Electrical studies of an off-shore oil and gas industrial electrical system

[Ariel Tavares]

Hello, I am a beginner and I am trying to model an electrical system of an offshore oil and gas plant in order to perform power flow studies and short-circuit calculations. I need to model a VSD (variable speed driver) as a starting method for large motors and I have not found anything about this in the OpenDSS manual or documents.

Could someone help me with this question?

[Davis Montenegro]

Hello,

Well, this is because OpenDSS is a general purpose frequency domain simulator, meaning some specifics need to be implemented by the users normally using external interfaces such as TCP/IP (OpenDSS-G), COM/DLL or modify the code inside.
For that, let me ask:

1. What is the purpose of the simulation? Is it QSTS? Dynamics? Harmonics?
2. What's the simulation resolution (model scale, detail?)
3. What is he phenomena you are trying to observe?

Let's start with those questions to better understand your request.

Best regards,

[Ariel Tavares]

Hello Davis,

Thanks for your reply.

In response to your questions,

The purpose of the study is to check the voltage levels on the system busbars, check the load on transformers and cables, and analyze the current profile and maximum currents to size the equipment.

The simplified diagram of the system in question is attached.

[Davis Montenegro]

Hello,

In this case you are probably aiming at a QSTS simulation. For that, the VSD can be modeled as a load model 5 (rectifier) it is a simple model and works well for modeling rectifiers like the ones you find in electronic loads. Then, with DSS-G you can record the voltages at the buses during the simulation (see https://www.youtube.com/watch?v=gxBrClL0jLY ), which will record all the voltages during the simulation. Otherwise, to record other values you can use monitors to focus on specific areas of the circuit.

Best,

[Ariel Tavares]

Thank you very much Davis, this was very helpful.

Still regarding this project, I would like to clarify a doubt about generators.

This electrical system I sent in the attachment is an isolated system powered only by 3 turbo generators. How can I model this system using only the generators as the power source.

[Davis Montenegro]

Hi,

Sure, this is doable. In this case one of the generators will work as reference and the other will be model as "PV" nodes (generator model 3). We have examples for this type of implementation using transmission systems, but the same principles will work in this case, check it out:

https://sourceforge.net/p/electricdss/code/HEAD/tree/trunk/Version8/Distrib/Examples/NCIM/Xmission_System_Kundur2Area/

Best,

[Ariel Tavares]

Davis, Thanks for the tip

I would also like to ask a question about short-circuit studies with OpenDSS.

1- First, I would like to know what is the difference between the short-circuit current values shown in the Fault Study Interface and in the report generated by the "show fault" command. Is it Initial symmetrical short-circuit current Ik" or steady-state short-circuit current Ik or another type of short-circuit current?
2- Second, I would like to know which standard OpenDSS uses to perform short-circuit calculations, IEC 60909-0, IEC 61363-1 or ANSI/IEEE Std 551?

[Davis Montenegro]

Hello,

The fault study differs from a standard snapshot analysis that includes a fault. On one hand, the fault study follows a simplified approach, where the circuit model is energized by a small current injection at the connection point of each PCE. Each PCE is represented within the Y bus matrix by its Y primitive matrix. By injecting approximately 1 Amp at each PCE connection point, the resulting power flow solution reveals the short-circuit impedance (Zsc) at that location, as it can be derived from known parameters. Further details on this methodology can be found here:

https://opendss.epri.com/FaultStudyModeEquations1.html

Alternatively, a snapshot simulation introduces a very small impedance at the fault location. However, this method also incorporates the nonlinearities of the surrounding PCEs, which adds complexity to the fault analysis. Despite these additional factors, the results remain fairly consistent with those obtained from the fault study. Further details can be found here:

https://opendss.epri.com/DefiningFaults.html

I hope this helps.

Best,

[Ariel Tavares]

Thanks, Davis.
Those references were really helpful in understanding the fault study mode methodology.

However, I still have a question (forgive my ignorance, I'm a beginner electrical engineering student): what type of short-circuit current is provided by the fault study mode report? Comparing it to the short-circuit current graph from IEC 60909-0, what value would it be? Would it be Ik'' (initial symmetrical short-circuit current), ip (peak short-circuit current), or Ik (permanent short-circuit current)?

From the methodology in the link you shared, I understand that the values provided would be compatible with Ik. Am I right?

[Davis Montenegro]

This analysis is made in the frequency domain and there are no dynamics involved. The results you are seeing in the report are in the steady statem which may fall into the Ik range. However, OpenDSS does not follow a specific standard for this analysis, it is up to you to verify how close it is to the standards you are studying.

Best,