Negative Sequence Voltage to DFR 1

[tefaber]

We have a client that is interested in taking a negative sequence voltage measurement back to their DFR. From looking at their schemes, they don't appear to have any relay or device capable of producing a negative sequence voltage output for use by the DFR.

1. Does anyone know of an old electromechanical relay (since that all that is installed off of the PTs) that would have such an output.
2. Is there a relatively cheap option for installing some sort of negative sequence filter/transducer?

Thanks for the input.

 

[davidbeach]

If the DFR has all three phase voltages and can output a COMTRADE file you can post process the COMTRADE file to calculate negative sequence.

 

[jghrist]

Define relatively cheap. A Schweitzer SEL-749M Motor Relay (base retail $1200) with the voltage option (pricing?) will output negative sequence voltage.

 

[davidbeach]

jghrist, yes, it will calculate negative sequence voltage and there may be even less expensive relays that will do that. But what I understood the original question to be, and I may have not understood the intent of the question, was that tefaber wanted to have a negative sequence voltage channel in the DFR output. If that is what is desired, I'm not aware of anything that will process three voltage inputs and then output a voltage proportional to the calculated negative sequence voltage.

 

[stevenal]

I never heard of a relay that would output analog quantities to a terminal, but the negative sequence voltage driving the induction disk of a negative sequence voltage relay might be possibly be tapped. See the ABB protection book, Fig 3-9 for diagram of the filtering used.

 

[dpc]

Was the OP looking for an ANALOG output proportional to negative seq voltage? I'm still confused (as usual).

 

[jghrist]

According to Section 5 (Metering and Monitoring) of the SEL-749M manual,

The relay provides the following methods to read the present meter values:
? Front-panel rotating display
? Front-panel menu
? EIA-232 serial ports (using SEL ASCII text commands or ACSELERATOR® QuickSet™ SEL-5030 Software
? Modbus® via Port 3 or Port 4
? DeviceNet port
? Analog output

and one of the measured values (with the voltage option) is negative-sequence voltage (3V2).

 

[davidbeach]

Oh, OK. But it would be a bit tough to deal with going into the DFR since it would be a 4-20mA signal. On the other hand, that brings up a different solution, the SEL-2411 has the ability to convert a calculate 3V2 value to a 0-10V (or +/- 10V) analog output. I'm not sure if that could go directly into the DFR or not, but probably comes closer than the 4-20mA of the 749M.

 

[jghrist]

So what is really needed is an AC voltage? Maybe it could be done with transformers. You need to put V_A in series with V_B, shifted 240°, and in series with V_C, shifted 120°, to get 3V2. Any ideas for cheap phase-shifting transformers?

 

[stevenal]

jghrist, davidbeach

I don't think your SEL solutions meet the OP's requirement number 1.

Sorry if I confused the issue with the word "analog." I didn't mean to imply the OP required a DC current loop. I assume the dfr would accept a secondary AC voltage level at its inputs (analog of the primary voltage, and not a digital input).

The figure I referenced is in the older Westinghouse version of the book also.

 

[dpc]

The old Basler 47 relay is an analog solid-state device. I'm sure somewhere before the comparator circuit there is an analog signal that is proportional to negative sequence voltage. It's not brought out to a terminal, but it might not be too hard to access.

 

[davidbeach]

Negative sequence was hugely expensive in the electromechanical days and was therefore limited to select machines. The phase shifting networks are not trivial, so I think the answer to #1 is "No". There is no harm in mixing in a new numeric relay with electromechanicals, particularly on the VT circuit. Unfamiliarity might be a problem, but no electrical issues.

I suppose that if one were daring, one could dig into a BE1-47N and try to find the output of the phase shifting network, but it would probably be at a very low level that would need to be amplified to go into the DFR.

 

[rcw retired EE]

I don't see much difference between the DFR recording an AC voltage signal representing the negative sequence voltage and using a post recording software package to calculate and display the negative sequence voltage.

Since it is a DFR, the signals are probably not going to be used for real time protection or control.

Is there a benefit to having a real time recording of negative sequence voltage?