The electrical power system in offshore oil & gas installation, consists of a large distribution network, generally operating in island mode i.e., without grid support. For a compact utility plate form design, multiple gas turbine-generators without generator transformers, feed directly to 11kV switchgear. Such a configuration however, introduces high capacitive charging current (Ico), which is more than the preferred high resistance grounding of generator neutral through 10A, 10sec resistor, to safeguard the generator core from damage during an earth fault. Therefore, some utility prefers to select low resistance grounding to limit the fault current above Ico; however this can cause severe damage to generator core. Generally, oil & gas installation is a customized design. So, earthing scheme of 11kV generating utility system should be selected judiciously at basic engineering stage to avoid equipment damage and protection mal-operation during operation. Different methods of earthing scheme are available to mitigate the same. One of the method is presented here in which generator neutral is connected to high resistance grounding and 11kV switchgear connected to low resistance grounding though zig-zag transformer, subject to single grounding operation at a time. Prior to synchronization or under complete load throw scenario, generator circuit breaker is opened. So, an earth fault in generator or evacuation system, create over-voltage or ferro-resonance conditions, stressing insulation of generator and associated system. This is mitigated by putting neutral earthing resistor into service at generator neutral. This paper presents the experience learned in designing neutral earthing scheme for off-shore utility plant in view of high capacitive charging current at 11kV voltage level, outlines impact on stator core damage, mitigation and conclusion

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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
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IJEET
© I A E M E
HYBRID GROUNDING IN OFF-SHORE UTILITY PLANT
PANKAJ KUMAR1, PANKAJ RAI2, NIRANJAN KUMAR3
1(Electrical Engg Department, BIT Sindri)
2(Electrical Engg Department, BIT Sindri/VBU, Hazaribag, India)
3(Electrical Engg Department, NIT, Jamshedpur, India)
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ABSTRACT
The electrical power system in offshore oil gas installation, consists of a large
distribution network, generally operating in island mode i.e., without grid support. For a compact
utility plate form design, multiple gas turbine-generators without generator transformers, feed
directly to 11kV switchgear. Such a configuration however, introduces high capacitive charging
current (Ico), which is more than the preferred high resistance grounding of generator neutral
through 10A, 10sec resistor, to safeguard the generator core from damage during an earth fault.
Therefore, some utility prefers to select low resistance grounding to limit the fault current above
Ico; however this can cause severe damage to generator core. Generally, oil gas installation is a
customized design. So, earthing scheme of 11kV generating utility system should be selected
judiciously at basic engineering stage to avoid equipment damage and protection mal-operation
during operation. Different methods of earthing scheme are available to mitigate the same. One of
the method is presented here in which generator neutral is connected to high resistance grounding
and 11kV switchgear connected to low resistance grounding though zig-zag transformer, subject to
single grounding operation at a time. Prior to synchronization or under complete load throw
scenario, generator circuit breaker is opened. So, an earth fault in generator or evacuation system,
create over-voltage or ferro-resonance conditions, stressing insulation of generator and associated
system. This is mitigated by putting neutral earthing resistor into service at generator neutral. This
paper presents the experience learned in designing neutral earthing scheme for off-shore utility
plant in view of high capacitive charging current at 11kV voltage level, outlines impact on stator
core damage, mitigation and conclusion.
Keywords: EDG (Emergency diesel Generator), FEED (Front End Engineering Design), GCB
(Generator Circuit Breaker), GRP (Generator Relay Panel), GTG (Gas Turbine Generator), GT
(Generator Transformer), HRG (High Resistance Grounding), LRG (Low Resistance Grounding),
NER (Neutral Earthing Resistor), NET (Neutral Earthing Transformer).

2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 12-19 © IAEME
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I. INTRODUCTION
Synchronous Generators are installed at Utility Plate form. They are driven by aero-derivative
gas turbine and/or industrial gas turbine diesel engines to supply un-interrupted reliable
power to different plate forms to meet process requirement. A typical single line diagram is shown in
Fig-1. NER with Breaker-C, CBCT 67N relay are not shown for simplicity, although applicable to
other generator.
It is imperative for System design engineer to pay particular attention to applications of
multiple generators connected directly to 11kV bus-bar without generator transformer (fig-1). Such
a configuration introduces high capacitive charging current (Ico), more than the preferred high
resistance grounding of generator neutral through 10A, 10sec NER, to safeguard the generator core
from damage during an earth fault. Therefore, some utility select low resistance grounding to limit
the fault current above Ico and try to mitigate the risk of core damage by reducing earth fault
protection clearing time.
Fig-1: Typical single line diagram with multiple generators
II. CAPACITIVE CHARGING CURRENT
Generator transformer, approximately equal to generator rating in MVA, occupies substantial
space weight on utility plate form. Necessary handling arrangement for GT maintenance further
adds to space/weight. Thus for a compact utility plate form design, GT is generally not considered,
unless technically required which results into a power system where multiple generators, feeding
directly to 11kV Switchgear, refer a typical single line diagram in Fig-1. Such a configuration

3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 12-19 © IAEME
however, increases the capacitive charging current (Ico), which needs to be mitigated through
equipment design and protection.
The electrical network at 11kV voltage level consists of 11kV cables, generators motors,
service transformers and feeders, spread to various plate forms, introducing significant capacitive
charging current, which could be of the order of 20A to 200A [1]. Thus, low resistance grounding is
an option, could be considered for further analysis for limiting the fault current.
Multiple generators can operate with unequal loading during parallel operation along with
low resistance grounding also contribute to increase in 3rd harmonics. The winding pitch of generator
could be 2/3rd or 5/6th; however both contribute to 3rd harmonic voltage, displaced by 3600 (electrical
degrees). The third harmonic fundamental phase voltages are co-phasal and their effect is felt in
the zero sequence circuit, in the form of a circulating current at the third harmonic frequency. The
magnitude of this current is determined by the third harmonic driving voltage and the third harmonic
impedance of the zero sequence circuit. The third harmonic current can circulate only if a closed zero
sequence path is available for the generator third harmonic voltage to drive it, refer fig-4 for
example. The magnitude of generated third harmonic voltage is [1]
14
U3=1.44+4.22 (Ia/In) – 2.72 (If/Ifn)
Where U3 (%) – is the measured third harmonic voltage,
Ia (Amp)-Armature current
In (Amp) – Rated armature current,
If – is calculated field current
Ifn – is the calculated field current at rated output power
Industry always prefers for a proven designed generator. Reducing the winding pitch to 2/3rd
reduces 3rd harmonic as compared to 5/6th winding pitch, however rotor pole surface losses is
increased by 6 times approx. and generator output reduced by 15%. Therefore for same output,
generator size needs to be increased, requiring more space weight and introducing large impact on
utility plate form design. It may be noted that in off-shore utility, sub-system are arranged
horizontally vertically, while in onshore plant the same is arranged horizontally. Hence, for a
standard proven generator, the manufacturer offers 5/6th winding pitch generator.
[A] GENERATOR CORE DAMAGE CURVE - Manufacturer’s damage curve of generator stator
should always be referred for the magnitude and duration of allowable earth fault current, so that iron
core is prevented from damage during fault. Core damage is considered more severe than winding
damage [7]. Fig. 2 is a typical set of damage curves for generator, showing three regions where there
are negligible, little, and serious core burning area.
15A, 10sec – Negligible burning to generator iron core
65A-200A for time duration selected according to the curve for little / slight damage to generator iron
core
Thus, earth fault current could be limited to 200A, subject to earth fault protection clearance
time is reduced to 100ms, to enable core to withstand higher fault current, in slight burning area. So,
for 90A fault current, the earth fault protection clearance time could be set for 800ms.
[B] NEUTRAL EARTHING RESISTOR - Due to high capacitive charging current and stringent
specification requirement for 11kV NER with IP54 protection, the size of NER becomes quite large.
Higher the degree of protection, higher is the size of NER because of heat dissipation. Thus NERs
needs more space, hence difficult to accommodate in compact utility plate form. Usually, short time

4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 12-19 © IAEME
rating of NER is 10sec. with temperature rise of 7600C [6]. In view of high temperature, it is
essential to place NER in safe area, not in hazardous area.
For Industrial generator, NER can be placed in Main terminal box of generator. However, in
case of ExnA generator, NER cannot be placed in Main terminal box or Line side cubicle of
generator, otherwise Exn certification cannot achieved due to temperature class limit – T4 i.e.,
2000C. Thus, it is imperative to judiciously select both continuous short-time rating and degree of
protection of NER.
Fig.-2: Typical curve for arc burning on generator stator core lamination
III. CHOICE OF GROUNDING METHODS
The choice of grounding method should provide safety, reliability, and continuity of service
desired for the oil gas distribution system. IEEE Standard [8] lists several reasons for limiting the
ground fault current by resistance grounding:
1. To reduce burning and melting effects in faulted electrical equipment, such as switchgear,
transformers, cables, and rotating machines.
2. To reduce mechanical stresses in circuits and apparatus carrying fault currents.
15

5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 12-19 © IAEME
3. To reduce electrical-shock hazard to personnel caused by stray ground fault currents in the
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ground return path.
4. To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who
happen to be in close proximity to the ground fault.
5. To reduce the momentary line voltage dip occasioned by the occurrence and clearing of a
ground fault.
6. To secure control of transient over-voltages while at the same time avoiding the shutdown of a
faulty circuit on the occurrence of the first ground fault (high resistance grounding).
For directly connected parallel operating generators, the system neutral grounding scheme
should be selected carefully because of high capacitive charging current of 75A at 11kV. Selection of
system grounding scheme should ensure that no circulating 3rd harmonic current be allowed in the
neutral circuits of the generators when they are operated in parallel.
Generally, high resistance grounding (HRG) is preferred for generators to minimize generator
core damage by using NER of 10A, 10sec however, low resistance grounding (LRG) is also used in
off-shore installation where Ico is high. Due to 75A capacitive charging current, HRG is not
recommended.
Low resistance grounding (LRG) through NER - Higher fault current is good for sensitive
selective relaying, limiting transient over-voltages to moderate values, and potential cost savings
over other grounding methods. However, the main drawback is the possibility of significant burning
of the generator stator core (Refer Fig-2). In addition, because of IP54 and generator core guarantee
for 75A fault current, this scheme is found not suitable as illustrated above (Refer II-B). There are a
certain issues, which needs a particular attention-
1. While using low resistance grounding it is recommended to have single NER in service at a
time, to reduce 3rd harmonic circulating current flow. So, with bus-coupler in closed condition (refer
fig-1), only one NET should be in service and other in switch-off condition. When bus-coupler is off,
then both NET should be in service. Hence NET should be designed for 2x100% rating. There
should not be parallel grounding of generators. Parallel grounding means generators shown in fig-1
are having their NER in service.
2. Even though there is no parallel grounding, there will still be capacitive leakage currents at
11kV voltage level due to generators and large network of 11kV cable length to motors, service
transformers and feeders, spread to various plate forms. This current will flow through the generator
neutral earthing resistor. Thus, for a ground fault in the stator winding occurring together with low
resistance grounding, the stator core will be severely damaged (fig-2).
In view of above, Hybrid grounding is a better option, combining best features of both low
resistance and high resistance grounding methods [2]. This requires 3 no NER (HRG) with degree of
protection defined to IP23 2 no Zig-Zag Grounding Transformer (LRG), which means more space
weight, however is insignificant and can be accommodated at Utility plate form. For Industrial
generator, NER can be installed within main terminal box of the generator. For ExnA generator [10],
NER cannot be placed within Main terminal box or Line side cubicle of generator otherwise Exn
certification cannot be achieved due to temperature class limit (T4=2000C), while NER temperature
can be up to 7600C [5]. In that scenario, 3 no NER along with 2 no NET are to be placed in safe area.
Generator neutral is earthed through 10A, 10sec NER with breaker for NER switch-in/off (fig-3).
During normal operation, only one Zig-Zag Grounding Transformer with resistor RG has to be kept
in service while generator NERs is kept switched off. Under bus-coupler closed condition, second
NET should be off (fig-1 fig-4). Prior to synchronization or under complete load throw scenario of
a generator, the corresponding NER should be put into service as GCB is opened.

6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 12-19 © IAEME
Fig-3: Hybrid Earthing scheme with Zig-Zag transformer
Neutral earthing transformer is connected in star/broken delta (fig-4). The primary winding is
solidly earthed and secondary in broken delta having loading resistor with Over-Voltage relay (59N)
[8]. The loading resistor is designed to limit the zero-sequence
Fig-4: Hybrid Earthing scheme showing fault current without NER
current in secondary to limit the earth fault current to 90A. Earthing transformer/loading resistor is
designed to withstanding the earth fault current for 10 sec (min).
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7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 12-19 © IAEME
Fault Scenario-1 - During an earth fault in 11kV switchgear or any of the outgoing feeders (fig-1),
the loading resistor across the NET broken delta restricts the fault current to 90A and allows over-voltage
protection 59N to detect the over-voltage to immediate tripping of the faulty circuit. In
addition, the loading resistor provides damping to over-voltage due to Ferro-resonance condition [3]
[4] [5].
Fault Scenario-2 – During an earth fault in generator or evacuation system, GRP (having
directional earth fault protection operation (67N) Instantaneous ground overcurrent protection
(50G), Generator Differential Protection (87G) and Over-voltage Protection (59N) - Part of
numerical Generator Protection) initiates tripping of GCB and Excitation Field Breaker, closing
of GTG shut-off valve and simultaneous closing of generator NER within 150ms through lock out
relay (86), so as to avoid build-up of stress on insulation of generator and associated system. Under
the above fault scenario, there are over voltages due to following-
18
1. Sudden load throw
2. Over-voltage due to single phase to ground
3. Ferro-resonance conditions [3] [4] [5].
Thus, fault is mitigated through employing Hybrid earthing scheme. Grounding scheme in
offshore installation should be finalized judiciously during basic engineering design or FEED.
Capacitive leakage current needs to be calculated [9] based on layout and similar plant database, to
be validated later during detailed engineering. Earth Fault protection clearing time should always be
derived from generator core damage curve. Degree of protection should be correctly defined;
otherwise NER size would be large, which requires more space at Utility plate form.
IV. CONCLUSION
Capacitive leakage current should be judiciously calculated during Front End Engineering
Design (FEED) or Basic engineering design stage. Earth Fault Protection clearing time should
always be obtained from generator manufacturer supplied core damage curve. It is imperative to
carefully select both continuous short-time rating and degree of protection of NER otherwise this
has impact on NER size, which can lead to a layout issue. While selecting earthing scheme, layout of
the utility plant in which generator electrical system including NER and NET with loading resistor
are placed, must be considered. NER and NET with loading resistor should always be installed in
Safe area (Non-hazardous area). During normal operation, one NET at 11kV bus is in service with
bus-coupler closed and all generator NERs are isolated. To avoid coordination problems, it may be
imperative to remove supplementary protection and NER (HRG), when the generator is operated in
connection with 11kV switchgear (i.e., normal mode) with LRG in service. Such a hybrid
arrangement offers the best features of both high resistance grounding and low resistance grounding
into the power system.
V. REFERENCES
[1] Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical
Industry - by Alan L. Sheldrake.
[2] Earth fault protection for synchronous Machines, International Application Treaty under
PCT, published on 13 May 2004.
[3] Grounding and ground fault protection of multiple generator installations on medium voltage
industrial and commercial power systems Part 1-4, An IEEE/IAS WG Report.

8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 12-19 © IAEME
[4] System Grounding and Ground-Fault Protection in the Petrochemical Industry: A need for a
Better Understanding, John P. Nelson, Fellow, IEEE Transaction on Industry on Industry
Applications, Vol. 38, No. 6, November / December 2002.
[5] State-of-the Art Medium Voltage Generator Grounding and Ground Fault Protection of
Multiple Generator Installations, David Shipp, Eaton Electrical, Warrendale, Pennsylvania.
[6] IEEE 32- IEEE Standard Requirements, Terminology, and Test Procedures for Neutral
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Grounding Devices.
[7] IEEE 142 - IEEE Recommended Practice for Grounding of Industrial and Commercial Power
Systems.
[8] IEEE 242-IEEE Recommended Practice for Protection and Coordination of Industrial and
Commercial Power Systems.
[9] Industrial Power System, Shoib Khan, CRC Press.
[10] IEC60079:15:2010 - Explosive atmospheres: Equipment protection by type of protection n.
[11] Sumit Kumar and Prof.Dr.A.A Godbole, “Performance Improvement of Synchronous
Generator by Stator Winding Design”, International Journal of Electrical Engineering
Technology (IJEET), Volume 4, Issue 3, 2013, pp. 29 - 34, ISSN Print: 0976-6545,
ISSN Online: 0976-6553.
[12] Archana Singh, Prof. D.S.Chauhan and Dr.K.G.Upadhyay, “Effect of Reactive Power
Valuation of Generators in Deregulated Electricity Markets”, International Journal of
Electrical Engineering Technology (IJEET), Volume 3, Issue 1, 2012, pp. 44 - 57,
ISSN Print: 0976-6545, ISSN Online: 0976-6553.
[13] Mosleh Maiet Al-Harthi and Sherif Salama Mohamed Ghoneim, “Measurements the Earth
Surface Potential for Different Grounding System Configurations using Scale Model”,
International Journal of Electrical Engineering Technology (IJEET), Volume 3, Issue 2,
2012, pp. 405 - 416, ISSN Print: 0976-6545, ISSN Online: 0976-6553.