Line Drop Compensation
Line Drop Compensation
(OP)
We have 2000kft of 750 MCM (UG conductor) and 6000kft of 556 AL. It's a 3 phase, wye connected regulator set at a base of 114/110. Currently it's set to X=0, R=2 with a ct ratio of 400/.2 and pt of 120/1. It runs at 13.2kV. It's a 750/937 KVA SFR reg. After all was said and done, I calculted it out to be about X=15, R=4 for my compensation to produce the 114/110 V base where needed. My questions are:
Why does the regulator currently have more bucking than boosting and allow people to have low voltage at heavy loads with even a R=2 volt line compensation?
Why does everyone say that we shouldn't set X compensation? There is no straight answer other than "it's too hard to understand" and "because of cap banks fuses blowing we try to avoid that due to t he sensitivity of the X compensation".
Any help would be greatly appreciated.
Why does the regulator currently have more bucking than boosting and allow people to have low voltage at heavy loads with even a R=2 volt line compensation?
Why does everyone say that we shouldn't set X compensation? There is no straight answer other than "it's too hard to understand" and "because of cap banks fuses blowing we try to avoid that due to t he sensitivity of the X compensation".
Any help would be greatly appreciated.






RE: Line Drop Compensation
We have seen systems where the substation voltage was 13.8 kV, the regulator nominal voltage was 13.2 kV and the actual feeder voltage was 12.47, so the regulator is always in buck mode regardless of the load.
We also see a lot of regulators with NO compensation of any kind, because it's easier.
If the regulator has "First House" protection turned on, this will limit the amount of voltage boost regardless of the compensation settings.
What type of control do you have?
RE: Line Drop Compensation
RE: Line Drop Compensation
RE: Line Drop Compensation
But I'm still wondering why they would shy away from setting the X compensation? Any thoughts?
RE: Line Drop Compensation
I assume that by a 114/110 setting, you mean a voltage setpoint of 112 with a 2 volt bandwidth.
With 13.2 kV at the station (110 volts), you would need to boost the voltage at the station to allow for voltage drop on the line.
If you set X=0, you will undercompensate for the line voltage drop and get lower load voltages. The load voltage will vary depending on the load power factor because the regulator setting X/R ratio is different than the line X/R ratio.
RE: Line Drop Compensation
You're suggesting not setting X is going to undercompensate for the line loss. Is there any way to raise the R to compensate for that? I'm told the X setting is too sensitive when cap bank fuses blow on the line.
RE: Line Drop Compensation
The capacitor bank will change the power factor of the line load, not the line impedance. If the R and X settings match the line impedance, then the load drop compensation will be correct regardless of the power factor (assuming that the capacitor bank is at the load center). You could raise R to compensate for setting X=0 for a particular load, but it would not give correct compensation for other loads. I don't see any reason for setting X=0. Can you get "everyone" to explain what capacitor fuses have to do with the X setting?
RE: Line Drop Compensation
I started this post simply because the things I'm being told was going against everything I thought and just wanted to get outside views on it because the answers I'm getting aren't making sense.
The initial setting though is a 114 volt setting on a 110 base with a 1.5 bandwith. You were right about 2000 ft instead of kft, sorry.
If there's anything else that you guys can say or suggest so that I can convince them to set the X value, that would be greatly appreciated. And thanks for the note about the K value.
RE: Line Drop Compensation
The voltage drop changes as the power factor of the load changes and you need both R and X settings to get it to operate correctly.
"First House" protection is found on newer regulator controls and allows a maximum voltage limit to be set that will override the Line Drop Compensation to avoid overvoltage at loads close to the substation.
RE: Line Drop Compensation
What is the system Ø-Ø and Ø-N voltage?
RE: Line Drop Compensation
Some manufactures want the X and R values in ohms, some use some cryptic pseudo-P.U. thing that is impossible to figure out on your own.
The reason why the X bit is left out is becasue it is 'hard' to compute properly. As already mentioned, this is almost always the dominant factor for compensation, and if you do not apply it correctly, you will get wrong results. It is for this reason that some POCOs avoid it altogether.
But it IS worth the trouble to accurately model your line impedances/capacitances to get your X values. If you follow consistant construction standards, you only have to work this out once.
RE: Line Drop Compensation
Strategies for R and X settings vary greatly and some examples were presented in earlier postings. If you have low (not just lower) service voltage on an express feeder during high-load times you can either raise the base voltage setting normally, increase the R and/or X compensation settings, add capacitors close to the load, or add an additional stage of regulation near the load. Recognize the X setting for overhead lines is a function of phase conductors size and spacing, if the line has several different construction types in the overhead segments you need to take the time to make detailed calculations of the X component of the impedance. Also, load power factor does affect the regulator output voltage if the X compensation is set to something other than 0 volts, something which was incorrectly stated earlier. So, large point loads with large reactive load swings or switched capacitor banks can affect your voltage regulator's output voltage if you use X compensation - something not many people want to do as there are too many variables to try and troubleshoot when you have a problem. Ask one of your peers if they have a copy of GE's old Omnitext for voltage regulation - the discussion of Line Drop Compensation should clear up any additional questions you might have.
RE: Line Drop Compensation
It may be simple to set X=0, but it isn't a very good approximation of the line reactance. The load voltage will change with load swings if you set X=0 (or any other bad approximation of the line reactance). If you want to use load drop compensation, why not set it so that it works as designed? There are good arguments for setting R=0 and X=0 with the setpoint voltage equal to the maximum allowable. With these settings, you will attain maximum possible voltage at all customers, but will have large voltage swings at the load center.
RE: Line Drop Compensation
jghrist - 2.5 volts does not look like a large voltage swing at the load center. Setting any level of reactive line drop compensation can cause voltage regulators to do unintended things from improperly operating capacitor banks for example. This causes service voltage imbalances which as we all know cause motor performance issues. That's one historical reason why many utilities don't use it; cap banks don't get maintained, fuses blow, oil switches hang up or fail and the results are service voltage complaints. At least that's been my experience.
RE: Line Drop Compensation