Choosing test SCR
Background
When running plant-level studies which use a Thevenin equivalent representation of the grid, different tests can choose different values for the Thevenin equivalent impedance, namely:
- Option 1: Infinite SCR (i.e. Zthev ≈ 0 Ω), or
- Option 2: Site-secific SCR (i.e. Zthev ≠ 0 Ω)
This pages provides an explanation of how to choose the test SCR.
How to choose test SCR
To understand which option to choose, we first need to explain the difference between the options.
Option 1: Infinite SCR
Infinite SCR is a purely fictitious representation of the grid, where the grid is infinitely strong. The Thevenin equivalent source voltage is directly bonded to the plant connection point voltage.
- Pros: This option is useful when you want to 'playback' a particular voltage profile to the plant level connection point.
- Cons: This option provides a less accurate representation of the plant dynamic performance. The plant-level connection point voltage does not change based on connection point active and reactive power flow.
Option 2: Site-specific SCR
Site-specific SCR is a more accurate representation of the grid, where Zthev is calculated based on a site-specific SCR and X/R ratio. The SCR and X/R ratio are provided by the network operator using this calculation methodology. Often a network operator will provide minimum and maximum site-specific values so that the plant settings can be more accurately tuned to a range of local grid conditions.
- Pros: This option provides a more accurate representation of the plant dynamic performance. The plant-level connection point voltage changes based on connection point active and reactive power flow.
- Cons: Due to the voltage change across the Thevenin equivalent impedance, it is often difficult to 'playback' a particular voltage profile to the plant level connection point.
Choosing the SCR for your test
The following is a guide to help you choose between the two options described above.
| If you care about | Performance assessment examples | Location on the network | Use |
|---|---|---|---|
| Final value only | Q(V) droop curveReactive power capabilityVoltage ride-through protection setting verification | Not relevant | Infinite SCR |
| Dynamic response during the event | Fault ride-through capabilityIq rise time during faultSettling time of Vref steps | Relevant | Site-specific SCR |