Determine the tap position of IEEE123 CASE!

[HuangWei]

Hello,i am studying how the regulator tap change after solve.I use Prof Kresting's book's method,but can't get the result as the opendss results show.I am just confused that what the secondary ct rating is.As i want to calculate the voltage drop in the compensator circuit,which is get by multiply the R+jX by the actual current of compensator. In the manual PDF,it said that the secondary rating of the compensator is 0.2A,but i can't get the result after calculating.The solving of IEEE123 test case shows that the reg1a tap is 6,how does OPENDSS get the tap position 6,i am very confused.

[Roger Dugan]

The 0.2 A rating is generally irrelevant. That is a typical design rating, but perhaps not universal. The CTRating property is relevant. When the line current reaches the value specfied for CTRating, you should expect the regulator to adjust the target voltage by the values in volts dialed into the R and X settings.

If you are running the typical 123-bus test feeder in the OpenDSS examples, OpenDSS changes the regulator tap one tap per control iteration until the voltage falls back into the specified band.

Some programs compute a tap that will get the voltage back to center band and assume it gets there. That's a different approach. Thus, you will see that Kersting computed tap 7 for that case while OpenDSS arrives at tap 6 because it stops when the voltage goes back into band.

Note that there are probably 3 possible tap positions that will yield a voltage within band. I would guess that in this case, they are 6, 7, and 8. What's the correct answer? Take your pick. I would suggest not designing a planning technique that requires the tap to be one or the other tap position because, it could end up on any one of the three depending on the voltage at previous time steps.

[HuangWei]

Thank for your quick reply.I just want to know what parameters cause the tap change,so I want to use the method to calculate the tap position as Mr Kresting's book told.But I can't get 6,even7 or 8.As i want to analyze the deep reason how the 4 regulators in IEEE123 case act,and i am very curious about how Mr Kresting calculate the tap position which i can't find in his book Distriution System modeling.

[HuangWei]

Actually i just want to know how opendss calculate the tap position,eg.in ieee123 test case,so i can use the method when load or other system condition changes,i can analyze the voltage variation.

[Roger Dugan]

I actually gave you the answer in my previous post. In OpenDSS the controls are adjusted outside the power flow solution loop. The tap is adjusted up or down until the voltage with the added voltage of the line drop compensator (LDC) is within band. Nothing magic. OpenDSS simulates it rather than computes it from some formula.

The pertinent code is found in RegControl.Pas (in the Controls folder). Here is the key snippet:

     // Check for LDC
     IF NOT UsingRegulatedBus and LDCActive Then
       Begin
            ControlledElement.GetCurrents(Cbuffer);
            ILDC  := CDivReal(CBuffer^[ControlledElement.Nconds*(ElementTerminal-1) + ControlledPhase], CTRating);
            If InReverseMode Then VLDC  := Cmul(Cmplx(revR, revX), ILDC) else VLDC  := Cmul(Cmplx(R, X), ILDC);
            Vcontrol := Cadd(Vcontrol, VLDC);   // Direction on ILDC is INTO terminal, so this is equivalent to Vterm - (R+jX)*ILDC
       End;

     Vactual := Cabs(Vcontrol);

     WITH   ControlledTransformer Do
       BEGIN
         // Check for out of band voltage
         IF (Abs(Vreg - Vactual) > Bandwidth / 2.0) Then TapChangeIsNeeded := TRUE
                                                    Else TapChangeIsNeeded := FALSE;

         If Vlimitactive Then
            If (Vlocalbus > Vlimit) Then TapChangeIsNeeded := TRUE;

         If TapChangeIsNeeded then
           BEGIN
                // Compute tapchange
                Vboost := (Vreg - Vactual);
                If Vlimitactive then If (Vlocalbus > Vlimit) then Vboost := (Vlimit - Vlocalbus);
                BoostNeeded      := Vboost * PTRatio / BaseVoltage[ElementTerminal];  // per unit Winding boost needed
                Increment        := TapIncrement[TapWinding];
                PendingTapChange := Round(BoostNeeded / Increment) * Increment;  // Make sure it is an even increment

See the calculation of ILDC and VLDC, which is added to Vcontrol. Vcontrol is initialized to the actual voltage measured by the PT and then VLDC is added to it (complex arithmetic -- Cadd function). Then if the voltage is outside the bandwidth the pending tap change is computed. The tap change may get modified later if MaxTapChange property is set to a smaller number, like 1, which I think it is if you ran the standed Run script for the IEEE 123 bus test case.

[HuangWei]

I learnt the pascal code,and i found the method in the code is like what I saw in Mr Kresting's book.I have few questions.
1.The parameter CTRating appears in the code,which I didn't find in opendss help.I only found the CTPrim,so how to calculate the ILDC.As far as i konw,the line current is transfered to the line compensator current by the CTprim/CTsecond.
2.I began to calculate the reg1a tap position of IEEE123test case,with the 3 other regulators set enabled=no,the simulation result of reg1a tap position is 6.First,the voltage and line current i used is power flow results with all 4 regulators set enabled=no,and i want to use voltage and line current of reg1a to get tap position(the voltage and current is without regcontrol),but i failed.

[HuangWei]

I think I can calculate the answer how opendss get.And when i calculate the result,i use the line current on the source terminal of regulator,for example,reg2a:buses=[9 9R],when i use the line current of bus9 or bus9R,the tap is a litter different of one.I think this is mainly due to the typical connection of regulator,which has typeA and typeB connection.With no regulator operating,the line current used in either terminal is approximately same.As you said above,6 or 7 or 8 is all OK,depending on what method you use.
And I found the secondary current rating of CT is irrelevant,because the secondary current rating can be eliminated during the process of calculating.
Thank Roger!

[Roger Dugan]

I'm glad you got it to work!

CTRating is the name of the variable in the code; CTPrim is the property name for the scripting language. We can change property names quite easily and they may not resemble variable names in the code.

[Ejike Ogada]

Hi,

I am running a voltage control study, and i am driving the dss through the COM interface in MATLAB.
My problem is, I have defined the RegControls to regulate voltage to a predefined Target of 1.0pu.

E.g New Transformer.T1A kVAs=[30000 30000] XHL=2.5
~ Wdg=1 Bus=100 Conn=Delta kV=132 %R=0
~ Wdg=2 Bus=302 Conn=Wye kV=33 numtaps=16 maxtap=1.15 mintap=0.95 %R=0 rneut=50 !~ %loadloss=1 %noloadloss=0 %imag=0
new regcontrol.creg1a transformer=T1A winding=2 vreg=120 band=2 ptratio=(33000 3 sqrt / 120 /) R=0.0001 X=0 bus=302 delay=15
Nominal tap to give 1.0 pu = 1.05
The idea is for the regcontrol to move to nominal tap, and I can vreg depending on the networkk conditions
However, when i run my code, I observe that the OLTC stays at the first converged Tap from snap solve throught out the simulation time of 60 minutes, in steps of one minute.

Is there anything wrong with the definition I have used for the regcontrol.
Thanks

[Roger Dugan]

You typically should not specify the Bus= property when it is the same bus the winding is connected to. Specifying Winding=2 is sufficient to control the transformer.

Some odd R and X settings. They're probably not doing anything, so omit them. (This is not the resistance and reactance of the control; look up documents on regulator controls.) Besides, these values get ignored when you specify Bus=302.

The 'Bus=' property is for regulating a remote bus.

Does the voltage on bus 302 vary more then +/- 1 V on a 120 V base? If not, the regulator tap won't change.

Does the transformer really have 16 taps between 0.95 and 1.15? I've seen some transformers with this range, but they had 32 taps over the whole range.

[LAU CHEIW YUN]

Hello All, I am studying the how the tap changer position move. The problem is the tap changer position doesn't move since I increase/decrease the load. The voltages will drop however the tap position still remain the same (either Max=16 or Min=-16),
Is it any wrong setting or error?

[Roger Dugan]

I think the PTRatio should be 52.9, not 20.

[LAU CHEIW YUN]

Thanks for the quick reply. By the way, how to get PTRation=52.9? I tried to follow Prof Kresting's book's method, but it still can't get the result.
I tried to vary the load, after PV injection, there is nothing happen on the tap position yet.

[Roger Dugan]

My bad. I didn't notice the transformer was delta. Should be =11000/.120

See how I modified your regulator definitions in the attached. Are the transformers really D-D?

The transformers default to tap from 0.9 to 1.1 in a total of 33 steps counting the neutral tap. If you're going to restrict the taps to a narrower range, you probably want to change the number of taps, too. Check the specs on your regulator.

[LAU CHEIW YUN]

Thank you for your kind help. It helps me. By the way, why should we put 11000/.120?

In our country, for the transformer in (33/11)kV side, is D-D.

However, for the LV side (11/0.4)kV, how do we fix the transformer tap? Because there is no OLTC for LV side in our country. If we need to fix the tap eg: 1.05% , how do we write in DSS format?

Thank you.