00010001

(1) [email protected]
EFFECT OF POWER TRANSFORMER NEUTRAL GROUNDING ON
HIGH VOLTAGE NETWORK SINGLE PHASE SHORT CIRCUIT LEVEL
A. ABADLIA(1) M. ELLEUCH(2)
(1) Société Tunisienne de l’Electricité et du Gaz (STEG) - Tunisia
(2) University of Tunis El Manar, ENIT-L.S.E.-BP 37-1002 Tunis le Belvédère,
Tunisia
SUMMARY
Authors present throughout this paper an approach for surveying single phase short-circuit
level on high voltage transmission network. The work is intended to study and quantify the
effect of grounding power transformers neutrals on High Voltage short-circuit currents level.
Simulations are carried out on a five nodes network.
Approach developed through this paper can be adapted to help solving single phase short
circuit problem already observed in Tunisian 90kV transmission network.
KEYWORDS
Transformer- Neutral- grounding – Network – Transmission - Short circuit.

2
1. INTRODUCTION
Oscillogram (Fig. 1) illustrates drained short circuit current generated by single phase
earth fault regarding occurred on 200 MVA autotransformer 225KV/90KV. Short-circuit
level has reached almost two times allowable value, generating consequently significant
damage (Fig.2).
High short circuit level is due to low value of zero sequence impedance, provided by
grounding whole power transformers neutral. Zero sequence impedance of Wye-Delta
transformer jumps to a huge larger value once its neutral is unearthed. This opportunity would
be a technical solution to mitigate single phase short circuit current level [1].
2. NETWORK MODELING AND FORMULATION
Tunisian electrical transmission network is exclusively equipped with HV/MV power
transformers; Wye-Delta connected. These latter will be however modeled, in zero sequence
system, by correspondent short circuit impedance or an almost infinite one, respectively if
neutral is grounded or not. HV network zero sequence impedance reaches its minimum level
once all transformers neutrals are grounded. This is the case of Tunisian network providing
highest presumed single phase short circuit level (1):
 (1)
With:
 V : Pre-fault network phase to neutral voltage,
 Zd, Zi and Z0: denote respectively positive, negative, and zero sequence impedance.
 ZF: fault impedance
Study is conducted on five-node network (Fig. 3), made of four power plants interconnected
through cross linking transmission lines. A generic fault node will help to consider multiple
fault location.
2.1 Modeling of network components
Transmission line could be presented by following models:
- Distributed parameter model; this is used for wave propagation studies and accurate
fault location [2].
- Π or T model; generally used for load flow computation [3].
- Series impedance model; it’s restrained for short circuit studies [2].
We will use the latter one to simulate short circuit current computation.
All HV/MV power transformers will be modeled by short circuit impedance for both positive
and negative system.
Fig. 1. Short circuit current oscillogram, Fig. 2. 90kV damaged transformer bushing
33
28
4
47
Short circuit current RMS (KA)
Time (ms)
-30 0 30 60 90
120
Phase 1
Phase 2
Phase 3
Neutral

3
Fig. 3. Studied network single line diagram Fig. 4. Symmetrical component illustration
With:
 Ucc: transformer short circuit voltage ; Un : transformer rating voltage ;
 Sn : transformer rating power
Transformer zero sequence impedance, as obviously mentioned, depends on its vector group
and neutral earthing:
- First case, neutral grounded: 
- Second case, neutral ungrounded ( zero sequence current through transformer
windings has a magnitude of magnetizing current, it will be neglected.
Regarding generators, positive and negative impedances are expressed in terms of alternator
power and voltage.
 ; 
Where:
 X’: transient direct reactance in %; U: generator rating voltage ; Sn : generator rating
power; Xd”:subtransient reactance d-axe in % ; Xq”:subtransient reactance q-axe in %
2.2 Short circuit current formulation
Faulted network can be studied by means of symmetrical components superposition [4]
(Fig. 4).
Nodes voltage and current are linked by equations’ system (2), (3) and (4):
(2)
(3)
(4)
This system could be presented by the following matrix representation:
(5)
(6)
(7)
Where:
 k=1,2, 3, 4 and F; Ek: pre-fault voltage at the terminals of kth generator.
 Vkd, Vki and Vk0: respectively positive, inverse and zeo sequence voltage at kth node.
Where :
 k=1,2, 3, 4 and F; Ek: pre-fault voltage at the terminals of kth generator.
1
3
2
4
.
.
.
.
5
3ZF
Icc

4
 Vkd, Vki and Vk0: respectively positive, inverse and zeo sequence voltage at kth node.
 Zkd, Zki, Zk0: respectively positive, inverse and zero sequence impedance of kth generator
associated to its step up transformer.
  Respectively positive, inverse and zero sequence current provided by kth
generator to supply fault.
; 
 ; 
 ;
The contribution of different plants, to short circuit current, depends on nodes voltage and
connection admittance, according to (4), ( 5) and (6).
(8)
(9)
(10)
Short circuit current flow, through network connections, is governed by (11), (12) and (13):
 (11)
 (12)
 (13)
With :
 Yknd , Ykni and Ykn0: respectively positive, negative and zero sequence admittance
connecting nodes ‘k and n’
; 
  .
Short circuit Current at fault location is but the sum of current provided by nodes directly
connected to faulty one according (14), ( 15) and (16).
(14)
(15)
(16)
This system could be presented by the following matrix representation:
(17)
(18)

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(19)
With:
  ; =
  ; =
   ; =
Resolution of equation system (5), (6), (7), (11), (12), (13), (17), (18) and (19) leads to find
out faulty nodes symmetrical voltage (20): 
 (20)
Where [M], 1, 1, 1, are detailed in Appendix.
Fault current, denoted Icc, will be then deduced by (21):
(21)
2.2 Effect of ungrounded neutral
Expressions (4) and (10), regarding nodes zero sequence current, lead to the following
equation system (22), (23), (24) and (25):
  (22)
  (23)
  (24)
  (25)
Analysis of systems (22), (23), (24) and (25) shows that fault current zero sequence could be
expressed as function of nodes voltage and admittance. (26):
 (26)
Hence, we deduct that single phase short circuit current will be null once all nodes zero
sequence admittances are null as well.
However, disconnecting all transformer neutral from the earth will mitigate single phase short
circuit level from its highest level to the lowest one.
Consequently, managing power transformer neutral grounding could be a technical solution to
survey fault current level.
3. SIMULATIONS AND INTERPRETATIONS
A program was developed in Matlab space, where all network data are specified and short
circuit current is simulated accordingly, as specified above and for different scenarios related
to transformers neutral earthing.
3.1 Network features
Considered network, object of this study, is made by four generators; each one is associated to
a step up transformer, interconnected by looped transmission network made of six
transmission lines.
Generators, transformers and transmission lines features are recorded in TABLE I: