EFFECTS OF MAGNETIC CORE CONFIGURATION AND

SATURATION IN EARTHING TRANSFORMERS

Copyright Material PCIC Europe
Paper No. PCIC Europe 25_05

Andrea Santarpia
SBM Offshore
11, Av. Albert II, 98000
Monaco
Andrea.Santarpia@sbmoffshore.com

Abstract – Resistive earthing of Medium Voltage systems With these concepts in mind, the paper is structured as
has been proven beneficial in limiting fault currents during follows: Section II reports the typical earthing scheme for
ground faults and suppressing resonance and surge FPSO installations and the sizing of earthing transformers
voltage conditions, which are crucial for safe operation. and resistors; Section III presents real cases in similar
Earthing can be achieved directly at power generators, installations where, upon earth faults, the transformers
through power transformers, or by means of dedicated failed to perform as expected, and investigates the
earthing transformers. This paper investigates the impact failures, identifying issues in the magnetic core
of magnetic core saturation in earthing transformers by configuration and saturation during earth faults that could
examining real cases on FPSO facilities where these have been detected with proper factory testing.
transformers failed to perform as expected. Analysis of
site data from disturbance recorders identified magnetic II. EARTHING SCHEME IN ALL ELECTRIC FPSO
core configuration and core saturation as key issues due
to incorrect design by the manufacturer. The findings Depending on field specificities, newly designed FPSOs
highlight the importance of rigorous engineering and may exhibit a total load demand of 70 MW to 120 MW
testing practices in the design of earthing transformers to during normal production [3]. Figure 1 shows a typical MV
ensure optimal performance. single line diagram. It includes two interconnected
systems at 11 kV: one dedicated to the Topside, with Gas
Index Terms — Resistive Earthing, Medium Voltage Generation, and the other to the Hull, with Essential
(MV) Systems, Ground Fault, Earthing Transformers, Generation (Diesel). Dedicated bus links interconnect the
Magnetic Core Saturation, Engineering Practices, Testing systems. Both systems can run, to a certain extent,
Practices, FPSO. independently.

NOMENCLATURE

MV Medium Voltage
LV Low Voltage
FPSO Floating Production Storage and Offloading
NER Neutral Earthing Resistor
ETR Earthing Transformer
IED Intelligent Electronic Device (Protection Relay)

I. INTRODUCTION

In offshore electrical installations, the safety and

reliability of the power system are fundamental. One
critical aspect of this is the effective management of Fig. 1. MV typical single line diagram
ground fault currents [1][2][4], which can pose significant
risks to both equipment and personnel. Resistive earthing The system is designed for modularity, allowing the two
of medium voltage (MV) systems has been proven MV networks to be installed at different times and
beneficial in limiting fault currents during ground faults and integrated later. For this reason, each bus is provided with
suppressing resonance and surge voltage conditions. a dedicated earthing transformer equipped with Neutral
Different earthing schemes are possible: earthing at the Earthing Resistor (NER). The Earthing Transformers
power generator star center, earthing at the power (ETR) have wye-neutral configuration on the MV side,
transformer, or using dedicated earthing transformers. with the secondary open delta closed on the NER at low
While all these solutions are effective, the latter is more voltage (LV) (see Figure 2).
flexible and safer. Allowing current to pass through the NER sizes are calculated for the Hull, considering its
generators’ windings can be harmful, and most power own capacitive current, and for Topside, considering the
transformers are designed with a 3-limb core for overall capacitive current of the FPSO (Topside and Hull),
economic reasons, which has a closing path for zero- following IEC 60287-1-1 [5].
sequence flux external to the core [6]. Dedicated earthing During normal operations, Topside 11kV system works
transformers, on the other hand, are purpose-designed with the bus-tie closed, while Hull 11kV system operates
and provide isolation between the MV system and the with the bus-tie opened. The Topside-Hull links are both
ground. closed. For operational reasons, including the
simplification of automatic logics in case of network 1) Incident 1: offloading pump motor earth fault on
separation, one Topside and one Hull earthing FPSO-1
transformer are connected to guarantee an earthing Network configuration is visible in Figure 5. The FPSO
reference to the system. was running with three gas turbines connected at Topside
with links to the Hull closed. Topside Earthing
Transformer under bus B and Hull Earthing Transformer
under bus A were connected.

Fig. 2. Earthing Transformers Wiring

III. REAL CASES: EARTH FAULTS AND SYSTEM

RESPONSE

With the above scheme in place, any earth fault current

should be limited to the desired value. During normal
operation, with Topside and Hull systems connected, the
earth fault current should result, in case of bolted earth
fault, in the sum of the current reclosed by the connected
earthing transformers.
In the analyzed cases, on two twin FPSOs, bolted
earthing currents of Topside and Hull are limited by the Fig. 3. Topside Earthing Transformer connection
related NER to respectively to 63A and 10A.

A. Introduction

As anticipated in Section II, in normal operations the

FPSO works with both Topside and Hull 11kV system
interconnected. The MV distribution of FPSO is designed
to maintain the highest reliability in case of failures. All
different feeder types, such as alternators incomers,
motors feeders, transformers feeders and distribution
feeders (links), are equipped with Intelligent Electronic
Devices (IEDs) to protect the individual feeder and
connected equipment/subsystem.
Earthing transformers are connected to the MV
switchboards by typical “fuses+contactor” feeder scheme,
as indicated in Figure 3 (Topside) and Figure 4 (Hull).
Fuses (for short-circuit) and the IED functions 26 (internal
temperature protection), 50/51 (overcurrent), and 51N
(earth fault) ensure protection. The 51N function
selectively discriminates real earth faults in the
transformer from earth faults elsewhere in the network,
which are protected by feeder dedicated overcurrent
protection 50G.
Characteristics of the earthing transformers connected
to the MV network under analysis are reported in Table I
(Topside Earthing Transformer Datasheet) and Table II
(Hull Earthing Transformer Datasheet). The next sections
will present two incidents out of the six earth fault cases
experienced in six motors failures affected by
manufacturing defects in two twin FPSOs.

B. Incidents Descriptions and Sequence of Events

Fig. 4. Hull Earthing Transformer connection
 Transformer under bus B and Hull Earthing Transformer
under bus A were connected.

TABLE II
DATA-SHEET OF HULL EARTHING TRANSFORMER

Primary winding Secondary winding

No. of phases 3 3
Rating AN 200kVA during fault (10.5A per phase @
11kV) for 10 seconds.
3A continuous current per phase on 11kV
side
Rated voltage 11kV (ph-ph) 220V (phase)
Connections Wye Delta (Open)
Maximum
12kV 1.1kV
system voltage
Fig. 5. Incident 1 – Single Line Diagram reference Power frequency
28kV 2.4kV
withstand
The bus-tie at Hull MV switchboard was open. The BIL (kV) 75kV -
failed offloading pump motor was connected under bus B Winding material Copper Copper
of the Hull. During the inrush of the offloading pump Insulation type Cast resin Cast resin
motor, phase L3 failed. The expected fault current was Insulation
73A (63A reclosing on Topside Transformer and 10A Temperature F F
Class
reclosing on Hull Transformer). However, the actual fault
Temperature rise
current reached about 280A, with 57A from the Topside limit
F F
Earthing Transformer (Figure 7.a.) and about 240A from NER - 6.9 ohm
the Hull Earthing Transformer (Figure 7.b.). The phase Earth fault
displacement between the earth fault current and zero- 10A (3.3A per phase) -
current
sequence voltage was about 65 degrees (1.135 rad), as No load losses Ambient
1.1kW 45degC
indicated in Figure 7.c. temperature
TABLE I Load losses (95
2.9kW Enclosure IP IP23
DATA-SHEET OF TOPSIDE EARTHING TRANSFORMER degC)
Impedance 3.5%
Cooling AN
Primary winding Secondary winding voltage (0~+10%)
No. of phases 3 3 Vector group YNd OPEN Installation Indoor
Rating AN 400kVA during fault (21A per phase @ 11kV) Frequency 60 Hz
for 10 seconds.
5A continuous current per phase on 11kV The bus-tie at Hull MV switchboard was open. The
side failed sea water lift pump motor was connected under bus
Rated voltage 11kV (ph-ph) 230V (phase) A of Topside.
Connections Wye Delta (Open) During the inrush of the sea water lift pump motor,
Maximum
12kV 1.1kV phase L3 failed. The expected fault current was 73A (63A
system voltage
Power frequency from the Topside Transformer and 10A from the Hull
28kV 3kV Transformer). However, the actual fault current reached
withstand
BIL (kV) 75kV - about 380A, with 56A from the Topside Earthing
Winding material Copper Copper Transformer (Figure 8.a.) and above 350A from the Hull
Insulation type Cast resin VPI impregnated Earthing Transformer (Figure 8.b.). This caused fuse L1
Insulation to blow in two cycles (around 35ms) and the contactor to
Temperature F H open. The earth current stabilized to about 56A after the
Class disconnection of the Hull Earthing Transformer, as
Temperature rise indicated in Figure 8.c.
F F
limit
NER size - 1.2 ohm
Earth fault
63A (21A per phase) -
current
No load losses Ambient
0.8kW 45degC
temperature
Load losses (120 1.5kW (@
Enclosure IP IP23
degC) 95.23kVA)
Impedance 6.5%
voltage (approx. @ Cooling AN
95.23kVA)
Vector group YNd OPEN Installation Indoor
Frequency 60 Hz

2) Incident 2: sea water lift pump motor earth fault

on FPSO-2
The network configuration is visible in Figure 6. The Fig. 6. Incident 2 – Single Line Diagram reference
FPSO was running with three gas turbines connected at
Topside with links to the Hull closed. Topside Earthing
Fig. 7.a. Incident 1 – Topside Earthing Transformer Disturbance Recorder

Fig. 7.b. Incident 1 – Hull Earthing Transformer Disturbance Recorder

Fig. 7.c. Incident 1 – Overall Fault Disturbance Recorder (“A15 Earthing” indicates the Hull Earthing Transformer)

Fig. 8.a. Incident 2 – Topside Earthing Transformer Disturbance Recorder

 Fig. 8.b. Incident 2 – Hull Earthing Transformer Disturbance Recorder

Fig. 8.c. Incident 2 – Overall Fault Disturbance Recorder (“A15 Earthing” indicates the Hull Earthing Transformer)
C. Design Assessment

The system response during earth faults did not meet

the intended design. While Topside Earthing Transformer
response was within the limits and proportional to the
zero-sequence voltage, the Hull Earthing Transformer
earth current was extremely high, causing a fuse to blow
in one case.
The first incident investigation focused mainly on the
failed motor. The second incident, occurring more than six
months later on a different installation, highlighted a
common issue in the earth fault current. Additionally, the
blown fuse on the Hull Earthing Transformer addressed Fig. 10. Impedance of Hull Earthing Transformer
the investigation in this direction.
After verifying the insulation resistance and connections During inspection and documentation analysis, it was
on-site, with satisfactory results and no anomalies found that the transformer was manufactured with 3-limbs
encountered, the first step was the design assessment of magnetic core, as reported in Figure 11.
the earthing system. Simulations were carried out for the
Hull Earthing System. Figure 9 shows the simulation
results of the Hull Earthing Transformer with an NER of
6.9 ohms in an equivalent network

Fig. 9. Equivalent network – Hull Earthing Transformer

with designed resistor
Fig. 11. Hull Earthing Transformer - Outline drawing
NER value was embedded into the Rcc% value of the
earthing transformer as per equation (1): 3-limbs core transformers are suitable for power
transformers due to their reduced dimensions and weight.
However, the zero-sequence impedance is limited
compared to 5 limbs core transformers, due to the
(1) reclosing zero-sequence flux in air [8][9], as indicated in
where Figure 12 and equations (2) and (3).
Rcc-zero% zero-sequence equivalent resistance
of windings;
Rcc% equivalent resistance of windings for
direct and inverse sequence;
Sn earthing transformer rated power
[MVA];
U2 earthing transformer rated secondary
voltage [kV].

IT should be noted that above equation does not

consider the transformer core configuration and
magnetization, which instead played a crucial role in this
case. The earthing transformer impedance for zero
sequence reflect the values displayed on Figure 10.
The results confirm that the problem was not in the
electrical circuit of the earthing transformer but in the
magnetic core. Fig. 12. 3-limb core transformer circuit
 Uph1 phase voltage on the winding [kV]
N1 primary winding coils
(2) k construction constant [kV*m/(A*Hz)]
f frequency [Hz]
Ø magnetic flux [A/m]
B magnetic induction [T]
(3) S magnetic core cross section [m2]
where
Rmo zero-sequence magnetic reluctance
Rfe magnetic reluctance of the core
Rair magnetic reluctance of the air
μfe permeability of the core
μair permeability of the air
lfe equivalent length of core limb and yoke
lair equivalent length of air closing circuit
Afe equivalent cross section area of core
Aair equivalent cross section area of air
Lmo zero-sequence magnetic inductance

In 5-limbs core transformers, the reclosing of zero-

sequence flux is in the external limbs of the core, resulting
in a much higher zero-sequence inductance due to the
higher permeability of the core.
The core-configuration has been identified as a root
cause of the high current; however, a second step was
reserved for the analysis of the disturbance recorders,
presented in below section III.D.

D. Disturbance Recorders Analysis

After assessing the equipment design, remaining efforts

were focused to the disturbance recorders. Fig. 13.a Incident 1 - Hull Earthing Transformer phase
With reference to Figures 7.a.b.c and Figures 8.a.b.c., currents harmonics (failure phase L3)
the following anomalies were identified in the response of
the Hull Earthing Transformer:

1) Earth current over 20-25 times the expected one.

2) Differences in phase currents, with major currents
on the healthy phases (L1 and L2).
3) Angle between zero sequences voltage and earth
current.
4) Distortion of currents waveform.

The currents harmonic contents for the Hull Earthing

Transformer, for the two incidents, are reported in Figure
13.a.b. and Figure 14.a.b. respectively. Current distortion
and phase imbalance at the Hull Earthing Transformer
has been identified as effects of core saturation. The
phase displacement between earth voltage and current
(above 65 degrees) reinforces the finding, suggesting that
the network is equivalently earthed with an inductive
impedance. Moreover, the value and the harmonic
distortion of the current of the faulted phase, which is
almost negligible in the first incident, confirmed the
assumption.
During the earth fault of one phase, the remaining
healthy phases increase the voltage to the phase-to-
phase level, as visible in Figure 15. The increase of
voltage results in an increase of the magnetic flux, as per Fig. 13.b Incident 1 – Earth currents harmonics
equations (4) and (5): (channel(4) Hull ETR – channel(12) Topside ETR –
channel(19) fault current)
(4)
(5)

where
 Fig. 15. Voltage diagram during earth fault in phase L2

A simple check of the core configuration in the

transformer outline drawing can verify the zero-sequence
impedance [6][7] at the factory. Prevention of core
saturation can be assessed on transformers size
calculation (if available from the manufacturer) or by
testing of the earthing transformer at full voltage (phase-
to-phase) on one phase. The two tests can be combined if
the manufacturer has the necessary testing facilities. A
reference scheme is reported in Figure 16.
Fig. 14.a Incident 2 - Hull Earthing Transformer phase
currents harmonics (failure phase L3)

Fig. 16. Full voltage testing scheme.

Implementing these checks and tests will ensure that

transformers are appropriately designed for earthing
applications.
Fig. 14.b Incident 2 – Earth currents harmonics IV. CONCLUSIONS
(channel(4) Hull ETR – channel(12) Topside ETR – fault
current not reported (fuse at Hull ETR blown in 2 cycles) The investigation into the earth faults and system
response of the two FPSOs under subject revealed critical
E. Investigation Outcome and Lessons Learnt design flaws in the earthing transformers. The primary
issue was identified in the core geometry, specifically the
It was assessed together with the manufacturer that the use of a 3-limb core, which is not suitable for earthing
transformer was designed for power applications and not applications due to its limited zero-sequence impedance.
for earthing power systems. The design flaw was This design flaw, combined with incomplete testing and
identified in the core geometry (3-limbs core) and the design reviews, led to significant anomalies in the
number of coils per phase. The area sections of the system's response during earth faults.
transformer core limbs and yokes were confirmed as The analysis of disturbance recorders highlighted
correct. several key issues, including earth currents exceeding
Additional casual factors, identified in design review expected values, phase imbalances, and waveform
and incomplete testing, were also noted in the distortions. These anomalies were traced back to core
investigation.
saturation effects, which were further confirmed through (100 % load factor) and calculation of losses –
simulations and harmonic analysis. General”
To address these issues, it is essential to implement [6] IEC 60076 series “Power Transformers”
thorough design assessments and testing protocols. [7] IEEE C57.12.00 “Standard for Requirements,
Verifying the core configuration and zero-sequence Terminology, and Test Procedures for Neutral
impedance at the factory, along with conducting full Grounding Devices”
voltage tests on earthing transformers, can prevent similar [8] J.S.Song, J.S.Kim, G.J.Cho, C.H.Kim, N.H.Cho,
problems in future installations. These measures will “Determination method for zero-sequence
ensure that transformers are appropriately designed for impedance of 3-limb core transformer”, Perpignan-
earthing applications, thereby enhancing system reliability France IPST 2019
and safety. [9] B.A.Mork, D.Ishchenko, F.Gonzalez, Sung D.Cho,
In summary, the lessons learned from this investigation “Parameter Estimation Methods for Five-Limb
underscore the importance of rigorous design and testing Magnetic Core Model”, IEEE Transactions on Power
processes in maintaining the integrity of offshore electrical Delivery, vol. 23, no. 4, Oct. 2008.
installations. By addressing the identified flaws and
implementing the recommended solutions, we can
significantly improve the safety and reliability of these II. VITA
critical systems.
Andrea Santarpia is the Electrical Fleet Operations
V. REFERENCES Technical Authority at SBM Offshore.
He earned his master’s degree with honors in electrical
[1] ABS “Rules for Building and Classing Facilities on engineering in 2010 from the University of Rome “La
Offshore Installations” – Chapter 3 (Floating Sapienza.” In 2010, he joined Technip Italy as an
Installations) Electrical Engineer, moved to Kinetics Technology (Maire
[2] ABS “Rules for Building and Classing Facilities on Tecnimont Group) in 2018 as an Electrical Project Lead,
Offshore Installations” – Chapter 4 (Fixed and subsequently joined SBM Offshore in 2019 as an
Installations) Asset Integrity Electrical Engineer in the Operations
[3] V.Sibille, A.Ashraf, A.Santarpia, “Direct-On-Line Department. During his tenure at SBM, he served as the
High Voltage Motor Starting Criteria for all-Electric EC&I Group Lead in Operations and Digital Solution
FPSO”, PCIC Europe 2024 Lead.
[4] IEC 61892 series “Mobile and fixed offshore units – He is also an individual member of the IEEE Industry
Electrical Installations” Applications Society (IAS)
[5] IEC 60287-1-1 “Electric cables - Calculation of the andrea.santarpia@sbmoffshore.com
current rating - Part 1-1: Current rating equations