Synchronous Motor Protection

[dkle40h]

Hi,
I am busy with a project to install new switchgear to supply two 2500HP, 6300V Synchronous Motors. As part of the project I thought of including a motor protection relay to provide stator protection for the motors, to complement the Multilin SPM (Synchronous Motor Protection) relays currently installed. I believe that the SPM relays control starting, synchronising, and provide rotor protection. GE Multilin recommend a 369/469 relay.
Other options considered include Alstom's (the switchgear supplier's) P225, a Cutler-Hammer MP 3000, Reyrolle RHO3. How does one objectively evaluate the performance of the relays? Cost, ease of programming, etc must also be taken into account and I am not sure where to start. Any advice will be appreciated.
Best Regards

 

[jbartos]

Suggestion: Sometimes what helps is to ask the relay sales reps for references that would include similar applications to yours. Then, try to contact the customers for experience with the product. There is nothing to be lost and a lot to be gained. In fact, if you happen to have a very similar application, the customers may become interested in the latest application/advancement, etc. in your Firm.

 

[jraef]

dkle40h,
The answer depends somewhat on how you are going to start the motors. Redundancy is a good thing in many systems but can be problematic in a protection system when it comes time to troubleshoot a fault or a nuisance trip. For instance if you are using a solid state soft starter, most of the ones on the market now include very excellent motor protection features comparable to what a Motor Protection Relay does, even with RTD input capabilities. The same will be true for a VFD. Adding the SPM will then give you a complete system for the synchronous motor. Adding an MPR to them means duplicating most if not all of the protection functions.

If you are using an Across-the-Line (DOL) starter, they typically come with bare minimum protection as standard. You had then better add the best MPR you can get because the strain on the motor and supply system will be tough to start with, leaving little room for errors.

Another thing to consider is the extent of protection you intend to utilize. At 2500HP+, many people add Differential Current tripping because it is significantly faster than inverse time/current tripping in the case of a fault. Of the choices you listed I beleive that only the GE/Multilin SR469 offers Differential Protection as a standard input. The others would require a separate DP relay and the associated coordination that goes with it.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Suggestion: Siemens also offers an excellent motor protection. Visit

http://www.sea.siemens.com

http://www.sea.siemens.com/power/product/ptsamms.html

for: Siemens Advanced Motor Master System (SAMMS)
Differential protection can be seen similar to the "Current unbalance." Once there is some above the set point current unbalance, there is the more serious motor problem.

 

[dkle40h]

Thank you for the comments. It would appear after my investigations that the majority of the top-end motor protection relays offer very similar protection features. Still remain at a loss to successfully adjudicate the best relay in terms of protection features, ease of programming, reliability, etc.

 

[jbartos]

Comment: The protection features may be compared via relay tech data sheet. The ease of programming is somewhat subjective. The comparison may be made by those who will actually program those relays. Software manuals may also be scrutinized. The reliability data are sometimes hard to obtain from the manufacturer since they can be used against the manufacturer in case of the relay malfunctions.

 

[Dopler]

We have been using GE relays for 17 years, electromechanical and digital, they work fine. We are introducing SEL relays, It look like will be the same history, but less cost.

 

[Rodmcm]

Get a reference list from the supplier and contact other users. Our experience is that the functionality is nearly the same for intergrated relays but the programming between them and the options for extracting data is very different between manufacturers

 

[jbartos]

Comment: Also, reliability data such as MTBF, MTTR, etc. vary. These do not seem to be readily available in catalogs and manuals.

 

[MVRockwell]

Greeting Everyone,

First let me state that I work for Rockwell/Allen-Bradley in the Medium Voltage Motor Control Center Division as a MV Engineer. This is not a selling pitch (at least I hope it is not), I just wish to help as this site contains a lot of informed people and good information.

Typically when I Engineer an MV synchronous machine controller there are 3 major question I have.
#1. Is this a Brush or Brushless type synchronous motor
#2. How is it to be starter (full voltage, reduced voltage autotransformer, Solid-State softstater, etc.
#3. Are there any special application requirements (i.e. dynamic braking)

As your question is how do I protect a motor, question 1 and 2 will be important. Not all synchronous protection relays are created equally. The top two relays I deal with are the GE SPM and the Rockwell SyncPro. Below is a quick summary of Pros and Cons

SyncPro
Pro
Easy to program (only 8 parameters)
Reliable hardware (based on PLC hardware)
Cost (approx 40% of SPM cost)
Can be used with reduced voltage controllers (with some tweaking)

Con
Supports Brush-Type motors only

SPM
Pro
Supports Brush and Brushless motors
Provide power factor regulation

Con
Every expensive
Quality issues (LCD display, micro-processor)
Long lead times
Recommended for full voltage starting only (per GE)
Overkill for Brushless motors as it only acts like a timer, DC ammeter/voltmeter, and PF relay.

However overall most synchronous relays do good job in protection and control of half of a synchronous motors. The other half of the protection is provided by a motor protection relay such as the
GE 269 Plus, 369, SR469
SEL 701, 749M
Cutler-Hammer MP3000
Startco MPS
Rockwell Bul. 825Plus

If you would like a detail review/comparison, just ask.

Obviously my preference is to use a Rockwell product, but I have used all of the above and more. The major differences between the units come down to a few things;
1. Price (GE most expensive, Rockwell and SEL least expensive)
2. Communication options (Rockwell most, C-H and GE least)
3. Extras such differential current transformer input (offered by GE SR469). Typically these make the relays more expensive and can be done with cheater and motor powerful discrete relays such as SEL-551 for differential protection, Rockwell PowerMonitor 3000 for metering , etc.

I hope this has been helpful. If you have any question please ask.

Regards

 

[aolalde]

MVRockwell (Electrical)

Could you provide a link to see details of your SyncPro?

 

[MVRockwell]

You can find a user manual at

http://www.theautomationbookstore.com/public.html

Look for publication 1901-5.0. You can also find some related information at

http://www.ab.com/mvb/literature.html

Regards,

 

[jraef]

MVRockwell,
Thanks for the info, and nice comparison.

Not to be too picky, but you mention the SEL-551 as offering Differential Protection and from what I see it does not, at least as defined by IEEE Function 87. The only one SEL seems to offer with that feature is the SEL-300G, which at $6000 is hardly economical. Is there anything else you have found for differential? The reason I ask is that I am always looking for a low-cost way of providing Fn 87 when a GE SR469 is unwarranted / duplicated. I have yet to find anything less than around $3500.

"Venditori de oleum-vipera non vigere excordis populi"

 

[MVRockwell]

Generally differential protection is not necessary when you're using an electronic motor protection relay (i.e GE 369, Rockwell Bul. 825, SEL-701) with accurate ground fault protection and current unbalance.

However when the situation comes that we're asked to provide differential protection on a motor and are not using the GE SR469, we look for an overcurrent relay (which is what the 469 is doing) The GE SR469 essentially is using the core-balance style differential CT inputs and performing an overcurrent function. With this in mind we can apply the use of other overcurrent relays, as long as the device can be set to low levels to trip. Other overcurrent relays we've use are Basler 50/51, ABB, and SEL-551. The good thing about SEL is that it's electronic and has very low CT burden (which means smaller CT, less$$ and space).

The SEL-551 is a good overcurrent/recloser relay with many functions but you don't have to all of it functions and hence make a great tool to provide differential protection (which is simply overcurrent on the core balance differential CTs)
From SEL
"Comprehensive Overcurrent Protection
Phase, negative-sequence, residual ground, and neutral overcurrent protection in a compact package. Complete set of instantaneous, definite-time, and time-overcurrent elements."

The best thing is that it's less than $1000 USD

If you're still concerned I have a couple of contacts as we do a lot of work with SEL. Let me know.

 

[jraef]

Well, what you are describing then is not true Fn 87 protection, that is actually just Fn 51. Fn 87 is more common for transformers and bus systems, but for motors requires 6 CTs in the motor connection box looking at each end of each winding connection and acting upon a differential in current from one end to the other (the GE SR469 has 6 CT inputs for doing this). 87 relays act much much faster than what you would see doing it the way you describe. I agree that they are not as prevalent on smaller motors, but I see specs requiring them on almost everything 2500HP and up now and a few on smaller motors. That is why I was looking for aditional resources. I'm surprised you don't see it more often in your capacity.

"Venditori de oleum-vipera non vigere excordis populi"

 

[MVRockwell]

From what I understand Fn87 is to measure/trip on a level that is based on a difference. The difference typically looked at is the current going into and out a motor. (I know that sounds very simplistic, I'm sorry) The use of an overcurrent is not what makes it Fn 87, it the way the 3 diff. CT's are configured at the motor. The SR469 does have 6 CT inputs (3 phase for metering and protection, and 3 diff for diff protection) The SR469 is looking for current at it diff CT inputs as in normal conditions there should be none. Each diff CT has a incoming power lead and a "wye motor lead" (I'm sure it called something else). If there is a problem the two lead fields don't cancel, similar to the idea of a zero sequence/core balance ground fault CT. You might say the diff CT's are acting as a per phase gfct and the 469 is acting like an overcurrent relay to detect and trip. Again this is only one form of providing fn87. You can also use 6 CT (3 phase, and 3 at the "wye motor leads") which you can connect as if you're doing a residue ground fault detection and then use an overcurrent to detect a difference. This is a similar idea for protection of a transformer (6 CT, 3 line, 3 load and measure a difference between in/out power with some correction factors).

As for seeing diff. protection in our project, it was the norm for motor over 1500hp, but it's becoming less and less required as the ground fault protection being offered by these electronic relays are very good and usually trip before the diff protection.





Joe Moura P.Eng.
Rockwell Automation
MV Project Engineer

 

[electricpete]

I agree with MVRockwell on terminology.

My spin:

Differential protection can be provided in at least two ways which do not include a differential relay. First we can use window CT's on each phase feeding overcurrent relay, or 2nd we can use 2 ct's per phase wired in a differential connection where the current difference is fed to an overcurrent relay.

The differential relay is the most sensitive since it provides ability for percent slope characteristic. With the other approaches the protection needs to be set loose to allow for max ct error at full load.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.