International Journal of Electronics, Electrical and Mechanical Fundamentals, (Vol. 16, Issue 01), August 2014 www.ijeemf.com (An Indexed, Referred and Impact Factor Journal) ISSN: 2278-3989
Design of Earthing Scheme in OffOff-Shore Utility Plant Pankaj Kumar1, Pankaj Rai2, Niranjan Kumar3 1
Electrical Engineering Deptt. BIT Sindri, VBU, Hazaribag, India
[email protected] 2
Associate Professor, Electrical Engineering Deptt. BIT Sindri, VBU, Hazaribag, India,
[email protected] India
3
Associate Professor, Electrical Engineering Deptt. NIT Jamshedpur, Jharkhand, India
[email protected]
relay are not shown for the sake of simplicity, although applicable to other 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 (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 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.
Abstract The power system in offshore installation consists of a large electrical distribution network, with multiple gas turbine-generators directly connected to 11kV switchgear. Such a configuration introduces high capacitive charging current (Ico); more than the preferred high resistance grounding of generator neutral from 10A, 10sec rating resistor, to safeguard the generator iron core lamination from damage during an earth fault. Hence, some utility selects low resistance grounding to limit the fault current above Ico, however it does not protect the core. This paper is an attempt to share the experience learned in designing neutral earthing scheme for off-shore utility in view of high capacitive charging current and outlines impact on stator core damage, its mitigation and conclusion.
Keywords: GCB (Generator Circuit Breaker), GTG (Gas Turbine Generator), HRG (High Resistance Grounding), LRG (Low Resistance Grounding), NER (Neutral Earthing Resistor), NET (Neutral Earthing Transformer)
2. Capacitive Charging Current Generator transformer (GT), approximately equal to generator rating in MVA, requires substantial space & weight on utility plate form. For a compact Utility plate form design, GT is generally not considered, unless defined as specification requirement. This result into a power system where multiple generators feed directly to 11kV Switchgear, refer a typical single line diagram in Fig-1. Such a configuration however, increases the capacitive charging current (Ico), which needs to be mitigated through equipment design and
1. 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 and 67N
IJEEMF www.ijeemf.com
1
International Journal of Electronics, Electrical and Mechanical Fundamentals, (Vol. 16, Issue 01), August 2014 www.ijeemf.com (An Indexed, Referred and Impact Factor Journal) ISSN: 2278-3989
protection. At 11kV voltage level, there are equipment like generators, motors, transformers and feeders along with a large network of 11kV cable length, spread to different 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, considered for further analysis.
magnitude of generated third harmonic voltage [2] is 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, 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, having impact on the 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.
2.1 Winding Pitch of Generator The winding pitch of generator could be 2/3rd or 5/6th; however both contribute to 3rd harmonic voltage, displaced by 3600 (electrical degrees).
2.2 Generator Core Lamination 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 laminated iron core is prevented from damage during fault.
Fig-1 – Typical Single line diagram Multiple generators may operate with unequal loading during parallel operation; also contribute to increase in harmonics, when connected with low resistance grounding at generator neutral. The third harmonic & fundamental phase voltages are co-phasal, however 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
IJEEMF www.ijeemf.com
2
International Journal of Electronics, Electrical and Mechanical Fundamentals, (Vol. 16, Issue 01), August 2014 www.ijeemf.com (An Indexed, Referred and Impact Factor Journal) ISSN: 2278-3989
10sec. with temperature rise of 7600C [5]. 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.
3. Selection of Grounding Methods The selection of grounding method should provide safety, reliability, and continuity of service desired for the oil & gas distribution system. IEEE Standard [7] 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. 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 60A 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
Fig-2 – Generator iron core damage curve 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. 12A, 10sec – Negligible/ damage 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 150ms, to enable core to withstand higher fault current, in little burning area. For 75A fault current, the earth fault protection clearance time could be set for 1000ms (1sec).
2.3 Neutral Earthing Resistor Due to high capacitive charging current and stringent specification requirement for 11kV NER like IP54 protection, the size of NER becomes quite large. Higher the degree of protection, higher is the size of NER for maintaining balance between heat generation and dissipation. Thus NERs needs more space, hence difficult to accommodate in compact utility plate form design. Usually, short time rating of NER is
IJEEMF www.ijeemf.com
3
International Journal of Electronics, Electrical and Mechanical Fundamentals, (Vol. 16, Issue 01), August 2014 www.ijeemf.com (An Indexed, Referred and Impact Factor Journal) ISSN: 2278-3989
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 60A capacitive charging current, HRG is not recommended. Low resistance grounding (LRG) through NER Higher fault current is good for sensitive & selective relaying, limiting transient overvoltages to moderate values, and a 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 as illustrated above (Refer 2.2). There are a certain issues, which needs a particular attention1. While using low resistance grounding it is recommended to have single NET/NER in service at a time, to reduce 3rd harmonic circulating current flow. So, with buscoupler in closed condition (refer fig-1), only one NET should be in service and others in switch-off condition. When buscoupler is off, then both NET should be in service. In addition, there should not be parallel grounding of generators. Parallel grounding means generators shown in fig-1 are having their NERs 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 Neutral Earthing 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 [9], NER cannot be placed within Main terminal box or Line side cubicle of generator otherwise Exn certification
cannot be achieved due to temperature class limitation (T4=2000C), while NER temperature can be up to 7600C [6]. 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 switchin/off (fig-3). During normal operation, only one NET with loading resistor has to be kept in service while Generator NERs are kept switchedoff.
Fig-3 – Earthing scheme with LRG & HRG
Fig-4 Earthing scheme with NET & loading resistor at 11kV 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 (refer fig-3) as GCB is opened to check the over-voltage condition due to fault. 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 current in secondary to limit the earth fault current to 75A. Earthing transformer/loading resistor is designed to
IJEEMF www.ijeemf.com
4
International Journal of Electronics, Electrical and Mechanical Fundamentals, (Vol. 16, Issue 01), August 2014 www.ijeemf.com (An Indexed, Referred and Impact Factor Journal) ISSN: 2278-3989
withstanding the earth fault current for 10 sec (min).
4. Conclusions Capacitive leakage current should be judiciously calculated during basic engineering design. Earth Fault Protection clearing time should always be derived from generator manufacturer supplied core damage curve. It is essential to carefully select both continuous & short-time rating and degree of protection of NER; otherwise has impact on NER size, which can lead to a layout problem on plate form. Therefore, 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.
Fault Scenario-1 & Mitigation 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 75A and allows over-voltage protection 59N to detect the over-voltage to trip the faulty circuit. In addition, the loading resistor provides damping to over-voltage due to Ferroresonance condition [3] [4] [5].
Fault Scenario-2 & Mitigation During an earth fault in generator or evacuation system, Generator Relay Panel (having directional Earth Fault relay operation (67N) & Instantaneous ground overcurrent protection (50G), Generator Differential Protection (87G) and Over-voltage Protection (59N) - Part of numerical Generator Protection-GRP) initiates tripping of GCB and Excitation & Field Breaker, closing of GTG shut-off valve to cutoff gas supply to gas turbine 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 following1. Sudden load throw 2. Ferro-resonance conditions during fault [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 Front End Engineering Design (FEED). Capacitive leakage current needs to be calculated [10] based on layout and similar off-shore 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.
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 [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 IJEEMF www.ijeemf.com
5
International Journal of Electronics, Electrical and Mechanical Fundamentals, (Vol. 16, Issue 01), August 2014 www.ijeemf.com (An Indexed, Referred and Impact Factor Journal) ISSN: 2278-3989
[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] IEC60079:15:2010 - Explosive atmospheres Part 15: Equipment protection by type of protection "n" [10] Industrial Power System, By Shoaib Khan, CRC Press, Taylor & Francis Group
IJEEMF www.ijeemf.com
6
