Protocols Blog

DCS 480V Breaker control via mMint and MODBUS - Part 1 (mMint) 

Friday, June 8, 2012 4:27:00 PM Categories: digitrip eaton mmint modbus ovation

OVERVIEW

So for this project it was decided that for all the LV breakers we were going to try and save money and serially open and close the breakers remotely from the DCS.  This makes sense and we are seeing this more and more because of arc flash and not wanting personnel to be close to the breakers when they are closed or opened.  Wiring the breakers to I/O costs alot more than if you are able to serially control them.  Now generally most companies I have been at don't like to use modbus for control but this control was not being used for interlocking or automation;  it was mainly going to be used for maintenance and startup.

EQUIPMENT INVOLVED

480V Circuit Breakers with Digitrip 1150 Trip Units

Circuit Breaker  

mMint INCOM to MODBUS convertor

mMint

Reference:   I.L.66A7508H07    

Ovation 1C31166G01 serial link card

   

 

ARCHITECTURE

All of the breakers were Eaton MDS616 or MDS632 with Digitrip 1150 Trip Units.  We ordered an mMint for each BUS with the idea that they would connect to an Ovation DCS via their 1C31166G01 serial link card.

 mMint Wiring

 

The mMint is actually a passive device that converts the protocol INCOM that the Digitrips use to MODBUS.  So, when the DCS which is the MODBUS Master polls for a Digitrips Breaker set to INCOM address 3 for example, the mMint would convert that INCOM address 3 to MODBUS slave address 3.  As far as the DCS goes though it is as if it is talking directly to the Digitrip .  So you don't need to know anything about INCOM to make this work.  In the mMint documentation they talk alot about the 'INCOM Product' which can be confusing and really just means the MODBUS slave device.

All of the data that the digitrip 1150's map to MODBUS registers is on page 10 of the mMint manual.  You have two different data types for the same information; IEEE Floats and Fixed Points.  We used IEEE Floats as then we didn't have to worry about scaling.  As long as the master can handle this data type and there are not alot of points coming across this seems to work fine.

The one thing to watch out for is that the table on page 10 lists a bunch of parameters that the Digitrip 1150 doesn't have.  Eaton made a common mapping for all of their "INCOM Products".  So if the MODBUS master polls for registers that aren't supported by the Digitrip 1150 an error will be thrown.  For example: The Digitrip 1150 supports Ia, Ib, Ic, Ig, In but not Iavg.  The IEEE Float register for Ia is 404611 and since IEEE Float are 32 bits it requires two registers so 404611 and 404612.  So if you poll for register 404611 and 2 registers you will get a valid response back.  So if you want to get the currents mentioned above you would poll starting at 404611 and end at 404620; so 10 registers.  This would give a valid response and return the data.  But if you brought back 12 registers starting at 404611 you would get an error as register 404621 Iavg is not supported by the Digitrip 1150. 

The hardest part was figuring out the 'Control to INCOM Product' which is  detailed on Figure 8, Page 7 of the mMint pdf.

The Eaton engineers designed breaker commands to not be able to be given accidently.  You have to load, using function 16, 3 specific registers with 3 specific values.

To figure out what each of the registers needs to be you have to get the 'Slave Action Number'.  These are shown on Table 12 on page 17 of the user manual.

On Table 12 you can see that the Slave Action Number to Open the breaker is:

Byte0 = 0

Byte1 = 0

Byte2 = 1

and to Close the breaker is:

Byte0 = 1

Byte1 = 0

Byte 2 = 1

So, using Figure 8 above you can see that to Open the breaker the value that you will need to write into the control registers are:

For Register 42901 it is simply Slave Action Byte0 and Byte1 so..

00000000-00000000 or 000016 HEX or 0INT

For Register 42902 it is Slave Action Byte2 and the 1's Complement of Slave Action Byte1.

Well what is the 1's complement of something?  It is simply the exact opposite or binary reverse of it.

So... since Byte 0 is 00000000 this would be

11111111-00000001 or FF0116 HEX or 65281INT

For Register 42903 it is the 1's complement of Slave Action Byte1 and Byte2 so..

 11111110-11111111 or FEFF16 HEX or 65279INT

So, if you want to send an open command to the breaker you write the following using function code 16:

42901 -> 0

42902 -> 65281

42903 -> 65279

If you want to send a close command it is Slave Action Number 101 or..

42901 ->  1

42902 -> 65025

42903 -> 65279

At the FAT for the switchgear we were unable to test the mMint to DCS communications adequately but we were able to check the mMint functionality using   modscan32 as a MODBUS master running on a laptop.

In fact, there is a ton of configuration that you can do to the Digitrip using modscan32 scripts.  I will write more on that and the DCS side of things in Part 2.

Along with a laptop that has a COM port, you will need a RS-232 to RS-485 convertor.  I like the 485SD9TB by B&B Electronics as it is port powered and just works.

If you download the trial version of Modscan32 you should be able to use it as a modbus master to run scripts from your laptop via the mMint to open and close your breakers.

If you login you should be able to download the scripts at the bottom of the protocols page.

If there is an interest and you want to know exactly how to run the scripts in modscan32 leave a comment and I will put something up.