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. 12-19 IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2014): 6.8310 (Calculated by GISI) www.jifactor.com IJEET I A E M E HYBRID GROUNDING 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 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). 12
I. INTRODUCTION Synchronous Generators are installed at Utility Plate form. They are driven by aeroderivative 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 13
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 3 rd harmonics. The winding pitch of generator could be 2/3 rd or 5/6 th ; however both contribute to 3 rd harmonic voltage, displaced by 360 0 (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] 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 5/6 th 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 5/6 th 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 14
rating of NER is 10sec. with temperature rise of 760 0 C [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., 200 0 C. 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
3. To reduce electrical-shock hazard to personnel caused by stray ground fault currents in the 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 3 rd 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=200 0 C), while NER temperature can be up to 760 0 C [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 R G 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. 16
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). 17
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 overvoltage 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- 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. 18
[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 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. 19