INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET)

INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 6545(Print), 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 KUMAR 1, PANKAJ RAI 2, NIRANJAN KUMAR 3 1 (Electrical Engg Department, BIT Sindri) 2 (Electrical Engg Department, BIT Sindri/VBU, Hazaribag, India) 3 (Electrical Engg Department, NIT, Jamshedpur, India) 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 switchgearend. 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). 1

I. INTRODUCTION Synchronous Generators are installed at Utility Plate form. They are driven by aeroderivative 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. 2

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]. 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 5/6 th 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). 3

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 760 0 C [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 3 rd 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 3 rd 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 [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-nonlinear 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 6

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 180 0 (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 nonlinearity 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]. Fig-6: Capacitive current and system voltage 8

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. 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 9

. Fig-9a: Voltage Plot Fig-9b: Arcing crrent Plot. Fig-10a: Voltage Plot Fig-10b: Current Plot 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. 10

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 overvoltage 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. 11