Lightninarrester
Member
For a 1250KW 13.8kv 0.8P.F, how do you size the generator neutral grounding transformer? how do you size grounding resistor across the secondary of the grounding transformer? Details appreciated. Thanks in advance.
Netral Grounding Transformer and Resistor sizing
- Thread starter Lightninarrester
- Start date
- Status
Re: Netral Grounding Transformer and Resistor sizing
It sounds like you need some help specifying this system. Below are links to some manufacturers that can help.
http://www.avtron.com/neutral_grounding_resistors.htm
http://www.postglover.com/hand/index.htm
http://www.ipc-resistors.com/
It sounds like you need some help specifying this system. Below are links to some manufacturers that can help.
http://www.avtron.com/neutral_grounding_resistors.htm
http://www.postglover.com/hand/index.htm
http://www.ipc-resistors.com/
physis
Senior Member
Re: Netral Grounding Transformer and Resistor sizing
Thanks Nick. I see that there are indeed ground resistors.
This from Avtron:
My second reaction is, where did they come up with this typical 20,000 to 40,000 amps fault? This thing goes between the service and the grounding electode system. In other words, it's inserted into the GEC. Not the EGC. Or do I have it wrong?
0.006 Ohms?
Edit: I made a nonsensical sentence.
[ June 24, 2005, 08:25 PM: Message edited by: physis ]
Thanks Nick. I see that there are indeed ground resistors.
This from Avtron:
This is interesting. My first reaction is would this be 250 compliant?
My second reaction is, where did they come up with this typical 20,000 to 40,000 amps fault? This thing goes between the service and the grounding electode system. In other words, it's inserted into the GEC. Not the EGC. Or do I have it wrong?
0.006 Ohms?
Edit: I made a nonsensical sentence.
[ June 24, 2005, 08:25 PM: Message edited by: physis ]
O
oliver100
Guest
Re: Netral Grounding Transformer and Resistor sizing
So sizing the transformer IMO is just XX/5 amps with consideration for the system voltage (13.8 KV). Sizing the resistor should be based on the IR^2 for limited (clearing the fault) time.
It is interesting how to determine the tolerable ground fault - 5 amps or 10,000 amps. The basis should be the nominal current for a particular generator or transformar.
As far as resistor, across a current transformer - what is the purpose?
A current transformer in the grounding line provides signal to the relay protection of the generator. On the other, hand the resistor in the grounding circuit limits the ground fault.
So sizing the transformer IMO is just XX/5 amps with consideration for the system voltage (13.8 KV). Sizing the resistor should be based on the IR^2 for limited (clearing the fault) time.
It is interesting how to determine the tolerable ground fault - 5 amps or 10,000 amps. The basis should be the nominal current for a particular generator or transformar.
As far as resistor, across a current transformer - what is the purpose?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Netral Grounding Transformer and Resistor sizing
Impedance Grounded Neutral Systems for over 600V applications is addressed under 250.186.
Properly determining the size of a loading resistor across a grounding transformer is a detailed design function that requires consideration of several conditions. The information given so far is insufficient and can?t be addressed with any degree of relevance.
IEEE Std. 142 Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) gives considerable guidance.
Impedance Grounded Neutral Systems for over 600V applications is addressed under 250.186.
Properly determining the size of a loading resistor across a grounding transformer is a detailed design function that requires consideration of several conditions. The information given so far is insufficient and can?t be addressed with any degree of relevance.
IEEE Std. 142 Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) gives considerable guidance.
rcwilson
Senior Member
- Location
- Redmond, WA
Re: Netral Grounding Transformer and Resistor sizing
Several engineering decisions need to be made in sizing the transformer or resistor for generator grounding. Very few of them relate to the size of the generator. Most are concerned with the level of allowable ground fault damage and how the design intent of the complete electrical system. The ANSI/IEEE Standard C37.101 ?Guide for Generator Ground Protection? is a good resource.
You need to look at several things for sizing generator neutral high impedance grounding.
#1 - What current will you allow during a ground fault?
On large generators (>5 MVA) a 10 amps limit is typical to minimize generator stator iron arcing damage during an internal fault. Sometimes as much as 600 amps is needed to operate any ground fault overcurrent relays on downstream equipment.
#2 - What other ground source is on the same bus?
Other ground sources affect the total ground fault current, the amount of damage and the relaying selected. If a delta-wye, solidly grounded transformer or another generator is on the same bus, the high resistance grounding values and fault current contributions have to be coordinated.
#3 - What is the system capacitance?
The cables, bus, transformers and generator windings all have capacitance to ground that suck a charging currents through the neutral transformer/resistor. If the grounding system impedance is too high, not enough charging current can flow and there is a possibility of unstable neutral to ground voltages.
#4- How long will the ground fault current be allowed to flow?
Continuous (alarm only), 0.5 seconds, 30 seconds, 5 minutes? This will determine the KVA rating of the neutral grounding system.
#5. What type of protective relaying is on the generator and downstream devices? Is the relay using current or voltage to detect a ground fault? What is the minimum pickup of the downstream ground fault relays?
With answers to these questions and a complete one line of the system, the ratings of the equipment can be determined.
Several engineering decisions need to be made in sizing the transformer or resistor for generator grounding. Very few of them relate to the size of the generator. Most are concerned with the level of allowable ground fault damage and how the design intent of the complete electrical system. The ANSI/IEEE Standard C37.101 ?Guide for Generator Ground Protection? is a good resource.
You need to look at several things for sizing generator neutral high impedance grounding.
#1 - What current will you allow during a ground fault?
On large generators (>5 MVA) a 10 amps limit is typical to minimize generator stator iron arcing damage during an internal fault. Sometimes as much as 600 amps is needed to operate any ground fault overcurrent relays on downstream equipment.
#2 - What other ground source is on the same bus?
Other ground sources affect the total ground fault current, the amount of damage and the relaying selected. If a delta-wye, solidly grounded transformer or another generator is on the same bus, the high resistance grounding values and fault current contributions have to be coordinated.
#3 - What is the system capacitance?
The cables, bus, transformers and generator windings all have capacitance to ground that suck a charging currents through the neutral transformer/resistor. If the grounding system impedance is too high, not enough charging current can flow and there is a possibility of unstable neutral to ground voltages.
#4- How long will the ground fault current be allowed to flow?
Continuous (alarm only), 0.5 seconds, 30 seconds, 5 minutes? This will determine the KVA rating of the neutral grounding system.
#5. What type of protective relaying is on the generator and downstream devices? Is the relay using current or voltage to detect a ground fault? What is the minimum pickup of the downstream ground fault relays?
With answers to these questions and a complete one line of the system, the ratings of the equipment can be determined.
- Status