Alternator Ground

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Ground-Fault Detection, Charging Current and Neutral-Grounding Resistor SelectionLittelfuse Startco 3714 Kinnear Place Saskatoon, Saskatchewan Canada S7P 0A6AbstractChoosing appropriate ground-fault protection and neutral-grounding resistor (NGR) letthrough current requires careful consideration of several factors including system voltage, ground-fault relay co-ordination, tripping action, harmonics, and system charging current. In order to facilitate system design and troubleshooting, this paper reviews charging current and sympathetic tripping, protective relay coordination, and the factors that affect NGR selection for alarming and tripping applications. A systematic method is proposed for selecting an NGR, and setting ground-fault protection. I. INTRODUCTION Resistance grounding has been used in three-phase industrial applications for many years. Properly designed resistance grounding resolves many of the problems associated with solidly grounded and ungrounded systems while retaining many of their benefits. Resistance grounding can limit point-of-fault damage, eliminate transient overvoltages, reduce the risk of an arc flash, limit personnel exposure to voltage, allow continuity of service with a ground fault, and provide adequate current for ground-fault detection and selective coordination. However, proper sizing of the NGR is critical in order to provide system stability. In this paper resistance grounding refers to a resistive element connected between the transformer or generator neutral and the system ground on the secondary side. There are two levels of resistance grounding, highresistance grounding and low-resistance grounding. The difference between these levels is a matter of perception and, therefore, is not well defined. Generally speaking high-resistance grounding refers to a system in which the NGR let-through current is less than 50 to 100 A. Lowresistance grounding indicates that NGR current would be above 100 A. As an example a 480 V system with a 100 A NGR would be considered low-resistance grounding because 5 A is a common let-through current for a 480 V system. A 35 kV system with a 100 A NGR could be considered high-resistance grounding. A better distinction between the two levels might be alarm only and tripping. An alarm-only system continues to operate with a single ground fault on the system for an unspecified amount of time. In a tripping system a ground fault is automatically removed by protective relaying and circuit interrupting devices. Alarm-only systems usually limit NGR current to 10 A or less. II. SYSTEM VOLTAGE CONSIDERATIONS In the application of resistance grounding the system voltage and NGR let-through current are the primary considerations that influence whether to deploy an alarmonly or tripping system. The NGR let-through-current rating in an alarm-only system should be no more than 10 A. This value of fault current is usually small enough to prevent burning of the insulation and the subsequent escalation to a phase-tophase fault, and burning of core material on motors, generators and transformers [1]. This level is the maximum let-through current for an alarm-only system as defined by section 10-1102 of the Canadian Electrical Code. Alarm-only systems are seldom used at system voltages above 5 kV because higher system capacitance necessitates an NGR let-though current greater than 10 A, and because higher voltage increases the probability of a ground-fault escalation to a phase-to-phase fault. [2] On a tripping system, an NGR with a higher let-through-current rating can be used and NGRs of 15 A and 25 A are common. Higher-voltage systems, which must trip on a ground fault, can have let-through currents up to hundreds of amps. Designers comfortable with solidly grounded systems often choose NGRs with let-through currents much larger than necessary for either system stability or selective coordination [3]. This practice increases potential damage to equipment and danger to personnel during a ground fault. III. UNDERSTANDING CHARGING CURRENT Each phase of a distribution system has capacitance to ground. Although a system may be ungrounded in that none of its current-carrying conductors are intentionally connected to ground, an ungrounded system has its neutral point established by distributed system capacitance as shown in Fig. 1. For simplicity, the distributed capacitance of the system is shown as a single capacitor per phase. When the system is energized current flows between the phase conductors and the capacitive neutral; this current is referred to as charging current. Extensive damage can occur when an ungrounded power-distribution system experiences a transient overvoltage condition caused by an intermittent ground fault [3].

Don Selkirk, P.Eng

Ground-Fault Detection, Charging Current and Neutral-Grounding Resistor Selection

Page 2

Fig 1: Ungrounded System On an ungrounded system a transient overvoltage can result from a re-striking ground fault. When the strike occurs, charging current only flows for a short time and is then extinguished. This results in a capacitive charge that elevates system voltage-to-ground for all phases and neutral. Phase-to-phase and phase-to-neutral voltages are not affected. The repetition of the re-strike process over several cycles can result in a voltage built up of 5 or 6 times the nominal voltage [4]. To prevent this transient overvoltage condition in a resistance-grounded system, the grounding resistor must have a let-through-current rating large enough to drain the charge between successive restrikes. This is especially important in an alarm-only system that continues to operate with a ground fault. The minimum let-through-current value is generally accepted to be equal to system charging current. IV. SYMPATHETIC TRIPPING Fig. 2 shows a system similar to Fig. 1 with a 5.0-A NGR added. Voltages and currents in the unfaulted case are the same as in the ungrounded system. If phase A is faulted to ground, voltages and currents in phases B and C are also the same as in the ungrounded system; however, fault current is the vector sum of NGR current and system charging current. If the system in Fig. 2 has three equal feeders, currents will be as shown in Fig. 2. Meters A1 and A3 on the unfaulted feeders each read 1.0 Athe charging current of their respective feeders. NGR current and fault current remain unchanged at 5.0 A and 5.8 A respectively. Meter A2 on the faulted feeder will read 5.4 Athe vector sum of NGR current and the charging currents of the unfaulted feeders.

Fig. 2: Resistance-Grounded System Sympathetic tripping is when unfaulted feeders trip in response to a ground fault elsewhere in the system, and is an undesirable situation. Sympathetic tripping can occur if the operating value of the feeders ground-fault relay is less than the feeders charging current. Sympathetic tripping cannot occur, regardless of the

Don Selkirk, P.Eng

Littelfuse Startco May 2008

Ground-Fault Detection, Charging Current and Neutral-Grounding Resistor Selection

Page 3

relative feeder sizes, if an operating value above the charging current of the largest feeder is used for all groundfault relays in the system. Personnel-level ground-fault protection is difficult to achieve in a three phase system because of the magnitude of charging currents. V. TRIPPING RATIO AND NGR SELECTION Tripping ratio is defined as the ratio of prospective ground-fault current to the operating value of the groundfault protection. An adequate tripping ratio ensures that sufficient ground-fault current is available for detection when a ground fault occurs. Reference [5] shows that a tripping ratio of at least 7 is necessary to detect a two phase-to-ground fault. It can be argued that this type of fault should be cleared by overcurrent devices and that ground-fault detection does not require a tripping ratio of 7. On the other hand, a higher tripping ratio is required to provide machine-winding ground-fault protection. Reference [2] states that the generally accepted protection philosophy is based on protecting 90 percent of a wye connected winding, and that the probability of a ground fault on the last 10 percent nearest the neutral is small. A tripping ratio of 10 is required to meet this protection philosophy; however, tripping ratios of 5 are common. If the operating value of the ground-fault relays is greater than the charging current of the largest feeder, and if a tripping ratio of 5 is selected to ensure adequate tripping levels and machine-winding ground-fault protection, the let-through current of the NGR must be greater than 5 times the charging current of the largest feeder. Charging current is a function of system voltage and can be measured on an existing system or estimated from tables. Typically, charging current will be 0.5 A per 1000 kVA on low voltage systems and 1.0 A per 1000 kVA on medium voltage systems. Consequently, 5-A, 15-A, and 25-A grounding resistors are common. VI. RELAY COORDINATION Resistance-grounded facilities benefit from orderly ground-fault clearing. On a solidly grounded system a bolted fault can result in a relay race in that several protective devices, both overcurrent and ground fault, detect the fault and begin to trip at the same time. The fault is then cleared by the device that operates the fastest rather than the device that would remove the least amount of equipment from the system. This problem can be solved using zone-selective interlocking but zone-selective interlocking requires a great deal of control wiring. By contrast a resistance-grounded system can be designed so that the available ground-fault current is much smaller than the pickup setting of overcurrent devices thus only ground-fault relays will respond to a ground fault. Since ground-fault current is controlled to a level that will reduce fault damage, and an arc flash will not o