ABOUT TERTIARY WINDING INDISPENSABILITY CASE OF TUNISIAN TSO POWER AUTOTRANSFORMERS A. ABADLIA(1) A. SELMI(2) F. GHODBANE(3) M. ELLEUCH(4) (1),(2) Société Tunisienne de l’Electricité et du Gaz (STEG) - Tunisia (3),(4) University of Tunis El Manar, ENIT-L.S.E.-BP 37-1002 Tunis le Belvédère, Tunisia SUMMARY This paper sums up work carried at STEG (Tunisian company of the electricity and the gas) regarding technical specifications dealing with autotransformer to be installed at STEG transmission network. These technical specifications require systematically power autotransformers with tertiary winding; while it is unnecessary and sometimes harmful. Specific features of STEG transmission network allows avoiding this tertiary winding requirement. This work shows that the obvious mentioned requirement, extended over decades, is technically unfounded. Moreover, analysis of international standards show that they do not require this additional winding; but they indicate particular cases requiring its use. Updating technical specifications will allow significant reduction of the costs of these devices. KEYWORDS Autotransformer- winding- Tertiary – Sequence - Homompolar- Earth- Short circuit. (1) [email protected] 1. INTRODUCTION Any electrical network consists of cascades of structures operating at different voltage level, and interconnected. Thus, energy produced by power plants is delivered to subscribers through these cascades of structures, while undergoing numerous voltage level adaptations. In normal diet and balanced operation, network involves only positive sequence of current and voltage. For cons, in case of unbalanced or faulted diet, all sequence components are involved in the network. Transformers must be able to withstand electrical and mechanical stresses generated during abnormal conditions. Network earth faults are tagged by the zero-sequence component which is among the most severe constraint applied to power transformers. Indeed, the positive and negative components of currents will be systematically transferred between networks connected to transformers. But transfer of zero sequence component of current remains dependent on transformers vector group and grounding. However, transformers should have a vector group allowing the transfer of zero sequence current between networks connected to it. Transformers may include two or three coils operating at different voltage levels; namely primary, secondary and tertiary winding. Technical specifications of Tunisian Electric Utilities "STEG" require systematically power autotransformers fitted with tertiary winding delta connected and sized to one third of rating power of the autotransformer, in one hand. In the other hand, international standards foresee tertiary winding only for particular applications. These obvious applications do not correspond to the case of STEG. This paper aims to understand the use of tertiary winding and determine its appropriateness. In fact, this winding constitutes an unjustified over-cost and affects "STEG" network performance and reliability; as it increases single phase short circuit. 2. ASSESSMENT OF THE NEED OF TERTIARY WINDING In the following, we will present the reasons leading to require autotransformers equipped with tertiary winding, on the one hand. On the other hand, we will check if these reasons are applicable to autotransformers already in operation and those to be installed on the transmission network of STEG. 2.1 Cases of autotransformers requiring a tertiary winding The autotransformer presents the specificity of common neutral between its primary and secondary coils; therefore the problem of transferring homopolar current does not arise. Power Transformers have to be provided with tertiary winding, delta connected, to match following requirements: To feed with electrical energy an auxiliary transformer or a bus-bar connected to a medium voltage distribution network, In cases of transformer cannot insure transferring homopolar current, under Ampereturn balance, additional delta connected equalizer winding will be required. This tertiary winding will serve to constitute a path to homopolar current (Fig. 1). This situation concerns only YNy or Yyn transformer. If the homopolar current is not transferred under Ampere turn balance "Fig. 2,", the zero sequence voltage, developed in network, will be fully applied to the magnetization branch of transformer, resulting in an excessive heating ; due to homopolar fluxes flowing through the transformer tank. Fig1. Transfer of homopolar current Fig.2. Homopolar current being not transferred 2 To allow the feeding of a high-voltage single-phase load connected to a transformer having a vector group : Yyn or YNy. Only in this case, the tertiary must be sized to one third of the power of transformer [1]. The single phase load involves the three sequence components of current with same RMS value. If the transformer is not equipped with tertiary winding, the zero sequence current won't be compensated, thereby leading to prevent the flow of load current " Fig. 3,". Added tertiary winding provides a path to zero-sequence current and only positive and negative sequence current will be transferred between networks through transformer. Reduce the zero sequence impedance of the connected system and thereby its earth fault factor. A consequence is that prospective earth fault current increases. The autotransformer zero sequence impedance is represented by Figures " Fig. 4a," and " Fig. 4b,", respectively with and without tertiary winding [2], [3] and [4]. Z12, Z23, Z13 designate series impedance respectively between primary-secondary, secondary-tertiary and primary-tertiary windings As result, autotransformer zero sequence impedance is equal to : Z12 or 0.92*Z12, respectively for the cases of autotransformer with or without tertiary winding. Hence the presence of the tertiary winding reduces the zero-sequence impedance with approximately 8%. 2.2 Case of STEG network The STEG transmission network is made of interconnected grids operating at different voltage levels: 400 kV, 225 kV, 150 kV and 90 kV. Grids interconnection is achieved by autotransformers having a common neutral between primary and secondary windings. Consequently, the whole sequence components of current, including the zero sequence one, are transferred between grids. As for medium-voltage networks, it's supplied by HV/MV power transformers having a vector group: YNd11. As consequence, only positive and negative components of current are ILoad1=0 Single phase Load I’Load=0 Id=Ii=Io=ILoad1/3 Fig. 3. Case of feeding single phase HV load a) Without tertiary b) With tertiary Fig. 4. Homopolar impedance of autotransformer 3 transferred from distribution network to transmission one. Unlike positive and negative sequence components, the zero-sequence current wont never be transferred to the transmission network ; Indeed it's trapped through an earthing transformer designed to create the MV artificial neutral. The HV/MV transformers have the particularity to be represented in homoplar mode with their short circuit impedance, during transmission network ground fault. Thus, they ensure the transfer of homopolar currents, under ampere-turns balance, between networks connected to autotransformers. To justify the need of tertiary, for the case of STEG network, we will check if any of the requirements mentioned above is expressed: Transfer of Homopolar current: Case of autotransformers without tertiary winding. The single line diagram of figure " Fig. 5," represents an autotransformer not equipped with tertiary winding and connected to transmission network This scheme shows that the autotransformers can transfer homopolar currents between the networks to which they are connected. This is due to the transformers specificity HV/MV, who presents their short circuit impedance during homopolar mode. Besides distribution networks 30, 15 and 10 KV are fed by HV/MV transformers having group vector: YNd11. This type of transformers ensures transferring of homopolar currents between the networks connected to autotransformers. However, they substitute tertiary winding, from this point of view. Feeding external networks In trial to do not impede normal operation of autotransformers, by distribution network outages, STEG has opted to discern between HV and MV networks. However, STEG does never uses autotransformers tertiary winding to supply MV bus-bars. Case for supplying a high voltage single phase load Transformers Yyn, neither YNy are never used in transmission network. But only transformers YNy type are used in power plants as ultimate solution for auxiliaries supply and are equipped with equalizer winding. Furthermore, auto-transformers are characterized by a common neutral between primary and secondary coils. Hence, they are treated as YNyn transformers "Fig. 6," Consequently, autotransformers can be used to supply a high voltage single phase load, without any recourse to the tertiary winding. ∑фi< 3фo J0/kT I0’ I0 I0’ I0 I0 I0’ j0 j0 j0 V0 Fig. 5 Case of autotransformers without tertiary winding 4 Fig. 6 Single phase load supplied by autotransformer not equipped with tertiary winding 10,0% Single phase short circuit current variation (%) Zo=3Zd 8,0% 6,0% Zo=Zd 4,0% 2,0% Zo=0.5Zd HV Network short circuit power "Scc/Sn" 0,0% 5 10 15 20 25 30 35 40 Fig 7. Simulation of tertiary winding effect on single phase short circuit current Effect on short circuit current level: Results of short circuit current simulation [5], with and without tertiary, are illustrated in Figure 7. Simulations were performed for a network short-circuit power; within a range from 5 to 41 times the rated power of the autotransformer, and network’s zero sequence impedance ranging from 0.5 to 3 times its direct impedance. These results show that increasing of short circuit current, by adding tertiary winding, cannot exceed 9%, in case of a low source impedance ratio; i.e. a source characterized by lower short circuit, and a zero-sequence impedance around three times its direct impedance. For the case of interconnected networks of STEG, the effect of adding tertiary winding on increasing single phase short circuit current, cannot exceed 3%; despite it generates a reduction on zero sequence impedance of the autotransformer of 8% It should be noted that single phase short circuit current occurring in transmission network, has already exceeded the maximum tolerated limit value. for instance, at Goulette substation, single phase earth fault current has reached 47,4 kA during outage occurred at autotransformer (Fig. 8a); thus generating damage of material (Fig. 8b), designed to withstand 25 kA 3. BENSHMARKING AND FAISABILITY The objective is to compare our technical decision with other global companies; to be inspired and reassured This comparison allows awareness regarding best practices, and helps for its adaptation. TABLE I lists companies around the world, already using autotransformers without tertiary Table I. References of using autotransformer without tertiary winding Operator ONE STADTWERKE FLENSBURG ELECTROSUL ISA CDE CLEMESY/KYRGHYSTAN Country MAROC GERMANY BRASIL COLOMBIA DOMINIC REPUBLIC KYRGYSTAN TNB and SARAWAK QVC NPC and HYUNDAI DEWA MALYZIA QATAR PHILIPPINE UAE Operator GECOL TEAŞ & TEK INGENDESA ICE EELPA EURO TECHNO LIMITED UNION FENOSA WAPDA PC2 and EVN Country LYBIE TURKEY CHILIE COSTA RICA ETHIOPIA KYRGYSTAN SPAIN PAKISTAN VIETNAM 5 a) b) Fig. 8. GOULETTE substation a) Recorded short circuit current b) Damaged 90kV bushing and lightning arrestor 4. EXPECTED GAIN VERSUS RISK From financial point of view, avoiding additional tertiary windings, prevents STEG treasurer a waste of one million Euros; it looks around two million of dinars. Extension of requiring tertiary winding, over the past decades, may lead someone to suspect that its elimination will generate risks. For this, we propose to compare the expected gain to likely risk. Table II shows barrier ensuring that cumulated weighted cost of risk remains less than the expected gain. Discounting rate and accounting life are respectively 6,21% and 25 years. No risk can be attributed to the lack of tertiary; consequently its elimination is estimated 124 thousand dinars perpetuity Table II. Barrier of Annual risk cost favoring elimination of tertiary winding Probability of Barrier of Annual risk cost occurrence: P(Rn) P(Rn)*R(kDT) 100% 160 90% 177 80% 199 70% 228 60% 266 50% 319 40% 399 30% 532 20% 798 10% 1 596 5. CONCLUSION This work shows that the autotransformers to be installed on STEG transmission network do not require any tertiary winding ; for the following reason: The transfer of homopolar currents, through autotransformer, is ensured by HV/MV transformers; characterized by their vector group YNd11, Only transformers having vector group YNy or Yyn, and at a time supplying single phase high voltage level, require a tertiary winding sized to one third of their rating power. Such kind of transformer is not used in STEG transmission network 6 Single phase short circuit current level has exceeded the maximum allowable limits; generating however significant damage on transmission network equipment. Consequently, no additional tertiary winding has to be foreseen, for future autotransformer; to prevent increasing improperly short circuit current. Indeed, it is rather timely to act for reducing the single-phase fault current level, Distribution networks is not supplied from tertiary winding ; in order to prevent impeding STEG transmission network operation, resulting from medium voltage equipment failures. Moreover, the unfounded requirement of tertiary winding generates an unjustified additional cost; estimated at slightly more than two million dinars, for the twelfth equipment plan where 6 new autotransformers are foreseen. BIBLIOGRAPHY [1] IEC International Standard 'Power Transformers - Application guide' IEC 60078-8-1997. [2] Dalibor Filipovic, Bozidar Filpovic, Kosyenko Capuder, "Modeling of three-phase autotransformer for short-circuit studies", International Journal of Electrical Power Energies Systems, Volume 56, March 2014,page 228-234. Elseiver. [3] G.A.CIVI djian, Gh calin, Dorin Papa, Alin Dolan, "Impedance voltage of power multiwinding autotransformer", Plovdiv. Siela 2005.doc [4] Vladimir Volcko, Zaneta ELESCHOVA, Anton Belan, Peter Janiga, Dominik Viglas, Mikroslava Smikkova, "Modeling of power autotransformer" Recent advances in energy, environement and economic development”, ISSBN. 978-1-61817-138-5. [5] Liu Jingyan, Liu Wenying, and Li Xiaorong, "Method for restricting GridbSingle-phase Short Circuit Current", International Journal of Computer and Electrical Engineering, Vol.5, N°2, April 2013. 7
