00010008

(1) [email protected]
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.

2
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 Ampere-
turn 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

3
 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 auto-
transformers 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
Fig. 3. Case of feeding single phase HV load
a) Without tertiary b) With tertiary
Fig. 4. Homopolar impedance of autotransformer
Single phase
Load
ILoad1=0
I’Load=0
Id=Ii=Io=ILoad1/3

4
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.
Fig. 5 Case of autotransformers without tertiary winding
∑фi< 3фo
V0
I0
I0
I0
j0
j0
j0
I0’
I0’
I0’
J0/kT

5
Fig. 6 Single phase load supplied by autotransformer not equipped with tertiary winding
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
Country
Operator
Country
ONE
MAROC
GECOL
LYBIE
STADTWERKE FLENSBURG
GERMANY
TEAŞ & TEK
TURKEY
ELECTROSUL
BRASIL
INGENDESA
CHILIE
ISA
COLOMBIA
ICE
COSTA RICA
CDE
DOMINIC REPUBLIC
EELPA
ETHIOPIA
CLEMESY/KYRGHYSTAN
KYRGYSTAN
EURO TECHNO
LIMITED
KYRGYSTAN
TNB and SARAWAK
MALYZIA
UNION FENOSA
SPAIN
QVC
QATAR
WAPDA
PAKISTAN
NPC and HYUNDAI
PHILIPPINE
PC2 and EVN
VIETNAM
DEWA
UAE
0,0%
2,0%
4,0%
6,0%
8,0%
10,0%
5
10
15
20
25
30
35
40
Zo=3Zd
Zo=Zd
Zo=0.5Zd
HV Network short circuit power "Scc/Sn"
Single phase short circuit current variation (%)