Motor Protection system use only 2 Current Transformer 1

[iphapppi]

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

 

[waross]

This was once done in North America. It was found that it did not provide adequate protection to motors.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I have heard it was an old practice to have overloads on 2 of 3 phases, and no longer in favor as Bill says.

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)' ?

 

[electricpete]

I should clarify, ground fault protection often involves use of 3 CT's. How do you do it with two?

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(2B)+(2B)' ?

 

[electricpete]

Or maybe there's a window CT for ground fault protection ?

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(2B)+(2B)' ?

 

[jraef]

Actually, it depends on what else is in the protection scheme. Bill hit it on the nose in terms of why it was outlawed, but even here in North America we allow an exception to that rule if there are " other means of protection". So for example if you have thermistor or RTD thermal sensors, they will provide the missing thermal protection necessary when using only 2 sensors. the issue is, current imbalance will heat a motor disproportionately to the amount of current flow, so current monitoring becomes a less effective means of motor protection in that case, and having only 2 sensors makes complete imbalance protection very difficult. So adding back in some other form of thermal protection, especially something more direct such as RTDs or even Thermistors, is a way of compensating for that shortcoming.

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.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[iphapppi]

Waross : for motor larger than 2 MW (like ID Fan), it also provided with differential protection, the motor relay management is also provided with voltage protection like under/over voltage. what do you think?

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

 

[electricpete]

Having overload protection on only two of three phases has been discussed here many times before, was well covered by Bill, and I fully agree.

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)' ?

 

[dpc]

There are a lot of old ungrounded 4160 V and 6900 V systems still in service. These typically had only two CTs for overcurrent protection and no ground fault protection. For grounded systems, a third relay was used for ground fault via a flux summation CT.

 

[electricpete]

Thanks, that’s kind of what I figured would be required (although not particularly for ungrounded system… I didn’t realize there were ungrounded medium voltage systems).

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)' ?

 

[DTR2011]

Interesting enough, when reviewing the latest version of "Protective Relaying Theory and Applications" 2nd Ed - ABB, 2 CT's are shown in the schemes for motors above 1500 HP, feeding relays for ANSI Dev # 46,50,51. It also shows RTD, a Zero Sequence CT(50G) and 87 protection with all required CT's. This information is a bit dated as I don't see it covering a comprehensive motor management protection scheme with a single relay.

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.

 

[Marke]

There are two ways that the two CTs can be used.
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

 

[waross]

Thank you for that insight Mark. I have used two CTs for revenue metering on three phases.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

The application that Mark Empson is talking about works on a high impedance grounded (or ungrounded) system. I've seen a number of older mills with impedance grounded systems where there were just two CTs and two relays. No redundancy what so ever, but it was cheap. When it came time to replace the electromechanical relays with numeric it was necessary to sum the two phases and then run that backward through the third phase of the relay.

 

[iphapppi]

thank you all for your explanation. I really appreciate and Sorry because I've been in remote area with no internet acces for a few days ago.