Avalable onlne at www.scencedrect.com ScenceDrect Proceda Engneerng (17) 159 167 4th Internatonal Colloquum "Transformer Research and Asset Management Frequency response and harmonc dstorton testng of nductve voltage used for power qualty measurements Dalbor Flpovć-Grčć a, Boždar Flpovć-Grčć b *, Danjel Krajtner c a Končar Electrcal Engneerng Insttute, Fallerovo šetalšte, 1 Zagreb, Croata b *nversty of Zagreb, Faculty of Electrcal Engneerng and Computng, nska 3, 1 Zagreb, Croata c Končar Instrument Transformers Inc., Jospa Mokrovća 1, 1 Zagreb, Croata Abstract Internatonal standards related to power qualty measurements defne methods and accuraces for the measurng nstruments, but do not specfy the accuracy of nstrument s. Therefore, t s not possble to specfy an overall accuracy for such measurements. Inductve voltage s (IVTs) whch are used for power qualty measurements should be tested to determne the rato correcton factors (RCFs) and phase angle errors at hgher frequences. In ths paper, harmonc dstorton and frequency response tests are performed to determne the level of harmoncs generated by IVT and ts ablty to transform harmoncs from hgh voltage (HV) to low voltage (LV) sde. Complex voltages consstng of fundamental voltage and certan amount of supermposed harmonc are used to check the frequency response of IVT. RCFs and phase angle errors of the IVT are determned and can be appled to power qualty montors for compensaton of errors that occur at hgh harmonc frequences. Dfferent test crcuts are proposed for generaton of HV consstng of fundamental voltage and harmonc voltages wth ampltudes n range 5-15 % of the appled fundamental voltage. In order to mprove the testng capabltes at hgher voltage levels (for equpment wth 13 kv m 4 kv), a compensaton for both fundamental and harmonc voltages s proposed wth a specal connecton through blockng and pass flters. 17 The Authors. Publshed by Elsever Ltd. Peer-revew under responsblty of the organzng commttee of ICTRAM 17. Keywords: Inductve voltage ; frequency response; harmonc dstorton; power qualty; hgh voltage testng. 1. Introducton Voltage harmonc dstorton level s one of the sgnfcant parameters of power qualty n the power system. Hgher harmoncs cause the followng effects n electrcal networks: addtonal heatng and losses on power system elements (such as transmsson lnes, s, compensaton devces, etc.), unwanted voltage dstorton, ncreased flow of crculatng currents through groundng wre, decrease of rated power and nterference wth conventonal telecommuncaton lnes. Numerous problems related to voltage and current harmonc effects n power systems are commonly observed nowadays due to growth of large ndustral consumers wth non-lnear loads and power electronc equpment [1]. Voltage harmoncs may dsturb senstve loads connected to the grd and therefore they should be lmted. Levels and spectral content of voltage dstortons njected nto electrc grds are tendng to ncrease even though the acceptable levels are determned by numerous regulatons. IEC standards * Correspondng author. Tel.: +3851619714; fax: +385161989. E-mal address: bozdar.flpovc-grcc@fer.hr 1877-758 17 The Authors. Publshed by Elsever Ltd. Peer-revew under responsblty of the organzng commttee of ICTRAM 17. 1.116/j.proeng.17.9.73
16 Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 []-[4] defne compatblty levels or plannng levels for voltage harmoncs n LV, MV and HV networks. Furthermore, European standard [5] defnes, descrbes and specfes the man characterstcs of the voltage at a network user's supply termnals n publc LV, MV and HV networks under normal operatng condtons. Accordng to [6], power qualty and voltage dstorton measurements are carred out by hgh accuracy measurng equpment of class A. In MV and HV grds ths measurng equpment s commonly connected to secondary sde of IVTs nstalled wthn the substatons. So, an overall accuracy of the measurement depends on the accuracy of IVTs. The accuracy requrements n [6] are defned only for the measurng equpment but not for IVTs. The standard for IVTs [7] defnes the accuracy lmts at rated frequency, but t doesn t defne the accuracy lmts at hgher frequences. The accuracy of IVTs for frequences hgher than the nomnal frequency s usually not known. It s therefore dffcult to specfy an overall accuracy not only for harmonc voltage measurements but also for measurement of any knd of transent contanng a wde harmonc spectrum. Lterature survey [8]-[17] showed that conventonal IVTs may have sgnfcant dfference between the accuracy at rated frequency and the accuraces at hgher frequences. Therefore, a frequency response of IVTs n the concerned range of frequency should be known to use them for harmonc measurements n power system. Dfferent measurng setups and calculaton procedures for frequency response of IVTs are presented n [18]-[1]. Sgnfcant dfferences between IVT rato at rated frequency and at hgher frequences can be observed, caused by resonances wthn the IVT. Also, a lttle research has been conducted concernng the phase angle characterstcs of IVTs operatng wth non-snusodal waveforms. To determne the behavour of the IVT s rato and phase angle at hgher frequences two separate tests should be performed: frequency response test and harmonc dstorton test. Although dfferent crcut setups for testng the harmonc responses of IVTs are descrbed n lterature, only a few of them could acheve the voltage level above kv. Besdes, the equpment used n the test crcut setups and test methods are also largely unspecfed and dffer from each other. In [], a test crcut s proposed for determnng the harmonc responses of IVTs. The frequency response assessment of a sngle phase 4 kv IVT and a sngle phase 75 kv capactor voltage n a range of frequency from 5 Hz up to 5 khz s presented. However, at 4 kv level the test crcut has a lmtaton regardng the harmonc njecton ablty. The maxmum ampltudes of generated voltage harmoncs are 1 % wth respect to the ampltude of the fundamental voltage up to 1 khz, whle the harmonc ampltudes from 1 khz up to 5 k Hz are lower than. %. The harmonc njecton ablty of the test crcut s lmted by maxmum power capacty of the amplfer whch was used as a harmonc power source. In ths paper, three dfferent test crcuts are proposed for generaton of HV whch conssts of fundamental voltage and harmonc voltages wth ampltudes n range 5-15 % of the appled fundamental voltage. Ths s of great mportance snce hgh sgnal to nose ratos reduce measurement uncertanty. The selecton of approprate test crcut depends on hghest voltage m for equpment under test and ts requred actve and reactve power. In order to mprove the testng capabltes at voltage levels 13 kv m 4 kv, a compensaton for both fundamental and harmonc voltages s proposed wth a specal connecton through blockng and pass flters. A test method s presented for frequency response and harmonc dstorton testng of IVTs used for power qualty measurements. Expermental verfcaton s demonstrated n case of medum voltage IVT. RCFs and phase angle errors of the IVT are determned at fundamental frequency and at each harmonc frequency from nd to 5 th harmonc. These correcton factors can be appled to power qualty montors connected to the secondary termnals of the IVT.. crcuts for generaton of hgh voltages contanng hgher harmoncs.1. crcut wth a sngle source and wthout compensaton The frst test setup shown n Fg. 1 s n most cases sutable for testng of medum voltage equpment. In ths test crcut, both fundamental and harmonc voltages are generated from the same source whch conssts of arbtrary waveform generator (AWG), low frequency amplfer and test. In ths case a sngle source s generatng both actve and reactve power requred by test object and capactor voltage dvder whch s used as a reference measurng system. Fg. 1. crcut wth sngle source wthout compensaton.
Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 161 Combned voltages are generated wth AWG from a fle whch contans tme-voltage data and are defned as: AWG = 1 sn( ω t ) + sn( ωt ), (1) where 1 s ampltude of fundamental harmonc and s ampltude of -th harmonc. AWG sgnal s amplfed by low frequency amplfer and afterwards by the test. However, test rato s frequency dependent and therefore t s necessary to adjust for each harmonc. Ths can be done by performng a separate automated test. Software controls AWG to produce sne sgnals wth frequences from fundamental up to 5 th harmonc, reads voltages on LV and HV sde of test and calculates frequency dependent rato p of test. Ratos for dfferent frequences shown n Fg. are expressed n p.u. of the fundamental frequency rato p 1. p /p 1 (p.u.) 8 7 6 5 4 3 1 5 1 15 5 3 35 4 45 5 Harmonc Fg.. Frequency dependency of test rato. In order to obtan X % ampltude of hgher harmoncs wth respect to fundamental the followng expresson s used: AWG X 1 p1 = 1 sn( ωt) + sn( ωt). () 1 p voltage waveforms are generated usng () and mported to AWG... crcut wth a sngle source and wth compensaton crcut wth a sngle source and wth compensaton s shown n Fg. 3. The same harmonc source s used as n the frst test crcut. AWG Amplfer Blockng flter for th harmonc L b Compensaton of fundamental harmonc C 1 C b Blockng flter for fundamental harmonc Compensaton of th harmonc L 1b L C 1b C object Measurng cable Capactor voltage dvder HV measurng system Measurng equpment Fg. 3. crcut wth a sngle source and wth compensaton. The reactve power requred by test object can be n some cases hgher than the power capacty of the source. To overcome ths lmtaton, a compensaton for both fundamental (C 1 ) and harmonc voltages (L or C ) s proposed. Snce the reactve power at the fundamental harmonc s often an addtonal burden for the hgher harmoncs and vce versa, t s necessary to connect the compensaton for fundamental and harmonc voltages through sutable blockng flters for fundamental harmonc (L 1b, C 1b ) and
16 Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 for hgher harmoncs (L b, C b ). Capactances and nductances used for desgn of blockng flters and compensaton of hgher harmoncs are varable elements whch should be adjusted accordng to reactve power requred by the test object for each harmonc. Compensaton of fundamental harmonc requres capactve reactve power snce the test crcut has an nductve character (for example when test object s IVT). Compensaton for hgher harmoncs can be nductve for lower frequences, but normally s capactve. The test crcut shown n Fg. 3 s n most cases sutable for testng of HV equpment wth m up to 13 kv..3. crcut wth two sources and wth compensaton crcut wth two sources and wth compensaton s shown n Fg. 4. LV network V Blockng flter for th harmonc Regulatng L b Matchng Branch 1: generaton of fundamental voltage harmonc (5 Hz) C 1p C b C 1 Pass flter for th harmonc Pass flter for fundamental harmonc AWG Amplfer L 1b Matchng L p L Branch : generaton of hgher voltage harmoncs C 1b Blockng flter for fundamental harmonc C object Measurng cable Capactor voltage dvder HV measurng system Measurng equpment Fg. 4. crcut wth two sources and wth compensaton. In case when harmonc power source s unable to supply all actve power requred by the test crcut, a fundamental harmonc s generated from the separate source. Voltage of fundamental frequency (5 Hz) s generated drectly from low voltage network ( V) through regulatng and matchng (branch 1). In ths branch, a blockng flter (L b, C b ) for hgher harmoncs and a pass flter (L b, C b, C 1p ) for fundamental harmonc are connected. Compensaton capactance C 1 s connected n parallel wth matchng. Hgher harmoncs are generated from AWG sgnal whch s amplfed by low frequency amplfer and afterwards stepped up by the matchng (branch ). Matchng n ths branch can be omtted for hgher frequences. In ths branch, a blockng flter (L 1b, C 1b ) for fundamental harmonc and a pass flter (L 1b, C 1b, L p ) for hgher harmoncs are connected. Compensaton nductance L or capactance C s connected n parallel wth matchng. The test crcut shown n Fg. 4 s sutable for testng of HV equpment wth m up to 4 kv. 3. Frequency response and harmonc dstorton testng of medum voltage nductve voltage s 3.1. Capactor dvder as a reference measurng system A capactor voltage dvder for AC voltages up to 5 kv s used as a reference measurng system for measurement of prmary voltage on IVT, snce t has suffcent accuracy and stablty to be appled for the approval of other measurng systems. Scale factor (rato) and phase dsplacement of capactor dvder are determned accordng to [3] for frequences 5 Hz.5 khz, n 5 Hz steps. Measurement results are shown n Fg. 5. Phase dsplacement δ (mnutes) -1 - -3-4 -5-6 Phase dsplacement 4.5-7 Rato 4-8 41.5 5 4 75 11 145 18 15 5 f (Hz) Fg. 5. Dvder rato and phase dsplacement versus frequency. 45.5 45 44.5 44 43.5 43 Rato
Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 163 3.. Frequency response test The purpose of the test s to evaluate the rato and phase angle errors of the IVT from nd to 5 th harmonc. Determnaton of errors at the fundamental frequency s a routne testng and therefore t s not dscussed n ths paper. Each test s carred out wth 5 Hz component of the appled prmary voltage equal to the nomnal prmary phase-to-ground voltage of IVT, at 3 secondary termnal burdens ( %, 5 % and 1 % of rated burden, cosφ=1). The magntudes of the appled harmonc voltages are 1 % of the appled fundamental voltage to produce hgh sgnal to nose ratos at each harmonc. Each harmonc s ndvdually supermposed onto fundamental 5 Hz voltage and the combned voltage s appled to the prmary of IVT. For each of the 3 specfed burdens measurements of the magntude and phase angle of the harmonc voltages are performed at both the prmary and the secondary termnals of the IVT. crcut for performng both frequency response and harmonc dstorton test s shown n Fg. 6 a) and photograph of the test setup s shown n Fg. 6 b). IVT Measurng system PC controlled Fg. 6. (a) crcut for frequency response and harmonc dstorton testng; (b) photograph of the test setup. RCF s defned as rato of voltage on HV sde measured wth reference measurng system and wth IVT under test: RCF p d d =, (3) p LV where p s rated rato of IVT under test, LV voltage on LV sde of IVT, p d rato of capactor dvder and d voltage on LV sde of capactor dvder. The phase angle error δ of IVT s defned as angle dfference between -th voltage harmoncs at HV and LV sde of IVT. δ s determned by usng the followng expresson: δ = δ + δ, (4) d d LV where δ d s angle dfference between harmoncs measured at HV and LV sde of capactor voltage dvder, δ d-lv angle dfference between harmoncs measured at LV sde of capactor voltage dvder and LV sde of IVT. Fgs. 7 and 8 show frequency dependency of RCF and δ for two secondary wndngs of IVT. RCF 1.5 1..999.996.993.99.987 5 VA 5 VA VA 5 1 15 5 δ (mnutes) 5 15 1 5-5 -1 5 VA 5 VA VA 5 1 15 5 Fg. 7. (a) RCF versus frequency - secondary wndng 1a-1n; (b) δ versus frequency - secondary wndng 1a-1n.
164 Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 RCF 1. 1.15 1.1 1.5 1 5 VA 5 VA VA.995 5 1 15 5 δ (mnutes) 15 1 Fg. 8. (a) RCF versus frequency - secondary wndng a-n; (b) δ versus frequency - secondary wndng a-n. The results of these tests provde an nformaton regardng frequency dependent correcton factors for both rato and phase angle. These correcton factors can be appled to power qualty montors connected to the secondary termnals of the IVT. Harmonc dstorton (generaton of hgher harmoncs on LV sde by IVT when pure sne voltage of fundamental frequency s appled on HV sde) can be neglected durng frequency response test because appled harmonc ampltudes are much hgher than those orgnatng from dstorton. Expanded measurement uncertanty of RCF can be determned usng the followng expresson: 5-5 5 VA 5 VA VA -1 5 1 15 5 u RCF RCF u pd RCF + u d RCF + u p RCF + u LV = pd d p LV, (5) where u pd s expanded measurement uncertanty of capactor voltage dvder rato (.5 %), u d s expanded uncertanty of voltage measurement at LV sde of capactor dvder (.1 %), u p s expanded uncertanty of IVT rato (.3 %), u LV s expanded uncertanty of voltage measurement at LV sde of IVT (.1 %). Maxmum value of RCF expanded uncertanty s.57 %. Expanded uncertanty of phase angle error δ s.8 mnutes. 3.3. Harmonc dstorton test The purpose of test s to determne the level of harmonc voltages (up to 5 th harmonc) at the secondary termnals generated by the IVT tself when a harmonc free 5 Hz voltage s appled to the prmary. The tests are carred out at appled 5 Hz (prmary) voltage levels beng 1. and 1.1 tmes the nomnal IVT prmary phase-to-ground voltage and at 3 secondary termnal burdens ( %, 5 % and 1 % of rated burden, cosφ=1). Snce the appled 5 Hz prmary voltage had a low content of hgher harmoncs, no flter was appled on HV sde. results show that IVT ntroduces very low harmonc content on LV sde voltage. Snce the appled prmary s not completely free of hgher harmoncs, harmonc magntudes ntroduced by IVT are calculated usng followng expresson: p d pd 1 Δ = ( ) + d d cos % p RCF ϕ, (6) p RCF LV 1 where Δ s calculated level of -th voltage harmonc generated by the IVT (Fg. 9) expressed as percentage of the fundamental voltage, s voltage of -th harmonc measured at IVT secondary termnals, LV1 s voltage of fundamental harmonc measured at secondary termnals, p s rato of the IVT, φ s the angle between and d whch s reduced to secondary sde of IVT wth approprate RCF determned from frequency response test. φ d pd p RCF Δ Fg. 9. Vector dagram showng Δ generated by the IVT Fgs. 1 and 11 show frequency dependency of Δ for two secondary wndngs of IVT.
Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 165 Δ (%).7.6 5 VA 5 VA VA.5.4.3..1 5 1 15 5 Fg. 1. (a) Δ versus frequency - secondary wndng 1a-1n, voltage n; (b) Δ versus frequency - secondary wndng 1a-1n, voltage 1.1 n. Δ (%).6.5.4.3..1 5 VA 5 VA VA 5 1 15 5 5 1 15 5 Fg. 11. (a) Δ versus frequency - secondary wndng a-n, voltage n; (b) Δ versus frequency - secondary wndng a-n, voltage 1.1 n. The results of these tests can be used to determne approprate offset correcton factors (for each harmonc) that should be appled to power qualty montors connected to the secondary termnals of the IVT to compensate for the small fxed levels of harmonc voltages expected to be generated by the non-lnearty of the IVT. In that case measurement uncertanty of Δ can be determned from (6), whle angle φ wth the correspondng measurement uncertanty should be also consdered. In the case of IVT consdered n ths paper, Δ s much lower than harmonc ampltudes appled n the frequency response test, so t can be excluded from further analyss. 4. ng of equpment m =4 kv wth fundamental voltage and supermposed hgher harmonc Δ (%) Δ (%) Ths example shows a crcut for generaton of fundamental and supermposed hgher harmonc from two sources as descrbed n secton.3. In partcular case only generaton of thrd harmonc s descrbed. For other harmoncs, flter components and compensaton need to be adjusted. The test crcut generates a HV on test object composed of the fundamental (5 Hz) harmonc wth RMS value 4/ 3 kv and supermposed 15 % of thrd harmonc (15 Hz) correspondng to RMS value 36.4 kv. crcut s shown n Fg. 1. RMS values whch correspond to fundamental frequency are marked blue, whle red ones correspond to 15 Hz..3.5..15.1.5.1.1.8.6.4. 5 VA 5 VA VA 5 VA 5 VA VA 5 1 15 5 LV network V Blockng flter for 3 rd harmonc Regulatng Branch 1: generaton of fundamental voltage harmonc (5 Hz) 66. ma 41.5 A 5.6 A Pass flter 4.7 A Blockng flter L 3b C 3b for 3 rd harmonc L 1b C 1b for fundamental 56.3 A 87.1 A harmonc Pass flter 3.7 A for fundamental 56.6 A C 1p harmonc 36.8 A L 3p 17 A 4 A 8 A 173. V 3.1 A 519.6 V C 1 AWG 3.4 V L 3 36 A 77.9 V Amplfer Matchng Matchng Branch : generaton of 3 rd voltage harmonc object Measurng cable Capactor voltage dvder HV measurng system Measurng equpment ng equpment data are shown n Table 1. Fg. 1. crcut used for testng of 4 kv equpment.
166 Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 Table 1. ng equpment data. Equpment Rated parameters Amplfer rated power 75 W Regulatng -5 V, 1 A Matchng 4/1 V, 13 kva 75 V / 35 kv, 1 A C 1; C 1p; L 3b; C 3b 141 µf;. µf; 3.75 H;.7 µf L 3b rated current 4 A L 3; L 3p; L 1b; C 1b.3 mh;.3 mh; 19 mh; 58 µf Capactor voltage dvder rato 1:59 Measurng equpment Power analyzer DEWE-571- PNA, accuracy ±.1 %; ±.5 mn Rated power of the amplfer s sgnfcantly lower than the rated power of source for fundamental harmonc. An open-core test s used, so compensaton of fundamental harmonc s mandatory because consumes relatvely hgh magnetzng current on the low voltage sde. The current of the fundamental harmonc 3.7 A s flowng through branch 1,.e. through capactor C 1p whch n seres wth L 3b -C 3b parallel provdes a low mpedance path for the fundamental harmonc. Slghtly hgher current 56.3 A s flowng through nductor L 3b (blockng flter for thrd harmonc). Capactors are generally more avalable for ths range of currents compared to nductors. So, the nductor L 3b s the most crtcal element of the flter branch regardng the current loadng. In order to reduce the current loadng of flters n branch 1, flters were not connected drectly nto test crcut as shown n Fg. 1, but va power kv / 4 V, Dyn5, 63 kva whch was avalable n HV laboratory, as shown n Fg. 13. The rated rato of power s p r =86.6. 1/3 1/3 1 V W /3 1/3 /3 1/3 1V 1W L 3b C 1p C 3b Impedance of flters Z f I Fg. 13. Connecton of flters n branch 1 through power. Besdes the current loadng of the flters, another mportant parameter that should be consdered s ther mpedance. Impedance of pass flter for fundamental harmonc should be as low as possble, whle the mpedance of blockng flter for thrd harmonc should be as hgh as possble. The mpedance of flter as seen from the test crcut sde (LV sde of power ) Z f s nversely proportonal to rato p: Z 1 ' f Z f = (7) p A drawback of ths connecton s that the mpedance of blockng flter for thrd harmonc reduces. Therefore, an optmum rato of power should be selected satsfyng both current loadng and acceptable flter mpedance. A specal connecton of flter to test crcut through power s shown n Fg. 13 and the acceptable rato of 8.9 s acheved. In ths way, a current through nductor L 3b reduced from 56.3 A to 8.9 A, whle the mpedance of blockng flter s stll hgh enough to effectvely block the thrd harmonc. Current of the thrd harmonc through L 1b s 4.7 A, whle current of the fundamental harmonc s 87.1 A. Inductance of 19 mh satsfyng the requred current loadng s acheved by usng a sngle-phase power 7.5/11 kv, 15 MVA whch was avalable n HV laboratory. HV sde of power s short-crcuted whle LV sde s connected to test crcut. Expermental verfcaton successfully confrmed all calculated voltages and currents n the test crcut. 5. Conclusons The voltage qualty s ganng more mportance due to the wdespread use of power electronc devces needed for example to connect renewable energy sources and HVDC transmsson lnes to power networks. Hence network operators, customers and
Boždar Flpovć-Grčć et al. / Proceda Engneerng (17) 159 167 167 regulators carry out power qualty measurements more frequently ncludng harmoncs at all voltage levels. In ths paper, a method for frequency response and harmonc dstorton testng of IVTs used for power qualty measurements s presented. RCFs and phase angle errors of the IVT are determned at fundamental frequency and at each harmonc frequency from nd to 5 th harmonc. These correcton factors can be appled to power qualty montors connected to the secondary termnals of the IVT. Dfferent test crcuts are proposed for generaton of HV consstng of fundamental voltage and harmonc voltages wth ampltudes n range 5-15 % of the appled fundamental voltage. Ths s of great mportance snce hgh sgnal to nose ratos reduce measurement uncertanty. The selecton of approprate test crcut depends on m of the test object and ts requred actve and reactve power. In order to perform tests on equpment wth m 13 kv, a compensaton for both fundamental and harmonc voltages s proposed wth specal connecton through blockng and pass flters. ng of equpment for m =4 kv wth fundamental voltage and supermposed hgher harmonc s successfully performed. Some practcal solutons are proposed such as reducng the current loadng of blockng flters for hgher harmoncs. References [1] J. K. Phpps, J. P. Nelson, P. K. Sen, Power qualty and harmonc dstorton on dstrbuton systems, IEEE Transactons on Industry Applcatons, vol. 3, no., pp. 476-484, Aprl 1994. [] IEC 61--: Electromagnetc compatblty (EMC) - Part -: Envronment - Compatblty levels for low-frequency conducted dsturbances and sgnallng n publc low-voltage power supply systems, Second edton, Internatonal Electrotechncal Commsson, Geneva, Swtzerland, March. 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