Pge 5 of 58/L N Tripthy

26 [ ] [ [ Current unblnce = or ] ] Problem : The Symmetricl components of set of unblnced 3-phse voltges re.69,.3,.8 3 Obtin the originl phsors. b c c b POWER N UNBALANCED SYSTEM P 3 phse jq 3 phse 3 ( ) 3 3 b ( b ) 3 3 c ( ) Pge 6 of 58/L N Tripthy

27 n single phse systems P Rel prt [. Q mg. prt [. ] ] Wveform distortion t is defined s stedy-stte devition from the idel sine wve of power frequency nd five types of distortions occur in power network: () DC offset (b) Hrmonics (c) nter hrmonics (d) Notching (e) Noise (f) oltge Flicker () DC offset The presence of dc voltge or current in n AC power system is known s dc offset. This occurs due to geomgnetic disturbnce, fults nd hlf-wve rectifier lods on the supply system. Direct currents in AC networks cn cuse (i) sturtion in trnsformers (ii) Heting nd Loss of trnsformer life (iii) electrolytic erosion of grounding electrodes The dc offset in AC voltge is represented s ( t) e or t dc ( t) e t i m sin t m sin( t ) includes DC offset oltge wveform including DC offset Pge 7 of 58/L N Tripthy

28 (b) Hrmonics They re sinusoidl voltges or currents hving frequencies tht re integer multiples of the frequency t which the supply systems is designed to operte. Fundmentl supply voltge frequency = 5 Hz or 6 Hz (t) = m sin (t + ) f (t) = m3 sin(3t+ 3 ) + m5 sin (5t + 5 ) + mi sin (t + ) t contins fundmentl component of pek vlue m, freq. f, phse ngle 3 rd hrmonic of pek vlue m3, freq. 3f nd 3 5 th hrmonic component of pek vlue m5, phse 5 nd freq. 5f. (t) Fundmentl (t) 5 th Hrmonic 3 rd Hrmonic Pge 8 of 58/L N Tripthy

29 oltge nd Current Hrmonics Lods: Hrmonic distortions originte in the nonliner chrcteristics of devices nd (i) (ii) (iii) Rectifiers nd nverters in the AC systems Energiztion of Trnsformers Cpcitor Energiztion ~ Supply ~ Lod (Distribution bus) ~ Tr C Lod Lod Supply Bus L L C Pge 9 of 58/L N Tripthy

30 (iv) Nonliner Lods Air Conditioners, Chillers, pumps, motors, etc. (iv) Power Electroniclly controlled Lods (Adjustble Speed drives) Hrmonic distortions re described by the complete hrmonic spectrum with mgnitude nd phse ngles of ech individul hrmonic component. distortion. T.H.D. or simply (THD) is mesure of the effective vlue of totl hrmonic Fig. wveform nd hrmonic spectrum for typicl Adjustble speed drive. s Supply bus PE Converter ~ M There re two distortion fctors: THD, THD THD where = Fundmentl RMS voltge 3 = 3 rd Hrmonic RMS voltge 5 = 5 th Hrmonic RMS voltge Similrly THD THD is limited to 5% to % (.5 to. pu) nd THD is s significnt s THD Pge 3 of 58/L N Tripthy

31 THD could be s high s 6% (c) nterhrmonics oltges nd Currents hving frequency components tht re not integer multiples of 5 Hz or 6 Hz re clled interhrmonics. nterhrmonics wveform distortion re due to: (i) use of sttic frequency converters (ii) Cycloconverters (iii) induction motors (iv) rcing devices (v) Power-line crrier signls Exmple: 5 Hz f 5 Hz 3f 45Hz, 35 Hz, 55 Hz, 75 Hz, 55 Hz inter-hrmonics (d) Notching it is periodic voltge disturbnce cused by the norml opertion of power electronic devices when current is commutted from one phse to nother. The frequency components ssocited with notching cn be quite high. Fig. shows the voltge Notching from 3-phse converter tht produces continuous dc current. Notches occur when the current commuttes from one phse to the other. Notches (t) time Fig. Pge 3 of 58/L N Tripthy

32 (e) Noise t is defined by considering the spectrl content of signl (lower thn khz superimposed over 5 Hz signl) which is found in power system voltge or current. (i) (ii) (iii) (iv) (v) These re cused by power electronic circuits switched mode power supplies solid stte rectifiers or inverters bsed Lods rcing devices improper grounding in distribution circuits Noise is n unwnted distortion. Other hrmonics or trnsients (f) oltge Flicker Lods, which cn exhibit continuous, rpid vritions in the lod current mgnitude, cn cuse voltge vritions tht re known s oltge flicker. (i) Effect of voltge fluctutions on the incndescent lmp re perceived by the humn eye s flicker. Mgnitude of flicker to % Frequency.5 Hz to Hz or 3 Hz.5% voltge flicker hving frequency 6 to 8 Hz produces perceptible lmp flicker. Power Frequency ritions The norml frequency chnge in power system occurs when (i) Lod chnges in system (prticulrly in the genertion system) (ii) Fults on Trnsmission systems (iii) Lrge source of genertion going off-line. f norml frequency is 5 Hz or 6 Hz, vrition up to 3Hz is tolerted. (iv) Lods supplied from n isolted generting system cn cuse frequency fluctutions. Pge 3 of 58/L N Tripthy

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35 POWER QUALTY FUNDAMENTALS AND MODELNG oltge sg nd nterruptions: oltge sgs nd interruptions (typiclly.5 to 3 cycles) re relted Power qulity phenomen oltge Sg: (i) short-durtion (ii) Typiclly.5 to 3 cycles durtion (iii) Amplitude of RMS voltge t point of the power system reduces to vlue within to 9% of the norml vlue. (iv) Occurs due to fults on the power system nd strting of lrge lods such s electric motors. Momentry interruption: (i) Durtion to 5 seconds (ii) Cuses complete loss of voltge (ii) Clernce of fults on power system cuses voltge interruption Dist. ~ CB Trns. CB CB 3 A CB 4 From the bove figure if fult occurs t A, CB is opened nd voltge flls down to zero t point of the Power system network. (iv) Sustined interruptions > min. re due to permnent fults. Pge 35 of 58/L N Tripthy

36 oltge sgs cuse severe problems for customers hving i) computer system ii) power electroniclly controlled lods iii) computer controlled mnufcturing systems iv) Adjustble Speed Drives Sources of Sgs nd interruptions: (A) FAULT NDUCED OLTAGE SAG oltge sgs nd interruptions re generlly cused by fults (short-circuits) on the utility system oltge in one-phse of power system An exmple of voltge sg is shown in fig... The voltge mplitude drops to vlue bout % of its pre-even vlue for bout two nd hlf cycles, fter which the voltge recovers gin. Sg = % nd durtion =.5 cycles. The mgnitude of the voltge sg is determined from the RMS voltge. Using one cycle window Fig.. A voltge sg rms ( k) N i k ik N ( i) N = number of smples per cycle nd (i) is the smpled voltge in the time domin. Pge 36 of 58/L N Tripthy

37 . pu Smpled vlues cycle window t t p.u voltge = 3 volt oltge (pu) Time in Cycles 6 One cycle RMS voltge for the voltge sg shown in fig.. To understnd the origin of voltge sg consider the Distribution network. Cse() Fult t position will cuse deep sgs t customers A, B, C, D. Sg t A could be somehow mitigted by genertors boosting up the voltge t the bus. Cse() A fult t position will not cuse much voltge drop t A. However it will cuse deep sg t both subtrnsmission substtions nd hence customers B, C, D. Cse (3) A fult t 3 cuses short or long durtion interruption for customer D. Customer C will experience deep sg. Customer B shllow sg, Customer A none Cse (4) For fult t 4 Customer C deep sg, Customer D shllow sg Cse(5) For fult t 5 Customer C shllow sg Customer D deep sg Pge 37 of 58/L N Tripthy

38 n both these cses customers A nd B will not experience ny significnt drop in voltge. G Trnsmission ~ Subtrnsmission A 3 Distribution B C D 5 Fult Position:,,3,4,5 Lods A,B, C, D 4 Fig..3 Distribution Network Pge 38 of 58/L N Tripthy

39 oltge Sg Mgnitude (clcultion) Consider simple circuit shown ~ Z s sg Z F E pcc Fig..4 oltge divider model Fult Lod For rdil type distribution system, Z s is the source impednce t the point of common coupling (pcc) Z F = impednce between pcc t the fult Here the lod current before the fult nd fter is neglected in comprison to short-circuit current. The voltge t pcc is found s Z F Z Z sg. s F E From this eqution we find, For Z F smll, sg becomes deep (fults electriclly closer to the customer). Z d Z Z sg. s d Z = impednce of the feeder per unit length d = distnce between the fult nd pcc. E ~ E Z s sg pcc d F Lod Fig..5 Pge 39 of 58/L N Tripthy

40 For typicl overhed line short-circuit level = E Z s Z s = j.6 for the 75 MA source. E = K voltge supply = 75 MA, MA, 5 MA (vried) For different short-circuit level, Z s is clculted For Fult level of 75 MA Z s =.6 mpednce of overhed line.7 + j.35 /km Similrly Z s cn be clculted for 75 MA nd MA levels. For SCL = 75 MA, Z s =.6 x =.6 The feeder impednce is clculted knowing the distnce of the fult..9.7 MA 75 MA.5 75 MA.3. km (d criticl) Km Fig..6 Sg mgnitude s function of fult distnce (km) Pge 4 of 58/L N Tripthy

41 d crit. = Z s Z = voltge sg mgnitude Z= feeder impednce f =.5E Sgs lso propgte to the higher voltge level from the lower voltge ones t which they hve occurred. Dist. ~ Trns. L System Fig..7 Propgtion of voltge sgs to higher voltge levels point-of-common coupling t: Fult t 4 k 33k 3k 4k 4-9% 98% 99% % k % 93% 99% 33k % 95% 3k % Pge 4 of 58/L N Tripthy

42 Criticl Distnce The definition of criticl distnce is such tht ech fult within the criticl distnce will cuse the equipment to trip. The criticl distnce is shown below for different voltge levels using typicl fult levels nd feeder impednces. (Criticl voltge = 5%, =.5 pu) Nominl oltge Short-circuit level Feeder mpednce Criticl distnce 4 MA 3m / km 35m k MA 3m /km km 33k 9MA 34m /km 4km 3k 3MA 45 m /km 3km 4k MA 9 m /km 55km Problem Let us consider circuit s shown below: E s ~ Z s s Substtion L L Z f Lod Let Z s = nd s is sinusoidl source = 7.6 k RMS, Two Line segments represented by inductnces L nd L re L = 3.7 mh, L =.7 mh Lod = 5 HP motor t power fctor.8 lgging. The lod is modeled s resistor nd inductor. Pge 4 of 58/L N Tripthy

43 To simulte the presence of fult, fult impednce Z f is connected s shown. The equivlent circuit is L L s Z f Lod Z f.9 Find the voltge cross the lod fter the fult hs occurred nd lso Disturbnce Power nd Disturbnce Energy. Solution Without ny fult Z f Cos Totl inductnce between the Source nd the Lod = L ) rds / sec. 3 L L ( 3.7.7) H 6.4 ( L ) L. 4-3 H ( L L voltge t the lod k RMS The instntneous vlue is L(RMS) Sin t Power Flowing to the Lod before the Fult =P(t) f S is represented s Pge 43 of 58/L N Tripthy

44 S m sin t L m sin( t - ) P( t) P(t) m sint. m sin( t - ) = sint. sin t. cos cos t sin m m m RMS m cos m m RMS cos - cos t sin.sin t cos cos t cos sin sin t RMS RMS t sin t kw RMS RMS When fult occurs Fult Current is found in the following wy : Z Z s Z f G B Step : Find G nd B of the lod * P jq, (G jb) G P,B - Q G (mh ) B (7.53) (mh ) Thus the Circuit is Z Z F L s Z f Z L Pge 44 of 58/L N Tripthy

45 Z j.39, Z j. Z L G jb 6.7 j5 S Z F 7.6 k.9 Find L Z s Z f Z Z L Z Z Z f L oltge t the Fult F S Z S Z S Z Z Z f Z Z f Z Z Z Z Z f L L Fult Current (Stedy Stte) F Z F oltge cross the Lod = Z L F F Z Z Hence L cn be found out. Pge 45 of 58/L N Tripthy L Substituting vlues of Z f, ZL, Z, Z, we get L (RMS) fter the fult = 6.6 k RMS nd the instntneous power delivered to the Lod P( t) f cost 76.3 sin t Compre nd Note : Before fult L = 7.53 k RMS L (Lod voltge) nst. Power kw After fult =6.6 k RMS nst. Power = cost kw cost 76.3 kw

46 6.6 Sg = %.8 p.u Note how the criticl distnce increses for higher voltge level. oltge Sg durtion oltge sg ppers during the fult nd once the fult is clered voltge does not come bck to norml immeditely. Thus the durtion of the sg is more thn the fult clering time. Power qulity monitors record sg, if the voltge is below 9% (threshold). They compute the RMS vlue once every cycle. A mgnitude durtion plot is common tool to show the qulity of supply in prticulr loction. % Motor Strting 8% Trnsmission Network Mgnitude 5% Fuses %.s sec locl M network interruptions Durtion (B) Fig..8 Motor strting Sgs Motors hve the undesirble effect of drwing severl times their full lod current while strting. By flowing through system impednces, this lrge current will cuse voltge sg which my dim lights, cuse contctors to drop out, nd disrupt sensitive equipment. Time to ccelerte to its rted speed increses with the mgnitude of the sg. Pge 46 of 58/L N Tripthy

47 Phse A - B Time (s) Fig..9 Typicl Motor strting voltge Sg Motor KA in % of Trnsformer Trnsformer KA (Sg = 9%) Fig.. 8 Fig. shows the strting of induction motor under full voltge strting method, where voltge sgs nd then mkes grdul recovery. However, if the motor is strted by Auto trnsformer strter, or Str Delt Strter when initilly less voltge is pplied, the sgs cn be controlled to some extent. Fig.. shows typicl motor versus trnsformer size for voltge sg of 9%. Equipment oltge Tolernce Generlly the electricl equipment prefers constnt rms voltge. For ech piece of equipment there is mximum interruption durtion fter which it will continue to operte correctly. Fig.. shows the voltge tolernce curve for n T equipment. 8 Sg Mgnitude % Durtion in cycles Fig.. oltge-tolernce requirement for T equipment Pge 47 of 58/L N Tripthy

48 From the figure it is found tht n T equipment hs to withstnd voltge sg down to zero for. cycle, down to 7% for 3 cycles nd for ny voltge >9%, the equipment opertes normlly. Adjustble Speed Drives Sg Mgnitude % Adjustble Speed Drives Time(ms) Fig.. Averge voltge tolernce curve for ASD t is found from Fig.. tht the djustble speed drive cn operte indefinitely on 85% of the norml voltge. For other voltges the durtion cn be found out. Performnce of PCs 3 AC Regulted dc voltge oltge Controller Fig..3 Typicl Power Supply to sensitive equipment oltge tolernce of personl computers vries over wide rnge : from 3-7 ms, 5 to 7% being the rnge contining hlf of the models. PLCs, motor contctors, etc. re more sensitive to voltge Sgs thn PCs. Pge 48 of 58/L N Tripthy

49 oltge tolernce of PLCs in the process industries shows very high sensitivity nd they ll tripped between to 3 cycles. The minimum voltge vried between 5% to 8% for most devices. For Adjustble Speed drives, they operte indefinitely for 85% of norml voltge. oltge tolernce is defined s utomtic speed recovery without reching zero speed. 8 oltge tolernce of PCs Effect of oltge Sgs on Rectifiers : Single-Phse Rectifiers The power supply to computer, process control equipment, consumer electronics, etc. consists of single-phse (four-pulse) rectifier together with cpcitor nd DC DC converter (Fig..3). 3 P U oltge (Mgnitude) Ripples Time in Cycles Fig..4 Dc-bus voltge Pge 49 of 58/L N Tripthy

50 3- Phse Rectifiers b c 4 olts AC-supply 3-phse Rectifier Equipment AC Bus oltge p u DC Bus oltge.4 3 Cycles Fig..5 For sg of 5%, =. 5 pu, the DC voltge drops s shown. Mitigtion of oltge Sgs Severl things cn be done by the utility, end user, nd equipment mnufcturer to reduce the number nd severity of oltge Sgs nd reduce the sensitivity of the equipment to voltge sgs. The solutions :. Equipment Specifictions The end-user should specify the equipment rtings nd methods to ride through voltge sgs.. Customer Solution Pge 5 of 58/L N Tripthy

51 (i) Providing UPS (un-interruptble power supply) to the mchine instlled by the end-user (ii) Provide protection 3. Overll Protection inside Plnt 4. Utility Solution The utilities cn provide the following : Reducing the number of Short-Circuit fults Reducing the fult clering time Chnging the system such tht the short-circuit fults result in less severe events t the equipment terminls or t the customer interfce. Connecting mitigtion equipment between the sensitive equipment nd power supply mproving the immunity of the equipment. Reducing the number of Short-Circuit fults : Replce Overhed lines by under ground cbles Use specil wires for over hed lines mplement strict policy of tree trimming nstll dditionl shielding wires ncrese mintennce nd inspirtion frequencies. Reducing Fult Clering Time Reducing the fult clering time does not reduce the number of events, but only their severity. The ultimte reduction in fult clering time is chieved by using (i) Current limiting fuse (ii) Fult current limitors (iii) Sttic circuit brekers (iv) Grding mrgins Typicl fult clering time vries from Pge 5 of 58/L N Tripthy cycle to 3 cycles. All these devices operte only for L or Medium voltge systems up to k / (33 k).

52 3. Chnging the Power System By implementing chnges in the supply system, the severity of the events cn be reduced. Here gin the costs my be very high, especilly for trnsmission nd subtrnsmission voltge levels. nstll Motor-Genertor set ner the sensitive lod. The genertor will boost the voltge for remote sg. Split bus or substtions in the supply pth to limit the number of feeders in the exposed re. Feed the bus with the sensitive equipment from two or more substtions. A voltge sg in one substtion will be mitigted by infeed from other substtions. Substtion - Substtion - Fig nstlling Mitigtion Equipment Some exmples of mitigtion equipments re : UPS : This is the most commonly used device to protect low power equipment (computers, etc.) ginst voltge sgs nd interruptions. During sg or interruption, the power supply is tken over by n internl bttery which cn supply the lod typiclly for 5 to 3 minutes. Pge 5 of 58/L N Tripthy

53 Norml Power Line Supply Line Rectifier Chrger nverter Lod Bttery Bnk Fig..7 Stndby UPS Ferroresonnt Trnsformers (CTs) CTs re very ttrctive for constnt low power lods. rible lods with high inrush currents present more of problem for CTs becuse of the tuned circuit on the output. Ferroresonnt trnsformers re bsiclly : trnsformer which re excited high on their sturtion curves, thereby providing n output voltge which is not significntly ffected by input voltge vritions. Motor Genertor Sets : Motor genertor sets re the clssicl solution for sg nd interruption mitigtion with lrge equipment. But they re not suitble s they produce noise nd mintennce problems. Modern Methods : - Dynmic oltge Restorers c Distribution Feeder Substtion Supply Bus Pge 53 of 58/L N Tripthy DR Fig..8 Lods

54 Dynmic oltge Restorer uses modern power electronic components to insert series voltge between the supply nd lod. The voltge source c compenstes for the voltge drop due to Sg. constnt. The STATCOM supplies rective power to the Supply Bus nd holds its voltge ref + P STATCOM s Controller Firing ngle control Fig.. STATCOM injects Rective Current to the Supply Bus - Universl Power Line Conditioners Distribution DR Lods STATCOM Fig.. Sub-Sttion Rectifier S - Sttic Compenstor Fig..9 Dynmic voltge restorer Pge 54 of 58/L N Tripthy

55 S.S S SC Sttic Compenstor Fig.. Sttic Compenstor comprises S (voltge source inverter) nd DC source connected cross it or cpcitor (chrged to voltge dc ). TRANSENT EROLTAGES Sources Lightning Cpcitor Switching Lod Switching t the customer site Trnsient over-voltges re chrcterized by either high frequency (Lod switching nd Lightning), Medium frequency (cpcitor switching), or Low frequency.. Lightning : A potent source of mpulsive trnsients A chrged cloud dischrges directly into power conductor either in the trnsmission side or Distribution side. Cloud Dist. ~ Trnsmission Gen. Pge 55 of 58/L N Tripthy Direct Stroke Dist.

56 Cl Cl Trnsmission Line ndirect Stroke (Cloud Cloud Conductor) The lightning voltge cn be modeled by voltge source t t E K e e K vries from k upwrd. nd re prmeters. t f t ( s) ( s). 5 s s s s s 6 5 A typicl Lightning voltge is represented s k 5 k.s Pge 56 of 58/L N Tripthy Fig. 5s Lightening voltge

57 To model lightning voltge, the following circuit is used : + S C K R e e t t R C nd depends on the prmeters C, C, R, nd R. A direct stroke to phse generlly cuses line flshover ner the strike point. Not only this genertes n impulsive trnsient, but it lso cuses fult ccompnying voltge sgs & interruptions. To protect ginst lightning surge rrestors re instlled ner the substtion where costly equipments re instlled. SA Surge rrestors re bsiclly vrible resistors, provide low resistnce pth to lightning currents nd high resistnce pth to power frequency currents. The chief power qulity problem ssocited with the lightning stroke current entering the ground system re :. They rise the potentil of the locl ground to few ks bove other grounds in the vicinity. Thus computer system connected to the other ground my fils. Pge 57 of 58/L N Tripthy

58 . Lightning my strike nerby ground nd cn still induce n impulse voltge on the phse conductor in the vicinity. 3. nduce high voltges in phse conductors s they pss through cbles on the wy to better ground. Principles of Lightning Over-voltge Protection. Limit the voltge cross sensitive insultion.. Divert the surge current wy from the lod. 3. Block the surge current entering the lod. 4. Reduce or prevent, surge current from flowing between grounds. 5. Crete low-pss filter using limiting nd blocking principles. 6. soltion Trnsformers C Line Lod C Fig. soltion Trnsformer They re used to ttenute high frequency trnsients nd noise s they ttempt to pss from one side to other. Also lod trnsients do not pss to the other sections of the power network. Pge 58 of 58/L N Tripthy

59 The Low-pss filter is shown below : Line Protected Line Gp-type protector LPF MO LPF = Low pss filter MO = Metl Oxide rristor. Cpcitor Switching Trnsients Cpcitors re used to provide rective (vrs) power (i) to correct power fctor connected cross the Lods to reduces losses (switched) (ii) to support the voltge on the system when required. One drwbck of Cpcitor switching is tht they interct with the inductnce of the power system to produce oscilltory trnsients when switched. (i) (ii) Some cpcitors re energized ll the times (fixed). Some re switched ccording to Lod levels. Trnsient t lod loction Cpcitor switched Pge 59 of 58/L N Fig. Tripthy Cpcitor-switching opertion

60 5 % olts Time (ms) Fig. oltge wveform t observed loction The cpcitor switch is closed ner the system voltge pek when the voltge cross the cpcitor is zero t the instnce. The cpcitor voltge will overshoot nd trnsient over-voltge is generted between. nd. per unit depending on system dmping. For this cse Cp. Over-voltge =.34 pu (mximum vlue =. pu) The phse current flowing in the feeder peks t nerly 4 times the lod current. 4 Amps - Pge 6 of 58/L N Tripthy -4 4 Time (ms) 6

61 Mgnifiction of cpcitor switching trnsients A potentil side effect of dding power fctor correction cpcitors t the customer loction is tht they my increse the impct of utility cpcitor switching trnsients on end-use equipment. Source L Distribution Feeder L C C Lod L L Equivlent circuit of clculting Pge 6 of 58/L N Tripthy resonnt frequency

62 Trnsient over-voltge (when C is switched on the utility side) on the Lod bus t C my rech s high s 3. to 4. pu, which might dmge customer equipments. Switching frequency f L C Nturl frequency of the customer resonnt circuit = f L C cn be used. To reduce this effect n inductnce in series with the power fctor correction bnk Cpcitor switched here Trnsient t lod loction One line digrm of cpcitor-switching opertion Pge 6 of 58/L N Tripthy

63 Typicl electric utility cpcitor switching trnsient reching 34 percent voltge, observed upline from the cpcitor. Feeder current ssocited with cpcitor switching event Source L Customer Trnsformer Distribution Feeder L Substtion Cpcitor oltge mgnitude t customer cpcitor Pge 63 of 58/L due N Tripthy to energizing cpcitor on utility system C C Cpcitor Lod

64 Switching frequency f L C Ntionl frequency of customer resonnt circuit f L C oltge mgnifiction of cpcitor bnk switching Arrestor energy duty cused by mgnified trnsient. Source kar Pge 64 of 58/L N Tripthy MA 5 MW Distribution Feeder Substtion Cpcitor Bnk Customer Trnsformer Cps PWM ASD 5 ka 48 Lod 3 kw dc cpcitor

65 Pge 65 of 58/L N Tripthy

66 Cpcitor switching Bus Lod Reduction of trnsient over-voltge by inserting n inductnce (3%). An djustble speed drive is very much susceptible to such over-voltge nd momentrily drws very high current, chrges the DC link voltge to higher level thn norml nd ASD shuts down. Lod Switching Trnsient Problems. Nuisnce tripping of ASDs Most ASDs typiclly use oltge Source nverter (S design) with cpcitor in the dc link. The controls re sensitive to dc over-voltges nd my trip the drive t Pge 66 of 58/L N Tripthy

67 level s low s 7%. Since utility cpcitor switching produces >3% over-voltge t the Lod buses, the nuisnce tripping of ASDs is high. Eliminte this by using series inductnce with the ASD (reduces trnsient overvoltge cross DC link).. Trnsient from Lod Switching De-energizing inductive circuits with ir-gp switches, such s relys nd contctors cn generte burst high frequency impulses. Such fst trnsients produces spikes up to k Air conditioners elevtors up to 3 k Arc Welders Motor Strters High frequency filters nd isoltion trnsformers cn be used in such cses. 3. Trnsformer Energiztion Energizing trnsformer from supply bus produce inrush currents tht re rich in hrmonic components. They lst up to s. f the system hs prllel resonnce ner one of the injected current frequencies dynmic over-voltge condition results cuse filure of rrestors nd equipments. p.u. 5 Time Mgnetizing inrush current s Pge 67 of 58/L N Tripthy

68 HARMONC ANALYSS Hrmonic distortion is cused by nonliner devices nd power electroniclly controlled lods in power system. A nonliner device is one in which the current is not proportionl to the pplied voltge. i(t) (t) ~ R(t) i(t) t ncresing the voltge by few % the current my double. Power electronic circuits nd nonliner lods produce voltge nd currents which re periodic but not Sinusoidl tht is when the wveform is identicl from one cycle to the next. This wveform cn be expressed s sum of sinusoids using Fourier series, in which the frequency of ech sinusoid is n integer multiple of the fundmentl frequency of the distorted wve. This multiple is clled Hrmonic of the fundmentl. Pge 68 of 58/L N Tripthy

69 6 Hz n = 3 Hz 8 Hz n =3 n =5 4 Hz 54 Hz 66 Hz 78 Hz n =9 n = n =3 When both positive nd negtive hlves of the wveform re identicl, only odd hrmonics ( n 3, 5, 7...) re present in the wveform. Higher order hrmonics beyond 3rd to 49th re negligible in power systems. A non-sinusoidl periodic wveform is mthemticlly represented s t n n ( t) [ cosn t bn sin n ] where T T T ( t) dt T n ( t) cosn t dt T T T bn ( t) sin n t dt T T Since sines nd cosines cn be combined into one sinusoid s they hve the sme frequency we get Pge 69 of 58/L N Tripthy

70 ( t) cos( n n n t n ) where n n b n nd n tn b n n n n t n or ( t) sin( n ) nd for n= n b, tn n n n b = mplitude of the fundmentl = phse ngle of the fundmentl component The rms. vlue of (t) is computed s n n rmsn n n 3 5 (...) For device or power circuit if periodic voltge nd currents re represented s t) cos( n t ) ( n n nd i t) cos( n t ) ( n n The verge power is P vg T ( t) i( t) dt or simply P n P n P n n, rms n, rms n n n,mx, n cos( ),mx n n n cos( ) Note tht the totl verge power is the sum of the powers t the frequencies in the Fourier series. Pge 7 of 58/L N Tripthy

71 f,, 3, 5, 7 hrmonics re present in wveform, the power is,,..., --- rms. vlues 3 3 P cos( ) cos( ) cos( ) cos( ) Problem : Nonliner source nd liner lod (t) Lod ( t) cos(377t 5 ) 3 cos(4.6t ) This voltge is connected to lod which is 5 resistor nd 5 mh inductor connected in series. Determine the power bsorbed by the lod. DC current is AC currents R 5 Amp R j L 5 j 6(.5) 5 R j L j 6.5 Lod current cn then be expressed s i ( t).65cos( 6t 73.5 ).43cos(4 6t 46. ) A Power t ech frequency in the Fourier series is determined by dc term : P.A W rds / sec Pge 7 of 58/L N Tripthy A A

72 .65 P cos( ) 7.4W rds / sec 3.43 P cos( 46 ) 4.8W Totl Power = = 5. W Totl Power bsorbed by the Lod = P rms R 5.W Sinusoidl source nd Nonliner Lod n this cse the current drwn by the lod will be non-sinusoidl. ( t) sin( t ) Current is represented by the Fourier series s n i( t) sin ( n nt n The verge power bsorbed by the lod P. n,mx n ) cos( ),mx n n,mx cos( ) n n n n cos( ) cos( ),, cos( ) rms rms Note tht only non-zero power term is t the frequency of the pplied voltge. The power fctor s pf P S rms P rms, rms, rms, rms, rms, rms. cos( ) cos( ) rms nd rms n n, rms n n Note tht for sinusoidl voltge nd sinusoidl current pf cos( ) Pge 7 of 58/L N Tripthy

73 nd for non-sinusoidl cse,rms rms is defined s DF, the distortion fctor DF, rms rms The DF represents the reduction in power fctor due to non-sinusoidl property of the current. The Pf is expressed s pf DF. cos( ) Totl Hrmonic Distortion THD is nother term used to quntify the non-sinusoidl property of wveform. THD is the rtio of the rms vlue of ll the non-fundmentl frequency terms to the rms vlue of the fundmentl frequency term : THD n n, rms n, rms THD is equivlently expressed s n, rms, rms THD rms, rms, rms THD is often pplied in situtions where the dc term is zero, in which cse THD is THD n n Another wy to express the distortion fctor s DF ( THD) Rective Power Rective power for sinusoidl voltge nd non-sinusoidl current cn be expressed s Q sin( ) With P nd Q defined for the non-sinusoidl cse, pprent power S must include term to ccount for the current t frequencies which re different from the voltge frequency. The term distortion volt-mps D is used for computtion of S Pge 73 of 58/L N Tripthy

74 S P Q D where D Q k, rms n n, rms order of hrmonic k k sin k n n, Other terms used for non-sinusoidl current or voltge re form fctor or crest fctor Form fctor Crest fctor Problem rms vg pek rms A sinusoidl voltge source of ( t) cos 377 t volts is pplied to nonliner lod, resulting in non-sinusoidl current of the form i ( t) 8 5 cos (377t 3 ) 6 cos (.377t 45 Determine (i) the power bsorbed by the lod (ii) pf of the lod (iii) the distortion fctor (iv) THD of lod current Solution (kw) ) cos (3.377t (i) P 8. cos 3 cos 45 cos wtt (ii) rms 7. 7 ) Amps rms A The power fctor is Pge 74 of 58/L N Tripthy

75 pf Alterntively (iii) (iv) pf P S, rms P rms rms cos( ) rms Distortion fctor DF, rms rms (7.7)(4.) THD of the lod current cos ( 3 4. ).66 THD rms, rms, rms % Similr definitions hold good for oltge distortion. THD v rms, rms, rms POWER N 3 - PHASE CRCUTS Apprent power in unblnced 3-phse circuits is currently clculted using severl definitions tht led to different power fctor levels. The following definitions re used : (i) S P Q D p k k k k, k =, b, c where P for -phse k P S p P b for b-phse, nd P c for c-phse k, b, c P k Q k D k P= ctive power, Q = rective power, D = distortion power Pge 75 of 58/L N Tripthy

76 Pge 76 of 58/L N Tripthy c c c b b b p D Q P D Q P D Q P S (ii) ) ( ) ( ) ( c b c b c b p D D D Q Q Q P P P S (iii) c c b b cd c bd b d p Q P Q P Q P S ( where cd bd d Q, Q, Q c c b b D Q D Q D Q (iv). c b c b p S Power Fctor = S p P Power fctor of lod or system is ccepted s mesure of the power trnsfer efficiency nd is the rtio of the rel power / pprent power. Problems Cse (i) phse /. R, 5 c b R R R Non-sinusoidl voltge, blnced lod t t sin sin.5 98 Z b Z c Z R R R c Line b Lod n Three phse System

77 rms ( rms vlue) i 59.7 sin t 5.97 sin 5 t A P =3.(98.5).(59.7) + 3.(9.85).(5.97) = 54. kw Power loss in the line = 3.(6).(.).6 kw Cse (ii) Unblnced lod, 4 wires R R b 5, Rc 5 i 98.5 sin t 9.85 sin 5 t 59.7 sin t 5.97 sin 5 t (rms vlue) 6A i b 59.7 sin( t ) 5.97 sin(5 t ) b 6A (rms vlue) i c.94 sin( t ).9 sin(5 t ) c (rms) A P P loss f the lod is unblnced rective power definitions do not exist. b b c c = 396 (6.3636) (6.3636) (.3636). =73.3 Wtts =.73 kw Here K=.3636 = (54 / 396) This is done for comprison. P Power fctor = S p Pge 77 of 58/L N Tripthy

78 Cse (iii) Unblnced lod, 3 wires R R b 5, R 5, R. c U 98.5 sin t 9.85 sin 5 t i 53.4 sin( t.5) 5.34 sin(5 t.5), 5. 6 A i b 5.34 sin( t 4.5) 5.34(5 t 4.5), 5.6 A b i c 6.34 sin( t ).68 sin(5 t ), c 6.36 A Line losses normlized to 54 kw = nd pf =.75 Cse (iv) : sinusoidl, nonliner lod, blnced R., U 3 sin t i 6 sin t 6 sin 5 t b c 6.3 A n this cse the lod non-linerity produces hrmonic component not seen in the voltge. pf =, P = 54 kw Cse (v) : nonliner bd, unblnced system R. / phse U 98.5 sin t 9.85 sin 5 t i 59.7 sin t, 59. 7A ib 59.7 sin( t ) 5.97 sin(5 t ), b 6A i c 59.7 c 6.88A sin( t ).94, P loss 74 sin(5 t ), Pge 78 of 58/L N Tripthy

79 oltge s. Current Distortion rious equipments like djustble speed drives nd induction furnces do not function properly becuse of hrmonics. The resons re :. Hrmonics voltges re too gret for the control to properly determine firing ngles.. Trnsformers nd mchines operte t low power levels becuse of the higher mgnitude of current hrmonics, the overll rms vlue becoming high. 3. Hrmonic voltges re too high. Hrmonic producing lods re treted s current sources nd they inject currents (hrmonics) to the distribution system. Consider n exmple below : ~ E S + oltge Drop Distorted oltge Distorted Current Although the source voltge is sinusoidl, the voltge drop in the source impednce is non-sinusoidl, the voltge t the lod bus is distorted. The rectifier injects current ( t) dc sin t. 3 sin 3 t 5 sin 5 t... Pge 79 of 58/L N Tripthy

80 The voltge distortion is due to the distorted voltge drop in the series impednce. The voltge distortion is of the order of 5%, where s the lod current distortion cn be s high s 6%. Hrmonics re not trnsients s hrmonics re stedy stte phenomen nd persist continuously depending on the nture of the lod. Doubt re integer multiples of fundmentl frequency. The wveform distortion tht produces hrmonics is present continully t lest for severl seconds. Trnsients re ssocited with chnges in the system such s switching, lightning nd energiztion nd they re dissipted fst within few cycles. Hrmonic Producing Lods or Devices. Single Phse Power Supply AC or 3 AC C Switch & Control DC SMPS Electronic power converter lods with their cpcity to produce hrmonic currents constitute the most importnt clss of nonliner lods in power systems. Equipments include : djustble speed motor drives, electronic power supplies, dc motor drives, bttery chrges, bllsts, nd mny other rectifier / inverter pplictions. Pge 8 of 58/L N Tripthy

81 Pge 8 of 58/L N Tripthy

82 6-pulse thyristor rectifier (t) A B C rmture field my be supplied from sme or seprte power supply Six-pulse dc ASD. SMPS (switch mode power supply) use dc/dc conversion technique to chieve smooth dc output with smll, light weight components. Personl computers, printers, copiers, nd most electronic equipments use SMPS where c dc high frequency c dc rectified to rectified to Becuse there is no lrge c-side inductnce, input current to the power supply comes in very short pulses s the cpcitor C regins its chrge on ech hlf cycle. SMPS finds pplictions in electronic bllsts for fluorescent lighting systems. The high frequency controlled output voltge tht is possible with trnsistorized inverters increses fluorescent tube efficiency nd permits more sophisticted light control. ncresed hrmonic genertion from fluorescent lighting cn be round 4 to 6 % of commercil building lod. Pge 8 of 58/L N Tripthy

83 3 Phse Power Converters Typicl use of such converters pertin to ASDs djustble speed drives. The current hrmonic spectrum shown in the figure exhibits the production of 5 th hrmonic current s high s 4% of the fundmentl. Dc Drives Compred to c drives, the dc drives offer wide rnge of speed control nd higher strting torque. Most DC drives use 6-pulse rectifiers nd 5th nd 7th hrmonics re pre-dominnt in the current wveform. AC Drives n AC drives, the rectifier output is inverted to produce vrible frequency c voltge for the motor. nverters re either voltge source (S) or current source types. A S requires constnt dc voltge input to the inverter stge. This is chieved with cpcitor or LC filter in the dc link. n the CS fed motor, constnt current input is required t the inverter input stge nd hence n inductor is plced in the dc link. PWM technique is used for best S & CSs. S drives offer best energy efficiency over wide speed rnge. Effects of Hrmonic Distortion (i) mpct of Hrmonics on ASDs The hrmonics current distortion is ASDs is not constnt nd it vries for different speed nd torque output. For exmple the wveform t 4% of rted speed is much more Pge 83 of 58/L N Tripthy

84 distorted proportiontely, the drive injects considerble mount of higher mgnitude hrmonic currents t rted speed. THD of current for ASD = 9 % This cn be reduced to 3 to 4% rnge using choke 3% bsed on ASD ka rting. (ii) Arcing Devices Pge 84 of 58/L N Tripthy

85 Pge 85 of 58/L N Tripthy

86 This ctegory includes rc furnces, rc welders nd dischrge type lighting (fluorescent, sodium vpour, mercury vpour) with mgnetic bllsts. (t) Equivlent Circuit for n rcing device Pge 86 of 58/L N Tripthy

87 The voltge current chrcteristics of n electric rc is nonliner. n fluorescent lmps bllsts or chokes re used to limit current within the cpbilities of the fluorescent tube nd stbilize the rc. Currents exceeding 6 A re seen in rc furnces. t is gret source of hrmonics. (iii) Sturble Devices Trnsformers with steel core nd induction or dc motors. generted due to nonliner mgnetizing chrcteristics of the steel. Hrmonics re Typiclly exciting current for trnsformers t norml operting voltge is less thn % of the full rted current. Triplen hrmonic currents (f =5 C/s ) re found in the erly morning hours when the lod is low nd the voltge rises. Hrmonic distortions (voltge) from trnsformer over excittion is generlly found under light lod conditions. Pge 87 of 58/L N Tripthy

88 Hrmonic Percent Phse(deg) Fund Motors when overexcited lso exhibit hrmonic distortions. Some frctionl HP motors show tringulr current wveforms with significnt 3rd hrmonic content. Str connected trnsformers show wveform distortions in the line current. Pge 88 of 58/L N Tripthy

89 (iii) mpct on Cpcitors The following ANS / EEE stndrd is followed for power cpcitors. 35 % of nmeplte KAR % rted rms voltge ( includes hrmonics only but not trnsients ) 8 % rted rms current % pek voltge including hrmonics For exmple, the full lod current for KAR cpcitor bnk is c KAR3 c 5. A 3k 33.8 The cpcitor is subjected to 5th nd 7th hrmonics where voltge distortion is 4% of fifth hrmonic nd 3% seventh hrmonic. This results in % 5th hrmonic nd % seventh hrmonic current. (iv) mpct on trnsformer There re three effects tht result in incresed trnsformer heting when the lod current includes hrmonic components. () rms current : Hrmonic current my result in the totl rms current being higher thn its norml vlue, which will increses losses. (b) Eddy current losses : These re induced currents in trnsformer cused by mgnetic fluxes. The eddy losses to the (frequency) nd produce heting in trnsformers. (c) Core losses : ncresed voltge distortion my increse the eddy currents in the core lmintions (depends on the thickness of lmintions nd the qulity of the core steel. The Lod Loss P R loss eddy current loss LL P EC P EC K EC h Pge 89 of 58/L N Tripthy

90 P LL = lod current, h = hrmonic frequency K EC = proportionlity constnt h h h P ECR where P LL is the per-unit full lod loss under hrmonic current conditions. PEC R = eddy current loss fctor under rted conditions. h = hrmonic current of h-hrmonic h = hrmonic number K h h h This fctor is defined s the K-fctor nd concerns with derting of trnsformers. The rms vlue of the distorted current is h PEC K P R EC R pu Thus the trnsformer derting cn be estimted by knowing the per-unit eddy current loss fctor. Trnsformer designer provides the K-fctor Typicl vlues of PEC R Type MA oltge % PEC R Dry k H k H Oil filled.5 48 L.5 to 5 48 L 5 >5 48 L 9 5 mpct on Motors Hrmonic voltge distortions t the motor terminl is trnslted into hrmonic fluxes within the motor. Hrmonic fluxes do not contribute to Motor torque, but produce Pge 9 of 58/L N Tripthy

91 high frequency currents in the rotor. Decresed efficiency, vibrtion, heting, high pitched noises re symptoms of hrmonic distortion. There is no need to derte the motor s THD limits re below 5%. Motors pper to be prllel with the power system impednce nd generlly shifts the system resonnce higher by cusing inductnce to decrese. mpct on system mpednce At the fundmentl frequency power systems re primrily inductive, nd equivlent impednce is clled short-circuit rectnce. From the Short-Circuit results, we get Z sc R sc j X sc k MA sc sc 3 k The Z sc = short-circuit impednce R sc = short-circuit resistnce X sc = short circuit rectnce k = phse-to-phse voltge MA sc = three-phse short-circuit MA sc = short-circuit current X sc rectnce chnges linerly with frequency X h h X h = hrmonic number X = fundmentl frequency rectnce X h = rectnce t the hth hrmonic Cpcitor impednce Shunt cpcitors either t the customer loction, or t the utility distribution system, drmticlly lter the system impednce vrition with frequency X K K. c fc MAR KAR For =3.8 k, MAR =. Pge 9 of 58/L N Tripthy

92 X c Prllel resonnce HS ~ Hrmonic X S Source X T X c Xs X c X T Prllel resonnce cn occur t certin hrmonic frequency. KAR KA cp trnsformer 5 4 5% 3% % Z h No Cpcitor Hrmonic Number Pge 9 of 58/L N Tripthy

93 Effect on cpcitor size on prllel resonnt frequency. The resonnt frequency is computed s f r LC h X r sc X h r h r X X c sc MA MAR sc cp h r = resonnt hrmonic X c = cpcitor rectnce X sc = system short-circuit rectnce For n industril lod, where the trnsformer impednce is dominnt, the resonnt hrmonic for 5 ka, 6% trnsformer impednce nd 5 kar cpcitor bnk is h r ka kar tr cp Z % tr % Resistive Lod Z X k tr Z tr % h Effect of resistive lod on prllel resonnce As little s % resistive lod in prllel with cpcitor cn hve drmtic effect in suppressing resonnce. Hence the resistive lods will dmp resonnce nd in turn cn reduce hrmonic distortion. Hrmonic Control Fundmentlly hrmonics becomes problem, if (i) the source of hrmonic currents is too gret. (ii) results in high voltge distortion or telephone interfce. (iii) the response of the system ccentutes one or more hrmonics. Pge 93 of 58/L N Tripthy

94 The principles of hrmonic control include : (i) Reduce the hrmonic currents produced by the lod (ii) Add filters to either siphon the hrmonic currents off the system, block the currents from entering the system or supply the hrmonic currents loclly. (iii) Alter the frequency response of the system by filters, inductors nd cpcitors. Reducing hrmonic currents in lods () An overexcited trnsformer cn be brought bck into norml opertion by reducing the pplied voltge. () Arcing devices nd electronic power converter re locked into their designed chrcteristics. (3) PWM drives tht chrge the dc bus cpcitor directly from the line cn use smll choke to significntly reduce hrmonics. (4) Trnsformer connections cn be used in 3-phse systems to reduce hrmonics. (5) -pulse power converters cn be used in plce of 6-pulse ones, thereby drsticlly reducing 5th & 6th hrmonics. (6) Delt connected trnsformers cn block the flow of zero-sequence hrmonic currents (typiclly 3rd hrmonic ones) from the line. (7) Zigzg nd grounding trnsformers cn divert the 3rd hrmonic currents from the line. Filtering (i) Shunt filters : which divert the hrmonic currents from the source of genertion. This keeps the hrmonic currents wy from the supply system. (ii) Series filters : block the hrmonic currents. This is prllel tuned circuit tht offers high impednce to the hrmonic current. One common ppliction is in the neutrl of grounded wye cpcitor bnk to block the flow of triplen hrmonics, while retining good ground t fundmentl frequency. There re generlly two clsses of filters : (i) Pssive filters Pge 94 of 58/L N Tripthy

95 (ii) Active filters (i) Pssive Filters Pssive filters re mde of inductnce, cpcitnce, nd resistnce elements. They re reltively inexpensive compred with other mens for eliminting hrmonic distortions. They re used either to shunt the hrmonic currents off the line or to block their flow between prts of the system by tuning the elements to crete resonnce t selected hrmonic frequency. Fig. below shows severl types of common filter rrngements. Single-tuned st order High-Pss nd order High-Pss 3 rd order High-Pss The most common type of filter is the single-tuned Notch filter. This is the most economicl type nd frequency used filter which effectively suppresses hrmonics. t is connected in shunt with the power system. The notch filter is series tuned to present low impednce to prticulr hrmonic current. X C X f X SC X C /3 X f Pge 95 of 58/L N Tripthy

96 5 4 5 th Hrmonic Notch Cpcitor Only th Hrmonic Notch Filter Notch filters cn provide power fctor correction in ddition to hrmonic suppression. For delt connected notch filter h notch X 3X c f f we use phse-to-phse voltge nd 3-phse KAR to compute by 3. X c, we would not divide Filters re commonly tuned slightly lower thn the frequency of the hrmonic to be filtered to provide mrgin of sfety in cse there is some chnge in the system prmeters. When tuned the filter cretes shrp resonnce point t frequency below the notch frequency. Alwys filters re dded to the system strting with the lowest problem hrmonic. (i) (ii) Suppose 7th hrmonic filter is to be instlled. A 5th hrmonic filter is lso required to be instlled. The notch filter (delt) does not llow zero-seq. 3rd hrmonic currents. Pge 96 of 58/L N Tripthy

97 (ii) Active Filters Active filters re reltively new types of devices. They re bsed on sophisticted power electronics nd re much more expensive compred to pssive filters. However, they present the following dvntges : (i) (ii) (iii) (iv) (v) No resonnce with system cpcitnce n cse prllel resonnt circuits, they offer distinct dvntge. They cn ddress more thn one hrmonic t time. They cn be used to reduce voltge flicker. Prticulrly suitble where lrge distorted lods re fed from reltively wek points on power system. PCC ~ Lod X S T r S S (Non- Liner) An ctive filter is voltge source inverter which injects current to the PCC bus to cncel the hrmonic portion of the current flowing into the lod. st 3rd 5th S produces [ + ] negtive of 3rd+5th hrmonic current nd cncels them from the totl current. Also the voltge t the PCC cn be smoothened out. Active filters typiclly correct the power fctor nd hrmonics. Pge 97 of 58/L N Tripthy

98 Hrmonic filter design 48 olt Bus b c Filter Rector A notch filter is to be designed to suppress 5th hrmonic component. Bus Notch Filter Ground Cpcitnce Bnk rting = 5 KAR KAR ctul n this cse ssume KAR ctul KAR KAR rted Pge 98 of 58/L N Tripthy rted K K ctul rted 5 KAR The fundmentl frequency current for the cpcitor bnk is FLcp KAR ctul K ctul.48 k 3 kctul A 3.48 The equivlent single-phse impednce of the cpcitor bnk is

99 X c k rted MAR The filter rector impednce X R X n c rted Here n = ctully 5, but for sfety mrgin, n is tken s 4.7. FL filter 3 bus X X c R A Due to the fct tht the filter drws more fundmentl current thn the cpcitor lone, the supplied KAR compenstion is lrger thn the cpcitor rting nd is determined s kar sup plied 3 bus FL filter kar Hence the detiled filter prmeters cn now be found out s Cpcitor rting = 5 kar Cpcitor mgnitude (equivlent Y) = F f = 6 Hz Find the delt vlue. C F Rector rting : Hrmonic current HARMONC STUDES (i) Most hrmonic nlysis is performed using stedy stte, liner circuit solution techniques. Hrmonic sources which re nonliner elements, re generlly considered to be injection sources into the liner network models. For most hrmonic flow studies, it is suitble to tret hrmonic sources s simple sources of hrmonic currents. Pge 99 of 58/L N Tripthy

100 ~ Nonliner lod ~ Z Source Z feeder Z tr lues of injected current should be determined by mesurement. For rectifier Lod, the hrmonic currents re : h 4 3 cos(3 t) cos 5 t cos 7 t cos Typicl % Hrmonic distortion of common Hrmonic sources : t Current Source Hrmonic 6 pulse PWM Arc SMPS ASD Drive Lighting Pge of 58/L N Tripthy

101 Resonnce Frequency nd Hrmonic Distortion Consider the circuit shown below : ~ h Hrmonic Source Sttion Cpcitnce. Determine the resonnt frequency. f the resonnt frequency is ner potentilly dmging hrmonic, either the cpcitor must be chnged or filter redesigned.. Estimte the voltge distortion due to current injection h. The equivlent circuit is L R C h =,3,4 where j R jl. C j R jl C h h R jl LC jrc R, nd L = source nd trnsformer resistnce nd rectnce C = sttion cpcitnce f Prcticl distribution systems re lrge nd need computer simultion studies. Dt nd circuit configurtion re required for this study. Dt needed include : Pge of 58/L N Tripthy

102 (i) (ii) (iii) (iv) (v) Line nd Trnsformer impednces Trnsformer connections Cpcitor vlues nd loctions Hrmonic spectr from non-liner lods Power source voltges These vlues re entered into the progrm which utomticlly djusts the impednces for frequency nd computes the hrmonic flow throughout the system. Hrmonic Power Flow The distribution of voltge nd current hrmonics throughout liner power network contining one or more hrmonic current sources is normlly crried out using nodl nlysis. The following steps re followed : Step : The Power network tken for nlysis is shown below. Bus Bus h ~ 3 3 Lod L Bus 3 Lod L The nodl current equtions t fundmentl frequency is written s : 3 Y Y Y 3 Y Y Y 3 Y Y Y At ny hrmonic frequency h, the dmittnce mtrix is reformed s () Pge of 58/L N Tripthy

103 h h 3h n eqution () nd Y Y Y h h 3h Y Y Y h h 3h Y Y Y h, h, 3h Y h 3h 3h 33h h h h 3h () (3) G h j B h nd similr expression re used for other R jh X dmittnce element. For ll inductive elements, the frequency is modified using the hrmonic number. For h =, 3, 4,, n, the solution of eqution () is ttempted s h h 3h Y Y Y h h 3h Y Y Y h h 3 h Y Y Y 3h 3h 33h - h (4) The voltges t fundmentl frequency (h=) re found from eqution () using Newton s Rhpson method / (Guss Seidel for smll networks). For hrmonics eqution (4) is used. After the convergence is chieved ( rms)... n ( rms)... n (5) 3 ( rms) n Hrmonic power flow softwre pckges re now stndrd nd hve the cpbility of solving severl hundred nodes, inclusion of sequence networks, multiphse models, voltge nd current hrmonic sources, severl models of trnsformer connections, etc. Pge 3 of 58/L N Tripthy

104 oltge Flicker Fluctutions in the system voltge (more specificlly in its rms. vlue) cn cuse perceptible (low frequency) light flicker depending on the mgnitude nd frequency of the vrition. Fig. shows voltge flicker, where the c voltge is mplitude modulted by sine wve seen s the envelope of the voltge wveform. The voltge cn be expressed s m ( t) ( m cos t) cos t ().c. system fundmentl frequency frequency of the modulting sine wve m = the nominl mplitude of the c voltge m = = modultion fctor Eqution () is simplified s m m ( t) cos t cos( m) t cos( m) t () Here m is the flicker frequency nd m is the flicker mplitude. From the figure it is found. 7 (7Hz frequency) nd eqution () exhibits to side-bnd spectrl m components 43 Hz nd 57 Hz in the voltge wveform. The vlue of f (flicker m frequency) vries between 3 Hz bnd nd mgnitude between. to %. The flicker frequency refers to the spectrl component of the enveloping wveform nd not to the voltge. Non-periodic events cn lso cuse perceptible light flicker. Any potentilly perceptible chnge in brightness should be termed s light flicker. Pge 4 of 58/L N Tripthy

105 Cuses of Flicker. The min cuses of flicker re lods drwing lrge nd highly vrible currents. These currents impednce of the power network produce mplitude modultion of the voltge t the lod bus nd remote buses.. Another common source of flicker is the strting of electric motors. Electricl motors drw hevy currents cycliclly due to vrying torque nd speed requirements in smll domestic pplinces (freeze, drills, mixers) nd lrger ones s in steel mill, het pumps, rolling mills, etc. Lrge induction motors strting cuses voltge flicker. The flicker wve shpes re minly tringulr nd my be periodic s in reciprocting compressors (or refrigertors) nd non periodic s in infrequent strting of lrge motors. 3. nter hrmonics present in the voltge spectrum t lod bus cn produce low frequency light flicker. For exmple : Hz hrmonic + 9 Hz inter hrmonic Hz light flicker Arc furnce voltge flicker Pge 5 of 58/L N Tripthy

106 Effects of Flicker. Flickering of electric lights cuses nnoynce to humn observers. ncndescent light flicker is perceptible for modultion fctors s smll s.5%. Humn eye is very sensitive to brightness.. Modern compct fluorescent lmps (CFL) operte t high frequencies using solid stte bllsts. Thus flicker due to.c. opertion is not perceptible to humn eye. 3. Reduced life of electronic, incndescent, fluorescent, nd cthode-ry tube devices. 4. Mlfunction of phse locked loops. 5. Loss of synchronism in UPSs. 6. Mlopertion of electronic controllers nd protective devices. oltge Fluctution nd Flicker oltge fluctution is described s cyclicl vrition of the voltge envelope or series of rndom voltge chnges (up to % ). The rte of occurrence rnges from 5 per second to one per minute nd it should be differentited from the norml slow vrition ( %). Generlly it is chrcterized by the mplitude of voltge chnges nd the rte of repetition. Mny voltge flicker curves give the percentge mgnitude of voltge pulstion nd pulstion frequency tht leds to perceptible flicker. Pge 6 of 58/L N Tripthy

107 Different curves provide different tolerble pulstion mgnitudes nd frequencies but they ll llow similr trend shown in the figure below where the most sensitive frequency is round 8 Hz. Pge 7 of 58/L N Tripthy

108 Lmp behviour nput oltge oltge dpter Squrelw Bndpss Weighti ng filter Squrin g & Sttistic l p st ; P lt Humn visul perception nstntneous flicker senstion Fig. Flicker Curves Pge 8 of 58/L N Tripthy

109 Long Durtion voltge ritions Service voltge supplied to n end user should be within 5% (ANS stndrd permits +6 % to 3 % vrition). oltge regultion is n importnt problem in power systems nd it chiefly occurs due to high impednce of the source or trnsmission nd distribution systems. oltge regultion Principles of regulting voltge f the impednce of the power system is high, the voltge drops too low when lod is hevy. Conversely when the source voltge is boosted, over-voltge occurs when the lod drops too low. For improving voltge regultion :. Add voltge regultors. Add shunt cpcitors to reduce current Z=R+jX oltge Supply R X Lod 3. Add series cpcitors to cncel the inductive impednce drop X. R X X c Lod Z = R + j (X X c ) Pge 9 of 58/L N Tripthy

110 4. Add Sttic AR compenstors cross the Lod. Fixed cpcitor, rible rector combintion 5. Re-conductor lines to lrger size to reduce the impednce Z. 6. mprove voltge regultion. Drwbcks. Hrmonic problems : resonting with system rectnce.. Switching trnsients my be generted such s mgnifiction of utility cpcitor switching trnsients. Pge of 58/L N Tripthy

111 Loction of Power Fctor Correction Cpcitors ncoming C Utiliztion or Distribution Bus C M C3 Distributed Lods C4 Fig. Loction of PF cpcitor Pge of 58/L N Tripthy

112 Devices for voltge regultion. Tp chnging Trnsformers. soltion devices with seprte voltge regultion 3. mpednce compensting devices such s cpcitors. Tp Chnging Trnsformers nput (primry) Secondry Lod (i) Mechnicl (ii) Electronic Tp chnging is performed under Lod conditions. Most tp chnging trnsformers re uto trnsformers.. soltion Devices (i) UPS (ii) Ferroresonnt (Constnt voltge trnsformers) (iii) Motor Genertor sets These devices isolte the lod from the power supply system, often cuse losses nd hrmonic problems. 3. Shunt cpcitors - Mintin voltge by reducing the current in the line. - nject rective power to the buses nd boost up voltge - mproves power fctor - Switched in steps in conjunction with the Lod Pge of 58/L N Tripthy

113 Utility step-voltge regultor cn regulte the voltge from to + percent of the incoming line voltge in 3 steps of 5/8%. Distribution substtion trnsformers commonly hve 3-phse lod tp chngers. These re slow devices nd tkes 3-45s for voltge regultion. Ferroresonnt Trnsformers. They protect the user from voltge sgs nd mintin very good voltge regultion ( % output). t mintins constnt output voltge even if the voltge is reduced to 5%of its initil vlue, beyond which, the output voltge collpses. However, ferroresont trnsformers re lossy nd inefficient. End User Cpcitor Appliction. Minly used for power fctor correction. mproves power qulity s well 3. Reduces the electric utility bill 4. Reduces R losses, therefore heting in lines nd trnsformers 5. To reduce current in the lines nd trnsformers, llowing dditionl lod to be served. Performnce clcultions (i) oltge rise The voltge rise relized with the instlltion of cpcitors % KARcp Z KAtr % = % voltge rise tr % KAR cp = cpcitor bnk rting KA tr = Step-down trnsformer rting Z tr % = % of impednce of Step-down trnsformer This formul ssumes tht the trnsformer impednce is the totl source impednce from source to the point where the cpcitor is instlled. Pge 3 of 58/L N Tripthy

114 Note f the voltge t the lod bus is low, cpcitors provide n increse in voltge so tht the lod bus voltge comes to tolerble limit. Sustined overvoltge might occur, if the cpcitor is left energised when the lod is switched off. (ii) Reduction in power System Losses Losses (Power fctor) % Loss PF PF originl corrected % Loss Reduction PF PF Reduction of Line Current cos % cos before fter % = percent current reduction originl corrected before = Power fctor ngle before correction fter = Power fctor ngle fter correction True Power Fctor (TPF) Hrmonic distortion in the voltge nd current cused by nonliner lods on the system chnges the wy power fctor must be clculted. TPF KW KA P rms rms KA = Hrmonic distortion is included to clculte olt-mperes n S = n = pprent power = S n n rms... n Pge 4 of 58/L N Tripthy

115 ,,... rms voltges rms... n n = order of hrmonics or = fundmentl voltge or current component TPF THD Assuming the THD v for the voltge is zero, the true power fctor is expressed in terms of THD of current s shown bove. Selection of cpcitor The KAR rting of power fctor correcting cpcitor is selected s KAR KW(tnφ orig - tnφ new ) PF PF orig new KW = rel power in KW orig = originl power fctor ngle new = Desired power fctor ngle PF orig = originl power fctor PF new = new power fctor WRNG AND GROUNDNG Mny power qulity vritions tht occur within customer fcilities re relted to wiring nd grounding problems. 8% of PQ problems re ground relted NEC (ntionl electric code) provides minimum stndrd for wiring nd grounding Resons for grounding. Sfety (Personl sfety) For fult on the equipment where high touch voltge exists, grounding of the equipment is must. Pge 5 of 58/L N Tripthy

116 SUPPLY SYSTEM OERCURRENT PROTECTON METALLC CONDUCTOR ENCLOSURE NEC 5- LOAD L N G Bond NEC 5 6 (e) N G Equipment Grounding Conductors N G Grounding Electrode Conductor NEC 5-6(b) Grounding Electrode NEC 5-6(c) Erth or some Conducting Mteril Fig. Terminology used in NEC definitions Ungrounded Cbinet Line Neutrl Fult Lod Sfety Ground Dngerous Touch Potentil System Ground Fig. High-touch voltge creted by improper grounding. Pge 6 of 58/L N Tripthy

117 . Grounding to ensure protective device opertion. A ground fult return pth to the point where the power source neutrl conductor is grounded is n essentil sfety feture. An insultion filure of phse wire my result in the phse wire touching the enclosure of the equipment nd power source neutrl 3. t includes trnsients from ll sources. This in turn cretes n equipotentil ground system. Potentil difference between different ground loctions cn cuse stress to insultion, crete circulting ground currents interference to other sensitive equipments in the vicinity Typicl Wiring nd Grounding Problems Problems with conductors nd connectors: (i) nspection is required for pnels nd sub-pnels for problems with conductors or connections. A bd connection (fulty, loose or resistive connection) results in heting, possible rcing nd burning of insultion. (ii) Missing sfety ground: f the sfety ground is missing, the equipment will get chrged to the potentil of the phse wire. (iii) Multiple Neutrl-to-ground connections The neutrl nd ground should be kept seprte t ll pnel bords nd junction boxes. Also if neutrl to ground loop is there t the pnels, improper opertion of protective devices might occur. (iv) Ungrounded Equipment: solted grounds cuse noise problems. Used for sensitive equipments Pge 7 of 58/L N Tripthy

118 (v) Additionl ground rods should be prt of grounding system. They provide pths for lightening currents to flow to the ground. (vi) Ground loops: Circulting currents flow between grounds cusing noise problems in dt processing nd telecom networks (vii) nsufficient Neutrl Conductors Switch mode power supplies nd fluorescent lighting with electronic bllsts hve high 3 rd hrmonic components nd rise the rtings of the neutrl conductors. Neutrl currents in SMPS 4 to 7% of 6 Hz/5Hz frequency currents. Thus hrmonics from computer power supplies need to be diverted to ground Filters or /Y trnsformers need to be instlled for 3 rd hrmonic currents. Solutions to Wiring nd Grounding Problems: Proper grounding prctices:. Grounding electrode (rod) The grounding rod provides the electricl connection from the power system ground to erth (ground rod resistnce is importnt) Electrode resistnce: Resistnce of the wire connecting ground rod nd erth Rod-erth contct resistnce due to interfce between soil nd erth Ground resistnce Due to the resistivity of the soil in the vicinity of the ground rod. content. The resistive of the soil vries over wide rnge due to soil type / moisture lightening. Resistnce of the ground rod influences trnsients due to switching nd Pge 8 of 58/L N Tripthy

119 POWER COMPANY TRANSFORMER Grounding Electrode Grounded Service Conductor BULDNG SERCE EQUPMENT N FEEDER Phse Conductor (Hot) Grounded Conductor (Neutrl) Bonding Jumper nsulted Ground G Conductor (Green wire) PANEL BOARD N G BRANCH CRCUT solted Ground Receptcle LOAD Grounding Electrode Conductor Building Grounding Electrode Conduit / Enclosure Receptcle Fig. 7.3 Bsic elements of properly grounded electricl system Pge 9 of 58/L N Tripthy

120 POWER COMPANY TRANSFORMER Grounding Electrode Grounded Service Conductor BULDNG SERCE EQUPMENT N Bonding Jumper G FEEDER Phse Conductor (Hot) Grounded Conductor (Neutrl) nsulted Ground Conductor (Green wire) PANEL BOARD N G BRANCH CRCUT solted Ground Receptcle LOAD Grounding Electrode Conductor Building Grounding Electrode Conduit / Enclosure Note: Ornge in color Fig. Grounding Configurtion for n solted ground Ground Ring provides better equipotentil ground for the grounding electrode. solted ground: Sensitive equipments use isolted ground receptcles to improve noise performnce. Lod currents should never flow to the ground under ny circumstnces. There should be n equipotentil reference for ll devices nd loctions in the system. High touch potentil sfety. Pge of 58/L N Tripthy

121 POWER QUALTY MONTORNG Power qulity investigtions often require monitoring to identify the exct problem nd to verify the solutions tht re implemented. Before extensive monitoring it is impertive to mke site survey to hve n understnding of the Customer fcility Equipments being ffected Wiring nd Grounding prctice Operting considertions Site Survey. Nture of the PQ problem t the customer site () Dt loss (b) Nuisnce trips (c) Component filure (d) Control system Mlfunctions etc.. Chrcteristics of the sensitive equipment experiencing PQ problem. 3. Time of occurrence of the problem 4. Cpcitor use nd its switching opertion 5. Possible sources of PQ vrition (i) Motor Strting (ii) Cpcitor Switching (iii) Fults (iv) Power Electronic Equipment Opertion (v) Arcing opertion 6. Existing Power conditioning Equipment being used 7. Electricl system Dt () One-line digrms (b) Trnsformer Sizes & mpednces (c) Lod informtion (d) Cpcitor nformtion (e) Cble dt Pge of 58/L N Tripthy

122 Detiled PQ Monitoring Power qulity monitoring beyond initil site survey is performed to chrcterize PQ vritions t specific system loctions over period of time For exmples: oltge sgs during remote fults require monitoring over long period Cpcitor switching dys Hrmonic Distortions week How Hrmonics vry with Lod chnges Choosing Monitoring Loctions (i) Strt monitoring t loctions s close s possible to the sensitive equipment being ffected by PQ vritions (ii) Choose the lod bus which experiences voltge sgs over period of time (iii) Min service entrnce could be n importnt loction to identify PQ vritions. Quntities to Mesure. When monitoring PQ disturbnces, monitor system voltges () RMS vritions (b) Pek vlue (c) Crest Fctor. To chrcterize hrmonics monitor both voltges nd currents: () RMS vlues of currents (b) THD of both voltge nd current (c) Current mesurements t individul lods, feeder circuits (d) Hrmonic oltge distortions (e) Hrmonic power flows Pge of 58/L N Tripthy

123 nterprettion of the results Correlte the chrcteristics of the PQ disturbnce with possible cuses of disturbnce. knowledge of PQ disturbnce wveform Threshold mgnitudes of PQ prmeters nd indices. Some typicl PQ indices re:. RMS nd Pek vlue. Crest Fctor 3. Form Fctor 4. THD (totl hrmonic disturbnce) 5. Telephone interference fctor Finding the source of PQ disturbnce. For High frequency disturbnces PQ monitor should be locted close to the source of disturbnce. Power interruptions cuse brupt chnge in voltge 3. Hrmonic voltge distortions occur close to the cpcitor loctions due to Resonnce conditions Power Qulity Mesurement Equipment (Anlog Types) Bsic PQ monitoring instruments is:. Wiring nd grounding test devices. Multimeters 3. Oscilloscopes 4. Disturbnce Anlyzers 5. Hrmonic Anlyzers / Spectrum Anlyzers 6. Combintion of Hrmonic nd Spectrum Anlyzers 7. Flicker Meters 8. Energy Monitors Pge 3 of 58/L N Tripthy

124 Besides other instruments like i) The nfrred meters detect loose connection nd overheting of conductors. ii) Mgnetic Guss meters mesure mgnetic field strengths for inductive coupling concerns iii) Electric field meters For selecting prticulr PQ instrument, the following fctors re considered:. Number of chnnels for voltge nd current (6). Temperture 3. Ruggedness of the instrument 4. nput voltge rnge ( 6 volt) 5. Power requirements 6. Ability to mesure 3-phse voltges nd currents 7. nput isoltion 8. Housing of the instrument (portbility, rck mount, etc.) 9. Ese of use (user interfce, grphics cpbility, etc.)]. Documenttion. Communiction cpbility (modem, network interfce, etc.). PQ Anlysis Softwre Wiring nd Grounding Monitoring Devices. isul inspection of wiring connections nd pnel boxes. Test devices for grounding 3. Detection of solted grounds 4. Ground impednce nd neutrl impednce mesurement kit. 5. Detection of open ground, open neutrl or open hot wire. 6. Detection of neutrl or ground reversls 7. 3-phse wiring testers for phse rottion nd phse to phse voltges Pge 4 of 58/L N Tripthy

125 Check the following:. Phse to- ground voltges. Phse to- neutrl voltge 3. Neutrl to ground voltge 4. Phse Currents 5. Neutrl Currents Multimeters All the commonly used multimeters re clibrted to give RMS indiction for the mesured signl. The three most common methods clculte RMS vlue is:. Pek Method: Meter reds pek pek pek rms =.44. Averging Method: The meter determines the verge vlue of rectified signl. pek pek rms rms verge verge. verge 3. True RMS Averging the signl on smple smple bsis nd clculte RMS vlue digitlly. rms n k ( k) Pge 5 of 58/L N Tripthy

126 Meter Type True RMS Pek Method Averge Responding Circuit Type RMS Converter PEAK/.44 Sine Avg. X. Sine Wve % % % Squre Wve % 8% % Tringle Wve % % 96% ASD Current % 7% 86% PC Current % 84% 6% Light Dimmer % 3% 84% Comprison of Meter reding for vrious Wveforms Pge 6 of 58/L N Tripthy

127 Oscilloscopes Oscilloscopes cn record voltge nd current wveforms which provide much informtion bout the vrious PQ disturbnces. Digitl oscilloscopes with dt storge fcilities re vilble to nlyze recorded wveforms i) clculte their spectrum nd energy contents ii) to uplod PC for further nlysis iii) Hnd held oscilloscopes tht perform signl processing nd hrmonic disply rc vilble Disturbnce Anlyzer Mesures i) Short durtion trnsient voltges ii) Long durtion outges iii) under nd over voltges iv) Thresholds re set for recording Two ctegories of DA re vilble ) Conventionl nlyzer tht summrizes events with specific informtion such s over / under voltges sgs / surge mgnitude nd durtion, etc. b) Grphic bsed nlyzer tht sve nd print the ctul wveform long with the descriptive informtion which would be generted by one of the conventionl nlyzers. Spectrum Anlyzers nd Hrmonic Anlyzers: These nlyzers hve hrmonic nlysis cpbilities unlike DAS nd re equipped with FFT (Fst Fourier Trnsform) clcultion softwre. They re cpble of i) Mesuring both voltge nd current nd thus Hrmonic power flows ii) Mesuring both mgnitude nd phse ngles of voltge nd current hrmonics iii) Synchroniztion nd smpling rte fst enough to provide ccurte mesurement of hrmonic signls up to 37 th hrmonic. Pge 7 of 58/L N Tripthy

128 iv) Chrcterize the sttisticl nture of hrmonic distortion levels (tht chnge with both lod nd system conditions). Two ctegories of SA nd HA re vilble:. Simple Meters A simple portble meter to quickly check the Hrmonic distortion level t PQ bus. Bsiclly this is hnd held instrument which uses Microprocessor bsed instrumenttion scheme to mesure up to 5 th hrmonics. t evlutes THD nd TF (telephone interference fctor).. Generl purpose Spectrum Anlysis ) Performs spectrum nlysis of wveforms b) Use Signl processing techniques for wveform mesurements c) Are not designed specificlly for fundmentl frequency wveforms nd hence my not ccurtely nlyze hrmonics Combining disturbnce nd hrmonic nlyzers cn produce wide spectrum nlysis on n input set of dt remotely gthered over telephone lines. Pge 8 of 58/L N Tripthy

129 t monitors 3-phse voltges nd currents simultneously nd stores the dt for downloding to PC. Pge 9 of 58/L N Tripthy

130 oltge Trnsducers (Ts). Ts should be sized to prevent mesured disturbnces from inducing sturtion. For trnsients this generlly requires tht knee point of the trnsducer sturtion curve be t lest % of nominl system voltge. Exmple: Distribution Feeder rting =.47k Nominl voltge cross primry of the T (for L-G fult) = 7. k RMS T output voltge is limited to = volts Hence T turns rtio = 7. / = 6 Pge 3 of 58/L N Tripthy

131 . T Frequency Response oltge rtio should not be ffected over wide frequency rnge. The frequency response of T depends on the type of T nd its burden. The burden should be of high impednce, so tht Rtio correction fctor (RCF) remins close to unit upto 5kHz Mesuring very high frequency components in the voltge requires cpcitor divider. (CCT = Cpcitively coupled voltge divider. input C C output Pge 3 of 58/L N Tripthy

132 Q = C input CC C C C output Q C Cinput C C The min points bout Ts re:. At medium voltge, ll the trnsformers perform dequtely up to khz, while only 6% of them mnge to cover the whole hrmonic spectrum.. At high voltge, the trnsformer response deteriortes quickly for frequencies bove 5Hz unless specil designs re introduced. 3. The conventionl voltge trnsformers of mgnetic type do not provide ccurte informtion bout hrmonic orders bove 5 th. Trnsducer Requirements Trnsducers for PQ mesurements should obtin cceptble voltge nd current signl levels. i) Most PQ trnsducers mesure input voltges up to 6 ii) CT current outputs up to 5A iii) Signl levels should use the full scle of the instrument without distorting or clipping the desired signl iv) Frequency response of the trnsducer is importnt for detecting trnsients nd hrmonics contining high frequency components Current Trnsducers Selecting proper trnsducer for currents is more difficult. The current chnges most often in ny system nd with greter mgnitude thn voltge. The proper CT rting nd turns rtio depends on the mesurement objective. (i) CT is sized by to 3 times for mesuring fult nd inrush current thn norml lod current. (ii) f hrmonics nd lod chrcteriztion re importnt CTs should be selected ccording to lod current mgnitude. Pge 3 of 58/L N Tripthy

133 Exmple: Desired CT current = to A rms. Feeder current = A Assume A CT current CT turns rtio = : Mnufcturer of CTs list CT turns rtio on bse of 5 Hence primry rting of CT is clculted s CT pri pri CT sec sec 5 6 Thus 6:5 should be used. Frequency Response Stndrd metering clss CTs re generlly dequte for frequencies upto KHz (phse error my become significnt before this). For higher frequencies window type CTs with high turns rtio (split core, br-type, clmp-on) should be used. The following min points for CTs re:. f CT is multi-secondry type, the highest rtio should be used. Higher rtios require lower mgnetizing current nd tend to be more ccurte.. Lrge turns rtios :5 or greter 3. The current trnsformer burden should be of very low impednce 4. Window type CTs re preferred 5. Smll remnent flux % of core sturtion vlue 6. Lrge core re. The more steel is used in the core, the better is the frequency response. 7. The burden power fctor should be high so s to reduce mgnetizing current errors. 8. Secondry winding resistnce nd lekge rectnce should be smll. Pge 33 of 58/L N Tripthy

134 Summry of T nd CT recommendtions for PQ monitoring Tble 8. T nd CT Options Loction T CT Substtion Metering Ts Metering CTs Specil purpose cpcitive or Relying CTs resistive dividers Clibrted bushing tps Overhed lines Metering Ts Metering CTs Underground loctions Metering Ts Metering CTs Pd-Mounted trnsformer Specil-purpose dividers Secondry sites Service entrnce Direct connection Metering CTs Clmp-on CTs n fcility Direct connection Clmp-on CTs Pge 34 of 58/L N Tripthy