Thevenin Equivalent Utility Impedances

[nightfox1925]

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.

 

[antigfk]

X1/R1 is the positive sequence reactance-to-resistance ratio. X0/R0 is the zero-sequence ratio. The fault currents you plug in to SKM are LLL (3-phase) and LG (single-line-to-ground). If the utility guys gave you good data, when you plug in the thevenin impedances in the utility component editor SKM should spit out the calculated fault currents. Check to see if these fault currents match the values you were given.

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.

 

[Kiribanda]

nightfox1925,

Why do you need Thevenin's driving point SC impedance at the common coupling point to run a load flow?

 

[redfurry]

Agree with Kiribanda. You don't normally want to use the impedance for load flow purposes.

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.

 

[nightfox1925]

Does this mean that for a Load Flow study, we should always consider 1.0 P.U. voltage at the in-feed or

unless the transmission line impedances are given and then we have to model them into our SLD model?

 

[redfurry]

For a load flow study, you need to know what the supply voltage is. It will not normally be 1.0 p.u. This is an additional piece of information the utility must provide to you along with the source impedance.

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.

 

[nightfox1925]

We putting up a new substation for a pump station facility. The new main transformer is a 10/13MVA, 25kV-4.16/2.4kV, Z=5.06% power transformer fed from a 25kV Utility Line.

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?

 

[redfurry]

The information you have been given is suitable for use in a fault study but is not adequate to help you determine the primary (25kV) supply voltage you will see at your station.

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.

 

[nightfox1925]

The problem we are facing is that this 3.7% 25kV line drop affects the input primary voltage of our transformer (96.3% 25kV nominal) and consequently drops the secondary voltage (including transformer impedance) of our pump station main transformer which is where we are getting the 4.16kV Pump Station power distribution. We can only adjust the tap changer up to -5% tap but we will have an excessive calculated overvoltage (104.8% nominal 4160V) if no motors are running.

 

[davidbeach]

First off, 104.8% is not an excessive overvoltage. Second, the actual voltage on the utility 25kV could well be over 26kV. Assume a 1.05 pu voltage from the utility and see how things work out. Some place the utility is regulating voltage based on load and you don't get the swings that studies often predict. I believe that you can turn off use of the source impedance during load flow studies in SKM as has been mentioned, that will give results more like what you will see in the field.

 

[nightfox1925]

I looked at one SKM training manual and it stated that if the "system impedance" check box is selected, the SKM program will use the system short circuit capacity to calculate the equivalent positive sequence impedance and the swing bus voltage drop is calculated given the total power supplied by the swing bus generator (with this calculated positive sequence impedance).

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.

 

[davidbeach]

If the utility says 25kV, they will deliver something between 23.75kV and 26.25kV most of the time. Most of the rest of the time the voltage will be between 22.5kV and 27.5kV. Out side that range the excursions will be very short, except for when it is zero volts.

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.