Ground faults in generator stator and field/rotor circuits are serious events that can lead to damage, costly repair, extended outage and loss of revenue.
This paper explores advances in field/rotor circuit ground fault and stator ground fault protection. These advanced protection strategies employ AC injection and other tactics to provide benefits in security, sensitivity and speed.
Generator and Transformer Protection (PART 1)Dr. Rohit Babu
Part 1. Generator Protection
Protection of generators against stator faults
Rotor faults and abnormal conditions
Restricted earth fault and inter-turn fault protection
Numerical examples
Tutorial on Distance and Over Current ProtectionSARAVANAN A
Contents
• Protection Philosophy of ERPC
• Computation of Distance Relay Setting
• System Study to Understand Distance Relay
Behaviour
• DOC and DEF for EHV system
Generator and Transformer Protection (PART 1)Dr. Rohit Babu
Part 1. Generator Protection
Protection of generators against stator faults
Rotor faults and abnormal conditions
Restricted earth fault and inter-turn fault protection
Numerical examples
Tutorial on Distance and Over Current ProtectionSARAVANAN A
Contents
• Protection Philosophy of ERPC
• Computation of Distance Relay Setting
• System Study to Understand Distance Relay
Behaviour
• DOC and DEF for EHV system
Protection of transmission lines (distance)Rohini Haridas
This gives idea about necessity of protection of transmission line and protection based on time grading as well as on current grading. Also includes three step distance protection of transmission line
To sense/detect the fault occurrence and other abnormal conditions at the protected equipment/area/section.
To operate the correct circuit breakers so as to disconnect only the faulty equipment/area/section as quickly as possible, thus minimizing the damage caused by the faults.
To operate the correct circuit breakers to isolate the faulty equipment/area/section from the healthy system in the case of abnormalities like overloads, unbalance, undervoltage, etc.
To clear the fault before the system becomes unstable.
To identify distinctly where the fault has occurred.
Bus Bar protection Schemes,Simple Current differential scheme,Need for bus bar protection,requirement of bus bar protection,recommendations for providing bus bar protection,basics of busbar protection,Types of bus-bar protections,High speed differential protection
Protection of transmission lines (distance)Rohini Haridas
This gives idea about necessity of protection of transmission line and protection based on time grading as well as on current grading. Also includes three step distance protection of transmission line
To sense/detect the fault occurrence and other abnormal conditions at the protected equipment/area/section.
To operate the correct circuit breakers so as to disconnect only the faulty equipment/area/section as quickly as possible, thus minimizing the damage caused by the faults.
To operate the correct circuit breakers to isolate the faulty equipment/area/section from the healthy system in the case of abnormalities like overloads, unbalance, undervoltage, etc.
To clear the fault before the system becomes unstable.
To identify distinctly where the fault has occurred.
Bus Bar protection Schemes,Simple Current differential scheme,Need for bus bar protection,requirement of bus bar protection,recommendations for providing bus bar protection,basics of busbar protection,Types of bus-bar protections,High speed differential protection
Main equipment in the power plant is Generator. It's cost is much higher than any other equipment so we will have to protect the generator from all the possible faults and errors.
In a generating station the generator and transformer are the most expensive equipments and hence it is desirable to employ protective system to isolate the faulty equipment as quickly as possible to keep the healthy section in normal operation and to ensure uninterruptable power supply.
Rotor earth fault protection of electric generatorCS V
As the field is operated ungrounded, a single fault does not cause any flow of current or affect the operation of the electric generator. However, a single rotor earth fault increases the stress to the ground in the field
Rotor earth fault protection of electric generatormytechinfo
As the field is operated ungrounded, a single fault does not cause any flow of current or affect the operation of the electric generator. However, a single rotor earth fault increases the stress to the ground in the field
The use of wireless technology in industrial automation systems offers a number of potential benefits, from the obvious cost reduction brought about by the elimination of wiring to the availability of better plant information, improved productivity and better asset management.
However, its practical implementation faces a number of challenges: not least the present lack of a universally agreed standard. This whitepaper looks at some of these challenges and presents the approach being taken by Yokogawa.
NIVELCO EasyTREK SP-500 Ultrasonic Level Transmitters for LiquidsArjay Automation
The newest generation EasyTREK SP-500 series level transmitters are based on NIVELCO’s 35 years of experience with ultrasonic level measurement. The IP68 rated units have their transducer and processing electronics incorporated in one single housing. The new EasyTREK transmitters utilize HART® 7 communication so they can be used in multidrop systems connected to MultiCONT process controller/display, or to a PC with the help of the UNICOMM HART®-USB modem or similar. The transmitters can be remotely programmed also with Handheld Field Communicator, and can be connected wirelessly to a PC with the SAT-504 Bluetooth® HART® modem.
BLH Nobel 1756 WM Dual-channel Plug-in Weighing Module for the Rockwell 1756 ...Arjay Automation
The 1756-WM is a weighing module designed to directly plug into the Allen-Bradley 1756 ControlLogix chassis.
Intended for inventory measurement and process control, the 1756-WM offers users simple configuration and is suitable for tank, silo, vessel and hopper weighing in the United States and Canadian markets.
The weighing module seamlessly integrates weighing into the Allen-Bradley PLC by directly fitting into a single slot in the 1756 ControlLogix chassis via a screw connection plug. The weighing module is powered from the input/output chassis backplane and needs only the load cells for connection.
EasyTREK / EchoTREK Ultrasonic Level Transmitters for LiquidsArjay Automation
Ultrasonic level metering technology is based on the principle of measuring the time required for the ultrasound pulses to cover the distance from the sensor to the level of the media being measured and back. Echoes bouncing back from the surface of the process media reach the sensor surface after the time of flight of the ultrasonic impulse. With the help of the customizable tank dimensions or the pre-programmed flume / weir parameters, the time of flight of the reflected signal is measured and processed by the level control electronics, and presented as distance, level, volume or flow proportional data. The intelligent QUEST+ process adaptive signal processing software system ensures that the electronics identifies and validates the liquid surface signal, giving reliable level monitoring.
Since 1968 Moore Industries has been providing interface solutions with a variety of instrumentation products. Signal converters, isolators, temperature transmitters, alarm modules, I/P and P/I transducers, temperature sensors, distributed I/O, fieldbus products and HART monitoring devices are key product groups.
The GM10 modular data acquisition and logging system utilizes an expandable housing linked to the module base. This design enables the use of a wide range of I/O modules, which can be expanded by one or more at a time. This data can be logged and changed in real time without additional software.
Non contact and guided wave radar liquid level transmitters for level measurement of liquids, slurries, emulsions and other chemicals in water, wastewater, pharmaceutical, chemical, food and energy industries.
For high precision level measurement tasks
The THZ3 Dual Input Smart HART® Temperature Transmitter in DIN Rail Mount housing with the Associated Intrinsically-Safe -AIS option is the newest member of the Moore Industries' Intrinsically Safe family of products.
The CA Station measures the total and static pressure components of airflow. The combination and dispersion of total and static pressure sensing ports minimizes the effect of directional airflow, and the addition of the honeycomb airflow processing cell makes the CA Station extremely effective at accurately measuring airflow in limited straight duct runs.
Nivelco Instrumentation for Chemical and Pharmaceutical IndustriesArjay Automation
Overview of the various process measurement instruments manufactured by Nivelco for the chemical and pharmaceutical industries. Applications and solutions for level measurement.
Hoffer Flow Controls has been designing and manufacturing quality turbine flowmeters and related process instrumentation. Hoffer Flow Controls manufactures high precision turbine flowmeters, not only for the cryogenic industry but is a world leader in turbine flowmeter technology for the measurement of clean liquids and gases throughout the processing industries.
Nivelco specializes in level measurement. Founded in 1982 to concentrate on the manufacture of industrial level measurement and control products, specifically level transmitters, using microwave, guided wave radar, ultrasonic, capacitive, and magnetostrictive technologies as well as level switches utilizing float, conductive, magnetic, and vibrating technologies.
Yokogawa manufactures several variants of sanitary pressure transmitters for industrial applications in food, pharma, and other industries. Leading edge measurement technology.
Yokogawa, globally recognized leader in a number of process control fields, has authored an e-book which provides useful insight into how operators of combustion based equipment and systems can improve efficiency and enhance safety by employing modern technology.
Programmable Limit Alarm Trips with Intrinsically-Safe Field ConnectionsArjay Automation
The universal SPA2IS Programmable Limit Alarm Trips provide on/off control, warn of unwanted process conditions, alarm on rate-of-change and provide emergency shutdown. Very versatile, they accept signal inputs from transmitters and temperature
sensors that are located in hazardous areas where the method of protection implemented by the plant or facility is Intrinsic Safety.
Continuous Conductivity Monitoring Instrument for Industrial ApplicationsArjay Automation
AMI CACE is an economical, low-maintenance monitor that continuously measures conductivity - delivering reliability, efficiency and productivity for consistent measuring and gap free trend analyses.
The on-line monitor AMI CACE continuously measures conductivity, before (specific / total conductivity) and after (acid / cation conductivity) cation exchange (CACE) as well as determining the pH value of the sample and alkalizing reagent based on differential conductivity measurement. With a measuring range of 0.055 to 1000 μS/cm, the AMI CACE is the perfect instrument dedicated to high quality conductivity measurements of feedwater, steam and condensate.
Thermo Scientific™
Reliably measure density or density related variables in the oil and gas, petrochemical and power industries with Thermo Scientific™ Sarasota Gas Density Meters. These meters offer near real-time control signal availability and ensure plant efficiency and vitality.
Moore Industries is proud to announce the new SFY Functional Safety Frequency-to-DC Transmitter with display including SIL 3 capability. The 2-wire (loop-powered) SFY is ideal for use in your Safety Instrumented System (SIS) in a wide range of process and factory automation monitoring applications and as part of the FS Functional Safety Series providing reliable and accurate monitoring of frequency or pulse signals in SIS. The SFY is approved for single use in SIS up to SIL 2 and in a redundant architecture (1oo2, 2oo3, etc.) up to SIL 3.
Industrial Wireless Network Components and EquipmentArjay Automation
A broad range of industrial wireless products delivers comprehensive solutions for reaching network devices, streamlining operations, improving productivity and safety, and reducing overall cost.
Hydrostatic Liquid Level Transmitters for Industrial Process MeasurementArjay Automation
Founded in 1982 to concentrate on the manufacture of industrial level measurement and control products, specifically level transmitters, using microwave, guided wave radar, ultrasonic, capacitive, and magnetostrictive technologies as well as level switches utilizing float, conductive, magnetic, and vibrating technologies.
Final project report on grocery store management system..pdfKamal Acharya
In today’s fast-changing business environment, it’s extremely important to be able to respond to client needs in the most effective and timely manner. If your customers wish to see your business online and have instant access to your products or services.
Online Grocery Store is an e-commerce website, which retails various grocery products. This project allows viewing various products available enables registered users to purchase desired products instantly using Paytm, UPI payment processor (Instant Pay) and also can place order by using Cash on Delivery (Pay Later) option. This project provides an easy access to Administrators and Managers to view orders placed using Pay Later and Instant Pay options.
In order to develop an e-commerce website, a number of Technologies must be studied and understood. These include multi-tiered architecture, server and client-side scripting techniques, implementation technologies, programming language (such as PHP, HTML, CSS, JavaScript) and MySQL relational databases. This is a project with the objective to develop a basic website where a consumer is provided with a shopping cart website and also to know about the technologies used to develop such a website.
This document will discuss each of the underlying technologies to create and implement an e- commerce website.
Welcome to WIPAC Monthly the magazine brought to you by the LinkedIn Group Water Industry Process Automation & Control.
In this month's edition, along with this month's industry news to celebrate the 13 years since the group was created we have articles including
A case study of the used of Advanced Process Control at the Wastewater Treatment works at Lleida in Spain
A look back on an article on smart wastewater networks in order to see how the industry has measured up in the interim around the adoption of Digital Transformation in the Water Industry.
COLLEGE BUS MANAGEMENT SYSTEM PROJECT REPORT.pdfKamal Acharya
The College Bus Management system is completely developed by Visual Basic .NET Version. The application is connect with most secured database language MS SQL Server. The application is develop by using best combination of front-end and back-end languages. The application is totally design like flat user interface. This flat user interface is more attractive user interface in 2017. The application is gives more important to the system functionality. The application is to manage the student’s details, driver’s details, bus details, bus route details, bus fees details and more. The application has only one unit for admin. The admin can manage the entire application. The admin can login into the application by using username and password of the admin. The application is develop for big and small colleges. It is more user friendly for non-computer person. Even they can easily learn how to manage the application within hours. The application is more secure by the admin. The system will give an effective output for the VB.Net and SQL Server given as input to the system. The compiled java program given as input to the system, after scanning the program will generate different reports. The application generates the report for users. The admin can view and download the report of the data. The application deliver the excel format reports. Because, excel formatted reports is very easy to understand the income and expense of the college bus. This application is mainly develop for windows operating system users. In 2017, 73% of people enterprises are using windows operating system. So the application will easily install for all the windows operating system users. The application-developed size is very low. The application consumes very low space in disk. Therefore, the user can allocate very minimum local disk space for this application.
Quality defects in TMT Bars, Possible causes and Potential Solutions.PrashantGoswami42
Maintaining high-quality standards in the production of TMT bars is crucial for ensuring structural integrity in construction. Addressing common defects through careful monitoring, standardized processes, and advanced technology can significantly improve the quality of TMT bars. Continuous training and adherence to quality control measures will also play a pivotal role in minimizing these defects.
About
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
• Remote control: Parallel or serial interface.
• Compatible with MAFI CCR system.
• Compatible with IDM8000 CCR.
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
• Easy in configuration using DIP switches.
Technical Specifications
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
Key Features
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
• Remote control: Parallel or serial interface
• Compatible with MAFI CCR system
• Copatiable with IDM8000 CCR
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
Application
• Remote control: Parallel or serial interface.
• Compatible with MAFI CCR system.
• Compatible with IDM8000 CCR.
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
• Easy in configuration using DIP switches.
Courier management system project report.pdfKamal Acharya
It is now-a-days very important for the people to send or receive articles like imported furniture, electronic items, gifts, business goods and the like. People depend vastly on different transport systems which mostly use the manual way of receiving and delivering the articles. There is no way to track the articles till they are received and there is no way to let the customer know what happened in transit, once he booked some articles. In such a situation, we need a system which completely computerizes the cargo activities including time to time tracking of the articles sent. This need is fulfilled by Courier Management System software which is online software for the cargo management people that enables them to receive the goods from a source and send them to a required destination and track their status from time to time.
Automobile Management System Project Report.pdfKamal Acharya
The proposed project is developed to manage the automobile in the automobile dealer company. The main module in this project is login, automobile management, customer management, sales, complaints and reports. The first module is the login. The automobile showroom owner should login to the project for usage. The username and password are verified and if it is correct, next form opens. If the username and password are not correct, it shows the error message.
When a customer search for a automobile, if the automobile is available, they will be taken to a page that shows the details of the automobile including automobile name, automobile ID, quantity, price etc. “Automobile Management System” is useful for maintaining automobiles, customers effectively and hence helps for establishing good relation between customer and automobile organization. It contains various customized modules for effectively maintaining automobiles and stock information accurately and safely.
When the automobile is sold to the customer, stock will be reduced automatically. When a new purchase is made, stock will be increased automatically. While selecting automobiles for sale, the proposed software will automatically check for total number of available stock of that particular item, if the total stock of that particular item is less than 5, software will notify the user to purchase the particular item.
Also when the user tries to sale items which are not in stock, the system will prompt the user that the stock is not enough. Customers of this system can search for a automobile; can purchase a automobile easily by selecting fast. On the other hand the stock of automobiles can be maintained perfectly by the automobile shop manager overcoming the drawbacks of existing system.
Industrial Training at Shahjalal Fertilizer Company Limited (SFCL)MdTanvirMahtab2
This presentation is about the working procedure of Shahjalal Fertilizer Company Limited (SFCL). A Govt. owned Company of Bangladesh Chemical Industries Corporation under Ministry of Industries.
Democratizing Fuzzing at Scale by Abhishek Aryaabh.arya
Presented at NUS: Fuzzing and Software Security Summer School 2024
This keynote talks about the democratization of fuzzing at scale, highlighting the collaboration between open source communities, academia, and industry to advance the field of fuzzing. It delves into the history of fuzzing, the development of scalable fuzzing platforms, and the empowerment of community-driven research. The talk will further discuss recent advancements leveraging AI/ML and offer insights into the future evolution of the fuzzing landscape.
1. Advanced Generator Ground Fault Protections
Wayne Hartmann
Beckwith Electric Company
WHartmann@BeckwithElectric.com
Abstract
Ground faults in generator stator and field/rotor circuits are serious events that can lead to
damage, costly repair, extended outage and loss of revenue.
This paper explores advances in field/rotor circuit ground fault and stator ground fault
protection. These advanced protection strategies employ AC injection and other tactics to
provide benefits in security, sensitivity and speed.
Introduction
Field/Rotor ground fault
Traditional field/rotor circuit ground fault protection schemes employ DC voltage detection.
Schemes based on DC principles are subject to security issues during field forcing and other
sudden shifts in field current.
To mitigate the security issues of traditional DC-based rotor ground fault protection
schemes, AC injection-based protection may be used. AC injection-based protection ignores
the effects of sudden DC current changes in the field/rotor circuits and attendant DC
scheme security issues.
Stator ground fault
Traditional stator ground fault protection schemes include neutral overvoltage and various
third harmonic voltage-dependent schemes. These exhibit sensitivity, security and clearing
speed issues that may subject a generator to prolonged low level ground faults that may
evolve into damaging faults.
To mitigate the sensitivity, security and speed issues of traditional stator ground fault
protection schemes, sequence-component-supervised protection, transient detection
schemes and low frequency AC injection-based protection may be used.
Sequence component supervised protection is used to discriminate against out-of-zone
ground faults and accelerate ground overvoltage schemes for in-zone faults.
A transient fault detection scheme is used to identify fleeting arcing faults which may
quickly evolve into permanent phase-to-ground or multiphase faults.
Low frequency AC injection-based protection is used to identify ground faults regardless
of operational mode or power level that cause difficulties with other schemes.
Field/Rotor Ground Fault Protection
1. Description and damage mechanism
The field/rotor circuit of a generator is an ungrounded DC system. The effect of one ground
in the field/rotor circuit establishes a reference to ground on the normally ungrounded
system. The voltage gradient to other parts of the field/rotor circuit increases as you move
away from the ground reference point in the circuit. If weakened insulation exists, it is more
likely to break down where the voltage gradient is now greater.
While an initial field/rotor circuit ground establishes a ground reference, the generator
remains operational. In the event of a second ground fault, however, part of the field/rotor
circuit is shorted out, and the resultant shorted portion of the rotor causes unequal flux in
2. the air gap between the rotor and the stator with the rotor at rated speed. The unequal flux
in the air gap causes torsional stress and vibration, and can lead to considerable damage in
the rotor and the bearings. In extreme cases, rotor contact with the stator is possible. A
second rotor ground fault also produces rotor iron heating from the unbalanced currents,
which results in unbalanced temperatures causing rotor distortion and vibration. Field/rotor
ground faults should be detected and affected generators alarmed with high resistance
levels and tripped for low resistance levels.
This protection may be employed on brushed excitation systems and brushless excitation
systems. The terms brushed and brushless refer to the use or nonuse of power
commutation brushes to provide field current to the rotor.
In the case of a brushed exciter, the ground fault protection system is coupled to the
field/rotor circuit through power commutation brushes and the grounding brush as seen
in Fig. 1.
In the case of brushless exciter, the ground fault protection system is coupled to the
field/rotor circuit through a measurement brush that is connected to the field/rotor
circuit and the grounding brush as seen in Fig. 2.
Fig. 1 “Brushed” Exciter
Fig. 2 “Brushless” Exciter
2. Traditional protection
Traditional field/rotor ground protection systems employ DC voltage injection and
monitoring. The intent of the protection is to alarm or trip for ground faults in the field/rotor
circuit.
The scheme shown in Fig. 3 employs a DC source in series with an overvoltage relay coil
connected between the negative side of the generator field/rotor circuit and ground. A
ground anywhere in the field causes current through the relay.
3. The scheme shown in Fig. 4 employs a voltage divider connected across the field and a
sensitive voltage relay between the divider midpoint and ground. The voltage divider is
composed of two standard resistors (R1 and R2) and one nonlinear resistor (RVarister).
Maximum voltage is impressed on the relay by a ground on either the positive or negative
side of the field circuit. In order for a fault at the midpoint to be detected, the nonlinear
resistor is applied. The nonlinear resistor’s value changes with voltage, varying the nullpoint
from the midpoint of the field/rotor circuit.
Fig. 3 DC Source 64F Fig. 4 Voltage Divider 64F
As both of these schemes use DC voltage, they are prone to insecure operation from DC
transients in the excitation and field circuits.
3. Advanced AC injection method
The scheme in Fig. 5 employs low frequency (0.1 to 1.0 Hz) ±15 volt square wave injection.
The square wave signal is injected into the field/rotor circuit through a coupling network.
The return signal waveform is modified because of the field winding capacitance. The
injection frequency setting is adjusted to compensate for field/rotor circuit capacitance and
relay-to-coupler lead length. Using the input and return voltage signals, the relay calculates
the field insulation resistance. The element setpoints are in ohms, typically with a 20-kilohm
alarm and 5-kilohm trip or critical alarm.
4. Fig. 5 AC Injection 64F
Using AC injection, the scheme is secure against the effects of DC transients in the
field/rotor circuit. An issue with generator plant operator regarding field ground fault
protection is the DC systems are prone to false alarms and false trips, so they sometimes
are ignored or rendered inoperative, placing the generator at risk. The AC system offers
much greater security so this important protection is not ignored or rendered inoperative.
Another benefit of the low frequency AC-injection system is that it can detect a rise in
impedance which is characteristic of grounding brush lift-off. In brushless systems, the
measurement brush may be periodically connected for short time intervals. If brush lift-off
protection is applied, the brush lift-off function must be blocked during the time interval the
measurement brush is disconnected.
Stator Ground Fault
1. Description and damage mechanism
Ground faults in a generator (stator winding) can cause considerable and severe damage as
the level of fault current increases. Depending on the ground fault current available, the
damage may be repairable or non-repairable. Generators are subject to prolonged exposure
to stator ground fault damage due to the fact that even if the system connection and
excitation are tripped, stored flux remains and contributes to the arc as the generator
coasts down. Due to the exposure to this damage, several types of generator grounding are
employed. The stator circuit of a generator may be ungrounded, impedance grounded, or
solidly grounded. Some descriptions follow.
A. Ungrounded
Ungrounded generators are not typically applied in utility systems, and find special
application in industrial systems (Fig. 6).
5. G System
Fig. 6 Ungrounded Generators
Advantages:
1. The first ground fault on a system causes only a small ground current to flow, so
the system may be operated with a ground fault present, improving system
continuity.
2. Arcing with a ground fault on the system may be greatly reduced, which is seen
as an advantage in certain industries (e.g., mining)
3. No expenditures are required for grounding equipment or grounded system
conductors.
4. Generator damage is minimal if ground fault is in the generator (stator winding)
unless resonance occurs with arcing.
Disadvantages:
1. Ground fault detection is more complicated than that in grounded generators.
2. There is decreased safety once an initial ground fault is established.
3. There is the possibility of excessive overvoltages that can occur due to restrikes
in the generator breaker clearing the ground fault.
4. Detection and location of ground faults is more difficult when they do occur.
B. Low impedance grounded
Low impedance grounded generators are typically applied in industrial systems, and
find special application in utility systems (Fig. 7).
G System
Grounding
Resistor
Fig. 7 Low Impedance Grounded Generator
Advantages:
1. Reduces burning and melting effects in faulted electric equipment, such as
switchgear, transformers and cables.
2. Reduces mechanical stresses in circuits and apparatus carrying fault currents.
3. Reduces electric-shock hazards to personnel caused by stray ground-fault
currents in the ground return path.
4. Reduces 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. Reduces the momentary line-voltage dip occasioned by the occurrence and
clearing of a ground fault.
6. Controls transient overvoltages.
Disadvantages:
1. Generator damage occurs with a ground fault in the generator (stator winding)
C. High impedance grounded
High impedance grounded generators are typically applied in utility systems and
some industrial systems (Fig. 8). With a “unit connection,” the only ground source
for the generator, the bus and the primary of the GSU is the high impedance ground
formed by the grounding transformer and the reflected impedance of the grounding
resistor.
6. GSU
Transformer
G System
Neutral
Grounding
Transformer
Secondary
Neutral
Grounding
Resistor
Fig. 8 High Impedance Grounded Generator (Unit Connection)
Advantages:
1. Reduces burning and melting effects in faulted equipment, such as switchgear,
transformers and cables.
2. Reduces mechanical stresses in circuits and apparatus carrying fault currents.
3. Reduces 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.
4. Reduces the momentary line-voltage dip while clearing a ground fault.
5. Controls transient overvoltages.
6. Generator damage is minimal if ground fault is in the generator (stator winding).
Disadvantages:
1. Requires unit connection and specialized grounding equipment.
2. Full (100%) ground fault coverage may require advanced protection techniques.
D. Hybrid impedance grounded
Hybrid impedance grounded generators are gaining acceptance in industrial systems.
Hybrid impedance grounding, or hybrid grounding as it is commonly called, offers
advantages of a low impedance grounded system during unfaulted generator
operation, and high impedance grounding when a fault is detected within the
generator to minimize damage to the stator. The grounding impedance is switched
depending on the presence of a ground fault within the generator (Fig. 9).
G
87
GD
51
G
59
G
VS
Trip
Excitation
&
Prime Mover
52
G
52
F
52
S
System
3Y
1
51
51
N
System
Fig. 9 Hybrid Grounded Generator
7. Advantages:
1. Provides a low impedance grounded system during normal operation for reliable
ground fault detection.
2. Controls transient overvoltages.
3. Generator damage is minimized with a ground fault in the generator (stator
winding) due to switched-in high impedance ground.
Disadvantages:
1. A small time duration for high level fault current in the stator winding exists until
the fault is detected and the grounding impedance switched from low to high
impedance.
2. Requires specialized grounding equipment and ground switching equipment.
3. Requires advanced protection techniques.
The balance of this section addresses high impedance grounded generators.
2. Traditional protection
With high impedance grounding, the grounding resistor provides a measurable voltage for
faults in the upper 95% of the stator winding (with the generator terminals designated
100% of the stator winding). The traditional protection scheme for 95% stator coverage
employs a 59G element that measures fundamental voltage. Use of the fundamental voltage
ensures only voltage produced from a ground fault is measured, as opposed to harmonic
voltages which may be present in the generator neutral circuit (Fig. 10).
59
G
90-95% Coverage
NGT
NGR
System
GSU Transformer
Fig. 10 59G Scheme
Selectivity and sensitivity issues with this protection develop for system ground faults
outside the generator zone. These system ground faults, due to the capacitive coupling of
the generator step-up transformer (GSU), can cause current to flow through the generator
neutral, and therefore cause a voltage to be detected across the grounding resistor by the
59G element (Fig. 11).
To provide security for this possibility, two steps of ground overvoltage protection is
typically employed (Fig. 11).
8. 59
G-1
90-95% Coverage
NGT
NGR
System
Capacitive Coupling on
System Ground Fault
59
G-2
59
G-1
GSU Transformer
Fig. 11 GSU Capacitive Coupling for System Ground Faults
59G-1 is set for 95% stator coverage with a time delay set to coordinate with the
longest possible uncleared ground fault on power system. This is dependent upon the
speed of the line relays off the generating station, with backup and breaker failure time
considered in the setting (Fig. 12).
59G-2 is set for a level that is greater than the maximum calculated interference voltage
from a ground fault in the system, with a short time delay (Fig. 12).
To cover the last 5% of the stator winding, use is made of the fact that generators typically
produce small and sometimes measureable third harmonic voltage at the neutral and
terminal ends of the stator winding (Figs. 13 and 14). The presence or absence of these
third harmonic voltages can be used to provide detection of ground faults near the ends of
the stator winding.
Fig. 12 Use of Dual 59-G Elements
Fig. 13 Capacitive Model of Stator
9. Fig. 14 Stator 3rd
Harmonic Profiles and Terminal Connections
As the terminal end of the stator winding is easily covered by the 59G element, of interest is
detecting and declaring ground faults at the neutral end of the stator winding. In high
impedance-grounded generators, the ground fault current is typically limited to 5-20A with
a ground fault at 100% of the stator winding. As the ground fault location moves to the
neutral end of the stator winding, the current decreases proportionally. A great concern of
operating a generator with a ground fault near the neutral is even though the resultant fault
current is very small, the high impedance used to limit ground fault currents is effectively
shunted, and if a second ground fault develops, the resultant ground fault current will be
very large as there is not any ground impedance to limit such current (Fig. 15).
GSU
Transformer
System
Neutral
Grounding
Transformer
Neutral
Grounding
Resistor
Fig. 15 Fault near Stator Neutral Shunting Grounding Impedance
Two schemes using 3rd
harmonic voltage are commonly employed to detect faults near the
generator neutral. The first technique applies an undervoltage element tuned to the 3rd
harmonic voltage. This element, 27TN, operates on the fact that the 3rd
harmonic voltage is
shunted by a ground fault near the neutral (Fig. 16).
10. 59
G
0-15% Coverage
27
TN
59
NGT
NGR
GSU Transformer
Fig. 16 27TN Scheme
The second technique applies 3rd
harmonic voltage detection elements to the neutral and
terminal ends of the generator. This element, 59R or 59D, operates on the fact that the 3rd
harmonic voltage is shunted by a ground fault near the neutral or terminal end of the
stator, thereby changing the ratio of neutral and terminal 3rd
harmonic quantities (Fig. 17).
Fig. 17 59D or 59R Scheme
Both of these 3rd
harmonic implementations can be rendered inoperable, or worse, insecure,
by the fact that 3rd
harmonic voltages produced by a given generator can vary widely over
various modes of operation (generating or motoring, static starting), real power output and
reactive power output (field forcing, absorbing VArs for voltage control). Depending on the
generator and the system to which it is connected, use of 3rd
harmonic-based protections
may be severely limited or not applicable (Fig. 18).
11. Fig. 18 Generator Power Output vs. 3rd
Harmonics at the Neutral
3. Advanced methods
Sequence Component Supervision of 59G Element
To better cope with issues from capacitive coupling due to ground faults in the system side
of the GSU, a 59G acceleration scheme can be employed using sequence component
supervision. This method has been documented in two works [7] [8] and employs the fact
that ground faults outside of the unit connection produce levels of negative sequence
current and voltage. Either of these quantities (I2 or V2) may be used to declare the ground
fault is outside of the unit-connected generator, thereby employing a longer time delay on
the 59G element than applied on the primary, backup and breaker failure protection for the
ground fault outside the generator zone. If a negative sequence current or voltage is not
detected, the ground fault is presumed to be in the generator zone and a short delay for the
59G element is employed (Fig. 19).
A
V2 < sp
V0 > sp
59G > sp
59G-1
(Short Delay)
59G-2
(Long Delay)
A
I2 < sp
59G > sp
59G-1
(Short Delay)
59G-2
(Long Delay)
Fig. 19 Sequence Component Supervision of the 59G Element
Transient Ground Fault Retentive Timer for 59G and 27TN
Transient ground faults are characterized by existing for a short duration, then
extinguishing. Over time, repeated transi ent ground faults can break down insulation and
evolve into permanent ground faults, and perhaps evolve further into multiphase faults (Fig.
20).
12. Fig. 20 Transient Ground Fault Evolving into Multiphase Faults
To detect these ground faults, an interval timing scheme can be employed on the 59G or
27TN elements. If either the 59G or 27TN elements pick up and quickly drop out, a timer is
started. If two or more (settable by delay manipulation) transient ground faults occur within
the scheme timing window, a ground fault is declared and the 59G element trips (Fig. 21).
A
27TN pu < sp
V1 > 80%
59G pu > sp
O
Interval Timer
IN
5 cycles
Delay Timer
10 cycles
OUT
IN Pick Up
Drop Out
3 cycles
OUT
Trip
59G/27TN
Arcing
Fig. 21 Transient Ground Fault Timing Scheme Logic
100% Stator Ground Fault by Subharmonic Injection
To overcome limitations of the 100% stator ground fault protection offered by combining
the 59G and 3rd
harmonic elements (27TN, 59R, 59D), ground fault detection by
subharmonic injection may be used. Subharmonic injection is used (versus superharmonic)
because the capacitive coupling induced current effect is reduced by using a lower injection
frequency (Fig. 22).
This example system uses a 20Hz injection signal. The signal is connected to the generator
secondary ground circuit through a coupling network consisting of a low pass filter. This
filter prevents high level fundamental voltage from the power system impacting the injector.
When the injector is energized, the voltage is measured. That measured voltage ensures the
injector is working. The 20Hz current flows through the grounding transformer, and in the
normal state (a ground fault does not exist in the unit-connected generator zone), a small
13. amount of current flows due to the naturally occurring capacitance to ground in the
generator and isophase bus, plus capacitance of applied surge capacitors. A CT is used to
measure current resulting from the injection. This current demonstrates the system is
functioning, and ground circuit integrity is maintained. If a primary connection to ground
opens, or the secondary ground circuit opens, the capacitive current drops and an alarm is
issued (self-diagnostic). If a true resistive ground fault develops, the real component of the
ground current increases. Detecting the real component, versus the total component, offers
much greater sensitivity for fault detection.
Coupling Filter
Voltage
Injector
Measurements
I
Natural Capacitance
Notes:
Ø Subharmonic injection frequency = 20 Hz
Ø Coupling filter tuned for subharmonic frequency
Ø Measurement inputs tuned to respond to subharmonic frequency
V
Voltage
Injector
20Hz
Fig. 22 100% Stator Ground Fault Protection Using Subharmonic Injection
The system is independent of generator operation, and will reliably function when the
generator is offline, starting and during power system-connected operation. It does not rely
on 3rd
harmonic signatures, so generator loading (real, reactive) has no effect. By use of an
injection signal, this method is independent of the generator and power system conditions.
If the frequency of the connected power system approaches or is at the injection frequency,
interference will not occur if the connected source is balanced three phase. In the cases of
combustion turbine static starting, pumped hydroelectric plant starting and rotor warming,
the source is balanced three phase power; therefore zero sequence current does not flow,
and the neutral circuit is not affected. The current sensed by the injection system CT is not
affected by positive sequence current of any frequency being impressed at the generator
terminals (Fig. 23).
14. GSU
Transformer
G System
Neutral
Grounding
Transformer
Neutral
Grounding
Resistor
52
Static Frequency
Converter
V1
No V0, therefore no I0
No current flow through neutral
No interference with 20Hz injected signal
Fig. 23 Static Starting of a Combustion Gas Turbine
Summary and Conclusions
Field/Rotor Ground Fault
Use of AC injection offers greater security than traditional DC systems, and also affords
brush lift-off protection.
95% Stator Ground Fault Protection
Use of the 59G element is a time-tested method of protecting 95% of the stator for
generator ground faults.
The traditional approach to cope with GSU capacitive coupling and interference with the
59G element is using two elements, one long with a long time delay coordinated system
ground protection, and the other with a short time delay for in-zone ground faults.
An advanced method of using sequence component supervision allows determination of
external ground faults, and allows the 59G element to quickly clear ground faults in the
generator zone.
100% Stator Ground Fault Protection
3rd
harmonic protection implementations are available to complement the 59N element
to provide 100% stator ground fault protection. It should be noted that 3rd
harmonic
protections may not work with all generators, and may not work at all times on a given
generator. The 3rd
harmonic values available to the protection vary with operational
mode and power (real and reactive) output. Both security and dependability issues may
develop.
Transient ground faults can be detected with the use of an interval timing scheme on the
59G and 27TN protections. This enhancement affords the ability to detect transient
ground faults before a permanent ground fault develops.
The use of subharmonic injection affords the ability to detect ground faults anywhere in
the stator or in the unit-connected zone regardless of the generator operation and
loading. If the element uses the real component for fault declaration, it is very sensitive.
As long as external signals at or near the subharmonic injected frequency are balanced,
the element is highly secure. The element only responds to zero sequence current in the
generator neutral, not positive sequence current from an external balanced system such
as another generator during back-to-back starting or static converter employed in
starting combustion gas turbine generators.
15. References
1. IEEE Guide for Generator Ground Protection, ANSI/IEEE C37.101-2006.
2. IEEE Guide for AC Generator Protection, ANSI/IEEE C37.102-2006.
3. IEEE Tutorial on the Protection of Synchronous Generators, Second Edition, 2010;
Special Publication of the IEEE Power System Relaying Committee.
4. IEEE Recommended Practice for Grounding of Industrial and Commercial Power
Systems, IEEE Std. 142-1991.
5. Protection Considerations for Combustion Gas Turbine Static Starting; Working Group J-
2 of the Rotating Machinery Subcommittee, Power System Relay Committee.
6. Protective Relaying for Power Generation Systems; Donald Reimert, CRC Press 2006;
ISBN#0-8247-0700-1.
7. Practical Improvement to Stator Ground Fault Protection Using Negative Sequence
Current; Russell Patterson, Ahmed Eltom; IEEE Transactions Paper presented at the
Power and Energy Society General Meeting (PES), 2013 IEEE.
8. Behavior Analysis of the Stator Ground Fault (64G) Protection Scheme; Ramón
Sandoval, Fernando Morales, Eduardo Reyes, Sergio Meléndez and Jorge Félix,
presented to the Rotating Machinery Subcommittee of the IEEE Power System Relaying
Committee, January 2013.
Author Biography
Wayne Hartmann is Vice President of Protection and Smart Grid Solutions for Beckwith
Electric. He provides customer and industry linkage to Beckwith Electric’s solutions, as well
as contributing expertise for application engineering, training and product development.
Before joining Beckwith Electric, Wayne performed in application, sales and marketing
management capacities with PowerSecure, General Electric, Siemens Power T&D and Alstom
T&D. During Wayne's participation in the industry, his focus has been on the application of
protection and control systems for electrical generation, transmission, distribution, and
distributed energy resources.
Wayne is very active in the IEEE as a Senior Member and has served as a Main Committee
Member of the IEEE Power System Relaying Committee for 25 years. He is presently Chair
of the Working Group “Investigation of the Criteria for the Transfer of Motor Buses.” His
IEEE tenure includes having chaired the Rotating Machinery Protection Subcommittee (’07-
’10), contributing to numerous standards, guides, transactions, reports and tutorials, and
teaching at the T&D Conference and various local PES and IAS chapters. He has authored
and presented numerous technical papers and contributed to McGraw-Hill's “Standard
Handbook of Power Plant Engineering, 2nd Ed.”
16. ANNEX 1: High Side of GSU Ground Fault and Influence on 59G Voltage
The following calculations are for the 59G elements.
“59G V” is used for 95% stator winding coverage and would either be set to coordinate
with high side GSU ground fault (long time delay), or use I2 or V2 inhibit and a short
time delay.
“59G V max coupled” is set to be blind to the influence of a high side GSU ground fault
and employs a short time delay. Some margin would be added to the voltage setting in
this calculation.
System 1-Line:
G
System
R
Aux Load
GSU
UAT
Vnom = 13.8kV
0.01uf to
GND
0.12uf to
GND
0.24uf to
GND
1.27uf to
GND
1.69 ohm
33.2:1
115kV:
13.8kV
0.013uf
Interwind
Select R ground pri to equal Xct to limit transient overvoltage
R ground pri= 1,864 W
Calculate NGR based on R ground pri
NGR = R ground pri / (NGT ratio)^2
NGR= 1.69 W
Calculate Max Primary Ground Fault Current
GFC pri max = V L-N / R ground pri
GFC pri max = 4.28 A
Calculate Max Secondary Ground Fault Current
GFC sec max = V sec max / NGR
GFC sec max = 142 A
Calculate NGT/NGR Power Dissipation
kW = (V sec max * GFC sc max)/1000
kW = 34.1 kW
Calculate Worse Case Coupling Voltage
GSU High side V= 115,000 V
GSU interwinding capacitance = 0.013 uf
GFC pri coupled = V L-N highside / (1/6.28 * f * C)
GFC pri coupled = 0.33 A
59G V max coupled = (GFC pri coupled * R ground pri) / NGT ratio
59G V max coupled = 18.3 V
Calculate Generator Line-Neutral Voltage
V L-N rated = V L-L rated / 1.73
V L-L rated = 13,800 V
V L-N rated = 7,977 V
Calculate Total Capacitance
Ct = Cgen + Clead + Cgsu + Cuat + Csurge (uf)
Cgen + Clead 1.27 uf
Csurge 0.12 uf
Cgsu 0.01 uf
Cuat 0.24 uf
Ct = 1.64 uf
Calculate Total Capacitive Reactance (ohms)
Xct = 1/2*(3.14)*f*Ct
Xct = 1,864 W
Select NGT Ratio
Desired V sec = 240 V
NGT Ratio =V L-G/V sec max
NGT Ratio= 33.2
Calculate 95% 59G Setting
Desired Coverage % (from terminal end) 95 %
59G V = V sec max * ((100% - % Desired Coverage)/100)
59G V = 12 V
17. ANNEX 2: 64S Element Security Calculations
A 64S ground fault scheme using subharmonic injection can be made secure by using both
the real and total components of the monitored 20Hz current resultant currents with proper
setting and margin.
Real component: Used to detect and declare stator ground faults through the entire
stator winding (and the isophase and GSU/UAT windings), except at the neutral or faults
with very low (near zero) resistance.
Total component: A fault at the neutral or with very low resistance results in very
little/no voltage (VN) to measure, therefore the current cannot be segregated into
reactive and real components, so the total current is used as it does not require a
voltage reference. In addition, presence of total current provides a diagnostic check that
the system is functional and continuity exists in the ground primary and secondary
circuits.
A typical stator resistance (not reactance) to ground is >100k ohm, and a resistive fault in
the stator is typically declared in the order of <=5k ohm.
The two areas of security concern are when the generator is being operated at frequencies
of 20 Hz and 6.67 Hz. All other operating frequencies are of no concern due to the 20 Hz
filter and tuning of the element response for 20 Hz values.
For our analysis, we use data from a generator in the southeastern USA outfitted with a
64S, 20 Hz subharmonic injection system.
Case 1: Generator Operating at 20 Hz
If the generator is operating as a generator at 20 Hz without an external source (e.g.,
drive, LCI, back to back hydro start), there is no concern as the 20 Hz at the terminals is
at or very close to balanced; therefore, 20 Hz zero-sequence current will not flow
through the neutral circuit.
If the generator is being operated as a motor with an external source (e.g., drive, LCI,
back to back hydro start), the phase voltages are balanced or very close to balanced.
18. Coupling Filter
Measurements
I
Natural Capacitance
Notes:
Ø Subharmonic injection frequency = 20 Hz
Ø Coupling filter tuned for subharmonic frequency
Ø Measurement inputs tuned to respond to subharmonic frequency
V
Injector
20Hz
0.2W
25V
20kV
20,000V:240V
400:5 Relay
8W
343 MVA
1-Line Diagram
Generator Breaker Closed
Generator plus isophase, surge caps and GSU delta winding
Metered values, including observed 20 Hz values, no fault conditions.
VN 20 Hz = voltage across the neutral grounding resistor
IN 20 Hz (mA) = total current (combined real and reactive) measured by the relay
Real 20 Hz (mA) = real component of current measured by the relay
19. Calculate the CT primary currents:
IN pri (total) = 14.1 A * 10-3
* CTR
IN pri (total) = 14.1 A * 10-3
* 80
IN pri (total)= 1.128A
IN pri (real) = 2.8 A * 10-3 * CTR
IN pri (real) = 2.8 A * 10-3 * 80
IN pri (real) =0.224 A
The currents and voltages at the grounding transformer primary:
IN pri (total) =1.128 A / NGT ratio
IN pri (total) =1.128 A / 83.33
IN pri (total) =0.013536 A
IN pri (real) = 0.0224 A / NGT ratio
IN pri (real) = 0.0224 A / 83.33
IN pri (real) = 0.002688 A
VN pri = V sec * NGT ratio
VN pri = V sec * NGT ratio
VN pri = 25 V
3rd
harmonic voltage measured at relay = 0.75 V
V pri = V sec * NGT ratio
V pri = 0.75 V * 83.33
V pri =62.5 V
Assuming a zero sequence unbalance of 0.1% of the nominal at 60 Hz
V pri unbalance = % unbalance / 100 * V L-L rated / 3
V pri unbalance = (0.1% / 100) * (20,000 V / 1.73)
V pri unbalance = 11.5V
V sec unbalance = V pri unbalance / NGT ratio
V sec unbalance = 11.5 V / 83.33
V sec unbalance = 0.14 V
Assuming V/Hz is kept constant in LCI or back-to-back generator start. The voltage at 20 Hz
frequency is 20 Hz voltage during the start. Assuming 1pu V/Hz 120/60 = 2 = 1pu
Frequency divisor: 60 Hz / 20 Hz = 3. Voltage divisor is 3.
V sec unbalance (20 Hz) = V sec unbalance (60 Hz) / 3
V sec unbalance (20 Hz) = 0.14 V / 3 = 0.0466 V
20 Hz current flowing through NGR:
NGR I 20 Hz = V sec unbalance (20 Hz) * NGR W
NGR I 20 Hz = 0.0466 / 0.2 = 0.223 A
20. Relay measured 20 Hz current:
I 20Hz Relay = NGR I 20 Hz * CTR
I 20Hz Relay = 0.223 A / 80
I 20Hz Relay = 0.0029 A = 2.9 mA
Using pickup values are 20 mA total and 6 mA real, the element remains secure.
Note the margins:
Total current calculated: 2.9 mA
Total current setting: 20 mA
Margin: 17.1 mA
Total current calculated: 2.9 mA
Real current setting: 6.0 mA
Margin: 3.1 mA
Case 2: 6.67 Hz voltage at the generator terminals, assume 3rd
harmonic (20 Hz)
created in the neutral
In this case, we are assuming the generator under study is being started with a drive, LCI
or back to back hydro start. The generator is acting like a motor and the unbalance is
originating from the source.
Using typical values from a generator operating under full load, 3rd
harmonic can be
expected to be approximately 5X no load value.
3rd
V 60 Hz NGT pri = 5 * (no load 3rd
harmonic) * NGT ratio
3rd
V 60 Hz NGT pri = 5 * 0.75 V * 83.33
3rd
V 60 Hz NGT pri = 312.498 V
The frequency during the start is reduced to 6.67 Hz (3 * 6.67 Hz= 20 Hz).
Assuming the V/Hz is kept as constant, the 3rd
harmonic voltage is reduced.
3rd
V 20 Hz NGT pri = 6.67 Hz / 60 Hz * 312.498 V (without reduction in capacitance)
3rd
V 20 Hz NGT pri = 34.74 V (without reduction in capacitance)
Since the frequency is 20 Hz and not 180 Hz, there is a further reduction in 3rd
harmonic
current due to the capacitance at 1/9th of the 60 Hz value. (180/20=9)
The model is complex and the relationship is not straightforward, so we assume a reduction
of 1/5th instead of 1/9th
3rd
V 20 Hz NGT pri = 34.74 V / 5 = 6.9 V
Voltage at NGT secondary:
NGT V sec = 3rd
V 20 Hz NGT pri / NGT ratio
NGT V sec = 6.9 V / 83.33 = 0.0828 V
21. Current through NGR:
NGR I 20 Hz = NGT V sec / NGR W
NGR I 20 Hz = 0.0828 / 0.2 = 0.414 A
Relay measured 20 Hz current:
I 20Hz Relay = NGR I 20 Hz * CTR
I 20Hz Relay = 0.414 A / 80
I 20Hz Relay = 0.005175 A = 5.175 mA
Note the margins:
Total current calculated: 5.175 mA
Total current setting: 20 mA
Margin: 14.825 mA
Total current calculated: 5.175 mA
Real current setting: 6.0 mA
Margin: 0.825 mA
Below is an oscillograph from a CGT during static start at 6.67 Hz. Note the VN is zero (the
waveform seen under VN is noise).
By in-situ observation of the quiescent (non-faulted) real and total currents, and also
observing the total current with a fault placed on the neutral during commissioning (on a
deenergized and isolated generator), proper values can be selected with adequate margin to
effect a coordinated protection scheme that is dependable, sensitive and secure.
VN =
ZERO