Essential-Service Motor Defeating some important motor protection ? 4

[sparky1976]

Hello every body,

I have problems with my motor protection lots of them, often false tripping and no cause found try hundred times to to trouble shot or probably incorrect settings but everything seem OK.
When I found technical paper from GE chapter 10 A-C Generator and motor protection on page 202 its rather confuse me its seem the motor is poorly protected, is it a usual pratice ?

If I want to disable the 46 that protect motor from single phasing trip, is there someone can advise me the brand that sell single function 46 device (the relay I got is one package) or is it safe enough and leave it to thermal as back up?

 

[busbar]

You are dealing with the age-old quandary of a short-term fix versus longer-term cure.

Disabling of a protective function needs careful review, understanding and acceptance by your management. Up the ladder a bit, production management will at times be at odds with maintenance management over preventive and breakdown repair efforts.

Remember that single phasing and phase loss are extreme cases of phase imbalance. A lot of stator damage can occur with less than a simple open phase. Thermal-overload relays (or their numerical equivalent) are generally much less sensitive to imbalance conditions and their consequences.

The 46 device is a negative-sequence current relay that is sensitive to current imbalance. Insulation failure from excess-heat generation in the stator can easily lead to serious repair costs and loss of production. Consider removal / transport / reinstall labor and motor-shop expenses -– possibly at a premium overtime rate.

Clearly understand system and component operating sequence. What external mechanical and electrical conditions influence electrical characteristics the relay is measuring? Is there an event report that the multifunction protective device can furnish? At what level were settings determined -- in-house or by an outside engineering firm? Are you able to verify that the intended settings are indeed programmed into the protective device? Can the protective relay package be tested removed from normal service? At some point, instrument-transformer and other control wiring needs careful verification.

It may be time to contemplate retaining some outside help or test equipment rental. Weigh using a multisource protective relay set versus a simple OL relay test box; i.e., primary versus secondary injection. An apparatus-repair or electrical-testing firm should have some track record with the particular protection package at hand.

These are some valid questions for what can be a very frustrating situation; that is, being under pressure to quickly and reliably troubleshoot a problem that is causing immediate production losses. The term “essential-service motor” should not be taken lightly. Bet on this being a good learning experience.

 

[sparky1976]

Busbar I give you astar there.
We already invite the relay vendor to site and they bring some fancy test equipment and still cannot find whats wrong with their relays.
Our manager already tired dicking around the vendor and think to buy some very standard motor protection and not to use this relays on our next project.

 

[busbar]

One standalone relay is a Basler Electric BE1-46N. It has optional analog output proportional to AC negative-sequence current.

 

[jbartos]

Suggestion: Reference:
1. ANSI/IEEE Std 242-1986 "An American National Standard IEEE Recommended Practice for Protection of Industrial and Commercial Power Systems"
1)Reference 1 page 148 par 4.8 "Phase Balance Current Relay - Device No. 46" indicates two different kinds of 46:
1a) Current balanced relay that operates when the difference in the magnitude of rms currents in two phases exceeds a given percentage value. This one will leave you some margins to consider.
1b) The negative sequence current relay operates on magnitude of negative sequence current, but is calibrated in terms of (I2**2)xt, the thermal energy produced by the negative sequence current. In order to set the negative sequence relay, the (I2**2)xt characteristic of the machine must be specified. This is not addressed in the above postings. The solid state 46Negative provides an alarm at sensitive pretrip value of (I2**2)xt in addition to the trip setting.
2. Normally, the phase unbalance protection is applied to motors 1000hp and up.
3. If you consider any delay in motor trip, e.g. 2 or 3 seconds, the motor designer should be contacted.
4. Reference 1, Section 9 "Motor Protection" practices are recommended. Notice that 46 Phase Current Balance may have inherent time delay. 46 Negative-sequence voltage operates instantaneously and may need timer. 46 Negative-sequence current is not intended for motors since it has a relatively high pick up. This statement cautions to use any unsubstantiated recommendation as far as this relay is concerned.

 

[BJC]

How big are these motors? What is the load? Does the load ocassionally drive the motor? Conveyor belts running down hill are the extream case of this, some times pumps can act as turbines. Would these conditions trip a negative sequence relay?

 

[sparky1976]

BJC,
The motor rating vary from 500 hp to 1000 hp. These motors drive fans.

 

[busbar]

Negative-sequence overcurrent and negative-sequence overvoltage relays can be desirable for ‘smaller’ motors. One example is a set of submersible pumps that serve water to a research facility. The cost of removal of one of these motors for inspection was greater than that of the motor itself, and exceeded the installed price of the protection scheme by an order of magnitude. ANSI C37.96 §2.7

 

[sharmen]

Busbar is correct. To get a motor that size rewound will be a significant cost. In addition, you will have the labour involved in removing and installing it, and the lost production associated with an OOC unit. You may want to consider retrofitting your system with a fully integrated solid state motor protection unit? Many of the modern units have multiple functions including 37, 46, 49, 50, 51, 51G, &86, just to name a few. Also, many manufacturers have engineering services that will do the engineering, specification, installation, testing, and commissioning for you.

I, personally, have never had much luck with stand alone Negative Sequence Relays, especially the electro-mechanical type.

To answer your original question. ABB, Basler, & GE have stand alone 46 devices. However, if you are going to have someone do retrofit work, you may want to investigate a full motor protection unit. The following are some manufacturers sites.

Cutler-Hammer

https://www.ch.cutler-hammer.com/docs/Fred/apscpgh/htmL/products/mp3000/mp3000.htm

GE

http://www.geindustrial.com/industrialsystems/pm/catalog/indexa.htm

ABB

http://www.abb.com/

Siemens

http://www.e.ptd.siemens.com/webapp/EVWebApp/index_en.jsp

Startco

http://www.startco.ca/products.html

Basler

http://www.basler.com/html/pcsdf-25.htm#MPS100

 

[BJC]

What kind of relays do you have in the system?
and is it possible for the system to push air through a fan and put the motor in an unloaded position or in a power generating mode.
I have worked on systems with parallel fans and faulty dampers. One fan would cause the other to run backwards and make it difficult to start.
If you had a situation like an FD and ID fan in a boiler the ID fan could suck enough air through and ID fan to cause it to unload and even act as a generator.
Some relays are smart enough to determine this even though you may know know the feature is active. It's for conditions where a pump impeller could be unscrewed from a pump shaft by flowing water.
I may be off here but I would like to study this problem more when I have time. Thanks

 

[electricpete]

I'm a little skeptical about whether a light-load or revserse power situation would actuate a negative-sequence relay. The relay is intended to detect unbalance currents. It is true that negative sequence stator currents do correspond to stator field rotating in the reverse direction. However if a motor should go in to reverse power situation, presumably all three phases would undergo a 180 degree phase shift (with respect to the terminal voltage reference). Mathematically, this does not change the rotation sequence (abc or acb). In physical terms, it should be apparent that the direction of rotation of stator field will be the same as the direction of rotation of the rotor whether the machine is acting as an inducation motor or an induction generator. Imbalance and harmonics can add components of the field which rotate in opposite direction, but the fundamental field must travel in the same direction for power transfer to occur.

 

[don01]

yamin
I can't comment directly on the details of these protection relays. I simply don't know the units. But you are saying you are having lots of trips - how many per motor say per day or week etc??.

If your problem extends across the range then you have a systemic problem. not indivudual motors or units. eg The set up is wrong the units are wrong you may even have an operating proceedure on your plant which has changed or needs review.

to try and help I suggest the following questions be asked -
when did the problem start.
what was changed at this time
what is the problem. (yeh the motor trips but how and which one)
when does the problem occur (always 3am ? only at start up?)

did some of this info up and post it for us it may help.


hi electricpete - I do agree with you about the low risk of reverse generation but I have seen motors "howling along " when the pump were down and they have tripped the incomers when started because the operator forgot to close the dampers before start up - just simply on the reverse flow or flood back. (i have heard of but not seen, shafts sheared by the same process)

Regards to all
Don

 

[electricpete]

Don - I have heard of but not seen what you describe as well (attempting motor start when the motor was rotating backwards). It's an interesting subject. Here are my thoughts about it.

The common scenario would be two fans or pumps feeding a common discharge header. The one that is not running tends to be spun backwards by the reverse flow (if not check valve, backdraft damper or anti-reverse-rotation ratchet device is present). Here's my understanding of what happens when we attempt to start it:

The starting current and starting torque can go either direction (above normal or below normal). The reason is that there are two competing effects when below zero speed (slip is more than 1):
A - Induced rotor current has a higher frequency so the rotor and rotor reactance is higher and tends to reduce inrush current and initial torque.
B - Induced rotor voltage is higher (proportional to relative speed). This tends to increase inrush current and initial torque.

Another way to visualize the torque when below zero speed is to extrapolate the torque-speed curve below zero. The extrapolation should be relatively smooth. Depending on the slope of the torque speed curve as it crosses zero speed (can be positive or negative depending on motor), it's easy to see that the starting torque under reverse rotation conditions migt be either greater or less than under normal conditions.

The starting time will always be longer than normal, since the rotor/load has to accelerate further. Particularly important for a fan which is a high-inertia load to begin with.

 

[BJC]

electricpete and don01.
My point in asking yamin what kind of relay he had was to see if some of the senarios discussed above could be happing and no one was aware of it. Some solid state relays have so many funtions that you may not know which of them are programed on. If he has current input ( and I assume he does) reverse power flow could be sensed.
There are applications of generator "anti-motoring" relays used on motors to limit their contribution ot a fault, but it seems to me they would not be fast enough to do any good.
When yamin said "fans" I was alerted to potential system configurations that be contributing to his problem. Fan systems are notorious for these kinds of problems. Even with an airflow diagram you have analyze the system and assume some dampers leak.
Yamin could have a fan system where the air flow trys to put power into the system or the air overloads the fans. Most fans can overload motors if inlet and outlet dampers are tweeked just right.
I worked on a fan system that an axial flow fan that ran backwards and triped on time/oc just because it took so long to accelerate. The long term fix was to repair leaky dampers, the short term fix was dynamic braking by running DC current through two windings.

 

[electricpete]

BJC - Sorry to misunderstand your point. I initially assumed that you were concluding that reverse-rotating torque resulted in negative sequence stator currents (which sounded plausible on the surface, but is probably not correct).

Sorry about that. Next time I'll think twice before assuming the other guy is wrong.

 

[BJC]

electricpete
No need to apologize.
I think the guy has an interesting problem. On which has potential for some learning. If we can get all the facts that is.

 

[sparky1976]

Guys,
Sorry for late response, I just got back from vacation.
Sharmen tanks for the links, My motor protection right now is one package(multifunction). The option to remedy the problem (right now with my conclusion) is to change to another brand multifunction relay (will cost a lots) or start disabling the function that initiates false tripping and cover the function by installing stand alone relay.

Don01 to answer your quetions:
1. The problem start since startup, I involved in start up.

2. We change the relay to newer serial # or upgrade the old relay, change the setting that considered too conservative.

3. The problem is the relay initiate false tripping like, ground fault or short circuit or under voltage or single phasing trip. Like I said before we've already reroute the wiring hundred times, do standard test and inspection cannot find anything wrong.

4. The problem occurs randomly, I donot see a pattern related to swithing, weather.


I have about 35 large motors and statistic of false tripping is once a week. All motor starts DOL.

I hope you guys can help me on this.

Thanks
Ilyas

4.

 

[peterb]

Yamin,
This has turned into a really interesting thread, with some great responses. I fully endorse what Busbar has had to say, and would add my two cents worth as follows -

It seems to me that your problem may be related to the installation rather than the setting of the relays, based on the random operations experienced. One point that may be well worth checking is the grounding of the relays and instrument transformer circuits.
Note that the relays are most likely equipped with grounding terminals that should preferably be connected to a "clean" instrument ground. CT secondary wiring must be grounded at one point only, multiple grounds can cause relay mal-operation for faults on other circuits (primarily ground fault element trips, one of the factors that you note).
Also, look at cable shielding and grounding practices - could there be noise coming in on the relay circuits? This would apply to instrument transformer and power supply cabling.
You say that the relay vendor has visited the site and done tests. Have they not been able to offer you any tangible support? If not, then you really do need to think about chaging vendors (and maybe advising us here of their identity). I spent several years working with one of the major relay manufacturers and we would have been extremely anxious to ensure that our product was being properly applied if such a condition as yours had arisen - I would hope that you would get a supportive response from them to resolve your problem.

 

[BJC]

Peterb has some good points and they should all be checked. If all the fans are doing it then I suspect it is a systemic design problem or:
I still betting money on a system problem independant of the relays or the motors. The more sophisciated
relays have no load trips that sense when the motor is at no load and trip it off line. The theory being that it found a broken shaft.
Fans systems can behave stangely especially if the ductwork has parallel and series connections and or/complicated damper arrangements. You can overload a squirrel cage blower by fulling opening the inlet damper, if you full close it the fan will go to no load. Axial flow fans have different patterns but can go to overload or no load also by flow control. A fan that is blowing no air, being forced to move to much air can trip.

 

[electricpete]

Yamin -
Do the motor trips occur only while starting? Or after the motor has been running several minutes also?

Does your relay give you any indication of which protective feature caused the trip?

Does your relay capture conditions at the time of the trip (love them SEL's).