Motor Protection system use only 2 Current Transformer
Motor Protection system use only 2 Current Transformer
(OP)
I work on a Coal fired power plant project now. We use 6 kV system for the large motor like ID Fan, FD Fan, etc. We use relay management system from China. this protection system only use 2 Current Transformer (CT)for the protection (phase a & c). none in phase b. do you think it has negative efect on the protection scheme because we just can monitor 2 out of 3 phase. this system is applied to all large motor in this power plant





RE: Motor Protection system use only 2 Current Transformer
When faced with a high voltage on one phase, that phase can overheat and burn out even though the current drawn by the other two phase windings is less than full load current.
For the last 50 years or so, three phase monitoring and protection has been demanded by North American codes.
Check your local codes. They may speak to this condition.
By the way, differential protection is also a good idea in most installations.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Protection system use only 2 Current Transformer
Here, you have a 6 kV system with only two CT's. I wonder if you even have ground fault protection on the 3rd phase?
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(2B)+(2B)' ?
RE: Motor Protection system use only 2 Current Transformer
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(2B)+(2B)' ?
RE: Motor Protection system use only 2 Current Transformer
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(2B)+(2B)' ?
RE: Motor Protection system use only 2 Current Transformer
What often happens however is that people ASSume that the thermal device system is optional, don't use it, and lose the motor, then wonder why. So to that end, it is better to start out with a 3 CT solution, then add the thermal devices if you feel it is necessary. That's what most of us do.
By the way, 6kV motors are typically on something VERY important in a plant. Deciding on using the cheapest junk motor protector available (as evidenced by having only 2 CTs) can end up being the most costly decision they have made.
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RE: Motor Protection system use only 2 Current Transformer
electricpete : I have learn about this protection scheme, it is provide with zero sequence CT to detect the ground fault, is it ok to use it?
Jraef: it is one of my concern because this relay is provide with complete protection such. but while commisioning, i found that many of these protection fuction disabled, like motor jam protection and thermal relay. is it danger? because almost all our large motor is using this scheme.. thank you all
RE: Motor Protection system use only 2 Current Transformer
The point I wanted to bring out is that that since this is a medium voltage machine with CT's integrated into the protection, the implications of having only two CT's may extend beyond overload protection.
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(2B)+(2B)' ?
RE: Motor Protection system use only 2 Current Transformer
RE: Motor Protection system use only 2 Current Transformer
It seems like it would be cheaper and better to provide three individual phase current CT's with residual-connection for ground fault detection rather than two phase current CT's with window CT for ground fault detection, wouldn't it?
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(2B)+(2B)' ?
RE: Motor Protection system use only 2 Current Transformer
How are the CT's connected and how many CTs are shown in the installation manual for the relay?
The concept of using one less instrument transformer is covered by Blondel's theorem and is still quite common in metering applications. Using 2 VT's in an open delta configuration is still quite common on generator protection.
The OP also mentioned relay functions that are not enabled. I would advise that you bring this to the attention of the Engineer that issued the settings. Typically with a digital relay there are many functions, however that does not mean that they all need to be implemented. What this does show, however is that you are paying attention to the schemes and details. Rarely is this considered a poor question to ask.
RE: Motor Protection system use only 2 Current Transformer
In one scheme, the theory is that as the motor is a three wire device and the current in the third phase is the vector sum of the first two, then measuring the current on two phases and using the vector sum to create the third phase actually does work very well provided that there is not a high leakage level to ground (effectively a four wire system!!)
In the other system, the magnitude only on the two phases is measured and the third phase is assumed to be equal and this can be very wrong if there is a significant phase shift in one phase.
Three phaase monitoring with two CTs does work, but three phase monitoring with three CTs is better.
Two phase monitoring with two CTs is very inadequate.
Best regards,
Mark
Mark Empson
Advanced Motor Control Ltd
RE: Motor Protection system use only 2 Current Transformer
I was thinking about the protection schemes where a CT directly feeds a single pole overload relay. If the third overload relay is connected between the ungrounded terminals of the two CTs it will see a current equivalent to the current in the third phase as long as there is not a ground fault internal to the motor. It will also protect against more than 60% of internal ground faults.
But the purpose of the overload devices is to protect against overloads, not ground faults, and the time delay aspect of an overload device makes it inherently unsuitable for ground fault protection.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Protection system use only 2 Current Transformer
RE: Motor Protection system use only 2 Current Transformer