40220140507001

INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 7, July (2014), pp. 01-11 © IAEME
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 7, July (2014), pp. 01-11
© IAEME: www.iaeme.com/IJEET.asp
Journal Impact Factor (2014): 6.8310 (Calculated by GISI)
www.jifactor.com
IJEET
© I A E M E
UNGROUNDED GENERATOR OPERATION 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)
1
ABSTRACT
In offshore oil gas installation, the electrical power system consists of a large distribution
network, generally operating in island mode i.e., without grid support. For reliability and a
compact utility plate form, power system is designed with multiple gas turbine and diesel
generators, directly connected to 11kV switchgear, without generator transformers. This kind of
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 iron core lamination from damage during an earth fault. In view of high Ico, some
utility prefers to select low resistance grounding, to achieve more sensitivity for generator earth
fault protection; however this cannot guarantee the core from damage during fault. Due to IP54
protection, 11kV neutral earthing resistor requires more space at utility plate form. So, other
method is to select ungrounded generator neutral with low resistance earthing at 11kV switchgear-end.
Prior to synchronization or under complete load throw scenario, an earth fault in generator or
evacuation system, create over-voltage or ferro-resonance conditions, stressing insulation of
generator, bus-duct, voltage-transformer cubicle and generator circuit breaker. 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,
ungrounded generator operation, mitigation and conclusion.
Keywords: GCB (Generator Circuit Breaker), GTG (Gas Turbine Generator), EDG (Emergency
diesel Generator), NET (Neutral Earthing Transformer), NER (Neutral Earthing Resistor),
Power Plant, LSC (Line Side Cubicle), NSC (Neutral side Cubicle).

2
I. INTRODUCTION
Synchronous Generators are installed at Utility Plate form. They are driven by aero-derivative
gas turbine (aircraft turbine derivative applied to industrial application) and/or industrial
gas turbine diesel engines to supply un-interrupted reliable power to different plate forms to meet
process requirement, refer to a typical single line diagram in Fig-1. During normal operation, GTGs
supply power to the entire complex while all other DGs are not kept in operation, except for
periodical operational testing. VT and 67N are not shown on other generators for the sake of
simplicity.
Fig-1: Typical single line diagram with multiple ungrounded generators
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 (refer 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. Hence, some utility prefers to select low resistance grounding to
limit the fault current above Ico and attempt to mitigate the risk of core damage by reducing earth
fault protection clearing time. Due to IP54 protection, 11kV NER at generator neutral requires more
space at utility plate form. So, other method is to unground generator neutral, supplemented with
over-voltage protection (59N) in broken delta VT to detect earth fault indirectly and provide low
resistance earthing at 11kV switchgear-end for normal operation, refer fig-1.

II. CAPACITIVE CHARGING CURRENT
Generator transformer, approximately equal to generator rating, adds substantial footprint
weight. Necessary handling arrangement also needs to be added for maintenance. Thus, for a
compact utility plate form design, GT is generally, not considered, unless technically required as per
specification. This results into a power system where, multiple generators feed directly to 11kV
Switchgear (Fig-1). Such configuration however, increases the capacitive charging current (Ico),
which needs to be mitigated through equipment design and protection.
At 11kV voltage level, there are equipments consisting of generators, motors, transformers,
feeders and a large network of 11kV cables length, spread to various plate forms and towers,
introducing significant capacitive charging current (Ico), could be of the order of 20A to 200A [1].
Thus, low resistance grounding option is considered for further analysis to limit the fault current in
different configuration, refer fig-1 fig-3.
Multiple generators may operate with equal or unequal loading during parallel operation,
however during unequal loading, with low resistance NER at generator neutral together with winding
pitch contribute to increase in 3rd harmonics. The magnitude of generated third harmonic voltage
is [2].
3
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
In off-shore installation, space and weight of equipment are important for plate form design
having sub-system arranged horizontally vertically, unlike the onshore plant where horizontal
placement of sub-system is not a concern. Industry always prefers a proven designed generator.
Reducing the winding pitch to 2/3rd reduces 3rd harmonic, however rotor pole surface loss is
increased by 6 times approx. and generator output reduced by 15%. Therefore for same output,
generator size needs to be increased, which requires more space weight at plate form i.e., having
impact on overall plate form design. For a typical 32MVA generator with 5/6th winding pitch, the
3rd harmonic content is as follows – Phase to neutral Voltage is 2.97% and Phase to phase voltage is
0.06%. Hence, for proven standard generator, the manufacturer offers an optimum designed
generator with 5/6th winding pitch.
III. GENERATOR IRON 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 core is prevented from damage during fault
through core. Core damage is considered more severe than winding damage [9]. Fig. 2 is a typical set
of damage curves for generator, showing three regions where there is negligible, slight, and severe core
burning area. The curves show that earth fault current could be limited to 50-200A, subject to
protection clearance time is reduced to 1600-150ms, to enable core to withstand higher fault current,
in slight burning area. For 75A fault current, the earth fault protection clearance time could be set for
1000ms (1sec).

Fig.-2: Typical curve for arc burning on generator stator core lamination
IV. SELECTION OF GROUNDING METHODS
Higher the degree of protection of 11kV, 75A NER, higher is the size of NER, required more
space in a compact utility plate form design. However, resistor (e.g., VT guard) on VT broken delta
including VT can be easily accommodated in 11kV indoor switchgear panel. Usually, short time
rating of NER is 10sec. with temperature rise of 7600C [7]. In view of high temperature, it is
essential to place NER in safe area, not in hazardous area. Thus, system design engineer should
judiciously select both continuous short-time rating and degree of protection of NER.
There are different earthing schemes to detect earth faults in synchronous generator, found in
various books, literature and papers. When selecting the earthing scheme, the dimension is
important, to be considered from layout view point at utility plant. Selection of system earthing
scheme should ensure that no circulating 3rd harmonic current be allowed in the neutral circuits of the
generators when they are operated in parallel.
High-resistance grounding is a good choice for minimizing damage to a generator core,
however, is not selected due to high capacitive charging current of 60A.
Low resistance grounding 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 60A fault current, this scheme is found not suitable.
Hybrid grounding is a good option, combining best features of both low resistance and high
resistance grounding methods but it needs 3 no NER (HRG) 3 no Earthing Transformer, which
4

means more space are required at plate form. Due to compact design, the other option is to unground
the generator neutral with low resistance earthing at 11kV switchgear through 2 no NET,
supplemented through protection.
Fig-3(a) – NET with loading resistor at 11kV switchgear
Fig-3(b) – Zig-Zag grounding transformer
With the use of neutral earthing scheme as depicted in Fig-3a Fig-3b at the 11kV
switchgear bus sections, NERs at generator neutral is eliminated. Thus, generator neutral is
ungrounded and impedances at generator neutral become infinite (very high in terms of Giga Ohm),
because there are apparently no paths for zero sequence currents between the windings. This
eliminate 3rd harmonic circulating current flowing through the generators windings, however when
GCB is opened during synchronization or under sudden load throw scenario, then generator operates
in ungrounded condition. Adequate protection is provided to detect and discriminate the fault.
Option at Fig-3a offers more compact and cost-effective solution than Fig-3b. Neutral
earthing transformer shown in fig-3a is connected in star/broken delta. The primary winding is
5

solidly earthed and secondary in broken delta having loading resistor with Over-Voltage relay (59N).
The loading resistor is designed to limit the zero-sequence current in secondary to limit the earth
fault current to 75A. Earthing transformer/loading resistor is designed to withstanding the earth fault
current for 10 sec (min). During earth fault in 11kV voltage, the loading resistor across the NET
restricts the fault current and allows over-voltage protection 59N to detect the over-voltage due to
earth fault and trip the faulty circuit. The non-linear loading resistor (VT guard) provides damping to
over-voltage or Ferro-resonance during lightly loaded VT secondary.
Generator and associated electrical system like, LSC NSC, bus-duct, multiple VTs up to
GCB operate in ungrounded condition during synchronization or when GCB tripped subsequent to
protection operation or sudden load throw. Under this scenario, a single phase to earth fault can
develop transient over-voltage, being discussed in subsequent section.
V. UNGROUNDED GENERATOR OPERATION
For those systems, where service continuity of process is of primary concern for productivity,
the ungrounded system is sometimes, preferred than grounded system. There is a perception that
ungrounded systems have higher service continuity which is based on the argument that the ground
fault current is small [5] and hence will cause negligible damage i.e., burning or heating to generator,
motor and other electrical system, even if the fault persists for some time, enabling plant operator to
complete the critical process, before shutdown. Afterwards, line to ground fault is traced and cleared.
Here, the basis of selection is due to high capacitive charging current over specified IP54
Protection of 11kV NER. Hence, generator neutral made unearthed i.e., without NER as illustrated
above, and low resistive grounding using NET (Refer fig-3a) is introduced. During synchronization
or under load throw scenario, the generator operates in ungrounded condition. An earth fault in this
scenario is analyzed below. Practically, a vast majority of faults start as low level arcing ground
faults, except the bolted fault, So, when there is arcing ground faults, then following conditions may
arise:-
[1]. Multiple Ground Faults, [2] Resonant Conditions, [3] Transient Over voltages
6
[1] MULTIPLE GROUNDS FAULTS
Multiple ground faults can occur on ungrounded systems. While a ground fault on one phase of
an ungrounded system does not cause an outage. That is why some critical process gets time for
completion (e.g., rolling mill product is not cobbled, otherwise, is a substantial loss to productivity).
Longer the ground fault allowed persisting in the electrical system, greater is the likelihood of a second
ground fault occurring on another phase because the un-faulted phases have line-to-line voltage
impressed on their line-to-ground insulation. In other words, the insulation is overstressed by 73
percent. If not isolated, then there is an accelerated degradation of the insulation system due to the
collective overvoltage impinged upon it, through successive ground-faults over a period into years [4]
[5].
[2] RESONANT GROUND
Resonant conditions may result in ungrounded systems when one phase is grounded
through an inductance, for example, a ground within the winding of a generator VT (Refer Fig-1,
assuming a fault in VT winding). When this happens, the high circulating currents result in high
voltages across the un-faulted phases [4] [5].
VTs are essentially connected to an ungrounded generator, so the possibility of Ferro-non-linear
resonance is almost a certainty. To suppress ferro-resonance, a non-linear resistance (VT
guard) is connected across broken delta for damping (Refer Fig-1). Resistance loading equal to VT

thermal rating is required. To detect the ground faults, Over voltage relay (59N) is connected in
parallel to resistor. Both resistor (59N) are connected to broken delta. In normal operation, the
summation of co-phasal zero sequence components 3Io are zero. However, during abnormal
scenario, 3Io 3Vo come into picture and activates the protection.
To avoid coordination problems, it may be necessary to remove this supplementary
protection when the unit is operated in normal mode. In addition, the resistance loading applied to
suppress ferro-resonance should be removed when the generator is reconnected.
[3] TRANSIENTS OVER-VOLTAGES DUE ARCING GROUND FAULTS
In oil gas installation, turbo-generator is, usually connected to 11kV indoor switchgear
through short length cast resin encapsulated non-segregated bus-duct, connected at one end in line
side cubicle (LSC) of generator and other end at 11kV switchgear. There are so many
instrumentations involved in LSC, like current transformers, toroidal CT, Partial discharge coupler,
surge arrester and/or NER VT. So, practically the possibility of single phase to earth fault in bus
duct is remote as compared to that in terminals.
During ungrounded generator operation, an arcing ground fault on generator or evacuation
system (which includes terminal box, bus duct, VT up to GCB) offer transient over voltages. This
can be explained from zero-sequence circuit based on symmetrical components concept. The
insulation of generator and evacuation system constitutes capacitance to ground, which forms a zero
sequence reactance component. The positive and negative sequence reactance of generator is
represented by equivalent impedances. The resistance of generator bus-duct are much less
compared to above reactance, hence are practically, not considered.
For an arcing ground fault on R-Ø (say), the circuit diagram is shown as consisting of
positive and negative sequence reactance of generator in series with the zero-sequence circuit (Refer
Fig. 4). However, zero sequence reactance is always high as compared to positive and negative
sequence reactance. Hence for all practical purpose, the approximate equivalent circuit is simplified
as shown in Fig-5.
Transient overvoltage due to restriking or intermittent ground faults develops substantial over
voltages on ungrounded electrical systems with respect to ground.
Fig-4 Fig-5
7

There have been many documented cases within industry e.g., failure of 2 no air cooled
generator, 15625kVA, 13.8kV, Wye connected and grounded through its own 400A grounding
resistor [3] [11]. Similarly, multiple equipment failures (e.g.-motors) over an entire 480V system
have occurred while trying to find and locate a ground fault. Measured line-to-ground voltages of
1200V – 1500V or higher in these instances are not that uncommon. In all instances, the cause has
been traced to a low-level intermittent arcing ground fault on an ungrounded system [3] [4] [5].
The mechanism explaining how this occurs is explained from Fig-7. At instant A, prior to a
ground fault, the generator neutral is at or near ground potential (i.e., 0 Volt) because of the
electrostatic charge on the systems' shunt capacitance to ground (insulation, surge capacitors) under
balanced load conditions.
As soon as there is an earth fault to R-Ø, the system voltages are displaced as illustrated at
point B. At the instant after the fault occurs, when the R-Ø capacitive charging current (Ioc) passes
through zero, it is extinguished leaving a trapped charge on the shunt capacitance to ground on R-Ø.
With no path to dissipate this trapped charge, (fixed electrostatic DC voltage) the generator system
tends to stay in the position shown at point B. So, neutral of generator is now shifted to new position
at B.
As the three AC sine wave phases continues to rotate on a 50 cycle basis. One-half cycle
means 1800 (electrical degree) of phase rotation. Hence, R-Y-B Ø rotates to new position above
ground at point C. Now, the instantaneous voltage across the R-Ø ground fault is twice the normal
line-to-neutral crest voltage (2.0 PU) relative to ground potential. This voltage (2.0 PU) on R- Ø
restrikes across the fault gap to ground and suddenly pulls R-Ø to ground potential. The non-linearity
of the arc (high frequency components) will tend to excite the inductance/capacitance of the
system resulting in a high frequency oscillation between (+200%) and (-200%). When the arc
extinguishes, the system voltage relationships will tend to remain in a new position with respect to
ground potential as shown in the lower part of C, due to transient oscillation from positive maximum
to negative maximum [11]. Since the arc current has been extinguished, a new trapped DC
electrostatic charge on the R-Ø capacitance of (-200%) now applies [4]. In the next 1/2 cycle the AC
sine wave will rotate the voltage vectors from point C to the lower part of D (-200% to -400%) at
which time the arc restrikes, and the mechanism repeats itself between -400% and +400%. At this
point in time the voltages to ground are shown at 6 per unit on the un-faulted phases. If the arc
extinguishes, the mechanism will continue. Theoretically, the overvoltage could go on unlimited
except for the fact that the insulation on the un-faulted phases will probably fail, creating a fault on a
second phase. This will result is a phase-to-phase fault and most likely clearing of the fault through a
fuse or breaker. From a practical standpoint, voltages in excess of 700% are rare since most
insulation systems break down between 600% and 700%. Bear in mind that these over voltages are
superimposed on the entire electrical system i.e., LSC NSC, bus duct, SP/VT cubicle and up to
GCB, thereby explaining accelerated insulation deterioration and increasing incidence of ground
fault occurrences over time [4].
8

Fig-6: Capacitive current and system voltage

Fig-7: Transient Voltages at R-Phase restriking to ground fault
Although fig. 7 shows a maximum deviation in just 3 arcs, it might take considerably longer
to reach these levels since the arc may restrike before reaching maximum voltages on the AC sine
wave. Also, if the fault becomes a bolted fault at any time during the process, the trapped DC
electrostatic charges will be dissipated leaving the voltage relationships as shown at point B.
9
VI. PSCAD SIMULATION
A PSCAD Plot for ungrounded system with R-Ø fault is shown below. Fig-8a is a voltage
plot, with R-Ø having ground fault and voltage of healthy Y B- Ø raised to 3 times i.e., 15.5kV.
Fig-8b showing arcing current for 200ms.
Fig-9a is a voltage plot, with R-Ø having ground fault for fault duration of 0.1sec. Fig-9b
showing arcing current strike at 200ms, restrike at 125ms and again re-striking after 100ms.
Fig-10a is a voltage plot, with R-Ø having earth fault and voltage of healthy Y B-Ø voltage
reduced to 10kV by employing surge arrester with 85% compensation. Fig-10b showing arcing
current with 85% compensation.
Thus surge arrester is an option, which can be installed at generator neutral, in addition to
phase.
Fig-8a: Voltage Plot Fig-8b: Arcing current Plot

Fig-9a: Voltage Plot Fig-9b: Arcing crrent Plot
Fig-10a: Voltage Plot Fig-10b: Current Plot
10
.
.
VII. MITIGATION
Ungrounded generator and associated electrical system operates only during synchronization,
protection operation or under load throw scenario, which can develop transient over-voltage during
single phase to earth-fault or ferro-resonanace through VT, lightly loaded. The over-voltage, as
illustrated above can be reduced by using surge arrester at generator neutral phase. Generator main
terminal box (LSC NSC separate or combined) can accommodate one no surge arrester at neutral
and one each in phase side. The cost of surge arrester are insignificant, however is recommended to
be finalized during pre-order engineering. To control ferro-resonance, VT guard (Resistor) in broken
delta is used. Both VT resistor can be easily accommodated in safe area in 11kV switchgear
without having any space constraint as compared to NER. Over-voltage protection (59N) in broken
delta, detect the over-voltage and trip the generator excitation through the lock out relay (86).
Generator ( 20 MVA) may take 5 to 20 seconds to stop [3]. So, Generator protection through lock
out relay trips both excitation and close the shut-off valve to cut of gas supply to gas turbine. To
avoid coordination problems, it may be necessary to remove this supplementary protection when the
generator is operated in connection with 11kV switchgear (i.e., normal mode). The supplier should
also design the generator for ungrounded operation.

11
VIII. CONCLUSION
Capacitive leakage current and degree of Protection should be judiciously calculated during
basic engineering design. While selecting earthing scheme, layout of the utility plant in which
generator electrical system including NET with loading resistor are placed, must be considered.
All VT should be installed in safe area at 11kV switchgear end. Earth Fault Protection clearing time
should always be obtained from generator manufacturer supplied iron core lamination damage curve.
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 otherwise both in
operation.
Surge arrester is recommended for installation at neutral and phase side to reduce the over-voltage
during single phase to earth fault. To avoid coordination problems, it may be necessary to
remove this supplementary protection when the generator is operated in connection with 11kV
switchgear (i.e., normal mode).
IX. 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 to 4, An IEEE/IAS working group Report.
[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] Compensation of Earth Faults in Ungrounded Systems, Tanu Rizvi, M.T Deshpande,
International Journal of Emerging Technology and Advanced Engineering, published in
August 2012.
[7] IEEE Std-32: 1972 Requirements, Terminology Test Procedures for Neutral Grounding
Devices.
[8] IEEE 142 - IEEE Recommended Practice for Grounding of Industrial and Commercial Power
Systems.
[9] IEEE 242-IEEE Recommended Practice for Protection and Coordination of Industrial and
Commercial Power Systems.
[10] Industrial Power System, Shoib Khan, CRC Press.
[11] Protective Relaying Theory and Application, By Walter A Elmore, Mercer Dekker Inc.
[12] 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.
[13] 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.

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