Performance testing low-voltage ground-fault protection

The National Electrical Code (NEC) requires ground-fault protection of equipment by devices intended to detect and interrupt low-current arcing phase-to-ground faults on many solidly grounded low-voltage systems having current ratings of 1000 A or greater. The NEC also dictates field performance testing of ground-fault protection, but does not prescribe specific techniques to be used. This paper reviews the types of ground-fault protection equipment and systems in use today, ranging from simple bolted-pressure switch and relay combinations to complex multiple-source systems and zone-selective-interlocked systems. Conditions that can cause misoperation of ground-fault protection are reviewed, including a summary of problems identified during field testing of actual projects. Available testing techniques are reviewed and primary current injection is recommended as the most reliable means of field verification for most ground-fault protection systems. Required test equipment setup for each system configuration is described, and cautions to be observed during this type of testing are presented. This paper will benefit engineers and technicians responsible for acceptance testing of electrical equipment.     

Ground-Fault Protection of Low-Voltage Equipment for Solidly Grounded Wye Electrical Services

Ground-fault protection sensors on 277/480-V systems have not been able to provide adequate ground-fault protection-costly equipment burndowns are still occurring. Also ground-fault protection through the use of ground-fault (GF) sensors produce a coordination problem where complete shut down becomes a probability.     

Equipment Grounding for Reliable Ground-Fault Protection in Electrical Systems Below 600 V

Equipment grounding is one of the most important, but least understood, requirements for reliable ground-fault protection. This paper defines the basic objectives of equipment grounding and analyzes the role of equipment grounding conductors in providing ground-fault protection for electrical systems below 600 V.     

System grounding and ground-fault protection in the petrochemical industry: a need for a better understanding

This paper provides an in-depth discussion of system grounding and ground fault protection on systems from 480 V and above. The paper also discusses modeling of ground faults, the proper design for ground-fault protection, and common problems associated with ground-fault protection. The paper address many real-life problems associated with system grounding and ground-fault protection, including safety issues and how to avoid those problems. The topics included in the paper include low-voltage systems, under 600 V, through high-voltage transmission systems.     

Proper grounding of on-site electrical power systems

There are a number of factors that should be considered when grounding on-site power systems. Simply conforming to minimum code requirements will not necessarily assure the degree of reliability required for such systems. Thorough consideration should be given to protecting against power disruption within the building or facility and providing adequate ground-fault protection. Techniques for both equipment and system grounding should provide optimum safety and assure maximum continuity of power to essential loads. This includes proper grounding and ground-fault sensing when the transfer switch is in the emergency as well as normal position     

中性点经消弧线圈瞬时并联小电阻接地研究

从配电网供电的安全可靠性、电气设备和线路的绝缘水平、继电保护的选择性和灵敏性以及对通信系统的干扰等方面,综合说明了采用中性点经消弧线圈瞬时并联小电阻的接地方式可充分发挥消弧线圈和小电阻接地方式的优点,建议在配电网中采用中性点经消弧线圈瞬时并联小电阻的接地方式。     

The effects of very-high-resistance grounding on the selectivity ofground-fault relaying in high-voltage longwall power systems

With the advent of high-voltage (greater than 1 kV) utilization circuits on longwall mining equipment in the late 1980s, the Mine Safety and Health Administration (MSHA) initially required maximum ground-fault current limits of 3.75 A for 4160 V systems and 6.5 A for 2400 V systems. (It should be noted that the Code of Federal Regulations defines low voltage, medium voltage, and high voltage for mine power systems as 0-660 V, 661-1000 V, and greater than 1000 V, respectively.) Ground-fault relay pickup settings were not permitted to exceed 40% of the maximum ground-fault current. Shortly thereafter, the MSHA began, and presently continues, requiring a much lower maximum ground-fault current limit of 1.0 A, or even 0.5 A, with ground-trip settings of 100 mA. Shielded cables, which have significantly more capacitance than their unshielded counterparts, are required for high-voltage applications in the mining industry. In an earlier paper, the author showed that with the long cable runs of a high-voltage longwall system, capacitive charging currents could easily exceed grounding-resistor currents under ground-fault conditions. As a result, overvoltages from inductive-capacitive resonance effects can occur. Because of the large system capacitance and low ground-trip setting, the relay selectivity of the ground-fault protection system may also be compromised. Therefore, an analysis of a typical 4160-V longwall power system that utilizes very-high-resistance grounding (ground-resistor-current limit of 0.5 A) is performed to determine if potential problems exist with the selectivity of ground-fault relaying     

Analysis of very-high-resistance grounding in high-voltage longwallpower systems

The application of very sensitive ground-fault protection in underground coal mines was demonstrated in the early 1980s for low- and medium-voltage utilization circuits (less than 1 kV), but its commercial application did not occur until the advent of high-voltage utilization circuits on longwalls in the late 1980s. With these high-voltage systems (greater than 1000 V), the Mine Safety and Health Administration initially required a maximum ground-fault resistor current limit of 3.75 A for 4160 V systems and 6.5 A for 2400 V systems in 101-c Petitions for Modification. However, more recent Petitions for Modification have been required to limit maximum ground-fault resistor currents to 1.0 A, or even 0.5 A. Standard practice in other industries generally requires high-resistance grounding to be designed so that the capacitive charging current of the system is less than or equal to the resistor current under a ground-fault condition. The intent of this practice is to prevent the system from developing some of the undesirable characteristics of an ungrounded system, such as overvoltages from inductive-capacitive resonance effects and intermittent ground faults. Shielded cables, which have significantly more capacitance than their unshielded counterparts, are required for high-voltage applications in the mining industry. Thus, with the long cable runs of a high-voltage longwall system, capacitive charging currents may exceed grounding-resistor currents under ground-fault conditions. An analysis of a typical 4160 V longwall power system that utilizes very-high-resistance grounding (grounding-resistor-current limit of 0.5 A) is performed to determine whether or not potential problems exist     

Effect of adjustable-speed drives on the operation of low-voltageground-fault indicators

The petroleum and chemical industry has found increasing favor with 60 Hz low-voltage (⩽600 Vac) power systems that utilize a high-resistance grounded (HRG) neutral philosophy. Historically, the older generation of adjustable-speed drives (ASDs) had little or no effect on the normal operation of ground-fault indicators (GFIs) used with the installed HRG systems. This paper focuses on nuisance GFI alarms that may occur when present-generation ASDs are retrofitted into the existing plant. The paper first reviews possible neutral grounding systems, with emphasis on the types of HRG systems possible and GFI alarm philosophy. The paper then discusses how ASDs may generate zero-sequence high-frequency noise currents in the HRG neutral circuit, which may cause nuisance ground-fault alarms and potentially mask a legitimate ground fault. GFI noise current magnitude is defined for both present and older ASD technologies. The effect this transient zero-sequence noise current magnitude has on GFI operation is described. Mitigation methods used at the drive to reduce ASD noise current magnitude to acceptable nonalarm levels is investigated. Filter solutions located at the HRG/GFI meter that reduce nuisance alarms are also investigated. The pros and cons of at the drive or at the meter filter solutions are supported with laboratory and field test data. Application guidelines are given to help avoid nuisance problems with a plant ground-fault protection scheme, which needs to successfully operate in the presence of multiple ASDs     

A new digital ground-fault protection system for generator–transformer unit

Ground faults are one of most often reasons of damages in stator windings of large generators. Under certain conditions, as a result of ground-fault protection systems maloperation, ground faults convert into high-current faults, causing severe failures in power system. Numerous publications in renowned journals and magazines testify about ground-fault matter importance and problems reported by exploitators confirm opinions, that some issues concerning ground-fault protection of large generators have not been solved yet or have been solved insufficiently. In this paper a new conception of a digital ground-fault protection system for stator winding of large generator was proposed. The process of intermittent arc ground fault in stator winding has been briefly discussed and actual ground-fault voltage waveforms were presented. A new relaying algorithm, based on third harmonic voltage measurement was also drawn and the methods of its implementation and testing were described.     

Sensitive ground-fault relaying

Sensitive ground-fault protection refers to the concept of detecting low levels of ground-fault current that might cause electrocution of any human that becomes part of the ground-current path and providing warnings. The concepts behind sensitive ground-fault relaying for use on AC and DC low-voltage utilization systems are covered. The background for these relaying types is presented, and it is shown that the critical component in sensitive ground-fault relay design is the current sensor. Zero-sequence devices for three-phase industrial utilization systems and a saturable-transformer device for DC utilization are discussed. Relaying schemes for both AC and DC systems are presented. This study is aimed at mine power systems but could be applicable to any portable low-voltage portion of industrial power systems that involve handling by personnel     

Grounding the Neutral of Electrical Systems Through Low-Resistance Grounding Resistors: An Application Case

It seems common knowledge that three-phase short-circuit currents are the worst possible scenario for electrical systems. In reality, single-phase ground-fault currents may be significantly more intense than three-phase fault currents. With the occurrence of high single-phase fault currents, severe damage could be caused to the iron cores of electric machines included in the zero-sequence fault loop. The method of neutral low-resistance grounding will be discussed by applying a step-by-step calculation procedure to an actual case, in order to properly size the grounding resistor, thereby limiting the fault current.      

Some Significant Effects of the New National Electrical Code on Our Industries

The increased concern for human safety is one of several factors which makes the National Electrical Code increasingly important to engineers involved in plant electrical systems. New requirements are extending into areas not previously covered by the Code. This paper discusses some selected major changes which are of most interest to industries using large amounts of power, and some changes which are apt to be controversial. Included are: ground-fault protection, service entrance arrangements, autotransformers, transformer primary and secondary protection, separately-derived systems, and new product possibilities.     

接地距离保护采用自适应控制消除过渡电阻的影响

针对距离保护存在的一些问题,详细分析了单相接地短路故障时产生的附加阻抗,说明了过渡电阻对接地方向阻抗继电器性能的影响。提出了接地距离保护运用人工神经网络作为自适应控制手段来消除过渡电阻的思想。     

Ground-Fault Protection of an Emergency or Stand-by Power System with Multiple Neutral-to-Ground Connections

SYNPOSIS

In the operation of an engine-generator set, when used for emergency power, it is required that the neutral conductor be connected to the nearest available effectively grounded electrode. Therefore, any three-phase, four-wire system with properly grounded engine generator sets will have multiple neutral-to-ground connections. One connection will be at the service entrance with the other connection at each engine-generator set.

When an electrical system has multiple neutral-to-ground connections, there may be problems in obtaining proper sensing of ground fault currents as well as nuisance tripping with unbalanced loads. These problems persist with the system in either its normal or emergency supply modes of operation or both. This paper discusses some of these problems and the methods of solution. A new and special ground-fault sensing scheme which can overcome these problems for such a multiple neutral-to-ground system is also presented.     

电力系统接地距离保护零序补偿系数分析

本文主要分析讨论接地距离保护在有零序互感线路上应用的一些特殊问题，尤其对零序补偿系数的不同情况进行了分析。详细推导了不同运行条件下零序互感耦合线路的零序补偿系数的计算公式，并附有实例对各种运行情况进行比较，确定在整定计算中采用的算法。     

More about standby generator grounding, GFP, and currents that gobump in the night

The issue of generator grounding, separately derived systems, and the application of three-pole and four-pole transfer switches continues to be misunderstood and misapplied by design engineers, in spite of several papers on the subject. Incorrect generator grounding and/or incorrect transfer switch selection can result in noncode-complying installations, intermingling of ground and neutral currents, and possible incorrect ground-fault sensing. In order to properly ground multiple service and multiple generator installations, the electrical design engineer must have an in-depth understanding of all of the variables involved     

Grounding transformer applications and associated protectionschemes

The use of grounding transformers on three-phase ungrounded governmental, industrial, and commercial distribution systems is addressed. Ground-fault protection schemes that provide selective and reasonably fast tripping are often incorporated with these grounding transformers. The authors first review the state of the art of grounding transformers to assist electric power systems engineers in the proper understanding of the use and applications of these devices and then discuss two governmental system case studies illustrating improperly applied grounding transformers and/or associated ground-fault protection systems. In addition, they describe remedial actions taken to correct these deficiencies. The objective in presenting improper grounding transformer applications is to highlight protection concerns that are often ignored. It is stressed that a single grounding transformer is not adequate for use with a multibus configuration. A unique protection scheme for use on a multibus arrangement is presented     

Influence of difference system parameters on operating conditions of third harmonic ground-fault protection system of unit-connected generator

In the paper, the influence of unbalance of the measuring element of the unit-connected generator stator ground-fault protection system excited by the ratio of third harmonic voltage to ground in the generator neutral and at the generator terminals on the value of the voltage supplied to the measuring element during ground faults is analyzed. The analysis is made for currently used ground-fault protection systems in which the measuring element is balanced by an autotransformer voltage divider or an impedance bridge.     

Ground-fault protection and the problem of nuisance tripping ofcritical feeders

National Electrical Code (NEC) ground-fault protection (GFP) requirements are examined in light of the various risk factors. The risk factors related to arcing ground faults and the possibility of nuisance tripping on critical feeders are examined. The adequacy of state-of-the-art ground-fault protection equipment is also examined. Special attention is given to process industries and health care facilities. It is concluded that those responsible for designing and installing GFP systems need to apply them more effectively and that manufacturers and code writers need to look at alternatives to present GFP schemes