TRANSFORMERLESS INVERTERS AND RCD: WHAT'S THE PROBLEM?

Transcription

1 25th Europen hotovolti Solr Energy Conferene nd Exhiition / 5th World Conferene on hotovolti Energy Conversion, 6- Septemer 2, Vleni, Spin TRASFORMERLESS IVERTERS AD RCD: WHAT'S THE ROBLEM? T. Trn-Quo, H. Colin 2, C. Duvuhelle 3, B. Giddon 4, C. Kieny, C. LE Thi Minh, S. Bh 5, S. Aissnou 5, G. Moine 6, Y. Tnguy 6 - IDEA G2el, B. 46, 3842 Sint Mrtin d Hères, Frne 2 - IES/CEA, B 332, 5 venue du l Lémn, F Le Bourget-du-l, Frne 3 - EDF R&D -, venue du Générl de Gulle 924 Clmrt Cedex - Frne 4 - HESUL, 4 oulevrd du novemre 98, F-69 Villeurnne, Frne 5 - G2el, B. 46, 3842 Sint Mrtin d Hères, Frne 6 - TRASEERGIE SA, 3D, llée Clude Deussy, 693 Eully, Frne ABSTRACT: Considering the struture of V systems, stry pitne n pper etween the V rrys nd the ground. When trnsformerless inverters re used, this pitne n use lekge urrents to the ground. Aording to the Frenh stndrds, Residul-Current Devie (RCD) hs to e instlled t the AC side of the V instlltion, for the protetion of individuls. Yet, when the vlue of the lekge urrents rehes threshold (3 ma in homes in Frne), the RCD my swith off nd unintended disonnetions of the V instlltion our with ompnying prodution losses. Bsed on simultions nd experimentl tests, this projet ims t giving informtion to V system designers nd inverter mnufturers out the est suitle type of RCD to use for severl V system onfigurtions. This issue is relevnt for ountries with TT grounded networks like Frne where mny opertions of V systems with unexplined disonnetions of RCD hve een reported. ITRODUCTIO When trnsformerless V inverters re used, the stry pitne etween the V rrys nd the ground n use lekge urrents to the ground (Fig. ): - These lekge urrents flow from the onnetion of the V struture to the ground (neessry for sfety resons nd to prevent lightning dmges) - The stry pitne is formed from the module eletrilly tive lyers nd the surrounding metlli strutures [], thus the pitne mgnitude will depend on prmeters suh s the module surfes, the distne etween the eletri hrges nd metlli strutures, nd the nture of the insultion mteril - The pitive urrent is reted from this stry pitne nd the lternting voltges of polrities - Trnsformerless inverters do not isolte the DC from the AC side, nd llow the urrent to irulte vi the ground onnetions nd through the inverter Aording to the Frenh stndrds, Residul- Current Devie (RCD) hs to e instlled t the AC side of the V instlltion, for the protetion of individuls. Yet, when the vlue of the lekge urrents rehes threshold (3 ma in homes in Frne), the RCD disonnets the V instlltion. TT grounding systems nd RCD re topis tht hve not een explored in detiled so fr. The purpose of this study, whih is prt of reserh projet funded y the Frenh Ageny for Environment nd Energy Mngement (ADEME), is to fully hrterise pitive dishrge urrents tht our with trnsformerless inverters, in order to determine whih type of RCD should e used to design sfe nd effiient V systems. This pper presents modelling tht hve een undertken so fr s well s detils on experimentl tests tht hve een done with the intention to hve etter understnding on tht issue, in prtiulr: - Theoretil: simultions investigting the influene of different prmeters on the lekge urrent (topologies of inverters, types of V modules, resistne to the ground,...) V pnel - Experimentl: tests with different types of inverters nd RCDs to quntify the lekge urrents nd study the disonnetion tions. Inverter Grid Ground Figure : Shemti of lekge urrent ording to the grounding system in Frne 2 WHY TO USE RESIDUAL CURRET DEVICES For V system, it is neessry to tke two different mesures of protetion: - Bsi protetion: protetion ginst diret ontts, - rotetion ginst fults: protetion in se of isoltion fult etween live ondutor nd the ground (indiret ontt). In Frne, F C 5- (or IEC 6479-) stndrd requires the instlltion of protetive devies tht will over those risks. The use of these devies will nturlly led the neutrl grounding sheme. For TT grounded networks (suh s in Frne), this devie is the si unit of the protetion of persons, its use is mndtory to ensure sfety throughout the eletril instlltion. The RCD is protetive devie tht monitors the residul urrent resulting from the vetor sum of urrents within ondutors. In norml ondition, the sum of the urrents of ll ondutors (phse neutrl ground) is zero [8]. The RCD is defined y the IEC 6755 interntionl stndrd whih provides different types of protetions, s well s disonnetion threshold or sensitivities: - Clss AC (sinusoidl lternting urrent) 4554

2 - Clss A (sinusoidl lternting urrent or pulsed DC omponent) - Clss B: this RCD is devie designed to protet networks with retified lterntion nd filtered y pitive lod - Clss HI (High Immunity): RCD used for eletronis or for sensitive devies. 3 AROACH This projet ims t giving onrete nswers to V system designers nd inverter mnufturers out the est suitle type of RCD to use for severl V system onfigurtions. This issue is relevnt for ountries with TT network like Frne where mny opertions of V systems with unexplined disonnetions of RCD hve een reported. This uses loss of prodution, loss of finnil enefit for the owner of the instlltion, loss of onfidene in the V tehnology nd most of ll sfety prolem sine system owners my e tempted to suppress the RCD or inrese the disonnetion threshold to higher vlue, generting potentilly dngerous sitution. T: Full-ridge inverter, no DC/DC onverter, without trnsformer V rry This prolem of lekge urrents hs lredy een investigted in the literture [-7]. Experimentl tests done within previous reserh projets suh s [9] onluded tht the lekge urrent mgnitude minly depends on the inverter typology ut without ny reommendtion onerning the type of RCD tht should e used to mke V systems perform well nd sfe. First, simultions hve een performed with the help of EMT-RV softwre in order to identify the ftors tht influene the lekge urrents. Different types of inverters nd modules hve een designed in different onfigurtions. The influene of stry pitne vlue, pitne distriution on oth poles (DC nd DC-) nd the influene of the resistne of the neutrl (impedne to ground) re lso studied. Experimentl tests hve een then onduted on rel omponents. Severl types of inverters omined with different types of RCD (AC, A, B ) will e tested in order to mesure the mximl dmissile pitne efore the trigger tion of the RCD nd then to determine the est suitle type of RCD for eh onfigurtion. T2: Full-ridge inverter, DC/DC oost onverter, without trnsformer V rry T3: Hlf-ridge inverter, DC/DC oost onverter, without trnsformer V rry?s T4: Full-ridge inverter, DC/DC forwrd onverter, with HF trnsformer V rry?i D D3 25th Europen hotovolti Solr Energy Conferene nd Exhiition / 5th World Conferene on hotovolti Energy Conversion, 6- Septemer 2, Vleni, Spin Tr_ Tr_2 D2 D4 T5: Full-ridge inverter, DC/DC full-ridge onverter, with HF trnsformer V rry R2 D D3 T6: Full-ridge inverter, no DC/DC onverter, with LF trnsformer V rry LF Trnsf. Tr_ 2 D2 D4 2 Figure 2: Different inverter models developed with EMT-RV for studying the lekge urrent 4555

3 25th Europen hotovolti Solr Energy Conferene nd Exhiition / 5th World Conferene on hotovolti Energy Conversion, 6- Septemer 2, Vleni, Spin LV2 LV3 LV4 LV5 2 kv etwork 2/.42 LV LV7 LV6 L6 V-3kW LV9 LV4 LV8 LV LV LV2 LV3 V V V V Figure 3: LV network with single phse V simulted with EMT-RV Finlly, solution tht llows to differentite the pitne lekge urrent from the fult urrent is proposed. This solution n e used to void the undesirle disonnetion of V y RCD. Suggestions nd solutions for reduing the urrent mgnitude re lso investigted. 4 MODELLIG AD SIMULATIO 4. Modelling In this prt, severl models of inverter were developed in EMT-RV in order to study the influene of inverter typology on lekge urrent (see Fig. 2): - Full-ridge inverter, no DC/DC onverter, without trnsformer (Typology T.) - Full-ridge inverter, DC/DC oost onverter, without trnsformer (Typology T.2) - Hlf-ridge inverter, DC/DC oost onverter, without trnsformer (Typology T.3) - Full-ridge inverter, DC/DC forwrd onverter, with HF trnsformer (Typology T.4) - Full-ridge inverter, DC/DC full-ridge onverter, with HF trnsformer (Typology T.5) - Full-ridge inverter, no DC/DC onverter, with LF trnsformer (Typology T.6). Then, model of rel LV network with V system hs een modelised (Fig. 3). This LV network is supplied y 2/.4 kv trnsformer of 6 kva with 4 uses, lods nd V system (3 kw). A three phse lod is onneted t us 3 nd single phse lods re onneted to other uses. The influene of stry pitne vlues, pitne distriution on oth poles (DC nd DC-) nd the influene of the resistne of the neutrl network (impedne to ground) on the lekge urrent re studied. 4.2 Simultions Tle I: Lekge urrent mgnitude ording to inverter typology (Module pitne: µf nd R_erth = 3 Ω) Topologies T : Full-ridge inverter no DC/DC onverter- trnsformerless T2 : Full-ridge inverter DC/DC oost onverter - trnsformerless T3 : Hlf ridge inverter DC/DC oost onverter - trnsformerless T4 : Full-ridge inverter DC/DC Forwrd onverter HF trnsformer T5 : Full-ridge inverter DC/DC full-ridge onverter HF trnsformer T6 : Full-ridge inverter no DC/DC onverter LF trnsformer Lekge urrents (ma) 38 The simultions show tht for 3 kwp V system using trnsformerless inverter, the lekge urrent ould reh 39 ma (Tle I nd Fig. 4), ording to the hypothesis defined

4 25th Europen hotovolti Solr Energy Conferene nd Exhiition / 5th World Conferene on hotovolti Energy Conversion, 6- Septemer 2, Vleni, Spin Current (A ) i_rcd i_rcd_rms t (ms) Figure 4: 39 ma lekge urrent otined y simultion for T2 typology Results otined from simultions indite tht the lekge urrent strongly depends on the inverter typology: - for inverter with trnsformer, lekge urrent is negligile (Topologies T4 to T6); - for trnsformerless hlf ridge inverter, lekge urrent is very smll (9mA, T3) - So we note tht lekge urrents re not neessrily linked with ll trnsformerless inverters, indeed proper hoie in the inverter typology will strongly redue this urrent. This omes from the redued voltge vrition mplitude of the polrities. - for trnsformerless inverter, with V rry pitne of,8 µf, the lekge urrent n use the trigger tions of the RCD (Tle II). Voltge (V) VDC_eg VDC_os t (ms) Figure 6: Voltge on DC nd DC- of trnsformerless inverter The lekge urrent does not depend on the distriution of pitne on oth positive nd negtive polrities (Tle III). This urrent depends on the grid grounding system resistne: in dmp wether onditions this resistne rehes very low vlues, nd this n led to the disonnetion of the RCD. In our simultions, this vrition is reltively smll (% vrition when the erth resistne vries from Ω to 3 Ω). But sometimes, this vrition is signifint nd n led to disonnetion of the V system. Simultions hve een rried out with onstnt.7µf pitne nd with severl erth resistne vlues. While Ω erth resistne doesn t generte ny ritil lekge urrent, n erth resistne vlue lower to Ω genertes lekge urrent upper to the limit of 3 ma (see Tle IV). Tle II: Influene of V module pitne C (µf) i_lekge (ma) Tle III: Influene of reprtition of V pitne on 2 polrities C (µf) i_lekge (ma) V DC (.5) & V DC- (.5) 39 V DC () & V DC- () 39 V DC () & V DC- () 39 Figure 5: Hrmonis of lekge urrent Fig. 5 shows the result of the hrmoni nlysis (FFT) for wve form of the lekge urrent shown in Fig. 4. It shows tht the fundmentl omponent (5Hz) is very importnt while other hrmonis re very smll. Fig. 6 shows the voltge on oth poles of DC prt of trnsformerless inverter (DC nd DC- ; output of V rrys for T2 typology see Fig. 2). We n notie tht we hve n importnt fundmentl omponent while this urrent is diret. The other hrmonis re due to the RLC iruit in the system. As the lekge urrents hve just little DC omponents, stndrd AC RCD type should orretly protet trnsformerless-sed V systems. Tle IV: Influene of the neutrl erth resistne (.7µF) R_erth (Ω) i_lekge(ma) Conlusion of the simultions The simultions show tht in generl ses, there is no prolem of lekge urrent; this is why the proility of disonnetion of RCD is very smll. 4557

5 25th Europen hotovolti Solr Energy Conferene nd Exhiition / 5th World Conferene on hotovolti Energy Conversion, 6- Septemer 2, Vleni, Spin In se where the lekge urrent is importnt (> 3 ma), following solutions n e rried-out: - Using inverters with trnsformer or trnsformerless hlf ridge inverter - Using V mono or polyrystlline modules (these modules with smll try pitne) - Using n dvned ontrol mode inverter. 5 EXERIMETAL TESTS Bsed on the theoretil results otined so fr, severl experimentl tests hve een onduted with rel system omponents (inverters nd RCD) in order to hve onrete evlution of lekge urrents nd see how RCD operte in V system. Suh experimentl tests took ple in lortory fility equipped with different types of trnsformerless inverters ville on the mrket nd most ommon RCD types : AC, A, B, (Fig. 7). Three types of trnsformerless inverters re used for these experimentl tests: - Inverter # : the sme s T in Fig. 2 - Inverter # 2: the sme s T in Fig. 2 with H5 topology - Inverter # 3: the sme s T2 in Fig. 2. Tests re undertken to give empiril nswers on 3 topis : - The RMS lekge urrent vlue for different trnsformerless inverters with regrd to the power delivered, - The ehviour of severl types of RCD in se of pitive lekge urrent, - The impt of using high immunity RCD on humn sfety in se of diret ontt with live ondutor. All inverters were supplied y 5 kwp V rry of polyrystlline type. Their nominl power is rnging from 3,8 to 4,6 kva. The results presented in Figure 8 show in this system onfigurtion nd the ville test onditions tht: - The lekge urrent vlue is rther proportionl to the inverter power ( to 2 ma/kva), - Extrpolted vlue for nominl power led to mximum of 9,4 ma for inverter #2: RMS lekge urrent (ma) Lekge urrents versus power Linéire (Inverter 3) Inverter power (W) Inverter Inverter 2 Inverter 3 Linéire (Inverter 2) Linéire (Inverter ) Figure 8: Lekge urrent versus inverter power 5.2 Behviour of RCD The ojetive of this experimentl test is to nlyse the ehviour of most RCD types ville in se of lekge urrent. For this test, 4 different RCD types re onneted in string to see whih RCD first trips for different levels of lekge urrents (see Fig. 9). The vlue of the lekge urrent is set y inresing mnully the pitne vlue (see Fig. 7). Figure 9: Severl RCD types under test Figure 7: Inverter under test with pitne etween the positive nd negtive poles nd the ground to inrese the lekge urrent 5. RMS lekge urrent The ojetive of this experimentl test is to quntify the RMS lekge urrent vlue of three trnsformerless inverters ville on the mrket. First results show tht: - The order of disonnetions of the RCD my vry (ording to the onditions of testing), - RCD s ehviour strongly depends on the inverter: for instne, inverter #3 is very sensitive nd disonnets efore ny tion of RCD; for the two others, the disonnetion sequene is different from eh other. 4558

6 25th Europen hotovolti Solr Energy Conferene nd Exhiition / 5th World Conferene on hotovolti Energy Conversion, 6- Septemer 2, Vleni, Spin 5.3 Lekge urrent nd humn sfety The third test dels with humn sfety. The inverters re tested with the most immunised RCD (i.e. the one disonneting for the highest vlue of urrent). A virtul diret ontt is simulted with the help of resistne pled in series with the vrile pitors (set to the highest vlue efore tripping), either on DC or DCpole, to hek if the opertion nd tripping of the RCD omplies with humn sfety requirements. Here s well, results depend on the inverter type: with inverter #3, whih detets the defult, RCD trips in se of diret ontt nd would thus perform its duty; with inverters # nd #2, RCD s ehviour differs depending on whether the resistne is onneted on DC or DC-: in prtiulr sitution no tion is oserved, whih would endnger humn sfety. 6 COCLUSIOS This study imed t giving some hints to V system designers nd inverter mnufturers out the est suitle type of RCD to use for severl V system onfigurtions. This issue is relevnt for ountries with TT grounded networks like Frne where mny opertions of V systems with unexplined disonnetions of RCD hve een reported. The simultions mde within this projet showed tht in generl ses, there is no prolem of lekge urrent; this is why the proility of disonnetion of RCD is very smll. In se where the lekge urrent is importnt (> 3 ma), following solutions n e rried out: - Using inverters with trnsformer or trnsformerless hlf ridge inverter - Using V mono or polyrystlline modules (these modules hve smll stry pitne) - Using n dvned ontrol mode inverter. 7 REFERECES [] J.C Hernndez,.G. Vidl, A. Medin, Chrteriztion of the insultion nd lekge urrents of V genertors: Relevne for humn sfety, Renewle Energy, ELSEVIER, o. 35, 2 [2] Lin M, Fen Tng, Fei Zhou, Xinmin Jin nd Yiin Tong, Lekge urrent nlysis of single-phse trnsformer-less V inverter onneted to the grid, IEEE Conferene, 27 [3] Roerto González, Jesús López, lo Snhis, nd Luis Mrroyo, Trnsformerless Inverter for Single- hse hotovolti Systems, IEEE Trnstion on ower Eletronis, Vol. 22, o. 2, Mrh 27 [4] Roerto González, Eugenio Guí, Jesús López, nd Luis Mrroyo, Trnsformerless Single-hse Multilevel-Bsed hotovolti Inverter, IEEE Trnstion on industril Eletronis, Vol. 55, o. 7, July 28 [5] Kerekes, T.; Teodoresu, R.; Borup, U Trnsformerless hotovolti Inverters Conneted to the Grid, AEC 27 onferene, Ferury 27, Anheim, Cliforni USA [6] Osr Lopez, Frniso D. Freijedo, Alejndro G. Yepes, lo Fernndez-Comesn, Jno Mlvr, Remus Teodoresu, nd Jesus Dovl-Gndoy, Eliminting ground urrent in trnsformerless photovolti pplition, IEEE Trnstion on power onversion, Vol. 25, mrh 2 [7] Cournts de déhrge pitifs - Remrques onernnt l oneption des onduleurs sns trnsformteur, Informtion tehnique, SMA [8] Shneider Eletri, L protetion différentielle dns les instlltions életriques sse tension, Inter Setions, mgzine de Shneider Eletri de l enseignement tehnologique et professionnel [9] DISOWER projet, Identifition of generl sfety prolems, definition of test proedures nd designmesures for protetion, 26 The first experiments lerly showed the omplexity of the study s results depend on inverter types nd testing onditions. They lso emphsized the relevne of this topi regrding prodution sustinility nd sfety. In the ontinution of the projet, further experimentl tests will e rried out in order to omplete the first results. 4559