Thevenin Equivalent Utility Impedances
Thevenin Equivalent Utility Impedances
(OP)
I have received a Utility Data for me to model the Utility in the SKM program.
The Data gave me the LLL, LL and LG maximum utility fault contribution. It also specified the "Thevenin Impedance" in X1, R1 and Xo, Ro at 100MVA utility Base. I assume that these impedances are in P.U.
However, if I take the X/R ratio say X1/R1, it equates to 1.2 which is very unlikely for a utility line..so I think I'm not understanding something right.
I put these values on my SKM editor for the Utility and chose the program to consider utility impedance in the calculation and it gave me a 3.7% voltage drop (load flow) at the utility end which is going to cause me a lot of trouble in my main transformer secondary. What I am doubting is if I am using these "Thevenin Impedances" correctly.
The Data gave me the LLL, LL and LG maximum utility fault contribution. It also specified the "Thevenin Impedance" in X1, R1 and Xo, Ro at 100MVA utility Base. I assume that these impedances are in P.U.
However, if I take the X/R ratio say X1/R1, it equates to 1.2 which is very unlikely for a utility line..so I think I'm not understanding something right.
I put these values on my SKM editor for the Utility and chose the program to consider utility impedance in the calculation and it gave me a 3.7% voltage drop (load flow) at the utility end which is going to cause me a lot of trouble in my main transformer secondary. What I am doubting is if I am using these "Thevenin Impedances" correctly.






RE: Thevenin Equivalent Utility Impedances
It has happened to me several times that the utility gave me thevenin impedances and fault currents that didn't match up. You might need to explain your situation to the utility and ask them politely to recalculate the data and give it back to you. FYI, in my experience most utility X/R ratios 230kV and below has been between 8 and 25.
RE: Thevenin Equivalent Utility Impedances
Why do you need Thevenin's driving point SC impedance at the common coupling point to run a load flow?
RE: Thevenin Equivalent Utility Impedances
For analysis purposes the utility source is usually specified as a voltage and a Thevenin impedance.
The voltage level at the source is normally used as a fixed voltage source (slack bus) for load flow purposes.
The Thevenin impedance is normally only considered for fault calculations.
Agree that an X/R of 1.2 is very unusual. Sounds like bad data from the utility.
RE: Thevenin Equivalent Utility Impedances
unless the transmission line impedances are given and then we have to model them into our SLD model?
RE: Thevenin Equivalent Utility Impedances
Again, generally you will not use the transmission system impedances unless you want to actually model the transmission system itself, which you would only want to do if you know for some reason that the system is especially weak and the supply voltage at your location will be strongly affected by load variations in your study. It's usually reasonable to assume that the voltage at the start (source) point of the study is fixed regardless of loading.
Maybe if you were to give some details on the extent of your study (type of system, total load, voltages, etc) further advice could be provided.
RE: Thevenin Equivalent Utility Impedances
The Utility provided us these information,
System Information At the Transfromer Primary (our 10/13MVA transformer):
Voltage : 25kV
Fault Current LLL: 10.343kA
Fault Curent LL: 8.957kA
Fault Current LG: 3.526kA (0 ohm)
Fault Current LG: 667A (20 ohm)
System Thevenin Impedance at 100MVA base,
R1: 0.1475 P.U.
X1: 0.1684 P.U.
R0: 0.3065 P.U.
X0: 1.5386 P.U.
These values were entered as Utility Data in the SKM's Utility Editor and a Load Flow was simulated with the Utility System Impedance included (this is an option in the LF study editor). To our surprise, there is a 3.7% Vd at our new transformer primary side (25kV bus).
Any comments?
RE: Thevenin Equivalent Utility Impedances
You do not want to use the 'Include Source Impedance' option on your load flow.
The only way to determine what voltage levels you will see at 25kV is to model the utility 25kV distribution feeder including major loads and any voltage regulation such as cap banks, regulators or station LTC.
The Thevenin impedance you are using for calculations represents the impedances of the 25kV feeder, the impedance of the substation transformer and the impedance of the transmission system behind that.
Saying there is a nominal 3.7% voltage drop between a power plant or plants 100's of miles away and your local 25kV supply point doesn't really tell you anything unless you know the voltage setting on the power plant generators is nominal and assume that every substation between them and you is set to a nominal tap position! (and there's no other load on the entire grid!)
The 3.7% drop is probably an accurate calculation it's just not meaningful for anything.
Realistically, if you're adding 10MW of load at 25kV it shouldn't cause much of a voltage problem along the feeder unless the feeder is very long (i.e 20km+) or is already heavily loaded.
RE: Thevenin Equivalent Utility Impedances
RE: Thevenin Equivalent Utility Impedances
RE: Thevenin Equivalent Utility Impedances
The problem is, at what sending voltage does the program see. I think it is looking on a 1.0 P.U. since this is what is modelled as a utility data in the program.
The problem of increasing this P.U. value is that it might affect the short circuit value as well unless the SKM can be set with different utility P.U. voltage between load flow and short cirucit calculations...any comments from long time SKM users? (I am more familiar with ETAP).
The other problem is that it might be a struggle for us to get the Utility Sending Voltage.
RE: Thevenin Equivalent Utility Impedances
Frankly you should be able to handle the load flow at 22.5kV and the fault current at 27.5kV. If either of those give you a problem, you need to change your design.