A New Protection Technique Against Overvoltages in Low-Voltage Electrical Networks. Experimental Results

Transcription

1 160 A New Prtectin Technique Against Overvltages in Lw-Vltage Electrical Netwrks. Experimental Results Mircea TĂMAŞ, Emil MERDAN and Călin MUNTEANU Abstract: This paper prpses a new apprach ver the electrical prtectins in lw-vltage netwrks, starting with TDP (0.4 kv transfrmer pint distributin bard) and ending with the custmer s branch. This paper utlines the practical results btained in real wrking cnditins tests. 1. Intrductin In a new apprach f the prtectins against industrial frequency ver vltages in lw-vltage electrical netwrks, we wrk n tw levels: A new apprach f the prtectin elements frm functinal pint f view; Detecting the prtectins that the netwrk requires and the accmplishment f these prtectins. We start frm the knwn fact that in TN 0.4 kv electrical netwrks ccurs the fllwing industrial frequency vervltages: U TOV temprary vervltage in nrmal wrking cnditin due t the size f the vltage at surce f supply s branches (MT/0.4 kv); U TOV temprary vervltage and U Δ null vervltage in unbalanced wrking cnditins due t the interruptin f the PEN cnductr in netwrk r in transfrmer pint and in case f PE cnductrs interruptin at the cnsumers that are cnnected t the netwrk. 2. The prpsed slutin Cnsidering the wrds abve results that in new cnceptin the prtectins against 50 Hz industrial frequency vervltages can be accmplished like this: Against U TOV temprary vervltages ccurred in nrmal wrking cnditin after the increase f the vltage at surce f supply s branches can be used the PMTT maxim vltage prtectin in transfrmer pint s structure; Against U TOV temprary vervltages and U Δ null vervltages ccurred in unbalanced wrking cnditin due t the interruptin f PEN cnductr PEN, PE is enugh t be used the null prtectin (PNB) at the cnsumer and PNT null prtectin at transfrmer pint. Fr the accmplishment f PNB prtectin at the cnsumers cnnected t the 0.4 kv electrical netwrks and f the maximal vltage prtectin PMTT at transfrmer pint, in new cnceptin, we suggest the fllwings: the accmplishment f BMPM and BMPT measurement and prtectin blcks at the cnsumers cnnected t 0,4 kv electrical netwrk int a new structure. the accmplishment f the prtectin blck at transfrmer pint, prvided with the fllwing prtectins: maximal vltage prtectin PMTT; null interruptin prtectin PNT; phase interruptin prtectin.

2 Vlume 48, Number 2, Slving the prblems at transfrmer pint TDP level will have a higher efficiency regarding the prtectin against human and/r animal accidents and against damages f the equipments assciated with fire ft. Any vervltage caused by unbalances will be able t be slved with the help f cntrlling the vltages n the null in transfrmer pint area and at the cnsumer s branch. Industrial frequency vervltages caused by sme lad hles r by bad adjustments are very rare (they ccur in special cnditins), s their cntrl at TDP is sufficient. The maximal vltage prtectin at all cnsumers branches is unjustified in this case. By applying the imprved prtectins will be cnsiderable reduced the expenses and cmpensatins that are supprted by Electrica units in the fllwing cases: the damage f the equipments at cnsumers due t vervltage in lw vltage electrical netwrk; accidents caused by the electrcutin f humans and animals due t accidental vltages in lw vltage installatin; lng unbalanced wrking cnditins caused by null interruptin r phase interruptin; lng wrking cnditins with ne phase bund accidentally t the earth. Als, the csts f the prtectin at the cnsumer s branch will be cnsiderable reduced by renuncing t the maximal vltage prtectin at the cnsumer, being sufficient the TDP prtectin fr the entire electrical netwrk supplied frm aerial transfrmer pint PTA; see the facts abve. In transfrmer pint area, may accidentally ccur the fllwing interruptins f null cnductrs: the interruptin between the neutral f the transfrmer in the lw-vltage part and the null utlet terminal, respectively an interruptin inside the medium/lw vltage transfrmer; the interruptin between transfrmer s terminal and the null bar r terminal f the transfrmer pint s distributin bard, TDP; the interruptin between the null bar r terminal f the TDP and the pint where the null cnductr and the grund plate are cnnected, in the transfrmer pint area. Each case f null interruptin in the transfrmer pint area is equivalent t an accidentally wrking with an islated neutral (symbl I), which has specific particularities twards the neutral bund t the grund (symbl T). In these cnditins, the unsymmetrical lad n the phases and the different equivalent impedance between phases and null cnductr can cause: vltages ver the admitted limit at the electrical receivers terminals (with values ver the admitted maximum) which determine ver currents that can destry the receivers; dangerus vervltages can ccur especially at the electrical circuits n a phase with the impedances relatively high in prprtin t the impedances n the ther phases; vltages between phases are unequally distributed n the different impedances, s in case f relatively higher impedances, the vltages have values between U r and U n (U r is the vltage n the phase and U n is the vltage between phases); the damages f the electrical equipments can be ften assciated with fire. the appearance f the dangerus vltages n the null cnductr U Δn, in case f using the null and prtectin cnductr (PEN), these vltages are transmitted t the electrical receivers carcass and electrcutin accidents can ccur if the values exceed the admitted limits; the vltage n the null cnductr U Δn can have values between 0 Ur depending n the impedances relatin. 3. Checking and recrding the wrking f the prtectin device designed Testing the wrking f prtectin blck at null and phase interruptin, BPNTT-1

3 162 maximal vltage prtectin and BPNA-1 prtectin blck fr accidental currents signalizing, in this wrk, cnsist in checking the wrking f the prtectin assembled in real explitatin installatin and cmparing it with the netwrk pattern. The interrupted phase tests, accidental currents thrugh earth binding installatin have been achieved n a netwrk with custmers cnnected t it. The null interrupted test are dangerus, therefre they have been achieved n a real netwrk but with nly ne cnsumer cnnected t it. This cnsumer has been frmed by cmbinatin f equivalent impedance thrugh resistive, inductive and capacitive elements. The tests have been achieved in installatin that belng t Electrica Satu Mare - Satu Mare Explitatin Center at PTA 5612 Ardud Clny, situated in Ardud city. One f the situatin analyzed, is that with a resistive cnsumer at the end f the electrical netwrk. The recrd presented is fr the cde in the scheme belw (unbalanced diphase lad). 4. Final cnclusins The slutin prpsed in this paper, is cnceived as a necessity demanded by the reality frm energetic installatins. Practically, until this phase has been a fcus n strict necessities at cnsumer, neglecting Fig. 1. The mnfilar electric scheme f PTA 5612 Ardud Clny with BPNTT-1 and BPNA-1 prtectin blck. the events and their effects in the prtin situated upstream. The new slutin prpses a new view ver the prtectins in lw-vltage electrical netwrk cnsidering the necessities in electrical installatin and the technlgical slutins that can be achieved. Cnsidering these general principles Fig. 2. The null interruptin with resistive lad electric scheme.

4 Vlume 48, Number 2, Fig. 3. Wrking f BPNTT-1 at null interruptin with unsymmetrical resistive lad. results that in new cnceptin the prtectins against 50 Hz industrial frequency vervltages can be achieved: Against vervltages at surces f supply s branches the maximal vltage prtectin in transfrmer pint structure is used. Against temprary and null vervltages ccurred in unbalanced wrking cnditin caused by null cnductr interruptin the null prtectin at cnsumers and the null prtectin at transfrmer pint is sufficient. Slving these prblems at TDP level will have a higher efficiency regarding the prtectin against human and/r animal accidents and against damages f the equipments r fires. By applicatin f the imprved prtectins will be cnsiderably reduced the expenses and cmpensatins which are supprted by Electrica units in the fllwing cases: the damage f the equipments at cnsumers due t vervltage in lw Fig. 4. The implemented prtectin device.

5 164 vltage electrical netwrk; accidents caused by the electrcutin f humans and animals wing t accidental vltages in lw vltage installatin; lng unbalanced wrking cnditins caused by null interruptin r phase interruptin; lng wrking cnditins with ne phase bund accidentally t the earth. Als, the csts f the prtectin at the cnsumer s branch will be cnsiderable reduced by renuncing t the maximal vltage prtectin at the cnsumer, being sufficient the TDP prtectin fr the entire electrical netwrk supplied frm the transfrmer pint. References 1. Sufrim Mauriciu and thers Mdern ways f prtectin against accidental vltage in lwvltage electrical installatin. Oradea: 2000; 2. Traian G. Inescu, Olga Pp Electrical energy distributin system enginery. Technical Publishing, Bucharest-1998; 3. Edith Clarke - Circuit analysis f A-C pwer systems. Technical Publishing, Bucharest; 4. Electrical installatin manual Schneider Electric Technics cllectin; 5. Technical cde f electric energy distributin netwrks ANRE decisin number 101/2000; Mircea TĂMAŞ Emil MERDAN SC Electrica Transilvania Nrd SA Ilie Macelaru 28A, Cluj-Napca, Rmania Călin MUNTEANU Department f Electrtechnics, Technical University f Cluj-Napca Baritiu 26-28, Cluj-Napca, Rmania