Choosing the Right Scaling Factors for Bitronics® IEDs

 

Q:    My SCADA software gives me one field to enter a scale factor and a field to enter a zero offset for converting each DNP analog point into primary engineering units.  The Bitronics configuration software asks me to enter a CT scale factor and a VT scale factor, but it doesn’t mention anything about what scaling to use for power, frequency, or other kinds of points besides the current and voltage.  How do I choose the best scale factors to enter in the Bitronics software?  Then what should I enter in SCADA?

A:    The settings for Modbus and DNP scaling on the 70 Series IED are designed to make most analog points work out for an optimum balance between range and resolution if you enter the same value for the CT and VT scale factors that you use for the CT and VT ratio settings.  That is, if the current and voltage are optimized, then the watts, VArs and VA will also work out.  Some measurements, like power factor and frequency do not depend on the CT and VT scale factors at all, and are handled a little differently.  And some measurements, like line-to-line volts may require you to choose a different value for the VT scaling factor than you use for the VT ratio under certain circumstances.

 

To keep this from getting confusing, let’s break the answer into three parts.  In the first part, I’ll describe the typical case where the scaling factors are the same as the instrument transformer ratios.  In the issue of TechTalk we’ll handle exceptional cases, where you might want to use a scale factor that’s different from the transformer ratios.  Then in the final part, we’ll describe points where the scaling factor doesn’t have anything to do with the CT and VT ratios.  At the conclusion of the three-part series, we’ll concatenate all three parts into a single white paper that covers all scaling issues comprehensively.  I’ll even throw in a primer on binary integer math and the significance of “two’s-complement” encoding.

 

DNP Scaling, Part 1:  The Typical Case

 

The 70 Series Configurator software has one page (Figure 1, top) to enter the CT and VT ratio settings.  There is another page (Figure 1, bottom) for each protocol where the CT and VT Scale Factors are entered. That way Modbus and DNP protocols can be scaled independently.  For the IED to work properly, the CT and VT Ratio settings must always be set equal to the actual turns ratios of the instrument transformers connected to the voltage and current terminals of the IED.  It is never necessary to “trick the meter” by multiplying a ratio times root-three, or any other value under any circumstances.  But the CT and VT Scale Factor settings can be manipulated to adjust the range and the resolution of various measurements according to the unique circumstances that may arise in a substation.

Figure 1

 

In the typical case, you would enter a CT Scale Factor equal to the CT Ratio setting and the VT Scale Factor equal to the VT Ratio setting.  There is an equation on the DNP Points page (Figure 2) that can be used to calculate the scale factor you should use in SCADA.

For most analog points, such as amps, volts, watts and VArs, the equation has the general form:
PEU = RV/32,768 * FSV * SF
Where:
PEU is the primary engineering units.  This is the “answer” you’re generally looking for.
RV is the raw DNP integer value (“Register Value” shown in figure 2).
FSV defines the full scale value of the parameter being measured in secondary units.  (The “10” shown in the equation in Figure 2 is 10A on the low side of the CTs.  That is, 5A full-scale times a transformer rating factor of 2.)
SF is the scaling factor (the CT Scale Factor in Figure 2).
 

Figure 2

 

Notice a couple things about the equation shown in Figure 2:
The equation is dynamic.  That is, the equation changes when you click on different DNP points in the point list above.  So the FSV term may be 10 for current, but 150 for voltage, and 4500 for power.  The SF term is the CT scale factor for current, the VT scale factor for voltage, and for power, SF if the CT scale factor times the VT scale factor.
Notice the equation is a direct proportion.  That is, it takes the form Y = m X.  There is no zero offset (such as would be in the form Y = m X + b).  So the zero offset (b) is zero, and the scale factor that you enter into SCADA (m) is given by:
m = FSV * SF / 32,768

 

So, using the term “m” (above) for the SCADA scale factor, the equation in the Configurator program becomes:  PEU = RV * m, which is exactly what SCADA expects to see. This equation will produce primary engineering units having the best possible resolution for a scaled 16-bit integer as long as the product of FSV * SF is close to the maximum value you ever expect the engineering units to have. The IED can never produce a measurement higher than FSV * SF, because RV can never be greater than 32,768.
 

Let’s work an example:

Say you’ve got a typical 13kV distribution substation station.  Your PT ratio is 70:1, and your CT ratio is 800:5.  The system is connected in a wye.  On a particular day the line-neutral voltage is 7,765V and the current is 670A.  Let’s say the system is well balanced and the power factor is 0.97 lagging so total watts, P = 15.75MW and total VArs, Q = 3.93MVAr.  In SCADA you want to read the line-neutral voltage, the current, watts and VArs.  How do you set up the Scale factors in the 70 Series IED and in your SCADA software?

 

On the Ratios page in the 70 Series Configurator program, set the CT ratio to 800 on all three phases making sure the radio button indicates 800 means 800:5 and not 800:1.  Set the VT ratio on all three phases to 70.  See Figure 1, top.

 

On the DNP Scaling Factors page in the 70 Series Configurator program, set the CT Scale factor to 160 (800/5=160 and the scale factor setting must be reduced).  Set the VT scale factor to 70.  See Figure 1, bottom.

 

Go to the DNP Points page in the Configurator and click on any of the points that are configured to represent amps (AI:2, 3, or 4 shown in Figure 2).  Look at the equation in the lower right corner of the page.  The equation tells you that the primary current, I = RV/32768 * 10 * 160.  (See how the Scale factor is identified as 160 immediately below the box?)  The scale factor to use in SCADA is:  10 * 160 / 32768 = 0.048828.  In other words, SCADA polls the IED, which reports point AI:2 is a raw DNP Integer of 13,722.  So SCADA needs to multiply 13,722 counts by the scale factor 0.048828 to get 670A.  If you prefer, and your SCADA software supports it, you could take the reciprocal of 0.048828 (which is 20.48) and set your SCADA software to divide the raw integer 13,722 by 20.48.  You would get the same results, 670A.  That’s it for Amps.

 

Volts is similar to Amps with just a couple slight differences.  When you click on the point AI:7 in the point list in the Configurator, the equation changes to RV/32768 * 150 * 70, where 150 is FSV and 70 is the VT scale factor.  You calculate your SCADA scale factor as 150 * 70 / 32768 = 0.320435.  Now when SCADA reads the IED, it reports AI:7 is 24,233 counts.  When SCADA multiplies 24,233 by 0.320435, you calculate the voltage as 7,767V which is right, but you probably prefer SCADA to display voltage in terms of kV.  To do that just requires you divide the SCADA scale factor for voltage by 1000.  So the scale factor becomes 0.000320.  Again, it is probably more convenient to take the reciprocal of the scale factor and set up SCADA to divide the raw DNP integer by 3,120.76 rather than to multiply by such a very small number.

 

The SCADA scale factors used for the Watts and VArs are both the same.  In this case, the equation in the Configurator changes the FSV to 4500 and the scale factor is now the product of  the CT scale factor times the VT scale factor.  When you’ve done that, the engineering units will be presented in terms of watts, and you probably prefer SCADA to read in terms of MW, so you will have to divide the scale factor that you calculate by one million, just as we divided the scale factor for voltage by 1000 to get it to be expressed in terms of kV in number 4, above.  For Watts, refer to Figure 3. 

 

Clicking on point AI:24 in the Configurator makes the equation change to:  RV/32,768 * 4,500 * 11,200 (because 11,200 = 70 * 160.)  So the scaling factor you use for SCADA is 4,500 * 11,200 / 32,768 which is 1,538.09.  You want the result in MW, so next divide that by one million to get 0.001538.  That’s a very small number, so taking the reciprocal gives you 650.159 .  Now just set up your SCADA to divide the raw DNP integer by 650.159 to get primary MW or MVAr.  Polling the IED produces the integer 10,238 for watts (AI:24) and 2,553 for VArs (AI:28).  So 10,238 / 650.159 = 15.75MW, and 2,553 /650.159 = 3.93MVAr, which is the answer.
 

Figure 3

 

In the next installment in this series, we’ll deal with situations when you want the CT or VT Scale factor to be different from the corresponding CT or VT Ratio.