# How to calculate the accurate size of the NGR

#### khalidgee1986

I am designing a power plant have three generators with similar capacity. The output of each generator is 11kV, 2 MW, 50HZ, and the nominal current of each generator is 132A. all three generators will be run in parallel on a common bus bar. I want to install an NGR to protect the generator from the earth's fault current. kindly anyone guides me on how can I calculate the NGR resistance and current carrying capacity.

#### PhilCorso

]khalidgee
Welcome to Control.com!
I understand the economic and operational advantages of Single and Multiple NGR schemes! But you should chose one that produces 2 major goals:
a) stator-winding damage, not stator-iron damage!
b) operator simplicity.
Thus, I recommend you consider Differential-Ground-Fault or Restricted Earth-Fault protection! Both are more costly! But, more importantly, either become a major contributor to plant-asset performance and operator-safety!
After all, isn't that the responsibility of the System Designer! Let me know if you want additional information!
Regards, Phil Corso

#### khalidgee1986

Thanks
PhilCorso
the client wants to install NGR protection to restrict the earth fault current of the generator. I have to calculate the specification of NGR which is suitable for the generator as the generator output V=11kV, 50 HZ, and nominal Current= 132A, can you guide how can calculate the accurate size of NGR? or what is the procedure to calculate the required NGR specification?

#### PhilCorso

khalidgee1986
1) Are the generators Wye or Delta connected?
2) Assuming Wye, I'm suggesting you familiarize yourself with what is called in the USA, the Distribution-Transformer & Resistor Grounding (Europeans call it the Earthing-Transformer & Resistor Earthing).
Unfortunately, other important factors must be known like:
a) What is Generator Winding Capacitance?
b) What is connected System Cable Capacitance?
c) What is Distribution-Transformer Capacitance?
d) What is Surge Arrestor Capacitance?
e) Are the Generators equipped with Field-Forcing?
Can you obtain the data required?
Regards, Phil Corso

#### khalidgee1986

I will try to get the data as most I can and then share it with you. Thanks

#### khalidgee1986

the generator is connected to why the connection. here I attached the alternator data sheet. is the above-mentioned information required for the calculation of NGR? if yes can you provide the formula so I can go through it.
Thanks

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#### PhilCorso

khalidgee1986
The data shown in the attachment you provided is that of a Low-Voltage 380/460-V, 50/60-Hz, Stanford PM Generator! There is NO information that can extracted for your goal of providing an NGR!!!
Regards, Phil

#### PhilCorso

khalidgee1986
Do you no longer want to continue your original request for an Generator NGR?
Phil Corso

#### khalidgee1986

thanks for your support I have resolved the matter and I am sharing some information.
Following are the Alternator nominal values:
kW=2012
V= 11000 VL-L
A= 132
In order to safeguard the generator's winding against the harmful of fault currents, we shall restrict the fault current to a magnitude not exceeding 105 A, as compared to the nominal current.
The Neutral Earthing Resistor (NER) value can find out through the utilization of the subsequent formula:
R= NER Resistance =?
VL= Line Voltage = 11000 V
If= Fault Current = 100 A
N= Neutral
R = VL-N/ If = (VL-L/√3) / If
(11000/1.732) / If
R = V/I
R = VL-N/ If
R= 6350/100
R=63.5 Ohm
 System Voltage 11 kV Rated Voltage 11/√3 kV Max. Operation Voltage 12/√3 kV Rated Frequency 50 HZ Ohmic Value (at 25 °C) 60 Ω Rated Short Time Current 105 A Rated Time 10 s Main Standard IEEE Std. C57.32a-2020 Resistor Temperature Rise After Operation < 760 °C Resistor Tolerance ±10 % Current Transformers (IEC 61869-2) 150/5 A, 5P10 10 VA 1 Disconnector Switch Vacuum contactors 4 Over Current Relay (50/50N) 1 Protection Degree (EN 60529) Resistor compartment: IP23 Switchgear compartment: IP54 Installation Outdoor Enclosure Aluminised zinc treated mild steel enclosure for outdoor service, with lockable access doors

#### khalidgee1986

khalidgee1986
Do you no longer want to continue your original request for an Generator NGR?
Phil Corso
I feel that most of the time persons here ask questions over a question and didn't provide a solution. sorry to say that.

#### WTF?

We are very sorry you feel that way, and wish you the best in your future endeavors.

Blessed day.

#### PhilCorso

@khalidgee1986 & WTF?
1. You have not accounted for various component-capacitance!
2. You have not provided time-coordination with downstream earth-fault devices!
3. 100-A earth-fault current for 10 sec will damage stator-iron!
Regards, Phil Corso

#### WTF?

@khalidgee1986

Not a single question in that response (but a lot of exclamation points).

This is a technical forum and as such we (many of us) try to provide meaningful and concise answers to what are, often and increasingly, posts that have little or no useful information. There is an also increasing sense that with little or no information experts can immediately discern the problem and provide concise information and solutions. We often get questions (exactly) like, “Why did the machine trip?” with little or no information about the circumstances prior to the trip OR the alarms annunciated when the machine tripped—because the experts just know why it tripped.

We (most of us) try to get people to understand and use the tools they have so that they can understand and troubleshoot problems in the future—or at least provide better information if they write for help in the future. (“Give a man a fish; feed him for a day. Teach a man to fish, feed him for life.”) Sometimes it works; most often it doesn’t. Sometimes we get incorrect information in replies to the questions. (Sound familiar?) In today’s world many people just want to be told what to do—they aren’t really interested in learning anything, they just want their problem solved for them without doing anything more than posting an incomplete message to invisible experts who are omniscient.

This is a free forum—meaning you don’t pay to ask your question, AND we don’t get paid to answer. (That’s right; we volunteer our time and experience.) And, AGAIN, it is a technical forum so most of the time we need information to be of assistance.

Blessed day.

#### PhilCorso

@khalidgee1986, @ WTF?
The error was mine. I thought you wanted the best. I failed to realize you wanted "An" NGR. Taken literally, that means "One" Resistor.
Thus, WTF?'s answers, Af=100 Amperes, NGR=63.5 Ohms, Fault-DurationTime Ft=10 seconds, can be used.

I apologize, for having misunderstood your position. But, If you want to know how much of the winding is protected, let me know!
Blessed life,
(And only 1 exclamation point) Phil